Basic Research And Technical Study On Bioleaching Of Zinc Sulfide Ore Bearing High Content Of Iron

The processing of zinc sulfide ore bearing high content of iron involves high unit cost and low metal recoveries. Zinc ferrite(ZnO · Fe₂O₃) is formed when the concentrate of Zn bearing high content of iron is submitted to the roast-leach electrolysis process, which decreases the leaching rate of Zn and makes it difficult treated. One alternative for the treatment of the refractory zinc sulfide ore or zinc concentrate is bioleaching. The bioleaching of the marmatite sample, which was obtained from Dachang mine in Guangxi province, was carried out to investigate the bioleaching process for the refractory zinc sulfide ore or flotation concentrate.The bacteria applied in this research are a mixed culture isolated from the mine water of Dachang mine. Characterization of this culture showed that the main bacterial strains were Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans. The influences of various ions, surfactants and various cultures on the oxidation activities of the bacteria and the bioleaching of marmatite were studied. The mechanism of bioleaching of marmatite and the leaching behavior of Zn and Fe were discussed. On the bases of these researches, the bioleaching of marmatite ores and the flotation concentrate was conducted. The solvent extraction of zinc from the bioleaching solution was also studied. A basic technical process of bioleaching-solvent extraction-electrowinning for zinc sulfide was proposed.The ions and surfactants have influences on the oxidation activities and leaching capacity of bacteria. A certain concentration of copper ions or OPD can accelerate the S oxidation rate during bioleaching while the Fe oxidation activities of bacteria were hardly influenced. As a result, the bioleaching rate of marmatite was enhanced. The copper ions replace the zinc ions in the crystal lattice of marmatite as CuS or CuS₂, which also has electrochemical catalysis on the bioleaching of marmatite.The bacteria cultured on various mediums have different distributing of strains and show different oxidation activities of Fe and S accordingly. The bacteria that both have strong oxidation activities of Fe and S showed high leaching capacity. The bacteria cultured on the magnetic pyrrhotite, which were characterized by strong iron and sulfur oxidation activities and adsorption activities, and the oxidation activities of S were not weakened, showed high leaching capacity.The bioleaching of marmatite follows the shrinking core product layer model. SEM and EDXA analysis of the leaching residue indicated that the components of the product layer is a mixture of FeS, S and jarosite. When the product layer increased and became insoluble, the leaching rate is controlled by the diffusion through the product layer. In the case of being leached by bacteria with strong oxidation activities of Fe and S, the product layer was oxidized and decreased or became a porous layer, then the leaching rate was controlled by chemical reaction. The electrochemical aspects on bioleaching of marmatite were studied. In the case of low potential, a passivation film was formed on the surface of marmatite electrode. The presence of bacteria decreased the passivation and increased the leaching rate.According the energy band model for the leaching of marmatite, marmatite can be leached by Fe3+ ions and FT protons, and the reaction products were Fe ions and elemental S. Therefore, the marmatite can be leached by the bacteria with the only oxidizing ability of Fe or S. The role of the bacteria was to oxidize ferrous ions to ferric ions and oxidize elemental sulfur to sulfuric acid. The adsorbed bacteria on the marmatite surface firstly obtain the growth medium and grow well, and the metabolizing substances of the adsorbed bacteria accelerate the leaching rate of marmatite.The crystal structures of ZnS and FeS in marmatite are similar, but the crystal field stable energy of Fe in octahedral sites is greater than that of Zn in tetrahedral sites, so Zn is more likely to be leached than Fe. Bioleaching of marmatite ores and flotation concentrate with the bacteria cultured on pyrrhotite were carried out in shaking experiments.In the optimized conditions, the leaching rate of zinc reached above 96%, while the leaching rate of iron was only about 18%. The zinc in marmatie was selectively extracted.D2EHPA (di (2-ethylhexyl) phosphoric acid) was used as zinc extractant diluted in 260 # kerosene to extract zinc from the bioleaching solutions. A purified zinc sulfate solution was obtained through the process of solvent extraction, washing and stripping. A basic technical process of bioleaching-solvent extraction-electrowinning for zinc sulfide was proposed. The process is feasible on technique for zinc sulfide ores or the flotation concentrates.