Determination and Correlation of Phase Equilibria of Quaternary System KCl–KH2PO4–CO(NH2)2–H2O and Its Ternary Subsystems at 323.15 and 333.15 K

Abstract

Solid-liquid phase equilibrium of the quaternary system KCl–KH₂PO₄–CO(NH₂)₂–H₂O and its subsystems KH₂PO₄–CO(NH₂)₂–H₂O and KCl–KH₂PO₄–H₂O at 323.15 and 333.15 K were investigated through the isothermal solution equilibrium method in this study. The equilibrium solid phases were determined by Schreinemaker’s wet residue method and X-ray powder diffraction method. The isothermal dissolution phase diagrams of the ternary systems and isothermal dry salt phase diagrams and water diagrams of the quaternary system were plotted based on the measured solubility data. The results indicates that there is one invariant point, two univariant curves, three crystallization regions and one undersaturated solution region in the isothermal dissolution phase diagrams of ternary subsystems KH₂PO₄–CO(NH₂)₂–H₂O and KCl–KH₂PO₄–H₂O. The isothermal dry salt phase diagrams of quaternary system KCl–KH₂PO₄–CO(NH₂)₂–H₂O contain an invariant point, three univariant curves, three crystallization regions. The solubilities of the three systems were correlated and calculated by the extended Pitzer model based on the electrolyte solution theory. The maximum values of relative average deviation (RAD) and root mean square deviation (RMSD) of calculated values and experimental values of the ternary system KCl–KH₂PO₄–H₂O at two temperatures are 1.75%, 0.34, respectively; that of the ternary system KH₂PO₄–CO(NH₂)₂–H₂O are 0.71%, 0.19, respectively; that of the quaternary system KCl–KH₂PO₄–CO(NH₂)₂–H₂O are 2.06%, 0.52, respectively; which indicates the model can be used for the correlation calculation of these systems due to good consistency in this study.