Biological treatment of solid waste materials from copper and steel industry Vestola, E.A. 1, 2, Kuusenaho, M.K.3, Närhi, H.M.3, Tuovinen, O.H.3,4, Puhakka, J.A.3, Plumb, J.J.2, Kaksonen, A.H.2, and Merta, E.S.A.1 1
VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland, e-mail
[email protected], tel. +358 40 843 1697 2
CSIRO Land and Water, Private Bag No. 5, Wembley, Western Australia 6913, Australia
3
Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101 Tampere, Finland
4
Department of Microbiology, Ohio State University, Columbus, OH 43210, U.S.A.
Abstract The aim of this work was to evaluate the feasibility of bioleaching for the solubilisation of metals from solid waste streams and by-products of copper and steel industries. The leaching experiments were carried out in shake flasks in mineral salts media inoculated with iron and sulphur oxidising acidophiles at 25°C. The experiments tested the effects of the inoculum, pH, supplemental ferrous iron and sulphur, sodium chloride, and the type of waste material. Solubilisation of metals was mainly achieved through acid attack due to the formation of sulphuric acid by sulphur oxidising bacteria. Addition of ferrous iron and chloride ions did not enhance metal solubilisation.
1. Introduction Solid waste streams from the mining and metallurgy, energy production and recycling industries may contain relatively high levels of metals that are harmful if released to the environment (Brandl et al. 2006). These waste streams can be considered as potentially valuable sources of metals (Bosecker 2001, Solisio et al. 2002). Traditionally, metals have been solubilised from solid waste materials in chemical leaching processes with strong acids. However, these methods are favourable only when recoverable metals are present at relatively high levels (Solisio et al. 2002). Bioleaching may be an alternative treatment method for those solid waste materials that have relatively low levels of valuable metals or if the material is otherwise difficult to handle or treat (Brandl et al. 2006). For sulphide minerals, bioleaching is an indirect process whereby ferric ion and oxygen in acid solutions are the primary oxidizing agents for the leaching of metal T. Böllinghaus et al. (eds.), Materials Challenges and Testing for Supply of Energy and Resources, DOI 10.1007/978-3-642-23348-7_26, © Springer-Verlag Berlin Heidelberg 2012
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sulphides. Ferrous iron, elemental sulphur and other reduced sulphur species are formed in these reactions and are oxidized to Fe3+ and sulphate. Thus, the role of the micro-organisms is to produce sulphuric acid and regenerate Fe3+ during the leaching of metal (denoted as M and M2+) sulphides (Sand et al. 2000, Suzuki 2001). MS + 2 Fe3+ ൺ M2+ + 2 Fe2+ + S0 (1)
2 Fe2+ + 0.5 O2 + 2 H+ ൺ 2 Fe3+ + H2O (2)
S0 +1.5O2 + H2O ൺ SO42- + 2H+ (3)
MS + 2O2 ൺ M2+ + SO42- (4) For industrial solid waste materials, conventional bioleaching with ferric iron in the central role may not be feasible because metals are present mainly as oxides, carbonates and silicates rather than sulphides. Metal oxides in these materials may be leached via sulphuric acid generated by Acidithiobacillus thiooxidans and other S-oxidizing acidophiles, according to the reaction (3). The results presented in this paper are based on a previous article by Vestola et al. (2010) (doi:10.1016/j.hydromet.2010.02.017). In the study the solubilisation of metals from solid waste materials was evaluated with acidophilic iron and sulphur oxidising cultures. This was a preliminary study to screen the suitability of the waste materials for the bioleaching and to test experimental variables for the solubilisation of metals in shake flask experiments.
