USBMRI9181LeachingGoldSilverWithCyanideAndThiourea

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Bureau of Mines Report of lnvestig8tionsll988

Leaching Gold-Silver Ores With Sodium

Cyanide and Thiourea Under

Comparable Conditions

By J. A. Eisele, A. H. Hunt. and D. L. Lampshire

UNITED STATES DEPARTMENTOF THE INTERIOR





Report of Investigations 9181

Leaching Gold-Silver Ores With Sodium

Cyanide and Thiourea UnderComparable Conditions

By J. A. Eisele, A. H. Hunt, and D. L. Lampshire

UNITED STATES DEPARTMENT OF THE INTERIORDonald Paul Hodel, Secretary

BUREAU OF MINEST S Ary, Director



Librafy of Congress Cataloging in Publication Data:

Eisele, J. A. (Judith A.)

Leaching gold-sihhr o r s with sodium cyanide and th io um under comparable

conditions.

(Report of investigations; 9181)

Bibliography: p. 67

Supt. of D m . no .: 1 28.U:9181.

1. Gold-Metallurgy. 2. Silver-Metallurgy. 3. Leaching. 4. Cyanides.

5. Thiourea. I. Hunt, A. H. (A1 H.). 11. Lampshire, D. L. (Dan L.).

111. Title. IV. Series: Report of in ~s tig at io ns (United States. Bureau of

Mines); 9181.

TN23.U43 [TN693.G6] 622 s [669'.22] 88-600111



CONTENTS

Abstract .......................................................................Introduction ...................................................................Experimental procedures........................................................Results and discussion.........................................................Conclusions....................................................................References .....................................................................

TABLE

1. Analyses and ex tra cti on of gold and si l v e r from ores .......................



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

g gram mV m i l l i v o l t

g/L gram per l i t e r Pet percent

h hour

lb / s t pound per short ton

mL m i l l i l i t e r

t r o z/ st troy ounce per

short ton

Y year



LEACHING GOLD-SILVER ORES WITH SODIUM CYANIDE

AND THIOUREA UNDER COMPARABLE CONDITIONS

By J. A. ~isele, ' A. E. ~ u n t , ~nd D. L amps shire^

ABSTRACT

There i s c u r r e n t l y m c h i n t e r e s t i n how e x t r a c t i o n of g o l d and s i l v e r

f rom o res wi th ac id i c th iou rea so lu t io n compares wi th ex t r ac t io n u s ing

a l ka l i ne cyan ide so lu t ion . Ag i t a t i on l each ing t e s t s were per fo rmed by

t h e Bureau of Mines on 14 pre cio us metal or es us in g sodium cyanide

(NaCN) and th io ur ea [(NH2)2CS] a s ex tr ac ta nt s. The ob je ct iv e was t o

compare t h e r e s u l t s of th e two ex tr ac ta nt s. NaCN was used a t a concen-

t r a t i o n of 2 g/L with and without H202 as an oxidant. (NH2)2CS was used

a t concen tra t ions of 2 and 20 g /L, and th e so lu t i on po te n t ia l was con-

t r o l l e d a t 390 t o 420 mV s tan dard hydrogen e le c t r od e (Eh) wi th

Fe2(S04)3. With both reage nts a t 2 g/L e xt ra ct an t le v e ls , NaCN ex-

t r a c te d more go ld and s i l v e r than (NH2)$S f rom a l l o f th e ores . A t

20 g / L (NH2)$S compared w i th 2 g/L NaCN, (NH2)2CS e x t r a c t e d more go ld

t h a n NaCN from two o re s , t h e same amount from one or e, and more s i l v e r

from one ore. NaCN e x t r a c t e d more go ld th an (NHZ)2CS from 10 ore s andmore s i l v e r from 4 ores .

'Superv isory chemical engineer .

2phys ica l sc i ence t echn ic ian .

Reno Res ear ch Cen te r, Bureau of Mines, Reno, NV.



