Ultrasonic enhanced preferential leaching process of Li from waste lithium-ion batteries using oxalic acid focused on optimization and mechanism

It's crucial for recycling valuable metals from waste lithium-ion batteries (LIBs) simply and efficiently, especially preferential leaching process is worth being deeply studied. In this study, we introduce the ultrasonication to facilitate the oxalic acid (OA) leaching process of selective Li extraction from waste LIBs. Based on available study, OA can preferentially leach Li without reductants because of the differences in oxalate solubility, and ultrasonic power can accelerate the dissolution of metals within the solid. The orthogonal design is used to optimize leaching parameters resulting in a synthetical optimal solution between leaching rate and selectivity, which is identified as 120 W ultrasonic power, 0.6 M OA, 20 g/L, 90 degrees C and 100 min. The validation experiments indicate that the leaching rate of Li is up to 98.46 %, while the selectivity is also beyond 98 %. The leaching process is described with chemical reaction analysis theoretically, while the fitting of kinetic models and the characterization of leaching residue reveal the specific essence quantitatively. The mechanism of preferential leaching includes the decomposition of stable LiNixCoyMnzO2 crystal structure and the versatile effects of OA, and ultrasound could promote these processes by cavitation, specifically manifested in particle size decrease, removal of layered structure, sufficient solid-liquid interaction, and thorough reduction of high-valence metals. The obtained results demonstrate that ultrasonication can significantly enhance the single organic acid leaching process of targeted metal extraction from the mixture of waste LIBs, and this application may provide a promising and effective approach of recycling valuable metals from waste LIBs.