Preparation of α-High-Strength Hemihydrate from Flue Gas Desulfurization Gypsum in AlCl3–MgCl2 Solution at Atmospheric Pressure

Abstract

An industrially feasible process was proposed to produce α-calcium sulfate hemihydrate (α-HH) from FGD gypsum without any additives. In this process, an AlCl3 solution was used to promote the conversion of calcium sulfate dihydrate (DH) to hexagonal prismatic α-HH with high strength at atmospheric pressure. The phase transition kinetics of FGD gypsum was experimentally investigated under conditions: AlCl3 (2–2.5 m), MgCl2 (0–0.4 m) existing in raw FGD gypsum, and the solid–liquid ratio (or solid content %) range of 0.1–2 at 363 K. It was found that increasing the concentrations of MgCl2 in AlCl3 solution accelerated the conversion. The uniform large hexagonal prism HH product was obtained at the folowing optimal conditions: 2.5 mol/kg AlCl3, 0.4 mol/kg MgCl2, solid–liquid ratio of 2, and equilibration time of 2 h at 363 K. The phase diagram of the AlCl3-MgCl2-H2O system was determined by measuring solubility of solids. The E-NRTL activity model was used to construct the phase transition diagram of CaSO4 in AlCl3–MgCl2 media and to judge the stable range of DH and HH.