(2006) 126–132www.elsevier.com/locate/hydromet

Hydrometallurgy 82

Ammoniacal thiosulphate leaching of gold in the presence of pyrite

D. Feng ⁎, J.S.J. van Deventer

Department of Chemical Engineering, University of Melbourne, Victoria, 3010, Australia

Available online 27 April 2006

Abstract

The effect of pyrite on gold dissolution was investigated in the ammoniacal thiosulphate leaching system using pure gold foils.Special emphasis was placed on gold leaching in association with pyrite dissolution, thiosulphate decomposition and gold leachingpassivation. The presence of pyrite retarded gold dissolution and this detrimental effect became more pronounced at higher pyritecontents. Pyrite catalysed the decomposition of thiosulphate to trithionate. The addition of sulphate enhanced gold leaching byretarding the dissolution of pyrite in ammoniacal thiosulphate solutions. XPS analysis indicated the presence of iron hydroxidespecies at the gold surfaces in the presence of pyrite, which was likely responsible for the reduced gold dissolution. SEM analysisindicated a lesser extent of gold dissolution occurring in the presence of pyrite.© 2006 Elsevier B.V. All rights reserved.

Keywords: Gold leaching; Thiosulphate; Pyrite; Passivation

1. Introduction

Thiosulphate as an alternative lixiviant for gold hasreceived much attention in recent years due to thegrowing environmental concerns over the use ofcyanide. Acceptable gold leaching rates using thiosul-phate were achieved in the presence of ammonia withthe cupric ion acting as the oxidant (Abbruzzese et al.,1995; Langhans et al., 1992; Tozawa et al., 1981;Kerley, 1981, 1983; Block-Bolten and Torma, 1986;Hemmati et al., 1989; Groudev et al., 1996; Marchbanket al., 1996; Wan, 1997; Wan et al., 1994; Wan andBrierley, 1997; Thomas et al., 1998; Zipperian et al.,1988). The leaching of gold in thiosulphate solutions isan electrochemical reaction, with the constituent halfreactions being the oxidation of gold to gold thiosul-

⁎ Corresponding author. Fax: 61 3 8344 4153.E-mail address: dfeng@unimelb.edu.au (D. Feng).

0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2006.03.006

phate, and the reduction of Cu(II) ammine to Cu(I)thiosulphate, as shown in Eqs. (1) and (2), respectively.

Au þ 2S2O2−3 →AuðS2O3Þ3− þ e− ð1Þ

CuðNH3Þ2þ4 þ 3S2O2−3 þ e−→CuðS2O3Þ5−3

þ 4NH3ð2Þ

The thiosulphate leaching system is complicated bythe homogeneous reduction of Cu(II) by thiosulphateaccording to the simplified overall reaction, Eq. (3).

2CuðNH3Þ2þ4 þ 8S2O2−3 →2CuðS2O3Þ5−3 þ 8NH3

þ S4O2−6

ð3Þ

Gong et al. (1993) suggested that the kinetics ofleaching gold from an auriferous pyrite concentratecould be controlled by a corrosion reaction of the am-monia-thiosulphate–copper system on pyrite. From X-ray powder diffraction studies, Qian et al. (1993)observed less chalcopyrite present in thiosulphate lea-ched residues than in the original ore. They concluded

127D. Feng, J.S.J. van Deventer / Hydrometallurgy 82 (2006) 126–132

that an ammonia-thiosulphate solution containing cop-per, leached chalcopyrite by a corrosive process. Scan-ning electron microscope showed clear scarring onleached chalcopyrite grains. These authors also observedsome dissolution of pyrite in the residues. Coppersulphide minerals, other than chalcopyrite, also dis-solved readily in an aerated thiosulphate leach solution,particularly when ammonia was present. Other sulphideminerals also showed varied extents of dissolution inthiosulphate leach solutions (Feng and Van Deventer,2002). However, the relationship between gold dissolu-tion and the dissolution of sulphides has not yet beeninvestigated in detail in the ammoniacal thiosulphatesystem. It is the objective of this investigation to conducta systematic study of the gold dissolution in the presenceof pyrite in ammoniacal thiosulphate systems.

