(174az) Determination and Modelling of the Reciprocal Quaternary Solid-Liquid Phase Equilibrium for the Systems K+,NH4+//Cl-,H2PO4- -H2O at 283.15K

Determination and Modelling of The Reciprocal Quaternary Solid-liquid Phase Equilibrium for The Systems K⁺,NH₄⁺//Cl^-,H₂PO₄^- -H₂O at 283.15K

Qiang Wu, Xue Hu, Jing Zhu, and Tianxiang Li*

School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou 550025, P. R. China

Solid-liquid equilibrium data are the basis of crystallization and co-crystallization processes, which is critical for solid form manufacture and separation. For provided equilibrium data for crystallization and co-crystallization of the reciprocal quaternary system K⁺,NH₄⁺//Cl^-,H₂PO₄^- -H₂O, the solid-liquid phase equilibrium of this system and four ternary sub-systems KH₂PO₄-KCl-H₂O, NH₄H₂PO₄-NH₄Cl-H₂O, KCl-NH₄Cl-H₂O, KH₂PO₄-NH₄H₂PO₄-H₂O were determined by an isothermal saturation method and the composition and structure of the solid in equilibrium were identified by wet residue method and XRD. The result shows that the ternary system KH₂PO₄-KCl-H₂O is a simple eutectic type and no double salt and solid solution formed. Based on the solubility data, the phase diagram of the system was plotted, consisting of one invariant point and two solubility branches which divide the diagram into four regions. Each region possesses diverse functions on the solid salt crystallization. For example, there is strong salting-out that potassium chloride has effect on potassium dihydrogen phosphate.

Similarly, the ternary system NH₄H₂PO₄-NH₄Cl-H₂O is also a simple eutectic type, there is an invariant point and no double salt and solid solution formed. The phase diagram is divided into four phase zones by two univariate curves, and ammonium chloride has strong salting-out effect on ammonium dihydrogen phosphate. It is founded that the system KCl-NH₄Cl-H₂O is of solid solution type in which two solid solutions are formed in the system, KCl-based solid solution and NH₄Cl-based solid solution. The phase diagram has six phase zones, including pure KCl crystallization region, pure NH₄Cl crystallization region, KCl-based solid solution crystallization region, NH₄Cl-based solid solution crystallization region and the mixed crystallization region of KCl-based solid solution, the unsaturated region. The system KH₂PO₄-NH₄H₂PO₄-H₂O is also of solid solution type. Two solid solutions are formed in the system, KH₂PO₄-based solid solution and NH₄H₂PO₄-based solid solution. The phase diagram has six phase zones, including pure KH₂PO₄ crystallization region, pure NH₄H₂PO₄ crystallization region, KH₂PO₄-based solid solution crystallization region, NH₄H₂PO₄-based solid solution crystallization region and the mixed crystallization region of KH₂PO₄-based solid solution, the unsaturated region. In the reciprocal quaternary system K⁺,NH₄⁺//Cl^-,H₂PO₄^--H₂O, it is founded that KCl and NH₄Cl formed solid solution and KH₂PO₄ and NH₄H₂PO₄ formed solid solution. In the phase diagram, this quaternary system has five univariant curves, four crystal domains, two invariant points. The crystallization field of KH₂PO₄-based solid solution is the largest that it extends to half of the area of the phase diagram, so the solubility is the smallest, while the field of NH₄Cl-based solid solution is the smallest, so the solubility is the largest.

Two thermodynamic models, Wilson and NRTL, were employed to correlate and calculate the mutual solubility data of ternary NH₄H₂PO₄-(NH₂)₂CO-H₂O and (NH₂)₂CO-KCl-H₂O. The binary interaction energy parameters of urea-water, monoammonium phosphate-water and potassium chloride-water; monoammonium phosphate-urea, potassium chloride-urea were obtained. The theoretical solubility data of the system is calculated. The relative average deviation of the Wilson model and the NRTL model fitting ternary system NH₄H₂PO₄-CO(NH₂)₂-H₂O are 0.0037 and 0.0071, respectively, and the root-mean-square deviation are 0.20 and 0.29, respectively. The Wilson model provided the relative average deviation and root-mean-square deviations are 0.008 and 0.2, respectively, the NRTL model are 1.27 and 2.51, respectively. The Pitzer model was applied into ternary system of KH₂PO₄-KCl-H₂O and NH₄H₂PO₄-NH₄Cl-H₂O. The mixing electrolyte ion-interaction parameters between K⁺, Cl^-, H₂PO₄^-and NH₄⁺, Cl^-, H₂PO₄^- were obtained. At the same time, the equilibrium constant is obtained and the theoretical solubility data of the system is calculated. The RAD value for the ternary system of KH₂PO₄-KCl-H₂O was 0.046, and the value of RMSE was 0.29, it was 0.044 and 0.31 for the ternary system of NH₄H₂PO₄-NH₄Cl-H₂O, respectively. The model calculation value was in good agreement with experimental solubility. All the fitted equations can provide an effective guidance for the relevant salt crystallization and screening process design.

*Corresponding Author

E-mail: txli@gzu.edu.cn

Acknowledgements

The authors are grateful for the financial support from Natural Science Foundation of P. R. China (No. 21868010) and Science and Technology Planning Project of Guizhou Province, P. R. China (No. [2018]5781）