The Formation Mechanism Of Passivation Layer And Its Elimination Way During Bioleaching Of Chalcopyrite

The formation of passivation layer is one of the major reasons for the low copper extraction and low extraction rate during bioleaching of chalcopyrite. This thesis focus on the investigation of the formation mechanism of passivation layer during bioleaching of chalcopyrite from four aspects:biology, ecology, electrochemistry and mineralogical, and the investigation results would provide the theoretic evidence to find a way to eliminate or reduce the passivation layer on the mineral surface. At the end, column and small heap bioleaching were carried out to confirm the feasibility of this way.A mixed culture of moderately thermophilic microorganisms with good performance of chalcopyrite bioleaching was enriched from AMD (acid mine drainage) in several copper mines in China. After adaptation to high pulp density of chalcopyrite, this culture could get high copper extraction. During the bioleaching of chalcopyrite by adapted moderate thermophiles, the high concentration of total iron would benefit the copper extraction. The pH value kept stable in the early stage through the addition of sulphuric acid, but after then pH value decreased in a large scale. The redox potential increased due to the decrease of pH value and increase of ferric iron concentration. The bioleaching microorganisms increased slowly at the early stage, but in the middle stage, the cell density would increase largely and this would promote the copper extraction. The ore residue after bioleached was analysed by XRD (x-ray diffraction). The result showed that during bioleaching of chalcopyrite by moderate thermophiles, the major component of passivation layer on the mineral surface was jarosite, but not sulphur.AFM (atomic force microscopy) was used to detect the formation and development process of EPS (extracellular polymeric substances) by the attached microorganisms on the mineral surface during bioleaching of chalcopyrite. The bioleaching microorganisms firstly attached onto the mineral surface, and then produced EPS rapidly. The EPS could cover the whole attached cell and then spread onto the mineral surface, finally a layer of biofilm would form. The major components of EPS were sugar and lipid, but the content of protein was relative small. The total amount of EPS on the mineral surface increased quickly at the early stage of bioleaching and achieved to the maximum at the middle stage. The large number of EPS could concentrate enough ferric iron to oxidize chalcopyrite, and so promote the copper extraction rate. However, once the EPS was produced, it was difficult to eliminate or reduce the amount of EPS in the bioleaching system. As a result, the large number of EPS would remain on the mineral surface, and mediate the precipitation of jarosite and therefore inhibit the bioleaching of chalcopyrite.ARDRA (amplified ribosomal DNA restriction analysis) method was used to analyse the community structure of the mixed culture during bioleaching of chalcopyrite. The results showed that only four species of bacteria were detected and they are A. caldus, L. ferriphilum, S. thermosulfidooxidans and S. acidophilus. Furthermore, only one archaea was found and it was F. thermophilum, which can be always detected during the whole bioleaching process. Real-time PCR method was used to monitor the community dynamics of the three kind of important microorganisms:A. caldus, L. ferriphilum, F. thermophilum during bioleaching of chalcopyrite. It was shown that both in the solution and on the mineral surface, A. caldus was the dominant species at the early stage and L. ferriphilum was the predominant species at the later stage. The amount of the same species exhibited different variation trends in the solution and on the mineral surface. The amount of attached L. ferriphilum increased faster than that of the free one, and the amount of attached F. thermophilum kept at a stable level while the free one increased rapidly at the end. Furthermore, at the late stage of bioleaching, when the passivation layer was formed on the mineral surface, the growths of the free and attached cells were both inhibited, and so their cell density relatively decreased.The electrochemical response of massive chalcopyrite electrodes bioleached after different days was studied using cyclic voltammetry. The results of cyclic voltammetry showed that the oxidation peak of chalcopyrite decreased gradually at the early and middle stage, and disappeared at the final stage, while the oxidation peak of intermediate species (CuxS(1<x<2)) existed throughout the whole bioleaching process. Furthermore, as bioleaching progressed, the anodic and cathodic current signals decreased and the anodic peak moved gradually from low potential to high potential, which indicates that a passivation layer formed on the mineral surface. The results of SEM/EDX (scanning electron microscope/energy-dispersive X-ray) analysis of the electrodes indicate that at the end of bioleaching, the surface of chalcopyrite was covered with EPS and jarosite, indicating that these were the major components of the passivation layer that blocked continuous copper extraction.EIS (electrochemiscal impedance spectroscopy) is a good way to investigate the passivation behaviour on the mineral surface during bioleaching of chalcopyrite. There were mainly two kinds of impedance to affect the dissolution of chalcopyrite:surface ion transfer impedance and passivation layer impedance. At the early stage of bioleaching, the amount of jarosite and sulphur on the mineral surface was small, so the passivation layer impedance originated from the formation of EPS produced by attached microorganisms. At this time, the impedance value by EPS was low. However, in the middle and the end stage of bioleaching, when the amount of jarosite increased largely, the impedance value by passivation layer would also increase largely. The increase of passivation layer impedance would inhibit the transfer of ion between the mineral surface and bioleaching solution, and therefore the ion transfer impedance increased rapidly, when the dissolution of chalcopyrite almost stopped.Synchrotron XRD was used to analyse the ore residue after bioleached different days during bioleaching of chalcopyrite. In the ore residue, the Cu-S intermediate species such as CuS, Cu2S, CuxS(1<x<2) were not detected by synchrotron XRD. This indicates that in the bioleaching environment, these intermediate species would be easy to be oxidized by acid or bioleaching mciroorganisms, and so these species would not be the major components of passivation layer during bioleaching.According to the investigation and analysis of microbiology, ecology, electrochemistry and mineralogy during bioleaching of chalcopyrite, the formation mechanism of passivation layer on the mineral surface was established and the feasible way to eliminate or reduce the formation of passivation layer was found and shown as below: 1) the addition of heterotrophic or facultative heterotrophic microorganisms can partly consume the inorganic substance during bioleaching system; 2) the thermophiles such as moderate thermophiles could oxidize suphur more effectively than mesophiles, and so could reduce the formation of sulphur layer on the mineral surface; 3) the acid adaptation of bioleaching microorganisms could promote the growth and bioleaching ability of cells at low pH value, and reduce the formation of jarosite, and thus reduce the inhibition of these species to copper extraction.The mixed culture of mesophiles and moderate thermophiles after acid adaptation for about 10 months was used to bioleach low grade chalcopyrite ore in the column reactor and in the small heap. A relative high copper extraction could be achieved. Furthermore, the ore residue after bioleached in the heap bioleaching was analysed by XRD. It was shown that the contents of jarosite and sulphur in the residue were low, and this indicates that using the mixed culture after acid adaptation to bioleach chalcopyrite ore could reduce the formation of passivation layer like jarosite and so promote the copper extraction.