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Microscopic observations snd Coulter counter measurements of the secondary nuclei produced when a parent crystal of potash alum was contacted with a solid rod suggest that these nuclei are born by an attrition mechanism which appears to be dependent on the growth rilte of the crystal. The nuclei can be up to 50 pm in size. The nuclei smaller than about 4 pm are relatively insensitive to supersaturation, whereas the number greater than about 4 pm is approximately linear in supersaturation. JOHN SCOPEAlthough secondary nucleation appears to be the major source of nuclei in industrial crystallizers, little is known about the mechanism by which such nuclei are produced. Scale-up of secondary nucleation kinetics is therefore difpopulation balance techniques to crystallizer design. Most have been obtained from observations on comparatively large crystals, nucleation rates then being inferred from the number of these crystals, Results of recent studies showing anomalous growth behavior of small crystals imply that secondary nucleation rates measured in this way may seriously underestimate the true kinetics,In the study reported here, the production of secondary ficdt and is a major of the published data on secondary nucleation kinetics to the further Of nuclei as a result of contacting a solid rod with a crystal surface is Observed using Optical microscopy* In addition, the size distribution of secondary nuclei, within a few seconds of being Produced from a Parent crystal, is measured by using a Coulter counter. CONCLUSIONS AND SIGNIFICANCEBoth the microscopic observations and the Coulter counter measurements indicate that secondary nuclei of potash alum are produced directly into size ranges up to 50 pm, although the majority are less than about 20 pm. The nuclei appear to be formed by an attrition mechanism in which particles are detached from the growing crystal surface* The number Of produced depends On the supersaturation. Although the number and distribution of sizes less than about 4 pm is relatively independent of supersaturation, the total number greater than 4 pm increases in an approximate linear manner with supersaturation, and more larger particles are produced at higher supersaturations.Because of the ease with which nuclei appear to be produced by attrition, it is likely that nucleation rates in industrial crystallizers are dominated by such a mechanism. Successful scale-up, therefore, requires detailed knowledge of the relation between frequency and energy of crystal collisions and of the hydrodynamics of crystallizers and information describing the variation of nuclei production rate with energy of contact.The process of secondary nucleation appears to be the major source of nuclei in industrial crystallizers. It is thus apparent that application of mathematical models to crystallizer design requires a knowledge of the wa lizers operating parameters. Over recent years, many studies have been directed at understanding and explaining the phenomenon of secondary nucleation and measuring kinetic data. In s...

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