Solvent extraction is a critical hydrometallurgical process for separating and purifying cobalt and nickel from mixed-metal solutions in battery recycling. The method relies on the selective transfer of metal ions from an aqueous phase to an organic phase using specific extractants, followed by stripping to recover high-purity metals. Key extractants such as D2EHPA, Cyanex, and Versatic Acid exhibit distinct selectivity mechanisms, enabling efficient separation of cobalt and nickel. Innovations in reagent formulations and process optimization have enhanced industrial scalability and metal recovery rates.
The solvent extraction process begins with leaching black mass from spent lithium-ion batteries, yielding a sulfate or chloride solution containing cobalt, nickel, manganese, lithium, and impurities like iron and aluminum. After impurity removal via precipitation or ion exchange, the solution undergoes solvent extraction for cobalt-nickel separation. The choice of extractant depends on the solution chemistry, pH, and target metal selectivity.
D2EHPA (Di-2-ethylhexyl phosphoric acid) is widely used for cobalt extraction due to its strong affinity for divalent metals. It operates effectively in acidic conditions (pH 2–4), where cobalt is preferentially extracted over nickel. The extraction mechanism involves cation exchange, where hydrogen ions from D2EHPA are replaced by cobalt ions in the aqueous phase. The reaction can be represented as:
Co²⁺ (aq) + 2(HA)₂ (org) → CoA₂·(HA)₂ (org) + 2H⁺ (aq)
Here, HA represents the dimeric form of D2EHPA. Nickel extraction is minimal under these conditions, allowing for high selectivity. However, D2EHPA also co-extracts manganese and zinc, necessitating scrubbing steps.
Cyanex 272 (bis(2,4,4-trimethylpentyl) phosphinic acid) offers superior cobalt-nickel separation compared to D2EHPA due to its higher selectivity. It operates at a slightly higher pH range (4–6), where cobalt extraction is favored while nickel remains in the aqueous phase. The extraction equilibrium for Cyanex 272 follows:
Co²⁺ (aq) + 2(HA)₂ (org) → CoA₂·(HA)₂ (org) + 2H⁺ (aq)
The steric hindrance of Cyanex 272’s branched alkyl groups reduces nickel extraction, enhancing cobalt purity. Innovations such as modifier additives have improved phase separation kinetics and reduced organic phase degradation, increasing industrial viability.
Versatic Acid (a branched carboxylic acid) is employed for nickel extraction from cobalt-depleted solutions. Unlike phosphoric acid-based extractants, Versatic Acid operates in near-neutral to alkaline conditions (pH 6–8), where nickel forms stable complexes. The extraction reaction involves proton exchange:
Ni²⁺ (aq) + 2RH (org) → NiR₂ (org) + 2H⁺ (aq)
Versatic Acid’s selectivity for nickel over cobalt is moderate, requiring careful pH control. Recent advances include synergistic systems combining Versatic Acid with other extractants to improve nickel selectivity and reduce co-extraction of impurities.
Scrubbing is essential to remove co-extracted impurities before stripping. For D2EHPA and Cyanex systems, dilute sulfuric acid or hydrochloric acid scrubs manganese, calcium, and magnesium from the loaded organic phase. In Versatic Acid systems, selective scrubbing with ammonia solutions removes residual cobalt. Process optimization has reduced scrubbing stages through tailored reagent blends, lowering operational costs.
Stripping recovers purified metals from the organic phase. For cobalt-loaded D2EHPA or Cyanex, strong hydrochloric acid (4–6 M) efficiently strips cobalt as CoCl₄²⁻, while sulfuric acid (1–2 M) is used for sulfate systems. Nickel stripping from Versatic Acid requires dilute sulfuric or nitric acid (0.5–1 M). Innovations like multi-stage counter-current stripping have increased metal recovery efficiency above 99%.
Industrial scalability hinges on extractant stability, phase disengagement rates, and reagent consumption. Modern solvent extraction plants employ mixer-settlers or centrifugal contactors for high throughput. Continuous monitoring and automation ensure consistent pH control and phase ratios, minimizing cobalt and nickel losses. Recent developments include:
- Chelating extractants with higher selectivity and lower acid consumption.
- Solvent-resistant polymers to reduce organic phase entrainment.
- Closed-loop systems for reagent regeneration, cutting waste generation.
The environmental footprint of solvent extraction is mitigated by recycling organic phases and optimizing acid usage. Compared to precipitation methods, solvent extraction achieves higher purity (99.9% Co and Ni) with fewer solid wastes. However, challenges persist in handling chloride solutions due to corrosion and in separating nickel from cobalt in highly concentrated feeds.
Future directions focus on novel extractants with higher pH tolerance and reduced sensitivity to feed variability. Research into bio-based extractants and ionic liquids aims to improve sustainability. Industrial adoption will depend on balancing reagent costs, metal recovery efficiency, and compliance with evolving environmental regulations.
In summary, solvent extraction remains indispensable for cobalt-nickel separation in battery recycling. D2EHPA, Cyanex, and Versatic Acid provide versatile platforms for selective metal recovery, while innovations in scrubbing, stripping, and process design enhance scalability. As demand for battery metals grows, optimized solvent extraction processes will play a pivotal role in sustainable resource recovery.