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UBC Theses and Dissertations

Reduction leaching of chalcopyrite 1983

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R E D U C T I O N L E A C H I N G O F C H A L C O P Y R I T E b y R A L P H P E T E R H A C K L B . S c . ( C h e m i s t r y ) , 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 , 1 9 7 7 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L 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 T H E D E G R E E O F M A S T E R O F S C I E N C E i n T H E F A C U L T Y O F G R A D U A T E ( D e p a r t m e n t o f M e t a l l u r g i c a l W e 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 S T U D I E S E n g i n e e r i n g ) 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 J u n e , 1 9 8 3 © R a l p h P e t e r H a c k l , 1 9 8 3 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 the requirements f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make 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 study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department or by h i s o r her r e p r e s e n t a t i v e s . I t i s understood t h a t copying 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 not be allowed without my w r i t t e n p e r m i s s i o n . Department o f < ^ £ ^ C ^ ? U > ^ < ^ ^ L ^ &*^A/*\£JZA-*'**-^ The U n i v e r s i t y o f B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date JM^e £ 2 j H)$>Z DE-6 (3/81) - i i - ABSTRACT This study has been concerned with deve 1 op i ng- nove 1 reduction leach methods for chalcopyr i te (CuFeS2) copper con- centrates. It has been found that chalcopyr i te can be essen- t i a l l y completely converted to cha lcoc i te (Cu2S) by leaching in strong copper sulphate solut ions at elevated temperatures, using either pressurized hydrogen gas or me ta l l i c copper pow- der as reductants. The essent ia l leach reactions appear to obey the fol lowing overa l l stoichiometry: CuFeS 2 + 3 C u 2 + + 2H 2 >2Cu2S + F e 2 + + W+ CuFeS 2 + C u 2 + + 2Cu° >2Cu2S + Fe 2 + . Reaction rates are increased by increasing leach temperature, decreasing concentrate and copper powder pa r t i c l e s i ze , and the presence of a cuprous-stabi1iz ing agent. The sulphide minerals bornite (Cu^FeS/^) and pyr i te (FeS 2 ) , comnonly found in copper concentrates, are also quant i ta t i ve ly converted by reduction leaching under these condi t ions . M i c r o s c o p i c ev idence i n d i c a t e s that c h a l c o c i t e forms as l a ye r s which c rack and s p a l l away from the r e a c t i n g s u l p h - i d e s , a l l o w i n g complete e x t r a c t i o n of i r on to take p l a c e . The mechanism for r e a c t i o n probab ly i nvo l ve s t r a n s p o r t of cuprous ions both in aqueous s o l u t i o n and in the s o l i d s t a t e (CU2S), and s o l i d s t a t e d i f f u s i o n of i r on outward. Cuprous ions are formed as an i n te rmed i a te spec ie s dur ing the l e a c h , e i t h e r by r e a c t i o n of c u p r i c ions w i th hydrogen or with copper meta l . This species then probab ly reac t s w i th the c h a l c o p y r i t e m i n e r a l as f o l l o w s : CuFeS 2 + 4Cu+ >2Cu 2 S + F e 2 + + C u 2 + . P o t e n t i a l methods of i n c o r p o r a t i n g r e d u c t i o n l e a c h i n g i n to a h y d r o m e t a l 1 u r g i c a l process for t r e a t i n g copper con - c e n t r a t e s are d i s c u s s e d . - i v - T A B L E O F C O N T E N T S P a g e A b s t r a c t i i T a b l e o f C o n t e n t s i v L i s t o f T a b l e s v i i L i s t o f F i g u r e s i x A c k n o w l e d g e m e n t s x i C h a p t e r 1 I N T R O D U C T I O N . 1 1 . 1 B A C K G R O U N D 1 1 . 2 E X I S T I N G C O P P E R H Y D R O M E T A L L U R G I C A L T E C H N O L O G Y * 1 . 3 C H L O R I D E R O U T E S 6 1 . 3 . 1 T h e U . S . B . M . C o p p e r P r o c e s s 7 1 . 3 . 2 T h e C L E A R C o p p e r P r o c e s s 9 1 . 3 . 3 T h e C y p r u s C o p p e r P r o c e s s 1 3 1 . 3 . 4 O t h e r C h l o r i d e P r o c e s s e s 1 7 1 . 4 A M M I N E R O U T E S 1 9 1 . 4 . 1 T h e A r b i t e r C o p p e r P r o c e s s 2 0 1 . 5 N I T R A T E R O U T E S 2 4 1 . 5 . 1 T h e N i t r i c - S u l p h u r i c C o p p e r P r o c e s s 2 6 1 . 6 O X I D A T I V E S U L P H A T E R O U T E S 3 2 1 . 6 . 1 C o n c e n t r a t e d S u l p h u r i c A c i d 3 2 1 . 6 . 2 D i l u t e S u l p h u r i c A c i d & O x y g e n 3 3 1 . 6 . 3 D i l u t e S u l p h u r i c A c i d & F e r r i c I r o n . 3 4 1 . 6 . 4 B i o l o g i c a l S y s t e m s 3 5 1 . 7 S U M M A R Y 3 7 2 R E D U C T I O N L E A C H I N G A S A M E T H O D O F I R O N R E M O V A L . . 3 9 2 . 1 B A C K G R O U N D 3 9 - V - 2 . 2 T H E S H E R R I T T - C O M I N C O C O P P E R P R O C E S S 4 5 2 . 2 . 1 C o p p e r - I r o n S e p a r a t i o n S t a g e s 4 7 2 . 2 . 2 J a r o s i t e P r e c i p i t a t i o n 4 9 2 . 2 . 3 C o p p e r R e c o v e r y S t a g e s 5 0 2 . 2 . 4 P r e c i o u s M e t a l s a n d S u l p h u r R e c o v e r y 5 3 2 . 2 . 5 E n e r g y C o n s u m p t i o n 5 3 2 . 2 . 6 E c o n o m i c s 5 4 2 . 2 . 7 S u m m a r y 5 5 2 . 3 P U R P O S E & S C O P E O F T H E P R E S E N T I N V E S T I G A T I O N 5 6 3 E X P E R I M E N T A L 5 9 3 . 1 M A T E R I A L S 5 9 3 . 1 . 1 C o p p e r C o n c e n t r a t e s 5 9 3 . 1 . 2 S u l p h i d e M i n e r a l s 6 1 3 . 1 . 3 R e a g e n t s 6 2 3 . 2 A P P A R A T U S 6 3 3 . 2 . 1 P a r r A u t o c l a v e 6 3 3 . 2 . 2 S h a k i n g A u t o c l a v e 6 5 3 . 3 E X P E R I M E N T A L P R O C E D U R E 6 6 3 . 3 . 1 M a k e - U p o f S t a r t i n g L e a c h i n g S o 1 u t i o n s 6 6 3 . 3 . 2 P a r r A u t o c l a v e 6 7 3 . 3 . 3 S h a k i n g A u t o c l a v e 6 8 3 . 4 A N A L Y T I C A L M E T H O D S 6 9 3 . 4 . 1 S o l u t i o n A n a l y s e s 6 9 3 . 4 . 2 S o l i d s A n a l y s e s 6 9 3 . 5 M A T E R I A L B A L A N C E S A N D E X T R A C T I O N C A L C U L A T I O N S 6 9 4 R E S U L T S A N D D I S C U S S I O N 7 1 4 . 1 R E D U C T I O N L E A C H I N G W I T H H Y D R O G E N G A S 7 1 4 . 2 R E D U C T I O N L E A C H I N G W I T H C O P P E R P O W D E R U N D E R N I T R O G E N 7 6 4 . 2 . 1 E f f e c t o f R e s i d e n c e T i m e a t 1 4 0 ° C . . 7 7 4 . 2 . 2 E f f e c t o f V a r y i n g t h e S t a r t i n g M o l a r C u 2 + / ( F e + Z n ) a n d C u ° / ( F e + Z n ) R a t i o s . 8 0 4 . 2 . 3 E f f e c t o f C o p p e r P o w d e r P a r t i c l e S i z e o n I r o n E x t r a c t i o n 8 3 4 . 3 P O S T U L A T E D R E A C T I O N M E A C H A N I S M S : G A L V A N I C V E R S U S C U P R O U S - M E D I A T E D 8 4 - v i - 4 . 3 . 1 G a l v a n i c M e c h a n i s m 8 4 4 . 3 . 2 C u p r o u s - M e d i a t e d M e c h a n i s m 8 8 4 . 4 R E D U C T I O N L E A C H I N G W I T H C O P P E R P O W D E R U N D E R C A R B O N M O N O X I D E 9 0 4 . 4 . 1 E f f e c t o f C a r b o n M o n o x i d e o n R e a c t i o n K i n e t i c s 9 0 4 . 4 . 2 E f f e c t o f L e a c h T e m p e r a t u r e o n R e a c t i o n K i n e t i c s 9 5 4 . 4 . 3 E f f e c t o f I n i t i a l C o n c e n t r a t e P a r t - i c l e S i z e o n I r o n E x t r a c t i o n 9 9 4 . 5 C O M P A R I S O N O F R E A C T I V I T Y O F S E V E R A L S U L P H I D E M I N E R A L S T O R E D U C T I O N A N D N E U T R A L L E A C H I N G 1 0 1 4 . 6 T H E M O R P H O L O G Y O F R E D U C T I O N L E A C H I N G 1 0 3 4 . 7 K I N E T I C M O D E L S A N D M E C H A N I S M S 1 0 8 5 A P P L I C A T I O N S , C O N C L U S I O N S A N D R E C O M M E N D A T I O N S . . . 1 1 2 5 . 1 A P P L I C A T I O N S O F R E D U C T I O N L E A C H I N G 1 1 2 5 . 1 . 1 I n c o r p o r a t i o n o f R e d u c t i o n L e a c h i n t o S . C . P r o c e s s w i t h n o R o a s t i n g . . 1 1 2 5 . 1 . 2 I n c o r p o r a t i o n o f R e d u c t i o n L e a c h i n t o S . C . p r o c e s s w i t h P a r t i a l R o a s t i n g 1 1 7 5 . 2 C O N C L U S I O N S 1 2 0 5 . 3 R E C O M M E N D A T I O N S F O R F U R T H E R W O R K 1 2 1 R E F E R E N C E S 1 2 3 A P P E N D I X 1 1 3 0 A P P E N D I X 2 1 3 3 - v i i - L IST OF TABLES Tab 1e Page 1 Chemis t ry of the CLEAR Copper Process 11 2 Chemis t ry of the Cyprus Copper Process 15 3 Chemis t ry of the A r b i t e r Copper P r o c e s s . . . . 22 4 R e l a t i v e R e a c t i v i t y and E lementa l Sulphur Y i e l d of Pure M i n e r a l s Leached w i th N i t r i c A c i d 3 8 27 5 Chemis t ry of the N i t r i c - S u 1 p h u r i c Copper Process 29 6 Comparison of Energy Requirements 54 7 Copper Concent ra te s Used 60 8 " P u r e " Su lph ide M i n e r a l s Used 62 9 Reduct ion Leach ing of Phoenix Concen t r a te With Hydrogen Gas 73 10 E f f e c t of Va ry ing S t a r t i n g Molar C u ° / ( F e + Z n ) and C u 2 + / ( F e + Z n ) R a t i o s on Reduc t i on Leach - ing of Fox Lake Concen t r a te 82 11 E f f e c t of Copper Powder P a r t i c l e S i ze on Reduct ion Leach ing of Fox Lake C o n c e n t r a t e . 85 12 Res idence Time R a t i o s at D i f f e r e n t Iron E x t r a c t i o n s - From F i g u r e 14 98 13 E f f e c t of I n i t i a l Concen t r a te P a r t i c l e S i ze on the Reduct ion Leach ing of Bethlehem Con- c e n t r a t e w i th Copper under 1 MPa CO 100 14 Reduct ion and Neut ra l Leach ing Chemis t ry of Severa l Su lph ide M i n e r a l s 102 15 R e s u l t s of Reduc t ion and Neutra l Leach Runs on Severa l Su lph ide M i n e r a l s 104 16 Molar Volumes of Su lph ide Phases 107 - v i i i - 17 Chemis t ry of T ranspor t Processes Dep i c ted in F i g u r e 16 110 18 Chemis t ry of Reduct ion Leach Process w i th no Roa s t i n g 114 - i x - L IST OF FIGURES F i g u r e Page 1 Schematic F lowsheet of the U.S.B.M. Copper P r o c e s s ^ 8 2 Schematic Flowsheet of the CLEAR Copper Process** 10 3 Schematic Flowsheet of the Cyprus Copper P r o c e s s 2 * 14 4 Schematic F lowsheet of the A r b i t e r Copper P r o c e s s 2 9 21 5 Schematic F lowsheet of the N i t r i c - S u l p h u r i c Copper P r o c e s s ^ 28 6 Pourba ix Diagram of the C u - F e - S - r ^ O System of 25 °C ( C o n d i t i o n s : 0.1 M F e , S S p e c i e s ; 0.01 M Cu Spec ies ) 5 9 40 7 S i m p l i f i e d Schematic F lowsheet of the Sher r i 11-Comi nco Copper P r o c e s s ^ 46 8 Reduct ion Leach ing of Ruttan Concen t r a te w i th F i n e Copper Powder at 140 °C - E f f e c t of Res idence Time 78 9 Schematic R e p r e s e n t a t i o n of the C u F e S 2 - C u ° G a l v a n i c Coup le in S u l p h u r i c A c i d S o l u t i o n 6 1 86 10 Schematic R e p r e s e n t a t i o n of a P o s t u l a t e d C u F e S 2 - C u ° Coup le in Copper Su lphate So 1 ut i on 87 11 Schematic R e p r e s e n t a t i o n of a Cuprous - Mediated R e a c t i o n between C h a l c o p y r i t e and Copper 89 12 E f f e c t of CO on the Reduc t i on Leach ing of Bethlehem Concen t r a te w i th Copper at 120 °C 92 13 Reduc t ion Leach ing of Fox Lake C o n c e n t r a t e w i th Copper at 120 °C under 1 MPa CO 94 - X - 1 4 E f f e c t o f T e m p e r a t u r e o n R e d u c t i o n L e a c h i n g o f B e t h l e h e m C o n c e n t r a t e w i t h C o p p e r u n d e r 1 M P a C O 9 6 1 5 O p t i c a l M i c r o s c o p e P i c t u r e s o f C o n v e r s i o n P r o d u c t s f r o m t h e N e u t r a l a n d R e d u c t i o n L e a c h E x p e r i m e n t s o n C h a 1 c o p y r i t e / B o r n i t e 1 0 6 1 6 M o r p h o l o g y o f C h a l c o c i t e F o r m a t i o n b y R e d u c t i o n L e a c h i n g - S o l i d S t a t e a n d S o l u t i o n T r a n s p o r t P r o c e s s e s 1 0 9 1 7 S c h e m a t i c F l o w s h e e t o f R e d u c t i o n L e a c h P r o c e s s w i t h n o R o a s t i n g 1 1 3 1 8 S c h e m a t i c F l o w s h e e t o f R e d u c t i o n L e a c h P r o c e s s w i t h P a r t i a l R o a s t i n g 1 1 9 - x i - ACKNOWLEDGEMENTS I would l i k e to extend my s i n c e r e a p p r e c i a t i o n to Professor Ernie Peters for h i s constant guidance and encourage- ment throughout the course of t h i s s tudy. Thanks are a l s o extended to a l l members of the Department of M e t a l l u r g i c a l E n g i n e e r i n g for t h e i r c o o p e r a t i o n and a s s i s t - ance w i th exper imenta l p rocedure s . The f i n a n c i a l support of the Na t i ona l Sc iences and E n g i n e e r i n g Research C o u n c i l of Canada, and the B.C. Sc ience C o u n c i l , i s g r a t e f u l l y acknowledged. CHAPTER 1 INTRODUCTION 1.1 BACKGROUND Approx imate ly 90% of the w o r l d ' s pr imary copper o r i g i n - ates in low-grade depos i t s of s u l ph ide m i n e r a l s , the most important one being c h a l c o p y r i t e (CuFeS2), fo l l owed by bornite ( C u ^ F e S ^ ) and c h a l c o c i t e ( C u 2 S ) ^ . As the copper c o n t e n t in an orebody is t y p i c a l l y on ly Yi to 2% copper , s u l p h i d e ores are upgraded by f r o t h f l o t a t i o n methods to achieve copper c o n c e n t r a t e s c o n t a i n i n g t y p i c a l l y 20 to 3 0% c o p p e r , w i t h a minimum of gangue m a t e r i a l s . Iron and sulphur are always present as major i m p u r i t i e s . Any copper recovery process from su lph ide concen t r a te s must be concerned p r i m a r i l y w i th the q u a n t i t a t i v e s e p a r a t i o n and removal of i r on and sulphur from copper . At present t h i s is accompl i shed almost e x c l u s i v e l y by pyrometa 11urgica1 m e t h o d s . The s u l p h i d e c o n c e n t r a t e s or p a r t i a 1 1 y - r o a s t e d concen t ra te s are me l ted at 1150-1250 °C to produce a copper- r ich Cu2S-FeS mat te , which is in turn blown w i th a i r to s e l e c t i v e l y o x i d i z e most of the i r on and su lphur wh i l e c o n v e r t i n g the copper s u l p h i d e s to l i q u i d m e t a l l i c copper in a crude (98.5- 99.5% Cu) " b l i s t e r " copper form. Th i s crude copper must be f u r t h e r f i r e - and e 1 e c t r o - r e f i n e d be fo re i t is s u i t a b l e for use. - 2 - Copper pyrometa 11urgy, a very o l d and e s t a b l i s h e d techno logy , has the advantages of high copper recovery (98%), r e c o v e r y of most m inor m e t a l s , and v i r t u a l l y 100% recovery of p r e c i o u s m e t a l s by r e f i n i n g . S m e l t i n g and c o n v e r t i n g r e a c t i o n s a r e r a p i d and go to comple t ion at the h igh process temperatures employed. However , i t has l ong been recogn i zed that the s i n g l e g rea te s t p r o b l e m w i t h copper py rometa 11urgy , and f o r t h a t m a t t e r s m e l t i n g in g e n e r a l , is the p roduc t i on of l a rge quan- t i t i e s of noxious su lphur d i o x i d e c o n t a i n i n g ga s ; a d i r e c t c o n s e q u e n c e of o x i d i z i n g s u l p h u r at h i g h t e m p e r a t u r e . A c h a l c o p y r i t e c o n c e n t r a t e p r o d u c e s v e r y n e a r l y two t o n n e s of S O 2 gas per tonne of py rometa11urg i ca11y -ex t rac ted copper . As S O 2 emi s s ion c o n t r o l s become i n c r e a s i n g l y s t r i n gen t , copper p lant des i gner s are f i n d i n g that a s i g n i f i c a n t p o r t i o n of c a p i t a l cos t s are a s s o c i a t e d w i th i n s t a l l i n g s o p h i s t i c a t e d SO2 c o l l e c t i o n systems. Th i s is p a r t i c u l a r l y the case in the U n i t e d S t a t e s , where dur ing the 1970's an es t imated 22 cents per k i l o g r a m copper was added to the domestic production costs of the primary copper i n d u s t r y through improved p o l l u t i o n c o n t r o l s 2 . One study conc luded that whi le cap i t a l expenditures by most U.S. copper companies have i nc reased s h a r p l y , a s i g - n i f i c a n t p o r t i o n (25%) has been a l l o c a t e d for n o n - p r o d u c t i v e p o l l u t i o n abatement-^. Sulphur d i o x i d e emi t ted from a roa s te r is r e l a t i v e l y concentrated (5-15%) and can be f i x e d as s u l p h u r i c a c i d , but S O 2 emi t ted from a sme l t ing furnace is of low con - centrat ion (0 .1-4%) , which for economic reasons can be n e i t h e r conver ted to a c i d nor n e u t r a l i z e d w i th l ime. In a d d i t i o n , - 3 - smel ters have a d i f f i c u l t time t r e a t i n g the lower-grade c o n c e n t r a t e s , that is those c o n t a i n i n g high l e v e l s of p y r i t e or meta l s such as a r s e n i c , b i smuth, antimony, lead and z i n c . The c o n t i n u i n g a i r p o l l u t i o n problems a s s o c i a t e d w i th copper pyrometa11urgy have p rov ided a great impetus for de- velopment of t e c h n i c a l l y and e c o n o m i c a l l y v i a b l e hydrometa l - l u r g i c a l a l t e r n a t i v e s , in which sulphur can be r e j e c t e d in an i n e r t s o l i d form. In the past 15 y e a r s , no less than 20 major hydrometa 11urg ica I p rocesses capable of t r e a t i n g copper su l ph ide concen t r a te s have been developed and patented in North Amer ica; yet on ly two of these have ever s u c c e s s f u l l y reached the commercial o p e r a t i n g s tage. These are the Anaconda Company's " A r b i t e r " Process, which is based on an oxygen-arrmon i a l e a c h ; and Duval C o r p o r a t i o n ' s "CLEAR" ch1 o r i d e - 1 each process. The Anaconda p l a n t , an i n s t a l l a t i o n capable of the p r o d u c t i o n of 33,000 tonnes per year of cathode copper^, was in operation from September 1974 to December 1977, at which time i t was shut down due to the depressed copper market at the t ime^. The Duval p lant s t a r t e d corrmercial p roduc t i on at 29, 000 tonnes copper in 1978, and is s t i l l in production at a current capacity of 36,000 tonnes copper per y e a r 6 . The copper i n d u s t r y has always viewed copper hydrometal- l u r g i c a l processes as being unable to compete t e c h n i c a l l y or e c o n o m i c a l l y w i th e x i s t i n g pyrometa 11urgica1 p r a c t i c e , d e s p i t e the f o r m e r ' s appeal of be ing non -a i r p o l l u t i n g . For example, one expert has s t a ted that to be conrmer c i a 1 1 y v i a b l e , a new h y d r o m e t a l 1 u r g i c a l route must demonstrate 30% lower c a p i t a l cost and at l ea s t 20% lower o p e r a t i n g cos t s _ IL _ t h a n c o m p e t i n g p y r o m e t a 11 u r g i c a 1 r o u t e s ' ' , t a r g e t f i g u r e s w h i c h h a v e y e t t o b e d e m o n s t r a t e d b y c o p p e r h y d r o m e t a 1 1 u r g i s t s . D e s p i t e t h e c o p p e r i n d u s t r y ' s n e g a t i v e o u t l o o k o n h y d r o m e t a l l u r g i c a l p r o c e s s e s , r e s e a r c h i n t o i m p r o v i n g e x i s t i n g p r o c e s s e s a n d d e v e l o p i n g n e w o n e s c o n t i n u e s . I t i s t h e o b j e c t - i v e o f t h i s t h e s i s t o p r e s e n t t h e r e s u l t s o f r e s e a r c h i n t o a n e w l y - d e v e l o p e d r e d u c t i o n l e a c h m e t h o d f o r c o p p e r c o n c e n - t r a t e s . S u g g e s t i o n s f o r i n t e g r a t i n g t h e r e d u c t i o n l e a c h i n t o o n e o f t h e m o r e p r o m i s i n g r e c e n t l y d e v e l o p e d h y d r o m e t a l - l u r g i c a l p r o c e s s e s , w i t h t h e o b j e c t i v e o f i m p r o v i n g s a i d p r o c e s s t o m a k e i t m o r e a t t r a c t i v e f r o m a c o m m e r c i a l s t a n d p o i n t , w i l l b e p r e s e n t e d . 1.2 E X I S T I N G C O P P E R H Y D R O M E T A L L U R G I C A L T E C H N O L O G Y A n y h y d r o m e t a 1 1 u r g i c a 1 p r o c e s s f o r c o p p e r c o n c e n t r a t e s s h o u l d b e a b l e t o t r e a t c h a l c o p y r i t e i f i t i s t o g a i n c o m m e r c i a l a c c e p t a n c e . U n f o r t u n a t e l y , c h a l c o p y r i t e i s , i n g e n e r a l , t h e m o s t r e f r a c t o r y o f t h e c o p p e r s u l p h i d e m i n e r a l s t o l e a c h i n g m e t h o d s . T h e f o l l o w i n g d i s c u s s i o n o f h y d r o m e t a 1 1 u r g i c a 1 p r o c e s s e s w i l l t h e r e f o r e f o c u s o n c h a l c o p y r i t e , b u t i t s h o u l d b e n o t e d t h a t a n y p r o c e s s e f f e c t i v e i n t r e a t i n g c h a l c o p y r i t e s h o u l d b e a p p l i c a b l e t o a l l t h e o t h e r c o m m o n c o p p e r s u l p h i d e m i n e r a l s . G e n e r a l l y s p e a k i n g , t h e i d e a l c o p p e r 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 w o u l d s a t i s f y a l l o f t h e f o l l o w i n g r e q u i r e m e n t s : - 5 - 1. The a b i l i t y to ach ieve high o v e r a l l copper r e c o v e r i e s (at l e a s t 98%) from w i d e l y - v a r y i n g grades of copper c o n c e n t r a t e s . 2. High c o n v e r s i o n of su lph ide sulphur to a marketab le e lementa l form. 3. P roduc t i on of w i r e - b a r grade cathode copper wi thout a separate r e f i n i n g s t ep . 4. High recovery of p rec i ou s meta l s ( s i l v e r and go 1d ) . 5. Sepa ra t i on of i r on as a marketab le p roduc t , or sa fe d i s p o s a l as an innocuous s o l i d . 6. Recovery or sa fe d i s p o s a l of minor meta l s such as se len ium, a r s e n i c , ant imony, b i smuth, l e a d , z i n c , molybdenum, n i c k e l and c o b a l t . 7. Use of i nexpens i ve l i x i v i a n t s , p r e f e r a b l y ones which can be regenera ted i n - s i t u and are n o n - c o r r o s i v e . 8. Low energy consumption by u t i l i z i n g exothermic leach r e a c t i o n s and chemica l reductant s ( ra ther than e1ec t rowinn ing ) for copper w inn ing . R e g r e t t a b l y , no e x i s t i n g hyd rometa l 1u r g i c a l process can c l a i m to meet a l l of the above c r i t e r i a , a l though some have the p o t e n t i a l to be c l o s e to i d e a l . Copper hydrometa 11urgica1 routes can be c l a s s i f i e d in a number of ways; one conmnon procedure is to c a t e g o r i z e them a c c o r d i n g to the type of l i x i v i a n t system used in the - 6 - l e a c h s t e p . H i s t o r i c a l l y , t h e m o s t c o m m o n l y u s e d l i x i v i a n t s i n c o p p e r h y d r o m e t a l 1 u r g y a r e : a c i d i c o x i d a t i v e c h l o r i d e m e d i a , a l k a l i n e o x i d a t i v e a r r m i n e m e d i a , a c i d i c o x i d a t i v e n i t r a t e o r n i t r a t e - s u 1 p h a t e m e d i a , a c i d i c o x i d a t i v e s u l p h a t e m e d i a . 