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Iron oxidation and precipitation in a simulated heap leaching solution in a Leptospirillum ferriphilum dominated biofilm reactor
Hydrometallurgy 88 (2007) 67 – 74 www.elsevier.com/locate/hydromet
Iron oxidation and precipitation in a simulated heap leaching solution in a Leptospirillum ferriphilum dominated biofilm reactor Bestamin Ozkaya ⁎, Erkan Sahinkaya, Pauliina Nurmi, Anna H. Kaksonen, Jaakko A. Puhakka Institute of Environmental Engineering and Biotechnology, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland Received 3 November 2006; received in revised form 7 February 2007; accepted 14 February 2007 Available online 13 April 2007
Abstract Iron is a key element influencing bacterial growth, oxidation efficiency and precipitate formation for most industrial applications using bioleaching and biooxidation processes. In this study, iron oxidation by an enrichment culture dominated by Leptospirillum ferriphilum was studied in a simulated heap leaching solution containing (g/L); Fe2+ (20); Mn2+ (3); Mg2+ (4); Al3+ (0.1); Na+ (3.6); Ca2+ (0.6). Initially, studies were conducted in batch bottles at 25 °C in order to determine possible toxicity effect and settling properties of precipitates produced at different pHs. Settling characteristics including interface height, zone settling velocity and sludge volume index were determined. The precipitates had good settling ability. Thereafter, a continuous-flow fluidized-bed reactor (FBR) was operated at 37 °C. The percent iron oxidation in the FBR decreased gradually from 98.5% to around 60% within 20 d due to precipitate formation. After installing a gravity settler to the recycle line of the FBR, the iron oxidation rate increased from 2 to 4 g Fe2+/L·h within 15 d. The maximum Fe2+ oxidation rate was 10 g Fe2+/L·h at a HRT of 2 h and optimum oxidation performance was achieved at a loading rate of 10.7 g Fe2+/ L·h. The oxygen mass transfer limited the Fe2+ oxidation corresponding to an oxygen transfer rate of 35 kg O2/m3·d. This study reveals that a FBR combined with a gravity settler in the recycle line has potential for Fe3+ regeneration in heap leaching of sulfidic minerals. © 2007 Elsevier B.V. All rights reserved. Keywords: Heap leaching; Fluidized-bed reactor; Iron oxidation; Iron oxide precipitation; Leptospirillum ferriphilum
1. Introduction Ferric ion as an oxidant is extremely important for biohydrometallurgical processes where sulfidic minerals such as pyrite and chalcopyrite are oxidized. The main mechanism of bacterial catalysis in the dissolution of sulfidic minerals is based on the bacterial oxidation of ferrous ion (Fe2+), with oxygen as electron acceptor.
⁎ Corresponding author. Tel.: +358 3 3115 2758; fax: +358 3 3115 2869. E-mail addresses: [email protected] (B. Ozkaya), [email protected] (J.A. Puhakka). 0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2007.02.009
Microbial oxidation of Fe2+ may occur 105–106 times faster than chemical oxidation at pH of 2–3 (Bosecker 1997). Fe3+ is biologically generated followed by chemical oxidation of sulfidic mineral (MS) by Fe3+ and finally the resulting Fe2+ is biologically regenerated to Fe3+ (Ehrlich 2001, Haddadin et al., 1995, Hansford and Vargas 2001, Kinnunen and Puhakka 2004). Main bioleaching techniques are in situ, dump, heap, vat, and reactor leaching (Bosecker 1997, Brandl et al., 2001, Ehrlich 2001, Kinnunen 2004). Heap leaching is the most common bioleaching process used mainly for bioleaching of copper and refractory gold-bearing ores (Rawlings 2002, Rawlings et al., 2003).
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Heap bioleaching processes offer little control over the prevailing operation conditions. Moreover, parameters such as temperature and pH can vary widely temporarily and spatially within the heap. The relatively large quantity of gangue compared to valuable materials within a typical heap, continuous recycle of the solution, and the protracted times of exposure can results in the release of considerable concentrations of gangue cations into the heap leach solution to the point where they exceed toxicity limits to Fe2+ oxidizing microorganisms (Ojumu et al., 2006). Furthermore, high temperature and pH increase the precipitation of jarosites and iron oxides, which may further impede the microbial Fe2+ oxidation. A typical pregnant leach solution may contain (g/L) Mg (3–10), Na (0.8–1.7), Ca (0.4–0.5), Al (0.3–12.2), Mn (0.2–0.7), SO42− (35–117) and pH 2.0–2.2 (Ojumu et al., 2006; Sandström and Mattsson, 2001). During bioleaching of sulfidic minerals (e.g. chalcopyrite), the precipitation of jarosite on the mineral surface prevents bacterial access and restricts the mass transfer of oxidant to the surface (Stott et al., 2000). Under moderately acidic conditions, Fe3+ may precipitate in a variety of mineral forms, including both oxides and hydroxy-sulfates (Sasaki and Konno, 2000; Daoud and Karamanev, 2006; Wang et al., 2007). Therefore, iron is one of the key elements influencing bacterial growth, oxidation efficiency and the formation of precipitates that can greatly impact the overall leaching process efficiency. The role of widely studied Acidithiobacillus ferrooxidans in bioleaching applications and natural environments has often been over-estimated, whereas Leptospirillum spp. are clearly more dominant in certain bioleaching applications (Kinnunen and Puhakka 2004; Kinnunen and Puhakka 2005; Dopson et al., 2003; Okibe et al., 2003; Pizarro et al., 1996). In the present study, the combined effect of a mixture of inorganic ions and pH on Fe2+ oxidation of a Leptospirillum ferriphilum dominated fluidized-bed reactor (FBR) culture was determined with a batch assay. The settling characteristics of the produced iron precipitate were studied at different pHs. Moreover, the FBR was operated at different hydraulic retention times (HRT) and various feed pHs — with and without a gravity settler which continuously removed the precipitate from the solution.
