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Bioleaching of complex zinc sulphides using mesophilic and thermophilic bacteria: comparative importance of pH and iron
Hydrometallurgy 73 (2004) 293 – 303 www.elsevier.com/locate/hydromet
Bioleaching of complex zinc sulphides using mesophilic and thermophilic bacteria: comparative importance of pH and iron H. Deveci a,*, A. Akcil b, I. Alp a b
a Mineral Processing Division, Department of Mining Engineering, Karadeniz Technical University, Trabzon TR 61080, Turkey BIOMIN Group, Mineral Processing Division, Department of Mining Engineering, Suleyman Demirel University, Isparta TR 32260, Turkey
Received 16 September 2003; received in revised form 8 December 2003; accepted 9 December 2003
Abstract This study investigates the bioleaching of the complex Pb/Zn ore/concentrate using mesophilic (at 30 jC), moderate (at 50 jC), and extreme thermophilic (at 70 jC) strains of acidophilic bacteria. The effects of bacterial strain, pH, iron precipitation, and external addition of Fe2 + on the extraction of zinc were evaluated. The results have shown that the ore is readily amenable to the selective extraction of zinc and lead using the acidophilic strains of bacteria [i.e., majority of lead (>98%) reports to the residue]. Moderate thermophiles displayed superior kinetics of dissolution of zinc compared with the other two groups of bacteria. The pH was found to exert a profound effect on the leaching process controlling the bacterial activity and precipitation of ferric iron mainly as K-jarosite. The K+ released presumably from the alteration of the silicate phases such as K-feldspar present in the ore appeared to promote the formation K-jarosite in moderately thermophilic leaching systems. The external addition of iron was shown to be required for the bacteria to efficiently drive the extraction of zinc from the bulk concentrate. These findings place the emphasis on the prime importance of ferric iron for the dissolution of zinc and of mineralogical properties (i.e., iron and silicate content) of an ore/concentrate to be treated via bioleaching processes. D 2004 Elsevier B.V. All rights reserved. Keywords: Mesophilic bacteria; Thermophilic bacteria; Bioleaching; Complex sulphides; Zinc sulphide; K-jarosite
1. Introduction Complex sulphide ores such as the McArthur River ore in Australia prove difficult to treat by conventional extraction processes (Barbery et al., 1980). Since its discovery in the 1950s, extensive research has been undertaken to develop a commercially feasible process for the McArthur River ore (Buchanan, 1984). However, the extremely fine dissemination of lead and zinc * Corresponding author. Tel.: +90-462-3773681; fax: +90-4623257405. E-mail address: [email protected] (H. Deveci). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2003.12.001
sulphides and their intimate association with noneconomic minerals such as dolomite and pyrite had prevented the commercial exploitation of the deposit until the mid 1990s due to the failure to produce bulk or separate lead and zinc flotation concentrates of sufficient grade for extraction processes such as Imperial Smelting Furnace (ISF) and electrolytic processes at reasonable recoveries (Buchanan, 1984; Chadwick, 1996). In the 1990s, the significant developments in fine grinding technology with the improvements in flotation and Imperial Smelting Furnace (ISF) technology enabled the production of a bulk flotation concentrate of 42 –46% Zn, 10– 14% Pb, and 120 –
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150 g/t Ag at recoveries of 87% Zn and 62% Pb and subsequent treatment of such concentrate in the Imperial Smelting Furnace (ISF) process as blended with high-grade concentrates (Chadwick, 1996; Enderle et al., 1997; van Os, 1994). Bioleaching may be utilised as an alternative process for the extraction of base metals from complex ores/concentrates (Carta et al., 1980; Gilbertson, 2000; Miller et al., 1999; Steemson et al., 1997). Mesophilic bacteria such as Acidithiobacillus ferrooxidans (formerly known as Thiobacillus ferrooxidans), Acidithiobacillus thiooxidans (formerly known as Thiobacillus thiooxidans), and Leptosprillium ferrooxidans operating at f 40 jC are the most extensively used microorganisms for the bioleaching of sulphide minerals with commercial interest (Dew et al., 1997). However, the commercial use of thermophilic bacteria with their ability to operate at temperatures exceeding 45 jC has great potential for improving the kinetics of metal extraction from sulphide minerals (Konishi et al., 1998a; Norris et al., 2000; Rawlings et al., 2003). Several processes for commercial bioleaching using thermophilic bacteria for base metals (Cu, Ni, and Zn) are in development with reports of imminent commercial realisation (Gilbertson, 2000; Miller et al., 1999). Bioleaching of sulphide minerals is based on the exploitation of the ability of the acidophilic bacteria to oxidise ferrous iron (Eq. (1)) and/or reduce sulphur compounds (Eq. (2) for elemental sulphur). The actual role of bacteria in the bioleaching process has not been completely resolved (Bosecker, 1997; Fowler and Crundwell, 1999; Sand et al., 2001; Suzuki, 2001; Tributsch, 2001); albeit recent findings (Fowler and Crundwell, 1999; Sand et al., 2001) suggest that the oxidation of sulphide minerals occurs mainly via the chemical attack by ferric iron and/or acid (Eqs. (3) and (4)), which are generated by bacteria: bacteria
2FeSO4þ1=2O2þH2 SO4 ! Fe2 ðSO4 Þ3þH2 O bacteria
ð1Þ
S0 þ 3=2O2 þ H2 O ! H2 SO4
ð2Þ
ZnS þ 1=2O2 þ H2 SO4 ! ZnSO4 þ H2 O þ S0
ð3Þ
ZnS þ Fe2 ðSO4 Þ3 ! ZnSO4 þ 2FeSO4 þ S0
ð4Þ
PbS þ Fe2 ðSO4 Þ3 ! PbSO4ðsÞ þ 2FeSO4 þ S0
ð5Þ
The oxidation of galena (Eq. (5)) leads to the formation of lead sulphate, which is ‘‘insoluble’’ (0.045 g/L at 25 jC) (Forward and Peters, 1985) under bioleaching conditions and reports to the residues (Gomez et al., 1995). This raises the possibility of the selective extraction of metals such as zinc from complex ores/concentrates. There are several important factors such as temperature, pH, availability of nutrients, sulphide minerals, O2 and CO2, solid ratio, metal toxicity, etc., that affect the growth of bacteria and hence the dissolution process (Akcil and Ciftci, 2003; Bosecker, 1997; Deveci, 2002; Deveci et al., 2003). The presence of some ions such as K+, Na+, and SO42 in bioleaching environments could promote the formation of solid products such as ferric precipitates (e.g., K-jarosite) which is controlled by pH (Tuovinen and Bhatti, 1999). The limited extraction of metals has often been attributed to the formation of these secondary phases during bioleaching (Ahonen and Tuovinen, 1995; Gomez et al., 1999; Hiroyoshi et al., 1999). In this study, the potential amenability of the McArthur River complex Pb/Zn ore to the selective extraction of zinc using strains of mesophilic, moderately thermophilic, and extremely thermophilic bacteria was evaluated. The effect of pH, iron precipitation, and external addition of ferrous iron on the bioleaching of the ore/concentrate and extraction of zinc was examined using the selected strains of bacteria.
2. Materials and methods 2.1. Ore and concentrate samples The McArthur River ore and bulk concentrate samples, kindly provided by MIM Holdings and Britannia Zinc, respectively, were used in this study. The chemical and mineralogical analyses of the ore and concentrate are shown in Table 1. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses of the samples revealed that sphalerite, galena, and pyrite are the main sulphide phases. Sphalerite and galena, in particular in the ore, were found to be very fine-grained and intimately associated with each other to such a degree that even fine grinding ( < 10 Am) would not produce
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complete liberation. This currently justifies only the production of a bulk concentrate as a commercial product and further treatment in a pyrometallurgical process route for the recovery of contained values. Particle sizes of d80 = 250 Am and d80 = 85 Am for the ore (as ground in a tema mill) and d80 = 20 Am for the concentrate (as received) were used in the experiments.
