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.

Investigation on the fluorine recovery mechanism by air stripping for synthetic and industrial wet process phosphoric acid
    Abstract
    1 Introduction
    2 Experimental
        2.1 Materials
        2.2 Solution preparation
        2.3 Air stripping process
        2.4 Analytical methods
            2.4.1 Elemental analysis
            2.4.2 Fourier transformed infrared spectroscopy
            2.4.3 MAS nuclear magnetic resonance
    3 Results and discussion
        3.1 Air stripping of F in synthetic phosphoric acid solution
            3.1.1 H3PO4-H2SiF6 solution
            3.1.2 H3PO4-HF solution
            3.1.3 H3PO4-H2SiF6-HF-Al3+ solution
        3.2 Air stripping of F in industrial WPA solution
            3.2.1 WPA solution
            3.2.2 Influence of Al3+ on air stripping of F in the WPA solution
        3.3 Influence of the existence form of F on air stripping in the WPA
    4 Conclusions
    Anchor 22
    Acknowledgements 
    References

• Binbin He(Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, People’s Republic of China, Yunnan Provincial Key Laboratory of Energy Saving in Phosphorus Chemical Engineering and New Phosphorus Materials, Kunming 650500, People’s Republic of China, Center for Development and Utilization of Phosphate Resources, National Engineering and Technology Research, Kunming 650600, People’s Republic of China)
• Yun Zu(Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, People’s Republic of China, Yunnan Provincial Key Laboratory of Energy Saving in Phosphorus Chemical Engineering and New Phosphorus Materials, Kunming 650500, People’s Republic of China)
• Yunxiang Nie(Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, People’s Republic of China, Yunnan Provincial Key Laboratory of Energy Saving in Phosphorus Chemical Engineering and New Phosphorus Materials, Kunming 650500, People’s Republic of China)
• Yi Mei(Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, People’s Republic of China, Yunnan Provincial Key Laboratory of Energy Saving in Phosphorus Chemical Engineering and New Phosphorus Materials, Kunming 650500, People’s Republic of China)