Solid-liquid equilibrium, structural features and separation process of ammonium potassium dihydrogen phosphate solid solution

Abstract

• Solid-liquid equilibrium of the NH 4 + , K + //H 2 PO 4 − -H 2 O system at 283.15 K and 313.15 K was investigated. • By characterization determined infinite substitutional solid solutions. • Two solid solutions are analyzed by FTIR, TG-DSC, EDS (mapping). • A multistage purification process was developed and proved feasibility for AQ-SS. Solid-liquid equilibrium for NH 4 H 2 PO 4 + KH 2 PO 4 + H 2 O ternary system was investigated at 283.15 K and 313.15 K, which revealed complete mixed crystals in the solid phase. By analysis of XRD and the formation rule of the substituted solid solution, it is further confirmed infinite substitutional solid solution. Then, from the analysis of FT-IR, TG-DSC and mapping of [K 0.78 (NH 4 ) 0.22 ]H 2 PO 4 and [K 0.11 (NH 4 ) 0.89 ]H 2 PO 4 solid solution, we can conclude that two solid solutions are similar, but the absorption position have slight shift to low wave number direction as the increasing of K + content; the homogeneous distribution of K + and NH 4 + elements. Finally, applying the experimental solid-liquid equilibrium data, multistage fractional crystallization experiment to obtain the purity of the KH 2 PO 4 is close to 97% after five times operation and purity of the NH 4 H 2 PO 4 reaches about 98% after four times operation, which proves multistage fractional purification is verified as an effective purification method for AQ-SS.