Table 2 Acidophilic strains of bacteria used in bioleaching experiments Type
Bacterial strain
Code
Mesophiles (30 jC)
Acidithiobacillus ferrooxidans Mixed cultures
DSM 583
Moderate (50 jC)
2.2. Microorganisms and nutrient solution Mesophilic, moderately thermophilic, and extremely thermophilic strains of acidophilic bacteria were used in this study (Table 2). The mesophilic enrichment culture designated WJM (an isolate from the acid mine drainage waters of Wheal Jane Mine, Cornwall, UK) consisted of iron and sulphur oxidisers such as A. ferrooxidans, and heterotrophic bacteria (Jordan, 1993). The other mixed mesophile MES1 was composed of mainly A. ferrooxidans, and to a minor extent L. ferrooxidans and A. thiooxidans (Deveci, 2001). The moderately thermophilic mixed culture MOT6 had a composition of Sulfobacillus acidophilus, Sulfobacillus yellowstonensis, Sulfobacillus thermosulfidooxidans and, to a lesser extent, Acidithiobacillus caldus (Deveci, 2001). All the strains of mesophilic, moderately thermophilic, and extremely thermophilic bacteria were grown/maintained on the ore (1 – 2% wt/vol) prior to use as inoculum in the experiments. The growth of all the strains was conducted in an enriched salt solution containing MgSO47H2O (0.4 g/L), (NH4)2SO4 (0.2 g/ L), K2HPO43H2O (0.1 g/L), and KCl (0.1 g/L). Yeast extract (YE; 0.02% wt/vol) was also added to support the growth of moderate thermophiles.
Table 1 Chemical and mineralogical compositions of the ore and bulk concentrate samples Sample
Zn Fe Pb (%) (%) (%)
Ore
16.2 7.95
Cu S Ag Mineralogical (%) (%) (g/t) content
59 Sphalerite, galena, pyrite, Concentrate 43.3 2.89 11.17 0.89 26.8 145 chalcopyrite, quartz, dolomite, and various silicate phases such as ortochlase
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Extreme thermophiles (70 jC)
Sulfobacillus thermosulfidooxidans Sulfobacillus acidophilus Sulfobacillus yellowstonensis Mixed culture Acidanus brierleyi
MES1 and WJM TH1 THWX YTF1 MOT6 DSM 1651
2.3. Bioleaching experiments Bioleaching experiments were carried out in 250mL Erlenmeyer flasks. Enriched salt solution (90 mL) adjusted to the required pH was transferred into each flask to which 1– 2 g of the ore/concentrate sample (corresponding to 1 – 2% wt/vol pulp density) was added. The flasks were then autoclaved at 1 atm and 121 jC for 20 min. Following autoclaving, each flask was inoculated under aseptic conditions with a 10-mL aliquot of the selected culture producing a final volume of slurry of approximately 100 mL. To facilitate mixing of the contents and transfer of O2 and CO2, the flasks were shaken on the orbital shakers controlled at growth temperatures of 30 jC for mesophiles, 50 jC for moderate thermophiles, and 70 jC for extreme thermophiles. Each flask was sampled daily by removing a 1-mL aliquot of the leach solution, which was then used for analysis of metals (Zn, Fe, and Pb) by atomic absorption spectrometry (AAS) and for monitoring pH and redox potential. The pH was adjusted using 18 M H2SO4 when it deviated towards neutrality from the initial preset values.
3. Results and discussion
5.60 0.27 15.2
3.1. Comparison of the bioleaching performance of mesophilic and thermophilic bacteria Fig. 1 illustrates the extraction of zinc from the ore by mesophilic (WJM strain), moderately thermophilic
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Fig. 1. Extraction of zinc from the ore using mesophilic (WJM strain), moderately thermophilic (S. yellowstonensis), and extremely thermophilic (A. brierleyi) bacteria (1% wt/vol, d80 = 250 Am at 30, 50, and 70 jC, respectively).
(S. yellowstonensis), and extremely thermophilic (Acidianus brierleyi) bacteria at 30, 50, and 70 jC, respectively. Over 90% extraction of the zinc was achieved by all the strains (groups) of bacteria used. However, the dissolution of lead was minimal, with >98% of lead having reported to the residues most likely in the form of either ‘‘insoluble’’ lead sulphate or undissolved galena irrespective of the strain of bacteria. This indicates the selective nature of bioleaching of zinc from the complex ore. The extraction of zinc by moderate thermophile S. yellowstonensis was rapid, resulting in a zinc extraction of f 93% over a bioleaching period of only 113 h compared with >200 h required for similar extractions by the mesophilic and extreme thermophilic bacteria. The findings also indicated the superior bioleaching capacity of thermophilic bacteria compared with mesophilic bacteria presumably due to the positive effect of elevated temperature on the dissolution of sulphide minerals. This was consistent with the reports for the improved dissolution kinetics of sulphides by extremely thermophilic bacteria (Dew et al., 1999; Konishi et al., 1998a; Witne and Phillips, 2001). It was also evident in Fig. 1 that the extraction of zinc by S. yellowstonensis (50 jC) and A. brierleyi (70 jC) was similar over the initial period of f 37 h; thereafter, A. brierleyi could not maintain its initial bioleaching performance compared with that of S. yellowstonensis. This could be attributed to the superior oxidising capacity of S. yellowstonensis as the
most rapidly growing bacteria reported to date (Johnson et al., 2001) and also, and probably more importantly, to decreasing availability of O2 and CO2 at a relatively high temperature of 70 jC for growth (Boon and Heijnen, 1998). In this regard, S. yellowstonensis could have readily utilised the yeast extract (0.02% wt/vol) added as a source of cellular carbon while the growth of A. brierleyi would have been limited by the availability (transfer) of atmospheric CO2 in the medium. Bacterial growth rates were not determined in this study. The current study was carried out at 1% wt/vol since further increase in the pulp density would probably lead to the increase in the CO2 demand of bacteria (i.e., the aggravation of CO2 limitation). Konishi et al. (1998b) observed a significant improvement in the growth of A. brierleyi and hence the biooxidation rate of pyrite at 65 jC when the growth media were supplemented by 0.005 – 0.25% wt/vol yeast extract. Increasing the availability of CO2 in the bioleaching medium may also improve the growth and hence bioleaching performance of A. brierleyi (Witne and Phillips, 2001; Boon and Heijnen, 1998). 3.2. Effect of pH on the bioleaching activity of mesophilic and thermophilic bacteria Fig. 2 shows the effect of pH in the range of 1.0– 2.0 on the dissolution rate of zinc from the ore by mesophilic A. ferrooxidans, moderately thermophilic
Fig. 2. Effect of pH on the initial (maximum) dissolution rate of zinc from the ore by mesophilic A. ferrooxidans, moderately thermophilic S. yellowstonensis and S. thermosulfidooxidans, and extremely thermophilic A. brierleyi (1% wt/vol, d80 = 250 Am).
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Fig. 3. Effect of pH on the extraction of zinc from the ore by S. yellowstonensis (1% wt/vol, d80 = 250 Am at 50 jC and 0.02% wt/vol yeast extract).
S. yellowstonensis and S. thermosulfidooxidans, and extremely thermophilic A. brierleyi. The bioleaching activity of mesophile A. ferrooxidans, as indicated by the dissolution rate of the zinc (Fig. 2), was adversely affected with decreasing pH to V 1.4. But, above this pH, there seemed no significant difference in the dissolution rate and extent (91 –93%) of zinc, with an optimum bioleaching performance having been recorded at pH 1.8. In contrast to A. ferrooxidans, the extraction rate of zinc by the extremely thermophilic A. brierleyi (at 70 jC) was observed to increase with increasing acidity, as shown in Fig. 2. The extraction of zinc, although being comparable at pH V 1.4 (95 – 92%) over the bioleaching period, was recorded to decrease by f10% with a step increase in pH between 1.6 and 2.0. A trend of increase in the dissolution of iron with decreasing pH was also discerned (not shown), consistent with the increase in the bioleaching activity of the strain in response to the decrease in pH. It should be noted that the extraction of iron by A. brierleyi was relatively low (i.e., 43% at pH 1.0), in comparison with those (55 – 64%) obtained for A. ferrooxidans at pH z 1.6 for a similar extent of zinc extraction. These findings were consistent with those of Jordan (1993). The minimal precipitation of ferric iron was observed for A. brierleyi even at pH 1.8 – 2.0 probably due to the slow accumulation of ferric iron in solution coupled with the rapid reduction of ferric iron by the sulphides such as sphalerite at 70 jC. However, high temperatures would promote the precipitation of ferric iron
297
even at low solution pH values (Arslan and Arslan, 2003; Dutrizac, 1983a,b; Konishi et al., 1998a; Welham et al., 2000) and the ability of extreme thermophiles to operate at low pH values could become important for the bioleaching process so that the formation of potentially deleterious precipitates could be minimised. The effect of acidity (pH 1.2 –2.0) on the dissolution of zinc from the ore by moderately thermophilic S. yellowstonensis and S. thermosulfidooxidans (Figs. 3 and 4) was similar in character to that by mesophilic A. ferrooxidans. The increase in the acidity to pH 1.2 –1.4 led to a decrease in the oxidising activity of both strains, indicating the inhibitory effect of increased acidity on the strains. The dissolution rate of zinc by both strains was observed to peak at pH 1.6; thereafter, a negligible decrease was recorded (Fig. 2). Bioleaching activity of both strains was limited with varying extent at pH values V 1.4. The increase in the acidity (to pH 1.2) was noted to severely inhibit the oxidising activity of S. yellowstonensis as indicated by a sharp decrease in the solubilisation of zinc (Fig. 3). Similarly, a severe limitation of the bioleaching activity of S. thermosulfidooxidans was also observed even at pH 1.4 (Fig. 4). This indicates the varying response of both strains to acidity although the extent of the inhibitory effect on the activity of both strains was similar at pH 1.2. The dissolution pattern of iron for both strains at pH 1.2– 2.0 was also similar in character as illustrated in
Fig. 4. Effect of pH on the extraction of zinc from the ore by S. thermosulfidooxidans (1% wt/vol, d80 = 250 Am at 50 jC and 0.02% wt/vol yeast extract).
