Hydrogen driven reduction of DRI-grade iron ore above 1500 °C: a “melt-then-reduce” approach

Hydrogen (H2) reduction of hematite iron ore is often studied either in the solid state or in the liquid state. In this study, a “melt-then-reduce” approach is investigated by analyzing iron oxide (FeO) reduction at 1500 °C, 1550 °C, and 1590 °C, where in-situ imaging highlights new phenomena in iron ore reduction. At 1500 °C and 1550 °C, iron (Fe) nucleates as dendrites via supersaturation-driven anisotropic growth within a liquid slag pool, mechanism usually reported during cooling events rather than H2 reduction of iron ore. The solid Fe layer, however, inhibits reduction and the maximum reduction degree reached after 20 min is 70%. In contrast, at 1590 °C, Fe precipitates exclusively as spherical droplets, marking a transition to fully liquid-state reduction. Convection driven by temperature-induced surface tension gradients, possibly Marangoni convection, facilitates Fe droplet migration toward cooler crucible regions, enabling a distinct slag-metal separation. Additionally, while this mechanism is know in metallurgical processes, this is the first report of it during H2-driven iron ore reduction. DFT-based molecular dynamics reveal a ~61% increase in the diffusivity of oxygen atoms in liquid FeO at 1500 °C compared to solid-state conditions at 1300 °C, supporting the observed enhancement in reduction kinetics once FeO is in the liquid state. This is reflected in the high reduction degree of ~90%, metallization degree of ~86%, and microstructural uniformity achieved at 1590 °C. Collectively, these findings demonstrate that the melt-then-reduce pathway offers an efficient and operationally integrated strategy for H2-based ironmaking, advancing the understanding and prospects for cleaner steel production.