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Miner a l s Eng ineer ing

Vol. 7, No. 10, pp. 1213-1241, 1994

~ ) Copyright ~ 1994 Elsevier Science Ltd

Pergamon Printed in Great Britain. All rights r e se rve

0892-6875/94 7.00+0.00

0892-6875(94)00063-8

R E V IE W O F G O L D E X T R A C T IO N F R O M O R E S

S . R . L a B R O O Y , H . G . L IN G E'I a n d G . S . W A L K E R *

A.J. Par ker Cooperative Research Centre for Hydrometallurgy, Australia

Mineral Sc ience , M urdoch Un ivers i ty , Murdoch, Austral ia , 6150

1 CSIRO DM P, Cu rtin Universi ty , Bentley , Australia , 6102

.'1: CSIR O DM P , Floreat Pa rk Labo ratories, Wem bley, Australia,

6 0 1 4

(Received 15 March 1994, accepted 26 April 1994)

A B S T R A C T

Australia is now an important gold producer in the worM. The nature of Australian goM

production is briefly reviewed and the hydrometalhu'gy of goM extraction is considered.

The choice ofprocessing routes fo r free milling, complex and refi'actoo, ores is discussed.

For free milling ores, cyanidation and recovely by the Carbon-in-Pulp/Carbon-in-Leach

process (CIP/CIL) is the primary and proven treatment process. Copper containing ores

are discussed its some detail as they interfere in the C1P/CIL process. Oxygen consuming

and preg-robbing ores are also described. Five different classes of process options fo r

pretreating rejS'actory ores are considered in detail. These options include: ultrafine

grinding, acid and alkaline pressure oxidation; and a variety of chemical pretreatments

such as: Activox process, HMC process and Electrolytic oxidation process, h~ Australia,

the two pre-eminent options fo r refractory gold ore pretreatment ate roasting attd

biooxidation and this development is reported. The trial of pressure cyanidation o f

stibnite concentrates at the Golden Spec mine its Australia is described. Pyrolysis, the

Nitrox/Redox process, the Artech/Cashman process and the Caro's acid process have not

gained commercial status so far.

The potential or resin-in-pulp (RIP) to replace CIP/CIL is discussed. The use of cyanide

has generated environmental concerns because of its toxici~ and therefore research on

alternative goM re.covety processes using non-toxic reagents is considered. The key

candidates are: ammoniacal thiosulphate, thiourea and halide solutions. The chemistry

of these le.aching systems is briefly described and proposed flowsheets are referenced.

The future prospect fo r biohydrometallurgical goM recovely is indicated.

K e y w o r d s

Australian go ld prod uction ; carbo n-in-pu lp; carbon-in-leach; resin-in-pulp; go ld ore

process ing; review o f gold extract ion; f ree mill ing gold ore; ref ractory g old ore; sulphidic

gold ore; arsenopyr i t ic gold ore; gold recovery process ; biohydrometal lurgical gold

recovery .

I N T R O D U C T I O N

Australia

i s exper iencing the third gold boo m in its his tory. In the f irs t gold bo om in 1856

Australia

prod uced 88 tonnes of gold; in the second boo m in 1903 116 tonnes and in 1990 produc t ion reached 24 5

tormes [1]. To put this prod uction into persp ective in the wo rld scene: in 1992 Au stralia ranke d third

1213

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1214 s . R . LA BROOY

t a l

b e h i n d S o u t h A t 'r ic a an d U S A a n d w a s t h e d o m i n a n t p r o d u c e r i n i ts r e g i o n [ 2 ]. A u s t r a l ia ' s p r o d u c t i o n

c on t r i bu t e s a r ound 12% to t he t o t a l wo r ld p r odu c t ion [ 2 ] .

I n A u s t ra l ia , m o s t o f th e m i n e s , i n c lu d i n g t h e l a rg e s t s in g l e o p e r a ti o n b a se d o n K a l g o o r l i e ' s s u p e r p i t ,

a r e s i t ua t e d i n W e s t e r n Aus t r a l i a . W e s t e r n Aus t r a l i a n ou tpu t c ons i s t e n t l y a c c oun t s f o r a bou t 75% of

A u s t r a li a 's p r o d u c t i o n [ 1 , 2 ]. T h e d e p o s i t s o n w h i c h t h e s e m i n e s a r e b a se d c a n b e d i v i d e d i n to t w o b r o a d

c a t e gor i e s . I n t he f ir s t c a t e gor y a r e t he ge ne r a l l y sm a l l , l ow g r a de , ox id i se d de pos i t s d i r e c t l y a s soc i a t e d

w i t h p r e v i o u s l y m i n e d d i s c o v e r ie s d u r i n g t h e p e r i o d 1 8 9 0 - 1 9 1 0 . T h e s e a r e n o w e c o n o m i c to m i n e d u e

t o a c o m b i n a t i o n o f f a c to r s s u c h a s : e f f i c i e n c y o f c u r r e n t e x t r a c t io n m e t h o d s s u c h a s C a r b o n - i n - P u l p

( C I P ) / C a r b o n - i n - L e a c h ( C I L ) , w h i c h c a n s u c c e s s fu l l y u s e s a li n e g r o u n d w a t e r as f o u n d in th e o u t b a c k

o f A u s t r a li a ; u s e o f lo w c o s t , o p e n - p i t m i n i n g r a th e r t h a n u n d e r g r o u n d m i n i n g ; a n d f i n a ll y an a d e q u a t e

go ld p r i c e . F igu r e 1 i l l us tr a t e s a t yp i c a l ope n p i t ope r a t i on i n whic h t he sha de d p or t i ons r e pr e se n t l e nse s

o f h i g h - g r a d e p r i m a r y g o l d m i n e r al i sa t i o n .

xide or

T m n s i U o n or

r i m a r y s u l p h i d e o ~ e

Fig . 1 Cr oss se c t i on o f go ld - or e p i t

U n f o r t u n a t e l y , a s t h e p i ts g r o w d e e p e r r e a d i l y - l ea c h e d o x i d i s e d g o l d - o r e i s g r a d u a l ly b e i n g r e p l ac e d b y

p r i m a r y o r e t h a t m a y b e r e f r a c t o r y , i . e . m a y n o t g i v e e c o n o m i c a l l y a c c e p t a b le g o l d r e c o v e r y b y

c onv e n t ion a l c ya n ida t i on . P r e t r e a tm e nt o f the r e f r a c to r y m a te r ia l w i l l r a i se p r oc e ss ing c os t s a nd w i l l ne e d

to be o f f se t by h ig he r go ld g r a de s . I t i s e st im a te d t ha t m os t o f t he ox id i se d o r e b od ie s i n t h is c a t e go r y

w i l l b e e x h a u s t e d w i t h in a f ew y e a r s . T h e s u r v i v in g o p e r a t io n s w i ll n e e d t o a d o p t s o m e f o r m o f

r e f r a c t o r y o r e p ro c e s s i n g t e c h n o l o g y f o r p ro c e s s in g t h e p r i m a r y s u l p h i d e zo n e .

I n t h e s e c o n d c a t eg o r y o f d e p o s i ts a r e th e g r e e n f i e l d d i s c o v e r ie s . T h e s e r e s u lt fr o m m o d e r n e x p l o r a t i o n

e f l b r ts , i n c l u d i n g r e m o t e s e n s in g t e c l m iq u e s a n d h ig h r e s o l u t i o n a e r o m a g n e t i c s u r v e y s . M a n y o f th e s e

de pos i t s ha ve subs t a n t i a l a nd p r ove n l ong- t e r m r e se r ve s whic h w i l l s e e t he m ope r a t i ona l un t i l a t l e a s t t he

tu r n o f t he c e n tu r y . Ov e r t he la s t f i ve ye a r s t he r e ha s be e n a de c l i ne i n t he nu m be r o f m ine s a nd a

c o n c o m i t a n t c o n s o l i d a t i o n o f t h e c o m p a n i e s m a n a g i n g a n d o w n i n g t h e o p e ra t i o n s . W e s t e r n A u s t r a l ia ' s

p r o ve n r e se r ve s i nd i c a t e t ha t the s t a te w i l l r e m a in t he m a jo r p r od uc e r o f go ld i n Aus t r a l i a , un l e s s t he r e

i s a c o l l a pse o f t he go ld p r i c e .

T h i s r e v i e w c o n s i d e r s p r o c e s s in g o p t i o n s f o r g o l d e x t ra c t i o n a v a i la b l e f o r u s e i n t h e w e s t e r n w o r l d .

T h e r e i s l i tt l e p u b l i s h e d i n f o r m a t i o n a b o u t g o l d p r o c e s s i n g t e c h n o l o g y i n t h e f o r m e r U S S R a l t h o u g h g o l d

r e c ov e r y wa s a ppa r e n t ly ba se d on Re s in - in - Pu lp ( RI P) . I n Sou th Af r i c a , a sm a ll m ine i s ba se d on RI P

a s w i l l be r e v i e we d be low .

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Gold extraction from ores 1215

C H O I C E O F P R O C ES S IN G R O U T E

Ge n e ra l l y go l d o re s c a n be c l a s s i fi e d as free m i l l i ng ' , c omp l e x or r e f ra c t o ry , a s i l lus t ra t e d i n F i gu re

2 . F re e mi l l i ng o re s (s i z ed 80% < 7 5 I .t m) g i ve a go l d re c ove ry > 9 0% wi t h a c onv e nt i ona l 20-30 hours

c ya n i da t i on l e a c h . Suf f i c i e n t c ya n i de i s a dde d t o l e a ve a c onc e n t ra t i on o f 100-250 pp m a t a bout pH 10 ,

by t he e nd of the l e ac h . Ore s t ha t do no t p rov i de e c onom i c go l d re c ove ry wi t h c onve nt i ona l c ya n i da t i on

a re t e rme d re f ra c t o ry . ( In th i s r e v i e w, o re s t ha t g i ve a c c ep t a b l e e c onom i c go l d re c ove ry o n l y wi t h the

u s e o f s ig n i f i ca n t l y h i g h e r c y a n i d e o r o x y g e n r e q u ir e m e n t s w i ll b e r ef e r re d t o a s c o m p l e x ' , w h i l e th e

t e rm re f ra c t o ry wi l l be use d wh e n e nha nc e d rea ge n t a dd i t i on s t il l g i ve s inc om pl e t e go l d re c ove ry) .

G O L D O R E P R O ES S IN G

F R E E M I L L I N G C O M P L E X R E F R A C T O R Y

HIGHLY SEMI

O X I D E S U LP H ID E R E FR A C TO R Y R E F RA C T O RY

C Y A NID E O X Y G E N P R E G

C O N S UM I NG C O N S U M I N G R O B B I N G

F i g . 2 C h o i c e o f g o l d p r o c e ss i n g r o u te

Ore s m a y no t r e spond t o c onve nt i ona l c ya n i da t i on fo r th re e ba s ic r e a sons . F i r s t l y , i n h i gh l y re f ra c t o ry

ore s , t he go l d c a n be l oc ke d up i n t he mi ne ra l ma t r i x so t ha t l e a c h re a ge n t s a re una b l e t o re a c h i t .

Se c ond l y , i n c ompl e x ore s , r e a c t ive m i ne ra l s in t he o re c a n c on sum e t he l e a c h re a ge n t s in s i de re a c ti ons

a nd t he re m a y be i nsuf f i c i e n t c ya n i de a nd / o r oxyg e n i n the pu l p t o l ea c h t he go l d . Th i rd l y , c omp one nt s

of t he o re ma y a dsorb o r p re c i p i t a te t he d i s so l ve d go l d c ya n i d e c ompl e x so t ha t i t i s los t f rom t he l e ac h

l i quor . Som e ore s ma y h a ve c on t r i bu t i ons f rom e a c h of the se fa c t o rs , whi c h wi l l i n f l ue nc e t he p roc e ss ing

s t ra t e gy . Ore mi ne ra l og y de te rmi ne s t he de gre e o f r e f ra ct o r i ne ss a nd c a n be g ra de d a nd c l a s s i f ie d a s

shown i n Ta b l e 1 .

