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 supersaturated aqueous solutions was investigated in a closed system under conditions of constant supersaturation (supersaturation 1.88-3.39) at 25 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 time in seeded and unseeded precipitation according to the classical nucleation theory provides additional information on the mechanism of crystal growth and nucleation of calcium carbonate from supersaturated solutions, High pH values and relatively high supersaturations favor the precipitation of vaterite. The induction times were shorter in the seeded 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 relatively low surface energy calculated for vaterite from the kinetics data, and the very low nucleation rate constant, indicate that some degree of heterogeneous nucleation occurs. A model equation describing the kinetics of precipitation of vaterite was derived by using the rate constants for nucleation and growth. According to this equation, crystal growth and nucleation predominate at relatively low and high supersaturations, respectively. In all cases examined the process was controlled by the diffusion of the growth units of calcium carbonate on the surface of the supercritical nuclei and/or the crystals inoculating the supersaturated solutions. Finally, it was found that kinetics of precipitation is independent of pH over the range investigated. The effect of pH is restricted to the supersaturation ratio which increases with pH.