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Flotation characteristics of arsenopyrite Vreugde, Morris Johannes Aloysius 1982

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FLOTATION CHARACTERISTICS OF ARSENOPYRITE by M o r r i s John A l o y s i u s Vreugde B.A.Sc. The U n i v e r s i t y of B r i t i s h Columbia, 1971 M.A.Sc. The U n i v e r s i t y of B r i t i s h Columbia, 1973 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES Department of M i n i n g and M i n e r a l Process Engineering  We a c c e p t t h i s t h e s i s as c o n f o r m i n g to the required  standard  The U n i v e r s i t y of B r i t i s h Columbia October 1982  © M o r r i s John A l o y s i u s Vreugde, 1 982  In p r e s e n t i n g  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of  requirements f o r an advanced degree a t the  the  University  o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make it  f r e e l y a v a i l a b l e f o r reference  and  study.  I further  agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may  be granted by  department or by h i s o r her  the head o f  representatives.  my  It i s  understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain  s h a l l not be allowed without my  written  permission.  Department o f  M i n i n g and M i n e r a l P r o c e s s E n g i n e e r i n g  The  U n i v e r s i t y of B r i t i s h  1956  Main Mall  Vancouver, Canada V6T  1Y3  Date  DE-6  (3/81)  Columbia  Supervisor:  D r . G e o r g e W.  Poling  ABSTRACT  E l e c t r o c h e m i c a l methods, s u r f a c e s p e c t r o s c o p y t e s t s have been used t o study  the i n f l u e n c e of the o x i d a t i o n  a r s e n o p y r i t e on i t s f l o a t a b i l i t y w i t h C y c l i c voltammetric arsenopyrite  at  pH  and f l o t a t i o n  xahthate.  s t u d i e s i n d i c a t e d t h a t t h e o x i d a t i o n of  g r e a t e r than 7 r e s u l t s i n t h e f o r m a t i o n of  f e r r i c h y d r o x i d e d e p o s i t s on the s u r f a c e of the m i n e r a l . i s o x i d i z e d t o arsenate arsenate  and sulphur  i s incorporated  sulphate  diffuses  elemental  Arsenic  i s o x i d i z e d t o s u l p h a t e . The  i n the f e r r i c h y d r o x i d e d e p o s i t s  into  solution.  Below  pH=7,  sulphur  with  decreasing  while  soluble  s p e c i e s a r e formed and t h e s u r f a c e becomes i n c r e a s i n g l y with  of  pH.  iron  covered  Increasing  temperature has no i n f l u e n c e on the q u a n t i t y of h y d r o x i d e formed over the range 30° t o 45°C but r e s u l t s i n t h i c k , porous f i l m s a t temperature g r e a t e r than 45°C. The o x i d a t i o n of a r s e n o p y r i t e was demonstrated t o occur a t lower pyrite  although  this  oxidation  effect  potentials  decreased  with  than  for  increasing  temperature. Mixed required  potential  studies  f o r arsenopyrite  indicated  oxidation  that  could  the p o t e n t i a l s be  achieved  with  common o x i d i z i n g a g e n t s . S e l e c t i v e o x i d a t i o n of a r s e n o p y r i t e a  bulk  pyrite-arsenopyrite  concentrate  was  indicated  in  t o be  possible. The  formation  arsenopyrite  of i r o n h y d r o x i d e d e p o s i t s on t h e s u r f a c e  of  r e s u l t e d i n the i n h i b i t i o n of subsequent o x i d a t i o n  of x a n t h a t e t o d i x a n t h o g e n a t the m i n e r a l ' s ESCA s t u d i e s  confirmed  the  formation  surface. of  oxidized  iron  layers  at  the  surface  of  e s s e n t i a l l y a l l the a r s e n a t e in  these  arsenopyrite  and  which was formed  revealed  was  that  incorporated  l a y e r s . Sulphur became o x i d i z e d a t the pH s t u d i e d and  t o a l a r g e e x t e n t went i n t o s o l u t i o n . F l o t a t i o n s t u d i e s demonstrated the arsenopyrite  depression.  In  the  i n c r e a s i n g pH above pH = 7 r e s u l t e d depression a  of  oxidation for  presence  in  of  increased  oxidation, arsenopyrite  w h i l e i n c r e a s i n g temperature had l i t t l e e f f e c t u n t i l  temperature  arsenopyrite  use  of could  40°C be  was  exceeded.  depressed  Previously  activated  through the use of o x i d i z i n g  agents. Arsenopyrite  c o u l d be s e l e c t i v e l y d e p r e s s e d from a  pyrite-arsenopyrite  concentrate  agents.  through  the  bulk  use of o x i d i z i n g  ACKNOWLEDGEMENT  The a u t h o r wishes t o e x p r e s s s i n c e r e Poling  thanks  t o Dr.  G.W.  f o r h i s support and guidance d u r i n g t h e c o u r s e of t h i s  work. P a r t i c u l a r a p p r e c i a t i o n i s expressed without  whose  help  t o D r . W.G.  t h i s p r o j e c t c o u l d not have been  and who ensured t h e c o n t i n u e d support of  Bacon,  Bacon  initiated  Donaldson  and  Associates Ltd. The carry  i n c e n t i v e t o i n i t i a t e t h i s p r o j e c t and t h e endurance t o  i t through t o c o m p l e t i o n r e s u l t s from an unending  t o c a r r y my e d u c a t i o n t o t h e h i g h e s t a t t a i n a b l e l e v e l .  desire  My  most  h e a r t f e l t a p p r e c i a t i o n i s e x p r e s s e d t o my p a r e n t s f o r i n s t i l l i n g in  me  the desire  t o l e a r n and t o my w i f e , K a t e , whose t o t a l  support and o f t e n e s s e n t i a l encouragement enabled me t o  fulfill  my d e s i r e . Appreciation  i s also  expressed  t o Morny  w i t h o u t whose  u n s e l f i s h e f f o r t s t h i s manuscript c o u l d not have been completed. Mr. Stephen P i c k e t t  of  t h e Department  of  Chemistry  is  a p p r e c i a t e d f o r h i s time i n p e r f o r m i n g t h e ESCA a n a l y s e s . Financial  a s s i s t a n c e i n t h e form of a B.C. S c i e n c e C o u n c i l  Grant i s g r a t e f u l l y  acknowledged.  iv  Table of Contents  1  INTRODUCTION  1  2  LITERATURE REVIEW  8  2.1 S t a t e Of The A r t Of A r s e n o p y r i t e F l o t a t i o n  8  2.2 Nature Of Adsorbed Xanthate S p e c i e s 2.3 C r y s t a l S t r u c t u r e  10 *. .1 2  2.4 E l e c t r o c h e m i c a l O x i d a t i o n Of A r s e n o p y r i t e  13  2.5 A r s e n o p y r i t e C o m p o s i t i o n And Phase R e l a t i o n s  16  2.6 E l e c t r o p h y s i c a l P r o p e r t i e s Of A r s e n o p y r i t e  19  2.7 E l e c t r o p h y s i c a l E f f e c t s I n F l o t a t i o n  20  3  OBJECTIVES OF THE PRESENT INVESTIGATION  23  4  ELECTROCHEMICAL STUDIES  25  4.1 E l e c t r o d e P o t e n t i a l Measurements  25  4.1.1 E x p e r i m e n t a l  25  4.1.2 O x i d i z i n g Agents  28  4.2 R e s u l t s And D i s c u s s i o n  32  4.3 C y c l i c Voltammetry  40  4.3.1 E x p e r i m e n t a l  42  4.3.2 R e s u l t s And D i s c u s s i o n  45  A. S i n g l e Sweep Voltammograms  45  B. M u l t i p l e Sweep ( C y c l i c ) Voltammetry  57  C. I r r e v e r s i b i l i t y  70  Of A r s e n a t e  Formation  D. I n f l u e n c e Of D i s s o l v e d A r s e n i c On Voltammetry  73  E. E f f e c t Of Sweep Rate  76  F. E f f e c t Of Temperature  83  ( i ) Experimental  84  V  ( i i ) R e s u l t s And G.  Discussion  84  I n f l u e n c e Of Cyanide  89  H. Other M i n e r a l s In The  Fe - As - S System  95  ( i ) Experimental  95  ( i i ) Results  97  I . Ring D i s c Study J. Influence  101  Of  Hydroxide  Formation  On  Xanthate  Oxidation 4.4 5  6  105  Discussion  109  ESCA STUDIES  113  5.1  Experimental  114  5.2  R e s u l t s And D i s c u s s i o n  115  FLOTATION STUDIES 6.0.1  122  Rougher F l o t a t i o n  122  6.0.1.1 E x p e r i m e n t a l  122  6.0.2  R e s u l t s And  6.0.3  Depression  Discussion  124  Of P r e v i o u s l y A c t i v a t e d A r s e n o p y r i t e  6.0.3.1 E x p e r i m e n t a l 6.0.3.2 R e s u l t s And 6.0.4  Selective  Discussion  128  And  ..130  Results  130  ( i ) Equity Concentrate  130  ( i i ) Giant Y e l l o w k n i f e Concentrate  131  6.0.4.2 D i s c u s s i o n  134  7  CONCLUSIONS  8  RECOMMENDATIONS FOR  Appendix  .....126  F l o t a t i o n Of P y r i t e From A r s e n o p y r i t e  6.0.4.1 E x p e r i m e n t a l  ..126  I  -  138 FUTURE WORK  P o t e n t i a l / p H Diagrams For The  S u l p h u r - Water System  141 Iron - Arsenic 142  Appendix I I - E q u a t i o n s  Used  For  The  C o n s t r u c t i o n Of  Diagrams Appendix I I I - F o l d o u t Of Important Diagrams  References  The 150 156  157  vi i  T a b l e of F i g u r e s  1 M i n e r a l s i n the Fe-As-S system  4  2 A r s e n o p y r i t e from Hedley, B.C. (3600x)  5  3 G i a n t Y e l l o w k n i f e Mines Flowsheet  6  4 C r y s t a l S t r u c t u r e of A r s e n o p y r i t e  ( a f t e r Buerger ( 3 3 ) )  14  5 Atomic % A r s e n i c i n A r s e n o p y r i t e  18  6 Method of e l e c t r o d e c o n s t r u c t i o n  29  7 O x i d a t i o n s t a t e diagram f o r the c h l o r i n e system  33  8 O x i d a t i o n s t a t e diagram f o r manganese system  34  9 O x i d a t i o n s t a t e diagram f o r the oxygen system  35  10 Eh v e r s u s pH f o r a r s e n o p y r i t e  37  11 Eh v e r s u s pH f o r p y r i t e  38  12 C o n s t r u c t i o n of r o t a t i n g a r s e n o p y r i t e e l e c t r o d e  44  13 C o n t r o l and measurement c i r c u i t used f o r voltammetry  46  14 Voltammograms f o r a r s e n o p y r i t e a t i n c r e a s i n g pH v a l u e s  48  15 Voltammograms f o r a r s e n o p y r i t e a t h i g h pH  49  16 Voltammogram  for  arsenopyrite  at  pH = 8.2  showing  p o t e n t i a l s a c h i e v e d w i t h o x i d i z i n g agents  50  17 Voltammograms f o r p y r i t e and a r s e n o p y r i t e a t pH = 11 18 Eh - pH diagram f o r a r s e n o p y r i t e . A c t i v i t y f o r each taken t o be 1 0"  3  52 species  M  54  19 C u r r e n t - decay c u r v e f o r A r s e n o p y r i t e a t +0.0343 V 20 M u l t i p l e Sweep Voltammograms f o r S t a t i o n a r y  and  58 Rotating  E l e c t r o d e s a t pH = 10.6 21 M u l t i p l e  sweep  voltammograms  e l e c t r o d e s a t pH = 11.7  59 f o r s t a t i o n a r y and r o t a t i n g 61  vi ii  22 M u l t i p l e sweep voltammograms a t pH = 5.8  69  23 I n f l u e n c e of c a t h o d i c l i m i t on voltammogram  71  24 I n f l u e n c e of a n o d i c l i m i t on voltammogram  72  25 Voltammogram f o r g o l d e l e c t r o d e i n a r s e n i c s o l u t i o n  74  26 E f f e c t  of  arsenic  additions  on  arsenopyrite  potential  sweeps  75  27 Voltammograms a t i n c r e a s i n g sweep r a t e  78  28 I n f l u e n c e of sweep r a t e on peak p o t e n t i a l  79  29 P l o t of peak p o t e n t i a l as a f u n c t i o n of l o g scan r a t e  81  30 Log peak c u r r e n t v e r s u s l o g scan r a t e  82  31 Voltammograms a t 19°C and a t 60.5°C  85  32 I n f l u e n c e of temperature on peak p o t e n t i a l and peak c u r r e n t 86 33 M u l t i p l e sweep voltammogram a t 58.5°C 34 Comparison  of  pyrite  and  88  arsenopyrite  voltammograms a t  59.8°C 35 M u l t i p l e  90 sweep  voltammogram  presence of 2.82X10" 36 I n f l u e n c e  of  3  for arsenopyrite  in  the  M NaCN  93  c y a n i d e on f o r m a t i o n of i r o n h y d r o x i d e f i l m s  on a r s e n o p y r i t e  94  37 Voltammogram f o r p y r i t e i n t h e presence of 1.62x10"  3  M NaCN 96  38 Voltammograms f o r p y r i t e and m a r c a s i t e  a t pH = 10.6  98  39 M u l t i p l e sweep voltammogram f o r l o e l l i n g i t e a t pH = 10.6 ..100 40 Voltammogram f o r i r o n e l e c t r o d e 41 T r a n s p o r t p a t t e r n of  soluble  102 species  at  a  ring -  disc  electrode 42 I n f l u e n c e of a r s e n o p y r i t e o x i d a t i o n a t pH = 5.9 on x a n t h a t e  104  oxidation 43 I n f l u e n c e  107 of  arsenopyrite  oxidation  at  pH = 11.8  on  xanthate o x i d a t i o n  108  44 Comparison of a r s e n o p y r i t e  rest  potential  and  oxidation  peak p o t e n t i a l w i t h o p e r a t i n g p l a n t c o n d i t i o n s (70) 45 XPS  peaks  associated  with  the  111  i r o n 2p e l e c t r o n s of  the  various minerals 46 XPS  117  peaks a s s o c i a t e d w i t h the a r s e n i c 3d e l e c t r o n s  of  the  various minerals 47 XPS  peaks  118  a s s o c i a t e d w i t h the s u l p h u r 2d e l e c t r o n s of  the  various minerals 48 F l o a t a b i l i t y  119  of  arsenopyrite  at  increasing  pH  in  the  presence and absence of o x i d a t i o n  125  49 I n f l u e n c e of temperature on a r s e n o p y r i t e f l o a t a b i l i t y 50 I n f l u e n c e  of  oxidation  on  pyrite  and  127  arsenopyrite  f l o a t a b i l i t y a t i n c r e a s i n g pH 51 D e p r e s s i o n  of  arsenopyrite  133 from  bulk  concentrate  with  i n c r e a s i n g xanthate a d d i t i o n 52 D e p r e s s i o n  of  arsenopyrite  135 from  bulk  concentrate  with  i n c r e a s i n g permanganate a d d i t i o n 53 A r s e n o p y r i t e  stability  diagram  136 at  10"  M  6  activity  of  dissolved species 54 A r s e n o p y r i t e  stability  144 diagram  at  10"  3  M  activity  of  dissolved species  145  55 A r s e n o p y r i t e s t a b i l i t y diagram at 1 M a c t i v i t y of d i s s o l v e d species 56 L o e l l i n g i t e  146 stability  dissolved species  diagram  at  10~  3  M  activity  of 147  X  57 A r s e n o p y r i t e FeAs  2  stability  diagram  considering  FeS,FeS  as s t a b l e p r o d u c t s  58 S t a b i l i t y  r e g i o n of f e r r i c a r s e n a t e  2  and 148  at 1 M a c t i v i t y  149  L i s t of  Tables  1 Arsenic Minerals 2 Arsenic Emissions  2 In Canada,1972 ( 6 )  3  3 Arsenopyrite R e s i s t i v i t y Values  20  4 Cost Of O x i d i z i n g Agents  31  5 I n f l u e n c e Of Scan Rate On K i n e t i c Parameters  83  6 E l e c t r o n B i n d i n g E n e r g i e s And  Intensities  For  Elements  Various Minerals. 7 XPS  In 116  J  Intensity Ratios  120  8 F l o t a t i o n Conditions  128  9 F l o t a t i o n R e s u l t s U s i n g Hydrogen P e r o x i d e As An Oxidant ....129 10 F l o t a t i o n R e s u l t s U s i n g Sodium H y p o c h l o r i t e As An Oxidant  .130  11 F l o t a t i o n Test R e s u l t s With E q u i t y C o n c e n t r a t e  13.1  12 Thermodynamic Data At 25°C  143  1  Chapter 1  INTRODUCTION  A r s e n i c o c c u r s i n n a t u r e w i t h numerous number Table are  of  p r i m a r y and secondary  other  elements.  arsenic minerals are l i s t e d i n  1. Those a r s e n i c m i n e r a l s which l i e i n t h e Fe-As-S shown  A  system  i n F i g u r e 1. A r s e n o p y r i t e i s t h e most common m i n e r a l  c o n t a i n i n g a r s e n i c . I t i s found w i t h  silver  and  copper  ores,  g a l e n a , s p a l e r i t e and p y r i t e . In  certain  ores  arsenopyrite  has  c o n s i d e r a b l e economic  s i g n i f i c a n c e s i n c e i t c a r r i e s t h e major p o r t i o n of g o l d ore.  Such  i n the  g o l d may occur as d i s c r e t e g r a i n s between i n d i v i d u a l  c r y s t a l s of a r s e n o p y r i t e and as  such  be  recoverable  from  an  a r s e n o p y r i t e c o n c e n t r a t e by d i r e c t c y a n i d a t i o n ( 1 ) . Gold  may  also  occur  in solid (2)  inclusions i n arsenopyrite recovery  and  solution may  or  require  as  minute  more  exotic  ( 3 ) . An example of such an o c c u r r e n c e  procedures  the Hedley a r e a of B r i t i s h Columbia i s shown i n F i g u r e most  s u c c e s s f u l treatment  a c h i e v e d by f l o t a t i o n of which  i s subsequently  is i n operation  at  from  2.  The  of o r e s of t h i s type t o date has been an  arsenopyrite  bearing  concentrate  r o a s t e d and then c y a n i d e d . Such a p r o c e s s Giant  Yellowknife  Mines  Ltd.  ( 4 , 5 ) .  A  f l o w s h e e t f o r t h i s o p e r a t i o n i s shown i n F i g u r e 3 . While  this  process  f o r the recovery  a r s e n o p y r i t e i s very s u c c e s s f u l from a view,  there  are serious  of  metallurgical  gold point  from of  p o l l u t i o n consequences ( 6 ) . The data  2 Table 1 Arsenic Minerals Arsenic Loellingite Realgar Orpiment Arsenopyrite Glaucodot Cobaltite Gersdorffite Skutterudite Niccolite Enargite Proustite Pearcite Tennantite Sperrylite Allemontite Geocronite Scorodite Pitticite Pharmacosiderite Symplesite Erythrite Annabergite  As FeAs AsS As S FeAsS (Co,Fe)AsS CoAsS NiAsS (Co,Ni,Fe)As NiAs Cu AsS„ Ag AsS (Ag,Cu), As S,! (Cu,Fe,Zn,Ag), As S, PtAs AsSb Pb (Sb,As) S FeAsO «2H 0 Fe (As0 )(SO„)OH»2H 0 6FeAsO„«2Fe(OH) «12H 0 Fe As 0 «8H 0 C0 (AsO«) «8H 0 Ni (AsO„) «8H 0 2  2  3  3  3  3  3  6  2  2  5  2  a  result  3  2  8  2  2  2  3  3  2  3  from  2  2  3  2  2  3  2  2  shown i n T a b l e 2 i n d i c a t e t h a t 47.5% of Canada  fl  2  arsenic  emissions  in  t h e m e t a l l u r g i c a l p r o c e s s i n g of g o l d o r e s .  The predominant source of aqueous c o n t a m i n a t i o n  by  arsenic i s  also i n d u s t r i a l smelting operations (6). As a commodity, a r s e n o p y r i t e i s of minor consequence. While in  the  1920's  the m i n e r a l was viewed as a v a l u a b l e p o t e n t i a l  source of a r s e n i c f o r t h e c o n t r o l of  pests  such  as  w e a v i l ( 7 ) , a t p r e s e n t i t i s viewed as a troublesome base  metal  concentrates  (8,59).  the  impurity i n  While t h e a r s e n i c p r e s e n t i n  such c o n c e n t r a t e s i s r e c o v e r e d t o some e x t e n t , t h e t o t a l i n t h e U.S.  demand  for arsenic  tonnes)  (9) and t h e r e c o v e r y of a r s e n i c from  yearly  i s o n l y 15,000 tons (13,600 waste  streams  c o s t l y . Smelters at present p r e f e r t o receive concentrates are e s s e n t i a l l y f r e e of a r s e n i c .  boll  is which  3  The  presence  of  a r s e n o p y r i t e i n s u l p h i d e c o n c e n t r a t e can  Table 2 A r s e n i c e m i s s i o n s i n Canada,1972 (6)  SOURCE  EMISSIONS TONS PERCENT  INDUSTRY Primary copper and n i c k e l Primary l e a d p r o d u c t i o n Primary z i n c p r o d u c t i o n Primary i r o n and s t e e l M e t a l l u r g i c a l p r o c e s s i n g of g o l d M i s c e l l a n e o u s sources Subtotal  661 18 359 1041 1934 15 4028  FUEL COMBUSTION/STATIONARY SOURCES Power G e n e r a t i o n I n d u s t r i a l and commercial Domestic Subtotal Transportation S o l i d waste i n c i n e r a t i o n Pesticide application Total  16.2 0.4 8.8 25.6 47.5 0.4 98.9  25 13 <1 38 <1 1 6  0.6 0.3 <0.1 0.9 <0.1 <0. 1 0.2  4073  100.0  l e a d t o s e r i o u s h e a l t h hazards a p a r t from t h e c o n t a m i n a t i o n s m e l t e r gases w i t h a r s e n i c . The presence  of  of a r s e n o p y r i t e i n such  c o n c e n t r a t e has on o c c a s i o n r e s u l t e d i n a r s i n e g e n e r a t i o n d u r i n g gold  precipitation  in  cyanide  circuits  (10)  or  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 i n g and r e f i n i n g ( 1 1 ) . The particular  association interest.  of This  arsenopyrite association  with  pyrite  is  of  i s wide spread and can  r e s u l t i n s i g n i f i c a n t economic consequences. I f p y r i t e i s t o be recovered  f o r t h e p r o d u c t i o n of s u l p h u r i c a c i d t h e presence of  a r s e n o p y r i t e i s h i g h l y u n d e s i r a b l e due t o c o n t a m i n a t i o n  of t h e  4  Fe  orpiment  realgar  Figure 1 Minerals  i n t h e F e - A s - S S y s t e m (600°C)  5  I  Area Area Area Area  A - G o l d w i t h minor s i l v e r B - B i s m u t h and t e l l u r i u m C - G o l d and a n t i m o n y D, E - B i s m u t h Figure 2  Arsenopyrite  f r o m H e d l e y , B.C.  (3600x)  PRIMARY CRUSHING Underground  CRUSHING 3 Stogts  | SECONDARY FLOTATION  PRIMARY GRINDING Boll Mills  T*  ur  SECONDARY GRINDING K - i Boll MILLS  CLASSIFIERS  ^ CYCLONES <jr  I  PRIMARY FLOTATION  1  - BULK SULPHIDE CONCENTRATE -  TAILINGS<HSAND PLANT  S  P  [CONCENTRATE . STORAGE Thickening  >| CYCLONE  I  lit  CALCINE WASH  BACKFILL  aJZ!  HOT C0TTRELL  FLUOSOLIDS ROASTING 2 Slogei  CYCLONES |  [2nd CALCINE WASH  [CARBON PLANT] CONDITIONING Thickening CRUDE A. 0 2  CYANIDATION  3  H THICKENING LOADED CARBON Dried Shipped  STRIPPING  PREGNANT SOLUTION Storoge  -(FILTRATION ILARIFIOATION  ARSENIC SUPPRESSION I Llmi )  IPRECIPITATTONI pjlTHICKENING  ARSENIC TAILINGS <-) SUPPRESSION (Lime)  GOLD BULLION  Figure 3 G i a n t Y e l l o w k n i f e Mines  flowsheet  7  acid  by  arsenic  ( 1 1 ) . In  gold  ores  t h e g o l d v a l u e s may be  e n t i r e l y associated with either mineral. The r e c o v e r y or d e p r e s s i o n  of a r s e n o p y r i t e d u r i n g  sulphide  f l o t a t i o n i s of apparent i n t e r e s t . I n t h e case of g o l d o r e s , t h e maximum  recovery  of  a r s e n o p y r i t e may be d e s i r a b l e t o maximize  the r e c o v e r y of a s s o c i a t e d g o l d v a l u e s . On t h e o t h e r maximum metal  depression  of  arsenopyrite  hand, t h e  i s d e s i r a b l e d u r i n g base  production. S i n c e fundamental i n v e s t i g a t i o n s  chemistry  of  the aqueous  of a r s e n o p y r i t e have p r e v i o u s l y not been u n d e r t a k e n , a  comprehensive  literature  survey  relating  t o t h i s m i n e r a l was  c a r r i e d o u t . W h i l e c e r t a i n a s p e c t s of t h i s review may apparent  surface  not  have  s i g n i f i c a n c e t o t h e f l o t a t i o n response of a r s e n o p y r i t e  they a r e b e l i e v e d t o c o n t r i b u t e t o t h e o v e r a l l u n d e r s t a n d i n g i t s occurence and b e h a v i o u r .  of  8  Chapter 2  LITERATURE REVIEW  2.1  S t a t e of t h e A r t of A r s e n o p y r i t e In  reviewing  arsenopyrite from  whether  l i t e r a t u r e r e l a t i n g t o t h e f l o t a t i o n of  i t i s apparent t h a t the s e p a r a t i o n of  pyrite  considered.  the  i s among  the p r o d u c t i o n  t h e most s i g n i f i c a n t s e p a r a t i o n s  gold  of a pure p y r i t e p r o d u c t , f r e e  discussing  w i l l be  determine  of  arsenic,  receives frequent  mention.  the c h a r a c t e r i s t i c s  f l o t a t i o n c o n d i t i o n s , frequent of  to  of a r s e n o p y r i t e under  reference w i l l  be  made  t o the  since  the depression  of  arsenopyrite  o b v i o u s l y not be of any r e a l s i g n i f i c a n c e i f most o t h e r would  Consideration  the  p y r i t e under t h e same c o n d i t i o n s . T h i s comparison  useful  minerals  t o be  i s w i t h p y r i t e o r a r s e n o p y r i t e or f o r  s e p a r a t i o n of t h e s e two m i n e r a l s  behaviour  arsenopyrite  Whether i t i s f o r a n a l y t i c a l purposes  associated  In  Flotation  be  depressed  of t h e b e h a v i o u r  of  under pyrite  the  same  therefore  would sulphide  conditions. gives  at  l e a s t a l i m i t e d measure of whether c o n d i t i o n s f o r t h e d e p r e s s i o n of  a r s e n o p y r i t e a r e e x c e s s i v e . Another b e n e f i t of comparing t h e  response of  pyrite  arsenopyrite  has  to  that  received  of  arsenopyrite  i s that  while  o n l y very l i m i t e d s t u d y , p y r i t e has  been e x t e n s i v e l y i n v e s t i g a t e d . Sutherland which  flotation  and Wark ( 1 2 ) showed d i d not  the c r i t i c a l  pH  (above  o c c u r ) f o r f l o t a t i o n of a r s e n o p y r i t e  9  w i t h 25 mg. p e r l i t r e of e t h y l x a n t h a t e t o be pH=8.4 compared t o a v a l u e of pH=l0.5 f o r p y r i t e . Mitrofanov  (13) i n d i c a t e d  a r s e n o p y r i t e t o be a c h i e v e d Plaksin  the  maximum  recovery  i n t h e range of pH from 4 t o 6.  (14) and l a t e r G l e m b o t s k i e t a l (15) d i s c u s s e d t h e  i n f l u e n c e of c r y s t a l s t r u c t u r e on t h e o x i d a t i o n arsenopyrite. considered produce  The  position  of  p y r i t e and  of s u l p h u r atoms i n t h e p y r i t e was  t o be such t h a t they c o u l d i n t e r a c t w i t h  soluble  species  oxygen  and  which l e f t the s u r f a c e as new p y r i t e ,  s t i l l able t o react with reagents.  