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The transition towards a more sustainable society requires the urgent decarbonization of the steel industry. Indeed, this sector is responsible for around 5% of the CO2 emissions in the European Union and 7% worldwide [1]. In 2016, 70% of the world steel production was done via Blast Furnace or Basic Oxygen Furnace [2], using carbon as a reducing agent. Hydrogen-based direct reduction (HyDR) is a promising alternative to produce metallic iron with a lower CO2 impact [3]. However, the behavior of the various iron sources during direct reduction is not yet fully understood. In this work, we studied the HyDR kinetics of mill scale, an industrial iron-rich oxide waste produced during the thermo-mechanical processing of steel. Mill scale mainly consists of wüstite (about 80%) with small amounts of impurities (Mn-0.4, Al-0.05, Si-0.04, Cr-0.02, Cu-0.02, Ni-0.02 in wt%). The studied iron oxides fines have a particle size range between 45-106 µm. Isothermal reduction kinetics were analyzed using a thermogravimetric analyzer. The influence of both deep pre-oxidation (using dry air at 700°C, 800°C or 900°C) and reduction temperatures (400°C, 450°C, 500°C and 700°C) on crushed mill scale powder was studied in pure hydrogen. The evolution of the impurities within the powder during its reduction was measured by ICP, SEM-EDS and STEM-EDS analyses. Pre-oxidation consistently resulted in faster reduction rates, with the highest kinetics obtained with a pre-oxidation temperature of 800°C followed by a reduction at 500°C. Interrupted tests were carried out at 500°C, with and without pre-oxidation, to assess the reaction mechanisms of the intermediate steps during direct reduction. The eutectoid wüstite decomposition at 500°C was also quantified. This is done with microstructural analyses (SEM, EDX, EBSD, XRD) coupled with kinetic fittings. For each reaction mechanism, the rate constant and activation energy was calculated and compared with the HyDR behavior of other Fe sources. [1] IEA (2020), Iron and Steel Technology Roadmap, IEA, Paris https://www.iea.org/reports/iron-and-steel-technology-roadmap. [2] Ariyama, T., Sato, M., Nouchi, T., & Koichi, T. (2016). Isij International, 56, 1681-1696. [3] Wang, P., Ryberg, M., Yang, Y., Feng, K., Kara, S., Hauschild, M., Chen, W-Q. (2021). Nature Communications 12, 2066.
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Kuypers, B., Stevens, N., Hessels, C. J. M., Jacques, P., Finotello, G., & Choisez, L. (2025). Hydrogen-based direct reduction of steel by-product mill scale. Euromat, Granada. https://hdl.handle.net/2078.5/268687