Study on the recovery of purified MgSO_4centre dot7H_2O crystals from industrial solution by eutectic freezing

摘要

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.MgSO_4centre dot12H_2O was formed in the crystallizer and the recrystallization into magnesium sulphate heptahydrate(MgSO_4centre dot7H_2O)occurred spontaneously during the subsequent filtration and drying above 0.4 deg 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 mu m.The MgSO_4centre dot7H_2O crystals were rather needle-like with an average size of 275 mu m.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.