] ]

Ci\Ti\LYSED CYJ\N!Di\TION OF GOLD

RB.E.Trincladcl a nel A.J.Monhcmius2

Cyanidation is th e mos t \Vidcly uscd proccss to recover golcl from its ores . ln this operation. the needed gold oxidation is normally accomplished by t h c oxygen. in thc form of a ir o r bulk oxygen. Undcr m ost circumstanccs , howevcr. thc cyan idation process suffers from slow kinetic rates. m ai nly bccause of th e low solubility of ox"Ygen in water and its relat ive inertness . ln the present work. the introdu ction, of a n organic compound. n a mely an thra quinone-2-sulphonic a cid (AQ) . is proposed to improve thc clissolution kinetics during the gold cyanid a tion. Th e m ajor advantage of t h is is that the reagent is able to act as a n additiona l a nel recyclablc ox idi zing agent. with the result that it n ot only increases the solution rates. but also its co nccn tration remains constant throughout the process. playing thus the role of a catalyst. Results from experiments carried out with a rotating gold disc are also shown anel a mechanism to explain the role of the catalyst on thc gold dissolution is suggested.

CIANETAÇÃO CATALISADA DO OURO

Cianetação é o p rocesso mais largamen te utilizado para extração de ouro de seus min érios. Nest a operaçã o. a n eccessári a oxidaçã o do ouro é normalmente efetuada pelo oxigénio d o ar ou por injeção direta de oxigénio puro na polpa. Sob muitas circunstâncias. no entanto, o processo ele cianetação apresenta baixa taxa de dissolução. principalmente por caus a da pouca solubilidade do oxigénio na água. O presente trabalho propõe a introdução de um composto orgânico (antraquinona . AQ) a fim de aumentar a taxa de dissolução do ouro durante o processo d e cian etação. A principa l vantagem desta adição é qu e este reagente é capaz de agir como um agente oxidante adicional e reciclável. com o resultado que ele náo somente aumenta a taxa de dissolução de ouro. mas também su a concentração permanece constante durante todo o processo . caracteriza n do p ortanto o papel de um catalisador . São apresentados ta mb ém resultados de ensaios feitos com um dis co r otativo e um mecanismo capaz de explicar o papel desempenhado pelo catalisador é s ugerido .

1 Imperial College , Dept. of Mineral Resources Engineering 2 Imperial College, Dept. of Mineral Resources Engineering, Prince Consort Road. London SW7, 2BP, U.K.

3 4

INTRODUCTION

Most of the gold ores processed today are trea tccl by the cyanidation

process. a techniquc known sincc the cnd of thc last ccntury. when in

1887 a British p atcnt was granted to John Steward MacArthur[l] . Th e

discovery. named "Process of Obtaining Gold a nd Silver From Ores". soon

· bccame known as the cyanidation process following its success ful

industrial application . With the use of cyanid e (KCN or NaCN) in th e

ext ractive metallurgy of gold . the world output of the metal increased by

more than four times in the period from 1901 to 1950 compared to th c

period from 1851 to 1900[2]. Throughou t this century. much

investigation have bcen carried out in arder not only to achieve a deeper

understanding of the process as whole or increase its efficiency. bu t a lso

to look for alternative leaching reagents. such as thiourea, thiosulpha te .

bromine and thiocyanate, anel to overcome difficulties that have arisen

with the deple tion of high grade ores or the exploitation of the so-callcd

refractory gold ores (those which cannot b e easily extrac t ed by

conventional means)[2,3,4,5].

Some important reviews concerning the cyanidation process a nd its

main characteristics have been published[6,7,8,9). The basic aspects have

been established, and revealed that th e rate of gold dissoluti on is

controlled by the concentration of cyanide and/or oxygen in solution.

depending on their relative concentrations. and that the optimum cyanide

concentration is best attained by experimen t. The dissolution process is

considered to be an electrochemical reaction. in which the anodi c a nd

cathodic parts of the metallic surface are linked in short circuit. At the

anode the metal gives up electrons to the latter. with further formation of

the aurocyanide ion as

Au + 2CN- = Au(CN)2 - + e- 1

3 ')

\vhcrcas lhe rcduction of oxygcn to hydrogen pcroxidc takes place in the

cat.hodic arca.

