Research Into Crystallization Metastable Zone Property Of Calcium Sulfate

The practical utilization of FGD gypsum is converted intoα-calcium sulfate hemihydrate in salt solution at atmospheric pressure. It is essential to measure the solubility and supersaturation of calcium sulfate in some typical salt solutions to evalue the thermodynamic relationship in the phase transition process. The kinetics of nucleation was discussed on the basis of the metastble zone properties to form available control process technique for industrial crystallization.This study investigated the crystallization metastable zone of calcium sulfate in K-Ca-Mg-Cl-H2O system at 75~100℃by the laser light scattering method on reaction crystallization process. The effects of the concentrations of the salt solution and temperature were investigated. The phase transition of calcium sulfate was also discussed on the basis of the inner connection among the concentrations of the salt solutions, temperature and the crystallization metastable zone during the calcium sulfate formation from the salt solution. The crystallization metastable zone was used to guide the crystal growth and morphology in theory.The solubility and supersatuation of the calcium sulfate are decreased by the higher salt concentrations among 5~30 %.The salt concentrations is the major factor to influence the metastable zone width., which becomes smaller when the salt concentration higher than 18 %.The temperature does not impact on the metastable zone dramatically. But the rise of temperature causes the transition of the calcium sulfate from AH to HH,then the metastable zone width becomes broaden. The metastable zone of calcium sulfate was also discussed in the different single salt concentrations (CaCl2,MgCl2,KCl)at 95℃.The induction time of calcium sulfate in mixed salt solutions was measured to figure out the interfacial energy at different temperature. The classical nucleation theory was used to deduce the mechanism of the nucleation process .The kinetics of nucleation of calcium sulfate was discussed to control the crystal morphology and crystal growth.