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Microbial Dissolution of Zn-Pb Sulfide Minerals UsingMesophilic Iron and Sulfur-Oxidizing AcidophilesSina Ghassa a b , Zohreh Boruomand a , Marzie Moradian a , Hadi Abdollahi a b c & Ata Akcil ca Applied Research Center of Geology Survey of Iran , Nano-Bio Earth Lab , Karaj , Iranb School of Mining, College of Engineering , University of Tehran , Tehran , Iranc Department of Mining Engineering, Mineral Processing Division (Mineral-Metal Recovery andRecycling Research Group) , Suleyman Demirel University , Isparta , TurkeyAccepted author version posted online: 17 Mar 2014.Published online: 25 Sep 2014.

To cite this article: Sina Ghassa , Zohreh Boruomand , Marzie Moradian , Hadi Abdollahi & Ata Akcil (2015) MicrobialDissolution of Zn-Pb Sulfide Minerals Using Mesophilic Iron and Sulfur-Oxidizing Acidophiles, Mineral Processing and ExtractiveMetallurgy Review: An International Journal, 36:2, 112-122, DOI: 10.1080/08827508.2014.898302

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Microbial Dissolution of Zn-Pb Sulfide Minerals UsingMesophilic Iron and Sulfur-Oxidizing Acidophiles

SINA GHASSA1,2, ZOHREH BORUOMAND1, MARZIE MORADIAN1, HADI ABDOLLAHI1,2,3, and ATA AKCIL3

1Applied Research Center of Geology Survey of Iran, Nano-Bio Earth Lab, Karaj, Iran2School of Mining, College of Engineering, University of Tehran, Tehran, Iran3Department of Mining Engineering, Mineral Processing Division (Mineral-Metal Recovery and Recycling Research Group),Suleyman Demirel University, Isparta, Turkey

The purpose of this study is to test the feasibility of using mixed culture of iron and sulfur-oxidizing bacteria for the dissolution ofmetals from high-grade zinc and lead sulfide ore. Considering that the roll crusher could reduce the ore size to less than 2mm, thissize fraction was selected in order to study the possibility of removing mill circuit. Effects of parameters such as pulp density, initialpH, Fe2þ, oxidation–reduction potential (ORP), and pH fluctuations were investigated, as well. The maximum Zn dissolution wasachieved under the conditions of initial pH 2, initial 75 g=L FeSO4 � 7H2O, and pulp density of 50 g=L. The results indicated thatunder the optimum conditions, about 68.8% of zinc was leached during 24 days of bacterial leaching treatment. The lead recoverieswere low (about 1%), because of precipitation of Pb as lead arsenate chloride. Furthermore, the surface studies by using SEMimages showed that during chemical leaching the ore dissolution starts from surface discontinuities, but in bacterial leaching allsurface becomes involved. In addition, in another process the ore was leached separately with sulfuric acid and sodium hydroxide,and then final results were compared to the bacterial leaching tests in order to find the optimum hydrometallurgical method toextract zinc and lead from these ores.

Keywords: biohydrometallurgy, mesophilic acidophiles, microbial dissolution, Zn-Pb sulfide minerals

1. Introduction

The negative environmental impacts, complex processes, andhigh production cost of traditional methods of metal extrac-tion make research and development of new methodsof metal extraction a necessity. Considering that the biohy-drometallurgical techniques can reduce above-mentionednegative effects, many researches have been conducted onapplication of biotechnology for extracting metals fromlow-grade ores and concentrates since 1950s (Ehrlich 2001;Liao and Deng 2003; Ehrlich 2004; Rehman et al. 2009;Baba et al. 2011; Ilyas et al. 2012; Sarirchi et al. 2012;Manafi, Abdollahi, and Tuovinen 2013). Bacterial leachingtechnology, which is based on the ability of microorganismsto aid transformation of solid compounds into soluble andextractable elements, has been rapidly developed in recentdecades for its advantages, which include mild reactionconditions, low energy consumption, simple process, lowenvironmental impacts, and being suitable for low-grade

mine tailings and residues (Akcil 2004; Cheng et al. 2009;Akcil and Deveci 2010).

