Effect of F- and SiF2-6 on the Crystallization Nucleation and Morphology of α-Hemihydrate Calcium Sulfate

Abstract

Abstract: In this study, the crystallization process of type-α hemihydrate calcium sulfate (α-HH) in the hemihydrate wet-process phosphoric acid production was simulated. The crystallization induction time of α-HH was determined by monitoring the turbidity changes in 95 ℃, supersaturation from 1.64 to 2.10, 30%P₂O₅ phosphoric acid solution with different F^- and SiF^2-₆ concentration. The critical nucleation radius and nucleation rates of α-HH were calculated by classical nucleation theory equations, and the effects of F^- and SiF^2-₆ on the crystallization process of α-HH were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that with the increase of F^- and SiF^2-₆ concentration, the crystallization induction time, the surface energy, and the critical nucleation radius of α-HH crystals increase, but the nucleation rates of α-HH crystals decrease. When 0.06 mol·L^-1 F^- is introduced in the system at the supersaturation 1.64, the crystallization induction time of α-HH crystals is prolonged by 465 s and the nucleation rate is reduced to 0.403×10²⁹ crystal nuclei·cm^-3·s^-1. However, in the presence of 0.06 mol·L^-1 SiF^2-₆, the crystallization induction time of α-HH crystals is prolonged by 710 s, and the nucleation rate is reduced to 0.339×10²⁹ crystal nuclei·cm^-3·s^-1. The effect of SiF^2-₆ on nucleation inhibition of α-HH crystals is greater than that of F^-. The F^- and SiF^2-₆ hinder the growth of α-HH crystals along the C-axis, resulting in a decrease of the crystal aspect ratio and a change of the crystal morphology to short cylinder shape. F^- and SiF^2-₆ affect the intensity and crystallinity of α-HH crystals (200), (310), (400) crystal planes. The concentration levels of F^- and SiF^2-₆ in the semi-aqueous wet phosphoric acid are controlled to obtain short columnar α-HH crystals, which are favorable for filtration and washing.

Key words: type-α hemihydrate calcium sulfate, crystallization induction time, critical nucleation radius, nucleation rate, wet-process phosphoric acid, surface energy

CLC Number:

TQ132.3⁺2

ZHANG Yuanyong, YANG Lin, CAO Jianxin, LIU Rong, ZHANG Yu. Effect of F^- and SiF^2-₆ on the Crystallization Nucleation and Morphology of α-Hemihydrate Calcium Sulfate[J]. Journal of Synthetic Crystals, 2022, 51(8): 1396-1405.

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Effect of F^- and SiF^2-₆ on the Crystallization Nucleation and Morphology of α-Hemihydrate Calcium Sulfate

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