Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Thiosulfate degradation during gold leaching in ammoniacal thiosulfate solutions : a focus trithionate Ahern, Noelene 2005

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-ubc_2005-104600.pdf [ 10.24MB ]
Metadata
JSON: 831-1.0078764.json
JSON-LD: 831-1.0078764-ld.json
RDF/XML (Pretty): 831-1.0078764-rdf.xml
RDF/JSON: 831-1.0078764-rdf.json
Turtle: 831-1.0078764-turtle.txt
N-Triples: 831-1.0078764-rdf-ntriples.txt
Original Record: 831-1.0078764-source.json
Full Text
831-1.0078764-fulltext.txt
Citation
831-1.0078764.ris

Full Text

THIOSULFATE DEGRADATION DURING G O L D LEACHING IN AMMONIACAL THIOSULFATE SOLUTIONS : A F O C U S ON TRITHIONATE  By NOELENE AHERN B . S c , University of Natal, 1994 B.Sc. (Hons), University of Natal, 1995 M . A . S c , University of Cape Town, 1997  A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Faculty of Graduate Studies (Materials Engineering) The University of British Columbia  October 2005  © Noelene Ahern, 2005  ABSTRACT Thiosulfate h a s s h o w n c o n s i d e r a b l e p r o m i s e a s a n alternative to c y a n i d e for leaching.  H o w e v e r , o n e of the m a i n limitations of the thiosulfate s y s t e m is the high  c o n s u m p t i o n of thiosulfate. products  gold  of  thiosulfate  B e s i d e s i n c r e a s i n g the c o s t of the p r o c e s s , the d e g r a d a t i o n  have  been  claimed  to  passivate gold  surfaces and  the  polythionates often p r o d u c e d a r e l o a d e d onto r e s i n s p r o p o s e d for gold r e c o v e r y .  T h e thiosulfate d e g r a d a t i o n p r o c e s s is not c o m p l e t e l y u n d e r s t o o d .  O f the d e g r a d a t i o n  p r o d u c t s , trithionate is a c o n c e r n in the resin r e c o v e r y of gold a n d is persistent in gold l e a c h solutions.  V e r y little is k n o w n a b o u t the e x p e c t e d b e h a v i o u r of trithionate,  both  with r e s p e c t to its formation a n d its interaction with other solution s p e c i e s .  T h e f o c u s of this work w a s therefore to further the u n d e r s t a n d i n g of the b e h a v i o u r of trithionate in gold l e a c h solutions.  E x p e r i m e n t a l work w a s c a r r i e d out to d e t e r m i n e the  kinetics of trithionate d e g r a d a t i o n in s y s t e m s r e s e m b l i n g gold l e a c h i n g s o l u t i o n s , a n d a kinetic m o d e l w a s d e r i v e d for trithionate d e g r a d a t i o n .  T h e rate of d e g r a d a t i o n of  trithionate in a q u e o u s a m m o n i a c a l solutions w a s e x p r e s s e d by E q u a t i o n 1.  -d[S 0 -]/dt = (k [NH ] + k [NH ] + k,[OHl + k )[S O T 2  3  +  6  3  where k = 0.012 h"\ 0  4  [1]  2  2  = 0.74  3  0  IvV.h;, k 1  3  6  = 0 . 0 0 4 9 M " . h ' , k = 0.01 M " . ^ . 1  2  1  1  1  3  In s o m e c a s e s , the p r e s e n c e of lower c o n c e n t r a t i o n s of thiosulfate c a t a l y s e d the reaction while e x c e s s thiosulfate inhibited it.  H o w e v e r , u n d e r typical gold l e a c h i n g c o n d i t i o n s ,  thiosulfate w a s not e x p e c t e d to h a v e a significant effect s o w a s e x c l u d e d from E q u a t i o n 1.  C u p r i c c o p p e r w a s not f o u n d to h a v e a n y significant effect o n the rate of trithionate  d e g r a d a t i o n u n d e r the conditions t e s t e d .  T h i s o b s e r v e d trithionate d e g r a d a t i o n rate e q u a t i o n w a s integrated with k n o w n kinetic b e h a v i o u r of thiosulfate a n d tetrathionate b a s e d o n literature findings to d e v e l o p a n overall m o d e l for the thiosulfate d e g r a d a t i o n a n d the resulting solution s p e c i a t i o n of the sulfur o x y a n i o n s in the a b s e n c e of o r e s . T h e m o d e l w a s e v a l u a t e d a g a i n s t e x p e r i m e n t a l d a t a a n d its s h o r t c o m i n g s w e r e identified.  ii  The model parameters were adjusted to obtain a best fit to the experimental data. It was found that the best-fit parameters varied with the experimental conditions, indicating inadequacies in the model. The main concern was that the understanding of the thiosulfate degradation reactions is limited, and the way in which thiosulfate degradation was described had a major impact on the model output.  In particular, the effects of  copper species and pH on thiosulfate degradation have not been adequately addressed in the literature. Even taking into consideration the limitations of the model, based on the model output, decreasing the cupric concentration and increasing the ammonia concentration should help to minimise thiosulfate degradation. Solution recycle can also be used to minimise thiosulfate degradation but can result in a build up of trithionate. Limiting the reaction time would also be useful. This investigation has led to an improved understanding of the behaviour of trithionate in gold leach solutions and the model of the thiosulfate degradation system is a first step in developing a useful assessment method for thiosulfate degradation and solution speciation under gold leaching conditions. Further research is required to refine the model, particularly with respect to thiosulfate degradation to trithionate and tetrathionate.  iii  TABLE OF CONTENTS Abstract  "  T a b l e of C o n t e n t s . . . .  iv  List of T a b l e s  viii  List of F i g u r e s  x  List of S y m b o l s a n d A b b r e v i a t i o n s  xviii  Acknowledgements  xix  1. Introduction  1  2. Literature Review  3  2.1  Introduction  3  2.2  T h e a m m o n i a c a l thiosulfate gold l e a c h i n g s y s t e m  3  2.3  Structure a n d t h e r m o d y n a m i c properties of the sulfur o x y a n i o n s  5  2.4  T h i o s u l f a t e d e g r a d a t i o n in g o l d l e a c h i n g - e x p e r i m e n t a l o b s e r v a t i o n s  12  2.5  Thiosulfate chemistry and fundamental studies  18  2.6  2.7  2.5.1  O x i d a t i v e d e g r a d a t i o n in g o l d l e a c h i n g s y s t e m s  18  2.5.2  Disproportionation a n d reductive d e g r a d a t i o n of thiosulfate  32  2.5.3  T h i o s u l f a t e d e g r a d a t i o n inhibitors  33  Trithionate d e g r a d a t i o n  34  2.6.1  Interaction with w ater  35  2.6.2  Interaction with hydroxide  36  2.6.3  Interaction with a m m o n i a  36  2.6.4  Interaction with thiosulfate  36  2.6.5  Interaction with c o p p e r  37  Tetrathionate d e g r a d a t i o n  41  2.7.1  Interaction with hydroxide  41  2.7.2  Interaction with a m m o n i a  43  2.7.3  Interaction with c o p p e r  44  2.7.4  Interaction with thiosulfate  44  2.8  R e m o v a l of polythionates from solution  47  2.9  S u m m a r y of literature findings  48  2 . 1 0 S c o p e a n d objectives  49  iv  3. Analytical Methods and Synthesis  50  3.1  Introduction  50  3.2  A n a l y s i s of sulfur o x y a n i o n s - ion c h r o m a t o g r a p h y  50  3.2.1  D e s c r i p t i o n of m e t h o d  50  3.2.2  Stability of s t a n d a r d solutions  51  3.2.3  Effect of other solution c o m p o n e n t s o n ion c h r o m a t o g r a p h i c a n a l y s i s . 5 3  3.3  A n a l y s i s of s u l f a m a t e  54  3.4  A n a l y s i s of total a m m o n i a  55  3.5  S y n t h e s i s of s o d i u m trithionate  55  3.6  C h a r a c t e r i s a t i o n of s o d i u m trithionate  56  3.7  3.6.1  T o t a l sulfur  56  3.6.2  T o t a l s o d i u m content  57  3.6.3  Volatiles  57  3.6.4  Sulfate  58  3.6.5  Polythionates  59  3.6.6  O v e r a l l trithionate purity  61  T r a c e impurity a n a l y s i s of c h e m i c a l s u s e d  4. Kinetics of Trithionate Degradation - Methodology  62  63  4.1  Introduction  63  4.2  R e a c t i o n kinetics theory  63  4.3  Experimental method  64  4.4  Data analysis  66  5. Kinetics of Trithionate Degradation - Results  72  5.1  Introduction  72  5.2  Stoichiometry  72  5.3  Reproducibility  74  5.4  Water  75  5.5  Hydroxide  77  5.6  Ionic strength  79  5.7  Carbonate and bicarbonate  82  5.8  Ammonium and ammonia  83  v  5.9  pH  88  5.10 T h i o s u l f a t e  93  5.11 O x y g e n e x c l u s i o n  95  5.12 C u p r i c c o p p e r  97  5 . 1 3 Tetrathionate  99  5.14 E l e m e n t a l sulfur, sulfate a n d c o p p e r p o w d e r  100  5.15 T e m p e r a t u r e  101  6. Kinetics of Trithionate Degradation - Discussion and Modelling  104  6.1  Qualitative d i s c u s s i o n  104  6.2  M o d e l l i n g of trithionate d e g r a d a t i o n  108  7. Modelling of Sulfur Oxyanion Speciation During Thiosulfate Degradation.... 119 7.1  Introduction  119  7.2  Model setup  119  7.2.1  R1 - T h i o s u l f a t e d e g r a d a t i o n to tetrathionate  120  7.2.2  R 2 - Tetrathionate d e g r a d a t i o n  121  7.2.3  R 3 - Trithionate d e g r a d a t i o n  122  7.2.4  R 4 - T h i o s u l f a t e d e g r a d a t i o n to trithionate  123  7.2.5  R 5 - T h i o s u l f a t e d e g r a d a t i o n directly to sulfate  125  7.2.6  R 6 - T h i o s u l f a t e d e g r a d a t i o n to sulfide  126  7.2.7  Incorporation of the rate e q u a t i o n s into a m o d e l - m e t h o d and constraints..  7.3  7.4  126  M o d e l sensitivity to m o d e l p a r a m e t e r s  127  7.3.1  P r o p o r t i o n of thiosulfate forming tetrathionate v e r s u s trithionate  129  7.3.2  R a t e of reaction R1 - thiosulfate d e g r a d a t i o n to tetrathionate  133  7.3.3  R a t e of reaction R 4 - thiosulfate d e g r a d a t i o n to trithionate  134  7.3.4  R a t e of reaction R 2 - tetrathionate d e g r a d a t i o n  135  7.3.5  R a t e of reaction R 3 - trithionate d e g r a d a t i o n  137  7.3.6  S u m m a r y - effect of m o d e l p a r a m e t e r s  138  C o m p a r i s o n of m o d e l output with e x p e r i m e n t a l results in the a b s e n c e of ore  139  7.4.1  Experiments  139  7.4.2  Validation method  140  vi  7.4.3  T e s t 1 - no c o p p e r  142  7.4.4  T e s t s 2 a a n d 2 b - low c o p p e r , p H 10  143  7.4.5  T e s t 3 - low c o p p e r , p H 9, low a m m o n i a  148  7.4.6  T e s t 4 - low c o p p e r , p H 9, higher a m m o n i a  151  7.4.7  T e s t 5 - h i g h c o p p e r , p H 10  152  7.4.8  Summary  155  7.5  C o m p a r i s o n of e x p e r i m e n t a l results with a n d without ore  159  7.6  S c o p e of u s e of m o d e l  163  7.7  Impact of solution conditions  163  7.7.1  C o p p e r concentration  164  7.7.2  Total a m m o n i a concentration  167  7.7.3  pH  170  7.7.4  Dissolved oxygen  172  7.7.5  Thiosulfate concentration  173  8.  Conclusions  180  9.  Recommendations  187  10.  References  189  A p p e n d i x 1: T h e r m o d y n a m i c v a l u e s u s e d to construct E h - p H d i a g r a m s  199  A p p e n d i x 2: Thiosulfate d e g r a d a t i o n - literature d a t a  200  A p p e n d i x 3: Determination of sulfur o x y a n i o n s by ion c h r o m a t o g r a p h y  205  A p p e n d i x 4 : C h e m i c a l impurity a n a l y s i s  208  vii  LIST O F T A B L E S  2.1  S e l e c t e d G i b b s free e n e r g i e s of formation (298 K ) for s p e c i e s of interest in the c o p p e r - a m m o n i a c a l - t h i o s u l f a t e s y s t e m  11  2.2  S e l e c t e d s t a n d a r d oxidation potentials (298 K )  11  2.3  S u m m a r y of e x p e r i m e n t a l o b s e r v a t i o n s for thiosulfate d e g r a d a t i o n  13  2.4  Literature d a t a for trithionate a n d tetrathionate production  16  2.5  F o r m a t i o n of trithionate from thiosulfate in the p r e s e n c e of o x y g e n ( o x y g e n p r e s s u r e 7 2 5 m m H g , 3 0 °C, [Cu(ll)] = 1 m M , [NH ] = 0.2 M )  26  2.6  Trithionate d e g r a d a t i o n in water  38  2.7  Trithionate d e g r a d a t i o n in a m m o n i a  39  2.8  Trithionate d e g r a d a t i o n in the p r e s e n c e of thiosulfate  40  2.9  Tetrathionate d e g r a d a t i o n in alkaline solutions  46  2.10  Potential m e t h o d s for polythionate r e m o v a l  47  3.1  S u m m a r y of o b s e r v a t i o n s o n stability of s t a n d a r d s o l u t i o n s of  3  thiosulfate, trithionate a n d / o r tetrathionate  52  3.2  Effect of a d d e d s p e c i e s o n a n a l y s i s of sulfur o x y a n i o n s  54  3.3  S o d i u m trithionate characterisation  61  5.1  Reproducibility in rate constant determination for trithionate d e g r a d a t i o n for two buffer s y s t e m s using the initial rate m e t h o d  5.2  V a l u e s for the o b s e r v e d rate constant k b for trithionate 0  S  d e g r a d a t i o n in w a t e r u s i n g the initial rate m e t h o d (40 °C) 5.3  76  Effect of h y d r o x i d e concentration o n the o b s e r v e d rate constant k  5.4  75  o b s  f o r trithionate d e g r a d a t i o n using the initial rate m e t h o d (40 °C)  V a l u e s f o r the o b s e r v e d rate c o n s t a n t k  o b s  77  for trithionate d e g r a d a t i o n  in the p r e s e n c e of v a r i o u s salts u s e d to adjust t h e ionic strength using the initial rate m e t h o d 5.5  Effect of a m m o n i u m concentration o n o b s e r v e d rate constant k  5.6  80  o b s  for trithionate d e g r a d a t i o n  88  Effect of thiosulfate o n the o b s e r v e d rate c o n s t a n t k b f o r 0  S  trithionate d e g r a d a t i o n using the integrated rate m e t h o d  viii  94  5.7  Effect of limiting d i s s o l v e d o x y g e n o n the o b s e r v e d rate constant k  5.8  o b s  for trithionate d e g r a d a t i o n in 0.1 M h y d r o x i d e solution  95  Effect of limiting d i s s o l v e d o x y g e n o n the o b s e r v e d rate c o n s t a n t k  o b s  for trithionate d e g r a d a t i o n a m m o n i a / a m m o n i u m b i c a r b o n a t e  solution 5.9  96  Effect of limiting o x y g e n o n the o b s e r v e d rate c o n s t a n t k  o b s  for  trithionate d e g r a d a t i o n using the integrated rate m e t h o d 5.10  97  Effect of c u p r i c addition o n the o b s e r v e d rate c o n s t a n t k b for 0  S  trithionate d e g r a d a t i o n using the integrated rate m e t h o d 5.11  97  Effect of c u p r i c c o p p e r o n the o b s e r v e d rate c o n s t a n t k b for 0  S  trithionate d e g r a d a t i o n 5.12  Effect of e l e m e n t a l sulfur, sulfate a n d c o p p e r p o w d e r o n t h e o b s e r v e d rate constant k  5.13  98  o b s  for trithionate d e g r a d a t i o n  100  Effect of t e m p e r a t u r e o n the o b s e r v e d rate constant k b for 0  S  trithionate d e g r a d a t i o n at p H 8.8 - 10.1 5.14  102  Effect of temperature o n the o b s e r v e d rate c o n s t a n t k  o b s  for  trithionate d e g r a d a t i o n using the integrated rate m e t h o d  103  7.1  M o d e l p a r a m e t e r s u s e d to test m o d e l sensitivity  128  7.2  S t a n d a r d e x p e r i m e n t a l p a r a m e t e r s u s e d in modelling  129  7.3  E x p e r i m e n t a l conditions u s e d in tests for m o d e l validation  140  7.4  A d j u s t m e n t of m o d e l p a r a m e t e r s f o u n d to give i m p r o v e d a g r e e m e n t b e t w e e n m o d e l output a n d e x p e r i m e n t a l d a t a  156  7.5  A s s a y results for o r e from P l a c e r D o m e u s e d in l e a c h tests  159  7.6  Effect of recycling o n thiosulfate c o n s u m p t i o n a n d solution s p e c i a t i o n (Initial conditions 0.2 M S 0 * , 3 0 mg/l C u , 0.4 M N H , 2  2  3  3  2 5 °C, p H 10, with m o d e l p a r a m e t e r s s e t a s in T a b l e 7.2 for T e s t 2 ) . . . . 1 7 8 7.7  Effect of recycling o n thiosulfate c o n s u m p t i o n a n d solution s p e c i a t i o n (Initial conditions 0.2 M S 0 " , 100 mg/l C u , 0 . 3 M N H , 2  2  3  3  2 5 °C, p H 10, with m o d e l p a r a m e t e r s set a s in T a b l e 7.2 for T e s t 5 ) . . . . 179  ix  LIST OF FIGURES 2.1  E h - p H d i a g r a m for the a q u e o u s sulfur s y s t e m ( 1 M sulfur, 2 5 °C)  6  2.2  B a s i c structures of s e l e c t e d sulfur o x y a n i o n s  7  2.3  E h - p H d i a g r a m for the a q u e o u s sulfur s y s t e m with the following s p e c i e s omitted: S 0 " , H S 0 " , H S 0 4 . x H 0 ( 1 M sulfur, 2 5 °C) 2  4  2.4  4  2  2  9  E h - p H d i a g r a m for the a q u e o u s sulfur s y s t e m with the following s p e c i e s omitted: S 0 " , H S 0 " , H S 0 4 . x H 0 , S 0 , S 0 " , H S 0 " , 2  2  4  4  S 0 " , HSOsf, S 0 - , S 0 2  2  2  6  2  8  2  2 5  2  2  3  3  3  ' ( 1 M sulfur, 2 5 °C)  10  2.5  B i m o l e c u l a r n u c l e a r substitution at the sulfenyl sulfur of trithionate  35  3.1  T G A / D T A for s o d i u m trithionate batch 2  58  3.2  T G A / D T A for s o d i u m trithionate batch 3  58  3.3  C h a n g e of m e a s u r e d trithionate (indicative v a l u e s only) a n d tetrathionate c o n c e n t r a t i o n s with time with a n d without u s i n g d e a e r a t e d w a t e r in solution preparation  3.4  60  C h a n g e of m e a s u r e d trithionate (indicative v a l u e s only) a n d tetrathionate c o n c e n t r a t i o n s with time in the p r e s e n c e of 2 m M NH OH  60  4  3.5  C h a n g e of m e a s u r e d trithionate (indicative v a l u e s only), tetrathionate a n d thiosulfate c o n c e n t r a t i o n s with time in the p r e s e n c e of 10 mg/l S 0 2  4.1  2 3  " a n d 10 mg/l S 0 4  2 6  "  61  C o n c e n t r a t i o n s of trithionate a n d thiosulfate with time for a n integrated rate m e t h o d test  4.2  67  Determination of reaction stoichiometry u s i n g the integrated rate method  4.3  68  T y p i c a l c o n c e n t r a t i o n profile for trithionate a n d thiosulfate u s i n g the initial rate m e t h o d  4.4  69  D e t e r m i n a t i o n of the reaction order for the rate of trithionate d e g r a d a t i o n with r e s p e c t to trithionate  4.5  70  T y p i c a l plot of trithionate d e g r a d a t i o n rate v e r s u s initial trithionate concentration to d e t e r m i n e the rate c o n s t a n t k b f r o m 0  the s l o p e  S  71  x  5.1  C o n c e n t r a t i o n profiles for trithionate a n d thiosulfate  74  5.2  O b s e r v e d rate c o n s t a n t for trithionate d e g r a d a t i o n k b v e r s u s [OHT 0  S  for pH<11 5.3  79  Effect of ionic strength o n o b s e r v e d rate c o n s t a n t k  o b s  for  trithionate d e g r a d a t i o n u s i n g t h e initial rate m e t h o d 5.4  Effect of ionic strength of N a H C 0 rate c o n s t a n t k  o b s  / Na C0  3  2  3  82  buffer o n t h e o b s e r v e d  for trithionate d e g r a d a t i o n u s i n g the initial rate  method 5.5  83  Effect of a m m o n i u m c o n c e n t r a t i o n o n the o b s e r v e d rate c o n s t a n t k b 0  S  for trithionate d e g r a d a t i o n u s i n g t h e initial rate m e t h o d 5.6  84  Effect of a m m o n i a c o n c e n t r a t i o n o n the o b s e r v e d rate c o n s t a n t k  o b s  for trithionate d e g r a d a t i o n u s i n g t h e initial rate m e t h o d 5.7  85  Effect of a m m o n i u m c o n c e n t r a t i o n o n t h e o b s e r v e d rate c o n s t a n t k b 0  for trithionate d e g r a d a t i o n for ( N H ) S 0 / N H a n d N H H C 0 4  NH 5.8  3  2  4  3  4  3  S  /  buffer s y s t e m s u s i n g t h e initial rate m e t h o d  D e p e n d e n c y o f the o b s e r v e d rate c o n s t a n t k  o b s  87  for trithionate  d e g r a d a t i o n o n the a m m o n i u m c o n c e n t r a t i o n at v a r i o u s p H 5.9  D e p e n d e n c y of the o b s e r v e d rate c o n s t a n t k  o b s  for trithionate  d e g r a d a t i o n o n the a m m o n i a c o n c e n t r a t i o n at v a r i o u s p H 5.10  O b s e r v e d rate c o n s t a n t k  o b s  O b s e r v e d rate c o n s t a n t k  o b s  91  for trithionate d e g r a d a t i o n v e r s u s p H  at25°C 5.12  91  Effect of p H o n o b s e r v e d rate c o n s t a n t k  o b s  for trithionate d e g r a d a t i o n  in s o d i u m c a r b o n a t e / b i c a r b o n a t e m e d i u m 5.13  Effect of thiosulfate o n the o b s e r v e d rate c o n s t a n t k  92 o b s  for  trithionate d e g r a d a t i o n using the initial rate m e t h o d 5.14  93  Effect of sulfur, sulfate o r c o p p e r p o w d e r o n trithionate c o n c e n t r a t i o n profile  6.1  89  for trithionate d e g r a d a t i o n v e r s u s p H  for a r a n g e of a m m o n i a / a m m o n i u m c o n c e n t r a t i o n s 5.11  89  101  C o n c e n t r a t i o n profiles for a m m o n i a , a m m o n i u m ions a n d h y d r o x i d e i o n s at 4 0 ° C with varying p H , u s i n g arbitrary c o n c e n t r a t i o n units  xi  106  6.2  Plot of o b s e r v e d rate constant k  obs  for trithionate d e g r a d a t i o n at  4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 0.0081 M" .h" ) 1  111  1  2  6.3  Plot of o b s e r v e d rate c o n s t a n t k  obs  for trithionate d e g r a d a t i o n at  4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.5 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 0.0081 M" .h" ) 1  112  1  2  6.4  Plot of o b s e r v e d rate constant k  obs  for trithionate d e g r a d a t i o n at  4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.9 - 1.2 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 2  0.0081 M- .h" ) 1  6.5  112  1  Plot of c a l c u l a t e d rate c o n s t a n t kcaic v e r s u s o b s e r v e d rate c o n s t a n t k  obs  for trithionate d e g r a d a t i o n at 4 0 °C for all d a t a , u s i n g k = 0.0081 M- .h1  113  1  2  6.6  Plot of o b s e r v e d rate constant k  obs  for trithionate d e g r a d a t i o n at  4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 0 . 0 0 4 9 M" .h" ) 1  1  2  6.7  Plot of o b s e r v e d rate c o n s t a n t k  obs  114  for trithionate d e g r a d a t i o n at  4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.1 - 0.2 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 2  0.0049 M' .h' ) 1  6.8  114  1  P l o t of o b s e r v e d rate constant k  obs  for trithionate d e g r a d a t i o n at  4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.3 - 0.4 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 2  0.0049 6.9  IvrVh- ) 1  115  P l o t of o b s e r v e d rate c o n s t a n t k  obs  for trithionate d e g r a d a t i o n at  4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.5 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 2  0.0049 IVr .ir ) 1  6.10  115  1  Plot of o b s e r v e d rate constant k  obs  for trithionate d e g r a d a t i o n at  4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.7 - 0.8 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 2  0.0049 M- .lr ) 1  116  1  xii  6.11  Plot of observed rate constant k  obs  for trithionate degradation at  40 °C for an ammonia + ammonium concentration of 0.9 - 1.2 M against pH, with modelled trend superimposed (using k = 2  0.0049 M- .h- ) 1  6.12  116  1  Plot of observed rate constant k b for trithionate degradation at 0  S  40 °C for an ammonia + ammonium concentration of 1.8 - 2.6 M against pH, with modelled trend superimposed (using k = 2  0.0049 lvr .h- ) 1  6.13  117  1  Plot of calculated rate constant k  ca  i  c  versus observed rate constant k b 0  S  for trithionate degradation at 40 °C for all data, using k = 0.0049 M'Vh"  117  7.1  Schematic showing the basis for modelling  120  7.2  Logarithm of the oxygen consumption rate versus the  1  2  logarithm of ammonia concentration for thiosulfate degradation to trithionate in the presence of oxygen 7.3  124  Logarithm of the oxygen consumption rate versus the logarithm of thiosulfate concentration for thiosulfate degradation to trithionate in the presence of oxygen  7.4  124  Modelled output for sulfur oxyanion speciation during thiosulfate degradation. Assumed all thiosulfate degradation is via Reaction R1  7.5  130  Modelled output for sulfur oxyanion speciation during thiosulfate degradation. Assumed all thiosulfate degradation is via Reaction R4  7.6  131  Modelled output for sulfur oxyanion speciation during thiosulfate degradation. Assumed thiosulfate degradation is via Reaction R1 and Reaction R4 (a = 50 %, b = 50 %)  7.7  132  Modelled output for sulfur oxyanion speciation during thiosulfate degradation. Assumed thiosulfate degradation is via Reaction R1 and Reaction R4 (a = 80 %, b = 20 %)  7.8  133  Modelled output for sulfur oxyanion speciation during thiosulfate degradation - k  R1  increased by a factor 10  xiii  134  7.9  M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n - k  7.10  R 4  i n c r e a s e d by a factor 10  135  M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during 5 thiosulfate d e g r a d a t i o n - k -o i n c r e a s e d by a factor 10  136  R2  7.11  M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n - k - i i n c r e a s e d by a factor 10  137  R2  7.12  M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n - k , k 0  7.13  u  k a n d k i n c r e a s e d by a factor 10 2  3  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n  7.14  138  143  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.15  R 1  - 1 x, k  R 2  - 1 x, k  R 3  - 1 x,  -0x  R 4  144  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.16  R 1  - 10 x, k  R 2  - 1 x, k  - 1 x,  R 3  -0x  R 4  145  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.17  R 1  - 14 x, k  R 2  - 8 x, k  - 1 x,  R 3  -0x  R 4  146  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.18  R 4  R 1  - 15 x, k ^ - 8 x, k  - 5 x,  R 3  -0.05 x  147  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.19  R 4  R 1  - 1 x, k  R2  - 1 x, k  R 3  - 1 x,  -0 x  149  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  R 4  -0x  R 1  - 1 x, k ^ - 10 x, k  R 3  - 1 x, 150  xiv  7.20  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s adjusted by the following factors: a k bk  7.21  R 4  R 1  - 1 x,  - 10 x, k  R3  - 5 x,  -0.1 x  151  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s adjusted by the following factors: a k bk  7.22  R 4  R 1  - 1.5 x, k  R2  - 10 x, k  R3  - 5 x,  -0x  152  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s adjusted by the following factors: a k bk  7.23  R 4  R 1  - 16 x, k ^ - 8 x, k  R3  - 1 x,  -0x  153  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n  ^  s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s adjusted by the following factors: a k bk 7.24  R 4  R 1  - 12 x, k  R2  - 8 x, k  R3  - 1 x,  -0.15x  154  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s adjusted by the following factors: a k bk  7.25  R 4  R 1  - 12 x, kp^ - 8 x, k  R3  - 5 x,  -0.17x  155  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.26  R 4  R 1  - 15 x, k  R 2  - 8 x, k  R3  - 5 x,  -0.05 x  161  M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n . M o d e l p a r a m e t e r s adjusted by the following factors: a k bk  7.27  R 4  R 1  - 12 x, k ^ - 8 x, k  R3  - 5 x,  -0.17x  162  M o d e l l e d thiosulfate a n d trithionate concentration after 2 4 h o u r s at varying c o p p e r c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  R 4  R 1  - 15 x, k  -0.05 x  R 2  - 8 x, k  R3  - 5 x, 165  xv  7.28  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at varying c o p p e r c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k  7.29  - 1 x, k  R 1  R 2  - 10 x, k  - 5 x, b k  R3  R 4  - 0.1 X . . . . 1 6 6  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at varying c o p p e r c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.30  R 4  R 1  - 12 x, k  - 8 x, k  R 2  R 3  - 5 x,  -0.17x  167  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at v a r y i n g a m m o n i a c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.31  R 4  R 1  - 15 x, k  R2  - 8 x, k  R 3  - 5 x,  -0.05 x  168  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at varying a m m o n i a c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k  7.32  R 1  - 1 x, k  R2  - 10 x, k  R 3  - 5 x, b k  R 4  - 0.1 x . . . 169  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at varying a m m o n i a c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.33  R 4  R 1  - 12 x, k  R2  - 8 x, k  R 3  - 5 x,  -0.17x  170  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at v a r y i n g p H . M o d e l p a r a m e t e r s adjusted by the following f a c t o r s : ak  7.34  R 1  - 15 x, k  R 2  - 8 x, k  R3  - 5 x, b k  R 4  - 0.05 x  171  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at varying p H . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k i - 1 x, kpu - 10 x, k R  7.35  R3  - 5 x, b k  R 4  - 0.1 x  172  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at varying initial thiosulfate c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  7.36  R 4  R 1  - 15 x,  - 8 x, k  R 3  - 5 x,  -0.05 x  173  M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n after 2 4 h o u r s at v a r y i n g initial thiosulfate c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k bk  R 4  -0.1 x  R 1  - 1 x, k  R 2  - 10 x, k  R 3  - 5 x, 174  xvi  7.37  M o d e l l e d thiosulfate a n d trithionate concentration after 2 4 h o u r s at varying initial thiosulfate c o n c e n t r a t i o n s . M o d e l p a r a m e t e r s adjusted by the following factors: a k i - 12 x, k R  bk 7.38  R 4  R 2  - 8 x, k  R3  - 5 x,  -0.17x  175  M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n o v e r 7 d a y s  7.39  8.1  176  M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n with thiosulfate r e p l e n i s h e d e v e r y 2 4 h o u r s  177  T y p i c a l m o d e l output a g a i n s t e x p e r i m e n t a l d a t a  183  xvii  LIST OF SYMBOLS AND ABBREVIATIONS  D0  2  Dissolved oxygen  E°  Standard potential, volts  E  h  Redox potential, volts  E  a  Activation energy, kJ/mol  F AG  Faraday's constant, 96500 C/mol 0  Standard free energy of formation, kJ/mol  HPLC  High performance liquid chromatography  ICP  Inductively coupled plasma spectrophotometry  I  Ionic strength, mol/l  k  Rate constant  M  Molarity, mol/l  n  Number of electrons  RT  Room temperature  a  Standard deviation  SHE  Standard hydrogen electrode  t  Time  XVlll  ACKNOWLEDGEMENTS I w i s h to a c k n o w l e d g e support of this project by P l a c e r D o m e , A n g l o g o l d - A s h a n t i , T e c k C o m i n c o a n d Barrick G o l d , a s well a s the N a t i o n a l S c i e n c e a n d E n g i n e e r i n g R e s e a r c h Council ( N S E R C ) .  I w o u l d like to thank m y s u p e r v i s o r s D a v i d D r e i s i n g e r a n d G u s V a n W e e r t for their insightful a d v i c e .  T h a n k s to the H y d r o g r o u p , e s p e c i a l l y A n i t a L a m a n d B e r e n d W a s s i n k , a n d all the staff in the M a t e r i a l s E n g i n e e r i n g D e p a r t m e n t w h o h a v e m a d e life e a s i e r for m e .  T h a n k y o u to Dr M i c h a e l W o l f for the loan of the c o o l i n g reactor.  I thank m y parents for their e n c o u r a g e m e n t , in this a n d all the other e n d e a v o u r s I h a v e undertaken.  I a m e s p e c i a l l y grateful to m y h u s b a n d C h r i s w h o k n e w that I w a n t e d to g o b a c k to university s o m e d a y a n d actually got m e to d o s o m e t h i n g about it! H e is m y inspiration.  xix  INTRODUCTION  1.  C y a n i d a t i o n is a well e s t a b l i s h e d a n d effective p r o c e s s for g o l d r e c o v e r y from o r e s a n d concentrates.  H o w e v e r , there a r e i n c r e a s i n g e n v i r o n m e n t a l c o n c e r n s pertaining to the  u s e of c y a n i d e , a n d it is not suitable for certain ore t y p e s , particularly c o p p e r containing ores  a n d c a r b o n a c e o u s refractory  ores where  cyanide consumption  can  become  u n e c o n o m i c a l a n d / o r gold r e c o v e r y low. Thiosulfate h a s s h o w n c o n s i d e r a b l e p r o m i s e a s a n alternative to c y a n i d e for gold l e a c h i n g . H o w e v e r , o n e of the m a i n limitations of the thiosulfate s y s t e m is the high c o n s u m p t i o n of thiosulfate a n d the lack of r o b u s t n e s s in its applicability to a large variety of o r e s .  B e s i d e s the fact that high reagent c o n s u m p t i o n i n c r e a s e s the c o s t of the p r o c e s s , the d e g r a d a t i o n p r o d u c t s of thiosulfate h a v e b e e n c l a i m e d to p a s s i v a t e gold s u r f a c e s a n d the polythionates often p r o d u c e d are l o a d e d onto r e s i n s p r o p o s e d for g o l d r e c o v e r y (Muir a n d A y l m o r e ,  2002). D e g r a d a t i o n products m a y a l s o facilitate the precipitation of  g o l d , c o p p e r a n d silver s u l f i d e s , a n d insufficient thiosulfate lixiviant c a n c a u s e the precipitation of metallic gold a n d silver.  W h i l e thiosulfate  c o n s u m p t i o n is usually monitored  p r o c e s s is not c o m p l e t e l y u n d e r s t o o d .  a n d reported, the  degradation  It is k n o w n that trithionate, tetrathionate  and  sulfate a r e c o m m o n l y p r e s e n t in thiosulfate l e a c h s o l u t i o n s a s d e g r a d a t i o n p r o d u c t s , but the factors influencing the formation a n d interactions of t h e s e s p e c i e s a r e not clear. Sulfate  is  a  thermodynamically  stable  degradation  product,  but  trithionate  and  tetrathionate a r e m e t a - s t a b l e . Tetrathionate is l e s s s t a b l e at the a l k a l i n e p H v a l u e s u s e d in gold l e a c h i n g but trithionate is persistent in gold l e a c h solutions. A l s o , of the s p e c i e s of interest, v e r y little is k n o w n a b o u t the e x p e c t e d b e h a v i o u r of trithionate, both with r e s p e c t to its formation a n d its interaction with other solution s p e c i e s .  T h e f o c u s of this w o r k w a s therefore to further the u n d e r s t a n d i n g of the b e h a v i o u r of trithionate in gold l e a c h solutions.  E x p e r i m e n t a l work w a s carried out to d e t e r m i n e the  kinetics of trithionate d e g r a d a t i o n in s y s t e m s r e s e m b l i n g gold l e a c h i n g s o l u t i o n s , a n d a kinetic m o d e l w a s d e r i v e d for trithionate d e g r a d a t i o n .  T h i s m o d e l w a s integrated with  k n o w n kinetic b e h a v i o u r of thiosulfate a n d tetrathionate b a s e d o n literature findings to d e v e l o p a n overall m o d e l for the thiosulfate d e g r a d a t i o n a n d the resulting  1  solution  s p e c i a t i o n of the sulfur o x y a n i o n s .  T h e model was evaluated against experimental data  a n d its s h o r t c o m i n g s w e r e identified.  In this t h e s i s , a literature review of thiosulfate d e g r a d a t i o n  is g i v e n in C h a p t e r  A n a l y t i c a l a n d s y n t h e s i s m e t h o d s u s e d a r e d i s c u s s e d in C h a p t e r 3.  The  2.  methodology,  results a n d d i s c u s s i o n a n d modelling of trithionate d e g r a d a t i o n kinetics a r e d i s c u s s e d in C h a p t e r s 4, 5 a n d 6 respectively. In C h a p t e r 7, a m o d e l of thiosulfate d e g r a d a t i o n a n d the resulting solution s p e c i a t i o n is set up, b a s e d o n literature d a t a a n d o n the findings in this work.  Conclusions and recommendations  Chapters 8 and 9 respectively.  2  resulting f r o m this w o r k a r e g i v e n in  2.  LITERATURE REVIEW  2.1  INTRODUCTION  In this review, thiosulfate d e g r a d a t i o n in gold l e a c h i n g s y s t e m s a n d the stability of tetrathionate  and  particularly  trithionate  in  aqueous  systems  are  discussed  to  c o n s o l i d a t e the current u n d e r s t a n d i n g of sulfur o x y a n i o n b e h a v i o u r u n d e r c o n d i t i o n s relevant to g o l d l e a c h i n g .  After a g e n e r a l introduction to the p r o b l e m of thiosulfate  d e g r a d a t i o n in gold l e a c h i n g , s o m e g e n e r a l structural a n d t h e r m o d y n a m i c properties of the sulfur o x y a n i o n s a r e s u m m a r i z e d . Next, the thiosulfate d e g r a d a t i o n a n d formation of other sulfur o x y a n i o n s o b s e r v e d experimentally in gold l e a c h i n g s y s t e m s a r e d i s c u s s e d . T h i s is followed by m o r e detailed d i s c u s s i o n s o n the c h e m i s t r y of thiosulfate, trithionate a n d tetrathionate in s y s t e m s relevant to gold l e a c h i n g . Finally, the overall r e l e v a n c e of the a v a i l a b l e literature to the u n d e r s t a n d i n g of the sulfur c h e m i s t r y in gold l e a c h i n g a n d the a r e a s of s h o r t c o m i n g a r e d i s c u s s e d to justify the work carried out in this t h e s i s .  2.2  THE AMMONIACAL THIOSULFATE GOLD LEACHING SYSTEM  C y a n i d e h a s b e c o m e the industry s t a n d a r d lixiviant for g o l d . C y a n i d a t i o n is a robust a n d well u n d e r s t o o d p r o c e s s . H o w e v e r , the ineffectiveness of c y a n i d e for g o l d r e c o v e r y f r o m preg-robbing  ores  and  the  high  cyanide  consumptions  e x p e r i e n c e d with  c o n t a i n i n g o r e s h a s l e a d to a s e a r c h for a n alternative lixiviant.  copper  Cyanide also has a  n e g a t i v e public i m a g e with r e s p e c t to e n v i r o n m e n t a l a n d safety c o n c e r n s .  However,  there is a n industry c o m m i t m e n t of both gold p r o d u c e r s a n d c y a n i d e m a n u f a c t u r e r s to the International C y a n i d e M a n a g e m e n t C o d e ( E M J , 2 0 0 4 , M i n i n g M a g a z i n e , 2 0 0 4 ) that promotes  best  practice  in the  use and  management  of  cyanide, exceeding  r e q u i r e m e n t s of m o s t g o v e r n m e n t s a n d regulatory a g e n c i e s .  the  E v e n s o , there r e m a i n  e n v i r o n m e n t a l c o n c e r n s with the u s e of c y a n i d e , t h o u g h t h e s e m a y b e f o u n d e d m o r e o n public opinion than scientific fact.  Thiosulfate h a s b e e n s h o w n to b e a p r o m i s i n g  alternative lixiviant for gold a n d silver r e c o v e r y a n d m u c h r e s e a r c h h a s b e e n d e v o t e d to the a m m o n i a c a l thiosulfate s y s t e m in recent y e a r s .  Thiosulfate w a s first u s e d in p r e c i o u s m e t a l s r e c o v e r y in the 1 9 P r o c e s s for silver r e c o v e r y ( M o l l e m a n , 1998).  3  t h  century in the P a t e r a  A n a t m o s p h e r i c a m m o n i a c a l thiosulfate  l e a c h to r e c o v e r g o l d a n d silver f r o m a m m o n i a c a l oxidative p r e s s u r e l e a c h r e s i d u e s of c o p p e r sulfide c o n c e n t r a t e s w a s d e v e l o p e d in 1 9 7 8 ( B e r e z o w s k y a n d G o r m e l y , 1978), r e n e w i n g interest in the thiosulfate lixiviant.  S i n c e t h e n , industrial a n d a c a d e m i c interest  in the thiosulfate l e a c h i n g s y s t e m h a s g r o w n t r e m e n d o u s l y , with the m a i n f o c u s o n finding w a y s to e n h a n c e gold l e a c h i n g a n d m i n i m i s e d e g r a d a t i o n of the  thiosulfate  reagent.  T o l e a c h g o l d , a suitable oxidant a n d a suitable c o m p l e x i n g lixiviant is r e q u i r e d .  Using  thiosulfate a s a lixiviant in a m m o n i a c a l solutions, it h a s b e e n f o u n d that the p r e s e n c e of c o p p e r a s a n oxidant greatly e n h a n c e d the rate of reaction ( A y l m o r e a n d Muir, 2 0 0 1 a ) . T h e c h e m i s t r y of the s y s t e m is c o m p l e x a n d not yet fully u n d e r s t o o d .  A simplified representation of the gold l e a c h i n g s y s t e m is g i v e n in E q u a t i o n s 2.1 a n d 2 . 2 , with the overall reaction g i v e n in E q u a t i o n 2.3 ( A b r u z z e s e et a l . , 1 9 9 5 , W a n 1997). C u p r i c c o p p e r or m o r e specifically cupric t e t r a a m m i n e b e h a v e s a s the oxidant for g o l d . T h i o s u l f a t e c o m p l e x e s the gold ion a n d s t a b i l i s e s the gold ion in solution. In the p r o c e s s of gold l e a c h i n g , the c u p r i c is r e d u c e d to c u p r o u s , s o o x y g e n is required for  its  regeneration.  Au + 5 S 0 2  2 3  " + Cu(NH ) 3  2 Cu(S 0 ) ' + 8 NH 3  3  3  2  1  3  Overall: 2 A u + 4 S 0 2  -> A u ( S 0 ) " + 4 N H + C u ( S 0 ) "  + / 0  5  2  2 + 4  2  2 3  2  " + / 0  3  2  2  3  + H 0 ^ 2  [2.1]  5  2  + H 0 -> 2 C u ( N H )  1  2  3  2  2 + 4  3  3  + 2 OH" + 6 S 0 2  2 3  -  [2.2]  2 Au(S 0 ) - + 2 OH"  [2.3]  3  2  3  2  O n e of the major o b s t a c l e s to thiosulfate  l e a c h i n g b e c o m i n g a c o m m e r c i a l l y viable  process  thiosulfate  is the  rapid  degradation  of the  reagent,  which  is a  concern  e c o n o m i c a l l y , technically a n d environmentally.  T h i o s u l f a t e is a m e t a - s t a b l e s p e c i e s a n d c a n be o x i d i z e d ultimately to sulfate through a n u m b e r of reaction paths.  T h e p r e s e n c e of cupric c o p p e r , c o n s i d e r e d n e c e s s a r y by  m a n y to facilitate gold l e a c h i n g , e n h a n c e s thiosulfate oxidation.  The decomposition  p r o d u c t s f o r m e d during gold l e a c h i n g generally include trithionate ( S 0 " ) . tetrathionate 2  3  (S 0 4  2 6  ) a n d sulfate.  4  6  E v e n t h o u g h tetrathionate,  trithionate a n d sulfate h a v e v e r y little effect o n the gold  oxidation reaction ( C h u et a l . , 2 0 0 3 ) , the formation of trithionate a n d sulfate in particular r e p r e s e n t a n irreversible l o s s of thiosulfate u n d e r typical gold l e a c h i n g c o n d i t i o n s .  S i n c e activated c a r b o n is not suitable for recovering gold f r o m thiosulfate  solutions,  r e s i n s h a v e b e e n p r o p o s e d a s a f a v o u r a b l e alternative ( F l e m i n g et a l . , 2 0 0 0 , W e s t - S e l l s et a l , 2 0 0 3 , J i et a l , 2 0 0 3 ) .  H o w e v e r , e v e n low c o n c e n t r a t i o n s of trithionate a n d  tetrathionate (0.01 M ) l o a d strongly onto a n i o n e x c h a n g e r e s i n s , r e d u c i n g gold loading ( F l e m i n g et a l . , 2 0 0 3 , M u i r a n d A y l m o r e , 2 0 0 2 ) .  It h a s b e e n f o u n d that c o n c e n t r a t i o n s of  0 . 0 5 M c a n effectively r e d u c e gold loading to z e r o (Nicol a n d O ' M a l l e y , 2 0 0 2 ) . s p e c i e s w e r e f o u n d not to affect the initial rate of gold loading but b e c a m e  These  important  o v e r longer times w h e n l o a d e d gold could be d i s p l a c e d (Nicol a n d O ' M a l l e y , 2 0 0 2 ) .  T h i s a d s o r p t i o n is a s e r i o u s i s s u e for m o r e a g g r e s s i v e l e a c h i n g conditions w h e r e a c o n s i d e r a b l e a m o u n t of thiosulfate is c o n s u m e d during l e a c h i n g a n d a c o n s i d e r a b l e quantity of polythionates is p r o d u c e d .  In o n e c a s e it w a s c l a i m e d that mild l e a c h i n g  conditions (low thiosulfate, c o p p e r a n d a m m o n i a c o n c e n t r a t i o n s , neutral p H , short time) g a v e relatively low c o n c e n t r a t i o n s of tetrathionate a n d trithionate ( F l e m i n g et a l . , 2 0 0 3 ) . H o w e v e r , the a u t h o r s a l s o a c k n o w l e d g e d that in g e n e r a l , l e a c h liquors still c o n t a i n e d tetrathionate a n d trithionate after a f e w hours.  T h e p r e s e n c e of trithionate a n d tetrathionate is a l s o of c o n c e r n in effluent d i s p o s a l , a s t h e s e s p e c i e s h a v e the potential for a c i d g e n e r a t i o n o n c o m p l e t e oxidation to sulfate, g e n e r a t i n g 1.3 m o l e s of a c i d per m o l e of sulfur in trithionate a n d 1.5 m o l e s of a c i d per m o l e of sulfur in tetrathionate c o m p a r e d with the 1 m o l e p r o d u c e d per m o l e of sulfur in thiosulfate (Smith a n d H i t c h e n , 1976).  2.3  STRUCTURE AND THERMODYNAMIC PROPERTIES  OF THE SULFUR  OXYANIONS T h e r m o d y n a m i c a l l y , thiosulfate is a m e t a s t a b l e ion.  L i k e the other m e t a s t a b l e sulfur  o x y a n i o n s , thiosulfate n e e d s to l o s e or g a i n e l e c t r o n s to r e a c h sulfate or sulfide w h i c h a r e s t a b l e ( W i l l i a m s o n a n d Rimstidt, 1992). A typical E - p H d i a g r a m for a q u e o u s sulfur h  5  s y s t e m s , s h o w n in F i g u r e 2 . 1 , will therefore  not include thiosulfate.  A l l the E - p H h  d i a g r a m s s h o w n in this t h e s i s w e r e g e n e r a t e d using H S C C h e m i s t r y for W i n d o w s software ( R o i n e , 1 9 9 4 , v e r s i o n 5.0), but s p e c i e s with t h e r m o d y n a m i c d a t a predicted by calculation in work by W i l l i a m s o n a n d Rimstidt (1992) w e r e not i n c l u d e d a s they w e r e a c k n o w l e d g e d by the a u t h o r s of H S C C h e m i s t r y for W i n d o w s to be unreliable a n d g a v e large stability r e g i m e s for l e s s e r k n o w n ions s u c h a s H S 0 " ) . 7  3  T h e d a t a u s e d to  g e n e r a t e t h e s e d i a g r a m s is s h o w n in A p p e n d i x 1.  E h (Volts) 2.0 -  1.5 HS0 4  S0  1.0 0.5  2 4  -  -  0.0 -0.5  -  -1.0  H S 2  HS-  s2  -1.5 -2.0  8  10  12  14 PH  F i g u r e 2.1 : E h - p H d i a g r a m for the a q u e o u s sulfur s y s t e m (1 M sulfur, 2 5 °C)  T h i o s u l f a t e c a n be o x i d i z e d through tetrathionate ( S 0 " ) . trithionate ( S 0 2  4  6  3  2 6  ) , sulfite a n d  other sulfur o x y a n i o n s to sulfate a s the potential i n c r e a s e s , a n d is r e d u c e d to sulfur in a c i d or bisulfide in neutral or alkaline solutions ( A y l m o r e a n d Muir, 2 0 0 1 b ) .  Under  a l k a l i n e c o n d i t i o n s , a n u m b e r of m e t a s t a b l e sulfur s p e c i e s o c c u r , for e x a m p l e sulfur oxyanions and  polysulfides  (S ")2  n  E x a m p l e s of the  d i a g r a m m a t i c a l l y b e l o w in F i g u r e 2.2.  sulfur o x y a n i o n s a r e  shown  Sulfur is multivalent a n d therefore e a s i l y f o r m s  6  o  o  o  2-  II I  O-S-S-S-0  o-s-s  o  o Thiosulfate S 0 2  o  2 _  Trithionate S 0  3  3  2 _ 6  o  o  O-S-S-S-S-0 o  o  O-S-S-S-S-S-0 o  o  Tetrathionate S 0 - *  o  Pentathionate S 0 -  2  4  2-  o  2  6  5  6  2-  o o - s  o-s=o o  Sulfite S 0 "  Sulfate S 0 - *  2  2  3  4  " 0 0 "  o  O-S-S-0  o=s-s=o  o  o  2  2  2  4  O  o  Dithionate S 0 "  Dithionite S 0 2  2-  6  O "  I I I  O-S-O-O-S-0 o  o  Persulfate S 0 2  2  8  F i q u r e 2.2 : B a s i c structures of s e l e c t e d sulfur o x y a n i o n s * i n d i c a t e s s p e c i e s e a s i l y quantifiable by current m e t h o d s at U B C  7  this large r a n g e of i o n s , a s well a s polysulfides a n d colloidal precipitates ( A y l m o r e a n d Muir, 2 0 0 1 b ) .  It is well k n o w n that the properties of the sulfur a t o m s in m a n y of the s p e c i e s s h o w n in F i g u r e 2.2 a r e not equivalent ( A m e s a n d W i l l a r d , 1951).  T h e properties of sulfur in  thiosulfate s u g g e s t a p o s s i b l e c o m b i n a t i o n of sulfur with sulfite or sulfide with sulfur d i o x i d e , with the effective  oxidation  state of +2  per sulfur ( S u z u k i , 1999).  The  polythionates h a v e two t y p e s of sulfur a t o m - sulfenyl sulfur a n d sulfonate sulfur.  T h e rate of the m e t a s t a b l e sulfur o x y a n i o n s d e c o m p o s i t i o n to t h e r m o d y n a m i c a l l y s t a b l e sulfur ions is often l e s s than p r e d i c t e d .  T h i o s u l f a t e h a s b e e n f o u n d to b e p r o d u c e d  u n d e r alkaline c o n d i t i o n s , during pyrite l e a c h i n g . kinetic  h i n d r a n c e of further thiosulfate  oxidation  N o sulfite w a s d e t e c t e d , implying a (Webster,  1984).  T h e fact  that  thiosulfate h a s a significant stability u n d e r certain c o n d i t i o n s m a k e s it useful a s a lixiviant for g o l d .  Trithionate is often f o u n d to persist in gold l e a c h liquors a s a d e g r a d a t i o n  product, e v e n t h o u g h it is t h e r m o d y n a m i c a l l y u n s t a b l e ( L a m , 2 0 0 2 , N i c o l a n d O ' M a l l e y , 2 0 0 2 , J i et a l . , 2 0 0 1 ) .  A n e x a m p l e of a n E - p H d i a g r a m w h e r e the stable sulfate ion h a s b e e n omitted to s h o w h  the m e t a s t a b l e d o m a i n of thiosulfate a n d other s p e c i e s is s h o w n in F i g u r e 2 . 3 .  The  d i a g r a m s h o w s that thiosulfate is 'stable' in a p H r a n g e of g r e a t e r than 6 a n d r e d o x potential - 0 . 2 V to  0.2 V ( S H E ) , in the a b s e n c e of other s p e c i e s s u c h a s c o m p l e x i n g  a g e n t s . Tetrathionate exhibits metastability at lower p H .  8  It has been found that sulfite and dithionate (S 0 ") did not appear in certain gold leach 2  2  6  liquors (Aylmore and Muir, 2001b), so these species and other related species were omitted from the E -pH diagram, giving the diagram shown in Figure 2.4. h  In this  diagram, trithionate is the most stable polythionate at high E and high pH. Based on h  this diagram, direct formation of trithionate from thiosulfate should be possible, and this has indeed been suggested (see Section 2.5). However, tetrathionate is often reported in gold leach liquors implying that the kinetics of tetrathionate formation are favourable. The presence of oxygen (hence an increase in potential) should favour the formation of trithionate.  9  E h (Volte)  -i  1  1  1  1  r  1  1  1  1  1  r  so -  1.5  2  3  6  1.0 0.5  1  - ^ V ^ -  =====—-^JW"  -1.0  HS  HS-  2  S ; 2  -1.5 -2.0  -  12  10  14 PH  F i g u r e 2.4 : E h - p H d i a g r a m for the m e t a s t a b l e a q u e o u s sulfur s y s t e m with the following s p e c i e s omitted - SO/'.  H S O V . H S 0 . x H 0 . S C X S O / , H S O V . S Q« '. H S O V . S?0« '. 2  ?  4  ?  2  ?  s g£ 2  (1 M sulfur, 2 5 °C)  F o r r e f e r e n c e , a s e l e c t i o n of G i b b s free e n e r g i e s of formation f o r s p e c i e s of interest in the gold l e a c h i n g s y s t e m is s h o w n in T a b l e 2 . 1 . F r o m t h e s e , the s t a n d a r d potentials for a f e w pertinent thiosulfate oxidation reactions c o u l d b e c a l c u l a t e d (from the relation A G = -nFE°) a n d a r e s h o w n in T a b l e 2.2.  10  0  T a b l e 2.1 : S e l e c t e d G i b b s free e n e r g i e s of formation (298 K ) for s p e c i e s of interest in the c o p p e r - a m m o n i a c a l - thiosulfate s y s t e m ( A y l m o r e a n d Muir, 2 0 0 1 b ) Species  AG° (kJ/mol)  Species  S  0  Cu  0  Au  0  -486.5  Cu  +  50.2  Au  +  163.2  -*  -742.0  Cu  2 +  65.0  Au  3 +  433.5  '  -532.2  Cu(S 0 ) '  -1624.7  Au(S 0 ) -  -966.0  Cu(S 0 )  -1084.1  Au(NH ) Au(NH )  SG-3 2  S0  2 4  S 0 2  2 3  AG  5  2  3  3  0  (kJ/mol)  Species  AG  3  2  3  2  0  (kJ/mol)  -1050.2  so so so so so -  -958.0  Cu(S 0 )-  -541.0  -1040.4  Cu(NH )  +  -10.3  -956.0  Cu(NH )  2 +  14.5  O H *  -157.3  -600.6  Cu(NH )  -32.3  H 0  -237.2  S 0  -1115.0  Cu(NH )  -73.2  NH  -26.7  2  2  6  2  3  6  2  4  6  2  5  6  2  2 2  4  2 8  "  2  3  2  3 _ 2  3  3  3  3  3  Cu(NH ) 3  2 + 2  2 + 3  3  3  3 +  64.4  +  -41.4  4  2  2  3  -113.0  2 + 4  * From Weast, 1975.  T a b l e 2.2 : S e l e c t e d s t a n d a r d oxidation potentials (298 K )  Half R e a c t i o n 2 S 0  3  3 S 0  3  2  2  2  S 0 3  6  S 0  3  2  2  E  - -> S 0 4  2  2 6  - + 3 H 0 -» 2 S 0 3  " + 6 H 0 -> 3 S 0  2  2  2  - + 5 OH" ^ 2 S 0  4  2 4  o x i d  (V)  0.12  - + 2 e"  2  0  2 6  - + 6 H +8 e" +  ' + 12 H  " +5H  +  +  +8 e  + 2 e"  0.51 0.20 -0.86  T h e s t a n d a r d potentials imply that d e g r a d a t i o n of thiosulfate directly to sulfate s h o u l d o c c u r readily u n d e r t h e conditions required for gold l e a c h i n g , a n d a l s o e x p l a i n w h y tetrathionate is likely to b e f o r m e d , a s the s t a n d a r d potentials for t h e s e oxidation half r e a c t i o n s a r e g e n e r a l l y lower than t h o s e required to l e a c h g o l d . T o f o r m trithionate f r o m thiosulfate requires a higher potential (0.51 V ) . T h e d a t a in T a b l e 2.2 highlights the fact that the t h e r m o d y n a m i c s of the s y s t e m a l o n e c a n n o t b e u s e d a s a n indicative tool, a s kinetically w e s e e the formation a n d p e r s i s t e n c e of trithionate, a n d sulfate is only f o r m e d in a n y significant quantity after long times o r a s a product of polythionate d e g r a d a t i o n ,  11  contrary to the predictions of thermodynamics. There appear to be significant kinetic hindrances of trithionate degradation to sulfate in the gold leaching system. 