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Hydrometallurgy 105 (2011) 234239

Contents lists available at ScienceDirect

Hydrome

l sePretreatment of refractory ores/concentrates prior to cyanide leachingis therefore required to render the contained gold and silver readilyamenable to extraction. Roasting, pressure oxidation, biooxidationand, to a limited extent, ultrane grinding are used as pretreatmentmethods in practice, but their effectiveness depends largely on thenature of the refractoriness (Gunyanga et al., 1999; Iglesias andCarranza, 1994; Corrans and Angove, 1991).

Silver-bearing suldes including proustite, pyrargyrite, tennantiteand tetrahedrite (Table 1) are also refractory in character leading to

Cu3AsS4 s + 2 S22Cu1:5 S s + AsS

34 2

Sb2S3 s + 2 S2Sb2S

45 3

SbS2 + S2SbS33 4

Previous studies on an antimonial gold/silver ore from Akoluk,

very poor silver extractions (often10%) in2000). Alkaline sulphide leaching of tetrahe2003), stibnite (Ubaldini et al., 2000), enarCurreli et al., 2009) and jamesonite (Bal a

Corresponding author. Fax: +90 462 325 7405.E-mail address: ialp@ktu.edu.tr (. Alp).

0304-386X/$ see front matter 2010 Elsevier B.V. Adoi:10.1016/j.hydromet.2010.10.005yrite/arsenopyrite, golde mineralogy includinga Brooy et al., 1994).

Cu14Sb4S13 s + 2 S27Cu2S s + 4SbS

2 1as tellurides or stibnides, or reactive gangu

preg-robbing carbonaceous constituents (L1. Introduction

Gold-bearing ores can be classirefractory based on their metallurgica(Adams, 2005). While high gold recovores can be readily achieved, refcharacterized by the low gold extraccyanide leaching (Gupta and Mukherjgold ores can result primarily fromfeatures such as having locked gold weither free milling ornse to cyanide leachingN90%) from free millinggold ores are often

(b80%) by conventional0). The refractoriness ofinherent mineralogical

has proved to be a suitable process for the pretreatment of theseminerals to make silver available for subsequent cyanide leaching orto remove hazardous/penalty elements such as arsenic and antimonyfrom ores/concentrates (Table 1). Ubaldini et al. (2000) also noted thebenecial effect of the temperature on the dissolution of stibnite withthis system.

The decomposition of some arsenical and antimonial sulphides inalkaline sulphide leaching systems can be given by the followingreactions (Bal, 2000; Curreli et al., 2009).cyanide leaching (Bal,drite (Bal et al., 1998,gite (Bal et al., 2000;nd Achimoviov, 2006)

Turkey have inpresumably linkzinkenite (Pb9Sbbearing componal., 2006, 2009decomposition oand silver by aantimonial ore (

ll rights reserved.Refractory oreAlkaline sulphide leachingImproved gold and silver extraction fromwith alkaline sulphide leach

Oktay Celep, brahim Alp , Hac DeveciDepartment of Mining Engineering, Karadeniz Technical University, 61080, Trabzon, Turke

a b s t r a c ta r t i c l e i n f o

Article history:Received 18 February 2010Received in revised form 13 October 2010Accepted 14 October 2010Available online 21 October 2010

Keywords:GoldSilverCyanidationAntimony

The pretreatment of an antleaching. The ore assayed 20a lesser extent, sulphides suThe latter mineral was idensilver. Cyanide leaching ofconrming the refractory nawas shown to leach out up toAg in the subsequent cyanidthat alkaline Na2S leachingcyanidation for the refracto

j ourna l homepage: www.erefractory antimony ore by pretreatment

nial refractory gold and silver ore was investigated using alkaline sulphideAu and 220 g/t Ag and contained predominantly quartz/clay and barite, and tos pyrite, stibnite, sphalerite, zinkenite (Pb9Sb22S42) and andorite (Sb3PbAgS6).ed to be the most important sulphide phase for the occurrence of gold andore consistently resulted in low extraction of gold (49%) and silver (18%)e of the ore. Alkaline sulphide treatment of the ore under suitable conditions% Sb, which remarkably improved the extraction of silver from b18% up to 90%n step. Gold extraction was also enhanced by 2030%. These ndings suggestn be suitably used as a pretreatment method prior to the conventionalb-bearing ores.

