HOW TO PREDICT THE METASTABLE ZONE WIDTH

Industrial crystallizers hare to be operated at the optimum supersatur ation which is less than the metastable zone width to obtain a desired product quality, with respect to median particle sizer crystal size d istribution (CSD),shape and purity. Due to the time-consuming experime ntal determination of metastable zone widths, a theoretical model base d on the birth of nuclei on the surface of crystals present in the sol ution and their outgrowth has been developed. In a solution cooled dow n at a constant cooling rate (polythermal method), the surface-born nu clei have to grow to a certain size to be detected by means of a parti cle analysing system or the human eye. Calculations were carried out f or 28 inorganic systems. A comparison with results from the literature showed that the visible shower of nuclei occurred at a volumetric hol d-up of crystals between 10(-4) and 10(-3) and a corresponding size be tween 10 and 100 mu m. A simplified model is presented which lakes onl y into account the growth step.