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

In order to optimize the crystallization process of KH2PO4, the paper measure the solid-liquid phase equilibrium data and physico-chemical property(nD, ?, p, pH) of liquid-phase of the quinary system's subsystem with the method of isothermal dissolution. The paper researchs the solid-liquid equilibrium of the quinary system's subsystem including three quaternary systems and four ternary systems at 298.15 K and 313.15 K. Corresponding phase diagram, physic-chemical properties and water diagram are plotted based on the phase equilibrium data and physical-chemical data.With the knowledge of application of phase diagram, the paper optimizes the crystallization process of KH2PO4 with solvent extraction method with the basis of the phase diagram of K+//Cl-, SO42-, H2PO4- - H2O, Na+,K+//SO42-, H2PO4--H2O, ternary system Na+, K+//SO42- - H2O at 298.15 K, 313.15 K.The researched results show that the simple ternary system K+// Cl-,H2PO4- -H2O?K+//Cl-,SO42- -H2O?K+// SO42-, H2PO4- - H2O include one invariant point,two isothermal univariant curves and two crystallization zones. No double salt and crystalline hydrate are exiated at these systems. When the temperature increases from 298.15 K to 313.15 K, invariant point will move up and crystallization zones gradually decrease with the increasing trend of saturated crystalline region. The salt with big crystallization area would crystallize out first from mixture, while the salt with small crystallization area would crystallize out finally. The same ion effect can be explained the rule of the sobility different salt solubility and crystallization zones. The phase diagram of the ternary system Na+,K+//SO42--H2O at 313.15 K includes two invariant points, three invariant curves and three fields of crystallization of single salt corresponding to K2SO4, Na2SO4·3K2SO4, Na2SO4. The two invariant points are cosaturated with two salts,that is (K2SO4+ Na2SO4.3K2SO4) and (Na2SO4 + Na2SO4·3K2SO4) with the analyzing of XRD. The scope of areas of crystallization of salts is such that Na2SO4·3K2SO4 > K2SO4> Na2SO4.Physico-chemical properties of phase equilibrium solution vary regularly with the composition of salt and it shows big value at invariant point.The quaternary system can be divided into two types of simple and interaction. The simple quaternary system K+//Cl-,SO42-, H2PO4- -H2O includes one invariant point, three isothermal univariant curves and two crystallization zones. The crystallized zone of KCl is the smallest than KH2PO4 and K2SO4, which means it's difficult to crystallize out from mixture. Quaternary system Na+, K+// SO42-,H2PO4--H2O at 298.15 K includes four invariant points, six crystallization zones and nine isothermal univariant curves. When the temperature increases into 313.15 K, the phase diagram involves three invariant points, five crystallization zones and seven isothermal univariant curves. It mainly because the crystallized zone of Na2SO4·10H2O has totaly changes into Na2SO4. Quaternary system Na+,K+//CI-,SO42--H2O at 313.15 K has three invariant points,five crystallization zones and seven isothermal univariant curves. The area of the crystallization is showed that KCl < NaCl < [(Na2SO4·10H2O ) <Na2SO4] < K2SO4 < Gla. All the physical properties of the equilibrium quaternary system change regularly with concentration change of J(SO42-).Based on the application of phase diagram, the paper designs a theoretically crystallization purification process of KH2PO4 containing Na+,K+,Cl-,SO42-, H2PO4-. By comparing the phase diagram, 298.15 K is determined as the temperature of the mixture separation. At the same time, the paper investigates aseparation method with the mixture of a large number of KCl (KH2PO4) and a small amount of K2SO4. At the method, KH2PO4 is used as salting-out agent at 323.15 K. Changing the composition of mother solution until KCl totally crystallize out. And then, the solution is isothermal evaporated to separate the mixture of KH2PO4, KCl and K2SO4. Finally, analyzing the process of isothermal evaporation of the phase diagram of Na+, K+//S042- - H2Otemary system, which offers the theoretical foundation for the crystallization purification process of Na2SO4 or K2S04.All the results obtained can be used to describe the solubility of every salt and the law of salt out. It not only can be used to supplemented the vacancy of solubility data, but also guide the crystallization process research of KH2PO4 and the related thermodynamics model of development and utilization.