Where Found (Encyclopedia of Global Resources)
Molybdenum has been found associated with thirteen minerals, but it is relatively uncommon in bulk ore. The U.S. Colorado deposit of molybdenum disulfide (molybdenite) is the biggest producer, but China, Chile, Peru, and Canada are also commercial sources. Significant molybdenum is also extracted from the by-products of tungsten and copper smelting. Trace molybdenum is found in most soils and is critical to plant health.
(The entire section is 65 words.)
Primary Uses (Encyclopedia of Global Resources)
The primary use of molybdenum is as a hardening agent and corrosion inhibitor for steel and other metals and alloys, but it is also used for high-temperature components such as electrodes, filaments, resistive heaters, electrical contacts, and mesh, and as a mount for tungsten filaments in lightbulbs. Molybdenum compounds are used as pigments, catalysts, fertilizer supplements, lubricants, semiconductors, and coatings.
(The entire section is 60 words.)
Technical Definition (Encyclopedia of Global Resources)
Molybdenum (abbreviated Mo), atomic number 42 and atomic weight 95.94, belongs, with chromium and tungsten, to Group VIB of the periodic table of the elements. It is a hard, corrosion-resistant, silvery-white metal. Its melting and boiling points are, respectively, 2,610° and 5,560° Celsius. Its density is 10.22 grams per cubic centimeter at 20° Celsius.
(The entire section is 48 words.)
Description, Distribution, and Forms (Encyclopedia of Global Resources)
Molybdenum’s primary ore, molybdenite (MoS2), was once confused with graphite and galena. It is not found naturally in the metallic state but as ores with sulfur and oxygen. It has an abundance of 1.2 parts per million in the Earth’s crust and 0.01 part per million in seawater. Other sources include wulfenite, PbMoO4; molybdite, Fe2O3·3MoO3·7H2O; powellite, Ca(Mo1-x)O4; and copper and tungsten smelting by-products.
The product of ore smelting is molybdenum trioxide, MoO3. Metal powder is formed by high-temperature reduction of MoO3 or ammonium molybdate, (NH4)2 MoO4, with reducing agents such as hydrogen; subsequent powder metallurgy or arc-casting techniques form the bulk metal. Molybdenum alloys with up to 50 percent iron (ferromolybdenum) can be produced from the oxide by electrical furnace or thermite processes.
Molybdenum dissolves in hot, concentrated acids such as nitric, sulfuric, and hydrochloric acid, aqua regia, and molten oxidizers such as sodium peroxide, potassium nitrate, and potassium chlorate. Heating in air oxidizes the surface to molybdenum oxides. Its heats of fusion and vaporization are, respectively, 6.7 and 117.4 kilocalories per mole. Natural molybdenum consists of seven isotopes with the following approximate distribution by mass number: 92 (16 percent), 94 (10 percent), 95 (15 percent) 96 (16 percent), 97 (10 percent), 98 (23...
(The entire section is 775 words.)
History (Encyclopedia of Global Resources)
Carl Scheele of Sweden identified molybdenum as an ore of a new element in 1778, and the metal was produced by Peter Jacob Hjelm, also from Sweden, in 1782. Hjelm called the new element “molybdos,” Greek for “lead.” Molybdenum did not see significant application until there arose a need for stronger steels in the automotive industry. Most molybdenum is still alloyed with steel to improve its hardness, wear resistance, corrosion resistance, and high-temperature strength.
(The entire section is 72 words.)
Obtaining Molybdenum (Encyclopedia of Global Resources)
Molybdenum is not hardened by heat treatment alone; it also requires working. Rolled molybdenum has a tensile strength of 260,000 pounds per square inch (psi), or 1.8 billion pascals, with a Brinell hardness of 160 to 185, while unalloyed molybdenum has a tensile strength of 97,000 psi (669 million pascals). Its high thermal conductivity (twice that of iron), low thermal expansion coefficient, low volatility, and excellent corrosion resistance allow molybdenum to be used for high strength/high temperature parts in jet engines, missiles, turbines, and nuclear reactors.
(The entire section is 81 words.)
Uses of Molybdenum (Encyclopedia of Global Resources)
Molybdenum is hardened by alloying agents. Adding titanium at 0.5 percent yields a tensile strength of 132,000 psi (9 billion pascals) that decreases only to 88,000 psi (607 million pascals) at 466° Celsius. Zirconium may also be added to increase strength further. Such alloys are used for parts such as tubing that maintain rigidity up to the melting point. Other common molybdenum alloys include Hastelloy (with nickel), molybdenum-chromium (roughly 70 percent molybdenum, 29 percent chromium, and 1 percent iron), and molybdenum-tungsten (70 percent molybdenum and 30 percent tungsten).
Molybdenum finds application as a flame-resistant, wear-resistant, and corrosion-resistant coating. It may be arc-deposited, but better coatings are produced by hydrogen chloride reduction of molybdenum pentachloride (MoCl5) at 850° Celsius. Its adherence to steel, iron, and aluminum is good. This strong bonding is utilized as molybdenum serves as a substrate for deposition of other coatings, such as semiconductor layers in solar cells.
Molybdenum is among the most successful elements in steel for increasing strength, rigidity, and hardness. It improves other metals’ corrosion resistance, increases elastic limit, and reduces grain size. It reacts with carbon to form hard molybdenum carbides within steel. Molybdenum steels have from 0.1 to 1 percent molybdenum. Higher percentages are used in molybdenum-containing stainless steels...
(The entire section is 361 words.)
Further Reading (Encyclopedia of Global Resources)
Adriano, Domy C. “Molybdenum.” In Trace Elements in Terrestrial Environments: Biogeochemistry, Bioavailability, and Risks of Metals. 2d ed. New York: Springer, 2001.
Brady, George S., Henry R. Clauser, and John A. Vaccari. Materials Handbook: An Encyclopedia for Managers, Technical Professionals, Purchasing and Production Managers, Technicians, and Supervisors. 15th ed. New York: McGraw-Hill, 2002.
Greenwood, N. N., and A. Earnshaw. “Chromium, Molybdenum, and Tungsten.” In Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Hewitt, E. J., and T. A. Smith. Plant Mineral Nutrition. London: English University Press, 1975.
Krebs, Robert E. The History and Use of Our Earth’s Chemical Elements: A Reference Guide. 2d ed. Illustrations by Rae Déjur. Westport, Conn.: Greenwood Press, 2006.
Lide, David R., ed. CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data. 85th ed. Boca Raton, Fla.: CRC Press, 2004.
Patton, W. J. Materials in Industry. 3d ed. Englewood Cliffs, N.J.: Prentice-Hall, 1986.
Sigel, Astrid, and Helmut Sigel, eds. Molybdenum and Tungsten: Their Roles in Biological Processes. New York: Marcel Dekker, 2002.
Natural Resources Canada. Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews....
(The entire section is 204 words.)
Molybdenum (Chemical Elements)
Molybdenum was one of the first metals to be discovered by a modern chemist. It was found in 1781 by Swedish chemist Peter Jacob Hjelm (1746-1813). Hjelm's work on the element was not published, however, until more than a century later.
Molybdenum is a transition metal, placing it in the center of the periodic table. The periodic table is a chart that shows how chemical elements are related to one another.
Molybdenum is a hard, silvery metal with a very high melting point. It is used primarily to make alloys with other metals. An alloy is a mixture of two or more metals. The mixture has properties different from those of the individual metals. The most common alloys of molybdenum are those with steel. Molybdenum improves the strength, toughness, resistance to wear and corrosion, and ability to harden steel.
(The entire section is 1309 words.)