UBC Theses and Dissertations

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UBC Theses and Dissertations

Electrochemical investigations of various sulphides, xanthates systems and sulphides, iron, xanthates… Moon, Kwang Soon 1975

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ELECTROCHEMICAL INVESTIGATIONS OF VARIOUS SULPHIDES - XANTHATES SYSTEMS AND SULPHIDES - IRON - XANTHATES SYSTEMS by KWANG SOON MOON B . S c , Seoul N a t i o n a l U n i v e r s i t y , 1964 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n t h e Department o f MINERAL ENGINEERING We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA September, 1975 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e H e a d o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f M i n e r a l E n g i n e e r i n g The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, C a n a d a D a t e O c t o b e r 20, 1975 i ABSTRACT An e l e c t r o c h e m i c a l s t u d y was c a r r i e d o u t on v a r i o u s e l e c -t r o d e s o f s p h a l e r i t e , p y r i t e , p y r r h o t i t e , c h a l c o p y r i t e and g a l e n a i n deoxygen a t e d and a i r - s a t u r a t e d s o l u t i o n s a t d i f f e r e n t c o n c e n t r a t i o n s o f p o t a s s i u m e t h y l x a n t h a t e ( K E t X ) , i . e . 0 M, 1 0 ' 5 M, 3 x 10" 5M, 1 0 " 4 M, 3 x I O " 4 M, 10" 4M, 1 0 " 3 M and pH (1 t o 12) a t 25°C ± 1°C. The e f f e c t o f g a l v a n i c c o n t a c t between v a r i o u s s u l p h i d e s and m e t a l l i c i r o n on v a r i o u s e l e c t r o c h e m i c a l c h a r a c t e r i s t i c s o f t h e g a l v a n i c c o u p l e was a l s o i n v e s t i g a t e d . F o l l o w i n g a pseudo N e r n s t r e l a t i o n s h i p , t h e r e s t p o t e n t i a l s o f v a r i o u s e l e c t r o d e systems became more n e g a t i v e w i t h t h e h i g h e r c on-c e n t r a t i o n o f KEtX. The e l e c t r o c h e m i c a l s e r i e s o f t h e s u l p h i d e min-e r a l s i n v e s t i g a t e d , i n p o t a s s i u m e t h y l x a n t h a t e s o l u t i o n , i n t h e o r d e r o f t h e i r n o b i l i t y , were s p h a l e r i t e , p y r i t e , p y r r h o t i t e , c h a l c o p y r i t e and g a l e n a . A l l s u l p h i d e - x a n t h a t e - a i r systems d i s p l a y e d more n o b l e r e s t p o t e n t i a l s than t h e redox p o t e n t i a l o f p o t a s s i u m e t h y l x a n t h a t e a t n a t u r a l pH; however, t h e r e s t p o t e n t i a l s o f most s u l p h i d e s e x c e p t f o r s p h a l e r i t e became more n e g a t i v e than t h e redo x p o t e n t i a l o f p o t -a s s i u m e t h y l x a n t h a t e a t a pH o f 9. S o l u t i o n p u r g i n g w i t h a i r c a u s e d t h e r e s t p o t e n t i a l s o f a l l s u l p h i d e s i n v e s t i g a t e d t o s h i f t i n t h e n o b l e d i r e c t i o n b e c a u s e o f t h e oxygen r e d u c t i o n . - i i -V a r i o u s s u l p h i d e - i r o n g a l v a n i c c o u p l e s a c q u i r e d mixed p o t e n t i a l s which l a y between t h e r e s t p o t e n t i a l s o f s u l p h i d e s and i r o n . The e x a c t v a l u e o f t h e mixed p o t e n t i a l o f the p a r t i c u l a r e l e c t r o d e s y s t e m was v a r i e d d e p e n d i n g on the r e l a t i v e s u r f a c e a r e a o f t h e two e l e c t r o d e s i n g a l v a n i c c o n t a c t . The e x p e r i m e n t a l p o t e n t i a l - p H diagrams were c o n s t r u c t e d f o r t h e m i n e r a l s and m e t a l l i c i r o n i n the s o l u t i o n s o f 0 M and 1 0 ~ 5 M o f p o t a s s i u m e t h y l x a n t h a t e . 7 h r'fU, - i i i -ACKNOWLEDGMENTS The a u t h o r w i s h e s t o e x p r e s s h i s s i n c e r e g r a t i t u d e t o Dr. J . L e j a f o r h i s c o n t i n u e d i n t e r e s t , g u i d a n c e and encouragement t h r o u g h o u t t h e p e r i o d o f s t u d y . A p p r e c i a t i o n i s a l s o e x t e n d e d t o Dr. G. P o l i n g and Dr. S. R. Rao f o r many h e l p f u l d i s c u s s i o n s . The F r e d e r i c k Armand McDiarmid S c h o l a r s h i p and f i n a n c i a l s u p p o r t f r o m t h e N a t i o n a l R e s e a r c h C o u n c i l o f Canada a r e g r a t e f u l l y acknowledged. - iv -TABLE OF CONTENTS Page ABSTRACT i ACKNOWLEDGMENTS i i i LIST OF TABLES v i i LIST OF FIGURES v i i i LIST OF APPENDICES x CHAPTER 1. INTRODUCTION 1 1.1 Objective and Scope of the Investigation 1 1.2 Review of the Literatures 2 1.2.1 Mechanism of Xanthate F lotat ion 2 1.2.1.1 Physical and Chemical Properties of Sulphide Minerals 2 1.2.1.2 Character ist ics of Xanthates and the related Compounds 6 1.2.1.3 Nature of Surface Products in Xanthate F lotat ion of Sulphide Minerals 8 1.2.1.4 Interaction Mechanisms in Xanthate F lotat ion of Sulphide Minerals 12 1.2.2 Role of Iron in Xanthate F lotat ion of Sulphide Minerals 18 1.2.3 Electrochemistry and Thermodynamics 20 1.2.3.1 Electrochemical Reactions 20 1.2.3.2 Electrode Potential 21 1.2.3.3 Mixed Potential 25 1.2.3.4 Rest Potential 32 1.2.4 Implications of Electrochemistry in F lotat ion 34 - v -Pa^e CHAPTER 2. EXPERIMENTAL 37 2.1 E l ec t rode s 37 2.1.1 Saturated Calomel E l e c t r o d e ; Reference E l e c t r ode 37 2.1.2 P lat inum E l e c t r o d e ; A u x i l i a r y E l e c t r ode 38 2.1.3 Working E l e c t r o d e s ; Su lph ide M inera l and Iron E l ec t r ode s 38 2.1.4 Glass E l e c t r ode 42 2.2 Reagents 42 2.3 E l e c t r o l y t i c C e l l 43 2.4 E l e c t r o n i c Instruments 44 2.5 Exper imental Set up and Procedures 46 CHAPTER 3. EXPERIMENTAL RESULTS AND DISCUSSIONS 49 3.1 P r e l i m i n a r y I n ve s t i g a t i on s 49 3.1.1 Potassium Ethy l Xanthate S o l u t i o n 49 3.1.2 Rest P o t e n t i a l and Mixed P o t e n t i a l w i t h re spec t to Time 53 3.1.3 E f f e c t of S o l u t i o n S t i r r i n g on P o t e n t i a l s 55 3.1.4 Mixed P o t e n t i a l vs Distance between Anodic and Cathodic E lec t rodes 57 3.2 Rest P o t e n t i a l of Sulphide M ine ra l s and M e t a l l i c Iron 58 3.2.1 Rest P o t e n t i a l of Galena 59 3.2.2 Rest P o t e n t i a l of Other Su lph ide M ine ra l s 64 3.2.3 Rest P o t e n t i a l of M e t a l l i c Iron 70 - v i -Page 3.3 Mixed P o t e n t i a l o f S u l p h i d e M i n e r a l - M e t a l l i c I r o n G a l v a n i c C o u p l e s 71 3.4 Meaning o f the C o n c e n t r a t i o n c o r r e s p o n d i n g t o t h e same r e s t p o t e n t i a l o f two d i f f e r e n t e l e c t r o d e systems 75 3.5 I m p l i c a t i o n o f R e l a t i v e S u r f a c e A r e a s o f E l e c t r o d e s f o r G a l v a n i c C o u p l e i n F l o t a t i o n 78 3.6 P o t e n t i a l s as a F u n c t i o n o f S o l u t i o n pH 80 SUMMARY AND CONCLUSION 84 REFERENCES 87 APPENDICES 96 - v i i -L IST OF TABLES Page T a b l e 1.1 ; E l e c t r o p h y s i c a l and E l e c t r o c h e m i c a l P r o p e r t i e s o f S u l p h i d e M i n e r a l s 3 T a b l e 2.1 ; O r i g i n , G e o m e t r i c A r e a , R e s i s t a n c e and A n a l y s i s o f Working E l e c t r o d e Specimens 41 T a b l e 3.1 ; R e s u l t s o f S t a t i s t i c a l L i n e a r R e g r e s s i o n A n a l y s i s on R e s t P o t e n t i a l and Mixed P o t e n t i a l o f v a r i o u s E l e c t r o d e Systems as a F u n c t i o n o f M [ E t X ~ ] 69 LIST OF FIGURES Pag F i g u r e 1.1 ; S t a n d a r d Redox P o t e n t i a l o f XJP/X" c o u p l e as a F u n c t i o n o f Hydrocarbon C h a i n L e ngth 9 F i g u r e 1.2 ; F o r m a t i o n o f Mixed P o t e n t i a l 27 F i g u r e 1.3 ; Mixed P o t e n t i a l o f S u l p h i d e M i n e r a l (MS) - I r o n (Fe) c o u p l e i n A i r - S a t u r a t e d w a ter a t N e u t r a l pH ( s c h e m a t i c ) 29 3+ 2+ F i g u r e 1.4 ; Mixed P o t e n t i a l o f S u l p h i d e M i n e r a l - Fe /Fe System i n A c i d i c C o n d i t i o n ( s c h e m a t i c ) 31 F i g u r e 2.1 ; P r e p a r a t i o n o f Working E l e c t r o d e s f o r S u l p h i d e M i n e r a l s 39 F i g u r e 2.2; ; S c h e m a t i c Diagram o f a T y p i c a l E x p e r i m e n t a l S e t up 47 F i g u r e 3.1 ; pH o f KEtX S o l u t i o n i n t h e p r e s e n c e o f G a l e n a E l e c t r o d e 50 F i g u r e 3.2 ; C o n d u c t i v i t y o f KEtX S o l u t i o n a t 25°C 52 F i g u r e 3.3 ; R e s t P o t e n t i a l o f C h a l c o p y r i t e and I r o n , and Mixed P o t e n t i a l o f C h a l c o p y r i t e w i t h I r o n w i t h r e s p e c t t o Time 54 F i g u r e 3.4 ; The E f f e c t o f S o l u t i o n S t i r r i n g and t h e D i s t a n c e between t h e two Working E l e c t r o d e s on t h e Mixed P o t e n t i a l 56 F i g u r e 3.5 ; The S c h e m a t i c P r e s e n t a t i o n o f t h e R e s t P o t e n t i a l s o f a M e t a l S u l p h i d e (MS) i n t h e A e r a t e d P o t a s s i u m E t h y l X a n t h a t e S o l u t i o n s 61 F i g u r e 3.6 ; R e s t P o t e n t i a l o f G a l e n a vs C o n c e n t r a t i o n o f E t X - a t v a r i o u s pH and Gas B u b b l i n g 62 F i g u r e 3.7 ; R e s t P o t e n t i a l o f S u l p h i d e M i n e r a l s and M ixed P o t e n t i a l o f a S u l p h i d e M i n e r a l and I r o n vs M [ E t X - ] (400 cc/min Argon P u r g i n g , N a t u r a l pH, 25°C) 65 i x -Page F i g u r e 3.8 ; R e s t P o t e n t i a l o f S u l p h i d e M i n e r a l s and Mixed P o t e n t i a l o f a S u l p h i d e M i n e r a l and I r o n vs M [ E t X ~ ] (400 cc/min A i r P u r g i n g , N a t u r a l pH, 25°C) 66 F i g u r e 3.9 ; P o t e n t i o d y n a m i c P o l a r i z a t i o n o f C h a l c o p y r i t e , I r o n and Mixed System a t 0 M [ E t X - ] 72 F i g u r e 3.10 ; P o t e n t i o d y n a m i c P o l a r i z a t i o n o f C H a l c o p y r i t e , I r o n and Mixed System a t I O - 3 M [ E t X - ] 73 F i g u r e 3.11 ; Meaning o f t h e c o n c e n t r a t i o n c o r r e s p o n d i n g t o the same r e s t p o t e n t i a l o f two d i f f e r e n t e l e c t r o d e systems 76 - x -LIST OF APPENDICES Page Appendix 1.1 ; R e s t P o t e n t i a l o f P y r i t e ( F e S 2 ) 96 Appendix 1.2 ; R e s t P o t n e t i a l o f M a r c a s i t e ( F e S 2 ) 98 Appendix 1.3 ; R e s t P o t e n t i a l o f P y r r h o t i t e (Fe-j X S ) 98 App e n d i x 1.4 ; R e s t P o t e n t i a l o f G a l e n a (PbS) 99 Appendix 1.5 ; R e s t P o t e n t i a l o f C h a l c o p y r i t e ( C u F e S 2 ) 102 Appendix 1.6 ; R e s t P o t e n t i a l o f B o r n i t e (CUgFeS^) 102 Appendix 1.7 ; R e s t P o t e n t i a l o f C h a l c o c i t e ( C i ^ S ) 103 Appendix 1.8 ; Rest P o t e n t i a l o f C o v e l l i t e (CuS) 103 Appendix 1.9 ; R e s t P o t e n t i a l o f S p h a l e r i t e (ZnS) 105 Appendix 1.10 ; R e s t P o t e n t i a l o f M o l y b d e n i t e (MoS 2) 106 Appendix 1.11 ; R e s t P o t e n t i a l o f A r g e n t i t e ( A g 2 S ) 106 Appendix 1.12 ; Rest P o t e n t i a l o f S t i b n i t e ( S b 2 S 3 ) 106 Appendix 2.1 ; pH vs [ E t X ~ ] i n D e a e r a t e d S o l u t i o n 107 Appendix 2.2 ; pH vs [ E t X ~ ] i n A i r - s a t u r a t e d S o l u t i o n 109 Appendix 3 ; The Mixed P o t e n t i a l vs t h e D i s t a n c e between two E l e c t r o d e s 110 Appendix 4.1 ; P o t e n t i a l - p H Diagram f o r S p h a l e r i t e 112 Appendix 4.2 ; P o t e n t i a l - p H Diagram f o r P y r i t e 113 Appendix 4.3 ; P o t e n t i a l - p h Diagram f o r P y r r h o t i t e 114 Appendix 4.4 ; P o t e n t i a l - p H Diagram f o r C h a l c o p y r i t e 115 Appendix 4.5 ; P o t e n t i a l - p H Diagram f o r G a l e n a 116 Appendix 4.6 ; P o t e n t i a l - p H Diagram f o r I r o n 117 Appendix 4.7 ; P o t e n t i a l - p H Diagram f o r C h a l c o p y r i t e and I r o n G a l v a n i c C ouple 118 - 1 -CHAPTER 1 INTRODUCTION 1.1 O b j e c t i v e and Scope o f t h e I n v e s t i g a t i o n A l t h o u g h t h e r e c o v e r y o f s u l p h i d e m i n e r a l s by x a n t h a t e f l o t a t i o n has been a s u c c e s s i n t h e m i n e r a l p r o c e s s i n g i n d u s t r y s i n c e t h e 1930's, t h e u n d e r s t a n d i n g o f t h e b a s i c r e a c t i o n mechanism o f x a n t h a t e a d s o r p t i o n on t h e m i n e r a l s u r f a c e s which r e n d e r s t h e s u r f a c e s o f m i n e r a l s h y d r o p h o b i c i s s t i l l not f u l l y u n d e r s t o o d . One o f t h e developments i n t h e t h e o r y o f s u l p h i d e m i n e r a l f l o t a t i o n i s t h e e l e c t r o c h e m i c a l i n t e r p r e t a t i o n o f t h e x a n t h a t e ad -s o r p t i o n mechanism. Most o f t h e r e p o r t e d 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 o f f l o t a t i o n mechanisms have been c a r r i e d o u t , however, on t h e h i g h l y i d e a l i z e d systems o f m e t a l s , e.g., P t , Au and Cu o r g a l e n a ( P b S ) . T h e r e i s ample e v i d e n c e t h a t d i f f e r e n t m i n e r a l s m i g h t behave i n a c o m p l e t e l y d i f f e r e n t manner under i d e n t i c a l f l o t a t i o n c o n d i t i o n s . No s e l e c t i v e f l o t a t i o n would be p o s s i b l e i f a l l m i n e r a l s r e s p o n d e d i n t h e same way under t h e g i v e n c o n d i t i o n s . The f i r s t o b j e c t i v e o f t h i s i n -v e s t i g a t i o n i s t o p r o v i d e a s e t o f c o n s i s t e n t b a s i c e l e c t r o c h e m i c a l d a t a on t h e f i v e most i m p o r t a n t s u l p h i d e m i n e r a l s i n f l o t a t i o n , i . e . , g a l e n a , c h a l c o p y r i t e , p y r i t e , p y r r h o t i t e and s p h a l e r i t e . The d e t r i m e n t a l e f f e c t o f s t e e l g r i n d i n g medium i n s e l e c t i v e f l o t a t i o n such as i n l e a d - z i n c s e p a r a t i o n was o b s e r v e d by Rey and Formanek ( 1 ) . They c l a i m e d t h a t p e b b l e g r i n d i n g c o u l d be t h e remedy t o t h i s problem. The g a l v a n i c c o n t a c t - 2 -between m e t a l l i c i r o n and s u l p h i d e m i n e r a l s c o u l d p l a y a s i g n i f i c a n t r o l e i n f l o t a t i o n o f s u l p h i d e m i n e r a l s by c h a n g i n g t h e s u r f a c e p o t e n t i a l s o f s u l p h i d e m i n e r a l s . The s e c o n d o b j e c t i v e o f t h e c u r r e n t i n v e s t i g a t i o n i s t o s t u d y t h i s e f f e c t o f g a l v a n i c c o n t a c t between m e t a l l i c i r o n and s u l p h -i d e m i n e r a l s . 1.2 Review o f t h e L i t e r a t u r e s 1.2.1 Mechanism o f X a n t h a t e F l o t a t i o n S i n c e x a n t h a t e was adopt e d as a c o l l e c t o r f o r s u l p h i d e min-e r a l f l o t a t i o n , many i n v e s t i g a t o r s have a t t e m p t e d t o e x p l a i n t h e a d s o r p -t i o n mechanism o f x a n t h a t e on s u l p h i d e m i n e r a l s . Some o f t h o s e s i g -n i f i c a n t r e a c t i o n mechanisms have been r e v i e w e d r e c e n t l y by F l e m i n g and K i t c h e n e r ( 2 ) , Rao ( 3 ) , G r a n v i l l e , F i n k e l s t e i n and A l l i s o n ( 4 ) , G u t i e r r e z ( 5 ) , and L e j a ( 6 ) . 1.2.1.1 P h y s i c a l and Chemical P r o p e r t i e s o f S u l p h i d e M i n e r a l s In g e n e r a l , s u l p h i d e m i n e r a l s a r e s e m i c o n d u c t o r s w i t h v a r y i n g c o n c e n t r a t i o n s o f e l e c t r o n s and h o l e s as shown i n T a b l e 1.1, a d o p t e d f r o m G l e m b o t s k i i and Klimenko ( 7 ) . A comprehensive r e v i e w on t h e band t h e o r y o f s e m i c o n d u c t i v i t y was g i v e n by M u l a r ( 8 ) . G a l e n a i s a s e m i c o n d u c t o r w i t h a band g a p - w i d t h o f 0.37 eV a t room t e m p e r a t u r e . L e a d - r i c h g a l e n a e x h i b i t s n - t y p e e l e c t r o n s e m i -TABLE 1.1 : E l e c t r o p h y s i c a l and E l e c t r o c h e m i c a l P a r a m e t e r s o f S u l p h i d e M i n e r a l ( f r o m Ref. 7 ) S u l p h i d e M i n e r a l s S p e c i f i c C o n d u c t i v i t y (mho • cm" ) T h e r m o e l e c t r i c E f f e c t U V / ° C ) Ne*/Np** E l e c t r o c h e m i c a l i n 50 m l . d i s t i P o t e n t i a l (mV) l i e d w a t e r as i t i s A f t e r 30 min. 0? b u b b l i n q G alena (PbS) 0.763 x 1 0 " 3 -192 2.114 - 310 - 152 C h a l c o p y r i t e (CuFeS2) 3.59 x 1 0 " 3 -337 2.40 - 330 + 5 P y r i t e ( F e S 2 ) 4.23 x 1 0 " 3 +120 0.618 - 150 + 37 B o r n i t e ( C u 5 F e S 4 ) 1.84 x 1 0 " 3 + 78 0.602 - 290 - 136 C h a l c o c i t e ( C u 2 S ) 29.8 x 1 0 " 3 +164 0.643 - 200 - 96 P y r r h o t i t e ( F e , . x S ) 0.618 x 1 0 " 3 + 3.5 0.563 - 340 - 162 S p h a l e r i t e (ZnS) 8.90 x 1 0 " 8 1.0 -500 600 - 70 — — ^ — — — — — — — Ne i s t h e c o n c e n t r a t i o n o f c h a r g e c a r r y i n g e l e c t r o n s i n a m i n e r a l Np i s t h e c o n c e n t r a t i o n o f c h a r g e c a r r y i n g h o l e s i n a m i n e r a l . - 4 -c o n d u c t i v i t y due t o t h e e x c e s s e l e c t r o n s n e u t r a l i z i n g t h e i n t e r s t i t i a l l e a d i o n s . E l e c t r o n donors such as B i a t Pb s i t e s can d e v e l o p n-semi-c o n d u c t i v i t y a l s o . P-type h o l e semi c o n d u c t i v i t y can be c r e a t e d by i o n i z e d e l e c t r o n a c c e p t o r s such as h o l e s i n a s u l p h u r r i c h g a l e n a o r Ag a t Pb s i t e s . Thus b o t h t y p e s o f semi c o n d u c t i v i t y can be f o u n d i n g a l e n a : however, most o f t h e n a t u r a l g a l e n a e x h i b i t s l e a d - r i c h n - t y p e s e m i c o n d u c t i n g c h a r a c t e r i s t i c s . A c c o r d i n g t o L e j a ( 6 ) , a v a r i a t i o n o f 1 0 ~ 6 M p e r c e n t i n t h e c o m p o s i t i o n o f PbS r e p r e s e n t s t h e d i f f e r e n c e between l e a d s a t u r a t i o n ( n - t y p e ) where a c t i v i t y o f l e a d becomes u n i t y and s u l p h u r s a t u r a t i o n ( p - t y p e ) where a c t i v i t y o f s u l p h u r becomes u n i t y . P e t e r s and Majima (9) show t h a t t h e e l e c t r i c a l c o n d u c t i v i t y o f p y r i t e can v a r y w i d e l y because i t i s a t y p i c a l s e m i c o n d u c t o r w i t h l a r g e e f f e c t s f r o m i m p u r i t i e s such as c o p p e r and a r s e n i c . A c c o r d i n g t o S p r i n g e r ( 1 0 ) , p y r i t e , d e v i a t i n g s l i g h t l y f r o m t h e i d e a l s t o i c h i o m e t r i c c o m p o s i t i o n , can r e s u l t i n d i f f e r e n t t y p e s o f s e m i c o n d u c t i v i t i e s , i . e . , i r o n - r i c h n - t y p e s e m i c o n d u c t i v i t y and s u l p h u r - r i c h p - t y p e s e m i c o n d u c t i v i t y . S p r i n g e r f o u n d , however, t h a t l a t t i c e i m p u r i t i e s p l a y e d a more s i g n i f i c a n t r o l e i n d e t e r m i n i n g t h e t y p e s o f s e m i c o n d u c t i v i t i e s t h a n t h e s t o i c h i o m e t r i c dev-i a t i o n f o r t h e p y r i t e s t u d i e d . The e f f e c t s o f s e m i c o n d u c t i v i t y on t h e f l o t a t i o n o f s u l p h i d e m i n e r a l s have been s t u d i e d by S p r i n g e r ( 1 0 ) , S i m k o v i c h ( 1 1 ) , Chekanov ( 1 2 ) , S h a f e e v ( 1 3 ) , P l a k s i n e t a l ( 1 4 ) , S w i n k e l s and M u l a r ( 1 5 ) , P r o s s e r (16) and G u a r n a s h e l l i ( 1 7 ) . P l a k s i n and c o - w o r k e r s m a i n t a i n t h a t oxygen a d -s o r p t i o n on s u l p h i d e m i n e r a l s l o w e r s t h e Fermi l e v e l o f t h e m i n e r a l s so t h a t - 5 -xanthate ions can adsorb on them. L e p e t i c ' s study (18) on na tu ra l f l o t a b i l i t y o f dry-ground c h a l c o p y r i t e tends t o agree w i t h P l a k s i n ' s v iew. Chekanov (12) a t t r i b u t e d the high f l o t a b i l i t y of copper su lph ide s to the n-type e l e c t r o n s e m i c o n d u c t i v i t y . G u a r n a s c h e l l i (17) found t ha t p-type galena adsorbed xanthate th ree t imes more than n-type ga l ena . Both p h y s i c a l c r y s t a l ! o g r a p h i c p r o p e r t i e s and chemical im -p u r i t i e s have a more pronounced e f f e c t i n semiconductors than i n m e t a l l i c conductors due to the space-charge c h a r a c t e r i s t i c s of semiconductors . Le ja (20) s t r e s sed the d e f i n i t e dependence o f f l o t a b i l i t y and n o n - f l o t a b i l i t y of galena (PbS), c a s s i t e r i t e ( S n0 2 ) , d e s c l o i z i t e (Pb, Zn, V 2 0g m ine ra l s ) upon the i m p u r i t i e s i n the c r y s t a l l a t t i c e . V e t t e r (21) mentioned t h a t p o l y c r y s t a l l i n e su r f ace or chemical i m p u r i t i e s cou ld form a l o c a l g a l v an i c c e l l on a e l e c t r o d e su r face to g i ve a mixed p o t e n t i a l . Accord ing to Novak and Ban (22 ) , the r e s t p o t e n t i a l o f s p h a l e r i t e became l e s s noble e x p o n e n t i a l l y w i t h an i n c r e a s i n g content o f i r o n impu r i t y i n p l ace o f z i n c . (Appendix 1.9) For the p o s s i b l e means of s u r f a ce energy l e v e l m o d i f i c a t i o n , Carta e t a l (23) d i s cus sed the a c t i o n o f modulat ing agents , heat t rea tment , r a d i o - i s o t o p e i r r i d a t i o n , neutron or ion bombardment, doping by s o l i d d i f f u s i o n under c o n t r o l l e d c o n d i t i o n s , types of communition and t r i b o -e l e c t r i c charg ing of the minera l p a r t i c l e s . I t i s obvious t h a t m ine ra l s can be f l o a t e d b e t t e r i f t h e i r su r face energy l e v e l can be mod i f i ed i n a p r e f e r a b l e way by the va r i ou s means de sc r i bed above. - 6 -Thus, the i l l u m i n a t i o n of l i g h t , j u s t as any o t h e r form o f energy, can help xanthate to overcome the p o t e n t i a l b a r r i e r and to ad-sorb at the s u r f a c e o f s u l p h i d e m i n e r a l s . A t the same time, the l i g h t a l s o can enhance the a d s o r p t i o n of oxygen on the s u r f a c e o f s u l p h i d e minerals so t h a t xanthate a d s o r p t i o n i s a f f e c t e d . P l a k s i n and co-workers (14) s t u d i e d the enhanced f l o t a t i o n behavior of i l m e n i t e on i l l u m i n a t i o n . Guarnaschel1i (17) found t h a t a d s o r p t i o n o f xanthate on galena was i n c r e a s e d with i n c r e a s i n g the l i g h t photon energy above the energy gap value (0.37 eV) o f galena. 