iron ore

Iron ore is any rock or mineral deposit rich enough in iron compounds that extracting and processing it yields metal at a profit. The ore must contain iron oxides, carbonates, or silicates in sufficient concentration, typically at least 20 to 30 percent iron by weight, to justify mining, crushing, transport, and smelting costs. Unlike pure mineral specimens, iron ore is defined entirely by economics: the same geological formation might be ore in one region and waste rock in another, depending on proximity to markets, fuel costs, and metal prices.

The three dominant ore types are hematite (ferric oxide, Fe2O3), magnetite (ferrous-ferric oxide, Fe3O4), and limonite (hydrated ferric oxide, roughly Fe2O3.nH2O). Hematite is the most widely mined worldwide and contains up to 70 percent iron in pure form, though most deposits run 50 to 65 percent. Magnetite ores are often lower grade but respond well to magnetic concentration. Taconite, an iron-silicate rock mined heavily in the Lake Superior region of North America, contains only 20 to 30 percent iron but is processed into pellets to reach 65 percent concentration before shipment to blast furnaces.

Raw ore straight from the pit contains gangue, unwanted silicate and carbonate minerals that dilute iron content and increase smelting fuel demand. Most operations crush, grind, and beneficiate ore to raise iron content before transport. Crushing reduces ore to fist-sized lumps; grinding produces fine powder suitable for magnetic or flotation separation. Pelletization, firing crushed ore with a binder into balls 10 to 16 millimeters across, improves permeability in the blast furnace and reduces dust losses. High-grade direct-shipping ore (DSO), containing 65 percent iron or more, can feed directly into smelting without beneficiation, but such deposits are becoming scarce.

Iron ore moves in bulk: a single large carrier hauls 170,000 to 200,000 tonnes in raw ore equivalent. Mines are therefore situated near deep-water ports or rail corridors to markets. The term ore applies only to deposits that can be extracted, transported, and smelted competitively; unmined formations containing iron are termed mineralization. When prices fall sharply or new, richer deposits are discovered elsewhere, declared ore can revert to waste in accounting terms, though the rock itself remains unchanged.