| Abstract:Zinc ferrite is produced in conventional zinc hydrometallurgical process and its stable structure makes it insoluble in neutral leaching, which causes the difficulty of zinc-iron separation. Zinc ferrite and magnetite are stable spinel structure, zinc ferrite can be transformed to zinc oxide and magnetite by reduction roasting, so zinc is separated from zinc ferrite, while iron stabilized in magnetite can not be dissolved in leaching process. In this paper, reduction thermodynamics analysis of zinc ferrite was discussed, then selective reduction kinetics was studied using synthetic zinc ferrite. The study of reduction reaction process mechanism of zinc ferrite was carried out, TG、XPS、XRD were used to ascertain the weight loss and decomposition features, behavior of ionic migration, phase transformation process and morphological change of product layer, revealing the reduction decomposition process of zinc ferrite to zinc oxide and magnetite. High iron-bearing zinc calcine was selected as processing object, the effects of roasting conditions on selective reduction of zinc ferrite to zinc oxide and magnetite were researched under CO and CO2atmosphere by single factor experiment, then zinc was separated from zinc calcine by sulfuric acid leaching.The reaserach concludes as follow:Thermodynamic analysis showed that CO/(CO+CO2) ratio was the key to realize the selective reduction. CO/(CO+CO2) shoule be controlled between2.68%and36.18%. The study of isothermal kinetics showed that the reduction process of zinc ferrite was in accordance with shrink core model. Kinetic equation of reduction was described as follow:1-(1-α)1/3=0.667925exp(-35817/RT), reaction activation energy was35.817kJ/mol. The reduction of zinc ferrite was controlled by reaction process. Rising temperatue was conducive to improve reaction rate and reduction degree, the reduction degree was not enough for the slow reaction rate with the lower CO concentration, CO/(CO+CO2) had little influence on reaction rate, but over-reduction of magnetite could occur if CO/(CO+CO2) was over40%.The research of reduction reaction process of zinc ferrite showed that reduction reaction was caused by the oxygen loss, the weight loss was less than theoretical weight loss when reaction reached equilibrium finally. The reduced ferrous iron made zinc ferrite to decompose into zinc oxide and magnetite selectively. The content of zinc oxide had a significant linear correlation with the content of ferrous iron, the equation was:wZnO:=2.8014wFe2++0.32113. The reduced ferrous iron ions migrated into the inner zinc ferrite to substitute zinc ions, while zinc moved out and enriched on the surface, the supersaturation of zinc promoted the formation of zinc oxide. Zinc ferrite was transformed to magnetite for the displacement of zinc. During the process, the lattice parameter increased due to the intercalation of ferrous iron and then decreased due to the emigration of zinc. The reduction products were zinc oxide and zinc-containing magnetite. Reduction products coexisted with each other in product layer, and unreacted core of zinc ferrite was covered with the product layer.Using the content of soluble zinc and ferrous iron as the evaluation index of the reduction roasting, the optimum conditions were obtained as follows:roasting temperature (750℃), roasting time (60min), CO concentration (8%) and CO/(CO+CO2) ratio(20%). Under the optimum conditions, the content of soluble zinc in zinc calcine increased from79.64%to91.75%. With the leaching efficiency of zinc and iron as the evaluation index of the acid leaching, the best leaching conditions were obtained as follows:leaching temperature (normal temperature), leaching time (30min), sulfuric acid concentration (90g/L) and liquid to solid ratio (10:1). The leaching efficiency of zinc and iron under this condition is91.8%and7.17%respectively. |
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