The more complex s t r u c t u r e of  a r s e n o p y r i t e was b e l i e v e d t o r e s u l t i n slower than  of  oxidation  rates  f o r p y r i t e . P r o l o n g e d o x i d a t i o n was b e l i e v e d t o r e s u l t i n  d e c o m p o s i t i o n of t h e a r s e n o p y r i t e l a t t i c e w i t h both s u l p h u r  and  a r s e n i c being o x i d i z e d . The a r s e n i c o x i d e groups were p o s t u l a t e d t o remain a t t h e m i n e r a l ' s The  different  surface.  susceptibility  of  t h e two  minerals  o x i d a t i o n has been e x p l o i t e d by Nekrasov (16) and Machovic to  achieve  concentrate agent.  a was  separation. produced  and  A  bulk  conditioned  Nekrasov used an a d d i t i o n of Mn0  conditioning  period  Machovic  used  floating  pyrite.  with  pyrite  aeration  -  with  use  of  (17)  arsenopyrite an  oxidizing  f o l l o w e d by a two hour  2  t o depress  arsenopyrite.  a d d i t i o n s of KMnO„ t o depress a r s e n o p y r i t e Similar  to  permanganate  to  while  depress  a r s e n o p y r i t e and p y r r h o t i t e s e l e c t i v e l y from p y r i t e has been t h e s u b j e c t of p a t e n t s Glembotski concentration that  (18). et a l .  (15)  related  required for arsenopyrite  required  for  pyrite  the greater flotation  flotation  to  oxygen  compared  their  to  different  10  s u s c e p t i b i l i t y t o o x i d a t i o n . Rand (19) p r o v i d e d for  an  explanation  t h e d i f f e r e n t oxygen r e q u i r e m e n t s which i s more i n keeping  with current f l o t a t i o n theory. greater  He  that  pyrite  has a  oxygen r e d u c t i o n a c t i v i t y than has a r s e n o p y r i t e . P y r i t e  t h e r e f o r e r e q u i r e s lower oxygen provide balance  showed  the cathodic t h e anodic  oxygen  reduction  o x i d a t i o n of x a n t h a t e .  i n t e r p r e t a t i o n s were p r e s e n t e d The  use  concentration  of  in solution to  reaction Similar  results  and  by B i e g l e r e t a l . ( 2 0 ) .  magnesia  mixture  as  a  depressant  a r s e n o p y r i t e has been proposed ( 2 1 ) . The depressant prepared  required to  for  mixture  was  from magnesium c h l o r i d e , ammonium c h l o r i d e and ammonium  hydroxide  w i t h d i s t i l l e d w a t e r . The m i x t u r e was found t o g i v e a  h i g h degree of a r s e n o p y r i t e d e p r e s s i o n  a t pH v a l u e s g r e a t e r than  8. C h a l c o p y r i t e was a l s o found t o be d e p r e s s e d by  this  mixture  i f t h e m i n e r a l was f i r s t c o n d i t i o n e d w i t h A s l . P y r i t e was found 3  to  be  unaffected  depression  by  the mixture.  concluded  that  r e s u l t e d from t h e f o r m a t i o n of a s t r o n g l y h y d r o p h i l i c  compound, MgNH AsO(,6H 0 fl  2  on  d i r e c t e v i d e n c e t o support  the surface  Very  of  arsenopyrite.  No  t h i s c o n c l u s i o n was p r e s e n t e d .  2.2 Nature of Adsorbed X a n t h a t e  regard  I t was  Species  l i m i t e d work has been r e p o r t e d i n t h e l i t e r a t u r e w i t h  to  t h e nature  of  adsorbed  xanthate  species  on  a r s e n o p y r i t e . A l l i s o n e t a l . (22) r e p o r t e d d i x a n t h o g e n t o be t h e reaction  product.  The l i m i t s of d e t e c t i o n were i n d i c a t e d t o be  such t h a t up t o 5 p e r c e n t  of  t h e minor  product  (i.e.  ferric  x a n t h a t e ) c o u l d be p r e s e n t . Although  additional  i n v e s t i g a t i o n s i n t o t h e nature of t h e  11  r e a c t i o n p r o d u c t s of a r s e n o p y r i t e w i t h x a n t h a t e reported,  i t i s of  interest  to  note  have  collector  both  as t h e  product and both have f e r r i c x a n t h a t e as  the a l t e r n a t i v e c o l l e c t o r p r o d u c t (22,24). I n oxygen,  been  the s i m i l a r i t i e s w i t h  p y r i t e . Both m i n e r a l s were r e p o r t e d t o have d i x a n t h o g e n predominant  not  minerals  also  have  mixed  t h e presence  of  p o t e n t i a l s which a r e  g r e a t e r than t h e e q u i l i b r i u m p o t e n t i a l f o r x a n t h a t e o x i d a t i o n t o dixanthogen. In s p i t e of t h e controversy  has  seeming  continued  consistency as  to  of  these  t h e n a t u r e of the p r o d u c t s  formed by t h e i n t e r a c t i o n of p y r i t e w i t h x a n t h a t e Various  investigators  findings,  in  solution.  (23,24,25,26) have s t u d i e d t h e n a t u r e of  the c o l l e c t o r s p e c i e s on  pyrite.  c o n c l u d e d t h e predominant  s u r f a c e product t o be d i x a n t h o g e n . The  continued of  controversy  Each  of  these  studies  has  appears t o r e s u l t from t h e d e t e r m i n a t i o n  some i n v e s t i g a t o r s t o e s t a b l i s h a s i n g l e adsorbed p r o d u c t and  to n e g l e c t t h e r o l e p l a y e d  by  minor  c o n c e n t r a t i o n s of  other  adsorbed s p e c i e s (27,28). The m a j o r i t y of r e s e a r c h e r s now b e l i e v e t h a t d i x a n t h o g e n i s the  active  collector  species i n p y r i t e f l o t a t i o n while a t the  same time a d d i t i o n a l minor c o n t r i b u t i o n s t o be  made  by  ferric  hydroxyxanthate  or  h y d r o p h o b i c i t y can  by  elemental sulphur  r e s u l t i n g from m i n e r a l o x i d a t i o n . The  electrochemical oxidation  of  adsorbed  xanthate  to  d i x a n t h o g e n can be r e p r e s e n t e d by:  X  2  + 2e" == 2X"  (1)  The c o r r e s p o n d i n g c a t h o d i c r e a c t i o n i s g e n e r a l l y c o n s i d e r e d  12  to be t h e r e d u c t i o n of oxygen ( 2 3 ) . 1/2 0  0  2  2  + H 0 + 2e" == 20H'  (2)  2  + 4H  + 4e" == 2H 0  +  (3)  2  The above r e a c t i o n s o c c u r i n g v i a p e r o x i d e In  the  iron  system,  intermediate (23).  o x i d a n t s o t h e r than oxygen a r e a l s o  possible (24). 2Fe  3 +  + 2X" == 2 F e  2Fe(OH)  Although pyrite,  i t  3  + 6H  +  + X  (4)  2  + 2X" == 2 F e  the preceding appears  2+  review  consistent  2 +  + 6H 0 + X 2  was  based  t o assume  (5)  2  on  work  that  with  t h e same  c o n c l u s i o n s can be made f o r the a r s e n o p y r i t e - xanthate  - oxygen  system.  2.3 C r y s t a l S t r u c t u r e The s t r u c t u r e of Buerger  arsenopyrite  has  been  by  (33,34) and Morimoto and C l a r k ( 3 6 ) . Buerger d e t e r m i n e d  the s t r u c t u r e t o be marcasite  monoclinic,  and l o e l l i n g i t e  closely  (34,35).  arsenic  atoms  related  to  is a  triangle  and t h e o t h e r a t r i a n g l e of t h r e e  atoms. The a r s e n o p y r i t e s t r u c t u r e  that  of  Each i r o n atom i s surrounded  by a d i s t o r t e d o c t a h e d r o n of which one face three  investigated  i s complex  and  of  sulphur  i s further  13  complicated twins.  by t h e f a c t t h a t the m i n e r a l almost always o c c u r s as  The  e f f e c t of t w i n n i n g  i s t o g i v e the m i n e r a l a pseudo-  o r t h o r h o m b i c symmetry. The  s t r u c t u r e of a r s e n o p y r i t e as p r e s e n t e d  by  Buerger  is  shown i n F i g u r e 4. It  can be seen t h a t the s t r u c t u r e i s a body c e n t e r e d  cubic  s t r u c t u r e w i t h r e g a r d t o i r o n . Sulphur and a r s e n i c occur as As-S groups a l o n g c e l l edges. B u e r g e r ( 3 3 ) e x p l a i n e d observed  variations  in  interatomic  s p a c i n g on the b a s i s t h a t i r o n i n p y r i t e i s i n t h e f e r r o u s s t a t e while  iron i n marcasite,  ferric  state.  Although Glembotski arguments  l o e l l i n g i t e and a r s e n o p y r i t e i s i n t h e  (15) and  for oxidation  of  Plaksin  pyrite  and  (14) based  their  arsenopyrite  on the  r e l a t i v e a c c e s s i b i l i t y of the s u l p h u r atoms t o i n t e r a c t i o n oxygen,  it  appears  that  considerations  are  marcasite  arsenopyrite  than  and  pyrite  the  significantly  controlling  results  in  are  this  more  regard.  with  complex  While  both  known t o o x i d i z e more r e a d i l y  presented  by  those  investigators  indicate  that arsenopyrite  marcasite  i s not ( 3 7 ) . The v a r i a t i o n i n b e h a v i o u r of the v a r i o u s  minerals  under o x i d i z i n g  controlled  by  the  i s passivated during oxidation while  conditions  nature  of  therefore  appears  to  be  both s u l p h u r and i r o n o x i d a t i o n  products.  2.4 E l e c t r o c h e m i c a l O x i d a t i o n of A r s e n o p y r i t e The  e l e c t r o c h e m i c a l o x i d a t i o n of a r s e n o p y r i t e has  l i m i t e d s t u d y . K o s t i n a and Chernyak (29,30,31) p r e s e n t e d  received results  IRON ARSENIC SULPHUR  Figure 4 Crystal  s t r u c t u r e of a r s e n o p y r i t e  ( a f t e r Buerger  (33))  15  for  oxidation  experiments  a r s e n o p y r i t e . The  carried  out  with  o b j e c t i v e of t h e i r work was  under which a r s e n o p y r i t e  both p y r i t e and  to f i n d  would be r a p i d l y o x i d i z e d  conditions  and  thereby  r e l e a s e any entrapped g o l d . They demonstrated t h a t the of both m i n e r a l s changed  from  occurred  acid  oxidation  at lower p o t e n t i a l s as c o n d i t i o n s were  t o a l k a l i n e pH.  Arsenopyrite  demonstrated a  d i s t i n c t o x i d a t i o n peak i n a l k a l i n e (sodium h y d r o x i d e ) s o l u t i o n . Increasing  temperature  was  found  to  increase  the  rate  of  o x i d a t i o n at a g i v e n p o t e n t i a l . Pyrite  was  found  to  oxidize  p o t e n t i a l than d i d a r s e n o p y r i t e . difference potential  in  oxidation  (30).  This  at a lower r a t e at a  I t was  rates  contradicts  increased the  G l e m b o t s k i (15) as p r e v i o u s l y d i s c u s s e d Analysis caustic  of  solutions  solutions  conditions,  that  sulphur  that  with  in section  made  oxidation in  moderate  were leached  by  2.1.  prolonged  under  the  increasing  assumptions  following  showed  a r s e n i c and  a l s o observed  given  oxidation  from  arsenopyrite.  Almost e q u a l amounts of t r i - v a l e n t and p e n t a - v a l e n t  a r s e n i c were  formed. Sulphur o x i d a t i o n r e s u l t e d i n v a r y i n g r a t i o s of  sulphate  to t h i o s u l p h a t e with varying o x i d a t i o n p o t e n t i a l (30). Vakhontova and Grudnev (32) measured a r s e n o p y r i t e p o t e n t i a l as a f u n c t i o n of pH. relationship  were  related  decomposition products.  electrode  Observed v a r i a t i o n s i n the to  changes  in  the  Eh-pH  nature  of  Under a c i d c o n d i t i o n s i r o n was  concluded  t o go i n t o s o l u t i o n as f e r r o u s w h i l e under n e u t r a l and  alkaline  conditions  it  went  zone of ore d e p o s i t s under  acidic  into the  conditions  s o l u t i o n as f e r r i c . In the alteration were  product  of  supergene  arsenopyrite  found t o be s y m p l e s i t e  (ferrous  16  a r s e n a t e ) w h i l e under n e u t r a l or a l k a l i n e ( f e r r i c a r s e n a t e ) was  2.5  Arsenopyrite The  conditions  scorodite  formed.  C o m p o s i t i o n and Phase R e l a t i o n s  compositional  v a r i a t i o n s of both s y n t h e t i c and  a r s e n o p y r i t e s have been  considered  in  natural  several  investigations  relations  i n the Fe-As-S  (38,36,39). Clark  (38)  in  system observed  studying  that  arsenopyrite  c o n d i t i o n s of temperature and ideal  FeAsS  determine various  formula.  the  d  phase  bulk c o m p o s i t i o n  X-ray  diffraction  spacing  1 3 1  synthesized  of  under  varying  d e v i a t e d from the  data  were  arsenopyrite  used  to  synthesized  in  u n i v a r i a n t assemblages as a f u n c t i o n of t e m p e r a t u r e . I t  was  observed t h a t w h i l e at any  g i v e n temperature  in  arsenopyrite  i s s m a l l , throughout the range of  temperatures  composition  investigated,  significant.  Only  a  the  variation  qualitative  quantify  the  dependence  of  d  on  1 3 1  in  relation  however, s i n c e i n s u f f i c i e n t c o m p o s i t i o n a l  the  variation  composition was  is  established  data were a v a i l a b l e t o the  S/As  ratio  in  arsenopyrite. In  addition  Clark presented  to  compositional  data which show the l i m i t s of s t a b i l i t y  various  mineral  formation  of a r s e n o p y r i t e was  arsenic was  coexists  below 688° ±  assemblages.  with 3°C.  and  The  maximum  of  temperature  shown t o be 702° ± 3°C.  If  the of  native  a r s e n o p y r i t e the d e p o s i t i o n temperature The  arsenopyrite coexistence Morimoto  v a r i a t i o n s of a r s e n o p y r i t e ,  Clark  maximum  i s 491° (36)  ±  temperature  of  pyrite  12°C.  presented  chemical  analyses  for  17  sixteen n a t u r a l l y occuring arsenopyrites. which  were  considered  to  be  From  consistent  those  w i t h the  m i n e r a l assemblages they d e t e r m i n e d the c o m p o s i t i o n  majority  of  analyses  arsenide  with  the  FeAs^^o.j,.  were shown t o be on the  sulfur-rich  Kretschmar and determining to  a  Scott  they p r e s e n t e d  of  (39) made a  published  detailed  naturally occurring Analyses  were  of  considered  sulphide  -  to  type  be over  deposits.  the c o m p o s i t i o n  review  which was  predominance  - type m i n e r a l  of n a t u r a l l y  9  s i d e of the i d e a l FeAsS c o m p o s i t i o n consistent  associated  from F e A s o . S , . , t o  o c c u r r i n g a r s e n o p y r i t e to vary The  analyses  further  contribution  to  of n a t u r a l a r s e n o p y r i t e . In a d d i t i o n s t u d i e s on a r s e n o p y r i t e  analyses  for  31  composition  synthesized  and  54  arsenopyrites. c a r r i e d out by means of e l e c t r o n m i c r o p r o b e  determinations. The  r e l a t i o n between atomic % a r s e n i c and  peak p o s i t i o n was  determined to  the  d,  X-ray  3 1  be:  As = 866.67d, , - 1381.12 3  w i t h an e s t i m a t e d A was  standard  d e v i a t i o n of ±0.45% As.  temperature - c o m p o s i t i o n  p r e p a r e d (39)  is  shown  in  section for arsenopyrite Figure  c o n s t r u c t the diagram were o b t a i n e d a t v a r y i n g t e m p e r a t u r e s and  the  conditions  analyzed  and  data  used  if  an  for arsenopyrite  diagram  depending  diagram i s a l s o u s e f u l i n  arsenopyrite  is  accurately  i t s e q u i l i b r i u m assemblage i s d e t e r m i n e d then  t e m p e r a t u r e of f o r m a t i o n  to  arsenopyrite  e q u i l i b r i u m assemblages. The  of f o r m a t i o n . The  the c o n v e r s e sense i n t h a t  The  by s y n t h e s i z i n g  i l l u s t r a t e s the range of c o m p o s i t i o n s on  5.  which  can be e s t a b l i s h e d .  the  18  32 33 ' 34 35 36 A t o m i c % A r s e n i c in Arsenopyrite  Figure 5 Atomic % a r s e n i c i n a r s e n o p y r i t e  37  19  Arsenopyrite deficiency.  i s s l i g h t l y n o n - s t o i c h i o m e t r i c , showing an  Morimoto  and  Clark  (36) and Kretschmar and  (39) d e t e r m i n e d the Fe d e f i c i e n c y t o be l e s s than 0.7 The  limits  Kretschmar  of  and  corresponding  arsenopyrite  Scott  are  composition  Fe  Scott  atomic  %.  presented  FeAso.gS,,,  to  FeAs  t o an approximate range i n atomic % As of  1 < 5  by S .8 5 0  30%  to  38%. Clark  (38)  and  Kretschmar  and  S c o t t (39) observed t h a t  a r s e n o p y r i t e formed w i t h a g i v e n c o m p o s i t i o n re-equilibrate Arsenopyrite  if  subjected  present  therefore  be  formation  of the  in  a  highly  characteristic  metamorphosed  of  the  are  initial  almost  rims  while  in  c e n t r e s are A s - r i c h r e l a t i v e t o the  E l e c t r o p h y s i c a l P r o p e r t i e s of Arsenopyrite  about  set  of  readily  conditions.  deposits  will  conditions  always  0.2  eV  presented  assemblages  i s a s e m i c o n d u c t i n g m i n e r a l w i t h a band gap  values  i n Table  the  rims.  ( 4 5 ) . Shuey summarized the m i n e r a l as having  Arsenopyrite intrinsic  As-rich  reported  S-  Arsenopyrite  c a r r i e r m o b i l i t y and c o n s i s t e n t l y Resistivity  of  compositionally  ( 3 9 ) . In S - r i c h assemblages the c e n t r e of c r y s t a l s are  r i c h r e l a t i v e t o the  2.6  new  not  deposit.  Natural arsenopyrites zoned  to  does  by  high  carrier  several  a  of low  concentration.  investigators  are  3. near  stoichiometric  composition  is  an  semiconductor at room temperature. A r s e n i c d e f i c i e n c y  or the presence of i m p u r i t i e s make most a r s e n o p y r i t e n-type, but i n a r s e n i c - r i c h environments i t i s p - t y p e .  20 Table 3 Arsenopyrite R e s i s t i v i t y Values  Source 42 43 44 45 46  Krasnikov deposit  varied  R e s i s t i v i t y , ohm-cm. 2.0 X 10" - 3.0 X 10" 1.10 X 10" - 6.0 X 107.5 X 10" - 4.5 X 10" 3.0 X TO" 1.5 X 10- - 7.0 X 103  2  the  2  1  2  3  1  (40) d e t e r m i n e d t h a t a r s e n o p y r i t e i n a  gold  from p-type i n h i g h t e m p e r a t u r e , lower  t o n-type i n lower t e m p e r a t u r e , discussed  2  2  results  of  upper  horizons.  horizons  Favorov  a s t a t i s t i c a l treatment  ore  (41)  of s p e c t r a l  a n a l y s i s and thermo-emf measurements f o r 2000 samples of  pyrite  and a r s e n o p y r i t e . A r s e n o p y r i t e was found t o v a r y from p-type f o r high  temperature,  low s u l p h u r p a r t i a l p r e s s u r e a s s o c i a t i o n s t o  n-type f o r d e c r e a s i n g  temperature and i n c r e a s i n g s u l p h u r  partial  pressure a s s o c i a t i o n s . The p o s s i b l e i n f l u e n c e of semiconductor p r o p e r t i e s f l o t a t i o n behaviour  on  the  of a r s e n o p y r i t e must be c o n s i d e r e d .  2.7 E l e c t r o p h y s i c a l E f f e c t s i n F l o t a t i o n Sulphide  minerals  are  generally  semiconductors.  These  m i n e r a l s v a r y w i d e l y i n t h e i r e l e c t r o p h y s i c a l p r o p e r t i e s such as r e s i s t i v i t y , band gap, c a r r i e r c o n c e n t r a t i o n and  conductor-type  ( 4 5 ) . These v a r y i n g e l e c t r o p h y s i c a l p r o p e r t i e s c o u l d be e x p e c t e d to  influence  the  adsorption  molecules a t the mineral  or e l e c t r o o x i d a t i o n of c o l l e c t o r  surfaces.  P l a k s i n and Shafeev (46) were t h e f i r s t t o study the n a t u r e of charge c a r r i e r s i n s u l p h i d e m i n e r a l s  (in particular  galena)  21  and  the  effect  They c o n s i d e r e d surface  of  charge c a r r i e r type on x a n t h a t e  t h a t the  presence  of  l a y e r of s u l p h i d e s p r e v e n t s  bonds w i t h x a n t h a t e .  The  free  adsorption  electrons  the f o r m a t i o n  of  in  the  adsorption  a c t i o n of oxygen on g a l e n a was  believed  t o r e s u l t from an i n v e r s i o n of the s u r f a c e l a y e r from n-type  to  p-type. Subsequent  investigators  r o l e s f o r oxygen and were  not  correlation  that  with  that  semiconductor  regard  to  properties  xanthate  -  mineral  B i e g l e r (50) c o n c l u d e d t h a t t h e r e was  no  obvious  (47,48,49).  Recently,  p y r i t e and  concluded  controlling  interaction  i n t h i s f i e l d proposed a l t e r n a t e  between  k i n e t i c parameters f o r oxygen r e d u c t i o n  the nature  of s e m i c o n d u c t i o n .  He  further  on  concluded  i f i m p u r i t i e s were the p r i m a r y i n f l u e n c e on d i f f e r e n c e s i n  electrochemical  behaviour  their  influence  was  not  exerted  t h r o u g h t h e i r e f f e c t s on the s e m i c o n d u c t i n g p r o p e r t i e s . It  appears  semiconductors  therefore and  that  display  while  a l l the  sulphide electrical  a s s o c i a t e d w i t h t h i s c l a s s of c o n d u c t o r s , of  these s u l p h i d e m i n e r a l s  minerals  properties  oxygen  i s not c o n t r o l l e d p r i m a r i l y by  deduction  has can  b e h a v i o u r of a r s e n o p y r i t e as an e l e c t r o c a t a l y s t f o r has  been shown (20) t o be s i m i l a r t o t h a t of  o t h e r s u l p h i d e s such as p y r i t e and g a l e n a .  arsenopyrite  these  semiconducting  i n the l i t e r a t u r e , a r e a s o n a b l e  reduction  reasonable  response  on the e l e c t r o c h e m i c a l behaviour of a r s e n o p y r i t e  not been r e p o r t e d be made. The  properties  the f l o t a t i o n  e l e c t r i c a l p r o p e r t i e s . A l t h o u g h the i n f l u e n c e of  are  assumption will  that not  the be  It  is  semiconducting a  principal  therefore  a  p r o p e r t i e s of influence  on  22  electrochemical  behaviour  throughout  r e l e v a n t t o f l o t a t i o n of the m i n e r a l .  the  potential  range  23  Chapter 3  OBJECTIVES OF THE  The the  PRESENT INVESTIGATION  o b j e c t i v e of the p r e s e n t  surface  chemistry  r e s e a r c h program was  to  study  of a r s e n o p y r i t e so t h a t i t s f l o t a t i o n or  d e p r e s s i o n c o u l d be more e f f e c t i v e l y c o n t r o l l e d  than  has  been  p o s s i b l e to date. A  review  response  of  of  the  literature  arsenopyrite  had  r e v e a l e d t h a t the  received  only  flotation  limited  study.  Fundamental i n v e s t i g a t i o n s of the nature of s u r f a c e r e a c t i o n s of a r s e n o p y r i t e under f l o t a t i o n c o n d i t i o n s had not been c a r r i e d  out  previously. Information  obtained  from  i n v e s t i g a t i o n s f o r the p r e s e n t surface  the l i t e r a t u r e and p r e l i m i n a r y  r e s e a r c h program  indicated  o x i d a t i o n r e a c t i o n s c o u l d be s i g n i f i c a n t i n c o n t r o l l i n g  a r s e n o p y r i t e f l o t a t i o n . In a d d i t i o n , the a d s o r p t i o n of at  that  the  arsenopyrite  surface  involves  xanthate  electron  transfer  r e a c t i o n s . E l e c t r o c h e m i c a l i n v e s t i g a t i o n s have t h e r e f o r e been principal cyclic  research  voltammetry  method of  for  this  stationary  program.  and  a  In p a r t i c u l a r ,  rotating  arsenopyrite  e l e c t r o d e s has been used t o study s u r f a c e o x i d a t i o n r e a c t i o n s . E l e c t r o n spectroscopy surface  oxidation  (ESCA) has been used t o augment these  studies.  This  o x i d a t i o n f i l m s t o be a n a l y z e d . formation a r s e n i c and determined.  of  surface  iron  behaviour  addition  hydroxide,  s u l p h u r d u r i n g the The  In  technique  enables to  surface  verifying  f i l m s , the behaviour  formation  of a r s e n i c and  of  these  films  s u l p h u r c o u l d not  the of was be  24  r e s o l v e d through the use of voltammetry a l o n e . S i n c e a r s e n o p y r i t e i s most commonly r e c o v e r e d systems  employing  xanthate  was a l s o  Flotation  i t s interaction  experiments  Arsenopyrite  xanthate  as  with  considered. to  evaluate  the  e f f e c t i v e n e s s of  v a r i o u s o x i d a t i o n p r o c e d u r e s on a r s e n o p y r i t e have out.  flotation  as the c o l l e c t o r , the i n f l u e n c e of  the s u r f a c e o x i d a t i o n of a r s e n o p y r i t e on xanthate  in  well  which as  had  some  been  carried  not been f l o a t e d p r e v i o u s l y w i t h which  had  been  floated  was  used  for  not  been  investigated. Compositional electrochemical considered of  or  variations flotation  of  arsenopyrite  experiments  have  s i n c e these a r e not e x p e c t e d t o c o n t r o l the o x i d a t i o n  the m i n e r a l or i t s i n t e r a c t i o n w i t h x a n t h a t e ,  amounts of i m p u r i t i e s a r e not p r e s e n t .  