中度嗜热菌浸出黄铜矿过程表面产物解析-有色金属在线

简介概要

中度嗜热菌浸出黄铜矿过程表面产物解析

来源期刊：中国有色金属学报(英文版)2015年第8期

论文作者：赵红波王军覃文庆郑细华陶浪甘晓文邱冠周

文章页码：2725 - 2733

关键词：黄铜矿；表面产物；生物浸出；钝化；中度嗜热微生物

Key words：chalcopyrite; surface species; bioleaching; passivation; moderately thermophilic bacteria

摘要：采用X射线衍射(XRD)与X射线光电子能谱(XPS)研究黄铜矿在中度嗜热菌浸出过程中的表面产物变化。结果表明，在A. caldus, S. thermosulfidooxidans 与 L. ferriphilum 浸出过程中，一硫化物(CuS)、二硫化物(S₂^2-)、元素硫(S⁰)、多硫化物(S_n^2-)与硫酸盐(SO₄^2-)是黄铜矿表面的主要产物。在A. caldus浸出黄铜矿过程速率较慢，这主要是由于黄铜矿的不完全溶解产生多硫化物，限制了进一步的溶解。在S. thermosulfidooxidans 与 L. ferriphilum 浸出黄铜矿过程中，多硫化物与黄钾铁矾是钝化膜的主要成分。元素硫不是导致黄铜矿生物冶金过程钝化的主要物质。

Abstract: X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were carried out to investigate the surface species and interfacial reactions during bioleaching of chalcopyrite by different strains of moderately thermophilic bacteria (45 °C). Results show that monosulfide (CuS), disulfide (S₂^2-), polysulfide (S_n^2-), elemental sulfur (S⁰) and sulfate (SO₄^2-) are the main intermediate species on the surface of chalcopyrite during bioleaching by A. caldus, S. thermosulfidooxidans and L. ferriphilum. The low kinetics of dissolution of chalcopyrite in A. caldus can be mainly attributed to the incomplete dissolution of chalcopyrite and the passivation layer of polysulfide. Polysulfide and jarosite should be mainly responsible for the passivation of chalcopyrite in bioleaching by L. ferriphilum or S. thermosulfidooxidans. However, elemental sulfur should not be the main composition of passivation layer of chalcopyrite during bioleaching.

Trans. Nonferrous Met. Soc. China 25(2015) 2725-2733

Surface species of chalcopyrite during bioleaching by moderately thermophilic bacteria

Hong-bo ZHAO^1,2, Jun WANG^1,2, Wen-qing QIN^1,2, Xi-hua ZHENG^1,2, Lang TAO^1,2, Xiao-wen GAN^1,2, Guan-zhou QIU^1,2

1. School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China;

2. Key Laboratory of Bio-hydrometallurgy of Ministry of Education, Central South University, Changsha 410083, China

Received 29 December 2014; accepted 3 April 2015

Key words: chalcopyrite; surface species; bioleaching; passivation; moderately thermophilic bacteria

1 Introduction

As a promising technology in processing low grade ores, bio-hydrometallurgy has been successfully applied for the recovery of metals such as copper, nickel, zinc, and refractory gold [1,2]. Chalcopyrite (CuFeS₂) is the most abundant and widespread copper-bearing mineral, accounting for approximately 70% of the copper resources [3,4]. However, it is still difficult to be effectively extracted by bioleaching mainly due to the low kinetics [5].

It has been generally accepted that the main cause of low leaching efficiency of chalcopyrite in bioleaching is the formation of passivation layer, which can inhibit the further dissolution of chalcopyrite [6-9]. Many researchers have focused on interpreting the dissolution process and the compositions of passivation layer, and different conclusions were proposed. Among the proposed conclusions, elemental sulfur, metal-deficient polysulfide and iron-hydroxyl compounds mainly consisting of jarosite were considered as the main compositions of passivation layer, but the specific compositions of passivation layer and the dissolution pathway are still being debated [8,10]. This can be mainly attributed to the differences of bioleaching systems, especially the bacteria used in bioleaching.