2010 Elsevier B.V. All rights reserved.

tallurgy

v ie r.com/ locate /hydrometdicated that the ore is refractory in character,ed with the occurrence of the sulphides including22S42) and andorite (Sb3PbAgS6) as the main silver-ent in the ore (and gold to a lesser extent) (Celep et). Diagnostic leaching tests suggested that thef the sulphides could improve the extraction of goldbout 29% and about 57%, respectively from theCelep et al., 2009). Ultrane grinding or roasting of

sulphide leach tests. In the alkaline leaching tests, the effects of

Table 1Antimonyarsenic sulphide minerals treated by alkaline sulphide leaching.

Minerals Formula Process References

Tetrahedrite Cu12Sb4S13 Sb removal Bal et al., 1998Tennantite Cu12As4S13 As removal Bal et al., 2003Stibnite Sb2S3 Sb removal Ubaldini et al., 2000Proustite Ag3AsS3 As removal/Ag

recoveryBal, 2000

Pyrargyrite Ag3SbS3 Sb removal/Agrecovery

Bal, 2000

Jamesonite FePb4Sb6S12 Sb removal Bal and Achimoviov,2006

Enargite Cu3AsS4 As removal/Curecovery

Curreli et al., 2009

Andorite Sb3PbAgS6 Zinkenite Pb9Sb22S42

235O. Celep et al. / Hydrometallurgy 105 (2011) 234239particle size, temperature and concentrations of Na2S and NaOH onthe removal of antimony and the extraction of gold and silver insubsequent cyanide leaching were investigated.

2. Experimental

2.1. Materials

An antimonial refractory gold/silver ore sample obtained fromAkoluk (Ordu-Turkey) was used in this work. The ore sample wascrushed down to 4 mm using jaw and roll crushers and rifed toobtain 1 kg representative sub-samples. These were then ground in alaboratory rod mill or stirred media mill to the desired neness (80%passing size, d80=15, 10 and 5 m) prior to the leaching tests. Theparticle size analysis of grounded samples was performed by aMalvern Mastersizer laser particle size analyzer.

The chemical composition of the ore sample (Table 2) wasthe ore was reported to be ineffective to improve the cyanide leachingof gold and silver (Celep et al., 2009, 2010).

Despite the extensive alkaline sulphide leaching studies on manygold- and silver-bearing sulphides (Ubaldini et al., 2000; Ficeriov etal., 2002, 2005a,b; Bal et al., 2003), to our knowledge, no detailedstudies on ores/concentrates containing andorite and zinkenite havebeen reported. Therefore, this study was designed to evaluate alkalinesulphide leaching as a potential pretreatment process prior tocyanidation. Prior to the leaching tests, detailed mineralogical studieswere performed to identify the presence and association of gold andsilver within the ore. Because of the intimate association of gold andsilver found with antimony minerals, the leaching behavior ofantimony from the ore was also monitored during the alkalinedetermined by wet chemical analysis methods using ICP-AES

Table 2Chemical analysis of the ore samples.

Compound Content (%) Element Content (g/ton)

SiO2 52.2 Au 20.2BaSO4 29.1 Ag 220Al2O3 4.7 Cu 400Fe2O3 1.3 As 262Sb 1.6 Cd 62.7Zn 1.5 Zr 40.7Pb 0.4 Ni 6.0Sr 0.3Tot. S 6.9LOIa 4.6 Tot. C 500

a L.O.I. = Loss on ignition.(Inductively Coupled Plasma-Atomic Emission Spectroscopy) andNAA (Neutron Activation Analysis) after digestion in aqua regia. Thesample was determined to be rich in silver containing 220 g/t Ag and20 g/t Au. The ore consisted predominantly of quartz/illite (52.2%SiO2) and barite (29.1% BaSO4) with pyrite, stibnite, sphalerite,zinkenite, and andorite being present as sulphide phases (Celep etal., 2006).

