Why Is Sponge Iron Called "Sponge" Iron?

Sponge iron is called "sponge" iron because the removal of oxygen from solid iron ore leaves a porous, honeycombed microstructure: an interconnected network of metallic iron walls and open pores that closely resembles the cross-section of a bath sponge. The direct reduction process removes oxygen without melting the iron, so the pore network created by outgoing oxygen is locked permanently into the product. Porosity typically ranges from 20-40% by volume [1]. The name is a physical description, not a brand or grade designation.

This piece is the companion to sponge iron production process and covers the morphology question specifically.

The solid-state physics behind the pore network

The morphology is a direct consequence of how oxygen leaves the iron-oxide lattice. In a typical hematite (Fe2O3) feed, oxygen atoms make up roughly 30% of the mineral by mass. When CO and H2 strip those oxygens off the lattice at 800-1,050 C, the iron atoms left behind do not collapse into a dense solid: they reorganise into a body-centred cubic (BCC) iron crystal structure that occupies a smaller volume than the original oxide. The volume the oxygen used to occupy becomes void, and because the reaction front advances inward through the pellet rather than melting it, the voids stay open and interconnected. The mass loss is on the order of 27-30% [2], and the resulting porosity of 20-40% by volume [1] is what gives the product its sponge-like appearance under a microscope.

The geometry of the pore network is set by the reduction sequence Fe2O3 to Fe3O4 to FeO to Fe, which advances as a moving front through the pellet. Reducing gas has to diffuse inward through the pores that earlier oxygen atoms left behind, and the product gases (CO2 and H2O) have to diffuse back out through the same channels. This is why the pore network is interconnected rather than a set of isolated bubbles: the geometry is dictated by the requirement that gas keeps reaching the unreduced core. Microstructure studies in the metallurgical literature show iron walls a few microns thick separating pores of roughly the same scale, with the original pellet's external dimensions essentially preserved.

Why the structure matters for steelmakers

The porous geometry is what makes sponge iron commercially valuable rather than just metallurgically interesting. Internal surface area is large, on the order of square metres per gram, which is why a sponge-iron charge melts faster and more uniformly in an electric arc furnace than a dense scrap charge of equivalent mass. The pore network also gives carbon and slag-forming additions a fast diffusion path into the iron during melting, which simplifies heat-to-heat chemistry control. For flat-rolled products where residuals like copper, tin, and chromium have to stay below a few hundred parts per million, this combination of low residuals (sponge iron has none of the tramp elements that scrap brings) and predictable melting behaviour is the reason DRI is preferred over high-grade scrap.

The same porosity is the reason hot briquetted iron (HBI) exists. Loose sponge iron pellets present a high specific surface area to the atmosphere, and once shipped through a humid port they will slowly re-oxidise, losing metallic iron and giving off heat. Compacting hot DRI at around 650-700 C into pillow-shaped briquettes of roughly 50 mm × 30 mm × 110 mm collapses much of the open porosity, drops the specific surface area, and produces a denser product (around 5.0 t/m3 versus 1.6-1.9 t/m3 for cold DRI) that is internationally classed as non-hazardous for shipping. The trade-off is that HBI gives up some of the EAF-melting advantages of the original sponge structure in exchange for safe sea transport.

Why the same name covers DRI from very different routes

The "sponge" description applies whether the reduction was done in a rotary kiln using coal as the solid reductant or in a shaft furnace using reformed natural gas. In both cases the chemistry is solid-state reduction of iron oxide by CO and H2; the differences sit in temperature profile, residence time, and gas composition rather than in the resulting morphology. A coal-based pellet from a 100 TPD rotary kiln and a Midrex pellet from a 2.5 Mt/y shaft furnace will both show the honeycomb structure under a microscope, even though their carbon contents (typically 0.08-0.2% from the rotary kiln, and 1.5-2.5% from a carburised shaft) and their metallization rates differ. The coal-based sponge iron production and gas-based DRI direct reduction pieces cover the route specifics. India produced 54.7 Mt of sponge iron in 2024, the largest national output globally [3], and most of that came from coal-based rotary kilns.

The commercial bridge: why kiln seal integrity protects the sponge

In a working DRI plant, the sponge structure is only as clean as the reducing atmosphere that produced it. Metallization rate (the fraction of total iron present as metallic Fe, target 88-94% for steelmaking grades) depends on keeping the kiln gas predominantly CO and H2, with CO2 and H2O held low enough that the reduction equilibrium continues to favour metallic iron rather than wustite (FeO). False air leaking in through worn seals at the kiln inlet or discharge end shifts that equilibrium the wrong way: oxygen burns out reductant, raises CO2 partial pressure, and partially re-oxidises freshly reduced product before it leaves the kiln. The visible symptom is a metallization rate that drifts down from the 90% range toward 80%, and a finished pellet whose porosity is partially closed off by re-oxidation rims around each grain. The fix is a tight sealing system at both ends of the kiln, which is what kiln sealing for DRI plants covers in detail.