INTRODUCTION

Cyanidation has been used almost exclu-

s iv e ly f o r l e ach in g go ld and s i l v e r s i n ce

i t s int rod uc tio n 100 yr ago. Recent ly,

t h e r e h as been m c h i n t e r e s t i n t h e min-ing and meta l lur gica l process ing indus-

t r i e s i n t h io u re a a s a n e x t ra c t an t f o r

gold and s i l v e r (1-3,- - --14).3 Thiourea

ex tr a c ts gold i n an aci di c medium (pH 11,and a t a s o lu t i o n o x id at i o n p o t en t i a l

such th at the dimer, formamidine di su l-

f i d e , i s formed and i s t h e ex t r ac t an t .

F e r r i c s u l f a t e i s commonly used a s an

oxidant to achieve and maintain th e oxi-

d a t io n p o t e n t i a l of t he s o l u t i o n i n t h e

prop er range f o r dimer formation. The

di ss ol ut io n of gold by th iou rea (fo r-mamidine disulfide) i s expressed by

Au + 2(NH2)p3 + A U [ ( N H ~ ) ~ C ] ~ Se-, (A)+

w i th g old s o lu b i l i z ed a s a c a t i o n i c

complex.

C yanide ex t r ac t s g o ld i n an a lk a l i n e

medium, and al tho ug h no ox ida nt o th e r

t h an a i r f rom a g i t a t i o n i s g en e r a l l y

ap pl ie d, ad di ti on of an oxidant may be

be nef ic ia l (4) . The dis sol ut i on of goldby cyanide iy exp ress ed by

wi th go ld so lu b i l i zed as an an ion ic

complex.

EXPERIMENTAL

Thiourea leaching sol ut i ons were pre-

pared by di ss ol vi ng reagent-grade(NH2)2CS i n water and ac id if yi ng with

H2S04 t o pH 1. Fifty-gr am samples of or e

3~ nd er l i ne d umbers i n paren theses r e -

f e r t o i tems i n t h e l i s t of r e fe rences a t

th e end of th i s r e por t .

Thi s Bureau of Mines stu dy was und er-

taken t o compare th e ext rac t io n of gold

and s i lv e r from a var ie t y of p rec ious

metal or es under comparable co nd iti on s ofreagent conce ntra tion s, t ime of leac hing ,

and a t oxidant le ve ls th at enhance ex-

t r ac t i on fo r bo th cyan ide and th iourea .

A NaCN co nc en tr at io n of 2 g/L (4 l b / s t

s o lu t i o n o r 12 l b l s t o r e u nd er t e s t con-

di t i on s) was used because t h i s i s a com-

monly used le ve l i n cya nida tion of pre-

ciou s metal ores . For some or es t h a t

con ta in no cons t i tu en ts such as base

metal s u l f i d e s or carbonaceous compounds.

which in te r f e r e wi th cyan ida t ion (c lean

oxide or es ), a NaCN le ve l a s low a s0.17 g/L (0.33 lb / s t sol ut i on o r 1 l b / s t

or e ) may be su ff ic ie n t. However, f o r

o res con ta in ing cyan ic ides , mc h h igher

l ev e l s t h an 2 g/L may be ne ce ss ar y t o

obt ai n good precious metal e xtr ac t io n.

One ore used i n th i s s tudy req uire d

100 g/L (200 l b / s t s o lu t io n o r 600 l b / s t

or e) NaCN con cent rati on t o obt ain over

90 pct gold extract ion.

A (NHz)zCS concentration of 2 g/L was

used t o g ive a d ir ec t comparison with

NaCN. A second l e v e l of (NHZ)2CS concen-

t r a t i o n , 20 g/L, was a ls o used becausemost previous lea chin g st ud ie s had used

concen t ra t ions o f 1 pc t and high er, and

i t i s of in te re s t t o know i f incr eas i ng

th e r eag ent l ev e l t en f o ld g r ea t l y i n -

c r ea s e s t h e g o ld and s i l v e r ex t r ac t i o n .

