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UBC Theses and Dissertations

Continuous biological tank leaching of a refractory arsenical sulphide concentrate to enhance gold… Marchant, P. Brad 1986

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CONTINUOUS B I O L O G I C A L TANK LEACH ING OF A REFRACTORY A R S E N I C A L SULPH IDE CONCENTRATE TO ENHANCE GOLD EXTRACT ION B Y CYANIDAT ION A P I L O T PLANT AND F E A S I B I L I T Y STUDY AT EQU ITY S I L V E R MINES L I M I T E D b y P . B R A D M A R C H A N T B . S c . ( B i o c h e m i s t r y ) , U n i v e r s i t y o f New B r u n s w i c k , 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 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 S O F A P P L I E D 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 S T U D I E S ( M i n i n g a n d M i n e r a l P r o c e s s E n g i n e e r i n g ) 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 8 6 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 7 r e q u i r e d s t a n d a r d (c) P. BRAD MAR01ANT, 1986 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or pu b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date QZ-A B S T R A C T Bench and p i l o t scale testing has shown that biooxidation of a re f r a c t o r y a r s e n i c a l sulphide concentrate enhanced gold and s i l v e r extraction by cyanidation. Direct cyanidation of Southern T a i l bulk sulphide concentrate t y p i c a l l y resulted in less than 10% gold and s i l v e r recovery. B i o l o g i c a l pretreatment of the sulphide concentrate r e s u l t e d i n 70% gold extraction and at l e a s t 30% s i l v e r e x traction within economic process constraints. Biooxidation of a low grade a r s e n i c a l sulphide concentrate, 5.5 g Au/t, 80 g Ag/t, and 2-5% As, was economically a t t r a c t i v e due to apparent p r e f e r e n t i a l oxidation of the gold bearing suphide mineral assemblage(s). Selective sulphide oxidation minimized the bioleach residence time, the oxygen/CC>2 requirements, and the costs associated with n e u t r a l i z a t i o n of the a c i d i c byproducts of sulphide oxidation. Approximately 15% t o t a l sulphide oxidation was required to e f f e c t 70% gold extraction and 30-40% s i l v e r e x t r a c t i o n . Additional sulphide oxidation improved gold extraction s l i g h t l y and improved s i l v e r extraction s i g n i f i c a n t l y but was not economically a t t r a c t i v e . ( i i ) C o m p l e t e b a t c h a n d c o n t i n u o u s l a b o r a t o r y b i o l e a c h f a c i l i t i e s were c o n s t r u c t e d a t E q u i t y . A d e t a i l e d d e s c r i p t i o n o f a p p a r a t u s , m e t h o d s , a n d o p e r a t i n g d a t a i s p r e s e n t e d . C r i t i c a l v a r i a b l e s t h a t were s t u d i e d i n c l u d e d t e m p e r a t u r e , p H , p u l p d e n s i t y , p a r t i c l e s i z e , a i r / C 0 2 r e q u i r e m e n t s , n u t r i e n t s , b i o l e a c h a t e r e c y c l e , i n o c u l u m s o u r c e , d e g r e e o f s u l p h i d e o x i d a t i o n , c y a n i d a t i o n c o n d i t i o n s , a n d b i o l e a c h a t e t r e a t m e n t . T h e l a b o r a t o r y d a t a p r o v i d e d t h e d e s i g n a n d o p e r a t i n g c r i t e r i a f o r a c o n t i n u o u s b i o l e a c h p i l o t p l a n t . O p e r a t i o n o f t h e c o n t i n u o u s p i l o t c i r c u i t , w i t h a n o m i n a l d e s i g n c a p a c i t y o f 2 t o n n e s p e r d a y , showed t h e r e was no s i g n i f i c a n t e f f e c t o f s c a l e o n t h e b i o h y d r o m e t a l l u r g y o r r e s u l t a n t p r e c i o u s m e t a l s h y d r o m e t a l l u r g y . A d e t a i l e d d e s c r i p t i o n o f t h e p i l o t e q u i p m e n t a n d s t e a d y s t a t e o p e r a t i n g p a r a m e t e r s i s p r e s e n t e d . C o n s i d e r a t i o n s f o r s c a l e u p a n d • f o r - - p l a n t • s c a l e d e s i g n a n d o p t i m i z a t i o n a r e d i s c u s s e d . A p r e l i m i n a r y f e a s i b i l i t y s t u d y , b a s e d o n a c o n c e p t u a l d e s i g n f o r a n 800 t o n n e p e r d a y b i o l e a c h c i r c u i t , i s d e t a i l e d . T h e f e a s i b i l i t y a n a l y s i s s h o w e d t h a t i t i s e c o n o m i c a l l y a t t r a c t i v e t o s c a v e n g e g o l d a n d s i l v e r v a l u e s f r o m S o u t h e r n T a i l f l o t a t i o n t a i l i n g u s i n g b i o o x i d a t i o n t o e n h a n c e t h e g o l d a n d s i l v e r e x t r a c t i o n b y c y a n i d a t i o n . The b a s e c a s e c a s h f l o w a n a l y s i s s h o w e d a N e t P r e s e n t V a l u e p o t e n t i a l o f $ 4 , 9 7 5 x 1 0 6 ( 1985 ) a t 15% d i s c o u n t f a c t o r . ( i i i ) TABLE OF CONTENTS Page A b s t r a c t Table of Contents. L i s t of F i g u r e s . . . L i s t of Flowsheets L i s t of Tables.... L i s t of P l a t e s .... L i s t of Appendices Acknowledgements.. 1.0 INTRODUCTION 1 1.1 M e t a l l u r g i c a l Background 9 1.2 P r o j e c t O b j e c t i v e s . . 19 1.3 Review of Biohydrometallurgy 20 1.4 N o t i o n a l Flowsheet 2 5 2.0 SUMMARY OF LABORATORY SCALE BIOLEACH STUDY 28 2.1 Batch Laboratory T e s t i n g 28 2.1.1 Apparatus 28 2.1.2 Standard Procedures 29 2.1.3 Data C o l l e c t i o n and A n a l y s i s 34 2.1.4 Summary of Batch Test Results 36 Temperature 3 6 pH 3 6 _ .. . p a r t i c l e s i z e 36 . pulp d e n s i t y 41 a i r / C 0 2 requirements 41 n u t r i e n t s 44 b i o l e a c h a t e r e c y c l e 47 inoculum source 53 degree of o x i d a t i o n / c y a n i d a t i o n 53 b i o l e a c h a t e treatment 56 2.1.5 A p p l i c a t i o n of Results 60 2.2 Continuous Laboratory T e s t i n g 6 0 2.2.1 Apparatus 6 2 2.2.2 Standard Procedures 62 2.2.3 Data C o l l e c t i o n and A n a l y s i s 63 2.2.4 Summary of Continuous T e s t i n g 66 B i o l o g i c a l washout 66 b i o l e a c h a t e r e c y c l e 68 n u t r i e n t s 68 d e n s i t y 71 c y a n i d a t i o n 7 3 b i o l e a c h a t e c h a r a c t e r i s t i c s 73 2.2.5 A p p l i c a t i o n of Results 74 . . ( i v ) . . ( v i ) ( v i i i ) ( i x ) . (x) . . ( x i ) . ( x i i ) ( i v ) Page 3.0 SUMMARY OF THE PILOT SCALE BIOLEACH STUDY 7 5 3.1 P i l o t P l a n t D e s c r i p t i o n 75 3.2 P r o c e d u r e s 83 3.3 Data C o l l e c t i o n and A n a l y s i s 87 3.4 Summary o f M e t a l l u r g i c a l R e s u l t s 87 3.5 G e n e r a l Comments 9 8 3.6 C o s t R e c o n c i l i a t i o n 100 4.0 PLANT SCALE FEASIBILITY AT EQUITY SILVER MINES 102 4.1 S c a l e u p C o n s i d e r a t i o n s 104 4.2 D e s i g n C r i t e r i a and Equipment S i z i n g 115 4.3 F l o w s h e e t D e s c r i p t i o n 123 4.4 C a p i t a l C o s t E s t i m a t e D e t a i l s 133 4.5 O p e r a t i n g C o s t E s t i m a t e D e t a i l s 144 4.6 D i s c u s s i o n o f P r o c e s s D e s i g n and C o s t i n g 149 4.7 Revenue and Cash Flows 150 4.8 E x e c u t i v e Summary 154 a. M e t a l l u r g y 154 b. C a p i t a l and O p e r a t i n g C o s t E s t i m a t e . . 1 5 5 c. Revenue 156 d. Cash Flow S e n s i t i v i t y 156 5.0 ADDITIONAL APPLICATIONS 158 6.0 CONCLUSIONS 16 0 REFERENCES 164 APPENDIX I B i o l e a c h Heat B a l a n c e - P l a n t S c a l e Example 172 APPENDIX I I D e t a i l e d Cash Flow E s t i m a t e s 177 (v) L i s t of Figures Page 1.1 (a) E q u i t y S i l v e r Mines p i t l o c a t i o n 2 1.1 (b) E q u i t y S i l v e r Mines t a i l i n g pond l o c a t i o n 3 1.2 Testwork l o g i c 13 1.3 Sulphur c y c l e 21 1.4 The a c i d o p h i l i c l e a c h i n g b a c t e r i a 21 1.5 A c i d minewater c y c l e 24 2.1 2.5 l i t r e l a b o r a t o r y r e a c t o r 31 2.2 T y p i c a l batch e x t r a c t i o n curve., 35 2.3 E f f e c t of temperature on batch i r o n e x t r a c t i o n . 37 2.4 E f f e c t of i n i t i a l pH on batch i r o n e x t r a c t i o n . . 38 2.5 E f f e c t of r e g r i n d time on batch i r o n e x t r a c t i o n 39 2.6 E f f e c t of r e g r i n d time on batch i r o n e x t r a c t i o n 40 2.7 E f f e c t of pulp d e n s i t y on batch i r o n e x t r a c t i o n 42 2.8 E f f e c t of CO on batch i r o n e x t r a c t i o n 43 2.9 E f f e c t of n u t r i e n t s on batch i r o n e x t r a c t i o n . . . 45 2.10 E f f e c t of n u t r i e n t s on batch i r o n e x t r a c t i o n . . . 46 2.11 E f f e c t of n u t r i e n t s on batch i r o n e x t r a c t i o n . . . 48 2.12 E f f e c t of water source on batch i r o n e x t r a c t i o n 49 2.13 E f f e c t of water source on batch i r o n e x t r a c t i o n 50 2.14 E f f e c t of f e r t i l i z e r on batah i r o n e x t r a c t i o n . . 51 2.15 E f f e c t of b i o l e a c h a t e r e c y c l e on batch i r o n e x t r a c t i o n 52 (vi) Page 2.16 E f f e c t of inoculum source on batch i r o n e x t r a c t i o n 54 2.17 T y p i c a l cumulative precious metals e x t r a c t i o n p r o f i l e 57 2.18 Laboratory s c a l e continuous b i o l e a c h apparatus. 61 2.19 T y p i c a l continuous b i o l e a c h e x t r a c t i o n 6 5 2.20 E f f e c t of r e s i d e n c e time on continuous b i o l e a c h e x t r a c t i o n 67 2.21 E f f e c t of b i o l e a c h a t e r e c y c l e on continuous b i o l e a c h e x t r a c t i o n 69 2.22 E f f e c t of n u t r i e n t and water source on continuous b i o l e a c h e x t r a c t i o n 70 2.23 Continuous b i o o x i d a t i o n a t 25% s o l i d s 72 3.1 P i l o t p l a n t r e a c t o r d e t a i l s 82 3.2 P i l o t p l a n t e x t r a c t i o n - Run 1 89 3.3 P i l o t p l a n t e x t r a c t i o n - Run 2 91 3.4 P i l o t p l a n t e x t r a c t i o n - Run 3 95 4.1 E f f e c t of p y r i t e o x i d a t i o n on g o l d recovery....106 4.2 E f f e c t of p y r i t e o x i d a t i o n on s i l v e r recovery..107 4.3 General o p t i m i z a t i o n - a e r a t i o n , r e s i d e n c e , d e n s i t y 109 4.4 Process water cyanide species 112 4.5 S e n s i t i v i t y summary 157 ( v i i ) L i s t of Flowsheets Page 1.1 E q u i t y S i l v e r Mines concentrator crushing c i r c u i t 5 1.2 E q u i t y S i l v e r Mines conce n t r a t o r Southern T a i l f l o t a t i o n c i r c u i t 6 1.3 E q u i t y S i l v e r Mines c o n c e n t r a t o r Main Zone f l o t a t i o n c i r c u i t 7 1.4 E q u i t y S i l v e r Mines CIL - l e a c h i n g c i r c u i t 11 1.5 E q u i t y S i l v e r Mines CIL - carbon c i r c u i t 12 1.6 N o t i o n a l b i o l e a c h c i r c u i t 27 3.1 P i l o t p l a n t flowsheet 76 3.2 P i l o t p l a n t general arrangement..... 77 4.1 Conceptual p l a n t s c a l e b i o l e a c h i n g - general 103 4.2 Conceptual p l a n t s c a l e b i o l e a c h i n g - beach r e c l a i m . 125 4.3 Conceptual p l a n t s c a l e b i o l e a c h i n g - f l o t a t i o n 126 4.4 Conceptual p l a n t s c a l e b i o l e a c h i n g r egrind/reagents 127 4.5 Conceptual p l a n t s c a l e b i o l e a c h i n g - b i o l e a c h i n g . . . 128 4.6 Conceptual p l a n t s c a l e b i o l e a c h i n g f i l t r a t i o n / n e u t r a l i z a t i o n 129 4.7 Conceptual p l a n t s c a l e b i o l e a c h i n g - water balance. 130 4.8 Conceptual p l a n t s c a l e b i o l e a c h i n g general arrangement - r e c l a i m 131 4.9 Conceptual p l a n t s c a l e b i o l e a c h i n g general arrangement - b i o l e a c h i n g 132 ( v i i i ) L i s t of Tables Page 1.1 Relative mineral abundance 4 1.2 Baseline Southern T a i l bulk sulphide t e s t r e s u l t s . . 15 1.3 Microbial mechanisms for metal extraction 22 2.1 Batch cyanidation r e s u l t s 55 2.2 Typical continuous operating data 64 2.3 Continuous cyanidation r e s u l t s 73 3.1 P i l o t plant equipment d e s c r i p t i o n 78 3.2 Typical d a i l y p i l o t plant data 85 3.3 Typical night s h i f t p i l o t plant report 86 3.4 B a l l m i l l media t e s t i n g 87 3.5 P i l o t plant d e t a i l e d data - run 1 90 3.6 P i l o t plant d e t a i l e d data - run 2 92 3.7 P i l o t plant d e t a i l e d data - run 3 96 3.8 P i l o t cyanidation t e s t i n g 98 3.9 Cost r e c o n c i l i a t i o n 100 4.1 Special waste t e s t r e s u l t s . 113 (ix) L i s t of Plates Page 2.1 Walk-in fume hood and shaker apparatus 30 2.2 Walk-in fume hood and batch tank apparatus 30 3.1 P i l o t plant b a l l m i l l 81 3.2 P i l o t reactors 1-6 under construction 81 3.3 P i l o t b e l t f i l t e r 84 3.4 P i l o t vacuum receivers 84 (x) L i s t of Appendices Page Appendix I Bioleach Heat Balance - Plant Scale Example.172 Appendix II Detailed Cash Flow Estimates 17 7 (xi) ACKNOWLEDGEMENTS I would l i k e to express my sincere gratitude to the management of Equity S i l v e r Mines Limited and Placer Development Limited for t h e i r foresight and f i n a n c i a l support to expedite t h i s project. Special recognition i s deserving of a l l of the employees from the me t a l l u r g i c a l , operating, and plant services s t a f f of Equity S i l v e r Mines who contributed throughout the project and notably E. Robles, D. Robinson, and J. Somers f o r t h e i r invaluable technical assistance i n operating the p i l o t plant. I am indebted to Dr.R.W. Lawrence of B.C. Research for introducing me to biohydrometallurgy and for volunteering h i s expertise i n t h i s f i e l d as co-supervisor of my programme. I wish to thank Dr.G.W. Poling f or invaluable motivation and techni c a l d i r e c t i o n throughout the -research project and i n p a r t i c u l a r f o r h i s patience during t h i s thesis preparation. F i n a l l y , I would l i k e to thank E. O'Rourke for her un s e l f i s h donation of time and e f f o r t to in t e r p r e t and tra n s l a t e my handwritten language to that which might be read and understood by others. (xii) 1 1.0 INTRODUCTION E q u i t y S i l v e r Mines L i m i t e d open p i t s i l v e r mine, a 70% owned s u b s i d i a r y of P l a c e r Development L i m i t e d , i s l o c a t e d 37 k i l o m e t e r s s o u t h e a s t of Houston i n n o r t h c e n t r a l B r i t i s h Columbia. The d e p o s i t c o n s i s t s of two major ore zones, Main Zone and Southern T a i l , F i g u r e 1.1(a). F e a s i b i l i t y ore r e s e r v e s a r e 28 m i l l i o n tonnes g r a d i n g 106 g Ag/t, 0.96 g Au/t and 0.38% copper. A d e t a i l e d m i n e r a l o g i c a l d e s c r i p t i o n has been p r e s e n t e d (1,2) and a m i n e r a l o g i c a l comparison of the two o r e zones i s shown i n Table 1.1. The c o n c e n t r a t o r f l o w s h e e t c o n s i s t s of c o n v e n t i o n a l c r u s h i n g , g r i n d i n g , f l o t a t i o n , and dewatering c i r c u i t s p r o c e s s i n g 5,300 tonnes per day of new m i l l f e e d . The r e s u l t a n t 80-100 tonnes per day o f Cu/Ag/Au c o n c e n t r a t e i s t r u c k e d t o P r i n c e Rupert f o r shipment to market. Flowsheet 1.1 shows the c r u s h i n g / s c r e e n i n g c i r c u i t employed t o m a i n t a i n the -1.9cm f i n e ore s t o c k p i l e . " Flowsheet 1.2 r e p r e s e n t s the c o n c e n t r a t o r f l o w s h e e t d u r i n g Southern T a i l ore p r o c e s s i n g which was expanded i n 1983 t o p r o v i d e a d d i t i o n a l g r i n d i n g power and f l o t a t i o n / d e w a t e r i n g c a p a c i t y due t o f i n e r m i n e r a l i z a t i o n i n the Main Zone, Flowsheet 1.3. S o u t h e r n T a i l f l o t a t i o n c o n c e n t r a t e t y p i c a l l y graded 4,500-6,500 g o f s i l v e r per tonne, 20-22% copper, and 25 grams of g o l d per tonne. Main zone c o n c e n t r a t e has graded 3,000-4,500 Ag/t, 15-20% Cu, and 15 g PROPERTY LAYOUT mtirtt u r e 1.1 E q u i t y S i l v e r M i n e s p i t l o c a t i o n (a ) 3 F i g u r e 1.1 (b) E q u i t y S i l v e r M ines - t a i l i n g pond C o n c e n t r a t o r 4 T A B L E 1.1 R e l a t i v e M i n e r a l A b u n d a n c e a t E q u i t y S i l v e r M i n e s M I N E R A L M A I N Z O N E S O U T H E R N T A I L P y r i t e R u t i l e I l m e n i t e M a g n e t i t e P y r r h o t i t e M o l y b d e n i t e H e m a t i t e A r s e n o p y r i t e S p h a l e r i t e C h a l c o p y r i t e T e t r a h e d r i t e G o l d G a l e n a S u l p h o s a l t s M a r c a s i t e C h a l c o c i t e C o v e l l i t e W o l f r a m i t e S t i b n i t e T o u r m a l i n e x x x x x x x x x x x x x x x x x x x X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X x x x x x = v e r y a b u n d a n t x x x x = a b u n d a n t x x x = m o d e r a t e x x = m i n o r x = t r a c e 5 F l o w s h e e t 1 . 1 E q u i t y S i l v e r M ines C o n c e n t r a t o r - c r u s h i n g c i r c u i t EQUITY SILVER MINES LTD  C O N C E N T R A T O R F L O W S H E E T ITEM DESCRIPTION ITEM DESCRIPTION 1 YARIASLE JPEEO CONVEYORS Ul ' " " • t M a FLOTATION CELLS (1*1 1 CLEANER I WtlOH SCALE K> 2.6m FLOTATION CELLS (SI 2 CLEANER 1 < l » i l l » "OO MILL (KOFRERS) II 15m THICKENER * I« »*LL MILL (KOPRERJI 12 STOCK TANK s 0 I m CYCLONES 121 13 U m OIA. DISC FILTER • l ) » FLOTATION CELLS (IS) 14 DRYER (12m OIA.) l . t m i . f t m REORINO MILL • 0 .4m CYCLONE o w s h e e t 1 .2 E q u i t y S i l v e r M i n e s C o n c e n t r a t o r - S o u t h e r n T a i l f l o t a t i o n c i r c u i t EQUITY SILVER MINES LTD CONCENTRATOR FLOWSHEET TEM DESCRIPTION ITEM DESCRIPTION l VARIABLE SPEED CONVEYORS 12) II 0.4 m CYCLONE 2 WEIGH SCALE 12 2,»m' FLOTATION CCLL3(I«) I*1 CLEANER } 4l»lt:ii« ROO MILL (KOPPERS) II 2 Bm' FLOTATION CELLS (8) CLEANER 4 4l«n«.li« BALL MILL (KOPPERS) 14 t»m> FLOTATION CELLSIBI 3'' CLEANER 3 O.Bi. CYCLONES (2) 13 13 m THICKENER t 14 »• FLOTATION CELLS (III ROUCHCRS l« 3T0CK TANK 7 41m if.In BALL MILL (KVS) 12 >!• $> DISC FILTER • OS" CYCLONCS (MAX 12) IB Z «m <t> DISC FILTER • B3m" FLOTATION CELLS (IB) 3CAVENOEA3 IS DRYER (I2m(j)| 10 24.n4.9m RC0R1ND MILL o w s h e e t 1 . 3 E q u i t y S i l v e r M i n e s C o n c e n t r a t o r - M a i n Z o n e f l o t a t i o n c i r c u i t tt Au/t at lower o v e r a l l recoveries than experienced with Southern T a i l ore. T a i l i n g s disposal i s v i a closed c i r c u i t ponding from which process water i s continuously recycled, Figure 1.1(b). Gold recovery was t y p i c a l l y 30-35% to f l o t a t i o n concentrate during Southern T a i l processing while Main Zone gold recovery todate has been only s l i g h t l y higher. Considerable research by both Equity personnel and independent laboratories has been dedicated to improving gold recovery at Equity S i l v e r Mines. A number of methods for scavenging gold and s i l v e r from f l o t a t i o n t a i l i n g have been investigated. The most s a l i e n t r e s u l t todate has been the i n d i c a t i o n that the mode of occcurrence of gold l o s t to t a i l i n g from the Southern T a i l orebody i s quite d i f f e r e n t from that expected through the Main Zone orebody. Gold l o s t to t a i l i n g during Southern T a i l processing (completed 1984) was associated p r i m a r i l y with arsenopyrite while most of the gold l o s t i n Main Zone f l o t a t i o n t a i l i n g - i s associated with s i l v e r bearing minerals, p y r i t e and s i l i c e o u s gangue. Consequently, separate e f f o r t s were directed at improving gold recovery from the two areas with a notable di f f e r e n c e i n m e t a l l u r g i c a l response. 9 1.1 METALLURGICAL BACKGROUND MAIN ZONE Mining i n the Main Zone commenced i n March, 1984. There i s a closer a s s o c i a t i o n of gold with s i l v e r i n the Main Zone, when compared with the Southern T a i l , and therefore gold recovery w i l l be higher i n f l o t a t i o n . Consequently, the f l o t a t i o n t a i l i n g w i l l average 0.5 g Au/t. Regression analysis of h i s t o r i c a l data indicated that about 20% of the gold i n the t a i l i n g might be associated with p y r i t e and 70% with s i l v e r bearing minerals (3). There i s v i r t u a l l y no arsenopyrite present. S i l v e r losses to t a i l i n g expected i n the Main Zone vary from 30-50 g Ag/t, due to f i n e s i l v e r mineralogy. The strongest c o r r e l a t i o n was with antimony i n the t a i l i n g , i n d i c a t i n g that s i l v e r losses are due to or associated with one or more antimony bearing minerals. A poor c o r r e l a t i o n with te t r a h e d r i t e indicated the presence of other sulphosalts. Due to the lack of arsenopyrite i n Main Zone t a i l i n g i t was expected that the f l o t a t i o n t a i l i n g would be more amenable to cyanidation than the Southern T a i l t a i l i n g . Research e f f o r t s were f i r s t directed at bulk sulphide production from Main Zone t a i l i n g to scavenge gold and s i l v e r by cyanidation of the concentrate. The r e s u l t s were encouraging, however, gold and s i l v e r recoveries to the bulk sulphide concentrate were 50% 10 and 40% r e s p e c t i v e l y , i n d i c a t i n g t h a t some of the g o l d and s i l v e r l o s t to t a i l i n g might be a s s o c i a t e d w i t h non-sulphide gangue. D e t a i l s have been reported ( 4 ) . P r e l i m i n a r y r e s u l t s of d i r e c t c y a n i d a t i o n of Main Zone t a i l i i n g were encouraging (4) which p r e c i p i t a t e d a d e t a i l e d research programme (5,6) and f e a s i b i l i t y study ( 7 ) . Consequently, a 5,400 tonne per day carbon-in-leach (CIL) c i r c u i t was c o n s t r u c t e d a t E q u i t y to scavenge precious metals from Main Zone f l o t a t i o n t a i l i n g , Flowsheets 1.4 and 1.5. The c i r c u i t was commissioned i n January 1985. SOUTHERN TAIL Mining i n the Southern T a i l p i t commenced i n J u l y 1980 and was completed by March 1984, m i l l i n g a t a r a t e of 5,300 tpd. Concentrator t a i l i n g averaged 0.9 g Au/t and s l i g h t l y more than 20 g Ag/t over the l i f e of the p i t . H i s t o r i c data i n d i c a t e d t h a t unrecovered gold was a s s o c i a t e d w i t h a r s e n o p y r i t e (3) and the s i l v e r l o s s e s probably as u n l i b e r a t e d - t e t r a h e d r i t e or minor s i l v e r b e a r i n g s u l p h o s a l t s . The concentrator process s e l e c t i v e l y prevented a r s e n o p y r i t e f l o t a t i o n to minimize concentrate grade d i l u t i o n by p y r i t e and a s s o c i a t e d smelter p e n a l t i e s . Figure 1.2 shows the testwork l o g i c employed to determine the most economic route to scavenge a d d i t i o n a l g o l d and s i l v e r values from 7 m i l l i o n tonnes of Southern T a i l f l o t a t i o n t a i l i n g p r e s e n t l y impounded i n the t a i l i n g s pond. 1 X TO CAABOH/ttfMtnr CIRCUIT H£W CAABON TO CARSON ClflCUIT n t or m > . a i o w t *  ISOKW S43>*« AIM COMPftCSSON T.im 4 \ 7.0m 4 AJA K t A C T O R I I * t 3.0* SAFETY SCRttN CAW s o w n e A S u m w a TAN* (SI) I 10w VIMATINS SCREN 101 1 7*mm T O " Q U E - * L O W HJUW SUBMtHfltO CMC SCflCCNS 4 • t 2.t* CIL TANKS IH- 4 % n.