F-及SiF2-6对α型半水硫酸钙结晶成核及形貌的影响

本文模拟了半水法湿法磷酸生产过程中α型半水硫酸钙(α-HH)的结晶过程。在30%P₂O₅,反应温度95 ℃,过饱和度S=1.64~2.10条件下,通过浊度仪监测溶液中浊度变化,测定了不同F^-及SiF^2-₆浓度下α-HH结晶诱导时间,采用经典成核理论公式计算了α-HH的临界晶核半径及成核速率,并通过扫描电子显微镜(SEM)、X射线衍射(XRD)、X射线光电子能谱(XPS)表征分析了F^-及SiF^2-₆对α-HH结晶过程的影响。结果表明:随着F^-、SiF^2-₆浓度的升高,α-HH晶体的结晶诱导时间延长,表面能和临界晶核半径都增大,然而成核速率减小。当过饱和度S=1.64时,加入0.06 mol·L^-1 F^-,α-HH结晶诱导时间延长了465 s,成核速率减小到0.403×10²⁹ 晶核数·cm^-3·s^-1,然而,加入0.06 mol·L^-1 SiF^2-₆,α-HH结晶诱导时间延长了710 s,成核速率减小到0.339×10²⁹晶核数·cm^-3·s^-1。SiF^2-₆对α-HH晶体抑制成核作用大于F^-。F^-、SiF^2-₆阻碍了α-HH晶体沿C轴方向生长,使得晶体长径比减小,晶体形貌向短柱状变化。F^-、SiF^2-₆影响了α-HH晶体(200)、(310)、(400)晶面衍射峰强度和结晶度。控制半水法湿法磷酸中F^-及SiF^2-₆浓度水平,可以得到短柱状的α-HH晶体,有利于过滤洗涤。

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

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.