2

lt is generally accepted nowadays that the dissolution. in aera ted

cvanicle solution takes place a ccording to the followtng cquations:

41\u + I\CN + 0 2 + 2H20 = 4KJ\u(CN)2 + 4KOH 3

21\u + 4KCN + 0 2 + 2H20 = 2KAu(CN) 2 + H 2 0 2 + 2KOH 4

2,\u + 4KCN + I-!20 2 = 2KAu(CN) 2 + 2KOH 5

Reaction 3 was first proposed by Elsner, whcrcas the other two are

clu e to Bodlander (thc sum of th ese, a s can r cadily be secn. gives the

single Elsner's equation)[l,IO].

ln order to form soluble cyanide complexes, the gold must be a lso

oxidized: this operation is normally performed bv introducing

comprcssed a ir to use oxygen as the oxidizing agent. Under most

circumsta nces thc dissolution of gold may be hampercd by slow kinetic

rates . or even stop altogether. mainly because of th e low solubility of

o:-.:y gen in water. This situation is aggravated at high a ltitudes , with

increasing temperature and during certain industrial practices. such as

hcap lcaching for instance, where the oxygen may be almost totally absent

toward the bottom of the h eap. Much research has been carried out to

overcome this difficulty. and suggestions include the Introduction of

o:-..-ygen in an already dissolved form, such as adding inorga nic peroxides.

or injection of compressed air where heap leaching is being

used[ll,l2,13].

ln the present work the addition of an organic compound, namely

3G

anthraquinone-2-sulphonic acid (AQ). is proposcd to improve thc kinctic

rate of thc gold dissolution during the cyaniclation proccss. Tlw major

advantagc of using this organic compouncl is that it can acl as an

adclitional anel rccyclable oxidizing agcnt. with thc result that it not only

may increase the rate of golcl dissolution, but its concentration rcmains

practically constant throughout the clissolution process. playing tlms the

classical role of a catalyst. An aclditional feature of this cyclic rcaction is

that, during the reoxidation of the rccluced AQ, thcre occurs the "in-situ"

production of hydrogen peroxide, which may also act as an appropriate

oxidizing agent for the process[11].

ANTHRAQUINONE: A CATALYST FOR THE GOLD CYANIDATION

Quinones are substances in which two hyclrogen atoms of the benzene

structure are replaced by two m .. 'Ygen atoms, as in the figure below:

o

6 8

Figure 1 - 1,4 benzoquinone molecular structure

The quinone structure plays an important role in theories concerning

the relationship of chemical constitution to colours. Anthraquinones

contain the two O:\.'}'gen atoms on an anthracene structure. as shown on

the following page:

'i ;I

I

37

7

6

Figure 2 - 9,10 anthraquinone molecular structure

Thi s is th c m os t importa nl quinon c clcrivative of a nthracen e and thc

"()urcc of a large cl ass of dycs a nel pigm cnts .

Wh cn aromati c hydrocarbons or th c ir dcriva tivcs are tre ated \vi th

I I ~S0_1 . th ey y icld s ulphonic acid s in whic h a h ydrogen of th e b en zene

llll Clcus is rcpl a ccd by thc sulphonic group SO)T.

o

Figure 3- 9,10 anthraquinone-2-sulphonic acid molecular structure

Anthra quinone s ulphoni c a cid s are important intermediates in the

prcparation of dye-stuffs. The s ulphonic a cid group renders the AQ soluble

in wa tcr a nd imparts it a n affinity for wool and silk.

lt is well csta b lis hed tha t thc most commonly used process n owadays

lor thc produc tion of hydrogen p eroxide is the chemical autoxidation of

anthraquinon e d crivatives. often rc fcrr ed to as the quinon e process for

hydrop;cn peroxide[l4,15]. Gcncrically. t he m a in reactions involved in this

]l!"Cl CC SS a re:

.18

-hvclro<rena t ion:

01 R R

+ Hz (G)

-oxiclation:

01

R R

+02 +H2~

While the possibility of using m any quinone-type compounds has been

mentioned. the use of products containing sulphonic acid groups is

prefcrred a nd commonly uscd industrially because of their relatively high

solubilities in water[15,16].

