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Molybdenum (pronounced /ˌmɒlɪbˈdiːnəm/ MOL-ib-DEE-nəm
or /məˈlɪbdɨnəm/ mə-LIB-di-nəm),
is a Group 6 chemical
element with the symbol Mo and atomic
number 42. The name is from Neo-Latin Molybdaenum, from Ancient
Greek Μόλυβδος molybdos, meaning lead, since its ores were
confused with lead ores.[3]
The free element, which is a silvery metal, has the sixth-highest melting
point of any element. It readily forms hard, stable carbides, and for
this reason it is often used in high-strength steel alloys.
Molybdenum does not occur as the free
metal in nature, but rather in various oxidation
states in minerals. Industrially, molybdenum compounds
are used in high pressure and high temperature applications, as pigments and catalysts.
Detailed descrption:
Molybdenum
minerals have long been known, but the element was "discovered" (in
the sense of differentiating it as a new entity from minerals salts of other
metals) in 1778 by Carl Wilhelm Scheele. The metal was first
isolated in 1781 by Peter Jacob Hjelm.
Most
molybdenum compounds have low solubility in water, but the molybdate ion MoO42−
is soluble and will form if molybdenum-containing minerals are in contact with oxygen and water.
Recent theories suggest that the release of oxygen by early life was important
in removing molybdenum from minerals into a soluble form in the early oceans,
where it was used as a catalyst by single-celled organisms. This sequence may
have been important in the history of life, because molybdenum-containing
enzymes then became the most important catalysts used by some bacteria to break
into atoms the atmospheric molecular nitrogen,
allowing biological nitrogen fixation. This, in turn allowed
biologically driven nitrogen-fertilization of the oceans, and thus the
development of more complex organisms.
At
least 50 molybdenum-containing enzymes are now known in bacteria and animals,
though only the bacterial and cyanobacterial enzymes are involved in nitrogen
fixation. Due to the diverse functions of the remainder of the enzymes,
molybdenum is a required element for life in higher organisms (eukaryotes),
though not in all bacteria.
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Characteristics
Physical
In
its pure form, molybdenum is silvery white metal with a Mohs hardness of 5.5. It has a melting
point of 2,623 °C (4,753 °F); of the naturally occurring
elements, only tantalum,
osmium,
HYPERLINK "http://en.wikipedia.org/wiki/Rhenium" \o
"Rhenium" rhenium, tungsten and carbon have higher melting points.
History
Molybdenite—the
principal ore from which molybdenum is now extracted—was previously known as
molybdena. Molybdena was confused with and often implemented as though it were graphite. Even
when molybdena was distinguishable from graphite, it was still confused with a
common lead ore (now
called galena),
which took its name from Ancient Greek Μόλυβδος molybdos, meaning lead
[5]
In 1754, Bengt Andersson Qvist examined molybdenite
and determined that it did not contain lead and was thus not the same as
galena.[16]
It
was not until 1778 that Swedish chemist Carl Wilhelm Scheele realized molybdena was
neither graphite nor lead.[17][18]
He and other chemists then correctly assumed that it was the ore of a distinct
new element, named molybdenum for the mineral in which it was
discovered. Peter Jacob Hjelm successfully isolated
molybdenum using carbon
and linseed
oil in 1781.[5][19]
For
a long time there was no industrial use for molybdenum. Although apparent
deliberate alloying of molybdenum with steel in one 14th century
Japanese sword has been reported, that art was never employed widely, and later
lost.[20]
Upon its rediscovery, the French Schneider Electric company produced the first
molybdenum-steel armor plates in 1894. Until World War I,
most other armor factories also used molybdenum alloys. In World War I, some
British tanks were protected by 75 mm (3 in) manganese steel
plating, but this proved to be ineffective. The manganese steel plates were
then replaced with 25 mm (1 in) molybdenum-steel plating. These
allowed for higher speed, greater maneuverability, and, despite being thinner,
better protection.[5]
The high demand for molybdenum in World War I and World War
II and the steep decrease after the wars had a great influence on prices
and production of molybdenum.
Occurrence
Though molybdenum is found in such minerals as wulfenite
(PbMoO4) and powellite (CaMoO4), the main commercial source
of molybdenum is molybdenite (MoS2).
