Solid-liquid phase equilibrium for the system ammonium polyphosphate-urea ammonium nitrate-potassium chloride-water at 273.2 K

[Display omitted] Based on the dynamic method, a quaternary system of ammonium polyphosphate (APP)-urea ammonium nitrate (UAN, CO(NH2)2-NH4NO3)-potassium chloride (KCl)-H2O and its subsystems (APP-[CO(NH2)2-NH4NO3]-H2O, KCl-[CO(NH2)2-NH4NO3]-H2O and APP-KCl-H2O) were systematically investigated at the temperature of 273.2 K. Each ternary phase diagram contains one invariant point and three crystallization regions. The crystallization regions are: (1) (NH4)3HP2O7, (NH4)4P2O7 and ((NH4)3HP2O7 + (NH4)4P2O7) for APP-[CO(NH2)2-NH4NO3]-H2O diagram; (2) KCl, KNO3 and (KCl + KNO3) for KCl-[CO(NH2)2-NH4NO3]-H2O diagram and (3) (NH4)3HP2O7, KCl and ((NH4)3HP2O7 + KCl) for APP-KCl-H2O diagram. The quaternary phase diagram of APP-[CO(NH2)2-NH4NO3]-KCl-H2O has no quaternary invariant point but includes four solid phase crystallization regions, i.e., (NH4)3HP2O7, (NH4)4P2O7, KNO3 and KCl, in which the KNO3 region occupies the largest area. The maximum total nutrient content (N + P2O5 + K2O) existing as ionic forms in the APP-[CO(NH2)2-NH4NO3]-H2O, KCl-[CO(NH2)2-NH4NO3]-H2O, APP-KCl-H2O and quaternary systems is 44.70%, 32.86%, 45.56% and 46.23% (mass), respectively, indicating that the maximum nutrient content can be reached using raw materials of the corresponding systems to prepare liquid fertilizer. In the quaternary system, the content of NH4+-N ascends with the increase of the total nutrient content, while the contents of NO3--N and CO(NH2)2-N increase with elevated total N. This work can help optimize the operating parameters for the production, storage and transportation of liquid fertilizers.