1.2.1.2 C h a r a c t e r i s t i c s o f Xanthates and the R e l a t e d Compound Xanthate, a common name f o r the s a l t s o f a l k y ! d i t h i o c a r b o n i c a c i d , has the s t r u c t u r a l formula S (A) General P r o p e r t i e s R - 0 - C (1) where R i s non-polar a l k y ! group r e p r e s e n t i n g C n H 2 n + l + + a l k a l i metal i on such as K or Na which forms s a l t s . and M + i s u s u a l l y A l k a l i metal xanthates, e x c l u s i v e l y used i n the mineral f l o t a t i o n i n d u s t r y with much suc c e s s , are completely water s o l u b l e . They are near white and possess no odor i n the pure s t a t e . - 7 -X a n t h a t e s can be p r e p a r e d f r o m an a l c o h o l , p o t a s s i u m o r sodium h y d r o x i d e and c a r b o n d i s u l p h i d e so t h a t a s e r i e s o f x a n t h a t e homologs can be p r e p a r e d d e p e n d i n g on t h e a l c o h o l u s e d . R - OH + KOH + C S 2 z+±z R - OCSgK + H 20 (2) A c o m p r e h e n s i v e monograph on x a n t h a t e s and r e l a t e d compounds was w r i t t e n by Rao ( 3 ) . (b) Redox P o t e n t i a l o f a D i x a n t h o g e n - X a n t h a t e C o u p l e The r e d o x p o t e n t i a l o f t h e x a n t h a t e - d i x a n t h o g e n c o u p l e i s g i v e n by t h e f o l l o w i n g e q u a t i o n : X 2 + 2e =5=^ 2X" (3) A X RT A 2 E = E° + — In — (4) X" 2F ,2 The s i g n c o n v e n t i o n u s e d i n t h e p r e s e n t s t u d y i s i n a c c o r d w i t h IUPAC c o n v e n t i o n . I f t h e c o n c e n t r a t i o n o f x a n t h a t e ( [ X " ] ) i s low enough so t h a t t h e a c t i v i t y o f x a n t h a t e (Ax" ) can be t a k e n as e q u a l t o [ X " ] , and i f t h e s o l u t i o n i s s a t u r a t e d w i t h d i x a n t h o g e n a t 25°C, E = E° - 0.0591 l o g [ X " ] (5) - 8 -The s t a n d a r d r e d o x p o t e n t i a l (E°) o f x a n t h a t e - d i x a n t h o g e n c o u p l e has been s t u d i e d by many i n v e s t i g a t o r s such as K a k o v s k i i e t a l ( 2 4 ) , T o l u n and K i t c h e n e r ( 2 5 ) , Majima and Takeda (26) and W i n t e r and Woods ( 2 7 ) , a l l by t h e p o t e n t i o s t a t i c methods, and Du R i e t z (28) by t h e i o d i n e t i t -r a t i o n method. (See F i g . 1.1.). As can be seen i n F i g . 1.1, o n l y t h e two most r e c e n t i n v e s t -i g a t i o n s a g r e e w i t h each o t h e r i n t h e i r s t a n d a r d r e d o x p o t e n t i a l o f x a n t h a t e - d i x a n t h o g e n r e d o x c o u p l e . The a v e r a g e o f t h e f i v e r e p o r t e d s t a n d a r d r e d o x p o t e n t i a l s o f p o t a s s i u m e t h y l x a n t h a t e - d i x a n t h o g e n c o u p l e was -0.057 V o l t which a g r e e d w i t h W i n t e r and Woods' (27) r e s u l t . T h e r e -f o r e , t h e s t a n d a r d r e d o x p o t e n t i a l o f t h e p o t a s s i u m e t h y l x a n t h a t e s y s t e m i s t a k e n t o be -0.057 V o l t s i n t h e p r e s e n t i n v e s t i g a t i o n . T o l u n and K i t c h e n e r ( 2 5 ) , Majima and Takeda (26) and W i n t e r and Woods (27) a g r e e t h a t t h e r e d o x c o u p l e o f X 2/X~ i s r e v e r s i b l e and t h e s t a n d -a r d r e d o x p o t e n t i a l o f X 2/X~ c o u p l e i s d e c r e a s e d by 59 mV w i t h one o r d e r o f magnitude i n c r e a s e i n x a n t h a t e c o n c e n t r a t i o n as i n d i c a t e d by Eqn. ( 5 ) . 1.2.1.3 N a t u r e o f S u r f a c e P r o d u c t s i n X a n t h a t e F l o t a t i o n o f S u l p h i d e M i n e r a l s I t i s g e n e r a l l y a c c e p t e d t h a t t h e i n t e r a c t i o n between x a n t h a t e and s u l p h i d e m i n e r a l s i s i r r e v e r s i b l e ( 2 9 ) . I n f r a r e d and u l t r a v i o l e t s p e c t r o s c o p y have been used s u c c e s s f u l l y I L _J L L I I I i 1 2 3 4 5 6 7 8 9 10 n i n C n H 2 n + l 0 C S 2 K -F i a u r e 1.1: S t a n d a r d Redox P o t e n t i a l o f X^/X" CouDle as a F u n c t i o n o f H y d r o c a r b o n C h a i n L e n g t h - 10 -t o i n v e s t i g a t e t h e s u r f a c e r e a c t i o n p r o d u c t s by many i n v e s t i g a t o r s , n o t a b l y by L e j a and P o l i n g and c o - w o r k e r s (30, 31, 32, 33, 34, 35, 3 6 ) . The r e a c t i o n p r o d u c t s o f x a n t h a t e and g a l e n a were i d e n t i f i e d as l e a d x a n t h a t e by G r e e n ! e r ( 3 7 ) , N i c o l ( 3 8 ) , A l l i s o n and F i n k e l s t e i n ( 3 9 ) , and A l l i s o n e t a! (40) and as l e a d x a n t h a t e and d i x a n t h o g e n by L e j a and P o l i n g ( 31, 32, 3 3 ) , Abramov ( 4 1 ) , and P r a s a d and Rao ( 4 2 ) . L e j a and P o l i n g d i f f e r e n t i a t e s u r f a c e l e a d x a n t h a t e (PbX) f r o m b u l k l e a d x a n t h a t e ( P b X 2 ) as h a v i n g 1:1 c o o r d i n a t i o n . S h e i k h and L e j a (43) m e n t i o n e d t h a t t h e h y d r o p h o b i c c o a t i n g formed on t h e s u r f a c e o f c o p p e r s u l p h i d e s c o n t a i n s c o - a d s o r b e d c u p r o u s x a n t h a t e and d i x a n t h o g e n . Yamasaki and U s u i (46) r e a c h e d t h e s i m i l a r c o n c l u s i o n d u r i n g t h e i r s p e c t r o s c o p i c s t u d y on x a n t h a t e t r e a t e d s p h a l e r i t e . They c l a i m e d t h a t t h e s u r f a c e p r o d u c t s were c h e m i s o r b e d z i n c x a n t h a t e (ZnX) w i t h 1:1 c o o r d i n a t i o n and p h y s i c a l l y c o a d s o r b e d b u l k z i n c x a n t h a t e ( Z n X 2 ) . From an e l e c t r o c h e m i c a l s t u d y on g a l e n a , Tolum and K i t c h e n e r (25) and Woods (19, 44) c o n c l u d e d t h a t t h e a d s o r b e d d i x a n t h o g e n i s t h e s p e c i e s r e s p o n s i b l e f o r f l o t a t i o n and t h a t l e a d x a n t h a t e i s h y d r o p h i l i c . The non-f l o t a b i l i t y o f b u l k p r e c i p i t a t e d l e a d x a n t h a t e o b t a i n e d by M e l l g r e n (45) a l s o s u p p o r t s t h i s view. F o r p y r i t e , v a r i o u s i n v e s t i g a t o r s a g r e e d t h a t d i x a n t h o g e n was t h e s o l e r e a c t i o n p r o d u c t formed on t h e s u r f a c e . An e l e c t r o c h e m i c a l s t u d y by Majima and Takeda (26) and a s p e c t r o p h o t o m e t r y and e l e c t r o c h e m i c a l s t u d y by F u e r s t e n a u e t a l (47) and A l l i s o n e t a l (40) r e a c h e d t h e same - 11 -c o n c l u s i o n r e g a r d i n g d i x a n t h o g e n f o r m a t i o n on p y r i t e s u r f a c e . A l l i s o n e t a l (40) i n d i c a t e d t h a t , d e p e n d i n g on t h e r e s t p o t e n t i a l s o f v a r i o u s s u l p h i d e m i n e r a l s , e i t h e r m e t a l x a n t h a t e (MX) o r d i x a n t h o g e n ( X 2 ) was formed on t h e s u r f a c e s . I f t h e r e s t p o t e n t i a l o f t h e m i n e r a l was n o b l e w i t h r e s p e c t t o t h e r e d o x p o t e n t i a l o f X 2/X~ c o u p l e , t h e p r i n c i p a l s u r f a c e p r o d u c t formed was d i x a n t h o g e n . On t h e o t h e r hand, metal x a n t h a t e (MX) was t h e p r i n c i p a l r e a c t i o n p r o d u c t f o r m e d on t h e m i n e r a l when t h e r e s t p o t e n t i a l o f t h e m i n e r a l was more n e g a t i v e than t h e r e d o x p o t e n t i a l o f X 2/X~ c o u p l e . In t h e l i g h t o f t h e above o b s e r v a t i o n and c o n s i d e r i n g t h a t even one m i n e r a l can d i s p l a y v a r i o u s r e s t p o t e n t i a l s ( s e e S e c t i o n 1.2.3.4), t h e r e a c t i o n p r o d u c t s o f a s u l p h i d e m i n e r a l can v a r y d e p e n d i n g on t h e r e s t p o t e n t i a l o f t h e m i n e r a l under t h e p a r t i c u l a r c o n d i t i o n s . V a r i o u s i n v e s t i g a t o r s s t u d i e d the enhanced f l o t a b i l i t y o f s u l p h i d e m i n e r a l s by e l e m e n t a l s u l p h u r d u r i n g x a n t h a t e f l o t a t i o n s . I s h i h a r a and Kagami (48) r e p o r t e d t h a t f l o t a b i l i t y o f p y r r h o t i t e was enhanced by t h e p r e s e n c e o f e l e m e n t a l s u l p h u r on t h e s u r f a c e . E a d i n g t o n and P r o s s e r (49) f o u n d a s m a l l amount o f e l e m e n t a l s u l p h u r on s u l p h i d e m i n e r a l s . Majima (50) p r o p o s e d t h a t t h e d i f f i c u l t y o f s e l e c t i v e f l o t a t i o n o f p y r i t i c l e a d - z i n c o r e c o u l d be c a u s e d by e l e m e n t a l s u l p h u r formed on t h e l e a d - z i n c m i n e r a l s due t o t h e g a l v a n i c c o n t a c t w i t h t h e r e l a t i v e l y n o b l e p y r i t e . R e c e n t s t u d y by A l l i s o n (51) i n d i c a t e d , however, t h a t e l e m e n t a l s u l p h u r , was n o t t h e s o l e o r main c a u s e o f t h e f l o t a b i l i t y o f o x i d i z e d s u l p h i d e m i n e r a l s i n x a n t h a t e f l o t a t i o n . - 12 -1.2.1.4 I n t e r a c t i o n Mechanisms i n X a n t h a t e F l o t a t i o n o f S u l p h i d e M i n e r a l s (A) C h e m i c a l r e a c t i o n t h e o r y The c h e m i c a l r e a c t i o n t h e o r y i s one o f t h e f i r s t p r o p o s e d by T a g g a r t and co - w o r k e r s (52) i n the e a r l y 1930's. They t h e o r i z e d t h a t h y d r o -p h i l i c s u l p h i d e m i n e r a l s u r f a c e s a r e r e n d e r e d h y d r o p h o b i c as a r e s u l t o f lower s o l u b i l i t y o f metal x a n t h a t e formed by a well-known c h e m i c a l r e a c t i o n than t h e s o l u b i l i t y o f metal s u l p h i d e . However, t h i s r e a c t i o n i s n o t t h e r m o d y n a m i c a l l y f a v o u r a b l e . I t has been fou n d l a t e r t h a t t h e s o l u b i l i t y p r o d u c t o f t h e s u r f a c e r e a c t i o n p r o d u c t i s d i f f e r e n t f r o m t h a t o f t h e b u l k phase r e a c t i o n p r o d u c t ( 5 3 ) . T h i s c h e m i c a l r e a c t i o n mechanism a l s o f a i l s t o a c c o u n t f o r t h e n e c e s s a r y r o l e o f oxygen i n f l o t a t i o n (25, 31, 53, 54, 5 5 ) . (B) I o n Exchange T h e o r y The c h e m i c a l r e a c t i o n t h e o r y has been r e v i s e d by S u t h e r l a n d and Wark (29) and Gaudin (56) t o t a k e a c c o u n t o f t h e r o l e o f oxygen i n f l o t a t i o n . They p r o p o s e d t h a t oxygen o x i d i z e d t h e s u r f a c e o f metal s u l p h i d e t o form s u r -f a c e p r o d u c t s such as t h i o s u l p h a t e and s u l p h a t e . T h e s e o x i d i z e d s u r f a c e p r o d u c t s can be r e l e a s e d i n t o s o l u t i o n by a t h e r m o d y n a m i c a l l y f a v o u r a b l e i o n - e x c h a n g e mechanism w i t h x a n t h a t e i o n . The h y d r o p h o b i c i t y o f a m i n e r a l i s c a u s e d by t h e n o n p o l a r group o f x a n t h a t e o r i e n t e d towards t h e s o l u t i o n phase. M e l l g r e n ' s t h e r m o c h e m i c a l s t u d y (45) on t h e x a n t h a t e and g a l e n a s y s t e m and Rao's (57) s t u d y on p y r r h o t i t e a l s o s u p p o r t t h i s i o n - e x c h a n g e t h e o r y . - 13 -(C) N e u t r a l M o l e c u l e , X a n t h i c A c i d T h e o r y Cook and Nixon (58) p o i n t e d o u t l a t e r t h a t t h e p r e s e n c e o f a n e g a t i v e p o t e n t i a l b a r r i e r a t t h e m i n e r a l - s o l u t i o n i n t e r f a c e would n o t a l l o w t h e x a n t h a t e a n i o n s t o p e n e t r a t e t h e e l e c t r i c a l d o u b l e l a y e r . I n s t e a d o f x a n t h a t e i o n , t h e y p r o p o s e d t h a t t h e n e u t r a l m o l e c u l e o f x a n t h i c a c i d formed by h y d r o l y s i s o f x a n t h a t e m i g h t be t h e a c t i v e s p e c i e s . T h i s n e u t r a l m o l e c u l e t h e o r y o f x a n t h i c a c i d , however, has n o t been w i d e l y a c c e p t e d , as d i s c u s s e d by G a u d i n ( 5 6 ) . (D) N e u t r a l M o l e c u l e , D i x a n t h o g e n T h e o r y P o l i n g and L e j a (31) s u g g e s t e d a n o t h e r p o s s i b l e a d s o r p t i o n , mechanism which a g r e e d w i t h p o t e n t i a l b a r r i e r c o n c e p t , i . e . , t h e n e u t r a l m o l e c u l e o f d i x a n t h o g e n a d s o r p t i o n t h e o r y . They r e a s o n e d t h a t t h i s d i x a n t h o g e n c o u l d be p h y s i c a l l y a d s o r b e d a n d / o r d i s s o c i a t e d i n t o xan-t h a t e t o f o r m l e a d x a n t h a t e . F i n k e l s t e i n ( 5 9 ) , however, showed t h a t homogeneous o x i d a t i o n o f x a n t h a t e t o d i x a n t h o g e n by d i s s o l v e d oxygen d i d n o t o c c u r t o any a p p r e c i a b l e e x t e n t and t h a t o x i d a t i o n o f x a n t h a t e was c a t a l y s e d by heavy m e t a l s o r heavy metal i o n s ( 6 0 ) . (E) S e m i c o n d u c t o r T h e o r y P l a k s i n and S h a f e e v (13, 61, 62, 6 3 ) , c o n s i d e r i n g t h e s e m i -c o n d u c t o r p r o p e r t i e s o f m i n e r a l s , have p r o p o s e d a s e m i c o n d u c t o r mech-ansim. They i n d i c a t e d p r e f e r e n t i a l a d s o r p t i o n o f x a n t h a t e on t h e s p e c i f i c s i t e s o f a m i n e r a l p a r t i c l e , and e x p l a i n e d t h a t oxygen, w h i c h i s a good - 14 -e l e c t r o n a c c e p t o r , can l o w e r t h e Fermi l e y e l by r e d u c i n g t h e c o n c e n -t r a t i o n o f h o l e s i n t h e v a l e n c e band and c o n v e r t i n g f r o m a n - t y p e t o a p - t y p e s e m i c o n d u c t o r where t h e x a n t h a t e c o u l d be a d s o r b e d and even d i x a n t h o g e n formed. However, S p r i n g e r (1Q) f o u n d t h a t s e m i c o n d u c t i v i t y i s n o t an i m p o r t a n t f a c t o r i n e l e c t r o d i s s o l u t i o n o f g a l e n a , p y r i t e and c h a l c o -p y r i t e . S w i n k e l s ( 6 4 ) , S w i n k e l s and M u l a r (15) and Woods (19) a l s o i n d i c a t e d t h a t s e m i c o n d u c t i v i t y o f g a l e n a was n o t i m p o r t a n t i n x a n t h a t e f l o t a t i o n o f g a l e n a , b ut t h a t i t m ight p l a y a s i g n i f i c a n t r o l e f o r a v e r y low c o n d u c t i v e s p h a l e r i t e f l o t a t i o n . (F) E l e c t r o c h e m i c a l T h e o r y The n e c e s s i t y o f oxygen i n x a n t h a t e f l o t a t i o n o f s u l p h i d e s i s a g e n e r a l l y a c c e p t e d f a c t (25, 31, 53, 54, 55, 6 5 ) ; however, F i n k e l s t e i n (66) f o u n d t h a t x a n t h a t e s t i l l a d s o r b s on an o x y g e n - f r e e g a l e n a s u r f a c e which i s t r e a t e d by sodium s u l p h i d e . S w i n k e l s ( 6 4 ) , S w i n k e l s and M u l a r (T5) and Woods (44, 67) showed t h a t a g a l e n a s u r f a c e can be made h y d r o -p h o b i c i n a de o x y g e n a t e d x a n t h a t e s o l u t i o n by i m p o s i n g an e x t e r n a l p o t e n -t i a l . I n e f f e c t , t h e y s u g g e s t t h a t a d s o r p t i o n o f x a n t h a t e on g a l e n a c a n be an a n o d i c r e a c t i o n w h i l e oxygen p r o v i d e s t h e n e c e s s a r y c a t h o d i c r e -a c t i o n . T h i s i s an e l e c t r o c h e m i c a l e x p l a n a t i o n o f t h e x a n t h a t e a d s o r p t i o n mechanism. T h i s e l e c t r o c h e m i c a l i n t e r p r e t a t i o n o f t h e x a n t h a t e a d s o r p t i o n - 15 -mechanism was f i r s t s u g g e s t e d by Salamy and Ni x o n ( 6 8 ) . They p r o p o s e d t h a t oxygen r a i s e d t h e e l e c t r o c h e m i c a l p o t e n t i a l o f a m i n e r a l s u r f a c e by f o r m i n g a mixed p o t e n t i a l between a n o d i c o x i d a t i o n o f x a n t h a t e t o metal x a n t h a t e o r t o d i x a n t h o g e n and c a t h o d i c r e d u c t i o n o f oxygen. A n o d i c : 2X" + 2Hg H g 2 X 2 + 2e (6) o r 2X ^ = ± : X 2 + 2e (7) C a t h o d i c : 1/2 0 2 + H 20 + 2e ^ ± 20H" (8) T o l u n and K i t c h e n e r (25) a l s o a g r e e d t h a t t h e r o l e o f oxygen was t o p r o v i d e a c a t h o d i c r e a c t i o n and c o n v e r t g a l e n a f r o m n - t y p e s e m i c o n d u c t o r t o p - t y p e s e m i c o n d u c t o r . S i n c e t h e n , s e v e r a l o t h e r i n v e s t i g a t o r s s t u d i e d t h e x a n t h a t e a d -s o r p t i o n mechanism e l e c t r o c h e m i c a l l y and r e p o r t e d s l i g h t l y d i f f e r e n t c a t h o d i c and a n o d i c r e a c t i o n s . They a g r e e d g e n e r a l l y t h a t t h e c a t h o d i c r e a c t i o n i s a r e d u c t i o n o f oxygen. However, two d i f f e r e n t t y p e s o f r e d u c e d s p e c i e s o f ox-ygen have been r e p o r t e d i n t h e l i t e r a t u r e , i . e . , t h e 0 2/0H~ c o u p l e by Salamy and N i x o n ( 6 8 ) , F u e r s t e n a u e t a l ( 4 7 ) , Majima and Takeda ( 2 6 ) , Woods (44) and G u t i e r r e z (5) and t h e 0 2 / H 2 0 2 c o u p l e by S a t o ( 6 9 ) , T o p e r i and t o l u m ( 7 0 ) , Woods ( 1 9 ) , N a t a r a j a n and Iwasaki (71) and Chander and F u e r s t e n a u ( 7 2 ) . P e t e r s and Majima (9) r e p o r t e d t h e 0 2 / H 2 0 c o u p l e as a c a t h o d i c r e a c t i o n i n t h e i r s t u d y o f a c i d l e a c h i n g o f s u l p h i d e m i n e r a l s . The v a r i o u s a n o d i c r e a c t i o n mechanisms i n x a n t h a t e f l o t a t i o n o f s u l f i d e m i n e r a l s a r e : - 16 -T o p e r i and T o l u n ( 7 0 ) ; G a l e n a , 2PbS + 3H 20 + 4X~ z+± 2PbX 2 + S g O g " + 6H +4- 8e (9) S w i n k e l s and M u l a r (15, 6 4 ) , F i n k e l s t e i n ( 6 6 ) , and N i c o l ( 3 8 ) : G a l e n a PbS + 2X'~ + ± PbX 2 + S° + 2e (10) Woods (19, 4 4 ) : G a l e n a 2X" =f±. 2X +2e (7) li X 2 ( d i m e r i z a t i o n ) Majima and Takeda ( 2 6 ) : P y r i t e 2X~ =f±: X 2 + 2e (7) Woods ( 6 7 ) , however, e x p r e s s e d h i s o p i n i o n r e c e n t l y t h a t t h e a n o d i c r e a c t i o n , which i n v o l v e s PbX 2 and S° b u t n o t X 2 , c o u l d be r e s p o n -s i b l e f o r f l o t a t i o n o f g a l e n a , i f t h e r e s t p o t e n t i a l o f t h e s y s t e m i s l e s s a n o d i c w i t h r e s p e c t t o X 2/X" redo x p o t e n t i a l . Indeed, A l l i s o n , e t a l (40) f o u n d r e c e n t l y t h a t a l l m i n e r a l s on which dixanthogen-was formed as t h e p r i n c i p a l r e a c t i o n p r o d u c t had r e s t p o t e n t i a l s i n e x c e s s o f t h e e q u i l i -b r i u m r e d o x p o t e n t i a l o f X 2/X" c o u p l e , and t h o s e on w h i c h metal x a n t h a t e - 1 7 -was formed had p o t e n t i a l s below t h i s v a l u e , i n d i c a t i n g t h a t t h e r e a c t i o n was e l e c t r o c h e m i c a l i n n a t u r e . In summing up t h e above d i s c u s s i o n , t h e f o l l o w i n g c o n c l u s i o n s have been t e n t a t i v e l y drawn: ( i ) S u l p h i d e m i n e r a l s a r e c h a r a c t e r i s t i c a l l y impure s e m i -c o n d u c t o r s p o s s e s s i n g d i f f e r e n t p h y s i c a l and c h e m i c a l p r o p e r t i e s . T h e s e p r o p e r t i e s , d e p e n d i n g on t h e method o f t r e a t m e n t , can be d i f f e r e n t f o r a p p a r e n t l y t h e same m i n e r a l s y s t e m and even f o r one sample. ( i i ) The h y d r o p h o b i c i t y o f s u l p h i d e m i n e r a l s . i n x a n t h a t e f l o t a t i o n i s i n d u c e d m a i n l y by v a r i o u s s u r f a c e p r o d u c t s o f x a n t h a t e , e.g., metal x a n t h a t e , s p e c i f i c a l l y a d s o r b e d x a n t h a t e o r p h y s i c a l l y a d s o r b e d d i x a n t h o g e n . E l e m e n t a l s u l p h u r can h e l p h y d r o p h o b i c i t y i f i t i s p r e s e n t i n a p p r e c i a b l e amount on t h e m i n e r a l s u r f a c e . ( i i i ) The r o l e o f oxygen can be d u a l . Oxygen can f o r m , on t h e s u r f a c e o f metal s u l p h i d e s , v a r i o u s o x i d a t i o n p r o d u c t s such as t h i o -s u l p h a t e , s u l p h a t e and c a r b o n a t e i n t h e p r e s e n c e o f c a r b o n d i o x i d e f o l -lowed by m e t a t h e t i c i n t e r a c t i o n between t h e o x i d a t i o n p r o d u c t s and x a n t h a t e . Oxygen can a l s o p r o v i d e a n e c e s s a r y r e d u c t i o n mechanism f o r t h e e l e c t r o -c h e m i c a l r e a c t i o n s . ( i v ) I n t e r a c t i o n between s u l p h i d e m i n e r a l and x a n t h a t e can o c c u r t h r o u g h v a r i o u s m e t a t h e t i c d o u b l e d e c o m p o s i t i o n r e a c t i o n s as w e l l as t h r o u g h e l e c t r o c h e m i c a l r e a c t i o n s . Depending on t h e c o n d i t i o n o f t h e - 18 -system, v a r i o u s r e a c t i o n s can o c c u r s i m u l t a n e o u s l y o r c o n s e c u t i v e l y . In t h e l i g h t o f t h i s c o n c l u s i o n , i t i s c o n s i d e r e d t h a t t h e i n t e r a c t i o n mechanism f o r d i f f e r e n t m i n e r a l s i s not n e c e s s a r i l y i d e n t i c a l even under t h e same c o n d i t i o n s . 