T A B L E 1 C l a s s if i c a ti o n o f o r e r e f r a c t o r in e s s

< 50% re c ove ry Hi gh l y re f ra c t o ry

50-80% re c ove ry M ode ra t e l y re f ra c t o ry

80-90% re c ove ry M i l d l y re f ra c t o ry

90-100% re c ove ry Non - re f ra c t o ry (F re e M i l l ing)

In Aus t ra l i a [3 ,4 ] , o re s f rom t he we a t he re d prof i l e usua l l y c on t a i n mos t o f t he go l d a s me t a l l i c g ra i ns

wh i c h c a n be e xpose d w i t h norm a l g r ind i ng . Ve ry f i ne -gra i ne d go l d c a n be e xpose d w i t h u l t r a - f i ne

gr i nd i n g , a l t hough t he a dde d go l d re c ove ry m a y no t be wor t h t he a dd i t i ona l p roc e ss i ng c os t . In ox i d i se d

ore , t he go l d c a n be c oa t e d by a n i ron ox i de pha se whi c h c a n s i gn i f i c a n t l y s l ow t he ra t e o f c ya n i da t i on .

Al t e rna t i ve l y , go l d c a n be p re se n t i n a n a l l oy wi t h s i l ve r , a s e le c t rum, o r i n a c om bi ne d c he mi c a l fo rm,

e .g . i n te l l u r ide s suc h a s c a l ve r i te (A uT o) o r , l e ss o f t e n , a s a uros t i bn i t e (AuSb2) , o r oc c a s i ona l l y a s

rna l doni t e (Au2B i) . The se c h e mi c a l c omb i na t i ons o f go l d c a n a l so be phys i c a l l y l oc ke d wi t h i n su l ph i de s .

Eve n w he n a c c e ss i b le t o t he le a c ha n t , t he se go l d c omp ound s rea c t ve ry s l ow l y wi t h oxy ge n a nd c ya n i d e

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1216 S R LA Bnoov

e t a l

so t ha t go l d d i s so l u t ion i s i nc ompl e t e wi t h i n t he s t a nda rd l e a c h ing pe r i od , t hus re duc i ng t he g o l d re c ove ry .

Pr i m a ry ore s a re o f t e n re f ra c t o ry be c a use t he go l d i s l oc ke d wi t h i n t he su l ph i de ma t r i x , wh i c h ne e ds t o

be b roke n dow n t o e xpose the go l d .

F R E E M I L L I N G O R E S

The se o re s m a ke up t he bu l k o f t hose c ur re n t l y t r ea t e d . P roc e ss i ng op t i ons a re shown i n F i gu re 3 .

F R E E M I L L I N G S E M I -

NO CRUSH ~ CRUSH

i |

GRINOf

I \ / I E C O N C E N T R A T ' O

/ | / I GRAVITY FLOTATION

, . Y , I C A L /

GRAWTY ~ TAILING

- 6 i - - / T R E A T . e N T

A G G L O M E R A T IO N ~ f

~ % ,% G R A V IT Y N O N E

R E G IN D

Fi g .3 Gol d p roc e ss i ng op t i ons fo r f r ee mi l li ng a nd se mi - re f ra c t o ry o re s

Placer ores a re a spec ia l case where no pre t rea tmen t is requi red. Class ica l ly these has been reco vered

by phys i c a l me t hod s suc h as g ra v i t y tr e a t me nt wi t h sl u i ce s , o r on t o b la nke t s . M ode rn g ra v i t y t r ea t me nt

un i t s suc h a s Kne l son [5 ] o r Fa l c on [6 ] c onc e n t ra t o r s , a re i n use fo r go l d re c ove ry a nd suppl e me nt

gra v i t a ti ona l fo rc e wi t h c e n tr i fuga l fo rc e t o e na b l e c a p t ure o f f i ne r go l d pa r ti c le s . Ano t he r phys i c a l

p roc e ss t ha t ha s be e n propose d i s c oa l-o i l a gg l om e ra t i on [7 ,8 ] w i t h sugge s t e d a pp l i c a ti on t o a l l uv i a l o re s

as we l l as other essent ia l ly l ibera ted gold.

For c he mi c a l r e c ove ry rou t e s p re t re a t me nt fo r f r e e mi l l i ng o re s i s usua l l y l i mi t e d t o c rush i ng a nd

gr i nd i ng , t o ma ke t he go l d a c c e ssi b le t o c ya n i de , wi t hou t ne c e ssa ri l y be ing t o t a l ly li be ra te d . The l e a c h

ra te a nd u l t ima t e go l d re c ove ry va ry t o som e e x te n t wi t h f i ne ne ss o f g r i nd . Lo w gra de o re s a re more

l i ke l y t o be tr e a t ed by h e a p l e a c h i ng [9 ,10] . The go l d ne e ds t o be a c c e ss i b le t o c ya n i de so l u t i on fo r

l e a c h i ng , he nc e t he pe rme a bi l i t y o f t he o re , a mongs t o t he r t h i ngs , wi l l d i c t a t e t he de gre e o f o re

pre pa ra t i on re qu i re d . The l owe s t p roc e ss i ng c os t wi l l c om e wi t h run-of -mi ne dum p l e a c h i ng . Le ss

pe rme a bl e o re re qu i re s c rush i ng be fore he a p l e a c h i ng , whi l e f ine s ma y ne e d t o be i mm obi l i s e d by

a ggl om e ra t i on wi t h l ime , c e m e nt [11 ,12] o r po l ym e r i c re a ge n t s [13] t o p re ve n t pe rc o l a t ion p rob l e ms . The

l owe r c a p i t al a nd op e ra t ing c os t s o f he a p l e a c h ing a re some w ha t o f f se t by l owe r g o l d e x t ra c t ion c om pa re d

wi t h c onve nt i ona l mi l l ing [14], wi t h t yp i c a l he a p l e a c h re c ove r i e s o f 60-80% . C ur re n t he a p l e a ch

ope ra t i ons ha ve c u t -o f f g ra de s a s low a s 0 .2 g . t l [15]. P re gna n t run- of f f rom t he ba se o f t he he a p i s

ge ne ra l l y c on t a c t e d by a c t i va te d c a rbon i n c o l umns (C IC ) [16] wh i c h usua l l y re c ove rs go l d e f f i c ie n t l y .

Sma l l e r s c a l e ope ra t i ons wi t h sa l i ne wa t e r m a y be be t t er t r e a t ed b y va t l e a c h ing (pa r t i a l ly f l ood e d he a p)

a s t h i s r e duc e s sc a li ng p rob l e ms wi t h l i quor i r r i ga t ion .

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The ma j or p roc e s s i ng rou t e fo r f r e e mi l l i ng o re s i n Aus t ra l i a , Sou t h Af r i c a a nd N or t h Am e r i c a i s a g i ta t i on

c ya n i de l e a c h i ng wi t h go l d re c ove ry by C IP / C IL . Th i s p ra c t ic e ha s d i sp l a c e d z i nc c e me n t a t i on , e xc e p t

i n t he c a se o f h i gh s i l ve r o re s , a s t he re i s no ne e d fo r so l i d - l i qu i d se pa ra t i on , p re gn a n t so l u t i on

c l a r i f ic a t i on a nd de a e ra t ion . Ore p re p a ra t i on usua l l y c ons i s ts o f 2 -3 s t a ge s o f c rush i ng a nd 1 o r 2 s t a ge s

of SAG or rod a nd ba l l mi l l i ng t o p rodu c e a p roduc t typ i c a l l y 80 < 751a m. Exc e p t whe n t he pa r t i c le

s i z e o f t he go l d i s e x t re me l y f i ne , i t i s be ne f i c ia l fo r l e a c h ing t o be p re c e de d by a g ra v i t y r e c o ve ry s t ep .

In t h i s wa y , go l d i s r e c ove re d more c he a p l y t ha n c ya n i da t i on-C IP / C IL , l e a c h c i r c u i t r e s i de nc e t i me i s

mi n i mi se d , a nd t he go l d i s a va i l a b l e fo r s a l e wi t h a mi n i mum de l a y .

Go l d re c ove ry w i t h c a rbon i s usua l l y op ti rn i se d be t we e n pu re C IP a nd C IL , so t ha t t he re ma y be t yp i c a l l y

1 o r 2 l e a c h i ng s t a ges be fore t he pu l p i s c on t a c t e d by c a rbon [17]. Fo r a g i ve n ore t he re is a n op t i mu m

c i r c u it b e t w e e n t h e e x tr e m e s o f p u r e C I P a n d p u r e C I L . T h i s c o m p r o m i s e w i l l b e c l o s e r to C I P f o r h ig h

gra de / s l ow l e a c h i ng ore s a nd c l ose r t o pure C IL for l ow gra de / or f a s t l e a c h i ng / or p re g- robb i ng ore s [18].

Al t ho ugh go l d re c ove ry on t o c a rbon wa s pa t e n t e d a s e a r l y as 1894 [19] a nd use d a t t he Youa nm i mi ne

by 1917 , t he re wa s no p ra c t ic a l p roc e s s t o r e c yc l e t he c a rbon a f t e r r e c ove r i ng t he go l d un t i l a round 1950

[20]. The fu l l C IP proc e s s a s we now k now i t , w i t h ca rbon a dsorp t i on , r e c ove ry o f the go l d f rom t he

c a rbon a nd re c yc l i ng t he c a rbon a f t e r t he rma l r e ge ne ra t ion w a s f i r s t a pp l ie d a t a sma l l s c a le a t C r i pp l e

C re e k a round 1951 [21]; t he n , a t la rge r s ca l e by Hom e s t a ke i n 1973 [22] , bu t C IP d i d no t r e a l l y ta ke

of f un t i l it s wi de spre a d use i n S out h A f r i c a i n t he l at e 1970 ' s [23].

Som e pr i ma ry su l ph i de o re s a re e s se n t ia l l y f re e mi l l i ng a nd c a n be p roc e s se d s i m i l a r ly t o ox i de o re s , b u t

he a d gra de s a re ge ne ra l l y h i ghe r a nd of t e n t he re i s a r e f ra c t o ry c ompo ne nt in the o re . In some c a se s , t he

re f ra c t o ry c ompo ne nt ma y ma y re spond t o fi ne r g r i nd i ng [24 ,25] . Som e t i me s , pos t l e a c h i ng re t r e a tme nt

c a n be use d , e . g . p roduc e a g ra v i t y c onc e n t ra t e f rom t he l e a c h re s i due a nd gr i nd t h i s f i ne r be fore

re c yc l i ng t o t he be g i nn i ng o f t he l e a c h proc e s s [26].

W he n re a c t ive su l ph i de s a re p re sen t i n the pu l p t he re is a n e nha nc e d oxyge n de m a nd . Th i s c a n be

sa t is f i e d by use o f c ompre sse d a i r o r t he a dd i t i on o f s t ronge r ox i da n t s to t he le a c h t a nks . S i nc e 1988

a roun d 30 go l d p l a n t s i n Aus t ra l i a ha ve i ns t al l e d pure oxyg e n a dd i t i on dur i ng l e a c h i ng [27] , wh i l e a t l e a st

4 ha ve use d hyd roge n pe rox i de a dd i t ion . Pure o xyg e n a dd i t i on is al so use d wh e n e nha nc e d l e a c h k i ne ti c s

a re a b l e to o f f se t c i r c u i t r e s i de nc e ti me [28] o r shor t - c i rc u i t ing p rob l e ms ; o r , w he re c ya n i de o r l i me

sa v i ngs c ove r t he c os t o f oxy ge n a d d i t i on [29].

C O M P L E X O R E S

C o m p l e x o r e s m a y b e c h a r a ct e ri se d b y h i g h c y a n i d e o r o x y g e n c o n s u m p t i o n o r p r e g - r o b b i n g b e h a v i o u r

(F i gure 4 ) .

O M P L E X O R E S

C Y A N I D E

C O N S U M E R S

Copper and

react ive

s u I p h i d e s

O X Y G E N

C O N S U M E R S

Re a c t i v e

a u l ph l de a

P R E G

R O B B I N G

Ca r bona c e ous

e u l ph i de e

and clays

Fi g .4 C om pl e x ore c l a s s if i c a ti on

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1218 S.R . LA BROOy

e t a l

Cyanide Consuming Complex Ores

The occurrence of gold in electrum can lead to slower gold leaching kinetics and increased cyanide

consumption. More serious cyanide consumption can arise from the reaction of some oxides or sulphides.

