Phase Eqilibria On Crystallization Of Calcium Sulfate In Hydrometallurgical Processs Of Heavy Metals

:Crystallization of calcium sulfate in hydrometallurgical process of heavy metals (HM), such as Cu, Zn, Ni, Mn et al., harms production process and decreases production qualities by scaling on the wall of reactors or pipelines or crystallizing along with main crystal products as impurity. Reducing the harmness of the crystallization of calcium sulfate has become a world-wide difficult task. To develop a new approach to avoid the harm of calcium sulfate, profound understanding of the crystallization mechanism of calcium sulfate is necessary. Generally, it is believed that the crystallization behavior of calcium sulfate is affected by crystal types, temperature and concentrations of heavy metal sulfate and sulfuric acid. A relatively complete solubility phase diagram of gypsum (CaSO4·2H2O) and insoluble anhydrite (CaSO4) in the quaternary system CaSO4-MSO4-H2SO4-H2O (M=Cu, Zn, Ni, Mn, Mg) is desired. The main research contents and results are:(1) The solubility isotherms of gypsum and insoluble anhydrite in the ternary system CaSO4-H2SO4-H2O have been elaborately determined at T=(298.1,323.1,348.1, and363.1) K using the isothermal method and kinetic method. Based the newly obtained experimental solubility data, we reevaluated the solubility phase diagram of the above ternary system.(2) The solubility isotherms of gypsum in the complex electrolyte solutions involving the hydrometallurgical process of Copper, Zinc, Nickel and Manganese at low temperature have been determined: Solubility of gypsum in the quaternary system CaSO4-CuSO4-H2SO4-H2O (M=Cu, Zn, Ni, Mn, Mg) with different H2SO4concentrations (0.2mol·kg-1,0.5mol·kg-1,1.0mol·kg-1,1.5mol·kg-1,2.0mol·kg-1) at298.15K.(3) The solubility isotherms of insoluble anhydrite in the complex electrolyte solutions involving the hydrometallurgical process of Copper, Zinc, Nickel and Manganese at high temperature have been determined: Solubility of insoluble anhydrite in the quaternary system CaSO4-CuSO4-H2SO4-H2O (M=Cu, Zn, Ni, Mn, Mg) with different H2SO4concentrations (0.2mol·kg-1,0.5mol·kg-1,1.0mol·kg-1,1.5mol·kg-1) at348.15K and363.15K.(4) The Pitzer thermodynamic model was selected to simulate and predict the thermodynamic properties of the complex systems including soluble heavy metal sulfates and insoluble calcium sulfate. The experimental solubilities and water activities for the sub-binary and sub-ternary systems were used for model parameterization. Then the solubility isotherms of gypsum and insoluble anhydrite in the above quaternary systems and the quinary system CaSO4-MSO4-H2SO4-H2O (M=Cu, Zn, Ni, Mn, Mg) were predicted with these obtained binary and ternary model parameters using the Pitzer thermodynamic model. Good agreement between the experimental results and the model values supports the reliability of the predicted values. The stable fields for gypsum and insoluble anhydrite as functions of temperature and concentrations of H2SO4and MSO4(M=Cu, Zn, Ni, Mn, Mg) were also constructed using the model calculation.(5) Some industrial process designs of avoiding calcium sulfate scaling were developed based on the constructed phase diagram of the quaternary system.