Bioleaching And Mechanism Of Anilite, Covellite And Chalcopyrite

Anilite is a compound of the Cu-S group. It was discovered by Morimoto in 1969(Morimoto et al.,1969). Its matrix phase can be expressed as Cu1.75S in chemical formula. Anilite was found in the Zijinshan Copper Mine, Fujian Province of China by Xue and co-worker in 2000(Xue et al.,2000). The report of anilite bioleaching is not fund. This research was support by the National Basic Research Program (973 Program) of PR China (No.2004CB619204) and Chinese Science Foundation for Distinguished Group (No.50621063). Bioleaching and mechanism of anilite, covellite and chalcopyrite were studied in this paper. The main works and innovations were indicated as followed:(1) Isolation, identification and physiological-biochemical characters of bioleaching bacteria.Two strains of bioleaching bacteria were using enrichment medium and then purified on overlaid solid medium. They were Acidithiobacillus ferrooxidans(A. ferrooxidans) and Aicdthiobacillus caldus(A. caldus). The morphological, physiological-biochemical characters of A.ferrooxidans and A. caldus were studied. The effect of different heavy metals (Cu2+, Cr3+, Cd2+, Sn2+, Pb2+) on growth inhibition was investigated. A. ferrooxidans was able to oxidize ferrous iron and elemental sulfur. The optimum temperature and pHs of A. ferrooxidans were 35℃and 2.0-2.5. A. ferrooxidans exhibited a high degree of resistance to Cu2+,Cr3+,Cd2+ and Pb2+.A. caldus was capable of growth by using elemental sulfur or sodium thiosulfate as the sole energy sources. It was unable to oxidize ferrous iron. The optimum temperature of A. caldus was 45℃. The resistance to heavy metals of A. caldus was no better than A. ferrooxidans.(2) Bioleaching of Anilite, covellite and chalcopyriteAnilite, covellite and chalcopyrite were leached by pure and mixed culture of A.ferrooxidans and A. caldus. Four systems were investigated: the system without sulphur and ferrous sulphate, the system with sulphur, the system with ferrous sulphate, the system with sulphur and ferrous sulphate. Copper dissolution in the A. caldus culture and the sterile control leaching were almost same in all systems. A. caldus was unable to oxidize anilite, covellite and chalcopyrite. A. ferrooxidans and A. caldus was more efficient than a pure culture of A. ferrooxidans or A. caldus in leaching anilite, covellite and chalcopyrite.Supplemental sulphur could improve copper extraction of anilite, but could not improve copper extraction of CuS and chalcopyrite. The effect of supplemental iron clearly improved Cu leaching by the A. ferrooxidans culture and the mixed culture. Copper concentration of anilite bioleached by A. ferrooxidans was increased by quantity of supplemental iron. The rate of covellite bioleached by A. ferrooxidans was increased but copper extractiont was not increased by quantity of supplemental iron. Supplemental iron increased obviously copper extraction of chalcopyrite in the sterile control. However, it could not increase mardedly copper extraction of chalcopyrite by A. ferrooxidans leaching.(3) Microscopic mechanism of bioleaching of Anilite, covellite and chalcopyriteSolid residues of A. ferrooxidans leaching systems after 15 days were examined by Scanning Electronic Microscope (SEM). The results show that the bacteria attached to sites with visible surface scratches. Cell-sized and-shaped pits was found on the chalcopyrite surface, whereas no such pits were observed on the anilite and covellite surface. The chalcopyrite surface became rough with bacteria attachment, and the surface was smooth without bacteria attachment.In the system with ferrous sulphate, on the A. ferrooxidans leached anilite solid residues, copper dropped off and sulfur increased gradually with bioleaching. Sulfur coating layer was presented as a reaction product on the surface of the bacterially leached anilite. Solid residue of A. ferrooxidans leaching systems after 15 days was examined by x-ray diffraction patterns. Jarosite was confirmed as the main phase of solid residues in A. ferrooxidans bioleaching. Sulphur was also detected beside copper sulphide (CuS) on the solid residues. It could be concluded that leaching of anilite was performed in two stages:(a) formation of covellite (CuS), and (b) the oxidation of covellite (CuS). In the system with ferrous sulphate, the EDAX analysis of the A. ferrooxidans leached covellite solid residues indicated copper dropped off and sulfur increased gradually with bioleaching, but sulfur coating layer was not presented as a reaction product on the surface of the bacterially leached covellite. Solid residue of A. ferrooxidans leaching systems after 15 days was examined by x-ray diffraction patterns. Jarosite was detected beside copper sulphide (CuS) on the solid residues. However sulfur was not found on the residues.In the system without initial sulphur and ferrous sulphate, the EDAX analysis of the A. ferrooxidans leached chalcopyrite solid residues indicated copper dropped off, at the same time iron and sulfur increased gradually with bioleaching. Jarosite was presented as a reaction product on the solid residues.(4)Electrochemical behavior of anilite, covellite and chalcopyriteElectrochemical behavior of anilite, covellite and chalcopyrite was studied in the presence or absence of bacteria in 9 K media. The cyclic voltammograms of anilite-carbon paste electrode found that two anodic current peaks, A1 and A2, appeared at-0.65V and-0.25V respectively. The cathodic current peak C1 and C2 was observed, they were monitored at 0V and-0.7V respectively. The anodic peaks A1 was obviously shifted to the negative when bacteria was added.The cyclic voltammograms of covellite-carbon paste electrode in 9 K media found that three anodic current peaks, A1, A2 and A3, appeared at-0.05V,-0.15V and 0.72V respectively. The cathodic current peak C1 and C2 was observed, they were monitored at-0.55V and-0.13V respectively. A. ferrooxidans maked peak A3 current increase.The cyclic voltammograms of chalcopyrite-carbon paste electrode in 9 K media found that three anodic current peaks, A1, A2 and A3, appeared at 1V,0.47V and 0.07V respectively. The cathodic current peak C1 was observed and monitored at 0V. Intermediate products (Cu1-xFe1-yS2-z) formed on the mineral surface after chalcopydte had been oxidized under anodic current peak A2. It caused further oxidation very difficult. When potential was 0.7V and formed peak Al, Fe2+was oxidized to Fe3+. intermediate products chalcocite was oxidized at anodic current peak A3. In the anti-scanning process, sulfur was deoxidized to CuS at cathodic current peak C1. Anodic current peak A4 and cathodic current peak C2 were appeared when scanning rate was bigger. Products run up to on the surface of electrode and formed peak.