Aggregation during precipitation from solution. Kinetics for calcium oxalate monohydrate

This paper considers the kinetics of two size enlargement processes, growth and aggregation, during the precipitation of calcium oxalate monohydrate in a batch system. Experiments in shaken flasks were conducted to investigate the effect of supersaturation, and the agitation rate, on the rates of growth and aggregation. The effects of supersaturation and of composition were studied by increasing the initial oxalate ion concentration while maintaining a constant initial calcium ion concentration; four different initial oxalate ion concentrations were considered. It is found that the linear rate of growth is size independent. The growth rates from all the experimental conditions studied are described by a single equation, G = 1.45 x 10(-9)(S-1)(2)(ms(-1)), with a second-order dependence of the growth rate, G, on relative supersaturation, S. These findings are consistent with those reported in many other studies available in the literature Aggregation is best described by a size-independent aggregation kernel. It is found that the aggregation rate constant decreases as the agitation rate increases. It is also found that the aggregation rate constant for the conditions studied correlates with the oxalate ion concentration only, rather than the activity product, or supersaturation, as has previously been suggested. (C) 1997 Elsevier Science Ltd.