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Bureau of Mines Information Circular/l9?8

Processing Gold Ores Using HeapLeach-Carbon Adsorption Methods

U N IT E D ST A T E S D E PA R T ME N T OF TH E INTERIOR


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Circular 8770

Gold Ores Using HeapAdsorption Methods

H. J . Heinen, D. G. Peterson, and R. E. Lindstrom

T H E I N T E R I O RD. Andrus , Secretary

OF MINES


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This publication has been cataloged as follows:

Hein en , Haro ld JProcess ing gold ores us ing heap leachsarbon adsorpt ion

methods / by H. J. Hein en , D. G. Peters on , an d R. E.Lindstrom. [Washington] : U.S. Dept. of the Interior. Bureauof Mines, 1978.

21 p. : i l l . ; 2 7 cm. (Information circular - Bureau o f M i n e s ;8 7 7 0 )

Bibliograp hy: p. 19-21.1. Gold - Meral lurgy. 2 . Cyanid e pr oce ss . 3. G ol d ores. I.

Peterson, D a v i d G . , joinr author. 11. Lind stro m, Ro ald E . , joinrautho r. 111. Un ited Sr ate s. Bureau of Mines. 1% T i t l e . V. Se -r i e s : U nired St a t e s . Bureau of Mines. Informarion circu lar - Bu-reau of Mi nes ; 8 7 7 0 .TN23.U71 no. 8770 622.06173U S . Depr. of the Inr. Library


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CONTENTS

A b s t r ac t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .hemis t ry of cya nid at io n. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ineralogy of gold ores. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .m en ab i l i t y t e s t i n gCyanide heap- leach opera t ions ............................................. . . . . . . . . . . . . . . . . . . . . . . . .o l d - s i l v e r r e co v e ry fro m p r o ces s s o l u t i o n s..................................conomics of heap-leach operationsCyanide handl i ng and di sp os al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .R ecen t i n n o v a t i o n s i n g o l d - s i l v e r r e co v e r y f ro m cy ani d e p r o ces s s o l u t i o n sSummary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ILLUSTRATIONSLaboratory column lea chi ng u n i t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P i l o t - s ca l e h eap l e ach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Heap- leach cyanidat ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Leaching or e heaps by th e pending method . . . . . . . . . . . . . . . . . . . . . . . . . . . . .H y p o t h e t i c a l d i s t r i b u t i o n of g o l d i n a co n ti n u o u s ca r b o n -

a d s o r p t i o n o p e r a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Recovery of g old f rom ac t i va te d carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P ro po se d t r e a t m e nt o f c y a ni d e e f f l u e n t s h i gh i n s i l v e r c o n t e n t . . . . . . .Proposed heap leach-carbon a dso rpt io n process fo r gold ore s low. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .n s i l v e r

TABLEEs t imated co s t o f a commercial heap-leach cya n id a t io n ope ra t i on .


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PROCESSING GOLD ORES USING HEAP LEACH-CARBONADSORPTION hiETHODS

byH. J. Heinen,' D. G. Peterson,' and R. E. Lindstrom2

ABSTRACTThis Bureau of Mines re po rt reviews th e rec en t developments i n th e heapleach ing of go ld o res wi th d i lu t e cyan ide so lu t ion s and th e recovery of go ldand assoc ia t ed s i l ve r f rom th e pregnant e f f l ue n t s by a ca rbon adsorp t ion-deso rpt i on method. Minera log ica l cons ide ra t ions , exper imenta l work, f i e lds tu die s , and commercial opera t ions a re desc r ibe d. A l so p r es e n t ed a r e r e c e n tprocess advances developed by the Bureau, incl udin g the a pp l i ca t i on of

( 1) a l c o ho l a d d i t i v e s t o t h e c o n v en t io n al h ot a l k a l i n e c ya ni de s t r i p s o l u t i o nt o enhance gold desor pt io n from precious meta l - laden ac t i va te d carbon, and(2) Na,S p r ec ip i t a t io n o f the s i l ve r from go ld -s i l ve r cyan ide so lu t ion s as ameans of reducing th e in ventory of carbon requ ired f o r precious-meta l adsorp-t i on f rom heap leach e f f lu en ts and fo r the p roduc t ion of low-s i lve r b u l l i onfrom a c t i v a t e d c ar bo n s t r i p p in g s o lu t i o n s .

INTRODUCTION

Gold mineral iza t ion i s widespread i n the United S ta te s . In 1967 theBureau of Mines conducted an engine erin g a p p ra i sa l of more t ha n 1,300 lode andp l a c e r d e p o s i t s , r e p r e s e n t i n g a lm o s t a l l o f t h e N a t i on ' s known g old r e s e r v e s , t ode te rmine th e i r go ld p roduc t po ten t ia l , (28) .3 These dep os it s were estim atedt o cont a in over 400 mil l ion ounces of gold; however, only 2 pct of t h i s amountwas found t o be e xp lo i ta bl e a t $35 per ounce. A 4 00 -p ct i n c r e a s e i n t h e p r i c eof gold wi thi n a per iod of 1 o r 2 years caused a remarkable reeval uat i on ofthe type of mat e r ia l t h a t co ns t i tu te s o re . Low-grade mate r i a l s con ta in ing asl i t t l e a s 0.03 ounce o f g o ld p e r t o n, p r ov idin g s u f f i c i e n t to nn ag e i s a v a i l -abl e, ar e now being fi e l d te st ed f or preciou s-metal reco very . The Bureau ofMines and s ev er al mining companies have conducted ext en siv e h ydr ome tall urg icals t ud ies t o exp lo i t low-grade ores and mine waste ma te r i a l . The innovat ionsdeveloped inc lude heap-leach cy an ida tio n followed by precious -metal rec over yl M e t a l l u r g i s t ." ~ u ~ e r v i s o r ~hemical engineer .Authors ar e with th e Reno Me tall urgy Research Cen ter, Bureau of Mines, Reno,

Nev .3 ~ n d e r l i n e d umbers i n pa r en the s es r e f e r t o i t em s i n t h e l i s t o f r e fe r e n c e s a tt h e end of t h i s r e p o r t .


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fro m t h e r e s u l t a n t s o l u t i o n s b y ca r bo n a d s o r p t i o n o r by p r e c i p i t a t i o n on z i n c ,and new p rocedures f o r more e f f i c i e n t s t r i pp in g of go ld f rom ac t iv a te d ca rbonand s e p a r a t e r e c ov e r y of g o l d and a s so c i a t e d s i l v e r f rom e n r i ch e d s t r i p so l u -t i o n s . The p ur po se of t h i s r e p o r t i s t o r ev ie w t h e s t a t e - o f - t h e - a r t o f he ap -l e a c h p r o c e s s in g o f p r e c i o u s m e t a l o r e s and t h e a p p l i c a t i o n of g r a n u l a ra c t i v a t e d c h a r c o a l f o r p re c i o u s -m e t a l r e c o v er y fro m c y a n i d e so l u t i o n s .

Heap l each ing may be de f ined as th e pe r co la t io n l each ing o f p i l e s o f low-g r a d e o r e s o r mine w a s te t h a t h av e be en s t a c k e d o r p i l e d o n sp e c i a l l y p re p a re dw a t e r t i g h t d r a i na g e p ad s f o r p re g na n t l i q u o r c o l l e c t i o n . T h is pr o c e s s in g c o n-ce p t d a te s back t o about 1752 when the Span i sh miner s pe rco la t ed ac id so lu -t i o n s th rough l a r ge heaps o f ox ide copper o re on th e banks o f th e Rio Tin toR i v e r . S i n c e t h e n , t h i s p r o c e s s ha s b ee n u sed e x t e n s i v e l y t h ro u g ho u t t h ewor ld t o l each ox ide copper va lue s from mine s t r i p mate r i a l f rom open p i t min -in g of porphyry copper d ep o si t s . Uranium produ cers have a l s o used heap leac h-i n g s i n c e t h e l a t e 1 9 5 0' s f o r e x t r a c t i n g ur an iu m from su b -m i ll - gr a de o r e s andf o r e x p l o i t i n g r e l a t i v e l y sm a l l lo w-g ra de o r e d e p o s i t s . H ea p- le ac h c ya n i d a -t i o n i s a comparat i vely re ce nt development sug gested by Bureau of Mines meta l -l u r g i s t s i n 1967 a s a p o t e n t i a l l o w - c a p i t a l means f o r p r o c e s s i n g l i m e s t o n e andd o lo m i te s i l t s t o n e - t y p e o r e s c o n t a in i n g s u bm ic ro me te r p a r t i c l e s o f g o l d i n aporous hos t rock (15* -a 9 -2 ) .

CHEMISTRY OF CYANIDATIONThe b a s i c p r i n c i p l e o f t h e c y a n i d a t i o n p r o c e s s i s t h a t w eak a l k a l i n e

c y a n i d e s o l u t i o n s h ave a p r e f e r e n t i a l d i s s o l v i n g a c t i o n on t h e g ol d and s i l v e rc o n t a i n e d i n a n o r e . The r e a c t i o n ( E l sn e r ' s e q u a t i o n ) g e n e r a l l y a c c ep t ed f o rs e v e r a l d e ca de s a s r e p r e s e n t i n g t h e d i s s o l u t i o n of g o ld by c ya n id e s o l u t i o n i s

Recen t r e s ea r ch on th e mechan ism of cy an ida t ion , however, i n d i ca te s th i s r e ac -t i o n p ro c ee d s i n two s t a g e s (1, . 305). Most o f t he go ld d i s so lve s by ther e a c t o n

a nd a s m a ll b u t s i g n i f i c a n t p r o p o r ti o n d i s s o l v e s v i a r e a c t i o n 1. The goldd i s s o l u t i o n r a t e i s dependent on th e co nc en t ra t io n of NaCN and th e a l k a l i n i t yo f th e so lu t i on , the optimum pH being 1 0.3 ( 3 ) . F or e f f i c i e n t le ac h in g , t h eg o l d s ho ul d o cc ur a s f r e e , f i n e - s i z e , c l e a n p a r t i c l e s i n an o r e t h a t c o n t ai n sno " c ya n ic i de s " o r i m p u r i t i e s t h a t m ig ht d e s t r o y c y an i de o r o t h e rw i se i n h i b i tt h e d i s s o l u t i o n r e a c t io n . An adequ ate s upp ly of dis so lv ed oxygen must bep r e s e n t i n t h e cy a ni de s o l u t i o n t h ro u gh ou t t h e r e a c t i o n p e r i o d .

