Spontaneous precipitation of calcium sulfate at conditions of sustained supersaturation

Abstract

Calcium sulfate scale formation is an important problem in a number of industrially interesting applications. An understanding of the mechanism of its formation in aqueous supersaturated solutions, in which the precipitation takes place spontaneously due to the high supersaturation with respect to this salt, is of key importance for the control of undesirable scale deposits. In the present work, the spontaneous precipitation of equimolar calcium sulfate supersaturated solutions was investigated at conditions of sustained supersaturation over the temperature range 20–60°C. In all cases, the only phase forming was identified as calcium sulfate dihydrate. The kinetic parameters were measured accurately by a potentiometric technique involving probing the conductivity of the supersaturated solutions. From the induction periods, preceding the onset of calcium sulfate precipitation the stability diagrams of the system were constructed. Kinetic analysis of the rates, which depended strongly on the solution supersaturation, suggested a polynuclear mechanism and yielded an activation energy of 44 kcal mole −1, indicative of a surface-controlled mechanism. Finally, the surface energy of the forming phase was calculated at various temperatures from kinetic data.