Crystallization of Gypsum from Phosphoric Acid Solutions

Abstract

A computer model has been generated which predicts the behaviour of a continuous well mixed gypsum crystallizer fed with a slurry of hemihydrate crystals. In the crystallizer, the hemihydrate dissolves as the gypsum grows. The solution operating calcium concentration must lie in the solubility gap. Growth and dissolution rates are therefore limited. Measurements were undertaken of the solubility of each phase in acid solutions, of the growth rate of gypsum crystals and the dissolution rate of hemihydrate. The growth rate depends on the square of the supersaturation and on temperature with an activation energy of 64 kJ/mol. The nucleation rate appears to vary linearly with supersaturation. As the shape of the needle-like hemihydrate crystals changes as they dissolve, it is necessary to convert to the crystal width as a measure of size. In terms of this measure, the dissolution rate is first order with undersaturation and shows only a small temperature effect (activation energy of 10 kJ/mol). With this data, the computer program is capable of predicting the operation of a hemihydrate - dihydrate crystallizer installation.