镁盐沉淀法从钨酸盐溶液除磷的热力学研究

通过计算，绘制了25℃时Mg2＋-3-4PO -＋4NH -H2O体系的热力学平衡图，并对钨酸盐溶液除磷过程进行系统的热力学研究。结果表明：当采用磷酸镁盐法除磷时，溶液中游离总镁浓度从0.01 mol/L增加到1.0 mol/L，对应的最佳理论除磷pH值从9.8降到8.8，而溶液中残留的总磷含量基本维持在4.0×10^-6 mol/L；随着溶液pH值的升高，体系中稳定存在的沉淀组分依次为 MgHPO4、Mg3（PO4）2和 Mg3（PO4）2＋Mg（OH）2。当采用磷酸铵镁法除磷时，增大溶液游离总氨浓度有利于除磷，而增大游离总镁量对除磷深度基本无影响。计算所得除磷的最佳 pH值为9-10；当游离总氨浓度为5.0 mol/L时，溶液中残留的总磷为1.4×10^-7 mol/L。以自制的钨酸铵溶液（WO350 g/L，P 13 g/L）为原料，采用磷酸铵镁盐法除磷以对理论分析进行验证。结果表明：当氯化镁的加入量为理论量的1.1时，除磷的最佳pH值为9.5，与热力学分析结果一致。

Thermodynamic study on phosphorus removal from tungstate solution via magnesium salt precipitation method

The thermodynamic equilibrium diagrams of Mg2＋- 3-4PO - ＋4NH -H2O system at 298 K were established based on the thermodynamic calculation. From the diagram, the thermodynamic conditions for removing phosphorus from the tungstate solution by magnesium salt precipitation were obtained. The results show that when the concentration of total magnesium increases from 0.01 mol/L to 1.0 mol/L, the optimal pH for the phosphorus removal by magnesium phosphate decreases from 9.8 to 8.8. The residual concentration of total phosphorus almost keeps the level of 4.0×10-6 mol/L in the system. MgHPO4, Mg3（PO4）2 and the mixture of Mg3（PO4）2 and Mg（OH）2 are stabilized in these system, respectively. However, increasing the total concentration of magnesium has little effect on phosphorus removal by magnesium ammonium phosphate, while it is helpful for phosphorus removal by increasing the total ammonia concentration. The calculated results demonstrate that the residual concentration of total phosphorus can decrease to 5.0×10-7 mol/L as the total concentration of ammonia reaches 5.0 mol/L and the optimal pH value is 9-10. Finally, verification experiments were conducted with home-made ammonium tungstate solution containing 50 g/L WO3 and 13 g/L P. The results show that when the dosage of MgCl2 is 1.1 times of the theoretical amount, the optimum pH for removing phosphorus is 9.5, which matches with the results of the theoretical calculation exactly.