Pyrometallurgical recycling is a widely used method for recovering valuable metals from spent lithium-ion batteries. A critical aspect of this process is slag formation, which plays a pivotal role in separating metals from impurities. Slag, a molten oxide phase, is engineered to selectively absorb unwanted elements while allowing target metals like cobalt, nickel, and copper to settle in a molten alloy phase. The efficiency of metal recovery hinges on slag composition, viscosity, and thermodynamic interactions between the slag and metal phases.

Slag is primarily composed of fluxes such as silica (SiO2), calcium oxide (CaO), and alumina (Al2O3), which are added to adjust physicochemical properties. The ratio of these components determines the slag’s melting point, viscosity, and capacity to dissolve oxides. For instance, a basic slag with high CaO content is often preferred for its ability to capture phosphorus and sulfur, while acidic slags rich in SiO2 are effective in dissolving metal oxides. The binary basicity (CaO/SiO2 ratio) is a key parameter, typically maintained between 0.8 and 1.5 to ensure optimal fluidity and metal-slag separation.

Viscosity control is crucial for efficient pyrometallurgical processing. A slag that is too viscous impedes the settling of metal droplets, leading to entrapment and losses. Conversely, excessively fluid slag may result in poor impurity removal. The temperature and composition directly influence viscosity, with higher temperatures reducing viscosity but increasing energy costs. Industrial operations often operate between 1400°C and 1600°C to balance these factors. Additives like magnesium oxide (MgO) or iron oxide (FeO) can further fine-tune viscosity by modifying the slag’s network structure.

Metal-slag partitioning coefficients dictate the distribution of elements between the two phases. Cobalt and nickel exhibit high affinities for the metal phase, with distribution coefficients (Lm = [%M]metal/[%M]slag) often exceeding 100 under reducing conditions. In contrast, lithium predominantly reports to the slag due to its high oxygen affinity, with Lm values as low as 0.01. This presents a challenge, as lithium recovery from slag requires additional processing steps. Aluminum and manganese also tend to partition into the slag, necessitating post-treatment for recovery.

Lithium entrapment in slag is a major hurdle in pyrometallurgical recycling. Up to 95% of lithium may report to the slag phase, depending on process conditions. To mitigate losses, several strategies are employed. One approach involves adjusting slag chemistry to enhance lithium oxide (Li2O) solubility, followed by hydrometallurgical leaching. Water or dilute acid leaching can recover up to 80% of lithium from slag, though impurities like calcium and silicon may co-dissolve, requiring further purification. Another method is carbothermic reduction at elevated temperatures to volatilize lithium as lithium oxide vapor, though this is energy-intensive.

Thermodynamic modeling is indispensable for optimizing slag systems. FactSage and Thermo-Calc are widely used to predict phase equilibria, slag-metal partitioning, and oxygen potential effects. These models rely on databases of thermodynamic properties for oxides, sulfides, and molten alloys. For example, simulations can predict how varying CaO/SiO2 ratios affect cobalt recovery efficiency or how oxygen partial pressure influences nickel oxidation. Industrial practices often combine modeling with empirical data to refine slag recipes and operational parameters.

Industrial pyrometallurgical processes for battery recycling, such as Umicore’s smelting technology, employ tailored slag systems to maximize metal recovery. The process typically involves co-smelting battery scrap with a reductant (e.g., coke or coal) in a furnace, producing a cobalt-nickel-copper alloy and a lithium-rich slag. Post-smelting, the slag is crushed and leached to recover lithium, while the alloy undergoes refining to separate individual metals. Other operators may use electric arc furnaces or rotary kilns, with slag compositions adjusted based on feed material variability.

Challenges persist in achieving high purity in recovered metals while minimizing slag waste. Impurities like fluorine from electrolyte salts can corrode furnace linings or form hazardous gases, necessitating pre-treatment or specialized slag formulations. Research continues into advanced slag systems, such as those containing rare earth oxides or borates, to improve selectivity and lower processing temperatures. Additionally, efforts to integrate pyrometallurgy with hydrometallurgical steps aim to enhance overall recovery rates, particularly for lithium and manganese.

The environmental impact of slag disposal is another consideration. Slag must be stabilized to prevent leaching of residual metals into the environment. Vitrification—melting slag into a glassy, inert form—is one solution, though it adds cost. Some operators repurpose slag as construction material, provided it meets regulatory standards for heavy metal content. Life cycle assessments are increasingly used to evaluate the sustainability of different slag management strategies.

In summary, slag engineering is central to pyrometallurgical battery recycling, enabling the separation of valuable metals from complex waste streams. By carefully controlling composition, viscosity, and thermodynamic conditions, operators can optimize metal recovery while addressing challenges like lithium entrapment. Continued advancements in thermodynamic modeling and hybrid processing routes hold promise for further improving the efficiency and sustainability of slag-based recycling systems.