How to predict the metastable zone width

Industrial crystallizers hare to be operated at the optimum supersaturation which is less than the metastable zone width to obtain a desired product quality, with respect to median particle sizer crystal size distribution (CSD),shape and purity. Due to the time-consuming experimental determination of metastable zone widths, a theoretical model based on the birth of nuclei on the surface of crystals present in the solution and their outgrowth has been developed. In a solution cooled down at a constant cooling rate (polythermal method), the surface-born nuclei have to grow to a certain size to be detected by means of a particle analysing system or the human eye. Calculations were carried out for 28 inorganic systems. A comparison with results from the literature showed that the visible shower of nuclei occurred at a volumetric hold-up of crystals between 10(-4) and 10(-3) and a corresponding size between 10 and 100 mu m. A simplified model is presented which lakes only into account the growth step.