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An Introduction to Rhenium

When Mendeleev developed the periodic table back in 1969, not all the elements were known. He left gaps to accommodate these materials which were subsequently discovered.

One such element is Rhenium (Re) whose discovery was attributed to Tacke, Noddack and Berg in 1925. Looking for some of the missing elements they were able to detect it in platinum ores. It was also found in minerals such as gadolinite and molybdenite.

The name rhenium was derived from the Rhine River, which the discoverers named the element after.

Occurrence

Rhenium metal is not found as such in nature, nor do its compounds exist in as distinct minerals. It is a rare material found widely scattered throughout the earths crust.

Natural rhenium is composed of two isotopes, while a further 16 unstable isotopes are also known.

Production

Commercially, rhenium is obtained as a by-product of copper smelting from molybdenum bearing copper ores. In this operation, the molybdenite roaster flue dusts from roasting of the copper ore are separated out.

The rhenium is liberated from the molybdenite via high temperature roasting and is often converted and sold as the ammonium salt, ammonium perrhenate (APR), from which it may be further purified. Purification would involve reduction by hydrogen at high temperatures, yielding rhenium powder, which can be fabricated into shapes using powder metallurgical processes.

Commercial operations producing rhenium in this way can be found in Miami, Arizona and Utah in the USA, as well as Chile, Russia, Kazakhstan and the Ukraine.

Fabrication

  • Rhenium can be rolled and dawn into wires and rods. However, it work hardens and required annealing to relive stresses is significant section reduction is required.
  • Rhenium can be welded using inert gas or electron beam welding techniques, but must be protected from oxidation.
  • It can be machined using electrochemical machining, electron discharge machining and abrasive cutting and grinding techniques.

Properties

  • Rhenium is silvery white in appearance and has a metallic lustre.
  • It has an extremely high density of 21.04g/cm3
  • It’s melting point of 3180°C is exceeded only by that of tungsten and carbon.
  • It has high ductility and can be formed into shapes by most conventional forming processes such as bending and rolling. Furthermore, it does not have a ductile/brittle transition, remaining ductile up to its melting point.
  • It has high electrical resistivity over a wide temperature range
  • It has good high temperature strength
  • It has a high modulus of elasticity
  • It has an extremely high recrystallisation temperature (2800°C) giving it excellent creep resistance.
  • It can withstand a large amount of thermal cycling without deterioration.
  • It does not form carbides

Applications

Rhenium is used for:

  • Rocket thrusters for small satellites.
  • It can be used as an electrical contact material where it can withstand high wear and arcing environments.
  • Rhenium wire is used in flash photography
  • It is used in catalysts for hydrogenation of fine chemicals, hydrocracking, reforming and disproportionating olefins. It is suited to this application as it has exceptional resistance to poisoning from elements such as phosphorus, nitrogen and sulfur.
  • After being made radioactive via neutron bombardment it is used in medical applications to treat restenosis and liver tumours.
  • Rhenium is also used to make thermistors.

It is added to other metals to impart beneficial properties e.g.

  • Nickel-based superalloys for gas turbine engines to increase their operating temperature.
  • Rhenium tungsten alloys are used for high temperature thermocouples (up to 2200°C)
  • Rhenium-molybdenum alloys are superconducting below 10K and are used electronics, space and nuclear industries

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