Research On Stable Equilibria In Quinary System Na⁺,NH₄⁺//Cl^-,H₂PO₄^-,SO₄^2--H₂O And Its Sub-system

NaH2PO4 is one of the most important products in the phosphate industry,which is widely used in the fields of plating,boiled water treatment,and the tanning fields.Solvent extraction technology of industrial sodium dihydrogen phosphate is currently a promising technology.In order to optimize the preparation process of sodium dihydrogen phosphate by solvent extraction,the phase equilibria of the subsystems Na+,NH4+//Cl-,SO42-,H2PO4--H2O have been studied.Phase equilibria and phase diagrams are the theoretic foundation for the exploitation of brine resources.Studies on phase diagrams of the solid solution-containing salt-water systems are of great importance for the development of brine systems.The stable phase equilibria and physical property parameters(refractive index,density and viscosity)of the Na+,NH4+//Cl-,H2PO4-,SO42--H2O quinary system's subsystem including two quaternary system and four ternary system.Na+//Cl-,SO42-,H2PO4--H2O have been studied by the method of isothermal solution saturation.Corresponding phase diagram,physical property parameters and water diagram of NH4+//Cl-,H2PO4-,SO42--H2O?Na+,NH4+//Cl-,SO42--H2O?NH4+//Cl-,H2PO4--H2O?NH4+//Cl-,SO42--H2O?Na+,NH4+//Cl--H2O and Na+,NH4+//SO42--H2O were plotted,respectively.All results obtained in this work can be used for the stripping process of solvent extraction in the production of NaH2PO4 and provide SLE data for multistage countercurrent crystallization process.Neither complex salt nor solid solution is found in quaternary system NH4+//Cl-,H2PO4-,SO42--H2O and Its subsystem NH4+//Cl-,H2PO4--H2O and NH4+//Cl-,SO42--H20 at 313.15 K.The ternary phase diagram of NH4+//Cl-,H2PO4--H2O contains one invariant point,two uninvariant curves and three crystallization regions(NH4H2PO4,NH4Cl,and(NH4Cl+NH4H2PO4)).The ternary phase diagram of NH4+//Cl-,SO42-H2O is similar to the previous one,also contains one invariant point,two uninvariant curves and three crystallization regions((NH4)2SO4,NH4Cl,and(NH4Cl +(NH4)2SO4)).The quaternary phase diagram of NH4+//"Cl-,SO42-,H2PO4--H2O contains one quaternary invariant point,three uninvariant curves and three crystallization regions((NH4)2SO4,NH4Cl,and NH4H2PO4),crystallization field of NH4H2PO4 is the largest one of them.The translation method of determining phase equilibria and construction of phase diagrams for multicomponent systems was applied to investigate the phase equilibria of quaternary system NH4+//Cl-,SO42-,H2PO4C-H2O.Comparison of experimental phase diagrams and predicted phase diagram shows agreement.Studies on phase diagrams of the solid solution-containing salt-water systems are of great importance for the development of brine systems.313.15 K,containing solid solution of quaternary system Na+,NH4+//Cl-,SO42--H2O and Its subsystem Na+,NH4+//Cl--H2O and Na+,NH4+//SO42--H2O all belong to the complex system?The ternary phase diagram of Na+,NH4+//Cl--H2O consists of three invariant points,four monovariant curves,and five solid phase crystallization regions and is also a type of complex with three solid solutions Nax(NH4)yCl at 313.15K.The ternary phase diagram of Na+,NH4+//SO42*-H2O is similar to the previous one,is a type of complex with an congruent type complex salt Na2SO4(NH4)2SO4-4H2O and solid solutions Na2-x(NH4)xSO4 formed at 313.15K,which contains four invariant points and five crystallization regions.The quaternary system Na+,NH4+/Cl-,SO42--H2O phase diagram consists of three invariant points,seven monovariant curves and five crystallization regions,which contains(NH4,Na)2SO4 +(Na,NH4)2SO4+(Na,NH4)Cl+(NH4,Na)Cl solid solutions and an an congruent type complex salt Na2SO4·(NH4)2SO4·4H2O.Based on the application of phase diagram,the isothermal evaporation analysis and salt-out process of ternary system NH4+//Cl-,H2PO4--H2O phase diagram and quaternary system NH4+//Cl-,H2PO4-,SO42--H2O have been carried out,which offers the theoretical foundation for the crystallization process of NaH2PO4.Due to the crystallization area of the solid solution Nax(NH4)yCl and Na2-x(NH4)xSO4 is about 80%in Na+,NH4+//Cl-,SO42--H2O and Its subsystem Na+,NH4+//Cl--H2O,Na+,NH4+//SO42--H2O.Therefore,the multistage countercurrent crystallization separation model is proposed to separate Na-NH4 solid solution.The results of this paper not only fill the gap of solubility data of Na+,NH4+//Cl-,H2PO4-,SO42--H2O system,but also guide the crystallization processes of NaH2PO4 and by-products NH4Cl,NH4H2PO4.