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Figs. 5 and 6. The dissolution of iron by both strains at pH 1.6– 2.0 was rapid, consistent with the dissolution of zinc (Figs. 3 and 4). However, a decrease in the concentration of iron following a bioleaching period of 83 h was apparent for both strains at pH z 1.8, indicating the precipitation of ferric iron probably as it accumulated in the system. The cessation of the zinc release at pH 2.0 after 113 h (Figs. 3 and 4) could be attributed to the extensive precipitation of ferric iron occurring at this pH (i.e., the formation of a protective layer on the mineral surface by ferric precipitates, thus retarding the continued dissolution of zinc). These findings suggest that pH profoundly affects the overall bioleaching process, adversely influencing the activity of moderately thermophilic strains used at pH V 1.4 and dictating the solubility of ferric iron particularly at pH z 1.8. Furthermore, the current tests were carried out in a batch mode where the accumulation and precipitation of ferric iron would be expected to occur following a significant recovery of zinc. However, the precipitation of ferric iron may present severe problems during a continuous operation at high pH values. The effect of pH was studied only at a pulp density of 1% wt/vol. An increase in the pulp density would lead to the increase in the concentrations of ferric ion and sulphate within the system, which would further promote the formation of precipitates probably even at pH values as low as pH 1.4 – 1.5 (Deveci, 2001). This would necessitate operation probably at pH values below 1.4 at the expense of reduced bacterial activity
Fig. 5. Effect of pH on the dissolution of iron from the ore by S. yellowstonensis (1% wt/vol, d80 = 250 Am at 50 jC and 0.02% wt/vol yeast extract).
Fig. 6. Effect of pH on the dissolution of iron from the ore by S. thermosulfidooxidans (1% wt/vol, d80 = 250 Am at 50 jC and 0.02% wt/vol yeast extract).
in order to minimise the formation of the detrimental ferric precipitates. Adaptation of the strains to operate at pH V 1.4 may ameliorate the negative impact of acidity (Deveci, 2001; Porro et al., 1989). 3.3. Nature of ferric precipitates formed during moderately thermophilic bioleaching processes These tests were designed to further elucidate the precipitation of iron encountered above during the moderately thermophilic bioleaching of the ore at pH 1.8 –2.0 and 50 jC. Fig. 7 illustrates the dissolution of zinc and iron by S. acidophilus at two different pH
Fig. 7. Extraction of zinc and iron from the ore by S. acidophilus in low (pH 1.5 – 1.6) and high (pH 1.8 – 2.0) pH regimes (2% wt/vol, d80 = 85 Am at 50 jC and 0.02% wt/vol yeast extract).
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regimes. A similar dissolution trend for zinc was observed at low pH (1.5 – 1.6) and higher pH (1.8 – 2.0) conditions with a minimal difference in the dissolution rate of zinc (52 – 54 mg/L/h). The extraction of zinc was essentially complete over a bioleaching period of 62 h, which was significantly shorter than the time (f182 h) required by S. yellowstonensis and S. thermosulfidooxidans to achieve the same level of extraction (at pH z 1.6). This was probably mainly due to the positive effect of decreasing particle size of the ore (d80 = 85 Am, cf. 250 Am). The release of iron at higher pH regime was observed to peak at 62 h and then the precipitation of iron became evident with a decrease in the concentration of iron in solution in the following periods as noted earlier at pH z 1.8 (Figs. 5 and 6). This coincided with a decrease in the pH from 1.86 to 1.69 probably as a result of excessive production of acid due to the precipitation. There was a negligible difference in the extraction of zinc at both pH conditions, indicating minimal effect of precipitates on the release of zinc. In fact, f 97% of zinc had already been recovered at the onset of the apparent decrease in the concentration of iron at high pH regime. The decrease in the concentration of iron in solution (after 62 h) at higher pH regime coincided with the decrease in the concentration of K+ as shown in Fig. 8. This suggests that the ferric iron had precipitated mainly in the form of K-jarosite (Eq. (6)), which was the most common precipitate phase detected in
299
the XRD analysis of the bioleach residues during the bacterial leaching of the complex ore particularly at 50 jC (Deveci, 2001). A tendency for a decrease in the concentration of Na+ during these periods (after 62 h) was also noted. The behaviour of K+ and Na+ during the precipitation of iron was in accordance with the relative stability of K-jarosite over Na-jarosite, as suggested by Dutrizac (1983a), who reported that participation of alkali ions in the jarosite structure would be in the order of K+>NH4+>Na+: Xþ þ 3Fe3þ þ 2SO2 4 þ 6H2 O ! XFe3 ðSO4 Þ2 ðOHÞ6 þ 6Hþ
ð6Þ
where X could be monovalent cations such as K+, NH4+, Na+, Ag+, etc., or 1/2Pb2 +. Dutrizac (1983b) reported the minimum concentrations of Fe3 +, K+, and Na+ in solution to be 10 3, 0.02, and 0.05 M, respectively, for the precipitation of jarosites presumably at high temperatures (f95 jC). Despite the relatively low operating temperature (50 jC), the precipitation of K-jarosite was observed to occur from solutions containing f4.2 mM K+ (and 12 mM Na+) in the current study. The concentration of K+ in enriched salt medium was determined to be 0.08 g/L, which indicated that the remaining K+ was released from the ore. Bhatti et al. (1993) reported the weathering of various mica phases during the biological leaching of a black schist ore, resulting in the release of the cations such as K+ and Na+. On further examining the ore samples under SEM and XRD, various silicate phases such as Kfeldspar (ortochlase) and albite were identified in the ore. During the bioleaching process, these phases would probably undergo structural alteration, transforming into various mineral phases by chemical acid leaching. A detailed discussion of chemical weathering of silicate minerals including the feldspars can be found elsewhere (Faure, 1997). The following reactions may be presumed to occur for the alteration of K-feldspar and albite (Faure, 1997; Hiskey, 1994): 3KAlSi3 O8 þ 2Hþ ! KAl2 AlSi3 O10 ðOHÞ2
Fig. 8. Behaviour of K+ and Na+ during bioleaching of the ore by S. acidophilus in low (pH 1.5 – 1.6) and high (pH 1.8 – 2.0) pH regimes (2% wt/vol, d80 = 85 Am at 50 jC and 0.02% wt/vol yeast extract).
þ 6SiO2 þ 2Kþ
ðK-feldspar ! MuscoviteÞ ð7Þ
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2KAl2 AlSi3 O10 ðOHÞ2 þ 3H2 O þ 2Hþ ! 3Al2 Si2 O5 ðOHÞ4 þ 2Kþ
ðMuscovite ! KaoliniteÞ
ð8Þ
2NaAlSi3 O8 þ 9H2 O þ 2Hþ ! Al2 Si2 O5 ðOHÞ4 þ 4H4 SiO4 þ 2Naþ ðAlbite ! KaoliniteÞ
ð9Þ
The above reactions represent the transformation of K-feldspar and albite into kaolinite, although many other phases such as gibbsite, montmorillonite, etc., can form as a result of chemical weathering process (Faure, 1997). As can be seen from Eqs. (7) –(9), K+ and Na+ are released into solution, promoting the formation of jarosites in so far as the ferric iron generated by bacteria attains a certain level providing favourable conditions in conjunction with pH and temperature. These weathering reactions would also contribute to the acid consumption. 3.4. Effect of external addition of Fe2+ on the bioleaching of bulk concentrate and ore Preliminary studies on the bioleaching of the McArthur River bulk concentrate revealed that the contribution of bacteria (both mesophiles and moderate thermophiles) to the dissolution of zinc was marginal
Fig. 9. Effect of addition of ferrous iron on the extraction of zinc from the bulk concentrate using moderately thermophilic mixed culture MOT6 (at 2% wt/vol, 50 jC, pH 1.6, and 0.02% wt/vol yeast extract).
