Blast furnace

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Blast furnace in Sestao, Spain. The actual furnace itself is inside the centre girderwork.
Blast furnace in Sestao, Spain. The actual furnace itself is inside the centre girderwork.

A blast furnace is a type of metallurgical furnace used for smelting to produce metals, generally iron. Fuel and ore are continuously supplied through the top of the furnace, while air (or pure oxygen) is blown into the bottom of the chamber, so that the chemical reactions take place throughout the furnace as the material moves downward. The end products are usually molten metal and slag phases tapped from the bottom, and flue gases exiting from the top of the furnace.

This type of furnace is typically used for smelting iron ore to produce pig iron, an intermediate material used in the production of commercial iron and steel. However, blast furnaces are also used for non-ferrous smelting processes, particularly in the production of lead.

Contents

[edit] Iron

Blast furnaces existed in China from about the 5th century BC, and in the West from the High Middle Ages. They spread from the region around Namur in Belgium in the late 15th century, being introduced to England in 1491. The fuel used in these was invariably charcoal. The successful substitution of (coke) for charcoal is widely attributed to Abraham Darby in 1709. The efficiency of the process was further enhanced by the practice of preheating the blast, patented by James Beaumont Neilson in 1828.

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 artificially blown using bellows, but the term 'blast furnace' is normally reserved for furnaces where iron (or other metal) are refined from ore.

[edit] History

[edit] 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.[1] Also, the effectiveness of the Chinese blast furnace was enhanced during this period by the engineer Du Shi (circa 31 AD), who applied water-power (hydraulics) to piston-bellows in forging cast iron.

While it was long thought that the Chinese developed the blast furnace and cast iron as their first method of iron production, Donald Wagner (the author of the above referenced study) has published a more recent paper[2] that supersedes some of the statements in the earlier work; the newer paper still places the date of the first cast iron artifacts at the 4th and 5th century BC, but also provides evidence of earlier bloomery furnace use, which migrated in from the west. He suggests that early blast furnace and cast iron production evolved from furnaces used to melt bronze. Certainly, though, iron was essential to military success by the time the State of Qin had unified China (221 AD).

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 it 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.

[edit] Medieval Europe

The spread of the Cistercian monks allowed a transfer of blast furnace technology throughout medieval Europe.
The spread of the Cistercian monks allowed a transfer of blast furnace technology throughout medieval Europe.

The oldest known blast furnaces in the West were built in Dürstel in Switzerland, the Märkische Sauerland in Germany, and Sweden at Lapphyttan where the complex was active between 1150 and 1350.[3] At Noraskog in the Swedish county of Järnboås there have also been found traces of blast furnaces dated even earlier, possibly to around 1100.[4] 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.[5]

Knowledge of certain technological advances was transmitted as a result of the General Chapter of the Cistercian monks, including the blast furnace, as the Cistercians are known to have been skilled metallurgists.[6] According to Jean Gimpel, their high level of industrial technology facilitated the diffusion of new techniques: "Every monastery had a model factory, often as large as the church and only several feet away, and waterpower drove the machinery of the various industries located on its floor."[7] Iron ore deposits were often donated to the monks along with forges to extract the iron, and within time surpluses were being offered for sale. The Cistercians became the leading iron producers in Champagne, France, from the mid-13th century to the 17th century, also using the phosphate-rich slag from their furnaces as an agricultural fertiliser.[8]

Archaeologists are still discovering the extent of Cistercian technology.[9] At Laskill, an outstation of Rievaulx Abbey and the only medieval blast furnace so far identified in Britain, the slag produced was low in iron content.[10] Slag from other furnaces of the time contained a substantial concentration of iron, whereas Laskill is believed to have produced cast iron quite efficiently.[10][11] Its date is not yet clear, but it probably did not survive Henry VIII's Dissolution of the Monasteries in the late 1530s, as an agreement (immediately after that) concerning the 'smythes' with the Earl of Rutland in 1541 refers to blooms.[12] Consequently, the type of blast furnace pioneered there did not spread outside Rievaulx.

