UBC Theses and Dissertations

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UBC Theses and Dissertations

Electrochemical study of pyrrhotite Jibiki, Kyosuke 1971

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AN ELECTROCHEMICAL STUDY OF PYRRHOTITE  BY  KYOSUKE J I B I K I B.Eng.  Hokkaido U n i v e r s i t y , 1966.  M. Eng. Hokkaido U n i v e r s i t y , 1968.  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE  REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE  i n the Department of METALLURGY  We accept t h i s t h e s i s as conforming  t o the  required standard  THE  UNIVERSITY OF BRITISH COLUMBIA February, 1971  «4  In  presenting  this  an a d v a n c e d  degree  the L i b r a r y  shall  I  f u r t h e r agree  for  scholarly  by h i s of  this  written  thesis at  the U n i v e r s i t y  make  it  of  Metallurgy  A p r i l 28,  1971  Columbia  the  requirements  B r i t i s h Columbia, for  I agree  r e f e r e n c e and copying of  this  shall  that  not  copying  or  for  that  study. thesis  by t h e Head o f my D e p a r t m e n t  is understood  financial gain  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8 , Canada  Date  It  permission.  Department  of  for extensive  p u r p o s e s may be g r a n t e d  for  fulfilment of  freely available  that permission  representatives. thesis  in p a r t i a l  or  publication  be a l l o w e d w i t h o u t my  - i i-  ABSTRACT  The  r e s t p o t e n t i a l o f s u l p h i d e e l e c t r o d e s was  thermodynamic and k i n e t i c a s p e c t s . found  to be n e c e s s a r y  examined from  The k i n e t i c aspect has  both  been  f o r the i n t e r p r e t a t i o n of the e s t a b l i s h m e n t of the  mixed p o t e n t i a l i n p o l y e l e c t r o d e system, to which most s u l p h i d e systems belong. The p y r r h o t i t e e l e c t r o d e system was p o t e n t i a l w h i l e changing  s t u d i e d by measuring the  rest  the c o n c e n t r a t i o n s of f e r r o u s i o n , hydrogen  i o n and hydrogen s u l p h i d e i n the e l e c t r o l y t e , and  the c o m p o s i t i o n  of  pyrrhotite. A mixed p o t e n t i a l of p y r r h o t i t e c o n s i s t i n g of the  S (in pyrrhotite)  as a c a t h o d i c p r o c e s s and the  +  2H  +  +  2e —*-  reaction  H^S  reaction  [ [  Fe  ( i n p y r r h o t i t e ) —*• Fe  as an anodic p r o c e s s accounts  +  2e  f o r the dependence of the r e s t  on those i o n i c s p e c i e s i n the e l e c t r o l y t e and pyrrhotite electrodes.  the c o m p o s i t i o n  potential of  - iii  -  ACKNOWLEDGEMENT  The author wishes t o express h i s g r a t i t u d e t o Dr. E. P e t e r s f o r his continuing  guidance and i n t e r e s t i n t h i s p r o j e c t .  H i s thanks  i s extended to f e l l o w graduate s t u d e n t s and t h e t e c h n i c a l s t a f f o f the Department of M e t a l l u r g y  f o r t h e i r h e l p f u l d i s c u s s i o n s and  assistance. F i n a n c i a l support from t h e N a t i o n a l Research C o u n c i l o f Canada i n the form o f a Research A s s i s t a n t s h i p i s g r a t e f u l l y acknowledged.  - iv -  TABLE OF CONTENTS Page I.  INTRODUCTION  1  II.  SIGNIFICANCE OF THE PRESENT WORK  3  III.  A REVIEW OF THE LITERATURE  4  (1)  IV.  V.  VI.  D e s c r i p t i o n of F e r r o u s  Sulphide  4  1)  Fe-S phase diagram  4  2)  Thermodynamic diagrams  g  3)  p H - p o t e n t i a l diagrams f o r i r o n s u l p h i d e s  ...  -^0  3-1)  E q u i l i b r i u m p H - p o t e n t i a l diagram  10  3-2)  Meta-stable  10  p H - p o t e n t i a l diagram  (2)  E l e c t r o c h e m i c a l Study o f P y r r h o t i t e  13  (3)  Leaching  14  of P y r r h o t i t e  THE METAL SULPHIDE ELECTRODE  19  (1)  Thermodynamic Aspect  19  (2)  K i n e t i c Aspect  2  1  2  8  8  EXPERIMENTAL Cl)  Materials  2  (2)  X-Ray A n a l y s i s o f P y r r h o t i t e s  33  (3)  Sulphide  33  (4)  Electrolytic Cell  37  (5)  Reagents  37  (6)  Experimental  Electrode  Procedure  RESULTS AND DISCUSSION Cl)  E f f e c t of Ferrous  C2)  E f f e c t of pH  37 41  Ion C o n c e n t r a t i o n  41 43  - v -  Page (3)  E f f e c t o f Hydrogen S u l p h i d e  (4)  E f f e c t of Non-Stoichiometry  (5)  E f f e c t of R e s i d u a l Impurity i n the E l e c t r o l y t e  (6)  I n t e r p r e t a t i o n of the Measured Rest P o t e n t i a l  (7)  (8)  43 of P y r r h o t i t e  ......  47  .  52  ..  54  G a l v a n i c and P o l a r i z a t i o n E f f e c t on the Hydrogen Sulphide E v o l u t i o n  62  E l e c t r o c h e m i c a l Mechanism of L e a c h i n g R e a c t i o n s .  64  VII.  CONCLUSIONS  71  VIII.  SUGGESTIONS FOR FUTURE WORK  73  APPENDIX A.  74 Measurement  of the E q u i l i b r i u m P r e s s u r e of H^S  on P y r r h o t i t e  B. C.  (1)  Introduction  74  (2)  Experimental  76  (3)  R e s u l t s and D i s c u s s i o n  78  T a b l e VI. Dependence o f the r e s t p o t e n t i a l on f e r r o u s i o n c o n c e n t r a t i o n a t pH = 2.8, 25°C T a b l e V I I . Dependence of the r e s t p o t e n t i a l on H at [Fe++] = 0.01 M, 25°C P  D.  Table V I I I . V a r i a t i o n i n the r e s t p o t e n t i a l w i t h change i n composition of p y r r h o t i t e at 25°C, pH=.3, [Fe" "] = 0.01 M. . .. 1-1  REFERENCES  74  82 83  84 85  - vi-  LIST OF TABLES Table I  Page Measured p o t e n t i a l s  of the c e l l  (-)FeS/FeSO^.x M/KC1/  Calomel R.E. (+) a t 18°C II.  13  Comparison o f the r e s t p o t e n t i a l s measured w i t h the mounted e l e c t r o d e and the powder e l e c t r o d e  III.  Comparison of the r e s t p o t e n t i a l s measured w i t h and without reduction  IV.  Equilibrium 2/nM  V.  35  n+  of e l e c t r o l y t e  constants of M ^ S 2  + 2H  52 +  = H S(aq) + 2  f o r v a r i o u s s u l p h i d e s a t 25°C  Values o f P  59  _, [Fe"*" "], pH and K"  78  1  u  ri^b  VI.  Dependence o f the r e s t p o t e n t i a l on f e r r o u s centration  VII.  i o n con-  a t pH = 2.8, 25°C  82  Dependence o f the r e s t p o t e n t i a l on pH a t [Fe*" "] = 0.01 M, 25°C  .  1  VIII.  83  V a r i a t i o n i n the r e s t p o t e n t i a l w i t h change i n compo s i t i o n of p y r r h o t i t e 0.01 M  I[  a t 25°C, pH * 3, [Fe  ] =84  - vii LIST OF  FIGURES  Figure  Page  1  Tentative  2  D e t a i l s of the Fe-S diagram i n the v i c i n i t y t r o i l i t e and p y r r h o t i t e  of  Schematic diagram of a c t i v i t y of s u l p h u r i n  the  3  Fe-S  diagram of Fe-S  system at low  5  temperatures.  6  9  system  4  P o t e n t i a l - p H diagram f o r s t a b l e Fe-S  5  P o t e n t i a l - p H diagram f o r m e t a s t a b l e Fe-S  6  at 25°C under c o n d i t i o n s where p y r i t e i s not formed. Schematic diagram f o r c u r r e n t - d e n s i t y potential r e l a t i o n s h i p of s u l p h i d e e l e c t r o d e i n a c i d r e g i o n . . .  25  7  T o t a l vapour p r e s s u r e of s u l p h u r between 120  31  8  V a r i a t i o n i n Fe temperatures  9  10  Iron sulphide mounted;  n  systems  and  450°C  12  content w i t h d i f f e r e n t s u l p h u r b a t h 32  X-ray d i f f r a c t i o n p a t t e r n s of v a r i o u s i r o n s u l p h i d e s u s i n g Co-K radiation a  (a)  34  electrodes (b) powder  11  Sketch of e l e c t r o l y t i c c e l l  12  V a r i a t i o n i n the  13  Dependence of the concentration  14a  systems at 25°C.  36 38 40  r e s t p o t e n t i a l w i t h time r e s t p o t e n t i a l on  ferrous  ion 42  Dependence of the r e s t p o t e n t i a l on pH (a); pH-dependence of the r e v e r s i b l e p o t e n t i a l f o r [S] + 2H + 2e ^ HS c a l c u l a t e d from the N e r n s t equation  44  14b  Dependence of the  r e s t p o t e n t i a l on pH  45  14c  Dependence of the  r e s t p o t e n t i a l on pH  46  15a  Rest p o t e n t i a l changes i n d i f f e r e n t atmosphere  48  15b  Rest p o t e n t i a l changes i n d i f f e r e n t atmosphere  49  16  V a r i a t i o n i n the r e s t p o t e n t i a l w i t h change i n composition of p y r r h o t i t e  51  +  2  - viii  -  Figure  Page  17  Sketch  18  E x p e r i m e n t a l v a r i a t i o n i n the r e s t p o t e n t i a l of g a l e n a f o r low (Pb ) at pH = 0  55  C u r r e n t - d e n s i t y p o t e n t i a l r e l a t i o n s h i p s f o r the c e l l (a) p y r r h o t i t e | X - F e S 0 , y-H S0 |S.H.E. (25°C) (b) p y r r h o t i t e s | H S 0 , He or H S|S.H.E. (25°C)  57  C u r r e n t - d e n s i t y p o t e n t i a l r e l a t i o n s h i p s f o r the c e l l d i f f e r e n t p y r r h o t i t e s | F e S 0 , H S0 |S.H.E. (25°C) ...  63  V a r i a t i o n i n H S evolution rate with a galvanic c o n t a c t and a n o d i z a t i o n of p y r r h o t i t e  65  V a r i a t i o n i n H S evolution rate with p o t e n t i a l of p y r r h o t i t e e l e c t r o d e  66  19  o f the c e l l  f o r r e d u c t i o n of the e l e c t r o l y t e  4  2  20  2  21  22  23  24  /  4  4  2  4  53  2  4  2  2  I l l u s t r a t i o n of the form of s u l p h u r l e a c h i n g of s u l p h i d e m i n e r a l s  change i n  during o x i d i z i n g 70  Schematic i l l u s t r a t i o n of the equipment f o r H S p r e s s u r e measurement 2  25  Increase  in H S  26  Dependence of K"  2  pressure with on pH  time  • 77 79 81  I.  The  INTRODUCTION  e l e c t r o c h e m i c a l p r o p e r t i e s of sulphide minerals  s o l u t i o n , which were f i r s t  examined i n the l a s t c e n t u r y ,  i n aqueous have been  s t u d i e d mainly by g e o l o g i s t s who determined e l e c t r o d e p o t e n t i a l s o f many s u l p h i d e m i n e r a l s . succeeded i n a r r a n g i n g to  From a p r a c t i c a l p o i n t of view these works s u l p h i d e s i n a s e r i e s o f p o t e n t i a l s analogous  the e l e c t r o c h e m i c a l s e r i e s o f metals.  However, the p o t e n t i a l s o f  s u l p h i d e s measured were p o o r l y r e p r o d u c i b l e and i n c o n s i s t e n t w i t h the values  c a l c u l a t e d from thermochemical data, and i n f a c t , the meaning  of e l e c t r o d e p o t e n t i a l s o f s u l p h i d e s Meanwhile, these utilized  i n the study  i s n o t p r e c i s e l y understood  e l e c t r o c h e m i c a l p r o p e r t i e s o f s u l p h i d e s have been of v a r i o u s h y d r o m e t a l l u r g i c a l p r o c e s s e s , e.g.  e l e c t r o l y s i s , l e a c h i n g and f l o t a t i o n .  Anodic e l e c t r o l y s i s o f  s u l p h i d e s y i e l d s m e t a l i o n s and elementary s u l p h u r products.  today.  or sulphate  The e l e c t r o l y s i s o f n i c k e l matte i s a l r e a d y  and known as the H y b i n e t t e  process.  Galvanic  i o n s as  commercialized,  a c t i o n may occur between  p a r t i c l e s of d i f f e r e n t s u l p h i d e s i n a s l u r r y , analogous to g a l v a n i c c o r r o s i o n between d i f f e r e n t metals; g e o l o g i c s t u d i e s of m i n e r a l  t h i s was f i r s t  noticed  during  deposits.  Some attempts were made t o i n t e r p r e t l e a c h i n g r e a c t i o n s o f s u l p h i d e s as e l e c t r o c h e m i c a l p r o c e s s e s ,  s i m i l a r to the c o r r o s i o n  process  - 2 of m e t a l s which i s now r e a s o n a b l y w e l l understood. p H - p o t e n t i a l diagrams  The a p p l i c a t i o n o f  t o the s u l p h i d e systems by v a r i o u s workers  much i n f o r m a t i o n o f a thermodynamic k i n d .  yields  However, a l l o f the works  undertaken t o date f a i l e d t o show t h e e x p e r i m e n t a l v a l i d i t y o f the p H - p o t e n t i a l diagram.  One o f the p o s s i b i l i t i e s  f o r t h i s i s that  k i n e t i c c o n s i d e r a t i o n s were l a r g e l y i g n o r e d . A f u l l u n d e r s t a n d i n g o f the e l e c t r o c h e m i c a l k i n e t i c  behaviour  of s u l p h i d e s i s n e c e s s a r y b e f o r e the p r o p e r t i e s o f systems over a s h o r t time i n t e r v a l  studied  ( s h o r t e r than g e o l o g i c a l time ) can be  c o m p l e t e l y understood f o r e x t r a c t i v e m e t a l l u r g y purposes.  Thus some  p r o c e s s e s t h a t come t o e q u i l i b r i u m over a p e r i o d o f y e a r s , may be s a f e l y i g n o r e d i n d e t e r m i n i n g a u s e f u l diagram f o r e x t r a c t i o n but then the diagram i s one which may c o n t a i n m e t a s t a b l e thermodynamically p e r i o d s o f time.  purposes,  phases,  s p e a k i n g , which do n o t r e a c t a p p r e c i a b l y i n a l l o w e d  II.  SIGNIFICANCE OF THE  The p r e s e n t work was  PRESENT WORK  u n d e r t a k e n t o o b t a i n the  systematic  measurement of the r e s t p o t e n t i a l s of s u l p h i d e s w h i c h were i n t e r p r e t e d i n terms of e l e c t r o c h e m i c a l k i n e t i c s r a t h e r than f i n a l thermodynamic equilibria. I n a d d i t i o n t o t h e s e measurements, an attempt was  made t o o b t a i n  thermodynamic d a t a of s u l p h i d e s w h i c h were n e c e s s a r y f o r the more q u a n t i t a t i v e i n t e r p r e t a t i o n of e l e c t r o c h e m i c a l P y r r h o t i t e , Fe^_^S (a << the p r e s e n t work was important  1), was  undertaken.  