Study On The Co-crystallization Of NH₄H₂PO₄ And (NH₂)₂C

NH₄H₂PO₄and（NH₂）₂CO are important raw materials for water-soluble fertilizer.However,there are few basic theoretical studies on its eutectic.In this paper,the thermodynamics and kinetics were studied of the eutectic crystallization process of the ternary system NH₄H₂PO₄-（NH₂）₂CO-H₂O in the temperature range of 303.15 K to 353.15 K,and the process part was simply discussed.1 The phase equilibrium data at 303.15 K,323.15 K,333.15 K and 343.15 K were measured by isothermal dissolution equilibrium method,and the phase diagram at corresponding temperature was drawn.The composition of the solid phase at equilibrium was identified by Schreinermark wet slag method and X-ray diffraction method.At the co-saturation point,the solubility of NH₄H₂PO₄decreases with the increase of temperature,and the increase of the mass ratio of（NH₂）₂CO has an inhibitory effect on the solubility of NH₄H₂PO₄.The experimental solubility data of ternary system NH₄H₂PO₄-（NH₂）₂CO-H₂O were calculated by Wilson and NRTL models.The results showed that the RAD of Wilson and NRTL model correlation values were 2.68%and 1.78%for the NH₄H₂PO₄-（NH₂）₂CO-H₂O system,respectively;The RMSD values of Wilson and NRTL models were 0.11 and 0.71,respectively,and the theoretical calculation value of solubility is consistent with the experimental value.Wilson and NRTL models have good applicability for the ternary mutually soluble system composed of electrolyte and anelectrolyte in this paper.Finally,the obtained Wilson model and NRTL model parameters were used to calculate the solubility of 313.15 K and 353.15 K,and compared with the literature values.The results showed that the comparison effect was good,indicating that these two models can be used to calculate the phase equilibrium of the system at 303.15K～353.15 K.2 The metastable zone is measured by laser method.The width of the metastable zone is affected by temperature and different saturation concentration.At a fixed temperature,by changing the composition of the equilibrium solution,the minimum width of the metastable zone is 0.6 K,the maximum is 17.4 K,and the average is 7.1 K.Under the condition of fixed equilibrium solution solute mass ratio（m NH₄H₂PO₄/m（NH₂）₂CO）,the variation relationship of metastable zone width under three ratios was obtained,and the metastable zone width was fitted.The metastable variation relationship is obtained.3 Focused beam reflectometry and online image analyzer were used to study the co crystallization process of NH₄H₂PO₄and（NH₂）₂CO,determine the particle number,average particle size and other dynamic data in the crystallization process,and observe the crystal crystallization process.In view of the crystallization process of NH₄H₂PO₄and（NH₂）₂CO,the nucleation and growth kinetics during the cooling crystallization process were investigated.The effects of changes in crystallization parameters on crystal nucleation and growth were obtained,and corresponding kinetic models were established.The nucleation kinetics equation is B=1.37×10¹¹exp（-0.23/RT）△C₁^0.11△C₂^0.25M_T^1.03,and growth kinetics equation is G=1.68×10^-5exp（-0.03/RT）△C₁^0.98△C₂^0.24.4 The process crystallization was simply explored.By changing the proportion of initial raw materials,the yield of products will also change.During process production,high yield products can be obtained by increasing the nutrient ratio of（NH₂）₂CO in raw materials.And the nutrient ratio of the product changes with the proportion of the original raw material,so the crystal products with different proportions can be obtained by adjusting the proportion of raw materials.It provides theoretical guidance for the research of co-crystallization process of NH₄H₂PO₄and（NH₂）₂CO and the control of crystal particle size and crystal quality.