Effect of particle size on the transformation kinetics of flue gas desulfurization gypsum to α-calcium sulfate hemihydrate under hydrothermal conditions

The effect of particle size on the transformation kinetics of flue gas desulfurization (FGD)gypsum to α-calcium sulfate hemihydrate (α-HH)in calcium chloride (CaCl2)solutions was investigated to better guide value-added FGD gypsum use.Gypsum samples from different sources were sieved into several size groups,and their transformation rates were compared.The results showed that using FGD gypsum with a smaller particle size accelerated the transformation to α-HH.The size effect accelerated nucleation kinetics of α-HH rather than its crystal growth rate (that is,the thermodynamic driving force for the transformation changed little with particle size variation).Analysis using a kinetics model revealed that a smaller gypsum particle size lowered the overall activation energy barrier for the transformation.This is because the smaller gypsum particles had a larger relative specific surface area and thus provided more nucleation sites and crystalline defects to promote α-HH nucleation.A smaller particle size of FGD gypsum also gave a higher yield of fine α-HH particles because of the increased incidence of primary and secondary nucleation coupled with attrition.This paper indicates the transformation of FGD gypsum into α-HH could be effectively promoted by regulating FGD gypsum particle size.