These side-reactions cause excessive reagent consumption which increases the cost o f production as well

as decreasing gold recovery. In Australia the presence of copper in ores is a significant problem in some

deposits. Treatment options are illustrated in Figure 5. Complex ores usually contain a range of sulphide

minerals e.g. pyrrhotite (Fe(l_x)S), marcasite (FeS2), covellite (CuS), digenite (Cu 1.8S), chalcocite (Cu2S),

arsenic and antimony sulphides and zinc sulphide. In gold mines with significant copper sulphide

mineralisation (e.g. more than 1 ), gold recovery by cyanidation is usually uneconomic and recovery via

a copper flotation concentrate or chemical pretreatment to remove copper may be required.

C Y A N I D E C O N S U M I N G


/ P RE TRE A TM E N T . ~ ~ . ~

NO N E FLOTATION P R E -LE ACH

/ \ / ' , , , , - -

N T E C

D E X T E C

C Y M E T

-

A c id

C Y A N ID A T IO N O T H E R C O N C E N T R A TE T A IL - NH3

r om i n e

- N H 3 / C N S E L T

C Y A N I D E / T h io s u lp h a t e

M E TAL - T h i o u r w - C o p p e r

R E C O V E R Y Sl r o s m e l l

- 8 c e r e M n l . B r o m in e

V i l r o k e le D is c a r d I f - C h l o d n e

- C u T s c h l o w T h l o s u l p h a t e

- I o n F i o l a l l o n

e n o u g h

-

I o n Ex c h a n g e

g o l d g r a d e

Fig.5 Process options for cyanide consuming complex ores

Copper minerals can consume large quantities of cyanide to give various copper(I) cyanide complexes:

CuCN, Cu(CN)2-, Cu(CN)32-, Cu(CN)43-, depending on the cyanide to copper ratio in solution. Of

these, only Cu(CN)32-and Cu(CN)43- can dissolve gold [30]. In the absence of pretreatments to remove

copper, cyanide and copper recovery may produce sufficient process credits to make treatment economic

[31-36]. However, copper(II) minerals oxidise an equivalent amount of cyanide to cyanate (CNO-), only

hal f of which can be recovered as cyanide. Hence, even with cyanide recovery, there can still be

considerable cyanide losses. The only currently installed process for cyanide recovery is the Sceresini

process. In this process, copper cyanide is loaded onto carbon ahead of significant gold dissolution to

be recovered as copper sulphate or sulphide with cyanide regeneration [31]. Alternative processes, such

as the CuTech process [32] offer potential copper and cyanide recovery, but have not been used

commercially to date. The Vitrokele process has also been considered for cyanide recovery by loading

copper cyanide complexes from the barren leached pulp onto special ion exchange resins [33], followed

by cyanide and some further gold recovery. Alternatively ion flotation [37,38] or ion exchange resins

[39,40] have been suggested for selective gold recovery in the presence of copper cyanide complexes.

Generally the most economical approach for sulphide ores has been to produce a gold-copper flotation

concentrate for metal recovery by smelting [41]. In some cases, the feed material may benefit from

controlled potential sulphidisation [42-44] which can produce an acceptable copper grade for smelters.

However, the copper grade requirement is likely to lead to reduced gold recovery, so that the float tail

may well contain too much gold to be discarded without further treatment [41].

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Am mo ni a -c ya n i de [45-47] , b rom i ne [48] , t h i osu lpha t e [49] a nd t h i oure a [50 ,51] ha ve a l l be e n sugge s t e d

a s a l t e rna t ive r e a ge n ts fo r go l d d i s so lu t i on i n the p re se nc e o f c oppe r m i ne ra ls . B rom i ne a nd t h i osu l pha t e

ha ve some l i mi t e d po t e n t i a l fo r s e l e c t i ve l e a c h i ng of go l d ove r c oppe r , howe ve r t h i oure a o f fe r s l i t t l e

a dva n t a ge ov e r c ya n i de a s c oppe r m i ne ra l s ha ve s i mi l a r ra t e s o f d i s so l u t i on i n c ya n i de a nd t h i oure a . In

pra c t ic e , a m mo ni a -c ya n i de is su i t a b l e a s a l e a c ha n t fo r l ow c opp e r va l ue s t o ob t a i n r e a sona b l y se l e c t ive

go l d re c ove ry wi t h l i tt l e c oppe r l e a c h i ng , t hough go l d re c ove ry i s o f t e n l i mi t e d t o 60-80 [52]. A p l a n t

i n M a ur i t a n i a c ur re n t l y use s t h i s le a c h sy s t e m [53].

An a l t e rna ti ve a pproa c h i s t o use a n e l e c t ro l y t i c p re l e a c h , suc h a s t he In te c p roc e s s [54] , t o r e m ove t he

c oppe r mi ne ra l s p r io r t o c ya n i da t i on-C IP / C IL . Th i s op t i on wa s t r i e d a t the R ot hsa y mi ne i n W e s t e rn

Austral ia in 1990 [55].

M os t go l d p roc e s s i ng re g i ons i n Aus t ra l ia a re a r i d a nd m i ne s a re o f t e n fo rc e d t o use sa l ine g roun d w a t e r

fo r go l d p roc e s s i ng . Th i s wa t e r norma l l y c on t a ins h i gh c onc e n t ra ti ons o f ma gne s i um . The pre c i p i t a ti on

o f m a g n e s i u m h y d r o x i d e l i m i ts th e p H b e l o w t h e p Ka o f H C N ( p Ka = 9 .5 ) a nd c ya n i de c a n be l os t a s

H C N va po ur un l e s s e xc e s s a lka l i ( l ime , c a us t ic ) ha s be e n a dde d t o t he pu l p . The re su l t i ng low pu l p

c ya n i de c onc e n t ra t i on ma y de c re a se go l d re c ove ry a nd ha s a n i mpa c t on t he p roduc t i on c os t be c a use o f

t he h i gh a l ka l i c onsum pt i on a nd t he poss ib l e r e qu i re me nt t o use c he mi c a l modi f i e r s t o ove rc om e h i gh pu l p

v i sc os i t y due t o t he p re c i p i t a te d m a gne s i um hyd rox i de ge l [56].

O x y g e n C o n s u m i n g C o m p l e x O r e s

R e a c t i ve su l ph i de s suc h a s pyr rho t i t e c a n l e a d t o h i gh oxyge n re qu i re me nt s a s i ron( I I ) i s ox i d i se d t o

i ron( I l l ) a nd su l ph i de is ox i d i se d t o su l pha t e. Th i s oxy ge n de ma n d c a n be me t by t he a dd i t i on o f ox i da n t s

[57] , suc h a s pure oxyg e n [29], hyd roge n pe rox i de [58] o r c a l c i um pe rox i de [59]. Oxi da n t a dd i t i on ca n

be c on t ro l l e d by a d i s so l ve d oxy ge n e l e c t rode i n t he f ir s t le a c h t a nk [58] , bu t t he re c a n be p rob l e m s w i t h

me t e r c a l i b ra t ion pa r t i c u l a r l y wi t h sa l i ne p roc e s s wa t e r [60]. Su l ph i de s a lso r e a c t wi t h c ya n i de t o g i ve

t h i oc ya na t e [57 ,61] ; a nd proc e s s i ng op t i ons a re ou t li ne d in F i gure 6 .

OXYGEN

ONSUMING

J OMPLEXORES

(React ive aulphide8) ~

ALKALINE / AE RA TION

PRE-OXIDATION

A i r /

FLOTATION

- Air Pb(NO3)2

L / / C O N C E N T R A T E

CYANIDE LEACH

w i t h :

- air

o x y g e n

H 2 0 2

C a O 2

Discard

pyrrhotite

rich

tailing

Fi g .6 P roc e ss i ng op t i ons lrbr oxy ge n c onsumi ng c omp l e x ore s

I f t he r e a c t i ve su l ph i de s a re ba r re n , t he n se l e c ti ve f l o t a t ion a f te r a e ra t ion c a n be e m pl oye d t o r e j ec t t he

e a s i l y ox i d i se d su l ph i de s in t he f l oa t t a il . Th i s a pproa c h i s use d a t t he Thre e M i l e Hi l l go l d mi ne , a t

C oo l ga rd i e in We s t e rn A us t ra l ia [62] . Al t e rna t i ve l y , a t l owe r pyr rho t i t e l e ve ls a lka l i ne p re a e ra t ion m a y

be suf f i c i e n t t o pa s si va t e t he pyr rh o t i t e wi t h i ron ( I I I ) ox i d e / hyd rox i d e f i l ms . The se f i l ms d is so l ve more

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1220 S .R . LA BROOY

e t a l

s l o w l y i n c y a n i d e s o l u t i o n t h a n p y r r h o t i t e [ 6 3 ] , e n a b l in g c o n v e n t i o n a l c y a n i d e l e a c h i n g t o b e e f f e c ti v e .

L e a d a n d / o r o x i d a n t a d d i t io n s m a y b e b e n e f i c ia l d u r in g t h e p r e a e ra t i o n a n d l e a c h in g p r o c e s s [ 6 4 ].

P r e g - R o b b i n g O r e s

A n o t h e r cl a ss o f c o m p l e x o r e s a r e p r e g - ro b b i n g o r e s , w h i c h c a n b e p r o c e s s e d b y t h e o p t i o n s s h o w n i n

F i g u r e 7 .

P R E G - R O B B I N G


C L A Y S

\

H E A T A C I D

/

C Y A N I D E

L E A C H

- C I L

S U L P H I D E S

C A R B O N A C E O U S

C Y A N I D E C H L O R I N E

w i t h : I A

- p r e a e r a t i o n

- h i g h [ N a C N ]

L E A C H

- o x y g e n

- C l L - c y a n i d e O I L

- b r o m i n e

- c h l o r i n e

~ourea

F i g . 7 T r e a t m e n t o p t i o n s w i t h p r e g - r o b b i n g o r e s

P r e g - r o b b i n g b e h a v i o u r m a y a r i s e f r o m n a t u r a l c a r b o n a c e o u s m a t e r ia l s i n t h e o r e t h a t m a y a d s o r b g o l d .

T h e m o s t f a m o u s e x a m p l e s a r e t h e C a r l i n o r e s i n t h e U S A w h e r e t h e c a r b o n a c e o u s i m p u r i t y l o c k s

A u C N ) 2 [ 6 5 ]. P r e o x i d a t i o n o f C a r l in o r e w i t h C I 2 f la s h c h lo r i n a t io n ) d e a c ti v a t es t h e c a rb o n a c e o u s

m a te r i a l s [66] . A l t e r na t ive ly , r oa s t i ng o r ba c t e r i a l p r e t r e a tm e n t [ 67] c a n de s t r oy the p r e g- r o bb in g

b e h a v i o u r a n d g o l d c a n b e r e c o v e r e d b y c o n v e n t i o n a l c y a n i d e , b r o m i n e , c h l o r i n e o r t h i o u r e a l e a c h i n g .

P r ob le m s wi th c a r bo na c e o us m a te r ia l s i n o r e s a r e no t a s se r ious i n Aus t r a l i a a nd f l a sh c h lo r ina t ion i s no t

u s e d . M i l d e r p r e g - r o b b i n g c a n b e r e d u c e d b y a d d i t i o n o f d i e se l o r k e r o s e n e t o t h e l e a ch t o d e a c ti v a t e t h e

c a r b o n a c e o u s m a t t e r i n th e o r e . H o w e v e r , t h is p r o c e d u r e a f fe c t s t h e A u C N ) 2 - a d s o r p t i o n o n t o C I P / C I L

c a r bo n a nd the r e f o r e r e qu i r e s c a u t ious a pp l i c a t ion . Ke r ose ne a dd i t i on ha s be e n use d at S t a we l l i n

Vic to r i a . Gr a p h i t i c m a te r ia l i n the o r e s doe s no t ha ve the sa m e go ld l oa d ing c a pa c i ty as a c t i va t e d c a r bon

a s i t ha s a m u c h low e r su r f a c e a re a . Ne v e r the l e s s it m a y typ i c a l ly loa d up to 40 g . t o f go ld [ 68].

