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 α-calcium sulfate hemihydrate were studied under simulated dihydrate–hemihydrate two-step wet-process phosphoric acid, and Fe 3+, Al 3+, Mg 2+, SiF 6 2–, and mixed impurities were introduced to explore their influence on the values of kinetic parameters. The properties of the dehydration conversion product α-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 α-calcium sulfate hemihydrate crystals. Results showed that the dehydration process of calcium sulfate dihydrate to α-calcium sulfate hemihydrate follows the dispersive kinetic model in the wet-process phosphoric acid solution. After Fe 3+, Al 3+, Mg 2+, SiF 6 2–, and mixed impurities were introduced into the system, the activation enthalpy increased, and the activation entropy decreased due to the reduction of the kinetic parameters α and β, causing the nucleation barrier of α-calcium sulfate hemihydrate crystals to increase, thereby inhibiting the dehydration and transformation of calcium sulfate dihydrate to α-calcium sulfate hemihydrate. The order of inhibition is mixed impurities > Al 3+ > SiF 6 2– > Fe 3+ > Mg 2+ . Fe 3+, Al 3+, and Mg 2+ impurities were combined with phosphate ions to form phosphate precipitates, which were selectively covered on different crystal surfaces of α-calcium sulfate hemihydrate, thereby hindering the growth of α-calcium sulfate hemihydrate crystals. Several Ca 2+ ions existed on the (111) crystal plane in the c-axial direction, and SiF 6 2– ions were combined with Ca 2+ ions, thereby hindering the one-dimensional growth in the c-axial direction. Consequently, the aspect ratio of α-calcium sulfate hemihydrate crystal decreased, and the morphology gradually evolved to a short columnar structure.