Three main types of microorganisms were classified according to their optimum growth temperature, including mesophiles, moderately thermophilic bacteria and extreme thermophilic bacteria [6,7]. Chalcopyrite can be easily passivated under the temperature less than 35 °C, and the extreme thermophilic bacteria are sensitive to high shear force caused by high pulp density as the absence of cytoderm, which restrict the further industrial application [2,11]. Moderately thermophilic bacteria are suitable to grow in heap and tank bioleaching as the relatively high temperature in heap and bioreactor, and they have a more extensive application prospect compared with the other two types of microorganisms [2,12].

Additionally, the intermediate species are usually with trace amount, and the passivation layer is usually too thin to be directly detected by the normal X-ray diffraction method [13]. X-ray photoelectron spectroscopy (XPS) has been widely used to characterize the surface layers, and it can provide reliable data on the chemical states of the surface species [13-15].

Therefore, in the present work, the compositions of formed product layers on chalcopyrite surface in bioleaching by moderately thermophilic bacteria were investigated mainly by X-ray photoelectron spectroscopy (XPS) to interpret the electrochemical dissolution process and the mechanisms of passivation.

2 Experimental

2.1 Materials

Chalcopyrite sample was obtained from geological museum of Guangxi Province of China. X-ray diffraction analysis (XRD) (Fig. 1) showed that it was of high purity. The chemical analysis showed that the chalcopyrite sample contained 34.46% Cu, 31.53% Fe, and 33.12% S (mass fraction), respectively. Ore samples were ground and sieved to less than 0.074 mm before being used for leaching experiments. All chemicals used were of analytical purity in this work.

Fig. 1 XRD pattern of untreated chalcopyrite

2.2 Microorganisms and culture media

Moderately thermophilic bacteria, including Acidithiobacillus caldus (A. caldus ), Sulfobacillus thermosulfidooxidans (S. thermosulfidooxidans) and Leptospirillum ferriphilum (L. ferriphilum), were obtained from the Key Laboratory of Bio- hydrometallurgy of Ministry of Education, Central South University, China. Bacteria were cultured in 250 mL shake flasks in an orbital incubator with a stirring speed of 170 r/min at a temperature of 45 °C. The basic culture medium was composed of the following compositions: (NH₄)₂SO₄ (3.0 g/L), MgSO₄·7H₂O (0.5 g/L), K₂HPO₄ (0.5 g/L), KCl (0.1 g/L), Ca(NO₃)₂ (0.01 g/L). L. ferriphilum and A. caldus were sub-cultured into basal salts medium supplemented with 44.7 g/L ferrous sulphate (FeSO₄·7H₂O) and 10 g/L sulfur as energy sources, respectively. S. thermosulfidooxidans were sub-cultured into basal salts medium supplemented with 44.7 g/L ferrous sulphate (FeSO₄·7H₂O) and 10 g/L sulfur as energy sources. The resulting culture was used as inoculums for the bioleaching experiments.

2.3 Bioleaching experiments

10 mL of cell solution was inoculated into a 250 mL shake flask containing 90 mL of sterilized culture medium and 2 g of minerals. The initial cell concentration of bacteria was higher than 1.0×10⁷ cells/mL. The shake flasks were placed into an orbital shaker at 170 r/min and 45 °C, pH value was adjusted to 1.7 by sulfuric acid regularly, and water lost by evaporation was supplemented periodically by adding deionized water. Copper concentrations were determined by inductively coupled plasma-atomic emission spectrometer (ICP-AES) (America Baird Co. PS-6). The pH values were measured with a pH meter (PHSJ-4A) and the redox potentials of leaching solution were measured by a Pt electrode with reference to a Ag/AgCl electrode (3.0 mol/L KCl) (BPP-922). The mineralogical compositions of solid samples were examined by X-ray diffraction (XRD) (DX-2700).

