Iron ore is the foundational raw material for nearly all modern infrastructure, serving as the source of iron needed to produce steel. About 98% of the iron ore mined worldwide is dedicated to steel production, which is the backbone of construction, transportation, and machinery. Understanding what this material is made of requires examining its economic definition, its core mineral components, and the non-iron materials it naturally contains.
What Defines Iron Ore
Iron ore is not simply any rock containing iron, but rather a geological deposit from which metallic iron can be extracted at a profit. This economic distinction is determined by the material’s “grade,” which is the concentration of iron present in the ore. Ores with very high iron content (typically above 60%) are known as Direct Shipping Ore (DSO) because they require minimal processing before being fed into a furnace.
Lower-grade ores must undergo a process called beneficiation, where the iron-bearing minerals are physically separated from the waste material to create a concentrate. The ore is crushed, ground, and concentrated before being sintered or pelletized for use in a blast furnace. The subsequent smelting process uses heat and a reducing agent, like carbon, to strip the oxygen from the iron oxides, yielding molten pig iron.
The Principal Iron-Bearing Minerals
The metallic iron within the ore is bound up in several different mineral forms, primarily iron oxides, with the two most significant being hematite and magnetite. Hematite is the most common form in major global deposits and is an anhydrous iron oxide (Fe2O3). In its pure state, hematite can contain up to 70% iron by mass, and it is known for giving rocks a characteristic reddish hue.
Magnetite (Fe3O4) is a mixed-valence iron oxide that holds the highest theoretical iron content at 72.4%. Its strong natural magnetism allows it to be easily separated from non-magnetic waste material using magnetic separation techniques. This magnetic property often makes magnetite ores easier to process, even if their initial grade is lower than hematite ores.
Less common but commercially important are the hydrated iron oxide goethite (up to 63% iron) and siderite, an iron carbonate (FeCO3) with about 48% iron content. Siderite requires calcining to convert the carbonate into a usable oxide form. The specific mix of these minerals determines the ore’s quality and refinement steps.
Non-Iron Materials and Ore Types
Iron ore deposits are always accompanied by unwanted materials, collectively referred to as “gangue,” which must be removed before or during the smelting process. Common gangue minerals include silica (SiO2) and alumina (Al2O3), which are separated by reacting them with flux materials in the furnace to form a molten waste product called slag. Trace elements like phosphorus and sulfur are particularly detrimental to the quality of the final metal.
Phosphorus causes “cold brittleness,” weakening the steel at ambient temperatures. Sulfur, considered the most harmful element, induces “hot brittleness,” making the metal prone to cracking during hot-rolling or forging processes. The presence of these impurities increases the complexity and cost of steel production, necessitating additional refining steps.
The majority of the world’s iron ore originates from ancient geological formations known as Banded Iron Formations (BIFs). These are sedimentary rocks, mostly Precambrian in age, characterized by alternating layers of iron oxides (magnetite or hematite) and silica-rich chert. BIFs are the most substantial source of global iron reserves. Secondary deposits, such as laterites, are also mined, but these typically result in a lower-grade ore compared to BIF deposits.