Investigating battery black mass leaching performance as a function of process parameters by combining leaching experiments and regression modeling

The current paper investigates the leaching phenomena of industrially produced Li-ion battery waste in hydrometallurgical recycling processes. Specifically, it studies the leaching reactions of NMC111-type (LiNi_1/3Mn_1/3Co_1/3O₂) black mass, as well as the statistical behavior of cathode material leaching yields under varying process conditions. The investigated process variables include reductive agent concentrations (Fe²⁺, Cu, H₂O₂) as well as process temperature, whereas S/L ratio (200 g/L) and initial acidity (2 M H₂SO₄) were kept constant. At lower temperatures (T = 30 °C), copper was found to act as the predominant reductant, enabled by the presence of sufficient solution iron concentrations (≥0.4 g/L Fe). Conversely, at higher temperatures (T ≥ 50 °C), the reductive capability of aluminum was substantially increased due to its decreased tendency for passivation. In contrast to copper, dissolved iron did not notably affect the reductive behavior of aluminum. The efficiency of metallic reductants initially present within the black mass was high, reaching cathode metal leaching yields above 90 % at 70 °C. A predictive leaching model for black mass leaching yields was built via regression analysis and can be used to indicate pregnant leach solution (PLS) composition – via leaching yields – after two hours of processing as a function of the investigated process variables. The model showed leaching temperature to be the most impactful parameter, while also indicating a higher reductive efficiency of copper when compared to equimolar additions of H₂O₂. As a case example, LFP (LiFePO₄) cathode powder was also investigated as an alternative reductant/catalysis species (Fe²⁺) source in the system and was found to increase cathode metal leaching yields almost as much as FeSO₄ while also resulting in a remarkable increase in Al dissolution in the process.