molybdenum

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molybdenum

(məlĭb`dənəm) [Gr.,=leadlike], metallic chemical element; symbol Mo; at. no. 42; at. wt. 95.96; m.p. about 2,617°C;; b.p. about 4,612°C;; sp. gr. 10.22 at 20°C;; valence +2, +3, +4, +5, or +6. Molybdenum is a hard, malleable, ductile, high-melting, silver-white metal with a body-centered cubic crystalline structure. It is below chromium in Group 6 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. Molybdenum resists corrosion at ordinary temperatures. In forming compounds, as in oxides, sulfides, and halides, it exhibits variable valence. In its most important compounds, however, it has an oxidation state of +6, as in the trioxide, which forms a series of compounds known as the molybdates. Molybdenum does not occur uncombined in nature. Its chief ore is molybdenitemolybdenite
, a mineral, molybdenum disulfide, MoS2, blue-gray in color, with a metallic luster and greasy feel. It occurs in crystals of the hexagonal system but more commonly in scales, grains, or foliated or massive form.
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 (molybdenum disulfide, MoS2). It also occurs in wulfenite (a lead molybdate) and powellite (a calcium molybdate-tungstate). It is widely but sparingly distributed throughout the world; it is found in the United States, Canada, Europe, Australia, Chile, Russia, and China. Large amounts of molybdenite are mined at Climax, Colo. Molybdenum ore is also obtained as a byproduct of copper mining. The ores are usually concentrated by the flotation processflotation process,
in mineral treatment and mining, process for concentrating the metal-bearing mineral in an ore. Crude ore is ground to a fine powder and mixed with water, frothing reagents, and collecting reagents.
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 before being refined. The actual refining process depends on the ultimate use. The molybdenite may be purified for use in lubricants. Almost all molybdenum ore is converted by roasting to molybdic oxide, MoO3. The oxide may be added directly to steel or may be converted to ferromolybdenum by a thermal process; this alloy is used to add molybdenum to other iron and steel alloys. The oxide may be further purified by sublimation, or converting directly from the solid to vapor state, and then reduced to molybdenum powder by reaction with carbon, aluminum, or hydrogen. The oxide may be dissolved in ammonium hydroxide; the solution is filtered and evaporated to yield ammonium molybdate, (NH4)2Mo2O7. In alloy, 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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 molybdenum acts as a hardening agent and also improves the properties of the alloy at high temperatures; such alloys are used in making high-speed cutting tools, aircraft parts, and forged automobile parts. The pure metal in the form of thin sheets or wire is used in X-ray tubes, electronic tubes, and electric furnaces because it can withstand high temperatures. It was used in early incandescent light bulbs. Because it retains its strength and structure at very high temperatures, it has found use in certain critical rocket and missile parts. Useful compounds of molybdenum include molybdenum disulfide, used as a lubricant; ammonium molybdate, used in chemical analysis for phosphates; and lead molybdate, used as a pigment in ceramic glazes. Molybdenum was recognized as a distinct element in 1778 by K. W. Scheele; its ore had earlier been confused with lead ore, hence its name. The element was isolated by P. J. Hjelm in 1782.

Molybdenum

 

(Latin, Molybdaenum), Mo, a chemical element in Group VI of the Mendeleev periodic system. Atomic number, 42; atomic mass, 95.94. A light gray refractory metal. In nature, molybdenum is represented by seven stable isotopes with mass numbers 92, 94–98, and 100, of which 98Mo is the most common (23.75 percent). Until the 18th century, the principal molybdenum mineral, molybdenite, was not distinguished from graphite and galena, since they are very similar in external appearance. These minerals had the common name “molybdenum” (from the Greek molybdos, “lead”).

The element molybdenum was discovered in 1778 by the Swedish chemist K. Scheele, who isolated molybdic acid by treating molybdenite with nitric acid. In 1782, the Swedish chemist P. Hjelm was the first to obtain metallic molybdenum, by reduction of MoO3 with carbon.