*m LC ACM TANK Um • XOm THASH f C J U C N fVOTATION CELLS OCSCAIfT10M •CALC N.T.t. OATt BA - « - 1 0 EQUITY SILVER MINES LTD. CYANIDATION SCAVENGER PLANT CYANIDATION CIRCUIT FIUN. I6-I-2I94-B F l o w s h e e t 1 . 4 E q u i t y S i l v e r M i n e s C I L - l e a c h i n g c i r c u i t 12 F l o w s h e e t 1.5 E q u i t y S i l v e r M ines C I L - c a r b o n c i r c u i t 13 INVESTIGATE ALTERNATE PROCESSES F i g u r e 1 .2 P r e c i o u s m e t a l s s c a v e n g i n g t e s t w o r k l o g i c 14 Laboratory t e s t i n g suggested that approximately 80% of the gold and 60% of the s i l v e r can be recovered from Southern T a i l f l o t a t i o n t a i l i n g by producing a bulk sulphide f l o t a t i o n concentrate, grading 6-10 g Au/t, 80-100 g Ag/t and 3-5% As (8). A plant scale t r i a l i n 1983 produced approximately 30,000 tonnes of bulk sulphide concentrate which was impounded ,separately. The concentrate was not marketable as produced (8) . Much of the testwork c a r r i e d out at Equity and other laboratories was directed a t producing an arsenopyrite r i c h concentrate from a bulk sulphide concentrate. Selective depression of the arsenopyrite i n froth f l o t a t i o n , using various surface oxidants (9), elevated temperatures, and multiple regrinding, resulted i n poor separation of the p y r i t e and arsenopyrite. The reason(s) for poor f l o t a t i o n response remain unclear. A g r a v i t y concentrate (laboratory shaking table) produced from f l o t a t i o n t a i l i n g , representing 1.4% weight recovery, assayed 23 g Au/t, 83 g Ag/t, 8% As(10). This concentrate represented only 23% gold recovery and 8% s i l v e r recovery. Although economically a t t r a c t i v e , a market has not been secured todate for g r a v i t y concentrate (11). Table 1.2 summarizes the r e s u l t s of baseline hydro/pyrometallurgical testwork todate on Southern T a i l bulk sulphide concentrate. Cyanidation of the enti r e f l o t a t i o n 15 TABLE 1.2 Southern T a i l Bulk Sulphide Treatment Summary PROCESSING SUMMARY % RECOVERY Au Ag_ Direct CN (24 h) (1 kg NaCN/t 11 .3 8. .1 15 mins. . Regrind/Direct CN (2.4 h) (1 kg NaCN/t) 13 .1 2. .7 15 mins. . Regrind/Direct CN (48 h) (1 kg NaCN/t) 21. .4 20. .9 Direct CN (24 h) (2.5 kg NaCN/t) 7. .8 13. .2 Direct CN (24 h) (5.0 kg NaCN/t) 7 .8 13. .8 15 mins. . Regrind/Direct CN (24 h)(5.0 kg MaCN/t) 28. .5 30. .6 8 mins. . Regrind/Direct CN (24 h)(5.0 kg NaCN/t) 25. .0 29. .2 30 mins. . Regrind/Direct CN (24 h)(1 kg NaCN/t) 12 .7 18. .2 30 mins, . Regrind/Direct CN (24 h)(5 kg NaCN/t) 19. .7 36. .4 4 5 mins. Regrind/Direct CN (24 h)(5 kg NaCN/t) 22. .8 23. .8 10 g/L H 2S0 4 wash (1 h) /Direct CN (24 h) (1 kg NaCN/t) 7. .6 13. .8 10 g/L H2SOiJ wash (4 h)/Direct CN (24 h) 11. .7 13. .8 20 g/L H 2S0 4 wash (4 h)/Direct CN (24 h) 5. .0 13. .8 10 g/L H 2S0 4 wash §90°C (1 h)/Direct CN (24 h) 7. .0 17, .2 30 mins. . Regrind in 20 g/L H 2S0 4/ Direct CN (24 h)(1 kg NaCN/t) 20. .4 25, .0 30 mins. . Regrind in 20 g/L H2SO4/10 g/L H 2S0 4 @90°C (1 h) Direct CN 7. .0 5. .0 4 stage 15 mins. Regring/wash/Direct CN (6 h)(1 kg NaCN/t) 35. .9 26. .9 30 mins. . Regrind/ CS(NH 2) 2 (24 h)(1 kg CS(NH 2) 2 /t) 13. .6 0 30 mins. . Regrind/ CS(NH 2) 2 (24 h)(5 kg CS(NH 2) 2 /t) 23. .7 1. .2 10 mins. . Regrind/ CS(NH 2) 2 ( 8 h) (10 kg CS (NH2) /1) 32. .5 12. .5 Roast @ 500°C (2 h)/Direct CN (24 h) 70. .7 1. .6 Roast @ 550°C (2 h)/Direct CN (24 h) 57. .1 3. .4 Roast @ 550°C 13.Jh.)^DirectlCN (24 h) 65. .4 16. .2 15 mins. . Regrind / Roast @550°C (4 h)/Direct CN(24 h) 74. .1 65. .2 15 mins. Regrind/Roast @550°C (4 h)/Direct CS(NH 2) 2 (24 h) 72. ,9 58. ,5 Roast @ 600°C (2 h)/Direct CN (24 h) 83. .8 4. .4 Roast @ 700°C (2 h)/Direct CN (24 h) 59. .6 28. .1 Pressure oxidation (98% Fe/As)/wash/Direct CN 96. .4 N.D. 16 t a i l i n g returned very poor r e s u l t s , less than 10% Au recovery (8), and was therefore abandoned. Direct cyanidation of the bulk sulphide concentrate resulted i n approximately 10% Au and Ag recovery. Regrinding, prolonged leaching, and high reagent addi t i o n resulted i n approximately 20% Au and Ag recovery. Leaching i n the presence of thiourea resulted i n s l i g h t l y higher gold and s i l v e r extraction but remained sub-economic. Pretreatment with sulphuric acid did not r e s u l t i n a s i g n i f i c a n t improvement i n gold extraction by cyanidation. Staged regrinding and cyanidation resulted i n s i g n i f i c a n t improvement i n gold and s i l v e r recovery, however, t h i s method would not be p r a c t i c a l on a plant scale. The r e f r a c t o r y nature of the Southern T a i l bulk sulphide concentrate was evident from the roasting testwork. Roasting followed by cyanidation of the washed c a l c i n e resulted i n greatly improved precious metals recovery. Note that the roasting was c a r r i e d out i n a laboratory furnace and i t i s expected that roasting on a plant scale would be more e f f i c i e n t and e f f e c t i v e . Oxidative pretreatment by pressure leaching also resulted i n high gold recovery (2). However, subsequent analysis indicated that complete pyrite/arsenopyrite oxidation i s not a v i a b l e a l t e r n a t i v e (8,13), due to high operating and by-product treatment costs. A preliminary economic f e a s i b i l i t y comparing d i r e c t cyanidation, prolonged cyanidation a f t e r regrind, and roasting p r i o r to cyanidation was 17 conducted(8). The r e s u l t s of which indicated that none of the a l t e r n a t i v e s tested were economically favourable. A number of exotic pretreatments were tested (eg. e l e c t r o l y t i c a c t i v a t i o n , high i n t e n s i t y wet magnetic separation) that showed no encouraging r e s u l t s and were abandoned. Exploratory bioleach testwork at BC Research indicated that biooxidation of a pyrite/arsenopyrite bulk sulphide concentrate effected a s i g n i f i c a n t increase i n gold and s i l v e r extraction by subsequent cyanidation of the bioleach residue. These preliminary r e s u l t s and independent bioleach testwork indicated a l i n e a r r e l a t i o n s h i p between the degree of p y r i t e oxidation by b i o l o g i c a l metabolism and the gold and s i l v e r recovery by cyanidation. A f t e r 89% p y r i t e oxidation subsequent cyanidation of the bioleach residue showed 90% gold recovery and 86% s i l v e r recovery (14). A second stage, of, bioleach testwork was i n i t i a t e d at BC Research, the r e s u l t s of which are summarized: Preliminary Biooxidation Testing at BC Research P y r i t e Oxidation % Recovery a f t e r Cyanidation % Au Ag 10 25 50 65 70.1 74.3 76. 5 74.5 52.5 52.5 58. 2 32. 7 18 The r e s u l t s i n d i c a t e d t h a t there may be p r e f e r e n t i a l b i o o x i d a t i o n of the a r s e n o p y r i t e and s i l v e r bearing gangue m i n e r a l ( s ) . The r e s u l t s of confirmatory t e s t i n g a t BC Research can be summarized: Confimatory B i o o x i d a t i o n T e s t i n g a t BC Research P y r i t e O x i d a t i o n % Recovery a f t e r C y a n i d a t i o n % Au Ag 4 55.7 70. 7 13 75.4 84. 3 The s i g n i f i c a n c e of these r e s u l t s i s t w o f o l d . F i r s t , b i o o x i d a t i o n was p r e v i o u s l y thought to r e q u i r e p r o h i b i t i v e l y long b i o l e a c h residence times to e f f e c t reasonable g o l d recovery by c y a n i d a t i o n . The residence time r e q u i r e d f o r p a r t i a l p y r i t e o x i d a t i o n turned out to be t y p i c a l of residence times of • " h y d r o m e t a l l u r g i c a l o p e r a t i o n s . Second, p a r t i a l o x i d a t i o n of the p y r i t i c gangue reduces unwanted by-product c o n s t i t u e n t s i n s o l u t i o n thus minimizing c o s t l y secondary treatment steps and environmentally r e c a l c i t r a n t m a t e r i a l . I n i t i a l l y , i n 1982, stock c u l t u r e s of T h i o b a c i l l u s f e r r o o x i d a n s a t BC research d i d not respond w e l l t o Equity bulk s u l p h i d e concentrate, due to the high s i l v e r and a r s e n i c content. However, a f t e r s e r i a l c u l t u r i n g techniques were employed, l e a c h r a t e s improved s i g n i f i c a n t l y u n t i l acceptable b i o o x i d a t i o n r a t e s were achieved. This method of s t r a i n 19 a c c l i m a t i o n , where the b a c t e r i a a c q u i r e the metabolic requirements to o x i d i z e a p r e v i o u s l y t o x i c s u b s t r a t e , i l l u s t r a t e d the f l e x i b i l i t y f o r a d a p t a t i o n t h a t can be expected i n b i o l o g i c a l systems. 1.2 PROJECT OBJECTIVES I t was evident from the r e s u l t s of the p r e l i m i n a r y i n v e s t i g a t i o n s t h a t the economic v i a b i l i t y of scavenging g o l d and s i l v e r from Southern T a i l f l o t a t i o n t a i l i n g s was dependent on c o n f i d e n t p r o d u c t i o n of a high grade, a r s e n o p y r i t e - r i c h s u l p h i d e concentrate to minimize o x i d a t i v e pretreatment c o s t s , or d e f i n i t i o n of a method f o r p r e f e r e n t i a l o x i d a t i o n of the p y r i t e / a r s e n o p y r i t e to enhance c y a n i d a t i o n response of a bulk s u l p h i d e concentrate. I n s t a l l a t i o n of a c y a n i d a t i o n c i r c u i t based on Main Zone economics a l t e r e d the Southern T a i l f l o t a t i o n t a i l i n g scavenging s t r a t e g y s i g n i f i c a n t l y . J u s t i f i c a t i o n of the c a p i t a l expense of a cyanidaton c i r u c u i t was not necessary to j u s t i f y retreatment f o r Southern T a i l t a i l i n g . The p r e l i m i n a r y r e s u l t s a t BC Research j u s t i f i e d a d d i t i o n a l i n v e s t i g a t i o n t o d e f i n e f u r t h e r the f e a s i b i l i t y of b i o l e a c h i n g to p r e t r e a t Southern T a i l bulk s u l p h i d e concentrate. To evaluate b i o l e a c h i n g as a commercial a l t e r n a t i v e f o r a r s e n i c a l s u l p h i d e concentrate o x i d a t i v e pretreatment i t was necessary to d e f i n e and o p t i m i z e c r i t i c a l o p e r a t i n g v a r i a b l e s and an o p e r a t i n g s t r a t e g y o u t s i d e of the l a b o r a t o r y under l e s s than i d e a l c o n d i t i o n s . 20 The o b j e c t i v e s o f t h e b i o l e a c h p r o j e c t were: ( i ) c o n s t r u c t and o p e r a t e l a b o r a t o r y s c a l e b a t c h and c o n t i n u o u s b i o l e a c h t e s t f a c i l i t i e s a t E q u i t y S i l v e r M i nes, ( i i ) d e s i g n and c o n s t r u c t a b i o l e a c h p i l o t p l a n t based on bench s c a l e i n d i c a t o r s , ( i i i ) o p e r a t e a c o n t i n u o u s b i o l e a c h p i l o t f a c i l i t y a t E q u i t y t o i n d i c a t e t h e e f f e c t o f s c a l e on b i o o x i d a t i o n and p r o v i d e o p e r a t i n g d a t a o f c o n t i n u o u s b i o o x i d a t i o n i n an i n d u s t r i a l e n v i r o n m e n t , ( i v ) i n d i c a t e t h e p r e l i m i n a r y f e a s i b i l i t y o f a p l a n t s c a l e b i o o x i d a t i o n c i r c u i t t o t r e a t S o u t h e r n T a i l b u l k s u l p h i d e c o n c e n t r a t e a t E q u i t y S i l v e r Mines L i m i t e d . 1.3 REVIEW OF BIOHYDROMETALLURGY B i o h y d r o m e t a l l u r g y t a k e s advantage o f one o f t h e most w e l l d e f i n e d n a t u r a l c y c l e s , t h e s u l p h u r c y c l e , as shown i n ( 29) F i g u r e 1.3 . A s e c o n d a r y c y c l e w i t h i n t h e s u l p h u r c y c l e i s c h a r a c t e r i s e d by m i c r o o r g a n i s m s w h i c h c a t a l y s e t h e o x i d a t i o n o f s u l p h u r compounds and m e t a l s u l p h i d e s . There i s a l a r g e number o f b a c t e r i a t h a t c o u l d p l a y some r o l e i n e x t r a c t i v e m e t a l l u r g y , as i n d i c a t e d i n T a b l e 1 . 3 ^ ^ . The d i s c u s s i o n h e r e w i l l be l i m i t e d t o t h e a c i d o p h i l i c a u t o t r o p h s , n o t a b l y T h i o b a c i l l u s f e r r o o x i d a n s . A u t o t r o p h i c o r g a n i s m s o b t a i n t h e i r m e t a b o l i c n u t r i e n t s from i n o r g a n i c compounds, and t h e c h e m o s y n t h e t i c sub-group d e r i v e energy from o x i d a t i o n o f S U L P H U R - C O N T A J N b N G P R O T E I N O f P L A N T S A M O A N I M A L A S S I M I L A T I O N / X P U T R B V C T C N B Y P L A N T S / \WB*nB«A. S U L P H A T E S * S O j > S U L P H A T E - R E D U C ^ S U L P H I D E S S * ) B A C T E R I A . S U L P H U R \ / S U L P H I D E -O X I D I S I N G \ / O X I D I S I N G B A C T E R I A . \ / B A C T E R I A . S U L P H U R ( S ) . SULPHUR CYCLE. FIGURE 1.3 The S u l p h u r C y c l e Tlitubacillut ftrrooxidant Ltptuipmlium firmoxutans Suljolubui Thrrmophilic thjobacilli T. ftrroaxUans Trnvrmophilic th iobic i l l i Sulfoiobui MixcU c u l i u r n T. tkiooiuiani T. frrrooxklam T. thtrmoiulfuiooxidans Sulfoloitui Some •cLrrolroph. Otkcr UtoUciUi Fc^" 1 s \ M e u l tulprudet (e.g. F e S v . C u F e S ^ / / 1 — ' / M e u l + sulphate , ' V Elemental mlphur SoJubk inorganic ailptuir corapouadi C O * E n e r g y ( i i . A T T i n d N A £ K P ) H ) O r g a n i c SubtUnces andlot CMluUt carbon btoaynthcsu (tit Cartria cycle) Major or miaor com Ln button to< FIGURE 1.4 The A c i d o p h i l i c L e a c h i n g B a c t e r i a : t h e i r A c t i v i t i e s and B a s i c M e t a b l o i s m Microbes Matels removed Method Thiobacillus, Sullolobus Iron, sullur Oxidation Sphaerolllus, Leplolhrlx, Iron, manganese Oxidation Hyphomlcrobrlum, Gallionella Splrogyra, Oscillator!*, Molybdenum, selenium. Oxidation Hhizoclonlum, Chart uranium, radium Oesullovlbrio species Mercury, lead Reduction Scenedesmus, Synechococcus, Nickel Surface Osclllaloria, Chlamydomonas, Ion-exchange Euglena Saccharomyces cerevislae. Uranium, cesium, Surface Rhlzopus arrhlzus radium Ion-exchange Penicllllum digltatum Uranium Surface lon-exchange Ustllago sphaerogena Iron Surface chelation Aspergillus nlger Aluminum Surface chelation Cynanldlum caldarium Iron, copper, nickel, Surface aluminum, chromium precipitation Staphylococcus aureus, Cadmium, zinc, arsenate, Chemosmotlc Escherichia coll arsenlte, antimony efflux Pseudomonas aeruginosa Uranium, cesium, radium Intracellular trap Synechococcus Nickel, copper, cadmium Intracellular trap Clostridium cochlearium Mercury - . . . Blomethylatlon Pseudomonas species Tin Blomethylatlon TABLE 1.3 ( 6 9 ) M i c r o b i a l mechanisms for metal extracting/concentrating/recovery. 2 3 inorganic compounds. Figure 1 .4 shows the metabolic processes of the a c i d o p h i l i c metal sulphide leaching bacteria, including T h i o b a c i l l u s ferrooxidans. B i o l o g i c a l enzymes catalyse the iron-sulphide acid cycle, as shown i n Figure 1.5, by d i r e c t (iron-sulphide oxidation) or i n d i r e c t (ferrous i r o n oxidation to f e r r i c iron) mechanisms. Typical iron-sulphide acid cycle reactions shown for p y r i t e i n Figure 1.5 can be s i m i l a r l y represented for arsenopyrite. T h i o b a c i l l u s ferrooxidans w i l l oxidize a host of other sulphide compounds, e x h i b i t i n g a wide range of oxidation rates under various conditions. B i o l o g i c a l leaching i s p a r t i c u l a r l y a t t r a c t i v e where s e l e c t i v e precious metals bearing sulphide oxidation, or p r e f e r e n t i a l rate of oxidation, can be exploited as previously discussed. Bioleaching can also be employed where a l l of the sulphide gangue must be oxidized to e f f e c t a s i g n i f i c a n t improvement i n gold extraction by cyanidation. However, the c a p i t a l and operating costs increase accordingly and therefore would require a higher gold grade s t a r t i n g material to maintain favourable economics. A review of the l i t e r a t u r e and a patent search at the beginning of t h i s project and following p i l o t plant operation provided an i n d i c a t i o n of the "state-of-the-art" of biooxidation a p p l i c a t i o n s for precious metals processing. The review material was used p r i m a r i l y to i n d i c a t e laboratory techniques employed fo r a p p l i c a t i o n during t h i s project and to F I G U R E 1.5 Sequence of Reactions In Acid Minewater Generation pH <3.0 T. ferrooxidans (pH C4.5) T. ferrooxidans FeS-H20 3 FeSO, 7/2 0 . F e A S 0 4 + H 2 S ° < F e 2 ( S O , ) 3 + H 2 0 2 Fe(0H) 3 • 3 H 2 S0 4 , + H?0 ! t * ^ " p T. ferrooxidans r 2 S° H20 2 0 2 2 H + * 2 S04* 3 CL 2 H 20 -2 H 2 S 0 r J 25 provide an i n d i c a t i o n of the c r i t i c a l bioleach parameters to study. A detailed summary of the l i t e r a t u r e i s beyond the scope and objectives of t h i s t h e s i s . A l i t e r a t u r e guideline i s presented overleaf for reference by subject area r e l a t i n g to biohydrometallurgy. The primary and secondary groupings are e n t i r e l y subjective and were based on the author's terms of reference only. 1.4 NOTIONAL FLOWSHEET Based on the l i t e r a t u r e reviewed a notional flowsheet was derived f o r plant scale biooxidation of a r e f r a c t o r y a r s e n i c a l sulphide concentrate, Flowsheet 1.6. The research project was designed to define the c r i t i c a l parameters as indicated i n the notional flowsheet to aid i n p i l o t plant design and operation. 26 LITERATURE SUMMARY 1. review - primary 29,3 5 - secondary 15,16,40, 43,69 2. precious metals sulphides - primary 25,35,49,59,61 - secondary 20,22,40,43,45,60,62 3. copper sulphide 62 4. zinc sulphide 28,30,43,49, 5. lead sulphide 36,43 6. n i c k e l sulphide 35,40,43,56 7. molybdenum sulphide 3 5,40,43 8. uranium sulphide 23,40,43,49 9. miscellaneous substrates 38,41,45,52 10. underlying biochemistry - primary 18,29,3 5,40,44 - secondary 15,20,23,43,49,50 11. surface chemistry 20,29,53 12. microscopy 19,45,71 13. alternate microorganisms 17,29,35,43,44,56,57 14. alternate applications - waste treatment 35,58,66 - s e l e c t i v e f l o t a t i o n 66 - heap leaching 25,26,27,29 16. commercial a p p l i c a t i o n 27,28,41,51,53,55, 59,60,61 17. patents 57,62,64 Reagents ? Leachate Recirculation? RESIDENCE T1H£ - F (density, leach rate, degree ? ? ? • FEED - Bineraloglcal origin ? - flotation or gravity ? TO CYANIDE DESTRUCTION FLOWSHEET 1.6 NOTIONAL FLOWSHEET TO TAILING POND CYANIDE DESTRUCTION 28 2.0 SUMMARY OF THE LABORATORY SCALE BIOLEACH STUDY Laboratory scale investigations consisted of bioleach testwork i n batch shake fl a s k s and 2.5 l i t r e reactors and continuous t e s t i n g i n 2.5 L reactors. It was the objective of the laboratory study to indicate the important variables f o r consideration i n p i l o t c i r c u i t design and operation. The bulk sulphide sample employed throughout the laboratory and p i l o t scale t e s t i n g was sub-sampled from approximately 30,000 tonnes of impounded material produced during a plant scale t r i a l of bulk sulphide production. Approximately 5 tonnes was sub-sampled and stored i n a covered area for the duration of t h i s project. Hand coring and assay of the subsample indicated a grade of 5-6 g Au/t, 80-100 g Ag/t, 38-40% Fe, 38-42% S, 2-5% As. 2.1 BATCH LABORATORY TESTING  2.1.1 Apparatus. A 2.5m x 2.5m x 1.0m walk-in fume hood served as an "incubator" f o r stock culture development and shake f l a s k t e s t i n g . I n i t a l l y the environment was maintained at 32°C and 0.2% CO,,. The CO,, was c o n t r o l l e d by a p e r i s t a l t i c pump actuated by cycle timers which pumped CO2 from a c y l i n d e r and delivered a preset volume (pump speed) over a preset time i n t e r v a l . An e l e c t r i c heater was actuated by a dedicated c o n t r o l l e r setpoint with thermocouple feedback to provide constant temperature c o n t r o l . A l l shake flask tests were c a r r i e d out i n the incubator as well as a single batch 29 2.5L tank used for maintaining biomass stocks. Plates 2.1 and 2.2 show the incubator f a c i l i t y and apparatus. A 30 flask reciprocating shaker was employed to provide ag i t a t i o n / a e r a t i o n for a l l shake flask t e s t i n g . Wheaton 250 mL b a f f l e d shake f l a s k s , f i t t e d with foam stoppers to allow gas trans f e r , were used throughout the batch testwork. Figure 2.1 describes the 2.5L reactors and equipment employed for batch t e s t i n g . The 2.5L reactors were modified only s l i g h t l y for continuous b i o l o g i c a l tank leaching as described i n Section 2.2. Cyanidation testing was car r i e d out i n 600 mL beakers with magnetic s t i r r e r s or on a bo t t l e r o l l machine using 4 l i t r e nalgene narrow mouthed containers, depending on sample s i z e . A Corning model 125 pH/ORP meter was employed with combination electrodes, c a l i b r a t e d d a i l y . E x i s t i n g laboratory glassware and support equipment provided peripheral apparatus. 2.1.2 Standard Procedures. Batch shake flask t e s t i n g employed conventional b i o l o g i c a l c u l t u r i n g techniques as practiced at BC Research. Moist bulk sulphide concentrate was placed in 250 mL Wheaton flasks and repulped to approximately 10% s o l i d s with 9K (40) s a l t s solution (10 mL 1% KCl, 10 mL 0.144% Ca(N0 3) 2, 10 mL 30% (NH 4 ) 2 S 0 4 ' 1 0 m L 5 % MgS04.7H20, 10 mL 5% K 2HP0 4 into 1 L). The pH was adjusted to approximately pH 2 (using 12N H 2S0 4), the flask weight recorded, and shaken for 4 hours. The flasks were bulked up to P l a t e 2.1 W a l k - i n f u m e h o o d a n d b a t c h a p p a r a t u s P l a t e 2 .2 W a l k - i n f u m e h o o d a n d b a t c h a p p a r a t u s LABORATORY TANKS A I R / C O . Tank construction - plexiglass © Heavy duty stirrer- 115VAC, VM hp, variable speed © Immersion heater - 115VAC, 300W, c/w thermostatic control ® Shielded flowmeter - 0-2OOmL/minute © 316 ss 50mm o turbine propeller © 3l6ss 6.5 mm « tubing © 10x10x200mm plexiglass baffle F i g u r e 2 .1 L a b o r a t o r y 2 . 