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

New scientific data on the solubility sodium hexafluorosilicate in the Na₂SiF₆–H₃PO₄–H₂O 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 (H₂O) 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 H₅P₂O₈– 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.

1. Global Feed Phosphate Market – Industry Trends and Forecast to 2029. Available at: https://www.databridgemarketresearch.com/ru/reports/global-feed-phosphates-market (accessed 24 January 2025) (in Rusian).

2. Global Food Phosphate Market Report. Available at: https://www.valuemarketresearch.com/report/food-phosphate-market (accessed 24 January 2025).

3. Coy M. Mc. Nearing End Game. Chemical and Engineering News, 1999, vоl. 77, pp. 17–20.

4. Davister A., Peterbroek M. The prayon process for wet acid purification. Chemical and Engineering News, 1982, vol. 78, no. 9, pp. 35–39.

5. Angelova, M. A. Production and consumption of phosphate raw materials in the USA. Trudy NIUIF: sb. nauch. tr. [Proceedings of the National Research Institute]. Moscow, Research institute for fertilizers and insectofungicides, 2004, pp. 105–119 (in Russian).

6. Dormeshkin O. B., Hauryliuk A. N., Mokhart M. S., Byshyk A. A. Investigation of the process of defluorination of extractive phosphoric acid by the combined method. Himicheskaya promyshlennost’ segodnya = Chemical Industry Developments, 2024, no. 6, pp. 19–28 (in Russian).

7. Mozheyko F. F., Shulga N. V., Shevchuk V. V. Production of defluorinated phosphoric acid by precipitation of hexafluorosilicates. Vestsi Natsyyanal’nai akademii navuk Belarusi. Seryya khimichnykh navuk = Proceedings of the National Academy of Sciences of Belarus. Chemical series, 2008, vol. 60, no. 1, pp. 9–14 (in Russian).

8. Elyamani Y., Skafi M., Rifai A., Guendouzi M. Solubility behaviors of (Na+, K+ or NH4+) hexafluoridosilicatesin H2SO4, HF, H2SiF6, H3PO4 acidic aqueous solutions at T = 353.15 K. Journal of Fluorine Chemistry, 2021, vol. 247, art. 109796. https://doi.org/10.1016/j.jfluchem.2021.109796

9. Guendouzi M. E., Skafi M., Rifai A. Hexafluorosilicate Salts in Wet Phosphoric Acid Processes: Properties of X2SiF6–H2O with X = Na+, K+, or NH4+ in Aqueous Solutions at 353.15 K. Journal of Chemical & Engineering Data, 2016, vol. 61, no. 5, pp. 1728–1734. https://doi.org/10.1021/acs.jced.5b00866

10. Kogan V. B. Handbook of Solubility. Binary Systems. Publishing house of the USSR Academy of Sciences, 1961–1970. – Vol. 1: Binary systems. Book 2. Vol. 1. – 1962. – 961–1960 p. (in Russian).

11. Mel’nik B. D., Mel’nikov E. V. A short engineering guide to the technology of inorganic substances. Graphs and nomograms. Moscow, Khimiya Publishing, 1968. 432 p. (in Russian).

12. Zhao H. W., Feng H., Qingwen W., Chen C., Sibei M. Solubility Measurement and Correlation of Potassium Hexafluorosilicate (K2SiF6) in H2O, HCl, and H3PO4 Solutions at a Temperature Range of 5–70 °C. Journal of Chemical & Engineering Data, 2023, vol. 68, no. 9, pp. 2449–2459. https://doi.org/10.1021/acs.jced.3c00270

13. Frayret J., Castetbon A., Trouve G., Potin-Gautier M. Solubility of (NH4)2SiF6, K2SiF6 and Na2SiF6 in acidic solutions. Chemical Physics Letters, 2006, vol. 427, pp. 356–364. https://doi.org/10.1016/j.cplett.2006.06.044

14. Kozitsina A. N., Ivanova A. V., Glazyrina Yu. A., Gerasimova E. L., Svalova T. S., Malysheva N. N., Okhokhonin A. V. Electrochemical methods of analysis. Ekaterinburg, Ural University Publishing House, 2017. 128 p. (in Russian).

15. State Standard 20851.2-75. Mineral fertilizers. Methods for the determination of phosphates. Moscow, Publishing standards, 1975. 39 p (in Russian).

16. Kreshkov A. P. Fundamentals of analytical Chemistry. Moscow, Goskhimizdat Publishing, 1961. 635 p. (in Russian).

17. Anosov V. Ya., Ozerova M. I., Burmistrova N. P., Shchedrina A. P. Manual for practical exercises in physico-chemical analysis. Water-salt systems and some techniques for studying equilibria and phase transformations. Kazan, Kazan University Publishing House, 1969. 90 p. (in Russian).

18. Brdička R. Grundlagen der physikalischen Chemie. Berlinб Deutscher Verlag der Wissenschaften, 1990, pp. 859–867.

19. N. Andronov V. I., Brodskii A. A., Zabeleshinskii Yu. A., Ionass A. A., Ionova L. A., Klenitskii A. I., Mikhailin A. D., Postnikov N., Frenkel’ M. G., Chelebi G. A. Thermal phosphoric acid, salts and fertilizers based on it. Moscow, Khimiya Publishing, 1976. 336 p. (in Russian).

20. Mel’nikova G. B. Solutions of strong electrolytes [presentation]. Available at: https://elib.bspu.by/bitstream/doc/40573/1/Л%2011 (аccessed 24 January 2025) (in Russian).

21. Prodan I. E. Features of the formation of iron (III) phosphates with a given composition and structure [dissertation]. Minsk, 1988. 265 p. (in Russian).

22. Pepi F., Ricci A., Rosi M., Di Stefano M. Gaseous H5P2O8-ions: a theoretical and experimental study on the hydrolysis and synthesis of diphosphate ion. Chemistry, 2004, vol. 10, no. 22, pp. 5706–5716. https://doi.org/10.1002/chem.200400293

23. Kochetkov S. P., Smirnov N. N., Il’in A. P. Concentration and purification of extractive phosphoric acid. Ivanovo, 2007. 304 p. (in Russian).

24. Kulesh I. V., Valentyukevich O. I. Practicum on physical chemistry. Grodno: GGAU Publishing, 2013. 94 p. (in Russian).