Abstract:

The global transition toward electrification and low-carbon technologies has significantly increased demand for magnesium (Mg), a critical lightweight metal essential for transportation, energy storage, and advanced materials. However, Mg metal production remains highly energy-intensive and carbon-emitting. Currently, 85% of global Mg metal is produced in China via the silicothermic reduction of calcined dolomite (MgO.CaO), commonly known as the Pidgeon process. This vacuum-based, multi-step thermal process consumes large quantities of coal-derived energy (8.68 tce), generates substantial solid waste (5-6 t waste/ t Mg), and emits up to 26 t CO_2eq per ton of Mg. Therefore, developing alternative Mg metal production pathways that reduce energy consumption, eliminate vacuum operation, and lower carbon footprint is vitally important. Dolomitic limestone waste rock (DLWR), generated during phosphate mining, represents an abundant secondary resource for Mg metal recycling. Our previous work demonstrated that high-purity Mg(OH)₂ and MgO can be selectively recycled from DLWR via dilute H₂SO₄ leaching and stepwise NaOH precipitation. To further valorize this waste stream, this study introduces a novel methanothermic-assisted carbothermal reduction (MCTR) of Mg(OH)₂ using a microwave-heating reactor under atmospheric pressure. Unlike conventional carbothermal reduction (CTR), which requires a deep vacuum (30-100 Pa) to thermodynamically favor MgO reduction at around 1200 ˚C, our proposed non-conventional pyrometallurgical process employs methane (CH₄) as a hydrogen-carrier reductant and microwave radiation as a heating source to promote the in-situ generation of H₂ and CO via methane decomposition. This shifts the dominant reduction pathway from solid-solid to gas-solid reactions, enabling Mg production at atmospheric pressure. Microwave irradiation further intensifies the process yield through synergistic thermal effects (i.e., selective, rapid, and volumetric heating) and non-thermal effects (i.e., enhanced hydrogen evolution and oxygen-vacancy formation). As a result, we produced metallic Mg rapidly within 20 min at 1200 ˚C under atmospheric pressure. In parallel, the methanothermic reduction of Mg(OH)₂ generated hydrogen-rich syngas with an H₂/CO molar ratio of approximately 2.5. Co-production of hydrogen-rich syngas offers additional value as both a chemical feedstock and an energy carrier. Compared to the Pidgeon process and conventional CTR, our proposed technology simplifies reactor design, accelerates heat and mass transfer, and enables process intensification while reducing solid waste generation and conserving energy. In summary, this work demonstrates a microwave-driven, syngas-assisted pyrometallurgical pathway for Mg production that integrates methane cracking, carbothermal reduction, and waste valorization. By avoiding external pure hydrogen supply and vacuum operation, our process provides a practical transition toward low-carbon Mg metallurgy and promotes the circular economy for mining utilization residues.

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Prof. Jamal Chaouki