Characterization and precipitation mechanism of α-calcium sulfate hemihydrate growing out of FGD gypsum in salt solution

Abstract

Alpha-calcium sulfate hemihydrate (α-HH) has been prepared from flue gas desulfurization (FGD) gypsum with salt solution method under atmospheric pressure. X-ray diffraction (XRD), thermogravimetry and differential scanning calorimetry (TG-DSC), optical micrograph, X-ray photoelectron spectroscopy (XPS), energy dispersive spectrometry (EDS), and scanning electron microscopy (SEM) have been employed to characterize the α-HH crystals, based on which the formation and growth mechanisms of the α-HH crystals have been discussed. The results show that FGD gypsum can be successfully transformed into high purity α-HH in salt solution under mild conditions, and that a dissolution-recrystallization route is most probably adopted by this transition. The growth of α-HH crystals in salt solution demonstrates a preferred direction along [001] and results in a bundle-of-sheets or bundle-of-raphide texture. The characteristics revealed in this study can help to understand and control the growth of the α-HH crystal from solution.