The c h em i s t r y in v o lv e d i n t h e d i s s o l u t i o n of g o ld i n t h e h e a p- l ea c h c y a -n i d a t i o n t r e a t m e n t i s t h e same a s t h a t f o r t h e a g i t a t i o n - c y a n i d a t i o n p r o c e s s .I n heap l e a ch i ng , t h e oxyg en , e s s e n t i a l f o r t h e d i s s o l u t i o n of g ol d , i s i n t r o -d uc ed i n t o t h e c ya ni de s o l u t i o n a s i t i s b e i ng sp r i n k l e d upo n t h e o r e he a p.The adsorbed oxygen and c arbon d i oxi de f rom the a i r may a l s o cause chemical


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losses o f cyan ide accord ing to the fo l lowing rea c t ion s :

NaCN + CO, + H20 - HCN + NaHCO,. ( 4I n heap leachin g high ly oxidized ores , th e decomposi t ion of cyanide by carbond iox ide may b e a s g r e a t a s t h a t c au se d b y t h e a c i d c o n s t i t u e n t s o f t h e o r e .The decomposit ion of cyanide by carbon dioxid e, a s we ll a s by ground ac id s, i sminimized by usi ng s u f f ic i e n t a l k a l i such as l ime (CaO) or ca u s ti c soda (NaOH)i n t h e l e ac h s o l u t i o n t o m a in t ai n t h e a l k a l i n i t y a t a pH r an g e of 9 t o 11.

The minera l cons t i tue n t s of the o re and o th er fo re ign substances ca ninf luence th e cyan ida t ion p rocess i n a number of d i f f e r en t ways (1, p. 284-351; 1 2 ) . S i l v e r n o rm a lly o c cu r s w i th g o ld e i t h e r a s a n imp u r i ty i n t h e p a r -t i c l e r o f n a t i v e go ld o r a s s i l v er - b e ar i n g m i ne r al s . The occurrence of si lveri n gold o res may range f rom le s s than 1 ppm to se ve ra l hundred t imes th e le ve lof t h e g old p r e s e n t i n a n o r e . G e n er a ll y , s i l v e r i s a l s o d i s s o lv e d by t h ecyanide so lu t i on and fol lows th e gold through th e process sequence. I r on su l-f i de minera l s , which a re common co ns t i t ue n ts o f go ld o res , a r e ox id ized t osome exte nt du ring t he cyanide leach, t h u s r e s u l t i n g i n t h e f o rm at io n of a c i d .These ac i ds a re n eu t r a l i z ed by th e l ime used i n th e cyan ide leach sequence.Copper min eral s may be diss olv ed by the cy anide leach s ol ut io n and th us con-sume la rg e q u a n t i t i e s of NaCN and oxygen. Arseni c-bearin g mi nera ls may a ls oi n t e r f e r e w i t h c y an i da t io n . Rea lgar (Asas,) and orpime nt (As,S,) r e a c tr a p id ly w i th t h e c y an id e s o lu t i o n and i n h ib i t t h e d i s s o l u t i o n o f g o ld .Arsenopyri te (FeAsS) , however , ge ne ra l ly oxidi zes very s lowly i n an aer a t edc ya nide s o lu t i o n and ha s v e r y l i t t l e a d v e r se e f f e c t on t h e l e a c h in g of g o ld .S t i b n i t e (Sb2S,) s t r o n l y i n h i b i t s c y an id a t io n . The presence of ba se meta lI:ions such as Fea+, Fe3 ,~i", u2+, 2n2+, and ~ n ~ +n th e c y a nid e l e a c h s o lu -t i o n w i l l r e t a r d t h e c y a n id at i o n of g o ld . I n some c a s e s t h e n a tu r a l l y o c c u r-r ing ca rbonaceous mate r ia l s i n sed imenta ry- type go ld o res a c t a s adsorben tsfo r th e gold d i s so lve d by cyan ide so l u t io ns . Charred mine timbers have simi-l a r p rop er t i e s and cause p remature p r ec ip i t a t io n of go ld . Organ ic subs tancessuch as decayed wood, o i l , gre ase, and f l o t a t i o n re ag en ts slow down cyanida -t io n o f go ld by consuming the dis solved oxygen i n the leac h so lu t i on and a lsoin h ib i t s s u bs eq ue nt g o ld re c ov e ry from l e a c h s o lu t i o n b y p r e c i p i t a t i o n o f t h egold on z inc dust .

General ly , go ld and s i lv e r a r e recovered f rom pregnan t cyan ide so lu t io nse i t h e r by p r e c ip i t a t i o n on z in c d u s t o r by a d s o r p t i o n on a c t i v a t e d c a rb on .For p r ec ip i t a t io n of go ld on z inc , c l a r i f i c a t i o n of t h e p re gn an t s o l u t i o n i sr e qu i re d t o e l im in a t e t h e s us pe nd ed c l a y e y c o n s t i t u e n t s t h a t c a n c o a t t h e z in cp a r t i c l e s a nd r e t a r d p r e c ip i t a t i o n of t h e pr e c iou s m e ta ls . E l imin a t ion o f thed i ss o lv e d oxygen f rom th e p re gn an t s o lu t i o n i s e s s e n t i a l t o p r e v e n t t h e r e d i s -s o l u ti o n of t h e p r e c i p i t a t e d g ol d v i a r e a c t i o n 1 or 2 , and excess ive z incconsumption through i t s in te ra c t io n wi th th e oxygen remain ing i n s o l u t i o n .The pr ec ip i t a t io n of gold on z inc i s greatly improved by adding soluble leads a l t s , s uc h a s l e a d a c e t a t e o r l ea d n i t r a t e , t o c y an id e s o l u t i o n s t o form az in c- le ad c ou pl e o f g r e a t e r a c t i v i t y . The r e a c t i o n f o r p r e c i p i t a t i p n of g o ld


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by z inc may be rep rese nte d by th e fol lo wing equ at ion:

Na2Zn(CN), + Au + 1/2H, + NaOH. (5)Act ivated carbon has t he ca pa bi l i ty of adsorbing th e gold cyanide complex

from cyan id a t ion re ac t io n pu lps and un c la r i f i e d cyanide e f f lu en ts , the rebye l i m in a t i n g t h e l i q u i d - s o l i d s s e p a ra t i o n, c l a r i f i c a t i o n , and d e a e ra t i o n p r o-cess ing s t ep s t h a t a re employed i n a n a l l s l i m e co u n t e rc u r r e nt d e c a n t a t i on -z i n c p r e c i p i t a t i o n p l a n t. Al though ac t i va te d carbon has been used i ngo l d-s i lve r recovery f rom cyan ide so l u t io ns fo r s eve ra l decades , th e mechanismof go ld adsorp t ion on ac t iva t ed ca rbon i s s t i l l no t f u l l y u nd er st oo d. Onet he or y i s t h a t t h e go ld i s adsorbed as th e gold cyanid e complex AU(CN)~- ndth a t t h i s a d s o r p t i o n t a k e s p l a c e by i o n ex ch an ge (1,p. 341). Some f a c t s t h a ts u pp o rt t h i s t h e o r y a r e ( 1 ) CN- i o ns a r e a l s o a ds or be d by t h e a c t i v a t e d c a r -bon, (2) a dso rpt ion of AU(CN)~- on i s n ot accompanied by adsor pt io n of ade qu iv al en t q u a n ti t y of caa+ o r ~ a +ons f rom so lut io n, and (3) th e adsorbedgold can be desorbed or d ispla ced wi th OH- ions provided by hot a l ka l i ne so lu-t i o n s . A r e c e n t s t u d y ( 7 ) i n d i c a t e d t h a t t h e g o ld ad so rb ed o n to a c t i v a t e dcarbon i s e i th e r i n th e ca lc ium aurocyan ide o r hydrogen aurocyan ide form,depending on th e pH of t h e cyanide s ol ut io n and t h e co nc en tr ati on and cha ra c-t e r o f t h e " s p e ct a to r" c a t i o n s p r e s e n t .

MINERALOGY OF GOLD ORES

The manner of gold occurrence and i t s a ss oc ia t i on with th e gangue miner-a l s d ic ta te whether o r no t t he o re can be p rocessed by t he cyan ide heap- leachmethod. From a meta l lu rg ic a l s tandpo in t , go ld o res may be roughly c l as s i f i eda s ( 1) s im ple o x id e o r e s c o n t a in in g f i n e p a r t i c l e s of n a t i v e g o ld i n a q u a r t zo r l im es to ne g angue, ( 2 ) s im p le s u l f i d e o r e s i n w hich t h e g o ld i s a s s o c i a t e d~ i t h inor amounts of p y r i t e o r a r seno pyr i te , (3) p la ce r o r a l luvium mat e r ia l ,(4 ) complex o r re f r ac to ry o res i n which t h e go ld-bearing minera l spec ies a r eno t re ad i ly so lub le i n cyan ide so lu t ion , (5 ) complex base meta l o res i n whichth e p rec ious meta l s a r e impor tan t economic co ns t i t ue n t s , and (6) base meta lor es i n which th e prec ious metals a r e of minor valu e and a re byproducts oft reatme nt (27) . Of the se , only th e s imple oxide and su lf id e or es and ce r t a inp l a c e r s a r e s u i t a b l e f o r heap l e ac hi ng . These mate r ia l s mus t possess the fo l -low ing c h a r a c t e r i s t i c s : (1) Gold and s i l v e r va lues a re l eachab le by cyan ida-t i o n , ( 2) s i z e of t h e g ol d p a r t i c l e s i s e xt re me ly s m al l , ( 3 ) t h e h o s t r o c k i sporous t o cyan ide so lu t i on , and remains pe rmeable dur ing the r e l a t iv e l y longl e a c h c y c l e , (4) g ol d p a r t i c l e s i n o r e s o f low p o r o s i t y a r e l i b e r a t e d o rexposed by f ractur ing and crushing, (5) th e o re i s f r ee o f ca rbonaceous mate -r i a l which has th e ca pa b i l i t y of adsorbing go ld cyan ide and causes p rematurep r e c ip i t a t i o n of t h e g o ld , ( 6 ) t h e o r e i s r e l a t i v e ly f r e e of " c ya n ic ide s " o rs u bs t an c es t h a t d e s t r o y cy an id e o r i n t e r f e r e w i th t h e g o ld - c y a nid a t io n r e a c -t io n , such as pa r t i a l l y ox id ized su l f i de s o f an timony , z inc , i ron , copper , andar se ni c conta i ning minerals , (7) th e ore does not co nt a in excess i ve amounts of" f in e s" o r c l a y ey c o n s t i t u e n t s t h a t w i l l impede sol ut io n pe rco la t io n, and(8 ) t h e o r e i s r e l a t i v e l y f r e e of a ci d- fo rm in g c o n s t i t u e n t s t h a t c au se h ig hlime consumption.


