| Zinc oxidation hazardous wastes will be harmful to the environment, so harmless and resource treatment on these materials is necessary.In order to improve the environmental quality,alleviate the demand for increased sharply of zinc resource,overcome the defect of tradition wet mention,so processing zinc oxidation hazardous wastes for zinc production is becoming more attractive. Among possible alternative technologies, the alkaline leaching and electrolysis process deserves attention. The Zn in zinc oxide wastes and ores can be leached selectively in NaOH solution, and then be recovered as metallic zinc by electrowinning which offering a significant energy saving over the conventional sulfate process.In this work, the alkaline leaching and electrolysis process was in-depth studied. The leaching parameters were observed and optimized, the impurities in leach solution were removed, Zn recovered as high-value-added metallic zinc powder, and the integrated alkaline process was thus developed, using the oxidized zinc wastes and ores at industrial scale, with a good economic and environmental benefits. The main conclusions were as follows:(1) A thermodynamic model of the system of Zn(Ⅱ)-NaOH-H2O was set up, Zn(Ⅱ) is existed as Zn(OH)42-and ZnO22-in the Zn(Ⅱ)-NaOH-H2O system, the molde between the zinc equilibrium concentration of zinc and the concentration of alkaline was constructed. The absolutely average error between experimental values and theoretically calculated values of zinc equilibrium concentration was found to be0.1298%using the solubility test of ZnO in the alkaline solution, showing the model developed should be reliable.(2) The effects of sodium hydroxide concentration, phase ratios (v/w), reaction, temperature, leaching time, particle size, stirring speed on leaching rate of zinc in sodium hydroxide solution were investigated. The optimum conditions varied for different raw materials. For the wastes bearing zinc, the optimum conditions were found to be a temperature of90℃, NaOH concentration of6mol/L, phase ratios (v/w) of10:1, and leaching time of120minutes, stirring speed of300r/min; and for smithsonite, to be a temperature of90℃, NaOH concentration of6mol/L, phase ratios (v/w) of10:1, and leaching time of120min, less than0.125mm of particle size, stirring speed of450r/min; while for willemite, to be a temperature of90℃, NaOH concentration of8mol/L, phase ratios (v/w) of8:1, and leaching time of240min, less than0.125mm of particle size, stirring speed of450r/min.(3) The leaching reaction kinetics of waste bearing zinc, smithsonite, and willemite in alkaline solution were studied. For waste bearing zinc and smithsonite, the apparent activation energy was obtained to be42kJ/mol and43.15kJ/mol from an Arrhenius plot. For willemite, during the early stage of leaching reaction the apparent activation energy of13.59kJ/mol; while in the later stage, the apparent activation energy of31.86kJ/mol. Thus, it indicated that the leaching mechanism of willemitethe changes from inner diffusion control to mixed control, during the leaching process in alkaline solution.(4) In depth purification of leach solution was explored. Sodium sulphide, sodium silicate, ferric sulfate and lime were used as purification agents. When the weight ratio of sodium sulphide/Pb was1.8,1.5g/L sodium silicate,1g/L ferric sulfate and weight ratio of lime/sodium sulphide of0.8were used, a high purity of zinc powder can be electrowon.(5) The theoretical voltage of alkaline zinc electrowinning is much lower than acidic electrowinning process. The optimum conditions of electrodeposition was that Current density of800-1000A/m2, NaOH concentration of180-240g/L, temperature of30-50℃, Zn concentration of30-40g/L. The current efficiency was99%and consumption of Power was only2.38kWh/kg Zn under the optimum conditions. The morphology of Zn on cathode changed from grain shape to thin slice if increasing current density and reducing solution temperature, the morphology of Zn changed from thin slice to Layered if increasing alkaline concentration.(6) The effects of various cation metal ions and anion ions on the electrowinning of metallic zinc powder from the alkaline solution was studied in detail. It was found that top impurities contents in the electrowinning solution should be controlled so that a high quality of zinc powder can be obtained.(7) Causticization of spent zinc electrolyte was conducted, by adding caustic soda into the spent electrolyte to make the concentration of caustic soda to350g/L so that the sodium carbonate and impurity ions can be thus separated from the solution. The precipitates separated from the process was then dissolved using the zinc-powder-washing wastewaters, with the concentrations of NaOH and sodium carbonate in the solution at80-100g/L and above40g/L, as well as at the conditions of temperature of90℃, reaction time of30min, lime1.5-1.8times of the theoretical value. As a result,28kg NaOH can be re-generated from each cubic meter solution, while some Fe,Cu,Mn,Mg,Cr,Cd and As can be removed simultaneously.(8) The process developed has been successfully at several plants with a scale of2000tons of metallic zinc powder annually. The long-term industrial operations have been shown that the process can use any raw materials containing Cl, F, Si, Fe, As, Cu, etc., except for zinc sulfides. Hence, the process is quite different to the acidic process and can apply the raw materials that the acidic process is unable to apply.(9) Zinc oxidation hazardous wastes change into general solid wastes after alkali extraction and it fulfils the requirement of waste disposal in harmlessness and reuse.In a word, whether from the economic benefits, environmental benefits or social benefit the process of alkaline leaching-electrowinning-preparation zinc powder has more competitive advantagea than traditional Zinc powder production methods, it can deal with the materials containing fluorine, chlorine, silicon which acid method can't deal with, it is a wet-cleaner process and has a bright prospects for industrial application.
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