2. Experimental

2.1. Minerals and reagents

Pyrite (Py) and quartz (Qz) samples were obtainedfrom Geological Specimen Supplies, Australia. Theminerals were crushed, dry milled in a ring mill, sievedinto the size fraction of 45–75 μm and stored in air-tightplastic bags in a refrigerator to avoid any further oxi-dation. Quantitative XRD results indicated that themineral samples are of over 98% purity. Gold foils(99.99% Au, thickness 0.2 mm) were used in the expe-riments with a surface area of about 0.20 cm2. La-boratory grade ammonium thiosulphate, ammoniumsulphate and ammonia solution (25%) were provided byWestlab Chem Supply Pty Ltd, Australia. Analyticallypure cupric sulphate, hydrogen peroxide (30% w/v), andhydrochloric acid were obtained from Merck. Distilledwater was used in the experiments.

2.2. Analytical techniques

Elemental concentrations in solutions were deter-mined by ICP-OES, involving the oxidation of sulphurspecies as very stable sulphates prior to the analysis.After oxidation by hydrogen peroxide, HCl was addedto the solution, which was boiled to ensure completeconversion of the metal species to the chloride form. Thethiosulphate concentration was determined by iodo-metric methods. In order to eliminate the effect of thecupric–ammonia complex on iodine titration, a certainamount of acetic acid (10% solution) was added prior tothe titration with the indicator Vitex.

Polythionates were determined by ion-chromatographyon Dionex DX-500 (Sunnyvale, CA, USA) using ion-

interaction chromatography with conductivity and UVdetection, as described by O'Reilly et al. (2002).Polythionates were eluted within 18 min using an eluantcomprising an acetonitrile step gradient at 0min from15%to 28%v/v, 3mMTBAOHand 2.5mMsodiumcarbonate,operated using a Dionex NS1 column with guard.

A platinum electrode (M21Pt, Radiometer) was usedto measure the mixed solution potential with a double-junction reference electrode (Ag/AgCl, saturated KCl,Orion) to avoid the interference of thiosulphate with thereference electrode. All potentials are given with respectto SHE.

X-ray photon spectroscopy (XPS)was used to identifythe surface species of the leached gold foil and pyritesamples. XPS was performed on a nanoprobe (FisonsInstruments) at about 10−8 Torr (using monochromaticthe Al Ká X-ray at 1486.6 eV). The pyrite samples werefiltered and rinsed with distilled water for three times anddried under vacuum overnight for XPS analysis.Similarly, the leached gold foils were rinsedwith distilledwater and dried under vacuum overnight for XPS andSEM (Phillips XL30)/EDAX (Oxford) analysis.

2.3. Experimental method

Experiments were performed in a 400 mL open-topreactor using a magnetic stirrer at a rotating speed of400 min− 1. 250 mL of solution was added to desiredamounts of the quartz and pyrite samples. The gold platesurfaces were polished by 2000 grid sand papers, washedwith acetone twice and rinsed with de-ionised waterbefore each test. The gold plates were suspended in theupper part of the leaching reactor with a nylon thread,ensuring no contact with the reactor wall duringleaching. All experiments were performed at room tem-perature (20±1 °C). Samples were taken continuously atcertain intervals during a total retention time of 24 h. Thesamples were centrifuged and filtered for the subsequentiodine titration, polythionate determination and oxida-tion for ICP analysis. The gold dissolution rate wascalculated based on the dissolved gold mass per m2 ofthe gold plate surface. All the experimentswere conducted with the reagent dosages of 0.5 M(NH4)2S2O3, 0.012 M CuSO4·5H2O and 2.0 M NH4OHat pH 10.5.

3. Results and discussion

3.1. Dissolution of gold in the presence of pyrite

Fig. 1 shows gold dissolution in the presence of variedamounts of pyrite. In comparison, the effect of varied

0

70

140

210

280

350

0 5 10 15 20 25

Time, h

Go

ld d

isso

luti

on

, g/m

2

Qz (0.4)

Py (0.4)

Py (2.0)

Py (8.0)

Py (16)

Fig. 1. Effect of pyrite on gold dissolution in ammoniacal thiosulphate.Pyrite content: 0.4, 2.0, 8.0 and 16 g/L.

