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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 Mining and Mineral Process Engineering We accept t h i s t h e s i s as conforming 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 © Morris John A l o y s i u s Vreugde, 1 982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of 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 University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) S u p e r v i s o r : Dr. George W. P o l i n g ABSTRACT Ele c t r o c h e m i c a l methods, surface spectroscopy and f l o t a t i o n t e s t s have been used to study the i n f l u e n c e of the o x i d a t i o n of arse n o p y r i t e on i t s f l o a t a b i l i t y with xahthate. C y c l i c voltammetric s t u d i e s i n d i c a t e d that the o x i d a t i o n of arse n o p y r i t e at pH greater than 7 r e s u l t s i n the formation of f e r r i c hydroxide deposits on the surface of the m i n e r a l . Arsenic i s o x i d i z e d to arsenate and sulphur i s o x i d i z e d to sulphate. The arsenate i s incorporated i n the f e r r i c hydroxide d e p o s i t s while sulphate d i f f u s e s i n t o s o l u t i o n . Below pH=7, s o l u b l e i r o n species are formed and the surface becomes i n c r e a s i n g l y covered with elemental sulphur with decreasing pH. Increasing temperature has no i n f l u e n c e on the q u a n t i t y of hydroxide formed over the range 30° to 45°C but r e s u l t s i n t h i c k , porous f i l m s at temperature greater 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 to occur at lower o x i d a t i o n p o t e n t i a l s than for p y r i t e although t h i s e f f e c t decreased with i n c r e a s i n g temperature. Mixed p o t e n t i a l s t u d i e s i n d i c a t e d that the p o t e n t i a l s required f o r a r s e n o p y r i t e o x i d a t i o n could be achieved with common o x i d i z i n g agents. S e l e c t i v e o x i d a t i o n of ar s e n o p y r i t e i n a bulk p y r i t e - a r s e n o p y r i t e concentrate was i n d i c a t e d to be p o s s i b l e . The formation of i r o n hydroxide d e p o s i t s on the surface of arse n o p y r i t e 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 xanthate to dixanthogen at the mineral's surface. ESCA s t u d i e s confirmed the formation of o x i d i z e d i r o n l a y e r s at the surface of arsenopy r i t e and revealed that e s s e n t i a l l y a l l the arsenate which was formed was incorporated i n these l a y e r s . Sulphur became o x i d i z e d at the pH st u d i e d and to a lar g e extent 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 use of o x i d a t i o n for ar s e n o p y r i t e depression. In the presence of o x i d a t i o n , i n c r e a s i n g pH above pH = 7 r e s u l t e d i n increased a r s e n o p y r i t e depression while 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 a temperature of 40°C was exceeded. 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 could be depressed through the use of o x i d i z i n g agents. Arse n o p y r i t e could be s e l e c t i v e l y depressed from a bulk p y r i t e - a r s e n o p y r i t e concentrate through the use of o x i d i z i n g agents. ACKNOWLEDGEMENT The author wishes to express s i n c e r e thanks to Dr. G.W. P o l i n g f o r h i s support and guidance during the course 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 to Dr. W.G. Bacon without whose help t h i s p r o j e c t could not have been i n i t i a t e d and who ensured the continued support of Bacon, Donaldson and Associates L t d . The i n c e n t i v e to i n i t i a t e t h i s p r o j e c t and the endurance to car r y i t through to completion r e s u l t s from an unending d e s i r e to c a r r y my education to the highest a t t a i n a b l e l e v e l . 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 expressed to my parents f o r i n s t i l l i n g i n me the d e s i r e to lea r n and to my wife, Kate, whose t o t a l support and often e s s e n t i a l encouragement enabled me to f u l f i l l my d e s i r e . A p p r e c i a t i o n i s a l s o expressed to Morny without whose u n s e l f i s h e f f o r t s t h i s manuscript could not have been completed. Mr. Stephen P i c k e t t of the Department of Chemistry i s appreciated f o r h i s time i n performing the ESCA analyses. F i n a n c i a l a s s i s t a n c e i n the form of a B.C. Science C o u n c i l Grant i s g r a t e f u l l y acknowledged. i v Table of Contents 1 INTRODUCTION 1 2 LITERATURE REVIEW 8 2.1 State Of The Art Of Arsenopyrite F l o t a t i o n 8 2.2 Nature Of Adsorbed Xanthate Species 10 2.3 C r y s t a l S t r u c t u r e * . . 1 2 2.4 E l e c t r o c h e m i c a l Oxidation Of Arsenopyrite 13 2.5 Arsenopyrite Composition 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 Arsenopyrite 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 In F l o t a t i o n 20 3 OBJECTIVES OF THE PRESENT INVESTIGATION 23 4 ELECTROCHEMICAL STUDIES 25 4.1 Electrode P o t e n t i a l Measurements 25 4.1.1 Experimental 25 4.1.2 O x i d i z i n g Agents 28 4.2 Results And Discussion 32 4.3 C y c l i c Voltammetry 40 4.3.1 Experimental 42 4.3.2 Results And Discussion 45 A. Sin 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 Of Arsenate Formation 70 D. Influence Of Diss o l v e d Arsenic 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 esults And D i s c u s s i o n 84 G. Influence Of Cyanide 89 H. Other Min e r a l s In The Fe - As - S System 95 ( i ) Experimental 95 ( i i ) R e s u l t s 97 I . Ring Disc Study 101 J . Influence Of Hydroxide Formation On Xanthate Oxidation 105 4.4 Discussion 109 5 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 6 FLOTATION STUDIES 122 6.0.1 Rougher F l o t a t i o n 122 6.0.1.1 Experimental 122 6.0.2 Results And D i s c u s s i o n 124 6.0.3 Depression Of P r e v i o u s l y A c t i v a t e d Arsenopyrite ..126 6.0.3.1 Experimental .....126 6.0.3.2 Results And D i s c u s s i o n 128 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 Arsenopyrite ..130 6.0.4.1 Experimental And 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 Dis c u s s i o n 134 7 CONCLUSIONS 138 8 RECOMMENDATIONS FOR FUTURE WORK 141 Appendix I - P o t e n t i a l / p H Diagrams For The Iron - Arsenic -Sulphur - Water System 142 Appendix II - Equations Used For The Construction Of The Diagrams 150 Appendix I I I - Foldout Of Important Diagrams 156 References 157 v i i Table of Figures 1 Minerals i n the Fe-As-S system 4 2 Arsenopyrite from Hedley, B.C. (3600x) 5 3 Giant 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 Arsenopyrite ( a f t e r Buerger (33)) 14 5 Atomic % Arsenic i n Arsenopyrite 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 Oxidation s t a t e diagram for the c h l o r i n e system 33 8 Oxidation s t a t e diagram for manganese system 34 9 Oxidation s t a t e diagram f o r the oxygen system 35 10 Eh versus pH for arsenop y r i t e 37 11 Eh versus pH for p y r i t e 38 12 Construction 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 for voltammetry 46 14 Voltammograms for ars e n o p y r i t e at i n c r e a s i n g pH values 48 15 Voltammograms for ars e n o p y r i t e at high pH 49 16 Voltammogram f o r arsenop y r i t e at pH = 8.2 showing p o t e n t i a l s achieved w i t h o x i d i z i n g agents 50 17 Voltammograms for p y r i t e and a r s e n o p y r i t e at pH = 11 52 18 Eh - pH diagram for a r s e n o p y r i t e . A c t i v i t y f or each species taken to be 1 0"3 M 54 19 Current - decay curve f o r Arsenopyrite at +0.0343 V 58 20 M u l t i p l e Sweep Voltammograms for S t a t i o n a r y and Rotating Electrodes at pH = 10.6 59 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 at pH = 11.7 61 v i i i 22 M u l t i p l e sweep voltammograms at pH = 5.8 69 23 Influence of cathodic l i m i t on voltammogram 71 24 Influence of anodic l i m i t on voltammogram 72 25 Voltammogram f o r gold 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 ar s e n i c a d d i t i o n s on ar s e n o p y r i t e p o t e n t i a l sweeps 75 27 Voltammograms at i n c r e a s i n g sweep rate 78 28 Influence of sweep rate 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 functi o n of log scan rate 81 30 Log peak curren t versus l og scan rate 82 31 Voltammograms at 19°C and at 60.5°C 85 32 Influence of temperature on peak p o t e n t i a l and peak current 86 33 M u l t i p l e sweep voltammogram at 58.5°C 88 34 Comparison of p y r i t e and ar s e n o p y r i t e voltammograms at 59.8°C 90 35 M u l t i p l e sweep voltammogram for ar s e n o p y r i t e i n the presence of 2.82X10" 3 M NaCN 93 36 Influence of cyanide on formation of i r o n hydroxide 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 the presence of 1.62x10" 3 M NaCN 96 38 Voltammograms f o r p y r i t e and marcasite at pH = 10.6 98 39 M u l t i p l e sweep voltammogram for l o e l l i n g i t e at pH = 10.6 ..100 40 Voltammogram f o r i r o n e l e c t r o d e 102 41 Transport p a t t e r n of s o l u b l e species at a r i n g - d i s c e l e c t r o d e 104 42 Influence of ar s e n o p y r i t e o x i d a t i o n at pH = 5.9 on xanthate o x i d a t i o n 107 43 Influence of a r s e n o p y r i t e o x i d a t i o n 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 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 with operating plant c o n d i t i o n s (70) 111 45 XPS peaks a s s o c i a t e d with the i r o n 2p e l e c t r o n s of the v a r i o u s minerals 117 46 XPS 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 v a r i o u s minerals 118 47 XPS peaks a s s o c i a t e d with the sulphur 2d e l e c t r o n s of the v a r i o u s minerals 119 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 i n the presence and absence of o x i d a t i o n 125 49 Influence of temperature on ars e n o p y r i t e f l o a t a b i l i t y 127 50 Influence 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 f l o a t a b i l i t y at i n c r e a s i n g pH 133 51 Depression of a r s e n o p y r i t e from bulk concentrate with i n c r e a s i n g xanthate a d d i t i o n 135 52 Depression of a r s e n o p y r i t e from bulk concentrate with i n c r e a s i n g permanganate a d d i t i o n 136 53 A r s e n o p y r i t e s t a b i l i t y diagram at 10" 6 M a c t i v i t y of d i s s o l v e d species 144 54 Arsenopyrite s t a b i l i t y diagram at 10" 3 M a c t i v i t y of d i s s o l v e d species 145 55 Arsenopyrite 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 146 56 L o e l l i n g i t e s t a b i l i t y diagram at 10~ 3 M a c t i v i t y of d i s s o l v e d species 147 X 57 Arsenopyrite s t a b i l i t y diagram c o n s i d e r i n g FeS,FeS 2 and FeAs 2 as s t a b l e products 148 58 S t a b i l i t y region of f e r r i c arsenate at 1 M a c t i v i t y 149 L i s t of Tables 1 Arsenic Minerals 2 2 Arsenic Emissions 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 Influence Of Scan Rate On K i n e t i c Parameters 83 6 E l e c t r o n Binding Energies And I n t e n s i t i e s For Elements In Various M i n e r a l s . 116J 7 XPS I n t e n s i t y Ratios 120 8 F l o t a t i o n Conditions 128 9 F l o t a t i o n Results Using Hydrogen Peroxide As An Oxidant ....129 10 F l o t a t i o n Results Using Sodium Hyp 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 Equity Concentrate 13.1 12 Thermodynamic Data At 25°C 143 1 Chapter 1 INTRODUCTION Arsenic occurs i n nature with numerous other elements. A number of primary and secondary a r s e n i c minerals are l i s t e d i n Table 1. Those arsenic minerals which l i e i n the Fe-As-S system are shown i n Figure 1. Arsenopyrite i s the most common mineral c o n t a i n i n g a r s e n i c . I t i s found with s i l v e r and copper ores, galena, s p a l e r i t e and p y r i t e . In c e r t a i n ores a r s e n o p y r i t e has considerable economic s i g n i f i c a n c e since i t c a r r i e s the major p o r t i o n of gold i n the ore. Such gold 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 arsenopy r i t e and as such be recoverable from an ars e n o p y r i t e concentrate by d i r e c t c y a n i d a t i o n (1). Gold may a l s o occur i n s o l i d s o l u t i o n or as minute i n c l u s i o n s i n ars e n o p y r i t e ( 2 ) and may req u i r e more e x o t i c recovery procedures ( 3 ) . An example of such an occurrence from the Hedley area of B r i t i s h Columbia i s shown i n Figure 2 . The most s u c c e s s f u l treatment of ores of t h i s type to date has been achieved by f l o t a t i o n of an ars e n o p y r i t e bearing concentrate which i s subsequently roasted and then cyanided. Such a process i s i n operation at Giant Y e l l o w k n i f e Mines L t d . ( 4 , 5 ) . A flowsheet f o r t h i s operation i s shown i n Figure 3 . While t h i s process for the recovery of gold from 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 m e t a l l u r g i c a l point of view, there are seriou s p o l l u t i o n consequences (6). The data 2 Table 1 Arsenic M i n e r a l s Arsenic As L o e l l i n g i t e FeAs 2 Realgar AsS Orpiment A s 2 S 3 Arsenopyrite FeAsS Glaucodot (Co,Fe)AsS C o b a l t i t e CoAsS G e r s d o r f f i t e NiAsS S k u t t e r u d i t e (Co,Ni,Fe)As 3 N i c c o l i t e NiAs Enargite Cu3AsS„ P r o u s t i t e Ag 3AsS 3 (Ag,Cu), 6As 2S,! (Cu,Fe,Zn,Ag), 2As f lS, 3 P e a r c i t e Tennantite S p e r r y l i t e P t A s 2 A l l e m o n t i t e AsSb Geocronite P b 5 ( S b , A s ) 2 S 8 Scorodite FeAsO a«2H 20 P i t t i c i t e Fe2(As02)(SO„)OH»2H20 Pharmacosiderite 6FeAsO„«2Fe(OH)3«12H20 Symplesite Fe 3As 20 3«8H 20 E r y t h r i t e C0 3(AsO«) 2«8H 20 Annabergite Ni 3(AsO„) 2«8H 20 shown i n Table 2 i n d i c a t e that 47.5% of a r s e n i c emissions i n Canada r e s u l t from the m e t a l l u r g i c a l processing of gold ores. The predominant source of aqueous contamination by a r s e n i c i s a l s o 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 i n the 1920's the mineral was viewed as a valuable p o t e n t i a l source of a r s e n i c f o r the c o n t r o l of pests such as the b o l l weavil ( 7 ) , at present i t i s viewed as a troublesome impurity i n base metal concentrates (8,59). While the a r s e n i c present i n such concentrates i s recovered to some extent, the t o t a l y e a r l y demand f o r arsenic i n the U.S. i s only 15,000 tons (13,600 tonnes) (9) and the recovery of a r s e n i c from waste streams i s c o s t l y . Smelters at present p r e f e r to receive concentrates which are e s s e n t i a l l y free of a r s e n i c . 3 The presence of ars e n o p y r i t e i n sulphide concentrate can Table 2 Arsenic emissions i n Canada,1972 (6) SOURCE EMISSIONS TONS PERCENT INDUSTRY Primary copper and n i c k e l Primary lead production Primary z i n c production Primary i r o n and s t e e l M e t a l l u r g i c a l processing of gold Miscellaneous sources Subtotal FUEL COMBUSTION/STATIONARY SOURCES Power Generation 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 P e s t i c i d e a p p l i c a t i o n T o t a l lead to seriou s h e a l t h hazards apart from the contamination of smelter gases with a r s e n i c . The presence of ars e n o p y r i t e i n such concentrate has on occasion r e s u l t e d i n a r s i n e generation during gold p r e c i p i t a t i o n i n cyanide c i r c u i t s (10) or hy d r o m e t a l l u r g i c a l processing and r e f i n i n g (11). The a s s o c i a t i o n of ars e n o p y r i t e with p y r i t e i s of p a r t i c u l a r i n t e r e s t . This a s s o c i a t i o n 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 to be recovered f o r the production of s u l p h u r i c a c i d the presence of arse n o p y r i t e i s h i g h l y undesirable due to contamination of the 661 16.2 18 0.4 359 8.8 1041 25.6 1934 47.5 15 0.4 4028 98.9 25 0.6 13 0.3 <1 <0.1 38 0.9 <1 <0.1 1 <0. 1 6 0.2 4073 100.0 4 Fe orpiment realgar F i g u r e 1 M i n e r a l s i n the Fe-As-S System (600°C) I 5 Area A - G o l d w i t h minor s i l v e r A rea B - Bismuth and t e l l u r i u m A rea C - G o l d and antimony Area D, E - Bismuth F i g u r e 2 A r s e n o p y r i t e from Hedley, B.C. (3600x) PRIMARY CRUSHING Underground CRUSHING 3 Stogts PRIMARY GRINDING Boll Mills CLASSIFIERS T* ^ | SECONDARY FLOTATION ur SECONDARY GRINDING Boll MILLS K - i CYCLONES <jr I 1  PRIMARY FLOTATION - BULK SULPHIDE CONCENTRATE -TAILINGS<HSAND PLANT P S BACKFILL a J Z ! [CONCENTRATE STORAGE Thickening HOT C0TTRELL . >| CYCLONE I FLUOSOLIDS ROASTING 2 Slogei lit CALCINE WASH CYCLONES | [CARBON PLANT] CONDITIONING Thickening CRUDE A . 2 0 3 [2nd CALCINE WASH CYANIDATION LOADED CARBON Dried Shipped STRIPPING H THICKENING -(FILTRATION ARSENIC SUPPRESSION I Llmi ) pjlTHICKENING TAILINGS <-) ARSENIC SUPPRESSION (Lime) PREGNANT SOLUTION Storoge ILARIFIOATION IPRECIPITATTONI GOLD BULLION Figure 3 Giant Y e l l o w k n i f e Mines flowsheet 7 a c i d by a r s e n i c (11). In gold ores the gold values may be e n t i r e l y a s s o c i a t e d with e i t h e r m i n e r a l . The recovery or depression of ar s e n o p y r i t e during sulphide f l o t a t i o n i s of apparent i n t e r e s t . In the case of gold ores, the maximum recovery of ars e n o p y r i t e may be d e s i r a b l e to maximize the recovery of a s s o c i a t e d gold values. On the other hand, the maximum depression of ars e n o p y r i t e i s d e s i r a b l e during base metal production. Since fundamental i n v e s t i g a t i o n s of the aqueous surface chemistry of ar s e n o p y r i t e have p r e v i o u s l y not been undertaken, a comprehensive l i t e r a t u r e survey r e l a t i n g to t h i s mineral was c a r r i e d out. While c e r t a i n aspects of t h i s review may not have apparent s i g n i f i c a n c e to the f l o t a t i o n response of arsenopy r i t e they are b e l i e v e d to c o n t r i b u t e to the o v e r a l l understanding of i t s occurence and behaviour. 8 Chapter 2 LITERATURE REVIEW 2 . 1 State of the Art of Arsenopyrite F l o t a t i o n In reviewing the l i t e r a t u r e r e l a t i n g to the f l o t a t i o n of arse n o p y r i t e i t i s apparent that the separation of arsenopy r i t e from p y r i t e i s among the most s i g n i f i c a n t separations to be considered. Whether i t i s f o r a n a l y t i c a l purposes to determine whether a s s o c i a t e d gold i s with p y r i t e or ar s e n o p y r i t e or f o r the production of a pure p y r i t e product, free of a r s e n i c , the separation of these two minerals receives frequent mention. In d i s c u s s i n g the c h a r a c t e r i s t i c s of ar 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 , frequent reference w i l l be made to the behaviour of p y r i t e under the same c o n d i t i o n s . This comparison w i l l be u s e f u l since the depression of ar s e n o p y r i t e would obviously not be of any r e a l s i g n i f i c a n c e i f most other sulphide minerals would be depressed under the same c o n d i t i o n s . Consideration of the behaviour of p y r i t e t h e r e f o r e 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 for the depression of a r s e n o p y r i t e are excessive. Another b e n e f i t of comparing the response of p y r i t e to that of ar s e n o p y r i t e i s that while a r s e n o p y r i t e has received only very l i m i t e d study, 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 and Wark ( 1 2 ) showed the c r i t i c a l pH (above which f l o t a t i o n d i d not occur) for f l o t a t i o n of ars e n o p y r i t e 9 with 25 mg. per l i t r e of e t h y l xanthate to be pH=8.4 compared to a value of pH=l0.5 fo r p y r i t e . Mitrofanov (13) i n d i c a t e d the maximum recovery of ars e n o p y r i t e to be achieved i n the range of pH from 4 to 6. P l a k s i n (14) and l a t e r Glembotski et a l (15) discussed the in 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 the o x i d a t i o n of p y r i t e and ar s e n o p y r i t e . The p o s i t i o n of sulphur atoms i n the p y r i t e was considered to be such that they could i n t e r a c t with oxygen and produce s o l u b l e species which l e f t the surface as new p y r i t e , s t i l l able to react with reagents. The more complex s t r u c t u r e of arse n o p y r i t e was b e l i e v e d to r e s u l t i n slower o x i d a t i o n rates than for p y r i t e . Prolonged o x i d a t i o n was b e l i e v e d to r e s u l t i n decomposition of the ars e n o p y r i t e l a t t i c e with both sulphur and ar s e n i c being o x i d i z e d . The a r s e n i c oxide groups were p o s t u l a t e d to remain at the mineral's s u r f a c e . The d i f f e r e n t s u s c e p t i b i l i t y of the two minerals to 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 (17) to achieve a separ a t i o n . A bulk p y r i t e - ars e n o p y r i t e concentrate was produced and cond i t i o n e d with an o x i d i z i n g agent. Nekrasov used an a d d i t i o n of Mn02 followed by a two hour c o n d i t i o n i n g p e r i o d with a e r a t i o n to depress a r s e n o p y r i t e . Machovic used a d d i t i o n s of KMnO„ to depress a r s e n o p y r i t e while f l o a t i n g p y r i t e . S i m i l a r use of permanganate to depress ar 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 the subject of patents (18). Glembotski et a l . (15) r e l a t e d the greater oxygen concentration required for a r s e n o p y r i t e f l o t a t i o n compared to that required f o r p y r i t e f l o t a t i o n to t h e i r d i f f e r e n t 10 s u s c e p t i b i l i t y to o x i d a t i o n . Rand (19) provided an explanation fo r the d i f f e r e n t oxygen requirements which i s more i n keeping w i t h current f l o t a t i o n theory. He showed that p y r i t e has a greater oxygen reduction 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 concentration i n s o l u t i o n to provide the cathodic oxygen reduction r e a c t i o n required to balance the anodic o x i d a t i o n of xanthate. S i m i l a r r e s u l t s and i n t e r p r e t a t i o n s were presented by B i e g l e r et a l . (20). The use of magnesia mixture as a depressant for ars e n o p y r i t e has been proposed (21). The depressant mixture was prepared from magnesium c h l o r i d e , ammonium c h l o r i d e and ammonium hydroxide with d i s t i l l e d water. The mixture was found to give a high degree of arsenopy r i t e depression at pH values greater than 8. C h a l c o p y r i t e was a l s o found to be depressed by t h i s mixture i f the mineral was f i r s t c o n d i t i o n e d with A s l 3 . P y r i t e was found to be unaffected by the mixture. I t was concluded that depression r e s u l t e d from the formation of a s t r o n g l y h y d r o p h i l i c compound, MgNHflAsO(,6H20 on the surface of a r s e n o p y r i t e . No d i r e c t evidence to support t h i s c o n c l u s i o n was presented. 