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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.[1]
In a blast furnace, fuel (coke), ores, and flux (limestone) are continuously supplied through the top of the furnace, while a hot blast of air (sometimes with oxygen enrichment) 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 waste gases (flue gas) exiting from the top of the furnace.[2] 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.[3]
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.[4][5]
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.
The challenge set by the greenhouse gas emissions of the blast furnace is being addressed in an ongoing[when?] European Program called ULCOS (Ultra Low CO2 Steelmaking).[16] Several new process routes have been proposed and investigated in depth to cut specific emissions (CO2 per ton of steel) by at least 50%. Some rely on the capture and further storage (CCS) of CO2, while others choose decarbonizing iron and steel production, by turning to hydrogen, electricity and biomass.[17] In the nearer term, a technology that incorporates CCS into the blast furnace process itself and is called the Top-Gas Recycling Blast Furnace is under development, with a scale-up to a commercial size blast furnace under way.[needs update]
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.[18] 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 English 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 Scottish inventor James Beaumont Neilson in 1828.[19]
Archaeological evidence shows that bloomeries appeared in China around 800 BC. Originally it was thought that the Chinese started casting iron right from the beginning, but this theory has since been debunked[clarification needed] by the discovery of 'more than ten' iron digging implements found in the tomb of Duke Jing of Qin (d. 537 BC), whose tomb is located in Fengxiang County, Shaanxi (a museum exists on the site today).[20] There is however no evidence of the bloomery in China after the appearance of the blast furnace and cast iron. 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.[21]
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.[22] These early furnaces had clay walls and used phosphorus-containing minerals as a flux.[23] 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 [24]
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.[25] 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 a push bellow.[26][27] 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.[28] 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.[29][30]
The primary advantage of the early blast furnace was in large scale production and making iron implements more readily available to peasants.[31] 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.[32]
The simplest forge, known as the Corsican, was used prior to the advent of Christianity. Examples of improved bloomeries are the Stuckofen,[34] sometimes called wolf-furnace,[35]) 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.
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.[39] 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.[40] The Vikings are known to have used double bellows, which greatly increases the volumetric flow of the blast.[41]
The General Chapter of the Cistercian monks spread some technological advances across Europe. This may have included the blast furnace, as the Cistercians are known to have been skilled metallurgists.[44] 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 after a time surpluses were offered for sale. The Cistercians became the leading iron producers in Champagne, France, from the mid-13th century to the 17th century,[45] also using the phosphate-rich slag from their furnaces as an agricultural fertilizer.[46]
Archaeologists are still discovering the extent of Cistercian technology.[47] 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.[48] Slag from other furnaces of the time contained a substantial concentration of iron, whereas Laskill is believed to have produced cast iron quite efficiently.[48][49][50] Its date is not yet clear, but it probably did not survive until 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.[51] Nevertheless, the means by which the blast furnace spread in medieval Europe has not finally been determined.
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 Darby's son built a new furnace at nearby Horsehay, 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.[62][63][64] Darby's original blast furnace has been archaeologically excavated and can be seen in situ at Coalbrookdale, part of the Ironbridge Gorge Museums. Cast iron from the furnace was used to make girders for the world's first cast iron bridge in 1779. The Iron Bridge crosses the River Severn at Coalbrookdale and remains in use for pedestrians.
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