Biooxidation and precipitation for iron and sulfate removal from heap bioleaching effluent streams

Biooxidation and precipitation for iron and sulfate removal from heap bioleaching effluent streams
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:  Author's personal copy Biooxidation and precipitation for iron and sulfate removal from heap bioleachingef  fl uent streams Pauliina Nurmi a, ⁎ , Bestamin Özkaya a,1 , Keiko Sasaki b , Anna H. Kaksonen a,2 , Marja Riekkola-Vanhanen c ,Olli H. Tuovinen a,d , Jaakko A. Puhakka a a Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101 Tampere, Finland b Department of Earth Resources Engineering, Kyushu University, Motooka 744, Nishiku, Fukuoka 819-0395, Japan c Talvivaara Mining Company Plc., Ahventie 4 B 47, FI-02170 Espoo, Finland d Department of Microbiology, Ohio State University, Columbus, OH 43210, USA a b s t r a c ta r t i c l e i n f o  Article history: Received 19 August 2009Received in revised form 10 November 2009Accepted 10 November 2009Available online 18 November 2009 Keywords: Barren solutionBioleachingIron oxidationIron removalSulfate removal Ef  fl uents from bioleaching processes cause severe problems if dispersed in the environment since theytypically have very low pH values and high sulfate and ferric iron concentrations. Dissolved iron may alsointerfere with the metal recovery. In the bioleaching circuit, partial removal of dissolved iron and sulfate isneeded to alleviate process disturbances. In this study, an integrated, bench-scale process comprising a fl uidized-bed reactor (FBR) and a gravity settler was developed for controlled biological oxidation of ferrousiron and precipitative removal of ferric iron and sulfate for use in waste management of heap bioleachingprocesses. The FBR for iron oxidation by an enrichment culture dominated by  Leptospirillum ferriphilum  wasoperated at 37±2 °C. The FBR recycle liquor was partially neutralized with 10 M KOH or 50 g/L CaCO 3  slurryto promote ferric iron and sulfate precipitation. With 6±1.5 g Fe 2+ /L in the feed and KOH-adjusted pH 3.5,the oxidation rate of Fe 2+ was 3.7 g/L h and 99% precipitation of ferric iron was achieved in the process.Adjustment with CaCO 3  to pH 3.2 slightly decreased the oxidation rate to 3.3 g/L h and 98% of ferric ironprecipitated. With 15 g Fe 2+ /L in the feed, the oxidation rate was 7.0 g Fe 2+ /L h coupled with 96%precipitation of ferric iron. A solid solution of jarosite was the main product of ferric iron precipitation withKOH adjustment and with minor amounts of goethite at the higher pH range. Adjustment of the pH withCaCO 3  precipitated ferric iron also as a solid solution of jarosite, and sulfate precipitated also in the form of gypsum (CaSO 4 ·2H 2 O) especially at the higher pH values.© 2009 Elsevier B.V. All rights reserved. 1. Introduction In bioleaching processes biological oxidation of iron and sulfur isexploited to solubilize and recover metals from low-grade sul fi deores.SolutionsfromtheseprocessestypicallyhaveverylowpHvaluesand high sulfate and ferric iron concentrations. Other metals may alsobepresentbutthechemicalcompositionoftheseef  fl uentsvarieswiththe source. Dissolved iron and sulfate accumulate in heap bioleachingcircuits because of solution recirculation. The accumulation isproblematic and excess iron and sulfate need to be removed becausethey may otherwise interfere with process kinetics due to precipitateformation (Nemati et al., 1998; Watling, 2006) and with the subsequent metal recovery (Dutrizac and Riveros, 2006; Cunhaet al., 2008). The  fi nal ef  fl uents from these processes have to betreated effectively to neutralize the streams and to remove iron andsulfateasstableendproductsastheef  fl uentscausesevereproblemsif dispersed in the environment (for reviews, see Johnson, 2003, 2006). Iron is commonly removed through hydroxide precipitation byadding lime or limestone to increase the pH approximately to 3.This conventional treatment process creates major sludge handlingand disposal problems due to the generation of voluminous sludge(Cunha et al., 2008; Dutrizac and Riveros, 2006; White et al., 2006).  