Research On Phase Equilibrium Of Na⁺,Mg²⁺,NH₄⁺//Cl^-,SO₄^2--H₂O System And Its Application

Magnesium and its compounds are widely used in many fields,such as the pharmaceutical industry,machinery industry,chemical industry,building materials,food,transportation,agriculture and aerospace.There is a large amount of soluble magnesium in salt lake,and effective recovery of it can not only solve the environmental pollution but also recycle the resources.Magnesium and ammonium are also essential to plant growth and are commonly found in fertilizers.And studies show that compound fertilizer has a higher effect on plant growth than single fertilizer.Both the "magnesium damage" problem of salt lake and the production problem of compound fertilizer are based on the phase equilibrium of water and salt systems Na+,Mg2+,NH4+//Cl-,SO42--H2O.Therefore,it is of great significance to study the solid-liquid equilibrium of the water-salt system of quinary system Na+,Mg2+,NH4+//Cl-,SO42--H2O.In this paper,the phase equilibrium data of four ternary subsystems and two quaternary subsystems in the quinary water-salt system Na+,Mg2+,NH4+//Cl-,SO42--H2O have been determined by means of isothermal solution equilibrium method and wet slag method at 303.15 K.At the same time,the physical parameters(p,?)of the equilibrium liquid phase of the system have been determined.And draw the phase diagram and physical property-composition diagram of each system.The experimental results show that the systems of Na+,NH4+//Cl--H2O and Na+,Mg2+//Cl--H2O all have a total saturation point,two monovariational curves and three crystallization regions,respectively.The ternary systems Mg2+,NH4+//Cl--H2O have double salt,two saturation points,three monovariant curves and five crystallization regions.For the quaternary systems Na+,Mg2+,NH4+//Cl--H2O,there are two saturation points and four crystallization regions,namely three pure matter crystallization regions(NH4Cl?MgCl2·6H2O?NaCl)and one double salt crystallization region(MgCl2·NH4Cl·6H2O).For the interaction quaternary systems Mg2+,NH4+//Cl-,SO42--H2O,It contains seven total saturation points and nine crystallization regions(NH4Cl),MgCl2·NH4Cl·6H2O,MgCl2·6H2O,MgSO4·(NH4)2SO4·6H2O,MgSO4·7H2O,MgSO4·6H2O,MgSO4·5H2O,MgSO4·4H2O,(NH4)2SO4).On the basis of obtaining the phase equilibrium data,the "translation method" is used to predict the phase diagram of the multicomponent system.The phase diagrams of the quinary system of Na+,Mg2+,NH4+//Cl-,SO42-H2O were obtained.The system contains ten total saturation points and thirty-three univariate curves.In addition,the phase diagrams of the 303.15K quaternary systems Na+,Mg2+,NH4+//Cl--H2O and Mg2+,NH4+//Cl-,SO42--H2O were analyzed by variable temperature crystallization method.The process flow of making ammonium carnallite(MgCl2·NH4Cl·6H2O)and the new process of producing N-Mg compound fertilizer and ammonium chloride with MgCl2·6H2O and(NH4)2SO4 have been developed,which provides the basis for practical application.In this paper,the solubility and salting out law of the substances in the quinary system of Na+,Mg2+,NH4+/Cl-,SO42--H2O were revealed through the study and prediction of the quinary system,which provided a theoretical basis for the preparation of ammonium carnallite(MgCl2·NH4Cl·6H2O)and the production of N-Mg compound fertilizer.