The recovery of cobalt and nickel from black mass, a fine powder derived from shredded lithium-ion batteries, represents a critical step in battery recycling. These valuable metals are concentrated in the cathode materials of spent batteries, particularly in nickel-manganese-cobalt (NMC) and nickel-cobalt-aluminum (NCA) chemistries. The hydrometallurgical processing of black mass involves several stages, including leaching, purification, and metal recovery, with selective precipitation, solvent extraction, and electrowinning being the most widely adopted techniques.

Black mass is first subjected to leaching, typically using sulfuric acid as the primary lixiviant, often combined with hydrogen peroxide as a reducing agent to enhance metal dissolution. The resulting leachate contains cobalt, nickel, lithium, manganese, and other impurities such as iron, aluminum, and copper. The subsequent purification and recovery steps focus on separating cobalt and nickel from this complex mixture.

Selective precipitation is a common method for initial separation. By carefully controlling pH, cobalt and nickel can be sequentially precipitated as hydroxides or carbonates. For instance, raising the pH to around 3-4 with sodium hydroxide or calcium carbonate precipitates iron and aluminum as hydroxides, which are removed by filtration. Further pH adjustment to 7-8 precipitates cobalt hydroxide, while nickel remains in solution due to its higher solubility. This method is cost-effective but may yield products with lower purity due to co-precipitation of impurities. Optimization involves precise pH control, temperature management, and the use of chelating agents to minimize contamination.

Solvent extraction offers higher selectivity and purity. Organic extractants such as di-(2-ethylhexyl) phosphoric acid (D2EHPA) or Cyanex 272 are employed to selectively separate cobalt and nickel from the leachate. In a typical flowsheet, D2EHPA first removes impurities like manganese and residual iron, followed by Cyanex 272 to separate cobalt from nickel. The loaded organic phase is then stripped with sulfuric acid to recover high-purity cobalt sulfate solution, while nickel remains in the raffinate. Process optimization includes adjusting extractant concentration, phase ratios, and stripping conditions to maximize recovery and minimize cross-contamination. Solvent extraction is highly efficient but requires significant capital and operational expertise.

Electrowinning is the final step for producing pure metal cathodes. The purified cobalt or nickel sulfate solutions are electrolyzed in cells equipped with inert anodes and stainless-steel cathodes. For cobalt, typical conditions include a current density of 200-300 A/m² and a temperature of 50-60°C, yielding high-purity cobalt deposits. Nickel electrowinning operates at similar current densities but may require chloride additives to enhance deposition efficiency. Impurities such as zinc or lead must be reduced to trace levels to prevent cathode contamination. Electrowinning is energy-intensive but produces market-ready metal products.

Impurity removal is critical throughout the process. Iron and aluminum are typically removed by precipitation at low pH, while copper is eliminated via cementation with iron powder or solvent extraction. Manganese poses a challenge due to its chemical similarity to cobalt and nickel; selective oxidation or solvent extraction is often employed. Process optimization focuses on minimizing reagent consumption, reducing waste generation, and ensuring high recovery rates. For example, integrating solvent extraction with selective precipitation can improve overall efficiency.

Several hydrometallurgical flowsheets exist, each with trade-offs. A simplified approach might involve leaching followed by selective precipitation, offering lower capital costs but reduced purity. More complex flowsheets incorporate multiple solvent extraction stages, delivering higher purity at greater expense. The choice depends on feedstock composition, desired product specifications, and economic considerations. For instance, operations targeting battery-grade materials may prioritize purity over cost, while those focused on bulk metals may opt for simpler processes.

Industrial-scale operations provide valuable case studies. One prominent recycler employs a flowsheet combining leaching, solvent extraction, and electrowinning to recover cobalt and nickel from black mass. The process achieves over 95% recovery for both metals, with final purities exceeding 99.5%. Another operation uses selective precipitation followed by electrowinning, achieving slightly lower purity but with reduced operational complexity. Economic analysis shows that solvent extraction-based flowsheets have higher upfront costs but lower long-term operating expenses due to reduced reagent consumption and waste treatment needs.

The economic impact of these processes is significant. Cobalt and nickel account for a substantial portion of black mass value, and efficient recovery directly affects recycling profitability. Flowsheets with high recovery rates and low impurity levels command premium pricing for their products. However, the cost of reagents, energy, and waste management must be carefully balanced. For example, solvent extraction reagents are expensive but reusable, while precipitation reagents are cheaper but generate more waste. Process optimization aims to strike this balance while meeting market demands.

In summary, cobalt and nickel recovery from black mass relies on a combination of selective precipitation, solvent extraction, and electrowinning. Each technique has advantages and limitations, with process selection depending on desired outcomes and economic constraints. Industrial operations demonstrate the feasibility of high recovery rates and purities, though ongoing optimization is essential to improve efficiency and reduce costs. The continued evolution of these processes will play a pivotal role in the sustainable recycling of lithium-ion batteries.