Transformation of Flue-Gas-Desulfurization Gypsum to (α-Hemihydrated Gypsum in Salt Solution at Atmospheric Pressure

Direct phase transformation of flue gas desulfurization gypsum in hot salt solution at atmospheric pressure was investigated. The effects of temperature, salt species, salt concentration, solids content, pH and modifier were examined. The crystals obtained under different conditions and solubility of calcium sulfate in contact with solid gypsum were also determined. α-Calcium sulfate hemihydrate crystals of stubby columnar shape and regular pentahedral sides were obtained under the following conditions: salt concentration 20%–30%, operation temperature 95–100 °C, solids mass content in the slurry 10%–30% and neutral pH. Thermodynamic analysis revealed that phase transformation of calcium sulfate dihydrate to α-calcium sulfate hemihydrate occurs because of the difference in solubilities between the two solid gypsum phases in this system.