provided  large  25  Chapter 4  ELECTROCHEMICAL STUDIES  4.1 E l e c t r o d e  P o t e n t i a l Measurements  The p o t e n t i a l a c h i e v e d i n t h e absence of  oxidizing  agents  g i v e s an i n d i c a t i o n of the tendency of the m i n e r a l t o o x i d i z e i n the  presence  of  aeration  (51). Potentials  achieved  presence of o x i d i z i n g agents can be c o r r e l a t e d w i t h electrochemical  investigations  to  predict  r a t e s as w e l l as t h e n a t u r e of o x i d a t i o n  results  relative  the  of  oxidation  products.  Mixed p o t e n t i a l measurements were c a r r i e d out i n determine  i n the  order  to  p o t e n t i a l a c h i e v e d by the m i n e r a l i n t h e absence  of any w e l l d e f i n e d  redox c o u p l e and then t o observe t h e e f f e c t s  of v a r i o u s o x i d i z i n g agents on e l e c t r o d e  potential.  Measurements were c a r r i e d out a c r o s s t h e range of pH v a l u e s commonly encountered i n s u l p h i d e f l o t a t i o n c i r c u i t s  (pH = 4  to  pH = 12).  4.1.1  Experimental Electrode  potential  measurements  spherical glass c e l l containing  were  carried  out i n a  1.25 l i t r e s of s o l u t i o n a t  room  temperature (25-28°C). Test s o l u t i o n s were p r e p a r e d from s i n g l e distilled desired  water  w i t h a s t a n d a r d a d d i t i o n of 0.1 molar KC1. The  pH v a l u e f o r each t e s t was a c h i e v e d through the  use  of  26  sodium  tetraborate  (0.05  M)  w i t h sodium h y d r o x i d e or  a c i d to r a i s e or lower the pH Solutions B u b b l i n g was source  of  respectively.  were deoxygenated by b u b b l i n g argon through them.  c o n t i n u e d throughout the solution  stirring.  test  Traces  and  was  the  only  of oxygen (3 to 5  ppm),  p r e s e n t i n the argon were removed by p a s s i n g the gas pyrogallol  through  a  solution.  Potential  measurements  were  s a t u r a t e d c a l o m e l e l e c t r o d e and by  sulphuric  taking  carried  out  relative  to a  c o n v e r t e d to the hydrogen  the p o t e n t i a l of the SCE  scale  to be -0.243 v o l t s r e l a t i v e  t o a hydrogen e l e c t r o d e a t 25°C. P o t e n t i a l s were measured w i t h a Beckman E l e c t r o s c a n potential  having  period  generally  was  was  recorded  ±5 mV.  v a l u e was  drift  of l e s s than 2 mV  achieved within  after  been a c h i e v e d i n within  a  5  30.  over a ten minute  minutes.  The  potential  20 minutes t o ensure s t a b l e c o n d i t i o n s  each The  case.  Potentials  could  be  determined  p l a c e d i n d i s t i l l e d water and  agent was  a  stable  was  potential.  added to the  from two One indicated  a l l o w e d a 20 minute p e r i o d  After  s o l u t i o n and  a l l o w e d b e f o r e the p o t e n t i a l was E l e c t r o d e s were  pH  mV.  In t e s t s i n v o l v i n g o x i d i z i n g agents the e l e c t r o d e was  achieve  had  r e p r o d u c i b i l i t y of p o t e n t i a l s at a g i v e n  found to be ±20  A  constructed  t h i s period  the  first to  oxidizing  a g a i n a 20 minute p e r i o d  was  recorded. using  arsenopyrite  crystals  sources. sample  was  t o come from  a n a l y z e d as  follows:  obtained Parral,  from  Ward's S c i e n t i f i c and  Chihuahua.  This  material  was was  27  Measured  Theoretical  As Fe S Co Ni Sb Cu Pb Zn  46.01 34.30 19.69  = 46.0 ±0.2% = 34.0 ±1.0% =19.8 ±0.1% = not d e t e c t e d = not d e t e c t e d = not d e t e c t e d = not d e t e c t e d = 0.27% = 0.06%  (<0.02%) (<0.02%) (<0.04%) (<0.01%)  T h i s c o m p o s i t i o n shows a s l i g h t i r o n d e f i c i e n c y compared t o stoichiometric  composition,  as  has  generally  been noted f o r  a r s e n o p y r i t e ( 3 9 ) . The c o m p o s i t i o n a l s o shows a s l i g h t excess i n the s u l p h u r t o a r s e n i c r a t i o and would be expected t o be type  an  n-  semiconductor. The e l e c t r o d e prepared from t h i s sample had an exposed a r e a  of a p p r o x i m a t e l y 0.7 cm . 2  The  second  sample of a r s e n o p y r i t e was s u p p l i e d by Mr. Joe  Nagel of t h e UBC Department of G e o l o g i c a l S c i e n c e s and came from R i o n d e l , B.C. There was  insufficient  sample  to  carry  c h e m i c a l a n a l y s i s of t h i s m a t e r i a l but a c o m p a r a t i v e the  sample  from  The  a  analysis to  Mexico was c a r r i e d out by means of a s c a n n i n g  e l e c t r o n microscope (SEM-EDX).  out  equipped w i t h an energy d i s p e r s i v e  analyzer  a n a l y s i s gave s i m i l a r i r o n , a r s e n i c and s u l p h u r  peaks as t h e Mexican m a t e r i a l and  d i d not  exhibit  any  peaks  i n d i c a t i v e of e x c e s s i v e c o n c e n t r a t i o n s of any i m p u r i t i e s . The  electrode  prepared  from t h i s m a t e r i a l had an exposed  a r e a of a p p r o x i m a t e l y 0.74 cm . 2  The means  semiconductor  type of each m a t e r i a l was  of a s i m p l e d e t e r m i n a t i o n employing  determined  by  t h e Seebeck e f f e c t . A  m i l l i v o l t m e t e r was used t o measure t h e p o t e n t i a l  across  a  hot  probe and a c o l d probe i n c o n t a c t w i t h the a r s e n o p y r i t e c r y s t a l .  28  In  each case the hot j u n c t i o n was found t o be p o s i t i v e r e l a t i v e  t o the c o l d j u n c t i o n i n d i c a t i n g the samples t o be  n-type  semi-  conductors . A  pyrite  Hanaoka,  electrode  Japan.  The  was  prepared  polished  using  electrode  material was  from  examined  m i c r o s c o p i c a l l y and was d e t e r m i n e d t o be f r e e from i n c l u s i o n s of other  s u l p h i d e m i n e r a l s . The exposed area of t h i s e l e c t r o d e was  approximately  0.38  cm . 2  The method of e l e c t r o d e c o n s t r u c t i o n i s shown i n F i g u r e A  6.  s e c t i o n of the m i n e r a l was mounted i n epoxy c o n s i s t i n g of one  p a r t d i e t h y l e n e t r i a m i n e t o 10 p a r t s Epon 828. Immediately a f t e r p l a c i n g t h e l i q u i d epoxy on the sample  i t was p l a c e d i n a vacuum  t o e l i m i n a t e a i r b u b b l e s and t o e n a b l e t h e epoxy t o s e a l in  the  sample.  Both  s i d e s of t h e mounted s e c t i o n were ground  down t o expose the m i n e r a l . One s i d e then had a gold  vacuum  deposited  A second epoxy s e c t i o n through  one  layer  was  side.  prepared  fastened  with  a  glass  The w i r e s from the m i n e r a l  glued together using a f a s t s e t t i n g  of  cement.  were i n s e r t e d t h r o u g h t h e g l a s s tube and the two  Prior  thin  i n two a r e a s . A copper w i r e was  t o each a r e a w i t h s i l v e r c o n d u c t i n g  inserted  cracks  sections  tube sample were  epoxy.  t o each experiment t h e s e c t i o n was wet ground on 600  mesh paper and then r i n s e d w i t h i n s e r t e d i n t o the t e s t  distilled  water  before  being  solution.  4.1.2 O x i d i z i n g Agents A v a r i e t y of o x i d i z i n g agents a r e used i n i n d u s t r y t o c a r r y out  reactions  r a n g i n g from p u l p b l e a c h i n g t o e f f l u e n t c o n t r o l .  29  WIRE  //-> GLASS TUBE  GOLD PLATING SILVER CEMENT  Figure 6 Method of e l e c t r o d e  construction  EADS  30  Only l i m i t e d use i s made of processing-  industry.  c h l o r i n a t i o n of leaching  of  oxidizing  These  cyanide  uranium  a r e used  solutions  ore  i n the  primarily  and  ( 5 4 ) . Use  p o t a s s i u m permanganate t o i n c r e a s e during  agents  as  mineral  for a l k a l i n e  oxidants  has a l s o  f o r the  been  the o x i d a t i o n  of  made of  pyrrhotite  g r i n d i n g and t h e r e b y d e c r e a s e i t s f l o a t a b i l i t y  (52).  In o r d e r f o r an o x i d i z i n g agent t o be a c c e p t a b l e f o r use i n controlling  the f l o t a t i o n  several c r i t e r i a Among  response  i n addition  of  t o being  m i n e r a l s i t must meet  an  effective  oxidant.  these c r i t e r i a a r e t h a t i t must be c o s t c o m p e t i t i v e ,  not be e x c e e d i n g l y  must  d i f f i c u l t or dangerous t o handle and mustnnot  r e s u l t i n p o l l u t i o n problems. The selected  oxidizing  agents  i n the present  on t h e b a s i s of e i t h e r a l r e a d y  in processing non-polluting, oxidizing  plants  present  subsequent  2  such as c h l o r i n e or Caro's a c i d ( H S 0 ) c o u l d 2  the range of mixed p o t e n t i a l s  series  of  sections.  the a v a i l a b l e  t h e number  of  5  achieved  by  agents was found t o cover t h e range of  The  current  experiments (1982)  electrons  described  costs Table  o x i d i z i n g u n i t s per kilogram.  are t h e p r o d u c t of the number of moles of o x i d a n t times  a  2  r e a g e n t s used i n t h e study a r e shown i n T a b l e 4. shows  t o be  reagent ( H 0 ) . W h i l e a d d i t i o n a l  i n t e r e s t as determined by e l e c t r o c h e m i c a l in  were  h a v i n g some a p p l i c a t i o n  4  cost competitive  agents  study  (NaCIO, KMn0 ) or b e i n g c o n s i d e r e d  a l s o be a p p r o p r i a t e , the  used  f o r the 4  also  These u n i t s  per  kilogram  t r a n s f e r r e d assuming a l k a l i n e  conditions. The  v a r i a t i o n i n o x i d i z i n g power of these r e a g e n t s  can be  d e t e r m i n e d from a s e r i e s of o x i d a t i o n s t a t e diagrams (53). These  31 Table 4 Cost of O x i d i z i n g Agents  OXIDIZING AGENT  COST $/kg. O x i d a n t  Hydrogen P e r o x i d e (50% S o l u t i o n ) P o t a s s i u m Permanganate Sodium h y p o c h l o r i t e (12% S o l u t i o n )  diagrams  graphically  present  1.10 3.75 3.48  -  1.32 4.75 5.61  58.8 25.3 26.8  the v o l t e q u i v a l e n t of a compound  or i o n as a f u n c t i o n of i t s o x i d a t i o n s t a t e . The i s the product relative  OXIDIZING units/kg  of the o x i d a t i o n s t a t e and  t o the element i n i t s s t a n d a r d  the  volt  equivalent  redox  potential  s t a t e and  the f r e e energy f o r the s p e c i e s a c c o r d i n g  to  AG  i s r e l a t e d to =  -nFE.  The  g r a d i e n t of the l i n e j o i n i n g two p o i n t s on such a diagram i s the redox p o t e n t i a l of the couple positive  gradient  represented  represents  a  s t r o n g o x i d i z i n g c o u p l e and  large negative gradient, a strong reducing Diagrams systems both  for  are  acid  (a  variation  of  the  chlorine,  shown  in  F i g u r e s 7,8  = l)  and  alkaline  o H +  the  oxidizing  by the p o i n t s . A l a r g e  couple.  manganese  and  and 9. The (a  power  o M  . = l)  with  oxygen-water  diagrams i n c l u d e conditions.  pH  is  an  at  pH  values  greater  The  important  c o n s i d e r a t i o n s i n c e f l o t a t i o n of s u l p h i d e s i s g e n e r a l l y out  a  carried  than pH=4 and more commonly i n the  range of pH=8 t o pH=11. Thus the diagram f o r manganese i n d i c a t e s that while oxidizing  in agent  acid  media  capable  permanganate of  being  represents  reduced  to  Mn  a l k a l i n e media a much lower g r a d i e n t i s apparent oxides  and h y d r o x i d e s  are formed. The  c o u l d be expected t o r e s u l t i n high  pH  this  use of Mn0  reasonable  and 2  a ++  strong ions, in  manganese  i n a c i d media  oxidation  while  reagent would be i n e f f e c t i v e as an o x i d a n t .  at This  32  c o u l d i n p a r t e x p l a i n t h e two Nekrasov  (16)  required  hour  when  conditioning  using  Mn0  2  period  which  for arsenopyrite  oxidation. In  t h e case  of  the c h l o r i d e  system  hypochlorite  is  i n d i c a t e d t o be a s t r o n g e r o x i d i z i n g agent than c h l o r a t e . While  the  diagrams  are useful  o x i d a t i o n p o t e n t i a l of v a r i o u s reagents  f o r representing  the  i t must be h e l d i n mind  t h a t p a r t i c u l a r l y i n a heterogeneous system, k i n e t i c f a c t o r s may in fact control t h e i r r e l a t i v e effectiveness.  4.2 R e s u l t s and D i s c u s s i o n Preliminary  experiments  with  a  platinum  electrode  in  deoxygenated d i s t i l l e d water gave t h e r e l a t i o n Eh = 850 - 62.3 pH r = -1.00 Since t h i s i s i n reasonable  ,mV  agreement w i t h t h e r e l a t i o n s h i p  Eh = 800 - 59 pH  ,mV  which has been d e v e l o p e d elsewhere (55) t h e r e c o r d i n g was c o n s i d e r e d t o be c a p a b l e A  variation  expected  of  because  accurately  of p r o v i d i n g r e l i a b l e measurements.  up t o 100 mV i n t h e i n t e r c e p t p o t e n t i a l can be while  controlled,  t h e pH the  i s buffered potential  c o n t r o l l e d by k i n e t i c f a c t o r s and thermodynamically achieved potential  may  and  i s to  therefore  and  then  measuring  therefore  some deviate  d e r i v e d v a l u e s . The e x p e r i m e n t a l  by v a r y i n g t h e pH without  equipment  extent from  r e l a t i o n was  the  electrode  r e p o l i s h i n g o r c l e a n i n g t h e e l e c t r o d e between  measurements. Slow k i n e t i c s w i t h r e g a r d t o e q u i l i b r a t i o n of t h e system c o u l d account f o r t h e d e v i a t i o n of both t h e i n t e r c e p t and  33  - 1 0  I 2 3 4 Oxidation s t a t e  5  6  Figure 7 O x i d a t i o n s t a t e diagram f o r the c h l o r i n e  system  7  34  -41  0  1  1  I  2  1  3  Oxidation  I  4  I  i  i  5  6  7  state  Figure 8 O x i d a t i o n s t a t e diagram f o r manganese system  35  Figure 9 O x i d a t i o n s t a t e diagram f o r the oxygen  system  36  s l o p e of t h e e x p e r i m e n t a l Mixed  potentials  r e l a t i o n from t h e o r e t i c a l  measured  with  arsenopyrite  values. and p y r i t e  e l e c t r o d e s a r e shown i n F i g u r e s 10 and 11. The p o t e n t i a l measured w i t h  arsenopyrite  in  deoxygenated  water over t h e pH range from 3.8 t o 11.2 g i v e s t h e r e l a t i o n Eh = 586 - 48.2 pH r = -0.9976  ,mV  At pH g r e a t e r than 11.2 t h e p o t e n t i a l s f a l l below t h i s with  line  i n c r e a s i n g d e v i a t i o n as t h e pH i n c r e a s e s . At t h e same time  an e l e c t r o d e which was a l l o w e d t o reach a s t a b l e p o t e n t i a l a t pH g r e a t e r than 11.2 was observed oxide  layer.  This  to  develop  a  visible  surface  s u r f a c e d e p o s i t had a brown appearance when  the e l e c t r o d e was a l l o w e d t o d r y . Another o b s e r v a t i o n was t h a t i f t h e pH of t h e s o l u t i o n  was  lowered a f t e r t h e e l e c t r o d e had reached a s t a b l e p o t e n t i a l a t pH greater  than  11.2,  the electrode p o t e n t i a l followed along the  same c u r v e as f o r f r e s h l y p o l i s h e d e l e c t r o d e points  obtained  in this  way  are  shown  measurements. i n Figure  The  10. The  d e v i a t i o n from a s t r a i g h t l i n e a t pH g r e a t e r than 11.2 i n d i c a t e s a r s e n o p y r i t e t o be mineral  inherently  decomposes,  leaving  unstable a  the  same  relation  as  of  obtained  this  but  i s too  thin  composition  of t h i s s u r f a c e  The  to  oxide  be  with  readily  will  visible  oxide  for a freshly polished  e l e c t r o d e , t h e same o x i d e f i l m must be p r e s e n t electrode  region.  surface oxide l a y e r . Since the  e l e c t r o d e p o t e n t i a l s i n t h e presence follow  in this  be  the  fresh  observed.  discussed  The  i n the  s e c t i o n on voltammetry. There  are  several  differences  p y r i t e e l e c t r o d e compared t o t h e  i n the  arsenopyrite  behaviour electrode.  of t h e Over  37  J 2  1  4  I 6  I 8 PH  Q  I n d i v i d u a l measurement  0  T i t r a t i o n point Figure  10  Eh v e r s u s pH f o r a r s e n o p y r i t e  I 10  L 12  F i g u r e 11 Eh v e r s u s pH f o r  pyrite  39  the pH range from 7 t o 12.2  the p o t e n t i a l f o l l o w e d the  Eh = 643 - 49.0 r = -0.993 indicating Unlike  pH  p y r i t e t o be a more noble m i n e r a l than a r s e n o p y r i t e .  arsenopyrite,  relation  at  high  pyrite  pH.  The  shows  no  deviation  r e l a t i o n down t o pH = 3.4.  l e s s than 7  measured  fall Similar  below  the  with the  a  Eh-pH  v i s i b l e for  c l e a n p y r i t e e l e c t r o d e at  line  represented  have  by  the  pH  above  been made f o r p y r i t e and  i n v e s t i g a t o r s ( 5 5 ) . The  r e l a t i o n below pH = 7 was  had  12.2.  observations  other s u l p h i d e s by other  this  f o l l o w e d the same  Only a s l i g h t t a r n i s h was  the p y r i t e e l e c t r o d e a t pH = Potentials  from  p o t e n t i a l of an e l e c t r o d e which  reached a s t a b l e p o t e n t i a l at pH = 12.2  relation.  relation  deviation  from  e x p l a i n e d on the b a s i s t h a t below  t h i s pH the p a s s i v a t i n g o x i d e l a y e r  was  not  forming  and  the  electrode i s in a corrosion region. The tests  c o n c e n t r a t i o n of each o x i d i z i n g agent which was  involving  these  additions  was  p r e l i m i n a r y f l o t a t i o n e x p e r i m e n t s . The i n d i c a t e d t h a t these c o n c e n t r a t i o n s  based on the r e s u l t s of f l o t a t i o n experiments  of o x i d i z i n g agent  the range which c o u l d be r e q u i r e d f o r a r s e n o p y r i t e Potential  used i n  were  case  measurements made w i t h an a r s e n o p y r i t e  electrode  The  10.  In  of p o t a s s i u m c h l o r a t e the p o t e n t i a l s i n the r e g i o n of  pH from 7 t o 10 were found t o f a l l on the obtained  in  depression.  i n the presence of o x i d i z i n g agents are shown i n F i g u r e the  had  i n the absence of any remaining  W i t h 160 mg/1  same  line  o x i d i z i n g agents.  p o t e n t i a l s f o l l o w the Hydrogen P e r o x i d e Eh = 688 - 46.9 r = -0.9886.  pH  relations:  as  those  40  With 57 mg/1  With 54.3  A  few  P o t a s s i u m Permanganate Eh = 663 - 34.1 pH r = -0.9533  mg/1  Sodium H y p o c h l o r i t e Eh = 1037 - 82.9 pH r = -0.9966  measurements  were  e l e c t r o d e i n the presence of permanganate.  As  in  the  carried  hydrogen case  of  out  with  peroxide  the  and  potassium  a r s e n o p y r i t e the p o t e n t i a l  a c h i e v e d w i t h p y r i t e i n the presence of permanganate than with, p e r o x i d e  4.3  over the pH r e g i o n of  cited  higher  interest.  i n the l i t e r a t u r e review on  f l o t a t i o n i n d i c a t e t h a t the s u r f a c e could  play  oxidation  arsenopyrite  of  the  a major r o l e i n c o n t r o l l i n g i t s f l o t a t i o n  r e s u l t s of e l e c t r o d e measurements p r e s e n t e d  section  indicate  that  surface  oxide  is  therefore  of  mineral response.  i n the  deposits  previous  are formed on  a r s e n o p y r i t e a t h i g h pH or under moderate o x i d i z i n g It  is  C y c l i c Voltammetry References  The  pyrite  conditions.  i n t e r e s t t o determine the nature of  these  s u r f a c e o x i d e d e p o s i t s so t h a t t h e i r f o r m a t i o n and t h e r e f o r e the f l o t a t i o n response of a r s e n o p y r i t e can be b e t t e r c o n t r o l l e d than has p r e v i o u s l y been p o s s i b l e . C y c l i c voltammetry, i n a d d i t i o n t o e n a b l i n g the of e l e c t r o d e s xanthate  to  with be  dissolved  electroactive  species  such  s t u d i e d , p r o v i d e s a means f o r s t u d y i n g  o x i d a t i o n r e a c t i o n s of e l e c t r o d e s . By scans  interaction  carrying  out  as  surface  repetitive  the i n f l u e n c e on s u r f a c e changes of e x c u r s i o n s t o v a r i o u s  p o t e n t i a l ranges can be o b s e r v e d . C y c l i c voltammetry has been used t o study the o x i d a t i o n  of  41  ethyl  xanthate  and  pyrite electrodes w i t h (57) and  the  ( 2 3 ) . The  interaction  of  flotation  reagents  the s u r f a c e o x i d a t i o n (58) of g a l e n a has a l s o been  s t u d i e d by t h i s  technique.  Electrochemical  reactions  the absence of c h e m i c a l using  i n f l u e n c e of f l o t a t i o n d e p r e s s a n t s on  noble  metal  i n v o l v i n g p a r t i c u l a r species in  r e a c t i o n mechanisms  electrodes  (23,49)  have  and  been  studied  employing  cyclic  voltammetry. C y c l i c voltammetry i s s i m i l a r t o l i n e a r sweep p o l a r i z a t i o n but  with  the  added  f e a t u r e t h a t the p o t e n t i a l v a r i a t i o n w i t h  time i s r e v e r s e d at some p o i n t potential.  In  this  way  both  and  returned  the  to  o x i d a t i o n and  e l e c t r o a c t i v e s p e c i e s at the e l e c t r o d e s u r f a c e can For  example,  if  xanthate  is  oxidized  to  starting  r e d u c t i o n of be  studied.  d i x a n t h o g e n at an  e l e c t r o d e d u r i n g an a n o d i c p o t e n t i a l sweep and reversed, the:reduction  the  the sweep i s then  r e a c t i o n can be m o n i t o r e d . By  measuring  both anodic and c a t h o d i c c u r r e n t s , i t can be d e t e r m i n e d t h a t  the  dixanthogen  and  formed  at the e l e c t r o d e remains at the s u r f a c e  does not d i f f u s e i n t o the b u l k s o l u t i o n . S e v e r a l f i g u r e s which are e s s e n t i a l t o this Figure  thesis  are  the  the e s s e n t i a l f e a t u r e s of a voltammogram. an  anodic  potential  sweep  In  49 t o the  starting  illustrate  presence a t -0.4V  exceeded. Beyond t h i s p o t e n t i a l t h e r e t o a peak, f o l l o w e d by a g r a d u a l below  0 V  dixanthogen  is  i s a rapid r i s e in current  decline. is  of  shows  e s s e n t i a l l y no c u r r e n t b e i n g passed u n t i l a p o t e n t i a l of 0.2V  falls  in  i n c l u d e d on a f o l d - out page as Appendix I I .  1 i n Appendix I I i s taken from r e f e r e n c e  xanthate  discussion  reduced,  Once at  the  potential  which  point a  42  c a t h o d i c peak i s o b s e r v e d . xanthate  The  - dixanthogen couple  xanthate  used. The  anodic  reversible i s 0.15  peak  potential  for  the  V a t the c o n c e n t r a t i o n of  can  be  associated  with  the  o x i d a t i o n of xanthate  t o d i x a n t h o g e n w h i l e the c a t h o d i c peak can  be  the  associated  xanthate,  with  reverse  r e a c t i o n . In the presence of  hydrogen and oxygen a d s o r p t i o n i s i n h i b i t e d and  peaks  a s s o c i a t e d w i t h these s p e c i e s are t h e r e f o r e not o b s e r v e d . The  height  stirring  of  the  concentration. of xanthate  of  the  xanthate  solution  Both  or  by  peaks would be i n c r e a s e d increasing  the  by  xanthate  of these a c t i o n s would i n c r e a s e the  supply  t o the e l e c t r o d e s u r f a c e .  Although  voltammetric  sweeps are sometimes c a r r i e d  out  at  slow scan speeds ( l e s s than 1mV/sec.) i n g e n e r a l the scan speeds employed  are  greater  than 1 mV/sec. At i n c r e a s i n g scan speeds  the c u r r e n t peaks due  t o charge t r a n s f e r r e a c t i o n s are  and may  studied.  be more f u l l y  For a f i x e d sweeprate and height  is  associated  with  current an  scale,  increased  increased  increased  reaction  rate.  i n c r e a s e i n the a r e a under a peak i n d i c a t e s an i n c r e a s e r e a c t i o n products  4.3.1  peak  in  An the  ( i . e . a greater t o t a l current i s passed).  Experimental Voltammetry  was  carried  out  r o t a t i n g e l e c t r o d e s . For s t a t i o n a r y electrodes  were  the  with  both  electrode  stationary  and  experiments  the  same ones as used f o r e l e c t r o d e p o t e n t i a l  measurements. The 12.  An  r o t a t i n g e l e c t r o d e was arsenopyrite  disc  c o n s t r u c t e d as shown  in  was  a  prepared  using  Figure diamond  43  impregnated g l a s s d r i l l . The c r y s t a l used f o r t h i s e l e c t r o d e was the same one from R i o n d e l , B.C. as was used f o r the s t a t i o n a r y electrode.  