New Technology And Kinetics Of Deoxidation Of Zinc Oxide

Zinc oxide fumes are very important secondary resources in hydrometallurgy zinc electrolysis industry. However, almost all the zinc oxide fume has different levels of chlorides, which strongly affects the zinc electro winning process. The present removal of chloride processes own lots of problems, such as low efficiency of dechlorination, high energy consumption and environmental pollutions. This necessitates development of a clean, efficient, energy saving process for removal of Cl elements. In the microwave fields, the zinc and lead chlorides own a better ability of absorbing microwave compared with zinc and lead oxides. Based on this character, the new removal of Cl process by microwave roasting from zinc oxide fume is proposed, combined with the advantage of microwave selective heating.In this paper, zinc oxide fumes obtained from zinc dross were investigated as raw materials. The main elements and main phases of zinc oxide fume were investigated firstly. Then thermodynamic analysis of chlorides from the zinc oxide fume was secondly calculated. Then the effects of roasting temperature, holding time, air flow, stirring speed on removal of Cl were investigated, and response surface methodology experiments were conducted to study the effects of microwave roasting temperature, holding time, air flow and stirring speed on the removal efficiency of Cl. The dynamics of chlorides volatile reaction in high temperature was studied in the last. The details as follows:(1) The main chlorides of zinc oxide fume were ZnCl2and PbCl2. Thermodynamic analysis of chloride shows that the dechlorination reactions between550℃～650℃are ZnCl2(I)=ZnCl2(g) and PbCl2(l)=PbCl2(g).(2) With the improvement of roasting temperature, extension of holding time, increasing of air flow and magnifying of stirring speed, the dechlorination rate of zinc oxide fumes increases significantly. Response surface methodology was applied to analyze the relationship between variables and the response factor, a mathematical model and optimization of process parameters were obtained. The optimization parameters were roasting temperature of626℃, holding time of25min, stirring speed of54rpm and air flow of300L/h, and the dechlorination rate will reached90.13%.(3) The kinetics of dechlorination reaction was investigated between550℃～650℃. The pre-exponential factor of dechlorination reaction is35.4and the activation energy is46.6kJ/mol, while the mechanism function is f(α)=(1-α). The activation energy is small, which shows that the volatile reaction is easy to carry out and realize the aim of dechlorination by microwave roasting between550℃～650℃.