Phase Equilibrium And Supersaturation Of The Interactive Quaternary System K⁺, NH₄⁺//H₂PO₄^-, Cl^--H₂O At Different Temperature

Due to the need of producing multi-component water-soluble fertilizers by co crystallization technology,this paper measured the phase equilibrium data of the interactive quaternary system K⁺,NH₄⁺//H₂PO₄^-,Cl^--H₂O,and its four ternary subsystems NH₄Cl-NH₄H₂PO₄-H₂O,KCl-NH₄Cl-H₂O,KH₂PO₄-NH₄H₂PO₄-H₂O and KH₂PO₄-KCl-H₂O at 303.15 K,323.15 K,333.15 K,and 343.15 K were measured by isothermal dissolution equilibrium method.The corresponding isothermal phase diagrams and water diagrams were drawn.For equilibrium solids,their composition is determined by wet slag method and X-ray diffraction analysis.The crystallization temperature point and metastable zone width of the interactive quaternary system at313.15K to 353.15K were measured by laser method.The solubility data of four ternary systems at 303.15 K,323.15 K,333.15 K,and 343.15 K were calculated using the Pitzer-Harvie-Wear model.The research results show that:（1）The ternary systems KH₂PO₄-KCl-H₂O and NH₄Cl-NH₄H₂PO₄-H₂O all have a co saturation point at experimental temperature,and there are four phase regions（three crystallization regions and one unsaturated region）in the phase diagram.The three crystallization regions of the system KH₂PO₄-KCl-H₂O are the pure material crystallization regions of potassium dihydrogen phosphate and potassium chloride,and the eutectic regions of both.The crystallization region of potassium dihydrogen phosphate is much larger than that of potassium chloride,indicating that potassium chloride has a high solubility in the system and is not easy to precipitate crystals,while potassium dihydrogen phosphate has a low solubility and is easy to crystallize,precipitate,and separate.The three crystallization zones of the system NH₄Cl-NH₄H₂PO₄-H₂O are:ammonium dihydrogen phosphate crystallization zone,ammonium chloride crystallization zone,and the co crystallization zone of ammonium dihydrogen phosphate and ammonium chloride.The crystallization area of ammonium dihydrogen phosphate is larger than that of ammonium chloride,indicating that the solubility of ammonium chloride is higher than that of ammonium dihydrogen phosphate,and ammonium chloride is more prone to precipitation in the mixture.The ternary systems KH₂PO₄-KCl-H₂O and NH₄Cl-NH₄H₂PO₄-H₂O are all simple systems at 283.15K to 353.15K.Under different temperature conditions,only the solubility of each component in the phase diagram of the above system has changed,without affecting the overall structure of the solution phase diagram.（2）The ternary systems KCl-NH₄Cl-H₂O and KH₂PO₄-NH₄H₂PO₄-H₂O formed solid solutions at 303.15 K,323.15 K,333.15 K,and 343.15 K.The phase diagram of the ternary system KCl-NH₄Cl-H₂O has six phase regions（five crystalline regions and one unsaturated region）.The phase diagram of the system includes potassium chloride and ammonium chloride crystallization regions,as well as（K,NH₄）Cl and（NH₄,K）Cl solid solution crystallization regions,indicating that it is a partially miscible system at the above temperatures.The ternary system KH₂PO₄-NH₄H₂PO₄-H₂O has four phase regions（three crystalline regions and one unsaturated region）.The system includes a（K,NH₄）H₂PO₄solid solution crystallization region dominated by potassium dihydrogen phosphate and a（NH₄,K）H₂PO₄solid solution crystallization region dominated by ammonium dihydrogen phosphate.There are no separate potassium dihydrogen phosphate and ammonium dihydrogen phosphate crystallization regions,indicating that the system is a fully miscible system.（3）The solubility data of the ternary system KCl-NH₄Cl-H₂O,KH₂PO₄-KCl-H₂O,KH₂PO₄-NH₄H₂PO₄-H₂O and NH₄Cl-NH₄H₂PO₄-H₂O at 303.15K,323.15K,333.15K,and 343.15K were calculated using the Pitzer-Harvie-Wear model.For the ternary system KCl-NH₄Cl-H₂O at 303.15K,323.15K,333.15K,and 343.15K,the maximum RAD value between the model calculation and the experimental value is 0.02,and the maximum RMSD value is 0.56.For the ternary system KH₂PO₄-NH₄H₂PO₄-H₂O,at303.15K,323.15K,333.15K,and 343.15K,the maximum RAD value between the model calculation and the experimental value is 0.03,and the maximum RMSD value is 0.75.For the ternary system KH₂PO₄-KCl-H₂O at 323.15K,333.15K,and 343.15K,the maximum RAD and RMSD values of the model calculation and experimental values are 0.03 and 0.59,respectively.For the ternary system NH₄Cl-NH₄H₂PO₄-H₂O at 303.15K,323.15K,333.15K,and 343.15K,the maximum RAD value of the model calculation and the experimental value is 0.03,and the maximum RMSD value is 1.26.This indicates that the Pitzer-Harvie-Wear model has a good correlation effect on the solubility data of four ternary systems.（4）The phase diagram of the interactive quaternary system K⁺,NH₄⁺//H₂PO₄^-,Cl^--H₂O at 303.15K to 343.15K contains four solid solution crystallization regions:（K,NH₄）Cl,（NH₄,K）Cl,（NH₄,K）H₂PO₄and（K,NH₄）H₂PO₄solid solution crystallization regions.The crystallization area of the two solid solutions formed by potassium dihydrogen phosphate and ammonium dihydrogen phosphate is much larger than that of the solid solution formed by potassium chloride and ammonium chloride.The interactive quaternary system includes two co saturation points at various temperatures,namely,the mixing of potassium chloride,ammonium chloride,potassium dihydrogen phosphate,（K,NH₄）Cl,（K,NH₄）H₂PO₄,and（NH₄,K）H₂PO₄,and the mixing of potassium chloride,ammonium chloride,potassium dihydrogen phosphate,（NH₄,K）H₂PO₄,（K,NH₄）Cl,and（NH₄,K）Cl.（5）The metastable zone width of the interactive quaternary system K⁺,NH₄⁺//H₂PO₄^-,Cl^--H₂O at the same temperature and different solute ratios is0.90～6.63K at 313.15K,1.10～7.43K at 323.15K,1.50～8.97K at 333.15K,1.13～9.20K at 343.15K,1.37～9.70K at 353.15K.The metastable zone width range is narrower at low temperatures and wider at high temperatures at fixed temperatures.The changes in the metastable zone at 313.15K to 353.15K were studied by selecting the co saturation point E（w（K⁺）/w（total salt）=0.25）,co saturation point F（w（K⁺）/w（total salt）=0.12）,and the midpoint of the connection curve between the two co saturation points（w（K⁺）/w（total salt）=0.19）.The results show that the width of the metastable zone widens with increasing temperature at a fixed solute ratio.