One  face  of t h e d i s c was g o l d c o a t e d and t h i s  face  was f a s t e n e d t o t h e end of t h e b r a s s r o d which formed t h e c e n t r e of t h e e l e c t r o d e . The space between surrounding  teflon  tube  was  the a r s e n o p y r i t e  filled  and t h e  i n t h r e e stages u s i n g a  hypodermic s y r i n g e f i l l e d w i t h epoxy. The e l e c t r o d e was equipped w i t h a g o l d r i n g which was s i l v e r s o l d e r e d leading  to  the  electrode  connections.  f e a t u r e s and manner of c o n n e c t i o n  was as  to  a  brass  The remaining for a  sleeve physical  standard  Pine  Instruments r o t a t i n g e l e c t r o d e ( 8 4 ) . The  measurement  c i r c u i t employed f o r voltammetry i s shown  i n F i g u r e 13. The p o t e n t i o s t a t Cyclic  potential  Princeton  Applied  Currents  functions  were  Research  Model  were r e c o r d e d  multimeter Current  in series -  potential  All  was  a  Wenking  controlled 175  by  68  TS10.  means  Universal  of  a  programmer.  by means of a K e i t h l e y Model 171 d i g i t a l with scans  I n s t r u m e n t s Model 520 XY carried  used  the platinum  counter  electrode.  were  recorded  with  an  Electro  recorder.  Rotating  experiments  were  out by means of a P i n e I n s t r u m e n t s Model ASR 2 r o t a t o r .  instruments  were plugged i n t o a m u l t i - o u t l e t  extension  to  e l i m i n a t e ground - l o o p problems. Tests  were  carried  out i n a c o n v e n t i a l H - c e l l w i t h the  working e l e c t r o d e compartment h a v i n g a being  separated  from  the counter  volume  of  850  cc and  e l e c t r o d e by a f r i t t e d  glass  d i s c . A l l t e s t s were c a r r i e d out a t room temperature (22±2 °C). A  s t a n d a r d a d d i t i o n of 0.1M p o t a s s i u m c h l o r i d e was made t o each  t e s t . The pH was a d j u s t e d by means of  sodium  tetraborate  with  BRASS  TEFLON  SILVER  EPOXY  GOLD  CEMENT  RING  ARSENOPYRITE  r, = 3-l75mm r =3-99 mm r =4-40 mm 2  3  F i g u r e 12 C o n s t r u c t i o n of r o t a t i n g a r s e n o p y r i t e  electrode  45  sodium  hydroxide  or  sulphuric  acid.  A l l reagents  p r e p a r a t i o n of s o l u t i o n s or f o r a d d i t i o n of s p e c i f i c  used f o r ions  were  reagent or a n a l y t i c a l grade.  4.3.2 R e s u l t s and D i s c u s s i o n  A. S i n g l e Sweep Voltammograms By  earring  out  voltammetry  v a r i a t i o n s i n both anodic observed.  By  position with  noting pH,  and c a t h o d i c  both  peak  possible these  across  associated  with  potentials  for postulated  a  peak  positions  elecrochemical  peaks  can  be  reactions  wide range of pH, positions  be  and s h i f t s i n peak surface  postulated. can  can  be  reactions Reversible  calculated  and  c o r r e l a t e d w i t h peak p o s i t i o n s . Voltammograms  r e s u l t i n g from the a p p l i c a t i o n of t r i a n g u l a r  p o t e n t i a l c y c l e s t o a r s e n o p y r i t e a c r o s s the range of pH  =  5.85  t o pH = 11.95 a r e shown i n F i g u r e s 14 and 15. Between pH = 6.6 and 7.3 an anodic  peak becomes apparent i n  the r e g i o n of 0.6 t o 0.7 v o l t s . T h i s peak becomes more prominent and  shifts  to  more  cathodic  potentials  with  i n c r e a s i n g pH  a c c o r d i n g t o the r e l a t i o n . Ep = 1216 - 78.6 pH  mV.  r = - 0.99 The scans shown i n f i g u r e s 14 and 15 s t a r t w e l l below potential  of a r s e n o p y r i t e . A d d i t i o n a l anodic  is  not  associated  with products  rest  scans were c a r r i e d  out s t a r t i n g from t h e r e s t p o t e n t i a l t o ensure t h a t peak  the  the  anodic  r e s u l t i n g from c a t h o d i c  c u r r e n t s observed a t the s t a r t of the scans.  Anodic  peaks  for  46  PROGRAMMER RECORD  E  POTENTIOSTAT CONTROL* W •• R •• C GROUND*  REF  C  WE  MULTIMETER =:RECORD I  COMMON ROTATOR  F i g u r e 13 C o n t r o l and measurement c i r c u i t used f o r voltammetry  47  scans  s t a r t i n g a t the r e s t p o t e n t i a l were found t o occur a t  same p o t e n t i a l and t o be of the  same  magnitude  as  the  those  for  scans s t a r t i n g a t the lower p o t e n t i a l s . The  anodic  peak  lies  in  the  r e l e v a n t t o f l o t a t i o n systems. The were  measured  in  the  electrode  presence  of  permanganate are i n d i c a t e d i n F i g u r e II,  on  a  voltammogram  achieved w i t h reaction  these  associated  p o t e n t i a l r e g i o n which i s  obtained  potentials  hydrogen  pH = 8.2.  oxidizing  agents  should  with  anodic  peak  the  peroxide  16 and F i g u r e at  which  2,  and  Appendix  The p o t e n t i a l s result  in  proceeding  the at  a  significant rate. Since a  KC1  connections the  to  electrode  potentials  supporting  was  used  and  good  the e l e c t r o d e were made and the r e s i s i t i v i t y material  are  electrolyte  is  believed  low  ( i e . 10"  2  ohm  -  t o be a c c u r a t e even a t the  cm)  of  peak  relatively  h i g h c u r r e n t s (800 microamperes) o b s e r v e d f o r some scans. The  r e p e a t a b i l i t y of  m i l l i v o l t s and was accurately  the  scans  was  found  to  be  within  10  l i m i t e d p r i m a r i l y by the a b i l i t y t o determine  peak  position  on  the  c u r r e n t s were found t o be s e n s i t i v e  to  chart the  recorder.  condition  Peak  of  the  g r i n d i n g paper used t o p r e p a r e the e l e c t r o d e s u r f a c e . P r o v i d e d fresh  area  preparation,  of  g r i n d i n g paper was  currents  could  be  a  used t o f i n i s h the e l e c t r o d e reproduced  to  within  20  microamperes. A s m a l l anodic -0.1  V  at  prewave becomes apparent a t  pH = 9.6  in  Figure  apparent over the pH range from 9.6 over the range of pH  11.05  approximately  14. T h i s prewave becomes more t o 10.55  t o 11.95. The  and  then  decreases  peak p o s i t i o n s h i f t s t o  pH  5-85  pH  6-6  pH 7-3 pH 9-2  pH 9*6  I  -0-8  I  I  -0-6 -0-4 -0-2 Potential /  0 V  0-2 0-4 vs SHE  0-6  0-8  F i g u r e 14 Voltammograms f o r a r s e n o p y r i t e a t i n c r e a s i n g pH v a l u e s  49  I  1  1  I  I  -0-8 -0-6 -0-4 -0-2 0 Potential / V vs  I  I  0-2 0-4 SHE  I  0-6  F i g u r e 15 Voltammograms f o r a r s e n o p y r i t e a t h i g h pH  50  F i g u r e 16 Voltammogram f o r a r s e n o p y r i t e a t pH = 8.2 showing a c h i e v e d w i t h o x i d i z i n g agents  potentials  51  more  cathodic  potentials  with  increasing  pH.  Such prewaves  p r e v i o u s l y have been i n t e r p r e t e d as being a s s o c i a t e d o x i d a t i o n l a y e r s ( 7 9 ) . In the p r e s e n t to  result  from  the  presence  of  with  thin  study the peak i s b e l i e v e d i r o n h y d r o x i d e f i l m s formed  d u r i n g e l e c t r o d e p r e p a r a t i o n as w i l l s u b s e q u e n t l y be d i s c u s s e d . The  behaviour  potential  region  of of  arsenopyrite interest  o x i d a t i o n of a r s e n o p y r i t e approximately oxidation.  200 mV The  significant  pyrite  i s compared  i n Figure  that  pyrite  those oxidation  further  Possible  indicates  reactions  a r s e n o p y r i t e can be  to  becomes  a t p o t e n t i a l s g r e a t e r than t h a t of the a r s e n o p y r i t e  that  s e l e c t i v e f l o t a t i o n or d e p r e s s i o n  used  17. The  for pyrite  reaction  by  with  considerations  the  diagram  for arsenopyrite  to  the  considering  f o r a r s e n o p y r i t e shown i n F i g u r e construct  these  two  control  the  oxidation  of  of a r s e n o p y r i t e .  associated  determined  of  the a n o d i c peak observed f o r  a r s e n o p y r i t e c o u l d be s i g n i f i c a n t i n a t t e m p t i n g  diagram  the  potentials  required  o x i d a t i o n peak. T h i s d i f f e r e n c e i n the b e h a v i o u r minerals  across  i s o b s e r v e d t o commence a t  lower  main  and  and are  the  stability  18. Thermodynamic additional  presented  data  stability  i n Appendix I .  These diagrams were c o n s t r u c t e d as p a r t of the p r e s e n t  study.  At pH = 4 t o pH = 7  1/2 A s S  + H  +  E = - 0.012 - 0.0295 l o g ( H S ) /  2  2  3  + Fe  + +  1  2  (Fe  +  +  2  - 0.0197 pH  )  or i f o x i d a t i o n t o form H A s 0 3  + 3e" = FeAsS + 1/2 H S  3  and SO*,"" i s c o n s i d e r e d  (6)  52  —'  0-4  "  -0-2  1  1  0  0-2  1  0-4  Potential / V vs SHE 20 mV s-' , pH=  110  F i g u r e 17 Voltammograms f o r p y r i t e and a r s e n o p y r i t e a t pH = 11.  u  0-6  53  Fe  + +  + H As0 3  3  + SO -- + 11H a  +  + 11e" = FeAsS + 7H 0 (7) 2  E = 0.283 + 0.005 l o g ( F e ) ( H A s 0 ) ( S O , " " ) -  0.0591  + +  3  3  pH  At  pH  g r e a t e r than 7  Fe(OH)  2  + H As0 3  + SO,,"- + 13H  3  + 1 1 e" = FeAsS+9H 0 (8)  +  2  E = 0.346 + 0.005 l o g ( H A s 0 ) ( S 0 " " ) - 0.070 pH 3  3  4  The appearance of t h e a n o d i c peak a t 0.6 V between pH = 6.6 and 7.3 can be r e l a t e d t o the f o r m a t i o n of F e ( O H ) than  approximately  7.  stable r e l a t i v e to F e  + +  The  exact  a t pH g r e a t e r  2  pH a t which F e ( O H )  depends on t h e a c t i v i t y  of  2  Fe  becomes a t the  s u r f a c e of t h e e l e c t r o d e . The  observed  anodic  peak  or  half  wave  potentials are  c o n s i d e r a b l y a n o d i c t o the r e v e r s i b l e p o t e n t i a l s c a l c u l a t e d the above e q u a t i o n s . Assuming a c t i v i t i e s of 10" M  for  3  from  example  e q u a t i o n 8 p r e d i c t s a r e v e r s i b l e p o t e n t i a l of -0.422 v o l t s . Such a  shift  i n peak p o t e n t i a l may be due t o i r r e v e r s i b i l i t y of t h e  r e a c t i o n , t o t h e f o r m a t i o n of a p a s s i v e l a y e r or t o b o t h . At t h e o b s e r v e d peak p o t e n t i a l s the s h o u l d be formed i s Fe(OH) equation  8.  The  oxidation  i s g i v e n by Fe(OH)  3  3  rather  reversible  + H  E = 0.28 - 0.059 pH  +  potential  which  hydroxide  than the F e ( O H )  + e" = F e ( O H )  from  iron  a  2  2  f o r Fe(OH)  + H0  i n d i c a t e d by 2  t o Fe(OH)  3  (9)  2  value  which  of  E = -0.342 V  is  o b t a i n e d a t pH = 10.55. It  can  be  seen  i n Figure  18 t h a t a t the o b s e r v e d peak  54  Figure  18  Eh - pH diagram f o r a r s e n o p y r i t e . A c t i v i t y f o r each taken t o be 10" M. 3  species  55  p o t e n t i a l s a r s e n i c i s e x p e c t e d t o be o x i d i z e d sulphur  to  arsenate  and  i s e x p e c t e d t o be o x i d i z e d t o s u l p h a t e .  The  formation  of  Fe(OH)  during  3  the  anodic  process  is  c o n s i s t e n t w i t h c e r t a i n a d d i t i o n a l f e a t u r e s of the voltammograms shown i n F i g u r e s 14 and near  the  peaks  for  greater  15. A c a t h o d i c p o t e n t i a l sweep  rest  potential  the  complete  currents  than  increased  cathodic  indicates  that  i s shown a t pH = 11.95. The potential  those  peak  species  starting  for  show  cathodic  current formed  sweeps  for during  significatly  sweeps  the  cathodic  only.  complete  the anodic  The  sweeps  p a r t of  the  p o t e n t i a l c y c l e are being reduced. These c a t h o d i c peaks occur  at  p o t e n t i a l s c o n s i s t e n t with equation  to  represent  the  r e d u c t i o n of F e ( O H )  9 and can be i n t e r p r e t e d 3  formed d u r i n g the o x i d a t i o n  reaction. The actually  c a t h o d i c peak a t pH g r e a t e r than 11.0 a  double  peak.  r e d u c t i o n of F e ( O H )  3  could  represent  or c o u l d i n d i c a t e t h a t  such as s u l p h u r are being Voltammograms  This  in Figure  from  hydroxide  formation  is  no anodic  evident.  electrode  peak is  t h i s c a t h o d i c peak r e s u l t s from the f o r m a t i o n  of  oxidation during  sweep.  of a s m a l l amount of Fe(OH)  electrode  d i s c u s s e d i n Chapter Figure  in  It  s m a l l prewave o b s e r v e d d u r i n g the anodic  the  species  reduced.  s u l p h u r or a s u l p h u r compound d u r i n g the anodic The  step  a t lower pH and p a r t i c u l a r l y a t pH = 5.85  associated with iron that  is  two  additional  F i g u r e 14 show a prominent c a t h o d i c peak a l t h o u g h  believed  a  15  polishing  and  scan 2  results  formed on  preparation  the as  5.  19 shows a c u r r e n t - decay curve  f o r an  arsenopyrite  56  electrode at  which  was stepped.from the r e s t p o t e n t i a l t o +343 mV  pH = 10.6. T h i s  potential  potential  represents  the  anodic  peak  a t t h i s pH. The c u r r e n t decays from t h e i n i t i a l  value  of s e v e r a l thousand microamps t o l e s s than 100 microamps i n l e s s than one minute. Such a c u r r e n t decay build-up The  of o x i d a t i o n p r o d u c t s  i s associated  passivates  difficult.  This  the  a t t h e s u r f a c e of t h e e l e c t r o d e .  presence of these o x i d a t i o n p r o d u c t s  electrode  with  at the surface  of t h e  i t , making f u r t h e r r e a c t i o n i n c r e a s i n g l y  rapid  decay  of  current  confirms  o x i d a t i o n of a r s e n o p y r i t e r e s u l t s i n s o l i d p r o d u c t s  that  the  which remain  at the e l e c t r o d e s u r f a c e . An  estimate  was  made  of  t h e q u a n t i t y of i r o n  formed on t h e s u r f a c e d u r i n g an anodic anodic  peak  hydroxide  scan. The area under  the  a t pH = 11.55 was determined i n o r d e r t o determine  the c u r r e n t passed. From t h e s t a r t of the o x i d a t i o n peak t o t h e inflection  i n t h e curve  approximately  past  t h e peak,  with equation  every mole of F e ( O H )  2  per  mole  of  arsenopyrite  8,  oxidized,  9, which i n d i c a t e s an e x t r a e l e c t r o n f o r  o x i d i z e d t o F e ( O H ) , t h e q u a n t i t y of FeAsS 3  which i s o x i d i z e d d u r i n g t h e scan i s 0.102 X 1 0 a stochiometric composition, iron  a c u r r e n t of  11700 micro coulombs was passed. U s i n g e q u a t i o n  which shows 11 e l e c t r o n s together  just  - 7  mole. Assuming  t h i s means t h a t t h e same amount  of  i s o x i d i z e d t o the f e r r i c s t a t e . Using the p r o j e c t e d area  of t h e e l e c t r o d e of 28.3 s q . mm. and assuming a s o l i d product of Fe 0 2  3  w i t h a d e n s i t y of 5 (60) an o x i d e t h i c k n e s s  determined.  This  surface  area  40  A° i s  i s an o r d e r of magnitude e s t i m a t e o n l y s i n c e  the s t r u c t u r e c o u l d d i f f e r from t h e assumed one real  of  and  since  can be e x p e c t e d t o be s e v e r a l times  the  greater  57  than the p r o j e c t e d a r e a . Taking  i n t o account the r e a l s u r f a c e area c o u l d  result  in  an e s t i m a t e of the o x i d e t h i c k n e s s as low as 10 A . A l e s s dense 0  iron  hydroxide  structure  than  has  been  assumed  c a l c u l a t i o n on the o t h e r hand c o u l d r e s u l t i n the  in  actual  the oxide  t h i c k n e s s b e i n g s e v e r a l times the c a l c u l a t e d v a l u e .  B. M u l t i p l e Sweep ( C y c l i c ) Voltammetry By  allowing  the  triangular  potential  c y c l e t o repeat a  number of t i m e s , changes w i t h time i n the s u r f a c e c o m p o s i t i o n  of  the e l e c t r o d e can be d e t e c t e d .  of  In  this  way  the  build-up  s u r f a c e o x i d e s or o t h e r e l e c t r o a c t i v e s p e c i e s can be d e t e c t e d . Cyclic  voltammograms  for  stationary  and  rotating  a r s e n o p y r i t e e l e c t r o d e s a t pH = 10.6 a r e shown i n F i g u r e 20. The anodic peak a t 0.35 V i s observed cycling  while  the  to  decrease  with  i n i t i a l l y s m a l l anodic peak a t -0.025 V and  the c a t h o d i c peak a t about -0.4 V s t e a d i l y i n c r e a s e These  continued  in  height.  o b s e r v a t i o n s a r e c o n s i s t e n t w i t h the f o r m a t i o n of F e ( O H )  d u r i n g the anodic decomposition  3  peak.  The o x i d a t i o n peak a t 0.35 V d e c r e a s e s w i t h each subsequent c y c l e due t o the e l e c t r o d e becoming p a s s i v a t e d by  the  of  c a t h o d i c peak  surface  hydroxide.  increases i n height since hydroxide  At  the there  same is  t o be reduced from F e ( O H )  amount of F e ( O H )  2  3  thus formed r e s u l t s  time an  the  increasing  build-up  amount  of  t o F e ( O H ) . The i n c r e a s i n g 2  in  the  anodic  peak  at  -0.25 V i n c r e a s i n g i n h e i g h t w i t h subsequent c y c l e s . Similar  peaks  and changes w i t h c y c l i n g were o b t a i n e d  a r s e n o p y r i t e e l e c t r o d e s from the two d i f f e r e n t  localities.  with  O CVJ  CVJ  io — VUJ  o — / ju a J J n 3 Figure  Current  10  6  o  19  - decay c u r v e f o r a r s e n o p y r i t e a t +0.0343 V.  59  -0.8  -0.6  1  -0.4 -0.2 0 Potentiol / V vs 20 mV s - ' ,  0.2SHE  0.4  pH = l0.6  F i g u r e 20 M u l t i p l e sweep voltammograms f o r s t a t i o n a r y and r o t a t i n g e l e c t r o d e s a t pH = 10.6  0.6  60  The peaks  e f f e c t of r o t a t i n g the e l e c t r o d e i s t h a t the  a t 0.35  i n F i g u r e 20 are i n c r e a s e d i n h e i g h t . At the same  time the a n o d i c peak  at  -  0.25  a d d i t i o n a l anodic  peak a t -0.35  cycling.  additional  This  Fe(OH) /Fe(OH) 2  additional  oxidation  redox  3  iron  becomes  narrower  V becomes e v i d e n t w i t h anodic  reaction  species  V  peak  results  than  suggests  in  the  and  an  continued that  the  formation  these h y d r o x i d e s .  The  of  e f f e c t of  r o t a t i o n i s d i s c u s s e d more f u l l y f o l l o w i n g the d i s c u s s i o n  of  a  are shown i n F i g u r e  21  r e a c t i o n mechanism. Cyclic  voltammograms  at  pH = 11.7  and F i g u r e 3, Appendix I I . The represents  greater  is  to  pH  =  in  the  the  effect  currents  this  of  rotating  associated  the  with  and  r e g i o n of i r o n h y d r o x i d e 11.7  o x i d a t i o n and  are  much  V  at  more The  i s explained  chapter.  An a d d i t i o n a l f e a t u r e of the voltammogram f o r the electrode  the  the f o r m a t i o n of a d d i t i o n a l anodic  these peaks are more apparent a t h i g h pH  later in this  the  0.35  than those which r e s u l t from s t i r r i n g a t lower pH.  reason why  0.35  figure  peak at -  r e d u c t i o n . These a d d i t i o n a l peaks a t pH = apparent  in  V, t o r e s u l t i n the a n o d i c  V becoming narrower and, peaks  10.6  increase  o x i d a t i o n peak a t 0.35  cathodic  scale  c u r r e n t s than t h a t of F i g u r e 20. As f o r the  r e s u l t s obtained at electrode  vertical  pH =  decreases  11.7 from  is the  that first  rotating  w h i l e the o x i d a t i o n peak a t to  the  second  cycle,  it  i n c r e a s e s on subsequent c y c l e s . For  each  of  the  e l e c t r o d e became covered multiple  potential  tests  shown  i n F i g u r e s 20 and  w i t h a v i s i b l e brown o x i d e  21,  during  the the  c y c l e s . T h i s o x i d e l a y e r became i r r i d e s c e n t  61  F i g u r e 21 M u l t i p l e sweep voltammograms f o r s t a t i o n a r y and r o t a t i n g e l e c t r o d e s a t pH = 11.7  62  i n c o l o r , v a r y i n g from b l u e a t low pH t o y e l l o w - green- a t pH.  Such  high  c o l o r s have been shown t o r e l a t e t o i r o n o x i d e l a y e r s  i n t h e range of 200 t o 400 angstroms i n t h i c k n e s s ( 6 0 ) . S i n c e t h e peaks  have  species,  been a  related  reaction  to  redox  reactions  involving  iron  sequence from Fe t o i r o n  hydroxides  w i l l be c o n s i d e r e d . Such a r e a c t i o n sequence has been  proposed f o r an i r o n e l e c t r o d e ( 6 1 ) . In  the  following  intermediates  whose  equations  surface  brackets  coverage  i s of  f r a c t i o n of a monolayer and b r a c e s i n d i c a t e related  to  the  formation  denote  reaction  t h e order of a  species  eventually  of new phases and which may undergo  ageing (61). [Fe(OH)]ad + e = Fe + OH"  (10)  [ F e ( O H ) ] a d + e = [Fe(OH)]ad  (11)  {Fe(OH) } = [ F e ( O H ) ] a d + OH"  (12)  HFe0 " + H 0 = {Fe(OH) } + OH"  (13)  F e 0 " + H 0 = HFe0 " + OH"  (14)  +  +  2  2  2  2  2  2  2  2  {FeOOH} + H 0 + e = {Fe(OH) } + OH"  (15)  {Fe 0  (16)  2  2  3  2  • H 0} = {FeOOH} + {FeOOH} 2  63  The  above e q u a t i o n s  hydroxide The  while  a r e w r i t t e n w i t h FeOOH as t h e o x i d i z e d  the d i s c u s s i o n so f a r has c o n s i d e r e d  p r e c i s e form present  from  results  of  on t h e e l e c t r o d e cannot  the present  be  FeOOH.  The  two  referring to f e r r i c  forms  i n v e s t i g a t i o n . Throughout  will  be  from  used  the  structure  interchangeably  as  hydroxide.  In c o n s i d e r i n g the r e a c t i o n cycle  3  determined  l i t e r a t u r e r e l a t i n g t o i r o n e l e c t r o d e s the p r e f e r r e d is  Fe(OH) .  the cathodic  sequence  during  a  potential  l i m i t t o t h e a n o d i c l i m i t and back t o  the s t a r t i n g p o i n t , t h e r e a c t i o n s would occur i n t h e f o l l o w i n g order as shown i n F i g u r e 21 and i n F i g u r e 3, Appendix I I . 1.  The i n i t i a l anodic p o t e n t i a l sweep r e s u l t s i n t h e o x i d a t i o n of  a r s e n o p y r i t e t o form FeOOH. The o x i d a t i o n i s a s s o c i a t e d  w i t h peak I I I . At t h e same  sulphate  arsenic  arsenate  ions  Arsenate  i s e l e c t r o i n a c t i v e and  participate  and  time  i n reactions  i s r e l e a s e d as s u l p h a t e therefore  associated  with  peaks. T h i s w i l l be more f u l l y d i s c u s s e d Based on t h e r e s u l t s  of  ESCA  hydroxide  complex  so  electroinactive  i s not  and  known.  ions. not  t h e remaining  presented  in  p r e c i p i t a t e s i n the  d e p o s i t s . The c o m p o s i t i o n  formed  does  as  i n s e c t i o n 4.3.2C.  experiments  Chapter 5, i t i s b e l i e v e d t h a t a r s e n a t e ferric  i s released  of t h e a r s e n i c  Sulphate  is  also  i s b e l i e v e d t o d i f f u s e s l o w l y through  the f e r r i c h y d r o x i d e l a y e r . 2.  On  the  according  reduction  cycle  FeOOH  i s reduced  to  Fe(OH)  t o r e a c t i o n 15. T h i s r e a c t i o n i s a s s o c i a t e d  2  with  peak I V . 3.  A f u r t h e r r e d u c t i o n of t h e F e ( O H )  2  according  to  equations  64  12,  11  and  10  i s indicated  by peak V. T h i s peak has a  somewhat g r e a t e r c u r r e n t a s s o c i a t e d w i t h i t than does IV  because  although  some F e ( O H )  t o r e a c t i o n s 13 and 14, e q u a t i o n s electron  i s solubilized  2  according  10 and 11 i n v o l v e  t r a n s f e r r e a c t i o n while equation  peak  a  two  15 i n v o l v e s o n l y  one. 4.  The  second  and  conjugated  successive  anodic  scans  involve  the  r e a c t i o n t o t h a t a s s o c i a t e d w i t h peak V as peak  I , namely t h e f o r m a t i o n  of  Fe(OH)  2  from  reduced  iron -  hydroxy s p e c i e s . 5.  The F e ( O H ) to  formed a t peak I i s o x i d i z e d t o FeOOH a c c o r d i n g  2  reaction  15 a t peak I I . As f o r peaks IV and V, peak I I  i s again l e s s  than  peak  I  since  only  a  one  electron  transfer i s involved. 6.  At  high  pH  the hydroxide  concentration  c o n s i d e r a b l e amount of F e ( O H ) reactions  13  and  2  i s dissolved  according  to  14, so t h a t t h e c u r r e n t a s s o c i a t e d w i t h  peak I I I i n c r e a s e s on subsequent scans. stationary  i s such t h a t a  e l e c t r o d e these p r o d u c t s  as a consequence t h e peak c u r r e n t  I n the case  of  a  a r e not swept away and  becomes  depressed  with  c o n t i n u e d c y c l i n g due t o a g r e a t e r b u i l d - u p of h y d r o x i d e a t the s u r f a c e . The  influence  of e l e c t r o d e r o t a t i o n and i n c r e a s i n g pH can  now be c o r r e l a t e d t o t h e above r e a c t i o n can be r e w r i t t e n i n terms of p r o t o n FeOOH + H and  +  + e = Fe(OH)  2  sequence.  