The diagram depicted in FIGURE 4 on the following page is the

supcrimposcd Eh - pH diagrams for thc system Au-CN-H2 0 2 -AQ in

aqucous solution. It m ay be secn that it is thcrm odynamically fcasiblc for

(7)

39

the Aq lo acl as an oxidizing agenl in thc Au-CW systcm over a pl-1 range

from approximalcly H.O upwards (th c diagram a lso discloses lhe region

wlwrc 1\Q. ll :/) 2 and A11(C N)2- may coexis t).

I h

t5

0.5

o. o

-o.s

- 1.0

ll CN

o 2 3 4 5 6 13 14 pH

Figure 4 - Eh-pH diagrarn for the system Au-CN-H202-AQ-~O at 25°C;

[H20~=10- 1M, [CW]=IO-IM, [Au]=I0-4M, [AQ]=IO-:Y.

Th e ove ra ll reactions involved with the gold dissolution in aerated

KCN and in lhe prcscnce of AQ as a catalytic oxidizing agent could then b e

writtcn as:

2Au + 4KCN + 0 2 + 2H2 0 = 2KAu(CN) 2 + H2 0 2 +2J\OH

2Au + 4ECN + H20 2 = 2KAu(CN) 2 + 2KOH

8

9

40

2Au + 4J<CN + AQ + 2H2 0 = 2KAu(CN) 2 + A;QB- + 2KOJ! + IJ+ 10

and thc oxid a tion stcp

J'J,.)H- + 0 2 + u+ = AQ + I I2 0 2 11

conlinuous ly gcneraUng bolh oxiclizing agents.

APPARATUS AND MATERIALS

ln ordcr to investigate thc use of AQ as a catalyst for thc cyanidation of

gold. a systcm consisting of a rotating clisc (1.4 cm dia meter) of pure gold

(99.9%). rotated by a precision speecl motor (Oxford Electrodes). was

choscn. This approach. for kinctic and mcchanistic studics. h as bcen thc

subject of cxtensivc investigations and its use incrcasing v.ridely[ 17 ,18] .

Bcfore each experiment. the clisc was carcfully polished with 1 .0~

diamond compound (Hyprez Five Star). using a silk cloth and a water

soluble polishing lubricant liquid. The gold disc with a mirror-like

appearance was then rinsed and immersed in a strong chromic sulphate

solution for about l minute: afteDvards the disc was rinscd again with

doublc distilled water before being used for an experim cnt. With this care.

rcproducibility could be achieved within 5% relative variation .

A volume of 250 ml reagcnt grade potassium cyanide was uscd in a li

tria ls. The salt was dissolved in double distilled deionized water and

solutions were freshly prepared shortly bcfore cach expcriment. Analytical

grade (BDH) 9.10-anthraquinonc-2-sulphonic ac id ("'silver salt"') (AQ) was

usccl as the organic catalyst. A stock solution of l5mM AQ in lmM I<OH

was prepared anel diluted accordingly to givc the concentrati on required

for cach expcriment. The reactor was a. 300 ml split-top. round-bottom

41

glass reac tor. n onnally uscd in hydromctallurg;i cal invcstip,a u on s of this

typc. FIGURE 9 on the last p agc of this work dcpi cts thc cquipmcnl u sed

for thcse tcs ts .

