Iron ore is a rock containing usable amounts of iron, often found with silicates. To create pure iron, the ore must be deoxygenated using carbon, creating carbon dioxide and carbon monoxide. Limestone is added to create cast iron, which can be further processed to create steel. Iron is abundant and plays a crucial role in human civilization, with 98% of ore used in iron or steel production. Ancient civilizations with ironwork had advantages over those with bronze and copper weaponry.
Iron ore is any rock that contains a usable amount of iron. Common mineral ores include hematite, magnetite, limonite, and siderite, and these are often found alongside assorted silicates. Although iron is not found in its pure form in nature, some types of ore contain up to 70% iron atoms. Iron ore consists of oxygen and iron atoms bonded together in molecules. To create pure iron, it is necessary to deoxygenate the ore, leaving behind only iron atoms, which is the essence of the refining process.
To pull the oxygen atoms away from the mineral requires heat and an alternate atomic partner for the oxygen to bind to. Carbon fulfills this role well and is readily available in the form of everyday charcoal, or coke, a form of carbon made from charcoal. The carbon atoms bind with the oxygen in the mineral to create carbon dioxide and carbon monoxide, gases that escape from a chimney. Since iron ore typically contains silicates, which do not bond with carbon, these remain in the iron after it is refined, creating wrought iron, a malleable and strong form of metal used by blacksmiths throughout history.
To create an even purer form of iron, known as cast iron, limestone must be added to the slurry and the heat increased. This is done simultaneously in the silo-like structure known as a blast furnace. The calcium in the limestone binds to the silicates of the mineral, creating a material called slag, which floats on top of the pure liquid iron. The iron is periodically drained into a mold from a door at the bottom of the blast furnace, where it cools. Cast iron can then be converted to wrought iron by mixing it with silicon or further processed to create steel.
Steel is a form of iron mixed with 0.5% – 1.5% carbon but no oxygen, silicates or other impurities. This metal is much more difficult to work than wrought iron, but is much stronger. Iron can be mixed with various other elements to create alloys with desired properties, such as light weight or resistance to rust (stainless steel).
Because iron is so common (composing 5% of the earth’s crust), strong, and relatively easy to work with, it plays a very intimate role in human civilization. About 98% of all ore shipped worldwide is used in the production of iron or steel. Surface deposits are abundantly available in most geographic areas. Ancient civilizations that achieved the threshold level of technology required to smelt iron ore enjoyed decisive advantages over their competitors, whose bronze and copper weaponry could not compete with ironwork.
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