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Types of gold-b ear ing de po si t s found amenable t o heap leachin g ar e a sfo l lows : (1) Limey s i l t s t o n e conta in ing submic romete r- s ize gold p a r t i c l e s andm inu te am ounts of p y r i t e , ga l e na , c i nna ba r , and s t i b n i t e , ( 2 ) s i l i c i f i e d s i l t -s t o n e s co n t a i ni n g m ic r om e te r -s i ze p a r t i c l e s o f g o l d , o f t e n a s s o c i a t ed w i t hr e s i d u a l i r o n o x i de s, (3) s ande d dolom it e o r e i n w hic h f i ne go ld pa r t i c l e so c cu r o n t h e i n t e r g r a n u l a r s u r f a c e s , ( 4 ) v e i n q u a r t z o r e i n wh ich g ol d o c c u rsi n l i m o n i te - r ic h c a v i t i e s and f r a c t u r e s , (5) i gneous hos t m a t e r i a l c u t w i thsma l l qua r tz ve ins c ont a in i ng f r ee gold and minor amounts of p yr i t e , ( 6 ) as c h i s t c o n t ai n i n g f r e e g o ld i n t h e l a m i n a t i o n of t h e ro c k s , and ( 7 ) s i l i c e o u sq u a r t z s i n t e r of h o t - sp r i n g o r i g i n .

PumpBarren cyanide

solutionF I G U R E 1. - Laboratory colunin leaching un i t .

AMENABILITY TESTINGEven though t h e miner-

a l o g i c a l s t u d i e s may i n d i -c a t e t h a t t h e go ldm i n e r a l i z a t i o n i s a s s o c i a te dw i th h o s t r o ck s t h a t a r es u i t a b l e f o r heap l e a c h i n g ,it i s p r u d e nt t o c o nd u ct l a b -o r a t or y and p i l o t t e s t s f o rv e r i f i c a t i o n . The t e c h n o l -ogy for de termining whethero r no t a go ld o r e i s a men-a b l e t o p e r c o l a t i o n l ea ch in gwi th weak cyanide so lu t io nsi s w e l l e s t a b l i s h e d . I n i -t i a l l y , b o t t l e c y an id a t io nt e s t s a re conducted on 200-to 500-gram charges ofground o re t o de te rmine thede gr e e o f go ld e x t r a c t i onand re ag en t consumption. I ft he go ld c on t e n t o f t h e o r ei s l ea ch ab le a t a r e l a t i v e l ycoa rse gr in d , co lumn pe rco la -t i o n t e s t s a r e made o n o r ec ru sh ed t o v a r i o u s s i z e s ,ran gi ng from minus 2 - t ominus 1/4 -inc h fe ed . Thet e s t a ppa r a tus i s shown inf i g u r e 1 . About 60 poundsof c rushed ore i s mixed wit hs u f f i c i e n t l im e t o p r ov id ep r o t e c t i v e a l k a l i n i t y d u ri ngcya nid a t io n . This ore - l imem i xt u re i s t r a n s f e r r e d t o acolumn about 6 inches i ndi am ete r t o make a bed about!, f e e t h igh . Twelve l i t e r so f s o l u t i o n c o n t a in i n g 1 . 0


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pound NaCN per t on i s pumped t o t h e top of th e column, and th e f l ow ra t e i sc o n t r o l l e d so t h a t t he s o l u t i o n t r i c k l e s s l ow ly downward t h rough t he o r e . Thepregnant e f f l ue nt f rom th e co lumn i s th en passed upward through thr ee s ma l lco lu mn s c o n t a i n i n g g r a n u l a r a c t i v a t e d c ar bo n and p l a c ed i n s e r i e s . The barrens o l u t i o n f ro m t h e a d s o r p t i o n columns i s r e cy c l e d f o r a d d i t i o n a l l e a c h i n g a f t e rmaking any necessa ry ad jus tments i n the cyanide and l ime concent ra t ion . Leach-i n g i s c on tinued u n t i l no f u r t h e r s i g n i f i c a n t amount o f go ld i s ext rac ted f romthe o r e . P i l o t - s c a l e he a p - l e ac h t e s t s may be c onduct ed on a tonna ge s c a l e i fit i s d e s i r a b l e t o c o n fi rm r e s u l t s o b ta in e d i n l a b o r a t o r y c ol um n -p e rc ol at io nt e s t s , o r t o f u r t h e r q u a n t i f y r e a g e nt r eq u ir em e nt s ( f i g . 2 ) . The pe rcola t ionr a t e of s o l u t io n through t he o re can be s t be measured by conduc t ing tonnage-sc a l e expe r iments i n columns conta in ing a bed of ore 15 t o 20 f e e t h ig h .

F I G U R E 2. - Pilot-scale heap leach.


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CYANIDE HEAP -LEACH OPERATIONSCyanide heap leaching i s a co m p ar a t iv e l y r ec en t h y d r o m e t a l l u r g i ca l d ev e l -

opment f o r ex p l o i t i n g low-grade gold o res , mine was te ma te r i a l , o r dep os i t sto o s m a l l t o j u s t i f y c o n s t r u c t i o n of m i l l i n g f a c i l i t i e s . A lth ou gh t h e p ro c es si s an al o go u s t o h eap l e ach i n g o f co p p er o r e s , it i s s t i l l c o ns id e re d t o b e i nt h e d ev e lo p m en ta l s t a g e b ecause many p r o ces s v a r i a b l e s i n f l u e n c i n g so l u t i o np e r c o l a t i o n and d i s s o l u t i o n of t h e g o l d a r e n o t f u l l y u n d er s to o d . Each com-m er c i a l i n s t a l l a t i o n h a s d eve lo ped an o p e r a t i n g t e ch n i q u e f o r pad and h eapp r ep a r a t i o n and l e ach i n g p r o ced u re t h a t m ee ts t h e req u i r em en ts o f t h e o r e .The t echn ique s e l ec te d depends l a rg e l y on the manner of occur rence o f th ego ld , p h y s i ca l ch a r ac t e r i s t i c s and m i n e r a l co m p o si t io n o f t h e o r e , and t h es c a l e o f o p e r a t i o n .

A l l commerc ia l cyan ide heap- leach ope ra t i ons a r e be ing conduc ted on mate -r i a l s t ack ed on im pe rm eable p ad s . Wa t e r t i g h t p ad s o r b a se s a r e r eq u i r ed t oc o l l e c t t h e p re g na n t s o l u t i o n s and t o e l i m i n a t e t h e p o s s i b i l i t y o f l o s i n g g oldand s i l v e r cy an i de so l u t i o n s t o t h e g r ou nd and co n t am i n a t i n g l o c a l s t r e am s andu nd er gr ou nd w a t e r r e so u r ce s . The t y p es o f m a t e r i a l s u sed f o r co n s t r u c t i n g t h eimperv ious pads inc lude (1 ) compac ted t a i l i n g s mixed wi t h ben to n i t e ,( 2 ) asp h a l t ( b l ack t o p ) o r l i g n i n su l f o n a t e mi x p l aced o n co mp ac ted g r av e l andc ov ere d w i t h a n a s p h a l t s e a l e r , ( 3 ) r e i n f o r c e d c o n c r e t e p a d s , and ( 4) p l a s t i co r rubber she e t i ng l a id on a smooth excava ted a r ea and covered wi th 2 t o 3f e e t of washed sand and grav el .

Ba s ic a l ly , two methods o f heap- leach c yan ida t io n a r e used commerc ia l ly :(1) sho r t - t e rm leach ing of c rushed o re , and (2 ) long- te rm leach ing of run-o f -mine mate r i a l .

\

I n p r a c t i c i n g s h o r t - t e rm , h e ap - le a ch c y a n i d a t io n , t h e o r e i s c r u sh ed t o asm a l l s i z e , s t a ck ed 4 t o 8 f e e t h ig h on pe rm an en t p ad s, e ach w i t h a c ap a c i t yranging from 1,000 to 10,000 ton s, and leached by sp r i nk l i ng th e top of t heheap w i t h d i l u t e c y a ni de s o l u t i o n ( f i g . 3 ) . C ya nid e s o l u t i o n p e r c o l a t e st hr ou gh t h e h eap d i s so l v i n g t h e g o l d and s i l v e r v a l u e s . I t i s subsequen t lyco l l e c t ed o n t h e w a t e r t i g h t p ad , w h ich i s s l o p ed t o p e r m i t t h e p r egn an t so l u -t i o n t o f l ow i n t o ch an n e l s f o r t r an s f e r t o a s t o r a g e pond o r t an k . The f eedm a t e r i a l i s c ru sh ed t o a s i z e t h a t w i l l g i v e g ood l i b e r a t i o n o r ex p o su re o fth e go ld m i n e r a l i z a t i o n t o c y an id e s o l u t i o n and s t i l l o b t a i n r e a so n a bl e p e r co -l a t i o n r a t e s . I n t y p i c a l o p e r a t io n s , o r e s a r e c ru sh ed t o minus t h r e e - f o u rt h si nch, and a s f i n e a s m inus o n e - f o u r t h i n ch i n t h e ca se of a g o l d -q u a r t z o r e .The l each cyc le i s measured i n days, ge ne ra l ly f rom 7 to 30. When th e l eachcy c le i s co m p l e ted , t h e w ast e i s removed from the pad and a new batch ofc ru sh ed o r e i s ap p l i ed . The Ca r l i n Gold M in in g Co. ( 21 ) s t a r t e d t h i s t y p e o fl e ach i ng o n m ine cu t o f f m a t e r i a l c ru sh ed t o t h r e e - f o u r t h s i n ch i n 19 71 . TheSmoky Val le y Mining Co. has completed th e i n s t a l l a t i o n of a 8 ,000- tonldaysh o r t -t e r m h eap l e ach - ca r b on ad so r p t i o n - e l ec t r o w i n n i n g f a c i l i t y a t RoundMountain t h a t went on-st ream December 1976 ( 2 9 ) .