1.3 C H L O R I D E R O U T E S T h e m a j o r i t y o f r e c e n t h y d r o m e t a 1 1 u r g i c a 1 r e s e a r c h a n d d e v e l o p m e n t h a s f o c u s s e d o n c h l o r i d e l e a c h r o u t e s . F e r r i c c h l o r i d e a n d c u p r i c c h l o r i d e h a v e p r o v e n t o b e t h e m o s t p o p u l a r o f t h e c h l o r i d e l i x i v i a n t s d u e t o t h e i r a b i l i t y t o r a p i d l y d e c o m p o s e c h a l c o p y r i t e a n d c o n v e r t t h e s u l p h i d e p o r t i o n t o e l e m e n t a l s u l p h u r . T h e c h e m i s t r y a n d m e c h a n i s m o f c h l o r i d e l e a c h i n g o f c o p p e r s u l p h i d e s h a s b e e n s t u d i e d e x t e n s i v e l y a n d i s s t i l l t h e s u b j e c t o f c o n s i d e r a b l e c o n t r o v e r s y . T h e r e a d e r i s r e f e r r e d t o s e v e r a l e x c e l l e n t r e c e n t p a p e r s o n t h e s u b j e c t 8 " ^ . F o r t h e p u r p o s e o f t h i s s t u d y , i t i s d e s i r a b l e t o r e v i e w t h r e e o f t h e m o r e p r o m i s i n g ( f r o m a c o r r m e r c i a l s t a n d p o i n t ) c h l o r i d e l e a c h p r o c e s s e s d e v e l o p e d i n t h e p a s t d e c a d e , a n d b r i e f l y m e n t i o n i m p o r t a n t a s p e c t s o f s o m e o f t h e o t h e r i n t e r e s t - i n g o n e s . - 7 - 1.3.1 The U.S.B.M. Copper Process In 1971 Haver and Wong, w h i l e working for the U.S. Bureau of Mines, became the f i r s t i n v e s t i g a t o r s to demonstrate that very h igh copper e x t r a c t i o n s cou ld be ach ieved us ing s t rong f e r r i c c h l o r i d e s o l u t i o n as a 1 i x i v i a n t 1 2 " 1 ^ . They developed what became known as the U.S.B.M. Copper P roces s . A s i m p l i f i e d schemat ic f lowsheet of t h i s process is reproduced in Figure 1. The leach chemi s t ry is t y p i f i e d by the f o l l o w i n g react i on: CuFeS 2 + 3 F e C l 3 — >CuCl + 4 F e C l 2 + 2 S ° . (1) Haver and Wong were ab le to o b t a i n 99.9% copper e x t r a c t i o n in two hours when l e a c h i n g f i n e l y ground (-325 mesh) c h a l - c o p y r i t e at 106 ° C 1 3 . P y r i t e , mo lybden i te and go ld remained una t t acked , but 96% s i l v e r d i s s o l u t i o n was a c h i e v e d . The f e r r i c c h l o r i d e l i x i v i a n t is regenerated by oxidat ion of f e r r o u s c h l o r i d e in a separate step 1 2 F e C l 2 + 3 0 2 + xH 2 0 >8FeC l 3 + 2 F e 2 0 3 - x H 2 0 (2) with excess i r on d i sposed of as an impure f e r r i c ox ide p r e c i p - i t a t e . Copper i s e lec t rowon in a diaphragm c e l l , and sulphur is recovered from leach re s i due in an ammonium su l ph ide leach s t ep . The U.S.B.M. Process succeeded in a c h i e v i n g a very h igh copper r e c o v e r y . Copper is e lec t rowon from the cuprous Ballmilled Concentrate OXIDATION I S FeCI3 EVAPORATION Gangue + Sulphur CONDENSING DRYING AND MELTING T Copper f+Ag] To Electrorefining F e 2 0 3 xH 20 To Tailings Pond (NH}2S LEACHING SULPHUR PRECIPITATION Sulphur Make up (NH4)2S * To Tailings Pond FIGURE 1 Schematic Flowsheet of the U.S.B.M. Copper Process14 - 9 - s t a t e at the r e l a t i v e l y low power consumption of about 1.5 kWh/kg C u ^ . Th i s can be compared to the power consumption for e l e c t r o w i n n i n g copper from the c u p r i c s t a te in su lphate s o l u t i o n s , which i s o f t e n quoted as being about 2.6 kWh/kg Cu. However, the U.S.B.M. Process has made no p r o v i s i o n for r e c o v e r i n g p r e c i o u s or minor m e t a l s , and the copper product r e q u i r e s f u r t h e r r e f i n i n g to ach ieve adequate p u r i t y . An economic e v a l u a t i o n conc luded that the process was competitive w i th e x i s t i n g pyrometa 11urgica1 p r a c t i c e 1 ^ , but as yet the U.S.B.M. Process has not been used corrrner c i a 11 y . 1.3.2 The CLEAR Copper Process D u v a l C o r p o r a t i o n ' s CLEAR P r o c e s s (an ac ronym f o r Copper L e a c h E l e c t r o l y s i s and R e g e n e r a t i o n ) , p a t e n t e d i n 1 9 7 4 1 6 , has r e a c h e d c o m m e r c i a l p r o d u c t i o n of 36,000 tonnes copper per y e a r 6 and as such must be r e g a r d e d as the most s u c c e s s f u l of the c h l o r i d e l e a c h p r o c e s s e s . A s c h e m a t i c f l o w s h e e t of the CLEAR P r o c e s s i s p r e s e n t e d in F i g u r e 2, and the p e r t i n e n t chemis t ry is surrmarized in Tab le 1. The CLEAR Process d i f f e r s from the U.S.B.M. Process in that two s u c c e s s i v e leach steps are employed to e x t r a c t 99.5% of the copper from a c h a l c o p y r i t e f e e d , and s i m u l t a n - eous ly d i spose of i r on as a g o e t h i t e - j a r o s i t e m i x t u r e . As w i th the U.S.B.M. P r o c e s s , p y r i t e and mo lybden i te remain una t t acked , but a s u b s t a n t i a l p o r t i o n of any s i l v e r would be s o l u b i l i z e d . The f i r s t - s t a g e l e a c h , conducted at 107 °C for VA hours , is e s s e n t i a l l y a c u p r i c c h l o r i d e leach make up NaCI, — KCI Concentrate i _ FIRST-STAGE LEACH St OXIDATION AND REGENERATION -PURGE J 7> SULPHUR RECOVERY Cement Copper REDUCTION ELECTROLYSIS S i 1 I o Copper S Basic Iron Salts Gangue FIGURE 2 Schematic Flowsheet of the CLEAR Copper Process6 - 11 - TABLE 1 Chemis t ry of the CLEAR Copper Process F i r s t - S t a g e Leach (1-1) 2CuFeS 2 + 3 C u C l 2 5>4CuCl + F e C l 2 + 2S° + CuFeS 2 P r e l i m i n a r y Reduc t ion (1-2) C u C l 2 + Cu »2CuCl E l e c t r o l y s i s (1-3) 4CuCl + F e C l 2 , »2Cu + 2 C u C l 2 + F e C l 2 Second-Stage Leach (Ox ida t i on and Regenera t i on -Purge ) (1-4) CuFeS 2 + 2 S° + 2 C u C l 2 + F e C l 2 + 3/2 0 2 + 3H 2 0 > 3 C u C l 2 + 2Fe(OH) 3 + 4 S° (1-5) 1 2 F e C l 2 + 30 2 + 2H 2 0 >4FeOOH + 8 F e C l 3 (1-6) 6FeSO^ + 1 2 F e C l 2 + 9/2 0 2 + 3KC1 + 9H 2 0 >3KFe 3 (SO^) 2 (OH) 6 + 9 F e C l 3 - 12 - which s o l u b i l i z e s about 50% of the c h a l c o p y r i t e copper as cuprous c h l o r i d e (equat ion 1-1). The pregnant l i q u o r is d i r e c t e d to a p r e l i m i n a r y r e d u c t i o n stage where some cement copper is added to reduce any remaining c u p r i c c h l o r i d e to the cuprous s t a t e (equat ion 1-2). The l i q u o r is then ready for en t ry to the diaphragm-1ype e1ec t rowinn ing c e l l s where h a l f of the copper is won as product at the cathode, and half is ox id ized back to c u p r i c c h l o r i d e at the anode (equat ion 1-3). The a n o l y t e from e1ec t row inn ing is r e c y c l e d back to the second-s tage l e a c h , which i s conducted at 140 °C under 276 kPa oxygen pres sure for lYi hours . Th i s leach serves a t h r e e - f o l d purpose - to d i s s o l v e the remain ing cha l copyr i te , to complete r e g e n e r a t i o n of the leach s o l u t i o n r e q u i r e d for the f i r s t - s t a g e l e a c h , and to p r e c i p i t a t e i r on as a ba s i c sa l t for d i s p o s a l . Equat ion 1-4 d e p i c t s the o v e r a l l chemi s t ry of t h i s s t ep . A minor amount of sulphur is oxidized to sulphate which is purged from s o l u t i o n as potass ium j a r o s i t e (equat ion 1-6). The remain ing i r on tends to p r e c i p i t a t e as the e a s i l y f i l t e r a b l e goethite (equation 1-5) rather than f e r r i c h y d r o x i d e . Regard ing su lphur r e c o v e r y , the l i t e r a t u r e i s vague and mentions on ly that " i t appears p o s s i b l e to produce a marketab le su lphur product from the re s i due by f r o t h f l o t a t - i o n " 6 . U n l i k e the U.S.B.M. P r o c e s s , the CLEAR Process has a p p a r e n t l y so l ved the problems of p rec i ou s meta l s recovery and minor meta l s r e c o v e r y / d i s p o s a l to y i e l d a r e l a t i v e l y pure copper p roduc t , but no d e t a i l s are a v a i l a b l e as y e t . The copper product almost c e r t a i n l y r e q u i r e s f u r t h e r r e f i n i n g - 13 - be fore i t is s u i t a b l e for use. The process is a l s o c l a imed to be economically compet i t ive w i th pyrometal 1 urg i c a l p r a c t i c e , but no cost data are a v a i l a b l e to support t h i s c l a i m . 1.3.3 The Cyprus Copper Process The Cyprus M e t a l l u r g i c a l Processes Corporation developed a novel process in 1972 whereby c h a l c o p y r i t e was a n o d i c a l l y decomposed in c h l o r i d e s o l u t i o n 1 ^ . Th i s p roce s s , named the "Cymet" P r o c e s s , had the d i s t i n c t i v e f e a t u r e of being ab le to r e c o v e r bo th copper and i r o n e 1 e c t r o 1 y t i c a 1 1 y, and was in f a c t demonstrated in a 23 tonne per day c h a l c o p y r i t e con - c e n t r a t e p l a n t i 8 > 1 9 . U n f o r t u n a t e l y the Cymet Process suffered from very high power consumption, problems w i th the e l e c t r o l y t i c c e l l des ign and the i n a b i l i t y to produce a pure copper product. It was abandoned in 1975 and an improved process which d i d away w i th the e l e c t r o l y t i c s t ep , renamed the "Cyprus" Process, was developed and is now being p i l o t e d at a 100 kg/h copper r a t e 2 0 ' 2 1 . A s i m p l i f i e d schemat ic f lowsheet of the Cyprus Process is p resented in F i g u r e 3 and the important r e a c t i o n chemi s t ry i s surrmarized in Tab le 2. Copper concen t ra te s are d i s s o l v e d in a m ixed f e r r i c ch 1 o r i d e - c u p r i c c h l o r i d e s o l u t i o n i n a counter cur rent two-stage l e a c h . The f i r s t leach stage serves to s o l u b i l i z e about ha l f the c h a l c o p y r i t e copper as cuprous c h l o r i d e (equat ions 2-1 and 2-2) which i s d i r e c t e d to the copper recovery s tage . A minor amount of su lphur is o x i d i z e d to su lphate (equat ion 2 -3 ) . The second leach stage i s e s sen - Concentrate 1 OXYDROLYSIS TAILINGS TREATMENT F T _w Jarosite.Gangue To Tailings Pond FLUID BED REACTOR \ MEL1 FURh riNG JACE Copper •HC1 To Oxydrolysis MoS 2 FIGURE 3 Schematic Flowsheet of the Cyprus Copper Process - 15 - TABLE 2 Chemis t ry of the Cyprus Copper Process Leach 1 (2-1) CuFeS 2 + 4 F e C l 3 (2-2) CuFeS 2 + 3 C u C l 2 (2-3) S ° + 6 C u C l 2 + 4H 2 0 Oxydro l y s i s (2-4) 4CuCl + 0 2 + 4HC1 >4CuC l 2 + 2H 2 0 (2-5) 4 F e C l 2 + 0 2 + 4HC1 > 4 F e C l 3 + 2H 2 0 (2-6) 3 F e C l 3 + 2Na 2SO^ + 6H 2 0 > N a F e 3 ( S O ^ ) 2 ( O H ) 6 + 3NaCl + 6HC1 (2-7) F e C l 3 + 6H 2 0 > F e ( O H ) 3 ( H 2 0 ) 3 + 3HC1 Leach 2 (2-8) CuCl + F e C l 3 > F e C l 2 + C u C l 2 (2-9) CuFeS 2 + 4 F e C l 3 > C u C l 2 + 5 F e C l 2 + 2S° Meta l Reduc t i on » C u C l 2 + 5 F e C l 2 + 2S° -*4CuCl + F e C l 2 + 2 S° >6CuCl + H 2SO^ + 6HC1 (2-10) CuCl + 1/2 H 2 ^ •> Cu + HC1 - 16 - t i a l l y a f e r r i c leach and d i s s o l v e s the remain ing cha lcopyr i te w h i l e r e g e n e r a t i n g l i x i v i a n t r e q u i r e d for the f i r s t leach ( e q u a t i o n s 2-8 and 2 - 9 ) . An o v e r a l l copper e x t r a c t i o n of 97.5% is o b t a i n e d , w i th p y r i t e and mo lybden i te remain ing unat t a cked . Cuprous c h l o r i d e from the f i r s t leach stage i s recovered by f l a s h i n g the hot pregnant l i q u o r to about 40 °C to c r y s t a l l i z e C u C l . The spent l i q u o r from c r y s t a l - l i z a t i o n i s d i r e c t e d to an o x y d r o l y s i s s t a ge where F e C l 3 is regenerated for the second-s tage leach (equat ion 2-5) and excess i r on i s removed as a m ix tu re of hydrated f e r r i c ox ide and j a r o s i t e (equat ions 2-6 and 2 -7 ) . Copper is recovered from CuCl in a unique process in which the c r y s t a l s are reduced w i th hydrogen in a f l u i d - bed reac to r at 510 °C (equat ion 2-10) . Copper forms as nodules c o v e r i n g sand p a r t i c l e s , which are me l ted in a c o n v e n t i o n a l f u r n a c e , s lagged to remove i m p u r i t i e s , po led to remove oxygen and cast i n t o w i r e b a r s . The w i re produced from these bars is a p p a r e n t l y of a p u r i t y comparable to e l e c t r o r e f i n e d copper. The Cyprus Copper Process thus e x h i b i t s a d i s t i n c t advantage over the U.S.B.M. and CLEAR Processes - i t avo ids the energy i n t e n s i v e e1ec t row inn ing steps of the l a t t e r two and produces a pure copper p roduc t . The t o t a l energy r e q u i r e - ment of the process i s c l a imed to be 37.1 MJ/kg Cu. The process r e s o r t s to a copper winn ing s t e p tha t i s p y r o m e t a l - l u r g i c a l ra ther than h y d r o m e t a l 1 u r g i c a l , but i l l u s t r a t e s the d e s i r a b i l i t y of seek ing chemica l r e d u c t i v e methods as a l t e r n a t i v e s to e 1 e c t r o w i n n i n g . A v a i l a b l e l i t e r a t u r e on - 17 - the Cyprus Process mentions that methods of recovery of S, MoS2> Au, Ag and other minor metals have been developed, but no de ta i l s are ava i lab le as yet . 1.3.4 Other Chlor ide Processes A number of other ch lor ide leach processes exist which never reached the p i lo t plant stage, but have in teres t ing features nonetheless. A French p r o c e s s 2 2 leaches chalcopyrite with cupr ic ch lor ide in the normal manner, but the leached copper, complexed as CuCl 2 ~ > is then oxidized to cupric chloride while copper is solvent extracted simultaneously by a LIX reagent, and str ipped from the organic by H 2 S O 4 to y i e ld a pure CuSO^ e l e c t r o l y t e . Copper is then electrowon in the normal matter. The process obviously u t i l i z e s solvent ex t rac t - ion to get around the d i f f i c u l t i e s of electrowinning in chloride media and to gain a pure copper e l ec t ro l y te . But i t is doubtful whether the added c o s t s of incorporat ing a solvent extract ion step, plus the extra power consumption inherent in e lectro- winning from sulphate media, is j u s t i f i e d . The Un ivers i ty of B r i t i s h Columbia, in conjunction with Cominco L td . patented a ch lor ide leach p r o c e s s ^ ' ^ in which the problems of copper winning and precious metals recovery were s p e c i f i c a l l y addressed. In the U.B.C. - Cominco process, cha lcopyr i te is leached by a standard chlor ide route and product is c r y s t a l l i z e d as CuCl , which contains s i l ve r - 18 - i n s o l i d s o l u t i o n . A n o v e l m e t h o d o f s i l v e r r e c o v e r y w a s d e v e l o p e d w h i c h c o n s i s t e d o f d i s s o l v i n g C u C l i n N H ^ C l t o g e n e r a t e t h e c u p r o u s a m m i n e c o m p l e x C u ( N H 3 ) 2 + , w h i c h i s a s t r o n g e n o u g h r e d u c i n g a g e n t t o p r e c i p i t a t e s i l v e r a s f o l l o w s : C u ( N H 3 ) 2 + + 2 N H 3 + A g C l 2 _ » A g ° + C u ( N H 3 ) ^ 2 + + 2 C 1 - . ( 3 ) T h e s u p e r n a t a n t s o l u t i o n i s t h e n s t e a m s t r i p p e d b a c k t o n e u t r a l p H t o y i e l d a C u N H 3 C l c o m p l e x , w h i c h c o u l d b e t h e r m a l l y d e c o m - p o s e d t o C u C l a n d N H 3 a t 2 5 0 ° C . T h e U . B . C . - C o m i n c o P r o c e s s a l s o o f f e r e d a g o l d r e c o v e r y m e t h o d . T h i s m e t h o d c o n s i s t e d o f s o l u b i l i z i n g g o l d f r o m l e a c h r e s i d u e w i t h f e r r i c c h l o r i d e , a c c o r d i n g t o 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 : A u + 3 F e 3 + + 4 C 1 " > A u C l / 4 - + 3 F e 2 + . ( 4 ) A s m a l l a m o u n t o f c h l o r i n e g a s w a s u s e d t o e n s u r e t h a t F e C l 2 w a s r e - o x i d i z e d . G o l d e x t r a c t i o n s o f 9 1 . 5 % w e r e o b t a i n e d a t 6 0 ° C . O t h e r v e r y r e c e n t c h l o r i d e l e a c h p r o c e s s e s f o r w h i c h l i t t l e i s y e t k n o w n , b u t w h i c h d e s e r v e m e n t i o n a r e t h e G r e a t C e n t r a l M i n e s P r o c e s s , d e v e l o p e d b y t h e V a n c o u v e r , B . C . f i r m o f B a c o n , D o n a l d s o n a n d A s s o c i a t e s L t d . 2 ^ , a n d t h e D e x t e c P r o c e s s , d e v e l o p e d b y a n A u s t r a l i a n c o m p a n y 2 6 . T h e G r e a t C e n t r a l M i n e s P r o c e s s a p p a r e n t l y r e c o v e r s i r o n i n s a l e a b l e p o w d e r f o r m w i t h 9 9 . 5 % e f f i c i e n c y a n d b o a s t s a n i n n o v a t i v e - 19 - e1ec t row inn ing c e l l des ign produc ing a purer product at lower c o s t . The Dextec Process a n o d i c a l l y decomposes c h a l c o p y r i t e in a s p e c i a l e l e c t r o l y t i c c e l l to produce, in a s i n g l e s tep , copper powder, e lementa l su lphur and p r e c i p i t a t e d i ron ox ides (mainly g o e t h i t e ) . A c e l l vo l t a ge of as l i t t l e as 0.8 v o l t s and a power consumption of 1 kWh/kg Cu are c l a i m e d . 1.4 AVMINE ROUTES Commercial a p p l i c a t i o n of ammonia o x i d a t i v e l e a c h i n g of sulphide concentrates has best been demonstrated by S h e r r i t t Gordon Mines L t d . w i th i t s now c l a s s i c process for t r e a t i n g n ieke 1 -copper -coba11 s u l ph ide m i n e r a l s at Fo r t Saskatchewan, A l b e r t a 2 ^ . As a p p l i e d to pure copper hydrometa 11urgy, the on ly a p p l i c a t i o n of importance is the Anaconda Company's A r b i t e r P r o c e s s ; developed to t r e a t copper concen t r a te s and used commerc i a l l y from 1974-1977^> 5 . The chemi s t ry and mechanism of anrmonia ox idat ive leaching of copper su lph ide s is q u i t e complex and has been s t u d i e d by a number of i n v e s t i g a t o r s 2 8 ' 3 ^ " 3 ^ . Leach ing is p o s s i b l e owing to the s t a b i l i z a t i o n of the c u p r i c ion in an a l k a l i n e s o l u t i o n as the tetraarrmine complex, Cu(NH3)^ 2 + . Su lph ide is o x i d i z e d u l t i m a t e l y to su lphate and r e j e c t e d as gypsum, but goes through a number of intermediates such as thiosulphate* •*2®3^~ > t r i t h i o n a t e , S 3 0 ^ 2 _ ; t e t r a t h i o n a t e , S ^ O g 2 - ; and sulphamate, N H 2 S O 3 - . Iron i s o x i d i z e d and rejected as hydrated i r on o x i d e . - 20 - 1.4.1 The Arb i ter Copper Process A sympl i f ied schematic flowsheet of the Arbiter Process is presented in Figure 4 2 9 , with the overall reaction chemistry out l ined in Table 3. Copper sulphide minerals such as c h a l - copyr i te and cha lcoc i te are dissolved by an oxidiz ing arrmonia- ammonium sulphate leach (equations 3-1 and 3-2) in a series of separate closed leaching tanks at temperatures ranging from 50-80 ° C . By optimizing mixing, the required oxygen par t i a l pressure is kept low at about 35 kPa. Leach time is t yp i c a l l y 5 hours. A second leach step would be required to obtain high copper ex t rac t ion , but the Arb i ter Process was o r i g i n a l l y designed to extract only about 80% of the concentrate copper value. Pyr i te and molybdenite remain unattacked. The remaining copper and any precious and minor metals are recovered by f l o t a t i on into a medium-grade con- centrate and fed to the Anaconda smelter. Iron is rejected during the leach as hydrated iron oxide. The pregnant l iquor is pur i f i ed by solvent extract ion using LIX 65N (equations 3-3 and 3-4) to y ie ld a copper sulphate e l ec t ro l y te which is electrowon by conventional methods. Apparently some re f in ing is required to obtain a pure product. The ra f f ina te contains excess ammonium sulphate which is prec ip i ta ted with lime and disposed of as gypsum, and ammonia is recovered for recycle back to the leach (equation 3-5). The Arb i ter Process enjoyed br ie f commercial success mainly because it could be used in conjunction with a smelter to treat successfu l l y a par t i cu lar concentrate. The process 1 1 PRIMARY LEACH [ N H J 2 S O 4 SECONDARY LEACH FLOTATION t 1 Secondary T a j | j n g s Concentration Lime Concentrates NH 3 AMMONIA RECOVERY Gypsum to Disposal SOLVENT EXTRACTION Loaded Organic Raffinate J Stripped Organic SOLVENT STRIPPING Strong Electrolyte Spent Electrolyte ELECTROWINNING Cathode Copper FIGURE 4 Schematic Flowsheet of the Arbiter Copper Process1* - 22 - TABLE 3 Chemis t ry of the A r b i t e r Copper Process Pr imary and Secondary Leaches (3-1) 2CuFeS 2 + 12NH 3 + 17/2 0 2 + (n + 2)H 20 » 2Cu(NH 3)4SO^ + 2(NH^) 2SO^ + Fe 20 3 -nH 204, (3-2) CCi2S + 6NH 3 + (NH^) 2SO^ + 5/2 0 2 > 2Cu(NH3)^SCty + H 20 So lvent E x t r a c t i o n (3-3) Cu(NH 3 ) / f SO^ + 2RH > CuR 2 + (NH^) 2SO^ + 2NH 3 So lvent S t r i p p i n g (3-4) CuR 2 + H 2SO^ >CuSO^ + 2RH Ammonia Recovery , Gypsum Format ion (3-5) (Nrty^SCty + CaO — ^ — ^ C a S O ^ + 2NH 3 + H 20 E l e c t r o w i n n i n g (3-6) CuSO^ + H 20 >Cu° + H 2SO^ + 1/2 0 2 - 23 - had a number of weaknesses inherent in atrmine leaching; namely slow leach rates and d i f f i c u l t i e s in obtaining 98% copper e x t r a c t i o n . A l s o , su lph ide sulphur o x i d a t i o n through to sulphate is i ne f f i c i en t for two reasons: high oxygen consumption high lime neut ra l i za t ion requirements to f i x sulphate as gypsum. In add i t ion , e f f ec t i ve recovery of precious metals, and r ecove ry/d i sposa 1 of minor me ta l s , was never proven. The process r e l i e d on an expensive so lvent e x t r a c t i o n step to transfer copper from an ammine medium to sulphate medium so as copper could be electrowon. A l l of the above def ic iences were re f lec ted in a high energy consumption requirement, which one study^6 estimated at 72.4 MJ/kg Cu. This f igure is roughly double the energy requirements of the chlor ide leach processes. To their c r ed i t , the Arb i ter Process developers have subsequently suggested improvements. Arbi ter and Mi l l igan-^ studied the f e a s i b i l i t y of using sulphur dioxide to reduce the Cu(NH3)/42+ complex d i r e c t l y to copper metal , thereby bypassing the energy-intensive solvent extract ion and e lectro- winning steps of the old process. The proposed reduction would be car r ied out in two stages: - 24 - 2Cu(NH 3)^SO^ + 2S0 2 + 4H2Q 3 0 °C > Cu 2 SO3 • (NHzf) 2 SO3 + 3(NH^) 2SO^ (5) C u 2 S Q 3 - ( N H / t ) 2 S Q 3 , ° C > 2 C u ° + (NH^) 2SO^ +so2. (6) A purer product than that ob ta ined by so lvent e x t r a c t i o n - e l ect r owi nn i ng i s c l a i m e d , however, f u r t h e r r e f i n i n g would s t i l l be nece s sa ry . 