Non-isothermal Hydrogen-Based Reduction Swelling Behavior of Ultra-High-Grade Pellets
摘要
Ultra-high-grade pellets are prone to swelling during hydrogen-based direct reduction, which adversely affects production. Clarifying their swelling characteristic during non-isothermal reduction is essential. In this study, an in situ visualization reduction method was employed to investigate the non-isothermal reduction swelling behavior of ultra-high-grade pellets, including the effects of heating rate and reducing atmosphere. The results indicate that the reduction swelling occurs in two stages. The first stage, at 500 °C–800 °C, is attributed to lattice expansion during the reduction of Fe2O3 to Fe3O4. The second stage, above 950 °C, results from directional growth of iron grains during FeO reduction. Abnormal swelling predominantly occurs during the second stage. A lower heating rate has little effect on the reduction swelling index (RSI) in the first stage but promotes layered accumulation of primary iron in the second stage, thereby decreasing RSI. Increasing the H2/(H2+CO) ratio suppresses both microcracks propagation and iron whiskers growth, decreasing RSI in the first and second stages. Specifically, maintaining H2/(H2+CO) ≥ 0.6 keeps the RSI in the first stage below 15 pct. Furthermore, limiting the heating rate to no more than 15 °C/min lowers the RSI in the second stage below 15 pct, thereby meeting the operational requirements for hydrogen-based direct reduction.