MINIMUM SEED CRYSTAL SIZE FOR SECONDARY NUCLEATION OF POTASSIUM ALUM IN A STIRRED-VESSEL CRYSTALLIZER

The minimum seed crystal size or the minimum size, usually defined as the critical size greater than which seed crystals can produce secondary nuclei, was measured for potassium alum secondary nucleation using a seeded stirred-vessel crystallizer. The value was lowered with increasing stirrer speed both for stainless steel and acrylic resin impellers. The former impeller gave a smaller minimum size than the latter. The minimum size was newly interpreted as a measure of difficulty of nucleation, by defining it as the seed size at which the nucleation rate, being increased as the seed crystals grow, reached a certain detectable level. The behavior of the experimental results and literature data (effects of stirrer speed, supersaturation, impeller materials, impurities and system size) for laboratory crystallizers were reasonably but qualitatively explained in terms of secondary nucleation caused by crystal-impeller collisions. It was concluded that nucleation data obtained in a laboratory, using a small crystallizer, would be difficult to apply to industrial crystallizers because crystal-impeller collisions are unlikely to occur in large-scale industrial systems.