Bio-oxidation of arsenopyrite（姜涛,李骞,杨永斌,李光辉,邱冠周《Transactions of Nonferrous Metals Society of China》2008.6）.pdf

Bio-oxidation of arsenopyrite JIANG Tao , LI Qian , YANG Yong-bin, LI Guang-hui, QIU Guan-zhou School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Received 20 September 2008; accepted 5 November 2008 Abstract Oxidation of arsenopyrite with Acidithiobacillus ferrooxidans was studied. The electrochemical results show that arsenopyrite is firstly oxidized to As2S2 at the potential of 0.20.3 V vs SHE and As2S2 covers the electrode and retards the process continuously. While at higher potential over 0.3 V vs SHE, As2S2 is oxidized to H3AsO3, and H3AsO3 is then oxidized to H3AsO4 at 0.8 V vs SHE. The leaching results show that the addition of FeS2 can promote the oxidation of As3 to As5 and increase the activity of the bacteria. The best bio-oxidation technical parameters are the initial pH of 1.82.0, particle sizes less than 0.074 mm, temperature in the range of 2530 and rotating speed of the orbital incubator of 100160 r/min. The results provide theoretical and technological supports of bio-oxidation arsenopyrite for pretreating refractory arsenic gold ores. Key words arsenopyrite; electrochemistry; bio-oxidation; Acidithiobacillus ferrooxidans 1 Introduction At present, free milling gold ores are gradually diminishing, the low grade and refractory ores will be the main resources for extracting[1]. Arsenopyrite is the typical sulfide mineral carrying gold, and gold is generally trapped as submicroscopic particles in arsenopyrite[2]. If the Fe, As and S of the arsenopyrite are effectively oxidized by pretreating with bio-oxidation, gold will be naked and its recoveries will be increased[3]. The arsenic may be as As or As in the solution during bio-leaching processes. It is one of the important assignments for bacteria cultivation and technical study that the arsenic tolerance of the bacteria and oxidation state of the arsenic during bio-pretreatment, because As and As are all toxicity to bacteria[4]. All arsenic solubilized is in the of As and a stable ferric arsenate product will be precipitated upon neutralization of the leach solution[5]. The reaction is as follows[6] 4FeAsSFe2SO4310.5O23H2O 6FeSO44HAsO2H2SO4 1 2Fe3As32Fe2As5 2 Some researchers report that arsenic is initially dissolved as As, but it is oxidized to As by contacting with water, oxygen, or ferric iron during the biooxidation process[7]. However, other researchers suggest that arsenic is dissolved as As during bio-oxidation and arsenic will remain in the trivalent oxidation state unless a stronger oxidant, such as ozone, is in presence. It is reported that the resistance of the bacteria to As is higher than that of As, which is 18 g/L and 6 g/L, respectively[89]. Therefore, it is very important to oxidize As to As during bioleaching. In this work, electrochemical aspects of oxidation of arsenopyrite in the presence of Acidithiobacillus ferrooxidans were studied. At the same time, the effects of temperature, initial slurry concentration, pH value, and particle size of ores on the activity of bacteria and arsenopyrite oxidation were studied. 2 Materials and s 2.1 Characterization of sample and bacteria The compositions of arsenopyrite for electrochemical test are As 46.0, Fe 34.36, S 19.64. The main chemical compositions and the distribution of Foundation item Project50321402 supported by the National Natural Science Foundation of China; Project2004CB619204 supported by the National Basic Research Program of China Corresponding authors JIANG Tao; Tel 86-731-8877656; E-mail jiangtao JIANG Tao, et al/Trans. Nonferrous Met. Soc. China 182008 1434 arsenic in different mineral phases of the ore for bio- oxidation are listed in Table 1 and Table 2, respectively. It can be seen from Table 1, the grade of the iron, arsenic and sulphur is high. The main compositions of the arsenopyrite, based on the electron microprobe analyses, are As 37.25, Fe 30.62 and S 18.37. Arsenic is mainly as arsenopyrite phase, which is about 90.07 of the total arsenic in the sample Table 2. In addition, in some experiments pyrite was added during the bio-oxidation process. Fe content is 41.66 according to the electron microprobe analysis results, and the particle size is under 0.074 mm. The bacteria applied in the experiment are Acidithiobacillus ferrooxidans. The culture medium used during bacteria training and bioleaching is named 9K culture medium, and the compositions are listed in Table 3. Table 1 Main chemical compositions of sample mass fraction, Fe As S SiO2 Others 30.62 37.25 18.37 3.75 10.01 Table 2 Arsenic distribution of sample mass fraction, Oxidic arsenic Arsenate Arsenopyrite Arsenic Total 0.67 0.43 90.07 8.83 100 Table 3 Compositions of culture medium g/L Composition Content