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Selective leaching of molybdenum mixed copper-molybdenum sulfides Ismay, Arnaldo Andrés 1976

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SELECTIVE LEACHING OF MOLYBDENUM FROM MIXED COPPER-MOLYBDENUM SULFIDES by ARNALDO ANDRES ISMAY I n g . M e t a l u r g i c o , U n i v e r s i d a d T e c n o l d g i c a N a c i o n a l , A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF • MASTER IN APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES We a c c e p t t h i s t h e s i s a s 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 C o r d o b a , A r g e n t i n a , 1973 C t h e D e p a r t m e n t Me. nk**/ f*f£. o f METALLURGY? THE UNIVERSITY OF BRITISH COLUMBIA September 1976 §i) Arnaldo Andres Ismay SELECTIVE LEACHING OF MOLYBDENUM FROM MIXED OTPERHWBDENUM SULFIDES BY ARNALDO ANDRES ISMAY Ing. Metalurgico, Universidad Tecnologica Nacional, CordoBa, Argentina, ]973 A THESIS SUBMITTED IN PARTIAL FUlflLFENT OF THE FEQUIREMENTS FOR THE DEGREE OF MASTER IN APPLIED SCIENCE IN THE FACULTY OF GRADUATE STUDIES (DEPT. OF METALLURGY) We accept this thesis as conforming to the required standard • i i i • i i • • • • • i • i i • • i i • • i • • i • • i • • • • i • • • • • • • • • • • • • • • • • • • i i i i i i • i i i i i i i i i i i i i i i i i i i i i • i • • • • i i • • • • i • • • • • • i • i 11 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i • i i i • • • > • THE UNIVERSITY OF BRITISH COLUMBIA SEPTEMBER 1976 Arnaldo Andres Ismay 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 r 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 and 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 Me>J*flurry  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 2075 Wesbrook P l a c e Vancouver, Canada V6T 1WS D a t e 0rf.4-m ABSTRACT A s t u d y has been made o f the o x i d a t i o n o f m o l y b d e n i t e i n s o l u t i o n s o f h y p o c h l o r i t e . The r e a c t i o n was found t o be f i r s t o r d e r i n r e a g e n t c o n c e n t r a t i o n w i t h a r e a c t i o n r a t e c o n s t a n t o f 1.90 x 1 0 " 2 m i n - 1 c m - 2 . a t pH 9 and 45°C. The o x i d a t i o n r a t e i n c r e a s e s w i t h i n c r e a s i n g pH and l a t e r d e c r e a s e s i f t h e pH i s r a i s e d above 9.5. An a c t i v a t i o n e n e r g y o f 6.3 ± 0.8 k c a l / m o l e was o b s e r v e d f o r t h i s r e a c t i o n which was con-t r o l l e d by e i t h e r c h e m i c a l o r mixed d i f f u s i o n and c h e m i c a l r e a c t i o n s . O t h e r f a c t o r s s t u d i e d were t h e a g i t a t i o n , s u r f a c e a r e a , s u r f a c e c h a r -a c t e r i s t i c s and e f f e c t o f s u l f a t e s , c h l o r i d e s , c h l o r i t e s and c h l o r a t e s . I t was o b s e r v e d t h a t h y p o c h l o r i t e i s c a p a b l e o f s e l e c t i v e l y l e a c h i n g m o l y b d e n i t e from c o p p e r s u l f i d e s and p o s i t i v e r e s u l t s were o b t a i n e d when a p p l y i n g t h i s p r o p e r t y t o t h e e x t r a c t i o n o f m o l y b d e n i t e from c o p p e r r o u g h e r c o n c e n t r a t e s . The f o r m a t i o n o f o x i d e s d u r i n g d r y i n g a nd/or l e a c h i n g o f copper s u l f i d e s was found t o be d e t r i m e n t a l t o t h e p r o c e s s because t h e y a c t as c a t a l y s t s i n t h e d e c o m p o s i t i o n r e a c t i o n o f h y p o c h l o r i t e . Rates o f normal d e c o m p o s i t i o n o f t h e r e a g e n t were s t u d i e d o v e r t h e range 35 t o 60°C and pH 7 t o 10 and found t o be n e g l i g i b l e as compared t o t h e r a t e s o f o x i d a t i o n o f m o l y b d e n i t e . The l e a c h i n g o f ro u g h e r c o n c e n t r a t e s w i t h h y p o c h l o r i t e was found not t o a f f e c t c o n s i d e r a b l y t h e subsequent f l o t a t i o n o p e r a t i o n . i i Data o f o n - s i t e h y p o c h l o r i t e g e n e r a t o r p l a n t s and f a c t o r s t h a t a f f e c t p r o d u c t i o n and d e c o m p o s i t i o n , have been p r e s e n t e d w i t h t h e purpose o f p r o p o s i n g a p r o c e s s u s i n g the i n f o r m a t i o n o b t a i n e d i n t h i s work f o r t h e e x t r a c t i o n o f m o l y b d e n i t e i n t h e Copper C o n c e n t r a t i o n P l a n t s . i i i TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v i i i LIST OF FIGURES i x ACKNOWLEDGMENTS x i i Cha p t e r 1 INTRODUCTION 1 1.1 Ge n e r a l 1 1.2 P r o d u c t i o n o f M o l y b d e n i t e C o n c e n t r a t e s 2 1.3 P r o d u c t i o n o f Rhenium 7 1.4 A l t e r n a t i v e H y d r o m e t a l l u r g i c a l P r o c e s s 8 1.5 L i t e r a t u r e Review 10 2 SCOPE OF PRESENT INVESTIGATION 17 3 EXPERIMENTAL 19 3.1 M a t e r i a l s 19 3.2 A p p a r a t u s 22 3.3 P r o c e d u r e 23 3.4 Chemical A n a l y s i s 24 4 RESULTS AND OBSERVATIONS 26 4.1 O x i d a t i o n o f M o l y b d e n i t e by Sodium H y p o c h l o r i t e . ... 26 i v C h a p t e r Page 4.1.1 R e p r o d u c i b i l i t y 26 4.1.2 E f f e c t o f S t i r r i n g Rate 27 4.1.3 E f f e c t o f Temperature 27 4.1.4 E f f e c t o f S u r f a c e A r e a 33 4.1.5 MoS 2 C r y s t a l i n i t y and S u r f a c e C h a r a c t e r i s t i c s 33 4.1.6 Morphology o f MoS 2 L e a c h i n g 37 4.1.7 E f f e c t o f NaOCl C o n c e n t r a t i o n on the Rates o f Mo E x t r a c t i o n . 41 4.1.8 E f f e c t o f pH on t h e Rate o f R e a c t i o n 44 4.1.9 E f f e c t o f S u l f a t e C o n c e n t r a t i o n on t h e R e a c t i o n Rate . 44 4.1.10 E f f e c t o f NaCl on t h e R e a c t i o n Rate. ... 47 4.1.11 E f f e c t o f C10 2~ and C 1 0 3 = on the Rate o f R e a c t i o n 49 4.1.12 E f f e c t o f Mo l y b d a t e Ions on t h e Rates o f R e a c t i o n . 49 4.2 O x i d a t i o n o f C h a l c o p y r i t e by Sodium H y p o c h l o r i t e 53 4.2.1 L e a c h i n g o f C h a l c o p y r i t e C o n c e n t r a t e 53 4.2.2 L e a c h i n g o f M a s s i v e Specimens 54 4.2.3 C o m p a r a t i v e O x i d a t i o n and D i s s o l u t i o n o f MoS 2 and C u F e S 2 i n S o l u t i o n s o f NaOCl 57 4.2.4 E f f e c t o f C h a l c o p y r i t e on t h e Rates o f NaOCl Consumption and MoS 2 D i s s o l u t i o n . . 59 4.3 L e a c h i n g o f M o l y b d e n i t e from Copper Rougher C o n c e n t r a t e s 62 v C h a p t e r Page 4.3.1 I n f l u e n c e o f P a r t i c l e S i z e on t h e E x t r a c t i o n o f Molybdenum 95 4.3.2 Q u a n t i t y o f NaOCl R e q u i r e d f o r T o t a l Molybdenum E x t r a c t i o n 67 4.3.3 E f f e c t o f P r e t r e a t i n g the Rougher C o n c e n t r a t e w i t h S u l f u r i c A c i d ....... 67 4.3.4 E x t r a c t i o n o f M o l y b d e n i t e from " I n - P u l p " Rougher C o n c e n t r a t e . . . . . . . 68 4.3.5 F l o t a t i o n o f Leached Rougher C o n c e n t r a t e s 69 4.3.6 F l o t a t i o n o f " I n - P u l p " Rougher C o n c e n t r a t e s 73 4.3.7 S t a b i l i t y o f Sodium H y p o c h l o r i t e S o l u t i o n s 75 5 DISCUSSION 78 5.1 O x i d a t i o n o f M o l y b d e n i t e 78 5.2 O x i d a t i o n o f C h a l c o p y r i t e 87 5.3 L e a c h i n g o f M o l y b d e n i t e from Copper Rougher C o n c e n t r a t e s 89 5.4 F l o t a t i o n o f Leached Rougher C o n c e n t r a t e s 91 5.5 P r o d u c t i o n o f Sodium H y p o c h l o r i t e 91 5.6 K i n e t i c s o f Sodium H y p o c h l o r i t e D e c o m p o s i t i o n 94 5.6.1 E f f e c t o f E x c e s s C h l o r i d e on t h e D e c o m p o s i t i o n Rates 100 5.6.2 E f f e c t o f C a t i o n s on D e c o m p o s i t i o n Rates 100 5.6.3. C a t a l y t i c Agents i n H y p o c h l o r i t e D e c o m p o s i t i o n 101 5.7 H y p o c h l o r i t e L e a c h i n g P r o c e s s f o r the E x t r a c t i o n o f M o l y b d e n i t e From Copper Rougher C o n c e n t r a t e s 102 v i C h a p t e r Page 6 CONCLUSIONS 108 7 SUGGESTIONS FOR FUTURE WORK 110 REFERENCES I l l v i i LIST OF TABLES T a b l e Page 1 Comparison between P r o d u c t i o n from T o t a l A v a i l a b l e S o u r c e s and Copper P o r p h y r i e s 2 2 I n t e r n a t i o n a l Market Grade o f M o l y b d e n i t e C o n c e n t r a t e s 3 3 Copper, Molybdenum and Rhenium P r o d u c t P r i c e s 9 4 Chemical A n a l y s i s and X-ray D i f f r a c t i o n A n a l y s i s o f M o l y b d e n i t e C o n c e n t r a t e 20 5 Chemical C o m p o s i t i o n and X - r a y D i f f r a c t i o n P a t t e r n o f Copper Rougher C o n c e n t r a t e 21 6 Comparison o f S/Cu r a t i o s f o r Leached and N o n l e a c h e d C h a l c o p y r i t e . 56 7 V a l u e s f o r D e t e r m i n a t i o n o f S u r f a c e A r e a f o r M o l y b d e n i t e and C h a l c o p y r i t e 59 8 Oxide Copper C o n t e n t f o r D r i e d and I n - P u l p Rougher C o n c e n t r a t e s 93 9 Economic Assumptions f o r t h e E s t i m a t i o n o f C o s t s i n t h e O p e r a t i o n o f a H y p o c h l o r i t e G e n e r a t o r 107 10 Consumption o f A c i d f o r t h e R e d u c t i o n o f pH o f Molybdenum C o n t a i n i n g H y p o c h l o r i t e S o l u t i o n s v i i i LIST OF FIGURES F i g u r e Page 1 S i m p l i f i e d g e n e r a l f l o w s h e e t o f a c o p p e r - b y p r o d u c t m o l y b d e n i t e c o n c e n t r a t i o n p r o c e s s 4 2 R e p r o d u c i b i l i t y o f t h e r e s u l t s f o r t h e l e a c h i n g o f m o l y b d e n i t e p a r t i c l e s 27 3 E f f e c t o f s t i r r i n g on t h e r a t e o f o x i d a t i o n o f m o l y b d e n i t e 29 4 E f f e c t o f s t i r r i n g on t h e r a t e o f o x i d a t i o n , o f m o l y b d e n i t e ( r a t e v s . r.p.m.) 30 5 E f f e c t o f t e m p e r a t u r e on t h e o x i d a t i o n o f m o l y b d e n i t e 31 6 A r r h e n i u s p l o t o f t e m p e r a t u r e d a t a 32 7 E f f e c t o f s u r f a c e a r e a on t h e r a t e o f o x i d a t i o n o f m o l y b d e n i t e 34 8 L a y e r e d s t r u c t u r e o f m o l y b d e n i t e p a r t i c l e s 35 9 M o l y b d e n i t e s t r u c t u r e 36 10 View o f t h e "FACE" s u r f a c e o f m o l y b d e n i t e 38 11 M o l y b d e n i t e p a r t i c l e s o x i d i z e d o v e r i n c r e a s i n g p e r i o d s 39 12 M o l y b d e n i t e p a r t i c l e s a f t e r 10 minutes o f r e a c t i o n i n sodium h y p o c h l o r i t e . 40 i x F i g u r e Page 13 E f f e c t o f sodium h y p o c h l o r i t e on t h e r a t e o f o x i d a t i o n o f m o l y b d e n i t e 42 14 P l o t o f r e a c t i o n r a t e s v s . i n i t i a l h y p o c h l o r i t e c o n c e n t r a t i o n 43 15 E f f e c t o f pH on the r a t e o f o x i d a t i o n o f m o l y b d e n i t e 45 16 P l o t o f r e a c t i o n r a t e v s . pH 46 17 E f f e c t o f s u l f a t e s on the r a t e o f o x i d a t i o n o f m o l y b d e n i t e 48 18 E f f e c t o f c h l o r i t e s and c h l o r a t e s on the r a t e o f o x i d a t i o n o f m o l y b d e n i t e 50 19 E f f e c t o f molybdates-MoS 2 i n t e r a c t i o n 52 20 E x t r a c t i o n o f c o p p e r and consumption o f h y p o c h l o r i t e d u r i n g l e a c h i n g o f c h a l c o p y r i t e 55 21 S u r f a c e roughness o f m o l y b d e n i t e and c h a l c o p y r i t e 58 22 E x t r a c t i o n o f c o p p e r and molybdenum d u r i n g l e a c h i n g o f equal i n i t i a l s u r f a c e a r e a s o f m o l y b d e n i t e and c h a l c o p y r i t e 60 23 E f f e c t o f c h a l c o p y r i t e on t h e r a t e s o f m o l y b d e n i t e d i s s o l u t i o n . . . . . . . . 61 24 E x t r a c t i o n o f molybdenum and c o p p e r from a c o p p e r r o u g h e r c o n c e n t r a t e 63 25 E f f e c t o f p a r t i c l e s i z e on r a t e s o f e x t r a c t i o n o f molybdenum from r o u g h e r c o n c e n t r a t e s 65 x F i g u r e Page 26 Molybdenum e x t r a c t i o n and h y p o c h l o r i t e consumption f o r the l e a c h i n g o f ground and normal r o u g h e r c o n c e n t r a t e s 66 27 M o l y b d e n i t e d i s s o l u t i o n and h y p o c h l o r i t e consumption f o r the l e a c h i n g o f t r e a t e d and i n - p u l p r o u g h e r c o n c e n t r a t e s , 70 28 R e c o v e r y - g r a d e c u r v e s f o r t h e s e c o n d a r y f l o t a t i o n o f l e a c h e d r o u g h e r c o n c e n t r a t e s ( a f t e r t r e a t m e n t ) 72 29 R e c o v e r y grade c u r v e s f o r t h e s e c o n d a r y f l o t a t i o n o f l e a c h e d r o u g h e r c o n c e n t r a t e s ( s t o r e d i n - p u l p ) 74 30 E f f e c t o f pH on the d e c o m p o s i t i o n o f h y p o c h l o r i t e a t 60°C 76 31 E f f e c t o f pH on the d e c o m p o s i t i o n o f h y p o c h l o r i t e a t 35°C 77 32 H y p o c h l o r i t e and h y p o c h l o r o u s a c i d c o n c e n t r a t i o n s as a f u n c t i o n o f pH 80 33 E p o t e n t i a l o f m o l y b d e n i t e as a f u n c t i o n o f pH 82 34 L o g - l o g c u r v e o f r a t e s o f o x i d a t i o n v s . c o n c e n t r a t i o n o f h y p o c h l o r i t e . . 86 35 L o g - l o g c u r v e o f r a t e o f d e c o m p o s i t i o n o f h y p o c h l o r i t e v s . h y p o c h l o r i t e c o n c e n t r a t i o n 98 36 P o s i t i o n o f p r o p o s e d l e a c h i n g p r o c e s s i n t h e g e n e r a l c o p p e r c o n c e n t r a t i o n f l o w - s h e e t 103 37 Flow s h e e t o f t h e proposed method f o r e x t r a c t i n g m o l y b d e n i t e from c o p p e r r o u g h e r c o n c e n t r a t e s 104 x i ACKNOWLEDGMENTS I would l i k e t o e x p r e s s my g r a t i t u d e f o r t h e a d v i c e and encouragement o f my r e s e a r c h s u p e r v i s o r Dr. Ian H. Warren t h r o u g h o u t t h e c o u r s e o f t h i s r e s e a r c h p r o j e c t . Thanks a r e a l s o e x t e n d e d t o th e o t h e r members o f the department and f e l l o w g r a d u a t e s t u d e n t s f o r making t h i s p e r i o d b o t h u s e f u l and p l e a s a n t . The h e l p r e c e i v e d f r o m Dr. James King and t h e use o f f a c i l i t i e s a t the Re s e a r c h D i v i s i o n o f P l a c e r Development L i m i t e d f o r th e accomplishment o f p a r t s o f t h i s p r o j e c t a r e v e r y much a p p r e c i a t e d . A s p e c i a l thanks goes t o M i r t h a f o r a l l t h e n o n - t e c h n i c a l h e l p and p a t i e n c e . F i n a n c i a l a s s i s t a n c e r e c e i v e d from t h e Canadian I n d u s t r i e s L i m i t e d ( C I L ) i s g r a t e f u l l y acknowledged. x i i C h a p t e r 1 INTRODUCTION 1.1 Ge n e r a l M o l y b d e n i t e (MoS 2) i s o f t e n a s s o c i a t e d w i t h copper s u l f i d e m i n e r a l s such as c h a l c o p y r i t e ( C u F e S 2 ) and c h a l c o c i t e ( C u 2 S ) as a minor component o f many g i g a n t i c , low grade d i s s e m i n a t e d c o p p e r d e p o s i t s c a l l e d " c o p p e r p o r p h y r i e s . " The most v a l u a b l e components o f t h e s e d e p o s i t s a r e co p p e r and molybdenum. The e x t r a c t i o n o f g o l d and s i l v e r i s a l s o s i g n i f i c a n t . , Rhenium i s a by p r o d u c t o f p o r p h y r y p r o c e s s i n g and a l t h o u g h t o t a l w o r l d p r o d u c t i o n i s o n l y o f t h e o r d e r o f 7400 l b s . [ 1 , 2 ] , t h e m o l y b d e n i t e c o n c e n t r a t e o b t a i n e d from c o p p e r p o r p h y r i e s ' i s t h e s o l e s o u r c e o f t h i s m etal a t p r e s e n t . A t y p i c a l p o r p h y r y o r e m i g h t - c o n t a i n : ; Cu = 0.8% Au = 0.01-0.02 o z / t o n Re - 0.038% o f MoS 2 c o n c e n t r a t e Mo = 0.015% Ag = 0.05-0.1 o z / t o n Due t o t h e l a r g e tonnage o f p o r p h y r y m a t e r i a l p r o c e s s e d (540 x 10 6 t o n s i n 1973) t h e p r o d u c t i o n f i g u r e s f o r t h e molybdenum and rhenium 1 2 a r e v e r y s i g n i f i c a n t as shown i n T a b l e 1. Roughly 1-1.5 oz o f Au and 3-10 oz o f Ag can be r e c o y e r e d p er t o n o f co p p e r e x t r a c t e d . T a b l e 1 Comparison Between P r o d u c t i o n from T o t a l A v a i l a b l e S o u r c e s and Copper P o r p h y r i e s (based on 1973 s t a t i s t i c s ) Metal T o t a l World P r o d u c t i o n Output from Copper P o r p h y r i e s % Produced from Copper P r o p h y r i e s Cu 8,100,000 3,500,000 43% Mo 87,500 37,400 37% Re 3.7 3.7 100% From 1967 t o 1972 t h e p r o d u c t i o n o f MoS 2 as a b y p r o d u c t o f co p p e r p o r p h y r i e s i n the U.S.A. r o s e o v e r 42% [ 1 ] . I t has been e s t i m a t e d [ 3 ] t h a t t h e s e d e p o s i t s a c c o u n t f o r some 5.5 x 1 0 6 t o n s o f Mo, which r e p r e s e n t 17% o f the w o r l d ' s molybdenum r e s e r v e s . 1.2 P r o d u c t i o n o f M o l y b d e n i t e C o n c e n t r a t e s Molybdenum consumed i n i t s v a r i o u s end p r o d u c t s i s i n t h e form o f m o l y b d i c o x i d e ( 6 6 % ) , ferromolybdenum ( 2 0 % ) , ammonium and sodium molybdate ( 2 % ) , and o t h e r m a t e r i a l s ( 1 2 % ) . .To o b t a i n t h e s e , a m o l y b d e n i t e concen-t r a t e i s produced by f l o t a t i o n and th e n r o a s t e d t o o x i d i z e t h e MoS 2 t o Mo0 3, which i s t h e s t a r t i n g compound f o r a l l subsequent p r o d u c t s . 3 The t a s k o f p r o d u c i n g s p e c i f i c a t i o n MoS 2 c o n c e n t r a t e s [ T a b l e 2) from o r e s w i t h an aver a g e o f 150 ppm Mo i n which c o p e r and i r o n m i n e r a l s outweigh MoS 2 by r a t i o s o f about 140 t o 1 i s q u i t e f o r m i d a b l e . The p r o c e s s i s l o n g and t h e r e c o v e r i e s a r e low. T a b l e 2 I n t e r n a t i o n a l Market Grade o f M o l y b d e n i t e C o n c e n t r a t e s MoS 2 80% (min) Cu 0.5% (max).. Pb 0.3% (max) P, Sn, As 0.2% (max t o t a l ) A g e n e r a l i z e d d e s c r i p t i o n o f t h e common p r a c t i c e o f t r e a t i n g t h e s e o r e s i s p r e s e n t e d below w i t h t h e i n t e n t i o n o f showing why MoS 2 r e c o v e r y i s so low. A l t h o u g h t h e r e a r e v a r i a t i o n s i n t h e 30 p l a n t s f o r whi c h t h e r e i s a v a i l a b l e d a t a i t i s f e l t t h a t t h e y do not a f f e c t t h i s a n a l y s i s . F i g u r e 1 i s a s i m p l i f i e d f l o w s h e e t o f the method d e s c r i b e d , i n d i c a t i n g t h e e f f i c i e n c y o f the s u c c e s s i v e s t a g e s and t h e c u m u l a t i v e d r o p i n r e c o v e r y . MoS 2 i s f l o a t e d c o l l e c t i v e l y w i t h t h e Cu s u l f i d e s i n t o a r o u g h e r copper c o n c e n t r a t e which c o n t a i n s the bul k o f Cu and Mo. T h i s r o u g h e r c o n c e n t r a t e has a h i g h p r o p o r t i o n o f m i d d l i n g s and i s t h e r e f o r e s u b j e c t e d t o a c a r e f u l s i z e r e d u c t i o n i n o r d e r t o a v o i d t h e f o r m a t i o n o f s l i m e s because o f t h e ease w i t h which MoS 2 g r i n d s . I t i s t h e n r e f l o a t e d i n c l e a n e r and r e c l e a n e r c i r c u i t s t o produce a copper c o n c e n t r a t e which c o n t a i n s 25-50% Cu and 0.2-1% MoS 2. Run of Mine SIZE REDUCTION 80-85 70-75 >-z : UJ o u_ u_ LU I-< r r UJ o_ O 70 15% RQUBHER FLOTATION 1 Rougher 1 Concentrate 1 CYCLONE 8 REGRINDING 1 CLEANING a RECLEANING 1 25% >0.2% T Cu Flotation Concentrate <0.2% Mo THICKENING >-r r LU > o o LU ^ 33%. in in o LU > in in UJ r r o r r o_ 50% ROUGHER FLOTATION -»»MoS^ Rougher Concentrate CLEANING CYCLONE a REGRINDING RECLEANING MoS Flotation Concentrate 2 >0.5% Cu LEACHING THICKENING FILTERING S DRYING Cu Concentrate Mo Concentrate FIG. I- SIMPLIFIED GENERAL FLOWSHEET OF A COPPER BYPRODUCT MoS 2 CONCENTRATE PROCESS. 