The recovery of cobalt and nickel from black mass, a mixture of shredded lithium-ion battery components, involves a series of carefully designed processes to maximize yield and purity. These metals are critical due to their high economic value and role in cathode materials. The recovery process typically includes pretreatment steps to prepare the black mass for metallurgical extraction, followed by hydrometallurgical or pyrometallurgical methods to isolate and refine cobalt and nickel.
**Pretreatment of Black Mass**
Before cobalt and nickel can be extracted, the black mass must undergo pretreatment to remove interfering materials and improve the efficiency of subsequent recovery steps.
1. **Sieving and Physical Separation**
The black mass consists of a heterogeneous mixture of cathode and anode materials, metals, plastics, and other residues. Sieving separates fine particles from larger debris, enabling more efficient processing. Magnetic separation can remove ferrous materials, while eddy current separation recovers non-ferrous metals like aluminum and copper. These steps reduce the load on downstream processes and improve metal recovery rates.
2. **Pyrolysis for Organic Removal**
Binders, electrolytes, and other organic components in black mass can hinder metal recovery. Pyrolysis, a thermal treatment conducted in an oxygen-free environment, decomposes these organics into volatile gases and carbonaceous residues. Typical pyrolysis temperatures range between 400°C and 800°C, effectively removing plastics and electrolytes while preserving metal oxides. The remaining char can be separated mechanically, leaving a cleaner feedstock for metallurgical processing.
3. **Leaching Preparation**
After pyrolysis, the black mass is often ground into a fine powder to increase surface area for leaching. Some processes employ roasting to convert metal oxides into more soluble forms. For example, sulfation roasting at 500°C–700°C with sulfuric acid converts cobalt and nickel oxides into water-soluble sulfates, enhancing leaching efficiency.
**Hydrometallurgical Recovery of Cobalt and Nickel**
Hydrometallurgy is widely used due to its selectivity and ability to achieve high purity. The process involves leaching, purification, and precipitation or electrowinning.
1. **Acid Leaching**
Sulfuric acid is the most common leaching agent due to its effectiveness and cost efficiency. A typical leaching process uses 2–4 M sulfuric acid at 60°C–90°C, often with a reducing agent like hydrogen peroxide to improve dissolution of cobalt and nickel oxides. Leaching efficiency can exceed 95% for both metals under optimized conditions.
Alternative leaching agents include hydrochloric acid or organic acids like citric acid, which offer lower environmental impact but may require longer processing times.
2. **Impurity Removal**
The leachate contains not only cobalt and nickel but also impurities like iron, aluminum, manganese, and lithium. Solvent extraction is the primary method for selective separation.
- **Iron and Aluminum Removal**: Adjusting the pH to 3–4 precipitates iron and aluminum as hydroxides, which can be filtered out.
- **Manganese and Lithium Separation**: Solvent extraction using extractants like di-(2-ethylhexyl) phosphoric acid (D2EHPA) selectively removes manganese, while lithium remains in the aqueous phase for later recovery.
3. **Cobalt and Nickel Separation**
The most challenging step is separating cobalt and nickel due to their similar chemical properties. Phosphinic acid-based extractants, such as Cyanex 272, selectively extract cobalt over nickel at a pH of 5–6. After extraction, cobalt is stripped from the organic phase using dilute sulfuric acid, while nickel remains in the raffinate.
Alternatively, selective precipitation can be employed. Adding sodium hypochlorite oxidizes cobalt to Co³⁺, which precipitates as cobalt hydroxide, leaving nickel in solution.
4. **Metal Recovery**
The purified cobalt and nickel solutions are processed into marketable products:
- **Electrowinning**: Cobalt and nickel are electrodeposited onto cathodes, producing high-purity metal sheets (≥99.8%).
- **Precipitation**: Adding oxalic acid precipitates cobalt as cobalt oxalate, which can be calcined to produce cobalt oxide. Nickel sulfate crystals can be obtained through evaporation.
**Pyrometallurgical Recovery of Cobalt and Nickel**
Pyrometallurgy is energy-intensive but effective for bulk processing, often used in conjunction with hydrometallurgy.
1. **Smelting**
Black mass is fed into a furnace with fluxes (e.g., silica, limestone) and a reducing agent (e.g., coke). At temperatures above 1400°C, cobalt and nickel are reduced to a metallic alloy, while impurities form a slag phase. The alloy typically contains 40–60% cobalt and nickel, along with copper and iron.
2. **Refining**
The alloy undergoes further refining:
- **Sulfidation**: Adding sulfur converts cobalt and nickel into sulfides, which can be separated from iron.
- **Electrorefining**: The sulfides are dissolved electrolytically, yielding high-purity cobalt and nickel.
**Yield Optimization and Impurity Control**
Maximizing cobalt and nickel recovery requires careful control of process parameters:
- **Leaching Efficiency**: Higher acid concentration, temperature, and agitation improve dissolution rates but must be balanced against corrosion and cost.
- **Solvent Extraction Selectivity**: Optimizing pH, extractant concentration, and phase ratios minimizes co-extraction of impurities.
- **Smelting Conditions**: Adjusting flux composition and temperature ensures clean separation of metal alloys from slag.
Impurities like manganese and lithium can reduce cathode material quality if not adequately removed. Advanced purification techniques, such as ion exchange or membrane filtration, may be employed for high-purity applications.
**Integration of Processes**
Many recycling operations combine pyro- and hydrometallurgical steps for optimal efficiency. For example, smelting produces a crude alloy that undergoes hydrometallurgical refining, while slag may be processed to recover residual metals. Alternatively, hydrometallurgical leaching residues can be smelted to recover additional metals.
The choice of method depends on feedstock composition, desired product purity, and economic considerations. Continuous process monitoring and adaptive control systems further enhance recovery rates and product quality.
By optimizing pretreatment, leaching, and refining steps, recyclers can achieve cobalt and nickel recovery rates exceeding 90%, ensuring a sustainable supply of these critical battery materials.