2.4  THIOSULFATE  DEGRADATION  IN  GOLD  LEACHING  -  EXPERIMENTAL  OBSERVATIONS  It has been found that thiosulfate degradation in the gold leaching system is often not reported in detail in the literature. Reasons for this could be that much of the reported literature has focussed on gold recovery, or that suitable analytical methods were not available to easily measure thiosulfate consumptions. However, from the literature data available, it has been attempted to compile thiosulfate degradation data in this section to identify the main factors influencing the degradation. The table in Appendix 2 shows a compilation of thiosulfate consumption data. In many cases, the available data was not completely quantitative or was given as a range. Qualitative information is not included in the appendix but is given in the summary in Table 2.3. The observations in Appendix 2 and in Table 2.3 were for a large range of conditions and in many instances it was not clear why particular conditions were selected. Due to the interdependency of the various parameters and the difficulty in comparing data from different sources with incomplete data sets and different experimental regimes, only general trends are presented. There was some contradiction in the assessment of how different parameters affect thiosulfate degradation. While it is acknowledged that the concentrations of thiosulfate, ammonia and copper have a significant effect, there is not complete agreement in how these parameters affect the degradation and not much discussion as to how the combination of these species affects degradation. It is generally agreed that an increase in temperature and reaction time, and the presence of ore increases the thiosulfate degradation. The effect of sulfite and sulfate addition has not been clearly established. A more detailed discussion of the chemistry of the thiosulfate degradation inferred in Table 2.3 is given in Section 2.5.  12  Table 2.3 : Summary of experimental observations for thiosulfate degradation Parameter Temperature  Effect Inc => increase in degradation  References A, B, C, D  c  Ammonia Inc => concentration increase in degradation Inc => decrease in degradation  D, E F G H A  Optimum required Copper Inc => concentration decrease in degradation Inc => increase in degradation  G C, H, 1  Thiosulfate Inc => concentration increase in degradation Inc => decrease in degradation Ore Presence of sulfides => mineralogy increase in degradation Ultrafine milling of pyrite => no significant effect Time Inc => increase in degradation Sulfite addition  B, C, H, J G, 1 K, L, M G G, K  If present => decrease in degradation If present => inconclusive  C, N,0 P  13  General Comment Often implied by decrease in gold leaching, assumed to be due to loss of cupric and thiosulfate reagents. Ammonia acts as stabiliser for cupric copper, hence it has often been postulated that ammonia inhibits the thiosulfate - copper reaction. Postulated that once a certain level of cupric tetraammine is reached, excess cupric oxidizes the leach reagent rather than catalyzing the leach reaction. The dependency has been found to vary with other solution parameters e.g. copper concentration. Important aspect, but often difficult to quantify and difficult to compare data. Expected trend as thiosulfate is not a thermodynamically stable species. Likely to be highly dependent on ore type, and also known to decrease extent of leaching.  T a b l e 2.3 ( c o n t i n u e d ) : S u m m a r y of e x p e r i m e n t a l o b s e r v a t i o n s for thiosulfate degradation  Effect  Parameter  p H > 11.4 required minimum degradation  PH  References B  for  General Comment Inconclusive.  i  Inc p H 8 . 5 - 1 0 ^ i n c r e a s e in d e g r a d a t i o n Inc p H 9 . 8 - 1 1 . 4 ^ d e c r e a s e in d e g r a d a t i o n  Q, R  Oxygen concentration  Inc => i n c r e a s e in d e g r a d a t i o n  Q  Anions (phosphate)  Inc => d e c r e a s e in d e g r a d a t i o n  Q, R  C e r t a i n a n i o n s s a i d to inhibit thiosulfate coordination with copper (II) ammine complexes  R e f e r e n c e s to T a b l e 2.3 A B C D E F G H I J K L M N O P Q R  A b b r u z z e s e et a l . , 1 9 9 5 B r e u e r a n d Jeffrey, 2 0 0 0 Rett etal.,1983 Jeffrey, 2001 A y l m o r e a n d Muir, 2 0 0 1 a Byerley e t a l . , 1973a A y l m o r e , 2001 Y e n e t a l . , 1999 Langhans e t a l . , 1992 Zipperian and R a g h a v a n , 1988 W a n , 1997 X u and S c h o o n e n , 1995 F e n g and V a n Deventer, 2002b K e r l e y a n d B e r n a r d , 1981 J i et a l . , 2001 Li e t a l . , 1996 B r e u e r a n d Jeffrey, 2 0 0 3 a B r e u e r a n d Jeffrey, 2 0 0 3 b  A significant a m o u n t of thiosulfate d e g r a d a t i o n d a t a w a s g i v e n by A y l m o r e a n d M u i r (2001a).  A l s o reported by t h e m w a s a c o m p a r i s o n of the c o n s u m p t i o n of c o p p e r with  time ( c o p p e r m e a s u r e d by I C P ) a n d thiosulfate ( m e a s u r e d by H P L C ) .  ( C u p r i c c o p p e r is  k n o w n to o x i d i z e thiosulfate - s e e S e c t i o n 2.5.) T h i s is not r e p r o d u c e d in this s u m m a r y . H o w e v e r , it is interesting that although c o p p e r c o n s u m p t i o n g e n e r a l l y i n c r e a s e d a s thiosulfate c o n s u m p t i o n i n c r e a s e d , the two r e a g e n t s w e r e not c o n s u m e d at a c o n s t a n t ratio.  T h i s c o u l d imply precipitation of c o p p e r or oxidation of thiosulfate by a n o t h e r  o x i d i z i n g a g e n t s u c h a s o x y g e n , a s well a s by c o p p e r . In other recent work ( B r e u e r a n d  14  Jeffrey, 2 0 0 3 a , b) it w a s a l s o r e a l i s e d that thiosulfate c o n s u m p t i o n a n d c u p r i c c o p p e r c o n c e n t r a t i o n s h o u l d b e c o n s i d e r e d s e p a r a t e l y . H o w e v e r , in o l d e r work, the thiosulfate d e g r a d a t i o n w a s implied by the m e a s u r e d c o p p e r c o n c e n t r a t i o n ( B y e r l e y et a l . , 1 9 7 3 a ) a n d e v e n t h o u g h u n d e r certain conditions this c a n give a g o o d a p p r o x i m a t i o n , it m a y not a l w a y s b e entirely appropriate.  A n u m b e r of effects o n thiosulfate d e g r a d a t i o n w e r e d i s c u s s e d in a silver l e a c h i n g s y s t e m (Flett et a l . , 1983). In l e a c h tests o n synthetic argentite, both the thiosulfate a n d tetrathionate in solution w e r e m e a s u r e d . A l t h o u g h it is c o m m o n l y a c c e p t e d or a s s u m e d that tetrathionate is the major d e g r a d a t i o n product of thiosulfate u n d e r typical gold a n d silver l e a c h c o n d i t i o n s , it w a s f o u n d that the l o s s of thiosulfate w a s slightly h i g h e r than the g a i n in tetrathionate, implying the formation of other sulfur o x y a n i o n s p e c i e s .  In  m a n y c a s e s only tetrathionate w a s m e a s u r e d , w h i c h c o u l d give m i s l e a d i n g c o n c l u s i o n s .  T h e s e o b s e r v a t i o n s highlight the i m p o r t a n c e of appropriate a n a l y s i s a n d that s o m e reported results a n d c o n c l u s i o n s are not b a s e d o n direct a n a l y s i s .  V e r y little e x p e r i m e n t a l d a t a o n trithionate a n d tetrathionate pertaining to gold extraction.  production is  reported  It m a y b e that in m o s t reported c a s e s , only the c h a n g e in  thiosulfate c o n c e n t r a t i o n w a s m e a s u r e d , without determining the s p e c i a t i o n of sulfur oxyanion  degradation  concentrations  products.  observed  in  gold  A  few  leach  values  liquors  for  are  trithionate shown  and  in T a b l e  tetrathionate 2.4.  While  pentathionate h a s o c c a s i o n a l l y b e e n noted (qualitatively) in gold l e a c h s o l u t i o n s ( D e J o n g , 2 0 0 4 ) , it is not a persistent s p e c i e s u n d e r t h e s e c o n d i t i o n s .  15  T a b l e 2.4 - Literature d a t a for trithionate a n d tetrathionate production  S 0 -(mM)  L e a c h conditions  2  2  3  S 0 4  2 6  - (mM)  0.05 M S 0 " 0.2 M N H 2 0 mg/l e a c h of Pb, Zn, C u , A g , A u 4 8 hrs (Nicol a n d O'Malley, 2002)  S 0 3  2 6  " (mM)  2  2  3  3  10  Continuous RIP initial 0 . 0 5 M S 0 " 0.8 M N H 1 mM S0 " p H 9.5 (Nicol a n d O'Malley, 2002) 2  2  3  3  2  0.089  0.1  3  L e a c h d i s c h a r g e slurry p H 6.9 (Ji et a l . , 2 0 0 1 )  75  2.3  3  P r e g n a n t l e a c h solution p H 10 (Ji e t a l . , 2 0 0 1 )  70  3.8  7.7  In l e a c h tests at the University of British C o l u m b i a ( L a m , 2 0 0 2 ) at r o o m t e m p e r a t u r e , u s i n g initial solution c o n c e n t r a t i o n s of 0.1 - 0.6 M S 0 " , 0.16 - 4 . 7 m M C u (II) a n d 0.3 2  2  3  0.5 M N H , typical c o n c e n t r a t i o n s of the sulfur o x y a n i o n s in the l e a c h s o l u t i o n s after 2 4 3  to 4 8 h o u r s w e r e :  S 0 2  3  S 0  6  3  S 0 4  S0  "  14-567mM  2  -  2 9 - 140mM  2  "  0-17mM  6  2 4  2  -  6-139mM  Trithionate in the final l e a c h liquor a c c o u n t e d for 7 - 8 3 % of the initial thiosulfate a d d e d .  The  formation of trithionate a n d tetrathionate  in s y s t e m s r e s e m b l i n g g o l d l e a c h i n g  s y s t e m s to e x a m i n e the effect of s p e c i f i c m i n e r a l s o n the thiosulfate d e g r a d a t i o n kinetics h a s recently b e e n reported ( D e J o n g , 2 0 0 4 ) .  V a r i o u s m i n e r a l s w e r e a l l o w e d to react  with a n a m m o n i a c a l thiosulfate solution (0.2 M ( N H ) S 0 at p H 10) with a n d without 4  added copper.  2  2  3  T h e author p r o p o s e d that trithionate w a s f o r m e d by h y d r o l y s i s of the  tetrathionate p r o d u c e d during thiosulfate d e g r a d a t i o n rather t h a n directly f r o m thiosulfate  16  (see Section 2.5). This was not proven as the thiosulfate degradation kinetics were not measured directly in this work due to an unreliable analysis method. Without added copper, the formation of trithionate increased in the order no mineral ~ hematite ~ galena ~ arsenopyrite < chalcopyrite < pyrrhotite < pyrite < chalcocite. It is not clear why trithionate formation would be enhanced in the presence of minerals in the absence of copper, but is likely to be a complex effect of surface catalysis, pH, redox potential and possibly the presence of soluble trace elements.  The formation of  tetrathionate increased in the order no mineral ~ hematite ~ galena ~ arsenopyrite ~ chalcocite < pyrite < pyrrhotite < chalcopyrite. The authors proposed that certain minerals enhanced the degradation of tetrathionate to trithionate, for example chalcocite. Where copper was initially present, all the minerals except for chalcocite showed less trithionate formation than the no-mineral condition.  The formation of trithionate  decreased in the order chalcocite > no mineral ~ hematite > pyrite > arsenopyrite > galena > pyrrhotite ~ chalcopyrite. The formation of tetrathionate increased in the order chalcocite < no mineral ~ hematite < arsenopyrite < pyrite ~ galena ~ chalcopyrite < pyrrhotite. Both with and without initial copper, the presence of chalcocite caused the formation of a significant amount of trithionate. The initial rapid formation of trithionate corresponded to rapid initial copper extraction from the chalcocite. The tetrathionate concentration in this case was very low.  Since the author had postulated that trithionate formed from  tetrathionate, this observation lead to the suggestion that copper accelerates the decomposition of tetrathionate to trithionate. To better understand the effects of minerals a more comprehensive sulfur species analysis to include thiosulfate and sulfate would be required. More fundamental studies investigating the degradation of thiosulfate in the absence of ores are discussed in Section 2.5.  17  THIOSULFATE CHEMISTRY AND FUNDAMENTAL STUDIES  2.5  E v e n t h o u g h the stability of thiosulfate in a q u e o u s solutions is affected by m a n y factors ( D h a w a l e , 1993), thiosulfate in solution, p r e p a r e d in freshly boiled d o u b l e distilled water, is v e r y s t a b l e s t o r e d in air tight bottles ( A y l m o r e a n d Muir, 2 0 0 1 a ) .  A i r oxidation of  thiosulfate at n o r m a l temperature a n d p r e s s u r e is v e r y s l o w . A t p H 7 s o l u t i o n s a e r a t e d for 4 m o n t h s u n d e r sterile conditions h a d l e s s than concentration.  10 % c h a n g e in the  thiosalt  T h e r e is significant oxidation at higher t e m p e r a t u r e a n d air or o x y g e n  p r e s s u r e , indicating kinetic control. ( R o l i a a n d C h a k r a b a r t i , 1982).  T h e d e g r a d a t i o n of thiosulfate that c a n b e e x p e c t e d in gold l e a c h s y s t e m s is d i s c u s s e d below.  2.5.1  Oxidative Degradation in Gold Leaching Systems  A c c o r d i n g to t h e r m o d y n a m i c s , thiosulfate  would  eventually  one would  be  oxidized  e x p e c t that u n d e r to  sulfate  (see  oxidizing  Figure  2.1).  conditions, This  is  d e m o n s t r a t e d in E q u a t i o n 2.4.  S 0 2  2 3  " + 2 0  2  + H 0 -» 2 S 0 2  2 4  " + 2 H  [2.4]  +  H o w e v e r , a n u m b e r of m e t a s t a b l e oxidation p r o d u c t s c a n b e e x p e c t e d .  In the g o l d  l e a c h i n g s y s t e m , w h e r e thiosulfate, c o p p e r a n d a m m o n i a a r e present, tetrathionate h a s often b e e n q u o t e d a s a primary oxidation product.  T h i o s u l f a t e oxidation to tetrathionate by o x y g e n c a n b e d e m o n s t r a t e d by E q u a t i o n 2.5 ( W a n , 1 9 9 7 , Li et a l . , 1 9 9 6 , A y l m o r e a n d Muir, 2 0 0 1 a ) .  2 S 0 2  2 3  " + H 0 + / 0 1  2  2  2  -» S 0 4  2 6  - + 2 OH"  [2.5]  T h e thiosulfate l e a c h s y s t e m is c o m p l i c a t e d by the reduction of c o p p e r by thiosulfate ( B y e r l e y et a l . , 1 9 7 3 a , B r e u e r a n d Jeffrey, 2 0 0 0 ) . T h e reaction of the c u p r i c t e t r a a m m i n e  18  c o m p l e x with thiosulfate to form tetrathionate  is s h o w n in E q u a t i o n 2.6 ( A y l m o r e a n d  Muir, 2 0 0 1 a , B r e u e r a n d Jeffrey, 2 0 0 0 ) .  2 Cu(NH ) 3  2 + 4  + 8 S 0 2  2 3  - -» 2 C u ( S 0 ) - + S 0 5  2  3  3  4  2 6  - + 8 NH  [2.6]  3  T h e cupric t e t r a a m m i n e c o m p l e x is u s e d in the reaction e q u a t i o n rather than s i m p l y the c u p r i c ion a s it h a s b e e n often reported that it is the c o m p l e x that is r e s p o n s i b l e for reacting with thiosulfate ( L a m , 2 0 0 1 , B r e u e r a n d Jeffrey, 2 0 0 0 ) . H o w e v e r , it h a s a l s o b e e n s u g g e s t e d that C u ( N H ) 3  2 + 3  rather  than  Cu(NH ) 3  2 + 4  oxidizes S 0 2  2 _ 3  to  S 0 4  2 _ 6  ( B y e r l e y et a l . , 1 9 7 3 a ) . T h i s is d i s c u s s e d later in this s e c t i o n . T h e reaction of thiosulfate with c u p r i c c o p p e r is rapid in a q u e o u s solution but s l o w e r with a m m o n i a  present  ( A y l m o r e a n d Muir, 2 0 0 1 a ) .  In alkaline solution, the reaction b e t w e e n c u p r i c c o p p e r a n d thiosulfate d o e s not require o x y g e n ( A y l m o r e a n d Muir, 2 0 0 1 a ) . H o w e v e r , o x y g e n still p l a y s a n important role in the overall extent a n d rate of thiosulfate d e g r a d a t i o n . In the gold l e a c h i n g s y s t e m , o x y g e n is u s e d to convert c u p r o u s c o p p e r to cupric, but d e p e n d i n g o n the a m o u n t of o x y g e n , s o m e direct oxidation of thiosulfate to tetrathionate  a n d trithionate c a n a l s o o c c u r .  In the  p r e s e n c e of o x y g e n , the r e d o x potential rises a n d the oxidation of c u p r o u s to cupric is m o r e rapid (as is the oxidation of thiosulfate, w h e r e the reaction rate h a s b e e n q u o t e d to be at least forty t i m e s higher in the p r e s e n c e of o x y g e n (Byerley et a l . , 1973b)).  E v e n t h o u g h in m a n y cited e x a m p l e s , tetrathionate is the m a i n oxidation product referred to ( W a n 1 9 9 7 , M u i r a n d A y l m o r e , 2 0 0 2 ) , it must be noted that s o m e t i m e s it h a s only b e e n a s s u m e d that a l o s s in thiosulfate  implies the production of tetrathionate,  or  s o m e t i m e s tetrathionate w a s the only d e g r a d a t i o n product m e a s u r e d , l e a d i n g to p o s s i b l e misinterpretation.  Trithionate a n d sulfate h a v e a l s o b e e n reported (Byerley et a l . , 1 9 7 3 b ,  Muir and Aylmore, 2002).  B e s i d e s c o p p e r a n d o x y g e n , other oxidants, if present, c a n a l s o d e g r a d e thiosulfate.  If  iron is present, it c a n d i s s o l v e at p H l e s s than 9.5 a n d d e g r a d e thiosulfate, producing tetrathionate w h i c h h a s n o lixiviating action for gold or silver ( A y l m o r e a n d Muir, 2 0 0 1 a , P e r e z a n d G a l a v i z , 1987).  H e n c e a high p H stabilizes thiosulfate by minimizing iron  dissolution ( P e r e z a n d G a l a v i z , 1987).  Also A g , H g +  19  2 +  and cyanide can degrade  thiosulfate (Kelly a n d W o o d , 1994). T h e addition of the metal ions of N i , Z n , P b , C d , C o a n d C r to the g o l d l e a c h i n g s y s t e m all h a d the effect of d e c r e a s i n g the free thiosulfate c o n c e n t r a t i o n , with the largest effect by C r a n d least by Ni ( F e n g a n d D e v e n t e r , 2 0 0 2 a ) . It w a s d e d u c e d that oxidation (especially for C r * a n d C o ) a s well a s c o m p l e x a t i o n 6  3 +  (especially for C d ) p l a y e d a role. 2 +  F e w s t u d i e s h a v e b e e n d o n e to determine the m e c h a n i s m or kinetics of degradation,  particularly  in  the  gold  leaching  system.  A  significant  thiosulfate amount  of  f u n d a m e n t a l w o r k w a s d o n e in the 1 9 7 0 s by B y e r l e y et a l . ( 1 9 7 3 a , 1 9 7 3 b , 1975) a n d a l s o v e r y recently by B r e u e r a n d Jeffrey ( 2 0 0 3 a , 2 0 0 3 b ) .  Copper (II) Oxidation of Thiosulfate in Absence of Oxygen B y e r l e y et a l . (1973a) studied the m e c h a n i s m of thiosulfate d e g r a d a t i o n by c u p r i c c o p p e r in a m m o n i u m hydroxide in the a b s e n c e of o x y g e n . T h e m e c h a n i s m p r o p o s e d w a s for a s y s t e m slightly primarily  related  different to that e x p e c t e d to sulfide  leaching  u n d e r gold l e a c h i n g conditions  in a m m o n i a  solutions, where  and  was  thiosulfate  was  produced.  W h e n thiosulfate w a s a d d e d to a c o p p e r - a m m o n i a s y s t e m , there w a s a n  immediate  i n c r e a s e in the a b s o r b a n c e of the solution prior to the c o m m e n c e m e n t of the reaction. But the position of the absorption m a x i m u m did not c h a n g e at this initial time, nor during the reaction. T h e i n c r e a s e in a b s o r b a n c e w a s interpreted a s a n a s s o c i a t i o n b e t w e e n the thiosulfate a n d the c o p p e r a m m o n i a c o m p l e x e s . T h e a s s o c i a t i o n w a s s u g g e s t e d a s b e i n g at o n e of the axial positions, therefore giving a strong polarizing effect of cupric c o p p e r o n thiosulfate.  T h e equatorial s q u a r e planar structure of the c o p p e r c o m p l e x  remained essentially unchanged.  T h e p r o g r e s s of the reaction w a s m e a s u r e d by m e a s u r i n g the cupric  concentration.  S o l u t i o n s a n a l y s e d for thiosulfate by iodometric titration a n d qualitatively for tetrathionate directly after the reaction (when it w a s o b s e r v e d that all c u p r i c h a d b e e n c o n s u m e d ) s h o w e d that overall o n e m o l e of c o p p e r (II) w a s c o n s u m e d for e a c h m o l e of thiosulfate, m a t c h i n g the stoichiometry of E q u a t i o n 2.7.  20  2 Cu  +2S 0 -  2+  2  2  3  S 0 - + 2 Cu 2  4  [2.7]  +  6  If higher copper (-0.03 M) or thiosulfate (-0.15 M) concentrations were used, or if the solutions were stored, trithionate and regenerated thiosulfate were produced as products of tetrathionate degradation (Byerley et al., 1973a). Tetrathionate chemistry is discussed later. At pH > 10, the cupric-ammine complex exists in equilibrium with hydroxo species, so some tri-ammine cupric species co-exist with the tetraammine species. Since the rate of the copper thiosulfate reaction was faster in aqueous solution, it was assumed that the tri-ammine cupric complex was more reactive than the tetraammine complex (Byerley et al., 1973a). Hence, a mechanism suggested from kinetic results implied substitution of thiosulfate into the co-ordination sphere of a copper (II) tri-ammine complex prior to the electron transfer step.  An electron transfer from thiosulfate to cupric copper in an  intermediate tri-ammine cupric-thiosulfate complex gives cuprous copper and thiosulfate, which dimerises to give tetrathionate. The mechanism outline is shown in Equations 2.8 to 2.11.  Cu(NH ) 3  2+ 4  + H 0 - Cu(NH ) (H 0) + N H 2  3  3  2  [2.8]  3  Cu(NH ) (H 0) + S 0 " -> Cu(NH ) (S 0 )+ H 0 2+  3  3  2  2  2  Cu(NH ) (S 0 ) + e 3  2S 0 2  2  3  3  2  3  3  3  3  2  3  Cu(NH ) + S 0 2+  3  2  3  S 0 " + 2e  [2.11]  2  4  [2.9] [2.10]  :  3  2  6  It was determined that the involvement of free radicals in the rate determining step was unlikely as addition of a free radical inhibitor, mannitol, had no effect on the reaction rate (Byerley et al., 1973a). In Byerley's work plots of log [Cu ] vs time were linear with the same slope, indicating 2+  first order dependency, assuming ammonia and thiosulfate concentrations were essentially constant during the reaction. Thiosulfate concentrations up to 0.10 M gave a rate of first order dependence. At higher thiosulfate concentrations (above 0.15 M), the reaction order was higher. There was a linear dependence of the rate on 1/[NH ] for 3  ammonia concentrations of 0.1 - 1 M.  21  U n d e r t h e s e c o n d i t i o n s , the rate of d e c r e a s e of c u p r i c c o p p e r c o n c e n t r a t i o n by reaction with thiosulfate in the a b s e n c e of o x y g e n w a s g i v e n by E q u a t i o n 2 . 1 2 . -d[Cu ]/dt = k [Cu ][S 0321/[NH ] 2+  [2.12]  2+  2  3  w h e r e k = 8.5 x 10" s ' at 3 0 °C. 4  1  T h e activation e n e r g y w a s 1 0 2 . 5 k J / m o l ( b a s e d o n  t h e r m o d y n a m i c p a r a m e t e r s at 3 0 °C).  M o r e recent work by B r e u e r a n d Jeffrey h a s d e v e l o p e d a further u n d e r s t a n d i n g of the s y s t e m ( B r e u e r a n d Jeffrey, 2 0 0 0 , 2 0 0 3 b ) .  T h e m a i n a i m for this s t u d y of the factors  affecting the gold l e a c h i n g kinetics u n d e r a n a e r o b i c c o n d i t i o n s w a s to allow for the effect of d e c r e a s i n g c o p p e r (II) (and h e n c e c u p r i c to c u p r o u s ratio) to b e investigated with a m i n i m a l d e c r e a s e in thiosulfate c o n c e n t r a t i o n . A l l the c o m p o n e n t s e x c e p t for c o p p e r (II) a n d a m m o n i a w e r e p u r g e d with a r g o n , then a c o n c e n t r a t e d c o p p e r (II) -  ammonia  solution w a s injected into the v e s s e l . After m i x i n g , a s a m p l e w a s transferred to a s e a l e d U V c e l l , with n o air contact at a n y time. T h e g e n e r a l conditions u s e d w e r e 0.1 M s o d i u m thiosulfate, 0.4 M a m m o n i a a n d 10 m M c o p p e r sulfate at 3 0 ° C , p H 11.4.  Complete  e x c l u s i o n of air w a s f o u n d to be e s s e n t i a l a s e v e n trace quantities w e r e s h o w n to i n c r e a s e the c o p p e r (II) reduction rate. T h e c u p r i c a m m i n e c o m p l e x e s a b s o r b i n g at 6 0 5 nm were m e a s u r e d using UV-visible spectrophotometry.  P l o t s of the c u p r i c a m m i n e c o m p l e x concentration v e r s u s time s h o w e d that the relation of 1/[Cu(ll)] v e r s u s time w a s fairly linear, while that of log[Cu(ll)] v e r s u s time w a s not. T h i s w o u l d imply that the rate determining step w a s s e c o n d order, not first o r d e r a s s u g g e s t e d by B y e r l e y et a l . (1973a).  A deviation f r o m linearity at l o n g e r times w a s  attributed to further c o p p e r (II) reduction by tetrathionate.  T h e addition of up to 0.8 M sulfate w a s f o u n d to d e c r e a s e the rate of c o p p e r reduction c o n s i d e r a b l y .  (II)  B y c h a n g i n g the e x p e r i m e n t a l p r o c e d u r e to inject a thiosulfate  solution into a solution of c o p p e r (II), a m m o n i a a n d sulfate, further information o n the m e c h a n i s m of the action of sulfate ions c o u l d b e o b t a i n e d . In this i n s t a n c e , a n o t i c e a b l e induction period o c c u r r e d , after w h i c h the reaction rate i n c r e a s e d to r e a c h a similar v a l u e to that obtained w h e n a c o p p e r (II) - a m m o n i a solution w a s injected into a thiosulfate sulfate solution.  22  -  It w a s noted by B y e r l e y et a l . (1973a) a n d B r e u e r a n d Jeffrey (2003b) that in the reaction of thiosulfate with c o p p e r (II),  it is likely that thiosulfate c o m p l e x e s with the c u p r i c  a m m i n e c o m p l e x at a n axial site, b a s e d o n a b s o r b a n c e m e a s u r e m e n t s . t h u s o c c u r s v i a a n inner s p h e r e m e c h a n i s m .  It w a s p o s t u l a t e d that sulfate c o m p e t e s  with thiosulfate for co-ordination, h e n c e s l o w i n g the c o p p e r (II) thiosulfate.  T h e reaction  reduction  rate  by  W h e r e thiosulfate w a s a d d e d to a c o p p e r (II) a m m o n i a sulfate solution, the  sulfate w a s a b l e to c o - o r d i n a t e before the addition of the thiosulfate, giving a n induction period for thiosulfate to r e p l a c e sulfate before the reaction c o u l d o c c u r .  T h e effect of other a n i o n s (chloride a n d nitrate) a n d e s p e c i a l l y p h o s p h a t e w a s a l s o to r e d u c e the c o p p e r (II) reduction rate, w h i c h is c o n s i s t e n t with a n inner s p h e r e reduction reaction.  P h o s p h a t e in particular is thought to readily c o m p l e x with c o p p e r (II) at the  axial position, inhibiting the substitution of thiosulfate into the inner s p h e r e .  T h e effect of p H o n the reaction m e c h a n i s m w a s investigated by u s i n g a m m o n i u m thiosulfate i n s t e a d of s o d i u m thiosulfate, with the s a m e c o n c e n t r a t i o n of a m m o n i a a n d s a m e ionic strength.  T h i s h a d the effect of r e d u c i n g the p H f r o m 11.4 to 9.8.  The  reaction o r d e r of c o p p e r (II) reduction at p H 9.8 s h o w e d a first o r d e r rate limiting s t e p , not s e c o n d o r d e r a s f o u n d at p H 11.4, w h i c h c o r r e l a t e s with the o b s e r v a t i o n by B y e r l e y et a l . (1973a). T h e d e c r e a s e in reaction o r d e r f r o m two to o n e at lower p H c o u l d b e d u e to the lack of a n i o n s (hydroxide) to c o m p e t e with thiosulfate for axial c o o r d i n a t i o n .  This  v i e w w a s s u p p o r t e d e x p e r i m e n t a l l y by a n o b s e r v e d return to s e c o n d o r d e r kinetics w h e n sulfate w a s a d d e d at p H 9.8.  A rate law w a s not g i v e n in this work.  It w a s s u g g e s t e d that the m e c h a n i s m w a s  different a n d m o r e c o m p l i c a t e d than that s u g g e s t e d by B y e r l e y et al ( 1 9 7 3 a ) .  A n o t h e r m e c h a n i s m related to the l e a c h i n g of silver sulfide in the a b s e n c e of air h a s b e e n p o s t u l a t e d , but with little detail g i v e n (Kelly a n d W o o d , 1994). First the reduction of c o p p e r by thiosulfate in the bulk solution ( E q u a t i o n 2.13) w a s p o s t u l a t e d , followed b y the substitution of c o p p e r for silver in the sulfide m o l e c u l e , within the m i n e r a l ( E q u a t i o n 2.14).  23  particle  5 ( N H ) 2 S 0 3 + 2 C u S G - 4 -» C u S 0 . 2 ( N H ) S 0 4  2  2  Cu S 0 .2(NH ) S 0 2  2  3  4  2  2  3  2  3  4  2  2  3  + 2 (NH ) S0 + (NH ) S 0 4  2  + A g S -» C u S + A g S 0 . 2 ( N H ) S 0 2  2  2  2  3  4  2  2  4  4  2  4  6  [2.13]  [2.14]  3  Copper (II) Oxidation of Thiosulfate in Presence of Oxygen T h e reaction b e t w e e n c o p p e r a m m i n e solutions a n d thiosulfate in the p r e s e n c e of o x y g e n but the a b s e n c e of o r e s w a s s t u d i e s by B y e r l e y et al (1975).  T h e y f o u n d that  rather than tetrathionate b e i n g f o r m e d (as s u g g e s t e d a b o v e ( D e J o n g , 2 0 0 4 ) a n d a s noted w h e n n o o x y g e n w a s p r e s e n t (Byerley et al.,1973a)), trithionate a n d sulfate w e r e f o r m e d , in ratios varying with the initial thiosulfate c o n c e n t r a t i o n a n d p H . S i n c e both the trithionate a n d sulfate f o r m e d w e r e s t a b l e with r e s p e c t to further oxidation u n d e r the conditions t e s t e d , it w a s p r o p o s e d that the trithionate w a s not a n intermediate to sulfate formation.  It w a s p r o p o s e d that two reactions w e r e o c c u r r i n g , a s in E q u a t i o n s 2 . 1 5 a n d  2 . 1 6 . C o p p e r d o e s not a p p e a r in t h e s e e q u a t i o n s a s it w a s p r o p o s e d that c o p p e r a c t e d a s a catalyst, a s d i s c u s s e d later.  3 S 0 2  S 0 2  2 3  2 3  " + 2 0  - + 2 0  2  2  + H 0 -> 2 S 0 2  2  3  + 2 O H " -> 2 S 0  6  2 4  - + 2 OH"  [2.15]  " + H 0  [2.16]  2  T h e reaction rate w a s monitored by monitoring the o x y g e n c o n s u m p t i o n . T h e rate of o x y g e n c o n s u m p t i o n s h o w e d a n initial induction p e r i o d , i n c r e a s i n g reaction rate then a linear m a x i m u m rate. T h e initial thiosulfate concentration h a d a significant influence o n the attainment of the m a x i m u m rate. A l s o , the rate of o x y g e n c o n s u m p t i o n w a s virtually identical for a c o p p e r (I) or a c o p p e r (II) a m m o n i a thiosulfate s y s t e m , s o it w a s a s s u m e d that in the former, c o p p e r (I) w a s rapidly o x i d i z e d to c o p p e r (II) a n d then the s y s t e m s w e r e e s s e n t i a l l y equivalent.  T h e initial o x y g e n c o n s u m p t i o n region w a s postulated to involve a build up of a catalytically active c o p p e r -  oxygen complex, which eventually  c o n c e n t r a t i o n at the m a x i m u m reaction rate.  reached a  steady  It w a s f o u n d that at different parts of the  o x y g e n c o n s u m p t i o n c u r v e , different y i e l d s of trithionate a n d sulfate w e r e o b t a i n e d . T h e  24  highest yields of trithionate were observed in the initial part of the oxygen consumption curve, and for the highest initial thiosulfate concentrations (> 0.02 M) and lowest pH values (<10).  Under gold leaching conditions, one would expect a predominantly  trithionate product. At the maximum oxygen consumption rate both trithionate and sulfate were formed concurrently.  The rate of sulfate formation was calculated using the total oxygen  consumption minus the rate of oxygen consumption for trithionate formation. The percentage conversion of thiosulfate to trithionate at the end of oxidation is shown in Table 2.5.  As the initial thiosulfate concentration increased, the relative yield of  trithionate versus sulfate increased. Also as the pH decreased, the yield of trithionate increased. It was also shown that the initial thiosulfate concentration had an effect on the oxygen consumption rate (Byerley et al., 1975).  However, in more recent work  where the thiosulfate oxidation rate was measured directly, the rate was found to be independent of the thiosulfate concentration (Breuer and Jeffrey, 2003a). It is likely that the oxygen consumption rate is not directly related to thiosulfate oxidation (Breuer and Jeffrey, 2003a). While Byerley et al. discussed possible mechanisms for the formation of trithionate from thiosulfate, these were postulated to involve Cu(ll) -  oxygen complexes as  intermediates. This is not discussed here as it has since been shown that the existence of such complexes is not likely (Breuer and Jeffrey, 2003a). While no rate law was given, it was noted by Byerley et al., that the rate of oxygen consumption was proportional to the copper and oxygen concentrations.  25  T a b l e 2.5 - F o r m a t i o n of trithionate from thiosulfate in the p r e s e n c e of o x y g e n ( o x y g e n p r e s s u r e 7 2 5 m m H g . 3 0 °C. fCu(ll)1 = 1 m M . TNH 1 = 0.2 M ) (Bverlev et a l . . ?  1975)  Initial S 0 (mM) 2  PH  2 _ 3  % S c o n v e r t e d to  so 2  3  6  5  11.2  30.6  15  11.2  42.2  25  11.2  50.4  50  11.2  64.1  75  11.2  66.6  100  11.2  75.6  25  11.2  54.0  25  10.0  64.6  25  9.6  81.0  25  9.3  100.0  B r e u e r a n d Jeffrey (2003a) investigated the s y s t e m in a different w a y . T e s t s w e r e d o n e in a flow through s y s t e m , monitoring the c o p p e r (II) a m m i n e c o m p l e x e s using U V - v i s i b l e spectrophotometry.  A c o n c e n t r a t e d c o p p e r ( l l ) - a m m o n i a solution w a s injected into the  v e s s e l a n d after a mixing time the solution w a s continually p u m p e d through a U V cell. T h e cell w a s t e m p e r a t u r e controlled, a n d a b s o r b a n c e w a s m e a s u r e d at 6 0 5 n m .  The  thiosulfate c o n c e n t r a t i o n w a s d e t e r m i n e d by r e m o v i n g a s a m p l e a n d a n a l y s i n g it u s i n g a rotating e l e c t r o c h e m i c a l q u a r t z crystal m i c r o b a l a n c e , by m e a s u r i n g the m a s s c h a n g e s to a silver e l e c t r o d e ( B r e u e r et a l . , 2 0 0 2 ) . T h e s t a n d a r d initial r e a g e n t c o n c e n t r a t i o n s w e r e 0.4 M a m m o n i a , 0.1 M s o d i u m thiosulfate a n d 10 m M c o p p e r sulfate. W h e r e c o p p e r (I) w a s i n v e s t i g a t e d , a n a m m o n i a solution w a s injected into a solution of thiosulfate a n d c o p p e r (I).  M i x t u r e s of c o p p e r (II) a n d thiosulfate, with a n d without air s p a r g i n g , s h o w e d a n initial d e c r e a s e in c o p p e r (II) concentration with time, m o r e s o in the p r e s e n c e of air, while the thiosulfate concentration d e c r e a s e d .  T h i s result w a s c o n s i d e r e d s u r p r i s i n g a s o x y g e n  readily o x i d i z e s c o p p e r (I) to c o p p e r (II).  H e n c e it w a s s h o w n that c o p p e r exists a s both  26  c o p p e r (I) a n d c o p p e r (II) during the oxidation of thiosulfate.  T h i s c o n t r a s t s with the  m e c h a n i s m by B y e r l e y et a l . ( 1 9 7 3 a ) w h e r e it w a s c l a i m e d that there w a s n o d e c r e a s e in c o p p e r (II) c o n c e n t r a t i o n . In B r e u e r a n d Jeffrey's work, s i n c e both the rate of thiosulfate oxidation a n d the rate of copper(ll) reduction w e r e i n c r e a s e d in the p r e s e n c e of o x y g e n , a n alternative m e c h a n i s m to B y e r l e y et al.'s w a s c o n s i d e r e d n e c e s s a r y .  T h e oxidation of thiosulfate w a s slightly faster w h e n the initial c o p p e r w a s p r e s e n t a s c o p p e r (I), a n d the initial i n c r e a s e of c o p p e r (II) w a s rapid.  T h i s s u g g e s t e d that the  reaction products of c o p p e r (I) oxidation m a y b e involved in the oxidation of thiosulfate. T h u s the m e c h a n i s m s of the c o p p e r (I) oxidation by o x y g e n w e r e c o n s i d e r e d .  It is s a i d to b e well e s t a b l i s h e d that the first s t e p s in c o p p e r (I) oxidation by o x y g e n involve the formation of c o p p e r (II) a n d p e r o x i d e , a s d e m o n s t r a t e d in E q u a t i o n 2 . 1 7 .  2Cu  +  + 0  2  ^ 2 C u  2  +  + H 0 " + OH-  [2.17]  2  H o w e v e r , the m e c h a n i s m s for this p r o c e s s a r e not c l e a r .  T h e kinetics s h o w that the  autooxidation of C u ( l ) c o m p l e x e s o c c u r s v i a inner s p h e r e m e c h a n i s m s involving d i o x y g e n a d d u c t s f o r m e d a c c o r d i n g to E q u a t i o n 2 . 1 8 .  Cu  +  + 0  2  ^ C u  +  0  [2.18]  2  Alternatively this m a y b e v i e w e d a s a c o p p e r (II) s u p e r o x i d e c o m p l e x C u 0 " . 2 +  2  T h e fact  that n o s u p e r o x i d e h a s b e e n identified during c o p p e r (I) autooxidation is not surprising g i v e n that its reactivity is c l o s e to diffusion rates  T e s t s to investigate the effect of peroxide o n thiosulfate oxidation w e r e conducted.  therefore  A d d i t i o n of 5 0 m M h y d r o g e n peroxide to 0.1 M thiosulfate a n d 0.4  M  a m m o n i a (without c o p p e r ) g a v e l e s s than 3 0 % thiosulfate oxidation after 1 hour. H o w e v e r , in the p r e s e n c e of c o p p e r (II) a m u c h faster reaction o c c u r r e d .  A  rapid  reaction a l s o o c c u r r e d w h e n p e r o x i d e w a s a d d e d to a thiosulfate solution containing c o p p e r (I) a n d to a thiosulfate, c o p p e r (II) a n d a m m o n i a l e a c h solution.  H e n c e it w a s  implied that the intermediates of peroxide d e c o m p o s i t i o n by c o p p e r o x i d i z e thiosulfate, a n d not peroxide directly. P e r o x i d e h a s b e e n c o n s i d e r e d p r e v i o u s l y to b e i n v o l v e d in the  27  formation of tetrathionate, for e x a m p l e in the electrolytic oxidation of thiosulfate w h e r e p e r o x i d e w a s a s s u m e d to b e active at the a n o d e ( G l a s s t o n e a n d H i c k l i n g , 1954).  A reaction s c h e m e w a s s u g g e s t e d to d e s c r i b e the c o p p e r c a t a l y s e d d e c o m p o s i t i o n of p e r o x i d e taking part in thiosulfate oxidation. T h i s is d e s c r i b e d in E q u a t i o n s 2.19 to 2.26. T w o thiosulfate oxidation p a t h s w e r e predicted - the copper(ll) thiosulfate  reaction  c a t a l y s e d by o x y g e n , evident by the higher initial rate of c o p p e r (II) reduction in the p r e s e n c e of o x y g e n , a n d the oxidation of thiosulfate by s u p e r o x i d e or hydroxide r a d i c a l s a s a result of c o p p e r (I) oxidation.  Cu  2 +  Cu  +  + S 0 2  + 0  2  2 3  " (+ 0 ) -> C u + X (X = u n s p e c i f i e d sulfur containing s p e c i e s )  [2.19]  Cu  [2.20]  +  2  2 0 " + H 0 -> 0 2  Cu  2 +  Cu  +  Cu  +  + GY  2 +  2  2  + H0 " + OH"  [2.21]  2  + H0 " + OH' ~ Cu + 0 " + H 0  [2.22]  +  2  2  + H0 - + H 0 ^ Cu 2  2 +  2  + O H -» C u  2 +  2  + OH + 2 OH"  [2.23]  + OH"  [2.24]  O H + H 0 " -> H 0 + 0 "  [2.25]  R + S 0  [2.26]  2  2  The  2 3  2  2  - -» P ( R = 0 " or O H , P unspecified) 2  effect of o x y g e n concentration w a s investigated by s p a r g i n g the solution with  o x y g e n , air or a g a s mixture containing 1.9 % o x y g e n in nitrogen.  Increased oxygen  c o n c e n t r a t i o n i n c r e a s e d the rate of thiosulfate oxidation a n d c o p p e r (I) oxidation, a n d a s s o c i a t e d reactions a c c o r d i n g to the peroxide reaction s c h e m e . H o w e v e r , the c o p p e r (II) to c o p p e r (I) ratio w a s hardly affected. A l s o the rate of thiosulfate oxidation i n c r e a s e d with i n c r e a s e d air s p a r g e rate, a n d it w a s s u g g e s t e d that the reaction w a s likely to b e limited by the rate of m a s s transfer of o x y g e n into solution.  A n i n c r e a s e in thiosulfate concentration f r o m 0.05 M to 0 . 1 5 M s h o w e d similar m a x i m u m thiosulfate oxidation rates. H o w e v e r , the c o r r e s p o n d i n g c o p p e r (II) concentration profiles s h o w e d that i n c r e a s i n g the thiosulfate concentration i n c r e a s e d the rate of c o p p e r (II) reduction a n d r e d u c e d the c o p p e r (II) to c o p p e r (I) ratio in solution.  28  A d e c r e a s e in c o p p e r concentration by a factor five d e c r e a s e d the thiosulfate oxidation rate by a factor two.  H o w e v e r it a l s o h a d a significant effect o n the solution potential,  w h i c h d e c r e a s e d a s the c o p p e r concentration d e c r e a s e d .  T h e thiosulfate oxidation rate w a s similar for 0.2 M or 0.4 M a m m o n i a but the initial rate of c o p p e r (II) reduction i n c r e a s e d with d e c r e a s i n g a m m o n i a .  T h e effect of p H (at c o n s t a n t a m m o n i a concentration) w a s s h o w n b y c o m p a r i n g reaction rates using a m m o n i u m thiosulfate a n d s o d i u m thiosulfate, with the s a m e a m m o n i a concentration.  It w a s s h o w n ( B r e u e r a n d Jeffrey, 2 0 0 3 d ) that at low p H v a l u e s , e v e n  t h o u g h the a m m o n i a c o n c e n t r a t i o n w a s c o n s t a n t , c o p p e r (II) w a s m u c h m o r e reactive t o w a r d s thiosulfate. S o it w a s not surprising that the c o p p e r (II) concentration d r o p p e d to a lower m i n i m u m level at lower p H . T h i o s u l f a t e oxidation w a s a l s o faster at p H 9.8 than 11.4.  T h e p r e s e n c e of a n i o n s w a s s h o w n to r e d u c e the rate of c o p p e r (II)  reduction by  thiosulfate with p h o s p h a t e b e i n g the m o s t effective. T h i s w a s s h o w n for the s y s t e m with or without o x y g e n present ( B r e u e r a n d Jeffrey, 2 0 0 3 a , b).  T h i s is c o n s i s t e n t with the  theory of a n i o n s c o m p e t i n g for coordination at the c o p p e r (II) axial s i t e s a n d thus r e d u c i n g the rate of c o p p e r (II) reduction by thiosulfate involving o x y g e n .  N o kinetic l a w w a s g i v e n in this c a s e .  Copper (II) Oxidation of Thiosulfate - General Considerations It w a s p r o p o s e d by B y e r l e y et a l . ( 1 9 7 3 a , b, 1975) a n d B r e u e r a n d Jeffrey (2003b) that in the oxidation of thiosulfate by c u p r i c c o p p e r , thiosulfate b e c a m e c o - o r d i n a t e d to the cupric-ammine complex.  S e n a n a y a k e (2004) recently revisited this r e a c t i o n , u s i n g  literature d a t a a n d t h e r m o d y n a m i c a n d kinetic a n a l y s i s .  B a s e d o n p u b l i s h e d stability  c o n s t a n t s , he c o n s i d e r e d the s p e c i e s distribution of the c u p r i c - a m m o n i a - thiosulfate h y d r o x i d e s y s t e m o v e r a r a n g e of p H a n d a m m o n i a a n d thiosulfate c o n c e n t r a t i o n s . O v e r a l l , f r o m p H 7 to Cu(NH ) 3  2 + 4  12, the s p e c i e s C u ( S 0 ) - , 2  2  w e r e predominant.  3  2  Cu(NH ) 3  2 + 3  3  2  3  3  +  and  T h i s s u p p o r t s B y e r l e y et a l ' s v i e w that a m i x e d c o m p l e x  of the type C u ( N H ) ( S 0 ) ° c a n b e involved in thiosulfate oxidation. 3  , Cu(NH ) OH  3  29  A s s u m i n g the g e n e r a l rate law in E q u a t i o n 2 . 2 7 , the author d e t e r m i n e d the  reaction  o r d e r s q , r a n d I to b e 1, -1.1 a n d 1.2 respectively, b a s e d o n a n a n a l y s i s of  literature  data.  -d[Cu ]/dt = k  [2.27]  [Cu ] [NH ] [S 03 -]'  2+  2+  Cu  q  r  2  3  2  H o w e v e r , B r e u e r a n d Jeffrey (2003b) p r o p o s e d that the c u p r i c c o u l d b e s e c o n d o r d e r with r e s p e c t to c u p r i c c o n c e n t r a t i o n .  thiosulfate  reaction  It w a s s u g g e s t e d  by  S e n a n a y a k e (2004) that the reaction c o u l d b e either first or s e c o n d o r d e r with o n e d o m i n a t i n g u n d e r certain conditions.  B a s e d o n work d o n e in the a b s e n c e of a m m o n i a , S e n a n a y a k e f o u n d that the oxidation of thiosulfate by c u p r i c c o p p e r took p l a c e via the m i x e d c o m p l e x C u ( S 0 ) ( H 2 0 ) 2  a n d the rate determining  3  n  p  s t e p w a s the d e c o m p o s i t i o n of this c o m p l e x to  H e n c e two s c e n a r i o s w e r e c o n s i d e r e d in the p r e s e n c e of a m m o n i a -  2 ( n  -  1 )  products.  a first o r d e r  d e c o m p o s i t i o n a n d a s e c o n d order d e c o m p o s i t i o n a s the rate determining s t e p .  F o r the first o r d e r s c e n a r i o , the reaction in E q u a t i o n 2.28 w a s c o n s i d e r e d . Cu(NH )p(S 0 )n- 2(n  2  3  [2.28]  products  1)  3  V a r y i n g the v a l u e s of p (2-3) a n d n (1-2) a n d c o m p a r i n g the p r o p o s e d kinetic law with literature  data,  k  Cu  was  C u ( N H ) ( S 0 ) 2 ~, a n d k  found  2  2  3  be  4  x  10"  s"  4  for  1  Cu(NH ) (S 0 )° 3  = 17 x 10" s" for C u ( N H ) ( S 0 ) 2 '.  2  3  to  4  Cu  1  2  3  3  2  3  3  2  3  and  T h e s e values are  s i m i l a r to that o b t a i n e d by B y e r l e y et al (1973a).  F o r the s e c o n d o r d e r s c e n a r i o , the reaction in E q u a t i o n 2 . 2 9 w a s c o n s i d e r e d a s the rate determining step. 2 Cu(NH )p(S 0 ) 2  3  3  n  2 ( r v 1 )  -» products  [2.29]  F o r this reaction the rate c o n s t a n t k  w a s 0.1 - 0.2 M" .s" for the d e c o m p o s i t i o n of 1  Cu  Cu(NH ) (S 0 )°, Cu(NH ) (S 0 )2 -and Cu(NH )2(S 0 ) . 2  3  3  2  3  3  2  2  0  3  3  30  2  3  1  Pyrite Catalysed Thiosulfate Oxidation in Presence of Oxygen C a t a l y s i s by pyrite h a s a l s o b e e n u s e d to e x p l a i n thiosulfate oxi dati on. In the a b s e n c e of c o p p e r , the rate of tetrathionate formation in a thiosulfate l e a c h s y s t e m w a s directly proportional to the  pyrite  pyrite  surface concentration (Xu and S c h o o n e n ,  implies a s u r f a c e m e d i a t e d  The  dependency  on  mechanism.  F o r pyrite to act a s a catalyst, there h a d to b e interaction of thiosulfate,  pyrite a n d o x y g e n .  surface concentration  1995).  reaction  T h i s w a s d i s c u s s e d in t e r m s of m o l e c u l a r orbital theory.  Since  negligible tetrathionate w a s f o r m e d in a s y s t e m of thiosulfate held u n d e r nitrogen, it w a s a s s u m e d that o x y g e n w a s the terminal electron a c c e p t o r in the oxidation reaction. A l s o , negligible thiosulfate oxidation to tetrathionate o c c u r r e d without pyrite present.  Since  both pyrite a n d thiosulfate h a v e b e e n f o u n d to react slowly with m o l e c u l a r o x y g e n , it w a s postulated that a s u r f a c e c o m p l e x with interaction b e t w e e n the m o l e c u l a r orbitals of thiosulfate, pyrite a n d o x y g e n w a s required for the reaction to p r o c e e d . A m e c h a n i s m w a s d e d u c e d b a s e d o n the following three conditions u s e d to d e t e r m i n e w h e t h e r a r e d o x reaction is likely to o c c u r a c c o r d i n g to m o l e c u l a r orbital theory. 1.  T h e e n e r g y of the lowest u n o c c u p i e d m o l e c u l a r orbital ( L U M O ) m u s t b e l e s s t h a n that of the highest o c c u p i e d m o l e c u l a r orbital ( H O M O ) or within 6 e V a b o v e that of the H O M O .  2.  T h e s y m m e t r i e s of the H O M O a n d L U M O m u s t b e the s a m e to e n s u r e p r o p e r o v e r l a p of the orbitals.  3.  T h e e l e c t r o n transfer m u s t yield a stable e n d product.  T h e reaction m e c h a n i s m postulated involved three electron transfer s t e p s .  T h e first  transfer w a s f r o m thiosulfate to a n a n o d i c site o n pyrite, the s e c o n d w a s from a n a n o d i c site to a c a t h o d i c site o n the pyrite s u r f a c e via its c o n d u c t i o n b a n d , a n d the third w a s f r o m the c a t h o d i c site to the terminal electron a c c e p t o r , o x y g e n ( X u a n d S c h o o n e n , 1995).  31  2.5.2  Disproportionation and Reductive Degradation of Thiosulfate  Disproportionation  of  thiosulfate  to  sulfur  and  sulfate,  or  sulfide  and  sulfite,  is  r e p r e s e n t e d in E q u a t i o n s 2 . 3 0 a n d 2 . 3 1 .  3 S 0 2  2 3  - + H 0 ^ 2  or 3 S 0 2  2 3  [2.30]  2 SO4 - + 4 S + 2 O H " 2  - + 6 O H " -» 4 S 0  2 3  [2.31]  " + 2 S '+ 3 H 0 2  2  Disproportionation is e x p e c t e d to o c c u r in o x y g e n deficient or low potential solutions, or w h e r e there is a high c o p p e r concentration ( A y l m o r e a n d Muir, 2 0 0 1 a ) .  T h i s type of  d e c o m p o s i t i o n of thiosulfate l e a d s to precipitation of e l e m e n t a l sulfur, g o l d , or c o p p e r , g o l d or silver s u l f i d e s (Li et a l , 1996).  E l e m e n t a l sulfur a n d c u p r i c sulfide h a v e b e e n  o b s e r v e d experimentally in the gold thiosulfate l e a c h s y s t e m ( W a n , 1 9 9 7 , A y l m o r e a n d Muir, 2 0 0 1 a ) .  R e d u c t i v e d e c o m p o s i t i o n c a n be r e p r e s e n t e d by E q u a t i o n s 2 . 3 2 , or 2 . 3 3 a n d 2 . 3 4 , in the p r e s e n c e of c o p p e r (Li et a l , 1996).  S 0 2  2 3  - + 8 H + 8 e  2 HS" + 3 H 0  +  Cu + S 0 2 +  2  2 3  2 Cu + S 0  " + 6 H + 6 e ^  2  S + CuS + 3 H 0  +  +  2 3  [2.32]  2  [2.33]  2  - + 6 H + 6 e ^ S + Cu S + 3 H 0  [2.34]  +  2  2  B e s i d e s the l o s s of the thiosulfate lixiviant a n d c o p p e r , i n c r e a s i n g the operating cost, this reaction m a y lead to precipitation of silver or block the s u r f a c e for further l e a c h i n g (Li et a l , 1996).  T h e p a t h w a y for the disproportionation reaction of thiosulfate in the p r e s e n c e of c o p p e r h a s b e e n s u g g e s t e d (Muir a n d A y l m o r e , 2 0 0 2 ) .  It w a s s u g g e s t e d that a fast r e d o x  reaction b e t w e e n c u p r i c c o p p e r a n d thiosulfate w a s followed by s l o w e r s i d e reactions of the c u p r o u s p r o d u c e d , forming C u S , a s in E q u a t i o n 2 . 3 5 . 2  2 Cu + S 0 +  2  2 3  - + H 0 2  Cu S + S0 2  2 4  " + 2 H  +  32  [2.35]  2.5.3  Thiosulfate Degradation Inhibitors  Based  on  Equation  2.31,  it  has  been  suggested  that  sulfite  d e c o m p o s i t i o n (Kerley a n d B e r n a r d , 1 9 8 1 , H e m m a t i et a l , 1989).  inhibits  thiosulfate  Sulfite is c l a i m e d to  prevent the formation of free sulfide (S ") a n d h e n c e precipitation of gold or silver (Kerley 2  a n d B e r n a r d , 1 9 8 1 , Z i p p e r i a n a n d R a g h a v a n , 1988). V e r y low c o n c e n t r a t i o n s ( - 0 . 0 5 %) of sulfite h a v e b e e n u s e d to stabilise thiosulfate but sulfite addition a l s o lowers the potential, w h i c h r e d u c e s c u p r i c in solution ( W a n , 1 9 9 7 , A y l m o r e a n d Muir, 2 0 0 1 a , J i et al, 2001).  Sulfite c a n a l s o b e o x i d i z e d by c u p r i c c o p p e r or o x y g e n to sulfate a n d  dithionate, d e p e n d i n g o n the conditions.  Without sulfite, the following c a n o c c u r in the p r e s e n c e of the o x i d e s of c a l c i u m , a s s h o w n in E q u a t i o n 2 . 3 6 , a n d of iron, a l u m i n i u m , m a n g a n e s e a n d c o p p e r (Kerley a n d B e r n a r d , 1981).  CaO + Ag S 0 2  2  -» A g S + C a S 0  3  2  [2.36]  4  Sulfite is a l s o k n o w n to react with polythionates v i a the g e n e r a l reaction in E q u a t i o n 2.37.  G e n e r a l l y this reaction equilibrium lies to the right ( F o s s a n d K r i n g l e b o t n , 1961).  E q u a t i o n 2.38 s h o w s  how ammonium  sulfite  r e a c t s with tetrathionate to  thiosulfate a n d sulfate (Flett et a l , 1983) thus regenerating thiosulfate.  produce  Trithionate,  h o w e v e r , is s t a b l e in the p r e s e n c e of sulfite ( F l e m i n g et a l , 2 0 0 3 ) .  S0  2 _ 3  + S 0 -  S^Oe + S 0  2  x  2:  2-  6  2  [2.37]  3  ( N H ) S 0 + 2 N H O H + ( N H ) S 0 -» 2 ( N H ) S 0 + ( N H ) S 0 + H 0 4  2  3  4  4  2  4  6  4  2  2  3  4  2  4  2  [2.38]  S u l f a t e h a s b e e n s u g g e s t e d a s alternative to sulfite a c c o r d i n g to E q u a t i o n 2 . 3 9 ( H u a n d G o n g , 1991).  S0  2 4  - + S -+ H 0 2  2  S 0 2  2 3  [2.39]  - + 20H"  T h i s is unlikely a s sulfate is v e r y s t a b l e ( A y m o r e a n d Muir, 2 0 0 1 a ) .  33  C o n v e r s i o n of trithionate a n d tetrathionate to thiosulfate u s i n g sulfide (e.g. N a H S ) or polysulfides after r e m o v a l o f ' g o l d from the solution h a s a l s o b e e n s u g g e s t e d (Ji et a l , 2 0 0 1 , F l e m i n g et a l , 2 0 0 3 ) . T h i s is d e m o n s t r a t e d in E q u a t i o n s 2 . 4 0 a n d 2.41 ( F l e m i n g et al, 2003).  