5 A t t h i s p o i n t MoS 2 i s s e p a r a t e d f r o m t h e Cu m i n e r a l s by s e l e c -t i v e f l o t a t i o n , e i t h e r d e p r e s s i n g t h e Cu and Fe m i n e r a l s and f l o a t i n g MoS 2 o r v i c e v e r s a . The p r o d u c t o f t h i s o p e r a t i o n i s a "moly" r o u g h e r c o n c e n t r a t e which i s th e n s u b j e c t e d t o r e g r i n d i n g and r e f l o a t i n g i n a c l o s e d - c o u n t e r c u r r e n t c l e a n i n g c i r c u i t to produce a MoS 2 c o n c e n t r a t e . I f t h i s p r o d u c t has i n e x c e s s o f 0.5% Cu i t must be f u r t h e r p r o c e s s e d by l e a c h i n g t o produce a m a r k e t a b l e c o n c e n t r a t e . The c u t o f f p o i n t f o r o p e r a t i n g a b y p r o d u c t MoS 2 f l o t a t i o n c i r c u i t i s c o n s i d e r e d e c o n o m i c a l l y t o be 0.2% MoS 2 i n t h e Cu c o n c e n t r a t e . However i n many c a s e s p l a n t s w i l l o n l y o p e r a t e i f t h e a s s a y i s above 0.7% MoS 2. Any Mo and Re not r e c o v e r e d by f l o t a t i o n a r e u n r e c o v e r a b l e i n t h e sub-sequent p y r o m e t a l l u g i c a l t r e a t m e n t . ' The f a c t o r s t h a t a f f e c t MoS 2 r e c o v e r y [ 4 ] a r e : 1. The p r i n c i p a l aim o f the o p e r a t i o n i s maximum Cu r e c o v e r y and t h i s may r e s u l t i n l e s s t h a n optimum c o n d i t i o n s f o r MoS 2 e x t r a c t i o n as may be seen i n t h e f o l l o w i n g examples: A. The a d d i t i o n o f l i m e which i s n e c e s s a r y t o d e p r e s s p y r i t e i n o r d e r to i n s u r e good Cu f l o t a t i o n may h i n d e r MoS 2 f l o t a t i o n by f l o c c u l a t i n g t h e gangue and t r a p p i n g t h e f i n e MoS 2 p a r t i c l e s . B. Hydrocarbon o i l s which r a i s e t h e MoS 2 r e c o v e r y a r e added o n l y t o t h e e x t e n t t h a t t h e r e i s no a d v e r s e e f f e c t on c o p p e r m i n e r a l s . The m i n e r a l i z a t i o n o f t h e o r e M i n e r a l i z a t i o n i s o f g r e a t e r s i g n i f i c a n c e i n MoS 2 f l o t a -t i o n t h a n i n o t h e r f l o t a t i o n o p e r a t i o n s due t o t h e v e r y low grade o f o r e as shown i n t h e next examples: A. Due t o t h e c h a r a c t e r i s t i c s o f copper p o r p h y r i e s i n which MoS 2 has a v e r y uneven d i s t r i b u t i o n , w h i l e t h e Cu grade o f the head may remain f a i r l y c o n s t a n t t h e MoS 2 g r a d e may be e r r a t i c and v a r y by s e v e r a l hundred p e r c e n t i n s h o r t p e r i o d s o f ti m e . B. MoS 2 can be p r e s e n t as w e l l formed c l e a n c r y s t a l s , o r as l e s s w e l l d e f i n e d m a t e r i a l w i t h v e r y a l t e r e d s u r f a c e s , w i t h o r w i t h o u t a t h i n o x i d e c o a t i n g . A l s o i t may o r may not be a s s o c i a t e d w i t h o t h e r h y d r o p h o b i c m a t e r i a l s such as g r a p h i t e , t a l c o r carbonaceous p r o d u c t s . C. M o l y b d e n i t e can a l s o be c o a t e d w i t h a c o l l o i d a l c h a l c o p y r i t e s l i m e which p r e v e n t s i t s f l o t a t i o n and causes a "drop o u t " i n t h e c l e a n e r c i r c u i t . D. T h e r e i s a s o - c a l l e d "crowding o u t phenomena" which can be e x p l a i n e d as a c o u n t e r e f f e c t on t h e d i f f e r e n t i a l f l o t a t i o n o f two m i n e r a l s p e c i e s produced by an e x t r a o r d i n a r y d i f f e r e n c e i n t h e number o f p a r t i c l e s o f each s p e c i e s . 7 3. C o s t o f Reagents The c o s t o f r e a g e n t s f o r d e p r e s s i n g t h e o t h e r m i n e r a l s p e c i e s impedes the e x t r a c t i o n o f MoS 2 from Cu c o n c e n -t r a t e s i n which Mo c o n t e n t i s v e r y low. A l t h o u g h c o n s i d e r a b l e r e s e a r c h has been d i r e c t e d towards s o l v i n g many of. t h e s e problems and i n c r e a s i n g t h e e f f i c i e n c y o f MoS 2 e x t r a c t i o n from c o p p e r p o r p h y r i e s s p r e s e n t t e c h n o l o g y does n o t a c h i e v e much more tha n 50% r e c o v e r y . T h i s means t h a t a p p r o x i m a t e l y 18,000 t o n s o f Mo r e p r e -s e n t i n g about 0.3% o f t h e i d e n t i f i e d r e s e r v e s o f Mo i n c o p p e r p o r p h y r i e s o f the w o r l d , a r e l o s t a n n u a l l y i n v a r i o u s p a r t s o f the p r o c e s s . 1.3 P r o d u c t i o n o f Rhenium To produce rhenium a MoS 2 c o n c e n t r a t e . i s r o a s t e d a t tempera-t u r e s between 540°C and 650°C and about 90% o f the Re i t c o n t a i n s i s v o l a t i l i z e d . The gases a r e s c r u b b e d u s i n g a c o m b i n a t i o n o f c y c l o n e s , e l e c t r o s t a t i c p r e c i p i t a t o r s and a h i g h energy V e n t u r i t y p e s c r u b b e r t o remove Re a l o n g w i t h some S 0 2 and s m a l l amounts o f Mo, Cu and Fe. By i n t e n s i v e r e c i r c u l a t i o n t h e p r e g n a n t s o l u t i o n i s c o n c e n t r a t e d t o 0.2 t o 0.5 g/1 o f Re. I t i s then c o n d i t i o n e d by a d d i n g C l 2 t o o x i d i z e Mo, Re and Fe v a l u e s and s u b s e q u e n t l y Fe and Cu a r e p r e c i p i t a t e d as c a r b o n a t e s . Re i s then a b s o r b e d i n ion-^exchange columns o r removed by s o l v e n t e x t r a c t i o n . . The e l u t r i a t i o n p r o d u c t i s t r e a t e d w i t h H2S. t o p r e c i p i -t a t e Re as ReS 2 which i s then t r e a t e d w i t h H 2 0 2 and.iNhU OH t o form a s o l u b l e p e r r h e n a t e which i s l a t e r c r y s t a l l i z e d . 8 A l t h o u g h t h e p r o c e s s i s r e l a t i v e l y s i m p l e i t has s e v e r a l d e f i c i e n c i e s which r e d u c e t h e t o t a l Re p r o d u c t i o n c a p a c i t y . These a r e : 1. R e d u c e d Re i n f e e d d u e t o l ow r e c o v e r i e s i n t h e p r e v i o u s M o S 2 c o n c e n t r a t i o n p r o c e s s 2 . P r o d u c t i o n o f S 0 2 3. L o s s e s o f Re i n t h e s c r u b b i n g o p e r a t i o n s The a v e r a g e r e c o v e r y o f rhenium from m o l y b e d e n i t e c o n c e n t r a t e s i s 60%. I f p r e s e n t t e c h n o l o g y c o n t i n u e s t o be used i n the f u t u r e t h i s would mean t h a t from t h e e s t i m a t e d 4.2 m i l l i o n l b s . o f Re r e s e r v e s c o n -t a i n e d i n copper p o r p h y r i e s o n l y 1.5 m i l l i o n can be c o n s i d e r e d r e c o v e r a b l e 1.4 A l t e r n a t i v e H y d r o m e t a l 1 u r g i c a l P r o c e s s A l i s t o f p r i n c i p a l commodities produced from c o p p e r p o r p h y r i e s and t h e i r r e s p e c t i v e p r i c e s i s p r e s e n t e d i n T a b l e 3. Any new p r o c e s s which a c h i e v e d g r e a t e r r e c o v e r i e s o f both molybdenum and rhenium c o u l d be o f s i g n i f i c a n t economic advantage s p e c i a l l y i n t h o s e c o p p e r p o r p h y r i e s i n which t h e Cu grade i s e x t r e m e l y low such as th e ones i n B r i t i s h Columbia. I f MoS 2 were p r e f e r e n t i a l l y l e a c h e d from copper s u l f i d e m a t e r i a l s t h e a p p l i c a t i o n o f a h y d r o m e t a l 1 u r g i c a l p r o c e s s t o an e a r l y s t a g e o f the co p p e r c o n c e n t r a t i o n c i r c u i t might r e s u l t i n a f a v o u r a b l e a l t e r n a t i v e t o t h e p r e s e n t t e c h n o l o g y . T h i s p r o c e s s c o u l d have t h e f o l l o w i n g a d v a n t a g e s : T a b l e 3 Copper, Molybdenum and Rhenium P r o d u c t s P r i c e s (1975) Copper P r i m a r y Cu .61 $ / l b R e f i n e d Cu . .64 $ / l b Rhenium a l l rhenium p r o d u c t s i n the range o f 1000-1200 $ / l b . Molybdenum MoS 2 C o n c e n t r a t e $ / l b . c o n t . Mo 2.20-.255 T e c h n i c a l Mo03 2.60 Mo0 3 B r i q u e t e s 2.65 F e r r o Molybdenum 2.30 Mo P e l l e t s 2.90 Mo Powder 3.10 10 1. H i g h e r M o S 2 r e c o v e r i e s 2. P r o d u c t i o n o f h i g h e r m a r k e t v a l u e p r o d u c t s , i . e . Mo s a l t s , Mo m e t a l p o w d e r 3. H i g h e r Re r e c o v e r y 4 . E l i m i n a t i o n o f t h e r o a s t i n g s t e p a n d t h u s p r o d u c t i o n o f Mo a n d Re w i t h o u t S 0 2 e m i s s i o n a n d w i t h o u t .^ c a p i t a l e x p e n d i t u r e o n r o a s t i n g e q u i p m e n t . Aqueous s u s p e n s i o n s o f MoS 2 must be o x i d i z e d t o produce molybdate i o n s t o e f f e c t T e a c h i n g ; ' a n d , depending on the p H , t h e s e ions" w i l l e i t h e r be HMoOit" o r MoO^ - [6 -9] . In s i m i l a r aqueous o x i d a t i o n , t h e s u l f i d e s o f Cu and Fe form c a t i o n s which under some c o n d i t i o n s o f pH, by h y d r o l y s i s , produce i n s o l u b l e o x i d e s o r h y d r o x i d e s . T h e r e f o r e t h e s e l e c t i v i t y o f a l e a c h i n g system might not o n l y depend on the r e l a t i v e o x i d a t i o n r a t e s o f m o l y b d e n i t e and the c o p p e r s u l f i d e s but on t h e s o l u b i l i t y o f the s p e c i e s formed, t he n a t u r e o f any s o l i d p r o d u c t s formed and t h e i r i n t e r a c t i o n w i t h t h e r e a g e n t employed. 1.5 L i t e r a t u r e Review S t u d i e s on the o x i d a t i o n o f aqueous s u s p e n s i o n s o f m o l y b d e n i t e have been r e p o r t e d as e a r l y as 1952. E.S. U s a t a y a et al. [11] i n v e s t i g a t e d t he e f f e c t o f a c i d s , b a s e s , s a l t s and o x i d i z i n g a g e n t s on MoS 2 c o n c l u d i n g t h a t t h e r a t e o f d e c o m p o s i t i o n i n c r e a s e s w i t h i n c r e a s i n g pH and t h a t s o l i d p r o t e c t i v e l a y e r s a r e formed i n the a c i d o r weakly a l k a l i n e r e g i o n s . The p r e s e n c e o f s t r o n g o x i d i z i n g a gents such as H 2 0 2 i n a s t r o n g a l k a l i n e s o l u t i o n was o b s e r v e d t o p r e v e n t the f o r m a t i o n o f such a c o a t i n g . D . M . Yukhtanov and K.D. L e o n t ' e v a [12] ( i n 1953) were t h e f i r s t t o use NaOCl a t normal and h i g h t e m p e r a t u r e s t o o x i d i z e MoS 2. 11 W.H. D r e s n e r et al. [7] i n 1956 s t u d i e d t h e k i n e t i c s o f MoS 2 l e a c h i n g i n a l k a l i n e s o l u t i o n s i n t h e t e m p e r a t u r e range o f 100-175°C. They r e p o r t e d t h a t MoS 2 c o u l d l e a c h under moderate p r e s s u r e s o f oxygen (100-200 p . s . i . ) - The o v e r a l l r e a c t i o n b e i n g : MoS 2 + | 0 2 + 6 OH" •> Mo0 4 = + 2 S0.f + 3 H 2 0 (1) The end p r o d u c t s a f t e r complete o x i d a t i o n were MoOiT and S0iT~ i o n s . The p r e s e n c e o f S 2 0 3 ~ was d e t e c t e d o n l y as an i n t e r m e d i a t e u n s t a b l e s p e c i e s . G. Monev and S. M i r e v a [13] i n 1958 made t h e f i r s t a t t e m p t s t o e x t r a c t Mo from poor copper-molybdenum c o n c e n t r a t e s and o r e s u s i n g NaOCl s o l u t i o n s . H. Cox and A.K: S e h e l l i n g e r [8] i n 1958 s t u d i e d t h e l e a c h i n g o f Mo from low grade o r e s , c o p p e r c o n c e n t r a t e s and h i g h grade m o l y b d e n i t e c o n c e n t r a t e s u s i n g NaOCl. A l t h o u g h t h e y o b t a i n e d e x c e l l e n t e x t r a c t i o n s l ow g r a d e m o l y b d e n u m o r e ( 0 . 0 1 5 $ M o S 2 ) R e c o v e r y = 9 3 . 3 $ c o p p e r c o n c e n t r a t e ( 1 . 0 5 $ M o S 2 ) " = 9 0 . 6 $ H i g h g r a d e m o l y b d e n u m c o n c e n t r a t e ( 9 9 . 9 $ MoS 2 .) " = 9 9 . 9 $ t h e y d i d n o t p e r f o r m a s y s t e m a t i c s t u d y o f the v a r i a b l e s t h a t a f f e c t t h e r a t e o f o x i d a t i o n . The b e s t c o n d i t i o n s t h e y recommended f o r l e a c h i n g were: 3% NaOCl s o l u t i o n , room t e m p e r a t u r e and 30 m i n u t e s l e a c h i n g t i m e . The e q u a t i o n s u g g e s t e d by t h e s e i n v e s t i g a t o r s was: 7 Na 0C1 + MoS 2 + 4e" + MoO^ + S 2 0 3 " + 7 NaCl (2) 12 K.V. Ioardanov and A.N. Z e l i k m a n [14] made an e x t e n s i v e s t u d y o f the p h y s i c o - c h e m i c a l bases o f the o x i d a t i o n o f MoS 2 i n NaOCl s o l u t i o n s . They used pure MoS 2 p r e s s e d i n t o compact samples and f i n e l y ground m a t e r i a l . They assumed MoS 2 was o x i d i z e d a c c o r d i n g t o t h e r e a c t i o n : MoS 2 + 9 NaOCl + 6 NaOH -> Na 2 Mo0„ + 2 N a 2 S 0 4 + 9 NaCl + 3 H 20 (3) The c o n c l u s i o n s r e a c h e d i n t h e i r s t u d y were: a ) T h e r a t e o f o x i d a t i o n i n c r e a s e s w i t h a n i n c r e a s e i n t e m p e r a t u r e a n d t h a t t h e r e a c t i o n i s d i f f u -s i o n c o n t r o l l e d . b) T h e r a t e i s d i r e c t l y p r o p o r t i o n a l t o t h e i n i t i a l c o n c e n t r a t i o n o f N a O C l . They proposed a mechanism f o r t h e o x i d a t i o n o f MoS 2. The f i r s t s t a g e i s the a d s o r p t i o n o f 0C1 i o n s on t h e s u r f a c e o f t h e MoS 2, th e second s t a g e i s the i n t e r a c t i o n o f the ad s o r b e d O C l" and t h e s o l i d t o form an a c t i v e complex OCl ( s o l n ) «- OCl (ads) (4) MoS 2 ( s o l i d ) + O C l ( a d s ) + |MoS2 + ••• + OCl"] a c t i v e complex (5) Then t he f o r m a t i o n o f r e a c t i o n p r o d u c t s , oxygen r e p l a c i n g s u l f u r atoms and f o r m i n g i n t e r m e d i a t e compounds and c h l o r i d e i o n s . |MoS2 + ... + O C l - ] a c t > c o m p l e x + OCr + [ M O S 2 0 + + O C l - ] a c t > c o m p l e x + cr | M o S 2 0 + + O C T a c t > c o m p l e i ( + O C T -[MoS0 2 + -.. + O C T - | a 9 t . complex + C 1 " + 5 [ M o S 0 2 + + 0 C l - ] a c t _ c o m p l e x + O C l " -|Mo03 + •*• + OCT"] + C l " + S [ M 0 O 3 + ••• + O C l " ] 2 M 0 O 3 + O C l ~ and f i n a l l y M 0 O 3 + 2 OH~ ^  MOOL>= + H 20 The s u l f u r atoms a r e o x i d i z e d by O C l " , a c c o r d i n g t o t h e r e a c t i o n s 2 S + 2 O C l " 2 S 2 0 2 = + 2 C l " 14 S 2 0 2 + 0C1 -«- S 2 0 3 + C l (12) 3 OC1 ^ 2 SOk + 3C1 + H 20 (13) In an i n d e p e n d e n t s t u d y R.B. Bhappu et al. [6] d e t e r m i n e d t h a t t h e c o r r e c t m o l ar r a t i o between NaOCl and MoS 2, when the NaOCl i s i n excess, i s 9:1. They showed t h a t i f s u l f u r was formed d u r i n g t h e l e a c h i n g p r o c e s s by the e x c e s s o f OCT p r e s e n t such as would be t h e c a s e i n l e a c h i n g low gr a d e m a t e r i a l s would o x i d i z e i t t o SOi* as f o l l o w s : t h e r e f o r e i f OCT i s i n e x c e s s t h e o n l y s u l f u r s p e c i e s p r e s e n t i s SO^ -. By combining r e a c t i o n s (14) and (15) t h e y s u g g e s t e d t h e o v e r a l l r e a c t i o n i d e n t i c a l t o the one used by K.V. Ioardanov and A.N. Z e l i k m a n [ 1 4 ] , which a l s o a g r e e s w i t h t h e s t o i c h i o m e t r y o f the o x i d a t i o n o f MoS 2 i n aqueous K0H-0 2 systems [ 7 ] . R.B. Bhappu et al. [ 6 ] a l s o r e p o r t e d t h a t : a ) T h e r e i s p r a c t i c a l l y no c o n s u m p t i o n o f NaOC l by g a n g u e m a t e r i a l s ( q u a r t z a n d a p l i t e i n t h e i r e x p e r i m e n t s ) . b) T h e r e i s a d e c r e a s e i n t h e l e a c h i n g r a t e s b o t h i n c o n c e n t r a t e s a n d l a r g e r s p e c i m e n s o f 6 0C1 + MoS2 + 4 OH HoOk + S° + S0h + 6 C l " + 2H 20 (14) 3 OCT + S° + H 20 1 SO^ + 3C1" + 2H (15) 15 M o S 2 w h i c h t h e y a t t r i b u t e d t o d i s s o l u t i o n o f f i n e r p a r t i c l e s i n t h e f i r s t c a s e a n d t o c o n -c e n t r a t i o n p h e n o m e n a i n t h e s e c o n d . c ) Due t o e c o n o m i c c o n s i d e r a t i o n s i t was n e c e s s a r y t o p r o d u c e t h e NaOC l i n s i t u e i t h e r by c o m b i n i n g p u r c h a s e d c a u s t i c s o d a a n d c h l o r i n e o r by e l e c t r o l y s i s o f N a C I . F o r t h i s s e c o n d s y s t e m t h e y s u g g e s t e d t h e r e c y c l i n g o f l e a c h s o l u t i o n i n o r d e r t o r e u s e t h e NaCI p r o d u c e d i n r e a c t i o n £3), . K.A. S h a p i r o and B.B. Kulakeva [15] a p p l i e d NaOCl d i g e s t i o n t o l e a n i n t e r m e d i a t e p r o d u c t s c o n t a i n i n g 2-6% Mo formed i n t h e c o n c e n t r a t i o n o f d i s s e m i n a t e d o r e . No i n d i c a t i o n was g i v e n as t o t h e o t h e r m i n e r a l s i n t h o s e i n t e r m e d i a t e p r o d u c t s . They r e p o r t e d h a v i n g t o consume 400 and 500% t h e s t o i c h i o m e t r i c amount o f NaOCl f o r complete e x t r a c t i o n o f Mo. No e x p l a n a t i o n was g i v e n o f the cause* o f t h i s e x c e s s consumption. R.B. Bhappu et al. [16,17] s t u d i e d t h e d i s s o l u t i o n o f MoS 2 u s i n g s e v e r a l o x i d i z i n g a g e n t s ; NaOCl, ozone, o x y g e n - a l k a l i ; a c i d perman-g a n a t e , p e r s u l f a t e , a c i d - f e r r i c c h l o r i d e , n i t r i c a c i d , c h l o r i n e d i o x i d e , a c i d - c h l o r a t e , manganese d i o x i d e - s u l f u r i c a c i d and b a c t e r i a l l e a c h i n g . They r e p o r t e d t h a t NaOCl i n b a s i c s o l u t i o n s p r e s e n t e d t h e f a s t e s t r a t e o f l e a c h i n g and h i g h s e l e c t i v i t y towards MoS 2. The d i s a d v a n t a g e s i n d i c a t e d were i t s h i g h p r o d u c t i o n c o s t s , i t s i n s t a b i l i t y and i t s t r o u b l e -some r e g e n e r a t i o n . A c i d - N a C 1 0 3 a l s o l e a c h e s MoS 2 but n o t as f a s t as NaOCl. In com p a r i s o n NaC10 3 i s l e s s e x p e n s i v e and e a s i e r t o r e g e n e r a t e than NaOCl., but i s not as s e l e c t i v e . B . J . S c h e i n e r and R.E. L i n d s t r o m [18] i n v e s t i g a t e d t h e p o s s i b l i t y o f l e a c h i n g MoS 2 from low grade o r e s by an e l e c t r o o x i d a t i o n t e c h n i q u e t h a t c o n s i s t s o f g e n e r a t i n g NaOCl i n s i t u by e l e c t r o l y s i s o f a b r i n e - o r e s l u r r y . 