Rhenium

Rhenium is a chemical element in the periodic table that has the symbol Re and atomic number 75. A silvery-white, rare, heavy, polyvalent transition metal, rhenium resembles manganese chemically and is used in some alloys. Rhenium is obtained as a by-product of molybdenum refinement and rhenium-molybdenum alloys are superconducting. This was the last naturally-occurring element to be discovered and belongs to the ten most expensive metals on Earth.

1 Notable characteristics
2 Applications
3 History
4 Occurrence
5 Isotopes
6 Precautions
7 References
8 External links

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Notable characteristics

Rhenium is a silvery white metal, lustrous, and has one of the highest melting points of all elements, exceeded by only tungsten and carbon. It is also one of the most dense, exceeded only by platinum, iridium, and osmium. The oxidation states of rhenium include -1,+1,+2,+3,+4,+5,+6 and +7 oxidation states. The oxidation states +7,+6,+4,+2 and -1 are the most common.

Its usual commercial form is a powder, but this element can be consolidated by pressing and resistance-sintering in a vacuum or hydrogen atmosphere. This procedure yields a compact shape that is in excess of 90 percent of the density of the metal. When annealed this metal is very ductile and can be bent, coiled, or rolled. Rhenium-molybdenum alloys are superconductive at 10 K.

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Applications

This element is used in platinum-rhenium catalysts which in turn are primarily used in making lead-free, high-octane gasoline and in high-temperature superalloys that are used to make jet engine parts. Other uses:

Widely used as filaments in mass spectrographs and in ion gauges.
An additive to tungsten and molybdenum-based alloys to give them useful properties.
Rhenium catalysts are very resistant to chemical poisoning, and so are used in certain kinds of hydrogenation reactions.
Electrical contact material due to its good wear resistance and ability to withstand arc corrosion.
Thermocouples containing alloys of rhenium and tungsten are used to measure temperatures up to 2200 °C.
Rhenium wire is used in photoflash lamps in photography.

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History

Rhenium (Latin Rhenus meaning "Rhine") was the last naturally-occurring element to be discovered. The existence of an as-yet undiscovered element at this postion in the periodic table had been predicted by Henry Moseley in 1914. It is generally considered to be discovered by Walter Noddack, Ida Tacke, and Otto Berg in Germany. In 1925 they reported that they detected the element in platinum ore and in the mineral columbite. They also found rhenium in gadolinite and molybdenite. In 1928 they were able to extract 1 g of element by processing 660 kg of molybdenite.

The process was so complicated and the cost so high that production was discontinued until early 1950 when tungsten-rhenium and molybdenum-rhenium alloys were prepared. These alloys found important applications in industry that resulted in a great demand for the rhenium produced from the molybdenite fraction of porphyry copper ores.

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Occurrence

Rhenium is not naturally found free in nature or even as a compound in a distinct mineral species. This element is widely spread through the earth's crust at approximately 0.001 ppm. Commercial rhenium is extracted from molybdenum roaster-flue dusts from copper-sulfide ores. Some molybdenum ores contain 0.002% to 0.2% rhenium. The metal form is prepared by reducing ammonium perrhenate with hydrogen at high temperatures.

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Isotopes

Naturally occurring rhenium is a mix of one stable isotope and one radioactive isotope with a very long half-life. There are twenty six other unstable isotopes recognized.

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