NH42SO4 3.00 KCl 0.10 KH2PO4 0.50 MgSO4.7H2O 0.50 CaNO32 0.01 FeSO4.7H2O 44.2 2.2 Experimental s The electrolytic cell and all electrodes are shown elsewhere[10]. Electrolyte is 9K culture medium containing bacteria, the initial pH is 2.0 and the amount of bacteria is 109 cells/mL. Tests were carried out in 250 mL conical flask bottle continuously shaking on an orbital incubator at required speed. At the start of each experiment, the 9K culture medium 80 mL was injected into the reactor. A certain proportion of arsenopyrite and pyrite was added to the solution and the pH was adjusted to a given value with H2SO4. Inoculum 20 mL was put into the bottle and the pH was readjusted. The electrical potential of the solution in the reactor was recorded periodically and liquid sample was collected. The sample was analyzed for As, total Fe and Fe2. The reduced solution was supplied with 9K culture medium without Fe2. The oxidizing slag was washed with distilled water, dried, and then analyzed for As after leaching. 3 Results and discussion 3.1 Anodic processes of arsenopyrite with bacteria Under the conditions of temperature of 20 , pH 2.0 and scan speed of 20 mV/min, the anodic process of arsenopyrite with bacteria is shown in Fig.1. Fig.1 Anodic process of arsenopyrite with bacteria It can be seen from Fig.1 that there is a peak at the potential of 0.20.3 V. At the potential of 0.8V, the current density increases abruptly. The substance on the surface at 0.20.3 V was analyzed by XRD Fig.2, suggesting that the arsenopyrite is firstly oxidized to As2S2, which covers on the surface and retards the further reaction that is Reaction 3. With the further rising potential, As2S2 is oxidized to arsenious acid, then to arsenic acid, and Fe2 to Fe3 FeAsSFe21/2As2S22e 3 As2S214H2O2H3AsO32SO4222H18e 4 H3AsO3H2OH3AsO42H2e 5 Fe2Fe3e 6 3.2 Effect of slurry density The sample is crashed under 0.074 mm. The temperature in the bioreactor maintains at 30 . The initial pH is 2.0, and the inoculum is 20. The rotating speed of the orbital incubator is 140 r/min and the oxidation time is 16 d. The effect of slurry density on bio-oxidation was studied and the results are shown in Fig.3, Fig.4 and Table 4. JIANG Tao, et al/Trans. Nonferrous Met. Soc. China 182008 1435 Fig.2 XRD patterns of arsenopyrite electrode surface at 0.2 0.3 V Fig.3 indicates the effect of slurry density on the dearsenization by bacteria. The oxidation of arsenopyrite quickly decreases with the slurry density increasing. When the slurry density of arsenopyrite increases to 5, the oxidation rate reduces to only 24. Fig.3 Effect of slurry density on oxidation Fig.4 shows the effect of slurry density on the electrical potential of solution. The electrical potential gradually declines at the same time with the increase of slurry density, and the electrical potential firstly increases then decreases with the bioleaching time increasing at the constant slurry density. When the slurry density is 0.5, the electrical potential decreases after 12 d, while it is 5, the electrical potential decreasing begins at the 6th day. The electrical potential of the solution reflects the activity of the bacteria in solution. The electrochemical results show that when the pH is 2.0 and the electrical potential is under 800 mV, arsenic is as tervalent arsenic in the solution. The bioleaching potential is 450480 mV Fig.4. This result is in accordance with the results obtained by MIN[11]. These conclusions are further coned in Table 4. Fig.4 Effect of slurry density on potential of liquor Table 4 Arsenic concentration at different slurry density Arsenopyrite concentration/ As3/gL1 As5/gL1 0.5 1.168 0.02 1.0 2.060 0.03 1.5 2.890 0.05 2.0 2.940 0.04 3.0 4.180 0.05 5.0 4.360 0.06 Furthermore, with the increase of slurry density, the concentration of tervalent arsenic increases, detailed in Table 4, thus the activity of the bacteria reduces and E of the solution decreases. 3.3 Effect of pyrite When two or more sulfide ores contact each other, they will microcell based on the electrochemical theory[1215]. Oxidation of sulfide ores by bacteria is actually an electrochemical process. When different sulfides contact each other, the one with low electrostatic potential is more reactive, and it will give its electron and be continuously corroded in acidic solution containing bacteria. Pyrite has the highest electrostatic potential among all sulfides, and it often associates with arsenopyrite. Therefore, in this study pyrite was chosen as addition agent to investigate its effect on the bioleaching of arsenopyrite. The conditions of the test are as follows. The temperature in the bioreactor maintains at 30 . The initial pH is 2.0 and inoculum is 20. Slurry density is 1.5. The rotating speed of the orbital incubator is 140 r/min and