16 T h i s method had been p r e v i o u s l y used i n t r e a t i n g c a r b o n a c e o u s g o l d o r e s [19,20] and f o r r e c o v e r i n g mercury from c i n n a b a r o r e s [ 2 1 ] . They c l a i m e d a 90-99% e x t r a c t i o n o f molybdenum, good s e l e c -t i v i t y o f Mo o v e r Cu and a power consumption o f 16-24 kwh/lb o f Mo e x t r a c t e d from a low grade MoS 2 o r e . However i n a r e c e n t p u b l i s t i o n D.S. B a r r et al. [22] i n d i c a t e d t h a t the p r o d u c t i o n o f b y p r o d u c t NaC10 3 d u r i n g the e l e c t r o o x i d a t i o n , i n t e r f e r e s w i t h t h e r e c o v e r y o f molybdenum and c o n s e q u e n t l y h i n d e r s t h e economics o f the p r o c e s s . To m i n i m i z e NaC10 3 p r o d u c t i o n t h e s e a u t h o r s s u g g e s t e d l e a c h i n g a t a low pH (4.0-5.0) and low t e m p e r a t u r e (<30°C). The use o f NaOCl as an o x i d i z i n g - l e a c h i n g r e a g e n t has a l s o been r e p o r t e d f o r s t i b n i t e [ 2 3 ] , s p h a l e r i t e [24] and c h a l c o p y r i t e [ 2 5 ] . D. Stump and Y. Berube [26] c l a i m e d t h a t t h e o x i d a t i o n r a t e s o f c h a l c o p y r i t e and m o l y b d e n i t e i n an a l k a l i n e medium a r e s i m i l a r and t h a t i t i s u n l i k e l y t h a t s e l e c t i v e l e a c h i n g o f molybdenum from c o p p e r con-c e n t r a t e s c o u l d be a c c o m p l i s h e d . C h a p t e r 2 SCOPE OF PRESENT INVESTIGATION The r e s e a r c h r e p o r t e d i n t h i s t h e s i s i s p r i m a r i l y aimed a t s t u d y i n g t h e p o s s i b i l i t i e s o f e x t r a c t i n g molybdenum from low g r a d e - c o p p e r s u l f i d e c o n t a i n i n g m a t e r i a l s by a d i r e c t l e a c h i n g o p e r a t i o n u s i n g sodium h y p o c h l o r i t e as r e a g e n t . The p r e v i o u s work on t h i s s u b j e c t s u g g e s t e d t h a t a p h y s i c o -c h e m i c a l s t u d y was r e q u i r e d i n o r d e r t o d e t e r m i n e t h e f a c t o r s t h a t govern t he o x i d a t i o n and d i s s o l u t i o n o f MoS 2 i n s o l u t i o n s o f NaOCl. The v a r i a b l e s s e l e c t e d f o r s t u d y were: a g i t a t i o n r a t e , t e m p e r a t u r e , pH, NaOCl c o n c e n t r a t i o n , s u r f a c e a r e a , n e u t r a l s a l t s , C 1 0 2~ and C10 3~ a d d i t i o n . A c o m p a r a t i v e s t u d y was made f o r l e a c h i n g r a t e s and t o t a l l e a c h i n g time between C u F e S 2 and MoS 2 under i d e n t i c a l e x p e r i m e n t a l con-d i t i o n s t o d e t e r m i n e i f s e l e c t i v e l e a c h i n g u s i n g NaOCl was p o s s i b l e . T h i s b e i n g s a t i s f a c t o r y i t was i n t e n d e d t o d e s i g n a s i m p l e f l o w s h e e t which would d e s c r i b e t h e p r a c t i c a l a p p l i c a t i o n o f t h i s t e c h n i q u e and a t th e same ti m e p r o v i d e a p r e l i m i n a r y economic b a s i s on which t o j u d g e t h e f e a s i b i l i t y o f the p r o c e s s . As a l r e a d y o u t l i n e d , p r e v i o u s workers had e s t a b l i s h e d t h e r e q u i r e m e n t o f p r o d u c i n g NaOCl i n - s i t u t o r e n d e r t h e p r o c e s s e c o n o m i c a l l y 17 18 competent, t h e r e f o r e t h e s e l e c t i o n o f a d i r e c t NaOCl l e a c h f o r o u r s t u d y , i n s t e a d o f a s i m u l t a n e o u s e l e c t r o o x i d a t i o n p r o c e d u r e , meant t h a t a v a i l a b l e d a t a on NaOCl g e n e r a t i n g systems would have t o be c o l l e c t e d . A n o t h e r s p e c i f i c o b j e c t i v e was t o e s t a b l i s h i f s o l u t i o n s o f NaOCl were s t a b l e under l e a c h i n g c o n d i t i o n s and i f any o f t h e m a t e r i a l s used o r p r o d u c t s o f r e a c t i o n formed c o u l d c a t a l y z e t h e d e c o m p o s i t i o n o f t h i s r e a g e n t . C h a p t e r 3 EXPERIMENTAL The e x p e r i m e n t a l work can be d i v i d e d i n t o f o u r s e c t i o n s : A. D e t e r m i n a t i o n o f t h e k i n e t i c s o f d i s s o l u t i o n o f m o l y b d e n i t e and c h a l c o p y r i t e i n NaOCl . s o l u t i o n s . B. Study o f the l e a c h i n g o f m o l y b d e n i t e from r o u g h e r c o p p e r c o n c e n t r a t e s . C. D e t e r m i n a t i o n o f the f l o t a t i o n c h a r a c t e r i s t i c s o f l e a c h e d r o u g h e r c o n c e n t r a t e s . D. E s t a b l i s h m e n t o f the s t a b i l i t y o f NaOCl s o l u t i o n s and f a c t o r s t h a t cause i t s d e c o m p o s i t i o n . 3.1 M a t e r i a l s a. M o l y b d e n i t e With a few e x c e p t i o n s , m o l y b d e n i t e used i n s e c t i o n A c o n s i s t e d o f a c l e a n e d m o l y b d e n i t e c o n c e n t r a t e (Brenda M i n e s ) . The c h e m i c a l a n a l y s i s o f t h e MoS 2 c o n c e n t r a t e i s g i v e n i n T a b l e 4. To p r e p a r e t h i s m a t e r i a l , t h e c o a r s e s t p a r t o f the MoS 2 c o n c e n t r a t e was s e p a r a t e d , washed i n a c e t o n e t o remove t h e o r g a n i c r e a g e n t s l e f t from t h e f l o t a t i o n o p e r a -t i o n , d r y e d a t 80°C and wet s c r e e n e d t o the s i z e -70 + 140 mesh. 19 20 T a b l e 4 Chemical A n a l y s i s o f M o l y b d e n i t e C o n c e n t r a t e Element Weight % Element Weight % T o t a l Molybdenum 52.57 Lead 0.02 Molybdenum ( o x i d e ) Mo0 3 0.099 I r o n 0.90 S u l f u r 37.37 C a l c i u m 0.19 Copper 0.59 Alumin a 0.65 S i 0 2 3.80 X-ray D i f f r a c t i o n P a t t e r n o f t h e M o l y b d e n i t e C o n c e n t r a t e o I/I i -- S p e c i e s I n d e n t i f i e d L i n e dA MoS 2 C u F e S 2 Mg 3 S n O i o ( O H ) 2 1 9.35 100 2 6.75 u n i d e n t i f i e d 3 6.18 100 4 4.67 30 5 3.11 80 6 3.06 100 7 2.73 10 8 2.60 30 9 2.72 20 10 2.05 20 11 1.86 80 12 1.82 25 13 1.54 60 80 14 1.34 20 15 1.23 5 16 1.08 30 17 1.03 20 Large specimens o f MoS 2 were p u r c h a s e d from David-New M i n e r a l s Providence, Utah and c o n s i s t e d o f l e a f y f ragments a d h e r i n g t o q u a r t z r o c k from which t h e y were e a s i l y s e p a r a t e d and i n some e x p e r i m e n t s ground t o f i n e powder o f c o m p o s i t i o n 99.8% MoS 2. 21 b. Copper Rougher C o n c e n t r a t e T h i s m a t e r i a l was s u p p l i e d by P l a c e r Development Inc. and was o b t a i n e d from the normal o u t p u t o f t h e r o u g h e r f l o t a t i o n o p e r a t i o n a t G i b r a l t a r ( B . C . ) . Chemical and x - r a y a n a l y s i s a r e p r e s e n t e d i n T a b l e 5. T a b l e 5(a) Chemical C o m p o s i t i o n o f Copper Rougher C o n c e n t r a t e T o t a l Copper 12.40% Molybdenum 9.3 % " S u l f u r 21.89% I r o n 32.29% " CaO 0.21% T a b l e 5(b) X-ray D i f f r a c t i o n P a t t e r n o f the Rougher C o n c e n t r a t e L i n e o dA I/I i - S p e c i e s I d e n t i f i e d C u F e S 2 F e S 2 1 3.43 40 2 3.40 u n i d e n t i f i e d 3 3.03 100 4 2.70 100 5 2.42 25 6 2.31 20 7 1.91 15 8 1.87 40 9 1.85 70 10 1.75 40 11 1.59 40 60 12 1.50 5 13 1.07 u n i d e n t i f i e d 22 c. C h a l c o p y r i t e The p a r t i c u l a t e m a t e r i a l was a Phoenix c o n c e n t r a t e c o n s i s t i n g a l m o s t e n t i r e l y o f c h a l c o p y r i t e . No p r e p a r a t i o n t r e a t m e n t was r e q u i r e d . Some e x p e r i m e n t s i n v o l v e d u s i n g m a s s i v e c h a l c o p y r i t e f o r which samples were c u t and mounted i n Quik-mount and then p o l i s h e d . d. Sodium H y p o c h l o r i t e The sodium h y p o c h l o r i t e used i n a l l e x p e r i m e n t s was s u p p l i e d by Chernetics I n t e r n a t i o n a l and was produced i n an e x p e r i m e n t a l membrane t y p e h y p o c h l o r i t e g e n e r a t o r . The v a r i o u s b a t c h e s o b t a i n e d v a r i e d o v e r a wide range o f c o n c e n t r a t i o n s o f both NaOCl and NaCI. In a l l c a s e s t h e pH was o v e r 12. 3. O t h e r M a t e r i a l s A l l o t h e r c h e m i c a l s used were r e a g e n t grade q u a l i t y . 3.2 A p p a r a t u s A l l d i s s o l u t i o n e x p e r i m e n t s were done i n a 1 I. g l a s s v e s s e l e q u i p p e d w i t h f o u r T e f l o n b a f f l e s f i x e d t o t h e w a l l o f the r e a c t o r a t e q u a l s p a c i n g s . A g i t a t i o n was p r o v i d e d by a f o u r b l a d e r a d i a l f l o w t u r b i n e . Both v e s s e l and a g i t a t o r d i m e n s i o n s were d e t e r m i n e d u s i n g g e o m e t r i c a l r e l a t i o n s t h a t c o r r e s p o n d t o S t a n d a r d Tank C o n f i g u r a t i o n [ 2 7 ] . S t i r r i n g r a t e was c o n t r o l l e d w i t h a F i s h e r Dyna-Mix. The v e l o c i t y o f t h e t u r b i n e was measured u s i n g a T e c l o c k hand tachometer. 23 Temperature u n i f o r m i t y was o b t a i n e d by p l a c i n g t h e r e a c t o r i n a water bath heated e l e c t r i c a l l y and c o n t r o l l e d by a Thermistemp YSI Model 71 t e m p e r a t u r e c o n t r o l l e r . The t e m p e r a t u r e was m a i n t a i n e d w i t h i n ±0.5°C o f the r e q u i r e d v a l u e . 3.3 P r o c e d u r e E x p e r i m e n t s i n s e c t i o n s A.and B were done i n t h e f o l l o w i n g way: A r e q u i r e d volume o f c o n c e n t r a t e d s t o c k NaOCl was added t o a p p r o x i m a t e l y 800 ml o f H 20 and t h e pH lowered t o t h e r e q u i r e d v a l u e f o r t h e p a r t i c u l a r e x p e r i m e n t . A weighed amount o f b u f f e r components (Na 2C0 3/NaHC0 3) was i n t r o d u c e d and t h e s o l u t i o n made up t o 1000 ml. The NaOCl s o l u t i o n was t r a n s f e r r e d t o the r e a c t o r and i n t r o -duced i n t h e wa t e r b a t h , slow a g i t a t i o n was i n d u c e d u n t i l t h e r e a g e n t r e a c h e d t h e e x p e r i m e n t t e m p e r a t u r e . A t t h i s t ime a g i t a t i o n was i n c r e a s e d , the f i r s t sample e x t r a c t e d f o r NaOCl a n a l y s i s and b l a n k s f o r molybdenum and copper a n a l y s i s . , t h e MoS 2 sample t o be l e a c h e d i n t r o d u c e d and t h e chronometer s t a r t e d . Samples were withdrawn from t he r e a c t o r w i t h a p i p e t t e and f i l t e r e d u s i n g a f r i t t e d g l a s s tube o r a f i l t e r paper. The pH was measured u s i n g an Expandomatic pH-meter. For t h e c o m p a r a t i v e l e a c h i n g e x p e r i m e n t s w i t h c h a l c o p y r i t e and m o l y b d e n i t e , both m i n e r a l s were wet s c r e e n e d t o a s i z e f r a c t i o n -140 + 200 mesh and examined m i c r o s c o p i c a l l y t o d e t e r m i n e t h e i r shape and s u r f a c e c o n d i t i o n s . Based on t h i s i n f o r m a t i o n a shape f a c t o r was s e l e c t e d and u s i n g a nomogram f o r t he D e t e r m i n a t i o n o f S p e c i f i c S u r f a c e s [28] t h e i r s u r f a c e a r e a was d e t e r m i n e d . 24 E x p e r i m e n t s i n s e c t i o n C were done a t P l a c e r Development and c o n s i s t e d o f s t a n d a r d f l o t a t i o n t e s t s on samples o f l e a c h e d r o u g h e r con-c e n t r a t e s t o d e t e r m i n e t h e i r f l o t a b i l i t y and r e a g e n t consumption. 3.4 Ch e m i c a l A n a l y s i s a. Molybdenum Molybdenum was d e t e r m i n e d by a c o l o r i m e t r i c method [29] u s i n g the brown-red H 2 Mo02(CNS) 3 complex. The o p t i c a l d e n s i t y was measured u s i n g a Beckman Model B S p e c t r o p h o t o m e t e r s e t a t a w a v e l e n g t h o f 460 my. The c o n c e n t r a t i o n o f Mo was r e a d d i r e c t l y f r o m a c a l i b r a t i o n c u r v e p r e p a r e d by u s i n g s t a n d a r d molybdenum s o l u t i o n s . When copper b e a r i n g m a t e r i a l s were l e a c h e d s i m u l t a n e o u s l y t he a n a l y s i s o f molybdenum was done by a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y u s i n g a wa v e l e n g t h o f 313.3 nm. To overcome any i n t e r f e r e n c e sample s o l u t i o n s were made up t o be 10% i n aluminum c h l o r i d e and 5% i n ammonium c h l o r i d e . b. Copper Copper i n l e a c h i n g s o l u t i o n s was d e t e r m i n e d by a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y u s i n g t he l i n e o f wa v e l e n g t h 324.7 nm. In a n a l y z i n g s o l i d m a t e r i a l s t h e d e t e r m i n a t i o n o f c o p p e r was done e l e c t r o g r a v i m e t r i c a l l y i n which c a s e t h e i n t e r f e r e n c e o f molybdenum was n u l l i f i e d by a r e d o p o s i -t i o n o f c o p p e r . The d i s s o l u t i o n o f m o l y b d e n i t e , c h a l c o p y r i t e and c o p p e r r o u g h e r c o n c e n t r a t e s f o r t h e purpose o f a n a l y s i s , was found t o be com p l e t e 25 f o l l o w i n g t r e a t m e n t w i t h a m i x t u r e o f c o n c e n t r a t e d n i t r i c a c i d and p o t a s s i u m c h l o r a t e a t low t e m p e r a t u r e . . c. Sodium H y p o c h l o r i t e D e t e r m i n a t i o n o f sodium h y p o c h l o r i t e was done u s i n g the KI-T h i o s u l f a t e method a c c o r d i n g t o ASTM-D-62-T [ 3 0 ] . P o t a s s i u m i o d i d e was added i n e x c e s s t o an a c i d i f i e d sample and t h e h y p o c h l o r i t e i m m e d i a t e l y t i t r a t e d u s i n g a 0.1 N N a 2 S 2 0 3 s o l u t i o n u n t i l t h e d i s a p p e a r a n c e o f t h e s t a r c h b l u e c o l o u r as e n d - p o i n t . T h i s method does not d i f f e r e n t i a t e between ( i n - i o n i z e d NaOCl and i o n i z e d h y p o c h l o r i t e O C l " , nor c h l o r i n e d i o x i d e C 1 0 2 . T h e r e f o r e a p o t e n t i o m e t r i c method was used f o r some e x p e r i m e n t s t h a t r e q u i r e d more p r e c i s e d a t a . T h i s method [31] i s based on d i f f e r e n t r e d u c i n g r a t e s o f h y p o c h l o r i t e , and c h l o r i t e by a r s e n i t e . The r e a c t i o n w i t h c h l o r i t e i s slow and may be d e l i b e r a t e l y a c c e l e r a t e d by a d d i t i o n o f OsO.* which works as a c a t a l y s t . C h l o r a t e i s d e t e r m i n e d on the same sample by b r o m a t o m e t r i c t i t r a t i o n and c h l o r i d e may be t i t r a t e d u s i n g AgN0 3. I t was t h e r e f o r e p o s s i b l e t o d e t e r m i n e t h e c o n c e n t r a t i o n o f O C l _ + HC10 by h o l d i n g the sample o f pH 12 and then d r o p p i n g the pH t o 8.5 t o d e t e r m i n e t h e c o n c e n t r a t i o n o f C 1 0 2 . C h a p t e r 4 RESULTS AND OBSERVATIONS 4.1 O x i d a t i o n o f M o l y b d e n i t e by Sodium H y p o c h l o r i t e 4.1.1 R e p r o d u c i b i l i t y The f a c t o r s t h a t c o u l d a f f e c t t h e r e p r o d u c i b i l i t y were: a ) R e a c t i v i t y o f r e a c t o r m a t e r i a l t o NaOC l s o l u t i o n s . b ) A n a l y t i c a l p r o c e d u r e s f o r Mo a n d N a O C l . To t e s t i f any o f t h e r e a c t o r m a t e r i a l s ( b a f f l e s o f T e f l o n and T i s t i r r e r c o v e r e d w i t h a f i l m o f M i c r o s t o p ) r e a c t e d w i t h NaOCl a run was made o f 40°C u s i n g a s o l u t i o n w i t h an i n i t i a l c o n c e n t r a t i o n o f 5 g/1 b u f f e r e d a t pH 9.3. Four samples were removed e v e r y 30 m i n u t e s and a n a l y z e d f o r NaOCl. The c o n c e n t r a t i o n o f NaOCl remained c o n s t a n t d u r i n g t h e e n t i r e run i n d i c a t i n g t h a t t h e m a t e r i a l s used n e i t h e r r e a c t n o r c a t a l y t i c a l l y decompose t h e r e a g e n t . The c o n s i s t e n c y o f sample e x t r a c t i o n and a n a l y s i s were t e s t e d by r u n n i n g two e x p e r i m e n t s under i d e n t i c a l c o n d i t i o n s . The m o l y b d e n i t e used i n t h e s e e x p e r i m e n t s was t h e pure m a t e r i a l o b t a i n e d by g r i n d i n g the m a s s i v e specimens. The r e s u l t s o f Mo d i s s o l u t i o n and NaOCl consumption a r e shown i n F i g u r e 2 showing good agreement f o r both r u n s . 26 o o run I * * run 2 5 8 10 TIME (min) 15 FIG- 2 - REPRODUCIBILITY OF THE RESULTS FOR THE LEACHING OF MOLYBDENITE PARTICLES. 28 4.1.2 E f f e c t o f S t i r r i n g Rate I t was e s t a b l i s h e d i n p r e l i m i n a r y e x p e r i m e n t s t h a t t h e r a t e o f r e a c t i o n between MoS 2. and NaOCl s o l u t i o n s was dependent on t h e a g i t a t i o n . F i n a l l y a b a f f l e d v e s s e l was made and e x p e r i m e n t s were c a r r i e d o u t v a r y i n g t h e r a t e o f t h e t u r b i n e a g i t a t o r w h i l e m a i n t a i n i n g c o n s t a n t t h e o t h e r v a r i a b l e s . Runs were made a d d i n g 0.3 g o f MoS 2 c o n c e n t r a t e t o 1000 ml o f a s o l u t i o n c o n t a i n i n g 5.1 g/1 o f NaOCl b u f f e r e d a t pH 9.6 by a 0.1 N Na 2C0 3-0.1 N NaHC0 3 s o l u t i o n . The t e m p e r a t u r e o f a l l e x p e r i m e n t s was 35°C ± 0.5°C. R e s u l t s o f t h e e x t r a c t i o n r a t e s o f molybdenum i n t o s o l u t i o n a r e p r e s e n t e d i n F i g u r e 3 f o r 5 a g i t a t i o n s peeds. I t was c o n c l u d e d from the above r e s u l t s t h a t t h e r e a c t i o n r a t e was ind e p e n d e n t o f s t i r r i n g above 750 rpm ( F i g u r e 4 ) . 4.1.3 E f f e c t o f Temperature The r e a c t i o n was s t u d i e d o v e r t h e t e m p e r a t u r e range 9-70°C. The e x p e r i m e n t a l c o n d i t i o n s f o r t h e s e t e s t s were: pH o f b u f f e r e d s o l u t i o n = 9; s t i r r i n g r a t e = 780 r.p.m. C o n c e n t r a t i o n .of NaOCl = 8 . 3 5 g/1; amount o f MoS 2 c o n c e n t r a t e added = 0.3g. The e x t r a c t i o n c u r v e s a r e p r e s e n t e d i n \ F i g u r e 5. An A r r h e n i u s p l o t t a k e n f o r a c o n c e n t r a t i o n o f 15 mg/1 Mo was c o n s t r u c t e d . The s l o p e o f t h i s l i n e ( F i g u r e 6) was found t o be l i n e a r o v e r t h e e n t i r e t e m p e r a t u r e range, and t h e e x p e r i m e n t a l a c t i v a t i o n e n e r g y was c a l c u l a t e d t o be 6.3 ± 0.8 k c a l / m o l e . 29 5 10 15 20 TIME (MIN) FIG-3- EFFECT OF AGITATION ON THE RATE OF OXIDATION-30 w , 1 1 1 5 6 7 8 RPM x 100 FIG- 4- EFFECT OF AGITATION ON THE RATE OF OXIDATION OF MOLYBDENITE-31 FIG. 5 - EFFECT OF TEMPERATURE ON THE OXIDATION OF MOLYBDENITE . 32 1.2 2.9 3.1 33 35 l/T(°K) x IOOO FIG. 6 - ARRHENIUS PLOT OF TEMPERATURE DATA. 33 T h i s v a l u e f o r t h e a c t i v a t i o n energy i s i n good agreement w i t h t h e e x p e r i m e n t s done by Ioardanov and Z e l i k m a n [14] u s i n g compressed MoS 2 powder specimens i n the t e m p e r a t u r e range 20-80°C g i v i n g a v a l u e o f 5.25 k c a l / m o l e . 4.1.4 E f f e c t o f S u r f a c e A r e a Two e x p e r i m e n t s were done v a r y i n g t h e amount o f MoS 2 p a r t i c l e s o n l y and m a i n t a i n i n g c o n s t a n t t h e o t h e r p a r a m e t e r s . As shown i n F i g u r e 7 t h e r a t e o f e x t r a c t i o n o f m o l y b d e n i t e was found t o v a r y d i r e c t l y w i t h t h e s u r f a c e a r e a . T h i s i n d i c a t e s t h a t t h e r a t e o f l e a c h i n g i s c o n t r o l l e d by a heterogeneous r e a c t i o n on t h e s u r f a c e o f t h e MoS 2 o r by d i f f u s i o n o f t h e h y p o c h l o r i t e o r molybdate i o n s t h r o u g h t h e s o l u t i o n boundary l a y e r on t h e s u r f a c e o f t h e MoS 2. 4.1.5 MoS 2 C r y s t a l i n i t y and S u r f a c e C h a r a c t e r i s t i c s L e a c h i n g o f MoS 2 p a r t i c l e s would be e x p e c t e d t o be a n i s o t r o p i c s i n c e t h e y c o n s i s t o f a l a y e r e d s t r u c t u r e ( F i g u r e 8 ) . In each l a y e r Mo(IV) atoms a r e s u r r o u n d e d by s i x S atoms f o r m i n g a t r i a n g u l a r p r i s m c o n f i g u r a t i o n s h a r i n g v e r t i c a l edges w i t h one a n o t h e r t o form S-Mo-S p l a n e s normal t o the c - a x i s . The r e p e t i t i o n o f t h e s e complete l a y e r s a c c o r d i n g t o r e q u i r e m e n t s o f hexagonal c l o s e p a c k i n g r e s u l t s i n a MoS 2 c r y s t a l ( F i g u r e 9 ) . The bonding w i t h i n each l a y e r i s about 80% c o v a l e n t and t h e m e t a l l i c l u s t e r i t has, i s a t t r i b u t e d t o some m e t a l l i c bonding [ 3 2 ] . 34 NoOCI = 5 g/l T= 30° C pH= 8.5 £ Q UJ I-O < or X U J 200} I0C4 0.6 g. • — 0.3 g. TIME (min) FIG. 7 - EFFECT OF SURFACE AREA ON THE RATE OF OXIDATION OF MOLYBDENITE. 