2.2 Nature of Adsorbed Xanthate Species Very l i m i t e d work has been reported i n the l i t e r a t u r e with regard to the nature of adsorbed xanthate species on ar s e n o p y r i t e . A l l i s o n et a l . (22) reported dixanthogen to be the r e a c t i o n 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 to be such that up to 5 percent of the minor product ( i . e . f e r r i c xanthate) could be present. Although a d d i t i o n a l i n v e s t i g a t i o n s i n t o the nature of the 11 r e a c t i o n products of ar s e n o p y r i t e with xanthate have not been reported, i t i s of i n t e r e s t to note the s i m i l a r i t i e s w ith p y r i t e . Both minerals were reported to have dixanthogen as the predominant c o l l e c t o r product and both have f e r r i c xanthate as the a l t e r n a t i v e c o l l e c t o r product (22,24). In the presence of oxygen, both minerals a l s o have mixed p o t e n t i a l s which are greater than the e q u i l i b r i u m p o t e n t i a l for xanthate o x i d a t i o n to dixanthogen. In s p i t e of the seeming consistency of these f i n d i n g s , controversy has continued as to the nature of the products formed by the i n t e r a c t i o n of p y r i t e with xanthate i n s o l u t i o n . Various i n v e s t i g a t o r s (23,24,25,26) have s t u d i e d the nature of the c o l l e c t o r species on p y r i t e . Each of these s t u d i e s has concluded the predominant surface product to be dixanthogen. The continued controversy appears to r e s u l t from the determination of some i n v e s t i g a t o r s to e s t a b l i s h a s i n g l e adsorbed product and to neglect the r o l e played by minor concentrations of other adsorbed species (27,28). The m a j o r i t y of researchers now be l i e v e that dixanthogen i s the a c t i v e c o l l e c t o r species i n p y r i t e f l o t a t i o n while at 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 to hydrophobicity can be made by f e r r i c hydroxyxanthate or by elemental sulphur r e s u l t i n g from mineral o x i d a t i o n . 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 adsorbed xanthate to dixanthogen can be represented by: X 2 + 2e" == 2X" (1) The corresponding cathodic r e a c t i o n i s g e n e r a l l y considered 12 to be the reduction of oxygen (23). 1/2 0 2 + H 20 + 2e" == 20H' (2) 0 2 + 4H + + 4e" == 2H 20 (3) The above r e a c t i o n s occuring v i a peroxide intermediate (23). In the i r o n system, oxidants other than oxygen are a l s o p o s s i b l e (24). 2 F e 3 + + 2X" == 2Fe 2 + + X 2 (4) 2Fe(OH) 3 + 6H + + 2X" == 2 F e 2 + + 6H 20 + X 2 (5) Although the preceding review was based on work with p y r i t e , i t appears c o n s i s t e n t to assume that the same conclusions can be made f o r the ars 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 ars e n o p y r i t e has been i n v e s t i g a t e d by Buerger (33,34) and Morimoto and Clark (36). Buerger determined the s t r u c t u r e to be monoclinic, c l o s e l y r e l a t e d to that of marcasite and l o e l l i n g i t e (34,35). Each i r o n atom i s surrounded by a d i s t o r t e d octahedron of which one face i s a t r i a n g l e of three a r s e n i c atoms and the other a t r i a n g l e of three sulphur atoms. The ar s e n o p y r i t e s t r u c t u r e i s complex and i s f u r t h e r 13 complicated by the f a c t that the mineral almost always occurs as twins. The e f f e c t of twinning i s to give the mineral a pseudo-orthorhombic symmetry. The s t r u c t u r e of ar s e n o p y r i t e as presented by Buerger i s shown i n Figure 4. I t can be seen that the s t r u c t u r e i s a body centered cubic s t r u c t u r e with regard to i r o n . Sulphur and a r s e n i c occur as As-S groups along c e l l edges. Buerger(33) explained observed v a r i a t i o n s i n interatomic spacing on the basis that i r o n i n p y r i t e i s i n the ferrous s t a t e while i r o n i n marcasite, l o e l l i n g i t e and ars e n o p y r i t e i s i n the f e r r i c s t a t e . Although Glembotski (15) and P l a k s i n (14) based t h e i r arguments f o r o x i d a t i o n of p y r i t e and ars e n o p y r i t e 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 sulphur atoms to i n t e r a c t i o n with oxygen, i t appears that s i g n i f i c a n t l y more complex c o n s i d e r a t i o n s are c o n t r o l l i n g i n t h i s regard. While both marcasite and ar s e n o p y r i t e are known to o x i d i z e more r e a d i l y than p y r i t e the r e s u l t s presented by those i n v e s t i g a t o r s i n d i c a t e that a r s e n o p y r i t e i s passivated during o x i d a t i o n while marcasite i s not (37). The v a r i a t i o n i n behaviour of the var i o u s minerals under o x i d i z i n g c o n d i t i o n s therefore appears to be c o n t r o l l e d by the nature of both sulphur and i r o n o x i d a t i o n products. 2.4 El e c t r o c h e m i c a l Oxidation of Arsenopyrite 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 ars e n o p y r i t e has received l i m i t e d study. Kostina and Chernyak (29,30,31) presented r e s u l t s IRON ARSENIC SULPHUR F i g u r e 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 ) ) 1 5 f o r o x i d a t i o n experiments c a r r i e d out with both p y r i t e and a r s e n o p y r i t e . The o b j e c t i v e of t h e i r work was to f i n d c o n d i t i o n s under which arsenop y r i t e would be r a p i d l y o x i d i z e d and thereby release any entrapped gold. They demonstrated that the o x i d a t i o n of both minerals occurred at lower p o t e n t i a l s as c o n d i t i o n s were changed from a c i d to 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 hydroxide) 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 given p o t e n t i a l . P y r i t e was found to o x i d i z e at a lower rate at a given 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 . I t was a l s o observed that the d i f f e r e n c e i n o x i d a t i o n rates increased with i n c r e a s i n g p o t e n t i a l (30). This c o n t r a d i c t s the assumptions made by Glembotski (15) as p r e v i o u s l y discussed i n s e c t i o n 2.1. A n a l y s i s of s o l u t i o n s f o l l o w i n g prolonged o x i d a t i o n i n c a u s t i c s o l u t i o n s showed that under moderate o x i d a t i o n c o n d i t i o n s , a r s e n i c and sulphur were leached from a r s e n o p y r i t e . Almost equal amounts of t r i - v a l e n t and penta-valent 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 varying 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 ars e n o p y r i t e e l e c t r o d e p o t e n t i a l as a f u n c t i o n of pH. Observed v a r i a t i o n s i n the Eh-pH r e l a t i o n s h i p were r e l a t e d to changes i n the nature of decomposition products. Under a c i d c o n d i t i o n s i r o n was concluded to go i n t o s o l u t i o n as ferrous while under n e u t r a l and a l k a l i n e c o n d i t i o n s i t went i n t o s o l u t i o n as f e r r i c . In the supergene zone of ore deposits the a l t e r a t i o n product of a r s e n o p y r i t e under a c i d i c c o n d i t i o n s were found to be symplesite (ferrous 16 arsenate) while under n e u t r a l or a l k a l i n e c o n d i t i o n s s c o r o d i t e ( f e r r i c arsenate) was formed. 2.5 Arsenopyrite Composition and Phase R e l a t i o n s The compositional v a r i a t i o n s of both s y n t h e t i c and n a t u r a l a r s e n o p y r i t e s have been considered i n s e v e r a l i n v e s t i g a t i o n s (38,36,39). Clark (38) i n studying phase r e l a t i o n s i n the Fe-As-S system observed that a r s e n o p y r i t e synthesized under varying c o n d i t i o n s of temperature and bulk composition deviated from the i d e a l FeAsS formula. X-ray d i f f r a c t i o n data were used to determine the d 1 3 1 spacing of a r s e n o p y r i t e synthesized i n v a r i o u s u n i v a r i a n t assemblages as a f u n c t i o n of temperature. I t was observed that while at any given temperature the v a r i a t i o n i n a r s e n o p y r i t e composition i s s m a l l , throughout the range of temperatures i n v e s t i g a t e d , the v a r i a t i o n i n composition i s s i g n i f i c a n t . Only a q u a l i t a t i v e r e l a t i o n was e s t a b l i s h e d however, since i n s u f f i c i e n t compositional data were a v a i l a b l e to q u a n t i f y the dependence of d 1 3 1 on the S/As r a t i o i n a r s e n o p y r i t e . In a d d i t i o n to compositional v a r i a t i o n s of a r s e n o p y r i t e , Clark presented data which show the l i m i t s of s t a b i l i t y of the various mineral assemblages. The maximum temperature of formation of a r s e n o p y r i t e was shown to be 702° ± 3°C. I f n a t i v e a r s e n i c c o e x i s t s with a r s e n o p y r i t e the d e p o s i t i o n temperature was below 688° ± 3°C. The maximum temperature of p y r i t e a r s e n o p y r i t e coexistence i s 491° ± 12°C. Morimoto and Clark (36) presented chemical analyses for 17 s i x t e e n n a t u r a l l y occuring a r s e n o p y r i t e s . From those analyses which were considered to be c o n s i s t e n t with the a s s o c i a t e d mineral assemblages they determined the composition of n a t u r a l l y o c c u r r i n g a r s e n o p y r i t e to vary from FeAso. 9S,., to F e A s ^ ^ o . j , . The m a j o r i t y of analyses were shown to be on the s u l f u r - r i c h side of the i d e a l FeAsS composition which was considered to be c o n s i s t e n t with the predominance of sulphide - type over arsenide - type mineral d e p o s i t s . Kretschmar and Scott (39) made a f u r t h e r c o n t r i b u t i o n to determining the composition 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 to a review of published s t u d i e s on a r s e n o p y r i t e composition they presented d e t a i l e d analyses for 31 synthesized and 54 n a t u r a l l y o c c u r r i n g a r s e n o p y r i t e s . Analyses were c a r r i e d out by means of e l e c t r o n microprobe determinations. The r e l a t i o n between atomic % a r s e n i c and the d , 3 1 X-ray peak p o s i t i o n was determined to be: As = 866.67d, 3, - 1381.12 with an estimated standard d e v i a t i o n of ±0.45% As. A temperature - composition s e c t i o n f o r a r s e n o p y r i t e which was prepared (39) i s shown i n Figure 5. The data used to c o n s t r u c t the diagram were obtained by s y n t h e s i z i n g a r s e n o p y r i t e at v a r y i n g temperatures and e q u i l i b r i u m assemblages. The diagram i l l u s t r a t e s the range of compositions for a r s e n o p y r i t e depending on the c o n d i t i o n s of formation. The diagram i s a l s o u s e f u l i n the converse sense i n that i f an a r s e n o p y r i t e i s a c c u r a t e l y analyzed and i t s e q u i l i b r i u m assemblage i s determined then the temperature of formation can be e s t a b l i s h e d . 18 32 33 ' 34 35 36 37 Atomic % Arsen ic in Arsenopyri te Figure 5 Atomic % arsenic i n arsenopyrite 19 Arsenopyrite 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 Fe d e f i c i e n c y . Morimoto and Clark (36) and Kretschmar and Scott (39) determined the Fe d e f i c i e n c y to be l e s s than 0.7 atomic %. The l i m i t s of a r s e n o p y r i t e composition presented by Kretschmar and Scott are FeAso.gS,,, to F e A s 1 < 5 S 0 . 8 5 corresponding to an approximate range i n atomic % As of 30% to 38%. Clark (38) and Kretschmar and Scott (39) observed that a r s e n o p y r i t e formed with a given composition does not r e a d i l y r e - e q u i l i b r a t e i f subjected to a new set of c o n d i t i o n s . Arsenopyrite present i n h i g h l y metamorphosed deposits w i l l t h e r e f o r e be c h a r a c t e r i s t i c of the i n i t i a l c o n d i t i o n s of formation of the d e p o s i t . N a t u r a l a r s e n o p y r i t e s are almost always c o m p o s i t i o n a l l y zoned (39). In S - r i c h assemblages the centre of c r y s t a l s are S-r i c h r e l a t i v e to the rims while i n A s - r i c h assemblages the centres are A s - r i c h r e l a t i v e to the rims. 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 Arsenopyrite Arsenopyrite i s a semiconducting mineral with a band gap of about 0.2 eV (45). Shuey summarized the mineral as having a low 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 high c a r r i e r c o n c e n t r a t i o n . R e s i s t i v i t y values reported by s e v e r a l i n v e s t i g a t o r s are presented i n Table 3. Arsenopyrite near s t o i c h i o m e t r i c composition i s an i n t r i n s i c semiconductor at room temperature. Arsenic 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 ars 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-type. Table 3 Arsenopyrite R e s i s t i v i t y Values 20 Source R e s i s t i v i t y , ohm-cm. 42 43 44 45 46 2.0 X 10" 3 -1.10 X 10" 2 -7.5 X 10" 2 -3.0 X TO"2 3.0 X 10" 2 6.0 X 10- 2 4.5 X 10" 1 1.5 X 10- 3 - 7.0 X 10- 1 Krasnikov (40) determined that a r s e n o p y r i t e i n a gold ore deposit v a r i e d from p-type i n high temperature, lower horizons to n-type i n lower temperature, upper horizons. Favorov (41) discussed the r e s u l t s of a s t a t i s t i c a l treatment 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 p y r i t e and a r s e n o p y r i t e . Arsenopyrite was found to vary from p-type f o r high temperature, low sulphur p a r t i a l pressure a s s o c i a t i o n s to n-type f o r decreasing temperature and i n c r e a s i n g sulphur p a r t i a l 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 on the f l o t a t i o n behaviour of ar s e n o p y r i t e must be considered. 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 g e n e r a l l y semiconductors. These minerals vary widely 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 oncentration and conductor-type (45). These varying 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 could be expected to i n f l u e n c e the adsorption 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 molecules at the mineral surfaces. P l a k s i n and Shafeev (46) were the f i r s t to study the nature of charge c a r r i e r s i n sulphide minerals ( i n p a r t i c u l a r galena) 21 and the e f f e c t of charge c a r r i e r type on xanthate adsorption They considered that the presence of free e l e c t r o n s i n the surface l a y e r of sulphides prevents the formation of adsorption bonds with xanthate. The a c t i o n of oxygen on galena was b e l i e v e d to r e s u l t from an i n v e r s i o n of the surface l a y e r from n-type to p-type. Subsequent i n v e s t i g a t o r s i n t h i s f i e l d proposed a l t e r n a t e r o l e s for oxygen and concluded that semiconductor p r o p e r t i e s were not c o n t r o l l i n g with regard to xanthate - mineral i n t e r a c t i o n (47,48,49). Recently, B i e g l e r (50) concluded that there was no obvious c o r r e l a t i o n between k i n e t i c parameters f o r oxygen reduction on p y r i t e and the nature of semiconduction. He f u r t h e r concluded that i f i m p u r i t i e s were the primary i n f l u e n c e on d i f f e r e n c e s i n e l e c t r o c h e m i c a l behaviour t h e i r i n f l u e n c e was not exerted through t h e i r e f f e c t s on the semiconducting p r o p e r t i e s . I t appears th e r e f o r e that while sulphide minerals are semiconductors and d i s p l a y a l l the e l e c t r i c a l p r o p e r t i e s a s s o c i a t e d with t h i s c l a s s of conductors, the f l o t a t i o n response of these sulphide minerals i s not c o n t r o l l e d p r i m a r i l y by these e l e c t r i c a l p r o p e r t i e s . Although the i n f l u e n c e of semiconducting p r o p e r t i e s 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 has not been reported i n the l i t e r a t u r e , a reasonable deduction can be made. The behaviour 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 for oxygen reduction has been shown (20) to be s i m i l a r to that of other sulphides such as p y r i t e and galena. I t i s therefore a reasonable assumption that the semiconducting p r o p e r t i e s of a r s e n o p y r i t e w i l l not be a p r i n c i p a l i n f l u e n c e on 22 e l e c t r o c h e m i c a l behaviour throughout the p o t e n t i a l range relevant to f l o t a t i o n of the mineral. 23 Chapter 3 OBJECTIVES OF THE PRESENT INVESTIGATION The o b j e c t i v e of the present research program was to study the surface chemistry of a r s e n o p y r i t e so that i t s f l o t a t i o n or depression could 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 of the l i t e r a t u r e revealed that the f l o t a t i o n response of a r s e n o p y r i t e had received only l i m i t e d study. Fundamental i n v e s t i g a t i o n s of the nature of surface 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 p r e v i o u s l y . Information obtained from the l i t e r a t u r e and p r e l i m i n a r y i n v e s t i g a t i o n s f o r the present research program i n d i c a t e d that surface o x i d a t i o n r e a c t i o n s could 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 adsorption of xanthate at the a r s e n o p y r i t e surface i n v o l v e s e l e c t r o n t r a n s f e r r e a c t i o n s . 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 the r e f o r e been a p r i n c i p a l research method for t h i s program. In p a r t i c u l a r , c y c l i c voltammetry of s t a t i o n a r y and 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 s has been used to study surface 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 (ESCA) has been used to augment these surface o x i d a t i o n s t u d i e s . This technique enables surface o x i d a t i o n f i l m s to be analyzed. In a d d i t i o n to v e r i f y i n g the formation of surface i r o n hydroxide, f i l m s , the behaviour of a r s e n i c and sulphur during the formation of these f i l m s was determined. The behaviour of a r s e n i c and sulphur could not be 24 resolved through the use of voltammetry alone. Since a r s e n o p y r i t e i s most commonly recovered i n f l o t a t i o n systems employing xanthate as the c o l l e c t o r , the i n f l u e n c e of the surface o x i d a t i o n of a r s e n o p y r i t e on i t s i n t e r a c t i o n with xanthate was a l s o considered. F l o t a t i o n experiments to evaluate the e f f e c t i v e n e s s of various o x i d a t i o n procedures on arsenop y r i t e have been c a r r i e d out. Arsenopyrite which had not been f l o a t e d p r e v i o u s l y with xanthate as w e l l as some which had been f l o a t e d was i n v e s t i g a t e d . Compositional v a r i a t i o n s of ar s e n o p y r i t e used for el e c t r o c h e m i c a l or f l o t a t i o n experiments have not been considered since these are not expected to c o n t r o l the o x i d a t i o n of the mineral or i t s i n t e r a c t i o n with xanthate, provided large amounts of i m p u r i t i e s are not present. 