1.2.2 R o l e o f I r o n i n X a n t h a t e F l o t a t i o n o f S u l p h i d e M i n e r a l s I r o n can p l a y a s i g n i f i c a n t r o l e d u r i n g f l o t a t i o n o f s u l p h i d e m i n e r a l s w i t h x a n t h a t e b o t h as m e t a l l i c i r o n and as numerous d i s s o l v e d s p e c i e s . The s e r i o u s p r o b l e m o f poor s e l e c t i v i t y between s u l p h i d e min-e r a l s a f t e r g r i n d i n g i n a s t e e l m i l l i s f r e q u e n t l y e n c o u n t e r e d i n s e l -e c t i v e f l o t a t i o n p r o c e s s e s . T h i s p oor s e l e c t i v i t y p r o b l e m i s u s u a l l y overcome by c o n v e r t i n g s t e e l g r i n d i n g t o autogeneous p e b b l e g r i n d i n g and t h u s t h e i r o n g r i n d i n g medium i s blamed f o r t h e p oor s e l e c t i v i t y ( 1 , 7 4 ) . Rey and Formanek (1) f o u n d t h a t s t e e l g r i n d i n g d e p r e s s e d g a l e n a i n x a n t h a t e f l o t a t i o n compared w i t h p o r c e l a i n g r i n d i n g , and c o n -s e q u e n t l y r e s u l t e d i n a p o o r s e l e c t i v i t y between g a l e n a and s p h a l e r i t e . T h o r t o n ' s (74) s t u d y on Cu-Pb-Zn complex s u l p h i d e o r e s f r o m v a r i o u s l o c a l i t i e s showed marked improvement a f t e r autogeneous g r i n d i n g i n s e l e c t i v i t y among t h e s u l p h i d e s . P o r c e l a i n g r i n d i n g w i t h i r o n powder gave t h e same poor s e l e c t i v i t y between s u l p h i d e s as i n s t e e l g r i n d i n g . W e l l s (75) showed t h a t s p h a l e r i t e was d e p r e s s e d by i r o n and a c t i v a t e d by p y r i t e . He a l s o f o u n d t h a t o n l y r e d u c e d i r o n s p e c i e s were - 1 9 -e f f e c t i v e i n r e n d e r i n g p y r i t e s u s c e p t i b l e t o c o p p e r a c t i v a t i o n w h i l e any i r o n s p e c i e s was c a p a b l e o f d e p r e s s i n g s p h a l e r i t e . He p r o p o s e d t h a t t h e a c t i v a t i o n o f p y r i t e by c o p p e r s u l p h a t e was a c t u a l l y t h e a c t -i v a t i o n o f m e t a l l i c i r o n which i s smeared on t h e r e l a t i v e l y h a r d p y r i t e s u r f a c e d u r i n g g r i n d i n g . T h i s t y p e o f s m e a r i n g and a b r a s i v e i n t e r -p e n e t r a t i o n between two m a t e r i a l s under c o m p r e s s i v e and/or s h e a r i n g l o a d i s a well-known phenomenon i n f r i c t i o n e n g i n e e r i n g , as d i s c u s s e d by K r a g e l s k i i (76) and R a b i n o w i c z ( 7 7 ) . A r e c e n t s t u d y by Rao (78) i n d i c a t e d t h a t t h e s h a r p d r o p i n t h e q u a n t i t y o f t h i o s u l p h a t e r e l e a s e d by g a l e n a i n t h e p r e s e n c e o f i r o n was m a i n l y due t o t h e c o nsumption o f oxygen by i r o n . F u e r s t e n a u and h i s c o - w o r k e r s (79, 80, 81, 82) s t u d i e d t h e r o l e o f i r o n i n s u l p h o n a t e f l o t a t i o n o f v a r i o u s s i l i c a t e s . They o b s e r v e d t h a t q u a r t z was a c t i v a t e d by i r o n u n i n t e n t i o n a l l y d u r i n g g r i n d i n g i n a s t e e l - m i 11 o r f l o t a t i o n i n s t e e l c e l l s even a f t e r p o r c e l a i n g r i n d i n g . They a l s o n o t e d t h a t t h e r o l e s o f m e t a l l i c i r o n and d i s s o l v e d i r o n i o n s were d i f -f e r e n t , i . e . , q u a r t z f l o a t e d w e l l o v e r a wide ra n g e o f pH a f t e r s t e e l - m i l l g r i n d i n g w h i l e f e r r i c i r o n f u n c t i o n e d as an a c t i v a t o r i n t h e narrow r a n g e o f pH where f e r r i c i r o n h y d r o l y z e d t o i t s h y d r o x y complexes. Woodcock and J o n es (83, 84) and some R u s s i a n w o r k e r s (53) who measured oxygen c o n c e n t r a t i o n i n t h e p u l p a f t e r s t e e l - m i 11 g r i n d i n g f o u n d t h a t oxygen c o n t e n t was o n l y 0-60% s a t u r a t i o n due t o t h e o x i d a t i o n . o f f e r r o u s . i r o n and c o l l o i d a l s u l p h i d e s d u r i n g g r i n d i n g . As a r e s u l t , t h e p u l p was i n a more r e d u c e d s t a t e . - 20 -A c c o r d i n g t o Rao ( 3 ) , f e r r i c i o n , as has been shown f o r many o t h e r m e t a l l i c i o n s , i s c a p a b l e o f c a t a l y t i c o x i d a t i o n o f x a n t h a t e t o d i x a n t h o g e n i n t h e p r e s e n c e o f oxygen and c a r b o n d i s u l p h i d e . In s u m m a r i z i n g t h e r o l e o f i r o n i n x a n t h a t e f l o t a t i o n o f s u l p h i d e m i n e r a l s : ( i ) M e t a l l i c i r o n can a f f e c t t h e r e s t p o t e n t i a l o f s u l p h i d e m i n e r a l s by d i r e c t c o n t a c t d u r i n g g r i n d i n g i n a s t e e l m i l l a n d / o r as i r o n smeared on s u l p h i d e m i n e r a l s by f o r m i n g l o c a l g a l v a n i c c e l l s and r e s u l t i n g i n g a l v a n i c p o l a r i z a t i o n phenomena. ( i i ) F e r r o u s i o n s w i l l consume oxygen i n t h e p u l p by t h e i r o x i d a t i o n , and f e r r i c i o n s w i l l c a t a l y z e o x i d a t i o n o f x a n t h a t e t o d i x a n t h o g e n i n t h e p r e s e n c e o f oxygen and c a r b o n d i s u l p h i d e . B oth f e r r o u s and f e r r i c i o n s w i l l f o r m f e r r o u s and f e r r i c x a n t h a t e s and i n c r e a s e t h e c o n s u m p t i o n o f x a n t h a t e . The change i n t h e c o n c e n t r a t i o n o f oxygen, x a n t h a t e and o t h e r s p e c i e s due t o t h e r e a c t i o n w i t h i r o n s p e c i e s can a l t e r t h e r e d o x p o t e n t i a l o f t h e whole s y s t e m , and t h u s f l o t a t i o n o f s u l p h i d e m i n e r a l s w i l l be a f f e c t e d a c c o r d i n g l y . 1.2.3 E l e c t r o c h e m i s t r y and Thermodynamics 1.2.3.1 E l e c t r o c h e m i c a l R e a c t i o n s An e l e c t r o c h e m i c a l r e a c t i o n i s a r e a c t i o n i n v o l v i n g e l e c t r o n t r a n s f e r . However, a pure c h e m i c a l r e a c t i o n which i n v o l v e s no n e t e l e c t r o n - c\ -t r a n s f e r can a l s o be c a r r i e d o u t i n an e l e c t r o c h e m i c a l c e l l . Such n e t r e a c t i o n s i n an e l e c t r o c h e m i c a l c e l l a r e f o r m a l l y i d e n t i c a l t o t h e f a m i l i a r , t h e r m a l l y i n d u c e d , p u r e l y c h e m i c a l r e a c t i o n s i n which m o l e c u l e s c o l l i d e w i t h each o t h e r and f o r m new s p e c i e s w i t h new bonds. In e l e c t r o c h e m i c a l r e a c t i o n s , however, t h e r e a c t a n t s c o l l i d e n o t w i t h each o t h e r b u t w i t h s e p a r a t e d " c h a r g e - t r a n s f e r c a t a l y s t s " , namely, t h e e l e c t r o d e s . T hese e l e c t r o d e s can be two s e p a r a t e d ones as i n e l e c t r o l y t i c c e l l s o r a s i n g l e p i e c e o f c o n d u c t i n g m a t e r i a l where l o c a l e l e c t r o c h e m i c a l c e l l s can be e s t a b l i s h e d due t o t h e l o c a l p o t e n t i a l d i f f e r e n c e s . The a p p l i c a t i o n o f e l e c t r o c h e m i s t r y i n a f l o t a t i o n s t u d y i s n o t s u r p r i s i n g i f one c o n s i d e r s t h a t a l m o s t a l l s u l p h i d e m i n e r a l s a r e e l e c t r i c a l s e m i c o n d u c t o r s and an e l e c t r i c a l d o u b l e l a y e r i s f o u n d a t t h e i n t e r f a c e between s u l p h i d e m i n e r a l s and t h e aqueous phase. 1.2.3.2 E l e c t r o d e P o t e n t i a l The t h e o r y o f f o r m a t i o n o f t h e e l e c t r i c a l d o u b l e l a y e r a t t h e i n t e r f a c e o f an e l e c t r i c a l c o n d u c t o r i n c o n t a c t w i t h an e l e c t r o l y t e s o l -u t i o n has been d e v e l o p e d f r o m t h e i n i t i a l i d e a s o f H e l m h o l t z , Gouy, and S t e r n and became e s t a b l i s h e d by Graham ( 8 5 ) . S u l p h i d e m i n e r a l s b e i n g s e m i c o n d u c t o r s , a s u b s t a n t i a l p o t e n t i a l d r o p and a s p a c e - c h a r g e r e g i o n can be d e v e l o p e d w i t h i n t h e s e m i c o n d u c t o r phase c l o s e t o t h e i n t e r f a c e f o r a s e m i c o n d u c t o r - e l e c t r o l y t e s y s t e m due t o the lwo c o n c e n t r a t i o n o f c h a r g e c a r r i e r s ( e l e c t r o n s and h o l e s ) and t h e low - 22 -d i e l e c t r i c c o n s t a n t o f t h e s e m i c o n d u c t o r s . In o t h e r words, an e l e c t r i c a l d o u b l e l a y e r may a l s o e x i s t on t h e s o l i d s i d e o f t h e i n t e r f a c e . T h i s s p a c e - c h a r g e becomes n e g l i g i b l e when t h e c o n c e n t r a t i o n o f f r e e c h a r g e c a r r i e r s i s l a r g e . The e l e c t r i c a l d o u b l e l a y e r formed a t t h e i n t e r f a c e between s u l p h i d e s and t h e e l e c t r o l y t e c o n s i s t s o f a c c u m u l a t e d c h a r g e s o f o p p o s i t e s i g n i n b o t h phases and o f o r i e n t e d d i p o l e s i n t h e i n t e r f a c e s . T h ese s e p a r a t i o n o f c h a r g e s and o r i e n t a t i o n o f d i p o l e s c a u s e a d i f f e r -ence i n t h e G a l v a n i c p o t e n t i a l between the c h a r g e - f r e e i n t e r i o r o f b o t h phases t h a t a r e i n c o n t a c t . f e r t a k e s p l a c e a t an e l e c t r o d e s u r f a c e , t h e e l e c t r o d e p o t e n t i a l i s d e t e r -mined by t h e c h a r g e t r a n s f e r between t h e e l e c t r o c h e m i c a l r e a c t i o n i n e q u i l i b r i u m . T h e r e can be no d r i v i n g f o r c e i n e i t h e r d i r e c t i o n and t h e n e t change i n e l e c t r o c h e m i c a l f r e e e n e r g y accompanying e i t h e r a n o d i c o x i d a t i o n o r c a t h o d i c r e d u c t i o n w i l l be z e r o . T h e r e f o r e , When an e l e c t r o c h e m i c a l r e a c t i o n which i n v o l v e s e l e c t r o n t r a n s -AG 0 = - z F E (11) where AG° n e t change i n c h e m i c a l f r e e e n e r g y ( c a l / m o l e ) E° E l e c t r o d e p o t e n t i a l ( V o l t ) z Number o f e l e c t r o n s i n v o l v e d F F a r a d a y (23,060 c a l / V o l t / g ' e q u i v . ) a t 298K * The s i g n used i n t h i s s t u d y i s i n a c c o r d w i t h I.U.P.A.C. c o n v e n t i o n The p o t e n t i a l E° i s c a l l e d t h e thermodynamic e q u i l i b r i u m p o t e n t i a l and i s a c h a r a c t e r i s t i c o f t h e metal e l e c t r o d e c o n c e r n e d . Because d i r e c t measure-ment o f t h i s e q u i l i b r i u m p o t e n t i a l i s i m p o s s i b l e , i t i s always measured as a - 23 -r e l a t i v e v a l u e a g a i n s t a r e f e r e n c e e l e c t r o d e p o t e n t i a l . The c o n v e n t i o n a l r e f e r e n c e e l e c t r o d e i s t h e s t a n d a r d hydrogen e l e c t r o d e (SHE) and t h e e q u i -l i b r i u m p o t e n t i a l o f t h e s t a n d a r d hydrogen e l e c t r o d e (SHE) i s d e f i n e d as z e r o v o l t s . The p o t e n t i a l measured a g a i n s t t h e s t a n d a r d hydrogen e l e c t r o d e i s c a l l e d t h e h a l f c e l l p o t e n t i a l (Eh) i n t h e s t a n d a r d hydrogen s c a l e . In t h i s s t u d y , t h e s a t u r a t e d c a l o m e l e l e c t r o d e has been used as a r e f e r e n c e e l e c t r o d e and t h e measured p o t e n t i a l was r e p o r t e d b o t h i n t h e s a t u r a t e d c a l o m e l e l e c t r o d e s c a l e (E) and the s t a n d a r d hydrogen e l e c t r o d e s c a l e ( E h ) . The g e n e r a l e q u a t i o n f o r a l l e l e c t r o c h e m i c a l h a l f c e l l r e a c t i o n s i n aqueous s o l u t i o n can be e x p r e s s e d by the f o l l o w i n g e q u a t i o n , i . e . , a 0 + b B ± mH + + z e =3=fc cR + dD (12) where 0 ; O x i d i z e d S p e c i e s R ; Reduced S p e c i e s B and D ; A u x i l i a r y S p e c i e s a, b , c, d, m and z ; c o e f f i c i e n t s The f r e e e n e r g y change ( AG,) i n terms o f a c t i v i t i e s (A) o f r e a c t a n t s and p r o d u c t s i s AG = 4G° * RT In W r 1 C ^ - 24 -where AG 0 i s t h e f r e e e n e r g y change a t t h e e q u i l i b r i u m s t a t e . S i n c e E = AG/ -zF (14) Then E becomes, ( A D ) C ( A n r AG° RT E = -2H_.--&!_ l n ( 1 5) " Z F Z F ( A 0 ) A ( A B ) B ( A H + ) ± M E = E° + 2 , 3 R T l o g zF I f a l l s p e c i e s a r e i n t h e s t a n d a r d s t a t e , t h e a c t i v i t i e s o f a l l s p e c i e s become u n i t y and hence E becomes t h e s t a n d a r d e q u i l i b r i u m p o t e n t i a l (E°) where E° = AG° / - ZF (17) The s t a n d a r d s t a t e r e f e r s t o one atmosphere and 298.15 K ( 2 5 ° C ) , and u n i t a c t i v i t y has been a s s i g n e d f o r p u r e s o l i d s and l i q u i d s under t h i s s t a n d a r d s t a t e . The a c t i v i t y o f gases i s a l s o u n i t y under one a t m o s p h e r i c p a r t i a l p r e s s u r e a t 298.15 K. I f c o n c e n t r a t i o n s o f a l l s p e c i e s a r e v e r y low, t h e a c t i v i t y c o e f f i c i e n t becomes u n i t y and t h u s c o n c e n t r a t i o n s can be used i n p l a c e o f a c t i v i t i e s . S q uare b r a c k e t s r e f e r t o t h e c o n c e n t r a t i o n o f t h e p a r t i c u l a r s p e c i e s . Then, s i n c e R = 1.98726 c a l / m o l e K and T = 298.15 K, a t 25°C Eqn. (16) becomes: ( A 0 ) A ( A B ) B ( A H + ) ™ ( 1 6 ) C A R ) C ( A D ) D - 25 -E = E° 7 0.0591 > pH - °-f?? log IS£_[l£ (18) T h i s Eqn. (18) i s a for m o f N e r n s t e q u a t i o n f o r t h e g e n e r a l i z e d e l e c t r o c h e m i c a l h a l f c e l l r e a c t i o n i n t h e f o r m o f Eqn. ( 1 2 ) . T h i s N e r n s t e q u a t i o n d e s c r i b e s o n l y a un i q u e p o i n t under t h e e q u i l i b r i u m c o n d i t i o n s where t h e n e t c u r r e n t f o r t h e h a l f c e l l r e a c t i o n i s z e r o , and i t i s n o t a p p l i c a b l e t o any o t h e r p o t e n t i a l . T h e r e f o r e , t o s t u d y t h e r e a c t i o n a t a c e r t a i n p o t e n t i a l o t h e r t h a n e q u i l i b r i u m p o t e n t i a l , a p o l a r i z a t i o n s t u d y has t o be c a r r i e d o u t . On t h e p o t e n t i a l -pH d i a g r a m , t h e s l o p e o f t h e l i n e i s always -0.0591 m/z v o l t s p e r u n i t o f pH, u n l e s s t h e hydrogen i o n s must be p l a c e d on t h e r i g h t hand s i d e o f t h e Eqn. (12) i n which c a s e t h e s l o p e becomes p o s i t i v e . I f no e l e c t r o n s a r e i n v o l v e d i n Eqn. (12) ( i . e . , z = 0 ) , t h e n t h e l i n e becomes v e r t i c a l , and , when no hydrogen i o n s a r e i n v o l v e d ( i . e . , m = 0 ) , t h e l i n e becomes h o r i z o n t a l i n t h e p o t e n t i a l -pH d i a g r a m . 1.2.3.3 M i x e d P o t e n t i a l "The p r i n c i p l e o f a d d i t i v e c o m b i n a t i o n o f a l l p a r t i a l p r o c e s s e s " a t an e l e c t r o d e s u r f a c e t o o b t a i n t h e t o t a l c u r r e n t - v o l t a g e c u r v e was f i r s t f o r m u l a t e d by Wagner and T r a u d (86) i n 1938. The s t e a d y - s t a t e open c i r c u i t p o t e n t i a l t h u s formed by t h e t o t a l c u r r e n t - p o t e n t i a l c u r v e i s c a l l e d t h e mixed p o t e n t i a l . Tiie f o r m a t i o n o f a mixed p o t e n t i a l has been d i s c u s s e d e x t e n s i v e l y by F o n t a n a and Greene ( 8 7 ) , V e t t e r ( 2 1 ) , West ( 8 8 ) , and C h a r i o t e t a l ( 8 9 ) . - 26 -The b a s i c c o n c e p t o f t h e f o r m a t i o n o f myxed p o t e n t i a l i s shown s c h e m a t i c a l l y i n F i g . 1.2 as a s i m p l i f i e d c u r r e n t - p o t e n t i a l c u r v e (a) and p o t e n t i a l - l o g c u r r e n t [b] form. I f two d i f f e r e n t e l e c t r o c h e m i c a l r e a c t i o n systems w i t h e q u i l i b r i u m p o t e n t i a l s o f and Eg a r e i n e q u i l i b r i u m , c a t h o d i c r e d u c t i o n w i l l t a k e p l a c e f o r t h e n o b l e c o u p l e A and a n o d i c o x i d a t i o n w i l l t a k e p l a c e f o r t h e b a s i c c o u p l e B. The mixed p o t e n t i a l o f t h e two s ystems w i l l t h e n be t h e p o t e n t i a l E mix ( i R = 0) where a n o d i c c u r r e n t and c a t h o d i c c u r r e n t a r e e q u a l . Now i f we assume t h a t t h e a n o d i c c u r v e f o r B and t h e c a t h o d i c c u r v e f o r A i n F i g . 1.2 r e p r e s e n t t o t a l a n o d i c and c a t h o d i c r e a c t i o n r a t e s r e s p e c t -i v e l y f o r t h e mixed sys t e m o f two e l e c t r o d e s i n g a l v a n i c c o n t a c t , two d i f f e r -e n t mixed p o t e n t i a l s a r e e s t a b l i s h e d on each s i d e o f A e l e c t r o d e (E mix.A) and B e l e c t r o d e (E mix.B ) due t o t h e s o l u t i o n iR d r o p between t h e two e l e c t -r o d e s as d i s c u s s e d by V e t t e r ( 2 1 ) . T h i s iR d r o p becomes n e g l i g i b l e when t h e c o n d u c t i v i t y o f t h e e l e c t r o l y t e i s v e r y h i g h . T h u s, t h e mixed p o t e n t i a l o f a g a l v a n i c c o u p l e i s d e t e r m i n e d by t h e thermodynamic e q u i l i b r i u m p o t e n t i a l s o f t h e a c t u a l p r o c e s s e s i n v o l v e d , t h e i r exchange c u r r e n t d e n s i t i e s and T a f e l s l o p e , and t h e r e l a t i v e a r e a s o f t h e two e l e c t r o d e s . The mixed p o t e n t i a l o f a g a l v a n i c c o u p l e w i l l be c l o s e t o t h e e q u i l i b r i u m p o t e n t i a l o f an e l e c t r o d e w i t h g r e a t e r exchange c u r r e n t d e n s i t y , l a r g e r s u r f a c e a r e a and s m a l l e r T a f e l s l o p e . A c c o r d i n g t o F o n t a n a and Greene ( 8 7 ) , C h a r i o t , e t a l (89) and V e t t e r ( 2 1 ) , i f t h e r e a r e no w e l l - d e f i n e d r e d o x c o u p l e s i n s i g n i f i c a n t amount, t h e mixed p o t e n t i a l o f a mixed sy s t e m o f two metal e l e c t r o d e s i s - 27 -Figure 1.2 ; Formation of Mixed Potential (a) i vs E i i E mix (iR=o) ^mix . A U—'—1 R—*^******~g^ Emix,B » (-) *• Potential (E) + ( + ) (b) E vs log i log i " " " B A mix mix (iR=o) log i -* - 28 -formed by an a n o d i c o x i d a t i o n r e a c t i o n on t h e l e s s n o b l e e l e c t r o d e and t o t a l c a t h o d i c r e d u c t i o n o f H * / ^ o r 0 2/H 20 C 0 U P l e s o n t h e n o b l e e l e c t r o d e and t h e l e s s n o b l e one. They emphasize t h a t t h e c a t h o d i c r e a c t i o n mechanism i s n o t th e c a t h o d i c r e d u c t i o n o f n o b l e metal i o n / n o b l e metal c o u p l e , b u t t h e t o t a l c a t h o d i c r e d u c t i o n o f t h e hydrogen o r t h e oxygen c o u p l e on b o t h o f t h e e l e c t -r o d e s s i n c e t h i s h ydrogen r e d u c t i o n o r o x y g e n r e d u c t i o n i s t h e a c t u a l r e d u c t -i o n r e a c t i o n o c c u r r i n g a t both o f t h e e l e c t r o d e s . The f o r m a t i o n o f a mixed p o t e n t i a l o f a s u l p h i d e m i n e r a l (MS) i n c o n t a c t w i t h m e t a l l i c i r o n (Fe) i n a i r - s a t u r a t e d w a t e r a t n e u t r a l pH i s shown s c h e m a t i c a l l y , i n F i g . 1.3. The r e s t p o t e n t i a l o f a s u l p h i d e m i n e r a l ( E 2 ) i s a mixed p o t e n t i a l formed by an a n o d i c d i s s o l u t i o n o f t h e m i n e r a l and a c a t h o d i c r e d u c t i o n o f 0 2 / H 2 0 2 c o u p l e . The r e s t p o t e n t i a l o f i r o n (Eg) i s formed i n t h e same manner. F o l l o w i n g t h e a d d i t i v e p r i n c i p l e o f a l l p a r t i a l r e a c t i o n s , t h e mixed p o t e n t i a l o f t h e whole mixed sys t e m (Eg) i s formed by th e t o t a l a n o d i c r e a c t i o n s o f MS/M and Fe/Fe c o u p l e s and t h e t o t a l c a t h -o d i c r e a c t i o n s o f 0 2 / H 2 0 2 c o u p l e on a s u l p h i d e m i n e r a l and on i r o n e l e c t r o d e s . The a n o d i c and c a t h o d i c r e a c t i o n r a t e s f o r a s i n g l e m i n e r a l e l e c t r o d e s y s t e m c o r r e s p o n d t o t h e c o r r o s i o n c u r r e n t o f i g , w h i l e i g r e -p r e s e n t s t h e c o r r e s p o n d i n g r e a c t i o n r a t e s f o r a s i n g l e i r o n e l e c t r o d e . The t o t a l r e a c t i o n r a t e s f o r t h e MS-Fe g a l v a n i c c o u p l e c o r r e s p o n d s t o t h e c o r r o s i o n c u r r e n t o f i ^ ; S u l p h i d e m i n e r a l , b e i n g a c a t h o d e i n t h i s g a l v a n i c c o u p l e , t h e a o d i c o x i d a t i o n r a t e o f t h e MS/M + + c o u p l e i s r e d u c e d f r o m i g t o i 2 and c a t h o d i c r e d u c t i o n r a t e o f 0 2 / H 2 0 2 c o u p l e on t h e m i n e r a l i s enhanced f r o m i g t o i g . On t h e o t h e r hand, f o r t h e i r o n e l e c t r o d e , t h e a n o d i c r e a c t i o n r a t e o f F e / F e + + c o u p l e i s i n c r e a s e d from i g t o i - ^ w h i l e t h e c a t h o d i c r e d u c t i o n o f 0 2 / H 2 0 2 c o u p l e on the i r o n e l e c t r o d e i s d e c r e a s e d f r o m i g t o i 7 c o r r e s p o n d i n g l y . -29 -F i g u r e 1.3; Mixed, P o t e n t i a l o f S u l p h i d e M i n e r a l ( M S ) - I r o n ( F e ) Couple i n A i r - S a t u r a t e d w a t e r a t N e u t r a l pH ( S c h e m a t i c ) l o g i (Amp.)—>-Thermodynamic E q u i l i b r i u m P o t e n t i a l o f G^/H,^ Co u p l e R e s t P o t e n t i a l o f MS Mixed P o t e n t i a l o f MS-Fe G a l v a n i c System Thermodynamic E q u i l i b r i u m P o t e n t i a l o f MS/M + + C o u p l e R e s t P o t e n t i a l o f Fe Thermodynamic E q u i l i b r i u m P o t e n t i a l o f F e / F e + + Couple ++ '2 ; [3 ; ! 4 ; '5 ; 'a ' '9 ; 'io; Exchange C u r r e n t o f MS/M Co u p l e o n l y .++ A n o d i c O x i d a t i o n Rate, o f MS/M Couple a t Mixed P o t e n t i a l ( E 3 ) Exchange C u r r e n t o f 0 2 / H 2 0 2 Couple on Iro n Exchange C u r r e n t o f F e / F e + + C o u p l e o n l y Exchange C u r r e n t o f O^/H^O^ Couple on MS C o r r o s i o n C u r r e n t o f MS a t R e s t P o t e n t i a l o f MS ( E 2 ) R e d u c t i o n Rate o f O^H^O^ Couple on I r o n a t Mixed P o t e n t i a l (E^) C o r r o s i o n C u r r e n t o f Fe a t Rest P o t e n t i a l o f Fe (E^) R e d u c t i o n Rate o f Couple on MS a t Mixed P o t e n t i a l (E^) O x i d a t i o n Rate o f F e / F e + + Couple a t Mixed P o t e n t i a l ( E 3 ) T o t a l R e a c t i o n Rate o f MS-Fe G a l v a n i c Couple a t Mixed P o t e n t i a l ( E 3 ) - 30 -D i s s o l v e d i r o n s p e c i e s such as f e r r o u s and f e r r i c i o n s i n t h e e l e c t r o l y t e s o l u t i o n f o r m t h e mixed p o t e n t i a l w i t h s u l p h i d e m i n e r a l s i n a d i f f e r e n t way f r o m m e t a l l i c i r o n . However, t h e a d d i t i v e p r i n c i p l e o f Wagner and T r a u d (86) i s s t i l l a p p l i e d , as shown s c h e m a t i c a l l y i n F i g . 1.4. E-|, Eg and Eg a r e the thermodynamic e q u i l i b r i u m p o t e n t i a l s and 3+ 2+ l ' l , ^ and i 2 a r e t h e exchange c u r r e n t d e n s i t i e s f o r t h e Fe /Fe r e d o x c o u p l e , + 2+ t h e H /H 2 c o u p l e and t h e MS/M c o u p l e r e s p e c t i v e l y . The r e s t p o t e n t i a l o f 3+ 2+ a s u l p h i d e m i n e r a l (E^) i n t h e a b s e n c e o f t h e Fe /Fe c o u p l e i s t h e mixed p o t e n t i a l o f a h y d r o g e n - s u l p h i d e m i n e r a l mixed s y s t e m as d i s c u s s e d i n t h e above. The mixed p o t e n t i a l o f t h e whole s y s t e m formed by t h e t o t a l a n o d i c and c a t h o d i c r e a c t i o n s becomes Eg w i t h c o r r e s p o n d i n g t o t a l r e a c t i o n r a t e o f i ^ . Thus t h e a n o d i c r e a c t i o n r a t e on t h e m i n e r a l s u r f a c e i s i n c r e a s e d f r o m i g t o iy, w h i l e c a t h o d i c r e d u c t i o n o f t h e H +/H 2 3+ 2+ c o u p l e on t h e m i n e r a l i s r e d u c e d f r o m i g t o i ^ when t h e Fe /Fe c o u p l e i s i n t r o d u c e d t o t h e s y s t e m . The H /H 2 r e d u c t i o n r a t e ( i ^ ) and t h e Fe /Fe r e d u c t i o n r a t e ( i g ) on t h e m i n e r a l s u r f a c e make up t h e t o t a l r e d u c t i o n r a t e ( i ^ ) . The above d i s c u s s i o n s a r e o n l y a s c h e m a t i c p r e s e n t a t i o n o f how t h e mixed p o t e n t i a l forms as a complex e l e c t r o c h e m c i a l s y s t e m . The r e l a t i v e p o s i t i o n o f each p o t e n t i a l - l o g c u r r e n t c u r v e f o r d i f f e r e n t e l e c t r o c h e m i c a l r e a c t i o n s i n t h e F i g u r e s 1-3 and 1-4 w i l l depend on t h e e f f e c t i v e s u r f a c e a r e a o f each e l e c t r o d e and t h e c o n c e n t r a t i o n s o f d i s s o l v e d m e t a l l i c i o n s . 