The most common microorganisms for bacterial leachingbelong to the genera Acidithiobacillus and Leptosprillium,which are acidophilic and chemolithoautotrophic meso-philes. Three mechanisms of bacterial action have beensuggested by recent studies: (1) an indirect mechanism,wherein the bacteria oxidize Fe2þ to Fe3þ in bulk solution(according to Equation (1)) and then the Fe3þ oxidize thesulfide minerals; (2) an indirect contact mechanism, whereinthe bacteria attach to the mineral surface oxidize ferrousions to ferric ions within the biofilm, which is comprisedof bacteria and exo-polymeric material, and then the ferricions generated within this layer oxidize the sulfide minerals;and (3) a direct contact mechanism, wherein bacteria attachto the mineral surface and it oxidizes through enzymaticreaction without requiring iron ions (Tributsch 2001;Watling 2006; Deveci and Ball 2010; Xingyu et al. 2010;Abdollahi et al. 2013). In the presence of bacteria, the fer-rous ions oxidize to ferric ions through following biochemi-cal reaction:

2Fe2þ þ 0:5O2 þ 2Hþ����!bacteria

2Fe3þ þH2O ð1Þ

Boon et al. (1998) have postulated that the microbialleaching of galena or sphalerite often proceeds through an

Address correspondence to Ata Akcil, Department of MiningEngineering, Mineral Processing Division (Mineral-MetalRecovery and Recycling Research Group), Suleyman DemirelUniversity, Isparta, TR 32260, Turkey. E-mail: ataakcil@sdu.edu.tr

Color versions of one or more of the figures in the articlecan be found online at www.tandfonline.com/gmpr.

Mineral Processing & Extractive Metall. Rev., 36: 112–122, 2015

Copyright # Taylor & Francis Group, LLC

ISSN: 0882-7508 print/1547-7401 online

DOI: 10.1080/08827508.2014.898302

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indirect mechanism according to the following relation:

MeSþ 2Fe3þ ! Me2þ þ Sþ 2Fe2þ ð2Þ

where MeS signifies sulfide minerals in the form of Me2þS2�

(e.g., PbS or ZnS).Two last reactions show the importance of iron in

bacterial leaching process. Previous studies show the greatinfluence of Fe in bacterial ore treatment (Sand et al. 2001;Deveci, Akcil, and Alp 2004).

Oxidation of the added elemental sulfur and=or oxidationof present pyrite by Acidithiobacillus produce acid (Vilcaez,Suto, and Inoue 2008) through following biochemicalreaction:

S0 þ 1:5O2 þH2O ����!Bacteria2Hþ þ SO4

2� ð3Þ

The microbiological studies show the influence of pH andiron ions in bacterial growth and activity, too. Previousstudies also confirm the importance of pulp density in orebiodissolution (Haghshenas et al. 2009; Sasaki, Takatsugi,and Hirajima 2011). Most of bioleaching studies have beencovering low-grade ores, ore concentrates, and secondaryresources such as mineral processing tailings and smeltingslag. However, few studies have been conducted on bacterialleaching of high-grade ores. In most of previous studies, fineparticles (less than 100–200 mm) have been used for shakingflasks tests. These tests then could be scaled up to designindustrial bioreactors. In all of these tests, mill circuits wererequired for grinding the ore particles. However, roll crushercan reduce the particle size to less than 2mm and if orebecome process-able in this size, then it becomes possibleto remove the mill circuit. This study was designed to inves-tigate the kinetics behavior of biodissolution of medium-sizefraction (less than 2mm), high-grade zinc (Zn) and lead (Pb)ore. Dissolution behavior of arsenic (As) and antimony (Sb)during Zn–Pb bacterial leaching process was also examined.Furthermore, this investigation tried to optimize the initialpH, Fe2þ concentration, and pulp density in order to achievethe highest dissolution rate.

