An industrially feasible process was proposed to produce α-calcium sulfate hemihydrate (α-HH) from FGD gypsum without any additives. In this process, an AlCl 3 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: AlCl 3 (2−2.5 m), MgCl 2 (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 MgCl 2 in AlCl 3 solution accelerated the conversion. The uniform large hexagonal prism HH product was obtained at the folowing optimal conditions: 2.5 mol/kg AlCl 3 , 0.4 mol/kg MgCl 2 , solid−liquid ratio of 2, and equilibration time of 2 h at 363 K. The phase diagram of the AlCl 3 -MgCl 2 -H 2 O system was determined by measuring solubility of solids. The E-NRTL activity model was used to construct the phase transition diagram of CaSO 4 in AlCl 3 −MgCl 2 media and to judge the stable range of DH and HH.

Lithium extraction from spodumene (LiAlSi2O6) is of great importance to meet the increasing demand for lithium-ion batteries used in electric energy storage. However, conventional processing of spodumene involves enormous consumption of H2SO4 during the neutralization step. We propose a new flowsheet to avoid wasting H2SO4 by adding a cooling crystallization step to crystallize lithium sulfate as Li2SO4·H2O. To advance the process development, the solubilities of Li2SO4·H2O in the system H2SO4–MgSO4–H2O from 283.15 to 353.15 K were measured experimentally via a dynamic method. Chemical modeling for the solubility was carried out by regressing the mixed solvent electrolyte model parameters with the average absolute relative deviation of less than 2%. The newly built model is capable of calculating the solubility of Li2SO4·H2O in aqueous H2SO4–MgSO4 solutions in wide concentration and temperature ranges. Cooling crystallization of Li2SO4·H2O in concentrated H2SO4 was conducted and its feasibility was proved.