S 0 3  2 6  4 S 0 4  - + S -» 2 S 0 2  2  2 6  2 3  [2.40]  -  - + 2 S - + 6 O H " -» 9 S 0 2  2  2 3  [2.41]  - + 3 H O z  C o n v e r s i o n rates of up to 9 9 % of both trithionate a n d tetrathionate h a v e b e e n a c h i e v e d (Ji et a l , 2 0 0 1 ) . reduce  Sulfide addition w a s a d v o c a t e d a b o v e sulfite addition a s sulfite c a n n o t  trithionate at  common  operating  temperatures  and  pressures and  sulfide  p r o d u c e s n o sulfur containing b y - p r o d u c t s . A l s o s p a r g i n g of a n o n - o x i d i s i n g g a s s u c h a s H S or S 0 2  2  h a s a l s o b e e n u s e d to control the formation of polythionates.  Alternative  reductants s u c h a s h y d r o g e n , fine reactive e l e m e n t a l sulfur a n d c a r b o n m o n o x i d e h a v e a l s o b e e n s u g g e s t e d (Ji et a l , 2 0 0 1 ) . A l l of t h e s e required prior g o l d r e m o v a l to e n s u r e n o g o l d l o s s e s by precipitation.  O t h e r d e g r a d a t i o n inhibitors s u c h a s p h o s p h a t e a n d e t h y l e n e d i a m i n e tetraacetic a c i d ( E D T A ) h a v e a l s o b e e n m e n t i o n e d briefly ( B r e u e r a n d Jeffrey, 2 0 0 3 a ) .  T h e u s e of s u c h additives is e x p e c t e d to b e limited to tank l e a c h i n g a p p l i c a t i o n s a n d not to b e of particular u s e in h e a p l e a c h i n g ( W a n , 1997).  2.6  TRITHIONATE DEGRADATION  T h e polythionates a r e well k n o w n for the e a s e with w h i c h they u n d e r g o  heterolytic  c l e a v a g e in r e a c t i o n s with nucleophilic r e a g e n t s , usually at a sulfenyl sulfur a t o m d u e to the electrophilic c h a r a c t e r of the divalent sulfur a t o m s of the c h a i n s (Ritter a n d K r u e g e r 1970).  D e p e n d i n g o n the properties of the n u c l e o p h i l e , trithionate c a n u n d e r g o  n u c l e o p h i l i c attack at either the sulfenyl sulfur a t o m or the sulfonate sulfur a t o m .  Direct  b i m o l e c u l a r n u c l e o p h i l i c substitution ( S 2 ) at the sulfenyl sulfur a t o m is s h o w n in F i g u r e N  2 . 5 (Ritter a n d K r u e g e r , 1970).  It h a s b e e n c l a i m e d that of the two potential s i t e s for  n u c l e o p h i l i c attack, the sulfenyl sulfur a t o m is m u c h m o r e reactive than the sulfonate  34  sulfur a t o m .  T h e sulfonate sulfur a t o m resists losing its s u l f u r - o x y g e n b o n d s ( S s e k a l o  a n d B a m u w a m y e , 1993) a n d is generally only subject to attack by hydroxide or other h a r d b a s e s (Ritter a n d K r u e g e r , 1970).  Soft polarizable b a s e s a r e e x p e c t e d to attack  trithionate at the sulfenyl sulfur a t o m .  H a r d b a s e s of low polarizability a n d  high  electronegativity (e.g. O H ' , ( C H ) N ) a r e m u c h l e s s reactive t o w a r d s trithionate (Ritter 2  5  3  a n d K r u e g e r , 1970).  ~Nu  O  II  +  0 = S-S-Nu  o = s - s - s = o _  ll  I  I  O  '  o  o  V 5  O  o  II  + o - s  J  I  o  o  F i g u r e 2.5 - B i m o l e c u l a r nucleophilic substitution at sulfenyl sulfur of trithionate  S p e c i f i c e x a m p l e s of nucleophilic substitution reactions relevant to gold l e a c h i n g a r e d i s c u s s e d in following s e c t i o n s .  2.6.1  Interaction with Water  K u r t e n a c k e r et  al  (1935)  showed  that the  following  two  equations  held for  the  d e c o m p o s i t i o n of trithionate in a q u e o u s solutions.  S 0 3  2 6  - + H 0 -» S 0  2 S 0 3  2  2 6  2  2 _ 3  + S0  - + 3 H 0 -> S 0 2  2  2 3  2 _ 4  + 2 H  - + 4 S0  2 3  [2.42]  +  - + 6 H  [2.43]  +  E q u a t i o n 2 . 4 2 w a s f o u n d to o c c u r f r o m p H 5.6 to p H 12 at 5 0 °C while E q u a t i o n 2 . 4 3 w a s f o u n d to o c c u r at p H 13.4 at 50 °C. W h i l e t h e s e e q u a t i o n s h a v e b e e n well a c c e p t e d in the literature, the effects, if a n y , of the s p e c i e s u s e d for p H a d j u s t m e n t s hydroxide or c a r b o n a t e ions) w e r e not g i v e n a n y c o n s i d e r a t i o n .  Kinetic testwork o n trithionate d e g r a d a t i o n in w a t e r is s u m m a r i z e d in T a b l e 2.6.  35  (e.g.  2.6.2  Interaction with Hydroxide  B a s e d o n K u r t e n a c k e r et a l ' s findings (1935) the fact that trithionate d e g r a d a t i o n in w a t e r follows a different reaction at high p H ( s e e E q u a t i o n s 2 . 4 2 a n d 2.43) implies that the h y d r o x i d e c o n c e n t r a t i o n h a s a n effect o n the d e g r a d a t i o n path.  In his work, t h e  d e g r a d a t i o n rate w a s h i g h e r for p H v a l u e s w h e r e E q u a t i o n 2 . 4 3 w a s v a l i d , than for Equation 2.42.  R o l i a a n d C h a k r a b a r t i (1982) did not notice a n y effect of hydroxide  c o n c e n t r a t i o n , but the highest p H that they u s e d w a s p H 11 (at 7 0 ° C ) , lower than that w h e r e E q u a t i o n 2 . 4 3 is e x p e c t e d to p r e d o m i n a t e .  It h a s b e e n s u g g e s t e d that hydroxide  w o u l d attack trithionate directly at the sulfonate sulfur a t o m (Ritter a n d K r u e g e r , 1970).  2.6.3  Interaction with Ammonia  It h a s b e e n s u g g e s t e d that E q u a t i o n 2 . 4 4 h o l d s for the interaction b e t w e e n trithionate a n d a m m o n i a (Naito et a l , 1975). T h e p r e s e n c e of s u l f a m a t e w a s not c o n f i r m e d in t h e s e tests.  T h e stoichiometry of the a m m o n o l y s i s reaction w a s verified by the authors in  a n h y d r o u s liquid a m m o n i a , but not p u b l i s h e d . P r e v i o u s l y s u l f a m a t e h a d b e e n m e a s u r e d during the d e g r a d a t i o n of trithionate at 1 1 0 ° C at a m m o n i a c o n c e n t r a t i o n s of 4 to 8 N ( S h i e h et a l , 1965).  B a s e d o n this, it w a s a s s u m e d that s u l f a m a t e w o u l d b e a likely  reaction product at 4 0 °C to 8 0 °C a n d a n a m m o n i a c o n c e n t r a t i o n b e t w e e n a b o u t 0.5 and 2.75 M.  It is not k n o w n h o w likely it is for a m m o n o l y s i s to o c c u r in a q u e o u s  a m m o n i a s o l u t i o n s of low c o n c e n t r a t i o n .  Kinetic testwork o n trithionate d e g r a d a t i o n in  a m m o n i a solutions is s u m m a r i z e d in T a b l e 2 . 7 .  S 0 3  2 6  2.6.4  - + 2 N H -> S 0 3  2  2 3  ' + NH S0 " + NH 2  3  [2.44]  + 4  Interaction with Thiosulfate  E x c h a n g e r e a c t i o n s a r e c o m m o n a m o n g s t the sulfur o x y a n i o n s .  T h e kinetics of the  e x c h a n g e b e t w e e n trithionate a n d thiosulfate is s u m m a r i s e d in T a b l e 2 . 8 , a l o n g with the kinetics of the effect of thiosulfate o n trithionate d e g r a d a t i o n in w a t e r a n d a q u e o u s ammonia  solutions.  T h e rate  of the e x c h a n g e reaction  between  trithionate  and  thiosulfate w a s f o u n d to i n c r e a s e with the concentration a n d c h a r g e of the positive i o n s , h e n c e it w a s p r o p o s e d that the interaction w a s b e t w e e n a n ionic c o m p l e x of thiosulfate,  36  trithionate  or both, rather than b e t w e e n free ions ( F a v a a n d P a j a r o , 1954).  As a  n u c l e o p h i l e , thiosulfate is e x p e c t e d to interact with trithionate through the sulfenyl sulfur of trithionate.  2.6.5  Interaction with Copper  C u p r i c c o p p e r h a s b e e n reported to d e g r a d e trithionate (Kelly a n d W o o d , 1994).  S*  s h o w s a radioactively m a r k e d isotope of sulfur to g i v e a n indication of the reaction pathway, in E q u a t i o n 2 . 4 5 . T h e conditions for this reaction w e r e not g i v e n .  [0 S-S*-S0 ] - + Cu 2  3  2 +  3  + 2 H 0 - CuS* + 2 S0 2  2 4  " +4 H  [2.45]  +  In a study of the reaction b e t w e e n c o p p e r (II) a n d thiosulfate in a m m o n i a c a l solutions in the a b s e n c e of o x y g e n , it w a s p r o p o s e d that the thiosulfate ion b e c a m e c o - o r d i n a t e d to the c o p p e r - a m m o n i a c o m p l e x before c o p p e r reduction ( B r e u e r a n d Jeffrey, 2 0 0 3 b ) . A d d i t i o n of trithionate to this s y s t e m i n c r e a s e d the c o p p e r reduction rate, s o it w a s a s s u m e d that trithionate c a n r e d u c e c o p p e r but the p o s s i b l e reaction o c c u r r i n g u n d e r t h e s e c o n d i t i o n s w a s not d i s c u s s e d . r e d u c e d the reaction rate.  A d d i t i o n of sulfate at the start of the test greatly  It w a s d e d u c e d that the c o p p e r -  trithionate  reaction  p r o c e e d e d v i a a n inner s p h e r e m e c h a n i s m , w h e r e trithionate m u s t first d i s p l a c e sulfate f r o m the c o p p e r co-ordination s p h e r e in order to react.  It w a s c l a i m e d that the reactions b e t w e e n c u p r i c c o p p e r a n d trithionate o r tetrathionate a r e f a s t e r than thiosulfate. gold leach solutions.  H o w e v e r , w e still s e e significant a m o u n t s of trithionate in  T h i s m a y b e a n indication that there is insufficient c o p p e r (II)  a v a i l a b l e o r that the reaction b e t w e e n c o p p e r a n d trithionate  is not a s rapid a s  s u g g e s t e d , o r h a s a m o r e c o m p l e x d e p e n d e n c y o n other c o n d i t i o n s ( e . g . thiosulfate concentration).  A l s o , in recent work investigating the effects of m i n e r a l s o n thiosulfate  d e g r a d a t i o n ( D e J o n g , 2 0 0 4 ) , the formation rate of trithionate g e n e r a l l y i n c r e a s e d in the p r e s e n c e of c o p p e r , w h i c h is contrary to what o n e w o u l d e x p e c t if c o p p e r r e a c t e d f a s t e r with trithionate than thiosulfate.  37  T a b l e 2.6 - Trithionate degradation in water Ref.  Naito U s e d e t h a n o l / ?i?h w a t e r mixtures 1975 to vary [ H 0 ] . 2  so 2  3  6  degradation determined by measuring  [ S 0 3 2 1 and 2  a s s u m i n g the reaction in Equation 2.42.  Rolia C o n s t a n t p H fjji tests using ^ N a O H addition. i a B  Hof- D e c o m p o s i t i o n mannin s o d i u m Bang, 1950 a c e t a t e , p H 5 -  9. S 0 " measured to infer S 0 degradation from Equation 2.42. 2  2  3  2 _  3  6  - d [ S C V - ]/dt = k [ H 0 ] [ S C V T 3  w  2  3  ?7n fln^ 3  - ou o )  k k k  = 3.56 x 1CV M - . m i n at 4 0 °C = 9 . 8 0 x 10" M . m i r V at 5 0 °C = 7.04 x 10" M - . m i n at 7 0 °C 6  w  1  6  w  5  w  1  -1  S 0 3  2 6  -+H 0^ S O .H 0 2  2  3  e  2  3  6  2  3  2  not h a v e appropriate. Indirect determination o f  v a , i d  +  4  1  b  e  e  m  a  v  n  2 s t e p rate determining, W a t e r w a s e x p e c t e d to attack the sulfonate sulfur a t o m directly.  kinetics.  91.7 kJ/mol (70 - 8 5 °C)  A s s u m e d Naito's mechanism.  T e m p e r a t u r e s higher than gold l e a c h i n g .  86.4 kJ/mol ( 2 5 - 4 0 °C)  Not d i s c u s s e d .  W o r k related to iodinea z i d e reaction.  n d  1  P H not g i v e n - the reaction a s s u m e d to b e  2  2  A d d i t i o n of s a l t s g a v e a n a v e r a g e value of k =1.08 x 10" M - . m i n at 5 0 °C. 6  2  S 0 .H 0 " -» S 0 - + S 0 - + 2 H  1  1  Discussion  Mechanism  Activation energy  Kinetics  Experimental  1  w  T h i s effect w a s not d i s c u s s e d b y the a u t h o r s . - d [ S 0 - ]/dt = k [ H 0 ] [ S 0 - ] (assumed) k = 6 . 7 8 x 1 0 M- .min (70 ° C , p H 11) 1U« — - MC d [ S r\ 0 "2- ]/dt = k [H 0][S 0<n (assumed) 2  3  2  6  w  2  - 5  1  3  6  1  w  3  k k  6  w  6  w  3  = 0 . 8 3 x 10" M . m i n at 2 5 °C = 4 . 5 9 x 10" M - . m i n at 4 0 °C 6  w  2  1  1  1  1  T a b l e 2.7 - Trithionate degradation in a m m o n i a Ret. Naito etal, 1975  -d[S 0 -]/dt = (k [H 0] +  Discussion  Mechanism  Activation energy  Kinetics  Experimental 2  3  degradation d e t e r m i n e d by measuring  [S 0 3 2 1 and 2  a s s u m i n g the reaction in E q u a t i o n 2.44. [NH ] 0.5 - 2 . 7 5 M. 3  6  w  2  k [NH ])[S 0 -] a  3  3  Sn oi o°[ S 0 (40 — 8 0 C ) 5  k  m  3  2  6  2 6  - NH +  3  -  S 0 .NH " 2  3  6  3  S 0 .NH " ->S 0 -+NH S0 +H 2  k = 5.18 x 1 0 M . m i n at 4 0 °C k = 3.41 x 10" lvr .mirr at 6 0 °C 5  1  1  a  4  1  3  6  3  2  2  3  2  +  3  1  a  2  step rate determining.  A m m o n o l y s i s reaction p r o c e e d s in competition with hydrolysis reaction.  T h e stoichiometry w a s not verified u n d e r the conditions u s e d . The p H o f t h e a m m o n o l y s i s tests w a s not indicated. Indirect determination of kinetics.  T a b l e 2.8 - Trithionate d e g r a d a t i o n in the p r e s e n c e of thiosulfate Activation energy 56 k J / m o l (25 - 51 °C)  Kinetics  Experimental  Ref.  Fava E x c h a n g e reaction and b e t w e e n S 0 and Pajaro, S 0 ' m e a s u r e d using 1954  Rate = k [ S 0 ] [ S 0 ] 2  3  3  6  2 _  2  3  2  3  6  radioactively m a r k e d S *  o s-s-so +s*-so 2  3  O3S-S' - S 0  -1^ o  Naito et al, 1975  2  3  3  2 3  +S-S0  2 3  "  S 0 degradation d e t e r m i n e d by measuring [ S 0 T and a s s u m i n g the reaction in E q u a t i o n 2 . 4 2 . 3  6  R a t e not a p p r e c i a b l y affected by p H (7 to 10) o r b y a 10-fold i n c r e a s e in the s u r f a c e to v o l u m e ratio. k = v a r i e d with concentration of inert s a l t s , e s p e c i a l l y concentration a n d c h a r g e of positive i o n s . - d l S a O ^ / d t = ( k [ H 0 ] + k [NH ] + k [S 0 l)[S 0 -] w  2  t  2  a  2  3  2  3  3  6  Independent of the type or concentration of inert salt.  53 kJ/mol (40 - 8 0 °C)  Discussion  Mechanism -OaS-S-SOy+S^SOa'-  ~o s-s-s*-so +so 2  3  3  3  ^ 0 - S - S * - S 0 + S - S O 23  3  P r o p o s e d that reaction w a s b e t w e e n trithionate a n d ionic c o m p l e x of thiosulfate (due to effect of positive ions). Thiosulfate essentially b e h a v e s a s a catalyst.  2  2  3  k, = 4 . 1 5 x 1 0 - * M- .min- at 4 0 °C k, = 2 . 0 5 x 10" M " . m i n at 6 0 ° C 1  2  1  1  1  s o "+s o -^ 2  3  2  6  2  3  S3O6  .S 0 2  S 0 ".S 0 "+H 0 -*2 S 0 -+S0 "+2H 2  3  2  6  2  3  2  2  2  1  Rolia et al, 1982  C o n s t a n t p H tests, p H 5.5 - 8, 8 5 °C  S 0 " (up to 1 0 m M ) i n c r e a s e d S 0 ' d e g r a d a t i o n rate. 2  2  3  2  3  6  st  3  3  2  +  4  step rate determining.  Naito et a l ' s m e c h a n i s m a s s u m e d (1975)  The mechanism proposed w o u l d involve a large transition state - it w a s not e x p l a i n e d w h y this w o u l d be f e a s i b l e or w h y the reaction rate would increase.  2.7  TETRATHIONATE DEGRADATION  A s d i s c u s s e d in the s e c t i o n o n trithionate ( S e c t i o n 2.6) the polythionates a r e well k n o w n for their reactions with nucleophilic r e a g e n t s .  T h i s s e c t i o n e x a m i n e s s o m e of the  r e a c t i o n s b e t w e e n tetrathionate a n d other solution c o m p o n e n t s .  2.7.1  Interaction with Hydroxide  Tetrathionate  is k n o w n to be unstable in alkaline solutions.  Tetrathionate  c a n be  d e c o m p o s e d to v a r i o u s p r o d u c t s by i n c r e a s i n g the p H (Muir a n d A y l m o r e , 2 0 0 2 , R o l i a a n d C h a k r a b a r t i , 1 9 8 2 , Naito et a l , 1 9 7 0 b , L y o n s a n d N i c k l e s s , 1968).  D e g r a d a t i o n of  tetrathionate in a q u e o u s a m m o n i a s h o w e d the following (Naito et a l , 1 9 7 0 b , S m i t h a n d H i t c h e n , 1976).  p H 8.9  2 S 0  6  p H 11.5  4 S 0  6  2  4  3  2  4  - -> S 0  2 6  - + S 0  2  5  - + 6 OH" ^  6  2 S 0  '  [2.46] 2  3  6  " + 5 S 0 2  2 3  " + 3 H 0 2  [2.47]  Further d e c o m p o s i t i o n of the trithionate a n d pentathionate f o r m e d is s h o w n in E q u a t i o n s 2 . 4 8 to 2 . 5 0 ( R o l i a a n d C h a k r a b a r t i , 1 9 8 2 , Naito et a l , 1970b).  p H 12  S 0  p H 13  2 S 0  6  2 S 0  6  3  2 6  - + 2 0 H " -> S 0  3  5  2  2  2  3  " + 6 O H " -» S 0  3  " + 6 OH" ^  2  2  2  " + S0 2  2 4  " + H 0  " + 4 S0  5 S 0  2 3  [2.48]  2  2 3  " + 3 H 0 2  " + 3 H 0 2  [2.49] [2.50]  H e n c e , if tetrathionate is a g e d in strongly alkaline s o l u t i o n s , it will finally f o r m thiosulfate a n d sulfite (Naito et a l , 1970b) a s s h o w n in E q u a t i o n 2 . 5 1 .  2 S 0 4  2 6  " + 60H"  3 S 0 2  2 3  " + 2 S0  2 3  " + 3 H 0 2  [2.51]  O t h e r reaction s c h e m e s for the alkaline d e c o m p o s i t i o n of tetrathionate h a v e a l s o b e e n suggested.  Tetrathionate  h a s b e e n c l a i m e d to d e g r a d e to thiosulfate, sulfite  and  sulfoxylic a c i d ( S ( O H ) ) , with further d e g r a d a t i o n of the sulfoxylic a c i d to sulfide (Muira 2  a n d K o h , 1983). T h e reaction e q u a t i o n s a r e s h o w n below. It w o u l d be e x p e c t e d that the sulfide a n d sulfite in E q u a t i o n 2.54 w o u l d c o m b i n e to p r o d u c e thiosulfate.  41  10 S 0 4  2 6  - + 2 0 O H " -» 10 S 0  6 S(OH) + 6 O H - > 3 S 0 2  2  2 3  " + 10 S ( O H ) + 10 S 0  2  2  3  2  2  [2.52] [2.53]  - + 9 H 0 2  4 S ( O H ) + 8 O H " -» % S - + / S 0 2  2; 3  8  3  2 3  [2.54]  " + 8 H 0 2  T h e r e h a v e b e e n s o m e s t u d i e s o n the kinetics of tetrathionate d e c o m p o s i t i o n in alkaline s y s t e m s , with details g i v e n in T a b l e 2.9.  In the a b s e n c e of a m m o n i a , c o p p e r a n d o x y g e n , the rate of the alkaline d e c o m p o s i t i o n of tetrathionate w a s d e s c r i b e d by Z h a n g a n d D r e i s i n g e r (2002).  N o build-up  of  trithionate w a s f o u n d in this testwork, implying that either trithionate w a s not a part of the m a i n reaction m e c h a n i s m or that it d e c o m p o s e d at a similar rate to tetrathionate.  The  p o s s i b l e catalytic effect of the thiosulfate product o n further tetrathionate d e g r a d a t i o n w a s not c o n s i d e r e d .  R o l i a a n d C h a k r a b a r t i (1982) did similar work but in the p r e s e n c e of o x y g e n , a n d found that while the rate law c o u l d be e x p r e s s e d in the s a m e w a y a s that of Z h a n g a n d D r e i s i n g e r ( s e e T a b l e 2.9), the rate constant w a s m o r e than 10 t i m e s s m a l l e r .  It w a s  p r o p o s e d by Z h a n g a n d D r e i s i n g e r that the p r e s e n c e of o x y g e n in the w o r k of R o l i a et a l . a n d the a b s e n c e of o x y g e n in their work w a s probably the r e a s o n for this d i s c r e p a n c y , a n d that oxidants c o u l d p o s s i b l y retard the rate of tetrathionate d e g r a d a t i o n .  However,  similar w o r k by B r e u e r a n d Jeffrey (2004) s h o w e d that tetrathionate d e g r a d a t i o n u n d e r air-saturated  conditions  gave  no m e a s u r a b l e  difference  in kinetics to tests  under  nitrogen. T h e i r results w e r e similar to t h o s e of Z h a n g a n d Dreisinger.  It w a s s u g g e s t e d that rather than the p r e s e n c e of oxidants being r e s p o n s i b l e for the difference  in kinetics m e a s u r e d by the two previous g r o u p s , variations in the  ionic  strength of the test solutions w a s r e s p o n s i b l e ( B r e u e r a n d Jeffrey, 2 0 0 4 ) . Increasing the ionic strength w a s f o u n d to h a v e a significant effect o n the initial rate of tetrathionate decomposition.  42  2.7.2  Interaction with Ammonia  T h e effect of a m m o n i a o n the rate of tetrathionate d e g r a d a t i o n w a s m e a s u r e d at 5 0 to 8 0 °C by Naito et al (1970b).  T h e a m m o n i a c o n c e n t r a t i o n w a s f o u n d to h a v e a  significant effect o n the rate, with tetrathionate d e c o m p o s i n g m u c h f a s t e r at higher a m m o n i a concentrations.  In addition to the trithionate, thiosulfate a n d pentathionate  f o r m e d during the alkaline d e c o m p o s i t i o n of tetrathionate, the formation of s u l f a m a t e ( S 0 N H " ) h a s a l s o b e e n p r o p o s e d in the p r e s e n c e of a m m o n i a (without c o p p e r ) . After 3  2  trithionate w a s p r o d u c e d by the alkaline d e g r a d a t i o n of tetrathionate ( E q u a t i o n 2.47), s u l f a m a t e w a s reportedly p r o d u c e d (Equation 2.55) (Naito et a l , 1 9 7 0 b , 1975).  S 0 3  2 6  ' + N H + OH"  S0 NH - + S 0  sulfamate  d e p e n d e d o n the  3  The  3  yield  2  2  2  3  " + H O  [2.55]  z  ammonia  concentration.  c o n c e n t r a t i o n w a s l e s s than 2 N , hardly a n y s u l f a m a t e f o r m e d .  If the  ammonia  If there w a s not e n o u g h  a m m o n i a , the solution b e c a m e neutral or w e a k l y a c i d i c with the precipitation of sulfur and  the formation of higher polythionates.  W i t h e x c e s s free a m m o n i a , the overall  reaction g a v e s u l f a m a t e instead of sulfate. ( E q u a t i o n s 2.56 a n d 2.57)  4 S 0 4  4 S 0 4  2 6  ' + 10 O H "  2 _ 6  7 S 0 2  2 3  " + 2 S0  + 2 N H + 8 O H " -» 7 S 0 3  2  2 3  2 4  " + 5 H 0  [2.56]  2  - + 2 S0 NH - + 5 H 0 3  2  [2.57]  2  In the p r e s e n c e of air or o x y g e n a similar set of e q u a t i o n s c o u l d b e written ( E q u a t i o n s 2 . 5 8 a n d 2.59).  3 S 0  6  3 S 0  6  4  4  2  2  - + 10 O H " + % 0  2  -> 4 S 0  " + 4 N H + 6 OH" + % 0 3  2  2  ^  2 3  " + 4 S0 4 S 0 2  2 3  2 4  " + 5 H 0  [2.58]  2  " + 4 S0 NH 3  2  + 5 H 0  [2.59]  2  B r e u e r a n d Jeffrey (2004) f o u n d that tetrathionate d e c o m p o s i t i o n in the p r e s e n c e of a n a m m o n i u m sulfate / a m m o n i a buffer w a s f a s t e r than for a c a r b o n a t e buffer at the s a m e p H , e v e n t h o u g h the ionic strength w a s lower.  A similar o b s e r v a t i o n w a s m a d e for  p h o s p h a t e buffers in c o m p a r i s o n with a n a m m o n i a c a l s y s t e m .  T h e s e authors  p r o p o s e d that a n i n c r e a s e in ionic strength w o u l d i n c r e a s e the rate of d e g r a d a t i o n . T h i s o b s e r v a t i o n w a s not e x p l a i n e d .  43  had  tetrathionate  2.7.3  Interaction with Copper  In a n investigation of the reaction b e t w e e n thiosulfate a n d c u p r i c c o p p e r in the p r e s e n c e of o x y g e n ( B r e u e r a n d Jeffrey, 2 0 0 3 b ) , the effect of tetrathionate a n d trithionate o n the reaction w e r e a l s o investigated briefly. A d d i t i o n of tetrathionate i n c r e a s e d the c o p p e r (II) reduction rate, implying that tetrathionate w a s a l s o o x i d i z e d by c o p p e r (II).  However,  d e p e n d i n g o n the initial c o n d i t i o n s , the tetrathionate c o n c e n t r a t i o n w a s e x p e c t e d to r e a c h a s t e a d y state, w h e n its production (by thiosulfate oxidation) a n d c o n s u m p t i o n (by c o p p e r (II) reduction) w e r e e q u a l .  If sulfate w a s a d d e d with tetrathionate  to the  thiosulfate s y s t e m , the initial rate of c o p p e r (II) reduction w a s not affected, h e n c e it w a s c o n c l u d e d that tetrathionate did not n e e d to c o m p l e x with c o p p e r (II) for reduction to o c c u r . S i n c e the addition of sulfate h a d b e e n f o u n d to s l o w the reaction b e t w e e n c o p p e r (II) a n d thiosulfate ( a n d h e n c e the production of tetrathionate), then the a m o u n t of tetrathionate at s t e a d y state w o u l d b e lower in this c a s e .  Similarly, if addition of  a m m o n i u m i o n s (reduction of p H ) i n c r e a s e d the c o p p e r (II) reduction r e a c t i o n , the s t e a d y state c o n c e n t r a t i o n of tetrathionate w o u l d b e e x p e c t e d to b e higher.  S o m e solid c o p p e r c o m p o u n d s h a v e b e e n s a i d to i n c r e a s e the rate of tetrathionate oxidation. ( C h a n d a a n d R e m p e l , 1 9 8 6 ) C h a l c o p y r i t e , covellite a n d c h a l c o c i t e h a v e all b e e n s u g g e s t e d to c a t a l y s e oxidation of tetrathionate by air.  In the c a s e of c u p r o u s  o x i d e c a t a l y s i s , only sulfate w a s f o r m e d a s a product. N o reaction e q u a t i o n w a s g i v e n .  2.7.4  Interaction with Thiosulfate  T h e d i s p l a c e m e n t reaction b e t w e e n thiosulfate a n d the polythionates is well k n o w n ( F o s s , 1 9 6 1 , F o s s a n d K r i n g e l b o t n , 1 9 6 1 , F a v a a n d B r e s a d o l a , 1955).  Tetrathionate  c a n u n d e r g o n u c l e o p h i l i c attack at the sulfenyl sulfur, with either sulfite or thiosulfate d i s p l a c e d , a s in E q u a t i o n s 2 . 6 0 a n d 2.61 (where S * d e n o t e s a m a r k e d sulfur atom).  [ 0 S S S 0 ] " + [ S * S 0 ] " -» [ 0 S S * S S 0 ] " + S 0 2  3  2  3  2  3  [0 SSS 0 ] - + [S*S0 ] 2  3  2  3  2  3  2-:  2  3  2  3  [0 SS*S0 ] 2  3  3  3  + [s o|2-r 2  3  [2.60] [2.61]  Similarly sulfite c a n react with tetrathionate, d i s p l a c i n g thiosulfate, a s in E q u a t i o n 2 . 6 2 .  44  [o sss o ] - + so - -> 2  3  2  2  3  [0 SSS0 ] 2  3  3  3  +[ s o ] 2  2  [2.62]  3  T h e kinetics of E q u a t i o n 2 . 6 0 w e r e investigated in the p r e s e n c e of f o r m a l d e h y d e a s a sulfite a c c e p t o r ( F o s s a n d K r i n g e l b o t n , 1961), at neutral p H . A s u m m a r y of the kinetics findings is s h o w n in T a b l e 2.9. T h e positive influence o n the rate by positive ions implied that the intermediate c o m p l e x for the reaction w a s likely f o r m e d not b e t w e e n free ions but ionic c o m p l e x e s of tetrathionate, thiosulfate or both.  T h e e q u a t i o n s a b o v e (2.60 a n d 2.62) c a n be u s e d to e x p l a i n the catalytic effect of thiosulfate  on  tetrathionate  degradation.  Under  non-oxidising  conditions,  some  thiosulfate (about 6 2 . 5 %) c a n be r e g e n e r a t e d from the d e c o m p o s i t i o n of tetrathionate to higher or lower polythionates through the formation of trithionate ( A y l m o r e a n d Muir, 2001a,  Marsden and  House,  1992).  T h i s is highly  c a t a l y s e d by thiosulfate,  as  r e p r e s e n t e d in E q u a t i o n s 2 . 6 3 to 2.66 (Byerley et a l , 1 9 7 3 a , G e l v e s et a l , 1996).  S 0 4  S0  2 3  S 0 5  2 6  - + S 0 2  - + S 0 4  2 6  2 3  2 6  -  S 0 5  - -» S 0 3  - + S0  3  ' + S 0  3  2 6  2 6  2  - + 3 O H " -» / S 0 5  2  Overall: 2 S 0 4  2 6  2  2 3  - + 3 OH"  2  -  [2.63]  2  -  [2.64]  - + %  [2.65]  H 0 2  % S 0 2  2 3  " + S 0 3  2 6  " + %  H 0 2  [2.66]  T h e effect of thiosulfate w a s a d d r e s s e d in a study by R o l i a a n d C h a k r a b a r t i (1982) w h o d e r i v e d a rate e q u a t i o n for tetrathionate d e g r a d a t i o n in the a b s e n c e of a m m o n i a at p H 1 1 . T h i o s u l f a t e w a s f o u n d to i n c r e a s e the rate of tetrathionate d e g r a d a t i o n . T h e kinetics results a r e s h o w n in T a b l e 2.9.  H o w e v e r , B r e u e r a n d Jeffrey (2004) p r o p o s e d that the a p p a r e n t i n c r e a s e in rate w h e n thiosulfate w a s p r e s e n t w a s d u e to the i n c r e a s e in ionic strength that thiosulfate addition w o u l d represent rather than the thiosulfate itself.  45  T a b l e 2.9 - Tetrathionate degradation in neutral to alkaline solutions Ref.  Z h a n g Alkaline et a l , d e g r a d a t i o n . 2 0 0 2 A b s e n c e of o x y g e n . In bicarbonate/ hydroxide o r HPCVhydroxide buffer,  Activation energy 98.5 kJ/mol (22-40 °C)  Kinetics  Experimental  -d[S cV- ]/dt =  k[OHl[S 0 "] 2  4  4  k = 0 . 3 8 x 1 0 M" .s" (22 °C, p H 1 0 - 1 1 . 5 ) 3  1  6  1  Discussion  Mechanism S u g g e s t e d p r e s e n c e of dissolved oxygen and/or c o p p e r had a role in mechanism.  p H a n d temperature dramatically affect degradation rate in alkaline solution.  N o n e given.  R a t e constant 10 times s m a l l e r than Zhang.  d e a e r a t e d with N , under N 2  0\  Rolia et a l , 1982  2  - d [ S 0 " ]/dt = (k + k [ S 0 1 ) [ O H 1 [ S 0 l  Naito et a l , 1970b  Alkaline d e g r a d a t i o n in p r e s e n c e of thiosulfate. Constant pH tests u s i n g N a O H addition. O x y g e n present. Ammoniacal d e g r a d a t i o n at 5 0 - 8 0 °C  Foss et a l , 1961  Thiosulfate exchange reactions  - d [ S 0 - ]/dt = k [ S 0 1 [ S 0 T  2  4  6  2  2  1  2  2  = 0.022 M \ s " k = 2 . 7 7 M" . s (25 ° C , p H 11) 2  4  3  6  115.5 k J / m o l (15-45 °C, p H 11)  1  1  2  None given  2  4  2  2  6  2  k = 1.3 x IO" M " . s ' (25 ° C , l = 1.15 M ) 3  1  1  3  4  6  so 2  so 2  50.2 kJ/mol S 0 " + 2 3 5 6 ( 2 0 - 4 0 °C, I + S 0 ' via nucleophilic = 1.15 M ) d i s p l a c e m e n t 2  4  6  2  3  In p r e s e n c e of > 2 M a m m o n i a , sulfamate formed f r o m further d e g r a d a t i o n of trithionate product.  REMOVAL OF POLYTHIONATES FROM SOLUTION  2.8  Polythionates are detrimental in gold recovery from thiosulfate leach solutions using adsorbants such as resins.  Besides the reactions already discussed to remove  polythionates from solution, by hydrolysis, ammonolysis or interchange between various sulfur species, it is possible to remove polythionates in other ways. While many of these methods do not allow for recovery of thiosulfate, this may not be of importance in cases where reduction of thiosulfate consumption is not critical.  There is also an  environmental concern of discharge of polythionates, as these can produce acid on further oxidation to sulfate under suitable conditions. A few potential methods are given in Table 2.10. This summary simply gives an indication of the types of processes available and has not been studied in any detail for this review. Table 2.10 : Potential Methods for Polythionate Removal Method Bacterial oxidation M n 0 oxidation 2  HOCI oxidation  Reference Hansford and Vargas, 2001 De Jong et al, 1997 Sand etal, 1995  A number of bacteria, generally requiring acidic  Schlppers and Jorgenson, 2001 Horvarth and Nagypal, 2000  Oxidation to sulfate.  Sulfite addition  Kerley and Bernard, 1981 Ji etal, 2001  Sulfide addition  Ji etal, 2001  Alkaline treatment Cyanide  Zhang and Dreisinger, 2002 Nor and Tabatabai, 1975 Koh,1990 Mizoguchi and Okabe, 1975  conditions, can oxidize various polythionates to sulfate.  Oxidation to sulfate. Reversal to thiosulfate and other lower polythionate. Reversal to thiosulfate. Reversal to thiosulfate and other polythionates. Thiosulfate and sulfate produced, and other unwanted products. Anion exchange resins can be used to adsorb  Selective resin adsorption  Applicability  Wassink, 2002  thiosulfate and polythionates, with selective elution of thiosulfate.  47  2.9  SUMMARY OF LITERATURE FINDINGS  T h e r e is a limited u n d e r s t a n d i n g of the d e g r a d a t i o n of thiosulfate in g o l d l e a c h i n g systems.  W h i l e thiosulfate c o n s u m p t i o n is u s u a l l y d o c u m e n t e d in l e a c h tests, the  d e g r a d a t i o n p r o d u c t s a r e not often identified or quantified. T h e r e h a v e b e e n a n u m b e r of f u n d a m e n t a l s t u d i e s o n the d e g r a d a t i o n of thiosulfate to predominantly trithionate a n d / o r tetrathionate.  It s e e m s that o n e c a n e x p e c t trithionate to form directly w h e n  o x y g e n is present, rather than a s a d e c o m p o s i t i o n product of tetrathionate,  while  tetrathionate is e x p e c t e d in the a b s e n c e of o x y g e n . T h e m e c h a n i s m s by w h i c h t h e s e s p e c i e s f o r m a n d the w a y s in w h i c h the solution c o n d i t i o n s affect their rates of formation a n d their proportions f o r m e d relative to other sulfur o x y a n i o n s is not u n d e r s t o o d . H o w e v e r , there is currently a fair a m o u n t of r e s e a r c h b e i n g d e v o t e d to the study of thiosulfate d e g r a d a t i o n u n d e r g o l d l e a c h i n g conditions.  T h e s u b s e q u e n t b e h a v i o u r of  the thiosulfate d e g r a d a t i o n products is not well u n d e r s t o o d .  Tetrathionate h a s b e e n identified in gold l e a c h s o l u t i o n s , but it is u s u a l l y q u i c k to d e g r a d e u n d e r the alkaline conditions u s e d in g o l d l e a c h i n g . T h i o s u l f a t e c a t a l y s e s this d e g r a d a t i o n , a n d it is postulated that c o p p e r c a n a l s o i n c r e a s e the d e g r a d a t i o n rate. Tetrathionate is not a s persistent a s trithionate u n d e r m a n y g o l d l e a c h i n g c o n d i t i o n s .  Trithionate is m o r e persistent in gold l e a c h i n g solutions but very little is k n o w n about this s p e c i e s in the context of g o l d l e a c h i n g . trithionate h a s not  b e e n directly  M u c h of the a v a i l a b l e literature c o n c e r n i n g  related to g o l d  l e a c h i n g : the  older  literature  is  f u n d a m e n t a l in nature a n d other s t u d i e s h a v e b e e n p u b l i s h e d in the context of sulfur oxidation p a t h w a y s during the a m m o n i a c a l treatment of s u l f i d e s . T o b e a b l e to a s s e s s thiosulfate  degradation  in  a  larger  context,  it  is  e s s e n t i a l to  develop  a  better  u n d e r s t a n d i n g of the b e h a v i o u r of trithionate.  v Alleviating the p r o b l e m of thiosulfate d e g r a d a t i o n is critical to the s u c c e s s of the thiosulfate l e a c h i n g p r o c e s s for g o l d . T h i s review h a s s h o w n that while the d e g r a d a t i o n of thiosulfate is not fully u n d e r s t o o d , there is a fair a m o u n t of r e s e a r c h currently u n d e r w a y to investigate f u n d a m e n t a l s of this d e g r a d a t i o n p r o c e s s . H o w e v e r , to b e a b l e to u n d e r s t a n d the s y s t e m in its entirety, it is n e c e s s a r y to u n d e r s t a n d the b e h a v i o u r of the other sulfur o x y a n i o n s e x p e c t e d to b e p r e s e n t , mainly trithionate a n d tetrathionate.  48  O f t h e s e two s p e c i e s , trithionate often a p p e a r s to b e m o r e persistent in the alkaline s o l u t i o n s u s e d , but v e r y little is u n d e r s t o o d about this s p e c i e s . t h u s d i s c u s s e x p e r i m e n t a l work  T h e following c h a p t e r s  carried out to better u n d e r s t a n d the b e h a v i o u r of  trithionate, followed by the integration of the findings with the e x p e c t e d b e h a v i o u r of thiosulfate a n d tetrathionate b a s e d o n literature o b s e r v a t i o n s .  2.10  SCOPE AND OBJECTIVES  T h e s p e c i f i c s c o p e a n d objectives of this study are outlined below:  •  T o further the u n d e r s t a n d i n g of trithionate solution c h e m i s t r y u n d e r conditions relevant to gold l e a c h i n g by thiosulfate by e x p e r i m e n t a l l y determining the kinetics of trithionate d e g r a d a t i o n a n d identifying the effects of v a r i o u s solution conditions o n the kinetics.  •  T o incorporate the findings  into a m o d e l to predict trithionate  degradation  kinetics. •  T o u s e literature d a t a a n d rate e q u a t i o n s d e s c r i b i n g thiosulfate a n d tetrathionate r e a c t i o n s , a s well a s the experimentally d e r i v e d trithionate d e g r a d a t i o n  rate  e q u a t i o n to d e v e l o p a s i m p l e kinetic m o d e l to d e t e r m i n e the e x p e c t e d s u l p h u r o x y a n i o n solution s p e c i a t i o n during gold l e a c h i n g u s i n g thiosulfate. •  T o e v a l u a t e the sensitivity of the m o d e l to p a r a m e t e r s in the rate e q u a t i o n s .  •  T o e v a l u a t e the ability of the m o d e l to a d e q u a t e l y d e s c r i b e e x p e r i m e n t a l d a t a .  •  T o identify  limitations  of the m o d e l to d e t e r m i n e w h e r e further r e s e a r c h is  required to improve the m o d e l . •  To  suggest  ways  in  which  thiosulfate  degradation  polythionates c a n b e m i n i m i s e d during gold l e a c h i n g .  49  or  the  formation  of  3  ANALYTICAL METHODS AND SYNTHESIS  3.1  INTRODUCTION  In this chapter, the analytical m e t h o d s u s e d in this w o r k are d e s c r i b e d .  A l s o , the  s y n t h e s i s a n d c h a r a c t e r i s a t i o n of the s o d i u m trithionate u s e d in the kinetic work a n d for analytical calibration p u r p o s e s is d e s c r i b e d . T h e e x p e r i m e n t a l p r o c e d u r e for the kinetic testwork is d e s c r i b e d in C h a p t e r 4.  3.2  ANALYSIS OF SULFUR OXYANIONS - ION CHROMATOGRAPHY  3.2.1  Description of Method  S o l u t i o n s a m p l e s w e r e a n a l y s e d for thiosulfate, trithionate, tetrathionate a n d sulfate u s i n g high p e r f o r m a n c e liquid c h r o m a t o g r a p h y ( H P L C ) using a D i o n e x S e r i e s 6 0 0 system.  A n a l y s i s of thiosulfate, trithionate a n d tetrathionate involved s e p a r a t i o n of the  s p e c i e s o n a n O m n i P a c P A X - 1 0 0 c o l u m n , a n d a n a l y s i s of the s e p a r a t e d s p e c i e s u s i n g U V - v i s i b l e a b s o r p t i o n s p e c t r o m e t r y at 2 0 5 n m .  Sulfate w a s s e p a r a t e d from the other  sulfur o x y a n i o n s u s i n g a n l o n P a c A S 4 A - S C c o l u m n a n d m e a s u r e d by detection.  conductivity  B e c a u s e the determination of sulfate w a s by a different m e t h o d , it w a s not  p o s s i b l e to a n a l y s e a s i n g l e s a m p l e for all the s p e c i e s of interest s i m u l t a n e o u s l y .  More  details of the c h r o m a t o g r a p h y m e t h o d s a r e given in A p p e n d i x 3.  C h r o m a t o g r a p h y requires the u s e of s t a n d a r d solutions to calibrate the instrument. preparation a n d s t o r a g e of t h e s e solutions is d i s c u s s e d in S e c t i o n 3.2.2.  The  All samples  a n d s t a n d a r d s w e r e diluted to the required concentration r a n g e (generally l e s s than 2 0 mg/l) using ultra-pure d e i o n i s e d water a n d a n a l y s e d immediately.  l o d o m e t r i c titration is often u s e d to d e t e r m i n e thiosulfate, but it is difficult to d e t e r m i n e thiosulfate a n d the polythionates individually in mixtures. C o p p e r a l s o interferes with this method (Wassink, 2002).  50  3.2.2  Stability of Standard Solutions  Experimental Work C a l i b r a t i o n s t a n d a r d solutions of s o d i u m thiosulfate ( S I G M A , a n h y d r o u s , >99% pure), s o d i u m trithionate ( s y n t h e s i s e d , s e e S e c t i o n 3.5) a n d s o d i u m tetrathionate dihydrate) w e r e p r e p a r e d in d e - i o n i s e d water.  (SIGMA,  T h e salts w e r e m a d e up to 1 0 0 0 mg/l  s o l u t i o n s of e a c h a n i o n , then diluted to 100 mg/l s o l u t i o n s , a n d then to 2 , 10, 15 a n d 2 0 mg/l solutions to b e u s e d in calibration. T h e calibration s t a n d a r d s w e r e a n a l y s e d within two h o u r s of preparation.  T o test the stability of s t a n d a r d solutions o v e r l o n g e r t i m e s a n d u n d e r different s t o r a g e c o n d i t i o n s , 100 mg/l a n d 10 mg/l solutions of thiosulfate, trithionate a n d tetrathionate (individually a n d in c o m b i n a t i o n s ) w e r e m a d e up in the s a m e w a y a s the calibration s t a n d a r d s . T h e 100 mg/l solutions w e r e diluted to 10 mg/l i m m e d i a t e l y before a n a l y s i s .  S a m p l e s of the v a r i o u s 100 mg/l a n d 10 mg/l solutions w e r e s t o r e d in g l a s s bottles at r o o m t e m p e r a t u r e o n the laboratory b e n c h , in the dark, with the h e a d s p a c e p u r g e d with nitrogen, in the fridge a n d in the f r e e z e r (defrosted at r o o m t e m p e r a t u r e i m m e d i a t e l y prior to a n a l y s i s ) .  S o m e solutions w e r e a l s o s t o r e d in plastic bottles at  room  t e m p e r a t u r e o n the laboratory b e n c h .  After set times up to about three w e e k s , the solutions w e r e a n a l y z e d for thiosulfate, trithionate a n d tetrathionate.  Results Results  are  general and  quantitative.  S o m e instrumental  problems were  being  e x p e r i e n c e d at the time of this w o r k giving s p u r i o u s results in c a s e s . T h e overall trends a r e s u m m a r i s e d in T a b l e 3 . 1 .  51  T a b l e 3.1 : S u m m a r y of o b s e r v a t i o n s o n stability of s t a n d a r d solutions of thiosulfate, trithionate a n d / o r tetrathionate  Initial solution concentration 10 mg/l S 0 " 2  2  3  1 0 0 mg/l S 0  2  2  3  "  10mg/IS O 2  4  6  100 mg/l S 0 4  10 mg/l S 0 3  2 6  z 6  -  C h a n g e s to thiosulfate < 10 % d e g r a d a t i o n over 22 days. Slightly m o r e s t a b l e in f r e e z e r . < 10 % d e g r a d a t i o n over 22 days. Similar to 10 mg/l solution. Negligible.  Negligible.  Formed as d e g r a d a t i o n product c o r r e s p o n d i n g to 1:1 ratio of S 0 " degraded.  "  2  3  10 mg/l S 0 10 mg/l S 0 2  4  2 3 2 6  100 mg/l S 0 100 mg/l S 0 2  4  10 mg/l S 0 10 mg/l S 0 10 mg/l S 0  2  2  3  4  6  3  2  2 6  100 mg/l S 0 100 mg/l S 0 100 mg/l S 0  " "  D e s t a b i l i s e d by p r e s e n c e of S 0 \ Typically 30 - 50 % d e g r a d a t i o n in 2 2 days. Freezing stabilised. 2  4  2 3 2 6  " -  2 3  4  6  3  6  2  2  6  Negligible degradation. D e g r a d a t i o n in similar r a n g e a s other solutions. S t a b l e o n freezing.  " "  2  6  " "  C h a n g e s to tetrathionate Formed as S 0 " d e g r a d a t i o n product.  C h a n g e s to trithionate None.  Seen as S 0 " d e g r a d a t i o n product  None.  < 10 % d e g r a d a t i o n o v e r 18 d a y s . F r e e z i n g e n h a n c e d d e g r a d a t i o n to - 2 0 % in 18 d a y s . < 10 % d e g r a d a t i o n o v e r 18 d a y s . F r e e z i n g m o r e stable than for 10 mg/l solution - - 1 3 % d e g r a d a t i o n in 18 d a y s . S m a l l a m o u n t present. A n exchange between S 0 " and S 0 " was noted for solutions of S 0 " in w a t e r ( s e e S e c t i o n 3.6). I n c r e a s e d a c c o r d i n g to S 0 " degradation. F r e e z i n g c a u s e d rapid d e g r a d a t i o n of S 0 " by - 6 0 % in 8 d a y s .  N e g l i g i b l e e x c e p t for that f o r m e d f r o m S 0 " d e g r a d a t i o n for f r o z e n s a m p l e in 1:1 ratio. N e g l i g i b l e e x c e p t for that f o r m e d f r o m S 0 " d e g r a d a t i o n for f r o z e n s a m p l e in 1:1 ratio.  2  2  3  2  2  3  2  3  2  6  4  6  2  3  6  2  2  3  2  4  6  4  6  2  - 4 0 % d e g r a d a t i o n in 14 d a y s . S t a b i l i s e d slightly in fridge to g i v e - 2 0 % d e g r a d a t i o n in 14 d a y s . Formed from S 0 " d e g r a d a t i o n in 1:1 ratio for f r o z e n s a m p l e . 2  4  6  2  4  6  Negligible c h a n g e e x c e p t for f r e e z i n g w h e r e - 5 0 % d e g r a d e d in 8 d a y s . I n c r e a s e s e x c e p t for freezing where - 20 % d e g r a d e d in 1 d a y .  Formed from S 0 " d e g r a d a t i o n in 1:1 ratio for f r o z e n s a m p l e . D e g r a d e s at similar rate to S 0 " a l o n e . Increases on freezing from S 0 " d e g r a d a t i o n in 1:1 ratio. 2  4  6  2  3  6  2  4  Stable.  S i m i l a r to 10 mg/l solution.  6  Slightly m o r e stable than 10 mg/l solution. Increases on freezing from S 0 " d e g r a d a t i o n in 1:1 ratio. 2  4  52  6  In g e n e r a l , the conditions u n d e r w h i c h the sulfur o x y a n i o n s o l u t i o n s w e r e s t o r e d did not significantly affect their stability, with the e x c e p t i o n of refrigeration w h i c h s t a b i l i s e d trithionate, a n d f r e e z i n g w h i c h significantly e n h a n c e d tetrathionate d e g r a d a t i o n .  In  g e n e r a l , the m o r e c o n c e n t r a t e d solutions w e r e m o r e s t a b l e .  It w a s c o n c l u d e d that calibration s t a n d a r d solutions s h o u l d b e u s e d i m m e d i a t e l y after preparation for the best a c c u r a c y .  3.2.3  Effect of Other Solution Components on Ion Chromatographic Analysis  A n u m b e r of non-sulfur s p e c i e s e x p e c t e d to be p r e s e n t in the solutions in the kinetic tests w e r e a d d e d to solutions of thiosulfate, trithionate a n d tetrathionate to d e t e r m i n e the effect  of t h e s e  recommended  components to  use  on  'matrix  the  ion  chromatographic  matching'  of  standard  method. solutions  U s u a l l y it used  for  is ion  c h r o m a t o g r a p h y calibration, h o w e v e r , in this c a s e m a n y of the c o m p o n e n t s tested w e r e k n o w n to or s u s p e c t e d to react with the analyte s p e c i e s , s o matrix m a t c h i n g c o u l d not used.  T y p i c a l test s o l u t i o n s in the kinetic testwork ( C h a p t e r 4) w e r e e x p e c t e d to contain 2-7 g/l trithionate,  0.1-0.4  g/l  thiosulfate  and  negligible  tetrathionate.  Other  solution  c o m p o n e n t s e x p e c t e d to b e present (individually or in c o m b i n a t i o n ) w e r e a m m o n i a , a m m o n i u m , b i c a r b o n a t e , c a r b o n a t e , chloride a n d p o t a s s i u m (from p o t a s s i u m chloride added). the  F o r the p u r p o s e s of testing the effects of t h e s e c o m p o n e n t s o n the a n a l y s i s of  sulfur  o x y a n i o n s , a solution  tetrathionate w a s p r e p a r e d . sulfur  oxyanion  was  of  100 mg/l e a c h of thiosulfate,  trithionate  and  F r o m this solution a set of solutions of 10 mg/l of e a c h  prepared,  c o n c e n t r a t i o n s s h o w n in T a b l e 3.2.  containing  the  components  of  interest  at  the  T h e c o n c e n t r a t i o n s c h o s e n r e p r e s e n t typical levels  of e a c h c o m p o n e n t relative to levels e x p e c t e d in the test s o l u t i o n s (taking into a c c o u n t the dilution required for ion c h r o m a t o g r a p h y ) .  E a c h s a m p l e w a s a n a l y s e d at least three times s i n c e it is r e c o g n i s e d that there is s o m e variability in the ion c h r o m a t o g r a p h i c m e t h o d .  T h e m e a n a n d s t a n d a r d deviation for  e a c h analyte is s h o w n in T a b l e 3.2. T h e largest difference in m e a s u r e d c o n c e n t r a t i o n c o m p a r e d with a s t a n d a r d solution with no extra c o m p o n e n t s a d d e d w a s 2.7 % for  53  thiosulfate, 1.7 % for trithionate and 2.4 % for tetrathionate.  The error expected in  preparing the diluted samples was expected to be larger than this, hence it was concluded that the addition of ammonia, ammonium, bicarbonate, carbonate or potassium chloride to the levels indicated in Table 3.2 did not affect the analysis of thiosulfate, trithionate and tetrathionate.  The effect of these species on the analysis of  sulfate was not tested as sulfate was only measured occasionally and was not used directly in deriving the kinetic results shown in Chapter 5. The effect of copper on analysis was not tested as it is known that cupric copper reacts readily with thiosulfate and that cuprous copper is readily oxidized to cupric in the presence of oxygen. Table 3.2 : Effect of added species on analysis of sulfur oxyanions Species  Concentration  Mean measured value and standard deviation for  added  of  nominal 10 mg/l of each of S 0 ' , S 0 ' and S 0 '  added  species (mM)  2  2  s o *2  2  3  3  4  6  S 0 "  S 0^  2  4  3  3  2  6  6  mean  0  mean  a  mean  0  None  -  10.0581  0.0169  10.2042  0.0111  10.1115  0.0151  NH  3  1  10.2037  0.0292  10.3763  0.0670  10.4557  0.1214  NH  4  1  10.0196  0.0469  10.2162  0.0448  9.8734  0.0846  1.3  10.0502  0.0067  10.1683  0.0287  10.1256  0.0151  0.9  10.3319  0.0432  10.3315  0.0193  10.2516  0.0929  1.9  10.0702  0.0224  10.1519  0.0169  10.0504  0.0154  as  +  (NH ) S0 4  2  4  HCCV as NaHC0  3  C0 '  as  2  3  Na C0 2  3  KCI  3.3  ANALYSIS OF SULFAMATE  Sulfamate (NH S0 ') is a potential species of interest in ammoniacal thiosulfate systems. 2  3  Two methods were considered to analyse for sulfamate. A titration method used by Sherritt (Liebovitch, 2005) was found to be unsuitable as unreliable results were expected for sulfamate levels below 1 g/l (which is much higher than anticipated levels in the test solutions) and thiosulfate is known to interfere with the determination.  54  It is  s u s p e c t e d that trithionate w o u l d a l s o interfere, e s p e c i a l l y in the s o l u t i o n s of interest where  it  is  expected  to  be  present  in  excess  compared  with  sulfamate.  A  c h r o m a t o g r a p h i c m e t h o d u s i n g conductivity detection (under the s a m e conditions a s that for sulfate determination) w a s f o u n d to be suitable to d e t e r m i n e s u l f a m a t e a n d sulfate s i m u l t a n e o u s l y , but in the p r e s e n c e of a m m o n i a the s u l f a m a t e p e a k w a s o b s c u r e d .  In  addition, s i n c e the s u l f a m a t e concentration in the test s o l u t i o n s w a s e x p e c t e d to b e v e r y low  (or  even  non-existent)  the  likelihood  of  being  able  to  optimise  the  ion  c h r o m a t o g r a p h i c m e t h o d to d e t e r m i n e s u l f a m a t e w a s c o n s i d e r e d low.  3.4  ANALYSIS OF TOTAL AMMONIA  A s t a n d a r d distillation m e t h o d w a s u s e d to a n a l y s e for total a m m o n i a . S o d i u m hydroxide (1M) w a s transferred to a r o u n d - b o t t o m e d flask c o n n e c t e d to a c o n d e n s o r with a s p r a y tube. A suitable aliquot of a m m o n i a - c o n t a i n i n g solution a n d a n t i - b u m p i n g g r a n u l e s w e r e a d d e d to the flask. T h e flask w a s h e a t e d a n d the a m m o n i a g a s p r o d u c e d w a s c o l l e c t e d in a hydrochloric a c i d solution ( 1 M HCI).  Distillation w a s c o n t i n u e d until the v o l u m e in  the r o u n d - b o t t o m e d flask w a s r e d u c e d by about half a n d n o m o r e g a s b u b b l e s w e r e o b s e r v e d to b e entering the a c i d solution.  T h e e x c e s s a c i d w a s titrated a g a i n s t a  s t a n d a r d s o d i u m hydroxide solution to d e t e r m i n e the total a m m o n i a c o n c e n t r a t i o n . ammonia  concentration  was  determined  by  subtracting  the  known  The  ammonium  c o n c e n t r a t i o n of the original solution f r o m the total a m m o n i a content d e t e r m i n e d by distillation.  R e p l i c a t e tests involving dilution of a stock a m m o n i a solution followed by  a n a l y s i s of the diluted solution for total a m m o n i a c o n c e n t r a t i o n g a v e up to a 10 % r a n g e in results.  3.5  SYNTHESIS OF SODIUM TRITHIONATE  S o d i u m trithionate is not c o m m e r c i a l l y a v a i l a b l e , s o w a s s y n t h e s i s e d a c c o r d i n g to a m e t h o d d e s c r i b e d by K e l l y a n d W o o d (1994). T h e m e t h o d involved oxidation of s o d i u m thiosulfate u s i n g h y d r o g e n p e r o x i d e at low t e m p e r a t u r e (near 0 °C).  A n u m b e r of  b a t c h e s w e r e s y n t h e s i z e d a c c o r d i n g to the following m e t h o d , b a s e d o n E q u a t i o n 3 . 1 .  2 S 0 2  2 3  ' + 4 H 0 2  2  S 0 3  2 6  - + S0  2 4  " + 4 H 0 2  55  [3.1]  S o d i u m thiosulfate pentahydrate (150 g) w a s d i s s o l v e d in 9 0 ml d e - i o n i s e d w a t e r in a g l a s s b e a k e r . T h e b e a k e r w a s p l a c e d in a cooling reactor to r e d u c e the temperature to a b o u t 1 °C. W i t h c o n t i n u o u s stirring, 140 ml 3 0 % (w/v) h y d r o g e n p e r o x i d e w a s a d d e d d r o p w i s e (over a f e w hours), taking c a r e that the t e m p e r a t u r e r e m a i n e d b e l o w 2 0 °C. Stirring w a s c e a s e d a n d the b e a k e r w a s left at about 0 °C for 1 to 2 h o u r s , allowing for crystallization of s o d i u m sulfate. T h e s o d i u m sulfate w a s r e m o v e d by filtration through a W h a t m a n N o . 1 filter p a p e r .  T h e sulfate w a s w a s h e d o n the filter with 100 ml e t h a n o l  w h i c h w a s a l l o w e d to mix with the filtrate.  T h e filtrate w a s transferred to a b e a k e r at  a b o u t 3 °C, 2 5 0 ml i c e - c o l d e t h a n o l w a s a d d e d a n d the solution left at 0 - 3 °C for o n e hour.  T h e resulting precipitate (mostly sulfate) w a s a g a i n r e m o v e d by filtration, a n d  w a s h e d o n the filter with 2 0 0 ml i c e - c o l d e t h a n o l , w h i c h w a s a l l o w e d to mix with the filtrate.  T h e filtrate w a s transferred to a b e a k e r containing 1 I i c e - c o l d e t h a n o l , a n d 100  ml e t h a n o l w a s u s e d to rinse the filtrate from the flask into the b e a k e r . T h e mixture w a s stirred thoroughly a n d left at 0 - 3 °C for 1 - 2 h o u r s . S o d i u m trithionate f o r m e d a n d w a s s e p a r a t e d by filtration a n d w a s h e d with 5 0 ml e t h a n o l , 5 0 ml a c e t o n e a n d dried in a desiccator.  