Melt Crystallization - Inclusion Kinetics of Mineral Ions during Ice Formation
Crystallization technology is one of the powerful methods to produce pure materials in solid state. Crystal purity fundamentally depends on the phase equilibrium of materials. However, impurity in mother liquor is frequently incorporated in crystal because crystallization occurs under supersaturated condition, that is, non-equilibrium condition. In this study an extended work will be reported on ice formation from water including the mineral ions (K+, Na+, Mg+2 and Ca+2). The inclusion kinetics of mineral ions in ice layer was studied experimentally and theoretically. We formed an ice layer on a rotating cold cylinder from the mineral water in an annular cylindrical vessel. We controlled the growth rate of ice layer by the rotation speed and the temperature of the cold cylinder. The shape of the ice layer formed on the rotating cold cylinder depended on the flow structure of Taylor vortices in the water region. Effective distribution coefficients of mineral components were obtained from the measurements of concentrations of mineral species in ice. The distribution coefficient of mineral ions was correlated well with the growth rate of ice. Pure ice without mineral inclusion was formed at the growth rate lower than critical growth rate. These measurements validated the theory of interfacial solute distribution proposed by the present authors [Chem. Eng. Sci. 57, 3133 (2002)] that was useful to predict the crystal purity during melt crystallization.