Nickel recovery from spent batteries and production scraps is a critical step in establishing a circular economy for battery materials. The refining of recovered nickel to battery-grade nickel sulfate hexahydrate (NiSO4·6H2O) requires precise process engineering to meet the stringent purity standards for precursor synthesis in lithium-ion battery cathode production. This article details the key unit operations, impurity control strategies, and analytical methods involved in producing high-purity nickel sulfate.
The process begins with the dissolution of nickel-containing black mass or intermediate products in sulfuric acid, typically at concentrations between 15-25% w/w at 60-80°C. The resulting leachate contains nickel along with impurities such as iron, zinc, cobalt, calcium, and magnesium. Initial purification involves pH adjustment to 2.5-3.5 with controlled hydroxide addition to precipitate iron and aluminum as hydroxides while keeping nickel in solution. Filtration removes these solids, yielding a clarified solution for further refining.
Solvent extraction (SX) serves as the primary purification step for nickel separation from other transition metals. Versatic 10 acid is commonly employed for bulk impurity removal due to its selective extraction of divalent cations in the order Cu>Zn>Co>Ni>Fe>Mn>Mg>Ca. Operating at pH 4.0-5.5, Versatic acid extracts zinc and calcium while leaving nickel in the raffinate. For cobalt separation, phosphinic acid extractants like Cyanex 272 are used at pH 5.0-6.0, showing over 10,000:1 selectivity for cobalt over nickel. A typical SX circuit consists of 2-4 extraction stages followed by stripping with dilute sulfuric acid.
Following solvent extraction, residual impurities are removed through sulfide precipitation. Sodium sulfide or hydrogen sulfide gas is added in controlled amounts to precipitate remaining copper, zinc, and cobalt as sulfides. The process requires careful control of sulfide dosage to avoid nickel loss, with optimal performance achieved at redox potentials between -100 to -200 mV vs SHE. Filtration yields a purified nickel solution containing less than 10 ppm total impurities.
Crystallization of nickel sulfate hexahydrate is the final purification and product formation step. The process employs evaporative crystallization at 50-70°C with careful control of supersaturation to produce uniform crystals meeting particle size specifications. Key parameters include evaporation rate (typically 10-15% water removal per hour), cooling rate (0.5-1.0°C/min), and seed crystal loading (5-10% w/w). The resulting crystals are separated by centrifugation and dried at 40-50°C to prevent dehydration.
Two primary process routes exist for nickel sulfate production: sulfate and chloride. The sulfate route, described above, is dominant due to its compatibility with most battery cathode synthesis processes. The chloride route involves leaching with hydrochloric acid followed by solvent extraction and crystallization, producing NiCl2·6H2O. While the chloride route offers higher leaching efficiency for some feed materials, it requires additional conversion steps to produce sulfate for battery applications and faces challenges in removing chloride residues below 100 ppm specifications.
Quality control for battery-grade nickel sulfate follows strict standards. ASTM E1177-14 and UBM standards specify maximum impurity levels:
Element Maximum (ppm)
Fe 5
Co 100
Zn 5
Ca 10
Cu 5
Cr 5
Na 20
Cl 100
Analytical methods ensure compliance with these specifications. Inductively coupled plasma optical emission spectroscopy (ICP-OES) provides quantitative multi-element analysis with detection limits below 0.1 ppm for most metals. X-ray diffraction (XRD) verifies crystal structure and phase purity, with battery-grade material showing >99% NiSO4·6H2O phase content. Particle size distribution is measured by laser diffraction, targeting D50 values of 100-300 μm for optimal cathode precursor synthesis.
The integrated purification train typically follows this sequence:
1. Leaching: Sulfuric acid dissolution of nickel feed
2. Iron removal: pH adjustment and filtration
3. Solvent extraction: Versatic acid for Zn/Ca removal
4. Solvent extraction: Cyanex 272 for Co removal
5. Sulfide polishing: Residual impurity precipitation
6. Crystallization: Evaporative crystallization
7. Solid-liquid separation: Centrifugation
8. Drying: Low-temperature fluid bed drying
9. Packaging: Moisture-controlled environment
Process optimization focuses on maximizing nickel recovery while minimizing reagent consumption. Overall recoveries above 98% are achievable with proper control of solvent extraction isotherms and crystallization yields. Energy consumption ranges from 8-12 kWh per kg of nickel sulfate produced, with the crystallization step accounting for approximately 60% of total energy use.
The production of battery-grade nickel sulfate represents a critical link in the sustainable battery value chain. By combining hydrometallurgical purification techniques with rigorous quality control, recyclers can transform recovered nickel into high-purity products meeting the exacting standards of cathode manufacturers. Continued process improvements aim to reduce costs while further enhancing purity levels to support next-generation battery technologies.