Rhenium Information
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Rhenium Element
Rhenium is a chemical element with the symbol Re and atomic
number 75. It is a silvery-gray, 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.
Rhenium has the third-highest melting point and second-highest boiling point of
any stable element at 5903 K.[4] Rhenium resembles manganese and technetium
chemically and is mainly obtained as a by-product of the extraction and
refinement of molybdenum and copper ores. Rhenium shows in its compounds a wide
variety of oxidation states ranging from −1 to +7.
Discovered in 1908, rhenium was the second-last stable element
to be discovered (the last being hafnium). 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. The second-most important use is as a catalyst: rhenium is an
excellent catalyst for hydrogenation and isomerization, and is used for example
in catalytic reforming of naphtha for use in gasoline (rheniforming process).
Because of the low availability relative to demand, rhenium is expensive, with
price reaching an all-time high in 2008/2009 of US$10,600 per kilogram
(US$4,800 per pound). Due to increases in rhenium recycling and a drop in
demand for rhenium in catalysts, the price of rhenium has dropped to US$2,844
per kilogram (US$1,290 per pound) as of July 2018.
Rhenium occurs up to about 20 parts per million in molybdenite
and to a lesser extent in sulfidic copper ores. The recovery of rhenium is
aided by the concentration of its volatile heptoxide (Re2O7) in the flue dust
and gases given off during the smelting of molybdenite ore or from its
concentration with the platinum metals in the anode sludge during electrolytic
copper refining. The black metal powder is extracted from the gases and dust by
leaching or scrubbing them with water to dissolve the oxide, Re2O7, which in
turn can be converted to ammonium perrhenate, NH4ReO4, and then reduced to the
metal with hydrogen. The powder may be compressed and sintered into bars in
hydrogen at elevated temperatures. Cold-working and annealing permit the
fabrication of wire or foil.
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