Hydrogen Plasma for Low-Carbon Extractive Metallurgy: Oxides Reduction, Metals Refining, and Wastes Processing

Abstract

Carbon-rich sources, such as coal and carbon monoxide gas, have been extensively used in the metal industry as the reducing agent of metal oxides and as the energy source for metal production. Consequently, the extractive metal sector contributes to approximately 9.5% of global greenhouse gas emissions. Hydrogen gas offers a promising alternative to using carbon in metallurgical processes as an eco-friendly reductant and energy provider that produces water vapor as a by-product. However, molecular hydrogen has some barriers to implementation. These primarily concern the thermodynamics and kinetics of metal oxide reduction. To address these issues, researchers have explored the use of hydrogen plasma, which is generated by applying high energy to molecular hydrogen to produce atomic, ionic, and excited hydrogen species. Hydrogen plasma has thermodynamic and kinetic advantages over molecular hydrogen and carbon-based reductants since it exhibits a lower standard Gibbs free energy of reaction for H2O formation and a lower activation energy. Hydrogen plasma is also a versatile reductant as it is proven on a laboratory scale to produce metal in fewer steps, process a wide range of oxides feed and feed sizes, and be used to refine metals. There are, however, some limitations to using hydrogen plasma in extractive metallurgy. These include the cost of electricity, potential back reactions or reoxidation, and industrial scale-up challenges such as heat utilization or heat loss minimization. This study undertakes a comprehensive review of prior research on the use of hydrogen plasma for metal oxides reduction and reviewing state-of-the-art techniques for its use in extractive metallurgy applications. An overview of hydrogen plasma utilization for producing and refining several metals from primary or secondary feed materials, the many types of plasma reactors, and the commonly used parameters for each metal production process are also presented. Prospects and potential feasibility of the hydrogen plasma route are also discussed.