PROCEDURES

and 150 mL of leachin g so lu t i on were

ag i t a t ed i n a beaker by magnetics t i r -

r ing. Fe rr ic su lf a t e was added as needed

t o m a in t ain t h e p o t e n t i a l of t h e s l u r r y

between 390 and 420 mV Eh, which i s a

s u i t ab l e r an ge f o r g old e x t r a c t i o n by

th io u r ea (1). eaching was a t ambient

tempera tu re fo r 5 h.



Cyanide leach ing t e s t s were performed

by ad di ng reage nt -g ra de NaCN and NaOH t o

50% samples of o re and 150 mL of water

t o b ri ng t he pH of t he s l u r r y t o about

11. Sl u r r i e s were ag i t a t ed a s above f o r

5 h a t ambient temperature . I n some cya-

nide leaches, hydrogen peroxide was added

i n sma l l i n c remen t s t o b r in g th e r ed ox of

t h e s o l u t i o n t o t h e d e s i r e d l e ve l .

A f t e r t h e l e a ch i n g p e r io d , s l u r r i e s

were f i l t e r e d and th e r e s id u es were

washed. Prec ious metal co nt en ts of head

samples and re si du es were determined by

f i r e a ssay . The g o ld and s i lv e r co n ten t

of pregnant so lu ti on s was determined by

atomic ab so rp ti on spectrometry . When

u s in g a to mic ab so rp t io n an a ly s i s ca r e

must be taken t o ob ta i n co r re c t va lues(2). Background ab so rp ti on can cause a

fa ls e , h igh read ing un less background

cor rec t io ns a r e p roper ly made, e spe c ia l ly

i n th e ca se of th io u rea so lu t io n s con-

t a i n i n g d i l u t e s u l f u r i c ac id . A not he r

so u rce of p o ss i b le e r ro r i n a to mic ab-

so r p t i o n an a ly s i s was th e fo l lo win g:

a f t e r prolonged use of an atomic absorp-

t i o n an a ly ze r fo r g o ld an a ly ses , mos tly

i n a l k a l i n e c ya ni de s o l u t i on , t h e s t a i n -

l e s s s t e e l i n t e r n a l l i n i n g of t h e nebu-

l i z e r was go ld p la ted . Th is can re su l t

i n e r r o r i n two ways, f i r s t , low v a lu e sf o r th e sample from which gold i s

cemented, and second, pos sib le dis sol u-

t i o n of gold by fol lo wing samples and

re su l t an t f a l s e h ig h v alue . The p o ss ib le

d i s s o l u t i o n i s more l i ke ly by a thi our ea

so lu t ion sample because th iourea i s known

t o d i s so lv e a g o ld d i s k more r ap id ly

than cyan ide ( 7 ) . The us e of a Tef lo n4

4 ~ e f e r e n c e o s p e c i f i c p ro du ct s do es

n o t imply endorsem ent by t h e Bureau of

Mines.

f luor ocarb on polymer neb uli zer avoids

such a problem.

Percen t ex t ra c t io n was ca lcu la te d f rom

th e amount of gold i n the pregnant solu -

t i o n and i n t h e c a l c u l a t ed h ea d; i . e . ,

th e amount i n th e p regnant so l u t io n

determined by atomic abso rptio n plus th e

amount i n th e residue determined by f i r e

assa y. When t h e amount i n th e pregn ant

so lu t i on was below th e de t ec t ion l i m i t ,

ex t r ac t i on was considered t o be zero .

Values of l e s s than 0.5 t r oz /s t Ag were

n o t c o n s id e re d i n e x t r a c t i o n c a l c u l a t i o n s

b ecau se ma te r i a l b a lan ces a r e d i f f i c u l t

t o ob tai n and th e monetary value i s very

low.

A l l oxida t ion po ten t ia l measurements

were made wi th Ag-AgC1 and Pt e l e c t ro d e s ,but a re repor ted i n terms of Eh.

