Leaching strategies for Si and Al impurity management during industrial black mass processing

Understanding impurity behavior during hydrometallurgical recycling of lithium-ion batteries is essential for producing high-purity battery-grade products and enabling graphite purification. This study investigates the leaching behavior of silicon (Si) and aluminum (Al) during black mass processing in sulfuric (H 2 SO 4 ) and hydrochloric (HCl) acid systems, considering the influence of acidity, temperature, reaction time, and H 2 O 2 addition. The results show that increasing acid concentration from 2 M to 4 M enhances Si dissolution by ~20%-units in both leaching systems at 60 °C. The role of H 2 O 2 strongly depends on the acid medium. In H 2 SO 4 systems, H 2 O 2 increases Si extraction from ~43% to ~54%, indicating a significant oxidant-enhanced dissolution effect. In contrast, in HCl systems, H 2 O 2 decreases Si extraction from ~37% to ~17%, suggesting the formation of surface–stabilized or passivating conditions under chloride-rich environments. Lowering the temperature to 20 °C significantly suppresses Si leaching to below 15%, highlighting strong temperature dependence. Al dissolution shows weaker sensitivity to acid concentration and type, indicating limited reactivity and possible surface stabilization effects; however, H 2 O 2 increases Al extraction by ~10%-units. From an industrial perspective, impurity transfer can be tuned through acid selection and operating conditions. HCl leaching is favorable for minimizing Si and Al dissolution under mild conditions (short leaching time and ≤ 60 °C), while H 2 SO 4 systems under more aggressive conditions promote impurity release from graphite–rich residues. These findings provide quantitative insights into oxidant- and acid-dependent impurity control and support process design for recycling Si– and Al–rich battery waste.