carbon in pulp process

~::'.'

t..'I~",~

~

Synopsis

one qfthe main bottle-necks in the carbon-in-pulp process for therecovery qf gold is theeludon procedure, whichtypicallY requires theheadng qf causdccyanide eluants to hightemperatures for 16 to24 hours. The presentwork demonstrates thatsodium sulphide soludonas an alternadve eluantcan dfect completeeludon in about 4 hoursat ambient temperatures.

Eludon dfidendes qfaround 100 per centwere obtained in 4 hourswith a single pass qfeluant containing0,2 M Na2S and0,4 NaOH~ut 10bed-volumes qf eluantThe inidal rate was slowover thejirst hour qfeludon, probablY becausethe acdvated carboncatalYsed the oxidadonqf sulphide topolYsulphide. Eludondfidendes qf around100 per cent were alsoobtained in less than4 hours during the batcheludon qf carbon atliquid-to-solid rados qfabout 100. Lowerliquid-to-solid rados resulted inthe re-adsorpdon qfgold,probablY owing to theoxidadon qf sulphide topolYsulphide, with theresultantfonnadon qfgold complexes that wereeluted less readilY.

.Mintek,PrivateBagX3015, Randhurg2125.

@ The South 4frican

Institute qf Mining and

Metallurgy, 1994. SA

ISSN 0038-223X/3.00

+ 0.00. Paper received,Sep. 1993; revised paper

received, Mar. 1994.

The elution of gold from activated.

carbon at room temperature usingsulphide solutionsby M.D. Adams*

Introduction

The carbon-in-pulp (CIP) process is the mostwidely used method for the recovery of gold in

new plants. However, despite the commercialsuccess of the process as a whole, there remains

much scope for improvement. One such area is

the elution of gold cyanide from loaded carbon. In

the Zadra process!, a hot solution of causticcyanide (typically comprising about 0,5 per cent

NaCN and 1,0 to 2,0 per cent NaOH) is

recirculated through an elution column andseveral electrowinning cells in, series. More recent

modifications to this processz employ pressurized

vessels or steam heat-exchangers to produce thehigher column temperatures (about 130°C)

required for more efficient elution. Between 16

and 24 hours are typically required before anacceptable degree of elution is attained, and this

not only represents a bottleneck in the CIPprocess, but also involves a large consumption of

energy and chemicals (cyanide has been shown

to decompose rapidly under these conditions3). Itis for these reasons that the recent trend towards

low-cyanide elution has arisen.

A popular alternative to the Zadra process

was developed by the Anglo American Research

Laboratories (AARL)4, in which pre-treatment of

the carbon with a hot solution of caustic cyanide

is followed by elution with hot de-ionized water.However, the total time required for an

acceptable degree of elution is similar to that for

the Zadra process.

Recent efforts5 have been directed towardsthe development of elution procedures thatemploy organic solvents such as acetonitrile or

ethanol. which have been shown to result in

efficient elution in about 8 hours at temperatures

between 25 and 70°C. The Micron distillationprocedure6 involves the refluxing and recycling

of hot solvent vapours and condensates through

a bed of carbon that acts as a fractionatingcolumn. The solvent elution procedures suffer

from the disadvantage that the solvent vapours

are invariably toxic, and may also represent a

fire hazard. The mining community is therefore

reluctant to employ these procedures on a largescale.

The Joumal of The South African Institute of Mining and Metallurgy

Early work by Feldtman7 and Gross andScott8 indicated the potential of using sodiumsulphide for desorbing gold. In 1950, Zadra9, ina pilot-plant study, demonstrated the technicaland commercial viability of sodium sulphidesolutions for the elution of gold at ambienttemperatures. He showed the carbon to be elutedefficiently in 4 hours at 25°C, and both thecarbon and the eluate were found to be re-usable. There are several reasons why thisprocess is not in use today. One is that adsorbedsilver and base metals are not eluted, beingimmobilized as insoluble silver and base-metalsulphides. However, this may not be a greatproblem, since an equilibrium situation may bereached. The other problem is that the elutedgold is sometimes re-adsorbed onto the carbon,and it is this aspect in particular that this paperaddresses.

Experimental Procedure

Approximately 1,1 kg of activated carbon (LeCarbone G210 AS) was screened to a particlesize of +1,16-2,07 mm and thoroughly washedwith de-ionized water to remove fines andsoluble impurities. It was then contactedovernight with 1,44 g of KAu(CN)z in approxi-mately 1 litre of de-ionized water to achieve therequired gold loading.

Batch Elution Experiments

In each experiment, sulphide eluant was freshlymade up by the use of NazS'9HzO. Typically,S gof loaded carbon was contacted with theappropriate volume of solution in a round-bottomed magnetically stirred flask with refluxcondenser for 48 hours. The solution wassampled periodically and analysed for gold byatomic-absorption spectrophotometry (AAS).The carbon was assayed for gold at the end ofthe experiment by X-ray fluorescence (XRF)analysis.

AUGUST 1994 187 ....

Elution of gold using sulphide solutions

Improved rates Q/elution were obtained athigher sulphide concen-trations and pH valuesgreater than about 13.Higher temperaturesincreased the initialelution rate, but loweredthe overall extraction~dency, probablYbecause Q/thedeposition Q/ elementalgold on the carbon.Variation Q/the ionicstrength by the additionQ/ NaCl had no dfect onthe elution, whichco'lfirms that the elutionmechanism in the caseQ/ sulphide is different

from that when cyanideor I1)1droxide is used asthe eluant.

