Hydrometallurgy 108 (2011) 109–114

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Implementation of sodium hydroxide pretreatment for refractory antimonial goldand silver ores

Oktay Celep a, İbrahim Alp a,⁎, Doğan Paktunç b, Yves Thibault b

a Mining Engineering Department, Karadeniz Technical University, 61080, Trabzon, Turkeyb CANMET Mining and Mineral Sciences Laboratories, 555 Booth Street, Ottawa, Ontario, Canada

⁎ Corresponding author. Fax: +90 4623257405.E-mail address: ialp@ktu.edu.tr (İ. Alp).

0304-386X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.hydromet.2011.03.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 January 2011Received in revised form 11 March 2011Accepted 13 March 2011Available online 3 April 2011

Keywords:GoldSilverCyanidationAntimonyRefractory oreAlkaline leaching

Alkaline pretreatment of a refractory gold–silver ore containing antimony minerals such as stibnite, andorite(Sb3PbAgS6) and zinkenite (Pb9Sb22S42) was tested using sodium hydroxide in order to determine itseffectiveness in improving the recovery of gold and silver. Mineralogical investigations show that silver waspresent as andorite and Au/Ag alloy. Gold particles have been observed as associated with quartz andinclusions within the antimonyminerals. Increasing the sodiumhydroxide concentrations from 0.5 to 5 mol/L,increasing the temperature from 20 to 80 °C, and reducing the particle size from 50 to 5 μm enhanced theremoval of antimony from the ore. Up to about 75.5% Sb removalwas achieved by alkaline pretreatment, whichin turn remarkably improved the extraction of silver from levels of less than 18.7% to 90% and gold from lessthan 49.3% to 85.4% during subsequent cyanidation. These findings, consistent with mineralogical results,suggest that alkaline leaching can effectively be used as chemical pretreatment method as an alternate to thealkaline sulfide leaching in the processing of refractory antimonial gold–silver ores.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The Akoluk ore deposit, hosted by volcanic–sedimentary rocks ofthe Eastern Pontides, is situated near Ordu in northeastern Turkey.The ore deposit contains a variety of sulfide and oxide minerals, aswell as native gold (Tüysüz and Akçay, 2000; Yaylalı-Abanuz andTüysüz, 2010). Total reserves are estimated to be about 1 milliontonnes (Anon, 1993). The most significant trace and minor mineralsare associated with stibnite and zinkenite (Pb–Sb sulfosalt), whichalong with sphalerite are the most widespread ore minerals in thedeposit (Ciftci, 2000). According to Aslaner and Ottemann (1972),gold occurs as inclusions in zinkenite (PbSb2S4). Recently, Celep et al.(2009) reported that the ore consisted predominantly of quartz, theillite/kaolinite group of clay minerals and barite with lesser amountsof pyrite, stibnite, sphalerite, zinkenite and andorite. Gold occurs assmall particles ranging from 1 to 88 μm in association with sulfideminerals and quartz. Cyanide leaching tests of 24-h duration showedthat metal extractions were consistently low, at less than 47% forgold and less than 19.2% for silver. Diagnostic leaching tests suggestedthat the decomposition of the sulfide could improve the extractionof gold and silver by about 29.5% and 56.7%, respectively. Detailedmineralogical characterization of the Akoluk gold–silver ore hasindicated that the ore contains antimony sulfides, including andorite(Sb3PbAgS6) and zinkenite (Pb9Sb22S42), as the main silver and gold

carriers (Alp et al., 2010; Celep et al., 2011). Ultrafine grinding androasting of the ore were ineffective as pretreatment for improvingthe recovery of gold and silver by cyanidation (Celep et al., 2010).Cyanidation tests also showed that lead nitrate addition had a limitedeffect on gold and silver extractions. The low gold and silverextractions from the ore indicated its refractory nature to cyanideleaching and the need for a suitable chemical pretreatment process toimprove metal dissolution.