2. Materials and Methods

2.1 Ore Characterization

A representative high-grade Zn–Pb sample was suppliedfrom Angouran mine in Zanjan province of Iran. Accordingto the Iran’s Zinc development Mine Co., this mine withaverage zinc sulfide grade 27.7%, has one of the highest zincgrades in the world (Gilg et al. 2006). A double roll crusher(Danesh Faravaran, Iran) and a 2-mm ASTM sieve were

used to reduce the ore size in a close circuit. This mechanismwas designed to remove the necessity of mill circuit, whichconsumes the highest energy in mineral processing systems.The chemical composition of the ore is listed in Table 1.The total Zn content was 40.71% and 24 h acid leaching withsulfuric acid showed that H2SO4-soluble Zn-content was15.64%. The ore sample also contains 12.4% lead. Theoptical mineralogical studies and X-ray diffraction (XRD)analysis (Inel EQUINOX3000, USA) showed that the mainminerals were sphalerite (ZnS) and galena (PbS). Inaddition, ore sample contained carbonate minerals, whichwere mainly smithsonite (Zn(CO3)) and cerussite (Pb(CO3)).Also calcite (CaCO3) was identified as gangue mineral. TheXRD graph is shown in Figure 1.

2.2 Microorganisms

Source of bacteria was acid mine drainages (AMD) and soilsamples, which were collected from Angouran lead and zincmine. Mesophilic acidophilic chemolithotrophic consortia(MACC) was obtained by inoculating the water and soilsamples in 9K medium containing Ca(NO3)2 0.01 g=L,KCl 0.1 g=L, K2HPO4 0.5 g=L, MgSO4 � 7H2O 0.5 g=L,(NH4)2SO4 3.0 g=L, and FeSO4 � 7H2O 44.22 g=L (Silvermanand Lundgren 1959). The pH of the medium was set closeto 1.8 by sulfuric acid. In addition, 1 g of elemental S wasadded to each flask to promote the growth of sulfur-oxidizing bacteria (Wei-Min and Shi-fei 2007). The flaskswere incubated in shaking incubator (Shin Saeng SKIR-601,Korea) at 32�C temperature and 120 rpm for 5 days and thenbacteria culture was transferred to new media. The bacterialgrowth was observed by counting bacterial cells underoptical microscope using Neubauer Counting Chamber.The medium containing bacteria was transferred to newmedia every 5–7 days (according to their growth) and afterfive transfers, mixed culture of bacteria were stored at 4�Cfor future inoculation and bacterial leaching experiments.The average microbial concentration was enumerated 4� 107

cells=mL by counting under optical microscope (KRUSS,Germany). The bacterial cultures were then used for SEM(KYKY-EM3900M, China) studies. Figure 2 shows thebacillus shape of extracted bacteria from soils, which werecollected from the mine.

Furthermore, a culture of bacteria was transferred to thenew media to study the bacterial growth pattern. Generally,the bacteria that used in bioleaching process obtain energyfrom oxidation of either ferrous ions to ferric, or sulfur com-pounds to sulfuric acid (Rawlings 2002; Rohwerder et al.2003, Haghshenas et al. 2012). In most acidic bacterialleaching systems, the oxidation–reduction potential (ORP)predominantly reflects the Fe3þ=Fe2þ ratio and increasing

Table 1. Chemical analysis of the ore sample

ComponentZn Pb Fe As Sb Cu Ag Cd(%) (%) (%) (mg=kg) (mg=kg) (mg=kg) (mg=kg) (mg=kg)

Amount 40.7 12.4 1.1 3621 115 500 32.14 2340

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in ORP reflected the activity of iron-oxidizing bacteria(Manafi, Abdollahi, and Tuovinen 2013). On the otherhand, oxidation of added elemental sulfur to sulfuric acidby Acidithiobacillus (Equation (3)) causes a decrease in pH.However, the bacterial activity and growth can be monitoredwith regular measurement of pH and redox potential.Figure 3 shows the pH and ORP fluctuation during 30 daysincubating of bacteria in 32�C. The curves are drawn on thebasis of regression method by using SigmaPlot 12.0. Figure 3shows that the highest growth and activity rate of bacteriawas between fifth and tenth days.