35 FIG. 9 - MOLYBDENITE STRUCTURE 37 The l a y e r s a r e h e l d t o g e t h e r by Van der-Waal bonds y i e l d i n g l a r g e S-S i n t e r l a y e r d i s t a n c e s and p e r m i t t i n g t h e d i s t i n c t i v e c l e a v a g e a l o n g t h e (0001) p l a n e . T h e r e f o r e MoS 2 p a r t i c l e s have two t y p e s o f s u r f a c e s each w i t h d i f f e r e n t c h e m i c a l a c t i v i t y . Those formed from r u p t u r e o f Van der-Waal bonds a r e c a l l e d " f a c e s " . t h e y a r e h y d r o p h o b i c and a r e c o n s i d e r e d i n a c t i v e and t h e ones formed from b r e a k i n g S-Mo c o v a l e n t bonds, c a l l e d edges, which a r e h y d r o p h i l i c and c h e m i c a l l y a c t i v e . However e x a m i n a t i o n o f t h e " f a c e s " o f a r e c e n t l y c l e a v e d MoS 2 p a r t i c l e w i t h t h e S.E.M. r e v e a l s a s u r f a c e which has m u l t i p l e s t e p l i k e hexagonal p i t s as shown i n F i g u r e 10, i n d i c a t i n g t h a t t h e " a c t i v e " s u r f a c e which can c o n t r i b u t e t o l e a c h i n g i s much g r e a t e r than t h e s u r r o u n d i n g "edge" s u r f a c e o f t h e p a r t i c l e s . 4.1.6 Morphology o f MoS 2 L e a c h i n g C o a r s e m o l y b d e n i t e c o n c e n t r a t e (-70 + 140 mesh) was l e a c h e d a t room t e m p e r a t u r e (26°C) i n a s o l u t i o n o f 5 g/1 NaOCl and s o l i d s were removed a f t e r 5, 10, 15, 20 and 30 m i n u t e s . A comparison between t h e s u r f a c e o f t h e p a r t i c l e s l e a c h e d f o r d i f f e r e n t p e r i o d s i s p r e s e n t e d i n F i g u r e 11. The u n l e a c h e d p a r t i c l e s a r e smooth and do not p r e s e n t t h e p i t s o f a normal MoS 2 p a r t i c l e such as shown i n F i g u r e 10, t h i s i s l i k e l y t o be due t o the g r i n d i n g a c t i o n d u r i n g s i z e r e d u c t i o n . However, i m m e d i a t e l y a f t e r l e a c h i n g b e g i n s t h e appearance o f such a c t i v e s i t e s i s o b s e r v e d and t h e y c o n t r i b u t e l a r g e l y t o t h e r e a c t i o n as can be o b s e r v e d a t a h i g h e r m a g n i f i c a t i o n i n F i g u r e 12. I t can a l s o be o b s e r v e d t h a t a f t e r FIG. 10- VIEW OF THE "FACE" SURFACE MoS 2 39 FIG. 12 - MoS 2 PARTICLES AFTER 10 MIN. OF REACTION IN NaOCl . 41 30 minutes o f l e a c h i n g under moderate c o n d i t i o n s o f c o n c e n t r a t i o n and te m p e r a t u r e , MoS 2 p a r t i c l e s have d i s s o l v e d a l m o s t c o m p l e t e l y . The r e a c t i o n o f h y p o c h l o r i t e on MoS 2 o c c u r s t h e r e f o r e n o t o n l y on t h e "edges" o f t h e p a r t i c l e s but a l s o on t h e s t e p s o f hexagonal p i t s formed on t h e " f a c e s " o f t h e p a r t i c l e s and i s a i d e d by a f r e e f l o w o f r e a g e n t deep i n t o t h e MoS 2 p a r t i c l e s t h r o u g h i n t e r l a y e r s p a c i n g s u c h as can be o b s e r v e d i n F i g u r e 12c. M o l y b d e n i t e may have up t o a p p r o x i m a t e l y 5% Mo0 3 on i t s s u r f a c e due t o d r y o x i d a t i o n . Under a l k a l i n e c o n d i t i o n s o n l y , Mo0 3 and o t h e r non-s t o i c h i o m e t r i c o x i d e s a r e s o l u b l e . I t has been r e p o r t e d [ 3 3 ] t h a t some Mo0 2 may a l s o be p r e s e n t on t h e MoS 2 s u r f a c e i n v e r y s m a l l amounts and i s not s o l u b l e under a l k a l i n e c o n d i t i o n s . 4.1.7 E f f e c t o f NaOCl C o n c e n t r a t i o n on t h e Rates o f Mo E x t r a c t i o n Samples o f m o l y b d e n i t e were l e a c h e d a t 45°C i n s o l u t i o n s o f NaOCl b u f f e r e d w i t h Na 2C0 3-NaHC0 3 a t pH 9 o f d i f f e r e n t i n i t i a l c o n c e n t r a -t i o n s . The e x t r a c t i o n c u r v e s a r e p l o t t e d i n F i g u r e 13 and i n d i c a t e t h a t t h e r e a c t i o n r a t e s i n c r e a s e w i t h h i g h e r i n i t i a l NaOCl c o n c e n t r a t i o n s . A p l o t o f r e a c t i o n r a t e s a t 15 mg / l t Mo a g a i n s t i n i t i a l NaOCT c o n c e n t r a t i o n ( F i g u r e 14) g i v e s a s t r a i g h t l i n e o f s l o p e 2 one 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 between MoS 2 and NaOCl i s f i r s t o r d e r w i t h r e s p e c t t o sodium h y p o c h l o r i t e . T h i s i s i n agreement w i t h p r e v i o u s work r e p o r t e d by Ioardanov and Z e l i k m a n [14] and Bhappu et al. [ 6 ] . 5 10 15 20 TIME (min) Fl G 13 - EFFECT OF NoOCI CONCENTRATION ON THE OXIDATION OF MOLYBDENITE • INITIAL NaOCl CONCENTRATION (g/1) FIG. 14 - PLOT OF REACTION RATES VS. INITIAL HYPOCHLORITE CONCENTRATION . 44 4.1.8 E f f e c t o f pH on the Rate o f R e a c t i o n The e f f e c t o f [ H + ] on the r a t e o f the r e a c t i o n between MoS 2 and NaOCl was s t u d i e d o v e r t h e pH range o f 6.7 t o 12. T h i s range was s e l e c t e d on t h e b a s i s t h a t NaOCl s o l u t i o n s a r e s t o r e d a t a pH o f 12 t o p r e v e n t any d e c o m p o s i t i o n and t h a t t h e y a r e s t a b l e o n l y i n a l k a l i n e s o l u t i o n s . In t h e s e e x p e r i m e n t s t h e pH was m a i n t a i n e d e i t h e r by p r e p a r i n g a b u f f e r s o l u t i o n ( h i g h e r pH's) o r by a d d i n g N a 2 C 0 3 t o t h e s o l u t i o n s a t i n t e r v a l s i n o r d e r t o keep t he s o l u t i o n s w i t h i n a narrow pH r e g i o n . I t was o b s e r v e d t h a t MoS 2 d i s s o l v e s f a s t e r w i t h i n c r e a s i n g pH up t o a p p r o x i m a t e l y pH 8.5 and t h a t t h e r a t e s d e c r e a s e f o r pH's above 10 ( F i g u r e 15). A p l o t o f r e a c t i o n r a t e s a g a i n s t pH o f t h e s o l u t i o n ( F i g u r e 16) i n d i c a t e s a peak between pH 8.3 and 9.8 i n which t he r e a c t i o n r a t e i s a maximum. T h i s o b s e r v a t i o n has n o t been r e p o r t e d by p r e v i o u s w orkers a l t h o u g h t h e same r e g i o n o f pH was s e l e c t e d f o r working p r o b a b l y due t o t h e s t a b i l i t y o f the NaOCl a t t h i s pH. Ioardanov and Z e l i k m a n [ 1 4] r e p o r t e d f i n d i n g no e f f e c t on t h e r a t e o f o x i d a t i o n when the c o n c e n t r a t i o n o f NaOH was above t h e t h e o r e t i c a l l y r e q u i r e d amount. 4.1.9 E f f e c t o f S u l f a t e C o n c e n t r a t i o n on t h e R e a c t i o n Rate When t h e r e i s s u f f i c i e n t h y p o c h l o r i t e a l l t h e s u l f u r i n the MoS 2 w i l l be o x i d i z e d t o S0i+~. T h e r e f o r e t h e e f f e c t o f s u l f a t e on t h e r e a c t i o n r a t e was i n v e s t i g a t e d . 5 10 15 20 TIME (min) FIG. 15 - EFFECT OF pH ON THE RATE OF OXIDATION OF MOLYBDENITE. 46 FIG. 1 6 - PLOT OF REACTION RATES VS. pH. 47 E x p e r i m e n t s were s e t up m a i n t a i n i n g i d e n t i c a l r e a c t i o n c o n d i -t i o n s by a d d i n g an e x c e s s amount o f Na2S04. t o t h e s o l u t i o n b e f o r e i n t r o d u c -i n g the"MoS 2. Two runs w i t h one and t e n ti m e s t h e s t o i c h i o m e t r i c amount o f SOtr t o be produced a r e compared i n F i g u r e 17 w i t h an e x t r a c t i o n c u r v e n o t c o n t a i n i n g any i n i t i a l SOir • The s l i g h t v a r i a t i o n o f r e a c t i o n r a t e s a r e w i t h i n e x p e r i m e n t a l e r r o r and t h e i n c r e a s e i n [SOi^] can be r e g a r d e d as h a v i n g no e f f e c t on t h e r e a c t i o n . S i n c e t h e [ S C \ = ] examined i s much l a r g e r t h a n what would be produced i n a MoS2 l e a c h , i t may be c o n c l u d e d t h a t t h e Na 2S0it formed i n the c o u r s e o f t h e r e a c t i o n i s a l s o w i t h o u t e f f e c t . 4.1.10 E f f e c t o f NaCl on the R e a c t i o n Rate Sodium c h l o r i d e i s always p r e s e n t i n s o l u t i o n s o f NaOCl i n g r e a t e r o r s m a l l e r c o n c e n t r a t i o n s a c c o r d i n g t o the method used t o g e n e r a t e i t , and t h e s a l t u t i l i z a t i o n i f i t i s produced by NaCl e l e c t r o l y s i s . I t i s a l s o formed as a p r o d u c t o f t h e r e a c t i o n between NaOCl and MoS 2 and i t was t h e r e f o r e c o n s i d e r e d i m p o r t a n t t o e s t a b l i s h i t s e f f e c t i f any upon t h e r e a c t i o n r a t e . To t h i s end an e x p e r i m e n t was c o n d u c t e d a d d i n g 50 g o f NaCl t o a s o l u t i o n o f 7.9 g/1 NaOCl a l r e a d y c o n t a i n i n g 70 g/1 o f NaCl. MoS 2 c o n c e n t r a t e was l e a c h e d i n t h i s s o l u t i o n and compared t o a run under i d e n t i c a l c o n d i t i o n s w i t h no NaCl-added:. No e f f e c t w hatsoever was d e t e c t e d on t h e r e a c t i o n r a t e . 5 10 15 20 TIME (min) FIG. 17 - EFFECT OF SULFATES ON THE OXIDATION OF MOLYBDENITE \ 49 4.1.11 E f f e c t o f C102 and C10 3 on t h e Rate o f R e a c t i o n S o l u t i o n s o f sodium h y p o c h l o r i t e a r e u n s t a b l e t o a c e r t a i n d e gree even i n t h e a l k a l i n e r e g i o n and decompose i n t o sodium c h l o r a t e and sodium c h l o r i d e . Most o f t h e mechanisms proposed t o e x p l a i n i t s d e c o m p o s i t i o n show the f o r m a t i o n o f C 1 0 2 i o n s as an i n t e r m e d i a t e s t e p . E x p e r i m e n t s were done t o e s t a b l i s h i f t h e s e s p e c i e s have any e f f e c t on t h e r a t e o f o x i d a t i o n o f MoS 2 by NaOCl. S o l u t i o n s o f NaOCl o f 8.0 g/1 were p r e p a r e d and b u f f e r e d a t pH 9 and d i f f e r e n t amounts o f NaC10 2 and N a 2 C 1 0 3 were added and the r a t e s o f e x t r a c t i o n compared. The i n f o r m a t i o n o b t a i n e d was p l o t t e d i n F i g u r e 18. I t can be o b s e r v e d t h a t C 10 2~ i o n s have no e f f e c t on t h e r a t e s o f e x t r a c t i o n o f Mo. The p r e s e n c e o f C10 3~ however lowers t h e r a t e o f Mo e x t r a c t i o n . In both c a s e s t h e amounts added were f a r i n e x c e s s o f what would be e x p e c t e d from t h e normal d e c o m p o s i t i o n o f NaOCl under t h e c o n d i t i o n s o f o u r l e a c h i n g s t u d i e s and t h e r e f o r e t h e e f f e c t s o b s e r v e d would not h i n d e r t h e l e a c h i n g r e a c t i o n . The p r e s e n c e o f c h l o r a t e would be hazardous o n l y i n the c a s e i n which s o l u t i o n s o f NaOCl had been s t o c k e d f o r l o n g p e r i o d s o f time under u n f a v o u r a b l e c o n d i t i o n s . 4.1.12 E f f e c t o f Molybdate Ions on t h e Rates o f R e a c t i o n Two e x p e r i m e n t s a re r e p o r t e d i n t h i s s e c t i o n . In the f i r s t one the e f f e c t o f [ M o O ^ ] on t h e r a t e o f r e a c t i o n was d e t e r m i n e d a t a f i x e d pH v a l u e . In t h e second, t h e e f f e c t o f MoS 2 p a r t i c l e s on-the [ M o 0 4 _ ] was e s t a b l i s h e d f o r d i f f e r e n t 0H~ c o n c e n t r a t i o n s . ( 5 0 5 10 15 20 TIME (min) FIG. 18- EFFECT OF CHLORITE AND CHLORATE ON THE OXIDATION OF MOLYBDENITE. 51 As mentioned i n 1.5, i t has been r e p o r t e d t h a t i n the p r e s e n c e o f e x c e s s o x i d i z i n g agent i n a l k a l i n e s o l u t i o n s t h e Mo(IV) i n MoS 2 i s e n t i r e l y o x i d i z e d t o (VI) i n t h e MoCV - i o n s . I t was t h e r e f o r e d e c i d e d t o i n v e s t i g a t e i f t h e p r e s e n c e o f l a r g e r c o n c e n t r a t i o n s o f MoO^ a f f e c t e d t h e r e a c t i o n . A run was made u s i n g a b u f f e r e d s o l u t i o n (pH 9.5) o f 7.5 g/1 NaOCl a t 35°C, l e a c h i n g 0.3 g MoS 2 c o a r s e p a r t i c l e s i n t h e p r e s e n c e o f an i n i t i a l [M0O4. ] f i v e t i m e s t h e amount t h a t would be produced by t h e t o t a l r e a c t i o n o f t h e MoS 2 employed. The., d i s s o l u t i o n c u r v e s were th e n compared t o an i d e n t i c a l run w i t h no i n i t i a l MoO^ - p r e s e n t . In t h i s e x p e r i m e n t MoOi*- i o n s e v i d e n t l y had no e f f e c t on t h e l e a c h i n g r e a c t i o n . From the s t o i c h i o m e t r y o f r e a c t i o n (3) i t c o u l d be e x p e c t e d t h a t t h e r a t e o f t h e r e a c t i o n s h o u l d depend on the c o n c e n t r a t i o n o f h y d r o x i d e ; however e x p e r i m e n t s r e p o r t e d i n 4.1.8 i n d i c a t e an i n c r e a s e i n r a t e s o f e x t r a c t i o n up t o pH 8.5 but a d e c r e a s e i n the r e a c t i o n r a t e s when t h e pH exceeds 10.5. C o n s e q u e n t l y i t was d e c i d e d t o examine i f t h e r e was any r e a c t i o n between Mo04~ i o n s and MoS2 p a r t i c l e s which would d e c r e a s e the molybdate c o n c e n t r a t i o n a t h i g h e r pH's. T h r e e runs were made w i t h s o l u t i o n s o f 10 g/1 Na 2Mo0i f a t 26°C w i t h t h e pH c o n t r o l l e d a t 8.5, 10.5 and 11.5. In a l l c a s e s 50 g o f MoS2 f i n e c o n c e n t r a t e were added. In F i g u r e 19 i t can be o b s e r v e d t h a t w h i l e t h e [MO0L>~] remains c o n s t a n t a f t e r 60 min. a t the lower pH, t h e runs a t pH 10.5 and 11.5 i n d i c a t e a d e c r e a s e i n t h e [MoOiT] i n t h e f i r s t 20 min. and th e n t h e c o n c e n t r a t i o n remains c o n s t a n t . The o c c u r r e n c e o f a r e a c t i o n was a l s o P 8 + o Si or h-LU O O o 5{ _ A o o pH 8.5 * ^ 10.5 • — . . „ 5 T = 2 6 ° C 10 20 30 TIME (min) -A • • 40 50 60 FIG. 19 - EFFECT OF MOLYBDATES - MoS2 INTERACTION 53 o b s e r v a b l e when an a d d i t i o n a l run was made w i t h an i n i t i a l pH o f 10.5 and t he s o l u t i o n was n o t b u f f e r e d . T h e r e was a d e c r e a s e i n t h e pH i n th e f i r s t 30 min. o f r e a c t i o n . No d i f f e r e n c e i n the s u r f a c e o f the MoS 2 p a r t i c l e s were o b s e r v e d a f t e r t h e r e a c t i o n ; the o b s e r v a t i o n s i n t h i s s e c t i o n however, i n d i c a t e t h a t t h e r e i s some i n t e r a c t i o n between t h e Mo0 4 i n s o l u t i o n and the MoS 2 p a r t i c l e s a t h i g h e r 0H~ c o n c e n t r a t i o n s which r e s u l t s i n a d e c r e a s e o f t h e [MoG\ =]. 4.2 O x i d a t i o n o f C h a l c o p y r i t e by Sodium H y p o c h l o r i t e S i n c e c h a l c o p y r i t e was t h e major copper m i n e r a l i n t h e r o u g h e r c o n c e n t r a t e and because a p p a r e n t l y i t was an i m p o r t a n t c o n t r i b u t o r t o t h e e x c e s s consumption o f h y p o c h l o r i t e as d e t e r m i n e d from p r e l i m i n a r y l e a c h i n g e x p e r i m e n t s o f t h e r o u g h e r c o n c e n t r a t e , i t was d e c i d e d t o i n v e s t i g a t e t h e e x t e n t o f i t s r e a c t i o n w i t h s o l u t i o n s o f sodium h y p o c h l o r i t e . 4.2.1 L e a c h i n g o f C h a l c o p y r i t e C o n c e n t r a t e Samples o f a Phoenix c h a l c o p y r i t e ground t o -200 mesh were l e a c h e d i n s o l u t i o n s o f sodium h y p o c h l o r i t e under s i m i l a r c o n d i t i o n s as t h o s e f o r t h e b e s t e x t r a c t i o n from m o l y b d e n i t e c o n c e n t r a t e s . In each run t h e c o n c e n t r a t i o n o f c o p p e r i o n s and sodium h y p o c h l o r i t e was d e t e r m i n e d . In a l l c a s e s i t was o b s e r v e d t h a t t h e r e a c t i o n was accompanied by a d e c r e a s e i n t h e [ H + ] . Some runs were then r e p e a t e d m a i n t a i n i n g t h e pH c o n s t a n t by b u f f e r i n g t h e s o l u t i o n s . 54 The r e s u l t s o f one run a r e p r e s e n t e d i n F i g u r e 20. I t can be o b s e r v e d t h a t t h e c o n c e n t r a t i o n o f Cu i n c r e a s e d r a p i d l y and t h e n remained c o n s t a n t t h r o u g h t h e e n t i r e e x p e r i m e n t . In c o n t r a s t the c o n c e n t r a t i o n o f NaOCl d e c r e a s e d c o n t i n u o u s l y a l m o s t r e a c h i n g z e r o a f t e r 24 h o u r s . In t h i s run t h e pH had d e c r e a s e d i n the l a s t 12 hours and t h e d e c r e a s e i n h y p o c h l o r i t e c o n c e n t r a t i o n may have been i n f l u e n c e d by t h e h i g h e r H + c o n c e n t r a t i o n . As was o b s e r v e d i n a l l o t h e r l e a c h i n g e x p e r i m e n t s u s i n g c h a l c o p y r i t e , i t s s u r f a c e was s l i g h t l y darkened by t h e r e a c t i o n . T h i s a p p a r e n t c o a t i n g was e a s i l y removed by d i l u t e s u l f u r i c a c i d . 4.2.2 L e a c h i n g o f M a s s i v e Specimens To i n v e s t i g a t e i f t h e consumption o f NaOCl was due t o any t o p o t a c t i c r e a c t i o n w i t h i n t h e c h a l c o p y r i t e p a r t i c l e s , l a r g e C u F e S 2 specimens were l e a c h e d and t h e i r c r o s s - s e c t i o n s l a t e r examined u s i n g t h e e l e c t r o n m i c r o p r o b e . In p r e p a r a t i o n f o r t h e s e t e s t s t h e C u F e S 2 specimens were p r e -v i o u s l y p o l i s h e d and a r e a s f r e e o f f o r e i g n i m p u r i t i e s s p e c i a l l y F e S 2 were d e t e c t e d and marked. They were t h e n immersed i n s o l u t i o n s o f NaOCl f o r d i f f e r e n t p e r i o d s and e x p e r i m e n t a l c o n d i t i o n s and f i n a l l y s e c t i o n e d t h r o u g h the marked a r e a s . A s i m i l a r t a r n i s h i n g o f t h e s u r f a c e was o b s e r v e d as i n t h e c a s e o f the f i n e p a r t i c l e s . The C u F e S 2 a r e a s a f t e r l e a c h i n g were brown and t r a n s p a r e n t w h i l e the F e S 2 a r e a s formed a b l a c k - o p a q u e c o a t i n g . 54 cn O O o 4 i E ©25 e - A A—Theor. NaOCl consumption. ^-Experim. II T- 26°C CuFeS - 10g pH - 9.2 10 30 TIME (min) 50 to —- 8 3 • o .4,2 FIG. 20 - EXTRACTION OF COPPER AND CONSUMPTION OF HYPOCHLORITE DURING LEACHING OF CHALCOPYRITE. 56 U s i n g t h e x ^ r a y . e n e r g y a n a l y z e r o f t h e SEM on t h e l e a c h e d C u F e S 2 a r e a s i t was p o s s i b l e t o d e t e r m i n e a d e c r e a s e i n the s u l f u r - c o p p e r r a t i o when compared t o u n t r e a t e d C u F e S 2 . An example o f t h e s e o b s e r v a t i o n s i s p r e s e n t e d i n T a b l e 6. I t was a l s o o b s e r v e d t h a t t h e S/Cu r a t i o d i d not change s i g n i f i c a n t l y w i t h t h e d u r a t i o n o f l e a c h i n g t h e specimens. T a b l e 6 Comparison o f S/Cu R a t i o s f o r ^ L e a c h e d and Nonleached C h a l c o p y r i t e Leaching Conditions NaOCl = 15g/l Temp = 31°C pH = 9.6 ( c o n t r o l l e d ) Time = 4 hours S/Cu r a t i o u n t r e a t e d C u F e S 2 = .79 " l e a c h e d " = .37 T h i s r e d u c t i o n i n t h e S/Cu r a t i o c o u l d be i n t e r p r e t e d as an o x i d a t i o n o f t h e C u F e S 2 s u r f a c e . However as t h e r e was s u l f u r r e m a i n i n g a f t e r t h e r e a c t i o n , a l t h o u g h t h e r e was an e x c e s s o f o x i d i z i n g r e a g e n t i n s o l u t i o n , t h i s p r o c e d u r e c o u l d n o t d i s c r i m i n a t e between an i n c o m p l e t e o x i d a t i o n o f the CuF e S 2 s u r f a c e and t h e i n t e r f e r e n c e o f l o w e r ' C u F e S 2 l a y e r s e x c i t e d by t h e x - r a y beam p e n e t r a t i o n . The m i c r o p r o b e a n a l y s i s o f l e a c h e d c r o s s - s e c t i o n s i n d i c a t e d t h a t a l i m i t e d t o p o t a c t i c r e a c t i o n had o c c u r r e d . The s u r f a c e o f t h e C u F e S 2 was a l t e r e d o n l y t o a maximum de p t h o f 20 m i c r o n s and i t was n o t p o s s i b l e t o d e t e r m i n e t h e e x t e n t o f t h e r e a c t i o n between the NaOCl and th e C u F e S 2 w i t h i n t h i s depth because o f the e r r a t i c s i g n a l s f o r Cu and S o r i g i n a t i n g from t he e m i s s i o n o f d i f f r a c t e d beams from t h e uneven edge o f the specimen. 