Reaction  15  t r a n s f e r as f o l l o w s . (17)  65  H  The  +  + OH" = H 0  (18)  2  f o r m a t i o n of f e r r i c h y d r o x i d e  then r e s u l t s i n f o r m a t i o n of H formed  i s neutralized  d i f f u s i o n of H  +  +  a c c o r d i n g t o r e a c t i o n 17  i n a d d i t i o n t o FeOOH. The H  according  to  reaction  either  by  away from t h e r e a c t i o n s i t e due t o c o n c e n t r a t i o n  g r a d i e n t s or by d i f f u s i o n of OH" t o the s i t e . promote  18  so  +  the d i f f u s i o n  or  transport  of  Conditions  which  these s p e c i e s  should  t h e r e f o r e i n c r e a s e t h e r a t e of FeOOH p r o d u c t i o n . T h i s i s i n f a c t o b s e r v e d s i n c e t h e e f f e c t of r o t a t i o n i s t o i n c r e a s e t h e of  peak  height  I I I . At t h e same time i n c r e a s i n g pH i n c r e a s e s t h e peak  h e i g h t s i n c e a g r e a t e r c o n c e n t r a t i o n of OH" i s a v a i l a b l e f o r H  +  neutralization. Reaction  10  can  similarly  be w r i t t e n i n terms of p r o t o n  t r a n s f e r and t h e same arguments as p r e s e n t e d  above  explain  the  appearance of peaks I and V w i t h s t i r r i n g o r i n c r e a s i n g pH. Similar  arguments  have  been p r e s e n t e d  the f o r m a t i o n of n i c k e l h y d r o x i d e  elsewhere (62) f o r  films according t o .  NiOOH + H 0 + e = N i ( O H ) 2  2  + OH"  (19)  or NiOOH + H  +  + e = Ni(OH)  (20)  2  and H  +  (21)  + OH" = H 0 2  To t h i s p o i n t a p a r t from t h e a r s e n o p y r i t e (peak  I I I ) no  peaks  have been a s s i g n e d  oxidation  peak  t o a r s e n i c or s u l p h u r  66  s p e c i e s . At the peak p o t e n t i a l s observed peak  for arsenopyrite  (  peak  f o r t h e main  oxidation  I I I ) a r s e n i c o x i d a t i o n t o the  a r s e n a t e s t a t e and s u l p h u r o x i d a t i o n t o the s u l p h a t e  state  are  is  well  expected. The  irreversibility  of  sulphate  formation  e s t a b l i s h e d and r e d u c t i o n peaks a s s o c i a t e d w i t h t h i s s p e c i e s a r e not e x p e c t e d .  In the case of p y r i t e  sulphur  been  has  shown  pyrrhotite,  of s u l p h u r was shown t o  the f o r m a t i o n of FeS d u r i n g c o n t i n u e d c y c l i n g  the f o r m a t i o n of s u l p h u r reasonable  during  expectation  elemental  t o be formed even a t h i g h pH and h i g h  o v e r p o t e n t i a l ( 6 3 ) . The presence in  and  there  arsenopyrite i s no  result  (63). Although  oxidation  evidence  is a  i n the p r e s e n t  r e s u l t s t h a t t h i s i s i n f a c t o c c u r i n g i n the pH range where i r o n hydroxide are  f i l m s a r e formed s i n c e no peaks  associated  with  FeS  observed. In  addition  to  the  formation  e l e m e n t a l s u l p h u r c o u l d be  of  expected  to  FeS,  the presence of  result  in increasing  t h i o s u l p h a t e f o r m a t i o n w i t h i n c r e a s i n g pH a c c o r d i n g t o S 0 2  2 3  - + 6H  + 4e = 2S + 3H 0  +  E = + 0.26 + 0.015 l o g ( S 0 - ) 2  2  2 S O - + 10 H 2  3  2  E = + 0.04 - 0.007 l o g ( S 0 " ) 2  2  3  (so, -) 2  peaks  - 0.089 pH  + 8 e = S 0  +  a  The  associated  2 3  - + 5 H0  from  the  (23)  2  - 0.074 pH  2  with  t h e above r e a c t i o n s would l i e  w i t h i n t h e peaks a s s i g n e d t o i r o n s p e c i e s resulting  (22)  2  as  would  any  peaks  r e d u c t i o n of t h i o s u l p h a t e t o s u l p h i d e . The  67  e f f e c t of e l e c t r o d e r o t a t i o n would be t o reduce peaks a s s o c i a t e d w i t h t h i o s u l p h a t e r e d u c t i o n and subsequent No  peaks  whose  height  decreases  sulphide  oxidation.  with electrode rotation are  observed and t h e r e f o r e t h i o s u l p h a t e and s u l p h u r do not appear t o be produced. The also  f o r m a t i o n of a r s e n a t e  irreversible  during arsenopyrite oxidation  and peaks a s s o c i a t e d w i t h a r s e n a t e  are not expected.  is  reduction  T h i s w i l l be d i s c u s s e d more f u l l y i n t h e  next  section. The in  i r r e v e r s i b i l i t y of both s u l p h a t e and a r s e n a t e  part  accounts  f o r the d e v i a t i o n  potential  f o r arsenopyrite  reversible  value.  Since  both  arsenic  oxidation  and  of  formation  t h e observed  from  the  peak  calculated  s u l p h u r form s o l u b l e s p e c i e s t h e  e f f e c t of s t i r r i n g or e l e c t r o d e r o t a t i o n i s e x p e c t e d t o be  that  the  This  anodic  observation  peak  height  increases,  as  i s observed.  i s t h e r e f o r e c o n s i s t e n t with the o x i d a t i o n  of a l l  t h r e e elements i n a r s e n o p y r i t e . Consideration f e r r i c arsenate  has  also  been  given  t o t h e p o s s i b l i t y of  p r e c i p i t a t i o n at the electrode surface  according  to FeAsO„ + 3H  +  = H AsO, + F e  3 +  3  (24)  pH = 0.12 - 1/3 l o g ( H A s O ) ( F e ) 3 +  3  a  or FeAsO, + 3H 0 = F e ( O H ) 2  pH = 5.30 + log(H AsO„") 2  3  + H AsO„- + H 2  (25)  68  At t h e pH v a l u e s b e i n g c o n s i d e r e d  i n the p r e s e n t  formation  of f e r r i c a r s e n a t e  would not be expected.  solubility  of f e r r i c a r s e n a t e  l i e s at  greater  pH  about  i t decomposes t o g i v e F e ( O H )  3  work  the  The minimum  pH =  2.2  and  at  and H AsO " a c c o r d i n g 2  ft  to r e a c t i o n 25 ( 6 4 ) . The o x i d a t i o n of a r s e n o p y r i t e t o form h y d r o x i d e the  release  of  H.  Equation  +  f o r m a t i o n of 13 moles of H oxidized.  8  for  instance  f o r every mole  +  of  results  i n d i c a t e s the  FeAsS  which  the  bulk  solution  lower  pH. F e r r i c a r s e n a t e c o u l d form a t the  e l e c t r o d e s u r f a c e i n t h e presence of such h i g h H  +  concentration.  Voltammograms produced a t d e c r e a s i n g pH v a l u e s s h o u l d enhanced  peaks due t o t h e f o r m a t i o n of FeAsO  fl  22  shows  a  surface.  m u l t i p l e sweep voltammogram c a r r i e d out a t  pH = 5.8. The absence of t h e peaks which a r e o b s e r v e d a t pH  have  s i n c e the bulk pH  would be a p p r o a c h i n g t h e pH g e n e r a t e d a t the e l e c t r o d e Figure  is  At the r a p i d sweep r a t e s (20 mV/sec.) employed the pH  at the e l e c t r o d e s u r f a c e c o u l d t h e r e f o r e be s i g n i f i c a n t l y than  in  higher  a t -0.35V i n d i c a t e s t h a t those peaks a r e not a s s o c i a t e d w i t h  FeAsO  a  formation.  The c a t h o d i c l i m i t of the p o t e n t i a l c y c l e s were kept w i t h i n the s t a b i l i t y l i m i t s was  f o r FeAsS which a r e shown i n F i g u r e 18.  r e c o g n i z e d however t h a t the a c t i v i t i e s  a t the e l e c t r o d e  could  construction  the  of  be  different  c o u l d t h e r e f o r e be o c c u r r i n g  i f the  of d i s s o l v e d s p e c i e s  from  diagram. C a t h o d i c  It  those  assumed  decomposition  limits  of  in  reactions  cycling  were  a c t u a l l y beyond the s t a b i l i t y l i m i t s of the m i n e r a l . F i g u r e 23 shows voltammograms produced w i t h c a t h o d i c of  -0.7 V  and -0.55 V. The same peaks a r e e v i d e n t  limits  i n each case  69  » -0.6  ' -0.4  I -Q2  I 0  I 02  Potential / V vs 20 mV s-'  f  I 0.4  SHE  pH = 5.8  Figure 2 2 M u l t i p l e sweep voltammograms a t pH = 5 . 8 .  L_ 0.6  70  although  the shape of the peaks  currents  change  differ  and  somewhat  the due  rate to  at  which  peak  a decrease i n t o t a l  c a t h o d i c c u r r e n t passed w i t h the l e s s n e g a t i v e c a t h o d i c l i m i t . A l e s s c a t h o d i c l i m i t does not r e s u l t i n e l i m i n a t i o n of any F i g u r e 24 shows a voltammogram limit  of  -0.55  peaks at -0.2 absent  in  and  an  with  in Figure  23  are  cathodic  completely  in  h e l d a t the c a t h o d i c l i m i t f o r 5 minutes  Figure  peaks  at  is  -0.2  C. I r r e v e r s i b i l i t y  (66,67).  V  are  associated is  with  anodic  has  been  determination  voltammetric  oxidation  reported of  25  oxidation  demonstrates  of  and  arsenic  As  to  ( I I I ) t o As  cathodic  representing  peak  to  peak  a r s e n i c onto the g o l d  the  10.7.  o x i d a t i o n of As  corresponding  to by  be  solution  electroinactive  polarographic  or  methods r e q u i r e s the r e d u c t i o n of a r s e n i c (V) w i t h  f o r m a t i o n a t pH = 9.2 the  that  0.6V.  r e d u c i n g agents p r i o r t o d e t e r m i n a t i o n Figure  2  of A r s e n a t e F o r m a t i o n  (V)  The  also  t o assume  W h i l e a r s e n i c ( I I I ) can be o x i d i z e d or reduced i n arsenic  the  24. Only a minor peak a s s o c i a t e d w i t h F e ( O H )  p r o d u c t s when the anodic l i m i t  (65),  anodic  F i g u r e 24. A p o r t i o n of a scan c a r r i e d out a f t e r  o x i d a t i o n i s e v i d e n t . I t i s t h e r e f o r e reasonable the  a  a n o d i c l i m i t of +0.24 V. The  V which are e v i d e n t  e l e c t r o d e was shown  V  produced  peaks.  B.  As  (67).  irreversibilty  of  Peak A i n t h i s f i g u r e  arsenate represents  ( I I I ) w h i l e peak B r e p r e s e n t s  the  ( V ) . I t can be seen t h a t t h e r e i s no As(V)  reduction  to  As  (III)  Peak C r e p r e s e n t s the d e p o s i t i o n of  electrode.  The  effect  of  holding  the  71  I  -0-6  1  1  I  -0-4 -0-2 Potential  I  0 0-2 / V vs SHE  I  0-4  20 mV s - ' , pH = 10.6  F i g u r e 23 I n f l u e n c e of c a t h o d i c l i m i t on voltammogram  |  0-6  72  after 5 min  I -0.6  I -0.4  L_ -0.2  Potentiai 20 mV s - ' ,  I 0 / V vs  I 0.2 SHE  pH = I 0.6  F i g u r e 24 I n f l u e n c e of a n o d i c l i m i t on voltammogram  1 0.4  73  electrode  at the c a t h o d i c l i m i t f o r 5 minutes p r i o r t o c a r r y i n g  out the p o t e n t i a l sweep i s t o  deposit  a  greater  quantity  a r s e n i c onto the e l e c t r o d e . T h i s i s made e v i d e n t by the  of  increase  i n the h e i g h t of peak A i n t h i s c a s e . Peak B at the same time i s unnaffected  since  o x i d a t i o n t o As  D.  the  c o n c e n t r a t i o n of As  (III) available for  (V) i s not changed.  I n f l u e n c e of D i s s o l v e d A r s e n i c on Voltammetry The  as  8  o x i d a t i o n of a r s e n o p y r i t e a c c o r d i n g  shows  soluble  arsenic  as  a  to  product.  equations The  such  i n f l u e n c e of  i n c r e a s i n g d i s s o l v e d a r s e n i c i n s o l u t i o n s h o u l d t h e r e f o r e be d e p r e s s the a n o d i c peak and t o s h i f t At  the  same  time  by  i t to higher p o t e n t i a l s .  carrying  out  presence of d i s s o l v e d a r s e n i c , peaks due r e d u c t i o n should be enhanced. The will  to  voltammetry  i n the  t o a r s e n i c o x i d a t i o n or  absence  of  peak  enhancement  c o n f i r m the p r e v i o u s c o n c l u s i o n t h a t a l l observed peaks a t  pH g r e a t e r than 7 are due F i g u r e 26 shows solution.  As  i s diminished anodic  to i r o n  the  species.  influence  of  arsenic  additions  to  e x p e c t e d the a r s e n o p y r i t e o x i d a t i o n peak at 0.35V by such a d d i t i o n as w e l l as b e i n g  potentials.  As  a  s h i f t e d to  consequence of the d i m i n i s h e d  r e a c t i o n , the c a t h o d i c peak  associated  with  ferric  more anodic  hydroxide  r e d u c t i o n a l s o becomes d i m i n i s h e d . None  of  the  observed  a r s e n i c a d d i t i o n s . The associated  with  peaks  are  enhanced by i n c r e a s i n g  c o n c l u s i o n t h a t none  arsenic  oxidation  or  of  the  peaks  are  reduction i s therefore  F i g u r e 25 Voltammogram f o r g o l d e l e c t r o d e i n a r s e n i c s o l u t i o n  75  I  i  -0-6  I  -0-4 -0-2 Potential / 20  mV  s-',  i  0 V vs  i  i  0-2 SHE  0-4  pH=IO-6 , 1000  Figure  u  0-6  rpm  26  E f f e c t of a r s e n i c a d d i t i o n s on a r s e n o p y r i t e p o t e n t i a l sweeps  76  supported. Peaks due t o the o x i d a t i o n of a r s e n i c I I I t o a r s e n i c V not  observed  although  considerable  s o l u t i o n . T h i s i s expected s i n c e As(III) lie  oxidation  which  are  arsenic I I I i s present i n  the  potentials  required  a r e p r e s e n t e d i n the p r e v i o u s  i n the r e g i o n where c u r r e n t s due t o  arsenopyrite  for  section  oxidation  are a l r e a d y s u b s t a n t i a l .  E. E f f e c t of Sweep Rate. The  model  which  arsenopyrite  across  mineral  oxidized  is  hydroxide  which  has  the pH range of i n t e r e s t i s t h a t i r o n i n the to  a r s e n a t e and s u l p h a t e formation  hydroxide  layer  behaviour  of  form  increases  time a r s e n i c and sulphur  The  been developed f o r the o x i d a t i o n of  a  surface  deposit  of  ferric  i n t h i c k n e s s w i t h t i m e . At the same  i n the m i n e r a l  are o x i d i z e d to  soluble  respectively. of  has  such  an  ever  implications  arsenopyrite.  The  to  -  thickening the  hydroxide  i n h i b i t activation controlled reactions  and  surface  electrochemical l a y e r w i l l tend t o further  oxidation  w i l l become d i f f u s i o n l i m i t e d . By  analyzing  the  i n f l u e n c e of sweep r a t e on peak  and p o t e n t i a l , k i n e t i c parameters and the e x t e n t  to  current  which  they  are a f f e c t e d by the h y d r o x i d e l a y e r may be d e t e r m i n e d . • Under  linear  diffusion  conditions  reversible  charge  t r a n s f e r p r o c e s s e s g i v e peak p o t e n t i a l s which a r e independent of sweep r a t e . Voltammograms  were o b t a i n e d  a t pH = 10.6  for  sweep  rates  77  from  1 mV/sec. t o 80 mV/sec. The r e s u l t s a t 2, 30 and 80 mV/sec  are shown i n F i g u r e 27 and a l l t h e r e s u l t s a r e " p l o t t e d i n F i g u r e 28.  I t i s apparent t h a t t h e a n o d i c peak a s s o c i a t e d w i t h  oxidation  i s greatly  affected  by  mineral  sweep r a t e i n d i c a t i n g t h i s  o x i d a t i o n p r o c e s s t o be i r r e v e r s i b l e . The c a t h o d i c is  peak a s s o c i a t e d w i t h the r e d u c t i o n of  insensitive  to  sweep  rate  the  fact  that  i t represents  the  cathodic  in  the  section  on  peak  a two stage r e a c t i o n ,  d i s p l a y i n g a double peak. T h i s i s c o n s i s t e n t w i t h presented  the argument  m u l t i p l e sweep voltammetry t h a t  i m p r o v i n g the c o n d i t i o n s f o r p r o t o n or h y d r o x i d e d i f f u s i o n make the f e r r o u s h y d r o x i d e r e d u c t i o n If  the  irreversible determining  anodic  reaction  electrode  r e a c t i o n more  i s considered  process  involving  to a  + (RT//3naF) (0.78  0  Ep - E p  2  + ln(Db)  v 6  be a t o t a l l y single  5  2  A D/ 1  2  rate  (56,68)  - InKs)  = 1 .857 (RT//3naF) = 0.048//Jna  i p = 3 X I0 n(/3na) V  will  favoured.  step, the f o l l o w i n g r e l a t i o n s should apply  Ep = E  3  i n d i c a t i n g t h i s r e a c t i o n t o be  r e v e r s i b l e , as e x p e c t e d . At low sweep r a t e s exhibits  Fe(OH)  (26)  (27)  C°  (28)  where Ks  = rate constant  n  =  total  number  when e l e c t r o d e of  electrons  i s a t E° involved  in  the e l e c t r o d e  process na=number of e l e c t r o n s i n v o l v e d i n the r a t e -  determining  step  pH  10-6  F i g u r e 27 Voltammograms a t i n c r e a s i n g sweep r a t e  79  pH =  10-6  -L  10  20  30 Scan  Figure  40  50  rate /  60 mV  70 s"  28  I n f l u e n c e of sweep r a t e on peak p o t e n t i a l  1  80  80  of the e l e c t r o d e p r o c e s s , f o r a s i n g l e s t e p p r o c e s s n = na C°  = bulk c o n c e n t r a t i o n of the r e a c t a n t  <x  = transfer  E  = E l e c t r o d e p o t e n t i a l = E° + vt  E°  = standard electrode p o t e n t i a l  D  = diffusion  A  = e l e c t r o d e area  Ep  = peak p o t e n t i a l c o r r e s p o n d i n g t o the o x i d a t i o n peak  Ep  2  coefficient  constant  = h a l f peak p o t e n t i a l where i = 1/2 i p  ip  = maximum c u r r e n t i n the c u r r e n t peak  0  = 1 -c*  b  = /3naFv/RT  v  = scan r a t e According  to  equation  26,  a  plot  of  Ep  vs. log v i s  expected  t o be a s t r a i g h t l i n e . The p l o t shown i n F i g u r e  clearly  not  29  l i n e a r and e q u a t i o n 26 t h e r e f o r e does not a p p l y t o  the o x i d i z i n g a r s e n o p y r i t e . At the two h i g h e s t scan r a t e s the  is  shown  peaks were very broad and a r e t h e r e f o r e r e p r e s e n t e d by bars  r a t h e r than by p o i n t s . Additional consideration  of  the  results  indicates  that  e q u a t i o n s 27 and 28 a l s o do not a p p l y . V a l u e s of na 0 d e t e r m i n e d according  t o e q u a t i o n 27 a r e shown i n Table 5. W h i l e a c o n s t a n t  v a l u e s h o u l d be o b t a i n e d f o r an a c t i v a t i o n the r e s u l t s a r e observed  controlled  process,  t o d e c r e a s e c o n t i n u o u s l y over the range  of sweep r a t e s c o n s i d e r e d . Equation  28  indicates  t h a t a p l o t of l o g i p v e r s u s l o g v  should give a s t r a i g h t l i n e having a slope  of  0.5.  shows a s t r a i g h t l i n e but h a v i n g a s l o p e of 0.77.  Figure  30  Figure Plot  29  of peak p o t e n t i a l as a f u n c t i o n of l o g scan r a t e  e^ej  U B O S  6ox s n s j S A luaaano ifeed 6oq  0£ 8^061,3  83  I t i s apparent t h a t the model used f o r t h e s e e q u a t i o n s does not  apply  i n the p r e s e n t c a s e . D e v i a t i o n from the model may i n  p a r t r e s u l t from the e x i s t e n c e of more than one r a t e d e t e r m i n i n g s t e p . A more s i g n i f i c a n t likely  results  from  formation  of  the  oxidation  results  cause  the  ferric in  of  deviation  proposed  from  oxidation  hydroxide  surface  the  model  mechanism. deposit  The  during  a p r o c e s s c o n t r o l l e d by d i f f u s i o n r a t h e r  than by a c t i v a t i o n p r o c e s s e s .  I n f l u e n c e of  Table 5 Scan Rate on, K i n e t i c  Scan Rate mV/sec  I max  Ep  Ep  mA.  V  V  1 2 5 10 20 30 40 50 60 70 80  .055 .105 .185 .260 .450 .630 .695 .880 .990 1 .200 1 .1 50  .223 .243 .301 .343 .408 .443 .463 .493 .533 (.543) (.603)  . 1 38 . 1 48 . 1 77 .201 .223 .248 .250 .263 .278 .286 .310  Parameters Ep-Ep  2  2  .085 .095 .124 .142 . 185 .195 .205 .230 .255 .267 .268  n/3  0.56 0.51 0.39 0.34 0.26 0.25 0.23 0.21 0.19 0.18 0.18  F. E f f e c t of Temperature Since  the  flotation  of  arsenopyrite  appears  c o n t r o l l e d by the f o r m a t i o n of s u r f a c e o x i d a t i o n l a y e r s , of  interest  to  determine  be  i t is  the i n f l u e n c e of temperature on the  f o r m a t i o n of these l a y e r s . The indicate  to  results  of  this  study  should  t h e e x t e n t t o which temperature can be used t o c o n t r o l  84  the  f l o t a t i o n of a r s e n o p y r i t e .  (i) Experimental The same p o l a r i z a t i o n equipment and t e s t s e t -  up  as  was  d e s c r i b e d i n the s e c t i o n on voltammetry was used f o r e x p e r i m e n t s at  different  temperatures.  In t h i s case the c e l l was  equipped  w i t h a water j a c k e t connected t o a C o l o r a Type K t h e r m o s t a t . The s o l u t i o n i n the c e l l c o u l d be c o n t r o l l e d t o w i t h i n ±  1°C.  ( i i ) R e s u l t s and D i s c u s s i o n A s e r i e s of voltammograms was o b t a i n e d a c r o s s the range temperature  from 19°C  t o 60.5°C. The s o l u t i o n used f o r the t e s t  sequence was pH = 10.75 a t 22°C. The voltammograms the  two  temperature  and  obtained  at  extremes a r e shown i n F i g u r e 31. The most  apparent d i f f e r e n c e i n the two shift  of  increased  peak  voltammograms current  is  the  associated  cathodic with  the  a r s e n o p y r i t e o x i d a t i o n peak. The peak p o t e n t i a l , h a l f wave p o t e n t i a l  and  peak  current  a c r o s s the temperature range a r e shown i n F i g u r e 32. The peak p o t e n t i a l d e c r e a s e s w i t h temperature a c c o r d i n g t o Ep = 479 - 4T r = - 0.9957 where T = t e m p e r a t u r e , °C W h i l e the h a l f wave p o t e n t i a l d e c r e a s e s a c c o r d i n g t o Ep  2  = 274.2 -  2.8T  r= - 0.9756 The d i f f e r e n c e i n s l o p e s of the two r e l a t i o n s i n d i c a t e s the peak t o become s t e e p e r w i t h i n c r e a s i n g t e m p e r a t u r e . The  dependence  of  peak  current  on  complex r e l a t i o n s h i p . I f the n a t u r e  of  layer  temperature  was  consistent  across  the  the  temperature shows a surface  hydroxide  range the peak  85  pH  -0-6  10-75  -0-4  -0-2  Potential  0 /  0-2  V vs SHE  F i g u r e 31 Voltammograms a t 19°C and a t 60.5°C.  0-4  0-6  86  T e m p e r a tu re  /  °C  F i g u r e 32 I n f l u e n c e of temperature on peak p o t e n t i a l and peak c u r r e n t  87  current  could  control.  be  The  expected  present  to  be  results  constant  indicate  under  diffusion  a d e v i a t i o n from t h i s  behaviour. At temperatures below a p p r o x i m a t e l y development i s incomplete with  increasing  approximately  and  the  temperature.  45°C a c o n s t a n t  30°C the h y d r o x i d e  current  Across  therefore  the  range  increases  from 30°C t o  f i l m t h i c k n e s s i s d e v e l o p e d . Above  45°C an i n c r e a s e i n c u r r e n t i s observed i n d i c a t i n g the b a r r i e r to be d i m i n i s h e d .  I t i s postulated  that  diffusion  this  increase  r e s u l t s from a change i n the morphology of the h y d r o x i d e more  porous  rapidly in process  film  being  thickness,  film, a  d e v e l o p e d . Thus w h i l e the f i l m b u i l d s  its  porous  nature  allows  the  voltammogram  by F i g u r e 33 which shows a m u l t i p l e sweep  obtained  at  58.5°C. While t h e r e i s a d e c r e a s e i n  c u r r e n t a s s o c i a t e d w i t h the anodic  peak ( I I I ) from the  1st.  to  c y c l e s , on subsequent c y c l e s the c u r r e n t s t a y s e s s e n t i a l l y  constant  while  potentials.  the  Both  peak  these  shifts  to  observations  slightly are  more  anodic  consistent with  presence of a s t e a d i l y t h i c k e n i n g but porous f i l m . The in  anodic  to c o n t i n u e .  This i s confirmed  2nd.  film  the  electrode  increase  peak h e i g h t of the peaks I and V i s c o n s i s t e n t w i t h  presence of an at  increasing  amount  the c o m p l e t i o n  of  surface  hydroxide.  of t h i s experiment was  the  the The  observed t o  be v e r y h e a v i l y t a r n i s h e d . The  f o r m a t i o n of a porous f i l m r e s u l t s i n  peaks  I  and  V  becoming more prominent than peaks I I and IV. T h i s i s c o n s i s t e n t with  the e f f e c t s noted upon e l e c t r o d e r o t a t i o n or i n c r e a s i n g pH  (section  4.3.2  -B)  and  confirms  improved  conditions  for  88  V I  -0-8  I  -0-6  I  -0-4  I  -0-2  Potential /  I  0  1  0-2  V vs S H E  2 0 m V s-» , pH =10-75 F i g u r e 33 M u l t i p l e sweep voltammogram a t 58.5°C  1—  0-4  89  d i f f u s i o n of s p e c i e s t o and from t h e e l e c t r o d e . Anodic  scans f o r p y r i t e and a r s e n o p y r i t e a t 60°C a r e shown  i n F i g u r e 34. The d i f f e r e n c e i n p o t e n t i a l between t h e two c u r v e s at the h a l f  peak  difference  potential  observed  at  f o r arsenopyrite  the  same  i s 30 mV.  The  p o i n t a t 22°C and pH 11 was  190 mV ( F i g u r e 1 7 ) .  G. I n f l u e n c e of Cyanide Cyanide i s used sulphides  such  as  as  The  flotation  pyrite.  