Molybdenum is mined as a principal ore, and is also recovered as a byproduct of
copper and tungsten mining.[3]
Large mines in Colorado
(such as the Henderson mine and the now-inactive Climax mine)[24]
and in British Columbia yield molybdenite as their
primary product, while many porphyry
copper deposits such as the Bingham Canyon Mine in Utah and the Chuquicamata
mine in northern Chile
produce molybdenum as a byproduct of copper mining. The Knaben mine in
southern Norway
was opened in 1885, making it the first molybdenum mine. It remained open until
1973.[25]
Molybdenum
is the 54th most abundant element in the Earth's crust and the 25th most
abundant element in the oceans, with an average of 10 parts per billion;
it is the 42nd most abundant element in the Universe.
Production
The
molybdenite is first heated to a temperature of 700 °C (1,292 °F) and
the sulfide is oxidized into molybdenum(VI) oxide by air:[13]
2 MoS2 + 7 O2
→ 2 MoO3 + 4 SO2
The
oxidized ore is then either heated to 1,100 °C (2,010 °F) to
sublimate the oxide, or leached with ammonia which
reacts with the molybdenum(VI) oxide to form water-soluble molybdates:
MoO3 + 2 NH4OH
→ (NH4)2(MoO4) + H2O
Copper,
an impurity in molybdenite, is less soluble in ammonia. To completely remove it
from the solution, it is precipitated with hydrogen
sulfide.[13]
Pure
molybdenum is produced by reduction of the oxide with hydrogen, while the
molybdenum for steel production is reduced by the aluminothermic reaction with addition of
iron to produce ferromolybdenum. Molybdenum has a value of
approximately $30,000 per tonne as of August 2009. It maintained a price at or near
$10,000 per tonne from 1997 through 2003, and reached, due to increased demand,
a peak of $103,000 per tonne in June 2005.[27]
In 2008 the London Metal Exchange announced that
molybdenum would be traded as a commodity on the exchange.[28]
Applications
In alloys
The
ability of molybdenum to withstand extreme temperatures without significantly
expanding or softening makes it useful in applications that involve intense
heat, including the manufacture of aircraft parts, electrical contacts,
industrial motors and filaments.[5][29]
Most
high-strength steel alloys (example 41xx steels)
contain 0.25% to 8% molybdenum.[3]
Despite such small portions, more than 43,000 tonnes of molybdenum are used as
an alloying agent each year in stainless
steels, tool
steels, cast
irons and high-temperature superalloys.[4]
Molybdenum
is also used in steel alloys
for its high corrosion
resistance and weldability.[4][21]
Molybdenum contributes further corrosion resistance to "chrome-moly"
type-300 stainless steels (high-chromium steels that are
corrosion-resistant already due to their chromium content) and especially so in
the so-called superaustenitic stainless steels (such as alloy AL-6XN). Molybdenum
acts by increasing lattice strain, thus increasing the energy required to
dissolve out iron atoms from the surface.
Biological role
Biochemistry
The
most important use of the molybdenum in living organisms is as a metal heteroatom
at the active site in certain enzymes. In nitrogen
fixation in certain bacteria, the nitrogenase
enzyme, which is involved in the terminal step of reducing molecular nitrogen,
usually contains molybdenum in the active site (though replacement of Mo with
iron or vanadium is also known). The structure of the catalytic center of the
enzyme is similar to that in iron-sulfur proteins: it incorporates a Fe4S3
and multiple MoFe3S3 clusters.[36]
In
2008, evidence was reported that a scarcity of molybdenum in the Earth's early
oceans was a limiting factor in the further evolution of eukaryotic life
(which includes all plants and animals) as eukaryotes cannot fix nitrogen and
must acquire it from prokaryotic bacteria. The scarcity of molybdenum resulted from the relative lack of oxygen in the
early ocean. Oxygen dissolved in seawater helps dissolve molybdenum from
minerals on the sea bottom. However, although oxygen may promote nitrogen
fixation via making molybdenum available in water, it also directly poisons
these nitrogenase
enzymes, so that organisms which continued to fix nitrogen in aerobic
conditions were required to isolate their nitrogen-fixing enzymes in heterocysts,
or similar structures.
PRICE
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