S o m e c l a y s p r e s e n t i n g o l d o r e s c a n a l so a d s o r b A u C N ) 2 - [ 6 9 - 7 1 ]. C l a y s c a n a l s o h i n d e r t h e

c y a n i d a t i o n p r o c e s s t h r o u g h c o a t in g s o r s l im e s [ 7 0 ] . M i l d p r e g - r o b b i n g b e h a v i o u r c a n b e r e d u c e d b y u s e

o f C I L , a s t h e a c t iv a t e d c a r b o n m a y h a v e f a s t e r g o l d l o a d in g k i n e ti c s th a t t h e p r e g - ro b b e r s [ 1 8 , 7 2 ] . T h e

p r o b l e m i s g e n e ra l ; e . g . t h e c a ti o n i c th i o u r e a c o m p l e x i s e v e n m o r e s u s c e p t ib l e t o p r e g - r o b b i n g b y c l a y s

t h a n t h e a n i o n i c a u r o c y a n i d e c o m p l e x [ 7 3 ].

P r e g - r o b b i n g c a n a l s o o c c u r a t l o w c y a n i d e c o n c e n t r a t io n s w i t h s o m e s u l p h i d e s , e . g p y r r h o t i t e a nd p y r i t e

[ 7 4 ]. U n d e r t h e se c o n d i t io n s , m e t a ll ic g o l d c a n c e m e n t o u t o n t o t h e s u l p h i d e m i n e r a l s [ 7 5 ]. T h e p r o b l e m

c a n b e c o n t r o l l e d b y i n c r ea s i n g c y a n i d e l e v e ls a n d / o r m a k i n g c o n d i t i o n s m o r e o x i d i s i n g w i t h p r e a e ra t i o n .

R E F R A C T O R Y G O L D O R E S

A m i n e r a l o g i c a l e x a m i n a t i o n o f t h e o r e n o r m a l l y s u g g e s t s r e a s o n s f o r t h e r e f r a c t o r i n e s s o f t h e o r e .

Va ugha n [ 76] i de n t i f i e d spe c i f i c m ine r a log ic a l t e c hn ique s , whic h c a n be use d f o r spe c i f y ing o r e t ype a nd

f or l oc a t ion o f t he go ld i n t he o r e . T he se t e c hn ique s a r e l i s te d i n T a b le 2 .

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Gold extraction from o r s

T A B L E 2 M i n e ra l o g i ca l t e ch n iq u es fo r l o ca ti n g g o l d in t h e o re s

22

Technique Comment

Opt ical Microsc opy Suitable for coarse gold (> 0.5 I .tm).

X-ray Diffract ion Ident i f icat ion of free mineral grains.

Scanning Elect ron Microsc opy Elemental analysis .

Elect ron Microp robe Low er detect ion l imi ts for gold in

solid solution (10-20 ppm).

Ion Microp robe M ore sensi tive for solid solut ion gold,

claimed to lppm.

L o c k e d G o l d

Gold can be locked in the hos t minera l in a n umb er of ways .

Physical locking in sulphides, oxides, si l icates, etc.

Chemical locking as gold al loys or compounds e.g. elect rum, gold tel lurides,

(aurostibnite), Au2 Bi (ma idonite), etc.

Go ld substitution in the sulphide lat t ice e.g. solid solution gold [4] in arsen opy rite.

Gold surface passivation due to formation of a chemical layer.

uSI~

In A ust ral ia , arseno pyri te , pyri te and chalcop yri te are the principal primary sulphide minerals locking gold

(or al loyed gold) in refractory ore [77]. The opt ions for processing refractory sulphidic ores are com plex

and de pend great ly on locat ion and mineralogy . Pret reatment opt ions for deal ing wi th refractory ores are

il lustrated in Figure 8 and described in detail in the subsequent sections.

THERMALTREATMENT:

Roast ing

Pyrolysis

PRESSUREOXIDATION:

Aci d ] A l ka l ine

CHEMICALTREATMENT: ~ , ~ ~ ' ~ ' ~ , . ~ . ,

J BIOLOGICAL

TREATMENT

Artech / Cashm an Process

Redox / Ni t rox Processes

AcUvox Process

Caro s Ac i d

Electrochemical

f

HYSICAL

TREATMENT

Rne gdnding

Fig.8 General process opt ions for pretreat ing refractory ores

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1222 s. R . LA BRoov

e t a l

P h y s i c a l P r e t r e a t m e n t

T he on ly op t ion i n t h i s c a t e gor y i s t o g r ind f i ne r , e .g . f i ne g r ind ing w i th a c onve n t iona l m iU ( 100

< 381 . t i n ) [ 62] , o r u l t r a f i ne g r ind ing w i th d i f f e r e n t t ype s o f n f i l l s p r oduc ing pa r t i c l e s w i th s i z e s i n t he

r a nge o f 1 - 20 p .m . T he r e a r e t h r e e t ype s o f m i l ls f o r u l t r a f i ne g r ind ing , na m e ly : v ib r a to r y m i l l s , s t i r r e d

m i l l s a nd j e t m i l l s . O f t he se , t he s t i r r e d m i l ls a r e t he m os t c os t - e f f e c t i ve in t e r m s o f t he e ne r gy i npu t

r e qu i r e d t o p r od uc e a g ive n g r a in s iz e [ 24 ,25] , w i th m a in a pp l i c a t i on t o se m i - r e f r a c to r y m a te r i a ls . T h e

e n e r g y i n t e n s iv e n a t u r e o f t h e p r o c e s s g e n e r a l l y l i m i t s it s a p p l i c at i o n t o h i g h e r g r a d e c o n c e n t r a te s r a t h e r

t h a n w h o l e o r e s . A s t i r re d m i l l i s c u r r e n t l y u s e d a t N e w c r e s t 's N e w C e l e b r a t i o n o p e r a t io n n e a r

K a l g o o r l i e i n W e s t e r n A u s t r a li a . T h i s m i l l r e g ri n d s a r e f r a c t o r y p y r i t e g r a v i t y c o n c e n t r a te o b t a i n e d f r o m

the l e a c h r e s idue whic h i s t he n r e c yc l e d t o t he l e a c h [ 26] .

C h e m i c a l P r e t r e a t m e n t

A v a r i e ty o f w e t - o x i d a t i o n p r o c e s s e s c a n b e u se d t o o x i d i s e m e t a l s u l p h id e s . T h e b e s t k n o w n p r o c e s s i n

th i s c a t e go r y i s p r e s sur e ox ida t i on . H ig h p r e s sur e p r oc e sse s a r e r e v i e w e d se pa r a t e ly i n a no th e r se c t i on

w h i l e p r o c e s s e s o p e r a t in g a t s l ig h t l y e n h a n c e d p r e s s u r e a r e c o v e r e d i n th i s s e c ti o n . T h e N i t r o x / R e d o x

p r o c e s s e s , A r t e c h / C a s h m a n p r o c e s s , A c t i v o x p r o c e s s , C a r o ' s a c id p r o c e s s , H M C p i p e r e a c t o r p r o c e s s

a n d e l e c t r o c h e m i c a l s l u rr y p r o c e s s a l l i n v o l v e a ci d m e d i a t o b r e a k d o w n t h e s u l p h i d e m a t r ix t o e x p o s e t h e

go ld . As a n a l te r na t i ve t o ne u t r a l i s a t i on a nd c ya n id e l e a c h ing , t he se p r oc e sse s c ou ld be c oup le d w i th

l e a c h in g u n d e r a c id c o n d i t i o n s w i t h t h i o u r e a, c h l o r i n e o r b r o m i n e a s l i x iv i a n ts . H o w e v e r t h e l e a c h r e s id u e

w o u l d s ti ll h a v e t o b e n e u t ra l i se d b e f o r e d i s p o sa l . T h e A c t i v o x , N i t r o x / R e d o x a n d t h e A r t ec h / C a s h m a n

pr oc e sse s ha ve b e e n i nve s t i ga t e d up t o p i l o t p l a n t l e ve l a nd i nv o lve l es s v igor ou s ox id i s ing c on d i t i ons t ha n

t h o s e in h i g h p r e ss u r e o x i d a t io n p ro c e s s es . T h e A c t i v o x p ro c e s s u se s u lt r a f in e g r in d i n g t o o b t a i n

s u f f ic i e n t m a t r i x a t ta c k u n d e r t h e m i l d e r c o n d i t io n s . T h e N i t r o x / R e d o x p r o c e s s e s a r e b a s e d o n o x i d a t i o n

wi th n i t r i c a c id a nd t he Ar t e c h /Ca shm a n pr oc e ss i s a hydr oc h lo r i c a c id p r e s sur e l e a c h ing p r oc e ss , i n t he

pr e se nc e o f c a l c ium ions . T h e use o f Ca r o ' s a c id (H2SO5) a s a n ox ida n t has be e n t e s t e d a t a l a bor a to r y

sc a l e t o ox id i se m e ta l su lph ide s a t a tm osp he r i c p r e s sur e a t 60- 80 C. E l e c t r oc he m ic a l p r oc e s s ing o f

a r se nopyr i t e s l u r r i e s ha s be e n i nve s t i ga t e d a t l a bor a to r y l e ve l .

N i t r o x / R e d o x P r o ce s s es

T h e o v e r a l l c h e m i s t r y o f t h e N i t r o x / R e d o x p r o c e s s e s i s th e s a m e b u t t h e r e a r e d i f f e r e n c e s in t h e o p e r a t i n g

t e m pe r a tu r e s a nd p r e s sur e s [ 78] . T h e ox ida n t i n t he se p r oc e sse s i s n i t r i c a c id , whic h i s r e c yc l e d . T h e

Ni t r ox p r oc e ss use s so lub l e c a l c ium n i t r a t e a nd t he Re dox p r oc e ss r e ge ne r a t e s spe n t n i t r i c a c id w i th

o x y g e n . T h e e s s e n t ia l s c h e m a ti c o f t h e f l o w sh e e t f o r t h e N i t r o x / R e d o x p r o c e s s is s h o w n i n F i g u r e 9 .

For pyr i t e a nd a r se nopyr i t e , t he ox ida t i on r e a c t i ons a r e a s f o l l ows :

3 F e S 2 + 1 8 H N O 3 ~ F e2 (S O 4)3 + Fe ( NO 3) 3 + 3H2SO 4 + 6 H 2 0 + 1 5 N O

3 F e A s S + 1 4 H N O 3 + 2 H 2 0 - --, 3 F e A s O 4 . 2 H 2 0 + 3 H 2 S O 4 + 1 4 N O

R e c y c l i n g o f t h e a c id i n v o l v e s r e o x i d a t i o n o f N O t o n i tr i c a c id :

2 N O + 02 - .- , 2 N O 2

3 N O 2 + H 2 0 - - . 2 H N O 3 + N O

T h e o p e r a t in g c o n d i t io n s f o r b o t h p r o c e s s e s a r e a r o u n d 1 0 0 - 4 0 0 k P a , 9 0 - 1 1 0 C a n d a p H < 1 . T h e

r e a c t i on i n t he p ipe r e a c to r i s ve r y f a s t a nd s inc e t he n i t r i c a c id i s r e ge ne r a t e d a nd r e c yc l e d , t he r e i s a

c o s t s a v i n g . A l s o , l o w e r o x y g e n c o n c e n t r a t io n s a r e i n v o l v e d th a n i n h i g h p r e s s u r e o x i d a t i o n b e c a u s e

n i t ri c a c id i s a m o r e p o w e r f u l o x i d a n t t h a n o x y g e n .

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F i g . 9

P i p e

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y a n i d a t i o n I P

J N i t r i c dd I

R e g e n e r a t i o n

T h e e s s e n ti a l s te p s o f t h e N i t r o x / R e d o x p r o c e s s e s

H M C P r o c e s s

T h i s p r oc e s s i s a s ing l e s ta ge n i t r i c - a c id sa l t l e a c h p r oc e ss w hic h i s c a r r i e d ou t a t 10( YC a nd up to 1400

k P a i n a p i p e r e a c t o r [ 7 9 ] . T h e r e a c t o r c o n s i st s o f c h e a p p o l y e t h y l e n e c o i l s e n c a s e d i n c o n c r e t e i n a t a n k .