2+ In g e n e r a l , however, t h e e l e c t r o d e p o t e n t i a l o f t h e MS/M c o u p l e ( e . g . 0.18V a t 10" 6M [ P b 2 + ] and -0.09V a t 1 0 " 1 5 M [ P b 2 + ] f o r PbS) i s l e s s F i g u r e 1.4 Mixed P o t e n t i a l o f S u l p h i d e M i n e r a l (MS) System i n A c i d i c C o n d i t i o n ( S c h e m a t i c ) ( f r o m Ref. 87 ) - 31 -- F e 3 + / F e 2 + i l ^2 i'3 M i 7 l o g c u r r e n t ( i ) Thermodynamic E q u i l i b r i u m P o t e n t i a l o f F e 3 + / F e 2 + Couple o f H +/H 2 C o u p l e Mixed P o t e n t i a l o f S u l p h i d e M i n e r a l - F e 3 + / F e 2 + System Rest P o t e n t i a l o f S u l p h i d e M i n e r a l W i t h o u t F e 3 + / F e 2 + C o u p l e Thermodynamic E q u i l i b r i u m P o t e n t i a l o f Ms/M C o u p l e •p Exhange C u r r e n t o f F e + 3 / F e + 2 C o u p l e on MS 2' 3 ; o f MS/M 2 + C o u p l e o f H +/H2 C o u p l e on MS 4; R e d u c t i o n C u r r e n t o f H +/H 2 C o u p l e on MS a t Mixed P o t e n t i a l ( E 3 ) o f H +/H2 C o u p l e on MS a t R e s t P o t e n t i a l ( E 4 ) 3+ 2+ o f Fe /Fe C o u p l e on MS a t Mixed P o t e n t i a l ( E 3 ) i y ; T o t a l R e a c t i o n C u r r e n t o f M S - F e 3 + / F e 2 + System - 32 -n o b l e t h a n t h e s t a n d a r d r e d o x p o t e n t i a l o f t h e F e J T / F e t T c o u p l e (0.77V) 2+ and more n o b l e t h a n t h a t o f t h e Fe/Fe c o u p l e (-0.44V). A c c o r d i n g t o T o p e r i and T o l u n ( 7 0 ) , Eh o f 0 2 / H 2 0 2 c o u p l e f o r a i r s a t u r a t e d s y s t e m i s +540 mV. T h e r e f o r e , t h e d i s s o l v e d i r o n s p e c i e s make t h e r e s t p o t e n t i a l o f a s u l p h i d e m i n e r a l e l e c t r o d e more n o b l e ( F i g . 1.4), w h i l e m e t a l l i c i r o n i n g a l v a n i c c o n t a c t w i t h t h e m i n e r a l s h i f t s t h e r e s t p o t e n t i a l o f t h e m i n e r a l e l e c t r o d e t o l e s s n o b l e one ( F i g . 1.3). 1.2.3.4. R e s t P o t e n t i a l The r e s t p o t e n t i a l i s d e f i n e d as an e x p e r i m e n t a l l y measured p o t e n t i a l between a w o r k i n g e l e c t r o d e and a r e f e r e n c e e l e c t r o d e i n an e l e c t r o l y t e s o l u t i o n when t h e n e t c u r r e n t f l o w i s z e r o . Thus t h e r e s t p o t e n t i a l i s t h e p o t e n t i a l where t o t a l a n o d i c and c a t h o d i c p o l a r i z a t i o n c u r v e s meet i n t h e p o t e n t i a l - c u r r e n t diagram. The p r i n c i p a l f a c t o r which d e t e r m i n e s t h e e l e c t r o d e r e s t p o t e n t i a l i s t h e e l e c t r o c h e m i c a l r e a c t i o n s t a k i n g p l a c e a t t h e e l e c t r o d e s u r f a c e s . I f t h e r e a c t i o n i s a s i n g l e r e v e r s i b l e r e a c t i o n a t e q u i l i b r i u m , t h e e l e c t r o d e p o t e n t i a l becomes t h e thermodynamic e q u i l i b r i u m p o t e n t i a l ; however, t h i s thermodynamic e q u i l i b r i u m p o t e n t i a l can be o b t a i n e d o n l y when a s u f f i c i e n t amount o f r e a c t a n t s o f t h e e l e c t r o c h e m i c a l r e a c t i o n s a r e p r e s e n t i n t h e s y s t e m . The s p e c i e s n o t i n v o l v e d i n p o t e n t i a l d e t e r m i n i n g e l e c t r o c h e m i c a l r e a c t i o n s may a l s o i n f l u e n c e t h e r e s t p o t e n t i a l by c h a n g i n g t h e a c t i v i t y o f t h e s p e c i e s w hich a r e d i r e c t l y i n v o l v e d i n t h e e l e c t r o c h e m i c a l r e a c t i o n s . The measured r e s t p o t e n t i a l o f an e l e c t r o d e becomes a mixed p o t e n -t i a l i f e x t r a r e d o x systems a r e p r e s e n t i n t h e e l e c t r o l y t e o r t h e e l e c t r o d e i s i n g a l v a n i c c o n t a c t w i t h o t h e r e l e c t r o d e s . In t h i s c a s e , t h e exchange c u r r e n t d e n s i t y , t h e T a f e l s l o p e and t h e r e l a t i v e s u r f a c e a r e a o f t h e e l e c t -- 33 -r o d e s as w e l l as t h e thermodynamic r e v e r s i b l e p o t e n t i a l o f each e l e c t r o -c e m i c a l s y s t e m p l a y an i m p o r t a n t r o l e i n d e t e r m i n i n g t h e m i x e d p o t e n t i a l . L o c a l g a l v a n i c c e l l s can a l s o f o r m mixed p o t e n t i a l s on t h e s u l p h i d e m i n e r a l s . These l o c a l p o t e n t i a l d i f f e r e n c e s can be d e v e l o p e d f o r v a r i o u s r e a s o n s , i . e . , t h e p o l y c r y s t a l l i n e c h a r a c t e r o f t h e m i n e r a l ( S z e g l o w s k i (90) and Pomianowski and C z a r n e c k i (91)), s u r f a c e h e t e r o g e n i t i e s ( P a l k s i n and S h a f e e v (62) ), d i f f e r e n t i a l a e r a t i o n (West ( 8 8 ) ) , l a t t i c e i m p u r i t i e s (Novak and Ban (22)), m i d d l i n g s and even i n pure random b a s i s . S a t o (92), Wood (19) and B r o d i e (93) d i s c u s s e d t h e o r e t i c a l a s p e c t s o f t h e e f f e c t o f s t o i c h i o m e t r i c d e v i a t i o n on t h e r e s t p o t e n t i a l o f s u l p h i d e m i n e r a l s . The v a r i a t i o n o f r e s t p o t e n t i a l w i t h r e s p e c t t o t h e s t o i c h i o m e t r i c d e v i a t i o n has been measured f o r g a l e n a (PbS) by T r u m p l e r (94) and B r o d i e (93), f o r c h a l c o p y r i t e ( C u F e S 2 ) by Noddack and Wrabetz (95), f o r p y r i t e ( F e S 2 ) by Springer(-|Q) and f o r n i c k e l S u l p h i d e ( N i S ) by Umetzu e t a l (96). The i n t e n s i t y o f e l e c t r o l y t e s t i r r i n g ( P e t e r s and Veltman (97)), and changes i n t e m p e r a t u r e and pH w i l l a f f e c t t h e r e s t p o t e n t i a l i f t h e p o t e n t i a l d e t e r m i n i n g e l e c t r o c h e m i c a l r e a c t i o n s a r e under d i f f u s i o n c o n t r o l . D u r i n g g r i n d i n g , changes i n s u r f a c e a c t i v i t y , s t o i c h -i o m e t r i c c o m p o s i t i o n and s e m i c o n d u c t i v i t y and t h e f o r m a t i o n o f g a l v a n i c c e l l s w i t h i r o n ( c o n d u c t i o n p o l a r i z a t i o n ) can o c c u r t o a l t e r t h e r e s t p o t e n t -i a l o f s u l p h i d e m i n e r a l s . P i e z o e l e c t r i c p o l a r i z a t i o n e f f e c t s c a u s e d by m e c h a n i c a l s t r e s s d u r i n g g r i n d i n g can a l s o i n f l u e n c e t h e r e s t p o t e n t i a l . - 34 -T h u s , t h e r e s t p o t e n t i a l o f a s u l p h i d e m i n e r a l depends on t h e o r i g i n and h i s t o r y o f t h e sample, methods o f p r e t r e a t m e n t and v a r i o u s p h y s i c a l and c h e m i c a l c o n d i t i o n s o f t h e e l e c t r o c h e m i c a l s y s t e m s . T h e r e f o r e , t h e r e p o r t e d r e s t p o t e n t i a l s v a r y w i d e l y f o r a p p a r e n t l y t h e same m i n e r a l system. R e p o r t e d v a l u e s o f r e s t p o t e n t i a l were c o m p i l e d i n A p p e n d i c e s 1.1, , 1.12 f o r v a r i o u s s u l p h i d e m i n e r a l s . 1.2.4 I m p l i c a t i o n s o f E l e c t r o c h e m i s t r y i n F l o t a t i o n As L e j a (6) m e n t i o n e d , e l e c t r o c h e m i c a l s t u d i e s on s u l p h i d e m i n e r a l f l o t a t i o n w i t h x a n t h a t e can p r o v i d e v a l u a b l e i n s i g h t i n t o r e a c t i o n s t h a t p r e c e d e t h e a t t a c h m e n t o f t h e s u l p h i d e s u r f a c e s t o a i r b u b b l e s i n f l o t a t i o n . T h i s e l e c t r o c h e m i c a l a p p r o a c h t o g e t h e r w i t h o t h e r w e l l - e s t a b l i s h e d t e c h n i q u e s can be u s e f u l f o r t h e i n v e s t i g a t i o n o f i n t e r a c t i o n mechanisms between x a n t h a t e and s u l p h i d e m i n e r a l s i n f l o t a t i o n . V a r i o u s i n v e s t i g a t o r s s t u d i e d e l e c t r o - f l o t a t i o n r e c e n t l y , i . e . , f l o t a t i o n i n a c o m b i n a t i o n o f an e l e c t r o l y t i c c e l l and a f l o t a t i o n c e l l . I f e x t e r n a l p o t e n t i a l i s a p p l i e d t o f l o t a t i o n p u l p t h r o u g h e l e c t r o d e s , , t h e r e i s no doubt t h a t t h e s u r f a c e e n e r g y l e v e l o f s u l p h i d e p a r t i c l e s i n t h e p u l p i s a l t e r e d due t o t h e i n d u c t i o n p o l a r i z a t i o n e f f e c t . However, the d e t a i l e d t h e o r y o f t h i s e l e c t r o - f l o t a t i o n i s as y e t t o be d e v e l o p e d . B a n e r j i 0 1 0 ) d e s c r i b e d v a r i o u s a d v a n t a g e s o f s p h a l e r i t e a c t -i v a t i o n by e l e c t r o l y t i c a l l y p r o d u c e d c o p p e r u s i n g c o p p e r e l e c t r o d e s i n s t e a d o f c o p p e r s u l p h a t e a c t i v a t i o n . S h a f e e v e t a l (111) f o u n d i n t h e i r e l e c t r o -- 35 -c h e m i c a l t r e a t m e n t o f x a n t h a t e s o l u t i o n s t h a t e l e c t r o c h e m i c a l l y o x i d i z e d x a n t h a t e s o l u t i o n y i e l d s improved f l o t a t i o n r e c o v e r y o f p y r i t e ( F e S 2 ) . g a l e n a (PbS) and C h a l c o p y r i t e (CuFeS,,). G o r o d e t s k i i e t a l (112) r e p o r t e d improved f l o t a t i o n r e c o v e r y o f s u l p h i d e s l i m e s by e l e c t r o c h e m i c a l t r e a t m e n t o f t h e p u l p . G l e m b o t s k i i and Klimenko (7) f o u n d i n t h e i r s t u d y on t h e m i x t u r e o f s u l p h i d e m i n e r a l s i n x a n t h a t e s o l u t i o n t h a t s u l p h i d e s w i t h l e s s n o b l e r e s t p o t e n t i a l s removed x a n t h a t e f r o m t h o s e w i t h more n o b l e p o t e n t i a l ; t h e y r e a s o n e d t h a t a r e d i s t r i b u t i o n o f a d s o r b e d x a n t h a t e f r o m a r e l a t i v e l y more n o b l e m i n e r a l t o a l e s s n o b l e one was c a u s e d by e l e c t r o c h e m i c a l r e a c t i o n s o c c u r r i n g when t h e y were i n g a l v a n i c c o n t a c t . W i n t e r and Woods (27) o b s e r v e d i n c r e a s e d f l o t a t i o n e f f i c i e n c y w i t h t h e e a s e o f o x i d a t i o n o f homologous x a n t h a t e s , i . e . , g a l e n a f l o a t e d b e t t e r w i t h i n c r e a s i n g c h a i n l e n g t h w h i l e t h e r e d o x p o t e n t i a l o f t h e d i x a n t h o g e n / x a n t h a t e c o u p l e became more n e g a t i v e due t o t h e l o w e r s o l u b i l i t y o f a l o n g e r c h a i n d i x a n t h o g e n . The r e d o x p o t e n t i a l o f f l o t a t i o n p u l p can be u t i l i z e d f o r p r o c e s s c o n t r o l i n f l o t a t i o n s i m i l a r t o t h e c o n t r o l o f f e r r i c i o n c o n c e n t r a t i o n i n some l e a c h i n g p r o c e s s e s . Woodcock and Jones (83, 84) and N a t a r a j a n and Iwaskaki (113) measured t h e redo x p o t e n t i a l o f a f l o t a t i o n p u l p a t v a r i o u s p o i n t s i n many f l o t a t i o n p l a n t s . They f o u n d t h a t t h e change i n r e d o x p o t e n t i a l o f f l o t a t i o n p u l p s was c l o s e l y f o l l o w e d by t h e change i n t h e c o n c e n t r a t i o n o f d i s s o l v e d oxygen i n t h e p u l p s , i . e . , t h e r e d o x p o t e n t i a l o f an o x y g e n - d e f i c i e n t g r i n d i n g d i s c h a r g e was t h e most n e g a t i v e and t h e p o t e n t i a l became more p o s i t i v e - 36 -as t h e p u l p p r o c e e d e d t h r o u g h t h e c i r c u i t s p i c k i n g up more oxygen f r o m a e r a t i o n . S i n c e oxygen p l a y s a g r e a t r o l e i n s u l p h i d e f l o t a t i o n w i t h x a n t h a t e , m o n i t o r i n g t h e r e d o x p o t e n t i a l o f f l o t a t i o n p u l p s can be v e r y u s e f u l f o r p r o c e s s c o n t r o l . They d i s c u s s e d t h e u t i l i t y o f r e d o x p o t e n t i a l measured by a p l a t i n u m e l e c t r o d e d i p p e d i n t o t h e f l o t a t i o n p u l p f o r m o n i t o r i n g t h e changes i n t h e s o l u t i o n c o m p o s i t i o n c a u s e d by o r e , added r e a g e n t s and v a r i o u s r e a c t i o n s and f o r p r o v i d i n g a u s e f u l g u i d e t o p r o c e s s c o n t r o l . S u l l i v a n and Woodcock (114) r e c e n t l y d e v e l o p e d a s i m p l e o n - s t r e a m x a n t h a t e m o n i t o r i n g d e v i c e u t i l i z i n g u l t r a - v i o l e t p h o t o s p e c t r o s c o p y . The l o g a r i t h m i c r e l a t i o n s h i p between x a n t h a t e c o n c e n t r a t i o n and r e d o x p o t e n t i a l o f t h e d i x a n t h o g e n / x a n t h a t e c o u p l e can be u t i l i z e d f o r t h e same p u r p o s e i n t h e o r y . However, p o i s o n i n g o f t h e p l a t i n u m e l e c t r o d e (115, 116) and a mixed p o t e n t i a l f o r m a t i o n w i t h o t h e r r e d o x c o u p l e s d e r i v e d f r o m o r e and r e a g e n t s make i t more d i f f i c u l t f o r x a n t h a t e m o n i t o r i n g by r e d o x p o t e n t i a l . Woods' (67) s t u d y i n d i c a t e d t h a t a smooth s u r f a c e p l a t i n u m e l e c t -r o d e i n g a l e n a p u l p showed t h e r e s t p o t e n t i a l o f g a l e n a w h i l e a h i g h l y p l a t i n i z e d p l a t i n u m e l e c t r o d e r e t a i n e d i t s o r i g i n a l r e s t p o t e n t i a l i n t h e same g a l e n a p u l p owing t o t h e v e r y l a r g e t r u e s u r f a c e a r e a which was n o t i n c o n t a c t w i t h g a l e n a ( s u r f a c e a r e a f a c t o r was c a . 2,000). T h e r e f o r e , P l a t i n i z e d p l a t i n u m e l e c t r o d e s m i g h t be more u s e f u l i n m o n i t o r i n g c o n c e n -t r a t i o n s o f d i s s o l v e d s p e c i e s w h i l e smooth p l a t i n u m e l e c t r o d e s m i g h t g i v e a r e d o x p o t e n t i a l c l o s e r t o t h e r e s t p o t e n t i a l o f s u l p h i d e s . - 37 -CHAPTER 2 EXPERIMENTAL 2.1 E l e c t r o d e s 2.1.1 S a t u r a t e d Calomel E l e c t r o d e ; R e f e r e n c e E l e c t r o d e A s a t u r a t e d c a l o m e l e l e c t r o d e (SCE) was used t h r o u g h o u t t h i s s t u d y as a r e f e r e n c e e l e c t r o d e . A c c o r d i n g t o Moore ( 1 1 7 ) , t h e m o b i l i t y o f K i s 7. 62 x 10" m • 1 - 1 - 8 2 1 - 1 s e c " -v and t h e m o b i l i t y o f Cl i s 7.91 x 10" m ' s e c " -v , hence t h e t r a n s f e r number becomes 0.49 and 0.51 f o r K + and C l " , r e s p e c t i v e l y . S i n c e K + and C l " have a l m o s t t h e same m o b i l i t i e s i n aqueous s o l u t i o n , t h e s a t -u r a t e d c a l o m e l e l e c t r o d e c a n p r o v i d e t h e most s a t i s f a c t o r y r o l e o f r e f e r e n c e e l e c t r o d e w i t h minimum j u n c t i o n p o t e n t i a l . A c c o r d i n g t o Shoemaker and G a r l a n d ( 1 1 8 ) , t h e h a l f - c e l l p o t e n t i a l o f t h e SCE a t 25°C becomes 0.2444 v o l t . The t e m p e r a t u r e c o e f f i c i e n t o f t h e SCE may be n e g l e c t e d so l o n g as t h e e x p e r i m e n t a l t e m p e r a t u r e r e m a i n s w i t h i n a few d e g r e e s o f 25°C.(see S e c t i o n 1.2.3.4) The p o t e n t i a l o f t h e SCE used i n t h i s s t u d y was f o u n d t o be +235 mV ~ + 255 mV vs t h e SHE i n x a n t h a t e s o l u t i o n (0M - 1 0 " 3 M) a t 25°C. The a v e r a g e v a l u e o f +245 mV was used t o c o n v e r t t h e measured p o t e n t i a l f r o m t h e SCE s c a l e t o t h e SHE s c a l e and a l l t h e e l e c t r o d e p o t e n t i a l s measured have been r e p o r t e d both i n t h e SCE and t h e SHE s c a l e . - 38 -2.1.2 P l a t i n u m E l e c t r o d e ; A u x i l i a r y E l e c t r o d e A smooth s u r f a c e p l a t i n u m s h e e t e l e c t r o d e has been used as an a u x i l i a r y e l e c t r o d e f o r t h e t h r e e - e l e c t r o d e s y s t e m d u r i n g p o l a r -i z a t i o n s t u d y . F o r use i n t h e hydrogen e l e c t r o d e , t h e p l a t i n u m e l e c t r o d e s h o u l d have p l a t i n u m b l a c k on i t i n o r d e r t o i n c r e a s e t h e s u r f a c e a r e a ; however, t h i s p l a t i n i z a t i o n i s u s u a l l y n o t n e c e s s a r y f o r an a u x i l i a r y e l e c t r o d e ( 1 1 8 ) . The p l a t i n u m e l e c t r o d e has been c l e a n e d i n h o t aqua r e g i a s o l -u t i o n (one p a r t c o n c e n t r a t e d HN0 3 t o t h r e e p a r t s c o n c e n t r a t e d HC1) and washed t h o r o u g h l y w i t h d o u b l e d i s t i l l e d w a t e r , as has been recommended by Shoemaker and G a r l a n d ( 1 1 8 ) . 2.1.3 Working E l e c t r o d e s ; S u l p h i d e M i n e r a l and I r o n E l e c t r o d e s Working e l e c t r o d e s o f v a r i o u s s u l p h i d e m i n e r a l s have been p r e -p a r e d as shown i n F i g . 2.1. A c r y s t a l o f s u l p h i d e m i n e r a l t o be t e s t e d was c u t f r o m a m a s s i v e specimen w i t h a diamond saw t o a cube a p p r o x i m a t e l y 1 cm i n each d i m e n s i o n . I t was t h e n p l a c e d i n a mold t o g e t h e r w i t h an L-shaped soda g l a s s t u b e (8mm d i a . ) and cemented u s i n g "Koldmount", a s e l f -c u r i n g epoxy r e s i n , t o expose o n l y one f a c e o f t h e m i n e r a l cube. A s m a l l p i e c e o f a p l a t i n u m w i r e which was s o l d e r e d t o a c o p p e r w i r e was d i p p e d i n t o m ercury which was i n t u r n was i n c o n t a c t w i t h s u l p h i d e m i n e r a l t o p r o v i d e a good e l e c t r i c a l c o n t a c t . The c o p p e r w i r e was s e c u r e d i n s i d e t h e g l a s s - 39 -S o l d e r i n g i Space P l a t i n u m Wire M e r c u r y *— I n s u l a t e d C o p per Wi G l a s s Tube Koldmount +— S u l p h i d e M i n e r a l F i g u r e 2.1 ; P r e p a r a t i o n o f Working E l e c t r o d e s f o r S u l p h i d e M i n e r a l s - 40 -t u b e w i t h "Koldmount". The e l e c t r i c a l c o n t a c t between a metal and a s e m i - c o n d u c t o r needs s p e c i a l a t t e n t i o n . To e n s u r e ohmic c o n t a c t w i t h o u t any r e c t i -f i c a t i o n e f f e c t , many i n v e s t i g a t o r s , e.g., S a t o ( 9 8 ) , T o l u n and K i t c h e n e r ( 2 5 ) , Woods ( 4 4 ) , A l l i s o n e t a l ( 4 0 ) , N i c o l ( 3 8 ) , and Chander and F u e r s t e n a u (72) used t h i s m e r c u r y c o n t a c t f o r e l e c t r i c a l c o n t a c t between p l a t i n u m w i r e and s u l p h i d e m i n e r a l s . A m e t a l l i c i r o n w o r k i n g e l e c t r o d e was p r e p a r e d i n a s i m i l a r way t o s u l p h i d e e l e c t r o d e s e x c e p t t h a t e l e c t r i c a l c o n t a c t was e s t a b l i s h e d t h r o u g h d i r e c t c o n t a c t between a p i e c e o f i r o n and i r o n w i r e . A f t e r p r e p a r a t i o n , t h e w o r k i n g e l e c t r o d e s were c l e a n e d u l t r a -s o n i c a l l y i n a d e t e r g e n t s o l u t i o n and r i n s e d t h o r o u g h l y w i t h d o u b l e d i s -t i l l e d w a t e r . P r i o r t o each e x p e r i m e n t , t h e w o r k i n g e l e c t r o d e s were r e -p o l i s h e d on a Emery paper and c l e a n e d a g a i n u s i n g o n l y d o u b l e d i s t i l l e d w a t e r t o remove s u r f a c e p r o d u c t s formed d u r i n g p r e v i o u s r u n s . The i r o n s pecimen used was' h i g h p u r i t y (99.95% Fe) Armco i r o n . The a n a l y s e s and o r i g i n o f t h e w o r k i n g e l e c t r o d e s a r e g i v e n i n T a b l e 2.1 t o g e t h e r w i t h t h e g e o m e t r i c s u r f a c e a r e a s and e l e c t r o d e r e s i s t -a n c e s w h i c h were measured i n a mer c u r y b a t h . M i c r o s c o p i c s t u d y r e v e a l e d t h a t t h e major i m p u r i t y i n t h e s p h a l e r i t e sample was s i l i c e o u s gangue. TABLE 2.1 : O r i g i n , G e o m e t r i c A r e a , R e s i s t a n c e and A n a l y s i s o f Working E l e c t r o d e Specimens Working O r i g i n G e o m e t r i c A r e a (Cm ) R e s i s t a n c e A n a l y s i s E s t i m a t e d E l e c t r o d e s ( a ) * Pb Cu Zn Fe P u r i t y ** G a l e n a (PbS) Kansas, USA 1.685 0.15 86.5 0.08 0.005 0.30 99.9% PbS C h a l c o p y r i t e ( C u F e S 2 ) O s a r i z a w a , Japan 1.157 4.0 0.01 33.4 0.270 29.6 96.5% C u F e S 2 P y r i t e ( F e S 2 ) O s a r i z a w a , J a p a n 1.511 5.5 0.01 0.01 0.005 46.0 98.6% F e S 2 P y r r h o t i t e < F e l - x S > C h i c h i b u , Japan 1.324 0.05 , 0.02 0.07 0.010 59.7 95.0% F e 0 . 9 6 4 S S p h a l e r i t e (ZnS) Balmot, N.Y. 1.219 ~ 1 08 0.09 0.04 50.5 6.3 85.0% ( Z n F e ) S I r o n ( F e ) Armco 1.285 0.01 99.95 99.95% Fe » 9 R e s i s t a n c e o f w o r k i n g e l e c t r o d e s i n c l u d i n g ohmic c o n t a c t r e s i s t a n c e , measured i n m e r c u r y b a t h u s i n g K e i t h l e y Model 61 OB E l e c t r o m e t e r . ( T h i c k n e s s o f each working e l e c t r o d e s was a p p r o x i m a t e l y 1.0 cm) E s t i m a t e d from major components. - 42 -2.1.4 G l a s s E l e c t r o d e A s p e c i a l g l a s s e l e c t r o d e w i t h a d o u b l e s h i e l d e d l e a d (Beckman p a r t No. 39004) was u s e d i n c o n n e c t i o n w i t h " E l e c t r o s c a n ™ 3 0 " f o r pH measurement, as recommended by t h e manual o f " E l e c t r o s c a n 30" ( 1 1 9 ) , t o m i n i m i z e s t r a y i n t e r f e r e n c e and t o a l l o w o p e r a t i o n w i t h grounded s o l u t i o n s . 