the experimental time is 12 d. The results are shown in Figs.5 and 6. The arsenopyrite oxidation increases with the addition of pyrite. When the relative rate of pyrite is in the range of 5065, the oxidation reaches maximum JIANG Tao, et al/Trans. Nonferrous Met. Soc. China 182008 1436 Fig.5 Effect of pyrite concentration on oxidation of arsenopyrite Fig.6 Electrical potential at different pyrite concentrations and keeps unchanged. When the concentration of pyrite is 65, the oxidation reaches 79.23 after being oxidized for 12 d. The result increases by 15.83 compared with pure arsenopyrite oxidized after 16 d, of which the concentration of arsenopyrite is 0.5. It is assumed that adding pyrite will microcell and comparatively active arsenopyrite will be first oxidized. Meanwhile, As is converted to As under the combined action of pyrite and Fe3. Consequently, the bacteria keep high activity, which can be confirmed from Fig.6. The electrical potential of solution ascends quickly and does not decrease. It is found that the concentration of bacteria keeps higher in the course of study. 3.4 Effect of initial pH The temperature maintains at 30 . Inoculum is 20. The rotating speed of the orbital incubator is 140 r/min. The slurry density is 1.5. The concentration of pyrite is 30 and the leaching time is 12 d. Under these conditions, the effect of initial pH of the solution on arsenopyrite oxidation rate was studied. The results are shown in Fig.7. Fig.7 Effect of initial pH on oxidation of arsenopyrite The arsenopyrite oxidation firstly rises then decreases with pH increasing. When pH is 1.8, the oxidation rate reaches its maximum as 79.58. When pH is 1.5 and 3.0, the oxidation rate is only about 10. It is found that when pH is 1.5, electrical potential is low and not changeable during the whole bioleaching process, which indicates that the bacteria are difficult to survive. When pH is 3.0, although the bacteria are alive, the color of the residue is yellow, and the yellow substance is arrosite show in Fig.8, it covers on the surface of arsenopyrite and blocks bacteria from further oxidizing. When pH is 1.8, electrical potential of solution is higher and Fe3 is difficult to deposit, which are advantageous for transing of As to As, consequently, the oxidation of arsenopyrite is higher. Fig.8 XRD pattern of residue at pH 3.0 3.5 Effect of particle size The temperature maintains at 30 . The initial pH is 2.0 and inoculum is 20. The rotating speed of the orbital incubator is 140 r/min. The slurry density is 1.5. JIANG Tao, et al/Trans. Nonferrous Met. Soc. China 182008 1437 The concentration of pyrite is 30 and the leaching time is 12 d. Under these conditions, the effect of particle size on arsenopyrite oxidation rate was studied. The results are shown in Table 5. Table 5 Effect of particle size on arsenopyrite oxidation Particle size/mm Oxidation rate/ 0.1060.148 26.98 0.0740.106 68.89 0.074 76.42 0.0460.074 68.66 0.046 68.92 The arsenopyrite oxidation rate firstly rises then decreases with particle sizes increasing. When the particle size is under 0.074 mm, the oxidation rate reaches maximum. Lager particle size leads to a decrease of bacteria adsorbed on the surface. However, when the size is too small, the probability of particle impacting with each other increases, as well as with bacteria, it causes cellular structure of bacteria to be damaged and affects the growth of bacteria. Furthermore, the viscosity of slurry is increased and the retardation of the air is enhanced if the size is too small, which leads to decrease the oxidizing ability of bacteria. 3.6 Effect of temperature The inoculum is 20, and the initial pH is 2.0. The rotating speed of the orbital incubator is 140 r/min. The slurry density is 1.5. The concentration of pyrite is 30 and the leaching time is 12 d. Under these conditions, the effect of different temperatures in the bioreactor on arsenopyrite oxidation rate was studied. The results are shown in Fig.9. Fig.9 Effect of temperature on oxidation of arsenopyrite Thiobacillus ferrooxidans survives at temperature of 2535 . The oxidation ratio decreases if temperature is over 35 . It is found that the oxidation speed of ferrous ion and E of the solution are the highest at 30 , which is advantageous for the transation of As to As. 3.7 Effect of rotating speed The temperature maintains at 30 , and the initial pH is 2.0. Inoculum is 20. Slurry density is 1.5. The concentration of pyrite is 30 and the leaching time is 12 d. Under these conditions, the effect of rotating speed on arsenopyrite oxidation rate was studied. The results are shown in Fig.10. Fig.10 Effect of rotating speed on oxidation of arsenopyrite It is disadvantageous for bacteria to oxidize arsenopyrite if the rotating speed is too high or too low. Bacteria obtai