Four teen samples used f o r the t e s t s had

a range of gold and s i l v e r values and of

base metal cont ent. Analyses of the or es

a r e g iv en i n t a b l e 1. P a r t i c l e s i z e of

t h e sam ples was nomi nall y 100 mesh. The

minera ls l i s t e d were id en t i f i ed by X-ray

d i f f r ac t i o n , which d e t ec t s o nly th o se

presen t i n la r ge amounts ; smal l quan t i -

t i e s would go unreported. For example,

sample 8, which contains 2 .8 p c t A s and

2.0 pct S, i s a ma te r i a l f ro m an a r e a

n o ted fo r i t s orpiment and re al ga r min-e r a l s , a l tho u g h none were id en t i f i ed i n

t h e X-ray scan. Ta bl e 1 shows some were

g ol d o r e s w i t h very l i t t l e s i l v e r , one

was a s i l v e r ore with no gold, and sev-

e r a l c o nt a in e d s i g n i f i c a n t v a l ue s of

both. Samples 1 thro ugh 7 r ep re sen t

c le an ox ide o res , which should be r ea d i ly

amenable t o cyanida tion. Samples 8

through 14 con tai n met al l i c and non-

me ta l l ic e lements th a t a re known t o

i n t e r f e r e w i t h c ya ni da ti on .



TABLE 1. - Analyaea and extra ct ion of gold and s i l ve r f rom ores-ample-.....

2.....

3.....

4.....

5.....

6.....

7.....

8.....

9.....

10.. ..l l . . . .

12....

13....

14....

Analysis

Minerals

i d e n t i f i e d

Extra

With 1

~ ~ -~n, pct -

h-EUAp

VAp

VAp

NAP

NAP

100

NAP

NAP

7 7

65

28

10

NAP

348-

o z l to nr-

aC N

No ox-u

92

82

{ 2

82

100

{ 2

67

67

( 2 1

{ 264

4

32

7

29L

NAP..............

NAP....... .......NAP...... ........NAP........ ......NAP..............

NAP........... ...NAP...... ........NAP..............

Sp h a le r i te . . .....P y r r h o t i t e ,

ch a lco p y r i te .

Arsenopyr ite ,

marcas i te -p y r i te

Pyr i te . . .........C er ru s i te , g a len a

. N ..'N O oxi dan t added, Eh of sl ur ry above 390 mV a t s t a r t and d u r in g leach ; su b seq uen t te s t w i th o x id an t b rou g ht Eh of s lu r ry

t o around 500 mV , g o l d -s i l v e r e x t r a c t i o n d i d no t i n c r e a s e.

2 ~ p l i c a t ee s t , Eh of sl ur ry i n 110- t o 220-mV range.

3 ~ o xid ant added, Eh of sl ur ry above 390 mV.

4 ~ x i d a n t d d i t i o n t o r a i s e Eh t b 500 mV ex tr ac te d 50 pct Au, 11 pc t Ag.

5 ~ a t e r i a l alance bad, t a i l i n g s a s s a y 0 . 0 1 t r o z ls t , b u t n o Au d e t e c t e d i n s o l u t i o n.

boxidant addi t io n to ra is e Eh above 500 mV did not incr eas e Au, Ag e x t r a c t i o n .



RESULTS AND DISCUSSION

The r e s u l t s of ex t r ac t io n t e s t s a r e

g iv en i n t a b l e 1. Cyanide le ac he s were

done a t a co nc en tr at io n of 2 g/L NaCN (12

l b l s t or e) and two condi tions . One con-

d i t i o n was th at no oxid ant was added

o t h e r th an t h e a i r c o n t a c ti n g t h e s l u r r y

dur ing ag i ta t ion . Since add i t i on of an

ox id an t may improve p re ci ou s me tal

ex tra ct io n, th e second con dit ion was with

H202 added to the slurry. A t e s t was run

on ore th a t d id no t respond wel l t o

cyan ida t ion t o de termine what le ve l of

HZOZ ad di t i on would be be ne fic ia l . The

t e s t showed th at when th e redox of th e

s lu r ry was maintained i n the range be-

tween 110 and 220 mV Eh, g o l d e x t r a c t i o nincrea sed f rom th e base l ine va lue of le s s

th an 30 pct t o more th an 50 pct . Above

250 mV Eh ex tr ac t i on decreased, and,

above 290 mV Eh, cy an id e was consumed and

no f re e cyan ide was de te c ted i n so lu t io n .

