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Molybdenum
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Molybdenum (from the Greek word for the metal "lead"), is a Group 6 chemical element with the symbol Mo and atomic number 42. It has the eighth-highest melting point of any element. It readily forms hard, stable carbides, and for this reason it is often used in high-strength steel alloys. Molybdenum is found in trace amounts in plants and animals, although excess molybdenum can be toxic in some animals. Molybdenum was discovered in 1778 by Carl Wilhelm Scheele and first isolated in 1781 by Peter Jacob Hjelm.
bdenum is a transition metal with an electronegativity of 1.8 on the Pauling scale and an atomic mass of 95.9 g/mole.
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Encyclopedia
Molybdenum (from the Greek word for the metal "lead"), is a Group 6 chemical element with the symbol Mo and atomic number 42. It has the eighth-highest melting point of any element. It readily forms hard, stable carbides, and for this reason it is often used in high-strength steel alloys. Molybdenum is found in trace amounts in plants and animals, although excess molybdenum can be toxic in some animals. Molybdenum was discovered in 1778 by Carl Wilhelm Scheele and first isolated in 1781 by Peter Jacob Hjelm.
Characteristics
Molybdenum is a transition metal with an electronegativity of 1.8 on the Pauling scale and an atomic mass of 95.9 g/mole. It does not react with oxygen or water at room temperature. At elevated temperatures, molybdenum trioxide is formed in the reaction 2Mo + 3O2 ? 2MoO3.
In its pure metal form, molybdenum is silvery white with a Mohs hardness of 5.5, though it is somewhat more ductile than tungsten. It has a melting point of 2623°C, and, of the naturally-occurring metals, only tantalum, osmium, rhenium, and tungsten have higher melting points. Molybdenum burns only at temperatures above 600°C. It also has the lowest heating expansion of any commercially used metal.
Molybdenum has a value of approximately $65,000 per tonne as of 4 May 2007. It maintained a price at or near $10,000 per tonne from 1997 through 2002, and reached a high of $103,000 per tonne in June 2005.
Isotopes
There are 35 known isotopes of molybdenum ranging in atomic mass from 83 to 117, as well as four metastable nuclear isomers. Seven isotopes occur naturally, with atomic masses of 92, 94, 95, 96, 97, 98, and 100. Of these naturally occurring isotopes, five are stable, with atomic masses from 94 to 98. All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium.
Molybdenum-92 and molybdenum-100 are the only naturally occurring isotopes that are not stable. Molybdenum-100 has a half-life of approximately 1×1019 y and undergoes double beta decay into ruthenium-100. Molybdenum-98 is the most common isotope, comprising 24.14% of all molybdenum. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of approximately .15 µs.
Occurrence
The world's largest producers of molybdenum materials are the United States, Canada, Chile, Russia, and China.
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Though molybdenum is found in such minerals as wulfenite (PbMoO4) and powellite (CaMoO4), the main commercial source of molybdenum is molybdenite (MoS2). Molybdenum is mined as a principal ore, and is also recovered as a byproduct of copper and tungsten mining. Large mines in Colorado (such as the now inactive Climax mine) and in British Columbia yield molybdenite, while many porphyry copper deposits such as the Chuquicamata mine in northern Chile produce molybdenum as a byproduct of copper mining. The Knaben mine in southern Norway was opened in 1885, making it the first molybdenum mine. It remained open until 1973. In 2007, Knaben Molybden AS reopend the mine and produce 100,000 kg of molybdenum disulfide a year.
Molybdenum is the 42nd-most-abundant element in the universe, and the 25th-most-abundant element in Earth's oceans, with an average of 10.8 mt/km³. The Russian Luna 24 mission discovered a single molybdenum-bearing grain (1 × 0.6 µm) in a pyroxene fragment taken from Mare Crisium on the Moon.
A side product of molybdenum mining is rhenium. As it is always present in small varying quantities in molybdenite, the only commercial source for rhenium is molybdenum mines.
Production
The molybdenite is first heated to a temperature of 700°C and the sulfide is oxidized into molybdenum(IV) oxide by air:
- 2MoS2 + 5O2 ? 2MoO3 + 2SO2
The oxidized ore is then either heated to 1100°C to sublimate the oxide, or leached with ammonia which reacts with the molybdenum(IV) oxide to form water soluble molybdates:
- MoO3 + NH4OH ? (NH4)2(MoO4) + H2O
Copper, an impurity in molybdenite, is less soluble in ammonia. To completely remove it from the solution, it is precipitated with hydrogen sulfide.
