Carbothermal reduction of hematite involving biogenic carbon sourced from CO2

• Thermodynamic and experimental analysis of carbothermal Fe 2 O 3 reduction was achieved. • Carbon resources can derive from CO 2 using solar pyrolysis of biomass or biomethane. • The reaction was studied in TG/MS and tubular reactor coupled with online gas analyzer. • A stepwise reduction pathway was confirmed from analysis of evolved CO/CO 2 gases. • Formation of pure Fe was evidenced at C/Fe 2 O 3 ratio ≥3 for all studied carbon types. The reduction of hematite (Fe 2 O 3 ) to metallic iron via a carbothermal process was investigated with the aim of using carbon derived from captured or recycled CO 2 . This CO 2 -sourced carbon can be obtained by pyrolysis either directly from biomass or methane (issued from biogas or via a synthetic methanation route, through the Sabatier reaction). The carbon can ultimately serve as the reducing agent in a carbon-neutral solar thermal process. A comparative study was conducted to assess the thermodynamic and kinetic behavior of Fe 2 O 3 reduction using four distinct carbon materials: carbon black, activated charcoal, graphite, and biochar. The influence of heating rate and carbon-to-oxide molar ratio (C/Fe 2 O 3 ) was analyzed. Thermogravimetric analysis was employed to examine the reaction kinetics and mechanisms, while independent experiments in a tubular reactor further validated the results. These techniques enabled the synthesis of metallic iron and provided valuable insights into the reaction sequence, gas evolution rates (CO, CO₂), and phase transformations under controlled conditions. The expected phase transformation sequence: Fe 2 O 3 → Fe 3 O 4 → FeO → Fe was evidenced by gas phase analysis. X-ray diffraction analysis of the solid products confirmed nearly complete conversion of Fe 2 O 3 to Fe at a C/Fe 2 O 3 molar ratio of 3.0 or higher. The heating rate did not significantly affect reduction efficiency, whereas a carbon excess promoted near-complete reduction. At a fixed C/Fe 2 O 3 ratio of 4.5, conversion profiles showed slight variations based on the carbon type used. Graphite exhibited a sharp conversion increase at ∼1110 °C, while the other reductants displayed more gradual and similar conversion profiles with stepwise release of CO 2 and CO according to a three-stage mechanism. Notably, a lower onset reduction temperature (∼959 °C) was observed with biochar, indicating the potential of bio-sourced materials conversion for green ironmaking.