Although the metal was known to ancient cultures, and its mineral forms were confused with graphite and the lead ore galena for at least 2,000 years, molybdenum was not formally discovered and identified until 1778, when the Swedish chemist and pharmacist Carl Wilhelm Scheele produced molybdic oxide by attacking pulverized molybdenite (MoS2) with concentrated nitric acid and then evaporating the residue to dryness. Following Scheele's suggestion, another Swedish chemist, Peter Jacob Hjelm, produced the first metallic molybdenum in 1781 by heating a paste prepared from molybdic oxide and linseed oil at high temperatures in a crucible. During the 19th century, the German chemist Bucholtz and the Swede Jöns Jacob Berzelius systematically explored the complex chemistry of molybdenum, but it was not until 1895 that a French chemist, Henri Moissan, produced the first chemically pure (99.98 percent) molybdenum metal by reducing it with carbon in an electric furnace, thereby making it possible to conduct scientific and metallurgical research into the metal and its alloys.[i]
Molybdenum, symbol Mo,
is a very useful 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. In addition 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.[ii]
Those physical properties have made molybdenum an attractive metallic element. In addition strength at high temperature, high stiffness, excellent thermal conductivity, low coefficient of thermal expansion, and chemical compatibility with a variety of environments also add to the demand for molybdenum and its alloys.[iii] Also to be considered in molybdenum’s unique advantages are high electrical and thermal conductivity, ease of fabrication, plus corrosion resistance, dimensional stability, and outstanding wear characteristics.
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.[iv]
The main ore in
which molybdenum is found is molybdenite and the only commercially viable mineral in the production of molybdenum is
its bisulfide (MoS2), found in molybdenite. Almost all ores are
recovered from porphyry-disseminated deposits in Colorado, Arizona, British
Columbia, Chile, Peru, Russia, and Kazakhstan. These are either primary
molybdenum deposits or complex copper-molybdenum deposits from which molybdenum
is recovered as a co product or byproduct. Primary deposits, containing between
0.1 and 0.5 percent molybdenum, are extensive. Copper porphyries also are very
large deposits, but their molybdenum content varies between 0.005 and 0.05
percent. Roughly 40 percent of molybdenum comes from primary mines, with the
other 60 percent a by-product of copper (or, in some cases, tungsten).[v]
More than 2,000
pounds of ore must be mined underground, crushed and milled to recover some
four to six pounds of molybdenum.[vi]
Molybdenum and copper-molybdenum
porphyries are mined by open-pit or by underground methods. Once the ore has
been crushed and ground, the metallic minerals are then separated from gangue
minerals (or the molybdenum and copper from each other) by flotation processes,
using a wide variety of reagents. The concentrates contain between 85 and 92
percent MoS2 and small amounts of copper (less than 0.5 percent) if
the molybdenum is recovered as a by-product of copper.
About 97 percent of
MoS2 must be converted into technical molybdic oxide (85-90 percent
MoO3) in order to reach its commercial destination. Such conversion
is almost universally carried out in Nichols-Herreshoff-type multiple-hearth
furnaces, into which molybdenite concentrate is fed from the top against a current
of heated air and gases blown from the bottom. Each hearth has four air-cooled
arms rotated by an air-cooled shaft; the arms are equipped with rabble blades
that rake material to the outside or center of the roaster, where the material
drops to the next hearth. In the first hearth, the concentrate is preheated and
the flotation reagents ignite, initiating the transformation of MoS2
into MoO3. This exothermic reaction, which continues and intensifies
in the following hearths, is controlled by adjustment of the oxygen and by
water sprays that cool the furnace when necessary. The temperature should not
rise above 650º C (1,200º F), the point at which MoO3 sublimates, or
vaporizes directly from the solid state. The process is finished when the
sulfur content of the calcines falls below 0.1 percent.