1.5 NITRATE ROUTES The a c t i o n of n i t r i c a c i d and mixed n i t r i c - s u 1 p h u r i c acids on copper s u l p h i d e m i n e r a l s has been s tud ied by a number of i n v e s t i g a t o r s 3 8 - ^ 0 . H a b a s h i 3 9 reported the react ion between c h a l c o p y r i t e and n i t r i c a c i d as being h i g h l y exo thermic , and rep re sen ted by the f o l l o w i n g equat ion 3CuFeS 2 + 2OHN03 >3Cu(N0 3 ) 2 + 3 F e ( N D 3 ) 3 + 6S° + 5N3>|v + 10H 2O (7) w i th a p o r t i o n of the su lphur being o x i d i z e d : S ° + 2HN0 3 »H 2 SCty + 2NOyjc . (8) If a s t o i c h i o m e t r i c amount of a c i d is used, i r on w i l l p r e - c i p i t a t e as f e r r i c ox ide when the f r e e a c i d l e v e l becomes low: - 25 - 6CuFeS 2 + 22HND3 » 6 C u ( N D 3 ) 2 .+ 3 F e 2 0 3 + 12S° + lONO^ + 11H 2 0. (9) If mixed n i t r i c - s u 1 p h u r i c a c i d s o l u t i o n s are used, n i t r i c a c i d p lays the r o l e of ox idant w h i l e s u l p h u r i c a c i d ac t s as a su lpha te source to s o l u b i l i z e the c a t i o n i c s p e c i e s . From a p r a c t i c a l s t a n d p o i n t , t h i s route is more a t t r a c t i v e than us ing s o l e l y n i t r i c a c i d due to the d i f f i c u l t y of winning copper from n i t r a t e s o l u t i o n s . The d i s s o l u t i o n of c h a l c o p y r i t e in a mixed n i t r i c - s u 1 p h u r i c a c i d s o l u t i o n can be represen ted as f o l l o w s 3 8 ' ^ 0 6CuFeS 2 + 22HN0 3 + 9H 2SO^ ->6CuSO^ + 3 F e 2 ( S O ^ ) 3 + 22ND^ + 6S° + 20H 2 0 (10) with elemental sulphur undergoing p a r t i a l o x i d a t i o n as d e p i c t e d by equat ion (10). If the f r e e a c i d l e v e l f a l l s low enough, i r on w i l l p r e c i p i t a t e as hydronium j a r o s i t e : 3 F e 3 + + 2SO/4 2" + 7H 2 0 > H 3 O F e 3 ( S O ^ ) 2 ( O H ) 6 + 5 H + . (11) The extent of su lphur o x i d a t i o n to su lpha te , as opposed to o x i d a t i o n to e lementa l su lphur depends on a number of f a c t o r s . P ra te r et a l . 3 8 r epo r ted that i n c r e a s i n g the concen- t r a t i o n of e i t h e r H 2SO^ or HND 3 much beyond the s to ich iometr ic r e q u i r e m e n t , or i n c r e a s i n g l e a c h t e m p e r a t u r e , a p p e a r s to i n c r e a s e the extent of su lphate f o r m a t i o n . A l s o , n i t r i c - 26 - or n i t r i c-su1phur ic media attack most sulphide minerals includ- ing pyr i te and molybdenite. Mineral r e a c t i v i t y , a long with e lemental sulphur y i e l d va r i es w ide ly as shown in Table 4. P y r i t e g ives a very low S ° y i e l d , and chalcopyr i te follows the general t rend of being the most d i f f i c u l t su lph ide to 1 each. 1.5.1 The N i t r i c-Su lphur ic Copper Process A n i t r i c-su1phur ic leach process was patented by the E.I. duPont de Nemours Company in and evaluated extensively by the Kennecott Copper C o r p o r a t i o n ^ 2 ' ^ 3 . A schematic flowsheet of the process is presented in Figure 5, and the important r e a c t i on chemist ry is summarized in Table 5. The process u t i l i z e s N02 gas as the chief oxidant in a two-stage counter cur rent leach conducted at 100 °C (equations 5-1 to 5-3), to obtain 98% copper extract ion in a tota l 5 hour residence time. By-product sulphides such as pyr i te and molybdenite are leached quant i ta t i ve ly along with c h a l c o p y r i t e , and roughly 20% of the su lph ide values are converted to e lemental sulphur (when t r e a t i n g a mixed CuFeS2 - FeS2 concentrate) . NZ>2 dissolves revers ib ly in aqueous ac id i c media according to the fol lowing step-wise mechanism^ 3: 2ND2(g) ^==^ 2ND2(aq) 2ND2(aq) ^ NO+ + ND 3" (12a) (12b) - 27 - TABLE 4 R e l a t i v e R e a c t i v i t y and E lementa l Sulphur Y i e l d of Pure M i n e r a l s Leached w i th N i t r i c Ac id38 M i n e r a l Re la t ive S ° Y i e l d , % of React i v 1 t y Reacted Sulphur C u 3 S b S 3 10 40 Cu 2 S 8 75 FeS 7 60 F e S 2 7 3 CU9S5 6 55 CuS 6 50 Cu^FeS^ 5 60 CuFeS 2 2 45 HN03 to 2nd-Stage Leach CONDENSER NO r OXIDIZER Concentrates NO.HzO srs NO.HjQ r FIRST-STAGE LEACH sV -r1\ SECOND-STAGE LEACH S \ L Au and Ag Recovery V V Au Ag s o G a n g U e Waste Product NH3 M0O3 \ t i 1 NO Mo Nitrate Recovery Removal NHa LA Purge Liquor Waste A Scrap Fe Zinc Recovery J Copper Purge Recovery i ^ CaCO Sulphate Removal Zn Product Cement Cu Iron Removal Sele Rem nium oval . Jarosite Waste Gypsum Waste Se(Cu) ro co ELECTROWINNING t Cathode Copper FIGURE 5 Schematic Flowsheet of the Nitric-Sulphuric Copper Process' - 29 - TABLE 5 Chemis t ry of the N i t r i c - S u l p h u r i c Copper Process Leaches (5-1) 2CuFeS 2 + 5N0 2 + 5H 2SO^ >2CuSO^ + F e 2 ( S 0 4 ) 3 + 4S° + 5H 2 0 + 5NO^ (5-2) 2FeS 2 + 3N0 2 + 3 H 2 S 0 4 > F e 2 ( S O f f ) 3 + 4S° + 3H 2 0 + 3NO^ (5-3) So + 3N0 2 + H 2 0 >H 2SO^ + 3NO^ O x i d i z e r (Reagent Regenera t ion ) (5-4) 2NO + 0 2 >2N0 2 N i t r a t e Removal (5-5) N 0 3 " + 4H+ + 3 F e 2 + >NO^ + 2H 2 0 + 3 F e 3 + I ron Remova1 (5-6) 3 F e 3 + + NH 3 + 2S0^2- + 6H 2 0 > NH^Fe 3 ( SO^ ) 2 (OH) 6 + 5H + Se len ium Removal (5-7) H 2 S e Q 3 + 4Cu° + 4H+ >Cu 2Se+ 2 C u 2 + + 3H 2 0 - 30 - TABLE 5 Chemis t ry of the N i t r i c - S u l p h u r i c Copper Process (contd.) E1ect r owi nn i ng; (5-8) CuSCty + H 2 0 >Cu° + H2SO/4 + 1/2 0 2 Su lphate Removal (5-9) H 2SO^ •+ CaC0 3 + H 2 0 >CaSO^-2H 2 0 + C 0 2 - 3 1 - ND+ + H 2 0 (H 2 ND 2 + ) HND 2 + H+ (12c) HND 2 + N3 2 (aq) ^ HND 3 + ND(g) . (12d) The h i g h l y r e a c t i v e N0 + ion forms as an i n te rmed i a te and is most l i k e l y the r e a c t i v e spec ie s dur ing l e a c h i n g . The NO gas formed dur ing the leach is c o l l e c t e d , r e - o x i d i z e d to ND 2 and r e c y c l e d back to the l e a c h . Because the n i t r i c - s u 1 p h u r i c system s o l u b i l i z e s such a wide v a r i e t y of m e t a l s , pregnant leach l i q u o r s must undergo a s e r i e s of r i go rous p u r i f i c a t i o n steps be fore copper can be r e c o v e r e d . Re s i dua l n i t r a t e is removed by r e d u c t i o n to NO w i th f e r r o u s i r o n ; i ron is p r e c i p i t a t e d as a j a r o s i t e (which a l s o removes A l , As , B i , Sb, and p a r t i a l l y removes S e , T e , S ) ; s e l e n i u m and t e l l u r i u m a re cemented out w i t h copper m e t a l ; and the purge l i q u o r s t i l l con ta in s r e s i d u a l Cd , Co, Mg, Mn, Ni and Zn. Copper i s e lec t rowon at high cu r ren t d e n s i t y (650 A/m 2 ) in a i r - a g i t a t e d c e l l s to ach ieve a product which is comparable in p u r i t y to e 1 e c t r o r e f i n e d copper . The n i t r i c - s u 1 p h u r i c leach process has the advantage of be ing ab le to r a p i d l y s o l u b i l i z e a l l types of copper s u l p h i d e s . U n f o r t u n a t e l y t h i s process produces a gaseous e f f l u e n t (from the n i t r i c a c i d recovery p l a n t ) which must be d e a l t w i t h . The e f f i c i e n c y of sulphide to sulphur conversion is decreased for p y r i t i c c o n c e n t r a t e s . The p r o c e s s i s a l s o a high energy consumer w i th an estimated 81.2M3/kg Cu required, which is at l e a s t 2.5 times g rea te r than that r e q u i r e d for f l a s h s m e l t i n g ^ . High energy consuming ope ra t i on s are e l e c - - 32 - t r i c a l energy for e1ec t row inn ing (43% of t o t a l ) and steam genera t i on for the p u r i f i c a t i o n steps (25% of t o t a l ) . The c a p i t a l cost for a 45,000 tonne Cu/year f a c i l i t y was estimated to be $100.5 m i l l i o n (1978 U.S. d o l l a r s ) ; the o p e r a t i n g cost was es t imated to be 67.9^/kg Cu. 1.6 OXIDATIVE SULPHATE ROUTES The o x i d a t i v e d i s s o l u t i o n of copper sulphide concentrates in va r i ous su lpha te media has been s tud ied e x t e n s i v e l y and i s w e l l documented in the l i t e r a t u r e . Processes have been proposed based on a number of d i f f e r e n t s u l p h a t e s y s t e m s , and can be grouped i n to four d i f f e r e n t c a t e g o r i e s - hot con - c e n t r a t e d s u l p h u r i c a c i d , d i l u t e s u l p h u r i c a c i d and oxygen, d i l u t e s u l p h u r i c a c i d and f e r r i c i r o n , and b i o l o g i c a l systems. 1.6.1 Concen t ra ted S u l p h u r i c A c i d P ra te r et a l . * * * s t u d i e d the chemi s t ry of s u l p h a t i n g , or a c i d - b a k i n g c o p p e r - i r o n s u l p h i d e s , w i th hot concen t ra ted H2SO4 (93-98% p u r e ) . Us ing c h a l c o p y r i t e as an example, the o v e r a l l r e a c t i o n chemis t ry can be d e s c r i b e d as b e i n g i n t e r - mediate between the f o l l o w i n g two r e a c t i o n s CuFeS 2 + *H2SC>4 ^CuSO^ + FeSO^ + 2S° + 2S0 2 +4H 20 (13) - 33 - CuFeS 2 + 5H2SO^ >CuSO^ + 1/2 Fe 2 (SO^) 3 + 2S° + 5/2 S0 2 + 5H 20 (14) with the l a t t e r r e a c t i o n , d e p i c t i n g o x i d a t i o n of i r o n to the f e r r i c s ta te , being favoured by higher reaction temperatures (200-265 °C ) and longer retention times (greater than 2 hours). Sulphation is carr ied out in an externa l ly heated rotary k i l n , with 99% chalcopyr i te decomposition achieved in about one hour. Pyr i te and molybdenite are not attacked. If the reaction temperature is kept below 230 ° C and only the stoich- iometric ac id requirement is used, S ° oxidat ion is kept to a minimum. The Anaconda Company p i lo ted a process based on cha l - copyr i te sulphation (the Anatread Process) in 1971 which treated 5 tonne/day concentrate^ 5 . The process suffered from the usual copper-iron separation problem and in the regeneration of process sulphuric ac id , but was claimed to be economically competitive with smelting at the time. 1.6.2 D i lu te Sulphuric Acid and Oxygen Oxygen under pressure in weakly ac id i c so lut ion can act as an e f fec t i ve oxidant for cha lcopyr i te . V i zso l y i and co-workers^ at Sherr i t t Gordon Mines developed an acid pres- sure leach process for chalcopyr i te concentrates. In this process, chalcopyr i te was ground to -325 mesh and leached with 95 g/L H 2SO^ under 1.4-3.5 MPa 0 2 pressure; at 115 ° C for 2-3 hours to obtain 98% copper recovery and 85% sulphur - 34 - recovery as S ° . The overa l l leach reaction can be approximated as fo l lows: CuFeS 2 + H 2 S C y + 5/4 0 2 + 1/2 H 2 0 >CuSO^ + Fe(OH) 3 + 2S ° . (15) Pyr i te and molybdenite were unattacked. Leaching with a 25-50% sto ich iometr ic excess of con- centrate over acid was recommended to ensure hydrolys is of i ron . However, unleached concentrate must then be recycled back to the leach step a f t e r S ° removal . Other problems with this process were - pyr i te would bui ld up in the sulphide recycle stream, the iron prec ip i ta te was gelatinous and enta i led a d i f f i c u l t l i qu id-so l i d separat ion, and high copper and precious metals losses were incurred in iron s l imes. 1.6.3 D i lu te Sulphuric Acid and Fe r r i c Iron The mechanism of a c id i c f e r r i c sulphate leaching of chalcopyr i te has been the subject of widespread in te res t , and as with f e r r i c ch lor ide leaching, is s t i l l shrouded in c o n t r o v e r s y 8 ' 9 ' ^ . The leach react ion is usual ly expressed as CuFeS 2 + 2Fe 2 (SO^) 3 >CuSO^ + 5FeSO^ + 2S° (16) with some sulphur oxidation to sulphate occurr ing . In general , the copper leach rate or extract ion is not nearly as high as with f e r r i c ch lor ide l e a c h i n g 8 ' 9 . This has been p a r t i a l l y - 35 - at t r ibuted to the observation that a tenacious, impermeable sulphur layer forms around par t ia 11y-1eached chalcopyr i te grains during f e r r i c sulphate leaching, which severely impedes the reaction progress. One solut ion to this problem was p roposed by Beckstead et a l . ^ 8 who found that a t t r i t i o n gr inding of c h a l c o p y r i t e concent ra te to a median p a r t i c l e s ize of 0.5 um allowed 90% copper extract ion to be attained in a 3 hour leach at 93 ° C . However, such gr inding is energy intensive (7.9 M3/kg Cu) and is l i k e l y to r e s u l t in severe l i qu id-so l i d separation problems. A process for t reat ing copper concentrates by f e r r i c sulphate leaching was developed and p i lo ted in Polan However, the concent ra te feed was p r i m a r i l y a m ix tu re of the less re f ractory minerals chalcoc i te and bornite. Retention time in the two-stage countercurrent leach was s t i l l 9 hours at 90-95 ° C . Thus, while weakly a c id i c f e r r i c sulphate would be attractive from a corrmercial standpoint because it is r e l a t i v e l y cheap, non-corrosive and easy to regenerate, it is not an aggressive enough l i x i v i an t for cha lcopyr i te . 1.6.4 B io log ica l Systems The oxidat ion of reduced sulphur compounds and ferrous iron by the leaching bacterium Th i obac i11 us fer rooxi dans, resu l t ing in metals s o l u b i l i z a t i o n and weak acid generation, has long been recognized as a natura l ly occurr ing process that can be exploi ted in mining o p e r a t i o n s 5 0 * 5 * . This unique - 36 - bacterium derives energy for growth by the oxidation of sulphide minerals and soluble ferrous i ron . The bacterium requires a source of ammonia nitrogen as well as small amounts of phosphate, calcium and magnesium, and f ixes carbon from carbon d iox ide in the a i r . It is able to withstand extremely high concentrations of such metals as copper (60 g/L) and zinc (100 g/L) without apparent i l l e f f e c t , and functions best at a c id i c pH's (1.5-2.5) and a temperature of 35 ° C . Much evidence exists which suggests that T. ferrooxidans can attack sulphide minera 1s by d i rect attachment and' oxidat ion of the sulphide m o i e t y 3 2 - 3 3 . Using chalcopyr i te and pyr i te as examples, the overa l l reactions can be represented as 12CuFeS2 + 510 2 + 22H 20 >12CuSO^ + 4H 3OFe 3 (SO^) 2 (OH) 6 + 4H2SO^ (17) 12FeS2 + 450 2 + 34H20 > 4H 3OFe 3 ( SO^ ) 2 (OH) g + 16H2S04 (18) with iron p rec ip i t a t i ng as hydronium ja ros i te in the pH range favoured by the bac te r ia . Although bacter ia l leaching has been considered most app l i cab le , from a prac t i ca l standpoint, to the treatment of low-grade ores by dump or heap leaching, at least two copper concentrate bioleach processes have been proposed 3 6 ' 3 ^. However, concentrate bioleaching suffers from a number of pr ob 1 ems : - 37 - low leach ra tes (300-600 mg/L-h C u ) . low one-pass e x t r a c t i o n s (40-80%), making c o n c e n t r a t e r e - g r i n d i n g and r e - c y c l i n g nece s sa ry . s u l ph ide va lues are o x i d i z e d to s u l p h a t e , r e s u l t i n g in l a r ge volumes of weak a c i d which must be n e u t r a l i z e d . On the p o s i t i v e s i d e , b i o l e a c h i n g is c a r r i e d out at near ambient temperature (35 °C) and p res sure in a c h e m i c a l l y n o n - a g g r e s s i v e l i x i v i a n t . A r e c e n t s t udy i n d i c a t e d t h a t a b i o l e a c h process may be economical ly competitive with smelting i f used on a smal l s c a l e (25,000 tonne/year Cu or l e s s ) 5 8 . 1.7 SUMMARY From the f o r g o i n g d e s c r i p t i o n of copper hydrometa l - l u r g i c a l p r o c e s s e s , i t i s c l e a r t ha t the c h l o r i d e r o u t e s a r e s u p e r i o r f r om a p r a c t i c a l s t a n d p o i n t . The s u p e r i o r i t y of the c h l o r i d e r o u t e s can be a t t r i b u t e d to a c o m b i n a t i o n of the e x c e l l e n t l e a c h r a t e s and e x t r a c t i o n s o b t a i n e d , and to the lower o v e r a l l ene r gy r e q u i r e m e n t s . However , a l l of the processes have a common major weakness inherent in ox idat ive l e a c h i n g - d i f f i c u l t y in a c h i e v i n g an e f f e c t i v e c o p p e r - i r o n s e p a r a t i o n . O x i d a t i v e l e a c h i n g , whether i t be in c h l o r i d e , ammine, n i t r a t e or su lpha te med ia , d i s s o l v e s copper and i r on s imu l t aneous l y w i th i r on then p r e c i p i t a t e d from solut ions - 38 - high in copper. As a r esu l t , up to 5% of the copper is lost due to co-prec ip i ta t ion with i ron . In add i t ion , any precious metals usual ly end up as d i lu te constituents in the iron residue, making recovery d i f f i c u l t or impossible. A potent ia l so lut ion to the copper-iron separation problem is to leach chalcopyr i te under reducing condi t ions , in which iron can be se l ec t i ve l y dissolved. Reduction leaching of chalcopyr i te has only received interest in the past 10 years, with the majority of studies being of an academic nature. For this reason it was decided to study the p rac t i ca l appl icat ions of reduction leaching. CHAPTER 2 REDUCTION LEACHING AS A METHOD OF IRON REMOVAL 2.1 BACKGROUND F i g u r e 6 po r t r ay s the Pourba ix d iagram for the C u - F e - S - H 2 0 system at 25 ° C 5 9 . It can be seen that at near n e u t r a l or nega t i ve p o t e n t i a l s at pH's l e s s than about 2, f e r rou s i r on w i l l tend to be s t a b i l i z e d in s o l u t i o n , l e a v i n g copper in a c o p p e r - r i c h s u l p h i d e form such as b o r n i t e , c h a l - c o c i t e , o r ; at low enough p o t e n t i a l s , m e t a l l i c copper . Thus, based on pu re l y thermodynamic c o n s i d e r a t i o n s , l e a c h i n g of c h a l c o p y r i t e under reduc ing c o n d i t i o n s should s e l e c t i v e l y remove i r o n . The c a t h o d i c r e a c t i o n for c h a l c o p y r i t e decom- p o s i t i o n to c h a l c o c i t e can be expressed as f o l l o w s : c h a l c o p y r i t e was f i r s t d e m o n s t r a t e d by McGau ley et a l . who leached copper concentrates with copper su lphate at e l e v a t e d tempera tures . The e s s e n t i a l chemica l r e a c t i o n s appeared to be: 2CuFeS 2 + 6H+ + 2 e - > C u 2 S + 2 F e 2 + + 3H 2 S. (19) H i s t o r i c a l l y , the s e l e c t i v e l each ing of i r on from CuFeS 2 + CuSO^ CuFeS/j + CuSO^ >2CuS + FeSO^ »2CuS + 2Cu 2 S + FeSO^. ( 2 0 ) (21) - 40 - FIGURE 6 Eh-pH Diagram of the Cu-Fe-S-H20 System at 25°C (Conditions: 0.1 M Fe,S Species; 0.01 MCu Species) 5 9 - 41 - However, leaching under these near neutral condit ions did not prove very e f f e c t i v e , as iron extract ions achieved were low even when using high temperatures (180-200 °C ) and long residence times (4-12 h). Severa l inves t iga tors have studied the acid reduction leaching of chalcopyr i te with metals having lower rest potent- i a l s , such as copper , i ron or l e a d 6 1 - 6 5 . They found that the react ion was contro l led by galvanic in te rac t ion of c h a l - c o p y r i t e wi th the m e t a l . This ga l van i c mechanism w i l l be discussed in greater deta i l in Chapter 4. Hiskey and Wadsworth 6 * s tud ied the k inet i cs of reduction leaching with f i ne copper powder in the temperature range 34-90 ° C . The reaction observed was: CuFeS 2 + Cu + 2H+ >Cu 2S + F e 2 + + H 2S^ (22) They found that r e l a t i v e l y high iron ext rac t ions , in the range 80-90%, could be obtained in as l i t t l e as 1 hour when leaching at very low pulp densi t ies (1%) and using s to i ch - iometric excesses of acid and copper. McKay and Swinkels patented a copper process based on react ion ( 2 2 ) 6 2 . The process successfu l ly treated a wide range of -325 mesh concentrates containing 7 to 41% Cu and 14-31% Fe,, 'using -400 mesh copper powder. Optimum leach condit ions were reported to be pH 1.0-1.5, 90-95 °C leach temperature and a residence time of 1-6 h. Iron extract ions obtained ranged from 66-87% depending on the type of con- centrate used. The formed cha lcoc i te is th,en separated from - 42 - the ferrous sulphate so lut ion by f i l t r a t i o n and is readi ly dissolved by oxygen pressure leaching, as fo l lows: Cu 2 S + 2H2SO^ + 0 2 >2CuSO^ + S ° + 2H 20. (23) Iron is prec ip i ta ted as a pure ja ros i te in a separate step. The formed H 2S gas is absorbed in a copper sulphate so lut ion to form copper sulphide C u 2 + + H 2S >CuS + 2H+ (24) which is directed to the oxygen pressure leach step. Sulphides such as pyr i te and molybdenite are not leached, and end up in the f inal leach residue along with sulphur and a l l precious me t a 1s. The McKay/Swinke1s process has the advantage of p rec ip i t a t i ng iron in a separate step, from a copper-free so lut ion to y i e ld a pure ja ros i te which is environmentally acceptable for d i sposa l . A l so , precious metals are not t ied up in d i lu te form in the prec ip i ta ted i ron , as is the case with most oxidat ive processes, and as such are more eas i l y recoverable. Sulphur is recovered in the pre ferent ia l elemental form. Drawbacks to the process are that i t produces noxious H 2S gas and doesn't achieve a complete cha lcopyr i te to chalcocite conversion under the process condit ions employed. Shirts et a l . 6 3 » 6 ^ and N i c o l 6 5 studied the acid re- duction leaching of cha lcopyr i te with copper, iron and lead. - 43 - Reactions observed when using iron and lead were: 2CuFeS2 + Fe + 6H+ >Cu 2S + 3Fe 2 + + 3H2S (25) CuFeS 2 + Fe + 4H+ >Cu + 2 F e 2 + + 2H2S (26) 2CuFeS2 + Pb + 6H+ >Cu2S + 2Fe2+ + P b 2 + + 3H2S (27) CuFeS 2 + Pb + 4H+ > Cu + F e 2 + + P b 2 + + 2H 2S. (28) Me ta l l i c copper general ly gave a more rapid rate of decomp- os i t i on in the f i r s t 15 minutes, but at times greater than 1 hour metal l ic iron or lead gave a more complete decomposition. Sohn and Wadsworth 6 6 have invest igated the reduction leaching of chalcopyr i te in copper sulphate solut ions using gaseous S0 2 as the reductant. The overa l l react ion could be expressed as: CuFeS 2 + 3Cu 2 + + 2S0 2 + 4H 20 >2Cu2S + 6H+ + 2HSCV + Fe 2 + . (29) The leach was found to be electrochemical in nature, with the cathodic reactions involv ing formation of an intermediate bornite product, as fo l lows: Cathodic React ions: 2CuFeS2 + 3Cu2+ + 4e" v "•Cu^FeS^ + Fe 2 + Cu 5 FeS 4 + 3Cu2+ + 4e" v s 4Cu2S + Fe 2 + (30) (31) - 44 - Anod ic R e a c t i o n : S0 2 + 2H zO k N 3H+ + HSO^- + 2e" (32) C h a l c o p y r i t e c o n v e r s i o n was i ncomp le te , w i th a maximum 82% i r on e x t r a c t i o n ob ta ined on ly when l e a c h i n g very f i n e (50% -2um) c o n c e n t r a t e at 90 °C for 4 h. B i e g l e r et a l . 