Calcium sulfate hemihydrate, an important form of mineral scale, may deposit in saline water evaporators. This metastable crystal modification is more soluble than anhydrite (the stable phase above 100 °C.) but solubility of both forms decreases sharply with increasing temperature. Pure solid phase hemihydrate can remain in contact with its dissolved ions for about 17 hours at 100 °C., but the rate of conversion to anhydrite increases with the temperature. Conversion rates are also faster in solutions with high concentrations of background ions. Because of the relatively short “life” of hemihydrate in saline water backgrounds at elevated temperatures, it is difficult to measure its solubility by conventional methods of equilibration. An experimental technique for hemihydrate solubility measurement in saline water solutions at conditions approaching equilibrium is described. The method is based on visual observation of incipient crystal formation in a glass pressure vessel at various temperatures and pressures just above the saturation vapor pressure. Measurements were carried out on sea water and Roswell water over a range of 100° to 150 °C. Experimental data on scaling threshold are compared with literature calculations based on Debye-Htickel Theory. The rate of conversion of hemihydrate to anhydrite has also been studied. © 1969, American Chemical Society. All rights reserved.
