Nickel recovery from spent lithium-ion batteries through hydrometallurgical methods has gained prominence due to its efficiency, lower energy requirements, and ability to produce high-purity products compared to traditional pyrometallurgical approaches. As demand for nickel rises—particularly for cathodes in nickel-manganese-cobalt (NMC) and nickel-cobalt-aluminum (NCA) batteries—developing optimized recycling processes is critical for sustainability and resource security. Hydrometallurgy offers a versatile framework for nickel extraction, encompassing leaching, purification, and recovery stages, each with distinct advantages and challenges.
**Leaching Techniques**
The first step in hydrometallurgical nickel recovery involves dissolving the metal from battery black mass, a mixture of cathode materials, conductive additives, and binders. Acid leaching is the most common method, utilizing inorganic acids such as sulfuric acid (H₂SO₄), hydrochloric acid (HCl), or nitric acid (HNO₃). Sulfuric acid is preferred due to its cost-effectiveness and efficiency, with optimal leaching occurring at concentrations of 1-4 M and temperatures between 60-90°C. Reductants like hydrogen peroxide (H₂O₂) are often added to enhance dissolution, particularly for metals in higher oxidation states. Studies show nickel recovery rates exceeding 95% under these conditions.
Bioleaching presents an alternative using microorganisms such as *Acidithiobacillus ferrooxidans* or *Aspergillus niger* to generate organic acids that solubilize metals. While energy-efficient and environmentally benign, bioleaching suffers from slower kinetics and lower yields compared to chemical leaching, with nickel recoveries typically ranging between 70-85%. Research is ongoing to improve bacterial strains and optimize conditions for industrial scalability.
**Solvent Extraction and Purification**
Following leaching, solvent extraction (SX) selectively isolates nickel from other metals like cobalt, lithium, and manganese. Extractants such as di-(2-ethylhexyl) phosphoric acid (D2EHPA) or Cyanex 272 are employed due to their high selectivity for nickel at specific pH levels. The process involves mixing the leachate with an organic phase containing the extractant, where nickel ions transfer into the organic phase. Subsequent stripping with a dilute acid, often sulfuric or hydrochloric, recovers nickel in a purified aqueous solution.
SX efficiency depends on pH control, extractant concentration, and phase ratios. Impurities like iron and aluminum must be removed beforehand through precipitation or ion exchange to prevent interference. Modern systems employ multi-stage counter-current extraction to maximize recovery and purity, with industrial operations achieving nickel purities above 99.5%.
**Precipitation and Electrowinning**
Nickel can be recovered from purified solutions through chemical precipitation or electrowinning. Precipitation with sodium hydroxide (NaOH) or oxalic acid (H₂C₂O₄) yields nickel hydroxide (Ni(OH)₂) or nickel oxalate (NiC₂O₄), respectively. These intermediates are calcined to produce nickel oxide (NiO) or reduced to metallic nickel powder. While simple, precipitation may introduce impurities if other metals remain in solution.
Electrowinning is favored for producing high-purity nickel cathodes. The process involves passing an electric current through the nickel-rich solution, causing nickel ions to deposit onto stainless-steel cathodes. Operating parameters such as current density (200-300 A/m²), temperature (50-60°C), and pH (3-4) influence deposit quality and energy efficiency. Electrowinning achieves purities exceeding 99.8%, suitable for direct reuse in battery manufacturing.
**Advantages Over Pyrometallurgy**
Hydrometallurgy outperforms pyrometallurgy in several key areas. Pyrometallurgical smelting, which operates above 1400°C, consumes substantial energy and often produces mixed alloys requiring further refining. In contrast, hydrometallurgical processes operate at near-ambient temperatures, reducing energy use by up to 50%. Additionally, hydrometallurgy enables selective recovery of individual metals, minimizing cross-contamination and yielding higher-value products. Environmental benefits include lower greenhouse gas emissions and reduced slag waste.
**Challenges and Innovations**
Despite its advantages, hydrometallurgy faces hurdles. Reagent costs, particularly for extractants and acids, impact economic viability. Wastewater treatment is another concern, as spent leachates contain residual acids and metal ions requiring neutralization or recycling. Process optimization is complicated by varying cathode compositions—NMC (LiNiₓMnₓCoₓO₂) and NCA (LiNiₓCoₓAlO₂) require tailored leaching conditions to avoid co-dissolution of unwanted elements.
Emerging innovations aim to address these challenges. Selective leaching using tailored organic acids or deep eutectic solvents minimizes reagent use and enhances metal specificity. Direct recycling approaches, where cathode materials are regenerated without full dissolution, are also under exploration. Advanced separation technologies like membrane filtration or adsorption resins show promise for reducing solvent extraction steps.
**Industrial Implementations**
Several companies have commercialized hydrometallurgical nickel recovery. Umicore’s Hoboken plant combines pyro- and hydrometallurgy to process mixed battery waste, achieving nickel recovery rates above 98%. American Manganese Inc. employs a closed-loop leaching and electrowinning process for NMC cathodes, emphasizing minimal waste generation. In China, Brunp Recycling’s facility integrates solvent extraction with precipitation to produce battery-grade nickel sulfate.
These cases highlight the scalability of hydrometallurgy, though further optimization is needed to handle the growing diversity of battery chemistries. Automation and real-time monitoring are being adopted to improve consistency and reduce operational costs.
**Conclusion**
Hydrometallurgical nickel recovery stands as a sustainable and efficient alternative to pyrometallurgy, aligning with circular economy goals. While challenges like reagent management and process adaptability persist, ongoing advancements in selective leaching and purification are enhancing its feasibility. As battery recycling infrastructure expands, hydrometallurgy will play a pivotal role in securing nickel supply chains and reducing reliance on primary mining.