Tantalum

Tantalum

For weights.

History

(Gr. Tantalos, mythological character, father of Niobe) Discovered in 1802 by Ekeberg, but many chemists thought niobium and tantalum were identical elements until Rowe in 1844, and Marignac, in 1866, showed that niobic and tantalic acids were two different acids. The early investigators only isolated the impure metal. The first relatively pure ductile tantalum was produced by von Bolton in 1903. Tantalum occurs principally in the mineral columbite-tantalite.

Sources

Tantalum ores are found in Australia, Brazil, Mozambique, Thailand, Portugal, Nigeria, Zaire, and Canada.

Production

Separation of tantalum from niobium requires several complicated steps. Several methods are used to commercially produce the element, including electrolysis of molten potassium fluorotantalate, reduction of potassium fluorotantalate with sodium, or reacting tantalum carbide with tantalum oxide. Twenty five isotopes of tantalum are known to exist. Natural tantalum contains two isotopes.

Properties

Tantalum is a gray, heavy, and very hard metal. When pure, it is ductile and can be drawn into fine wire, which is used as a filament for evaporating metals such as aluminum. Tantalum is almost completely immune to chemical attack at temperatures below 150oC, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulfur trioxide. Alkalis attack it only slowly. At high temperatures, tantalum becomes much more reactive. The element has a melting point exceeded only by tungsten and rhenium. Tantalum is used to make a variety

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Tantalum

of alloys with desirable properties such as high melting point, high strength, good ductility, etc.

Tantalum has a good "gettering" ability at high temperatures, and tantalum oxide films are stable and have good rectifying and dielectric properties.

Uses

Scientists at Los Alamos have produced a tantalum carbide graphite composite material, which is said to be one of the hardest materials ever made. The compound has a melting point of 3738oC. Tantalum is used to make electrolytic capacitors and vacuum furnace parts, which account for about 60% of its use. The metal is also widely used to fabricate chemical process equipment, nuclear reactors, aircraft, and missile parts. Tantalum is completely immune to body liquids and is a nonirritating material. It has, therefore, found wide use in making surgical appliances. Tantalum oxide is used to make special glass with high index of refraction for camera lenses. The metal has many other uses.

Cost

The price of (99.9%) tantalum in Dec. 1988 was about $50/oz.

Sources: CRC Handbook of Chemistry and Physics and the American Chemical Society.

Last Updated: 12/19/97, CST Information Services Team

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Rhenium

Rhenium

For jewelry plating.

History

(L. Rhenus, Rhine) Discovery of rhenium is generally attributed to Noddack, Tacke, and Berg, who announced in 1925 they had detected the element in platinum ore and columbite. They also found the element in gadolinite and molybdenite. By working up 660 kg of molybdenite in 1928 they were able to extract 1 g of rhenium.

Sources

Rhenium does not occur free in nature or as a compound in a distinct mineral species. It is, however, widely spread throughout the earth's crust to the extent of about 0.001 ppm. Commercial rhenium in the U.S. today is obtained from molybdenum roaster-flue dusts obtained from copper-sulfide ores mined in the vicinity of Miami, Arizona, and elsewhere in Arizona and Utah.

Some molybdenum contain from 0.002% to 0.2% rhenium. More than 150,000 troy ounces of rhenium are now being produced yearly in the United States. The total estimated Free World reserve of rhenium metal is 3500 tons. Rhenium metal is prepared by reducing ammonium perrhentate with hydrogen at elevated temperatures.

Isotopes

Natural rhenium is a mixture of two stable isotopes. Twenty six other unstable isotopes are recognized.

Properties

The element is silvery white with a metallic luster; its density is exceeded only by that of platinum , iridium, and osmium , and its melting point is exceeded only by that of tungsten and

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Rhenium

carbon .

The usual commercial form of the element is powder, but it can be consolidated by pressing and resistance-sintering in a vacuum or hydrogen atmosphere. This process produces a compact shape in excess of 90 percent of the density of the metal.

Annealed rhenium is very ductile, and can be bent, coiled, or rolled. Rhenium is used as an additive to tungsten and molybdenum -based alloys to impart useful properties.

Uses

It is widely used as filaments for mass spectrographs and ion gauges. Rhenium-molybdenum alloys are superconductive at 10 K.

Rhenium is also used as an electrical contact material because it has good wear resistance and withstands arc corrosion. Thermocouples made of Re-W are used for measuring temperatures up to 2200C, and rhenium wire is used in photoflash lamps for photography.

Rhenium catalysts are exceptionally resistant to poisoning from nitrogen , sulfur , and phosphorus , and are used for hydrogenation of fine chemicals.

Costs

In 1928 rhenium cost $10,000/g. The price today is about $250/troy oz.

Hazards

Because little is known about its toxicity, it should be handled with care until more data becomes available.

Sources: CRC Handbook of Chemistry and Physics and the American Chemical Society.

Last Updated: 12/19/97, CST Information Services Team

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