6 ^ - 6 9 have i n v e s t i g a t e d the e l e c t r o - l y t i c r e d u c t i o n of c h a l c o p y r i t e in HC1 s o l u t i o n , and con - f i rmed the fo rmat ion of c h a l c o c i t e or d j u r l e i t e a c c o r d i n g to the r e a c t i o n 2CuFeS 2 + 6H+ + 2e" ;==^ Cu 2 S + 3H 2S + 2 F e 2 + (33) for which the c a l c u l a t e d s tandard p o t e n t i a l i s -0.14 V. Cur ren t e f f i c i e n c y decreased d r a s t i c a l l y as c h a l c o p y r i t e r e d u c t i o n proceeded, and was ma in l y a t t r i b u t e d to hydrogen e v o l u t i o n on the forming c h a l c o c i t e . A copper e x t r a c t i o n process was proposed based on cont inuous e l e c t r o l y t i c r e d - u c t i o n in a s l u r r y e l e c t r o l y s i s c e l l 6 9 . I n v e s t i g a t o r s in the r e d u c t i o n l e a c h i n g of cha lcopyr i te have shown that the method can ach ieve an e f f e c t i v e , s imple c o p p e r - i r o n s e p a r a t i o n . However, r e l a t i v e l y l i t t l e work has been done in d e v e l o p i n g r e d u c t i v e leach processes as p r a c t i c a l a l t e r n a t i v e s to the o x i d a t i v e leach p r o c e s s e s . Two companies, S h e r r i t t Gordon Mines L t d . and Cominco L t d . , d i d deve lop a a process in which i r on was p r e f e r e n t i a l l y separa ted from - 45 - c h a l c o p y r i t e by a combinat ion of pyrometa 11urgica1 and hydro - m e t a l l u r g i c a l methods. To i l l u s t r a t e the p r a c t i c a l b e n e f i t s of a c h i e v i n g an e a r l y c o p p e r - i r o n s e p a r a t i o n , t h i s process w i l l be d e s c r i b e d in some d e t a i l next . 2.2 THE SHERRITT-GOMI NCO COPPER PROCESS The S h e r r i t t - C o m i n c o Copper Process (herea f te r re ferred to as the S.C. P roces s ) is the product of a j o i n t re sea rch development e f f o r t by S h e r r i t t Gordon Mines L t d . and Cominco L t d 7 0 . P i l o t e d in 1975-1976 at For t Saskatchewan, A l b e r t a 7 0 , the process was demonstrated to have a number of i n n o v a t i v e f e a t u r e s which make i t a p o t e n t i a l l y a t t r a c t i v e and v i a b l e a l t e r n a t i v e to the best hydrometa 11urgica1 methods a v a i l a b l e . A s i m p l i f i e d schemat ic f lowsheet of the S.C. Process i s p resented in F i g u r e 7. The process so lves the c o p p e r - i r o n s e p a r a t i o n problem in a novel manner - i r on i s p r e f e r e n t i a l l y e x t r a c t e d from copper in a s e r i e s of three un i t o p e r a t i o n s , and d i sposed of as j a r o s i t e in a separa te s t ep . The copper - r i c h leach re s i due is then leached w i th weak s u l p h u r i c a c i d under oxygen p re s su re f o l l o w e d by p u r i f i c a t i o n of the pregnant leach l i q u o r and copper e 1 e c t r o w i n n i n g . Sulphur is recovered p r i m a r i l y in the e lementa l form. The process a l lows for recovery of p rec i ou s meta l s and d i s p o s a l or recovery of any minor m e t a l s . The f o l l o w i n g process d e s c r i p t i o n i s based on p i l o t p l an t r e s u l t s on a c h a l c o p y r i t e / p y r i t e concen t r a te a s s ay ing 23.7% Cu, 27.6% Fe, 3.16% Zn and 31.5% S. - 46 - COPPER CONCENTRATES THERMAL ACTIVATION 1%h Retention Time I T PRECIOUS METALS CONCENTRATES SOLID SLURRY COPPER METAL FIGURE 7 Simplified Schematic Flowsheet of the Sherritt-Cominco Copper Process7 1 - 47 - 2.2.1 Copper - I r on Sepa ra t i on Stages Iron i s separa ted from copper in three stages cons i s t ing of thermal a c t i v a t i o n , a c i d l e a c h i n g and a c t i v a t i o n l e a c h i n g to a t t a i n an o v e r a l l 90% or g r e a t e r i r o n r e j e c t i o n be fo re the copper leach s tep . Iron is d i sposed of as j a r o s i t e . (a) Thermal A c t i v a t i o n Thermal a c t i v a t i o n i s e s s e n t i a l l y a pyrometa 11urgica1 step in which p e l l e t i z e d copper concen t r a te is p r e t r e a t e d at 700 °C in a 12-stage m u l t i p l e hear th r oa s te r to conver t c h a l c o p y r i t e to b o r n i t e and any p y r i t e to p y r r h o t i t e . The e f f e c t i v e s t o i c h i o m e t r y of the r e a c t i o n can be rep re sen ted as f o i l o w s : - » 5 C u F e S K 8 + S 0 2 t (34) - > F e 7 S g + 6 S 0 2 / r (35) >Cu^FeS^ + 4FeS + H2S^ (36) *7FeS + H 2 S f . (37) Depending on the p y r i t e content of the f e e d , about 25% of the su lphur i s removed as S0 2 and/or H 2 S and recovered as H 2SO^ and S° r e s p e c t i v e l y , by c o n v e n t i o n a l methods. U n f o r t u n a t e l y the c o n v e r s i o n r e a c t i o n s do not go to comple t ion at 700 °C , so that t he rma l l y p r e t r e a t e d concentrate Roaster Top C 5CuFeS 2 + 0 2 ( g ) — - S 0 2 p rod . C 7FeS 2 + 60 2 ( g ) Roaster BottomC 5 C u F e S 1 > 8 + H 2 ( g ) -H 2 S p rod . L F e 7 S g + H 2 ( g ) - 48 - is not comp le te l y f r ee of c h a l c o p y r i t e . A r s e n i c , i f present in the f e e d , is not v o l a t i l i z e d s u b s t a n t i a l l y at 700 °C and remains in the a c t i v a t e d s o l i d s . (b) A c i d Leach The a c t i v a t e d s o l i d s are a c i d leached at 85 °C w i th weak ac id obta ined from j a r o s i t e p r e c i p i t a t i o n , the a c t i v a t i o n leach and the a c i d p l an t ( o v e r a l l 200-300 g/L H 2 SO^), in a two-stage c o u n t e r c u r r e n t system to d i s s o l v e p y r r h o t i t e FeS + H 2SO^ »FeSO^ + H 2 S ^ (38) and produce a l i q u o r h igh in i r on (65 g/L F e + + ) and low in a c i d (30 g/L H 2 SO^). O v e r a l l i r on e x t r a c t i o n at t h i s stage is about 75%. If z i n c is p r e s e n t , i t w i l l be p a r t i a l l y d i s s o l v e d and can be recovered by p r e c i p i t a t i o n as ZnS w i th H 2 S . (c) A c t i v a t i o n Leach To o b t a i n an o v e r a l l 90% or g rea te r i r on e x t r a c t i o n be fo re copper is l eached , the S.C. Process u t i l i z e s a so- c a l l e d " a c t i v a t i o n " or " n e u t r a l " leach on the re s i due from the a c i d l e a c h . The a c t i v a t i o n leach can be o r i g i n a l l y a t t r i b u t e d to McGauley et a l . 6 0 As a p p l i e d to the S.C. P r o c e s s , the e s s e n t i a l chemica l r e a c t i o n s a r e : - 49 - C u 5 F e S ^ + CuSO^ >2CuS + 2Cu 2 S + FeSO^ (39) CuFeS 2 + CuSO^ > 2CuS + FeSO^ (40) Copper su lpha te l i x i v i a n t (52 g/L C u 2 + , 12 g/L F e 2 + , 54 g/L H 2SO^) i s s u p p l i e d as a b leed stream from s o l u t i o n from the o x i d a t i o n l e a c h . An u n d e s i r a b l e s i de r e a c t i o n a l s o takes pi ace 5CuS + 3CuSO^ + 4H 2 0 >4Cu 2S + 4H 2SO^ (41) which r e q u i r e s that the CuSO^ r e c y c l e be 20-30% in s t o i c h - i o m e t r i c exce s s . The a c t i v a t i o n leach i s c a r r i e d out at 156 °C for 4 hours , and i nc rea se s o v e r a l l i r o n e x t r a c t i o n from 75% to 91% to y i e l d a r e s i due a s s ay ing 4% Fe and 53% Cu. Z inc w i l l be p a r t i a l l y s o l u b i l i z e d , i f p r e s e n t , and can be recovered by H 2 S p r e c i p i t a t i o n . P y r i t e w i l l not be a t t a cked by t h i s l e a c h . S o l u t i o n from the a c t i v a t i o n leach (3 g/L C u 2 + , 30 g/L F e 2 + , 79 g/L H 2SO^) i s r e c y c l e d back to the a c i d leach a f t e r z i n c r e c o v e r y . If the copper c o n c e n t r a t e i s r i c h in b o r n i t e and/or c h a l c o c i t e ra ther than p y r i t e , the a c t i v a t i o n leach on i t s own may be ab le to produce an adequate c o p p e r - i r o n separat ion, wi thout the need for thermal pretreatment and a c i d l e a c h i n g . 2.2.2 J a r o s i t e P r e c i p i t a t i o n S o l u t i o n from the a c i d l e a c h , s l i g h t l y d i l u t e d from - 50- - re s i due washing, is sub jec ted to conven t i ona l j a r o s i t e p r e - c i p i t a t i o n in a four-compartment au toc l a ve w i th the r e a c t i o n temperature i n c r e a s e d from 120 °C in compartment 1 to 190 °C in compartment 4. The o v e r a l l r e a c t i o n i s : 6FeSO^ + 2NH4OH + 7H20 + 3/2 0 2 >2NH^Fe 3(SO^) 2(OH) 6 + 2H2SO^. (42) An oxygen p a r t i a l p re s su re of 0.7-1.8 MPa is used; t o t a l r e t e n t i o n time i s 45 m i n u t e s . S o l u t i o n from j a r o s i t e p r e - c i p i t a t i o n (5 g/L Fe , 50 g/L H 2SO^) i s r e c y c l e d to the a c i d l e a c h . About 33% of the i n i t i a l feed sulphur is l o s t to j a r o s i t e . 2.2.3 Copper Recovery Stages (a) O x i d a t i o n Leach Res idue from the a c t i v a t i o n leach c o n s i s t s p r i m a r i l y of b o r n i t e , c h a l c o c i t e and c o v e l l i t e , w i th minor amounts c h a l c o p y r i t e and p y r i t e . To recover copper , t h i s r e s i d u e is ac id leached under oxygen in a cont inuous two-stage c o u n t e r - c u r r e n t o p e r a t i o n , f o l l owed by a f i n a l batch leach under more a g g re s s i ve c o n d i t i o n s to scavenge as much copper as p o s s i b l e . The o v e r a l l l each r e a c t i o n s can be summarized as f o i l o w s : - 51 - C u 5 F e S ^ + 6H2SO4 + 3 0 2 ^5CuSO^ + FeSO^ + 4 S ° + 6H 2 0 (43) Cu 2 S + 2H 2SO^ + 0 2 >2CuSO^ + S° + 2H 2 0 (44) CuS + H 2SO^ + 1/2 0 2 ^CuSO^ + S° + H 2 0 . (45) A c i d is s u p p l i e d by r e t u r n e l e c t r o l y t e from copper e l e c t r o - winn ing and i s supplemented w i th c o n c e n t r a t e d a c i d . The o x i d a t i o n leach i s conducted at 100 °C for a t o t a l r e t e n t i o n time of 7'A hours ; oxygen pres sure is 1.05 MPa. A c i d c o n c e n t r a t i o n s are 150 g/L for the cont inuous phase and 250 g/L for the batch scaveng ing l e a c h . O v e r a l l copper e x t r a c t i o n is 98 .4%, w i th most of the remain ing i r o n and z i n c ( i f p re sent ) a l s o s o l u b i l i z e d . The pregnant copper s o l u t i o n (85 g/L C u 2 + , 6 g/L Fe , 20 g/L H 2SO^) i s d i r e c t e d to the p u r i f i c a t i o n s t ep . (b) S o l u t i o n P u r i f i c a t i o n Pregnant leach l i q u o r from the o x i d a t i o n leach must undergo p u r i f i c a t i o n to remove t r a c e i m p u r i t i e s such as Se, Te, As, Sb, Bi and Pb which would c o - d e p o s i t w i t h copper du r ing e 1 e c t r o w i n n i n g . A l s o i t i s d e s i r a b l e to lower the i r o n l e v e l as much as p o s s i b l e , as i r on reduces current e f f i c - iency in e 1 e c t r o w i n n i n g . The p u r i f i c a t i o n method chosen was to s u b j e c t the s o l u t i o n to h igh temperature o x y d r o l y s i s ( s i m i l a r to j a r o s i t e p r e c i p i t a t i o n ) w h i c h w i l l remove i r o n as Fe 2 0-j a long w i th adsorbed t r a c e i m p u r i t i e s . - 52 - P r i o r to o x y d r o l y s i s , however, any s o l u b i l i z e d selenium must be q u a n t i t a t i v e l y reduced to the t e t r a v a l e n t s t a te as the s e l e n i t e anion ( S e 0 3 2 - ) because the hexavalent form (SeCty 2 -) w i l l not p r e c i p i t a t e dur ing o x y d r o l y s i s . Se lenium r e d u c t i o n i s c a r r i e d out in the f i r s t chamber of the four-compartment oxydrolys i s a u t o c l a v e . By e x c l u d i n g oxygen from t h i s chamber, the s e l e n a t e an ion can be reduced to s e l e n i t e by f e r rou s i r o n : SeO^ 2 " + 2 F e 2 + + 2H + >Se03 2 " + 2 F e 3 + + H 2 0 . (46) R e a c t i o n c o n d i t i o n s of 200 °C for about 5 minutes are adequate to complete the r e d u c t i o n . The s o l u t i o n then undergoes o x y d r o l y s i s at 200 °C under 0.35 MPa 0 2 for a f u r t h e r 15 minutes to p r e c i p i t a t e i r on as f e r r i c ox ide (probab ly h y d r a t e d ) : 2FeSO^ + 1/2 0 2 + 2H 2 0 > 2H 2 SO i f + F e 2 0 3 . (47) Minor i m p u r i t i e s are p r e c i p i t a t e d or adsorbed concur rent w i th i r on p r e c i p i t a t i o n . The p u r i f i e d s o l u t i o n con ta in s t y p i c a l l y on l y 1 g/L Fe and 0.4 mg/L Se-r and i s ready for copper e 1 e c t r o w i n n i n g . (c) Copper E l e c t r o w i n n i n g Copper i s e lec t rowon from the p u r i f i e d pregnant leach l i q u o r (85 g/L C u 2 + , 30 g/L H 2SO^) in a i r - s p a r g e d c e l l s at - 53 - high cur rent density (330-650 A/rrr) and elevated temperature (50 ° C ) . Spent e l e c t r o l y t e con t a i n i ng 30 g/L C u 2 + and 130 g/L H2SO4 is r e c y c l ed to the oxidat ion leach. Pure cathode copper su i table for sale is produced at a current e f f i c i e n c y of 90% or greater . 2.2.4 Precious Metals and Sulphur Recovery Residue from the oxidat ion leach contains elemental sulphur together with precious metals, gangue and any unreacted sulphides ( inc luding M0S2 which is not so lub i l i z ed by any of the leaches) . Gangue is rejected by f l o t a t i o n , fo l lowed by sulphur removal by solvent extract ion (xylene at 100 ° C ) . The end product is a r ich precious metals concentrate con- t a i n i n g t y p i c a l l y 6,600 g/t Ag and 275 g/t Au, and can be sold or further processed. Recovery of Ag and Au based on concentrates is 96%. 2.2.5 Energy Consumption Energy consumption for the S.C. Process is contrasted with f lash smelting in Table 6 7 2 . As can be seen, the S.C. Process follows the trend of most hydrometa11urgica1 processes in being energy intens ive , and cannot compete with f lash smelting if the la t ter has an acid market. However, if SO2 produced by a smelter must be f ixed as elemental sulphur or neutra l ized sulphate, then the energy gap between the two processes is v i r t u a l l y e l iminated. - 54 - TABLE 6 Comparison of Energy Requirements Fos s i1 F u e l * k c a l / k g Cu E l e c t r i c Powe r # Total S.C. Copper Process 6320 3650 9970 F l a s h S m e l t i n g / E l e c t r o - r e f i n i n g w i th A c i d P roduc t i on 3290 1280 4570 F l a s h S m e l t i n g / E l e c t r o - r e f i n i n g w i th Sulphur P roduc t i on f r om SO2 9300 to 11000 * Natura l gas c a l o r i f i c va lue = 8900 k c a l / s t d m 3 # E l e c t r i c power = 860 kcal/kWh Reasons for the h igher energy requirement for the S.C. Process are - the e1ec t row inn ing step which accounts for v i r t u a l l y a l l of the e l e c t r i c power requ i rement , the heating of large so lut ion volumes, and the i n e f f i c i e n t recovery and r e c y c l e of process heat . 2.2.6 Economi cs A d e t a i l e d cost e s t imate was prepared for the S.C. Process and compared to f l a s h sme 11 i ng/e lect roref in i ng co s t s ' 7 2 . The es t imate was based on a 68,000 tonne Cu/year f a c i l i t y b u i l t at a Western Canada l o c a t i o n w i th cos t s in 1977 Canadian d o l l a r s . No c r e d i t s were g iven for the e lementa l su lphur - 55 - recovered in the S.C. Process or the by-product acid produced in a f lash smeIter. The analys is concluded that an S.C. plant could be bu i l t for 172 m i l l i o n do l la rs versus 195-205 m i l l i o n for a comparable f lash sme 11er/e1ectroref inery. Operating cost for the S.C. Process was estimated at 31.5^/kg versus 28.4^/kg for sme l t ing with acid product ion, or 40.0^/kg for smelting with acid n e u t r a l i z a t i o n . A major ope ra t ing cost expense for the S.C. Process is energy consumption, which is about 9.1f£/kg Cu as opposed to 4.2^/kg for smelt- ing/el ect r or ef i n i ng with acid product ion. 2.2.7 Surrmar y The S.C. Copper Process retains a l l the key advantages boasted by the best of the other copper hydrometa 11urgica1 processes, namely 98% or greater recovery of copper, conversion of sulphur to S ° , disposal of iron as an innocuous s o l i d , f l e x i b i l i t y in t reat ing varying grades and types of copper concentrates, and recovery/disposal of minor metals. However, the S.C. Process can claim these further advantages: Negl ig ib le copper loss to j a ros i t e . Pure cathode copper is eas i l y obtained. Precious metals are recovered quant i ta t i ve ly into a P.M. concentrate, from which they can eas i l y be recovered. Copper is leached under mild condi t ions . - 56 - These advantages can a l l be a t t r i b u t e d , d i r e c t l y or i n d i r e c t l y , to the f a c t that i r on is s e l e c t i v e l y e x t r a c t e d from copper . However, three un i t ope ra t i on s are r e q u i r e d to e x t r a c t 91% of the i r on from the copper , i n c l u d i n g an o b j e c t i o n a l pyrometa 11urgica1 f r o n t - e n d s tep . The process cannot be c a l l e d pu re l y "hydrometa11urg i ca1 " , and as such may not be as c l ean as other hydrometa 11urgica1 p roce s se s . The number of un i t ope ra t i on s r e q u i r e d for the process i s r e f l e c t e d in the h i g h o p e r a t i n g and c a p i t a l c o s t . C l e a r l y , i f the c o p p e r - i r o n s e p a r a t i o n procedure cou ld be s i m p l i f i e d , t h i s would lower s u b s t a n t i a l l y c a p i t a l and o p e r a t i n g c o s t s . Maschmeyer et a l . 7 2 e s t imated c a p i t a l and o p e r a t i n g cos t s cou ld both be reduced by 15% i f thermal a c t i v a t i o n and a c i d l e a c h i n g of i r on cou ld be e l i m i n a t e d , which i s proposed for feeds h igh in b o r n i t e . The other obvious improvement to the S.C. Process would be to r e p l a c e the e1ec t row inn ing step wi th hydrogen r e d u c t i o n . Th i s cou ld reduce energy requirements by roughly 20-25%. 2.3 PURPOSE AND SCOPE OF THE PRESENT INVESTIGATION Prev ious s tud ie s on the r e d u c t i o n l e a c h i n g of c h a l - c o p y r i t e have i n d i c a t e d that t h i s method shows promise as an e f f e c t i v e method of s e p a r a t i n g i r on from copper , p r i o r to copper d i s s o l u t i o n . However, s tud ie s to date have been m a i n l y of academic i n t e r e s t . L i t t l e work has been done to - 57 - develop optimal leach condit ions which could be incorporated into an overa l l process. The prac t i ca l advantages of se l ec t i ve l y extract ing iron from copper concentrates in a hydrometa 11urgical process have been demonstrated by the S.C. Copper Process. However, the S.C. Process resorted to using three unit operations to remove i ron , one of which was pyrometa 11urgical . Reduction leaching is seen to be a potent i a l l y v iable a l te rnat ive which could be incorporated into the S.C. Process. Therefore, i t was decided to invest igate two novel methods of reduction leaching of cha lcopyr i te . Referr ing back to Figure 6, it can be seen that there is a small area above the hydrogen-water l i ne , at about pH O or l ess , in which cha lcoc i te and ferrous iron are stable with respect to cha lcopyr i te . This region of s t a b i l i t y is expanded to s l i g h t l y higher pH's at higher temperatures, as shown by Baratin?3 j n n - i S 200 ° C Pourbaix diagram. This suggests that hydrogen gas can act as a reducing agent for cha lcopyr i te . Furthermore, i f the reduction leach is carr ied out in copper sulphate so lu t ions , evolut ion of H 2S gas can be avoided. The overa l l react ion can be postulated as fo l lows: CuFeS 2 + 3 C u 2 + + 2H 2 >2Cu2S + F e 2 + + 4H+. (48) In addition, metal l ic copper should be an a l te rnat i ve reductant under s imi lar condi t ions : CuFeS 2 + C u 2 + + 2Cu° >2Cu2S + F e 2 + . (49) - 58 - This work w i l l be concerned with studying the potential p rac t i ca l appl icat ions of the above two react ions. The focus w i l l be on optimizing iron leach rates and ext rac t ions ; for this reason experiments w i l l be conducted in strong copper sulphate solut ions at elevated temperatures and pressures. CHAPTER 3 EXPERIMENTAL 3.1 MATERIALS 3.1.1 Copper Concent ra te s Reduc t ion leach exper iments were c a r r i e d out on four d i f f e r e n t copper c o n c e n t r a t e s , ob ta ined from the Phoenix (now shut down) and Bethlehem mines located in B r i t i s h Columbia, and from the Fox Lake and Ruttan mines in Man i toba . The as - r e c e i v e d c o n c e n t r a t e samples were kept s t o red in a i r t i g h t t i n s or p l a s t i c bags when not in use, to m in imize su r f a ce a i r o x i d a t i o n . In format ion s u p p l i e d by the r e s p e c t i v e mines i n d i c a t e d that a l l c oncen t r a te s con ta ined p r i m a r i l y c h a l c o p y r i t e . In a d d i t i o n , the Bethlehem c o n c e n t r a t e con ta i ned s u b s t a n t i a l b o r n i t e whereas the Phoenix, Fox Lake and Ruttan concen t ra te s con ta i ned p y r i t e . The l a t t e r two a l s o con ta ined minor amounts of s p h a l e r i t e (ZnS). Tab le 7 presents r e s u l t s of e lementa l a n a l y s e s , c a l c u l a t e d approximate su lph ide mineral compositions, and screen analyses (Tyler s e r i e s s i e v e s ) on the c o n c e n t r a t e s . Approximate m i n e r a l compos i t i ons were c a l c u l a t e d from Cu and Fe ana ly ses by assuming that on ly the above mentioned s u l p h i d e m i n e r a l s were present . For example, with the Bethlehem c o n c e n t r a t e , l e t x = moles CuFeS2 present and y = moles Cu^FeS^ p r e s e n t . Then, on a 100 g b a s i s : - 6 0 - T A B L E 7 C o p p e r C o n c e n t r a t e s U s e d P h o e n i x B e t h 1 e h e m F o x L a k e R u t t a n E l e m e n t a l A n a l y s i s % C u 2 4 . 3 3 2 . 7 2 6 . 4 2 8 . 2 % F e 3 2 . 9 2 0 . 0 3 3 . 1 3 1 . 7 % S 3 1 . 0 * 2 6 . 0 3 5 . 6 3 5 . 9 % Z n - - 3 . 7 5 2 . 0 2 C a l c u l a t e d M i n e r a l C o n t e n t * * % C u F e S 2 7 0 5 9 7 5 8 2 % F e S 2 2 5 - 2 0 1 4 % C u 5 F e S ^ - 2 0 - - % Z n S - - 5 3 P a r t i c l e S i z e D i s t r i b u t i o n (%) M e s h + 1 0 0 0 . 2 0 . 3 5 . 0 0 . 2 S i z e + 1 4 0 1 2 . 7 5 . 9 - - ( T y l e r + 2 0 0 1 8 . 7 1 0 . 7 - - S e r i e s ) + 2 7 0 1 9 . 9 2 3 . 7 - - + 4 0 0 1 3 . 8 2 3 . 8 - - - 4 0 0 3 4 . 7 3 5 . 6 7 2 . 4 6 4 . 9 T o t a l 1 0 0 . 0 1 0 0 . 0 S u l p h u r a s s a y i s l o w e r t h a n e x p e c t e d b e c a u s e e x t e n s i v e s u r f a c e o x i d a t i o n w i t h t i m e h a d o c c u r r e d f o r t h i s v e r y o l d c o n c e n t r a t e . D o e s n o t v a l u e s . c o n s i d e r m i n o r s u l p h i d e m i n e r a l s a n d g a n g u e - 61 - t o t a l moles Cu = x + 5y = 32.7/63.54 t o t a l moles Fe = x + y = 20 .0/55.85. S o l v i n g for x and y, we get : x = 0.319 moles CuFeS 2 = (0.319 m) (183.52 g/m) = 59% CuFeS 2 y = 0. 039 moles Cu 5 FeS^ = (0. 