It is proposed thatthe elution Q/gold bysulphide solutionsproceeds by means Q/ aninitial step that involvesthe reaction Q/polYsulphide ions withthe adsorbedaurocyanide spedes,

forming AuCN on thecarbon and thiocyanatein solution. This step is

followed by theformation Q/poorlYadsorbed complexeswith sulphide ions, suchas AUS32-The presenceQ/polY sulphides,whether generated insitu by the catalYticoxidation dfect Q/activated carbon or bythe addition Q/elemental sulphur,reduces the elution rateand ~dency drama-ticallY. This is probablYdue to theformation Q/complexes such as AUS"and AUS:;, which have ahigh adsorption qlJlnity.

Single-pass Elution Experiments

The procedure was identical to that described byAdams and Nicopo in a study of the elution ofgold from activated carbon using cyanide andhydroxide solutions. Fresh eluant was pumpedinto a reactor similar to that used for the batchexperiments, which contained a fIXed mass ofcarbon and volume of solutions. The eluate wasremoved by pumping from the top of the solutionat the same rate. The eluate fractions werecollected and analysed for gold by MS. In someinstances, the DV-VIS spectra of eluate fractionswere measured with a Beckman MIV DV-visiblespectrophotometer. Unless specified otherwise,the conditions for the elution were as follows:

Temperature 25 :!: 3°CMass of carbon 26,7 g (75 ml wet-settled)Flowrate 200 mllh

~~~ ~2~

Silver-stripping Experiments

A batch of activated carbon was loaded to1000 g/t of gold and 200 g/t of silver by amethod similar to that already described. Thecarbon then underwent a batch elution by theuse of 0,2 M Na2S at 25°C. After 6 hours, thecarbon was filtered off and analysed for silver byXRFanalysis.

Results and Discussion

Speciation Calculations for Po/ysulphideSolutions

The relevant reactions and stability constants forsulphides and polysulphides in aqueous solutionare listed in Table I. These 'best' values wereselected from the most recent literature on thesubject1H3. No stability constants for the S~-and

higher species were available, and these aretherefore not included.

Stability constants for polysulphides in aqueous solution

Reaction

2W (aq) + S2- (aq) H H2S (aq)

W (aq) + S2- (aq) H HS-(aq)S2- (aq) + S H s~- (aq)

S2- (aq) + 2S H s;- (aq)S2- (aq) + 3S H s~- (aq)

S2- (aq) + 4S H s;- (aq)

W (aq) + s~- (aq) H HS~ (aq)

W (aq) + HS:; (aq) H H2S4 (aq)

W (aq) + s;- (aq) H HSs (aq)

W (aq) + HSs (aq) H H2S5 (aq)

W (aq) +OW(aq) H H2O

~ 188 AUGUST 1 994

Distribution diagrams for the species wereprepared fromcalculationsgenerated by theHALTAFALL programme14. The species distri-bution for a 0,1 M Na2Ssolution is shown inFigure 1. The predominant species are H,S (aq)at pH valuesbelow7 and HS- (aq) at pHvaluesabove 7. It is only at pH values greater than 13that a small fractionof S'- (aq) becomes evident.

The situation is somewhat similar in the caseof a 0,1 M Na,S2 solution, as shown in Figure 2.Smallfractionsof S~- (aq) species becomeevident only at pH values greater than 13. It isinteresting to note that, at pH 14, the fractions ofS~-speciesincrease in the order

S2- < S;- < S~-< S~-< S~-« HS-.

The protonated HS4, H2S4'HSs, and H2Ssspeciesare present to only a negligible extent.

When the solution under considerationcontains 0,2 M Na2Swith 0,6 MS, Le.with astoichiometry equivalent to Na2S4'the S~-speciesbegin to play a more predominant role, as shownin Figure3. Abovea pH valueof 12, thesespecies are present in significant amounts, withfractions increasing in the order

S2-< S~-< S~- < S~- < S;- < HS-.

At a pH value of 14, the S~-species constituteabout half of the total sulphide species insolution.

Batch Elution with Sulphide Solutions

Effect of Polysulphides at Low Liquid-to-solid Ratios

Zadra9 has reported that, during an elutionwithsodium sulphide, gold was initially re-adsorbed,followedby a slow release of gold from thecarbon. Noexplanation for this behaviour wasgiven. In the present work, batch elutionexperiments were performed with

(a) a fresh solution of sodium sulphide madeup from Na2S.9H2O

(b) a polysulphidesolution of average Na2S2composition, made up with a stoichio-metric mixture of Na2S.9Hp andelemental sulphur.

Figure 4 shows that a trend similar to thatreported by Zadra occurred in the batch elutionof 5 g of carbon loaded to 12 800 g/t of goldusing 50 ml of solution. The effect wasevenmore enhanced when a polysulphide solution ofaverage 0,1 M Na2S2 compositionwas used.


0,01

0,008

0,008

0,004

0,002

~o 11 12pH


Fraction of total soluble sulphide1

0,8

0,6

0,4

0,2

00

0,01

0,008

0,006

0,004

2-

qo 13 14

Figure 1-Distribution of sulphide species in the S2- -H2O system with variation in pH value(0,1 MS"

11 12pH

52-

\2 6 12 148 104

pH


0,8

0,6

0,4

0,2

H5

Figure 2-Distribution of polysulphide species in the s-s2--H2O system with variation in pH

value (0,1 M S~-)

5'.S';

° °

5~-\

2 4 6 8 10 12 14pH

The Journal of The South African Institute of Mining and Metallurgy

Effect of Liquid-ta-solid Ratio

Further light may be shed on the re-adsorptionphenomenon by consideration of the effect of theliquid-to-solid ratio on the batch elution of goldfrom activated carbon using fresh sulphidesolutions. The results in Figures 5 and 6 showthat re-adsorption occurs when the liquid-to-solid ratio is low, and not when the solutionvolume exceeds about 200 ml for a 5 g batch ofloaded carbon.

Effect of Na2S Concentration

The effect of Na2Sconcentration on the batchelution of gold from activated carbon at highliquid-to-solid ratios is shown in Figure 7.Complete elution was achieved in about 1 hourfrom a 1,0 M Na2S solution, and in 36 hoursfrom a 0,1 M Na2S solution. No re-adsorptioneffect was evident under these conditions.

Figure 7 shows that re-adsorption occurredafter 36 hours when 0,01 Na2Swas used, whichsuggests that the re-adsorption phenomenon isrelated to the ratio of activated carbon tosulphide concentration in the solution, as well asto the liquid-to-solid ratio. The fact that the pHshifts slightly is probably also a factor. It isevident that re-adsorption is associated withsmall amounts of sulphide present in relation tothe amount of gold in the system. No elutionwas obtained in the case of 0,001 M Na2Ssolution. Possibly a greater degree of oxidationof sulphide to polysulphide occurs when theinitial sulphide concentration is low:

2 HS- (aq) + 1/2O2H H2O+ S~-(aq). [1]

This type of reaction occurs slowly 15 when