5 l i t r e b i o l e a c h r e a c t o r 32 t h e i r o r i g i n a l weight with d i s t i l l e d water as required, to correct for evaporation. I f the pH was stable ( ApH<0.2) the f l a s k s were inoculated with a stock culture of T h i o b a c i l l u s ferrooxidans, developed at BC Research on Equity sulphide concentrate. The f i n a l s o l u t i o n pH, Eh, and f l a s k weight was recorded and the f l a s k s were returned to the shaker for culture development and t e s t monitoring. The volume of inoculum varied considerably depending on the t e s t but was generally 5% by volume of the t e s t pulp. Batch tank t e s t i n g employed a s i m i l a r procedure except tank volume was c a l i b r a t e d and maintained constant rather than using t o t a l weight to correct for evaporation. The batch procedure was modified as each t e s t objective varied. The f l a s k s , or tanks, were sampled d a i l y to monitor the progress of biooxidation. A 1 mL s o l u t i o n sample was drawn, d i l u t e d 1:10 with pH 2.0 water and assayed by atomic absorption spectrophotometry (A.A,) for -dissolved metal concentration. The s o l u t i o n pH and Eh were recorded and the f l a s k s returned to the shaker. The % extraction for each t e s t was calculated, a f t e r c o r r e c t i o n for i n i t i a l dissolved Fe added by inoculum, using: % Ext. = (Fe i n sol'n x sol'n wt.) x 100 (Fe i n s o l i d x sample wt.) The e x t r a c t i o n data for As, Cu, etc., can be derived i n a s i m i l a r fashion. 33 Shake fl a s k and batch tank t e s t i n g was carried out concurrently to study: Apparatus Variables Tested Shake fl a s k Grind (p80) I n i t i a l pH Nutrient requirements Density 2.5 L tanks Temperature Air/CC^ requirements Degree of oxidation for enhanced extraction Cyanidation testwork, on the washed bioleach residues, followed conventional laboratory methods. The bioleach residue was repulped to 3 5% s o l i d s with process water. The pH was adjusted to 10.5 with Ca(OH) 2« A predetermined amount of NaCN was added which varied with each t e s t . The pulp was agitated v i o l e n t l y = for aeration. The pulp was sampled every hour and the r e s u l t a n t sample s o l u t i o n assayed f o r Au, Ag, Fe, Cu, free cyanide, and t o t a l cyanide. After a 6 hour leach, the f i n a l pregnant s o l u t i o n was f i l t e r e d from the s o l i d s , the s o l i d s washed, and a l l products assayed f o r the same elements as the hourly a l i q u o t s . The cumulative e x t r a c t i o n curves were back calculated using the hourly s o l u t i o n assays and f i n a l residue a n a l y s i s . 34 2.1.3 Data C o l l e c t i o n and Analysis. Figure 2.2 describes a t y p i c a l batch ex t r a c t i o n curve. The lag phase i s t y p i c a l only of batch t e s t i n g and represents an adjustment period for enzyme adaptation, as required, and population development. The accelerated oxidation phase i s quite d i s t i n c t from the lag phase and i s i n i t i a t e d following c r i t i c a l biomass development and s o l u t i o n redox p o t e n t i a l . The dormant phase occurs when the b i o l o g i c a l system i s l i m i t e d by substrate a v a i l a b i l i t y , nutrient l e v e l (such as oxygen) or when the t o x i c i t y of the environment becomes i n h i b i t o r y (such as high dissolved i r o n levels) (15,29,68). Accelerated biooxidation can be restored by adjusting the environment as required. The rate of biooxidation was indicated by the slope of the e x t r a c t i o n curve. It was the objective of the i n i t i a l batch testwork to maximize the rate of sulphide oxidation. Note that the pH declines as biooxidation progresses, due to H 2 S O 4 production, and the s o l u t i o n redox p o t e n t i a l generally varied inversely with pH. The e x t r a c t i o n curves f o r various t e s t conditions can be compared to determine the best conditions for rapid biooxidation. The lag phase was not p a r t i c u l a r l y s i g n i f i c a n t i n the comparative pl o t s as a lag period does not occur i n continuous biooxidation and therefore was not an important factor i n t h i s testwork. The author recommends that the comparative e x t r a c t i o n curves be adjusted for equal lag periods for ease of comparison and extrapolation of the r e s u l t s to continuous systems. Fe Extraction Curve (typical) / dormant / phase /biological Extraction / growth lag / ^ phase / Time (days) F i g u r e 2.2 T y p i c a l b a t c h e x t r a c t i o n c u r v e 36 2.1.4 Summary of Exploratory Batch Test Results Exploratory batch t e s t i n g was conducted to approximate the operating range of various bioleach parameters to provide an i n d i c a t i o n of p i l o t scale design requirements. Temperature. Figure 2.3 shows the e f f e c t of elevated temperature on Fe extraction. These tests were c a r r i e d out i n the 2.5 L tanks and encountered very long l a g periods, due to small inoculum dosage. The extraction curves indicated that 30 ° C, 35 ° C and 40°C a l l e f f e c t s i m i l a r leach rates a f t e r the lag period. At 4 5 ° C considerable foaming was experienced possibly i n d i c a t i n g b a c t e r i a l k i l l . It was not possible to t e s t lower temperatures with the laboratory apparatus as the oxidation r e a c t i o n maintained the tank temperature between 28-30°C with i n i t i a l heating provided by a g i t a t o r energy and warm a i r introduced by p e r i s t a l i c pump. pH. Figure 2.4 shows the e f f e c t of i n i t i a l pH on i r o n extraction. I n i t i a l pH did not a f f e c t the ultimate leach rate s i g n i f i c a n t l y . The most s a l i e n t e f f e c t of pH was the i n d i c a t i o n that between pH 1.3 and 1.5 the accelerated leach rate was i n i t i a t e d . Also, 10-20% sulphide oxidation (expected requirement f o r cyanidation enhancement) occured c o n s i s t e n t l y at approximately pH 1.0. P a r t i c l e Size. Figures 2.5 and 2.6 in d i c a t e that only a mild regrind step was necessary. The b i o l o g i c a l leach rates were i n s e n s i t i v e to regrind times over 10 minutes. Regrind times between 2 and 8 minutes showed s i m i l a r leach c h a r a c t e r i s t i c s to the 10 minute regrind except lag periods EFFECT OF TEMPERATURE 80 10 20 30 Time-days Figure 2.3 E f f e c t of temperature on batch iron extraction EFFECT OF pH Time-days Figure 2.4 E f f e c t of i n i t i a l pH on batch iron extraction 39 EFFECT OF REGffiND TIME 80 7<H Time-days F i g u r e 2.5 E f f e c t o f -regfind t i m e o n b a t c h i r o n e x t r a c t i o n E F F E C T O F R E G R I N D T I M E 80 T i m e - d a y s F i g u r e 2 . 6 E f f e c t o f r e g r i n d time o n b a t c h i r o n e x t r a c t i o n 41 were extended at the coarser grinds, probably due to reduced sulphide surface area a v a i l a b l e for biomass attachment. I t was decided that a 6 minute regrind was s u f f i c i e n t to allow reproducible laboratory bioleaching. Screen a n a l y s i s showed: 4 minute regrind - 80% passing 71 micrometers 6 minute regrind - 80% passing 56 micrometers 8 minute regrind - 80% passing 49 micrometers The p i l o t plant was designed to produce a regrind product sized at 80-90% passing 74 micrometers (200 mesh), adjusted by b a l l charge and m i l l throughput. Pulp Density. Figure 2.7 shows the e f f e c t of pulp density on Fe extraction rate, i n shake f l a s k t e s t s . Between 10.5% s o l i d s (w/w) and 13.6% there was a s i g n i f i c a n t decrease i n Fe e x t r a c t i o n rate. The "optimum" density i s dependent on other operating variables ( C G2' dissolved oxygen, agitation) and capital/operating costs and therefore w i l l be addressed i n d e t a i l during the f e a s i b i l i t y a n a l y s i s . I n i t i a l tank s i z i n g for p i l o t t e s t i n g was based on 10% s o l i d s which was the best estimate of an optimum given exploratory laboratory r e s u l t s and i s consistent with conditions reported i n the l i t e r a t u r e . Air/CCU Requirements. Figure 2.8 i n d i c a t e s that only a very small increase i n CO^ concentration resulted i n a s i g n i f i c a n t increase i n Fe extraction r a t e . Overdosing with CO2/ up to 360 mL/min/L pulp, did not appear to have any e f f e c t of Fe extracton rate, b e n e f i c i a l or detrimental. However, the increase i n extraction rate may not be 42 EFFECT OF PULP DENSITY 80 10 20 Time-days F i g u r e 2 . 7 E f f e c t o f p u l p d e n s i t y o n b a t c h i r o n e x t r a c t i o n EFFECT OF C 0 2 important at low extraction l e v e l s (eg.10%) and therefore the addition of CC^ must be evaluated on the basis of the incremental capital/operating costs of CC^ addition versus residence time. Aeration rate ( t y p i c a l l y 0.1 L/min/L pulp) w i l l a f f e c t CC^ requirements and therefore should be addressed in detailed f e a s i b i l i t y p i l o t testing where aeration and ag i t a t i o n more c l o s e l y represent plant conditions. It i s doubtful that CC^ w i l l be required on a plant scale. Small increases in the aeration rate and a i r d i f f u s i o n e f f i c i e n c y w i l l probably be more cost e f f e c t i v e than adding pure CC^. It was decided that testing aeration rate was more meaningful during continuous laboratory and p i l o t scale studies, which are discussed in Sections 2.2 and 3.2 resp e c t i v e l y . Nutrients. The extraction curves i n Figure 2.9 show that the leach rates were not p a r t i c u l a r l y s e n s i t i v e to 9K s a l t s olution d i l u t i o n s , including a 20% 9K so l u t i o n in tap water (chlorinated), although the lag period was uncommonly long. The r e s u l t s indicate that some nutrient was required to i n i t i a t e accelerated b i o l o g i c a l growth. It was decided to eliminate some of the components of the 9K so l u t i o n . Figure 2.10 showes that leach rates were in s e n s i t i v e to removal of Ca(NC>3)2, KCl, and MgSO^ . Eliminating K 2HSO^ resulted in a long lag period but comparable leaching rate. Removal of ( N H 4 ) 2 S O 4 apparently retarded b i o l o g i c a l leaching and therefore was studied further to determine the minimum requirements in fresh water. E F F E C T OF NUTRIENTS 70 10 20 Time-days F i g u r e 2 . 9 E f f e c t o f n u t r i e n t s o n b a t c h i r o n e x t r a c t i o n EFFECT OF NUTRIENTS u r e 2 . 1 0 E f f e c t o f n u t r i e n t s o n b a t c h i r o n e x t r a c t i o n Figure 2.11 indicates that very low concentrations of (NH^)2S0^were s u f f i c i e n t to promote accelerated i r o n e x t r a c t i o n . The e f f e c t of the water source was not anticipated and was studied further. Figure 2.12 indicates that water source could have a s i g n i f i c a n t e f f e c t on bioleach rates. Further shake f l a s k t e s t i n g i ndicated that water source was not a s i g n i f i c a n t f a c t o r , Figure 2.13. I n i t i a l r e s u l t s using various water sources might have been misleading due to i n s u f f i c i e n t b i o l o g i c a l adaptation. It was apparent that Fe extraction was not p a r t i c u l a r l y s e n s i t i v e to a r t i f i c i a l nutrient additions and therefore a common garden f e r t i l i z e r was tested as a replacement for 9K s a l t s s o l u t i o n . Figure 2.14 indicates that, i n the early stages of Fe e x t r a c t i o n (<20%), a f e r t i l i z e r (21.7.14) provided s u f f i c i e n t n u t r i e n t for low lev e l s of Fe extraction. Subsequent tests showed that 21.0.0 ((NH 4) 2S0 4) f e r t i l i z e r grade was e f f e c t i v e even at addition rates 0.1 g/L. Bioleachate Recycle. The r e s u l t s shown i n Figure 2.15 ind i c a t e that bioleachate recycle had a s i g n i f i c a n t e f f e c t on Fe extraction r a t e . It was apparent that r e c y c l i n g dissolved Fe and As had a deleterious e f f e c t on biooxidation. It i s advantageous to recycle at l e a s t some of the bioleachate to use the contained a c i d to adjust the pH of new feed on a continuous basis. These preliminary r e s u l t s may have been misleading due to i n s u f f i c i e n t b i o l o g i c a l adaptation to the high dissolved metals EFFECT OF NUTRIENTS F i g u r e 2 . 1 1 E f f e c t o f n u t r i e n t s o n b a t c h i r o n e x t r a c t i o n 49 EFFECT OF WATER SOURCE ure 2.12 E f f e c t of water source on batch iron extraction 3 U EFFECT OF WATER SOURCE 70 8 12 20 Time - days F i g u r e 2 . 1 3 E f f e c t o f w a t e r s o u r c e o n b a t c h i r o n e x t r a c t i o n 51 EFFECT OF FERTILIZER • control 9K . 2 g/L 21-7-14 o 10 " • 20 •• A 40 10 20 30 Time-days F i g u r e 2 . 1 4 E f f e c t o f f e r t i l i z e r o n b a t c h i r o n e x t r a c t i o n 52 EFFECT OF BIOLEACHATE RECYCLE 70 Figure 2.15 Ef f e c t of bioleachate recycle on batch iron extraction 5 3 i n s o l u t i o n . Additional t e s t i n g was c a r r i e d out on a continuous basis and i s summarized i n Section 2.2. Note that where complete oxidation of the contained sulphides i s necessary i t may be advantageous to exchange some of the bioleachate with fresh water during the biooxidation step to maintain a reasonable oxidation rate. The l i t e r a t u r e suggests 25 g Fe/L and 10 g As/L i s deleterious to b i o l o g i c a l growth (15) . Inoculum Source. After a few months of batch t e s t i n g the o v e r a l l e xtraction rate, for most batch t e s t s , appeared to decrease. It was f e l t that b i o l o g i c a l s t r a i n evolution may have been responsible for a decrease i n b i o l o g i c a l a c t i v i t y . Comparison of shake f l a s k tests using a stock culture developed at Equity with that maintained at BC Research i s summarized i n Figure 2.16. There was no s i g n i f i c a n t d i f f e r e n c e between the Fe extraction rates of the two s t r a i n s . The o v e r a l l change i n extraction rate during batch t e s t i n g was probably due to feed sulphide variance due to sampling of the impounded concentrate. Degree of Oxidation and Cyanidation. The bioleach residues from eight batch tank leaches were further processed to determine the cyanidation reponse at various degrees of p y r i t e oxidation as measured by i r o n extraction. The bioleachate was f i l t e r e d from the bioleach residue for assay by A.A. and for further processing. The t e s t results are summarized i n Table 2.1. 5 4 EFFECT OF INOCULUM SOURCE Time - days F i g u r e 2 . 1 6 E f f e c t o f i n o c u l u m s o u r c e o n b a t c h i r o n e x t r a c t i o n 55 TABLE 2.1 E f f e c t of oxidation l e v e l on cyanidation response. BIOLEACH EXTRACTION CYANIDATION RECOVERY % Fe % As % Au % Ag 6.0 44.5 66.6 27 .3 17.6 86.4 70.5 29 .1 30.9 94.8 65.2 41.4 32.0 95.1 69.5 39.1 34.6 96.0 69.0 63.5 60.7 97.8 66.9 8.4* 62.6 97.7 78.9 61.6 65.5 98.3 70.3 19.7* * high cyanide consumption, incomplete bioleach residue washi ng. Arsenic d i s s o l u t i o n appeared to be p r e f e r e n t i a l and e s s e n t i a l l y complete a f t e r only 30% of the t o t a l iron sulphide (including arsenopyrite) had been oxidized (as measured by Fe ex t r a c t i o n ) . The arsenic l e v e l in s o l u t i o n w i l l probably be the c o n t r o l l i n g parameter on a plant scale to determine the degree of sulphide oxidation necessary for enhanced gold extraction. The gold recovery by cyanidation appeared i n s e n s i t i v e to the degree of pyrite oxidation, whereas s i l v e r recovery appeared highly dependent on Fe extraction. It i s advantageous to minimize the degree of oxidation, at the expense of s i l v e r recovery, to minimize treatment costs of a c i d i c byproduct bioleachate constituents. Optimization of the s i l v e r recovery must be addressed i n d e t a i l during detailed f e a s i b i l i t y a n a l y s i s . 56 P i l o t scale design was based on 10% Fe extraction to enhance gold recovery. Typical leach rates i n batch tests showed 0.25% Fe/h extraction and therefore a 40 hour leach residence was incorporated into the p i l o t plant design. Washing the bioleach residue u n t i l the wash water was pH 3 appeared s u f f i c i e n t to prevent r e - d i s s o l u t i o n of acid soluble species following n e u t r a l i z a t i o n and cyanidation. The major constituent i n the cyanide s o l u t i o n other than gold and s i l v e r was copper which should not present any problems during gold r e f i n i n g steps downstream (CIP, CIL, or zinc p r e c i p i t a t i o n ) . NaCN requirements less than 1 kg NaCN/t concentrate were indicated as very low consumptions were experienced at the lower Fe oxidation le v e l s ( < 30% Fe e x t r a c t i o n ) . Higher Fe extractions would require a much more intense washing step p r i o r to cyanidation to minimize cyanide consumption. Cyanide leach durations following bioleaching w i l l probably be 4 hours or l e s s ; - as prolonged leaching did not e f f e c t a d d i t i o n a l extraction. An example of the gold and s i l v e r e x traction p r o f i l e t y p i c a l of cyanidation t e s t i n g a f t e r bioleaching i s shown i n Figure 2.17. Bioleachate Treatment. Following biooxidation the bioleachate must be washed from the bioleach residue p r i o r to cyanidation. Due to the low p y r i t e oxidation required the p a r t i c l e s i z e of the reground sulphide concentrate did not change s i g n i f i c a n t l y . Therefore conventional s o l i d / l i q u i d separation techniques were employed. 57 FIGURE 2.17 Au Ag 1.0 10i 2 TIME h 4 F i g u r e 2.17 T y p i c a l cumulative p r e c i o u s metals e x t r a c t i o n p r o f i l e by d i r e c t c y a n i d a t i o n of washed b i o l e a c h res idue 58 The most important requirements of the dewatering equipment are high throughput and e f f i c i e n t cake washing. For t h i s p a r t i c l e s i z e range (80% passing 50 urn) a drum or b e l t f i l t e r can be selected. For f i n e r sizes or more viscous material (high l e v e l of biooxidation) a pressure f i l t e r would probably be employed. Note that sol u t i o n v i s c o s i t y increases as the I^SO^ content of the bioleachate increases. A t y p i c a l bioleachate solution, a f t e r 10% p y r i t e oxidation, might assay: 0.01 mg/L Au 0.10 mg/L Ag 150 mg/L Cu 5-10.5 g/L Fe 3 g/L As 50 mg/L Sb 30-40 g/L S0 4(as H2SC>4) The precious metal content of the bioleachate increases as the degree of biooxidation increases, probably due to an increase i n the biomass content of the s o l u t i o n and bioaccumulation of precious metals by the s o l u t i o n biomass. Normally, the biomass i s r e s t r i c t e d to the s o l i d s f r a c t i o n of the mixture. The dissolved metal l e v e l s , with the exception of arsenic, are s i m i l a r to acid mine drainage presently co l l e c t e d and neutralized with lime at Equity. Following hydroxide sludge removal the water can be discharged to the surrounding 59 watershed or recycled to the m i l l i n g process. Recent testwork at Equity has shown that mixing acid mine drainage sludge with m i l l t a i l i n g s l u r r y at a r a t i o of 1:10, or possibly higher, provides a mixture that i s exempt under the Special Waste Act of B r i t i s h Columbia, and therefore can be stored i n a closed t a i l i n g system as produced. The arsenic, i n the bioleaching environment, dissolves as a highly soluble ortho arsenidus acid (H 2 AsO 3 ) which i s oxidized i n the presence of f e r r i c i r o n to ortho arsenic acid ( H 2 A s 0 4 ) i f the pH<3 and the Eh>370 mV (S.C.E.)(7.2) . The reaction products can be summarized. (Pyrite) F e + 2 — F e S 0 4 ^ F e 2 (S0 4 ) 3 Fe (OH) 3 Z HFe 3(OH) 6(S0 4) 2 , J - ^ A s 2 O 5 ^ 2 0 _ H 3 A s 0 4 The arsenate species i n so l u t i o n represents p o t e n t i a l l y 30 kg As/t sulphide concentrate oxidized, which requires s p e c i a l a t t e ntion environmentally. The po t e n t i a l f o r a marketable arsenic co-product i s small. The simplest, and cheapest, route to pursue i s disposal of arsenic as a low s o l u b i l i t y arsenate compound. Calcium arsenate, produced by adding lime to H^As04 i n so l u t i o n , must be maintained i n a licensed disposal area due to high s o l u b i l i t y i n water. F e r r i c arsenate (FeAsO 4 ), on the other hand, has a s o l u b i l i t y of 0.15 mg/L at pH 7 and therefore i s stable for disposal with t a i l i n g s l u r r y and a c i d mine drainage sludge at reasonably low mixing r a t i o s . 60 In the presence of excess f e r r i c i r o n ( at l e a s t 3:1 + 3 + 5 Fe :As ) n e u t r a l i z a t i o n of the bioleachate to pH 3 w i l l p r e c i p i t a t e FeAsO^, further excess a l k a l i n i t y to pH 9.5 w i l l remove the remaining metals i n solution, i n the presence of f e r r i c iron, as a l k a l i n e stable hydroxide p r e c i p i t a t e s . The i r o n i n s o l u t i o n for arsenic cementation can be controlled during the biooxidation step. Additional information on the hydrometallurgy of arsenic by-products and disposal considerations i s referenced (72-85). 2.1.5 Application of Results The batch laboratory t e s t r e s u l t s were used to provide an i n d i c a t i o n of the operating range of a continuous bioleach c i r c u i t . The r e s u l t s suggested the important variables f o r control and the approximate operating values that would maintain accelerated b i o l o g i c a l growth. It could only be assumed at t h i s stage of the project that batch shake f l a s k and 2.5L tank t e s t i n g provided a reasonable estimate of bioleaching on a continuous basis. 2.2 Continuous Laboratory Testing The 2.5L reactors used for batch testwork were modified s l i g h t l y to allow continuous overflow and gravity t r a n s f e r between reactors for continuous t e s t i n g . The objective of the continuous laboratory scale testwork was to determine the e f f e c t of scaleup from batch shake flask and tank t e s t i n g to continuous bioleaching on the biohydrometallurgy and resultant precious metals hydrometallurgy. The continuous tank t e s t i n g 61 LABORATORY APPARATUS air 1 Nutrient solution stock tank 15Omm0 x 150mm 2 Concentrate stock tank 15Omm0 x 220mm 3 Variable speed peristaltic pumps a concentrate slurry b nutrients c pulp recycle d air sparging 4 Bioleach reactors 5 Residue receiver 6 Timers 7 pH/ORP meter F i g u r e 2 . 1 8 L a b o r a t o r y s c a l e c o n t i n u o u s b i o l e a c h a p p a r a t u s 62 was also important i n defining an operating strategy and acquiring operating experience to a i d i n p i l o t plant design and operation. 2.2.1 Apparatus The i n i t i a l continuous laboratory scale t e s t apparatus i s described i n Figure 2.18. I t was thought, i n i t i a l l y , that adequate pH and biomass control could be maintained by r e c i r c u l a t i n g a portion of bioleach pulp from the l a s t reactor to the f i r s t . However, t h i s strategy d i d not appear p r a c t i c a l for plant scale operations. Instead, the i n i t i a l pH was controlled by adjusting the d i l u t i o n water (nutrient solution) to a desired pH using H 2 S O 4 and the residence time of the f i r s t reactor controlled for biomass maintenance. Two gravity-flow-through reactors (2.5 L each) were employed f o r continuous t e s t i n g . A stock tank of reground concentrate s l u r r y (30% s o l i d s , p80 = 75 jam) and a nutrient sp.lution tank provided continuous feed at 10% s o l i d s v i a variable speed p e r i s t a l t i c pumps. The bioleach residue was colle c t e d continuously for subsequent processing as required. The basic apparatus and operating conditions were modified as required f o r each t e s t objective. 