Direct reduced iron, also called sponge iron, is produced from the direct reduction of iron ore to iron by a reducing gas or elemental carbon produced from natural gas or coal. 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 in the presence of the reducing gas syngas, a mixture of hydrogen and carbon monoxide.
Process
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 drive off the oxygen contained in various forms of iron ore, in order to convert the ore to metallic iron, without melting it. 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 using electric arc furnaces to take advantage of the heat produced by the DRI product.
Benefits
Direct reduction processes were developed to overcome the difficulties of conventional blast 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. India is the world’s largest producer of direct-reduced iron. Many other countries use variants of the process. Factors that help make DRI economical:
Direct-reduced iron has about the same iron content as pig iron, typically 90–94% total iron so it is an excellent feedstock for the electric furnaces used by mini mills, allowing them to use lower grades of scrap for the rest of the charge or to produce higher grades of steel.
Hot-briquetted iron is a compacted form of DRI designed for ease of shipping, handling and storage.
Hot direct reduced iron is DRI that is transported hot, directly from the reduction furnace, into an electric arc furnace, thereby saving energy.
The direct reduction process uses pelletized iron ore or natural "lump" ore. One exception is the fluidized bed process which requires sized iron ore particles.
The direct reduction process can use natural gas contaminated with inert gases, avoiding the need to remove these gases for other use. However, any inert gas contamination of the reducing gas lowers the effect of that gas stream and the thermal efficiency of the process.
Supplies of powdered ore and raw natural gas are both available in areas such as Northern Australia, avoiding transport costs for the gas. In most cases the DRI plant is located near a natural gas source as it is more cost effective to ship the ore rather than the gas.
Direct reduced iron is highly susceptible to oxidation and rusting if left unprotected, and is normally quickly processed further to steel. The bulk iron can also catch fire since it is pyrophoric. Unlike blast furnace pig iron, which is almost pure metal, DRI contains some siliceous gangue, which needs to be removed in the steel-making process.
History
Producing sponge iron and then working it was the earliest method used to obtain iron in the Middle East, Egypt, and Europe, where it remained in use until at least the 16th century. There is some evidence that the bloomery method was also used in China, but China had developed blast furnaces to obtain pig iron by 500 BCE. The advantage of the bloomery technique is that iron can be obtained at a lower furnace temperature, only about 1,100 °C or so. The disadvantage, relative to a blast furnace, is that only small quantities can be made at a time.
Chemistry
The following reactions successively convert hematite to magnetite, magnetite to ferrous oxide, and ferrous oxide to iron by reduction with carbon monoxide or hydrogen. 3 Fe2O3 + CO/H2 -> 2 Fe3O4 + CO2/H2O Fe3O4 + CO/H2 -> 3 FeO + CO2/H2O FeO + CO/H2 -> Fe + CO2/H2O Carburizing produces cementite. 3 Fe + CH4 -> Fe3C + 2H2 3 Fe + 2CO -> Fe3C + CO2 3 Fe + CO +H2 -> Fe3C + H2O
Uses
Sponge iron is not useful by itself, but can be processed to create wrought 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, oxidise any carbon or carbide and weld 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. 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 upon scrap metal.