2.2 Reagents A l l r e a g e n t s used were a n a l y t i c a l r e a g e n t g r a d e . F o r t h e p r e p -a r a t i o n o f e l e c t r o l y t e s o l u t i o n s , d o u b l e d i s t i l l e d w a t e r was used t h r o u g h -o u t t h e e x p e r i m e n t . The d o u b l e d i s t i l l e d w a t e r was n o t d e i o n i z e d a l t h o u g h many i n v e s t i g a t o r s used d e i o n i z e d w a t e r i n t h e i r e l e c t r o c h e m i c a l s t u d y . Greene (120) c a u t i o n e d on t h e p o s s i b i l i t y o f o r g a n i c c o n t a m i n a t i o n f r o m d e i o n i z a t i o n . A l l g ases ( a r g o n , a i r and hydrogen) were o b t a i n e d f r o m C a n a d i a n L i q u i d A i r L t d . . Argon was p a s s e d t h r o u g h a l k a l i n e p y r o g a l l o l s o l u t i o n and hydrogen was p a s s e d t h r o u g h Deoxo i n o r d e r t o e l i m i n a t e t r a c e s o f oxygen f r o m t h e g a s e s . A l k a l i n e p y r o g a l l o l s o l u t i o n was p r e p a r e d by d i s s o l v i n g 15g o f p y r o g a l l i c a c i d ( C g H 3 ( 0 H ) 3 ) 1n 25 ml o f d o u b l e d i s t i l l e d w a t e r and m i x i n g w i t h 150 ml o f 30% K0H s o l u t i o n . Deoxo i s a c a t a l y t i c p u r i f i e r f o r e l e c t -r o l y t i c h y d r o g e n . P a l l a d i u m CPd) c o n t a i n e d i n Deoxo c a t a l y t i c a l l y combines oxygen w i t h hydrogen t o f o r m w a t e r . - 43 -F o l l o w i n g t h e normal p r o c e d u r e , p u r e normal p o t a s s i u m e t h y l x a n t h a t e ( KEtX) was p r e p a r e d by a d d i t i o n o f c a r b o n d i s u l p h i d e t o a s o l -u t i o n o f p o t a s s i u m h y d r o x i d e i n t h e normal e t h y l a l c o h o l and p u r i f i e d by m u l t i p l e r e c r y s t a l l i z a t i o n f r o m a c e t o n e . M o s t o f t h e i n v e s t i g a t i o n s have been c a r r i e d o u t a t n a t u r a l pH. However, t h e pH o f t h e e l e c t r o l y t e s o l u t i o n was c o n t r o l l e d by d i l u t e KOH s o l u t i o n and HgSO^ s o l u t i o n i n t r o d u c e d w i t h g l a s s s y r i n g e s d u r i n g t h e s t u d y o f p o t e n t i a l — p H r e l a t i o n s h i p s . S u l p h u r i c a c i d was chos e n i n t h i s i n v e s t -i g a t i o n b e c a u s e o f t h e wide a p p l i c a t i o n o f t h e a c i d i n t h e f l o t a t i o n i n d u s t r y . 2.3 E l e c t r o l y t i c C e l l The t y p i c a l e l e c t r o l y t i c c e l l used i n t h i s i n v e s t i g a t i o n i s shown i n F i g . 2.2 t o g e t h e r w i t h t h e e x p e r i m e n t a l s e t - u p . The e l e c t r o -l y t i c c e l l i s a c o n v e n t i o n a l two compartment H - c e l l formed by j o i n i n g two 1000 ml p y r e x b e a k e r s (10 cm d i a . ) and s e p a r a t e d by a f r i t g l a s s which i s p l a c e d i n t h e m i d d l e t o p r e v e n t e x c e s s i v e m i x i n g o f t h e e l e c t r o l y t e . One o f t h e two compartments c o n t a i n s a w o r k i n g e l e c t r o d e ( s u l p h i d e e l e c t r o d e a n d / o r i r o n e l e c t r o d e ) , a s a t u r a t e d c a l o m e l e l e c t r o d e p l a c e d on a T e f l o n cup which i s i n t u r n c o n n e c t e d t o a L u g g i n c a p i l l a r y p o i n t i n g t o t h e w o r k i n g e l e c t r o d e , a g l a s s e l e c t r o d e , a gas d i s p e n s e r , a thermometer and a T e f l o n c o a t e d magnet. The L u g g i n c a p i l l a r y was p r e -p a r e d by d r a w i n g o u t 8 mm p y r e x t u b i n g and f i r e p o l i s h i n g t h e t i p . The e l e c t r o l y t e can be made a l m o s t oxygen f r e e o r a i r s a t u r a t e d by c o n t i n u o u s - 44 -p u r g i n g w i t h a r g o n ( A r ) o r a i r , r e s p e c t i v e l y . The o t h e r compartment o f t h e H - c e l l c o n t a i n s a p l a t i n u m e l e c t r o d e , a hydrogen gas d i s p e r s e r and a T e f l o n c o a t e d magnet. D u r i n g t h e p o l a r i z a t i o n s t u d y o n l y , t h i s a u x i l i a r y p l a t i n u m e l e c t r o d e i s us e d t o g e t h e r w i t h a s a t u r a t e d c a l o m e l e l e c t r o d e and a w o r k i n g e l e c t r o d e i n th e o t h e r compartment. The r e s t p o t e n t i a l o f a w o r k i n g e l e c t r o d e can a l s o be o b t a i n e d f r o m t h i s p o l a r i z a t i o n s t u d y . 2.4 E l e c t r o n i c I n s t r u m e n t s A l l 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 have been c a r r i e d o u t w i t h TM a " E l e c t r o s c a n 30" m a n u f a c t u r e d by Beckman I n s t r u m e n t s , I n c . . The Beckman TM E l e c t r o s c a n 30 i s a s e l f - c o n t a i n e d e l e c t r o a n a l y t i c a l s y s t e m c a p a b l e o f p e r f o r m i n g v a r i o u s measurements i n t h e two main c a t e g o r i e s o f p o t e n t i o m e t r y and v o l t a m e t r y , e.g., t h e f u n c t i o n s o f pH-meter, h i g h impedance m i l l i -v o l t m e t e r , p o t e n t i o s t a t and g a l v a n o s t a t ( 1 1 9 ) . D i r e c t measurement o f an e l e c t r o d e r e s t p o t e n t i a l can be done i n t h e h i g h impedance m i l l i v o l t mode o p e r a t i o n w i t h two e l e c t r o d e s y s t e m , i . e . , a w o r k i n g e l e c t r o d e and a r e f e r e n c e e l e c t r o d e . S i n c e t h e impedance o f t h e i n s t r u m e n t i s v e r y h i g h , i t draws v i r t u a l l y no c u r r e n t ( l e s s t h a n 0.001 yA) making i t c a p a b l e o f m e a s u r i n g open c i r c u i t p o t e n t i a l . In t h e p o t e n t i o s t a t mode o p e r a t i o n f o r p o l a r i z a t i o n s t u d y , t h e i n s t r u m e n t compares t h e p o t e n t i a l between t h e w o r k i n g and r e f e r e n c e e l e c -t r o d e s w i t h t h e i n p u t p o t e n t i a l . The n e g a t i v e f e e d b a c k c i r c u i t , i n c l u d i n g - 45 -w o r k i n g and a u x i l i a r y e l e c t r o d e s , a c t s t o r e s t o r e t h i s d i f f e r e n c e t o z e r o . The iR drop i n t h i s f e e d b a c k c i r c u i t i s measured and d i s p l a y e d by t h e b u i l t - i n r e c o r d e r as t h e e l e c t r o l y s i s c u r r e n t . A c c o r d i n g t o t h e manual f o r t h e i n s t r u m e n t ( 1 1 9 ) , t h e i n -s t r u m e n t o p e r a t e s on power o f 115 V and 60 Hz, and t h e v o l t a g e s c a n r a t e can be v a r i e d f r o m ± 0.5 mV/sec t o ± 500 mV/sec. The l e a k a g e c u r r e n t f o r m i l l i v o l t measurements i s l e s s t h a n 10 amperes and t h e a c c u r a c y o f v o l t a g e s c a n i s ± 0.5% o f t h e span r a n g e . The i n s t r u m e n t was c a l i b r a t e d w i t h an i n d e p e n d e n t e l e c t r o -meter and pH meter f o r m i l l i v o l t measurement and pH mode o p e r a t i o n r e s p e c t i v e l y . The K e i t h ! e y Model 610B E l e c t r o m e t e r i s a v e r s a t i l e i n s t r u m e n t which can measure a wide ra n g e o f dc v o l t a g e s , c u r r e n t s , r e s i s t a n c e s and / 14 \ c h a r g e s . The i n s t r u m e n t i s an improved h i g h impedance (10 ohms) vaccum t u b e - s o l i d s t a t e v o l t m e t e r w i t h a c c u r a c y o f ± 1% o f v o l t m e t e r f u l l s c a l e . T h i s e l e c t r o m e t e r was used t o check t h e p e r f o r m a n c e o f t h e TM "Beckman E l e c t r o s c a n 30" and t o measure t h e r e s i s t a n c e o f v a r i o u s w o r k i n g e l e c t r o d e s . The C o r n i n g r e s e a r c h t y p e Model 10 pH me t e r was use d t o c a l -TM i b r a t e t h e pH-mode o p e r a t i o n o f t h e Beckman E l e c t r o s c a n 30. S p e c i f i c c o n d u c t i v i t y o f t h e e l e c t r o l y t e s o l u t i o n was measured - 46 -u s i n g a c o n d u c t i v i t y meter m a n u f a c t u r e d by R a d i o m e t e r , Copenhagen. 2.5 E x p e r i m e n t a l S e t Up and P r o c e d u r e s F i g u r e 2.2 i l l u s t r a t e s s c h e m a t i c a l l y a t y p i c a l e x p e r i m e n t s e t - u p used f o r 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 d u r i n g t h i s i n v e s t i g a t i o n . The gases ( H 2 , A r o r A i r ) were s u p p l i e d f r o m h i g h - p r e s s u r e c y l i n d e r s t h r o u g h p r e s s u r e r e g u l a t o r s and r o t a m e t e r s w i t h n e e d l e v a l v e s . The f l o w r a t e o f a l l gases was m a i n t a i n e d a t 400 ml p e r m i n u t e . Compressed a i r was h u m i d i f i e d by b u b b l i n g t h r o u g h a b u b b l e r c o n t a i n i n g d o u b l e d i s -t i l l e d w a t e r . Argon was f i r s t p u r i f i e d f o r a t r a c e o f oxygen by b u b b l i n g t h r o u g h a l k a l i n e p y r o g a l l o l s o l u t i o n and t h e n h u m i d i f i e d p r i o r t o a d -m i s s i o n t o t h e a u x i l i a r y p l a t i n u m e l e c t r o d e compartment o f t h e H - c e l l . The f l o w o f e i t h e r a i r o r a r g o n was c o n t r o l l e d w i t h t h e three-way s t o p -c o c k . The t u b i n g s used f o r gas l i n e s were t r a n s p a r e n t Tygon t u b i n g s , a compound o f p o l y v i n l y c h l o r i d e w i t h l i q u i d p l a s t i c i z e r s . Each compartment o f t h e H - c e l l c o n t a i n e d 850 ml o f e l e c t r o l y t e s o l u t i o n , a n d , d e p e n d i n g on t h e p u r p o s e o f t h e i n v e s t i g a t i o n s , t h e s o l u t i o n was c o n t i n u o u s l y s t i r r e d and purged w i t h d i f f e r e n t g a s e s . The temp-e r a t u r e o f t h e s o l u t i o n was m a i n t a i n e d a t 25° ± 1°C u s i n g a m a g n e t i c s t i r r e r - h e a t e r p l a c e d u n d e r n e a t h t h e H - c e l l . E l e c t r o l y t e s o l u t i o n f i l l e d t h e L u g g i n c a p i l l a r y and T e f l o n ( p o l y t e t r a f l u o r j e t h y l e n e ) cup where t h e s a t u r a t e d c a l o m e l e l e c t r o d e was p l a c e d . The s a t u r a t e d KCL s o l u t i o n i n s i d e t h e L u g g i n c a p i l l a r y was f o u n d t o have no s i g n i f i c a n t , e f f e c t on t h e e l e c t r o c h e m i c a l measurements. Thermometer P o t e n t i o m e t e r ( E l e c t r o s c a n 1 1 30) Gas O u t l e t Working E l e c t r o d e Luggin C a p i l l a r y 4=> Rotameter F r i t t e d G l a s s (Hydrogen) (Argon) ( A i r ) F i g u r e 2.2 ; S c h e m a t i c Diagram o f a T y p i c a l E x p e r i m e n t a l S e t Up - 48 -TM The p o t e n t i o m e t e r , Beckman E l e c t r o s c a n 30, measured t h e r e s t p o t e n t i a l o f t h e w o r k i n g e l e c t r o d e a g a i n s t t h e r e f e r e n c e e l e c t r o d e and measured pH o f t h e s o l u t i o n w i t h s a t u r a t e d c a l o m e l and g l a s s e l e c t r o d e s u t i l i z i n g o n l y one compartment o f t h e H - c e l l . The p o l a r i z a t i o n s t u d y was done w i t h t h e p o t e n t i o m e t e r c o n n e c t e d t o t h e w o r k i n g r e f e r e n c e and a u x i l i a r y e l e c t r o d e s , i n t h e t h r e e - e l e c t r o d e system. - 49 -CHAPTER 3 EXPERIMENTAL RESULTS AND DISCUSSIONS 3.1 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 3.1.1 P o t a s s i u m E t h y l X a n t h a t e S o l u t i o n W i t h i n c r e a s i n g c o n c e n t r a t i o n s o f x a n t h a t e , pH and t h e s p e c i f i c c o n d u c t i v i t y o f t h e x a n t h a t e s o l u t i o n s were f o u n d t o i n c r e a s e . T h e r e f o r e , x a n t h a t e can i n f l u e n c e an e l e c t r o c h e m i c a l r e a c t i o n which i s t a k i n g p l a c e on t h e s u r f a c e o f e l e c t r o d e s i n t h r e e d i f f e r e n t ways, i . e . , by d i r e c t i n -v o l v e m e n t i n t h e r e a c t i o n and by t h e i n d i r e c t r o l e s o f a l t e r i n g t h e pH and t h e c o n d u c t i v i t y o f t h e e l e c t r o l y t e s o l u t i o n . H i g h c o n d u c t i v i t y o f t h e s o l -u t i o n w i l l l o w e r t h e s o l u t i o n i R d r o p and w i l l i n c r e a s e t h e r e a c t i o n r a t e s . (A) pH o f P o t a s s i u m E t h y l X a n t h a t e (KEtX) S o l u t i o n N a t u r a l pH o f b u l k s o l u t i o n (850 ml) a t d i f f e r e n t c o n c e n t r a t i o n s o f p o t a s s i u m e t h y l x a n t h a t e was measured i n t h e p r e s e n c e o f g a l e n a e l e c t r o d e s ( S u r f a c e A r e a : 1.585 cm ) w i t h t h e s o l u t i o n purged w i t h a r g o n and a i r a t a t e m p e r a t u r e o f 25°C. The i n i t i a l pH o f t h e d o u b l e d i s t i l l e d w a t e r u s e d f o r t h e p r e p a r a -t i o n o f KEtX s o l u t i o n p urged w i t h a r g o n was 5.6. Double d i s t i l l e d w a t e r , used d u r i n g i n v e s t i g a t i o n s p u r g e d w i t h a i r , showed pH o f 4.9, which was p r o b a b l y due t o a b s o r p t i o n o f c a r b o n d i o x i d e f r o m t h e atmosphere. F i g u r e 3.1 i n d i c a t e s t h a t pH o f KetX s o l u t i o n i s a l o g a r i t h m i c f u n c t i o n o f t h e c o n c e n t r a t i o n o f KEtX, as has been shown by Pomianowski and - 50 -F i g u r e 3.1 ; pH o f KEtX S o l u t i o n i n t h e P r e s e n c e o f G a l e n a E l e c t r o d e ( A f t e r 15 Min. Gas P u r g i n g , 2 5 ° C ) - 51 -L e j a ( 3 5 ) , and F u e r s t e n a u , e t a l ( 4 7 ) . When t h e KetX s o l u t i o n was purged w i t h a r g o n g a s , an i n c r e m e n t o f c a . 0.5 u n i t i n pH was o b s e r v e d f o r a t e n - f o l d i n c r e a s e o f E t X " c o n -c e n t r a t i o n . T h i s change i n pH w i t h c o n c e n t r a t i o n o f E t X " can be e x p l a i n e d as t h e r e s u l t o f x a n t h a t e h y d r o l y s i s t o f o r m x a n t h i c a c i d . (See A p p e n d i x 2.1) On t h e o t h e r hand, f o r t h e s o l u t i o n purged w i t h a i r , t h e s l o p e o f pH vs KEtX c o n c e n t r a t i o n was one pH u n i t p e r decade o f KEtX c o n c e n t r a t i o n . T h i s was e x p l a i n e d as a r e s u l t o f t h e x a n t h a t e o x i d a t i o n t o d i x a n t h o g e n under s o l u t i o n p u r g i n g w i t h a i r . (See A p p e n d i x 2.2) (B) S p e c i f i c C onductance o f P o t a s s i u m E t h y l X a n t h a t e (KEtX) S o l u t i o n S p e c i f i c c o n d u c t i v i t y was measured a t 25°C f o r p o t a s s i u m e t h y l x a n t h a t e s o l u t i o n s a t d i f f e r e n t c o n c e n t r a t i o n s p r e p a r e d i n d o u b l e -d i s t i l l e d w a t e r h a v i n g a s p e c i f i c c o n d u c t i v i t y o f 1.8 y mho • c n f ^ . S i n c e p o t a s s i u m e t h y l x a n t h a t e (KEtX) i s a s t r o n g e l e c t r o l y t e w i t h a p o l a r f u n c t i o n a l g r o u p , i t i s a l m o s t c o m p l e t e l y s o l u b l e i n a l l p o l a r s o l v e n t s , i . e . , i n w a t e r i n t h e p r e s e n t s t u d y . As a r e s u l t , t h e c o n -d u c t i v i t y o f KEtX s o l u t i o n i n c r e a s e d l i n e a r l y w i t h i n c r e a s i n g c o n c e n -t r a t i o n o f KEtX. ( s e e F i g . 3.2) The s p e c i f i c c o n d u c t i v i t y o f KEtX s o l u t i o n was c a . 12 and 120 y -1 -4 -3 mho cm a t 10 M and 10 M, r e s p e c t i v e l y . O c t a v i e c (121) f o u n d t h e same l i n e a r S p e c i f i c C o n d u c t i v i t y , k (y mho-cm" 1) O ro 4> CTi CD o O O O O o ro o o r t X o I -Pi - 29 -- 53 -r e l a t i o n s h i p between s p e c i f i c c o n d u c t i v i t y and x a n t h a t e c o n c e n t r a t i o n . S o l u t i o n p u r g i n g w i t h Argon o r a i r r e s u l t e d i n no s i g n i f i c a n t change i n s o l u t i o n c o n d u c t i v i t y . 3.1.2 R e s t P o t e n t i a l and Mixed P o t e n t i a l w i t h R e s p e c t t o Time An e l e c t r o d e d i p p e d i n t o e l e c t r o l y t e s o l u t i o n u s u a l l y t o o k some ti m e f o r t h e e l e c t r o d e t o r e a c h a s t e a d y s t a t e p o t e n t i a l , as T o p e r i and T o l u n (70) f o u n d . Some i n v e s t i g a t o r s showed t h a t even a f t e r s e v e r a l days the r e s t p o t e n t i a l d r i f t e d s t e a d i l y . A s i m i l a r d r i f t o f p o t e n t i a l was o b s e r v e d i n t h e p r e s e n t i n -v e s t i g a t i o n s . T h e r e f o r e , t h e p o t e n t i a l was t a k e n , a r b i t r a r i l y , as e s t -a b l i s h e d when t h e p o t e n t i a l d r i f t e d l e s s t h a n 5 mV f o r a 10 m i n u t e p e r i o d . I t was f o u n d t h a t , d e p e n d i n g on t h e s u l p h i d e m i n e r a l i n v e s t i g a t e d , a b o u t 15 t o 30 mi n u t e s were r e q u i r e d t o r e a c h such a s t e a d y s t a t e o f p o t e n t i a l ; however, m e t a l l i c i r o n r e a c h e d such a s t e a d y s t a t e - i n l e s s t h a n 5 m i n u t e s . I t u s u a l l y t o o k a l o n g e r t i m e i n a i r - s a t u r a t e d s o l u t i o n t h a n i n a de-ox y g e n a t e d s y s t e m . F i g u r e 3.3 i l l u s t r a t e s how t h e mixed p o t e n t i a l o f t h e c h a l c o -p y r i t e - i r o n g a l v a n i c s y s t e m a p p r o a c h e d t h e s t e a d y s t a t e p o t e n t i a l i n t i m e . The s t u d y was done i n deoxyge n a t e d d o u b l e d i s t i l l e d w a t e r a t a n a t u r a l pH o f 5.4 and a t 25°C. G a l v a n i c c o n t a c t between t h e two e l e c t r o d e s was made on and o f f u s i n g a s p e c i a l snap s w i t c h i n o r d e r n o t t o d i s t u r b t h e e l e c t r o -c h e m i c a l s y s t e m a f t e r t h e r e s t p o t e n t i a l o f t h e two e l e c t r o d e s r e a c h e d s t e a d y s t a t e . 400 R e s t P o t e n t i a l o f C h a l c o p y r i t e 600 200 -200 -400 -600 0 M [ E t X " ] N a t u r a l pH (5.4) 25°C Argon P u r g i n g Mixed P o t e n t i a l o f C h a l c o p y r i t e and I r o n R e s t P o t e n t i a l o f I r o n -800 -L ± _L 10 20 30 60 70 40 50 Time (min.) -»• F i g u r e 3.3; R e s t P o t e n t i a l o f C h a l c o p y r i t e and I r o n , a n d Mixed P o t e n t i a l o f 80 400 200 > -200 -400 90 - 55 -The mixed p o t e n t i a l o f a c h a l c o p y r i t e - i r o n g a l v a n i c c o u p l e a p p r o a c h e d a s t e a d y s t a t e f r o m t h e c h a l c o p y r i t e s i d e . B e i n g d i s c o n n e c t e d f r o m each o t h e r , t h e r e s t p o t e n t i a l o f c h a l c o p y r i t e r e c o v e r e d s l o w l y t o t h e v a l u e below t h e o r i g i n a l one w h i l e i r o n r e c o v e r e d a l m o s t i n s t a n t a n e o u s l y t o i t s o r i g i n a l r e s t p o t e n t i a l - The above two o b s e r v a t i o n s i n d i c a t e d t h a t c h a l c o p y r i t e was b e i n g p o l a r i z e d by some s u r f a c e r e a c t i o n s c a u s e d by g a l -v a n i c c o n t a c t w i t h i r o n . 3.1.3 E f f e c t o f S o l u t i o n S t i r r i n g on P o t e n t i a l s A p r e l i m i n a r y s t u d y d u r i n g t h i s i n v e s t i g a t i o n i n d i c a t e d t h a t t h e r e s t p o t e n t i a l s o f a l l s u l p h i d e m i n e r a l s were s h i f t e d t o l e s s n o b l e p o t e n t i a l s due t o t h e s o l u t i o n s t i r r i n g , e.g., t h e r e s t p o t e n t i a l o f g a l e n a i n t h e S H E s c a l e was s h i f t e d f r o m +225 mV t o +185 mV a t 0 M [ E t X " ] and f r o m +155 mV t o +135 mV a t I O " 4 M [ E t X " ] f o r t h e "no s t i r r i n g " and " s o l u t i o n s t i r r i n g " c o n d i t i o n s r e s p e c t i v e l y . T h i s i n d i c a t e d t h a t t h e e l e c t r o c h e m i c a l r e a c t i o n s o c c u r i n g on t h e m i n e r a l s u r f a c e were under d i f f u s i o n c o n t r o l . On t h e o t h e r hand, t h e r e s t p o t e n t i a l o f m e t a l l i c i r o n i n t h e SHE s c a l e was s h i f t e d f r o m -10 mV t o +175 mV i n d o u b l e d i s t i l l e d w a t e r and f r o m -380 mV t o -220 mV a t 1 0 " 4 M [ E t X " ] " w i t h o u t " and " w i t h s t i r r i n g " , r e s p e c t i v e l y . T h i s l a r g e p o s i t i v e change i n t h e r e s t p o t e n t i a l o f i r o n i n d i c a t e d t h a t t h e c a t h o d i c r e a c t i o n was s i g n i f i c a n t l y d i f f u s i o n c o n t r o l l e d . The mixed p o t e n t i a l o f a s u l p h i d e - i r o n g a l v a n i c s y s t e m b e h a v i n g s i m i l a r l y t o i r o n i n r e s p o n s e t o s t i r r i n g I n d i c a t e d t h a t t h e r e s t p o t e n t i a l o f i r o n p l a y e d a more s i g n i f i c a n t r o l e i n d e t e r m i n i n g t h e nitxed p o t e n t i a l ( F i g u r e 3.4) - 56 -F i g u r e 3.4 ; The E f f e c t o f S o l u t i o n S t i r r i n g and the D i s t a n c e Between the two Working E l e c t r o d e s on t h e Mixed P o t e n t i a l ( C h a l c o p y r i t e and I r o n System) 0.1 -300 -320 -340 -360 -380 ^ -400 U J C J oo £ -420 E U J -440 -460 -480 -500 -520 --540 --560 —1 1—1—r 0 M [EtX"' ] N a t u r a l pH Ar Top P u r g i n g . 25°C 0.1 T—I I I I— T r 10.0 T 1—' ' 1 ' I -40 -60 -80 -100 -120 -140 -160 -180 -200 -220 -240 -260 -280 -300 J I I I 1 1 t 1 J I 1 • • • • I 0.1 1.0 10.0 D i s t a n c e between C h a l c o p y r i t e and Iron E l e c t r o d e (cm) - 57 -The e f f e c t o f s o l u t i n s t i r r i n g became l e s s s i g n i f i c a n t w i t h i n c r e a s i n g t h e c o n c e n t r a t i o n o f EtX". T h i s m i g h t be an i n d i c a t i o n t h a t the e l e c t r o c h e m i c a l r e a c t i o n i n v o l v i n g E t X " was under d i f f u s i o n c o n t r o l . The above i n v e s t i g a t i o n was c a r r i e d o u t a t n a t u r a l pH w i t h s o l u t i o n t o p p u r g i n g because s o l u t i o n p u r g i n g w i t h gas a l s o p r o v i d e d t h e e f f e c t o f s t i r r i n g a t t h e same t i m e . Based on t h e s e 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 , a l l e l e c t r o c h e m i c a l measurements r e p o r t e d i n t h i s t h e s i s were c a r r i e d o u t w i t h s t i r r i n g a t a c o n t r o l l e d gas f l o w r a t e o f 400 c c / m i n . 3.1.4 Mixed P o t e n t i a l vs D i s t a n c e between A n o d i c and C a t h o d i c E l e c t r o d e s S i n c e mixed p o t e n t i a l i s i n f l u e n c e d by t h e s o l u t i o n i R d r o p , t h e mixed p o t e n t i a l measured on a s u l p h i d e e l e c t r o d e can be v a r i e d d e p e n d i n g on t h e d i s t a n c e between anode and c a t h o d e . ( F i g . 1.2) The mixed p o t e n t i a l o f t h e c h a l c o p y r i t e - i r o n c o u p l e was measured w i t h the v a r i a t i o n i n d i s t a n c e between t h e two e l e c t r o d e s i n d e o x y g e n a t e d d o u b l e - d i s t i l l e d w a t e r a t n a t u r a l pH and a t 25°C. F i g u r e 3.4 i l l u s t r a t e s t h a t t h e mixed p o t e n t i a l thus measured i s a l o g a r i t h m i c f u n c t i o n o f d i s t a n c e s between t h e two e l e c t r o d e s . The m a t h e m a t i c a l e x p l a n a t i o n f o r t h i s r e l a t i o n s h i p i s g i v e n i n A p p e n d i x 3. The p r o b l e m o f s o l u t i o n iR d r o p a r i s e s because o f t h e v e r y low c o n d u c t i v i t y o f d o u b l e d i s t i l l e d w a t e r w i t h o u t any s u p p o r t i n g e l e c t r o l y t e . - 58 -To e l i m i n a t e t h e e f f e c t o f 1R drop and t o make t h e measurement o f t r u e mixed p o t e n t i a l ( F i g u r e 1 . 