Simultaneous determination of H2SiF6, HF and total fluoride in fluorosilicic acid recovered from wet phosphoric acid production by potentiometric titration

Fluorosilicic acid is generally obtained unintentionally in phosphate fertilizer manufacturers, particularly during wet phosphoric acid and triple superphosphate (TSP) production from fluorapatite. Since sustainability is a major environmental concern, phosphoric acid production factories have a fluorine recovery process as fluorinated products, mainly consisting of fluorosilicic acid (H2SiF6). In other words, to optimize the control of the fluorine recovery during the phosphate chemical process and to investigate the valorization alternatives of the concomitantly produced fluorosilicic acid, the main fluorine-based by-products have to be analyzed. The present work describes the development of a potentiometric analytical method for the major constituents of fluorosilicic acid produced during the chemical phosphate process, i.e., H2SiF6, HF and total fluoride. Therefore, a preliminary speciation study was investigated on industrial and synthetic fluorosilicic acid samples using fluorine-19 (19F) and silicon-29 (29Si) nuclear magnetic resonance (NMR) spectroscopies. Interestingly, we observed the presence of two forms of fluoride in a highly acidic medium (pH = 1.80) ascribed to HF and H2SiF6, as well as a single form of silicon which is assigned to H2SiF6. Moreover, a potentiometric titration of fluorosilicic acid with known H3PO4 content, using a 0.5 mol.L−1 NaOH solution as a titrant, allowed us to determine the H2SiF6 concentration as well as the amount of total acid content. Then, the amount of hydrofluoric acid is calculated from the difference between total acid, phosphoric acid and fluorosilicic acid contents. The total fluoride content is deduced from H2SiF6 and HF contents. The results obtained with this method are compared to other standard methods, e.g., silicon determination by flame atomic absorption spectroscopy and total fluoride determination by fluoride ion selective electrode (F-ISE). It is worth noting that, based on this comparison study, the developed method provides consistent results. Furthermore, the analysis is fast and accurate, and is therefore applicable to the control of recovery process as well as to the characterization of industrial fluorosilicic acid.