Optimization of carbon deposition process during the pre-reduction of fine iron ore in a fluidized bed

• Composition and structure variations of carbon-coated iron ore during reduction were investigated.
• The Ccritical value of iron ore decreased with the increase of its metallization ratio.
• The Ccritical value reduced to 3.0–3.5 wt.% by process optimization, much lower than the reported value in the literature.

Coating carbon on the iron ore powder is a quite promising way to prevent defluidization during its high-temperature direct reduction in the fluidized bed reactor, but usually causes excessive carbon content of the DRI production, thus the present study focuses on decreasing the critical carbon content (Ccritical value) needed for defluidization prevention by the optimization of the pre-reduction process. It was for the first time found that increasing the metallization ratio of the pre-reduced iron ore significantly could reduce the Ccritical value, since it inhibited the carbon consumption and formation of metallic iron during the early stage of high temperature reduction. Based on this result, optimization principle to reduce the Ccritical value was further proposed, i.e. increasing the pre-reduction rate (Rpre-reduction) and reducing the carbon deposition rate (Rcarbon deposition) during the pre-reduction, such as by increasing the temperature and the H2 mole fraction in CO–H2 mixture gas. And such principle was verified by the experiment results, where for the iron ore pre-reduced in CO–H2 mixture gas with the xH2 ≥ 0.9 at 600 °C, its corresponding Ccritical value was less than 3.5 wt.%, much lower than that of other conditions or the reported value in the literature.

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