Direct reduced iron

Direct reduced iron is metallic iron produced by removing chemically bound oxygen from iron ore without melting the iron. Its porous structure gives rise to the name sponge iron. Gas-based processes can use hydrogen and carbon monoxide as reducing agents; the product can subsequently be melted during steelmaking.^[ref]

For the idealized hydrogen reaction $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\to 2Fe+3{\mathrm{H}}_2\mathrm{O}$, the removed oxygen leaves in water. This reaction changes the direct chemical product relative to a carbon-based reduction, but the full environmental comparison also depends on how the hydrogen, process heat and electricity are supplied. Solid-state reduction and final steel melting remain distinct operations.