Retarding effect of impurities on the transformation kinetics of FGD gypsum to α-calcium sulfate hemihydrate under atmospheric and hydrothermal conditions

Abstract The influence of impurities in FGD gypsum (>90% CaSO 4 ·2H 2 O) on the kinetics of its transformation to α-calcium sulfate hemihydrate (α-HH, CaSO 4 ·0.5H 2 O) has been seldom investigated. This work demonstrates that CaCO 3 and CaSO 3 in FGD gypsum retard the transformation kinetics under hydrothermal conditions at atmospheric pressure, while SiO 2 produces little effect. The induction time elapsed to initiate the transformation is significantly prolonged mainly because of CaSO 3 , which origins from its ability to decrease the driving force and simultaneously block α-HH nucleation sites on gypsum crystals. Transformation of FGD gypsum to α-HH can be significantly boosted by washing away the CaSO 3 . Enlarging the driving force via regulating temperature, salt solution concentration and acidity can also accelerate the transformation. The results indicate FGD process optimization, especially by a complete oxidation of CaSO 3 , would advance the utilization of FGD gypsum for α-HH production.