engleski

Batch crystallization of KCl: effect of stirrer type, mixing and cooling rate on crystal size distribution and crystal habit

The influence of the type of stirrer (turbine and propeller), stirrer speed and the cooling rate on crystallization kinetics of KCl, crystal size distribution and obtained crystal habit, has been investigated on a laboratory scale. At low mixing rates, crystals are not agitated uniformly, so they are exposed to different hydrodynamics conditions. Consequently, wider range of crystal size distribution is obtained. Well mixing of all present crystals (high mixing rates) results with production of bigger crystals with more regular shape and narrower range of crystal sizes. The cooling rate influences strongly the crystallization kinetics. At higher cooling rates crystallization starts earlier, for all investigated stirrer speeds. Consequently, nucleation starts at lower temperature and less stable shapes and smaller crystals are produced. Population density of crystal nuclei and overall linear growth rate are evaluated according to procedure suggested by Stone and Randolph. With regard to the influence of the cooling rate and mixing intensity on the supersaturation, conditions that expands the metastable zone, (fastest cooling rate and lower agitation) results with higher supersaturation and higher linear growth rate. The influence of cooling rate is more intense than the influence of mixing rate. Population density of crystal nuclei is increased for higher intensity of agitation. Different process conditions results with different crystal shapes. Rapid cooling of a solution at low mixing rates causes the preferential growth of a crystal in one particular direction, leading to the formation of needles. For a given intensity of mixing slower controlled rate of cooling results in the production of crystals of regular size (cubic). Quite different results are obtained when crystallization is carried out without mixing. If solution is cooled rapidly large number of needle shape crystal is produced. For slow, natural (at room temperature) cooling large crystals are obtained.

crystal size distribution; crystal habit; crystallization

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