2.2. Detailed mineralogical characterization of the ore

Earlier studies (Celep et al., 2006, 2009) have revealed that gold ispresent as particles ranging from 1 to 88 m in size associated withsulphide minerals and quartz. Further detailed mineralogical analysisof the ore sample was performed to determine gold and silver-bearingphases. Hand-picked pieces of the ore were used to prepare thepolished sections for mineralogical examination under an oremicroscope (Leitz Wetzlar). Scanning electron microscopy (SEM)studies were also carried out using a FEI Quanta 400MK2 Model SEMequipped with the EDAX Genesis 4XMI Model a light elementenergy dispersive system to obtain backscattered electron micro-graphs (BSE). Elemental analysis of the grains was performed withwavelength dispersive (WDS) and energy dispersive (EDS) spectros-copy systems. WDS analyses were carried out by a JEOL JXA 8900electron probe X-ray microanalyzer (EPMA). This system has vewavelength dispersive spectrometers (WDS) operated at 20 kVwith aprobe current of 20 to 30 nA.

The occurrence and association of gold and silver are illustrated inFig. 1 where the sections were initially observed by ore microscopy(Fig. 1a, b, and c) and the same areas were then analyzed under SEM-WDS (Fig. 1d, and e) and SEM-EDS (Fig. 1f) to conrm the phases.Table 3 shows the results of microprobe analyses of the phases withinthe numbered spots in Fig. 1.

Themajority of the gold particles were observed to be smaller than3 m in size (Fig. 1). Andorite was determined to be the main gold-and silver-bearing component in the ore (Fig. 1 and Table 3). Goldparticles containing silver also occurred associated with quartz and asinclusions within the minerals such as andorite (Fig. 1 and Table 3).Framboidal pyrite consisted of concentric zones having antimonyconcentrations accompanying silver (Fig. 1b, e and Table 3).

2.3. Leaching tests

In these tests, the effects of concentrations of sodium sulde (0.54.1 mol/L Na2S) and sodium hydroxide (13.75 mol/L NaOH), tem-perature (2070 C) and particle size (d80: 515 m) were studied(Table 4). The ground samples (d80: 15 m) were leached in a 1-Lglass vessel, which was mechanically stirred at 750 rpm and placed inawater bath to control the leaching temperature within2 C. A 200-ml leach solution (Na2S+NaOH)was added to the vessel, followed bya 70 g ore sample. After a leaching period of 150 min solid and liquidphases were separated by ltration and the ltrates were analyzed forantimony, gold and silver. The residues were dried, and also analyzedfor gold, silver and antimony after acid digestion to establish a massbalance. The residues were then subjected to cyanide leaching so as toassess the effectiveness of alkaline sulphide leaching pretreatment forthe extraction of gold and silver, and its correlation with the removalof antimony based upon the residue analysis.

All cyanide leaching tests (24 h) were performed in a 1-L glassreactor equipped with a pitched-blade turbine impeller rotating at750 rpm. A 200-ml leach solution (1.5 g/L NaCN) was added to thevessel. The reactor was aerated at 0.3 L/min. Table 4 presents theconditions for cyanide leaching tests. In all tests, 10-ml samples weretaken from the leach pulp at pre-determined time intervals and then,centrifuged to obtain clear aliquots for the determination of Au, Agand free cyanide in the solution. Free CN concentration was

determined by titration with a silver nitrate solution (0.02 mol/L)

236 O. Celep et al. / Hydrometallurgy 105 (2011) 234239using p-dimethylamino-benzal-rhodanine (0.02% w/w in acetone) asthe indicator. The consumption of cyanide was recorded. Theconcentration of free cyanide was maintained at 1.5 g/L over theleaching period by the addition of a 5% NaCN solution. On thetermination of cyanide leaching tests, the residues were digested inacid (HCl, HNO3, HClO4 and HF) to determine the undissolved metalcontent. Analysis of gold, silver and antimony from the solutions wascarried out using an atomic adsorption spectrometer (AAS-PerkinElmer AAnalyst 200). The extraction of metals was calculated basedon the metal content of leaching residues.

