Crystallization Process Of Calcium Sulfate In Ca-Mg-K-Cl-H₂O Salt Solution System

Flue gas desulfurization gypsum, as the industrial by-product of flue gas desulfurization process, are mostly made of calcium sulfate dehydrate (DH). The conversion of flue gas desulfurization gypsum (FGD) into a-calcium sulfate hemihydrate (a-HH) is the effective measure of resource utilization for FGD. The solubility, metastable zone and the induction period data in salt solution system under the condition of different temperatures were determined in this work. It is the foundation of the research into the thermodynamics and kinetics process for the transformation of DH into a-HH. From the industrial production perspective, it has theoretical guidance meaning to the transformation process as well.The solubility change of a-HH in CaCl2-MgCl2-KCl salt solution system in the temperature range of80～98℃was determined by the isothermy dissolution equilibrium method in our work. The time for a-HH in salt solution to attain an equilibrium state at different temperatures was determined. Through the comparison with the solubility of DH, the transformation mechanism of DH into a-HH in salt solution system was discussed from the perspective of thermodynamics. It turned out that the dissolution equilibrium time of a-HH at high temperature (90～98℃) is much below than that of a-HH at low temperature (80～85℃). The main factors which influence on the solubility of a-HH are the temperature, species and concentrations of salt solution. The influences on the solubility of a-HH in CaCl2, MgCl2and KCl solution are the common ion effect of Ca2+, the association effect of Mg2+and the salt effect of KCl. The common ion effect inhibits the dissolution process of a-HH solid, and then the solubility of a-HH in CaCl2solution is decreased. While the solubility of a-HH in the latter two solution is increased. Due to the combined action of the three effect on the solubility of a-HH in the mixed salt solution, especially the dominant action of common ion effect, the solubility of a-HH is decreased with the concentration increasing of the mixed chloride salt solution.In mixed salt solution with mass concentration of22to30%, the metastable zone and the induction period of calcium sulfate were measured by reactive crystallizing. Based on the classic nucleation theory, solid-liquid interfacial tensions and the nucleation rate of calcium sulfate were estimated. The results show that the main effectors on the metastable zone width of calcium sulfate in salt solution are the concentration of mixed salt solution and the metastable zone width is narrowed down in high concentration range. As for the induction period, solid-liquid interfacial tensions and the nucleation rate, the main influences are the supersaturation(1.4～3.8), the temperature and the concentration of salt solution. The nucleation induction period and interfacial tension of calcium sulfate is decreased with the increasing supersaturation, increased with the decreasing temperature and increasing concentration of salt solution. While the change rules of nucleation rate are opposite. The crystal habit of a-HH formed in salt solution at higher temperature and lower concentration range tend to be ideal short columnar crystals, and the size distribution is more uniform.