The Influence of Impurities on the Dehydration and Conversion Process of Calcium Sulfate Dihydrate to α-Calcium Sulfate Hemihydrate in the Two-Step Wet-Process Phosphoric Acid Production

The dehydration process and kinetics of calcium sulfate dihydrate to a-calcium sulfate hemihydrate were studied under simulated dihydrate-hemihydrate two-step wet-process phosphoric acid, and Fe3+, Al3+, Mg2+, SiF62-, and mixed impurities were introduced to explore their influence on the values of kinetic parameters. The properties of the dehydration conversion product alpha-calcium sulfate hemihydrate were characterized and analyzed by using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric and differential scanning calorimetry to reveal the interaction mechanism between impurities and alpha-calcium sulfate hemihydrate crystals. Results showed that the dehydration process of calcium sulfate dihydrate to alpha-calcium sulfate hemihydrate follows the dispersive kinetic model in the wet-process phosphoric acid solution. After Fe3+, Al3+, Mg2+, SiF62-, and mixed impurities were introduced into the sytem, the activation enthalpy increased, and the activation entropy decreased due to the reduction of the kinetic parameters alpha and beta, causing the nucleation barrier of a-calcium sulfate hemihydrate crystals to increase, thereby inhibiting the dehydration and transformation of calcium sulfate dihydrate to alpha-calcium sulfate hemihydrate. The order of inhibition is mixed impurities > Al3+ > SiF62- > Fe3+ > Mg2+. Fe3+, Al3+, and Mg2+ impurities were combined with phosphate ions to form phosphate precipitates, which were selectively covered on different crystal surfaces of alpha-calcium sulfate hemihydrate, thereby hindering the growth of a-calcium sulfate hemihydrate crystals. Several Ca2+ ions existed on the (111) crystal plane in the c-axial direction, and SiF62- ions were combined with Ca2+ ions, thereby hindering the one-dimensional growth in the c-axial direction. Consequently, the aspect ratio of alpha-calcium sulfate hemihydrate crystal decreased, and the morphology gradually evolved to a short columnar structure.