Effects Of Microorganisms On Surface Properties Of Chalcopyrite And The Stepwise Dissolution Mechanism Of Chalcopyrite During Bioleaching

Abstract:The speed of chalcoprite bioleaching by mesophilic bacteria and the leaching rate are slow and low, which largely restricts the application of bioleaching in the processing of chalcopyrite. On the contrary, chalcopyrite bioleached by thermophilic bacteria can be significantly improved, the mechanism in which has attracted increasingly attention from researchers. Based on thermomechanical analysis, this paper studied the effects of mesophile and thermophile on surface properties of chalcopyrite by using adsorption, zeta-potential, contact angle and bioleaching tests. XRD, Raman, SEM/EDS analysis and electrochemical experiments were employed to compare the different bioleaching process by mesophile and thermophile, deeply investigate the reasons for the different leaching efficiencies, further analyze the influencing factors of chalcopyrite decomposition mechanism, and reveal the stepwise dissolution mechanism of chalcopyrite.This paper presented the reason for different leaching efficiencies when chalcopyrite bioleached by mesophile and thermophile. The decomposition mechanism of chalcopyrite does not have a direct relation to the changes of bacteria or temperature. The dissolution of chalcopyrite is stepwise, and it can be transformed into Cu2S. Cu2S accelerated leaching process, which is the main reason for the different leaching efficiency. During leaching process by thermophilic bacteria, Fe3+is easy to be converted into jarosite, which can result in and keep a low redox potential, promoting the formation of Cu2S and leaching. While bioleaching by mesophilic bacteria, the redox potential maintaining on a high level hinders the generation of Cu2S and bioleaching. However, the generation of sulfur, jarosite precipitation is not the restraints for chalcopyrite dissolution.The main conclusions are as follows:(1) The thermodynamic analysis results show that chalcopyrite can be converted into Cu5FeS4, CuS, Cu2S copper sulfur compounds under the acidic conditions, eventually release the Cu2+ions. The existence range of components is nearly the same under different temperature conditions. In the acidity condition at high temperature, Fe3+ions are likely to generate KFe3(SO4)2(OH)6precipitation. In addition, the high ion concentration in the solution also promotes Fe3+ions converting into KFe3(SO4)2(OH)6precipitation. The precipitation process can reduce the redox potential of leaching system, which affects the decomposition of chalcopyrite.(2) Due to a similar impact to chalcopyrite surface, the effect of mesophile and thermophile on chalcopyrite surface properties is not the key factor to influence the different leaching efficiency. While in the leaching system, the changes of contact angle have a tight connection with the stepwise dissolution mechanism of chalcopyrite. Due to the formation of hydrophobic elemental sulfur, copper sulfides and hydrophilic jarosite, the contact angle of chalcopyrite increased before they are reduced.(3) The key factor to reduce chalcopyrite to Cu2S is determined by the redox potential of leaching system, which restricts the leaching rate of chalcopyrite. During bioleaching by mesophile, the redox potential remained at high value (more than550mV vs. SCE) except the first few days. The leaching residue during the initial days is detected to be Cu2S and the bioleaching rate was relatively fast. However, when the potential rose to a high value, Cu2S cannot be detected in the residue, which approved that the reduction of chalcopyrite is restrained and leaching speed is hindered. Throughout the whole bioleaching process of chalcopyrite by mesophilic bacteria, the redox potential of the system keep in high value, which is not beneficial to the reduction reaction, hindering the leaching of chalcopyrite. During chalcopyrite bioleaching by thermophile, the redox potential has stabilized at a relative low level (380-450mV vs. SCE), which enhanced the leaching efficiency of chalcopyrite.(4) During microbial leaching process, the role of microbes is to determine the oxidation of ferric ions or sulfur. Therefore, mesophilic bacteria and thermophilic bacteria showing different patterns of chalcopyrite leaching effects should be based on its iron oxide or sulfur capacity differences. For thermophilic bacteria using elemental sulfur as energy material, its ability of oxidizing ferrous is significantly weaker than the ability of bacteria cultured at medium temperature. In addition, when the temperature rises, Fe3+ions are easily transformed into jarosite and Fe3+ions consumption ensues. This is the critical factor that results in different redox potential at high temperature and medium temperature leaching systems.(5) The electrochemical research results present that the changes of temperature or bacteria do not alter the decomposition mechanism of chalcopyrite, but they enhanced the decomposition. In addition, during bioleaching by mesophilic bacteria, the open circuit potential of chalcopyrite sharply declined after5days, and then rose and kept at a high value. By contrast, during the bioleaching process by thermophilic bacteria, the open circuit potential of chalcopyrite dropped rapidly and kept at lower level. Obviously, chalcopyrite can be easily decomposed only in the first few days of bioleaching by mesophile but it become tough afterwards. However, it can easy be dissolved during the whole process when bioleached by thermophile.(6) During bioleaching of chalcopyrite, sulfur and jarosite precipitation are not the restraints for chalcopyrite dissolution. During bioleaching by mesophilic bacteria, when add sulfur-oxidizing A. thiooxidans in the leaching system by iron-oxidizing L. ferriphilum, the sulfur membrane disappeared, but copper leaching rate was not significant improved. During bioleaching, the jarosite precipitate was loose and fall off from chalcopyrite surface. Hence, it cannot hinder the leaching. In addition, during bioleaching by thermophilic bacteria, the leaching effect has been largely enhanced compared to bioleaching by mesophilic bacteria, though sulfur and jarosite has been detected in the leaching residue. It further illustrates that chalcopyrite dissolution cannot be restricted by sulfur and jarosite precipitation.