Novel insights into the reoxidation of direct reduced iron (DRI) during ball-mill treatment: A combined experimental and computational study

Abstract High quality direct reduced iron (DRI) plays a fundamental role in determining the functional properties of high-quality steel. DRI powder with a metallization ratio of 82.82% was obtained by reduction-grinding separation of low-grade iron ore, and iron resource was recycled (recovery ratio: 87.96%). However, a serious reoxidation phenomenon occurs in the grinding system, which is also a common problem in extraction metallurgy based on this process. Therefore, a combined experimental and computational study was executed to investigate the reoxidation. Phase evolution analysis shows that Fe0 in bulk iron particles mainly shift to Fe2+ during the grinding process, while Fe3+ are the main valence state on the nano-surface, indicating the reoxidation occurs via diffusion from the nano-surface to the interior. The cleavage surfaces (Fe100/110) occurred in grinding process are more easily to form Fe O ionic bonds with O2, especially when surface defects appear, as calculated by DFT. Further, the deeper iron atoms are more exposed after the defected iron surfaces combine with O2, greatly increasing contact opportunities of Fe O2. Therefore, the reduction time should be appropriately extended to decrease the surface defects of new iron, and other wet ball mills should be selected for the treatment of DRI produced by this process.