Abstract:

The solid-liquid phase equilibrium of ternary system of （NH₂）₂CO-NH₄H₂PO₄-H₂O was studied byisothermal dissolution equilibrium method at 10 ℃.The composition and structure of the equilibrium solid phase were identified by the combination of wet slag method and X-ray powder diffraction.The result showed that the ternary system was a simple co-saturated type and no double salt or solid solution,the mass composition（in terms of mass fraction） of invariant points was of urea 40.03%,monoammonium phosphate 13.92%,water 46.05%.There were two univariate curves in the phase diagram,which divided the diagram into four regions including the unsaturated region,（NH₂）₂CO crystallization region,N₄H₂PO₄ crystallization region and the mixed crystallization region of（NH₂）₂CO-NH₄H₂PO₄.In addition,two thermodynamic models,Wilson and NRTL,were employed to correlate and calculate the mutual solubility data.The calculated values agreed well with the experimental values,the RAD of the calculated and experimental values of the two thermodynamic models were 0.37% and 0.71% respectively,and the RMSD were 0.20 and 0.29 respectively.

Key words: urea, monoammonium phosphate, solid-liquid phase equilibrium