Valence variation of arsenic in bioleaching process of arsenic-bearing gold ore

Available online at www.sciencedirect.comTransactions of骂PScienceDirectNonferrous MetalsScienceSociety of ChinaELSEVIER PressTrans. Nonferrous Met. Soc. China 20(2010) 1171-1176www .nmsc.cnValence variation of arsenic in bioleaching process ofarsenic-bearing gold oreCUI Ri-cheng(崔日成), YANG Hong-ying(杨洪英), CHEN Sen(陈森), ZHANG Shuo(张硕), LI Ke-feng(李科峰)School of Materials Science and Metallurgy, Northeastern University, Shenyang 10004, ChinaReceived 6 July 2009; accepted 25 December 2009Abstract: The concentration and variational trend of As3+ and As'*, the bacterial resistance for the As+ and As$+ and convertingconditions from As' to As'+ were analyzed. The additive was used to prompt the bacterial leaching efficiency by changing valencestate of arsenic. The results show that the concentration ofAs3+ is larger than that of As' in the lag phase. The concentration of As'+decreases in the log phase, and is lower than that of As't. HQ-0211 typed bacteria express better resistance for As3+ and AsS+ andremain growing when the concentrations of As+ and As+ are above 6.0 g/L and 12.0 g/L, respectively. It is found that Fel* cannotoxidize As singly as strong oxidant in the leaching system, but can cooperate with pyrite or chalcopyrite to do that. The oxidation ofAs3+ is prompted with addition of H2O2. The bacterial activity is improved in favor of baterial leaching ficincy. NaClO restrainsthe bacterial growth to depress leaching eficiency because of the chloric compounds affecting bacterial activity.Key words: As3+; As"; bacterial leaching; arsenic resistance; oxidant; arsenic-bearing gold oreRefractory gold ore with high arsenic concentration has1 Introductiontwo impending problems to be solved, i.e. improvingbacterial leaching efficiency and shortening oxidationArsenic-bearing gold ore is a common refractoryperiod. Arsenic is the most influential compound on thegold ore. Arsenic of gold ore is very unfavorable tobacterial activity in the leaching system and it is highlycyanide leaching[1- 2]. Arsenic wraps gold grain in thetoxic substances. There are two types ofAs'+ and As'+ inform of arsenopyrite, which severely cuts off the cyanidethe solution. The toxicity of As'+ is much higher thanfrom gold grain. Arsenic-bearing gold ore is easy toAs*[9-11]. Accelerating the transition from As* to As+produce AsS3}, CNS, S205, AsO3 and AsO inis very important for decreasing the concentration of As't,cyanide solution. These products decrease the leachingimproving bacterial activity and increasing leachingefficiency because they exhaust cyanide and formefficiency. The present research of bacterial oxidation ofcompact film on the surface of gold grain to scparatearsenic-bearing gold ore is emphasized on the bacterialCN and O2 from the grains in the leaching process. Theadsorption, bacterial extracellular polysaccharide andoxidation pretreatment is used to depress or remove thedifferent single-mineral studies. In this work, a series ofarsenic in the gold ore in order that the gold is denudedtheoretical researches are emphasized on the valencebeforecyanide leaching[3-4].Now pretreatmentstate distribution of arsenic in the bacterial leachingtechnologies for the ore are roasting oxidation, pressureprocess, bacterial resistance for As'+and Asoxidation and bacteria oxidation. Among them, thtransitional conditions from As'+ to AsSt and oxidant forbacteria oxidation is famous for low cost, high efficiencyconverting As3+ and depressing bacterial toxicity. Theand environmental protection[5- 6]. Iron and sulfur in theresearch provides important theoretical consults for moreore are energy sources for bacteria to meet their metabolicmature, fast, steady, economic and high-performancedemand. Then ore grade will be increased[7-8].bacterial oxidation pretreatment of the high arsenicFoundation item: Prizets(50674029, 50874030) spprted by the National Naturalthe National Hightech Rescarch and Development Program of Chin::YHCNMHGcialized Research Fund forthe Doctoral Program of Higher Education, ChinaCorresponding author: YANG Hong-ying; Tel: +86 24-83680373; E-mail: yanghy@smom.neu.eu.cnDOI: 10.1016/51003-6326(09)6027401172CUI Ri-cheng, et al/Trans. Nonferrous Met. Soc. China 20(2010) 1171-1176refractory gold ore.measured by hypophosphite titrimetric method[14].2.2.2 Arsenic resistance of bacteria2 Materials and experimental10% of HQ-0211 typed bacteria with high activitywere injected into the 200 mL of 9K medium with2.1 Materialsdifferent