A white crystalline material w a s p r o d u c e d a n d stored in a g l a s s bottle in a refrigerator. Storing of s u c h s o l i d s at low temperature h a s b e e n r e c o m m e n d e d ( M i u r a , 2 0 0 3 ) .  3.6  CHARACTERISATION OF SODIUM TRITHIONATE  3.6.1  Total Sulfur  T h e total sulfur content w a s d e t e r m i n e d by c o m p l e t e l y o x i d i s i n g all the sulfur s p e c i e s to sulfate a n d a n a l y s i n g sulfate.  C o m p l e t e oxidation of trithionate to sulfate by boiling in  p e r o x i d e w a s c o n s i d e r e d q u e s t i o n a b l e (Tan a n d R o l i a , 1985) a n d instead F e n t o n ' s r e a g e n t w a s u s e d to facilitate oxidation ( D r u s c h e l , 2 0 0 3 ) .  F e n t o n ' s r e a g e n t a l l o w s for  h y d r o x y r a d i c a l s to b e p r o d u c e d in situ from the reaction of h y d r o g e n p e r o x i d e o n ferrous ions. F e r r o u s chloride (1.35 g F e C I . x H 0 ) w a s d i s s o l v e d in h y d r o c h l o r i c a c i d (50 ml 0.1 2  2  M HCI). A n y u n d i s s o l v e d iron w a s r e m o v e d using a s y r i n g e filter. A k n o w n m a s s of the s o d i u m trithionate  (about 0.2 g) w a s a d d e d to the ferrous solution a n d  hydrogen  p e r o x i d e (10 ml 3 0 % (w/v)) w a s a d d e d . T h e reaction w a s left to g o to c o m p l e t i o n a n d  56  the solution w a s diluted to 1 0 0 m l .  T h e diluted solution w a s then diluted further a s  a p p r o p r i a t e a n d a n a l y s e d for sulfate by ion c h r o m a t o g r a p h y .  3.6.2  Total Sodium Content  A k n o w n m a s s of s o d i u m trithionate w a s h e a t e d to a b o u t 7 9 0 °C in a f u r n a c e until n o m o r e f u m e s w e r e emitted, a n d then left at that t e m p e r a t u r e for a n o t h e r hour.  The  trithionate w a s c o n v e r t e d to s o d i u m sulfate ( R o l i a a n d C h a k r a b a r t i , 1982) a c c o r d i n g to E q u a t i o n 3.2, a n d the s o d i u m content d e t e r m i n e d by the m a s s c h a n g e .  A correction  w a s m a d e for the quantity of a n y volatiles a n d a n y s o d i u m sulfate p r e s e n t a s a n impurity in the trithionate.  Na S 0 2  3.6.3  3  6  + 0  2  -> N a S 0 2  4  + 2S0  [3.2]  2  Volatiles  T h e r m o g r a v i m e t r i c a n a l y s i s ( T G A ) w a s d o n e o n two trithionate b a t c h e s .  The sample  w a s h e a t e d at 10 °C p e r minute in a n a l u m i n a crucible in h e l i u m a t m o s p h e r e to 8 0 0 °C. T h e results a r e s h o w n graphically for two b a t c h e s of trithionate in F i g u r e s 3.1 a n d 3.2. T h e m a s s d r o p b e l o w 100 °C w a s likely d u e to a l o s s of e t h a n o l a d s o r b e d to the s o l i d . ( E t h a n o l w a s u s e d e x t e n s i v e l y in the s y n t h e s i s . ) c o u l d b e d u e to d e c o m p o s i t i o n of the trithionate.  57  T h e m a s s c h a n g e at a r o u n d 2 5 0 °C  0  200  400  600  800  Temperature (°C) Figure 3.1 : TGA/DTA for sodium trithionate batch 2  0  200  400  600  800  Temperature (°C) Figure 3.2 : TGA/DTA for sodium trithionate batch 3  3.6.4  Sulfate  A solution of the trithionate was analysed for sulfate by ion chromatography.  3.6.5 Polythionates A solution of the trithionate w a s a n a l y s e d for polythionates by ion c h r o m a t o g r a p h y . T h e s o l u t i o n s w e r e a n a l y s e d i m m e d i a t e l y after dilution a n d w e r e a n a l y s e d a n u m b e r of t i m e s in s u c c e s s i o n . prepared  T h e concentration profile with time is s h o w n in F i g u r e 3.3 for solutions  using deaerated water  and  in w ater without  deaeration.  Although  the  m e a s u r e d trithionate c o n c e n t r a t i o n w a s in q u e s t i o n h e r e , the v a l u e s o n the g r a p h are still indicated a s mg/l trithionate,  based on analysis against a  mixed  trithionate,  tetrathionate a n d thiosulfate s t a n d a r d m a d e u s i n g a different b a t c h of trithionate. a c t u a l trithionate c o n c e n t r a t i o n s are indicative only.  The  W h i l e n o thiosulfate w a s f o u n d to  b e p r e s e n t , the initial a n a l y s i s at time z e r o s h o w e d a significant c o n c e n t r a t i o n of tetrathionate (up to 3 mg/l for a n e s t i m a t e d 10 mg/l trithionate concentration).  Repeat  a n a l y s e s of the s a m e solution s h o w e d a continually d r o p p i n g tetrathionate c o n c e n t r a t i o n a n d a c o r r e s p o n d i n g i n c r e a s e in the trithionate c o n c e n t r a t i o n .  T h e c a u s e of this p h e n o m e n o n is not clear.  H o w e v e r , it w a s f o u n d that a n a l y s i n g  trithionate in a l k a l i n e solution (containing 2 m M a m m o n i a for 10 mg/l S 0 3  2 _ 6  , s e e Figure  3.4) or w h e r e thiosulfate a n d tetrathionate w e r e p r e s e n t (Figure 3.5), this p h e n o m e n o n did not o c c u r .  It is p o s s i b l e that dilute solutions of trithionate in s u c h a n unbuffered  s y s t e m (water) w e r e m o r e subject to variations in s p e c i a t i o n .  59  10  CO  o II  e  I  9  3 CQ  E CN CD  "  o  A  A  A Trithionate (with nitrogen)  *  A  A  CO  CO 0 H  •  A Trithionate (no nitrogen)  •  • Tetrathionate (with nitrogen)  0  i  100  200  • Tetrathionate (no nitrogen)  300  Time (min)  Figure 3.3 : Change of measured trithionate (indicative values onlv) and tetrathionate concentrations with time with and without using deaerated water in solution preparation 11 10 CO  9  O o 3to  •  CM CD  o  8  CQ  CO  CO  7  • Trithionate  6  • Tetrathionate  20  40  60  80  100  Time (min) Figure 3.4 : Change of measured trithionate (indicative values onlv) and tetrathionate concentrations with time in the presence of 2mM N h U O H  60  11 A  10  CM  o CN  9  _8_  • A  • A  CO  CO  CN  co  o  8  •* CO  7  o  6  A Trithionate • Tetrathionate x Thiosulfate  CD  CO  40  20  CO  60  80  100  Time (min) Figure 3.5 : Change of measured trithionate (indicative values onlv), tetrathionate and thiosulfate concentrations with time in the presence of 10 mg/l SgOg ' and 10 mg/l S 0g2  4  3.6.6  Overall Trithionate Purity  Table 3.3 gives comparative data for all the trithionate batches.  The purity was  calculated as follows: % purity = (total S (%) - sulfate S (%)) x 100 / 40.3 Table 3.3 : Sodium trithionate characterisation Batch  S0 2  Na  STOT  Volatiles  Purity (based  (%)  (%)  (%)  (%)  on total S and  4  S 0 ) (%) 2  4  Theoretical  0  19.3  Batch 0  1.34  Batch 1  1.69  Batch 2  0.67  36.0  Batch 2a  4.27  39.1  Batch 3  1.13  40.0  16.8  Batch 4 *  40.3  100  40.1  98.4  36.1  87.2 10  88.8 93.4  3  98.3 96.0*  Standardised against Batch 2 61  3.7  TRACE IMPURITY ANALYSIS OF CHEMICALS USED  S o l u t i o n s of the m a i n c h e m i c a l s u s e d in this testwork, n a m e l y s o d i u m trithionate ( B a t c h 4),  s o d i u m thiosulfate,  ammonia  and  ammonium  bicarbonate, were  analysed  by  inductively c o u p l e d p l a s m a s p e c t r o p h o t o m e t r y to s c a n for the p r e s e n c e of t r a c e e l e m e n t impurities. T h e d a t a is s h o w n in A p p e n d i x 4.  62  4  KINETICS OF TRITHIONATE DEGRADATION - METHODOLOGY  4.1  INTRODUCTION  T w o s t a n d a r d m e t h o d s a r e u s e d to d e t e r m i n e reaction kinetics - the integrated rate m e t h o d a n d the initial rate m e t h o d ( B r e z o n i k , 1994).  B o t h m e t h o d s w e r e u s e d in this  w o r k to d e t e r m i n e the kinetics of trithionate d e g r a d a t i o n . In this c h a p t e r , the b a s i c theory b e h i n d e a c h m e t h o d is s u m m a r i z e d , a n d the e x p e r i m e n t a l m e t h o d s u s e d to collect the n e c e s s a r y d a t a a r e d e s c r i b e d . E x a m p l e d a t a a n d the interpretation thereof a r e g i v e n for each method.  4.2  REACTION KINETICS THEORY  F o r a g e n e r i c irreversible reaction ( E q u a t i o n 4.1), the rate e q u a t i o n is typically g i v e n b y Equation 4.2.  A +Byproducts  [4.1]  R a t e = k[A] [B]  [4.2]  a  b  E l e m e n t a r y reaction kinetics a l l o w s for the determination of the reaction o r d e r s with r e s p e c t to the v a r i o u s reactants (a a n d b in E q u a t i o n 4.2) a n d the rate c o n s t a n t (k).  In  the c a s e of trithionate d e g r a d a t i o n , E q u a t i o n 4.2 c a n b e e x p r e s s e d a s E q u a t i o n 4 . 3 .  -d[S 0 -]/dt = k 2  3  6  where k b 0  [4.3]  [S 0 T 2  o b s  S  3  6  is the o b s e r v e d rate constant a n d is e x p e c t e d to b e d e p e n d e n t o n  v a r i o u s reaction conditions a n d other reactant c o n c e n t r a t i o n s .  T h e initial rate m e t h o d involves m e a s u r i n g the rate of a reaction o v e r short t i m e s , before a n y significant c h a n g e s in concentration of the reactants o c c u r .  It is a s s u m e d that the  reaction rate is c o n s t a n t o v e r the initial time p e r i o d , i.e. the c o n c e n t r a t i o n v e r s u s time profile for the reactant (trithionate) is linear.  T h e reaction o r d e r is d e t e r m i n e d by c o m p a r i n g the initial rates m e a s u r e d at different initial trithionate  concentrations.  F r o m E q u a t i o n 4 . 3 , if only the c o n c e n t r a t i o n  63  of  trithionate is c h a n g i n g , the reaction o r d e r with r e s p e c t to trithionate c a n be e x p r e s s e d by Equation 4.4.  R e a c t i o n o r d e r = In ( r / r ) / l n ( [ S 0 - M S 0 - ] ) 2  1  2  3  6  [4.4]  2  3  6  2  w h e r e ^ is the initial rate at initial concentration [ S 0 ' ]i a n d r is the initial rate at initial 2  3  concentration [ S 0 ' ] . 3  6  6  2  A plot of ln(r) v e r s u s l n [ S 0 ' ] g i v e s the reaction o r d e r a s the  2  2  2  3  6  slope.  T h e rate c o n s t a n t c a n be d e t e r m i n e d by plotting the m e a s u r e d initial rate a g a i n s t the initial c o n c e n t r a t i o n of trithionate  (raised to the p o w e r of the reaction order).  The  gradient of s u c h a plot g i v e s the o b s e r v e d rate constant, a c c o r d i n g to E q u a t i o n 4 . 3 .  T h e integrated rate m e t h o d involves d a t a collection o v e r longer times.  A s s u c h , the  concentration of reactants a n d products c h a n g e s significantly o v e r the duration of the measurements.  T o a c c o u n t for the continually c h a n g i n g c o n c e n t r a t i o n s , a n integrated  form of E q u a t i o n 4 . 3 is u s e d . F o r a reaction rate first o r d e r with r e s p e c t to the trithionate c o n c e n t r a t i o n , the equivalent integrated rate equation is s h o w n in E q u a t i o n 4 . 5 .  In [ S 0 1 , = - k 2  3  6  t + In [ S O - ]  [4.5]  2  obs  3  6  0  T h u s for a first o r d e r d e p e n d e n c y , a plot of In [ S ^  2  ^ v e r s u s time g i v e s a straight line  with the o b s e r v e d rate constant a s the s l o p e a n d In [S 06 -]o (the initial 2  3  concentration) a s the intercept.  trithionate  S u c h a plot c a n b e u s e d a s a d i a g n o s t i c tool in  determining the reaction order.  T h e theory for s e c o n d a n d higher order reactions is not d i s c u s s e d h e r e , but is readily a v a i l a b l e in the literature ( B r e z o n i k , 1994).  4.3  EXPERIMENTAL METHOD  S o d i u m trithionate w a s s y n t h e s i z e d a c c o r d i n g to the m e t h o d in C h a p t e r 3 for u s e in t h e s e tests.  T h e a i m of the e x p e r i m e n t a l p r o g r a m m e w a s to e s t a b l i s h the factors  influencing trithionate  d e g r a d a t i o n kinetics.  T h e kinetics w e r e e x a m i n e d in s i m p l e  a q u e o u s solutions with v a r i o u s c o m p o n e n t s present. M o s t tests w e r e d o n e in a buffer of  64  ammonium bicarbonate / ammonia, since the buffer pH of this system was maintained between pH 9.3 and 10.3, which is the pH range of interest. An ammonium sulfate / ammonia buffer was also considered, but in that system small concentrations of sulfate produced during the degradation of trithionate could not be accurately measured. The test solutions were made up to the required concentrations using deionised water. integrated Rate Method The ammonium bicarbonate or ammonium sulfate solution was made up (as above) and aqueous ammonia was added to adjust the pH. The amount of ammonia added was not measured. Most tests were done at room temperature unless otherwise stated. In tests where the temperature was controlled, the solutions were brought to the required temperature in a water bath. For each test, the required mass of sodium trithionate was diluted using the buffer to give a nominal concentration of 8.5 g/l or 4.5 g/l trithionate, and in some cases sodium thiosulfate and/or sodium tetrathionate were added to give 10 g/l thiosulfate and 2 g/l or 10 g/l tetrathionate. These concentrations were selected based on concentrations typically found in gold leach liquors (see Chapter 2). The solutions were transferred to glass vessels and stoppered with rubber stoppers. The ionic strength was not adjusted. At set times, samples were withdrawn by pipette and diluted for analysis for thiosulfate, trithionate and tetrathionate concentration by ion chromatography. The solutions were also analysed for sulfate level at the termination of the test. The tests were typically run for 36 to 54 days. Initial Rate Method Three or four aliquots (50 ml) of each test solution containing all solution components except sodium trithionate were placed in sealed flasks in a water bath for at least 45 minutes to reach the required temperature. Tests were done at 40 °C unless otherwise stated. Solid sodium trithionate was added to each of three conical flasks and at time zero, one aliquot of test solution was added to each flask and mixed. The amount of  65  s o d i u m trithionate a d d e d w a s to give n o m i n a l c o n c e n t r a t i o n s of 2 g/l, 5 g/l a n d 7 g/l trithionate, b a s e d o n e x p e c t e d v a l u e s in gold l e a c h solutions ( s e e C h a p t e r 2).  In s o m e  c a s e s a 'blank' test w a s d o n e w h e r e trithionate w a s e x c l u d e d to d e t e r m i n e the p H c h a n g e in the a b s e n c e of trithionate.  T h e f l a s k s w e r e s e a l e d with r u b b e r s e p t u m s a n d  kept at c o n s t a n t temperature in a w a t e r bath.  S a m p l e s w e r e r e m o v e d at set times either by r e m o v i n g the rubber s e p t u m or by using a s y r i n g e to s a m p l e through the s e p t u m if a significant a m o u n t of a m m o n i a w a s present or if the s y s t e m w a s u n d e r nitrogen a t m o s p h e r e . appropriate  S a m p l e s w e r e i m m e d i a t e l y diluted a s  a n d a n a l y s e d for trithionate a n d thiosulfate  (and  o c c a s i o n a l l y sulfate)  c o n c e n t r a t i o n using the ion c h r o m a t o g r a p h y m e t h o d s d i s c u s s e d earlier. Total a m m o n i a w a s a n a l y s e d a c c o r d i n g to the m e t h o d in C h a p t e r 3. T h e tests w e r e typically run for 3 to 4 hours.  4.4  DATA ANALYSIS  Integrated Rate Method  T h e d a t a e v a l u a t i o n for a typical d a t a set o b t a i n e d u s i n g the integrated rate m e t h o d is e x p l a i n e d in this s e c t i o n by e x a m p l e , for a test w h e r e 9 g/l S 0 3  initially p r e s e n t .  2 6  ' a n d 10 g/l S 0 2  2 3  ' were  T h e d a t a f r o m all tests w e r e treated in a similar w a y a n d the findings  a r e s u m m a r i z e d in C h a p t e r 5.  T h e trithionate a n d thiosulfate c o n c e n t r a t i o n s a s well a s the logarithm of the trithionate c o n c e n t r a t i o n a r e plotted in F i g u r e 4 . 1 . T h e r e w a s a g o o d straight line fit to the l n [ S 0 1 2  3  v e r s u s time d a t a points.  6  E l e m e n t a r y reaction kinetics s h o w s that a relation of this type  c o r r e s p o n d s to p s e u d o first o r d e r kinetics, s h o w n in E q u a t i o n 4 . 5 , w h e r e k  o b s  is the  o b s e r v e d rate constant ( B r e z o n i k , 1994). T h e e x p e c t e d relationships for other reaction o r d e r s did not fit the e x p e r i m e n t a l d a t a , s o it w a s d e d u c e d that the d e g r a d a t i o n of trithionate follows first o r d e r kinetics.  T h e rate c o n s t a n t w a s d e t e r m i n e d by u s i n g the  b e s t fit c u r v e of this type.  66  3  20  4 eg  6  U  12  CM  CO °  •—  6  °  o  fD  1  O  f  2  o  0  ™CD  o  co  CO  8  •  CM  o CO  A  X  m  4 -2  I  500  1000 T i m e (hr)  1500  A Trithionate 0  Thiosulfate  * In (trithionate)  F i g u r e 4.1 : C o n c e n t r a t i o n s of trithionate a n d thiosulfate with time for a n integrated rate m e t h o d test ( R o o m t e m p e r a t u r e . N H H C Q = 0 . 0 3 M . NIK, a d d e d to adjust initial DH to - 9 . 7 ) 4  3  Plotting [S203 ] o r [ S 0 ' ] a g a i n s t the d e c r e a s e in trithionate c o n c e n t r a t i o n g a v e the 2-  2  4  stoichiometric co-efficients of thiosulfate a n d sulfate in the reaction e q u a t i o n a s the gradients.  T h e d a t a w a s plotted in this m a n n e r in F i g u r e 4 . 2 , u s i n g e x p e r i m e n t a l l y  o b s e r v e d c o n c e n t r a t i o n s for thiosulfate a n d sulfate, a n d u s i n g the best-fit e x p o n e n t i a l e q u a t i o n (from F i g u r e 4.1) to d e t e r m i n e the trithionate c o n c e n t r a t i o n s .  67  0.15  o E, •  CM  o - OO - io" o  0.10  o  h o .  CO CM  0.05 o Thiosulfate!  o  CM  CO  • Sulfate 0.00 0.00  0.01  0.02  0.03  0.04  0.05  Decrease in S 0 " (mol/l) 3  6  F i g u r e 4 . 2 : Determination of reaction stoichiometry u s i n g the integrated rate m e t h o d ( R o o m t e m p e r a t u r e . N h L H C O g = 0.03 M . N H a d d e d to adjust initial p H to - 9 . 7 ) 3  D a t a from the other tests w e r e treated in a similar w a y . W h e r e tetrathionate w a s initially a d d e d (to s i m u l a t e a p o s s i b l e r e c y c l e solution), the tetrathionate w a s f o u n d to d e g r a d e v e r y q u i c k l y u n d e r the alkaline c o n d i t i o n s , c a u s i n g a n initial i n c r e a s e in trithionate a n d thiosulfate c o n c e n t r a t i o n s .  In t h e s e c a s e s , the best fit of the e x p o n e n t i a l e q u a t i o n for  trithionate d e g r a d a t i o n e x c l u d e d the initial d a t a w h e r e tetrathionate w a s still p r e s e n t , a n d the e x t r a p o l a t e d trithionate concentration at time z e r o w a s d e r i v e d from the trithionate c o n c e n t r a t i o n at the time w h e n the initial tetrathionate h a d d i s a p p e a r e d .  Initial Rate Method T y p i c a l profiles of trithionate c o n c e n t r a t i o n a n d thiosulfate c o n c e n t r a t i o n with time a r e s h o w n in F i g u r e 4 . 3 for the initial rate m e t h o d .  A linear d e c r e a s e in  trithionate  c o n c e n t r a t i o n a n d a c o r r e s p o n d i n g linear i n c r e a s e in thiosulfate concentration w e r e f o u n d , a s e x p e c t e d for this m e t h o d . O v e r the duration of the test, typically l e s s than 10 % of the trithionate h a d r e a c t e d .  68  Q  1  2  3  I a Thiosulfate  T i m e (h)  F i g u r e 4 . 3 : T y p i c a l c o n c e n t r a t i o n profile for trithionate a n d thiosulfate u s i n g the initial rate m e t h o d (40 °C. p H 9.9. 0.05 M N h U H C O a . 0 . 6 8 7 M N h k total ionic strength 1 M )  T h e reaction o r d e r w a s d e t e r m i n e d g r a p h i c a l l y u s i n g the relation in E q u a t i o n 4 . 4 . A n e x a m p l e s h o w i n g this d e p e n d e n c y is g i v e n in F i g u r e 4 . 4 , w h e r e the a v e r a g e s l o p e w a s 1.  T h e rate e q u a t i o n for the d e g r a d a t i o n of trithionate c a n thus b e e x p r e s s e d a s in  E q u a t i o n 4 . 6 , w h e r e the o b s e r v e d rate constant, k  obs  , is e x p e c t e d to b e a function of the  c o n c e n t r a t i o n s of other solution c o m p o n e n t s affecting the trithionate d e g r a d a t i o n rate.  - d [ S 0 - ]/dt = k 2  3  6  [S 0 1  [4.6]  2  o b s  3  6  69  i  2  1.5  0.5  -2  y = 1.0x-4.2  in s cy3  F i g u r e 4 . 4 : Determination of the reaction o r d e r for the rate of trithionate d e g r a d a t i o n with r e s p e c t to trithionate  (40 °C. NHJHCOg = 0.2 M . initial p H 9.2)  T h e concentration of trithionate v e r s u s time w a s plotted a n d u s i n g a trendline, the b e s t line fit through the d a t a w a s d e t e r m i n e d , giving the initial reaction rate a s the s l o p e a n d the initial trithionate concentration a s the intercept.  A s t a n d a r d statistical m e t h o d (Miller  a n d Miller, 1988) to d e t e r m i n e the s t a n d a r d deviation in the s l o p e a n d intercept of a r e g r e s s i o n line w a s u s e d to d e t e r m i n e the s t a n d a r d d e v i a t i o n s of the initial rate a n d initial trithionate c o n c e n t r a t i o n .  T h e initial rate w a s plotted a g a i n s t the initial trithionate  c o n c e n t r a t i o n for e a c h test s e r i e s , usually c o m p r i s e d of three d a t a s e t s , to d e t e r m i n e the a v e r a g e o b s e r v e d rate constant, a c c o r d i n g to E q u a t i o n 4 . 6 . T o a c c o u n t for e x p e r i m e n t a l variability, the e s t i m a t e d m a x i m u m rate (initial rate plus the s t a n d a r d deviation) w a s plotted a g a i n s t the m i n i m u m initial concentration (initial c o n c e n t r a t i o n m i n u s the s t a n d a r d deviation), a n d the m i n i m u m rate a g a i n s t the m a x i m u m c o n c e n t r a t i o n . R e g r e s s i o n lines w e r e g e n e r a t e d for all three plots, a n d f o r c e d through the origin. regression  lines r e p r e s e n t e d the  average, maximum  and  T h e s l o p e of the  minimum  observed  c o n s t a n t s . A typical plot to d e t e r m i n e the rate c o n s t a n t k is s h o w n in F i g u r e 4 . 5 .  rate In all  s u b s e q u e n t figures s h o w i n g the o b s e r v e d rate c o n s t a n t , the error b a r s indicate the e s t i m a t e d m i n i m u m a n d m a x i m u m rate c o n s t a n t s d e t e r m i n e d in this w a y .  70  0.16  • A v e r a g e rate constant • M i n i m u m rate! constant A Maximum rate c o n s t a n t  Initial S3O6 " Concentration (g/l) F i g u r e 4 . 5 : T y p i c a l plot of trithionate d e g r a d a t i o n rate v e r s u s initial trithionate c o n c e n t r a t i o n to determine the rate constant knhs from the s l o p e (40 °C. PH 9.9. 0.05 M N H 4 H C O 3 , 0.687 M NH . total ionic strength 1 M ) A  The  reaction stoichiometry w a s d e t e r m i n e d in the s a m e w a y a s for the integrated rate  method.  71  5.  KINETICS O F TRITHIONATE DEGRADATION - R E S U L T S  5.1  INTRODUCTION  B o t h the integrated rate m e t h o d a n d the initial rate m e t h o d w e r e u s e d to investigate the kinetics of trithionate d e g r a d a t i o n . T h e e x p e r i m e n t a l m e t h o d s a n d the w a y s in w h i c h the d a t a w e r e interpreted u s i n g t h e s e m e t h o d s w e r e d i s c u s s e d in C h a p t e r 4 .  T h e initial rate  m e t h o d is e x p e c t e d to give m o r e reliable d a t a than the integrated rate m e t h o d , a s by the nature of this m e t h o d , the reaction conditions are held m o r e c o n s t a n t a n d t h e reactions of p r o d u c t s a r e e x p e c t e d to b e m i n i m i z e d .  H o w e v e r , the integrated rate m e t h o d is useful  to give a n indication of t h e factors e x p e c t e d to b e of s i g n i f i c a n c e in t h e trithionate d e g r a d a t i o n reactions, a n d a l s o to s e r v e a s confirmation of effects noted u s i n g the initial rate m e t h o d .  In this chapter, the trithionate degradation kinetics u s i n g both m e t h o d s are  d i s c u s s e d together. M o s t of the results w e r e obtained u s i n g the initial rate m e t h o d a n d the quantitative c o n c l u s i o n s  were based  primarily o n this d a t a .  Results from the  integrated rate m e t h o d are indicative a n d confirmatory.  T h e a i m of this work w a s to e s t a b l i s h t h e factors influencing trithionate d e g r a d a t i o n , starting with s i m p l y trithionate in a q u e o u s solution a n d e x a m i n i n g solution c o m p o n e n t s s y s t e m a t i c a l l y to identify individual effects.  T h e effects w e r e m a n i f e s t e d in c h a n g e s to  the o b s e r v e d rate constant, k b . in E q u a t i o n 5 . 1 . 0  -d[S O - ]/dt = k 2  3  5.2  e  S  [S 0 1  [5.1]  2  obs  3  6  STOICHIOMETRY  In m u c h of the r e s e a r c h reported in the literature, the rate of reaction of various sulfur o x y a n i o n s h a s b e e n d e d u c e d b a s e d o n a n a l y s i s of thiosulfate a n d a n a s s u m e d reaction stoichiometry, rather than direct m e a s u r e m e n t of the a s s u m e d reaction p r o d u c t s . In this work, both thiosulfate a n d trithionate w e r e m e a s u r e d at the s a m e time.  O n e would  e x p e c t , b a s e d o n E q u a t i o n 5 . 2 ( b a s e d o n prior work, s e e C h a p t e r 2), that the rate of trithionate d e g r a d a t i o n w o u l d b e e q u a l to the rate of thiosulfate production.  S 0 3  2 6  - + 20H"  S 0 2  2 3  " + S0  2 4  " + H 0  [5.2]  2  72  F o r both the initial rate m e t h o d a n d the integrated rate m e t h o d , the ratio of thiosulfate f o r m e d to trithionate d e g r a d e d w a s g e n e r a l , a ratio of 1:1,  m e a s u r e d at 4 0 ° C a n d at r o o m t e m p e r a t u r e . In  c o r r e s p o n d i n g to E q u a t i o n 5.2 w a s f o u n d .  sulfate a l s o g a v e c l o s e to a 1:1  T h e limited d a t a for  stoichiometric relationship with trithionate, w h i c h is a l s o  c o n s i s t e n t with the reaction in  Equation  5.2.  Specific cases  where  a  different  s t o i c h i o m e t r y f r o m E q u a t i o n 5.2 w a s o b s e r v e d a r e d i s c u s s e d s e p a r a t e l y .  T h e r e w e r e a n o m a l i e s noted in s o m e initial rate tests. thiosulfate  concentrations  were  measured,  but  in  In t h e s e tests, the trithionate a n d most  cases,  c o n c e n t r a t i o n , a s a different ion c h r o m a t o g r a p h i c m e t h o d w a s  not  the sulfate  n e e d e d , s i n c e sulfate  c o u l d not be d e t e r m i n e d s i m u l t a n e o u s l y .  In g e n e r a l it w a s f o u n d that the thiosulfate formation rate w a s linear e x c e p t for the initial s a m p l i n g point.  It is p o s s i b l e that slight impurities in the trithionate solution or the  n e c e s s a r y re-equilibration of the ion c h r o m a t o g r a p h y c o l u m n w h e n the first s a m p l e of the s e r i e s w a s injected m a y h a v e h a d s o m e influence h e r e .  It is relevant to note that  e s p e c i a l l y at the start of the reaction, the concentration of thiosulfate w a s  negligible  c o m p a r e d with trithionate, but it w a s n e c e s s a r y to a n a l y s e for both s p e c i e s at the s a m e time using the s a m e dilution factor for a n a l y s i s .  T h i s is not ideal for a c c u r a t e a n a l y s i s of  the low c o n c e n t r a t i o n of thiosulfate. E x c l u d i n g this initial point, the thiosulfate formation stoichiometry w a s a s e x p e c t e d .  T h e sulfate formation stoichiometry w a s a l s o a s e x p e c t e d f o r the limited d a t a a v a i l a b l e . In addition, the sulfur b a l a n c e s for the m e a s u r e d s p e c i e s of trithionate, thiosulfate a n d sulfate w e r e typically b e t w e e n 95% a n d 105%,  implying that t h e s e s p e c i e s w e r e the only  sulfur s p e c i e s p r e s e n t in significant quantities a n d no other p r o d u c t s (e.g. pentathionate) needed consideration.  It h a s b e e n s u g g e s t e d (Naito et a l . , 1975)  but not proven that s u l f a m a t e could f o r m in the  p r e s e n c e of a m m o n i a under similar conditions to t h o s e t e s t e d in this work.  If s u l f a m a t e  w a s p r e s e n t a s a reaction product f r o m trithionate d e g r a d a t i o n , its c o n c e n t r a t i o n w o u l d be e x p e c t e d to be trace u n d e r the conditions tested here, a n d it c o u l d not be d e t e r m i n e d in the s a m p l e matrix.  73  5.3  REPRODUCIBILITY  Integrated Rate Method  T h e c o n c e n t r a t i o n profiles for two very similar tests a r e s h o w n in F i g u r e 5 . 1 . W h i l e the test c o n d i t i o n s w e r e nominally the s a m e , there w a s a s m a l l difference in the  initial  trithionate c o n c e n t r a t i o n a n d the tests w e r e d o n e at r o o m t e m p e r a t u r e at different t i m e s , s o there w e r e likely a l s o t e m p e r a t u r e variations.  W i t h i n t h e s e constraints, the results  w e r e r e p r o d u c i b l e - the o b s e r v e d rate c o n s t a n t s for the tests w e r e 0 . 0 0 1 6 h" a n d 0 . 0 0 1 7 1  h" . 1  12 10 • CM  CO  o  CM  CO  ii  8 '.A  A A  AAA  A Trithionate  AA  6  A  A CM  CD  o  CO  CO  A A  A  • Thiosulfate  *  4  A  2  4M-  A  O A  ^ AA  " A  A  •  o8»°°  0  o A  i  l  I  I  i  200  400  600  800  1000  1200  T i m e (hr)  F i g u r e 5.1 :  C o n c e n t r a t i o n profiles for trithionate a n d thiosulfate  ( r o o m t e m p e r a t u r e . NH4HCO3/NH3 buffer. f N H / 1 = 0.03 M . initial PH 9.7. o p e n s y m b o l s r e p r e s e n t o n e test, c l o s e d s y m b o l s represent a r e p e a t e d test)  Initial Rate Method  S i n c e two different buffer s y s t e m s w e r e u s e d p r e d o m i n a n t l y in this w o r k (the a m m o n i a ammonium  b i c a r b o n a t e s y s t e m , a n d the  sodium bicarbonate -  sodium  carbonate  s y s t e m ) the reproducibility of the initial rate m e t h o d w a s t e s t e d for e a c h buffer s y s t e m at  74  40 °C.  T h e o b s e r v e d rate c o n s t a n t s (and their potential error r a n g e s ) a r e s h o w n in  Table 5.1.  F o r both buffer s y s t e m s , the potential range in the rate c o n s t a n t m e a s u r e d f o r e a c h test was  significant.  measuring  T h i s is to b e e x p e c t e d s i n c e determination of t h e rate  involved  s m a l l c h a n g e s of concentration o v e r time, a n d e a c h rate c o n s t a n t w a s  d e t e r m i n e d f r o m only four i n d e p e n d e n t d a t a points, a s d i s c u s s e d in t h e m e t h o d o l o g y section.  F o r this r e a s o n , the d a t a is a l w a y s s h o w n including the potential m i n i m u m a n d  maximum values,  s h o w n a s horizontal b a r s in all g r a p h s .  However, the agreement  b e t w e e n tests for e a c h buffer s y s t e m w a s well within the error range for o b s e r v e d for e a c h rate constant.  T a k i n g into a c c o u n t the appropriate r a n g e s , the initial rate m e t h o d  p r o v i d e d r e p r o d u c i b l e results for the o b s e r v e d rate c o n s t a n t s .  T a b l e 5.1 : Reproducibility in rate constant determination for trithionate d e g r a d a t i o n for two buffer s y s t e m s using the initial rate m e t h o d  Buffer  Total  contribution  Buffer  ionic  system  strength (M)  Na C0 / 2  3  NaHC0  5.4  PH  kobs  max  min  (h- )  (h- )  0.0103  0.0125  0.0084  0  0.0110  0.0129  0.0093  1  0.84  0.0168  0.0181  0.0155  2  0.89  0.0159  0.0174  0.0144  [NH ]  kbs  no.  (M)  Oh")  1  0  2  strength  3  0  1  1  1  (M)  0.5  0.15  8.5  1  0.032  10.2  3  NH4HCO3 /NH  to ionic  kobs  Replicate  3  WATER  T h e d e g r a d a t i o n of trithionate in w a t e r w a s investigated at 4 0 ° C u s i n g the initial rate m e t h o d only.  E i t h e r s o d i u m p e r c h l o r a t e o r p o t a s s i u m c h l o r i d e w a s u s e d to adjust the  ionic strength (I).  R e s u l t s a r e s h o w n in T a b l e 5 . 2 .  c o n s t a n t s a r e not s h o w n in this c a s e .  T h e m a x i m u m a n d m i n i m u m rate  B e c a u s e t h e r e a c t i o n rate w a s v e r y s l o w a n d f e w  d a t a points w e r e t a k e n , t h e c a l c u l a t e d deviation of the initial trithionate c o n c e n t r a t i o n  75  w a s often unrealistically large.  H o w e v e r , the o b s e r v e d rate c o n s t a n t s w e r e c o n s i s t e n t  for the four tests carried out.  T a b l e 5.2 : V a l u e s for the o b s e r v e d rate c o n s t a n t l w for trithionate d e g r a d a t i o n in w ater u s i n g the initial rate m e t h o d (40 °C)  1 adjusted  l(M)  kobs (h" )  0.1  0.010  1.0  0.012  0.1  0.012  1.0  0.013  1  with: KCI  NaCI0  T h e u s e of K C I or N a C I 0  4  4  from 0.1 M to 1.0 M did not h a v e a significant effect.  The  o b s e r v e d rate c o n s t a n t s in w a t e r w e r e very similar to t h o s e e x t r a p o l a t e d f r o m work by N a i t o et al (1975).  U s i n g their  rate c o n s t a n t at 4 0 °C a n d a s s u m i n g a  c o n c e n t r a t i o n of 5 5 . 5 M , a v a l u e for k  obs  of 0.012 h'  1  water  w a s o b t a i n e d , v e r y c l o s e to the  v a l u e s m e a s u r e d here. W h i l e it h a s b e e n p r o p o s e d that the c o n c e n t r a t i o n of w ater h a s a n influence o n the rate (Naito et a l . , 1975), this w a s not tested in this work.  It w a s  p r o p o s e d by Naito et a l . (1975) that w ater w a s e x p e c t e d to attack the sulfonate sulfur a t o m of trithionate directly. T h i s w a s d i s c u s s e d in the literature review in C h a p t e r 2 .  It is important to note that the natural p H of trithionate in w ater w a s a r o u n d 4 . 5 (at 4 0 °C), a n d d e c r e a s e d with time to a r o u n d p H 2 after 2 4 h o u r s at 4 0 °C.  T h e reactions  o c c u r r i n g at t h e s e low p H v a l u e s are not n e c e s s a r i l y the s a m e a s t h o s e at the higher p H v a l u e s relevant to gold l e a c h i n g .  T h e overall stoichiometry of thiosulfate f o r m e d to  trithionate r e a c t e d w a s typically 0.5-0.6 : 1. A t neutral a n d higher p H the stoichiometry is e x p e c t e d to b e 1:1 a c c o r d i n g to E q u a t i o n 5.2 ( K u r t e n a c k e r , 1935) but at the low p H v a l u e s o b s e r v e d h e r e , it is likely that s o m e of the thiosulfate f o r m e d w a s d e g r a d e d to e l e m e n t a l sulfur, w h i c h w a s o b s e r v e d in the s o l u t i o n s .  76  further  T h i s d a t a s h o w s that trithionate will u n d e r g o a s l o w d e g r a d a t i o n in a q u e o u s solutions. T h i s inherent instability is c o m m o n to the polythionates.  In s u m m a r y , the trithionate  d e g r a d a t i o n rate in w a t e r at 4 0 ° C c a n b e e x p r e s s e d b y E q u a t i o n 5 . 3 .  - d [ S 0 - ] / d t = 0.012h" [ S 0 - ] = k 2  3  5.5  1  6  2  3  6  [5.3]  [S 0 '] 2  0  3  6  HYDROXIDE  Trithionate d e g r a d a t i o n in s o d i u m hydroxide solution w a s investigated using the initial rate m e t h o d .  A l l tests w e r e d o n e at 4 0 ° C a n d a total ionic strength of 1 M , a d j u s t e d  u s i n g s o d i u m perchlorate. T h e results a r e s h o w n in T a b l e 5.3.  T a b l e 5.3 : Effect of hydroxide concentration o n the o b s e r v e d rate constant k„hg for trithionate d e g r a d a t i o n using the initial rate m e t h o d (40 ° C )  [NaOH]  Average  Ratio S 0 2  2 3  formed : S 0 3  " 2 6  "  kobs  kobs min  kbs m a x  Ch")  (h-)  (h-)  1  1  0  1  (M)  initial p H  0.5  12.62  0.2-0.4  0.729  *  *  0.1  11.96  0.3-0.4  0.143  0.132  0.156  0.01  11.06  0.7-0.8  0.0205  0.0184  0.0224  0.006  10.88  0.8-0.9  0.0146  0.0133  0.0159  0.001  9.76  0.8-0.9  0.0123  0.0098  0.0149  d e g r a d e d (M:M)  * F o r the test at 0 . 5 M N a O H , the reaction rate w a s very high, s o the initial rate m e t h o d c o u l d not b e u s e d to determine the o b s e r v e d rate constant.  Instead, a n  e x p o n e n t i a l fit c o n s i s t e n t with first order kinetics w a s u s e d to d e r i v e the rate c o n s t a n t (i.e. the integrated rate method).  T h e error margin w a s not d e t e r m i n e d , h e n c e n o  m i n i m u m o r m a x i m u m v a l u e for the rate constant is reported.  A t h i g h e r hydroxide c o n c e n t r a t i o n s (> 0.1 M ) a n d c o r r e s p o n d i n g higher p H s ( - 1 2 a n d higher), the d e g r a d a t i o n of trithionate w a s very fast. A l s o t h e ratio of thiosulfate f o r m e d to trithionate r e a c t e d c o r r e s p o n d s (within error) to literature o b s e r v a t i o n s of the reaction of trithionate at high p H a c c o r d i n g to E q u a t i o n 5.4 ( K u r t e n a c k e r , 1935) f o r reaction a t 5 0 °C a n d p H 1 3 . 4 - 1 3 . 7 .  77  2 S 0 3  2 6  - + 3 H 0 ^ 2  S 0 2  2 3  ' + 4 SO, " + 6 H 2  [5.4]  +  In the context of gold l e a c h i n g , slightly lower p H s a r e of m o r e interest.  The expected  trithionate d e g r a d a t i o n for p H ~ 6 to - 1 1 c a n b e e x p r e s s e d by E q u a t i o n 5.2 a n d the m e a s u r e d reaction stoichiometry under t h e s e conditions s u p p o r t s this.  T h e sulfate  c o n c e n t r a t i o n w a s not m e a s u r e d . It s h o u l d b e noted that the tests using 0.001 M N a O H h a d a significant p H drop of 3 to 6 p H units o v e r the duration of the test.  H o w e v e r this  did not a p p e a r to affect the reaction stoichiometry.  T h e data- for p H v a l u e s of a r o u n d 11 a n d l e s s c o r r e s p o n d i n g to E q u a t i o n 5.2 w e r e interpreted in the following w a y .  It w a s s h o w n p r e v i o u s l y in S e c t i o n 5.4 that trithionate  h y d r o l y s e s in w a t e r at a rate c o r r e s p o n d i n g to E q u a t i o n 5.5 at 4 0 °C.  - d [ S 0 - ] / d t = 0.012 h " [ S 0 - ] = k 2  3  1  6  2  3  6  [S 0 -]  [5.5]  2  0  3  6  It is p r o p o s e d that the overall o b s e r v e d rate c o n s t a n t in the p r e s e n c e of hydroxide c o n s i s t s of a c o m p o n e n t for the d e g r a d a t i o n in water plus a c o m p o n e n t for d e g r a d a t i o n i n f l u e n c e d by the p r e s e n c e of e x c e s s hydroxide. T h e g r a p h in F i g u r e 5.2 s h o w s a plot of the o b s e r v e d rate c o n s t a n t v e r s u s the hydroxide c o n c e n t r a t i o n for tests with a hydroxide c o n c e n t r a t i o n of < 0.01 M , including tests in w a t e r only.  B a s e d o n this g r a p h , the  o b s e r v e d rate c o n s t a n t c a n b e e x p r e s s e d a s E q u a t i o n 5.6 at 4 0 °C. T h e intercept is in a g r e e m e n t with the e x p e c t e d rate constant in water.  - d [ S 0 - ] / d t = (0.0121V + 0 . 7 4 M " l r [ O H - ] ) [ S 0 - ] = (k + k OH"]) [ S 0 2  3  1  6  1  1  2  3  6  0  1L  3  2 6  ^  [5.6]  It is noted f r o m T a b l e 5.3 that the o b s e r v e d stoichiometry of thiosulfate f o r m e d  to  trithionate r e a c t e d w a s slightly lower than o n e w o u l d e x p e c t , b a s e d o n either E q u a t i o n 5.4 or E q u a t i o n 5.2.  It is p o s s i b l e that part of the thiosulfate f o r m e d g o e s o n to react  itself. A l s o it s h o u l d b e noted that the concentration of the thiosulfate in the s o l u t i o n s is v e r y m u c h lower than that of the trithionate, a n d it is p o s s i b l e for significant analytical errors to o c c u r in this r a n g e .  78  0.04  0.03  0.01  — i 0.005  0 H 0  ,  1  :  0.01  0.015  [OH"] M F i g u r e 5.2 : O b s e r v e d rate constant for trithionate d e g r a d a t i o n I w v e r s u s TOHI for pH<11  In the literature, R o l i a a n d C h a k r a b a r t i (1982) c l a i m e d that for trithionate  degradation  b e t w e e n p H 10 a n d 11 at 7 0 to 8 5 °C, the hydroxide c o n c e n t r a t i o n did not affect the trithionate d e g r a d a t i o n rate.  H o w e v e r , no results w e r e g i v e n to support this a s s e r t i o n ,  a n d the conditions u s e d w e r e quite different from t h o s e u s e d in this w o r k a n d of r e l e v a n c e to gold l e a c h i n g .  It h a s b e e n s u g g e s t e d (Ritter a n d K r u e g e r , 1970) that  h y d r o x i d e w o u l d attack trithionate directly at the s u l f o n a t e sulfur a t o m a s it is a hard b a s e . T h i s w a s d i s c u s s e d in the literature review in C h a p t e r 2 .  5.6  IONIC S T R E N G T H  It is well k n o w n that in a q u e o u s reactions, the ionic strength (I) of the solution c a n affect the reaction rate ( B r e z o n i k , 1994), s o it w a s c o n s i d e r e d important to investigate the ionic strength in this work.  T h e c h o i c e of salt to b e u s e d in setting the ionic strength w a s  investigated using the initial rate m e t h o d .  It w a s s h o w n ( S e c t i o n 5.4) that for a solution  of trithionate in water, the p r e s e n c e of variable a m o u n t s of K C I or N a C I 0 the  o b s e r v e d trithionate  degradation  rate  79  constant  appreciably.  4  did not affect  H o w e v e r , in  the  p r e s e n c e of a m m o n i a , the c h o i c e a n d concentration of salt h a d a n effect o n t h e rate of degradation.  A s e r i e s of tests at 4 0 ° C w a s d o n e in t h e p r e s e n c e of a m m o n i a , u s i n g K C I , N a C I o r NaCI0  4  to adjust t h e ionic strength.  N o a m m o n i u m w a s a d d e d a n d t h e p H (>10.6) w a s  not a d j u s t e d . R e s u l t s a r e s h o w n in T a b l e 5.4.  T a b l e 5.4 : V a l u e s for the o b s e r v e d rate c o n s t a n t I w for trithionate d e g r a d a t i o n in the p r e s e n c e of v a r i o u s s a l t s u s e d to adjust the ionic strength u s i n g t h e initial rate m e t h o d  Naito et  I  [NH ]  adjusted  (M)  with:  (meas)  3  l(M)  kobs  kobs min  kobs m a x  al. est  (h- )  (h- )  Ch")  kobs  1  1  1  (h- r i  0.1  0.0166  0.0141  0.0192  0.013  1.0  0.0232  0.0205  0.0260  0.013  0.1  0.0175  0.0158  0.0192  0.013  1.0  0.0160  0.0137  0.0185  0.013  0.39  0.1  0.0116  0.0098  0.0134  0.014  0.37  0.5  0.0173  0.0161  0.0186  0.014  0.39  1.0  0.0236  0.0208  0.0264  0.014  NaCI  0.41  1.0  0.0169  0.0142  0.0197  0.014  NaCICU  0.49  1.0  0.0171  0.0154  0.0188  0.014  KCI  NaCIC-4  KCI  0.15  0.13  * T h e s e o b s e r v e d rate c o n s t a n t s w e r e e s t i m a t e d only, a s this d a t a w a s not s u p p l i e d by Naito et al (1975). Instead they u s e d a n estimation of t h e w a t e r c o n c e n t r a t i o n (not given) to e x p r e s s k  o b s  in t e r m s of k , k a n d k for h y d r o l y s i s , a m m o n o l y s i s a n d t h e w  a  t  reaction with thiosulfate respectively.  G e n e r a l l y t h e results o b t a i n e d g a v e higher rate c o n s t a n t s than t h o s e extrapolated f r o m N a i t o et a l ' s rate e q u a t i o n , introduced in C h a p t e r 2 . H o w e v e r , there w e r e a n u m b e r of d i f f e r e n c e s b e t w e e n the m e t h o d u s e d by Naito et a l . a n d in this work.  A t a l o w ionic  strength (0.1 M ) , the u s e of K C I g a v e rate c o n s t a n t s similar to solutions containing N a C I 0 , but w h e n h i g h e r levels of K C I w e r e u s e d , t h e trithionate d e g r a d a t i o n 4  80  rate  i n c r e a s e d significantly.  N a C I did not h a v e a n y effect o n the rate at a n ionic strength of  1 M , implying that rather than the chloride a n i o n b e i n g r e s p o n s i b l e for the i n c r e a s e d rate w h e n K C I w a s u s e d , the p o t a s s i u m ion w a s r e s p o n s i b l e .  P o t a s s i u m affected the trithionate d e g r a d a t i o n rate only in the p r e s e n c e of a m m o n i a . H o w e v e r , in the a b s e n c e of a m m o n i a (see S e c t i o n 5.4), the p H w a s m u c h lower a n d the reaction stoichiometry w a s different, s o this situation c a n n o t b e directly c o m p a r e d with the s y s t e m of interest, in the p r e s e n c e of a m m o n i a , e x c e p t to note that p o t a s s i u m d o e s not affect the b a s e l i n e d e g r a d a t i o n in water.  T h e influence of positive ions o n the  reaction rate implies the formation of a n activated c o m p l e x b e t w e e n the positive ion a n d o n e of the reactants.  In this set of tests, the only s p e c i e s p r e s e n t w e r e s o d i u m  trithionate a n d a m m o n i a , a n d the s p e c i e s u s e d to set the ionic strength.  S i n c e the  c o n c e n t r a t i o n of s o d i u m did not h a v e a n y significant effect it w o u l d s e e m that a n e x c e s s of s o d i u m w h e r e the trithionate w a s a l r e a d y a s s o c i a t e d with s o d i u m did not provide a n y c o m p e t i n g interaction.  H o w e v e r , the addition of p o t a s s i u m , w h i c h h a s a larger ionic  r a d i u s than s o d i u m , introduced a c o m p e t i n g cation for c o m p l e x a t i o n with trithionate. p r e s e n c e of p o t a s s i u m m a d e the trithionate p r e s e n c e of a m m o n i a .  The  m o r e a m e n a b l e to d e g r a d a t i o n in the  T h e e x c h a n g e reaction b e t w e e n trithionate a n d thiosulfate h a s  b e e n f o u n d to b e influenced by the concentration a n d c h a r g e of c a t i o n s p r e s e n t ( F a v a a n d P a j a r o , 1954) but it w a s not evident w h e t h e r trithionate or thiosulfate w a s the m o s t likely to form a c o m p l e x with the positive ion. S i n c e ionic c o m p l e x e s of thiosulfate h a d b e e n reported previously, it w a s a s s u m e d by F a v a a n d P a j a r o (1954) that trithionate r e a c t e d with a n ionic c o m p l e x of thiosulfate in this c a s e .  T h e results reported in this  t h e s i s imply that trithionate too c a n form ionic c o m p l e x e s with c a t i o n s (e.g. M S 0 ' ) . 3  6  T h i s is d i s c u s s e d further in C h a p t e r 6.  B a s e d o n t h e s e results, N a C I 0 w a s u s e d in all further tests to adjust the ionic strength. 4  It is important to realise that in Naito et al.'s work, the u s e of K C I for ionic strength a d j u s t m e n t s m a y h a v e h a d a significant influence o n the results a n d w a s not t a k e n into consideration.  T h e effect of ionic strength o n trithionate d e g r a d a t i o n w a s investigated further u s i n g s o d i u m perchlorate at slightly lower p H w h e r e both a m m o n i a a n d a m m o n i u m w e r e  81  p r e s e n t . Trithionate solutions in N H H C 0 4  3  / N H at p H 9 - 9.2 at 4 0 °C w e r e not affected 3  by ionic strength a d j u s t m e n t s up to about 1.8 M , a s s e e n in F i g u r e 5.3.  F i g u r e 5.3 : Effect of ionic strength o n o b s e r v e d rate constant I w for trithionate d e g r a d a t i o n using the initial rate m e t h o d (0.15 M NHdHCOa at p H 9-9.2 bv N H addition. 4 0 °C) a  5.7  CARBONATE AND BICARBONATE  S i n c e m o s t tests w e r e carried out in a n a m m o n i u m b i c a r b o n a t e -  ammonia  buffer  s y s t e m , it w a s n e c e s s a r y to e n s u r e that the b i c a r b o n a t e a n d c a r b o n a t e ions did not a p p r e c i a b l y affect the rate.  It h a d a l r e a d y b e e n e s t a b l i s h e d that the concentration of  s o d i u m in the f o r m of s o d i u m perchlorate did not affect the trithionate d e g r a d a t i o n rate (Figure 5.3). T h e effect of c h a n g i n g the concentration of a s o d i u m b i c a r b o n a t e / s o d i u m c a r b o n a t e buffer w a s investigated using the initial rate m e t h o d . T h e results a r e s h o w n in F i g u r e 5.4 for p H 9.0-9.2 at 4 0 °C.  T h e r e a p p e a r e d to b e no influence of  c o n c e n t r a t i o n of c a r b o n a t e / b i c a r b o n a t e u n d e r t h e s e conditions.  the  S i m i l a r trends h a v e  b e e n noted for s o d i u m b i c a r b o n a t e solutions at p H ~ 8 (not s h o w n ) w h e r e i n c r e a s i n g the c o n c e n t r a t i o n f r o m 0.5 M to 0.7 M b i c a r b o n a t e h a d no influence o n the rate of trithionate degradation.  T h e u s e of a m m o n i u m b i c a r b o n a t e w a s thus d e e m e d a c c e p t a b l e in  82  investigating  the  system,  particularly  to  investigate  the  effect  of  ammonium  concentration.  Figure  5.4 : Effect  of ionic strength of  NaHCOg / Na?COg  buffer o n the o b s e r v e d rate  c o n s t a n t I w for trithionate d e g r a d a t i o n using the initial rate m e t h o d (40 °C. p H 9.03 - 9.20. total ionic strength 0.51  5.8  M closed symbols. 1 M open symbol)  AMMONIUM AND AMMONIA  Initial Rate Method T h e trithionate d e g r a d a t i o n rate in solutions of a m m o n i u m b i c a r b o n a t e a n d a m m o n i u m sulfate of v a r i o u s c o n c e n t r a t i o n s w a s d e t e r m i n e d to find the effect of c o n c e n t r a t i o n o n the rate.  ammonium  A plot of the o b s e r v e d rate c o n s t a n t v e r s u s the a m m o n i u m  c o n c e n t r a t i o n for the two s y s t e m s is s h o w n in Figure 5.5.  T h e o b s e r v e d rate c o n s t a n t i n c r e a s e d linearly with a m m o n i u m c o n c e n t r a t i o n for both s y s t e m s , but w a s g r e a t e r at higher a m m o n i u m c o n c e n t r a t i o n s in the sulfate m e d i u m than the b i c a r b o n a t e m e d i u m .  It s h o u l d be pointed out that in the a m m o n i u m b i c a r b o n a t e  s y s t e m , the p H w a s fairly stable (around neutral) while in the a m m o n i u m sulfate s y s t e m , the initial p H w a s 4 to 5, d r o p p i n g v e r y rapidly throughout the test to a s low a s p H  83  1.1.  At pH values as low as this, other factors could come into play and the observed dependency on ammonium concentration was probably not an isolated effect. The data measured in the ammonium bicarbonate system is more suitable to judge the effect of ammonium concentration.  0.04 o  —  0.03  S  u  o  0.02  a  o  o pH 7.4  ammonium bicarbonate!  o  •  • pH4 ammonium sulfate  0.01 0.00  i  0.5  J  1  1  1  1.5  i  2  [NH ] (M) +  4  Figure 5.5 : Effect of ammonium concentration on the observed rate constant l w for trithionate degradation using the initial rate method (40 °C. total ionic strength 1 to 2 M. natural pH) Taking into consideration the degradation of trithionate in water and the gradient of the ammonium bicarbonate data in Figure 5.5, the extent of the dependency on the ammonium concentration in the bicarbonate system could be expressed as Equation 5.7 at 40 °C. This expression of the rate equation is similar to that for hydroxide solutions, discussed in Section 5.5. -d[S 0 -]/dt = (0.012h + 0.01M- h- [NH ])[S O T 2  3  6  1  1  1  +  2  4  3  6  [5.7]  The ammonium ion had a very similar effect to the potassium ion, discussed in Section 5.6. These ions have similar radii, with ammonium having a radius of 0.143 nm and potassium having a radius of 0.138 nm (Brodbelt and Liou, 1993). This is discussed further in Chapter 6.  Since gold leaching by thiosulfate generally takes place in  ammoniacal medium, the effect of ammonium was quantified as in Equation 5.7.  84  T h e effect of a m m o n i a o n the trithionate d e g r a d a t i o n rate w a s m e a s u r e d , in the a b s e n c e of a m m o n i u m ions at high p H (>10.5), using the initial rate m e t h o d .  T h e d e p e n d e n c y of  the o b s e r v e d rate c o n s t a n t o n t h e a m m o n i a c o n c e n t r a t i o n is s h o w n in F i g u r e 5 . 6 . T h e d a t a in F i g u r e 5.6 s h o w s the o b s e r v e d rate constant a d j u s t e d for the effect of hydroxide c o n c e n t r a t i o n , a s d e t e r m i n e d in S e c t i o n 5 . 5 .  0.04  Q  0.03  i  X  N-  0.02  O  i  S 0.01  J  y=0.0081x+0.012 0.00 0  0.5  [NH ](M) 3  1  1-5  F i g u r e 5.6 : Effect of a m m o n i a concentration o n the o b s e r v e d rate constant I w for trithionate d e g r a d a t i o n using the initial rate m e t h o d (40 ° C . total ionic strength 1 M using N a C l O j )  The  data shows  a slight i n c r e a s e  in the o b s e r v e d  rate c o n s t a n t with  ammonia  c o n c e n t r a t i o n . T w o trend lines a r e indicated o n the g r a p h . T h e best-fit trend line implies that the o b s e r v e d rate constant c a n b e e x p r e s s e d a s E q u a t i o n 5 . 