determine t h e e x t e n t depressing  a  to  It  which  depressant  i s therefore cyanide  may  of be  interest to  e f f e c t i v e at  arsenopyrite. i n t e r a c t i o n of c y a n i d e w i t h p y r i t e has been s t u d i e d by  measuring t h e z e t a p o t e n t i a l and mixed p o t e n t i a l v a r y i n g pH w i t h i n c r e a s i n g a d d i t i o n of c y a n i d e , ferricyanide  adsorb  measurements determined  Fe,[Fe(CN) ] . therefore  chemically  for pyrite to  6  of  pyrite  at  f e r r o c y a n i d e and  ( 6 9 ) . The z e t a p o t e n t i a l was observed t o decrease  with increasing cyanide a d d i t i o n s i n d i c a t i n g species  f o r gangue  3  on  that  pyrite.  i n t h e presence  the  Mixed of  cyanide  potential  cyanide  were  l i e i n t h e r e g i o n of s t a b i l i t y of t h e compound The f o r m a t i o n  concluded  to  be  of s u r f a c e f e r r i c f e r r o c y a n i d e responsible  was  f o r t h e d e p r e s s i o n of  p y r i t e by c y a n i d e . These i n v e s t i g a t o r s (69) a l s o determined t h a t v a r i a t i o n s i n the degree of d e p r e s s i o n to  various.  solubility.  pyrite More  achieved samples  soluble  w i t h equal resulted  pyrite  samples  cyanide  from were  additions  varying  pyrite  found  t o be  —I -0-6  I I -0-4 -0-2 Potential Figure  /  I J 0 0-2 V vs SHE  I 0-4  34  Comparison of p y r i t e and a r s e n o p y r i t e voltammograms a t 59.8°C  91  depressed  to  a  l e s s e r degree by c y a n i d e than was l e s s s o l u b l e  p y r i t e . More s o l u b l e p y r i t e was p o s t u l a t e d t o r e s u l t levels  of  cyanide  higher  dissolved iron i n s o l u t i o n , r e s u l t i n g i n i n e f f e c t i v e consumption.  investigations  into  The  on  pyrite  results  pyrite  i n t e r p r e t e d on the b a s i s formed  in  -  cyanide  that  inhibited  of  the the  interaction  ferric  ferrocyanide  The  cyanide  sodium  4  concentration voltammograms o x i d a t i o n of  for arsenopyrite  in  of i n c r e a s i n g c y a n i d e c o n c e n t r a t i o n a t pH = 10.6. concentration  8 X lO' M  which  floatability.  A s e r i e s of voltammograms was o b t a i n e d presence  (23) were  e l e c t r o c h e m i c a l o x i d a t i o n of  x a n t h a t e and thus r e s u l t e d i n d i m i n i s h e d  the  electrochemical  cyanide.  the only is  was  Across  significant  that  ferrous  varied  the  this  of  to  cyanide  multiple  associated  ferric  M  -  in  peak  to  1 X 10 " range  change  anodic  hydroxide  from  sweep  w i t h the  hydroxide  becomes  diminished with increasing cyanide concentration. F i g u r e 35 shows a m u l t i p l e sweep voltammogram o b t a i n e d a t a cyanide  concentration  concentration completely.  of  2.8 X 1 0  the f e r r o u s h y d r o x i d e  - 3  molar.  At  oxidation  t h i s cyanide  peak  disappears  Comparison of F i g u r e 35 w i t h F i g u r e 20 r e v e a l s t h a t  i n t h e presence of c y a n i d e the e l e c t r o d e becomes p a s s i v a t e d t o a much l e s s e r degree than i n the absence of  cyanide.  It  appears  t h e r e f o r e t h a t c y a n i d e a c t s t o d i s s o l v e f e r r o u s h y d r o x i d e formed at  the  surface  which a r e  formed  of the e l e c t r o d e . Any i r o n - c y a n i d e complexes are  less  effective  inhibitors  to  further  o x i d a t i o n than i s the h y d r o x i d e . Curve  A  in  Figure  36  represents  a  p o t e n t i a l sweep on  a r s e n o p y r i t e c a r r i e d out a f t e r the e l e c t r o d e had  been  held  at  92  +343  mV  f o r 15 m i n u t e s . T h i s p o t e n t i a l i s j u s t below t h e peak  p o t e n t i a l f o r a r s e n o p y r i t e o x i d a t i o n . The e f f e c t of h o l d i n g  the  electrode  the  at  Fe(OH) /Fe(OH) 2  3  this  anodic  peaks  while  potential  i s to  diminishing  enhance  the  arsenopyrite  o x i d a t i o n peak. The  peak  f o r t h e o x i d a t i o n of f e r r o u s h y d r o x i d e t o f e r r i c  h y d r o x i d e i s b e l i e v e d t o r e s u l t from t h e f o r m a t i o n  of  a  small  amount of f e r r o u s h y d r o x i d e a t t h e s t a r t of t h e p o t e n t i a l sweep. Curves  B  and  c a r r i e d out a f t e r minutes  C  i n F i g u r e 36 r e p r e s e n t  holding  the e l e c t r o d e  i n t h e presence  Increasing  cyanide  hydroxide  oxidation  of  2X10"  concentration  4  at  +343 mV  M and 2.8X10"  results  in  peak becoming d i m i n i s h e d ,  peak becoming enhanced and t h e f e r r i c h y d r o x i d e being  p o t e n t i a l sweeps  3  f o r 15  M cyanide.  the  ferrous  the arsenopyrite reduction  peak  unaffected. Curve  D  represents  a  potential  h o l d i n g t h e e l e c t r o d e a t +343 mV conditioning 1.63X10'  3  the electrode  sweep c a r r i e d out a f t e r  f o r 15  for 5  minutes  followed  by  minutes i n t h e presence of  M c y a n i d e w i t h no a p p l i e d p o t e n t i a l . Compared t o c u r v e  A t h e f e r r o u s peak i s d i m i n i s h e d  while  the a r s e n o p y r i t e  and  f e r r i c peaks a r e enhanced. It  i s apparent  t h a t t h e a c t i o n of c y a n i d e i s t o d i s s o l v e  f e r r o u s h y d r o x i d e from t h e e l e c t r o d e . T h i s d e c r e a s e s the e f f e c t of  oxidation  by  removing  the  oxidation  products  s u r f a c e . The a r s e n o p y r i t e o x i d a t i o n c u r r e n t s a f t e r electrode  at  a  from  the  holding  the  corroding p o t e n t i a l are therefore increased i n  the presence of c y a n i d e . The  implication  of  this  removal  of  surface  hydroxide  93  I  -0.8  1  -0.6  1  I  I  -0.4 -0.2 0 Potential / V vs SHE  I  02  2 0 mV s-», pH = !0.6 , 2 . 8 2 x | 0 " M 3  t  0.4  •  0.6  Na CN  F i g u r e 35 M u l t i p l e sweep voltammogram f o r a r s e n o p y r i t e i n t h e presence of 2.82X10' M NaCN 3  94  Potential  Figure  /  V vs  SHE  36  I n f l u e n c e of c y a n i d e on f o r m a t i o n of i r o n h y d r o x i d e f i l m s on arsenopyrite  95  d e p o s i t s from a r s e n o p y r i t e by c y a n i d e an  activating  agent  rather  i s t h a t c y a n i d e may  than  as  a  act  depressant  as for  arsenopyrite. A similar presence 37.  of  elimination cyanide  of  ferrous  hydroxide  2  1 . 6 3 X 1 0 "  peak. I t was  peaks  current at  400MA  w h i l e i n the absence of c y a n i d e  3  M c y a n i d e does not  V i n the presence of i t was  cyanide  a d e c r e a s e i n anodic  The  640MA.  c y a n i d e complexes  known  are  be  formed  only cyanide pyrite.  c u r r e n t i n the presence of c y a n i d e  not observed f o r a r s e n o p y r i t e . The to  the  show  was  t h e r e f o r e a c t s as an o x i d a t i o n i n h i b i t o r i n the case of Such  in  determined however t h a t the  anodic  0 . 4 4 3  3  i s observed f o r p y r i t e as shown i n F i g u r e  P y r i t e i n the presence of  any  Fe(OH) /Fe(OH)  on  pyrite  which  is are  a p p a r e n t l y not formed on  arsenopyrite.  H. Other M i n e r a l s i n the Fe - As - S System. M u l t i p l e sweep voltammograms were o b t a i n e d a t pH = 1 0 . 6 f o r s e v e r a l other minerals i r o n . The whether  i n the Fe - As - S system as w e l l as  purpose of t h i s s e r i e s of experiments was ferric  hydroxide  to determine  formed d u r i n g the o x i d a t i o n of  m i n e r a l s . D i f f e r e n c e s i n f l o a t a b i l i t y of these m i n e r a l s can be r e l a t e d t o the v a r i a t i o n i n s u r f a c e (i)  these then  composition,  Experimental The  previously marcasite was  for  pyrite  electrode  described.  The  sample o b t a i n e d  was  the  marcasite  same  electrode  e l e c t r o d e was  from Wards' S c i e n t i f i c .  i r r e g u l a r i n shape w i t h an exposed s u r f a c e of  as  was  made from a  The  electrode  approximately  96  F i g u r e 37 Voltammogram f o r p y r i t e i n the presence of 1 . 6 2 X 1 0  -3  M NaCN  97  4 square m i l l i m e t e r s . The  loellingite  sample was  of G e o l o g i c a l S c i e n c e s and was Ontario.  The  electrode  a r e a of a p p r o x i m a t e l y sample was  were  square  found  t r a c e c o n s t i t u e n t which was  come  Department  from  Cobalt,  i r r e g u l a r i n shape w i t h an exposed millimeters.  a n a l y z e d by means of an SEM  constituents  from the UBC  i n d i c a t e d to  was  59  obtained  to  be  - EDX  as was  loellingite  a n a l y s i s . The  a r s e n i c and  d e t e c t e d was  E l e c t r o d e s were prepared  The  major  i r o n and the o n l y  antimony.  previously  described  but  w i t h o n l y one w i r e connected t o them. The  iron  i r o n . The  electrode  e l e c t r o d e was  approximately  was  made  square i n shape w i t h an exposed area  14 square m i l l i m e t e r s . T h i s e l e c t r o d e was  s o l d e r i n g a copper w i r e t o one encasing  the  w i r e was (ii)  from a p i e c e of Armco pure  f a c e of the i r o n  assembly i n epoxy. The  ground down t o 600  cube  made by  and  f a c e o p p o s i t e the  g r i t paper p r i o r t o  then  attached  use.  Results Voltammograms f o r p y r i t e and m a r c a s i t e are shown i n  38. The in  of  r e s u l t s f o r p y r i t e are s i m i l a r t o those  that  the  Fe(OH) /Fe(OH) 2  t h a t no anodic apparent.  peak  As  due  previously  to  3  Figure  for arsenopyrite  peaks are prominent but d i f f e r  in  oxidation  is  discussed,  of  the  pyrite  itself  oxidation  of p y r i t e  o c c u r s a t h i g h e r p o t e n t i a l s than does o x i d a t i o n of a r s e n o p y r i t e . Peak A f o r p y r i t e has been a t t r i b u t e d (63) t o the of s u l p h u r d u r i n g p y r i t e o x i d a t i o n . T h i s become d i m i n i s h e d w i t h c o n t i n u e d The  results  Fe(OH) /Fe(OH) 2  3  for  peak  is  formation  observed  to  cycling.  marcasite  show  less  significant  peaks but enhanced peaks a s s o c i a t e d w i t h reduced  I  I  I  I  -0.8  -0.6  -0.4  -0.2  Potential 20mV  I  I  0  0,2  —I  1  0.4  0.6  / V vs SHE  s - , pH = 10.6  F i g u r e 38 Voltammograms f o r p y r i t e and m a r c a s i t e a t pH = 10.6  99  i r o n - hydroxy s p e c i e s . The more anodic tendency  more  to  passivate with continued  soluble  observation to  of  marcasite  occurs  at  p o t e n t i a l s than p y r i t e . M a r c a s i t e shows a much lower  or a r s e n o p y r i t e . The of  oxidation  scanning  than does p y r i t e  r e s u l t s are c o n s i s t e n t w i t h  iron species during marcasite  the  formation  o x i d a t i o n . This  i s c o n s i s t e n t w i t h the f a c t t h a t m a r c a s i t e  decompose  with  the f o r m a t i o n of f e r r o u s s u l p h a t e  r a t e of o x i d a t i o n of m a r c a s i t e times as f a s t as p y r i t e Figure  39  Fe(OH) /Fe(OH) 2  associated  shows peaks  3  with  a  apparent  loellingite  oxidation  been  reported  voltammogram  are  with  has  known  (37).  to  be  The nine  (37).  o x i d a t i o n peak i s a p p r o x i m a t e l y arsenopyrite  is  peak  as  loellingite.  is  an  oxidation.  100 mV under  for  more  The  anodic The  peak  loellingite  anodic  than  the  s i m i l a r c o n d i t i o n s (Figure  20). A minor r e v e r s i b l e r e a c t i o n i s apparent i n F i g u r e 39 a t to 0.3 the  V. The  peaks r e s u l t from a s p e c i e s which i s  f i r s t anodic  sweep and  in  the  antimony was  d e t e c t e d as a  was  originated  considered  concentrations  minor  l o e l l i n g i t e , the peak p o t e n t i a l s cannot be  r e l a t e d t o antimony r e a c t i o n s . C o n s i d e r i n g loellingite  on  then r e v e r s i b l y reduced and o x i d i z e d  on subsequent sweeps. A l t h o u g h constituent  oxidized  0.2  possible  the  i n a s i l v e r producing that  the  peaks  fact  that  the  area i n Canada i t  were  due  to  trace  of s i l v e r which had not been d e t e c t e d d u r i n g  the  sample a n a l y s i s . A voltammogram f o r a g o l d dissolved  silver  electrode  in  i s shown i n F i g u r e 39. The  the  presence  of  peak p o s i t i o n s show  good agreement w i t h those observed f o r l o e l l i n g i t e .  100  F i g u r e 39 M u l t i p l e sweep voltammogram f o r l o e l l i n g i t e a t pH = 10.6  101  F i g u r e 40 shows a voltammogram f o r was c a r r i e d out w i t h o u t KC1 and  formation  in solution  2  voltammogram  3  experiment  since excessive currents  peaks  are  apparent  although  i s more complex than those f o r m i n e r a l s . T h i s  i n c r e a s e d c o m p l e x i t y i s assumed t o physical  This  of s u r f a c e h y d r o x i d e s p e c i e s were encountered i n  i t s p r e s e n c e . The F e ( O H ) / F e ( O H ) the  iron.  result  from  the  different  n a t u r e of i r o n h y d r o x i d e f i l m s formed on i r o n compared  to those formed on m i n e r a l s . Based on t h i s s e r i e s regarding  the  variable  c o n s i d e r e d . The similar  to  characteristics more  of  unstable to  possibilities  of  loellingite  a r s e n o p y r i t e and  is  be  very  similar  flotation  M a r c a s i t e shows  behaviour  t o t h a t of p y r i t e w i t h the e x c e p t i o n t h a t more  s o l u b l e i r o n - hydroxy  difficult  some  of t h e s e m i n e r a l s can  behaviour  would be a n t i c i p a t e d .  comparable  this  experiments,  floatability  oxidation that  of  s p e c i e s a r e formed.  behaviour float  with  of  marcasite  xanthate  and  It is  expected  that  would  make  it  more  would  make  it  more  d i f f i c u l t t o d e p r e s s w i t h c y a n i d e than i n the case of p y r i t e . None  of  the m i n e r a l s c o n s i d e r e d show b e h a v i o u r s i m i l a r t o  t h a t of i r o n . A l t h o u g h i r o n h y d r o x i d e d e p o s i t s of some form developed  are  i n each c a s e , each m i n e r a l shows a n o d i c d e c o m p o s i t i o n  c u r r e n t s i n the r e g i o n where i r o n i s p a s s i v a t e d .  I . R i n g D i s c Study The gold  rotating arsenopyrite electrode  ring  purpose  of  to  detect  these  electroactive  experiments  was  was  equipped  oxidation to  determine  with  a  products.  The  whether  any  F i g u r e 40 Voltammogram f o r i r o n e l e c t r o d e  103  o x i d a t i o n p r o d u c t s which had p r e v i o u s l y not been c o n s i d e r e d escaping  into  solution.  pH = 10.6  so t h a t the p o s s i b l e f o r m a t i o n of e l e m e n t a l  lower pH v a l u e s was  Experiments  were c a r r i e d out o n l y a t sulphur  and the t r a n s p o r t p a t t e r n of s o l u b l e s p e c i e s formed a t the Such  at  not i n v e s t i g a t e d .  F i g u r e 41 shows the s o l u t i o n flow from the d i s c t o the  (71).  were  ring -  ring disc  d i s c e l e c t r o d e s have been used t o study  the  a n o d i c d e c o m p o s i t i o n of galena (72) as w e l l as the c o r r o s i o n  of  dental alloys The  (73).  efficiency  released  at  according  to  the  with  disc  is  r  N = ir/id =  r  -  3  r 1  r  3  ring on  collects  electrode  species geometry  2/3  (29)  2  r1  3  the  dependent  r  3  r  l_  which  r  3  where r , = r a d i u s of d i s c r  2  = i n n e r r a d i u s of  ring  r  3  = o u t e r r a d i u s of  ring  i r = ring current id = disc current The  geometry  of  the a r s e n o p y r i t e e l e c t r o d e g i v e s a v a l u e  f o r the c o l l e c t i o n e f f i c i e n c y of 0.77. due  to  the  fact  This value i s  very  high  t h a t the g o l d r i n g i s very. wide. Such a wide  r i n g i s s u b j e c t t o e x c e s s i v e n o i s e p i c k - u p . An attempt was  made  to  determine the a c t u a l c o l l e c t i o n e f f i c i e n c y of the e l e c t r o d e .  The  a r s e n o p y r i t e was  found  to  give  high  background  currents  104  Figure  41  T r a n s p o r t p a t t e r n of s o l u b l e s p e c i e s a t a r i n g - d i s c  electrode  105  however and a m e a n i n g f u l r e s u l t c o u l d not be Two  experiments  pH = 10.6.  One  were  carried  out w i t h t h i s e l e c t r o d e a t  experiment i n v o l v e d h o l d i n g the a r s e n o p y r i t e d i s c  a t the a n o d i c peak p o t e n t i a l w h i l e scanning second  achieved.  experiment  the g o l d  and  second a t a c a t h o d i c  is  consistent  with  ring  first  at  an  potential.  In n e i t h e r experiment were any result  The  i n v o l v e d c a r r y i n g out a t r i a n g u l a r p o t e n t i a l  sweep of the a r s e n o p y r i t e w h i l e h o l d i n g the anodic  ring.  the  ring currents detected. formation  of  This  sulphate  and  a r s e n a t e , both of which are e l e c t r o i n a c t i v e , at the a r s e n o p y r i t e disc.  The  formation  of  solid  elemental  s u l p h u r or A s S  of  experiments.  2  products  such  would be c o n s i s t e n t w i t h the  p r e v i o u s l y have been d i s c u s s e d show no e v i d e n c e of these  species  are  voltammetry  results which  they  The  as  experiments  and  these  2  oxidation  not b e l i e v e d t o be p r e s e n t . W h i l e more e l a b o r a t e  e x p e r i m e n t s c o u l d have been c a r r i e d o u t , i t seemed u n l i k e l y t h a t these would c o n t r i b u t e s i g n i f i c a n t l y t o the t h i s a r e a t h e r e f o r e was  J.  present  and  not pursued f u r t h e r .  I n f l u e n c e of Hydroxide F o r m a t i o n on Xanthate O x i d a t i o n . It  is  generally  c o l l e c t o r species adsorbed  in  xanthate  accepted pyrite species  t h a t d i x a n t h o g e n i s the a c t i v e  flotation on  i n v e s t i g a t e d but i t i s e x p e c t e d t h a t active collector Since  the  (27).  is  The  arsenopyrite dixanthogen  nature  has will  not be  of been the  species. oxidation  of  arsenopyrite  r e s u l t i n a r a p i d b u i l d - u p of f e r r i c h y d r o x i d e it  study  has on  been shown t o the  surface,  of i n t e r e s t t o determine the i n f l u e n c e of t h i s  hydroxide  106  b u i l d - u p on the o x i d a t i o n of xanthate (i)  to  dixanthogen.  Experimental. Anodic o x i d a t i o n scans were c a r r i e d out a t a scan r a t e of 5  mV per  second.  X a n t h a t e used f o r these e x p e r i m e n t s potassium  ethyl  acetone and  xanthate  which  was  consisted  of  Hoechst  p u r i f i e d by d i s s o l v i n g i n  r e c r y s t a l l i z i n g by a d d i t i o n of e t h e r .  ( i i ) R e s u l t s and  Discussion  F i g u r e 42 shows the r e s u l t s of experiments c a r r i e d pH = 5.9.  An anodic  electrode  after  it  p o t e n t i a l of + 460 mV. curve  for  achieved treatment Two  had  been  held  shown  for  arsenopyrite  and  represents  the  for  the  5 minutes a t a  T h i s p o t e n t i a l i s p a r t way  up the  anodic  p o t e n t i a l region  i n the presence of permanganate a t t h i s pH.  The  anodic  i s observed t o r e s u l t i n o n l y a minor p a s s i v a t i o n . scans  are  shown  p o t a s s i u m e t h y l x a n t h a t e . One while  at  scan i s shown f o r an a r s e n o p y r i t e e l e c t r o d e  which had been f r e s h l y p o l i s h e d . A scan i s a l s o same  out  in  the  scan  presence is  for  a  of  2.6  fresh  X 10"  3  M  electrode  the o t h e r i s f o r an e l e c t r o d e as d e s c r i b e d above. At  this  pH the o x i d a t i o n of a r s e n o p y r i t e has a n e g l i g i b l e e f f e c t on  the  oxidation  of  xanthate  the view t h a t h y d r o x i d e a r s e n o p y r i t e at t h i s Figure experiments  43  to dixanthogen.  f i l m s are not formed by the o x i d a t i o n of  pH.  shows  the  results  c a r r i e d out a t pH = 11.8.  of f e r r i c h y d r o x i d e  This i s consistent with  for  a  similar  At t h i s pH,  set  visible  of films  are formed.  S e v e r a l d i f f e r e n c e s w i t h the r e s u l t s o b t a i n e d are a p p a r e n t . W h i l e a t pH = 5.9  at  pH =  the o x i d a t i o n of xanthate  5.9  occurs  107  5 mV s-' 1000 rpm pH 5-9  I  I  0  Fresh  I  I  I  0-2 0-4 0-6 Potential / V vs SHE •  Fresh FeAsS, 2-6 x 10" M KEtX Oxidized FeAsS,2-6xiO- M KEtX 5  8  F i g u r e 42 I n f l u e n c e of a r s e n o p y r i t e o x i d a t i o n a t pH = 5.9 on x a n t h a t e oxidation  108  0-4i  5 m V s~ 1000 rpm pH 118  l  0-3 Fresh/ / FeAsS / / no X" / 0-2|  <  oxidized FeAsS no X-  E  ~ 0-1  0  J_ -0-2  0  0-2  0-4  Potent i ai / V vs SHE Fresh Fe AsS , 2-6X|0" M KEtX —Oxidized FeAsS , 2-6x I0~ M KEtX 3  3  Figure 4 3 I n f l u e n c e of a r s e n o p y r i t e o x i d a t i o n a t pH = 11.8 on x a n t h a t e oxidation  109  at  potentials  which a r e c a t h o d i c t o a r s e n o p y r i t e o x i d a t i o n , a t  pH = 11.8 these two c u r v e s a r e oxidation  of  arsenopyrite  reversed. will  be  This  indicates  favored  over  that  xanthate  o x i d a t i o n a t t h i s pH. A  further  electrode  difference  was  potential,  hardly  at  significant  affected  pH = 11.8  degree.  i s that  the  The  by  while holding  electrode  oxidation  at  pH = 5.9  at  an  the  oxidizing  i s passivated  of  xanthate  to  a  i s greatly  i n h i b i t e d by the presence of t h i s p a s s i v a t i n g h y d r o x i d e l a y e r . W h i l e these e x p e r i m e n t s c l e a r l y show t h a t t h e thick  ferric  hydroxide  layers  such  f l o t a t i o n c o n d i t i o n s ( i . e . 5 minutes will  prevent  the  formation  presence  of  as c o u l d be formed under conditioning  with  KMnO«)  of dixanthogen a t t h e s u r f a c e , t h e  a d s o r p t i o n of x a n t h a t e i o n s a t the h y d r o x i d e l a y e r has not  been  ruled out.  4.4 D i s c u s s i o n The  oxidation  conditions  has  of  been  a r s e n o p y r i t e under m o d e r a t e l y o x i d i z i n g shown  to  result  in  the  formation  r e l a t i v e l y t h i c k s u r f a c e l a y e r s of f e r r i c h y d r o x i d e . time  the  sulphate elemental The  arsenic  on  hydroxide  sulphur  are  oxidized  At the same  t o a r s e n a t e and  r e s p e c t i v e l y . At the lower end of the pH range  studied  s u l p h u r can be expected t o be a product of o x i d a t i o n . influence  arsenopyrite layer  and  of  i s exerted  the layer  dixanthogen.  of  oxidation  on  the  through the b u i l d - u p  surface  of  inhibits  The m i n e r a l  the the  mineral. oxidation  of  floatability the  of  hydroxide  The presence of t h i s of  xanthate  to  i s t h e r e f o r e not r e n d e r e d h y d r o p h o b i c .  110  At t h e same time t h e h y d r o x i d e l a y e r i t s e l f can be be  strongly  hydrophilic  and t h e m i n e r a l  expected  to  i s therefore strongly  depressed. The  oxidation  oxidation  potentials  required  to  bring  about  of a r s e n o p y r i t e have been shown t o be a c h i e v e d  the  i n the  presence of s e v e r a l common o x i d i z i n g a g e n t s . In  addition,  arsenopyrite  the o x i d i z i n g  depression  potentials  required  for  a r e e n c o u n t e r e d i n some p l a n t s even i n  the absence of o x i d i z i n g a g e n t s . F i g u r e 44 shows v a r i o u s Eh - pH c o n d i t i o n s which were measured i n p l a n t s (70) as  well  as t h e  r e s t p o t e n t i a l and o x i d a t i o n peak p o t e n t i a l f o r a r s e n o p y r i t e . I t is  apparent  t h a t i n some c a s e s t h e c o n d i t i o n s r e q u i r e d f o r t h e  o x i d a t i o n and t h u s the d e p r e s s i o n  of  arsenopyrite  are  being  achieved. Current  peaks  increase with therefore  increasing  expected  flotation  should  arsenopyrite  be  deposits  influence  arsenopyrite to  30°C,  more  not  build-up  greater  with  difficult.  of  hydroxide  increasing  Maximum  is  pH  and  flotation  of  form  and  where  the  formation  of  s u l p h u r would f u r t h e r c o n t r i b u t e  of  temperature  on  the  oxidation  of  showed a complex v a r i a t i o n . Over t h e range of 15°C  of  temperature  surface  hydroxide.  resulted  in  while  at  temperature  increasing  Over t h e temperature range  30°C t o 40°C temperature has no apparent i n f l u e n c e on build-up  oxidation  of t h e m i n e r a l .  