Bessemer process

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Bessemer process

(bĕs`əmər) [for Sir Henry BessemerBessemer, Sir Henry
, English engineer and inventor, b. Charleton, Hertfordshire. He made experiments to obtain stronger material for gun manufacture and discovered the basic principle of the Bessemer process.
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], industrial process for the manufacture of steel from molten pig iron. The principle involved is that of oxidation of the impurities in the iron by the oxygen of air that is blown through the molten iron; the heat of oxidation raises the temperature of the mass and keeps it molten during operation. The process is carried on in a large container called the Bessemer converter, which is made of steel and has a lining of silica and clay or of dolomite. The capacity is from 8 to 30 tons of molten iron; the usual charge is 15 or 18 tons. The converter is egg-shaped. At its narrow upper end it has an opening through which the iron to be treated is introduced and the finished product is poured out. The wide end, or bottom, has a number of perforations (tuyères) through which the air is forced upward into the converter during operation. The container is set on pivots (trunnions) so that it can be tilted at an angle to receive the charge, turned upright during the "blow," and inclined for pouring the molten steel after the operation is complete. As the air passes upward through the molten pig iron, impurities such as silicon, manganese, and carbon unite with the oxygen in the air to form oxides; the carbon monoxide burns off with a blue flame and the other impurities form slag. Dolomite is used as the converter lining when the phosphorus content is high; the process is then called basic Bessemer. The silica and clay lining is used in the acid Bessemer, in which phosphorus is not removed. In order to provide the elements necessary to give the steel the desired properties, another substance (often spiegeleisen, an iron-carbon-manganese alloy) is usually added to the molten metal after the oxidation is completed. The converter is then emptied into ladles from which the steel is poured into molds; the slag is left behind. The whole process is completed in 15 to 20 min. The Bessemer process was superseded by the open-hearth process (see steelsteel,
alloy of iron, carbon, and small proportions of other elements. Iron contains impurities in the form of silicon, phosphorus, sulfur, and manganese; steelmaking involves the removal of these impurities, known as slag, and the addition of desirable alloying elements.
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). See also metallurgymetallurgy
, science and technology of metals and their alloys. Modern metallurgical research is concerned with the preparation of radioactive metals, with obtaining metals economically from low-grade ores, with obtaining and refining rare metals hitherto not used, and with the
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.

Bessemer Process

 

one of the ways of transforming molten pig iron into steel without the consumption of fuel.

The process was proposed by Henry Bessemer in 1856 in connection with the growing demands for steel created by increased construction of railroads, ships, and machines; for its time it was a progressive method of obtaining cast steel. The first factory trials for the production of Bessemer steel in Russia date to the late 1850’s (the Ural plants at Kushva, Nizhnee Isetskoe, Sysert’, Vsevolodo-Vil’va, and others). In organizing the Bessemer process on an industrial scale, Russian metallurgists (D. K. Chernov at the Obukhov plant in 1872 and, almost simultaneously, K. P. Polenov at the Nizhniaia Salda plant) followed independent paths and developed a particular method of transforming low-silicon pig irons in a Bessemer converter which came to be called the Russian Bessemer process. This method was characterized by the high heating of the pig iron in a cupola furnace (Obukhov plant) or reverberatory furnace (Nizhniaia Salda plant) before pouring it into the converter. The Bessemer process is usually carried out in converters by blowing air through tuyeres set in the bottom of the converter. Compressed air—often atmospheric, and sometimes enriched with oxygen—is blown through the molten pig iron that has been poured into the converter. Because of the blowing, the impurities in the pig iron (silicon, manganese, and carbon) are oxidized, giving off a considerable amount of heat; as a result of this, the temperature of the metal is increased at the same time as the impurity content is lowered. The metal is thus kept in a molten state. In the production of steel for mold casting, a small converter with side blowing is used. This process has been called the small Bessemer process.