2.3 Bacterial Leaching Experiments

Bacterial leaching experiments were performed in 250mLErlenmeyer flasks containing 90mL sterile and free Fe2þ

9K medium and 10mL of inoculums of culture. In orderto study the effects of initial medium pH, Fe2þ concen-tration, and pulp density on the bacterial leaching process,a series of flasks were arranged with the 9K pH at 1, 1.5,and 2, initial ferrous sulfate concentration at 25, 50, and

75 g=L and pulp density at 50, 100, and 150 g=L. Theoriginal 9K medium contained 50 g=L FeSO4 � 7H2O, whichacted as Fe2þ source. In this study, one higher level and onelower level of FeSO4 � 7H2O were selected to study the effectof ferrous ions on bacterial leaching. Design-Expert 7 (DX7)software was used for designing experiments. Factorial Menuand General Factorial methods were used to design the tests.

Fig. 1. X-ray diffraction of ore before bacterial leaching experiments.

Fig. 2. The bacillus shape of indigenous bacteria underelectronic microscope (SEM).

Fig. 3. Changes in ORP (A) and pH (B) with time in a culturemedia.

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Three influential factors at three different levels were selectedto study the above-mentioned parameters. Table 2 shows theoutput of DX7 software. In addition, 1 g elemental S wasadded to each flask. Control tests were carried out byaddition of 97mL deionized water and 3mL formaldehyde37% (as bactericide). The extraction of zinc (Zn), lead(Pb), arsenic (As), antimony (Sb), and iron (Fe) was theninvestigated after 24 days processing and under the optimumconditions obtained from above-mentioned factors. Allflasks were shaken in a shaker incubator at 32�C and120 rpm. The test with highest zinc recovery then wasrepeated again with presence and absence of bacteria for30 days to study the metals concentration fluctuations. Foranalysis of metals, the suspended solids were allowed to set-tle for 30min before sampling of the solution phase. Sampleswere filtered (0.45 mm) for analysis of dissolved zinc, lead,arsenic, antimony, and total dissolved iron (total Fed).The pregnant leach solutions (PLS) were analyzed for zinc,lead, iron, copper, and silver by atomic absorption spec-troscopy (AAS) (Varian AA 240Z, Germany) and arsenicand antimony by atomic fluorescence spectroscopy (AFS)(AF-640A, China). The conditions of bacterial leachingexperiments were presented in Table 2.

2.4 Chemical Leaching Experiments

The sample used in this study contains carbonate mineralssuch as cerussite and smithsonite. Mesophilic bacteriaplay no direct role on leaching of these minerals and theirdissolution follows the chemical leaching mechanism. Thecarbonate minerals dissolve if they are attacked by acid oralkaline. For studying the acidic and alkali leachable partof the ore and compare the results of chemical leachingand bacterial leaching process, a series of acid and alkaline

leaching experiments were designed based on previousleaching studies (Aydogan, Aras, and Canbazoglu 2005;Chen et al. 2009). Leaching tests were performed in 200mLbeakers in 32�C at pulp density of 50 g=L and �2mm oreand 300 rpm shaking speed. Two similar ore samples wereleached for 24 h, one in 30% acid sulfuric and the other in5.5M NaOH in separate beakers. Deionized water at pH 2was used for chemical leaching as control test (Table 3).