increasing  development  be  arsenopyrite  The  of e l e m e n t a l  to the hydrophobicity The  to  pH.  with  w i t h x a n t h a t e i s e x p e c t e d a t pH l e s s than 7, where  f e r r i c h y d r o x i d e does surface  associated  greater  hydroxide  than 40°C, i n c r e a s i n g  Ill  Figure 4 4 Comparison of a r s e n o p y r i t e r e s t p o t e n t i a l and o x i d a t i o n peak p o t e n t i a l w i t h o p e r a t i n g p l a n t c o n d i t i o n s (70)  112  temperature r e s u l t s i n a f e r r i c hydroxide.  rapid  increase  i n the q u a n t i t y  of  The i n f l u e n c e of temperature on t h e d e p r e s s i o n  of a r s e n o p y r i t e by o x i d a t i o n i s t h e r e f o r e e x p e c t e d t o be m i n i m a l u n t i l a temperature of 40°C i s exceeded. Above t h i s t e m p e r a t u r e , t h i c k , h y d r o p h i l i c l a y e r s of f e r r i c h y d r o x i d e a r e formed. Differential the  use  of  temperature  flotation  oxidizing  of a r s e n o p y r i t e and p y r i t e through  agents  should  be  possible  Increasing  than  does  temperature can be e x p e c t e d t o d i m i n i s h t h e  e f f i c i e n c y of t h e d i f f e r e n t i a l f l o t a t i o n of t h e s e elevated  the  r e g i o n near 20°C. At t h i s t e m p e r a t u r e , a r s e n o p y r i t e  o x i d i z e s a t s i g n i f i c a n t l y lower o x i d a t i o n p o t e n t i a l s pyrite.  in  temperature  (60°C)  the p o t e n t i a l s  minerals.  At  a t which t h e two  minerals o x i d i z e at a s i g n i f i c a n t rate are c l o s e r together  than  at low t e m p e r a t u r e s . The  addition  of  cyanide  to  solution  resulted  d i s s o l u t i o n of t h e h y d r o x i d e s u r f a c e d e p o s i t s p r e v i o u s l y on  arsenopyrite.  It  formed  i s e x p e c t e d t h a t c y a n i d e a d d i t i o n s would  r e s u l t i n increased arsenopyrite e f f e c t s of o x i d a t i o n .  in a  f l o a t a b i l i t y by d i m i n i s h i n g t h e  113  Chapter 5  ESCA STUDIES  X-ray p h o t o e l e c t r o n s p e c t r o s c o p y known  as  ESCA,  i s an e x p e r i m e n t a l  (XPS)  or  more  t e c h n i q u e which p e r m i t s  a n a l y s i s of a s u r f a c e l a y e r on a s o l i d sample. The the  surface  layer  which  commonly  w i l l be a n a l y z e d may  the  thickness  of  vary from a  few  angstroms t o 50 angstroms. The method c o n s i s t s of bombarding the sample t o be with  nearly  monoenergetic  photons  and  studied  measuring the  energy d i s t r i b u t i o n of the e j e c t e d e l e c t r o n s ( 7 4 ) . Each  kinetic element  w i l l have a c h a r a c t e r i s t i c s e t of p h o t o e l e c t r o n peaks due different  electronic  l e v e l s . The  e l e c t r o n s a r e i n the range of 1 KeV The  photon e n e r g i e s used t o e j e c t or more.  b i n d i n g energy of e l e c t r o n s i n a g i v e n e l e c t r o n i c  can be measured  with  resulting  differences  from  sufficient  precision  i n the c h e m i c a l  to  detect  a  metal  Similarly,  will  be  variations  different in  than  oxidation  for  state  level shifts  s t a t e of the atom.  For i n s t a n c e , the energy f o r e l e c t r o n s i n a g i v e n for  t o the  the will  energy  state  metal  oxide.  result  in  v a r i a t i o n of the b i n d i n g energy of core e l e c t r o n s . ESCA  studies  have been c a r r i e d out on p y r i t e t o determine  the n a t u r e of s u f a c e compounds formed d u r i n g f l o t a t i o n was  determined  that  iron  hydroxide  formed  (75).  on p y r i t e d u r i n g  g r i n d i n g and a t h i g h pH d u r i n g f l o t a t i o n . For v a l u e s of pH than  7,  surface  hydroxide  It  less  f i l m s are d i s s o l v e d l e a v i n g a clean  p y r i t e s u f a c e . Only l i m i t e d q u a n t i t i e s of e l e m e n t a l  s u l p h u r were  114  d e t e c t e d even a t pH =  3.0.  T h i s o b s e r v a t i o n t h a t l i m i t e d sulphur was t h a t the method may elemental  of  suggests  be i n s e n s i t i v e t o s u r f a c e c o n c e n t r a t i o n s  sulphur since sulphur  oxidation  detected  pyrite  i s known t o form d u r i n g the a c i d  ( 7 6 ) . W h i l e no e x p l a n a t i o n has p r e v i o u s l y  been o f f e r e d i n the l i t e r a t u r e i t i s proposed t h a t any sulphur present  on the s u r f a c e of p a r t i c l e s  the h i g h vaccuum ( 1 0 " i t can be In  7  is  elemental  volatilized  detected. the  present  formation  a r s e n i c and  study,  the  results  of  electrochemical  of f e r r i c h y d r o x i d e  sulphur to arsenate  and  with concurrent sulphate  indicating o x i d a t i o n of  respectively.  r e s u l t of ESCA e x p e r i m e n t s w i l l be used to c o n f i r m the of  i r o n hydroxide  arsenate  5.1  at  t o r r ) encountered during a n a l y s i s , before  e x p e r i m e n t s on a r s e n o p y r i t e have been i n t e r p r e t e d as the  of  and  s u r f a c e f i l m s and  The  existence  t o show the e x t e n t t o which  s u l p h a t e are i n c o r p o r a t e d i n these  films.  Experimental Samples of a r s e n o p y r i t e , p y r i t e and l o e l l i n g i t e were of  same o r i g i n as those used mineral  samples  were  for  electrochemical  obtained  studies.  the  Other  from Ward's S c i e n t i f i c and were  v i s u a l l y judged t o be f r e e of o t h e r m i n e r a l i m p u r i t i e s . A l l samples were p u l v e r i z e d w i t h pestle  to  minus 74 m i c r o n s and  a  porcelain  mortar  stored in sealed v i a l s p r i o r  and to  use. The Table  VI  a r s e n o p y r i t e which i s shown i n f i g u r e s 45 t o 47 and to  have  been  treated  s t i r r i n g 2 grams of sample i n  100  a t pH = 11.5 ml.  of  was  water  prepared adjusted  in by to  115  pH. = 11.5  with  sodium  hydroxide.  c o n d i t i o n i n g the sample was  Following  15  f i l t e r e d and d r i e d  minutes  of  argon  at  under  35°C. Samples  to  be  analyzed  were  dusted  onto  double  sided  c e l l u l o s e tape which c o u l d be f a s t e n e d t o the sample h o l d e r i n s e r t i o n i n t o the XPS  spectrometer.  measurements  spectrometer  equipped  were with  recorded at approximately  carried a  10 "  out u s i n g a V a r i a n IEE-15  magnesium 7  anode.  Spectra  were  torr.  From ten t o f o r t y scans were made of each m i n e r a l . The points  were  collected  in  d i g i t a l form and a G a u s s i a n  a p p l i e d t o the data n o r m a l i z e d t o overcome the of  5.2  data  f i t was  variable  number  scans.  R e s u l t s and D i s c u s s i o n The  iron,  arsenic  and  sulphur  peaks  m i n e r a l s i n c l u d e d i n the study a r e shown i n 47.  for  for  the  Figures  various  45,46  and  The b i n d i n g e n e r g i e s and i n t e n s i t y data a s s o c i a t e d w i t h the  peaks are summarized i n Table 6. I n t e n s i t i e s a r e counts  per  second  based  on  the  at the peak. B i n d i n g e n e r g i e s r e l a t e t o the  i r o n 2p, a r s e n i c 3d and s u l p h u r 2p o r b i t a l e l e c t r o n s . The purpose of i n c l u d i n g the v a r i o u s arsenopyrite  in  minerals  other  the study i s t o p r o v i d e a b a s i s f o r comparison  of peak p o s i t i o n s and r e l a t i v e i n t e n s i t y of o x i d i z e d and s p e c i e s f o r each m i n e r a l . I n t e n s i t i e s f o r the different  m i n e r a l s can be expected  compositional r a t i o for surface  area  than  that  same  reduced  element  in  t o vary i n d e p e n d e n t l y of the  element.  Factors  such  as  and volume per u n i t c e l l , t a k i n g i n t o account  real the  116  number of formula u n i t s per u n i t c e l l , w i l l ratios  between  m i n e r a l s . S i n c e XPS  influence  intensity  a n a l y z e s a t h i c k n e s s of  the  Table 6 E l e c t r o n b i n d i n g e n e r g i e s and i n t e n s i t i e s f o r elements i n various minerals. Mineral  Element  FeOOH AS 2 S 3  Fe S AS Fe  FeS  (pyrite)  2  B i n d i n g Energy eV  (marc.)  2  Fe S  FeAs  Fe  2  As FeAsS  (dry)  Fe As S  FeAsS (pH  11.5)  Fe AS S  o r d e r of 50 angstroms the  result  thinner  represent  product  than and  this  21 1 37 7406 4403 758 5758 1 352 5395 1879 5949 1692 4187 1707 2347 1546 2289 5646 2761 1 1 67 525 549 1234 1 1 921 1322 2067 281 837.8 831 .4  713.6 163.4 44.1 712.3 709.4 169.7 163.7 713.7 709.5 170.9 164.9 713.9 709.5 46.9 43.8 712.9 708.6 47.2 44.1 170.6 165.0 712.8 708.5 47.1 43.4 171.1 165.5  S FeS  Counts/sec.  will  for  surface  product  layers  a composite of the  surface  the s u b s t r a t e .  I n t e n s i t y r a t i o s of o x i d i z e d t o t o t a l s u r f a c e pyrite,  marcasite  and  species  a r s e n o p y r i t e are summarized i n Table  for 7.  117  Figure 45 XPS  peaks a s s o c i a t e d w i t h the i r o n 2p e l e c t r o n s of the minerals  various  118  FeAs, FeAsS-  FeAsS  A s  33-3  ±  a 3  36-8  S  ±  40-4  BINDING  439  ENERGY  p e a k s  a s s o c i a t e d  w i t h  the  v a r i o u s  47-5 /  F i g u r e XPS  JL  510  54-5  eV  46 a r s e n i c  m i n e r a l s  3d  e l e c t r o n s  of  the  119  I  I 162-6  I 165-6  I 168-5  BINDING  I 171-5  ENERGY /  I 174-5  I 177-5  eV  Figure 4 7 XPS  peaks a s s o c i a t e d w i t h the s u l p h u r 2d e l e c t r o n s of various minerals  the  120  S e v e r a l c i t e d the in  t r e n d s  in  t h e s e  c h a r a c t e r i s t i c s  e l e c t r o c h e m i c a l t h i s  d o e s  g r e a t e r  s u r f a c e  XPS  s p e c i e s  o x i d e  r e s u l t s  r a t e s  a  a r s e n o p y r i t e  w i t h to  p r e v i o u s l y  p y r i t e  w h i c h  t h o s e  p r e s e n t .  of  c o n s i s t e n t  w i t h  p y r i t e  and  a r e  w i t h  p r e s e n t e d  t h e s e an  m i n e r a l s  even  a r s e n o p y r i t e  was  was  T h i s  f o r m e d .  of  the  a p p r o x i m a t e l y  h i g h e r T h e s e  pH  p H ,  t h a t  l e s s  i r o n  than  i n d i c a t i n g  r e s u l t s  c a n  now  M i n e r a l  ox  ox  p r e v i o u s l y  h e l p s  to  o x i d i z e d  i s a  a p p a r e n t  d r y  f o r m ,  a  e x p l a i n  the  e x p e r i m e n t s .  The  h y d r o x i d e same  o b e y e d  of  was  not  t i m e the  r e c o n c i l e d  the  same  s u r f a c e on  formed r e s t  r e l a t i o n l a y e r  the  to  b a s i s  7  I n t e n s i t y  F e / F e  It in  c o n s i s t e n t  be  T a b l e XPS  w i t h  c r u s h e d  the 7.0  a  s h o w i n g  m a r c a s i t e .  At  7.0.  m a r c a s i t e  p r o p o r t i o n  e l e c t r o c h e m i c a l  i n d i c a t e d  at  or  s p e c i e s  (77).  g r e a t e r  p y r i t e  o x i d i z e d  and  (37)  the  m e a s u r e d at  f o r  p r o p o r t i o n  e i t h e r  some  of  p o t e n t i a l s  be  c o m p a r e d  than  r e s u l t s  pH  i s  than  shows  l a y e r  of  v o l t a m m e t r y  as  f o r  g r e a t e r  T h i s  r e s u l t s  a l t h o u g h  s u r f a c e  a  p y r i t e .  A r s e n o p y r i t e  below  c o n s i s t e n t  m a r c a s i t e  r e s u l t s  shows  o x i d a t i o n  p u b l i s h e d  t h a t  f o r  a r e  s t u d y .  M a r c a s i t e than  d a t a  r a t i o s  S / S  ox  A s / A s  F e / A s  F e / S  Pyrite  0..12  0..20  -  0..97  Marcasite  0..24  0..29  -  1 .33 .  Arseno. - DRY  0,.67  0..31  0..69  5.0  4..71  Arseno - p H = 11.5  0..90  0..50  0,.88  5.6  7..93  t h a t  in  e a c h  c a s e  a  s u r f a c e  l a y e r  of  i r o n  h y d r o x i d e  was  formed  121  d u r i n g the e l e c t r o d e p r e p a r a t i o n . T h i s h y d r o x i d e electrode  potential  measurements  t h i c k n e s s t o a f f e c t the voltammetry As  expected,  measurements,  based  subjecting  r a p i d decomposition three  on  elements  results  While  of  arsenopyrite  of  insufficient  electrode  potential  t o pH = 11.5 r e s u l t s i n  lattice.  For  each  of the  7 shows the r a t i o of o x i d i z e d t o reduced  atoms t o i n c r e a s e a f t e r o x i d a t i o n due pH = 11.5.  was  the  experiments.  of the m i n e r a l  Table  but  controlled  the  to  ratio  of  iron  s i g n i f i c a n t l y a f t e r treatment,  the  ratio  dissolved to  oxygen  sulphur  of  iron  at  increases to  arsenic  i n c r e a s e s o n l y a s m a l l amount. These r e s u l t s i n d i c a t e t h a t a t t h e same time as t h e s u r f a c e layer  of  ferric  hydroxide  i s formed, much of t h e s u l p h u r goes  i n t o s o l u t i o n presumably as s u l p h a t e , w h i l e t h e a r s e n i c at  the  surface  i n a o x i d i z e d s t a t e . I t i s proposed t h a t t h i s  a r s e n i c i s adsorbed on the f e r r i c h y d r o x i d e that  since  i t i s known  a r s e n i c can be removed from s o l u t i o n i n t h i s way ( 7 8 ) . The  e x i s t e n c e of such a s u r f a c e o x i d a t i o n product with  remains  the  existence  pitticite  of  such  secondary  ( F e ( A s 0 ) ( S 0 ) OH«2H 0) 2  4  4  2  i s also consistent  arsenic and  minerals  as  pharmacosiderite  (6FeAsO„«2Fe(OH) '12H 0). 3  The  results  conclusions  drawn  2  of  this  spectroscopic  study  from e l e c t r o c h e m i c a l e x p e r i m e n t s .  v a l u e s , a r s e n o p y r i t e decomposes t o form i r o n deposits.  Arsenic  i n the form of a r s e n a t e  the form of s u l p h a t e a r e i n c o r p o r a t e d i n t h i s Such  hydroxide  depression  confirm  layers  can  be  expected  the  At h i g h pH  hydroxide  surface  and some sulphur i n hydroxide to  of a r s e n o p y r i t e d u r i n g f l o t a t i o n w i t h  result  layer. i n the  xanthate.  122  Chapter  6  FLOTATION STUDIES  F l o t a t i o n s t u d i e s were c a r r i e d out w i t h r e a l ore samples t o v e r i f y t h e i n t e r p r e t a t i o n s made of  electrochemical  as they r e l a t e t o the f l o t a t i o n response Three type,  types  of experiments  conditions  were  experiments  of a r s e n o p y r i t e .  were c a r r i e d o u t . In t h e f i r s t  controlled  to  observe  the  flotation  operation. In  t h e second t y p e , a rougher  prepared and  i t s subsequent  a r s e n o p y r i t e c o n c e n t r a t e was  depression  through  the  use  of  o x i d i z i n g agents was s t u d i e d . In were  t h e t h i r d t y p e , bulk p y r i t e - a r s e n o p y r i t e c o n c e n t r a t e s  prepared.  selectively  The  extent  depressed  to  which  arsenopyrite  could  be  from t h i s c o n c e n t r a t e through the use of  o x i d i z i n g agents was e x p l o r e d .  6.0.1 Rougher F l o t a t i o n  6.0.1.1 E x p e r i m e n t a l . Rougher f l o t a t i o n t e s t s were c a r r i e d out on grade  samples  Hedley,  B.C.  from The  the sample  property contained  of  G.M. 51.2  selected  Resources percent  by  high  L t d . at weight  a r s e n o p y r i t e i n a s i l i c e o u s gangue. A m i c r o s c o p i c e x a m i n a t i o n of the  material  indicated  only  minor  p y r i t e t o be p r e s e n t . The  123  a r s e n o p y r i t e was assayed t o c o n t a i n 270 t o 340 grams Au  per  tonne  i n t h e form shown i n F i g u r e 2. Minor c o n c e n t r a t i o n o f o t h e r  metals a r e a l s o i n d i c a t e d i n F i g u r e 2 as determined by SEM - EDX analysis. For each t e s t , 1000 grams of o r e was ground i n a l a b o r a t o r y rod m i l l t o a p p r o x i m a t e l y  92% minus 200 mesh. Vancouver tapwater  having a pH of 5.5 was used f o r a l l t e s t s . For t e s t s c a r r i e d out a t v a r y i n g 24±1°C.  pH  t h e temperature  The pH was observed t o i n c r e a s e from t h e water v a l u e of  pH=5.5 t o a n a t u r a l v a l u e of pH=8.0 increase  i n pH  during  grinding.  natural  hydroxide  Such  r e s u l t s from t h e presence of s l i g h t l y  rock forming c o n s t i t u e n t s i n t h e o r e . The pH was the  was  value  of  8.0  to  an  alkaline  adjusted  from  t h e d e s i r e d v a l u e u s i n g sodium  o r s u l p h u r i c a c i d . F o l l o w i n g pH a d j u s t m e n t s t h e s l u r r y  was c o n d i t i o n e d f o r 15 minutes w i t h an a i r f l o w  of  2.4  litres  per minute i n an A g i t a i r f l o t a t i o n c e l l a t 1000 RPM. F o l l o w i n g t h i s c o n d i t i o n i n g p e r i o d w i t h a i r , an a d d i t i o n of 250  grams  per tonne of i s o p r o p y l xanthate  was made and a l l o w e d  to c o n d i t i o n f o r 2 m i n u t e s . Rougher f l o t a t i o n was c a r r i e d out f o r 15  minutes  with  an  a d d i t i o n of 20 grams p e r tonne of Dowfroth 250 f r o t h e r . Tests  carried  out a t  varying  temperature  were  a t the  n a t u r a l pH of 8.0. Some t e s t s were a l s o c a r r i e d out w i t h t h e pH adjusted  to  carried  out  9.0 u s i n g sodium h y d r o x i d e . Temperature t e s t s were in a  jacketed  stainless  steel  cell  temperature c o n t r o l l e d by means of a C o l o r a Type - K Once  with  the  thermostat.  t h e temperature had been s t a b i l i z e d the c e l l a i r was  t u r n e d on and c o n d i t i o n i n g was c a r r i e d out as f o r the t e s t s a t  124  v a r y i n g pH.  6.0.2 R e s u l t s and D i s c u s s i o n The r e s u l t s of t e s t s c a r r i e d out a t v a r y i n g pH a r e shown i n Figure  48. The r e c o v e r y of a r s e n o p y r i t e i s 90% o r g r e a t e r  approximately steadly  pH=7.5  at  which  u n t i l approximately  continues  point  i t starts  t o decrease but a t a lower  of  decrease  pH=l0.5. Above pH=l0.5 t h e r e c o v e r y r a t e than below t h i s pH.  R e s u l t s a r e a l s o shown f o r two t e s t s absence  to  carried  o x i d a t i o n . The r e s u l t s of these t e s t s ,  out  i n the  particularly  a t pH = 10 i n d i c a t e t h a t t h e d e c r e a s e i n r e c o v e r y observed increasing  pH  i s not  r e s u l t i n g from a e r a t i o n  the r e s u l t during  b r i n g about a r s e n o p y r i t e The  conditioning  i s necessary  to f e r r i c h y d r o x i d e pH = 7.  obtained and  peak a s s o c i a t e d w i t h a r s e n o p y r i t e o x i d a t i o n t o appear i n t h e v i c i n i t y  flotation tests  amount of f e r r i c h y d r o x i d e  of  approximately  i s observed  the r e s u l t  of  the i n c r e a s i n g  a s s o c i a t e d w i t h these  i n c r e a s i n g peak  to lead to decreasing  f l o a t a b i l i t y of t h e  m i n e r a l . F o r maximum r e c o v e r y of a r s e n o p y r i t e a pH of l e s s 7.0 s h o u l d be used w h i l e f o r maximum d e p r e s s i o n 12.0  15  The o x i d a t i o n peak h e i g h t s i n c r e a s e w i t h i n c r e a s i n g pH.  In t h e p r e s e n t  heights  to  depression.  f l o t a t i o n t e s t r e s u l t s agree w i t h t h e r e s u l t s  t h e anodic  with  of pH a l o n e . The o x i d a t i o n  w i t h voltammetry. The voltammograms shown i n F i g u r e s 14 indicate  until  would  be  than  a pH a p p r o a c h i n g  a p p r o p r i a t e . The pH used f o r d e p r e s s i o n  i n most  cases would be d i c t a t e d by t h e pH dependance of t h e f l o a t a b i l i t y of o t h e r m i n e r a l s b e i n g  recovered.  The r e s u l t of t e s t s c a r r i e d out a t  increasing  temperature  Figure  48  F l o a t a b i l i t y of a r s e n o p y r i t e at i n c r e a s i n g pH and absence of o x i d a t i o n  i n the presence  126  are shown i n F i g u r e 49. The r e c o v e r y i n c r e a s e s somewhat over the temperature decreases the  range  7°C  to  40°C. Above 40°C t h e r e c o v e r y  w i t h i n c r e a s i n g temperature.  absence  temperatures desired  from  of with  there  oxidation  was  oxidation.  If  would  be  no  The r e c o v e r y a t  higher  arsenopyrite  benefit  s l u r r y s i n c e p l a n t temperatures  than  at  60°C the  in  lower  depression  was  d e r i v e d from h e a t i n g the  can n o r m a l l y be expected  to  be  i n the range of 10°C t o 20°C. The  r e s u l t s a c h i e v e d a t pH = 9.0 show s i m i l a r behaviour t o  those a c h i e v e d a t pH = 8.0 w i t h i n c r e a s i n g temperature.  In  each  case the c o n c e n t r a t e grade was i n s e n s i t i v e t o temperature.  6.0.3 D e p r e s s i o n  6.0.3.1  of P r e v i o u s l y A c t i v a t e d A r s e n o p y r i t e  Experimental  Tests  were  c a r r i e d out u s i n g ore samples d e s c r i b e d i n the  p r e v i o u s s e c t i o n . Two s e r i e s of using  hydrogen  peroxide  as  tests the  were  oxidant  carried  out,  one  and t h e o t h e r u s i n g  sodium h y p o c h l o r i t e . In each laboratory  case rod  a  mill  two  kilogram  sample  was  ground  in a  t o 74 p e r c e n t p a s s i n g 200 mesh. A c l e a n e d  a r s e n o p y r i t e c o n c e n t r a t e was p r e p a r e d a c c o r d i n g t o t h e f l o t a t i o n c o n d i t i o n s shown i n Table 8. The rougher out  at  the  natural  f i l t e r e d and s p l i t  pH  of  8.0.  flotation  was  carried  The c l e a n e d c o n c e n t r a t e was  i n t o p o r t i o n s of 120 grams f o r t h e i n d i v i d u a l  t e s t s . These t e s t s were c a r r i e d out i n a 500 gram a t 800 RPM, a temperature  of 20° and pH = 8.0.  Agitair  cell  - J  10  —1  —  J  J  20 30 40 Temperature /  1  50 °C  I  60  Figure 49 I n f l u e n c e of temperature on a r s e n o p y r i t e  floatability  128 Table 8 F l o t a t i o n Conditions Stage  Isopropyl Xanthate g/tonne  Conditioning Rougher 1st. Cleaner 2nd. Cleaner  O x i d a t i o n was  Amyl/ Xanthate g/tonne  Dowfroth 250 g/tonne  Time minutes  50 10 10  5 17.5 2.5 2.5  2 15 10 8  150 37.5 20 10  —  c a r r i e d out a t pH = 8.0  t h e r e s u l t s . F l o t a t i o n was  allowed  f o r the time shown i n  t o proceed f o r 5 minutes.  A f r e s h l y p o l i s h e d a r s e n o p y r i t e e l e c t r o d e was the f l o t a t i o n c e l l f o r each t e s t and monitored r e l a t i v e to a calomel The was  shown f o r each t e s t i s t h a t which  w i t h no a d d i t i o n of x a n t h a t e f o l l o w i n g o x i d a t i o n .  6.0.3.2 R e s u l t s and The  the e l e c t r o d e p o t e n t i a l was  electrode.  a r s e n o p y r i t e recovery  achieved  Discussion  r e s u l t s of f l o t a t i o n t e s t s u s i n g hydrogen  an o x i d a n t are shown i n Table 9. The hydrogen  peroxide  resulted  arsenopyrite f l o a t a b i l i t y .  in  a  significant  in  these  Figure  reveals  decrease  by  the  arsenopyrite  the  potentials  lie  partway  a r s e n o p y r i t e o x i d a t i o n peak. For  the  test  shown  up  in the  i n T a b l e 9, an a d d i t i o n of up t o  grams per tonne of e t h y l x a n t h a t e gave no recovery  in  floatability.  t e s t s w i t h the voltammogram a t pH = 8.2 that  as  I n c r e a s i n g the c o n d i t i o n i n g time from  Comparison of the p o t e n t i a l s a c h i e v e d electrode  peroxide  a d d i t i o n of 357 m g / l i t r e of  5 t o 15 minutes gave a f u r t h e r d e c r e a s e i n  14  immersed i n t o  w i t h 357 m g / l i t r e H 0 2  2  further  increase  and an i n c r e a s e of 3.1%  with  900 in 214  129  mg/litre H 0 . 2  2  Table 9 F l o t a t i o n r e s u l t s u s i n g hydrogen p e r o x i d e as an o x i d a n t Test No.  Conditions  1 2  No o x i d a t i o n 357 m g / l i t r e H 0 cond. 5 min. 357 m g / l i t r e H 0 cond. 15 min. 214 m g / l i t r e H 0 cond. 15 min.  3 4  The  results  97.1 1 1 .6  2  213 t o 233 363  2  2  368  3.3  2  2  323 t o 353  6.0  using  sodium  h y p o c h l o r i t e as an o x i d a n t a r e i n t h e absence of  oxidation  slight  was produced from t h e same  variations  i n t h e rougher  caused t h e v a r i a t i o n i n f l o a t a b i l i t y . As significant  decrease  i n arsenopyrite  t h r o u g h t h e use of h y p o c h l o r i t e . The required  f o r depression  required  peroxide  ore  f o r the peroxide,  a  f l o a t a b i l i t y was a c h i e v e d addition  of  hypochlorite  t o be a c h i e v e d i s much lower than t h e  addition.  