Refractory gold ores do not respond to direct cyanidation;therefore, such ores have to be pretreated prior to cyanidation toliberate the contained gold and silver so that they are readilyamenable to extraction (La Brooy et al., 1994). Roasting, pressureoxidation, bio-oxidation and, to a limited extent, ultrafine grindinghave been commercially applied to increase gold recoveries fromrefractory ores (Corrans and Angove, 1991; Iglesias and Carranza,1994; Gunyanga et al., 1999). Alkaline sulfide leaching is a suitablepretreatment process for antimonial ores and concentrates (Ubaldiniet al., 2000; Baláž and Achimovičová, 2006; Curreli et al., 2009; Aweand Sandström, 2010; Awe et al., 2010) by making the silver availableto cyanide solutions or removing the hazardous or penalty elementssuch as As and Sb from the ores and concentrates. Alp et al. (2010) andCelep et al. (2011) have already shown that alkaline sulfide leachingis an effective pretreatment method ahead of cyanide leaching forthe extraction of gold and silver from antimonial refractory ores.Recoveries as high as 90% Ag and 82.6% Au were achieved afterthe removal of 95% Sb in an alkaline pre-treatment stage under theconditions of 4 mol/L Na2S and NaOH,≤15 μm particle size, and 80 °C(Alp et al., 2010). In addition, increasing NaOH concentrations

110 O. Celep et al. / Hydrometallurgy 108 (2011) 109–114

enhanced the dissolution of Sb-bearing minerals. This finding wasconsistent with reports on leaching of antimonial sulfides such asstibnite (Anderson and Krys, 1993; Ubaldini et al., 2000; Smincákováand Komorová, 2005; Smincáková, 2009). Smincáková (2009)showed that the leaching of stibnite by sodium hydroxide waspossible. Baláž (2000) showed that proustite (Ag3AsS3) and pyrargrite(Ag3SbS3) decompose during the alkaline leach, which makeshigh silver recovery possible during subsequent cyanidation. Nosuch studies on alkaline pretreatment of antimonial ores containingandorite (Sb3PbAgS6) and zinkenite as the main gold/silver bearingminerals have been reported in the literature.

Recent studies (Alp et al., 2010, Celep et al., 2011) indicated thatgold losses (up to 13%) occur during the alkaline sulfide leachingand the losses increase at high temperatures and high concentra-tions of sulfide. Furthermore, considering the problematic healthissues associated with the sulfide leaching, this study was designed toevaluate alkaline leaching using sodium hydroxide as a potentialpretreatment process alternative to alkaline sulfide leaching.

2. Experimental

2.1. Materials

The ore used in this study is an antimony-rich refractory gold–silver ore from Akoluk (Ordu-Turkey). The ore samples were reducedin size by crushing and grinding. A laboratory-scale stirred media millwas used for fine grinding (80% passing size, d80=50, 15 and 5 μm)prior to the leaching tests. Particle size analysis of the ore samples wasperformed by a Malvern Mastersizer 2000 model laser analyzer.The earlier chemical and mineralogical studies (Celep et al., 2009)indicated that it is a high grade gold and silver ore assaying at 220 g/tAg and 20 g/t Au. In addition, the ore contained 52.2% SiO2, 17.1% Ba,4.7% Al2O3, 6.9% S, 1.6% Sb, 1.5% Zn, 1.3% Fe2O3, 0.4% Pb, 0.02% Asand 0.04% Cu (all in weight percent). Quartz, the illite/kaolinitegroup clay minerals and barite are the predominant phases in the ore.Pyrite, stibnite, sphalerite, zinkenite and andorite are the main sulfideminerals identified in the ore (Celep et al., 2011).

2.2. Mineralogical characterization of the ore

Mineralogical analysis of the ore sample was performed to deter-mine gold and silver bearing phases. Characterization studies werecarried out using a FEI Quanta 400MK2 Scanning electron microscopy(SEM) equipped with EDAX Genesis 4XMI at the Mineral Researchand Exploration Institute of Turkey and a HITACHI variable-pressureSEM with a Link microanalysis system at CANMET. Microanalysis ofthe mineral grains was performed by a JEOL JXA 8900 electronprobe X-ray microanalyzer (EPMA) at CANMET utilizing five wave-length dispersive spectrometers (WDS) operated at 20 kV with aprobe current of 20 to 30 nA.