[edit] 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.[13]

The first British 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.[14]

Representation of blast furnaces and other ironmaking processes from the 19th century
Representation of blast furnaces and other ironmaking processes from the 19th century

[edit] Coke blast furnaces

In 1709, at Coalbrookdale in Shropshire, England, 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.[15] Darby's 'old blast furnace' has been archaeologically excavated and can be seen in situ at Coalbrookdale as part of the Ironbridge Gorge Museums.

A further important development was the change to hot blast, patented by James Beaumont Neilson at Wilsontown Ironworks 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 (containing CO) from the furnace was directed and burnt. The resultant heat was used to preheat the air blown into the furnace.[16]

[edit] Modern furnaces

The blast furnace remains an important part of modern iron production. Modern furnaces are highly efficient, including Cowper stoves to pre-heat the blast air and employ recovery systems to extract the heat from the hot gases exiting the furnace. Competition in industry drives higher production rates, and the largest blast furnaces produce around 60,000 tonnes of iron per week. This is a great increase from the typical 18th century furnaces, which averaged about 400 tons per year. Variations of the blast furnace, such as the Swedish electric blast furnace, have been developed in countries which have no native coal resources.

[edit] Process

Blast furnace diagram1. Hot blast from Cowper stoves 2. Melting zone (bosh)3. Reduction zone of ferrous oxide (barrel) 4. Reduction zone of ferric oxide (stack) 5. Pre-heating zone (throat)6. Feed of ore, limestone and coke 7. Exhaust gases 8. Column of ore, coke and limestone 9. Removal of slag 10. Tapping of molten pig iron 11. Collection of waste gases
Blast furnace diagram
1. Hot blast from Cowper stoves
2. Melting zone (bosh)
3. Reduction zone of ferrous oxide (barrel)
4. Reduction zone of ferric oxide (stack)
5. Pre-heating zone (throat)
6. Feed of ore, limestone and coke
7. Exhaust gases
8. Column of ore, coke and limestone
9. Removal of slag
10. Tapping of molten pig iron
11. Collection of waste gases

Modern furnaces are equipped with an array of supporting facilities to increase efficiency, such as ore storage yards where barges are unloaded. The raw materials are transferred to the stockhouse complex by ore bridges, or rail hoppers and "ore transfer cars". Rail-mounted scale cars or computer controlled weigh hoppers weigh out the various raw materials to yield the desired hot metal and slag chemistry. A "skip car" powered by winches brings these to the top of the furnace.[17]

The ironmaking blast furnace itself is built in the form of a tall chimney-like structure lined with refractory brick. Coke, limestone flux and iron ore (iron oxide) are charged into the top of the furnace, with a precise filling order to control gas flow and chemical reactions inside the furnace. Four "uptakes" allow the hot, dirty gas to exit the furnace dome, while "bleeder valves" protect the top of the furnace from sudden gas pressure surges. The coarse particles in the gas settle in the "dustcatcher", and are dumped into a railroad car or truck for disposal, while the gas itself flows through a Venturi scrubber and a gas cooler to reduce the temperature of the hot but clean gas.[17]

The hot blast temperature can be from 900°C to 1300 °C (1600°F to 2300°F) depending on the stove design and condition.[17] The hot blast is directed into the furnace through water-cooled copper nozzles called "tuyeres" near the base. The temperatures they deal with may be 2000 °C to 2300 °C (3600°F to 4200°F).[17] Oil, tar, natural gas, powdered coal and oxygen can also be injected into the furnace at tuyere level to combine with the coke to release additional energy which is necessary to increase productivity.[17]

The "casthouse" at the bottom half of the furnace contains the bustle pipe, tuyeres and the equipment for casting the liquid iron and slag. Once a "taphole" is drilled through the refactory clay plug, liquid iron and slag flow down a trough through a "skimmer" opening, separating the iron and slag. Modern, larger blast furnaces may have as many as four tapholes and two casthouses.[17]