from a FBR fed with 7 g Fe2+/L at pH 0.9) was studied in 250 mL bottles at different pH values. Inorganic cations were added as sulfate salts to the feed solution. The initial pH was adjusted with H2SO4 to pH 0.9–2.5 and the initial Fe2+ concentration of 20 g/L was used. The bottles were inoculated with 10 mL carrier material from the FBR and incubated at 150 rpm on a rotary shaker at 25 °C. Settling characteristics of the sludge produced in the batch experiments were determined from an uniform suspension of a known solids concentration. The variation of solid/liquid interface height with time was measured with a 100 mL measuring cylinder and zone settling velocities (ZSV) were calculated from the slope. The sludge volume index (SVI), i.e., the volume in milliliters occupied by 1 g of a suspension after 30 min settling, was determined. The suspended solid concentration of the well mixed liquor was determined after drying the samples in an aluminum dish at 105 °C overnight. 2.2. Continuous-flow fluidized-bed reactor experiments The FBR, in some cases called an expanded bed reactor, relies on the attachment of microorganisms to particles that are fluidized by a high-upward flow rate of the liquid to be treated (Fig. 1). In our study, FBR with activated carbon (Kaiser 0.4–1.4 mm) as biomass carrier was used for continuous-flow experiments at 37 °C. Total and fluidized-bed volumes of the reactor were
2. Materials and methods 2.1. Batch experiments The influence of a mixture of inorganic cations (g/L); Mn2+, (3); Mg2+, (4); Al3+, (0.1); Na+, (3.6); Ca2+, (0.6) on Fe2+ oxidation by the enrichment culture (obtained
Fig. 1. Schematic diagram of fluidized-bed reactor (FBR) used for iron oxidation.
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500 mL and 340 mL, respectively. The upward velocity of the fluid must be sufficient to fluidize the carrier. The bed expansion or fluidization ratio was adjusted to 30%. In order to increase the biomass concentration in the FBR, between days 0 and 40, the reactor was fed with 7 g Fe2+/L in a growth medium consisting of (g/L); (NH4)2SO4, 3.0; Na2SO4, 1.5; KCl, 0.1; K2HPO4, 0.05; MgSO4·7H2O, 0.5; Ca(NO3)2, 0.01 at pH 0.9 and 25 °C. After day 40, the FBR was fed with simulated multimetal ore bio-heap leaching solution at 37 °C from which Cu, Zn, Ni and Co had been removed. For the characteristics of the multi-metal ore, see Dopson et al. (in press). Again the solution contained (g/L); Fe2+ (20); Mn2+ (3); Mg2+ (4); Al3+ (0.1); Na+ (3.6); Ca2+ (0.6) as corresponding sulfate salts in the growth medium described above. The feed pH of the FBR was gradually increased from pH 1.5 to 2.5 by decreasing the amount of H2SO4 added to the feed. The HRT in the fluidize-bed was kept constant at 5 h for 120 d for studying the effect of pH on the FBR performance. Thereafter, the FBR was operated with HRT between 1.5 and 5 h in order to determine the effect of HRT on FBR performance at pH 2. Effluent recycle was used to aerate and to dilute the feed, as well as to maintain a fully mixed conditions and constant up-flow rate. Air was used for aeration and the aeration system was placed in a separate column connected to the recycle flow line of the FBR (Fig. 1). Sampling point was the effluent stream of the FBR. The average dissolved oxygen concentration in the FBR was 3.5 ± 0.5 mg/L. A settling tank with the capacity of 25 L (HRT 2.4 h) was installed on day 50 and used thereafter.
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2.4. Analyses The Fe2+ concentration was determined using the Shimadzu UV 1601 spectrophotometer (Shimadzu, Japan) by the colorimetric ortho-phenantroline method, according to modified 3500-Fe method (Anon. 1992). The iron oxidation rate in the FBR as g/L∙h was calculated as a difference of the Fe2+ concentration in the feed solution and in the FBR effluent for the (effective) activated carbon occupied volume for a given feed rate. The pH and dissolved oxygen (DO) were measured using WTW pH 330i® pH meter and WTW OXI96 dissolved oxygen meter (Weilheim, Germany), respectively. Oxygen transfer rate (OTR) was calculated from the stoichiometric O2 requirement for oxidized Fe2+ as kg O2/m3∙d. Total suspended solid (TSS) concentration in settling tank effluent was determined monthly by filtering a 100 mL sample and drying the filter in an oven at 105 °C. All analytical measurements were performed at least as duplicates. 3. Results 3.1. Iron oxidation in batch assays The effect of a simulated heap leaching solution on Fe2+ oxidation was investigated in batch assays at initial pH of 1.5. Iron oxidation was almost complete both in the presence and absence of the cations (Fig. 2). Their presence, decreased the iron oxidation rates from 6.3 to 2.2 g Fe2+/L·d. The oxidation of Fe2+ in simulated heap leaching solution was studied at different initial pH values and the settling characteristics of the precipitates produced
2.3. Enrichment culture and Denaturing Gradient Gel Electrophoresis (DGGE) The acidophilic iron-oxidizing culture was obtained from a FBR, long-term fed with 7 g Fe2+/L and nutrient medium containing (g/L); (NH4)2HPO4 (0.35), K2CO3 (0.05) and MgSO4 (0.05) at pH 0.9. The microbial community was monitored by phase contrast microscopy and by using denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplified partial 16S rRNA genes as previously described (Ozkaya et al., in press). DGGE analysis was performed with total DNA extracted from the operating FBR carrier at different time intervals to reveal possible changes in bacterial community over time. The sequencing of the purified products was performed at DNA Sequencing Facility, Institute of Biotechnology, Helsinki University, Finland.
Fig. 2. Fe2+ oxidation at pH 1.5 in the absence ( ) and presence ( ) of a mixture of cations (g/L), Mn2+ (3), Mg2+ (4), Al3+ (0.1), Na+ (3.6) Ca2+ (0.6) by the L. ferriphilum dominated culture.
