氟硅酸钾(钠)在湿法磷酸中的结晶动力学研究

Crystallization Kinetics of Potassium(Sodium) Fluosilicate in Wet-Process Phosphoric Acid

在MSMPR结晶器中系统地研究了氟硅酸钾(钠)在湿法磷酸中的结晶动力学及主要杂质F -、Mg2+、Fe3+、Al3+ 的影响。结果表明，在纯硫磷混酸中，Na2SiF6为八面体晶体、K2SiF6为由立方体晶体聚在一起的聚晶；SO42-、Mg2+ 对氟硅酸钾(钠)晶体生长有促进作用，F - 的影响不明显，Fe3+、Al3+ 能抑制其生长; 四种杂质共存的模拟湿法磷酸及磷矿萃取磷酸中氟硅酸钾(钠)晶体生长速率、平均粒径均明显减小，形态发生很大变化。

Potassium (sodium) fluosilicate scaling on the filtration system is the main operating problem in wet-process phosphoric acid (WPA) production from phosphate ore with high potassium and sodium content. Knowledge of potassium (sodium) fluosilicate crystallization and the effect of impurities in phosphate ore would considerably be helpful in predicting and minimizing scaling. In this paper, the crystallization kinetics of Na2SiF6 and K2SiF6 in WPA, and the effects of F-, Mg2+, Fe3+ and Al3+ on crystallization were studied in a MSMPR crystallizer, and the possible crystallization mechanisms were discussed. It is found that Na2SiF6 forms hexagonal crystals and K2SiF6 forms cluster aggregated by cubic crystals in the pure mixed acid solution. Mg2+ and SO42- promote whereas Fe3+ and Al3+ inhibit the crystal growth, and F- has little effect. The synergistic effect of impurities in phosphate ore appears to inhibit the crystal growth process, and Mg2+, Fe3+ and Al3+ are the major impurities which influence crystallization. It is suggested that potassium (sodium) fluosilicate scale could be inhibited by addition of soluble iron and aluminum compounds. In the real WPA, the shapes of Na2SiF6 and K2SiF6 change greatly and nearly form fine plate, grain-shaped crystals, which would form hard deposits in filter circuits during filtration of gypsum slurry.