Fig. 1. (a, b, and c) Presence and association of gold and silver in ore under ore microscopyscanning electron microscopy (SEM) equipped with wavelength dispersive spectrometers (W7: Au particles); (e) Au particle in the quartz matrix with framboidal pyrite (1:Au, 2, 3, and 4:3. Results and discussion

3.1. Cyanidation without pretreatment

Cyanidation tests revealed that the extraction of gold and silverfrom the ore was low (49% Au and 18% Ag) even at a neness of grindof d80: 15 m (Fig. 2), conrming the earlier ndings (Celep et al.,2009). The extraction of gold and silver occurred most extensivelyover an initial period of 13 h. Thereafter, the dissolution of goldappeared to be arrested while a signicant decrease in the extraction

(Au: Gold; Py: Pyrite; Qz: Quartz; And: Andorite; and SbAg-S: SbAg sulphide) andDS): (d) Au disseminates within the andorite mineral (1, 2, 3, and 4: andorite; 5, 6, andpyrite); (f) SbAg sulphideminerals within quartz (1, and 2: SbAg sulphide; 3:quartz).

the release of antimony is shown in Fig. 5. A substantial dissolution ofSb (54%)was observed to occur even at 20 C. However, increasing thetemperature to 50 and 70 C improved the release of antimony to~71% and ~84%, respectively.

The effect of the particle size of the ore (d80: 515 m) on thealkaline sulphide leaching process was also studied at 70 C, 4.1 mol/LNa2S and 1.875 mol/L NaOH. Decreasing the particle size (d80) from

the ore under different conditions on the subsequent cyanideextraction of gold and silver. A remarkable 69% increase in the

Table 3Microprobe phase analyses from spots in Fig. 1.

Spot wt.% 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Au 84.8 88.2 85.5 84.8Ag 6.7 10.1 11.4 9.9 14.6 11.9 12.7 14.6 0.6 0.5 0.7 12.7 13.1Cu 0.3 1.3 1.5 1.4 0 b0.1 b0.1 0.2 0.3 0.3Fe 41.2 35.4 40.8Sb 32.9 40.3 42.8 40.8 4.2 13.4 3.3 51.7 52.2Pb 38.3 26.0 22.0 25.9 1.2 1.4 1.1As 0.7 0.8 0.7Zn b0.1 0.4 b0.1S 20.2 21.8 22.4 21.9 48.5 37.2 48.4 34.2 34.3Si b0.1 b0.1 b0.1 45.4O 3.69 9.3 2.1 52.7

98.3 99.4 100.5 98.7 97.4 98.6 99.6 98.1

237O. Celep et al. / Hydrometallurgy 105 (2011) 234239of silver was observed. Zhang et al. (1997) reported a similar decreasein the extraction of silver after 2 h leaching despite high free cyanidelevels of N50 mmol/L. Based on the thermodynamic analysis, theysuggested that the precipitation of Ag2S occurs in the presence ofsoluble sulphide since its oxidation is kinetically unfavourable undercyanide leaching conditions. It may be relevant to note that no suchtrend of decrease in the extraction of silver from the Akoluk ore wasobserved during the cyanide leaching of the roasted ore samplespresumably due to the oxidative removal of sulphide (Celep et al.,2010).

3.2. Antimony leaching during pretreatment

The leaching behavior of antimony from the ore was evaluatedduring the alkaline sulphide leach tests performed under differentconditions (Figs. 3 and 4). At a xed level of 2.05 mol/L Na2S, thedissolution of antimony was observed to increase with increasing theconcentration of NaOH in the range of 13.75 mol/L (Fig. 3). This trendwas also conrmed at a higher concentration of Na2S (4.1 mol/L) inwhich the extraction of antimony was also consistently higher thanthat in 2.05 mol/L Na2S. The enhancing effect of increasing concentra-tions of both Na2S and NaOH on the release of antimony is also shownin Fig. 4. This can be attributed to the decomposition of andorite,stibnite and zinkenite in the ore as represented by:

2Sb3PbAgS6 s + 6 S2Ag2S s + 2PbS s + 3Sb2S

45 : 5

Themaintenance of alkaline conditions is required tominimise thehydrolysis of S2 (S2+H2O=HS+OH) and hence to increasethe effectiveness of sulphide leaching (Bal, 2000; Curreli et al.,2009). This is consistent with the benecial effect of increasing theNaOH concentration on the release of antimony. Furthermore, theleaching of stibnite would also be expected to occur by NaOH(Ubaldini et al., 2000; Sminckov, 2009).

