Solid-liquid phase equilibria of quaternary system NH4+//C1−, SO42−, H2PO4−–H2O and its subsystems NH4+//C1−, SO42−–H2O, NH4+//C1−, H2PO4−–H2O at 313.15 K

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Solid–liquid phase equilibrium and physical properties of ternary systems NH4Cl-NH4H2PO4-H2O, NH4Cl-(NH4)2SO4-H2O and quaternary system NH4Cl-(NH4)2SO4-NH4H2PO4-H2O at 313.15K and atmospheric pressure were obtained by means of an isothermal solution method. The solid phases formed in the systems studied were determined by the Schreinemaker wet residue method. Based on the measured data, corresponding phase diagrams and the diagrams of physical properties versus composition were plotted. Neither complex salt nor solid solution is found in these systems at 313.15K. The ternary phase diagram of NH4+//C1−, H2PO4−–H2O contains one invariant point, and three crystallization regions (NH4H2PO4, NH4Cl, and (NH4Cl+NH4H2PO4)). The ternary phase diagram of NH4+//C1−, SO42−–H2O is similar to the previous one, also contains one invariant point, three crystallization regions ((NH4)2SO4, NH4Cl, and (NH4Cl+(NH4)2SO4)). The quaternary phase diagram of NH4+//C1−, SO42−, H2PO4−–H2O contains one quaternary invariant point, three crystallization regions ((NH4)2SO4, NH4Cl, and NH4H2PO4), and crystallization field of NH4H2PO4 is the largest one of them. All results obtained in this work can be used for the stripping process of solvent extraction in the production of potassium dihydrogen phosphate.