Dehydration behavior of FGD gypsum by simultaneous TG and DSC analysis

The dehydration behaviors of FGD gypsums from three power plants were investigated at N-2 atmosphere (autogenous and negligible partial pressure of water, P-H2o) in non-isothermal and isothermal condition. The dehydration of gypsum proceeded through one step, i.e., CaSO4 center dot 2H(2)O -> gamma-CaSO4 (gamma-anhydrite) or two steps, i.e., CaSO4 center dot 2H(2)O -> CaSO4 center dot 0.5H(2)O (hemihydrate) -> gamma-CaSO4 depending on temperature and P-H2O. The discrepancies of three FGD gypsums on dehydration behavior were very likely due to the different crystalline characteristics (size and habit) and impurities, such as fly ash and limestone. Experimental data of non-isothermal analysis have been fitted with two 'model-free' kinetic methods and those of isothermal analysis have been fitted with Avrami and linear equation. The apparent empirical activation energies (E (a)) suggest that the transition from gypsum to hemihydrate is mainly controlled by nucleation and growth mechanism, while the transition from gypsum to gamma-anhydrite is mostly followed by phase boundary mechanism.