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Refractory Metals
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Molybdenum, symbol Mo, metallic element with chemical
properties similar to those of chromium.
Molybdenum is one of the transition elements
of the periodic table. The atomic
number of molybdenum is 42.
Molybdenum was discovered in 1778 by the Swedish chemist Carl Wilhelm Scheele.
It is a silvery white, tough, malleable metal. Molybdenum is dissolved by
dilute nitric acid and
aqua regia, and is attacked by fused
alkalies; it is not attacked by air at ordinary temperatures, but at
elevated temperatures it oxidizes to form molybdenum oxide. Molybdenum
melts at about 2610° C (about 4730° F), boils at about 4640° C (about
8380° F), and has a specific gravity, or relative density, of 10.2.
Molybdenum does not occur free in nature, but in the form
of its ores, the most important of which are molybdenite
and wulfenite.
It ranks 56th in order of abundance of the elements in the crust of the
earth and is an important trace element in soils, where it contributes to
the growth of plants.
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Niobium
or Columbium, symbol Nb, steel-gray,
lustrous, ductile, and malleable metallic element. The atomic number of
niobium is 41. Niobium is one of the transition elements
of the periodic table.
This metal was discovered in 1801 by the British chemist
Charles Hatchett. Niobium burns when heated in air and combines with
nitrogen, hydrogen, and the halogens. It resists the actions of most
acids. Its principal use is as an alloying element in stainless steel, to
which it lends additional corrosion resistance, particularly at high
temperatures.
Niobium ranks about 32nd in natural abundance among the
elements in crustal rock. It occurs, associated with the similar element tantalum,
in various minerals, the most important of which is called columbite
or tantalite, depending on which of the two elements predominates. Pure
niobium has excellent characteristics as a construction material in
nuclear power plants.
Niobium melts at about 2468° C (about 4474° F), boils at
about 5127° C (about 9261° F), and has a specific gravity of 8.57. The
atomic weight of niobium is 92.906.
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Tantalum, symbol Ta, white, ductile, malleable
metallic element. Tantalum is one of the transition elements
of the periodic table. The atomic
number of tantalum is 73. Tantalum was first obtained in pure form in 1820
by the Swedish chemist
Baron Jöns Jakob Berzelius, who heated
potassium tantalifluoride, K 2TaF7,
with excess potassium.
Properties and Occurrence
Tantalum is soluble in fused alkalies, insoluble in
sulfuric, hydrochloric, and nitric acids, and soluble in hydrofluoric
acid. It ignites in air to form tantalum pentoxide (sometimes referred to
as tantalum(V) oxide or tantalic acid anhydride), Ta2O5, a white infusible
substance that combines with metallic oxides or hydroxides to form
compounds called tantalites. Tantalic acid (also know as tantalum(V) oxide
hydrate), Ta2O5 . xH2O, is a gelatinous precipitate that has a varying
number of water molecules associated with each tantalum pentoxide
molecule. Tantalum melts at about 2996° C (about 5425° F), boils at about
5425° C (about 9797° F), and has a specific gravity of 16.6. The atomic
weight of tantalum is 180.948.
Tantalum belongs to the group of metals that includes vanadium
and niobium.
It occurs mainly in the mineral tantalite (see Columbite),
FeTa 2O6.
Tantalum ranks about 53rd among the elements in natural abundance in the
earth's crust. Principal deposits of the metal occur in Australia and
Scandinavia. In the United States, small deposits are found associated
with the pegmatite veins of the Black Hills of South Dakota. Most tantalum
minerals contain some niobium metal, which is separated by solvent
extraction or selective-crystallization procedures. Commercially, tantalum
is prepared by the electrolysis of fused potassium tantalifluoride or of
tantalum compounds dissolved in dilute sulfuric acid.
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Titanium, symbol Ti, silver-white metallic element
used principally to make light, strong alloys.
Titanium is one of the
transition elements of the
periodic table. The atomic number of
titanium is 22.
Titanium was discovered in 1791 in the mineral menachanite
by the British clergyman William Gregor, who named the new element
menachite. Four years later, the German chemist Martin Heinrich Klaproth
rediscovered the element in the mineral rutile and named it titanium in
allusion to the strength of the mythological Greek Titans. The metal was
isolated in 1910.
Properties and Occurrence
Pure titanium is soluble in concentrated acids, such as
sulfuric and hydrofluoric acids, and insoluble in water. The metal is
extremely brittle when cold, but is readily malleable and ductile at a low
red heat. Titanium melts at about 1660° C (about 3020° F), boils at about
3287° C (about 5949° F), and has a specific gravity of 4.5. The atomic
weight of titanium is 47.88.
Titanium burns in oxygen at 610° C (1130° F) to form
titanium dioxide, and it burns in nitrogen at 800° C (1472° F) to form
titanium nitride, TiN. Titanium exhibits valences of 4, 3, and 2, and
forms the salts titanium tetrachloride, TiCl 4;
titanium trichloride, TiCl3;
and titanium dichloride, TiCl2.
It ranks ninth in abundance among the elements in the crust of the earth
but is never found in the pure state. It occurs as an oxide in the
minerals ilmenite,
FeTiO3; rutile,
TiO2;
and sphene, CaO · TiO2
· SiO2.
To obtain titanium oxide, the mineral is ground and mixed
with potassium carbonate and aqueous hydrofluoric acid to yield potassium
fluorotitanate, K 2TiF6.
The fluorotitanate is extracted with hot water and decomposed with ammonia.
The resulting ammoniacal hydrated oxide, when ignited in a platinum
vessel, yields titanium dioxide, TiO2.
Titanium is obtained in the pure form by first treating the oxide with
chlorine to form titanium tetrachloride, a volatile liquid, and then
reducing the liquid with magnesium in a closed iron chamber to yield
metallic titanium. The metal is then melted and cast into ingots.
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Tungsten, symbol W (from the earlier name, wolfram),
metallic element that has the highest melting point of any metal. Tungsten
is one of the
transition elements of the
periodic table. The atomic number of
tungsten is 74.
Some credit the Swedish chemist Carl Wilhelm Scheele
with the discovery of tungsten in 1781, while others name the Spanish
D'Elhuyar brothers Juan José and Fausto as its discoverers, in 1783. Pure
tungsten is silver-white in color and is ductile; the more easily obtained
impure form is steel-gray and is hard and brittle. Tungsten is insoluble
in hot and cold water and in alcohol, slightly soluble in ammonia and
nitric acid, and soluble in hot, concentrated potassium hydroxide.
Tungsten melts at about 3410° C (6170° F), boils at about 5660° C (10,220°
F), and has a specific gravity of 19.5. The atomic weight of tungsten is
183.85.
Tungsten ranks 57th in abundance among the elements in the
crust of the earth. It is never found free in nature, but occurs in
combination with other metals, notably in the minerals scheelite
and wolframite,
which are the important tungsten ores. Mines in South Korea, Portugal,
Austria, and Australia produce more than half of the world's supply of
these ores.
To separate the element from its ore, the ore is first
fused with sodium carbonate to yield sodium tungstate, Na 2WO4.
The soluble sodium tungstate is then extracted with hot water and treated
with hydrochloric acid to yield tungstic acid, H2WO4.
The latter compound is washed and dried to produce the oxide WO3,
which is reduced by hydrogen in an electric furnace. The resulting fine
powder is reheated in molds in an atmosphere of hydrogen and pressed into
bars, which are hammered and rolled at high temperature to compact them
and make them ductile.
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