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were also determined. At initial pH of 1.5 and 3.2, the Fe2+ oxidation was almost complete (Fig. 3a) and the initial pH had a minor impact on the oxidation rate which was determined as 2.2 and 1.7 g Fe2+/L·d, respectively (Fig. 3b). The variation of sludge characteristics measured as interface height with time, and ZSV at different initial pH is shown in Fig. 4. As the initial pH increased, the settling velocity of the precipitate increased. The ZSV increased from 2.2 to around 2.9 cm/h with the increase of initial pH from 1.5 to 3.0. The SVI was almost constant about 26.7 ±1.2 mL/g when initial pH was 1.5– 3.0 but increased to 33 mL/g with the increase of initial pH to 3.2 (Fig. 5). The suspended solid concentration of the precipitate was 34 ± 3.5 g/L at the end of the experiment and the concentration did not correlate with the initial pH. 3.2. Fluidized-bed reactor performance Initially, the FBR was fed with synthetic solution containing 7 g Fe2+/L (pH 0.9) for 40 d to increase the biomass concentration. The effluent Fe2+ concentration remained constant during this period being around
Fig. 4. Effect of initial pH on sludge characteristics measured as (a) interface height with time and (b) zone settling velocity (ZSV) in batch assays.
150 mg/L (Fig. 6a). After switching to the simulated heap leaching solution on day 40, the feed pH was gradually increased from 1.5 to 2.5 (Fig. 6c) while the HRT was kept constant at 5 h. The change of feed to the simulated heap leaching solution (pH 1.5–2.5) significantly increased the effluent Fe2+ concentration
Fig. 3. Effect of initial pH on (a) Fe2+ oxidation and (b) oxidation rates in batch assays.
Fig. 5. The effect of initial pH on sludge volume index (SVI) of sludge in batch assays.
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(Fig. 6a) and was associated with inorganic precipitate formation within the FBR. The iron oxidation rate decreased from 3.5 to around 2.0 g Fe2+/L·h on days 40 and 50, respectively (Fig. 6b). A settling tank was then installed to the recycle line and used from day 50 onwards, to continuously remove
Fig. 7. (a) Fe2+ concentration (feed: ; effluent: ) and hydraulic retention time (HRT) (—), (b) pH (feed: ; effluent: ), (c) Fe2+ loading (—) and oxidation ( ) rate (g Fe2+/L·h) (d) dissolved oxygen ( ) and oxygen transfer rate ( ) in a fluidized-bed reactor operated at different HRTs. Fig. 6. (a) Fe2+ concentrations (feed: ; effluent: ), (b) Fe2+ oxidation rates, (c) pH (feed: ; effluent: ) and (d) iron ) in fluidized-bed reactors at a hydraulic retention precipitation ( time of 5 h.
the precipitate from the solution. With the use of settling tank, the Fe2+ concentration decreased from 8000 mg/L to 400 mg/L within 15 d and resulted in almost complete
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Table 1 Optimum operating conditions for Fe2+ oxidation in FBR and separation of the precipitates formed during the regeneration of simulated heap leaching solution Operating conditions
FBR
Settling tank
HRT pH Volume
2h 2.0 0.34 L (effective volume) –
2.4 h – 25 L
37 °C 30% Air 10 g/L·h 35 kg O2/m3·d
N.A. N.A. N.A. N.A. N.A.
Settling tank cleaning frequency Temperature Bed fluidization Aeration Maximum Fe2+ oxidation rate Maximum O2 transfer rate
2 weeks
N.A.: not applicable.
Fe2+ oxidation (98.5%) — even at pH 2.5 (Fig. 6a). Simultaneously, the iron oxidation rate (g Fe2+/L·h) increased from 2.0 to 4.0 (Fig. 6b). With the initial pH of simulated solution of 1.5, the pH of FBR effluent varied in the range of 1.9–1.6. At feed pH of 1.8 and 2, the effluent pH was about 1.6 and 1.8, respectively. Finally when the pH was increased to 2.5, the effluent pH was around 2 (Fig. 6c). The precipitation of Fe3+ even at pH of 1–1.5 cannot be completely avoided. The incorporation of a gravity settler to the system solved the clogging problems caused by the iron and other inorganic precipitates. Fig. 6d shows the concentration of iron precipitate within the settling tank at elevated pH. The iron precipitate concentrations varied between 2000 and 6000 mg/L within the settling tank at pH 2.0–2.5. The HRT in the fluidized bed was kept constant at 5 h for 120 d. Thereafter, the effect of the HRT (1.5–5 h) on Fe2+ oxidation was studied for 60 d at pH 2 (Fig. 7). At HRT above 2 h and loading rates below 10.7 g Fe2+/L·h, the Fe2+ oxidation was almost independent of HRT and loading rate and remained constant above 96% (days 117–147) (Fig. 7). At HRT below 2 h and loading rate above 10.7 g Fe2+/L·h, the Fe2+ oxidation rate decreased due to the oxygen mass transfer limitation. During this period, DO sharply decreased from 2.50 to 0.25 mg/L and accordingly effluent Fe2+ concentration increased from 800 to 9500 mg/L. Due to the oxygen mass transfer limitation, Fe2+ conversion was 55% during this period. The optimum operating conditions of FBR including the gravity settling unit are summarized in Table 1. The microbial community was monitored by phase contrast microscopy and molecular methods. As the results of PCR-DGGE followed by partial sequencing of 16S rRNA gene of the Fe2+ oxidizing culture showed,
the FBR operation at different pH values did not affect the biofilm composition and the bacterial community remained dominated by L. ferriphilum (100% similarity). 4. Discussion Heap leaching solutions typically contain elevated concentrations of inorganic ions and jarosite formation is possible in the acidic environment containing high concentrations of dissolved Fe3+ that can cause deposition and scaling of pipes, emitters and in the heap itself, thereby reducing leaching efficiency and metal recovery (Hiroyoshi et al., 1999). Jarosites have an important influence on the regulation of the pH and dissolved ferric iron in aqueous environments (Dutrizac and Jambor, 2000; Catalan et al., 2002, Casas et al., 2007). Jarosite formation is also a convenient way of eliminating dissolved iron accumulated in solutions obtained in bioleaching of mineral sulfides (Kawano and Tomita, 2001). 