2.3. Experimental work

Tests were designed to evaluate the effects of sodium hydroxideconcentrations, temperature and particle size (d80: 5–50 μm). Theground samples (d80: ≤50 μm) were leached in a 1-L glass reactorimmersed in a water bath to control the leaching temperature (20–80 °C) within ±2 °C. The vessel with 200-mL leach solution (NaOH)and 70 g ore sample (solids 35% w/vol) was continuously stirredat 750 rpm. Sodium hydroxide (NaOH, assay 99.9% Merck) in therange of 0.5–5 mol/L NaOH was used to maintain the alkalinity. Theleach solution was sampled (10 mL) at regular time intervals forthe analysis of antimony. At the end of leaching after 120 min, solidand liquid phases were separated by filtration and the filtrateswere analyzed for Sb, Au, Ag, Pb, Zn, Cu and Fe. The residues were air-dried, and sampled for analysis to determine the metal recoveries.

Cyanide leaching of the residues was then carried out to deter-mine the effects of alkaline leaching pretreatment on the extractionof gold and silver. Glass reactors (1 L) were used for cyanide leachingof the residues. The reactors were mechanically agitated withpitched-blade turbine impellers and aerated at a flow rate of 0.3 L/min (Alp et al., 2010; Celep et al., 2011). NaCN (Merck) and NaOHwere used to adjust the pH at 10.5 during cyanidation. In all tests,10-mL samples were taken from the leach pulp at pre-determinedtime intervals and then centrifuged to obtain clear aliquots for thedetermination of Au, Ag and free cyanide in solution. Silver nitratetitration in the presence of p-dimethylamino-benzal-rhodanine(0.02% w/w in acetone) as the indicator was used to determinethe concentration of free CN− in samples (Celep et al., 2011). Ifrequired, concentrated cyanide solution (5% NaCN) was added tomaintain free CN− concentration at the initial level of 1.5 g/L NaCN,and consumption of NaCN was recorded (Celep et al., 2011). On thetermination of cyanide leaching tests, the residues were digestedin acid (HCl, HNO3, HClO4 and HF) to determine the undissolvedmetal content. Analysis of gold, silver and antimony from the solu-tions was carried out using an atomic absorption spectrometer (AAS-Perkin Elmer AAnalyst 400). The extraction of metals was calculatedbased on the metal content of leaching residues.

3. Results and discussion

3.1. Alkali pretreatment

Alkaline leaching has a significant advantage over the alkalinesulfide leaching due to lower reagent costs. In addition, a small amountof gold can be dissolved in alkaline sulfide solutions (Anderson, 2001;Jeffrey and Anderson, 2003; Alp et al., 2010; Celep et al., 2011). Thissituation will bring additional costs in the course of gold recoveryfrom solution. The alkaline sulfide leaching has also potential envi-ronmental issues related to the formation of H2S gasses.

3.1.1. Dissolution of metals during pretreatmentAlkali pretreatment of the ore at 15 μm (d80 particle size) using

3 mol/L NaOH at 80 °C in sodium hydroxide solution caused thedissolution of elements in the following proportions: 64.4% Sb, 0.5%Ag, 0.06% Fe, 1.5% Cu, 0.05% Zn and 0.03% Pb. The pretreatment had noeffect on the dissolution of gold. The results indicate that sodiumhydroxide leaching is highly selective for the removal of antimonyfrom the ore.

Although alkaline leaching using alkali metal hydroxides ispotentially applicable to elements that form anionic complexes suchas Al, Sb, As, Cu, Fe and Pb, the higher metal extractions would requirehigh pressure or temperatures under oxidizing conditions (Gupta andMukherjee, 1990; Filippou et al., 2007). The alkaline leaching (3 mol/LNaOH, 80 °C, d80: 15 μm) had no important effect on the dissolutionof metals with the exception of antimony. For this reason, the effectson the removal of antimony of NaOH concentration, temperatureand particle size were investigated.