25 Chapter 4 ELECTROCHEMICAL STUDIES 4.1 Electrode P o t e n t i a l Measurements The p o t e n t i a l achieved i n the absence of o x i d i z i n g agents gives an i n d i c a t i o n of the tendency of the mineral to o x i d i z e i n the presence of a e r a t i o n (51). P o t e n t i a l s achieved i n the presence of o x i d i z i n g agents can be c o r r e l a t e d with r e s u l t s of el 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 to p r e d i c t r e l a t i v e o x i d a t i o n rates as w e l l as the nature of o x i d a t i o n products. Mixed p o t e n t i a l measurements were c a r r i e d out i n order to determine the p o t e n t i a l achieved by the mineral i n the absence of any w e l l defined redox couple and then to observe the e f f e c t s of various o x i d i z i n g agents on el e c t r o d e p o t e n t i a l . Measurements were c a r r i e d out across the range of pH values commonly encountered i n sulphide 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 p o t e n t i a l measurements were c a r r i e d out i n a s p h e r i c a l g l a s s c e l l c o n t a i n i n g 1.25 l i t r e s of s o l u t i o n at room temperature (25-28°C). Test s o l u t i o n s were prepared from s i n g l e d i s t i l l e d water with a standard a d d i t i o n of 0.1 molar KC1. The de s i r e d pH value f o r each t e s t was achieved through the use of 26 sodium t e t r a b o r a t e (0.05 M) with sodium hydroxide or s u l p h u r i c a c i d to r a i s e or lower the pH r e s p e c t i v e l y . S o l u t i o n s were deoxygenated by bubbling argon through them. Bubbling was continued throughout the t e s t and was the only source of s o l u t i o n s t i r r i n g . Traces of oxygen (3 to 5 ppm), present i n the argon were removed by passing the gas through a p y r o g a l l o l s o l u t i o n . P o t e n t i a l measurements were c a r r i e d out r e l a t i v e to a saturated calomel e l e c t r o d e and converted to the hydrogen scale by t a k i n g the p o t e n t i a l of the SCE to be -0.243 v o l t s r e l a t i v e to a hydrogen el e c t r o d e at 25°C. P o t e n t i a l s were measured with a Beckman E l e c t r o s c a n 30. A p o t e n t i a l having a d r i f t of l e s s than 2 mV over a ten minute pe r i o d was g e n e r a l l y achieved w i t h i n 5 minutes. The p o t e n t i a l was recorded a f t e r 20 minutes to ensure s t a b l e c o n d i t i o n s had been achieved i n each case. P o t e n t i a l s could be determined w i t h i n ±5 mV. The 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 given pH value was found to be ±20 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 f i r s t placed i n d i s t i l l e d water and was allowed a 20 minute per i o d to achieve a s t a b l e p o t e n t i a l . A f t e r t h i s p e r i o d the o x i d i z i n g agent was added to the s o l u t i o n and again a 20 minute per i o d was allowed before the p o t e n t i a l was recorded. E l e c t r o d e s were constructed using a r s e n o p y r i t e c r y s t a l s from two sources. One sample was obtained from Ward's S c i e n t i f i c and was i n d i c a t e d to come from P a r r a l , Chihuahua. This m a t e r i a l was analyzed as f o l l o w s : 27 Measured T h e o r e t i c a l As = 46.0 ±0.2% Fe = 34.0 ±1.0% S =19.8 ±0.1% 46.01 34.30 19.69 Co = not detected (<0.02%) Ni = not detected (<0.02%) Sb = not detected (<0.04%) Cu = not detected (<0.01%) Pb = 0.27% Zn = 0.06% This composition shows a s l i g h t i r o n d e f i c i e n c y compared to s t o i c h i o m e t r i c composition, as has g e n e r a l l y been noted for arsenopyr i t e (39). The composition a l s o shows a s l i g h t excess i n the sulphur to arsenic r a t i o and would be expected to be an n-type semiconductor. The e l e c t r o d e prepared from t h i s sample had an exposed area of approximately 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 the UBC Department of G e o l o g i c a l Sciences and came from Ri o n d e l , B.C. There was i n s u f f i c i e n t sample to c a r r y out a chemical a n a l y s i s of t h i s m a t e r i a l but a comparative a n a l y s i s to the sample from Mexico was c a r r i e d out by means of a scanning e l e c t r o n microscope equipped with an energy d i s p e r s i v e analyzer (SEM-EDX). The 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 sulphur peaks as the Mexican m a t e r i a l and d i d not e x h i b i t any peaks i n d i c a t i v e of excessive concentrations of any i m p u r i t i e s . The e l e c t r o d e prepared from t h i s m a t e r i a l had an exposed area of approximately 0.74 cm 2. The semiconductor type of each m a t e r i a l was determined by means of a simple determination employing the Seebeck e f f e c t . A m i l l i v o l t m e t e r was used to measure the p o t e n t i a l across a hot probe and a c o l d probe i n contact with the a r s e n o p y r i t e c r y s t a l . 2 8 In each case the hot j u n c t i o n was found to be p o s i t i v e r e l a t i v e to 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 to be n-type semi-conductors . A p y r i t e e l e c t r o d e was prepared using m a t e r i a l from Hanaoka, Japan. The p o l i s h e d e l e c t r o d e was examined m i c r o s c o p i c a l l y and was determined t o be free from i n c l u s i o n s of other sulphide minerals. 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 Figure 6. A s e c t i o n of the mineral was mounted i n epoxy c o n s i s t i n g of one part d i e t h y l e n e triamine to 10 p a r t s Epon 828. Immediately a f t e r p l a c i n g the l i q u i d epoxy on the sample i t was placed i n a vacuum to e l i m i n a t e a i r bubbles and to enable the epoxy t o s e a l cracks i n the sample. Both sides of the mounted s e c t i o n were ground down to expose the mi n e r a l . One side then had a t h i n l a y e r of gold vacuum deposited i n two areas. A copper wire was fastened to each area with s i l v e r conducting cement. A second epoxy s e c t i o n was prepared with a glass tube i n s e r t e d through one s i d e . The wires from the mineral sample were i n s e r t e d through the g l a s s tube and the two s e c t i o n s were glued together using a f a s t s e t t i n g epoxy. P r i o r to each experiment the s e c t i o n was wet ground on 600 mesh paper and then r i n s e d with d i s t i l l e d water before being i n s e r t e d i n t o the t e s t s o l u t i o n . 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 are used i n in d u s t r y to c a r r y out r e a c t i o n s ranging from pulp bleaching to e f f l u e n t c o n t r o l . 29 WIRE //->EADS t G L A S S TUBE GOLD PLATING SILVER CEMENT Method of Figure 6 e l e c t r o d e c o n s t r u c t i o n 30 Only l i m i t e d use i s made of o x i d i z i n g agents i n the mineral processing- i n d u s t r y . These are used p r i m a r i l y for a l k a l i n e c h l o r i n a t i o n of cyanide s o l u t i o n s and as oxidants f o r the leachi n g of uranium ore (54). Use has a l s o been made of potassium permanganate to increase the o x i d a t i o n of p y r r h o t i t e during g r i n d i n g and thereby decrease i t s f l o a t a b i l i t y (52). In order for an o x i d i z i n g agent to be acceptable for use i n c o n t r o l l i n g the f l o t a t i o n response of minerals i t must meet se v e r a l c r i t e r i a i n a d d i t i o n to being an e f f e c t i v e oxidant. Among these c r i t e r i a are that i t must be cost c o m p e t i t i v e , must not be exceedingly d i f f i c u l t or dangerous to handle and mustnnot r e s u l t i n p o l l u t i o n problems. The o x i d i z i n g agents used i n the present study were s e l e c t e d on the basis of e i t h e r already having some a p p l i c a t i o n i n processing p l a n t s (NaCIO, KMn04) or being considered to be a no n - p o l l u t i n g , cost competitive reagent ( H 2 0 2 ) . While a d d i t i o n a l o x i d i z i n g agents such as c h l o r i n e or Caro's a c i d (H 2S0 5) could a l s o be app r o p r i a t e , the range of mixed p o t e n t i a l s achieved by the present s e r i e s of agents was found to cover the range of 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 experiments described i n subsequent s e c t i o n s . The current (1982) cost s f o r the reagents used i n the study are shown i n Table 4. Table 4 a l s o shows the a v a i l a b l e o x i d i z i n g u n i t s per kilogram. These u n i t s are the product of the number of moles of oxidant per kilogram times the number of e l e c t r o n s t r a n s f e r r e d assuming a l k a l i n e c o n d i t i o n s . The v a r i a t i o n i n o x i d i z i n g power of these reagents can be determined 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 COST OXIDIZING AGENT $/kg. Oxidant u n i t s / k g Hydrogen Peroxide (50% S o l u t i o n ) 1.10 - 1.32 58.8 Potassium Permanganate 3.75 - 4.75 25.3 Sodium h y p o c h l o r i t e (12% S o l u t i o n ) 3.48 - 5.61 26.8 diagrams g r a p h i c a l l y present the v o l t equivalent of a compound or ion 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 v o l t equivalent i s the product of the o x i d a t i o n s t a t e and the redox p o t e n t i a l r e l a t i v e to the element i n i t s standard s t a t e and i s r e l a t e d to the free energy f o r the species according to AG = -nFE. The gradient 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 represented by the p o i n t s . A large p o s i t i v e gradient represents a strong o x i d i z i n g couple and a large negative g r a d i e n t , a strong reducing couple. Diagrams f o r the c h l o r i n e , manganese and oxygen-water systems are shown i n Figures 7,8 and 9. The diagrams include both a c i d ( a o H + = l ) and a l k a l i n e ( a o M . = l ) c o n d i t i o n s . The v a r i a t i o n of the o x i d i z i n g power with pH i s an important c o n s i d e r a t i o n since f l o t a t i o n of sulphides i s g e n e r a l l y c a r r i e d out at pH values greater than pH=4 and more commonly i n the range of pH=8 to pH=11. Thus the diagram f o r manganese i n d i c a t e s that while i n a c i d media permanganate represents a strong o x i d i z i n g agent capable of being reduced to Mn + + ions, i n a l k a l i n e media a much lower gradient i s apparent and manganese oxides and hydroxides are formed. The use of Mn02 i n a c i d media could be expected to r e s u l t i n reasonable o x i d a t i o n while at high pH t h i s reagent would be i n e f f e c t i v e as an oxidant. This 32 could i n part e x p l a i n the two hour c o n d i t i o n i n g p e r i o d which Nekrasov (16) req u i r e d when using Mn0 2 f o r ar s e n o p y r i t e o x i d a t i o n . In the case of the c h l o r i d e system h y p o c h l o r i t e i s i n d i c a t e d to be a stronger o x i d i z i n g agent than c h l o r a t e . While the diagrams are u s e f u l f o r representing the o x i d a t i o n p o t e n t i a l of various reagents i t must be held i n mind that 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 f a c t c o n t r o l t h e i r r e l a t i v e e f f e c t i v e n e s s . 4.2 Results and Di s c u s s i o n P r e l i m i n a r y experiments with a platinum e l e c t r o d e i n deoxygenated d i s t i l l e d water gave the r e l a t i o n Eh = 850 - 62.3 pH ,mV r = -1.00 Since t h i s i s i n reasonable agreement with the r e l a t i o n s h i p Eh = 800 - 59 pH ,mV which has been developed elsewhere (55) the recording equipment was considered to be capable of p r o v i d i n g r e l i a b l e measurements. A v a r i a t i o n of up to 100 mV i n the i n t e r c e p t p o t e n t i a l can be expected because while the pH i s buffered and therefore a c c u r a t e l y c o n t r o l l e d , the p o t e n t i a l i s to some extent 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 may t h e r e f o r e deviate from thermodynamically der i v e d values. The experimental r e l a t i o n was achieved by varying the pH and then measuring the ele c t r o d e p o t e n t i a l without r e p o l i s h i n g or c l e a n i n g the ele c t r o d e between measurements. Slow k i n e t i c s with regard to e q u i l i b r a t i o n of the system could account for the d e v i a t i o n of both the i n t e r c e p t and 33 - 1 0 I 2 3 4 5 6 7 Oxidation state F i g u r e 7 Oxidation s t a t e diagram for the c h l o r i n e system 34 - 4 -1 1 1 1 I I i i 0 I 2 3 4 5 6 7 Oxidation state Figure 8 Oxidation s t a t e diagram f o r manganese system 35 Figure 9 Oxidation s t a t e diagram f o r the oxygen system 36 slope of the experimental r e l a t i o n from t h e o r e t i c a l values. Mixed p o t e n t i a l s measured with a r s e n o p y r i t e and p y r i t e e l e c t r o d e s are shown i n Figures 10 and 11. The p o t e n t i a l measured with a r s e n o p y r i t e i n deoxygenated water over the pH range from 3.8 to 11.2 gives the r e l a t i o n Eh = 586 - 48.2 pH ,mV r = -0.9976 At pH greater than 11.2 the p o t e n t i a l s f a l l below t h i s l i n e with i n c r e a s i n g d e v i a t i o n as the pH increases. At the same time an el e c t r o d e which was allowed to reach a s t a b l e p o t e n t i a l at pH greater than 11.2 was observed to develop a v i s i b l e surface oxide l a y e r . This surface deposit had a brown appearance when the e l e c t r o d e was allowed to dry. Another observation was that i f the pH of the s o l u t i o n was lowered a f t e r the 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 at pH greater than 11.2, the ele c t r o d e p o t e n t i a l followed along the same curve as for f r e s h l y p o l i s h e d e l e c t r o d e measurements. The p o i n t s obtained i n t h i s way are shown i n Figure 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 at pH greater than 11.2 i n d i c a t e s a r s e n o p y r i t e to be i n h e r e n t l y unstable i n t h i s region. The mineral decomposes, l e a v i n g a surface oxide l a y e r . Since the elec t r o d e p o t e n t i a l s i n the presence of t h i s v i s i b l e oxide f o l l o w the same r e l a t i o n as obtained f o r a f r e s h l y p o l i s h e d e l e c t r o d e , the same oxide f i l m must be present with the f r e s h e l e c t r o d e but i s too t h i n to be r e a d i l y observed. The composition of t h i s surface oxide w i l l be discussed i n the s e c t i o n on voltammetry. There are s e v e r a l d i f f e r e n c e s i n the behaviour of the p y r i t e e l e c t r o d e compared to the ar s e n o p y r i t e e l e c t r o d e . Over 37 J 1 I I I L 2 4 6 8 10 12 PH Q I n d i v i d u a l measurement 0 T i t r a t i o n p o i n t Figure 10 Eh versus pH f o r ars e n o p y r i t e Figure 11 Eh versus pH for p y r i t e 39 the pH range from 7 to 12.2 the p o t e n t i a l followed the r e l a t i o n Eh = 643 - 49.0 pH r = -0.993 i n d i c a t i n g p y r i t e to be a more noble mineral than a r s e n o p y r i t e . Unlike a r s e n o p y r i t e , p y r i t e shows no d e v i a t i o n from t h i s r e l a t i o n at high pH. The p o t e n t i a l of an e l e c t r o d e which had reached a s t a b l e p o t e n t i a l at pH = 12.2 followed the same Eh-pH r e l a t i o n down to pH = 3.4. Only a s l i g h t t a r n i s h was v i s i b l e f o r the p y r i t e e l e c t r o d e at pH = 12.2. P o t e n t i a l s measured with a clean p y r i t e e l e c t r o d e at pH l e s s than 7 f a l l below the l i n e represented by the above r e l a t i o n . S i m i l a r observations have been made for p y r i t e and other sulphides by other i n v e s t i g a t o r s (55). The d e v i a t i o n from the r e l a t i o n below pH = 7 was explained on the b a s i s that below t h i s pH the p a s s i v a t i n g oxide l a y e r was not forming and the e l e c t r o d e i s i n a c o r r o s i o n region. The concentration of each o x i d i z i n g agent which was used i n t e s t s i n v o l v i n g these a d d i t i o n s was based on the r e s u l t s of p r e l i m i n a r y f l o t a t i o n experiments. The f l o t a t i o n experiments had i n d i c a t e d that these concentrations of o x i d i z i n g agent were i n the range which could be required f o r a r s e n o p y r i t e depression. P o t e n t i a l measurements made wit h an a r s e n o p y r i t e e l e c t r o d e i n the presence of o x i d i z i n g agents are shown i n Figure 10. In the case of potassium c h l o r a t e the p o t e n t i a l s i n the region of pH from 7 to 10 were found to f a l l on the same l i n e as those obtained i n the absence of any o x i d i z i n g agents. The remaining p o t e n t i a l s f o l l o w the r e l a t i o n s : With 160 mg/1 Hydrogen Peroxide Eh = 688 - 46.9 pH r = -0.9886. 40 With 57 mg/1 Potassium Permanganate Eh = 663 - 34.1 pH r = -0.9533 With 54.3 mg/1 Sodium Hypochlorite Eh = 1037 - 82.9 pH r = -0.9966 A few measurements were c a r r i e d out with the p y r i t e e l e c t r o d e i n the presence of hydrogen peroxide and potassium permanganate. As i n the case of a r s e n o p y r i t e the p o t e n t i a l achieved w i t h p y r i t e i n the presence of permanganate i s higher than with, peroxide over the pH region of i n t e r e s t . 4.3 C y c l i c Voltammetry References c i t e d i n the l i t e r a t u r e review on a r s e n o p y r i t e f l o t a t i o n i n d i c a t e that the surface o x i d a t i o n of the mineral could play 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 response. The r e s u l t s of e l e c t r o d e measurements presented i n the previous s e c t i o n i n d i c a t e that surface oxide deposits are formed on a r s e n o p y r i t e at high pH or under moderate o x i d i z i n g c o n d i t i o n s . I t i s t h e r e f o r e of i n t e r e s t to determine the nature of these surface oxide deposits so that t h e i r formation and therefore 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 to enabling the i n t e r a c t i o n of e l e c t r o d e s with d i s s o l v e d e l e c t r o a c t i v e species such as xanthate to be s t u d i e d , provides a means fo r studying surface 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 c a r r y i n g out r e p e t i t i v e scans the i n f l u e n c e on surface changes of excursions to various p o t e n t i a l ranges can be observed. C y c l i c voltammetry has been used to study the o x i d a t i o n of 41 e t h y l xanthate and the i n f l u e n c e of f l o t a t i o n depressants on p y r i t e e l e c t r o d e s (23). The i n t e r a c t i o n of f l o t a t i o n reagents with (57) and the surface o x i d a t i o n (58) of galena has a l s o been studi e d by t h i s technique. E l e c t r o c h e m i c a l r e a c t i o n s i n v o l v i n g p a r t i c u l a r species i n the absence of chemical r e a c t i o n mechanisms have been studied using noble metal e l e c t r o d e s (23,49) and employing c y c l i c voltammetry. C y c l i c voltammetry i s s i m i l a r to l i n e a r sweep p o l a r i z a t i o n but with the added feature that the p o t e n t i a l v a r i a t i o n with time i s reversed at some point and returned to the s t a r t i n g p o t e n t i a l . In t h i s way both the o x i d a t i o n and reduction of e l e c t r o a c t i v e species at the e l e c t r o d e surface can be studied. For example, i f xanthate i s o x i d i z e d to dixanthogen at an e l e c t r o d e during an anodic p o t e n t i a l sweep and the sweep i s then reversed, the:reduction r e a c t i o n can be monitored. By measuring both anodic and cathodic c u r r e n t s , i t can be determined that the dixanthogen formed at the e l e c t r o d e remains at the surface and does not d i f f u s e i n t o the bulk s o l u t i o n . Several f i g u r e s which are e s s e n t i a l to the d i s c u s s i o n i n t h i s t h e s i s are included on a f o l d - out page as Appendix I I . Figure 1 i n Appendix II i s taken from reference 49 to i l l u s t r a t e the e s s e n t i a l features of a voltammogram. In the presence of xanthate an anodic p o t e n t i a l sweep s t a r t i n g at -0.4V shows e s s e n t i a l l y no current being passed u n t i l a p o t e n t i a l of 0.2V i s exceeded. Beyond t h i s p o t e n t i a l there i s a r a p i d r i s e i n current to a peak, followed by a gradual d e c l i n e . Once the p o t e n t i a l f a l l s below 0 V dixanthogen i s reduced, at which point a 42 cathodic peak i s observed. The r e v e r s i b l e p o t e n t i a l f o r the xanthate - dixanthogen couple i s 0.15 V at the c o n c e n t r a t i o n of xanthate used. The anodic peak can be a s s o c i a t e d with the o x i d a t i o n of xanthate to dixanthogen while the cathodic peak can be a s s o c i a t e d with the reverse r e a c t i o n . In the presence of xanthate, hydrogen and oxygen adsorption i s i n h i b i t e d and peaks a s s o c i a t e d with these species are t h e r e f o r e not observed. The height of the xanthate peaks would be increased by s t i r r i n g of the s o l u t i o n or by i n c r e a s i n g the xanthate c o n c e n t r a t i o n . Both of these a c t i o n s would increase the supply of xanthate to 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 general the scan speeds employed are greater than 1 mV/sec. At i n c r e a s i n g scan speeds the current peaks due to charge t r a n s f e r r e a c t i o n s are increased and may be more f u l l y s t u d i e d . For a f i x e d sweeprate and current s c a l e , increased peak height i s a s s o c i a t e d with an increased r e a c t i o n r a t e . An increase i n the area under a peak i n d i c a t e s an increase i n the r e a c t i o n products ( i . e . a greater t o t a l current i s passed). 4.3.1 Experimental Voltammetry was c a r r i e d out with both 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 . For s t a t i o n a r y e l e c t r o d e experiments the e l e c t r o d e s were the same ones as used for e l e c t r o d e p o t e n t i a l measurements. The r o t a t i n g e l e c t r o d e was constructed as shown i n Figure 12. An a r s e n o p y r i t e d i s c was prepared using a diamond 43 impregnated gla 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 Ri o n d e l , B.C. as was used f o r the s t a t i o n a r y e l e c t r o d e . One face of the d i s c was gold coated and t h i s face was fastened to the end of the brass rod which formed the centre of the e l e c t r o d e . The space between the ars e n o p y r i t e and the surrounding t e f l o n tube was f i l l e d i n three stages using a hypodermic syringe f i l l e d with epoxy. The e l e c t r o d e was equipped with a gold r i n g which was s i l v e r soldered to a brass sleeve leading to the ele c t r o d e connections. The remaining p h y s i c a l features and manner of connection was as f o r a standard Pine Instruments r o t a t i n g e l e c t r o d e (84). The measurement c i r c u i t employed f o r voltammetry i s shown in Figure 13. The p o t e n t i o s t a t used was a Wenking 68 TS10. C y c l i c p o t e n t i a l f u n c t i o n s were c o n t r o l l e d by means of a Princeton A p p l i e d Research Model 175 U n i v e r s a l programmer. Currents were recorded by means of a K e i t h l e y Model 171 d i g i t a l multimeter i n s e r i e s with the platinum counter e l e c t r o d e . Current - p o t e n t i a l scans were recorded with an E l e c t r o Instruments Model 520 XY recorder. Rotating experiments were c a r r i e d out by means of a Pine Instruments Model ASR 2 r o t a t o r . A l l 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 - loop problems. Tests were c a r r i e d out i n a c o n v e n t i a l H - c e l l w ith the working e l e c t r o d e compartment having a volume of 850 cc and being separated from the counter e l e c t r o d e by a f r i t t e d g l a s s d i s c . A l l t e s t s were c a r r i e d out at room temperature (22±2 °C). A standard a d d i t i o n of 0.1M potassium c h l o r i d e was made to each t e s t . The pH was adjusted by means of sodium t e t r a b o r a t e with EPOXY BRASS TEFLON SILVER CEMENT GOLD RING ARSENOPYRITE r, = 3-l75mm r2=3-99 mm r3=4-40 mm Figure 12 Construction 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 45 sodium hydroxide or s u l p h u r i c a c i d . A l l reagents used for preparation 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 ions were reagent or a n a l y t i c a l grade. 