2. Materials and methods
2.1 Material characterization Two different metal-containing solid waste samples were tested in this study: final slag from copper smelting and converter sludge from steel production. Samples were sub sampled using a riffle splitter and a rotating divider to obtain representative samples for analyses and bioleaching studies. Their chemical composition was determined using inductively coupled plasma atomic emission spectroscopy
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and is summarised in Table 1. Mineral composition of the materials was determined using Xray diffraction. The final smelter slag was composed mainly of fayalite (FeSiO4), calcite (CaCO3) and chromite (FeCr2O4), and the converter sludge contained chromite, calcite, magnetite (Fe3O4) and iron oxides (Fe2O3). The final slag and converter sludge samples were freeze-dried. Table 1. Chemical composition (% dry weight) of the sample materials. Sample
% Composition Fe
Cu
Ni
Zn
Co
Pb
S
C
40.7
0.35
0.08
1.7
0.05
0.08
0.19
0.09
Converter sludge 60.2
0.02
0.02
1.7
0.01
0.09
0.06
0.78
Final slag
2.2 Leaching experiments The leaching of the samples was evaluated in 250 mL Erlenmeyer flasks containing 200 mL mineral salts medium (MSM), trace elements (Dopson 2004) and 1% or 10% (w/v) solids. The media were inoculated (10% v/v) with a mixed culture, which was enriched from a sulphide ore mine site and contained Acidithiobacillus spp. and Leptospirillum spp. The pH was adjusted to pH 1.5, 1.0 or 0.5 with sulphuric acid. Cultures were supplemented with sterile S0 (1% w/v) and/or ferrous sulphate (4.5 g/L Fe2+). In some experiments, 5 g/L NaCl was added to the media to test for chloride enhancement of metal leaching. The possible inhibitory effect of chloride on bacterial sulphur and iron oxidation was tested separately. Various un-inoculated controls were included in the experiments and chemical leaching experiments were also carried out in ddH2O at pH 0.5. Microscopic observations were made to ensure the sterility of un-inoculated controls. All cultures were incubated at 25 °C and at 150 rpm. Samples were taken at intervals for analysis of pH and redox potential and for measurement of Fe2+ and dissolved metals.
2.3 Inhibition experiments For testing possible inhibitory effects of leachates on the bacterial activity, solid waste samples (10% w/v) were shaken in MSM with trace elements (pH 1.0) for three days. The suspensions were filtered through 0.45 ȝm membrane filter and supplemented with Fe2+ and S0. The filtrates were inoculated (10% v/v) with the mixed test culture and ferrous iron and sulphur oxidation was monitored over the subsequent time course of incubation. Iron and sulphur oxidation rates were determined with and without the leachate samples.
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2.4 Analytical procedures Samples were taken at intervals and filtered (0.45 ȝm) for chemical analysis. Ferrous iron concentrations were determined using the colorimetric orthophenanthroline method according to the 3500-Fe APHA-standard (Anonymous, 1992). Dissolved metal concentrations (Cu, Ni, Fe, and Zn) were measured by atomic absorption spectroscopy. The pH was measured using a WTW 315i pH meter and redox potential using a Hamilton Pt-ORP platinum electrode.
2.5 Molecular characterization of the mixed culture The mixed culture was characterised by polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) followed by partial sequencing of the 16S rRNA gene. Total genomic DNA was extracted from samples using a phenol chloroform extraction protocol (Plumb et al. 2001). 16S rRNA genes from purified extracts were amplified and sequenced as described previously (Kinnunen and Puhakka 2004). The amplified fragments were resolved by DGGE and sequenced (Kinnunen and Puhakka 2004) and identified by BLAST analysis using the GenBank database (NCBI website).
3. Results
3.1 Sulphur oxidation In the converter sludge and final slag suspensions the pH decreased in the S0 supplemented cultures (Fig. 1), indicating sulphur oxidation. The pH in the uninoculated control flasks increased slightly, indicating dissolution of alkaline constituents in the samples. The pH did not affect biological sulphur oxidation at pH 11.5. Sulphur oxidation was negligible at pH 0.5. The addition of NaCl (5 g/L) did not affect sulphur oxidation.
Biological treatment of solid waste materials from copper and steel industry
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Ƈ- Inoculated + Fe + S; pH 0.5 Ÿ-Control + Fe + S; pH 1.5 -'- Control + Fe + S; pH 0.5
Fig. 1. Changes in the pH over time in shake flask cultures amended with 1% final slag sample (A and B) and 1% converter sludge (C and D). The initial pH value was 1.5 unless otherwise indicated. The vertical bars indicate standard deviations.
3.2 Iron oxidation Iron oxidation was monitored by measuring changes in Fe2+ concentration (Figure 2) and redox potential over time. The cultures received 4.5 g/L Fe2+, but the final slag and converter sludge samples also contained relatively high levels of iron
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which was solubilised as Fe2+. Iron oxidation was relatively slow, 4.5 g/L Fe2+ in 21 days, under these conditions (Fig. 2). The pH did not affect the iron oxidation at pH 1-1.5. At pH 0.5 and with 10% w/v solids, iron oxidation was partially suppressed. The addition of NaCl (5 g/L) did not enhance metal solubilisation or inhibit iron oxidation. B
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Fig. 2. Changes in Fe concentrations over time in shake flask cultures amended with 1% final slag sample (A and B) and 1% converter sludge (C and D). The initial pH value was 1.5 unless otherwise indicated. The vertical bars indicate standard deviations.