concentrations of As'+ and AsS and wereBacteria HQ-0211 used by test, which are mixedshake-flask cultured in constant temperature shakingwith leaching bacteria dominated by Thiobacillusincubator at 44 C. The concentration gradient of As'ferrooxidans. They are screened and have beenwas 0, 1.5, 3.0, 4.5 and 6.0 g/L. The concentrationdomesticated for a long time in the laboratory.gradient ofAs't was 0, 3.0, 6.0, 9.0 and 12.0 g/L.The medium is 9K that was composed of 3.00 g/L2.2.3 Bacterial leaching of gold concentrate ore(NH4)2SO4, 0.10 g/L KCl, 0.50 g/L K2HPO4, 0.50 g/L10% of HQ-0211 typed bacteria were injected intoMgSO:7H2O，0.01 g/L Ca(NO3)2 and 44.30 g/L500 mL of shake flask with 200 mL of 9K medium inFeSO:7H20[12]. pH is 1.8 regulated by sulfuric acid.constant temperature shake incubator at 44 C. AfterThe ore sample comes from gold concentrate mineactivation of bacteria, 10 g of gold concentrate ore wasin Hunan Province, China, and the sample with size lessadded into the pulp.than 0.038 mm is over 90%. The phase analysis showsthat arsenic exists as arsenopyrite in the sample as shown2.3 Analytical methodin Fig.1. Table 1 lists the result of main elementalThe elctric potential, pH and concentration ofFe2*,analysis.TFe, As and AsS+ are determined in leach liquor inbioleaching and oxidant experiments per 24 h. Fe2* andTFe are determined by dichromate method. The.一AsFeS- FeS2concentration of As+ and AsS are determined by toluene- SiO2extraction-separation method.一FezOkThe electric potential, pH and concentration of Fe'*and TFe are determined in arsenic resistance experimentper 24 h. The bacterial reproduction is counted in bloodcounting chamber under the microscope. Growth curve isdrawn.3 Discussionjwhlhi0 To2030 4050 703.1 Concentration change of arsenic20/(*)Arsenic is the key negative factor in theFig,1 XRD pattem of gold concentratebiooxidation. The research of arsenic and its derivativesis the hotspot in the biohydrometallurgy. As3+ becomesTable1 Result of main element analysisthe hotspots including its indigenous time, concentrationAssie_change and transition to the arsenic in high valencebecause the toxicity of As* is stronger than that of As"129.10 g/t16.05%30.31% 21.60% 17.71 %[15-16]. The toluene extraction- separation method isused to pursue the concentration changes of As'+ andThe chemicals used in the experiments includedAsS+ in the arsenic-bearing gold concentrate bioleachingAs2O3, Na2HAsO37H2O, H2O2, NaClO, NaOH, HCl,process. Due to ferrous ion and sulfur element of ore asCgHs-CH3 and I2. All the chemicals were analyticalenergy source for leaching bacteria, they obtained energyreagents. All the aqucous solutions were prepared withby oxidation them to grow, so the system redox potentialthe ditilld water.and pH value changes can be indirectly reaction to thegrowth of bacteria and the disintegration of minerals.2.2 ExperimentalBacteria adapted to the ore. Lag phase of bacteria only2.2.1 Extractive separation of As3# and As'+lasts three days. The fourth day is logarithm. At this time,The toluene was used to extract the As+ from thearsenic-bearing ores are quickly oxidized andinorganic phase to the organic phase quantificationally indec中国煤化工T end,and pH value isthe condition of strong acid. The AsS* remained in thedeclirh day are the stableorganic phase. The As'+ was back extracted to aqueousphaseMYHC N M H Gelement are oxidized,phase from the inorganic phase and titrated by iodine1o increase in potential. But bacteria are still in thetiter in the condition of weak base[13]. The As$+ wasproduction of acid phase, and pH value is declining asCUI Ri-cheng, et alTrans. Nonferrous Met. Soc. China 20(2010) 1171-11761173shown in Fig.2. The concentration of As'+ is higher than3.2 As3+ and As+t resistances of bacteriathat of AsS+ in the former three days, namely the lagBecause As+ and AsS+ coexist in the bioleachingphase. The concentration of As3* reaches the climax, 4.63system, As and AsS resistances of bacteria wereg/L, in the 3rd day. The concentration of As' steps downstudied further. This work will help with formulations ofuntil below AsS+ in the log phase of bacteria as shown inpulp density and ore blending in bioleaching process.Fig.3. The concentration of As'+ increases then decreasesThe results of experiment show that the bacterial growthcompared with the escalation of As'+ in bioleachingrate and the activity decrease with increasingprocess. The change of arsenic phase is AsS2 →As'+- +concentrations of As3+ and As'*. Bacterial activity isAsS+ as shown in Fig.3. As'+ is produced and oxidized inseriously affected with the gradual increase in thebioleaching process. In the lag phase of bacteria, theconcentration of As*. Ferrous ion as energy cannot beindigenous rate is higher than the oxidized rate of theoxidized, resulting in potential increased slowly. WhenAs+ because of the weak activity of bacteria andthe concentration of As is 6.0 g/L, the bioleaching timeoxidization. In the flushing log phase and stable phase ofof bacterial lag phase is 264 h. No restrain happens whenbacteria, escalating concentration of Fe3* leads to thethe concentration of As3+ is below 1.5 g/L, as shown inhigher oxidizing rate than indigenous rate. The As'+ hasFig.4 and Fig.5. As'+ also has a great impact on thehigher concentration than As*. In other words, thebacteria, but its toxicity is weaker than As"*. When theactivity of bacteria increases with the decrease of As'concentration and increase of As concentration. Thus,concentration of AsS+ is 12.0 g/L, the bioleaching time oftransition from As'+ to As'+ has great influence on thebacterial lag phase is 264 h. No restrain happens whenbiooxidation pretreatment. A large number ofthe concentration of As'+ is below 3.0 g/L, as shown inFig.6 and Fig.7. The lag phase is extended with thearsenopyrite，pyrite and other sulfide minerals areoxidized in bioleaching process. At the end of theincrease of arsenic concentration because the bacteriaexperiment, the mass loss rate is 61.8% and thedearsenization rate is 98.2%.650.860050+要550Blank50t.4500-250.一Potential 1.3Time/d450Fig.4 Potential change with bioleaching time at diferent As3+concentrationsFIg.2 Potential and pH changes with bioleaching timel4-■- Blank▲- 3.0g/L2'●一1.5 g/L◆一6.0 g/L:二锅s中国煤化工-10 12CHCNMH GFig.3 Concentration changes of As*, As'+ and TAs withFig.5 Bacteria count change with bioleaching time at differeatbioleaching timeAs3+ concentrations1174CUI Ri-cheng, et alTrans. Nonferrous Met. Soc. China 20(2010) 1171-1176bacterial leaching process. The direct oxidation and650-indirect oxidation of the main body are bacteria and Fe'in bacterial oxidation system, therefore, to analyze As+550-to As conversion conditions respectively, the bacteria,- 3.0gLthe medium of Fe2+ and pyrite often associated with gold.- 6.0g/Lg 450-- 9.0g/Lore were studied. The activated bacterial liquid is placed◆- 12.0g/Linto four shake-flasks of 500 mL with 200 mL of 2 g/LAs3* standard liquid, as listed in Table 2.2 350Table 2 Different conditions of oxidation test for As+250Concentration ofNo.ConditionAs*/(g:L)1505立468101"2Primitive bacteria solutionTime/dFig.6 Potential change with biolaching time at dfferent As'*2*Sterile conditionconcentrations32 g pyrite4*4.64 g FeSO4:7H2O14一Blank12.一6.0g/LThe converting of As'+ in the bacteria system icompared with asepsis after the bacteria are filtrated in210●- 12.0g/L/'the 2" flask through microporous membrane. The8conversion ratios are zero in the 1# and 2" flasks afterthree days germiculture. This shows that the leaching6bacteria cannot convert As'+ to higher valence state in岳4the simplex 9K culture medium and Fe'* does not playits role as oxidant, as listed in Table 3. 2 g pyrite withFe+ as strong oxidant added into the 3# flask iscompared with 4# flask with 4.64 g FeSO47H2O. This is02下4681the same amount of iron in 2 g pyrite. The conversionratio of As'+ reaches 35.61% and progressively rises inFig.7 Bacteria count change with bioleaching time at difercatthe 3" flask after being cultured for three days, on theAs$+ concentrationscontrast, none in the 4" flask, as listed in Table 3. 2 gFeSO& 7H2O is added into the 4# flask to ensure the moleratio of iron to arsenic to convert As3* to AsS+ in thecoenzyme and intermediate to adapt to new environment.solution. After one day's culture, As" isn't oxidized inIn this phase, the bacterial growth is slow and thethe ratio of 10:1 for iron to arsenic, and 480 mV in the 3"oxidizing efficiency is low for ferrous ion. The bacteriaflask, which proves high concentration of Fe+ and highmay meet the high arsenic gold concentrate ore withelectric potential are not preconditions to convert As+ to16.05% arsenic in the section 3.1 because the resistancesAs'+ in the leaching system. A part of As'* is alsoof As3+ and As+ are above 6.0 g/L and 12.0 g/L as shownoxidized after addition of chalcopyrite. O2 in the air is anin Figs.3, 5 and 7. Toxicity of As3+ is 60 times higherelectron acceptor in addition to Fe'+ in leaching system.than that of AsS+ for the human body and animalTo analyze the electron acceptor in the