iron furnace

A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being supplied above atmospheric pressure.

In a blast furnace, fuel ( coke), ores, and flux (limestone) are continuously supplied through the top of the furnace, while a hot blast of (sometimes oxygen enriched) air is blown into the lower section of the furnace through a series of pipes called tuyeres, so that the chemical reactions take place throughout the furnace as the material falls downward. The end products are usually molten metal and slag phases tapped from the bottom, and flue gases exiting from the top. The downward flow of the ore along with the flux in contact with an upflow of hot, carbon monoxide-rich combustion gases is a countercurrent exchange and chemical reaction 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.

Blast furnaces are estimated to have been responsible for over 4% of global greenhouse gas emissions between 1900 and 2015, and are difficult to decarbonize.

Process engineering and chemistry

Blast furnaces operate on the principle of chemical reduction whereby carbon monoxide converts iron oxides to elemental iron.

Blast furnaces differ from bloomeries and reverberatory furnaces in that in a blast furnace, flue gas is in direct contact with the ore and iron, allowing carbon monoxide to diffuse into the ore and reduce the iron oxide. The blast furnace operates as a countercurrent exchange process whereas a bloomery does not. Another difference is that bloomeries operate as a batch process whereas blast furnaces operate continuously for long periods. Continuous operation is also preferred because blast furnaces are difficult to start and stop. Also, the carbon in pig iron lowers the melting point below that of steel or pure iron; in contrast, iron does not melt in a bloomery.

Silica has to be removed from the pig iron. It reacts with calcium oxide (burned limestone) and forms silicates, which float to the surface of the molten pig iron as slag. Historically, iron was produced with charcoal to prevent sulfur contamination.

In a blast furnace, a downward-moving column of ore, flux, coke (or charcoal) and their reaction products must be sufficiently porous for the flue gas to pass through, upwards. To ensure this permeability the particle size of the coke or charcoal is of great relevance. Therefore, the coke must be strong enough so it will not be crushed by the weight of the material above it. Besides the physical strength of its particles, the coke must also be low in sulfur, phosphorus, and ash.

The main chemical reaction producing the molten iron is:
:Fe₂O₃ + 3CO → 2Fe + 3CO₂

This reaction might be divided into multiple steps, with the first being that preheated air blown into the furnace reacts with the carbon in the form of coke to produce carbon monoxide and heat:

:2 C_(s) + O_2(g) → 2 CO_(g)

Hot carbon monoxide is the reducing agent for the iron ore and reacts with the iron oxide to produce molten iron and carbon dioxide. Depending on the temperature in the different parts of the furnace (warmest at the bottom) the iron is reduced in several steps. At the top, where the temperature usually is in the range between 200 °C and 700 °C, the iron oxide is partially reduced to iron(II,III) oxide, Fe₃O₄.

:3 Fe₂O_3(s) + CO_(g) → 2 Fe₃O_4(s) + CO_2(g)

The temperatures 850 °C, further down in the furnace, the iron(II,III) is reduced further to iron(II) oxide:

:Fe₃O_4(s) + CO_(g) → 3 FeO_(s) + CO_2(g)

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 and decompose the limestone to calcium oxide and carbon dioxide:

:CaCO_3(s) → CaO_(s) + CO_2(g)

The calcium oxide formed by decomposition reacts with various acidic impurities in the iron (notably silica), to form a fayalitic slag which is essentially calcium silicate, :
:SiO₂ + CaO → CaSiO₃

As the iron(II) oxide moves down to the area with higher temperatures, ranging up to 1200 °C degrees, it is reduced further to iron metal:

:FeO_(s) + CO_(g) → Fe_(s) + CO_2(g)

The carbon dioxide formed in this process is re-reduced to carbon monoxide by the coke:

:C_(s) + CO_2(g) → 2 CO_(g)

The temperature-dependent equilibrium controlling the gas atmosphere in the furnace is called the Boudouard reaction:

::2CO CO₂ + C

The pig iron produced by the blast furnace has a relatively high carbon content of around 4–5% and usually contains too much sulphur, making it very brittle, and of limited immediate 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 and sulphur content and produce various grades of steel used for construction materials, automobiles, ships and machinery. Desulphurisation usually takes place during the transport of the liquid steel to the steelworks. This is done by adding ''calcium oxide'', which reacts with the iron sulfide contained in the pig iron to form calcium sulfide (called ''lime desulfurization''). In a further process step, the so-called basic oxygen steelmaking, the carbon is oxidized by blowing oxygen onto the liquid pig iron to form ''crude steel''.