alkali sulphide solution stands in air. Activatedcarbon is known16,17to catalyse the reaction,with the ultimate formation of products such aselemental sulphur and S20;~ This is in directagreement with the observations made duringthe present experiments: that the initiallycolourless Na2S solutions rapidly turned yellowin colour owing to the formation of intermediatepolysulphides18 (S~-to S~- are yellow in colour,S~- to S~-are orange, and S~- to S~-are red). Theyellow colour tended to disappear after about 1to 2 hours of reaction, which suggests that eitherthe carbon eventually reduces the polysulphideback to sulphide, or that elemental sulphur wasdeposited on the carbon surface in a secondaryreaction, e.g.

S~-Activated

Hcarbon

[2]Sads + S2- (aq).

AUGUST 1994 189 ~



0,8

0,6

0,4

0,2

°° 2 6 10 124 8pH

Figure 3-Distribution of polysulphide species in the S- S2--H2O system with variation in pHvalue (0,2 M S;-J

400

INB~1M NB~1MI

300

These reactions are substantiated bymeasurements of the visible spectra of various

solutions. Giggenbach19 measured such spectra

and assigned bands to individual polysulphides,

as detailed in Table Il.

Table 11

Visible absorbance bands due topolysulphide ions in aqueous solution

(after Giggenbach 19)

Wavelength, Ilm

229

250

358

417; 303

368; 303

375; 299

0:::~E

15

'5200~.5"Cl"0Cl

100

\ /. //

\ /

\ /

\ //...

00 20 30

rlmB, h40 50

The peak maxima of the various solutionsmeasured in the present study are shown inTable Ill. Large amounts of polysulphides werefound to be present in Na2S+ S mixtures, partic-ularly after being boiled. No evidence forpolysulphide species was found in fresh 0,1 MNa2Ssolution; however, some polysulphides,mainly S~~were detected in the solution after ithad been exposed to the atmosphere for 1 week.

Polysulphides were detected in 0,1 M Na2Ssolutions that had been contacted with activatedcarbon for periods of up to 24 hours. However,no polysulphides were found to be present after96 hours.

In the eluates of batch sulphide elutions withand without gold present, polysulphides weredetected after 2 hours, but not at 24 hours and48 hours, which is consistent with reactions [1]and [2].

Despite the re-adsorption that occurs undercertain conditions, a 95 per cent elution of goldfrom carbon at room temperature in less than1 hour in a batch system is a significantimprovement on that achieved under comparableconditions with the conventional caustic cyanideeluant.

10

Figure 4-Effect of polysulphides on the batch elution of gold from activated carbon at Iow

liquid-to-solid ratios

~ 190 AUGUST 1994 The Joumal of The South Afncan Institute of Mining and Metallurgy


60L:S 40.L:S 10.

50L:S 5

...

40

'#.>.ucell'u=

30CIIc0'Siii

~.-----..~-

/

;'I

/

""/

20

10

-------00 2 4 24

Time,h48

'#.

~ 60ell'u!i

.~40

IiJ

100

[Na~J

1,0 M(pH 13,51

.-- -,--- -- - - - "~:"

;

,.;'

,0,1 M

(pH 13,01.

0,01 M(pH12.51

...

,;

80 ,,,..

0,001 M(pH 11,01

0

20

_/-~--~- \

\\\\

\\\,\\

00 4 24

Time, h482

Figure 5-Effect of liquid-ta-solid ratio on the batch elution of gold fromactivated carbon at high initial gold loading

Figure 7-Effect of Na2S concentration on the batch elution of gold fromactivated carbon

60L:S 40.

60",,

//

/,.'I

II

II

II

II

II

II

II

II

I

L:S 10.L:S £>

...

40~

~i'u 30~c

~iD

20

------

10

00 4 24

Tlme,h482

Effect of pH Value

The form in which the sulphide is presentdepends on the pH value of the solution, asshown in Figure 1. The free S2-species is presentto any significant extent only at values above13; the HS- species predominates between pHvalues of 6 and 13; and the H2Sspecies predom-inates at pH values below 6. The effect of eluantpH on the efficiency of gold elution is shown inFigure 8. A high degree of elution is achievedonly at pH values above 13, which suggests thatit is the free S2-ion that reacts most readily withthe adsorbed aurocyanide species. This result isin contrast to those obtained by Green et al.20 ona different system-the elution using thiourea ofgold that had been adsorbed as aurocyanide ontostrong-base ion-exchange resins. In that work2O,it was found that acidic solutions were necessaryto achieve acceptable elution efficiencies. In thatinstance, acid destabilization of the Au(Cn);complex is necessary in order to replace thestrongly held cyanide ligand (log ~2Au(Cn); =39,7) with the less strongly held thiourea ligand(log ~2 Au(SC(NH2Jzn = 23,3):

Figure 6-Effect of liquid-ta-solid ratio on the batch elution of gold from activated carbon

at Iow initial gold loading

The Journal of The South African Institute of Mining and Metallurgy AUGUST 1994 191 ~

80

*'.;.u 60cGI'u!EGIC0'5 40W


Table 11/

Visible spectral data for sulphide and polysulphide solutions

.v-very strong, s-strong, w-weak, vw-very weak

100~---.

I~"--'"~fr

-~--- /-~ /

-~- ///

II

/.I

.I.I

I/

/.I

pH 14.pH 13.pH 9

A

pH 50

20

00

A- - - - - - - - - - - -

_0 "

,,.A.---;:,---

4 24 48Time, h

2

Figure 8-Effect of pH on the batch elution of gold from activated carbon

~ 192 AUGUST 1994

vw

Au(CN)2 + W + H AuCN + HCN [3]

AuCn + SC (NH2h H NCAuSC(NH2h [4]2NCAuSC(NH2h H Au(CN)2 + AU(SC(NH2h);. [5]

The stability constants for gold complexes

with several sulphide ligands were measured,

and these complexes were found to have a muchhigher degree of stability than the gold thioureacomplex21 (Table IV), which is consistent with

the notion that gold sulphides may form insulphide eluate solutions.

Seward21 and Renders and Seward22 have

demonstrated the predominance of the Au(SH);complex between pH 4 and 10, and the

increasing predominance of species such as

Au2S;at pH values greater than 10 (Figure 9