4.3.2 Results and Discussion A. S i n g l e Sweep Voltammograms By e a r r i n g out voltammetry across a wide range of pH, v a r i a t i o n s i n both anodic and cathodic peak p o s i t i o n s can be observed. By noting both peak p o s i t i o n s and s h i f t s i n peak p o s i t i o n with pH, p o s s i b l e elecrochemical surface r e a c t i o n s a s s o c i a t e d with these peaks can be p o s t u l a t e d . R e v e r s i b l e p o t e n t i a l s for po s t u l a t e d r e a c t i o n s can be c a l c u l a t e d and c o r r e l a t e d with 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 to arsenop y r i t e across the range of pH = 5.85 to pH = 11.95 are shown i n Figures 14 and 15. Between pH = 6.6 and 7.3 an anodic peak becomes apparent i n the region of 0.6 to 0.7 v o l t s . This peak becomes more prominent and s h i f t s to more cathodic p o t e n t i a l s with i n c r e a s i n g pH according to 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 the rest p o t e n t i a l of a r s e n o p y r i t e . A d d i t i o n a l anodic scans were c a r r i e d out s t a r t i n g from the r e s t p o t e n t i a l to ensure that the anodic peak i s not a s s o c i a t e d with products r e s u l t i n g from cathodic c u r r e n t s observed at the s t a r t of the scans. Anodic peaks for 46 PROGRAMMER POTENTIOSTAT CONTROL* W •• R •• C GROUND* ROTATOR RECORD E REF C WE MULTIMETER =:RECORD I COMMON Figure 13 Co 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 at the r e s t p o t e n t i a l were found to occur at the same p o t e n t i a l and to be of the same magnitude as those f o r scans s t a r t i n g at the lower p o t e n t i a l s . The anodic peak l i e s i n the p o t e n t i a l region which i s relevant to f l o t a t i o n systems. The e l e c t r o d e p o t e n t i a l s which were measured i n the presence of hydrogen peroxide and permanganate are i n d i c a t e d i n Figure 16 and Figure 2, Appendix I I , on a voltammogram obtained at pH = 8.2. The p o t e n t i a l s achieved w i t h these o x i d i z i n g agents should r e s u l t i n the r e a c t i o n a s s o c i a t e d with the anodic peak proceeding at a s i g n i f i c a n t r a t e . Since a KC1 supporting e l e c t r o l y t e was used and good connections to 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 of the e l e c t r o d e m a t e r i a l i s low ( i e . 10" 2 ohm - cm) peak p o t e n t i a l s are b e l i e v e d to be accurate even at the r e l a t i v e l y high c u r r e n t s (800 microamperes) observed f o r some scans. The r e p e a t a b i l i t y of scans was found to be w i t h i n 10 m i l l i v o l t s and was 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 to determine a c c u r a t e l y the peak p o s i t i o n on the chart recorder. Peak currents were found to be s e n s i t i v e to the c o n d i t i o n of the g r i n d i n g paper used to prepare the e l e c t r o d e s u r f a c e . Provided a f r e s h area of g r i n d i n g paper was used to f i n i s h the e l e c t r o d e p r e p a r a t i o n , c u r r e n t s could be reproduced to w i t h i n 20 microamperes. A small anodic prewave becomes apparent at approximately -0.1 V at pH = 9.6 i n Figure 14. This prewave becomes more apparent over the pH range from 9.6 to 10.55 and then decreases over the range of pH 11.05 to 11.95. The peak p o s i t i o n s h i f t s to pH 5-85 pH 6-6 pH 7-3 pH 9-2 pH 9*6 I I I -0-8 -0-6 -0-4 -0-2 0 0-2 0-4 0-6 0-8 Potential / V vs SHE Figure 14 Voltammograms for ars e n o p y r i t e at i n c r e a s i n g pH values 49 I 1 1 I I I I I -0-8 -0-6 -0-4 -0-2 0 0-2 0-4 0-6 Potential / V vs SHE Figure 15 Voltammograms f o r ar s e n o p y r i t e at high pH 50 Figure 16 Voltammogram f o r ars e n o p y r i t e at pH = 8.2 showing p o t e n t i a l s achieved with o x i d i z i n g agents 51 more cathodic p o t e n t i a l s with i n c r e a s i n g 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 with t h i n o x i d a t i o n l a y e r s (79). In the present study the peak i s b e l i e v e d to r e s u l t from the presence of i r o n hydroxide f i l m s formed during e l e c t r o d e preparation as w i l l subsequently be discussed. The behaviour of a r s e n o p y r i t e and p y r i t e across the p o t e n t i a l region of i n t e r e s t i s compared i n Figure 17. The o x i d a t i o n of ar s e n o p y r i t e i s observed to commence at p o t e n t i a l s approximately 200 mV lower that those required for p y r i t e o x i d a t i o n . The main p y r i t e o x i d a t i o n r e a c t i o n becomes s i g n i f i c a n t at p o t e n t i a l s greater than that of the arsenop y r i t e o x i d a t i o n peak. This d i f f e r e n c e i n the behaviour of these two minerals f u r t h e r i n d i c a t e s that the anodic peak observed f o r arsenopy r i t e could be s i g n i f i c a n t i n attempting to c o n t r o l the s e l e c t i v e f l o t a t i o n or depression of a r s e n o p y r i t e . P o s s i b l e r e a c t i o n s a s s o c i a t e d with the o x i d a t i o n of ars e n o p y r i t e can be determined by c o n s i d e r i n g the s t a b i l i t y diagram for a r s e n o p y r i t e shown i n Figure 18. Thermodynamic data used to con s t r u c t the diagram and a d d i t i o n a l s t a b i l i t y c o n s i d e r a t i o n s f o r a r s e n o p y r i t e are presented i n Appendix I . These diagrams were constructed as part of the present study. At pH = 4 to pH = 7 1/2 A s 2 S 3 + F e + + + H + + 3e" = FeAsS + 1/2 H 2S (6) E = - 0.012 - 0.0295 lo g ( H 2 S ) 1 / 2 - 0.0197 pH (Fe + +) or i f o x i d a t i o n to form H 3As0 3 and SO*,"" i s considered 52 — ' " 1 1 1 u 0-4 -0-2 0 0-2 0-4 0-6 Potential / V vs SHE 20 mV s-' , pH= 110 Figure 17 Voltammograms for p y r i t e and ar s e n o p y r i t e at pH = 11. 53 F e + + + H 3As0 3 + SOa-- + 11H+ + 11e" = FeAsS + 7H 20 (7) E = 0.283 + 0.005 l o g ( F e + + ) ( H 3 A s 0 3 ) ( S O , " " ) - 0.0591 pH At pH greater than 7 Fe(OH) 2 + H 3As0 3 + SO,,"- + 13H+ + 1 1 e" = FeAsS+9H20 (8) E = 0.346 + 0.005 l o g ( H 3 A s 0 3 ) ( S 0 4 " " ) - 0.070 pH The appearance of the anodic peak at 0.6 V between pH = 6.6 and 7.3 can be r e l a t e d to the formation of Fe(OH) 2 at pH greater than approximately 7. The exact pH at which Fe(OH) 2 becomes st a b l e r e l a t i v e to F e + + depends on the a c t i v i t y of Fe at the surface of the e l e c t r o d e . The observed anodic peak or h a l f wave p o t e n t i a l s are considerably anodic to 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 from the above equations. Assuming a c t i v i t i e s of 10" 3 M f o r example equation 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 s h i f t i n peak p o t e n t i a l may be due to i r r e v e r s i b i l i t y of the r e a c t i o n , to the formation of a passive layer or to both. At the observed peak p o t e n t i a l s the i r o n hydroxide which should be formed i s Fe(OH) 3 rather than the Fe(OH) 2 i n d i c a t e d by equation 8. The r e v e r s i b l e p o t e n t i a l for Fe(OH) 2 to Fe(OH) 3 o x i d a t i o n i s given by Fe(OH) 3 + H + + e" = Fe(OH) 2 + H 20 (9) E = 0.28 - 0.059 pH from which a value of E = -0.342 V i s obtained at pH = 10.55. I t can be seen i n Figure 18 that at the observed peak 54 Figure 18 Eh - pH diagram for a r s e n o p y r i t e . A c t i v i t y for each species taken to be 10" 3 M. 55 p o t e n t i a l s a r s e n i c i s expected to be o x i d i z e d to arsenate and sulphur i s expected to be o x i d i z e d to sulphate. The formation of Fe(OH) 3 during the anodic process i s 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 features of the voltammograms shown i n Figures 14 and 15. A cathodic p o t e n t i a l sweep s t a r t i n g near the r e s t p o t e n t i a l i s shown at pH = 11.95. The cathodic peaks for the complete p o t e n t i a l sweeps show s i g n i f i c a t l y greater c u r r e n t s than those for cathodic sweeps only. The increased cathodic peak current f o r the complete sweeps i n d i c a t e s that species formed during the anodic part of the p o t e n t i a l c y c l e are being reduced. These cathodic peaks occur at p o t e n t i a l s c o n s i s t e n t w i t h equation 9 and can be i n t e r p r e t e d to represent the reduction of Fe(OH) 3 formed during the o x i d a t i o n r e a c t i o n . The cathodic peak at pH greater than 11.0 i n Figure 15 i s a c t u a l l y a double peak. This could represent a two step reduction of Fe(OH) 3 or c o u l d i n d i c a t e that a d d i t i o n a l species such as sulphur are being reduced. Voltammograms at lower pH and p a r t i c u l a r l y at pH = 5.85 i n Figure 14 show a prominent cathodic peak although no anodic peak a s s o c i a t e d with i r o n hydroxide formation i s evident. I t i s b e l i e v e d that t h i s cathodic peak r e s u l t s from the formation of sulphur or a sulphur compound during the anodic sweep. The small prewave observed during the anodic scan r e s u l t s from the o x i d a t i o n of a small amount of Fe(OH) 2 formed on the e l e c t r o d e during e l e c t r o d e p o l i s h i n g and preparation as discussed i n Chapter 5. Figure 19 shows a current - decay curve for an a r s e n o p y r i t e 56 e l e c t r o d e which was stepped.from the r e s t p o t e n t i a l to +343 mV at pH = 10.6. This p o t e n t i a l represents the anodic peak p o t e n t i a l at t h i s pH. The current decays from the i n i t i a l value of s e v e r a l thousand microamps to l e s s than 100 microamps i n l e s s than one minute. Such a current decay i s a s s o c i a t e d with the build-up of o x i d a t i o n products at the surface of the e l e c t r o d e . The presence of these o x i d a t i o n products at the surface of the ele c t r o d e p a s s i v a t e s i t , making fu 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 d i f f i c u l t . This r a p i d decay of current confirms that the o x i d a t i o n of arsenopy r i t e r e s u l t s i n s o l i d products which remain at the electro d e surface. An estimate was made of the qu a n t i t y of i r o n hydroxide formed on the surface during an anodic scan. The area under the anodic peak at pH = 11.55 was determined i n order to determine the current passed. From the s t a r t of the o x i d a t i o n peak to the i n f l e c t i o n i n the curve j u s t past the peak, a current of approximately 11700 micro coulombs was passed. Using equation 8, which shows 11 e l e c t r o n s per mole of ar s e n o p y r i t e o x i d i z e d , together with equation 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 for every mole of Fe(OH) 2 o x i d i z e d to Fe(OH) 3, the q u a n t i t y of FeAsS which i s o x i d i z e d during the scan i s 0.102 X 1 0 - 7 mole. Assuming a stochiometric composition, t h i s means that the same amount of i r o n i s o x i d i z e d to the f e r r i c s t a t e . Using the pr o j e c t e d area of the electro d e of 28.3 sq. mm. and assuming a s o l i d product of F e 2 0 3 with a d e n s i t y of 5 (60) an oxide t h i c k n e s s of 40 A° i s determined. This i s an order of magnitude estimate only since the s t r u c t u r e could d i f f e r from the assumed one and since the r e a l surface area can be expected to be s e v e r a l times greater 57 than the p r o j e c t e d area. Taking i n t o account the r e a l surface area could r e s u l t i n an estimate of the oxide thickness as low as 10 A 0. A l e s s dense i r o n hydroxide s t r u c t u r e than has been assumed i n the c a l c u l a t i o n on the other hand could r e s u l t i n the a c t u a l oxide thickness being s e v e r a l times the c a l c u l a t e d value. B. M u l t i p l e Sweep ( C y c l i c ) Voltammetry By a l l o w i n g the t r i a n g u l a r p o t e n t i a l c y c l e to repeat a number of times, changes with time i n the surface composition of the e l e c t r o d e can be detected. In t h i s way the build-up of surface oxides or other e l e c t r o a c t i v e species can be detected. C y c l i c voltammograms f o r s t a t i o n a r y and 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 s at pH = 10.6 are shown i n Figure 20. The anodic peak at 0.35 V i s observed to decrease with continued c y c l i n g while the i n i t i a l l y small anodic peak at -0.025 V and the cathodic peak at about -0.4 V s t e a d i l y increase i n height. These observations are c o n s i s t e n t with the formation of Fe(OH) 3 during the anodic decomposition peak. The o x i d a t i o n peak at 0.35 V decreases with each subsequent c y c l e due to the el e c t r o d e becoming passivated by the build-up of surface hydroxide. At the same time the cathodic peak increases i n height since there i s an i n c r e a s i n g amount of hydroxide to be reduced from Fe(OH) 3 to Fe(OH) 2. The i n c r e a s i n g amount of Fe(OH) 2 thus formed r e s u l t s i n the anodic peak at -0.25 V i n c r e a s i n g i n height with subsequent c y c l e s . S i m i l a r peaks and changes wi t h c y c l i n g were obtained with 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 l o c a l i t i e s . CVJ O CVJ io o 10 o — — 6 V U J / ju a J J n 3 Figure 19 Current - decay curve for a r s e n o p y r i t e at +0.0343 V. 59 1 -0.8 -0.6 0.4 -0.4 -0.2 0 0.2-Potentiol / V vs SHE 20 mV s - ' , pH = l0.6 Figure 20 M u l t i p l e sweep voltammograms for 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 at pH = 10.6 0.6 60 The 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 that the o x i d a t i o n peaks at 0.35 i n Figure 20 are increased i n height. At the same time the anodic peak at - 0.25 V becomes narrower and an a d d i t i o n a l anodic peak at -0.35 V becomes evident with continued c y c l i n g . This a d d i t i o n a l anodic peak suggests that the Fe(OH) 2/Fe(OH) 3 redox r e a c t i o n r e s u l t s i n the formation of a d d i t i o n a l i r o n species than these hydroxides. The e f f e c t of r o t a t i o n i s discussed 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 r e a c t i o n mechanism. C y c l i c voltammograms at pH = 11.7 are shown i n Figure 21 and Figure 3, Appendix I I . The v e r t i c a l s c a l e i n t h i s f i g u r e represents greater c u r r e n t s than that of Figure 20. As f o r the r e s u l t s obtained at pH = 10.6 the 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 to increase the c u r r e n t s a s s o c i a t e d with the o x i d a t i o n peak at 0.35 V, to r e s u l t i n the anodic peak at - 0.35 V becoming narrower and, the formation of a d d i t i o n a l anodic and cathodic peaks i n the region of i r o n hydroxide o x i d a t i o n and r e d u c t i o n . These a d d i t i o n a l peaks at pH = 11.7 are much more apparent than those which r e s u l t from s t i r r i n g at lower pH. The reason why these peaks are more apparent at high pH i s explained l a t e r i n t h i s chapter. An a d d i t i o n a l feature of the voltammogram fo r the r o t a t i n g e l e c t r o d e at pH = 11.7 i s that while the o x i d a t i o n peak at 0.35 V decreases from the f i r s t to the second c y c l e , i t increases on subsequent c y c l e s . For each of the t e s t s shown i n Figures 20 and 21, the e l e c t r o d e became covered with a v i s i b l e brown oxide during the m u l t i p l e p o t e n t i a l c y c l e s . This oxide l a y e r became i r r i d e s c e n t 61 Figure 21 M u l t i p l e sweep voltammograms for 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 at pH = 11.7 62 i n c o l o r , v a r y i n g from blue at low pH to yellow - green- at high pH. Such c o l o r s have been shown to r e l a t e to i r o n oxide l a y e r s i n the range of 200 to 400 angstroms i n thickness (60). Since the peaks have been r e l a t e d to redox r e a c t i o n s i n v o l v i n g i r o n species, a r e a c t i o n sequence from Fe to i r o n hydroxides w i l l be considered. 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 (61). In the f o l l o w i n g equations brackets denote r e a c t i o n intermediates whose surface coverage i s of the order of a f r a c t i o n of a monolayer and braces i n d i c a t e species e v e n t u a l l y r e l a t e d to the formation of new phases and which may undergo ageing (61). [Fe(OH)]ad + e = Fe + OH" (10) [Fe(OH)] +ad + e = [Fe(OH)]ad (11) {Fe(OH) 2} = [Fe(OH)] +ad + OH" (12) HFe0 2" + H 20 = {Fe(OH) 2} + OH" (13) F e 0 2 2 " + H 20 = HFe0 2" + OH" (14) {FeOOH} + H 20 + e = {Fe(OH) 2} + OH" (15) { F e 2 0 3 • H 20} = {FeOOH} + {FeOOH} (16) 63 The above equations are w r i t t e n with FeOOH as the o x i d i z e d hydroxide while the d i s c u s s i o n so f a r has considered Fe(OH) 3. The p r e c i s e form present on the electro d e cannot be determined from r e s u l t s of the present i n v e s t i g a t i o n . Throughout the l i t e r a t u r e r e l a t i n g to i r o n e l e c t r o d e s the p r e f e r r e d s t r u c t u r e i s FeOOH. The two forms w i l l be used interchangeably as r e f e r r i n g to f e r r i c hydroxide. In c o n s i d e r i n g the r e a c t i o n sequence during a p o t e n t i a l c y c l e from the cathodic l i m i t to the anodic l i m i t and back to the s t a r t i n g p o i n t , the r e a c t i o n s would occur i n the f o l l o w i n g order as shown i n Figure 21 and i n Figure 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 the o x i d a t i o n of a r s e n o p y r i t e to form FeOOH. The o x i d a t i o n i s as s o c i a t e d with peak I I I . At the same time a r s e n i c i s released as arsenate ions and sulphate i s released as sulphate ions. Arsenate i s e l e c t r o i n a c t i v e and ther e f o r e does not p a r t i c i p a t e i n r e a c t i o n s a s s o c i a t e d with the remaining peaks. This w i l l be more f u l l y discussed i n s e c t i o n 4.3.2C. Based on the r e s u l t s of ESCA experiments presented i n Chapter 5, i t i s b e l i e v e d that arsenate p r e c i p i t a t e s i n the f e r r i c hydroxide d e p o s i t s . The composition of the ars e n i c complex so formed i s not known. Sulphate i s a l s o e l e c t r o i n a c t i v e and i s b e l i e v e d to d i f f u s e slowly through the f e r r i c hydroxide l a y e r . 2. On the reduction c y c l e FeOOH i s reduced to Fe(OH) 2 according to r e a c t i o n 15. This r e a c t i o n i s as s o c i a t e d with peak IV. 3. A f u r t h e r reduction of the Fe(OH) 2 according to equations 64 12, 11 and 10 i s i n d i c a t e d by peak V. This peak has a somewhat greater current a s s o c i a t e d with i t than does peak IV because although some Fe(OH) 2 i s s o l u b i l i z e d according to r e a c t i o n s 13 and 14, equations 10 and 11 involve a two e l e c t r o n t r a n s f e r r e a c t i o n while equation 15 involves only one. 4. The second and successive anodic scans involve the conjugated r e a c t i o n to that a s s o c i a t e d with peak V as peak I, namely the formation of Fe(OH) 2 from reduced i r o n -hydroxy species. 5. The Fe(OH) 2 formed at peak I i s o x i d i z e d to FeOOH according to r e a c t i o n 15 at peak I I . As for peaks IV and V, peak I I i s again l e s s than peak I since only a one e l e c t r o n t r a n s f e r i s invo l v e d . 