Production of food phosphates: selective crystallization of sodium and potassium salts from solutions of refined ammonium orthophosphate

The work is devoted to the determination of technologically appropriate conditions for preparation of chemically pure salts of sodium and potassium orthophosphates based on solutions formed after the purification of orthophosphate acid from impurities of toxic elements by the method of complete ammonization with parallel co-precipitation with dicalcium phosphate. Based on the calculated solubility polytherms of mixtures of orthophosphates and chlorides of sodium, potassium, and ammonium at temperatures of 0–1000С, the conditions of their selective crystallization were determined. The results of calculations are compared with experimental data on crystallization of salts under laboratory conditions. The phase compositions of the sediments were confirmed by the XRD method. It was shown that the following chemically pure salts can be prepared from solutions of mixtures of orthophosphates and ammonium chlorides using the methods of isothermal-isohydric crystallization and conversion: Na2HPO4 (with a yield of up to 96%), NaH2PO4 (with a yield of up to 34%) and KH2PO4 (with a yield of up to 41%). The yield of crystalline KH2PO4 can be increased to 64% by converting NaH2PO4 with KCl. Measurements of the content of toxic compounds Cd(II), As(III), Cu(II), Pb(II) and fluorine in the synthesized salts showed that they were at levels lower than the current requirements for maximum permissible concentrations, therefore the synthesized salts can be used as food additives.