and Table IV). However, this is not the onlypossible species present in the alkaline region

since the data were scattered and somewhatambiguous in that region21. Other complexes thatmay be present under alkaline conditions21

include Au2S~~ AU2SHS32-,AUS32-,Au2(SHjzS2-,AuSHS2-, AU2(SH-3, AU2SHS-. and AuS-. The

presence of multi-charged gold complexes such

as AuS~- in alkaline sulphide solutions is

therefore consistent with the present experi-

mental results.

It is proposed that the elution of gold bysulphide solutions proceeds by means of an

initial step that involves the polysulphide ion,expressed as S~- for clarity:

Au(CN); + S~- H AuCN + SCW + S2-. [6]



Acknowledgements

This paper is published bypermission of Mintek. Theexcellent technical assistanceof Mr C. Tolken, Mrs O.L.Wellington, and Ms K.T.Wintle is gratefullyacknowledged.

References

1. ZADRA, J.B.. ENGEl,A.L., and HEINEN,

H.J. A process for recovering gold

and silver from activated carbon by

leaching and electrolysis.

Washington, US Bureau of Mines,

R/4843.1952.32pp.

-3log mAu, t

-4

-5ms,t =0.0125 °C

-6

-7

-8

-92 4

Table IV

Stability constants for several Au(l) complexes at 25°C (after Seward2')

II

6

Figure 9- The solubility of Au2S at 25'C as a function of pH (after Renders and Seward22)

pH

2. BAILEY,P.R. Application of activated

carbon of gold recovery. The

extractive metallurgy 91gold in

South Africa. Stanley, G.G. (ed.).johannesburg, South African

Institute of Mining and Metaliurgy,1978. pp. 379-614.

3. ADAMS,M.D. The chemistry ofcyanide in the extraction of gold. H.

Mechanisms of cyanide loss in the

carbon-in-pulp process.;'5.IJ'r.ll1$t.MinMetall.,vo!.90.1990.pp. 67-73.

4. DAVIDSON, R.J., and DUNcANsoN, D.

The elution of gold from activated

carbon using deionized water. Ibid.,

vo!. 77. 1977. pp. 254-261.

5. MulR,D.M. HINCHUfF<, W.D.,

TSUCHIDA,N., and RUANE,M. SOlvent

elution of gold from CIP carbon.Hydrometallurgy, vo!. 14, no. 1.

1985. pp. 47-65.

6. MUlR, D.M. HINCHUffE, W.D., and

GIDfAN, A. Elution of gold from car-bon by the Micron solvent distil-

lation procedure. Hydrometallurgy,

vo!. 14.1985. pp.151-169.

7. FELDTMAN,W.R. The precipitating

action of carbon in contact with

auriferous cyanide solutions. TriJllS.

ll1$m. Min. Metal/., vo!. 24.1914.

pp.329-371.

This mechanism is substantiated by evidencefrom current work for the presence of AuCN oncarbons that have been eluted by sulphidesolutions, and by the presence of SCNin theeluates as measured by ion chromatography.

The elution behaviour of gold depends onthe relative concentrations of sulphide andpolysulphide ions in solution. In the presence ofa small amount of polysulphide, it is postulatedthat gold is converted into a multi-charged anionthat has little affinity for activated carbon, and iseluted:

2 AuCN + 2 S2- H AuS~- + Au(CN)z.

When a large amount polysulphide ispresent, the gold is proposed to be converted intoa singly charged anion that has a relatively highaffinity for activated carbon, and is re-adsorbed:

AuCN+ 2 S~- H AuSz+ SCW + S2-. [8]

[7]


Complex log ~

AU2S~-Au(HS)2

AuHSo

AU(S20:J~-AuCH2N(SNH2)~

41,1

30,1

24.526,0

22.2

The type of re-arrangement reaction,represented by [7] and [8], between gold

cyanide and another ligand has been

demonstrated previously for thiosulphate23:

AuCN + S205-H NCAuS205- [9]

2 NCAuS205- H AU(S203)~- + Au(CN)z [10]

and for thiols (RS-)24:

Au(CN)z + RSH H RSAuCW [11]

2 RSAuCN- H Au(RS)z + Au(CN)z. [12]

Effect of Initial Gold Loading on Carbon

The initialgold loading on carbon does not havea great effect on the elution efficiency, as shownin Figure 10. However, high gold loadings ofabout 10 000 to 50 000 g/t depress the elutionslightly when compared with an initial loading of1000 g/t. This is consistent with the resultsshown in Figures 5 and 6, and confirms thathigh molar ratios of gold to sulphide depress theelution of gold.

Effect of Polysulphides at High Liquid-ta-solidRatios

The effect of polysulphides on the elutionefficiency at high gold loadings (12800 g/t) andrelatively low liquid-to-solid ratios (about 10)are shown in Figure 4. A comparison of theseresults with those obtained at low gold loadings(1245 g/t) and high liquid-to-solid ratios (about100) is interesting, as shown in Figure 11. Thereis now no evidence of re-adsorption from0,1 M Na2S solution with no added polysul-phides. The re-adsorption effect in the presenceof polysulphides is still evident at high liquid-to-solid ratios but is much less marked, with the re-adsorption equilibrium predominating only after48 hours, as compared with 1 hour in the lowliquid-to-solid experiment.

AUGUST 1994 193 ....


100

80

~>-uc.1u

is'5iD

60

40

20

00

,..---.

Initial Au oncarbon, g/t

49000.

Effect of Na~ Concentration