6. At high pH the hydroxide c o n c e n t r a t i o n i s such that a considerable amount of Fe(OH) 2 i s d i s s o l v e d according to rea c t i o n s 13 and 14, so that the current a s s o c i a t e d with peak I I I increases on subsequent scans. In the case of a s t a t i o n a r y e l e c t r o d e these products are not swept away and as a consequence the peak current becomes depressed with continued c y c l i n g due to a greater build-up of hydroxide at the su r f a c e . The i n f l u e n c e of ele 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 to the above r e a c t i o n sequence. Reaction 15 can be r e w r i t t e n i n terms of proton t r a n s f e r as f o l l o w s . FeOOH + H + + e = Fe(OH) 2 (17) and 65 H + + OH" = H 20 (18) The formation of f e r r i c hydroxide according to r e a c t i o n 17 then r e s u l t s i n formation of H + i n a d d i t i o n to FeOOH. The H + so formed i s n e u t r a l i z e d according to r e a c t i o n 18 e i t h e r by d i f f u s i o n of H + away from the r e a c t i o n s i t e due to concentration gradients or by d i f f u s i o n of OH" to the s i t e . Conditions which promote the d i f f u s i o n or tr a n s p o r t of these species should t h e r e f o r e increase the rate of FeOOH production. This i s i n f a c t observed since the e f f e c t of r o t a t i o n i s to increase the height of peak I I I . At the same time i n c r e a s i n g pH increases the peak height since a greater 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 + n e u t r a l i z a t i o n . Reaction 10 can s i m i l a r l y be w r i t t e n i n terms of proton t r a n s f e r and the same arguments as presented above e x p l a i n the appearance of peaks I and V with s t i r r i n g or i n c r e a s i n g pH. S i m i l a r arguments have been presented elsewhere (62) for the formation of n i c k e l hydroxide f i l m s according t o . NiOOH + H 20 + e = Ni(OH) 2 + OH" (19) or NiOOH + H + + e = Ni(OH) 2 (20) and H + + OH" = H 20 (21) To t h i s point apart from the ars e n o p y r i t e o x i d a t i o n peak (peak I I I ) no peaks have been assigned to a r s e n i c or sulphur 66 species. At the peak p o t e n t i a l s observed f o r the main o x i d a t i o n peak for arsenop y r i t e ( peak I I I ) a r s e n i c o x i d a t i o n to the arsenate s t a t e and sulphur o x i d a t i o n to the sulphate s t a t e are expected. The i r r e v e r s i b i l i t y of sulphate formation i s w e l l e s t a b l i s h e d and reduction peaks a s s o c i a t e d with t h i s species are not expected. In the case of p y r i t e and p y r r h o t i t e , elemental sulphur has been shown to be formed even at high pH and high o v e r p o t e n t i a l (63). The presence of sulphur was shown to r e s u l t i n the formation of FeS during continued c y c l i n g (63). Although the formation of sulphur during a r s e n o p y r i t e o x i d a t i o n i s a reasonable expectation there i s no evidence i n the present r e s u l t s that t h i s i s i n f a c t occuring i n the pH range where i r o n hydroxide f i l m s are formed since no peaks a s s o c i a t e d with FeS are observed. In a d d i t i o n to the formation of FeS, the presence of elemental sulphur could be expected to r e s u l t i n i n c r e a s i n g t h i o s u l p h a t e formation with i n c r e a s i n g pH according to S 2 0 3 2 - + 6H + + 4e = 2S + 3H 20 (22) E = + 0.26 + 0.015 l o g ( S 2 0 3 2 - ) - 0.089 pH 2SO a 2- + 10 H + + 8 e = S 2 0 3 2 - + 5 H 20 (23) E = + 0.04 - 0.007 l o g ( S 2 0 3 2 " ) - 0.074 pH ( s o , 2 - ) 2 The peaks a s s o c i a t e d with the above r e a c t i o n s would l i e w i t h i n the peaks assigned to i r o n species as would any peaks r e s u l t i n g from the reduction of t h i o s u l p h a t e to sulphide. The 6 7 e f f e c t of el e c t r o d e r o t a t i o n would be to reduce peaks a s s o c i a t e d with t h i o s u l p h a t e reduction and subsequent sulphide o x i d a t i o n . No peaks whose height decreases with e l e c t r o d e r o t a t i o n are observed and therefore t h i o s u l p h a t e and sulphur do not appear to be produced. The formation of arsenate during a r s e n o p y r i t e o x i d a t i o n i s a l s o i r r e v e r s i b l e and peaks a s s o c i a t e d with arsenate reduction are not expected. This w i l l be discussed more f u l l y i n the next s e c t i o n . The i r r e v e r s i b i l i t y of both sulphate and arsenate formation i n part accounts f o r the d e v i a t i o n of the observed peak p o t e n t i a l f o r ars e n o p y r i t e o x i d a t i o n from the c a l c u l a t e d r e v e r s i b l e value. Since both a r s e n i c and sulphur form s o l u b l e species the e f f e c t of s t i r r i n g or electro d e r o t a t i o n i s expected to be that the anodic peak height i n c r e a s e s , as i s observed. This observation i s therefore 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 three elements i n a r s e n o p y r i t e . Consideration has a l s o been given to the p o s s i b l i t y of f e r r i c arsenate p r e c i p i t a t i o n at the ele c t r o d e surface according to FeAsO„ + 3H + = H 3AsO, + F e 3 + (24) pH = 0.12 - 1/3 log(H 3AsO a) ( F e 3 + ) or FeAsO, + 3H 20 = Fe(OH) 3 + H2AsO„- + H pH = 5.30 + log(H2AsO„") (25) 68 At the pH values being considered i n the present work the formation of f e r r i c arsenate would not be expected. The minimum s o l u b i l i t y of f e r r i c arsenate l i e s at about pH = 2.2 and at greater pH i t decomposes to give Fe(OH) 3 and H 2AsO f t" according to r e a c t i o n 25 (64). The o x i d a t i o n of ar s e n o p y r i t e to form hydroxide r e s u l t s i n the release of H +. Equation 8 f o r instance i n d i c a t e s the formation of 13 moles of H + for every mole of FeAsS which i s o x i d i z e d . At the r a p i d sweep rates (20 mV/sec.) employed the pH at the e l e c t r o d e surface could t h e r e f o r e be s i g n i f i c a n t l y lower than the bulk s o l u t i o n pH. F e r r i c arsenate could form at the ele c t r o d e surface i n the presence of such high H + c o n c e n t r a t i o n . Voltammograms produced at decreasing pH values should have enhanced peaks due to the formation of FeAsOfl since the bulk pH would be approaching the pH generated at the e l e c t r o d e surface. Figure 22 shows a m u l t i p l e sweep voltammogram c a r r i e d out at pH = 5.8. The absence of the peaks which are observed at higher pH at -0.35V i n d i c a t e s that those peaks are not a s s o c i a t e d with FeAsO a formation. The cathodic 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 f o r FeAsS which are shown i n Figure 18. I t was recognized however that the a c t i v i t i e s of d i s s o l v e d species at the e l e c t r o d e could be d i f f e r e n t from those assumed i n c o n s t r u c t i o n of the diagram. Cathodic decomposition r e a c t i o n s could t h e r e f o r e be o c c u r r i n g i f the l i m i t s of c y c l i n g 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 mi n e r a l . Figure 23 shows voltammograms produced with cathodic l i m i t s of -0.7 V and -0.55 V. The same peaks are evident i n each case 69 » ' I I I I L_ -0.6 -0.4 -Q2 0 02 0.4 0.6 Potential / V vs SHE 20 mV s-' f pH = 5.8 Figure 2 2 M u l t i p l e sweep voltammograms at pH = 5 . 8 . 70 although the shape of the peaks and the rate at which peak currents change d i f f e r somewhat due to a decrease i n t o t a l cathodic current passed with the l e s s negative cathodic l i m i t . A l e s s cathodic 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 peaks. Figure 24 shows a voltammogram produced with a cathodic l i m i t of -0.55 V and an anodic l i m i t of +0.24 V. The anodic peaks at -0.2 V which are evident i n Figure 23 are completely absent i n Figure 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 the electrode was held at the cathodic l i m i t f o r 5 minutes i s a l s o shown i n Figure 24. Only a minor peak a s s o c i a t e d with Fe(OH) 2 o x i d a t i o n i s evident. I t i s t h e r e f o r e reasonable to assume that the peaks at -0.2 V are a s s o c i a t e d with anodic o x i d a t i o n products when the anodic l i m i t i s 0.6V. C. I r r e v e r s i b i l i t y of Arsenate Formation While a r s e n i c ( I I I ) can be o x i d i z e d or reduced i n s o l u t i o n (65), a r s e n i c (V) has been reported to be e l e c t r o i n a c t i v e (66,67). The determination of a r s e n i c by polarographic or voltammetric methods re q u i r e s the reduction of a r s e n i c (V) with reducing agents p r i o r to determination (67). Figure 25 demonstrates the i r r e v e r s i b i l t y of arsenate formation at pH = 9.2 and 10.7. Peak A i n t h i s f i g u r e represents the o x i d a t i o n of As to As ( I I I ) while peak B represents the o x i d a t i o n of As ( I I I ) to As (V). I t can be seen that there i s no cathodic peak representing As(V) reduction to As ( I I I ) corresponding to peak B. Peak C represents the d e p o s i t i o n of arsenic onto the gold e l e c t r o d e . The e f f e c t of h o l d i n g the 71 I 1 1 I I I | -0-6 -0-4 -0-2 0 0-2 0-4 0-6 Potent ia l / V vs SHE 20 mV s - ' , pH = 10.6 Figure 23 Influence of cathodic l i m i t on voltammogram 72 after 5 min I I L_ I I 1 -0.6 -0.4 -0.2 0 0.2 0.4 P o t e n t i a i / V vs SHE 20 mV s - ' , pH = I 0.6 Figure 24 Influence of anodic l i m i t on voltammogram 73 e l e c t r o d e at the cathodic l i m i t f o r 5 minutes p r i o r to c a r r y i n g out the p o t e n t i a l sweep i s to deposit a greater q u a n t i t y of a r s e n i c onto the e l e c t r o d e . This i s made evident by the increase i n the height of peak A i n t h i s case. Peak B at the same time i s unnaffected since the concentration of As ( I I I ) a v a i l a b l e f o r o x i d a t i o n to As (V) i s not changed. D. Influence of D i s s o l v e d Arsenic on Voltammetry The o x i d a t i o n of a r s e n o p y r i t e according to equations such as 8 shows so l u b l e a r s e n i c as a product. The 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 should therefore be to depress the anodic peak and to s h i f t i t to higher p o t e n t i a l s . At the same time by c a r r y i n g out voltammetry i n the presence of d i s s o l v e d a r s e n i c , peaks due to a r s e n i c o x i d a t i o n or reduction should be enhanced. The absence of peak enhancement w i l l confirm the previous c o n c l u s i o n that a l l observed peaks at pH greater than 7 are due to i r o n species. Figure 26 shows the i n f l u e n c e of a r s e n i c a d d i t i o n s to s o l u t i o n . As expected 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 i s diminished by such a d d i t i o n as w e l l as being s h i f t e d to more anodic p o t e n t i a l s . As a consequence of the diminished anodic r e a c t i o n , the cathodic peak a s s o c i a t e d with f e r r i c hydroxide reduction a l s o becomes diminished. None of the observed peaks are enhanced by i n c r e a s i n g a r s e n i c a d d i t i o n s . The conclusion that none of the peaks are a s s o c i a t e d with a r s e n i c o x i d a t i o n or reduction i s therefore Figure 25 Voltammogram f o r gold 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 7 5 I i I i i i u -0-6 -0-4 -0-2 0 0-2 0-4 0-6 Potential / V vs SHE 20 mV s - ' , pH=IO-6 , 1000 rpm Figure 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 to the o x i d a t i o n of a r s e n i c I I I to a r s e n i c V are not observed although considerable a r s e n i c I I I i s present i n s o l u t i o n . This i s expected since the p o t e n t i a l s required for A s ( I I I ) o x i d a t i o n which are presented i n the previous s e c t i o n l i e i n the region where c u r r e n t s due to ar s e n o p y r i t e o x i d a t i o n are already s u b s t a n t i a l . E. E f f e c t of Sweep Rate. The model which has been developed f o r the o x i d a t i o n of ar s e n o p y r i t e across the pH range of i n t e r e s t i s that i r o n i n the mineral i s o x i d i z e d to form a surface deposit of f e r r i c hydroxide which increases i n thickness with time. At the same time a r s e n i c and sulphur i n the mineral are o x i d i z e d to sol u b l e arsenate and sulphate r e s p e c t i v e l y . The formation of such an ever - t h i c k e n i n g surface hydroxide l a y e r has i m p l i c a t i o n s to 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 . The hydroxide l a y e r w i l l tend to i n h i b i t a c t i v a t i o n c o n t r o l l e d r e a c t i o n s and f u r t h e r o x i d a t i o n w i l l become d i f f u s i o n l i m i t e d . By a n a l y z i n g the i n f l u e n c e of sweep rate on peak current and p o t e n t i a l , k i n e t i c parameters and the extent to which they are a f f e c t e d by the hydroxide l a y e r may be determined. • Under l i n e a r d i f f u s i o n c o n d i t i o n s r e v e r s i b l e charge t r a n s f e r processes give peak p o t e n t i a l s which are independent of sweep r a t e . Voltammograms were obtained at pH = 10.6 f o r sweep rates 77 from 1 mV/sec. to 80 mV/sec. The r e s u l t s at 2, 30 and 80 mV/sec are shown i n Figure 27 and a l l the r e s u l t s are "plotted i n Figure 28. I t i s apparent that the anodic peak a s s o c i a t e d with mineral o x i d a t i o n i s g r e a t l y a f f e c t e d by sweep rate i n d i c a t i n g t h i s o x i d a t i o n process to be i r r e v e r s i b l e . The cathodic peak a s s o c i a t e d with the reduction of Fe(OH) 3 i s i n s e n s i t i v e to sweep rate i n d i c a t i n g t h i s r e a c t i o n to be r e v e r s i b l e , as expected. At low sweep rates the cathodic peak e x h i b i t s the f a c t that i t represents a two stage r e a c t i o n , d i s p l a y i n g a double peak. This i s c o n s i s t e n t with the argument presented i n the s e c t i o n on m u l t i p l e sweep voltammetry that improving the c o n d i t i o n s for proton or hydroxide d i f f u s i o n w i l l make the ferrous hydroxide reduction r e a c t i o n more favoured. I f the anodic r e a c t i o n i s considered to be a t o t a l l y i r r e v e r s i b l e e l e c t r o d e process i n v o l v i n g a s i n g l e rate determining step, the f o l l o w i n g r e l a t i o n s should apply (56,68) Ep = E 0 + (RT//3naF) (0.78 + l n ( D b ) v 6 - InKs) (26) Ep - Ep 2 = 1 .857 (RT//3naF) = 0.048//Jna (27) i p = 3 X I05n(/3na) V2 A D1/2 C° (28) where Ks = rate constant when el e c t r o d e i s at E° n = t o t a l number of e l e c t r o n s involved i n the e l e c t r o d e process na=number of e l e c t r o n s involved i n the rate - determining step pH 10-6 Figure 27 Voltammograms at i n c r e a s i n g sweep rate 79 pH = 10-6 -L 10 20 30 40 50 60 70 80 Scan rate / mV s" 1 Figure 28 Influence of sweep rate on peak p o t e n t i a l 80 of the e l e c t r o d e process, for a s i n g l e step process n = na C° = bulk concentration of the reactant <x = t r a n s f e r c o e f f i c i e n t E = El e c t r o d e p o t e n t i a l = E° + vt E° = standard e l e c t r o d e p o t e n t i a l D = d i f f u s i o n constant A = e l e c t r o d e area Ep = peak p o t e n t i a l corresponding to the o x i d a t i o n peak Ep 2 = h a l f peak p o t e n t i a l where i = 1/2 i p i p = maximum current i n the current peak 0 = 1 - c * b = /3naFv/RT v = scan rate According to equation 26, a p l o t of Ep vs. log v i s expected to be a s t r a i g h t l i n e . The p l o t shown i n Figure 29 i s c l e a r l y not l i n e a r and equation 26 th e r e f o r e does not apply to 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 highest scan rates shown the peaks were very broad and are the r e f o r e represented by bars rather than by p o i n t s . A d d i t i o n a l c o n s i d e r a t i o n of the r e s u l t s i n d i c a t e s that equations 27 and 28 a l s o do not apply. Values of na 0 determined according to equation 27 are shown i n Table 5. While a constant value should be obtained for an a c t i v a t i o n c o n t r o l l e d process, the r e s u l t s are observed to decrease continuously over the range of sweep rates considered. Equation 28 i n d i c a t e s that a p l o t of log i p versus 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. Figure 30 shows a s t r a i g h t l i n e but having a slope of 0.77. P l o t of Figure 29 peak p o t e n t i a l as a f u n c t i o n of log scan rate e^ej U B O S 6ox snsjSA luaaano ifeed 6oq 0£ 8^ 061,3 83 I t i s apparent that the model used for these equations does not apply i n the present case. D e v i a t i o n from the model may i n part r e s u l t from the existence of more than one rate determining step. A more s i g n i f i c a n t cause of d e v i a t i o n from the model l i k e l y r e s u l t s from the proposed o x i d a t i o n mechanism. The formation of the f e r r i c hydroxide surface deposit during o x i d a t i o n r e s u l t s i n a process c o n t r o l l e d by d i f f u s i o n rather than by a c t i v a t i o n processes. Table 5 Influence of Scan Rate on , K i n e t i c Parameters Scan I max Ep Ep 2 Ep-Ep 2 n/3 Rate mV/sec mA. V V 1 .055 .223 . 1 38 .085 0.56 2 .105 .243 . 1 48 .095 0.51 5 .185 .301 . 1 77 .124 0.39 10 .260 .343 .201 .142 0.34 20 .450 .408 .223 . 185 0.26 30 .630 .443 .248 .195 0.25 40 .695 .463 .250 .205 0.23 50 .880 .493 .263 .230 0.21 60 .990 .533 .278 .255 0.19 70 1 .200 (.543) .286 .267 0.18 80 1 .1 50 (.603) .310 .268 0.18 F. E f f e c t of Temperature Since the f l o t a t i o n of a r s e n o p y r i t e appears to be c o n t r o l l e d by the formation of surface o x i d a t i o n l a y e r s , i t i s of i n t e r e s t to determine the i n f l u e n c e of temperature on the formation of these l a y e r s . The r e s u l t s of t h i s study should i n d i c a t e the extent to which temperature can be used to 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 set - up as was described i n the s e c t i o n on voltammetry was used for experiments at d i f f e r e n t temperatures. In t h i s case the c e l l was equipped with a water jacket connected to a Colora Type K thermostat. The s o l u t i o n i n the c e l l could be c o n t r o l l e d to w i t h i n ± 1°C. ( i i ) R esults and Discussion A s e r i e s of voltammograms was obtained across the range of temperature from 19°C to 60.5°C. The s o l u t i o n used for the t e s t sequence was pH = 10.75 at 22°C. The voltammograms obtained at the two temperature extremes are shown i n Figure 31. The most apparent d i f f e r e n c e i n the two voltammograms i s the cathodic s h i f t and increased peak current a s s o c i a t e d 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 across the temperature range are shown i n Figure 32. The peak p o t e n t i a l decreases with temperature according to Ep = 479 - 4T r = - 0.9957 where T = temperature, °C While the h a l f wave p o t e n t i a l decreases according to Ep 2 = 274.2 - 2.8T r= - 0.9756 The d i f f e r e n c e i n slopes of the two r e l a t i o n s i n d i c a t e s the peak to become steeper with i n c r e a s i n g temperature. The dependence of peak current on temperature shows a complex r e l a t i o n s h i p . I f the nature of the surface hydroxide l a y e r was c o n s i s t e n t across the temperature range the peak 85 pH 10-75 -0-6 -0-4 -0-2 0 0-2 0-4 0-6 Potential / V vs SHE Figure 31 Voltammograms at 19°C and at 60.5°C. 86 T e m p e r a tu re / °C Figure 32 Influence of temperature on peak p o t e n t i a l and peak current 87 current could be expected to be constant under d i f f u s i o n c o n t r o l . The present r e s u l t s i n d i c a t e a d e v i a t i o n from t h i s behaviour. At temperatures below approximately 30°C the hydroxide f i l m development i s incomplete and the current t h e r e f o r e increases with i n c r e a s i n g temperature. Across the range from 30°C to approximately 45°C a constant f i l m thickness i s developed. Above 45°C an increase i n current i s observed i n d i c a t i n g the d i f f u s i o n b a r r i e r to be diminished. I t i s p o s t u l a t e d that t h i s increase r e s u l t s from a change i n the morphology of the hydroxide f i l m , a more porous f i l m being developed. Thus while the f i l m b u i l d s r a p i d l y i n t h i c k n e s s , i t s porous nature allows the anodic process to continue. This i s confirmed by Figure 33 which shows a m u l t i p l e sweep voltammogram obtained at 58.5°C. While there i s a decrease i n current a s s o c i a t e d with the anodic peak ( I I I ) from the 1st. to 2nd. c y c l e s , on subsequent c y c l e s the current stays e s s e n t i a l l y constant while the peak s h i f t s to s l i g h t l y more anodic p o t e n t i a l s . Both these observations are c o n s i s t e n t with the 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 increase i n the peak height of the peaks I and V i s c o n s i s t e n t with the presence of an i n c r e a s i n g amount of surface hydroxide. The e l e c t r o d e at the completion of t h i s experiment was observed to be very h e a v i l y t a r n i s h e d . The formation 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 II and IV. This i s c o n s i s t e n t with the e f f e c t s noted upon el 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 ( s e c t i o n 4.3.2 -B) and confirms improved c o n d i t i o n s f o r 88 V I I I I I 1 1— -0-8 -0-6 -0-4 -0-2 0 0-2 0-4 Potential / V vs SHE 20mV s-» , pH =10-75 Figure 33 M u l t i p l e sweep voltammogram at 58.5°C 89 d i f f u s i o n of species to and from the e l e c t r o d e . Anodic scans for p y r i t e and ars e n o p y r i t e at 60°C are shown i n Figure 34. The d i f f e r e n c e i n p o t e n t i a l between the two curves at the h a l f peak p o t e n t i a l f o r ars e n o p y r i t e i s 30 mV. The d i f f e r e n c e observed at the same point at 22°C and pH 11 was 190 mV (Figure 17). G. Influence of Cyanide Cyanide i s used as a f l o t a t i o n depressant f o r gangue sulphides such as p y r i t e . I t i s therefore of i n t e r e s t to determine the extent to which cyanide may be e f f e c t i v e at depressing a r s e n o p y r i t e . The i n t e r a c t i o n of cyanide with p y r i t e has been studied by measuring the zeta p o t e n t i a l and mixed p o t e n t i a l of p y r i t e at var y i n g pH with i n c r e a s i n g a d d i t i o n of cyanide, ferrocyanide and f e r r i c y a n i d e (69). The zeta p o t e n t i a l was observed to decrease with i n c r e a s i n g cyanide a d d i t i o n s i n d i c a t i n g that the cyanide species adsorb chemically on p y r i t e . Mixed p o t e n t i a l measurements f o r p y r i t e i n the presence of cyanide were determined to l i e i n the region of s t a b i l i t y of the compound F e , [ F e ( C N ) 6 ] 3 . The formation of surface f e r r i c ferrocyanide was the r e f o r e concluded to be res p o n s i b l e f o r the depression of p y r i t e by cyanide. These i n v e s t i g a t o r s (69) a l s o determined that v a r i a t i o n s i n the