3. Results and Discussions

3.1 Oxidation–Reduction Potential (ORP) and pH

Oxidation–reduction potential is one of the most importantfactors during bacterial leaching process, which shows theoxidation rate and bacterial activities, so the ORP changeswere monitored during 24 days bacterial leaching experi-ments. Figure 4 shows fluctuations of redox potentialaccording to test number. Generally, the control flasks (tests16, 17, 18) had the lowest final redox potential. The resultsshowed that the final redox potential was in a direct relationwith metals solubilization. Figure 4C indicates that the high-est final redox potential was reported for test 13, which hadthe highest zinc recovery. On the other hand, the lowest finalredox potential was for test 4 that had the lowest zinc recov-ery. As shown in Figure 4, the flasks with initial pH 2 hadhigher final redox potential and ORP decreased withreduction in the initial pH due to higher bacterial count inhigher pH. In fact, the extracted bacteria showed tendencytoward pH near 2. The redox potential fluctuated between136 and 536 (mV). In most of the flasks, the ORP passedthrough a decreasing period and after that the redox poten-tial started to increase. The ORP has direct relation withFe3þ=Fe2þ ratio, as mentioned before. As time passed andthe bacterial count increased, the ferrous ions oxidized toferric ions by bacteria and redox potential started toincreased. However, the redox potential in control flaskshad smooth fluctuation due to absence of bacteria.

The pH changes are illustrated in Figure 5. According toFigure 5A, tests with initial pH 1, passed an increasing per-iod of pH, and after almost 15 days the pH started todecrease and then stabilized on around 2. The pH increaseddue to dissolution of the carbonate minerals and using Hþ inreaction with (CO3)

2�. But when the bacterial activitystarted, they oxidized the added elemental S to sulfuric acidand this, in turn decreased the pH, which thereafter becamestabilized. The pH of tests with initial pH 1.5, 2 raised andstayed at around 2.5. The pH of control tests rose during24 days treatment. The slope of changes in these tests wassharp at the beginning of the experiment but it reduceddue to dissolving of most of the carbonate minerals. The

Table 2. The condition of mixed mesophilic bacterial leachingtests

Testnumber

InitialpH

FeSO4 � 7H2O(g=L)

Pulp density(g=L) Type

1 1 25 50 Bioleaching2 1 75 50 Bioleaching3 1 50 100 Bioleaching4 1 25 150 Bioleaching5 1 75 150 Bioleaching6 1.5 50 50 Bioleaching7 1.5 25 100 Bioleaching8 1.5 50 100 Bioleaching9 1.5 75 100 Bioleaching10 1.5 50 150 Bioleaching11 2 25 50 Bioleaching12 2 50 100 Bioleaching13 2 75 50 Bioleaching14 2 25 150 Bioleaching15 2 75 150 Bioleaching16 1 50 100 Control test17 1.5 50 100 Control test18 2 50 100 Control test

Table 3. The condition of leaching experiments

Test number Leaching agent Concentration pH

19 Sulfuric acid 30% (v=v) –20 Sodium hydroxide 5.5M 1321 Water – 2

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carbonate minerals dissolve in acidic or alkaline condition,first. In the presence of the carbonate phases, acid consumesand pH increases.

3.2 Zinc Solubilization and Recovery

After 24 days treatment, the pregnant leach solutions wereanalyzed for zinc, lead, iron, silver, copper, arsenic, andantimony. Figure 6 shows the zinc recoveries in differentbacterial leaching conditions. The highest zinc recoverywas 68.8%, obtained from the flask with pulp density¼50 g=L, ferrous sulfate concentration¼ 75 g=L, and initialpH 2 (test no 13). The zinc concentration in this flask was

14 g=L. The zinc dissolution in control test with above-mentioned conditions was 25.87%. According to results,the Zn recovery increased 42.93% due to bacterial activity.On the other hand, the lowest zinc recovery was 33.66%and obtained from the test with pulp density 150 g=L,ferrous sulfate concentration 25 g=L, and initial pH 1 (testno 4). The recovery of zinc in control flask with pH 1 was18.08%. Generally, a decrease in pulp density caused anincrease in zinc recovery, and an increase in ferrous concen-tration and initial pH caused a decrease in recovery. In fact,using low pH culture media caused a decrease in the growthof this specific bacteria and it could be the main reason forindirect relation between zinc dissolution and initial pH.