As observed previously, the rate of elution isenhanced at higher sulphide concentrations. Atlower sulphide concentrations, the elution is veryslow, as shown in Figure 12. After a time that isapproximately inversely proportional to the Na2Sconcentration, there is a sudden and dramaticincrease in the elution rate. For example, with0,05 M Na2S, this increase occurs after 2,3hours; with 0,1 M Na2S, after 1,2 hours; andwith 0,2 M Na2S, after 0,5 hours. The batchelution experiments showed that, at Iowsulphide-to-gold ratios, there is a predominanceof polysulphide in solution, and the re-adsorption equilibrium [9] predominates. Whenthe polysulphide concentration has beensufficiently lowered by adsorption onto theactivated carbon, reaction [2], there is asignificant amount of sulphide ion in solution,and the elution equilibrium, reactions [7] and[8], predominates. The application ofuv-visiblespectrophotometry to the eluates revealed thepresence of polysulphides (absorbance at295 nm) in regions of high elution rate. Theseresults are consistent with the batch results, andcorroborate the hypothesis that polysulphidesare associated with re-adsorption effects. Eluantscontaining 0,4 M Na2Sor higher were found toresult in the elution of about 98 per cent of thegold from the carbon in about 4 hours at theseflowrates.

Adams and Nicopo, using an identical experi-mental arrangement with cyanide and hydroxidesolutions, found the kinetics of elution to be firstorder and to be described by the rate equation

,,

A

,,~ ..., /',, ,-8

" "I

,,

/M /,

~~",~~/~ ,/,

~

",/,

---8'

9400.1300

A

2 4Time, h

24 48

Figure 10-Effect of initial gold loading on the batch elution of gold from activated carbon

8. GROSS,J., and Scorr, j.W. Precipita-

tion of gold and silver from cyanidesoiution on charcoal. Washington,

US Bureau of Miues, Terhnica/

Paper 378. 1927. 78 pp.

9. ZADRA, j.B. A process for the

recovery of goid from activated

carbon by ieaching and eiectrolysis.

Washiugton, US Bureau of Mines,

R14fJ72.1950.47pp.

10. ADAMS, M.D., and NICOl, M.J. The

kinetics of elution of goid from acti-

vatedcarbon. Gdd100.Proceedings

0/ the International (()1J/erence on

Go/d. Johannesburg, South African

iustitute of Miniug and Metallurgy,

1986. vol. 2, pp. 111-121.

11. RAMACHANDRA,RAO, S., and HEPlER,

C.G. Equilibrium constants and

thermodynamics of ionization of

aqueous hydrogen suiphide.

/(ydrometa//urgy, vol. 2. 1978.

pp. 293-299.

12. LICHT,S., FOROUZAN,F., and LoNGO,

K. Differential densometric analysis

of equilibria in highiy concentrated

media: determination of the

aqueous second acid dissociation

constant ofH,S. Anal. chem., vol.

62.1990. pp. 1356-1360.

13. SIllEN, LG., and MARTEIL, A.E.

(eds.).StabiliryronstanlS. London,

The Chemical Society, 1964.

~ 194

There is very little information availableregarding the complexes that are formed

between gold and polysulphide in aqueoussolution, although polysulphides have beenshown25 to leach gold from the arsenical stibnite

concentrate. Mellor26 states that the solubility ofgold in polysulphide solution is due to the

formation of AuS;3and AuS2 species. The

predominance of species like these in the earlystages of elution, where the S~- polysulphide

species were found to be present, would account

for the re-adsorption phenomenon mentionedearlier, since these singly negatively charged

complexes are likely to have a high affinity for

activated carbon27.

Single-pass Elution of Gold Cyanide withSodium Sulphides

The batch experiments that were discussed inthe previous section may be useful in theinterpretation of some of the effects that wereobserved in the single-pass elution experiments.

AUGUST 1994

-dcldt=k(C-K5), [13]

where C and S are the concentrations of gold onthe carbon and in solution, respectively, and kand K are constants. The incorporation of thisequation in the mass balance yields the relation

InC=lnCo-k't, [14]

where Cois the initial concentration of gold onthe carbon, t is the time from the start of elution,and k' depends on the experimental conditionsand diffusion coefficients.

Whereas data from the equivalent cyanideand hydroxide elutions yielded linear relationswhen In Cwas plotted against t, this was not thecase for the sulphide elutions, as shown inFigure 13. The initial region with the slower ratecorresponds to the region where polysulphideswere detected in the eluates. This results in adelayed onset of elution.



100

80

'#.

>-uC41'u!E41C0.~w

60

40

20

00

JI- - --e/

//

fI/

/'/

//

./

/

//

/

/

~~The results in Figure 15 show that the ionic

strength had a negligible effect on the elutionwith sulphide solution; so, the simple equation[15] does not hold in this case. Owing to thevery stable gold sulphide complexes that form inaqueous solution (Table I), and the tendency22 ofthiolligands for the displacement of cyanidefrom Au(CN)2 , it is suggested that themechanism of sulphide elution involves a similartype of ligand displacement reaction, with theformation of a multi-charged gold complex thathas little affinity for carbon23,24.

Effect of Temperature

The results in Figure 16 show that the initial rateof elution is enhanced at higher temperatures.However, increasing amounts of adsorbed goldare presumably deposited as an insoluble form,such as Auo, resulting in the plateau effect in thehigh-temperature curves in Figure 16. Thispremise could not be confirmed by XRD analysisowing to the low gold loading that was present.

Effect of Other Anions

The addition of sulphite and, to a greater extent,thiosulphate ions to the eluant results inenhanced rates of elution, as shown inFigure 17. These ions may assist by reducingany polysulphides back to sulphide ion.Figure 17 also shows the slight depressing effectof 0,2 M S, and the dramatic effect when 0,6 M Sis added to the eluant, which virtually completelyeliminated the elution. The 0,2 M Na2Sconditions are equivalent to the 0,1 M S~-conditions used in the species distributiondiagram in Figure 2. At pH 13, there aresubstantial concentrations of polysulphides(comparable with the concentration of S2-),which would result in a depression of the elutionkinetics. This would also be affected by theadsorption of polysulphides via reaction [2].