Jarositeprecipitationisalsoacommonchemicalironremovalmethodespecially in zinc industry. Jarosite precipitation produces ironprecipitates with relatively good settling,  fi ltering and washingcharacteristics, but various other metal ions may co-precipitate, andelevated temperatures are preferred for the process (Tamargo et al.,1996; Ismael and Carvalho, 2003). Biological and combined biologicaland chemical iron and sulfate removal systems have been examined,especiallyforthetreatmentofacidminedrainage(AMD)asreviewed,for example, by Johnson and Hallberg (2005), Gaikwas and Gupta(2008) and Kaksonen et al. (2008). These AMD treatment systems often help to decrease the chemical costs and improve the handling Hydrometallurgy 101 (2010) 7 – 14 ⁎  Corresponding author. Present address: MTT Agrifood Research Finland, Biotech-nology and Food Research, FI-31600 Jokioinen, Finland. Tel.: +358 50 351 8211. E-mail address: (P. Nurmi). 1 Present address: Department of Environmental Engineering, Faculty of CivilEngineering, Davutpasa Campus, Yildiz Technical University, TR-34342 Istanbul, Turkey. 2 Presentaddress:CSIROLandandWater,UnderwoodAve,Floreat,WA6014,Australia.0304-386X/$  –  see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2009.11.004 Contents lists available at ScienceDirect Hydrometallurgy  journal homepage:  Author's personal copy characteristics of the sludge produced. Examples of active treatmentprocesses employing iron-oxidizing microorganisms include aeratedlagoons and stirred tanks (Umita, 1996), packed-bed bioreactors (Diz and Novak, 1999) and rotating biological contactors (Olem and Unz,1980; Nakamura et al., 1986). High-rate iron oxidation has beenachieved with bioreactors using immobilized iron-oxidizing micro-organisms, e.g. packed-bed bioreactors (Diz and Novak, 1999) androtating biological contactors (Olem and Unz, 1980; Nakamura et al.,1986).Inthelaboratoryscale,atreatmentprocesshasbeendevelopedconsisting of a packed-bed bioreactor for iron oxidation and a fl uidized-bed reactor for chemical precipitation of iron onto thesurface of seed particles (Diz, 1998; Diz and Novak, 1998). These applications have been developed for treating AMD and, in contrast, avery limited amount of information has been published on thetreatmentofbioleachingsolutions.Inourpreviousstudy(Nurmietal.,2009) a bioprocess was developed based on a  fl uidized-bed reactor(FBR) for Fe 2+ oxidation by a  Leptospirillum ferriphilum  dominatedbio fi lm coupled with a gravity settler for precipitative removal of ferric iron. Only few patents have addressed biological iron oxidationfor iron removal; e.g., Das and Das (2004) described a processthat included simultaneous oxidation and precipitation of iron inhydrometallurgical leach liquors in the presence of   Acidithiobacillus ferrooxidans , and Maree and Johnson (1999) designed a bioprocessscheme for the treatment of acidic  “ raw ”  (unde fi ned) water contain-ing high concentrations of dissolved ferrous ions.The solubility of Fe 3+ is a function of the pH in leach solutions.Generally, Fe 3+ has an extremely low solubility at pH above 