The recovery of nickel from polymetallic solutions is a critical step in battery recycling, particularly for sustaining the supply chain of nickel-rich cathodes used in lithium-ion batteries. Recent advances in solvent extraction have introduced novel extractants that improve selectivity, efficiency, and operational stability. Among these, phosphonium-based ionic liquids such as Cyphos IL 104 and hydroxyoxime-based reagents like LIX 87QN have demonstrated superior performance in nickel separation.
Cyphos IL 104, a quaternary phosphonium ionic liquid, operates through anion exchange mechanisms, selectively extracting nickel as an anionic chlorocomplex from acidic chloride media. Extraction isotherms indicate near-quantitative recovery at equilibrium pH values between 2.5 and 3.5, with a loading capacity of approximately 12 g/L in the organic phase. The extraction follows a slope of ~2 in log-log plots, suggesting the involvement of two extractant molecules per nickel ion. In contrast, LIX 87QN, a modified hydroxyoxime, extracts nickel via cation exchange, showing optimal performance in sulfate media at pH 4.0–5.0. Its extraction isotherm exhibits a steep rise in distribution coefficients above pH 4.0, with a maximum loading capacity of 8–10 g/L.
Stripping efficiency is a crucial parameter for process economics. For Cyphos IL 104, sulfuric acid concentrations above 1.5 M achieve over 98% nickel recovery from the loaded organic phase. However, excessive acid concentrations (>3 M) lead to increased co-extraction of impurities such as cobalt and iron. LIX 87QN requires milder stripping conditions, with 0.5–1.0 M sulfuric acid sufficient for ≥99% nickel recovery. Phase disengagement rates differ significantly between the two systems. Cyphos IL 104, being an ionic liquid, forms stable emulsions with aqueous phases, requiring 5–10 minutes for complete phase separation in mixer-settler units. LIX 87QN, with its lower viscosity, achieves faster disengagement in under 3 minutes.
Equipment selection impacts both extraction kinetics and operational efficiency. Mixer-settlers, the conventional choice, provide high throughput but suffer from extended residence times and potential crud formation with viscous extractants like Cyphos IL 104. Centrifugal contactors, though more energy-intensive, enhance mass transfer and reduce phase disengagement times to under 30 seconds, making them preferable for ionic liquid-based systems. Pilot-scale trials using centrifugal contactors with Cyphos IL 104 demonstrated nickel purity of 99.94% from a feed solution containing Ni, Co, Mn, and Fe in a 5:2:2:1 ratio.
Organic phase degradation is a persistent challenge. Cyphos IL 104 undergoes progressive loss of extraction efficiency due to anion exchange with sulfate during stripping, necessitating periodic reconditioning with hydrochloric acid. LIX 87QN is susceptible to oxidative degradation, particularly in the presence of iron(III), leading to the formation of ketone byproducts that reduce extraction capacity. Mitigation strategies include the addition of antioxidants like nonylphenol and maintaining reducing conditions (Eh < 0.4 V) in the aqueous phase.
Pilot plant data from a 100 L/day continuous operation confirmed the robustness of these extractants. Using a three-stage extraction and two-stage stripping configuration, nickel recovery exceeded 99.5%, with final electrolyte assays showing impurity levels below 50 ppm for cobalt and 10 ppm for iron. The organic phase maintained stable performance over 200 cycles with less than 5% loss in extraction efficiency when proper mitigation measures were applied.
In conclusion, both Cyphos IL 104 and LIX 87QN offer viable pathways for high-purity nickel recovery, with selection dependent on feed composition and process constraints. Centrifugal contactors improve efficiency for viscous systems, while mixer-settlers remain cost-effective for traditional extractants. Degradation management is critical for long-term viability, with ongoing research focused on enhancing reagent stability under industrial conditions.