T h e o x i d i s i n g s o l u t i o n i s 1 0 - 3 0 w / w H N O 3 a n d 1 0 w / w s o d i u m c h l o r i d e s o l u t io n . T y p i c a l o r e

c o n s t i tu e n t s ( c h a l c o p y r i t e , p y r i t e a n d a r s e n o p y r i t e ) a n d g o l d a r e s o l u b i li s ed w i t h i n 1 h o u r f o r a f i n e ( < 5 0

I. tm ) 3 0 - 6 0 w / w s l u r r y . T h e g o l d co m p l e x i n s o l u t io n i s p r e s u m a b l y A u ( C 1 ) 4 - a n d p l at e s a s m e t a ll i c

g o l d o n c a r b o n p a rt i c le s . T h e l o a d e d c a r b o n i s b u r n t t o r e c o v e r th e g o l d . H i g h g o l d re c o v e r i e s ( > 9 0 )

ha v e be e n c l a im e d f o r va r ious t e s t o r e s and the p r oc e ss ha s be e n pa t e n t e d in se ve r a l c oun t r i e s ( e .g , Aus t .

P a t 5 8 0 7 6 8 , 4 / 2 / 8 9 ) ; b u t , t h e r e i s n o k n o w n c o m m e r c i a l a p p l i c a t i o n o f t h i s t e c h n o l o g y .

T h e A r t e c h / C a s h m a n P r o ce s s

T h e A r t e c h / C a s h m a n p r o c e s s w a s d e v e l o p e d t o t re a t b a s e m e t al f l u e d u s t s c o n t a in i n g a rs e n i c . T h e p r o c e s s

use s C a 2+ a s a r e a c t a n t i n a c id i c c h lo r ide so lu t ion at 110C a nd 350 kPa . T h e a u toc l a v e is l i ne d w i th

t i ta n i u m t o p r e v e n t c o r r o s i o n o f t h e r e a c t o r b y H C 1 . O p e r a t in g p H i s 1 - 2 a n d r e a c t i o n t i m e is 0 . 5 - 1 h [ 8 0 ].

I n t h i s e n v i r o n m e n t , a r s e n i c i s f i x e d i n a c o m b i n e d a r s e n i c -i r o n - c a lc i u m r e s i d u e w h i c h i s c l a i m e d t o h a v e

e n v i r o n m e n t a l l y a c c e p t a b le a r s e n ic r e t e n t io n . T h e p r o c e s s c o u l d t h e r e f o r e b e s u i ta b l e f o r tr e a ti n g

a r s e n o p y r i t i c g o l d o r e i n e n v i r o n m e n t a l l y s e n s i ti v e a r ea s . H o w e v e r , d o u b t h a s b e e n e x p r e s s e d a b o u t t h e

l o n g t e r m s t a b i l i t y o f t h e a r s e n i c - i r o n - c a l c i u m r e s i d u e d u e t o i t s c o n v e r s i o n t o c a l c i u m c a r b o n a t e b y

r e a c t ion w i th C O 2 in a i r r e l e a s ing the a r se n i c [ 81] .

T h e C a r o ' s A c i d P ro c e s s

T h e C a r o ' s a c i d p r o c e s s [ 8 2 , 8 3 ] a p p e a r s t o h a v e b e e n e x p l o r e d o n l y a t l a b o r a to r y sc a l e. T h e s u l p h i d e

i s s lu r r i e d i n su lphu r i c a c id a nd Ca r o ' s a c id is i n t r oduc e d a s the ox ida n t . Ca r o ' s a c id c on ta ins t he

s p e c i e s H 2 S O5 a n d H 2 0 2 a n d i s a s o u r c e o f f r e e o x y g e n i n s o l u t i o n , a s e x p r e s s e d i n t h e f o l l o w i n g

e qua t ions :

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1224 S .R . LA BROOVet al

H 2 S O 5 + H 2 0 2 - - - . H 2 S O 4 + H 2 0 + 0 2

2 H 2 0 2 ~ H 2 0 + 0 2

T h e p r o d u c t s o f t h e o x i d a t i o n o f t h e su l p h i d e s a r e s u l p h a t e s a n d a r se n a t e s a n d s o m e e l e m e n t a l s u l p h u r .

E l e c t r o c h e m i c a l P r o c e s s i n g o f O r e S l u r r i e s

M e t a l s u l p h i d e a n d t e l l u r id e o r e p a r t ic l e s a r e e l e c t r i c a ll y c o n d u c t i n g a n d e l e c t r o c h e m i c a l p r o c e s s i n g i s

t h e r e f o r e p o s s i b l e [ 4 ] . S u l p h i d e o x i d a t i o n to e i t h e r s u l p h u r o r s u l p h a t e a n d g o l d te l l u r i d e o x i d a t i o n c n

b e d e s c r i b e d a s f o l l o w s :

M S - - - , M + + S O + e -

M S + 4 H 2 0 - -- , M + 5 0 4 2 n - 8 H + + 7 e -

A u T e 2 + 4 H 2 0 ---, A u + 2 H T e O 2 + 6 H + + 6 e -

A p r a c t i c a l p r o c e s s t h a t w a s p u t i n p l a c e i n th e l a te 1 9 5 0 ' s i s t h e I n c o n i c k e l m a t t e a n o d e p r o c e s s i n

C a n a d a w h i c h t r e a te d c a s t a n o d e s o f N i3 S 2 [ 8 4 ] . I t i s u s u a l l y d i f f i c u l t t o p r o d u c e c o m p a c t e d o r c a s t

e l e c t r o d e s f r o m o r e s a n d d i re c t e l e c t ro l y s i s o f s lu r r i e s i s m o r e p r a c ti c a l . T h e m i n e r a l p a r t i c l e s c a n b e

s u s p e n d e d t o m a k e d i r e c t c o n t a c t w i t h a n i n e r t f e e d e r e l e c t r o d e i n t h e c e ll o r r e a c t w i t h a s o l u t i o n r e d o x

c o u p l e , w h i c h i s c o n t i n u o u s l y b e i n g r e g e n e r a t e d a t t h e f e e d e r e l e c t r o d e . E x p e r i e n c e t o d a t e s h o w s t h a t

d i r e c t p a r t i c l e - e l e c t r o d e c o n t a c t i s le s s e f f i c i e n t th a n t h e r e a c t i o n o f o r e p a r t ic l e s w i t h s o l u t i o n r e d o x

s p e c ie s [ 7 7 ] . T h i s m e c h a n i s m is su m m a r i s e d d i a g r a m m a t i c a l l y i n F i g u r e 1 0 f o r a n o x i d a t io n r e ac t io n .

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M i n e r a l M i n e r a l I o n d i ff u s io n n o d e F e e d e r

P a r t i d e O x i d a tio n in e le c t r o ly t e R e a c t i o n e l e c l r o d e

R e a c t io n a n d d r c u i t

t o c a t h o d e

F i g . 1 0 A n o d e r e a c t io n m o d e s f o r o x id a t i v e sl u r r y e le c t ro l y s is . O d e n o t e s t h e o x i d a n t f o r m o f th e

r e d o x c o u p l e i n s o l u ti o n a n d R d e n o t e s t h e re d u c e d f o r m o f th e r e d o x c o u p l e i n s o lu t i o n .

T h e m a i n a d v a n t a g e s o f s l u r r y e l e c tr o l y s is a r e a s f o l l o w s :

T h e m a t e r i a l w h i c h i s t o b e r e a c t e d c a n b e u s e d i n i ts a s r e c e i v e d f o r m , i .e . a s a p o w d e r .

T h e s u r f a c e a r e a o f t h e s lu r r i e d p a r t i c le s i s m u c h g r e a t e r t h a n t h a t o f c a s t e l e c t r o d e s . T h i s f e a t u r e

r e d u c e s r e t e n t i o n t i m e a n d t h e ca p i t a l c o s t o f t h e p l a n t .

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Gold extracti on from ores 225

S l u r r y e l e c t r o l y s i s h a s b e e n a p p l i e d a t a l a b o r a t o r y , p i l o t p la n t a n d p l a n t l e v e l i n v o l v i n g t h e f o l l o w i n g

s y s t e m s : c h a l c o p y r i t e ( C u F e S 2 ) r e d u c t io n , p y r o l u s i t e ( l~ - M n O 2 ) r e d u c t i o n , c o a l o x i d a t i o n a n d c h a l c o p y r i t e

o x i d a t i o n [ 5 4 , 5 5 , 7 7 ] .

A r s e n o p y r i t e s l u r r ie s h a v e b e e n o x i d i s e d w i t h a n o d e - g e n e r a t e d f e r r i c s u l p h a t e o r c h l o r i n e i n a l a b o r a t o r y

s c a le d i a p h r a g m c e l l . T h e es t im a t e d p o w e r c o n s u m p t i o n w a s 2 . 9 k W h . k g 1 a r s e n o p y r i t e . D i s s o l v e d

As( I I I ) wa s de p os i t e d a s As on the c a thode [ 85] . I n th i s sys t e m , t he r e wa s i nsuf f i c i e n t ox id i s in g s t r e ng th

i n t h e a n o l y t e a n d t h e s l u r r y r e t e n t io n t i m e w a s s i m i l ar t o b i o o x i d a t i o n [8 6 ] . M u c h f a s te r o x i d a t i o n r at e s

h a v e b e e n o b t a i n e d i n sa l t s o l u t io n i n a m e m b r a n e c e l l, p r o d u c i n g c h l o r i n e a t t h e a n o d e . I n t h is s y s t e m ,

p o w e r c o n s u m p t i o n w a s i n c r e a s e d t h r e e f o l d b u t t h e c o s t o f t h is p o w e r c a n b e r e c o v e r e d f r o m t h e s a le

o f b y - p r o d u c t s o d i u m h y d r o x i d e . I n a d d i t io n , a ll d i s s o l v e d A s w a s p r e s e n t a s A s ( V ) an d c o u l d t h e r e f o r e

b e c o n t a i n e d i n th e s a m e w a y a s t h e e f f l u e n t f r o m b i o o x i d a t i o n . T h i s s t r a te g y c o u l d m a k e t h e s l u r ry

e l e c t ro l y s i s o f a r s e n o p y r i t e e c o n o m i c a l l y c o m p e t i t i v e w i t h , b u t m u c h f a s t er t h a n , b i o o x i d a t i o n [ 8 7 ] .

A c t i v o x P r o c e s s

T h i s p r o c e s s u t i li s e s u l t r a f i n e g r i n d i n g a s a p r e t r e at m e n t t o o b t a i n s u l p h i d e d i s s o l u t io n u n d e r m o r e

m o d e r a t e c o n d i t io n s t h a n r e q u i r e d f o r h i g h p r e s su r e o x i d a t i o n . S u l p h i d e c o n c e n t ra t e s a r e m i l le d i n a

ve r t i c a l s t i r r e d m i l l t o a ds0 o f a bou t 5 I z m , a t a n e ne r gy r e qu i r e m e nt o f 50- 120 k W h. t -1 o f fe e d . T h e

a c t iv a t e d m a t e r ia l i s le a c h e d a t t e m p e r a t u re s b e l o w 1 0 0 C a n d o x y g e n o v e r p r e s s u r e s b e l o w 9 0 0 k P a ,

w i t h a r e t e n t i o n t i m e o f 0 . 5 - 1 h . I n t h e p r o c e s s , s o m e s u l p h u r is f o r m e d b u t i t d o e s n o t c o a t g o l d o r

m i n e r a l s u r f ac e s a s t h e p r o c e s s o p e r a t e s b e l o w th e m e l t in g p o i n t o f s u l p h u r ( 1 2 0 C ) . T h e m i l d c o n d i t i o n s

a l low the use o f r ubb e r o r p l a s t i c li ne d m i ld s t e e l, o r s t a in l e s s st e e l p l a n t . L a b or a to r y t e s ts ha v e g ive n

9 8 - 9 9 % g o l d r e c o v e r y f r o m p r e t r e a te d p y r i t e / a r s e n o p y r i t e c o n c e n tr a t e s [ 8 8 ] .

