The recovery of cobalt from spent lithium-ion batteries through hydrometallurgical methods has gained significant attention due to the increasing demand for this critical metal and the environmental necessity of recycling. Hydrometallurgy offers a selective and efficient approach to extract cobalt while minimizing energy consumption and emissions compared to pyrometallurgical routes. The process typically involves leaching, solvent extraction, precipitation, and electrowinning, each stage optimized to maximize cobalt recovery and purity.

Leaching is the first and most critical step, where cobalt is dissolved from the cathode material into a liquid medium. Acidic leaching using inorganic acids such as sulfuric acid, hydrochloric acid, or nitric acid is the most common method. Sulfuric acid is preferred due to its cost-effectiveness and high leaching efficiency. The leaching reaction is influenced by parameters such as acid concentration, temperature, solid-to-liquid ratio, and the presence of reducing agents like hydrogen peroxide. For example, using 2M sulfuric acid with 5% hydrogen peroxide at 80°C can achieve over 95% cobalt leaching efficiency from lithium cobalt oxide cathodes within two hours. Alkaline leaching, though less common, is explored for its selectivity, using ammonia or sodium hydroxide to dissolve cobalt while leaving impurities like aluminum in the solid residue. Bioleaching employs microorganisms such as Acidithiobacillus ferrooxidans to facilitate metal dissolution, offering an environmentally friendly alternative but with slower kinetics and lower efficiency compared to chemical leaching.

Following leaching, the solution contains cobalt along with other metals like nickel, manganese, and lithium. Solvent extraction is employed to selectively separate cobalt from the leachate. Organic extractants such as di-(2-ethylhexyl) phosphoric acid (D2EHPA), bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272), and trioctylamine (TOA) are commonly used. Cyanex 272 is particularly effective for cobalt-nickel separation due to its high selectivity for cobalt at optimal pH conditions. The extraction efficiency depends on factors like pH, extractant concentration, and the organic-to-aqueous phase ratio. Stripping the loaded organic phase with sulfuric acid yields a purified cobalt-rich solution, ready for further processing.

Precipitation is an alternative or supplementary method to solvent extraction, where cobalt is selectively precipitated as a hydroxide, carbonate, or oxalate. Sodium hydroxide or ammonium hydroxide is often used to adjust the pH and precipitate cobalt hydroxide. However, this method may co-precipitate other metals unless careful pH control is maintained. Oxalate precipitation is more selective, forming cobalt oxalate, which can be thermally decomposed to produce cobalt oxide or further processed into metallic cobalt. The purity of the precipitated product depends on the initial leachate composition and the precipitation conditions.

Electrowinning is the final step to obtain high-purity cobalt metal from the purified solution. The process involves passing an electric current through the solution, reducing cobalt ions at the cathode. Key parameters include current density, temperature, and electrolyte composition. A typical electrowinning setup uses stainless steel cathodes and lead anodes, with a cobalt sulfate solution at pH 3-4. The resulting cobalt deposits can achieve purity levels exceeding 99.9%, suitable for reuse in battery manufacturing.

Comparing different hydrometallurgical routes reveals trade-offs between efficiency, cost, and environmental impact. Acidic leaching with solvent extraction and electrowinning is the most established method, offering high recovery rates and purity but requiring careful handling of corrosive reagents. Bioleaching presents a greener alternative but faces challenges in scalability and processing time. Alkaline leaching is less corrosive but may require additional steps to achieve high cobalt purity. The choice of method depends on the specific battery chemistry, desired product quality, and economic considerations.

Environmental impact is a crucial factor in hydrometallurgical cobalt recovery. While the process generates less greenhouse gas emissions than pyrometallurgy, it produces acidic or alkaline wastewater containing residual metals. Proper treatment, such as neutralization and metal precipitation, is essential to prevent contamination. Recycling reagents and optimizing water usage can further reduce the environmental footprint. Life cycle assessments indicate that hydrometallurgical methods, when coupled with efficient waste management, offer a sustainable pathway for cobalt recovery.

In summary, hydrometallurgical methods provide a versatile and effective means to recover cobalt from spent lithium-ion batteries. By optimizing leaching, solvent extraction, precipitation, and electrowinning processes, high-purity cobalt can be reclaimed with minimal environmental impact. Continued research into reagent efficiency, waste reduction, and process integration will further enhance the sustainability and economic viability of cobalt recycling.