2.5. Insulfate-richenvironmentsFe 3+ precipitatesmainlyasjarosites(Fe(III)-hydroxysulfates, MFe 3 (OH) 6 (SO 4 ) 2  where M can be Na + , K + , NH 4+ orH 3 O + oradivalentmetalionatambienttemperaturesandpressures)atpH values b 4 and as oxyhydroxides and oxides at higher pH values.Goethite,anFe(III)-oxyhydroxide( α -FeOOH),orschwertmannite,anFe(III)-hydroxysulfate(ideallyFe 16 O 16 (OH) 12 (SO 4 ) 2 · n H 2 O)canalsoformat similar pH ranges. Jarosite formation is enhanced at high concentra-tionsofmonovalentcations,Fe 3+ andSO 42 − (Nematietal.,1998;Grampetal.,2008).ThelowerthresholdpHforjarositeformationisclosetopH1.5, but this depends on the temperature and ionic composition of thesolution.The purpose of this study was to develop a high-rate, high-ef  fi ciency process for iron oxidation and precipitation together withsulfate. A combined iron and sulfate removal bioprocess wasconceived by employing ferrous sulfate oxidation by iron-oxidizingbacteria followed by precipitation through partial neutralization. Theprocess was developed for the treatment of heap bioleachingef  fl uents. Solid phase samples were characterized in an effort torelate precipitate properties to pH adjustments with two differentneutralizing chemicals. 2. Materials and methods  2.1. Reactor setup A  fl uidized-bed reactor (Fig. 1) based system was used for theexperiments at 37±2 °C. The reactor setup was as described in Nurmi Fig. 1.  Schematic diagram of the  fl uidized-bed reactor (FBR) and gravity settler system. 1: Feed pump, 2: FBR, 3: Base addition, 4: Settling tank, 5: Aeration pump, 6: Aeration unit,7: Recycle pump, 8: Final system ef  fl uent, 9: Precipitate removal. Not drawn to the scale. Fig. 2.  Operational conditions during the seven experimental periods of the combined biological iron oxidation and removal system.8  P. Nurmi et al. / Hydrometallurgy 101 (2010) 7  – 14  Author's personal copy et al. (2009) except that the settling tank had a 40 L capacity andretention time of 1 h interfaced with a pH adjustment unit (capacity of 4.5 L, retention time of 0.12 h). The surface loading rate of the settlingtank ( fl ow of solution through the settling tank divided by the surfacearea of the settling tank) was 0.29 m/h throughout the experiments.Prior to these experiments, the FBR was fed for 160 days with a barrenheap leaching solution at different retention times of the FBR. Theoptimal Fe 2+ oxidation performance with 99% oxidation ef  fi ciency andaverageoxidationrateof8.2 gFe 2+ /Lhwasachievedataretentiontimeof 1 h. The concurrent Fe 2+ oxidation and partial precipitative ironremoval were maximized at an FBR retention time of 1.5 h,with oxidation rate of 5.1 g Fe 2+ /Lh and Fe 3+ precipitation rate of 15 g Fe 3+ /day, which corresponded to 37% iron removal (Nurmi et al.,2009). In the present study, the FBR was fed with barren heapbioleaching solution supplemented with mineral salts. The barrenleach solution was from a multimetal heap leaching pilot plant atTalvivaara, Finland, retrieved after the recovery of target metals. Theleachsolutioncontained(mg/L)Fe(6000±1500),S(15,800±600),Na(1000±50), K (20±0), Mg (3800±150), Mn (1450±60), Ca (470±10), Al (35±5), Cu (1±0), Zn (20±0), Ni (90±0) and Co (2±0), allanalyzed by ICP. The relatively high amount of Na had been dissolvedfromganguematerialassociatedwiththemultimetalore.Alltheironinthe feedwas in the form of Fe 2+ . The pH and the redox potentialof thefeedsolutionvariedfrom2.8to3.6andfrom280to360 mV(Ag/AgClasthe reference electrode), respectively.The operational conditions during the seven different experimentalperiods are summarized in Fig. 2. To determine the effect