KINETICS OF PRECIPITATION OF CALCIUM-CARBONATE IN ALKALINE PH AT CONSTANT SUPERSATURATION - SPONTANEOUS AND SEEDED GROWTH

The seeded and unseeded precipitation of calcium carbonate from supers aturated aqueous solutions was investigated in a closed system under c onditions of constant supersaturation (supersaturation 1.88-3.39) at 2 5 degrees C and at pH 9.0 and 10.0. The analysis of our results based on expressions for the supersaturation dependence of the induction tim e in seeded and unseeded precipitation according to the classical nucl eation theory provides additional information on the mechanism of crys tal growth and nucleation of calcium carbonate from supersaturated sol utions, High pH values and relatively high supersaturations favor the precipitation of vaterite. The induction times were shorter in the see ded precipitation compared to those in the unseeded precipitation and displayed a marked inverse dependence on the solution supersaturation. Crystal growth proceeds via the spiral growth mechanism. The relative ly low surface energy calculated for vaterite from the kinetics data, and the very low nucleation rate constant, indicate that some degree o f heterogeneous nucleation occurs. A model equation describing the kin etics of precipitation of vaterite was derived by using the rate const ants for nucleation and growth. According to this equation, crystal gr owth and nucleation predominate at relatively low and high supersatura tions, respectively. In all cases examined the process was controlled by the diffusion of the growth units of calcium carbonate on the surfa ce of the supercritical nuclei and/or the crystals inoculating the sup ersaturated solutions. Finally, it was found that kinetics of precipit ation is independent of pH over the range investigated. The effect of pH is restricted to the supersaturation ratio which increases with pH.