15th International Mineral Processing Symposium

15th International Mineral Processing Symposium, Istanbul-Turkey, October 19-21, 2016

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BIOLEACHING OF ZINC SULPHIDE CONCENTRATES USING MESOPHILIC BACTERIA H. Deveci Hydromet B&PM Group, Mineral&Coal Proces. Div., Mining Eng. Dept., Karadeniz Technical University, 61080, Trabzon, Turkey ([email protected])

Abstract: Bioleaching can be suitably exploited for the treatment of difficult-to-treat ores and concentrates. In this study, bioleaching of two different concentrates (a bulk Zn/Pb concentrate and zinc concentrate with high iron content) was studied to demonstrate the importance of mineralogical characteristic with particular reference to iron content. Bioleaching tests have shown that iron content of zinc concentrates is of practical importance since the external addition of iron is required to enhance the extraction of zinc from the concentrate with a low iron content. Bench scale bioleaching tests in stirred tank reactors have revealed that increasing pulp density (up to 10% w/w) tends to adversely affect bioleaching process due apparently to the excessive increase in surface area and concomitantly, the inability of bacteria to maintain strong oxidising conditions required. Dissolution rates as high as 862 mg/L/h Zn at 10% w/w were recorded. The ability of mesophilic culture to operate at high levels of Zn in solution (up to 100 g/L) was demonstrated. Keywords:

bioleaching,

acidophilic

bacteria,

complex

sulphide

ore,

zinc,

hydrometallurgy. INTRODUCTION Complex sulphide ores are the prime source of zinc and lead in addition to copper and silver to a lesser extent (Barberly et al. 1980). The production of separate concentrates of contained metals, i.e. zinc and lead may impose the utilisation of expensive techniques i.e. ultrafine grinding to achieve satisfactory response to flotation (Logan et al., 1993). In such cases, a bulk flotation concentrate may be produced with the extraction of metal values (i.e. zinc and lead) only through smelting processes. However, this route endures stringent environmental regulations and high operating and capital costs as well as low quality of end product (Gordon 1985).

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Bioleaching is potentially one of the most promising options as it enables the treatment of low grade and complex ores with reduced environmental impact and lower fixed capital costs. The amenability of zinc sulphide minerals/concentrates to bioleaching processes has been demonstrated in laboratory studies and pilot scale trials (Torma et al., 1970; Sandstrom et al., 1997; Deveci et al., 2003a, 2004a,b; RiekkolaVanhanen, 2013; Ahmadi and Musavi, 2015). In bacterial leaching systems, the oxidation of sphalerite may be expressed as follows: ZnS + 1/2O2 + H2SO4  ZnSO4 + H2O + S0

(1)

The findings (Kandemir, 1985; Sand et al., 2001; Deveci et al., 2004b; Deveci and Ball, 2010) suggest that it occurs chemically by acid attack. The only involvement of bacteria in the dissolution of sphalerite would therefore be the oxidation of elemental sulphur formed possibly via intermediate polysulphides. Garcia et al. (1995) observed that At. ferrooxidans and At. thiooxidans enhanced the solubilisation of zinc from a research grade sphalerite. The gradual decrease observed in pH was presumably the indication of the oxidation of the elemental sulphur formed: bacteria S0 + 3/2O2 + H2O  H2SO4

(2)

Bioleaching of sphalerite is a relatively slow process. It has been suggested (Fowler and Crundwell, 1998; Deveci et al., 2004b, Deveci and Ball, 2010) that the rate of the oxidation of sphalerite is enhanced in the presence of ferric iron. The oxidation of sphalerite by ferric iron can be represented by the reaction below: ZnS + Fe2(SO4)3  ZnSO4 + 2FeSO4 + S0

(3)

In this study, bench scale bioleaching of two zinc concentrates (a bulk Zn/Pb concentrate and zinc concentrate with high iron content) with different mineralogical characteristics was undertaken to show the importance of mineralogy with particular reference to iron content. Effect of pulp density on bioleaching process was examined