Fig. 4. The ORP changes in different tests during 24 daystreatment.

Fig. 5. The pH changes in different tests during 24 daystreatment.

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On the other hand, zinc recovery for chemical leachingprocess was less than bacterial leaching. The recovery forleaching with sulfuric acid was 15.64% with 3.54 g=L inPLS. For alkaline leaching with NaOH, the solubilizationwas 2.448 g=L with 12.81% zinc recovery, and for leachingwith H2O in pH 2, the recovery was 0.92%. The dissolvedmetals during the chemical leaching process were releasedinto PLS mainly because of the reaction between carbonateminerals and leaching solvent. Carbonate components gen-erally dissolve in acid or alkali in a very short period of time.Therefore, the maximum leachable zinc was 15.64% andresidual ores mostly were sulfide minerals. These mineralsdissolve in acid or alkali at a very slow rate. Practically,the sulfide minerals cannot leach with this leachate. Here,two different methods can be considered. First, bacterialleaching method can be used for leaching of mixture ofcarbonate and sulfide minerals (which is a one-step leach-ing). Second, dissolving carbonate minerals by leachingand then dissolve sulfide minerals by bacterial leachingprocess (two-step leaching).

3.3 Lead Solubilization and Recovery

Figure 8A shows the lead recoveries in different testsconditions. The control flask contained 0.06 g=L with0.54% lead recovery. However, the highest lead recoverywas 1.08% in the flask with initial pH 1, pulp density¼50 g=L, and Fe2þ sulfate concentration¼ 75 g=L. The leadrecovery is much more less than zinc recovery due to PbSlow solubilization in acidic environments. The XRD analysisof bacterial leaching residual (Figure 7) showed that the leaddissolved and then reacted with chloride and arsenic andprecipitated as lead arsenate chloride according to followingequation:

5Pb2þ þ 3AsO43� þ Cl� $ Pb5ðAsO4Þ3Cl ð4Þ

The arsenic transferred to solvent because of dissolutionof arsenic minerals and the source of Cl� was KCl thatwas added to 9K media. Thus, lead and zinc cannot bebioleached simultaneously from ore. To increase the leadrecovery, PbS might be concentrated by mineral processingmethods such as flotation or gravity separation, and avoidtransferring arsenic minerals to lead concentrate, then leadand zinc concentrates might be leached separately. However,mineral processing separation usually requires grinding andthis step increases the production expenses. In acid leachingwith sulfuric acid, the lead recovery was just 0.022% and foralkaline leaching, the recovery was 3.57% and for leachingwith H2O, it was 0.025%. Both arsenic and lead dissolutionswere higher in acid leaching but As and Pb reacted togetherand made precipitation. Pb recoveries from water and acidwere almost equal, this shows that almost all dissolved Pbprecipitated as lead arsenate chloride. Figure 10B shows thisprecipitation (the Pb increased for 6 days and then started todecrease). Although, the lead recovery in alkaline leaching isvery low, results showed that alkaline leaching can improvelead recovery. Also sum of lead and zinc recovery was morefor alkaline leaching. In two steps leaching, it is possible to

Fig. 6. Zinc recovery in different conditions after 24 daystreatment.

Fig. 7. X-ray diffraction of ore after bacterial leaching experiments.

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leach ore with NaOH in first step to increase Pb recoveryand then use bacterial leaching for recovering of othermetals.

Presence of antimony, arsenic, and cadmium has thenegative effects on microorganism activity, especially inhigher concentration (data are not presented here). Thereis no obvious influence in dissolution of zinc and lead inthe presence of these elements. Among catalytic ions, silveris of special interest because since it has been proven effec-tive and selective in enhancing some elements such as Cu dis-solution through the biological oxidation of chalcopyrite(Ahonen and Tuovinen 1990a,b). The positive catalyticeffect of silver ions on the bacterial leaching of chalcopyritehas been demonstrated in shake flasks and column bioreac-tors (Ahonen and Tuovinen 1990a,b; Munoz et al. 2007a,b,c;Cordoba et al. 2009; Feng et al. 2013).