039 m) (501.80 g/m) = 20% Cu 5 FeS^. Leach exper iments were c a r r i e d out mainly on as -received m a t e r i a l . However, to assess the e f f e c t s of p a r t i c l e s i z e on l e a c h i n g , some exper iments were performed on m a t e r i a l c a r e f u l l y screened to the f o l l o w i n g s i z e ranges: a) -140+270 mesh (53-105 um) b) -275 mesh (< 50 um) c) -400 mesh (< 38 um) 3.1.2 Su lph ide M i n e r a l s Reduct ion leach exper iments were a l s o performed on s i x d i f f e r e n t s u l p h i d e m i n e r a l s to assess t h e i r r e l a t i v e ra tes of r e a c t i v i t y . M i n e r a l s s t u d i e d were p y r i t e , b o r n i t e , c h a l c o p y r i t e , p e n t l a n d i t e (N iFeSj g), spha ler i te and pyrrhot i te (~Fen 9 S ) . Mass ive rock samples, s u p p l i e d by the Ward Museum and thought to be reasonab ly pure, were crushed and ground to -100 mesh s i z e , a f t e r which p o r t i o n s were c a r e f u l l y s i z e d to -270+325 mesh for use. Tab le 8 presents the r e s u l t s of e lementa l ana lyses on the m i n e r a l samples. With the exception - 62 - TABLE 8 " P u r e " S u l p h i d e M i n e r a l s U s e d M i n e r a l Or i g i n Cu % Fe % S % O t h e r % P y r i t e Quebec 43. 6 52. 2 B o r n i t e M o n t a n a 53.7 14. 2 28. 3 - Cha1 c o p y r i t e Quebec 20. 1 41. 2 36. 5 - P e n t 1 a n d i t e S u d b u r y - 31. 9 32. 1 29.5 Ni P y r r h o t i t e K i m b e r l e y - 58. 1 35. 1 - S p h a 1 e r i t e New Y o r k - - 34. 2 50.5 Zn o f t h e c h a l c o p y r i t e s a m p l e , t h e a n a l y s e s c o r r e l a t e r e a s o n a b l y w e l l w i t h e x p e c t e d v a l u e s . H o w e v e r , t h e c h a l c o p y r i t e s a m p l e c o n t a i n e d an i n o r d i n a t e l y h i g h amount of i r o n , w h i c h SEM and X - r a y d i f f r a c t i o n a n a l y s i s i n d i c a t e d was due p r i m a r i l y t o t h e p r e s e n c e of p y r i t e . 3.1.3 R e a g e n t s R e a g e n t g r a d e c o p p e r s u l p h a t e p e n t a h y d r a t e and armrionium s u l p h a t e w e r e u s e d f o r a l l l e a c h e x p e r i m e n t s . S u l p h u r i c a c i d u s e d was 9 4 - 9 8 % p u r e and was d i l u t e d as r e q u i r e d . R e a g e n t g r a d e c o p p e r powder was used f o r a l l e x p e r i m e n t s . In most c a s e s , v e r y f i n e c o p p e r s c r e e n e d t o -400 mesh was u s e d . To a s s e s s t h e e f f e c t o f i n i t i a l c o p p e r p a r t i c l e s i z e on l e a c h i n g , -100+200 mesh and 9 0 % -100+325 mesh c o p p e r s i z e s w e r e u s e d . - 63 - S t anda rd c y l i n d e r grade h y d r o g e n , n i t r o g e n and ca rbon monoxide gases were used t h r o u g h o u t . 3.2 APPARATUS Leach e x p e r i m e n t s were performed in two types of pressure a u t o c l a v e s ; a P a r r s e r i e s 4500 2L c a p a c i t y r e a c t o r , and a 107 mL c a p a c i t y s h a k i n g a u t o c l a v e . 3 .2 .1 P a r r A u t o c l a v e The Pa r r A u t o c l a v e assembly was equ ipped w i t h c o n t r o l s f o r s t i r r i n g , t e m p e r a t u r e , gas p r e s s u r e and s a m p l i n g . Due to the c o r r o s i v e n a t u r e of some of the r e d u c t i o n l e a c h e x p e r - i m e n t s , a 2L t i t a n i u m r e a c t o r , 10 cm i n d i ame te r and 25 cm i n h e i g h t ( i n t e r i o r d i m e n s i o n s ) , was u s e d . E a r l y e x p e r i m e n t s showed t ha t a t i t a n i u m s t i r r i n g assembly s t i l l tended to c o r r o d e . An assembly c o n s i s t i n g of a z i r c o n i u m s t i r r i n g s h a f t and t a n t a l u m i m p e l l o r s was t r i e d and proved to be a d e - q u a t e l y c o r r o s i o n r e s i s t a n t . The a u t o c l a v e s t i r r i n g assembly was powered by a b e l t - d r i v e n 1/15 hp motor w h i c h c o u l d p r o v i d e v a r i a b l e s t i r r i n g speeds of up to 1,000 rpm by changing p u l l e y s i z e s . For t e s t s i n w h i c h hydrogen gas under p r e s s u r e was used as the r e d u c t a n t , two 7 cm d i amete r downward t h r u s t i n g i m p e l l o r s , one s i t u a t e d 1 cm be low the s o l u t i o n l e v e l and the o t h e r s i t u a t e d 1 cm above the b o t t o m , were u s e d . A h i g h s t i r r i n g speed of 750 rpm was used to ensu re adequate gas d i s p e r s i o n i n t o s o l u t i o n . For t e s t s i n w h i c h copper - 64 - powder was the reductant, so lut ion pulp densi t ies were much higher and a d i f fe rent impellor design was necessary to ensure good so l ids suspension. A single 9 cm diameter upward thrusting impel lor , s i tuated Yi cm above the bottom of the reactor , was used. A s t i r r i n g speed of 250 rpm proved adequate for these tes t s . The reactor was heated by s l i d i ng it into an insulated, s ta in less steel sleeve which was e l e c t r i c a l l y heated and pressure a i r cooled for temperature control. Accurate automatic temperature control was provided by a Yellow Springs Instruments Thermistemp cont ro l l e r (Model 71), which was connected to a thermister probe inserted into the autoclave temperature we l l . This arrangement contro l led leach temperatures to i 1 ° C . The temperature was measured and monitored by a chrome 1-alumel thermocouple wire, one end of which was inserted into the temperature well and the other end connected to a Sargeant-Welch s t r i p chart recorder. If des i red , the leach solut ion could be sampled during the course of a run via a titanium valve connected to a sampling tube, the end of which was s i tuated about 3 cm above the autoclave bottom. To prevent so l ids from entering the sampling tube, a graphite plug or porous tef lon f i l t e r was f i t t e d to the end. Frequently, plugging problems were encountered during sampling, for two reasons - (a) leach solutions contained high concentrations of copper, iron and ammonium sal ts which tended to c r y s t a l l i z e upon cool ing and clog the sampling valve, and (b) considerable sulphide mineral pa r t i c l e s ize reduction occurred during leaching, and the u l t ra- f ine so l ids - 65 - (<5 um) clogged the graphite or tef lon f i l t e r s . This sampling problem was never completely overcome. Gas pressure was measured using a 0-6.5 MPa (0-1000 psi) gauge s i tuated on the top of the autoclave head. Pressures were corrected for the pressure of steam at the par t i cu la r leach temperature. 3.2.2 Shaking Autoclave Smaller-scale leach experiments were performed in a home-made shaking autoclave set-up. A 107 mL capacity reactor constructed of corrosion res is tant zirconium and equipped with temperature we l l , gas in let and outlet was used. The reactor was heated by s l i d i ng i t into an e l e c t r i c - a l l y heated aluminum jacket. The temperature monitoring and control system used was ident ica l to that used for the Parr autoclave, except pressurized a i r cool ing was not required due to su f f i c i en t natural cool ing by heat loss through the large autoclave head. Temperature control was again maintained within ± 1 ° C with th is system. Ag i ta t ion was provided by a l inear hor izontal shaking motion at a rate of 288-3.8 cm strokes per minute, with the autoclave t i l t e d at a 45° angle. Gas pressure was measured and monitored by a pressure transducer (Consolidated Electrodynamics Corporat ion, Model 4-313, 0-6.5 MPa range) coupled v ia a "T " connection to the gas in let tube. The output signal from this transducer was monitored by a Sargeant-Welch s t r i p ' cha r t recorder. The 66 t ransducer was c a l i b r a t e d aga ins t steam pres su re in the au to - c l a v e . The shak ing a u t o c l a ve had no p r o v i s i o n for s o l u t i o n sampl ing d u r i n g the course of a run . 3.3 EXPERIMENTAL PROCEDURE 3.3.1 Make-Up of S t a r t i n g Leach ing S o l u t i o n s For a l l s t ud i e s on r e d u c t i o n l e a c h i n g w i th hydrogen gas, a s t a r t i n g leach s o l u t i o n of 63.5 g/L C u 2 + , made w i th CuS04 '5H 2 0 , w a s used. For a l l s t ud ie s on r e d u c t i o n l e a c h i n g w i th copper powder, a s t a r t i n g leach s o l u t i o n c o n t a i n i n g 90 g/L C u 2 + (as CuSO^-5H 2 0) , 20 g/L H 2SO^ and 132 g/L (NH^^SO^ was used. D e i o n i z e d water was used throughout . The s o l u t i o n s were made s l i g h t l y a c i d i c to prevent h y d r o l y s i s and p r e c i p i t a t i o n of copper as a n t l e r i t e , CuSO^•2Cu(OH) 2 , at high tempera tures . In a d d i t i o n , some of the concen t ra te s and m i n e r a l s consumed a c i d i n i t i a l l y (probab ly due to the presence of minor amounts of a l k a l i n e gangue and o x i d e s ) . Arrmonium su lpha te was always added to b u f f e r the s o l u t i o n s , which in an o v e r a l l p roce s s , r e s u l t s in improved j a r o s i t e p r e c i p i t a t i o n of the i r o n and hydrogen r e d u c t i o n of copper . More w i l l be s a i d about t h i s in Chapter 5. As the maximum room tempera ture s o l u b i l i t y of copper su lpha te in aqueous s o l u t i o n i s about 90 g/L C u 2 + } i t was necessary to warm the ammonium su lphate b u f f e r e d s o l u t i o n s to about 50 °C to ach ieve complete d i s s o l u t i o n of a l l the - 67 - and ammonium s a l t s . Solutions were then made up acc- to the desired volume in volumetric f l a sks , and irrmed- charged to the autoclave. 3.3.2 Parr Autoclave For a l l runs in the Parr autoclave, the experimental procedure used was as fo l lows: 1) The requ i red amounts of so l ids and leach so lut ion were added to the autoclave, the autoclave was sealed, placed into the heater sleeve and s t i r r i n g was i n i t i a t e d . 2) The autoclave was then heated. During the 10 minute warm-up period required to reach 100 ° C temperature, nitrogen was flushed through the autoclave to purge a i r . At 100 ° C , nitrogen f lushing was stopped, the gas in let and bleed valves were c losed, and preparations were made to i n t ro - duce the appropriate gas. 3) At the des i r ed r eac t i on temperature, the approp- r i a t e gas ( e i the r n i t r o g e n , hydrogen or carbon monoxide) was introduced at the desired pressure. 4 ) Solution sampling, i f des i red, was carr ied out as fo l lows - at the appropr i a t e t ime, the sampling v a l v e was opened and about 10 mL of so lut ion was flushed through and d i s ca rded . Then, about 5 mL of sample was d i scharged into a 50 mL volumetric f l a sk . The sample was d i lu ted to 50 mL from a burette of deionized water ( a c id i f i ed to pH 2.0 with H 2 S O 4 to prevent metal hydro l ys i s ) . The o r ig ina l sample - 68 - volume was then ca lcu lated to the nearest 0.1 mL from the burette reading by d i f fe rence , and the d i l u t i on factor taken as 50 mL * sample volume. Sampling from the Parr autoclave was not done for runs in which the leach temperature was greater than 120 ° C , because of errors imposed by s ign i f i can t water loss from the sample due to f lash ing at higher temperatures. 5) To terminate a run, the gas in le t valve was shut, s t i r r ing was stopped, the reactor was removed from the heating jacket and quickly cooled by immersing in a pa i l of cold water. Once cooled, excess gas was vented from the reactor , the autoclave head removed and the contents pressure-fi l tered. The residue f i l t e r cake was washed thoroughly with pH 2.0 deionized water, dried at 60 ° C , weighed, ro l l ed thoroughly to homogenize and stored for subsequent ana lys i s . The volumes of the f i l t e r e d leach so lut ion and wash water were recorded, and the f i l t r a t e s stored for ana lys i s . 3.3.3' Shaking Autoclave The shaking autoclave proved to be idea l l y suited for sma 11er-sea 1e runs in which solut ion volumes were less than about 70 mL. The experimental procedure used was identical to that described for the Parr autoclave, except that the reactor contents were suet ion-f i1tered in a Buchner funnel , washed with pH 2.0 water, and the tota l f i l t r a t e made up to 250 mL volume in a volumetric f lask. Data on leach behaviour with varying residence times could be rapidly obtained due - 69 - to the ease and r a p i d i t y w i th which one run could be terminated and the next run s t a r t e d . 3.4 ANALYTICAL METHODS 3.4.1 S o l u t i o n Ana ly ses S o l u t i o n c o p p e r , i r o n , z i n c and n i c k e l v a l u e s were determined, a f t e r a p p r o p r i a t e sample d i l u t i o n , by atomic a b s o r p t i o n a n a l y s i s u s ing a Pe rk in Elmer model 306 s p e c t r o - photometer. Meta l c o n c e n t r a t i o n s repo r ted were back- c a l c u l a t e d to the o r i g i n a l s t a r t i n g leach s o l u t i o n b a s i s . Free a c i d c o n c e n t r a t i o n s were determined by t i t r a t i o n w i th IN NaOH us ing a Metrohm automat ic t i t r a t o r . 3.4.2 So 1 ids Ana 1yses C o n c e n t r a t e s , m i n e r a l s and leach re s idues were assayed for copper , i r o n , z i n c and n i c k e l by d i g e s t i o n of the sample in aqua r e g i a and bromine, f o l l owed by atomic a b s o r p t i o n a n a l y s i s of the s o l u b i l i z e d meta l v a l u e s . S o l i d s were ana lyzed for t o t a l sulphur by d i g e s t i o n of the sample in aqua r e g i a and bromine, f o l l owed by p r e c i p i - t a t i o n of su lphur as bar ium s u l p h a t e . 3.5 MATERIAL BALANCES AND EXTRACT ION CALCULATIONS The p r e c i s i o n and a c c u r a c y of s o l i d s and s o l u t i o n - 70 - analyses were checked by performing complete mater ia l balance ca l cu l a t i ons for each run . Examples of such c a l c u l a t i o n s are given in Appendix 1. If mater ia l balances did not agree to within 5%, the so l ids and s o l u t i o n s were re-assayed, or the run was repeated. Metal extract ion percentages were ca lculated in two ways - on a so lut ion bas is , that i s : % ext r . = tota l weight of dissolved metal x 100, head metal weight and, wherever poss ib le , on a residue bas is , that i s : % ext r . = head metal weight - residue metal weight x 100, head metal weight Extract ion values quoted are averages of values ca lcu lated on a solut ion basis and on a residue bas is , whenever both values were ava i l ab le . CHAPTER 4 RESULTS AND DISCUSSION 4. 1 REDUCTION LEACHING WITH HYDROGEN GAS E a r l y exper imenta l work was concerned w i th de te rmin ing how e f f e c t i v e l y i r on cou ld be e x t r a c t e d from a p y r i t i c copper c o n c e n t r a t e by r e d u c t i o n l e a c h i n g w i th hydrogen gas. The leach chemistry was pos tu la ted to p roceed, i f at a l l , a c c o r d i n g to the f o l l o w i n g s t o i c h i o m e t r y : CuFeS 2 + 3 C u 2 + + 2H 2. >2Cu 2S + F e 2 + + 4H + (50) F e S 2 + 4Cu 2 + + 3H 2 >2Cu 2S + F e 2 + + 6 H + . (51) Phoenix c o n c e n t r a t e , assumed to c o n t a i n 70% C u F e S 2 and 25% F e S 2 , was used for these t e s t s . The s t o i c h i o m e t r i c c u p r i c requirement for t h i s c o n c e n t r a t e , based on equat ions (50) and (51), i s : 70 x 3 + 25 x 4 = 1.98 moles C u 2 + per 183.52 119.98 100 g c o n c e n t r a t e , g i v i n g a s t o i c h i o m e t r i c molar C u 2 + / F e r a t i o o f : 1.98 - = 3 .4 /1 . 100 x .329 55. 85: - 72 - The runs were performed in the 2L Parr autoclave using as-received concentrate and 1L of a IM copper sulphate so lu t ion . In the hopes of promoting rapid leaching, the runs were carried out under r e l a t i v e l y high temperature, 180 ° C , and high pres- sure, 2.8 MPa hydrogen. Table 9 summarizes the results obta in - ed Runs #1-3 were performed under hydrogen; Run #4, the control run, had 2.8 MPa nitrogen subst i tuted for hydrogen. In Run #1, 50 g concentrate, representing the stoichio- metr ic requirement, was leached for 2 h. The iron extract ion achieved was 99.9% with only 0.017% Fe remaining in the leach residue. Therefore, iron was extracted quant i ta t i ve ly from both pyr i te and cha lcopyr i te . The f ina l leachate contained 15.0 g/L Fe and 0.57 g/L Cu, ind icat ing an excel lent copper- iron separation was achieved. The residue assayed 74.5% Cu, which is c lose to the composition of pure cha lcoc i te (79.9% Cu), as predicted by equations (50) and (51). However, no attempt was made to determine the exact stoichiometry of the product. Either digenite (Cu^gS) or djurleite (CujggS) may also have been present. Runs #2 and 3 show results obtained when a stoichiometric excess of concentrate was leached. In Run #2, 65g of concen- trate (~20% excess) was leached for VA h to give an iron extract ion of 91.7% with 7.0 g/L Cu le f t in so lu t ion . In Run #3, 80 g of concentrate (~35% excess) was leached for 2 h to give an iron extract ion of 94.5% with only 0.06 g/L Cu le f t in so lu t ion . In both cases iron extract ions were higher than theore t i ca l l y possible based on equations (50) and (51). - 73 - TABLE 9 Reduction Leaching of Phoenix Concentrate With Hydro gen Gas Run # 1 2 3 4 Feed Weight (g) 50 65 80 50 I n i t i a l Leachate: Vo1ume (mL) 1, 000 1, 000 1, 000 1, 000 g/L Cu 63. 5 63.5 63. 5 63. 5 I n i t i a l Mo 1 a r Cu 2 + /Fe Ratio 3.4 2.6 2. 1 3.4 Leach Cond i t ions : Temperature ( ° C ) 180 180 180 180 Pressure (MPa) 2.8 H 2 2.8 H 2 2.8 H 2 2.8 N 2 Time (h)* 2 lfc 2 2 F ina l Leachate: Vo 1 ume (mL) 1, 000 1 , 000 1 , 000 1 , 000 g/L Cu 0. 57 7.0 0. 06 40. 0 g/L Fe 15.0 n.d. 24. 8 5.2 Leach Residue: Weight (g) 89. 5 98.9 111.1 58. 1 % Cu 74. 5 70.6 74. 1 54. 1 % Fe 0.017 1. 8 1 . 1 16.8 Fe Extract ion (%)** 99. 9 91.7 95. 4 40.7 n.d. = not determined * does not include warm-up time (~25 min. to 180 °C ) ** based on head and residue iron assays - 74 - It is l i k e l y that side-react ions were taking place during the leach which lowered the i n i t i a l cupric requirement. One possible competing react ion contr ibut ing to chalcopyr i te conversion is the previously mentioned "neut ra l " or McGauley- type leach, which requires just one mole cupric per mole chalcopyr i t e : CuFeS 2 + C u 2 + >2CuS .+ F e 2 + . (52) Note that this leach generates cove l l i t e (CuS), not cha l coc i t e . Another p o s s i b i l i t y is that the Phoenix concentrate had under- gone s ign i f i can t surface air oxidat ion with time, rendering some of the contained copper acid-soluble CuO + 2H+ >Cu2+ + H 2 0 (53) thereby reducing the i n i t i a l cupr ic requirement. Run #4, representing a neutral or McGau1ey-type leach (equation 52), was i d e n t i c a l to Run #1 except n i t rogen was used rather than hydrogen. After 2 h leaching an iron extract- t i on of 40.7% was ob t a i ned , cons ide r ab l y lower than tha t achieved under reducing condi t ions . Thus, under comparable condi t ions , the neutral leach is considerably less e f f ec t i ve than the reduction leach in achieving a copper-iron separation. However, these resul ts showed that iron can be p a r t i a l l y extracted from chalcopyr i te under neutral leach condi t ions , supporting the above conjecture that a neutral leach may - 75 - be a supplementary side react ion during reduction leaching. It is well known that cupr ic-containing solut ions are rapid ly reduced to copper metal by hydrogen under the above- described condi t ions . Therefore, it can be presumed that the reduction leach might take place in two steps: 2 C u 2 + + 2H 2 >2Cu° + 4H+ (54a) CuFeS 2 + 2Cu° + C u 2 + ' >2Cu2S + F e 2 + (54b) Reaction (54a) is very rapid at 1 8 0 ° c , with C u 2 + being lowered from 6 0 - 9 0 g/L to 3 g/L in 3 0 minutes 7**. The overall reduction leach appears to take place at a rate comparable to hydrogen reduction of copper sulphate solutions. More wi l l be said about the k inet i cs l a t e r . These resul ts show that reduction leaching of copper concentrate with hydrogen gas is an e f fec t i ve means of achiev- ing a good copper-iron separat ion, with both pyr i te and cha l - copyrite being converted to cha l coc i t e . However, for p rac t i ca l purposes the use of hydrogen gas as a reductant has two major drawbacks: a) The requirement of three moles dissolved copper per mole chalcopyr i te means leach pulp densi t ies are res t r i c t ed to being unacceptably low at 10% or l ess . - 76 - b) The s o l u t i o n produced is q u i t e a c i d i c , c o n t a i n i n g at l ea s t 65 g/L H 2SO^. Removal of i r on from a c i d i c s o l u t i o n s is c on s i de red to be too d i f f i c u l t to be p r a c t i c a l l y f e a s i b l e . For these reasons no f u r t h e r s tud ie s on r e d u c t i o n l e a c h i n g w i th hydrogen were c a r r i e d out . 