5"7 These l a s t o b s e r v a t i o n s d e m o n s t r a t e t h a t t h e r e a c t i o n between NaOCl and C u F e S 2 o c c u r r e d o n l y a t t h e s u r f a c e f o r m i n g a p r o t e c t i v e l a y e r c o n s i s t i n g a t l e a s t i n p a r t by some form o f o x i d e o r h y d r o x i d e o f c o p p e r I I . I t was r e a s o n a b l e t o s p e c u l a t e t h e r e f o r e , t h a t t h e f u r t h e r d e c r e a s e i n [NaOCl] was due t o some d e c o m p o s i t i o n r e a c t i o n which i s c a t a l y z e d by th e a l t e r e d C u F e S 2 s u r f a c e . 4.2.3 Comparative O x i d a t i o n and D i s s o l u t i o n o f MoS 2 and C u F e S 2 i n  S o l u t i o n s o f NaOCl In a r o u g h e r c o n c e n t r a t e t h e w e i g h t r a t i o between C u F e S 2 and MoS 2 i s a p p r o x i m a t e l y 80. I t i s t h e r e f o r e c o n v e n i e n t t o d e t e r m i n e how each o f them behave when e x p o s i n g equal s u r f a c e a r e a s i n s o l u t i o n s o f NaOCl under t h e c o n d i t i o n s o f p r e v i o u s e x p e r i m e n t s . T h i s c o u l d then e s t a b l i s h i f s e l e c t i v e l e a c h i n g i s p o s s i b l e . MoS 2 and C u F e S 2 o f the same s c r e e n e d s i z e (-140 + 200 mesh) were examined u s i n g t h e SEM t o d e t e r m i n e t h e s u r f a c e r o u g h n e s s . Examples o f each a r e p r e s e n t e d i n F i g u r e 21. I t was o b s e r v e d t h a t t h e p r e s e n c e o f m u l t i p l e s t e p s and edges on t h e m o l y b d e n i t e p a r t i c l e s i n c r e a s e d i t s s u r f a c e a r e a . The s p e c i f i c a r e a was d e t e r m i n e d u s i n g a nomogram [28] on t h e b a s i s o f p a r t i c l e d i a m e t e r , shape f a c t o r and the m a t e r i a l s d e n s i t y . T h i s method o f s u r f a c e a r e a d e t e r m i n a t i o n i s not as a c c u r a t e as any o f the a d s o r p t i o n methods but i t was c o n s i d e r e d t o be adequate f o r c o m p a r a t i v e p u r p o s e s i n t h e l e a c h i n g e x p e r i m e n t . 58 FfG. 21 - SURFACE ROUGHNESS OF MoS2 AND CuFeS2 59 T a b l e 7 V a l u e s f o r D e t e r m i n a t i o n o f S u r f a c e A r e a f o r M o l y b d e n i t e and C h a l c o p y r i t e M a t e r i a l Average P a r t i c l e d i a m e t e r -(urn) Shape F a c t o r D e n s i t y S p e c i f i c S u r f a c e - -m2/g MoS 2 C u F e S 2 100 ll 5 4 4.8 5.1 .06 m 2/g .02 m 2/g Samples o f m o l y b d e n i t e and c h a l c o p y r i t e p a r t i c l e s c o r r e s p o n d i n g t o equal s u r f a c e a r e a s were l e a c h e d t o g e t h e r i n a s o l u t i o n o f NaOCl. The e x t r a c t i o n c u r v e s f o r molybdenum and co p p e r a r e shown i n F i g u r e 22. I t was p o s s i b l e t o o b s e r v e t h a t t h e r e l a t i v e d i s s o l u t i o n between t h e s e m i n e r a l s f a v o r s m o l y b d e n i t e by a t l e a s t a r a t i o 100:1 and t h a t t h e r e a c t i o n o f MoS 2 was complete. T h e r e f o r e t h e s e l e c t i v e l e a c h i n g o f MoS 2 from C u F e S 2 s h o u l d be p o s s i b l e . 4.2.4 E f f e c t o f C h a l c o p y r i t e on t h e Rates o f NaOCl Consumption and  MoS 2 D i s s o l u t i o n Runs i n t h i s s e c t i o n were made by a d d i n g p r o g r e s s i v e l y g r e a t e r amounts o f C u F e S 2 t o l e a c h i n g e x p e r i m e n t s i n which t h e i n i t i a l m o l y b d e n i t e and c o n c e n t r a t i o n o f sodium h y p o c h l o r i t e were i d e n t i c a l . R e s u l t s f o r m o l y b d e n i t e d i s s o l u t i o n a r e p r e s e n t e d i n F i g u r e 23. I t was o b s e r v e d t h a t g r e a t e r amounts o f c h a l c o p y r i t e ( s e v e r a l t i m e s t h e w e i g h t and s u r f a c e a r e a o f m o l y b d e n i t e ) d i d not have a s i g n i f i c a n t e f f e c t on t h e d i s s o l u t i o n r a t e s s i n c e t i m e s f o r t o t a l e x t r a c t i o n remained unchanged. .601 Q UJ I-O < or h-x LU .40 ,20 [NaOCl] = 7.2 g/1 pH =9 T =30°C Mo = 100 % extracted o o 30 TIME (min) a UJ I-< r r h-x UJ •3 V .006 004 •.002 FIG. 22 - EXTRACTION OF Cu AND Mo DURING LEACHING OF MoS2 AND CuFeS2 OF EQUAL INITIAlO SURFACE AREAS. 10 30 50 TIME (hr) FIG. 2 3 - E F F E C T OF CHALCOPYRITE ON THE RATE OF MOLYBDENITE DISSOLUTION. 62 4.3 L e a c h i n g o f M o l y b d e n i t e from Copper Rougher C o n c e n t r a t e s U s i n g t h e optimum l e a c h i n g c o n d i t i o n s e s t a b l i s h e d w i t h pure MoS 2, e x p e r i m e n t s were performed t o e x t r a c t molybdenum from c o p p e r m i l l p r o d u c t s . Some i n i t i a l e x p e r i m e n t s u s i n g a c o p p e r o r e showed e r r a t i c r e s u l t s which were a t t r i b u t e d t o t h e e x t r e m e l y low grade (<.01% Mo) o f t h e o r e sample. I t was then d e c i d e d t o use a co p p e r r o u g h e r c o n c e n t r a t e , which was d r i e d i n an oven a t 110-120°C, w i t h the purpose o f e l i m i n a t i n g most o f t h e a t t a c h e d f l o t a t i o n r e a g e n t s . The r e s u l t s o f p r e l i m i n a r y e x p e r i m e n t s u s i n g a c o n c e n t r a t i o n o f sodium h y p o c h l o r i t e up t o 10 t i m e s t h a t t h e o r e t i c a l l y r e q u i r e d t o o x i d i s e a l l t h e m o l y b d e n i t e p r e s e n t t o Mo0 4~ and SQk~ showed as was e x p e c t e d , t h a t t he e x t r a c t e d Mo/Cu r a t i o was v e r y h i g h . However some Cu was p r e s e n t i n t h e s o l u t i o n and the consumption o f NaOCl was f a r i n e x c e s s o f the t h e o r e t i c a l l y c a l c u l a t e d amount. The r e s u l t o f one such e x p e r i m e n t i s r e p o r t e d i n F i g u r e 24. The % e x t r a c t e d Mo and Cu r e f e r t o t h e t o t a l o f each i n t h e i n i t i a l sample. I t can be o b s e r v e d t h a t t h e r a t e s o f moybdenum e x t r a c t i o n a r e v e r y h i g h i n th e i n i t i a l p a r t (50% r e a c t e d i n 8 m i n u t e s ) but d e c r e a s e r a p i d l y t o z e r o a f t e r 40 m i n u t e s . A t t h i s time t h e c o n c e n t r a t i o n o f NaOCl had d e c r e a s e d t o z e r o and t h e t o t a l e x t r a c t i o n o f Mo was o n l y 83%. The d i s s o l u t i o n o f c o p p e r was o b s e r v e d t o go t h r o u g h a maximum and l a t e r remain c o n s t a n t a f t e r t h e NaOCl c o n c e n t r a t i o n r e a c h e d z e r o . In t h i s run t h e maximum Cu d e t e c t e d i n s o l u t i o n was 1.2%. In no c a s e d i d t h e Cu e x t r a c t i o n exceed 3%. I t i s assumed t h a t i n t h e p r e s e n c e o f an e x c e s s [NaOCl] the r e a c t i o n between c h a l c o p y r i t e and sodium h y p o c h l o r i t e w i l l have t h e f o l l o w i n g s t o i c h i o m e t r y : 63 NaOCl = 5.3 T=29°C pH = 8.9 FIG. 24 - EXTRACTION OF Mo AND Cu FROM A COPPER ROUGHER CONCENTRATE , 64 2 C u F e S 2 + 17 NaOCl + 8 NaOH + H 20 -»• 2 C u ( O H ) 2 + 2 F e ( 0 H ) 3 + 4 Na 2 SO* + 17 Na C l In the l e a c h i n g o f the r o u g h e r c o n c e n t r a t e , t h e a n a l y s i s o f [NaOCl] measured even i n t h e f i r s t samples i n d i c a t e d t h a t t h e consumption o f r e a g e n t was much g r e a t e r t h a n t h e o r e t i c a l l y r e q u i r e d t o c o m p l e t e l y o x i d i z e a l l t h e Mo and t h e Cu d i s s o l v e d a t t h o s e s t a g e s . As i n t h e c a s e o f l e a c h i n g C u F e S 2 , c l e a r f i l t e r e d s o l u t i o n s from t h e r o u g h e r c o n c e n t r a t e l e a c h were o b s e r v e d t o form a b l a c k f i n e p r e c i p i t a t e . I t was not p o s s i b l e t o c o l l e c t s u f f i c i e n t o f t h i s m a t e r i a l f o r NaOCl d e c o m p o s i t i o n s t u d i e s . 4.3.1 I n f l u e n c e o f P a r t i c l e S i z e on t h e E x t r a c t i o n o f Molybdenum The i n f l u e n c e o f p a r t i c l e s i z e was i n v e s t i g a t e d by s e p a r a t i n g two s i z e f r a c t i o n s o f t h e r o u g h e r c o n c e n t r a t e and s u b j e c t i n g them t o i d e n t i c a l l e a c h i n g e x p e r i m e n t s . The r e s u l t s a r e p r e s e n t e d i n F i g u r e 25. I t was o b s e r v e d t h a t t h e sample w i t h g r e a t e r s u r f a c e a r e a p r e s e n t e d a h i g h e r e x t r a c t i o n r a t e but the t o t a l m o l y b d e n i t e l e a c h e d was l o w e r . The r e a s o n f o r t h i s was b e l i e v e d t o be a f a s t e r d e c r e a s e i n NaOCl s t r e n g t h . T h i s was c o n f i r m e d i n t h e f o l l o w i n g e x p e r i m e n t s i n which a f i n e l y ground r o u g h e r c o n c e n t r a t e sample was l e a c h e d . A c o mparison o f t h e e x t r a c t i o n r a t e s and NaOCl consumption i s p r e s e n t e d i n F i g u r e 26, between the l e a c h i n g o f t h e "as r e c e i v e d " r o u g h e r c o n c e n t r a t e and t h e ground sample. A l t h o u g h the c o n c e n t r a t i o n o f NaOCl 65 Co ioot o UJ h-V < or h-X LU — o o (- 4 0 + 70) MESH — * * (- 140) • A. O 30 TIME (min) 50 FIG. 25 - EFFECT OF PARTICLE SIZE ON THE RATES OF Mo EXTRACTION FROM Cu ROUGHER CONCEN TRATES. 66 . 26 - Mo EXTRACTION AND NaOCl CONSUMPTION IN THE LEACHING OF GROUND AND NORMAL ROUGHER CONCENTRATES. 67 was o v e r t h r e e t i m e s t h e t h e o r e t i c a l l y r e q u i r e d t o l e a c h a l l the m o l y b d e n i t e p r e s e n t i t d e c r e a s e d t o z e r o i n l e s s than 10 m i n u t e s a t which time o n l y 46% o f Mo had been e x t r a c t e d from the ground sample. Equal c o n d i t i o n s e x t r a c t e d 84% o f t h e c o a r s e r c o n c e n t r a t e . 4.3.2 Q u a n t i t y o f NaOCl R e q u i r e d f o r T o t a l Molybdenum E x t r a c t i o n S e v e r a l runs were per f o r m e d 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 sodium h y p o c h l o r i t e w i t h the purpose o f d e t e r m i n i n g the minimum q u a n t i t y t h a t would e x t r a c t a l m o s t a l l the molybdenum. The e x p e r i m e n t a l c o n d i t i o n s were s e t a t : i n i t i a l pH 9.5, t e m p e r a t u r e 30°C. I t was d e t e r m i n e d t h a t f o r the complete e x t r a c t i o n o f MoS 2 from t h e r o u g h e r c o n c e n t r a t e used, t h e q u a n t i t y o f NaOCl r e q u i r e d was e q u i v a l e n t t o 4.2 t i m e s the t h e o r e t i c a l l y needed t o c o m p l e t e l y o x i d i z e t h e molybdenum and s u l f u r i n t h e sample. 4.3.3 E f f e c t o f P r e t r e a t i n g t h e Rougher C o n c e n t r a t e w i t h S u l f u r i c A c i d With the i n t e n t i o n o f e l i m i n a t i n g any o x i d e s p r e s e n t w i t h t h e c o p p e r s u l f i d e m i n e r a l s i n the r o u g h e r c o n c e n t r a t e which c o u l d be i n c r e a s i n g t h e NaOCl consumption, a p o r t i o n o f t h e c o n c e n t r a t e was l e a c h e d i n a d i l u t e s o l u t i o n o f s u l f u r i c a c i d (0.5%) and l a t e r l e a c h e d w i t h sodium h y p o c h l o r i t e under s i m i l a r c o n d i t i o n s as the u n t r e a t e d r o u g h e r c o n c e n t r a t e . I t was o b s e r v e d t h a t t h i s p r e t r e a t m e n t had a r e m a r k a b l e e f f e c t on t h e sodium h y p o c h l o r i t e consumption. C o n s e q u e n t l y s e v e r a l runs were perf o r m e d as i n s e c t i o n 4.3.2 and i t was d e t e r m i n e d t h a t t o t a l m o l y b d e n i t e 68 e x t r a c t i o n was o b t a i n e d u s i n g a s o l u t i o n o f NaOCl w i t h an i n i t i a l c oncen-t r a t i o n e q u i v a l e n t t o 1.5 t o 1.8 t i m e s the t h e o r e t i c a l l y r e q u i r e d t o d i s s o l v e t h e MoS 2 p r e s e n t . 4.3.4 E x t r a c t i o n o f M o l y b d e n i t e from " I n - P u l p " Rougher C o n c e n t r a t e The e x p e r i m e n t s i n s e c t i o n 4.3.3 were per f o r m e d s i m u l t a n e o u s l y w i t h the F l o t a t i o n s t u d i e s r e p o r t e d below. The r e s u l t s o f b o t h i n d i c a t e d t h a t the r o u g h e r c o n c e n t r a t e used c o u l d have an e x c e s s o f c o p p e r o x i d e as compared t o t h e u n p r e p a r e d " i n - p u l p " r o u g h e r c o n c e n t r a t e o b t a i n e d i n t h e p r i m a r y f l o t a t i o n c i r c u i t . T h i s was c o n f i r m e d by c h e m i c a l a n a l y s i s as i n d i c a t e d i n Tabe 8. T a b l e 8 Oxide Copper C o n t e n t f o r D r i e d and I n - P u l p Rougher C o n c e n t r a t e s G i b r a l t a r R.C. washed and d r i e d f o r L e a c h i n g E x p e r i m e n t s Oxide Copper = 1.26% G i b r a l t a r R.C. u n t r e a t e d " " = 0.52% The c a u s e o f t h e e x c e s s c o p p e r o x i d e was a t t r i b u t e d t o h i g h t e m p e r a t u r e s o f the d r y i n g p r o c e s s used i n p r e p a r i n g the samples f o r l e a c h i n g e x p e r i m e n t s and s t o r i n g t h e d r i e d r o u g h e r c o n c e n t r a t e i n c o n t a c t w i t h a i r f o r l o n g p e r i o d s o f time. C o n s e q u e n t l y a new b a t c h o f r o u g h e r c o n c e n t r a t e was o b t a i n e d from G i b r a l t a r Mines and m a i n t a i n e d as a dense p u l p . The a d d i t i o n o f r o u g h e r c o n c e n t r a t e t o the l e a c h i n g e x p e r i m e n t s r e p o r t e d i n t h i s s e c t i o n was by measured volumes a f t e r s t i r r i n g t h e p u l p o n e - h a l f hour. 69 S e v e r a l runs were performed w i t h t h e u n t r e a t e d r o u g h e r c o n c e n t r a t e u s i n g s i m i l a r c o n d i t i o n s as i n s e c t i o n 4.3 and 4.3.2 w i t h t h e purpose o f d e t e r m i n i n g t h e e f f e c t o f the exc e s s c o p p e r o x i d e on t h e e x t r a c t i o n r a t e s o f m o l y b d e n i t e and the consumption o f r e a g e n t . The r e s u l t s o f one run a r e p r e s e n t e d i n F i g u r e 27. A l s o p l o t t e d i s t h e e x t r a c t i o n c u r v e s o f the l e a c h i n g o f a t r e a t e d r o u g h e r c o n c e n t r a t e . I t i s not p o s s i b l e t o compare the a p p r o x i m a t e r e a c t i o n r a t e s from t h e s e two c u r v e s because o f the d i f f e r e n c e i n t h e i n i t i a l c o n c e n t r a t i o n o f the r e a g e n t . F u r t h e r m o r e t h e d e t e r m i n a t i o n o f r e a c t i o n r a t e s i n t h i s r e a c t i o n would have r e q u i r e d a more s o p h i s t i c a t e d s a m p l i n g t e c h n i q u e which was not ju d g e d c o n v e n i e n t f o r t h e purpose o f t h i s s t u d y . F o r t h e purpose o f a p r a c t i c a l a p p l i c a t i o n i t i s p o s s i b l e t o say t h a t t h e r e was no s i g n i f i c a n t change e i t h e r i n t h e r e a c t i o n r a t e s nor i n t h e t o t a l l e a c h i n g times between t h e s e two c o n c e n t r a t e s . However t h e NaOCl consumption was n o t i c e a b l y d i f f e r e n t . By u s i n g t h e p r o c e d u r e o f l e a c h i n g w i t h i n c r e a s i n g i n i t i a l NaOCl c o n c e n t r a t i o n and f o l l o w i n g the Mo e x t r a c t i o n u n t i l t o t a l NaOCl consumption i t was p o s s i b l e t o d e t e r m i n e t h a t 98% o f t h e MoS 2 i n t h e u n t r e a t e d r o u g h e r con-c e n t r a t e was e x t r a c t e d w i t h a NaOCl s o l u t i o n w i t h an i n i t i a l c o n c e n t r a t i o n 1.6 times t he t h e o r e t i c a l l y r e q u i r e d t o o x i d i z e a l l MoS 2 t o MoOiT and S 0 H = . 4.3.5 F l o t a t i o n o f Leached Rougher C o n c e n t r a t e s To compare t h e e f f e c t o f l e a c h i n g on t h e f l o t a b i l i t y o f r o u g h e r c o n c e n t r a t e s , t e s t s were per f o r m e d u s i n g t h r e e m a t e r i a l s : 70 T = 30° C pH = 9r.5 10 30 TIME (min) FIG. 2 7 - MOLYBDENITE DISSOLUTION AND HYPOCHLORITE CONSUMPTION IN THE LEACHING TREATED AND IN PULP ROUGHER CONCENTRATES 71 A . T r e a t e d R o u g h e r C o n c e n t r a t e d i r e c t l y l e a c h e d i n a s o l u t i o n o f NaOC l ( c o m p l e t e e x t r a c t i o n o f mo I y b d e n i t e ) . B. T r e a t e d R o u g h e r C o n c e n t r a t e p r e v i o u s l y l e a c h e d i n a \% H2S0^ a t 2 5 ° C f o r o n e h o u r a n d l a t e r l e a c h e d i n a s o l u t i o n o f NaOC l ( c o m p l e t e e x t r a c t i o n o f M o S 2 ) . C . T r e a t e d R o u g h e r C o n c e n t r a t e n o t l e a c h e d t o s e r v e a s c o m p a r i s o n . In a l l c a s e s the s t a n d a r d t e c h n i q u e f o r t h e p r e f e r e n t i a l f l o -t a t i o n o f c h a l c o p y r i t e was used, c a r e f u l l y c o n t r o l l i n g the pH a t which t h e s e v e r a l f l o t a t i o n and c l e a n i n g s t a g e s were p e r f o r m e d . Z200 was used as the c o l l e c t o r h a v i n g t h e p r o p e r t y o f s e l e c t i v i t y o f c h a l c o p y r i t e o v e r p y r i t e . The r e c o v e r y grade r e l a t i o n s h i p has been p l o t t e d i n F i g u r e 28. I t was o b s e r v e d t h a t the f l o t a b i l i t y o f t h e l e a c h e d r o u g h e r c o n c e n t r a t e was i n f e r i o r t o t h e u n l e a c h e d m a t e r i a l . In c o n t r a s t t h e r o u g h e r concen-t r a t e which r e c e i v e d t h e same NaOCl l e a c h but had been p r e v i o u s l y t r e a t e d w i t h a d i l u t e HzSOk had a s l i g h t l y b e t t e r f l o t a b i l i t y . In a l l c a s e s the consumption o f c o l l e c t o r was f a r i n e x c e s s o f the normal q u a n t i t i e s employed f o r the s e c o n d a r y f l o t a t i o n o f t h e r o u g h e r c o n c e n t r a t e s i n the normal c o p p e r c o n c e n t r a t i o n p r o c e s s and i t was s u s p e c t e d t h a t the s u r f a c e o f t h e m i n e r a l s c o u l d have been a l t e r e d d u r i n g the d r y i n g p r o c e s s i n p r e p a r a t i o n f o r l e a c h i n g , and t h a t t h i s c o n d i t i o n was r e s p o n s i b l e t o a l a r g e r degree f o r t h e poor f l o t a b i l i t y than any changes produced on the m i n e r a l s u r f a c e d u r i n g l e a c h i n g w i t h NaOCl. 72 PROGRESSIVE GRADE (% Cu) FIG. 28 - RECOVERY - GRADE CURVES FOR THE SECONDARY FLOTATION OF LEACHED ROUGHER CONCENT. 