Batch Cooling Crystallization of Potassium Sulphate from Water Solution

Batch cooling crystallization, at the rotation speed of 700 min(-1), of an aqueous solution of a potassium sulphate has been investigated on a laboratory scale. The effect of hydrodynamics conditions on the crystallization process were investigated by using different type of impellers. Two types of impellers were investigated; the four-pitched blade impeller which generates axi-al flow and the six-blades Rusthon turbine which generates radial flow. The experiments were performed at four different linear cooling rates in the range from 8-20 degrees C h(-1) for both types of impeller. The influence of the cooling rates on the metastable zone width, the crystallization kinetics and the granulometric properties of the obtained crystals were investigated. The experimental data show that higher cooling rate expands the metastable zone for all the types of impeller (Fig. 2) and influences the crystal size distribution (Fig. 7 and Fig. 8). At low cooling rates, supersaturation was kept at a constant value for a longer period. It resulted in improved conditions for mass transfer and the crystals grew. Bigger crystals were obtained at lower cooling rates (Fig. 7). It is stated that radial flow (Rusthon turbine) is particularly inappropriate for the nucleation process, and for crystallization. Nucleation started at a lower temperature and higher supersaturation (Fig. 3). These conditions resulted in a high nucleation's rate and large number of nucleation centres. Also, the obtained crystals settled on the wall of the reactor, baffles and stirrer. A great part of the obtained crystals was agglomerated. The nucleation order, n and coefficient of nucleation, k(n) were determined for different cooling rates (Fig. 5a). The nucleation order is higher at radial flow (nucleation started at higher supersaturation). The relation between the rate of concentration drop in a solution and supersaturation has been approximated with a power low equation (Fig. 5b). For the used impellers, reaction orders do not change much for different cooling rates, except at the greatest cooling rates that results in the smallest crystals (Tab. 1).