of pH on ironoxidation and on iron and sulfate precipitation, the pH of the recycledsolution (constant recycling rate of 38.5 L/h) was increased step-wise fi rst with 10 M KOH from pH 2.0 to 2.5, then 3.0 and  fi nally 3.5. Afterthese experiments thepH was adjusted with 50g/L CaCO 3  slurry to pH3.2, 3.0 and  fi nally 2.8. The pH was continuously controlled using anautomatic titrator (Metrohm 719S Titrino with LL Syntrode 1000electrode)connectedtoacomputer(withKOHexperiments)orusingapumpconnectedtoapHsteeredcontrolunit(withCaCO 3 experiments,Metrohm titrator system consisting of Metrohm 780 pH meter, 731Relay Box, LL Syntrode 1000 electrode and 6.2148.010 Remote Box).Base was added to the system prior to the settling tank in a separaterapidmixingunit( V  =4.5 L,retentiontimeof0.12 h)withamixingrateof 170 rpm (Fig. 1). Because of the high recycling rate the pH of theentire reactor system was approximately the same. The retention timeoftheFBRwasmaintainedat2 h( fl owrateofthefeedpump170 mL/h).The  fi nal experiment was conducted by adjusting the pH of the systemwithCaCO 3 to2.8andsimultaneouslyincreasingtheFe 2+ concentrationof feed solution from 6.0±1.5 g Fe 2+ /L to 14±0.5 g Fe 2+ /L by addingferrous sulfate (FeSO 4 ·7H 2 O) to the barren heap leaching solution.  2.2. Source and analysis of the enrichment culture The acidophilic iron-oxidizing mixed culture used in this studywas obtained from an FBR, which was long-term fed with 7 g Fe 2+ /L and nutrient medium containing (g/L) (NH 4 ) 2 HPO 4  (0.35), K 2 CO 3 (0.05) and MgSO 4  (0.05) at pH 0.9. The inoculum was from previousFBR experiments with enrichment cultures that were srcinallyderived from drainage water and sludge samples from the Pyhäsalmimine, Finland (Kinnunen and Puhakka, 2004, 2005). The bacterial community was monitored by using denaturing gradient gelelectrophoresis (DGGE) of PCR-ampli fi ed partial 16S rRNA genes aspreviously described (Özkaya et al., 2007). Fragments corresponding to nucleotide positions 341 – 926 of the  Escherichia coli  16S rRNA genesequence were ampli fi ed with the forward primer GC-BacV3f (5 ′ -CCTACG GGA GGC AGC AG-3 ′ ) with a GC clamp at the 5 ′– end, and thereverse primer 907r (5 ′ -CCG TCA ATT CMT TTG AGT TT-3 ′ ). DGGEanalysis was performed with total DNA extracted from the operatingFBR carrier at different time intervals. The carrier material (granularactivated carbon) sample taken from the FBR was sonicated 5 timesfor 1 min to detach the carrier bound cells. The DNA puri fi cation andsequencingofthepuri fi edPCRproductswereperformedbyMacrogen(Seoul, South Korea). For microscopic counts of bacterial cells,bacteria were sampled from the carrier material and in the FBR outletand stained as previously described (Nurmi et al., 2009). Fig. 3.  Effect of partial neutralization with KOH and CaCO 3  on the Fe 2+ oxidation rateand ef  fi ciency and ferric iron precipitation. (a): Feed ( ♦ ) and FBR outlet ( ○ ) Fe 2+ concentrations, (b): Redox potential, (c): Dissolved oxygen (DO) concentration,(d): Fe 2+ oxidation rate (g/L h) ( ○ ) and ef  fi ciency (%) ( ♦ ), and (e): Fe 3+ precipitationrate (g/day) ( ○ ) and ef  fi ciency (%) ( ♦ ). Precipitation ef  fi ciency was calculated aspercent Fe 3+ precipitated from the total amount of Fe 2+ in the feed. The vertical lineshows the periods when the pH of the reactor system remained stable at the target pH.Duringperiodsbetweendays50and87,FBRwasoperatedataretentiontimeof5 handpH 3.0 before starting the CaCO 3  experiments (data not shown).9 P. Nurmi et al. / Hydrometallurgy 101 (2010) 7  – 14  Author's personal copy  2.3. Analyses Sampleswere fi