T h e r m a l P r e t r e a t m e n t

T h e r m a l t r e a t m e n t c a n b e a p p l ie d t o b r e a k d o w n t h e s u l p h id e p h a s e t o m a k e g o l d a c c e s si b l e. I n t h e

p r e s e n c e o f o x y g e n , t h e s u l p h i d e c o n v e r t s t o a n o x i d e i n a p r o c e s s ca l le d r o a s t in g . L e s s c o m m o n l y , i n

t h e a b s e n c e o f o x y g e n , t h e s u l p h i d e t r a n s fo r m s i n t o a v a r i e ty o f r e a c t i o n p r o d u c t s a n d t h is t r e a t m e n t i s

d e s c r i b e d a s p y r o l y s i s . F i g u r e 1 1 il lu s t ra t e s th e p r o c e s s o p t io n s f o r t h e r m a l p r e tr e a t m e n t o f r e fr a c t o r y

or e s o r c onc e n t r a t e s .

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R o a s t i n g

R o a s t i n g h a s b e e n t h e t r ad i t io n a l m e t h o d o f p r e t r ea t i n g r e f r a c t o r y g o l d o r e s o r c o n c e n t r a t e s a n d i s o n e o f

the m os t c os t - e f f e c t i ve p r oc e ss ing op t ions ; bu t t he c os t goe s up s ig n i f i c a n t ly , i f su lph ur d iox ide s t a ck

e m i s s i o n s m u s t b e c o n t r o l l e d . F o r e x a m p l e , a t K a l g o o r l ie in W e s t e r n A u s t ra l ia , p y r i t i c - g o l d t e l lu r i d e

c onc e n t r a t e s a r e r oa s t e d i n c i r c u l a t i ng f l u id be d r oa s t e r s c o l~s tr uc te d by L ur g i Gm bH [ 89] , w i th SO 2

e m i s s i o n f r o m a 2 0 0 m h i g h s ta c k . A l t h o u g h t h is p r a c t ic e h e l p s m a n a g e S O2 p o l l u t i o n o f t h e n e a r b y

t o w n s h i p [ 9 0 ], t h e p r a c t i c e w o u l d n o t b e p e r m i t t e d i n l es s i s o la t e d a re a s , r e m o v i n g m u c h o f t h e c o s t

a dva n ta ge o f r oa s t i ng . I n Ka lgoor l i e , t he r e i s a shor t a ge o f f r e sh wa te r a nd sa l i ne g r ound wa te r i s use d

in go ld p r oc e ss ing . T h e sa l t c on t e n t o f t he c onc e n t r a t e f e e d f o r t he r oa s t e r m us t be l im i t e d to m in im i se

g o l d v o l a t il i s at i o n a n d s u b s e q u e n t g o l d d e p o s i t i o n w i t h i n t h e ro a s t e r, e . g . [ 9 1 ] . F o r a r s e n o p y r i t i c g o l d o r e ,

r oa s t i ng g ive s a va r i a b l e r e c ove r y un l es s the r oa s t e r ope r a t i on is c a r e f u l l y c on t r o l l e d . A pyr i t i c

c o n c e n t r a t e c o n t a in i n g 5 % a r s e n o p y r i t e is r o a s te d b y W e s t e r n M i n i n g C o r p o r a t i o n a t W i n d a r r a i n W e s t e r n

Aus t r a l i a i n a s i ng l e s t a ge , s t a t i ona r y f lu id be d r oa s t e r [ 92]. Ho we ve r , w i th a r se nopy r i t e , t he r e is a l so

t h e p r o b l e m o f e n s u r i n g e n v i r o n m e n t a l l y a c c e p t a b l e d i s p o s a l o f t h e a r s e n i c o x i d e b y - p r o d u c t ; h e n c e

a l t e r na t i ve ox ida t i on p r e t r e a tm e nt s c ou ld be m or e a t t r a c t i ve .

T he ob j e c t i ve s i n r oa s t i ng a r e [ 93] :

T he de s t r uc t i on o f t he m e ta l su lph ide pha se .

T h e e x p o s u r e o f g o l d i n t h e s u lp h i d e .

T h e p r o d u c t i o n o f a h i g h s u r f a c e a re a , h i g h p o r o s i t y h a e m a t i t e ( F e 2 0 3 ) c a l c in e .

M i n i m i s a t i o n o f t h e p r o d u c t i o n o f r e a c ti v e c o n s t it u e n t s i n t h e c a l c in e e . g . p y r r h o t i t e , f e r r o u s

s u l p h a te e t c . w h i c h c o u l d c o n s u m e r e a g e n ts .

T h e f i x a ti o n o f a r se n i c a n d s u l p h u r i n a n e n v i r o n m e n t a l l y a c ce p t a b le f o r m .

T h e t w o m o s t c o m m o n t y p e s o f s u l p h i d e o r e r o a s te r s a r e t h e s t a t io n a r y f l u id b e d a n d t h e c i r c u l a ti n g f l u id

be d r oa s t e r [ 90] . F igu r e 12 show s the e s se n t ia l d i f f e r e nc e s be twe e n the two r oa s t e r t ype s .

S l u r r y

S t a t i o n a r y H o t G a s C i rc u la t in g H o t G a s

F l ui d B e d C y c l o n e F lu id B e d C y c l o n e

R e a c t o r S y s t e m

To Evaporat ive . - . - . . . . . . . .. . _ . ._ ._

G a s C o o l e r ;

. ' - ' . ' - , ~ : .. . O o . o

I O o O - o ,

i : : : :

: . : . : . :

. : . : . :

o O o . .

o . o o O o 1 o O o .

A i r Q u e n c h

Roast ing Air

T o E v a p o r a t i v e

G a s C o o l e r

T o C a l c i n e

F l u i d Be d

C o o l e r

Fig . 12 Sc he m a t i c o f a s t a ti ona r y a nd c i r c u l a t i ng f l u id be d r oa s t e r s

I n t he s t a t i ona r y f l u id be d r oa s t e r , t he a i r f l ow r a t e a nd t he o r e f e e d r a t e a r e r e gu l a t e d so t ha t t he so l i ds

in t he r e a c to r a r e m a in t a ine d i n a un i f o r m f lu id i se d st a te . Coa r se c a l c ine i s r e m ove d th r o ugh a f l u id se a l

a t t he bo t tom o f t he r oa s t e r i n to t he c a l c ine que nc h t a nk a nd t he o f f ga ses a r e pa sse d i n to a c yc lon e whe r e

the f i ne r c a l c ine pa r t i c l e s a r e r e c ove r e d . T he ga s f l ow in t he r oa s t e r i s r e l a t i ve ly l ow a nd c on t r o l o f t he

t e m p e r a t u r e a n d g a s c o m p o s i t i o n w i t h in t h e r e l a ti v e l y d e n s e b e d i s d i f fi c u l t. T h e r e a r e p o c k e t s o f v e r y

h igh t e m pe r a tu r e , i n whic h t he o r e i s ove r - r oa s t e d a nd r e g ions de f i c i e n t i n oxyge n . I n p r a c t i c e ,

c o n v e n t i o n a l r o a s t e r s a r e t h e r e f o r e a s s o c ia t ed w i t h g o l d e n c a p s u l a ti o n b y f u s e d i r o n o x i d e s i n o v e r - r o a s te d

c a l ci n e . A l t e r n a t i v e l y , t h e c o m b u s t i o n o f t h e s u l p h i d e i n t h e o r e c a n b e i n c o m p l e t e . B o t h o f t h e s e f a c to r s

d e c r e a s e t h e g o l d e x t r a c t io n b y c y a n i d a t i o n - C I P / C I L .

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I n t he c i r c u l a t i ng f l u id be d r oa s t e r , t he c yc lo ne a nd r e a c to r c ha m b e r a r e i n t e gr a t e d a nd e na b l e a

c o n t i n u o u s c y c l i n g o f t h e o r e i n t h e r o a s t i n g c h a m b e r . T h i s i n c re a s e s t h e o v e r a l l t u r b u l e n c e i n t h e r o a s t er

c ha m be r a nd r e su l t s i n m u c h h ig he r he a t a nd m a ss t r a ns f e r r a t e s t ha n i n s t a t i ona r y f l u id be d r oa s t e r s . T he

g a s f e e d a n d o r e f e e d r a t e s c a n b e v a r i e d i n d e p e n d e n t l y to o p t i m i s e t h e p e r f o r m a n c e o f t h e r o a s te r . T h e

t e m p e r a t u r e p r o f i l e is u n i f o r m t h r o u g h o u t th e r o a s ti n g c h a m b e r a n d c y c l o n e , w h i c h e n a b l e s m o r e p r e c is e

c o n t r o l o f t h e ro a s t , e l i m i n a ti n g b o t h o v e r - ro a s t i n g a n d i n c o m p l e t e c o m b u s t i o n o f t h e o r e. C u r r e n t

p r a c t i c e a t K a l g o o r l i e a c h i e v e s 9 9 . 5 % e l im i n a t i o n o f s u l p h u r f r o m t h e c o n c e n t ra t e s , a c a l c in e h e a d g r a d e

o f 5 0 g . r -1 a n d a n o v e r a l l g o l d r e c o v e r y o f 9 4 % [ 9 0 ].

P y r i t e r o a s ti n g is n o r m a l l y c a r ri e d o u t i n t h e ra n g e 6 5 0 - 7 0 0 C a s a d e a d r o a s t . T h i s p r o c e d u r e c o n v e r t s

t h e p y r i t e i n s t a g e s t o h a e m a t i te ( F e 2 0 3 ) w h i c h i s i n s o l u b le i n c y a n i d e s o l u t i o n a n d s u f f i ci e n t l y p o r o u s t o

e x p o s e t h e g o l d p ar t ic l e s . T h e o v e r a l l e q u a t io n f o r t h e c o n v e r s i o n i s

4 F e S 2 + 1 1 0 2 ~ 2 F e o_ O3 + 8 S O 2

bu t t he a c tua l r e a c t i on t a ke s p l a c e i n two s t a ge s [ 94]. F i r s t l y , pyr r ho t i t e ( Fe S) is f o r m e d a nd t he

p y r r h o t i t e i s th e n c o n v e r t e d t o h a e m a t it e . T h e s u l p h u r i s v o l a t il i s ed a s S O2 . T h e t w o i m p o r t a n t

p a r a m e t e r s i n r o a s t in g a r e t h e p a rt i al p r e s s u r e o f o x y g e n a n d t h e t e m p e r a t u re i n t h e r o a s t i n g c h a m b e r [ 8 9 ].

R o a s t i n g a l so d e c o m p o s e s t h e g o l d - t e ll u r i d e s i n t h e p y r i t i c c o n c e n t r a te s a b o v e 4 5 0 C b y t h e r e a c ti o n [ 4 ]:

A u T e + 0 2 ~ A u + T e O2

Ar se nopyr i t e r oa s t i ng c a n be c a r r i e d ou t e i t he r a s a s i ng l e s t a ge o r two s t a ge r oa s t a t a bou t 650C in

w h i c h a r s e n o p y r i t e i s u l ti m a t e l y c o n v e r t e d t o h a e m a t i t e , a r s e n ic t r i o x i d e (A s 2 0 3 ) a n d S O 2 a c c o r d i n g t o

the ove r a l l r e a c t i on :

2 F e A s S + 5 0 2 - -- . F e 2 0 3 + A s 2 0 3 + 2 S O 2

T h e a r s e n i c t r i o x i d e i s c o o l e d a n d c o l l e c t e d a s a s o l id a n d S O 2 i s e m i t t e d t o t he a tm os phe r e . T h e pa r ti a l

p r e s s u r e o f o x y g e n a n d t h e t e m p e r a t u r e i n t h e r o as t in g c h a m b e r a r e a g a i n t h e m a j o r p r o c e s s v a r i a b l es

c on t r o l l i ng t he f i na l p r oduc t s .

