Direct reduced iron (DRI), also calledsponge iron,[1] is produced from the directreduction ofiron ore (in the form of lumps, pellets, or fines) into iron by areducing gas which contains elementalcarbon (produced fromnatural gas orcoal) and/orhydrogen. When hydrogen is used as the reducing gas no carbon dioxide is produced. Many ores are suitable for direct reduction.
Direct reduction refers to solid-state processes which reduce iron oxides to metallic iron at temperatures below the melting point of iron. Reduced iron derives its name from these processes, one example being heating iron ore in a furnace at a high temperature of 800 to 1,200 °C (1,470 to 2,190 °F) in the presence ofsyngas (a mixture ofhydrogen andcarbon monoxide) or pure hydrogen.[2]
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Direct reduction processes can be divided roughly into two categories: gas-based and coal-based. In both cases, the objective of the process is to remove theoxygen contained in various forms of iron ore (sized ore, concentrates, pellets, mill scale, furnace dust, etc.) in order to convert the ore to metallic iron, without melting it (below 1,200 °C (2,190 °F)).
The direct reduction process is comparatively energy efficient.Steel made using DRI requires significantly less fuel, in that a traditional blast furnace is not needed. DRI is most commonly made into steel usingelectric arc furnaces to take advantage of the heat produced by the DRI product.[3]
Direct reduction processes were developed to overcome the difficulties of conventionalblast furnaces. DRI plants need not be part of an integrated steel plant, as is characteristic of blast furnaces. The initial capital investment and operating costs of direct reduction plants are lower than integrated steel plants and are more suitable for developing countries where supplies of high grade coking coal are limited, but where steel scrap is generally available for recycling.[citation needed] Many other countries use variants of the process.
Factors that help make DRI economical:
Direct reduced iron is highly susceptible tooxidation andrusting if left unprotected, and is normally quickly processed further to steel.[5] The bulk iron[page needed] can also catch fire (it ispyrophoric).[6] Unlikeblast furnacepig iron, which is almost pure metal, DRI contains somesiliceousgangue (if made from scrap, not from new iron from direct reduced iron with natural gas), which needs to be removed in the steel-making process.
Producing sponge iron and then working it was the earliest method used to obtain iron in theMiddle East,Egypt, andEurope, where it remained in use until at least the 16th century.
Whilst bloomeries allowed for iron production at a lower furnace temperature (only about 1,100 °C or so), they produce iron at a much slower rate than a blast furnace.
The following reactions successively converthematite (fromiron ore) intomagnetite, magnetite intoferrous oxide, and ferrous oxide intoiron by reduction withcarbon monoxide orhydrogen.[7]
Carburizing producescementite (Fe3C):
India is the world’s largest producer of direct-reduced iron.[8]
Sponge iron is not useful by itself, but can be processed to createwrought iron or steel. The sponge is removed from the furnace, called a bloomery, and repeatedly beaten with heavy hammers and folded over to remove the slag,oxidize any carbon or carbide, andweld the iron together. This treatment usually creates wrought iron with about three percent slag and a fraction of a percent of other impurities. Further treatment may add controlled amounts of carbon, allowing various kinds of heat treatment (e.g. "steeling").
Today, sponge iron is created by reducing iron ore without melting it. This makes for an energy-efficient feedstock for specialty steel manufacturers which used to rely uponscrap metal.
Hydrogen-reduced iron is used as a source of food-gradeiron powder, forfood fortification and foroxygen scavenging. This elemental form is not absorbed as well as ferrous forms,[9] but the oxygen-scavenging function keeps it attractive. Purity standards for this use were established in 1977.[10]
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