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Growth kinetics of nickel sulfate α-hexahydrate crystals in a fluidized bed Phillips, Victor Roger


Crystal growth rates determined under controlled conditions can be used to test theories of the mechanism of crystallization, while dissolution rates determined under comparable conditions allow an attempt to assess the role of mass transfer in the overall growth process. Laboratory-scale studies of fluidized-bed crystallization are also of interest as aids to the design of full-scale crystallizers of the advantageous suspended-bed ("Krystal") type. Measurements on crystal growth rates under conditions simulating those in industrial crystallizers, especially "Krystal" crystallizers, are still relatively scarce. In this study, growth and dissolution rates of nickel sulfate α-hexahydrate were measured as functions of the concentration driving force in a laboratory-scale fluidized-bed crystallizer, for the temperature range 35-50°C and the crystal size range 0.5-4.0 mm. The growth of 1 mm crystals at 40°C was measured by two different methods. Dissolution rates at a given temperature and crystal size were first order in the undersaturation (c* - c). Growth rates were about one-quarter of dissolution rates and depended on a higher power (around 1.3) of the supersaturation (c - c*). This power had no significant dependence on crystal size, but decreased significantly as temperature increased. The apparent variation of growth rate itself with crystal size at constant temperature was slight. The nature of the dependence of the growth rate on temperature and on crystal size supports the diffusion theory concept of crystal growth as a two-step process, i.e. mass transfer of solute to the crystal surface, followed by integration into the solid lattice. In the present case, the growth rate appears to be mainly but not wholly controlled by the surface integration step. Making the assumption that the (unknown) rate constant for the mass transfer step of growth can be represented by the (known) rate constant for dissolution, the apparent kinetics of the surface integration step of growth have been extracted from the overall kinetics of growth. For the growth of 1 mm crystals at 40°C, growth rates found by the "Batch Method," in which the void fraction was 0.998, were in agreement with those found by the "Continuous Method" in which the void fraction was 0.80, close to industrial levels. Since other workers have found agreement for other systems between growth rates from the Batch Method and from single crystal tests, the implication is that only single crystal tests need be made to predict fairly closely the growth rate which would prevail in an industrial type fluidized bed. This should considerably simplify the problem of crystallizer design.

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