Study on the recovery of purified MgSO₄·7H₂O crystals from industrial solution by eutectic freezing

Eutectic freezing, characterized by a simultaneous crystallization of ice and salt, was applied to recover magnesium sulphate heptahydrate from a magnesium sulphate industrial stream emitted from flue gas desulphurization. Key aspects of the recovery process for further pilot-scale investigation were studied in a laboratory-scale batch crystallizer with focus on discovering the type of the hydrate product and the quality of the ice and salt crystals. MgSO₄·12H₂O was formed in the crystallizer and the recrystallization into magnesium sulphate heptahydrate (MgSO₄·7H₂O) occurred spontaneously during the subsequent filtration and drying above 0.4 °C. The ice and salt crystal products were reasonably pure (only a small amount of Mn was built into the heptahydrate crystals). Ice crystals were disk-shaped with an average size of 300 μm. The MgSO₄·7H₂O crystals were rather needle-like with an average size of 275 μm. Both products were easily filtered. The characteristics of the eutectic freezing process, i.e. the working area and the dynamics of the simultaneous crystallization, are demonstrated experimentally.