Study on the Crystallization Thermodynamics of Ammonium Dihydrogen Phosphate in H2O–Ethylene Glycol Binary System

Ammonium dihydrogen phosphate (MAP) is a highly efficient compound fertilizer and industrial raw material, yet commercial industrial-grade MAP often contains a substantial amount of impurities and presents issues with particle size distribution and shape. These factors make it unsuitable for use as battery-grade MAP, necessitating the study of MAP’s crystallization in H₂O–Ethylene Glycol (EG) binary systems to identify potential solutions. This paper explores the thermodynamic process of MAP crystallization in these systems. It reports the results of a comprehensive study on the effects of H₂O–EG binary system proportions on MAP’s thermodynamics. This research employs a range of established models, including the Apelblat model, Van’t Hoff model, and Redlich–Kister model, to correlate the experimental data and determine the thermodynamic parameters associated with MAP’s crystallization process. Furthermore, the Mersmann equation and Barata equation are utilized to calculate the solid-liquid surface tension (γ) and the crystal surface entropy factor (f) of MAP in the systems. The thermodynamic experimental results of this paper provide a preliminary research foundation for the crystallization kinetics of MAP. The findings indicate that the dissolution of MAP is an endothermic and non-spontaneous process, primarily driven by enthalpy changes. The Apelblat model appears to offer a superior description of MAP’s solubility. The findings of this study are of significant importance to designers engaged in the industrial crystallization process of MAP. They provide valuable information for the optimization and improvement of the production process of MAP, thereby enhancing its quality and efficiency.