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Open hearth furnaces are one of a number of kinds of furnace where excess carbon and other impurities are burnt out of pig iron to produce steel. Since steel is difficult to manufacture due to its high melting point, normal fuels and furnaces were insufficient and the open hearth furnace was developed to overcome this difficulty. Compared to Bessemer steel, which it displaced, its main advantages were that it did not expose the steel to excessive nitrogen (which would cause the steel to become brittle), was easier to control, and it permitted the melting and refining of large amounts of scrap iron and steel.
The open hearth furnace was first developed by German-born engineer Carl Wilhelm Siemens. In 1865, the French engineer Pierre-Émile Martin took out a license from Siemens and first applied his regenerative furnace for making steel. Their process was known as the Siemens–Martin process, and the furnace as an "open-hearth" furnace. Most open hearth furnaces were closed by the early 1990s, not least because of their slow operation, being replaced by the basic oxygen furnace or electric arc furnace.
Whereas the earliest example of open hearth steelmaking is found about 2000 years ago in the culture of the Haya people, in present day Tanzania, and in Europe in the Catalan forge, invented in Spain in the 8th century, it is usual to confine the term to certain 19th-century and later steelmaking processes, thus excluding bloomeries (including the Catalan forge), finery forges, and puddling furnaces from its application.
The open hearth process is a batch process and a batch is called a "heat". The furnace is first inspected for possible damage. Once it is ready or repaired, it is charged with light scrap, such as sheet metal, shredded vehicles or waste metal. The furnace is heated using burning gas. Once the charge has melted, heavy scrap, such as building, construction or steel milling scrap is added, together with pig iron from blast furnaces. Once all the steel has melted, slag-forming agents such as limestone are added. The oxygen in iron oxide and other impurities decarburizes the pig iron by burning excess carbon away, forming steel. To increase the oxygen content of the heat, iron ore can be added.
The process is far slower than that of the Bessemer converter and thus easier to control and sample for quality assessment. Preparing a heat usually takes eight to eight and a half hours, and (more) hours to finish the conversion into steel. As the process is slow, it is not necessary to burn all the carbon away as in the Bessemer process, but the process can be terminated at any given point when the desired carbon content has been achieved.
The furnace is tapped in the same way a blast furnace is tapped; a hole is drilled in the side of the hearth and the raw steel flows out. Once all the steel has been tapped, the slag is skimmed away. The raw steel may be cast into ingots, a process called teeming, or it may be used in continuous casting in the rolling mill.
The regenerators are the distinctive feature of the furnace and consist of fire-brick flues filled with bricks set on edge and arranged in such a way as to have a great number of small passages between them. The bricks absorb most of the heat from the outgoing waste gases and return it later to the incoming cold gases for combustion.
Sir Carl Wilhelm Siemens developed the Siemens regenerative furnace in the 1850s, and claimed in 1857 to be recovering enough heat to save 70–80% of the fuel. This furnace operates at a high temperature by using regenerative preheating of fuel and air for combustion. In regenerative preheating, the exhaust gases from the furnace are pumped into a chamber containing bricks, where heat is transferred from the gases to the bricks. The flow of the furnace is then reversed so that fuel and air pass through the chamber and are heated by the bricks. Through this method, an open-hearth furnace can reach temperatures high enough to melt steel, but Siemens did not initially use it for that.
In 1865, the French engineer Pierre-Émile Martin took out a license from Siemens and first applied his regenerative furnace for making steel. The most appealing characteristic of the Siemens regenerative furnace is the rapid production of large quantities of basic steel, used for example to construct high-rise buildings. The usual size of furnaces is 50 to 100 tons, but for some special processes they may have a capacity of 250 or even 500 tons.
The Siemens–Martin process complemented rather than replaced the Bessemer process. It is slower and thus easier to control. It also permits the melting and refining of large amounts of scrap steel, further lowering steel production costs and recycling an otherwise troublesome waste material. Its worst drawback is that melting and refining a charge takes several hours. This was an advantage in the early 20th century, as it gave plant chemists time to analyze the steel and decide how much longer to refine it. But by about 1975, electronic instruments such as atomic absorption spectrophotometers had made analysis of the steel much easier and faster. The work environment around an open-hearth furnace is said to be extremely dangerous, although that may be even more true of the environment around a basic oxygen or electric arc furnace.
Basic oxygen steelmaking eventually replaced the open hearth furnace. It rapidly superseded both the Bessemer and Siemens–Martin processes in western Europe by the 1950s and in eastern Europe by the 1980s. The open hearth steelmaking had superseded the Bessemer process in UK by 1900, but elsewhere in Europe, especially in Germany, the Bessemer and Thomas processes were used until the late 1960s when they were superseded by basic oxygen steelmaking. The last open-hearth furnace in the former East Germany was stopped in 1993. In the US, steel production using the Bessemer process ended in 1968 and the open-hearth furnaces had stopped by 1992. In Hunedoara steel works, Romania the last 420-tonne capacity open-hearth furnace was shut down on 12 June 1999 and demolished and scrapped between 2001 and 2003, but the eight smokestacks of the furnaces remained until February 2011. The last open-hearth shop in China was shut down in 2001. The nation with the highest share of steel produced with open-hearth furnaces (almost 50%) is Ukraine.The process is still in use in India and some parts of Ukraine. Russia retired its last hearth furnace in March 2018, and was considering preserving it as a museum artifact.
