Rhenium

Rhenium

Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust. The free element has the third-highest melting point and highest boiling point of any element. Rhenium resembles manganese chemically and is obtained as a by-product of molybdenum and copper ore's extraction and refinement. Rhenium shows in its compounds a wide variety of oxidation states ranging from −1 to +7.

Discovered in 1925, rhenium was the last stable element to be discovered. It was named after the river Rhine in Europe.

Nickel-based superalloys of rhenium are used in the combustion chambers, turbine blades, and exhaust nozzles of jet engines, these alloys contain up to 6% rhenium, making jet engine construction the largest single use for the element, with the chemical industry's catalytic uses being next-most important. Because of the low availability relative to demand, rhenium is among the most expensive of metals, with an average price of approximately US$4,575 per kilogram (US$142.30 per troy ounce) as of August 2011; it is also of critical strategic military importance, for its use in high performance military jet and rocket engines.

Occurrence

Rhenium is one of the rarest elements in Earth's crust with an average concentration of 1 ppb; other sources quote the number of 0.5 ppb making it the 77th most abundant element in Earth's crust. Rhenium is probably not found free in nature (its possible natural occurrence is uncertain), but occurs in amounts up to 0.2% in the mineral molybdenite (which is primarily molybdenum disulfide), the major commercial source, although single molybdenite samples with up to 1.88% have been found. Chile has the world's largest rhenium reserves, part of the copper ore deposits, and was the leading producer as of 2005. It was only recently that the first rhenium mineral was found and described (in 1994), a rhenium sulfide mineral (ReS2) condensing from a fumarole on Russia's Kudriavy volcano, Iturup island, in the Kurile Islands. Kudryavy discharges up to 20–60 kg rhenium per year mostly in the form of rhenium disulfide. Named rheniite, this rare mineral commands high prices among collectors.

Symbol		Re
Atomic Number		75
Atomic Weight		186.207
Oxidation States		+4,+6,+7
Electronegativity, Pauling		2.2
State at RT		solid
Melting Point, K		3453
Boiling Point, K		5900

Appearance and Characteristics

Harmful effects:

The toxicity of rhenium and its compounds is not well documented. Elemental rhenium has been described as ‘relatively inert’ in the body.

Characteristics:

• Rhenium is a rare, silvery-white, lustrous, dense metal.
• It resists corrosion and oxidation but slowly tarnishes in moist air.
• Of the elements, only carbon and tungsten have higher melting points and only iridium, osmium, and platinum are more dense.
• Rhenium is one of the five major refractory metals (metals with very high resistance to heat and wear).
• The other refractory metals are tungsten, molybdenum, tantalum and niobium.
• Rhenium compounds include oxides, halides and sulfides.

Uses of Rhenium

• Rhenium is used with platinum as catalysts in the production of lead-free, high-octane gasoline.
• The metal is used in alloys for jet engines and in tungsten and molybdenum based alloys.
• It is widely used as filaments for mass spectrographs.
• Rhenium is also used as an electrical contact material.
• Rhenium catalysts are exceptionally resistant to poisoning from nitrogen, sulfur and phosphorous and are useful in the hydrogenation of fine chemicals.