Nonequilibrium Hydrogen/Argon Plasma for a Swift Reduction of Cobalt Oxalate into Metal: Utilization of Radiofrequency and Microwave Plasma Reactors

Abstract Cobalt is a critical material for energy storage, catalysis, and advanced alloys, yet its recovery from end-of-life products with precursors such as cobalt oxalate remains limited even when cobalt resources are depleting. This study investigates the rapid reduction of cobalt oxalate dihydrate (Co(C2O4)·2H2O) to metallic cobalt using low-pressure nonequilibrium hydrogen and argon (H2/Ar) plasma in two reactor configurations: radiofrequency (RF, 40 MHz) and microwave (MW, 2.45 GHz). Experiments were conducted at transmitted powers of 550 and up to 800 W, with gas flow rates of 120 to 200 sccm with variable H2/Ar ratios and pressures ranging from 40 to 200 Pa. Under optimized conditions, complete reduction occurred in 5 min for the RF reactor. The MW reactor demonstrated superior performance that was attributed to higher power density and localized discharge, enabling full reduction within 2 min. Gas temperatures were estimated using optical emission spectroscopy, reaching temperatures of 700 to 1000 K, while the phase transformation and microstructural evolution of cobalt were confirmed through X-ray diffraction and scanning electron microscopy indicating the formation of 1 µm granulometry α-cobalt powder. Thermodynamic discussion indicates that plasma activation significantly enhances reaction kinetics, overcoming the limitations of conventional thermal reduction. These findings highlight cobalt oxalate dihydrate as a promising feed for cobalt secondary sourcing and nonequilibrium plasma as a promising route for fast cobalt recovery, with potential applications in battery recycling and advanced metallurgical processes.