73 4.3.6 F l o t a t i o n o f " I n - P u l p " Rougher C o n c e n t r a t e s F l o t a t i o n s t u d i e s r e p o r t e d i n t h i s s e c t i o n were pe r f o r m e d on t h e G i b r a l t a r c o p p e r r o u g h e r c o n c e n t r a t e kept as " i n - p u l p . " Samples o f t h i s m a t e r i a l were l e a c h e d w i t h o u t any p r e t r e a t m e n t i n s o l u t i o n s o f NaOCl and then f l o a t e d as i n 4.3.5. A sample o f u n l e a c h e d m a t e r i a l was f l o a t e d f o r c o m p a r i s o n . A l t h o u g h a s o l i d s / l i q u i d s e p a r a t i o n was used a f t e r l e a c h i n g , c a r e was t a k e n t o p r e v e n t the r e s i d u e from d r y i n g o r e x c e s s i v e c o n t a c t w i t h a i r . The same f i l t e r i n g p r o c e d u r e was a p p l i e d t o t h e u n l e a c h e d m a t e r i a l and t i m e s between S / l s e p a r a t i o n and f l o t a t i o n were a p p r o x i m a t e l y t h e same f o r both m a t e r i a l s . The r e c o v e r y - g r a d e r e l a t i o n s h i p was p l o t t e d i n F i g u r e 29. I t i s p o s s i b l e t o o b s e r v e t h a t t h i s r e l a t i o n s h i p i s s i m i l a r f o r both r o u g h e r c o n c e n t r a t e s . On the b a s i s o f r e a g e n t consumption i t was d e t e r m i n e d t h a t t h e f l o t a b i l i t y o f the l e a c h e d r o u g h e r c o n c e n t r a t e i s lower t h a n t h e u n t r e a t e d m a t e r i a l , i . e . the f l o t a t i o n r e c o v e r y i s l o w e r a l t h o u g h t h e c o n c e n t r a t e grade i s h i g h e r f o r i d e n t i c a l r e a g e n t consumption. I t i s l i k e l y t h a t t h e r e c o v e r y o f the l e a c h e d r o u g h e r c o n c e n t r a t e c o u l d be i n c r e a s e d by a d j u s t i n g t h e r e a g e n t s e i t h e r by i n c r e a s i n g t h e dosage o f the one used i n our e x p e r i m e n t s o r u s i n g a d i f f e r e n t t y p e . The d e t e r m i n a t i o n o f t h e a p p r o p r i a t e f l o t a t i o n p r o c e d u r e would have r e q u i r e d many more e x p e r i m e n t s and was n o t i n t h e scope o f t h i s p r o j e c t . However the i n f o r m a t i o n g a t h e r e d i s s u f f i c i e n t t o i n d i c a t e t h a t t h e l e a c h i n g o f a c o p p e r r o u g h e r c o n c e n t r a t e w i t h sodium h y p o c h l o r i t e does not d e c r e a s e 74 100 3 804 or w 6 0 O o or 8 4 0 3 O 2 0 - — o Rougher Con. Not L e a c h e d Leached — A £k H= ' —=±r 4 z+? 1 rA= 1 =h- 1-16 20 2'4 28 32 "^6 PROGRESSIVE GRADE (% Cu) FIG. 2 9 - RECOVERY- GRADE CURVES FOR THE SECONDARY FLOTATION OF LEACHED ROUGHER CONCENTRATES. 75 t o a g r e a t e x t e n t t he f l o t a b i l i t y o f the m a t e r i a l and t h a t t he a d d i t i o n a l c o s t s o f f l o t a t i o n r e a g e n t s i f a l e a c h i n g c i r c u i t were i n t r o d u c e d t o a c o n v e n t i o n a l c o n c e n t r a t i o n p r o c e s s might not be s i g n i f i c a n t . 4.3.7 S t a b i l i t y o f Sodium H y p o c h l o r i t e S o l u t i o n s S e v e r a l e x p e r i m e n t s were per f o r m e d t o d e t e r m i n e t h e s t a b i l i t y o f NaOCl under t he c o n d i t i o n s used i n t h e l e a c h i n g s t u d i e s . The r e g i o n o f pH i n f l u e n c e s t u d i e d was between 12 a t which NaOCl i s g e n e r a l l y s t o r e d f o r minimum d e c o m p o s i t i o n and 7 a t which t h e r e a c t i o n t o H0C1 i s r a p i d l y f a v o r e d . Two t e m p e r a t u r e s were s e l e c t e d f o r comparison o v e r t he same range as i n v e s t i g a t e d i n t h e m o l y b d e n i t e d i s s o l u t i o n , and t h e i n i t i a l c o n c e n t r a t i o n o f NaOCl was s i m i l a r i n a l l r u n s . [NaOCl] as used i n t h i s s e c t i o n i s a measure o f the t o t a l a c t i v e c h l o r i n e s p e c i e s , t h a t i s O C l " , H0C1 and C10 2~. R e s u l t s f o r some runs have been p l o t t e d i n F i g u r e 30 and 31. The d e t r i m e n t a l e f f e c t o f both h i g h e r t e m p e r a t u r e s and lower pH a r e o b s e r v e d . A t lower t e m p e r a t u r e s t h e r e i s no me a s u r a b l e d e c r e a s e i n t h e c o n c e n t r a t i o n o f NaOCl a t a hydrogen i o n c o n c e n t r a t i o n i n which t h e f a s t e s t r a t e s o f m o l y b d e n i t e d i s o l u t i o n were o b s e r v e d . In two runs, samples were a n a l y z e d u s i n g t he p o t e n t i o m e t r i c t e c h n i q u e w i t h t he purpose o f d i s t i n g u i s h i n g C 1 0 2 " from O C l " and HC10. No C 1 0 2 was o b s e r v e d a t h i g h pH and the c o n c e n t r a t i o n was v e r y s m a l l and e r r a t i c a t pH 7 o b s t r u c t i n g any q u a n t i t a t i v e a n a l y s i s . 76 ~ 12 pH 7 1 1 h 2 3 4 TIME (hs) FIG. 3 0 - EFFECT OF pH ON THE DECOMPOSITION OF HYPOCHLORITE AT 60° C. 77 12 FIG. 31- EFFECT OF pH ON THE DECOMPOSITION OF HYPOCHLORITE AT 35° C. C h a p t e r 5 DISCUSSION 5.1 O x i d a t i o n o f M o l y b d e n i t e The r e s u l t s o f the d i s s o l u t i o n e x p e r i m e n t s i n d i c a t e t h a t t he r e a c t i o n between m o l y b d e n i t e and aqueous s o l u t i o n s o f sodium h y p o c h l o r i t e goes t o c o m p l e t i o n v e r y r a p i d l y under t h e c o n d i t i o n s used i n t h i s s t u d y . T h i s was found t o be t r u e both w i t h a h i g h grade m o l y b d e n i t e c o n c e n t r a t e and c o p p e r r o u g h e r c o n c e n t r a t e s i n which t he MoS 2 c o n t e n t i s v e r y s m a l l . T o t a l d i s s o l u t i o n o f MoS 2 from d i l u t e p u l p s was o b t a i n e d i n a l l c a s e s w i t h i n 45 minutes l e a c h i n g t i m e . The o v e r a l l s t o i c h i o m e t r y o f t h e r e a c t i o n as d e t e r m i n e d by [6,14] i s : MoS 2 + 9 NaOCl + 6 NaOH -*• NazMoO^ + 2 N a 2 S 0 4 •+ 9 NaCI + 3 H 20 (3) which i s v a l i d i n t h e c a s e s i n which NaOCl i s i n e x c e s s and t h e r e f o r e both Mo and S can be assumed t o be o x i d i z e d t o t h e i r h i g h e s t s t a t e . T h i s r e a c t i o n was found t o a c c u r a t e l y r e p r e s e n t a l l t h e e x p e r i m e n t s u s i n g t h e MoS 2 c o n c e n t r a t e and a t l e a s t t h e i n i t i a l s t a g e s i n the d i s s o l u t i o n o f MoS 2 from t h e r o u g h e r c o n c e n t r a t e s . No attempts have been r e p o r t e d o f 78 79 e s t a b l i s h i n g t h e s t o i c h i o m e t r y o f t h e r e a c t i o n when M0S2 i s i n e x c e s s , as would be t h e c a s e i n t h e f i n a l s t a g e s o f t h e r o u g h e r c o n c e n t r a t e d i s s o l u t i o n when t h e [NaOCl] i s v e r y low, and was not a t t e m p t e d i n t h i s s t u d y because o f t h e c o m p l e x i t i e s o f p a r a l l e l r e a c t i o n s and c a t a l y z e d d e c o m p o s i t i o n o f the r e a g e n t . The A6°of r e a c t i o n (3) was c a l c u l a t e d as -670 k c a l , which p r e d i c t s t h e h i g h l y f a v o r a b l e thermodynamic tendency o f d i s s o l u t i o n o f MoS 2. A l i n e a r dependence was o b s e r v e d between t h e r a t e s o f d i s s o l u -t i o n o f MoS 2 and t h e s u r f a c e a r e a i n d i c a t i n g a heterogeneous r e a c t i o n . The a p p a r e n t a c t i v a t i o n e n e r g y o f t h e r e a c t i o n (6.3 k c a l / m o l e ) i s on the b o r d e r o f t h e r e g i o n c h a r a c t e r i s t i c o f d i f f u s i o n c o n t r o l l e d r e a c t i o n s . T h e r e f o r e t h e r a t e d e t e r m i n i n g s t e p c o u l d be e i t h e r under e x c l u s i v e c h e m i c a l o r p o s s i b l y mixed c h e m i c a l and d i f f u s i o n c o n t r o l . I t i s c o n s i d e r e d t h a t t h e a c t i v a t i o n energy o f 6.3 k c a l / m o l e o b t a i n e d i n t h i s work i s i n c l o s e agreement w i t h t h e v a l u e o f 5.25 k c a l / mole o b t a i n e d by [14] u s i n g compressed specimens o f h i g h grade MoS 2. I t - h a s been o b s e r v e d i n t h i s s t u d y , t h a t t h e r e a c t i o n between C u F e S 2 and NaOCl produces a p r o t e c t i v e c o a t i n g which p r e v e n t s f u r t h e r d i s s o l u t i o n . The r e a c t i o n o f NaOCl w i t h ZnS has a l s o been s t u d i e d [24] and t h e o b s e r v a -t i o n o f a z i n c h y d r o x i d e and z i n c f e r r i t e l a y e r on t h e s u r f a c e o f s p h a l e r i t e was c o n s i d e r e d t o be t h e cause o f t h e d e c r e a s e i n t h e r e a c t i o n r a t e . The a p p a r e n t a c t i v a t i o n e n e r g i e s have been measured i n both c a s e s ; 5 k c a l f o r t h e r e a c t i o n w i t h C u F e S 2 and 4.6 k c a l f o r the r e a c t i o n w i t h ZnS. 80 The s i m i l a r v a l u e s o f a c t i v a t i o n energy f o r r e a c t i o n s i n which the r e a c t i o n p r o d u c t s a r e e n t i r e l y d i f f e r e n t i n n a t u r e would s u p p o r t the c o n c e p t t h a t the r a t e c o n t r o l l i n g s t e p i s t h e d i f f u s i o n o f NaOCl t h r o u g h t h e boundary l a y e r . S i n c e NaOCl has a h i g h s o l u b i l i t y i t can be e x p e c t e d t h a t most o f i t . . i s i o n i z e d i n a l k a l i n e s o l u t i o n s even a t low t e m p e r a t u r e s . The q u a n t i t y o f O C T i o n s however w i l l be d e t e r m i n e d by t h e pH o f t h e s o l u t i o n s i n c e t h e r e e x i s t s an e q u i l i b r i u m between O C T and m o l e c u l a r H0C1 which i s i n f l u e n c e d d i r e c t l y by t h e [ H + ] as r e p r e s e n t e d by r e a c t i o n (17) Ka 0C1" + H + H0C1 (17) The p r o p o r t i o n o f each s p e c i e s has been p l o t t e d a g a i n s t pH i n F i g u r e 32. F i g . 32. H y p o c h l o r i t e and h y p o c h l o r o u s a c i d c o n c e n t r a t i o n s as a f u n c t i o n o f pH. 81 A l t h o u g h i t has been r e p o r t e d t h a t t h e o x i d i z i n g power o f t h e i o n i z e d and m o l e c u l a r s p e c i e s o f h y p o c h l o r i t e a r e s i m i l a r [34] i t was o b s e r v e d i n t h i s work t h a t t h e r a t e s o f o x i d i a t i o n o f MoS 2 i n c r e a s e w i t h i n c r e a s i n g pH u n t i l a maximum i s o b t a i n e d i n the r e g i o n pH 8.5-9.5 i n which a l l h y p o c h l o r i t e e x i s t s as 0C1~ i o n s s u g g e s t i n g t h a t i t i s t h e O C T i o n which p l a y s t h e major r o l e i n t h e o x i d a t i o n r e a c t i o n . The i n f l u e n c e o f [ H + ] on t h e r a t e s o f o x i d a t i o n i s complex as shown i n F i g u r e 10. I t would be e x p e c t e d from r e a c t i o n (3) t h a t any i n c r e a s e i n the [OH ] would f a v o r t h e f o r m a t i o n o f MoOt,-. T h i s was o b s e r v e d i n l e a c h i n g e x p e r i m e n t s v a r y i n g t h e pH from 7 t o 9.5. But any i n c r e a s e o v e r t h i s r e g i o n r e s u l t e d i n a d e c r e a s e o f t h e r e a c t i o n r a t e s . These r e s u l t s d i f f e r from t h e work o f Ioardanov [14] who r e p o r t e d t h a t t h e r a t e o f o x i d a t i o n i s not i n c r e a s e d by an i n c r e a s e i n f r e e sodium base above t h e t h e o r e t i c a l l y r e q u i r e d . A l t h o u g h most o f t h e o t h e r i n v e s t i g a -t i o n s on t h i s r e a c t i o n [6,8] have worked i n the h i g h a l k a l i n e r e g i o n i t i s f e l t t h a t t h i s was s e l e c t e d o n l y on t h e b a s i s o f r e a g e n t s t a b i l i t y . To e x p l a i n t h e d e c r e a s e i n the r a t e s o f o x i d a t i o n above pH 10 i t was c o n s i d e r e d t h a t t h e r e c o u l d e x i s t c o m p e t i t i o n between 0C1 a d s o r p -t i o n and some o t h e r s p e c i e s which d e c r e a s e d t h e c o n c e n t r a t i o n o f o x i d a n t on t h e s u r f a c e o f the MoS 2. The z e t a p o t e n t i a l o f m o l y b d e n i t e i n t h e absence o f any c o l l e c t o r s i s shown i n F i g u r e 33 i n d i c a t i n g an i n c r e a s e i n t h e n e g a t i v e v a l u e o f £ f o r both h i g h e r a c i d and h i g h e r a l k a l i n e s o l u t i o n s . S: Chander and D.W. F u e r s t e n a u [ 3 3 ] i n s t u d y i n g t h e n a t u r a l f l o t a b i l i t y o f m o l y b d e n i t e were a b l e t o o b s e r v e t h a t on a d d i t i o n o f MoO^ 82 ' 4 5 6 7 8 9 10 II 12 pH F i g . 33. E, p o t e n t i a l o f m o l y b d e n i t e as a f u n c t i o n o f pH. i o n s i n a l k a l i n e s o l u t i o n s t h e z e t a p o t e n t i a l was even more n e g a t i v e and s u g g e s t e d t h a t MoOV i o n s a r e s t r o n g l y s p e c i f i c a l l y a b s o r b e d on m o l y b d e n i t e . I t seems l i k e l y then t h a t i n a l k a l i n e s o l u t i o n s an i n c r e a s e i n the pH r e s u l t s i n some p r e f e r e n t i a l a d s o r p t i o n o f Mo0 4~ and p o s s i b l y OH" i n s t e a d o f O C l " . In the l e a c h i n g e x p e r i m e n t s performed a t h i g h e r pH's i t was o b s e r v e d t h a t lower r a t e s o f d i s s o l u t i o n o f MoS 2 were o b t a i n e d when M o 0 4 -i o n s were i n t e n t i o n a l l y added t o the h y p o c h l o r i t e s o l u t i o n s . Under t h e s e c o n d i t i o n s t h e lower r a t e s o f m o l y b d e n i t e d i s s o l u t i o n can be e x p l a i n e d i f i t assumed t h a t some MoS 2 s i t e s a d s o r b Mo0 4~ i o n s and r e a c t f o r m i n g an i n s o l u b l e porous o x i d e t h r o u g h which t he OCl" must p e n e t r a t e i n o r d e r t o r e a c t w i t h t h e MoS 2 and t h a t t h e d i f f u s i o n t h r o u g h i t r e t a r d s the v e l o c i t y o f d i s s o l u t i o n . 83 The p o s s i b l e r e a c t i o n s a r e : 7 Mo0\ = + MoS 2 + 4 OH" 2 Mo^On + 2 S 0 4 = + 2 H 20 (18) MoOiT + MoS 2 2 Mo0 2 + 2 S (19) Th e r e a p p e a r s t o be g r e a t d i s c r e p a n c y between t he r e s u l t s o b t a i n e d i n t h i s s t u d y and t h o s e o b t a i n e d w i t h e l e c t r o o x i d a t i o n l e a c h i n g o f m o l y b d e n i t e . In t h e i r f i r s t r e p o r t B.J. S c h e i n e r and R.E. L i n d s t r o n [18] d e t e r m i n e d t h e pH f o r optimum e x t r a c t i o n as b e i n g i n the range o f 6-8. L a t e r work on the c o n t r o l o f c h l o r a t e f o r m a t i o n d u r i n g e l e c t r o o x i d a -t i o n l e a d D.S. B a r r , R.E. L i n d s t r o n and J.T. H e n d r i x [ 2 2] t o s u g g e s t t h a t maximum m o l y b d e n i t e d i s s o l u t i o n i s o b t a i n e d a t a pH between 4- and 5. In t h e i r s t u d y t h e y o b s e r v e d a d e c r e a s e i n t h e r a t e o f m o l y b d e n i t e d i s -s o l u t i o n when i n c r e a s i n g t h e pH from 5.5 t o 8. I t i s i m p o s s i b l e t o compare both p r o c e s s e s d i r e c t l y as t h e r e i s no me n t i o n o f any s p e c i f i c r a t e s . However i t i s p o s s i b l e t o o b s e r v e i n [22] t h a t f o r equal c o n c e n t r a t i o n s o f h y p o c h l o r i t e , t h e e l e c t r o o x i d a t i o n p r o c e s s produces a t l e a s t 50 t i m e s more c h l o r a t e than a normal h y p o c h l o r i t e c o n t a i n i n g s o l u t i o n r e g a r d l e s s o f t h e pH. C o n t r a r y t o t h e b e l i e f t h a t because C 1 0 3 ~ i s s t a b l e i t does not a f f e c t t h e m o l y b d e n i t e d i s s o l u t i o n , i t was o b s e r v e d i n t h i s work ( F i g u r e 12) t h a t t h e p r e s e n c e o f C 1 0 3 " d e c r e a s e s the r e a c t i o n r a t e s . In t h i s s t u d y t h e l o w e s t pH a t which d i s s o l u t i o n o f MoS 2 was performed was 7 and i n t h e s e s o l u t i o n s , as can be o b s e r v e d from F i g u r e 33, 84 HC10 a c i d p r e d o m i n a t e s o v e r 0C1 . The r e a s o n s f o r not s t u d y i n g t h e e f f e c t s below t h i s r e g i o n a r e two: f i r s t as a l r e a d y p o i n t e d o u t , t h e r e i s e v i d e n c e t h a t i t i s the 0C1~ i o n s which p a r t i c i p a t e m o s t l y i n t h e d i s s o l u t i o n o f MoS 2 and second r e g a r d l e s s o f the ty p e o f mechanism adop t e d t o e x p l a i n h y p o c h l o r i t e d e c o m p o s i t i o n t o c h l o r a t e (which w i l l be d e a l t w i t h b e l o w ) , 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 i t i s s t r o n g l y dependent on HC10 a c i d and t h e r e f o r e from t h e o r e t i c a l c o n s i d e r a t i o n s i t would be e x p e c t e d t h a t t h e l o s s e s o f h y p o c h l o r i t e due t o d e c o m p o s i t i o n would p r o h i b i t i t s a p p l i c a t i o n . The s l o p e o f the s t r a i g h t l i n e i n the p l o t r e a c t i o n r a t e s f o r o x i d a t i o n o f MoS 2 a g a i n s t i n i t i a l h y p o c h l o r i t e c o n c e n t r a t i o n i s a p p r o x i -m a t e l y one i n d i c a t i n g a f i r s t o r d e r dependence on the r e a g e n t . T h i s i s i n agreement w i t h t h e work o f Ioardanov and Z e l i k m a n [14] and on e x t r a p o l a -t i o n o f Bhappu et al. [6] d a t a . I f i t i s assumed t h a t i n the pH r e g i o n 7-8.5 t h e c o n c e n t r a t i o n o f OH" i o n s does not a f f e c t t h e r a t e o f r e a c t i o n d i r e c t l y but t h r o u g h t h e a v a i l a b i l i t y o f 0C1~ as d e s c r i b e d i n r e a c t i o n (17) and t h a t i n t h e pH r e g i o n 8.5-9.5 t h e r a t e s a're inde p e n d e n t o f [0H~] then't'he r e a c t i o n between MoS 2 and NaOCl can be d e s c r i b e d as a second o r d e r r e a c t i o n , t h e r e f o r e , Rate o f O x i d a t i o n o f MoS 2 = d = k i [ M o S 2 ] [NaOCl] (20) The e x p e r i m e n t a l r a t e c o n s t a n t k i was d e t e r m i n e d u s i n g t h e d a t a i n F i g u r e 7 o b t a i n e d from l e a c h i n g e x p e r i m e n t s i n which t he i n i t i a l s u r f a c e a r e a o f m o l y b d e n i t e was i d e n t i c a l , by p l o t t i n g a d o u b l e l o g a r i t h m i c 85 c u r v e o f the r a t e s o f MoO^ - appearance a g a i n s t c o n c e n t r a t i o n o f NaOCl, F i g u r e 34. A c t u a l l y t h e v a r i a t i o n w i t h r e s p e c t t o [NaOCl] i s o n l y con-s i d e r e d , i n the form o f i u j o o * : - k 2 [ N a 0 C i ] but as the s u r f a c e a r e a was i d e n t i c a l a t t h e s t a r t o f e v e r y e x p e r i m e n t and r a t e s were o b t a i n e d o n l y i n t h e i n i t i a l s t a g e i t i s p o s s i b l e t o i n c l u d e t h e v a r i a t i o n w i t h r e s p e c t t o MoS 2 i n t h e e m p i r i c a l c o n s t a n t k 2 The v a l u e o f k 2 was d e t e r m i n e d as 2.29 m i n " 1 . As t h e s u r f a c e a r e a was c a l c u l a t e d as 120 cm 2 the s p e c i f i c r a t e c o n s t a n t k 2 = 1.90 x 1 0 " 2 m i n " 1 T h e r e e x i s t s no d a t a on r e a c t i o n r a t e c o n s t a n t s f o r t h i s system w i t h which t o compare t h i s r e s u l t . The upper l i m i t o f a r e a c t i o n r a t e i n a d i f f u s i o n c o n t r o l l e d r e a c t i o n i s r e a c h e d when i t i s assumed t h a t t h e c h e m i c a l r e a c t i o n i s i n s t a n t a n e o u s , i n which c a s e t h e c o n c e n t r a t i o n a t t h e i n t e r f a c e i s z e r o T h e r e f o r e d[NaQCl] = D.A.C. d t 6 i n which: D = D i f f u s i o n c o e f f i c i e n t o f h y p o c h l o r i t e i n aqueous media = 1.84 x 1 0 " 5 cm 2/sec [35] A = S u r f a c e a r e a o f m o l y b d e n i t e = 120 cm 2 C = C o n c e n t r a t i o n i n g/1 o f h y p o c h l o r i t e = 16 g/1 6 = d i f f u s i o n l a y e r t h i c k n e s s f o r an i n t e n s e l y a g i t a t e d system = 10" 3 cm [36] I I I 1 I I 1 1 I 1 1 I I I 1— 0 .2 .4 .6 .8 1.0 1.2 1.4 Log NaOCl (g/l) 34 - LOG - LOG CURVE OF THE RATES OF OXIDATION VS. [NaOCl] . 87 and t h e maximum r a t e o f h y p o c h l o r i t e consumption f o r t h e s e c o n d i t i o n s i s : d(NaOCl) _ 9 , 9 . . jj£ 2.12 g/rmn S i n c e t h e s t o i c h i o m e t r y o f t h e r e a c t i o n d i c a t e s a consumption o f 9 moles o f NaOCl p e r mole o f m o l y b d e n i t e , t h e maximum r a t e o f molybdate a p p e a r a n c e was c a l c u l a t e d a s : d(MoO i t =) cnr. / • ^ ' = 505 mg/min The e x p e r i m e n t a l r a t e f o r t h e s e c o n d i t i o n s was measured, and: dCMoOO c „ , . d t P = m g / m i n The c a l c u l a t e d d i f f u s i o n r a t e b e i n g t e n ti m e s f a s t e r t h a n t h e o b s e r v e d r a t e would s u g g e s t c h e m i c a l c o n t r o l o f t h e l e a c h i n g . However, t h e low a c t i v a t i o n energy and t h e marked dependency o f the l e a c h r a t e on a g i t a -t i o n c o u l d f a v o r t h e c o n c e p t o f mixed d i f f u s i o n and c h e m i c a l c o n t r o l o f th e l e a c h i n g r a t e b e i n g o p e r a t i v e . 