Magnetite is a naturally occurring iron oxide mineral with the chemical formula Fe₃O₄. It contains both iron(II) and iron(III) and exhibits strong magnetic properties, making it the most magnetic mineral found in nature. This unique characteristic allows it to be readily attracted to a magnet and even become a permanent magnet itself. Magnetite is a significant source of iron, primarily valued as an iron ore for steel production.
Extracting the Ore
Magnetite is typically found in large formations within the Earth’s crust, often in igneous, metamorphic, and sedimentary rocks, including banded iron formations. The most common method for removing magnetite-bearing ore from the ground is open-pit mining, especially for large, shallow deposits. This operation begins with drilling to access the ore body.
After drilling, explosives are used to blast the rock into manageable pieces. Heavy machinery, such as excavators and front-end loaders, then loads the broken ore onto large haul trucks for transport out of the pit. While open-pit mining is prevalent, underground mining techniques might be employed for deeper, high-grade deposits.
Concentrating the Magnetite
Once the raw magnetite ore is extracted, it contains significant amounts of waste rock, known as gangue. Initial processing separates the valuable magnetite from these impurities. The first step is crushing, where the ore undergoes primary, secondary, and sometimes tertiary stages to reduce its size. This process begins with coarse crushers, like jaw crushers, followed by fine crushers such as cone or hammer crushers, preparing the material for the next stage.
Following crushing, the ore is subjected to grinding, where rod mills and ball mills reduce the material to a fine powder, typically to a particle size of 25-35 micrometers. This fine grinding is essential to “liberate” the magnetite particles from the surrounding gangue minerals. The finely ground ore is then mixed with water to create a slurry.
The slurry proceeds to magnetic separation, which is the core process for magnetite. Strong magnets in devices like wet drum magnetic separators attract the magnetic magnetite particles, pulling them away from the non-magnetic waste material. The magnetic particles adhere to a rotating drum and are then washed off to form a concentrated magnetite slurry, while the non-magnetic tailings are discarded. This method effectively upgrades the iron content to over 65% iron.
Refining and Preparing for Use
After magnetic separation, the magnetite concentrate contains a significant amount of water. The next step involves dewatering, where excess water is removed from the concentrate. This is often achieved using filters, reducing the moisture content to approximately 8-10%. This ensures the concentrate is drier and easier to handle for subsequent steps.
The dewatered concentrate is then transformed into small, uniform pellets through a process called pelletizing. This involves mixing the fine magnetite concentrate with binders, such as bentonite or other organic binders, and sometimes additives like limestone. The mixture is rolled in balling drums or discs to form “green” or “wet” pellets, typically around 10 millimeters in diameter. These green pellets are then dried and subjected to high-temperature firing, often between 1250°C and 1350°C, to harden them and oxidize the magnetite to hematite, improving their strength and suitability for blast furnaces.
Pelletizing improves the handling and transport of the fine concentrate for steelmaking. The hardened pellets are then cooled. Finally, the finished magnetite pellets are transported to steel mills or other end-users, often by rail, truck, or bulk carriers.
Key Applications
Magnetite’s primary application is in the production of steel, where it is reduced in blast furnaces to produce pig iron or sponge iron. This iron then becomes the foundation for manufacturing steel, a widely used material.
Beyond steelmaking, magnetite finds specialized uses in other sectors. It is employed in dense media separation, particularly in coal washing, where its density helps separate valuable materials from waste. Magnetite also acts as a catalyst in processes like the Haber-Bosch and Fischer-Tropsch processes for producing ammonia and hydrocarbons. Furthermore, it is used as a pigment in paints and ceramics, in water treatment to remove impurities, and in magnetic materials.