Impurity control and post-crystallization purification of CoSO4•7H2O for battery-grade cobalt production

The increasing demand for battery-grade cobalt sulfate (CoSO 4 •7H 2 O) in lithium-ion battery production requires stringent control of impurity incorporation during crystallization. This study systematically investigates the mechanisms governing impurity uptake and removal in CoSO 4 •7H 2 O through a combination of evaporative crystallization, post-crystallization purification, and advanced characterization. A synthetic acidic feed containing controlled levels of impurities was crystallized under carefully managed supersaturation to produce CoSO 4 •7H 2 O crystals. Impurity mapping, SEM–EDS, and ToF-SIMS analyses revealed distinct incorporation behaviors depending on ionic size and charge. Divalent cations with radii close to Co 2+ (Ni 2+ , Cu 2+ , Mg 2+ , Mn 2+ ) exhibited linear impurity release profiles and spatial co-localization with Co, consistent with lattice substitution. In contrast, monovalent (Na + , Cs + ) and large divalent (Ca 2+ ) ions showed surface enrichment, confirming adsorptive or inclusion-type uptake. Sequential displacement and repulp washing removed up to ~80% of surface-associated Na, increasing Co purity from 99.71% to 99.88%, demonstrating the practical effectiveness of post-crystallization purification. However, lattice-incorporated impurities remained largely unchanged, demonstrating the limited efficacy of surface-targeted purification. The results highlight the need for dual impurity-control strategies, upstream removal of lattice-forming species and downstream washing for surface contaminants. This work establishes a mechanistic framework linking ionic properties, partitioning behavior, and washability during CoSO 4 •7H 2 O crystallization, providing actionable insights for process optimization and the production of high-purity cobalt sulfate for battery manufacturing.