The cxpcrim cnts shown wcre carri ecl oul at a cons tan \ tempcraturc of

2 5°C with thc a id of a watc r b a th. Samples werc collccled cvcry 15

mi nutes and tcs ts werc n ormally run for 9 0 min utes . unl css s pccifica lly

o t.lw rwisc stal ccl. Thc di ssoluti on potcntial observed on th e gold surfacc

(a nel meas urcd aga inst a Ag-AgC! e lcc trodc) typi cally ch a ngcel from -

420mV to -3 70mV for a convenli onal ae ra tcd solution. and fro m <350mV

to -320rnV when AQ was add eel to th e lea ching soluti on : pH wa s kcp t

bctwecn 10.5 anel 11.2 . The a m ounl of ga lei di s solved w;~ s a nalysec! by

a tomi c a bsorption in a ll cases (Pcrkin Elmer 1100 B). a nd th c r csults

calculatccl accorcling to thc following cquation:

o

M t = Ct(Vt-1 - S t-tl+I SiCi t~t - 1

where

Mt is thc amount of gold diss olved at time t (mg)

Ct the- atomic absorption reading in time t (mg/1)

V1 the solution volume in the reactor in time t (!)

S1 the volume of lhe sample taken (I)

[1 )

In the eva lua tion of the ra tes obtainec! experimenta lly. the use of this

equ a tion is important to make allowa nce for the decrcase in volume at

each s ample withdrawal.

~I li 'I ,j

,_·_ .. _ .. j·.:·. !•

., l ~~:·

4 2

RESULTS

Thc dissoluUon of go ld against time in 0.0:\M KC N is s hown in

1.-lGUI\E 5. The linearity of Lhe plot of gold cli ssolvcd anel time is an

indication that the surfacc ::1rca of thc gold disc rcmain cd constant

ihroughoul thc lcaching trial. The gold surfacc was cxamincd

microscopically at thc cncl of cach tcst. a nel no indi ca tion of uncvcn attack

or pitling was obscrvcd. The rale v\'as calculatccl from th c s lope of thi s

t.vpc of pl ot. All thc r es ults coulcl b e fiUccl by a straight lin c. with a

co rrclation factor cqu a l to or greatcr than 0 .980 in mo st cases . The

avc ragc rate value of thc rcproclucccl trials under 0 .031\:1 1\C:'\ anel rotation

spccd of 10Hz reached 2.R6 x 10 ' 1 mol/cm 2 .s

5

4 Ol

E rn 3 rn

'O

::J 2

<(

0 ~~-L--~----~--~--~--~----~--~--~--~J

0.0 0.5 1.0 1. 5

time (h)

Figure 5- Gold dissolution in cyanide 0.03M from a rotating disc (10Hz;

p02=2.13 x 104 Pa).

Thc effect of additi ons of AQ on the rate of gold clissolution in a

normally aerated 0.03M KCN solution is shown in FIGURE 6 on the

following page:

~ N

E u

o E

43

4.5.----------------------------------------------, c

4.0 ····C · ·· .

3 .5 ,, ,,, , ,, .,_ .. , ... - ----· ·------------- ---------· ---··-- ----- ··----- ------ --- - -------- --------c

2.oL-~--~--~--~--~~--~--~--~--~--~~--~

0.0 1. 0 2.0 3.0 4 .0 5.0 6 .0

AQ (mM)

Figure 6- Gold dissolution rate variation with AQ additions (KCN 0.03M;

p02=2.13 x 104 Pa)

f'rom thesc rcsults . it may bc sccn that an increase of up to 50% is

observcd . when 3.0mM AQ is prcscnt in the leaching s olution. Above this

i\Q conce n tration thcre is no further in crease in the rate of gold

clissolution.

The rate depcndence on KCN concentration at 3.0mM AQ is depicted

in FIGURE 7. It may be scen that for the conventional system (air flowing)

thcre is no improvement in the rate if the cyan id e concentra tion is

incrcased bcyond O.OlM. On the other hand. when 3.0mM AQ is presen t

in the same system. i. e .. when a supplementary oxidizing agent is added,

the rate increases wiU1 cyanide concentration up to 0.03M. This indicates

lhat with a supplementary oxidizing agent in a O.OlM cyanidc solution,

thc rate turns to bc controlled by the availability of cyanide.

