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A blast furnace is a type of furnace for smelting metal ore, usually iron ore. The combustion material and ore are supplied from the top while an air flow is supplied from the bottom of the chamber, so that the chemical reaction takes place throughout the ore, not only at the surface. This type of furnace is typically used for smelting iron ore to produce pig iron, the raw material for wrought and cast iron.
The blast furnace is to be distinguished from the bloomery in that the object of the blast furnace is to produce molten metal that can be tapped from the furnace, whereas the intention in the bloomery is to avoid it melting so that carbon does not become dissolved in the iron. Bloomeries were also articially blown using bellows, but the term 'blast furnace' is normally reserved for furnaces where iron (or other metal) was melted.
History
The Ancient World
The oldest known blast furnaces were built in Han China in the 1st century BC. However, cast iron artifacts found in China have been dated as early as the 5th century BC, so it is possible that the history of the blast furnace in China is older than presently known. These early furnaces had clay walls and used phosphorus-containing minerals as a flux.*In Europe, the iron was made in bloomeries by the Greeks, Celts, Romans, and Carthaginians in the ancient period; several examples have been found in France; and materials found in Tunisia suggest their use there as well as in Antioch during the Hellenistic Period. Though little is known of its use during the Dark Ages, the process probably continued in use. The improved bloomery named Catalan forge was invented in Catalonia, Spain during the 8th century. Instead of using natural draught, it relied on bellows for pumping the air in. This enabled to produce better quality iron and enlarge the capacity. It is assumed the Cistercian monks, who were skilled engineers and metallurgists, had managed to produce true cast iron and thus invented the blast furnace in Europe.
Medieval Europe
The oldest known blast furnaces in the West were built in Sweden at Lapphyttan, and the complex was active between 1150 and 1350. At Noraskog in the Swedish county of Järnboås there have also been found traces of blast furnaces dated even earlier, possibly to around 1100 * A. Wetterholm, 'Blast furnace studies in Nora bergslag '¨(Örebro universitet 1999, Järn och Samhälle) ISBN 91-7668-204-8 . It is unclear, and probably impossible to determine, whether the blast furnace was independently developed in medieval Sweden or if the knowledge somehow was transmitted from Asia. These early blast furnaces, as were the Chinese examples, were very inefficient compared to those used today. The iron from the Lapphyttan complex was used to produce balls of wrought iron known as osmonds, and these were traded internationally - a possible reference occurs in a treaty with Novgorod from 1203 and several certain references in accounts of English customs from the 1250s and 1320s. Other furnaces of the 13th to 15th centuries have been identified in Westphalia. * N. Bjökenstam, 'The Blast Furnace in Europe during the Middle Ages: part of a new system for producing wrought iron' in G. Magnusson, The Importance of Ironmaking: Technological Innovation and Social Change I (Jernkontoret, Stockholm 1995), 143-53 and other papers in the same volume.Knowledge of certain technological advances may have been transmitted as a result of the General Chapter of the Cistercians, including the blast furnace, as the Cistercians are known to have been skilled metallurgists. Certainly, a medieval blast furnace (the only one so far identified in Britain, believed to be as advanced as a modern blast furnace) has been identified by Gerry McDonnell, archeometallurgist of the University of Bradford. This was at Laskill, an outstation of Rievaulx Abbey, producing cast iron. Its date is not yet clear; certainly it did not survive Henry VIII's Dissolution of the Monasteries in the 1530s, and thus the type of blast furnace pioneered there did not spread outside Rievaulx.* R. W. Vernon, G. McDonnell and A. Schmidt, 'An integrated geophysical and analytical appraisal of early iron-working: three case studies' Historical Metallurgy 31(2) (1998), 72-5 79.
The date when the furnace operated is not clear, and it is possible that it did not survive that late, as an agreement concering the 'smythes' with the Earl of Rutland in 1541 refers to blooms.H. R. Schubert, History of the British iron and steel industry from c. 450 BC to AD 1775 (Routledge, London 1957), 395-7.
Early modern blast furnaces: origin and spread
The direct ancestor of those used in France and England was in the Namur region in what is now Belgium. From there, they spread first to the Pays de Bray on the eastern boundary of Normandy and from there to the Weald of Sussex, where the first furnace (called Queenstock) in Buxted was built in about 1491, followed by one at Newbridge in Ashdown Forest in 1496. They remained few in number until about 1530 but many were built in the following decades in the Weald, where the iron industry perhaps reached its peak about 1590. Most of the pig iron from these furnaces was taken to finery forges for the production of bar iron.* B. Awty & C. Whittick (with P. Combes), 'The Lordship of Canterbury, iron-founding at Buxted, and the continental antecedents of cannon-founding in the Weald' Sussex Archaeological Collections 140 (2004 for 2002), 71-81.The first furnaces outside the Weald were not built until the 1550s, but many were built in the remainder of that century and the following ones. The output of the industry probably peaked about 1620, and was followed by a slow decline until the early 18th century. This was apparently because it was more economic to import iron from Sweden and elsewhere than to make it in some more remote British locations. Charcoal that was economically available to the industry was probably being consumed as fast as the wood to make it grew.* P. W. King, 'The production and consumption of iron in early modern England and Wales' Economic History Review LVIII(1), 1-33.