3.1.2. Effect of NaOH concentrationThe effect of leaching time on antimony removal from the ore was

investigated at different reagent concentrations (0.5–5 mol/L NaOH)at the fixed slurry temperature of 80 °C. The experimental results arepresented in Fig. 1. Antimony dissolution varies between 10.4 and70.1%. Increasing leaching time resulted in the increase of antimonyextraction. Most of antimony dissolution had occurred within thefirst 5 min at high reagent concentrations. When the leaching timewas increased to 2 h, antimony removal was improved only slightly,even at 5 mol/L NaOH.

Fig. 1 shows the effect of NaOH concentration (0.5–5 mol/L) onthe removal of Sb from the ore (d80=≤15 μm) at 35% w/v solidsand 80 °C. The dissolution of Sb improved with increasing the

0

20

40

60

80

100

0 20 40 60 80 100 120Leach time, min.

Sb r

emov

al, %

0.5 M NaOH1 M NaOH3 M NaOH5 M NaOH

Fig. 1. Effect of leaching time and NaOH concentrations on the removal of antimonyfrom the ore (d80: 15 μm, 80 °C).

0

20

40

60

80

100

0 20 40 60 80 100 120

Leach time, min.

Sb r

emov

al, %

20°C40°C60°C80°C

Fig. 3. Effect of leaching time and temperature on the removal of antimony from the ore(3 mol/L NaOH, d80: 15 μm).

111O. Celep et al. / Hydrometallurgy 108 (2011) 109–114

concentration of NaOH. The highest removal of Sb at 70.1% wasachieved at the highest reagent concentration of 5 mol/L NaOH. Thebeneficial effect of sodium hydroxide pretreatment is attributed to thedecomposition of antimonial phases such as andorite, stibnite andzinkenite present in the ore. Based on the speciation of Sb (Fig. 2),decomposition of these phases by hydroxide would release antimonyin the form of species such as SbO2

−, Sb(OH)4−, Sb(OH)6−, SbOS−, andSbS2− (Baláž, 2000; Anon, 2005). Smincáková (2009) reported thatstibnite was dissolved as SbOS− and SbS2− species (Eq. (1)) in theleaching by sodium hydroxide (at 0.5–4 wt.% NaOH). In the case of thearsenical silver-bearing sulfide minerals such as proustite (Ag3AsS3)in alkaline leaching, it was indicated that silver could remain in Ag2S

Fig. 2. Eh–pH diagrams of the system Sb–O–H (Sb=10−10 mol/L, 25.15 °C, 105 Pa.)(JNC-TDB/GWB) (Anon, 2005).

(Eq. (2)) phase which is highly soluble within cyanide solutions(Baláž, 2000). It is affirmed that the antimonial minerals such asandorite and zinkenite as well as stibnite can be decomposed withsimilar mechanism.

Sb2S3 sð Þ + 2NaOH→NaSbOS aqð Þ + NaSbS2 aqð Þ + H2O ð1Þ

2Ag3AsS3 sð Þ+ 6NaOH→3Ag2S sð Þ+ Na3AsO3 aqð Þ+ Na3AsS3 aqð Þ + 3H2O

ð2Þ

3.1.3. Effect of temperatureThe influence of temperature (20–80 °C) on the release of antimony

during the NaOH alkaline pretreatment (3 mol/L NaOH, d80: ≤15 μmand 120 min.) is shown in Fig. 3. An increase of temperature from 20to 80 °C improved the antimony extraction. At 20 °C, only 23.1% ofthe antimony was solubilized from the ore. The extraction of Sb wassubstantially improved to 64.4% with an increase in the temperature

0

20

40

60

80

100

0 20 40 60 80 100 120

Leach time; min.

Sb r

emov

al, %

50 µm

15 µm

5 µm

Fig. 4. Effect of particle size on the removal of antimony from the ore (3 mol/L NaOH,80 °C).