On the ba s is o f th is t e s t , H202 was added

t o c ya ni de l e ac h i ng s l u r r i e s t o m ai nt ai n

t h e po te nt ia l between 110 and 220 mV Eh.

Id ea l ly , such a t e s t se r i e s would be

run fo r each ore sample t o de termine

th e proper le ve l of oxidant; however ,

t h i s l ev e l p ro bab ly gave a good f i r s t

approximation.For samples 3, 6, 9, 10, and 14 no ox i-

da nt was added t o th e seco nd NaCN le ac h

(wi th H202 column). Sin ce t h e redox

p o t e n t i a l f o r t h e s l u r r y was a l re a dy i n

t h e d e s i r e d ra ng e, t h e s e t e s t s a r e d u p li -

c a t e s of t h e f i r s t s e r i e s . A range of

ex t r ac t i on va lues a re observed when a

l e a c h i n g t e s t i s repeated se ve ra l weeks

la t e r . The d i f f e r e n ce fo r sample 3 i su n u s a ll y l a r g e f o r m a t e r i a l w i t h a r e l a -

t iv e l y high head value. For sample 9 th e

di ff er en ce between 67 and 100 pct extra c-

t i o n f o r g o l d i s l a r g e , b ut f o r m a t e r i a l

wit h such a low head assa y the percent

e x t r a c t i o n i s v er y s e n s i t i v e t o s m al l

changes. I n t h i s cas e, th e same amount

of gold was found i n each pregnant sol u-

t ion, 0 .02 t r o z / s t o r e , b ut t h e t a i l i n g s

assayed 0.01 t r o z /s t f o r t he f i r s t t e s t

and none i n t he second te s t .

The same type of s i t ua t i on was t ru e fo r

t h e s i l v e r e x t r a c t i o n o f sam ple 6 , t h e

same amount of s i l v e r was found i n each

pregnant so l u t i on , bu t , in one case , no

s i lv e r was found i n th e t a i l i n g s and

i n the oth er case a smal l amount,

0.2 t r oz/s t . In genera l , add i t ion of an

oxidant did not improve gold or s i l v e r

ex tr ac t i on, with the exception of sample

11, which contained py rr ho ti te and chal-

copyri te . The ores th at contained ars e-

nopyr i te , pyr i te , and ce r r us i t e responded

poorly t o cya nid ati on and were not helped

by th e oxidant. The t e s t conditions--

ag it at io n i n an open beaker--supplied

s u f f i c i e n t oxygen. For sample 14, in -

cr ea si ng t h e NaCN co nc en tr at io n t o 20 g/L

and leaching fo r 4 h ext rac ted 23 pct of

t h e Au and 51 pc t of the Ag. Inc re as in g

th e NaCN co nc en tr at io n t o 100 g/L (600l b / s t o re) and leach ing fo r 24 h ex-

tr ac te d 92 pct of the Au and 87 pct of

t h e Ag. Cyanide consumption was not

measured.

Thiourea leach es were done a t two

reagent concen t ra t ion leve ls wi th the

redox po te n t i a l of the s l u r ry main ta ined

between 390 and 420 mV Eh. The 2 g/L

thi oure a le ve l gave a di re ct comparison

of ex t ra c t ion wi th the cyan ide leach ing

t e s t s . On a molar ba si s th e (NH2)zCS

s o l u t i o n i s weaker (0.026M) than th e NaCN

(0.041M) becau se of t h e hi gh er mole cularweig ht of (NH2)2CS. If th e dimer i s con-

s i d e r e d t h e e x t r a c t a n t , t h e t h i o u re a s o l-

u t i o n would be appro xima tely 0.013M.

