Iron ore is a natural rock or mineral aggregate from which metallic iron can be extracted at a profit. It is fundamentally an oxide, carbonate, or hydrated compound of iron that forms the primary raw material for nearly all of the world’s steel production. The quality and type of the ore determine the complexity and cost of the process required to isolate the pure metal.
The Major Mineral Forms
The name “iron ore” collectively refers to several specific mineral compounds, defined by their unique chemical composition and iron concentration. The most common and commercially significant forms are the iron oxides, Hematite and Magnetite. Hematite, an iron(III) oxide (Fe2O3), is often called “red ore” because it leaves a reddish streak and can contain up to 70% iron by mass.
Magnetite is an iron(II,III) oxide (Fe3O4). It is recognizable due to its strong magnetic properties, which aids in processing. This mineral boasts the highest theoretical iron content of the common ores, reaching approximately 72%. Its magnetic nature makes it easier to separate from waste rock using low-cost magnetic separation techniques.
Less common, but commercially relevant, are the hydrated iron oxides and iron carbonate forms. Goethite, a hydrated iron oxide, has a theoretical iron content of about 63%. Limonite is often a mixture of various hydrated iron oxides, including Goethite, and typically has a lower iron concentration, not exceeding 60%. Siderite, an iron carbonate, is the least iron-rich of the primary ores, containing only about 48.2% iron, and is non-magnetic.
Geological and Commercial Classifications
Iron ore is classified by the geological structure of its deposit or by its commercial quality, referring to the entire rock body rather than a single mineral component. The Banded Iron Formation (BIF) is one of the most significant geological structures. BIFs are ancient sedimentary rocks characterized by alternating layers of iron-rich material and fine-grained silica, representing the source for most of the world’s iron ore resources.
Taconite, a specific type of BIF found in North America, is a low-grade iron-bearing rock containing 20% to 35% iron, often fine-grained Magnetite or Hematite. Although historically considered waste rock, processing advancements have made it an economically viable source. In contrast, Direct Shipping Ore (DSO) describes high-quality ore, typically over 60% iron, that is rich enough to be transported directly to a steel mill without needing significant concentration.
DSO is naturally high-grade and requires only crushing and screening before being shipped. Low-grade ores like Taconite require extensive upgrading before they can be used. These classifications help determine the necessary processing steps and the economic feasibility of mining a deposit.
Preparing Ore for Smelting
The various ore classifications dictate the preparation steps required to create a usable product for the blast furnace. The first step for low-grade ores is Beneficiation, which involves crushing, grinding, and separating the iron-rich minerals from the waste rock (gangue). This mechanical separation, often using magnetic techniques for Magnetite-rich ores, increases the iron content to a concentrated powder known as fines.
Since fine powder cannot be fed directly into a furnace due to poor gas flow, it must be agglomerated using one of two main methods. Pelletizing involves rolling the fine concentrate into small spheres, usually 10 to 20 millimeters in diameter. These “green pellets” are then heat-hardened through induration, creating a strong, durable, and uniform feed material.
The alternative method is Sintering, which involves mixing iron ore fines with flux and coke breeze, then heating the mixture to fuse it into a porous, lumpy material called sinter. Sintering is effective for consolidating fine particles and waste materials into a product typically ranging from 5 to 60 millimeters. Both pelletizing and sintering transform fine ore concentrates into a suitable feedstock for efficient reduction in steel-making.