Evaluating the performance of citric acid and maleic acid for mixed-acid leaching of critical metals from spent lithium-ion batteries

Leaching, especially using mixtures of organic acids, can reduce the chemical requirement of organic acid leaching of metals. This work investigates the performance of maleic acid and citric acid and their potential synergy in a mixed-acid leaching system for the recovery of valuable metals from the cathode material of spent lithium-ion batteries (LIBs). The influence of key leaching parameters such as acid concentration, temperature and reducing agent (fructose) were examined. As single acids, citric acid proved to be a stronger lixiviant than maleic acid. 83% Li, 84%Mn, 80% Co and 80%Ni was leached using 0.5 M citric acid at a temperature of 90 °C after 60 min. As mixtures, the leaching of the metals showed significant dependence on the ratio of the acids and increased with the proportion of citric acid, indicating that citric acid is the dominant lixiviant. However, the performance of the mixtures of citric acid and maleic acid was lower than that of the individual acids, thus demonstrating lack of synergy. Spectra analysis of the leachates confirmed the formation of metal complexes and interactions between citric and maleic acid, and explains the observed performance of the acid mixtures. Overall, this work reveals that not all organic acid mixtures are synergic.