2.2.2 Standard Procedures The continuous reactors were i n i t i a t e d as two batch reactors exactly as described i n Section 2.1.2, using fresh inoculum from shake flask tests i n an advanced state of sulphide oxidation. The batch phase was allowed to progress 63 well beyond target values of expected continuous oxidation lev e l s for best r e s u l t s . Target values were: Reactor 1 - 5g/L Fe, 3g/L As Reactor 2 - 10g/L Fe, 3.5g/L As and the rate by which these l e v e l s were reached was controlled by i n i t i a l inoculum dosage. After target values of dissolved i r o n and arsenic were attained the continuous feed was i n i t i a t e d . It was found that i n i t i a t i n g at a low feed rate provided more predictable continuous operating than s t a r t i n g at higher feed rates, close to the l i m i t of the apparatus. As the b i o l o g i c a l population developed the feed rate was slowly increased to the desired operating rate. The c r i t e r i a for successsful continuous operation was an i n d i c a t i o n of steady state operation for a period of 3-4 days. Unsuccessful operation was t y p i c a l l y indicated by b i o l o g i c a l 'washout'; a "rapid decline i n the l e v e l of sulphide oxidation, as indicated by pH, Eh, and dissolved Fe and As. 2.2.3 Data C o l l e c t i o n and Analysis Table 2.2 indicates t y p i c a l laboratory continuous bioleach data. Figure 2.19 i s a sample plot of the i r o n and arsenic extraction data from Table 2.2. In addition to the variables already indicated, the following parameters were monitored on a d a i l y basis: (i) feed rate, density and s o l i d s / s o l u t i o n assay (Fe,As) ( i i ) residue t o t a l SO. and s o l i d s Fe and As content TABLE 2.2 Typical Continuous Laboratory Bioleach Data DAY RESIDENCE FEED SOLIDS REACTOR 1 REACTOR 2 % EXTRACTION (h) %Fe %As pH Eh Fe As pH Eh Fe As Fe As 1 80 43. 3 2 4 1 . 1 590 4 480 1470 1 . 0 612 8600 1740 17 . 6 63 2 2 41. 4 2 9 1 . 1 583 4580 1390 1 . 0 610 8620 1710 18 . 4 52 0 3 39 . 0 2 2 1 . 1 591 3700 1320 1. 0 613 8200 1800 18. 5 73 7 5 39 . 0 2 2 1 . 1 595 4180 1350 1 . 0 616 8780 1700 19 . 9 68 8 8 38 . 8 2 1 1 . 1 605 4 280 1 480 1 . 0 620 8680 1780 19 . 6 72 7 9 60 38 . 0 2 2 1 . 1 595 4140 1490 1. 0 6 09 7920 1810 18 . 0 73 3 10 43. 5 2 4 1 . 1 603 4668 1 470 1 . 0 618 9240 1 480 21. 1 59 4 11 4340 1450 8 640 1800 19 . 7 73 3 1 2 39 . 0 2 2 1 . 1 606 4100 1400 1 . 0 624 8760 1790 19 . 9 72 3 15 1 . 1 610 5250 1190 0. 9 629 10050 1770 22. 9 71 3 18 38 . 7 2 3 1 . 1 611 4700 1450 0 . 9 629 93 20 1780 21 . 2 72 3 19 38 . 8 2 2 1 . 1 615 5010 1340 0 . 9 629 9900 1740 22. 6 70 3 22 38 . 8 2 2 1 . 1 618 5210 1360 0 . 9 629 9840 1870 22. 4 75 2 24 39 . 0 2 1 1 . 1 618 5600 1570 0 . 9 630 11000 1920 25. 0 78 2 25 40 40. 7 2 1 1 . 2 576 4 880 1560 1. 0 615 10960 2040 22. 1 82 0 26 40. 7 2 1 1 . 2 593 4080 1510 1. 0 616 9930 1980 20. 5 81 4 DENSITY = 10%, NUTRIENTS = 9K, FEED pH = 1.3, AIR = 300 mL/MIN/REACTOR, Eh(mV), Fe/As (mg/L) DECEMBER CONTINUOUS TANK BIOLEACHING 4 8 12 16 20 24 Time - days F i g u r e 2 . 1 9 T y p i c a l c o n t i n u o u s b i o l e a c h e x t r a c t i o n 66 ( i i i ) n utrient s o l u t i o n feed rate (iv) dissolved oxygen i n each reactor (intermittent depending on test) The extraction rate was calculated based on a simple unit mass balance i n the bioleach c i r c u i t , corrected for lag time as required due to variations i n feed s o l i d / s o l u t i o n assay and residence time. The e x t r a c t i o n data based on so l u t i o n assays, and so l u t i o n pH and Eh, provided the d a i l y indicators for c i r c u i t c o n t r o l . When increasing biooxidation l e v e l s were indicated (increasing metal extraction, decreasing pH, increasing Eh) then the feed rate was increased or some other variable of i n t e r e s t was adjusted. There was usually an i n i t i a l drop i n extraction l e v e l a f t e r a c i r c u i t change, possibly due to b i o l o g i c a l shock. However, steady state operation was usually restored and the te s t continued. 2.2.4 Summary of Exploratory Continuous Testing B i o l o g i c a l Washout. The f i r s t continuous test, a f t e r a number of p r a c t i c e runs for f a m i l i a r i z a t i o n , was conducted to estimate the b i o l o g i c a l 'washout' c h a r a c t e r i s t i c s i n the f i r s t reactor. Figure 2.20 summarizes the re s u l t s of t h i s f i r s t t e s t . P r e f e r e n t i a l As d i s s o l u t i o n i s apparent i n Figure 2.20. The r e s u l t s i n d i c a t e that with <20 hours residence i n the f i r s t reactor b i o l o g i c a l washout occurred. That i s , the residence time at washout i s less than the doubling time of the bacteria, r e s u l t i n g i n biomass washing from the reactor on a CONTINUOUS LAB BIOOXIDATION RESIDENCE / 8Q ,6C> 20 _^ 25 , 2 0 i 15 TANK ^ ^ 1 ^ 4 8 12 16 20 24 DAYS F i g u r e 2 . 2 0 E f f e c t o f r e s i d e n c e t i m e o n c o n t i n u o u s b i o l e a c h e x t r a c t i o n 68 continuous basis. These data provided the design basis for p i l o t and plant scale reactor configuration, i n d i c a t i n g a constraint on the residence time of the f i r s t reactor. Bioleachate Recycle. Figure 2.21 indicates the e f f e c t of bioleachate recycle on continuous biooxidation. Recycling bioleachate apparently shocked the b i o l o g i c a l system but recovery of accelerated sulphide oxidation was evident following an adaptation period. Approximately 50% bioleachate recycle would be s u f f i c i e n t to maintain new feed a c i d i t y i n the f i r s t reactor on a plant scale and therefore additional recycle was not tested. Note that s o l u t i o n assay corrections must be made during bioleachate recycle to account for c i r c u l a t i n g dissolved Fe and As p r i o r to extraction c a l c u l a t i o n s . Note also that b i o l o g i c a l systems v/here higher levels of p y r i t e oxidation are necessary may require bioleachate 'polishing' p r i o r to recycle to p r e c i p i t a t e a portion of the dissolved Fe and As so that a high c i r c u l a t i n g - l o a d of dissolved i r o n and arsenic does not i n h i b i t biooxidation. Nutrients. Figure 2.22 shows the r e s u l t s of a continuous laboratory bioleach t e s t where stepped removal of various nutrient s a l t s and the e f f e c t of water source was investigated. The data summarized i n Figure 2.22 confirms batch data, i n d i c a t i n g that most of the conventional nutrients can be removed without longterm adverse a f f e c t s on biooxidation. Fresh water appeared to 'shock' the system dramatically, as did phosphate removal. However, a short batch phase allowed 69 CONTINUOUS LAB BIOOXIDATION 0 10 20 30 DAYS Figure 2.21 E f f e c t of bioleachate recycle on continuous b i o l o g i c a l tank leaching, (a) 25% recycle, (b) 50% recycle 7 0 CONTINUOUS L A B BIOOXIDATION R E S I D E N C E < & - H - I H B A T C H (h) a b e d e f g 20 4 0 6 0 8 0 DAYS Figure 2 . 2 2 E f f e c t of nutrients and water source on continuous biooxidation. (a) Ca source removed, (b) Mg source removed, (c) K source removed, (d) PO^ source removed, (e) PO. restored, fresh water replaced d i s t i l l e d , (f) b i o l o g i c a l a c t i v i t y recovered,(g) no recovery indicated, flushed tanks 71 complete recovery of b i o l o g i c a l a c t i v i t y i n fresh water. Addition of process water, containing elevated thiocyanate l e v e l s appeared to i n h i b i t biooxidation i r r e v e r s i b l y , even a f t e r batch leaching for 5 days. The e f f e c t of process water on biooxidation i s discussed further i n Section 4.0. Density. Figure 2.23 shows the re s u l t s of a continuous t e s t from i n i t i a t i o n , during batch phase, to washout at 25% so l i d s (w/w). Previous tests were carried out at 10% s o l i d s . The data indicates that biooxidation i s p r a c t i c a l at 25% s o l i d s . However, 50 hours residence (25 h per tank) was required to maintain continuous oxidation at 25% s o l i d s , rather than 40 hours at 10% s o l i d s . The s a l i e n t r e s u l t from t h i s data was a s i g n i f i c a n t increase i n oxygen effeciency for sulphur oxidation: 0.10 L air/minute/L pulp at 10% solids (40h) = 12.3% O- e f f i c i e n c y 0.10 L air/minute/L pulp at 25% solids (50ft) = 26.7% 0 2 e f f i c i e n c y (based on 40% S feed, 10% t o t a l S oxidation during bioleach residence) The a d d i t i o n a l residence time required at 25% solids might be due to oxygen depletion or CO,, depletion or simply mechanical a t t r i t i o n of the biomass. Note that the nutrient addition rate was increased at 25% solids to maintain 0.6 kg (NH 4) 2S0 4/t of concentrate and 0.12 kg K 2HPC>4/t concentrate RESIDENCE (h) CONTINUOUS LAB BIOOXIDATION .BATCH — 8 Q m _ l 6Q - , 50 , 4 0 , WASH m IJ 8 0 20 -V 4 0 6 0 DAYS F i g u r e 2 . 2 3 C o n t i n u o u s b i o o x i d a t i o n a t 25% s o l i d s 73 Cyanidation. The r e s u l t s of cyanidation tests (bottle r o l l ) on the washed bioleach residues fom various continuous bioleach tests are summarized i n Table 2.3. TABLE 2.3 E f f e c t of oxidation on cyanidation res pons e. BIOLEACH EXTRACTION CYAN I DAT I ON RECOVERY %Fe %As %Au %Ag 5.4 71.9 69 39 5 . 1 52.0 66 34 18.4 68.6 52 54 19. 7 73.3 57 43 19.9 72.3 58 41 22.1 82. 0 69 39 22.6 70.3 56 30 Again, the bioleach residue was washed u n t i l the f i l t r a t e was pH 3, repulped to 3 5% s o l i d s , and leached i n the presence of 0.5 kg NaCN/t s o l u t i o n . Cumulative extraction p r o f i l e s indicated that cyanide extraction was e s s e n t i a l l y complete a f t e r 4 hours. A d d i t i o n a l cyanide and residence time did not improve extraction s i g n i f i c a n t l y . Incremental i r o n extraction did not r e s u l t i n an incremental improvement i n gold extraction and acutually appeared deleterious, the reason for which was not determined. Bioleachate C h a r a c t e r i s t i c s . The bioleachate from continuous bioleaching was s i m i l a r to that produced i n batch testwork, at comparative l e v e l s of o v e r a l l sulphide oxidation. Dewatering c h a r a c t e r i s t i c s of the continuous bioleach residue appeared consistent with batch testwork, as indicated on laboratory scale f i l t e r i n g apparatus. 7 4 2.2.5 Ap p l i c a t i o n of Results The r e s u l t s of continuous laboratory bioleach testwork indicated that shake flask t e s t i n g provided a reasonable estimate of continuous bioleach operating parameters. However, the r e s u l t s also indicated that continuous t e s t i n g provides more confident data where rapid b i o l o g i c a l adaptation i s required, such as bioleachate recycle and high density testwork. The author recommends that shake flask t e s t i n g be employed only for s t r a i n development due to space and cost l i m i t a t i o n of continuous tank t e s t i n g . Continuous laboratory t e s t i n g provides a better i n d i c a t i o n of continuous operating strategy and allows f o r rapid evaluation of various bioleach parameters without any lag period. Based on the continuous t e s t data the c r i t i c a l operating parameters f o r consideration during p i l o t t e s t i n g were: (-i)•••-< residence time i n the f i r s t reactor ( i i ) i n i t i a l pH control ( i i i ) aeration rate and pulp density (iv) t o t a l residence time and degree of sulphide oxidation (v) n u t r i e n t s . 75 3.0 SUMMARY OF THE PILOT SCALE BIOLEACH STUDY The laboratory t e s t r e s u l t s summarized i n Section 2.0 were used as the baseline operating parameters for p i l o t c i r c u i t design. The equipment s i z i n g ( p i l o t plant capacity) was constrained by budget allowance and a v a i l a b l e f l o o r space i n the Equity concentrator basement. The primary objectives of the p i l o t scale t e s t i n g were: (i) determine the e f f e c t of scale on biohydrometallurgy, ( i i ) produce s u f f i c i e n t bioleach product (residue and bioleachate) for downstream process d e f i n i t i o n and confident design and cost parameters for a plant scale f e a s i b i l i t y analysis, ( i i i ) gain operating experience with a large scale continuous biooxidation c i r c u i t , under less than i d e a l conditions, to determine the c r i t i c a l operating parameters and control strategy to a i d i n plant scale conceptual, design and costing. 3.1 PILOT PLANT DESCRIPTION Flowsheets 3.1 and 3.2 summarize the p i l o t plant flowsheet and general arrangement respectively. The o v e r a l l design constraint was budget allowance. By an i t e r a t i v e process the unit process s i z i n g , based on Flowsheets 3.1 and 3.2, was determined to maximize the p i l o t c i r c u i t capacity and meet budget constraints. Table 3.1 describes the major equipment items used i n the p i l o t c i r c u i t . The underlying design c r i t e r i a was 2 tonne per PILOT PLANT FLOWSHEET FEED 50% solids 4.5m] reactors c/w1.5hp agitators 4 C ft I • I 1 L liquid bioleachate roceiver STOCK TANK REGRIND MILL BELT FILTER wash water receiver J' water nulripnt -solution -H2S04 10%solids -air -air -water solid -water CYANIDATION CIRCUIT T>Teed MILL TAILINGS SUMP F l o w s h e e t 3 . 1 P i l o t p l a n t f l o w s h e e t PILOT PLANT GENERAL ARRANGEMENT owsheet 3.2 P i l o t plant general arrangeme 78 TABLE 3.1 P i l o t p l a n t equipment d e s c r i p t i o n and approximate cos t ITEM NO.REQ'D DESCRIPTION $CDN 1 1 76cm x 91cm b a l l m i l l c/w 7.5 hp motor on a d j u s t a b l e mounting, gear reducer, chain d r i v e n on r o l l e r , feed and d i s -charge chute, grade d i s c h a r g e . 93 24 2 9 Polyethylene c y l i n d r i c a l tank, 1 cm w a l l c/w (8) 2.5cm and (8) 5.0 cm PVC bulk head adapters. No l i d no reinforcement 8100 3 9 Por t a b l e clamp mount a g i t a t o r c/w 1.5 hp motor, 3.8 cm x 183 cm s h a f t , 316 s t a i n l e s s s t e e l , 350 rpm output 20700 4 18 30.5 cm, 3 p i t c h blade, open p r o p e l l e r s f o r 3.8 cm s h a f t , s t a i n -l e s s s t e e l , 55F rubber covered blades 4500 5 3 1.9 cm Denver v e r t i c a l sand pump, 1 hp, c/w rubber covered i m p e l l e r from P l a c e r Research 0 6 1 FISHER-PORTER i n d i c a t o r f l o w r a t e r model 10A3555A 530 7 1 YSI d i s s o l v e d oxygen meter c/w combination probe and membrane k i t 1000 8 8 S t a i n l e s s s t e e l immersion h e a t e r s , 5kW each c/w thermostat c o n t r o l 3 200 9 8 Pyrotenax heat tape (P/N D-R195-2-18-3200-240-3-14-X) 1400 10 8 Nopol a i r d i f f u s e r , model HKP 600-1 560 11 1 Feed d i s t r i b u t o r , f a b r i c a t i o n only ( d r i v e and c o n t r o l l e r from e x i s t i n g i nventory) 5 00 12 Various PVC f i t t i n g s 79 TABLE 3.1 (continued) ITEM NO.REQ'D DESCRIPTION $CDN 13 Various p o l y e t h y l e n e hose, diameter 5 cm 14 15 Various p o l y e t h y l e n e inventory) tanks (from pH probe, t r a n s m i t t e r , a n a l y z e r / c o n t r o l l e r , p u l s e d u r a t i o n t r a n s -m i t t e r , metering pump f o r H2SO4 (from inventory) 16 P e r i s t a l t i c pump f o r n u t r i e n t d e l i v e r y (from l a b o r a t o r y inventory) 17 Denver lm h o r i z o n t a l b e l t f i l t e r (316 L s t a i n l e s s s t e e l ) c/w 2 hp v a r i a b l e frequency d r i v e , a s s i s t u n i t w i t h 4 r e c e i v e r s , f i l i t r a t e pump w i t h 2 hp motor, and vacuum pump c/w 15 hp motor, s k i d mounted. P i p i n g not inc l u d e d 1400/mo 18 F i l t e r c l o t h f o r Denver lm h o r i z o n t a l b e l t f i l t e r . 4511 poly-propylene ^needle f e l t , 25-30 cfm, 16 oz. wt., 2RK65 C l i p p e r J o i n t 100 19 100kg Potassium phosphate (monobasic) 120 20 500kg Ammonium sulphate, f e r t i l i z e r grade 120 80 day c i r c u i t c a p a c i t y a t 10% s o l i d s and 40 hour residence, g r i n d i n g from 80% passing 200 um to 80% p a s s i n g 50 um. The b a l l m i l l was o v e r s i z e d t o allow use i n other p r o j e c t s a t E q u i t y . The g r i n d was adjusted by b a l l charge and throughput v a r i a t i o n . An open c i r c u i t g r i n d was chosen f o r ease of o p e r a t i o n on the s m a l l s c a l e . D e t a i l s of the b a l l m i l l are i n d i c a t e d i n P l a t e 3.1. The p o l y e t h y l e n e tanks were chosen based on c o s t , immediate a v a i l a b i l i t y , and s h o r t d u r a t i o n of the p r o j e c t . Polyethylene tanks do not normally perform w e l l i n an a b r a s i v e environment f o r extended p e r i o d s . A simple bridge of 5 cm angle i r o n was i n s t a l l e d on each tank f o r a g i t a t o r support. One a i r d i f f u s e r was mounted a t the base of each tank and the a g i t a t o r angled over the d i f f u s e r . Each a i r l i n e had a manual c o n t r o l v alve and was f i t t e d w i t h quick-connect a i r f i t t i n g s t o a l l o w i n s t a l l a t i o n of the f l o w r a t e r on each a i r l i n e as *req"uired ( r a t h e r than purchasing numerous f l o w r a t e r s ) . I n i t i a l l y , c o n v e n t i o n a l 5kW s t a i n l e s s s t e e l (316) immersion heaters were i n s t a l l e d i n the f i r s t f i v e tanks, however, severe c o r r o s i o n occurred a t the a i r - l i q u i d i n t e r f a c e . Subsequently, 20cm diameter x 2m s t a i n l e s s s t e e l pipes were f i t t e d i n the f i r s t f i v e r e a c t o r s , f i l l e d w i t h used t r a n s m i s s i o n f l u i d , and heated u s i n g 3.2 kW m i l d s t e e l heat tape (4m wrapped around a c o r e ) . D e t a i l s of the b i o l e a c h r e a c t o r s are shown i n F i g u r e 3.1. The arrangement of the f i r s t s i x r e a c t o r s i s i n d i c a t e d i n P l a t e 3.2. a t e 3 . 2 P i l o t p l a n t b i o l e a c h r e a c t o r s 1 -6 u n d e r c o n s t r u c t i o n 82 FIGURE 3.1 PILOT PLANT REACTOR (NTS) POLYETHYLENE TANK 1750 AGITATOR 350 rpm 1.5 hp 38 mm0 SHAFT 316ss 305 mm <f PBT 55F RUBBER COVERED NOPOL HKP 600-1 AIR DIFFUSER 5kW ss IMMERSION HEATER c/w THERMOSTAT CONTROL 6mm x 150mm PLYWOOD BAFFLE 19 mm PVC BALL VALVE •19mm QUICK CONNECT AIR FITTINGS H SUPPORT 50mm STEEL ANGLE 6mm PLYWOOD COVER F i g u r e - 3 . 1 P i l o t p l a n t b i o l e a c h r e a c t o r d e t a i l : 83 A l l s l u r r y t r a n s p o r t used PVC or polyethylene l i n e s . The 2 b i o l e a c h r e s i d u e reported t o a 1 m h o r i z o n t a l b e l t f i l t e r . This u n i t was g r e a t l y o v e r s i z e d f o r t h i s a p p l i c a t i o n but was chosen due t o the l i m i t e d a v a i l a b i l i t y of r e n t a l u n i t s . D e t a i l s of the u n i t are evident i n P l a t e s 3.3 and 3.4. Feed r a t e to the b a l l m i l l was c o n t r o l l e d by an a i r actuated p i n c h v a l v e on a c l o s e d loop w i t h the stock tank which was c y c l e d by s o f t timers from a Modicon Micro 84 programmable l o g i c c o n t r o l l e r . 3.2 PROCEDURES Due to the e x p l o r a t o r y nature of the p i l o t t e s t i n g there were no f i x e d o p e r a t i n g procedures. G e n e r a l l y , s t a r t u p followed procedures used i n the l a b o r a t o r y . The f i r s t four r e a c t o r s were f i l l e d w i t h f r e s h feed, n u t r i e n t added, and the pH adjusted w i t h F^SO^ and s t a b i l i z e d . Approximately 100 l i t e r s of stock inoculum, from a 500 l i t r e continuous stock development tank i n the l a b o r a t o r y , was added and the r e a c t o r s allowed t o batch. S i m i l a r to l a b o r a t o r y t e s t s , a f t e r a t a r g e t l e v e l of s u l p h i d e o x i d a t i o n was a t t a i n e d f r e s h feed was introduced t o the f i r s t four r e a c t o r s ( e s s e n t i a l l y Reactor 1) a t a slow r a t e and i n c r e a s e d as b i o l o g i c a l a c t i v i t y warranted. P l a t e 3 . 4 P i l o t p l a n t b e l t f i l t e r 85 TABLE 3.2 Bioleach p i l o t plant operating data - day s h i f t Date: 01 July Stock Tank: pH/Eh: 3.5/340 ppm Fe: 520 ppm As: 80 density (%): 53 B a l l M i l l : feed (mL/min): 1800 discharge density (%): 50 media charged (kg): 0 p a r t i c l e size (p80 urn): 70 Water: source: fresh feed (mL/min): 7900 Nutrient: s o l u t i o n : 21.0.0 + P0 4 feed (mL/mi n): 5 0 Bioleach: Tank A i r dO„ Temp Solution (ppm) Solid (%) No. (% flow) (ppm) ( C) pH Eh Fe As Ag Fe As 1 - . ,25. 2.1 28 1.2 2 25 1.4 30 1.1 3 25 3.8 . 27 1.2 4 25 3.6 30 1.1 5 25 1.4 30 1.1 6 7 25 3.0 26 1.1 1 8 535 5950 1230 0. 02 38 0 .4 529 7050 1510 0. 02 37 0 .8 518 7300 1840 0. 02 38 0 .8 530 9400 1855 0. 04 40 0 .7 557 8600 1985 0. 08 37 0 .5 584 9100 2105 0. 19 37 0 .4 AGITATOR DOWN -SPARE 8 6 TABLE 3.3 Bioleach p i l o t operating data - night s h i f t Da te: 01 July S h i f t : B nights Time Stock Tank B a l l M i l l Bioleach Agit Pump Loop Densi ty Wa ter Nutrient Agi t Flow pH 2000 + + + 80 8000 50 + + 1.5 2100 + + Plug 80 + + + + + 2200 + + + 60 + + + + + 2300 + + + 50 + + + + + 2400 + + + + + + + + + 0100 + + + + + + + + + 0200 + + + + + + + + + 0300 + + + + + + + + + 0400 + + + + + + + + + 0500 + + + + + + + + + 0600 + + + + + + + + + 0700 + + + + + + + + + 87 3.3 DATA COLLECTION AND ANALYSIS Two t e c h n i c i a n s were dedicated to p i l o t c i r c u i t o p e r a t i o n to provide o p e r a t i n g s u p e r v i s i o n 12 hours per day, 7 days per from the m i l l o p e r a t i n g crew checked the c i r c u i t each hour t o ensure continuous o p e r a t i o n . Table 3.2 and 3.3 are examples of d a i l y data c o l l e c t i o n d u r i n g continuous p i l o t o p e r a t i o n f o r day s h i f t and n i g h t s h i f t r e s p e c t i v e l y . The data were used t o c a l c u l a t e c i r c u i t throughput, d e n s i t y , residence time and l a g time t o f a c i l i t a t e continuous e x t r a c t i o n c a l c u l a t i o n s s i m i l a r to t h a t discussed i n S e c t i o n 2.2. The e x t r a c t i o n data, along w i t h pH and Eh, served as the d a i l y i n d i c a t o r of c i r c u i t s t a t u s . 3.4 SUMMARY OF METALLURGICAL RESULTS The b a l l m i l l media charge was s e t usi n g 50% s o l i d s feed to the g r i n d i n g m i l l a t 1.8 L/minute (2.1 t p d ) : -TABLE 3.4 B a l l m i l l media t e s t i n g B a l l Charge (kg) %-74um i n product week. During the n i g h t s h i f t the g r i n d i n g c i r c u i t o perator 80 57.4 90 64. 