Thermodynamic Analysis and Experimental Investigation of Al and F Removal from Sulfuric Acid Leachate of Spent LiFePO4 Battery Powder

Abstract

The co-precipitation thermodynamics of the Li⁺–Fe²⁺/Fe³⁺–Al³⁺–F⁻–SO₄²⁻–PO₄³⁻–H₂O system at 298 K is studied, aiming to understand the precipitation characteristics. Based on the principle of simultaneous equilibrium and the mass action law, the missing Ksp values of AlF₃ and FeF₃ were estimated. The results of thermodynamic calculation demonstrate that Al³⁺ and F⁻ in the sulfuric acid leachate could be preferentially precipitated in the form of AlPO₄ and FeF₃ by the precise adjustment of the final pH value. Only a small amount of P and Fe was lost by the precipitation of Fe₃(PO₄)₂·8H₂O, FePO₄, and Fe(OH)₃ during the purification process. Controlling the oxidation of ferrous ions effectively is of critical significance for the loss reduction of P and Fe. Precipitation experiments at different pH value indicated that the concentration of Al³⁺ and F⁻ in the leachate decreased as the final pH value rose from 3.05 to 3.90. When the final pH value was around 3.75, aluminum and fluoride ion impurities could be deeply purified, and the loss rate of phosphate ions and iron ions could be reduced as much as possible. Relevant research results can provide theoretical guidance for the purification of leachate in the wet recycling process of lithium-ion batteries.