As may b e secn from the results. the effect on the rate of gold

di s solution obt.ainccl with add itions of AQ were encouraging . Thus, in

ordcr to further invcstigate t11e action of AQ itself. tests were also

4 ·1

pcrformcd undcr oxy_t;cn-free atmospl1crcs. \Vhilc cvidcnt. it is import;1nt

to note that. 1mdcr thcsc circumstanccs. Aq is thc sole oxiclizin,L( a,t;cnt for

t h c procr:ss.

())

o

40

<f) o N

E u

2.0 o E

~ o

o 00

• • ----------· -----·~--------------

•

D o o

D D

o J (no AO)

• J (AO 3.0mM)

O. O 1 o 02 o 03 0.04 O. 05 O OG

KCN (M)

Figure 7 - Gold dissolution rate dependence on KCN and AQ

concentration (p02=2.13 x 104 Pa; f=lOHz).

Experimcnts With No Oxygcn

FIGURE 8 shows thc dissolution of gold in an aqueous cyanide

(I\.CN=O.O 1M) solution at four cliffcrent AQ conccntrations. An

introductory trial was carricd out uncler nitrogen but with no AQ to test

for any oxygen Jcakin,o; into thc reactor: in this experimental conclition. as

expeetcd. no cleteetable golcl was dissolvecl.

It may be seen from thc figure that the amount of golcl clissolution

increases with thc AQ concentration. up to a maximum bcyonc! which the

cffect starts to influcnce in thc oppositc way (i.e .. Jcss golcl is clissolvecl

with 12.0 mM AQ than with 6.0 mM AQ). llowcver. thc dissolution of gold

in thc complete abscnce of oxygcn inclicatcs that thc oxidation reaction

nc ccss;u·y fo r th c proccss was ca r ricd o ut l>v t h c 1\Ç) . Thi s may \)('

rcprc sc ntcd bv

wllich m c:an s that. if 1\.Q is p rcs c n t in th e cyan id c sys tcm. the d is s olution

of _t.; ol cl is p ossiblc. cvcn in t h c com p lete a bscncc of oxygcn . This is a n

in tc res tin ,t.; f:lct indccd. for it s h ows for llH' fi r st tim e tha t an oxidi zing

a _r.;c n t othcr th a n oxygcn o r in or,c;anic p c roxic!es is poss i b le to bc u sed in

th c cyan id ali on of ,t.;ol cl.

4.0

Õl -o--- AO (0 30 mM)

E 3. 0 AO (300mM)

-o A0(6.00mM)

(!) A0 (1 2.00mM) > o 2 .0 (/) (/)

-o

:::> <t 1.0

o o 0. 0 0 .5 1 .O 1 . 5

tim e (h )

Figure 8 - Amount of gold dissolved, under an oxygen-free atmosphere

and different AQ concentrations (KCN=O.OIM; f=lOHz).

Und e r t h c n o rmal a e ra t cd s ituati on. it h a s bee n s een that the

rcoxi dati on of thc reduccd form of AQ . the s o-callccl a nthraquinol. leads to

th c "in- s itu" production of h ydrogen p eroxid e. In the present case th e

rea ction may be rcpresented by

13

4 (,

Ilyclrogen pcroxiclc is a strong cnough oxidant to oxiclizc golcl anel

promotc th e formation of thc soluble complex Au(CN )2 -

2Au + 2CW + 1!})2 = 2Au(CN) 2 - + 20Ir 14

Equations 12 . 13 anel 14 show that AQ is not only capable of oxidizing

gold in a cyanide solution but also that it may be continuously regenerated.