2 1 p o s s i b l e , t h e s o l u t i o n r e s i s t a n c e between t h e two e l e c t r o d e s must be r e d u c e d e i t h e r by a d d i t i o n o f s u p p o r t i n g e l e c t r o l y t e s o r by b r i n g i n g t h e two e l e c t r o d e s v e r y c l o s e t o g e t h e r . D u r i n g t h e p r e s e n t i n v e s t i g a t i o n , t h e s o l u t i o n was made w i t h o u t s u p p o r t i n g e l e c t r o l y t e i n o r d e r t o e l i m i n a t e any p o s s i b l e e f f e c t s o f t h e s u p p o r t i n g e l e c t r o l y t e on t h e 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 . By k e e p i n g t h e d i s t a n c e between t h e two e l e c t r o d e s t o a minimum, t h e mixed p o t e n t i a l measured w i t h t h e L u g g i n probe on t h e m i n e r a l s was v e r y c l o s e t o t h e t r u e one, as was c o n f i r m e d by t h e f o l l o w i n g e x p e r i m e n t . The mixed p o t e n t i a l o f c h a l c o p y r i t e and i r o n was measured d i r e c t l y i n a i r s a t u r a t e d KEtX s o l u t i o n s a t n a t u r a l pH and a t 25°C. The mixed p o t e n t i a l o f t h e g a l v a n i c c o u p l e was a l s o o b t a i n e d g r a p h i c a l l y f r o m i n d e p e n d e n t p o t e n t i o d y n a m i c c a t h o d i c p o l a r i z a t i o n o f c h a l c o p y r i t e and a n o d i c p o l a r i z a t i o n o f i r o n . D i r e c t l y measured mixed p o t e n t i a l s and g r a p h -i c a l l y o b t a i n e d t r u e mixed p o t e n t i a l s on t h e SHE s c a l e were; +97 mV and +97 mV a t 0 M, +45 mV and +75 mV a t 10" 5M, +35 mV and +50 mV a t 3 x 10" 5M, -23 mV and -5 mV a t 10~ 4M, -75 mV and -60 mV a t 3 x 10" 4M and -97 mV a t 10" 3M o f l E t X " ] , r e s p e c t i v e l y . 3.2 R e s t p o t e n t i a l o f S u l p h i d e M i n e r a l s and M e t a l l i c I r o n R e s t p o t e n t i a l s o f v a r i o u s s u l p h i d e m i n e r a l s were measured a t - 59 -0 M, 10" 5M, 3 x 10" 5M, 10~ 4M, 3 x 10" 4M and 10~ 3M o f E t X " , c o v e r i n g t h e p r a c t i c a l r a n g e o f E t X " c o n c e n t r a t i o n i n f l o t a t i o n . Measurements were done a t n a t u r a l pH as w e l l as a t c o n t r o l l e d pH 0-0 - 12.0) a t a temp-e r a t u r e o f 25°C. Two s e r i e s o f measurements were c o n d u c t e d , i . e . , one i n deoxygena t e d s o l u t i o n s by p u r g i n g w i t h a r g o n and t h e o t h e r i n a i r - s a t u r a t e d s o l u t i o n s by p u r g i n g w i t h a i r . A l t h o u g h t h e a c c u r a c y o f t h e p o t e n t i o m e t e r used was v e r y h i g h (± 1 mV), r e p r o d u c i b i l i t y o f t h e r e s t p o t e n t i a l measurement d u r i n g t h e p r e s e n t s t u d y was abo u t ± 15 mV because o f many d i f f i c u l t i e s i n v o l v e d . 3.2.1 R e s t P o t e n t i a l o f G a l e n a The r e s t p o t e n t i a l o f g a l e n a (PbS) i n d o u b l e - d i s t i l l e d w a t e r a t n a t u r a l pH and a t 25°C was +350 mV and +400 mV vs SHE f o r d e o x y g e n a t e d and a i r s a t u r a t e d s o l u t i o n , r e s p e c t i v e l y . Upon a d d i t i o n o f p o t a s s i u m e t h y l x a n t h a t e i n t o t h e e l e c t r o l y t e s o l u t i o n , t h e r e s t p o t e n t i a l o f a l l s u l p h i d e m i n e r a l s was s h i f t e d a l m o s t i n s t a n t a n e o u s l y t o more n e g a t i v e v a l u e s . Majima and Takeda (26) e x p l a i n e d t h e n e g a t i v e s l o p e f o r t h e r e s t p o t e n t i a l vs c o n c e n t r a t i o n o f E t X " as b e i n g c a u s e d by t h e r e d u c i n g b e h a v i o r o f t h e x a n t h a t e , w h i l e N i c o l ( 3 8 ) , Woods (19, 44) and N a t a r a j a n and Iwaskai (116) e x p l a i n e d t h i s phenomenon b y i n h i b i t i o n o f oxygen r e d u c t i o n c a u s e d by x a n t h a t e a d s o r p t i o n . On t h e o t h e r hand, T o p e r i and T o l u n (70) me n t i o n e d t h a t x a n t h a t e d i s s o l v e s o x i d i z e d p r o d u c t s o f p a s s i v a t i n g f i l m s i n t o s o l u t i o n - 60 -e x p o s i n g t h e l e s s n o b l e s u r f a c e o f metal s u l p h i d e and r e s u l t i n g i n a l e s s n o b l e r e s t p o t e n t i a l i n x a n t h a t e s o l u t i o n . T h e r e f o r e , i t i s most l i k e l y t h a t t h e r e d u c i n g c h a r a c t e r o f t h e x a n t h a t e s o l u t i o n might be c a u s e d by t h e combined e f f e c t o f i n h i b i t i o n o f oxygen r e d u c t i o n and e x p o s u r e o f a l e s s n o b l e s u r f a c e . The e f f e c t o f KEtX on t h e r e s t p o t e n t i a l o f a s u l p h i d e m i n e r a l can be e x p l a i n e d a s : T, t h e p r e s e n c e o f E t X " i n t h e s o l u t i o n i n t r o d u c e s e x t r a e l e c t r o c h e m i c a l systems ( e . g . , MS/MX2 and X 2/X" c o u p l e s ) t o t h e metal s u l p h i d e (MS) - w a t e r syste m , and 2, t h e r e d u c t i o n r a t e o f t h e 0 2 / H 2 0 2 c o u p l e on t h e m i n e r a l i s d e p r e s s e d due t o t h e c o v e r a g e o f t h e m i n e r a l s u r -f a c e s by r e a c t i o n p r o d u c t s . F o l l o w i n g t h e a d d i t i v e p r i n c i p l e as shown i n F i g . 3.5, t h e r e s t p o t e n t i a l o f t h e s u l p h i d e m i n e r a l i n a e r a t e d x a n t h a t e s o l u t i o n becomes t h e mixed p o t e n t i a l formed by t o t a l a n o d i c and c a t h o d i c r e a c t i o n s on t h e m i n e r a l . C o n s e q u e n t l y , t h e r e s t p o t e n t i a l o f t h e s u l p h i d e m i n e r a l i s s h i f t e d i n t h e l e s s n o b l e d i r e c t i o n f r o m E-| t o E 2 and Eg due t o t h e f o r m a t i o n o f t h e mixed p o t e n t i a l , as t h e c o n c e n t r a t i o n o f E t X " i s i n -c r e a s e d . F i g u r e 3.5 s e r v e s o n l y t o e x p l a i n t h e b a s i c p r i n c i p l e s o f how t h e r e s t p o t e n t i a l s o f v a r i o u s s u l p h i d e m i n e r a l s a r e formed i n a e r a t e d x a n t h a t e s o l u t i o n . To p e r f e c t t h e f i g u r e , t h e d e t a i l e d 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 a s u l p h i d e m i n e r a l and x a n t h a t e as w e l l as t h e exchange c u r r e n t d e n s i t y and t h e T a f e l s l o p e o f each e l e c t r o c h e m i c a l c o u p l e i n t h e system must be known. In F i g u r e 3.6, t h e r e s t p o t e n t i a l o f g a l e n a (PbS) i s p l o t t e d as - 61 -F i g u r e 3.5 ; The S c h e m a t i c P r e s e n t a t i o n o f the Rest P o t e n t i a l o f a Metal S u l p h i d e ( M S ) i n the a e r a t e d P o t a s s i u m E t h y l X a n t h a t e S o l u t i o n s ( S c h e m a t i c ) (1) 0 2 / H 2 0 2 C o u p l e on MS a t 0 M [ E t X " ] ; Eh = +540 mV (*) (2) 0 2 / H 2 0 2 Couple on MS a t 10" 4M [ E t X " ] ; Eh = +540 mV (*) (3) 0 2 / H 2 0 2 Couple on MS a t 10" 3M [ E t X " ] ; Eh = +540 mV (*) (4) MS/M + + Couple ; e.g. Eh = 177 mV f o r PbS a t 1 0 ~ 6 M [ P b 2 + ] (5) MS/MX2 Couple a t 10" 4M [ E t X ' ] ; Eh = +112 mU (*) (6) MS/MX2 C o u p l e a t 10 " 3 M [ E t X ' ] ; Eh = +53 m y .(*) (7) X 2/X" Co u p l e on MS a t 10" 4M [ E t X - ] ; Eh = +155 mV (**) (8) X 2/X' Couple on MS a t 10" 3M [ E t X " ] ; Eh = +96 mV (**) *From T o p e r i and T o l u n (70) **From T o l u n and K i t c h e n e r (25) +100 -100 % -200 > •300 -400 (-, A i r , N a t u r a l = 5 . 0 ) ^ . ^ ^ A i r , N a t u r a l = 5 . M S ( - ^ r ^ U i f 5 . ^ ^ ^ ^ ^ O (kk , o 2 , 9 .1) (-, A i r , T'.Hifc „ " " " ^ • - i . - , A i r , N a t u r a l = 5 . 6 ) V ' *» " ^ A J - » A i r , N a t u r a l = 5.8) ^J&(-, A i r , N a t u r a l ( - , A r ^ N a t = ^ ? 3 4 — — ^ = 6 . 2 ) (-, A i r , 9 .0) (-, A i r , 10 .0) (-, A i r , 11 .0) ( - ,Ar,Nat=6T&)-8 (6k, 02, 9 . 0 ) O O (TO , 0 2 , i o . 7 ) (U0 , None, 7 .0) O (6k, o 2, 1 0 . 9 ) O (70 , A i r , 1 0 . 7 ) H +300 +200 +100 O ; C u r r e n t I n v e s t i g a t i o n O ; O t h e r I n v e s t i g a t i o n s ( R e f . No., Gas, pH) O (38, 0 2 , 9 .3) ° (6k, 0 2 , 12.1) o o (25 , N 2 , 9 .1) (38, 0 2 , 9oH X -100 -200 10" 10 "^  M [ E t X " ] 10 -3 to > c n F i g u r e 3. 6 ; Rest P o t e n t i a l o f G a l e n a vs C o n c e n t r a t i o n o f E t X " a t v a r i o u s pH and Gas b u b b l i n g . (With Redox P o t e n t i a l o f Xo /x" ("nun!o• - b J -a f u n c t i o n o f t h e c o n c e n t r a t i o n o f E t X " and summarizes t h e r e s u l t s o f the p r e s e n t i n v e s t i g a t i o n and many o t h e r i n v e s t i g a t i o n s r e p o r t e d . Redox p o t e n t i a l o f t h e Xg/X" c o u p l e a t a c o r r e s p o n d i n g c o n -c e n t r a t i o n o f E t X " i s a l s o shown i n F i g u r e 3.6 f o r c o m p a r i s o n p u r p o s e . The two d i f f e r e n t r e d o x p o t e n t i a l s o f t h e X 2/X~ c o u p l e p l o t t e d i n F i g u r e 3.6 a r e : E ° x jy- = -0.081 V g i v e n by Tolum and K i t c h e n e r (25) and a v e r a g e E ° x ^- = -0.057 V ( s e e S e c t i o n 1.2.1.2) A l t h o u g h t h e r e s t p o t e n t i a l o f g a l e n a seems t o v a r y w i d e l y ( F i g . 3 . 6), some s y s t e m a t i c p a t t e r n can be seen a f t e r c a r e f u l i n v e s t i -g a t i o n o f t h e F i g u r e . F i r s t , t h e r e s t p o t e n t i a l o f g a l e n a i s a l o g a r i t h m i c f u n c t i o n o f E t X " c o n c e n t r a t i o n i n d i c a t i n g t h a t i t f o l l o w s a pseudo N e r n s t r e l a t i o n -s h i p . T h i s a l s o i n d i c a t e d t h a t t h e r e s t p o t e n t i a l o b s e r v e d i s a mixed p o t e n t i a l . T o l u n and K i t c h e n e r ( 2 5 ) , T o p e r i and T o l u n (70) and Woods (44) r e p o r t e d a s i m i l a r r e l a t i o n s h i p f o r g a l e n a - x a n t h a t e s y s t e m . Second, t h e r e s t p o t e n t i a l o f g a l e n a i n a i r - s a t u r a t e d s o l u t i o n was a b o u t 40 - 50 mV n o b l e r t h a n t h a t i n d e o x y g e n a t e d x a n t h a t e s o l u t i o n due t o t h e f o r m a t i o n o f a mixed p o t e n t i a l w i t h t h e oxygen c o u p l e . The same o b s e r v a t i o n was made by T o l u n and K i t c h e n e r (25) and T o p e r i and T o l u n ( 7 0 ) . The r e s t p o t e n t i a l o f g a l e n a i n x a n t h a t e s o l u t i o n a t n a t u r a l PH was i n c l o s e agreement w i t h t h e r e d o x p o t e n t i a l o f t h e X 2/X" c o u p l e . The - 64 -r e s t p o t e n t i a l became l e s s n o b l e w i t h i n c r e a s i n g pH. T h i s i n d i c a t e d t h a t one o r more o f the e l e c t r o c h e m i c a l r e a c t i o n s were under d i f f u s i o n c o n t r o l . The p o t e n t i a l - p H r e l a t i o n s h i p measured f o r v a r i o u s s u l p h i d e m i n e r a l systems w i l l be d i s c u s s e d i n a l a t e r s e c t i o n (3.7).. F i g u r e 3.6 a l s o s e r v e s t o i l l u s t r a t e t h a t t h e r e s t p o t e n t i a l o f g a l e n a depends upon t h e o r i g i n and t h e p r e t r e a t m e n t o f t h e e l e c t r o d e , ( s e e S e c t i o n 1.2.3.4) The p r e s e n t i n v e s t i g a t i o n a g r e e s w i t h a l l o t h e r i n v e s t i g a t i o n s w i t h t h e one e x c e p t i o n o f Woods' (44) i n t h a t r e s t p o t e n t i a l o f g a l e n a i n a i r -s a t u r a t e d x a n t h a t e s o l u t i o n becomes more n e g a t i v e t h a n t h e redo x p o t e n t i a l o f X^/X" c o u p l e i n a l k a l i n e pH. In o t h e r words, no d i x a n t h o g e n f o r m a t i o n i s p o s s i b l e on t h e s u r f a c e o f g a l e n a i n a l k a l i n e pH; t h i s c o n f i r m e d t h e c o n c l u s i o n r e a c h e d by A l l i s o n e t a l ( 4 0 ) . 3.2.2 R e s t P o t e n t i a l o f O t h e r S u l p h i d e M i n e r a l s The r e s t p o t e n t i a l s o f s p h a l e r i t e ( Z n S ) , p y r i t e ( F e S 2 ) , p y r r h o t i t e ( F e ^ _ x S ) , c h a l c o p y r i t e (CuFeSg) and g a l e n a (PbS) i n KEtX s o l u t i o n s a r e i l l u s t r a t e d i n F i g u r e 3.7 f o r d e o x y g e n a t e d s y s t e m and i n F i g u r e 3.8 f o r a i r - s a t u r a t e d system. Due t o t h e i n c r e a s e d e f f e c t o f oxygen r e d u c t i o n on t h e f o r m a t i o n o f mixed p o t e n t i a l s , t h e r e s t p o t e n t i a l s o f v a r i o u s s u l p h i d e m i n e r a l s s h i f t e d i n t h e n o b l e d i r e c t i o n by 20 t o 50 mV d e p e n d i n g on t h e m i n e r a l s i n t h e a i r - s a t u r a t e d s o l u t i o n s . The r e s t p o t e n t i a l o f g a l e n a and c h a l c o p y r i t e i n deoxygen-a t e d KEtX s o l u t i o n a t n a t u r a l pH a p p e a r e d t o be more n e g a t i v e t h a n t h e + 4 U U +200 L S p h a l e r i t e +600 +400 LU to to > -200 -400 -600 -800 P y r i t e P y r r h o t i te +200 P y r i t e & I r o n « - . A « £ v r r h o t i t e ^ _ • "6h-alf <opyri t e — —-3; £*.Ljf on — o - f i a J g n a ^ & I r o n Ex 2/x" = - 8 ^ m V ( R e f . 2 5 ) S p h a l e r i t e & I r o n «-x " ~ — — 10 -5 3x10" ,-4 3x10 10"" 3 x l 0 " 4 1 0 " 3 M [ E t X " ] F i g u r e 3.7; R e s t P o t e n t i a l o f S u l p h i d e M i n e r a l s and Mixed P o t e n t i a l o f a S u l p h i d e M i n e r a l and I r o n vs M [ E t X " ] (400 cc/m i n Argon P u r g i n g , N a t u r a l pH, 25°C) -200 -400 UJ to to > cn +600 +200 t_> in > -200 -400 -600 — S p h a l e r i t e +400 8 7 ">V (Ref.25) P y r i t e P y r r h o t i t e C h a l c o p y r i t e G a l e n a — P y r i t e & I r o n +200 " - - A — . P y r r h o t i t e & I r o n " C h a l c o p y r i t e & I r o n °***«»Galena & I ron S p h a l e r i t e & I r o n I r o n O n l y -200 to > LU -400 -800 10 _L 3x10' 10' 10' T 3x10 3x10 " M [ E t X " ] F i g u r e 3.8 ; R e s t P o t e n t i a l o f S u l p h i d e M i n e r a l s and Mixed P o t e n t i a l o f S u l p h i d e M i n e r a l and I r o n vs M[ E t X " ] (400 cc/m i n A i r P u r g i n g , N a t u r a l pH, 25°C) cn - 6 7 -a v e r a g e redox p o t e n t i a l o f t h e X 2/X~ c o u p l e , w h i l e .that o f p y r r h o t i t e , p y r i t e and s p h a l e r i t e were r e l a t i v e l y more n o b l e . F o r a i r - s a t u r a t e d KEtX s o l -u t i o n a t n a t u r a l pH, t h e r e s t p o t e n t i a l s f o r a l l s u l p h i d e m i n e r a l s were r e l a -t i v e l y morenoble w i t h r e s p e c t t o t h e r e d o x p o t e n t i a l o f t h e X 2/X c o u p l e w i t h i n t h e r a n g e o f E t X " c o n c e n t r a t i o n s t u d i e d . The o r d e r o f n o b i l i t y f o r v a r i o u s s u l p h i d e s i s : S p h a l e r i t e , p y r i t e , p y r r h o t i t e , c h a l c o p y r i t e and g a l e n a . With t h e one e x c e p t i o n o f s p h a l e r i t e , p y r i t e was t h e most n o b l e m i n e r a l among t h e s u l p h i d e s i n v e s t -i g a t e d , c o n f i r m i n g Majima's (50) o b s e r v a t i o n . The v e r y n o b l e r e s t p o t e n t i a l o f s p h a l e r i t e (+465 mV vs SHE i n d e o x y g e n a t e d E t X " s o l u t i o n and +510 mV vs SHE i n a i r - s a t u r a t e d s o l u t i o n ) was p r o b a b l y due t o t h e v e r y low c o n d u c t -8 1 i v i t y o f t h e sample ( c a . 10" mho*cm" ) which r e s u l t e d i n a v e r y low a n o d i c r e a c t i o n r a t e . Thus t h e r e s t p o t e n t i a l o f s p h a l e r i t e m i ght be c o n t r o l l e d a l m o s t c o m p l e t e l y by t h e oxygen r e d u c t i o n c o u p l e . T h i s c o n s i d e r a t i o n i s a l s o s u b s t a n t i a t e d by t h e f a c t t h a t t h e r e s t p o t e n t i a l o f s p h a l e r i t e i s a l m o s t i n d e p e n d e n t o f t h e c o n c e n t r a t i o n o f E t X " . In most c a s e s , however, o t h e r s u l p h i d e m i n e r a l s showed r e s t p o t e n t i a l s w i t h i n 100 mV range f r o m t h e r e d o x p o t e n t i a l o f X 2/X" c o u p l e . I f i t i s i n d e e d t h e c a s e t h a t s t r o n g b u b b l e c o n t a c t i s a c h i e v e d a t t h e e q u i l i b r i u m p o t e n t i a l o f t h e m i n e r a l - d i x a n t h o g e n s y s t e m , as c l a i m e d by T o p e r i and T o l u n ( 7 0 ) , t h e v e r y n o b l e p o t e n t i a l o f s p h a l e r i t e w i t h r e s p e c t E X /X" m 9 n t e x P l a i n t h e d i f f i c u l t y i n t h e f l o t a t i o n o f s p h a l e r i t e w i t h o u t a c t i v a t i o n . The r e s t p o t e n t i a l o f e a c h s u l p h i d e m i n e r a l became l e s s n o b l e a t - 6 8 -h i g h e r c o n c e n t r a t i o n s o f E t X " both i n d e o x y g e n a t e d s o l u t i o n and i n a i r -s a t u r a t e d s o l u t i o n , as shown t n F i g u r e 3.7 and 3.8, r e s p e c t i v e l y , f o l l o w i n g a pseudo N e r n s t r e l a t i o n s h i p . T h i s dependence o f t h e ' r e s t p o t e n t i a l upon t h e l o g a r i t h m i c c o n c e n t r a t i o n o f E t X " has been o b t a i n e d f o r v a r i o u s systems by many o t h e r i n v e s t i g a t o r s , e.g., S a l amy and N i x o n (68) f o r m e r c u r y ; by T o l u n and K i t c h e n e r ( 2 5 ) , Woods (44) and T o p e r i and T o l u n (70) f o r g a l e n a , and by Majima and Takeda (26) and F u e r s t e n a u and c o - w o r k e r s (47, 72, 101) f o r P y r i t e . L e k k i and Laskowski (122) f o u n d a s i m i l a r r e l a t i o n -s h i p between z e t a p o t e n t i a l and l o g [ E t X ~ ] f o r CUgS t r e a t e d w i t h NagS. S t a t i s t i c a l l i n e a r r e g r e s s i o n a n a l y s e s have been done on a l l p o t e n t i a l s measured as a f u n c t i o n o f l o g [ E t X " ] , i . e . , Eh = a + b l o g [ E t X " ] (19) The r e s u l t s a r e summarized i n T a b l e 3.1. The c o n s t a n t " a " i s t h e p o t e n t i a l on t h e SHE s c a l e a t one m o l ar c o n c e n t r a t i o n o f E t X " and t h e c o e f f i c i e n t "b" i s t h e s l o p e i n F i g u r e s 3.7 and 3.8. I f t h e e l e c t r o c h e m i c a l h a l f - c e l l r e a c t i o n i n v o l v e s a M o f E t X " and B e l e c t r o n s , b becomes -0.059 x a/B, e.g., a i s t h e s t a n d a r d r e d o x p o t e n t i a l o f Xg/X" (-0.057 mV vs SHE) and b i s -0.059 mV p e r decade o f E t X " c o n c e n t r a t i o n f o r t h e r e d o x c o u p l e o f X 2/X" s i n c e t h e e l e c t r o c h e m i c a l r e a c t i o n i n v o l v e s one e l e c t r o n p e r mole o f E t X " , as e x p r e s s e d by Eqn. ( 3 ) . A n a l y s e s o f e x p e r i m e n t a l r e s t p o t e n t i a l o f v a r i o u s s u l p h i d e s g i v e n i n T a b l e 3.1, e x p r e s s e d as a f u n c t i o n o f l o g [ E t X ~ ] , i n d i c a t e d t h a t s p h a l e r i t e was a l m o s t i n d e p e n d e n t o f [ E t X ~ 3 w i t h s l o p e o f -9 mV and -5 mV - 69 -TABLE 3.1; R e s u l t s o f S t a t i s t i c a l L i n e a r R e g r e s s i o n A n a l y s i s on R e s t P o t e n t i a l and Mixed P o t e n t i a l o f V a r i o u s E l e c t r o d e Systems as F u n c t i o n o f M [ E t X - ] . E l e c t r o d e Deoxygenated S o l u t i o n A i r S a t u r a t e d S o l u t i o n Systems a * b * a * b * S p h a l e r i t e (ZnS) + 0.427 - 0.009 + 0.471 - 0.005 P y r i t e ( F e S 2 ) 0.054 - 0.076 - 0.141 - 0.072 ingl e P y r r h o t i t e (Fe, S) 1-x ' + 0.030 - 0.040 + 0.075 - 0.041 oo 4 - ts) O E CU C h a l c o p y r i t e ( C u F e S 2 ) -0.089 - 0.066 + 0.100 - 0.032 Rest Potential Electrode S.ysl G a l e n a (PbS) I r o n ( F e ) -0.053 0.504 - 0.051 - 0.005 -0.052 0.792 - 0.059 - 0.145 S p h a l e r i t e & I r o n - 0.637 - 0.050 - 0.817 - 0.155 Mixed Potential of Galvanic Couples P y r i t e & I r o n - 0.926 - 0.208 - 0.204 - 0.071 Mixed Potential of Galvanic Couples P y r r h o t i t e & I r o n C h a l c o p y r i t e & I r o n G a l e n a & I r o n -0.656 0.854 0.675 - 0.138 - 0.163 - 0.101 -0.423 0.471 0.692 - 0.108 - 0.112 - 0.148 Av o f Coi s r a g e Redox Pot. ( E t X ) 2 / E t X " j p l e - 0.057 - 0.059 - 0.057 - 0.059 " a " i s a c o n s t a n t and "b" i s a c o e f f i c i e n t f o r Eqn. ( 1 9 ) , Eh = a + b l o g [ E t X ~ ] ( V o l t ) - 70 -p e r decade o f [ E t X ~ ] i n d e o x y g e n a t e d and a i r - s a t u r a t e d s o l u t i o n s r e s p e c t -i v e l y . The s l o p e s f o r o t h e r s u l p h i d e s v a r i e d f r o m -30 mV t o -75 mV p e r decade o f [ E t X - ] . The s l o p e s f o r p y r i t e , p y r r h o t i t e , c h a l c o p y r i t e and g a l e n a were -76 mV,-50 mV, -66 mV and -51 mV p e r decade o f [ E t X - ] i n deoxygen-a t e d s o l u t i o n , and -72 mV, -41 mV, -32 mV and -59 mV p e r decade o f [ E t X " J i n a i r - s a t u r a t e d s o l u t i o n r e s p e c t i v e l y . The s l o p e s were n o t i n f l u e n c e d s i g n i f i c a n t l y by oxygen c o n c e n t r a t i o n i n t h e s o l u t i o n e x c e p t f o r c h a l c o -p y r i t e which showed -32 mV i n a i r - s a t u r a t e d s o l u t i o n vs -66 mV i n de-o x y g e n a t e d s o l u t i o n . 3.2.3 R e s t P o t e n t i a l o f M e t a l l i c I r o n The r e s t p o t e n t i a l o f t h e m e t a l l i c i r o n e l e c t r o d e was measured i n t h e same manner as t h e r e s t p o t e n t i a l o f a s u l p h i d e m i n e r a l . The r e s u l t s a r e summarized i n T a b l e 3.1 and p l o t t e d i n F i g u r e s 3.7 and 3.8 t o g e t h e r w i t h t h e r e s t p o t e n t i a l o f s u l p h i d e m i n e r a l s . In d e o x y g e n a t e d s o l u t i o n , t h e r e s t p o t e n t i a l o f t h e i r o n e l e c t r o d e was -470 mV vs SHE i n d o u b l e - d i s t i l l e d w a t e r and, s u r p r i s i n g l y , x a n t h a t e a d d i t i o n t o t h e s o l u t i o n d i d n o t make any a p p r e c i a b l e change i n t h e r e s t p o t e n t i a l i n d i c a t i n g t h a t no e l e c t r o c h e m i c a l r e a c t i o n i n v o l v i n g E t X " was t a k i n g p l a c e on t h e s u r f a c e o f t h e i r o n e l e c t r o d e . T h i s i s p r o b a b l y b e c a u s e t h e r e s t p o t e n t i a l o f t h e i r o n e l e c t r o d e i s r e l a t i v e l y f a r l e s s n o b l e , by 600-1000 mV w i t h r e s p e c t t o t h e r e d o x p o t e n t i a l o f X 2/X" s y s t e m w i t h i n t h e e x p e r i m e n t a l r a n g e o f E t X " c o n c e n t r a t i o n s . S i n c e t h e f o r m a t i o n o f d i -x anthogen i s n o t l i k e l y t o o c c u r i n d e o x y g e n a t e d x a n t h a t e s o l u t i o n , t h e r e d u c t i o n r e a c t i o n i n v o l v i n g x a n t h a t e s p e c i e s m i g h t n o t be p o s s i b l e on t h e - 71 -i r o n e l e c t r o d e t h u s r e s u l t i n g i n an a l m o s t c o n s t a n t r e s t p o t e n t i a l o f -500 mV vs SHE a t d i f f e r e n t c o n c e n t r a t i o n s o f E t X " . The r e s t p o t e n t i a l o f i r o n s h i f t e d t o t h e n o b l e d i r e c t i o n t hus r e s u l t i n g i n -5 mV and -225 mV vs SHE i n de o x y g e n a t e d d o u b l e d i s t i l l e d w a t e r and 10~ 4M [ E t X ~ ] s o l u t i o n , r e s p e c t i v e l y , due t o t h e f o r m a t i o n o f a mixed p o t e n t i a l w i t h t h e oxygen c o u p l e . The s l o p e f o r i r o n i n a i r - s a t u r a t e d s o l u t i o n was -145 mV p e r decade o f [ E t X " ] . 