Fig. 5. Au particle in quartz matrix with framboidal pyrite.

0

20

40

60

80

100

0 4 8 12 16 20 24

Leach time, hours

Ag

extr

actio

n, %

as-received oreafter alkaline leach

a

R2 = 0.9775

0

20

40

60

80

100

0 20 40 60 80 100

Sb removal, %

Ag

reco

very

, %

b

Fig. 6. (a) Effect of pretreatment on the silver extraction by cyanidation (3 mol/L NaOH,80 °C, d80: 5 μm) and (b) dependence of the cyanide extraction of silver on the removalof antimony by the alkaline pretreatment (d80: 15 μm, 80 °C).

112 O. Celep et al. / Hydrometallurgy 108 (2011) 109–114

to 80 °C. These findings suggest that temperature is themost influentialfactor in the alkaline treatment process.

3.1.4. Effect of particle sizeThe effect of particle size of the ore (d80: ≤5–15–50 μm) on the

alkaline leaching process was studied at a fixed reagent concentrationof 3 mol/L NaOH, and a slurry temperature of 80 °C. Decreasing theparticle size (d80) from 50 to 5 μm produced a positive effect on thesolubilization of Sb, which improved from 54 to 72.5% Sb removal(Fig. 4). In addition to increased surface area and “liberation” ofantimony sulfides, the beneficial effect of reducing particle size can bealso attributed to mechanical activation phenomena as reported byBaláž (2000).

It appears that not all the antimony is responding to alkalineleaching. Approximately ~25% antimony remained in the residue. Itappears that the framboidal pyrite with concentric enrichments ofantimony and silver is probably responsible for this behavior (Fig. 5,Table 1). In this case, unrecovered antimony would represent theamount tied to framboidal pyrite.

3.2. Cyanidation after treatment

The influence of the sodium hydroxide alkaline pretreatmentprior to cyanidation was shown to be effective for the decompositionof the most of antimony minerals. All cyanidation tests indicatedthat silver and gold recoveries improved after antimony removal

Table 1Microprobe phase analyses from spots in Fig. 5.

Spot 1 2 3 4 5

wt.% Gold Pyrite Pyrite Pyrite Pyrite

Au 85.57Ag 13.79 3.63 0.18 0.78 0.25Cu 0.32 0.36 0.41 0.34Fe 33.77 42.23 41.16 42.01Sb 8.32 1.76 1.67 1.77Pb 5.38 0.58 1.21 0.78As 0.62 0.69 0.57 0.58Zn 0.08 0.31 0.14 0.55S 44.5 49.09 48.85 48.81Si 0.09 0.06 0.26 0.05O 2.91 2.07 2.16 2.94Total 99.36 99.62 97.33 97.21 98.06

by alkaline leach. While less than 18.7% Ag was extracted from theuntreated ore, 90% of the silver was recovered following 75.5%antimony removal by alkaline leaching (Fig. 6). The highest silverrecovery (90% Ag) was obtained with 3 mol/L NaOH at 80 °C and5 μm particle size. Similarly, gold extraction was shown to improvefrom 49.3% to 55.8–85.4% following the pretreatment (Fig. 7). Thehigh overall Au and Ag extractions with pretreatment confirm thata large proportion of the gold and silver was refractory in naturebecause of their occurrence in the structures or as unliberated(i.e. locked) inclusions in the antimony minerals that are apparentlyinsoluble in cyanide solutions.

The occurrence and association of gold and silver are illustratedin Fig. 8 where sections of the feed sample were analyzed underSEM-WDS. Table 2 shows the results of microprobe analyses of thephases within the numbered spots in Fig. 8. Andorite is the maingold and silver-bearing mineral in the ore (Fig. 8). Gold particlescontaining silver also occurred associated with quartz and as in-clusionswithin theminerals such as andorite (Table 2). Andorite wereidentified to be the most important Ag bearing sulfide phase. Thesefindings indicate andorite (Sb3PbAgS6) and zinkenite (Pb9Sb22S42) asthe main gold and silver bearing and hosting minerals (Celep et al.2011) are not amenable to cyanide leaching without NaOH pretreat-ment process.

