Removal of phosphorus and fluorine from wastewater containing PF6−via accelerated decomposition by Al3+ and chemical precipitation for hydrometallurgical recycling of lithium-ion batteries†

During hydrometallurgical recycling of lithium-ion batteries (LIBs), one important challenge is the efficient treatment of wastewater containing LiPF₆ used as a lithium salt in the LIBs. The difficulty of the treatment is attributed to the persistence of PF₆⁻ in aqueous solutions. In this study, the accelerated decomposition of PF₆⁻ by Al³⁺ at an elevated temperature and the removal of phosphorus and fluorine by chemical precipitation were attempted. These reactions were analyzed using a pH electrode and fluoride-ion selective electrode, and by a distillation method for total fluorine analysis, ICP-AES, ion chromatography, XRD, and WDS. The results showed that when 10 mM LiPF₆ aqueous solution containing 100 mM Al₂(SO₄)₃ was kept at 90 °C for 24 h, more than 90% of the PF₆⁻ was decomposed into PO₄³⁻ and F⁻. The produced PO₄³⁻ and F⁻ were coprecipitated with Ca₆Al₂(SO₄)₃(OH)₁₂ (ettringite) by adding sufficient Ca(OH)₂. The concentrations of the total phosphorus and total fluorine in the supernatant after precipitation were 0.028 mM and 0.77 mM, respectively. Here, the pH after the decomposition of 10 mM PF₆⁻ decreases to around 1 due to the formation of H⁺ during the decomposition, which may be too low for some practical cases. For this problem, the decomposition of PF₆⁻ in various pre-mixed solutions of Al₂(SO₄)₃ and Ca(OH)₂ was also examined. As a result, when the prepared molar ratio was Al/Ca > 2/3, the decomposition of PF₆⁻ proceeded, and the pH decrease accompanying the decomposition was alleviated due to the buffer effect of the Al(OH)₃ precipitate.