Fundamental Research On Selective Reduction Of Zinc Calcine And Separation Of Zinc And Iron

Formation of zinc ferrite (ZnFe2O4) is unavoidable in traditional zinc hydrometallurgical of "roast-leach-electrowin" process due to the presence of iron in zinc sulfide concentrate, which leads to the enrichment of zinc in leaching residue. It’s hard to separate zinc and iron because the stable of zinc ferrite. The existing technology for zinc and iron separation is characterized by complex process, high energy consumption and serious environmental pollution, which significantly hampered the sustainable development of zinc production. Thus, the efficient separation of zinc and iron from zinc ferrite is critical for cleaner zinc production. Decomposing zinc ferrite to zinc oxide and ferroferric oxide selectively under reduction atmosphere was proposed, and acid leaching and magnetic separation processes were carried out to separate zinc and iron from reduced zinc calcine according to their characteristic difference. The decomposition mechanism, phase transformation and phase composition during zinc ferrite reduction process were detected. Besides, selective reduction of zinc calcine was carried out and the process features was identified. The main conclusions obtained are as follows:The thermodynamic predominace area for selective reduction of zinc ferrite under CO atmosphere was determined after the thermodynamic calculation and study of the decomposition mechanism. Thermodynamics analysis results show that the decomposition of zinc ferrite to zinc oxide and ferroferric oxide occurred spontaneously. The predominace area of CO/(CO+CO2) reduced from3.1%-38.5%to1.7%-30.7%at equilibrium when temperature increased from700to900℃. Zinc ferrite decompostion was a continuous zinc oxide precipitation process. Atmosphere have a great impact on the initial decomposition temperature of zinc ferrite. The zinc ferrite begins to decompose at1020℃and400℃under inert atmosphere and reduction atmosphere, respectively.The phase transformation behavior of zinc ferrite was conformed according to the weight loss, change of phase composition and microstructure in reduction process. At the initial stage of the reduction process, the zinc ferrite transformed to oxygen-deficient zinc ferrite by fast lossing oxygen at the crystal surface. As the reaction proceeds, the oxygen-deficient zinc ferrite began to precipitate zinc oxide by further lossing oxygen. The normal spinel structure of zinc ferrite gradually converted to inverse spinel structure of ferroferric oxide, and the continuous solid solution between zinc ferrite and ferroferric oxide was formed. The formation of zinc iron solid solution at750℃lead to the decrease of zinc oxide content in reduced products.The zinc ferrite was selectively decomposed to zinc oxide and ferroferric oxide by control of reduction conditions and the kinetic model of zinc ferrite decomposition was established. The key points for selective decomposition of zinc ferrite is to restrain the over reduction of zinc oxide and ferroferric oxide and the formation of zinc iron solid solution. Reduction of ferroferric oxide was restrained by controlling the CO/(CO+CO2) below30%, and the reduction of zinc oxide and the formation of zinc iron solid solution were restrained by controlling temperature below750℃. The unreacted shrinking core model was used to study the reduction kinetics of zinc ferrite, and the results showed that decomposition of zinc ferrite to zinc oxide and ferroferric oxide is controlled by chemical reaction, and the apparent activation energy is29.26kJ/mol. The long-range migration of zinc and iron ions in zinc ferrite crystal during reduction process is the main cause that contributes to the lower decomposition rate, The zinc oxide in reduced zinc ferrite mainly enriches at the surface of the particles.The parameters for selective reduction of zinc calcine were optimized, and the mineralogical properties of zinc calcine in reduction process were studied. The soluable zinc content in roasted product reached92.8%after roasting zinc calcine at8%CO with20%CO/(CO+CO2) under750℃for75min. However, the soluable zinc content in unroasted zinc calcine is only79%. The roasted zinc calcine mainly contains zinc oxide, ferroferric oxide, zinc sulfide, lead, zinc silicate and few unreduced zinc ferrite. The roasted zinc calcine particles have hollow structure with zinc oxide exists in the outmost layer, while zinc sulfide, ferroferric oxide and unreacted zinc ferrite particles form a hollow skeleton. The ferroferric oxide particles have clear baundaries with other phases are basically spherical in shape. The sulfur exists in zinc sulfide contributes to the lower soluable zinc content in roasted zinc calcine. The magnetic properties of zinc calcine incresed significantly after reduction roasting, which provide the basic condition for magnetic separation.The routes of magnetic separation and acid leaching were proposed to separate zinc and iron on the basis of mineralogical properties of roasted zinc calcine. The study of direct magnetic separation of roasted zinc calcine indicates that the separation efficiency of zinc and iron were limited due to the phase inclusions in the form of chemical and physical states. The acid leaching showed better effect on the separation efficiency of zinc and iron. The appropriate conditions were determined after leaching roasted zinc calcine in90g/L H2SO4at30℃with L/S of10:1for2min. Under these conditions, the zinc leaching rate achieved92%, while iron leaching rate was below7%and the iron concentration in leaching solution was only1.03g/L. The study results of mineralogical properties of leaching residue show that the main phase composition of leaching residue were ferroferric oxide, zinc sulfide, lead sulfate and lead sulfide. The presence of zinc sulfide was the main cause that lead to the high content of zinc in leaching residue. An iron grade of51.3%have been obtained after leaching residue treated by magnetic separation, and the valuable metals such as lead、indium and silver were enriched in magnetic separation tailings. The figures, tables and references are87,13and216, respectively.