Study On The Solid-liquid Equilibrium Of KCl-NH₄H₂PO₄-?NH₂?₂CO-H₂O System At 298.15K

The phase equilibrium data of KCl-NH₄H₂PO₄-?NH₂?₂CO-H₂O is the basis of studying the formation of eutectic and the possibility of Cocrystallization.In this paper,the solid-liquid equilibria of some subsystems of the quinary system KCl-NH₄H₂PO₄-?NH₂?₂CO-H₂O at 298.15 K are studied by means of isothermal solution equilibrium method.The solubility data of the related systems are determined.According to the experimental data and the corresponding equilibrium solid phase diagram is drawn.The composition and structure of the equilibrium solid phase of the system are jointly determined by wet slag method and X-ray diffraction method.The study found:?1?The ternary system NH₄Cl-NH₄H₂PO₄-H₂O is a simple eutectic type without complex salt and solid solution formation.Its phase diagram has four phase regions,two univariate curves and one ternary eutectic point,the compositional mass fraction of the saturated liquid phase is:NH₄Cl 25.76%,NH₄H₂PO₄ 5.69%,H₂O 68.55%.The four phase regions are unsaturated phase region,NH₄Cl crystal region,NH₄Cl and NH₄H₂PO₄ eutectic region,and NH₄H₂PO₄ crystal region.From the analysis of the phase diagram,NH₄Cl has salting out effect on NH₄H₂PO₄;The ternary system NH₄H₂PO₄-?NH₂?₂CO-H₂O is a simple eutectic type without complex salt and solid solution formation.Its phase diagram has four phase regions,two univariate curves and one ternary eutectic point,the compositional mass fraction of the saturated liquid phase is:?NH₂?₂CO 44.9%,NH₄H₂PO₄11.93%,H₂O 43.17%.The four phase regions are unsaturated phase region,?NH₂?₂CO crystal region,?NH₂?₂CO and NH₄H₂PO₄ eutectic region,and NH₄H₂PO₄ crystal region;The ternary system KH₂PO₄-NH₄H₂PO₄-H₂O is a partially soluble solid solution type.In this system,a small amount of NH₄H₂PO₄ is dissolved into KH₂PO₄ to form a solid solution mainly composed of KH₂PO₄ and a small amount of KH₂PO₄ is dissolved into NH₄H₂PO₄ to form a solid solution mainly composed of NH₄H₂PO₄.Its phase diagram has six phase regions,two univariate curves and one ternary eutectic point,the compositional mass fraction of the saturated liquid phase is:NH₄H₂PO₄17.55%,KH₂PO₄14.83%,H₂O 67.62%.The six phase regions are unsaturated phase region,KH₂PO₄ crystal region,NH₄H₂PO₄ crystal region,KH₂PO₄-based solid solution crystal region,NH₄H₂PO₄-based solid solution crystal region,and co-crystal region of two solid solutions.?2?In the Quaternary System NH₄⁺,K⁺//H₂PO₄^-,?NH₂?₂CO-H₂O,NH₄H₂PO₄and KH₂PO₄ form a solid solution.The phase diagram has three phase regions,three univariate curves and a quaternary co-saturation point.The mass fraction of the saturated liquid phase is:NH₄H₂PO₄ 17.55%,KH₂PO₄ 5.56%,?NH₂?₂CO 38.36%,H₂O 47.78%;In the quaternary system NH₄⁺//Cl^-,H₂PO₄^-,?NH₂?₂CO-H₂O,NH₄Cl and?NH₂?₂CO form the adduct?NH₂?₂CO?NH₄Cl.The phase diagram has four phase regions,four univariate curves,and two quaternary co-saturation points.The mass fractions of the saturated liquid phase are NH₄Cl 19.16%,NH₄H₂PO₄ 3.14%,?NH₂?₂CO 32.86%,H₂O 44.84%and NH₄Cl 14.43%,NH₄H₂PO₄ 3.22%,?NH₂?₂CO42.42%,H₂O 39.93%.In the quaternary system NH₄⁺,K⁺//Cl^-,?NH₂?₂CO-H₂O,NH₄Cl and?NH₂?₂CO form the adduct?NH₂?₂CO?NH₄Cl.The phase diagram has four phase regions,four univariate curves,and three quaternary co-saturation points.The mass fractions of the saturated liquid phase are NH₄Cl 15.73%,KCl 6.62%,?NH₂?₂CO 35.35%,H₂O 42.3%;NH₄Cl 11.21%,KCl 5.02%,?NH₂?₂CO 61.12%,H₂O22.65%and NH₄Cl 14.42%,KCl 5.38%,?NH₂?₂CO 47.26%,H₂O 32.94%;In the interactive quaternary system NH₄⁺,K⁺//Cl^-,H₂PO₄^--H₂O,NH₄H₂PO₄ and KH₂PO₄form solid solution,and NH₄Cl and KCl form solid solution.The phase diagram has four phase regions,five univariate curves,and two quaternary co-saturation points.The mass fractions of the saturated liquid phase are H₂PO₄^-4.25%,Cl^-17.6%,NH₄⁺7.03%,K⁺5.9%,H₂O 65.22%and H₂PO₄^-4.54%,Cl^-17.22%,NH₄⁺7.88%,K⁺ 3.73%,H₂O 66.63%.In the phase diagram,the crystalline area of the solid solution dominated by KH₂PO₄ is the largest,accounting for more than half of the total phase diagram area,so the solubility of KH₂PO₄ is the smallest,the crystalline area of the solid solution mainly composed of NH₄Cl is the smallest,so the solubility of NH₄Cl is the largest.?3?Solubility data of ternary system were correlated by Pitzer model.The theoretical calculation value of the ternary system NH₄H₂PO₄-NH₄Cl-H₂O is basically consistent with the experimental data.The maximum relative error of Cl^-is 2.274%,the minimum relative error is 0.05835%,and the average relative error is 1.023%;the maximum relative error of H₂PO₄^-is 2.235%,the minimum relative error is 0.06191%,and the average relative error is 1.397%.The theoretical calculation value of?NH₂?₂CO-NH₄H₂PO₄-H₂O is basically consistent with the experimental data.The maximum relative error of?NH₂?₂CO is 0.6191%,the minimum relative error is 0.4009%,and the average relative error is 0.5005%;the maximum relative error of NH₄H₂PO₄ is 0.6403%,the minimum relative error is 0.1176%,and the average relative error is 0.3450%.The theoretical calculation value of KH₂PO₄-NH₄H₂PO₄-H₂O in the ternary system containing solid solution is basically consistent with the experimental data.The maximum relative error of NH₄⁺is 2.426%,the minimum relative error is 0.03555%,and the average relative error is 0.5586%;the maximum relative error of K⁺is 1.295%,the minimum relative error is0.05647%,and the average relative error is 0.5652%.It shows that the Pitzer model is suitable for the theoretical calculation of solid solution of the ternary system.