Dynamic And Thermodynamic Research On Harmful Crystallizations In Zinc Hydrometallurgical Process

Elimination of the harm of calcium sulfate scaling in zinc hydrometallurgical process is a worldwide challenge for all metallurgists. In order to understand the crystallization mechanism and develop an economic and efficient method to eliminate harmful crystallization, we have performed the following work:The experiment measured the concentration of gypsum and anhydrous at different temperatures (25℃、40℃、60℃、80℃) in the zinc sulfate solution (95g/L、167g/L、161g/L) with equilibrium time of12h. The results showed that the concentration of gypsum and anhydrite decreases with increasing zinc sulfate concentration at a certain temperature. However, in a certain zinc sulfate concentration solution, the solubility of gypsum increases with increasing temperature while anhydrite decreases with increasing temperature. The crystallization trend in a short time obeys its thermodynamic solubility theory.The experiment has researched the mutual conversion rate between gypsum and anhydrite at different sulfuric acid concentration and temperatures. The experiment found that when sulfuric acid concentrations below3mol·kg-1at90℃, the spontaneously transformation from gypsum to anhydrite can be finished after36hours. Therefore, the data of anhydrite in sulfuric acid reported by Zdanovskii could not be regard as solubility. Because the time used in his work for the conversion from gypsum to anhydrite and the equilibrium between anhydrite is only6hours. It was also found that the conversion direction between gypsum and anhydrite was seriously affected by the amount of CO2dissolved in the electrolyte. The affection mechanism is waiting for further study.Using the Pitzer thermodynamic model, The experiment simulated and evaluated the solubility phase diagram of the (NH4)2SO4-ZnSO4-H2O system, and found that the data reported in the literatures (Journal of the Chemical Society,1933,30:943-946; Journal of the American Chemical Society,1938,60:1099-1104.) are reliable, and those in the literature (Guocheng Gongcheng Xuebao,2006,6:32-36.) are wrong. Based on the study, to avoid the harmful crystallization in the zinc hydrometallurgical process where the iron is removed as ammonium jarosite, one must decrease the concentration of ammonium sulfate rather zinc sulfate.