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Metal Profile: Rhenium

What is rhenium?

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Metal Profile: Rhenium

Rhenium alloys are used in the production of General Electric's CFM56® jet engines.

Copyright © CFM International

Rhenium is a rare metal that is highly resistant to heat and wear, making it ideal for use in superalloys, such as those used in the manufacture of jet engines.

Properties

  • Atomic Symbol: Re
  • Atomic Number: 75
  • Atomic Mass: 186.207g/mol1
  • Element Category: Transition Metal
  • Density: 21.04g/cm3 at 20°C
  • Melting Point: 5767°F (3186°C)
  • Boiling Point: 10105°F (5596°C)
  • Moh’s Hardness: 7.0

Characteristics
Rhenium is a very heavy (dense) metal that has the second highest melting point of all metals, behind only tungsten.

Like all refractory metals, rhenium is resistant to heat, wear and corrosion. It is particularly resistant to the deleterious effects of nitrogen, sulfur and phosphorus.

The silvery element is also ductile, malleable and resistant to creep (deformation under stress).

Rhenium has the unique ability to maintain ductility between absolute zero and its melting point 5767°F (3186°C). This makes it valuable as an alloying agent for hardening metal components, as well as for use under extreme temperatures.  It has both a high modulus of elasticity as well as a high electrical resistivity.

History
Rhenium was still an unknown element when the periodic table was introduced in 1870. It was not until 1925 when German chemists Walter Noddack, Ida Tack and Otto Berg detected trace amounts of rhenium in platinum ores, as well as columbite, gadolinite and molybdenite.

In 1928 the group successfully produced a small amount of rhenium metal from molybdenite, making it the last naturally occurring element to be isolated. Yet, the level of difficulty required to extract the metal, along with its rareness, resulted in rhenium having no commercial applications for another 30 years.

In 1956, Kennecott Copper began producing rhenium at its facility in Washington, Pennsylvania. But between 1942 and 1965, it is believed that no more than 4,200kgs of rhenium were produced in the Western World, most of which was used in research and laboratories.

This changed in the late 1960s when both Chevron Inc. and UOP Inc. developed bi-metallic platinum-rhenium catalysts for petroleum refining. Demand for rhenium drove prices up three-fold by the early 1970s.

Developments in the commercial use of rhenium metal began to take-hold during the 1980s when engine makers discovered that nickel-based alloys containing rhenium were able to keep strength at high temperatures, while also resisting wear, which could extend the life of many engine components. As a result, rhenium containing jet turbine blades were first used in the late 1980s.

By 2000, due to significant growth in demand from the aerospace sector, metallurgical applications had become the major consumer of rhenium.

Production
Rhenium doesn’t occur freely in nature, but is mainly found in poryphyry copper deposits. Consequently, it is often produced as a by-product of molybdenum refining (which, itself, is most often produced as a by-product of copper).

While molybdenum sulphide concentrates are roasted to produce molybdenum trioxide (MoO3), rhenium oxide (Re2O7) forms and passes up the flue stack with the sulphur emissions. When the flue dusts are scrubbed the rhenium then enters the resulting sulfurous acid and is, ultimately, precipitated out as ammonium perrhenate (NH4ReO4).

Ammonium perrhenate (APR), a white crystalline, non-hazardous powder that contains between 68.5-69.4% rhenium content, is a common tradeable form of the element that can be further refined into rhenium metal.

Once purified as a metal powder, rhenium is often sintered into small pellets for alloying. Catalyst applications, however, can convert APR directly into perrhenic acids.

An increasing proportion of rhenium now also comes from recycled materials, both metal alloys and catalysts.

General Electric, whose aviation division is a major consumer of rhenium, launched a program to recycle rhenium-containing jet turbines in 2006, while also making efforts to cut down on its consumption through the development of new superalloys, such as René™ alloys. Now, more than 10% of the company's rhenium supplies are from recycled sources.

Rhenium recycling is mainly focused in Germany and the US, but also occurs in Estonia and Russia.

The US Geological Survey (USGS) estimates that global rhenium mine production was roughly 49 metric tonnes in 2011. Accounting for 53% of this production, Chile is, by far, the largest producing country, followed by the US (12.8%), Peru (10.2%) and Poland (9.6%).

Chile's Molymet, the world's largest molybdenum producer, is also - not surprisingly - the largest producer of rhenium. Other producers include Freeport McMoran Copper & Gold Inc. (USA), Zhezkazganredmet (also known as Redmet, Kazakhstan) and KGHM (Poland).

Applications
Major uses for rhenium include:

  • Metallurgical applications - superalloys (~80%)
  • Catalysts (15-20%)
  • Other (5%)

Rhenium is an ideal metal for extreme conditions. Consequently, it is not surprising that nearly 80% of global rhenium consumption is in superalloys, alloys that can operate at extremely high temperatures.

Rhenium alloys are critical to the production of high pressure turbine (HPT) blades that are used in both civil and military jet engines. Rhenium-containing alloys allow the engine to be designed with closer tolerances and to operate at higher temperatures than other alloys, thereby, increasing the engines operating efficiency and prolonging its life.

Other applications for rhenium alloys include in:

  • nuclear power plants
  • gas turbines
  • rockets
  • rocket thrusters, chambers and nozzles
  • crucibles
  • electrical contacts
  • electromagnets
  • heating elements
  • metallic coatings
  • semiconductors
  • temperature controls
  • thermocouples
  • heating elements
  • vacuum tubes for x-rays
  • filaments in ion gauges mass spectrographs and photoflashes
  • electron tubes and targets
  • gyroscopes

The other major end-use from rhenium, although not in its metal form, is in the petroleum catalyst industry.

As noted, in the 1960s platinum-rhenium catalysts were developed for use in the production of high-octane, lead free gasoline. Rhenium-containing catalysts allow the gas production process to be conducted at lower pressures and higher temperatures, which improves yields and octane ratings.

Although rhenium catalysts are mainly used in the petroleum industry, they are also used in the production of benzene, toluence and xylene chemicals.

Sources:
US Geological Survey 2010 Minerals Yearbook. Rhenium (2010). Desiree E. Polyak.; US Geological Survey 2001 Minerals Yearbook. Rhenium (2002). John W. Blossom & Mineral Commodity Summary: Rhenium (2012).
Source: http://minerals.usgs.gov/minerals/pubs/commodity/rhenium/
HardAssetInvestor.com. Rhenium: Son of Moly. Tom Vulcan (2008). Source: www.mmta.co.uk
H. Cross Company. Rhenium & Alloys.
Source: http://hcrosscompany.com/refractory/rhenium.htm

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