3Fe3þ þ Naþ þ 2SO42− þ 6H2 O→NaFe3 ðSO4 Þ2 ðOHÞ6 þ 6Hþ ð1Þ Our work shows the combined effects of inorganic ions (Mn2+, Mg2+, Al3+, Na+, Ca2+), pH and HRT on Fe3+ regeneration in a FBR by L. ferriphilum dominated culture. This work further demonstrates successful use of a gravity settler connected to the recycle line of the FBR to remove the precipitates. Settling characteristics such as ZSV and the SVI produced at different pH showed good settling of the Fe3+ precipitate sludge. In related work, Malik et al. (2004) studied the separate effects of various concentrations of nickel, lead, zinc, manganese, aluminum and silicon on microbial Fe2+ oxidation by A. ferrooxidans. They demonstrated that as the concentration of aluminum was increased from 1.2 to 2.5 g/L, Fe2+ oxidation rate decreased from 2.8 to 0.05 g Fe2+/L·d. They also determined that manganese (0.05–0.5 g/L) did not affect the Fe2+ oxidation rates (2.7–2.9 g Fe2+/L·d). Our study on the combined effect of inorganic ions on Fe2+ oxidation by L. ferriphilum in FBR demonstrates that the optimum oxidation performance (96%) is achieved at a HRT of 2 h and loading rate of 10.7 g/L·h in the presence of various cations which can be present in heap leaching solutions. Ebrahimi et al. (2005) reported that the maximum Fe2+ oxidation rate corresponded to an oxygen transfer rate of 28 kg O2/m3 d in an airlift reactor. In our study, with a FBR the oxygen transfer rate was higher than 35 kg O2/m3·d (Fig. 7). Iron oxidation rates with FBR
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are generally higher than values reported for other types of bioreactors, as reviewed by Kinnunen and Puhakka (2004). In this study, iron oxidation by the enrichment culture dominated by L. ferriphilum did not significantly vary between pH of 1.5–2.5 as demonstrated in batch experiments and in continuous-flow FBR. In the batch assays, the initial pH had a minor impact on the Fe2+ oxidation rate. Iron oxidation consumes protons whereas the precipitation of Fe3+ produces protons. The combined effect of these reactions resulted in final pH values close to pH 1.6 in all batch assays. The precipitates produced showed good settling properties (SVI values about 30 mL/g). This was also confirmed this by the TSS analysis carried out for the effluent of the settling tank having a capacity of 25 L (HRT 2.4 h). Maximum and minimum TSS values of the settling tank effluent were 360 and 480 mg/L, respectively, with highest TSS at highest feed pH 2.5. As the initial pH increased, the settling velocity of the precipitate increased from 2.2 to around 2.9 cm/h. Jensen and Webb (1995) have reported a decrease of iron oxidation rate with activated carbon carrier material which has been proposed to partly result from precipitate deposition in pore spaces at pH 2. In our study, there was no adverse effect on iron oxidation rate at pH 2 since the produced precipitate was removed in the settling tank. However, precipitate accumulation within the FBR was observed at pH 2.5. Therefore, the highest suitable feed pH value to oxidize the iron in a heap leaching solution using a FBR is pH 2. Often the pH of leach liquors was higher than 2 and the regeneration is still used. The optimum feed pH range for Fe3+ regeneration was pH 1.5–2.0 with simulated heap leaching solution and percent of precipitated Fe3+ was below 30%. At higher pH (pH N 2), Fe3+ precipitation increases and results in partial loss of the oxidant and accordingly decreases in the leaching efficiency. FBR experiments performed at pH of 1.5–2.5 with a simulated heap leaching solution and a L. ferriphilum dominated biofilm, demonstrate that the iron oxidation performance decreased due to precipitate formation. After installing a settling tank for the recycled effluent, the iron oxidation rate significantly increased from 2 g Fe2+/L·h to 4 g Fe2+/L·h within 15 d and iron oxidation was almost complete (98.5%) even at pH 2.5. Although iron oxidation rate did not significantly vary at the feed pH range of 1.5–2.5, the amount of inorganic precipitation in the settling tank increased at high pH values. The effluent pH was around 2 when feed pH was 2.5 and feed pH was not further increased due to high amount of precipitate accumulation within the FBR.
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This study demonstrates that the maximum Fe 2+ oxidation rate was 10 g Fe2+/L h at a HRT of 2 h. The maximum iron oxidation rate for the simulated heap leaching solution obtained in our study is similar to iron oxidation rates obtained in other studies using mesophiles (Kinnunen 2004; for a reviews see Nemati et al., 1998; Jensen and Webb 1995). 5. Conclusions 1) Ferric ion is a key element in the bioleaching of sulfide minerals. This study covers Fe2+ oxidation by L. ferriphilum dominated enrichment culture at pH between 1.5 and 3.2 in FBR fed with simulated heap leaching solution. 2) PCR-DGGE followed by partial sequencing of 16S rRNA gene confirmed that the bacterial community of the FBR operated at different pH remains dominated by L. ferriphilum with the similarity of 100%. 3) Fe2+ is almost completely oxidized in simulated heap leaching solution in the presence of cations (g/L): Fe2+ (20); Mn2+ (3); Mg2+ (4); Al3+ (0.1); Na+ (3.6); Ca2+ (0.6). 4) With air aeration, the maximum oxidation rate in the FBR is 10 g/L·h at a HRT of 2 h, below which oxygen mass transfer becomes limiting (b0.5 mg/L). Oxygen mass transfer rate in the FBR corresponds to 35 kg O2/m3·d. 5) Optimum Fe2+ oxidation performance in FBR is achieved at a loading rate of 10.7 g Fe2+/L·h with the conversion efficiency of 96% but decreases due to precipitate formation. 6) For low Fe3+ precipitation (b 30%), the optimum pH range is between 1.5 and 2.0. 7) Installation of a gravity settler in the recycle line, significantly improves the oxidation rate. The Fe3+ precipitate has good settling characteristics and the FBR settling tank effluent has low turbidity with a suspended solid concentration of 360–480 mg/L at a HRT of 2.4 h. In summary, L. ferriphilum dominated FBR combined with a gravity settler in the recycle line has potential for Fe3+ regeneration in heap leaching of sulfidic minerals. Acknowledgements This research was funded by the Talvivaara Mining Company Ltd. Authors Bestamin Özkaya and Erkan Sahinkaya would like to thank the Scientific and
