The recovery of nickel from spent lithium-ion battery black mass requires precise process engineering to achieve high purity and yield while addressing challenges posed by complex feedstock composition. Mechanically processed black mass, derived from shredded NMC622 cathodes, contains nickel, cobalt, manganese, lithium, and residual organics, necessitating systematic pretreatment and extraction steps.

Pretreatment begins with sieving to remove coarse debris and homogenize particle size distribution. A 200-mesh sieve (75 µm) is typically employed to ensure consistent leaching kinetics. Magnetic separation follows, exploiting nickel's ferromagnetic properties to concentrate nickel-rich fractions. Eddy current separation may supplement this step to remove aluminum foils, though complete foil removal is rarely achieved, leaving trace aluminum as an impurity.

Redox-assisted leaching is critical for nickel extraction. Sulfuric acid (1.5-2.5 M) with hydrogen peroxide (3-5 vol%) as a reducing agent achieves over 95% nickel dissolution at 70-80°C within 2 hours. The peroxide facilitates the reduction of Ni(III) to Ni(II), enhancing solubility while suppressing manganese dioxide formation. Optimal solid-to-liquid ratios range from 1:8 to 1:12 to balance leaching efficiency and downstream processing volume. For NMC622 feedstock (60% Ni, 20% Co, 20% Mn), the leachate typically contains 25-30 g/L Ni, 8-10 g/L Co, and 8-10 g/L Mn.

Solid-liquid separation poses significant challenges due to carbon/binder residues. Polyvinylidene fluoride (PVDF) and conductive carbon form colloidal suspensions that reduce filtration rates by 40-60% compared to residue-free solutions. Pressure filtration at 3-5 bar with diatomaceous earth pre-coat improves clarity, though fouling remains a concern. Centrifugation offers an alternative but struggles with fine carbon particulates below 10 µm. Residual organics also complicate impurity profiles, with fluorides from PVDF degradation (50-200 mg/L) requiring subsequent precipitation steps.

Two flowsheet approaches dominate nickel recovery: single-stage and multi-stage extraction. The single-stage process combines all metals in one leachate, followed by solvent extraction or precipitation. A mass balance for 100 kg NMC622 black mass (60 kg Ni) yields:
- Leachate: 58.2 kg Ni (97% yield)
- Residue: 1.8 kg Ni (3% loss)
- Impurities: 0.5 kg Al, 2.1 kg Fe

Multi-stage extraction employs selective leaching, first targeting nickel at lower acid concentrations (0.5-1.0 M H2SO4), achieving 85-90% nickel recovery while leaving cobalt and manganese in the solid phase. The second stage uses stronger acid (2.0-3.0 M) for remaining metals. For the same feedstock:
- Stage 1 leachate: 51 kg Ni (85% yield)
- Stage 2 leachate: 6 kg Ni (10% yield)
- Residue: 3 kg Ni (5% loss)
- Impurities: 0.3 kg Al, 1.7 kg Fe

The multi-stage approach reduces downstream purification costs but increases process complexity and energy consumption by 15-20%. Carbon residues affect both methods similarly, though multi-stage systems allow for intermediate residue washing to reduce organic carryover.

Final nickel purification typically employs solvent extraction with di-(2-ethylhexyl) phosphoric acid (D2EHPA) at pH 2.5-3.5, achieving 99.5% Ni/Co separation efficiency. Residual manganese and aluminum are removed via pH-controlled precipitation with sodium hydroxide, maintaining nickel losses below 0.5%. The process must account for fluoride complexation with nickel, which can reduce extraction efficiency by 5-8% if not addressed through prior calcium precipitation.

Process optimization requires balancing nickel recovery, energy input, and reagent consumption. Single-stage systems favor throughput and simplicity, while multi-stage flowsheets provide superior purity for battery-grade nickel sulfate production. Both must incorporate robust solid-liquid separation and impurity management to meet the stringent specifications of cathode precursor synthesis.