In pyrometallurgical recycling of lithium-ion batteries, high-temperature processing is employed to recover valuable metals such as cobalt, nickel, and copper in the form of metallic alloys while lithium is directed into the slag phase. This separation is driven by differences in the thermodynamic affinities of these elements for oxygen and sulfur, as well as their miscibility in molten metal versus oxide phases. The process leverages well-established principles of extractive metallurgy, adapted to the unique composition of spent battery materials.

The feed material for pyrometallurgical processing typically consists of shredded battery components, often referred to as black mass, which contains a mixture of cathode metals, current collectors, and other battery materials. When subjected to temperatures between 1200°C and 1500°C in a reducing atmosphere, the oxides of cobalt, nickel, and copper are reduced to their metallic forms. These metals are mutually soluble in the liquid state, forming a molten alloy phase that separates from the less dense slag by gravity. Lithium, having a much stronger affinity for oxygen, remains oxidized and reports to the slag phase as lithium oxide or lithium carbonate.

Thermodynamically, the reduction of metal oxides follows the general reaction:
MO + C → M + CO
where M represents cobalt, nickel, or copper. The Gibbs free energy of these reactions becomes negative at elevated temperatures, making the reduction spontaneous under typical smelting conditions. The Ellingham diagram for metal oxides shows that cobalt and nickel oxides are reduced at similar temperatures, while copper oxide reduces at lower temperatures. Lithium oxide remains stable even at the highest temperatures used in pyrometallurgy, explaining its persistence in the slag.

The composition of the resulting alloy depends on the relative proportions of cobalt, nickel, and copper in the battery feedstock. Typical compositions from lithium-ion battery recycling range from 30-50% cobalt, 20-40% nickel, and 10-30% copper, with minor amounts of iron and other impurities. These ratios reflect the prevalence of layered oxide cathodes (LiCoO2, LiNiMnCoO2) and copper current collectors in battery designs. The phase diagram for the Co-Ni-Cu system shows complete miscibility in the liquid phase and the formation of solid solutions upon cooling, which facilitates alloy formation.

Industrial operations often employ electric arc furnaces or submerged arc furnaces for this process. The molten alloy is tapped from the bottom of the furnace and cast into anodes for further refining, while the lithium-containing slag is processed separately for lithium recovery. Some operations add fluxes such as silica or limestone to modify the slag properties and improve metal recovery. The reducing conditions are maintained by adding carbon in the form of coke, coal, or carbonaceous materials present in the battery waste itself.

The refining of battery-derived alloys typically involves electrolytic processes similar to those used in primary metal production. The impure alloy is cast into anodes and subjected to electrorefining in acidic sulfate or chloride electrolytes. During electrorefining, cobalt and nickel dissolve and plate out at the cathode with high purity, while copper and impurities either remain as anode slimes or plate out at different potentials. This step produces high-purity metals suitable for battery-grade applications.

Alternative refining routes include hydrometallurgical processing of the alloy through leaching with acids or ammoniacal solutions, followed by selective precipitation or solvent extraction. The choice between pyrometallurgical and hydrometallurgical refining depends on the desired product specifications and economic considerations. Some operations produce intermediate alloys like ferro-nickel or cobalt-nickel mattes that are sold directly to stainless steel producers or other metal consumers.

The slag phase, containing most of the lithium along with aluminum, manganese, and other oxides, represents a secondary resource that requires separate processing. While current industrial practice often treats this slag as waste, emerging processes aim to recover lithium through leaching or additional pyrometallurgical steps. The high lime content of some slags makes them suitable for construction applications after appropriate treatment.

Industrial examples of alloy production from battery recycling include operations by major smelters that process battery scrap alongside other cobalt and nickel-bearing materials. These facilities typically produce alloys that feed into existing metal refining infrastructure rather than making battery-grade materials directly. The trend toward higher nickel content in modern lithium-ion batteries is shifting the composition of recycled alloys toward nickel-rich formulations, which may require adjustments in refining processes.

The efficiency of metal recovery in pyrometallurgical processing depends on several factors including temperature, reducing conditions, slag chemistry, and the initial composition of the battery feed. Cobalt and nickel recoveries typically exceed 90%, while copper recovery can approach 95% in well-optimized systems. Lithium recovery to slag is nearly quantitative, though subsequent lithium extraction from slag remains challenging. The process generates emissions that require careful management, including CO2 from reduction reactions and potential volatilization of fluorine or other battery components.

Future developments in pyrometallurgical recycling may include process modifications to accommodate new battery chemistries with higher nickel or lower cobalt content, as well as improved slag treatment for lithium recovery. The integration of pyrometallurgy with hydrometallurgical steps in hybrid flowsheets shows promise for increasing overall metal recovery while maintaining the advantages of high-temperature processing for throughput and volume reduction.

The production of alloys from battery recycling creates a valuable intermediate product that bridges the gap between complex battery waste and high-purity battery materials. By concentrating multiple valuable metals into a single phase, pyrometallurgy simplifies subsequent refining steps and enables economies of scale. The process exemplifies how traditional extractive metallurgy can be adapted to meet the needs of modern energy storage systems while recovering critical materials for reuse.