History
Rhenium was an unknown element when the periodic table was introduced in
1870. The properties predicted for an element of atomic #75 turned out to
be in general agreement of those of rhenium when it was discovered in
1925. W. Noddack and O. Berg were the first to locate it in platinum ores
such as columbite and tungstate. J.G.F. Druce discovered it independently
in manganese sulphate. The ores from which rhenium was first extracted
commercially came from the region of the river Rhine (Latin: Rhenus) hence
the name rhenium was born.
Rhenium's melting point of 3180º C is second only to tungsten. Only
osmium, iridium, and platinum exceed its density of 21.04 g/cc. Because of
its high melting point, rhenium is a refractory metal. In that
classification, rhenium is unique. It is the only refractory metal that
does not form carbides. Its crystal structure is hexagonal close-packed (hcp),
while other refractory metals have a body centered cubic (bcc) structure.
Rhenium also does not have a ductile-to-brittle transition temperature it
maintains its ductility from absolute zero all the way to its melting
point and also has a high modulus of elasticity. This means that
structures made of rhenium will have very good stability and rigidity.
A high re-crystallization temperature is a pre-requisite for good creep
resistance and among refractory metals, rhenium is the highest. At
temperatures up to 2800º C and high stresses, the rupture life of rhenium
is longer than tungsten. The metal also accommodates wide swings in
temperature - large thermal expansions and contractions - without
incurring mechanical damage.
Distribution in Nature
Rhenium is distributed in extremely minute quantities throughout nature
but does not exist as a separate mineral. Approximately 4 parts per
billion of the earths crust contain rhenium and no ore contains more than
1 part per billion. Most rhenium today is produced as a by-product of the
copper mining industry. It is found in those copper ores contaminated with
molybdenum. When they purify the copper the rhenium is extracted with the
molybdenum in the form of sulfurous sludge. The high temperature roasting
of the sludge to form molybdenum oxide releases the rhenium and it is
collected and concentrated in special units. The next step is to make a
medium purity ammonium salt containing rhenium called ammonium perrhenate
(APR). This APR is sold around the world to various metal traders and
catalyst manufacturers.
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