Growth and aggregation of vaterite in seeded-batch experiments

Seeded-batch crystallization experiments allowing study of the growth and aggregation of the Vaterite modification of calcium carbonate are reported. Results are reported for initial relative supersaturation (sigma = S - 1) values in the range 1.66 to 6.70, stirrer speeds in the range 400 and 1200 rpm, and at temperatures of 25 and 40degreesC It is found that in all cases the linear rate of growth of the particles is size independent and depends on relative supersaturation squared. The aggregation process is apparently size independent with a rate constant that is directly proportional to the instantaneous growth rate and decreases with increasing stirrer speed. The behavior of the aggregation rate constant is very well described by the model recently proposed by Liew et al., in which the efficiency of the aggregation process is predicted to depend only on the dimensionless strength of the particles and the nature of the flow field. It is concluded that Vaterite forms aggregates with crystalline bridges having an effective strength of approximately 25% of that of calcite or calcium oxalate monohydrate. The fitted parameter Lsigma*/M-50 takes on a value of 0.18 +/- 0.02. The kinetics and associated model are capable of describing the evolving particle size distributions very well. Finally, a hypothesis is advanced to describe the aggregation, of small inorganic crystals in supersaturated solutions. (C) 2004 American Institute of Chemical Engineers.