0

7

14

21

28

[S3O

62-],

mM

Qz (0.4)Qz (16)Py (0.4)Py (2.0)Py (8.0)Py (16)

0 5 10 15 20 25

Time, h

Fig. 2. Variation of trithionate concentration during gold dissolution inthe presence of pyrite and quartz. Quartz content: 0.4 and 16 g/L.Pyrite content: 0.4, 2.0, 8.0 and 16 g/L.

128 D. Feng, J.S.J. van Deventer / Hydrometallurgy 82 (2006) 126–132

amounts of inert quartz on the rate of gold dissolutionwas also evaluated. The gold dissolution rates in thequartz slurries were used as standards to compare therates in the presence of pyrite. The gold dissolutionexhibited an approximately linear relationship with timein the presence of quartz. Quartz had only a marginaleffect on reducing gold dissolution at a higher content.The rate reduction is attributed to quartz slime in thesolutions partially blocking the gold plate surfaces. In thequartz slurry, the gold surface showed light brown afterleaching regardless of the quartz content in the slurries.In contrast, the gold plates appeared dark-brown in thepyrite systems and the leached gold surfaces becamedarker with an increase in the pyrite content. This indi-cates that leaching passivation occurred in the presenceof pyrite and became more pronounced at higher pyritecontents. This is in line with the leaching result where thegold dissolution and leaching kinetics were significantlyreduced with the addition of pyrite and this detrimentaleffect became more remarkable at higher pyrite contents(Fig. 1).

3.2. Effect of pyrite on thiosulphate oxidation

Thiosulphate is a metastable anion that tends toreadily undergo chemical decomposition in aqueoussolutions, particularly in acidic solutions. In the presenceof cupric ions, thiosulphate can be oxidised to tetra-thionate following the route in Eq. (3). Tetrathionate canfurther decompose to higher or lower polythionatesthrough the formation of trithionate (Byerley et al.,1973a,b, 1975). The reaction is catalysed by the presenceof thiosulphate with the overall reaction shown in Eq. (4):

2S4O2−6 þ 3OH−→ 5=2S2O

2−3 þ S3O

2−6 þ3=2H2O ð4Þ

Trithionate can also further decompose to givethiosulphate and sulphite, as shown in Eq. (5):

2S3O2−6 þ 6OH−→S2O

2−3 þ 4SO2−

3 þ 3H2O ð5ÞThe decomposition of tetrathionate to form trithio-

nate is much faster in alkaline solutions following Eq.(4), in comparison with the decomposition of trithio-nate as shown in Eq. (6) (Zhang and Dreisinger, 2002).Under the reagent conditions of this work, onlytrithionate was determined, as shown in Fig. 2. Clearly,the trithionate concentration increased almost linearlywith time. In terms of the changes of trithionate con-centrations, the presence of 0.4 to 16 g/L pyrite sig-nificantly increased the decomposition of thiosulphateand this effect became more remarkable with an in-crease in the pyrite content (Fig. 2). In contrast, thetrithionate concentration was only slightly increasedwhen the quartz content was increased from 0.4 to16 g/L.

Polythionates were found to reduce copper(II) ionsand hence decrease the leaching kinetics of gold (Chu etal., 2003). This could be one of the reasons causinglower gold dissolution in the presence of pyrite. Inaddition, the decomposition of thiosulphate has beenreported to form passivation layers of elemental sulphurand copper sulphide, which prevent thiosulphate solu-tions from diffusing to gold surfaces (Bagdasaryan et al.,1983; Pedraza et al., 1988). Therefore, the higher de-composition of thiosulphate could cause a larger extentof passivation in the presence of pyrite.

The slurry pH remained constant at 10.5 duringleaching, due to the buffering effect of NH3 and NH4

+.The mixed potential was in the range of 155 to 175 mV

129D. Feng, J.S.J. van Deventer / Hydrometallurgy 82 (2006) 126–132

in the presence of quartz. The mixed potentials wereslightly lower in the presence of pyrite and this trendbecame more obvious at higher pyrite contents. Intheory, the oxidative decomposition of thiosulphateshould be lower at a lower mixed potential. As aconsequence, the excessive consumption of thiosulphatewas likely due to the catalytic effect of pyrite on thedecomposition of thiosulphate. Xu and Schoonen(1995) proposed that the pyrite-surface-catalysed oxi-dation of thiosulphate to tetrathionate by dissolvedoxygen was the dominant thiosulphate decompositionmechanism in aqueous solution of pH 2.9–8.6 at 20 °C,and that the rate of tetrathionate formation was firstorder with respect to the pyrite surface concentration.