data.  chosen as the s u l p h i d e i n w h i c h  A l t h o u g h p y r r h o t i t e i s not  as p y r i t e f o r m e t a l l u r g i c a l purposes because of i t s r a r e r  o c c u r r e n c e i n n a t u r a l o r e s , i t i s o f t e n accompanied by n i c k e l o r e s and  as  sulphide  the e l e c t r o c h e m i c a l b e h a v i o u r seems to be c l o s e l y r e l a t e d t o  t h a t of p y r i t e .  Only a few works have been r e p o r t e d on  s t u d i e s of p y r r h o t i t e .  P y r r h o t i t e appears i n a  compound w i t h a wide range of Fe:S l a t t i c e , t h i s non-stoichiometry  electrochemical  non-stoichiometric  r a t i o forming  of p y r r h o t i t e may  an e f f e c t on the e l e c t r o c h e m i c a l b e h a v i o u r of the  an i r o n d e f i c i e n t be e x p e c t e d to have minerals.  III.  (1)  Description 1)  A REVIEW OF THE  LITERATURE  of P y r r h o t i t e  The Fe-S phase diagram  Although the Fe-S b i n a r y system has been s t u d i e d  in fair  detail  from l i q u i d u s temperatures down to about 300°C, the low  temperature  phase r e l a t i o n s h i p s are l e s s w e l l understood because of  difficulty  with sluggish reaction rates.  N e v e r t h e l e s s , i n F i g . 1 the phase  r e l a t i o n s h i p s of the system at low temperatures are shown based phases observed i n n a t u r e and on a l i m i t e d number of  on  laboratory  studies."'" A c c o r d i n g to F i g . 1 i t can be seen t h a t at low temperature  the  phase r e l a t i o n s h i p s are more c o m p l i c a t e d than those a t h i g h temperature. On c o o l i n g  t o 320 + 5°C, the h i g h temperature hexagonal  passes through an i n v e r s i o n t o low temperature hexagonal  pyrrhotite  pyrrhotite.  The temperature o f t h i s inversion does not seem to be i n f l u e n c e d  by the  c o m p o s i t i o n of the p y r r h o t i t e . F u r t h e r c o o l i n g near the FeS c o m p o s i t i o n l e a d s to a second i n v e r s i o n at 139°C and the t r o i l i t e phase becomes s t a b l e . i n F i g . 1 that  the t r o i l i t e  stability  I t i s noted  i s v e r y r e s t r i c t e d both i n r e g a r d  to c o m p o s i t i o n , which cannot d e v i a t e from the s t o i c h i o m e t r i c and i n r e g a r d to temperature.  FeS  In f a c t , a r e c e n t work done by R. Yund  2 and H. H a l l  showed t h a t t r o i l i t e  appears to be r e s t r i c t e d to the  -  5 -  Hex.= Hexagonal Mono. = Monoclinic H.T. » High temperature L.T. Low »  Po. Pyrrhotite Py,= Pyrite s  ?  600  H  500. Hex. H.T.  Hex. H.T. R>. +  Po.  O o  Py.  py. + liquid Sulphur  400. Hex. L.T. Po.  + Py. az  3 < or iii  300  Hex. L.T. Po.  310 rMono. Po. Mono. Po. + Py.  CL 200.  Py. + Mono. S  -Hex. L.T. Po.  UJ  +  Mono. Po  114.5  100 Tr.  TroUite  Hex.  Mono. Po.  +• Fe S Fe S + Py. 3  4  3  4  Po  oIL , 50  F i g u r e 1.  Fe^Q '  Fe S 3  4  40  102 Py. + orthorhombic S FeS2  ATOMIC PERCENT Fe  T e n t a t i v e diagram o f Fe-S system a t low temperatures.  30  - 6 -  140.  1204  OIOOJ  o  it! ?  CCD 80 Troilite  Troilite +  £  6 0j  Iron  I  +  H e x. L . T Pyrrhotite  H e x . L T. CL| Pyrrhotite  40l  Troilite  50  49  48  ATOMIC PERCENT Fe  F i g u r e 2.  Details and  o f t h e Fe-S d i a g r a m i n t h e v i c i n i t y  pyrrhotite.  47  of t r o i l i t e  s t o i c h i o m e t r i c FeS, shown i n F i g . 2. The u n i v a r i a n t f i e l d  e x i s t i n g below 139°C between t r o i l i t e and  the low temperature hexagonal p y r r h o t i t e i n c r e a s e s s i g n i f i c a n t l y i n width w i t h d e c r e a s i n g temperature.  These two phases commonly c o - e x i s t  i n many o r e s .  a A m o n o c l i n i c p y r r h o t i t e was f i r s t ores.  I t has g r a d u a l l y become apparent  common i n o r e d e p o s i t s .  found  i n a number o f Swedish  that t h i s mineral i s quite  M o n o c l i n i c p y r r h o t i t e has been s y n t h e s i z e d i n  the pure Fe-S system, b e i n g s t a b l e below 310°C i n t h e presence of s u l p h u r vapour. below 310°C. been found  In F i g . 1 i t i s t e n t a t i v e l y shown as a s t a b l e phase  The compositions  o f numerous m o n o c l i n i c p y r r h o t i t e s have  to vary only s l i g h t l y ;  p e r c e n t Fe.  the range i s 46.45 t o 46.70  atomic  M o n o c l i n i c p y r r h o t i t e and low-temperature hexagonal  p y r r h o t i t e form a common assemblage i n n a t u r a l ores as evidenced by X-ray powder d i f f r a c t i o n s t u d i e s . assemblages a r e found Fe^S^  composition  named smythite confirmed  Monoclinic pyrrhotite-marcasite  t o be q u i t e common i n o r e s .  A phase w i t h  and rhombohedral s t r u c t u r e was f i r s t  r e p o r t e d and  i n 1957. However, i t s s t a b i l i t y r e g i o n i s n o t  y e t , so i n F i g . 1 a breakdown o f t h e Fe^S^ compound i s  i n d i c a t e d t e n t a t i v e l y a t about 100°C. In n a t u r a l ores p y r i t e as w e l l as m a r c a s i t e i s v e r y common as t h e i r o n d i s u l p h i d e phase.  However, t h e p y r i t e - m a r c a s i t e r e l a t i o n has  been a p u z z l e f o r many y e a r s .  In composition  there i s a d i f f e r e n c e  between t h e two m i n e r a l s , i n d i c a t e d by numerous experiments, orthorhombic  i . e . the  m a r c a s i t e c o n t a i n s l e s s s u l p h u r than the c u b i c p y r i t e  which i s e s s e n t i a l l y s t o i c h i o m e t r i c FeS..  T h i s account  f o r the f a c t  - 8 t h a t , when m a r c a s i t e i s heated w i t h e l e m e n t a l s u l p h u r under c o n f i n i n g p r e s s u r e i t c o n v e r t s to p y r i t e i n a matter  of days,  at  temperatures  even as low as 150°C, forming a p y r i t e r i m around i n d i v i d u a l grains.  marcasite  S e v e r a l o t h e r s t u d i e s on these m i n e r a l s i n d i c a t e t h a t hydrogen  a p p a r e n t l y p l a y s an important  r o l e i n the f o r m a t i o n of m a r c a s i t e  because both m a r c a s i t e and p y r i t e form i n Fe-S-O-H experiments p y r i t e o n l y forms i n Fe-S-0 experiments. need to be done, i t now marcasite structure. phase i n the pure Fe-S  2)  appears  Although more e x t e n s i v e s t u d i e s  t h a t the H-S  bond may  stabilize  the  For these reasons m a r c a s i t e i s not shown as a system.  Thermodynamic diagrams  Thermodynamic c o n s i d e r a t i o n s y i e l d i n f o r m a t i o n on the phases i n the Fe-S  stable  system t o appear i n s e l e c t e d environments.  s u l p h u r as one o f components of the Fe-S its  but  Because  system i s a v e r y a c t i v e element,  a c t i v i t y i n the environment determines  the phase t o be  stabilized.  To date, a l t h o u g h many s t u d i e s have been made on the thermodyanmic p r o p e r t i e s of the Fe-S temperatures  system at h i g h temperatures,  are not a v a i l a b l e .  s t u d i e s at low  However, the d a t a a t h i g h  temperature  can be e x t r a p o l a t e d to approximate the thermodynamic p r o p e r t i e s o f s u l p h i d e s at low The  temperature.  s u l p h u r a c t i v i t i e s can be c a l c u l a t e d as a f u n c t i o n of  a c r o s s the c o m p o s i t i o n ranges,  composition  shown i n the phase diagram of F i g . 1,  by  * u s i n g the e x i s t i n g thermodynamic d a t a .  In F i g , 3 the s u l p h u r  activities  In t h i s work, u n l e s s o t h e r w i s e s t a t e , a l l thermodynamic d a t a were o b t a i n e d from " O x i d a t i o n P o t e n t i a l s " by L a t i m e r .  - 9 -  4 0 0 °C  FeS -20-L 50 F i g u r e 3.  RS2 r-  55  r —  60  ATOMIC PERCENT of S  I-J  65  Schematic diagram of a c t i v i t y o f s u l p h u r i n the Fe-S system.  -  10 -  were c a l c u l a t e d a t 400°C, and a r e shown s c h e m a t i c a l l y f o r lower temperature r e g i o n s c o r r e s p o n d i n g  3) The  t o d i f f e r e n t phase r e l a t i o n s h i p s .  p H - P o t e n t i a l diagrams f o r i r o n p H - p o t e n t i a l diagram  sulphide  shows a s t a b l e r e g i o n o f each phase i n  the Fe-S system i n aqueous environments from a thermodynamic p o i n t o f view. 3-1)  E q u i l i b r i u m p H - p o t e n t i a l diagram  F i g . 4 shows the p H - p o t e n t i a l diagram i n a c i d r e g i o n s made by H. Majima.  4  -3 The f e r r o u s i o n c o n c e n t r a t i o n i s 1 M and 10 M to make  the diagram more a p p l i c a b l e t o p r a c t i c a l c o n s i d e r a t i o n s , c o n c e n t r a t i o n s of o t h e r s o l u t e s a r e 1 M.  I n t h i s f i g u r e , the p y r r h o t i t e domain i s a  s m a l l r e g i o n , compared w i t h 3-2)  Meta-stable  pyrite. p H - p o t e n t i a l diagram  In p r a c t i c e the p y r r h o t i t e phase p e r s i s t s a t p o t e n t i a l s and pH's where p y r i t e i s s t a b l e because o f the extremely from p y r r h o t i t e .  Sulphur  that i s l e f t  slow f o r m a t i o n of p y r i t e  on p y r r h o t i t e d u r i n g o x i d a t i o n  does n o t r e a c t i n l a b o r a t o r y times w i t h unreacted pyrite.  p y r r h o t i t e t o form  I n F i g . 5 w i t h t h i s c o n s i d e r a t i o n the m e t a - s t a b l e  pH-potential  4 diagram f o r p y r r h o t i t e i n a c i d r e g i o n s i s shown by H. Majima.  This  diagram i s a p p l i c a b l e o n l y t o i r o n - s a t u r a t e d p y r r h o t i t e , which i s s t o i c h i o m e t r i c p y r r h o t i t e , because the f r e e e n t h a l p y v a l u e used i s o f p y r r h o t i t e saturated with i r o n .  For non-stoichiometric p y r r h o t i t e , i f  thermodynamic data a r e a v a i l a b l e , the'same m e t a l - s t a b l e  diagrams  can be drawn.  Fe^ g^S i s  The domain f o r p y r r h o t i t e of c o m p o s i t i o n  drawn t o i n d i c a t e the change i n s t a b i l i t y due t o c o m p o s i t i o n a l  changes  - 11 -  0.8H  r: 0.447 LU  X CO  to  "o  >  UJ r-  o CL  -0.4H  Fe + H S 2  i  -2  0  2  4  PH  I M (Fe ) 2+  „ „ _ F i g u r e 4.  , - M(Fe ) 3  0  P o t e n t i a l - p H diagram f o r s t a b l e Fe-S systems a t 25°C.  2+  - 12 -  1  -2  r  1  0  2 P  1  4  H  I  M (Fe " ") 2 1  IO" M(Fe ) 3  F i g u r e 5.  2+  P o t e n t i a l - p H diagram f o r m e t a s t a b l e Fe-S systems a t 25°C under c o n d i t i o n s where p y r i t e i s n o t formed.  - 13 -  of t h i s n o n - s t o i c h i o m e t r i c  compound.  The composition  F e . ^S i s near Q  that which corresponds t o e q u i l i b r i u m w i t h p y r i t e , i . e . , m o n o c l i n i c pyrrhotite,  (2)  Fe^S . / o 0  E l e c t r o c h e m i c a l Study of P y r r h o t i t e An  e a r l y e l e c t r o c h e m i c a l study  Wrabetz,"* as a p a r t o f e x t e n s i v e  i n p y r r h o t i t e was made by K.E.  contributions to electrochemical studies  of s u l p h i d e s by both h i m s e l f and h i s co-workers.  I n t h i s study the  s y n t h e s i z e d p y r r h o t i t e was used to i n v e s t i g a t e t h e e f f e c t o f f e r r o u s ion  c o n c e n t r a t i o n on t h e e l e c t r o d e p o t e n t i a l .  The data a r e shown i n -  Table I.  Table I.  Measured p o t e n t i a l o f the c e l l  (-)FeS/FeS0. •XM/KCl/Calomel R.E 4  at 18°C.  [Fe" " "] M 1  1  E  (mV) v s S.H.E.  [Fe" " "] M 1  1  E (mV) vs S.H.E.  0.358  396  0.0075  394  0.138  387  0.0020  402  0.042  394  0.00046  399  0.020  406  <average>  397  As seen i n Table  I , i t was concluded  t h a t t h e p o t e n t i a l d i d not depend  on f e r r o u s i o n c o n c e n t r a t i o n i n range o f 0.0004 ^ 0.3 M. the c e l l  Further,  using  (-)FeS/FeS0 , 0.1 M. H S 0 , 0.1 M/KCl/Calomel R.E. (+) the 4  2  4  p o t e n t i a l measured showed -0.40 ^ -0.45 V(S.H.E.) f o r the s y n t h e s i z e d  - 14 p y r r h o t i t e and +0.51 V(S.H.E.) f o r the n a t u r a l p y r r h o t i t e .  There was  a l a r g e d i f f e r e n c e i n p o t e n t i a l between those p y r r h o t i t e s .  This  d i f f e r e n c e i n p o t e n t i a l was not i n t e r p r e t e d i n h i s work. Then, M. Sato  p u b l i s h e d a s y s t e m a t i c work i n s e v e r a l s u l p h i d e s  t h a t was undertaken to measure the r e s t p o t e n t i a l of s u l p h i d e s i n changing the pH, and the c o n c e n t r a t i o n s s u l p h i d e i o n s i n the e l e c t r o l y t e .  of c o r r e s p o n d i n g  Unfortunately  m e t a l i o n s and  the p o t e n t i a l  measurement of p y r r h o t i t e i n the a c i d r e g i o n f a i l e d because of i t s poor r e p r o d u c i b i l i t y caused by the f o r m a t i o n and  f e r r o u s i o n s through the a c t i o n of a c i d s .  i n basic regions  sulphide  Nevertheless,  the data  showed t h a t the p o t e n t i a l f o r the n a t u r a l p y r r h o t i t e  was about 600 mV h i g h e r Recently  of hydrogen  than t h a t f o r the s y n t h e s i z e d p y r r h o t i t e .  