Fig. 10. Redox potential profiles produced during bioleaching of the bulk concentrate using moderately thermophilic mixed culture MOT6 in the presence of 0 – 4 g/L Fe2 + (at 2% wt/vol, 50 jC, pH 1.6, and 0.02% wt/vol yeast extract).
compared with the acid leaching in the absence of bacteria (i.e., the growth/subculture of bacteria on the concentrate was inefficient). Reevaluation of the mineralogy and the preliminary experimental data suggested that the limited contribution of the bacteria was probably due to the mineralogical properties with respect to the low iron content (2.89%). In order to confirm this, bioleaching of the bulk concentrate (2% wt/vol) was carried out using the mixed moderately thermophilic MOT6 culture (50 jC) in the presence of externally added ferrous iron (1 – 4 g/L). Fig. 9 illustrates the extraction of zinc by acid leaching (control) and bioleaching with the addition of 1 –4 g/L Fe2 +. Bioleaching of the concentrate (with no Fe2 + supplement) enhanced the extraction of zinc to a limited extent (by 12%). During the acid leaching, the addition of acid (18 M H2SO4) was necessary to control the pH at the predetermined value (1.6) and the acid consumption amounted to 2.4 g H2SO4 per gram of Zn solubilised. The acid consumption during the bioleaching with no Fe2 + addition was f 44% less than that during the acid leaching, suggesting that the main contribution of the bacteria was the elimination/conversion of sulphide sulphur. The extraction of zinc was slow and appeared to be limited by the availability of soluble iron [i.e., f 0.22 g/L after the addition of inoculum (10% v/v) grown on the ore] for the bacteria since a 3.4-fold increase in the dissolution rate of zinc was achieved by the supplementary addition of ferrous iron (1 g/L) as a source of ferric
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iron to be generated by the bacteria. Increasing the concentration of ferrous iron added from 1 to 2– 4 g/L resulted in a further improvement in the rate from 92.3 to f 114.5 mg/L/h. The utilisation of ferrous iron (i.e., production of ferric iron) by the strain was evident from the redox potential profiles, as shown in Fig. 10. These results were consistent with the reports (Kandemir, 1985; Sand et al., 2001) that the oxidation of sphalerite in the absence of ferric iron proceeds via the acid attack, resulting in the formation of polythionates (i.e., H2Sn), which are eventually oxidised chemically or biologically to produce elemental sulphur. Kandemir (1985) noted that, under bioleaching conditions, the oxidation of aqueous hydrogen sulphide (H2S) is a slow reaction in the absence of ferric iron and the overall oxidation of sphalerite is controlled by the removal of sulphur deposited on the mineral surface. In contrast to the bulk concentrate, the dissolution of zinc from the ore was similar in the presence of 1– 6 g/L Fe2 + externally added (Fig. 11), suggesting that the iron dissolved from the ore was sufficient to enhance the dissolution process. These findings for the ore and bulk concentrate emphasise the importance of ferric iron for the dissolution of zinc, and hence the iron content of the ore or concentrate to be treated via bioleaching processes. The bulk concentrate sample contained 2.89% iron, which apparently did not solubilize sufficient iron for the bacteria to effectively drive the dissolution of zinc in the absence
Fig. 11. Effect of addition of ferrous iron on the extraction of zinc from the ore using moderately thermophilic mixed culture MOT6 (2% wt/vol, d80 = 250 Am at 50 jC, pH 1.6, and 0.02% wt/vol yeast extract).
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of added Fe2 +. A rougher concentrate from the ore with higher iron content would be desirable for an efficient bioleaching operation. This would also allow the elimination of expensive stages of the flotation circuit [i.e., (fine) regrinding designed to remove pyrite], with significant reductions in the capital and operating costs and likely improvements in the overall metal recovery. Furthermore, K and Na content of the ore would be largely reduced in the rougher flotation concentrate, which would alleviate the formation of potentially detrimental jarosite precipitates during the bioleaching process.
4. Conclusions Bioleaching tests on the complex Pb/Zn ore/concentrate have shown that zinc can be readily/selectively extracted using mesophilic (at 30 jC), moderately thermophilic (at 50 jC), and extremely thermophilic (at 70 jC) strains of acidophilic bacteria while majority of lead (>98%) remains in the residue. Thermophilic bacteria, particularly moderate thermophiles, appear to be capable of significantly enhancing the kinetics of the extraction of zinc compared with mesophilic bacteria. Better bioleaching performance of the moderately thermophilic S. yellowstonensis than the extremely thermophilic A. brierleyi, despite the ability of the latter to operate at a higher temperature, could be attributed to its superior oxidising capacity coupled probably with the use of yeast extract to support its growth. pH is a significant operating parameter governing the oxidative activity of bacteria and solubility of ferric iron. The bioleaching activity of mesophiles and moderate thermophiles is adversely affected with increasing acidity at pH < 1.6, while the extremely thermophilic A. brierleyi can readily tolerate higher levels of acidity (pH 1.0 – 2.0). During bioleaching, monovalent cations (e.g., K+ and Na+) released from the alteration of silicate phases (e.g., K-feldspar) present in the ore may promote the precipitation of ferric iron mainly as K-jarosite under the suitable conditions of pH >1.6 and temperature of 50 jC. A sulphide ore/ concentrate should contain sufficiently high iron since the provision/availability of sufficient soluble iron in bioleaching environment is essential for the bacteria to generate ferric iron and efficiently drive the extraction of zinc from the complex sulphides.
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This study addresses the importance of: (i) thermophilic bacteria for improving the kinetics of metal extraction; (ii) ferric iron for the bioleaching of zinc; and (iii) mineralogical properties (i.e., iron and silicate content) of an ore/concentrate to be treated via bioleaching processes. Considering the inherent limitations of the current study (e.g., batch mode of operation, low pulp densities tested, and likely occurrence of O2 and CO2 limitation in shake flasks), further tests using stirred tank reactors in a continuous mode where optimum bioleaching conditions can be readily maintained are probably required to assess the bioleaching performance of moderately and extremely thermophilic bacteria at high pulp densities. These further investigations should focus on the bioleaching of a bulk concentrate with a sufficiently high iron content to be produced from the complex ore and on the use of CO2enriched air to improve the bacterial growth.
Acknowledgements The authors would like to thank the late Dr. C.V. Phillips for supervisory support of Dr. H. Deveci; the Analytical Laboratory staff and Mr. T. Ball (SEM and XRD; Camborne School of Mines) for their technical support with a range of chemical and mineralogical analyses; Dr. B. Johnson (School of Biological Sciences, University of Wales) for kindly providing the pure strains of moderate thermophiles; and the Turkish Ministry of Education for financial support.