degree of depression achieved with equal cyanide a d d i t i o n s to various. p y r i t e samples r e s u l t e d from varying p y r i t e s o l u b i l i t y . More s o l u b l e p y r i t e samples were found to be —I I I I J I -0-6 -0-4 -0-2 0 0-2 0-4 Potential / V vs SHE Figure 34 Comparison of p y r i t e and a r s e n o p y r i t e voltammograms at 59.8°C 91 depressed to a l e s s e r degree by cyanide 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 po s t u l a t e d to r e s u l t i n higher l e v e l s of d i s s o l v e d i r o n 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 cyanide consumption. The r e s u l t s of 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 i n t o p y r i t e - cyanide i n t e r a c t i o n (23) were i n t e r p r e t e d on the basis that the f e r r i c ferrocyanide which formed on p y r i t e i n h i b i t e d 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 xanthate and thus r e s u l t e d i n diminished f l o a t a b i l i t y . A s e r i e s of voltammograms was obtained f o r ars e n o p y r i t e i n the presence of i n c r e a s i n g cyanide concentration at pH = 10.6. The cyanide concentration was v a r i e d from 1 X 10 -" M to 8 X l O ' 4 M sodium cyanide. Across t h i s range of cyanide concentration the only s i g n i f i c a n t change i n m u l t i p l e sweep voltammograms i s that the anodic peak a s s o c i a t e d with the o x i d a t i o n of ferrous hydroxide to f e r r i c hydroxide becomes diminished with i n c r e a s i n g cyanide c o n c e n t r a t i o n . Figure 35 shows a m u l t i p l e sweep voltammogram obtained at a cyanide conce n t r a t i o n of 2.8 X 1 0 - 3 molar. At t h i s cyanide concen t r a t i o n the ferrous hydroxide o x i d a t i o n peak disappears completely. Comparison of Figure 35 with Figure 20 reveals that i n the presence of cyanide the el e c t r o d e becomes passivated to a much l e s s e r degree than i n the absence of cyanide. I t appears t h e r e f o r e that cyanide acts to d i s s o l v e ferrous hydroxide formed at the surface of the e l e c t r o d e . Any i r o n - cyanide complexes which are formed are l e s s e f f e c t i v e i n h i b i t o r s to f u r t h e r o x i d a t i o n than i s the hydroxide. Curve A i n Figure 36 represents a p o t e n t i a l sweep on ars e n o p y r i t e c a r r i e d out a f t e r the el e c t r o d e had been held at 92 +343 mV f o r 15 minutes. This p o t e n t i a l i s j u s t below the peak p o t e n t i a l f o r ars 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 holding the electrod e at t h i s anodic p o t e n t i a l i s to enhance the Fe(OH) 2/Fe(OH) 3 peaks while d i m i n i s h i n g the ars e n o p y r i t e o x i d a t i o n peak. The peak for the o x i d a t i o n of ferrous hydroxide to f e r r i c hydroxide i s b e l i e v e d to r e s u l t from the formation of a small amount of ferrous hydroxide at the s t a r t of the p o t e n t i a l sweep. Curves B and C i n Figure 36 represent p o t e n t i a l sweeps c a r r i e d out a f t e r holding the el e c t r o d e at +343 mV for 15 minutes i n the presence of 2X10" 4 M and 2.8X10" 3 M cyanide. Increasing cyanide conce n t r a t i o n r e s u l t s i n the ferrous hydroxide o x i d a t i o n peak becoming diminished, the ars e n o p y r i t e peak becoming enhanced and the f e r r i c hydroxide reduction peak being unaffected. Curve D represents a p o t e n t i a l sweep c a r r i e d out a f t e r h o l d i n g the ele c t r o d e at +343 mV f o r 15 minutes followed by c o n d i t i o n i n g the el e c t r o d e for 5 minutes i n the presence of 1.63X10'3 M cyanide with no a p p l i e d p o t e n t i a l . Compared to curve A the ferrous peak i s diminished while the ars e n o p y r i t e and f e r r i c peaks are enhanced. I t i s apparent that the a c t i o n of cyanide i s to d i s s o l v e ferrous hydroxide from the e l e c t r o d e . This decreases the e f f e c t of o x i d a t i o n by removing the o x i d a t i o n products from the surfac e . The ars e n o p y r i t e o x i d a t i o n currents a f t e r holding the elec t r o d e at a corroding p o t e n t i a l are ther e f o r e increased i n the presence of cyanide. The i m p l i c a t i o n of t h i s removal of surface hydroxide 93 I 1 1 I I I t • -0.8 -0.6 -0.4 -0.2 0 02 0.4 0.6 Potential / V vs SHE 20 mV s-», pH = !0.6 , 2 . 8 2 x | 0 " 3 M Na CN Figure 35 M u l t i p l e sweep voltammogram for ar s e n o p y r i t e i n the presence of 2.82X10' 3 M NaCN 94 P o t e n t i a l / V vs S H E Figure 36 Influence of cyanide on formation of i r o n hydroxide f i l m s on a r s e n o p y r i t e 95 deposits from a r s e n o p y r i t e by cyanide i s that cyanide may act as an a c t i v a t i n g agent rather than as a depressant f o r a r s e n o p y r i t e . A s i m i l a r e l i m i n a t i o n of Fe(OH) 2/Fe(OH) 3 peaks i n the presence of cyanide i s observed for p y r i t e as shown i n Figure 3 7 . P y r i t e i n the presence of 1 . 6 3 X 1 0 " 3 M cyanide does not show any ferrous hydroxide peak. I t was determined however that the anodic current at 0 . 4 4 3 V i n the presence of cyanide was only 4 0 0 M A while i n the absence of cyanide i t was 6 4 0 M A . The cyanide therefore 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 p y r i t e . Such a decrease i n anodic current i n the presence of cyanide i s not observed f o r a r s e n o p y r i t e . The cyanide complexes which are known to be formed on p y r i t e are apparently not formed on a r s e n o p y r i t e . 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 obtained at pH = 1 0 . 6 f o r s e v e r a l other minerals i n the Fe - As - S system as w e l l as f o r i r o n . The purpose of t h i s s e r i e s of experiments was to determine whether f e r r i c hydroxide formed during the o x i d a t i o n of these minerals. 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 minerals can then be r e l a t e d to the v a r i a t i o n i n surface composition, ( i ) Experimental The p y r i t e e l e c t r o d e was the same electro d e as was p r e v i o u s l y described. The marcasite e l e c t r o d e was made from a marcasite sample obtained from Wards' S c i e n t i f i c . The e l e c t r o d e was i r r e g u l a r i n shape with an exposed surface of approximately 96 Figure 37 Voltammogram f o r p y r i t e i n the presence of 1.62X10 - 3 M NaCN 97 4 square m i l l i m e t e r s . The l o e l l i n g i t e sample was obtained from the UBC Department of G e o l o g i c a l Sciences and was i n d i c a t e d to come from Cobalt, Ontario. The e l e c t r o d e was i r r e g u l a r i n shape with an exposed area of approximately 59 square m i l l i m e t e r s . The l o e l l i n g i t e sample was analyzed by means of an SEM - EDX a n a l y s i s . The major c o n s t i t u e n t s were found to be a r s e n i c and i r o n and the only t r a c e c o n s t i t u e n t which was detected was antimony. Electrod e s were prepared as was p r e v i o u s l y described but with only one wire connected to them. The i r o n e l e c t r o d e was made from a piece of Armco pure i r o n . The e l e c t r o d e was square i n shape with an exposed area of approximately 14 square m i l l i m e t e r s . This e l e c t r o d e was made by s o l d e r i n g a copper wire to one face of the i r o n cube and then encasing the assembly i n epoxy. The face opposite the attached wire was ground down to 600 g r i t paper p r i o r to use. ( i i ) R e s ults Voltammograms for p y r i t e and marcasite are shown i n Figure 38. The r e s u l t s for p y r i t e are s i m i l a r to those f o r a r s e n o p y r i t e i n that the Fe(OH) 2/Fe(OH) 3 peaks are prominent but d i f f e r i n that no anodic peak due to o x i d a t i o n of p y r i t e i t s e l f i s apparent. As p r e v i o u s l y discussed, the o x i d a t i o n of p y r i t e occurs at higher 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 for p y r i t e has been a t t r i b u t e d (63) to the formation of sulphur during p y r i t e o x i d a t i o n . This peak i s observed to become diminished with continued c y c l i n g . The r e s u l t s f o r marcasite show l e s s s i g n i f i c a n t Fe(OH) 2/Fe(OH) 3 peaks but enhanced peaks a s s o c i a t e d with reduced I I I I I I —I 1 -0.8 -0.6 -0.4 -0.2 0 0,2 0.4 0.6 Potential / V vs SHE 20mV s - , pH = 10.6 Figure 38 Voltammograms for p y r i t e and marcasite at pH = 10.6 99 i r o n - hydroxy species. The o x i d a t i o n of marcasite occurs at more anodic p o t e n t i a l s than p y r i t e . Marcasite shows a much lower tendency to passivate with continued scanning than does p y r i t e or a r s e n o p y r i t e . The r e s u l t s are c o n s i s t e n t with the formation of more s o l u b l e i r o n species during marcasite o x i d a t i o n . This observation i s c o n s i s t e n t with the f a c t that marcasite i s known to decompose with the formation of ferrous sulphate (37). The rate of o x i d a t i o n of marcasite has been reported to be nine times as f a s t as p y r i t e (37). Figure 39 shows a voltammogram for l o e l l i n g i t e . The Fe(OH) 2/Fe(OH) 3 peaks are apparent as i s an anodic peak as s o c i a t e d with with l o e l l i n g i t e o x i d a t i o n . The l o e l l i n g i t e o x i d a t i o n peak i s approximately 100 mV more anodic than the a r s e n o p y r i t e o x i d a t i o n peak under 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 Figure 39 at 0.2 to 0.3 V. The peaks r e s u l t from a species which i s o x i d i z e d on the f i r s t anodic sweep and then r e v e r s i b l y reduced and o x i d i z e d on subsequent sweeps. Although antimony was detected as a minor c o n s t i t u e n t i n the 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 to antimony r e a c t i o n s . Considering the f a c t that the l o e l l i n g i t e o r i g i n a t e d i n a s i l v e r producing area i n Canada i t was considered p o s s i b l e that the peaks were due to trace concentrations of s i l v e r which had not been detected during the sample a n a l y s i s . A voltammogram for a gold e l e c t r o d e i n the presence of d i s s o l v e d s i l v e r i s shown i n Figure 39. The peak p o s i t i o n s show good agreement with those observed f o r l o e l l i n g i t e . 100 Figure 39 M u l t i p l e sweep voltammogram for l o e l l i n g i t e at pH = 10.6 101 Figure 40 shows a voltammogram for i r o n . This experiment was c a r r i e d out without KC1 i n s o l u t i o n since excessive c u r r e n t s and formation of surface hydroxide species were encountered i n i t s presence. The Fe(OH) 2/Fe(OH) 3 peaks are apparent although the voltammogram i s more complex than those for minerals. This increased complexity i s assumed to r e s u l t from the d i f f e r e n t p h y s i c a l nature of i r o n hydroxide f i l m s formed on i r o n compared to those formed on minerals. Based on t h i s s e r i e s of experiments, some p o s s i b i l i t i e s regarding the v a r i a b l e f l o a t a b i l i t y of these minerals can be considered. The o x i d a t i o n behaviour of l o e l l i n g i t e i s very s i m i l a r to that of a r s e n o p y r i t e and s i m i l a r f l o t a t i o n c h a r a c t e r i s t i c s would be a n t i c i p a t e d . Marcasite shows behaviour more comparable to that of p y r i t e with the exception that more so l u b l e i r o n - hydroxy species are formed. I t i s expected that t h i s unstable behaviour of marcasite would make i t more d i f f i c u l t to f l o a t with xanthate and would make i t more d i f f i c u l t to depress with cyanide than i n the case of p y r i t e . None of the minerals considered show behaviour s i m i l a r to that of i r o n . Although i r o n hydroxide deposits of some form are developed i n each case, each mineral shows anodic decomposition c u r r e n t s i n the region where i r o n i s p a s s i v a t e d . I . Ring Disc Study The 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 was equipped with a gold r i n g to detect e l e c t r o a c t i v e o x i d a t i o n products. The purpose of these experiments was to determine whether any Figure 40 Voltammogram for i r o n e l e c t r o d e 103 o x i d a t i o n products which had p r e v i o u s l y not been considered were escaping i n t o s o l u t i o n . Experiments were c a r r i e d out only at pH = 10.6 so that the p o s s i b l e formation of elemental sulphur at lower pH values was not i n v e s t i g a t e d . Figure 41 shows the s o l u t i o n flow from the d i s c to the r i n g and the transport p a t t e r n of s o l u b l e species formed at the d i s c (71). Such r i n g - d i s c e l e c t r o d e s have been used to study the anodic decomposition of galena (72) as w e l l as the c o r r o s i o n of d e n t a l a l l o y s (73). The e f f i c i e n c y with which the r i n g c o l l e c t s species released at the d i s c i s dependent on e l e c t r o d e geometry according to N = i r / i d = r 3 - r 3 r 3 r 2 2/3 (29) r 3 r 3   1  1 l_ where r, = radius of d i s c r 2 = inner radius of r i n g r 3 = outer radius of r i n g i r = r i n g current i d = d i s c 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 gives a value fo 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. This value i s very high due to the f a c t that the gold r i n g i s very. wide. Such a wide r i n g i s subject to excessive noise pick-up. 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 Transport p a t t e r n of s o l u b l e species at a r i n g - d i s c e l e c t r o d e 105 however and a meaningful r e s u l t c ould not be achieved. Two experiments were c a r r i e d out with t h i s e l e c t r o d e at pH = 10.6. One experiment involved holding the a r s e n o p y r i t e d i s c at the anodic peak p o t e n t i a l while scanning the gold r i n g . The second experiment 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 while h o l d i n g the r i n g f i r s t at an anodic and second at a cathodic p o t e n t i a l . In n e i t h e r experiment were any r i n g c u r rents detected. This r e s u l t i s c o n s i s t e n t with the formation of sulphate and arsenate, 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 d i s c . The formation of s o l i d o x i d a t i o n products such as elemental sulphur or A s 2 S 2 would be c o n s i s t e n t with the r e s u l t s of these experiments. The voltammetry experiments which p r e v i o u s l y have been discussed show no evidence of these species and they are not b e l i e v e d to be present. While more elaborate experiments could have been c a r r i e d out, i t seemed u n l i k e l y that 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 to the present study and t h i s area t h e r e f o r e was not pursued f u r t h e r . J . Influence of Hydroxide Formation on Xanthate Ox i d a t i o n . I t i s g e n e r a l l y accepted that dixanthogen i s the a c t i v e c o l l e c t o r species i n p y r i t e f l o t a t i o n (27). The nature of adsorbed xanthate species on a r s e n o p y r i t e has not been i n v e s t i g a t e d but i t i s expected that dixanthogen w i l l be the a c t i v e c o l l e c t o r species. Since the o x i d a t i o n of a r s e n o p y r i t e has been shown to r e s u l t i n a r a p i d build-up of f e r r i c hydroxide on the s u r f a c e , i t i s of i n t e r e s t to determine the i n f l u e n c e of t h i s hydroxide 106 build-up on the o x i d a t i o n of xanthate to dixanthogen. ( i ) Experimental. Anodic o x i d a t i o n scans were c a r r i e d out at a scan rate of 5 mV per second. Xanthate used for these experiments c o n s i s t e d of Hoechst potassium e t h y l xanthate which was p u r i f i e d by d i s s o l v i n g i n acetone and 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 ether. ( i i ) R esults and D i s c u s s i o n Figure 42 shows the r e s u l t s of experiments c a r r i e d out at pH = 5.9. An anodic scan i s shown fo 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 shown for the same el e c t r o d e a f t e r i t had been held f o r 5 minutes at a p o t e n t i a l of + 460 mV. This p o t e n t i a l i s part way up the anodic curve f o r a r s e n o p y r i t e and represents the p o t e n t i a l region achieved i n the presence of permanganate at t h i s pH. The anodic treatment i s observed to r e s u l t i n only a minor p a s s i v a t i o n . Two scans are shown i n the presence of 2.6 X 10" 3 M potassium e t h y l xanthate. One scan i s f o r a f r e s h e l e c t r o d e while the other i s f o r an e l e c t r o d e as described above. At t h i s 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 o x i d a t i o n of xanthate to dixanthogen. This i s c o n s i s t e n t with the view that hydroxide f i l m s are not formed by the o x i d a t i o n of a r s e n o p y r i t e at t h i s pH. Figure 43 shows the r e s u l t s for a s i m i l a r set of experiments c a r r i e d out at pH = 11.8. At t h i s pH, v i s i b l e f i l m s of f e r r i c hydroxide are formed. Several d i f f e r e n c e s with the r e s u l t s obtained at pH = 5.9 are apparent. While at pH = 5.9 the o x i d a t i o n of xanthate occurs 107 5 mV s-' 1000 rpm pH 5-9 Fresh I I I I I 0 0-2 0-4 0-6 Potential / V vs SHE • Fresh FeAsS, 2-6 x 10" 5 M KEtX Oxidized FeAsS,2-6xiO- 8M KEtX Figure 42 Influence of ars e n o p y r i t e o x i d a t i o n at pH = 5.9 on xanthate o x i d a t i o n 108 0-4i 5 m V s~ l 1000 rpm pH 118 0-3 0-2| < E ~ 0-1 0 Fresh/ / FeAsS / / no X" / oxidized FeAsS no X-J_ -0-2 0 0-2 Potent i ai / V vs SHE 0-4 Fresh Fe AsS , 2-6X|0"3M KEtX —Oxidized FeAsS , 2-6x I0~3M KEtX Figure 4 3 Influence of ars e n o p y r i t e o x i d a t i o n at pH = 11.8 on xanthate o x i d a t i o n 109 at p o t e n t i a l s which are cathodic to ars e n o p y r i t e o x i d a t i o n , at pH = 11.8 these two curves are reversed. This i n d i c a t e s that o x i d a t i o n of arsenop y r i t e w i l l be favored over xanthate o x i d a t i o n at t h i s pH. A f u r t h e r d i f f e r e n c e i s that while at pH = 5.9 the ele c t r o d e was hardly a f f e c t e d by holding at an o x i d i z i n g p o t e n t i a l , at pH = 11.8 the el e c t r o d e i s passivated to a s i g n i f i c a n t degree. The o x i d a t i o n of xanthate i s g r e a t l y 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 hydroxide l a y e r . While these experiments c l e a r l y show that the presence of t h i c k f e r r i c hydroxide l a y e r s such as could be formed under f l o t a t i o n c o n d i t i o n s ( i . e . 5 minutes c o n d i t i o n i n g with KMnO«) w i l l prevent the formation of dixanthogen at the surface, the adsorption of xanthate ions at the hydroxide l a y e r has not been r u l e d out. 4.4 Dis c u s s i o n The o x i d a t i o n of arsenop y r i t e under moderately o x i d i z i n g c o n d i t i o n s has been shown to r e s u l t i n the formation of r e l a t i v e l y t h i c k surface l a y e r s of f e r r i c hydroxide. At the same time the ars e n i c and sulphur are o x i d i z e d to arsenate and sulphate r e s p e c t i v e l y . At the lower end of the pH range studied elemental sulphur can be expected to be a product of o x i d a t i o n . The i n f l u e n c e of o x i d a t i o n on the f l o a t a b i l i t y of ars e n o p y r i t e i s exerted through the build-up of the hydroxide l a y e r on the surface of the mineral. The presence of t h i s hydroxide l a y e r i n h i b i t s the o x i d a t i o n of xanthate to dixanthogen. The mineral i s therefore not rendered hydrophobic. 110 At the same time the hydroxide l a y e r i t s e l f can be expected to be s t r o n g l y h y d r o p h i l i c and the mineral i s ther e f o r e s t r o n g l y depressed. The o x i d a t i o n p o t e n t i a l s r e q u i r e d to b r i n g about the o x i d a t i o n of ars e n o p y r i t e have been shown to be achieved i n the presence of s e v e r a l common o x i d i z i n g agents. In a d d i t i o n , the o x i d i z i n g p o t e n t i a l s r e q u i r e d for ars e n o p y r i t e depression are encountered i n some p l a n t s even i n the absence of o x i d i z i n g agents. Figure 44 shows var 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 w e l l as the 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 for a r s e n o p y r i t e . I t i s apparent that i n some cases the c o n d i t i o n s r e q u i r e d for the o x i d a t i o n and thus the depression of arsenopy r i t e are being achieved. Current peaks a s s o c i a t e d with a r s e n o p y r i t e o x i d a t i o n increase with i n c r e a s i n g pH. The build-up of hydroxide i s there f o r e expected to be greater with i n c r e a s i n g pH and f l o t a t i o n should be more d i f f i c u l t . Maximum f l o t a t i o n of ar s e n o p y r i t e with xanthate i s expected at pH l e s s than 7, where f e r r i c hydroxide does not form and where the formation of surface d e p o s i t s of elemental sulphur would f u r t h e r c o n t r i b u t e to the hydrophobicity of the min e r a l . The i n f l u e n c e of temperature on the o x i d a t i o n of ars e n o p y r i t e showed a complex v a r i a t i o n . Over the range of 15°C to 30°C, i n c r e a s i n g temperature r e s u l t e d i n i n c r e a s i n g development of surface hydroxide. Over the temperature range 30°C to 40°C temperature has no apparent i n f l u e n c e on hydroxide build-up while at temperature greater than 40°C, i n c r e a s i n g I l l 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 with operating plant c o n d i t i o n s (70) 112 temperature r e s u l t s i n a r a p i d increase i n the q u a n t i t y of f e r r i c hydroxide. The in f l u e n c e of temperature on the depression of arsenopyrite by o x i d a t i o n i s therefore expected to be minimal u n t i l a temperature of 40°C i s exceeded. Above t h i s temperature, 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 hydroxide are formed. D i f f e r e n t i a l f l o t a t i o n of ars e n o p y r i t e and p y r i t e through the use of o x i d i z i n g agents should be p o s s i b l e i n the temperature region near 20°C. At t h i s temperature, a r s e n o p y r i t e o x i d i z e s at 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 than does p y r i t e . Increasing temperature can be expected to d i m i n i s h the e f f i c i e n c y of the d i f f e r e n t i a l f l o t a t i o n of these minerals. At elevated temperature (60°C) the p o t e n t i a l s at which the 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 temperatures. The a d d i t i o n of cyanide to s o l u t i o n r e s u l t e d i n a d i s s o l u t i o n of the hydroxide surface deposits p r e v i o u s l y formed on a r s e n o p y r i t e . I t i s expected that cyanide a d d i t i o n s would r e s u l t i n increased arsenopyrite f l o a t a b i l i t y by d i m i n i s h i n g the e f f e c t s of o x i d a t i o n . 