Tbe AQ conccntratiol1 was monitorcd after a11d during some experiments

(through on-li11e UV -visible spectroscopy) a11d no variatiol1 could be

detectcd. showing that its co11centration is incleccl kept constant

t:hroughout the lcaching process. This confirms the catalytic role played

by thc AQ cluring the cya11idatio11 of gold. since. as sec11 earlier. thcre has

bee11 an increase in the ra te of dissolution of gold whcn this reagent is

addcd to the leaching solutio11.

Moreover. it should be poi11ted out that while thc rate enhanceme11t

observcd in these tests may be attributed to the system outlined above.

this outcome is probably better u11derstood by examining a mechanism of

molecular adsorptio11 011 the metal surface. The charactcr of the reactions

taking place 011 thc gold dissolution in the leaching system comprised of

cyanide + m,:ygen + a11thraquinone is said to be probably regulated by the

adsorptive n ature of each one of these components[l9,20,21] . This would

characterize a picture of "competitive adsorption"[22], where the

rcacti11g m olecules would displace each other from sitcs on the gold

surface. Such a mechanism might possibly to explai11 the results obtained

where the benefit caused by the addition of AQ shows an optimum, beyond

which either no further enhanceme11t is achieved (Figure 6) or a decrease

in the rate actually occurs (Figure 8). This aspect of the system is

currently und er investigation anel will be published shortly.

47

CONCLUSIONS

1\cconlin g to th e currcntly a cccpted mechanism of gold cyanidation

a nel the proclu ction of hyclrogcn pcroxidc. and from the experimental

rcsults obtaincd in this work it may be concluclcd that: 1-the rate of go ld

clissolution in thc cyanicbtion proccss may be clefinitcly enhanced by about

50'Xl with th c aclclition of 1\Q . at con ce ntration up to 3.0mM in a 0.03M

f\:CN soluUon; 2-this cffcc t is noticecl up to a ccrta in AQ conccntration.

bc:y·ond which no furt.hcr cnha nce mcnt is observed: 3 -t.ests performed

und cr an oxygcn-frcc atmosphcrc dcmonstrated tha t 1\Q is ind ced ablc to

oxicl ize gold in a dilut e cyanide solution: the gold m ay be then dissolved by

lorming the auro -complcx . 1\part from oxygen anel hydrogen peroxide,

t hi s was thc first a ltcrnativc (anel recyclable) oxiclizing agcnt that was

s u cccssfully uscd in thc cya nidation proccss: 4-under the convcntional

a era tcd situation. thc rcox.icla tion of the reduced AQ is rca dily achicved by

o:-.:ygcn . with thc rcgencra ti on of the original AQ anel simultaneous "in­

s itu" production o f hydrogcn peroxidc; 5-the oxidizing effect of AQ is,

therefore. a cyclic one wit.h the continuous produced hydrogcn peroxid e

a lso bcing able to act a s an additional oxidizing a gent: 6-while the rate

enhan cement m ay bc attributcd to the presence of an additional o:xidizing

agent in the systcm. th c mechanism is probably better understood by

exa mining the adsorptivc n a ture of the reactions occurring on the gold

surface during cyanidation: 7-the addition of AQ to the gold cyanidation

process may prove intercs ting in some extreme situations such as at high

altitudes. with increa sing temperature. and during h eap leaching, for

instance. where t.he dissolution of gold may b e hampered by slow rates.

m a inly b ecausc of th e dccrcasing o>..-ygen solubility in water, or even stop

altogcther due to thc complete absence of o>..-ygcn toward the bottom of

the heap.

48

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4 ')

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5 0

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21. Zolotova. T.K. et ai.. Kinetics and mechanism of electroreduction of

anthraquinone-2.6-disulphonic acid. Elelctrokhimi!Ja. 1211. 9

(1973).

22. Laidler, K.J., The kinetics of electrode processes. J. Chem. Educ.,

600. 47 (1970).

nitrogenia1r iniE't

Figure 9 - Scheme representing the equipment used in this work: 1-rotating disc; Ag-AgCI electrode; 3-frit

4-nitrogen outlet; 5-thermostat; 6-rotating disc speed controller; 7 -water bath.

'-"-