BLAST FURNACE / REF / 37 /2014

                      

A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron, but also others such as lead or copper.

In a blast furnace, fuel, ore, and flux (limestone) are continuously supplied through the top of the furnace, while air (sometimes with oxygen enrichment) is blown into the lower section of the furnace, 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. The downward flow of the ore and flux in contact with an upflow of hot, carbon monoxide-rich combustion gases is a countercurrent exchange process.

In contrast, air furnaces (such as reverberatory furnaces) are 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, and the shaft furnaces used in combination with sinter plants in base metals smelting.^[1][2]

History

Blast furnaces existed in China from about 1st century AD and in the West from the High Middle Ages. They spread from the region around Namur in Wallonia (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 combustion air (hot blast), patented by James Beaumont Neilson in 1828.

China

The oldest extant 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, while 3rd century BC iron smelters employed an average workforce of over two hundred men. These early furnaces had clay walls and used phosphorus-containing minerals as a flux. The effectiveness of the Chinese blast furnace was enhanced during this period by the engineer Du Shi (c. 31 AD), who applied the power of waterwheels to piston-bellows in forging cast iron.

While it was long thought that the Chinese had 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 5th and 4th centuries 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). Usage of the blast and cupola furnace remained widespread during the Song and Tang Dynasties. By the 11th century AD, 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 woodland from felling. This may have happened as early as the 4th century AD.

The Chinese blast furnace remained in use well until the 20th century. The backyard furnaces favoured by Mao Zedong during the Great Leap Forward were of this type. In the regions with strong traditions of metallurgy, the steel production actually increased during this period. In the regions where there was no tradition of steelmaking or where the ironmasters knowing the traditional skills or the scientific principles of the blast furnace process had been killed, the results were less than satisfactory.

Elsewhere in the ancient world

In most places in the world other than in China, there is no evidence of the use of the blast furnace (proper). Instead, iron was made by direct reduction in bloomeries. These are not correctly described as blast furnaces, though the term is occasionally misused in referring to them. An exception would the Haya people of northwestern Tanzania, who are renowned for creating steel using a blast furnace process and refining process very similar to open hearth process for possibly as long as 2000 years.

In Europe, the Greeks, Celts, Romans, and Carthaginians all used this process. Several examples have been found in France, and materials found in Tunisia suggest they were used there as well as in Antioch (south-central Turkey, between Syria and the Mediterranean Sea) during the Hellenistic Period. Though little is known of it during the Dark Ages, the process probably continued in use.^{[citation needed]} Similarly, smelting in bloomery-type furnaces in West Africa and forging for tools appear in the Nok culture in Africa by 500 BC. The earliest records of bloomery-type furnaces in East Africa are discoveries of smelted iron and carbon in Nubia and Axum that date back between 1,000–500 BCE. Particularly in Meroë, there are known to have been ancient bloomeries that produced metal tools for the Nubians and Kushites and produced surplus for their economy.

Bloomeries have also been discovered and recorded to have been created in medieval West Africa with some of the metalworking Bantu civilizations such as the Bunyoro Empire and the Nyoro people.

Medieval Europe

Catalan forges

The simplest forge, known as the Corsican, was used prior to the advent of Christianity. Improved bloomeries as the Stückofen (sometimes called wolf-furnace) or the Catalan forge, which remained until the beginning of 19th century. The Catalan forge was invented in Catalonia, Spain, during the 8th century. Instead of using natural draught, air was pumped in by a trompe, resulting in better quality iron and an increased capacity. This pumping of airstream in with bellows is known as cold blast, and it increases the fuel efficiency of the bloomery and improves yield. The Catalan forges can also be built bigger than natural draught bloomeries.

Modern experimental archaeology and history re-enactment have shown there is only a very short step from the Catalan forge to the true blast furnace, where the iron is gained as pig iron in liquid phase. Usually, obtaining the iron in liquid phase is actually undesired, and the temperature is intentionally kept below the melting point of iron, since while removing the solid bloom mechanically is tedious and means batch process instead of continuous process, it is almost pure iron and can be worked immediately. On the other hand, pig iron is the eutectic mixture of carbon and iron and needs to be decarburized to produce steel or wrought iron, which was extremely tedious in the Middle Ages.

Oldest European blast furnaces

The oldest known blast furnaces in the West were built in Dürstel in Switzerland, the Märkische Sauerland in Germany, and at Lapphyttan in Sweden, where the complex was active between 1205 and 1300. At Noraskog in the Swedish parish 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.

One possibility involves technology transfer from China. Al-Qazvini in the 13th century and other travellers subsequently noted an iron industry in the Alburz Mountains to the south of the Caspian Sea. This is close to the silk route, so that the use of technology derived from China is conceivable. Much later descriptions record blast furnaces about three metres high. As the Varangian Rus' people from Scandinavia traded with the Caspian (using their Volga trade route, it is possible that the technology reached Sweden by this means. However, since blast furnace has also been invented independently in Africa by the Haya people, it is more likely the process has been invented in Scandinavia independently. The step from bloomery to true blast furnace is not big.

This Caspian region may also separately be the technological source for at furnace at Ferriere, described by Filarete. Water-powered bellows at Semogo in northern Italy in 1226 in a two-stage process. In this, the molten iron was tapped twice a day into water thereby granulating it.

Origin and spread of early modern blast furnaces

The direct ancestor of these used in France and England was in the Namur region in what is now Wallonia (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 appeared during the 1550s, and 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 Backbarrow blast furnace built in Cumbria in 1711 has been described as the first efficient example.

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 central Russia and then finally to the Urals.

Modern process

Blast furnace placed in an installation
1. Iron ore + limestone sinter
2. Coke
3. Elevator
4. Feedstock inlet
5. Layer of coke
6. Layer of sinter pellets of ore and limestone
7. Hot blast (around 1200 °C)
8. Removal of slag
9. Tapping of molten pig iron
10. Slag pot
11. Torpedo car for pig iron
12. Dust cyclone for separation of solid particles
13. Cowper stoves for hot blast
14. Smoke outlet (can be redirected to carbon capture & storage (CCS) tank)
15: Feed air for Cowper stoves (air pre-heaters)
16. Powdered coal
17. Coke oven
18. Coke
19. Blast furnace gas downcomer

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