Total 98.4 99.5 100.1 99.9 99.4 100.2The inuence of the temperature of Na2SNaOH alkaline pretreat-ment (4.1 mol/L Na2S+1.875 mol/L NaOH, d80: 15 m and 2.5 h) on

Table 4Experimental conditions for alkaline leaching and cyanidation of the ore.

Parameter Alkaline leaching Cyanidation

Na2S concentration, mol/L 0.54.1 NaOH concentration, mol/L 13.75 NaCN concentration, g/l 1.5Pulp density, (w/w), % 26 24.5Temperature, C 2050702 202Leach time, hour 2.5 24Agitation, rpm 750 750Particle size, d80: micron 51015 51015Aeration, l/min 0.3pH (NaOH) Not measured 10.50.3extraction of silver was noted to occur when the ore was subjected toalkaline sulphide pretreatment prior to cyanidation (Fig. 6). Similarly,gold extraction improved from 49% to 5578% after the pretreatment.The highest extraction of silver (87%) was recorded to occur with4.1 mol/L Na2S and 2.0 M NaOH (Fig. 6). The extraction of silver andgold tended to improve with increasing the concentration of Na2S atxed concentrations of NaOH (Fig. 7). It can be inferred from these

80

100

Au

Ag

%15 to 5 mdid not produce a discernable effect on the solubilisation ofSb, which remained at 8485% (data not shown).

During these alkaline sulphide leaching tests, the dissolution ofgold, in particular, and other metals from the ore was also monitored.The extent of gold extraction was determined to be 39% whichtended to increase with increasing temperature and concentrations ofNa2S and NaOH under the conditions tested.

3.3. Cyanidation after treatment

Figs. 68 illustrate the effect of alkaline sulphide pretreatment of0

20

40

60

0 2 6 8 10 12 14 16 18Leaching time, hours

2422204

Met

al e

xtra

ctio

n,

Fig. 2. Extraction of gold and silver from the as-received ore by cyanidation (1.5 g/LNaCN) over a period of 24 h, particle size (d80) 15 m).

020

40

60

80

100

0 1 2 3 4

2.05 mol/L Na2S

4.1 mol/L Na2S

Antim

ony

rem

oval

, %

NaOH concentration, mol/L

Fig. 3. The effect of the NaOH concentration on antimony removal (temperature: 70 C;leach time: 2.5 h; particle size (d80): 15 m).

0

20

40

60

80

100

0 1 2 3 4 5

3.75 mol/L NaOH

2.5 mol/L NaOH

Antim

ony

rem

oval

, %

Na2S concentration, mol/L

Fig. 4. The effect of the Na2S concentration on antimony removal (temperature: 70 C;leach time: 2.5 h; particle size (d80): 15 m).

0

20

40

60

80

100

20 30 40 50 60 70

Ant

imon

y re

mov

al ,

%

Temperature, C

Fig. 5. Effect of the temperature on antimony removal in alkaline sulphide pretreatment(particle size d80: 15 m; Na2S: 4.1 mol/L; NaOH: 1.8 mol/L).

Fig. 6. Cyanidation of the pretreated residues (2.05 and 4.1 mol/L Na2S) for gold andsilver recovery (1.5 g/L NaCN; pH 10.5; leach time: 24 h; particle size (d80): 15 m).

238 O. Celep et al. / Hydrometallurgy 105 (2011) 234239ndings that alkaline sulphide pretreatment renders the gold andsilver-bearing refractory phases amenable to cyanide leaching.

In a similar manner, the extraction of silver in the cyanide leachingwas enhanced from 66% to 87% with the increase in the temperatureof the alkaline sulphide pretreatment using 4.1 M Na2S and 1.8 MNaOH from 20 C to 70 C as illustrated in Fig. 8. These ndingssupport the conclusions that the extraction of silver can be correlatedwith the dissolution of silver-bearing antimony sulphides such asandorite. However, comparedwith Ag, only an 8% improvement in theextraction of gold was observed with this Na2S/NaOH compositiongiving a maximum of only about 60% overall recovery (Fig. 8). Withthis alkali sulphide composition, a decrease in the particle size of theore from 15 to 5 prior to leaching did not produce any signicanteffect on the extraction of gold (5562% Au) and silver (8791% Ag).