History

Cast iron has been found in China dating to the 5th century BC, but the earliest extant blast furnaces in China date to the 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 British inventor Abraham Darby in 1709. The efficiency of the process was further enhanced by the practice of preheating the combustion air (hot blast), patented by British inventor James Beaumont Neilson in 1828.

China

Archaeological evidence shows that iron-smelting techniques and bloomeries were brought to China by nomadic peoples around 800 BC. Wrought iron artifacts were originally only found in the northwest, but by the 6th century BC, luxury items such as wrought iron swords and knives were widespread in China. Bloomery iron co-existed with blast furnaces and cast iron, which were invented shortly after wrought iron technology entered China, for quite some time. Cast iron pieces have been found alongside wrought iron in Shanxi Province dating to the 9th-8th centuries BC, however it is uncertain if these cast iron pieces were accidental byproducts of the iron smelting process. Adzes made using cast iron with a decarburised layer of steel have been found in Luoyang dating to the 5th century BC. Cast iron farming tools that had undergone an annealing process to decrease brittleness were found in burials and smelting cites dating to the 4th century BC. Bloomery iron disappeared in China after the 3rd century AD with the exception of Xinjiang, where bloomery iron objects could be found as late as the 9th century AD. In China, blast furnaces produced cast iron, which was then either converted into finished implements in a cupola furnace, or turned into wrought iron in a fining hearth.

Although cast iron farm tools and weapons were widespread in China by the 5th century BC, employing workforces of over 200 men in iron smelters from the 3rd century onward, the earliest blast furnaces constructed were attributed to the Han dynasty in the 1st century AD.Ebrey, p. 30. These early furnaces had clay walls and used phosphorus-containing minerals as a flux. Chinese blast furnaces ranged from around two to ten meters in height, depending on the region. The largest ones were found in modern Sichuan and Guangdong, while the 'dwarf" blast furnaces were found in Dabieshan. In construction, they are both around the same level of technological sophistication.

The effectiveness of the Chinese human and horse powered blast furnaces was enhanced during this period by the engineer Du Shi (c. AD 31), who applied the power of waterwheels to piston-bellows in forging cast iron. Early water-driven reciprocators for operating blast furnaces were built according to the structure of horse powered reciprocators that already existed. That is, the circular motion of the wheel, be it horse driven or water driven, was transferred by the combination of a belt drive, a crank-and-connecting-rod, other connecting rods, and various shafts, into the reciprocal motion necessary to operate push bellows.. Donald Wagner 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, the Song dynasty Chinese iron industry made a switch of resources from charcoal to coke 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 primary advantage of the early blast furnace was in large scale production and making iron implements more readily available to peasants. Cast iron is more brittle than wrought iron or steel, which required additional fining and then cementation or co-fusion to produce, but for menial activities such as farming it sufficed. By using the blast furnace, it was possible to produce larger quantities of tools such as ploughshares more efficiently than the bloomery. In areas where quality was important, such as warfare, wrought iron and steel were preferred. Nearly all Han period weapons are made of wrought iron or steel, with the exception of axe-heads, of which many are made of cast iron.

Blast furnaces were also later used to produce gunpowder weapons such as cast iron bomb shells and cast iron cannons during the Song dynasty.

Medieval Europe

The simplest forge, known as the Corsican, was used prior to the advent of Christianity. Examples of improved bloomeries are the Stuckofen, sometimes called wolf-furnace, which remained until the beginning of the 19th 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 air in with bellows is known as ''cold blast'', and it increases the fuel efficiency of the bloomery and improves yield. They can also be built bigger than natural draught bloomeries.

Oldest European blast furnaces

The oldest known blast furnaces in the West were built in Durstel 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, traces of even earlier blast furnaces have been found, possibly from 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.

The technology required for blast furnaces may have either been transferred from China, or may have been an indigenous innovation. 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

HOME


Content is Copyleft
Website design, code, and AI is Copyrighted (c) 2014-2017 by Stephen Payne


Consider donating to Wikimedia



As an Amazon Associate I earn from qualifying purchases