S. Venkatachalam and R. M a l l i k a r j u n a n ' ' showed the  independence of the p o t e n t i a l of the p r e c i p i t a t e d f e r r o u s s u l p h i d e on f e r r o u s i o n c o n c e n t r a t i o n i n the range o f 0.001 ^ 0.5 M i n f e r r o u s ammonium s u l p h a t e s o l u t i o n . For the g e n e r a l e l e c t r o c h e m i c a l b e h a v i o u r of p y r r h o t i t e i n a c i d regions  i t can be d e s c r i b e d t h a t the r e s t p o t e n t i a l i s n o t i n f l u e n c e d  by f e r r o u s i o n c o n c e n t r a t i o n i n the e l e c t r o l y t e and the r e s t p o t e n t i a l of n a t u r a l p y r r h o t i t e i s more n o b l e than t h a t of s y n t h e s i z e d p y r r h o t i t e .  (3)  Leaching  of P y r r h o t i t e  I t i s w e l l known t h a t p y r r h o t i t e e a s i l y d i s s o l v e s i n t o a c i d s o l u t i o n forming according  f e r r o u s i o n s and hydrogen s u l p h i d e as r e a c t i o n  to the f o l l o w i n g  equation,  products  - 15  FeS  T h i s occurs  +  2H  --—*  +  -  Fe" " " 1  i n Kipp's generators  4  +  HS 2  (+)  (III-l)  t o produce hydrogen s u l p h i d e i n  standard  chemical l a b o r a t o r i e s . g H.A.  Pohl  proposed the f o l l o w i n g mechanism of hydrogen  sulphide  e v o l u t i o n from the p r e c i p i t a t e d f e r r o u s s u l p h i d e i n a c i d ,  FeS  +  H  +  •  Fe "*" 4  +  HS  +  H  +  •  HS  (+)  accounting  f o r the f a c t  2  t h a t FeS,  HS  (III-2)  (III-3)  CdS  and  ZnS  dissolve i n kinetically  f i r s t o r d e r r e a c t i o n s w i t h r e s p e c t to the c o n c e n t r a t i o n of hydrogen i o n , which suggests the step of  ( I I I - 2 ) as a r a t e - d e t e r m i n i n g  reaction.  In i n d u s t r y the hydrogen s u l p h i d e formed from p y r r h o t i t e can of i n t e r e s t product  to produce elemental  s u l p h u r as a commercially  be  valuable  by the o x i d a t i o n p r o c e s s , i . e .  HS 2  +  l/20  y  2  H0 2  +  S°  (III-4)  When p y r r h o t i t e i s d i r e c t l y o x i d i z e d i n aqueous media by oxygen, the f o l l o w i n g s t o i c h i o m e t r y of r e a c t i o n i s e s t a b l i s h e d ,  4FeS  +  30  2  >  2 F e  2°3  +  The mechanism of the o x i d a t i o n p r o c e s s  4 ?  of p y r r h o t i t e i s not  (IH-5)  fully  - 16 understood y e t .  9 K.W.  Downes and R.W.  Bruce  c a r r i e d out the o x i d a t i o n o f p y r r h o t i t e  at 110-125°C under h i g h oxygen p r e s s u r e . no  In a u t o c l a v e experiments  e l e m e n t a l s u l p h u r was observed except when the pH o f t h e s o l u t i o n  reached about 1.5.  The e v o l u t i o n o f hydrogen s u l p h i d e , when p y r r h o t i t e  i s added to a u t o c l a v e l i q u o r a t room temperature, has been  noticed.  These f a c t s l e a d t o the p o s t u l a t i o n o f the f o l l o w i n g mechanism o f r e a c t i o n ; water and p y r r h o t i t e i n the a u t o c l a v e r e a c t f i r s t ferrous  forming  sulphate,  FeS  +  20„ 2  y  This sulphate i s oxidized  6FeS0  +  4  FeSO. 4  (III-6)  to f e r r i c  1 l/20  sulphate,  > lle^SO^^  2  +  ^ °3 2  (III-7)  T h i s f e r r i c s u l p h a t e b e i n g u n s t a b l e i n n e u t r a l water h y d r o l y s e s to f e r r i c oxide and s u l p h u r i c  Fe (S0 ) 2  The  4  +  3  acid,  3H 0  •  2  2°3  ?e  +  3 H  2 °4 S  CIH-8)  s u l p h u r i c a c i d then d i s s o l v e s p y r r h o t i t e t o form H^S and f e r r o u s  sulphate,  FeS  •+  H S0. 2 4 o  v  FeSO. 4  +  H„S 2  CUI-9)  As a f o l l o w i n g s t e p , H^S form e l e m e n t a l  2  +  Fe (S0 )  HS  +  l/20  2  4  •  3  y  2  2FeS0  H0  +  2  +  4  ^SO^  +  S°  (111-10)  S°  (III-4)  G e r l a c h , H. Hahne and F. Pawlek"^ s t u d i e d the k i n e t i c s o f the  oxygen p r e s s u r e l e a c h i n g o f p y r r h o t i t e . and  s u l p h a t e or oxygen t o  sulphur,  HS  2  J.  is o x i d i z e d by f e r r i c  Sulphur, hydrogen s u l p h i d e  s u l p h a t e were d e t e c t e d as r e a c t i o n p r o d u c t s  of sulphur  during  l e a c h i n g , then, as a mechanism of r e a c t i o n the f o l l o w i n g s t e p s were proposed.  FeS  +  2H  HS 2  +  l/20  HS 2  +  20  HS 2  +  2Fe  HS  +  8Fe  2  A l s o , elemental  S°  +  HS  +  +  2  •  2  •  2  S°  S0  4  y  +  + + +  S  Fe"^ " 1  (III-l)  +  H0  (III-4)  +  2H  (III-ll)  2  +  4H 0  +  2Fe  y  2  S0  +  2H  +  4  ZYtt*  (111-12)  +  10H  +  (111-13)  sulphur r e a c t s w i t h oxygen to form s u l p h a t e i o n ,  3/20  2  +  H0 2  y  S0  4  +  2H  +  (111-14)  - 18 The o x i d a t i o n o f f e r r o u s t o f e r r i c i o n by oxygen o c c u r s slowly  i n s u l p h u r i c a c i d media, so the r e a c t i o n s  seem l e s s s i g n i f i c a n t , but the r e a c t i o n  (111-12) and  relatively (111-13)  ( I I I - l ) i s predominant because  most of the s u l p h u r was found as e l e m e n t a l s u l p h u r  (more than 70%).  IV.  (1)  THE METAL SULPHIDE ELECTRODE  Thermodynamic Aspect A metal s u l p h i d e e l e c t r o d e c o n s i s t s o f two components and t h e r e f o r e  i t s e q u i l i b r i u m p o t e n t i a l can be d e s c r i b e d components, i . e . a c c o r d i n g  i n terms o f e i t h e r o f i t s  t o the Nernst e q u a t i o n  f o r equilibrium  between m e t a l i n the s u l p h i d e and m e t a l c a t i o n i n the e l e c t r o l y t e ,  M^aq)  v  E and  M  +  ne ^  v°  =  ^—»•  E M  M° ( i n sulphide)  2.3RT  +  V**"  — - — log nF a  f o r e q u i l i b r i u m between s u l p h u r S° v E  S  ( i n sulphide) T7O,  - S E  +  ,_„  (IV-1)  B  +  2H  2.3RT  "IF—  l o g  where E° and E° a r e the standard M S  +  a  M  O  i n the s u l p h i d e and hydrogen +  2e  sulphide,  H S (aq) 2  2 H+ * S ° a  I^T  ( I V  "  2 )  electrode potentials.  When e q u i l i b r i u m i s reached between the e l e c t r o d e and the e l e c t r o l y t e , i.e.  the s u l p h i d e  i s i n a t o t a l s o l u b i l i t y e q u i l i b r i u m with  the e l e c t r o l y t e ,  the v a l u e of the p o t e n t i a l i s the same i n both c a s e s , because the e l e c t r o d e can e x e r t only one p o t e n t i a l . equations  (IV-1) and ( I V - 2 ) ,  Therefore,  a c c o r d i n g to  - 20 -  2 *  ° 4 . -3RT ,  E  =  E  V*  2  w  M "nT" +  l  o  7~  g  N L ?  is  S M O =  A  a  E  . 2.3RT _ ~~2F  +  b  l  Z  o  V  ' S° a  g  (  i  a  I  V  _  3  )  H S 2  obtained. A b a s i c thermodynamic  (  o  F  p r o p e r t y o f the m e t a l s u l p h i d e , ^2/n^l+  << 1 ) , i s t h a t the f r e e e n t h a l p y o f f o r m a t i o n o f t h e s u l p h i d e phase  d e f i n e s t h e r e l a t i o n s h i p between m e t a l and s u l p h u r a c t i v i t i e s , i . e .  2/nM°  A  F  (  4  +  (1 + <x)S°  ^  = -2.3RT l o g [ a  )  M  - 2.3RT l o g a ^  X/ s  Where  M  2 / n  S  7 ^ 2/n l+a  - -a,. g  s  . a  n  (IV-4)  1 - + a  o  ^  (IV-5)  =1 1 +  2/n l +  a  For a s o l u b i l i t y e q u i l i b r i u m  M  S-, + z/n l+a  2H  0 /  2/n  V+ K  >-  +  •«  a a S ' s°  '  (6) -  2 —  \ , S * 2/n l+a 1 +  V-  2/nM  H_S z  + .  S°  (IV-6)  2/n a a ' *H S " s°  -  2 H  ( I V  "  7 )  +  i s t h e e q u i l i b r i u m constant  a^ = 1 as mentioned b e f o r e . According 2/n l+a f o l l o w i n g e q u a t i o n can be o b t a i n e d : M  a  V+  a  i s o b t a i n e d , where  +  n+  f o r (IV-6) and  t o (IV-2) and ( I V - 7 ) , t h e  b  I? S E  =  E  w ° _L 2.3RT S If+  l  o  1 g  [a  2/n Mn+ * V r  l+a ,„ (6)  / K  J  ,  , ( I V  "  8 )  T T 7O N  - 21 From t h i s e q u a t i o n  v s E  =  E  =  =  y° 2.3RT S " ~^2F~ 1  E  E  0  8  K  and (IV-5)  2.3RT -2F~ V+  (6)  +  l  o  g  +  2/n , 1 , _o 2F ( 4 ) ' ( F  „o 2.3RT . „ S ' ~2F (6)  —W  _,o 2.3RT S " ~2F—  AF° (4) , 2.3RT . "2F— "nF- 8(V  l  l  i s obtained.  o  o  g  g  K  K  +  (6)  +  _  2  .  0 3  R  T l  o  V  g  " ( 4 ) , 2.3RT . 2/n 2.3RT ~2T*U*+ ~ ~2F +  l  +  L  o  g  l  O  , +  /  .  a M  o  o  2/n >  2/n  1  g  V  .  ,  )  (  I  V  _  9  T T 7n N  )  In (IV-9)  _o 2.3RT . S " -JTE  l  0  g  K  AF° . (4) ~~2F -  _ (6)  n  a  +  M  therefore, E  S=  This equation  (2)  E ;  M  +  —  l  0  g  i ^  shows the v a l i d i t y of (IV-3).  K i n e t i c Aspect The  r e v e r s i b l e p o t e n t i a l o f s u l p h i d e e l e c t r o d e s which can be  c a l c u l a t e d from thermochemical d a t a u s i n g the Nernst e q u a t i o n does n o t always agree w i t h t h a t o b t a i n e d  i n measurements.  most s u l p h i d e e l e c t r o d e systems belong a k i n e t i c c o n s i d e r a t i o n i s necessary sulphide electrode.  T h i s a r i s e s because  t o a p o l y e l e c t r o d e system where  t o i n t e r p r e t the p o t e n t i a l of the  In the a c i d r e g i o n the p o s s i b l e e l e c t r o c h e m i c a l  r e a c t i o n s i n t h i s p o l y e l e c t r o d e system i n c l u d e ; 1.  O x i d a t i o n o f m e t a l i n t h e s u l p h i d e to metal c a t i o n s M°(in s u l p h i d e )  • M  n +  +  ne;  i  al  - 22  2.  O x i d a t i o n of s u l p h u r i n the s u l p h i d e t o s u l p h a t e i o n s ,  S° ( i n s u l p h i d e )  3.  Reduction  S°  4.  +  Reduction  electrolyte,  M  6.  +  n +  into  +  4H 0  +  •  2  S0  4  +  +  + 6e;  i  a  2  +  2H  +  + 2e  *• H S;  i  2  of hydrogen i o n s i n t o hydrogen  2e  y  c  3  molecules,  H„; 2  i . c4  of c o r r e s p o n d i n g m e t a l i o n s , which are added to metal,  2e  •  M°;  i _ c5  O x i d a t i o n of hydrogen s u l p h i d e which i s d i s s o l v e d i n the  electrolyte,  H.S 2  i n t o elemental  y  S°  +  sulphur,  2H  +  +  2e;  i ,  ab  These r e d u c t i o n and o x i d a t i o n , i . e . c a t h o d i c and occur s i m u l t a n e o u s l y but s t a t i s t i c a l l y The  8H  of s u l p h u r i n the s u l p h i d e to hydrogen s u l p h i d e ,  ( i n sulphide)  Reduction  2H  5.  -  anodic p r o c e s s e s  independent  of one  r a t e of each r e a c t i o n , i . e . c u r r e n t d e n s i t y , i c  can  another. or i , can a  be d e s c r i b e d by the f o l l o w i n g e q u a t i o n s , a c c o r d i n g to e l e c t r o c h e m i c a l kinetics.  - 59 T a b l e IV.  E q u i l i b r i u m constants of  + 2H  = H S ( a q ) + 2/-n^  +  +  2  f o r v a r i o u s s u l p h i d e s a t 25°C =  2/n V +  Sulphide  a  2  . 2 H+  / a  log K  MnS FeS  • H S  .  CoS  Sulphide  log K  8.0  CdS  - 6.14  2.55  PbS  - 7.10  -0.33  CuS  -15.0 -18.9  NiS  (y)  -6.69  Cu S  ZnS  (Spal)  -4.12  Ag S  (Wurt)  -1.80  HgS  2  A c c o r d i n g to t h i s e q u a t i o n , a l t h o u g h i n t h e case when p -H- i s 10  -2  a  e  -15.58  2  g(aq')  -32.3  l  s  dependent on p + + a  e  2  o r l e s s than i t ,  t h e a c t i v i t y of -2  aqueous hydrogen s u l p h i d e a t e q u i l i b r i u m i s more than 3.55 x 10 which suggests  the continuous  e v o l u t i o n o f hydrogen s u l p h i d e i n t o t h e  He gas atmosphere which i s used  i n t h i s work.  On t h e o t h e r hand,  T a b l e IV s t a t e s t h a t CuS, C u S , Ag S and PbS have extremely 2  v a l u e s o f K.  2  small  T h i s i s a s s o c i a t e d w i t h n e g l i g i b l e H S e v o l u t i o n and 2  t h e r e f o r e promises  t h e p o s s i b i l i t y o f measuring t h e r e v e r s i b l e  potential  of the r e s p e c t i v e s u l p h i d e s , a n d indeed these have been e x p e r i m e n t a l l y o 1,1. b t a i• n e dA. > 6  1  6  When the r e s t p o t e n t i a l o f p y r r h o t i t e i s c o n t r o l l e d by the r e a c t i o n s of  ++ + [Fe] -y Fe + 2e as an anodic p r o c e s s and [S] + 2H + 2e •+ R^S  as a c a t h o d i c p r o c e s s , t h e c u r r e n t d e n s i t y o f each r e a c t i o n can be equated  a c c o r d i n g t o Equations  (IV-10) and (IV-11); f o r the anodic  process  - 23  for  anodic c u r r e n t  density,  n . a_  r ... a exp  cathodic  i  where k  current  =-ZFk  c  k  a" 1  c  •  V  • • • 3.  P  anodic and  cathodic  Faraday c o n s t a n t ;  q  and  are a c t i v i t i e s  cathodic  of c a t h o d i c  cathodic  of r e a c t a n t s  n, r , u, v, w,  of anodic  are orders  to each r e a c t a n t ;  and there  Z i s the  reactions, respectively; F is"the  p o t e n t i a l at the s u l p h i d e  of reactants  of  transfer  electrode.  the r a t e of each r e a c t i o n w i l l  steady s t a t e i s e s t a b l i s h e d and sum  f o r anodic and  e i s the p o t e n t i a l of the s u l p h i d e  the e l e c t r i c a l  activities  (IV-11)  i n each r e a c t i o n ; a, 0 are  involved  to these e q u a t i o n s ,  governed by  (IV-10)  e}  -ZBF RT  reactions with respect  c o e f f i c i e n t s f o r anodic and  constant,  6Xp  r e a c t i o n s , r e s p e c t i v e l y ; m,  number of e l e c t r o n s  According  w  are r e a c t i o n r a t e constants  cathodic  Za.F {• RT  density,  reactions, respectively: a , a and  r  P  2  for  -  be  electrode,  the t r a n s f e r c o e f f i c i e n t . i s no  rate If a  external disturbance,  r e a c t i o n r a t e s w i l l be e q u a l to the sum  of  the  anodic  reaction rates;  Zi  c  = Zi  Considering  (IV-12)  a  the topography of the e l e c t r o d e s u r f a c e  i n the  poly-  e l e c t r o d e system, i t i s not n e c e s s a r y to have s e p a r a t e macroscopic areas  - 24  -  which are e x c l u s i v e l y c a t h o d i c or anodic e i t h e r o p e r a t i o n a l l y or c o n c e p t u a l l y . d u r i n g one  i n s t a n t of time and  anodic  cathodic processes  and  adjacent  sites.  