Deep fluoride removal from the sulfate leaching solution of spent LIBs by complexation extraction with Al3+ loaded solvent

Fluoride contained in the leaching solution of spent lithium-ion batteries (LIBs) not only decreased product quality, but also led to severe equipment corrosion and potential environmental hazards. Therefore, a coordination extraction system using di-(2-ethylhexyl) phosphoric acid (D2EHPA) loaded with aluminum ions (Al3+) as extractant was designed to deeply remove fluoride from the sulfate leaching solution of spent LIBs that combined the excellent binding affinity of Al3+ to fluoride and the perfect chemical efficiency of solvent extraction. The effects of D2EHPA concentration, loaded Al3+ concentration, equilibrium pH and shaking time on the extraction efficiency of fluoride were investigated. Experimental results indicated that over 99 % of fluoride was selectively extracted and the fluoride concentration in the leaching solution of spent LIBs was reduced from 210 mg/L to 2 mg/L by single-stage extraction, meeting the requirement of fluoride content in the battery grade Ni/Co/Mn sulfate solution and the wastewater discharge standards. The loaded organic phase was scrubbed and stripped by diluted H2SO4, and fluoride could be fully stripped into the stripping solution. Further characterization studies revealed that Al3+ and D2EHPA may mainly coordinate in the form of AlL3·HL in the Al-loaded organic phase (HL denotes D2EHPA). After the Al-loaded organic phase was contacted with solution containing fluoride, a structure of AlFL2·HL coordinated with F- was formed. This study provides a novel technical approach to improve product quality and reduce fluoride wastewater pollution for the recycling process of spent LIBs.