113 Chapter 5 ESCA STUDIES X-ray photoelectron spectroscopy (XPS) or more commonly known as ESCA, i s an experimental technique which permits the a n a l y s i s of a surface l a y e r on a s o l i d sample. The thickness of the surface l a y e r which w i l l be analyzed may vary from a few angstroms to 50 angstroms. The method c o n s i s t s of bombarding the sample to be studied with nearly monoenergetic photons and measuring the k i n e t i c 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 (74). Each element w i l l have a c h a r a c t e r i s t i c set of photoelectron peaks due to the d i f f e r e n t e l e c t r o n i c l e v e l s . The photon energies used to e j e c t e l e c t r o n s are i n the range of 1 KeV or more. The binding energy of e l e c t r o n s i n a given e l e c t r o n i c l e v e l can be measured with s u f f i c i e n t p r e c i s i o n to detect s h i f t s r e s u l t i n g from d i f f e r e n c e s i n the chemical s t a t e of the atom. For instance, the energy f o r e l e c t r o n s i n a given energy s t a t e for a metal w i l l be d i f f e r e n t than for the metal oxide. S i m i l a r l y , v a r i a t i o n s i n o x i d a t i o n s t a t e w i l l r e s u l t i n v a r i a t i o n of the binding energy of core e l e c t r o n s . ESCA studi e s have been c a r r i e d out on p y r i t e to determine the nature of suface compounds formed during f l o t a t i o n (75). I t was determined that i r o n hydroxide formed on p y r i t e during g r i n d i n g and at high pH during f l o t a t i o n . For values of pH l e s s than 7, surface hydroxide 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 suface. Only l i m i t e d q u a n t i t i e s of elemental sulphur were 114 detected even at pH = 3.0. This observation that l i m i t e d sulphur was detected suggests that the method may be i n s e n s i t i v e to surface concentrations of elemental sulphur since sulphur i s known to form during the a c i d o x i d a t i o n of p y r i t e (76). While no explanation 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 that any elemental sulphur present on the surface of p a r t i c l e s i s v o l a t i l i z e d at the high vaccuum (10" 7 t o r r ) encountered during a n a l y s i s , before i t can be detected. In the present study, the r e s u l t s of e l e c t r o c h e m i c a l experiments 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 i n d i c a t i n g the formation of f e r r i c hydroxide with concurrent o x i d a t i o n of a r s e n i c and sulphur to arsenate and sulphate r e s p e c t i v e l y . The r e s u l t of ESCA experiments w i l l be used to confirm the existence of i r o n hydroxide surface f i l m s and to show the extent to which arsenate and sulphate are incorporated i n these f i l m s . 5.1 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 the same o r i g i n as those used for e l e c t r o c h e m i c a l s t u d i e s . Other mineral samples were obtained from Ward's S c i e n t i f i c and were v i s u a l l y judged to be free of other mineral 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 with a p o r c e l a i n mortar and p e s t l e to minus 74 microns and stored i n sealed v i a l s p r i o r to use. The 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 to 47 and i n Table VI to have been t r e a t e d at pH = 11.5 was prepared by s t i r r i n g 2 grams of sample i n 100 ml. of water adjusted to 115 pH. = 11.5 with sodium hydroxide. Following 15 minutes of c o n d i t i o n i n g the sample was f i l t e r e d and d r i e d under argon at 35°C. Samples to be analyzed were dusted onto double s i d e d c e l l u l o s e tape which could be fastened to the sample holder f o r i n s e r t i o n i n t o the spectrometer. XPS measurements were c a r r i e d out using a Varian IEE-15 spectrometer equipped with a magnesium anode. Spectra were recorded at approximately 10 " 7 t o r r . From ten to f o r t y scans were made of each m i n e r a l . The data p o i n t s were c o l l e c t e d i n d i g i t a l form and a Gaussian f i t was a p p l i e d to the data normalized to overcome the v a r i a b l e number of scans. 5.2 Results and D i s c u s s i o n The i r o n , a r s e n i c and sulphur peaks f o r the v a r i o u s minerals included i n the study are shown i n Figures 45,46 and 47. The binding energies and i n t e n s i t y data a s s o c i a t e d with the peaks are summarized i n Table 6. I n t e n s i t i e s are based on the counts per second at the peak. Binding energies r e l a t e to the i r o n 2p, a r s e n i c 3d and sulphur 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 various minerals other than arsen o p y r i t e i n the study i s to provide 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 reduced species for each mineral. I n t e n s i t i e s for the same element i n d i f f e r e n t minerals can be expected to vary independently of the compositional r a t i o for that element. Factors such as r e a l surface area and volume per u n i t c e l l , t a k i n g i n t o account the 116 number of formula u n i t s per u n i t c e l l , w i l l i n f l u e n c e i n t e n s i t y r a t i o s between minerals. Since XPS analyzes a thickness of the Table 6 E l e c t r o n binding energies and i n t e n s i t i e s for elements i n v a r i o u s minerals. M i n e r a l Element Binding Energy Counts/sec. eV FeOOH Fe 713.6 21 1 37 AS 2 S 3 S 163.4 7406 AS 44.1 4403 FeS 2 ( p y r i t e ) Fe 712.3 758 709.4 5758 S 169.7 1 352 163.7 5395 FeS 2 (marc.) Fe 713.7 1879 709.5 5949 S 170.9 1692 164.9 4187 FeAs 2 Fe 713.9 1707 709.5 2347 As 46.9 1546 43.8 2289 FeAsS (dry) Fe 712.9 5646 708.6 2761 As 47.2 1 1 67 44.1 525 S 170.6 549 165.0 1234 FeAsS (pH 11.5) Fe 712.8 1 1 921 708.5 1322 AS 47.1 2067 43.4 281 S 171.1 837.8 165.5 831 .4 order of 50 angstroms the r e s u l t for surface product l a y e r s thinner than t h i s w i l l represent a composite of the surface product and the substrate. I n t e n s i t y r a t i o s of o x i d i z e d to t o t a l surface species for p y r i t e , marcasite and a r s e n o p y r i t e are summarized i n Table 7. 117 Figure 4 5 XPS peaks a s s o c i a t e d with the i r o n 2p e l e c t r o n s of the various minerals 118 FeAs, F e A s S -FeAsS A s a S3 ± ± JL 33-3 36-8 40-4 4 3 9 47-5 BINDING E N E R G Y / eV 510 54-5 F i g u r e 4 6 X P S p e a k s a s s o c i a t e d w i t h t h e a r s e n i c 3 d e l e c t r o n s o f t h e v a r i o u s m i n e r a l s 119 I I I I I I I 162-6 165-6 168-5 171-5 174-5 177-5 BINDING ENERGY / eV Figure 4 7 XPS peaks a s s o c i a t e d with the sulphur 2d e l e c t r o n s of the various minerals 120 S e v e r a l t r e n d s i n t h e s e d a t a a r e c o n s i s t e n t w i t h p r e v i o u s l y c i t e d c h a r a c t e r i s t i c s f o r m a r c a s i t e c o m p a r e d t o p y r i t e a n d w i t h t h e e l e c t r o c h e m i c a l r e s u l t s f o r a r s e n o p y r i t e w h i c h a r e p r e s e n t e d i n t h i s s t u d y . M a r c a s i t e s h o w s a g r e a t e r p r o p o r t i o n o f o x i d i z e d s p e c i e s t h a n d o e s p y r i t e . T h i s i s c o n s i s t e n t w i t h m a r c a s i t e s h o w i n g g r e a t e r o x i d a t i o n r a t e s t h a n p y r i t e (37) a n d w i t h p r e v i o u s l y p u b l i s h e d X P S r e s u l t s f o r t h e s e m i n e r a l s (77). A r s e n o p y r i t e s h o w s a n e v e n g r e a t e r p r o p o r t i o n o f o x i d i z e d s u r f a c e s p e c i e s t h a n e i t h e r p y r i t e o r m a r c a s i t e . I t i s a p p a r e n t t h a t a l t h o u g h t h e a r s e n o p y r i t e w a s c r u s h e d i n a d r y f o r m , a s u r f a c e o x i d e l a y e r w a s f o r m e d . T h i s h e l p s t o e x p l a i n t h e r e s u l t s o f s o m e o f t h e 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 . T h e v o l t a m m e t r y r e s u l t s i n d i c a t e d t h a t i r o n h y d r o x i d e w a s n o t f o r m e d b e l o w a p H o f a p p r o x i m a t e l y 7.0. A t t h e s a m e t i m e t h e r e s t p o t e n t i a l s m e a s u r e d a t p H l e s s t h a n 7.0 o b e y e d t h e s a m e r e l a t i o n a s t h o s e a t h i g h e r p H , i n d i c a t i n g a c o n s i s t e n t s u r f a c e l a y e r t o b e p r e s e n t . T h e s e r e s u l t s c a n n o w b e r e c o n c i l e d o n t h e b a s i s T a b l e 7 X P S I n t e n s i t y r a t i o s M i n e r a l o x F e / F e o x S / S o x A s / A s F e / A s F e / S P y r i t e 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 i n e a c h c a s e a s u r f a c e l a y e r o f i r o n h y d r o x i d e w a s f o r m e d 121 during the e l e c t r o d e p r e p a r a t i o n . This hydroxide c o n t r o l l e d the ele c t r o d e p o t e n t i a l measurements but was of i n s u f f i c i e n t t h ickness to a f f e c t the voltammetry experiments. As expected, based on r e s u l t s of e l e c t r o d e p o t e n t i a l measurements, s u b j e c t i n g a r s e n o p y r i t e to pH = 11.5 r e s u l t s i n r a p i d decomposition of the mineral l a t t i c e . For each of the three elements Table 7 shows the r a t i o of o x i d i z e d to reduced atoms to increase a f t e r o x i d a t i o n due to d i s s o l v e d oxygen at pH = 11.5. While the r a t i o of i r o n to sulphur increases s i g n i f i c a n t l y a f t e r treatment, the r a t i o of i r o n to a r s e n i c increases only a small amount. These r e s u l t s i n d i c a t e that at the same time as the surface l a y e r of f e r r i c hydroxide i s formed, much of the sulphur goes i n t o s o l u t i o n presumably as sulphate, while the a r s e n i c remains at the surface i n a o x i d i z e d s t a t e . I t i s proposed that t h i s a r s e n i c i s adsorbed on the f e r r i c hydroxide since i t i s known that 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 (78). The existence of such a surface o x i d a t i o n product i s a l s o c o n s i s t e n t with the existence of such secondary a r s e n i c minerals as p i t t i c i t e ( F e 2 ( A s 0 4 ) (S0 4) OH«2H20) and pharmacosiderite (6FeAsO„«2Fe(OH)3'12H20). The r e s u l t s of t h i s spectroscopic study confirm the conclusions drawn from e l e c t r o c h e m i c a l experiments. At high pH values, a r s e n o p y r i t e decomposes to form i r o n hydroxide surface d e p o s i t s . Arsenic i n the form of arsenate and some sulphur i n the form of sulphate are incorporated i n t h i s hydroxide l a y e r . Such hydroxide l a y e r s can be expected to r e s u l t i n the depression of ars e n o p y r i t e during f l o t a t i o n with 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 with r e a l ore samples to v e r i f y the i n t e r p r e t a t i o n s made of el e c t r o c h e m i c a l experiments as they r e l a t e to the f l o t a t i o n response of a r s e n o p y r i t e . Three types of experiments were c a r r i e d out. In the f i r s t type, c o n d i t i o n s were c o n t r o l l e d to observe the f l o t a t i o n o p e r a tion. In the second type, a rougher a r s e n o p y r i t e concentrate was prepared and i t s subsequent depression through the use of o x i d i z i n g agents was stu d i e d . In the t h i r d type, bulk p y r i t e - a r s e n o p y r i t e concentrates were prepared. The extent to which a r s e n o p y r i t e could be s e l e c t i v e l y depressed from t h i s concentrate through the use of o x i d i z i n g agents was explored. 6.0.1 Rougher F l o t a t i o n 6.0.1.1 Experimental. 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 s e l e c t e d high grade samples from the property of G.M. Resources L t d . at Hedley, B.C. The sample contained 51.2 percent by 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 microscopic examination of the m a t e r i a l i n d i c a t e d only minor p y r i t e to be present. The 123 arse n o p y r i t e was assayed to con t a i n 270 to 340 grams per tonne Au i n the form shown i n Figure 2. Minor concentration of other metals are a l s o i n d i c a t e d i n Figure 2 as determined by SEM - EDX a n a l y s i s . For each t e s t , 1000 grams of ore was ground i n a l a b o r a t o r y rod m i l l to approximately 92% minus 200 mesh. Vancouver tapwater having a pH of 5.5 was used for a l l t e s t s . For t e s t s c a r r i e d out at va r y i n g pH the temperature was 24±1°C. The pH was observed to increase from the water value of pH=5.5 to a n a t u r a l value of pH=8.0 during g r i n d i n g . Such an increase i n pH r e s u l t s from the presence of s l i g h t l y a l k a l i n e rock forming c o n s t i t u e n t s i n the ore. The pH was adjusted from the n a t u r a l value of 8.0 to the de s i r e d value using sodium hydroxide or s u l p h u r i c a c i d . F o l l o w i n g pH adjustments the s l u r r y was co n d i t i o n e d for 15 minutes w i t h an a i r flow of 2.4 l i t r e s 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 at 1000 RPM. Follow 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 with 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 allowed to c o n d i t i o n f o r 2 minutes. Rougher f l o t a t i o n was c a r r i e d out for 15 minutes w i t h an a d d i t i o n of 20 grams per tonne of Dowfroth 250 f r o t h e r . Tests c a r r i e d out at va r y i n g temperature were at the na 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 with the pH adjusted to 9.0 using sodium hydroxide. Temperature t e s t s were c a r r i e d out i n a jacketed s t a i n l e s s s t e e l c e l l w i t h the temperature c o n t r o l l e d by means of a Colora Type - K thermostat. Once the temperature had been s t a b i l i z e d the c e l l a i r was turned on and c o n d i t i o n i n g was c a r r i e d out as for the t e s t s at 124 vary i n g pH. 6.0.2 Res u l t s and Dis 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 at varying pH are shown i n Figure 48. The recovery of ar s e n o p y r i t e i s 90% or greater u n t i l approximately pH=7.5 at which poi n t i t s t a r t s to decrease steadly u n t i l approximately pH=l0.5. Above pH=l0.5 the recovery continues to decrease but at a lower rate than below t h i s pH. Resu l t s are a l s o shown for two t e s t s c a r r i e d out i n the absence of o x i d a t i o n . The r e s u l t s of these t e s t s , p a r t i c u l a r l y at pH = 10 i n d i c a t e that the decrease i n recovery observed with i n c r e a s i n g pH i s not the r e s u l t of pH alone. The o x i d a t i o n r e s u l t i n g from a e r a t i o n during c o n d i t i o n i n g i s necessary to brin g about arsenop y r i t e depression. The f l o t a t i o n t e s t r e s u l t s agree with the r e s u l t s obtained with voltammetry. The voltammograms shown i n Figures 14 and 15 i n d i c a t e the anodic peak a s s o c i a t e d with a r s e n o p y r i t e o x i d a t i o n to f e r r i c hydroxide to appear i n the v i c i n i t y of approximately pH = 7. The o x i d a t i o n peak heights increase with i n c r e a s i n g pH. In the present f l o t a t i o n t e s t s the r e s u l t of the i n c r e a s i n g amount of f e r r i c hydroxide a s s o c i a t e d with these i n c r e a s i n g peak heights i s observed to lead to decreasing f l o a t a b i l i t y of the mine r a l . For maximum recovery of ars e n o p y r i t e a pH of l e s s than 7.0 should be used while f o r maximum depression a pH approaching 12.0 would be appr o p r i a t e . The pH used for depression i n most cases would be d i c t a t e d by the pH dependance of the f l o a t a b i l i t y of other minerals being recovered. The r e s u l t of t e s t s c a r r i e d out at i n c r e a s i n g 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 i n the presence and absence of o x i d a t i o n 126 are shown i n Figure 49. The recovery increases somewhat over the temperature range from 7°C to 40°C. Above 40°C the recovery decreases w i t h i n c r e a s i n g temperature. The recovery at 60°C i n the absence of o x i d a t i o n was higher than at the lower temperatures with o x i d a t i o n . I f ar s e n o p y r i t e depression was de s i r e d there would be no b e n e f i t derived from heating the s l u r r y since p l a n t temperatures can normally be expected to be i n the range of 10°C to 20°C. The r e s u l t s achieved at pH = 9.0 show s i m i l a r behaviour to those achieved at pH = 8.0 wi t h i n c r e a s i n g temperature. In each case the concentrate grade was i n s e n s i t i v e to temperature. 6.0.3 Depression of P r e v i o u s l y A c t i v a t e d Arsenopyrite 6.0.3.1 Experimental Tests were c a r r i e d out using ore samples described i n the previous s e c t i o n . Two s e r i e s of t e s t s were c a r r i e d out, one using hydrogen peroxide as the oxidant and the other using sodium h y p o c h l o r i t e . In each case a two kilogram sample was ground i n a lab o r a t o r y rod m i l l to 74 percent passing 200 mesh. A cleaned a r s e n o p y r i t e concentrate was prepared according to the 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 f l o t a t i o n was c a r r i e d out at the n a t u r a l pH of 8.0. The cleaned concentrate was f i l t e r e d and s p l i t i n t o p o r t i o n s of 120 grams f o r the 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 g i t a i r c e l l at 800 RPM, a temperature of 20° and pH = 8.0. - J —1 — J J 1 I 10 2 0 3 0 4 0 5 0 6 0 T e m p e r a t u r e / ° C Figure 49 Influence of temperature on arsenopyrite f l o a t a b i l i t y 128 Table 8 F l o t a t i o n Conditions Stage Isopropyl Amyl/ Dowfroth Time Xanthate Xanthate 250 minutes g/tonne g/tonne g/tonne Co n d i t i o n i n g 150 50 5 2 Rougher 37.5 10 17.5 15 1st. Cleaner 20 10 2.5 10 2nd. Cleaner 10 — 2.5 8 Oxidation was c a r r i e d out at pH = 8.0 for the time shown i n the r e s u l t s . F l o t a t i o n was allowed to 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 immersed i n t o the f l o t a t i o n c e l l for each t e s t and the e l e c t r o d e p o t e n t i a l was monitored r e l a t i v e to a calomel e l e c t r o d e . The a r s e n o p y r i t e recovery shown for each t e s t i s that which was achieved with no a d d i t i o n of xanthate f o l l o w i n g o x i d a t i o n . 6.0.3.2 Results and D i s c u s s i o n The r e s u l t s of f l o t a t i o n t e s t s using hydrogen peroxide as an oxidant are shown i n Table 9. The a d d i t i o n of 357 m g / l i t r e of hydrogen peroxide r e s u l t e d i n a s i g n i f i c a n t decrease i n a r s e n o p y r i t e f l o a t a b i l i t y . I ncreasing the c o n d i t i o n i n g time from 5 to 15 minutes gave a f u r t h e r decrease i n f l o a t a b i l i t y . Comparison of the p o t e n t i a l s achieved by the a r s e n o p y r i t e e l e c t r o d e i n these t e s t s with the voltammogram at pH = 8.2 i n Figure 14 reveals that the p o t e n t i a l s l i e partway up the a r s e n o p y r i t e o x i d a t i o n peak. -For the t e s t shown i n Table 9, an a d d i t i o n of up to 900 grams per tonne of e t h y l xanthate gave no f u r t h e r increase i n recovery with 357 m g / l i t r e H 20 2 and an increase of 3.1% with 214 129 m g / l i t r e H 20 2. Table 9 F l o t a t i o n r e s u l t s using hydrogen peroxide as an oxidant Test Conditions Arsenopyrite As No. Eh,mV Recovery % 1 No o x i d a t i o n 213 to 233 97.1 2 357 m g / l i t r e H 20 2 363 1 1 .6 cond. 5 min. 3 357 m g / l i t r e H 20 2 368 3.3 cond. 15 min. 4 214 m g / l i t r e H 20 2 323 t o 353 6.0 cond. 15 min. The r e s u l t s using sodium h y p o c h l o r i t e as an oxidant are shown i n Table 10. The recovery i n the absence of o x i d a t i o n i s lower i n t h i s t e s t s e r i e s than i n the s e r i e s shown i n Table 9. While the concentrate was produced from the same ore i n each case, s l i g h t v a r i a t i o n s i n the rougher f l o t a t i o n procedure caused the v a r i a t i o n i n f l o a t a b i l i t y . As f o r the peroxide, a s i g n i f i c a n t decrease i n ars e n o p y r i t e f l o a t a b i l i t y was achieved through the use of h y p o c h l o r i t e . The a d d i t i o n of h y p o c h l o r i t e required f o r depression to be achieved i s much lower than the requ i r e d peroxide a d d i t i o n . The reason f o r t h i s lower h y p o c h l o r i t e requirement i s not apparent from the el 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 which have been c a r r i e d out. I t i s b e l i e v e d that the two reagents 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 the p o t e n t i a l s . The p o t e n t i a l of the a r s e n o p y r i t e e l e c t r o d e with h y p o c h l o r i t e jumped to approximately +343 mV and then decreased during the c o n d i t i o n i n g p e r i o d . While at 389 m g / l i t r e NaCIO the a d d i t i o n of a large a d d i t i o n of xanthate f a i l e d to r e s u l t i n ars e n o p y r i t e f l o t a t i o n , 130 Table 10 F l o t a t i o n r e s u l t s using sodium h y p o c h l o r i t e as an oxidant Test Conditions Arsenopyrite As No. Eh,mV Recovery % 1 No o x i d a t i o n 203 to 223 83.1 2 389 m g / l i t r e NaCIO 343 to 283 0.0 cond. 5 min. 3 77.6 m g / l i t r e NaCIO 343 to 283 0.0 cond. 5 min. 4 19.4 m g / l i t r e NaCIO 343 to 193 28.7 cond. 5 min. 5 19.4 m g / l i t r e NaCIO 343 to 203 22.5 cond. 15 min. at the 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 percent to 30 percent a d d i t i o n a l a r s e n o p y r i t e recovery. 