[edit] Chemistry

Blast furnaces of Třinec Iron and Steel Works
Blast furnaces of Třinec Iron and Steel Works

The main chemical reaction producing the molten iron is:

Fe2O3 + 3 CO → 2Fe + 3CO2[18]

Preheated blast air blown into the furnace reacts with the carbon in the form of coke to produce carbon monoxide and heat. The carbon monoxide then reacts with the iron oxide to produce molten iron and carbon dioxide. Hot carbon dioxide, unreacted carbon monoxide, nitrogen from the air pass up through the furnace as fresh feed material travels down into the reaction zone. As the material travels downward, the counter-current gases both preheat the feed charge, decompose the limestone to calcium oxide and carbon dioxide, and begin to reduce the iron oxides in the solid state. The main reaction controlling the gas atmosphere in the furnace is called the Boudouard reaction:

C + O2 → CO2[18]
CO2 + C → 2 CO[18]

The decomposition of limestone in the middle zones of the furnace proceeds according to the following reaction:

CaCO3 → CaO + CO2[17]

The calcium oxide formed by decomposition reacts with various acidic impurities in the iron (notably silica), to form the slag which is essentially calcium silicate, CaSiO3.[18]

The "pig" iron produced by the blast furnace has a relatively high carbon content of around 4-5%, making it very brittle, and of little commercial use. Some pig iron is used to make cast iron. The majority of pig iron produced by blast furnaces undergoes further processing to reduce the carbon content and produce various grades of used for tools and construction materials.

Although the effiency of blast furnaces is constantly evolving, the chemical process inside the blast furnace remains the same. According to the American Iron and Steel Institute; "Blast furnaces will survive into the next millenium because the larger, efficient furnaces can produce hot metal at costs competitive with other iron making technologies."[17]

[edit] See also

[edit] Other Metals

Blast furnaces are used today to smelt lead from its oxide, after it has been desilvered.

[edit] Footnotes

  1. ^ Early iron in China, Korea, and Japan, Donald B. Wagner, March 1993
  2. ^ The earliest use of iron in China, Donald B. Wagner, 1999
  3. ^ Archaeological Investigations on the Beginning of Blast Furnace-Technology in Central Europe
  4. ^ A. Wetterholm, 'Blast furnace studies in Nora bergslag ' (Örebro universitet 1999, Järn och Samhälle) ISBN 91-7668-204-8
  5. ^ 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.
  6. ^ Thomas Woods, How the Catholic Church Built Western Civilization (2005), ISBN 0-89526-038-7; p 34
  7. ^ Jean Gimpel, The Medieval Machine: The Industrial Revolution of the Middle Ages (New York, Penguin, 1976), p 67. Cited by Woods
  8. ^ Gimpel, p 68; cited by Woods, p 35
  9. ^ Woods, p 36
  10. ^ a b Woods, p 37
  11. ^ 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</ref name="Woods">David Derbyshire, 'Henry "Stamped Out Industrial Revolution"', The Daily Telegraph (21 June 2002); cited by Woods.
  12. ^ H. R. Schubert, History of the British iron and steel industry from c. 450 BC to AD 1775 (Routledge, London 1957), 395-7.
  13. ^ 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.
  14. ^ P. W. King, 'The production and consumption of iron in early modern England and Wales' Economic History Review LVIII(1), 1-33; G. Hammersley, 'The charcoal iron industry and its fuel 1540-1750' Economic History Review Ser. II, XXVI (1973), 593-613.
  15. ^ 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)
  16. ^ * A. Birch, Economic History of the British Iron and Steel Industry , 181-9
    • C. K. Hyde, Technological Change and the British iron industry (Princeton 1977)
  17. ^ a b c d e f g h AISI
  18. ^ a b c d [1]

[edit] External links

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