However, t h i s shoul d pro vid e enough

ex t r ac tan t to ad equ a tely d i s so lv e th e

precious metals, because even th e hig hes t

gra de o r e a t 1 7t r o z/ st Ag c on ta in s

0.0003 mole of Ag i n 50 g of ore and

150 mL of th e th io u rea so l u t i o n co n ta in s

abo ut 0.002 mole of formamidine d i s u l -

f id e . One-h un dred -f if ty m i l l i l i t e r s of

cyanide so lu ti on con tai n 0.006 mole of

NaCN. If b ase me ta ls a r e consuming cya-

nide or thi our ea, the se le ve ls may be in-

adequate. This i s w e ll i l l u s t r a t e d by

th e cyanide l ev el required fo r sample 14,

100 g/L NaCN, ment ioned above. Th iourea

leach ing was a l so done wi th a so lu t io n

con tai nin g 20 g/L, which corresponds t o

le ve ls repor ted g iv ing good go ld ex t rac -

t io n , genera l ly between 1 and 5 p c t

th iourea .

Examination of the ex tr ac t i on values i n

t a b l e 1 shows th at a t th e 2-g/L le ve l



th i our ea had mixed success i n ex tr ac t i ng

gold from the clean oxidized ore s, sam-

pl es 1 through 7. Ex tr ac ti on was almost

th e same as cyanide f o r one ore , l e s s

than cyanide f o r two ores , and negl ig ib lef o r th e oth er ores. When th e thi our ea

le ve l was increased t o 20 g /L fo r th e

same ore samples, re s ul ts were again

mixed, al though gold ex tr ac t i on s gener-

a l ly in c reased . For a l l t h e samples of

t h i s group (1 th rough 7) , go ld ex t ra c t i on

a t 20 g/L th io u rea was l e s s th an a t 2 g/L

NaCN, ex ce pt f o r sample 4 where i t was

higher .

Examination of th e re fr ac to ry group of

samples (8 through 14) shows th a t a t th e

2-g/L le ve l th i our ea did not leach gold

o r s i l v e r from s e v e r a l o r e s t h a t h ad

responded t o cyan idati on, samples 8through 11. For the remaining th re e

or es (samples 12-14) ne it he r cyanide nort h i o u r e a e x t r a c t ed m c h g ol d o r s i l v e r .

A t 20 g/L thi our ea, gold and s i lv e r ex-

t r ac t i on s increased over the 2-g/L le ve l ,

but were le ss than with 2 g/L cyanide,

ex ce pt f o r sample 14. Ore sample 14.

con tai nin g a la rg e amount of le ad carbon-

a t e and si g n i f i c a n t amounts of Cu and Fe,

had almost hal f of i t s s i l v e r c o n t e n t

e x t r a c t e d ; g o ld e x t r a c t i o n was s i m i l a r t o

tha t ob ta ined wi th cyan ide .

CONCLUSIONS

A curso ry in ve st ig at io n was made t o

determine the e xt ra ct io n of gold and

s i l v e r from 14 ores wi th cyan ide o r

th iourea . The research ind ic a ted th a t i f

most or es cannot be cyanided they w i l l

not be leached any b e t t e r with thiourea.

Ores th a t a r e amenable t o cyan ida t ion

g en e ra l ly g iv e p o o re r ex t r ac t io n wi th

thiourea. Thiourea w i l l probably f ind

ap p l i ca t io n a s a l each an t f o r g o ld o nly

under sp ec ia l condi t ions; fo r example,

when an a c i d i c medium i s d e s i r e d t o co-

e x t r a c t a m et al t h a t i s not leached i n

al ka li ne cyanide, such a s uranium.

Another s pe ci al ca se where t hio ure a found

app l ic a t i on was i n leach ing a go ld-

beari ng antimony conc entra te (10). The

fa s te r leach ing ra t e of go ld , compared t o

ant imony, i n th iou rea so lu t i on a l lowed

th e g o ld to be ex t r ac ted b e fo re th e an t i -

mony was attacked.

REFERENCES

1. Bilston, D. W . , S. R. LaBrooy, and

J. T. Woodcock. Gold and S i l v e r Leachin g

From an Oxid ized Gold Ore With Thi our ea

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