Technical molybdic oxide is made into
briquettes that are fed directly into furnaces to make alloy steels and other
foundry products. They also are used to make ferromolybdenum, but if more
purified molybdenum products are desired, such as molybdenum chemicals or
metallic molybdenum, then technical MoO3 must be refined to
chemically pure MoO3 by sublimation. This is carried out in electric
retorts at temperatures between 1,200º and 1,250º C (2,200º and 2,300º F). The
furnaces consist of quartz tubes wound with molybdenum-wire heating elements,
which are protected from oxidation by a mixture of refractory-brick paste and
wood charcoal. The tubes are inclined 20º from the horizontal and rotated. The
sublimed vapors are swept from the tubes by air and collected by hoods leading
to filter bags. Two separate fractions are collected. The first corresponds to
vaporization of the initial 2-3 percent of the charge and contains most of the
volatile impurities. The last fraction is the pure MoO3. This must
be 99.95 percent pure in order to be suitable for the manufacture of ammonium
molybdate (ADM) and sodium molybdate, which are starting materials for all
sorts of molybdenum chemicals. Reacting chemically pure MoO3 with aqueous
ammonia or sodium hydroxide obtains these compounds. Ammonium molybdate, in the
form of white crystals, assays 81 to 83 percent MoO3, or 54 to 55
percent molybdenum. It is soluble in water and is used for the preparation of
molybdenum chemicals and catalysts as well as metallic molybdenum powder.[vii]
The
production of metallic molybdenum from pure MoO3 or ADM is carried
out in electrically heated tubes or muffle furnaces, into which hydrogen gas is
introduced as a countercurrent against the feed. Usually there are two stages
in which the MoO3 or ADM is first reduced to a dioxide and then to a
metal powder. The two stages may be carried out in two different furnaces with
cooling in between, or a two-zone furnace can be employed. (Sometimes, a
three-stage process is utilized beginning at a low temperature of 400º C, or
750º F, to avoid an uncontrolled reaction and prevent sintering.) In the
two-stage process, two long-muffle furnaces with molybdenum-wire heating
elements can be used. The first reduction is carried out in mild-steel
"boats" holding 5 to 7 kilograms (10 to 15 pounds) of oxide, which
are fed at intervals of 30 minutes. The temperature of the furnace is 600º-700º
C (1,100º-1,300º F). The product from the first furnace is broken up and fed at
the same rate in nickel boats to a second furnace operating at 1,000º-1,100º C
(1,800º-2,000º F), after which the metal powder is screened. The purest powder,
containing 99.95 percent molybdenum, is obtained by reduction of ADM.
Because of its extremely high melting point,
molybdenum cannot be melted into ingots of high quality by conventional
processes. It can, however, easily be melted in an electric arc. In one such
process, developed by Parke and Ham, molybdenum powder is continuously pressed
into a rod, which is partially sintered by electric resistance and melted at
the end in an electric arc. The molten molybdenum is deoxidized by carbon added
to the powder, and it is cast in a water-cooled, copper mold.
Technical molybdic
oxide is the least expensive agent for adding molybdenum to alloy steels and
irons, but for higher-grade alloy steels, in which the molybdenum content is
more than 1 percent, ferromolybdenum (FeMo) is preferred since it avoids having
to add oxygen to the heat.
Either a metallothermic process or a
carbon-reduction process in electric furnaces can produce Ferromolybdenum. Because
the latter process has the inherent disadvantage of introducing a high carbon
content into the FeMo alloy, the thermic process, in which aluminum and silicon
metals are used for the reduction of a charge consisting of a mixture of
technical molybdic oxide and iron oxide, is practically the only manufacturing
method used. Reduction takes place in a furnace consisting of a bottomless,
brick-lined steel shell or ring, approximately 180 centimetres (6 feet) in
diameter and 50 centimetres (18 inches) high that are placed on a sand bed in a
mold box. After the charge is fed into the pot and leveled, a dust hood is set
in place and the reaction started by ignition with a starting fuse (usually a
mixture of powdered aluminum, magnesium, iron oxide, and potassium nitrate).
The reduction reaction lasts between 2 and 20 minutes, during which time most
of the fumes produced are drawn from the hood to a dust-collecting train. After
the reaction is completed, the metal and slag are allowed to cool and solidify
for 4 to 16 hours, depending on the size of the heat and the melting practice.
The solidified metal and slag block is then removed from the mold and quenched
in water; this cools the metal, facilitates the separation of metal and slag in
two blocks, and produces fine fractures in the metal that make it easy to break
into pieces. The FeMo cake is hammered into 20-centimetre chunks and then
crushed and screened to sizes of 2.5, 1.9, and 1.6 centimetres. Specifications
for FeMo call for a minimum of 60 percent molybdenum, between 2 and 2.5 percent
carbon, and 1 percent or less copper, phosphorus, silicon, and sulfur, and the
rest iron.[viii]
Pure molybdic oxide is made by sublimation of technical oxide or by calcining ammonium molybdate. Pure oxide is suitable for use by chemical and catalyst manufacturers. Molybdenum metal powder is produced by hydrogen reduction of the pure molybdic oxide or ammonium molybdate. In turn, this molybdenum metal powder is consolidated into usable forms by vacuum melting or by pressing and sintering.[ix]
The demands of
industry are becoming constantly more severe. Engineers want stronger, tougher
materials with better hot strength, superior properties at low temperatures,
more corrosion resistance and added wear resistance so they can design and
build more efficient equipment to give us a better life. Molybdenum helps meet these
demands.