8 (at 4 0 ° C ) .  ko = 0 . 0 1 5 6 h " + 0 . 0 0 4 9 M - V [ N H ] + 0 . 7 4 i V T V [OH] 1  3  bs  [5.8]  H o w e v e r , the b a s e l i n e d e g r a d a t i o n rate of trithionate in w a t e r ( E q u a t i o n 5.3) w o u l d imply a n intercept of 0 . 0 1 2 h" . B y fixing the intercept of the g r a p h in F i g u r e 5.6 to 0 . 0 1 2 h"\ 1  the o b s e r v e d rate c o n s t a n t c a n b e e x p r e s s e d a s E q u a t i o n 5 . 9 (at 4 0 ° C ) .  85  k  = 0.012h" + 0.0081 M- h" [NH ] + 0.74 i V T V [OH"] 1  obs  1  [5.9]  1  3  A relatively small change in the intercept resulted in a larger variation to the gradient. It should be noted that the results indicating a dependency on ammonia concentration are strongly dependent on the accuracy of the ammonia analysis method.  Since the  ammonia concentration was found by difference between a total ammonia plus ammonium concentration, and an initial ammonium value, this method is prone to some error. Naito et al. (1975) assumed that ammonia was involved in a nucleophilic substitution reaction at the sulfonate sulfur atom of trithionate, producing thiosulfate and sulfamate as products. As discussed in the literature review (Chapter 2) the presence of sulfamate was not confirmed by these authors and this deduction was based on reactions known to occur under very different reaction conditions. A suitable method to analyse sulfamate in the presence of trithionate and thiosulfate could not be found (see Chapter 3 on analytical methods) so this assumption could not be confirmed in this work. Figure 5.7 shows the observed rate constant plotted against  the ammonium  concentration for both the ammonium bicarbonate / ammonia and the ammonium sulfate / ammonia systems at a constant pH of 9.1 - 9.2 (at 40 °C). Since the pH was constant, an increase in ammonium concentration also implied an  increase in ammonia  concentration. The data show that the two systems behave fairly similarly, within the error margins for each data point. It is assumed that the observed rate constant can be interpreted as being representative of the sum of various individual components, including the baseline degradation rate of trithionate in water and an enhanced degradation seen either in the presence of ammonia or ammonium (as noted above). The effect of hydroxide concentration was not included in this case as under these conditions it was expected to contribute to less than 0.1 % ofthe observed rate constant. The observed rate constant was thus expressed as a sum of its components, as in Equation 5.10.  86  0.04  0.03 y = 0.022x +0.013  4-  0.02  y = 0.014x + 0.012  0.01  0.2  0.4  0.6  0.8  • Ammonium sulfate / ammonia • Ammonium bicarbonate/ammonia  NH (M) +  4  Figure 5.7 : Effect of ammonium concentration on the observed rate constant l w for trithionate degradation for (NhUgSCu / NH and N H H C O a / N H buffer systems using 3  3  4  the initial rate method (40 °C, pH 9.13 - 9.19, ionic strength 0.51 M closed symbols. 1 M open symbols)  k  = 0.0121V + k [NH ] + k [NH ] 1  obs  [5.10]  +  2  3  3  4  For the data in Figure 5.7, at pH 9.1 to 9.2, the concentration of ammonia was typically 2.1 times greater than that of ammonium, given in Equation 5.11. k  = 0.012h" +2.1 k [NH ] + k [NH ] 1  obs  +  2  4  [5.11]  +  3  4  The slope in Figure 5.7 was thus 2.1 k + k . Taking k as 0.01M" .h" (see earlier) and k 1  2  3  1  3  2  as either 0.0049M"V (Equation 5.8) or 0.0081 M " V (Equation 5.9), one would expect 1  the slope in Figure 5.7 to be between 0.020 and 0.027 M" h" . This is in agreement with 1  1  the slope obtained for the data obtained in bicarbonate medium (0.022 M" h" ). 1  87  1  Integrated Rate Method C h a n g i n g the a m m o n i u m a n d a m m o n i a concentration for the s t a n d a r d  ammonium  b i c a r b o n a t e / a m m o n i a buffer s y s t e m a s well a s for a n a m m o n i u m sulfate / a m m o n i a buffer s y s t e m w a s a l s o investigated at 2 5 °C using the integrated rate m e t h o d .  Results  a r e s h o w n in T a b l e 5.5.  T a b l e 5.5 : Effect of a m m o n i u m concentration o n o b s e r v e d rate c o n s t a n t l w for trithionate d e g r a d a t i o n (25 °C. nominally 0 . 0 2 3 M S Q a  Buffer s y s t e m (NH ) S0 4  2  NH 4  F o r either  buffer  /  3  NH HC0 NH  4  3  /  3  2 f i  ' . a m m o n i a a d d e d to adjust p H to - 1 0 . 3 )  [ N H ] (M) +  4  kobs  (h" )  0.02  0.0022  0.09  0.0027  0.03  0.0026  0.15  0.0034  s y s t e m , i n c r e a s i n g the  concentration  a m m o n i a ) i n c r e a s e d the o b s e r v e d rate constant.  1  of a m m o n i u m  (and  hence  W h i l e the effects of a m m o n i a a n d  a m m o n i u m w e r e not e x a m i n e d individually u s i n g this m e t h o d , the trends m a t c h t h o s e f o u n d at 4 0 °C u s i n g the initial rate m e t h o d .  5.9  pH  A n u m b e r of c o m p a r a t i v e tests w e r e d o n e at c o n s t a n t p H using the initial rate m e t h o d . T h e total a m m o n i a a n d a m m o n i u m c o n c e n t r a t i o n s w e r e v a r i e d but kept at a c o n s t a n t ratio to maintain the d e s i r e d p H . T h e s a m e d a t a is s h o w n in two different w a y s - a s the o b s e r v e d rate c o n s t a n t plotted a g a i n s t the a m m o n i u m concentration a n d a g a i n s t the a m m o n i a c o n c e n t r a t i o n . S i n c e it h a s b e e n s h o w n that the hydroxide  concentration  i n f l u e n c e s the reaction rate, the effect of hydroxide ion c o n c e n t r a t i o n s h o u l d b e t a k e n into a c c o u n t a s the p H v a r i e s .  H o w e v e r , the v a l u e of this contribution w a s g e n e r a l l y  negligible, a n d e v e n at p H > 10.5 w a s l e s s than 4 % of the o b s e r v e d rate c o n s t a n t , w h i c h is l e s s than the level of uncertainty in the m e a s u r e m e n t s . t h u s not m a d e in the figures.  88  T h e adjustment w a s  0.04  o p H 7.4 • p H 8.8  0.03  JD  • p H 9.0  0.02  O  -  I  ~ A  p H 9.2  0.01 o p H 9.9 0.00 0.5  1  x pH10.2  1.5  [ N H ] (M) +  4  F i g u r e 5.8 : D e p e n d e n c y of the o b s e r v e d rate c o n s t a n t Iw  for trithionate d e g r a d a t i o n o n  a m m o n i u m concentration at v a r i o u s p H (40 °C)  0.04 SI  •  0.03  p H 8.8  • pH  9.0  •  0.02  J  o  A y  •  _  A  I  •  *o "-_ o si - X  "  x  •  —  A  pH9.2  O  pH9.9  £  0.01  X pH10.2  0.00  •  i  0  0.5  1  pH>10.5  1.5  [NH ] (M) 3  F i g u r e 5.9 : D e p e n d e n c y of the o b s e r v e d rate c o n s t a n t Iw  for trithionate d e g r a d a t i o n o n  a m m o n i a concentration at v a r i o u s p H (40 °C)  89  It is difficult to s e e a n y variations in the s l o p e of the o b s e r v e d rate c o n s t a n t v e r s u s a m m o n i a or a m m o n i u m c o n c e n t r a t i o n s with p H f r o m F i g u r e 5.8 or 5.9. T h e s a m e d a t a w a s therefore plotted in a different w a y , a s s e t s of d a t a at c o n s t a n t a m m o n i a plus a m m o n i u m concentration a g a i n s t the p H , a s in F i g u r e 5.10. S i n c e the effect of c h a n g i n g the ionic strength h a d not b e e n f o u n d to b e significant w h e n N a C I 0 w a s u s e d to adjust 4  it, the d a t a in F i g u r e 5.10 r e p r e s e n t s a r a n g e in ionic strengths.  E a c h data series  r e p r e s e n t s a s m a l l r a n g e of total a m m o n i a c o n c e n t r a t i o n a s indicated o n the g r a p h . Error b a r s w e r e e x c l u d e d for clarity.  T h e r e a p p e a r s to b e a m i n i m u m d e g r a d a t i o n rate  a r o u n d p H 10.  T h e following  g e n e r a l o b s e r v a t i o n s w e r e m a d e , taking  into c o n s i d e r a t i o n that  the  uncertainty levels for e a c h d a t a point w e r e significant ( s e e S e c t i o n 5.3): •  F o r a n y p H , a s the total a m m o n i a plus a m m o n i u m i n c r e a s e d , the o b s e r v e d rate constant increased.  •  T h e r e w a s a m i n i m u m in the trithionate d e g r a d a t i o n rate at a r o u n d p H 10. A t p H v a l u e s higher than a r o u n d 10 (at 4 0 °C), the o b s e r v e d rate c o n s t a n t i n c r e a s e d a s the p H i n c r e a s e d . A t p H v a l u e s lower than a r o u n d 10 (at 4 0 °C), the o b s e r v e d rate constant i n c r e a s e d a s the p H d e c r e a s e d .  T h i s effect is attributed to the  relative a m o u n t s of a m m o n i a a n d a m m o n i u m i o n s , rather than the p H (hydroxide concentration) directly in this p H r a n g e , a s d i s c u s s e d in C h a p t e r 6. •  T h e r e w a s a m i n i m u m trithionate d e g r a d a t i o n c o r r e s p o n d i n g to the d e g r a d a t i o n in water.  90  0.04 NH  3  + NH4  +  concentration 0.03  x OM A  • 0.1-0.2M  o  CO  n o  0.02  0.01  X A  x  A 0.3-0.4M m  • 0.5M x 0.7-0.8M  -X—  A 0.9-1.2M o 1.5-1.7M  0.00 11  13  o 1.8-2.6M  pH  F i g u r e 5.10 : O b s e r v e d rate constant l w for trithionate d e g r a d a t i o n v e r s u s p H for a r a n g e of a m m o n i a / a m m o n i u m c o n c e n t r a t i o n s (40 °C, v a r i o u s ionic strength)  T h e o b s e r v e d rate c o n s t a n t at 2 5 °C s h o w e d very little c h a n g e with a n i n c r e a s e in p H . H o w e v e r , only three d a t a points w e r e a v a i l a b l e a n d the m a g n i t u d e of the o b s e r v e d rate c o n s t a n t s w e r e m u c h l e s s than at 4 0 °C, s o it w o u l d b e m u c h m o r e difficult to notice trends.  F i g u r e 5.11 : O b s e r v e d rate constant l w for trithionate d e g r a d a t i o n v e r s u s p H at 2 5 °C (Total ionic strength = 1.1 M , 1 M N H / . a m m o n i u m b i c a r b o n a t e / a m m o n i a s y s t e m )  91  In o r d e r to investigate p H effects in the a b s e n c e of c o m p l i c a t i n g effects from a m m o n i a a n d a m m o n i u m , a s o d i u m b i c a r b o n a t e / s o d i u m c a r b o n a t e s y s t e m w a s u s e d u s i n g the initial rate m e t h o d at 4 0 °C.  It w a s s h o w n ( S e c t i o n 5.7) that the b i c a r b o n a t e  c a r b o n a t e s p e c i e s did not influence the trithionate reaction rate at p H 9-9.2.  and The  b i c a r b o n a t e to c a r b o n a t e ratio w a s varied to investigate the effect of p H without h a v i n g to introduce a m m o n i a s p e c i e s to the s y s t e m . T h e o b s e r v e d rate constant v e r s u s p H is s h o w n in F i g u r e 5.12. B e c a u s e the c o n c e n t r a t i o n s of b i c a r b o n a t e a n d c a r b o n a t e did not h a v e a n effect, the d a t a s h o w n in F i g u r e 5.12 include d a t a w h e r e varying c o n c e n t r a t i o n s of b i c a r b o n a t e / c a r b o n a t e w e r e u s e d (with the ionic strength from the b i c a r b o n a t e / c a r b o n a t e buffer ranging b e t w e e n 0.05 a n d 0.9 M ) .  N o o b v i o u s trend in reaction rate with p H w a s o b s e r v e d in this p H r a n g e . T h i s implies that rather than p H it is the relative c o n c e n t r a t i o n s of the a m m o n i u m a n d a m m o n i a w h i c h influence the trithionate d e g r a d a t i o n .  F i g u r e 5.12 : Effect of p H o n o b s e r v e d rate constant I w for trithionate d e g r a d a t i o n in sodium carbonate/bicarbonate medium (40 °C, Ionic strength 0.5 M for c l o s e d s y m b o l s . 1 M for o p e n s y m b o l s )  92  5.10  THIOSULFATE  Initial Rate Method The  effect  of  thiosulfate  on  the  trithionate  degradation  rate  was  investigated  in  a m m o n i a c a l b i c a r b o n a t e buffer u s i n g a r a n g e of a m m o n i u m c o n c e n t r a t i o n s a n d p H . T h e tests w e r e d o n e at 4 0 °C with a q u e o u s a m m o n i a a d d e d to adjust the p H to b e t w e e n 9 a n d 10. R e s u l t s a r e s h o w n in F i g u r e 5.13.  • 1M ammonium pH 9.4  0.04 0.03  • 0.5M  S  ammonium  0.02  o  0.01  pH 9.4 A  t  A  o 0.15 M ammonium  pH9  0.00 0.1  0.2  0.3  0.4  0.5  [S 0 -] (M) 2  2  3  A 0.024 M ammonium  pH 10  F i g u r e 5 . 1 3 : Effect of thiosulfate o n the o b s e r v e d rate c o n s t a n t I w for trithionate d e g r a d a t i o n u s i n g the initial rate m e t h o d (40 °C) (Total ionic strength for 1 M a m m o n i u m tests w a s 1 M ( o p e n s y m b o l s ) or 2 M ( c l o s e d s y m b o l s ) . T o t a l ionic strength for 0.5 M a m m o n i u m tests w a s 0.6 M ( o p e n s y m b o l ) or 1.8 M ( c l o s e d s y m b o l s ) . Total ionic strength for all other d a t a points w a s 1 M.)  A t low a m m o n i u m c o n c e n t r a t i o n (0.024 M) at p H 10, the addition of thiosulfate h a d a negligible effect o n the trithionate d e g r a d a t i o n , p e r h a p s c a u s i n g a slight inhibition of the d e g r a d a t i o n . H o w e v e r , from the o b s e r v a t i o n s of trithionate d e g r a d a t i o n at different p H , it w a s f o u n d that n e a r p H 10, the trithionate d e g r a d a t i o n w a s at a m i n i m u m ( s e e S e c t i o n 5.9). A n y effects of thiosulfate m a y h a v e b e e n m i n i m i s e d at this p H .  93  A t 0 . 1 5 M a m m o n i u m , pH 9, addition of thiosulfate i n c r e a s e d the trithionate d e g r a d a t i o n rate but c a u s e d inhibition at higher levels.  Increasing the a m m o n i u m concentration to  0.5 M (and the p H to 9.4) s h o w e d a similar trend w h e r e thiosulfate e n h a n c e d the rate at lower levels but inhibited it at higher levels.  Similarly, for 1 M a m m o n i u m (pH 9.4)  addition of thiosulfate i n c r e a s e d the trithionate d e g r a d a t i o n rate. thiosulfate t e s t e d , no inhibition w a s noted for this s e r i e s .  A t the levels of  P o s s i b l e r e a s o n s for t h e s e  o b s e r v a t i o n s a r e d i s c u s s e d in C h a p t e r 6.  Integrated Rate Method T h e effect of thiosulfate addition at r o o m temperature is s h o w n in T a b l e 5.6 for two s e t s of tests, u s i n g the integrated rate m e t h o d .  It is c l e a r that within the variability of the  e x p e r i m e n t a l c o n d i t i o n s , thiosulfate at 0.09 M (almost d o u b l e the initial a m o u n t trithionate a n d three times the significant effect.  initial a m m o n i u m  concentration)  did not h a v e  of any  T h e reaction stoichiometry w a s not affected by the p r e s e n c e of initial  thiosulfate. H o w e v e r at r o o m t e m p e r a t u r e , a n y differences in o b s e r v e d rate c o n s t a n t a r e e x p e c t e d to b e m u c h l e s s than at 4 0 °C, s o it is p o s s i b l e that the effects w e r e s i m p l y not m e a s u r a b l e at this temperature.  T a b l e 5.6 : Effect of thiosulfate o n the o b s e r v e d rate constant l w for trithionate d e g r a d a t i o n using the integrated rate m e t h o d (room t e m p e r a t u r e . rNH HCO l = 0 . 0 3 M . NHs a d d e d to p H - 9 . 7 ) 4  Initial S 0 3  2 6  - (M)  ?  Initial S 0 2  2 _ 3  kobs (h" ) 1  (M)  0.043  0  0.0017  0.041  0.083  0.0021  0.054  0  0.0016  0.053  0.091  0.0017  94  5.11  OXYGEN EXCLUSION  Initial Rate Method T h e effect of limiting the a m o u n t of o x y g e n in solution o n the trithionate d e g r a d a t i o n rate w a s investigated.  S o l u t i o n s w e r e s p a r g e d with nitrogen a n d a nitrogen-filled g l o v e b a g  w a s u s e d during preparation of the test. septum.  S a m p l e s w e r e r e m o v e d by s y r i n g e through a  W h i l e this m e t h o d m a y not h a v e c o m p l e t e l y r e m o v e d all o x y g e n from the  s y s t e m , the a m o u n t of o x y g e n present would h a v e b e e n significantly limited.  The  efficiency of o x y g e n r e m o v a l w a s not d e t e r m i n e d .  T h e effect of limiting o x y g e n in solution w a s investigated in the a b s e n c e of a m m o n i a for both the low alkaline p H reaction ( E q u a t i o n 5.2) a n d the high alkaline reaction ( E q u a t i o n 5.4).  A t high p H (pH ~ 12 at 4 0 °C) 0.1 M N a O H w a s u s e d .  T h e results a r e s h o w n in  T a b l e 5.7 w h e r e it c a n b e s e e n that limiting the o x y g e n in solution g a v e n o significant c h a n g e in the rate c o n s t a n t or the reaction stoichiometry with r e s p e c t to thiosulfate u n d e r t h e s e conditions, implying that d i s s o l v e d o x y g e n h a s no role in this d e g r a d a t i o n .  T a b l e 5.7 : Effect of limiting d i s s o l v e d o x y g e n o n the o b s e r v e d rate c o n s t a n t knhg for trithionate d e g r a d a t i o n in 0.1 M hydroxide solution  Ratio S 0 2  Atmosphere  2 3  "  Ave  formed :  kobs  initial p H  so -  (h- )  (h- )  Ch")  2  3  1  6  k  min  o b s  1  k bs 0  max 1  decayed air N  2  (0  2  limited)  11.96  0.3-0.4  0.143  0.132  0.156  12.09  0.2-0.4  0.133  0.124  0.141  T h e effect of nitrogen s p a r g i n g w a s a l s o tested at a two different p H s in the a m m o n i a / a m m o n i u m bicarbonate medium.  R e s u l t s are s h o w n in T a b l e 5.8, w h e r e a g a i n there  w a s n o significant effect. T h i s is not surprising a s the anticipated reaction, s h o w n in E q u a t i o n 5.2, d o e s not involve o x y g e n . T h e r e is likely to b e a n effect noted after longer  95  t i m e s , a s the thiosulfate p r o d u c e d from trithionate d e g r a d a t i o n c a n o x i d i s e slowly in the p r e s e n c e of o x y g e n .  T a b l e 5.8 : Effect of limiting o x y g e n o n the o b s e r v e d rate c o n s t a n t l w for trithionate d e g r a d a t i o n in a m m o n i a / a m m o n i u m b i c a r b o n a t e solution  Atmosphere  air N (0 2  2  limited) air N (0 2  2  limited)  [NH4HCO3]  (M)  [NH ] (M) 3  A v e initial PH  k kobs  (IT  1  )  o  b  s  rnin  k  o  b  max  s  Ch")  (h- )  1  1  0.5  0  7.4  0.0182  0.0163  0.0203  0.5  0  7.9  0.0185  0.0178  0.0192  0  0.492  10.9  0.0171  0.0154  0.0188  0  0.625  11.0  0.0190  0.0180  0.0200  Integrated Rate Method In similar tests u s i n g the integrated rate m e t h o d , the o x y g e n concentration w a s limited by u s i n g water that h a d b e e n s p a r g e d with nitrogen a n d filling the test flask h e a d s p a c e with nitrogen at the start of the test a n d at e a c h s a m p l i n g point.  R e s u l t s for trithionate  d e g r a d a t i o n both in the p r e s e n c e a n d a b s e n c e of initial thiosulfate a r e s h o w n in T a b l e 5.9.  R e d u c i n g the level of o x y g e n h a d n o effect o n the trithionate  confirming the findings using the initial rate m e t h o d .  96  degradation,  T a b l e 5.9 : Effect of limiting o x y g e n o n the o b s e r v e d rate constant I w for trithionate d e g r a d a t i o n using the integrated rate m e t h o d (room t e m p e r a t u r e . rNhUHCOJ = 0 . 0 3 M . N H s a d d e d to p H - 9 . 7 )  Test  Atmosphere  Initial S 0 3  2 6  " (M)  Initial S 0 2  2 3  "  kobs  (h" ) 1  (M)  5.12  14  air  0.054  0  0.0016  16  nitrogen  0.048  0  0.0016  15  air  0.053  0.091  0.0017  17  nitrogen  0.048  0.089  0.0016  CUPRIC COPPER  C u p r i c c o p p e r is c o n s i d e r e d n e c e s s a r y by m a n y to c a t a l y s e the thiosulfate l e a c h i n g of g o l d , but it is k n o w n to e n h a n c e the d e g r a d a t i o n of thiosulfate.  It w a s  therefore  c o n s i d e r e d important to e s t a b l i s h the effect of c o p p e r o n trithionate d e g r a d a t i o n .  Integrated Rate Method C u p r i c sulfate ( 0 . 0 0 4 5 M ) w a s a d d e d to a trithionate solution (0.049 M) to d e t e r m i n e the effect o n the trithionate d e g r a d a t i o n rate. T a b l e 5.10 s h o w s the effect o n the o b s e r v e d rate c o n s t a n t a n d o n the reaction stoichiometry.  T a b l e 5.10 : Effect of c u p r i c addition o n the o b s e r v e d rate constant k„hg for trithionate d e g r a d a t i o n using the integrated rate m e t h o d (room t e m p e r a t u r e . N H j H C O g / N H g buffer. T N H / 1 = 0.03 M . initial p H - 9 . 7 )  Cu Test  Stoichiometry  2 +  present (M)  p H after  p H after  4 3 2 hrs  6 0 0 hrs  kobs  (h" ) 1  S 0 2  2 3  S 0 3  ":  S0 ":  "  S 0  2 6  2  4  3  2 6  14  0  -  9.13  0.0017  0.9  0.9  20  0.0045  8.21  -  0.0018  0  3.3  97  "  E v e n with c o p p e r present, the trithionate d e g r a d a t i o n reaction still followed first o r d e r reaction kinetics.  A l t h o u g h the rate constant w a s similar either with o r without c o p p e r  p r e s e n t , the p H drop w a s m u c h m o r e significant with c o p p e r p r e s e n t ( b a s e d o n the limited d a t a a v a i l a b l e ) , a n d n o thiosulfate f o r m e d .  E v e n t h o u g h the rate c o n s t a n t s w e r e  similar, they c a n n o t b e directly c o m p a r e d b e c a u s e o v e r t h e s e long times  different  r e a c t i o n s o c c u r r e d with a n d without c o p p e r present.  Initial Rate Method T h e effect of c u p r i c c o p p e r (in the form of cupric sulfate) o n the trithionate d e g r a d a t i o n rate in water a n d in a m m o n i a / a m m o n i u m bicarbonate buffer w a s investigated. A i r w a s not e x c l u d e d from t h e s e tests, a s for all other tests e x c e p t for t h o s e reported in S e c t i o n 5 . 1 1 , a n d the temperature w a s maintained at 4 0 ° C . T h e results a r e s h o w n in T a b l e 5.11.  It s h o u l d b e noted that in s o m e tests 0.01 M c o p p e r w a s a d d e d . T h e trithionate  c o n c e n t r a t i o n u s e d in t h e s e tests w a s 0.01 M to 0 . 0 4 M , of w h i c h only a b o u t 10 % is e x p e c t e d to d e g r a d e o v e r the duration of the test.  H e n c e the ratio of c o p p e r to  trithionate w a s high c o m p a r e d to typical gold l e a c h i n g conditions. H o w e v e r , the a i m w a s to identify a n effect, if a n y , e v e n in e x a g g e r a t e d form.  T a b l e 5.11 : Effect of c u p r i c c o p p e r o n the o b s e r v e d rate constant  knhg  for trithionate  degradation  k min (hf )  k m a x (IV )  [ C u ] (M)  [NH4HCO3] (M)  A v e initial p H  kobs (h" )  0  0  4  0.012  0.01  0  4  0.027  0.021  0.029  0  0.54  9.4  0.022  0.021  0.023  0.001  0.5  9.7  0.025  0.023  0.026  0.01  0.5  9.6  0.026  0.025  0.027  0  0.02  9.6  0.013  0.011  0.015  0.01  0.02  10.2  0.013  0.012  0.014  2+  * N o t d e t e r m i n e d - s e e S e c t i o n 5.4  98  1  1  1  *  In water, there w a s a significant effect of c o p p e r o n the trithionate d e g r a d a t i o n rate. Precipitation of c o p p e r sulfides w a s a l s o evident, s o this result c a n n o t be directly c o m p a r e d to that in the a b s e n c e of c o p p e r or to higher p H tests w h e r e no precipitation w a s noted.  In the p r e s e n c e of a n a m m o n i a / a m m o n i u m buffer, c o p p e r did not h a v e a n y significant effect o n the o b s e r v e d rate constant. T h e stoichiometry w a s the s a m e a s in the a b s e n c e of c o p p e r a n d no precipitation w a s evident.  W h i l e the o b s e r v e d rate c o n s t a n t w a s not  affected for the integrated rate m e t h o d either, a different reaction stoichiometry a n d precipitation w a s o b s e r v e d . It m a y be that in the integrated rate m e t h o d tests, the m u c h longer test durations w e r e r e s p o n s i b l e for this.  It is p o s s i b l e that o v e r t h e s e t i m e s , the  significant p H d r o p in c o m b i n a t i o n with the p r e s e n c e of reaction p r o d u c t s in the solution c o u l d h a v e affected the overall reaction stoichiometry. T h e initial rate m e t h o d results are c o n s i d e r e d to be m o r e reliable.  It h a s b e e n p r o p o s e d by Jeffrey a n d his c o w o r k e r s that trithionate reacts with cupric c o p p e r ( B r e u e r a n d Jeffrey, 2 0 0 3 b ) .  In work f o c u s i n g o n the reaction b e t w e e n cupric  c o p p e r a n d thiosulfate, they m e a s u r e d the rate of reduction of c u p r i c c o p p e r , inferring f r o m this the thiosulfate d e g r a d a t i o n rate.  O n addition of trithionate to the s y s t e m , the  c u p r i c c o p p e r w a s r e d u c e d faster, s o it w a s d e d u c e d that cupric c o p p e r reacts with trithionate in a similar w a y to with thiosulfate. T h a t o b s e r v a t i o n d o e s not a g r e e with that f o u n d in t h e s e results.  It w a s f o u n d that for the thiosulfate-copper reaction, the a m o u n t of o x y g e n present is v e r y important ( B r e u e r a n d Jeffrey, 2 0 0 3 a ) . w a s not controlled.  In the current tests o n trithionate, o x y g e n  It m a y be n e c e s s a r y to carefully control the levels of d i s s o l v e d  o x y g e n to notice a n y effect s u c h a s that p r o p o s e d by Jeffrey et a l .  5.13  TETRATHIONATE  T h e effect of tetrathionate w a s only e x a m i n e d using the integrated rate m e t h o d .  When  tetrathionate w a s present at the start of a test, it w a s f o u n d to very rapidly d e g r a d e to lower than the a n a l y s i s detection limit, c a u s i n g a c o r r e s p o n d i n g s m a l l i n c r e a s e in the  99  trithionate  a n d thiosulfate levels.  T h i s w a s e x p e c t e d a s tetrathionate  is k n o w n to  d e g r a d e rapidly u n d e r alkaline conditions, a s s h o w n in E q u a t i o n 5.12.  S 0 4  2 6  ' + % O H ' -> / S 0 5  4  2  2 3  ' + / S 0 1  2  3  2 6  "+ % H 0  [5.12]  2  B e s i d e s this s m a l l initial effect, the kinetics of trithionate d e g r a d a t i o n w e r e not affected in a n y other w a y by the p r e s e n c e of tetrathionate.  5.14  ELEMENTAL SULFUR, SULFATE AND COPPER POWDER  A f e w exploratory tests w e r e carried out to investigate the effect of the p r e s e n c e of elemental  sulfur  (hydrophobic),  sulfate  and  elemental  d e g r a d a t i o n rate, using the integrated rate m e t h o d . 0.08 M S 0 2  2 3  ' and 0.009 M S 0 4  with time for e a c h test  2 6  ' were used.  copper  on  the  trithionate  Initial conditions of 0.05 M S 0 3  2 6  ,  T h e c h a n g e of trithionate concentration  is s h o w n in Figure 5.14.  T h e o b s e r v e d rate  constants  c o r r e s p o n d i n g to F i g u r e 5.14 d e t e r m i n e d from the point w h e n all the tetrathionate had d e g r a d e d are s h o w n in T a b l e 5.12.  T a b l e 5.12 : Effect of e l e m e n t a l sulfur, sulfate a n d c o p p e r p o w d e r o n the o b s e r v e d rate constant  knhs  for trithionate d e g r a d a t i o n  (room t e m p e r a t u r e , f N H H C Q l = 0 . 0 3 M , NHs a d d e d to pH - 9 . 7 , 4  3  nominally 0.05 M S?OR '. 0.08 M S?Oa '. 0.009 M S Q 2  2  4  Species added  kobs  0.0021  S (4.95 g/l)  0.0015  S0 "(15g/I)  0.0021  C u (8 g/l)  0.0010  2  )  (h" )  none  4  2 F I  1  W h i l e the addition of sulfate did not affect the o b s e r v e d rate, e l e m e n t a l sulfur m a y h a v e slightly  retarded  the  reaction, while  metallic  copper  powder  d e c r e a s e d the  rate  significantly. W h e r e c o p p e r p o w d e r w a s u s e d , the c o p p e r p o w d e r b e c a m e black, probably d u e to the formation of sulfides. Insufficient sulfate a n a l y s e s w e r e a v a i l a b l e to d r a w a definitive c o n c l u s i o n regarding the d e g r a d a t i o n stoichiometry in the p r e s e n c e of  100  c o p p e r p o w d e r , but indications are that higher levels of sulfate than e x p e c t e d a c c o r d i n g to E q u a t i o n 5.2 w e r e o b t a i n e d .  T h e s e effects w e r e not p u r s u e d further in this work, but future work m a y b e w a r r a n t e d .  10 9 ITT 8  i  7  Ui 6  5  £  X  A No added species A Sulfur  5  A  4  A Sulfate 3 2  x Copper powder  1 0 100  200  300  400  500  600  T i m e (hr)  F i g u r e 5.14 : Effect of sulfur, sulfate or c o p p e r p o w d e r o n trithionate concentration profile (room t e m p e r a t u r e , p H ~ 9.7)  5.15  TEMPERATURE  Initial R a t e M e t h o d M o s t of the kinetic tests w e r e d o n e at 4 0 °C, to e n s u r e that the reaction rates w e r e high e n o u g h for the initial rate m e t h o d to be u s e d with a r e a s o n a b l e d e g r e e of a c c u r a c y a n d to limit the scatter in the d a t a . A f e w c o m p a r a t i v e tests w e r e d o n e at 2 5 °C to quantify the effect of t e m p e r a t u r e o n the d e g r a d a t i o n rate.  T h e o b s e r v e d rate c o n s t a n t s for tests d o n e in a n a m m o n i u m b i c a r b o n a t e / a m m o n i a buffer of ionic strength 1.1 M for three initial p H v a l u e s at 2 5 °C a n d 4 0 °C are s h o w n in T a b l e 5.13.  B a s e d o n this d a t a , the activation e n e r g i e s w e r e c a l c u l a t e d using the  Arrhenius equation.  T h e a p p a r e n t activation e n e r g i e s r a n g e d b e t w e e n 7 2 . 9 a n d 7 8 . 9 v  k J / m o l b e t w e e n p H 8.8 a n d 1 0 . 1 . T h e p e r c e i v e d i n c r e a s e in activation e n e r g y with p H  101  w a s within the uncertainty of the m e a s u r e m e n t s of the rate c o n s t a n t s .  Activation  e n e r g i e s c a l c u l a t e d u n d e r similar conditions in the literature r a n g e d from 81 - 87 k J / m o l for a t e m p e r a t u r e r a n g e of 4 0 - 8 0 °C (Naito et a l . , 1975), to 9 1 . 7 k J / m o l m e a s u r e d at 7 0 -  8 5 °C ( R o l i a a n d C h a k r a b a r t i , 1982).  T h i s implies the reaction is u n d e r c h e m i c a l  reaction control.  T a b l e 5.13 : Effect of temperature o n the o b s e r v e d rate constant k ^ for trithionate d e g r a d a t i o n at p H 8.8 - 10.1 ( N h U H C O q a d d e d to give I = 1.1 M . variable rNH l) 3  PH  Temperature  kobs  (h" )  9.4  10.1  Activation e n e r g y (kJ/mol)  (°C) 8.8  1  25  0.0054  40  0.0221  25  0.0070  40  0.0294  25  0.0067  40  0.0308  72.9  74.2  78.9  Integrated Rate Method T h e s a m e temperature  effect w a s found  integrated rate m e t h o d .  T h e o b s e r v e d rate c o n s t a n t s a n d c a l c u l a t e d activation e n e r g y  are s h o w n in T a b l e 5.14.  in the p r e s e n c e of thiosulfate,  using the  It is important to note that b e c a u s e of the s p e e d of the  r e a c t i o n , very f e w d a t a points w e r e m e a s u r e d at 4 0 °C, s o the derivation of the rate c o n s t a n t w a s not likely to b e a s a c c u r a t e a s using the initial rate m e t h o d . A l s o , for the p u r p o s e s of the A r r h e n i u s c a l c u l a t i o n , it w a s a s s u m e d that the r o o m temperature tests w e r e at 2 2 °C. T h e e s t i m a t e d activation e n e r g y w a s about 71 k J / m o l .  102  T a b l e 5.14 : Effect of t e m p e r a t u r e o n the o b s e r v e d rate c o n s t a n t I w for trithionate d e g r a d a t i o n using the integrated rate m e t h o d d N H ^ H C O g l = 0 . 0 3 M . N H a d d e d to p H - 9 . 7 ) a  Temperature  Initial  Initial  S 0  so -  3  2 6  "  kobs  (h" )  2  2  Estimated E (kJ/mol)  3  (M)  (M)  RT  0.041  0.083  0.0021  4 0 °C  0.047  0.090  0.0110  103  1  70.6  a  6.  KINETICS OF THE DEGRADATION OF TRITHIONATE - DISCUSSION AND MODELLING  6.1  QUALITATIVE DISCUSSION  T h e o b s e r v e d d e g r a d a t i o n of trithionate c o n f i r m e d the literature.  in the p r e s e n c e of water or hydroxide ions  It is likely to p r o c e e d v i a nucleophilic attack at the sulfonate  sulfur a t o m (Naito et a l . , 1975). A n i n c r e a s e in the concentration of hydroxide ions (or p H ) i n c r e a s e d the trithionate d e g r a d a t i o n rate, a s e x p e c t e d , s i n c e hydroxide is k n o w n to react directly with trithionate (Equation 6.1).  S 0  2  3  6  - + 20H-  S 0 2  2 3  - + S0  2 4  " + H 0  [6.1]  2  A s o d i u m b i c a r b o n a t e / c a r b o n a t e buffer h a d no effect o n the rate, at constant p H (varying concentration) or with v a r y i n g p H (varying c a r b o n a t e / b i c a r b o n a t e ratio) from a r o u n d p H 7.8 to 11 (at 4 0 °C). A l t h o u g h the hydroxide c o n c e n t r a t i o n i n c r e a s e d the rate, this effect w a s very s m a l l at p H v a l u e s l e s s than 1 1 , s o varying the p H in this range did not h a v e m u c h effect.  T h e bicarbonate a n d c a r b o n a t e a n i o n s w e r e inert in this s y s t e m . T h i s is  not surprising a s positive i o n s , not negative ions, are likely to h a v e a n effect o n reactions b e t w e e n a n i o n s , i.e. trithionate a n d hydroxide in E q u a t i o n 6 . 1 .  Increasing the ionic strength using N a C I 0  4  did not h a v e a n y effect o n the rate of  trithionate d e g r a d a t i o n in the p r e s e n c e of a m m o n i a . H o w e v e r , addition of p o t a s s i u m or a m m o n i u m ions g e n e r a l l y i n c r e a s e d the rate. W h i l e no d a t a is a v a i l a b l e for trithionate, ion pairing of thiosulfate with c a t i o n s is k n o w n .  In fact, it h a s b e e n s h o w n that in the  p r e s e n c e of s o d i u m , p o t a s s i u m or a m m o n i u m i o n s , the concentration of the c o m p l e x e d thiosulfate ion ( M S 0 ) is the s a m e a s or higher than that of the free thiosulfate ion _  2  3  ( S e n a n a y a k e , 2 0 0 5 ) . T h i s r e s e a r c h e r f o u n d that there w a s a linear relationship b e t w e e n the logarithms of the a s s o c i a t i o n c o n s t a n t s for the s p e c i e s M S 0 2  _ 3  and M S 0  _ 4  for the  c a t i o n s ( M ) N a , K a n d H . B a s e d o n this relationship a n d the a s s o c i a t i o n c o n s t a n t of +  +  +  +  N H S 0 , the a s s o c i a t i o n constant of N H S 0 _  4  4  4  similar to that for K S 0 \ 2  3  2  _ 3  w a s c a l c u l a t e d a n d w a s f o u n d to be  T h e similarity b e t w e e n the b e h a v i o u r of p o t a s s i u m ions a n d  a m m o n i u m ions is likely d u e to their similar ionic radii.  104  It has been shown that in the sulfur exchange reaction between thiosulfate and trithionate, an increase in charge and concentration of positive ions increases the reaction rate (Fava and Pajaro, 1954). The authors therefore postulated this reaction to be between ionic complexes of thiosulfate, trithionate or both, rather than between free ions. Since the trithionate degradation rate varied with the concentration of positive ions, it is reasonable to believe that cations influence the reaction, possibly by forming ionic complexes with trithionate. An increase in sodium ion concentration did not give an increase in the trithionate degradation rate. However, since the trithionate salt was in the sodium form, sufficient sodium was always present to ensure complex ions with sodium, so one would only expect to notice an effect when a competing ion (potassium or ammonium) was introduced. It was interesting that the trithionate degradation rate continued to increase with the ammonium or potassium ion concentrations even when the cation concentration was much higher than the trithionate concentration, and did not reach a maximum over the range of cation concentrations tested. The presence of potassium ions in acidic solutions (the natural pH obtained for a solution of sodium trithionate in water) did not increase the trithionate degradation rate, but the reaction stoichiometry was not necessarily the same as in alkaline systems. With no ammonium present, an increase in ammonia concentration increased the rate of trithionate degradation. While it could not be proven that the reaction in Equation 6.2 forming sulfamate proceeded, it is possible that this reaction occurred to a small extent, or that the interaction between ammonia and trithionate facilitated the reaction between hydroxide ions and trithionate by altering the electronic properties of the trithionate. S 0 " + 2 NH 2  3  6  S 0 - + NH S0 " + N H 2  3  2  3  2  3  [6.2]  + 4  Based on the above discussion, the overall rate of degradation of trithionate is influenced by water, hydroxide and ammonia, and the reaction is enhanced by changing the electronic properties of a complex ion of trithionate by changing the cation type or concentration. It was assumed that the water concentration was constant in this case and that no thiosulfate was originally present. In an ammoniacal medium, which is of most interest to this study, the relative concentrations of ammonia, hydroxide ions and  105  a m m o n i u m ions are s h o w n d i a g r a m m a t i c a l l y in F i g u r e 6.1 with varying p H .  S i n c e the  d e g r a d a t i o n rate i n c r e a s e s with the concentration of e a c h of t h e s e s p e c i e s (but to different extents), it is f e a s i b l e b a s e d o n the d i a g r a m in F i g u r e 6.1 that the overall rate w o u l d v a r y with p H in a n a m m o n i a c a l s y s t e m , a n d there c o u l d b e a p H of m i n i m u m degradation  rate.  The way  in w h i c h t h e s e qualitative  effects  c a n be  combined  quantitatively is d i s c u s s e d in S e c t i o n 6.2.  4  7  PH  10  13  F i g u r e 6.1 : C o n c e n t r a t i o n profiles for a m m o n i a , a m m o n i u m ions a n d hydroxide ions at 4 0 °C with varying p H , using arbitrary concentration units  It w a s s h o w n that the effect of thiosulfate on the trithionate d e g r a d a t i o n rate w a s d e p e n d e n t o n the a m m o n i u m c o n c e n t r a t i o n . higher than the thiosulfate  W h e r e the a m m o n i u m concentration w a s  c o n c e n t r a t i o n , the trithionate d e g r a d a t i o n rate  i n c r e a s e d with a n i n c r e a s e in thiosulfate c o n c e n t r a t i o n .  generally  H o w e v e r , a s the thiosulfate  c o n c e n t r a t i o n e x c e e d e d the a m m o n i u m c o n c e n t r a t i o n , the trithionate d e g r a d a t i o n rate w a s unaffected or e v e n d e c r e a s e d with a further i n c r e a s e in thiosulfate.  T h i o s u l f a t e is  k n o w n to form c o m p l e x ions, a n d it h a s b e e n postulated that the ion N H S 0 " e x i s t s 4  (Senananyake, 2005).  2  3  O t h e r reactions h a v e b e e n p r o p o s e d to o c c u r via thiosulfate  c o m p l e x ions ( F a v a a n d P a j a r o , 1 9 5 4 , C h a n d r a a n d Jeffrey, 2 0 0 4 ) . A t high a m m o n i u m c o n c e n t r a t i o n s , the concentration of the a m m o n i u m c o m p l e x ion is e x p e c t e d to b e higher, facilitating the interaction b e t w e e n thiosulfate a n d trithionate.  H o w e v e r , a s the  thiosulfate c o n c e n t r a t i o n b e c o m e s c o m p a r a b l e with the a m m o n i u m ion c o n c e n t r a t i o n ,  106  the possibility of free thiosulfate ions (or the s o d i u m c o m p l e x , s i n c e thiosulfate w a s a d d e d a s s o d i u m thiosulfate) existing i n c r e a s e s . If free or s o d i u m thiosulfate ions d o not readily interact with trithionate, o n e w o u l d e x p e c t the trithionate d e g r a d a t i o n rate to p l a t e a u with a n y further i n c r e a s e in thiosulfate c o n c e n t r a t i o n .  T h e fact that a further  i n c r e a s e in thiosulfate concentration actually d e c r e a s e s the trithionate d e g r a d a t i o n rate implies a h i n d r a n c e of the reaction. If thiosulfate interacts v i a the sulfenyl sulfur a t o m of trithionate, w h e r e it a c t s to alter the electronic properties of trithionate to facilitate its reaction with h y d r o x i d e a n d a m m o n i a , then it is p o s s i b l e that too m u c h thiosulfate c o u l d sterically hinder t h e s e reactions.  T h e effect of thiosulfate o n the trithionate  d e g r a d a t i o n rate w a s not c o m p l e t e l y in  a g r e e m e n t with that f o u n d by Naito et al (1975). T h e r a n g e of thiosulfate c o n c e n t r a t i o n s t es t ed w a s b a s e d o n t h o s e to b e e x p e c t e d during gold l e a c h i n g , a n d w e r e m u c h higher than t h o s e u s e d by Naito et a l . (typically 0.02 M).  T h e trithionate d e g r a d a t i o n rate  d e c r e a s e d at higher levels of thiosulfate, rather than i n c r e a s i n g a s e x p e c t e d from Naito et al.'s rate e q u a t i o n .  Naito et a l . (1975) e x p l a i n e d the catalytic effect they o b s e r v e d  w h e n thiosulfate w a s p r e s e n t by p r o p o s i n g that thiosulfate f o r m e d a c o m p l e x with trithionate a n d the c o m p l e x r e a c t e d with w ater m o r e rapidly t h a n trithionate a l o n e .  In the p r e s e n c e of a m m o n i a , c u p r i c c o p p e r did not affect the reaction rate. In the c a s e s t e s t e d , the a m m o n i a w a s p r e s e n t in a molar ratio of m o r e than 4:1 to c u p r i c c o p p e r , s o it is e x p e c t e d that a significant a m o u n t tetraammine complex.  of the  cupric copper w a s  present a s  the  T h e p r e s e n c e of a m m o n i a is k n o w n to stabilise c u p r i c c o p p e r  a n d h e n c e inhibit the reaction b e t w e e n c u p r i c a n d thiosulfate. It h a s b e e n p r o p o s e d that thiosulfate joins the inner co-ordination s p h e r e of c u p r i c t e t r a a m m i n e for the reaction to p r o c e e d ( B y e r l e y et a l , 1 9 7 3 a , B r e u e r a n d Jeffrey, 2 0 0 0 , 2 0 0 3 b ) . A similar m e c h a n i s m m a y b e relevant for the interaction b e t w e e n trithionate a n d c o p p e r , but d u e to steric h i n d r a n c e s , co-ordination of trithionate to cupric t e t r a a m m i n e m a y not be a s f a v o u r a b l e (trithionate is larger than thiosulfate).  W h e r e c o p p e r is not primarily p r e s e n t a s the  c u p r i c t e t r a a m m i n e c o m p l e x , there m a y be a n influence o n the trithionate d e g r a d a t i o n rate.  107  MODELLING OF TRITHIONATE DEGRADATION  6.2  T w o t y p e s of e x p e r i m e n t a l m e t h o d w e r e u s e d to investigate the reaction kinetics of the d e g r a d a t i o n of trithionate: the integrated rate m e t h o d a n d the initial rate m e t h o d . T h e integrated rate m e t h o d g a v e useful information about the s y s t e m a n d s h o w e d trends c o n s i s t e n t with t h o s e f o u n d using the initial rate m e t h o d .  T h e a n a l y s i s w h i c h follows is  b a s e d primarily o n results d e r i v e d from the initial rate m e t h o d , a s in t h e s e tests the e x p e r i m e n t a l v a r i a b l e s could b e better controlled o v e r the short duration of e a c h test.  T h e effects noted in the testwork o n reaction kinetics w e r e c o m b i n e d to derive a m a t h e m a t i c a l m o d e l to e x p r e s s the trithionate d e g r a d a t i o n rate a s a function of the solution conditions.  It w a s f o u n d that the rate of trithionate d e g r a d a t i o n w a s first order with r e s p e c t to the trithionate c o n c e n t r a t i o n .  T h e o b s e r v e d rate could thus b e e x p r e s s e d by E q u a t i o n 6 . 3 .  T h e o b s e r v e d rate c o n s t a n t , k b > w a s f o u n d to d e p e n d o n other solution conditions a n d 0  S  the structure of this d e p e n d e n c y w a s d e t e r m i n e d by testing o n e solution variable at a time.  -d[S 0 -]/dt = k 6  [6.3]  [S 0 -]  2  3  2  o b s  3  6  F o r a solution of s o d i u m trithionate in w a t e r at 4 0 ° C , the o b s e r v e d rate constant w a s f o u n d to h a v e the m a g n i t u d e 0 . 0 1 2 h  _1  ( s e e S e c t i o n 5.4). T h e o b s e r v e d rate constant  kobs in E q u a t i o n 6.3 c a n thus b e e x p r e s s e d a s E q u a t i o n 6.4 u n d e r t h e s e conditions.  kobs = k  0  = 0.012  h  -1  [6.4]  T h e p r e s e n c e of hydroxide at p H - 1 1 a n d l e s s (at 4 0 °C) i n c r e a s e d the trithionate d e g r a d a t i o n rate.  In this c a s e , the d e p e n d e n c y of the o b s e r v e d rate constant o n the  h y d r o x i d e concentration c o u l d b e e x p r e s s e d by E q u a t i o n 6 . 5 ( s e e S e c t i o n 5.5).  k  -1  = M O K ] + k = 0.74 M" .h- [OH-] + 0.012 h 1  o b s  1  0  108  [6.5]  T h e p r e s e n c e of a m m o n i a a n d a m m o n i u m , w h i c h a r e of i m p o r t a n c e in the a m m o n i a c a l g o l d l e a c h i n g s y s t e m , w e r e f o u n d to influence the trithionate d e g r a d a t i o n rate.  Addition  of a m m o n i a to the s y s t e m a l s o i n c r e a s e d the p H . In deriving the d e p e n d e n c y of the o b s e r v e d rate c o n s t a n t o n the a m m o n i a c o n c e n t r a t i o n , it w a s thus n e c e s s a r y to correct for the d e p e n d e n c y o n the hydroxide c o n c e n t r a t i o n d e t e r m i n e d earlier.  In a plot of the  c o r r e c t e d rate c o n s t a n t a g a i n s t the a m m o n i a c o n c e n t r a t i o n , fixing the intercept a s the b a s e l i n e d e g r a d a t i o n rate in water, the o b s e r v e d rate c o n s t a n t c o u l d b e e x p r e s s e d by E q u a t i o n 6.6. ( S e e S e c t i o n 5.8)  kobs =  k [ N H ] + M O H ] + k = 0.0081 M- .h- [NH ] + 0 . 7 4 M ' . h - [ O H ] + 0 . 0 1 2 rf 1  2  3  1  1  1  3  0  1  [6.6]  T h e equivalent d e p e n d e n c y o n a m m o n i a c o n c e n t r a t i o n d e r i v e d b y Naito et a l . e x p r e s s e d in a similar w a y a s E q u a t i o n 6.6 s h o w e d a d e p e n d e n c y a s in E q u a t i o n 6 . 7 .  kobs  = 0.0031 M- .h- [NH ] + 0 . 0 1 2 h,-1 1  [6.7]  1  3  T h e s e a u t h o r s did not state the p H of their work nor did they investigate a n y effects of h y d r o x i d e c o n c e n t r a t i o n or p H o n the d e g r a d a t i o n rate. T h e i r a m m o n i a d e p e n d e n c y w a s m u c h l e s s than that o b t a i n e d in this work.  H o w e v e r , the v a l u e of the gradient in the plot  of the a d j u s t e d o b s e r v e d rate c o n s t a n t v e r s u s the a m m o n i a c o n c e n t r a t i o n w a s very s e n s i t i v e to the intercept v a l u e s e l e c t e d , a n d within the error r a n g e of e a c h d a t a point, a n u m b e r of trend lines c o u l d b e fit to the d a t a .  T h e a m m o n i a d e p e n d e n c y w a s of the  s a m e o r d e r of m a g n i t u d e .  T h e influence of the a m m o n i u m ion w a s to i n c r e a s e the rate of trithionate d e g r a d a t i o n . T h e p o t a s s i u m ion g a v e a very similar effect a n d the two ions a r e of a similar s i z e , s o it w a s p r o p o s e d that the a m m o n i u m ion's interaction w a s b a s e d o n its a s s o c i a t i o n with the trithionate ion ( s e e S e c t i o n 6.1). A t neutral p H in the a b s e n c e of a m m o n i a , the o b s e r v e d rate c o n s t a n t c o u l d b e e x p r e s s e d b y E q u a t i o n 6 . 8 .  In this c a s e , n o correction for  h y d r o x i d e c o n c e n t r a t i o n w a s n e c e s s a r y a s the p H w a s neutral.  kobs = k [ N H 3  + 4  ] + k = 0 . 0 1 M - . h - [ N H ] + 0 . 0 1 2 h,-1 1  0  1  +  4  109  [6.8]  O t h e r solution c o m p o n e n t s like b i c a r b o n a t e , c a r b o n a t e , sulfate a n d o x y g e n w e r e s h o w n to h a v e a negligible effect o n the trithionate d e g r a d a t i o n rate.  The  interaction b e t w e e n trithionate a n d thiosulfate w a s m o r e c o m p l e x , d e p e n d i n g o n  both the thiosulfate concentration a n d the a m m o n i u m c o n c e n t r a t i o n .  F o r the p u r p o s e s  of modelling the trithionate kinetics, the conditions e x p e c t e d to m o s t c l o s e l y m a t c h typical gold l e a c h i n g conditions w e r e c o n s i d e r e d .  T y p i c a l thiosulfate  concentrations  u s e d in gold l e a c h i n g are a r o u n d 0.2 M , a n d this concentration d e c r e a s e s with time.  At  the high p H s u s e d in gold l e a c h i n g , most of the total a m m o n i a present (typically 0.2 0.4 M) is e x p e c t e d to be present a s a m m o n i a , not a m m o n i u m i o n s , s o typically the a m m o n i u m ion concentration is e x p e c t e d to b e l e s s than 0.2 M . U n d e r t h e s e conditions, the effect of thiosulfate o n the o b s e r v e d rate constant for trithionate d e g r a d a t i o n w a s very s m a l l , a n d for the p u r p o s e s of modelling trithionate kinetics, the effect of thiosulfate w a s c o n s i d e r e d negligible.  No  interaction of trithionate with c o p p e r w a s a c c o u n t e d for, a s in the p r e s e n c e of  sufficient a m m o n i a , c u p r i c c o p p e r w a s f o u n d to h a v e no significant effect on  the  trithionate d e g r a d a t i o n rate.  The  d e g r a d a t i o n of trithionate w a s e x p r e s s e d a s the s u m of v a r i o u s  interactions,  o c c u r r i n g in parallel. U s i n g the data at 4 0 °C the v a r i o u s interactions w e r e c o m b i n e d a s in E q u a t i o n 6.9, using E q u a t i o n s 6.5, 6.6 a n d 6.8.  kobs = k [ N H ] + k [ N H ] + k,[OH] + k  [6.9]  +  3  4  2  3  0  = 0 . 0 1 M " . h " [ N H ] + 0.0081 M ' . h [ N H ] + 0.74 M- .h- [OrT] + 0.012 h" 1  1  +  1  4  1  1  1  1  3  T h e s a m e a p p r o a c h of parallel interactions w a s u s e d by Naito et a l . (1975) but they only e x a m i n e d the effects of water, a m m o n i a a n d thiosulfate, a n d at c o n c e n t r a t i o n s that w e r e not specifically relevant to gold l e a c h i n g .  T h e d a t a for the m e a s u r e d o b s e r v e d rate c o n s t a n t s for the initial rate tests are s h o w n plotted a g a i n s t p H in F i g u r e s 6.2 to 6.4 for three representative r a n g e s in total a m m o n i a plus a m m o n i u m c o n c e n t r a t i o n s . T h e o b s e r v e d rate constant a s c a l c u l a t e d by E q u a t i o n  110  6.9 is s h o w n s u p e r i m p o s e d for e a c h .  T o calculate  k  o b s  f r o m E q u a t i o n 6.9, the p H w a s  c o n v e r t e d to [ O H ] , a n d a l s o u s e d to c a l c u l a t e the ratio of a m m o n i a to a m m o n i u m , g i v e n the total a m m o n i a plus a m m o n i u m a n d using the p K v a l u e for a m m o n i a of 8.8 at 4 0 °C a  ( D e a n , 1992).  Qualitatively, the m o d e l l e d v a l u e s m a t c h the e x p e r i m e n t a l trends, s h o w i n g a local m i n i m u m in d e g r a d a t i o n rate at p H a r o u n d 10, but the e x p e r i m e n t a l d a t a s h o w s a lower m i n i m u m d e g r a d a t i o n rate at high a m m o n i a / a m m o n i u m c o n c e n t r a t i o n s than the m o d e l predicts (i.e. the m i n i m u m rate is lower).  F i g u r e 6.5 s h o w s the c a l c u l a t e d rate constant  plotted a g a i n s t the o b s e r v e d rate constant for all the d a t a o n a logarithmic s c a l e .  In  s o m e c a s e s the a m m o n i a concentration w a s not m e a s u r e d but e s t i m a t e d b a s e d o n the a m m o n i u m concentration a n d p H . T h i s is indicated in F i g u r e 6.5 w h e r e the correlation coefficient, R , is e q u a l to 0.6. 2  W h i l e the scatter is significant, the gradient is o n e ,  s h o w i n g a n overall qualitative m a t c h . T h e typical 10 % error m a r g i n s in the e x p e r i m e n t a l d a t a are thought to contribute to this scatter significantly.  0.04  Nhb + N H  4  concentration x 0M  ko =0.012 ki  = 0.74  k = 0.0081 2  ka  F i g u r e 6.2 : Plot of o b s e r v e d rate constant  knhg  = 0.01  for trithionate d e g r a d a t i o n at 4 0 °C for a n  a m m o n i a + a m m o n i u m concentration of 0 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k ? = 0.0081 M . h ) 1  111  1  0.04  N H + NH  4  concentration x 0.5 M  0.03  V 0.02  0.01  = 0.012  ki = 0.74 Ko  k = 0.0081  ka = 0.01 2  0.00  -1~  5  7  11  PH  13  F i g u r e 6.3 : Plot of o b s e r v e d rate constant k^. for trithionate d e g r a d a t i o n at 4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.5 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k ? = 0.0081 M ' . h ' ) 1  1  0.04  Ni-fe + 1NH4 concentration x 1M  0.03  0.02  0.01  0.00  = 0.012  ki = 0.74 ko  k = 0.0081  ka = 0.01 2  i  i  5  7  11  PH  F i g u r e 6.4 : Plot of o b s e r v e d rate constant  knhs  13  for trithionate d e g r a d a t i o n at 4 0 °C for a n  a m m o n i a + a m m o n i u m concentration of 0.9 - 1.2 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k ? = 0.0081 M ' . h ' ) 1  112  1  F i g u r e 6.5 : Plot of c a l c u l a t e d rate c o n s t a n t k i v e r s u s o b s e r v e d rate c o n s t a n t I w R a  r  trithionate d e g r a d a t i o n at 4 0 °C for all d a t a , u s i n g k? = 0.0081 M ' . h ' 1  for  1  If the d e p e n d e n c y of the o b s e r v e d rate constant o n a m m o n i a (k ) is c h a n g e d to 0 . 