3.4 Other Metals Solubilization

The sources of iron in PLS during bacterial leaching wereore and FeSO4 � 7H2O, which were added to flasks. Ironsolubilization in flask with highest zinc recovery was11.55 g=L. The highest iron solubilization, which was16.4 g=L, occurred in flaks with pH 1, pulp density 150 g=L,and FeSO4 � 7H2O concentration¼ 75 g=L. Figure 8B showsiron dissolution in different tests condition. Generally,results show that an increase in pulp density and initial Feconcentration causes an increase in iron solubilization and

an increase in pH causes a decrease of iron in PLS. Inthe leaching process, iron solubilization was stronglydependent on pH. The iron dissolution in the ore leachingwith sulfuric acid was 4.223 g=L, in the leaching with NaOHwas 0.006 g=L, and in leaching with H2O with pH 2 wasabout 0 g=L.

The atomic absorption spectroscopy (ASS) analyses offinal PLSs showed that the silver dissolution was less than0.1 ppm and copper solubilization was less than 0.5 ppm inall experiments.

Acid mine drainage (AMD) is one of the most importantproblems in mining industries. Dissolving metals fromtailings dump during this phenomenon can cause lots ofenvironmental damages. Removal of heavy metals canreduce the toxicity of these drainages. Arsenic and antimonyare two of the most toxic elements in nature and can causelots of negative impacts on life of all creatures, so monitor-ing them in soil and water is essential (Akcil and Koldas2006; Dinkla et al. 2013). During bacterial leaching process,arsenic and antimony were removed from ore and trans-ferred to PLS. For monitoring these two toxic metals, theirconcentrations were determined by atomic fluorescence spec-troscopy after 24 days bacterial leaching. Figure 8C showsthe dissolved arsenic (%) and Figure 8D shows the antimonyrecoveries trough the different tests condition. Test 13showed the highest As recovery (51%). This test, also, hadthe maximum Zn recoveries. In addition, during lead,arsenic, and chloride reaction, some part of arsenic sediment

Fig. 8. Recoveries of lead (A), iron (B), arsenic (C), and antimony (D) in different media after 24 days treatment.

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precipitated. Figure 9 shows Zn and As final recoveries,simultaneously. As this figure shows, there is a relationshipbetween As and Zn dissolution.

The highest Sb recovery was 38% in test 9. Figure 8Dindicates that the Sb removal in test with pH 2 was verylow and in test 13 (the test with maximum Zn and Assolubilization), Sb recovery was 3.13%.

3.5 Leaching Kinetics

The test with highest zinc recovery (test 13) was repeatedagain with bacteria and without bacteria for 30 days in orderto study the solution kinetics in presence and absence of

bacteria. The initial pH was set on 2, pulp density was50 g=L and 75 g=L FeSO4 � 7H2O was added as the optimumcondition. The pulp density which is low provides a con-dition that could show the leaching kinetic. This bacterialleaching test included 90mL 9K medium and 10mLenriched bacteria and were performed in 250mL Erlenmeyerflasks. The control flask includes 98mL 9K medium and2mL formaldehyde 37% (as bactericide). During these tests,samples of suspension were taken periodically and, afterphase separation, were analyzed for zinc, lead, arsenic, andiron. Sample volumes were replaced with sterile mineral saltssolution. The final results were correct because of thedilution effect caused by partial replacement of the nutrientsolution. Figure 10 shows metals dissolution during 30 daystreatment in optimum condition. Although the zinc quantityin test with bacteria increased during the first 25 days, it sta-bilized in control test after 10 days. This clearly shows thebacterial influence in zinc dissolution. The zinc dissolutionhad sharp slope in first days due to carbonate mineralsdissolution. The As dissolution pattern in bacterial leachingtest was similar to Zn. However, the arsenic dissolution incontrol test was very low, which shows the vital effect ofbacteria in arsenic leaching. The iron quantity changes inbacterial leaching test was almost smooth; however, itdropped during the chemical leaching due to iron precipi-tation, which probably took place under jarosite form. ThepH is one of the most important factors that control thejarosite precipitation (Leahy and Schwarz 2009) and in con-trol flasks iron precipitated due to increase of pH. The Pbdissolution was around a few mg=L due to lead insolubility.

