Mixing determines the environment in which crystals nucleate and grow and is therefore intrinsic to industrial crystallization. Individual nucleating and growing crystals respond directly to their microenvironment and not in a simple way to the macroenvironment, often thought of as the bulk or average environment. Because the growing crystal removes solute from solution and the dissolving crystal releases it, the solute concentration and therefore the supersaturation is in general different at the crystal surface than in the bulk. Crystals grow when the microenvironment is supersaturated, stop when it is just saturated, and dissolve when it is undersaturated. In most cases, impurities are rejected by growing crystals; therefore, each growing crystal face creates a zone of locally higher impurity concentration immediately adjacent to it. The growth rate and amount of impurity taken up by the growing crystal are functions of the impurity concentration where growth is occurring – at the crystal face itself. Mixing is the family of processes that links this local microenvironment to the macroscopic scale of the crystallizer by affecting the mass transfer between crystal and the larger environment and the dynamics of crystal suspension flow in the crystallizer. Mixing, therefore, to a large extent creates the crystal microenvironments. Furthermore, it determines the homogeneity of the macroenvironment, both temporally and spatially. Inhomogeneity in the macroenvironment affects the microenvironments around crystals, causing temporal variations as the crystals circulate from one zone to another inside the crystallizer. This is particularly important because local values of key variables such as supersaturation and solids concentration are often much more important in crystallization than the bulk or global averages of these quantities, as discussed below.
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