P y r o l y s i s

D r y a n d C o e t z e e [ 9 5 ] s t u d ie d t h e p y r o l y s i s o f p y r i t e a n d a r s e n o p y r i t e . S i m p l i f ie d e q u a t io n s f o r t h e

t r a ns f o r m a t ion o f e a c h m ine r a l a r e a s f o l l ows :

FeS 2 ( sol id) + hea t - -, FeS ( sol id) + S (gas)

Fe A sS ( so l i d ) + he a t . - -. Fe S (so l i d ) + As ( ga s )

T h e se r e a c t i on s l i be r a t e t he go ld a nd f o r m t r i o l i t e ( Fe S) , no n- s to i c h iom e t r i c py r r ho t i t e (Fe S l l + x l ), o r bo th .

T h e m e t a l s u l p h i d e s t h a t ar e p r o d u c e d r e ac t w i t h c y a n i d e s o l u t io n f o r m i n g f e r r i c y a n i d e - f e rr o c y a n i d e

s p e c ie s . T h e c o n s u m p t i o n o f c y a n i d e in c r e a se s t h e p ro c e s s i n g c o s t u n le s s t h e m e ta l s u l p h i d e c a n b e

s t a b i l is e d , I n a n e x t e ns ion o f th i s wo r k a t a l a bor a to r y sc a le , t he ga s a tm o sphe r e i n t he f u r na c e wa s

c o n t r o l l e d t o i n c l u d e N 2 an d S O 2 [96] . I n t he p r e se nc e o f N 2 t he r e a c t i on p r odu c t s we r e e s se n t i a l l y

p y r r h o t i t e a n d a r s e n ic . I n t h e p r e s e n c e o f S O2 t h e r e a c t i o n p r o d u c t s w e r e e s s e n t ia l l y p y r r h o t i t e , p y r i t e

a nd a r se n i c t r i ox ide .

i o l o g i c a l P r e t r e a t m e n t

B i o o x i d a t i o n is n o w a n a c c e p t e d t e c h n o l o g y f o r t h e p r e tr e a t m e n t o f r e f r a c t o r y s u l p h i d e g o l d o r e s w h i c h

c a n b e r e t r o f it t e d to c o n v e n t i o n a l c y a n i d a t i o n - C 1 P / C I L o p e r a ti o n s [ 9 7 ]. A t t h e H a r b o u r L i g h t s g o l d m i n e

a t l . , e onor a i n We s t e r n Aus t r a l i a , a r se nopyr i t i c c onc e n t r a t e s ha ve be e n p r oc e sse d s inc e June 1992 a t t he

r a t e o f 4 0 t . d -1 , u s in g G e n c o r ' s p r o p r i e t a r y B I O X e c h n o l o g y [ 8 6 ]. F u r t h e r n o r t h , a t W i l u n a , t h e s a m e

HE 7 10 B

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1228 S R LA BRooY e t a l

t e c hnol ogy i s now u se d t o p roc e s s 115 t . d -1 pyr i ti c / a r se nopyr i t i c c onc e n t ra te . Th i s B IOX plant i s

c ur re n t l y t he l a rge s t b i oox i da t i on ope ra ti on i n Aus t ra li a [98] . At Ge n c or ' s p re s sure ox i d a t i on p l a n t a t

Sao Bento in Braz i l , a com bine d BIOX/pressure oxid a t ion c i rcui t i s used. Th is com bina t ion resul t s in

l owe r p roc e s s i ng c os t s a nd e na b l e d p l a n t t h roughput t o be i nc re ase d i n s t a ge s by t a k i ng a dva n t a ge o f the

mo dul a r na t u re o f t he b i oox i da t i on p roc e s s [99]. Ano t he r B IOX pl a n t is unde r c on s t ruc t i on i n Gha na .

The p l a nne d t h rou ghput wi l l be 2 M t .y l o f r e f ra c to ry o re a nd i t i s s a id t o be t he l a rge s t B IOX g o l d

process ing plant in the world [100] .

B i ooxi da t i on ha s a l so be e n propose d a s a p re t re a tme nt fo r c a rbona c e ous p re g- robb i ng o re s [101 ,102] a nd

i s we l l e s ta b l ishe d a s a he a p / dum p l e a c h t e c hn i que fo r c oppe r r e c ov e ry f rom c ha l c opyr i t e . Ho we v e r , i n

t he go l d i ndus t ry t h i s op t i on i s s t il l unde r de ve l opm e nt a nd ma y we l l r e qu i re a n a c i d r e c o ve ry sys t e m for

t he go l d . The p roc e s s op t i ons fo r b i o l og i c a l p re t re a tme nt a re i l l us t r a te d i n F i gu re 13 .

B C T E R I L P R E O X I D T I O N

W H O L E O R E F L O T A TIO N C O N C E N T R A T E

H E A P A G I T A T E D

T R E A T M E N T T R E A T M E N T

N E U T R A L I S A T I O N

T H I O U R E A C Y A N I D E L E A C H

Fig. 13 Process op t ions for biologica l pre t rea tm ent

B i ooxi da t i on i nvo l ve s ba c t er i a l a t ta c k o f t he su l ph i de i n a e ra te d s l u r ri e s a t a m bi e n t t e mpe ra t u re s a nd

pre s sure i n a c i d i c so lu t i on . The b a c t e ri a d i s so lve t he su l ph i de by a n e xo t he rm i c ox i da t i on me c h a n i sm i n

t he pH ra nge 1 .0 -1 .5 , a c c ord i ng t o the ge ne ra l r e a c ti on (wr i t te n fo r a r se nopyr i t e ) :

Fe AsS + 2 .5 H2 0 + 3 .5 02 ~ 0 .5 Fe 203 + H2SO 4 + H3AsO 4

Va r i ous t yp e s o f ba c t e r ia e . g .

Thiobacillusferroox ans

a n d

Sulpholobus

c a n be use d i n t he b i oox i da t i on

s t ep . The m e c ha n i sms ope ra t i ng in b i oox i da t i on ha ve re c e n t l y be e n re v i ewe d [103] , a nd a de ta i le d

me c ha ni s t i c st udy of t he b i oox i da t ion o f a r se nopyr i t e ha s be en pub l i she d [104] . The re i s a c a t a l ys is o f

t he ox i da t i on o f t he su l ph i de sur fa c e wi t h d i sso l ve d oxyge n , whi c h n orm a l l y r e qu i re s h i g h t e mpe ra t u re

a nd pre s sure i n t he a bse nce o f ba c t er i a . The ba c t e ri a r e ge ne ra t e c on t i nu ous l y , u t il i s ing C O 2 f rom t he a ir

a nd nu t r i e n t s f rom t he so lu t i on . The ba c t e r i a c a n be k i l le d by t ox i ns e . g . d i s so l ve d C u , Zn , A s , Sb a nd

Ag; bu t t he y c a n a da p t t o som e de gre e t o t he se t ox i ns in t he pu l p .

A ma j or a dva n t a ge o f b i oox i da t i on i s t ha t fo r su l ph i de s in whi c h g o l d i s c onc e n t ra te d a t g ra i n boun da r i e s

or o t he r p re fe r re d s i te s , on l y pa rt ia l ox i da t i on o f the su l ph i de is r e qu i red . Th e ba c t e r i a p re fe re n t i a ll y

a t ta c k a t suc h s it e s li be ra ti ng go l d ve ry e f f i c i e n tl y . How e ve r , t h i s a dva n t a ge doe s no t a pp l y wh e n go l d

i s in sol id solut ion [4] wi th in the sulphid e phase , as i s of ten the case for a rsen opy ri te [97]. Oth er

a dva n t a ge s o f ba c t e r ia l o x i da t i on i nc l ude :

Modul a r p roc e s s wi t h e a sy sc a l e up by a dd i ng fu r t he r t a nks .

No a t mosphe r i c po l l u t i on .

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Disadvantage s of bacterial oxidat ion and considerat ions in cyanidat ion of bioox idised produ cts [105]

include the fol lowing:

Slow rate o f react ion, e .g. days are req uired for ef fect ive t reatment o f sulphide concentrates .

Carefu l tem peratu re con trol required.

Cor ros ion due to low pH pulps .

The poss ib i l ity of carbon deac t iva t ion in the CIP sec tion of the p lan t due to fou l ing by d i s so lved

organic residues from the bacterial reaction.

Poss ib le foaming problems .

The relat ive economics for t reat ing sulphide ores by biooxidat ion compared to pressure oxidat ion and

roas t ing ( including the projected cos t of SO2 recovery ) have be en given for a 100 t .d 1 plant in Table 3,

showing that biooxidation is the cheapest option [97].

T A B L E 3 C o s t co m p ar i s o n o f p r o ces s o p t i o n s ( a f te r [ 97 ])

II Biooxidation

Pressure Roast ing

Capital ( US mill) 3.8 7.5 4.9

Operat ing Cost 43 43 56

( US/tonne)

P r e s s u r e O x i d a t i o n P r e t r e a t m e n t

Proce ss opt ions for high pressure oxidat ion are il lus trated in Figure 14. W hils t the normal route for gold

recov ery fol lowing pressure oxidat ion is s ti ll cyanidat ion-CIL /CIP, much research has bee n carr ied o ut

on acid leach systems to avoid the need for pre-leach neutralisation.

P R E S S U R E T R E T M E N T

O R E P R E T R E T M E N T

P R E S S U R E

C Y N I D T I O N

F L O T T I O N

D J U S T C O N C E N T R T I O N O F

B S IC C O M P O N E N T S IN O R E

C I D t ~

P R E S S U R E

O X I D T I O N L K L I N E

O X I D T I O N

N E U T R L I S T I O N

T h i o u re a L E C H

C h l o r i n e C y a n i d e

B r o m i n e

T h i o e u l p h a t e

I o d i n e

Fig . 14 Pro ces s optio ns for pressu re pretreatment-leaching o f refracto ry materials

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1230 S R LA BROOYe t a l

P r e s s u r e o x i d a t i o n o f r e f r a c t o r y s u lp h i d e s i s a n e s ta b l is h e d t e c h n o l o g y a n d s e v e n c o m m e r c i a l p l a n ts h a v e

be e n bu i l t [ 77]. T h e t e c hn o log y i s m a tur e bu t i t i s st il l r e ga r de d a h igh t e c h no lo gy- h igh c a p i t a l c os t op t i on

[ 106] . T h e p r oc e ss i s be s t su i t e d t o o r e s t ha t r e qu i r e c om ple t e su lph ide de s t r uc t i on .

A c i d P r e s s u r e O x i d a t i o n

T y p i c a l o p e r a t i n g c o n d i t i o n s a r e t e m p e r a t u r e s o f 1 7 0 - 2 2 5 C w i t h a r e t e n t i o n t i m e o f 1 - 3 h i n a 4 o r 5

c o m p a r t m e n t a u t o c la v e [ 1 0 7 ]. O p e r a t i o n at a ro u n d 2 0 0 C m i n i m i s e s c o n s t r u c ti o n c o s t o f a u t o c la v e s

[ 1 0 8 ]. T e m p e r a t u r e s n e e d t o b e a b o v e 1 6 0 C t o e n s u r e i rr e v e r s i b l e o x i d a t i o n o f s u l p h u r t o s u l p h a te , a s

m o l t e n s u l p h u r c an o c c l u d e g o l d a n d c o n s u m e c y a n i d e in s u b se q u e n t c y a n i d a t i o n - C I P / C I L . I n t h e

a u toc l a ve , pyr i t e a nd a r se nopyr i t e i n i t i a l l y d i s so lve t o f o r m f e r r i c , su lpha t e a nd a r se na t e i ons whic h a r e

t r a ns f o r m e d by hy dr o ly s i s in to so l i ds suc h a s sc or od i t e , ha e m a t i t e , ba s i c i r on ( I II ) su lpha t es a nd j a r os i t e s

[ 1 0 7 , 1 0 8 ] .