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3.3 Leaching of metals The results of metal solubilisation from the final slag and converter sludge samples in the test cultures are summarized in Table 2. The pH values decreased after 10-20 days of contact (Fig. 1), when the acid production by biological sulphur oxidation became faster than the acid consumption by the materials. The highest metal recoveries were achieved in cultures supplemented with S0 at pH 1.0 and 0.5. Solubilisation of metals was mainly achieved through acid attack due to sulphuric acid formed upon bacterial oxidation of elemental sulphur. As a result of the low pH, precipitation of dissolved metals was not evident. Comparable metal yields were observed in chemical leaching tests, but these had higher acid consumption as compared to the biological leaching which generated sulphuric acid from the added sulphur. Additional Fe2+ or NaCl did not increase total metal solubilisation. The metals in the final slag and converter sludge samples are present mainly as oxides and their solubilisation does not involve a redox reaction. Table 2. Yields of metal solubilisation from the converter sludge and the final slag samples. The pulp density was 1% unless otherwise stated. Experimental condition
Length of incubation (days)
% Yield of con- % Yield of final slag leaching verter sludge leaching Zn
Fe
Zn
Fe
Cu
Ni
Inoculated; pH 1.5
42
11
11
26
29
88
53
Inoculated + Fe + S; pH 1.5
42
14
NAa
63
NA
100
100
Inoculated + Fe; pH 1.5
42
15
NA
35
NA
95
75
Inoculated + S; pH 1.5
42
36
32
64
100
100
100
Sterile control; pH 1.5
42
17
15
30
27
94
52
Inoculated + Fe + S, 10% solids; pH 1.5
79
10
NA
12
NA
45
19
Inoculated + Fe + S, 5 g NaCl L-1; pH 1.5
79
35
NA
52
NA
100
100
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79
48
NA
44
NA
84
100
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79
100
NA
63
NA
100
100
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79
15
NA
33
NA
100
87
Control + Fe + S; pH 0.5
79
97
NA
70
NA
100
100
Chemical leaching in ddH2O; pH 22 0.5a a NA = not applicable due to Fe2+ addition.
89
92
55
69
81
87
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3.4 Inhibition The results from the separate inhibition test indicated that Fe2+ oxidation rates were faster in the presence of final slag and converter sludge than in the absence of materials. Iron (4.5 g/L Fe2+) was oxidised in 7 days for the final slag and in 14 days for the converter sludge. Neither material inhibited biological sulphur oxidation under the conditions the inhibition test. However, metal solubilisation at 10% pulp density decreased the relative yields for both types of solids. In contrast to the experiments at 1% pulp density, it is plausible that the solids contained inhibitory substances which were increasingly dissolved at 10% pulp density. The pH values increased up to 3.0, which, although not inhibitory to iron and sulphur oxidizing acidophiles, this may have decreased the solubility of dissolved metals.
4. Discussion The results demonstrate that metals can be bioleached from solid waste materials and the solubilisation is due to acid attack as previously reported (Solisio et al. 2002). Acid formation in this study was achieved by biological oxidation of sulphur. Metal solubilisation yields for the final slag and converter sludge samples were averaging 30-80% depending on the test conditions. Highest metal recoveries were achieved in flasks supplemented with S0 at pH 1.0 and 0.5. Copper and nickel were solubilised almost completely in the flasks containing the final slag sample whereas the yields were lower with the converter sludge sample. It was concluded that the final slag sample did not contain inhibitory substances and the metal content was higher as compared to the converter sludge. High concentration of ferric iron or ferric oxide/hydroxide in the converter sludge may promote the formation of jarosites. Such Fe(III) hydroxyl-sulphate precipitates can form on reactive surfaces and form a diffusion barrier, slowing down fluxes of reactants and products (Nemati et al. 1998). For example, Mishra et al. (2007) reported that ferric iron precipitated with other metals in the leach medium, forming metal complexes and effectively preventing the solubilisation of metals from lithium ion batteries. Elemental sulphur is known to form a passivating layer on mineral surfaces, thereby resulting in slow metal leaching rates especially in the case of chalcopyrite (Schippers et al. 1999, Klauber et al. 2001, Rodriquez et al. 2003, Watling 2006, Klauber 2008). Different catalysts such as chloride ions may promote the formation of porous crystalline sulphur layer instead of passivating crypto-crystalline or amorphous sulphur and therefore enhancing the metals leaching (Lu et al. 2000, Vraþar et al. 2000). In this study, however, chloride ion did not enhance metal solubilisation. Metals leaching rates were slow compared to the previous studies with various metalcontaining solid waste materials (Solisio et al. 2002, Brombacher et al. 1998, Wang et al.2007, Ilyas et al. 2007, Brandl et al. 2001, Bakhtiari et al. 2008, Wong
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et al. 2004, Aung et al. 2005). The final slag and converter sludge samples were ground to