The catalysis of pyrite in this reaction originatedfrom its strong affinity for aqueous sulphur species andits semi-conducting properties. Pyrite was believed toform an interfacial intermediate complex with thiosul-phate, the aqueous electron donor, on anodic sites, andoxygen, the terminal electron acceptor, on cathodicsites. The electrons could transfer from the anodic site tothe cathodic site via the conduction band of pyrite. Thismechanism could also contribute to the catalytic effectof pyrite on the oxidative decomposition of thiosulphatein alkaline aqueous solutions. In the presence of thecopper(II)-ammine complex, the electron acceptorwould be preferentially the copper(II)-ammine complex.

3.3. Effect of sulphate on gold dissolution

Fig. 3 shows the effect of sulphate on gold dissolutionin the mineral slurry systems. With the addition of 0.5 MSO4

2−, the gold dissolution rate slightly increased in thequartz slurry, while it was significantly increased in thepyrite slurry. This observation was in accordance with

0

70

140

210

280

350

Go

ld d

isso

luti

on

, g/m

2

Qz

Qz (SO4)

Py

Py (SO4)

0 5 10 15 20 25

Time, h

Fig. 3. Effect of sulphate on gold dissolution in the presence of pyriteand quartz. Mineral content: 2 g/L; 0.5 M SO4

2−.

those of Hu and Qian (1991) and Hu and Gong (1989).The decomposition of thiosulphate was slightly reducedwith the addition of 0.5M SO4

2− in both quartz and pyritesystems. It was shown that sulphate competed withthiosulphate in complexing to Cu(II) at the axial sites,thus reducing the rate of thiosulphate oxidation by Cu(II)(Breuer and Jeffrey, 2003). The decrease in the de-composition of thiosulphate with the addition of sulphatecould reduce the formation of passivation layers ofelemental sulphur and copper sulphide and hence,increase the dissolution of gold.

Oxidative dissolution of pyrite in the ammoniacalthiosulphate leach is expected to form the final productof sulphate. The addition of sulphate depressed thedissolution of pyrite in terms of the variation of thesulphur concentrations in solutions. The released sul-phur concentration from pyrite was reduced from 21.3 to12.5 mg/L in the presence of 2 g/L pyrite. Perez andGaraviz (1987) reported that at pH levels lower than 8.0,metallic iron (from grinding media) and iron salts dis-solve in solution resulting in a decrease in gold dis-solution. The formation of colloidal iron oxide orhydroxide particles at gold surfaces could retard golddissolution. This could be one of the reasons why pyriteretarded the gold dissolution. Therefore, the reduceddissolution of pyrite with the addition of sulphate mayreduce the tendency to form the passivation layers ofiron species at gold surfaces and enhance the dissolutionof gold as a result.

4. XPS studies

The XPS analysis of sulphur and iron speciation atthe pyrite and gold surfaces after contact with ammo-niacal thiosulphate solutions by using XPS allows abetter understanding of the mechanism for pyrite re-ducing gold dissolution. The XPS spectra were recordedfor Fe 2p and S 2p for these samples.

Fig. 4 shows the S 2p spectra for the sulphide samplesbefore and after contact with solution. The S 2p spectrumfor the original sulphide was composed of three peaks ataround 163.2, 164.2 and 169.3 eV, while the S 2p spec-trum for the sulphide after contact in the ammoniacalsolution had two clear peaks at around 163.2 and 169.3 eVand a broad peak in the range of 163.9 and 166.4 eV,which was de-convoluted to two peaks at around 164.2and 164.9 eV, respectively. The peak at around 164.2 eVwas assigned to elemental sulphur S8 (Wagner, 1990), thepeak at around 163.2 eV was originated from the S2

2− inpyrite (Descostes et al., 2001), and the peak at around169.3 eV resulted from SO4

2− (Wagner, 1990). The peakat around 164.9 eV was attributed to intermediate

30

40

50

60

70

700706712718724

Binding energy, eV

kCP

S (X

1000

)

Fig. 6. Fe 2p spectra for gold leached in the presence of 16 g/L pyrite.