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H. Deveci et al. / Hydrometallurgy 73 (2004) 293–303 Forward, F.A., Peters, E., 1985. Leaching principles. In: Weiss, N.L. (Ed.), SME Mineral Processing Handbook, vol. 2. SME, New York, pp. 13, 6 – 12. Fowler, T.A., Crundwell, F.K., 1999. Leaching of zinc sulphide by Thiobacillus ferrooxidans: bacterial oxidation of sulphur product layer increases the rate of zinc sulphide dissolution at high concentrations of ferrous iron. Applied and Environmental Microbiology 65 (12), 5285 – 5292. Gilbertson, B., 2000. Creating value through innovation: biotechnology in mining. IMM Transactions, C 109, 61 – 67. Gomez, C., Roman, E., Blazquez, M.L., Ballester, A., Gonzalez, F., 1995. SEM and AES studies of a lead sulphide bioleaching in presence of catalytic ions. Minerals Engineering 8 (12), 1503 – 1512. Gomez, C., Blazquez, M.L., Ballester, A., 1999. Bioleaching of a Spanish complex sulphide ore-bulk concentrate. Minerals Engineering 12 (1), 93 – 106. Hiroyoshi, N., Hirota, M., Hirajima, T., Tsunekawa, M., 1999. Inhibitory effect of iron-oxidizing bacteria on ferrous-promoted chalcopyrite leaching. Biotechnology and Bioengineering 64 (4), 478 – 483. Hiskey, J.B., 1994. In-situ leaching recovery of copper—what’s next? Biomine ’94, International Conference and Workshop Applications of Biotechnology to the Minerals Industry. Australian Mineral Foundation, Adelaide. Johnson, D.B., Body, D.A., Bridge, T.A.M., Bruhn, D.F., Roberto, F.F., 2001. Biodiversity of acidophilic moderate thermophiles isolated from two sites in Yellowstone National Park and their roles in the dissimilatory oxido-reduction of iron. In: Resenbach, A.L., Voytek, A. (Eds.), Biodiversity, Ecology and Evolution of Thermophiles in Yellowstone National Park. Plenum, New York, pp. 23 – 39. Jordan, M.A., 1993. The oxidation of base metal sulphides and mechanisms and preferential release of ferrous iron. PhD Thesis, Camborne School of Mines, University of Exeter, UK. Kandemir, H., 1985. Fate of sulphide sulphur in bacterial oxidation of sulphide minerals. In: Clum, J.A., Haas, L.A. (Eds.), Microbiological Effects on Metallurgical Processes. AIME, Warrendale, PA, pp. 51 – 64. Konishi, Y., Nishibura, H., Asai, S., 1998a. Bioleaching of Sphal-
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Bioleaching of complex zinc sulphides using mesophilic and thermophilic bacteria: comparative importance of pH and iron H. Deveci a,*, A. Akcil b, I. Alp a b
a Mineral Processing Division, Department of Mining Engineering, Karadeniz Technical University, Trabzon TR 61080, Turkey BIOMIN Group, Mineral Processing Division, Department of Mining Engineering, Suleyman Demirel University, Isparta TR 32260, Turkey
Received 16 September 2003; received in revised form 8 December 2003; accepted 9 December 2003
Abstract This study investigates the bioleaching of the complex Pb/Zn ore/concentrate using mesophilic (at 30 jC), moderate (at 50 jC), and extreme thermophilic (at 70 jC) strains of acidophilic bacteria. The effects of bacterial strain, pH, iron precipitation, and external addition of Fe2 + on the extraction of zinc were evaluated. The results have shown that the ore is readily amenable to the selective extraction of zinc and lead using the acidophilic strains of bacteria [i.e., majority of lead (>98%) reports to the residue]. Moderate thermophiles displayed superior kinetics of dissolution of zinc compared with the other two groups of bacteria. The pH was found to exert a profound effect on the leaching process controlling the bacterial activity and precipitation of ferric iron mainly as K-jarosite. The K+ released presumably from the alteration of the silicate phases such as K-feldspar present in the ore appeared to promote the formation K-jarosite in moderately thermophilic leaching systems. The external addition of iron was shown to be required for the bacteria to efficiently drive the extraction of zinc from the bulk concentrate. These findings place the emphasis on the prime importance of ferric iron for the dissolution of zinc and of mineralogical properties (i.e., iron and silicate content) of an ore/concentrate to be treated via bioleaching processes. D 2004 Elsevier B.V. All rights reserved. Keywords: Mesophilic bacteria; Thermophilic bacteria; Bioleaching; Complex sulphides; Zinc sulphide; K-jarosite
1. Introduction Complex sulphide ores such as the McArthur River ore in Australia prove difficult to treat by conventional extraction processes (Barbery et al., 1980). Since its discovery in the 1950s, extensive research has been undertaken to develop a commercially feasible process for the McArthur River ore (Buchanan, 1984). However, the extremely fine dissemination of lead and zinc * Corresponding author. Tel.: +90-462-3773681; fax: +90-4623257405. E-mail address: [email protected] (H. Deveci). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2003.12.001
sulphides and their intimate association with noneconomic minerals such as dolomite and pyrite had prevented the commercial exploitation of the deposit until the mid 1990s due to the failure to produce bulk or separate lead and zinc flotation concentrates of sufficient grade for extraction processes such as Imperial Smelting Furnace (ISF) and electrolytic processes at reasonable recoveries (Buchanan, 1984; Chadwick, 1996). In the 1990s, the significant developments in fine grinding technology with the improvements in flotation and Imperial Smelting Furnace (ISF) technology enabled the production of a bulk flotation concentrate of 42 –46% Zn, 10– 14% Pb, and 120 –
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150 g/t Ag at recoveries of 87% Zn and 62% Pb and subsequent treatment of such concentrate in the Imperial Smelting Furnace (ISF) process as blended with high-grade concentrates (Chadwick, 1996; Enderle et al., 1997; van Os, 1994). Bioleaching may be utilised as an alternative process for the extraction of base metals from complex ores/concentrates (Carta et al., 1980; Gilbertson, 2000; Miller et al., 1999; Steemson et al., 1997). Mesophilic bacteria such as Acidithiobacillus ferrooxidans (formerly known as Thiobacillus ferrooxidans), Acidithiobacillus thiooxidans (formerly known as Thiobacillus thiooxidans), and Leptosprillium ferrooxidans operating at f 40 jC are the most extensively used microorganisms for the bioleaching of sulphide minerals with commercial interest (Dew et al., 1997). However, the commercial use of thermophilic bacteria with their ability to operate at temperatures exceeding 45 jC has great potential for improving the kinetics of metal extraction from sulphide minerals (Konishi et al., 1998a; Norris et al., 2000; Rawlings et al., 2003). Several processes for commercial bioleaching using thermophilic bacteria for base metals (Cu, Ni, and Zn) are in development with reports of imminent commercial realisation (Gilbertson, 2000; Miller et al., 1999). Bioleaching of sulphide minerals is based on the exploitation of the ability of the acidophilic bacteria to oxidise ferrous iron (Eq. (1)) and/or reduce sulphur compounds (Eq. (2) for elemental sulphur). The actual role of bacteria in the bioleaching process has not been completely resolved (Bosecker, 1997; Fowler and Crundwell, 1999; Sand et al., 2001; Suzuki, 2001; Tributsch, 2001); albeit recent findings (Fowler and Crundwell, 1999; Sand et al., 2001) suggest that the oxidation of sulphide minerals occurs mainly via the chemical attack by ferric iron and/or acid (Eqs. (3) and (4)), which are generated by bacteria: bacteria
2FeSO4þ1=2O2þH2 SO4 ! Fe2 ðSO4 Þ3þH2 O bacteria
ð1Þ
S0 þ 3=2O2 þ H2 O ! H2 SO4
ð2Þ
ZnS þ 1=2O2 þ H2 SO4 ! ZnSO4 þ H2 O þ S0
ð3Þ
ZnS þ Fe2 ðSO4 Þ3 ! ZnSO4 þ 2FeSO4 þ S0
ð4Þ
PbS þ Fe2 ðSO4 Þ3 ! PbSO4ðsÞ þ 2FeSO4 þ S0
ð5Þ
The oxidation of galena (Eq. (5)) leads to the formation of lead sulphate, which is ‘‘insoluble’’ (0.045 g/L at 25 jC) (Forward and Peters, 1985) under bioleaching conditions and reports to the residues (Gomez et al., 1995). This raises the possibility of the selective extraction of metals such as zinc from complex ores/concentrates. There are several important factors such as temperature, pH, availability of nutrients, sulphide minerals, O2 and CO2, solid ratio, metal toxicity, etc., that affect the growth of bacteria and hence the dissolution process (Akcil and Ciftci, 2003; Bosecker, 1997; Deveci, 2002; Deveci et al., 2003). The presence of some ions such as K+, Na+, and SO42 in bioleaching environments could promote the formation of solid products such as ferric precipitates (e.g., K-jarosite) which is controlled by pH (Tuovinen and Bhatti, 1999). The limited extraction of metals has often been attributed to the formation of these secondary phases during bioleaching (Ahonen and Tuovinen, 1995; Gomez et al., 1999; Hiroyoshi et al., 1999). In this study, the potential amenability of the McArthur River complex Pb/Zn ore to the selective extraction of zinc using strains of mesophilic, moderately thermophilic, and extremely thermophilic bacteria was evaluated. The effect of pH, iron precipitation, and external addition of ferrous iron on the bioleaching of the ore/concentrate and extraction of zinc was examined using the selected strains of bacteria.