In the case of 0,6 M S, the conditions areequivalent to the 0,2 M S~-conditions used inthe species distribution diagram in Figure 3. Inthat case, the concentrations of polysulphideswere very much larger than the concentration ofS2-ions, and the solution remained yellow-

orange in colour throughout the experiment. Theratio of AuS~-to AuSi species was presumablyvery low, as evidenced by the negligible elutionrate.

.

8\-- \\

\\

\\

\\

\

\

'8

2 24 48

Figure 11-Effect of polysulphides on the batch elution of gold from activated carbon athigh liquid-to-solid ratios

14. INGffi,N., KAKOLOWlcz,W., SILLEN,LG., and WARNQVlST,B. High-speed

computers as a supplement to

graphical methods. V. Haltafall, a

general program for calculating the

composition of equilibrium mix-

tures. Talanta, vo1. 14. 1967.pp. 1261-1286.

15. REMv, H., and ANDERSON,).5.

Treatise on inorganic chemistry.

Amsterdam, Elsevier, 1956. vo1. 1,

p.738.

16. MORI, T., TAKEUCH',5., and MATSUDA,

S. Oxidation of sulphide ion by

molecular oxygen in the presenCe of

a water -repeDent catalyst. Nippon

Kugaku Kaishi, 1989. pp. 204-208.(ChemAbstr., no. 110, 199893m).

17. HAVA, S. and ONO,T. The hnproved

white liquor oxidation process with

the new catalyst. Kami Fa Gikyosh(

vo1. 42, no. 1. 1988. pp. 46-51.(ChemAbstr, no. 108, 152387t).

18. THORNE,D.L.c., and RoBERTS,E.R.

Inorganic chemistry. Edinburgh,

Oliver and Boyd, 1954. p. 565.

19. GIGGENBACH,W.Opticalspectraand

equilibrium distribution of polysul-

phide ions in aqueous solution at

2'.lnorg. Chem., vo1.11, no. 6.

1972. pp. 1201-1207.

4Time,h

Effect of NaOH Concentration

The pH value of a 0,2 M Na2S solution is approx-imately 13. The addition of NaOH causes a shiftto a higher pH region, where the fraction of S2-species is higher. This results in a concomitantlyhigher elution rate, as shown in Figure 14.

Effect of NaC! Concentration

The ionic strength has a marked effect on theelution of gold from activated carbon withcyanide or caustic solutions because of the effectof the Mn+ cation on the equilibrium:

Mn+ [Au(CN);]n (ads) H

Mn+ (aq) + Au(CN)2 (aq).

The Journal of The South African Institute of Mining and Metaliurgy

[15]

AUGUST 1994 195 ....

6,8=

_....~6,6

-t::ca

c.i..E

6,4


#.

~ 60c.'u==.c0~iD 40

100

[Na;zSJ

0.005M-+-0.05M--0.1 M- A-

0.2M~

0.4M-B-

0.6M-A -

1.0M--2.0M

4

80

20

.8

....----

--00 2

Tlme.h3

Figure 12-Effect of Na~ concentration on the single-pass elution ofgold cyanide from activated carbon

7

6,2

[Nal>1

0.05 M-+-

0.1 M---

46

0 2Tlme,h

3

Figure 13-Plot of In C versus t for the elution of gold from activatedcarbon at different sulphide concentrations

~ 196 AUGUST 1994

~

>-~ 60QI

U!EGIC0

"5 40iii

100

(NaOHI

0,0 molll--.-

0,4 molll-+-

80

20

00 3 42

Time, h

Figure 14-Effect of NaOH concentration on the single-pass elution ofgold cyanide from activated carbon

~

>-uCQI'u!EQI

C0

+I:I

iii

100

(NaCI]

0,0 molll--.-

0,4 mol/l--

0,6 mol/l- .-

0,8 molll~

80

60

40

20

00 2

Time, h3 4

Figure 1~Effect of NaCI concentration on the single-pass elution ofgold cyanide from activated carbon



Temp.

22"C 100100 --+-

30.C

.e-�T"°~""----40"C 80

80I?

~...~.:e - ..-~~.---- SO.C 'I.-B-#. 'I ~8'

'O.C >.>. ~. g 60u 60

p, /J{.JI-- -EJ-c ..

rifl ,~~ 9S.C '13U -8 - !ESi

f~.l' .

cc 00 ~40-g 40m ~iiJ

r;J

20 20

00 2

Time. h3 4

00 2

Time.h3 4

Figure 16-Effect of temperature on the single-pass elution of goldcyanide from activated carbon

100

80

'#>.~ 60.!!u

'ic0~ 40iii

20

00 1

BJDB-O-O2 3

Time. h

Figure 18-Effect of pretreatment on the single-pass elution of goldcyanide from activated carbon

Eluant

Na:; 0.2 MNa:;p:s0.2 M

~

Na:; 0.2 MNaJ)~~2 M

NaJ) 0.2 M- A-

Na:; 0.2 MSO.2M

-e-Na~ 0,2 M

SO.6M-�I-

4

Effect of SulphidePretreatmentFollowedbyElution with De-ionized Water

In an attempt to extend the concept of sulphideelution to the AARL method, loaded carbon waspretreated with 0,2 M Na2Sfor 4 hours. beforebeing drained and eluted with de-ionized water.The experiment was unsuccessful, as shown inFigure 18. This is attributable to the fact thatalkaline solution is necessary21 for the stabiliza-tion of gold in the elutable AU2S~-form. Figure15 shows that Au(HS)"2is the predominant formbetween pH 4 and 10, and this singly negativelycharged species would probably27,28be stronglyadsorbed by activated carbon.

Stripping of Silver from Sulphide-elutedCarbons

Several experiments were carried out on thefeasibility of stripping the silver, in particular,from the carbon in a step subsequent to sulphideelution. Gold was first eluted batchwise fromcarbon containing 1000 g/t of gold and 200 g/tof silver with a 0,2 M Na2S solution. The resultsobtained by several different stripping methodsare shown in Table V, and indicate that it ispossible to remove most of the silver fromsulphide-eluted carbon. These procedures couldundoubtedly be optimized further.Figure 17-Effect of other anions on the single-pass elution of gold cyanide from activated

carbon

The Journal of The South Afncan Institute of Mining and Metallurgy AUGUST 1994 197 ...


20. GREEN, B.K, ASHURST, K.G., and

CHANTSaN, T.E. A dedicated resin

for gold-the stimulus needed for