Steel is an alloy of iron and carbon, and sometimes other elements like chromium. Because of its high tensile strength and low cost, it is a major component used in buildings, infrastructure, tools, ships, trains, automobiles, machines, appliances, and weapons.
The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities from the iron by oxidation with air being blown through the molten iron. The oxidation also raises the temperature of the iron mass and keeps it molten.
Pig iron is an intermediate product of the iron industry, also known as crude iron, which is obtained by smelting iron ore in a blast furnace. Pig iron has a very high carbon content, typically 3.8–4.7%, along with silica and other constituents of dross, which makes it very brittle and not useful directly as a material except for limited applications.
Wrought iron is an iron alloy with a very low carbon content in contrast to cast iron. It is a semi-fused mass of iron with fibrous slag inclusions, which gives it a "grain" resembling wood that is visible when it is etched or bent to the point of failure. Wrought iron is tough, malleable, ductile, corrosion-resistant and easily welded.
Steelmaking is the process of producing steel from iron ore and/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon are removed from the sourced iron, and alloying elements such as manganese, nickel, chromium, carbon and vanadium are added to produce different grades of steel. Limiting dissolved gases such as nitrogen and oxygen and entrained impurities in the steel is also important to ensure the quality of the products cast from the liquid steel.
A furnace is a device used to heat and melt metal ore to remove gangue. The name derives from Latin word fornax, which means oven. The heat energy to fuel a furnace may be supplied directly by fuel combustion, by electricity such as the electric arc furnace, or through induction heating in induction furnaces.
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 "forced" or supplied above atmospheric pressure.
Basic oxygen steelmaking, also known as Linz–Donawitz-steelmaking or the oxygen converter process is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing oxygen through molten pig iron lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as basic because fluxes of burnt lime or dolomite, which are chemical bases, are added to promote the removal of impurities and protect the lining of the converter.
An electric arc furnace (EAF) is a furnace that heats charged material by means of an electric arc.
A reverberatory furnace is a metallurgical or process furnace that isolates the material being processed from contact with the fuel, but not from contact with combustion gases. The term reverberation is used here in a generic sense of rebounding or reflecting, not in the acoustic sense of echoing.
An ironworks or iron works is a building or site where iron is smelted and where heavy iron and steel products are made. The term is both singular and plural, i.e. the singular of ironworks is ironworks.
An air preheater (APH) is any device designed to heat air before another process with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil.
Puddling is a step in the manufacture of high-grade iron in a crucible or furnace. Known by the 1st century AD in the Han Dynasty of ancient China, it was advanced in Great Britain during the Industrial Revolution. There molten pig iron in a reverberatory furnace was stirred with consumable rods resulting in a less brittle, more purified steel. It was one of the most important processes of making the first appreciable volumes of valuable and useful bar iron. Eventually, the furnace would be used to make small quantities of specialty steels.
An induction furnace is an electrical furnace in which the heat is applied by induction heating of metal. Induction furnace capacities range from less than one kilogram to one hundred tonnes, and are used to melt iron and steel, copper, aluminium, and precious metals.
Hot blast refers to the preheating of air blown into a blast furnace or other metallurgical process. As this considerably reduced the fuel consumed, hot blast was one of the most important technologies developed during the Industrial Revolution. Hot blast also allowed higher furnace temperatures, which increased the capacity of furnaces.
A crematorium or crematory is a venue for the cremation of the dead.
Steel, Peech and Tozer was a large steel maker with works situated at Ickles and Templeborough, in Rotherham, South Yorkshire, England.
Brown Bayley Steels was a steel-making company established in Sheffield, England in 1871, as Brown, Bayley & Dixon. They occupied a site on Leeds Road which was later occupied by the Don Valley sports stadium. The firm was founded by George Brown, Nephew of "John Brown" of the firm John Brown & Company. The firm manufactured Bessemer steel and railway tracks.
A cupola or cupola furnace is a melting device used in foundries that can be used to melt cast iron, Ni-resist iron and some bronzes. The cupola can be made almost any practical size. The size of a cupola is expressed in diameters and can range from 1.5 to 13 feet. The overall shape is cylindrical and the equipment is arranged vertically, usually supported by four legs. The overall look is similar to a large smokestack.
The Corex Process is a smelting reduction process created by Siemens VAI as a more environmentally friendly alternative to the blast furnace. Presently, the majority of steel production is through the blast furnace which has to rely on ever decreasing amounts of coking coal. That is coal which has been cooked in order to remove impurities so that it is superior to coal. In addition, the Blast furnace requires a sinter plant too in order to prepare the iron ore for reduction. Unlike the Blast Furnace, smelting reduction processes are typical smaller and use coal and oxygen directly to reduce iron ore into a usable product. Smelting reduction processes come in two basic varieties, two-stage or single-stage. In a single-stage system the iron ore is both reduced and melted in the same container. Meanwhile, in a two-stage process, like Corex, the ore is reduced in one shaft and melted and purified in another. Plants using the Corex process have been put use in areas such as South Africa, India, and China.
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