2. Materials and methods 2.1. Ore and concentrate samples The McArthur River ore and bulk concentrate samples, kindly provided by MIM Holdings and Britannia Zinc, respectively, were used in this study. The chemical and mineralogical analyses of the ore and concentrate are shown in Table 1. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses of the samples revealed that sphalerite, galena, and pyrite are the main sulphide phases. Sphalerite and galena, in particular in the ore, were found to be very fine-grained and intimately associated with each other to such a degree that even fine grinding ( < 10 Am) would not produce
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complete liberation. This currently justifies only the production of a bulk concentrate as a commercial product and further treatment in a pyrometallurgical process route for the recovery of contained values. Particle sizes of d80 = 250 Am and d80 = 85 Am for the ore (as ground in a tema mill) and d80 = 20 Am for the concentrate (as received) were used in the experiments.
Table 2 Acidophilic strains of bacteria used in bioleaching experiments Type
Bacterial strain
Code
Mesophiles (30 jC)
Acidithiobacillus ferrooxidans Mixed cultures
DSM 583
Moderate (50 jC)
2.2. Microorganisms and nutrient solution Mesophilic, moderately thermophilic, and extremely thermophilic strains of acidophilic bacteria were used in this study (Table 2). The mesophilic enrichment culture designated WJM (an isolate from the acid mine drainage waters of Wheal Jane Mine, Cornwall, UK) consisted of iron and sulphur oxidisers such as A. ferrooxidans, and heterotrophic bacteria (Jordan, 1993). The other mixed mesophile MES1 was composed of mainly A. ferrooxidans, and to a minor extent L. ferrooxidans and A. thiooxidans (Deveci, 2001). The moderately thermophilic mixed culture MOT6 had a composition of Sulfobacillus acidophilus, Sulfobacillus yellowstonensis, Sulfobacillus thermosulfidooxidans and, to a lesser extent, Acidithiobacillus caldus (Deveci, 2001). All the strains of mesophilic, moderately thermophilic, and extremely thermophilic bacteria were grown/maintained on the ore (1 – 2% wt/vol) prior to use as inoculum in the experiments. The growth of all the strains was conducted in an enriched salt solution containing MgSO47H2O (0.4 g/L), (NH4)2SO4 (0.2 g/ L), K2HPO43H2O (0.1 g/L), and KCl (0.1 g/L). Yeast extract (YE; 0.02% wt/vol) was also added to support the growth of moderate thermophiles.
Table 1 Chemical and mineralogical compositions of the ore and bulk concentrate samples Sample
Zn Fe Pb (%) (%) (%)
Ore
16.2 7.95
Cu S Ag Mineralogical (%) (%) (g/t) content
59 Sphalerite, galena, pyrite, Concentrate 43.3 2.89 11.17 0.89 26.8 145 chalcopyrite, quartz, dolomite, and various silicate phases such as ortochlase
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Extreme thermophiles (70 jC)
Sulfobacillus thermosulfidooxidans Sulfobacillus acidophilus Sulfobacillus yellowstonensis Mixed culture Acidanus brierleyi
MES1 and WJM TH1 THWX YTF1 MOT6 DSM 1651
2.3. Bioleaching experiments Bioleaching experiments were carried out in 250mL Erlenmeyer flasks. Enriched salt solution (90 mL) adjusted to the required pH was transferred into each flask to which 1– 2 g of the ore/concentrate sample (corresponding to 1 – 2% wt/vol pulp density) was added. The flasks were then autoclaved at 1 atm and 121 jC for 20 min. Following autoclaving, each flask was inoculated under aseptic conditions with a 10-mL aliquot of the selected culture producing a final volume of slurry of approximately 100 mL. To facilitate mixing of the contents and transfer of O2 and CO2, the flasks were shaken on the orbital shakers controlled at growth temperatures of 30 jC for mesophiles, 50 jC for moderate thermophiles, and 70 jC for extreme thermophiles. Each flask was sampled daily by removing a 1-mL aliquot of the leach solution, which was then used for analysis of metals (Zn, Fe, and Pb) by atomic absorption spectrometry (AAS) and for monitoring pH and redox potential. The pH was adjusted using 18 M H2SO4 when it deviated towards neutrality from the initial preset values.
3. Results and discussion
5.60 0.27 15.2
3.1. Comparison of the bioleaching performance of mesophilic and thermophilic bacteria Fig. 1 illustrates the extraction of zinc from the ore by mesophilic (WJM strain), moderately thermophilic
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Fig. 1. Extraction of zinc from the ore using mesophilic (WJM strain), moderately thermophilic (S. yellowstonensis), and extremely thermophilic (A. brierleyi) bacteria (1% wt/vol, d80 = 250 Am at 30, 50, and 70 jC, respectively).
(S. yellowstonensis), and extremely thermophilic (Acidianus brierleyi) bacteria at 30, 50, and 70 jC, respectively. Over 90% extraction of the zinc was achieved by all the strains (groups) of bacteria used. However, the dissolution of lead was minimal, with >98% of lead having reported to the residues most likely in the form of either ‘‘insoluble’’ lead sulphate or undissolved galena irrespective of the strain of bacteria. This indicates the selective nature of bioleaching of zinc from the complex ore. The extraction of zinc by moderate thermophile S. yellowstonensis was rapid, resulting in a zinc extraction of f 93% over a bioleaching period of only 113 h compared with >200 h required for similar extractions by the mesophilic and extreme thermophilic bacteria. The findings also indicated the superior bioleaching capacity of thermophilic bacteria compared with mesophilic bacteria presumably due to the positive effect of elevated temperature on the dissolution of sulphide minerals. This was consistent with the reports for the improved dissolution kinetics of sulphides by extremely thermophilic bacteria (Dew et al., 1999; Konishi et al., 1998a; Witne and Phillips, 2001). It was also evident in Fig. 1 that the extraction of zinc by S. yellowstonensis (50 jC) and A. brierleyi (70 jC) was similar over the initial period of f 37 h; thereafter, A. brierleyi could not maintain its initial bioleaching performance compared with that of S. yellowstonensis. This could be attributed to the superior oxidising capacity of S. yellowstonensis as the
most rapidly growing bacteria reported to date (Johnson et al., 2001) and also, and probably more importantly, to decreasing availability of O2 and CO2 at a relatively high temperature of 70 jC for growth (Boon and Heijnen, 1998). In this regard, S. yellowstonensis could have readily utilised the yeast extract (0.02% wt/vol) added as a source of cellular carbon while the growth of A. brierleyi would have been limited by the availability (transfer) of atmospheric CO2 in the medium. Bacterial growth rates were not determined in this study. The current study was carried out at 1% wt/vol since further increase in the pulp density would probably lead to the increase in the CO2 demand of bacteria (i.e., the aggravation of CO2 limitation). Konishi et al. (1998b) observed a significant improvement in the growth of A. brierleyi and hence the biooxidation rate of pyrite at 65 jC when the growth media were supplemented by 0.005 – 0.25% wt/vol yeast extract. Increasing the availability of CO2 in the bioleaching medium may also improve the growth and hence bioleaching performance of A. brierleyi (Witne and Phillips, 2001; Boon and Heijnen, 1998). 3.2. Effect of pH on the bioleaching activity of mesophilic and thermophilic bacteria Fig. 2 shows the effect of pH in the range of 1.0– 2.0 on the dissolution rate of zinc from the ore by mesophilic A. ferrooxidans, moderately thermophilic
Fig. 2. Effect of pH on the initial (maximum) dissolution rate of zinc from the ore by mesophilic A. ferrooxidans, moderately thermophilic S. yellowstonensis and S. thermosulfidooxidans, and extremely thermophilic A. brierleyi (1% wt/vol, d80 = 250 Am).
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Fig. 3. Effect of pH on the extraction of zinc from the ore by S. yellowstonensis (1% wt/vol, d80 = 250 Am at 50 jC and 0.02% wt/vol yeast extract).