lteredthrougha0.45µmpolysulfonemembrane fi lter(Whatman, Kent, UK) and diluted with 0.7 M HNO 3 . The Fe 2+ concentration was determined colorimetrically (Shimadzu UV 1601spectrophotometer,Shimadzu,Kyoto,Japan)withortho-phenanthrolineaccording to modi fi ed 3500-Fe method (APHA, 1992). The retentiontimesandtheparametersdescribingtheironoxidationandprecipitationperformance of the reactor system were calculated as previouslydescribed (Nurmi et al., 2009a) except that the rates of ferric iron andsulfate precipitation were reported as the amounts of ferric iron andsulfate precipitated per day. The rate of ferrous iron oxidation wascalculated in two ways: (i) as customary for FBR based processes, thevolumeofthe fl uidized-bed(340 mL)wastakenasthereferencevolume,and (ii) as the iron oxidation continued in the settling tank, forcomparison, the total volume of the reactor system (45.5 L) was takenas another reference volume. The pH and dissolved oxygen (DO) weremeasured using a WTW pH 330i pH meter and a WTW OXI96 dissolvedoxygenmeter(WTW,Weilheim,Germany),respectively.RedoxpotentialwasmeasuredusingPlatinCombinationElectrodeBlueLine31Rx(Schott,Mainz, Germany) with Ag/AgCl as the reference electrode. Sulfateconcentrationsweremeasuredbyionchromatography(DX-120,Dionex,Sunnyvale,CA,USA).Totaldissolvedironconcentrationwasdeterminedby using atomic absorption spectrophotometry (PerkinElmer 1100,Waltham, MA, USA).To characterize precipitates retrieved from the solid – liquidseparation, 5 L of settled precipitate was drained as a slurry fromthe gravity settler. Aliquots (20 mL) were washed in distilled waterand centrifuged (5000×  g  ) for 10 min. This step was repeated threetimes before the samples were dried at room temperature (22±2 °C)for X-ray diffraction (XRD) analysis. XRD patterns were collectedusing an XRD Multi Flex instrument (Rigaku, Akishima, Japan) withmonochromatized CuK α  radiation and a source power of 40 kV and20 mA with °2 Θ min − 1 scan speed and 0.02° step scan.  Table 1 Performance of the combined reactor system at different pH values.Period pH Number of measurements a Fe 2+ oxidationrate (g/L h) b,c Fe 2+ oxidationrate (mg/L h) b,d Fe 2+ oxidation% b Fe 3+ precipit.rate (g/day) b Fe 3+ precipit.% b SO 42 − precipit.rate (g/day) b SO 42 − precipit.% b Base consumption(kg/kg Fe 3 removed) b Base consumption(meq/mol Fe 3 removed) b KOH  1 2.5 4 3.3 25 99.4 19 70 18 13 N.d. N.d.2 3.0 4 3.5 26 99.7 26 92 24 17 2.2 2.23 3.5 4 3.7 27 99.5 30 99 13 11 1.7 1.7 CaCO  3 4 3.2 6 3.1 23 99.6 25 98 53 34 1.8 2.05 3.0 5 2.7 20 99.5 21 94 36 28 2.1 2.46 2.8 6 3.0 23 99.7 23 93 59 40 1.4 1.67 2.8 6 7.0 53 99.7 55 96 150 66 0.69 0.77N.d. not determined. a Number of measurements based on which the average values are calculated. b Average value. c Fluidized-bed volume as the reference volume. d Total volume of the reactor system as the reference volume. Fig. 4.  (a) Effect of partial neutralization with KOH and CaCO 3  on the sulfateprecipitation rate (g/day) ( ○ ) and ef  fi ciency (%) ( ♦ ). Precipitation ef  fi ciency wascalculated as percent sulfate precipitated from the total amount of sulfate in the feed.(b) Base consumption in iron and sulfate precipitation with KOH (10 M solution) andCaCO 3  (50 g/L slurry). Fig. 5.  An X-ray diffractogram of precipitate from gravity settler without pHadjustment. All peaks with labeled  d -values (in Å) represent a solid solution of Na-and H 3 O-jarosites (JCPDS 11-302 and 31-650, respectively). The vertical bar shows thescale of relative counts.10  P. Nurmi et al. / Hydrometallurgy 101 (2010) 7  – 14
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