0 0 4 9 2  M" .h" 1  1  (as indicated by the best fit line through the a m m o n i a d e p e n d e n c y d a t a points,  F i g u r e 5.6, S e c t i o n 5.8), the fit of the m o d e l a p p e a r s better, a s is s e e n for different total a m m o n i a c o n c e n t r a t i o n s in F i g u r e s 6.6 to 6.12.  H o w e v e r , in the plot of the c a l c u l a t e d  v e r s u s the o b s e r v e d rate c o n s t a n t in F i g u r e 6 . 1 3 , the correlation coefficient R 0.6.  A s d i s c u s s e d earlier, the  s l o p e of the  line  in F i g u r e 5.6 for the  2  is a g a i n ammonia  d e p e n d e n c y w a s very s e n s i t i v e to the c h o s e n intercept a n d g i v e n the error m a r g i n s for e a c h d a t a point, there w e r e lines of m a n y s l o p e s that w e r e c o n s i s t e n t with the d a t a .  If  w e refer b a c k to E q u a t i o n 5.11 w h e r e the d e p e n d e n c y of the rate c o n s t a n t o n both the a m m o n i a a n d a m m o n i u m c o n c e n t r a t i o n s w a s m e a s u r e d , a v a l u e for k of 0 . 0 0 4 9 M" .h" 1  2  w a s m o r e c o n s i s t e n t with the e x p e r i m e n t a l d a t a .  113  1  0.04  NFfe + NH.4 concentration xOM  Ko = ki  0.012  = 0.74  k = 0.0049 2  ks =  0.01  F i g u r e 6.6 : Plot of o b s e r v e d rate constant k ^ for trithionate d e g r a d a t i o n at 4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k? = 0 . 0 0 4 9 M ' . h ' ) 1  0.04  1  Nhb + N H concentration 4  x 0.15 M  0.03  0.02  0.01  ko  = 0.012  ki  = 0.74  k = 0.0049 2  0.00  ka  = 0.01  F i g u r e 6.7 : Plot of o b s e r v e d rate constant k ^ for trithionate d e g r a d a t i o n at 4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.1 - 0.2 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k? = 0 . 0 0 4 9 M ' . h ' ) 1  114  1  0.04  Nhfe + N H  4  concentratbn  x 0.35 M  0.03  V 0.02  0.01  ko =0.012 ki  = 0.74  k = 0.0049 2  0.00 7  9  11  ks  13  = 0.01  PH F i g u r e 6.8 : Plot of o b s e r v e d rate constant k^  for trithionate d e g r a d a t i o n at 4 0 °C for a n  a m m o n i a + a m m o n i u m concentration of 0.3 - 0.4 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k ? = 0 . 0 0 4 9 M" .h" ) 1  0.04  1  Nhb + N H concentratbn 4  x 0.5 M  0.03  V 0.02 .a  X  j  0.01  ko  = 0.012  ki  = 0.74  k = 0.0049 2  0.00 7  9  11  ka  13  = 0.01  PH  F i g u r e 6.9 : Plot of o b s e r v e d rate constant  knhg  for trithionate d e g r a d a t i o n at 4 0 °C for a n  a m m o n i a + a m m o n i u m concentration of 0.5 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k ? = 0 . 0 0 4 9 M" .h" ) 1  115  1  0.04  NH3 + N H  4  concentration x 0.75 M  Ko = 0.012 ki = 0.74  k = 0.0049 2  0.00 11  13  kg = 0.01  PH  F i g u r e 6.10 : Plot of o b s e r v e d rate constant  knhg  for trithionate d e g r a d a t i o n at 4 0 °C for  a n a m m o n i a + a m m o n i u m concentration of 0.7 - 0.8 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k = 0 . 0 0 4 9 M . h ) 1  1  2  0.04  NH3  +  N H 4  concentration x 1M  0.03  T  0.02  0.01  ko = 0.012 ki = 0.74  k = 0.0049 2  0.00  KB = 0.01  F i g u r e 6.11 : Plot of o b s e r v e d rate constant k ^ for trithionate d e g r a d a t i o n at 4 0 °C for a n a m m o n i a + a m m o n i u m concentration of 0.9 - 1.2 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k ? = 0 . 0 0 4 9 M ' . h ' ) 1  116  1  0.04  NHs + NH  4  concentration  x2.1 M  0.03  V  0.02  0.01  ko  = 0.012  ki  = 0.74  k = 0.0049 2  0.00  kg  F i g u r e 6.12 : Plot of o b s e r v e d rate constant  knhg  = 0.01  for trithionate d e g r a d a t i o n at 4 0 °C for  a n a m m o n i a + a m m o n i u m concentration of 1.8 - 2.6 M a g a i n s t p H , with m o d e l l e d trend s u p e r i m p o s e d (using k ? = 0 . 0 0 4 9 M" .h" ) 1  117  1  T o s u m m a r i z e , b a s e d o n the e x p e r i m e n t a l work, the d e g r a d a t i o n of trithionate  to  thiosulfate a n d sulfate at 40 °C c o u l d be a d e q u a t e l y m o d e l l e d using the rate equation a n d p a r a m e t e r s e x p r e s s e d in E q u a t i o n 6.10. T h e s l o w reaction of trithionate with water, r e p r e s e n t e d by k , will p r e d o m i n a t e in typical gold l e a c h i n g solutions. T h i s rate equation 0  w a s u s e d in c o m b i n a t i o n with rate e q u a t i o n s for other sulfur s p e c i e s to m o d e l the overall sulfur s p e c i a t i o n e x p e c t e d during thiosulfate d e g r a d a t i o n during gold l e a c h i n g . T h i s is d i s c u s s e d in C h a p t e r 7, w h e r e it will be s h o w n that although the m o d e l for trithionate d e g r a d a t i o n is prone to a fair a m o u n t of variability, this d o e s not affect the overall s y s t e m s p e c i a t i o n to a n y significant d e g r e e .  - d [ S 0 - ] / d t = ( k [ N H ] + k [ N H ] + k,[OH] + k ) [ S O ] 2  3  6  w h e r e at 40 °C  +  3  2  4  2  3  0  k = 0.012 h'  1  0  ^ = 0.74 iVr .h' 1  1  k = 0.0049 IVT.IY  1  2  k = 0.01 ivr .h1  1  3  118  3  6  [6.10]  7  MODELLING OF SULFUR OXYANION SPECIATION DURING THIOSULFATE DEGRADATION  7.1  INTRODUCTION  T h e d e g r a d a t i o n of thiosulfate in gold l e a c h i n g s y s t e m s , w h i c h a l s o involve a m m o n i a c a l a n d c o p p e r c o m p o n e n t s , is c o m p l e x a n d not fully u n d e r s t o o d . It h a s b e e n p r o p o s e d that thiosulfate c a n d e g r a d e directly to tetrathionate, trithionate or sulfate, but tetrathionate a n d trithionate t h e m s e l v e s u n d e r g o further d e g r a d a t i o n .  N o p u b l i s h e d m o d e l for this  s y s t e m h a s b e e n found in the public d o m a i n . T h i s c h a p t e r s h o w s h o w a s i m p l e m o d e l w a s set up for thiosulfate d e g r a d a t i o n in the a b s e n c e of o r e s a n d s h o w s the m o d e l sensitivity to both the m o d e l p a r a m e t e r s a n d e x p e r i m e n t a l conditions.  T h e model w a s  tested a g a i n s t a set of e x p e r i m e n t a l d a t a in the a b s e n c e of ore a n d c o m p a r e d with the e x p e r i m e n t a l b e h a v i o u r in the p r e s e n c e of a n ore.  B a s e d o n this e v a l u a t i o n , the  s h o r t c o m i n g s a n d s c o p e of u s e of the m o d e l w e r e identified. show  some  expected  effects  of  changing  the  solution  T h e m o d e l w a s u s e d to conditions  on  thiosulfate  degradation.  7.2  MODEL SETUP  F i g u r e 7.1 s h o w s a simplified s c h e m a t i c of s o m e of the p o s s i b l e reaction p a t h w a y s for thiosulfate in a n a m m o n i a c a l , c o p p e r - c o n t a i n i n g solution.  T h i s s c h e m a t i c w a s u s e d to  set up a b a s i c kinetic m o d e l of the s y s t e m , using rate e q u a t i o n s derived from literature or from e x p e r i m e n t a l work in this t h e s i s (for trithionate). this  s c h e m a t i c is not  exhaustive  but  the  reactions  the  It s h o u l d b e noted that  represented  are expected  to  c h a r a c t e r i s e the s y s t e m sufficiently well to b e a b l e to u s e the m o d e l to identify the factors playing the m o s t significant role in thiosulfate d e g r a d a t i o n a n d sulfur o x y a n i o n speciation.  In the s c h e m a t i c the reactions are labelled R1 to R 6 , with fractions a , b, c a n d d representing the fraction of thiosulfate at a n y time reacting through e a c h of the p a t h w a y s shown.  It is r e c o g n i s e d that the v a l u e s for a , b, c a n d d c o u l d c h a n g e with time, but this  w a s not incorporated into the m o d e l .  119  S 0 2  2 3  - + Cu  R1  -> / S 0  2 +  1  2  2  4  6  - + Cu  +  R2  t  S 0 - + / OH2  4  •a  3  6  5  2  s o -  / S 0 - + / S 0 2  4  2  2  3  2 6  " + / 3  4  H 0 2  R3  2  2  1  3  3  S O - + 2 O H " -> S 0 2  3  e  2  2 3  " + S0  2 4  " + H 0 2  R4  \ \ S 0 " + V H 0 + / 0 2  2  3  2  3  2  3  -» / S 0 2  2  3  3  2 6  " + / OH" 2  3  d \ V S 0 " + 2 0 + 2 O H " -> R6 2 S0 " + H 0 S 0 " + 2 O H - + / C u -> / S 0 " + H 0 + / C u S 2  2  3  2  2  4  2  2  2  3  2 +  2  4  3  2  3  2  3  2  3  F i g u r e 7.1 : S c h e m a t i c s h o w i n g the b a s i s for modelling  T h e w a y in w h i c h e a c h of reactions R1 to R 6 w e r e h a n d l e d in setting up the m o d e l is d i s c u s s e d in turn below.  7.2.1  R1 - Thiosulfate Degradation to Tetrathionate  s o - + Cu 2  2  3  2 +  -»  so '+ 2  y  4  2  6  cu  +  [7.1]  T h e rate e q u a t i o n for the reaction b e t w e e n thiosulfate a n d c o p p e r w a s t a k e n f r o m work by B y e r l e y et al (1973a).  T h e rate of d e c r e a s e of c u p r i c c o p p e r concentration by  reaction with thiosulfate in the a b s e n c e of o x y g e n w a s g i v e n by E q u a t i o n 7.2.  -d[Cu ]/dt = k 2+  where k  = 8.5 x 10" s" at 3 0 °C. 4  R1  1  [Cu ][S 0 -]/[NH ] 2+  R1  2  2  3  3  [7.2]  U s i n g the A r r h e n i u s e q u a t i o n a n d the activation  e n e r g y of 102.5 k J / m o l , the rate constant at 2 5 °C w a s extrapolated to be 4.2 x 10" s ' . 4  1  B a s e d o n the reaction stoichiometry for R e a c t i o n R 1 , the rate of thiosulfate d e g r a d a t i o n at 2 5 °C could b e e x p r e s s e d by E q u a t i o n 7.3.  120  - d [ S 0 l / d t = 4 . 2 x 10" [ C u ] [ S 0 " ] / [ N H ] M . s ' 2  2  4  2+  2  3  2  3  [7.3]  1  3  T h e rate c o n s t a n t w a s found to i n c r e a s e up to 4 0 t i m e s in the p r e s e n c e of o x y g e n , but it is a l s o k n o w n that other reactions c a n o c c u r in the p r e s e n c e of o x y g e n (e.g. R 4 a n d R 5 ) .  7.2.2  R2 - Tetrathionate Degradation  S 0 - + % OH" 2  4  5  6  / S 0 - + V2 S 0 2  2  4  3  3  2 6  " + / H 0  [7.4]  3  4  2  In the a b s e n c e of a m m o n i a , c o p p e r a n d o x y g e n , the rate of the alkaline d e c o m p o s i t i o n of tetrathionate w a s d e s c r i b e d by Z h a n g a n d D r e i s i n g e r (2002) in E q u a t i o n 7 . 5 .  -d[S 0 -]/dt = k [ S 0 ] [ O H ] 2  4  [7.5]  2  6  4  6  w h e r e k = 1.38 x 1 0 M" .h" at 2 2 ° C a n d the activation e n e r g y w a s 9 8 . 5 k J / m o l . H e n c e 3  1  at 2 5 ° C , k = 5 . 7 4 x 1 0 -  1  IvrVs .  1  -1  T h e effect of c o p p e r w a s not c o n s i d e r e d by Z h a n g a n d D r e i s i n g e r in the derivation of this rate equation a n d c o p p e r h a s b e e n s u g g e s t e d to react with tetrathionate at a significant rate ( B r e u e r a n d Jeffrey, 2 0 0 3 b ) .  A l s o , the p o s s i b l e catalytic effect of  thiosulfate o n the d e g r a d a t i o n w a s not c o n s i d e r e d .  W h i l e Naito et a l . (1970b) e x a m i n e d the reactions of tetrathionate s y s t e m s , n o rate equation w a s d e r i v e d .  in a m m o n i a c a l  T h e effect of thiosulfate w a s a d d r e s s e d in a  study by R o l i a a n d C h a k r a b a r t i (1982) w h o derived a rate equation for tetrathionate d e g r a d a t i o n in the a b s e n c e of a m m o n i a a n d c o p p e r at p H 1 1 . T h e rate equation in the p r e s e n c e of thiosulfate at 2 5 °C w a s e x p r e s s e d a s E q u a t i o n 7.6.  -d[S 0 "]/dt = (k . + k . [ S 0 l ) [ S 0 l [ O H ] 2  4  2  6  R2  w h e r e k . = 0 . 0 2 2 M" .s" a n d 1  R2  0  1  k  R 2  0  .i  R 2  1  2  2  3  4  6  [7.6]  = 2.77 M" .s" . T h e rate c o n s t a n t s w e r e d e t e r m i n e d 2  1  u s i n g the p u b l i s h e d rate c o n s t a n t s at 3 5 °C a n d the activation e n e r g y in the a b s e n c e of thiosulfate (115.5 kJ/mol).  121  In the a b s e n c e of thiosulfate, the rate e q u a t i o n in E q u a t i o n 7.6 is r e d u c e d to E q u a t i o n 7.5, but it s h o u l d b e noted that the rate c o n s t a n t k -o is m o r e than 10 t i m e s s m a l l e r than R2  that f o u n d by Z h a n g a n d D r e i s i n g e r .  It w a s p r o p o s e d by Z h a n g a n d D r e i s i n g e r that the  p r e s e n c e of o x y g e n in the work of R o l i a a n d C h a k r a b a r t i a n d the a b s e n c e of o x y g e n in their o w n work w a s p r o b a b l y the r e a s o n for this d i s c r e p a n c y , a n d that o x i d a n t s c o u l d p o s s i b l y retard the rate of tetrathionate d e g r a d a t i o n .  B r e u e r a n d Jeffrey (2004) f o u n d  that the ionic strength h a d a n effect o n tetrathionate d e g r a d a t i o n a n d p r o p o s e d that the difference  between  the  rate  constants  of  Zhang  and  Dreisinger  and  Rolia  and  C h a k r a b a r t i w a s b e c a u s e the solution ionic strengths w e r e different.  S i n c e the m o d e l l e d s y s t e m w a s for thiosulfate d e g r a d a t i o n in the p r e s e n c e of o x y g e n a n d thiosulfate, the rate e q u a t i o n p r o p o s e d by R o l i a a n d C h a k r a b a r t i ( E q u a t i o n 7.6) w a s u s e d in the m o d e l .  7.2.3  R3 - Trithionate Degradation S 0 3  2 6  " + 2 O H " -> S 0 2  2 3  " + S0  2 4  [7.7]  " + H 0 2  W h i l e limited literature d a t a is a v a i l a b l e for the d e g r a d a t i o n of trithionate in a m m o n i a c a l s o l u t i o n s , the m e t h o d o l o g y a n d conditions u s e d to derive the kinetics w e r e not entirely s u i t a b l e for g o l d l e a c h i n g conditions.  T h e rate e q u a t i o n d e r i v e d in this t h e s i s ( s e e  C h a p t e r 6) w a s u s e d in modelling the s y s t e m , a s s h o w n in E q u a t i o n 7.8. T h e limitations of this rate e q u a t i o n w e r e d i s c u s s e d in C h a p t e r 6. T h i s rate e q u a t i o n is m o s t suitable for p H v a l u e s w h e r e the a m m o n i u m to thiosulfate ratio is s m a l l , a s d i s c u s s e d in C h a p t e r s 5 a n d 6.  It s h o u l d b e noted that the rate e q u a t i o n in E q u a t i o n 7.8 i n c l u d e s the h y d r o l y s i s  of trithionate, r e p r e s e n t e d by k . 0  -d[S 0 3  2 6  " ]/dt = (k + HOH]  + k [NH ] + k [NH ])[S 0 - ] +  0  2  w h e r e at 4 0 °C k = 3.3x10-  6  s  k, = 2.1 x l O "  4  M' .s  0  k = 2  1  1  .-1 .-1  1.3 x 10" IvT.s 6  ,-1  k = 2 . 7 x 1 0 " IVTVS 6  3  122  3  3  4  2  3  6  [7.8]  U s i n g the activation e n e r g y of 7 5 . 3 k J / m o l , the p a r a m e t e r s at 2 5 °C w e r e c a l c u l a t e d to be  k = 1.4 x 1 0 " s " 8  1  0  ki = 5 . 8 x 1 0 - M . s "  1  k = 3.1 x 1 0 " M - . s  1  k = 6.4x10- M- .s  1  5  1  7  1  2  7  1  3  7.2.4  R4 - Thiosulfate Degradation to Trithionate S 0 2  2 3  " + / H 0 + / 0 1  2  3  2  3  -» / S 0 2  2  3  3  2 6  " + / OH"  [7.9]  2  3  T h e d e g r a d a t i o n of thiosulfate directly to trithionate in the p r e s e n c e of c o p p e r a m m i n e s a n d o x y g e n a s p r o p o s e d by B y e r l e y et a l . (1975) w a s d i s c u s s e d in the literature review in C h a p t e r 2.  T h e reaction kinetics of thiosulfate d e g r a d a t i o n w e r e m e a s u r e d by  m e a s u r i n g the o x y g e n c o n s u m p t i o n .  It w a s f o u n d that u n d e r certain c o n d i t i o n s , the  initial rate of o x y g e n c o n s u m p t i o n g e n e r a l l y c o r r e s p o n d e d to the formation of trithionate only, while after longer times w h e n the m a x i m u m o x y g e n c o n s u m p t i o n w a s r e a c h e d , both trithionate a n d sulfate w e r e f o r m e d . B y e r l e y et a l . did\not d e d u c e a c o m p r e h e n s i v e rate e q u a t i o n using their d a t a , a s the s y s t e m a p p e a r e d quite c o m p l e x . H o w e v e r , for the p u r p o s e s of this m o d e l , the kinetic d a t a for the initial rate of o x y g e n c o n s u m p t i o n m e a s u r e d by B y e r l e y et a l . w a s u s e d to derive a n a p p r o x i m a t e rate e q u a t i o n , a s follows.  A s noted by B y e r l e y et a l . , the rate of o x y g e n c o n s u m p t i o n w a s f o u n d to be proportional to the c o p p e r a n d o x y g e n c o n c e n t r a t i o n s . T h e d e p e n d e n c y o n thiosulfate a n d a m m o n i a concentrations  was  not  straightforward.  Assuming  that  the  rate  of  thiosulfate  d e g r a d a t i o n w a s e q u a l to the rate of o x y g e n c o n s u m p t i o n (as a s s u m e d by B y e r l e y et al.) the logarithm of this rate w a s plotted a g a i n s t the logarithm of a m m o n i a concentration in F i g u r e 7.2 a n d a g a i n s t the logarithm of the thiosulfate concentration in Figure 7.3.  123  -11.0 CD -t—»  ro  S  Q. E  -11.5  CO  c o o  CM  o  -12.0  -12.5 -2.5  -2.0  -1.5  -1.0  -0.5  0.0  In NH, F i g u r e 7.2 : L o g a r i t h m of the o x y g e n c o n s u m p t i o n rate v e r s u s the logarithm of a m m o n i a c o n c e n t r a t i o n for thiosulfate d e g r a d a t i o n to trithionate in the p r e s e n c e of o x y g e n - D a t a from B y e r l e y et al  (1975)  -10.0  c  -12.0  H  -4.0  1 -3.5  . -3.0  1 -2.5  1  -2.0  In S 0 2  2  3  F i g u r e 7.3 : L o g a r i t h m of the o x y g e n c o n s u m p t i o n rate v e r s u s the logarithm of thiosulfate c o n c e n t r a t i o n for thiosulfate d e g r a d a t i o n to trithionate in the p r e s e n c e of o x y g e n - D a t a from B v e r l e v et al (1975)  124  It w a s f o u n d that the rate of o x y g e n c o n s u m p t i o n h a d a reaction o r d e r of a p p r o x i m a t e l y -1  with  respect  to  thiosulfate  (for  concentrations  greater  than  0.025  M)  and  a p p r o x i m a t e l y z e r o with r e s p e c t to a m m o n i a (in the r a n g e of about 0.2 M to 0.6 M ammonia).  T h e rate w a s i n d e p e n d e n t of the a m m o n i a c o n c e n t r a t i o n for a m m o n i a  c o n c e n t r a t i o n s w h e r e the c u p r i c tetraammine s p e c i e s w a s e x p e c t e d to be at a m a x i m u m concentration relative to the c u p r i c c o n c e n t r a t i o n s u s e d in this work, a n d d e c r e a s e d at higher or lower a m m o n i a c o n c e n t r a t i o n s .  T h e r e f o r e B y e r l e y et a l . p r o p o s e d that the  c u p r i c t e t r a a m m i n e s p e c i e s is the active c u p r i c s p e c i e s in the formation of trithionate from thiosulfate.  B a s e d o n this preliminary evaluation of B y e r l e y et al's d a t a , the overall rate e q u a t i o n w a s e x p r e s s e d a s E q u a t i o n 7.10.  -d[S 0 "]/dt = k 2  2  T h e rate c o n s t a n t k  R4  3  [Cu ][0 ]/[S 0 -] 2+  R4  [7.10]  2  2  2  3  w a s e s t i m a t e d for e a c h set of d a t a , b a s e d o n the m e a s u r e d rate  a n d the test conditions, a n d the a v e r a g e rate constant w a s f o u n d to be 1.03 + 0.17 s" (at 1  p H 11.2, 3 0 °C at w h i c h m o s t of the d a t a w a s a v a i l a b l e ) . measured  but  given  the  limited  a c c u r a c y of  N o activation e n e r g y w a s  determining  the  rate  using  oxygen  c o n s u m p t i o n a n d of deriving the rate e q u a t i o n s h o w n in E q u a t i o n 7.10, the rate constant at 2 5 °C is e x p e c t e d to b e of the s a m e order of m a g n i t u d e a s that at 3 0 °C. H e n c e using the rate constant e s t i m a t e d at 3 0 °C w a s c o n s i d e r e d sufficiently a c c u r a t e within t h e s e limitations.  A c o n c e r n in using this rate e q u a t i o n is the fact that the o x y g e n c o n s u m p t i o n rate is not likely to b e a direct indicator of the thiosulfate d e g r a d a t i o n rate. A l s o , the rate e q u a t i o n a n d rate constant w e r e d e r i v e d u n d e r very s p e c i f i c conditions w h i c h m a y not a l w a y s b e a p p l i c a b l e to thiosulfate l e a c h i n g conditions.  7.2.5  R5 - Thiosulfate Degradation Directly to Sulfate  S 0 2  2 3  " + 2 0  2  + 2 O H 2 S0  125  2 4  " +H 0 2  [7.11]  It h a s b e e n f o u n d that sulfate c a n form directly f r o m thiosulfate in the p r e s e n c e of o x y g e n (Byerley et a l , 1975).  H o w e v e r , u n d e r conditions typical in gold l e a c h i n g , the  proportion of sulfate f o r m e d (via R e a c t i o n R 5 ) to trithionate f o r m e d is e x p e c t e d to b e very low, a s d o c u m e n t e d in the literature review in C h a p t e r 2.  S i n c e the extent of this  reaction is e x p e c t e d to be negligible c o m p a r e d with c o m p e t i n g reactions a n d no rate e q u a t i o n w a s a v a i l a b l e , R e a c t i o n R 5 w a s e x c l u d e d from the m o d e l .  7.2.6  R6 - Thiosulfate Degradation to Sulfide S 0 2  2 3  " + 2 OH" + / C u 2  2 +  3  % S0 " + H 0 + / CuS 2  3  2  2  3  [7.12]  W h i l e reaction R 6 s h o w i n g the formation of c o p p e r sulfide from the reaction b e t w e e n c o p p e r a n d thiosulfate w a s i n c l u d e d in F i g u r e 7.1 for c o m p l e t e n e s s , kinetic d a t a o n this reaction w a s not a v a i l a b l e . A l s o during gold l e a c h i n g , b a s e d o n m a s s b a l a n c e s of the sulfur s p e c i e s , sulfide d o e s not a p p e a r to be f o r m e d in a n y significant quantities, if at all ( L a m , 2 0 0 2 ) . H e n c e this reaction w a s not included in the m o d e l .  7.2.7  Incorporation of the Rate Equations into a Model - Method and Constraints  T h e m o d e l w a s set up in a s i m p l e s p r e a d s h e e t format. T h e concentration profiles of the v a r i o u s sulfur o x y a n i o n s p e c i e s with time w e r e d e t e r m i n e d iteratively u s i n g time intervals of 0.05 hours o v e r 2 4 h o u r s .  T h e c h a n g e in concentration of e a c h of the s p e c i e s  thiosulfate, trithionate, tetrathionate a n d sulfate w a s d e t e r m i n e d u s i n g the rate e q u a t i o n s in E q u a t i o n s 7.3, 7.6, 7.8 a n d 7.10 a n d the reaction stoichiometries of E q u a t i o n s 7 . 1 , 7.4, 7.7 a n d 7.9.  T h e s e e q u a t i o n s a c c o u n t e d for all the sulfur s p e c i e s in the m o d e l s o  the total sulfur p r e s e n t r e m a i n e d constant.  The  c o n c e n t r a t i o n s of the sulfur o x y a n i o n s at time t a s a function of the  solution  c o n d i t i o n s , reaction rates a n d reaction stoichiometry are s h o w n in E q u a t i o n s 7.13 to 7.16.  126  [ S 0 - ] = [ S 0 - ] - i + (% r2 + r3t - M - r4 )At  [7.13]  [S 0 -], = [ S 0 ^ - i + (Vl,  [7.14]  2  2  2  3  t  2  3  2  4  t  t  4  t  - r2,)At  2  6  t  6  [ S 0 " ] = [ S 0 e l t . i + ( / r 2 + / r 4 - r3 )At  [7.15]  [S0 "] = [ S 0 - ]  [7.16]  2  3  2  6  t  2  t  2  2  2  4  1  3  4  M  t  3  t  t  + (r3t)At  where  M, = ak  R 1  [7.17]  [SzOg^t-iICult-i/ENHalM  [7.18]  r2 = (k ^, + k ^ I S z O a ^ t - i ) [OHIt-i [ S 0 - ] - i 2  t  R2  4  6  t  r3t = (k + k ^ O H j , , + k [ N H ] „ + k [ N H ] ) [ S 0 - ] „  [7.19]  r4, = b k  [7.20]  2  0  2  3  3  4  M  3  6  [CulnIOzlt-i/ISaOa !,., 2  R 4  S t a n d a r d v a l u e s for the rate c o n s t a n t s in E q u a t i o n s 7.17 to 7.20 a n d initial conditions a r e d i s c u s s e d in S e c t i o n 7.3.  T h e m o d e l w a s subject to the following constraints: T h e only c h e m i c a l reactions a s s u m e d to be o c c u r r i n g w e r e t h o s e s h o w n in F i g u r e 7.1 a s d i s c u s s e d a b o v e . It w a s a s s u m e d that a n y c u p r o u s c o p p e r f o r m e d w a s i m m e d i a t e l y r e - o x i d i s e d to cupric, s o that the cupric concentration r e m a i n e d constant. It w a s a s s u m e d that the solutions c o n t a i n e d 10 mg/l d i s s o l v e d o x y g e n . The a m m o n i a concentration, pH and dissolved oxygen concentration were a s s u m e d to r e m a i n constant. T h e p H w a s u s e d to calculate the hydroxide ion c o n c e n t r a t i o n a n d the ratio of a m m o n i a to a m m o n i u m g i v e n the total a m m o n i a c o n c e n t r a t i o n . T h e effects of ore w e r e ignored. T h e effects of other a n i o n s (e.g. sulfate) w e r e i g n o r e d .  7.3  MODEL SENSITIVITY TO MODEL PARAMETERS  T h e m o d e l output is d e p e n d e n t o n the f o r m of the rate e q u a t i o n s , the v a l u e s of the rate c o n s t a n t s a n d the solution conditions.  B e f o r e testing the validity of the m o d e l a g a i n s t  127  e x p e r i m e n t a l d a t a , the sensitivity of the m o d e l to c h a n g i n g the m o d e l p a r a m e t e r s w a s investigated.  It w a s anticipated that the m o d e l a s set up in S e c t i o n 7.2 m a y not a l w a y s  b e suitable to d e s c r i b e e x p e r i m e n t a l o b s e r v a t i o n s during gold l e a c h i n g a s the conditions u n d e r w h i c h the v a r i o u s m o d e l rate e q u a t i o n s w e r e derived w e r e not a l w a y s related to gold  leaching.  understanding  By of  identifying  the  effects  possible shortcomings  of  the  of the  various  model  parameters,  in d e s c r i b i n g  a  better  experimental  situations could b e g a i n e d .  In this s e c t i o n , the forms of the rate e q u a t i o n s w e r e not c h a n g e d , but the qualitative effects of c h a n g i n g the rate c o n s t a n t s for e a c h rate equation ( s e e T a b l e 7.1) a n d a l s o of v a r y i n g the proportion of thiosulfate reacting to form tetrathionate  v e r s u s trithionate  ( R e a c t i o n R1 v e r s u s R e a c t i o n R 4 ) w e r e investigated. A s a s t a n d a r d condition, the rate c o n s t a n t s d i s c u s s e d in S e c t i o n 7.2 w e r e u s e d , a n d it w a s a s s u m e d that 8 0 % of the thiosulfate r e a c t e d to form tetrathionate v i a R e a c t i o n R1 a n d 2 0 % r e a c t e d to form trithionate via R e a c t i o n R 4 . T h e s e p e r c e n t a g e s w e r e s e l e c t e d s i m p l y a s e x a m p l e s , a n d a r e c o n s i s t e n t with the o b s e r v a t i o n ( s e e later in S e c t i o n 7.4) that a greater p e r c e n t a g e of thiosulfate a p p e a r s to react via R e a c t i o n R1 to form tetrathionate.  T a b l e 7.1 : M o d e l p a r a m e t e r s u s e d to test m o d e l sensitivity Parameter  Standard value  R a n g e tested  80%  0-100%  20%  0-100%  a (proportion of thiosulfate reacting via R1 to form tetrathionate) b (proportion of thiosulfate reacting via R 4 to f o r m trithionate) km  4 . 2 x 1 0 ^ s"  1  4 . 2 X10"  kp.2-0  0.022 M ^ . s  1  0.022 M " . s - x 1 0  kp.2-1  2.77 M" .s"  k  0  1.4 x 10" s  ki:  5.8x10- M- .s-  k  2  3.1 x 1 0 " M . s  k  3  6.4xl6' M- .s  2  8  5  1.03 s"  kp.4  128  1  1  1  2 . 7 7 M " . s x 10 2  1  1.4 x 10" s" x 10 8  1  1  1  7  1  1  1  1  7  s x10  4  1  1  1  5.8x10- M- .s' x10 5  3.1  x  10  1  1  M ^ . s " x 10  7  1  6 . 4 x IO" M" .s" x 10 7  1.03 s  1  1  1  x 10  T h e solution conditions in T a b l e 7.2 w e r e s e l e c t e d a s s t a n d a r d conditions for the p u r p o s e s of this e v a l u a t i o n .  T a b l e 7.2 : S t a n d a r d e x p e r i m e n t a l p a r a m e t e r s u s e d in m o d e l l i n g L e a c h conditions  Standard value  Temperature  2 5 °C  [S 0 ]  0  0.2 M  [NH ] .  0.4 M  [Cu ]  3 0 mg/l  PH  10  2  2  3  3  t0  2+  D0  7.3.1  10 mg/l  2  Proportion of Thiosulfate Forming Tetrathionate versus Trithionate  T h e proportion of thiosulfate reacting directly to f o r m tetrathionate ( R e a c t i o n R 1 ) v e r s u s that reacting directly to form trithionate ( R e a c t i o n R 4 ) (i.e. a a n d b in T a b l e 7.2) w a s c o n s i d e r e d . In F i g u r e 7.4 it is a s s u m e d that thiosulfate d o e s not form trithionate directly, only tetrathionate (i.e. a = 1 0 0 % ) while in F i g u r e 7.5, only trithionate is f o r m e d (i.e. b = 100%).  T w o intermediate c o m b i n a t i o n s of t h e s e two reaction paths are s h o w n in  F i g u r e s 7.6 a n d 7.7.  129  0.10  0.08  0  CD —\  GO  0.06  c co  0.04  "O CD  o  CD' CO  0.02 •Thiosulfate  0.00 12  24  18  T i m e (hrs)  - Tetrathionate | -Trithionate Sulfate  F i g u r e 7.4 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  A s s u m e d all thiosulfate d e g r a d a t i o n is via R e a c t i o n R1 (a = 100 % , b = 0%).  •  Initial conditions: 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  2 +  3  3  130  0.10  0.08  —i  CD  co  0.06  g. co  T3 4-  0.04  CD  o  CD"  co 0.02  -Thiosulfate Tetrathionate  0.00 12  24  18  T i m e (hrs)  Trithionate -  Sulfate  F i g u r e 7.5 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  A s s u m e d all thiosulfate d e g r a d a t i o n is via R e a c t i o n R 4 (a = 0 %, b = 100  •  Initial c o n d i t i o n s : 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  %).  2 +  3  3  F i g u r e s 7.4 a n d 7.5 s h o w the e x t r e m e s of the m o d e l with r e s p e c t to the thiosulfate d e g r a d a t i o n pathway.  F r o m this it is o b v i o u s that the m o d e l output is very sensitive to  the ratio of thiosulfate a s s u m e d to react to form tetrathionate v e r s u s the thiosulfate that f o r m s trithionate directly.  Qualitatively, w h e r e R e a c t i o n R1 to tetrathionate is f a v o u r e d ,  the thiosulfate d e g r a d a t i o n c u r v e is c o n c a v e . W h i l e this is not i mmedi atel y c l e a r from F i g u r e 7.4, it is a p p a r e n t if the thiosulfate concentration axis is c h a n g e d .  Where  R e a c t i o n R 4 to trithionate is f a v o u r e d , the thiosulfate d e g r a d a t i o n c u r v e is c o n v e x a n d the trithionate formation c u r v e is m u c h m o r e c o n c a v e . T h e s e o b s e r v a t i o n s w e r e u s e d to a s s e s s the m o d e l a g a i n s t e x p e r i m e n t a l d a t a in S e c t i o n 7.4.  131  F i g u r e 7.6 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  A s s u m e d thiosulfate d e g r a d a t i o n is v i a both R e a c t i o n R1 a n d R e a c t i o n R 4 (a =  50 %, b = 50 %). •  Initial conditions: 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  2 +  3  3  132  0.10  0.08 CD  0.06  co  c  co  4-  0.04  TD CD O  CD' CO  0.02 -Thiosulfate - Tetrathionate I  1 0.00 24  12  - Trithionate  T i m e (hrs)  Sulfate  F i g u r e 7.7 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  A s s u m e d thiosulfate d e g r a d a t i o n is via both R e a c t i o n R1 a n d R e a c t i o n R 4 (a =  80 %, b = 20 %). •  Initial conditions: 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  2 +  3  3  F o r the p u r p o s e s of demonstrating the sensitivity of the m o d e l to other p a r a m e t e r s , it w a s a s s u m e d that a r o u n d 8 0 % of the thiosulfate reacts via R e a c t i o n R1 a n d 2 0 % r e a c t s via R e a c t i o n R 2 , s i m p l y a s a n e x a m p l e . All other figures s h o w i n g the sensitivity of the m o d e l to the m o d e l p a r a m e t e r s c a n be c o m p a r e d to F i g u r e 7.7.  7.3.2  Rate of Reaction R1 - Thiosulfate Degradation to Tetrathionate  T h e rate constant for R e a c t i o n R 1 , k , w a s i n c r e a s e d by a factor of 10 a n d the m o d e l R1  output is s h o w n in F i g u r e 7.8. i n c r e a s i n g the v a l u e of k  R1  W h e n c o m p a r e d with F i g u r e 7.7, it c a n be s e e n that  h a s a significant effect o n the m o d e l output, i n c r e a s i n g the  rate of thiosulfate d e g r a d a t i o n , the rate of trithionate formation, a n d the concentration.  T h e s t a n d a r d v a l u e for k  R1  133  tetrathionate  s h o w n in T a b l e 7.1 w a s d e r i v e d in the  a b s e n c e of o x y g e n a n d it is k n o w n that the rate of this reaction c a n i n c r e a s e up to 4 0 t i m e s in the p r e s e n c e of o x y g e n (Byerley et a l , 1973b).  0.05  •Thiosulfate  12  18  Tetrathionate I  24  Trithionate  T i m e (hrs) -  Sulfate  F i g u r e 7.8 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n  - kgi increased by a factor 10 •  A s s u m e d thiosulfate d e g r a d a t i o n is via both R e a c t i o n R1 a n d R e a c t i o n R 4 (a = 8 0 % , b = 2 0 %).  •  7.3.3  Initial c o n d i t i o n s : 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  2 +  3  3  Rate of Reaction R 4 - Thiosulfate Degradation to Trithionate  T h e rate c o n s t a n t for R e a c t i o n R 4 , k , w a s i n c r e a s e d by a factor of 10 a n d the m o d e l R4  output is s h o w n in F i g u r e 7.9.  T h i s h a s a t r e m e n d o u s effect o n the output, giving very  rapid thiosulfate d e g r a d a t i o n a n d trithionate formation, a c c e n t u a t i n g the c o n v e x nature of the thiosulfate d e g r a d a t i o n c u r v e a n d c o n c a v e nature of the trithionate formation c u r v e . Very  little tetrathionate  is f o r m e d .  N o t e that the d a s h e d line for the  thiosulfate  c o n c e n t r a t i o n profile after a b o u t 16 hours indicates that the rapid d e c l i n e of thiosulfate to z e r o c o n c e n t r a t i o n m a y not be reliable a n d is a manifestation of the i n v e r s e d e p e n d e n c y of the thiosulfate d e g r a d a t i o n rate o n the thiosulfate c o n c e n t r a t i o n (Equation 7.10).  134  The  rate c o n s t a n t d e s c r i b i n g this reaction w a s i n c r e a s e d by a factor of 10 in F i g u r e 7 . 9 , w h i c h is m u c h higher than w o u l d b e e x p e c t e d b a s e d o n c o m p a r i s o n with e x p e r i m e n t a l d a t a ( s e e S e c t i o n 7.4).  F i g u r e 7.9 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n  - kp increased by a factor 10 4  •  A s s u m e d thiosulfate d e g r a d a t i o n is via both R e a c t i o n R1 a n d R e a c t i o n R 4 (a = 8 0 %, b = 2 0 %).  •  7.3.4  Initial conditions: 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  2 +  3  3  Rate of Reaction R 2 - Tetrathionate Degradation  T h e rate c o n s t a n t s for R e a c t i o n R 2 , k -o a n d k . i , w e r e i n c r e a s e d by a factor of 10 R2  R2  (separately) a n d the m o d e l output for e a c h situation is s h o w n in F i g u r e s 7.10 a n d 7 . 1 1 . T h e s c a l e h a s b e e n c h a n g e d to s h o w the differences in tetrathionate concentration m o r e clearly.  W h i l e adjusting t h e v a l u e of k . did not h a v e m u c h effect, the tetrathionate R2  0  c o n c e n t r a t i o n d e c r e a s e d significantly by i n c r e a s i n g k . i . R2  135  F i g u r e 7.10 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n - k ^ increased bv a factor 1 0 R?  • •  A s s u m e d thiosulfate d e g r a d a t i o n is via both R e a c t i o n R1 a n d R e a c t i o n R 4 (a = 8 0 %, b = 2 0 %). Initial conditions: 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  2 +  3  3  136  F i g u r e 7.11 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n - k .i R?  •  increased by a factor 10  A s s u m e d thiosulfate d e g r a d a t i o n is via both R e a c t i o n R1 a n d R e a c t i o n R 4 (a = 8 0 % , b = 2 0 %).  •  7.3.5  Initial c o n d i t i o n s : 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  2 +  3  3  Rate of Reaction R3 - Trithionate Degradation  T h e rate c o n s t a n t s for R e a c t i o n R 3 , k , k i , k a n d k , w e r e all i n c r e a s e d by a factor of 10, 0  a n d the m o d e l output  2  is s h o w n in F i g u r e 7.12.  3  Increasing the rate of  trithionate  d e g r a d a t i o n h a d the overall effect of lowering the rate of trithionate formation, a n d the formation of sulfate w a s e n h a n c e d .  T h e trithionate c o n c e n t r a t i o n c u r v e b e c a m e m o r e  convex.  137  0.05  0.04 CD  4-  cn  tz  0.03  cn TD CD  0.02  o CD'  cn  4- 0.01 -Thiosulfate - Tetrathionate I  0.00  - Trithionate Sulfate  F i g u r e 7.12 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n - kn. k i . k? a n d k i n c r e a s e d b v a f a c t o r 1 0 3  A s s u m e d thiosulfate d e g r a d a t i o n is v i a both R e a c t i o n R1 a n d R e a c t i o n R 4 (a = 80%, b = 20%). Initial conditions: 0.2 M S 0 " , 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C. 2  2  7.3.6  2 +  3  3  Summary - Effect of Model Parameters  T h e p e r c e n t a g e of thiosulfate reacting to form tetrathionate v e r s u s that reacting to form trithionate h a d a significant impact o n the m o d e l output, a s did the m a g n i t u d e of the rate c o n s t a n t s for t h e s e  reactions.  W h e r e tetrathionate  formation  was favoured,  thiosulfate c o n c e n t r a t i o n profile w a s m o r e c o n c a v e , while w h e r e trithionate  the  formation  w a s f a v o u r e d it w a s c o n v e x . C h a n g i n g the rate c o n s t a n t s for tetrathionate d e g r a d a t i o n h a d a significant effect o n the tetrathionate c o n c e n t r a t i o n , a l s o affecting the thiosulfate a n d trithionate c o n c e n t r a t i o n s slightly, a s t h e s e s p e c i e s a r e products of that d e g r a d a t i o n . T h e rate of d e g r a d a t i o n of trithionate affected the trithionate c o n c e n t r a t i o n , a s well a s h a v i n g a n impact o n the thiosulfate a n d sulfate c o n c e n t r a t i o n s . trithionate  Increasing the rate of  d e g r a d a t i o n c a u s e d the trithionate c o n c e n t r a t i o n profile to b e c o m e m o r e  c o n v e x . O v e r a l l , the m o d e l w a s m o s t s e n s i t i v e to the rate of d e g r a d a t i o n of thiosulfate to  138  tetrathionate a n d / o r trithionate.  A c o m p a r i s o n of the m o d e l output with e x p e r i m e n t a l  d a t a ( S e c t i o n 7.4) w a s u s e d to better estimate t h e s e m o d e l p a r a m e t e r s .  7.4  COMPARISON OF MODEL OUTPUT WITH EXPERIMENTAL RESULTS IN THE ABSENCE OF ORE  The  aim  of  this  section w a s  to  determine  whether  the  model  qualitatively  and  quantitatively w a s a b l e to d e s c r i b e e x p e r i m e n t a l d a t a in the a b s e n c e of ore, a n d to identify s h o r t c o m i n g s in the m o d e l .  7.4.1  Experiments  A f e w e x p e r i m e n t s w e r e carried out to provide a d a t a set a g a i n s t w h i c h to test the m o d e l . A solution containing a m m o n i a a n d c o p p e r ( a d d e d a s c u p r i c sulfate) w a s m a d e up a n d the p H a d j u s t e d using sulphuric a c i d (to p H 9 or 10). T h e solution w a s transferred to a b e a k e r in a w a t e r j a c k e t a n d brought to 2 5 °C. A n aliquot of s o d i u m thiosulfate solution w a s a d d e d s o that the final solution w o u l d h a v e the required concentration of thiosulfate (0.2 M ) , c o p p e r (30 or 100 mg/l) a n d a m m o n i a (0.3 - 0.4 M ) . T h e h e a d s p a c e a b o v e the solution w a s p u r g e d with o x y g e n a n d the b e a k e r s e a l e d with plastic film.  T h e solution  w a s stirred using a m a g n e t i c stirrer a n d kept at 2 5 °C. T h e test conditions a r e s h o w n in T a b l e 7.3.  Samples were  removed  periodically by syringe c o n n e c t e d to a  tube  e x t e n d i n g into the solution a n d diluted a s required for a n a l y s i s for thiosulfate, trithionate a n d tetrathionate c o n c e n t r a t i o n s by ion c h r o m a t o g r a p h y , a s d i s c u s s e d in C h a p t e r 3. Sulfate w a s not m e a s u r e d a s it could not b e d e t e r m i n e d at the s a m e time a s the other sulfur o x y a n i o n s a n d storing of the s a m p l e for later a n a l y s i s of sulfate w a s  not  c o n s i d e r e d d u e to the s o m e t i m e s rapid c h a n g e in solution c o n c e n t r a t i o n s . T h e r e f o r e no sulfur b a l a n c e c o u l d b e carried out.  T h e total a m m o n i a concentration w a s a n a l y s e d by  titration, a s d i s c u s s e d in C h a p t e r 3.  139  T a b l e 7.3 : E x p e r i m e n t a l conditions u s e d in tests for m o d e l validation  Initial thiosulfate  Copper  Total a m m o n i a  (M)  (mg/l)  (M)  1  0.2  0  0.42  10  2a  0.2  32  0.42  10  2b  0.2  32  0.38  10  3  0.2  30  0.33  9  4  0.2  30  0.40  9  5  0.2  101  0.30  10  Test  pH  T h e solution w a s kept u n d e r o x y g e n s i n c e the m o d e l a s s u m e d that there w a s sufficient o x y g e n present s o that all c u p r o u s c o p p e r w a s re-oxidised to c u p r i c c o p p e r .  In a m o r e  e l a b o r a t e m o d e l , it w o u l d b e n e c e s s a r y to take c o p p e r s p e c i a t i o n into c o n s i d e r a t i o n . A s s u m i n g that only thiosulfate w a s likely to react with c u p r i c c o p p e r (and not a n y trithionate or tetrathionate that formed), the m a x i m u m o x y g e n requirement to maintain all the c o p p e r in the cupric form could be d e t e r m i n e d  b a s e d o n the total  thiosulfate  c o n c e n t r a t i o n . A c c o r d i n g to E q u a t i o n 7 . 1 , e a c h m o l e of thiosulfate o x i d i z e d requires o n e m o l e of c u p r i c c o p p e r .  H e n c e for 100 ml 0.2 M thiosulfate solutions, 2 0 m m o l c u p r i c  c o p p e r w o u l d be required for c o m p l e t e oxidation of thiosulfate to tetrathionate.  Even  with low levels of c o p p e r present (30 - 100 mg/l) continual r e p l e n i s h m e n t of the cupric c o u l d facilitate this.  T h e a m o u n t of o x y g e n required to o x i d i s e 2 0 m m o l of c u p r o u s  c o p p e r is 5 m m o l . B a s e d o n the s t a n d a r d g a s v o l u m e s at a t m o s p h e r i c p r e s s u r e , this is equivalent  to  about  112  ml  oxygen.  The  h e a d s p a c e in  the  vessel  used  was  a p p r o x i m a t e l y 160 ml, allowing for sufficient o x y g e n to b e present s o a s not b e limiting.  7.4.2  Validation Method  W h i l e the f o r m s of the rate e q u a t i o n s w e r e not c h a n g e d , the m a g n i t u d e of the rate c o n s t a n t s a n d the proportion of thiosulfate reacting to form tetrathionate or trithionate (i.e. a or b in F i g u r e 7.1) w e r e adjusted to fit e x p e r i m e n t a l d a t a to identify s h o r t c o m i n g s of the m o d e l .  B e c a u s e the c h a n g e of thiosulfate concentration w a s g i v e n by the  appropriate rate e q u a t i o n multiplied by the p e r c e n t a g e of thiosulfate reacting either v i a R e a c t i o n R1 or R 4 , the product of this p e r c e n t a g e a n d the appropriate rate constant for  140  e a c h of t h e s e reactions w a s c o n s i d e r e d a s a s i n g l e p a r a m e t e r in this e v a l u a t i o n . H e n c e the following four p a r a m e t e r s w e r e adjusted a s n e c e s s a r y to attempt to find a g o o d fit of the m o d e l to the e x p e r i m e n t a l d a t a : a x k  R1  for R e a c t i o n R 1 , w h e r e a = 1 for the s t a n d a r d c a s e  k - a n d k . i (adjusted together by the s a m e factor a s k ) for R e a c t i o n R 2 R2  k, k 0  R 2  0  1t  b x k  R4  R2  k a n d k (adjusted together by the s a m e factor a s k ) for R e a c t i o n R 3 2  3  R3  for R e a c t i o n R 4 , w h e r e b = 1 for the s t a n d a r d c a s e  T h e s e p a r a m e t e r s w e r e multiplied by a factor a s n e c e s s a r y to give the best fit. factor for a k  R 1  Hence a  of 10 w o u l d imply that the p e r c e n t a g e of thiosulfate reacting via R e a c t i o n  R1 multiplied by the rate constant w o u l d b e 10 times higher than if all the thiosulfate r e a c t e d via R e a c t i o n R1 (a = 1) a n d the rate constant k  R1  w a s its s t a n d a r d v a l u e a s in  T a b l e 7 . 1 . T h i s m e t h o d c a n n o t distinguish w h e t h e r a n overall i n c r e a s e of, 10 times (for a k ) is d u e to a 10 fold i n c r e a s e in the rate constant only, for e x a m p l e , or a 2 0 fold R 1  i n c r e a s e in the rate constant but with only 5 0 % of the thiosulfate reacting via this reaction.  T w o a p p r o a c h e s w e r e c o n s i d e r e d in c o m p a r i n g the e x p e r i m e n t a l d a t a to the m o d e l output.  In the first a p p r o a c h , it w a s attempted to u s e a m a t h e m a t i c a l m e t h o d to adjust  the m o d e l p a r a m e t e r s to obtain the best fit.  T h e s u m of the s q u a r e s of the errors  b e t w e e n the e x p e r i m e n t a l d a t a points a n d the m o d e l l e d v a l u e s w a s m i n i m i s e d adjusting the m o d e l p a r a m e t e r s .  by  H o w e v e r , this a p p r o a c h g a v e a large variation in the  best fit m o d e l p a r a m e t e r s , giving w h a t a p p e a r e d to be m e a n i n g l e s s s c e n a r i o s at t i m e s . A l s o the final o u t c o m e of the adjusted p a r a m e t e r s w a s strongly d e p e n d e n t o n the starting  values selected.  T h e c o n c e n t r a t i o n s of thiosulfate  a n d trithionate  e x p e r i m e n t a l l y w e r e g e n e r a l l y higher than t h o s e of tetrathionate a n d sulfate.  found Hence  minimising the s u m of the s q u a r e s of errors for all s p e c i e s c o u l d b e e a s i l y b i a s e d d e p e n d i n g o n w h i c h s p e c i e s w e r e present in higher c o n c e n t r a t i o n s a n d o n w h e t h e r the a b s o l u t e or relative errors w e r e u s e d .  T h e s e c o n d a p p r o a c h u s e d v i s u a l inspection a n d adjustment of p a r a m e t e r s a n d g a v e m o r e realistic results. T h i s m e t h o d u s e d w h a t w a s learnt in e x a m i n i n g the sensitivity of the m o d e l to the m o d e l p a r a m e t e r s in S e c t i o n 7.3 a n d the p a r a m e t e r s w e r e adjusted b a s e d o n the e x p e c t e d s h o r t c o m i n g s of parts of the m o d e l a n d a v i s u a l best fit.  141  The  c o n c a v e a n d c o n v e x natures of the concentration profiles w e r e u s e d to e s t i m a t e w h e t h e r the s y s t e m w a s b i a s e d t o w a r d s thiosulfate d e g r a d a t i o n to tetrathionate or trithionate, a n d the m a g n i t u d e s of the rate c o n s t a n t s w e r e a d j u s t e d to allow for a n i m p r o v e d m o d e l fit to  match  both the  m e a s u r e d c o n c e n t r a t i o n s a n d the  qualitative  s h a p e of  the  concentration profiles. T h e results obtained w e r e m o r e realistic a n d c o n s i s t e n t using this approach.  T h e five s e t s of test results are first c o n s i d e r e d s e p a r a t e l y a n d the overall findings are s u m m a r i s e d thereafter. In all figures, the m o d e l output is s h o w n by c o n t i n u o u s lines while e x p e r i m e n t a l d a t a is s h o w n a s individual d a t a points. S i n c e n o e x p e r i m e n t a l d a t a for sulfate c o n c e n t r a t i o n s w e r e a v a i l a b l e , the m o d e l l e d sulfate c o n c e n t r a t i o n s are not shown.  7.4.3 Test 1 - No Copper T h e g r a p h in F i g u r e 7.13 s h o w s the m o d e l output a n d e x p e r i m e n t a l d a t a points for a test with no c o p p e r present. O v e r the duration of the test there w a s no significant thiosulfate degradation. appropriate  T h e m o d e l a s s u m e s that c o p p e r is present. T h e s e results s h o w that it is to  require  copper  in  the  model  to  have  d e g r a d a t i o n , for t h e s e conditions a n d test duration.  0  142  any  significant  thiosulfate  0.04 0.2  o ~  CD cz O o  0.1 2  ' -Thiosulfate  O CN  CO  Trithionate  0.0  » T  0  2  •  4  6  1  1  1  1  1  1  1  r  8  10  12  14  16  18  20  22  0.00 Tetrathionate  24  T i m e (hrs) F i g u r e 7.13 : M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  Initial c o n d i t i o n s : 0.2 M S 0 " , 0.42 M N H , 0 mg/l C u , p H 10, 2 5 °C, s e a l e d 2  2  v e s s e l with 0  7.4.4  2  2 +  3  3  in h e a d s p a c e  T e s t s 2a a n d 2b - L o w C o p p e r , p H 10  T h e following conditions w e r e u s e d in T e s t 2 (a a n d b): C u  2 +  (added a s C u S 0 ) - 32 4  mg/l, N H (total) - 0.38 - 0.42 M , p H 10. T h e g r a p h in F i g u r e 7.14 s h o w s the test d a t a . 3  T h e thiosulfate c o n c e n t r a t i o n c u r v e is c o n c a v e while the trithionate c o n c e n t r a t i o n c u r v e is c o n v e x . T h e initial d a t a point for the thiosulfate c o n c e n t r a t i o n is lower than e x p e c t e d , p r o b a b l y d u e to p o o r mixing. T h e tetrathionate c o n c e n t r a t i o n r e a c h e s a m a x i m u m within the first 2 hours of reaction then slowly d e g r a d e s . B a s e d o n t h e s e qualitative trends a n d the qualitative b e h a v i o u r of the m o d e l , it is r e a s o n a b l e to a s s u m e that m o s t of the thiosulfate reacts via reaction R1 to form tetrathionate. T h e m o d e l output a s s u m i n g that all thiosulfate reacts via this p a t h w a y a n d n o n e r e a c t s to form trithionate directly is a l s o s h o w n in F i g u r e 7.14.  In F i g u r e 7.14, the m o d e l predicts m u c h s l o w e r thiosulfate  143  d e g r a d a t i o n than o b s e r v e d experimentally a n d c o r r e s p o n d i n g l y m u c h s l o w e r trithionate a n d tetrathionate formation.  0.06 0.2  4>  0.05 •  •<> o  CD  0.04 CO rz  c o  CD  V  0.03 %  0.1  CD O CD' CO  0.02 <=;  4-  CO  0.01  o •  0.00  0.0 0  2  4  6  A •  8 10 12 14 16 18 20 22 24 26  •Thiosulfate -Trithionate Tetrathionate  T i m e (hrs) F i g u r e 7.14 : M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  R 1  - 1 x, k  R 2  - 1 x, k  R 3  - 1  - Ox.  Initial conditions: 0.2 M S 0 2  2 3  \ 0 . 4 2 M N H ( c l o s e d s y m b o l s ) or 0 . 3 8 M N H 3  (open s y m b o l s ) , 3 2 mg/l C u , p H 1 0 , 2 5 °C, s e a l e d v e s s e l with 0 2 +  2  3  in h e a d  space. •  A s s u m e d that all thiosulfate reacts v i a reaction R 1 .  Multiplying the rate of thiosulfate d e g r a d a t i o n v i a R e a c t i o n R 1 ( a k ) b y a factor of 10 R1  a l l o w e d a m u c h better fit to the thiosulfate d e g r a d a t i o n d a t a .  It is k n o w n that this  r e a c t i o n p r o c e e d s m u c h f a s t e r in the p r e s e n c e of o x y g e n than in its a b s e n c e (for w h i c h the rate e q u a t i o n w a s derived), s o i n c r e a s i n g the rate is justifiable.  T h i s effect is s h o w n  in F i g u r e 7.15. H o w e v e r , although the thiosulfate d a t a w a s a d e q u a t e l y d e s c r i b e d b y t h e  144  m o d e l , the tetrathionate  c o n c e n t r a t i o n predicted w a s too  h i g h , a n d the  trithionate  c o n c e n t r a t i o n too low.  0.06  -Thiosulfate -Trithionate - Tetrathionate  F i g u r e 7.15 : M o d e l output v e r s u s e x p e r i m e n t a l data for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: ak x, b k  •  R 4  R1  -10  x, k  - 1 x, k  R3  - 1  - 0 x.  Initial c o n d i t i o n s : 0.2 M S 0 " , 0.42 M N H ( c l o s e d s y m b o l s ) or 0.38 M N H 2  2  3  3  (open s y m b o l s ) , 3 2 mg/l C u , p H 10, 2 5 °C, s e a l e d v e s s e l with 0 2 +  •  R 2  2  3  in h e a d s p a c e  A s s u m e d that all thiosulfate reacts via reaction R 1 .  T h e rate of tetrathionate d e g r a d a t i o n predicted by the m o d e l w a s too low. increasing  the  rate  of  tetrathionate  degradation  would  affect  the  net  However, thiosulfate  c o n c e n t r a t i o n , a s thiosulfate is a product of tetrathionate d e g r a d a t i o n . H e n c e , i n c r e a s i n g the rate of tetrathionate  d e g r a d a t i o n w o u l d a l s o require a further  i n c r e a s e of the  thiosulfate d e g r a d a t i o n rate to maintain a g o o d fit for the thiosulfate d a t a .  F i g u r e 7.16  s h o w s the m o d e l output w h e r e the tetrathionate d e g r a d a t i o n rate ( k ) is i n c r e a s e d by a R2  factor of 8 a n d the thiosulfate d e g r a d a t i o n rate to tetrathionate ( a k ) is i n c r e a s e d by a R1  145  factor of 14. This produces a better fit in general, however, the trithionate concentration curve is not quite convex enough to fit the experimental data.  0.06  —Thiosulfate  — Trithionate  - - Tetrathionate  Figure 7.16 : Model output versus experimental data for sulfur oxyanion speciation during thiosulfate degradation •  Model parameters adjusted by the following factors: ak  R1  - 1 4 x, k  R2  - 8 x, k  R3  -  1 x, bk - 0 x. R4  •  Initial conditions: 0.2 M S 0 \ 0.42 M NH (closed symbols) or 0.38 M N H 2  2  3  3  3  (open symbols), 32 mg/l C u , pH 10, 25 °C, sealed vessel with 0 in head space 2+  2  •  Assumed that all thiosulfate reacts via reaction R1.  