5.2 O x i d a t i o n o f C h a l c o p y r i t e Both c h a l c o p y r i t e and p y r i t e were o b s e r v e d t o r e a c t . w i t h sodium h y p o c h l o r i t e but t h e e x t e n t o f t h e r e a c t i o n s a r e v e r y s m a l l i n comparison w i t h m o l y b d e n i t e . A l t h o u g h t h e f a s t e l i m i n a t i o n o f t h e c o a t i n g formed on C u F e S 2 u s i n g d i l u t e H2SO4 s u g g e s t e d t h a t i t was an o x i d e , o b s e r v a t i o n s of t h e 88 p r e c i p i t a t i o n o f c o p p e r out o f s o l u t i o n when the c o n c e n t r a t i o n o f NaOCl d e c r e a s e d i n d i c a t e d t h a t t h e n a t u r e o f the p r o d u c t s formed on t h e s u r f a c e o f C u F e S 2 was complex. I t i s r e p o r t e d i n the l i t e r a t u r e [37] t h a t Cu p r e c i p i t a t e s f r o m a l k a l i n e s o l u t i o n s as compounds which a r e p r o b a b l y s o l i d s o l u t i o n s o f CuO and C u ( 0 H ) 2 , t h e appearance b e i n g s i m i l a r t o t h e p r e c i p i t a t e s o b t a i n e d from t h e l e a c h i n g o f C u F e S 2 and r o u g h e r c o n c e n t r a t e s . Both s p e c i e s a r e t h e r m o d y n a m i c a l l y s t a b l e i n t h i s pH r e g i o n . T h i s c o a t i n g formed on C u F e S 2 e f f e c t i v e l y i n h i b i t s any f u r t h e r r e a c t i o n and t h e r e s u l t s from m i c r o p r o b e a n a l y s i s on c r o s s s e c t i o n s o f l e a c h e d p a r t i c l e s d e m o n s t r a t e s t h a t t h e d i s t r i b u t i o n o f e l e m e n t s i n the n e x t t o s u r f a c e l a y e r s i s u n a l t e r e d . The c o n t i n u e d consumption o f r e a g e n t i s t h e r e f o r e e x c l u s i v e l y o f a d e c o m p o s i t i o n n a t u r e . The r a t e o f f o r m a t i o n o f t h e o x i d e c o a t i n g i s d i f f i c u l t t o c a l c u l a t e from the d a t a o b t a i n e d s i n c e no r e d i s s o l u t i o n e x p e r i m e n t s were pe r f o r m e d . The o b s e r v a t i o n s o f t h e l e a c h i n g time e f f e c t on t h e S/Cu r a t i o change on t h e C u F e S 2 s u r f a c e i n d i c a t e s t h a t the f o r m a t i o n o f t h e o x i d e l a y e r i s f a s t e r t h a n t h e d i s s o l u t i o n o f c o p p e r i n t h e l e a c h s o l u t i o n . T h e r e f o r e i t i s p r o b a b l e t h a t the o x i d a t i o n r a t e o f C u F e S 2 i s s i m i l a r t o t h e o x i d a t i o n r a t e s o f MoS 2 when equal s u r f a c e a r e a s a r e exposed. T h i s s i m i l a r i t y o f t h e o x i d a t i o n r a t e s o f t h e s e two m i n e r a l s was a l s o s u g g e s t e d by B\-Stump and Y. Berube [26] working w i t h a d i s s o l v e d o x y g e n - c a u s t i c system. However t h e i r c o n c l u s i o n t h a t because o f t h e s i m i l a r o x i d a t i o n r a t e s , p r e f e r e n t i a l l e a c h i n g cannot be a c h i e v e d i s not s u p p o r t e d by t h i s work. 89 The p r i n c i p a l d i f f e r e n c e between t h e o x i d a t i o n o f C u F e S 2 and MoS 2 i s t h a t w h i l e t h e f i r s t forms a v e r y e f f e c t i v e p r o t e c t i v e l a y e r , t h e l a t e r forms a r e a d i l y s o l u b l e i o n i c s p e c i e s i m m e d i a t e l y e x p o s i n g f r e s h s u r f a c e . T h i s was c o n f i r m e d by l e a c h i n g e q u a l s u r f a c e a r e a s o f C u F e S 2 and MoS 2 i n a s o l u t i o n w i t h a g r e a t e x c e s s o f NaOCl, F i g u r e 22. The f a v o r a b l e d i s s o l u t i o n o f m o l y b d e n i t e i n t h i s e x p e r i m e n t d e m o n s t r a t e s t h a t i f a m a t e r i a l which c o n s i s t s m a i n l y o f t h e s e two m i n e r a l s i s ground t o i t s l i b e r a t i o n s i z e and a g i t a t e d v i g o r o u s l y i n a s o l u t i o n o f NaOCl t h e p r e f e r -e n t i a l e x t r a c t i o n o f molybdenum s h o u l d be p o s s i b l e . F u r t h e r m o r e t h e r e s u l t s p l o t t e d i n F i g u r e 23 i n d i c a t e t h a t a l t h o u g h t h e p r e s e n c e o f C u F e S 2 l o w e r s t h e MoS 2 d i s s o l u t i o n r a t e s s l i g h t l y , t h e r e i s not a g r e a t v a r i a t i o n i n t h e t o t a l l e a c h i n g t i m e s . T h i s f a c t w i l l be i m p o r t a n t i n t h e d e s i g n o f the c a p a c i t y o f l e a c h i n g equipment as t h e r e a r e c o n t i n u e d v a r i a t i o n s < i n t h e MoS 2 c o n t e n t . o f r o u g h e r c o n c e n t r a t e s . 5.3 L e a c h i n g o f M o l y b d e n i t e from Copper Rougher C o n c e n t r a t e s The r e s u l t s o f t h e e x c e s s i v e consumption o f h y p o c h l o r i t e when l e a c h i n g r o u g h e r c o n c e n t r a t e s was a t t r i b u t e d i n l a t e r e x p e r i m e n t s t o be due p r i n c i p a l l y t o t h e h i g h c o n t e n t o f c o p p e r o x i d e produced i n i t s p r e p a r a t i o n f o r e x p e r i m e n t s . The p r o c e s s o f d r y i n g t h e ro u g h e r c o n c e n t r a t e i n o r d e r t o e l i m i n a t e f l o t a t i o n r e a g e n t s and t o c o n v e n i e n t l y add mass samples t o t h e e x p e r i m e n t s , was u n d e r t a k e n w i t h o u t much c o n s i d e r a t i o n o f t h e change i t would produce i n i t s r e a c t i v i t y . N e i t h e r was i t known 90 a t t h i s s t a g e , t h a t t h e o x i d e s o f some heavy m e t a l s a c t e d c a t a l y t i c a l l y on t h e d e c o m p o s i t i o n o f h y p o c h l o r i t e s . T h e r e f o r e t h e d a t a o b t a i n e d from l e a c h i n g e x p e r i m e n t s o f the p r e t r e a t e d r o u g h e r c o n c e n t r a t e i s o f l i t t l e s i g n i f i c a n c e t o t h e g e n e r a l aim o f t h i s t h e s i s , and the most u s e f u l i n f o r m a t i o n d e r i v e d from i t , i s t h a t s p e c i a l p r e c a u t i o n s would have t o be ta k e n i n a p r a c t i c a l p r o c e s s i n o r d e r t o m a i n t a i n a low c o n t e n t o f o x i d i z e d c o p p e r m i n e r a l s i n the f e e d . In t h e s e e x p e r i m e n t s w i t h a c o n t e n t a v e r a g i n g 1.6% CuO i t was o b s e r v e d t h a t complete e x t r a c t i o n o f MoS 2 c o u l d o n l y be o b t a i n e d w i t h a consumption o f a t l e a s t 400% t h e t h e o r e t i c a l l y r e q u i r e d . S i m i l a r h y p o c h l o r i t e consumption was r e p o r t e d by K .A . S h a p i r o et al. [ 9 ] f o r the l e a c h i n g o f a low molybdenum i n t e r m e d i a t e c o n c e n t r a t e . No e x p l a n a t i o n was g i v e n o f t h i s e x c e s s i v e c o n s u m p t i o n , however t he c o n c e n t r a t e r e c e i v e d a d r y i n g p r o c e s s s i m i l a r t o t h e one used i n t h i s work. The e x p l a n a t i o n o f t h i s g r e a t NaOCl consumption i s t h a t t h e co p p e r o x i d e formed on the s u r f a c e o f CuF e S 2 d u r i n g d r y i n g a c t s as a c a t a l y s t i n t h e d e c o m p o s i t i o n o f t h e reagent, a c c e l e r a t i n g t h e r a t e o f d e c o m p o s i t i o n t o a v a l u e s i m i l a r t o the r a t e o f MoS 2 o x i d a t i o n . The r e s u l t o f m a i n t a i n i n g t h e r o u g h e r c o n c e n t r a t e i n a p u l p t o m i n i m i z e c o n t a c t w i t h t h e a i r , r e d u c e d t h e h y p o c h l o r i t e consumption con-s i d e r a b l y . In the e x p e r i m e n t s w i t h a c o p p e r o x i d e c o n t e n t o f 0.52%, t o t a l r e c o v e r y was o b t a i n e d w i t h o n l y 150% o f the t h e o r e t i c a l l y r e q u i r e d h y p o c h l o r i t e . I f a l e a c h i n g p r o c e s s i s i n c o r p o r a t e d i m m e d i a t e l y a f t e r t h e p r i m a r y f l o t a t i o n , i t s h o u l d be p o s s i b l e t o c o n t r o l t h e o x i d e c o n t e n t below 0.5% s e c u r i n g t h e low r e a g e n t consumptions o b t a i n e d i n t h e s e b a t c h e x p e r i m e n t s . 91 5.4 F l o t a t i o n o f Leached Rougher C o n c e n t r a t e s As the s u r f a c e s o f t h e component m i n e r a l s ( n o t i c e a b l y C u F e S 2 and F e S 2 ) i n the c o p p e r r o u g h e r c o n c e n t r a t e a r e o x i d i z e d d u r i n g t h e h y p o c h l o r i t e l e a c h i n g o f m o l y b d e n i t e , i t was n e c e s s a r y t o d e t e r m i n e i f t h e r o u g h e r c o n c e n t r a t e c o u l d be r e f l o a t e d e c o n o m i c a l l y i n t h e s e c o n d a r y f l o t a t i o n i n o r d e r t o o b t a i n t h e f i n a l c o n c e n t r a t e . The r e s u l t s shown i n F i g u r e 29 i n d i c a t e t h a t the l e a c h e d r o u g h e r c o n c e n t r a t e s t i l l has a s i m i l a r r e c o v e r y - g r a d e c o r r e s p o n d e n c e as t h e non-l e a c h e d m a t e r i a l , the d i f f e r e n c e i s i n t h e consumption o f c o l l e c t o r . In t h e s e e x p e r i m e n t s i n which the same c o l l e c t o r was used f o r both m a t e r i a l s , t h e net a d d i t i o n f o r the s e c o n d a r y f l o t a t i o n would be 2 kgms/ton o f concen-t r a t e . The c o s t o f the e x t r a c o l l e c t o r dosage i s v e r y s m a l l compared t o t h e molybdenum r e c o v e r e d [ 3 8 ] . I t i s a l s o n e c e s s a r y t o i n d i c a t e t h a t e x p e r i m e n t a t i o n w i t h o t h e r r e a g e n t s more s p e c i f i c t o o x i d i z e d s u l f i d e s u r f a c e s c o u l d d e c r e a s e t h i s c o s t even more. The i n f o r m a t i o n g a t h e r e d i s s u f f i c i e n t t o i n d i c a t e t h a t t h e l e a c h i n g o f copper r o u g h e r c o n c e n t r a t e s w i t h NaOCl does n o t d e c r e a s e t o a g r e a t e x t e n t t h e f l o t a b i l i t y and t h a t t h e a d d i t i o n a l c o s t s o f f l o t a t i o n r e a g e n t s i f a l e a c h i n g c i r c u i t were i n t r o d u c e d t o a c o n v e n t i o n a l c o n c e n t r a -t i o n p r o c e s s might n o t be s i g n i f i c a n t . 5.5 P r o d u c t i o n o f Sodium H y p o c h l o r i t e A p r o c e s s d e s i g n e d to e x t r a c t molybdenum from c o p p e r r o u g h e r c o n c e n t r a t e u s i n g NaOCl would r e q u i r e t h a t the r e a g e n t be produced i n - s i t u . P u r c h a s i n g , t r a n s p o r t i n g and s t o r i n g e i t h e r sodium h y p o c h l o r i t e o r t h e 92 r e a g e n t s r e q u i r e d f o r i t s d i r e c t p r o d u c t i o n , c h l o r i n e and c a u s t i c most l i k e l y would r e n d e r t he p r o c e s s u n e c o n o m i c a l . Not c o n s i d e r i n g any s i d e r e a c t i o n s nor d e c o m p o s i t i o n o f sodium h y p o c h l o r i t e , t h e consumption o f t h i s r e a g e n t as i n d i c a t e d by s t o i c h i o m e t r y o f t h e r e a c t i o n between MoS 2 and NaOCl i s 7 kg per kg o f Mo e x t r a c t e d . The p r i c e o f pu r c h a s e d NaOCl i s between 20 and 32 c e n t s p er kg which would a l l o w a narrow margin under t h e p r i c e o f c o n t a i n e d Mo which i s a p p r o x i m a t e l y 3 $/kg. Sodium h y p o c h l o r i t e i s produced by r e a c t i n g c h l o r i n e and sodium h y d r o x i d e , both t h e s e r e a g e n t s a r e g e n e r a t e d s i m u l t a n e o u s l y by e l e c t r o l y z -i n g a sodium c h l o r i d e b r i n e . T h e r e a r e two a l t e r n a t i v e s i n t h e s e l e c t i o n o f e l e c t r o c h e m i c a l c e l l s f o r o n - s i t e g e n e r a t i o n o f sodium h y p o c h l o r i t e based on t h e s e p a r a -t i o n o f t h e a n o d i c and c a t h o d i c compartments. They a r e t h e diaphragm and t h e membrane c e l l s . Diaphragm c e l l t e c h n o l o g y has been employed f o r many y e a r s i n the p r o d u c t i o n o f c h l o r i n e w h i l e t h e membrane t e c h n o l o g y has been c o m m e r c i a l l y proven o n l y i n s m a l l s c a l e . T h e r e a r e no s i g n i f i c a n t d i f f e r e n c e s i n c u r r e n t e f f i c i e n c y between t h e s e two p r o c e s s e s [48]. The advantages o f t h e membrane c e l l a r e a h i g h e r c o n c e n t r a t i o n o f NaOCl and b e t t e r s a l t u t i l i z a t i o n . In b o t h c a s e s t h e c h l o r i n e formed a t t h e anode i s s e p a r a t e d from t h e d e p l e t e d b r i n e and hydrogen i s removed from t he c a u s t i c formed i n th e c a t h o d e compartment. C h l o r i n e and c a u s t i c i m m e d i a t e l y combine i n the r e a c t o r t o form sodium h y p o c h l o r i t e f o l l o w i n g t h e r e a c t i o n 2 NaOH + C l 2 •* NaOCH NaCI + H 20 (23) 93 The p o s s i b i l i t y o f f o r m i n g NaOCl d i r e c t l y i n t h e c e l l s by c a r e f u l l y c o n t r o l l i n g c o n d i t i o n s t o m a i n t a i n some s e c o n d a r y r e a c t i o n s t h a t p roduce NaOCl i s not e c o n o m i c a l l y p r a c t i c a l because s o l u t i o n s p r o d u c e d t h i s way have a low c o n c e n t r a t i o n o f NaOCl and the e l e c t r i c a l e f f i c i e n c y i s low. The o p e r a t i n g c o s t s o f a h y p o c h l o r i t e g e n e r a t o r depend p r i n c i p a l l y on the s a l t u t i l i z a t i o n and t h e power consumption. The t h e o r e t i c a l consumption o f NaCl i s 1.6 Kg per kg o f NaOCl produced. In membrane c e l l s s a l t u t i l i z a t i o n can r e a c h 80% o f t h e t o t a l NaCl f e d t o the p r o c e s s . The minimum v a l u e s f o r c u r r e n t and v o l t a g e a r e 755 amp hr per kg o f c h l o r i n e and 2.15 v o l t s p e r c e l l . Data from o p e r a t i n g o n - s i t e g e n e r a t o r s o f h y p o c h l o r i t e f o r t h e t r e a t m e n t o f water [39] i n d i c a t e s a consumption o f 3.52 Kwh and 2.1 kg o f NaCl per kg o f NaOCl produced. P r o j e c t e d c o s t s f o r a p l a n t p r o d u c i n g NaOCl a t a r a t e o f o v e r 500 kg/day w i t h the assumptions s t a t e d i n T a b l e 9 i s about 12tt/kg which i s between 1/2 and 1/3 o f t h e p u r c h a s e d p r i c e . T a b l e 9 Economic Assumptions f o r t h e E s t i m a t i o n o f C o s t s i n the O p e r a t i o n o f a H y p o c h l o r i t e G e n e r a t o r [39] 1) power c h a r g e i n c l u d i n g demand charge = 2.8it/Kwh 2) s a l t ( s t a n d a r d g r a d e ) b u l k d e l i v e r y = 15$/Ton 3) p r o c e s s water - n e g l i g i b l e 4) l a b o r = 5.71 $/hr 5) l e n g t h o f a m o r t i z a t i o n 20 y e a r s 6) i n t e r e s t 5% 94 I t i s p o s s i b l e t o compare t h e power consumption between t h i s p r o c e s s and the e x t r a c t i o n o f m o l y b d e n i t e by e l e c t r o o x i d a t i o n , a l t h o u g h t h e d a t a f o r the l a t e r was o b t a i n e d from l e a c h i n g a low grade m o l y b d e n i t e o r e i n which t h e c o n t e n t o f Cu was o n l y 0.01% and t h e r e f o r e t h e consump-t i o n o f h y p o c h l o r i t e due t o c a t a l y t i c d e c o m p o s i t i o n s h o u l d be e x p e c t e d t o be n e g l i g i b l e . To a r r i v e a t t h e power consumption o f t h i s p r o c e s s i t has been e s t i m a t e d t h a t t o t a l r e c o v e r y f r o m a c o p p e r r o u g h e r c o n c e n t r a t e can be a c h i e v e d w i t h 160% o f t h e t h e o r e t i c a l amount o r 11.2 kg NaOCl/kg o f Mo and t h a t the power r a t i o i s 3.52 kwh/kg o f NaOCl. T h e r e f o r e : Power consumption e s t i m a t e d f o r t h i s p r o c e s s = 40 kwh/kg molybdenum Power consumption o b t a i n e d i n e l e c t r o o x i d a t i o n p r o c e s s = 53 kwh/kg molybdenum 5.6 K i n e t i c s o f Sodium H y p o c h l o r i t e D e c o m p o s i t i o n The p r o c e s s by which sodium h y p o c h l o r i t e s o l u t i o n s decompose has been s t u d i e d e x t e n s i v e l y . I t i s known t h a t t h e r e a c t i o n can be spontaneous and c a t a l y z e d even i n s l i g h t l y a l k a l i n e s o l u t i o n s . The p r o d u c t s o f r e a c t i o n a r e c h l o r i d e s , c h l o r a t e and oxygen. However t h e mechanisms remain c o n t r o -v e r t i a l p r i n c i p a l l y due t o a n a l y t i c a l d i f f i c u l t i e s . The r e a c t i o n s t h a t most g e n e r a l y d e s c r i b e t h e d e c o m p o s i t i o n o f NaOCl s o l u t i o n s a r e : HC10 + 2 0C1" + C 1 0 3 " + 2 . G l " + H + (24) 95 2 HC10 + OCT -> C10 3 + 2 Cl + 2H (25) HCIO + OCl' 2 C l + 0 2 + H (26) R e a c t i o n s (24) and (25) r e p r e s e n t t h e same p r o c e s s , when the s o l u t i o n i s a l k a l i n e O C l " pr e d o m i n a t e s and i t w i l l f o l l o w ( 2 4 ) , when the s o l u t i o n i s a c i d i f i e d i t f o l l o w s ( 2 5 ) . As t h e r e was no .agreement w i t h r e s p e c t t o t h e o r d e r o f t h e r e a c t i o n n o r w i t h r e g a r d t o r a t e c o n s t a n t s , i t was not p o s s i b l e t o d e t e r m i n e t h e o r e t i c a l l y the r a t e o f d e c o m p o s i t i o n . . f o r v a r i a t i o n s i n such v a r i a b l e s as pH and t e m p e r a t u r e . T h e r e f o r e a few e x p e r i m e n t s on d e c o m p o s i t i o n were pe r f o r m e d t o e s t a b l i s h t h e r e g i o n i n which t h e r e a g e n t c o u l d be used s a f e l y . The NaOCl used was t h e same as t h a t used i n some l e a c h i n g e x p e r i m e n t s , i t had been g e n e r a t e d i n t h e same e x p e r i m e n t a l c e l l as a l l t h e NaOCl s o l u t i o n s used i n t h i s work and c o u l d d i f f e r from o t h e r b a t c h e s i n t h e c o n c e n t r a t i o n o f c h l o r i d e . No p u r i f i c a t i o n was per f o r m e d b e f o r e t h e d e c o m p o s i t i o n r e a c t i o n s . The r a t e s o b t a i n e d from F i g u r e 32 were p l o t t e d on a d o u b l e l o g a r i t h m i c p l o t , l o g r a t e o f d e c o m p o s i t i o n i n mg/l/min v s . l o g c o n c e n t r a -t i o n o f NaOCl i n g/1. The r e s u l t s were a good f i t t i n g s t r a i g h t l i n e w i t h a s l o p e o f 2.5. T h i s o r d e r i s s i m i l a r t o the t h i r d o r d e r r e p o r t e d by s e v e r a l o t h e r w o r k e r s . As i t i s not the i n t e n t i o n o f t h i s work t o s t u d y t he mechanism o f NaOCl d e c o m p o s i t i o n a b r i e f summary o f some o f the most common pap e r s i n t h i s s u b j e c t i s p r e s e n t e d t o i n d i c a t e t h e wide v a r i a t i o n s i n r e s u l t s and i n t e r p r e t a t i o n s . 96 1. N.V.S. Knibbs and H. Pa l f r e e m a n [40] s u g g e s t e d t h a t the r a t e d HftC 1 = K [ H C 1 0 ] 2 [OCT'] (27 and t h a t c h l o r a t e formed by th e r e a c t i o n 0C1" + 2 H0C1 C 1 0 3 " + 2 H + + 2 C l " (25 wi t h a f a s t r e f o r m a t i o n o f H0C1 by 2 0C1" + 2 H + + 2 C l " -> 2 HOC! + 2 C l " (28 2. M.W. L i s t e r [41] and B.P. N i k o l s k i i et al. [ 4 2 ] . Both a g r e e t h a t i t i s a second o r d e r r e a c t i o n and c o r r o b o r a t e e a r l i e r work by F. F o e r s t e r [50] t h a t t h e r e a c t i o n goes t h r o u g h t h e i n t e r m e d i a t e f o r m a t i o n o f c h l o r i t e , t h e r e f o r e C 1 0 3 ~ i s formed i n a l k a l i n e s o l u t i o n s by the r e a c t i o n s HC10 + CIO" + C1 0 2 " + C l " + H + C 1 0 2 " + CIO" + C I O 3 " + C l " HC10 + 2 CIO" + C I O 3 " + 2C1" + H + (29 3. J . D'ans and H.E. Freund [43] d e s c r i b e the f o r m a t i o n o f c h l o r a t e v i a the f o r m a t i o n o f a complex o f two moles o f HC10 and one mole o f 0C1~ f o l l o w e d by the r e a r r a n g e m e n t o f t h e complex a c c o r d i n g t o 97 a f i r s t o r d e r r a t e c o n t r o l l i n g r e a c t i o n t o form c h l o r a t e . No c h l o r i t e was f o u n d i n t h e i r work which shows t h a t i t i s a t h i r d o r d e r r e a c t i o n . 