Solid-liquid phase equilibria in aqueous solutions of four common fertilizers at 303.2 K and atmospheric pressure

Application of liquid fertilizers can significantly improve the fertilizer utilization efficiency and labor efficiency. Phase equilibrium of various fertilizers in aqueous solution is the basis of manufacturing liquid fertilizers. The solid-liquid phase equilibria of ternary systems NH4H2PO4 + CO(NH2)2 + H2O, (NH4)2HPO4 + CO(NH2)2 + H2O, K2SO4 + CO(NH2)2 + H2O, K2SO4 + NH4H2PO4 + H2O and K2SO4 + (NH4)2HPO4 + H2O at 303.2 K were investigated by isothermal solution saturation method. The maximum nutrient content of (NH4)2HPO4 + CO(NH2)2 + H2O solution is 31.78 wt% at the saturated solution of (NH4)2HPO4, and the mass ratio of N: P2O5 is 1: 2.53. In the solutions of NH4H2PO4 + CO(NH2)2 + H2O, K2SO4 + CO(NH2)2 + H2O and K2SO4 + (NH4)2HPO4 + H2O, the corresponding maximum of nutrient contents all are obtained at the corresponding invariant points as 33.82 wt%, 27.32 wt% and 32.92 wt% respectively. And the mass ratios of nutrients are N: P2O5 of 2.69: 1, K2O: N of 1:15.66 and N: P2O5: K2O of 1.64: 4.15: 1, respectively. The maximal nutrient content in the K2SO4 + NH4H2PO4 + H2O saturation solution is 25.14 wt% and the corresponding mass ratio of N: P2O5: K2O is 1.26: 6.38: 1. Based on the phase equilibrium diagrams, the species and nutrient contents in liquid fertilizers all are adjustable by choosing different raw materials or modulating the prescription. The results provide fundamental data for the preparation of liquid fertilizers. It is also helpful to optimize the operation parameters in the manufacturing process, storage process or application process to avoid the precipitation of solid.