Just
like other common alloying elements such as chromium and nickel, molybdenum
additions give alloy steel and iron a combination of strength, toughness and
wear resistance not possible with unalloyed steels. Its extensive use is proof
that, under many conditions, its inclusion (alone or with other alloys) results
in a more economical and serviceable part. Moreover, molybdenum makes a unique
contribution to hot strength, corrosion resistance and toughness.
Increasing temperature raises the efficiency of most types of equipment from steam turbines in central power stations to gas turbines in jet planes and eventually automobiles. Relatively small molybdenum additions are in many cases the best means of increasing hot strength. This applies not only to steel but also to the nonferrous super-alloys with nickel or cobalt as a base. In some aerospace and metalworking applications, molybdenum metal – either pure or with small additions of other alloys – is needed as it stands up even at temperatures where steel melts.
Molybdenum additions give stainless steel greater corrosion resistance. Molybdenum-containing stainless steel is now specified in automotive trim for long life even along the seacoast and in contact with de-icing salts. In other grades, the added corrosion resistance resulting from molybdenum makes chemical processes industrially feasible that would otherwise be confined to the laboratory.
Small amounts of molybdenum confer toughness to most steel including grades used at cryogenic temperatures for handling and containing liquid gases. Because of molybdenum’s contribution to strength and toughness, low-alloy molybdenum containing steels offer safety and economy in pipelines for oil and natural gas even under arctic conditions.[x]
The following uses for molybdenum are gathered from a number of sources as well as from anecdotal comments.[xi]
- Valuable alloying agent (contributes to the harden ability and toughness of quenched and tempered steels). Almost all ultra-high strength steels contain molybdenum in amounts from 0.25 to 8%
- Improves the strength of steel at high temperatures
- Electrodes for electrically heated glass furnaces
- Nuclear energy applications
- Missile and aircraft parts
- Valuable catalyst in petroleum refining
- Filament material in electrical applications
- Essential trace element in plant nutrition. Some soils are barren for lack of this element in the soil
- Molybdenum disulphide is a good lubricant, especially at high temperatures where normal oils decompose
While metals account for the larger part of molybdenum’s consumption, its chemical and lubrication uses in the form of chemical compounds are also important.
|
Abundance |
ppb by weight |
ppb by atoms |
|
Universe |
5 |
0.1 |
|
Sun |
9 |
0.1 |
|
Meteorite
(carbonaceous) |
1200 |
250 |
|
Crustal
rocks |
1100 |
230 |
|
Sea water |
10 |
0.64 |
|
Stream |
0.8 |
0.008 |
|
Human |
100 |
7 |
General Properties of Molybdenum
|
Atomic Number |
42 |
|
Atomic Weight |
95.95 |
|
Atomic Volume |
9.41 |
|
Lattice Type |
Body Centered Cube |
|
Lattice Constant at 20°C |
3.1468 |
|
Natural Isotopes |
92, 94, 95, 96, 97, 98, 100 |
|
|
|
|
Density at 20°C (grams/cc) |
10.2 |
|
|
|
|
Melting Point °C |
2610 |
|
Boiling Point °C (at 14.7 psi) |
4830 |
|
Linear Coefficient of Expansion per °C |
4.9 x 10^-6 |
|
Thermal Conductivity at 20°C |
0.35 cal/cm^2/cm°C/sec |
|
Specific Heat at 20°C |
0.061 |
|
|
|
|
Electrical Conductivity, % IACS |
30 |
|
Resistivity, microhm-cm at 20°C |
5.7 |
|
Temp. Coeff. Of Elect. Resistivity per °C (0-100°) |
0.0046 |
|
|
|
|
Tensile Strength at Room Temperature, psi |
120,000 - 200,000 |
|
Tensile Strength at 500°C, psi |
35,000 - 65,000 |
|
Tensile Strength at 1000°C, psi |
20,000 - 30,000 |
|
|
|
|
Young's Modulus of Elasticity (lb/in.^2) at 20°C |
46 x 10E-6 |
|
Young's Modulus of Elasticity (lb/in.^2) at 500°C |
41 x 10E-6 |
|
Young's Modulus of Elasticity (lb/in.^2) at 1000°C |
36 x 10E-6 |
|
|
|
|
Poisson's Ratio |
0.321 |
|
|
|
|
Spectral Emissivity (1000°C, ~0.65µ wavelength) |
0.37 |
|
Total Emissivity at 1500°C |
0.19 |
|
Total Emissivity at 2000°C |
0.24 |
|
|
|
|
Working Temperature °C |
</= 1600 |
|
Recrystalization Temperature °C |
900 - 1200 |
|
Stress Relieving Temperature °C |
800 |