It was shown in determining the model sensitivity to the model parameters that increasing the rate of trithionate degradation would give a more convex trithionate concentration profile. Thiosulfate is produced during the degradation of trithionate, so increasing the trithionate degradation rate would required an increase in the rate of thiosulfate degradation to ensure a satisfactory fit remains for the thiosulfate data. Also, since it has been shown that trithionate forms from thiosulfate in the presence of copper  146  a n d o x y g e n , it is not realistic to a s s u m e that n o thiosulfate r e a c t s v i a R e a c t i o n R 4 . A l l o w i n g f o r a s m a l l fraction o f thiosulfate to react v i a this route ( b k the m o d e l fit slightly. thiosulfate  R 4  = 0.05) e n h a n c e s  In Figure 7 . 1 7 , the m o d e l output is s h o w n w h e r e the rate of  degradation  is i n c r e a s e d by a factor  of 1 5 , the rate  of  tetrathionate  d e g r a d a t i o n is i n c r e a s e d by a factor of 8, the rate of trithionate d e g r a d a t i o n is i n c r e a s e d b y a factor of 5, a n d s o m e reaction of thiosulfate to trithionate i s a l l o w e d . T h i s p r o d u c e s a slightly m o r e satisfactory fit to the e x p e r i m e n t a l d a t a than in F i g u r e 7 . 1 6 .  0.06  o  | c  CD O  rz o o  CO  o  -Thiosulfate  CM  CO  o • "l  0  2  1  1  1  4  6  8  1  i  1  1  1  r  -Trithionate  A  A  10 1 2 14 16 18 2 0 2 2 2 4 2 6  o  T i m e (hrs)  Tetrathionate |  F i g u r e 7 . 1 7 : M o d e l l e d v e r s u s e x p e r i m e n t a l data for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: ak  R1  - 1 5 x, k  R2  - 8 x, k  R3  -  5 x, bk - 0.05 x. R4  •  Initial conditions: 0.2 M S 0 ~ , 0.42 M N H ( c l o s e d s y m b o l s ) or 0 . 3 8 M N H 2  2  3  3  (open s y m b o l s ) , 3 2 mg/l C u , p H 10, 2 5 °C, s e a l e d v e s s e l with 0 2 +  •  A s s u m e d that thiosulfate reacts v i a reaction R1 a n d R 4 .  147  2  3  in h e a d s p a c e  7.4.5  Test 3 - Low Copper, pH 9, Low Ammonia  T h e following conditions w e r e u s e d in T e s t 3: C u  ( a d d e d a s C u S 0 ) - 3 0 mg/l, N H 4  3  (total) - 0 . 3 3 M , p H 9. T h e g r a p h in Figure 7.18 s h o w s that the thiosulfate c o n c e n t r a t i o n curve is c o n c a v e while the trithionate concentration curve is c o n v e x , but l e s s s o than for T e s t 2 at higher p H .  T h e overall thiosulfate d e g r a d a t i o n w a s l e s s a n d l e s s trithionate  w a s f o r m e d . T h e m a x i m u m tetrathionate c o n c e n t r a t i o n r e a c h e d w a s higher than that at higher p H . T h e m o d e l output s h o w n in F i g u r e 7.18 a s s u m e s that the thiosulfate r e a c t s only via R e a c t i o n R1 to form tetrathionate, b a s e d on the qualitative trends of the d a t a . W h i l e the thiosulfate d e g r a d a t i o n predicted a d e q u a t e l y fits the d a t a , e s p e c i a l l y at s h o r t e r times, the tetrathionate c o n c e n t r a t i o n s predicted w e r e too high while the trithionate c o n c e n t r a t i o n s w e r e too low, a s w a s f o u n d in T e s t 2.  Increasing the tetrathionate  d e g r a d a t i o n rate provided a better fit, a s s h o w n in F i g u r e 7.19.  H o w e v e r , the trithionate c o n c e n t r a t i o n curve in F i g u r e 7.19 w a s convex.  concave  instead of  T o alter the s h a p e of the trithionate c o n c e n t r a t i o n curve, the rate of trithionate  d e g r a d a t i o n could be i n c r e a s e d .  Increasing the rate of trithionate d e g r a d a t i o n in this  c a s e w o u l d improve the s h a p e of the m o d e l l e d trithionate profile, but the v a l u e s w o u l d be too low c o m p a r e d with e x p e r i m e n t a l d a t a .  T o i n c r e a s e the trithionate c o n c e n t r a t i o n ,  the reaction of thiosulfate d e g r a d a t i o n to trithionate w a s a s s i g n e d  a non-zero  value.  F i g u r e 7 . 2 0 s h o w s the m o d e l output w h e r e it is a s s u m e d that part of the thiosulfate r e a c t s v i a R e a c t i o n R 4 to f o r m trithionate, a n d a l s o that the trithionate d e g r a d a t i o n rate is 5 t i m e s higher t h a n predicted by the original m o d e l . much improved.  148  T h e fit to the e x p e r i m e n t a l d a t a is  0.02  o  zr  CD —I  co rz  CZ  o  #— -2* rz  CO  0.1  0.01 "a  H  CD  o  CD O  CD" CO  c o o CN CO  o  CM  -Thiosulfate  CO  0.0  — i  0  2  1  i  i  4  6  8  1  1  i  i  10  12  14  16  1  1  1  18 20  22  24  - Trithionate  0.00  1— 26  Tetrathionate  T i m e (hrs) F i g u r e 7.18 : M o d e l output v e r s u s e x p e r i m e n t a l data for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  - 1 x, k  R2  - 1 x, k  R3  - 1  - 0 x.  Initial conditions: 0.2 M S 0 " , 0.33 M N H , 30 mg/l C u , p H 9, 2 5 °C, s e a l e d 2  2  v e s s e l with 0 •  R 1  2  2 +  3  3  in h e a d s p a c e  A s s u m e d that all thiosulfate reacts via reaction R 1 .  149  0.02  CD Cfl  rz  c o  2' »-—  0.01  fZ  cu o cz o o  tn TD  CD O CD'  CO  -Thiosulfate  - Trithionate  0.00  Tetrathionate  Time (hrs) Figure 7.19 : Model output versus experimental data for sulfur oxyanion speciation during thiosulfate degradation •  Model parameters adjusted by the following factors: ak - 1 x, k R1  R2  - 10 x, k  R3  x, bk - 0 x. R4  •  Initial conditions: 0.2 M S 0 " , 0.33 M NH , 30 mg/l C u , pH 9, 25 °C, sealed 2  2  2+  3  3  vessel with 0 in head space. 2  •  Assumed that all thiosulfate reacts via reaction R1.  150  - 1  0.02  CD CO  c o  c «  CO  CO  •a  c 0 o c o o  CD  o  CD CO  CO  •Thiosulfate Trithionate 0  2  4  6  8  10 12 14 16 18 20 22 24 26 — - — - Tetrathionate T i m e (hrs)  F i g u r e 7.20 : M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  R 1  - 1 x, k  R 2  - 10 x, k  R 3  - 5  - 0.1 x.  Initial conditions: 0.2 M S 0 " , 0.33 M N H , 3 0 mg/l C u , p H 9, 2 5 °C, s e a l e d 2  2  v e s s e l with 0 •  2  2 +  3  3  in h e a d s p a c e .  A s s u m e d that all thiosulfate reacts via reaction R 1 .  7.4.6  Test 4 - Low Copper, pH 9, Higher Ammonia  T e s t 4 w a s very similar to T e s t 3, e x c e p t that the a m m o n i a c o n c e n t r a t i o n w a s higher at 0.40 M . T h e following conditions w e r e u s e d in T e s t 4: C u NH  3  2 +  ( a d d e d a s C u S 0 ) - 30 mg/l, 4  (total) - 0.40 M , p H 9. Fitting of the m o d e l required similar adjustments a s for T e s t  3, a n d a satisfactory fit is s h o w n in F i g u r e 7 . 2 1 .  151  0.02  —Thiosulfate  — Trithionate  - - Tetrathionate  F i g u r e 7.21 : M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: a k 5 x, b k  •  R 4  7.4.7  - 1.5 x, k  R2  - 10 x, k  R3  -  - 0.1 x.  Initial c o n d i t i o n s : 0.2 M S 0 2  v e s s e l with 0 •  R 1  2  2 3  \ 0.40 M N H , 3 0 mg/l C u , p H 9, 2 5 °C, s e a l e d 2 +  3  in h e a d s p a c e  A s s u m e d that all thiosulfate r e a c t s v i a reaction R 1 .  Test 5 - H i g h Copper, pH 1 0  T h e following conditions w e r e u s e d in T e s t 5: C u  2 +  ( a d d e d a s C u S 0 ) - 101 mg/l, N H 4  3  (total) - 0.30 M , p H 10. A s s h o w n in F i g u r e 7 . 2 2 , the thiosulfate c o n c e n t r a t i o n c u r v e is initially c o n c a v e while the trithionate concentration c u r v e is initially c o n v e x . T h e higher c o p p e r concentration c a u s e d a m u c h more rapid thiosulfate d e g r a d a t i o n a n d trithionate production than in T e s t 2.  T h e tetrathionate r e a c h e d a m a x i m u m c o n c e n t r a t i o n then  d e g r a d e d m u c h m o r e rapidly than for T e s t 2 at lower c o p p e r c o n c e n t r a t i o n .  152  B a s e d on  the b a s i c s h a p e of the c u r v e s , it w a s first a s s u m e d that no thiosulfate r e a c t e d via R e a c t i o n R 4 to f o r m trithionate directly.  A s in the previous e x a m p l e s , it w a s n e c e s s a r y  to i n c r e a s e the rate of thiosulfate d e g r a d a t i o n to tetrathionate  a n d of  tetrathionate  d e g r a d a t i o n to i m p r o v e the m o d e l fit. T h i s m o d e l output is s h o w n in F i g u r e 7.22. W h i l e the m o d e l fit w a s a d e q u a t e at shorter t i m e s , it did not d e s c r i b e the d a t a at longer t i m e s .  Thiosulfate Trithionate Tetrathionate  F i g u r e 7.22 : M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: ak 1 x, b k  •  R 4  x, k  R2  - 8 x, k  R3  -  - 0 x.  Initial conditions: 0.2 M S 0 " , 0.30 M N H , 101 mg/l C u , p H 10, 2 5 °C, s e a l e d 2  2  v e s s e l with 0 •  -16  R1  2  2 +  3  3  in h e a d s p a c e  A s s u m e d that all thiosulfate reacts via reaction R 1 .  A m o r e linear trend for both thiosulfate concentration a n d trithionate concentration w a s required.  A l l o w i n g for thiosulfate d e g r a d a t i o n via both R e a c t i o n R1 a n d R e a c t i o n R 4  w a s f o u n d to give this type of trend.  F i g u r e 7.23 s h o w s the m o d e l output w h e r e this is  t a k e n into a c c o u n t .  153  Thiosulfate Trithionate Tetrathionate  F i g u r e 7 . 2 3 : M o d e l output v e r s u s e x p e r i m e n t a l data for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  - 0.15  - 12x, kR2 - 8 x, k  R 3  - 1  x.  Initial conditions: 0.2 M S 0 " , 0.30 M N H , 101 mg/l C u , p H 10, 2 5 °C, s e a l e d 2  2  v e s s e l with 0 •  R 1  2  2 +  3  3  in h e a d s p a c e .  A s s u m e d that all thiosulfate reacts via reaction R 1 .  S i m i l a r output is o b t a i n a b l e w h e r e the rate of trithionate d e g r a d a t i o n is i n c r e a s e d 5 fold to b e c o n s i s t e n t with that required for T e s t s 2, 3 a n d 4. T h i s is s h o w n in F i g u r e 7.24.  154  Thiosulfate Trithionate Tetrathionate  F i g u r e 7.24 : M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: a k 5x, bk  •  R 4  R 1  - 1 2 x, k  R 2  - 8 x, k  R 3  -  -0.17x.  Initial conditions: 0.2 M S 0 " , 0.30 M N H , 101 mg/l C u , p H 10, 2 5 °C, s e a l e d 2  2  v e s s e l with 0 •  2  2 +  3  3  in h e a d s p a c e  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  7.4.8  Summary  W h i l e a d j u s t m e n t s to the m o d e l w e r e b a s e d only o n v i s u a l i n s p e c t i o n , the extent of s u c h a d j u s t m e n t s required to give a better fit to the e x p e r i m e n t a l d a t a give useful insight into the  shortcomings  of the  model.  E x a m p l e s of adjustments  found  to i m p r o v e  a g r e e m e n t b e t w e e n the m o d e l a n d e x p e r i m e n t a l d a t a are s h o w n in T a b l e 7.4. be noted that t h e s e m o d e l p a r a m e t e r adjustments a r e not unique.  155  the  It s h o u l d  T a b l e 7.4 : A d j u s t m e n t of m o d e l p a r a m e t e r s f o u n d to give i m p r o v e d a g r e e m e n t b e t w e e n m o d e l output a n d e x p e r i m e n t a l d a t a  Test  2a, 2b  [Cu]  [NH ]  (mg/l)  (M)  32  Multiplication  3  factor for  PH  ak  0.380.42  Multiplication  Multiplication  factor for k  factor for k  (k .i R 2  and  R2  (k , k 0  Multiplication factor for  k and  1t  R 1  R3  2  l<R2-2)  k)  bk  R 4  3  10  15  8  5  0.05  3  30  0.33  9  1.5  10  5  0.10  4  30  0.40  9  1.0  10  5  0.10  5  101  0.30  10  12  8  5  0.17  T h e following c o n c l u s i o n s c a n b e d r a w n from this validation of the m o d e l :  T h e m o d e l c a n b e u s e d to qualitatively d e s c r i b e the d e g r a d a t i o n of thiosulfate a n d the subsequent  formation  and  degradation  of  containing c o p p e r in a very g e n e r a l m a n n e r .  polythionates  in  ammoniacal  systems  Both the m o d e l a n d e x p e r i m e n t a l results  s h o w that a s thiosulfate d e g r a d e s , both tetrathionate a n d trithionate are f o r m e d initially. The  tetrathionate  itself  degrades  at  an  a p p r e c i a b l e , rate,  so  the  tetrathionate  c o n c e n t r a t i o n r e a c h e s a m a x i m u m , varying with the p H a n d c o p p e r c o n c e n t r a t i o n , before d e g r a d i n g to v e r y low l e v e l s . T h e trithionate concentration continually i n c r e a s e s o v e r the time f r a m e of interest (24 hours).  H o w e v e r , the m o d e l in its s t a n d a r d form is not a d e q u a t e to quantitatively d e s c r i b e the d e g r a d a t i o n of thiosulfate a n d the s p e c i a t i o n of the sulfur o x y a n i o n s . T h e m o d e l c a n b e m a d e to fit the e x p e r i m e n t a l d a t a by multiplying e a c h of the a p p l i c a b l e rate e q u a t i o n s by a factor, h o w e v e r , the v a l u e of e a c h factor is not c o n s i s t e n t for all the conditions t e s t e d , a s c a n b e s e e n in T a b l e 7.4. E v e n using the best fit factors, the m o d e l d o e s not predict the tetrathionate d e g r a d a t i o n s e e n after the m a x i m u m c o n c e n t r a t i o n of tetrathionate is r e a c h e d , a n d the trithionate c o n c e n t r a t i o n s predicted in the early s t a g e s of the tests are not high e n o u g h c o m p a r e d with the e x p e r i m e n t a l d a t a .  156  T h e tetrathionate d e g r a d a t i o n rate predicted by the m o d e l w a s lower than that s e e n experimentally.  Increasing the tetrathionate d e g r a d a t i o n rate c o n s t a n t s by a c o n s i s t e n t  factor of a b o u t 8 to 10 w a s beneficial in allowing the m o d e l to better d e s c r i b e the experimental observations. was  mentioned  that  In the s e c t i o n d i s c u s s i n g the m o d e l s e t u p ( S e c t i o n 7.2), it  Zhang  and  Dreisinger  (2002)  found  c o r r e s p o n d i n g to k - for the d e g r a d a t i o n of tetrathionate R2  0  that  the  rate  constant  in a l k a l i n e solutions in the  a b s e n c e of thiosulfate w a s 10 t i m e s higher than that f o u n d by R o l i a a n d C h a k r a b a r t i (1982), w h o s e rate e q u a t i o n w a s u s e d for this a n a l y s i s . H o w e v e r , R o l i a a n d C h a k r a b a r t i a l s o i n c l u d e d a m u c h stronger d e p e n d e n c y o n the thiosulfate c o n c e n t r a t i o n .  Using  Z h a n g a n d D r e i s i n g e r ' s rate e q u a t i o n for tetrathionate d e g r a d a t i o n instead of R o l i a a n d C h a k r a b a r t i ' s w o u l d not h a v e a l l o w e d for a better fit of the m o d e l to the e x p e r i m e n t a l d a t a s i n c e the rate c o n s t a n t for the thiosulfate d e p e n d e n c e ( k . i ) w a s of m u c h m o r e R2  s i g n i f i c a n c e . It is p o s s i b l e that tetrathionate reacts with c u p r i c c o p p e r a n d that this c o u l d e n h a n c e the reaction rate.  T h e tetrathionate  c o n c e n t r a t i o n profile predicted by the  m o d e l s h o w s a n i n c r e a s e in tetrathionate c o n c e n t r a t i o n to a m a x i m u m c o n c e n t r a t i o n with very little further d e g r a d a t i o n o v e r the time f r a m e t e s t e d .  H o w e v e r , e s p e c i a l l y for  T e s t 5 in the p r e s e n c e of higher c o p p e r c o n c e n t r a t i o n , the tetrathionate  concentration  d e c r e a s e d m u c h m o r e rapidly than predicted by the m o d e l after r e a c h i n g a m a x i m u m . T h i s d e g r a d a t i o n is e x p e c t e d to b e a function of the c o p p e r c o n c e n t r a t i o n a n d is not a c c o u n t e d for in the m o d e l . It h a s a l s o b e e n p r o p o s e d that the ionic strength affects the tetrathionate d e g r a d a t i o n rate ( B r e u e r a n d Jeffrey, 2 0 0 4 ) .  G i v e n that the  tetrathionate  c o n c e n t r a t i o n is g e n e r a l l y low, the rate e q u a t i o n d e s c r i b i n g tetrathionate d e g r a d a t i o n is a d e q u a t e u n d e r the conditions tested w h e n the rate c o n s t a n t is i n c r e a s e d by a factor of 8 to 10.  T o a l l o w for a n i m p r o v e d m o d e l fit, the rate of trithionate d e g r a d a t i o n w a s i n c r e a s e d c o n s i s t e n t l y by a factor of 5. predicts trithionate  T h i s implies that the rate e q u a t i o n d e r i v e d in this work  d e g r a d a t i o n rates that a r e too low.  It is thought that either the  p r e s e n c e of c o p p e r or of thiosulfate c o u l d b e the r e a s o n for this.  In the rate e q u a t i o n  d e r i v e d for trithionate d e g r a d a t i o n , c o p p e r a n d thiosulfate c o n c e n t r a t i o n s w e r e a s s u m e d to h a v e no significant effect o n the rate (see C h a p t e r 6).  H o w e v e r , there w e r e certain  c o n d i t i o n s for w h i c h thiosulfate i n c r e a s e d the trithionate d e g r a d a t i o n rate, a n d the effect of c o p p e r w a s not c o m p r e h e n s i v e l y t e s t e d , a n d not t e s t e d in the p r e s e n c e of e x c e s s oxygen.  S i n c e the factor required to i n c r e a s e the trithionate d e g r a d a t i o n rate w a s the  157  s a m e for both c o p p e r c o n c e n t r a t i o n s t e s t e d , it is m o r e likely that the o b s e r v e d effect w a s d u e to the role of thiosulfate.  T h e d e g r a d a t i o n of thiosulfate w a s the m o s t important in determining the m o d e l output. Not only c o u l d the p e r c e n t a g e of thiosulfate reacting v i a two routes to form either tetrathionate or trithionate  b e v a r i e d , but the rate c o n s t a n t s for the rate e q u a t i o n s  d e s c r i b i n g t h e s e two r e a c t i o n s c o u l d b e v a r i e d . T h e overall factors giving a satisfactory m o d e l fit s h o w n in T a b l e 7.4 w e r e not c o n s i s t e n t a c r o s s all tests. F o r the d e g r a d a t i o n of thiosulfate to tetrathionate ( R e a c t i o n R 1 ) , there w a s s o m e c o n s i s t e n c y b e t w e e n the tests at p H 9 a n d t h o s e at p H 10, but the d e g r a d a t i o n rate w a s higher at higher p H (or higher free a m m o n i a concentration). T h e c o p p e r concentration did not h a v e a n y notable effect (as s e e n in a c o m p a r i s o n of T e s t s 2 a a n d 2 b , a n d T e s t . 5 ) .  H o w e v e r , for the d e g r a d a t i o n of thiosulfate to trithionate ( R e a c t i o n R 4 ) , a c o m p a r i s o n of the factors required for the tests at p H 10 at different c o p p e r c o n c e n t r a t i o n s s h o w that the p r e s e n c e of c o p p e r d o e s influence the best fit factor. T h i s factor i n c r e a s e d with a n i n c r e a s e in c o p p e r c o n c e n t r a t i o n , by a similar factor.  T h i s c o u l d imply a stronger  d e p e n d e n c y o n the c o p p e r concentration than u s e d in the m o d e l . C o m p a r i n g T e s t s 2 a a n d 2 b with T e s t 4 s h o w s a n i n c r e a s e in the best fit factor with a d e c r e a s e in p H , implying either a d e p e n d e n c e of the d e g r a d a t i o n rate o n the p H or indirectly o n the free a m m o n i a concentration.  It is likely that the p e r c e n t a g e of thiosulfate reacting to form tetrathionate  versus  trithionate c h a n g e s with time. T h i s w a s not a c c o u n t e d for in the m o d e l a n d m a y a c c o u n t for the under-prediction of trithionate concentration for the initial t i m e s in the m o d e l . S i n c e the formation of trithionate from thiosulfate d e p e n d s o n the c o n c e n t r a t i o n of d i s s o l v e d o x y g e n , the availability of o x y g e n is e x p e c t e d to influence the relative rates of formation of trithionate a n d tetrathionate from thiosulfate.  A l t h o u g h the rates of d e g r a d a t i o n for tetrathionate a n d trithionate h a d a significant influence o n the m o d e l output, the rates of thiosulfate d e g r a d a t i o n either to tetrathionate or to trithionate w e r e the determining factors in the m o d e l output.  This exercise has  highlighted h o w the w a y in w h i c h thiosulfate is a s s u m e d to d e g r a d e h a s the greatest i m p a c t o n the predicted solution s p e c i a t i o n . T h e d e f i c i e n c i e s in the rate e q u a t i o n s u s e d  158  for t h e s e reactions h a v e a l s o b e e n highlighted. T h e r e is a n e e d to better a c c o u n t for the effect of c o p p e r c o n c e n t r a t i o n , p e r h a p s by using the active c o p p e r s p e c i e s , p o s s i b l y c u p r i c t e t r a a m m i n e , in the rate e q u a t i o n s rather than the total c u p r i c c o n c e n t r a t i o n . T h e r e is a l s o a p o s s i b l e effect of p H or free a m m o n i a concentration that is  not  adequately addressed.  T h e i n a d e q u a c y of the rate e q u a t i o n s d e s c r i b i n g thiosulfate d e g r a d a t i o n s u g g e s t s a s e r i o u s s h o r t c o m i n g in the m o d e l , a s the m o d e l o u t c o m e is v e r y s e n s i t i v e to t h e s e rate e q u a t i o n s . A better u n d e r s t a n d i n g is required of h o w thiosulfate d e g r a d e s to trithionate a n d tetrathionate.  7.5  COMPARISON OF EXPERIMENTAL RESULTS WITH AND WITHOUT ORE  Limited c o m p a r a b l e d a t a w a s a v a i l a b l e w h e r e o r e s w e r e l e a c h e d in thiosulfate solutions. A s e r i e s of l e a c h tests w a s carried out by L a m (2002) o n a n o r e p r o v i d e d by P l a c e r Dome.  T h e ore w a s relatively free of c o p p e r with the c o a r s e gold r e m o v e d by gravity  separation.  R e s u l t s of a n a n a l y s i s of the ore are s h o w n in T a b l e 7.5.  S u b s a m p l e s of  the o r e w e r e a n a l y s e d at different laboratories at different t i m e s , a n d s a m p l e variability m a y b e the r e a s o n for the apparently w i d e r a n g e in sulfide c o n c e n t r a t i o n s .  T a b l e 7.5 : A s s a y results for ore from P l a c e r D o m e u s e d in l e a c h tests  Species  Concentration  Au  1 . 1 4 - 1 . 2 0 g/t  Cu  0.004 - 0.006 %  Fe  3.84 %  S (total)  0.52 - 0.58 %  S as S 0  2 4  < 0.01 %  "  S°  <0.01 %  S*  0.07 - 0.51 %  C (total)  1.62%  C (organic)  0.47 %  159  T h e tests w e r e d o n e at 3 0 % s o l i d s at r o o m temperature in a s e a l e d v e s s e l with o x y g e n in the h e a d s p a c e a n d o x y g e n w a s r e p l e n i s h e d after s a m p l i n g . manually.  T h e p H w a s adjusted  T h e test duration w a s typically 2 4 h o u r s during w h i c h four s a m p l e s w e r e  r e m o v e d for a n a l y s i s of thiosulfate, trithionate, tetrathionate a n d sulfate c o n c e n t r a t i o n s . T h e a n a l y s i s of thiosulfate, trithionate a n d tetrathionate  w a s i m m e d i a t e , but  sulfate  a n a l y s e s w e r e g e n e r a l l y carried out at least four hours after s a m p l i n g a n d therefore t h e s e v a l u e s a r e not i n c l u d e d in this a n a l y s i s . A m a x i m u m gold l e a c h i n g of about 8 0 % w a s a c h i e v e d ( c o m p a r e d with 7 5 - 88 % gold extraction during cyanidation), but it w a s found  that gold  could  not  be  l e a c h e d efficiently  or  c o m p l e t e l y without  sufficient  thiosulfate or c u p r i c t e t r a a m m i n e present.  T w o s u c h l e a c h tests w e r e c o m p a r e d with the tests in the a b s e n c e of ore - T e s t 2 a a n d T e s t 5 respectively. N o d a t a w a s a v a i l a b l e for a test c o m p a r a b l e with T e s t s 3 or 4. T h e results from the l e a c h tests are s u p e r i m p o s e d o n the d a t a from the solution-only tests a n d the m o d e l predictions, using the s t a n d a r d m o d e l with the p a r a m e t e r s a d j u s t e d a s in T a b l e 7.4.  T e s t 2 a n d the c o r r e s p o n d i n g l e a c h test for 3 0 mg/l c o p p e r a r e s h o w n in  F i g u r e 7 . 2 5 , a n d T e s t 4 a n d the c o r r e s p o n d i n g l e a c h test for 100 mg/l c o p p e r are s h o w n in F i g u r e 7.26.  160  0.06  —Thiosulfate  — Trithionate  -Tetrathionate  F i g u r e 7.25 : M o d e l output v e r s u s e x p e r i m e n t a l d a t a for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  .  R 4  - 15 x, k  R 2  - 8 x, k  R3  - 5  - 0.05 x.  Initial conditions: 0.2 M S 0 " , 0.4-0.42 M N H , 3 0 - 3 2 mg/l C u , p H 10, 2 5 °C, 2  2  s e a l e d v e s s e l with 0 •  R 1  2  2 +  3  3  in h e a d s p a c e  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  ( L i n e s represent m o d e l l e d trends. O p e n s y m b o l s r e p r e s e n t e x p e r i m e n t a l v a l u e s in the a b s e n c e of o r e , c l o s e d s y m b o l s represent e x p e r i m e n t a l v a l u e s in the p r e s e n c e of ore.)  161  — Thiosulfate — Trithionate — - Tetrathionate  F i g u r e 7.26 : M o d e l output v e r s u s e x p e r i m e n t a l data for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  - 12 x, k  R2  - 8 x, k  R3  - 5  - 0.17 x.  Initial conditions: 0.2 M S 0 " , 0.3 - 0.4 M N H , 1 0 0 - 1 0 1 mg/l C u , p H 10, 2 5 2  2  °C, s e a l e d v e s s e l with 0 •  R 1  2  2 +  3  3  in h e a d s p a c e  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  ( L i n e s r e p r e s e n t m o d e l l e d trends. O p e n s y m b o l s r e p r e s e n t e x p e r i m e n t a l v a l u e s in the a b s e n c e of ore, c l o s e d s y m b o l s r e p r e s e n t e x p e r i m e n t a l v a l u e s in the p r e s e n c e of ore.) F o r both l e a c h tests, the thiosulfate d e g r a d a t i o n w a s faster in the p r e s e n c e of ore than in the solution only tests.  T h e p r e s e n c e of ore is likely to e n h a n c e the d e g r a d a t i o n of  thiosulfate, a n d there h a v e b e e n s o m e limited investigations into the effects of ore o n thiosulfate d e g r a d a t i o n ( W a n , 1 9 9 7 , X u a n d S c h o o n e n , 1 9 9 5 , D e J o n g , 2 0 0 4 ) .  F o r the  test at 3 0 mg/l c o p p e r , m o r e trithionate w a s p r o d u c e d than in the a b s e n c e of o r e , while for the 100 mg/l c o p p e r test, l e s s trithionate w a s p r o d u c e d . T h e r e c o u l d be a n effect of o r e o n the trithionate d e g r a d a t i o n rate.  162  T h e differences b e t w e e n tests in the p r e s e n c e a n d a b s e n c e of ore s h o w that it is n e c e s s a r y to test the effects of o r e s o n the v a r i o u s sulfur o x y a n i o n reactions to b e a b l e to fully u n d e r s t a n d the s y s t e m .  7.6  SCOPE OF USE OF MODEL  B a s e d o n the d i s c u s s i o n in S e c t i o n 7.4.7 o n the s h o r t c o m i n g s of the m o d e l in predicting thiosulfate d e g r a d a t i o n a n d the s u b s e q u e n t sulfur o x y a n i o n s p e c i a t i o n in the a b s e n c e of o r e s , a n d the o b s e r v e d difference in solution c h e m i s t r y in the p r e s e n c e of o r e (Section 7.5), the s c o p e of u s e of the m o d e l in its current form is s o m e w h a t limited a n d c a n n o t b e c o n s i d e r e d quantitative.  H o w e v e r , the m o d e l d o e s s h o w that o u r u n d e r s t a n d i n g of the  thiosulfate d e g r a d a t i o n p a t h w a y s in particular is limited, highlighting the n e e d for further w o r k in this a r e a . T h e m o d e l c a n b e u s e d to e s t i m a t e g e n e r a l trends in sulfur o x y a n i o n s p e c i a t i o n during thiosulfate  d e g r a d a t i o n , provided that the m o d e l p a r a m e t e r s  are  a d j u s t e d a c c o r d i n g to the empirical factors given in T a b l e 7.4. W h i l e the m o d e l w a s not o p t i m i s e d for solutions containing o r e s a n d there is a difference in results w h e n o r e s are present, the g e n e r a l trends are similar.  E v e n though the m o d e l is in n e e d of significant  refining, involving m u c h testwork, s i n c e the qualitative trends it predicts a r e similar to t h o s e s e e n experimentally, the m o d e l is useful to identify factors w h i c h h a v e a significant influence o n the sulfur o x y a n i o n s p e c i a t i o n .  T h e impact of solution conditions o n the  thiosulfate d e g r a d a t i o n a n d sulfur o x y a n i o n s p e c i a t i o n a s predicted by the m o d e l is d i s c u s s e d in S e c t i o n 7.7.  7.7  IMPACT OF SOLUTION CONDITONS  T h e m o d e l w a s u s e d to predict the sulfur o x y a n i o n s p e c i a t i o n for c h a n g i n g solution conditions. T h e effects of c o p p e r c o n c e n t r a t i o n , a m m o n i a c o n c e n t r a t i o n , p H , d i s s o l v e d o x y g e n c o n c e n t r a t i o n , initial thiosulfate concentration a n d solution r e c y c l e o n the m o d e l output w e r e e x a m i n e d . E a c h of t h e s e p a r a m e t e r s w a s v a r i e d in turn a n d input into the m o d e l . B e c a u s e the best-fit m o d e l p a r a m e t e r s in T a b l e 7.4 v a r i e d with the e x p e r i m e n t a l c o n d i t i o n s , three s c e n a r i o s w e r e m o d e l l e d for e a c h of the solution p a r a m e t e r s t e s t e d , u s i n g the best-fit m o d e l p a r a m e t e r s a n d solution conditions for T e s t 2, T e s t 4 a n d T e s t 5. T h e solution s p e c i a t i o n after 2 4 hours predicted by the m o d e l w a s plotted a g a i n s t the parameter being tested.  163  T h i s evaluation s h o w s h o w the m o d e l c o u l d b e u s e d to highlight potential c o n c e r n s regarding thiosulfate d e g r a d a t i o n during gold l e a c h i n g , a n d h o w solution conditions c a n b e o p t i m i s e d to m i n i m i s e thiosulfate d e g r a d a t i o n .  7.7.1  Copper Concentration  T h e c o p p e r concentration w a s i n c r e a s e d from 10 mg/l to 9 0 - 1 2 0 mg/l for the three m o d e l s c e n a r i o s b a s e d o n T e s t s 2 , 4 a n d 5. In all c a s e s , i n c r e a s i n g the c o p p e r concentration i n c r e a s e d the thiosulfate d e g r a d a t i o n rate a s e x p e c t e d , s i n c e thiosulfate d e g r a d a t i o n to both tetrathionate a n d trithionate h a s a rate d e p e n d e n t o n the c o p p e r c o n c e n t r a t i o n . A c c o r d i n g to the m o d e l , neither trithionate d e g r a d a t i o n n o r tetrathionate d e g r a d a t i o n a r e affected b y c o p p e r , s o the overall rate of formation of t h e s e s p e c i e s i n c r e a s e d with a n i n c r e a s e in c o p p e r c o n c e n t r a t i o n .  T h e c o n c e n t r a t i o n s of thiosulfate a n d trithionate after 2 4 h o u r s , a s well a s the m o l a r ratio of trithionate to thiosulfate a r e s h o w n in F i g u r e s 7.27, 7.28 a n d 7 . 2 9 for the three m o d e l scenarios  corresponding  concentration  w a s found  to T e s t s 2 , 4 a n d 5 respectively. to vary  much  The  m o r e with the p H than  tetrathionate  with the c o p p e r  c o n c e n t r a t i o n , a n d s i n c e the tetrathionate c o n c e n t r a t i o n s w e r e g e n e r a l l y m u c h lower than the other s p e c i e s , they a r e not s h o w n in this e v a l u a t i o n . In all c a s e s , the trithionate to thiosulfate ratio i n c r e a s e d with a n i n c r e a s e in c o p p e r c o n c e n t r a t i o n , with the extent of this  i n c r e a s e i n c r e a s i n g with c o p p e r concentration  a s well.  Limiting  the c o p p e r  c o n c e n t r a t i o n is e s s e n t i a l to limit thiosulfate d e g r a d a t i o n , but s i n c e c o p p e r is required to facilitate  gold l e a c h i n g , the s y s t e m n e e d s to b e o p t i m i s e d for both l e a c h i n g a n d  thiosulfate d e g r a d a t i o n .  164  o CO  Q)  o' O Q)  O cn c_ —t,  — I »0  20  40  60  80  120  100  140  CD  C o p p e r (II) (mg/l) - • — T h i o s u l f a t e after 24 hrs (M) -A—Trithionate after 24 hrs (M) -e—Trithionate to thiosulfate molar ratio F i g u r e 7.27 : M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 hours at varying c o p p e r c o n c e n t r a t i o n s •  M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k x, b k  •  R 4  R 1  R2  - 8 x, k  - 5  R3  - 0.05 x.  Initial c o n d i t i o n s : 0.2 M S 0 " , 0.4 M N H , p H 10, 2 5 °C, s e a l e d v e s s e l with 0 2  2  3  3  head space. •  - 15 x, k  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  165  2  in  0  20  40  60  80  100  120  140  C o p p e r (II) (mg/l) —•^Thiosulfate  after 24 hrs (M)  —A— Trithionate after 24 hrs (M) —e—Trithionate to thiosulfate molar ratio  F i g u r e 7.28 : M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 hours at varying c o p p e r c o n c e n t r a t i o n s •  M o d e l p a r a m e t e r s adjusted by the following factors: a k i - 1x, k R  x, b k •  R 4  - 10 x, k  - 5  R3  -0.10x.  Initial conditions: 0.2 M S 0 " , 0.4 M N H , p H 9, 2 5 °C, s e a l e d v e s s e l with 0 2  2  3  3  head space. •  R2  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  166  2  in  ro c o  0.15  — i g o 2 CD c  ro ca o o  0.05  40  20  60  C o p p e r (II) (mg/l) - • — T h i o s u l f a t e after 24 hrs (M) - A — Trithionate after 24 hrs (M) -e—Trithionate to thiosulfate molar ratio  F i g u r e 7.29 : M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 24 h o u r s at varying c o p p e r concentrations •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 1  - 12 x, k  R 2  - 8 x, k  - 5  R3  - 0 . 1 7 x.  R 4  Initial conditions: 0.2 M S 0 ~ , 0.3 M N H , p H 10, 2 5 °C, s e a l e d v e s s e l with 0 2  2  3  3  2  in  head space. •  7.7.2  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  Total Ammonia Concentration  T h e effect of the a m m o n i a concentration o n the thiosulfate a n d trithionate c o n c e n t r a t i o n s m o d e l l e d after 2 4 hours is s h o w n in F i g u r e s 7.30, 7.31 a n d 7.32 for the three m o d e l scenarios.  While  there  are  quantitative  differences  depending  on  which  model  p a r a m e t e r s w e r e u s e d , in all c a s e s the m o d e l s h o w s that it is beneficial to i n c r e a s e the a m m o n i a concentration to limit thiosulfate d e g r a d a t i o n a n d the ratio of trithionate thiosulfate.  to  It is k n o w n that free c o p p e r reacts very rapidly with thiosulfate a n d that  a m m o n i a retards this reaction a s e x p r e s s e d in the rate e q u a t i o n for R e a c t i o n R1 in E q u a t i o n 7.3.  T h e optimal a m m o n i a concentration will d e p e n d o n the other solution  c o n d i t i o n s ( c o p p e r concentration a n d pH).  It s h o u l d b e noted that the rate e q u a t i o n for  R e a c t i o n R 4 (thiosulfate d e g r a d a t i o n to trithionate) w a s derived u n d e r conditions of  167  a b o u t 0.2 to 0.6 M a m m o n i a . A t higher a m m o n i a c o n c e n t r a t i o n s , the rate of thiosulfate d e g r a d a t i o n to trithionate w a s d e p e n d e n t o n a n i n v e r s e of the a m m o n i a c o n c e n t r a t i o n (see F i g u r e 7.2), s o the trends noted in F i g u r e 7.30, 7.31 a n d 7.32 w o u l d b e e v e n m o r e pronounced.  0  0.5  1  1.5  2  A m m o n i a (M) —•—Thiosulfate after 24 hrs (M) —A— Trithionate after 24 hrs (M) —e—Trithionate to thiosulfate molar ratio F i g u r e 7.30: M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 hours at varying ammonia concentrations •  M o d e l p a r a m e t e r s a d j u s t e d by the following factors: a k x, b k  •  R 4  R 1  R 2  - 8 x, k  R3  - 5  - 0.05 x.  Initial c o n d i t i o n s : 0.2 M S 0 " , 3 0 mg/l C u , p H 10, 2 5 °C, s e a l e d v e s s e l with 0 2  2  2 +  3  in h e a d s p a c e . •  - 15 x, k  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  168  2  ro c o c o o o  fS  0.1  3 c ro CD O  g  c o o  0.4  0.6  A m m o n i a (M) - • — T h i o s u l f a t e after 24 hrs (M)  -tr— Trithionate after 24 hrs (M) -©—Trithionate to thiosulfate molar ratio  F i g u r e 7.31 M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 hours at varying a m m o n i a concentrations •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  R 1  R2  - 10 x, k  R3  - 5  -0.10x.  Initial conditions: 0.2 M S 0 ~ , 3 0 mg/l C u , p H 9, 2 5 °C, s e a l e d v e s s e l with 0 2  2  2 +  3  in h e a d s p a c e . •  - 1 x, k  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  169  2  0  0.5  1  1.5  A m m o n i a (M) — • — T h i o s u l f a t e after 24 hrs (M) —*—Trithionate after 24 hrs (M) —©—Trithionate to thiosulfate molar ratio  F i g u r e 7.32: M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 hours at varying a m m o n i a concentrations •  M o d e l p a r a m e t e r s adjusted by the following factors: a k i - 12 x, k R  x, b k •  7.7.3  - 8 x, k  R3  - 5  - 0.17 x.  Initial conditions: 0.2 M S 0 " , 100 mg/l C u , p H 10, 2 5 °C, s e a l e d v e s s e l with 2  2  0 •  R 4  R2  2  2 +  3  in h e a d s p a c e .  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  pH  T h e effect of the p H o n the thiosulfate, trithionate  a n d tetrathionate  concentrations  m o d e l l e d after 2 4 hours is s h o w n in F i g u r e s 7.33 a n d 7.34 for two m o d e l s c e n a r i o s , c o r r e s p o n d i n g to T e s t s 2 a n d 4 respectively.  D a t a is not s h o w n c o r r e s p o n d i n g to the  T e s t 5 s c e n a r i o s i n c e at p H v a l u e s less than 10, all the thiosulfate w a s predicted to h a v e d e g r a d e d in l e s s than 2 4 h o u r s .  T h e m a i n impact of c h a n g i n g p H w a s to c h a n g e the  tetrathionate c o n c e n t r a t i o n , with greater tetrathionate c o n c e n t r a t i o n s being r e a c h e d at lower p H v a l u e s . It w a s s h o w n that for either s c e n a r i o , the trithionate concentration w a s not affected m u c h by a c h a n g e in p H , but the overall polythionate to thiosulfate ratio w a s h i g h e r at lower p H d u e to the p r e s e n c e of tetrathionate.  T h e fact that the m o d e l g a v e a n  i n c r e a s e d thiosulfate d e g r a d a t i o n rate at lower p H is d u e mainly to the c h a n g e s in free  170  a m m o n i a concentration with p H for a given total a m m o n i a c o n c e n t r a t i o n . T h e variability in the best-fit m o d e l p a r a m e t e r s for T e s t 2 a n d for T e s t 4 (at different p H v a l u e s ) m a y be d u e to the w a y in w h i c h a m m o n i a is t a k e n into c o n s i d e r a t i o n in the rate e q u a t i o n s or b e c a u s e p H is not directly included in the rate e q u a t i o n s for thiosulfate d e g r a d a t i o n . T h i s highlights a s h o r t c o m i n g in the m o d e l .  8  8.5  9  9.5  10.5  10  11  11.5  PH * — T h i o s u l f a t e after 24 hrs (M) •A—Trithionate after 24 hrs (M) •m—Tetrathionate after 24 hrs (M) ^ — (Trithionate + tetrathionate) to thiosulfate molar ratio  F i g u r e 7.33: M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 hours at varying  pJH •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  - 15 x, k  R2  - 8 x, k  R3  - 0.05 x.  R 4  Initial conditions: 0.2 M S 0 ~ , 3 0 mg/l C u , 0.4 M N H , 2 5 °C, s e a l e d v e s s e l 2  2  with 0 •  R 1  2  2 +  3  3  in h e a d s p a c e .  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  171  - 5  PH -Thiosulfate after 24 hrs (M) -Trithionate after 24 hrs (M) -Tetrathionate after 2 4 hrs (M) - (Trithionate + tetrathionate) to thiosulfate m o l a r ratio  F i g u r e 7.34: M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 hours at varying pH •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  7.7.4  - 1 x, k  R 2  - 10 x, k  R 3  - 5  - 0 . 1 0 x.  R 4  Initial conditions: 0.2 M S 0 " , 3 0 mg/l C u , 0.4 M N H , 2 5 °C, s e a l e d v e s s e l 2  2  with 0 •  R 1  2  2 +  3  3  in h e a d s p a c e .  A s s u m e d that thiosulfate reacts v i a reaction R1 a n d R 4 .  Dissolved Oxygen  A c c o r d i n g to the m o d e l , only the d e g r a d a t i o n of thiosulfate to trithionate is affected by the d i s s o l v e d o x y g e n c o n c e n t r a t i o n . A s s e e n in T a b l e 7.4, this reaction s e e m e d to o c c u r to  the greatest  concentration).  extent  under  conditions  corresponding  to T e s t  5  (high  copper  T h e m o d e l w a s tested using the best fit conditions for this test varying  the d i s s o l v e d o x y g e n c o n c e n t r a t i o n .  W i t h 2 0 mg/l d i s s o l v e d o x y g e n , 0.2 M thiosulfate  d e g r a d e d c o m p l e t e l y within 2 2 h o u r s , producing 0 . 1 1 8 M trithionate.  With 2 mg/l  d i s s o l v e d o x y g e n , 0.2 M thiosulfate took 2 5 hours to c o m p l e t e l y d e g r a d e , p r o d u c i n g 0 . 1 1 3 M trithionate.  T h e level of d i s s o l v e d o x y g e n did not lead to a significant difference  in the time t a k e n for c o m p l e t e thiosulfate d e g r a d a t i o n nor the solution s p e c i a t i o n . H o w e v e r , it is p o s s i b l e that the d i s s o l v e d o x y g e n concentration c o u l d h a v e h a d a n effect  172  o n other r e a c t i o n s , particularly the d e g r a d a t i o n of thiosulfate to tetrathionate, but this w a s not t a k e n into a c c o u n t in the m o d e l .  7.7.5  Thiosulfate Concentration  T h e g r a p h s in F i g u r e 7 . 3 5 , 7 . 3 6 a n d 7 . 3 7 s h o w h o w the c o n c e n t r a t i o n s of thiosulfate a n d trithionate at 2 4 hours a r e affected by the initial thiosulfate c o n c e n t r a t i o n , a n d the effect o n the ratio of trithionate to thiosulfate after 2 4 h o u r s , for the three different scenarios.  model  F r o m t h e s e g r a p h s it is c l e a r that if the initial thiosulfate concentration is too  low, the relative a m o u n t of trithionate f o r m e d is very large.  A t higher initial thiosulfate  c o n c e n t r a t i o n s , e v e n t h o u g h the m a g n i t u d e of the trithionate c o n c e n t r a t i o n f o r m e d is higher, s o is the remaining thiosulfate concentration.  0.8 O  co  0.6  2  5'  c  o  0.4  o  8 0.2  5'  E  CO  3-  9» CD  0 0  0.2  0.4  0.6  0.8  Initial thiosulfate (M) -Thiosulfate cone after 24 hrs (M) -Trithionate cone after 24 hrs (M) - Ratio trithionate : thiosulfate  F i g u r e 7 . 3 5 : M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 h o u r s at varying initial thiosulfate c o n c e n t r a t i o n s •  M o d e l p a r a m e t e r s adjusted by the following factors: a k i - 15 x, k R  x, b k •  R 4  - 8 x, k  R 3  - 5  - 0.05 x.  Initial c o n d i t i o n s : 0.4 M N H , 3 0 mg/l C u , p H 10, 2 5 °C, s e a l e d v e s s e l with 0 2 +  3  head space. •  R 2  A s s u m e d that thiosulfate reacts v i a reaction R1 a n d R 4 .  173  2  in  0.8  r 1  n  Initial thiosulfate (M) — • — T h i o s u l f a t e c o n e after 24 hrs (M) — A — T r i t h i o n a t e c o n e after 24 hrs (M) —Q—Ratio trithionate : thiosulfate  F i g u r e 7.36 : M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 h o u r s at varying initial thiosulfate c o n c e n t r a t i o n s •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  R 1  R 2  - 10 x, k  R3  - 5  -0.10x.  Initial conditions: 0.4 M N H , 3 0 mg/l C u , p H 9, 2 5 °C, s e a l e d v e s s e l with 0 2 +  3  head space. •  - 1 x, k  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  174  2  in  F i g u r e 7.37 : M o d e l l e d thiosulfate a n d trithionate c o n c e n t r a t i o n s after 2 4 hours at varying initial thiosulfate c o n c e n t r a t i o n s •  M o d e l p a r a m e t e r s adjusted by the following factors: a k i - 12 x, k R  x, b k •  R 4  R 2  - 8 x, k  R3  - 5  - 0.17 x.  Initial conditions: 0.3 M N H , 100 mg/l C u , p H 10, 2 5 °C, s e a l e d v e s s e l with 0 2 +  3  2  in h e a d s p a c e . •  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  T h i s effect is e n h a n c e d in F i g u r e 7.37, w h i c h g i v e s the m o d e l s c e n a r i o c o r r e s p o n d i n g to the high c o p p e r situation of T e s t 5.  In this c a s e , the reaction of thiosulfate d e g r a d a t i o n  to trithionate plays a significant role, a n d the rate e q u a t i o n for this reaction involves a n i n v e r s e d e p e n d e n c y o n the thiosulfate c o n c e n t r a t i o n .  L o w e r thiosulfate c o n c e n t r a t i o n s  will thus give m o r e rapid d e g r a d a t i o n after longer t i m e s , a s w a s s e e n in F i g u r e 7.24.  The  effect  of  recycling the  solution with a n d without  c o n c e n t r a t i o n w a s investigated using the m o d e l .  replenishing the  thiosulfate  T h e g r a p h in F i g u r e 7.38 s h o w s the  m o d e l output o v e r 7 d a y s , a s s u m i n g recycling of the l e a c h solution with no adjustment to the reagent c o n c e n t r a t i o n s . C o n d i t i o n s a n d m o d e l best-fit p a r a m e t e r s c o r r e s p o n d i n g to T e s t 2 w e r e u s e d , a c c o r d i n g to T a b l e 7.4. In F i g u r e 7.39, the effects of replenishing the  175  thiosulfate concentration to its initial concentration e v e r y 2 4 h o u r s are s h o w n .  The  findings are s u m m a r i s e d in T a b l e 7.6, a l o n g with a c o m p a r i s o n to a situation with no recycling w h e r e it w a s a s s u m e d that fresh l e a c h solution w a s u s e d e v e r y 2 4 hours.  F i g u r e 7.38 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate degradation over 7 days •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  - 15 x, k  R2  - 8 x, k  R3  - 0 . 0 5 x.  Initial conditions: 0.2 M S 0 " , 0.4 M N H , 30 mg/l C u , p H 10, 2 5 °C, s e a l e d 2  2  v e s s e l with 0 •  R 1  2  2 +  3  3  in h e a d s p a c e .  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  176  - 5  0.20  0.20  O r—t-  — i CO zr  CD  c  0.10  0.10 o  CO  TD  CD O CD' CO  CNI  CO  -Thiosulfate - Tetrathionate I  0.00  0.00 K~-—'— 24  48  72  96  120  144  168  - Trithionate  T i m e (hrs)  Sulfate  F i g u r e 7.39 : M o d e l l e d output for sulfur o x y a n i o n s p e c i a t i o n during thiosulfate d e g r a d a t i o n with thiosulfate r e p l e n i s h e d e v e r y 2 4 hours •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  - 15 x, k  R2  - 8 x, k  R3  - 0.05 x.  Initial conditions: 0.2 M S 0 2  v e s s e l with 0 •  R 1  2  2 3  ' , 0.4 M N H , 30 mg/l C u , p H 10, 2 5 °C, s e a l e d 2 +  3  in h e a d s p a c e .  A s s u m e d that thiosulfate reacts via reaction R1 a n d R 4 .  177  - 5  T a b l e 7.6 : Effect of recycling o n thiosulfate c o n s u m p t i o n a n d solution s p e c i a t i o n (Initial conditions 0.2 M S c V ' . 3 0 mg/l C u , 0.4 M N H , . 2 5 °C. p H 10, with m o d e l p a r a m e t e r s ?  set a s in T a b l e 7.2 for T e s t 2) Total S 0 2  Condition  2 3  S o l u t i o n c o n c e n t r a t i o n after 7  "  d a y s (M)  consumed "  so -  S 0  "  SCV"  0.357  0.149  0.002  0.0287  0.006  0.141  0.059  0.002  0.0503  0.122  0.224  0.177  0.002  0.0897  0.i74  (M)  S 0 2  2 3  2  4  6  3  2 6  N o r e c y c l e - 7 x 2 4 hr l e a c h e s , e a c h with  fresh  thiosulfate-only  leach  solution Recycle  -  7 day  r e c y c l e with  no  additional reagent addition R e c y c l e with r e a g e n t r e p l e n i s h m e n t 7  days  recycle  concentration  with  thiosulfate  replenished every  24  hrs  T h e total thiosulfate c o n s u m p t i o n w a s the highest w h e r e there w a s n o solution r e c y c l e . H o w e v e r , the thiosulfate to polythionate ratio in the solution after e a c h 2 4 hour l e a c h w a s the highest, s o this s c e n a r i o could b e f a v o u r e d in situations w h e r e resin r e c o v e r y of gold is u s e d a n d the polythionate concentration is ideally kept a s low a s p o s s i b l e . S o l u t i o n r e c y c l e with r e p l e n i s h m e n t of the thiosulfate reagent e v e r y 2 4 h o u r s g a v e higher thiosulfate c o n s u m p t i o n a n d higher trithionate concentration in the final solution after  7 d a y s recycling than w h e r e  replenishment.  the  solution w a s  r e c y c l e d with  no  thiosulfate  H o w e v e r , the final ratio of polythionates to thiosulfate w a s l o w e r than  w h e r e n o reagent addition o c c u r r e d .  Similarly, the thiosulfate c o n s u m p t i o n for a higher c o p p e r s c e n a r i o c o r r e s p o n d i n g to T e s t 5 with a n d without recycling is s h o w n in T a b l e 7.7.  In this c a s e , the thiosulfate reagent  w a s c o m p l e t e l y d e p l e t e d after 2 4 hours, s o only two situations w e r e c o n s i d e r e d . In the first, it w a s a s s u m e d that there w a s no recycling a n d that f r e s h l e a c h solution w a s u s e d every 24 hours.  In the s e c o n d c a s e it w a s a s s u m e d that the thiosulfate reagent w a s  r e p l e n i s h e d to its initial concentration e v e r y 2 4 h o u r s .  178  T a b l e 7.7 : Effect of recycling o n thiosulfate c o n s u m p t i o n a n d solution s p e c i a t i o n (Initial c o n d i t i o n s 0.2 M S ? Q ' . 100 mg/l C u , 0.3 M N H . 2 5 °C. p H 10. with m o d e l p a r a m e t e r s 2  3  3  set a s in T a b l e 7.2 for T e s t 5) Total S 0 2  Condition  Solution concentration after 7  2 _ 3  consumed  days(M)  (M)  so -  so -  so -  S0  1.4  0  0.006  0.1104  0.019  0.81  0.117  0.007  0.3297  0.624  2  2  3  2  4  6  2  3  6  2 4  '  N o r e c y c l e - 7 x 2 4 hr l e a c h e s , e a c h with  fresh  thiosulfate-only  leach  solution R e c y c l e with r e a g e n t r e p l e n i s h m e n t 7  days  recycle  concentration  with  thiosulfate  replenished every  24  hrs  In this c a s e , the thiosulfate  c o n s u m p t i o n w a s a g a i n d e c r e a s e d by recycling with  r e p l e n i s h m e n t of thiosulfate c o m p a r e d with no recycling at all.  T h e effect of recycling on gold l e a c h i n g n e e d s to be t a k e n into c o n s i d e r a t i o n a s well.  179  CONCLUSIONS  8.  A review of the literature s h o w e d that while there is a significant a m o u n t of literature o n gold l e a c h i n g in a m m o n i a c a l thiosulfate solutions, the topic of thiosulfate d e g r a d a t i o n during l e a c h i n g h a s not b e e n given the attention it d e s e r v e s , e s p e c i a l l y s i n c e thiosulfate d e g r a d a t i o n is o n e of the major c o n c e r n s in the thiosulfate l e a c h i n g p r o c e s s .  Where  thiosulfate d e g r a d a t i o n h a s b e e n d i s c u s s e d , often the reagent c o n s u m p t i o n is g i v e n without reference to w h i c h d e g r a d a t i o n products h a v e b e e n f o r m e d , or their quantities. T h e two m o s t c o m m o n m e t a s t a b l e thiosulfate d e g r a d a t i o n products a r e trithionate a n d tetrathionate, with trithionate being particularly persistent in gold l e a c h solutions. It w a s found in the review that little w a s k n o w n about this s p e c i e s , particularly in the context of gold l e a c h i n g .  T h e r e f o r e , in this work, the reactions of trithionate in solution w e r e  investigated experimentally, a n d the kinetics of trithionate d e g r a d a t i o n w e r e d e r i v e d . T h e rate equation derived w a s u s e d in a m o d e l of the overall thiosulfate d e g r a d a t i o n system.  T h e reaction stoichiometry of trithionate d e g r a d a t i o n in alkaline solutions w a s found to m a t c h that of E q u a t i o n 8.1.  S 0 3  2 6  " + 20H-  Based  o n the  S 0 2  2 3  " + S0 " + H 0  [8.1]  2  4  quantitative  2  experimental  o b s e r v a t i o n s , the  rate  of d e g r a d a t i o n  of  trithionate in a q u e o u s a m m o n i a c a l solutions could be e x p r e s s e d by E q u a t i o n 8.2.  -d[S 0 "]/dt = (k [NH ] + k [NH ] + M O H ] + k )[S O -] 2  3  6  w h e r e at 4 0 °C  +  3  [8.2]  2  4  2  3  0  k = 0.012 h"  3  6  1  0  k, = 0.74 M" .h1  1  k = 0 . 0 0 4 9 M" .h1  1  2  k = 0.01 iVr .lY 1  1  3  T h e d e g r a d a t i o n of trithionate  in water, r e p r e s e n t e d by k  a g r e e m e n t with that found by others.  0  in E q u a t i o n 8.2, w a s in  A n i n c r e a s e in hydroxide concentration i n c r e a s e d  the trithionate d e g r a d a t i o n rate.  180  T h e p r e s e n c e of a m m o n i a w a s f o u n d to i n c r e a s e the rate of trithionate d e g r a d a t i o n to a s m a l l extent.  It h a s b e e n s u g g e s t e d previously that in the p r e s e n c e of a m m o n i a ,  s u l f a m a t e f o r m s instead of sulfate a s o n e of the reaction p r o d u c t s .  W h i l e this h a s not  b e e n p r o v e n either by p r e v i o u s r e s e a r c h e r s u n d e r the conditions relevant h e r e , or in this work, it is p o s s i b l e that e v e n if a m m o n i a itself d o e s not react to form s u l f a m a t e , it could still interact with trithionate  a n d facilitate the latter's d e g r a d a t i o n by hydroxide, for  example.  