The  reason  is  not  for  this  requirement  electrochemical  i n v e s t i g a t i o n s which have been c a r r i e d  apparent  lower  from  the  out. I t  b e l i e v e d t h a t the two r e a g e n t s r e s u l t e d i n d i f f e r e n t degrees  of o x i d a t i o n not r e f l e c t e d by t h e p o t e n t i a l s . The the  i n each  f l o t a t i o n procedure  hypochlorite  is  is  i n t h i s t e s t s e r i e s than i n t h e s e r i e s shown i n Table 9.  While the concentrate case,  As Recovery %  2  shown i n T a b l e 10. The r e c o v e r y lower  Arsenopyrite Eh,mV  arsenopyrite  electrode  with  p o t e n t i a l of  hypochlorite  jumped  to  a p p r o x i m a t e l y +343 mV and then d e c r e a s e d d u r i n g t h e c o n d i t i o n i n g p e r i o d . W h i l e a t 389 m g / l i t r e NaCIO  the a d d i t i o n  of  a d d i t i o n of x a n t h a t e f a i l e d t o r e s u l t i n a r s e n o p y r i t e  a  large  flotation,  130 T a b l e 10 F l o t a t i o n r e s u l t s u s i n g sodium h y p o c h l o r i t e as an o x i d a n t Test No.  Conditions  Arsenopyrite As Eh,mV Recovery %  1 2  No o x i d a t i o n 389 m g / l i t r e NaCIO cond. 5 min. 77.6 m g / l i t r e NaCIO cond. 5 min. 19.4 m g / l i t r e NaCIO cond. 5 min. 19.4 m g / l i t r e NaCIO cond. 15 min.  203 t o 223 343 t o 283  83.1 0.0  343 t o 283  0.0  343 t o 193  28.7  343 t o 203  22.5  3 4 5  at  t h e lower NaCIO a d d i t i o n s xanthate a d d i t i o n s r e s u l t e d i n 20  p e r c e n t t o 30 p e r c e n t a d d i t i o n a l a r s e n o p y r i t e r e c o v e r y .  6.0.4 S e l e c t i v e F l o t a t i o n of P y r i t e From A r s e n o p y r i t e S e l e c t i v e f l o t a t i o n t e s t s were c a r r i e d out on bulk p y r i t e a r s e n o p y r i t e c o n c e n t r a t e s from G i a n t Y e l l o w k n i f e Mines and  from  E q u i t y S i l v e r Mines L i m i t e d .  6.0.4.1 E x p e r i m e n t a l and R e s u l t s .  (i) Equity Tests  Concentrate were  carried  out on a bulk c o n c e n t r a t e s u p p l i e d by  E q u i t y M i n e s . The c o n c e n t r a t e had been produced a t t h e s i t e flotation  of t h e s i l v e r c i r c u i t  ( t e t r a h e d r i t e and c h a l c o p y r i t e )  t a i l i n g . The c o n c e n t r a t e c o n t a i n e d a p p r o x i m a t e l y much  five  times  p y r i t e a s a r s e n o p y r i t e . Some o x i d a t i o n was observed  concentrate refloated  and with  i t was 85  grams  therefore per  by  tonne  reground,  as  on t h e  filtered  i s o p r o p y l xanthate.  and This  131  r e f l o a t e d c o n c e n t r a t e was then f i l t e r e d and s p l i t for the i n d i v i d u a l The  tests  into  portions  tests. o u t i n an A g i t a i r f l o t a t i o n  cell  u s i n g 150 grams c o n c e n t r a t e i n a 500 gram (1.4 l i t r e ) c e l l .  The  repulped  were  carried  c o n c e n t r a t e was found t o have a pH of 5.7 and t h i s was  a d j u s t e d t o pH = 9.2 u s i n g NaOH. A f t e r the pH was  adjusted  the  o x i d i z i n g agent was added and c o n d i t i o n e d . F l o t a t i o n was c a r r i e d out  for 3  minutes  with  an  addition  Downfroth 250. The o x i d a n t used was  of  H 0 . 2  2  5  grams p e r tonne  A l l tests  were  at  20°C. The t e s t r e s u l t s a r e shown i n Table 11.  Table 11 F l o t a t i o n Test R e s u l t s w i t h E q u i t y  Concentrate  H 0 mg/litre  Conditioning Time minutes  Arsenopyrite Recovery %  Pyrite Recovery %  0 357 178 178 71 71  5 5 0 5 15  92.3 17.5 18.5 35.1 23.6 26.3  92.4 61.4 55.2 63.2 53.2 49.2  2  2  ( i i ) Giant Y e l l o w k n i f e Concentrate Several  a  bulk  c o n c e n t r a t e produced a t t h e m i n e s i t e . T h i s c o n c e n t r a t e had  been  produced Microscopy was  series  of  tests  were  out on  w i t h a d d i t i o n s of copper s u l p h a t e as w e l l as x a n t h a t e . of t h i s m a t e r i a l r e v e a l e d numerous m i d d l i n g s  therefore  reground  used. The reground  and i t  t o .94% minus 200 mesh p r i o r t o b e i n g  c o n c e n t r a t e was r e f l o a t e d w i t h an a d d i t i o n of  25 grams p e r tonne i s o p r o p y l x a n t h a t e with  carried  and  cleaned  twice  more  no f u r t h e r a d d i t i o n s . The c l e a n e d c o n c e n t r a t e was f i l t e r e d  132  and  split  i n t o 90 gram p o r t i o n s f o r i n d i v i d u a l  tests.  The pH f o r each t e s t was a d j u s t e d u s i n g H S O 2  required.  The  or  a  NaOH  as  c o n d i t i o n i n g p e r i o d f o r each t e s t was 15 minutes  and t h e temperature was 20°C. The r e s u l t s of t e s t s u s i n g both a i r and h y p o c h l o r i t e as the o x i d a n t a r e shown i n F i g u r e 50. W h i l e w i t h a i r t h e recovery  i s somewhat  the two m i n e r a l s  arsenopyrite  lower than f o r p y r i t e , w i t h h y p o c h l o r i t e  show e s s e n t i a l l y the same b e h a v i o u r .  of these t e s t s when compared w i t h those shown  The r e s u l t  i n Table  11  as  w e l l as w i t h those f o r t h e a r s e n o p y r i t e t e s t e d i n d i v i d u a l l y make it  apparent  that  each  ore o c c u r r e n c e can be expected t o show  some v a r i a t i o n i n response factors  to  depression  oxidation.  Such  as the r a t i o of p y r i t e t o a r s e n o p y r i t e and the p r e v i o u s  f l o t a t i o n h i s t o r y of t h e samples can be the  by  expected  to  influence  results. Additional  tests  were  carried  out on a bulk  concentrate  p r e p a r e d i n the l a b o r a t o r y from G i a n t Y e l l o w k n i f e o r e . The c o n c e n t r a t e  was p r e p a r e d by g r i n d i n g 2 k i l o g r a m s  in a laboratory rod m i l l carried  t o 80% p a s s i n g  of ore  200 mesh. F l o t a t i o n  was  out w i t h 65 grams per tonne i s o p r o p y l x a n t h a t e 25 grams  per tonne amyl x a n t h a t e and 10 grams per tonne Dowfroth  250  pH = 6.0.  further  The  concentrate  was  a d d i t i o n s and was then f i l t e r e d for  cleaned and s p l i t  t w i c e without into  at  three  portions  with  constant  testing. Two  test  series  were  carried  o u t , one  p o t a s s i u m permanganate a d d i t i o n and the second permanganate  addition  and  constant  with  increasing  x a n t h a t e a d d i t i o n . I n each  case a f i v e minute c o n d i t i o n i n g p e r i o d was  used  following  the  133  !00 80 >*60 0 FeAsS,77.6mg r'NaCIO  > O  «40  • FeS  2  ,7 76 mg r'NaCIO  D FeAsS , 2.4 Ipm air  EC  20h  1 FeS , 2.4 Ipm air 2  8  9 PH  10  12  F i g u r e 50 I n f l u e n c e of o x i d a t i o n on p y r i t e and a r s e n o p y r i t e a t i n c r e a s i n g pH  floatability  134  permanganate  a d d i t i o n . At the end of t h i s p e r i o d n e i t h e r p y r i t e  nor a r s e n o p y r i t e were found t o f l o a t . The per l i t r e Dowfroth 250 were added and out.  The  addition  of  frother  xanthate  and  f l o t a t i o n was  alone  12  mg.  then c a r r i e d  f a i l e d t o r e s u l t i n any  flotation.  6.0.4.2 D i s c u s s i o n The  result  hydrogen  of  peroxide  tests  on  peroxide  additions  can  floatability  Equity  show g r e a t e r d e p r e s s i o n  p y r i t e . Maximum d e p r e s s i o n high  the  concentrate,  of a r s e n o p y r i t e than  of a r s e n o p y r i t e i s  achieved  addition  (357  m g . / l i t r e ) . Much lower  be  but  with  used  resulting.  increasing  Increasing  with  a  peroxide  arsenopyrite  P y r i t e appears to be l e s s f l o a t a b l e i n  the presence of low p e r o x i d e a d d i t i o n s than w i t h additions.  using  conditioning  time  high  peroxide  does not improve the  s e p a r a t i o n of p y r i t e from a r s e n o p y r i t e . The show  r e s u l t s of t e s t s  both  pyrite  hypochlorite depression  while  and air  with  Giant  Yellowknife  concentrate  a r s e n o p y r i t e t o be e q u a l l y d e p r e s s e d by oxidation  results  in  preferential  of a r s e n o p y r i t e .  While  the  addition  of  p o t a s s i u m permanganate, d e p r e s s e d  both p y r i t e and a r s e n o p y r i t e , a subsequent a d d i t i o n of resulted in preferential pyrite The  complete  depression  xanthate  flotation. of  both  p r e f e r r e n t i a l a c t i v a t i o n of the p y r i t e  minerals with  followed  xanthate  has  by not  been a t t e m p t e d w i t h o x i d a n t s o t h e r than permanganate. The  selective  arsenopyrite  flotation  concentrate  of  appears  pyrite to  be  from  a bulk  feasible  pyritealthough  Figure  51  D e p r e s s i o n of a r s e n o p y r i t e from bulk c o n c e n t r a t e xanthate a d d i t i o n  with increasing  136  IOOI  ;  71 m g r«  NaEtX  80-  ^60a>  S40-  o  rr  250 KMn04  /  m g I"1  F i g u r e 52 D e p r e s s i o n of a r s e n o p y r i t e from b u l k c o n c e n t r a t e w i t h i n c r e a s i n g permanganate a d d i t i o n  137  c o n s i d e r a b l e e f f o r t would be r e q u i r e d t o o p t i m i z e the c o n d i t i o n s f o r s e p a r a t i o n . These optimum c o n d i t i o n s can be expected t o vary depending on the p y r i t e t o a r s e n o p y r i t e r a t i o and on the h i s t o r y of the c o n c e n t r a t e . The  results  of  floated arsenopyrite  the has  various been  tests  depressed  o x i d i z i n g agents r e v e a l t h a t a g r e a t e r is  in  which p r e v i o u s l y  though  i n f l u e n c e on  the  use  of  floatability  e x e r t e d by the f e r r i c h y d r o x i d e s u r f a c e d e p o s i t s than by the  adsorbed c o l l e c t o r  layer.  138  Chapter 7  CONCLUSIONS  1. C y c l i c v o l t a m m e t r i c arsenopyrite formation  across  of  concurrent  s t u d i e s have r e v e a l e d the  ferric  of  oxidation  surface  arsenic  to  deposits  the  with  arsenate  the  s t a t e and  s u l p h u r t o the s u l p h a t e s t a t e . A c r o s s the pH range s t u d i e d , hydroxide At  the  f i l m t h i c k n e s s i n c r e a s e s w i t h i n c r e a s i n g pH. temperatures below 30°C, i n c r e a s i n g temperature r e s u l t s  in i n c r e a s i n g hydroxide range  of  pH range from 7 - 12 t o r e s u l t i n the  hydroxide  oxidation  the  from  30°C  to  f i l m t h i c k n e s s . Across 45°C,  film  development  the  temperature  appears  independent of t e m p e r a t u r e . At temperatures g r e a t e r  to  than  be  45°C,  f i l m t h i c k n e s s increases r a p i d l y w i t h i n c r e a s i n g temperature. Below  pH = 7, f e r r i c h y d r o x i d e  the f o r m a t i o n of e l e m e n t a l  d e p o s i t s a r e not formed but  sulphur a t the electrode  surface  is  indicated. 2. ESCA s t u d i e s have r e v e a l e d t h a t most of t h e a r s e n a t e  and some  of  hydroxide  the  sulphate  i s incorporated  i n the  ferric  deposits. 3. E l e c t r o d e p o t e n t i a l measurements i n t h e presence oxidizing  agents  f e r r i c hydroxide  indicate formation  w i l l be a c h i e v e d w i t h these 4.  The  that  the  (as i n d i c a t e d by c y c l i c  Dixanthogen  required for voltammetry)  d e p o s i t s on t h e s u r f a c e of  t h e o x i d a t i o n of xanthate  i s believed  several  agents.  p r e s e n c e of f e r r i c h y d r o x i d e  arsenopyrite inhibits  potentials  of  to  dixanthogen.  t o be the a c t i v e c o l l e c t o r s p e c i e s on  139  arsenopyrite. 5.  Flotation  depressed  studies  with  conditioned  show  arsenopyrite  increasing  pH  i n t h e presence  when of  to  be  the ore  aeration  increasingly  s l u r r y has been  prior  to  xanthate  a d d i t i o n . T h i s i n f l u e n c e of pH h o l d s t r u e over t h e pH range from 7 t o 12. At pH l e s s than 7, a r s e n o p y r i t e shows h i g h due  to  t h e absence  elemental  of  depression  greater  influence  floatability  of  interpretations  of  than  40°C  both  pH  arsenopyrite  result  in  increased  depression  arsenopyrite  temperature  correlate  of  arsenopyrite  of f e r r i c h y d r o x i d e s u r f a c e  with  and  well  on t h e  with  the  of e l e c t r o c h e m i c a l e x p e r i m e n t s . I t i s c o n c l u d e d  that the f l o a t a b i l i t y  floated  and t h e presence of  of a r s e n o p y r i t e .  The  The  films  sulphur.  Temperatures  formation  hydroxide  floatability  of  xanthate  i s controlled  by t h e  deposits.  a r s e n o p y r i t e which has p r e v i o u s l y been and  the  from bulk c o n c e n t r a t e s  selective  depression  of  through t h e use of o x i d i z i n g  agents has been demonstrated. I t i s t h e r e f o r e c o n c l u d e d t h a t t h e f l o a t a b i l i t y of t h e m i n e r a l  i s i n f l u e n c e d t o a g r e a t e r degree by  the f e r r i c h y d r o x i d e d e p o s i t s formed by o x i d a t i o n of t h e m i n e r a l than by adsorbed c o l l e c t o r l a y e r s . 6. V o l t a m m e t r i c s t u d i e s i n t h e presence of c y a n i d e i n d i c a t e t h a t cyanide  does  not  contribute  t o t h e development of d e p r e s s a n t  f i l m s on a r s e n o p y r i t e and may i n f a c t r e s u l t i n some  degree  of  activation. 7.  Electrochemical  studies  of  other  minerals  i n t h e Fe-As-S  system r e v e a l s i g n i f i c a n t d i f f e r e n c e s i n t h e n a t u r e  of  surface  140  hydroxide  deposits  which  correlate  d i f f e r e n c e s i n m i n e r a l s o l u b i l i t y and may their f l o a t a b i l t y with  xanthate.  well  with  observed  explain variations  in  141  Chapter 8  RECOMMENDATIONS FOR FUTURE WORK  Additional arsenopyrite  work  can be  t o be  carried  divided  into  out on t h e f l o t a t i o n of fundamental  and  applied  categories. Fundamental r e s e a r c h the  growth  s h o u l d be d i r e c t e d a t f u r t h e r study of  and morphology of h y d r o x i d e f i l m s on a r s e n o p y r i t e .  The i n f l u e n c e of v a r i o u s d e p r e s s a n t s and the  structure  of  i n v e s t i g a t e d . While contribute  the  hydroxide  additional  the use of s p e c t r o s c o p i c t e c h n i q u e s  Applied utilization arsenopyrite some e x t e n t specific  research of  should  surface  layers  agents  on  also  be  should  electrochemical  much t o t h e u n d e r s t a n d i n g  complete a p p r e c i a t i o n of t h e i r  activating  studies  of these s u r f a c e  could  deposits,  w i l l be r e q u i r e d t o g i v e  a  nature. be  directed  oxidation  for  at  the  optimizing the depression  of  from o t h e r m i n e r a l s , p a r t i c u l a r l y p y r i t e . W h i l e t o t h e optimum  conditions  f o r each m i n e r a l o c c u r r e n c e ,  can be  expected  t o be  a s i g n i f i c a n t common base  i s a n t i c i p a t e d . The i n f l u e n c e of o t h e r d i s s o l v e d s p e c i e s such as calcium  i o n or  soluble  s h o u l d a l s o be e v a l u a t e d .  silicates  on  arsenopyrite  depression  142  Appendix I P o t e n t i a l / p H diagrams f o r the I r o n - A r s e n i c - S u l p h u r - Water System  Potential/pH a r s e n i c - sulphur  diagrams  have  been  p r e p a r e d f o r the i r o n -  - water system a t 25°C. The  assumptions  made  f o r each diagram a r e as f o l l o w s : F i g u r e 53. A r s e n o p y r i t e and  sulphur  s t a b i l i t y diagram assuming i r o n , a r s e n i c  a c t i v i t i e s of 10'  6  M. A s S 2  2  and A s S 2  3  a r e shown as  s t a b l e s p e c i e s . S o l u b l e a r s e n i c s p e c i e s are not shown.  F i g u r e 54. A r s e n o p y r i t e  s t a b i l i t y diagram assuming i r o n , a r s e n i c  and s u l p h u r a c t i v i t i e s of 1 0  M. A s S  - 3  2  and A s S  2  2  are  3  shown  as  stable species.  F i g u r e 55. A r s e n o p y r i t e and  sulphur  s t a b i l i t y diagram assuming i r o n , a r s e n i c  activities  of  1  M. A s S 2  2  and A s S 2  3  a r e shown as  s t a b l e s p e c i e s . S o l u b l e a r s e n i c s p e c i e s a r e not shown.  F i g u r e 56.  Loellingite  stability  diagram  assuming  iron  a r s e n i c a c t i v i t i e s of 10~  3  Figure  57. A r s e n o p y r i t e  s t a b i l i t y diagram c o n s i d e r i n g F e S ,  and F e A s  2  and  M.  2  FeS  as p o s s i b l e s t a b l e s p e c i e s . S t a b i l i t y domains of A s S  are i n d i c a t e d . S o l u b l e  2  arsenic  and  sulphur  shown. A c t i v i t y of each d i s s o l v e d s p e c i e s = 10~  species 6  M.  are  2  not  143  Ferric  arsenate  i s not shown i n F i g u r e s 53 t h r o u g h 57. At  low i r o n and a r s e n i c a c t i v i t y region  near  the  ferric  i t has  ion -  only  a  narrow  f e r r i c hydroxide  boundary. The  s t a b i l i t y r e g i o n f o r f e r r i c a r s e n a t e a t 1 molar a c t i v i t y and a r s e n i c i s shown i n F i g u r e 58.  T a b l e 12 Thermodynamic Data a t 25°C Spec i e s HS"  s 2  HS0 S0 " HS 4 2  4  s  2  H AsO„ H AsO HAsOa " AsO„ ' H As0 H As0 " HAs0 ~ AsH AS As S As S FeAsS FeAs FeAsOo Fe Fe HFe0 " FeS FeS Fe(OH) Fe(OH) 3  _  2  ft 2  3  3  3  2  3  2  3  3  2  2  2  3  2  3 +  2 +  2  2  2  3  State aq aq aq aq aq s aq aq aq aq aq aq aq aq s s s s s s aq aq aq s s s s  G°f Kcol/mole 2.88 20.5 -180.69 -177.97 -6.66 0 -184.0 -181.0 -171.5 -155.8 -154.4 -141.8 -125.3 23.8 0 -17.0 -23.0 -26.2 -12.5 -185.18 -1.1 -18.85 -90.6 -24 -38.3 -116.3 -116.5  stability  298°K 80 80 80 80 80 80 82 82 82 82 82 82 82 82 82 81 81 81 81 83 80 80 80 80 80 80 80  of i r o n  144  Figure Arsenopyrite  stability  53  d i a g r a m at 10~ spec i e s  6  M activity  of  dissolved  145  Figure Arsenopyrite  stability  54  diagram at 10" species  3  M activity  of  dissolved  146  Figure Loellingite  stability  55  diagram at 10" spec i e s  3  M activity  of  dissolved  147  Figure  56  A r s e n o p y r i t e s t a b i l i t y diagram at 1 M a c t i v i t y spec i e s  of d i s s o l v e d  148  Figure Arsenopyrite  stability  57  diagram c o n s i d e r i n g FeS,FeS stable products  2  and F e A s  2  as  Figure Stability  r e g i o n of  ferric  58 arsenate at  1 M  activity  Appendix I I E q u a t i o n s Used For The C o n s t r u c t i o n Of The Diagrams  1. FeAsS + 2H  = Fe  +  + H S + As.  + +  2  -0.030 = 0.0591 l o g ( F e ) ( H S ) + 0.118 pH + +  2  2. F e  2 +  + H As0 3  3  + HS + H  + 3e" = FeAsS + 3H 0  +  2  2  E = 0.236 - 0.0197 l o g ( F e ) ( H A s 0 ) ( H S ) - 0.0197 pH 2 +  3  3. F e ( O H )  2  +  H3ASO3  + SO,"  + 13H  3  2  + 1 l e " = FeAsS + 9H 0  +  2  E = 0.396 + 0.005 l o g ( H A s 0 ) ( S O , " " ) - 0.070 pH 2  4. F e ( O H )  2  3  + H A s 0 - + SO,'" + 14H 2  + 11e" = FeAsS + 9H 0  +  3  2  E = 0.396 +0.005 l o g ( H A s 0 " ) ( S O , " " ) - 0.075 pH 2  5. F e ( O H )  3  + HAs0 " + SO,-" + 15H 2  2  3  +  + l i e ' = FeAsS + 9H 0 2  E = 0.461 + 0.005 l o g ( H A s 0 - ) ( S O , - - ) - 0.080 pH 2  3  6. F e ( O H )  + HAs0 2  2  3  +  HS" + 6H  + 3e" = FeAsS + 5H 0  +  2  E = 1.025 + 0.0197 l o g ( H A s 0 " ) ( H S " ) - 0.118 pH 2  3  7. FeAsS + 5H  +  + 3e" = A s H  3  + Fe  + +  + HS 2  E = - 0.354 - 0.0197 l o g ( A s H ) ( F e ) ( H S ) - 0.0985 pH + +  3  8. FeAsS + 5H  +  + 5e~ = A s H  3  2  + H S + Fe 2  E = -0.376 - 0.0118 l o g ( A s H ) ( H S ) - 0.0591 pH 3  2  151  9. FeAsS + 4H  + 5e" = A s H  +  +HS" + Fe  3  E = - 0.459 - 0.0118 l o g ( A s H ) ( H S - ) - 0.0473 pH 3  10. FeAsS + H  + 2e" = A s H  +  + HS" + Fe  3  E = - 0.631 - 0.0295 log(HS") - 0.0295 pH  11. 2 H A s 0 3  3  + 3HSO - + 27H  + 24e" = A s S  +  q  2  E = 0.348 + 0.0025 l o g ( H A s 0 ) ( H S 0 " ) 2  3  12. 2 H A s 0 3  3  + SO„-- + 30H  3  2  2  + 18H 0  3  2  E = 0.363 + 0.0025 l o g ( H A s 0 ) ( S O " " ) 2  3  13. A s S 2  + 4H  2  +  3  + 4e" = 2As + 2H S 2  - 0.0591 pH  2  2  2  + 2H  2  +  + 4e" = 2As + 2HS"  E = -0.0247 - 0.0148 l o g ( H S " )  15. 2 H A s 0 3  3  + 2SO„-- + 22H  - 0.0295 pH  2  + I8e" = A s S  +  2  + 14H 0  2  2  E = 0.352 + 0.0033 log(H As0 ) (SO«"-) 2  3  16. 2 H A s 0 " + 2SO "- + 24H 2  - 0.074 pH  3  f t  E = - 0.040 - 0.0148 l o g ( H S )  14. A s S  - 0.066 pH  3  4  + 24e" = A s S  +  + 18H 0  3  3  + I8e" = A s S  +  ft  2  + 14 H 0  2  2  E = 0.412 + 0.0033 log(H As0 ") (SO„"') 2  2  17. H A s 0 - + 2HS" + 6H 2  3  +  2  2  2  2  3  + 2H  +  + 2e" = A s S 2  2  3  + HS 2  - 0.0788 pH  + 6H 0  E = 1.719 + 0.0295 l o g ( H A s 0 ) ( H S " )  18. A s S  2  3  + 2e" = A s S 2  - 0.0722 pH  2  3  2  2  - 0.1773 pH  152  E = 0.0143 -0.0148 l o g ( H S )  - 0.0591 pH  2  2  19. A s S 2  + H  3  + 2e" = A s S  +  2  + HS'  2  E = - 0.193 - 0.0295 log(HS") - 0.0295 pH  20. A s S 2  2  + S 0 " - + 3H  + 6e" = A s S  +  4  2  + 4H 0  3  2  E = 0.395 + 0.0098 l o g ( S O - " ) - 0.0788 pH a  21. 1/2 A s S 2  3  + Fe  + H  ++  +  + 3e" = FeAsS + 1/2 H S 2  E = -0.012 - 0.0295 l o g ( H S ) V * - 0.0197 pH 2  (Fe ) ++  22. 1/2 A s S 2  2  + Fe  + 2e" = FeAsS  + +  E = -0.025 +0.0295 l o g ( F e ) + +  23. 2 H A s 0 3  3  + Fe(OH)  2  + 8H  + 8e" = F e A s  +  E = 0.220 + 0.007 l o g ( H A s 0 ) 3  24. F e  + 2As + 2e" = F e A s  + +  2  + 8H 0 2  - 0.059 pH  2  3  2  E = -0.138 + 0.0295 l o g ( F e ) + +  25. 2 H A s 0 3  3  + Fe  + +  + 6H  +  + 8e" = FeAs + 6H 0 2  E = 0.135 + 0.007 l o g ( H A s 0 ) 3  26. FeAs2 + 6H  +  E = -0.434 - 0.010 l o g ( A s H ) 3  27. F e A s  2  + 6H  +  + 4e" = F e  + +  ( F e ) - 0.044 pH  2  + +  3  + 6e" = Fe + 2AsH  2  3  2  - 0.059 pH  + 2AsH  3  153  E = - 0.447 - 0.015 l o g ( A s H )  ( F e ) - 0.089 pH  2  + +  3  28. 2 H A s 0 - + Fe(OH) 2  3  2  + 10H  + I8e" = F e A s  +  E = 0.358 + 0.007 l o g ( H A s 0 " ) 2  29. 2 H A s 0 " + Fe(OH) 2  3  2  + 12H  +  + 8e" = F e A s 2  2  3  30. 1/2 A s S 2  + Fe  3  + +  2  - 0.074 pH  2  3  E = 0.537 + 0.007 l o g ( H A s 0 - )  + H 0  2  2  + 8H 0 2  - 0.089 pH  + 2e" = FeAsS + 1/2 S  E = - 0.090 + 0.0295 l o g ( F e ) + +  31. F e  2 +  + 2SO, - + 14e" = F e S  + 8H 0  2  2  2  E = 0.362 + 0.0042 l o g ( F e ) (SO,, ") - 0.068 pH 2 +  32. F e S  2  + 4H  +  + 2e" = F e  2  2  + 2H S  2 +  2  E = - 0.133 - 0.0296 l o g ( F e ) ( H S ) 2 +  2  33. F e S  2  + 2H  +  2  - 0.118 pH  + 2e" + As = FeAsS + H S 2  E = -0.118 - 0.0295 l o g ( H S ) - 0.059 pH 2  34. F e S  2  + As + H  +  + 2e" = FeAsS + HS"  E = -0.325 - 0.0295 log(HS') - 0.0295 pH  35. F e ( O H )  2  + 2SO "- + 18H tt  +  + 14e" = F e S  E = 0.412 + 0.0042 l o g ( S O " - ) a  36. F e S  2  + H A s 0 " + 5H 2  3  +  2  2  + 10H 0 2  - 0.076 pH  + 5e" = FeAsS + HS" + 3H 0  E = 0.115 - 0.012 l o g (HS~)  2  - 0.059 pH  154  (H As0 ~) 2  37. F e S  + H  2  +  3  + 2e~ = FeS + HS"  E = - 0.372 - 0.0295 pH - 0.0296 l o g HS"  38. F e ( O H )  2  + HS" + H  +  = FeS + 2H 0 2  1.039 = 0.059 log(HS') - 0.059 pH  39. FeAsS + As + 2H  + 2e" = F e A s  +  + HS  2  2  E = - 0.153 - 0.0295 l o g ( H S ) - 0.059 pH 2  40. FeAsS + As + H  + 2e" = F e A s  +  + HS"  2  E = -0.359 - 0.0295 log(HS') - 0.0295 pH  41. FeS + 2 H A s 0 - + 9H 2  3  +  + 8e" = F e A s  E = 0.229 - 0.007 l o g (HS") (H As0 -) 2  42. HFe0 " + 2 H A s 0 " + 13H 2  2  2  + HS" + 6H 0  - 0.066 pH 2  3  + 8e" = F e A s  +  3  2  + 8H 0  2  2  E = 0.677 + 0.007 l o g ( H A s 0 " ) ( H F e 0 " ) - 0.096 pH 2  2  3  43. FeAsO« + 3H  +  = H AsO„ + F e  2  3 +  3  pH = 0.12 - 1/3 l o g ( H A s O , , ) ( F e ) 3+  3  44. FeAsO« + 3H 0 = F e ( O H ) 2  3  + H AsO„- + H 2  pH = 5.30 + l o g ( H A s O - ) 2  45. FeAsO„ + 3H  +  + e- = F e  fl  2 +  + H AsO 3  a  +  E = 0.766 - 0.0.59 l o g ( F e ) (H AsO«) - 0.177 pH 2+  3  FeAsO  fl  + 5H  +  + 3e" = H A s 0 3  + Fe  3  2 +  + H 0 2  E = 0.647 -0.020 l o g ( H A s 0 ) ( F e ) - 0.098 pH 2 +  3  3  A P P E N D I X  • Figure  156  I I I  100  1.  oxidation  Voltammograms f o r p l a t i n u m i n presence and absence of xanthate ( 4 9 ) .  < 3.  Q I—  £ -too a. 3  -150  Reduction j i_ -04  0  04  08  POTENTIAL/ V (vs. S H.E.)  Figure  2.  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