The course of the Bessemer process is controlled primarily by the chemical composition and temperature of the pig iron poured into the converter. An important role is played by silicon, which through its oxidation at the beginning of the process facilitates an increase in the temperature in the period during which it is still insufficiently high for a decarbonization reaction. The greater the degree of heating of the pig iron above the melting point, the lower the silicon content. Bessemer pig iron is classified into three groups according to the silicon content: cold (under 1.0 percent Si), chemically normal (1.0–1.5 percent Si), and chemically hot (over 1.5 percent Si). According to the degree of superheating of the pig iron poured into the converter, it is classified as hot (1350° C and above), physically normal (1250°-1350° C), and physically cold (below 1250° C). By adjusting the ratio of the factors (chemical composition—chiefly the silicon content—and temperature of the pig iron), a heat balance is struck in the Bessemer process that determines its normal course and the correct properties of the final product—steel. The course (that is, the sequence of the oxidation reactions of the pig iron’s impurities) of the process is determined by the temperature regime. The temperature is regulated by changing the amount of the blast or by making additions to the metal in the converter. Steel scrap, ore, or scale is usually introduced in order to lower the temperature of the metal. When there is too little heat, an addition of ferroalloys—which are rich in silicon—is made. The metal’s temperature at tapping time is about 1600° C. The blown metal—the so-called Bessemer steel—contains an excess of oxygen in solution in the form of ferrous oxide (FeO). Consequently, the final stage of the blow is the deoxidation of the metals by means of ferroalloys.

During the blowing of the cast iron, nonvolatile oxides of its component elements (silicon and manganous and ferrous oxides—SiO2, MnO, and FeO) are obtained; together with components of the eroded refractory lining, they form slag having a chemical composition that varies during the blowing. The approximate chemical composition of slag for a normally executed operation in the production of low-carbon steel is 60 percent SiO2, 3 percent Al2O3,, 15 percent FeO, 17 percent MnO, and an insignificant amount of the compound CaO + MgO. The pronounced acidic nature of slags and the additional presence of an acid refractory converter lining offers no means of removing harmful impurities such as phosphorus and sulfur from the metal in the Bessemer process. Only an insignificant part of the phosphorus is volatilized with the gases into a vapor state. An imperative requirement for Bessemer pig irons is purity with regard to sulfur and phosphorus. Only special “Bessemer” ores containing no more than 0.025–0.03 percent phosphorus—of which there are very limited supplies—are suitable for smelting into Bessemer pig iron.

The high nitrogen content in the blasting has a considerable effect on the heat balance in the Bessemer process: approximately 630 kilojoules (150 kilocalories) of heat is expended per kg of blown pig iron in the heating of inert nitrogen (a basic component of the flue gases at an average temperature of 1450° C). In addition, the presence of nitrogen in the metal, in which it is partially soluble, sharply reduces the quality of the steel.

The ever-increasing requirements imposed on steel, together with a substantial reduction in the supplies of “Bessemer” ores, have resulted in a pronounced curtailment of Bessemer production. The limited capacity of bottom-blown converters (up to 50 tons) has also contributed to this. The production of Bessemer steel as a percentage of the total steel production is 1.5 in the USSR, 0.2 in the USA, 0.3 in France, and 0.06 in England. The open-hearth process and, in the last decade, the oxygen-converter process are more promising than the Bessemer process.

REFERENCES

Afanas’ev, S. G. Issledovanie bessemerovskogo protsessa. Moscow, 1957.
Lapitskii, V. I., N. I. Stupar’, and O. I. Legkostup. Metallurgiia stali. Moscow, 1963.
Levin, S. L. Staleplavil’nye protsessy. Kiev, 1963.
Staleplavil’noe proizvodstvo: Spravochnik, vol. 1. Moscow, 1964.

S. G. AFANAS’EV

Bessemer process

[′bes·ə·mər ′präs·əs]
(metallurgy)
A steelmaking process in which carbon, silicon, phosphorus, and manganese contained in molten pig iron are oxidized by a strong blast of air.