on a s u l p h i d e e l e c t r o d e ,  Any  one  s i t e may  be  anodic  c a t h o d i c d u r i n g another i n s t a n t , and can occur s i m u l t a n e o u s l y  on a t o m i c a l l y .  T h i s homogeneous s u r f a c e c o n d i t i o n w i l l become  e s p e c i a l l y important  when we  c o n s i d e r c u r r e n t d e n s i t y i n s t e a d of  c u r r e n t i n a q u a n t i t a t i v e understanding  of the  electrochemical  behaviour of the e l e c t r o d e . Here, i t i s h e l p f u l to use a c u r r e n t - d e n s i t y p o t e n t i a l diagram i n order  to understand b e t t e r the c u r r e n t - d e n s i t y p o t e n t i a l r e l a t i o n s h i p  i n the p o l y e l e c t r o d e system. are s c h e m a t i c a l l y p l o t t e d . parameters such as k r  a  In F i g . 6 the p o s s i b l e r e a c t i o n s , l ) - 6 ) The  l o c a t i o n s of each l i n e depends on  , k , a , a . c p q  on the v a l u e of a or 0 and  Z.  The  A l s o the s l o p e of each l i n e depends e f f e c t of c o n c e n t r a t i o n  which w i l l be s i g n i f i c a n t at h i g h c u r r e n t - d e n s i t y i n t h i s i s not accounted f o r i n order  the  to s i m p l i f y the d i s c u s s i o n .  polarization diagram In  this  s u l p h i d e p o l y e l e c t r o d e system where each p o s s i b l e r e a c t i o n i s independent and  a l t e r n a t i v e , the p o t e n t i a l of the e l e c t r o d e i s l a r g e l y determined by  the coupled  r e a c t i o n s which have the h i g h e s t  highest possible current d e n s i t i e s , i ^ ^ as the i n t e r s e c t i o n s of c a t h o d i c and p o t e n t i a l s of the e l e c t r o d e E^^ and  and  anodic E^-  current-density. i  ^  a  r  e  The  shown i n F i g . 6,  l i n e s coordinated with  S t r i c t l y speaking,  this  i n t e r p r e t a t i o n f o r the p o t e n t i a l from the i n t e r s e c t i o n of both i s not  c o r r e c t , because e q u a t i o n  intersection. potential E  m  the  lines  (IV-12) can not be s a t i s f i e d at  the  However, i f the other minute c u r r e n t d e n s i t i e s at  the  or E  n 9  were n e g l e c t e d ,  equation  (IV-12) y i e l d s  - 26 -  i  a  = i c  (IV-13)  When both of the coupled r e a c t i o n s d e t e r m i n i n g  the p o t e n t i a l o f  the s u l p h i d e a r e i d e n t i c a l , as the example shown i n F i g . 6, the p o t e n t i a l is  called  the r e v e r s i b l e or e q u i l i b r i u m p o t e n t i a l .  In e i t h e r o f these  c a s e s , the r e v e r s i b l e p o t e n t i a l f o r the metal-metal from  x  01  =  (IV-10),  a  x  =  _  1  c =  (IV-11),  n  F  k  m V  a  cation equilibrium,  (IV-13) and Z = n  naF _ ~RT 0 1  , c  r  6  X  P  {  E  } =  n  F  k  V +  e  x  p  {  " "RT  • , n£F _ 01 E  ,  }  (IV-14) i s obtained.  J  Equation  (IV-14) y i e l d s  RT (a+B)nF  01  „  k  c k  a  a  *  Mn+ a M  O  Here, a+g = 1 i n the case when anodic and c a t h o d i c p r o c e s s e s a r e i d e n t i c a l and k /k  C ne  -—>-«-  M°.  E  = K which i s the e q u i l i b r i u m constant f o r M  n +  +  SL  Therefore,  RT „ . RT „ = — An K + — In nF nF a v  n i  01  „o  = M E  .  M  O  2.3RT .  + ~~~o~F~  , _. . „ T  ~^yp~  T h i s e q u a t i o n i s i d e n t i c a l w i t h t h e Nernst s e c t i o n on thermodynamic sulphur-hydrogen  considerations.  ( I V _ 1 5 )  e q u a t i o n d e r i v e d i n the I n the same manner f o r the  s u l p h i d e e q u i l i b r i u m the r e v e r s i b l e  potential  - 27 -  02 = E, +  i s derived.  2.3RT 2F  3  log  H+  (IV-16)  V.  (1)  EXPERIMENTAL  Materials N a t u r a l and  s y n t h e s i z e d p y r r h o t i t e s were used  i n this  experiment.  N a t u r a l m i n e r a l s were o b t a i n e d from the S u l l i v a n Mine i n Kimberley, and  the C h i c h i b u Mine i n Japan.  M i c r o s c o p i c o b s e r v a t i o n s d i d not show  any o t h e r phase except p y r r h o t i t e . s t a b l e s t a t e thermodynamically  N a t u r a l p y r r h o t i t e s are i n a more  than s y n t h e s i z e d p y r r h o t i t e s .  they i n v a r i a b l y c o n t a i n i m p u r i t y elements such as N i , Co, For e x p e r i m e n t a l purposes  i t i s very d i f f i c u l t  Cu, As e t c .  The s y n t h e s i z e d  p y r r h o t i t e s are r e q u i r e d e s p e c i a l l y to examine the Fe:S Three methods were used  However,  to o b t a i n p y r r h o t i t e s  having a s y s t e m a t i c a l l y v a r y i n g range of c o m p o s i t i o n .  ratio.  B.C.  composition  i n t h i s work to s y n t h e s i z e p y r r h o t i t e s .  Method I . Sulphur  lump c r y s t a l s  0.022 cm diameter)  ( c h e m i c a l pure) and  i r o n wire  (99.9%,  r e s p e c t i v e l y were weighed out to correspond  to an  a p p r o p r i a t e c o m p o s i t i o n of p y r r h o t i t e , then p l a c e d t o g e t h e r i n t o a s i l i c a g l a s s ("Vycor") tube, 0.8  cm o u t e r - d i a m e t e r , which  evacuated  tube was  heated  and  sealed.  The Vycor  t o 500°C f o r one day and  c o o l e d to room temperature. was  produced.  was  p l a c e d i n the f u r n a c e ,  t o 700°C f o r two  In each Vycor  96%  days,  then  furnace  tube about one gram p y r r h o t i t e  At 700°C the s u l p h u r decomposition  pressure  equilibrated  - 29  w i t h p y r r h o t i t e was  -  n e g l i g i b l y s m a l l up to the c o m p o s i t i o n of the  p y r r h o t i t e of 48 atomic p e r c e n t Fe, i . e . l e s s than 0.1 assumed t h a t a l l s u l p h u r put i n t o the Vycor  atm,  so i t was  tube r e a c t e d w i t h  However, below about 48 atomic p e r c e n t Fe the s u l p h u r  iron.  decomposition  p r e s s u r e of p y r r h o t i t e can not be n e g l e c t e d i n the m a t e r i a l b a l a n c e of sulphur.  T h e r e f o r e t h i s method was  not used  m a t e r i a l of l e s s than 48 atomic p e r c e n t  for preparing pyrrhotite  Fe.  Method I I . The "Dew a Vycor iron  tube, 1.5  (99.99%) and  evacuated, two  p o i n t method"; the apparatus cm o u t e r - d i a m e t e r  and  f o r t h i s method c o n s i s t e d of  30 cm l e n g t h , i n which sheet  s u l p h u r were p l a c e d a t each end,  then t h i s tube  was  s e a l e d and p l a c e d i n f u r n a c e system which c o n s i s t s of  s e p a r a t e l y heated  zones.  On the i r o n s i d e the temperature  was  kept constant a t 700°C, w h i l e on the s u l p h u r s i d e the temperature a d j u s t e d i n the range 110°C p r e s s u r e of s u l p h u r .  was  to 450°C t o e s t a b l i s h a chosen p a r t i a l  In t h i s method, by weighing  the i r o n samples  b e f o r e and a f t e r each run the i r o n content i n s u l p h i d e can be  calculated.  Thus the d e t e r m i n a t i o n of Fe content i n the p y r r h o t i t e does not c o n t a i n any e r r o r due  to incomplete  b e i n g s u l p h i d i z e d was to 450°C was  r e a c t i o n of s u l p h u r .  However, when the  kept a t 700°C, a s u l p h u r temperature  between  iron 110°C  too h i g h to s y n t h e s i z e the p y r r h o t i t e c o n t a i n i n g more  than 48 atomic p e r c e n t Fe.  Thus methods I and  the s y n t h e s i s of p y r r h o t i t e s w i t h wide range of  I I i n combination  permitted  composition.  In t h i s method each run took 4 days to complete s u l p h i d i z a t i o n of sheet i r o n  (0.04  cm t h i c k n e s s ) and  to homogenize the r e s u l t i n g  pyrrhotite.  - 30 The vapour p r e s s u r e o f s u l p h u r i s known from the work o f W.  West and  13 A; Menzies,  shown i n F i g . 7.  I t was  assumed t h a t by steady s t a t e  c o n d i t i o n s the t o t a l s u l p h u r p r e s s u r e s at both ends i n the Vycor tube were e q u a l but not the p a r t i a l p r e s s u r e s of the d i f f e r e n t m o l e c u l a r species.  The vapour d e n s i t y o f the gas i n c r e a s e d markedly  hot zone a t 700°C, where the gas c o n s i s t e d of m a i n l y  from the  m o l e c u l e s , to  the c o l d e r p a r t h e l d i n the range 110-450°C, where i t c o n s i s t e d of Sg, S^,  S^,  S,., S^,  S^ and  molecules.  In F i g . 8 the v a r i a t i o n i n  Fe content w i t h d i f f e r e n t s u l p h u r bath temperatures i s shown. P y r r h o t i t e made a t 700°C was electrodes i n electrochemical  f a i r l y massive and c o u l d be used f o r studies.  Method I I I . T h i s method can be c a l l e d the "Melt method". p u r i t y , and s u l p h u r powder were mixed t h i s m i x t u r e was  i n a weight r a t i o o f 1:1;  held f o r 5 hours.  A f t e r t h a t the temperature  i n c r e a s e d to 1250°C, above the m e l t i n g p o i n t  where molten FeS was  a p e r i o d o f 10 days.  (1190°C) of FeS,  kept f o r a h a l f an hour, then c o o l e d to 750°C,  from t h i s temperature the sample was  c o o l e d to room temperature over  A l l p r o c e s s e s of h e a t i n g , m e l t i n g and  were undertaken i n an i n e r t atmosphere supposed  then  g r a d u a l l y heated i n a g r a p h i t e c r u c i b l e to 700°C at  which temperature i t was was  I r o n powder, 99%  of He f l o w .  cooling  This pyrrhotite  to be i r o n s a t u r a t e d or l e s s excess s u l p h u r p y r r h o t i t e .  t e c h n i q u e was  was  This  e s s e n t i a l f o r the p r o d u c t i o n of lumps of p y r r h o t i t e .  B e s i d e s p y r r h o t i t e s , o t h e r m a t e r i a l s o c c a s i o n a l l y used were p y r i t e , i r o n powder and c h a l c o c i t e . was  from Montana, U.S.A.  The s o u r c e of p y r i t e was  I r o n powder used was  not known, c h a l c o c i t e  of 99% p u r i t y .  -5-J  100 F i g u r e 7.  .  1  200  •  TEMPERATURE  .  •  300  :  400  °C  T o t a l vapour p r e s s u r e o f sulphur between 120 and 450 C. C  CD  47,§) U, Z  U J  ce U J C L  O  § 45" o  100  F i g u r e 8.  200  300  SULPHUR BATH TEMPERATURE V a r i a t i o n i n Fe content w i t h d i f f e r e n t  °C  s u l p h u r bath  400  temperatures.  - 33 -  (2)  X-Ray A n a l y s i s of P y r r h o t i t e s P y r r h o t i t e s used  i n t h i s study were examined by X-ray  to i d e n t i f y the phases p r e s e n t .  A Debye-Scherrer  take the powder d i f f r a c t i o n p a t t e r n s w i t h a CoK  diffraction  camera was X-ray  used  tube.  to  In F i g . 9  a the X-ray  d i f f r a c t i o n p a t t e r n s are p r e s e n t e d .  ASTM cards f o r p y r r h o t i t e ,  p y r i t e , and m a r c a s i t e are i n c l u d e d on l i n e s 1, 10 and f o r comparison w i t h r e s u l t s o b t a i n e d .  11,  respectively  A c c o r d i n g to these d a t a , i t can  be concluded  t h a t p y r r h o t i t e s s y n t h e s i z e d by methods d e s c r i b e d  corresponded  t o p y r r h o t i t e , w h i l e n e i t h e r p y r i t e nor m a r c a s i t e were  present.  The  d i f f r a c t i o n l i n e r e p r e s e n t e d by  h i g h e s t i n t e n s i t y , changed s l i g h t l y  (102), which has  i n p o s i t i o n due  the  to the extent 14  of n o n - s t o i c h i o m e t r y of p y r r h o t i t e as r e p o r t e d by M.  Haraldsen.  However, d a t a o b t a i n e d i n t h i s work were too s c a t t e r e d to e s t a b l i s h a reliable relationship.  In p a t t e r n s 12 and  13,  the powder p y r r h o t i t e s  b e f o r e and a f t e r the r e s t p o t e n t i a l measurement were examined t o check the p o s s i b i l i t y of a phase change; however, X-ray p i c t u r e s  indicated  that no such phase change o c c u r r e d , because b o t h X-ray p a t t e r n s were essentially identical. not apparent (3)  The phase r e l a t i o n s h i p s d e s c r i b e d e a r l i e r  i n t h i s X-ray  was  study.  Sulphide Electrodes In t h i s study two k i n d s of s u l p h i d e e l e c t r o d e s were used.  them was  made i n the f o l l o w i n g way;  a m i n e r a l plaque was  s e t t i n g p l a s t i c r e s i n , "Koldmount", w i t h the two resin.  flat  of  mounted i n s e l f -  sides free  A mercury c o n t a c t i n g column c o n t a i n i n g a copper  formed i n a r e s i n mount on one  One  lead wire  from was  of the f r e e s i d e s of the e l e c t r o d e , which  j_L LA jj  I  Sullivan Po.  J»—M—I  I  -  Synthesized Po.50at%Fe  i  J I • ii Li  Jj  L_j  J_J  I I i i »  I — A -  ±  • 49.67 =  i_ •jl  •48.72  =  •47.72  =  -46.39 = 45.42  .in  i.»  i  • Pyrite  1111 • I  • »  ASTM Py.  • IM,  S  *  .  ASTM Marcasite 45.36 =  I 8  -mm  m  before run after run  30  60  90  120  26 F i g u r e 9.  X-ray d i f f r a c t i o n p a t t e r n s o f v a r i o u s i r o n s u l p h i d e s Co-K.  a  radiation.  using  - 35 -  remained i s o l a t e d from the s o l u t i o n .  