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Iron oxidation and precipitation in a simulated heap leaching solution in a Leptospirillum ferriphilum dominated biofilm reactor Bestamin Ozkaya ⁎, Erkan Sahinkaya, Pauliina Nurmi, Anna H. Kaksonen, Jaakko A. Puhakka Institute of Environmental Engineering and Biotechnology, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland Received 3 November 2006; received in revised form 7 February 2007; accepted 14 February 2007 Available online 13 April 2007
Abstract Iron is a key element influencing bacterial growth, oxidation efficiency and precipitate formation for most industrial applications using bioleaching and biooxidation processes. In this study, iron oxidation by an enrichment culture dominated by Leptospirillum ferriphilum was studied in a simulated heap leaching solution containing (g/L); Fe2+ (20); Mn2+ (3); Mg2+ (4); Al3+ (0.1); Na+ (3.6); Ca2+ (0.6). Initially, studies were conducted in batch bottles at 25 °C in order to determine possible toxicity effect and settling properties of precipitates produced at different pHs. Settling characteristics including interface height, zone settling velocity and sludge volume index were determined. The precipitates had good settling ability. Thereafter, a continuous-flow fluidized-bed reactor (FBR) was operated at 37 °C. The percent iron oxidation in the FBR decreased gradually from 98.5% to around 60% within 20 d due to precipitate formation. After installing a gravity settler to the recycle line of the FBR, the iron oxidation rate increased from 2 to 4 g Fe2+/L·h within 15 d. The maximum Fe2+ oxidation rate was 10 g Fe2+/L·h at a HRT of 2 h and optimum oxidation performance was achieved at a loading rate of 10.7 g Fe2+/ L·h. The oxygen mass transfer limited the Fe2+ oxidation corresponding to an oxygen transfer rate of 35 kg O2/m3·d. This study reveals that a FBR combined with a gravity settler in the recycle line has potential for Fe3+ regeneration in heap leaching of sulfidic minerals. © 2007 Elsevier B.V. All rights reserved. Keywords: Heap leaching; Fluidized-bed reactor; Iron oxidation; Iron oxide precipitation; Leptospirillum ferriphilum
1. Introduction Ferric ion as an oxidant is extremely important for biohydrometallurgical processes where sulfidic minerals such as pyrite and chalcopyrite are oxidized. The main mechanism of bacterial catalysis in the dissolution of sulfidic minerals is based on the bacterial oxidation of ferrous ion (Fe2+), with oxygen as electron acceptor.
⁎ Corresponding author. Tel.: +358 3 3115 2758; fax: +358 3 3115 2869. E-mail addresses: [email protected] (B. Ozkaya), [email protected] (J.A. Puhakka). 0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2007.02.009
Microbial oxidation of Fe2+ may occur 105–106 times faster than chemical oxidation at pH of 2–3 (Bosecker 1997). Fe3+ is biologically generated followed by chemical oxidation of sulfidic mineral (MS) by Fe3+ and finally the resulting Fe2+ is biologically regenerated to Fe3+ (Ehrlich 2001, Haddadin et al., 1995, Hansford and Vargas 2001, Kinnunen and Puhakka 2004). Main bioleaching techniques are in situ, dump, heap, vat, and reactor leaching (Bosecker 1997, Brandl et al., 2001, Ehrlich 2001, Kinnunen 2004). Heap leaching is the most common bioleaching process used mainly for bioleaching of copper and refractory gold-bearing ores (Rawlings 2002, Rawlings et al., 2003).
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Heap bioleaching processes offer little control over the prevailing operation conditions. Moreover, parameters such as temperature and pH can vary widely temporarily and spatially within the heap. The relatively large quantity of gangue compared to valuable materials within a typical heap, continuous recycle of the solution, and the protracted times of exposure can results in the release of considerable concentrations of gangue cations into the heap leach solution to the point where they exceed toxicity limits to Fe2+ oxidizing microorganisms (Ojumu et al., 2006). Furthermore, high temperature and pH increase the precipitation of jarosites and iron oxides, which may further impede the microbial Fe2+ oxidation. A typical pregnant leach solution may contain (g/L) Mg (3–10), Na (0.8–1.7), Ca (0.4–0.5), Al (0.3–12.2), Mn (0.2–0.7), SO42− (35–117) and pH 2.0–2.2 (Ojumu et al., 2006; Sandström and Mattsson, 2001). During bioleaching of sulfidic minerals (e.g. chalcopyrite), the precipitation of jarosite on the mineral surface prevents bacterial access and restricts the mass transfer of oxidant to the surface (Stott et al., 2000). Under moderately acidic conditions, Fe3+ may precipitate in a variety of mineral forms, including both oxides and hydroxy-sulfates (Sasaki and Konno, 2000; Daoud and Karamanev, 2006; Wang et al., 2007). Therefore, iron is one of the key elements influencing bacterial growth, oxidation efficiency and the formation of precipitates that can greatly impact the overall leaching process efficiency. The role of widely studied Acidithiobacillus ferrooxidans in bioleaching applications and natural environments has often been over-estimated, whereas Leptospirillum spp. are clearly more dominant in certain bioleaching applications (Kinnunen and Puhakka 2004; Kinnunen and Puhakka 2005; Dopson et al., 2003; Okibe et al., 2003; Pizarro et al., 1996). In the present study, the combined effect of a mixture of inorganic ions and pH on Fe2+ oxidation of a Leptospirillum ferriphilum dominated fluidized-bed reactor (FBR) culture was determined with a batch assay. The settling characteristics of the produced iron precipitate were studied at different pHs. Moreover, the FBR was operated at different hydraulic retention times (HRT) and various feed pHs — with and without a gravity settler which continuously removed the precipitate from the solution.