2.4 X-ray photoelectron spectroscopy (XPS)

The samples in different stages of bioleaching (7, 15, 22 d) were filtered and rinsed with deionized water three times, then transferred to vacuum drying oven (DZF-6050) to dry before X-ray photoelectron experiment (XPS) measurements.

X-ray photoelectron experiment (XPS) measure- ments were carried out on the model of ESCALAB 250Xi. Spectra were recorded at constant pass energy of 20 eV and energy step size of 0.1 eV, with Al K_α X-ray as the source. Binding energy calibration was based on C 1s at 284.6 eV. Thermo Avantage 5.52 software was used for fitting the XPS peaks. The Shirley method was chosen for obtaining the background of spectra, and the S 2p spectra were fitted by Gaussian-Lorentzian line (SGL) function.

3 Results and discussion

3.1 Bioleaching behaviors

Figure 2 shows that redox potential of bioleaching by A. caldus maintained at a low and steady value throughout the bioleaching process, during which the copper extraction continued to increase slowly and steadily. Redox potential of bioleaching by S. thermo- sulfidooxidans started to increase sharply from the 5th day, and reached a high value of about 600 mV on the 10th day. Correspondingly, the copper extraction kept increasing significantly in the first 10 d, and then stopped. For bioleaching of chalcopyrite by L. ferriphilum, redox potential started to increase sharply from the 10th day, and arrived at the value of more than 600 mV on the 15th day. Accordingly, the copper extraction continued increasing significantly in the first 15 d, and then stagnated. This distinctly revealed that kinetics of bioleaching by A. caldus was low, and chalcopyrite was easy to be passivated in bioleaching by S. thermo- sulfidooxidans and L. ferriphilum.

Fig. 2 Bioleaching of chalcopyrite by moderately thermophilic bacteria

Redox potential was mainly determined by the concentration ratio of ferric to ferrous ions (Fe³⁺/Fe²⁺), so the sharp increase of redox potential can be mainly attributed to the oxidation of ferrous to ferric ions by S. thermosulfidooxidans and L. ferriphilum. In addition, redox potential was an important, even a determining factor in bioleaching of chalcopyrite. Many researchers have proposed that dissolution of chalcopyrite can be accelerated by controlling redox potential at a relatively low value, and extremely high redox potentials can easily cause the passivation of chalcopyrite, thus inhibiting its further dissolution [16-19]. To further investigate the causes of low kinetics and passivation of chalcopyrite in bioleaching, XRD and XPS were used to analyze the intermediate species on chalcopyrite surface in different stages of bioleaching.

Fig. 3 Mineralogical compositions of samples leached by different strains of bacteria

Figure 3 presents the mineralogical compositions of samples after bioleaching by A. caldus, S. thermo- sulfido oxidans and L. ferriphilum for 7, 15, and 22 d, respectively. It can be found that elemental sulfur formed at the later stage of bioleaching by A. caldus, while significant amount of jarosite formed at the later stage of bioleaching by S. thermosulfidooxidans and by L. ferriphilum.

Fig. 4 Survey (full range) XPS spectra of samples leached by different strains of bacteria

3.2 X-ray photoelectron spectroscopy (XPS) analysis

XPS can provide reliable data to determine the trace amounts of intermediate species during the bioleaching process. Hence, the samples leached for 7, 15 and 22 d were analyzed by XPS, and the obtained survey XPS spectra are shown in Fig. 4. The Cu 2p, Cu LMM and S 2p peaks were further analyzed. For convenience, the stage of the first 7 d was described as the initial stage, 7-15 d was defined as the middle stage, and 15-22 d was represented by the later stage.

