The Nucleation Kinetics of Calcium Sulfate Dihydrate in NaCl Solutions up to 6 m and 90°C

Abstract

Induction periods for the nucleation of calcium sulfate dihydrate were experimentally determined as a function of supersaturation in NaCl solutions from 0 to 6 m at 25°C and in 3.0 m NaCl solutions at temperatures from 25 to 90°C. Using classic nucleation theory, the interfacial tension between gypsum and 3.0-m NaCl solution was estimated to be about 39 mJ/m 2 at 25°C and increases with increasing temperature (to 63 mJ/m 2 at 90°C). It was observed that the induction period decreases with increasing NaCl concentrations up to 3 m and then starts to increase slightly at a given temperature and saturation state. The electrolyte effect on nucleation was proportional to the enhanced solubility of calcium sulfate dihydrate in NaCl solutions. This phenomenon may result from decreased crystal-solution interfacial tensions caused by increased gypsum solubility in electrolyte solutions. Although anhydrite is the thermodynamically stable phase at elevated temperatures and in concentrated solutions, only gypsum was nucleated.