2 110 77.1 120 77.9 8 8 Obviously, a t lower feed r a t e s the g r i n d would be f i n e r . However, the c i r c u i t was c a l i b r a t e d f o r approximately 2 tpd throughput. L a b o r a t o r y t e s t i n g i n d i c a t e d t h a t the b i o o x i d a t i o n r a t e was not a f f e c t e d s i g n i f i c a n t l y by f i n e r g r i n d i n g . During b a l l m i l l t e s t i n g a number o f b e l t f i l t e r t e s t s were c a r r i e d out t o determine the f i l t e r c a p a c i t y a t v a r i o u s g r i n d s . The c o n c l u s i o n s from t h i s testwork a r e d i s c u s s e d below. F i g u r e 3.2 summarizes the e x t r a c t i o n d a t a f o r the f i r s t continuous p i l o t run. D e t a i l e d data i s shown i n Table 3.5. The f i r s t 36 days showed a slow batch p e r i o d , r e q u i r i n g i n t e r m i t t e n t s u l p h i d e feed and the i n s t a l l a t i o n o f immersion h e a t e r s t o a c c e l e r a t e the batch phase. S i m i l a r t o l a b o r a t o r y r e s u l t s , a c c e l e r a t e d b i o o x i d a t i o n was i n d i c a t e d a t <pH 1.5 and >500 mV (SCE). P r e f e r e n t i a l As d i s s o l u t i o n was apparent. However, f o l l o w i n g continuous feed i n i t i a t i o n the As e x t r a c t i o n was lower than expected. Temperature c o n t r o l was determined c r i t i c a l and oxygen d e p l e t i o n was noted d u r i n g a c c e l e r a t e d — b a t c h l e a c h i n g , j u s t p r i o r t o continuous b i o o x i d a t i o n . The t e s t was conducted a t 10% s o l i d s u s i n g 21-7-14 f e r t i l i z e r as the o n l y n u t r i e n t source w i t h f r e s h water. No r e g r i n d was i n c o r p o r a t e d d u r i n g t h i s t e s t . F i g u r e 3.3 summarizes Fe and As e x t r a c t i o n from the second p i l o t run, as d e t a i l e d i n Table 3.6, u s i n g reground feed and a pH c o n t r o l l o o p i n the f i r s t 4 tanks ( f i r s t r e a c t o r ) . N u t r i e n t s were (NH4)2S04 and K H 2 P O 4 f e r t i l i z e r grade i n f r e s h water. At 80 h r e s i d e n c e the l e v e l o f As and Fe e x t r a c t i o n 89 F i g u r e 3.2 P i l o t p l a n t e x t r a c t i o n d a t a - r u n no. 1. (a) i m m e r s i o n h e a t e r s i n s t a l l e d , i n t e r m i t t e n t s u l p h i d e f e e d , (b) c o n t i n u o u s f e e d i n i t i a t i o n . Table 3.5 P H O T P L A N T DATA - RUN 1 E X T R A C T ION OAT pH Eh (mV) F f A s (rj/U ^ 0 , •c Fe A s 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 1.7 1.7 1.7 1.7 0.4 0.4 0 .5 0 4 0.16 0.15 0.17 0.14 8 5 8 8 8.2 8 1 16 16 16 16 1 9 1 9 2 4 1 .9 12 2 11 4 13 D 10.7 2 1.7 1.8 1.8 1.8 0 .5 0 .5 0.6 0 5 0.22 0.22 0.26 0.21 8 1 8 0 8.2 8 1 16 16 16 16 2 4 2 .4 2 9 2 4 16 8 16 8 19 8 16.0 3 i .7 1 .8 1.8 1.8 0.5 0 .5 0.6 0 5 0 .18 0.19 0.20 0 .18 - - - - - 2 4 2 4 2 9 2 4 13 7 14 5 15 2 13.7 e 2.0 1.9 2 . 0 1.9 0.7 0 .9 0 .8 0 6 0.23 0.25 0.24 0.17 7 7 7 6 7.8 8 1 16 16 16 16 3 3 4 3 3 8 2 9 17 5 19 8 ie 3 13 .0 15 2.1 1.9 2 . 0 1.9 0.4 0.6 0 .8 0 6 0.16 0.25 0.25 0.25 7 8 7 8 7.6 7 7 17 17 17 17 1 9 2 9 3 8 2 9 12 2 19 0 19 0 19.0 22 2.1 1.9 1.9 1.9 0.6 0.6 0 .8 0 6 0 .99 0.21 0.23 0.25 2 9 2 9 3 8 2 9 37 3 16 0 17 5 19 .0 29 1.9 1.9 1.9 1.9 1.2 1.2 1.1 0 9 0.73 0.66 0.19 0.43 - 25 26 25 27 5 7 5 7 5 2 4 3 55 6 50 3 14 5 32.8 34 1.7 1.7 2 . 0 1.7 1.3 1.4 1.2 1 1. 0 .89 0.99 0.90 0.83 5 4 6 9 6.7 6 7 24 27 27 24 6 2 6 7 5 7 2 67 8 75 4 68 6 63.2 36 1.6 1.6 1.8 1.6 2.0 2.1 1.8 1 6 1.16 1.13 1.16 0 .99 5 0 5 8 6.3 6 8 24 28 27 26 9 5 10 0 8 6 7 6 e8 4 B6 1 88 4 75.4 CONTINUOUS FEED - 80 h RESIDENCE 1 -4 1-4 46 1.5 1.5 1.6 1.5 553 557 517 497 3.1 2 .9 2.1 2 7 1.76 1.74 2.07 1.50 5 4 5 1 7.7 6 2 23 27 15* •26 5 2 52 4 50 1.5 1.5 1.7 1.5 486 484 472 479 4 .8 4 .5 2.2 1.86 1.85 2.16 1.48 6 3 5 6 7.4 6 2 25 29 16 26 7 4 54 5 53 1.6 1.6 1.7 1.6 477 477 464 468 3.2 2 .9 1.8 2 4 1.53 1.51 1.84 1.16 6 1 6 4 6.2 5 0 22 26 19 26 4 9 44 e 57 3.6 3.3 1.8 3 0 1.68 1.65 1.82 1.37 5 0 5 6 6.6 5 I 22 24 19 26 5 6 48 3 60 - - - - - - - - 2.3 2 .8 2 .3 1.52 1.6? 1.67 1.25 5 6 4 . 6 5.9 4 8 22 27 19 25 4 7 45 3 63 1.4 1.4 1.5 1.4 463 54 8 433 441 3.6 3.4 4 .1 2 9 1.S4 1.96 1.81 1.44 5 1 4 . 6 5.5 5 1 24 27 17 2 5 " 6 7 52 2 66 •1 .4 1.4 1.5 1.4 481 554 427 444 3.3 3.4 3.9 2 7 1.98 2.27 1.73 1.52 - - - - - 6 4 55 6 69 1.5 1.4 1.5 1.5 523 544 420 452 5.9 6.4 6 .2 4 . 8 1.94 2.14 1.44 1.42 - - 23 23 27 24 11 2 51 4 73 1.6 1.4 1.6 1.6 531 570 544 55! 5.4 7.6 . 4 . 2 2.05 2.41 1.53 1.52 4 9 4 . 6 4.1 5. 1 23 27 30 21 11 0 55 6 NEW F E E D - R E G R I N D - B A T C H L E A C H I N G * I m m e r s i o n H e a t e r s I n s t a l l e d * L i m e S p i l l * H e a t e r F a i l u r e 91 F i g u r e 3.3 P i l o t p l a n t e x t r a c t i o n d a t a - r u n n o . 2. ( a ) s u b s t r a t e l i m i t e d , ( b ) i n t e r m i t t e n t f r e s h f e e d , ( c ) C O „ d e p l e t i o n , ( d ) f e e d c i r c u i t m e c h a n i c a l d i f f i c u l t i e s , ( e ) p r o c e s s w a t e r a d d e d , ( f ) f e e d r a t e i n c r e a s e d , ( g ) b a t c h l e a c h i n g t o r e c o v e r - n o s u c c e s s , f l u s h e d t a n k s . Table 3.6 P I L O T P L A N T D A T A - R U N 2A I E X T R A C T I O N PAY pH Eh (mV) Et (q/L) Ai ( y L ) dO, (mg/L) °C ft As_ 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 4 1 4 1 - - - - - - - - 3.6 5.7 3 6 3.1 2.04 2.40 2.04 1.58 - - - - - - 5 7 45 7 3 1.6 1.4 1.6 1.6 537 559 451 539 3.7 6.1 3 6 3.4 2.13 2.48 .130 1 .64 5.2 4.4 5 2 4 9 28 30 31 30 5 7 42 8 6 1.5 1.2 1.6 1.4 539 564 499 554 6.5 11.2 6 1 6.1 2.44 2.92 1.7B 1.93 4.9 4.7 4 0 4 4 27 30 35 30 6 0 51 4 10 1.5 1.3 1.6 1.4 547 560 502 562 5.8 11.3 6 1 - 2.16 3.03 1.89 1.61 5.1 4.4 4 2 4 8 27 30 34 29 10 6 49 2 17 1.6 1.6 1.8 1.6 542 54 7 503 557 6.8 10.1 5 5 6.2 2.32 2.42 1.7B 1.60 4.8 5.6 t 0 4 4 30 17 35 30 11 0 46 0 20 1.6 1.5 1.8 1.6 553 549 506 576 7.4 9.7 5 7 6.5 2.48 2.66 1.84 1.60 9.0 4.8 4 9 4 1 32 29 25 30 10 1 48 1 24 1.7 1.7 2 . 0 » 1.6 507 548 412 575 9.2 9.8 6 0 7.4 2.26 2.35 1.08 1.60 4.7 4.8 5 7 3 9 33 28 19 29 10 4 41 3 27 1.7 1.6 2.0 1.6 491 540 388" 4 9 7 « 9.1 - - 2.46 - - 4.6 4.8 4 9 4 5 32 29 31 29 11 5 31 1.6 1.5 1.9 1.5 545 545 398 479 10.6 9.4 7 9 - 2.33 2.40 0.48 1.53 4.1 4.7 4 3 4 8 33 29 31 28 38 2 3S 1.6 1.5 2.1 1.5 534 535 386 452 9.8 10.1 7.3 2.57 2.56 0.21 1.49 4.3 4.9 5 1 4 6 33 29 31 28 13 2 38 7 3B 1.6 1.5 1.9 1.5 548 550 501 460 12.0 11.9 . 7.9 3.26 3.26 1.10 1.82 4.4 4.8 5 0 4 6 31 29 30 28 15 0 53 5 41 1.5 1.5 1.8 1.6 563 555 4 94 493 11.3 10.8 - 2.99 1.30 1.64 4.9 4.9 5 1 4 8 29 28 30 28 15 7 44 8 44 1.6 1.6 1.7 1.6 565 563 516 525 12.5 12.5 7.6 3.56 3.73 2.16 2.36 4.9 4.9 5 2 4 9 27 28 30 27 15 4 66 S 46 1.4 1.4 1.6 1.5 566 570 518 542 12.9 13.3 7.8 3.73 2.29 2.08 2.21 4.9 4.8 3 4 4 8 30 32 31 30 16 1 5E 4 CONTINUOUS FEEO - 80 h RESIDENCE 49 1.4 1.4 1.6 1.5 576 580 534 54 9 14.0 14.4 7.9 2.88 2.06 2.22 4.8 4.8 3 1 4 8 29 31 32 30 17 1 54 1 S3 1.4 1.3 1.5 1.4 583 588 552 560 18.3 18.5 9 5 10.7 3.25 3.23 3.00 3.01 1.4 1.4 1 9 2 1 35 34 33 34 20 2 70 8 65 1.3 1.3 1.4 1.3 586 592 558 564 17.3 18.1 10 5 12.1 3.31 3.18 3.26 3.67 20 5 76 0 58 1.2 1.2 1.3 1.2 592 594 567 569 22.5 25.1 13 6 17.3 3.69 3.88 4.02 4.34 1.3 1.4 1 9 1 9 35 35 33 34 27 8 90 3 61 1.2 1.1 1.2 1.1 584 596 572 584 22.0 23.4 13 5 20.0 3.36 3.36 3.70 4.24 4.0 4.1 4 0 2 6 33 32 32 32 27 9 83 1 BATCH - FEED OFF 65 1.2 1.1 1.2 1.1 - 19.4 19.7 12 9 20.0 3.30 3.32 3.77 3.95 4.8 4.1 4 6 4 4 30 30 31 30 25 5 81. 3 * L ime S p i l l •* Substrate Limited - Intermittent Sulphide Feed To Increase Density to Table 3.6 (cont'd) PILOT PLANT DATA - RUN 26 PH rh (mV) Ft (q/L) M ( l / L ) RECLAIM WATER REPLACED FRESH WATER CONTINUOUS FEED - 80 h RESIDENCE (4 TANKS) i C l T C i C T I O N 69 1.2 1.1 : 1.3 1.2 506 SIO 472 494 20.4 20.1 13.5 20.0 3. .87 3.69 - 4. .14 5. 1 4. .4' 4. .9 4. .6 30 31 31 30 26. .2 88. 4 72 1.2 1.1 1 1.3 1.2 SOS S08 468 494 20.0 21.1 13.4 20.0 3. .44 3.24 3.65 3. .52 5. 3 4, .6 5. .1 4. .8 31 30 31 31 26. .4 78. .5 75 1.2 1.1 1 1.3 1.1 497 "98 4S0 482 20.5 21.6 13.8 17.3 3. .85 3.32 3.67 4 .47 4. 9 4. .6 5. ,3 4 .9 31 31 32 31 25. .9 86 .7 76 1.2 1.1 1.3 1.1 496 496 449 482 20.6 21.5 13.8 17.2 3. :&3 3.33 3.97 4. .11 5. 3 5. ,1 5. 3 4. .9 32 32 32 32 25. .9 85, 2 PILOT PLANT DATA - RUN 2C I EXTRACTION  pH EL (HV) Fe (g/L) At (g/L) Fe As 1/4 5 6 7 1/4 5 6 7 1/4 5 6 7 1/4 5 6 7 1 - 4 1 - 7 1 - 4 1 - 7 CONTINUOUS FEED - 80 h RESIDENCE (7 TANKS) 77 1.3 1.3 1.3 1.3 4 70 625 519 680 16.0 18.6 20.0 14.4 3.50 2.85 3.80 2 .35. 22. .6 20. 4 79 .3 -80 1.4 1.4 1.4 1.4 455 482 464 517 10.5 15.5 12.3 16.4 2.10 2.90 2.60 3 .0 14, .9 23. 2 47. .6 68.0 82 1.4 1.4 1.4 1.4 440 444 454 465 7.0 8.5 11.1 13.3 1.20 1.70 2.20 2. .59 9, .9 16. 6 27, ,2 58.7 85 1.6 1.5 1.5 1.4 400 415 426 435 3.0 4.1 5.3 6.7 0.07 0.90 1.11 4, .3 9. ,5 15. .9 -BATCH - FEEO OFF 90- 1.3 1.2 1.3 1.3 41S 419 424 429 3.5 3.7 3.9 4.7 1.10 1.20 1.20 1 .30 5.0 6.7 24.9 29.5 95 1.3 1.2 1.3 1.4 410 421 422 426 3.5 4.0 3.9 4.7 1.10 1.20 1.10 1.20 5.0 6.7 24.9 27.2 FLUSHED TANKS ' I n t e r m i t t e n t S u l p h i d e Feed To Provide Fresh S u r f a c e And N u t r i e n t 94 continued to r i s e and l e v e l o f f at 85% As extraction and 15% Fe extraction, i n d i c a t i n g the e f f e c t of residence on degree of oxidation. Note that dissolved s i l v e r became evident at higher oxidation l e v e l s (>30% t o t a l sulphide oxidized). Addition of process water at Day 69 caused an immediate drop in Eh of almost 100 mV with l i t t l e change in pH. At Day 76 the feed rate was increased i n the f i r s t four tanks to maintain 80 hours residence over seven tanks, as the c i r c u i t appeared stable. However, s i m i l a r to continuous laboratory t e s t s , the process water apparently i n h i b i t e d further biooxidation, even a f t e r batch leaching with intermittent fresh feed. The bioleach reactors were flushed and restocked with fresh sulphide, nutrient, acid, and inoculum and allowed to batch leach. Figure 3.4 shows the extraction data of the f i n a l p i l o t run, d e t a i l e d i n Table 3.7, from batch i n i t i a t i o n to steady state continuous biooxidation. These data are an excellent example of a t y p i c a l startup and s t a b i l i z a t i o n of a continuous c i r c u i t , operating under the following conditions at steady state: 40 hours residence t o t a l , 27 hours f i r s t reactor 13% s o l i d s , 2.1 tpd throughput, 80% passing 70 urn 0.05 L air/minute/L pulp, 30°C pH 1.2 in f i r s t reactor, pH 1.1 at discharge 520 mV (SCE) i n f i r s t reactor, 550 mV (SCE) at discharge 0.6 kg (NH 4) 2S0 4/t and 0.12 kg KH 2P0 4/t concentrate 70% As extraction, 14% Fe extraction 95 F i g u r e 3 . 4 P i l o t p l a n t e x t r a c t i o n data - run no. 3 . (a) continuous i n t i a t i o n , (b) d 0 „ d e p l e t i o n , (c) a c c e l e r a t e d b i o o x i d a t i o n , (d) steady s t a t e . Table 3.7 PILOT PLANT DATA • RUN 3 •• EXTRACTION DAT pH [h (mV) rt ( ,A.) »- im) JO; (™>M. 1/4 5 6 1/4 5 6 1/4 5 6 1/4 5 6 1/4 5 6 1/4 5 6 1 4 1 4 1 2 .0 400 0 .7 0.14 6.0 30 1 2 3 8 2 1.9 400 0 .7 0.15 6.0 31 1 2 4 0 5 1.9 415 0 .9 0.17 5.8 29 1 5 4 6 10 1.7 462 1.7 0.67 5.8 29 2 8 16 0 15 1.6 523 4.3 1.44 3.5 31 7 2 38 6 20 1.3 553 8.3 2.24 1.9 31 13.9 60 1 CONTINUOUS - 80 h 1 - 4 1_ _6 1 - 4 1 - 6 22 1.3 1.3 1.3 543 421 424 9.6 4 .0 4.3 2.38 1.10 1.14 2.4 5.8 7.4 33 29 20 16.1 7 2 63 8 30.6 23 1.3 1.3 1.3 551 441 423 10.4 4.5 4.4 2.69 1.34 1.22 2.7 6.2 7.7 31 29 20 17.4 7 4 72 1 32. 7 24 1.3 1.2 1.2 540 464 439 9 .6 6.1 4.4 2.42 1.61 1.22 2.4 5.9 7.4 31 30 21 16.1 7 4 64 9 32.7 25 1.5 1.3 1.3 518 520 470 8.4 8.7 7.3 2.02 2.14 1.88 2.3 4.0 5.5 30 30 25 14.1 12 2 54 2 50.4 CONTINUOUS - 40 h 26 1.5 1.3 1.3 518 545 469 7.8 8.5 7.5 2.00 2.04 1.69 2.3 3.4 5.7 30 31 24 13.1 12 6 53 6 50.7 26 1.2 1.2 1.2 530 552 550 8.4 9.4 9.5 1.92 2.04 2.10 2.3 1.5 4.1 30 31 20 14.1 15 9 51 5 56.3 30 1.1 1.0 1.0 530 560 572 8.2 9.7 9.7 2.18 2.44 2.44 1.9 1.0 5.4 32 32 21 13.7 16 3 50 5 65.4 32 1.2 1.1 1.1 520 532 549 6.3 7.4 8.9 1.34 1.63 1.90 3.0 1.5 2.1 29 30 27 10.6 14 9 35 9 51.0 34 1.2 1.1 1.1 520 536 554 5.5 6.6 8.3 1.51 1.84 2.21 2.4 1.2 1.7 30 31 28 9.2 13 9 40 5 59.3 36 1.2 1.1 1.1 520 531 550 5.4 6.2 8.4 1.20 1.45 2.65 1.9 1.1 1.5 31 32 29 9.1 14 1 32 2 71.1 38 1.2 1.1 1.1 520 534 549 5.5 6.0 8.0 1.33 1.60 2.60 2.0 1.1 1.4 31 31 29 9.2 13 4 35 7 69.7 FEED OFF 41 1.2 1.2 1.2 540 561 568 9.1 10.1 10.0 1.72 2.46 2.63 - - - 32 32 28 15.3 16 £ 46 1 70.5 46 1.1 1.0 1.0 550 571 604 13.3 - - 2.24 - - - - 32 - - 24.4 - 60 0 -97 Based on l a b o r a t o r y and p i l o t t e s t i n g , f i l t e r c a p a c i t y was estimated a t 400 kg/h/m r e q u i r i n g 0.28m of air/m a t 20 inches of vacuum. Laboratory t e s t i n g and p l a n t s c a l e treatment of a c i d mine drainage i n d i c a t e d t h a t lime requirements to n e u t r a l i z e the a c i d i c b i o l e a c h a t e t o pH 7 approximate 1 Ca0:2 d i s s o l v e d s o 4 . Cyanidation of the washed p i l o t p l a n t residues i n d i c a t e d t h a t 0.5 kg NaCN/t was s u f f i c i e n t to e x t r a c t the cyanide s o l u b l e g o l d and s i l v e r i n four hours or l e s s showing e x t r a c t i o n curves s i m i l a r t o Figu r e 2.17. Organic f o u l i n g of carbon a d s o r p t i o n c i r c u i t s (CIL or CIP) subsequent to c y a n i d a t i o n should be examined f o r d e t a i l e d f e a s i b i l i t y a n a l y s i s as much of the biomass w i l l remain w i t h the s o l i d f r a c t i o n even a f t e r vacuum f i l t r a t i o n . Precious metals e x t r a c t i o n was dependent on the combined l e v e l of Fe and As o x i d a t i o n , as dis c u s s e d i n d e t a i l i n S e c t i o n 4.0. The r e s u l t s of c y a n i d a t i o n t e s t i n g on washed b i o l e a c h residue samples are summarized i n Table 3.8. 98 TABLE 3.8 P i l o t p l a n t b i o l e a c h residue c y a n i d a t i o n r e s u l t s BIOLEACH EXTRACTION CYANIDATION RECOVERY % Fe % As % Au % Ag 2.8 18.0 29 .8 28. 7 7 . 2 38. 6 44. 7 37. 3 14.0 59.0 70.0 51.9 15.9 56. 3 66.8 27. 3 16.0 62.0 62.0 24.9 16. 3 65.4 61.4 25.0 11.0 44.0 51.4 36.6 7.0 32.0 44.4 29.0 13.4 69. 7 65. 2 59.8 15.3 46.1 64.3 34.5 16.8 70. 5 68.9 37.7 24.4 60. 0 70.1 34.1 Higher pulp d e n s i t i e s were not t e s t e d on a p i l o t s c a l e due to p i l o t p l a n t a g i t a t o r l i m i t a t i o n s and budget c o n s t r a i n t s . B i o l e a c h a t e c h a r a c t e r i s t i c s remained c o n s i s t e n t w i t h l a b o r a t o r y t e s t i n g . P i l o t s c a l e b i o l e a c h a t e provided s u f f i c i e n t s o l u t i o n f o r treatment s t u d i e s t o p r e c i p i t a t e metals from s o l u t i o n as des c r i b e d i n S e c t i o n 2.1. The r e s u l t a n t metal p r e c i p i t a t e was subjected to environmental s t a b i l i t y t e s t i n g , the r e s u l t s of which are summarized i n S e c t i o n 4.0. 3.5 GENERAL COMMENTS P i l o t s c a l e t e s t i n g of a e r a t i o n r a t e and method, pulp d e n s i t y , and degree of a g i t a t i o n would be meaningful f o r b e t t e r d e f i n i t i o n of these parameters and t h e i r i n t e r a c t i o n . However, the p i l o t p l a n t was not designed to al l o w f o r f l e x i b i l i t y of these parameters. S i g n i f i c a n t a d d i t i o n a l c a p i t a l was required t o i n c o r p o r a t e the necessary equipment f o r larg e range v a r i a t i o n of these o p e r a t i n g v a r i a b l e s . 99 The author recommends tha t f o r d e t a i l e d f e a s i b i l i t y -a n a l y s i s these parameters are more c o n f i d e n t l y defined on a s c a l e a t l e a s t as l a r g e as the Equity p i l o t c i r c u i t . T e s t i n g and o p t i m i z i n g these parameters on a sm a l l e r s c a l e might not provide meaningful data. Most of the op e r a t i n g problems encountered duri n g p i l o t t e s t i n g were mechanical i n nature such as: feed l i n e plugging, a g i t a t o r f a i l u r e , vacuum pump f a i l u r e . The outstanding m e t a l l u r g i c a l d i f f i c u l t y encountered was the d e l e t e r i o u s e f f e c t of process water, as discussed i n Se c t i o n 4.0. Future p i l o t o p e r a t i o n should i n c l u d e more f l e x i b i l i t y i n design f o r a g i t a t i o n , a e r a t i o n , and pulp d e n s i t y changes. A d d i t i o n a l i n s t r u m e n t a t i o n and c i r c u i t alarming might reduce manpower requirements f o r continuous o p e r a t i o n . A d d i t i o n a l research i s r e q u i r e d t o d e f i n e automatic c o n t r o l i n s t r u m e n t a t i o n and s t r a t e g y f o r p l a n t s c a l e continuous b i o l e a c h o p e r a t i o n s . The p i l o t c i r c u i t maintained one spare r e a c t o r f o r inoculum maintenance t o provide an immediate supply of f r e s h inoculum a f t e r a c i r c u i t d i s r u p t i o n . This would not be necessary on a p l a n t s c a l e as d r a s t i c experimentation would not be ongoing i n the p l a n t s c a l e c i r c u i t but r a t h e r i n a p a r a l l e l l a b o r a t o r y or p i l o t c i r c u i t f o r t e s t purposes. 100 3.6 COST RECONCILIATION The p r o j e c t c o s t , i n c l u d i n g l a b o r a t o r y s t u d i e s , i s summarized i n Table 3.9. TABLE 3.9 Research p r o j e c t c o s t summary ($CDN) ITEM ACTUAL BUDGET BETTER (WORSE) THAN BUDGET Lab Equipment 9705 10000 295 Ser v i c e s Purchased 17051 16600 (451) P i l o t Equipment 54639 50600 (4039) P i l o t L a bour/Material 47661 11400 (36261) Freight/Duty 4398 5700 1302 Taxes 0 4500 4500 TOTAL 133,454 98, 800 (34654) The major c o n t r i b u t e r t o the p r o j e c t c o s t overrun was P i l o t P l a n t L a b o u r / M a t e r i a l . The overrun was the r e s u l t p r i m a r i l y of an underestimation of p r o j e c t schedule. Commissioning of a car b o n - i n - l e a c h c i r c u i t d u r i n g December 1984 - March 1985 caused severe manpower c o n s t r a i n t s and a slow down i n p i l o t p l a n t r e s e a r c h a c t i v i t y . The e f f e c t of process water was not a n t i c i p a t e d , t h i s consumed about two months of the p i l o t c i r c u i t o p e r a t i n g p e r i o d . The sc h e d u l i n g e r r o r a l s o r e s u l t e d i n an over payment of b e l t f i l t e r r e n t a l . Constant s u p e r v i s i o n of the c i r c u i t by t e c h n i c i a n s was not a n t i c i p a t e d but was a n e c e s s i t y , r e s u l t i n g i n approximately $20,000 i n labour c o s t s . Mechanical and e l e c t r i c a l items not a n t i c i p a t e d i n the cos t estimate i n c l u d e d immersion heater purchase and i n s t a l l a t i o n , b e l t f i l t e r w i r i n g r e p a i r s , and b e l t f i l t e r 101 r e c e i v e r pump s e a l r e p a i r s . M o d i f i c a t i o n steelwork to the b a l l m i l l discharge chute, b e l t f i l t e r discharge chute, and feed d i s t r i b u t o r d u r i n g p i l o t p l a n t o p e r a t i o n c o n t r i b u t e d to the c o s t overrun. Note t h a t the c o s t estimate and budget was deri v e d p r i o r t o any b i o l e a c h testwork a t E q u i t y and t h e r e f o r e , i n h i n d s i g h t , should have i n c l u d e d a considerable contingency f a c t o r due t o the e x p l o r a t o r y nature of the p r o j e c t . 102 4.0 P l a n t Scale F e a s i b i l i t y a t E q u i t y S i l v e r Mines The l a b o r a t o r y and p i l o t p l a n t s t u d i e s summarized h e r e i n have only shown t h a t the scie n c e of biotechnology i s a p p l i c a b l e to E q u i t y Southern T a i l bulk s u l p h i d e concentrate. Much of the research reported was dedicated to modifying the b i o l e a c h r e a c t i o n to maximize the subsequent c y a n i d a t i o n response. The biohydrometallurgy of the p r o j e c t i s only a s m a l l p a r t of the p r o j e c t f e a s i b i l i t y to r e c l a i m gold and s i l v e r values l o s t to Southern T a i l f l o t a t i o n t a i l i n g . There are many other f a c t o r s t h a t must be addressed f o r a meaningful p l a n t s c a l e f e a s i b i l i t y a n a l y s i s . Flowsheet 4.1 i s a conceptual p l a n flowsheet f o r Southern T a i l f l o t a t i o n t a i l i n g retreatment a t Eq u i t y , i n c o r p o r a t i n g a b i o l e a c h step a t r e l a t i v e l y low o v e r a l l o x i d a t i o n l e v e l s . The e x i s t i n g c a rbon-in-leach c i r c u i t would be a v a i l a b l e f o r precious metal e x t r a c t i o n , recovery and r e f i n i n g . Based on the l a b o r a t o r y and p i l o t s c a l e testwork, and the conceptual arrangement shown i n Flowsheet 4.1, more d e t a i l e d m e t a l l u r g i c a l flowsheets were developed. Flowsheets 4.2 t o 4.9 and the r e l e v a n t equipment d e s c r i p t i o n s provided the b a s i s f o r c a p i t a l and o p e r a t i n g c o s t e s t i m a t i o n , as d e t a i l e d i n Sections 4.2, 4.3, 4.4, and 4.5. 103 CONCEPTUAL PLANT FLOWSHEET H2SO4 or AMD IMPOUNDMENT NUTRIENT H,S04 or AMD FRESH WATER CONDITIONERS F l o w s h e e t 4 .1 C o n c e p t u a l p l a n t s c a l e b i o l e a c h i n g - g e n e r a l 104 4.1 Scaleup Co n s i d e r a t i o n s T a i l i n g sand r e c l a m a t i o n i s a s i g n i f i c a n t f a c t o r f o r p l a n t s c a l e design c o n s i d e r a t i o n . Approximately 8.5 x 10 tonnes of Southern T a i l f l o t a t i o n t a i l i n g , p r e s e n t l y impounded as c o n s o l i d a t e d sand a t 15% moisture, must be reclaimed and reprocessed, t o produce a bulk s u l p h i d e concentrate, a t a r a t e of 6000 tonnes per day to a v o i d encroachment of the more d i l u t e Main Zone f l o t a t i o n c y a n i d a t i o n t a i l i n g . A number of r e c l a i m methods have been proposed, such as bucket wheel dredging, high pressure water r e p u l p i n g and h i g h c a p a c i t y sand pumping. Based on the corporate experience of P l a c e r Development L i m i t e d i t was decided t h a t r e c l a m a t i o n using a h i g h c a p a c i t y heavy d e n s i t y s l u r r y pump, f i t t e d w i t h a s i t e s p e c i f i c c u t t e r head, as proposed by Nelmaco (86), provided the a l t e r n a t i v e w i t h the lowest r i s k economically. The bulk s u l p h i d e concentrate i s produced i n an a c i d environment and t h e r e f o r e process •• equipment; pumps, p i p i n g , f l o t a t i o n c e l l s , r e g r i n d m i l l media and l i n e r s , cyclones, pumpboxes, and i n s t r u m e n t a t i o n , must be a c i d r e s i s t a n t which has a s i g n i f i c a n t e f f e c t on c a p i t a l and o p e r a t i n g c o s t e s t i m a t i o n . B i o l e a c h i n g and f i l t r a t i o n are a l s o c a r r i e d out i n an a c i d i c environment a t a notably higher o x i d a t i o n p o t e n t i a l , again a s i g n i f i c a n t c o n s i d e r a t i o n i n c a p i t a l and o p e r a t i n g c o s t i n g and d e s i g n . Laboratory and p l a n t s c a l e t e s t i n g provided the design b a s i s f o r f l o t a t i o n and r e g r i n d i n g c i r c u i t s . 