Everything about Blast Furnace totally explained
A
blast furnace is a type of
metallurgical furnace used for
smelting to produce metals, generally iron.
In a blast furnace,
fuel and
ore are continuously supplied through the top of the furnace, while
air (sometimes with oxygen enrichment) 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.
Blast furnaces are to be contrasted with air furnaces (such as
reverberatory furnaces), which were naturally aspirated, usually by the convection of hot gases in a chimney flue. According to this broad definition,
bloomeries for iron,
blowing houses for
tin and
smelt mills for
lead would be classified as blast furnaces. However, the term has usually been limited to those used for smelting
iron ore to produce
pig iron, an intermediate material used in the production of commercial
iron and
steel.
Certain modern furnaces used for non-
ferrous smelting processes are known as blast furnaces, and are particularly in the production of
lead and
copper. However this article (except its final section) will concentrate on furnaces for the production of pig iron.
History
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 doesn't 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.
The Ancient World
The oldest existent blast furnaces were built during the
Han Dynasty of
China in the 1st century BC. However,
cast iron farm tools and weapons were widespread in China by the 5th century BC, These early furnaces had
clay walls and used
phosphorus-containing minerals as a
flux. Also, the effectiveness of the Chinese blast furnace was enhanced during this period by the engineer
Du Shi (circa 31 AD), who applied the power of
waterwheels (
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 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 during the beginning of the Chinese
Bronze Age of the late
Longshan culture (2000 BC). 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 BC). By the 11th century, the
Song Dynasty Chinese iron industry made a remarkable switch of resources from
charcoal to
bituminous coal in casting iron and steel, sparing thousands of acres of prime
timberland from felling.
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.
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. At Noraskog in the Swedish county of Järnboås there have also been found traces of blast furnaces dated even earlier, possibly to around 1100. These early blast furnaces, like 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.
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. 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." 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 fertilizer.
Archaeologists are still discovering the extent of Cistercian technology. 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. Slag from other furnaces of the time contained a substantial concentration of iron, whereas Laskill is believed to have produced
cast iron quite efficiently. Its date isn't yet clear, but it probably didn't 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. Nevertheless, the means by which the blast furnace spread in medieval Europe hasn't finally been determined.
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.
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.
The first blast furnace in
Russia opened in 1637 near
Tula and was called the Gorodishche Works. The blast furnace spread from here to the center of Russia and then finally to
Urals.
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. 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.
A further significant development was the application of raw
anthracite coal to the blast furnace, first tried successfully by George Crane at
Yniscedwyn ironworks in south
Wales in 1837. This was taken up in America by the
Lehigh Crane Iron Company at
Catasauqua, Pennsylvania (q.v.) in 1839.
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. The largest blast furnaces have a volume around 5580 m
3 (190,000 cu ft) and can produce around 80,000 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.
Modern process
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 weight 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.
There are different charging systems: With the classical bell top the position of the charged material can only be modified within small limits. More possibilities and precise charging are offered by "bell less top" systems: the
Distribution Chute and the
Gimbal Top.
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 in a precise filling order which helps control gas flow and the 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 cleaned gas.
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, and 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 + O
2 → CO
2
CO
2 + C → 2CO
The decomposition of limestone in the middle zones of the furnace proceeds according to the following reaction:
» CaCO
3 → CaO + CO
2
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.
Although the efficiency 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 millennium because the larger, efficient furnaces can produce hot metal at costs competitive with other iron making technologies."Other metals
Blast furnaces are used today to smelt lead from its oxide, after it has been desilvered.
One less commonly used method for smelting zinc is through the blast furnace process.
Further Information
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