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.MATERIAL AND METHODS Concentrate Samples A bulk Zn/Pb concentrate (MAR-BC) and a zinc rougher concentrate (ERC) were used in bioleaching tests. Mineralogical examination of the samples using Scanning Electron Microscope (SEM) (Jeol 840 SCE) interfaced with an Electron Dispersive Spectrophotometer (Oxford Instruments) and an X-Ray Diffractometer (XRD) (Siemens D5000) showed that sphalerite (ZnS), galena (PbS) and pyrite (FeS2) as major sulphides and quartz are present in both concentrates. Relatively coarse sulphide grains and discrete sulphide mineralisation were apparent in ERC compared with MAR-BC. The particle size analysis of concentrates using the Malvern Mastersizer indicated extremely fine particle size distribution of the concentrates as received i.e. 70% passing 10 µm for MAR-BC and 70% passing 20 µm for ERC. The chemical analysis of the ores and concentrates are presented in Table 1. Table 1. Chemical composition of the ores and concentrates Sample

Zn,%

Fe,%

Pb,%

Cu,%

S,%

Ag,(g/t)

MAR-BC

43.3

2.89

11.17

0.89

26.8

145

ERC

45.4

8.95

2.94

0.46

29.0

66

Bioleaching Tests A mixed mesophile, designated as MES1 (mainly At. ferrooxidans, to a minor extent L. ferrooxidans and At. thiooxidans) was used in bioleaching experiments (Deveci et al., 2004b). The culture was continuously maintained on a Zn/Pb complex ore at 30°C. A modified salt solution (MS: MgSO4.7H2O (0.5 g/l), (NH4)2SO4 (1.5 g/l), K2HPO4.3H2O (0.5 g/l) and KCl (0.1 g/l)) was used as the growth medium. Bioleaching tests were performed in jacketed glass reactors (an operating volume: 673 ml) fitted with four blade, 45° pitched blade turbine (PBT) impellers (D=4.75 cm) to agitate the reactor contents at 1000 rpm. Temperature was maintained at 30-35°C. Air

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was supplied to reactors at a flow rate of 0.5 L/min. The bacteria grown on the ore (510% w/w) in the reactors were used as inoculum (109 cells/ml). The progress of bioleaching was monitored by periodic sampling of reactors (1.3 ml samples) and analysis of samples for metals (Zn, Fe and Pb) using an Atomic Absorption Spectrometer (AAS). Evaporation losses were compensated for by the addition of double distilled water prior to sampling. pH, redox potential and dissolved oxygen concentration (DO2) were also followed at sampling intervals. pH was controlled at 1.4-1.7 by the addition of 18 M H2SO4 or 5-10% w/v CaCO3 slurry. XRD and chemical analysis of bioleach residues were also undertaken. RESULTS AND DISCUSSION Bioleaching of the Zn/Pb Bulk Concentrate It had been demonstrated (Deveci et al., 2004b) that the contribution of bacteria to the oxidation of the bulk concentrate (MAR-BC) was limited in connection with its low iron content (2.89%) and hence the external addition of iron was required to enhance the extraction of zinc. Therefore, in the current tests the addition of ferrous iron at the start of the bioleaching experiments was made to increase the initial iron level. Figure 1 illustrates the extraction of zinc from MAR-BC at 5-10% w/w.

Fig. 1. Extraction of zinc from the Zn/Pb bulk concentrate (MAR-BC) using MES1 strain at 5-10% w/w, 35°C and 0.5 l air/min 676


The inoculum for B1 (5% w/w) was prepared by growing the culture at 5% w/w (MARBC). At the start of B1, the concentration of iron was increased to 4.1 g/l by the addition of ferrous iron. During B1, the pH was maintained between 1.57-1.33 and hence the precipitation of iron was minimal. The iron in solution varied between 4.0-4.3 g/l over the bioleaching period. The dissolved oxygen concentration was 4.3 mg/l during B1. The dissolution of zinc in B1 was rapid during the initial 21 h where 72% of the zinc was extracted at a rate of 740 mg/l/h. During this initial period, the addition of acid was required to control the pH. However, in the following periods where the zinc release slowed down the process became acid producing presumably due to the oxidation of elemental sulphur formed on the surface of mineral particles. Between 21-71 h, the addition of CaCO3 slurry (10% w/v) was made in an attempt to keep the pH above 1.4. The final extraction of zinc was determined to be 97% after 71 h. At the start of B2, the concentration of zinc in solution was 55.1 g/l which increased to 100 g/l corresponding to 93% zinc recovery over the bioleaching period of 92 h. The dissolution rate of zinc was determined to be 862 mg/l/h. The concentration of iron in solution was between 6.6-6.7 g/l over the bioleaching period. During B2, pH was maintained between 1.62-1.29 and the minimum concentration of dissolved oxygen was recorded to be 2.7 mg/l. Following the termination of B2, the solid residue was separated and analysed for the metal content. The residue was found to contain 4.2% Zn and 27.4% Pb indicating that 7.1% of the original zinc remained in the residue which was consistent with the extraction of zinc recorded (93%). High lead content suggested the concentration of the lead in the solid phase presumably in the form of PbSO4. During B1 and B2, the number of bacteria in solution was also monitored as shown in Figure 2.