The o t h e r f r e e s i d e then became the  a c t i v e e l e c t r o d e s u r f a c e i n c o n t a c t w i t h the e l e c t r o l y t e .  A drawing of  t h i s "mounted e l e c t r o d e " i s shown i n F i g . 1 0 ( a ) . The  other e l e c t r o d e c o n s i s t e d of s u l p h i d e powder f l o a t i n g on a  mercury p o o l .  A g l a s s U-tube was  which the s u l p h i d e powder was  f i l l e d w i t h mercury on one  floated.  A copper  lead wire  entered  mercury from the o t h e r s i d e f o r an e l e c t r i c a l c o n n e c t i o n . c a l l e d a "powder e l e c t r o d e " , i s shown i n F i g . 10(b). as a thermodynamically  s i d e of the  This electrode,  Mercury i s known  n o b l e metal whose s t a n d a r d s i n g l e e l e c t r o d e  p o t e n t i a l i s +0.789 v o l t s and has a v e r y h i g h hydrogen o v e r y o l t a g e :  a s s o c i a t e d w i t h a v e r y low exchange c u r r e n t d e n s i t y , i 2 A/cm  = 10  -13  -10  -11  15 .  In a d d i t i o n , p y r r h o t i t e w i l l not r e a c t w i t h mercury because  of more n e g a t i v e s t a n d a r d f r e e e n t h a l p y of f o r m a t i o n of s u l p h i d e s f o r FeS, -23.32 Kcal/mole,  than t h a t f o r HgS,  f a c t o r s were c o n s i d e r e d i n the experiments  -11.05 Kcal/mole.  These  comparing the r e s t  potentials  when measured w i t h the mounted e l e c t r o d e s as compared to the powder electrode.  In T a b l e I I the r e s u l t s f o r both n a t u r a l p y r r h o t i t e  and  c h a l c o c i t e are shown. Table I I .  Comparison of the r e s t p o t e n t i a l measured w i t h the mounted e l e c t r o d e and the powder e l e c t r o d e .  C h i c h i b u p y r r h o t i t e , 25°C pH = 2.85, [Fe* -] = 0.01 4  mounted e l e c t r o d e +161 +143  mV  (S.H.E.)  powder e l e c t r o d e +186 +124  mV  (S.H.E.)  M  C h a l c o c i t e , 25°C pH = 1.35 [Cu"""] = 0.1 4  1  mounted e l e c t r o d e +433 mV  (S.H.E.)  powder e l e c t r o d e +439 mA  (S.H.E.)  M  - 36 -  •Copper lead wire —Glass tube  Mercury rResin Sulphide  (a) F i g u r e 10.  Iron sulphide  electrodes  Ca) mounted;  (b) powder.  pSulphide powder Mercury  According  to these  data, the powder e l e c t r o d e i s s u i t a b l e f o r the  s u l p h i d e e l e c t r o d e , although  the v a l u e s  appear more s c a t t e r e d w i t h the  powder e l e c t r o d e .  (4)  Electrolytic The  Cell  rubber bung a c t i n g as the top of the c e l l  d i s p e r s e r , the  sulphide e l e c t r o d e , a Pt-counter  contained  a gas  e l e c t r o d e , a Luggin  c a p i l l a r y w i t h r e f e r e n c e e l e c t r o d e , and a gas o u t l e t tube, and was fitted  t o a 400-ml ( t a l l s t y l e ) beaker.  U s u a l l y 250 ml of e l e c t r o l y t e  was p l a c e d i n the beaker and a g i t a t e d m i l d l y w i t h In F i g . 11 the s k e t c h f o r the c e l l  i s shown.  were made w i t h the KC1 s a t u r a t e d calomel a s s o c i a t e d w i t h a Luggin c a p i l l a r y .  a magnetic  stirrer.  P o t e n t i a l measurements  e l e c t r o d e as a r e f e r e n c e  The end of the gas o u t l e t tube  was water s e a l e d , so the s m a l l p o s i t i v e p r e s s u r e  i n the c e l l  caused  by the water s e a l r e s u l t e d i n an improved c o n t a c t of the s u l p h i d e  with  mercury on which the s u l p h i d e powder was f l o a t e d .  (5)  Reagents The  for  e l e c t r o l y t e s o l u t i o n c o n s i s t e d of 1 M Na_S0, as a b u f f e r , H„S0 2 4 2 •  pH c o n t r o l of the s o l u t i o n and FeSO^ of the d e s i r e d f e r r o u s i o n  concentration s o l u t i o n . Helium and hydrogen s u l p h i d e gas used were d i r e c t l y passed from both gas c y l i n d e r s without  (6)  Experimental Before  purification.  Procedure  each run the e l e c t r o l y t e was deoxygenated by b u b b l i n g  helium  - 38 -  I  2  4  5  I- Gas out-let tube 2. Pt counter electrode 3. Gas disperser 4. Sulphide electrode 5. Luggin capillary 6. Stirrer magnet  Figure 11.  Sketch of e l e c t r o l y t i c  cell.  - 39 -  gas through i t f o r at l e a s t  two hours.  mounted e l e c t r o d e , which was e l e c t r o d e , which was was  first  first  ground  Then an e l e c t r o d e , e i t h e r  the  p o l i s h e d on emery paper, or the powder i n a ceramic mortar under  immersed i n t o the e l e c t r o l y t e .  methanol,  A f t e r a c e r t a i n p e r i o d both  e l e c t r o d e s were c a t h o d i z e d f o r 30 minutes  a t around -400  mV.  This  c a t h o d i c e x c u r s i o n c o u l d not be expected to change the c o m p o s i t i o n o f p y r r h o t i t e , because  the c u r r e n t d i d not exceed about 1 coulomb.  A f t e r the c a t h o d i c e x c u r s i o n the r e s t p o t e n t i a l was i n t e r v a l s u n t i l a s t a b l e p o t e n t i a l v a l u e was  read a t  o b t a i n e d , i . e . , 1-5  days.  To read p o t e n t i a l and to p o l a r i z e the e l e c t r o d e , a Wenking Standard P o t e n t i o s t a t Model 68 TS10 was of  used.  F i g . 12 shows the r e l a t i o n s h i p s  p o t e n t i a l w i t h time d u r i n g the measurement.  A l l p o t e n t i a l s were  measured a g a i n s t the K C l - s a t u r a t e d calomel e l e c t r o d e , which was to  be +0.241 v o l t s r e l a t i v e to the s t a n d a r d hydrogen  taken  e l e c t r o d e a t room  temperature, and the p o t e n t i a l s are r e p o r t e d on the s t a n d a r d p o t e n t i a l s c a l e i n t h i s work. The  temperature o f the s o l u t i o n was  not e s p e c i a l l y measured and  c o n t r o l l e d i n room temperature experiments. the  e l e c t r o l y t e was  concentration.  B e f o r e and a f t e r each run  u s u a l l y a n a l y s e d to determine pH and f e r r o u s i o n  However, i n most cases no s i g n i f i c a n t  v a l u e s were observed.  changes i n these  REST POTENTIAL : Volts (SHE)  -  0*7 -  VI. RESULTS AND  (1)  E f f e c t of F e r r o u s Ion The  DISCUSSION  Concentration  f e r r o u s i o n e f f e c t on the r e s t p o t e n t i a l was i n v e s t i g a t e d  i n the range of c o n c e n t r a t i o n of 0.001-0.1 M o b t a i n e d by a d d i t i o n of FeSO^.7H 0 a t approximate pH 2  i o n c o n c e n t r a t i o n was  2.8 w i t h He b u b b l i n g .  The  ferrous  checked b e f o r e and a f t e r each run.  most cases no s i g n i f i c a n t  change was  detected.  However, i n  F i g . 13 shows d a t a  o b t a i n e d f o r f o u r d i f f e r e n t s t o i c h i o m e t r i e s of p y r r h o t i t e . ferrous ion effect  on the r e s t p o t e n t i a l i s obscure  because of  s c a t t e r e d d a t a ; n e v e r t h e l e s s no e f f e c t of f e r r o u s i o n may f o r the l i m i t i n g pyrrhotites. 0.01  The  M from 0.001  0.001  M showed no  F i g . 13.  I t may  compositions  50 atomic  be  percent  seen of  i n c r e a s e d to  M a f t e r the measurement of the r e s t p o t e n t i a l i n change i n the p o t e n t i a l , as i n d i c a t e d by arrows i n be concluded  t h a t f e r r o u s i o n does not a f f e c t T h i s i s supported  S. Venkatachalam e t a l . ^ who  f e r r o u s i o n than 0.1  by K.E.  the  Wrabetz"*  found no e f f e c t of f e r r o u s i o n on  the r e s t p o t e n t i a l of the p y r r h o t i t e .  electrolyte,  and  experiment i n which f e r r o u s i o n was  rest potential s i g n i f i c a n t l y . and  of 46.2  The  M at about 2.8  At h i g h e r c o n c e n t r a t i o n s of  of pH a p r e c i p i t a t e formed i n the  so such c o n c e n t r a t i o n s were not  used.  - 42 -  0.2 (46.20 at%Fe)  o o  J  JL  c g b - - " - - ^ 8 LU X  0-  if)  (49.26 at%Fe ) (49.86 at%Fe)  A  o  > -0.2-  < Z LU  O CL -0.4-  (50  at%Fe)  •2-}h-»  10M(Fe* )  pH^2.8  +  LU  3  -I  -2  LOG. ( Fe ) : M 24  Figure 13.  Dependence of the rest potential on ferrous ion concentration.  - 43 -  (2)  E f f e c t of pH In changing pH by a d d i t i o n o f s u l p h u r i c a c i d the r e s t  potential  of p y r r h o t i t e s was measured i n the presence o f f e r r o u s i o n i n t h e electrolyte.  The measurements were made on four d i f f e r e n t  m e t r i c s of the p y r r h o t i t e .  stoichio-  F i g . 14(a)-(c) p r e s e n t the d a t a o b t a i n e d .  I t i s c l e a r t h a t the r e s t p o t e n t i a l decreases s h a r p l y as pH i n c r e a s e s . The dependence of pH ranged  from about -150 t o -350 mV/pH.  From these  dependences o f the r e s t p o t e n t i a l on f e r r o u s i o n and pH i t i s e v i d e n t t h a t e q u i l i b r i u m between i r o n i n s u l p h i d e and f e r r o u s i o n i n t h e e l e c t r o l y t e i s n o t e s t a b l i s h e d a t l e a s t i n these ranges.  I f equilibrium  were reached, the p o t e n t i a l would depend on the f e r r o u s i o n c o n c e n t r a t i o n and would n o t depend on pH, a c c o r d i n g t o the Nernst e q u a t i o n (IV-1).  (3)  E f f e c t o f Hydrogen S u l p h i d e The next experiment  bubbled  was c a r r i e d out w i t h hydrogen s u l p h i d e  through the e l e c t r o l y t e .  S i n c e mercury r e a c t s w i t h  hydrogen  s u l p h i d e t o form HgS, the powder e l e c t r o d e c o u l d n o t be used i n t h i s experiment,  and o n l y the mounted e l e c t r o d e was used.  Initially  the r e s t p o t e n t i a l was measured i n a h e l i u m atmosphere, then ^ S was i n t r o d u c e d and the r e s t p o t e n t i a l was a g a i n measured a t a s u i t a b l e interval.  R e s u l t s are shown i n F i g . 15(a) and ( b ) .  These data were  o b t a i n e d a t pH = 3.01 w i t h o u t f e r r o u s i o n i n the e l e c t r o l y t e .  There  are sharp drops i n the p o t e n t i a l f o r the n a t u r a l C h i c h i b u p y r r h o t i t e and the s y n t h e s i z e d 47.49 atomic p e r c e n t Fe p y r r h o t i t e .  However,  no change i n p o t e n t i a l was found f o r the 50 atomic p e r c e n t s y n t h e s i z e d  - 44 -  \  REST P O T E N T I A L : Vol ts (SHE)  0.2-  \V  o-  \  -0.2"  -0.4-  ( a ) \  46.20 at%Fe (Fe > O.OIM 24  \ •  2  r  \ i  3  4  PH F i g u r e 14a.  Dependence of the r e s t p o t e n t i a l on pH Ca);  pH-dependence of the r e v e r s i b l e p o t e n t i a l f o r  IS] + 2 H  +  +  2e  t  H 0  S  calculated  from the Nernst e q u a t i o n .  - 45  -  0.2  -0.6-J  —  r  I  ,  ,  r  2  3  4  PH Figure  14b.  Dependence of  the  rest potential  on  pH.  - 46  -  0.2H  Figure  14c.  Dependence of  the  rest potential  on  pH.  - 47 -  p y r r h o t i t e even a f t e r l ^ S i n t r o d u c t i o n . RyS  bubbling  I n these  experiments  the o u t l e t gas from the c e l l f o r the 50 atomic  before  percent  Fe p y r r h o t i t e c o n t a i n e d B^S (as d e t e c t e d by s m e l l ) , but no E^S was detected  from the c e l l s  c o n t a i n i n g the o t h e r p y r r h o t i t e s .  A l l exhaust  gases were passed through a s o l u t i o n c o n t a i n i n g 1 M Cd i o n s o r Ag i o n s , and i n a l l cases although percent  y e l l o w i s h CdS o r brown Ag^S p r e c i p i t a t e s formed,  t h e p r e c i p i t a t i o n r a t e was much g r e a t e r f o r t h e 50 atomic synthesized p y r r h o t i t e .  According  t o t h i s experiment, i t i s p o s s i b l e t o make the f o l l o w i n g  c o n c l u s i o n ; f o r the n a t u r a l and 47.49 atomic p e r c e n t  Fe s y n t h e s i z e d  p y r r h o t i t e s the e f f e c t o f hydrogen s u l p h i d e on the p o t e n t i a l i s l a r g e because o f a low hydrogen s u l p h i d e e v o l u t i o n r a t e from the e l e c t r o d e s . On the o t h e r hand, f o r the 50 atomic p e r c e n t  Fe p y r r h o t i t e the e f f e c t  of hydrogen s u l p h i d e i s n o t d e t e c t a b l e because o f a h i g h i n i t i a l  rate  of hydrogen s u l p h i d e e v o l u t i o n from the e l e c t r o d e .  (4). E f f e c t o f N o n - S t o i c h i o m e t r y of P y r r h o t i t e The  a c t i v i t i e s of sulphur  and i r o n i n the p y r r h o t i t e as w e l l as  the a c t i v i t i e s o f i o n s i n the e l e c t r o l y t e can a f f e c t t h e r e s t a c c o r d i n g t o the Nernst e q u a t i o n  potential,  (IV-1) and ( I V - 2 ) .  In t h i s work the r e s t p o t e n t i a l was measured w i t h  different  I |  compositions  of p y r r h o t i t e a t pH =  3  and [Fe  ] = 0.01 M.  The  * The e f f e c t o f a c t i v i t y of the components i n a s i n g l e phase-two component e l e c t r o d e has been i g n o r e d i n most p u b l i s h e d works on sulphide electrochemistry. These a c t i v i t i e s a r e v e r y s e n s i t i v e to composition i n the s i n g l e phase r e g i o n , and as a r e s u l t cause d r a s t i c changes i n the p o t e n t i a l when c o m p o s i t i o n i s changed.  -  48  so  TIME F i g u r e 15a.  Rest p o t e n t i a l  -  :  Hrs  changes i n d i f f e r e n t  Too atmosphere.  - 49 -  TIME: Figure 15b.  Hrs  Rest p o t e n t i a l changes i n d i f f e r e n t atmosphere.  - 50 -  r e s u l t s are varies  shown i n F i g . 16.  