Preparation of α-calcium sulfate hemihydrate from FGD gypsum in K, Mg-containing concentrated CaCl2 solution under mild conditions

To prepare α-calcium sulfate hemihydrate (α-HH) from flue gas desulfurization (FGD) byproduct under atmospheric pressure, the dehydration of FGD gypsum in a concentrated Ca and Mg chlorides solution medium at 95 °C was investigated, and the effects of K+ ions were focused on. The crystal water content variation, material phases, and morphological characteristics of the dehydration products were examined and compared. The solubilities of α-HH in the mixed salt solutions with different K+ ion concentrations were also determined. It was found that with low concentration of K+ ions in the mixed salt solution (i.e. 0.001–0.035 M), the dehydration rate was enhanced, while with higher K+ ion concentration (i.e. 0.087–0.263 M), this process was retarded. α-HH was the dehydration product when K+ ion concentration was at appropriate level, while anhydrite (AH) occurred when K+ ion concentration was above 0.173 M. K+ ions were assumed to behave as a capping agent to control the morphology of the α-HH, resulting in crystals of lower aspect ratio. FGD gypsum can be successfully transformed into α-HH in concentrated Ca and Mg chloride solution under mild conditions, and K+ ions play an important role in the process.