105 The research data i n d i c a t e d that s c a l e d i d not a f f e c t the biohydrometallurgy or precious metals hydrometallurgy of the E q u i t y t e s t c i r c u i t . The parameters such as pH, Eh, n u t r i e n t s , water source, temperature, degree of o x i d a t i o n f o r enhanced c y a n i d a t i o n , b i o l e a c h a t e c h a r a c t e r i s t i c s and treatment, dewatering and washing, and c y a n i d a t i o n c o n d i t i o n s appeared t o be i n s e n s i t i v e to s c a l e . For example, the degree of sulphide o x i d a t i o n to e f f e c t enhanced c y a n i d a t i o n , based on batch and continuous l a b o r a t o r y t e s t s and p i l o t p l a n t r e s u l t s , i s represented i n Figures 4.1 and 4.2. I t i s apparent t h a t combined Fe + As e x t r a c t i o n >80-90% i s not j u s t i f i e d f o r a d d i t i o n a l gold recovery w h i l e s i l v e r recovery continues t o improve as sulphide o x i d a t i o n continues. These r e l a t i o n s h i p s served as the b a s i s f o r b i o l e a c h c i r c u i t s i z i n g and o p e r a t i n g parameters to maintain 80-90% combined Fe + As o x i d a t i o n as present s i l v e r p r i c e does not warrant the a d d i t i o n a l c a p i t a l and o p e r a t i n g expense to increase s i l v e r e x t r a c t i o n . I t was apparent from continuous t e s t i n g t h a t an i n c r e a s e i n d e n s i t y improved the p r o j e c t economics by reducing r e a c t o r volume ( c a p i t a l cost) and i n c r e a s i n g the oxygen e f f i c i e n c y (reduced o p e r a t i n g cost) but increased the residence time ( c a p i t a l and o p e r a t i n g costs) a t a f i x e d a e r a t i o n r a t e . Further o p t i m i z a t i o n of these v a r i a b l e s must be conducted p r i o r to d e t a i l e d f e a s i b i l i t y a n a l y s i s , on a s c a l e at l e a s t as 106 GOLD RECOVERY loo 80' y 5 a ( x ) b o o o o RECOVERY Au 6a ° • + • oo o o RECOVERY Au 40-A a* 20 O LAB BATCH A LAB CONTINUOUS « PILOT 40 80 120 160 % EXTRACTION (Fe.As) Figure 4.1 The e f f e c t of combined Fe and As e x t r a c t i o n by b i o o x i d a t i o n on gold e x t r a c t i o n by c y a n i d a t i o n . 107 F i g u r e 4 . 2 T h e e f f e c t o f c o m b i n e d Fe and As e x t r a c t i o n by b i o o x i d a t i o n o n g o l d e x t r a c t i o n by c y a n i d a t i o n . 108 l a r g e as the p i l o t c i r c u i t . I t i s reasonable to assume from r e s u l t s todate t h a t 0.05 L air/minute/L pulp at 25% s o l i d s and 60 hours residence w i l l p rovide s u f f i c i e n t a e r a t i o n f o r the sul p h i d e o x i d a t i o n r e q u i r e d (45% oxygen e f f i c i e n c y ) . See Fig u r e 4.3 ( c o n c e p t u a l ) . The e f f e c t of r e a c t o r design on c a p i t a l and op e r a t i n g c o s t o p t i m i z a t i o n must be addressed f o r a d e t a i l e d f e a s i b i l i t y study. One of the s a l i e n t o p e r a t i n g cost items i n b i o l e a c h i n g i s a e r a t i o n . Innovative r e a c t o r design i s re q u i r e d i n l a r g e r e a c t o r s to make e f f i c i e n t use of lower pressure ( <_ 70 kPa) blower a i r . H i g h l y e f f i c i e n t a i r d i f f u s e r mechanisms were employed i n the p i l o t p l a n t r e a c t o r s to improve a i r e f f i c i e n c y . These are i n c l u d e d i n the conceptual p l a n t design f o r t h i s p r e l i m i n a r y f e a s i b i l i t y a n a l y s i s . E f f i c i e n t mixing i n the presence of l a r g e q u a n t i t i e s of a i r r e q u i r e s s p e c i f i c engineering depending on the r e a c t o r dimensions. A height:diameter r a t i o <1.0- i s recommended t o al l o w e f f i c i e n t a i r d i f f u s i o n and a g i t a t o r mechanism design. A d d i t i o n a l r e a c t o r scaleup c o n s i d e r a t i o n s are referenced (87,88,89). I t i s recommended t h a t the b i o l e a c h r e a c t o r s are sealed f o r fume removal. Considerable noxious fuming was evident d u r i n g p i l o t i n g , mainly due t o the a c i d i c nature of the b i o l e a c h environment and moisture entrainment i n the a i r . C l o s i n g the tops of the tanks w i l l a l s o a l l o w a more p r e d i c t a b l e heat balance. The a c i d i c fumes and r e t u r n a i r from 109 G E N E R A L OPTIMISATION P U L P SOLIDS (%w/ w) Figure 4.3 Conceptual o p t i m i z a t i o n of d e n s i t y , a e r a t i o n r a t e and residence time. 110 b i o l e a c h i n g can be employed i n cyanide d e s t r u c t i o n to reduce o x i d a t i v e reagent costs or p o s s i b l y employed f o r cyanide r e g e n e r a t i o n . P i l o t s c a l e t e s t i n g i n d i c a t e d t h a t tank h e a t i n g was c r i t i c a l t o m a i n t a i n the b i o o x i d a t i o n r e a c t i o n . However, p l a n t s c a l e heat balance c a l c u l a t i o n s , d e t a i l e d i n Appendix I , i n d i c a t e d a net heat gain d u r i n g b i o o x i d a t i o n f o r an 800 tpd p l a n t , which must be exchanged (cooled) to m a i n t a i n c o n t r o l of r e a c t o r temperature. A maximum of 30°C i n the r e a c t o r s was assumed f o r the heat balance c a l c u l a t i o n s . The maximum o p e r a t i n g temperature i s dependent on the l i m i t a t i o n s of the b i o l o g i c a l system. Recently, l a b o r a t o r y s c a l e t e s t i n g i n d i c a t e d t h a t o p e r a t i n g a t temperatures g r e a t e r than 45°C i s p r a c t i c a l f o r continuous op e r a t i o n , f o l l o w i n g biomass adaptation, w i t h enhanced o x i d a t i o n r a t e s i n d i c a t e d . Further t e s t i n g i s r e q u i r e d t o b e t t e r i d e n t i f y the temperature l i m i t a t i o n s of b i o l o g i c a l systems f o r s e l e c t i v e sulphide o x i d a t i o n . The heat exchange c a l c u l a t i o n shown i n Appendix I i s only an example as the process i s i t e r a t i v e and warrants o p t i m i z a t i o n a t a more d e t a i l e d design stage. B i o l e a c h r e s i d u e dewatering can be c h a r a c t e r i z e d q u i t e c o n f i d e n t l y d u r i n g p i l o t t e s t i n g or continuous l a b o r a t o r y t e s t i n g . Therefore, f e a s i b i l i t y design parameters f o r f i l t r a t i o n were based on the l a b o r a t o r y and p i l o t s c a l e t e s t I l l r e s u l t s . S i m i l a r l y , r e s i d u e washing r e q u i r e m e n t s ( e g . wash r a t i o and s t a g e s ) can be s c a l e d p r e d i c t a b l y from l a b o r a t o r y o r p i l o t s c a l e c o n t i n u o u s o p e r a t i o n s . B i o l e a c h a t e c h a r a c t e r i s t i c s d i d not v a r y w i t h s c a l e and t h e r e f o r e b i o l e a c h a t e n e u t r a l i z a t i o n p r o c e d u r e s can be d e t e r m i n e d i n t h e l a b o r a t o r y and t h e p a r a m e t e r s c o n f i d e n t l y s c a l e d as r e q u i r e d . C y a n i d a t i o n r e q u i r e m e n t s can be d e t e r m i n e d i n t h e l a b o r a t o r y and s c a l e d as any c y a n i d a t i o n c i r c u i t development. The e x i s t i n g c a r b o n - i n - l e a c h c i r c u i t a t E q u i t y w o u l d be s u i t a b l e f o r b i o l e a c h r e s i d u e p r o c e s s i n g . To a v o i d p o t e n t i a l c y a n i c i d a l e f f e c t s o f t h e p y r i t e c o n c e n t r a t e t h e b i o l e a c h r e s i d u e s h o u l d be d i r e c t e d t o t h e f o u r t h c y a n i d e r e a c t o r , t o m i n i m i z e t h e r e s i d e n c e t i m e o f t h e b i o l e a c h r e s i d u e and p o t e n t i a l s u l p h i d e d i s s o l u t i o n . A t p r e s e n t o p e r a t i n g c a p a c i t y t h i s would p r o v i d e a p p r o x i m a t e l y f i v e h o u r s r e s i d e n c e w h i c h i s s u f f i c i e n t f o r e f f e c t i v e p r e c i o u s m e t a l e x t r a c t i o n , based on l a b o r a t o r y e x t r a c t i o n c u r v e s . The b i o l e a c h i n g / f i l t r a t i o n / w a s h i n g s t e p s w i l l r e q u i r e a c o n s i d e r a b l e r e c y c l a b l e w a t e r s u p p l y . A p r o p e r t y w a t e r b a l a n c e i n d i c a t e d t h a t p r o c e s s w a t e r must be used t o m a i n t a i n a r e a l i s t i c w a t e r b a l a n c e . However, p r o c e s s w a t e r , c o n t a i n i n g r e s i d u a l c y a n i d e s p e c i e s , appeared t o i n h i b i t b i o o x i d a t i o n . See F i g u r e 4 .4 . I t a ppears t h a t t h i o c y a n a t e (CNS) was t h e p e r s i s t e n t c o n s t i t u e n t i n h i b i t i n g b i o o x i d a t i o n , p r o b a b l y c o m b i n i n g w i t h d i s s o l v e d i r o n i n t h e b i o l e a c h a t e t o produce f e r r o t h i o c y a n a t e 1 1 2 1201 CN SPECIES IN PROCESS WATER 100 80-• CN TOTAL A CNS O CNO i / \ 5 a. a. 60 40 / / O7' / / / / / / y s \ \ 2a .JV' y M MONTH M F i g u r e 4.4 Process water cyanide c o n s t i t u e n t s s i n c e c y a n i d a t i o n c i r c u i t s t a r t u p , (a) s u c c c e s s f u l shake f l a s k , (b) s u c c e s s f u l shake f l a s k , (c) u n s u c c e s s f u l shake f l a s k , continuous l a b and p i l o t , (d) u n s u c c e s s f u l shake f l a s k . compounds, Fe(SCN) 3 or Fe 2(SCN) 6 , as a l l b i o l e a c h a t e s o l u t i o n s f o l l o w i n g process water a d d i t i o n e x h i b i t e d a b r i g h t orange c o l o u r . Recent shake f l a s k t e s t s u s i n g 100 mg/L ' s a l t e d ' CNS s o l u t i o n confirmed e a r l i e r r e s u l t s . I t i s reasonable to assume t h a t T h i o b a c i l l u s ferrooxidans can be adapted to a CNS environment by s l o w l y i n t r o d u c i n g the CNS t o continuous b i o l e a c h r e a c t o r s (90). This testwork has been i n i t i a t e d . P e r m i t t i n g f o r safe d i s p o s a l of the t r e a t e d b i o l e a c h a t e sludge must a l s o be considered on a p l a n t s c a l e . T e s t i n g a t E q u i t y showed the sludge was c l a s s i f i e d as a s p e c i a l waste as produced (91). However, mixing the sludge w i t h c y a n i d a t i o n t a i l i n g s l u r r y provided an e x c e l l e n t b u f f e r f o r the sludge. Mixing r a t i o s as h i g h as 10 p a r t s t a i l i n g : 1 p a r t sludge (v/v) provided a s a f e l y d i s p o s a b l e product i n a closed t a i l i n g system, see Table 4.1. TABLE 4.1 S p e c i a l waste t e s t r e s u l t s on 10:1 t a i l i n g : b i o l e a c h a t e sludge mixture ELEMENT As Cd Cr Cu Ni Pb Sb Zn E x t r a c t a b l e l i m i t (ug/g) 10.0 10:1 mixture (ug/g) 0.5 T a i l i n g only (ug/g) 0.3 1.0 10.0 50.0 50.0 10.0 10.0 500.0 0.8 8.8 3.8 3.8 0.2 0.1 28.4 0.1 0.1 22.6 0.8 0.2 0.3 12.2 114 Long term t a i l i n g pond s i m u l a t i o n t e s t i n g has i n d i c a t e d t h a t mixing b i o l e a c h a t e sludge w i t h t a i l i n g s l u r r y does not s i g n i f i c a n t l y a f f e c t the long term s t a b i l i t y c h a r a c t e r i s t i c s of the c o n s o l i d a t e d t a i l i n g sand. The long term s t a b i l i t y of the t a i l i n g s , b i o l e a c h r e s i d u e and b i o l e a c h a t e products, must be addressed f o r r e s p o n s i b l e waste management. The e f f e c t of a b i o l e a c h i n g c i r c u i t on environmental p e r m i t t i n g i s s i t e s p e c i f i c and should be in c l u d e d i n the d e t a i l e d f e a s i b i l i t y study. Any p e r m i t t i n g amendments or a p p l i c a t i o n s should be addressed i n the very e a r l y stages of the p r o j e c t . A patent search should be c a r r i e d out as there are a number of processes employing biotechnology t h a t are patented. In g e n e r a l , previous patents should not present a problem f o r a s p e c i f i c a p p l i c a t i o n of biotechnology such as t h a t being pursued a t E q u i t y . The f o l l o w i n g i s an excerpt from an a r t i c l e t h a t appeared i n Biotechnology and Bioengineering i n 1 9 7 5 ( 5 7 ) . " I t i s d o u b t f u l whether patent r i g h t s should be extended t o these b a c t e r i a ( t h i o b a c i l l i ) and t h e i r metabolic r e a c t i o n . Perhaps t h i o b a c i l l i may be regarded as components e s s e n t i a l but not novel t o the i n d u s t r i a l process, i n which the t e c h n i c a l l a y o u t and c o n s t r u c t i o n are w e l l s p e c i f i e d and thus covered by patent r i g h t s . These organisms may t h e r e f o r e be used without i n f r i n g i n g of any patent r i g h t s a l r e a d y i s s u e d , i n processes which i n v o l v e t e c h n i c a l c o n s t r u c t i o n s t h a t are d i f f e r e n t from the patented ones though employing s i m i l a r m i c r o b i o l o g i c a l p r i c i p l e s . " 115 4.2 Design C r i t e r i a and Equipment S i z i n g a. T a i l i n g Reclaim Reclaim 6000 tpd (S.G. 2.9) T a i l i n g sand c o n s o l i d a t e d t o 85% s o l i d s Makeup water t o 50% s o l i d s f o r s l u r r y t r a n s f e r F l o t a t i o n feed d i l u t i o n t o 42% s o l i d s Booster pump re q u i r e d on r e c l a i m barge t o feed f l o t a t i o n c i r c u i t b. F l o t a t i o n 6000 tpd a t 42% s o l i d s 15 minute residence, bulk concentrate 430 m3/h * 0.25 * 1.18 = 127m3 c e l l volume ( i n c l u d i n g a i r / f r o t h ) 3 18 m / c e l l / m i n u t e a i r r e q u i r e d 800 tpd concentrate a t 40% s o l i d s 5200 tpd t a i l i n g a t 40% s o l i d s Concentrate grade = 5.5g Au/t, 90g Ag/t Concentrate recovery =81.5% Au, 54.5% Ag, 13.3% weight T a i l i n g flows by g r a v i t y to t a i l i n g pond pH 6.0 w i t h I^SO^ (Epoxy p a i n t f l o t a t i o n c e l l s ) O.lg/L H 2S0 4 50 g/t PAX 20 g/t MIBC 3 300,000m sealed impoundment area f o r concentrate i n t e r i m storage 116 c. Regrinding M i l l F80 = 200 um P80 = 75 )iim Wi = 25 kWh/t W =11.2 kWh/t tph = 34 Power i n p u t = 381 kW = 510 HP Motor power = 6 00 HP M i l l S i z e = 9 1/2' x 16' HP= ABCL = 49.6 * 5.02 * 0.16 * 16 = 637 HP B a l l charge = 50 tonnes, s t a i n l e s s M i l l discharge temperature = 15°C (approximate) Cyclones Feed = 24 L/sec, 24% s o l i d s by volume One 18" cyclone o p e r a t i n g a t 8 p s i g i n l e t pressure (per pump) Pumps 540 USGPM a t 1.72 s l u r r y S.G. 55 f t dynamic head - design f o r 800 USGPM 6 x 6 SRL pumps ( 2 ) , 1000 RPM, 30 bhp (design 40 i n s t a l l e d ) V a r i a b l e speed d r i v e system (1 only) 117 d. B i o l e a c h i n g 800 tpd a t 25% s o l i d s ( s o l i d s S.G.= 4.0) 60h t o t a l r e s i d e n c e a v a i l a b l e - 10% Fe, 70% As o x i d a t i o n >_ 25h residence i n f i r s t stage 3 3 2600m /24h * 60h = 6500 m a c t i v e residence volume 3 >_ 2700 m i n f i r s t r e a c t o r Reactors <_ 10m i n h e i g h t t o use blower a i r Reactor dimensions 0.8-0.9 height/diameter f o r e f f i c i e n t mixing 50 m /minute /tank a i r a t 10.2 p s i g ( 5.5m below pulp l e v e l ) 11m© x 10m tank, 4 i n p a r a l l e l and 4 i n s e r i e s , w i t h by-passing 3 3 3 (810m pulp + 50m a i r + 90m freeboard = 9 50m3 tank) 3 f i r s t r e a c t o r c a p a c i t y = 4 tanks 3 240m 3 or = 5 tanks 4050m 3 t o t a l r e a c t o r c a p a c i t y = 6480 m Tank temperature c o n t r o l l e d by a i r temperature and c o o l i n g c o i l s Fine a i r d i f f u s e r g r i d a t 6m from top of tank H y d r o f o i l , s i n g l e a g i t a t o r , 100 hp each Epoxy coated tank, rubber covered a g i t a t o r , p l a s t i c a i r p i p i n g 118 Sealed tank top i n t e g r a t e d i n t o a g i t a t o r bridge /walkway-V e n t i l a t i o n of each tank ( p l a s t i c l i n e ) to cyanide d e s t r u c t i o n or m i l l t a i l i n g box A l l S c l a i r p i p e p i p i n g f o r s l u r r y t r a n s f e r A i r d i f f u s e r s : 36" © header 12" © secondary header 4" © d i f f u s e r headers (4 per tank) 10 scfm max./diffuser 2000 scfm max./tank 2 200 d i f f u s e r s /tank i n 95m area 2 = 0.5m / d i f f u s e r 2 (design c o n s t r a i n t = >0.23m / d i f f u s e r ) 3 Stock tank - 250m , 7m© x 7m, 50 hp a g i t a t o r (2) pumps, 6x6x15 SRL, approx. 100ft. tdh 3 A c i d holdtank - 100m , 5m© x 5m, curbed containment, m i l d s t e e l (2) metering pumps, 1.5L/min c a p a c i t y , l h p 3 N u t r i e n t mix tank - 5m , 2m© x 2m, 25% s o l u t i o n , mixed d a i l y (2) metering pumps, 1.5L/min c a p a c i t y , l h p Reagents - 500kg/d ( N H 4 ) 2 S 0 4 f e r t i l i z e r grade 100 kg/d KH 2P0 4 f i r t i l i z e r grade 1 g/L H„SO. t o new feed i n stock tank 119 e. Heat Exchange Heat r e q u i r e d t o r a i s e 2600m3/d from 15°C to 30°C = 1862 kW Heat s u p p l i e d by a i r (110°C i n , e q u i l i b r i u m a t 3 0°C) = 690kW Heat s u p p l i e d by sulphur o x i d a t i b n =5811 kW Net heat t o be removed = 6501-1862 = 4639 kW Assumed l i n e a r sulphur o x i d a t i o n during r e a c t o r r e s i d e n c e Assumed no heat l o s s through tank w a l l s 2 Require 76m s u r f a c e area, 350m of 7.6cm o.d. 316 L s t a i n l e s s c o o l i n g c o i l C o o l i n g water = 340m /h i n closed c i r c u i t w i t h c o o l i n g tower Cool i n g water s p l i t e q u a l l y to each of 8 r e a c t o r s C o o l i n g water i n = 16°C Coo l i n g water out = 27°C (2) C o o l i n g water pumps = 340m3/h (1500 USGPM) t o t a l dynamic head = 30m 55 hp, design 60 hp Coolin g tower, r a t e d c a p a c i t y = 1330 i c e tons (2) 10 hp convection fans 120 f. F i l t r a t i o n / W a s h i n g and S o l u t i o n N e u t r a l i z a t i o n 720 tpd a t 25% s o l i d s - g r a v i t y feed from b i o l e a c h r e a c t o r 80% p a s s i n g 50 jam 3 F i r s t stage remove 50m /h a c i d f i l t r a t e , 3 2g S0 4/L 3 WR = 2.0 based on t o t a l feed = 200m /h F i l t e r cake = 15% moisture B e l t f i l t e r = 400 kg/h/m2 = 75m2 f i l t e r (75hp) 10scfm/m 2 = 7500 scfm (400hp a t 20 i n . vacuum) 3 10 m /h gland water 3 Repulp tank - 2m N e u t r a l i z a t i o n tanks -3 (2) 5m - 2m (Zi x 2m FRP c/w 2hp a g i t a t o r s (two stage, 1 minute residence each) Primary f i l t r a t e (50 m3/h) = pH 1.1, 550mV (SCE) Secondary f i l t r a t e (250m3/h) = pH 2.0, 350mV (SCE) 250m3/h a t 16.0g / L S 0 4 = 8g CaO/L to pH 6-7 = 48t CaO/d Slak e r = 2.0 t CaO/h, design 4.0 t CaO/h 3 = 2 stage s l a k e r , lm /chamber, l h p a g i t a t o r s 3 = 18m /h f r e s h water (10% s l u r r y ) Lime s i l o -200t c a p a c i t y c/w vent sock, screw feeder (lhp) 3 Lime day tank - 20m , 3m<z> x 3m, 2hp a g i t a t o r Repulp to 50% s o l i d s , 96kg SO^/h to n e u t r a l i z e = 96kg CaO/h to pH>10.0 = 2.3 t CaO/d pump 40m /h repulp t o c y a n i d a t i o n CIL #4 S.G. s l u r r y = 1.6, 80* dynamic head 3x3x10 SRL-C ( 2 ) , 1500rpm, 7bhp - design lOhp Cy a n i d a t i o n 720 tpd b i o l e a c h e d concentrate Pump repulped concentrate to CIL #4 Approximately 8% of CIL #4 new feed volume 0.5 kg NaCN/t s o l u t i o n a t 50% s o l i d s = 360 kg NaCN/d 70% Au e x t r a c t i o n , 30% Ag e x t r a c t i o n i n 4 hours Residence i n CIL #4 and #5 i s 2.4h/stage base case = 2.2h/stage w i t h b i o l e a c h , which has i n s i g n i f i c a n t impact on base case recovery Assume 95% d i s s o l v e d precious metals to b u l l i o n Cyanide d e s t r u c t i o n costs incremental R e f i n i n g c o s t s incremental 122 h. TAILING DISPOSAL Treated f i l t r a t e (pH 6-7) t o m i l l t a i l i n g box ( g r a v i t y ) f o r d i s p o s a l Leached concentrate i n t e g r a t e d i n base case c y a n i d a t i o n t a i l i n g 25m3/h a t 12.8g/L S0 4, 3.25g/L Fe - 600m3/d sludge 3 T o t a l m i l l t a i l i n g = 13000m /d Therefore d i l u t i o n = 13000/600 = 22:1 which i s safe d i s p o s a l d i l u t i o n A d d i t i o n a l t o t a l t a i l i n g to m i l l t a i l i n g box ( i n c l u d i n g s o l u t i o n ) = 6900m 3/d Does not overload h y d r a u l i c c a p a c i t y of e x i s t i n g c i r c u i t . 123 4.3 FLOWSHEET DESCRIPTION To f a c i l i t a t e c o s t e s t i m a t i o n , p r e l i m i n a r y process flowsheets were developed from the conceptual p l a n t flowsheet shown i n Flowsheet 4.1. Flowsheets 4.2 to 4.9 are s e l f - e x p l a n a t o r y and are augmented by the equipment design c r i t e r i a and s i z i n g d e s c r i b e d i n S e c t i o n 4.2. As d i s c u s s e d above, the process design was constrained by the requirement to r e c l a i m 8.5 x 10 tonnes of impounded Southern T a i l / M a i n Zone t a i l i n g a t a r a t e of 6,000 tpd beginning i n 1985, to avoid encroachment and d i l u t i o n , by Main Zone m i l l t a i l i n g . The r e s u l t a n t 800 tpd concentrate, grading 5.5 g Au/t and 90 g Ag/t, must be impounded f o r a t l e a s t two years to p r o v i d e s u f f i c i e n t time to c a r r y out d e t a i l e d f e a s i b i l i t y a n a l y s i s , design, and c o n s t r u c t i o n of the b i o l e a c h c i r c u i t . 800 tpd s u l p h i d e concentrate w i l l r e p o r t to a r e g r i n d c i r c u i t where n u t r i e n t s , r e c y c l e d b i o l e a c h a t e , and makeup a c i d are added. The reground concentrate w i l l r e p o r t t o one of four f i r s t stage r e a c t o r s from which common overflows w i l l r e p o r t t o the f i r s t of f o u r cascaded r e a c t o r s i n s e r i e s . F l e x i b l e and e a s i l y operable bypassing must be i n c o r p o r a t e d i n the r e a c t o r design f o r maintenance and c o n t r o l of the r a t e of b i o o x i d a t i o n and of the utimate degree of s u l p h i d e o x i d a t i o n . A nested r e a c t o r arrangement was employed i n the conceptual design t o reduce the i n t e r s t a g e e l e v a t i o n d i f f e r e n t i a l and bypass p i p i n g requirements. The residence 124 time and r a t e of b i o o x i d a t i o n could be c o n t r o l l e d by r e a c t o r bypassing and a e r a t i o n r a t e c o n t r o l a t each r e a c t o r . Each conceptual r e a c t o r i n c o r p o r a t e d independent temperature c o n t r o l using a c o o l i n g water loop i n closed c i r c u i t w i t h a common c o o l i n g tower. The b a s i c process c o n t r o l i n s t r u m e n t a t i o n i s i n d i c a t e d . A d e t a i l e d a n a l y s i s of process c o n t r o l requirements i s not j u s t i f i e d w i t hout f u r t h e r o p e r a t i n g experience, however, complete i n d i c a t i n g i n s t r u m e n t a t i o n , expandable f o r process c o n t r o l a p p l i c a t i o n , i n c l u d i n g programmable c o n t r o l l e r s , was i n c l u d e d i n the process design f o r c o s t i n g purposes. Complete in s t r u m e n t a t i o n of the b i o l e a c h c i r c u i t i s recommended by the author to a i d i n s t a r t u p t r o u b l e s h o o t i n g . The b i o l e a c h product w i l l r e p o r t t o a continuous b e l t f i l t e r where approximately h a l f of the b i o l e a c h a t e w i l l be separated f o r r e c y c l e . The b i o l e a c h r e s i d u e w i l l be washed, repulped , and then cyanide leached i n the e x i s t i n g carbon-in-leach c i r c u i t . The d i l u t e b i o l e a c h a t e w i l l be n e u t r a l i z e d i n two stages and r e p o r t t o c y a n i d a t i o n t a i l i n g f o r d i s p o s a l (or p o s s i b l y used to p r e t r e a t c y a n i d a t i o n t a i l i n g p r i o r to cyanide d e s t r u c t i o n ) . Note t h a t p l a s t i c p i p i n g ( a i r , s o l u t i o n s , s l u r r y t r a n s p o r t ) , a c i d r e s i s t a n t l i n e r s and media f o r g r i n d i n g , epoxy l i n e d tanks, and rubbber l i n e d s l u r r y t r a n s f e r boxes, d i v e r t e r s and pumps were i n c l u d e d where a p p l i c a b l e i n the equipment design. 