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Direct reduction of iron ore in hydrogen (25% H2/N2) between 450–1050 °C and subsequent room temperature reoxidation of the reduced iron in air was systematically examined using a TGA. The reduced samples with various degrees of metalization were cooled to room temperature in N2 and then exposed to air. Weight gain due to iron reoxidation was observed. Cyclic reduction and reoxidation runs were also performed at selected reduction temperatures (Tr). Iron samples obtained at low reduction temperatures (450–850 °C) incurred pyrophoricity, with weight gain increasing with decreasing Tr. The weight gain behavior varied with reduction times (tr) at different Tr. XRD characterization showed wustite, fayalite, and hercynite as intermediate mineral phases during reduction. BET results indicated that increasing Tr and tr decreased the surface area and weight gain during reoxidation. The surface area and porous structure predominantly determine the pyrophoricity of the H2-reduced iron.
Okoye et al. (Tue,) studied this question.