Carbothermic reduction is a pyrometallurgical process used to recover valuable metals such as cobalt, nickel, and lithium from spent lithium-ion batteries. This method relies on high-temperature reactions between metal oxides and carbon-based reductants to reduce the oxides into their metallic forms. The process is energy-intensive but offers scalability and compatibility with existing smelting infrastructure, making it a viable option for large-scale battery recycling.

The core principle of carbothermic reduction involves the use of carbon as a reducing agent to strip oxygen from metal oxides. Common reductants include coke, charcoal, and coal, which react with metal oxides at elevated temperatures to produce pure metals or alloys, along with carbon monoxide or carbon dioxide as byproducts. The general reaction for a metal oxide (MO) can be represented as:
MO + C → M + CO
For multi-component systems like those found in spent batteries, the reactions become more complex due to the presence of multiple metal oxides.

In the context of lithium-ion battery recycling, the cathode materials typically consist of lithium cobalt oxide (LiCoO2), lithium nickel manganese cobalt oxide (NMC), or lithium iron phosphate (LiFePO4). During carbothermic reduction, these compounds decompose and react with carbon. For LiCoO2, the reaction proceeds as:
2LiCoO2 + 3C → 2Co + Li2O + 3CO
Similarly, for NMC cathodes, nickel, manganese, and cobalt oxides are reduced to their metallic states. Lithium, however, tends to form lithium oxide (Li2O), which often reports to the slag phase due to its high affinity for oxygen.

The process typically operates within a temperature range of 1200°C to 1600°C, depending on the specific metals being targeted. Higher temperatures favor the reduction of more stable oxides but also increase energy consumption and the risk of lithium volatilization. The choice of reductant affects the efficiency of the process. Coke, due to its high carbon content and low impurities, is commonly used in industrial settings, while charcoal may be preferred in smaller-scale operations for its lower sulfur content.

Fluxing agents such as silica (SiO2) or lime (CaO) are often added to facilitate slag formation and improve metal recovery. The slag acts as a solvent for impurities and lithium oxide, allowing the desired metals to separate into a molten alloy phase. For instance, adding silica can help form a lithium silicate slag, which can later be processed to recover lithium. The composition of the slag is critical, as it influences the viscosity, melting point, and metal partitioning.

The purity of recovered metals depends on the initial feedstock and process conditions. Cobalt and nickel can be recovered as a mixed alloy with purities exceeding 95%, which can then be refined further through hydrometallurgical or electrochemical methods. Lithium recovery, however, remains a challenge due to its tendency to partition into the slag. Pilot studies have shown that only 50-70% of lithium is recoverable from the slag phase, often requiring additional leaching steps.

Scalability is one of the key advantages of carbothermic reduction. The process can be integrated into existing smelting facilities with minimal modifications, making it attractive for large-scale operations. Commercial plants, such as those operated by Umicore and Sumitomo Metal Mining, utilize similar pyrometallurgical techniques to process thousands of tons of spent batteries annually. These facilities often combine carbothermic reduction with downstream refining to achieve high-purity metal products.

Energy efficiency is a significant concern, as the process requires sustained high temperatures. The use of electric arc furnaces or submerged arc furnaces can improve energy utilization, but the overall energy demand remains high compared to hydrometallurgical methods. Innovations such as pre-treatment steps to remove organic materials or the use of renewable reductants could mitigate some of these energy challenges.

A major limitation of carbothermic reduction is the loss of lithium to slag, which reduces overall recovery rates. While lithium can be extracted from the slag through additional processing, this adds complexity and cost. Another drawback is the generation of greenhouse gases, primarily CO and CO2, from the reduction reactions. Efforts to capture and utilize these gases are underway but are not yet widely implemented.

Pilot plants have demonstrated the feasibility of carbothermic reduction for battery recycling. For example, the ReLieVe project in Europe has explored the use of pyrometallurgical methods to recover metals from electric vehicle batteries. Similarly, commercial operations in Asia have successfully implemented the process at scale, though lithium recovery remains a focus for further optimization.

In summary, carbothermic reduction is a robust and scalable method for recovering cobalt, nickel, and other valuable metals from spent batteries. While it offers high metal purity and compatibility with existing infrastructure, challenges such as lithium loss and high energy consumption must be addressed to improve sustainability. Ongoing research and industrial advancements aim to refine the process, particularly in enhancing lithium recovery and reducing environmental impact.