Solubility study sodium hexafluorosilicate in the system Na2SiF6–H3PO4–H2O

New scientific data on the solubility sodium hexafluorosilicate in the Na 2 SiF 6 –H 3 PO 4 –H 2 O system in the range of changes in the content of orthophosphoric acid 40–65 wt.% under isothermal and polythermal conditions are presented. A decrease in the solubility of sodium hexafluorosilicate in solutions of orthophosphoric acid was established, with an increase in its concentration from dilute to content solutions over the entire range of temperature changes from 20 to 80 °C. This phenomenon, according to the authors, is due to a decrease in the amount of solvent (H 2 O) as the concentration of the acid increases on the one hand, as well as an increase in the number of water molecules for ion hydration, in particular H 5 P 2 O 8 – formed during the dissociation of orthophosphoric acid molecules, the dipoles of which do not participate in the process of salt dissolution. A change in the content of an acid has a much greater effect on the change in solubility compared to temperature, which led to the conclusion that defluorination of concentrated solutions of orthophosphoric acid is more preferable, since it allows to achieve a higher degree of defluorination when using alkali metal salts as a precipitating reagent. The analysis of the presented data on the effect of temperature and content of orthophosphoric acid is the basis for the subsequent selection of optimal conditions for the technological process of defluorination of extractive phosphoric acid by the method of precipitation using alkali metal salts and the prediction of the achieved residual content of fluoride ions.