Long- te rm leach ing i s used p r im ar i l y t o ex t r ac t go ld f rom uncrushed ,porous, sub-mi l l -g rade m at e r i a l f rom open p i t op era t ion s . The o re chargesar e run-of-mine m at er ia l generate d by b la s t i ng , and may co nt ai n some lar ge


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Barrencyanidesolution

PregnantcyanideeffluentFIGURE 3. - Heap-leach cyanidation.

boul:l. .rs, bu t most of th e feed i s minus 6 i nc he s i n s i z e . The t onna ge o f t heheaps under t re a tm ent usu al ly rang es f rom 10,000 t o 2,000,000 to n s . Mostl e a c h he aps a r e s ha pe d t o re se m ble a t r un c a t e d pyram id 20 t o 30 f e e t h igh , a sde p i c t e d i n f i g u r e 3 . The he ig h t i s gover ne d by f a c t o r s s uc h a s o r e per me a-b i l i t y and t h e ma in te na nc e of p r o t e c t i v e a l k a l i n i t y , c y an i de s t r e n g t h , ands u f f i c i e n t d i s s o l v e d o xygen i n t h e l ea ch s o l u t i o n a s i t percola tes downwardthrough th e heap . The leach cy c le i s measured i n months . These heaps a r el e a c h e d u n t i l i t i s no l o n g er p r o f i t a b l e t o c o n t i n u e t h e o p e r a t i o n , w hich mayb e f o r y e a r s . Upon t e r m i n a t io n o f t h e l e a c h , t h e r e s i d u e i s l e f t o n t h e pad .Co rte z Gold Mines has completed heap le ac hi ng ap prox imate ly 2 m i l l i o n t o n s o fr un -o f- mine c u to f f m a te r i a l (8-9; 20, p p. 4 0 -4 2 ). C o rt ez o p e ra t ed t h e f i r s tknown in t e g r a t e d he a p - le a ch c ya n ida t ion - c a r bon a ds o r p t i on - e l e c t r ow inn ing p l a n ta t i t s Gold A cr es p r o p e r t y , a bo u t 8 m i le s f rom the main c ya n ide p l a n t .

G e ne r a l l y , t h e c ya n ide l e a c h s o lu t i on s a r e i n t roduc e d on to t he hea ps bys p r ay i n g from p e r f o r a t e d p l a s t i c p i p e s , by s p r i n k l i n g u s i ng p l a s t i c s p r i n k l e rh e a d s, o r by p o n di n g. Ty i c a l r a t e s of a p p l i c a t i o n of t h e l e a c h s o l u t i o n srange f rom 5 t o 75 g a l / f t of s u r f a c e a r e a p e r d ay . Pond l e a c h i n g , a s i l l u s -t r a t e d i n f i g u r e 4 , was a pp l i e d s uc c e s s f u l l y by t he I da ho Min ing C orp . (30)t o e x t r a c t t h e r e a d i l y l e a c h a b le g ol d v a l u e s fro m a s an ded d o l o mi te o r e t h a t


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n arrencyanidesolutionImpervious pad

PregnantcyanideeffluentFIGURE 4. - Leaching ore heaps by the ponding method.

e x h i b i t s s lo w p e r c o l a t i o nc h a r a c t e r i s t i c s . one hun-dred and s i x t y thousand tonsof o re was stac ked abo ut 25f e e t h i gh and l ev e l ed ; t hef l a t s u r f a c e o f t h e p i l e wasd i v i ded by sm a l l d i kes i n t on i n e ponds t o f a c i l i t a t ec o n t r o l of l e a ch i n g s o l u -t i o n s . The heap was washedw i t h ba r ren so l u t i ons f romt h e c a r b o n a d s o r p t i o n p l a n t .

In commercial appl ica-t i o n , s o l u t i o n s t r e n g t hsrang e from 0.5 t o 1.0 poundof sodium cyan ide pe r to n ofso lu t i on , and the pH i smainta ined a t about 10 wi tht he app l i ca t i o n of l im e(CaO) o r c a u s t i c soda (NaOH)Gold-Silver Recovery From

Process Solu t ionsBased on th e av a i l a b i l -

i t y of e x i s t i n g c o nv e n ti o n alcvanide m i l l f a c i l i t i e s w itha z in c p r e c i p i t a t i o n c i r c u i t , t h e g ol d and s i l v e r c o nt ai ne d i n h e ap - le a ch

pregnant s ol ut io ns may be recovered from so lu t io ns by conv ent io nal M er ri l l -Crowe pr ec ip i t a t io n . I f m i l l f a c i l i t i e s a r e no t a v a i l ab l e o r i f t he c oncen-t r a t i o n of go l d i n so l u t i on i s below a nom inal 0 .05 -0z / t0n so l u t i on , t h epref erre d method fo r recover ing t he prec ious metal value s from heap-leache f f l ue n t s i s by adso rp t i on on ac t i va t ed carbon . Not on l y i s t he carbona d s or p t io n p r oc e ss more e f f i c i e n t t h a n zi n c p r e c i p i t a t i o n f o r t r e a t i n g d i l u t ego l d - s i l ve r cyan ide so l u t i o ns , bu t t h e method a l s o en t a i l s l ow er c a p i t a l andope ra t i ng cos t s (5-5, 4).

The carb on ad so rp tio n method became p r a c t i c a l on development of a usa blemethod f o r s t r ip p i ng t he prec ious meta l va lue s f rom th e ac t i va t ed carbon so itcould be recyc led i n the sys t em. Today, th e use of ac t iv a t ed carbon i n con-junc t ion wi th heap l each ing i s a s imple , economica l p rocess t h a t i s s u i t a b l e...fo r exp l o i t i ng l ean r e sou rces o r sm a l l o r e bod i e s . The ca p i t a l i nvest men t fo rt h i s p r o c e s s i s e s t im a t ed t o b e a b ou t 20 t o 25 pct of the c o st of a conven-t i on a l coun t e rcu r ren t decan t a t i on cyan ide p l an t , and ope ra t i ng c os t s a r e about40 pct (6-5, 24) of a conven t i ona l cyan i de p l a n t .

In typ ic a l con t inuous carbon-adsorp tion oper a t ion , t h e go ld cyanideef f l ue nt s from heap leaching a r e pumped upward through from thr ee to f iv ecolumns or t anks i n s e r i e s which conta in gran ular ac t iv a te d carbon . Theharder carbons , manufactured f rom coconut sh e l l s , a re p re f e r r ed f o r use


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because th e r e i s l e s s t endency f o r breakage or abras io n . The amount of go ldand s i l v e r t h a t c a n be l oa de d o n t h e c a rb o n w i l l v a r y g r e a t l y , depending onth e t ype o f o r e a nd t he t e no r o f t he cya n ide l e a c h s o lu t i o n . Some f a c t o r st h a t i n f l u e n c e t h e l o ad i ng a r e ( 1 ) t h e c o n c e n t ra t i o n of g o ld and s i l v e r i nc ya ni de l e ac h s o l u t i o n s , ( 2 ) r a t i o o f g ol d t o s i l v e r , ( 3 ) pH of lea ch so lu -t i o n , (4) c o n c e n t r a t i o n o f i m p u r i t i e s , (5) f lo w r a t e , and ( 6 ) type and pa r -t i c l e s i z e o f g r a n u l a r c ar b o n em plo yed . T y p i c a l l o a di n g s o b t a i n e dcommercia l ly range f rom 200 t o 800 ounces of gold , or comb inat ion of gold ands i l v e r , pe r to n of carbon. However, th e ads orp t io n phenomena i s no tr e s t r i c t e d t o g o ld and s i l v e r c y a n i de co mp le xe s. A c t iva t e d c a r bon i s c a p a b l eo f a d s o r bi n g a l a r g e v a r i e t y o f o r g a n i c s u b s t a n ce s , a nd i n o r g a n i c c o n s t i t u e n t ss u c h a s s i l i c i c a c i d , and m e t a l l i c i o n s s u ch a s c al ci u m, c o pp e r, n i c k e l , mer-c u r y , and i r o n , t h u s r e d u c i n g t h e number o f s i t e s a v a i l a b l e f o r t h e p r e c io u sm e t a l s . The p r e f e r r e d pH of t h e l e ac h s o l u t i o n t o o b t a i n e f f i c i e n t g o lda ds o r p t i o n r a nge s f rom 9 t o 11. The c onc e n t r a t i on o f f r e e c ya n ide i n t h el e a c h s o l u t i o n s h ou ld b e m ai nt ai ne d a t a v a l u e t h a t i s as low as p oss ib l e ands t i l l be c o n s i s t e n t w i t h s a t i s f a c t o r y m e ta l e x t r a c t i o n f ro m t h e o r e . F i n e rs i z e ca r b on p a r t i c l e s a d so r b g r e a t e r amounts o f g o l d and a t a f a s t e r r a t e t h a ncoa rse ca rbon s ; however, th e mesh s i z e should not be so sma l l th a t it r e s u l t si n p a r t i c u l a t e c a rb o n l o ss e s t o t h e p ro ce ss s o l u t i o n s d u ri n g t h e a d s o r p ti o ns t e p . The a ds o r p t i v e c a p a c i t y o f a c t i va t e d c a r bon m a nuf ac tu r ed from c oconu ts h e l l s f o r g o ld c ya ni de i s g e n e r a l l y g r e a t e r t h a n t h a t o f c a rb o ns p r ep ar edf rom pe t ro leum coke , wood, o r co a l . A lso , h ig he r co nce nt r a t i on s of go ld i np r e gna n t c ya n ide s o lu t i o ns p r oduc es h ighe r go ld l oa d ing on t h e c a rbon . Simi-l a r l y , h ighe r go ld l oa d ing on a c t i va t e d c a rbon and t h e p r oduc t i on of lowe rgo ld - be a r ing ba r r e ns a r e a ch i e ve d by us ing lower f l ow r a t e s .

There a r e two methods f o r loadin g ac t i va te d carbon. I n one method, t h eg o l d - b e a r i n g c y a n i d e s o l u t i o n i s pe rc ol at ed downward throu gh a f i xe d bed ofa c t i v a t e d c ar b on . I n t h e o t h e r , t h e p re g na n t cy a ni d e s o l u t i o n i s pumpedupward t h ro u g h t h e ca rb o n a t a v e l o c i t y s u f f i c i e n t t o m a i n t ai n t h e bed o f c a r -b on i n a f l u i d i z e d s t a t e o r su sp en ded i n t h e s o l u t i o n s t r ea m w i th o ut b ei ngc a r r i e d o u t o f t h e s y st em . The choi ce of load ing techn ique depends on th eamount of t u r b i d i t y o r s l i m e s p r e s e n t i n t h e h e a p -l e ac h e f f l u e n t s . The f i x e dbed s o r gacked carbo n columns ar e l im it ed t o a maxirnum so lu t i o n f low of about5 g p m lf t of c r o s s - s e c t i o n a l a r e a . The f e ed s o l u t i on s must be f r e e o f pa r t i c -u l a t e m a t t e r be ca use t he bed o f c a rbon behaves a s a s a n d - f i l t e r and w i l le ve n tu a l l y become p lugge d i f s l im e s a r e p r e s e n t . The on ly advantage of th ef i xe d - be d c a rbon i s t h a t t h e amount o f c a r bon r e qu i r e d i s l e s s t ha n t h a tr e q u i r e d f o r a f l u id i z e d s ys te m p r oc e s s ing t h e same amount o f s o lu t i on .

