Investigation on the fluorine recovery mechanism by air stripping for synthetic and industrial wet process phosphoric acid

Fluorine (F) recovery from wet process phosphoric acid (WPA) is essential for sustainable resource utilization and environmental protection. This work systematically investigates the F recovery mechanism by air stripping from three simulated systems: H₃PO₄-H₂SiF₆-H₂O, H₃PO₄-HF-H₂O, H₃PO₄-H₂SiF₆-HF-Al³⁺-H₂O, and from two industrial systems: WPA and WPA-Al³⁺ under different stripping temperatures (60–110 ℃) and stripping times (0–120 min). The influence on the existence form of F, the content of Al³⁺ cations and the addition of active silica on the F removal rate in the phosphoric acid solution is studied by analyzing the changes in the contents of F, P and Si. The results indicate that the F in the form of H₂SiF₆ is more easily released from the phosphoric acid solution than that in the form of HF. While, the release of F is inhibited in the presence of the Al³⁺ in the solution due to the formation of Al-F complexes that are characterized by ¹⁹F NMR, ³¹Si NMR and FTIR techniques. Interestingly, the addition of active silica can promote the conversion of HF to H₂SiF₆ in the solution and significantly improve the release rate of F. The researching results can provide an important guidance for industrial practice of WPA.