Role of Temperature in the Spontaneous Precipitation of Calcium Sulfate Dihydrate

Abstract

The spontaneous precipitation of calcium sulfate in supersaturated solutions over the temperature range between 25.0 and 80.0 °C was investigated by monitoring the solution specific conductivity during desupersaturation. From measurements of the induction times preceding the onset of precipitation the surface energy of the forming solid, identifield as gypsum, was found between ca. 12 and 25 mJ m-2 for the temperature range between 80.0 and 25.0 °C, respectively. Kinetics analysis showed that over 50 °C it is possible that anhydrous calcium sulfate is forming as a transient phase converting into the more stable calcium sulfate dihydrate. The linear dependence of the rates of precipitation on the relative solution supersaturation suggested a mechanism according to which the growth units are integrated into the active sites of the supercritical nuclei by surface diffusion. According to the morphological examination of the crystals it is possible that crystal growth occurs by the advancement of steps.