7

10

13

16

19

22

25

28

160162164166168170172

Binding energy, eV

kCP

S (X

1000

)

Before

After

Fig. 4. S 2p spectra for pyrite before and after 24 h contact withthiosulphate leach solution.

130 D. Feng, J.S.J. van Deventer / Hydrometallurgy 82 (2006) 126–132

sulphur species between elemental sulphur and sulphitesuch as thiosulphate (Descostes et al., 2001). The pres-ence of the thiosulphate-like intermediate sulphurspecies at the pyrite surface was possibly the result ofthe interaction between the adsorbed thiosulphate andthe pyrite surface. Clearly, the elemental sulphur con-centration became higher after leaching, indicating thatthe decomposition of pyrite partially formed some ele-mental sulphur (Fig. 4). Both sulphur and copper specieswere detected at the surfaces of the gold foils leached inthe presence of quartz and pyrite. The dominant sulphurspecies were found to be thiosulphate and the dominantcopper species were in copper(I) form.

The Fe 2p spectra for pyrite in Fig. 5 indicated that thepyrite surface after leaching in ammoniacal thiosulphatesolutions was oxidised to form FeOOH showing a peakat∼711.8 eV (Harvey and Linton, 1981). In comparison,the original pyrite only showed an iron peak at∼708 eV,

30

55

80

105

704707710713716

Binding energy, eV

CP

S (X

1000

) Before

After

Fig. 5. Fe 2p spectra for pyrite before and after 24 h contact withthiosulphate leach solution.

which was Fe2+ in FeS2. The Fe 2p spectra for the goldleached in the presence of pyrite showed a clear peak at∼711.8 eV, which was the characteristic peak forFeOOH (Fig. 6). This could be caused by the

Fig. 7. Typical SEM images of the leached gold surfaces in thepresence of (a) quartz and (b) pyrite. Mineral content: 2 g/L; leachingtime: 24 h.

131D. Feng, J.S.J. van Deventer / Hydrometallurgy 82 (2006) 126–132

precipitation of pyrite fines at the gold surfaces and thesubsequent oxidation of pyrite. The formation of ironhydroxide at the gold surfaces could be the main reasoncausing the decrease of gold dissolution in the presenceof pyrite.

5. Topological studies on the leached gold foils

In order to study the surface morphology of the goldfoils after leaching, the gold foils were subjected to SEManalysis coupled with EDAX. Fig. 7a and b shows theSEM images of the leached gold surfaces in the presenceof quartz and pyrite, respectively. Clearly, the leachedgold foil surfaces show varied degrees of erosion, anddifferent morphological structures. In the absence ofsulphides, the leached gold foil surface showed a regularnet-work structure as depicted in Fig. 7a. The leachedgold surface in the presence of pyrite appeared to berelatively smooth, and a very thin waterprint-like filmwas spotted unevenly at the surfaces. The composition ofthe film could not be identified by the EDAX due to itslow sensitivity. The film is possibly an iron oxide formedin the dissolution of pyrite, which retarded the furthergold dissolution.

6. Conclusions

• The presence of pyrite significantly reduced golddissolution in ammoniacal thiosulphate solutions andthis detrimental effect became more prominent athigher pyrite contents. Pyrite had a catalytic effect onthe decomposition of thiosulphate. The decomposi-tion of thiosulphate mainly formed trithionate underthe experimental conditions.

• The addition of sulphate in the pyrite system in-creased the gold dissolution rate because of the de-pression of pyrite dissolution in the leaching system.

• XPS results indicated the formation of iron hydrox-ide species at the gold surfaces in the presence ofpyrite, which likely contributed to the reduced golddissolution.

• Topological studies by SEM demonstrated that golddissolved to a lesser extent in the presence ofpyrite.

Acknowledgements

The financial support from Newcrest Mining Limit-ed, Placer Dome Technical Services Limited, and theAustralian Research Council is gratefully acknowl-edged. Appreciation is also expressed to Hui Tan forassistance with the experimental work.

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