S. yellowstonensis and S. thermosulfidooxidans, and extremely thermophilic A. brierleyi. The bioleaching activity of mesophile A. ferrooxidans, as indicated by the dissolution rate of the zinc (Fig. 2), was adversely affected with decreasing pH to V 1.4. But, above this pH, there seemed no significant difference in the dissolution rate and extent (91 –93%) of zinc, with an optimum bioleaching performance having been recorded at pH 1.8. In contrast to A. ferrooxidans, the extraction rate of zinc by the extremely thermophilic A. brierleyi (at 70 jC) was observed to increase with increasing acidity, as shown in Fig. 2. The extraction of zinc, although being comparable at pH V 1.4 (95 – 92%) over the bioleaching period, was recorded to decrease by f10% with a step increase in pH between 1.6 and 2.0. A trend of increase in the dissolution of iron with decreasing pH was also discerned (not shown), consistent with the increase in the bioleaching activity of the strain in response to the decrease in pH. It should be noted that the extraction of iron by A. brierleyi was relatively low (i.e., 43% at pH 1.0), in comparison with those (55 – 64%) obtained for A. ferrooxidans at pH z 1.6 for a similar extent of zinc extraction. These findings were consistent with those of Jordan (1993). The minimal precipitation of ferric iron was observed for A. brierleyi even at pH 1.8 – 2.0 probably due to the slow accumulation of ferric iron in solution coupled with the rapid reduction of ferric iron by the sulphides such as sphalerite at 70 jC. However, high temperatures would promote the precipitation of ferric iron
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even at low solution pH values (Arslan and Arslan, 2003; Dutrizac, 1983a,b; Konishi et al., 1998a; Welham et al., 2000) and the ability of extreme thermophiles to operate at low pH values could become important for the bioleaching process so that the formation of potentially deleterious precipitates could be minimised. The effect of acidity (pH 1.2 –2.0) on the dissolution of zinc from the ore by moderately thermophilic S. yellowstonensis and S. thermosulfidooxidans (Figs. 3 and 4) was similar in character to that by mesophilic A. ferrooxidans. The increase in the acidity to pH 1.2 –1.4 led to a decrease in the oxidising activity of both strains, indicating the inhibitory effect of increased acidity on the strains. The dissolution rate of zinc by both strains was observed to peak at pH 1.6; thereafter, a negligible decrease was recorded (Fig. 2). Bioleaching activity of both strains was limited with varying extent at pH values V 1.4. The increase in the acidity (to pH 1.2) was noted to severely inhibit the oxidising activity of S. yellowstonensis as indicated by a sharp decrease in the solubilisation of zinc (Fig. 3). Similarly, a severe limitation of the bioleaching activity of S. thermosulfidooxidans was also observed even at pH 1.4 (Fig. 4). This indicates the varying response of both strains to acidity although the extent of the inhibitory effect on the activity of both strains was similar at pH 1.2. The dissolution pattern of iron for both strains at pH 1.2– 2.0 was also similar in character as illustrated in
Fig. 4. Effect of pH on the extraction of zinc from the ore by S. thermosulfidooxidans (1% wt/vol, d80 = 250 Am at 50 jC and 0.02% wt/vol yeast extract).
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Figs. 5 and 6. The dissolution of iron by both strains at pH 1.6– 2.0 was rapid, consistent with the dissolution of zinc (Figs. 3 and 4). However, a decrease in the concentration of iron following a bioleaching period of 83 h was apparent for both strains at pH z 1.8, indicating the precipitation of ferric iron probably as it accumulated in the system. The cessation of the zinc release at pH 2.0 after 113 h (Figs. 3 and 4) could be attributed to the extensive precipitation of ferric iron occurring at this pH (i.e., the formation of a protective layer on the mineral surface by ferric precipitates, thus retarding the continued dissolution of zinc). These findings suggest that pH profoundly affects the overall bioleaching process, adversely influencing the activity of moderately thermophilic strains used at pH V 1.4 and dictating the solubility of ferric iron particularly at pH z 1.8. Furthermore, the current tests were carried out in a batch mode where the accumulation and precipitation of ferric iron would be expected to occur following a significant recovery of zinc. However, the precipitation of ferric iron may present severe problems during a continuous operation at high pH values. The effect of pH was studied only at a pulp density of 1% wt/vol. An increase in the pulp density would lead to the increase in the concentrations of ferric ion and sulphate within the system, which would further promote the formation of precipitates probably even at pH values as low as pH 1.4 – 1.5 (Deveci, 2001). This would necessitate operation probably at pH values below 1.4 at the expense of reduced bacterial activity
Fig. 5. Effect of pH on the dissolution of iron from the ore by S. yellowstonensis (1% wt/vol, d80 = 250 Am at 50 jC and 0.02% wt/vol yeast extract).
Fig. 6. Effect of pH on the dissolution of iron from the ore by S. thermosulfidooxidans (1% wt/vol, d80 = 250 Am at 50 jC and 0.02% wt/vol yeast extract).
in order to minimise the formation of the detrimental ferric precipitates. Adaptation of the strains to operate at pH V 1.4 may ameliorate the negative impact of acidity (Deveci, 2001; Porro et al., 1989). 3.3. Nature of ferric precipitates formed during moderately thermophilic bioleaching processes These tests were designed to further elucidate the precipitation of iron encountered above during the moderately thermophilic bioleaching of the ore at pH 1.8 –2.0 and 50 jC. Fig. 7 illustrates the dissolution of zinc and iron by S. acidophilus at two different pH
Fig. 7. Extraction of zinc and iron from the ore by S. acidophilus in low (pH 1.5 – 1.6) and high (pH 1.8 – 2.0) pH regimes (2% wt/vol, d80 = 85 Am at 50 jC and 0.02% wt/vol yeast extract).
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regimes. A similar dissolution trend for zinc was observed at low pH (1.5 – 1.6) and higher pH (1.8 – 2.0) conditions with a minimal difference in the dissolution rate of zinc (52 – 54 mg/L/h). The extraction of zinc was essentially complete over a bioleaching period of 62 h, which was significantly shorter than the time (f182 h) required by S. yellowstonensis and S. thermosulfidooxidans to achieve the same level of extraction (at pH z 1.6). This was probably mainly due to the positive effect of decreasing particle size of the ore (d80 = 85 Am, cf. 250 Am). The release of iron at higher pH regime was observed to peak at 62 h and then the precipitation of iron became evident with a decrease in the concentration of iron in solution in the following periods as noted earlier at pH z 1.8 (Figs. 5 and 6). This coincided with a decrease in the pH from 1.86 to 1.69 probably as a result of excessive production of acid due to the precipitation. There was a negligible difference in the extraction of zinc at both pH conditions, indicating minimal effect of precipitates on the release of zinc. In fact, f 97% of zinc had already been recovered at the onset of the apparent decrease in the concentration of iron at high pH regime. The decrease in the concentration of iron in solution (after 62 h) at higher pH regime coincided with the decrease in the concentration of K+ as shown in Fig. 8. This suggests that the ferric iron had precipitated mainly in the form of K-jarosite (Eq. (6)), which was the most common precipitate phase detected in
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the XRD analysis of the bioleach residues during the bacterial leaching of the complex ore particularly at 50 jC (Deveci, 2001). A tendency for a decrease in the concentration of Na+ during these periods (after 62 h) was also noted. The behaviour of K+ and Na+ during the precipitation of iron was in accordance with the relative stability of K-jarosite over Na-jarosite, as suggested by Dutrizac (1983a), who reported that participation of alkali ions in the jarosite structure would be in the order of K+>NH4+>Na+: Xþ þ 3Fe3þ þ 2SO2 4 þ 6H2 O ! XFe3 ðSO4 Þ2 ðOHÞ6 þ 6Hþ
ð6Þ
where X could be monovalent cations such as K+, NH4+, Na+, Ag+, etc., or 1/2Pb2 +. Dutrizac (1983b) reported the minimum concentrations of Fe3 +, K+, and Na+ in solution to be 10 3, 0.02, and 0.05 M, respectively, for the precipitation of jarosites presumably at high temperatures (f95 jC). Despite the relatively low operating temperature (50 jC), the precipitation of K-jarosite was observed to occur from solutions containing f4.2 mM K+ (and 12 mM Na+) in the current study. The concentration of K+ in enriched salt medium was determined to be 0.08 g/L, which indicated that the remaining K+ was released from the ore. Bhatti et al. (1993) reported the weathering of various mica phases during the biological leaching of a black schist ore, resulting in the release of the cations such as K+ and Na+. On further examining the ore samples under SEM and XRD, various silicate phases such as Kfeldspar (ortochlase) and albite were identified in the ore. During the bioleaching process, these phases would probably undergo structural alteration, transforming into various mineral phases by chemical acid leaching. A detailed discussion of chemical weathering of silicate minerals including the feldspars can be found elsewhere (Faure, 1997). The following reactions may be presumed to occur for the alteration of K-feldspar and albite (Faure, 1997; Hiskey, 1994): 3KAlSi3 O8 þ 2Hþ ! KAl2 AlSi3 O10 ðOHÞ2
Fig. 8. Behaviour of K+ and Na+ during bioleaching of the ore by S. acidophilus in low (pH 1.5 – 1.6) and high (pH 1.8 – 2.0) pH regimes (2% wt/vol, d80 = 85 Am at 50 jC and 0.02% wt/vol yeast extract).