The f l u id i z e d - be d a ds o r p t i o n s yst em i s g e ne r a l l y u se d i n c om m er ci alp r a c t i c e f o r a d s o rb i n g g o ld c y an i de v a l u e s f ro m u n c l a r i f i e d l e a c h s o l u t i o n scon ta in ing minor amounts of s l ime s . Four impor tan t process v a r ia b le s to bec o n s i d e r e d i n t h e d e s i g n of a f l u i d i z e d bed a c t i v a t e d c a r b o n l o ad i n g s ys te ma r e ( 1 ) f l ow r a t e o f t h e f e e d s o l u t i o n , ( 2) a v e ra g e d a i l y p r o d u ct i o n ofp r e c i o u s m e t a l s , ( 3 ) maximum amount of go ld t h a t ca n be loaded on t he car bo n,and (4) t h e p a r t i c l e s i z e o f ca r bon em ployed . I te m 1 i s based on the volumeof pregnant s o l u t i o n g e ne r at e d d a i l y f rom t h e he ap - le ac h o p e r a t i o n . I tem 2may be based on a weighed maximum gold co nc en tra t io n i n so lu t i on s obt a inedf ro m s e v e r a l hea ps p la ce d i n o p e r a t io n a t s t a g e d i n t e r v a l s . The inventory of


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ca rbon r equ i r ed i n t he p l an t and t he s i ze o f ad so rp t i on and s t r i p p i n g equ ip -ment can be min imized by u t i l i z in g th e adso rp t ive capa c i ty of th e carbon fo rgo ld t o t h e f u l l e s t e x t e n t t h a t i s p r a c t i c a l . A loading of 400 ounces ofprec ious meta l s per ton i s cons idered de s i ra b l e by th e indus t ry . However, i fthe incoming so lu t io n cont a ins l e ss than 0 .05 ounce of go ld per ton , t h e load-i n g u s u a l l y w i l l not exceed 200 oz / to n of ca rbon . The p a r t i c l e s i z e of ca rbonu se d c o m e r c i a l l y i n he ap -l ea ch c y a n i d a t io n i s e i t h e r minus 6 plu s 16 or minus12 p l u s 30 mesh. G ene ra l ly , t he so l u t i on f low r a t e r equ i r ed t o m a i n t a inf l u i d i za t i on i n a bed of ca rbon cont a in in g minus 6- p lus 16-mesh pa r t i c l e s i sabout 25 gpm/fta of cr os s- se ct io na l ar ea of th e column, whereas fo r th e minus12- plu s 30-mesh carbon, th e req ui r ed f low r a t e i s 15 gpm/f$. Under t h e s econd i t ions , th e carbon bed expands about 50 p c t . The depth of t h e bed of c a r -bon a t re s t should no t be more th an th re e t imes th e d iameter o f the co lumn.The heig ht of the column should be 2.5 t o 3 times t he hei ght of t he carbon a tr e s t t o a l low f or p roper expans ion of the bed and t o provide enough f re e boardto a l low fo r so l u t io n surg es . The amount o f ca rbon requ i red f or a f lu i d i ze d-bed ad so rp tio n system employing minus 12- pl us 30-mesh carb on p a r t i c l e s i sabout 10 t imes the da i l y amount of carbon s t r ip pe d. For the minus 6- plu s 16-mesh carbon, th e carbon invent ory should be about 15 t imes t he d a i ly amount ofcarbon s t r i ppe d because o f h i ghe r s o l u t i o n ve l o c i t y and sh o r t e r con t ac t t i m e .In du s t r i a l exper i ence has shown t h a t t he carbon charge should be equ a l l y d i s -t r i b u te d th ro ug h f o u r o r f i v e c olumns o r t an ks i n s e r i e s f o r a n e f f i c i e n t c a r -bon ads orp t io n system.

Dur ing s t a r t up of a coun tercur ren t ca rbon adso rp t io n sys tem fo r p roces s-i ng h e ap - le a ch cy an id e e f f l u e n t s , t h e f i r s t o r le ad column of ca rb on mayadsorb a l l o f t h e p rec i ous m e t al va l ue s . A f t e r t he ca rbon has been i n s e rv i cefor some time and c a r r ie s about 20 ounces of gold pe r ton, a gr adua l break-through of gold w i l l occu r i n the e f f l u en t from t he f i r s t column and beadsorbed by the succeeding bed of carbon. As th e carbon i n the f i r s t columnbecomes loaded w i t h more go ld and i t s adso rp t i ve capac i t y dec rea se s , t h e go ldcon ten t of t he column e f f l ue n t so l u t i on g radua l l y i n c rea se s . I t i s obviousfrom m on it or in g t h e p r og r es s o f a d s o r p t i on t h a t t o o b t ai n e f f i c i e n t u t i l i z a -t ion of th e carbon go ld , it i s advantageous t o use se ve ra l charges of ca rboni n s e r i e s . E f f l u e n t s am pl es sh ou ld be c o l l e c t e d a t r e g u l a r i n t e r v a l s and a na -lyzed, p re f e rab ly by th e a tomic abs orp t io n t echnique t o ob ta in rap id go ld ands i l v e r a s s ay s a t t h e p l a n t s i t e . When th e a n al y s es i n d i c a t e t h a t t h e l e a dcharge of c arbo n has adsorbed the d es ir ed amount of gold o r becomes sa tu ra te d,a po r t i on o f t h i s c a rbon cha rge ( r ep re sen t i ng t he d a i l y p roduc ti on of loadedcarbon) i s removed, and an equ iv al en t amount from each of t he o th er columns i sadvanced one column. An equivale nt amount of fr es h ac t iv at ed carbon i s addedt o t h e l a s t column t o i n su re t h e p roduc ti on of ba r r en s o l u t i o ns low i n gol dcon ten t fo r r ecyc l i ng t o t h e heap. F ig ur e 5 i l l u s t r a t e s t h e h y p o th e t ic a ld i s t r i b u t i o n of g ol d a t t a i n a b l e i n process ing 5 ,000 tons of cyanide so lu t io ncon tai nin g 0.10 ounce of go ld per to n and employing fo ur 1 -ton beds of carboni n s e r i e s .

The prec ious metal va lue s pre se nt ly a r e recovered from loaded carbon bytwo ra d ic a l ly d i f f e r en t t echniques . In one pr ac t i c e , adopted by some smal lcompanies wi th l imi t ed c ap i t a l fo r inves tment i n the requ i red equipment, t h eloaded carbon i s sh ipped to sm el t e rs where the produc t i s burned to recover a


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Pregnant solution0.10 oz/fonPu

uSolution storageond makeup tank

F IGURE 5. - Hypothetical distribution of gold i n a con.

00002 oz Au

B a r r e nSolution

NoCN ICaO

dore' bu l l io n by smel t in g theashed res idu e . Typica lsmel t e r charges inc lud ingth e nonpayment fo r 7 . 5 pc to f t h e t o t a l g ol d c o n te n tw i l l average about $3,500per d ry ton of ca rbon con-taining 300 ounces of goldpe r t on . A dd i t i ona l co s t st o be cons ide red a re t hepu rchase p r i ce o f t h e ac t i -vat ed carbo n and haulagecharg es. Thus, a reaso nablec o s t f i g u r e f o r p r o c es s in ga to n of loaded carbon bythe smel t ing t echnique i s$5,500 to $7,000. I n th epr ef er re d method, goldrecovery f rom ac t iva ted car -bon i s a ccom ~ l i shedbv.tinuous carbon-adsorption operation. desorbing the gold anb s i l -ver values from the carbonand e l ec t row i nn ing t he va l ues from t he r e su l t a n t s t r i p so l u t i o ns . T h is pe r -mi ts repeated reu se of the carbon, which gr ea t l y enhances the economics of theca rbon cyan i da t i on p roces s . The f i r s t p r ac t i ca l de so rp t i on p roces s was deve l -oped by the Bureau of Mines i n 1952 ( 2 63 -1 ) . The pro ces s (commonly r e f e r r e dt o as th e Zadra proces s) employs a 1 . 0 pc t NaOH-0.1 p c t NaCN s t r i p so lu t i on a t

93" C and a t a tmospher ic p ressure t o desorb the go ld and s i l ve r f rom the ca r -bon . The go l d-s i lve r desor p t ion s t ep , which i s dependent upon the concent ra -t i o n of a l k a l i (NaOH) and t h e t em pera t ure o f t he s t r i p so l u t i on , i s g re a t l yenhanced by the ad di t ion of an a l cohol such as e thanol o r methanol t o th ed e s or b e nt o r s t r i p p i n g s o l u t i o n . The prec ious meta l va lues conta ined i n thes t r i p s o l u t i o n a r e e l e ct ro w on from t h e s o l u t i o n , and t h e b a r r e n s o l u t i o nr e c y c l ed f o r a d d i r i o n a l s t r i p p i n g . C u rr en t i n d u s t r i a l a p p l i c a t i o n of t h eZadra desorp t ion process (10-11)- nd i ca t e s t ha t 50 t o 100 hour s a r e r equ i r edt o s t r i p th e carbon from a loading of 300 ounces of gold t o le ss than 5 ounceso f gol d pe r t on o f carbon . The Bureau a t t he S a l t Lake Ci ty Metal lurgyResearch Center deve loped a pressure s t r ip p in g method th a t g r ea t l y reducesst r ip pi n g t ime and reag ent consumpt ion (3, 5).