4.2 REDUCTION LEACHING WITH COPPER POWDER UNDER NITROGEN From a p r a c t i c a l s t a n d p o i n t , i t was des i rab le to develop a r e d u c t i o n leach method which would work at h igher pulp d e n s i t i e s (20% or g r e a t e r ) and not produce a c i d . Reduct ion l e a c h i n g w i th copper powder seemed the obvious c h o i c e , as the a n t i c i p a t e d leach r e a c t i o n s for c h a l c o p y r i t e and p y r i t e r e q u i r e on ly one mole c u p r i c per mole m ine ra l i r o n , and do not produce a c i d . The f o l l o w i n g s e r i e s of experiments were des igned to study the f e a s i b i l i t y of us ing copper powder as a reduc ing agent . The o b j e c t i v e s were to produce, at reasonable temperature and re s i dence t ime, a re s idue c o n t a i n i n g le s s than 1% Fe and a l eacha te c o n t a i n i n g le s s than 3 g/L Cu . CuFeS 2 + C u 2 + + 2Cu° F e S 2 + C u 2 + + 3Cu° • ->2Cu 2 S + F e 2 + •>2Cu2S + F e 2 + (55) (56) - 77 - 4.2.1 Ef fect of Residence Time at 140 ° C Reduction leach experiments on the as-received Ruttan concentrate, in which the leach residence time was var ied , were carr ied out at 140 ° C in the shaking autoclave. The leach solut ion used in each test contained i n i t i a l l y 90 g/L C u 2 + , 20 g/L H2SO4 and 132 g/L (NH^^SO^. Fine copper powder (-400 mesh) was used as a reducing agent. Assuming the Ruttan concentrate to contain 82%CuFeS2» 14% FeS 2 and 3% ZnS; and. the predominant leach reactions to be, CuFeS 2 + C u 2 + + 2Cu° ; r->2Cu2S + F e 2 + (57) FeS 2 + C u 2 + + 3Cu°- >2Cu2S + F e 2 + (58) ZnS + C u 2 + + Cu°, *Cu 2S + Z n 2 + (59) the s to ich iometr ic molar Cu 2 +/(Fe+Zn) ra t io is 1/1 and the sto ich iometr ic molar Cu°/(Fe+Zn) is 2.15/1. On this bas is , 11.8 g of Ruttan concentrate and 9.4 g of -400 mesh copper powder were leached under 1 MPa N2 pressure for residence times ( inc luding the warm-up period) of 12, 22, 42 and 102 minutes. Figure 8 shows the resul ts of iron and zinc leaching, and acid consumption as a function of residence time. The i n i t i a l rate of iron and zinc d i sso lu t ion was very rap id , with 68% iron extract ion and 20% zinc extract ion obtained during the 12 minute warm-up per iod. After 22 minutes, iron leaching had slowed considerably, and reached 93.4% extract ion  - 79 - a f t e r 102 m inu te s . Z inc leached s t e a d i l y but reached a f i n a l ext ract ion of on ly 55.8% a f t e r 102 m inu te s . The f i n a l r e s i due s o l i d s assayed 0.6% Fe and 0.54% Zn, and the f i n a l leach s o l u t i o n con ta ined 15.4 g/L C u 2 + . Dur ing the l e a c h , the f r e e s u l p h u r i c a c i d concentrat ion decreased from 20.0 g/L to 14.1 g/L, i n d i c a t i n g that a c i d consuming r e a c t i o n s such as CuO + 2H+ > C u 2 + + H 2 0 (60) F e 2 0 3 + Cu° + 6H + : >2Fe + + + 3H 2 0 '+ Cu + + (61) may be o c c u r r i n g to a s l i g h t e x t e n t . A l s o , the f i n a l C u 2 + concentrat ion is higher than expected based on the i ron e x t r a c t - ion a c h i e v e d . Th i s may be due to su r f a ce a i r ox ida t ion having generated copper ox ides which are a c i d s o l u b l e , as shown by equat ion (60). The h igh i r on e x t r a c t i o n ob ta ined i n d i c a t e s that py r i te must a l s o be l eached , as was the case when r e d u c t i o n l e a c h i n g w i th hydrogen. Z inc was on ly p a r t i a l l y e x t r a c t e d , showing that the contained spha ler i te is not as r e a c t i v e as c h a l c o p y r i t e and p y r i t e to r e d u c t i o n l e a c h i n g under these c o n d i t i o n s . The " t a r g e t " l e v e l of 1% Fe in s o l i d s was a c h i e v e d , but the s o l u b l e copper l e v e l in the f i n a l l eachate remained too h i g h . - 80 - 4.2.2 Ef fect of Varying the Start ing Molar Cu 2 +/(Fe+Zn) and Cu°/(Fe+Zn) Ratios It has been demonstrated that reduction leaching of a copper concentrate with f ine copper powder at 140 ° C can achieve a high iron ex t rac t ion . However, the ca lculated s to ich iometr ic condit ions used, based on mineral composition est imat ions, were not ideal because the f ina l leach solut ion s t i l l contained approximately 17% of the in i t i a l cupric content. It was des i rab le , i f poss ib le , to achieve a low f ina l cupr ic level ("^3 g/L) while maintaining a high iron ext rac t ion , by determining the optimum re la t i ve amounts of C u 2 + , Cu° and concentrate to use. To achieve t h i s , a ser ies of reduction leach experiments were performed on the as-received Fox Lake concentrate in which the i n i t i a l molar Cu 2 +/(Fe+Zn) and Cu°/(Fe+Zn) rat ios were s to ich iometr i c , and varied from stoichiometry by plus or minus 25%. The Fox Lake concentrate was assumed to contain 75% CuFeS 2 , 20% FeS 2 and 5% ZnS, and reactions (57)-(59) were assumed to be the predominant leach react ions. On this bas is , the s ta r t ing sto ich iometr ic molar Cu 2 +/(Fe+Zn) and Cu°/(Fe+Zn) rat ios for Fox Lake concentrate are 1/1 and 2.12/1, res pect i ve1y. Leach tests were performed in the shaking autoclave using f ine copper powder (-400 mesh) and a s tar t ing leach so lut ion containing 90 g/L C u 2 + , 20 g/L H2SO^ and 132 g/L (NH/j^SO^. To vary the C u 2 + /(Fe + Zn) r a t i o , the concentrate - 81 - weight was held f ixed and the solut ion volume was var ied . S im i l a r l y , to change the Cu°/(Fe+Zn) ra t io the weight of copper powder used was var ied . A l l leach tests were performed at 140 ° C under 1 MPa N 2 pressure for 1 h (not including a 12 minute warm-up time). Table 10 summarizes the resul ts obtained. Run #1 gives the resul ts of the sto ich iometr ic tes t , and shows that 97% iron extract ion was obtained ( ind icat ing again that pyr i te must also be leached) but 22.0 g/L Cu remained in so lu t ion . Iron extract ions were d r a s t i c a l l y lowered, to the 80-82% range, only when the Cu°/(Fe+Zn) ra t io was de- creased by 25% from stoichiometry (Runs #5, #7 and #8). The runs which produced the lowest iron content in residues and the lowest so lut ion copper values were Runs #3 and #9, where the Cu 2 +/(Fe+Zn) molar ra t io was decreased by 25% from stoichiometry. End solut ion copper values, at 0.4 and 0.5 g/L, were well below the suggested target value of 3 g/L. Residue iron values, at 1.1% and 1.2%, were very c lose to the 1% target value. In a l l runs, z inc was only p a r t i a l l y extracted (52-57%). The resul ts of this series of experiments have shown that, for the Fox Lake concentrate, the optimum Cu 2 +/(Fe+Zn) s ta r t ing molar ra t io is about 20-25% less than ca lculated stoichiometry. This suggests that the mineral composition estimation was not accurate espec ia l l y with respect to neglect - ing the oxidized species. The Cu 2 +/(Fe+Zn) ra t io can be decreased by decreasing the so lut ion volume rather than the cupr ic concentrat ion, meaning the reduction leach can be Run// 1 2 3 4 5 6 7 8 9 Cone. Weight (g) 10.8 10.8 10.8 10.8 10.8 10.8 10.8 10.8 10.8 Copper Weight (g) 9.4 9.4 9.4 11.8 7.1 11.8 7.1 7.1 11.8 Molar Cu°/(Fe+Zn) 2.12 2.12 2.12 2.65 1.59 2.65 1.59 1.59 2.65 Initial Leachate: Volume (mL) 50 62.5 37.5 50 50 62.5 37.5 62.5 37.5 Molar Cu++/(Fe+Zn) 1.00 1.25 0.75 1.00 1.00 1.25 0.75 1.25 0.75 Leach Residue: Weight (g) 19.7 20.0 18.8 22.6 18.5 22.8 18.5 18.4 22.8 % Cu 80.6 78.3 75.5 80.6 70.8 79.2 71.6 71.2 79.1 % Fe 0.9 0.6 1.1 0.5 3.1 0.7 3.6 3.8 1.2 % Zn 0.95 0.95 0.91 0.85 1.00 0.83 0.96 1.01 0.83 % S 19.6 19.1 19.3 16.5 22.5 16.7 20.6 22.1 18.2 Final Leachate: g/L Cu 22.0 34.7 0.4 20.1 47.7 30.1 1.8 42.2 0.5 g/L Fe 70.8 54.7 88.7 70.5 56.3 54.1 78.0 45.0 90.0 g/L Zn 4.55 3.52 6.03 4.48 4.40 3.33 5.55 3.14 5.44 Extraction (96): Fe 97 96 94 98 81 95 82 80 93 Zn 55 54 57 54 54 52 54 52 52 TABLE 10 Effect of Varying Starting Molar Cu°/(Fe+Zn) and Cu2+/(Fe+Zn) Ratios on Reduction Leaching of Fox Lake Concentrate - 83 - run at 20-25% higher pulp density without decreasing the overa l l iron ex t rac t ion . The optimum Cu°/(Fe+Zn) molar ra t io appears to be the calculated stoichiometric value, as increasing th is value had no benef i c i a l e f fect whereas decreasing the value resulted in dramatical ly reduced iron ext rac t ions . To determine i f these "opt imized" condit ions could be appl ied to other copper concentrates, 237 g of the as- received Ruttan concentrate was reduction leached in the Parr autoclave with 189 g of -400 mesh copper powder, in 800 mL (a 20% def ic iency ) of leach solut ion containing i n i t - i a l l y 90 g/L C u 2 + , 20 g/L H2SO^ and 132 g/L (NH^)2SO^. The leach was performed at 140 °C for V/i h (not inc luding a 17 minute warm-up period) under 1 MPa N 2 . The dried residue so l ids weighed 413 g and contained 76.2% Cu, 0.85% Fe, 0.68% Zn and 18.6% S. The f ina l leach solut ion combined with wash water contained, on an 800 mL bas is , 83.7 g/L Fe, 4.77 g/L Zn, 0.26 g/L Cu and 9.6 g/L H2SO^ (pH = 1.5). This represents an iron extract ion of about 92.2% and zinc extract ion of about 55.7%. These resul ts show that an excel lent copper-iron separation in the Ruttan concentrate can be made by reduction leaching with f ine copper powder and a 20% def ic iency in cupric copper. 4.2.3 Ef fect of Copper Powder Pa r t i c l e Size on Iron Extract ion To determine whether the copper powder pa r t i c l e s ize - 84 - affected the k inet ics of reduction leaching, Fox Lake con- centrate was leached as described for Table 10, Run #1, except that copper powder ca re fu l l y sized to -100+200 mesh was used. Reaction condit ions and results are summarized in Table 11. The resul ts from Table 10, Run #1 are also reproduced in Table 11 for comparative purposes. The use of coarser copper powder resulted in a sharply reduced iron ex t rac t ion , from 97% to 81%, after leaching at 140 ° C for 1 h. Zinc extraction was also decreased s l i g h t l y , from 55% to 48%. Thus, as might be expected, the use of f iner pa r t i c l e s ize copper improved react ion k i ne t i c s . 4.3 POSTULATED REACTION MECHANISMS: GALVANIC VERSUS CUPROUS-MEDIATED 4.3.1 Galvanic Mechanism Hiskey and Wadsworth postulated that reduction leaching of cha lcopyr i te with copper powder in acid so lu t ions , which produced cha lcoc i te and hydrogen sulphide gas, was contro l led by galvanic in teract ion of the chalcopyr i te mineral with copper 6 ^. Electrochemical experiments performed by Nicol later confirmed a galvanic mechanism 6^. Chalcopyr i te and copper can couple ga lvan ica l l y because of the excel lent e l ec - t ron ic conduct iv i ty of a l l phases. The pr inc ipa l cathodic and anodic ha l f -ce l l reactions a re 6 ^ : - 85 - TABLE 11 Effect of Copper Powder Pa r t i c l e Size on Reduction Leaching of Fox Lake Concentrate -400 mesh Cu° - 100 + 200 mesh Cu° Concentrate (g) 10.8 10.8 Copper Powder: Wt (g) 9.4 9.4 I n i t i a l Leachate: Vo 1 ume (mL) 50 50 Cu (g/L) 90 90 H 2SO^ (g/L) 20 20 (NH^)2SO^ (g/L) 132 132 Reaction Condi t ions ; Temp ( ° C ) : 140 140 Time (h) 1 1 Leach Residue: Weight (g) 19.7 20. 1 % Cu 80. 6 75. 1 % Fe 0.9 3.7 % Zn 0. 95 1.01 % S 19.6 20.3 F ina l Leachate: g/L Cu 22. 0 30. 8 g/L Fe 70.8 59.6 g/L Zn 4.55 3.75 Extract ion (%) Fe 97 81 Zn 55 48 - 86 - Cathod i c : 2CuFeS 2 + 6H+ + 2e" v ^Cu2S + 2 F e 2 + + 3H 2S (62a) Anod i c : 2Cu° + H 2 S ^Cu 2 S + 2H+ + 2e" (62b) which g i ve the o v e r a l l r e a c t i o n : CuFeS 2 + Cu° + 2H + >Cu 2S + F e 2 + + H 2 S . (63) F i g u r e 9 d e p i c t s s c h e m a t i c a l l y the g a l v a n i c CuFeS 2 -Cu c o u p l e . C h a l c o c i t e forms as a porous layer which surrounds FIGURE 9 Schematic R e p r e s e n t a t i o n of the C u F e S ? - C u ° G a l v a n i c Coup le in S u l p h u r i c A c i d S o l u t i o n - 87 - both the Cu and CuFeS2 p a r t i c l e s . H i skey and Wadsworth con - firmed the presence of a c h a l c o c i t e layer by d i r e c t o b s e r v a t i o n of p o l i s h e d sec t i on s of re s idue p a r t i c l e s under a m i c r o s c o p e . One can p o s t u l a t e that reduction leaching of cha lcopyr i te w i th copper in copper su lphate s o l u t i o n s might a l s o be con - t r o l l e d by a g a l v a n i c mechanism. In t h i s c a se , however, no cha l coc i te should grow on the m e t a l l i c copper p a r t i c l e s because su lphur is not t r a n s p o r t e d through the e l e c t r o l y t e . Such a m o d i f i e d g a l v a n i c model is d e p i c t e d s c h e m a t i c a l l y in F i g u r e 10. The anodic r e a c t i o n now is the d i s s o l u t i o n of copper 2 C u ° ^ ^ 2 C u 2 + + 4e" (64a) Fe porous FIGURE 10 Schematic R e p r e s e n t a t i o n of a P o s t u l a t e d CuFeS? -Cu 0 Couple in Copper Su lphate S o l u t i o n - 88 - while the cathodic react ion to form cha lcoc i te i s : CuFeS 2 + 3 C u 2 + + 4e" v N 2 C u 2 S + F e 2 + . (64b) More C u 2 + is consumed by the cathodic reaction than is produced by the anodic react ion , to give the overa l l reac t ion : CuFeS 2 + 2Cu° + C u 2 + > 2Cu2S + Fe 2 + . (64) The above galvanic mechanism could p a r t i a l l y account for the observed chemistry of reduction leaching in copper sulphate so lu t ion . However, under s to ich iometr ic condit ions galvanic reactions do not go to completion due to the lack of perfect mixing of so l id phases. Hiskey and Wadsworth found that iron extract ions from chalcopyr i te rarely exceeded 80% by their galvanic reac t ion . In add i t ion , they showed that the i n i t i a l presence of cupric ions severely inh ib i ted the react ion . McKay and Swinkels also observed incomplete iron extract ion when reduction leaching chalcopyr i te with copper in acid s o l u t i o n 6 2 . Thus it is unlikely that a galvanic mechanism can to t a l l y account for the observed chemistry of reduction leaching in copper sulphate so lu t ion . 4.3.2 Cuprous-Mediated Mechanism It is known that the cuprous ion is in sulphate solut ions by the equ i l ib r ium: si ight ly stabi1ized 89 - C u 2 + + Cu° 5= 2Cu + (65) The observed va lue of Kg^ in d i l u t e s o l u t i o n s at room temp- e r a t u r e is I O - 6 lyĵ  k u t t h i s va lue i nc reases w i th i n c r e a s i n g temperature, leading to much h igher cuprous ion c o n c e n t r a t i o n s . The measured va lue for Kg<j at 0.1 M C u 2 + c o n c e n t r a t i o n at 160 °C was found to be about 0 . 0 3 5 ' 7 5 . The cuprous ion cou ld mediate the r e a c t i o n between c h a l c o p y r i t e and copper , e l i m - i n a t i n g the need for g a l v a n i c c o n t e n t , i e : CuFeS 2 + 4 C u + >2Cu 2 S + C u 2 + + F e 2 + (66) Such a cuprous mediated mechanism can be represen ted s c h ema t i c a l l y as sh own in F i g u r e 11. If t h i s me c h a n i sm is FIGURE 11 Schematic R e p r e s e n t a t i o n of a Cuprou s-Med i a ted Reac t i on between C h a l c o p y r i t e and Copper - 90 - o p e r a t i n g at e l e v a t e d temperatures , the r e a c t i o n should be improved by a weak cuprous s t a b i l i z i n g agent such as carbon monoxide. Carbon monoxide under p res sure w i l l act as a comp- lexing agent to s t a b i l i z e the cuprous ion in su lphate s o l u t i o n s , i f C u ° is p r e s e n t , v i a the f o l l o w i n g e q u i l i b r i u m 7 6 : 2Cu° + 2 C u 2 + + 4CO(g) ^==& 4Cu(CO) + . (67) The cuprous-car bony 1 complex could then react w i th c h a l c o p y r i t e : CuFeS 2 + 4Cu(C0)+ ->2Cu 2 S + F e 2 + + C u 2 + + 4CO . (68) Under the modera te ly a c i d i c c o n d i t i o n s that the r e d u c t i o n leach is c a r r i e d ou t , carbon monoxide w i l l not act as a reduc ing a g e n t 7 7 and should be q u a n t i t a t i v e l y r egenera ted , as copper in s o l u t i o n is d e p l e t e d . 4.4 REDUCTION LEACHING WITH COPPER POWDER UNDER CARBON MONOXIDE 4.4.1 E f f e c t of Carbon Monoxide on Reac t i on K i n e t i c s To determine whether r e d u c t i o n l e a c h i n g w i th copper powder in copper su lphate s o l u t i o n s cou ld be e x p l a i n e d by a cuprous -med ia ted mechanism, Bethlehem concen t r a te was leached under both carbon monoxide and n i t r o g e n . The Bethlehem - 91 - c o n c e n t r a t e was assumed to c o n t a i n 59% CuFeS2 and 20% Cu^FeS^ as the main s u l p h i d e m i n e r a l s . The o v e r a l l r e a c t i o n s were assumed to be: CuFeS 2 + 2Cu° + C u 2 + — • • > 2Cu 2 S + F e 2 + (69) C u 5 F e S Z t + 2Cu° + C u 2 + > 4Cu 2 S + F e 2 + . (70) On t h i s b a s i s , the s t o i c h i o m e t r i c molar C u 2 + / F e r a t i o i s 1/1 and the C u ° / F e r a t i o is 2/1. S t o i c h i o m e t r i c amounts of c o n c e n t r a t e (391.6 g, -275 mesh) and coarse copper powder (180 g of 90% -100+325 mesh m a t e r i a l ) were leached in the Parr au toc l ave at 120 °C w i th a leach s o l u t i o n c o n t a i n i n g i n i t i a l l y 90 g/L C u 2 + , 20 g/L H 2SO^ and 132 g/L (NH i f ) 2 SO i f . A lower leach temperature and coar ser copper powder were used to d e l i b e r a t e l y slow down the r e a c t i o n k i n e t i c s so that e f f e c t i v e comparisons cou ld be made. In one t e s t , the leach was p r e s s u r i z e d w i th 1 MPa N 2 du r ing the 18 minute warm-up p e r i o d , the s o l u t i o n was sampled when the temperature reached 120 °C, whereupon the n i t r o g e n was vented and rep l a ced w i th 1 MPa carbon mon- o x i d e . In another t e s t , the r eac to r was kept p r e s s u r i z e d w i th 1 MPa N 2 throughout the l e a c h . T o t a l r e s i dence time for both runs was 6 h (not i n c l u d i n g the 18 minute warm-up per iod ) . F i g u r e 12 p o r t r a y s g r a p h i c a l l y the r e s u l t s of these runs in terms of percent Fe e x t r a c t e d versus r e s i dence t ime. I n i t i a l l y the i ron leach ra tes were s i m i l a r for both runs, - 92 - -i — r 1 T j - 2 3 4 5 6 Leach Time - hours FIGURE 12 Effect of CO on the Reduction Leaching of Bethlehem Concentrate with Copper at 120°C - 93 - but after 45 minutes reduction leaching under nitrogen slowed down dramatical ly to give 64% iron extract ion after 6.3 h. By contrast , reduction leaching under carbon monoxide continued at a reasonable rate to give a f i na l iron extract ion of 99%. These resul ts strongly suggest that the k inet ics of reduction leaching can be explained by a cuprous-mediated mechanism in which the rate of cuprous formation can be rate control l ing. In another leach tes t , 218.4 g of Fox Lake concentrate was leached with 187.6 g of copper powder, again at 120 ° C under 1 MPa CO. Leach condit ions were ident ica l to the above, with the exception that as-received concentrate (72.4% -400 mesh) and f ine copper powder (-400 mesh) were used. Figure 13 portrays graphica l l y the resul ts obtained, in terms of % Fe and % Zn extracted versus residence time. The Fox Lake concentrate was i n i t i a l l y even more react ive than the Beth- lehem concentrate, both before and after CO was added at the 18 minute mark. This can probably be a t t r ibuted to the increased rate of formation of cuprous ions, due to the use of copper powder of much f iner pa r t i c l e s i z e . It is also possible that mi nera logica l cha rac te r i s t i c s of the Fox Lake concentrate make i t more react ive to reduction leaching under these cond i t ions , although studies to date had not indicated d i f fe rent concentrates to vary substant ia l l y in r eac t i v i t y under s imi lar leach condi t ions . Iron and zinc extract ions reached 93% and 54%, respect ive ly after 6.3 h leaching. - 94 - FIGURE 13 Reduction Leaching of Fox Lake Concentrate with Copper at 1206C under 1MPa C6 - 95 - 4.4.2 Ef fect of Leach Temperature on Reaction K inet i cs To evaluate the temperature s ens i t i v i t y of reduction leaching under carbon monoxide, a series of experiments were performed on Bethlehem concentrate in which the leach temp- erature was varied between 100 ° C and 140 ° C . Tests were carr ied out in the shaking autoclave for residence times varying from 0.75 h to 9 h. In each tes t , 21.5 g of con- centrate, c a r e f u l l y s i zed to -140+270 mesh p a r t i c l e s i z e , was leached with 9 g of coarse copper powder (90% -100+325 mesh), using 50 mL of a leach solut ion containing i n i t i a l l y 90 g/L C u 2 + , 20 g/L H2SO^ and 132 g/L (NH^SO^. These con- d i t ions represented an approximate 10% excess of concentrate over ca lcu lated stoichiometry. After a leach warm-up period which varied from 3-12 minutes depending upon the reaction temperature to be maintained, the reactor was pressurized with 1 MPa CO gas. Figure 14 portrays graph ica l l y the resul ts in terms of both iron extract ions and iron remaining in leach residues. It can be seen that the react ion k inet i cs are very temperature sens i t i v e . An approximate estimation of the experimental a c t i v a t i o n energy can be made as fo l lows. It can be assumed that the rate law is uniform at d i f fe rent temperatures for equal percent conversions. Therefore an approximate activation energy can be extracted from the Figure 14 plot by assuming that - 96 - FIGURE 14 Effect of Temperature on Reduction Leaching of Bethlehem Concentrate with Copper under 1MPa CO - 97 - k 120 TTioo k100 Vl20 where kj20 a n d k 100 a r e t n e r e a c t i on rates at 120 ° C and 100 ° C respec t i ve ly ; and X\2Q a n d "^100 a r e t h e times required, at 120 °C and 100 ° C , to reach the same iron extraction value. Table 12 gives thetTioO a n c ' f l 2 0 v a l u e s * o r d i f fe rent percent iron ext rac t ions , from 35% to 65% , as determined from Figure 14. It can be seen that in this range,f100 /T i20 is v i r t u a l l y constant and has an average value of 4.14. The Arrhenius equation k = A exp (-Ea/RT) (71) k i on can be re-written in terms of , that i s : k 100 k120 A120 e x P (-Ea/RT 1 2 0) (72) k100 A100 e x P (-Ea/RT 1 0 0) Assuming A to be essen t i a l l y constant with temperature, and taking the natural log of both s ides , we get: In f k 120 N ' k100/ Ea Ea _ . (73) R T 1 0 0 R T 1 2 0 This equation can be expressed in terms of Ea as : I - 98 - TABLE 12 Residence Time Ratios at D i f ferent Iron Extract ions - From Figure 14 Leach Residence Time (h) Tioo k 1 20 Fe ext r . at 100 ° C at 120 ° C ' = (%) (fioo) ( f i2 0> f l 2 0 k 100 35 2.25 0. 55 4. 09 40 2.70 0. 65 4. 15 45 3. 20 0.75 4. 27 50 3. 80 0.90 4. 22 55 4.55 1.10 4. 14 60 5. 40 1.35 4. 00 65 6.60 1. 60 4. 13 Avg. = 4. 14 - 99 Ea ln 120 \ k 100 R T l 2 0 T100 T l 2 0 " T100 (74) Subst i tut ing numbers into this expression Ea = ln (4.14) (8.314 3K" 1 mo l " 1 ) (393°K) (373°K) (1000 3/k3) (393°K-373°K) (75) = 87 kJ/mol. This value is considerably higher than the ac t i va t ion energy for the galvanic conversion of chalcopyr i te as determined by Hiskey and Wadsworth 6*, which was 48 k3/mol. 4.4.3 Ef fect of I n i t i a l Concentrate Pa r t i c l e Size on Iron Extract ion To determine the ef fect of i n i t i a l concentrate particle s ize on the k ine t i cs of reduction leaching under CO, two experiments were performed on Bethlehem concentrate ca re fu l l y sized to -140+270 and -400 mesh. The experiments were performed on the shaking autoclave with a l l other reaction condit ions held constant. Reaction condit ions and resul ts are summarized in Table 13. A f t e r l each ing for 12 h at 100 ° C , the -140+270 mesh m a t e r i a l had reached 73% Fe e x t r a c t i o n , whereas the -400 mesh mater ia l had reached 84% Fe ext rac t ion . As might be expected, iron extract ions were improved by leaching con- - 100 - TABLE 13 Effect of I n i t i a l Concentrate Pa r t i c l e Size on the Reduction Leaching of Bethlehem Concentrate wi th Copper under 1 MPa CO -140+270 mesh size -400 mesh size Concentrate (g) 21. 5 21. 5 Copper Powder: Wt (g) Par t i c1e S i ze 9.0 90% -100+325 9.0 90% -100+325 I n i t i a l Leachate: Vo 1 ume (mL) Cu (g/L) H2SO^ (g/L) (NH^)2SO^ (g/L) 50 90 20 132 50 90 20 132 Reaction Condi t ions : Temp ( ° C ) Time (h) 100 12 100 12 Leach Residue: Weight (g) % Fe Pa r t i c l e Size n. d. 3.8 81% -270 27. 1 2.8 F ina l Leachate: g/L Fe 62. 8 73. 