from a FBR fed with 7 g Fe2+/L at pH 0.9) was studied in 250 mL bottles at different pH values. Inorganic cations were added as sulfate salts to the feed solution. The initial pH was adjusted with H2SO4 to pH 0.9–2.5 and the initial Fe2+ concentration of 20 g/L was used. The bottles were inoculated with 10 mL carrier material from the FBR and incubated at 150 rpm on a rotary shaker at 25 °C. Settling characteristics of the sludge produced in the batch experiments were determined from an uniform suspension of a known solids concentration. The variation of solid/liquid interface height with time was measured with a 100 mL measuring cylinder and zone settling velocities (ZSV) were calculated from the slope. The sludge volume index (SVI), i.e., the volume in milliliters occupied by 1 g of a suspension after 30 min settling, was determined. The suspended solid concentration of the well mixed liquor was determined after drying the samples in an aluminum dish at 105 °C overnight. 2.2. Continuous-flow fluidized-bed reactor experiments The FBR, in some cases called an expanded bed reactor, relies on the attachment of microorganisms to particles that are fluidized by a high-upward flow rate of the liquid to be treated (Fig. 1). In our study, FBR with activated carbon (Kaiser 0.4–1.4 mm) as biomass carrier was used for continuous-flow experiments at 37 °C. Total and fluidized-bed volumes of the reactor were
2. Materials and methods 2.1. Batch experiments The influence of a mixture of inorganic cations (g/L); Mn2+, (3); Mg2+, (4); Al3+, (0.1); Na+, (3.6); Ca2+, (0.6) on Fe2+ oxidation by the enrichment culture (obtained
Fig. 1. Schematic diagram of fluidized-bed reactor (FBR) used for iron oxidation.
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500 mL and 340 mL, respectively. The upward velocity of the fluid must be sufficient to fluidize the carrier. The bed expansion or fluidization ratio was adjusted to 30%. In order to increase the biomass concentration in the FBR, between days 0 and 40, the reactor was fed with 7 g Fe2+/L in a growth medium consisting of (g/L); (NH4)2SO4, 3.0; Na2SO4, 1.5; KCl, 0.1; K2HPO4, 0.05; MgSO4·7H2O, 0.5; Ca(NO3)2, 0.01 at pH 0.9 and 25 °C. After day 40, the FBR was fed with simulated multimetal ore bio-heap leaching solution at 37 °C from which Cu, Zn, Ni and Co had been removed. For the characteristics of the multi-metal ore, see Dopson et al. (in press). Again the solution contained (g/L); Fe2+ (20); Mn2+ (3); Mg2+ (4); Al3+ (0.1); Na+ (3.6); Ca2+ (0.6) as corresponding sulfate salts in the growth medium described above. The feed pH of the FBR was gradually increased from pH 1.5 to 2.5 by decreasing the amount of H2SO4 added to the feed. The HRT in the fluidize-bed was kept constant at 5 h for 120 d for studying the effect of pH on the FBR performance. Thereafter, the FBR was operated with HRT between 1.5 and 5 h in order to determine the effect of HRT on FBR performance at pH 2. Effluent recycle was used to aerate and to dilute the feed, as well as to maintain a fully mixed conditions and constant up-flow rate. Air was used for aeration and the aeration system was placed in a separate column connected to the recycle flow line of the FBR (Fig. 1). Sampling point was the effluent stream of the FBR. The average dissolved oxygen concentration in the FBR was 3.5 ± 0.5 mg/L. A settling tank with the capacity of 25 L (HRT 2.4 h) was installed on day 50 and used thereafter.
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2.4. Analyses The Fe2+ concentration was determined using the Shimadzu UV 1601 spectrophotometer (Shimadzu, Japan) by the colorimetric ortho-phenantroline method, according to modified 3500-Fe method (Anon. 1992). The iron oxidation rate in the FBR as g/L∙h was calculated as a difference of the Fe2+ concentration in the feed solution and in the FBR effluent for the (effective) activated carbon occupied volume for a given feed rate. The pH and dissolved oxygen (DO) were measured using WTW pH 330i® pH meter and WTW OXI96 dissolved oxygen meter (Weilheim, Germany), respectively. Oxygen transfer rate (OTR) was calculated from the stoichiometric O2 requirement for oxidized Fe2+ as kg O2/m3∙d. Total suspended solid (TSS) concentration in settling tank effluent was determined monthly by filtering a 100 mL sample and drying the filter in an oven at 105 °C. All analytical measurements were performed at least as duplicates. 3. Results 3.1. Iron oxidation in batch assays The effect of a simulated heap leaching solution on Fe2+ oxidation was investigated in batch assays at initial pH of 1.5. Iron oxidation was almost complete both in the presence and absence of the cations (Fig. 2). Their presence, decreased the iron oxidation rates from 6.3 to 2.2 g Fe2+/L·d. The oxidation of Fe2+ in simulated heap leaching solution was studied at different initial pH values and the settling characteristics of the precipitates produced
2.3. Enrichment culture and Denaturing Gradient Gel Electrophoresis (DGGE) The acidophilic iron-oxidizing culture was obtained from a FBR, long-term fed with 7 g Fe2+/L and nutrient medium containing (g/L); (NH4)2HPO4 (0.35), K2CO3 (0.05) and MgSO4 (0.05) at pH 0.9. The microbial community was monitored by phase contrast microscopy and by using denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplified partial 16S rRNA genes as previously described (Ozkaya et al., in press). DGGE analysis was performed with total DNA extracted from the operating FBR carrier at different time intervals to reveal possible changes in bacterial community over time. The sequencing of the purified products was performed at DNA Sequencing Facility, Institute of Biotechnology, Helsinki University, Finland.
Fig. 2. Fe2+ oxidation at pH 1.5 in the absence ( ) and presence ( ) of a mixture of cations (g/L), Mn2+ (3), Mg2+ (4), Al3+ (0.1), Na+ (3.6) Ca2+ (0.6) by the L. ferriphilum dominated culture.