Coke blast furnaces
In 1709, at Coalbrookdale in Shropshire in the UK, Abraham Darby began to fuel a blast furnace with coke instead of charcoal. Coke iron was initially only used for foundry work, making pots and other cast iron goods. Foundry work was a minor branch of the industry, but his son built a new furnace at Horsehay (nearby), and began to supply the owners of finery forges with coke pig iron for the production of bar iron. Coke pig iron was by this time cheaper to produce than charcoal pig iron. The use of a coal-derived fuel in the iron industry was a key factor in the British Industrial Revolution.* A. Raistrick, A Dynasty of Ironfounders (1953; York 1989)- C. K. Hyde, Technological Change and the British iron industry (Princeton 1977)
- B. Trinder, The Industrial Revolution in Shropshire (Chichester 2000)
A further important development was the change to hot blast, patented by James Beaumont Neilson in Scotland in 1828. This further reduced production costs. Within a few decades, the practice was to have a 'stove' as large as the furnace next to it into which the waste gas from the furnace was directed and burnt. The resultant heat was used to preheat the air blown into the furnace.* A. Birch, Economic History of the British Iron and Steel Industry , 181-9
Process
The blast furnace relied on the fact that the unwanted silicon and other impurities were lighter than the molten iron, pig iron, that was its main product. The furnace was built in the form of a tall chimney-like structure lined with refractory brick. Coke, limestone and iron ore (iron oxide) are poured in the top. Air is blown in through tuyeres near to the base. This "blast" allows combustion of the fuel there. This reduces the oxide to the metal, which being heavier sank to the bottom of the furnace. The exact nature of the reaction is:
More precisely, the compressed air blown into the furnace reacts with the carbon in the fuel to produce carbon monoxide, which then mixes with the iron oxide, reacting chemically to produce iron and carbon dioxide, which leaks out of the furnace at the top.
The temperature in the furnace typically runs at about 1500°C, which is enough to also decompose limestone (calcium carbonate) into calcium oxide and additional carbon dioxide:
CaCO3 → CaO + CO2
The calcium oxide reacts with various acidic impurities in the iron (notably silica), forming a slag containing calcium silicate, CaSiO3 which floats on the iron.
The pig iron produced by the blast furnace is not useful for most purposes due to its high carbon content, around 4-5%, making it very brittle. Some pig iron is used to make cast iron goods, often being remelted in a foundry cupola.
For other purposes further processing is needed to reduce the carbon content to enable iron to be used for tools or as a construction material. There have been various processes for this. The earliest process was conducted in the finery forge. In the late 18th century, this began to be displaced by 'potting and stamping', but the most successful new process of the industrial revolution period was puddling.
This is now done by forcing a jet of high-pressure oxygen into a special rotating container containing the pig iron. Some of the carbon is oxidised into carbon monoxide, CO, and carbon dioxide, CO2. This also oxidizes impurities in the pig iron. The container is rotated and the processed pig iron can be separated from the oxidised impurities. Before the mid 19th century, pig iron from the blast furnace was made into wrought iron, which is commercially pure iron. At that period, if steel was needed, particularly pure varieties of iron were heated with charcoal in a cementation furnace to produce blister steel (with about 1-2% carbon). This might be further purified using the crucible technique, but steel was too expensive to use on a large scale. However with the introduction of the Bessemer process in the late 1850s and then other processes, the production of steel was dramatically increased. By the early 20th century most iron was being converted to steel before use.
The blast furnace remains an important part of modern iron production. Modern furnaces include Cowper stoves to pre-heat the blast air to high temperatures in order to avoid cooling (and thus having to re-heat) the mix, and use fairly complex systems to extract the heat from the hot carbon dioxide when it escapes from the top of the furnace, further improving efficiency. The largest blast furnaces produce around 60,000 tonnes of iron per week, enough for about four cars per minute. This is a great increase from the 18th century, when charcoal blast furnaces averaged 400 tons per year.
Footnotes
- Darby's old blast furnace has been archaeologically excavated and can be seen in situ at Coalbrookdale as part of the Ironbridge Gorge Museums.
External links
- Blast Furnace animation
- How a Blast Furnace works Illustrated.
- Precursors of the Blast Furnace
- Extensive picture gallery about all methods of making and shaping of iron and steel in North America and Europe. In German and English.
- Blast Furnace Museum Radwerk IV
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"Blast furnace"