0 Fe Extract ion (%) 73 84 n.d. = not determined - 101 - c e n t r a t e of f i n e r p a r t i c l e s i z e . Leach re s i due from the o r i g i n a l -140+270 mesh m a t e r i a l underwent a sc reen a n a l y s i s which showed, s u r p r i s i n g l y , that 81% was now -270 mesh. Th i s indicates that s i gn i f i can t p a r t i c l e s i z e r e d u c t i o n had occu r red dur ing l e a c h i n g . Th i s phenomenon w i l l be d i s c u s s e d in g rea te r d e t a i l l a t e r . 4.5 COMPARISON OF REACTIVITY OF SEVERAL SULPHIDE MINERALS TO REDUCTION AND NEUTRAL LEACHING As was ment ioned in the S.C. copper process de sc r i p t i on , n e u t r a l or "McGau1ey-type" l e a c h i n g of a c t i v a t e d copper con - c e n t r a t e s i s used to i n c r e a s e the o v e r a l l i r on e x t r a c t i o n from 75% to about 91%. To eva lua te the f e a s i b i l t y of us ing r e d u c t i o n l e a c h i n g as an a l t e r n a t i v e to n e u t r a l l e a c h i n g , a ser ies of experiments was performed to determine the r e l a t i v e r e a c t i v i t i e s of s e v e r a l s u l ph ide m i n e r a l s commonly found in copper concen t r a te s to r e d u c t i o n l e a c h i n g and n e u t r a l l e a c h i n g . The m i n e r a l s used in this study were p y r i t e , born i te , c h a l c o p y r i t e , p e n t l a n d i t e ( ^ N i F e S ^ g ) , s p h a l e r i t e (ZnS) and p y r r h o t i t e (~FeS). The expected chemis t ry for r e d u c t i o n and n e u t r a l l e a c h i n g of these m i n e r a l s is compared in the equations equat ions shown in Tab le 14. Ground m i n e r a l samples were c a r e f u l l y screened to a narrow s i z e range of -270+325 mesh for use in leach t e s t s . A l l t e s t s were performed in the shaking au toc l ave us ing 50 mL of a leach s o l u t i o n c o n t a i n i n g 90 g/L Cu2+ } Mi neral Reduction Leaching Neutral Leaching FeS 2 CujjFeS^ CuFeS2 NiFeSi.g ZnS FeS FeS 2 + CuSO^ + 3Cu° > 2Cu2S + FeSO^ Cu5FeS^ + CuSO^ + 2Cu° »4Cu 2 S + FeSO/j CuFeS2 + Q1SO4 + 2Cu° »2Cu 2 S + FeSCty NiFeSi.8 + 2CuSO^ + 1.6 Cu° > 1.8 Cu 2S + FeSCty + MSO4 ZnS + CX1SO4 + Cu° » Cu2S + ZnSO^ FeS + CuSO^ + Cu° » Cu 2S + FeSO^ FeS 2 + CuSCv >CuS + S + FeSCty Cu 5FeSi f + CuSCty. >2CuS + 2Cu2S + FeSCty CuFeS2 + CuSCty >>2CuS + FeSCty NiFeSi.g + CuSCty >1.6 CuS + 0.2 Cu 2S + FeSCty + N1SO4 ZnS + CuSO^ > CuS + ZnSO^ FeS + CuSCty >CuS + FeSCty. TABLE 14 Reduction and Neutral Leaching Chemi Several Sulphide Minerals stry of - 103 - 20 g/L H2SO4 and 132 g/L (NH^) 2SO^. For the r e d u c t i o n leach runs, s t o i c h i o m e t r i c amounts of m i n e r a l s and coarse powder (90% -100+325 mesh), c a l c u l a t e d on the bas i s of the equat ions d e p i c t e d in Tab le 14, were used. For the n e u t r a l leach runs, s t o i c h i o m e t r i c amounts of m i n e r a l s were a l s o used. A l l runs were performed at 140 °C under 1 MPa n i t r o g e n for 1 h. The r e s u l t s of these runs are summarized in Tab le 15. P y r i t e , bornite and cha lcopyr i te are about e q u a l l y r e a c t i v e to r e d u c t i o n l e a c h i n g , w i th Fe e x t r a c t i o n s in the range 68-74% o b t a i n e d . By c o n t r a s t , a l l three m i n e r a l s are very much le s s r e a c t i v e to n e u t r a l l e a c h i n g . Pyr i te was e s s e n t i a l l y unat tacked w i th on ly 3% Fe e x t r a c t i o n o b t a i n e d , whereas c h a l - c o p y r i t e and b o r n i t e were on ly s l i g h t l y r e a c t i v e (8% and 19% Fe e x t r a c t i o n s , r e s p e c t i v e l y ) . For the other three m i n e r a l s , p e n t l a n d i t e , s p h a l e r i t e and p y r r h o t i t e , the d i f f e r e n c e s between r e d u c t i o n and n e u t r a l l e a c h i n g are less d r amat i c , a l though r e d u c t i o n l e a c h i n g d id e x t r a c t more i ron or z i n c than d id n e u t r a l l e a c h i n g . Th i s imp l i e s that the a c t u a l leach s t o i c h i o m e t r y as suggested in Tab le 14 is not c o r r e c t , w i th i r on be ing leached somewhat p r e f e r e n t i a l l y to n i c k e l . 4.6 THE MORPHOLOGY OF REDUCTION LEACHING Res idue p a r t i c l e s from both the r e d u c t i o n and n e u t r a l leach exper iments on b o r n i t e and c h a l c o p y r i t e were mounted FeS2 Cu^FeS^ CuFeS2 NiFeSi .8 ZnS FeS Red. Neut. Red. Neut. Red. Neut. Red. NeUt. Red. Neut Red. Neut. Mineral Weight (g) 9.07 9.07 27.93 27.93 9.60 9.60 6.20 6.20 9.17 9.17 6. 81 6.81 Copper Weight (g) 13.50 - 9.00 — 9.00 — 3.60 — 4.50 — 4. 95 — Residue: Wt. (g) 22.00 8.96 36.58 29.03 19.21 9.45 9.73 6.26 13.65 9.25 11. 99 6.98 % Cu 74.2 1.6 72.7 56.5 69.1 23.7 41.9 8.5 39.2 4.2 56 .8 5.4 % Fe 4.3 43.4 2.6 11.3 7.1 38.3 15.4 28.0 - - 22 .7 57.8 % Other _ _ _ - - - 19.5 Ni 30.2 Ni 29.4 Zn 44.2 Zn - - % S 18.5 53.6 21.4 26.5 18.7 36.1 . 20.8 32.2 23.7 34.4 21 .2 33.5 Final Leach Solution: g/L Cu 28.2 85.4 25.9 65.6 29.5 82.0 74.9 82.3 64.8 82.3 55 .0 82.4 g/L Fe 56.5 3.5 54.0 17.4 55.8 6.1 8.9 3.4 - - 27 .0 8.3 g/L Other - - - - - - 2.4 Ni 3.3 Ni 12.4 Zn 5.3 Zn — _ Fe Extr. (96) 74 3 72 19 68 8 23 10 33 11 "Other" Extr. (%) - - - - - - 7 M 9 Ni 13 Zn 9 Zn - - TABLE 15 Results of Reduction and Neutral Leach Runs on Several Sulphide Minerals - 105 - on epoxy, pol ished and viewed under a Zeiss opt ica l micro- scope. Figure 15 shows colour pictures depict ing cross- sect ional views of the conversion products from (a) the neutral leach and (b) the reduction leach. The d i f fe rent mineral phases are quite c l ea r , with par t i c l es of pyr i te (white), chalcopyrite (yellow), bornite (brown) and cha1cocite/cove11ite (blue/grey) being i d e n t i f i a b l e . Only bornite pa r t i c l es show s ign i f i can t conversion by neutral leaching, with chalcopyr i te and pyr i te remaining r e l a t i v e l y unattacked. By contrast , both chalcocopyr i te and pyr i te show s ign i f i can t conversion to cha lcoc i te by reduction leaching. A l l o r ig ina l bornite pa r t i c l es in Figure 15(b) have been completely converted to cha1coci te . The morphology of cha lcoc i te formation by reduction leaching is quite c l ea r l y evident as being an expansion of the product Cu 2S away from the o r ig ina l mineral due to rapid volume increases. The volume, per mole of sulphur, for the d i f fe rent mineral phases is given in Table 16. It is clear from this table that every mineral must expand if i ts sulphur content is quant i ta t i ve ly converted to cha l coc i t e . Mineral expansion is pa r t i cu l a r l y dramatic in the case of pyr i te con- v e r s i o n , as i t s molar volume inc rases from 11.95 to 27.92 cm3 /mo 1 . The conversion products were studied extensively under the e lectron microprobe for v isual and ana ly t i ca l evidence which would support a galvanic mechanism. However, copper par t i c l es were never observed to be coupled to any mineral phases, lending further support to a cuprous-mediated mechanism. - 1 0 6- K—50 um—* K—50 um—a (a) (b) Neu t ra l Leach Products Reduc t ion Leach Products Y e l l o w - c h a l c o p y r i t e Wh i te - p y r i t e Brown - born i te B l ue / g rey - c h a 1 c o c i t e / c o v e 11 i te (400x M a g n i f i c a t i o n ) FIGURE 15 O p t i c a l M ic roscope P i c t u r e s of Conver s i on Products from the Neut ra l and Reduct ion Leach Exper iments on Cha1copyr i te/Born i te - 107 - TABLE 16 Molar Volumes of Sulphide Phases Phase Nomi na 1 Formu 1 a Formu la Wt. , g/mol . Dens i t y * , g/cm3 Vol./Mole S, cm3 /mol Pyr i te FeS 2 119.98 5. 02 11.95 Bo r n i t e Cu -jFeS^ 501.80 5. 07 24. 74 Cha1 copyri te CuFeS 2 183.52 4.2 21.85 Pent 1 and i te N i F e S 1 < 8 172.27 4.8 19.94 Spha1er i te ZnS 97.44 4.0 24. 36 Pyr rhot i te F e 0 . 9 S 82. 33 4.6 17.90 Cha1coci te Cu 2S 159.14 5.7 27.92 Cove 11i te CuS 95. 60 4.7 20. 34 * Taken from: CRC Handbook of Chemistry and P h y s i c s , 54th ed. (1973-1974), pp. B192-B197. - 108 - 4.7 KI NET IC MODELS AND MECHANI SMS Postulated so l i d state and solut ion transport processes for cuprous-mediated reduction leaching of chalcopyr i te are i den t i f i ed in Figure 16, and summarized in Table 17. The mineral decomposition process takes place by (a) rearrangement of sulphur atoms to form the Cu 2S basic l a t t i c e , (b) diffusion of Cu + inward through this new l a t t i c e and (c) d i f f u s i on of F e 2 + outward through the Cu 2S phase. The d i f f us ion rate of Cu + in Cu 2S is r e l a t i v e l y f as t , and has been measured by E t i e n n e 7 8 to be 3.8 x 10~ 1 0 cm 2/sec at 75 °C , with a corres- ponding activation energy of about 49 kJ/mol. This is somewhat lower than the 87 kJ/mol ac t i va t ion energy estimated for the reduction leach in this study; therefore d i f f us ion of Cu + would not seem to be rate l i m i t i n g . In genera l , Cu 2 S is known to have a small Fe so l i d s o l u b i l i t y 7 3 ; therefore F e 2 + d i f f u s i o n should be much slower than that of C u + . However, as depicted in Figure 16, d i f fus ion paths for F e 2 + are probably shorter than for C u + because of spal l ing of the product layer. This would explain why essen t i a l l y complete removal of iron can be be achieved. The so l i d state process that forms Cu 2S and el iminates F e 2 + is d r i ven by the r e a c t i on with aqueous cuprous ions, that i s : CuFeS 2 + 4Cu+(aq) >2Cu2S + Cu 2 + (aq) + Fe 2 + ( aq ) . (76) The Cu + ions are formed by react ion of C u 2 + with Cu° or H 2 - 109 - FIGURE 16 M o r p h o l o g y of C h a l c o c i t e F o r m a t i o n by R e d u c t i o n L e a c h i n g - S o l i d S t a t e and S o l u t i o n T r a n s p o r t P r o c e s s e s - 1 1 0 - TABLE 17. C h e m i s t r y o f T r a n s p o r t P r o c e s s e s D e p i c t e d i n F i g u r e 16 O v e r a l l ( 1 7 - D C u F e S 2 + 4Cu + 2+ 2+ ->2Cu 2S + Fe + Cu C u F e S 2 - C u 2 S I n t e r f a c e Zone (17-2) 2S CuFeS. -> 2S C u 2 S (17-3) Cu CuFeS-. -*Cu C u 2 S (17-4) F e CuFeS. -*Fe C u 2 S C u 2 S - A q u e o u s I n t e r f a c e Zone (17-5) (17-6) (17-7) Fe C u 2 S 3 C u + ( a q ) C u + ( a q ) -»Fe 2 +(aq) + 2e" ( C u 2 S ) -»3Cu - 3e ( C u 2 S ) - > C u 2 + ( a q ) + e" ( C u 2 S ) A q u e o u s S o l u t i o n o r Cu I n t e r f a c e (17-8) C u 2 + ( a q ) + h H 2 ^ C u + ( a q ) + H + H y d r o g e n as r e - d u c t a n t . (17^9) C u 2 + ( a q ) + Cu° + -»2Cu (aq) M e t a l l i c c o p p e r as r e d u c t a n t - I l l - (equations 16-8 and 16-9). In view of the evidence that the meta l l i c copper pa r t i c l e s ize and the presence of a Cu + s t a b i l i z i n g agent af fect the k i ne t i c s , it is clear that under these circumstances the so l i d state processes mentioned above are not rate-determining, even though they must take place. Because a l l leach experiments had s ta r t ing condit ions close to stoichiometry, reaction k inet i cs were mixed in that the so l i d state d i f f us ion processes were dependent on, or "coupled" to, formation of C u + . reduction leaching was beyond the scope of this work. However, there is a temptation to apply parabol ic k inet i cs to the reduction leach (see Appendix 2 ) . This model is s u i t a b l e for the neutral leach where there is no evidence of product s p a l l i n g . However, the reduction leach k inet ics are probably c loser to being l i nea r , with a rate constant dependent on the C u + content of the so lu t ion . It was not possible to resolve the C u + dependence on the leaching rate because Cu + could not be d i r e c t l y measured. If it is f i r s t order, the react ion rate can probably be writ ten where u t is the l inear unreacted pa r t i c l e dimension. The integrated form of this equation would depend on (a) the pa r t i c l e s ize d i s t r i bu t i on and (b) the dependence of Cu + on time. Measurement of the la t ter function was not eas i l y access ib le within the range of this project . Rigorous evaluation of a k ine t i c rate equation for dt (77) CHAPTER 5 APPLICATIONS, CONCLUSIONS AND RECOMME NDATI ONS 5.1 APPLI CAT I ONS OF REDUCTI ON LEACH I NG 5.1.1 I n co rpo ra t i on o i Reduct ion Leach i n to S.C. Process w i th no Roa s t i n g Reduct ion l e a c h i n g , as a s i n g l e un i t o p e r a t i o n , cou ld in p r i n c i p l e be i n c o r p o r a t e d i n to the S.C. Process as an a l t e r n a t i v e to the thermal a c t i v a t i o n - a c i d 1 e a c h i n g - n e u t r a 1 l e a c h i n g s t e p s . One such h y p o t h e t i c a l f lowsheet for t r e a t i n g su l ph ide concen t r a te s in which r o a s t i n g is t o t a l l y e l i m i n a t e d is shown in F i g u r e 17, and the r e a c t i o n chemis t ry ( for c h a l - c o p y r i t e ) is summarized in Tab le 18. In t h i s h y p o t h e t i c a l p r o c e s s , copper c o n c e n t r a t e i s i n i t i a l l y r e d u c t i o n leached w i th a s o l u t i o n c o n t a i n i n g 80 g/L C u 2 + , 20 g/L H 2SO^ and 132 g/L (NH^) 2SO^, for 1J4 h at 140 °C , to o b t a i n 95% or g rea te r i r on e x t r a c t i o n . For c h a l - c o p y r i t e c o n c e n t r a t e s , the copper reagent burden is l a r g e - about two tonnes copper powder and one tonne copper as copper su lpha te leach s o l u t i o n is r e q u i r e d for each tonne of copper in the c o n c e n t r a t e (equat ion 18-1) . Any p y r i t e present w i l l a lso be leached, adding to the reagent requ i rement s . However, p y r i t e decompos i t ion at t h i s stage i s p r e f e r r e d i f i t i s associated with any gold and s i l v e r , as is f r e q u e n t l y the ca se , because p rec i ou s meta l s locked into pyr i te are usual ly d i f f i c u l t to recover . - 113 - FIGURE 17 Schematic Flowsheet of Reduction Leach Process with no Roasting Powdered Scrap &_ Cement Copper New Sulphuric Acid Copper Concentrates Reduction Leach Recycle Sulphides FeS04 Solution _Weak Acid Oxidizing B Leach . Recycle Pregnant Solution 1^ Recycle Copper Powder Residues Pregnant Solution Purification Purified Solution L*_ Return. Acid Recycle Copper ' Powder Hydrogen D Reduction L / S Split 0 2 NH 3 Oxydrolysis E H L Flotation Flotation Concentrate Sulphide Residues to P.M. Recovery Jarosite Residue Silicious Tails • 1 Sulphur Recovery Recycle Sulphides Split to Oxidizing Leach B Elemental Sulphur Melting and Casting Wire Bars to Market - 114 - TABLE 18 Chemis t ry of Reduct ion Leach Process w i th no Roa s t i n g A. Reduct ion Leach (18-1) CuFeS 2 + 2Cu° + CuSOzj > 2Cu 2 S + FeSO^ B. O x i d a t i o n leach (18-2) 2Cu 2 S + 4H 2SO^ + 2 0 2 ^ 4CuSO^ + 2S° + 4H 2 0 C. Pur i f i cat i on (18-3) H 2 S e 0 3 + 4Cu° + 2H 2SO^ > Cu 2 Se + 2CuSO^ + 3H 2 0 (18-4) H 2 SeO / f + 5Cu° + 3H 2SO^ >Cu 2 Se + 3CuSO i f '+ 4H 2 0 D. Hydrogen Reduc t ion (18-5) 3CuSO / f + 3H 2 >3Cu<> + 3H 2SO^ E. Oxyd r o l y s i s (18-6) FeSO^ + 1/3 NH 3 + 1/4 0 2 + 3/2 H 2 0 > 1/3 N H 4 F e 3 ( S O ^ ) 2 ( O H ) 6 + 1/3 H 2SO^ - 115 - The copper reagent burden would be much lower for bornite concentrates, because the copper requirement for bornite is only 0.4 tonnes metal and 0.2 tonnes copper as copper sulphate, for each tonne of copper contained as bornite (refer back to equat ion 70). Sol ids from the reduction leach contain less than 1% Fe and are subjected to acid oxygen pressure leaching under condit ions s imi lar to the S.C. Process ox id iz ing leach (equation 18-2). Acid feed to the ox id iz ing leach comes from recycled acid from the hydrogen reduction and oxydrolysis steps, and new acid (added as 96-100% H2SO4) which is required to replace the sulphate lost by ja ros i te p r e c i p i t a t i on . The leach so lut ion w i l l contain 0.8-1 M (NH^)2SO^ which is recycled from hydrogen reduction and oxydro lys is . The pregnant so lut ion from the ox id iz ing leach is s p l i t , with one quarter of the dissolved copper recycled to the reduction leach and the remaining three quarters directed to a pu r i f i c a t i on step. The solut ion may contain impurit ies such as Se, Te, As, Sb and B i . Of these, Se and Te wi l l be particularly troublesome as they w i l l p rec ip i ta te quant i ta t i ve ly with copper during hydrogen reduct ion, and so must be removed. The suggested pu r i f i c a t i on method is to prec ip i ta te these elements with copper powder destined for recyc l ing to the reduction leach,N just as proposed by Kennecott Copper for their n i t r i c sulphuric leach process^ 2 (equations 18-3 and 18-4). The pur i f i ed pregnant leach solut ion is subjected to hydrogen reduction (equation 18-5) to bring the copper - 116 - level in so lut ion from 80 g/L down to about 20 g/L. The buffer ing ef fect of 1 M (NH^)2SO^ wi 11 ensure that the reduction proceeds rapidly at 160-180 ° C 7 ^ . Of the precipitated copper, two thirds is recycled back to the reduction leach after being used for p u r i f i c a t i o n , and one th i rd is melted and cast into wire bars for sa le . Hydrogen reduction rather than e1ectrowinning of fers substant ia l l y reduced energy costs. Solution from the reduction leach is low in copper ( less than 3 g/L Cu) and high in iron (70-90 g/L Fe) , and undergoes oxydrolysis to prec ip i ta te iron as a clean ammonium ja ros i te (equation 18-6), su i table for d i sposa l . This step is e ssen t i a l l y the same as the iron removal step in the S.C. Process, except iron p rec ip i t a t i on should be more rapid and complete due to the buffer ing ef fect of ammonium sulphate. Jaros i te p rec ip i t a t i on w i l l remove about one th i rd of the o r ig ina l sulphide sulphur in the concentrate as sulphate. Residue from the ox id iz ing leach w i l l contain a l l precious metals, elemental sulphur, gangue and some unleached sulphide minera ls . This residue can be treated as in the S.C. Process, by (a) f l o t a t i on to reject gangue; (b) elemental sulphur removal by stream stripping or dissolution in a suitable organic solvent; and (c) s p l i t t i n g into a precious metals con- centrate for shipment, and a recycle sulphide concentrate to be returned to the ox id iz ing leach for enhanced overa l l copper ex t rac t ion . This proposed modi f i ca t ion to the S.C. Process has the advantage of se l ec t i ve l y extract ing iron from copper in one unit operation and a residence time of Vh h, versus - 117 - the three unit operations and a tota l residence time of over 10 h required for the S.C. Process. The process is en t i r e l y hydrometa 11urgica1, provides for high recovery of copper and elemental sulphur, and rejects iron as a r e l a t i v e l y pure ja ros i te compound. Gold and s i l ve r are recovered from the process as a precious metals concentrate which can be sold to a re f inery or subjected to a special hydrometal 1 urgical t reatment. However, this process suffers from a large c i r cu l a t i ng load of copper. The consequence of this c i r cu l a t i ng load is that for chalcopyr i te concentrates, 4 tonnes of copper are leached, and 3 tonnes of copper are hydrogen reduced, for every tonne of copper recovered for sa le . As a result the leaching plant is 4 times as large and the hydrogen re - duction plant 3 times as large as i f there were no circulat ing loads of copper sulphate or copper metal . 5.1.2 Incorporation of Reduction Leach into S.C. Process with Par t i a l Roasting One method of reducing the c i r cu l a t i ng load of copper in a reduction leach process is to el iminate part of the feed sulphur by roast ing to produce copper f e r r i t e CuFeS 2 + 13/4 0 2 >CuFe0 2 > 5 + 2S0 2 (78) followed by reduction leaching of a sulphide/calc ine mixture, using varying mixtures of hydrogen and copper metal as reduct- - 118 - ants. Experimental work conducted in our laborator ies has proven that iron can be extracted from s u 1 ph i de/ca 1 c i ne mixtures by reduction leaching, although not as eas i l y as from pure s u l p h i d e s 7 9 . A flowsheet for such a process is shown in Figure 18. The reduction leach has the fol lowing s t o i ch i omet r y : CuFeS 2 +°<.CuFe02 <5 + ̂ CuSO^ + VCu° + <SH2SO^ + JOH2 >2Cu2S + (ot+DFeSO^ + 2.5o<H20. (79) Here oM-/J+y= 3; ^+£=e/.+ l ; $+f> = 2.5«K. The l im i t ing condit ion for maximum ca lc ine u t i l i z a t i o n is for the case where = 3, ($ = 0, y= 0, S = 4 and f> = 3.5. In this case, 75% of the chalcopyr i te is roasted, which resul ts in a zero c i r cu l a t i ng load of copper in the process. An intermediate stoichiometry occurs when c< = 1 (50% of chalcopyr i te roasted). In this case ft = 0 to 2, v = 2 to 0, 8= 2 to 0 and f> = 0.5 to 2.5. The c i r cu l a t i ng load of copper is now about 1 tonne copper per tonne copper in the feed. Thus, by p a r t i a l l y e l iminat ing sulphur in a roaster , the amount of copper required to convert the remaining sulphur to cha lcoc i te by reduction leaching is substant ia l l y reduced. As a consequence, the sizes of the subsequent ox id iz ing leach and hydrogen reduction steps are reduced, resulting in improved process economics. The major drawback of this par t i a l c a l - c inat ion option is that the process is less acceptable environ- me n t a 11y. - 1 1 9 - FIGURE 18 Schematic Flowsheet of Reduction Leach Process with Partial Roasting Powdered Scrap & Cement Copper New Acid • H 2 J_ Copper Concentrates I Calcines Split Reduction Leach New Acid Acid Recycle Sulphides Recycle Pregnant Solution Split Purification L, Recycle _ Copper Powder Return_ ' Acid I^Copper Powder Split Roaster (fluid bed) * FeSOd „ Air 1 Solution °2 » Acid Leach B (Fe Scavenger) NH 3 _L_ Oxydrolysis F I S j - Weak Acid Oxidizing C Leach Residues Flotation Sulphur Recovery Hydrogen E Reduction Sulphides Recycle to Oxidizing Leach Split Melting & Casting Sulphide Residues to P.M. Recovery S02-containing gases to Acid Plant. Jarosite Residues Silicious Tails Elemental Sulphur Wire Bars To Market - 120 - 5.2 CONCLUSIONS This study has been concerned with developing novel reduction leach methods for chalcopyr i te concentrates. It has been found that chalcopyr i te can be essent i a l l y completely converted to cha lcoc i te by leaching in strong copper sulphate solut ions at elevated temperatures, using hydrogen gas or copper metals as reductants. The reactions are sens i t i ve to temperature, concentrate and copper pa r t i c l e s i z e , and to the presence of a cuprous-stabi1iz ing agent. The essent ia l leach reactions appear to obey the fol lowing overa l l stoich- i ome t ry : The sulphide minerals bornite and py r i t e , commonly found in copper concentrates, are also quant i ta t i ve ly converted to cha lcoc i te by reduction leaching under these condi t ions . Microscopic evidence indicates that cha lcoc i te forms as layers which crack and spa l l away from the reacting s u l - phides, al lowing complete extract ion of iron to take place. The mechanism for reaction probably involves transport of cuprous ions both in aqueous solut ion and in the so l i d state (Cu 2 S), ar>d so l id state d i f f us ion of ferrous ions outward. Cuprous ions are formed as an intermediate species during the leach react ions , either by reaction of cupric ions with hydrogen or with copper meta l : CuFeS 2 + 3Cu 2 + + 2H 2 - CuFeS 2 + Cu 2+ + 2Cu° >2Cu 2S + F e 2 + + 4H+ >2Cu2S + F e 2 + . (80) (81) - 121 - 2Cu 2 + + H 2 >2Cu+ + 2H+ (82) C u 2 + + Cu° >2Cu + . (83) This species then reacts with the sulphide mineral as fo l lows, using chalcopyr i te as an example: CuFeS 2 + 4Cu+ >2Cu2S + F e 2 + + C u 2 + (84) Reduction leaching is po tent i a l l y an e f fec t i ve method of separating iron from copper in a hydrometal 1 ur gi cal process. Various p rac t i ca l appl icat ions of reduction leaching have been proposed. In pa r t i cu l a r , reduction leaching as a s ingle unit operation of fers an a t t rac t i ve a l te rnat i ve to the thermal ac t iva t ion-ac id 1eaching-neutral leaching steps in the S.C. Copper Process. 