Economics of Heap-Leach OperationsPiz ar r o descr ibed i n d e t a i l t he 10,000- ton-per-month heap- l each ing opera-t i o n a t the Ca rl in Gold Mine, C arl i n , Nev. (21) . The heap-leach ef f l ue n ts

j o i n t he p regnan t so l u t i ons from t he C a r l i n mill f o r c l a r i f i c a t i o n f ollo we d byconven t i ona l z i nc dus t p rec i p i t a t i on fo r go l d r ecove ry . Reagent consumptionwas minima l, 0 . 1 pound NaCN and 1 . 0 pound CaO pe r t o n of o re . Pi zz ar o (2 1)and McQuiston (20 , pp . 19-25) presen ted sev era l exc e l l e n t t a b l es o f ca r l i n t sope rat ing costs- Based on the se data , the ca lcu la ted heap-leaching oper at io nc os t s a t Ca r l i n were $0 .96 per ton of o re i n 1973 and $1 .21 per ton i n 1974.


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Mountain S t a t e s Re sea rch and Development, Tucson, A ri z. (4-5J, made aneconomic ev al ua ti on on four pro ce sse s co-nly employed f o r rec ove rin g goldfrom low-grade ore s . The ex tr ac t i on proc esse s evalua t ed were (1) heap lea ch-ing wi th ca rbon adso rp t ion , ( 2 ) v a t l each ing wi th ca rbon adso rp t io n , (3 ) con -v e n t i o n a l c o u n t e r c u r r e n t d e c a n t a t i o n , and (4) c a r b o n - in - p u lp c y a n id a t i o n .Bhappu ( 4 ) i n d i c a t e d t h a t t h e h e a p- l ea c h p r o c e ss i s e c on o m ic a ll y j u s t i f i e d i nthe p roce ss in g of l ean o res ave rag ing 0 .04 ounce o f r ecov e rab le go ld pe r tona t g o ld p r i c e s o f $ 80 p e r oun ce f o r m u l t im i l l i o n - to n o r e b o d ie s . For t h esma l le r ope ra t ion s , l abo r cos t s would be p ro por t io na te l y much la rg e r pe r to n ,and t he co s t s f o r equipmen t and f a c i l i t i e s would d i f f e r wide ly depend ing ont h e a c c e s s i b i l i t y of t h e mine.

Ch isho lm (6 ) repor ted tha t a s m a ll min ing company i n New Mexico bro ugh tin to product ion-a 500- ton-per-day open p i t go ld mine and heap- leach op era t io na t a t o t a l c o s t o f $150,000. A breakdown of t h e c os ts f o r i tem s of equipmentpurchased and f o r mine f a c i l i t i e s was p resen ted .

A d d i t i o n a l c o s t f a c t o r s t h a t a p pe ar t o s u b s t a n t i a t e t h e f o re g oi n g c o s t swere developed a s a r e s u l t o f in fo rma t ion ga th e re? f rom se ve ra l sou r ces .Based on a 500,000- ton heap- leach o pe ra t ion , ty p i ca l cos t s fo r p repa r ing ana s p h a l t pad, i n c lu d in g p r e p a r a t i o n o f s i t e , s u bb as e, s e a l e r , a nd co v e r in g t h ea s p h a l t w i t h a l a y e r of g r a v e l a r e $ 0 .5 0 p er s qu ar e f o o t . T h is i s e q u i v a l en tt o $0 .50 per t on of o re , a s suming th a t a ton of o r e i s s tacked pe r squa re foo to f pad su r f ac e . The s tack ing of o re on a pad rep or t ed ly c os t s abou t $0 .10 pe rton . The lea chin g co s t s a r e dependent on rea ge nt consumption and l eng th ofpe r iod o f t r ea tmen t , and thus can va ry appre c iab ly . A r e a s o n a b l e l e a c h in gc o s t r e p o r t e d l y i s $ 0 . 50 p er t o n of o r e . Opera t ion of th e carbon deso rp t i onu n i t c o s t s a b o ut $ 0 . 0 2 p e r t o n of o r e , a ssu ming t h a t t h e o r e c o n t a in s 0 .0 5ounce of recoverable gold per to n and t h a t t h e c a rb o n i s loaded with 400o unc es o f go ld p e r t o n . P r e v a i l i n g p r a c t i c e i s t o r e g e n e r a t e t h e l e ac he d c a r -bon be fo re reu se by hea t ing th e ca rbon i n a steam atm osphere a t -700" C i n ar o t a r y k i l n ( 8- 10 ).- Based on c o s t f i g u r e s s t a t e d i n a r e c e n t a r t i c l e o n c a r -bon r e g e n e r a t i o n ( 161, t h e c o s t f o r r e a c t i v a t i n g t h e s t r i p p e d c a rb o n i s e s t i -mated t o be $0.02 per to n of o re . A sunnnary of th e co s t es t im at es , e xc ludingm in in g c o s t s , i s s hown i n t a b l e 1.

TABLE 1. - Est imated c os t of a commercial heap-l e a c h c y a n id a t i o n o p e r a t i o nItem 1 Cost per tonof oreCon stru c t io n of pad . . . . . . . . . . . . . . . I $0 .50

Stacking ore on pad . . . . . . . . . . . . . . .Heap leaching . . . . . . . . . . . . . . . . . . . . .Carbon desorp t ion . . . . . . . . . . . . . . . . .Carbon regenera t ion . . . . . . . . . . . . . . . .02r o t a 1 . . . . . . . . . . . . . . . . . . . . . . . 1.14


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CYANIDE HANDLING AND DISPOSALThe p rec iou s-met a l s min ing in du s t r y fo r many ye a r s has p romoted t he

h e a l t h a nd s a f e t y of i t s e mp lo ye es r e g a r d i n g t h e h a n d l i n g a nd u s e o f c y a n i d e .The i n d u s t r y h a s d em o ns tr at ed t h a t , w i t h pr o pe r t r a i n i n g a nd i n s t r u c t i o n s ,c ya ni de c a n b e us ed r o u t i n e l y i n l e ac hi ng g o l d - s i l v e r o r e s w i t h l i t t l e r i s kt o t h e w o r k e r . However, g ro w in g c o n c e r n a b o u t o c c u p a t i o n a l h a z a r d s an de n vi r on m en t al p o l l u t i o n ha s r e s u l t e d i n t h e pr o m ul g at i on of r e g u l a t i o n s t h a tr e q u i r e i n d u s t r y t o com ply w i t h s t a n d a r d s and g u i d e l i n e s e s t a b l i s h e d b yF e d e r a l , S t a t e , and c o u n ty r e g u l a t o r y b o d i e s . The O c c u p a ti o n a l S a f e t y a ndHea l th Ad mi n i s t r a t i on (OSHA) i n Oc tober 1976 pub l i sh ed i t s recommendationsd e si g ne d t o p r o t e c t t h e h e a l t h o f e mployees wo rk in g w i t h c y an i de s a l t s . Thep u b l i c a t i o n , " C r i t e r i a f o r a Recommended S t a n d a r d - 4 c c u p a t i o n a l E x po s ur e t oHydrogen Cyanide and Cyanide S a l t s [ N ~ c N , KCN, a nd c ~ ( c N ) , ] " c a n b e p u r c h as e df r o m t h e S u p e r i n t e n d e n t o f D oc um en ts , U . S . G ov ern me nt P r i n t i n g O f f i c e ,Washington, D . C . 20402.

E mp lo ye es w or k in g a t a h e a p - l e a c h c y a n i d a t i o n i n s t a l l a t i o n may be e x po s edt o c y a ni d e i n t h e fo rm o f d u s t and s o l u t i o n s , e s p e c i a l l y d u r i n g m ix in g of c o n -c e n t r a t e d s t o c k c ya ni de s o l u t i o n s . I n g e s t i o n o f a s l i t t l e a s 0.20 gram o fs od iu m c y a n i d e i s c o n s i d e r e d t o b e l e t h a l f o r human b e i n g s ( 1 7 ) . The he a p-l e a c h o p e r a t i o n i t s e l f i s c o n si d er a b l y l e s s h az ar do us b e c a u s e t h e l ea c h i sc o nd u ct e d i n a n o pe n a r e a w i t h maximum v e n t i l a t i o n . By m a i n t a i n i n g t h e a l k a -l i n i t y o f t h e l e ac h s o l u t i o n a t pH 1 0 t o 11, t h e p o s s i b i l i t y o f g e ne r a t i nghydrogen cya nid e gas (HCN) i s min imized , and on ly t r a c e amounts of HCN c a n b er e l e a s e d b y i n t e r a c t i o n o f NaCN an d COa i n t h e env i ronmen t . Measurements madeby Mining Enforcement and Sa f e t y Ad mi ni s t ra t i on (MESA) i ns pe ct or s (18) showt h a t t he HCN c o n c e n t r a t i o n i n t h e a i r c l o s e t o a w or ki ng h ea p i s c o n s i s t e n t l yo n l y 2 t o 3 ppm. This i s s i g n i f i c a n t l y l e s s t h a n t h e l i m i t of 1 0 ppm ( 2 )-e s t a b l i s h e d by OSHA f o r s u s t a i n e d b r e a t h i n g o f g a s e o u s c y a n i d e .

I n a w e l l- d es i g ne d h e ap - le a ch i n s t a l l a t i o n , t h e p re g na n t c ya n id e s o l u t i o ns e t t l i n g p on d, w hic h c a t c h e s t h e d r a i n a g e f ro m t h e h ea p, s h ou l d b e d e s i g n e d s ot h a t t h e c a p a c i t y of t h e pond i s s u f f i c i e n t t o accommoda te t h e maximum r a i n -f a l l and r un o ff t h a t c a n b e ex pe ct ed f o r t h a t p a r t i c u l a r l o c a l i t y , t h us p r e -v e n t i n g t h e d i s c h a r g e o f c y a ni d e s o l u t i o n t o t h e w at e rs h e d d u r i n g o p e r a t i o na n d a f t e r ab an do nm en t of t h e l e a c h e d o r e he a p. The s e t t l i n g p on ds a r e g e n-e r a l l y e a r t h f i l l s t r u c t u r e s t h a t a r e l i n e d w i t h w a t e r t i g h t po ly v i ny l ch l o r id eo r p o l y e t h y l e n e s h e e t i n g . P onds ho l d i ng c y a n i de s o l u t i o n s sh o ul d b e a d e -q u a t e l y p o st e d and f e nc ed t o r e s t r i c t a c c e s s t o t h e a r e a .