Fig. 10. Changes in concentration of dissolved zinc (g=L) (A), lead (g=L) (B), arsenic (g=L) (C), and iron (g=L) (D) with time in 30days bacterial leaching treatment and control flask in optimum conditions.

Fig. 9. Final Zn and As recoveries in different tests condition.

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3.6 SEM Images Analysis

Previous studies show the different surface changes duringchemical and bacterial leaching (Liu et al. 2003; Liu, Gu,and Xu 2011; Abdollahi et al. 2013b). Thus, scanning elec-tron microscopy (SEM) was carried out to study the surfacechanges in absence and presence of bacteria. Figure 11Ashows sphalerite surface before processing, Figure 11Bshows surface of an ore particle from control test, andFigure 11C shows SEM image of surface of an ore particlefrom a test with highest zinc recovery (from the test 13).According to Figure 11A, the particle surface was smoothbefore processing. But the surfaces of ore particles becamerough after chemical and bacterial leaching (Figure 11B,C).There were several holes 1–2 mm in depth on the sphaleritesurfaces after 24 days leaching in control test. In fact, inabsence of bacteria the leaching started from microfracturesand improved in surface and depth. Therefore, the numberof microfractures on surface has a great effect on leaching.Therefore, producing artificial microfractures by ultrasoniccan increase the leaching rate. Identification of microfrac-tures in the grain surface is so difficult in SEM images butin some photos, it is distinguishable. On the other hand,changes on ore surface during bacterial leaching processwere evenly distributed. As can be seen from Figure 11B,C,depth of holes grows more in the leached particle surfacesrather than bioleached particles. Thus, the total free surfaces

of particles have more important effect on bacterial leachingprocess in comparison with leaching process.

4. Conclusions

The behaviors of a high-grade Zn–Pb ore were studied dur-ing chemical and bacterial leaching. 15 bacterial leachingand 6 chemical leaching experiments with different con-ditions were designed and carried out. The highest Zn recov-ery (68.8%) was achieved in bacterial leaching with initial pH2, pulp density 50 g=L, and iron sulfate concentration 75 g=L. The zinc changes diagram shows that after 24 days bac-terial treatment, the Zn quantity in pregnant leach solutiondecrease due to zinc precipitation. However, the optimumretention time for achieving highest Zn recovery is 24 days.The Pb recoveries were very low (0.31%–1.08%). The resultsshowed that the dissolved lead reacted with As� and Cl� andprecipitated as lead arsenate chloride. However, if Pb oreinclude arsenic minerals, it is impossible to recover arsenicdue to mentioned phenomenon. The results showed a directproportionate between zinc and arsenic dissolution, and alsomore than 50% of arsenic dissolved during bacterial leachingunder optimum conditions. Moreover, the antimony dissol-ution was very sensitive to pH and an increase in initialpH reduced the Sb recovery. Finally, scanning electronmicroscope (SEM) images showed the free surface had great

Fig. 11. SEM image of sphalerite surface before leaching (A), after chemical leaching in control test (B), and after bacterial leachingtreatment (C).

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impact on bacterial leaching and also number of microfrac-tures in surface had important effect on leaching. Therefore,reducing the ore size particle can increase the bacterial leach-ing efficiency. On the other hand, making microfractures inore particles, by using ultrasonic wave, chemical additive, oretc., can improve the chemical leaching rate.

Funding

The authors appreciate the support of the Geology Survey ofIran (Grant no. 92-171-459).

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