4. Conclusions

Detailedmineralogical analysis of the ore revealed that gold occursas alloyed with silver (~1215% Ag) and mainly associated withandorite (Sb3PbAgS6) which is the most important antimonial silverphase. Alkaline sulphide leaching pretreatment was shown tosignicantly improve the extraction of gold and silver, in particular,by cyanidation. The temperature and relative concentrations of Na2Sand NaOH in the leach were identied as the most important factors0

20

40

60

80

100

0 1 2 3 4 5

Au (2.5 mol/L NaOH)Ag (2.5 mol/L NaOH)Au (3.75 mol/L NaOH)Ag (3.75 mol/L NaOH)

Met

al e

xtra

ctio

n, %

Na2S concentration, mol/L

Fig 7. Cyanidation of the pretreated residues (2.5 and 3.75 mol/L NaOH) for gold andsilver recovery (1.5 g/L NaCN; pH 10.5; leach time: 24 h; particle size (d80): 15 m).

References

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Ficeriov, J., Bal, P., Boldizarova, E., Jelen, S., 2002. Thiosulphate leaching of gold from

0

20

40

60

80

100

0 20 40 60 80 100Antimony removal, %

AgAu

20 C 50 oC

70 C M

etal

ext

ract

ion,

%

Fig. 8. Correlation between precious metal extraction and Sb removal at different

239O. Celep et al. / Hydrometallurgy 105 (2011) 234239affecting the extraction of antimony and the subsequent cyanideextraction of silver and gold. The highest gold and silver extractionswere 78% and 91%, respectively, corresponding to an increase of 29%and 74% compared to the untreated ore. The improvement in silverextraction could be attributed to the leaching of andorite since up to89% Sb was extracted in the alkaline sulphide leach. These ndingssupport other studies that indicate that alkaline Na2S leaching is asuitable pretreatment method prior to conventional cyanidation forrefractory antimonial gold and silver ores.

Acknowledgements

Sincere thanks and appreciation go to the Research Foundation ofKaradeniz Technical University (Project No: 2004.112.008.2) fornancial support, to the General Directorate of the Mineral Researchand Exploration of Turkey, to Prof. Dr. Doan Paktun and Dr. YvesThibault for support and to the Grelik Mining Trading Ind. Ltd. for

kindly providing the ore samples.a mechanically activated Cu/Pb/Zn concentrate. Hydrometallurgy 67 (13), 3743.Ficeriov, J., Bal, P., Boldizarova, E., 2005a. Combined mechano-chemical and

thiosulphate leaching of silver from a complex sulphide concentrate. Int. J. Miner.Process. 76 (4), 260265.

Ficeriov, J., Bal, P., Villachica, C.V., 2005b. Thiosulphate leaching of silver, gold andbismuth from complex sulde concentrate. Hydrometallurgy 77 (12), 3539.

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Gupta, C.K., Mukherjee, T.K., 1990. Hydrometallurgy in Extraction Processes, Volume I.CRC Press, Boston.

Iglesias, N., Carranza, F., 1994. Refractory gold-bearing ore: a review of treatmentmethods and recent advances in biotechnological techniques. Hydrometallurgy 34(3), 383395.

La Brooy, S.R., Linge, H.G., Walker, G.S., 1994. Review of gold extraction from ores.Miner. Eng. 7 (10), 12131241.

Sminckov, E., 2009. Leaching of natural stibnite using sodium hydroxide solution.JOM 61 (10), 3235.

Ubaldini, S., Veglio, F., Fornari, P., Abbruzzesse, C., 2000. Process ow-sheet for gold andantimony recovery from stibnite. Hydrometallurgy 57 (3), 187199.

Zhang, Y., Fang, Z., Muhammed, M., 1997. On the solution chemistry of cyanidation ofgold and silver bearing sulphide ores. A critical evaluation of thermodynamiccalculations. Hydrometallurgy 46 (3), 251269.temperatures (particle size (d80): 15 m; 4.1 mol/L Na2S; 1.8 mol/L NaOH).

Improved gold and silver extraction from a refractory antimony ore by pretreatment with alkaline sulphide leachIntroductionExperimentalMaterialsDetailed mineralogical characterization of the oreLeaching tests

Results and discussionCyanidation without pretreatmentAntimony leaching during pretreatmentCyanidation after treatment

ConclusionsAcknowledgementsReferences