B ec au se o f t h e a p p r e c i a b l e e v a p o r a t i o n l o s s e s t h a t o c cu r d u r i n g he apl e a c h in g , m ost o p e r a t o r s a r e a b l e t o m a i n t a i n c om p le te r e c y c l i n g o f t h e l e a c ha nd wash s o l u t i o n s . Thus t h e n ee d t o d i s c h a r g e p o t e n t i a l l y ha z a rd o us s o l u -t i o n s t o m a i n t a in t h e w at er b a l an c e f o r t h e l e ac h o p e r a t i o n i s c i r c u m v e n t e d .I f a b l e ed o f f s y s te m i s r e q u i r e d , i n t h e e v e n t o f a n ab no rm al ly he av y r a i n f a l l ,c y a n i d e re m o va l t e c h n i q u e s m us t be c o n s i d e r e d . The most widely used methodf o r r e d u ci n g f r e e c y a n i d e and h e av y -m e ta l c y a n i d e c o n c e n t r a t i o n s i n w a s t es t r e a m s i n vo l v e s ch e mi c al t r e a t m e n t w i t h c h l o r i n e o r h y p o c h l o r i t e . The


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react ion mechanism i s bel i eved to be as fo l lows:NaCN + 2NaOH + C1, -. NaCNO + 2NaC1 + H,O, ( 6 )

The av a i l a b l e ch l o r i ne may be fu rn i shed a s ch l o r i ne gas o r a hypoch l o r it esolut ion. Approximately 1 pound of calciu m hy poc hlo ri t e, Ca(OCl),, w i l l ox i-d i z e 1 pound of f r e e cyan ide. The term 'heavy metal s" ge ne ra l ly denotes thosem e ta ls t h a t a r e o f p a r t i c u l a r c on ce rn i n cyanide t rea tment such as copper ,s i l v e r , zi nc, cadmium, mercury, ar se ni c, chromium, manganese, i r on , and ni ck el .The main method employed fo r removal of th e heavy metal s a f t e r t he d es tr uc t i onof cyan ide i s by add i t i on of l im e t o p re c i p i t a t e t he m e t al s and t o promotef l occ u l a t i o n of t he p r ec i p i t a t e s . The heavy -m et al p re c i p i t a t e s would be f i l -t e r ed and sh ipped t o an app roved t ox i c - subs t ance l a nd f i l l f o r d i spos a l . TheU.S. Environmental Pr ot ec t io n Agency (EPA) regu la t io ns f or l i mi ta t i on s ofcyanide discharge i n waste s o lu t ion s i s 0 .02 ppm CN .

It i s c le a r l y advantageous from an economic s t and poi nt f o r t he mine oper-at or t o lower t he sol ubl e gold los ses i n the heap to a minimum by thoroughwashing of the l eached or e wi t h f r es h water . The washing s t e p r e s u l t s i nrecovery of most o f the d i sso lved go ld and a l a r ge por t io n of o th er cyanidesremaining i n th e heap as f r e e cyanide o r complexed wi th heavy metals . Heavy-m eta l cy an id e s a l t s a r e known t o p e r s i s t f o r s e v e r a l y e a rs , b u t r e s i d u a l f r e ecyanide i n abandoned heaps i s bel i eved t o e x i s t no more than 1 month, depend-ing on cl imat ic condi t ions; however, s c i e n t i f i c d a t a t o s u pp o rt t h i s c on te n-t i o n a r e l a ck i ng . The r e t en t i on and f a t e of r e s i d ua l cyani de i n heap leachedre s i dues i s be i ng sc r u t i n i z ed t o an i nc rea s i ng ex t en t by r egu l a t o r y agenc i e s .

Abandoned heap s a r e l e s s su sc ep t i b l e t o wind and w a t er e ro s i o n t hanfi ne ly ground t a i l i n g s impounded behind a dam. I n th e semiarid regio ns of t heWestern Uni ted S ta te s , where most of th e heap leach cy ani dat ion i s being prac-t i ced , i nvas ion of the abandoned heaps by na t iv e de se r t f lo ra has beenobserved t o occur wi th in 1 o r 2 y e a r s.

Under pr ov is io ns of the Fe de ra l Metal and Nonmetal lic Mine S af et y Act of1966 (P ub lic Law 89-577), MESA i s res po ns ib le f o r th e enforcement of th eh e a l t h and s a f e t y s t a n d a r d s p r e sc r ib e d t o p r o t e c t t h e w or ke rs a t a l l mines i t e s , which inc lud es heap-l each cyanid a t ion oper a t ion s . With th e s ign ing ofthe Fe de ra l Mine S af et y and Health Act of 1977 (Public Law 91-173), i t becamethe re sp on s i b i l i t y o f th e Secr e ta ry of Hea l th , Educat ion , and Wel fare and th eSecr etary of Labor t o develop and promulgate improved hea l t h and sa fe ty s ta n-dards f or persons working a t mining p rop er t i es . New and/or r ev is ed re g u la -t i ons w i l l be implemented by t he Mining Safe t y and Heal th ~ d m i n i d t r a t i o n(MSHA) under the Department of Labor. The ef fe ct iv e da te of these ru le s andre gu la t io ns i s expected to be March 9, 1978.

Under p ro vi si on s of th e Resource Con serv ation and Recovery Act of 1976(Public Law 94-580), EPA i s res po ns ib le f o r th e development of re gu la t i on s andguide l ines f or d i s pos a l and management o f a l l s o l id was tes , i nc lu d ing min ingwastes. EPA cu r r en t l y i s conduc ti ng a de t a i l e d s t udy on t h e adve rse e f f ec t s


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on th e environment of so l i d wastes f rom ac t i v e and abandoned su rf ac e andunderground mines. Guidel ines fo r the d i s pos a l of mining was tes a r e expectedfol lowing th e comple t ion of th e s tu dy i n June 1978. Meanwhi le , S ta te and/orcoun ty gwer nment s wher e heap- leach cyan i da t i on ope r a t ions e x i s t r eg u l a t e t h edi sp os al o r abandonment of cyanided or e heaps.

RECENT INNOVATIONS I N GOLD-SILVER RECOVERYFROM CYANIDE PROCESS SOLUTIONS

Recent ly, e f fo r t s have been di re ct ed toward th e development of lower costgold-process ing procedures tha t w i l l improve th e economics of t r e a t i n g lowerg r ade o r es and d epo s i t s t oo s ma l l t o war r an t cons t r uc t i o n of conven t i ona l m i l lf a c i l i t i e s .

The Bureau of Mines (14) developed a more e f f i c i e n t pro ces s f o r rec ov er-ing gol d and s i l v e r from l aden ac t i v a t ed ca rbon. Fac t o r s t h a t can a f f e c t t heper formance of the modif ied s t r i p so lu t io n inc lude t empera ture , and concent ra-t i o n s of NaOH and eth ano l . The s t r i p so lu t io n, which i s wate r con tai nin g1wt-pct NaOH and 20 vo l - pc t e t hano l , i s hea ted t o 80" C and c i r cu l a t edthrough a bed of loaded carbon t o desorb prec ious metals and cyanide io ns .The p r eci ous me ta l s a r e r ecover ed by e l e c t r o l y s i s , and t he dep l e t ed e l e c t r o -l y t e i s r e c y c l e d t o t h e c a rb on d e so r p t i o n colum n. F i v e t o s i x h ou rs o fs imul taneous s t r ip pi ng and e lec t rowinning w i l l desorb up t o 99 pc t of the

p r ec i ous me t a l va l ues c a r -r i e d by t h e a c t i v a t e d c a r -bon. A f low diagram for thep r o ce s s i s shown i n f i g u r e 6 .

Activated carbon, whichhad been used f o r carbon - in-pulp cyanida t ion of goldo r e s f o r s e v e r a l y e a r s, i snow playing an importantr o l e i n t h e re c ov e ry of g o ldfrom heap- leach cy ani dat ionef f l ue nt s . However, ap pl i -c a t i o n of t h i s t ec hn ol og y t o

le th e t r ea tment of gold ore s" t h a t a r e r i c h i n s i lv e rg r ea t l y i nc r eas es t he amoun t

of ac t iva ted carbon neededt o adso rb t h e equ i va l en td o l l a r v a l u e o f s i l v e r froman equa l volume of lea chs o l u t i o n . A process ingsequence was developed t h a tc i r cunwents the handl ing ofl a rg e q u a n t i t i e s of a c t i -

F IGURE 6. - Recovery of gold from activated carbon. va ted carbon (13) . I t con-s i s t s of r e co v e ri n g t h es i l v e r s e l e c t i v e l y a s a AgzS


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p r e c i p i t a t e u s i ng sodium s u l f i d e , and a f t e r f i l t r a t i o n , r e c ov e r in g t h e go l dfrom t h e f i l t r a t e b y c a rb o n a d s o rp t io n . The key t o t h i s p r o c e s s i s t h e f or ma-t i o n of r e a d i l y f i l t e r a b l e AgzS f l o c s by a g i t a t i n g t h e s l u r r y w i t h 0 .5 poundof CaO per to n of s ol ut io n f o r about 1 hour. The process sequence i s shown i nf i g u r e 7 . The Ag,S pr ec ip i ta t io n- ca rb on a ds or pt io n proc essin g method i s beingeva lua ted a t a heap- leach cyan i da ti on i n s t a l l a t i on by Sand Sp r ings C o. , Fa l l on ,N ev ., fo r p roces s i ng a s i l ve r o re low i n go l d con t en t .

The sod ium su l f i d e p re c i p i t a t i o n p rocedu re was a l s o shown t o be e f f e c t i v ef o r s e l e c t i v e l y p r e c i p i t a t i n g s i l v e r fro m h i g h l y c o n c en t r at e d g o l d - s i l v e re f f l u e n t s o b ta in e d i n t h e i n i t i a l s t a g e s of s t r i p p i n g p r e ci o u s m e ta l- la d enac t iv at ed carbon . The method provides a means f o r th e subsequent product ionof a l ow -s i lve r go ld bu l l i o n . S i l v e r i s p r e c i p i t a t e d s e l e c t i v e l y from s t r i pso l u t i on by t h e same p rocedu re used p rev i ous l y fo r s i l ve r p r ec i p i t a t i o n fromp re gn an t c y an id e le a c h s o l u t i o n s . F i l t r a t e s c o n ta i n in g l e s s t ha n 1 ppm Ag andUD t o 800 m m Au were o b ta in e d. The ~ r e c i ~ i t a t e s ,hich contain diatomaceousL .ea rth , l ime, s l im es, e t c . , as we l l as Ag,S, assa y 3,000 to 6 ,000 ounces ofs i l v e r p e r t o n and r e p r e s e n t 99 -pc t s i l v e r r e c o ve r y . The ent rained goldamounts t o l e s s t han 0 .1 pc t o f t he t o t a l p rec i ous m e t a l con t en t . The

NaAg(CNI2 solutic n from heap leach

Gold recovery

Corbon

Irecipitotion

.....

adsorptionLoadedcarbon I

wrensolution

Retention

I I uiltrationCorbonstripping I L , Silver productGold productFIGURE 7. - Proposed treatment of cyanide effluents high in silver content.