Fig. 5 XPS spectra of Cu peaks of chalcopyrite leached by A. caldus for different time

Figure 5(a) shows the Cu 2p peaks of samples after bioleaching by A. caldus for 7, 15, and 22 d, respectively. The two peaks of Cu 2p_3/2 and Cu 2p_1/2 were centered at 932.1 and 952.0 eV, respectively. Both of the Cu 2p_3/2 and Cu 2p_1/2 peaks were centered at lower binding energy than that of chalcopyrite (932.4, 952.1 eV) [20]. It has been well demonstrated that Cu 2p_3/2 peak with binding energy (933.0-933.8 eV) and the presence of a shake-up peak (939-944 eV) are the two major XPS characteristics of cupric species, while a lower binding energy of Cu 2p_3/2 peak (931.8-933.1 eV) and the absence of the shake-up peak are the characteristics of cuprous species, including Cu₂O (932.4 eV), CuS (932.0-932.4 eV) and Cu₂S (932.9 eV) [20-24]. By comparison, it can be speculated that CuS may be the possible cuprous species. Figure 5(b) shows that the Cu LMM peaks of the three samples were centered at 568.7 eV. The corresponding values of Cu LMM peaks of Cu₂O, Cu₂S and CuS were reported at 569.7, 569.5 and 568.5 eV, respectively [23,24]. Therefore, CuS can be the most plausible cuprous species during bioleaching of chalcopyrite by A. caldus.

Figure 6(a) presents the Cu 2p peaks of samples after bioleaching by S. thermosulfidooxidans for 7, 15, and 22 d, respectively. The Cu 2p_3/2 peaks of the three samples were centered at 931.8 eV, which was approximately equal to the value of Cu 2p_3/2 peak of CuS (932.0-932.4 eV) [20]. In addition, Fig. 6(b) shows that the Cu LMM peaks were centered at 568.5-568.7 eV, which was in consistent with that of Cu LMM peak of CuS (568.5 eV) [24]. Hence, CuS can be the most possible cuprous species during bioleaching of chalcopyrite by S. thermosulfidooxidans.

Fig. 6 XPS spectra of Cu peaks of chalcopyrite leached by S. thermosulfidooxidans for different time

Figure 7(a) compares the Cu 2p spectra of samples after bioleaching by L. ferriphilum for different time. The Cu 2p_3/2 peaks of the three samples were centered at 932.0 eV, which agreed well with that of Cu 2p_3/2 peak of CuS (932.0-932.4 eV) [20]. Figure 7(b) shows that the Cu LMM peaks of the three samples were centered at 568.5-568.7 eV, which was almost equal to that of Cu LMM peak of CuS (568.5 eV) [24]. Therefore, CuS can be the most receivable cuprous species during bioleaching of chalcopyrite by L. ferriphilum.

Fig. 7 XPS spectra of Cu peaks of chalcopyrite leached by L. ferriphilum for different time

According to the above results, it can be concluded that CuS is the main cuprous species on the surface of chalcopyrite during bioleaching by A. caldus, S. thermo- sulfidooxidans and L. ferriphilum. Similarly, in some other researches, the formation of CuS on the surface of chalcopyrite in leaching process and electrochemical measurements has been detected [9,13,25-28].

Sulfur species can be mainly classified as monosulfide (S^2-, binding energy: 161.1-161.8 eV; FWHM: 0.5-0.7 eV), disulfide (S₂^2-, binding energy: 162.3-162.4 eV; FWHM: 0.5-0.7 eV), polysulfide (S_n^2-, binding energy: 163.0-163.9 eV; FWHM: 1.1-1.3 eV), elemental sulfur (S⁰, binding energy: 163.05-164.7 eV; FWHM: 0.7-1.7 ), sulfite (SO₃^2-, binding energy: 166.4-166.5 eV), thiosulfate (S₂O₃, binding energy: 161.7-163.2 eV; 167.4-167.8 eV) and sulfate (SO₄^2-, binding energy: 168.0-169.0; FWHM: 0.9), respectively [8,29-32].

Figure 8 shows the XPS spectra of S 2p peaks of samples leached by A. caldus for 7, 15 and 22 d, respectively. Monosulfide, disulfide, polysulfide and elemental sulfur were the main sulfur species on the surface of chalcopyrite during bioleaching by A. caldus.