3.3 Mi x e d P o t e n t i a l o f S u l p h i d e M i n e r a l - M e t a l l i c I r o n G a l v a n i c C o u p l e s The mixed p o t e n t i a l s o f g a l v a n i c c o u p l e s f o r m e t a l l i c i r o n and v a r i o u s s u l p h i d e m i n e r a l s have been measured i n t h e same way as f o r t h e r e s t p o t e n t i a l measurements. The g a l v a n i c c o n t a c t between s u l p h i d e s and i r o n was e s t a b l i s h e d t h r o u g h e x t e r n a l e l e c t r o n i c c o n t a c t , and t h e d i s t a n c e between t h e two e l e c t r o d e s i n s o l u t i o n was k e p t t o a minimum t o e l i m i n a t e as much iR dr o p as p o s s i b l e . The mixed p o t e n t i a l o f t h e c h a l c o p y r i t e - i r o n c o u p l e measured d i r e c t l y was c o n s i s t e n t w i t h t h e mixed p o t e n t i a l o b t a i n e d g r a p h i c a l l y f r o m i n d e p e n d e n t p o l a r i z a t i o n s t u d i e s o f t h e two e l e c t r o d e s . ( F i g u r e s 3.9 and 3.10). T h i s i n d i c a t e d t h a t t h e p r e d o m i n a n t p o t e n t i a l d e t e r m i n i n g e l e c t r o c h e m i c a l r e a c t i o n s m i ght be t h e c a t h o d i c r e d u c t i o n on t h e s u l p h i d e m i n e r a l s and a n o d i c o x i d a t i o n on t h e m e t a l l i c i r o n . The d i r e c t l y measured mixed p o t e n t i a l s o f s u l p h i d e - i r o n c o u p l e s were p l o t t e d as a l o g a r i t h m i c f u n c t i o n o f E t X " c o n c e n t r a t i o n i n F i g u r e 3.7 f o r d e o x y g e n a t e d systems and i n F i g u r e 3.8 f o r a i r - s a t u r a t e d s y s t e m s . +600 +400 L, +200 h -200 -400 h -600 F i g u r e 3.9 C u r r e n t i (A) P o t e n t i o d y n a m i c P o l a r i z a t i o n o f C h a l c o p y r i t e , I r o n and Mixed System a t 0 M [ E t X " ] +800 H +600 +400 oo to > - J +200 E -A o J -200 r o +800 -4+200 +600 +400 H-200 ; P o t e n t i o d y n a m i c P o l a r i z a t i o n o f C h a l c o p y r i t e , I r o n and Mixed System a t 10~ M [ £ t X ~ 3 - 74 -S t a t i s t i c a l l i n e a r r e g r e s s i o n a n a l y s e s on t h e r e s u l t s were a l s o c a r r i e d o u t ; t h e r e s u l t s a r e summarized i n T a b l e 3 . 1 t o g e t h e r w i t h t h e r e s u l t s f o r t h e r e s t p o t e n t i a l o f each m i n e r a l . Because a l l t h e s u l p h i d e m i n e r a l s were 600 - 1000 mV more n o b l e than t h e i r o n e l e c t r o d e i n d e o x y g e n a t e d x a n t h a t e s o l u t i o n s and 300 - 800 mV more n o b l e i n a i r - s a t u r a t e d x a n t h a t e s o l u t i o n s ( F i g u r e s 3.7 and 3 . 8 ) , a s u l p h i d e e l e c t r o d e behaved as a c a t h o d e and an i r o n e l e c t r o d e d i s p l a y e d a n o d i c p o l a r i t y t o f o r m a mixed p o t e n t i a l when t h e y were i n g a l v a n i c c o n t a c t . The measured mixed p o t e n t i a l o f a metal s u l p h i d e ( M S ) - i r o n (Fe) g a l v a n i c c o u p l e was f o u n d t o f a l l i n between t h e r e s t p o t e n t i a l s o f MS and Fe. The e x a c t n u m e r i c a l v a l u e o f t h i s mixed p o t e n t i a l , however, w i l l v a r y d e p e n d i n g upon t h e r e l a t i v e t r u e s u r f a c e a r e a s o f t h e two e l e c t -r o d e s , T a f e l s l o p e s and t h e exchange c u r r e n t d e n s i t i e s as w e l l as t h e thermodynamic e q u i l i b r i u m p o t e n t i a l s d e t e r m i n e d by t h e e l e c t r o c h e m i c a l r e a c t i o n s ( d i s c u s s e d i n S e c t i o n 1.2.3.3). T h u s , t h e mixed p o t e n t i a l o f a g a l v a n i c c o u p l e w i l l be c l o s e r t o t h e r e s t p o t e n t i a l o f an e l e c t r o d e w hich has t h e l a r g e r t r u e s u r f a c e a r e a . The i m p l i c a t i o n o f t h i s r e l a t i v e s u r -f a c e a r e a i n t h e f o r m a t i o n o f a mixed p o t e n t i a l d u r i n g f l o t a t i o n w i l l be d i s c u s s e d i n S e c t i o n 3.5. D u r i n g t h e p r e s e n t i n v e s t i g a t i o n , t h e same i r o n e l e c t r o d e was used and t h e a p p a r e n t s u r f a c e a r e a s f o r a l l w o r k i n g e l e c t r o d e s were a b o u t t h e same. T h e r e f o r e , t h e c h a r a c t e r i s t i c s o f t h e mixed p o t e n t i a l formed by v a r i o u s m e t a l s u l p h i d e s and i r o n c o u l d be a n a l y z e d i n r e l a t i v e t e r m s . - 75 -E x c e p t f o r s p h a l e r i t e , a l l o t h e r s u l p h i d e s showed t h e same o r d e r o f n o b i l i t y i n mixed p o t e n t i a l w i t h i r o n as i n t h e r e s t p o t e n t i a l s o f t h e s u l p h i d e s , i . e . , p y r i t e - i r o n , p y r r h o t i t e - i r o n , c h a l c o p y r i t e - i r o n , g a l e n a - i r o n and s p h a l e r i t e - i r o n . A s p h a l e r i t e - i r o n g a l v a n i c c o u p l e formed t h e mixed p o t e n t i a l w h ich was v e r y c l o s e t o t h e r e s t p o t e n t i a l o f i r o n m a i n l y because o f t h e v e r y low c a t h o d i c r e a c t i o n r a t e ( i . e . , a v e r y s t e e p T a f e l s l o p e ) o f s p h a l e r i t e as has been c h e c k e d by an i n d e p e n d e n t p o l a r i z a t i o n s t u d y . F i g u r e s 3.7 and 3.8 i n d i c a t e d t h a t t h e mixed p o t e n t i a l s o f v a r i o u s MS-Fe g a l v a n i c c o u p l e were a l s o l o g a r i t h m i c f u n c t i o n s o f t h e Et X " c o n c e n t r a t i o n , and f o l l o w a pseudo N e r n s t r e l a t i o n s h i p as d i d t h e r e s t p o t e n t i a l s o f v a r i o u s s u l p h i d e m i n e r a l s . The s l o p e s o f t h e mixed p o t e n t i a l s (-100 t o -200 mV p e r decade o f t h e E t x " c o n c e n t r a t i o n ) , how-e v e r , which i n c r e a s e d 2 - 3 t i m e s i n c o m p a r i s o n w i t h t h e r e s t p o t e n t i a l s l o p e , i n d i c a t e d t h a t t h e r e l a t i v e r a t i o o f x a n t h a t e i o n s t o e l e c t r o n s i n v o l v e d i n t h e o v e r a l l e l e c t r o c h e m i c a l r e a c t i o n s f o r t h e MS-Fe g a l v a n i c c o u p l e was, due t o a d d i t i o n a l r e a c t i o n s , i n c r e a s e d t o t h e same e x t e n t . 3.4 Meaning o f t h e C o n c e n t r a t i o n c o r r e s p o n d i n g t o t h e Same R e s t P o t e n t i a l o f two d i f f e r e n t e l e c t r o d e systems F i g u r e 3.11 i l l u s t r a t e s s c h e m a t i c a l l y a c a s e i n which c u r v e s o f r e s t p o t e n t i a l (E) vs l o g [ E t X " ] f o r two d i f f e r e n t m i n e r a l systems c r o s s o v e r due t o t h e i r d i f f e r e n t s l o p e s . Around t h e c o n c e n t r a t i o n c o r r e s p o n d i n g t o the c r o s s o v e r p o i n t , t h e two e l e c t r o d e s (A and B e l e c t r o d e i n F i g u r e 3.11) behave d i f f e r e n t l y when t h e y a r e i n g a l v a n i c c o n t a c t . - 76 -F i g u r e 3.11 ; Meaning o f t h e c o n c e n t r a t i o n c o r r e s p o n d i n g t o t h e same r e s t p o t e n t i a l s o f two d i f f e r e n t e l e c t r o d e s y s t e m s . - 77 -F o r c o n c e n t r a t i o n s o f E t X " l o w e r t h a n the c o n c e n t r a t i o n c o r r e s p o n d i n g t o t h e c r o s s o y e r p o i n t , e l e c t r o d e "A" behaves as a c a t h o d e (where r e d u c t i o n i s o c c u r r i n g ) and "B" e l e c t r o d e as an anode (where o x i d a t i o n r e a c t i o n i s o c c u r r i n g ) s i n c e t h e r e s t p o t e n t i a l o f "A" i s always more n o b l e t h a n "B". On t h e o t h e r hand, t h e p o l a r i t y o f t h e two e l e c t r o d e s i n g a l v a n i c c o n t a c t i s r e v e r s e d i n t h e r e g i o n where c o n c e n t r a t i o n o f E t X " i s g r e a t e r t h a n t h e c o n c e n t r a t i o n c o r r e s -p o n d i n g t o t h e c r o s s o v e r p o i n t . In t h i s c a s e , most o f a n o d i c o x i d a t i o n i s t a k i n g p l a c e on e l e c t r o d e "A" w h i l e c a t h o d i c r e d u c t i o n i s o c c u r r i n g on e l e c t r o d e "B". As a c o n s e q u e n c e , the above phenomenon can have an i m p o r t a n t e f f e c t i n f l o t a t i o n as w e l l as i n c o r r o s i o n e n g i n e e r i n g . S i n c e t h e r e s t p o t e n t i a l can a f f e c t x a n t h a t e f l o t a t i o n o f s u l p h i d e m i n e r a l s by r e d i s t r i b u t i o n o f a d s o r b e d x a n t h a t e f r o m a r e l a t i v e l y n o b l e m i n e r a l t o a l e s s n o b l e m i n e r a l ( G l e m b o t s k i i and K l i m e n k o (7)), t h e r e v e r s e d p o l a r i t y o f t h e two m i n e r a l s a r o u n d t h e c o n c e n t r a t i o n c o r r e s p o n d i n g t o t h e c r o s s o v e r p o i n t can i n f l u e n c e t h e f l o t a t i o n o f t h e two m i n e r a l s d i f f e r e n t l y . A l l e l e c t r o c h e m i c a l s t u d i e s by v a r i o u s i n v e s t i -g a t o r s i n d i c a t e t h a t a d s o r p t i o n o f x a n t h a t e on t h e s u r f a c e s o f m i n e r a l s i s some k i n d o f a n o d i c o x i d a t i o n r e a c t i o n ( s e e S e c t i o n 1.2.1.5). I t i s e a s i l y f o r e s e e n t h a t t h e a n o d i c o x i d a t i o n on a l e s s n o b l e m i n e r a l (anode) can be enhanced by a mechanism s i m i l a r t o a g a l v a n i c c o r r o s i o n when i t i s i n g a l v a n i c c o n t a c t w i t h a r e l a t i v e l y n o b l e m i n e r a l . C o n v e r s e l y , a d e c r e a s e i n x a n t h a t e a d s o r p t i o n on t h e r e l a t i v e l y n o b l e m i n e r a l w i l l be a r e s u l t o f c a t h o d i c p r o t e c t i o n due t o a g a l v a n i c c o n t a c t w i t h a r e l a t i v e l y l e s s n o b l e m i n e r a l . - 78 -The above g a l v a n i c e f f e c t w i l l be s t r o n g e r when t h e d i f f e r e n c e between t h e c o n c e n t r a t i o n o f E t X " and t h e c o n c e n t r a t i o n c o r r e s p o n d i n g t o t h e c r o s s o v e r p o i n t i s b i g g e r i n e i t h e r d i r e c t i o n . T h i s g a l v a n i c e f f e c t m i ght be an e x p l a n a t i o n f o r t h e b e t t e r s e l e c t i v i t y between s u l p h i d e m i n e r a l s w hich i s u s u a l l y o b s e r v e d a t a s t a r v a t i o n c o n c e n t r a t i o n o f x a n t h a t e . T h i s c o n c e p t o f t h e e f f e c t o f g a l v a n i c c o n t a c t a r o u n d t h e c o n -c e n t r a t i o n c o r r e s p o n d i n g t o t h e c r o s s o v e r p o i n t m i ght a l s o be u s e f u l i n d e v e l o p i n g new c o l l e c t o r s and r e g u l a t i n g r e a g e n t s f o r a p a r t i c u l a r c o m p l e x - m i n e r a l s y s t e m . A n o t h e r i n t e r e s t i n g a p p l i c a t i o n o f t h e above phenomenon may be f o u n d i n c o r r o s i o n e n g i n e e r i n g . I f one can f i n d o r d e v e l o p a c h e m i c a l c o r r o s i o n i n h i b i t o r which i n d i c a t e s t h e c h a r a c t e r i s t i c c o n c e n t r a t i o n f o r a p a r t i c u l a r g a l v a n i c s y s t e m o f two m e t a l s , i t i s most p r o b a b l y t h a t t h e l e s s n o b l e metal can be c a t h o d i c a l l y p r o t e c t e d by making t h e c o n c e n t r a t i o n o f t h e c o r r o s i o n i n h i b i t o r h i g h e r t h a n t h e c h a r a c t e r i s t i c c o n c e n t r a t i o n . 3.5 I m p l i c a t i o n o f R e l a t i v e S u r f a c e A r e a s o f E l e c t r o d e s f o r G a l v a n i c C o u p l e i n F l o t a t i o n A c c o r d i n g t o West ( 8 8 ) , t h e e l e c t r o c h e m i c a l r e a c t i o n f o r a g a l -v a n i c c o u p l e i s g r e a t l y i n f l u e n c e d by t h e r e l a t i v e s u r f a c e a r e a o f t h e two e l e c t r o d e s . Thermodynamic e q u i l i b r i u m p o t e n t i a l i s n o t g o v e r n e d by t h e s u r f a c e a r e a o f an e l e c t r o d e ; however, t h e mixed p o t e n t i a l o f a - 79 -g a l v a n i c c o u p l e depends upon t h e r e l a t i v e s u r f a c e a r e a o f t h e two e l e c t -r o d e s , ( s e e S e c t i o n 1.2.3.3). T h e r e f o r e , t h e r e s u l t s p r o d u c e d by g a l v a n i c c o n t a c t between s u l -p h i d e m i n e r a l s (MS) and m e t a l l i c i r o n ( f e ) i n f l o t a t i o n can d i f f e r d e p e n d i n g on t h e a r e a s o f MS and Fe i n c o n t a c t . I f a c a t h o d i c s u l p h i d e m i n e r a l g r e a t l y exceeds a n o d i c i r o n i n s u r f a c e a r e a , as when a v e r y s m a l l p a r t i c l e o f i r o n i s d e p o s i t e d on t h e s u r f a c e o f a r e l a t i v e l y l a r g e s u l p h i d e m i n e r a l d u r i n g g r i n d i n g w i t h s t e e l media, a n o d i c r e a c t i o n s , such as d i s s o l u t i o n o f i r o n a n d / o r o x i d a t i o n o f x a n t h a t e t o d i x a n t h o g e n , w i l l be enhanced g r e a t l y on t h e s m a l l i r o n s u r f a c e b e c a u s e o f t h e g a l v a n i c e f f e c t . On t h e o t h e r hand, most o f t h e e l e c t r o n s g e n e r a t e d by a n o d i c o x i d a t i o n o f i r o n a r e f e d i n t o t h e n o b l e s u l p h i d e m i n e r a l p a r t i c l e s where c a t h o d i c r e d u c t i o n ( e . g . , oxygen r e d u c t i o n ) o c c u r s a l m o s t e x c l u s i v e l y w i t h a n e g l i g i b l e amount o f o x i d a t i o n t a k i n g p l a c e . As a c o n s e q u e n c e , t h e o x i d a t i o n r e a c t i o n i n v o l v i n g x a n t h a t e s p e c i e s w i l l be c o n f i n e d more o r l e s s t o t h e s m a l l i r o n s u r f a c e and make t h e p a r t i c l e l e s s f l o t a b l e . However, i f t h e o x i d i z e d p r o d u c t o f x a n t h a t e , such as d i -x a nthogen, can m i g r a t e f r o m i r o n t o s u l p h i d e s i n c o n t a c t , f l o t a b i l i t y o f t h e p a r t i c l e w i l l be enhanced t o t h e same e x t e n t . The mixed p o t e n t i a l o f t h i s g a l v a n i c s y s t e m w i l l be v e r y c l o s e t o t h e r e s t p o t e n t i a l o f t h e s u l -p h i d e m i n e r a l . The o t h e r extreme c a s e o f g a l v a n i c c o n t a c t o c c u r s when a s m a l l s u l p h i d e m i n e r a l s u r f a c e and a l a r g e s u r f a c e o f l e s s n o b l e i r o n a r e i n c o n t a c t even t e m p o r a r i l y but r e p e a t e d l y as t h e s m a l l s u l p h i d e m i n e r a l - 80 -p a r t i c l e s c o l l i d e w i t h the l a r g e s u r f a c e o f s t e e l g r i n d i n g media ( e . g . , s t e e l b a l l s , r o d s and l i n e r s ) o r p r o c e s s i n g machines ( e . g . , c o n d i t i o n e r , s t e e l p i p e and f l o t a t i o n m a c h i n e ) . Under t h e s e c i r c u m s t a n c e s , t h e mixed p o t e n t i a l o f t h e . g a l v a n i c c o u p l e w i l l become v e r y c l o s e t o t h e r e s t p o t e n t i a l o f i r o n . The p r e s e n t i n v e s t i g a t i o n was done w i t h s u l p h i d e s and i r o n e l e c t r o d e s h a v i n g a p p r o x i m a t e l y t h e same a r e a s . The mixed p o t e n t i a l s ob-t a i n e d d u r i n g t h e p r e s e n t i n v e s t i g a t i o n must be u n d e r s t o o d i n c o n n e c t i o n w i t h t h e r e l a t i v e s u r f a c e a r e a s o f t h e two e l e c t r o d e s . A n o t h e r i n t e r e s t i n g a s p e c t r e l a t e d t o t h i s s u b j e c t c o n c e r n s t h e d i f f i c u l t i e s e x p e r i e n c e d i n s l i m e f l o t a t i o n . As p r e v i o u s l y d i s c u s s e d , s m a l l p a r t i c l e s o f s u l p h i d e m i n e r a l s a c q u i r e a mixed p o t e n t i a l which i s c l o s e r t o t h e r e s t p o t e n t i a l o f i r o n and f u r t h e r removed f r o m t h e X^W r e d o x p o t e n t i a l t h a n t h a t o f l a r g e s u l p h i d e p a r t i c l e s . T o p e r i and T o l u n (70) c l a i m e d t h a t s u l p h i d e m i n e r a l s w i l l be d e p r e s s e d under t h i s c o n d i t i o n . 3.6 P o t e n t i a l s as a F u n c t i o n o f S o l u t i o n pH The r e s t p o t e n t i a l s o f v a r i o u s s u l p h i d e m i n e r a l s and m e t a l l i c i r o n , and mixed p o t e n t i a l s o f t h e c h a l c o p y r i t e - i r o n g a l v a n i c c o u p l e were measured as a f u n c t i o n o f s o l u t i o n pH. V a r i o u s p o t e n t i a l s o f each e l e c t r o d e s y s t e m were d e t e r m i n e d f i r s t a t n a t u r a l pH. The pH o f t h e s o l u t i o n was i n c r e a s e d u s i n g NaOH, f o l l o w e d by back t i t r a t i o n u s i n g H 2 S 0 4 , c o v e r i n g t h e pH r a n g e o f 1 - 12. S u l p h u r i c - 81 -a c i d was chosen d u r i n g t h e s e i n v e s t i g a t i o n s because o f t h e wide a p p l i c a i t o n o f t h e a c i d i n t h e f l o t a t i o n i n d u s t r y . F i f t e e n m i n u t e s were a l l o w e d t o pass a f t e r each a d j u s t m e n t o f pH b e f o r e t h e measurement o f p o t e n t i a l was made. The c o n c e n t r a t i o n o f -5 EtX i n v e s t i g a t e d was 0 M and 10 M. S o l u t i o n s were purged w i t h a i r a t th e r a t e o f 400 c c / m i n . t h r o u g h o u t t h e i n v e s t i g a t i o n s . The r e s u l t s a r e i l l u s t r a t e d i n A p p e n d i c e s 4.1, 4.5 f o r t h e v a r i o u s s u l p h i d e m i n e r a l s , i n Appendix 4.6 f o r t h e m e t a l l i c i r o n and i n Ap p e n d i x 4.7 f o r t h e c h a l c o p y r i t e - i r o n g a l v a n i c c o u p l e . F o r t h e purpose o f c o m p a r i s o n , some o f t h e v a l u e s o b t a i n e d by o t h e r i n v e s t i g a t o r s a r e a l s o i n c l u d e d i n the F i g u r e s . Some d i s c r e p a n c i e s o b s e r v e d i n t h e s e r e s u l t s c o u l d be a t t r i b u t e d t o t h e d i f f e r e n t h i s t o r i e s o f t h e s u l p h i d e e l e c t r o d e s as w e l l as t o t h e d i f f e r e n t e x p e r i m e n t a l c o n d i t i o n s . T h ese e x p e r i m e n t a l p o t e n t i a l - p H diagrams c o u l d n o t be compared d i r e c t l y w i t h t h e t h e o r e t i c a l p o t e n t i a l - p H diagrams because t h e e x p e r i m e n t a l p o t e n t i a l i s a mixed p o t e n t i a l formed by v a r i o u s e l e c t r o c h e m i c a l r e a c t i o n s o c c u r r i n g on t h e e l e c t r o d e s u r f a c e ( d i s c u s s e d i n S e c t i o n 1.2.3.4). In a l l c a s e s i n v e s t i g a t e d , NaOH t i t r a t i o n i n t h e a l k a l i n e d i r e c t i o n and H^SO^ back t i t r a t i o n i n the a c i d i c d i r e c t i o n r e s u l t e d i n h y s t e r e s i s on t h e p o t e n t i a l - p H d i a g r a m s , i . e . , p o t e n t i a l s o b t a i n e d by a c i d back t i t r a t i o n f o l l o w e d s i g n i f i c a n t l y d i f f e r e n t c o u r s e s f r o m t h e p o t e n t i a l s o b s e r v e d d u r i n g a l k a l i n e t i t r a t i o n . A c i d back t i t r a t i o n gave l o w e r p o t e n t i a l s f o r a l l c a s e s e x c e p t f o r s p h a l e r i t e . A l t h o u g h t h e - 82 -e x a c t r e a c t i o n mechanisms were n o t d e t e r m i n e d , t h i s h y s t e r e s i s i s t h o u g h t t o be c a u s e d by SO^"" i o n s . An i n d e p e n d e n t s t u d y i n d i c a t e d no s i g n i f i c a n t changes i n pH and t h e p o t e n t i a l s a f t e r 15 m i n u t e s . T h i s h y s t e r e s i s i m p l i e s t h a t s u l p h i d e m i n e r a l s c a n n o t a c q u i r e t h e o r i g i n a l r e s t p o t e n t i a l s a f t e r a t r e a t m e n t i n an a l k a l i n e s o l u t i o n f o l l o w e d by a c i d n e u t r a l i z a t i o n t o t h e o r i g i n a l pH. C o n d i t i o n i n g i n a l k a l i n e and a c i d s o l u t i o n i s a common p r a c t i c e i n s e l e c t i v e f l o t a t i o n o f v a r i o u s s u l p h i d e m i n e r a l s . -5 In g e n e r a l , t h e p r e s e n c e o f 10 M EtX i n t h e s o l u t i o n c a u s e d a s h i f t o f t h e whole p o t e n t i a l - p H d i a g r a m t o a l e s s n o b l e d i r e c t i o n . S p h a l e r i t e was a g a i n e x c e p t i o n a l i n t h a t i t s r e s t p o t e n t i a l s h i f t e d t o t h e n o b l e d i r e c t i o n by about 10 mV d u r i n g an a c i d back t i t r a t i o n . S p h a l e r i t e ( Appendix 4.1) i n d i c a t d an a l m o s t c o n s t a n t r e s t p o t e n t i a l t h r o u g h o u t t h e r a n g e o f pH i n v e s t i g a t e d w h i l e o t h e r s u l p h i d e s showed complex h o r i z o n t a l and s l o p i n g l i n e s ( A p p e n d i c e s 4.2,..4.5). I t i s assumed, c o n s i d e r i n g t h e a n a l o g y o f t h e e x p e r i m e n t a l p o t e n t i a l - p H diagrams f o r v a r i o u s e l e c t r o d e systems w i t h t h e r e s p e c t i v e Eh-pH diagrams o f s u l p h i d e s / w a t e r / o x y g e n s y s t e m , t h a t t h e h o r i z o n t a l p o r t i o n s i n t h e e x p e r i m e n t a l diagrams c o r r e s p o n d t o t h e c o r r o s i o n r e g i o n where t h e e l e c t r o d e systems r e s p o n d t o t h e r e s p e c t i v e metal i o n c o n c e n -t r a t i o n i n t h e s o l u t i o n . By t h e same r e a s o n i n g , t h e s l o p e d p o r t i o n s o f t h e diagrams i n t h e a l k a l i n e r e g i o n ( w i t h s l o p e s o f a b o u t -60 mV p e r pH u n i t ) i s c o n s i d e r e d t o r e p r e s e n t t h e e l e c t r o d e s u r f a c e s p a s s i v a t e d due - -t o t h e f o r m a t i o n o f o x i d e s o r h y d r o x i d e s . A l l p a s s i v a t e d e l e c t r o d e s r e s p o n d t o pH change i n t h e manner o f t h e pH e l e c t r o d e , as d i s c u s s e d by N a t a r a j a n and Iwasaki ( 7 1 ) . S a t o (69) a l s o o b s e r v e d i n h i s p o t e n t i a l - p H s t u d y o f v a r i o u s s u l p h i d e s t h a t t h e p o t e n t i a l s o f s u l p h i d e m i n e r a l s were dependent o n l y upon t h e a c t i v i t i e s o f t h e r e s p e c t i v e metal i o n s i n a c i d s o l u t i o n and o n l y upon pH i n a l k a l i n e s o l u t i o n . S a t o (123) f o u n d i n a n o t h e r i n v e s t i g a t i o n t h a t a change f r o m a h o r i z o n t a l p o r t i o n t o a s l o p e d p o r t i o n o c c u r r e d a t t h e p o t e n t i a l o f t h e Og/HgOg r e d o x c o u p l e r e g a r d l e s s o f t h e form i n which i r o n was p r e s e n t i n the s o l u t i o n o r whether t h e s y s t e m was homogeneous o r h e t e r o g e n e o u s . Some unusual b e h a v i o u r o f t h e i r o n e l e c t r o d e was o b s e r v e d i n t h e pH rang e o f 1 - 4.5 d u r i n g HgSO^ back t i t r a t i o n ( A ppendix 4 . 