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s i l ve r - f r e e f i l t r a t e ca nt h en be e l e c t r o l y z e d t o w int h e g o l d by d ep o s i t i o n ons t e e l wool ca thodes . Bar rene l e c t r o l y t e r e cy c le d t o th ed es o r p t i o n u n i t c an b e us edt o desorb more valu es f romt h e c a rb o n. F i n a l l y , t h eg o l d - lad en s t e e l w oo l c a t h -odes when mixed with properf l u x e s c a n b e r e f i n e d i n t og ol d b u l l i o n low i n s i l v e r .

Corbon columns Figure 8 shows a conceptualf lo w d iagr am d ep i c t i n g t h el Loaded mrbonI I heap l each-carbon adsorp t ion-Stripped carbon

L--------------------- I des orp t ion system and th e1 7 p r ec i o u s m e t a l s s ep a r a t i o nDesorplionunit

S t r iproluliOntank

and r ecovery cyc le .SUMMARY

Digestion precipitation 1 1. Au cyonida solution-% - + - + - . . . - +Fi l t ra t ion I H yE 1 e c l r o l ~ t i c R e c y c le d r l r ipThe r i s i n g f r ee -marke t

gold pr ice has given a newl e a s e o n l i f e t o t h e g o ldmining in du s t ry . The heapl each cy an i d a t i o n - ca r b o n

soIution ad s o r p t i o n - e l ec t r o w i n n i n g- ilvFr prod uct Gold product process developed by theF I GURE 8. - Proposed heap leach-carbon adsorption F ed e ra l Bureau of Mines hasprocess for gold ores low in silver. ? roved t o be an economica lmethod f o r ex pl oi t i ng low-g r ad e g o ld o r e s and s m a l l i s o l a t ed d ep o s i t s n o t s u i t ab l e f o r t r e a t m en t b y co n -v e n t i o n a l c y a n i d a t i o n p r o c ed u re s . Heap le ach ing may be th e most pr o f i ta b lem ethod f o r p r o ce s s i n g s e l ec t ed g o ld and s i l v e r o r e s t h a t d o n o t r eq u i r e f i n eg r i n d i n g and a r e r e ad i l y am enab le t o cy an i d a t i o n .


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REFERENCESAdamson, R . J . The Chemistry of th e E x tr ac ti on of Gold From I t s Ores.

Ch. i n Gold Meta l lurgy i n South Af r ica . Cape and Transvaal P r in te rsL t d ., Cape Town, South A fr i ca , 1972, 452 pp .

American Conference of Governmental I n d u st r ia l Hy gie nis t s . ThresholdLimit Values f o r Chemical Sub stances i n Workroom A i r Adopted by ACGIHf o r 1974. American Conference of Governmental In d u st r i a l Hy gien is ts ,Ci nc in na t i , Ohio, 1974, 94 pp.

Barsky, G . , S. J . Swainson, and N . Hedley. Di ss ol ut io n of Gold and S i l -ve r i n C ya nide So lu t i ons . T ra ns. Am. I n s t . Min. Meta l . Eng., v . 122,1962, p. 660.

Bhappu, R . B . , and F. M . Lewis. Gold E x t r a c t i o n From Low Grade Or es --Economic Ev alu ati on of Pr oc ess es. Min. Cong. J . , v . 61, No. 1,pp. 38-41.

%.

Bhappu, R . B . , M. F. Lewis, and J. A . M c al li st e r. Leaching of Low GradeGold Ores--Economic E va lu at io n of Av ail ab le Pr oc ess es. Pr es . a t AIMEAnn. Meetin g, D al la s, Tex., Feb . 23-28, 1974, AIME P r e p r i n t 74-AS-55,1974, 18 pp.

Chisholm, E . 0 . Canadians Opera ting Gold Leaching Op erat ion s i n NewMexico. The No rth ern Miner, v. 61, No. 27 , 1975, pp . 59-60.

Davidson, R . J . The Mechanism of Gold Adso rpti on on Ac tiv ate d Ch arc oa l.J . So. A fr . I n s t . Min. and M eta l. , v . 75, No. 4, 1974, pp. 67-76.

Duncan, D . M . Open P i t Gold Mining a t Corte z. Ch. i n Colo. Min. Assoc.Min eral s Yearbook 1974, pp. 92-94.

Duncan, D . M . , and T . J . S mo lick . How Co rt ez Gold Mines Heap-Leached LowGrade Gold Ores a t Two Nevada P r o p e r t i e s . Eng. and Min. J . , v. 178 ,No. 7, 1 977, pp . 65-6 9.

H a l l , K . B . Homestake Carbon-in-Pulp Pro ces s. Pr es . a t Am. Min. CongMeeting, Las Vegas, Nev. , O ct . 7-10, 1974, 16 pp. ; av ai la b l e uponre q u es t from Homestake Mining Co., Lead, S. Dak.. Homestake Uses Car bon -in- Pul p To Recov er Gold From Sl im es . World-in., v . 27, No. 12 , 1974, pp . 44-4 9.Hedley, N . , and T. Howard. Ch emi stry of Cy an id at io n. American CyanamidCo., Wayne, N . J . , 1968, 54 pp.

Heinen, W. J . , D. G . Pe te r son , and R . E . Lindst rom . S i l v e r Ex t r a c t i onFrom Marg inal Res ou rce s. P r e s . a t 1 04 th TMS-AIM!Z Ann. Me etin g, NewYork, Feb. 16-20, 1975, 14 p p. ; av a i la bl e upon re qu es t f rom Bureau ofMines, Reno M eta llu rg y Res earc h Cen ter, Reno, Nev.


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Heinen, H. J . , D. G . Peterson, and R . E. Lindstrom . Gold De sor pti on FromAc tiv ate d Carbon With Al kal ine Alcohol So lut ion s. Ch. i n World Miningand Metals Technology, v . 1. American In s t i t u t e of Mining, M etal lurg i-c a l , and Pe tro leu m Eng in eer s, New York, 1976, pp. 551-564.

Heinen, H . J . , and Bernard P o r te r. Experime ntal Leaching of Gold FromMine Waste . BuMines R I 7250, 1969, 5 pp.Loven, A . W . Pe rs pe ct iv es on Carbon Rege nera tion . Chem. Eng. Prog.,

v . 69, No. 11, 1973, pp. 56-62.Manufacturing Chemists Association. Chemical Safety Data Sheet SD-30--

Pro pe r t i es and Es se nt ia l Informat ion f o r Safe Handling and Use ofSodium Cyanide. Man ufac turi ng Chemists As so ci at io n, Washington, D.C . ,1967, 18 pp.

McAlexander, R. M . Pr i va t e cownunicat ion , 1977. Ava i lab l e fo r cons u l ta -t io n a t Mining Enforcement and Sa fet y Adminis t ra t ion, Reno Fie l d Of f ice ,Reno, Nev.

Merwin, R . W . , G . M . P o t t e r , a n d H. J . Hein en. Heap Leachin g of GoldOres i n Northeas tern Nevada. Pre s . a t AIME Ann. Meeting, Washington,D.C. , Feb. 16-20, 1969, AIME P r e p r i n t 60-AS-79, 1969, 13 pp.

McQuiston, F. W . , J r . , and R . S. Shoemaker. Fre e M il li ng Ores . Ch. i nGold and Si lv er Cyanidat ion Pla nt P ra ct ic e . American In s t i t u t e of Min-in g, M et al lu rg ic al , and Petr oleu m Eng in eer s, New York, 1975, 187 pp.

P i z a r r o , R., J . D . McBeth, and G . M . Po t t e r . Heap Leach ing Pr ac t ice a tth e C a rl in Gold Mining Co., C a rl i n , Nev. Ch. i n So lu ti on Mining Sym-posium. American I n s t i t u t e of Mining, Me ta ll ur gi ca l , and PetroleumEn gi ne er s, New York, 1974, pp. 253-267.

P o t t e r , G . M . Recovering Gold From S tr ip pi ng Waste and Ore by Pe rc ol a-t i o n Cyanide Leach ing . BuMines TPR 20, 1969 , 5 pp.

P o t t e r , G . M . , and H . B . Sal i sbur y. Innovat ions i n Gold Metallurgy.P r es . a t Am . Min. Cong. Min. Convention/Enviromnental Show, Denver,Colo., Se pt . 9-12, 1973, 11 pp. ; available from Bureau of Mines SaltLake Ci t y Metal lurgy Research Center , S al t Lake Ci ty , Utah.. Inno vati ons i n Gold Met allu rgy. Min. Cong. J . , v . 60, No. 7,-974, pp. 54-57.Ross, J . R . , H . B . Sal isbury, and G . M . P o t t e r . P r e s s u r e S t r i p p in g GoldFrom Activated Carbon. Pre s. a t AIME Ann. Conf., SME, Chicago, I l l . ,Feb. 26-Mar. 1, 1973, 15 pp .; av ai la bl e from Bureau of Mines S a l t LakeCity Metal lurgy Research Center , S a l t Lake Ci ty , Utah.

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27. Taggar t , A . F . Handbook of Min eral Dr ess in g. John Wiley & Sons , Inc . ,New York, 1945 , Sec. 2, p . 72.

28. U.S. Bureau of Mines S t a f f . Pro duc tion P o t e n t i a l of Known Gold Dep osit si n th e Uni ted S t a t e s . BuMines I C 8331 , 1967, 24 pp.29. White L. Heap Le ach ing W i l l Produce 85 ,000 OzIYear of ~ o r k u l l i o n f o r

Smoky V al le y Minin g. Eng. and Min. J . , v . 178, No. 7, 1977, pp. 70-72.30 . Wilson, W . L. The Eureka Wind fall Gold Mine. Pr es . a t th e Geol. Soc. of

Nev. and Mackay Sc ho ol of Mines Symp., Reno, N ev., Mar. 26, 1976 ;a u tho r a va i l a b l e f o r c on s u l t a t i o n a t I da ho Min ing C orp ., Grand J unc t i o n ,Colo. 81501.

31. Zadra, J . B. , A . L . Engel, and H . J . Heinen. Process f o r Recovering Goldand Si lv er From Act ivat ed Carbon by Leaching and E l ec tr o ly si s . BuMinesR I 4843, 1952, 39 pp.

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