þ 6SiO2 þ 2Kþ
ðK-feldspar ! MuscoviteÞ ð7Þ
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2KAl2 AlSi3 O10 ðOHÞ2 þ 3H2 O þ 2Hþ ! 3Al2 Si2 O5 ðOHÞ4 þ 2Kþ
ðMuscovite ! KaoliniteÞ
ð8Þ
2NaAlSi3 O8 þ 9H2 O þ 2Hþ ! Al2 Si2 O5 ðOHÞ4 þ 4H4 SiO4 þ 2Naþ ðAlbite ! KaoliniteÞ
ð9Þ
The above reactions represent the transformation of K-feldspar and albite into kaolinite, although many other phases such as gibbsite, montmorillonite, etc., can form as a result of chemical weathering process (Faure, 1997). As can be seen from Eqs. (7) –(9), K+ and Na+ are released into solution, promoting the formation of jarosites in so far as the ferric iron generated by bacteria attains a certain level providing favourable conditions in conjunction with pH and temperature. These weathering reactions would also contribute to the acid consumption. 3.4. Effect of external addition of Fe2+ on the bioleaching of bulk concentrate and ore Preliminary studies on the bioleaching of the McArthur River bulk concentrate revealed that the contribution of bacteria (both mesophiles and moderate thermophiles) to the dissolution of zinc was marginal
Fig. 9. Effect of addition of ferrous iron on the extraction of zinc from the bulk concentrate using moderately thermophilic mixed culture MOT6 (at 2% wt/vol, 50 jC, pH 1.6, and 0.02% wt/vol yeast extract).
Fig. 10. Redox potential profiles produced during bioleaching of the bulk concentrate using moderately thermophilic mixed culture MOT6 in the presence of 0 – 4 g/L Fe2 + (at 2% wt/vol, 50 jC, pH 1.6, and 0.02% wt/vol yeast extract).
compared with the acid leaching in the absence of bacteria (i.e., the growth/subculture of bacteria on the concentrate was inefficient). Reevaluation of the mineralogy and the preliminary experimental data suggested that the limited contribution of the bacteria was probably due to the mineralogical properties with respect to the low iron content (2.89%). In order to confirm this, bioleaching of the bulk concentrate (2% wt/vol) was carried out using the mixed moderately thermophilic MOT6 culture (50 jC) in the presence of externally added ferrous iron (1 – 4 g/L). Fig. 9 illustrates the extraction of zinc by acid leaching (control) and bioleaching with the addition of 1 –4 g/L Fe2 +. Bioleaching of the concentrate (with no Fe2 + supplement) enhanced the extraction of zinc to a limited extent (by 12%). During the acid leaching, the addition of acid (18 M H2SO4) was necessary to control the pH at the predetermined value (1.6) and the acid consumption amounted to 2.4 g H2SO4 per gram of Zn solubilised. The acid consumption during the bioleaching with no Fe2 + addition was f 44% less than that during the acid leaching, suggesting that the main contribution of the bacteria was the elimination/conversion of sulphide sulphur. The extraction of zinc was slow and appeared to be limited by the availability of soluble iron [i.e., f 0.22 g/L after the addition of inoculum (10% v/v) grown on the ore] for the bacteria since a 3.4-fold increase in the dissolution rate of zinc was achieved by the supplementary addition of ferrous iron (1 g/L) as a source of ferric
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iron to be generated by the bacteria. Increasing the concentration of ferrous iron added from 1 to 2– 4 g/L resulted in a further improvement in the rate from 92.3 to f 114.5 mg/L/h. The utilisation of ferrous iron (i.e., production of ferric iron) by the strain was evident from the redox potential profiles, as shown in Fig. 10. These results were consistent with the reports (Kandemir, 1985; Sand et al., 2001) that the oxidation of sphalerite in the absence of ferric iron proceeds via the acid attack, resulting in the formation of polythionates (i.e., H2Sn), which are eventually oxidised chemically or biologically to produce elemental sulphur. Kandemir (1985) noted that, under bioleaching conditions, the oxidation of aqueous hydrogen sulphide (H2S) is a slow reaction in the absence of ferric iron and the overall oxidation of sphalerite is controlled by the removal of sulphur deposited on the mineral surface. In contrast to the bulk concentrate, the dissolution of zinc from the ore was similar in the presence of 1– 6 g/L Fe2 + externally added (Fig. 11), suggesting that the iron dissolved from the ore was sufficient to enhance the dissolution process. These findings for the ore and bulk concentrate emphasise the importance of ferric iron for the dissolution of zinc, and hence the iron content of the ore or concentrate to be treated via bioleaching processes. The bulk concentrate sample contained 2.89% iron, which apparently did not solubilize sufficient iron for the bacteria to effectively drive the dissolution of zinc in the absence
Fig. 11. Effect of addition of ferrous iron on the extraction of zinc from the ore using moderately thermophilic mixed culture MOT6 (2% wt/vol, d80 = 250 Am at 50 jC, pH 1.6, and 0.02% wt/vol yeast extract).
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of added Fe2 +. A rougher concentrate from the ore with higher iron content would be desirable for an efficient bioleaching operation. This would also allow the elimination of expensive stages of the flotation circuit [i.e., (fine) regrinding designed to remove pyrite], with significant reductions in the capital and operating costs and likely improvements in the overall metal recovery. Furthermore, K and Na content of the ore would be largely reduced in the rougher flotation concentrate, which would alleviate the formation of potentially detrimental jarosite precipitates during the bioleaching process.
4. Conclusions Bioleaching tests on the complex Pb/Zn ore/concentrate have shown that zinc can be readily/selectively extracted using mesophilic (at 30 jC), moderately thermophilic (at 50 jC), and extremely thermophilic (at 70 jC) strains of acidophilic bacteria while majority of lead (>98%) remains in the residue. Thermophilic bacteria, particularly moderate thermophiles, appear to be capable of significantly enhancing the kinetics of the extraction of zinc compared with mesophilic bacteria. Better bioleaching performance of the moderately thermophilic S. yellowstonensis than the extremely thermophilic A. brierleyi, despite the ability of the latter to operate at a higher temperature, could be attributed to its superior oxidising capacity coupled probably with the use of yeast extract to support its growth. pH is a significant operating parameter governing the oxidative activity of bacteria and solubility of ferric iron. The bioleaching activity of mesophiles and moderate thermophiles is adversely affected with increasing acidity at pH < 1.6, while the extremely thermophilic A. brierleyi can readily tolerate higher levels of acidity (pH 1.0 – 2.0). During bioleaching, monovalent cations (e.g., K+ and Na+) released from the alteration of silicate phases (e.g., K-feldspar) present in the ore may promote the precipitation of ferric iron mainly as K-jarosite under the suitable conditions of pH >1.6 and temperature of 50 jC. A sulphide ore/ concentrate should contain sufficiently high iron since the provision/availability of sufficient soluble iron in bioleaching environment is essential for the bacteria to generate ferric iron and efficiently drive the extraction of zinc from the complex sulphides.
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This study addresses the importance of: (i) thermophilic bacteria for improving the kinetics of metal extraction; (ii) ferric iron for the bioleaching of zinc; and (iii) mineralogical properties (i.e., iron and silicate content) of an ore/concentrate to be treated via bioleaching processes. Considering the inherent limitations of the current study (e.g., batch mode of operation, low pulp densities tested, and likely occurrence of O2 and CO2 limitation in shake flasks), further tests using stirred tank reactors in a continuous mode where optimum bioleaching conditions can be readily maintained are probably required to assess the bioleaching performance of moderately and extremely thermophilic bacteria at high pulp densities. These further investigations should focus on the bioleaching of a bulk concentrate with a sufficiently high iron content to be produced from the complex ore and on the use of CO2enriched air to improve the bacterial growth.
Acknowledgements The authors would like to thank the late Dr. C.V. Phillips for supervisory support of Dr. H. Deveci; the Analytical Laboratory staff and Mr. T. Ball (SEM and XRD; Camborne School of Mines) for their technical support with a range of chemical and mineralogical analyses; Dr. B. Johnson (School of Biological Sciences, University of Wales) for kindly providing the pure strains of moderate thermophiles; and the Turkish Ministry of Education for financial support.
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