6 ) . The minimum r e s t p o t e n t i a l a t a pH o f ab o u t 3.5 was n o t i n a c c o r d w i t h P o u r b a i x ' s (73) Eh-pH d i a g r a m f o r i r o n . The r e a s o n f o r t h i s phenomenon rem a i n s t o be e x p l a i n e d . The e x p e r i m e n t a l mixed p o t e n t i a l - p H d i a g r a m f o r a c h a l c o -p y r i t e - i r o n g a l v a n i c c o u p l e f o l l o w e d more c l o s e l y t h e o u t l i n e o f t h e e x p e r i m e n t a l Eh-pH d i a g r a m f o r the i r o n e l e c t r o d e t h a n t h a t f o r c h a l c o -p y r i t e . T h i s i n d i c a t e d t h a t t h e r e s t p o t e n t i a l o f t h e i r o n e l e c t r o d e m i g h t be t h e d o m i n a t i n g f a c t o r i n d e t e r m i n i n g t h e mixed p o t e n t i a l o f t h e g a l -v a n i c c o u p l e when t h e y had a p p r o x i m a t e l y t h e same s u r f a c e a r e a . -84 -SUMMARY AND CONCLUSIONS The e l e c t r o c h e m i c a l c h a r a c t e r i s t i c s o f f i v e d i f f e r e n t s u l -p h i d e m i n e r a l s ( i . e . , s p h a l e r i t e , p y r i t e , p y r r h o t i t e , c h a l c o p y r i t e , and g a l e n a ) were s t u d i e d i n d e o x y g e n a t e d and i n a i r - s a t u r a t e d p o t a s s i u m e t h y l x a n t h a t e s o l u t i o n s a t v a r i o u s c o n c e n t r a t i o n s and pH. The e f f e c t o f a g a l v a n i c c o n t a c t between t h e s e s u l p h i d e m i n e r a l s and m e t a l l i c i r o n on t h e e l e c t r o c h e m i c a l c h a r a c t e r i s t i c s o f t h e g a l v a n i c c o u p l e was a l s o i n v e s t i g a t e d . The e x p e r i m e n t a l r e s u l t s i n d i c a t e t h a t : (1) P o t a s s i u m e t h y l x a n t h a t e can a c t on t h e e l e c t r o c h e m i c a l c h a r a c t e r i s t i c s o f v a r i o u s s u l p h i d e m i n e r a l s i n t h r e e d i f f e r e n t ways: 1, by a d i r e c t r e a c t i o n w i t h t h e s u l p h i d e m i n e r a l s ; 2, by c h a n g i n g t h e c o n d u c t -i v i t y o f t h e e l e c t r o l y t e s o l u t i o n s ; and 3, by c h a n g i n g t h e pH o f t h e e l e c t r o -l y t e s o l u t i o n s . (2) The s t i r r i n g o f t h e s o l u t i o n r e s u l t e d i n s i g n i f i c a n t changes o f t h e e l e c t r o c h e m i c a l c h a r a c t e r i s t i c s o f t h e v a r i o u s e l e c t r o d e s y s t e m s , t h u s i n d i c a t i n g t h a t t h e p o t e n t i a l - d e t e r m i n i n g e l e c t r o c h e m i c a l r e a c t i o n s were under d i f f u s i o n c o n t r o l . Depending on t h e e l e c t r o d e systems and t h e c o n c e n t r a t i o n o f x a n t h a t e , s o l u t i o n p u r g i n g w i t h a i r s h i f t e d t h e p o t e n t i a l s t o t h e n o b l e d i r e c t i o n by 20 - 50 mV f o r t h e r e s t p o t e n t i a l s o f s u l p h i d e rrnnera.ls; and by 200 - 500 mV f o r t h e mixed p o t e n t i a l s o f v a r i o u s -meta.l s u l p h i d e - i r o n g a l y a n i c c o u p l e s , as o b s e r v e d by o t h e r i n v e s t i g a t o r s . - 85 -(3) The order, o f t h e e l e c t r o c h e m i c a l s e r i e s f o r t h e v a r i o u s s u l p h i d e m i n e r a l s i n p o t a s s i u m e t h y l x a n t h a t e s o l u t i o n s was s p h a l e r i t e , p y r i t e , p y r r h o t i t e , c h a l c o p y r i t e and g a l e n a . T h i s o r d e r r e m a i n e d t h e same when t h e y were i n g a l v a n i c c o n t a c t w i t h m e t a l l i c i r o n e x c e p t f o r t h e s p h a l e r i t e -i r o n s y s t e m which showed t h e most n e g a t i v e mixed p o t e n t i a l . .In p o t a s s i u m e t h y l x a n t h a t e s o l u t i o n a t n a t u r a l pH, most o f t h e r e s t p o t e n t i a l s f o r v a r i o u s s u l p h i d e m i n e r a l s were more n o b l e t h a n t h e r e d o x p o t e n t i a l o f t h e d i x a n t h o g e n / x a n t h a t e c o u p l e ; however, t h e f o r m e r became l e s s n o b l e t h a n t h e l a t t e r i n t h e a l k a l i n e r e g i o n o f pH, e s p e c i a l l y when t h e y were i n g a l v a n i c c o n t a c t w i t h m e t a l l i c i r o n . T h i s i m p l i e d t h a t o n l y metal x a n t h a t e , n o t d i x a n t h o g e n , c o u l d be formed on t h e s u r f a c e o f t h o s e s u l p h i d e m i n e r a l s i n i n d u s t r i a l f l o t a t i o n u t i l i z i n g s t e e l g r i n d i n g media and a l k a l i n e pH. (4) The r e s t p o t e n t i a l s o f t h e s e s u l p h i d e m i n e r a l s and t h e mixed p o t e n t i a l s o f t h e s u l p h i d e - i r o n c o u p l e f o l l o w e d a pseudo N e r n s t r e -l a t i o n s h i p w i t h r e s p e c t t o t h e c o n c e n t r a t i o n o f p o t a s s i u m e t h y l x a n t h a t e i n d e o x y g e n a t e d as w e l l as i n a i r - s a t u r a t e d s o l u t i o n s . The s l o p e s f o r p o t e n t i a l vs l o g [ E t X ~ ] , however, were d i f f e r e n t d e p e n d i n g on t h e v a r i o u s e l e c t -r o d e s y s t e m s . The changes i n t h e r e s t p o t e n t i a l s i n t h e x a n t h a t e s o l u t i o n s were e x p l a i n e d i n terms o f t h e mixed p o t e n t i a l s o f v a r i o u s 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 x a n t h a t e . (5) The e x a c t v a l u e o f t h e mixed p o t e n t i a l f o r a g i v e n m e t a l s u l p h i d e - i r o n c o u p l e was a f u n c t i o n o f t h e d i s t a n c e between t h e two - ob e l e c t r o d e s and t h e r e l a t i v e s u r f a c e a r e a . The i m p o r t a n c e o f t h i s r e l a t i v e s u r f a c e a r e a i n f l o t a t i o n was d i s c u s s e d . (6) The p r e d o m i n a n t e l e c t r o c h e m i c a l r e a c t i o n which d e t e r -mined the mixed p o t e n t i a l s o f a s u l p h i d e - i r o n g a l v a n i c c o u p l e m i g h t be a c a t h o d i c r e d u c t i o n on t h e s u l p h i d e m i n e r a l and an a n o d i c o x i d a t i o n on t h e m e t a l l i c i r o n under t h e c o n d i t i o n s i n v e s t i g a t e d . The above c o n c l u s i o n s a r e a p p l i c a b l e f o r t h e samples and t h e e x p e r i m e n t a l c o n d i t i o n s i n v e s t i g a t e d . T h e s e 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 s t i l l p r o v i d e an i n c o m p l e t e view o f t h e h i g h l y complex f l o t a t i o n mechanisms. 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O k t a v i e c , M., " P o t e n t i o m e t r i c Methode f o r D e t e r m i n a t i o n o f X a n t h a t e s " , P r a c e I n s t . H u t n i e z y c h , 6, 1954, p i 8 4 ; Chem. Abs. V o l . 49, 1955, 5212d 122. L e k k i , J . and L a s k o w s k i , J . , "On t h e Dynamic E f f e c t o f F r o t h e r -C o l l e c t o r J o i n t A c t i o n i n F l o t a t i o n " , T r a n s . IMM, V o l . 80, S e p t . 1971, Cl74-180 123. S a t o , M. " O x i d a t i o n o f S u l p h i d e Ore B o d i e s - ( I ) Geochemica! E n v i r o n m e n t i n terms o f Eh and pH", Econo. Geo!., V o l . 55, 1960, p928 -961 APPENDIX 1.1;; R e s t P o t e n t i a l o f P y r i t e ( F e S j S o l u t i o n Gas pH Temp. (°C) Rest P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks — — — — + 37 7 — IM HC10 4 N 2 -- — - 30- + 120 10 p - t y p e IM HClOy. He _ — 25 +200- + 300 q c l e a n e d H c a t h o d i c a l l y KC1, N a 2 B 4 0 7 • 10H 20 He 9.1 25 + 170 26 --10" 3M F e S 0 4 , KOH A i r 9.0 25 + 300 98 --— 9.0 — + 540 99 IM HC10 4 N 2 — - 40- + 650 10 n - t y p e IM HC10 4 A i r 25 + 620 9 o x i d i z e d IM H 2 S 0 4 -- -- 25 + 630 100 — 10" 3M F e S 0 4 , HC1 A i r 2.0 25 + 640 98 — — 4.0 — + 658 99 - -0 M [ E t X " ] , N a 2 B 4 0 7 He 9.1 25 + 170 26 --10" 3M [ E t X " ] , N a 2 B 4 0 ? He 9.1 25 + 140 26 — 5 x 10" 3M [ E t X " ] , N a 2 B 4 0 7 He 9.1 25 + 80 26 — 10' 2M [ E t X " ] , N a 2 B 4 0 ? He 9.1 25 + 65 26 — 5 x 10" 2M [ E t X " ] , N a 2 B 4 0 7 He 9.1 25 + 15 26 ( c o n t ' d ) APPENDIX 1.1 ( c o n t ' d ) S o l u t i o n Gas PH Temp. (°C) Rest P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks 2 x 10" 5M [ E t X " ] , HCl — 4.0 25 + 650 47 P t . i n P y r i t e p u l p 2 x 10" 5M [ E t X " ] , KOH -- 9.4 25 + 507 47 P t . i n P y r i t e p u l p 10" 4M [ E t X " ] , 1 0 " 4 M K 4 F e ( C N ) 6 , H C l — 3.9 25 + 319 101 P t . i n P y r i t e p u l p 10~ 4M [ E t X ' ] , 10" 4M K 4 F e ( C N ) g 1 K O H — 9.9 25 + 234 101 P t . i n P y r i t e p u l p APPENDIX 1.2 ; R e s t P o t e n t i a l o f M a r c a s i t e ( F e S 9 ) S o l u t i o n Gas pH Temp. (°C) R e s t P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks 1 0 " 3 M F e S 0 4 , HCl A i r 1.98 26 + 573 93 ---- -- 4.0 — + 634 99 — 9.0 + 646 99 APPENDIX 1.3 ; Rest P o t e n t i a l o f P y r r h o t i t e (Fe, S) * I — X S o l u t i o n Gas P H Temp. (°C) Rest P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks 1 0 " 3 M F e S 0 4 , KOH 1 0 " 3 M F e S 0 4 , HCl A i r A i r 9.0 5.6 25 25 - 410 - 330 98 98 FeS, p p t . FeS, p p t . 100 g/1 H 2 S 0 4 - - • — 20 - 283 102 FeS, c a s t 10" 3 M F e S 0 4 , KOH A i r 9.0 25 - 162 + 200 7 98 N a t u r a l P y r r h o t i t e 1 0 " 3 M F e S 0 4 , HCl A i r 4.0 25 +350 93 it APPENDIX 1.4 ; Rest P o t e n t i a l o f Galena (PbS) S o l u t i o n Gas pH Temp. (°C) Rest P o t e n t i a l (mV vs SHE ) R e f e r e n c e Remarks 100 g/1 H 2 S 0 4 -- 20 - 32 102 PbS p p t . p r e s s e d 100 g/1 H 2 S 0 4 — — 20 + 36 102 PbS, c a s t 100 g/1 H 2 S 0 4 — -- 20 + 284 102 N a t u r a l G a l e n a 1 M HC1 -- — 25 + 40 103 --IM HC1 -- -- 25 + 290 103 --IM HC10 4 He — 25 + 230 104 — IM HC10 4 He -- 25 + 240 50 — IM HC10 4 Ar -- 25 + 180 -+250 93 — IM HC10 4, IM Pb ( C 1 0 4 ) 2 A r — 25 + 350 93 ' — 10" 4M P b ( C 2 H 3 0 2 ) 2 , HN0 3 A i r 4.0 25 + 360 98 --10" 4M P b ( C 2 H 3 0 2 ) 2 , KOH A i r 9.0 25 + 230 93 — 1 0 _ 1 M N a - t e t r a b o r a t e N2 9.1 25 - 460 44 S u l p h i z e d i n H 2S Borax B u f f e r A i r 8.0 + 130 70 --Borax B u f f e r 9.2 — - 40 25 Na 2S t r e a t e d H 20 9.0 — - 120 105 — ( c o n t ' d ) APPENDIX 1.4 ( c o n t ' d ) S o l u t i o n Gas PH Temp. (°C) Rest P o t e n t i a l (mV vs SHE) R e f e r e n c e Remarks H 20 -- 9.0 - 100 106 --H 20 9.0 — - 10 107 — H 20 -- 9.0 — + 160 103 — H 20 -- 9.2 — - 260 55 — H 20 N 2 9.0 25 + 40 64 p - t y p e g a l e n a H 20 N 2 9.0 25 - 150 64 n- t y p e g a l e n a — — — " 1 5 2 7 — — 4.0 — + 395 99 --— 9.0 -- + 328 99 ---- -- — + 49 ~+ 50 95 PbS, p p t . — — + 48 -+ 52 95 g a l e n a powder, p r e s s e d — -- — — + 52" + 58 95 g a l e n a 2 x 10" 3M [ E t X " ] , Borax °2 9.3 2.2 - 65 38 Na 2S t r e a t e d 10" 3M [ E t X " ] , Borax ° 2 10.7 — + 60 70 — 10 3M [ E t X " ] , Borax A i r 10.7 — - 5 70 --10" 3M [ E t X " ] , Borax N 2 9.1 — - 65 25 NaS t r e a t e d 6.24 x 10" 4M [ E t X " ] none 7.0 — + 60 40 — ( c o n t ' d ) APPENDIX 1.4 ( c o n t ' d ) S o l u t i o n Gas PH Temp. (°C) R e s t P o t e n t i a l (mV vs SHE) R e f e r e n c e Remarks 10" 4N [ E t X " ] °2 9.0 25 f 55 64 mm mm 10" 4M [ E t X ' ] °2 10.9 25 f 15 64 10" 4M [ E t X " ] °2 12.1 25 - 72 64 10" 4M [ E t X " ] °2 9.3 22 - 35 38 Na 2S t r e a t e d 10' 4M [ E t X " ] °2 9.1 — + 205 44 — APPENDIX 1 .5; R e s t P o t e n t i a l o f C h a l c o p y r i t e ( C u F e S j S o l u t i o n Gas PH Temp. (°C) R e s t P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks -- — -- + 5 7 — 9.0 — + 434 99 --0.1 N H 2 S 0 4 None — 20-25 + 462 109 — 1 M HC10 4 He — 25 + 530 50 — — — 4.0 — + 558 99 APPENDIX 1.6; R e s t P o t e n t i a l o f B o r n i t e ( C u v F e S J S o l u t i o n Gas pH Temp. (°C) Rest P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks — — -- - 136 7 — -- — 4.0 — +416- +448 99 --— — 9.0 — + 453 99 — APPENDIX 1.7 ; R e s t P o t e n t i a l o f C h a l c o c i t e ( C u 9 S ) S o l u t i o n Gas PH Temp. (°C) R e s t P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks „ _ - - -- — - 96 7 — 10" 4M C u S 0 4 , KOH A i r 9.0 25 + 230 98 — 10" 2M C u S 0 4 , KOH A i r 9.0 25 + 230 98 — 10" 4M C u S 0 4 , HCl A i r 4.0 25 + 380 98 — 10" 2M C u S 0 4 , HCl A i r 4.0 25 + 440 98 -- 9.0 — + 416 99 --100 g/1 H 2 S 0 4 — 20 + 442 102 --APPENDIX 1 .8 ; R e s t P o t e n t i a l o f C o v e l l i t e (CuS) S o l u t i o n Gas PH Temp. (°C) R e s t P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks 10" 4M C u S 0 4 , KOH A i r 9.0 25 + 390 98 E t c h i n g i n 5% H 2 0 2 10" 2M C u S 0 4 , KOH A i r 9.0 25 + 390 98 E t c h i n g i n 5% H 2 0 2 10" 4M C u S 0 4 , HCl A i r 4.0 25 + 480 98 E t c h i n g i n 5% H 2 0 2 ( c o n t ' d ) APPENDIX 1 .8 ( c o n t ' d ) S o l u t i o n Gas PH Temp. (°C) R e s t P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks 10" 2M C u S 0 4 , HCl A i r 4.0 25 + 540 98 E t c h i n g i n 5% H 2 ° 2 1M HC10 4 He 9.0 4.0 25 + 420 + 434 + 448 50 99 99 — APPENDIX 1 .9 ; Rest P o t e n t i a l o f S p h a l e r i t e (ZnS) S o l u t i o n Gas pH Temp. (°C) Rest P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks 100 g/1 H 2 S 0 4 — -- 20 - 242 102 ---- -- -- - 70 7 — 2 x 10" 2M Z n S 0 4 , KOH N 2 9.8 28 - 38 98 ZnS, p p t . 2 x 10' 2M Z n S 0 4 , HCl N, 2.7 28 + 166 98 ZnS, p p t . — -- 9.0 — + 181 99 — — 4.0 — + 459 99 --+ 900 22 66% Zn, 0.33% Fe. + 770 22 52% Zn, 1.70% Fe. + 660 22 63% Zn, 1.80% Fe + 510 22 63% Zn, 2.00% Fe + 220 22 60% Zn, 4.20% Fe + 180 22 54% Zn, 11.9% Fe + 210 22 43 % Zn, 12.0% Fe + 150 22 48% Zn, 14.4% Fe APPENDIX 1.10 ; R e s t P o t e n t i a l o f M o l y b d e n i t e (MoS ?) S o l u t i o n Gas pH Temp. (°C) Rest P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks — -- 4.0 — + 109 99 — 10" 3M KC1, 10" 4M M o l y d a t e None 9.0 25 + 300 72 — 10" 3M KC1, 10" 4M M o l y b d a t e None 4.0 25 + 380 72 — — — 9.0 -- + 411 99 — APPENDIX 1.11 ; R e s t P o t e n t i a l o f A r g e n t i t e ( A g ? S ) S o l u t i o n Gas PH Temp. (°C) Rest P o t e n t i a l (mV v s . SHE) R e f e r e n c e - Remarks — 4.0 — + 276 99 --10' 3M AgN0 3, HN0 3 A i r 4.0 25 + 646 98 — APPENDIX 1.12 ; Rest P o t e n t i a l o f S t i b n i t e ( S b 2 S 3 ) S o l u t i o n Gas pH Temp. (°C) Rest P o t e n t i a l (mV v s . SHE) R e f e r e n c e Remarks 9.0 — + 86 99 — — -- 4.0 — + 120 99 --- 107 -Appendix 2.1; pH vs [ E t X " ] i n D e a e r a t e d S o l u t i o n I f we assume t h e h y d r o l y s i s o f p o t a s s i u m e t h y l x a n t h a t e (KEtX) to f o r m e t h y l x a n t h i c a c i d ( H E t X ) ; Kh E t X " + H 20 OH" + HEtX ( A l ) A c c o r d i n g t o G l a s s t o n e ( 1 7 0 ) , t h e c o n j u g a t e b a s e , EtX , o f a r e l a t i v e l y weak x a n t h i c a c i d (HEtX) i s f a i r l y s t r o n g and i n t e r a c t s w i t h w a t e r a c t i n g as an a c i d , r e s u l t i n g i n t h e i n c r e a s i n g o f pH o f KEtX s o l u t i o n . S i n c e t h e a c t i v i t y o f w a t e r i s u n i t y , t h e h y d r o l y s i s c o n s t a n t (Kh) a t low coac e n t r a t i o n can be a p p r o x i m a t e d as f o l l o w s : Kh = [ 0 H ~ ] [ H E t X ] / [ E t X ~ ] (A2) where s q u a r e b r a c k e t s r e f e r t o c o n c e n t r a t i o n s . Assuming t h a t t h e c o n c e n t r a t i o n o f KEtX i s c moles p e r l i t e r and x i s t h e d e g r e e o f t h e h y d r o l y s i s , i . e . t h e f r a c t i o n o f each mole o f KEtX h y d r o l y s e d when e q u i l i b r i u m i s a t t a i n e d , t h e n t h e c o n c e n t r a t i o n o f u n h y d r o l y s e d KEtX becomes c ( 1 - x ) moles p e r l i t e r . S i n c e KEtX i s a l m o s t c o m p l e t e l y i o n i z e d i n t o K + and E t X " i o n s , t h e c o n c e n t r a t i o n o f E t X " becomes c ( l - x ) and t h e c o n c e n t r a t i o n s o f OH" and HEtX a r e cx moles p e r l i t e r . S u b s t i t u t i o n o f t h e s e v a l u e s i n t o Eqn. ( A 2 ) , g i v e s Kh = c X 2 / (1-X) (A3) I f d e g r e e o f h y d r o l y s i s , - x, i s v e r y s m a l l Kh = c x 2 (A4) - 108 -x = (Kh / c ) 1 / 2 (A5) S i n c e t h e r e l a t i o n s h i p s Kw = [ H + ] [OH"] (A6) and Kd = [H ] [ E t X " ] / [ H E t X ] (A7) f o r Kd HEtX H + E t X " ' (A8) must be o b s e r v e d w i t h Eqn. ( A 2 ) , Kh becomes Kh = Kw /Kd (A9) From E q u a t i o n (A5) and (A9) and t h e c o n c e n t r a t i o n o f H +, c x , w h i c h i s equal t o [OH"] ; pH = 1/2 (pKw + pKd) + 1/2 l o g c (A10) E q u a t i o n (A10) i n d i c a t e s t h a t pH o f t h e KEtX s o l u t i o n i s a l o g a r i t h m i c f u n c t i o n o f the c o n c e n t r a t i o n o f KEtX w i t h a s l o p e o f +0.5, m a t c h i n g w i t h t h e s l o p e o f t h e l i n e f o r d e o x y g e n a t e d s o l u t i o n i n F i g . 3.1. By s u b s t i t u t i n g o b s e r v e d pH o f 6.15 a t 1 0 " 4 moles o f [ E t X - ] p e r l i t e r i n t o E q u a t i o n ( A l 0 ) , t h e d i s s o c i a t i o n c o n s t a n t o f e t h y l x a n t h i c a c i d (Kd) i s f o u n d t o be c a . 5 x 1 0 . T h i s i s in good agreement w i t h many 3 -2 r e p o r t e d v a l u e s r a n g i n g f r o m 2 x 10 to 3.4 x 10 . - 109 -A p p e n d i x 2.2 ; p H v s [ E t X " ] i n A i r - s a t u r a t e d S o l u t i o n P o t a s s i u m e t h y l x a n t h a t e (KEtX) can be o x i d i z e d t o e t h y l d i x a n t h o g e n ( ( E t X ) 2 ) . K 2EtX" + 1/2 0 2 + H 20 i — * - ( E t X ) + 20H ( A l l ) Assuming t h a t c o n c e n t r a t i o n s o f KEtX and 0 2 a r e c and nc moles p e r l i t e r r e s p e c t i v e l y and x i s t h e d e g r e e o f r e a c t i o n , t h e c o n c e n t r a t i o n s f o r u n r e a c t e d E t X " and 0 2 become c ( l - x ) and c ( n - x ) / 4 , w h i l e t h e c o n c e n t r a t i o n o f ( E t X ) 2 and OH" become l / 2 c x and cx r e s p e c t i v e l y . T h e r e f o r e , K = x 2 / ( 2 ( l - x ) • ( n - x ) / 4 ) (A12) where n i s a c o n s t a n t . I f x i s v e r y s m a l l , K = x 2 / 2n (A13) x = ( 2 n K ) 1 / 2 (A14) S u b s t i t u t i o n o f OH" c o n c e n t r a t i o n ( c x ) i n t o E q u a t i o n (A6) g i v e s , pH = 14 + l o g x + l o g c (A15) S i n c e x i s a c o n s t a n t term e x p r e s s e d by E q u a t i o n ( A 1 4 ) , E q u a t i o n ( A l 5 ) i n d i c a t e s t h a t pH o f t h e KEtX s o l u t i o n purged w i t h a i r i s a l o g a r i t h m i c f u n c t i o n o f KEtX c o n c e n t r a t i o n w i t h a p o s i t i v e s l o p e o f one pH u n i t p e r decade o f KEtX c o n c e n t r a t i o n . - no -Appendix 3 ; The Mixed P o t e n t i a l vs t h e D i s t a n c e between two E l e c t r o d e s . The l o g a r i t h m i c dependence o f mixed p o t e n t i a l on t h e d i s t a n c e between anode and c a t h o d e may be e x p l a i n e d as f o l l o w i n g . I f we e x p r e s s t h e a n o d i c T a f e l e q u a t i o n o f i r o n and t h e c a t h o d i c T a f e l e q u a t i o n o f c h a l c o p y r i t e as •]a = a + b l o g i ; f o r i r o n ( A l 6 ) l c = c - d l o g i ; f o r c h a l c o p y r i t e ( A l 7 ) t h e n , by d e f i n i t i o n , i R d r o p becomes iR = f\c - l a (A18) i R = (c - a) - (b + d) l o g i = m - n l o g i (A19) where a, b, c and d a r e c o n s t a n t , and m = c - a and n = b + d. I f we t a k e t h e l o g a r i t h m o f E q u a t i o n ( A l 9 ) , l o g i + l o g R = l o g (m - n l o g i ) l o g R = - l o g i + l o g (m - n l o g i ) (A20) S i n c e i i s a v e r y s m a l l number o f t h e o r d e r o f 10" Amp., l o g i becomes v e r y l a r g e and c o n s e q u e n t l y l o g (m - n l o g i ) becomes v e r y s m a l l , hence l o g i i s t h e p r e d o m i n a t i n g term i n t h e E q u a t i o n ( A 2 0 ) . T h e r e f o r e , - I l l -l o g R becomes; l o g R - - l o g i (A21) S i n c e R i s p r o p o r t i o n a l t o t h e d i s t a n c e (D cm) between t h e two e l e c t r o d e s , R i s equal t o _p-D where f i s r e s i s t i v i t y . l o g i - - l o g R - - l o g f D - - l o g f - l o g D (A22) S u b s t i t u t i o n o f E q u a t i o n (A22) i n t o E q u a t i o n ( A l 7 ) r e s u l t s , lc = c - d ( - l o g f - l o g D) = c + d l o g ^  + d l o g D = e + d l o g D (A23) where e = c + d l o g 5 . E q u a t i o n (A23) c l e a r l y i n d i c a t e s t h a t a mixed p o t e n t i a l measured on a c a t h o d i c e l e c t r o d e i s a l o g a r i t h m i c f u n c t i o n o f d i s t a n c e (D) w i t h a p o s i t i v e s l o p e o f d. T h i s i s i n d e e d t h e c a s e f o r a c h a l c o p y r i t e e l e c t r o d e w h ich i s i n g a l v a n i c c o n t a c t w i t h i r o n e l e c t r o d e ( F i g u r e 3.4 ) . - 112 -T 1 1 r • S t a r t i n g P o i n t O NaOH T i t r a t i o n O H 2 S 0 4 T i t r a t i o n +800 0 M [ E t X " ] 10" 5K [ E t X " ] +700 +600 •» —. mm - — <~ «^p» U J - : oo > +500 +400 +300 _ L 6 PH 10 12 A D p u v i i x 4.1 ; P o t e n t i a l - pH Diagram f o r S p h a l e r i t e (25°C, A i r P u r g i n g ) - 113 -+500 +400 +300 co +200 > + 100 • S t a r t i n g P o i n t O NaOH T i t r a t i o n 0 rL^SO^ T i t r a t i o n 0 M [ E t X " ] l O " 5 M [ E t X " ] -100 J L_ 1 I L J — L_ 1 I I L 6 PH 10 12 A p p e n d i x 4.2 ; P o t e n t i a l - pH Diagram f o r P y r i t e (25°C, A i r P u r g i n g ) - 114 -pH A p p e n d i x 4.3 ; P o t e n t i a l - pH Diagram f o r P y r r h o t i t e (25°C, A i r P u r g i n g ) - 115 -Appendix 4.4; P o t e n t i a l - pH Diagram f o r C h a l c o p y r i t e (25°C, A i r P u r g i n g ) - 116 -+ 300 T r i I I i +200 O S t a r t i n g P o i n t O NaOH T i t r a t i o n • H2SO4 T i t r a t i o n — 0 M [ E t X - ] 10" 5M [ E t X " ] +500 +400 +100 LU o oo CO ' > E ,-100 -200 -300 (Ref.rs ; 0 M [E t X " ] , 1 0 " 4 M [ P b + 2 ] ) \ ,.^-4i^i_^Qooo-."™; +300 +200 (Ref.70 ; 0 M [ E t X - ] i n a e r a t e d Borax S o l u t i o n ) +100 -100 -400 J L J I I I I L_ I I 10 12 PH Appendix 4.5 *, P o t e n t i a l - pH Diagram f o r G a l e n a (25° C, A i r P u r g i n g ) - 117 -A p p e n d i x 4.6; P o t e n t i a l - pH Diagram f o r I r o n ( 2 5 ° C , A i r p u r g i n g ) - 118 -T 1 1 1 1 1 1 1 r -200 h • S t a r t i n g P o i n t O NaOH T i t r a t i o n 9 H^SO^ T i t r a t i o n - — — 0 M [ E t X " ] 1 0 " 5 M [ E t X " ] +100 e — — mm • # » — @— — -100 S > -200 -300 -400 I i » I I i I I 1 L_ 1 L 10 12 pH A p p e n d i x 4.7; P o t e n t i a l - pH Diagram f o r C h a l c o p y r i t e and I r o n G a l v a n i c C o u p l e (25°C, A i r P u r g i n g ) 

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