Evaluation of MgO on hematite swelling dynamics during CO-driven stepwise reduction

The significant effect of MgO in inhibiting the reduction swelling of iron ore pellets has been widely recognized. The swelling behaviors of pellets during the stepwise reduction by CO were assessed. The linear expansion of strip samples was measured using a linear dilatometer during the staged reduction process at a temperature of 900 °C. The existence states of MgO in hematite, magnetite, and wüstite were investigated through thermodynamic calculations. The magnetite strip samples were subjected to oxidizing roasting at 1250 °C for 30 min to produce hematite strip samples. The strip samples with 0.49, 1.49, 2.49, and 3.49 wt.% MgO were analyzed for length change. It was observed that the sample with 2.49 wt.% MgO exhibited the least significant length change. The lengths of the samples with the initial length being 20 mm before reduction changed during the reduction stages of Fe₂O₃ → Fe₃O₄, Fe₃O₄ → FeO, and FeO → Fe were 615, − 25, and − 378 μm, respectively. The volume expansion of hematite to magnetite was primarily attributed to the crystal transformation. During the reduction stage from wüstite to metallic iron, a substantial contraction occurred, while the slag phase was able to retain its original basic shape. The enclosed areas, as indicated by the expansion change curves of the samples with 0.49, 1.49, 2.49, and 3.49 wt.% MgO, were measured at 3.76 × 10⁶, 3.23 × 10⁶, 3.05 × 10⁶, and 3.17 × 10⁶ μm s, respectively.