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

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)4P2O7and ((NH4)3HP2O7+(NH4)4P2O7) for APP-[CO(NH2)2-NH4NO3]-H2O diagram;(2) KCl,KNO3and(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,KNO3and KCl,in which the KNO3region 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.