T h e r a t e o f r e a c t io n i s c o n t r o l le d b y o x y g e n m a s s t r a n s f e r to t h e m i n e r a l s u r f a ce . H e n c e , f i n e r g r i n d i n g

to i nc r e a se su r f a c e a r e a i nc r e a se s t he ra t e o f r e a c t i on . Ox yge n is b low n th r oug h the sys t e m to i nc r e a se

t u r b u l e n c e a n d a i d i t s d i s s o l u ti o n . O v e r a l l p re s s u r es o f 1 1 0 0 - 3 2 0 0 k P a a r e e m p l o y e d w i t h o x y g e n p a r ti a l

p r e s s u re s o f 3 5 0 - 7 0 0 k P a [ 1 0 7 ]. H i g h e r o x y g e n p a r ti a l p r e s s u re s w o u l d a l l o w m o r e r a p i d o x i d a t i o n b u t

w o u l d c a u s e o v e r h e a t i n g le a d i n g t o e q u i p m e n t f a il u re s , A u t o c l a v e p re s s u r e v e s s e l s a re c o n s t r u c t e d o f

c a r bo n s t e e l, p r o t e c t e d f r o m c he m ic a l a t ta c k by a le a d o r fi b r e g l a s s r e in f o r c e d v iny l e s t e r li n ing . T h i s

l i n ing i s i ts e l f t he r m a l ly i nsu l a t e d by l a ye r s o f a c id r e s is t a n t b r i c k . E x o t i c t i t a n ium ba se d m a te r i a l s a r e

g e n e r a l ly re q u i r e d f o r i n te r n a l c o m p o n e n t s w e t t e d b y t h e s l u r r y [ 1 0 9 ]. C o m b u s t i o n o f t i ta n i u m

c o m p o n e n t s w i t h i n t h e a u t o c l a v e c a n o c c u r w i t h o u t a d e q u a t e c o n t ro l o f o p e r a ti n g c o n d i t i o n s . T y p i c a l

o x y g e n u t il is a t io n e f f i c ie n c y is a ro u n d 5 0 - 9 0 % d e p e n d i n g o n h o w m u c h C O 2 ha s t o be ve n t e d [ 110] . On-

s i t e ox yge n ge ne r a t i on f a c i l it i e s a r e ge ne r a l l y r e qu i r e d . Au toge no us ope r a t i on i s poss ib l e w i th > 3 %

s u l p h i d e s u l p h u r i n t h e f e e d [ 1 1 1 ] b u t i s al s o d e p e n d e n t o n c a r b o n a t e c o n t e n t , n u m b e r o f p r e tr e a t m e n t

s t ag e s , p u l p d e n s i t y a n d o p e r a t i n g t e m p e r a t u r e [1 1 2 ] . T h e o r g a n i c c a r b o n c o n t e n t o f t h e f ee d d o e s n o t

c on t r i bu t e a he a t inpu t t o t he p r oc e ss [ 112] . Pu lp a c id i t y i s ge ne r a l l y c on t r o l l e d a t pH < 2 [ 110] to

m a in t a in a sa t is f a c to r y r e dox po t e n t i a l a nd p r e ve n t e xc e ss ive p r e c ip i t a t i on i n t he a u toc l a ve s . Oxid a t i on

of su lph ur i n t he f e e d to su lphur i c a c id ke e ps t he pH low . I f t he su lphur c on t e n t o f t he fe e d i s le s s t ha n

2% , a c id ha s t o be r e c yc l e d t o t he f e e d , o r fr e sh a c id a dde d [ 110] . T h e s i z e o f a u toc l a ve s i s de t e r m ine d

b y s u l p h u r o x i d a t i o n r e q u i r e m e n t s [ 1 0 9 ]; h e n c e t r ea t m e n t o f w h o l e o r e i s f e a s i b l e i f t h e o r e d o e s n o t

r e spond we l l t o f l o t a t i on .

A l k a l i n e P r e s su r e O x i d a t i o n

Ox ida t ion und e r ne u t r a l o r s l i gh t ly a lka l i ne c ond i t i ons c a n be p r a c t i s e d f o r h igh c a r bon a t e ( > 10% CO32

a n d l o w su l p h u r o r e s ( < 2 % ) a n d is u s e d a t B a r r i c k 's M e r c u r p l a n t i n U t a h , U S A [ 1 1 3 ] . T h i s p r o c e s s

o p e r a t e s a t 2 2 0 C w i t h a n o x y g e n p a r ti a l p re s s u r e o f 1 4 0 - 1 80 k P a a n d 3 3 0 0 k P a t o ta l p r e s s u r e a n d

d i s s o lv e s p y r i t e to f o r m i r o n o x i d e s / h y d r o x i d e s a n d s u l p h u r ic a c i d . A t M e r c u r , t h e ac i d g e n e r a te d i n t h e

pr oc e ss i s ne u t r a l is e d by t he c a r bona t e i n t he o r e ; t hus t h i s p r oc e ss wo uld a t t r a c t a dd i t i ona l r e a ge n t c os t s

in a m or e ge ne r a l a pp l i c a t i on . T h e c a p i t a l c os t o f the p l a n t f o r a lka l ine p r e s sur e o x ida t i on i s l e s s t ha n

f or a c id p r e s su r e ox ida t i on be c a use s t a in l e s s s t e el ( r a the r t ha n t i t a n ium ) c a n b e use d f o r va lve s a nd p ipe s

w i t h i n th e a u to c l a v e s ; h o w e v e r , g o l d r e c o v e r y is l o w e r [1 1 4 ] , a s in s o l u b l e i r o n o x i d e s / h y d r o x i d e s c a n

f or m a nd c oa t go ld a nd su lph ide m ine r a l su r f a c e s [ 110] .

P r e s s u r e C y a n i d a t i o n

Pie t sc h [ 115] de m o ns t r a t e d t ha t e l e va t e d ox yg e n p r e s sur e a nd i nc r e a se d t e m pe r a tu r e s dur ing c y a n ida t i on

c o u l d i m p r o v e t h e r a t e o f g o l d d i s s o lu t i o n . H o w e v e r , c y a n i d e d e c o m p o s e s r a p i d ly a b o v e 7 5 C w h i c h

l im i t s t he m a xim um t e m p e r a tu r e t ha t c a n be a pp l ie d i n p r a c t i c e . T h e r e a c t i on c a n be c a r r i e d ou t i n a n

a u t o c l a v e o r p i p e r e a c t o r . T h e t e c h n i q u e h a s b e e n a p p l i e d t o c y a n i d a ti o n o f s t ib n i t e c o n t a i n i n g o r e s o r

c o n c e n t r a t es , w h i c h g i v e p o o r g o l d re c o v e r y u n d e r a t m o s p h e r ic c o n d i t i o n s [6 3 ] . T h e p r o b l e m h a s b e e n

a s c r ib e d t o o x y g e n d e p l e t i o n a s s o c ia t e d w i t h t h e f o r m a t i o n o f a n t i m o n i t e s a n d t h i o a n t im o n i t e s , w i t h

s t ib n i t e d e c o m p o s i t i o n b e in g g r e a t e r a t p H > 1 0. P r e s s u r e c y a n i d a ti o n at p H < 1 0 h a s b e e n p a t e n t e d f o r

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19/29


gol d recove ry f ro m s t ibn i t e concent ra t e s [116] . Thi s p rocess gave 90% gol d recove ry f ro m a s ti bn i t e

concent ra t e bu t l ower recove ry f rom an a r sen ic r i ch mi ddl i ng . P re s sure cyan i da t i on of s ti bn i te

concent ra t e , u s i ng a ro t a t i ng cy l i ndr i ca l con t ac t or , w as prac t i c ed a t t he B l ue Spec m i ne i n Wes t e rn

Aus t ra l i a bu t f a i l ed t o ga i n commerc i a l s t a tus [117]. Labo ra t ory i nves t iga t i ons show ed t ha t p re s sure

cyan i da t i on of s t i bn i t e concent ra t e s was ab l e t o recove r t he f ree go l d , bu t r e cove ry o f encapsu l a t ed go l d

was on l y po ss i b l e a f t e r de s t ruc t ion o f t he s t i bn i t e [117] .

G O L D E X T R A C T I O N P R O C E S S E S

C y a n i d e

The norm a l go l d recove ry s tep i nvol ves a l ka l ine cyan i d a t i on-C IP / C IL . Thi s p rocess requ i re s

neut ra l i sa t ion of f e ed f rom b i ooxi da t i on or a c id pre ssure ox i da t i on . C ya ni da t i on -C IP / C IL was cove red

in de ta i l ear l ie r in thi s review.

Use of the R IP process in the wes t e rn wo r l d i s i n i ts i n fancy . R IP ha s on l y been used a t t he Go l den

Jubi l ee mi ne i n Sout h A f r i ca , whe re t he o re was heav i l y con t ami na t ed wi t h na t ura l hum a t e s and fu l va t e s

wh i ch compl e t e l y fou l ed C IP / C IL ca rbon [118]. The po t en t i a l o f R IP as a r ep l acement fo r C IP / C IL has

been ex t ens i ve l y rev i ewed by F l emi ng [119 ,120] . The m a i n advant ages of r e s ins a re : l e s s sens i t iv i t y t o

organ i c s , h i gh adsorp t i on ra t e , l ow t empe ra t ure e l u t i on and promi se of go l d spec i f i c i t y unde r c e r t a i n

cond i t i ons fo r r e s i ns s t il l und e r deve l opm ent . The d i sadvant ages of r e s i ns i nc l ude : sma l l e r pa r ti c l e s iz e s

requi r i ng f i ne r sc reens , poss i b l e sens i t i v it y t o osmot i c shock and rap i d i n -pu l p wea r o f c e r t a i n re s i ns .

R es i ns a re a l so t h ree t o f i ve ti mes more expens i ve t han ac t iva t ed ca rbon on a mass fo r mass ba s i s . In

v i ew of fa s t e r k i ne t i c s and more favourab l e equ i l i b r i um l oad i ngs a r e s i n p l an t i s l i ke l y t o be ope ra t ed

mu ch c l ose r t o equ i l i b r i um t han ac t i va t ed ca rbon , r equ i r ing t i gh t e r ope ra t i ng cont ro l o f t he p lan t [120] .

The prospec t fo r t he fu t ure app l i ca t i on of R IP i s s t i l l be i ng deba t ed [120 ,121] ; i n t e re s t i ng l y , R IP

t echnol ogy i s r epor t ed t o have had wi despread use i n t he former USSR [122 ,123] .

C ya ni de ha s g ene ra t ed envi ron men t a l conce rns because o f i ts t ox i c i t y and an a l t e rna ti ve non- t ox i c a lka l i ne

process t ha t ha s been cons i de red i s the ammon i aca l t h iosu l pha t e l e ach i ng of go l d . The re a re a l so a

num ber o f poss i b l e a c i d go l d recove ry op t i ons . The t e chn ol ogy is gene ra l l y i mm at ure , bu t cou l d e l i mi na t e

t he neu t ra l i sa ti on cos t s . The be s t r e sea rched ac i d i c op t i ons i nc l ude t h i ourea and ha l i de i ons .

A m m o n i a c a l T h i o s u l p h a t e

Go l d form s a s t ab l e an i on i c compl ex w i t h t h i osu l pha t e accord i ng t o t he fo l l owi ng reac ti on :

2 Au + 0.5 02 + 4 $2032 + H2 0 ~ 2 Au($203)23 + 2 OH-

Al ka l i ne so l u t i ons a re used t o p revent t he decompos i t i on of t h i osu l pha t e by ac i d a l t hough t he go l d

t h i osu l pha t e comp l ex i s qu i t e s t ab le once i t ha s fo rm ed. The f i rs t r epor ted use fo r t h i osu l pha t e fo r go l d

l each i ng was pa r t o f the von Pa t e ra p rocess fo r s i l ve r r e cove ry . In t h i s p rocess , o re s w e re g i ven a

ch l or i d i s i ng roa s t and t hen le ached wi t h sod i u m t h i osu l pha t e [49]. I t was sub sequent l y found t ha t coppe r

i on accel e ra ted t he d i s so l u t i on ra t e o f go l d t went y fo l d . P rob l ems a re enco unt e red due t o t he forma t i on

of a coppe r su l ph i de l aye r on t he go l d [124] , whi ch can be ove rcome by pre s sure l e ach i ng i n

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