Research On The Process Of The Preparation Of Zinc Oxide From Zinc-containing Dust

Abstract:In China, about60%of zinc oxide consumption was used in rubber industry as industrial material. Zinc oxide used in the rubber industry has strict requirements on the heavy metal content of impurity elements, especially the element of manganese because the zinc oxide of manganese exceeding standard will accelerate aging of the rubber. In the process to obtain zinc oxide by ammoniacal leaching process, the removal of iron and manganese is usually accomplished by the oxidation process. However the economic costs of oxidation process are high, and the removal of iron and manganese isn’t ideal. In order to solve the problems in the traditional process of preparation of zinc oxide from zinc-containing dust, the zinc-containing dust was oxidatively pretreated with calcium hypochlorite, subsequently leached with ammonia/ammonium bicarbonate solution to obtain the ammoniacal leach solution, and zinc oxide was produced by ammonia distillation of the ammoniacal leach solution and subsequent calcination. The effect of the pretreatment conditions on the concentrations of iron and manganese in ammoniacal leach solution was studied, and optimum pretreatment condition was obtained as follows:8(wt.%) of calcium hypochlorite addition and liquid/solid ratio of3ml/g at room te mperature for30min. The elements of iron and manganese could not be detected in ammoniacal leach solution obtained under optimum pretreatment conditions. The analyses of the valence states of iron and manganese in zinc-containing dust with or without pretreatments were performed by chemical analysis and X-ray photoelectron spectroscopy measurement (XPS), and the possible oxidation mechanism of the oxidative pretreatment for the zinc-containing dust was discussed. It is concluded that Fe2+oxide and Mn2+-oxide in zinc-containing dust were oxidized to Fe3+-oxide and/or its hydrate and Mn4+-oxide and/or its hydrate by calcium hypochlorite during the oxidative pretreatment process, respectively. Therefore, the elements of iron and manganese in pretreated zinc-containing dust became less soluble in ammonia/ammonium bicarbonate solution.After the process of oxidative pretreatment and ammonia was carried out, the ammoniacal leach solution was obtained, and was deeply purified by zinc-dust cementation. Firstly the concentrations of free ions of Zn2+, Cu2+, Cd2+and Pb2+in ammoniacal leach solution were obtained as3.22×10-10mol/L,3.42×10-12mol/L,3.2×10-12mol/L and2.23×10-12mol/L by theoretical calculation, respectively. And the calculated concentrations of Zn, Cu, Cd and Pb were7.83×10-34mol/L,2.7×10-13mol/L and1.14×10-19mol/L in purified solution obtained by zinc-dust cementation, respectively. Subsequently, the effect of the purification conditions on impurity element concentrations in purified solution was studied, and the optimum purification conditions(8times of zinc powder theoretical consumption, reaction time lh,60℃of reaction temperature and at a stirring speed of250rpm) were obtained. The elements of copper, cadmium and lead could not be detected in purified solution obtained under optimum purification conditions.From above experiments, the new process of oxidative pretreatment-ammonia leaching-purification-ammonia distillation-calcination was obtained for preparation of zinc oxide. The contents of iron, manganese, copper, cadmium and lead in zinc oxide produced by new process were0.84ppm,0.28ppm,0.12ppm,0.88pp and0.96ppm, respectively, which can meet the requirements of manganese content in zinc oxide used as rubber vulcanization additives.