Semi-batch evaporative crystallization and drying of cobalt sulphate hydrates

Semi-batch evaporative crystallization of CoSO₄ was investigated at various underpressures and with different heating powers. The underpressures for boiling aqueous CoSO₄ solutions at various temperatures and concentrations were measured and compared to predicted values obtained by the Aspen Plus Version 11 process simulation software. The presence of CoSO₄ solute decreased the underpressure for boiling solution in comparison to pure water. In the evaporative crystallization experiments, CoSO₄ crystallized as CoSO₄.7H₂O at evaporation temperatures of 30 °C and 40 °C, whereas CoSO₄.6H₂O crystallized at 60 °C and 80 °C. The crystals obtained at 30 °C and 40 °C were mainly agglomerates of octahedron shaped particles. Heating power and mixing speed had significant effects on the evaporation flux and particle size distribution of the products. Smaller crystals were produced with higher heating power and higher mixing speed due to the enhancement of evaporation. Dehydration of the formed cobalt sulfate heptahydrate in the drying process was also investigated with two variables: drying temperature and drying time. The crystallization at 40 °C yielded unstable CoSO₄.7H₂O, which tended to dehydrate in a temperature range between 23 °C and 60 °C; these crystals were used in the study of crystal drying. At a drying temperature of 23 °C, CoSO₄.7H₂O transferred and stabilized as CoSO₄.6H₂O. At drying temperatures of 40 °C and 60 °C, the stable hydrate form was CoSO₄.H₂O. Gravimetric analysis, crystal habit, XRD, and Raman analyses were successfully utilized for the investigation of the transformation of the hydrate form during drying.