Current State of Hydrometallurgical Processing for the Recovery of Nickel and Cobalt from Primary and Secondary Sources: Part II—Metal Separation and Purification Approaches

Nickel–cobalt hydrometallurgical flowsheets face significant challenges during the purification stage, where closely similar metal chemistries, complex speciation, and high impurity loads ultimately impact product quality, process stability, and operating costs. While upstream beneficiation and leaching control metal availability, it is the downstream separation and upgrading steps that determine the potential for producing high-purity, battery-grade or metallurgical products at scale. This review, presented as Part II of a two-part series, focuses exclusively on the metal separation and purification strategies. Solvent extraction is discussed in this part as the core downstream separation technique, given its central role in achieving selective nickel–cobalt separation in complex aqueous systems. Emphasis is placed on the role of metal speciation as a guiding principle for extractant selection and process optimization. In addition, the review critically examines complementary solid-formation and separation processes, which are often underrepresented in the literature despite their decisive contribution to achieving high-purity nickel and cobalt products and their integration within industrial hydrometallurgical flowsheets. By synthesizing dispersed knowledge into a coherent framework, this review provides practical insights to support the design of selective, efficient, and sustainable hydrometallurgical processes for nickel and cobalt recovery.