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 Arsenopyrite 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 -ars e n o p y r i t e concentrates from Giant Y e l l o w k n i f e Mines and from Equity S i l v e r Mines L i m i t e d . 6.0.4.1 Experimental and R e s u l t s . ( i ) Equity Concentrate Tests were c a r r i e d out on a bulk concentrate s u p p l i e d by Equity Mines. The concentrate had been produced at the s i t e by f l o t a t i o n of the 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 concentrate contained approximately f i v e times as much p y r i t e as a r s e n o p y r i t e . Some o x i d a t i o n was observed on the concentrate and i t was th e r e f o r e reground, f i l t e r e d and r e f l o a t e d with 85 grams per tonne i s o p r o p y l xanthate. This 131 r e f l o a t e d concentrate was then f i l t e r e d and s p l i t i n t o p o r t i o n s for the i n d i v i d u a l t e s t s . The t e s t s were c a r r i e d out i n an A g i t a i r f l o t a t i o n c e l l using 150 grams concentrate i n a 500 gram (1.4 l i t r e ) c e l l . The repulped concentrate was found to have a pH of 5.7 and t h i s was adjusted to pH = 9.2 using NaOH. A f t e r the pH was adjusted the o x i d i z i n g agent was added and co 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 f o r 3 minutes with an a d d i t i o n of 5 grams per tonne Downfroth 250. The oxidant used was H 20 2. A l l t e s t s were at 20°C. The t e s t r e s u l t s are shown i n Table 11. Table 11 F l o t a t i o n Test Results w i t h Equity Concentrate H 20 2 C o n d i t i o n i n g Arsenopyrite P y r i t e m g / l i t r e Time Recovery Recovery minutes % % 0 - 92.3 92.4 357 5 17.5 61.4 178 5 18.5 55.2 178 0 35.1 63.2 71 5 23.6 53.2 71 15 26.3 49.2 ( i i ) Giant Y e l l o w k n i f e Concentrate Several s e r i e s of t e s t s were c a r r i e d out on a bulk concentrate produced at the m i n e s i t e . This concentrate had been produced with a d d i t i o n s of copper sulphate as w e l l as xanthate. Microscopy of t h i s m a t e r i a l revealed numerous middlings and i t was t h e r e f o r e reground to .94% minus 200 mesh p r i o r to being used. The reground concentrate 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 per tonne i s o p r o p y l xanthate and cleaned twice more with no f u r t h e r a d d i t i o n s . The cleaned concentrate was f i l t e r e d 132 and s p l i t 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 t e s t s . The pH for each t e s t was adjusted using H 2SO a or NaOH as re q u i r e d . The c o n d i t i o n i n g p e r i o d for each t e s t was 15 minutes and the temperature was 20°C. The r e s u l t s of t e s t s using both a i r and h y p o c h l o r i t e as the oxidant are shown i n Figure 50. While with a i r the arsenopy r i t e recovery i s somewhat lower than for p y r i t e , with h y p o c h l o r i t e the two minerals show e s s e n t i a l l y the same behaviour. The r e s u l t of these t e s t s when compared with those shown i n Table 11 as w e l l as with those f o r the ars 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 i t apparent that each ore occurrence can be expected to show some v a r i a t i o n i n response to depression by o x i d a t i o n . Such f a c t o r s as the r a t i o of p y r i t e to arsenop y r i t e and the previous f l o t a t i o n h i s t o r y of the samples can be expected to i n f l u e n c e the r e s u l t s . A d d i t i o n a l t e s t s were c a r r i e d out on a bulk concentrate prepared i n the l a b o r a t o r y from Giant Y e l l o w k n i f e ore. The concentrate was prepared by g r i n d i n g 2 kilograms of ore in a l a b o r a t o r y rod m i l l to 80% passing 200 mesh. F l o t a t i o n was c a r r i e d out with 65 grams per tonne i s o p r o p y l xanthate 25 grams per tonne amyl xanthate and 10 grams per tonne Dowfroth 250 at pH = 6.0. The concentrate was cleaned twice without f u r t h e r a d d i t i o n s and was then f i l t e r e d and s p l i t i n t o three p o r t i o n s for t e s t i n g . Two t e s t s e r i e s were c a r r i e d out, one with constant potassium permanganate a d d i t i o n and the second with i n c r e a s i n g permanganate a d d i t i o n and constant xanthate a d d i t i o n . In 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 f o l l o w i n g the 133 ! 0 0 8 0 >*60 > O « 4 0 EC 2 0 h 0 FeAsS,77.6mg r'NaCIO • FeS2 ,7 76 mg r'NaCIO D FeAsS , 2.4 Ipm air 1 FeS2 , 2.4 Ipm air 8 9 P H 10 12 Figure 50 Influence of o x i d a t i o n on p y r i t e and ars e n o p y r i t e f l o a t a b i l i t y at i n c r e a s i n g pH 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 to f l o a t . The xanthate and 12 mg. per l i t r e Dowfroth 250 were added and f l o t a t i o n was then c a r r i e d out. The a d d i t i o n of f r o t h e r alone f a i l e d to r e s u l t i n any f l o t a t i o n . 6.0.4.2 Di s c u s s i o n The r e s u l t of t e s t s on the Equity concentrate, using hydrogen peroxide show greater depression of a r s e n o p y r i t e than p y r i t e . Maximum depression of a r s e n o p y r i t e i s achieved with a high peroxide a d d i t i o n (357 m g . / l i t r e ) . Much lower peroxide a d d i t i o n s can be used but with i n c r e a s i n g a r s e n o p y r i t e f l o a t a b i l i t y r e s u l t i n g . 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 peroxide a d d i t i o n s than with high peroxide a d d i t i o n s . Increasing c o n d i t i o n i n g time does not improve the separation of p y r i t e from a r s e n o p y r i t e . The r e s u l t s of t e s t s with Giant Y e l l o w k n i f e concentrate show both p y r i t e and a r s e n o p y r i t e to be e q u a l l y depressed by h y p o c h l o r i t e while a i r o x i d a t i o n r e s u l t s i n p r e f e r e n t i a l depression of a r s e n o p y r i t e . While the a d d i t i o n of potassium permanganate, depressed 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 xanthate r e s u l t e d i n p r e f e r e n t i a l p y r i t e f l o t a t i o n . The complete depression of both minerals followed by 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 with xanthate has not been attempted with oxidants other than permanganate. The 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 bulk p y r i t e -a r s e n o p y r i t e concentrate appears to be f e a s i b l e although Figure 51 Depression of a r s e n o p y r i t e from bulk concentrate with i n c r e a s i n g xanthate a d d i t i o n 136 IOOI ; 71 m g r« N a E t X 8 0 -^ 6 0 -a> S40-o rr 2 5 0 K M n 0 4 / m g I"1 Figure 52 Depression of arse n o p y r i t e from bulk concentrate with i n c r e a s i n g permanganate a d d i t i o n 137 considerable e f f o r t would be required to optimize 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 to vary depending on the p y r i t e to ar 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 concentrate. The r e s u l t s of the various t e s t s i n which p r e v i o u s l y f l o a t e d a r s e n o p y r i t e has been depressed though the use of o x i d i z i n g agents r e v e a l that a greater i n f l u e n c e on f l o a t a b i l i t y i s exerted by the f e r r i c hydroxide surface deposits than by the adsorbed c o l l e c t o r l a y e r . 138 Chapter 7 CONCLUSIONS 1. C y c l i c voltammetric s t u d i e s have revealed the o x i d a t i o n of ar s e n o p y r i t e across the pH range from 7 - 12 to r e s u l t i n the formation of f e r r i c hydroxide surface deposits with the concurrent o x i d a t i o n of a r s e n i c to the arsenate s t a t e and sulphur to the sulphate s t a t e . Across the pH range s t u d i e d , the hydroxide f i l m thickness increases with i n c r e a s i n g pH. At temperatures below 30°C, i n c r e a s i n g temperature r e s u l t s i n i n c r e a s i n g hydroxide f i l m t h i c k n e s s . Across the temperature range from 30°C to 45°C, f i l m development appears to be independent of temperature. At temperatures greater than 45°C, f i l m t hickness increases r a p i d l y with i n c r e a s i n g temperature. Below pH = 7, f e r r i c hydroxide deposits are not formed but the formation of elemental sulphur at the ele c t r o d e surface i s i n d i c a t e d . 2. ESCA s t u d i e s have revealed that most of the arsenate and some of the sulphate i s incorporated i n the f e r r i c hydroxide d e p o s i t s . 3. Ele c t r o d e p o t e n t i a l measurements i n the presence of se v e r a l o x i d i z i n g agents i n d i c a t e that the p o t e n t i a l s required for f e r r i c hydroxide formation (as i n d i c a t e d by c y c l i c voltammetry) w i l l be achieved with these agents. 4. The presence of f e r r i c hydroxide deposits on the surface of ar s e n o p y r i t e i n h i b i t s the o x i d a t i o n of xanthate to dixanthogen. Dixanthogen i s b e l i e v e d to be the a c t i v e c o l l e c t o r species on 139 a r s e n o p y r i t e . 5. F l o t a t i o n s t u d i e s show arsenopy r i t e to be i n c r e a s i n g l y depressed w i t h i n c r e a s i n g pH when the ore s l u r r y has been condi t i o n e d i n the presence of a e r a t i o n p r i o r to xanthate a d d i t i o n . This i n f l u e n c e of pH holds true over the pH range from 7 to 12. At pH l e s s than 7, ars e n o p y r i t e shows high f l o a t a b i l i t y due to the absence of hydroxide f i l m s and the presence of elemental sulphur. Temperatures greater than 40°C r e s u l t i n increased depression of a r s e n o p y r i t e . The i n f l u e n c e of both pH and temperature on the f l o a t a b i l i t y of ars e n o p y r i t e c o r r e l a t e w e l l w i t h the i n t e r p r e t a t i o n s of el e c t r o c h e m i c a l experiments. I t i s concluded that the f l o a t a b i l i t y of ar s e n o p y r i t e i s c o n t r o l l e d by the formation of f e r r i c hydroxide surface d e p o s i t s . The depression of ar s e n o p y r i t e which has p r e v i o u s l y been f l o a t e d with xanthate and the s e l e c t i v e depression of arse n o p y r i t e from bulk concentrates through the use of o x i d i z i n g agents has been demonstrated. I t i s therefore concluded that the f l o a t a b i l i t y of the mineral i s in f l u e n c e d to a greater degree by the f e r r i c hydroxide deposits formed by o x i d a t i o n of the mineral than by adsorbed c o l l e c t o r l a y e r s . 6. Voltammetric s t u d i e s i n the presence of cyanide i n d i c a t e that cyanide does not c o n t r i b u t e to the development of depressant f i l m s on ar 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 a c t i v a t i o n . 7. E l e c t r o c h e m i c a l studies of other minerals i n the Fe-As-S system reveal s i g n i f i c a n t d i f f e r e n c e s i n the nature of surface 140 hydroxide deposits which c o r r e l a t e w e l l with observed d i f f e r e n c e s i n mineral s o l u b i l i t y and may e x p l a i n v a r i a t i o n s i n t h e i r f l o a t a b i l t y with xanthate. 141 Chapter 8 RECOMMENDATIONS FOR FUTURE WORK A d d i t i o n a l work to be c a r r i e d out on the f l o t a t i o n of ars e n o p y r i t e can be d i v i d e d i n t o fundamental and a p p l i e d c a t e g o r i e s . Fundamental research should be d i r e c t e d at f u r t h e r study of the growth and morphology of hydroxide 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 various depressants and a c t i v a t i n g agents on the s t r u c t u r e of the hydroxide l a y e r s should a l s o be i n v e s t i g a t e d . While a d d i t i o n a l e l e c t r o c h e m i c a l s t u d i e s could c o n t r i b u t e much to the understanding of these surface d e p o s i t s , the use of spectroscopic techniques w i l l be required to give a complete a p p r e c i a t i o n of t h e i r nature. A p p l i e d research should be d i r e c t e d at o p t i m i z i n g the u t i l i z a t i o n of surface o x i d a t i o n for the depression of ars e n o p y r i t e from other 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 . While to some extent the optimum c o n d i t i o n s can be expected to be s p e c i f i c f o r each mineral occurrence, 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 other d i s s o l v e d species such as calcium ion or s o l u b l e s i l i c a t e s on ar s e n o p y r i t e depression should a l s o be evaluated. 142 Appendix I Po t e n t i a l / p H diagrams f o r the Iron - Arsenic - Sulphur - Water System Po t e n t i a l / p H diagrams have been prepared f o r the i r o n -ar s e n i c - sulphur - water system at 25°C. The assumptions made for each diagram are as f o l l o w s : Figure 53. Arsenopyrite s t a b i l i t y diagram assuming i r o n , a r s e n i c and sulphur a c t i v i t i e s of 10' 6 M. A s 2 S 2 and A s 2 S 3 are shown as s t a b l e species. Soluble arsenic species are not shown. Figure 54. Arsenopyrite s t a b i l i t y diagram assuming i r o n , a r s e n i c and sulphur a c t i v i t i e s of 1 0 - 3 M. A s 2 S 2 and A s 2 S 3 are shown as s t a b l e s p e c i e s . Figure 55. Arsenopyrite s t a b i l i t y diagram assuming i r o n , a r s e n i c and sulphur a c t i v i t i e s of 1 M. A s 2 S 2 and A s 2 S 3 are shown as s t a b l e s p e c i e s . Soluble arsenic species are not shown. Figure 56. L o e l l i n g i t e s t a b i l i t y diagram assuming i r o n and ars e n i c a c t i v i t i e s of 10~ 3 M. Figure 57. Arsenopyrite s t a b i l i t y diagram c o n s i d e r i n g FeS 2, FeS and FeAs 2 as p o s s i b l e s t a b l e species. S t a b i l i t y domains of A s 2 S 2 are i n d i c a t e d . Soluble arsenic and sulphur species are not shown. A c t i v i t y of each d i s s o l v e d species = 10~ 6 M. 143 F e r r i c arsenate i s not shown i n Figures 53 through 57. At low i r o n and ar s e n i c a c t i v i t y i t has only a narrow s t a b i l i t y region near the f e r r i c ion - f e r r i c hydroxide boundary. The s t a b i l i t y region for f e r r i c arsenate at 1 molar a c t i v i t y of i r o n and a r s e n i c i s shown i n Figure 58. Table 12 Thermodynamic Data at 25°C Spec i e s State G°f 298°K Kcol/mole HS" aq 2.88 80 s 2 - aq 20.5 80 HS0 4- aq -180.69 80 S 0 4 2 " aq -177.97 80 H 2S aq -6.66 80 s s 0 80 H3AsO„ aq -184.0 82 H2AsOft _ aq -181.0 82 HAsOa 2" aq -171.5 82 AsO„ 3' aq -155.8 82 H 3As0 3 aq -154.4 82 H 2As0 3" aq -141.8 82 HAs0 3 2 ~ aq -125.3 82 AsH 3 aq 23.8 82 AS s 0 82 A s 2 S 2 s -17.0 81 A s 2 S 3 s -23.0 81 FeAsS s -26.2 81 FeAs 2 s -12.5 81 FeAsOo s -185.18 83 Fe 3 + aq -1.1 80 Fe 2 + aq -18.85 80 HFe0 2" aq -90.6 80 FeS s -24 80 FeS 2 s -38.3 80 Fe(OH) 2 s -116.3 80 Fe(OH) 3 s -116.5 80 144 F i g u r e 53 A r s e n o p y r i t e s t a b i l i t y d i a g r a m a t 10~ 6 M a c t i v i t y o f d i s s o l v e d s p e c i e s 145 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 d i a g r a m a t 1 0 " 3 M a c t i v i t y o f d i s s o l v e d s p e c i e s 146 F i g u r e 55 L o e l l i n g i t e s t a b i l i t y d i a g r a m a t 1 0 " 3 M a c t i v i t y o f d i s s o l v e d s p e c i e s 147 Figure 56 Arsenopyrite s t a b i l i t y diagram at 1 M a c t i v i t y of dissolved spec ies 148 F i g u r e 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 FeS,FeS 2 and F e A s 2 as s t a b l e p r o d u c t s F i g u r e 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 a t 1 M a c t i v i t y Appendix II Equations Used For The Construction Of The Diagrams 1. FeAsS + 2H + = F e + + + H 2S + As. -0.030 = 0.0591 l o g ( F e + + ) ( H 2 S ) + 0.118 pH 2. F e 2 + + H 3As0 3 + H 2S + H + + 3e" = FeAsS + 3H 20 E = 0.236 - 0.0197 l o g ( F e 2 + ) ( H 3 A s 0 3 ) ( H 2 S ) - 0.0197 pH 3. Fe(OH) 2 + H 3 A S O 3 + SO," + 13H+ + 1le" = FeAsS + 9H 20 E = 0.396 + 0.005 log(H 2As0 3)(SO,"") - 0.070 pH 4. Fe(OH) 2 + H 2As0 3- + SO,'" + 14H+ + 11e" = FeAsS + 9H 20 E = 0.396 +0.005 log(H 2As0 3")(SO,"") - 0.075 pH 5. Fe(OH) 2 + HAs0 3 2" + SO,-" + 15H+ + l i e ' = FeAsS + 9H 20 E = 0.461 + 0.005 log(HAs0 3 2-)(SO,--) - 0.080 pH 6. Fe(OH) 2 + HAs0 3 2- + HS" + 6H + + 3e" = FeAsS + 5H 20 E = 1.025 + 0.0197 log(HAs0 3 2")(HS") - 0.118 pH 7. FeAsS + 5H + + 3e" = AsH 3 + F e + + + H 2S E = - 0.354 - 0.0197 l o g ( A s H 3 ) ( F e + + ) ( H 2 S ) - 0.0985 pH 8. FeAsS + 5H + + 5e~ = AsH 3 + H 2S + Fe E = -0.376 - 0.0118 log(AsH 3)(H 2S) - 0.0591 pH 151 9. FeAsS + 4H + + 5e" = AsH 3 +HS" + Fe E = - 0.459 - 0.0118 log(AsH 3)(HS-) - 0.0473 pH 10. FeAsS + H + + 2e" = AsH 3 + HS" + Fe E = - 0.631 - 0.0295 log(HS") - 0.0295 pH 11. 2 H 3As0 3 + 3HSOq- + 27H + + 24e" = A s 2 S 3 + 18H 20 E = 0.348 + 0.0025 l o g ( H 3 A s 0 3 ) 2 ( H S 0 4 " ) 3 - 0.066 pH 12. 2H 3As0 3 + SO„-- + 30H + + 24e" = A s 2 S 3 + 1 8 H 20 E = 0.363 + 0.0025 l o g ( H 3 A s 0 3 ) 2 ( S O f t " " ) 3 - 0.074 pH 13. A s 2 S 2 + 4H + + 4e" = 2As + 2H2S E = - 0.040 - 0.0148 log ( H 2 S ) 2 - 0.0591 pH 14. A s 2 S 2 + 2H + + 4e" = 2As + 2HS" E = -0.0247 - 0.0148 lo g ( H S " ) 2 - 0.0295 pH 15. 2H 3As0 3 + 2SO„-- + 22H + + I8e" = A s 2 S 2 + 14H 20 E = 0.352 + 0.0033 log(H 3As0 3) 2(SO«"-) 2 - 0.0722 pH 16. 2H 2As0 3" + 2SOft"- + 24H + + I8e" = A s 2 S 2 + 14 H 20 E = 0.412 + 0.0033 log(H 2As0 3") 2(SO„"') 2 - 0.0788 pH 17. H 2As0 3- + 2HS" + 6H + + 2e" = A s 2 S 2 + 6H 20 E = 1.719 + 0.0295 l o g ( H 2 A s 0 3 ) 2 ( H S " ) 2 - 0.1773 pH 18. A s 2 S 3 + 2H + + 2e" = A s 2 S 2 + H 2S 152 E = 0.0143 -0.0148 l o g ( H 2 S ) 2 - 0.0591 pH 19. A s 2 S 3 + H + + 2e" = A s 2 S 2 + HS' E = - 0.193 - 0.0295 log(HS") - 0.0295 pH 20. A s 2 S 2 + S0 4"- + 3H+ + 6e" = A s 2 S 3 + 4H 20 E = 0.395 + 0.0098 log(SO a-") - 0.0788 pH 21. 1/2 A s 2 S 3 + Fe +  + H + + 3e" = FeAsS + 1/2 H 2S E = -0.012 - 0.0295 log(H 2S)V* - 0.0197 pH (Fe + + ) 22. 1/2 A s 2 S 2 + F e + + + 2e" = FeAsS E = -0.025 +0.0295 l o g ( F e + + ) 23. 2H 3As0 3 + Fe(OH) 2 + 8H + + 8e" = FeAs 2 + 8H 20 E = 0.220 + 0.007 l o g ( H 3 A s 0 3 ) 2 - 0.059 pH 24. F e + + + 2As + 2e" = FeAs 2 E = -0.138 + 0.0295 l o g ( F e + + ) 25. 2H 3As0 3 + F e + + + 6H + + 8e" = FeAs 2 + 6H 20 E = 0.135 + 0.007 l o g ( H 3 A s 0 3 ) 2 ( F e + + ) - 0.044 pH 26. FeAs2 + 6H + + 6e" = Fe + 2AsH 3 E = -0.434 - 0.010 l o g ( A s H 3 ) 2 - 0.059 pH 27. FeAs 2 + 6H + + 4e" = F e + + + 2AsH 3 153 E = - 0.447 - 0.015 l o g ( A s H 3 ) 2 ( F e + + ) - 0.089 pH 28. 2H 2As0 3- + Fe(OH) 2 + 10H+ + I8e" = FeAs 2 + H 20 E = 0.358 + 0.007 log ( H 2 A s 0 3 " ) 2 - 0.074 pH 29. 2HAs0 3 2" + Fe(OH) 2 + 12H+ + 8e" = FeAs 2 + 8H 20 E = 0.537 + 0.007 log(HAs0 3 2-) 2 - 0.089 pH 30. 1/2 A s 2 S 3 + F e + + + 2e" = FeAsS + 1/2 S E = - 0.090 + 0.0295 l o g ( F e + + ) 31. F e 2 + + 2SO,2- + 14e" = FeS 2 + 8H 20 E = 0.362 + 0.0042 l o g ( F e 2 + ) (SO,, 2") 2 - 0.068 pH 32. FeS 2 + 4H + + 2e" = F e 2 + + 2H 2S E = - 0.133 - 0.0296 l o g ( F e 2 + ) ( H 2 S ) 2 - 0.118 pH 33. FeS 2 + 2H + + 2e" + As = FeAsS + H 2S E = -0.118 - 0.0295 log(H 2S) - 0.059 pH 34. FeS 2 + As + H + + 2e" = FeAsS + HS" E = -0.325 - 0.0295 log(HS') - 0.0295 pH 35. Fe(OH) 2 + 2SOtt"- + 18H+ + 14e" = FeS 2 + 10H 20 E = 0.412 + 0.0042 l o g ( S O a " - ) 2 - 0.076 pH 36. FeS 2 + H 2As0 3" + 5H + + 5e" = FeAsS + HS" + 3H 20 E = 0.115 - 0.012 l o g (HS~) - 0.059 pH 154 (H 2As0 3~) 37. FeS 2 + H + + 2e~ = FeS + HS" E = - 0.372 - 0.0295 pH - 0.0296 log HS" 38. Fe(OH) 2 + HS" + H + = FeS + 2H 20 1.039 = 0.059 log(HS') - 0.059 pH 39. FeAsS + As + 2H + + 2e" = FeAs 2 + H 2S E = - 0.153 - 0.0295 log(H 2S) - 0.059 pH 40. FeAsS + As + H + + 2e" = FeAs 2 + HS" E = -0.359 - 0.0295 log(HS') - 0.0295 pH 41. FeS + 2H 2As0 3- + 9H + + 8e" = FeAs 2 + HS" + 6H 20 E = 0.229 - 0.007 log (HS") - 0.066 pH ( H 2 A s 0 3 - ) 2 42. HFe0 2" + 2HAs0 3 2" + 13H+ + 8e" = FeAs 2 + 8H 20 E = 0.677 + 0.007 l o g ( H A s 0 3 2 " ) 2 ( H F e 0 2 " ) - 0.096 pH 43. FeAsO« + 3H + = H3AsO„ + F e 3 + pH = 0.12 - 1/3 log(H 3AsO,,)(Fe 3 +) 44. FeAsO« + 3H 20 = Fe(OH) 3 + H2AsO„- + H + pH = 5.30 + log(H 2AsO f l-) 45. FeAsO„ + 3H+ + e- = F e 2 + + H 3AsO a E = 0.766 - 0.0.59 l o g (Fe 2 +) (H3AsO«) - 0.177 pH FeAsOfl + 5H + + 3e" = H 3As0 3 + F e 2 + + H 20 E = 0.647 -0.020 l o g ( H 3 A s 0 3 ) ( F e 2 + ) - 0.098 pH A P P E N D I X I I I 156 • Figure 1. Voltammograms for platinum in presence and absence of xanthate ( 4 9 ) . 100 < 3. Q I— £ -too a. 3 -150 oxidation Reduction j i _ -04 0 0 4 0 8 POTENTIAL/ V (vs. S H.E.) F i g u r e 2. Voltammogram f o r a r s e n o p y r i t e a t pH = 8.2 showing p o t e n t i a l s a c h ieved with o x i d i z i n g agents, -0-6 -0-4 -0-2 0 0-2 0-4 0-6 Polentlol / V vi SHE 20mV ; p H - 8 2 , lOOOrpm F i g u r e 3. M u l t i p l e sweep voltammogram f o r r o t a t i n g e l e c t r o d e at pH = 11.7 5 0 0 / 1 A 20 mV s - i , PH=II.7 - 0 . 8 -0.6 -0.4 -0.2 0.2 0.4 O f i Potential / V vs S H E 157 REFERENCES Heinen, H.J., McClelland, G.E. and Lindstrom, R.E. 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