The Crystal Growth of Sparingly Soluble Salts

The mechanism of crystal growth of sparingly soluble salts from their supersaturated solutions is discussed in the light of a number of available growth theories. It is shown that in the case of both calcium sulphate and calcium carbonate, the observed quadratic dependence of the rate of crystallization upon the relative supersaturation is consistent with a surface controlled reaction. The reaction rates are unaffected by changes in the fluid dynamics within the crystallization cell, and the activation energy for both seeded growth and the linear growth of calcium sulphate is 15.0 ± 0.5 kcal mo1e-1• Evidence is discussed for a screw dislocation mechanism for calcium sulphate crystal growth. Under conditions of relatively high supersaturation, secondary nucleation of both calcium sulphate and calcium carbonate crystals is observed upon the addition of seed crystals. The striking effect of certain additives in reducing the rate of crystal growth is discussed in terms of the important factors involved and in a number of instances it is shown that a simple adsorption isotherm of the Langmuir type satisfactory describes the growth data