| Ferrite and sulphur dioxide were formed during the process of treating marmatite by traditional roasting-leaching-electrowinning technology. The produced ferrite can reduce the leaching rate of zinc. At the same time, sulphur dioxide can bring environmental problems. While, when marmatite was treated by bioleaching technology, it can avoid the generation of sulpher dioxide. Bioleaching also has the advantages of simple process and low cost. So it is worth of doing futher sutdies on marmatite bioleaching.The bioleaching of marmatite was carried out by shake flask leaching experiments. The bacteria applied were original L.ferrooxidans or S.thermosulfidooxidans. The influences of particle size, pulp densities, controlling pH and inoculation on marmatite bioleaching were studied. Also the effects of additional yearst extraction or ions of Fe2+, Fe3+or Cu2+were investigated. The methods of SEM, XRD, EDS and electrochemical measurements were used to make a further study on the mechanism of marmatite bioleaching.The bioleaching results show that the optimum particle size range for bioleaching was-0.074+0.043mm. It has a profound effect on obtaining higher leaching rate when reducing the pulp density or controlling the pH value at1.6during the experimental processes. The leaching rate of zinc was significantly decreased when Cu2+ions was added, since Cu2+ions can produnce a toxic effects on bacteria growth. Appropriate addition of Fe3+ions promoted the dissolution of marmatite, whereas high concentrations of ferric ions may inhibit the leaching rate. The stimulative effect of Fe2+ions on marmatite leaching was obvious in the first15days, and then it can be negligible. Increasing the initial inoculum or adding0.02%yearst extraction caused a promotion to the zinc extraction by S.thermosulfidooxidans. It also can be found that the original bacteria of S.thermosulfidooxidans are more sensitive to the mineral partical size and pulp density than L.ferrooxidans. Compared with L.ferrooxidans, the leaching rate of zinc was faster in the presence of S.thermosulfidooxidans under the pulp density of3%. The leaching rates of zinc were63.92%and70.33%, respectively.Marmatite bioleaching is an acid consuming process. The chemical oxidation of Fe3+ions played an important role on the dissolution of marmatite during the leaching process. The analyses results of SEM, XRD and EDS revealed that a product layer was formed on the particle surface, and reduced the dissolution rate of marmatite at the later leaching stage. The residues obtained were composed of elemental sulfur, jarosite and unreacted marmatite. Zinc in marmatite was selectively leached.The results of potentiodynamical anodic curves show that the dissolution of marmatite increased with the increace of applied potentials, whereas it became passivation when the applied potential was up to800mV vs. SHE. Tafel results demonstrate that the corrosion current density increased after the inoculation of L.ferrooxidans or S.thermosulfidooxidans, which indicated that marmatite dissolution was promoted in the presence of bacteria. The EIS studies demonstrate that there was a single capacitive arc when bacteria were inoculated into the solution, and the process was controlled by chemical reactions. Compared with the results obtained from the tests using different bacteria, the corrosion current density in the system with supplementary addition of Fe2+or Fe3+was higher than that without iron ions, implying the increase of marmateite dissolution rate. The capacitance arc in EIS was constantly depressed with the increase concentration of Fe2+ions, which meaned that the reaction resistence decreased and the marmatite dissolution was improved. Though the EIS spectra turned out same new features in the presence of L.ferrooxidans when Fe3+ions were added, the peocess was still controlled by chemical reaction. The shape of EIS spectra didn’t changed with additional Fe3+in the presence of S.thermosulfidooxidans, while the capacitance arc was compressed. |
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