Occurrence in nature. Molybdenum is a typical rare element; its content in the earth’s crust is 1.1 × 10−4 percent by mass. There are 15 molybdenum minerals, most of which are formed in the biosphere (various molybdates). In magmatic processes, molybdenum is associated mainly with acidic magma and with granitoids. There is little molybdenum in the earth’s mantle; the molybdenum content in ultrabasic rock is only 2 × 10−5 percent. The accumulation of molybdenum is related to hot internal waters, from which it precipitates as molybdenite, MoS2 (the main industrial molybdenum mineral), forming hydrothermal deposits. The most important agent for precipitating molybdenum from water is F2S.

The geochemistry of molybdenum in the biosphere is closely related to living matter and the products of its decompositionthe average molybdenum content in organisms is 1 × 10−5 percent. On the earth’s surface, especially under basic conditions, Mo(IV) is easily oxidized to molybdates, many of which are relatively soluble. In regions with a dry climate, molybdenum migrates easily, accumulating upon evaporation in salt lakes (up to 1 × 10−3 percent) and salt marshes. In wet climates and acidic soils, molybdenum is often relatively immobile; here molybdenum-containing fertilizers are required (for example, for legumes).

There is little molybdenum in river waters (10−7 to 10−8 percent). Molybdenum enters the ocean with water runoff and to some extent accumulates in seawater (the molybdenum content resulting from evaporation is 1 × 10−6 percent). Some molybdenum also precipitates, concentrating in clay mud rich in organic matter and H2S. In addition to molybdenum ores, some molybdenum-bearing copper and copper-lead-zinc ores are used to produce molybdenum. The extraction of molybdenum is increasing rapidly.

Physical and chemical properties. Molybdenum crystallizes in a cubic body-centered lattice with period a = 3.14 angstroms (Å). The atomic radius of molybdenum is 1.4 Å; the ionic radius of Mo4+ is 0.68 Å, and of Mo6+, 0.62 Å. Density at 20°C, 10.2 g/cm3; melting point, 2620° ± 10°C; boiling point, about 4800°C. Specific heat 20°-100°C, 0.272 kilojoule per (kg . °K), or 0.065 cal/g ⋅ deg; thermal conductivity at 20°C, 146.65 watts per (m ⋅ °K), or 0.35 cal/cm ⋅ sec ⋅ deg; thermal coefficient of linear expansion, (5.8−6.2) × 10−6at 25°-700°C; specific electric resistance, 5.2×10−8 ohm ⋅ m, or 5.2 × 10 ohm ⋅ cm; electron work function, 4.37 electron volts. Molybdenum is paramagnetic. Atomic magnetic susceptibility, about 90 × 10−6 at 20°C.

The mechanical properties of molybdenum depend on the purity of the metal and prior mechanical and thermal treatment. Thus, the Brinell hardness is 1,500–1,600 meganewtons per sq m (MN/m2), or 150–160 kilograms-force per sq mm (kgf/mm2), for a small sintered bar; 2,000–2,300 MN/m2 for a forged rod; and 1,400–1,850 MN/m2 for annealed wire. The ultimate strength of annealed molybdenum wire is 800–1,200 MN/m2. The elastic modulus of molybdenum is 285–300 GN/m2. Molybdenum is more malleable than tungsten. Recrystallization annealing does not lead to brittleness of the metal.

Molybdenum is stable in air at ordinary temperatures. The onset of oxidation (temper colors) takes place at 400°C. Rapid oxidation, with the formation of Mo3a, begins at 600°C. At temperatures above 700°C, water vapor rapidly oxidizes molybdenum to MoO2 Molybdenum does not react chemically with hydrogen up to its melting point. Fluorine acts on molybdenum at ordinary temperatures, and chlorine acts at 250°C, forming MoF6 and MoCl5, respectively. Molybdenum disulfide, MoS2, forms upon the action of sulfur and hydrogen sulfide vapor above 440° and 800°C, respectively. Above 1500°C, molybdenum forms a nitride with nitrogen (probably Mo2N). Solid carbon and hydrocarbons, as well as carbon monoxide, react with molybdenum metal at 1100°-1200°C to form the carbide Mo2C (which melts, with decomposition, at 2400°C). Molybdenum reacts with silicon above 1200°C, forming the silicide MoSia, highly stable in air up to 1500°-1600°C (microhardness, 14,100 MN/m2).