Figure 9 shows the percentages of sulfur species on the surface of chalcopyrite leached by A. caldus for different time. Monosulfide and polysulfide were the most abundant sulfur species throughout the bioleaching process. The percentage of polysulfide was kept at more than 20% throughout the bioleaching process. However, the percentage of elemental sulfur was always less than 6% during the bioleaching process. Therefore, it can be inferred that the formed polysulfide should be the main composition of passivation layer in bioleaching of chalcopyrite by A. caldus.

Fig. 8 XPS spectra of S 2p peaks of chalcopyrite leached by A. caldus for different time

Fig. 9 Percentages sulfur species on surface of chalcopyrite leached by A. caldus for different time

Figure 10 provides the XPS spectra of S 2p peaks of samples leached by S. thermosulfidooxidans for 7, 15 and 22 d, respectively. By fitting the peaks, monosulfide, disulfide and polysulfide can be considered as the main sulfur species on the surface of chalcopyrite leached by S. thermosulfidooxidans for 7 d. Except for monosulfide, disulfide and polysulfide, sulfate can be detected on the 15th and 22nd days. The formed sulfate mainly consisted of jarosite according to the analytical results of XRD analysis. The percentages of sulfur species on the surface of chalcopyrite leached by S. thermosulfidooxidans for different time are shown in Fig.11, which revealed that the percentage of polysulfide remained at more than 20% throughout the bioleaching process, and a high percentage of about 20% of sulfate was formed from the 15th day of bioleaching. However, the percentage of elemental sulfur was less than 5% throughout the bioleaching process. Therefore, polysulfide and sulfate should mainly be responsible for the passivation of chalcopyrite during bioleaching by S. thermo- sulfidooxidans.

Fig. 10 XPS spectra of S 2p peaks of chalcopyrite leached by S. thermosulfidooxidans for different time

Fig. 11 Percentages of sulfur species on surface of chalcopyrite leached by S. thermosulfidooxidans for different time

Figure 12 shows the XPS spectra of S 2p peaks of samples leached by L. ferriphilum for 7, 15 and 22 d, respectively. Sulfur species of monosulfide, disulfide and polysulfide can be detected on the surface of chalcopyrite leached for 7 d. Monosulfide, disulfide, polysulfide, elemental sulfur and sulfate mainly consisting of jarosite were the main sulfur species on the surface of chalcopyrite leached for 15 and 22 d. Figure 13 provides the compositions of sulfur species on the surface of chalcopyrite in different stages of bioleaching. It can be found that polysulfide kept at a relatively stable percentage of more than 18%, and a significant amount of sulfate species was formed from the middle stage of bioleaching. Moreover, the percentage of elemental sulfur was less than 5% during the whole bioleaching process. Therefore, the formed polysulfide and jarosite can be the main compositions of passivation layer in bioleaching of chalcopyrite by L. ferriphilum.

Fig. 12 XPS spectra of S 2p peaks of chalcopyrite leached by L. ferriphilum for different time

4 Conclusions

1) Monosulfide (CuS), disulfide (S₂^2-), polysulfide (S_n^2-), elemental sulfur (S⁰) and sulfate (SO₄^2-) are the main intermediate species on the surface of chalcopyrite during bioleaching by A. caldus, S. thermosulfidooxidans and L. ferriphilum.

2) For bioleaching of chalcopyrite by A. caldus, the selective dissolution of iron elements causes the formation of metal-deficient polysulfide, which should be the main composition of the passivation layer. Polysulfide and jarosite can be mainly responsible for the passivation of chalcopyrite in bioleaching by L. ferriphilum or S. thermosulfidooxidans. Moreover, elemental sulfur should not be the main cause of passivation of chalcopyrite during bioleaching.

Fig. 13 Percentages of sulfur species on surface of chalcopyrite leached by L. ferriphilum for different time

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