0

20

40

60

80

100

0 4 8 12 16 20 24

Leach time, hours

Au

extr

actio

n, %

as-received oreafter alkaline leach

a

R2 = 0.9457

0

20

40

60

80

100

0 20 40 60 80 100

Sb removal, %

Au

reco

very

, %

b

Fig. 7. (a) Effect of pretreatment on the gold extraction by cyanidation (3 mol/L NaOH,80 °C, d80: 5 μm) and (b) dependence of the cyanide extraction of gold on the removalof antimony by the alkaline pretreatment (d80: 15 μm, 80 °C).

Fig. 8. Backscattered electron images showing the locations of microprobe analyseslisted in Table 2. (a) Disseminated gold grains (white) and Sb–Pb–Ag sulfide(whitish gray) in andorite grains (light gray) (b) Sb–Ag sulfide grains (white) inquartz (gray).

113O. Celep et al. / Hydrometallurgy 108 (2011) 109–114

The consumption of cyanide was determined to be 4.8–5 kg NaCNper ton of the ore. In comparison with the consumption of 9.1 kg/tfor the untreated ore (Celep et al., 2009), this figure represents asignificant reduction in cyanide consumptionwith important cost andenvironmental implications, including the treatment costs of residualcyanide in the tailings pond water. Thus, the alkaline pretreatmentprocess is considered to be an appropriate method for processingof the antimonial refractory gold–silver ores.

4. Conclusion

Sodium hydroxide pretreatment of the refractory gold–silverore from the Akoluk deposit showed that up to 75.5% of theantimony can be removed by adjusting the molar concentration ofthe leaching solution. Temperature, particle size and NaOHconcentrations were identified to be the most important factorsaffecting the extraction of antimony and subsequent cyanideextraction of silver and gold. Whereas the direct cyanidationresulted in gold recoveries of about 49.3%, the alkaline pretreat-ment process significantly improved the gold recovery to 85.4%.With the removal of about 75.5% Sb during pretreatment, Ag

recoveries reached 90% from very low levels of 5–10%. A significantincrement in gold and silver extractions corresponded to theremoval of antimony from the ore during the alkaline pretreatment.Based on mineralogical investigation, silver was present as andoriteand Au/Ag alloy. Gold particles containing silver have beenobserved as associated with quartz and inclusions within theantimony minerals such as andorite. These findings were consistentwith the alkali pretreatment results. Experimental results demon-strated that sodium hydroxide pretreatment is a viable extractivemetallurgy technique for the processing of refractory antimonialgold and silver ores. Mineralogical characterization of materialsresulting from alkaline pre-treatments could shed further light onmechanisms of Au and Ag liberation and will be discussed in detailsat another paper.

Acknowledgements

Sincere thanks and appreciation go to the General Directorate ofthe Mineral Research and Exploration of Turkey (MTA) for SEM-EDXanalysis and to Anatolia Minerals Development Ltd. for kindlyproviding the ore samples.

Table 2Microprobe analyses (wt.%) of andorite, an unknown Sb–Ag sulfide, gold and quartz grains shown in Fig. 8.

Spot 1 2 3 4 5 6 7 8 9 10

wt.% Sb–Pb–Ag sulfide Andorite Andorite Andorite Gold Gold Gold Sb–Ag sulfide Sb–Ag sulfide Quartz

Au 84.8 88.2 85.5Ag 6.7 10.1 11.4 9.9 14.6 11.9 12.7 12.7 13.1Cu 0.3 1.3 1.5 1.4 0 b0.1 b0.1Sb 32.9 40.3 42.8 40.8 51.7 52.2Pb 38.3 26.0 22.0 25.9S 20.2 21.8 22.4 21.9 34.2 34.3Si 45.4O 52.7Total 98.4 99.5 100.1 99.9 99.4 100.2 98.3 98.6 99.6 98.1

114 O. Celep et al. / Hydrometallurgy 108 (2011) 109–114

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