Mechanism of growth rate enhancement by micro-crystals for the potash alum-water system

Phenomenon of growth rate enhancement (GRE) caused by newly born micro-crystals was experimentally measured and quantitatively analyzed for the system of potash alum and water. Micro-crystals were prepared by mixing stoichiometric amounts of aqueous solutions of aluminum and potassium sulphates, and were fed as slurry. The size and number were controlled by the age and volume of the slurry, respectively. The age showed a substantial effect on the growth enhancement in such a way that the younger the age greater the effect. The degree of enhancement was found to be proportional to the square of the slurry volume added to the mother liquor. An empirical correlation was obtained among the degree of GRE, the slurry age and the slurry volume added. The size and number of micro-crystals were then estimated from the analysis based on a simple nucleation - growth model. Although the maximum effective size of micro-crystals could not be to determined due to the lack of reliable values of surface tension for the present system, it was estimated to be several micrometers from the experimental observations. In addition, the growth rate of such micro-crystals was found to be much slower than that of coarse ones. (C) 2001 Elsevier Science Ltd. All rights reserved.