| Zinc oxide has excellent catalytic,thermoelectric,piezoelectric and photoelectric characteristics.Zinc oxide has broad application prospects of the fields such as sensors,solar cells and varistor,and is an important basic industrial material.The preparation methods of zinc oxide materials can be divided into solid-phase method,liquid-phase method and gas-phase method,which have the problems of high equipment requirements,harsh production conditions,small output,large energy consumption,heavy pollution and high cost.At present,the main method of industrial separation and extraction of zinc oxide resources is sulfuric acid leaching,which produces a large number of hazardous wastes with low resource utilization rate and heavy environmental pollution.Zinc ores resources are basically leached after roasting to zinc oxide,and the secondary zinc resources are mainly zinc oxide.Therefore,this paper proposes a new method to use sodium hydroxide solution to extract zinc oxide directly and obtain sodium zincate solution,and zinc oxide and zinc hydroxide products were prepared by controlling the hydrolysis crystallization of sodium zincate solution to realize the integration of zinc oxide resource separation and product preparation.This method has the advantages of simple process,media can be recycled,no by-product,no pollution,short process,low energy consumption,low cost and easy to scale production.However,there is little research on the solution properties,crystallization process mechanism and kinetic of Na2O-ZnO-H2O system.Therefore,hydrolysis crystallization mechanism and kinetic of Na2O-ZnO-H2O system were studied in the paper.The innovative results obtained are as follows.Process conditions for the preparation of zinc oxide and zinc hydroxide were obtained by hydrolysis crystallization from sodium zincate solution.Based on the classic nucleation theory,nucleation and growth mechanisms of ZnO and Zn(OH)2 were analyzed.The interfacial free energies of Zn(OH)2 and ZnO were calculated respectively.The effect of temperature on the sodium zincate solution structure was investigated.When temperature is equal or greater than 50?,Zn element exists as Zn(OH)x2-x(x ? 3),and Zn element mainly exists as Zn(OH)+ and a little as Zn(OH)x2-x(x?3)when temperature is less than or equal to 35?.It deepens the understanding of the structure of sodium zincate solution and provides a theoretical basis for the hydrolysis crystallization of sodium zincate solution.Batch dynamic method was used to research the hydrolysis crystallization of sodium zincate solution,and kinetic data was obtained.Based on the population balance model,kinetic data was treated by size independent model and moment transformation,and nucleation rate,growth rate and agglomeration rate equations of ZnO were obtained at last.It founds that the growth mechanism of ZnO during the hydrolysis crystallization of sodium zincate solution is controlled by diffusion process and surface reaction.Suspension density of crystals promotes the agglomeration of ZnO.Nucleation rate of ZnO decreases with increasing suspension density,and increases with increasing agitation speed.When the temperature is below 60?,impurity silicon can not only promote the crystallization process of sodium zincate solution,but also inhibit the hydrolysis crystallization process of sodium zincate solution.Impurity silicon affects the phase of the crystalline products also.The structure analysis of sodium zincate solution showed that impurity silica was beneficial to Zn2+existed as Zn(OH)+in the sodium zincate solution,but the effect is small when temperature is above 50 ?.Hydrolysis crystallization of sodium zincate solution was investigated systematically.Parameters such as seed,agitation speed,supersaturation and temperature had obviously influence on the conversion and particle size distribution of crystallization.It founds that Zn(OH)2 seed had effect on the hydrolysis crystallization ratio of sodium zincate solution at 60?,and ZnO seed had effect on the hydrolysis crystallization ratio of sodium zincate solution at 35?.The narrowest particle size distribution of hydrolyzed zinc oxide products is at 50?.Under experimental conditions,the conversion of sodium zincate solution was affected by agitation speed below 300 rpm.On the basis of the above research,this paper further carried out the experimental study of zinc oxide in the secondary resource nickel cobalt residue produced by the wet process of zinc smelting with sodium hydroxide solution.The optimal leaching condition was 120?,30%NaOH(w/w),1 h,L/S=10:1.The leaching rate of zinc and cobalt were 96%and 4%in the optimal leaching condition.Effective separation of zinc and cobalt was achieved.The sodium zincate solution was further hydrolyzed and crystallized to prepare zinc hydroxide. |
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