through a wide range between -350  changes from 50  to 46  atomic p e r c e n t .  measured f o r i r o n powder on Point  (B)  i n F i g . 16  (C) i n F i g . 16  +150  In F i g . 16  rest  mV  as  the  potential the  Fe  potential  shown as p o i n t  containing  two  p y r r h o t i t e powder.  pyrrhotite  phases; Fe and  FeS.  of Point  Point  Points  (D)  shows the  (E) show the  rest  and  rest potentials  specimens from C h i c h i b u .  Generally natural  s u l p h i d e s have more p o s i t i v e p o t e n t i a l s  synthesized sulphides.  Pyrrhotite  conforms to t h i s  excess  s u l p h u r which i s more s t a b l e under an o x i d i z i n g atmosphere. p u r r h o t i t e which has  than  generalization.  Most s u l p h i d e s tend towards a n o n - s t o i c h i o m e t r y c o n t a i n i n g  and  (A).  r e s t p o t e n t i a l of the p y r r h o t i t e  p o t e n t i a l measured f o r p y r i t e and  natural  content  shows the p o t e n t i a l f o r the m i x t u r e of i r o n powder  i r o n saturated  of n a t u r a l  and  a mercury p o o l was  shows the  52.8, atomic p e r c e n t Fe and  the  From these d a t a , the  been formed at h i g h s u l p h u r  Therefore,  activities  l a t e r exposed to o x i d i z i n g atmospheres w i l l always show a more  p o s i t i v e p o t e n t i a l than s y n t h e s i z e d p y r r h o t i t e  formed at h i g h i r o n  activities. I f these r e s t p o t e n t i a l s measured corresponded to potentials, could  be  the  reversible  a c t i v i t y of each component, i . e . , s u l p h u r and  calculated,  Duhem r e l a t i o n s h i p .  a c c o r d i n g to the Nernst e q u a t i o n and  the  However, the p o s s i b i l i t y of measuring  the  r e v e r s i b l e p o t e n t i a l has a c t i v i t i e s were not  a l r e a d y been shown to be  calculated  from the  rest  poor, and  potentials.  so  iron, Gibbs-  - 51 -  0.4 D  0.2H O O  LU X CO  9Qf  8  •OA  %f/  co  o  O > -J <  LU IO CL  9§  o  0  /  ° / A, Iron  -0.21  B, C, D, E,  8  B  /  C  1/  -0.4H rloA CO  52.8 at%Fe A+ B Pyrite Natural Pyrrhotite  (Fe )=0.0l M PH =. 3 2+  LU 0C  -tt  100^ 53  r  50  48  46  ATOMIC PERCENT Fe F i g u r e 16.  V a r i a t i o n i n the r e s t p o t e n t i a l w i t h change i n c o m p o s i t i o n of  pyrrhotite.  - 52 -  (5)  E f f e c t o f R e s i d u a l Impurity i n the E l e c t r o l y t e In e l e c t r o c h e m i c a l experiments  o x i d a n t o r r e d u c t a n t i n the r o l e t o determine  The  former  e l e c t r o l y t e sometimes p l a y s an important  the p o t e n t i a l  In t h i s experiment,  i t i s known t h a t t h e r e s i d u a l  o f the e l e c t r o d e even when v e r y  dilute.  p o s s i b l e o x i d a n t s a r e f e r r i c i o n and oxygen gas.  can come from  the f e r r o u s s u l p h a t e reagent and the l a t t e r  can s c a r c e l y be avoided from the atmosphere even w i t h He gas b u b b l i n g . The  experiment  was c a r r i e d  out i n the c e l l  o r d e r t o check the e f f e c t o f r e s i d u a l they e x i s t , -on the r e s t p o t e n t i a l . they can be reduced  tube t o p r e v e n t  o x i d a n t s i n the e l e c t r o l y t e , i f  I f oxidants e x i s t  on the Pt w i r e cathode  anode compartment i s i s o l a t e d  shown i n F i g . 17 i n  i n the e l e c t r o l y t e ,  during e l e c t r o l y s i s .  from the e l e c t r o l y t e w i t h a c a p i l l a r y  t h e m i g r a t i o n of o x i d a n t s p e c i e s formed on the anode  i n t o the b u l k o f e l e c t r o l y t e .  In T a b l e I I I d a t a o b t a i n e d i n t h i s  are compared w i t h those measured i n the o r d i n a r y c e l l . of e l e c t r o l y t e was c o n t i n u e d d u r i n g the r e s t p o t e n t i a l  Table I I I .  An  The c a t h o d i z a t i o n measurement.  Comparison o f the r e s t p o t e n t i a l s measured w i t h and without  reduction of e l e c t r o l y t e  r e d u c t i o n of e l e c t r o l y t e  measured p o t e n t i a l s  with  +101, +91  without  +139, +147, +91, +80, +150, +60, +50  (mV)  [Fe**] = 0.01 M, pH = 2.80 cathode 46.2  potential  a t % Fe  = -259 mV f o r  pyrrhotite.  cell  - 53 -  2 3  1. Gas out-let & Anode compartment 2. Pt cathode 3. Gas bubbler 4. Sulphide electrode 5. Luggin capillary 6. Stirrer magnet  F i g u r e 17.  Sketch of the c e l l f o r r e d u c t i o n of the  electrolyte.  - 54  -  Before and  a f t e r the run,  detected.  A c c o r d i n g to T a b l e I I I , i t may  significant  checked, but  change i n the p o t e n t i a l .  that e i t h e r there as e x i s t do not  (6)  the pH was  i s no  be  no  change i n pH  seen t h a t  Therefore,  i t may  there be  was  i s no  concluded  o x i d a n t i n the e l e c t r o l y t e or such o x i d a n t s  take p a r t i n the p o t e n t i a l determing  reaction.  I n t e r p r e t a t i o n of the Measured Rest P o t e n t i a l The  described  behaviour of the r e s t p o t e n t i a l of p y r r h o t i t e can as  follows:  1)  The  2)  In the presence of f e r r o u s  p o t e n t i a l does not  p o t e n t i a l decreases as pH 3)  The  be  H^S  depend on the  ferrous  ion  concentration.  i o n i n the e l e c t r o l y t e the  increases.  e f f e c t on the r e s t p o t e n t i a l i s not  p y r r h o t i t e s of a l l c o m p o s i t i o n s , t h a t i s , H^S f o r the p y r r h o t i t e s c o n t a i n i n g  excess s u l p h u r ,  consistent  for  a f f e c t s the p o t e n t i a l but  has  no  e f f e c t on  the s t o i c h i o m e t r i c p y r r h o t i t e . 4)  The  e f f e c t of n o n - s t o i c h i o m e t r y of p y r r h o t i t e s on  the  p o t e n t i a l i s s u b s t a n t i a l , i . e . as the excess s u l p h u r content i n pyrrhotites increases In t h i s r e s p e c t , character  i n the  electrodes,  first  the p o t e n t i a l s h i f t s towards more n o b l e the p y r r h o t i t e e l e c t r o d e  t i o n dependence was  is different in  t h r e e p o i n t s mentioned above from  i . e . f o r Cu-S,  Pb-S  and  Ag-S  always o b t a i n e d and  values.  sulphide  systems a m e t a l i o n c o n c e n t r a the observed p o t e n t i a l was  con-  s i s t e n t with an e q u i l i b r i u m between metal i o n s i n the e l e c t r o l y t e and metal i n the s u l p h i d e  phase; a l s o H^S  i n e q u i l i b r i u m w i t h s u l p h u r i n the above i s s i m i l a r to o b s e r v a t i o n s  i n the e l e c t r o l y t e was  sulphide.  by J . B r o d i e .  The 16  fourth  apparently  point  H i s measurements  are  reproduced i n F i g . 18. The curve i n F i g . 18 was o b t a i n e d f o l l o w i n g method; a galena  e l e c t r o d e was c a t h o d i z e d  with  by t h e a current  2 d e n s i t y 1 mA/cm  i n 1 M HCIO^ s o l u t i o n f o r 1 h r , then anodized i n 2  f r e s h l y deoxygenated 1 M HCTO^ s o l u t i o n w i t h During anodization  a c u r r e n t d e n s i t y 1 mA/cm .  t h e c u r r e n t was i n t e r r u p t e d f o r the measurement  of t h e r e s t p o t e n t i a l a f t e r s u c c e s s i v e s h o r t p e r i o d s . the e l e c t r o l y t e was sampled f o r Pb s i g n i f i e s t h a t galena  being  The curve  s a t u r a t e d w i t h Pb m e t a l by c a t h o d i z a t i o n was  g r a d u a l l y changed i n composition anodization,  ion analysis.  Meanwhile,  from m e t a l - r i c h t o s u l p h u r - r i c h by  e q u i l i b r i u m between the e l e c t r o d e and t h e e l e c t r o l y t e  reached to e s t a b l i s h the p o t e n t i a l of the galena  From t h i s curve the e f f e c t of c o m p o s i t i o n  of galena  electrode.  on the r e s t  p o t e n t i a l i s seen, a l t h o u g h a q u a n t i t a t i v e r e l a t i o n s h i p showing t h e precise s t o i c h i o m e t r y The  range c o u l d n o t be o b t a i n e d .  behaviour o f the p y r r h o t i t e e l e c t r o d e w i l l be i n t e r p r e t e d  s c h e m a t i c a l l y on the c u r r e n t - d e n s i t y p o t e n t i a l diagram earlier.  introduced  During t h e f o l l o w i n g i n t e r p r e t a t i o n i t i s assumed t h a t  concentration  p o l a r i z a t i o n w i l l n o t appear and the k i n e t i c  i.e.. k a , k c , a and g, remain constant  as p o t e n t i a l changed.  parameters, In  o t h e r words l i n e a r r e l a t i o n s h i p s o f l o g a r i t h m - c u r r e n t - d e n s i t y vs p o t e n t i a l are maintained.  A l t h o u g h these c o n d i t i o n s seem t o be  o v e r s i m p l i f i e d , i t i s e a s i e r to understand the s u l p h i d e  electrode  when these assumptions a r e made. (6-1)  Initially,  hydrogen i o n . for  l e t us c o n s i d e r  the e f f e c t s of f e r r o u s i o n and  In F i g . 19(a) the c u r r e n t - d e n s i t y p o t e n t i a l r e l a t i o n s h i p s  [Fe] -> Fe " " + 2e,* [S] + 2 H + 2e -> H S,* and Fe* " + 2e •+ Fe a r e 4  1  +  4  2  From here on, i r o n and s u l p h u r [Fe] and [S;] , r e s p e c t i v e l y .  i n s u l p h i d e phase a r e expressed as  POTENTIAL : Volts Figure  19.  (SHE)  Current-density  p o t e n t i a l r e l a t i o n s h i p s f o r the c e l l  Ca)  pyrrhotite  |X-FeS0  (b)  pyrrhotites  |H S0 O  4>  / | }  y-H^SO^|S.H.E. C25°C) He or H S|s.H.E. (25°C). 0  - 58 -  schematically described.  In F i g . 19(a),  the l i n e o f t h e c a t h o d i c  r e a c t i o n of f e r r o u s i o n which i s added 0.1 M as a maximum t o the e l e c t r o l y t e i n order  to o b t a i n t h e r e v e r s i b l e p o t e n t i a l o f Fe  + 2e  [Fe] i s l o c a t e d below the l i n e f o r the c a t h o d i c  r e a c t i o n of [S] + 2 H  2e -*- H^S.  coupled  Therefore,  the p o t e n t i a l d e t e r m i n i n g  c o n s i s t o f t h e anodic  r e a c t i o n o f [Fe] ->• Fe  r e a c t i o n o f [S] + 2 H + 2e +  In t h i s  + 2e and t h e c a t h o d i c  R^S i n the r e g i o n where t h e experiment  undertaken.  not  depend on the f e r r o u s i o n c o n c e n t r a t i o n but i t depends on pH  case t h e r e s t p o t e n t i a l o f p y r r h o t i t e does  because the l i n e f o r the c a t h o d i c p r o c e s s concentration.  concentration  i s a f u n c t i o n o f hydrogen  When the pH i s decreased, i . e . hydrogen i o n  i s i n c r e a s e d , t h e l i n e f o r the c a t h o d i c  upwards l i f t i n g  the i n t e r s e c t i o n with  the anodic  of hydrogen s u l p h i d e e v o l u t i o n from the c e l l  reaction  shifts  l i n e as a r e s u l t .  T h i s causes the i n c r e a s e i n p o t e n t i a l as pH d e c r e a s e s .  of  +  reactions  was  ion  +  The d e t e c t i o n  supports the p o s s i b i l i t y  [S] + 2 H + 2e -> Yl^S as a c a t h o d i c process +  of the p o t e n t i a l - d e t e r m i n i n g  reactions. A thermodynamic c o n s i d e r a t i o n f o r the r e a c t i o n o f FeS + 2H~*~ —»• Fe  + H S suggests t h e p o s s i b i l i t y o f [S] + 2H 2  c a t h o d i c process In T a b l e  i n the p o t e n t i a l - d e t e r m i n i n g  IV the e q u i l i b r i u m constants  of  + 2e -> H S as a 2  reactions of p y r r h o t i t e . + 2H  +  = H S ( a q ) + 2/nM  n+  2  3 which a r e c a l c u l a t e d from t h e f r e e enthalpy are presented.. a  .  Using  data from the L a t i m e r  the v a l u e K f o r p y r r h o t i t e , when pH i s 3,  . can be c a l c u l a t e d i n the f o l l o w i n g way;  3  H S(aq) 2  -4 = 3.55 x 10 x  1  (VI-1)  - 60 -  a n d f o r the c a t h o d i c p r o c e s s  *c  " ~  2  F k  V  c s a  *** <" ^  < "> VI  3  At t h e e q u i p o t e n t i a l on the p y r r h o t i t e e l e c t r o d e and i n t h e assumption of a steady s t a t e , from  \  " -c  =  2  F  k  a Fe a  (VI-2) and (VI-3) t h e e q u a t i o n  *** ^  -  2 F  k  c sV ^ a  {  ~^  < " VI  4)  i s o b t a i n e d , where E i s t h e p o t e n t i a l o f p y r r h o t i t e . Equation  E = -  ^  (VI-4) y i e l d s f o r the pH-dependence o f t h e p o t e n t i a l ,  0.059( H) -  |  P  ^  f  _  l  o  ^ c s  (VI-5)  g  A c c o r d i n g t o data of the dependence o f t h e r e s t p o t e n t i a l on pH i n t h i s work, i . e . (-150) to (-350) mV/pH, t h e sum o f the t r a n s f e r c o e f f i c i e n t s f o r the c a t h o d i c and anodic r e a c t i o n s w i l l be p r e d i c t e d to be l e s s than u n i t y .  (6-2)  Secondly,  t h e e f f e c t o f H^S on the r e s t p o t e n t i a l i s  d i s c u s s e d i n t h e same manner u s i n g t h e c u r r e n t - d e n s i t y p o t e n t i a l diagram  shown i n F i g . 19(b).  processes are;  [Fe] -> Fe  I j  I n F i g . 19(b), t h e p o s s i b l e e l e c t r o c h e m i c a l  + 2e, [S] + 2H  i_  + 2e  I  H S and H S -> 2H 2  2  + S + 2e.  -  B e f o r e H^S  61  -  b u b b l i n g , the r e s t p o t e n t i a l f o r the 5 0 atomic  Fe p y r r h o t i t e and n a t u r a l or 4 7 . 4 9 shown E Q ^ and r e a c t i o n s of H^S  EQ  ( Q2 E  2  ^01^' k i w  >  ++  [Fe] -*- Fe  atomic c  n  a  r  Then, by H^S  the l i n e of the anodic r e a c t i o n of H^S As a r e s u l t , when t h i s new  anodic l i n e of  by  the  + 2e as an anodic p r o c e s s and  as a c a t h o d i c p r o c e s s .  