Synergistic leaching of critical metals from spent lithium-ion battery black mass using citric and acetic acids

Spent lithium-ion batteries (LIBs) pose a significant environmental challenge, yet they remain a valuable source of critical metals. Hydrometallurgical recycling has been widely explored; however, the process relies heavily on strong inorganic acids, leading to chemical hazards and toxic effluents. Recent efforts to replace inorganic acids with organic acids have shown potential, but often necessitate the use of H₂O₂ as a reducing agent. The industrial application of H₂O₂ in these systems presents challenges due to its inherent instability, inefficient utilization and increased operational costs, particularly at elevated temperatures. This study addresses these limitations by investigating a full organic system, employing a synergy of citric and acetic acids for the recovery of Mn, Co, Ni, and Li from LIB black mass at ambient temperature (25 °C), thereby eliminating the need for H₂O₂. Process parameters were evaluated using response surface methodology, revealing that leaching time and solid-to-liquid ratio exert a stronger influence on metal dissolution than acid concentration within the investigated range. Under optimal conditions (1.6 M citric acid, 0.9 M acetic acid, 188 min, 155 g/L), leaching efficiencies reached 52.73% Mn, 5.90% Co, 1.75% Ni, and 31.85% Li. Kinetic analysis using shrinking core models showed that manganese dissolution is governed by external liquid-film diffusion, whereas Co, Ni and Li dissolution are controlled by surface chemical reactions. The synergy of citric and acetic acids provides both proton-assisted dissolution and ligand complexation, enabling metal extraction without adverse environmental impact. However, the relatively low recoveries for Co and Ni highlight that this dual organic acid system alone may not fully compensate for the absence of a strong reducing agent for all metals, suggesting avenues for further optimization. • Introduces an oxidant-free, fully organic citric–acetic acid system for LIB black mass leaching. • Achieves high Mn, Co, Ni, and Li recovery at room temperature and high pulp density. • Identifies solid-to-liquid ratio and time as the dominant leaching parameters. • Reveals diffusion-controlled Mn leaching and reaction-controlled Co, Ni, and Li kinetics. • Offers a low-hazard, scalable pathway for sustainable lithium-ion battery recycling.