In hydrochloric and sulfuric acids, molybdenum is only slightly soluble at 80°-100°C. Nitric acid, aqua regia, and hydrogen peroxide dissolve the metal slowly in the cold, and rapidly upon heating. A mixture of nitric and sulfuric acids is a good solvent for molybdenum. Tungsten does not dissolve in a mixture of these acids. Molybdenum is stable in cold alkaline solutions but corrodes somewhat upon heating. The outer electron configuration of the molybdenum atom is 4d55s1, and the most characteristic valence is 6. Compounds containing pentavalent, tetravalent, trivalent, and divalent molybdenum are also known.

Molybdenum forms two stable oxides—molybdenum trioxide, MoO3 (white crystals with a greenish hue; melting point, 795°C; boiling point, 1155°C), and molybdenum dioxide, MoC>2 (dark brown in color). In addition, intermediate oxides are known that correspond in composition to the homologous series MonO3n _ i (MO9O26, Mo8Cha, and Mo4O11); these compounds are thermally unstable and decompose above 700°C, with the formation of MoO3 and MoO2 Molybdenum trioxide forms simple (or normal) molybdic acids, the monohydrate H2MoC>4 and dihydrate H2MoO4 ⋅ H2O, and the isopolyacids H6Mo?O24, H4MO6O24, I^MogChe, and so on. The salts of the normal acids are called normal molybdates, and those of the polyacids are called polymolybdates. In addition to the oxides listed above, a few peracids of molybdenum are known, with the formula H2MoOjr (x = 5–8), as well as heteropoly compounds with phosphoric, arsenic, and boric acids. One of the common salts of heteropolyacids is ammonium phosphomolybdate, (NH4)3[P(Mo3Oio)4] ⋅ 6H 2O. Molybdenum fluoride, MoF6 (melting point, 17.5°C; boiling point, 35°C), and molybdenum chlo-ride, Mods (melting point, 194°C; boiling point, 268°C) are the most important molybdenum halides and hydroxyhalides. The compounds MoF6 and Mods may be purified easily by distillation and are used in the production of high-purity molybdenum.

The existence of three molybdenum sulfides, MoSs, MoS2, and Mo28 3, has been reliably established. Of these, MoSa and MoS2 are of practical importance. Molybdenum disulfide, MoS2, is found in nature as the mineral molybdenite and may be produced by the action of sulfur on molybdenum or by fusion of MoO3 with sodium bicarbonate and sulfur. Molybdenum disulfide is virtually insoluble in water, HC1, and dilute sulfuric acid; it decomposes above 1200°C, with the formation of Mo2S3. If hydrogen sulfide is introduced into heated acidified solutions of molybdates, MoSs precipitates.

Production. The main raw materials for the production of molybdenum and its alloys and compounds are standard molybdenite concentrates containing 47–50 percent molybdenum, 28–32 percent sulfur, 1–9 percent SiO2, and traces of other elements. The concentrate undergoes oxidizing roasting at 570°-600°C in multihearth roasters or fluidized-bed furnaces. The roasting product contains impure MoO3- Pure MoO3, which is required for the production of metallic molybdenum, is obtained from the roasting product by volatilization at 950°-1100°C or by a chemical method consisting of leaching the roasting product with ammonia water, which transfers the molybdenum into solution; separation of ammonium polymolybdates, mainly 3(NH4)2O ⋅ 7MoO3nH2O, from the ammonium molybdate solution (after removal of copper and iron impurities) by neutralization or evaporation with subsequent crystallization; and roasting of the paramolybdate at 450°-500°C to yield pure MoO3, with an impurity content of not more than 0.05 percent.

Metallic molybdenum is produced (initially in powder form) by reduction of molybdenum trioxide in a stream of dry hydrogen. The process is carried out in tube furnaces in two stages. The first stage is conducted at 550°-700°C, and the second, at 900°-1000°C. Molybdenum powder is converted into compact metal by powder metallurgy or smelting methods. In the former case, relatively small bars (with a cross section of 2–9 sq cm and length of 450–600 mm) are produced. Molybdenum powder is pressed in steel molds at 200–300 MN/m2 (2–3 tons-force per sq cm). After preliminary roasting at 1000°-1200°C in a hydrogen atmosphere, the bars undergo high-temperature sintering at 2200°-2400°C. The sintered moldings are forged, extruded, or rolled. Larger sintered bars (100–200 kg) are produced by hydrostatic pressing in an elastic shell.