Potassium  and  degradation.  ammonium  ions  were  found  to  i n c r e a s e the  rate  of  trithionate  T h e similar effect of t h e s e ions w a s likely d u e to their similar s i z e a n d  e q u a l c h a r g e . T h i s effect of positive ions o n the rate implied the formation of a positive c o m p l e x b e t w e e n the positive ions a n d trithionate,  p o s s i b l y of the form  NH4S3CV, w h i c h altered the electronic properties of the trithionate  KS 0 " 3  6  or  to e n h a n c e its  d e g r a d a t i o n . S u c h ionic c o m p l e x e s h a v e b e e n reported previously for thiosulfate.  T h e effect of p H (hydroxide ions) w a s only significant at high p H v a l u e s w h e r e the h y d r o x i d e ion c o n c e n t r a t i o n w a s high. H o w e v e r , the effects of a m m o n i a a n d a m m o n i u m i o n s g a v e a n a p p a r e n t p H effect. ammonium  c o n c e n t r a t i o n , the  A s the p H i n c r e a s e d , for a g i v e n total a m m o n i a plus  rate of trithionate  d e g r a d a t i o n d e c r e a s e d slightly  to  a r o u n d p H 9, then d e c r e a s e d faster to a r o u n d p H 10 w h e r e the rate r e a c h e d a m i n i m u m . U p o n further i n c r e a s e of the p H the rate i n c r e a s e d rapidly, d u e to the i n c r e a s e in both hydroxide and a m m o n i a concentration.  T h i o s u l f a t e w a s f o u n d to affect the trithionate d e p e n d i n g o n the a m m o n i u m c o n c e n t r a t i o n .  degradation  rate in different  ways,  T h e p r e s e n c e of lower c o n c e n t r a t i o n s of  thiosulfate c a t a l y s e d the reaction while e x c e s s thiosulfate inhibited it.  T h e a m o u n t of  thiosulfate  ammonium  required  concentration. cations.  to  see these  effects  was  dependent  on  the  ion  It is k n o w n that thiosulfate c a n form c o m p l e x ions with m o n o v a l e n t  Interaction of thiosulfate with trithionate at the sulfenyl sulfur a t o m of trithionate  is likely to c a u s e the catalytic effect o n trithionate  d e g r a d a t i o n , a s thiosulfate  and  trithionate a r e k n o w n to u n d e r g o a n e x c h a n g e reaction, involving b o n d breaking a n d reforming.  H o w e v e r , too high a thiosulfate concentration m a y sterically hinder the  181  a s s o c i a t i o n of trithionate with other s p e c i e s like hydroxide a n d a m m o n i a , giving the o b s e r v e d inhibitory effect.  Limiting the a m o u n t of o x y g e n in solution h a d no influence o n the trithionate d e g r a d a t i o n rate. T h i s is to b e e x p e c t e d b a s e d o n the reaction stoichiometry of E q u a t i o n 8.1.  C u p r i c c o p p e r w a s not f o u n d to h a v e a n y significant effect o n the rate of trithionate d e g r a d a t i o n u n d e r the conditions tested.  It is p o s s i b l e that there could b e a n effect  noted u n d e r a different e x p e r i m e n t a l r e g i m e .  The  activation  e n e r g y for the trithionate d e g r a d a t i o n  reaction  in the  p r e s e n c e of  a m m o n i a a n d a m m o n i u m w a s f o u n d to be about 7 5 k J / m o l b e t w e e n 2 5 a n d 4 0 °C.  A m o d e l w a s set up to integrate the o b s e r v e d trithionate d e g r a d a t i o n rate e q u a t i o n ( E q u a t i o n 8.2) into the overall thiosulfate d e g r a d a t i o n s c h e m e in gold l e a c h i n g s y s t e m s . Rate  equations  based on  e x p e r i m e n t a l findings  literature  reported  findings  in this t h e s i s .  were  used  in  conjunction  with  T h e model was evaluated  the  against  e x p e r i m e n t a l results for thiosulfate d e g r a d a t i o n a n d s u b s e q u e n t solution s p e c i a t i o n in the a b s e n c e of o r e s .  T h e m o d e l p a r a m e t e r s w e r e adjusted to obtain a best fit to the e x p e r i m e n t a l d a t a . It w a s f o u n d that the best-fit p a r a m e t e r s v a r i e d with the e x p e r i m e n t a l c o n d i t i o n s , indicating i n a d e q u a c i e s not only in the initial v a l u e s of the m o d e l p a r a m e t e r s but in the form of the rate e q u a t i o n s u s e d to set up the m o d e l .  T h e m a i n objective of the m o d e l w a s to i m p r o v e the u n d e r s t a n d i n g of the sensitivity of the  solution  speciation  to  the  various  reaction  pathways  involved,  to  s h o r t c o m i n g s in o u r u n d e r s t a n d i n g of s u l p h u r o x y a n i o n s p e c i a t i o n during  highlight thiosulfate  d e g r a d a t i o n . T h e m o d e l in its s t a n d a r d form w a s not a b l e to quantitatively d e s c r i b e the system.  T h e g r a p h in F i g u r e 8.1 s h o w s typical qualitative output for the m o d e l m a t c h e d by e x p e r i m e n t a l d a t a . T h e thiosulfate concentration rapidly d e c r e a s e s a n d there is a n initial i n c r e a s e in tetrathionate c o n c e n t r a t i o n .  H o w e v e r at the alkaline p H v a l u e s u s e d in gold  182  l e a c h i n g , the  tetrathionate  tetrathionate c o n c e n t r a t i o n .  rapidly d e g r a d e s , resulting  in a s l o w d e c r e a s e in  T h i s results in the formation of trithionate.  the  Trithionate is  a l s o f o r m e d directly from thiosulfate d e g r a d a t i o n .  0.06  Thiosulfate  Trithionate  Tetrathionate  F i g u r e 8.1 : T y p i c a l m o d e l output a g a i n s t e x p e r i m e n t a l data •  M o d e l p a r a m e t e r s adjusted by the following factors: a k x, b k  •  R 4  R 1  - 15 x, k  R2  - 8 x, k  R3  - 5  - 0.05 x.  Initial conditions: 0.2 M S 0 " , 0.42 M N H ( c l o s e d s y m b o l s ) or 0.38 M N H 2  2  3  3  ( o p e n s y m b o l s ) , 3 2 mg/l C u , p H 10, 2 5 °C, s e a l e d v e s s e l with 0 2 +  2  3  in h e a d s p a c e  T h e m o d e l h a d the following s h o r t c o m i n g s :  Tetrathionate concentration profile:  T h e tetrathionate concentration predicted by the  m o d e l w a s too high. Increasing the rate of tetrathionate d e g r a d a t i o n by a c o n s t a n t factor a c r o s s a r a n g e of e x p e r i m e n t a l conditions g a v e a m u c h m o r e satisfactory fit, but the m o d e l w a s not a b l e to predict the d e c r e a s e in tetrathionate concentration w h i c h o c c u r r e d  183  after its m a x i m u m concentration h a d b e e n r e a c h e d .  It is thought that the p r e s e n c e of  c o p p e r w a s r e s p o n s i b l e for this b e h a v i o u r a n d c o p p e r w a s not t a k e n into c o n s i d e r a t i o n in modelling tetrathionate d e g r a d a t i o n .  E v e n with t h e s e s h o r t c o m i n g s , given that the  tetrathionate concentration w a s v e r y low, it could be a d e q u a t e l y predicted by the m o d e l if the tetrathionate d e g r a d a t i o n rate w a s i n c r e a s e d by a factor of 8-10.  Trithionate concentration profile:  In g e n e r a l , the trithionate c o n c e n t r a t i o n s predicted by  the m o d e l at longer times w e r e too high.  Increasing the rate of trithionate d e g r a d a t i o n  by a c o n s t a n t factor for the r a n g e of e x p e r i m e n t a l conditions u s e d i m p r o v e d the fit.  The  rate of trithionate d e g r a d a t i o n m o d e l l e d in this work did not t a k e into a c c o u n t the effect of thiosulfate c o n c e n t r a t i o n , a s it w a s e x p e c t e d to b e negligible, a s d i s c u s s e d in C h a p t e r 5.  H o w e v e r , after longer times w h e r e less thiosulfate w a s a v a i l a b l e , conditions for  thiosulfate c a t a l y s i s of trithionate d e g r a d a t i o n could h a v e b e c o m e m o r e f a v o u r a b l e . E v e n i n c r e a s i n g the trithionate d e g r a d a t i o n rate by a c o n s t a n t factor did not give a perfect fit to the e x p e r i m e n t a l d a t a . T h i s is e x p e c t e d to b e d u e to i n a d e q u a c i e s in the prediction of trithionate formation from thiosulfate.  Thiosulfate concentration profile:  W h i l e the thiosulfate concentration profile c o u l d b e  predicted using the m o d e l , it w a s n e c e s s a r y to adjust the rates of thiosulfate d e g r a d a t i o n to both tetrathionate a n d trithionate, a n d the extent of t h e s e a d j u s t m e n t s v a r i e d with the e x p e r i m e n t a l conditions. T h e m o d e l l e d thiosulfate d e g r a d a t i o n h a d the m o s t significant effect o n the solution s p e c i a t i o n . But variation of the thiosulfate d e g r a d a t i o n rate with c o p p e r c o n c e n t r a t i o n a n d p H w e r e not a d e q u a t e l y a d d r e s s e d .  T h e m a i n limiting factor in the m o d e l is finding the best w a y to include the thiosulfate d e g r a d a t i o n reactions.  T h e rate e q u a t i o n s u s e d w e r e d e r i v e d from e x p e r i m e n t a l d a t a  that utilised non-reliable t e c h n i q u e s , a n d the d a t a w a s o p e n to interpretation to derive the kinetics. T h e r e is currently a significant a m o u n t of work b e i n g d o n e in this a r e a , a n d hopefully there will s o o n b e a better f u n d a m e n t a l u n d e r s t a n d i n g of this p r o c e s s . particular, the role of c u p r i c c o p p e r (in its v a r i o u s forms), d i s s o l v e d o x y g e n  In and  o r e s / m i n e r a l s n e e d s to be i n c l u d e d .  T h e r e a r e two w a y s to limit thiosulfate c o n s u m p t i o n - either thiosulfate d e g r a d a t i o n c a n b e inhibited, or thiosulfate c a n be r e g e n e r a t e d from its d e g r a d a t i o n products.  184  The  p r o b l e m with regeneration is that for c o m m e r c i a l r e a s o n s , g o l d m u s t first b e r e m o v e d f r o m the solution.  It is a l s o p o s s i b l e to r e g e n e r a t e part of the thiosulfate  from  tetrathionate a n d trithionate d e g r a d a t i o n , but part of the initial thiosulfate is then lost to sulfate. T h e f a v o u r e d route will vary d e p e n d i n g o n the extent of d e g r a d a t i o n u n d e r the l e a c h conditions u s e d , the e c o n o m i c s of the p r o c e s s a n d the p r o p o s e d p r o c e s s for gold r e c o v e r y from solution.  T h e c o m p l e t e r e m o v a l of trithionate e v e n at the e x p e n s e of  forming s o m e sulfate m a y b e g e n e r a l l y m o r e f a v o u r a b l e t h a n sacrificing gold l e a c h i n g to m i n i m i s e trithionate formation in the first p l a c e .  B a s e d o n the m o d e l output  u n d e r a n u m b e r of c o n d i t i o n s , d e c r e a s i n g the c u p r i c  c o n c e n t r a t i o n a n d i n c r e a s i n g the a m m o n i a concentration s h o u l d help to thiosulfate d e g r a d a t i o n .  minimise  Solution r e c y c l e c a n a l s o be u s e d to m i n i m i s e thiosulfate  d e g r a d a t i o n but often results in a build up of trithionate.  Limiting the reaction time w o u l d  a l s o be u s e f u l . If the a i m is to form the least a m o u n t of trithionate p o s s i b l e , it is better to e n c o u r a g e the d e g r a d a t i o n of thiosulfate to tetrathionate a n d allow for  tetrathionate  d e g r a d a t i o n to trithionate a n d to inhibit the direct d e g r a d a t i o n of thiosulfate to trithionate. T h e tetrathionate route g e n e r a t e s l e s s trithionate per m o l e of thiosulfate a n d a l s o the d e g r a d a t i o n of tetrathionate allows for the partial r e g e n e r a t i o n of thiosulfate. Limiting the a m o u n t of o x y g e n or i n c r e a s i n g the a m o u n t of initial thiosulfate m a y e n c o u r a g e this. B o t h d e g r a d a t i o n p a t h w a y s are directly related to the c o p p e r c o n c e n t r a t i o n , s o limiting the c o p p e r c o n c e n t r a t i o n w o u l d b e benefi ci al , a s long a s gold l e a c h i n g is not inhibited. A n i n c r e a s e in c u p r i c concentration l e a d s to m o r e rapid thiosulfate d e g r a d a t i o n a n d m o r e rapid formation of tetrathionate a n d trithionate.  A n i n c r e a s e in the a m m o n i a  c o n c e n t r a t i o n s l o w s the rate of thiosulfate d e g r a d a t i o n , a n d l e s s tetrathionate  and  trithionate are f o r m e d . A m m o n i a is k n o w n to c o m p l e x with c u p r i c c o p p e r , inhibiting the reaction b e t w e e n c u p r i c c o p p e r a n d thiosulfate.  C a r e f u l manipulation of the c u p r i c a n d  a m m o n i a c o n c e n t r a t i o n s will h a v e a significant impact o n the stability of thiosulfate.  A  d e c r e a s e in p H r e d u c e s the rate of tetrathionate d e g r a d a t i o n , in turn r e d u c i n g the formation rate of trithionate, but a l s o r e d u c i n g the regeneration rate of thiosulfate.  H o w e v e r , e v e n if l e a c h i n g conditions are adjusted to m i n i m i s e thiosulfate d e g r a d a t i o n , it is inevitable that trithionate will f o r m .  R a t h e r than minimising trithionate  formation,  a n o t h e r option c o u l d b e to r e m o v e trithionate from the l e a c h solutions by its d e g r a d a t i o n . H o w e v e r , c o m p a r e d to the other reactions i n v o l v e d , the d e g r a d a t i o n of trithionate is  185  e x t r e m e l y s l o w u n d e r typical l e a c h conditions.  A large i n c r e a s e in p H or a m m o n i a  c o n c e n t r a t i o n will i n c r e a s e the trithionate d e g r a d a t i o n rate, but probably not sufficiently to h a v e a n y major i m p a c t o n the solution s p e c i a t i o n . T h e effect of other c a t i o n s in the s y s t e m o n trithionate d e g r a d a t i o n are not k n o w n - it is p o s s i b l e that certain c a t i o n s will i n c r e a s e the d e g r a d a t i o n rate, while others m a y c a u s e precipitation of Similarly the  degradation  rate  may  be slowed down  by the  trithionate.  p r e s e n c e of  certain  c o m p o n e n t s (e.g. metallic c o p p e r ) . T h e effect of o r e s a n d of e l e m e n t s s o l u b i l i s e d from o r e s during gold l e a c h i n g o n the d e g r a d a t i o n rate of thiosulfate a n d the d e g r a d a t i o n p r o d u c t s n e e d s investigation. A m o r e suitable f o c u s for minimising trithionate in solution w o u l d b e to investigate w a y s to selectively r e m o v e trithionate from solution.  186  9.  RECOMMENDATIONS  T h i s investigation h a s led to a n i m p r o v e d u n d e r s t a n d i n g of the b e h a v i o u r of trithionate in gold l e a c h solutions a n d the m o d e l of the thiosulfate d e g r a d a t i o n s y s t e m is a first step in developing  a  useful  a s s e s s m e n t method  s p e c i a t i o n u n d e r gold l e a c h i n g conditions.  for  thiosulfate  degradation  and  solution  Further r e s e a r c h is required in the following  areas:  Trithionate degradation  •  T o confirm the effect of positive ions o n trithionate d e g r a d a t i o n , a larger r a n g e of ions s h o u l d b e t e s t e d , e . g . L i , C s , C a , M g , A u a n d A g . +  •  +  2 +  2 +  +  +  M o r e detail o n the effect of c o p p e r a n d particularly the v a r i o u s c o m p l e x e s of c o p p e r o n trithionate d e g r a d a t i o n is required.  •  T h e role of d i s s o l v e d o x y g e n s h o u l d b e investigated in the p r e s e n c e of c o p p e r .  •  T h e effect of g o l d , silver a n d o r e s o n trithionate d e g r a d a t i o n s h o u l d b e t e s t e d .  Modelling  •  T h e m o d e l output w a s most sensitive to the p a r a m e t e r s for the  thiosulfate  d e g r a d a t i o n rate. T o be a b l e to predict the rate of thiosulfate d e g r a d a t i o n a n d the reaction products, it is e s s e n t i a l to fully u n d e r s t a n d this s u b - p r o c e s s . T h e r e is currently a significant a m o u n t of work b e i n g carried out o n in this a r e a ( B r e u e r and  Jeffrey, v a r i o u s  references).  In  particular  the  role of  copper  (or  its  c o m p l e x e s ) a n d o x y g e n are c o n s i d e r e d important, a n d o n c e better u n d e r s t o o d , the effect of using h e t e r o g e n e o u s s y s t e m s a n d o r e s s h o u l d a l s o b e i n c l u d e d .  •  T h e effect of o r e s o n not only thiosulfate d e g r a d a t i o n but o n trithionate a n d tetrathionate d e g r a d a t i o n n e e d s to be included in the m o d e l . C e r t a i n ore/mineral t y p e s will likely h a v e a significant effect o n t h e s e s p e c i e s .  •  T h e sulfate c o n c e n t r a t i o n s predicted by the m o d e l s h o u l d b e validated a g a i n s t experimental data.  187  •  R e a s o n s for w h y the tetrathionate d e g r a d a t i o n rate a n d trithionate d e g r a d a t i o n rate predicted by the m o d e l a r e too low n e e d further investigation. In particular, the effect of v a r i o u s c o p p e r s p e c i e s a n d the p r e s e n c e of thiosulfate a r e e x p e c t e d to b e of i m p o r t a n c e .  Gold leaching  •  T h e sulfur d e g r a d a t i o n products s h o u l d a l w a y s b e d e t e r m i n e d during  gold  l e a c h i n g if w e a r e to b e a b l e to fully u n d e r s t a n d thiosulfate d e g r a d a t i o n .  •  To  m i n i m i s e thiosulfate  d e g r a d a t i o n , c o p p e r a n d o x y g e n required for  gold  oxidation s h o u l d b e m i n i m i s e d , a n d the a m m o n i a c o n c e n t r a t i o n s h o u l d  be  maximised,  be  optimised  without based on  inhibiting the  gold  leaching. Solution  specific process requirements  recycling to  should  either  minimise  thiosulfate d e g r a d a t i o n or m i n i m i s e the polythionate to thiosulfate ratio.  The  l e a c h i n g time s h o u l d be limited.  •  D e p e n d i n g o n the temperature d e p e n d e n c e of the gold l e a c h i n g kinetics, it m a y b e p o s s i b l e to s e l e c t a n o p t i m u m t e m p e r a t u r e to m a x i m i s e g o l d l e a c h i n g while m i n i m i s i n g thiosulfate d e g r a d a t i o n .  •  T h i s work h a s not a d d r e s s e d the u s e f u l n e s s of u s i n g other s p e c i e s (e.g. E D T A ) to inhibit the reaction b e t w e e n thiosulfate a n d c u p r i c c o p p e r . T h e r e is s c o p e for a n investigation of this type, but the effects of the additives o n g o l d l e a c h i n g a n d o n d o w n s t r e a m p r o c e s s i n g a n d w a s t e m a n a g e m e n t i s s u e s m u s t be i n c l u d e d .  •  W a y s to selectively r e m o v e trithionate (and tetrathionate)  from  gold  leach  solutions p o s s i b l y by high p H treatment, u s i n g resins a n d / o r bioreduction s h o u l d b e investigated further, a s t h e s e polythionates are a l w a y s likely to b e present.  188  REFERENCES  10.  A b b r u z z e s e C , Fornari P . , M a s s i d a R., V e g l i o F., Ubaldini S . (1995)  Thiosulphate  l e a c h i n g for gold hydrometallurgy', Hydrometallurgy, 3 9 , 2 6 5 - 2 7 6 .  Ames  D.P., Willard J . E .  (1951)  The  kinetics of the e x c h a n g e of sulfur  between  thiosulfate a n d sulfite', J o u r n a l o f t h e A m e r i c a n C h e m i c a l S o c i e t y , 7 3 , 1 6 4 - 1 7 2 .  A y l m o r e M . G . (2001)  'Treatment  of a refractory  g o l d - c o p p e r sulfide c o n c e n t r a t e  by  c o p p e r a m m o n i a c a l thiosulfate l e a c h i n g ' , M i n e r a l s E n g i n e e r i n g 14 (6), 6 1 5 - 6 3 7 .  A y l m o r e M . G , Muir D . M . (2001a) 'Thiosulfate l e a c h i n g of gold -  a review', M i n e r a l s  E n g i n e e r i n g , 14, 1 3 5 - 1 7 4 .  Aylmore  M . G . , Muir  D.M.  (2001b)  'Thermodynamic  a n a l y s i s of  gold  a m m o n i a c a l thiosulfate u s i n g E h / p H s p e c i a t i o n d i a g r a m s ' , M i n e r a l s a n d  leaching  by  Metallurgical  P r o c e s s i n g , 18 (4), 2 2 1 - 2 2 7 .  Berezowsky  R.M., Gormely  L . S . (1978)  ' R e c o v e r y of  precious  metals f r o m  metal  s u l f i d e s ' , U S patent 4 0 7 0 182, 24 J a n u a r y 1978.  B r e u e r P . L . , Jeffrey M.I. (2000) 'Thiosulphate l e a c h i n g kinetics of gold in the p r e s e n c e of c o p p e r a n d a m m o n i a ' , M i n e r a l s E n g i n e e r i n g , 13 (10-11), 1 0 7 1 - 1 0 8 1 .  B r e u e r P . L . , Jeffrey M.I., T a n E . H . K . , Bott A . W . (2002) ' T h e e l e c t r o c h e m i c a l  quartz  crystal m i c r o b a l a n c e a s a s e n s o r in the gold thiosulfate l e a c h i n g p r o c e s s ' , J o u r n a l of Applied Electrochemistry, 32, 1167-1174.  B r e u e r P . L . , Jeffrey M.I. (2003a) ' C o p p e r c a t a l y s e d oxidation of thiosulfate by o x y g e n in gold l e a c h solutions', M i n e r a l s E n g i n e e r i n g , 16, 2 1 - 3 0 .  189  B r e u e r P . L . , Jeffrey M.I. (2003b)  T h e reduction of c o p p e r (II) a n d the oxidation  of  thiosulfate a n d oxysulfur a n i o n s in gold l e a c h i n g solutions', Hydrometallurgy, 7 0 , 1 6 3 173.  B r e u e r P L , Jeffrey M l . (2003c) ' A review of the chemistry, e l e c t r o c h e m i s t r y a n d kinetics of the gold thiosulfate l e a c h i n g p r o c e s s ' , Hydrometallurgy 2 0 0 3 - Fifth Int C o n f , V o l . 1: L e a c h i n g a n d solution purification, 1 3 9 - 1 5 4 .  B r e u e r P . L . , Jeffrey M.I. (2003d) U n p u b l i s h e d work, p e r s o n a l c o m m u n i c a t i o n , J a n u a r y 2003.  B r e u e r P . L . , Jeffrey M.I. (2004) ' T h e effect of ionic strength a n d buffer c h o i c e o n the d e c o m p o s i t i o n of tetrathionate in alkaline solutions', H y d r o m e t a l l u r g y , 7 2 (3-4),  335-  338. B r e z o n i k P . L . (1994) ' C h e m i c a l Kinetics a n d P r o c e s s D y n a m i c s in A q u a t i c S y s t e m s ' , C R C Press.  Brodbelt J . S . , Liou C C . (1993) ' N e w frontiers in host-guest chemistry: the g a s p h a s e ' , P u r e a n d A p p l i e d C h e m i s t r y , 6 5 (3), 4 0 9 - 4 1 4 .  B y e r l e y J . J . , F o u d a S . A . , R e m p e l G . L . (1973a) 'Kinetics a n d m e c h a n i s m of the oxidation of thiosulphate ions by c o p p e r (II) ions in a q u e o u s a m m o n i a solution', J o u r n a l of the C h e m i c a l S o c i e t y , Dalton T r a n s a c t i o n s , 8 8 9 - 8 9 3 .  B y e r l e y J . J . , F o u d a S . A . , R e m p e l G . L . (1973b) ' T h e oxidation of thiosulfate in a q u e o u s a m m o n i a solution by c o p p e r (II) o x y g e n c o m p l e x e s ' , Inorganic a n d N u c l e a r C h e m i s t r y Letters, 9, 8 7 9 - 8 8 3 .  B y e r l e y J . J . , F o u d a S . A . , R e m p e l G . L . (1975)  'Activation  of  copper  (II)  ammine  c o m p l e x e s by m o l e c u l a r o x y g e n for the oxidation of thiosulphate ions', J o u r n a l of the C h e m i c a l S o c i e t y Dalton T r a n s a c t i o n s , 1329 - 1338.  C h a n d a M . , R e m p e l G . L . (1986) ' C u p r o u s oxide c a t a l y s e d air oxidation of thiosulfate a n d tetrathionate', A p p l i e d C a t a l y s i s , 2 3 , 1 0 1 - 1 1 0 .  190  C h a n d r a .1, Jeffrey M.I. (2003) ' C a n a thiosulfate l e a c h i n g p r o c e s s be d e v e l o p e d w h i c h d o e s not require c o p p e r a n d a m m o n i a ? ' , Hydrometallurgy 2 0 0 3 - Fifth Int C o n f , V o l . 1: L e a c h i n g a n d solution purification, 6 9 - 1 8 2 .  C h a n d r a I., Jeffrey M.I. (2004) ' A n e l e c t r o c h e m i c a l study of the effect of additives a n d electrolyte o n the dissolution of gold in thiosulfate solutions', Hydrometallurgy, 7 3 , 3 0 5 312.  C h u C . K . , B r e u e r P . L . , Jeffrey M.I. (2003) ' T h e impact of thiosulfate oxidation products o n the oxidation of gold in a m m o n i a thiosulfate solutions', M i n e r a l s E n g i n e e r i n g , 16, 2 6 5 271.  D e a n J . A . (1992) ' L a n g e ' s H a n d b o o k of C h e m i s t r y ' , 1 4  th  Edition, P u b l i s h e d M c G r a w - H i l l  Inc.  D e J o n g G . A . H . , H a z e u W . , B o s P . (1997) 'Polythionate d e g r a d a t i o n by tetrathionate h y d r o l a s e of T h i o b a c i l l u s f e r r o o x i d a n s ' , M i c r o b i o l o g y U K , 143 (2), 4 9 9 - 5 0 4 .  D e J o n g S . V . Z . (2004) " T h e effect of m i n e r a l o g y o n the a m m o n i a c a l  thiosulphate  extraction of g o l d " , P h . D . T h e s i s , University of M e l b o u r n e .  D h a w a l e S . W . (1993) 'Thiosulfate: a n interesting sulfur o x o a n i o n that is useful in both m e d i c i n e a n d industry-but is implicated in c o r r o s i o n ' , J o u r n a l of C h e m i c a l E d u c a t i o n . , 70, 12-14.  D r u s c h e l G . (2003) University of V e r m o n t , p e r s o n a l c o m m u n i c a t i o n .  E M J . (2004) ' G o l d p r o d u c e r s e n d o r s e c y a n i d e c o d e ' , E n g i n e e r i n g a n d M i n i n g J o u r n a l , July 2004, 38.  F a v a A . , B r e s a d o l a S . (1955) 'Kinetics of the catalytic r e a r r a n g e m e n t of tetrathionate', J o u r n a l of the A m e r i c a n C h e m i c a l S o c i e t y , 7 7 , 5 7 9 2 - 5 7 9 4 .  191  F a v a A . , P a j a r o G . (1954) ' C i n e t i q u e d e I'echange i s o t o p i q u e  thiosulfate-trithionate',  J o u r n a l of C h e m i c a l P h y s i c s , 5 1 , 5 9 4 - 5 9 9 .  F e n g D., V a n D e v e n t e r J . S . J . (2002a) ' T h e role of h e a v y m e t a l i o n s in g o l d dissolution in the a m m o n i a c a l thiosulfate s y s t e m ' , Hydrometallurgy, 6 4 , 2 3 1 - 2 4 6 .  F e n g D., V a n D e v e n t e r J . S . J . (2002b) ' L e a c h i n g b e h a v i o u r of s u l p h i d e s in a m m o n i a c a l thiosulfate s y s t e m s ' , H y d r o m e t a l l u r g y , 6 3 , 1 8 9 - 2 0 0 .  F l e m i n g C , W e l l s J . , T h o m a s K . G . (2000) ' P r o c e s s for r e c o v e r i n g g o l d f r o m thiosulfate l e a c h solutions a n d slurries with ion e x c h a n g e resin', U S P a t e n t 6 3 4 4 0 6 8 , 4 A p r i l .  F l e m i n g C . A . , M c M u l l e n J . , T h o m a s K . G . , W e l l s J . A . (2003) ' R e c e n t a d v a n c e s in the d e v e l o p m e n t of a n alternative to the cyanidation p r o c e s s : t h i o s u l p h a t e l e a c h i n g a n d resin in pulp', M i n e r a l s a n d Metallurgical P r o c e s s i n g , 20(1), 1-9.  Flett D . S . , Derry R., W i l s o n J . C . (1983) ' C h e m i c a l study of t h i o s u l p h a t e l e a c h i n g of silver s u l p h i d e ' , T r a n s a c t i o n of the Institution of Mining a n d Metallurgy, 9 2 , 2 1 6 - 2 2 3 .  F o s s O . , Kringlebotn I. (1961) ' D i s p l a c e m e n t of sulphite g r o u p s of polythionates  by  thiosulphate', A c t a C h e m i c a S c a n d i n a v i c a . 15 (7), 1 6 0 8 - 1 6 0 9 .  F o s s O . (1961) 'Thiosulfate c a t a l y s i s o n r e a r r a n g e m e n t s of higher p o l y t h i o n a t e s ' , A c t a C h e m i c a S c a n d i n a v i c a . , 15, 1 6 1 0 - 1 6 1 1 .  G e l v e s G . A . , A r i a s V . A . , P e d r a z a J . E . (1996) ' R e c o v e r i n g of refractory  gold  using  a m m o n i u m thiosulfate solutions', C l e a n T e c h n o l o g y for the M i n i n g Industry, e d . M A S a n c h e z , F V e r g a r a , S H C a s t r o , University of C o n c e p c i o n , C o n c e p c i o n , C h i l e , 4 7 7 - 4 8 7 .  G l a s s t o n e , H i c k l i n g . (1954) J o u r n a l of the C h e m i c a l S o c i e t y , 1 9 3 2 , p 2 3 4 5 , p 2 8 0 0 a n d 1933,  p 8 2 9 , cited in Partington J R , ' G e n e r a l a n d Inorganic C h e m i s t r y for University  Students', 2  n d  edition, M a c m i l l a n a n d C o Ltd., L o n d o n , p 7 2 6 .  192  H a n s f o r d G . S . , V a r g a s T. (2001) ' C h e m i c a l a n d e l e c t r o c h e m i c a l b a s i s of b i o l e a c h i n g p r o c e s s ' , Hydrometallurgy 5 9 (2-3), 1 3 5 - 1 4 5 .  Hemmati  M.,  Hendrix  J . L . , N e l s o n J . H . , Milosavljevic  E . B . (1989)  'Study  thiosulphate l e a c h i n g of gold f r o m c a r b o n a c e o u s ore a n d the quantitative  of  the  determination  of thiosulphate in the l e a c h e d solution', Extraction Metallurgy '89 S y m p o s i u m , Inst. M i n . Metall., L o n d o n U K , 6 6 5 - 6 7 8 .  H o f m a n - B a n g N. (1950) ' T h e i o d i n e - a z i d e reaction. V . T h e catalytic effect of trithionate ions a n d their d e c o m p o s i t i o n in a q u e o u s solution", A c t a C h e m i c a S c a n d i n a v i c a , 4 , 1 0 0 5 1013.  Horvath A . K . , N a g y p a l I. (2000)  'Kinetics a n d m e c h a n i s m of the reaction  between  h y p o c h l o r o u s a c i d a n d tetrathionate ion', International J o u r n a l of C h e m i c a l K i n e t i c s , 3 2 (7), 3 9 5 - 4 0 2 .  H u J . , G o n g Q . (1991)  'Substitute  sulfate for sulfite  during  extraction  of gold  by  thiosulfate solution'. R a n d o l G o l d F o r u m , C a i r n s , P u b . R a n d o l International Ltd, 1 2 6 - 1 3 2 .  Jeffrey M.I. (2001) 'Kinetic a s p e c t s of gold a n d silver l e a c h i n g in a m m o n i a - t h i o s u l p h a t e solutions', Hydrometallurgy, 6 0 , 7-16.  Ji J . , F l e m i n g C . A . , W e s t - S e l l s P . G . , H a c k l R . P . (2001) ' M e t h o d for thiosulfate l e a c h i n g of  precious  metal-containing  materials',  International  Patent  Application  PCT  WO  01/88212 A 2 , 22 November 2001.  J i J . , F l e m i n g C , W e s t - S e l l s P . G . , H a c k l R . P . (2003) ' A novel thiosulfate s y s t e m for leaching  g o l d ' , P r o c e e d i n g s Hydrometallurgy  2 0 0 3 , E d . C . A . Y o u n g et a l , T M S ,  Warrendale, p241.  Kelly  D.P., W o o d  A . P . (1994)  'Synthesis  and  determination  polythionates', M e t h o d s in E n z y m o l o g y , 2 4 3 , 4 7 5 - 5 0 1 .  193  of  thiosulfate  and  K e r l e y J . , B e r n a r d J . ( 1 9 8 1 ) ' R e c o v e r y of p r e c i o u s m e t a l s f r o m difficult o r e s ' , U S P a t e n t 4 269 622, 26 M a y 1981.  K o h T.  (1990)  'Analytical c h e m i s t r y  of  polythionates  a n d thiosulfate  -  a  review',  A n a l y t i c a l S c i e n c e s , 6, 3-14.  Kurtenacker A . , Mutschin A., Stastny  F . Z . (1935)  ' U b e r die  selbstersetzung  von  p o l y t h i o n a t l o s u n g e n ' , Zeitschrift fur A n o r g a n i s c h e und A l l g e m e i n e C h e m i e , 2 2 4 , 3 9 9 419.  L a m A . E . (2001) ' T h e thiosulfate l e a c h i n g of g o l d : introduction to p h a s e III',  Project  report, University of British C o l u m b i a , O c t o b e r 2 0 0 1 .  L a m A . E . (2002) ' T h e thiosulfate l e a c h i n g of g o l d :  A continuing investigation', Project  report, University of British C o l u m b i a , D e c e m b e r 2 0 0 2 .  L a n g h a n s J . W . , Lei K . P . V . , C a r n a h a n T . G . (1992) ' C o p p e r - c a t a l y s e d thiosulfate l e a c h i n g of low-grade gold o r e s ' , Hydrometallurgy, 2 9 , 1 9 1 - 2 0 3 .  Li J . , Miller J . D . , W a n R . Y . (1996) 'Important solution c h e m i s t r y factors that influence the c o p p e r - c a t a l y z e d a m m o n i u m thiosulfate l e a c h i n g of g o l d ' , 1 2 5  th  S M E Annual Meeting,  Phoenix, Arizona.  Liebovitch M . (2005) Sherrit M e t a l s , p e r s o n a l c o m m u n i c a t i o n .  Lyons  D.,  N i c k l e s s G . (1968)  'The  lower o x y - a c i d s of  sulfur'  in  Inorganic  Sulfur  Chemistry', ed. G Nickless, Elsevier Publishing C o . , 509-532.  M a r s d e n J . , H o u s e I. (1992) ' T h e C h e m i s t r y of G o l d Extraction', P u b . Ellis H o r w o o d Ltd.  Miller J . C . , Miller J . N . (1988) 'Statistics for A n a l y t i c a l C h e m i s t r y ' , S e c o n d Edition, Ellis H o r w o o d S e r i e s in A n a l y t i c a l C h e m i s t r y , P u b . Ellis H o r w o o d Ltd.  M i n i n g M a g a z i n e . (2004) ' C y a n i d e m a n a g e m e n t ' , S e p t e m b e r 2 0 0 4 , 3 4 - 3 5 .  194  M i z o g u c h i T., O k a b e T. (1975) ' T h e c h e m i c a l behavior of low v a l e n c e sulfur c o m p o u n d s . IX. P h o t o m e t r i c determination  of thiosulfate, trithionate a n d tetrathionate in  mixture',  Bulletin of the C h e m i c a l S o c i e t y of J a p a n , 4 8 (6), 1 7 9 9 - 1 8 0 5 .  M o l l e m a n E . (1998)  ' T h e treatment of c o p p e r - g o l d o r e s by a m m o n i u m  thiosulfate  l e a c h i n g ' , M A S c T h e s i s , D e p a r t m e n t of M e t a l s a n d M a t e r i a l s E n g i n e e r i n g , University of British C o l u m b i a .  M u i r D . M . , A y l m o r e M . G . (2002) ' T h i o s u l p h a t e a s a n alternative to c y a n i d e for gold processing - issues and impediments', G r e e n Processing Conference, A u s IMM, Cairns, M a y 2 0 0 2 , draft c o p y , n o p a g e n u m b e r s g i v e n .  M i u r a Y . (2003) D e p a r t m e n t of C h e m i s t r y , T o k a i University, P e r s o n a l c o m m u n i c a t i o n , May 2003.  M u i r a Y . , K o h T. (1983) ' S p e c t r o s c o p i c determination of tetrathionate by m e a n s of its alkaline d e c o m p o s i t i o n ' , N i p p o n K a g a k u K a i s h i , 1 1 , 1 5 9 7 - 1 6 0 1 .  Naito K., Y o s h i d a M . , S h e i h M . C . , O k a b e T. (1970a) ' T h e c h e m i c a l b e h a v i o u r of low v a l e n c e sulfur c o m p o u n d s . III.  P r o d u c t i o n of a m m o n i u m s u l f a m a t e by the oxidation of  a m m o n i u m thiosulfate', Bulletin of the C h e m i c a l S o c i e t y of J a p a n , 4 3 , 1 3 6 5 - 1 3 7 2 .  Naito K., S h i e h M . C . , O k a b e T. (1970b) ' T h e c h e m i c a l b e h a v i o u r of low v a l e n c e sulfur c o m p o u n d s . V . D e c o m p o s i t i o n a n d oxidation of tetrathionate in a q u e o u s  ammonia  solution', Bulletin of the C h e m i c a l S o c i e t y of J a p a n , 4 3 , 1 3 7 2 - 1 3 7 6 .  Naito K., H a y a t a H., M o c h i z u k i M . (1975) ' T h e reactions of polythionates. Kinetics of the c l e a v a g e of trithionate ion in a q u e o u s solutions', Inorganic a n d N u c l e a r C h e m i s t r y , 3 7 , 1453-1457.  N i c o l M . J . , O ' M a l l e y G , (2002) ' R e c o v e r i n g gold f r o m thiosulfate l e a c h pulps v i a ion e x c h a n g e " , J O M , 54 (10), 4 4 - 4 6 .  195  N o r Y . M . , T a b a t a b a i M . A . (1975) 'Colorimetric determination of m i c r o g r a m quantities of thiosulfate a n d tetrathionate', Analytical letters, 8 (8), 5 3 7 - 5 4 7 .  P e r e z A . E . , G a l a v i z H . D . (1987) ' M e t h o d for r e c o v e r y of p r e c i o u s metals f r o m difficult o r e s with c o p p e r - a m m o n i u m thiosulfate', U S P a t e n t 4 6 5 4 0 7 8 , 31 M a r c h 1987.  Ritter  R.D.,  displacement  Krueger  J . H . (1970)  'Nucleophilic  substitution  at  sulfur.  Kinetics  of  reactions involving trithionate ion', J o u r n a l of the A m e r i c a n C h e m i c a l  S o c i e t y , 92.(8), 2 3 1 6 - 2 3 2 1 .  R o i n e A . (1994) ' O u t o k u m p u H S C C h e m i s t r y for W i n d o w s ' software v e r s i o n 5.0.  R o l i a E . , C h a k r a b a r t i C . L . (1982) 'Kinetics of d e c o m p o s i t i o n of tetrathionate, trithionate a n d thiosulfate in alkaline m e d i a ' , E n v i r o n m e n t a l S c i e n c e a n d T e c h n o l o g y , 16, 8 5 2 - 8 5 7 .  S a n d W . , G e r k e R., H a l l m a n n R. (1995) 'Sulfur chemistry, biofilm, a n d the (in)direct attack m e c h a n i s m - a critical evaluation of bacterial l e a c h i n g ' , A p p l i e d M i c r o b i o l o g y a n d B i o t e c h n o l o g y , 4 3 (6), 9 6 1 - 9 6 6 .  S c h i p p e r s A . , J o r g e n s e n B . B . (2001) 'Oxidation of pyrite a n d iron sulfide by m a n g a n e s e dioxide in m a r i n e s e d i m e n t s ' , G e o c h i m i c a et C o s m o c h i m i c a A c t a , 6 5 (6), 9 1 5 - 9 2 2 .  S e n a n a y a k e G . (2004) ' A n a l y s i s of reaction kinetics, s p e c i a t i o n a n d m e c h a n i s m of gold l e a c h i n g a n d thiosulfate oxidation by a m m o n i a c a l c o p p e r (II) solutions', Hydrometallurgy, 75, 55-75.  S e n a n a y a k e G . (2005) 'Kinetic m o d e l for a n o d i c oxidation of gold in thiosulfate b a s e d o n the adsorption of M S 0 2  _ 3  media  ion-pair, Hydrometallurgy, 7 6 , 2 3 3 - 2 3 8 .  S h i e h M . C , O t s u b o H., O k a b e T. (1965) ' T h e c h e m i c a l b e h a v i o u r of low v a l e n c e sulfur compounds.  I. O x i d a t i o n  of e l e m e n t a l  sulfur with c o m p r e s s e d o x y g e n  in  aqueous  a m m o n i a solution', Bulletin o f t h e C h e m i c a l S o c i e t y of J a p a n , 3 8 (10), 1 5 9 6 - 1 6 0 0 .  196  S m i t h C . W . , H i t c h e n A . (1976)  ' A q u e o u s solution c h e m i s t r y of polythionates  thiosulphate: a review of formation  and degradation  pathways.', C A N M E T  and  Mineral  S c i e n c e s Laboratories Report M R P / M S L 76-223 (LS).  S s e k a l o H., B a m u w a m y e M . (1993) ' M e c h a n i s m s of interactions involving formation a n d rupture of  S - S bonds  in  inorganic s u l p h u r oxo-derivative  systems', R e s e a r c h on  C h e m i c a l Intermediates, 19 (6), 5 6 5 - 6 0 0 .  S u z u k i I. (1999) 'Oxidation of inorganic sulfur c o m p o u n d s : c h e m i c a l a n d e n z y m a t i c reactions', C a n a d i a n J o u r n a l of M i c r o b i o l o g y , 4 5 , 9 7 - 1 0 5 .  T a n K . G . , R o l i a E . (1985) ' C h e m i c a l oxidation m e t h o d s for the treatment of thiosaltcontaining mill effluents', C a n a d i a n Metallurgical Quarterly, 2 4 (4), 3 0 3 - 3 1 0 .  W a n R . Y . (1997)  'Importance of solution c h e m i s t r y for thiosulphate l e a c h i n g of g o l d ' ,  P r o c e e d i n g s of W o r l d G o l d ' 9 7 , S i n g a p o r e , S o c . M i n i n g Metallurgy a n d E x p l o r a t i o n , Littleton, 1 9 9 7 , 1 5 9 - 1 6 2 .  W a s s i n k B. (2002) University of British C o l u m b i a , p e r s o n a l c o m m u n i c a t i o n .  W e a s t R. (1975) (Ed.) ' C R C H a n d b o o k of C h e m i s t r y a n d P h y s i c s ' , 5 6  t h  Edition, P u b .  C R C Press.  W e b s t e r J . G . (1984) ' T h i o s u l p h a t e c o m p l e x i n g of gold a n d silver during the oxidation of a s u l p h i d e - b e a r i n g c a r b o n a t e lode s y s t e m , u p p e r ridges m i n e , P N G ' , A u s I M M P e r t h a n d K a l g o o r l i e b r a n c h e s , R e g i o n a l C o n f e r e n c e o n G o l d M i n i n g , Metallurgy a n d G e o l o g y , Pub. A u s IMM.  W e s t - S e l l s P . G . , J i J . , H a c k l R . P . (2003) ' A p r o c e s s for counteracting the effect of tetrathionate o n  resin gold a d s o r p t i o n from thiosulfate  l e a c h a t e s ' , P r o c e e d i n g s of  H y d r o m e t a l l u r g y 2 0 0 3 , E d . O A . Y o u n g et a l , T M S , W a r r e n d a l e , 2 4 5 - 2 5 6 .  197  W i l l i a m s o n M . A . , Rimstidt J . D . (1992) 'Correlation b e t w e e n structure a n d t h e r m o d y n a m i c properties of a q u e o u s sulfur s p e c i e s ' , G e o c h i m i c a et C o s m o c h i m i c a A c t a , 5 6 , 3 8 6 7 3880.  X u Y . , S c h o o n e n A . A . (1995) ' T h e stability of thiosulfate in the p r e s e n c e of pyrite in lowt e m p e r a t u r e a q u e o u s solutions', G e o c h i m i c a et C o s m o c h i m i c a A c t a , 5 9 (22), 4 6 0 5 4622.  Y e n W . T . , G u o H., D e s c h e n e s G . (1999) ' D e v e l o p m e n t in percolation l e a c h i n g with a m m o n i u m thiosulfate for g o l d extraction of a mild refractory o r e ' , 1 0  t h  Annual E P D  C o n g r e s s , S a n D i e g o , E d . B. M i s h r a , M i n e r a l s , M e t a l s a n d M a t e r i a l s S o c i e t y , 4 4 1 - 4 5 5 .  Z h a n g H . G . , D r e i s i n g e r D . B . (2002) ' T h e kinetics for the d e c o m p o s i t i o n of tetrathionate in alkaline solutions', Hydrometallurgy, 66 (1-30), 5 9 - 6 5 .  Z i p p e r i a n D., R a g h a v a n S . (1988) ' G o l d a n d silver extraction by a m m o n i a c a l thiosulfate l e a c h i n g from a rhyolite o r e ' , Hydrometallurgy, 19, 3 6 1 - 3 7 5 .  198  Appendix 1 : Thermodynamic Values used to construct Eh-pH Diagrams ( T a k e n f r o m best a v a i l a b l e v a l u e s f r o m H S C C h e m i s t r y for W i n d o w s , R o i n e ,  Species  AH kJ/mol  S J/mol/K  S  0  32.070  s-  33.095  -14.602  -395.765  256.772  2  S0  3  2  -  -635.550  -29.288  2  "  -909.602  18.828  "  -648.520  66.944  so -  -753.538  92.048  S2O5 -  -970.688  104.600  S0  3  S0  4  S 0 2  2 3  2  2  4  2  2  "  -1173.194  125.520  2  "  -1344.763  244.346  so -  -1167.336  138.072  so 4  6  -1224.238  257.316  S 0 5  6  S 0  6  S 0 2  6  S 0  8  2  2  3  6  2  2  "  -1175.704  167.360  2  "  -1381.000  321.323  HS"  -16.108  68.199  HS0 -  -626.219  139.746  HS0 -  -889.100  125.520  HSOs '  -775.63  212.129  6  3  4  2  199  1994)  consumed consumed .kg/ton %  i  A  Au ore, quartz, muscovite, 90 % <74  Stirred tank  25,40, 60  3  0.125-2  B  Quartz  Stirred tank  25  24  0.25  Stirred tank  25  Stirred tank Stirred tank  25  Na  0  1 -8  30-60  8.510.5  40 % solids  With NH OH  20  42  3%  With NH OH With NH OH  20  392  28%  20  252  18%  -80  lodometric titration  l> D  O CD  Pyrite Arsenopyrite Chalcopyrite pyrrhotite ro o o  C  Untreated pyrite cone  NH  4  0.25 M  1  6  so 2  4  4  24  0.25  NH  4  0.25 M  1  6  so 2  4  4  24  0.25  NH  4  0.25 M  1  6  so 2  25  24  0.25  NH  Stirred tank  25  24  0.25  NH  Stirred tank  RT?  4  0.25 M  4  0.25 M  1  6  With NH OH  20  168  12%  1  6  With NH OH  20  140  10%  so 2  so 2  0.4  96  Na  4  50  0.5  4  24  0.8  96  0.8  24  0  50  52  39%  4  50  30  14%  56  26%  0.8  1.5  50  39  18%  0.8  3  50  0.8  4  12.5  96 24  0.8  4  50  96 24  0.8  4  37.5  96 24 96  417  57%  96 24  80-180  61  96 24  10.2  0.8  4  62.5  o v> c_ r*  CD  4  4  3  a x  ST  4  4  96  3  4  4  lodometric titration with acetic acid added to eliminate Cu NH complex effect.  Immediate HPLC  o  CD  a  CD  a. I  128  59%  r~  99  46%  CO  115  54%  90  42%  166  77%  22  10%  99  46%  41  19%  137  64%  40  19%  80  37%  0)  •  M  Au ore, quartz, muscovite, 90 % <74  Stirred tank  25,40, 60  3  0.125-2  Na  0  B  Quartz  Stirred tank  25  24  0.25  NH  Stirred tank Stirred tank  25  0.25 M S0 ' 0.25 M  Pyrite Arsenopyrite Chalcopyrite pyrrhotite C  Untreated pyrite cone  4  1-8  30-60  1  6  0.25  NH  4  1  6  so " 2  24  0.25  NH  4  0.25 M S O 4  Stirred tank  25  Stirred tank  25  Stirred tank  RT?  24  0.25  NH*  2  1  6  0.25 M  1  6  2  NH  4  0.25 M S O 4  96  0.4  Na  0  2  lodometric titration  42  3%  20  392  28%  20  252  18%  lodometric titration with acetic acid added to eliminate Cu NH complex  With  3  With  20  168  12%  20  140  10%  417  61  57% 39%  1  6  With NH4OH  "  4  50  10.2  80-180  96  0.5  4  50  52  24  0.8  4  50  30  14%  96  0.8  56  26%  24  0.8  1.5  50  39  18%  0.8  3  50  0.8  4  12.5  96 24 96 24 96 24  96  59%  99  46%  115  54%  90  42%  166  77%  4  50  22  10%  99  46%  0.8  4  37.5  41  19%  137  64%  40  19%  80  37%  96 24  128  0.8  96 24  0.8  ettect.  NH4OH  4  0.25  20  40-50%  "  so 24  With  -80  NH4OH  4  25  40 % solids  NH4OH  2  4  24  1  8.510.5  4  62.5  Immediate HPLC  leached D  Synthetic Ag S 2  0  in air  -20  24  0.072  in air  -20  24  0.073  NH  9.92  Ag81.7  164  36%  2  -20  24  0.073  NH«  0 0  15  in N  15  9.92  Ag 85.3  114  25%  in air  -20  24  0.073  NH  3.5 mM  15  9.92  Ag66.4  114  25%  15  9.92  Ag 70.3  68  15%  15  9.92  Ag50.7  68  15%  15  9.92  Ag44.7  25  5%  83  0.40  in N  2  -20  24  0.073  NH  NH  4  4  4  so  3  4  3.5 mM S0 14 mM S0 14 mM S0 0-0.0125 M not specified  16  9.92  Ag86.7  192  43%  IX, also for  so 2  4  6  3  in air  -20  24  0.073  NH  4  3  in N  2  -20  24  0.073  NH  4  3  E  to  Low grade oxidized Au ore  Bottle roll  ambient  48  0.2  Na  1  IC for SjOa ", 2  S0 ". S 0 ' 2  3  Bottle roll  ambient  94  0.2  Na  0-0.0125 M not specified  1  90  1.71  Bottle roll  ambient  94  0.125  Na  0-0.0125 M not specified  1  85.8  3.60  Bottle roll  ambient  94  0.05  Na  0-0.0125 Mnot specified  1  <11  4.61  2  4  E  Rhyolite and andesite breccia matrix, 80 % < 200 mesh, Mn  to  F  Mn ore, 2.1 % Mn, difficult to teach  Stirred tank  25  28  0.15  NH  4  -31  8.5  40 % solids  8  32%  Stirred tank Stirred tank Stirred tank Stirred tank Stirred tank Stirred tank Stirred tank Stirred tank See text  25  28  0.23  NH  4  -31  8.5  40 % solids  12  32%  25  28  0.38  1NH4  -31  8.5  40 % solids  15  23%  25  28  0.15  NH  4  -31  9.5  40 % solids  13  52%  25  28  0.23  NH  4  -31  9.5  40 % solids  21  55%  25  28  0.38  NH  4  -31  9.5  40 % solids  33  52%  25  28  0.12  N1H4  -31  10  40 % solids  9  36%  25  28  0.23  NH  4  -31  10  40 % solids  19  50%  25  28  0.38  NH  4  -31  10  40 % solids  35  55%  18%?  NH  4  3%  2%  (NH )2  NH4OH  4  - 4 g/l  86.7  lodometric titration  3.6  SO3  G  Sulfide gold flotation cone with Cu, 90% 200 mesh  Stirred tank  60  Stirred tank  60  Stirred tank  60  Stirred tank  60  0.5  0.14  1NH4  0.8 M (NH fc S0  4  47  10.2  500  2.2  7%  0.8 M  4  47  10.2  500  2.2  7%  4  47  10.2  500  0.0  0%  4  47  10.2  500  6.6  21%  4  4  1  0.14  NH  4  (NH )2 SO4 4  1.5  0.14  NH  4  0.8 M (NH )2 4  SO« 2  0.14  NH  4  0.8 M (NH )2 SO4 4  unspecified  H  Refractory gold ore  H  Refractory gold ore  to  o  A B C D  Bottle roll Column  RT  24  0.5  NH  4  RT  50 d  0.1  NH  4  Column  RT  50 d  0.3  NH  4  Column  RT  50 d  0.5  NH  4  Column  RT  50 d  1  NH  4  Column  RT  50 d  NH  4  Column  RT  50 d  NH  4  Column  RT  50 d  NH  4  Column  RT  50 d  NH  Column  RT  50 d  Column  RT  50 d  Column  RT  50 d  0.3 maintained 0.3 maintained 0.3 maintained 0.3 maintained 0.3 maintained 0.3 maintained 0.5  Column  RT  50 d  0.5  References for Appendix 2 Abbruzzese et al, 1995 Fend and Van Deventer, 2002b Aylmore, 2001 Rett e t a l , 1983  E F G H  -  6  50  10.2  45 % solids  72  52.0  -  3  30  10.2  3000  45  15.5  -  3  30  10.2  3000  64.2  27.5  -  3  30  10.2  3000  64.9  30.3  -  3  30  10.2  3000  71.9  36.8  -  1  50  10.2  3000  57  35.8  -  3  50  10.2  3000  60.7  30.4  -  6  50  10.2  3000  71.7  28.1  4  -  3  50  10.2  3000  67.7  27.5  NH  4  -  3  50  10.2  3000  60.7  30.5  NH  4  -  3  50  10.2  3000  56.7  35.8  NH  4  -  3  30  10.2  830  38.6  -  3  30  10.2  5000  15.9  NH  4  Langhans et al, 1992 Kerley and Bernard, 1981 Cao e t a l , 1992 Yen e t a l , 1999  lodometric titration  Appendix 3 : Determination of Sulfur Oxyanions by Ion Chromatography (modified from a m e t h o d by L a k e f i e l d R e s e a r c h Ltd)  Analysis of thiosulfate. trithionate and tetrathionate Reagent Preparation Calibration standards P r e p a r e 1 0 0 0 mg/l thiosulfate solution by d i s s o l v i n g the required a m o u n t of s o d i u m thiosulfate a n h y d r o u s in ultrapure d e - i o n i s e d water. P r e p a r e 1 0 0 0 mg/l trithionate solution by d i s s o l v i n g the required a m o u n t of s o d i u m trithionate in ultrapure d e - i o n i s e d water. P r e p a r e 1 0 0 0 mg/l tetrathionate solution by d i s s o l v i n g the required a m o u n t of s o d i u m tetrathionate in ultrapure d e - i o n i s e d water. F r o m t h e s e s t a n d a r d s , p r e p a r e calibration s t a n d a r d s containing 2 , 10, 15 a n d 2 0 mg/l thiosulfate, trithionate a n d tetrathionate.  C a l i b r a t i o n s t a n d a r d s s h o u l d b e p r e p a r e d i mmedi atel y prior to u s e .  E l u a n t - S o d i u m perchlorate (0.038 M) with 10 % m e t h a n o l D i s s o l v e 1 0 . 6 7 5 g N a C I 0 . H 0 ( H P L C g r a d e ) in w ater a n d a d d 2 0 0 ml m e t h a n o l ( H P L C 4  grade).  2  Dilute to 2 litres u s i n g ultrapure d e - i o n i s e d water.  Filter the solution through a  0.22 u.m filter.  Procedure  S e t up the ion c h r o m a t o g r a p h a c c o r d i n g to the m a n u f a c t u r e r s instructions, u s i n g a n O m n i P a c P A X 100 g u a r d c o l u m n a n d a n O m n i P a c P A X 100 analytical c o l u m n . P u m p the eluant through the s y s t e m at 1 ml/min. Monitor the a b s o r b a n c e at 2 0 5 n m u s i n g a U V - v i s i b l e a b s o r p t i o n detector. A l l o w the instrument to equilibrate s o that the a b s o r b a n c e m e a s u r e d is c o n s t a n t (typically 2 hours). Inject the 2 mg/l calibration s t a n d a r d solution, followed by the 10 m g / l , 15 mg/l a n d 2 0 mg/l s t a n d a r d solution.  205  C h e c k that the calibration c u r v e is linear a n d then p r o c e e d to a n a l y s e the s a m p l e s o l u t i o n s , w h i c h m u s t b e diluted to within the calibration r a n g e immediately prior to analysis. T y p i c a l retention times are about 2 minutes for thiosulfate, 6 m i n u t e s for trithionate a n d 10 m i n u t e s for tetrathionate, d e p e n d i n g o n the condition of the c o l u m n s .  Analysis of sulfate Reagent Preparation Calibration standards P r e p a r e 1 0 0 0 mg/l sulfate solution by d i s s o l v i n g the required a m o u n t of s o d i u m sulfate in ultrapure d e - i o n i s e d water. F r o m this s t a n d a r d solution, p r e p a r e calibration s t a n d a r d s containing 2, 10, 15 a n d 2 0 mg/l sulfate.  Eluant -  S o d i u m c a r b o n a t e (1.8 m M ) -  s o d i u m b i c a r b o n a t e (1.7 m M ) with 10 %  methanol Dissolve 0.382 g N a C 0 2  ( H P L C grade).  3  a n d 0.286 g N a H C 0  3  in w a t e r a n d a d d 2 0 0 ml m e t h a n o l  Dilute to 2 litres using ultrapure d e - i o n i s e d water.  Filter the solution  through a 0.22 u,m filter.  Procedure  S e t up the ion c h r o m a t o g r a p h a c c o r d i n g to the m a n u f a c t u r e r s instructions, using a n l o n P a c A G 4 A - S C g u a r d c o l u m n a n d a n l o n P a c A S 4 A - S C analytical c o l u m n . P u m p the eluant through the s y s t e m at 2 ml/min. S e t the s u p p r e s s o r current at 2 7 m A . Monitor the conductivity using a conductivity detector. A l l o w the  instrument  to equilibrate s o that the conductivity  m e a s u r e d is c o n s t a n t  (typically 2 hours). Inject the 2 mg/l calibration s t a n d a r d solution, followed by the 10 m g / l , 15 mg/l a n d 2 0 mg/l s t a n d a r d solution.  206  C h e c k that the calibration c u r v e is linear a n d then p r o c e e d to a n a l y s e the s a m p l e solutions, w h i c h m u s t b e diluted to within the calibration r a n g e immediately prior to analysis. T y p i c a l retention times are a b o u t 8 minutes for sulfate, d e p e n d i n g o n the condition of the columns.  207  Appendix 4 - Chemical Impurity Analysis Impurity Analysis for a solution of Ammonium Bicarbonate Solution of 3 7 . 6 g/l N H H C 0 4  Element  Concentration (mg/l)  Al Sb As Ba Bi Cd Ca Cr Co Cu Fe La Pb Mg Mn  <0.2 0.3 0.3 <0.01 <0.1 <0.01 0.1 <0.01 <0.01 <0.01 <0.03 <0.05 <0.05 <0.1 <0.01 <0.05 <0.02 2.28 <0.1 <2 0.02 <0.02 7 <0.01 <0.2 <0.1 <0.1 <0.01 <0.01 <0.01  Hg Mo Ni P K Sc Ag Na Sr Tl Ti W V Zn Zr  208  3  Impurity Analysis for a solution of Ammonium Hydroxide Solution of 0.5 M N H Element Al Sb As Ba Bi Cd Ca Cr Co Cu Fe La Pb Mg Mn Hg Mo Ni P K Sc Ag Na Sr Tl Ti W V Zn Zr  3  Concentration (mg/l) <0.2 0.1 1.2 <0.01 <0.1 <0.01 1 <0.01 0.06 0.23 <0.03 0.09 <0.05 2.3 <0.01 <0.05 <0.02 1.54 <0.1 <2 <0.01 <0.02 48 <0.01 <0.2 <0.1 <0.1 <0.01 <0.01 <0.01  209  Impurity Analysis for a solution of Sodium Thiosulfate Solution of 1.5 g/l N a S 0 2  2  Element  Concentration (mg/l)  Al Sb As Ba Bi Cd Ca Cr Co Cu Fe La Pb Mg Mn Hg Mo Ni P K Sc Ag Na Sr Tl Ti  <0.2 <0.1 <0.2 <0.01 <0.1 <0.01 0.5 <0.01 0.09 <0.01 <0.03 0.22 0.31 1.2 <0.01 <0.05 <0.02 <0.02 0.9 20 <0.01 <0.02 529 <0.01 <0.2 <0.1 <0.1 <0.01 <0.01 <0.01  W V  Zn Zr  210  3  Impurity Analysis for a solution of Sodium Trithionate  Solution of 1.3 g/l Na S 0 Batch 4 2  Element Al Sb As Ba Bi Cd Ca Cr Co Cu Fe La Pb Mg Mn Hg Mo Ni P K Sc Ag Na Sr Tl Ti  2  3  Concentration (mg/l) <0.2 <0.1 <0.2 <0.01 <0.1 <0.01 0.8 <0.01 0.11 <0.01 <0.03 0.18 0.34 1 <0.01 <0.05 <0.02 <0.02 <0.1 20 <0.01 O.02 316 <0.01 <0.2 <0.1 <0.1 <0.01 <0.01 0.04  W V  Zn Zr  211  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0078764/manifest

Comment

Related Items