In the iron and steel industry, direct reduction is a set of processes for obtaining iron from iron ore, by reducing iron oxides without melting the metal. The resulting product is pre-reduced iron ore.
Historically, direct reduction was used to obtain a mix of iron and slag called a bloom in a bloomery. At the beginning of the 20th century, this process was abandoned in favor of the blast furnace, which produces iron in two stages (reduction-melting to produce cast iron, followed by refining in a converter).
However, various processes were developed in the course of the 20th century and, since the 1970s, the production of pre-reduced iron ore has undergone remarkable industrial development, notably with the rise of the . Designed to replace the blast furnace, these processes have so far only proved profitable in certain economic contexts, which still limits this sector to less than 5% of world steel production.
History
Bloomery
Historically, the reduction of iron ore without smelting is the oldest process for obtaining steel. Low-temperature furnaces, unable to reach the melting temperatures of iron alloys, produce a bloom, a heterogeneous agglomerate of metallic iron more or less impregnated with carbon, gangue, and charcoal. This process was gradually succeeded, from the 1st century in China and the 13th century in Europe, by the blast furnace, which simultaneously reduces and melts iron.(fr) Jacques Corbion (pref. Yvon Lamy), Le savoir… fer — Glossaire du haut-fourneau: Le langage… (savoureux, parfois) des hommes du fer et de la zone fonte, du mineur au… cokier d'hier et d'aujourd'hui, 2003, 5th ed. [publishing details] (read online archive) § Réduction directe.
Elaborate low furnaces, such as the tatara or the Catalan forge, survived until the early 19th century. Compared with the indirect process (reduction-melting in the blast furnace, followed by cast-iron refining), these processes only survived when they enjoyed at least one of the following two advantages:
ability to process ores that are incompatible with blast furnaces (such as iron sands that clog blast furnaces, or ores that generate slag that is too pasty to be drained); a more "reasonable" size than that of giant plants and their constraints (ore and capital requirements, production to sell off, etc.).
Modern direct reduction
More advanced direct reduction processes were developed at the beginning of the 20th century, when it became possible to smelt pre-reduced ores using the Martin-Siemens process or the electric arc furnace. Based on this technical and economic model, a number of processes were industrialized before World War II (the Krupp-Renn process adopted by the Shōwa Steel Works, the Chenot process, etc.). They remained confidential, however, and their profitability was generally debated.
Modern direct reduction processes, based on the use of natural gas instead of coal, were studied intensively in the 1950s. On December 5, 1957, the Mexican company Hylsa started up the first industrial production unit of this type in Monterrey, with the pre-reduced ore obtained destined for smelting in an electric arc furnace."4000 Years Later… a history of the direct reduction of iron ore ", Hylsa, 1997. As the production of pre-reduced ore with natural gas was economically viable, several plants were built in the late 1960s. As a cheap supply of natural gas was essential to their profitability, most plants were located in countries with gas deposits, in Latin America (where many were developed) and in the Middle East.Best Available Techniques (BAT) Reference Document for Iron and Steel Production, Direction régionale de l'environnement, de l'aménagement et du logement, February 28th, 2012, 597 p. (read online ), p. 523-202.
In 1970, worldwide production of pre-reduced iron ore reached 790,000 tonnes. The processes then in operation were the HYL process (680,000 tonnes produced), an SL/RN unit, a Purofer unit, and the first plant to use the Midrex process.
Although profitable and innovative, the processes invented did not ultimately prove to be a technological revolution capable of supplanting the traditional blast furnace-based process. However, the quantity of steel produced from pre-reduced materials grew steadily, outstripping world steel production:
in 1975, NML played a significant role in developing a ‘Direct Reduction Technology’ for producing sponge iron with solid fuel like non-metallurgical coal. This formed the basis of the first commercial sponge iron plant of India. in 1976, installations in service totalled less than 5 Mt; in 1985, annual production was 11 Mt for an installed capacity of around 20 Mt, the difference being explained by fluctuations in energy costs;(fr) Jacques Corbion (pref. Yvon Lamy), Le savoir… fer — Glossaire du haut-fourneau: Le langage… (savoureux, parfois) des hommes du fer et de la zone fonte, du mineur au… cokier d'hier et d'aujourd'hui, 2003, 5th ed. [publishing details] (read online archive), § Éponge (de fer). in 1991, production reached 20 Mt. in 1995, worldwide production of prereducts passed the 30 Mt mark for the first time.(fr) Jacques Corbion (pref. Yvon Lamy), Le savoir… fer — Glossaire du haut-fourneau: Le langage… (savoureux, parfois) des hommes du fer et de la zone fonte, du mineur au… cokier d'hier et d'aujourd'hui, 2003, 5th ed. [publishing details] (read online archive), § D.R.I. In 2010, 70 Mt were produced, 14% from HYL processes and 60% from the Midrex process. The latter accounts for most of the growth in natural gas-fired production of pre-reduced products, although since 2005 coal-fired processes have been making a strong comeback, mainly in India."World direct reduction statistics ", Midrex, 2012.
Packaging of pre-reduced iron ore is evenly divided between sponge iron and briquettes. Sponges are a highly porous metallic product, close to the original ore but highly pyrophoric, which limits their transport. They are therefore often subjected to hot compaction, which improves both product density and handling safety.Pablo Duarte, Klaus Knop and Eugenio Zendejas, "Technical and economic aspects of production and use of DRI in integrated steel works", millennium-steel, 2004. In 2012, 45% of prereducts were transformed into briquettes in this way.
