ON THE EFFECT OF SECONDARY NUCLEATION ON THE CRYSTAL SIZE DISTRIBUTION FROM A SEEDED BATCH CRYSTALLIZER

The results of two series of experiments concerning, respectively, the breakage and the batch crystallization of K2SO4 crystals suspended in liquids in the same apparatus are reported. A model, incorporating crystal breakage, to predict the performance of the seeded batch cooling crystallizer is also presented and the effect of breakage on fine crystal distribution obtained is examined in detail. It is confirmed that fragments of potassium sulphate crystals arise by breakage over the whole size range up to the seed size, and the generation rate by secondary nucleation is largely dependent on magma density and supersaturation. In consequence, a broadening of the product crystal size distribution is observed during seeded batch crystallization. This behaviour is well-simulated by the crystal breakage and growth model under moderate cooling conditions; at high cooling rates, however, a significant deviation in crystallizer fines distribution occurs, implying a change in the mechanism of secondary nucleation on moving from slightly to highly supersaturated solutions.