A novel approach of preparing ZnO from ammoniacal leaching solution with high chlorine levels based on thermodynamic analysis

Thermodynamic calculations for Zn(II)−NH3 − Cl−−H2O system have been presented based on free energy minimization algorithms and simultaneous equilibrium principle. Independent predominance diagrams was constructed as a function of Zn(II), NH3, Cl− concentrations versus pH, which indicates that Zn(NH3)2Cl2(s) is the most likely solid phase in near-neutral zinc solutions with high ammonia and chlorine levels. The precipitation trend of Zn(NH3)2Cl2(s) derived from experiments were consistent with the prediction by theoretical calculation. Pure ZnO(s) was successfully synthesized by a two-step hydrolysis approach, in which Zn(OH)1.6Cl0.4(s) was firstly produced from the hydrolytic deamination of Zn(NH3)2Cl2(s) and ZnO(s) was then obtained by the further hydrolytic dechlorination of Zn(OH)1.6Cl0.4(s). Both temperature and water amount played an important role on the hydrolysis procedure. Zn(NH3)2Cl2(s) can be transformed to Zn(OH)1.6Cl0.4(s) completely at room temperature with liquid-solid ratio higher than 9:1, and Zn(OH)1.6Cl0.4(s) can be thoroughly converted to ZnO(s) at 80 °C with liquid-solid ratio of 5:1.