Chemical reactions
Iron oxide reduction Iron oxides are reduced in the following sequence:Amit Chatterjee, Sponge Iron Production By Direct Reduction Of Iron Oxide, PHI Learning Private Limited, 2010, 353 p. (, read online archive) p. 41.
Each transition from one oxide to the next is due to two simultaneous high-temperature reduction reactions by carbon monoxide CO or dihydrogen H2: Main reduction reactions"Process technology followed for sponge iron", Environment Compliance Assistance Centre (ECAC)TemperatureCarbon monoxide reductionHydrogen reduction900 °C < T < 1 000 °CFe3O4 + H2 → 3 FeO + H2O1,000 °C < T < 1 050 °C These temperatures differ from those predicted by the Ellingham diagram. In reality, there is a coupling between carbon monoxide reduction and dihydrogen, so that these reactions work together, with hydrogen significantly improving the efficiency of CO reduction.
Reducing gas production
Coal-fired processes In coal-fired processes, part of the fuel is first burnt to heat the charge. The product of this combustion is CO2. When the temperature reaches 1,000 °C, the CO2 reacts with the unburned carbon to create CO:J. Feinman, "Direct Reduction and Smelting Processes ", The AISE Steel Foundation, 1999
CO2 + C ⇌ 2 CO when T > 1 000 °C (Boudouard reaction)
The production of H2 cannot be achieved by the thermal decomposition of water, as the temperatures involved are too low. Hydrogen is in fact produced along with carbon monoxide by the reaction:
H2O + C → H2 + CO when T > 1 000 °C
These two reducing gas production reactions, which consume 172.45 and 131.4 kJ/mol(de) Arnold F. Holleman and Nils Wiberg, Lehrbuch der Anorganischen Chemie, Berlin, de Gruyter, 2007, 102th ed., 2149 p. (), p. 897. respectively, are highly endothermic and operate by limiting charge heating.
Natural gas processes The reducing atmosphere, rich in CO and H2, can be created from the high-temperature cracking of natural gas at around 1100-1150 °C, in the presence of oxidized gases (H2O and CO2) from ore reduction reactors.
The system that generates the reducing gases is called a "reformer". In the Midrex process, it consists of tubes heated by the combustion of a portion (around a third) of the gas from the reactor.
Procedures Plants for the production of pre-reduced iron ore are known as direct reduction plants. The principle involves exposing iron ore to the reducing action of a high-temperature gas (around 1000 °C). This gas is composed of carbon monoxide and dihydrogen, the proportions of which depend on the production process.
Generally speaking, there are two main types of processes:
processes where the reducing gas is obtained from natural gas. In this case, the ore is reduced in tanks; processes where the reducing gas is obtained from coal. The reactor is generally an inclined rotary kiln, similar to those used in cement plants, in which coal is mixed with limestone and ore, then heated.
Another way of classifying processes is to distinguish between those where the reducing gases are produced in specific facilities separate from the reduction reactor - which characterizes most processes using natural gas - and those where the gases are produced inside the fusion reactor: coal-fired processes generally fall into this category. However, many "gas-fired" processes can be fed by gasification units producing a reducing gas from coal."MXCOL®: A breakthrough in coal-based direct reduction ", Midrex.
In addition, since the melting stage is necessary to obtain alloys, reduction-melting processes have been developed which, like blast furnaces, produce a more or less carburized liquid metal. Finally, many more or less experimental processes have been developed. Direct reduction processes
Bold indicates technically and commercially proven processes (i.e. operating viably in several economic contexts)"Ironmaking Process Alternatives Screening Study", Lockheed Martin, October 2002.Natural gasCoalTanksCyclicHYL I (the oldest industrial direct reduction process using natural gas(fr) ) and HYL IIRetorts (some processes of secondary importance, surviving only in specialized production)Continuous Midrex (2/3 of the world's pre-reduced production)
HYL III (competing process with Midrex)Variants of natural gas processes, where gas can be synthesized from coal in an additional unit.Fluidized bedsMany recent developments (FINMET, CIRCORED...) but limited industrial applications"Gas-based direct reduction in fluidized beds - FINMET process", 2011"Circored® for fine ore reduction in CFB", Outotec, 2013.Rotating furnaceRotary hearthOccasionally used as a supplement to coalMany processes developed in the 1990s without commercial successRotary drumsKrupp-Renn (developed in the 1930s, 38 furnaces in 1945.)
SL/RN (developed in 1964, 45% of pre-reduced coal production in 1997Amit Chatterjee, Sponge Iron Production By Direct Reduction Of Iron Oxide, PHI Learning Private Limited, 2010, 353 p. (, read online archive) p. 179-184.)
A number of other efficient but more confidential processes succeeded SL/RN: Kawasaki and Koho,Amit Chatterjee, Sponge Iron Production By Direct Reduction Of Iron Oxide, PHI Learning Private Limited, 2010, 353 p. (, read online archive) p. 216-219. Krupp-CODIRAmit Chatterjee, Sponge Iron Production By Direct Reduction Of Iron Oxide, PHI Learning Private Limited, 2010, 353 p. (, read online archive) p. 99-102.
Tank processes