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

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.