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 recovery and subsequent phosphoric acid purification. This paper firstly emphasizes on investigating the competitive complexation behavior between F species and metal cations (M = Al 3+ , Fe 3+ , Ca 2+ and Mg 2+ ) by designing a series of F-containing model solutions (as HF and/or H 2 SiF 6 ) 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 SiF 6 2- hydrolysis follows the order: Al 3+ > Fe 3+ > Ca 2+ > Mg 2+ . DFT calculation further confirms that the acid-soluble AlF 4 - and FeF 3 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 H 2 SiF 6 ) in industrial WPA for preferentially removing metal impurities in the forms of M−F precipitates (e.g., AlF 3 and CaF 2 ), 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.