Iron ore primarily consists of iron oxides like hematite or magnetite and is the starting material for creating steel. This raw material is not pure and contains significant amounts of unwanted elements, or gangue, such as silica, alumina, phosphorus, and sulfur. These impurities must be reduced to produce a metal with the strength and durability required for construction and manufacturing.
Refinement is a series of high-temperature steps designed to chemically separate the iron from oxygen and contaminants. Because iron ore must be chemically reduced from its oxidized state, this industrial sequence demands large inputs of energy and specific chemical agents. The ultimate goal is to transform the oxidized iron into a clean, metallic base suitable for alloying into usable steel.
Preparing the Raw Ore
Before the ore can be charged into any furnace, it must undergo preparation, known as beneficiation, to increase its iron concentration. This initial stage upgrades the material quality and creates a uniform size for efficient furnace operation.
The process begins with crushing and grinding the mined ore into smaller particles, increasing the surface area for subsequent processing. Concentration techniques are then employed to physically separate the iron oxides from the lighter gangue materials. Methods like magnetic separation are effective for magnetite ores, while flotation may be used for other iron oxide types.
This concentration enhances the iron content, but the resulting fine concentrate is unsuitable for direct use in a blast furnace. To create a feed material with the necessary physical properties, the fine ore is either sintered or pelletized. Sintering involves heating the fines with flux and fuel to create porous lumps, while pelletizing forms small, hard spheres that allow gases to pass through easily in the furnace.
Smelting Iron in a Blast Furnace
The blast furnace is the primary method for iron refinement, operating as a continuous, counter-current reactor. It is continuously fed from the top with three inputs: prepared iron ore (pellets or sinter), coke, and flux.
Coke, a carbon-rich substance derived from coal, serves a dual purpose. It acts as the fuel source to generate the intense heat needed for the process, and it supplies the reducing agent. Hot air, often enriched with oxygen, is blown into the lower section through nozzles called tuyeres, causing the coke to combust and produce carbon monoxide gas.
This carbon monoxide is the chemical agent responsible for the reduction, traveling upward and reacting with the descending iron oxides to strip away the oxygen. As the material descends into the hotter zones, the metallic iron melts and accumulates in the hearth at the bottom of the furnace.
The third input, flux (typically limestone or dolomite), removes non-metallic impurities. This material reacts with the silica and alumina gangue, forming a liquid byproduct called slag. Since slag is less dense than the molten iron, it floats on top of the liquid metal, allowing both the pig iron and the slag to be separately tapped. The resulting product, known as pig iron, is a molten metal containing about 3 to 5% carbon, which makes it brittle and unsuitable for most commercial applications.
Direct Reduction Ironmaking
Direct Reduction Ironmaking (DRI) operates at temperatures below the melting point of iron. This solid-state reduction process produces a product often called sponge iron due to its porous structure.
The DRI process relies on reducing gases, such as a mixture of hydrogen and carbon monoxide (syngas), derived primarily from reformed natural gas or coal. These reducing agents strip the oxygen from the iron ore pellets or lump ore, converting the iron oxides into metallic iron without the need for a blast furnace’s high heat.
This method is energy-efficient and well-suited for areas with abundant natural gas resources. The resulting product, Direct Reduced Iron (DRI), is a high-iron, low-impurity material typically fed directly into an Electric Arc Furnace for steelmaking. DRI can be produced in various forms, including hot briquetted iron (HBI) for easier transport and storage.
Converting Iron into Usable Steel
The iron products from the initial refinement stages—either molten pig iron or solid DRI—are not yet steel because they still contain excessive carbon and other impurities. Converting these intermediate products into commercially usable steel requires a final refinement process to adjust the chemical composition.
The most common method for refining molten pig iron is the Basic Oxygen Furnace (BOF). In the BOF, high-purity oxygen is blown into the molten iron, which rapidly oxidizes the excess carbon, silicon, and other impurities. This oxidation reaction releases heat and quickly reduces the carbon content to the levels required for steel, typically in under an hour.
For Direct Reduced Iron and scrap metal, refinement takes place in an Electric Arc Furnace (EAF). The EAF uses powerful electric arcs generated by graphite electrodes to melt the solid charge material. Fluxes are added to the molten bath to capture and remove remaining impurities like phosphorus and sulfur, while alloying elements are added to achieve the precise composition of the final steel product.