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were also determined. At initial pH of 1.5 and 3.2, the Fe2+ oxidation was almost complete (Fig. 3a) and the initial pH had a minor impact on the oxidation rate which was determined as 2.2 and 1.7 g Fe2+/L·d, respectively (Fig. 3b). The variation of sludge characteristics measured as interface height with time, and ZSV at different initial pH is shown in Fig. 4. As the initial pH increased, the settling velocity of the precipitate increased. The ZSV increased from 2.2 to around 2.9 cm/h with the increase of initial pH from 1.5 to 3.0. The SVI was almost constant about 26.7 ±1.2 mL/g when initial pH was 1.5– 3.0 but increased to 33 mL/g with the increase of initial pH to 3.2 (Fig. 5). The suspended solid concentration of the precipitate was 34 ± 3.5 g/L at the end of the experiment and the concentration did not correlate with the initial pH. 3.2. Fluidized-bed reactor performance Initially, the FBR was fed with synthetic solution containing 7 g Fe2+/L (pH 0.9) for 40 d to increase the biomass concentration. The effluent Fe2+ concentration remained constant during this period being around
Fig. 4. Effect of initial pH on sludge characteristics measured as (a) interface height with time and (b) zone settling velocity (ZSV) in batch assays.
150 mg/L (Fig. 6a). After switching to the simulated heap leaching solution on day 40, the feed pH was gradually increased from 1.5 to 2.5 (Fig. 6c) while the HRT was kept constant at 5 h. The change of feed to the simulated heap leaching solution (pH 1.5–2.5) significantly increased the effluent Fe2+ concentration
Fig. 3. Effect of initial pH on (a) Fe2+ oxidation and (b) oxidation rates in batch assays.
Fig. 5. The effect of initial pH on sludge volume index (SVI) of sludge in batch assays.
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(Fig. 6a) and was associated with inorganic precipitate formation within the FBR. The iron oxidation rate decreased from 3.5 to around 2.0 g Fe2+/L·h on days 40 and 50, respectively (Fig. 6b). A settling tank was then installed to the recycle line and used from day 50 onwards, to continuously remove
Fig. 7. (a) Fe2+ concentration (feed: ; effluent: ) and hydraulic retention time (HRT) (—), (b) pH (feed: ; effluent: ), (c) Fe2+ loading (—) and oxidation ( ) rate (g Fe2+/L·h) (d) dissolved oxygen ( ) and oxygen transfer rate ( ) in a fluidized-bed reactor operated at different HRTs. Fig. 6. (a) Fe2+ concentrations (feed: ; effluent: ), (b) Fe2+ oxidation rates, (c) pH (feed: ; effluent: ) and (d) iron ) in fluidized-bed reactors at a hydraulic retention precipitation ( time of 5 h.
the precipitate from the solution. With the use of settling tank, the Fe2+ concentration decreased from 8000 mg/L to 400 mg/L within 15 d and resulted in almost complete
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Table 1 Optimum operating conditions for Fe2+ oxidation in FBR and separation of the precipitates formed during the regeneration of simulated heap leaching solution Operating conditions
FBR
Settling tank
HRT pH Volume
2h 2.0 0.34 L (effective volume) –
2.4 h – 25 L
37 °C 30% Air 10 g/L·h 35 kg O2/m3·d
N.A. N.A. N.A. N.A. N.A.
Settling tank cleaning frequency Temperature Bed fluidization Aeration Maximum Fe2+ oxidation rate Maximum O2 transfer rate
2 weeks
N.A.: not applicable.
Fe2+ oxidation (98.5%) — even at pH 2.5 (Fig. 6a). Simultaneously, the iron oxidation rate (g Fe2+/L·h) increased from 2.0 to 4.0 (Fig. 6b). With the initial pH of simulated solution of 1.5, the pH of FBR effluent varied in the range of 1.9–1.6. At feed pH of 1.8 and 2, the effluent pH was about 1.6 and 1.8, respectively. Finally when the pH was increased to 2.5, the effluent pH was around 2 (Fig. 6c). The precipitation of Fe3+ even at pH of 1–1.5 cannot be completely avoided. The incorporation of a gravity settler to the system solved the clogging problems caused by the iron and other inorganic precipitates. Fig. 6d shows the concentration of iron precipitate within the settling tank at elevated pH. The iron precipitate concentrations varied between 2000 and 6000 mg/L within the settling tank at pH 2.0–2.5. The HRT in the fluidized bed was kept constant at 5 h for 120 d. Thereafter, the effect of the HRT (1.5–5 h) on Fe2+ oxidation was studied for 60 d at pH 2 (Fig. 7). At HRT above 2 h and loading rates below 10.7 g Fe2+/L·h, the Fe2+ oxidation was almost independent of HRT and loading rate and remained constant above 96% (days 117–147) (Fig. 7). At HRT below 2 h and loading rate above 10.7 g Fe2+/L·h, the Fe2+ oxidation rate decreased due to the oxygen mass transfer limitation. During this period, DO sharply decreased from 2.50 to 0.25 mg/L and accordingly effluent Fe2+ concentration increased from 800 to 9500 mg/L. Due to the oxygen mass transfer limitation, Fe2+ conversion was 55% during this period. The optimum operating conditions of FBR including the gravity settling unit are summarized in Table 1. The microbial community was monitored by phase contrast microscopy and molecular methods. As the results of PCR-DGGE followed by partial sequencing of 16S rRNA gene of the Fe2+ oxidizing culture showed,
the FBR operation at different pH values did not affect the biofilm composition and the bacterial community remained dominated by L. ferriphilum (100% similarity). 4. Discussion Heap leaching solutions typically contain elevated concentrations of inorganic ions and jarosite formation is possible in the acidic environment containing high concentrations of dissolved Fe3+ that can cause deposition and scaling of pipes, emitters and in the heap itself, thereby reducing leaching efficiency and metal recovery (Hiroyoshi et al., 1999). Jarosites have an important influence on the regulation of the pH and dissolved ferric iron in aqueous environments (Dutrizac and Jambor, 2000; Catalan et al., 2002, Casas et al., 2007). Jarosite formation is also a convenient way of eliminating dissolved iron accumulated in solutions obtained in bioleaching of mineral sulfides (Kawano and Tomita, 2001). 