Designing an efficient fluorine recovery strategy for Wet-Process phosphoric acid purification by disclosing competitive complexation behavior between fluorine species and metal cations

Fluorine (F) is a high-value resource that has been overlooked in the production of wet process phosphoric acid (WPA) due to its high susceptibility to complexation with metal cationic impurities, hindering F efficient re-covery and subsequent phosphoric acid purification. This paper firstly emphasizes on investigating the competitive complexation behavior between F species and metal cations (M = Al3+, Fe3+, Ca2+ and Mg2+) by designing a series of F-containing model solutions (as HF and/or H2SiF6) with fixed combinations of metal cations. The results reveal that the complexation ability between metal cations and "free" F- species from HF dissociation and/or SiF62-hydrolysis follows the order: Al3+ > Fe3+ > Ca2+ > Mg2+. DFT calculation further confirms that the acid-soluble AlF4- and FeF3 species with lower binding energies are the main obstacle to difficult recovery of F species in the WPA. Given the above facts, an efficient F recovery strategy is proposed by increasing initial F concentration (as HF and H2SiF6) in industrial WPA for preferentially removing metal impurities in the forms of M-F precipitates (e.g., AlF3 and CaF2), according to the variations in distributions of M-F species under different n(F)/n(M) ratios predicted through establishment of mathematical models. The ultimate F recovery rate is greater than 95% by stripping process when initial HF concentration added is about 7.49 mol/L. This novel strategy has been successfully applied in phosphoric acid purification process.