Electrochemically Assisted Leaching of Critical Metals from Spent NCA Batteries: Kinetic Study and Optimization

Recovery of critical metals from spent lithium-ion batteries commonly relies on single-pass chemical reductants, increasing reagent consumption and limiting opportunities for reductant reuse. In this study, electrochemically assisted leaching (EAL) with continuous regeneration of the Fe3+/Fe2+ redox mediator was investigated for nickel–cobalt–aluminum oxide (NCA)-containing black mass. The leaching kinetics of nickel, cobalt, and lithium were compared in H2SO4, H2SO4 + Fe2+, and in H2SO4 with continuous cathodic regeneration of Fe2+. Kinetic data were analyzed by nonlinear regression in fractional-conversion space, model selection using the corrected Akaike information criterion, and subsequent assessment of the physical plausibility of the selected kinetic schemes. Solid-state changes were examined by XRD, Raman spectroscopy, and SEM-EDS, while response surface methodology was used for multi-response process optimization. Nickel and cobalt were generally better described by two-stage kinetics, with a temperature-sensitive initial regime followed by a slower product-layer-affected stage. Reductive leaching was accompanied by reconstruction of the Ni-rich layered oxide toward a rock-salt NiO-type residual phase, which is considered a plausible contributor to the nearly temperature-independent cobalt extraction limit of approximately 69–74%. Multi-response optimization identified a mediator-to-solid ratio of approximately 0.8 mmol Fe2+ per gram of black mass, enabling sub-stoichiometric mediator use. At 45 °C and 120 min, leaching efficiencies reached 99.6% for lithium, 95.1% for nickel, and 70.4% for cobalt. These results demonstrate the feasibility of coupling chemical leaching with continuous electrochemical mediator regeneration while identifying the solid-state and mass-transfer limitations governing further process intensification.