Solvent extraction is a critical step in cobalt recovery from spent lithium-ion batteries, offering high selectivity and efficiency in separating cobalt from other metals like nickel, lithium, manganese, and copper. The process relies on organic extractants that selectively bind cobalt ions from aqueous leach solutions, followed by stripping to recover purified cobalt. Key extractants such as D2EHPA (Di-2-ethylhexyl phosphoric acid), Cyanex 272 (bis(2,4,4-trimethylpentyl) phosphinic acid), and PC-88A (2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester) are widely used due to their distinct selectivity profiles and operational advantages.

D2EHPA is a versatile extractant capable of recovering multiple metals but requires careful pH control for cobalt selectivity. It operates effectively in a pH range of 4.5 to 5.5, where cobalt extraction efficiency peaks. However, D2EHPA also co-extracts nickel and manganese, necessitating additional purification steps. Stripping is typically performed using strong acids like sulfuric or hydrochloric acid at concentrations of 1-2 M. The organic-to-aqueous (O:A) ratio is maintained between 1:1 and 3:1 to balance extraction efficiency and phase separation. Industrial applications often use multi-stage counter-current extraction to improve purity.

Cyanex 272 is highly selective for cobalt over nickel, making it ideal for battery recycling where these two metals dominate. Its optimal extraction occurs at pH 5.0-6.0, with minimal nickel interference. The selectivity arises from the steric hindrance of its branched alkyl groups, which favor cobalt complexation. Stripping is achieved with dilute sulfuric acid (0.5-1.0 M), and phase separation is rapid due to the extractant’s low solubility in water. The O:A ratio is typically 1:1 for high cobalt recovery. A notable industrial case involves a European recycler achieving 99% cobalt purity using Cyanex 272 in a four-stage extraction process.

PC-88A offers intermediate selectivity between D2EHPA and Cyanex 272, extracting cobalt efficiently while partially co-extracting nickel. Its optimal pH range is 4.0-5.0, and stripping requires stronger acids (1.5-2.5 M) compared to Cyanex 272. PC-88A is favored in Asian recycling facilities due to its lower cost and robust performance in mixed-metal systems. Phase separation is slower than Cyanex 272 but faster than D2EHPA, with O:A ratios of 2:1 commonly employed. A Japanese plant reported 97% cobalt recovery using PC-88A with minimal nickel contamination after two extraction stages.

The choice of extractant depends on the leachate composition and desired purity. For high-nickel feeds, Cyanex 272 is preferred, while D2EHPA and PC-88A are suitable for complex mixtures with manganese or calcium. Optimization involves adjusting pH, extractant concentration, and O:A ratio. Extractant concentration typically ranges from 10-30% v/v in a diluent like kerosene. Higher concentrations improve cobalt loading but may increase viscosity, impairing phase separation. Modifiers like TBP (tri-butyl phosphate) are sometimes added to prevent third-phase formation.

pH control is critical for selectivity. Automated pH adjustment using sodium hydroxide or lime ensures consistent extraction efficiency. Over-acidification reduces cobalt recovery, while under-acidification increases impurity co-extraction. Continuous monitoring and feedback loops are employed in industrial setups to maintain optimal pH. Temperature also influences extraction, with most processes operating at 25-40°C to balance kinetics and stability.

Stripping efficiency depends on acid type, concentration, and contact time. Sulfuric acid is most common, but hydrochloric acid may be used for higher cobalt concentrations. Stripping is typically performed in 2-3 stages to maximize recovery. The loaded organic phase is washed with water to remove entrained impurities before stripping. Industrial systems often integrate extraction and stripping in mixer-settler units for continuous operation.

Phase separation is facilitated by optimizing organic phase composition and mixing intensity. Long chain alcohols or sulfonates are added as surfactants to reduce emulsion formation. Mixing time is kept short (2-5 minutes) to minimize droplet size and enhance settling. Centrifugal separators are sometimes used for difficult-to-separate systems.

Industrial case studies highlight the scalability of solvent extraction. A North American recycler achieved 98.5% cobalt recovery using Cyanex 272 at pH 5.5, with nickel content below 0.5%. The process operated at an O:A ratio of 1:1 and 20% extractant concentration. Another example is a Chinese facility using PC-88A to process NMC (nickel-manganese-cobalt) black mass, attaining 96% cobalt purity after three extraction stages and two stripping stages. The O:A ratio was 2:1, with 15% PC-88A in kerosene.

Challenges include organic phase degradation due to prolonged exposure to acidic conditions and impurity buildup. Regular solvent regeneration or replacement is necessary to maintain performance. Advanced systems incorporate online monitoring of extractant health and automated dosing of fresh reagent.

Future developments focus on novel extractants with higher selectivity and lower environmental impact. However, D2EHPA, Cyanex 272, and PC-88A remain industry standards due to their proven efficacy and cost-effectiveness. Process optimization continues to target higher purity, lower energy consumption, and reduced chemical usage, ensuring solvent extraction remains a cornerstone of cobalt recovery in battery recycling.