Current State of Hydrometallurgical Processing for the Recovery of Nickel and Cobalt from Primary and Secondary Sources: Part I —Resources and Market Context, Physical Beneficiation, and Leaching Strategies

Nickel (Ni) and cobalt (Co) are critical metals with rapidly growing demand due to their essential role in battery cathodes for electric vehicles and large-scale energy storage systems. Ensuring a sustainable and high-purity supply of these metals presents major challenges, particularly in the context of hydrometallurgical processing, where efficiency, selectivity, operational cost, and environmental impact must be carefully balanced. This review, presented as Part I of a two-part series, provides a comprehensive and critically integrated overview of upstream hydrometallurgical strategies for Ni and Co recovery, with emphasis on resource characteristics, physical beneficiation, and pretreatment, and chemical dissolution and leaching. The article discusses ore types, mineralogical features, and feed preparation methods, highlighting their influence on leaching behavior and overall process performance. Conventional approaches such as atmospheric leaching and high-pressure acid leaching are evaluated alongside emerging techniques, including bioleaching and process-intensification strategies such as microwave- and ultrasound-assisted leaching. Particular attention is given to the trade-offs governing process selection, including acid consumption, energy demand, reaction kinetics, and scalability constraints. In addition, the integration of secondary feedstocks, such as tailings, metallurgical residues, and end-of-life products, is discussed within a unified flowsheet framework, emphasizing their role in enhancing resource efficiency and enabling circular processing pathways.