Pure molybdenum is produced by reduction of the oxide with hydrogen, while the molybdenum for steel production is reduced by the aluminothermic reaction with addition of iron to produce ferromolybdenum. Ferromolybdenum contains 60% of molybdenum.
Compounds
Molybdenum has several common oxidation states, +2, +3, +4, +5 and +6.
The highest oxidation state is common in the molybdenum(VI) oxide MoO3 while the normal sulfur compound is molybdenum disulfide MoS2. The broad range of oxidation states shows up in the chlorides of molybdenum:
Like chromium and some other transition metals molybdenum is able to form quadruple bonds. Mo2(CH3COO)4 is a example for a quadruple bond. This compound can be transformed into the chlorine compound Mo2Cl84-.
The oxidation state 0 is possible with carbon monoxide as ligand, such as in molybdenum hexacarbonyl, Mo(CO)6.
Biological role
The most important use of the molybdenum atom in living organisms is as a metal hetero-atom at the active site in certain enzymes. In nitrogen fixation in certain bacteria, the nitrogenase enzyme which is involved in the terminal step of reducing molecular nitrogen, usually contains molybdenum in the active site (though replacement of Mo with iron or vanadium is known). The structure of the catalytic centre of the enzyme is similar to that in iron-sulfur proteins, it incorporates a Fe4S3 and MoFe3S3 cluster.
In March 2008, researchers reported that they had found strong evidence for the hypothesis that a scarcity of molybdenum in the Earth's early oceans was a limiting factor in the further evolution of eukaryotic life (which includes all plants and animals) as eukaryotes cannot fix nitrogen and must acquire it from prokaryotic bacteria. The scarcity of molybdenum resulted from the relative lack of oxygen in the early ocean. Oxygen dissolved in seawater is the primary mechanism for dissolving molybdenum from minerals on the sea bottom.
Though molybdenum forms compounds with various organic molecules, including carbohydrates and amino acids, it is transported throughout the human body as MoO42-. Molybdenum is present in approximately 20 enzymes in animals, including aldehyde oxidase, sulfite oxidase, xanthine oxidase. In some animals, the oxidation of xanthine to uric acid, a process of purine catabolism, is catalyzed by xanthine oxidase, a molybdenum-containing enzyme. The activity of xanthine oxidase is directly proportional to the amount of molybdenum in the body. However, an extremely high concentration of molybdenum reverses the trend, and can act as an inhibitor in both purine catabolism and other processes. Molybdenum concentrations also affect protein synthesis, metabolism, and growth. These enzymes in plants and animals catalyse the reaction of oxygen in small molecules, as part of the regulation of nitrogen-, sulfur- and carbon cycles.
In a human body, there is approximately 9.3 mg of molybdenum, comprising .00001% of the total body mass. It occurs in higher concentrations in the liver and kidneys, and in lower concentrations in the vertebrae. Molybdenum is also present within human tooth enamel and may help prevent the decaying thereof. Pork, lamb, and beef liver each have approximately 1.5 parts per million of molybdenum. Other significant dietary sources include green beans, eggs, sunflower seeds, wheat flour, lentils, and cereal grain.
The average daily intake of molybdenum is .3 mg . Acute toxicity has not been seen in humans, and the toxicity depends strongly on the chemical state. Studies on rats show a median lethal dose (LD50) as low as 180 mg/kg for some Mo compounds. Although human toxicity data is unavailable, animal studies have shown that chronic ingestion of more than 10 mg/kg of molybdenum can cause diarrhea, growth retardation, sterility, low birth weight, and gout, as well as affecting the lungs, kidneys, and liver. Molybdenum deficiency is not usually seen in healthy people. Sodium tungstate is a competitive inhibitor of molybdenum. Dietary tungsten reduces the concentration of molybdenum in tissues.
Copper-molybdenum antagonism
High levels of molybdenum can interfere with the body's uptake of copper, producing copper deficiency. Molybdenum prevents plasma proteins from binding to copper, and it also increases the amount of copper that is excreted in urine. Ruminants that consume high amounts of molybdenum develop symptoms including diarrhea, stunted growth, anemia, and achromotrichia (loss of hair pigment). These symptoms can be alleviated by the administration of more copper into the system, both in dietary form and by injection. The condition can be aggravated by excess sulfur.
Applications
The ability of molybdenum to withstand extreme temperatures without significantly expanding or softening makes it useful in applications that involve intense heat, including the manufacture of aircraft parts, electrical contacts, industrial motors, and filaments. Molybdenum is also used in alloys for its high corrosion resistance and weldability. Moly contributes corrosion resistance to type 316 stainless steel by 'gettering' residual carbon, preventing the formation of chromium carbide at grain boundaries. Most high-strength steel alloys are .25% to 8% molybdenum. Despite being used in such small portions, more than 43 million kg of molybdenum is used as an alloying agent each year in stainless steels, tool steels, cast irons, and high-temperature superalloys.