4. H. Imagama et al. [44] a l s o i n d i c a t e s a t h i r d o r d e r r e a c t i o n , however the mechanism proposed i s t h a t HC10 r e a c t s w i t h C 1 0 2 _ l i b e r a t i n g c h l o r i n e d i o x i d e i n the f i r s t s t e p f o l l o w e d by a r e a c t i o n between C 1 0 2 and OCl t o form c h l o r a t e . In such a complex system i t i s d i f f i c u l t t o e x p l a i n t h e 2.5 o r d e r o b t a i n e d i n t h i s work, but as t h e e x p e r i m e n t s were pe r f o r m e d w i t h t h e r e a g e n t i n the same c o n d i t i o n s as was used i n the l e a c h i n g e x p e r i m e n t s , the r a t e c o n s t a n t s o b t a i n e d here a r e u s e f u l f o r comparing w i t h r a t e c o n s t a n t s f o r the l e a c h i n g o f MoS 2. From the i n t e r c e p t o f t h e l i n e s on F i g u r e 35 a t l o g NaOCl = 0 i t i s p o s s i b l e t o o b t a i n the d e c o m p o s i t i o n s r e a c t i o n s r a t e c o n s t a n t s k 3 f o r t h e d i f f e r e n t pH's a t 50°C. From t h i s d a t a i t i s p o s s i b l e t o d e t e r m i n e t h e e f f e c t o f pH on the d e c o m p o s i t i o n r a t e s . A t pH 8 t h e r a t e i s 13 t i m e s the v e l o c i t y a t pH9, and a t pH 7 t h e r a t e i s 60 t i m e s f a s t e r than a t pH 9. An A r r h e n i u s c u r v e was p l o t t e d u s i n g the r a t e s from the decom-p o s i t i o n a t pH 7 a t t h e two t e m p e r a t u r e s e x p e r i m e n t e d (60° and 3 5 ° C ) . The r e s u l t s show an a c t i v a t i o n e n e r g y o f a p p r o x i m a t e l y 20 k c a l . With the i n f o r m a t i o n o f the a c t i v a t i o n energy and t h e r a t e c o n s t a n t s f o r t h e d e c o m p o s i t i o n r e a c t i o n i t i s p o s s i b l e t o c a l c u l a t e t h e l o s s e s o f h y p o c h l o r i t e due t o d e c o m p o s i t i o n f o r a wide range o f o p e r a t i n g v a r i a b l e s . 98 .2 .4 .6 .8 1.0 1.2 Log [NaOCl] (g/1) FIG. 35 - LOG-LOG CURVES OF RATE OF DECOMPOSITION OF NaOCl VS. [NaOCl] . 99 The d e c o m p o s i t i o n r a t e c o n s t a n t was c a l c u l a t e d f o r the o p e r a t i n g c o n d i t i o n s a t which t h e r o u g h e r c o n c e n t r a t e was l e a c h e d , t h a t i s pH = 9 and T = 30°C. The v a l u e o f k 3= 4.89 x 1 0 - 3 ( m i n - 1 ) - H i g h e r t e m p e r a t u r e s o f l e a c h i n g c o u l d be ex p e r i m e n t e d i n the f u t u r e as i t was o b s e r v e d t h a t the maximum r a t e s o f d e c o m p o s i t i o n o f t h e r e a g e n t a t t h e s e h i g h pH's a r e slow i n com p a r i s o n t o the r a t e s o f consumption o f NaOCl i n t h e r e a c t i o n w i t h MoS 2. One example i s p r e s e n t e d i n which t h e f o l l o w i n g c o n d i t i o n s have been s e t : . I n i t i a l C o n c e n t r a t i o n o f NaOC l = 50 g / l W e i g h t o f M o S 2 l e a c h e a b l e w i t h t h i s a m o u n t o f NaOC l = 3 . 6 g I f c o m p o s e d o f p a r t i c l e s - 7 0 + 140 mesh t h e s u r f a c e a r e a = 1432 c m 2 O p e r a t i n g pH = 9 O p e r a t i n g T e m p e r a t u r e = 5 0 ° C V o l u m e o f R e a g e n t = 1000 c m 3 To c a l c u l a t e t h e d e c o m p o s i t i o n r a t e : m ^ M = k 3 [ N a O C l ] 2 " 5 a t pH 9 and 50°C k 3 = 0.0251 4^ -d-^CU= 0.0251 ( 1 5 ) 2 - 5 = 218.7 mg/l/min The r e a c t i o n r a t e i s c a l c u l a t e d a s : 100 d(NaOCl) D x A x C  (Max) = LNaOCl d t 6 x v d(NaOCl) _ 2 x 10 ~ 5 x 1432 x 15 x 60 _ ?t- 7 A n / 1 / m,_ dt 10-3 x 103 25.78 g / l / m i n From t h i s , i t i s p o s s i b l e t o o b s e r v e t h a t t h e r a t e o f consump-t i o n o f NaOCl w i l l be a p p r o x i m a t e l y lOOx f a s t e r i n t h e r e a c t i o n s t e p t h a n i n t h e d e c o m p o s i t i o n one even a t t h i s h i g h t e m p e r a t u r e . However t h e s e r e s u l t s must be t a k e n w i t h c a r e as t h e y do not r e p r e s e n t the i n f l u e n c e o f the c a t a l y s i s o f the c o p p e r o x i d e p r e s e n t i n r e a l s i t u a t i o n s . 5.6.1 E f f e c t o f E x c e s s C h l o r i d e on t h e D e c o m p o s i t i o n Rates The i n c r e a s e i n the r a t e o f d e c o m p o s i t i o n o f NaOCl w i t h i n c r e a s e s o f the C l - c o n c e n t r a t i o n i s now c o n s i d e r e d t o be due t o the i n c r e a s e i n i o n i c s t r e n g t h . o f the s o l u t i o n s and not any d i r e c t p a r t i c i p a t i o n o f t h e c h l o r i d e i o n i n t h e r e a c t i o n [41,42,43]. A l t h o u g h [ C l - ] was n o t measured i n t h e NaOCl s o l u t i o n s used, the i n f l u e n c e o f t h i s v a r i a b l e i s s m a l l compared t o t h e ones a l r e a d y c o n s i d e r e d and as a l r e a d y mentioned t h e same s o l u t i o n s were used f o r l e a c h i n g e x p e r i m e n t s and d e c o m p o s i t i o n e x p e r i m e n t s . 5.6.2 E f f e c t o f C a t i o n s on D e c o m p o s i t i o n Rates The d e c o m p o s i t i o n r a t e depends c o n s i d e r a b l y on t h e c a t i o n o f t h e a l k a l i n e s o l u t i o n . The o r d e r o f s t a b i l i t y o f h y p o c h l o r i t e s o l u t i o n s o f d i f f e r e n t base has been found t o be [ 4 6 ] . 101 NaOCl > K0C1 > C a ( 0 C l ) 2 5.6.3 C a t a l y t i c Agents i n H y p o c h l o r i t e D e c o m p o s i t i o n When a NaOCl s o l u t i o n has been p u r i f i e d from c a t a l y t i c a g e n t s , r e a c t i o n s (24) o r (25) c o n t r i b u t e much more th a n r e a c t i o n (26) i n t h e d e c o m p o s i t i o n o f t h e s o l u t i o n . The p l a t i n u m group m e t a l s have been r e p o r t e d t o c a t a l y z e both r e a c t i o n s and t h e mechanism o f d e c o m p o s i t i o n i n t h e p r e s e n c e o f m e t a l l i c c a t a l y s t s has been c l a i m e d t o be i d e n t i c a l t o t h e spontaneous r e a c t i o n [ 4 5 ] . Ox i d e s o f heavy m e t a l s (Co, Cu and N i ) a r e a l s o known t o c a t a l y z e NaOCl d e c o m p o s i t i o n . I t has been shown t h a t c o pper o x i d e does not a f f e c t t h e r e a c t i o n s l e a d i n g t o C10 3~ f o r m a t i o n , i n s t e a d c o p p e r and n i c k e l o x i d e s s t r o n g l y c a t a l y z e r e a c t i o n (26) a c c e l e r a t i n g t h e f o r m a t i o n o f oxygen [ 4 1 ] . N o r m a l l y t h e f o r m a t i o n o f oxygen and c h l o r a t e t a k e p l a c e i n p a r a l l e l [46,47] and i t was found t h a t t h e amount o f C10 3~ formed i n c r e a s e s w i t h t h e amount o f oxygen e v o l v e d . The maximum e v o l u t i o n was o b s e r v e d i n an a c i d medium [ 4 7 ] . A g a i n t h i s p o i n t i s c o n t r a d i c t o r y and M.W. L i s t e r [41] and I.E. F l i s et al. [47] found t h a t s u b s t a n c e s t h a t produce g r e a t i n c r e a s e s i n 0 2 p r o d u c t i o n do n o t i n c r e a s e C 10 3~ f o r m a t i o n . In t h i s a r e a t h e work p e r f o r m e d was l i m i t e d t o t h e o b s e r v a t i o n t h a t i n the p r o c e s s o f l e a c h i n g w i t h h y p o c h l o r i t e s o l u t i o n s , c o p p e r m i n e r a l s f o r m a c o a t i n g which c o n s i s t s o f c o p p e r o x i d e and t h a t t h i s compound was r e s p o n s i b l e f o r e x c e s s i v e l o s s e s due to- i t s c a t a l y t i c a c t i o n . No attempt was made o f e s t a b l i s h i n g a q u a n t i t a t i v e i n f l u e n c e . 102 5.7 H y p o c h l o r i t e L e a c h i n g P r o c e s s f o r t h e E x t r a c t i o n o f M o l y b d e n i t e  From Copper Rougher C o n c e n t r a t e s The e x p e r i m e n t a l work has e s t a b l i s h e d t h a t m o l y b d e n i t e can be s e l e c t i v e l y l e a c h e d from copper r o u g h e r c o n c e n t r a t e s w i t h o u t s i g n i f i c a n t changes i n the f l o t a t i o n p r o p e r t i e s o f t h e c o p p e r s u l f i d e m i n e r a l s . On t h i s b a s i s , a p r o c e s s i s proposed f o r the a p p l i c a t i o n o f t h i s t e c h n i q u e t o t h e e x t r a c t i o n o f molybdenum. The f l o w - s h e e t i n F i g u r e 36 shows the p o s i t i o n o f the l e a c h i n g p r o c e s s i n t h e c o p p e r c o n c e n t r a t i o n p l a n t . The molybdenum e x t r a c t i o n s h o u l d be i n c l u d e d i m m e d i a t e l y a f t e r t h e p r i m a r y f l o t a t i o n o p e r a t i o n . T h i s w i l l e n s u r e t h a t no more th a n 15% o f t h e t o t a l Mo i n t h e o r e w i l l be l o s t and p r e v e n t o x i d a t i o n o f t h e c o p p e r m i n e r a l s . The l e a c h e d r o u g h e r c o n c e n t r a t e i s washed and r e t u r n s t o the c o p p e r f l o t a t i o n c i r c u i t b e f o r e t h e r e g r i n d i n g s t a g e i n p r e p a r a t i o n f o r t h e s e c o n d a r y c l e a n i n g f l o t a t i o n . A s c h e m a t i c f l o w - s h e e t f o r the molybdenum e x t r a c t i o n p r o c e s s i s p r e s e n t e d i n F i g u r e 37. The fundamental u n i t o p e r a t i o n s a r e : 1) H y p o c h l o r i t e g e n e r a t o r 2 ) L e a c h i n g 3) S/L S e p a r a t i o n 4) M o l y b d e n u m r e c o v e r y Sodium h y p o c h l o r i t e s h o u l d be o f h i g h c o n c e n t r a t i o n (20-30 g/1) when emerging from the h y p o c h l o r i n a t o r s i n c e i t w i l l be d i l u t e d when c o n t a c t e d w i t h t he c o n c e n t r a t e s l u r r y . Due t o the h i g h v e l o c i t y o f the 103 Cu-Mo Ore SIZE REDUCTION PRIMARY FLOTATION Cu ROUGHER CONCENTRATE. HYPOCHLORITE LEACHING PROCESS REGRINDING Molybdenum 81 Rhenium SECONDARY FLOTATION Copper Concentrate FIG. 36 - POSITION OF PROPOSED LEACHING PROCESS IN THE GENERAL Cu CONCENTRATION FLOWSHEET. I Cu circuit „.Cu rougher concentrate j <40% solids-pH 8.0 104 THICKENING pH control LEACHING >50% solids-pH 9.5 NaOCl S/L leached Cu rougher concentrate pH adjustm. Cu circuit recycle H 2 REDUCTION IV l 0 FINAL TREATMENT Mo Products HYPOCHLORINATOR r — f Cl 2 NaOH anodic cothodic NaCI H 0 recycled 2 I NaOH/ H 2 separator EVAPORATOR H, SO^ REMOVAL •NoCI feed s o : 4 effluent FIG. 37 - FLOWSHEET OF THE PROPOSED METHOD FOR EXTRACTING MoS^ FROM COPPER ROUGHER CONCENTRATES. 105 l e a c h i n g r e a c t i o n , t h e r e a g e n t w i l l be added d i r e c t l y t o t h e l e a c h i n g r e a c t o r s . The a d d i t i o n o f c a u s t i c f o r pH c o n t r o l w i l l a l s o be added t o th e r e a c t o r s . R e s i d e n c e time i n t h e l e a c h s e c t i o n f o r a 40% s l u r r y was d e t e r m i n e d e x p e r i m e n t a l l y as 7-10 m i n u t e s , and t h e amount o f a l k a l i needed f o r m a i n t a i n i n g t he pH i n t h e narrow r e g i o n 9.5 t o 8.5 was between 3.0 and 3.5 g/1 when t h e r o u g h e r c o n c e n t r a t e has 0.3% Mo. A r e c y c l e c i r c u i t i s i n c l u d e d i n o r d e r t o b u i l d up t h e con-c e n t r a t i o n o f molybdenum. The amount o f c y c l e s w i l l be d e t e r m i n e d by t h e maximum t o l e r a n c e o f S 0 H ~ i n t h e e l e c t r o l y t i c c e l l s . C a l c i u m , magnesium and s u l f a t e i o n s a r e d e t r i m e n t a l i n t h e b r i n e e l e c t r o l y t i c c e l l as t h e y p r e c i p i t a t e on t h e diaphragm o r membrane [ 4 8 ] . I t w i l l be n e c e s s a r y t o i n c l u d e a S 0 4 ~ removal o p e r a t i o n i n any r e c y c l e c i r c u i t . The s o l u t i o n c o n t a i n i n g NaCl f r e e o f MoO^ - and S 0 4 _ must be c o n c e n t r a t e d i n o r d e r t o be r e c y c l e d as an a n o l y t e . T h i s would r e q u i r e an e v a p o r a t i o n s t a g e . The economics o f r e c y c l i n g t h e l e a c h i n g s o l u t i o n t o m i n i m i z e s a l t consumption r e q u i r e s f u r t h e r s t u d y . F o r c o s t e s t i m a t e s o f t h e p r o c e s s , t o t a l s a l t has been c o n s i d e r e d w i t h o u t any r e c y c l i n g . A major problem o f t h i s p r o c e s s i s t h e molybdenum r e c o v e r y from s o l u t i o n . The c o n c e n t r a t i o n o f MO0L>~ would be low due t o t h e r e c y c l i n g problems a l r e a d y o u t l i n e d . I n s t e a d c h l o r i d e s and s u l f a t e s w i l l be h i g h . T h i s s u b j e c t w i l l have t o be s t u d i e d i n f u t u r e i n v e s t i g a t i o n s . The i d e a l p r o c e s s would r e q u i r e t h a t molybdenum be removed from a l k a l i n e s o l u t i o n s . A l i t e r a t u r e s e a r c h has shown t h a t a l m o s t a l l molybdenum r e c o v e r y p r o c e s s e s u s i n g e i t h e r s o l v e n t e x t r a c t i o n o r i o n 106 exchange a r e worked on a c i d s o l u t i o n s . T h i s would be u n d e s i r a b l e i n t h e pr o p o s e d f l o w s h e e t because o f t h e a c i d consumed i n t h e pH ad j u s t m e n t f o r e x t r a c t i o n and a l k a l i a d d i t i o n f o r r e c y c l e . I t has been r e p o r t e d t h a t A l i q u a t 336, a q u a t e r n a r y amine i s c a p a b l e o f e x t r a c t i n g Mod* - i o n s from a l k a l i n e s o l u t i o n s a l t h o u g h w i t h a d e c r e a s e d l o a d i n g c a p a c i t y [ 4 9 ] . However t h i s r e a g e n t would not be a p p r o p r i a t e t o t h e p r o c e s s because i t s l o a d i n g c a p a c i t y i s d e c r e a s e d by the p r e s e n c e o f C l " i o n s i n a l k a l i n e s o l u t i o n s . A few at t e m p t s were made ( i n our work) o f r e d u c i n g MOOL, i o n s i n a l k a l i n e s o l u t i o n s w i t h o u t s u c c e s s . The r e d u c i n g a g e n t s e x p e r i m e n t e d were sodium d i t h i o n a t e , sodium f o r m a l d e h i d e s u l f o x y l a t e and . h y d r a z i n e . From the p r e s e n t i n f o r m a t i o n i t appea r s t h a t Mo would have t o be r e c o v e r e d from a c i d s o l u t i o n s . T h i s a l t e r n a t i v e can be a c c o m p l i s h e d e i t h e r by i o n exchange, s o l v e n t e x t r a c t i o n o r hydrogen r e d u c t i o n . On t h e b a s i s o f s i m p l i c i t y o f o p e r a t i o n and a v a i l a b i l i t y o f hydrogen from t h e e l e c t r o l y t i c c e l l t h e l a t e r o p t i o n has been s e l e c t e d and i n c l u d e d i n t h e f l o w s h e e t w i t h t h e c o n s i d e r a t i o n t h a t f u r t h e r s t u d i e s a r e r e q u i r e d . The d i s a d v a n t a g e t h a t hydrogen r e d u c t i o n has o v e r t h e o t h e r two o p t i o n s i s t h e d i f f i c u l t y i n s e p a r a t i n g rhenium, which because o f i t s c h e m i c a l s i m i l a r i t y w i l l l i k e l y f o l l o w molybdenum i n t h e e x t r a c t i o n . The consumption o f a c i d - was measured by a d d i n g s u l f u r i c a c i d t o s o l u t i o n s o f sodium h y p o c h l o r i t e a t an i n i t i a l pH o f 9.5 and l o w e r i n g t h e pH t o 4 and 3. I t was o b s e r v e d t h a t t h e r e was a d i f f e r e n c e o f con-sumption i n r e l a t i o n t o the c o n c e n t r a t i o n o f molybdate i o n s . T h e r e f o r e two c o n c e n t r a t i o n s were used a t 1 and 4 g/1 o f Mo. The i n f o r m a t i o n i s p r e s e n t e d i n T a b l e 10. 107 T a b l e 10 Consumption o f A c i d i n t h e R e d u c a t i o n o f pH o f Molybdenum C o n t a i n i n g H y p o c h l o r i t e S o l u t i o n s Consumption o f A c i d (as c o n c e n t r a t e d H2S0it) Reduced t o pH 4 Reduced t o pH 3 1 g/1 Mo-pH 9 .41 ml/1000 ml .50 ml/1000 ml 4 g/1 Mo-pH 9 1.54 ml/1000 ml 1.81 ml/1000 ml C h a p t e r 6 CONCLUSIONS 1. The r e a c t i o n between MoS 2 and. NaOCl. i s - f i r s t o r d e r i n h y p o c h l o r i t e . 2. The a p p a r e n t a c t i v a t i o n e n e r g y i s 6.3 k c a l / m o l e . 3. T h i s r e a c t i o n was p r o b a b l y c o n t r o l l e d i n t h e s e e x p e r i m e n t s by e i t h e r c h e m i c a l o r mixed d i f f u s i o n and c h e m i c a l r e a c t i o n s . 4. The r e a c t i o n r a t e c o n s t a n t was e s t i m a t e d as 1.90 x 10 " 2 m i n - 1 c m - 2 at.pH9 and 4 5 ° . 5. Sodium h y p o c h l o r i t e i s c a p a b l e o f s e l e c t i v e l y l e a c h i n g m o l y b d e n i t e from c h a l c o p y r i t e . 6. M i n e r a l s accompaning m o l y b d e n i t e i n t h e c o p p e r r o u g h e r c o n c e n t r a t e s form p r o t e c t i v e c o a t i n g s which p r e v e n t any s i g n i f i c a n t d i s s o l u t i o n . In t h e c a s e o f C u F e S 2 , t h i s l a y e r was d e s c r i b e d as a s o l i d s o l u t i o n o f CuO - C u ( 0 H ) 2 . 7. The p r e s e n c e o f CuO i s u n d e s i r a b l e i n t h e f e e d t o t h e h y p o c h l o r i t e l e a c h i n g system because i t i s a c a t a l y s t i n t h e decomposi-t i o n r e a c t i o n o f t h i s r e a g e n t . 108 109 9. The r a t e s o f d e c o m p o s i t i o n o f NaOCl a r e n e g l i g i b l e when compared w i t h t h e r a t e o f r e a c t i o n w i t h m o l y b d e n i t e when the s o l u t i o n i s a t pH 9 and ambient t e m p e a t u r e s . 10. The consumption o f NaOCl from c o p p e r r o u g h e r c o n c e n t r a t e s can be kept as low as 1.6 t i m e s t h e s t o i c h i o m e t r i c consumption o f t h e m o l y b d e n i t e p r e s e n t i f t h e c o n c e n t r a t e i s not a l l o w e d t o o x i d i z e b e f o r e l e a c h i n g . 11. With t h e i n f o r m a t i o n g a t h e r e d a f l o w s h e e t has been proposed f o r t h e a p p l i c a t i o n o f t h i s method. The g e n e r a l c o n c e p t i s t h a t l e a c h i n g o f m o l y b d e n i t e be a c c o m p l i s h e d i m m e d i a t e l y a f t e r t h e p r i m a r y f l o t a t i o n i n t h e c o p p e r c o n c e n t r a t i o n p l a n t , u s i n g NaOCl as t h e r e a g e n t which has been p r o d u c e d by an o n - s i t e g e n e r a t o r and t h a t molybdenum be r e c o v e r e d by a hydrogen r e d u c t i o n p r o c e d u r e . C h a p t e r 7 SUGGESTIONS FOR FUTURE WORK T h i s s t u d y has opened up s e v e r a l a r e a s o f work t h a t c o u l d be u s e f u l l y p e r s u e d : 1) Hydrogen r e d u c t i o n o f molybdenum from s o l u t i o n s w i t h h i g h c o n c e n t r a t i o n s o f c h l o r i d e s and s u l f a t e s such as would be o b t a i n e d from t h e h y p o c h l o r i t e l e a c h i n g o p e r a t i o n . 2) R e c i r c u l a t i o n o f l o a d e d s o l u t i o n t h r o u g h the e l e c t r o l y t i c c e l l t o o b t a i n b e t t e r s a l t u t i l i z a t i o n . T h i s s t u d y would a l s o d e t e r m i n e t h e a l l o w a b l e c o n c e n t r a t i o n o f d e t r i m e n t a l i o n s and what t y p e o f p r e -t r e a t m e n t i s r e q u i r e d . 3) F l o t a t i o n o f c o p p e r c o n c e n t r a t e s from r o u g h e r c o n c e n t r a t e s w h ich have been s l i g h t l y o x i d i z e d by h y p o c h l o r i t e . 110 REFERENCES 1. 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