Bars weighing 500–2,000 kg are produced by arc smelting in furnaces with a cooled copper crucible and a consumable electrode (a bundle of sintered bars). Electron-beam smelting is also used for molybdenum. Reduction of the sintered molybdenite concentrate by ferrosilicon in the presence of iron ore and steel chips is used for the production of ferromolybdenum, which is an alloy consisting of 55–70 percent molybdenum and the remainder iron. Ferromolybdenum is used to introduce molybdenum additives into steel.

Use. About 70–80 percent of the raw molybdenum is used in the production of alloy steels. The remainder is used in pure form and in molybdenum-based alloys and alloys with nonferrous and rare metals, as well as in chemical compounds. Metallic molybdenum is a very important construction material in the production of electric lights and electrovacuum devices (radio tubes, transmission tubes, and X-ray tubes). Molybdenum is used in the production of anodes, grids, cathodes, and holders for incandescent filaments in electric bulbs. Molybdenum wire and ribbon are widely used as heating elements for high-temperature furnaces.

After the production of large molybdenum bars became common, molybdenum came to be used (in pure form or alloyed with other metals) in cases requiring the maintenance of strength at high temperatures—for example, in the production of parts for rockets and aircraft. Coatings made of molybdenum silicide and heat-resistant enamels are used to protect molybdenum from oxidation at high temperatures. Molybdenum is used as a structural material in nuclear power reactors, since it has a relatively low thermal neutron capture cross section (2.6 barns). Molybdenum plays an important role as a component in heat-resistant and acid-resistant alloys, in which it is combined mainly with nickel, cobalt, and chromium.

A number of molybdenum compounds are used in technology. For example, molybdenum disulfide is used as a lubricating material for rubbing parts of mechanisms, molybdenum disilicide is used for producing heating elements for high-temperature furnaces, and Na2MoO4 is used in the production of paints and varnishes. Molybdenum oxides are used as catalysts in the chemical and petroleum industries.

A. N. ZELIKMAN

Molybdenum in the organism. Molybdenum is always present in plants, animals, and humans as a trace element; it participates mainly in nitrogen metabolism. Molybdenum is necessary for the activation of a number of oxidation-reduction enzymes (flavoproteins), which catalyze nitrate reduction and nitrogen fixation in plants (a great deal of molybdenum is found in the tubers of legumes), as well as in purine metabolism in animals.

In plants, molybdenum stimulates the biosynthesis of nucleic acids and proteins and increases the content of chlorophyll and vitamins. In the case of molybdenum insufficiency, legumes, oats, tomatoes, and lettuce develop a special type of blotch, become barren, and die. Thus, soluble molybdates are introduced into micronutrients. Animals usually do not experience molybdenum insufficiency. However, an excess of molybdenum in the feed of ruminants leads to chronic molybdenum toxicosis, accompanied by dysentery, loss of weight, and a disruption of copper and phosphate metabolism (biogeochemical provinces with a high molybdenum content are known on the Kulunda steppe and in the Altai and the Caucasus). The toxic action of molybdenum is eliminated by the introduction of copper compounds.

An excess of molybdenum in humans may lead to a disruption of metabolism, retardation of bone growth, and gout.

I. F. GRIBOVSKAIA

REFERENCES

Zelikman, A. N. Molibden. Moscow, 1970.
Molibden: Sbornik. Moscow, 1959. (Translated from English.)
Biologicheskaia rol’ molibdena. Moscow, 1972.

molybdenum

[mə′lib·de·nəm]
(chemistry)
A chemical element, symbol Mo, atomic number 42, and atomic weight 95.94.
(metallurgy)
A silvery-gray metal used in iron-based alloys.

molybdenum

a very hard ductile silvery-white metallic element occurring principally in molybdenite: used mainly in alloys, esp to harden and strengthen steels. Symbol: Mo; atomic no.: 42; atomic wt.: 95.94; valency: 2--6; relative density: 10.22; melting pt.: 2623?C; boiling pt.: 4639?C