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

- This paper confirms the retarding effect of the impurities in the FGD gypsum that inhibits the transformation to α-HH.
- CaSO3 proves to be the main retarding factor in comparison with CaCO3.
- CaSO3 impedes the transformation by depressing the driving force and probable occupation of the α-HH nucleation sites.
- Several possible solutions are proposed to overcome the retarding effect.

The influence of impurities in FGD gypsum (>90% CaSO4·2H2O) on the kinetics of its transformation to α-calcium sulfate hemihydrate (α-HH, CaSO4·0.5H2O) has been seldom investigated. This work demonstrates that CaCO3 and CaSO3 in FGD gypsum retard the transformation kinetics under hydrothermal conditions at atmospheric pressure, while SiO2 produces little effect. The induction time elapsed to initiate the transformation is significantly prolonged mainly because of CaSO3, 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 CaSO3. 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 CaSO3, would advance the utilization of FGD gypsum for α-HH production.