50℃时 P_2O_5-MgO-NH_3-H_2O 四元体系相平衡研究

Study on the Equilibrium of P_2O_5-MgO-NH_3-H_2O Quarternary System at 50℃

本文测定了 P_2O_5-MgO-NH_3-H_2O 四元体系在50℃时的相平衡数据,绘出了干盐相图.应用Pitzer 电解质溶液理论进行了活度系数关联,计算了所研究体系在50℃时的相平衡数据.

In this paper,the experimental data of solid-liquid phase equilibrium for P_2O_3-MgO-NH_3-H_2O quarternary system at 50℃ have been presented,and the phase diagramhas been drawed too.The phase diagram consists of five regions of crystallization:Mg (H_2PO_4)_2·4H_2O,MgHPO_4·3H_2O,NH_4H_2PO_4,(NH_4)_2HPO_4,MgNH_4PO_4·6H_2O.According to the phase diagram,the crystallization region of MgNH_4PO_4·6H_2O isthe largest.It means that this salt could crystallizes in wide range of compositions.This has a great importance for the production of MgNH_4PO_4·6H_2O.Some legitimate assumptions have been made in this paper when Pitzer's ele-ctrolyte solution theory to the quarternary system was applied.The assumptions arebased on the analysis for the particularities of the system.From Pitzer's theory,the activity coefficient models have been putted forward.These models could be used to the electrolyte and non-electrolyte mixed solutions.Theparameters which appear in the models have been regarded as adjustable whose valueshave been found by fit of the experimental equilibrium data.With these activity coefficient models,the solid-liquid phase equilibrium data ofP_2O_5-MgO-NH_3-H_2O system have been calculated.The resuls show satisfactory agre-ement between the experimental data and the calculation values.