3Fe3þ þ Naþ þ 2SO42− þ 6H2 O→NaFe3 ðSO4 Þ2 ðOHÞ6 þ 6Hþ ð1Þ Our work shows the combined effects of inorganic ions (Mn2+, Mg2+, Al3+, Na+, Ca2+), pH and HRT on Fe3+ regeneration in a FBR by L. ferriphilum dominated culture. This work further demonstrates successful use of a gravity settler connected to the recycle line of the FBR to remove the precipitates. Settling characteristics such as ZSV and the SVI produced at different pH showed good settling of the Fe3+ precipitate sludge. In related work, Malik et al. (2004) studied the separate effects of various concentrations of nickel, lead, zinc, manganese, aluminum and silicon on microbial Fe2+ oxidation by A. ferrooxidans. They demonstrated that as the concentration of aluminum was increased from 1.2 to 2.5 g/L, Fe2+ oxidation rate decreased from 2.8 to 0.05 g Fe2+/L·d. They also determined that manganese (0.05–0.5 g/L) did not affect the Fe2+ oxidation rates (2.7–2.9 g Fe2+/L·d). Our study on the combined effect of inorganic ions on Fe2+ oxidation by L. ferriphilum in FBR demonstrates that the optimum oxidation performance (96%) is achieved at a HRT of 2 h and loading rate of 10.7 g/L·h in the presence of various cations which can be present in heap leaching solutions. Ebrahimi et al. (2005) reported that the maximum Fe2+ oxidation rate corresponded to an oxygen transfer rate of 28 kg O2/m3 d in an airlift reactor. In our study, with a FBR the oxygen transfer rate was higher than 35 kg O2/m3·d (Fig. 7). Iron oxidation rates with FBR
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are generally higher than values reported for other types of bioreactors, as reviewed by Kinnunen and Puhakka (2004). In this study, iron oxidation by the enrichment culture dominated by L. ferriphilum did not significantly vary between pH of 1.5–2.5 as demonstrated in batch experiments and in continuous-flow FBR. In the batch assays, the initial pH had a minor impact on the Fe2+ oxidation rate. Iron oxidation consumes protons whereas the precipitation of Fe3+ produces protons. The combined effect of these reactions resulted in final pH values close to pH 1.6 in all batch assays. The precipitates produced showed good settling properties (SVI values about 30 mL/g). This was also confirmed this by the TSS analysis carried out for the effluent of the settling tank having a capacity of 25 L (HRT 2.4 h). Maximum and minimum TSS values of the settling tank effluent were 360 and 480 mg/L, respectively, with highest TSS at highest feed pH 2.5. As the initial pH increased, the settling velocity of the precipitate increased from 2.2 to around 2.9 cm/h. Jensen and Webb (1995) have reported a decrease of iron oxidation rate with activated carbon carrier material which has been proposed to partly result from precipitate deposition in pore spaces at pH 2. In our study, there was no adverse effect on iron oxidation rate at pH 2 since the produced precipitate was removed in the settling tank. However, precipitate accumulation within the FBR was observed at pH 2.5. Therefore, the highest suitable feed pH value to oxidize the iron in a heap leaching solution using a FBR is pH 2. Often the pH of leach liquors was higher than 2 and the regeneration is still used. The optimum feed pH range for Fe3+ regeneration was pH 1.5–2.0 with simulated heap leaching solution and percent of precipitated Fe3+ was below 30%. At higher pH (pH N 2), Fe3+ precipitation increases and results in partial loss of the oxidant and accordingly decreases in the leaching efficiency. FBR experiments performed at pH of 1.5–2.5 with a simulated heap leaching solution and a L. ferriphilum dominated biofilm, demonstrate that the iron oxidation performance decreased due to precipitate formation. After installing a settling tank for the recycled effluent, the iron oxidation rate significantly increased from 2 g Fe2+/L·h to 4 g Fe2+/L·h within 15 d and iron oxidation was almost complete (98.5%) even at pH 2.5. Although iron oxidation rate did not significantly vary at the feed pH range of 1.5–2.5, the amount of inorganic precipitation in the settling tank increased at high pH values. The effluent pH was around 2 when feed pH was 2.5 and feed pH was not further increased due to high amount of precipitate accumulation within the FBR.
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This study demonstrates that the maximum Fe 2+ oxidation rate was 10 g Fe2+/L h at a HRT of 2 h. The maximum iron oxidation rate for the simulated heap leaching solution obtained in our study is similar to iron oxidation rates obtained in other studies using mesophiles (Kinnunen 2004; for a reviews see Nemati et al., 1998; Jensen and Webb 1995). 5. Conclusions 1) Ferric ion is a key element in the bioleaching of sulfide minerals. This study covers Fe2+ oxidation by L. ferriphilum dominated enrichment culture at pH between 1.5 and 3.2 in FBR fed with simulated heap leaching solution. 2) PCR-DGGE followed by partial sequencing of 16S rRNA gene confirmed that the bacterial community of the FBR operated at different pH remains dominated by L. ferriphilum with the similarity of 100%. 3) Fe2+ is almost completely oxidized in simulated heap leaching solution in the presence of cations (g/L): Fe2+ (20); Mn2+ (3); Mg2+ (4); Al3+ (0.1); Na+ (3.6); Ca2+ (0.6). 4) With air aeration, the maximum oxidation rate in the FBR is 10 g/L·h at a HRT of 2 h, below which oxygen mass transfer becomes limiting (b0.5 mg/L). Oxygen mass transfer rate in the FBR corresponds to 35 kg O2/m3·d. 5) Optimum Fe2+ oxidation performance in FBR is achieved at a loading rate of 10.7 g Fe2+/L·h with the conversion efficiency of 96% but decreases due to precipitate formation. 6) For low Fe3+ precipitation (b 30%), the optimum pH range is between 1.5 and 2.0. 7) Installation of a gravity settler in the recycle line, significantly improves the oxidation rate. The Fe3+ precipitate has good settling characteristics and the FBR settling tank effluent has low turbidity with a suspended solid concentration of 360–480 mg/L at a HRT of 2.4 h. In summary, L. ferriphilum dominated FBR combined with a gravity settler in the recycle line has potential for Fe3+ regeneration in heap leaching of sulfidic minerals. Acknowledgements This research was funded by the Talvivaara Mining Company Ltd. Authors Bestamin Özkaya and Erkan Sahinkaya would like to thank the Scientific and
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