Molybdenum

• 1. Introduction

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  Molybdenum carries a name that is, in a sense, a mistake. It comes from the Ancient Greek word molybdos, meaning lead, because for centuries its dark ores were confused with the ores of lead. The element hides behind the symbol Mo and the atomic number 42. For all of recorded history its minerals were known, yet no one realized they held a distinct metal. People used one of those minerals to blacken surfaces, much as they used graphite, never suspecting it was something new. So how did a substance mistaken for lead and for graphite become essential to steel, to nuclear fuel, and even to life itself? And why does a metal so rare in the Earth's crust end up inside at least 50 enzymes across bacteria, plants, and animals? The answers run from an 18th-century Swedish laboratory to the floor of the ocean.

• 2. Mistaken For Lead

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  Molybdenite, the principal ore from which molybdenum is now extracted, was once called molybdena. It was confused with graphite and even used as if it were graphite, since both can blacken a surface or act as a solid lubricant. Bengt Andersson Qvist examined a sample of molybdenite in 1754 and found it contained no lead, so it was not galena. By 1778 the Swedish chemist Carl Wilhelm Scheele stated firmly that molybdena was neither galena nor graphite. Scheele proposed instead that it was the ore of a distinct new element, one that might be isolated from it. In 1781 Peter Jacob Hjelm succeeded, isolating a metal he called molybdaenum using carbon and linseed oil. Long before any of this, molybdenum was reportedly alloyed deliberately with steel in a single 14th-century Japanese sword, made around 1330. That art was never used widely and was later lost. The Greek word for lead itself has been proposed as a loanword from the Anatolian Luvian and Lydian languages.

• 3. A Metal Of Extremes

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  In its pure form molybdenum is a silvery-grey metal with a Mohs hardness of 5.5 and a standard atomic weight of 95.95 grams per mole. Its melting point reaches 2623 degrees Celsius, the sixth highest among naturally occurring elements. Only tantalum, osmium, rhenium, tungsten, and carbon melt at higher temperatures. The metal also has one of the lowest coefficients of thermal expansion among commercially used metals, meaning it barely changes size when heated. As a transition metal it has an electronegativity of 2.16 on the Pauling scale. At room temperature it does not visibly react with oxygen or water, but halogens and hydrogen peroxide attack it. Weak oxidation begins at 300 degrees Celsius, and above 600 degrees the metal oxidizes in bulk into molybdenum trioxide. Stranger still is its gaseous form. Gaseous molybdenum exists as the diatomic species Mo2, a molecule held together by a sextuple bond, one of the strongest chemical bonds known.

• 4. Steel And The World Wars

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  For a full century after Hjelm isolated it, molybdenum had no industrial use at all. It was scarce, the pure metal was hard to extract, and the metallurgy of the era was immature. Early molybdenum steel alloys promised greater hardness, but large-scale efforts ran into brittleness and recrystallization. William D. Coolidge filed a patent in 1906 for making molybdenum ductile, which led to its use as a heating element in high-temperature furnaces and as a support for tungsten-filament light bulbs. In 1913 Frank E. Elmore developed a froth flotation process to recover molybdenite from ores, and flotation remains the primary isolation process. During World War I demand spiked, as molybdenum served in armor plating and as a substitute for tungsten in high-speed steels. Some British tanks carried 75 millimeter manganese steel plating that proved ineffective. Replacing it with much lighter 25 millimeter molybdenum steel plates gave higher speed, greater maneuverability, and better protection. The Germans used molybdenum-doped steel for heavy artillery such as the super-heavy howitzer Big Bertha, because ordinary steel melted at the temperatures its one-ton shells produced. In World War II molybdenum again became strategically important as a substitute for tungsten in steel alloys.

• 5. Mined From Rock And Sea

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  Molybdenum is the 54th most abundant element in the Earth's crust, averaging 1.5 parts per million, and the 25th most abundant in the oceans at about 10 parts per billion. The Soviet Luna 24 mission even found a molybdenum-bearing grain in a pyroxene fragment taken from Mare Crisium on the Moon. The main commercial source is molybdenite, though the metal also occurs in wulfenite and powellite. It is mined as a principal ore and recovered as a byproduct of copper and tungsten mining. World production reached 250,000 tonnes in 2011, led by China with 94,000 tonnes, followed by the United States, Chile, Peru, and Mexico. Processing begins by roasting the ore in air at 700 degrees Celsius, yielding sulfur dioxide and molybdenum trioxide, which can be purified by sublimation at 1100 degrees Celsius. The price has swung wildly. It held near 10,000 dollars per tonne from 1997 through 2003, then peaked at 103,000 dollars per tonne in June 2005. In 2008 the London Metal Exchange announced that molybdenum would be traded as a commodity. The Knaben mine in southern Norway, opened in 1885, was the first dedicated molybdenum mine.

• 6. The Many Faces Of Mo

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  About 86 percent of molybdenum produced goes into metallurgy, with most of the rest used in chemical applications. Because the metal can withstand extreme temperatures without significantly expanding or softening, it appears in military armor, aircraft parts, electrical contacts, and industrial motors. Most high-strength steel alloys contain between 0.25 and 8 percent molybdenum, and more than 43,000 tonnes are used each year in stainless steels, tool steels, cast irons, and superalloys. Its lower density and steadier price let it substitute for tungsten, as in the M series of high-speed steels that replace the tungsten-bearing T series. One specialized alloy, TZM, resists molten fluoride salts above 1300 degrees Celsius and is used in torpedo engine valve bodies, rocket nozzles, and radiation shields. Molybdenum's compounds carry their own duties. Molybdenum disulfide serves as a solid lubricant and, with small amounts of cobalt, as a catalyst in the hydrodesulfurization of petroleum, one of the largest-scale applications of catalysis in industry. Lead molybdate yields a bright-orange pigment used with ceramics and plastics. The radioactive isotope molybdenum-99, itself a fission product, generates technetium-99m, a short-lived radionuclide used across medical imaging.

• 7. Essential To Life

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  Without molybdenum, nitrogen fixation would be greatly reduced, and much of biosynthesis as we know it would not occur. The element sits inside the most common nitrogenases, the bacterial catalysts that break the bond in atmospheric nitrogen. At least 50 molybdenum-containing enzymes have been identified, most of them in bacteria, including aldehyde oxidase, sulfite oxidase, and xanthine oxidase. In nearly all of them the metal is held by molybdopterin to form the molybdenum cofactor. Nitrogenase is the lone exception, using instead the FeMoco cofactor with the formula Fe7MoS9C. A 2008 research paper speculated that a scarcity of molybdenum in the Earth's early oceans may have strongly shaped the evolution of eukaryotic life. In humans, four molybdenum-dependent enzymes are known, and the body holds about 0.07 milligrams of molybdenum per kilogram of body weight, concentrated in the liver and kidneys. The metal even appears within tooth enamel and may help prevent decay. A congenital inability to make the cofactor, seen in infants, leaves high levels of sulfite and urate and causes neurological damage. The story of this element reaches even into the ocean's chemistry, where its 80,000-year residence time makes it one of the most reliable reference tracers marine geochemists have.