diagram.  p e r c e n t Fe p y r r h o t i t e s  determined  e  -»• 2 H  +  p o t e n t i a l are s t i l l  [Fe] -> Fe  f o r the n a t u r a l and  47.49  anodic l i n e i s lower  + 2e and  atomic  l i n e o f the r e a c t i o n H S-»- 2H so a new 2  E  3  the  p e r c e n t Fe p y r r h o t i t e s , the  -> 2 H  +  + S + 2e  However,  [Fe]  and  anodic  Fe  +  2e  determined  [ S ] + 2H  +  +  2e  shown i n F i g . 1 9 ( b ) , must be more  I f e q u i l i b r i u m between s u l p h u r i n s u l p h i d e and  than  + 2e t R^S  T h i r d l y , the behaviour  can be  1Q > 2  sulphur deposited  the anodic r e a c t i o n of hydrogen s u l p h i d e i s e s t a b l i s h e d , the  (6-3)  rest  2  2  +  the  the  + 2e -> H S .  n e g a t i v e than E Q , having a h i g h e r c u r r e n t d e n s i t y i ^ ^  p o t e n t i a l f o r [S] + 2 H  + 2e ->•  electrolyte  than  p o t e n t i a l i s e s t a b l i s h e d , as  by the coupled r e a c t i o n s of H S potential Q >  [ S ] + 2H  + S + 2e i s over the  2  T h i s new  the  +  + S + 2e appears on  p e r c e n t Fe p y r r h o t i t e , the coupled r e a c t i o n s d e t e r m i n i n g  UyS.  coupled  [ S ] + 2H  b u b b l i n g through  are  + 2e, as i s the case f o r the 5 0 atomic  [Fe] -> Fe  anodic r e a c t i o n l i n e ,  percent  from  equilibrium  obtained.  of the n o n - s t o i c h i o m e t r y  of  p y r r h o t i t e s w i l l be i n t e r p r e t e d u s i n g the c u r r e n t - d e n s i t y p o t e n t i a l diagram.  The  o b s e r v a t i o n s on the e f f e c t of the n o n - s t o i c h i o m e t r y  these; when the excess p o t e n t i a l i n c r e a s e s and decreases.  s u l p h u r i n the p y r r h o t i t e i n c r e a s e s , the  are rest  the r a t e of the hydrogen s u l p h i d e e v o l u t i o n  In c o n s i d e r a t i o n of these f a c t s ,  the c u r r e n t - d e n s i t y  - 62  -  p o t e n t i a l r e l a t i o n s h i p i s shown i n F i g . 20, stoichiometry  of p y r r h o t i t e .  pyrrhotite increases, 2e -»• H^S  and  When the  [Fe] -> Fe  As  of the c a t h o d i c and  and  The of  +  and  while  2  the Hydrogen S u l p h i d e  Evolution  p o t e n t i a l r e l a t i o n s h i p f o r the c a t h o d i c  p r e d i c t s t h a t the  r a t e of the hydrogen  a f f e c t e d i n the f o l l o w i n g manner;  a)  anodic  hydrogen s u l p h i d e  e v o l u t i o n , b)  p o l a r i z a t i o n should  hydrogen s u l p h i d e  evolution.  In consequence, when p y r r h o t i t e  a m a t e r i a l which i s lower i n p o t e n t i a l than the  evolution Two  cathodic  from the p y r r h o t i t e should be  attempts were c a r r i e d out  for a galvanic composition and  e f f e c t two  k i n d s of p y r r h o t i t e , one  of s t o i c h i o m e t r i c  electrically  of  To  test  stoichiometric  excess s u l p h u r were immersed i n  p o l a r i z a t i o n e f f e c t on the hydrogen s u l p h i d e  with an e l e c t r o d e  accelerate  accelerated.  a c e l l and the e l e c t r i c a l l e a d w i r e s from both specimens were The  of  pyrrhotite,  to t e s t these p r e d i c t i o n s .  the other c o n t a i n i n g  reaction  sulphide  s h o u l d reduce the r a t e  2  the  decreases when the  p o l a r i z a t i o n of the p y r r h o t i t e e l e c t r o d e  HS  +  towards  i n excess s u l p h u r i n  density  P o l a r i z a t i o n E f f e c t on  + 2e -> H S  e v o l u t i o n must be  contacts  +  increases.  current-density  [S]+2H  [S] + 2H  anodic l i n e s moves towards more n o b l e p o t e n t i a l s  the v a l u e of the exchange c u r r e n t  Galvanic  + 2e s h i f t  the  a r e s u l t the r e s t p o t e n t i a l shown as an i n t e r s e c t i o n  excess sulphur content  (7)  r e a c t i o n of  because the a c t i v i t y of s u l p h u r i n c r e a s e s ,  the a c t i v i t y of i r o n decreases w i t h i n c r e a s e pyrrhotite.  non-  content of excess s u l p h u r i n  the l i n e s f o r the c a t h o d i c  the anodic r e a c t i o n of  more noble v a l u e s ,  as a f u n c t i o n o f the  composition.  e v o l u t i o n was The  shorted. studied  r a t e of hydrogen  - 63 -  (S)+2H+2e-^H S 2  i  1  -0.5  0 P O T E N T I A L  F i g u r e 20.  Current-density  : Volts  (SHE)  p o t e n t i a l r e l a t i o n s h i p s f o r the c e l l  d i f f e r e n t p y r r h o t i t e s , | FeSO,., H S O j S . H . E .  (25°C).  - 64 -  s u l p h i d e e v o l u t i o n was measured i n the f o l l o w i n g way; He c a r r i e r gas from the c e l l was bubbled through the s o l u t i o n c o n t a i n e d  1 M of  CdCNO^)^ t o c o l l e c t H^S gas i n the form of CdS p r e c i p i t a t e . 1/2 hour i n t e r v a l s the CdS p r e c i p i t a t e was f i l t e r e d  i n a Gooch c r u c i b l e ,  washed, d i s s o l v e d i n t o 1:1 h y d r o c h l o r i c a c i d , and analysed an atomic a d s o r p t i o n  At 1 or  f o r Cd w i t h  spectrometer.  In F i g . 21 and 22, the r e s u l t s a r e shown.  A g a l v a n i c e f f e c t on  the H^S e v o l u t i o n i s o b v i o u s , and the a n o d i z a t i o n of p y r r h o t i t e decreases the ^ S evolution rate.  e v o l u t i o n and the c a t h o d i z a t i o n i n c r e a s e s Using  under open c i r c u i t  the ^ S  an H^S e v o l u t i o n r a t e o f 2.7 x 10 ^ mol/hr  c o n d i t i o n s i n a s o l u t i o n o f pH = 2.65 and an  2 e l e c t r o d e s u r f a c e a r e a o f 2.92 cm , the d i s s o l u t i o n e q u i v a l e n t  current  -4 2 d e n s i t y was c a l c u l a t e d to be 5.0 x 10 A/cm . T h i s v a l u e i s much —8 2 l a r g e r than the v a l u e o f 10 A/cm f o r an i r o n e l e c t r o d e i n 1 M FeSO^ s o l u t i o n o b t a i n e d  by R o i t e r e t al.'*"''  T h i s supports the view t h a t  [ |  the r e v e r s i b l e r e a c t i o n f o r Fe  + 2e -*• [Fe] i n the s t o i c h i o m e t r i c  p y r r h o t i t e , where a ^ = 1, can n o t be a p o t e n t i a l d e t e r m i n i n g  reaction  at t h i s pH.  (8)  E l e c t r o c h e m i c a l Mechanism o f L e a c h i n g When the l e a c h i n g p r o c e s s  the c a t h o d i c  Reactions  proceeds i n an o x i d i z i n g atmosphere,  reduction of oxidants  becomes p a r t of the s u l p h i d e  electrode  system.  F o r example, when p y r r h o t i t e i s l e a c h e d w i t h  oxidant,  the p o s s i b l e e l e c t r o c h e m i c a l r e a c t i o n s i n c l u d e the f o l l o w i n g ;  1)  oxygen as a  o x i d a t i o n of i r o n i n p y r r h o t i t e i n t o f e r r o u s i o n [Fe] ->- F e " + 2e 44  • e-FH*—G  i  Figure 21.  i  *-FM<—A—^-R  »  i  i 5  i  i  i  i  TIME : Hrs  i 10  i  <  Variation i n H^S evolution rate with a galvanic contact and anodization of pyrrhotite.  i  i  *  15  - 67 -  2)  o x i d a t i o n of sulphur [S]  3)  sulphide  +  +  2  sulphide  sulphur  S0. 4  +  +  2e  +  =  8H  +  +  6e  i n p y r r h o t i t e i n t o hydrogen  +  +  -y  4e  sulphide  -y  surface,  2H 0 2  oxidation of ferrous i o n i n s o l u t i o n i n t o f e r r i c  i o n on the  surface ^ 4-HFe  ->  , +  reduction of f e r r i c  e i o n i n s o l u t i o n i n t o f e r r o u s i o n on the  surface Fe  7)  2H  4H  _ ++ Fe 6)  +  r e d u c t i o n o f oxygen gas on the s u l p h i d e 0  5)  4H.0 2  r e d u c t i o n of sulphur [S]  4)  +  i n p y r r h o t i t e i n t o sulphate i o n  +  e  -> Fe  o x i d a t i o n of hydrogen s u l p h i d e i n s o l u t i o n i n t o e l e m e n t a l on the s u l p h i d e HS 2  2H  -y  +  +  surface S  +  2e  These a l l p o s s i b l e r e a c t i o n s must be c o n s i d e r e d ,  and some of these  r e a c t i o n s , i . e . 4 ) , 5 ) , 6) and 7) can combine as homogeneous e l e c t r o n t r a n s f e r r e a c t i o n s o c c u r r i n g remote from the s u l p h i d e s u r f a c e , which complicates  t h e s y s t e m s t i l l more. ;  However, the r e a c t i o n p o t e n t i a l  on the s u l p h i d e s u r f a c e i s determined by c o u p l i n g o f the p a r t i c u l a r cathodic  and anodic r e a c t i o n s which l e a d t o a maximum exchange c u r r e n t  d e n s i t y i n the system.  As a r e s u l t  the r e a c t i o n r a t e s , i . e . the  c u r r e n t d e n s i t i e s , f o r the slower r e a c t i o n s must be c o n t r o l l e d by this potential.  I n a p r a c t i c a l case the c o n c e n t r a t i o n  must a l s o be c o n s i d e r e d .  polarization effect  I f t h i s e f f e c t e x i s t s , the r a t e s o f e f f e c t e d  - 68  -  r e a c t i o n s a r e determined by d i f f u s i o n a l parameters r a t h e r than by electrode potential.  Therefore,  l e a c h i n g becomes v e r y Besides experimental  the a n a l y s i s of the p r o c e s s  the  of  complicated.  t h i s , an e l e c t r o c h e m i c a l study of o x i d a n t s must encounter difficulties,  because these o x i d a n t s  often react with  the e l e c t r o d e changing i t s s u r f a c e c o n d i t i o n and, t h a t are p o o r l y  l e a d i n g to  data  reproducible.  Nevertheless,  i n the a p p l i c a t i o n of e l e c t r o c h e m i c a l mechanisms to  leaching processes,  the form of sulphur as a r e a c t i o n product  can  be  anticipated; a) is  I f the p o t e n t i a l on the s u l p h i d e s u r f a c e d u r i n g  so low  product  t h a t the hydrogen s u l p h i d e e v o l u t i o n may  the l e a c h i n g  occur,  the  i s hydrogen s u l p h i d e , or when hydrogen s u l p h i d e can  sulphur  be  o x i d i z e d i n the s o l u t i o n homogeneously as a s e q u e n t i a l p r o c e s s i t causes the f o r m a t i o n b)  of e l e m e n t a l  sulphur  or s u l p h a t e  I f the p o t e n t i a l on the s u l p h i d e i s between t h a t of hydrogen  s u l p h i d e e v o l u t i o n and  t h a t of o x i d a t i o n to s u l p h a t e  s u l p h u r w i l l remain l i k e  This sulphur  to  recrystallized  f u r t h e r o x i d a t i o n , once i t has  s k e l e t o n form r e p r e s e n t i n g the s u l p h u r  sulphate  elemental  lattice  is particularly from the  of the  can be formed, s u l p h u r  i n the s u l p h i d e may  i o n , accompanying m e t a l  ' These p o t e n t i a l s , 0.81 were found.  V for pyrite  18  resistant  initial  mineral.  I f the p o t e n t i a l at the s u l p h i d e s u r f a c e i s so h i g h  form of s u l p h a t e *  ion,  an anode s l i m e a f t e r the d i s s o l u t i o n of  m e t a l from the s u l p h i d e l a t t i c e .  c)  ion.  that  d i s s o l v e i n the  dissolution.  and  about 1 V f o r  galena  16  - 69 In  this  elemental  case s u l p h u r i s o b t a i n e d as s u l p h a t e o r a mixture  s u l p h u r and  sulphate.  T h i s behaviour  of s u l p h u r  of  from  s u l p h i d e s i n an o x i d i z i n g l e a c h i n g p r o c e s s i s i l l u s t r a t e d i n F i g . 23.  I I  Sulphur in Sulphide i  o  ^Reaction at the sulphide surface >• Reaction remote from the sulphide surface F i g u r e 23.  Illustration of s u l p h i d e  o f the form of sulphur minerals.  during o x i d i z i n g  leaching  VII.  (1) ferrous  CONCLUSIONS  The r e s t p o t e n t i a l o f p y r r h o t i t e was independent o f the i o n c o n c e n t r a t i o n i n the e l e c t r o l y t e i n the range of 0.001 M -  0.1 M.  (2)  The r e s t p o t e n t i a l o f p y r r h o t i t e was dependent on pH i n the  range 2 t o 4 even i n the presence o f f e r r o u s  (3)  i o n i n the e l e c t r o l y t e .  The H^S i n the e l e c t r o l y t e a f f e c t e d  pyrrhotite  containing  the r e s t p o t e n t i a l of  excess s u l p h u r by r e d u c i n g the p o t e n t i a l , b u t  d i d n o t have an e f f e c t on the r e s t p o t e n t i a l o f s t o i c h i o m e t r i c pyrrhotite.  (4)  The e f f e c t of n o n - s t o i c h i o m e t r y of p y r r h o t i t e  p o t e n t i a l was s u b s t a n t i a l . the  Excess s u l p h u r i n p y r r h o t i t e  on the r e s t increased  rest potential.  (5)  A mixed p o t e n t i a l o f p y r r h o t i t e S° i n p y r r h o t i t e  +  2H  +  +  2e  c o n s i s t i n g o f the r e a c t i o n —»-  HS 2  as a c a t h o d i c p r o c e s s and the r e a c t i o n Fe  o  i n pyrrhotite  -H-  —*• Fe  +  2e  as an anodic p r o c e s s accounts f o r the c h a r a c t e r o f p y r r h o t i t e described  above.  electrodes  -  (6) c a t i o n may  The be  hydrogen  f r o m p y r r h o t i t e by  e x p l a i n e d as a n e l e c t r o c h e m i c a l  effect, i.e., a  a substance of d i f f e r e n t p o t e n t i a l  a p o l a r i z a t i o n on p y r r h o t i t e evolution.  -  sulphide evolution  contact of p y r r h o t i t e w i t h  H-S  72  that  e i t h e r accentuates or  acidifigalvanic imposes  suppresses  VIII.  (1)  The  SUGGESTIONS FOR  FUTURE WORK  r e s t p o t e n t i a l s measured were v e r y  s c a t t e r must be  scattered.  This  c o r r e c t e d or accounted f o r so as to i n t e r p r e t the  data q u a n t i t a t i v e l y .  (2)  The  p o l a r i z a t i o n s t u d i e s of p y r r h o t i t e e l e c t r o d e s  n e c e s s a r y f o r d i s c u s s i o n i n more d e t a i l . considered  t h a t the system of s u l p h i d e  However, i t must  electrodes  i s more  are be complicated  than t h a t of m e t a l e l e c t r o d e s , so the p o l a r i z a t i o n curves o b t a i n e d i n v o l v e those of more than one p o l a r i z a t i o n of a s u l p h i d e non-stoichiometric e s s e n t i a l problem.  reaction.  In a d d i t i o n , d u r i n g  may  the  such as p y r r h o t i t e which e x i s t i n l a r g e  ranges, the c o m p o s i t i o n can  change, and  t h i s i s an  Composition changes must be a v o i d e d f o r m e a n i n g f u l  measurements.  (3) can be mineral  The  study of n i c k e l - i r o n s u l p h i d e m i n e r a l s ,  undertaken i n a m e a n i n g f u l way, i s w e l l understood.  i . e . pentlandite,  o n l y when the p y r r h o t i t e  - 74 -  APPENDIX  A.  Measurement o f the E q u i l i b r i u m P r e s s u r e o f ^ S  (1)  on P y r r h o t i t e  Introduction When t h e r e i s no n e t c u r r e n t , because o f a b a l a n c e between anodic  and if  c a t h o d i c p r o c e s s e s , the o v e r a l l r e a c t i o n can a l s o be c o n s i d e r e d as i t were a s t r a i g h t f o r w a r d c h e m i c a l r e a c t i o n w i t h  characteristic  k i n e t i c s and e q u i l i b r i u m . In the f o l l o w i n g experiments FeS  i n p y r r h o t i t e was attempted  with acid s o l u t i o n s .  a measurement o f the a c t i v i t y of  on the b a s i s of c h e m i c a l  equilibrium  The d a t a on a c t i v i t i e s of FeS i n the non-  s t o i c h i o m e t r i c s u l p h i d e a r e u s e f u l f o r d e s c r i b i n g thermodynamic f u n c t i o n s a c r o s s the composition range. activity brium  i n s u l p h i d e s a c o n v e n t i o n a l method i s to measure the e q u i l i -  s u l p h u r vapour p r e s s u r e over the s u l p h i d e , • e i t h e r w i t h hydrogen-  hydrogen s u l p h i d e gas mixtures gas.  F o r the measurement of the  o r w i t h s u l p h u r gas i n an i n e r t  However, t h i s method can not be a p p l i e d a t low  temperatures  because of unmeasurably s m a l l e q u i l i b r i u m p r e s s u r e s of s u l p h u r s u l p h i d e s and p r o b a b l y v e r y slow e q u i l i b r a t i o n r a t e s . t h i s work a measurement of a c t i v i t y was attempted  carrier  over  Therefore, i n  utilizing  a reaction  of the s u l p h i d e w i t h an aqueous s o l u t i o n . P y r r h o t i t e may be c o n s i d e r e d as a b i n a r y compound o f the components FeS and S.  The component o f FeS w i l l r e a c t w i t h hydrogen i o n i n the  s o l u t i o n a c c o r d i n g to the f o l l o w i n g e q u a t i o n :  FeS  (in pyrrhotite)  +  2H  Fe  +  H S(aq) 2  (1)  forming  f e r r o u s i o n and hydrogen s u l p h i d e .  equilibrium  constant  can be expressed  For t h i s e q u a t i o n , the  i n the f o l l o w i n g manner;  "W"" ^ ( a q ) K  =  (2)  ^ a  FeS  a  H+  The hydrogen s u l p h i d e i n the s o l u t i o n w i l l e q u i l i b r a t e w i t h hydrogen s u l p h i d e i n gaseous phase;  H S(aq)  =  2  HS  =  K ( 3 )  (3)  (gas)  2  2  3  H S(aq) 2  So, f i n a l l y E q u a t i o n  (2) [Fe  KK/ON •  a  £  a  n  d  + +  ]P  =  a H  +  H  g  K-  •«  ( 3 )  where -p -H-  yields  w  =  K'  (5)  !  a r e assumed to be e q u i v a l e n t to c o n c e n t r a t i o n s o f  each i o n . A c c o r d i n g to E q u a t i o n activity  (5),  when [Fe  of FeS can be determined  ] and [H ] are known, the  i n measuring the p r e s s u r e o f H S 2  e q u i l i b r a t e d w i t h the system a f t e r f i x i n g a s t a n d a r d s t a t e . the a c t i v i t y o f S i n the FeS-S b i n a r y system can be c a l c u l a t e d the a c t i v i t y d a t a o f FeS and the composition  Consequently, from  o f p y r r h o t i t e , u s i n g the  Gibbs-Duhem i n t e g r a t i o n method f o r the b i n a r y system;  - 76 -  £n a_  where N„  (2)  and  N  =  N. FeS  /.  (6)  a. FeS  are mole f r a c t i o n of S and  FeS  FeS,  respectively.  Experimental Most thermodynamic s t u d i e s  at low  state. obtain  of s u l p h i d e s  done i n aqueous systems  temperatures have encountered e x p e r i m e n t a l d i f f i c u l t i e s because  of s l u g g i s h  r e a c t i o n r a t e s and  very small  diffusivities  in  In t h i s work, a s p e c i a l l y designed b a l l m i l l was an e q u i l i b r i u m  as soon as p o s s i b l e  c o m p o s i t i o n of p y r r h o t i t e from the The  b a l l m i l l was  containing  ^SO^  pyrrhotite  was  to a v o i d  and  nitrogen  gas  The  to the  ml volume).  system was  tube and  of powdered  put  deoxygenated. and  solution  i n the b a l l After  filled  took 5-10  mill  the  with of  started.  p r e s s u r e of H^S  was  measured w i t h an Hg  A f t e r a s t a b l e H^S  hrs,  the  s o l u t i o n was  pyrrhotite.  or o i l manometer at  p r e s s u r e was  measured, which  analysed f o r ferrous  e x p e r i m e n t a l system i s i l l u s t r a t e d  Sullivan  bulk.  10 gms  depressurizing  to  a heterogenity i n  at atmospheric p r e s s u r e , a h o r i z o n t a l s h a k i n g a c t i o n  certain intervals. usually  avoid  solid  used so as  of i t s c a p a c i t y w i t h  i n t o a pyrex g l a s s  deoxygenated by  and  surface  to 2/3  a r e a c t i o n b e f o r e the  the b a l l m i l l was The  filled  FeSO^, (100  sealed  whole system was  pH.  d£n  in Fig.  24.  ion  and  To air  To air  Porcelain ball mill  Sampling tube. V T o vac. p-To qas cylinder  Shaking table F i g u r e 24.  Water trap Hg Manometer  Oil Manometer  Schematic i l l u s t r a t i o n of the equipment f o r H^S p r e s s u r e measurement.  - 78 -  (3)  R e s u l t s and D i s c u s s i o n In  In  F i g . 25, t h e v a r i a t i o n i n R^S p r e s s u r e w i t h time i s p l o t t e d .  t h i s experiment  the i n i t i a l  s o l u t i o n c o n t a i n e d o n l y 3 c c / 1 R^SO^.  A c c o r d i n g t o these c u r v e s , t h e i n c r e a s e s i n H^S p r e s s u r e a r e f a s t a t the b e g i n n i n g o f each r u n and g r a d u a l l y decreased  to a stable pressure.  T a b l e V shows t h e d a t a on H^S p r e s s u r e , f e r r o u s i o n c o n c e n t r a t i o n and pH a t e q u i l i b r i u m f o r i n i t i a l  s o l u t i o n s f r e e o f f e r r o u s i o n and  c o n t a i n i n g 3 cc/1 H^SO^, and f o r 1 M o f f e r r o u s i o n and 6 c c / 1 H^SO^, respectively.  I n the same t a b l e , from these d a t a the v a l u e s o f K" =  ++ + 2 K'a_ = [Fe ] P „/[H ] a r e c a l c u l a t e d and p r e s e n t e d . FeS H.S ^ n  TT  3 6 The K" v a l u e s o b t a i n e d a r e s c a t t e r e d i n t h e range from 10 to 10 . T a b l e V.  Values o f P at  2  P  H S 2  4  no FeSO. 4  6 cc/1 H S 0 2  _, [Fe  ] , pH and K".  S u l l i v a n powder p y r r h o t i t e  25°C.  Initial solution 3 cc/1 H S 0  u  4  1 M FeSO. 4  •'  < C I  "  H  g  >  [ F e " ] gr/1  :  pH  K" atm/M  10.55  4.18  3.05  1.33 X i o  8.45  3.30  3.25  2.08 X 10  7.15  5.64  4.32  4.27 X i o  6  6.85  4.70  3.06  1.02 X 1 0  4  6.35  57.00  2.45  6.77 X i o  3  17.40  53.74  2.10  3.50 X i o  3  8.62  58.17  2.76  4.55 X i o  4  27.25  58.22  2.87  2.06 X i o  5  24.36  56.84  3.08  4.72 X i o  5  18.54  57.22  3.06  1.89 X i o  5  27.06  59.22  2.91  2.50 X i o  5  22.42  58.06  2.95  2.44 X i o  5  27.74  59.28  2.78  1.34 X i o  5  15.30  57.67  2.41  1.13 X i o  4  4  4  - 80 -  In  F i g . 26 these v a l u e s of K"  A c c o r d i n g t o F i g . 26,  i t i s seen t h a t the v a l u e s of K"  and v a r y w i t h the i n i t i a l the i n i t i a l  a r e p l o t t e d as a f u n c t i o n of  s o l u t i o n used.  depend on  The dependence of K"  s o l u t i o n of 3 cc/1 ^ S O ^  on pH was  not expected.  i n pH of the f i n a l s o l u t i o n s was s o l u t i o n s were  than  and no-FeSO^.  A l s o the wide v a r i a t i o n  not r e a s o n a b l e when the same i n i t i a l  used.  reason f o r these unexpected  problem i n sampling  the f i n a l  was  the sampling  d r a i n e d through  the b a l l m i l l ,  pH  The K" v a l u e s o b t a i n e d i n  s o l u t i o n of 1 M FeSO. and 6 cc/1 H„S0, are l a r g e r 4 2 4  those o b t a i n e d i n the i n i t i a l  The  pH.  solution.  base.  be t h a t t h e r e i s a  U s u a l l y the f i n a l  solution  tube a f t e r l e a d i n g n i t r o g e n gas  then the s o l u t i o n was  w i t h an asbestos f i l t e r  r e s u l t s may  filtered  into  i n a Gooch c r u c i b l e ,  During t h i s f i l t e r i n g p r o c e s s of  2-5 minutes the r e a c t i o n between a c i d and p y r r h o t i t e suspended i n the s o l u t i o n sampled would be p o s s i b l e , because t h i s f i l t e r a t i o n conducted If  was  under a hydrogen s u l p h i d e - f r e e atmosphere w i t h e v a c u a t i o n .  t h i s r e a c t i o n o c c u r r e d , the c o n c e n t r a t i o n s of hydrogen i o n  and f e r r o u s i o n i n the f i l t r a t e s o l u t i o n would d i f f e r  from those i n  the s o l u t i o n sampled, and v a r y w i t h d i f f e r e n t p e r i o d s i n which the sampled s o l u t i o n was Furthermore,  exposed t o a free-hydrogen  s u l p h i d e atmosphere. g  h i g h d i s s o l u t i o n r a t e s of f e r r o u s s u l p h i d e  and  zinc  19 sulphide  i n a c i d s o l u t i o n s are r e p o r t e d .  to be o b t a i n e d , • t h e avoidance and  a c i d d u r i n g .the sampling  The more a c c u r a t e  results  of p o s s i b l e r e a c t i o n between p y r r h o t i t e process i s necessary.  F i g u r e 26.  Dependence of K" on pH.  - 82 -  B. Table VI.  Dependence o f the r e s t p o t e n t i a l on f e r r o u s i o n c o n c e n t r a t i o n at pH = 2.8 a t 25°C ( f o r F i g . 1 3 ) .  [Fe "] M 44  Fe content in pyrrhotite  0.001 M  mV  mV  46.2 a t % Fe  0.1 M  0.01 M  mV  +055  , +046  +060  , +150  +151  +053  , +046  +050  , +147  +135  +139  +120  +091  +095  +080  +035  49.26 a t % Fe  -045  +016  -055  +009  -059  49.86 a t % Fe  -159  -099  -029  -129  -069  -179 -189  50.  a t % Fe  no [Fe**] -411  -425  -420  -440  - 83 -  Table VII.  Dependence o f the r e s t p o t e n t i a l on pH, a t 25°C, [Fe" " "] = 0.01 M ( f o r F i g . 14) 1  Fe content in pyrrhotite at % Fe 46.2  1  pH  2.8  the p o t e n t i a l (mV)  +150  Fe c o n t e n t i n pyrrhotite a t % Fe 49.86  pH  2.8  the p o t e n t i a l (mV)  -099  +147  -129  +139  -179  +091  -189  +080 +060  3.8  +050  3.8  -109  -339 -429  50.24  2.0  +051 +041  -259  +021 49.26  1.8  +041 2.8  2.0  000  -130 -279 -285  2.8  -045 -055 -059  3.8  -239 -274  - 84 -  D. Table VIII.  Variation i n the rest potential with change i n composition of pyrrhotite at 25°C, pH ^ 3 and [Fe" "] = 0.01 M 1-1  (for F i g . 16) Fe content at % Fe 46.02  46.20  46.43 46.80  46.95 50.24  the potential ' mV  Fe content at % Fe  the p o t e n t i a l mV  +116 +111 +091 +071 +061  47.26  +150 +147 +139 +091 +080. +060 +050  47.30  + 91 + 41  47.48  + 91 + 61 + 51  +116 +071 +116 +060 +038 +033 +126 +121 -139 -279 -285  (A) iron powder  -434  (B) 52.8  -354  at % Fe (C) (A) + (B)  -419.  (D) p r i t e (E) Chichibu pyrrhotite (natural)  +347 +186 +161 +143 +124  + + + + +  75 70 60 43 41  Fe content at % Fe  the p o t e n t i a l mV  48.72  +015 -027 -035  49.16  -025 -030 -035 -038 -215 -279  49.26  -045 -055 -059  47.57  + 41  47.72  + 36 + 33  49.39  -010 -030  48.41  -005 -010 -040 -139  49.67  -099 -129 -179 -189  - 85 -  REFERENCES  • 1.  K u l l e r u d , G. Research i n Geochemistry, Volume I I .  2.  Yund, R. and H a l l , H.  3.  L a t i m e r , W.M.  4.  Majima, H. u n p u b l i s h e d work.  5.  Wrabetz, K.E.  6.  Sato, M.  7.  Venkatachalam, Met.  Mat.  Res. B u l l .  79, C181  P o h l , H.A.  9.  Downes, K.W.  (1968).  Oxidation p o t e n t i a l s , P r e n t i c e H a l l Inc.  Z e i t . f u r Elektrochem. 60, 722  Economic Geology, 55, 1202  8.  3, 779  (1956).  (1960).  S. and M a l l i k a r j u n a n , R. T r a n s , o f I n s t , o f Min. (1970).  J . Amer. Chem. Soc. 76, 2182 and Bruce, R.W.  (1954).  T r a n s . CIMM 58, 77  (1955).  10.  G e r l a c h , J . , Hahne, H. and Pawlek, F.  11.  V e t t e r , K. E l e c t r o c h e m i c a l K i n e t i c s , Academic P r e s s .  12.  Conway, B.E.  E r z m e t a l l . 18, 73  (1965).  Theory and P r i n c i p l e s of E l e c t r o d e P r o c e s s e s , The  Ronald P r e s s  Co.  13.  West, W.A.  14.  H a r a l d s e n , H.  15.  Conway, B.E.  16.  B r o d i e , J . T h e s i s of M.A.Sc. U.B.C. (1969).  17.  R o i t e r , A c t a P h y s i c o c h i m , 10, 389  18.  P e t e r s , E. and Majima,.H. Canadian Met.  19.  Romankiw, L.T., Group A, p.  and  and Menzies, A.W.C.  J . o f Phys. Chem. 33, 1880  Z e i t . anorg. und a l l g e m . Chem. 246,  169  E l e c t r o c h e m i c a l Data, E l s e v i e r P u b l i s h i n g  45.  De Bruyn, P.L.  (1929).  (1941). Co.  (1939). Q u a r t e r l y 7, 111  (1968).  Unit Process i n Hydrometallurgy,  

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