F^(-)及SiF^(2-)_(6)对α型半水硫酸钙结晶成核及形貌的影响

Effect of F^(-)and SiF^(2-)_(6) on the Crystallization Nucleation and Morphology ofα-Hemihydrate Calcium Sulfate

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%P_(2)O_(5),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0.403×10^(29)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0.339×10^(29)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。

In this study,the crystallization process of type-αhemihydrate calcium sulfate(α-HH)in the hemihydrate wet-process phosphoric acid production was simulated.The crystallization induction time ofα-HH was determined by monitoring the turbidity changes in 95℃,supersaturation from 1.64 to 2.10,30%P_(2)O_(5)phosphoric acid solution with different F^(-)and SiF^(2-)_(6)concentration.The critical nucleation radius and nucleation rates ofα-HH were calculated by classical nucleation theory equations,and the effects of F^(-)and SiF^(2-)_(6)on the crystallization process ofα-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 ofα-HH crystals increase,but the nucleation rates ofα-HH crystals decrease.When 0.06 mol·L^(-1)F^(-)is introduced in the system at the supersaturation 1.64,the crystallization induction time ofα-HH crystals is prolonged by 465 s and the nucleation rate is reduced to 0.403×10^(29)crystal nuclei·cm^(-3)·s^(-1).However,in the presence of 0.06 mol·L^(-1)SiF^(2-)_(6),the crystallization induction time ofα-HH crystals is prolonged by 710 s,and the nucleation rate is reduced to 0.339×10^(29)crystal nuclei·cm^(-3)·s^(-1).The effect of SiF^(2-)_(6)on nucleation inhibition ofα-HH crystals is greater than that of F^(-).The F^(-)and SiF^(2-)_(6)hinder the growth ofα-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 ofα-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 columnarα-HH crystals,which are favorable for filtration and washing.