125 B E A C H R E C L A I M WATER i _ 0 , 8>U W k 2 5 0 2.8 86 3 4 5 4 2 3 4 5 5 9 5 1 JB 431 0 FLEXIBLE POWER CABLE (575V) 0 SCLAIRPIPE 0 CONTROL HOUSE 0 BARGE 0 TOYO MODEL DP-150B SUBMERSIBLE PUMP 0 BOOSTER PUMP, 12x10x25 SRL-C,75h:p. ^ TO REMOTE FLOTATION CIRCUIT Flowsheet 4.2 Conceptual p l a n t scale b i o l e a c h i n g - b i o l e a c h r e c l a i m 126 BULK SULPHIDE FLOTATION 150 2 9 as 345 42 345 595 1.38 431 © 'JCM HjS04 PAX M I BC 217 2.8 78 325 40 325 542 1J4 403 TO TAILING PONO 33 4.0 8 50 40 50 B3 1.43 58 0OUTOKUMPU OK-38-3U 0 B L O W E R , 5 0 h p , 35kPa 0 POLYETHYLENE REAGENT TANKS (2 m3) METERING PUMP 4 0 0 m m SCLAIRPIPE CONCENTRATE PUMP, 5x4x14 SRL-C,15hp 1 0 0 m m SCLAIRPIPE § IMPOUNDMENT DAM, 3 0 0 0 0 0 m 3  TQYQ PUMP  Flowsheet 4 . 3 Conceptual p l a n t s c a l e b i o l e a c h i n g - f l o t a t i o n 127 CONCENTRATE REGRIND 0 33 40 8 50 40 50 83 1.43 58 CM N U T . © 33 4.0 8 100 25 100 133 1.23 108 © 9 . 5 x 1 6 ' B A L L M I L L , 7 0 0 h p 0 6 x 6 S R L P U M P , V A R I A B L E S P E E D , 4 0 h p 0 4 5 0 m m H Y D R O C Y C L O N E 0 7 m x 7 m S T O C K T A N K , 5 0 h p A G I T A T O R © 6 x 6 S R L P U M P , 4 0 h p . 0 M E T E R I N G P U M P 0 M I L D S T E E L T A N K ( 5 m x 5 m ) © P O L Y E T H Y L E N E T A N K (2mx2m) C/ W 1 h p A G I T A T O R Flowsheet 4 . 4 Conceptual p l a n t s c a l e b i o l e a c h i n g - regrinding/reagents 128 BIOL EA CHING AIR CONTROL • ACIO ADDITION TO * FILTRATION 0 DISTRIBUTOR (7) 11m8x10m REACTORS C/W 100 hp AGITATORS 0 AIR DIFFUSERS 0 BLOWERS(2),700hp,70kPa 0 COOLING TOWER, 1330 i c e t o n c a p a c i t y 0 COOLING LOOP PUMP 6x18, 60hp 0 PROGRAMMABLE LOGIC CONTROLLER Flowsheet 4 . 5 Conceptual p l a n t scale b i o l e a c h i n g - b i o l e a c h i n g \ 129 FILTRATION/ NEUTRALIZATION TO MILL TAILINGS (80tpd SLUDGE) 0 7 5 m J BELT FILTER C / W 4 0 0 h p VACUUM 0 LIME SLAKER , 2 t C a O / h 0 R E P U L P T A N K , 2 m 3 0 LIME SLURRY TANK, 2 1 m 3 STEEL 0 FILTRATE TANK, 2 m 3 FRP © N E U T R A L I Z A T I O N REACTORS, 6 . 3 m 3 FRP 0 LIME SILO, 2 0 0 t CAPACITY Flowsheet 4 . 6 Conceptual p l a n t s c a l e b i o l e a c h i n g - f i l t r a t i o n / n e u t r a l i z a t i o n FRESH • WATER BALANCE 282 RECLAIM 30 6J 345 FLOTATION 50 I 325 50 BIOLEACH I 200 110 TWO STAGE WAS HING TWO STAGE NEUTRALIZATION IT REPULP 24 31 CYANIDE 32 CH DESTRUCTION RECYCLE 536 TAILING POND 10 10 HEAT EXCHANGE • REAGENTS VACUUM GLAND • PUMPS GLAND REAGENTS - REAGENTS CaO 228 TO 114 WATER SHEO Flowsheet 4 . 7 Conceptual pl a n t s c a l e b i o l e a c h i n g - water balance Flowsheet 4.8 Conceptual p l a n t s c a l e - general arrangement - r e c l a i m b i o l e a c h ing 132 0 CYANIDATION (exiSTC) 0 BELT FILTER 0 ACID TANK 0 LIME SILO 0 CARBON REFINERY ( K X I S T O ) Flowsheet 4 . 9 Conceptual p l a n t s c a l e - general arrangement - b i o l e a c h i n g b i o l e a c h i n g 133 4.4 EQUIPMENT LIST AND CAPITAL COST ESTIMATE a. BEACH RECLAIM INSTALLED ITEM NO REQ'D DESCRIPTION KW $ CDN 1 1 Toyo model DP-150 B submersible pump c/w barge, i n s t r u m e n t a t i o n , system engineering and t e s t fees 112 150,000 2 1 Booster pump, 12x10x25 SRL-C 12,000 3 1 75 hp motor f o r Item 2 56 5,000 4 1 E l e c t r i c cable and switchgear 20,000 5 500m 12" S c l a i r p i p e - s e r i e s 100 c/w fused ends and flange assemblies 30,000 6 500m 16" S c l a i r p i p e - s e r i e s 45 c/w fused ends and flange assemblies . 50,000 7 500m 4" S c l a i r p i p e - s e r i e s 45 c/w connectors (water l i n e ) 8,000 8 1000m 4" S c l a i r p i p e - s e r i e s 45 c/w connectors (concentrate l i n e ) 15,000 SUBTOTAL 290, 000 134 b. FLOTATION CIRCUIT (REMOTE) INSTALLED ITEM NO.REQ'D DESCRIPTION KW $ CDN 1 2 Outokumpu 0K-38-3u c/w d r i v e , feed and t a i l box, launders, epoxy p a i n t i n g , 75hp a g i t a t o r mechanisms and wear p a r t s 112 200,000 2 1 Blower, 54m3/min. a t 35 kPa 37 5,000 3 Power supply cable and switchgear, e l e c t r i c motors f o r F l o t a t i o n c e l l s 90,000 4 P i p i n g 10,000 5 1000m 4" S c l a i r p i p e - s e r i e s 45 c/w connectors (water l i n e ) 15,000 6 2 Concentrate pumps, 5x4x14 SRL-C 15,000 7 2 15hp motors f o r item 6 11 3,000 8 Reagent mixing, d e l i v e r y system f o r c o l l e c t o r , f r o t h e r , pH m o d i f i e r c/w tanks, a g i t a t o r s , pumps, p i p i n g 1 30,000 9 Instrumentation 30,000 10 P r e f a b r i c a t e d s t e e l b u i l d i n g , s i t e p r e p a r a t i o n , concrete, support f o r f l o t a t i o n c e l l s , l i g h t i n g , h e a t i n g , v e n t i l a t i o n 5 385,000 SUBTOTAL 166 683,000 135 c. REGRINDING INSTALLED ITEM NO.REQ'D DESCRIPTION KW $ CDN 1 1 B a l l m i l l (used), 9 l / 2 ' x l 6 ' , c/w 700hp d r i v e and motor, e l e c t r i c s , instrumentation, l u b r i c a t i o n , (concrete, rebar, engng.) 450 550,000 B a l l charge ( s t a i n l e s s ) 50 tonnes 40,000 L i n e r s (rubber) 90,000 2 2 18" cyclone 8,000 3 2 6x6 SRL pumps c/w d r i v e 12,000 4 2 40hp motors 30 4,000 5 1 V a r i a b l e speed d r i v e mechanism 12,000 6 E l e c t r i c a l 15,000 7 P i p i n g 10,000 8 S t e e l ( i n s t a l l e d ) 40,000 SUBTOTAL 480 781,000 136 d. BIOLEACHING INSTALLED ITEM NO REQ'D DESCRIPTION KW $ CDN 1 1 Stock tank - 7mo x 7m, m i l d s t e e l epoxy coated, c/w 50hp rubber covered a g i t a t o r , curbed containment 60,000 2 1 50 hp motor f o r Item 1 37 6,000 3 2 6x6 SRL pumps c/w d r i v e 12,000 4 2 40 hp motor f o r Item 3 30 5,000 5 8 11m x 10m b i o l e a c h r e a c t o r s , m i l d s t e e l , epoxy coated, covered, c/w br i d g e 2,000,000 6 9 A g i t a t o r mechanism f o r item 5, w i t h spare, rubber covered 600,000 7 9 100 hp motors f o r Item 6, w i t h spare 600 54,000 8 2 A i r blowers, 280m /minute (100%) each, 70.3 kPa o u t l e t pressure, 110°C discharge, 3 500 rpm, modulated surge c o n t r o l , base t h r o t t l e gates, check v a l v e s , f i l t e r s , s i l e n c e r s , couplings 130,000 9 2 700 hp motors f o r Item 8 1044 7 5,000 10 1600 A i r d i f f u s e r s , Wyss F l e x - a Tube 96,000 137 INSTALLED ITEM NO.REQ'D DESCRIPTION KW $ CDN 11 1 A c i d h o l d tank, 5m© x 5m, m i l d s t e e l , curbed containment 35,000 12 2 Metering pumps, a c i d r e s i s t a n t 1 4,000 13 1 N u t r i e n t mix tank, 2m0 x 2m, p l a s t i c 2,000 14 1 A g i t a t o r and motor (lhp) . f o r Item 13 1 2,000 15 A i r header - 200m 36" p l a s t i c pipe 50,000 - 100m 12" p l a s t i c pipe 16,000 - 1000m 4" p l a s t i c pipe 15,000 - F i t t i n g s , c o u p l i n g s , a c c e s s o r i e s 25,000 16 S l u r r y p i p i n g - S c l a i r p i p e -6 "0/ 200m s e r i e s 45 c/w f i t t i n g s , c o u p l i n g s , elbows, i n s u l a t e d 30,000 17 1 Feed box, rubber l i n e d , w e i r s , plugs 5,000 18 5 D i v e r t e r box, rubber l i n e d , plugs and a c t i v a t o r s 25,000 19 E l e c t r i c a l 200,000 20 Instrumentation 25,000 SUBTOTAL 1713 3,472,000 138 e. HEAT EXCHANGE INSTALLED ITEM NO REQ'D DESCRIPTION KW $ CDN 1 350 m s t a i n l e s s s t e e l c o o l i n g c o i l 20,000 2 2 C o o l i n g water pump, 6x18-13, d u c t i l e i r o n , 1750 rpm 15,000 3 2 60 hp motor f o r Item 2 45 8,000 4 1 Cooling tower - 1330 i c e ton c a p a c i t y - complete 100,000 5 2 Vent fans f o r Item 4, lOhp each 15 4,000 6 3 00m, 12" gS DWV water header pipe (feed and r e t u r n ) , feed s i d e i n s u l a t e d 25,000 7 Temperature c o n t r o l valves and in s t r u m e n t a t i o n , 8 independent loops 30,000 SUBTOTAL 60 202,000 139 f • FILTRATION / WASHING / NEUTRALIZATION INSTALLED ITEM NO REQ'D DESCRIPTION KW $ CDN 1 1 75m2 b e l t f i l t e r c/w 316 ss wetted p a r t s , c l o t h , r e c e i v e r s , f i l t r a t e pumps, i n t e r n a l p i p i n g , i n s t r u m e n t a t i o n , 75hp d r i v e 56 900,000 2 1 Vacuum pump, 7500 scfm a t 20 i n . vacuum, s i n g l e stage c/w base, f i t t i n g s 60,000 3 1 400 hp motor f o r Item 2 298 25,000 4 1 Repulp tank, 2rr? , m i l d s t e e l 5,000 5 2 C y a n i d a t i o n feed pumps, 3x3x10 SRL-C 6,000 6 2 lOhp motors f o r Item 5 8 2,000 7 2 N e u t r a l i z a t i o n tanks, ,; 2mo x 2m FRP, c/w a g i t a t o r mechanisms, 2 hp 20,000 8 2 2 hp motors f o r Item 7 2 1,000 9 1 Lime s l a k e r , 2 t CaO/h, 2 stage, c/w i n s t r u m e n t a t i o n , 1 hp a g i t a t o r s 2 20,000 10 1 2x2 SRL pump f o r lime loop 2,000 11 1 2 hp water pump f o r Item 10 2 1,000 140 f. FILTRATION / WASHING /NEUTRALIZATION Cont. INSTALLED ITEM NO.REQ'D DESCRIPTION KW $ CDN 12 1 Lime day tank, 3m<t> x 3m, m i l d s t e e l c/w rubber covered a g i t a t o r , 2 "hp 10,000 13 1 2 hp motor f o r Item 12 2 1,000 14 P l a s t i c p i p i n g ( f i l t r a t e s , t r e a t e d water, lime loop, concentrate) c/w f i t t i n g s , c o u p l i n g s , various s i z e s 10,000 15 E l e c t r i c a l 75,000 16 Instrumentation 20,000 17 1 200 tonne c a p a c i t y lime s i l o , c/w vent sock, support foundation, screw conveyor, steelwork, h a n d r a i l s 1 80,000 SUBTOTAL 371 1,238,000 cj CYANIDATION ITEM NO.REQ'D DESCRIPTION KW $ CDN •1 100M - 6" S c l a i r p i p e , c y a n i d a t i o n feed, i n s u l a t e d c/w connectors, f i t t i n g s , elbows 15,000 SUBTOTAL 0 15,000 141 h. TAILING DISPOSAL INSTALLED ITEM QUANTITY DESCRIPTION KW $ CDN A d d i t i o n a l power t o pump b i o l e a c h t a i l i n g t o t a i l i n g pond v i a e x i s t i n g t a i l i n g c i r c u i t 30 SUBTOTAL 3 0 j . CONCENTRATE IMPOUNDMENT ITEM QUANTITY DESCRIPTION KW $ CDN 1 1 Rock dyke, t i l l s ealed f o r concentrate impoundment (300,000m 3) 100,000 SUBTOTAL 0 100,000 142 a k. BUILDINGS, FOUNDATIONS, MISCELLANEOUS EQUIPMENT INSTALLED ITEM QUANTITY DESCRIPTION KW $ CDN 2 4000 rrr* S i t e p r e p a r a t i o n -excavation, back-f i l l and compaction, perimeter d r a i n s 100,000 2 1000 rr? Concrete - B i o l e a c h tank foundation Regrind / F i l t r a t i o n b u i l d i n g Reagent / stock tank foundation i n c l u d e s forming, rebar, g r o u t i n g 400,000 3 400 m2 Pr e f a b r i c a t e d s t e e l b u i l d i n g -complete s t e e l support - operator access, doors, plumbing, masonry, h e a t i n g and v e n t i l a t i o n , p a i n t i n g 500,000 4 6 Sump pumps c/w lOhp d r i v e and motors (45kW operating) 45 30,000 5 2 2 tonne crane 20,000 6 Miscellaneous - scrubbers, samplers, f i l l e r s , chutes, hoppers, spare p a r t s 100,000 SUBTOTAL 45 1,150,000 143 m. PROJECT OVERHEAD INSTALLED ITEM QUANTITY DESCRIPTION KW $ CDN 1 Co n t r a c t o r overhead 350,000 2 Design and Engineering -M e t a l l u r g i c a l Mechanical S t r u c t u r a l 600,000 3 C o n s t r u c t i o n management and Contractor Fees 400,000 SUBTOTAL 0 1,350,000 144 4.5 OPERATING COST ESTIMATE TAILING RECLAIM , ITEM DESCRIPTION / CRITERIA $/a $/t CONC . 1 Labour -1 operator a t $25/h continuous 219, 000 0. 75 0. 5 mechanic a t $25/h, 40h/wk 26, 000 0. 09 0. 25 e l e c t r i c i a n , as above 13, 000 0. 04 0. 25 instrument mech, as above 13, 000 0. 04 Consumables -Power (168 kW a t $0.028/kWh) 41,200 0.14 maintenance s u p p l i e s , l u b r i c a t i o n 5,200 0.02 SUBTOTAL 317,400 1.08 b. CONCENTRATE FLOTATION ITEM DESCRIPTION / CRITERIA $/a $/t CONC, 1 Labour -1 operator a t $25/h continuous 219,000 0.75 0.25 mechanic a t $25/h, 40h/wk 13,000 0.04 0.25 e l e c t r i c i a n / in s t r u m e n t a t i o n , as above 13,000 0.04 2 Consumables -Power (166 kW a t $0.028/kWh) 41,200 0.14 Xanthate (50g/t a t $1.60/kg) 175,200 0.60 MIBC (20g/t a t $1.76/kg) 77,100 0.26 145 b CONCENTRATE FLOTATION Cont. ITEM DESCCRIPTION / CRITERIA $ / a $/t CONC. H 2S0 4 (0.1 g/L a t $0.15/kg) Maintenance s u p p l i e s , l u b r i c a t i o n SUBTOTAL 4, 500 2,600 545,600 0.02 0.01 1.86 c. REGRINDING ITEM DESCRIPTION / CRITERIA $/a $/t CONC, Labour -0.5 operator a t $25/h continuous 109,500 0.38 0.25 mechanic a t $25/h, 40h/wk 13,000 0.04 0.25 e l e c t r i c i a n / i n s t r u m e n t a t i o n , as above 13,000 0.04 Consumables -power (480 kW a t $0.028/kWh) 117,700 0.40 g r i n d i n g media ($0.86/t cone.) 250,000 0.86 l i n e r s ($0.07/t cone.) 20,000 0.07 maintenance s u p p l i e s ($0.03/t) 8,800 0.03 SUBTOTAL 532,000 1.82 146 d. BIOLEACHING ITEM DESCRIPTION / CRITERIA $/a $/t CONC, Labour -1 operator a t $25/h continuous 219,000 0.75 0.20 mechanic a t $25/h, 40h/wk 10,400 0.04 0.25 e l e c t r i c i a n /instrument a t $25/h, 40h/wk 13,000 0.04 Consumables -power (1713 kW a t $0.028/kWh) 420,200 1.44 (NH 4 ) 2 S 0 4 (0.06 k g / t cone, a t $0.24/kg) 42,000 0.14 KH 2 P0 4 (0.12kg/t cone, a t $2.60/kg) 91,100 0.31 maintenance s u p p l i e s , l u b r i c a t i o n 1,000 0.01 SUBTOTAL 796,700 2.73 e. HEAT EXCHANGE ITEM DESCRIPTION /CRITERIA $/a . $/t CONC, Consumables -power (60kW a t $0.028/kWh) maintenance s u p p l i e s , l u b r i c a t i o n d e s c a l a n t , water c o n d i t i o n i n g SUBTOTAL 14,700 5,000 5,000 24, 700 0.05 0.02 0.02 0. 09 147 f• FILTRATION / WASHING / NEUTRALIZATION ITEM DESCRIPTION / CRITERIA $/a $/t CONC, Labour -0.5 operator, $25/h continuous 0.25 mechanic, $25/h, 40h/wk 0.25 e l e c t r i c i a n , as above 0.25 i n s t r u m e n t a t i o n mech, as above Consumables -power (371 kW a t $0.028/kWh) CaO (0.06t/t cone, a t $122/t) Maintenance s u p p l i e s ($0.20/t cone.) SUBTOTAL 109,500 13,000 13, 000 13, 000 91,000 2,137,400 58, 400 2,435,300 0. 38 0.04 0.04 0. 04 0.31 7.32 0. 20 8.33 g. CYANIDATION ITEM- DESCRIPTION / CRITERIA $/a $/t CONC. Consumables -NaCN (0.50kg/t cone, a t $1.80/kg) 262,800 CN d e s t r u c t i o n (20% of NaCN) 52,600 R e f i n i n g ($0.02/g Au+Ag produced, 3.66g Au/t, 25.7g Ag/t) 172,000 SUBTOTAL 487,400 0.90 0. 18 0.59 1.67 148 h. TAILING DISPOSAL ITEM DESCRIPTION / CRITERIA $/a $/t CONC. 1 Consumables -Incremental power to pump bioleach residues (treated water + cyanide t a i l i n g ) to t a i l i n g pond (30kW at $0.028/kWh) SUBTOTAL j . MISCELLANEOUS OPERATING EXPENSES ITEM DESCRIPTION / CRITERIA 7, 400 7, 400 0.03 0.03 $/a $/t CONC. 1 Supervisory Labour - 1 engineer 1 technician 2 Distributed Accounts -Assay 50, 000 35,000 24, 000 0.17 0.12 0. 09 Plant-Equipment / Supervision 24,000 0.08 Research Supplies 12,000 0.04 Services Purchased 6,000 0.02 Power -Sump Pumps (45kW at $0.028/kWh) 11,000 0.04 Lighting (40W/m2 x500m2= kW, as above) 4,900 0.02 Heating (natural gas -200 km3 gas) 30,000 0.10 Miscellaneous motors (lOOkW) 24,500 0.08 SUBTOTAL 2 21,400 0.7 6 149 4.6 DISCUSSION OF PROCESS DESIGN AND COSTING I t i s a n t i c i p a t e d t h a t the estimates described i n the previous s e c t i o n s are c o n s e r v a t i v e . The study todate was p r i m a r i l y e x p l o r a t o r y and t h e r e f o r e was not subjected to r i g o r o u s o p t i m i z a t i o n . O p t i m i z a t i o n of the b i o l e a c h c i r c u i t design ( c a p i t a l cost) and the f i l t r a t i o n /washing / n e u t r a l i z a t i o n o p e r a t i n g c o s t w i l l have the g r e a t e s t impact on the p r o j e c t r e t u r n or investment (see S e c t i o n 4.8 f o r comparative summary). Due to the p r e l i m i n a r y nature of the design and c o s t estimates there are a number of contingencies inherent i n the analys i s : ( i ) c o n s e r v a t i v e f l o t a t i o n estimates due t o the unknown sur f a c e s t a t e of the impounded t a i l i n g sand ( f l o t a t i o n r a t e , recovery, and re a g e n t s ) , ( i i ) c o n s i d e r a b l e f l e x i b i l i t y and overdesign of the b i o l e a c h r e a c t o r s due t o - u n c e r t a i n t y of exact o p e r a t i o n , ( i i i ) c o n s e r v a t i v e work index estimates f o r the bulk s u l p h i d e r e g r i n d , ( i v ) assumed no heat l o s s through b i o l e a c h r e a c t o r w a l l s , assumed summer c o o l i n g requirements f o r op e r a t i n g c o s t s , and assumed 30°C maximum o p e r a t i n g temperature where 40°C i s probably the upper l i m i t f o r o p e r a t i o n , (v) assumed t h a t none of the b i o l e a c h a t e s o l u t i o n w i l l be employed as a pretreatment f o r cyanide d e s t r u c t i o n and t h a t b i o l e a c h v e n t i l a t i o n a i r w i l l not reduce cyanide d e s t r u c t i o n o p e r a t i n g c o s t s , 150 ( v i ) assumed t h a t a c i d mine drainage (AMD) w i l l not be used as a c i d makeup, and ( v i i ) the a n a l y s i s w i l l not i n c l u d e any c o n t r i b u t i o n from a hi g h grade Main Zone bulk sulphide concentrate f l o a t e d from c y a n i d a t i o n t a i l i n g , f o r which b i o l e a c h testwork has been ongoing a t E q u i t y . Therefore a contingency value was not i n c o r p o r a t e d i n the c o s t estimates as s i g n i f i c a n t contingency i s evident i n the a n a l y s i s as d e s c r i b e d . 4.7 REVENUE AND CASH FLOWS The economic f e a s i b i l i t y a n a l y s i s i s based on the c a p i t a l and o p e r a t i n g c o s t estimates p r e v i o u s l y summarized, the precious metals e x t r a c t i o n estimates as presented, and c u r r e n t p r e c i o u s metals market p r i c e s . A "base case" s i t u a t i o n served as the s t a r t i n g p o i n t f o r cash flow e s t i m a t i o n , as described below. A b r i e f s e n s i t i v i t y of the cash flow about metal p r i c e , c a p i t a l c o s t , and o p e r a t i n g cost was conducted. 151 4.7.1 Revenue C a l c u l a t i o n - Base Case C a l c u l a t i o n c r i t e r i a -800 tpd concentrate t r e a t e d 350 d/a ope r a t i n g 5.5 g Au/t concentrate 90 g Ag/t concentrate 70% Au e x t r a c t i o n 30% Ag e x t r a c t i o n 95% net payment f o r metal e x t r a c t i o n $CDN 13.50/g Au $CDN 0.27/g Ag ( i ) Au: 1 tonne * 5.5g Au/t * 0.70 * 0.95 = 3.66g Au/t recovered 3.66g Au/t * $13.50/g Au = $49.4l/t Ag: 1 tonne * 90 Ag/t * 0.30 * 0.95 = 25.7g Ag/t recovered 25.7g Ag/t * $0.27/g Ag = $6.94/t T o t a l = $49.4l/t + $6.94/t = $56.35/t ( i i ) 800 tpd * 350 d/a * $56.35/t = $15,788 x 10 6/a 4.7.2 Cash Flow S e n s i t i v i t y The "base case" parameters f o r cash flow e s t i m a t i o n were: $CDN 13.50/g Au ($US 300/oz Au) $CDN 0.27/g Ag ($US 6/oz Ag) 152 7 YEAR PROJECT YEAR 0 - c a p i t a l expense f o r t a i l i n g r e c l a i m / f l o t a t i o n YEAR 1 - c a p i t a l expense f o r impoundment dams - o p e r a t i n g expense f o r r e c l a i m / f l o t a t i o n YEAR 2 - c a p i t a l expense f o r b i o l e a c h c i r c u i t - o p e r a t i n g expense f o r r e c l a i m / f l o t a t i o n YEAR 3 - o p e r a t i n g expense f o r r e c l a i m / f l o t a t i o n - o p e r a t i n g expense f o r b i o l e a c h / c y a n i d a t i o n YEAR 4 •-• o p e r a t i n g expense f o r r e c l a i m / f l o t a t i o n - o p e r a t i n g expense f o r b i o l e a c h / c y a n i d a t i o n YEAR 5 - o p e r a t i n g expense f o r b i o l e a c h / c y a n i d a t i o n YEAR 6 - o p e r a t i n g expense f o r b i o l e a c h / c y a n i d a t i o n Equipment c r e d i t a f t e r shutdown - assumed 0 Revenue - $15.778/a (Year 3-6) Operating expense - $823 ,x^l0 3'/a-for r e c l a i m / f l o t a t i o n (Year 1-4) - $4320 x 10 /a f o r b i o l e a c h i n g / c y a n i d a t i o n (Year 3-6) C a p i t a l Expense - $973 x 10 3 f o r r e c l a i m / f l o t a t i o n (Year 0) 3 - $100 x 10 f o r impoundment dams (Year 1) 3 - $8208 x 10 f o r b i o l e a c h i n g / c y a n i d a t i o n (Year 2) C a p i t a l a s s e t s 30% d e p r e c i a b l e on d e c l i n i n g balance (expansion asset) Taxation ( f e d e r a l , p r o v i n c i a l , resource) t o t a l 58% Discount f a c t o r = 15% 153 The base case cash flow s e n s i t i v i t y was conducted +20% on metal p r i c e (revenue), c a p i t a l cost, and operating cost: metal p r i c e +20% = $16.20/g Au, $0.32/g Ag -20% = $10.80/g Au, $0.22/g Ag c a p i t a l cost +20% = $1168; 120; 9850 x 10 3 = $11138 -20% = $778; 80; 6566 x 10 3 = $7424 x operating cost +20% = $988; 5184 x 10 3 -20% = $658/ 3456 x 10 3 A l l of the detailed cash flow estimates are included i n Appendix I I . 154 4.8 EXECUTIVE SUMMARY  a. METALLURGY ( i ) 8.5x10^ tonnes Southern T a i l /Main Zone t a i l i n g to r e c l a i m grading 0.90g Au/t and 22g Ag/t - 75% recoverable ( i i ) Reclaim 6000 tpd t a i l i n g to minimize f u r t h e r d i l u t i o n by Main Zone t a i l i n g ( i i i ) 800 tpd bulk s u l p h i d e concentrate produced, grading 5.5g Au/t, 90g Ag/t, r e p r e s e n t i n g 81.5% Au recovery, 54.5% Ag recovery and 13.3% weight recovery -concentrate i s s t o r e d pending b i o l e a c h c i r c u i t c o n s t r u c t i o n ( i v ) B i o l e a c h i n g f o r 60 hours to e f f e c t 10% Fe/S o x i d a t i o n and 70% As s u l p h i d e o x i d a t i o n . (v) Washed b i o l e a c h r e s i d u e i s n e u t r a l i z e d and cyanide leached f o r 4 hours t o e x t r a c t 70% Au and 30% Ag from the b i o l e a c h e d concentrate. 95% d i s s o l v e d metal recovery by CIL i s assumed. ( v i ) C y a n i d a t i o n t a i l i n g f o l l o w s e x i s t i n g d e s t r u c t i o n / d i s p o s a l c i r c u i t w h i l e b i o l e a c h a t e s o l u t i o n i s n e u t r a l i z e d w i t h lime and mixed w i t h t a i l i n g sand f o r d i s p o s a l . 155 b. COST ESTIMATE $ CDN % OF ( i ) CAPITAL (xlOOO) TOTAL ( i ) T a i l i n g Reclaim 290 3 . 