Rhenium

Rhenium (pronounced /ˈriːniəm/) is a chemical element with 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.^[1] This was the last naturally-occurring element to be discovered^[2] and belongs to the ten most expensive metals on Earth (over US$ 7500.-/kg).^[3]

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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. Rhenium has the widest range of oxidation states of any known element: -3, -1, +1, +2, +3, +4, +5, +6 and +7. 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; tungsten-rhenium alloys are also superconductive,^[4] around 4-8 K depending on the alloy. Rhenium metal superconducts at 2.4 K.^[5]

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 increase ductility in these alloys.
• An additive to tungsten in some x-ray sources.
• 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.
• Rhenium forms rhenium diboride with boron. It is a compound noted for its extreme hardness.^[6][7]

History

Rhenium (Latin Rhenus meaning "Rhine") was the next-to-last naturally occurring element to be discovered and the last element to be discovered having a stable isotope. The existence of a yet undiscovered element at this position in the periodic table had been predicted by Henry Moseley in 1914. It is generally considered to have been 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 the 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.

In 1908, Japanese chemist Masataka Ogawa announced that he discovered the 43rd element, and named it nipponium (Np) after Japan (which is Nippon in Japanese). However, later analysis indicated the presence of rhenium (element 75), not element 43. The symbol Np was later used for the element neptunium.

Occurrence

Rhenium is not found free in nature, and it was only recently that the first rhenium mineral was found. In 1994, Nature published a letter describing a rhenium sulfide mineral found condensing from a fumarole on Russia's Kudriavy volcano.^[8] This is not an economically viable source of the element. Rhenium is widely spread through the Earth's crust at approximately 1 ppb.

Chile has the world's largest reserves and was the leading producer as of 2005.^[9]

Commercial rhenium is extracted from molybdenum roaster-flue gas obtained from copper-sulfide ores. Some molybdenum ores contain 0.002% to 0.2% rhenium. Total world production is between 40 and 50 tons/year; the main producers are in Chile, USA and Kazakhstan. Recycling of used Pt-Re catalyst and special alloys allow the recovery of another 10 tons/year.

The metal form is prepared by reducing ammonium perrhenate with hydrogen at high temperatures.

Isotopes

Main article: Isotopes of rhenium

Naturally occurring rhenium is 37.4% ¹⁸⁵Re, which is stable, and 62.6% ¹⁸⁷Re, which is unstable but has a very long half-life. There are twenty-six other unstable isotopes recognized.

References