universal acceptance of a resin-in-pulp process. Bhappu, R.B., and

Harden, R.J. (eds.). Gold Forum on

Ted1nology and Praca'ces-World

Gold '89. SME, Colorado, 1989.pp. 339-346.

21. SEWARD,T.M. The hydrothennal

geochemistry of gold. Gold metal-

lurgy and exploradon. Foster, R.P.

led.). Glasgow, Blackie, 1991.pp. 37-62.

22. RENDERS,P.J" and SEWARD,T.M. The

stability of hydrosulphido- and

sulphido-complexes of Aull) andAg(l) at 25'(, Geochim. Casmochim.

Acta, vol. 53. 1989.pp. 245-253.

23. El-HINNAWI, M., PETER,lo, and

MmR, B. Raman spectra ofcopper(l), silver(!) and gold (I)

cyanides in aqueous solutions of

sodium thiosulphate.! Raman

Spectrosc., vol. 16, no. 4.1985.pp. 272-279.

24. LEWlS, G., and SHAW, C.F. Compe-

tition of thiols and cyanide forgold(l)./norg. Chem., vol. 25.

1986. pp. 58-62.

25. LDuw, N.J., EDWARDS,A.M" and

GUSSMAA,H.W. A new process to

extract gold and stibnite from

arsenical concentrates. Chem.SA,

vol. 3,1977. pp. 135-136.

26. MEllOR, J. Comprehensive treatise

on inorganic and thearedcal

chemistry. London, Longmans

Green, 1923. vol. 3, p. 613.

27. GAlLAGHER,N.P., HENDRIX,I.lo,

MllOSAVLIEVlC,E.B., NElSON, I.H.,

and SoWIIC, lo Affinity of activated

carbon towards some gold(l)

complexes. Hydrometallurgy, vol.

25.1990. pp. 305-316.

28. ADAMS, M.D. The chemistry of the

carbon-in-pulp process.

Johannesburg, University of the

Witwatersrand, Ph.D. thesis, 1989.

29. HEPEl, T., HEPEl, M., LES'KO, M.,

SEWERYNSKI, B., WOjTOWlCZ, J.,

MYCZKDWSKI, Z., GOTFRVD, lo,

SZOlOMICKI, Z., CAIS, A.,

KoUROVOWSKl,J"

aGU'A, A., and

BIESZCZAD,T. Hydrometailurgical

processing of materials containing

silver, lead and their compounds

resulting from leaching of copper

from sulphide concentrates. Pol.

Patent97320. 31 Jul., 1978.3 pp.

(ChemAbstr. 90: 15514OC: NIM-

TR-995).

30. SAND'URG, KG. and HUlATT, J.lo

Recovery of silver, gold, and lead

from a complex sulphide ore using

ferric chloride, thiourea, and btine

leach solutions. Washington, US

Bureau of Mines, R/9022. 1986.

14 pp.

31. GAlLAGHER,N.P., and LEI, K.P.V.

Recovery of lead and silver from

plumbojarosite by hydrothermal

sulfidation and chloride leaching.

Washington, US Bureau of Mines,

R/9277.1989.9pp.

~ 198

Table V

Stripping of silver from sulphide-eluted carbon by various methods

Stripping solution

5M NH4CI

90°CpH 7,0(HCI/NHa)S:L406h

0,5M Na2S20a25°CpH 7,0S:L406h

0,5M Na2S0a

25°CpH 6,0S:L406h

50 9/1 NaCI

90°CpH 7,0S:L406h

Zadra9 has shown that carbon can be

recycled between loading and stripping many

times without a loss in efficiency. It is conceiv-

able that the form of the silver and base-metalsulphides that are produced inside the carbon

during sulphide elution and subsequent regener-

ation is such that some cyanide leaching of thesespecies occurs during the subsequent adsorption

step. This could result in equilibrium loadings of

these elements eventually being reached, with no

further detrimental effect on gold recovery.

Conclusions

Elution efficiencies of around 96 per cent wereobtained with a single pass of eluant containing0,2 M Na2Sand 0,4 M NaOH in 4 hours-about10 bed-volumes of eluant. The initial rate wasslow over the first hour of elution, probablybecause the activated carbon catalysed theoxidation of sulphide to polysulphide.

Elution efficiencies of around 100 per centwere also obtained in batch elutions of carbon inless than 4 hours at liquid-to-solid ratios ofabout 100. Lower liquid-to-solid ratios resultedin re-adsorption of the gold, probably owing tothe oxidation of sulphide to polysulphide withthe resultant formation ofless elutable goldcomplexes.

No attempt was made to fully optimize theelution conditions, but several trends wereevident. Improved rates of elution were obtainedat higher sulphide concentrations and pH valuesgreater than about 13,

AUGUST 1994

30;this work

30;this work

30; 31

Increasing temperature raised the initialelution rate, but lowered the overall extractionefficiency, probably owing to the deposition ofelemental gold on the carbon.

Increased ionic strength, by means of NaCIaddition, had no effect on the elution, whichconfirms that the elution mechanism in the caseof sulphide is different from that when cyanideor hydroxide is used as the eluant.

The elution of gold by sulphide solutions isproposed to proceed by means of an initial stepthat involves the reaction of polysulphide ionswith the adsorbed aurocyanide species, formingAuCN on the carbon and thiocyanate in solution,This is followed by the formation of poorlyadsorbed complexes with sulphide ions, such asAuS~~The presence of polysulphides, whethergenerated in situ by the catalytic oxidation effectof activated carbon or by the addition ofelemental sulphur, serves to reduce the elutionrate and efficiency dramatically. This is probablydue to the formation of complexes such as AuS-and AuSi, which have a high adsorption affinity.

Sodium sulphide solutions are rapid andeffective for the elution of gold from activatedcarbon at room temperature. As silver and basemetals are not eluted, the technique is suitablefor applications where the carbon is not intendedfor re-use, for example, in the elution of goldfrom carbon fines. .


- ~-- -- -

carbon in pulp process

Documents

elution of gold cyanide

elution column

batch eludon qf carbon

eludon dfidendes qf

efficient elution

recovery qf gold

hour qf eludon

oxidadon qf sulphide