Following the addition of the concentrate in B1 and B2, there was a

significant decrease in the number of bacteria in solution within 1 h; the extent of decrease increased with increasing pulp density (i.e. 17% decrease in the cell numbers at 5% w/w (B1) compared with 71% at 10% w/w (B2)). The decrease in the number of bacteria in solution can be attributed to the attachment of the bacteria to mineral particles and the attrition of the bacterial cells by the solid 677


particles. Although the initial number of bacteria in B2 (10% w/w) was higher than that in B1 (5% w/w), in the following periods (1-46 h) the number of free cells in solution was consistently lower in B2 with a reduced capacity for the generation of ferric iron in solution compared with that in B1. Accordingly, the bacterial population in B2 could not maintain high redox potentials (Figure 2) and the extraction of zinc was slow compared with that in B1 (Figure 1).

Figure 2: Concentration of bacteria in solution and redox potential profiles during the bioleaching of MAR-BC at 5-10% w/w It can be inferred from these findings that, although the bioleaching rate would increase with increasing the surface area available i.e. via size reduction at low pulp densities (Deveci et al., 2003a), the excessive increase in the surface area at high pulp densities can produce an adverse effect on the dissolution of zinc due to the inability of the bacteria (i.e. due to the decrease in the number of free cells in solution) to produce ferric iron in sufficient quantity and to maintain the strong oxidising conditions required. Therefore, the residence time required for the extraction of zinc tends to increase with increasing pulp density and/or decreasing particle size at high pulp densities.

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Bioleaching of the Rougher Concentrate Figures 3 and 4 show the extraction of zinc, redox potential and pH profiles during the bioleaching of the rougher concentrate (ERC) using MES1 strain at different pulp densities. The experimental results are summarised in Table 2.

Figure 3 Extraction of zinc from the rougher concentrate (ERC) using MES1 strain at 3-5% w/w (30°C), 10% w/w (35°C) and 0.5 l air/min

Figure 4 Redox potential and pH profiles during the bioleaching of ERC using MES1 strain at 3-5% w/w (30°C), 10% w/w (35°C) and 0.5 l air/min 679


Table 2. A summary of the results for the bioleaching of the ERC concentrate using MES1 strain at 3-10% w/w, 30-35°C and 0.5 l air/min PD (% w/w) 3%

Temp.

Time(h)

Zn (%)

[Fe]i(g/l)

DO2 (mg/l)

72

Max. Rate Zn (mg/l/h) 250

30°C

82

1.2

5.6

5%

30°C

118

375

85

2.6

4.2

10%

35°C

106

762

72

2.6

3.8

Inoculum size ~4109 cells/ml; PD: Pulp density; DO2: Dissolved O2 concentration; i: initial

The final extraction of zinc at 3-10% w/w was lower than those obtained for the bulk concentrate varying between 72-85%. Despite the rapid dissolution of zinc during the initial 29-45 h, in the following periods the zinc release was severely limited at 5-10% w/w. The chemical analysis and the XRD examination of the residues at 5% w/w confirmed the presence of unreacted sphalerite (11% Zn i.e 15% of the original zinc remained in the residues consistent with the extraction of zinc determined (Table 2)). The limited dissolution of zinc following the initial stages of the process suggests the possible passivation of the unreacted sphalerite particles. There was negligible precipitation of iron at 5% w/w over the bioleaching period as also indicated by the XRD analysis of the residues. The pH at 5% w/w was readily controlled between 1.291.63 by the addition of acid during the initial 28 h (Figure 4). A consistent decrease in the pH was observed in the following periods where the addition of CaCO 3 slurry (10% w/v) at each sampling interval was necessary to increase pH above 1.4. A similar trend for pH was also observed at 10% w/w. This decrease in pH can be ascribed to the bacterial conversion of the elemental sulphur. Thus, the passivation of unreacted sphalerite particles could have been largely due to the elemental sulphur accumulating on the mineral surface. In addition, the low pH (