Because of its lower density and more stable price, molybdenum is implemented in the place of tungsten. An example is the 'M' series of high speed steels such as M2, M4, and M42 as substitution for the 'T' series of HSS. Molybdenum can be implemented both as an alloying agent and as a flame-resistant coating for other metals. Although its melting point is , molybdenum rapidly oxidizes at temperatures above , making it better-suited for use in vacuum environments.
Molybdenum 99 is used as a parent radioisotope to the radioisotope Technetium-99, which is used in many medical procedures.
Molybdenum disulfide (MoS2) is used as a solid lubricant and an extreme pressure (EP) antiwear agent. It forms strong films on metallic surfaces, and is highly resistant to both extreme temperatures and high pressure, and for this reason, it is a common additive to extreme pressure application greases; in case of a catastrophic failure, the thin layer of molybdenum prevents metal-on-metal contact.
Molybdenum trioxide (MoO3) is used as an adhesive between enamels and metals. Molybdenum powder is used as a fertilizer for some plants, such as cauliflower.
Lead molybdate (Wulfenite) co-precipitated with lead chromate and lead sulfate is a bright-orange pigment used with ceramics and plastics.
Also used in NO, NO2, NOx analyzers in power plants for pollution controls. At the element acts as a catalyst for NO2/NOx to form only NO molecules for consistent readings by infrared light.
History
Molybdenite (from the Ancient Greek , meaning lead), the principal ore from which molybdenum is now extracted, was previously known as molybdena. Molybdena was confused with and often implemented as though it were graphite. Even when the two ores were distinguishable, molybdena was thought to be a lead ore. In 1754, Bengt Qvist examined the mineral and determined that it did not contain lead.
It was not until 1778 that Swedish chemist Carl Wilhelm Scheele realized molybdena was neither graphite nor lead. He and other chemists then correctly assumed that it was the ore of a distinct new element, named molybdenum for the mineral in which it was discovered. Peter Jacob Hjelm successfully isolated molybdenum using carbon and linseed oil in 1781.
For a long time there was no industrial use for molybdenum. The French Schneider Electrics company produced the first steel molybdenum alloy armor plates in 1894. Until World War I most other armor factories also used molybdenum alloys. In World War I, some British tanks were protected by manganese plating, but this proved to be ineffective. The manganese plates were then replaced with molybdenum plating. These allowed for higher speed, greater maneuverability, and, despite being thinner, better protection. The high demand for molybdenum in World War I and World War II and the steep decrease after the wars had a great influence on prices and production of molybdenum.
Precautions
Molybdenum dusts and fumes, as can be generated by mining or metalworking, can be toxic, especially if ingested (including dust trapped in the sinuses and later swallowed). Low levels of prolonged exposure can cause irritation to the eyes and skin. The direct inhalation or ingestion of molybdenum and its oxides should also be avoided. OSHA regulations specify the maximum permissible molybdenum exposure in an 8-hour day to be 5 mg/m³. Chronic exposure to 60 to 600 mg Mo/m³ can cause symptoms including fatigue, headaches, and joint pains.
Supply and demand
Although current molybdenum production meets demand, refiners, or roasters, are expected to run into a shortfall between 2009 and 2015, depending on demand.
A roaster processes the molybdenum into a fine powder, pellets, or other forms. Total world molybdenum roaster capacity is currently 320 million pounds per year, barely enough to meet demand. There is not much excess roasting capacity, and no one is actively permitting for the production of any new roasters in the United States. Global roaster capacity also looks limited, and a future roaster shortage is predicted. The data above are based on the assumption that mines will be able to increase output.
Western demand is projected to increase by around 3 percent annually, while China and the CIS demand is projected to increase by around 10 percent annually, increasing overall global demand by around 4.5 percent annually. Increasing demand can be attributed to two main factors. Hydroprocessing catalysts are becoming essential for crude oil. The other contributing factor is the increase in nuclear reactor construction. There are 48 nuclear reactors to be built by 2013, and approximately 100 are to be built by 2020. The International Molybdenum Association (IMOA) says that an average reactor contains about of stainless steel alloy. Some larger reactors contain over 1 million feet of stainless steel alloy.
254,000 tons of molybdenum was discovered on Hainan, off the southern coast of China, on 6 October 2008. The mine is expected to produce 7,000 tons of molybdenum annually.
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