1 ( i i ) F l o t a t i o n 683 7.4 ( i i i ) R egrinding 781 8.4 ( i v ) B i o l e a c h i n g 3472 37.4 (v) Heat Exchange 202 2.2 ( v i ) F i l t r a t i o n / W a s h i n g / N e u t r a l i z a t i o n 1238 13.3 ( v i i ) C y a n i d a t i o n 15 0.2 ( v i i i ) T a i l i n g d i s p o s a l 0 0 ( i x ) B u i l d i n g s , Foundations, Misc. Equip. 1150 12.4 (x) P r o j e c t Overhead 1350 14.5 ( x i ) Concentrate impoundment 100 1.1 TOTAL 9281 100.0 ( i i ) OPERATING $CDN/t CONC. % OF TOTAL ( i ) T a i l i n g Reclaim 1.08 5.9 ( i i ) F l o t a t i o n 1.86 10. 1 ( i i i ) R egrinding 1.82 9.9 ( i v ) B i o l e a c h i n g 2.73 14.9 (v) Heat Exchange 0.09 0.5 ( v i ) F i l t r a t i o n / W a s h i n g / N e u t r a l i z a t i o n 8.33 45.3 ( v i i ) C y a n i d a t i o n 1.67 9.1 ( v i i i ) T a i l i n g D i s p o s a l 0.03 0.2 ( i x ) M iscellaneous D i s t r i b u t e d Accounts 0.76 4.1 TOTAL 18. 37 100.0 156 c. REVENUE (Base Case) At $13.50/g Au, $0.27/g Ag: ( i ) Au: 1 tonne * 5.5g Au/t * 0.70 * 0.95 = 3.66g Au/t recovered 3.66 g Au/t * $13.50/g Au = 49.41/t Ag: 1 tonne * 90g Ag/t * 0.30 * 0.95 = 25.7g Ag/t recovered 25.7g Ag/t * $0.27/g Ag = $6.94/t T o t a l = $49.4l/t + $6.94/t = $56.35/t ( i i ) 800 tpd * 350 d/a * $56.35/t = $15,778 x 10 6/a d. CASH FLOW SENSITIVITY A s e n s i t i v i t y a n a l y s i s about the base case f o r c a p i t a l c o s t s , o p e r a t i n g c o s t s , and metal p r i c e (revenue) i s summarized i n Figure 4.5. 157 CUMULATIVE NPV at 15 % 0 1 2 3 4 5 8 7 8 9 1 0 1 » •* i 1 -1 •20% + 20% Ravwnua • 20% Capital cost • 20% .20% Operating cost BASE CASE $13.50/gAu $0.27/gAg F i g u r e 4.5 S e n s i t i v i t y s u m m a r y 158 5.0 ADDITIONAL APPLICATIONS A l l of the researc h data summarized h e r e i n was s p e c i f i c t o a bulk s u l p h i d e concentrate f l o a t e d from Southern T a i l t a i l i n g . There are a d d i t i o n a l p o t e n t i a l a p p l i c a t i o n s of b i o o x i d a t i o n to enhance precious metals e x t r a c t i o n a t Equ i t y S i l v e r Mines. The Main Zone c l e a n e r t a i l i n g , which may represent up to 500 tpd s o l i d s , i s the source of con s i d e r a b l e r e g r i n d c i r c u i t l o a d i n g and f l o t a t i o n c i r c u i t pulp d i l u t i o n due t o r e c i r c u l a t i o n . Much of the c l e a n e r t a i l i n g e x i s t s as a mi d d l i n g f r a c t i o n which r e q u i r e s f u r t h e r comminution f o r economic upgrading. An a l t e r n a t e method of treatment t o upgrade c l e a n e r t a i l i n g i s t o t h i c k e n , b i o l e a c h , then e x t r a c t the precious metals i n the e x i s t i n g carbon-in-leach c i r c u i t . I n i t i a l c y a n i d a t i o n testwork on the Main Zone was d i r e c t e d a t producing a bulk s u l p h i d e concentrate from Main Zone f l o t a t i o n t a i l i n g and cyanide l e a c h i n g the concentrate to scavenge preci o u s metals from Main Zone f l o t a t i o n t a i l i n g . A s i g n i f i c a n t f r a c t i o n of the bulk s u l p h i d e p r e c i o u s metals was r e f r a c t o r y t o cyanide e x t r a c t i o n . C y a n i d a t i o n of the e n t i r e f l o t a t i o n t a i l i n g was more p r o f i t a b l e . I t may be advantageous t o remove a bulk s u l p h i d e f r a c t i o n of the Main Zone f l o t a t i o n t a i l i n g , b i o l e a c h the s u l p h i d e s , and r e t u r n the b i o l e a c h r e s i d u e to the CIL c i r c u i t feed, f o r precious metals e x t r a c t i o n . 159 Both of the a l t e r n a t i v e s d escribed above would r e q u i r e a d e t a i l e d research p l a n t o determine the preliminary-f e a s i b i l i t y . There i s very l i t t l e s i m i l a r i t y between the b i o l e a c h feed m a t e r i a l i n each case. Therefore assumptions based on the Southern T a i l testwork described i n t h i s r e p o r t are probably i n v a l i d . A p p l i c a t i o n of b i o o x i d a t i o n t o Main Zone m a t e r i a l i s p a r t i c u l a r l y a t t r a c t i v e due to the l a r g e r tonnage a v a i l a b l e f o r treatment. I t may be more a t t r a c t i v e t o delay Main Zone scavenging u n t i l the end of mine l i f e then r e c l a i m the t a i l i n g pond f o r a d d i t i o n a l p r e c i o u s metals e x t r a c t i o n using b i o o x i d a t i o n , depending on metal p r i c e f o r e c a s t s . T a i l i n g pond r e p r o c e s s i n g may prove b e n e f i c i a l i n the longer term f o r b a c k - f i l l i n g the spent p i t s and s e a l i n g the waste dumps f o r long term re c l a m a t i o n and a c i d mine drainage c o n t r o l . 160 6.0 CONCLUSIONS AND RECOMMENDATIONS Laboratory and p i l o t s c a l e r e s u l t s showed that s e l e c t i v e b i o o x i d a t i o n of the a r s e n i c bearing sulphide mineral(s) i n Southern T a i l bulk sulphide concentrate was e f f e c t i v e f o r enhanced gold and s i l v e r recovery by c y a n i d a t i o n . D i r e c t c y a n i d a t i o n of the concentrate r e s u l t e d i n approximately 10% gold and s i l v e r e x t r a c t i o n . B i o o x i d a t i o n of approximately 15% of the sulphide gangue r e s u l t e d i n 70% gold e x t r a c t i o n and 30-40% s i l v e r e x t r a c t i o n by subsequent c y a n i d a t i o n . Batch shake f l a s k t e s t i n g provided a p r e l i m i n a r y i n d i c a t i o n of the important b i o l e a c h parameters and t h e i r approximate range f o r s u c c e s s f u l b i o o x i d a t i o n . Batch l a b o r a t o r y s t i r r e d r e a c t o r t e s t i n g provided s i m i l a r b i o h y d r o m e t a l l u r g i c a l data to that of shake f l a s k t e s t i n g and a l s o provided the necessary q u a n t i t i e s of b i o l e a c h products f o r downstream process a n a l y s i s , such as c y a n i d a t i o n and biol e a c h a t e treatment and waste d i s p o s a l . Continuous l a b o r a t o r y s c a l e b i o l e a c h i n g provided the data necessary to determine the c r i t i c a l design areas f o r p i l o t p l a n t design and a l s o provided extensive continuous, s t a r t u p , and shutdown o p e r a t i n g experience which was i n v a l u a b l e i n p i l o t p l a n t o p e r a t i o n . The p i l o t s c a l e study provided meaningful opera t i n g data on a l a r g e s c a l e to i n d i c a t e the e f f e c t of scale on b i o h y d r o m e t a l l u r g i c a l operations as w e l l as i n d i c a t e c r i t i c a l areas f o r design c o n s i d e r a t i o n of l a r g e r s c a l e o p e r a t i o n s . The p i l o t p l a n t o p e r a t i o n provided the necessary opera t i n g 161 experience w i t h which to c o n f i d e n t l y propose a f e a s i b i l i t y study f o r p l a n t s c a l e b i o o x i d a t i o n a t E q u i t y S i l v e r Mines. There was no s i g n i f i c a n t e f f e c t of s c a l e on continuous b i o h y d r o m e t a l l u r g i c a l o p e r a t i o n and on subsequent preci o u s metals hydrometallurgy. However, f o r design purposes, the author recommends t h a t the optimum pulp d e n s i t y , a e r a t i o n r a t e , and residence time be defined on a p i l o t s c a l e a t l e a s t as l a r g e as the c i r c u i t t e s t e d a t E q u i t y . A heat balance should be c a l c u l a t e d f o r p l a n t s c a l e operations as the p i l o t c i r c u i t s d i d not p r o v i d e s u f f i c i e n t heat balance i n f o r m a t i o n by measurement. For f u t u r e p i l o t c i r c u i t s of t h i s s i z e the f o l l o w i n g m o d i f i c a t i o n s are recommended: ( i ) reduced residence time i n the stock tank or no stock tank a t a l l due t o the unknown e f f e c t s of concentrate p r e a e r a t i o n and c o n d i t i o n i n g , ( i i ) r e g r i n d m i l l i n closed c i r c u i t w i t h a c l a s s i f i e r f o r b e t t e r g r i n d c o n t r o l , ( i i i ) surge tank p r i o r t o the feed d i s t r i b u t o r t o maintain a more constant feed volume which would reduce p o t e n t i a l sanding d i f f i c u l t i e s , ( i v ) minimum l i n e v e l o c i t y of 2 meters per second f o r s u l p h i d e concentrates a t low d e n s i t y ( < 2 0 % s o l i d s ) , f o r pump s i z i n g , speed, and l i n e s i z i n g , to avoid sanding, 162 (v) v a r i a b l e speed a g i t a t o r s designed f o r a t l e a s t 30% s o l i d s o p e r a t i o n , ( v i ) more p r e c i s e a i r f l o w r a t e and bubble s i z e c o n t r o l , ( v i i ) more p r e c i s e wash water c o n t r o l and measurement and more p r e c i s e f i l t r a t e measurement and sampling. The most e f f e c t i v e r a t e c o n t r o l v a r i a b l e s to consider i n f u t u r e continuous c i r c u i t designs i n c l u d e p a r t i c l e s i z e , a e r a t i o n r a t e , pulp d e n s i t y , pH, r e a c t o r bypassing and temperature. The most important v a r i a b l e s t o address f o r p l a n t s c a l e design i n c l u d e r e a c t o r design ( i n c l u d i n g pulp d e n s i t y ) and c o n f i g u r a t i o n ( f o r bypassing), a e r a t i o n method and r a t e , m a t e r i a l s of c o n s t r u c t i o n , heat balance and c o n t r o l , water source and balance, waste treatment and d i s p o s a l , and process c o n t r o l s t r a t e g y . I t i s a n t i c i p a t e d t h a t l a r g e s c a l e b i o o x i d a t i o n c i r c u i t s w i l l employ conventional u n i t processes w i t h minor m o d i f i c a t i o n s , such as m a t e r i a l s of c o n s t r u c t i o n , to s u i t the s p e c i f i c needs of a continuous b i o l o g i c a l process. The g r e a t e s t c o s t f a c t o r s i n sul p h i d e b i o o x i d a t i o n w i l l probably be a c i d i c byproduct n e u t r a l i z a t i o n , s o l i d / l i q u i d s e p a r a t i o n , and power to supply a i r / C 0 2 t o the b i o l e a c h c i r c u i t . The author recommends t h a t h i g h d e n s i t y sludge technology be employed f o r b i o l e a c h a t e treatment to maximize the b e n e f i t s of the lime f o r n e u t r a l i z a t i o n and t o minimize the f i n a l volume of the r e s u l t a n t hydroxide sludge. 163 Areas t h a t r e q u i r e a d d i t i o n a l research and development to expedite widespread use of biotechnology f o r r e f r a c t o r y s u l p h i d e o x i d a t i o n i n c l u d e : ( i ) h i g h temperature b i o o x i d a t i o n using S u l f o l o b u s s p e c i e s , ( i i ) improved a i r d i f f u s i o n devices f o r s l u r r y systems to a l l o w use of low pressure a i r i n l a r g e r e a c t o r s , ( i i i ) improved intermediate s o l u t i o n exchange methods f o r b i o o x i d a t i o n r a t e c o n t r o l where a h i g h degree of s u l p h i d e o x i d a t i o n i s r e q u i r e d , ( i v ) h i g h e r c a p a c i t y s o l i d / l i q u i d s e p a r a t i o n devices f o r more e f f i c i e n t b i o l e a c h r e s i d u e washing, (v) more s e l e c t i v e s t r a i n s of b a c t e r i a f o r p a r t i c u l a r mineral assemblages, ( v i ) the p o t e n t i a l of t h i o u r e a as a p r e c i o u s metals l i x i v i a n t f o l l o w i n g b i o o x i d a t i o n . I t i s economically f e a s i b l e to r e c l a i m 8.5 x 10 tonnnes of E q u i t y Southern T a i l f l o t a t i o n t a i l i n g and scavenge g o l d and s i l v e r values by f l o t a t i o n / b i o l e a c h i n g / c y a n i d a t i o n of the r e f r a c t o r y s u l p h i d e s . However, due t o the time c o n s t r a i n t s , t o avoid d i l u t i o n by encroaching Main Zone t a i l i n g , i t may be more economically favourable to reprocess the e n t i r e t a i l i n g pond i n a s i m i l a r f a s h i o n to maximize the precio u s metals scavenged from E q u i t y f l o t a t i o n t a i l i n g . 164 REFERENCES 1. Cyr, J.B., Pease R.B., and Schroeter, T.G., "Geology and M i n e r a l i z a t i o n of E q u i t y S i l v e r Mine," Econ. Geol. V o l . 79, 1984 pp 947-968. 2. Bruce, D.E. and M i l l e r , J.H.L., "Equity S i l v e r Mines - A Success Story," presented a t the Northwest Assoc. 1983 Short Course, Nov 28-30, 1983, Spokane, Washington, U.S.A. 3. 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APPENDIX I B i o l e a c h Heat Balance P l a n t Scale Example BIOLEACH HEAT BALANCE - PLANT SCALE EXAMPLE T o t a l feed = 2600 m3/d x 1.23 t/m3 = 3198 t/d (i) B a s e l i n e design c r i t e r i a m3/ at 25% s o l i d s = (3198) x 0.25 = 800 tpd M a i n t a i n leach r e a c t o r s a t 30°C Assume no heat l o s s through tanks Blower a i r (14000 scfm) a t tanks = 110°C Assume f r e s h water a v a i l a b l e at 16°C f o r c o o l i n g (summer) Regrind m i l l d i s c h a r g e pulp = 15°C 120 kcal/mole S o x i d i z e d i n exothermic r e a c t i o n ( S h e r r i t t Gordon) ( i i ) Pulp h e a t i n g requirements: oQ = ra Cp nT + m, Cp, <=>T s r s s 1 1 1 where m = mass f l o w r a t e of s o l i d s (s) or l i q u i d (1) p o r t i o n of the s l u r r y Cp = s p e c i f i c heat o f the s o l i d s (s) or l i q u i d (1) p o r t i o n of the s l u r r y (Assume Cp i s e s s e n t i a l l y independent o f temperature over the narrow range being addressed i e : a constant) •aT = temperature changeof component (T f i n a l - T i n i t i a l ) C o n c e n t r a t i o n of s u l f u r i c a c i d can change the heat c a p a c i t y of the s o l u t i o n : 32t of Sulphur = 98t H 2S0 4 98/3198 x 100% = 3.06% H 2 S 0 4 s o l u t i o n Cp 3.5% H 2S0 4 = 0.9688 cal/g°C (Perry 5th e d i t i o n ) A more a p p r o p r i a t e f i g u r e assuming l i n e a r o x i d a t i o n would be an average v a l u e o f : Cpl.53% H 2S0 4 = 0.9855 c a l / g C (Perry by i n t e r p o l a t i o n ) 174 AQ = 800t/d x 0.25 cal/g°C x (30°C - 15°C) x 1 0 6 g / t + 2398 t/d x 0.9855 cal/g°C x (30°C - 15°C) x 10* g/t = 3.845 x I O 1 0 c a l / d = 3.845 x 10 7 k c a l / d = 1862 kW ( i i i ) Heat donated by sulphur o x i d a t i o n : 32 tS/d x 10 6mole/32t-S ! x 120 kcal/mole S = 1.2 x 10 8 k c a l / d = 5811 kW (iv) Heat donated by a i r : 14000 scfm x m 3/35.3147 f t 3 x 60 min/h x 24 h/d x 1 mole/0.0224lm 3 x 29g a i r / m o l e x 1 t / 1 0 6 g = 739 t a i r / d = 2.547 x 10 g moles a i r / d r Tf i Q a i r = M J Cp (t)dT a T i a where m = molar flow of a i r a Cp (t) = s p e c i f i c heat of the a i r which i s a a Q f u n c t i o n of temperature (J/mol c) — 3 — 6 2 = 28.94 + 4.147 x 10 T + 3.191 x 10 T -9 3 -1.965 x 10 T ( F e l d e r & Rosseau) where T i s i n °C T i = i n i t i a l temperature (110°C) Tf = f i n a l temperature (30°C) (Assumed i n e q u i l i b r i u m w i t h r e a c t o r s l u r r y ) 7 <s Q a i r = 2.547 x 10 moles/d x /3 0 110' / -0.4913 x 10 9 T 4 110/ 110 175 = 2.547 x 10 7 moles/d[2315.2 + 23.2 + 1.4 - O.i] = 2.547 x 10 7 moles/d [2339 .7 J/molel 7 = 5.959 x 10 J o u l e s / d (1 J o u l e = 0.23901 c a l ) = 1.424 x 10 7 k c a l / d = 690 kW (v) Summary of h e a t i n g a. Donated by S o x i d a t i o n = 5811 kW b. Donated by a i r = 690 kW c. Consumed to heat s l u r r y 15° = 1862 kW Net heat balance 4639 kW t h e r e f o r e , 4639 kW must be removed to m a i n t a i n 30°C b a l a n c e . C o o l i n g the a i r would not be e f f i c i e n t (only 15% of heat input) t h e r e f o r e , c o o l i n g c o i l s i n r e a c t o r s i n c l o s e d c i r c u i t w i t h a c o o l i n g tower i s more a p p r o p r i a t e . (vi) S u r f a c e area f o r c o o l i n g Q = h A AT o o where Q = heat to be removed h = heat t r a n s f e r c o e f f i c i e n t of the o u t e r c o i l o A = o u t e r s u r f a c e area a v a i l a b l e f o r heat t r a n s f e r o AT = temperature d i f f e r e n c e between the c o o l i n g water and the s l u r r y S i n c e the c o o l i n g water i s heated as i t t r a v e l s through the c o i l , an average temperature d i f f e r e n t i a l should ^e used. Assuming the p r o c e s s temperatue i s c o n s t a n t at 30 C. AT = 30°C - ( T c o o l i n g H_0 i n + T c o o l i n g H O out) 2 heat c o e f f i c i e n t - can be c a l c u l a t e d (Oldshue, Chem Engng, V o l . 90, No. 12 , 1983) h o d = 0 . 1 7 / D 2 N A ° - 6 7 / C P U N ° - 3 7 1 7 6 where h^ = o u t s i d e f i l m heat t r a n s f e r c o e f f i c i e n t (W/m2°c) d = tube diameter (3" i n t h i s example = 0.0762m) D = i m p e l l o r diameter (4 m) N = r o t a t i o n a l speed of a g i t a t o r (20 rpm) J~* = d e n s i t y of the s l u r r y (1230 kg/m ) = v i s c o s i t y of the s l u r r y (IcP = 0.01 g/cm'sec.) Cp = s p e c i f i c heat of the s l u r r y (0.80 cal/g°c) T = tank diameter (11m) y^s = v i s c o s i t y of the s l u r r y at the mean f i l m temperature (~yu a t s m a l l temperature d i f f e r e n c e s ) k = thermal c o n d u c t i v i t y of the f l u i d ( t hat of water = 0.620 W/m°C) m = e x p e r i m e n t a l v i s c o s i t y c o r r e c t i o n f a c t o r t h e r e f o r e , h =7170 W/m2 °c o from Q = h A A T o o A = 76 . 3 m 2 o Assuming c o o l i n g water a t 1500 USGPM (94.63 kg/sec.) AT = 11°C o. t h e r e f o r e , T f o r c o o l i n g H_0 =27 C (16 +11) t i n d i 2. I t can be argued t h a t the s u l p h u r o x i d a t i o n w i l l not be l i n e a r i n p a r a l l e l tanks. T h e r e f o r e , each r e a c t o r must have an independent temperature c o n t r o l loop, each s e r v i c e d by a common c o o l i n g tower. For a 3 i n c h c o o l i n g c o i l d i a m e t e r : A = 2 rr r L o 76.3 m2 = 2 -rr (0 . 0381m)L L= 319 m or an average of 319 . = 40 m per tank / ° = APPENDIX I I D e t a i l e d Cash Flow E s t i m a t e s CASH FLOW ESTIMATE BASE CASE (thousands of d o l l a r s , 1985) 198S YEAR 0 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 REUENUE - 0 0 15779 15779 15778 15778 OPERATING EXPENSES 923 923 5143 5143 4320 4320 INTEREST ON LOAN 97 199 1122 561 25 0 CASH FLOW BEFORE TAXES 0 0 9513 10074 11433 11458 DEPRECIATION &30>. d.b. 0 0 2794 1949 1364 955 TAXABLE INCOME BASE 0 0 S728 B125 10059 10503 INCOME TAX 0 G 3902 4712 5840 6092 CASH FLOW AFTER TAX 0 0 SB 10 5361 5593 5366 LOAN REPAYMENTS 0 0 5510 5361 252 0 NET CASH FLOW 0 0 0 0 5341 5366 NPU 015* 0 0 0 0 2655 2320 CUMULATIUE NPUSlST-i 0 0 0 0 2655 497S BALANCE OF LOAN 973 1993 11224 5613 252 0 0 BALANCE DEPRECIABLE E30* 973 1073 9291 6497 4549 3193 2228 co CASH FLOUJ EST I MATE B A S E C A S E +20% R E V E N U E Cthausands of d o l l a r s , 1SB53 1SQS YEAR 0 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 REUENUE 0 0 18334 18334 18334 18334 OPERATING EXPENSES 823 923 5143 5143 4320 4320 INTEREST ON LOAN 37 133 1122 423 0 0 CASH FLOUJ BEFORE TAXES 0 0 12663 13362 14614 14614 DEPRECIATION @30?; d.b. 0 0 27B4 1343 1364 355 TAXABLE INCOME BASE 0 0 3BB4 11413 13250 13653 INCOME TAX GSB^i 0 0 5733 6620 76B5 7322 CASH FLOUJ AFTER TAX 0 0 6336 6743 6323 6632 LOAN REPAYMENTS 0 0 6335 42BB 0 0 NET CASH FLOUJ 0 0 0 2455 6323 6632 NPU @15?s 0 0 0 1403 3445 2833 CUMULATIUE NPU@15"-i 0 0 0 1403 484B 7742 BALANCE OF LOAN (310JS S73 1333 11224 42BB 0 0 0 BALANCE DEPRECIABLE 030* S73 1073 92B1 6437 454B 3183 222B CASH FLOW ESTIMATE BASE CASE -20% REVENUE Cthausands of d o l l a r s , 1985) 1386 YEAR 0 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 REUENUE 0 0 12522 12622 12622 12622 OPERATING EXPENSES 823 823 5143 5143 4320 4320 INTEREST ON LOAN 37 133 1122 634 296 0 CASH FLOW BEFORE TAXES 0 0 5357 6785 8006 8302 DEPRECIATION C330* d.b. 0 0 2784 1349 1364 955 TAXABLE INCOME BASE 0 0 3572 4836 6642 7347 INCOME TAX 058* 0 0 2072 2B05 3852 4261 CASH FLOW AFTER TAX 0 0 4285 3380 4154 4041 LOAN REPAYMENTS 0 0 42B5 3380 2353 0 NET CASH FLOW 0 0 0 0 1195 4041 NPU GlS'i 0 0 0 0 534 1747 CUMULATIUE NPUG15*; 0 0 0 0 594 2341 BALANCE OF LOAN SlO'-i 373 1333 11224 5333 2353 0 0 BALANCE DEPRECIABLE 030* 973 1073 3281 S437 4548 31B3 2228 C A S H FLOW E S T I M A T E B A S E C A S E +20% C A P I T A L C O S T S (thousands of d o l l a r s , 19855 199B YEAR 0 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR B REUENUE 0 0 15778 15778 15778 15778 OPERATING EXPENSES 823 823 5143 5143 4320 4320 INTEREST ON LOAN • 117 223 1312 727 175 0 CASH FLOW BEFORE TAXES 0 0 9323 9908 11283 11459 DEPRECIATION 03OV. d.b. 0 0 3341 2339 1B37 1146 TAXABLE INCOME BASE 0 •0 5981 7569 9646 10312 INCOME TAX 058* 0 0 3469 4390 5594 5991 CASH FLOW AFTER TAX 0 0 5854 5518 5B88 5477 LOAN REPAYMENTS 0 0 5954 5519 1752 0 NET CASH FLOW 0 0 0 0 3936 5477 NPU 015* 0 0 0 0 1957 2368 CUMULATIUE NPU@15* 0 0 0 0 1957 4325 BALANCE OF LOAN 010* 1168 2228 13124 7270 1752 0 0 BALANCE DEPRECIABLE 030* 1168 128B 11138 7797 5458 3920 2674 CASH FLQuJ ESTIMATE BASE CASE -20 (thousands of d o l l a r s , 1985) REUENUE OPERATING EXPENSES INTEREST ON LOAN CASH FLOUJ BEFORE TAXES DEPRECIATION 030* d.b. TAXABLE INCOME BASE INCOME TAX 05B* CASH FLQUJ AFTER TAX LOAN REPAYMENTS NET CASH FLOUJ NPU 015* CUMULATIUE NPU015* 1986 YEAR 0 YEAR 1 0 883 7B 0 0 0 0 0 0 0 0 0 BALANCE OF LOAN 010* 778 1753 BALANCE DEPRECIABLE 030* 778 858 CAPITAL COSTS YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 0 15778 15778 15778 15778 833 5143 5143 4320 4320 176 332 336 0 0 0 3703 10233 11458 11458 0 2227 1559 1031 764 0 7475 8680 10367 10634 0 4335 5035 5013 6203 0 5367 5205 5445 5255 0 5367 3357 0 0 0 0 1248 5445 5255 0 0 714 2707 2272 0 0 714 3421 5633 3324 3357 0 0 0 7424 5137 3638 2546 1783 CASH FLOW ESTIMATE BASE CASE +20% OPERATING COST (thousands of d o l l a r s , 1385: 1386 YEAR 0 YEAR 1 YEAR 8 YEAR 3 YEAR 4 YEAR 5 YEAR 6 REUENUE 0 0 15778 1577B 15778 15778 OPERATING EXPENSES 9BB 3B8 6172 6172 5184 5184 INTEREST ON LOAN 37 216 1157 641 151 0 CASH FLOW BEFORE TAXES 0 0 8443 B365 10443 10534 DEPRECIATION (330* d.b. 0 0 2784 1343 1364 355 TAXABLE INCOME BASE 0 0 5665 7016 3073 3633 INCOME TAX 05B* 0 0 3285 4063 5266 5531 CASH FLOW AFTER TAX 0 0 5163 4B36 5177 5003 LOAN REPAYMENTS 0 0 5153 4B96 1511 0 NET CASH FLOW 0 0 0 0 3667 5003 NPU 015* 0 0 0 0 1823 2163 CUMULATIUE NPU015* 0 0 0 0 1823 3386 BALANCE OF LOAN 010* 373 215B 11570 6407 1511 0 0 BALANCE DEPRECIABLE 030* 373 1073 32B1 6437 4543 3183 2228 CASH FLOU ESTIMATE BASE CASE -20% OPERATING COST Cthousands of d o l l a r s , 1985) 1986 YEAR 0 YEAR 1 YEAR 2 YEAR 3 YEAR 4 YEAR 5 YEAR 6 REUENUE 0 0 15778 15778 15778 15778 OPERATING EXPENSES 658 658 4114 4114 3456 3456 INTEREST ON LOAN 97 183 1088 482 0 0 CASH FLOU BEFORE TAXES 0 0 10576 11182 12322 12322 DEPRECIATION 230* d.b. 0 0 2784 1949 1364 955 TAXABLE INCOME BASE 0 0 7792 9233 10958 11367 INCOME TAX 258* 0 0 4519 5355 6355 6593 CASH FLOW AFTER TAX 0 0 6057 5827 5967 5729 LOAN REPAYMENTS 0 0 6057 4820 0 0 NET CASH FLOUJ 0 0 0 1007 5967 5729 NPU 21s* 0 0 0 576 2966 2477 CUMULATIUE NPU215* 0 0 0 575 3542 6019 BALANCE OF LOAN 010* 973 1828 10877 4820 0 0 0 BALANCE DEPRECIABLE 230* 973 1073 9281 5497 4548 3163 2229 CO 

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