5.3 RECOMMENDATIONS FOR FURTHER WORK Because this study has focussed on prac t i ca l a p p l i - ca t ions , the react ion mechanisms of reduction leaching have not been f u l l y e luc idated . To gain a better understanding of the mechani sms involved, future research should be directed towards examining the so l i d state d i f fus iona l processes ocur- r ing . This could be accomplished by the fol lowing types of exper iments: a) Massive pol ished sulphide mineral specimens should be reduction leached, then sectioned and subjected - 122 - to a d e t a i l e d m i c r o s c o p i c a n a l y s i s of s t r u c t u r a l changes and compos i t i on of the c o n v e r s i o n product 1ayer s . A combinat ion of X - r ay d i f f r a c t i o n , e lec t ron micro- probe and energy d i s p e r s i v e a n a l y s i s of the con - v e r s i o n products should p rov ide enough i n f o r m a t i o n to determine whether the products are c h a l c o c i t e or some other copper su l ph ide such as d j u r l e i t e or d i g e n i t e . In a d d i t i o n , the d e t e c t i o n of i ron g r a d i e n t s in product l ayer s would c o n f i r m the presence of s o l i d s t a te d i f f u s i o n a l p roce s s . Measurement of a c t i v a t i o n energ ie s and other r a te dependencies dur ing r e d u c t i o n l e a c h i n g should be measured under c o n d i t i o n s of e s s e n t i a l l y s t a b l e s o l u t i o n c o m p o s i t i o n , which can be accompl i shed by per forming runs at low pulp d e n s i t i e s and w i th l a rge excesses of Cu 2 + a n d C u ° , r e l a t i v e to minera l . Under these c o n d i t i o n s , s o l i d s t a t e d i f f u s i o n would be r a t e - d e t e r m i n i n g ra ther than fo rmat ion of cuprous ions at the copper s u r f a c e . REFERENCES 1. B iswas, A.K. and Davenport , W.G. E x t r a c t i v e M e t a l l u r g y of Copper . Pergarrmon Press ( 1976TJ 438 pp. 2. Rov i g , D.A. and Doran, R.K. Copper - a decade of change and i t s meaning for the future. Min. Cong. Jour. , December (1980), pp. 32-40. 3. From: The p o t e n t i a l economic impact of U.S. r e g u l a t i o n s on the U.S. copper i n d u s t r y . Industry and Trade Admin- i s t r a t i o n , Department of Commerce, A p r i l 1979. 4. Anon. Engng. and M in . J o u r . , June (1975), pp. 104-105. 5. M in ing Annual Review. M in . Jou r . , June ( 1978), p. 358. 6. S chwe i t ze r , F.W. and L i v i n g s t o n , R.W. D u v a l ' s CLEAR 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 . C h l o r i d e E l e c t r o m e t a l l u r g y , Proceedings of a Symposium sponsored by the SME of AIME, he ld at the 111th AIME Annual Mee t ing , D a l l a s , Texas , February 15-16, 1982, 233 pp. 7. Agarwal , J . C . The f u t u r e of hydrometal1urgy in the copper i n d u s t r y . Paper p resented at the CIM Annual Mee t i n g , T o r o n t o , A p r i l 23, 1980. 8. Jones , D.L. and P e t e r s , E. The l e a c h i n g of c h a l c o p y r i t e w i th f e r r i c su lpha te and f e r r i c ch l o r i de . Ex t rac t i ve Met- a l l u r g y of Copper, Volume 2, J . C . Yannopoulos and J . C . Agarwa l , E d s . , AIME, N.Y., 1976, pp. 633-653. 9. D u t r i z a c , J . E . The d i s s o l u t i o n of c h a l c o p y r i t e in f e r r i c su lpha te and f e r r i c c h l o r i d e med ia . Meta l 1. T ran s . B, v o l . 12B, June (1981), pp. 371-378. 10. Palmer, B.R., Nebo, C O . , Rau, M.F. and F u e r s t e n a u , M.C. ' Rate phenomena i nvo l ved in the d i s s o l u t i o n of c h a l c o p y r i t e in c h l o r i d e - b e a r i n g l i x i v i a n t s . Meta 11. T ran s . B, v o l . 12B, September (1981), pp. 595-601. 11. W i l s o n , J . P . and F i s h e r , W.W. C u p r i c c h l o r i d e l e a c h i n g of c h a l c o p y r i t e . J . M e t a l s , February (1981), pp. 52-57. 12. Haver, F .P . and Wong, M.M. Recover ing e lementa l s u l f u r f r o m n o n f e r r o u s m i n e r a l s - f e r r i c c h l o r i d e l e a c h i n g of c h a l c o p y r i t e c o n c e n t r a t e . U.S.B.M. Report of Inves t - i g a t i o n s 7474, 1971. - 124 - 1 3 . H a v e r , F . P . a n d W o n g , M . M . R e c o v e r y o f c o p p e r , i r o n a n d s u l p h u r f r o m c h a l c o p y r i t e c o n c e n t r a t e u s i n g a f e r r i c c h l o r i d e l e a c h . J . M e t a 1 s , F e b r u a r y ( 1 9 7 1 ) , p p . 2 5 - 2 9 . 1 4 . H a v e r , F . P . , B a k e r , R . D . a n d W o n g , M . M . I m p r o v e m e n t s i n f e r r i c c h l o r i d e 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 . U . S . B . M . R e p o r t o f I n v e s t i g a t i o n s 8 0 0 7 , 1 9 7 5 . 1 5 . P h i l l i p s , T . A . E c o n o m i c e v a l u a t i o n o f a p r o c e s s f o r f e r r i c c h l o r i d e 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 . U . S . B . M . I n f o r m a t i o n C i r c u l a r 8 6 9 9 , 1 9 7 6 . 1 6 . A t w o o d , G . E . a n d C u r t i s , C . H . 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 f o r t h e p r o d u c t i o n o f c o p p e r . U . S . P a t e n t 3 , 7 8 5 , 9 4 4 , J a n u a r y 1 5 , 1 9 7 4 ; a n d U . S . P a t e n t 3 , 8 7 9 , 2 7 2 , A p r i l 2 2 , 1 9 7 5 . 1 7 . K r u e s i , P . R . P r o c e s s f o r t h e r e c o v e r y o f m e t a 1 s f r o m s u l f i d e o r e s t h r o u g h e l e c t r o l y t i c d i s s o c i a t i o n o f t h e s u l f i d e s . U . S . P a t e n t 3 , 6 7 3 , 0 6 1 , J u n e 2 7 , 1 9 7 2 . 1 8 . K r u e s i , P a u l R . C y m e t c o p p e r r e d u c t i o n p r o c e s s . M i n . C o n g . j o u r . , S e p t e m b e r ( 1 9 7 4 ) , p p . 2 2 - 2 3 . 1 9 . K r u e s i , P . R . , A l l e n , E . S . , a n d L a k e , J . L . C y m e t p r o c e s s - h y d r o m e t a 1 1 u r g i c a 1 c o n v e r s i o n o f b a s e - m e t a l s u l p h i d e s t o p u r e m e t a l s . C I M B u i 1 . , J u n e ( 1 9 7 3 ) , p . 8 1 - 8 7 . 2 0 . M c N a m a r a , J . H . , A h r e n s , W . A . a n d F r a n e k , J . B . A 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 f o r t h e e x t r a c t i o n o f c o p p e r . P a p e r p r e s e n t e d a t t h e 1 0 7 t h A I M E M e e t i n g , F e b r u a r y 2 6 , 1 9 7 8 , D e n v e r , C o l o r a d o . 2 1 . M c N a m a r a , J . H . , A h r e n s , W . A . a n d F r a n e k , J . H . A 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 f o r t h e e x t r a c t i o n o f c o p p e r . C I M B u i 1 . , M a r c h ( 1 9 8 0 ) , p p . 2 0 1 - 2 0 4 . 2 2 . D e m a r t h e , J . M . , G a n d o n , L . a n d G e o r g e a u x , A . A n e w h y d r o m e t a 1 1 u r g i c a 1 p r o c e s s f o r c o p p e r , E x t r a c t i v e M e t a l l u r g y o f C o p p e r , V o l u m e 2 , J . C . Y a n n o p o u l o s a n d J . C . A g a r w o l , E d s . , A I M E , N . Y . , 1 9 7 6 , p p . 8 2 5 - 8 4 8 . 2 3 . M i l n e r , E . F . G . , P e t e r s , E . , V i z s o l y i , A . a n d S w i n k e l s , G . M . C a n a d i a n P a t e n t 9 3 8 , 7 9 3 , D e c e m b e r 2 5 , 1 9 7 3 ; a n d U . S . P a t e n t 3 , 7 9 8 , 0 2 6 , M a r c h 9 , 1 9 7 4 . 2 4 . P e t e r s , E . , S w i n k e l s , G . M . a n d V i z s o l y i , A . C o p p e r r e c o v e r y f r o m s u l p h i d e c o n c e n t r a t e s b y t h e U . B . C . C o m i n c o f e r r i c c h l o r i d e l e a c h r o u t e . P r o c e s s a n d F u n d a m e n t a l C o n s i d e r a t i o n s o f S e l e c t e d H y d r o m e t a l l u r g i c a l S y s t e m s , M . C . K u h n , E d . , S M E o f A I M E , N . Y . , 1 9 8 1 , p p . 7 1 - 8 1 . - 125 - 2 5 . Tompk ins , J . Un ique new copper p r o c e s s i s ready f o r u n v e i l i n g . M i n i n g R e v i e w , M a r c h / A p r i l ( 1981 ) , pp. 29-31. 26 . E v e r e t t , P .K. The Dex t e c copper p r o c e s s . Ext r ac t i on M e t a l l u r g y ' 8 1 , I .M .M. Symposium ( 1 9 8 1 ) , pp . 149-156. 2 7 . F o r w a r d , F .A . and M a c k i w , V . N . C h e m i s t r y of the anrmonia p r e s s u r e p r o c e s s f o r l e a c h i n g Ni , Cu and Co from S h e r r i t t Gordon s u l p h i d e c o n c e n t r a t e s . J . Meta 1s , March ( 1955 ) , p p . 4 5 7 - 4 6 3 . 2 8 . Kuhn , M . C . , A r b i t e r , N. and K l i n g , H. Anaconda ' s A r b i t e r p r o c e s s f o r c o p p e r . C IM B u 1 1 . , F e b r u a r y ( 1 9 7 4 ) , pp . 62- 73 . 2 9 . Kuhn , M .C . and A r b i t e r , N. Recove r y of m e t a l s . U .S . P a t en t 4 , 0 2 2 , 8 6 6 , May 10, 1977. 30 . E vans , D . 3 . L . et a l . Treatment of copper-z inc concentrates by p r e s s u r e h y d r o m e t a l 1 u r g y . CIM Bu i 1 . , 628 ( 1964 ) , pp . 857-866 . 3 1 . S t a n c z y k , M .H . and Rampacek, C . O x i d a t i o n l e a c h i n g o f c o p p e r s u l p h i d e s i n a m m o n i a c a l p u l p s a t e l e v a t e d t empe ra tu r e s and p r e s s u r e s . U . S . B . M . Repor t of I n v e s t - i g a t i o n s 6808 , 1966. 32 . Tozawa, K . , Umetsu , Y . and S a t o , K. On c h e m i s t r y of anrmonia l e a c h i n g of copper c o n c e n t r a t e s . Ext r ac t i ve M e t a l l u r g y of C o p p e r , Volume 2, 3 .C . Yannopou los and 3 .C . A g a r w a l , E d s . , A IME, N . Y . , 1976, pp . 706-721 . 33 . B e c k s t e a d , L.W. and M i l l e r , 3 .D . Ammonia o x i d a t i o n l e a c h i n g of c h a l c o p y r i t e . M e t a 1 1 . T r a n s . B. , v o l . 7B, pp . 19-29 . 34 . B e c k s t e a d , L.W. and M i l l e r , J . D . Ammonia o x i d a t i o n l e a c h i n g of cha1 c o p y r i t e - s u r f a c e depos i t e f f e c t s . Meta l 1. T r a n s . B. , v o l . 8B, pp . 31-38 . 35 . W i l l i a m s , R .D . and L i g h t , S .D . Copper c o n c e n t r a t e d i s s o l u t i o n c h e m i s t r y and k i n e t i c s and an arrrnon i a-oxygen e n v i r o n m e n t . Fundamenta l A s p e c t s of Hydrometa 11u rg i c a1 P r o c e s s e s . , A . I . C h . E . Symposium S e r i e s , v o l . 74 , N . Y . , 1978, pp . 21-27 . 36 . P i t t , C H . and Wadswor th , M.E . An assessment of energy r e q u i r e m e n t s i n proven and new copper p r o c e s s e s . R e p o r t p r epa r ed f o r : U .S . Department of E n e r g y , D i v i s i o n of I n d u s t r i a l Energy C o n s e r v a t i o n , C o n t r a c t No. EM-78-S-07- 1743, December 3 1 , 1980, 362 pp . - 126 - 37. A rb i t e r , N. and M i l l i g a n , D.A. Reduction of copper anrmine solut ions to metal with sulfur d ioxide. Ext ract i ve Metal lurgy of Copper, Volume 2, 3.C. Yannopoulos and 3.C. Agarwal, Eds . , AIME, N.Y., 1976, pp. 974-993. 38. Prater, 3.D., Queneau, P.B. and Hudson, T . J . N i t r i c acid route to processing copper concentrates. Trans. SME- AIME, 3une (1973), pp. 117-122. 39. Habashi, F. Act ion of n i t r i c acid on cha lcopyr i te . Trans. SME-AIME, September (1973), pp. 224-231. 40. B jo r l i ng , G . , Fa ld t , I., L indgren, E. and Toromanov, I. A n i t r i c acid route in combination with solvent extract ion for hydr ometal lugical treatment of chalcopyrite. Extract ive Metal lurgy of Copper, Volume 2, 3.C. Yannopoulos and 3.C. Agarwol, Eds . , AIME, N.Y., 1976, pp. 725-737. 41. Brennecke, H.M. Recovery of Metal values from ore con- centrates. U.S. Patent 3,888,748, 3une 10, 1975. 42. Brennecke, H.M. et a l . N i t r i c-su1fur i c leach process for recovery of copper from concentrate. Min. Engng., August (1981), pp. 1259-1266. 43. Davies, D.S. et al . Ni tr ic-sul fur ic leach process improve- ments. Min. Engng., August (1981), pp. 1252- 1259. 44. Prater , 3.D., Queneau, P.B. and Hudson, T.3. The sulfation of copper-iron sulphides with concentrated sulfur ic acid. 3• Meta 1s, December ( 1970), pp. 23-27. 45. Subramanian, K.N. and Jennings, P.H. Review of the hydrometal lurgy of chalcopyrite concentrates. Can. Metal 1. Quar t . , 11, 2 (1972), pp. 387-400. 46. V i zso l y i et a l . Copper and elemental sulphur from cha l - copyr i te by pressure leaching. 3. Meta 1s, November (1967), pp. 52-59. 47. Baur , 3.P. Gibbs, H.L. and Wadsworth, M.E. Initial-stage su l f u r i c acid leaching k inet i cs of chalcopyr i te using radiochemical techniques. U.S.B.M. Report of Invest i - gations 7823, 1974. 48. Beckstead, L.W. et a l . Ac id i c f e r r i c sulphate leaching of at tr i tor-ground chalcopyr i te concentrates. Extract ive Metal lurgy of Copper, Volume 2, J . C . Yannopoulos and 3.C. Agarwal, Eds . , AIME, N.Y., 1976, pp. 611-632. - 127 - 49. Letowski, F. et a l . A new hydrometa 11urgica1 method for the processing of copper concentrates using f e r r i c sulphate. Hydromet. 4 (1979), pp. 169-184. 50. Malouf, E.E. and Prater, J .D . Role of bacter ia in the a l t e ra t ion of su l f ide minera ls . J . Meta 1s, 13, May (1961), pp. 353-356. 51. Malouf, E.E. The role of micro-organisms in chemical mining. Mi n. Engng., Nov. (1977), pp. 43-46. 52. Duncan, D.W. , Landesman, J . and Walden, C C . Role of Th i obac i11 us fer rooxi dans in the oxidat ion of su l f ide minera ls . Can. Jour. M i c r o b i o l . , 13 (1967), pp. 397-403. 53. Beck, J .V . and Brown, D.G. Direct su l f ide oxidat ion in the s o l u b i l i z a t i o n of su l f ide ores by Thiobaci1lus fer rooxidans. J . Bacter i o 1 . , 96 (4), October (1968), pp. 1433-1434. 54. Duncan, D.W. and Walden, C C Mic rob io log ica l leaching in the presence of f e r r i c iron. Developments in industrial microbio logy, v o l . 13 (1972), pp. 66-74. 55. Sakaguchi, H. , Torma, A .E . and Si lver, M. Microbiological oxidat ion of synthet ic cha lcoc i te and c o v e l l i t e by Xtli°ka.£iiIu.l_l.e.LL0.02Sic'a.n.s.* Appl . Environ. M i c r o b i o l . , M (1), January (19677, pp. 7-10. 56. Torma, A . E . e t . a l . M ic rob io log ica l leaching of a cha l - copyr i te concentrate and recovery of copper by solvent extract ion and e1ectrowinning. Meta 11. , 33, May (1979), pp. 479-484. 57. McElroy, R.O. and Bruynesteyn, A. Continuous b io log i ca l leaching of chalcopyr i te concentrates: demonstration and economic ana lys i s . Meta l lu rg ica l appl icat ions of bac ter ia l leaching and re lated microb io log ica l phenomena. Murr, L .E . , Torma, A . E . and Br ier ley , J .A . , Eds. Academic Press Publ ishers , New York, 1978, pp. 441-462. 58. Lawrence, R.W., Bruynesteyn, A. and Hackl , R.P. Recent developments in the bioleaching of su l f ide concentrates. Paper presented at the 28th Congress of the I . U . P . A . C , Vancouver, B.C. , August 16-22, 1981. 59. Peters, E. The physical chemistry of hydrometal1urgy. International Symposium on Hydrometal1urgy, Chicago, I l l i n o i s , February 25-March 1, 1973, pp. 205-228. Evans, D.J.I, and Shoemaker, R.S., Eds. AIME, New York, New York. - 128 - 60. McGauley, P.3. , Cove, G. and Roberts, E.S. Process for the recovery of copper from i ts ores and minera ls . U.S. Patent 2,568,963, September 25, 1951. 61. Hiskey, 3.B. and Wadsworth, M.E. Galvanic conversion of cha lcopyr i te . Metal 1. Trans. B., vo l . 6B ( 1975), pp. 183-190. 62. McKay, D.R. and Swinkels, G.M. Hydrometa 11urgica1 process for t reat ing copper-iron sulphides. Canadian Patent 953,925, September 3, 1974; and U.S. Patent 3,891,522, 3une 24, 1975. 63. Sh i r t s , M.B., Winter, 3.K., Bloom, P.A. and Potter , G.M. Aqueous reduction of chalcopyr i te concentrate with meta 1s. U.S.B.M. Report of Investigations 7953, 1974. 64. Sh i r t s , M.B. and Staker , L. Decomposition of chalcoprite. U.S. Patent 3,985,555, October 12, 1976. 65. N i co l , M.3. Mechanism of aqueous reduction of chalcopyrite by copper, iron and lead. I .M.M. Trans. C, v o l . 84 (1975) , pp. C206-C209. 66. Sohn, H.3. and Wadsworth, M.E. Reduction of chalcopyrite wi th S0 2 in the presence of c u p r i c i ons . 3. Me t a 1s, November (1980), pp. 18-22. 67. B ieg le r , T. and Swift, D.A. The e l e c t r o l y t i c reduction of cha lcopyr i te in acid so lu t ion . 3. Appl. Electrochem. 6 (1976) , pp. 229-235. 68. B ieg le r , T. and Constable, D.C. Upgrading and activation of cha lcopyr i te concentrates by s lur ry e l e c t r o l y s i s . I.M.M. Trans. C . , v o l . 85 ( 1976), pp. C23-C29. 69 B ieg le r , T. and Constable, D.C. Continuous e l e c t r o l y t i c reduction of chalcopyr i te s lu r r y . 3. Appl. Electrochem. 7 (1977) , pp. 175-179. 70. Swinkels, G.M. and Berezowsky, R.M.G.S. The Sherr i t t- . Cominco copper process - part I: the process. CIM Bui 1., February (1978), pp. 105-121. 71. Kawulka, P., K i rby , C R . and Bolton, G.L. The Sherr i t t- Cominco copper process - part II: p i lot-plant operation. CIM Bui 1., February ( 1978), pp. 122- 130. 72. Maschemeyer, D.E .G. , M i lner , E.F.G. and Parekh, B.M. The Sher r i t t-Comi nco copper process - part III: commercial impl ica t ions . CIM Bu11., February (1978), pp. 131-139. - 129 - 73. Barat in , F. An invest igat ion of the Cu-Fe-S-r^O system at 200 ° C . Ph.D. Thes is , U.B.C. Department of Metallurgy, Vancouver, B.C., October, 1980. 74. Stenhouse, J .H . Reduction of aqueous cupric sulphate by hydrogen, carbon monoxide, and their mixtures. M.Sc. Thes is , U.B.C. Department of Metal lurgy, Vancouver, B.C., Apr i 1 , 1982 75. von Hahn, E.A. and Peters, E. The role of Cu(I) in the 0 k inet ics of hydrogen reduction of aqueous cupric su l fa te so lu t ions . 3. Phys. Chem. 69, February (1965), pp. 547- 552. 76. Peters, E. e t . a l . A carbonyl-hydrometal1urgy method for re f in ing coper. Joint meeting of the MMI3-AIME, Tokyo, 1972. 77. Byerley, J . J . and Peters, E. K inet i cs and mechanisms of the react ion between carbon monoxide and copper (II) in aqueous so lu t ion . Can. Jour. Chem. 47 (1969), pp. 313- 321. 78. Et ienne, A. Electrochemical method to measure the copper ionic d i f f u s i v i t y in a copper sulphide scale. J . Electro- chem. S o c , July ( 1970), pp. 870-873. 79. Peters, E. and Hackl , R.P. Iron-copper separation by reduction leaching. Canadian Patent App l i c a t i on , 1981. - 130 - APPENDIX 1 EXAMPLES OF MATERIAL BALANCE CALCULATIONS FOR LEACH RUNS - 131 - APPENDIX 1 M a t e r i a l B a l a n c e C a l c u l a t i o n s f o r T a b l e 11 Runs ( s e e page 85) -400 mesh Cu° -100 + 200 mesh Cu° Head C o n c e n t r a t e : Wt. (g) 10.8 10.8 % Cu 26.4 26. 4 % Fe 33. 1 33. 1 % Zn 3.75 3.75 % S 35.6 35.6 I n i t i a l L e a c h a t e : Vo 1 ume (mL) 50 50 Cu ( g / L ) 90 90 C o p p e r P owder: W t . ( g ) 9.4 9.4 % Cu 99. 9 99.9 L e a c h R e s i d u e : Wt. (g) 19.7 20. 1 % Cu 80. 6 75. 1 % Fe 0.9 3.7 % Zn 0. 95 1.01 % S 19.6 20.3 F i n a l L e a c h a t e : Vo 1 ume (mL) 50 50 g/L Cu 22. 0 30. 8 g/L Fe 70. 8 59.6 g/L Zn 4. 55 3.75 - 132 - APPENDIX 1 Mater ia l Balance Ca lcu la t ions for Table 11 Runs (see page 85) (contd.) -400 mesh Cu° -100 + 200 mesh Cu° Weight In (g): Cu 16.75 16.75 Fe 3. 57 3. 57 Zn 0.41 0.41 S 3. 84 3. 84 Weight Out (g): Cu 16.98 16.64 Fe 3. 72 3.72 Zn 0.41 0. 39 S 3. 86 4. 08 Ext ract i on (So 1ut i on Bas i s ) : Fe 99.0 83. 4 Zn 56.2 49.9 Ext ract i on (Res i due Bas i s ) : Fe 95. 0 79. 2 Zn 53. 8 49.9 Ext ract ion (Average): Fe 97.0 81. 3 Zn 55.0 48. 1 - 133 - A P P E N D I X 2 A K I N E T I C MODEL FOR THE R E D U C T I O N L E A C H - 134 - APPENDIX 2 A KINETIC MODEL FOR THE REDUCTION LEACH 1. Assume that the process is parabol ic to a f i r s t approx- imation, with a shrinking core morphology. u t ^ — 5 — * ^ u , % o <S = thickness of o r ig ina l mineral that has disappeared (related to thickness of Cu 2S formed). Then, -d(u t ) k p dt S that i s , the rate-determining step is d i f fus ion through the so l i d product layer . 2. Develop an expression for u t based on constant geometry (the " sphe r i c a l " shrinking core model). 135 therefore Integrat ing , Therefore, or , u Q - u t = 2 5 from geometry. - d ( u t ) = + d ( u Q - u t ) = 2 d ( S ) , 2d(S) k n dt or 2 S d ( 6 ) = k p dt 6 2 = k p t j fi= (k p t )^ . u Q -u t = 2 ( k p t ) * = (kkpt)y2, u t = u Q - U k p t ) » 3. The percent iron extracted is proport ional to the volume f rac t ion of mineral converted to CU2S. 1 V t = c<u t- ? , o < = g7Tfor spheres of diameter u t . V 3 V t <* ut 3 V 0 ' O / [ u 0 - <4k pt ) » ] 3 1 - (kkpt)Y2 Fe extracted = 100 \1- — J , therefore, V t % Fe extr . V. 100 1- Rearranging, kp u o " u o •̂ % Fe ext 100 -)1 4t - 136 - 4. Now, using iron extract ion data from the neutral and reduction leach runs on born i te , chalcopyr i te and pyr i te (Table 14), we can ca lcu la te the respective parabol ic rate constants kpN a n d kp^. These resul ts are tabulated in the fol lowing tab le . Parabol ic Rate Constants for Neutral and Reduction Leaching* Neutral Leach i ng Reduct i on Leach i ng Mineral Fe extr k N Fe extr . k R kp R/k pN % (um)2h- 1 % (um) 2 h" 1 Cu ^FeS^ 19 2. 88 72 74.7 26 CuFeS 2 8 0. 470 68 62. 4 133 FeS 2 3 0.064 74 81. 8 1278 * Using a u Q value of 50 um and t = 1 h. 5. Therefore, assuming both leaching processes are para- b o l i c , the rate constants for reduction leaching of born i te , cha lcopyr i te and pyr i te are very much higher than the cor res - ponding neutral leach rate constants. However, for reduction leaching, a parabol ic approximation is poor because the Cu 2S product ruptures and spa l ls away due to i ts own expansion.

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