Issue 5, 2024

Removal of phosphorus and fluorine from wastewater containing PF6via accelerated decomposition by Al3+ and chemical precipitation for hydrometallurgical recycling of lithium-ion batteries

Abstract

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.

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

Supplementary files

Article information

Article type
Paper
Submitted
22 nov 2023
Accepted
28 mar 2024
First published
02 apr 2024
This article is Open Access
Creative Commons BY license

Environ. Sci.: Water Res. Technol., 2024,10, 1245-1255

Removal of phosphorus and fluorine from wastewater containing PF6via accelerated decomposition by Al3+ and chemical precipitation for hydrometallurgical recycling of lithium-ion batteries

T. Miyashita, K. Yasuda and T. Uda, Environ. Sci.: Water Res. Technol., 2024, 10, 1245 DOI: 10.1039/D3EW00854A

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