Calcium sulfate dihydrate crystal growth in aqueous solution at elevated temperatures

Abstract

The growth of calcium sulfate dihydrate seed crystals from supersaturated solutions at temperatures where this phase is thermodynamically stable has been shown, in previous studies, to involve a surface controlled rate determining step. In the present study, the investigation of calcium sulfate dihydrate crystallization has been extended up to temperatures at which phase transformation to the hemihydrate occurs, conditions under which the dihydrate is the thermodynamically unstable phase. Solubilities of both the dihydrate and hemihydrate have been determined from the results of both dissolution and crystallization experiments in order to establish the conditions for the dihydrate-hemihydrate phase transition in the salt free systems used. Calcium sulfate dihydrate seeded growth experiments at temperatures between 60–105°C showed no evidence of transition to the thermodynamically stable salt, anhydrous calcium sulfate, during the course of the experiments. The rate of reaction was proportional to the square of the relative supersaturation and an approximately constant number of growth sites were available during the crystallization experiments. The crystal growth activation energy was found to be 14 ± 2 kcal/mole. Seeded crystal growth experiments near the dihydrate-hemihydrate phase transition temperature (103°C in salt free solutions at 4 atm) show that phase transformation accompanies the seeded growth of the less stable modification. The mechanism of phase transformation appears to involve collission breeding of nuclei of the more stable phase followed by a dissolution growth process.