process of Ast+body[17-19]. But the result of this work is 2 times for- +As$+, biooxidation of pyrite is analyzed. XPS showsthe bacterial leaching activity experiments as shown inthat 02 exists on the surface of pyrite when the flakyFig.5 and Fig.7. Thus, the further research is needed forpyrite is oxidized by leaching bacteria as aerobic fromthe toxicity theory of arsenic ion for the leachinglow valence state to high valence state in the process ofbacteria.biooxidation. When the environment changes, EPS中国煤化工creted in the outer3.3 Analysis of converting conditions from As* to As+tlayer53% EDTA is used toThe bacterial activity steps up from As'+ to AsS+ inextraYHC N M H Gks. The results showthe process of biooxidation. Thus, it is important to studythat the EPS concentration in the 3" flask is two timesthe converting conditions from As3+ to As'+ in thehigher than that in the 4" flask because the secretion isCUI Ri-cheng, et al/Trans. Nonferrous Met. Soc. China 20(2010) 1171-11761175stimulated by the addition of pyrite. Fig.8 shows that theleaching oxidizing rate. The total arsenic concentrationbacteria are adhered to the surface of ore by the stickyexceeds 0.39 g/L which is larger than that of the blankEPS which controls the transference and exchangesample and trends to stabilize in the 3rd day afterbetween the bacterial surface and environment. SAND etreaching maximum dearsenization rate, as shown inal[20] found that Fe'*+ concentration in the EPS is muchFig. 10. The concentration of As'+ in the shake-flask withhigher than that in the solution which prompts theNaClO is lower than that of the blank sample, but theoxidation of elements in low valence state in the system,total arsenic concentration in the shake-flask with NaCl0and oxidoreductase secreted is congregated in the EPS tois also lower than that of the blank sample, as shown inaccelerate transference of electron in the bacterialFig.10. The results show that bacteria are unfit for theenzyme system and to cause redox reaction to processenvironment and the activity and the leaching efficiencyeasily. As exothermic reaction, the thermal energydecreases. The As'+ liberating from the ore leads to thelower concentration of As+ than the blank sample. Therelaxed by bacterial leaching catalyzes the reaction.observation through the microscope shows that theTable 3 Oxidation ratio of As'+ after being cultured forgrowth rate and activity are lower than those in the otherdifferent time (%)two shake-flasks. H2O2 remains more stable in the acidTime/d23environment than the alkaline, but lots of ferric ion,sulfur ion and arsenic ion catalyze the decomposition0rapidly in the solution at 44 C. The purpose is to depress22.92the toxicity of arsenic for the bacteria and to improve the31.71bacterial activity because the oxidation of H2O235.61disappears in a short time and does not restrain thebacteria. NaCl0 is decomposed to HCIO, Cl2 and CIe3■- Blankg:二NaOFig.8 SEM image of bacteria adhering to surface of ore63.4 Influence on bioleaching process by oxidantThe arsenic is notorious for toxicity and danger, andFig.9 Influence of leaching time on concentration change forthe oxidant is applied widely in the arsenic-bearingAs+ by H2O2 and NaClOindustrial wastewater. The section 3.1 shows that thetrivalent arsenic exists in the former half part of thebioleaching process. The addition of oxidant oxidizes一8As'+ rapidly to depress the toxicity of arsenic and shortenthe lag phase to enhance bacterial activity. It is veryimportant to deal with long bioleaching period. It is easyto remove AsSt through coprecipitation, flocculation andabsorption in the post processing. The oxidant has twosides because it promotes the oxidation of trivalentarsenic and restrains the bacterial growth. 30% of0.3 mL一BlankH2O2 and 10% of 0.5 mL NaClO are added into at the.一H93.▲一Nacio48th hour with high concentration of As3*. H2O2 leads to中国煤化工the lower concentration of As* than the blank samplefrom the 3rd day to the end of bacterial leaching, asDHCNMHGshown in Fig.9. The reduced As'+ leads to the enhancedFig.10 Influence of leaching time on total arsenic concentrationbacterial activity in the shake-flask with H2O2 and largechange by H2O2 and NaClO1176CUI Ri-cheng, et al/Trans. Nonferrous Met. Soc. China 20(2010) 1171-1176rapidly because of its instability and catalysis of otherreality Turkishmetalmining's future p Mincrals Engineering. 2004,ions. The chloric compounds restrain bacterial activity17(3): 477-480.6] WATLINGH R. 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