Molybdenum price, history, occurrence, extraction and use

Molybdenum (Greek Μόλυβδος molybdos "Lead") is a chemical element with the element symbol Mo and the atomic number 42. It is one of the transition metals, in the periodic table it is in the 5th period and the 6th subgroup (group 6) or chromium group.

Molybdenum, which is usually found in deposits as molybdenum luster (molybdenum disulphide), has long been confused with lead luster or graphite. In 1778 Carl Wilhelm Scheele succeeded in turning molybdenum luster into white molybdenum (VI) oxide (also molybdenum trioxide) MoO by treatment with nitric acid₃ (Water lead earth). In 1782 Peter Jacob Hjelm reduced the oxide to elemental molybdenum with coal. Because of its difficult machinability (pure molybdenum can be plastically deformed, but contamination with one ten-thousandth of a percent oxygen or nitrogen makes molybdenum extremely brittle), molybdenum was ignored for a long time. At the end of the 19th century, employees of the French company Schneider & Co. noticed the useful properties of molybdenum alloys in the manufacture of armored pipes. In the two world wars, the demand for the metal was great, after the Second World War, prices fell dramatically. The only Western European mine was operated in Knaben, Norway until 1973.

occurrence 

Molybdenum mostly comes as molybdenite (molybdenum gloss, MoS₂) with a concentration of about 0,3%. There is also wulfenite (yellow lead ore, PbMoO₄) and Powellit Ca (Mo, W) O₄. The co-product molybdenite, which arises from copper mining, is mainly used for smelting. The MoS₂-Concentrate, as it leaves the mines in the direction of the "roaster", contains approx. 50–60% molybdenum. Large deposits are found in the United States, Chile, China, Canada, and Peru. World production in 2007 was 211.000 tons (2006 179.000 tons). In 2007 the USA - as the largest producer - produced 62.000 tons, China 60.000 tons and Chile 45.000 tons.

Molybdenum in dignified, i.e. elemental form, could only be detected in four samples so far (status: 2011): on earth in a rock sample from the Korjakskaja Sopka volcano on the Russian peninsula of Kamchatka and in three rock samples from the moon from the Apollonius highlands (Luna 20 ), the Mare Crisium (Luna 24) and the Mare Fecunditatis (Luna 16).  However, since the discoveries were published without checking by the IMA / CNMNC, the status of molybdenum as a mineral has not yet been confirmed, even if the mineral system no. 1.AC.05 (based on the 9th edition of Strunz's mineral systematics).

Extraction and presentation

The majority of the molybdenum is obtained as a by-product in copper production and only approx. 30% directly from molybdenum ores. All ores are mainly processed into ammonium heptamolybdate. This is made by calcining at approx. 400 ° C in molybdenum trioxide MoO₃ convicted. The latter is reduced to pure molybdenum powder in two stages using hydrogen. The first stage leads to the metastable brown-violet molybdenum dioxide MoO at 500–600 ° C₂, the second stage leads to pure metal powder at approx. 1100 ° C. The compression into compact metal takes place by the HIP process, by remelting in an arc furnace under argon as a protective gas or in an electron beam furnace. Single crystals are produced using the zone melting process. The recovery of molybdenum from scrap is almost 100% as there are no oxidation losses.

Features 

Molybdenum is a 5th period transition metal. The high-strength, tough and hard metal has a silvery white sheen. It has the highest melting point of all the elements of the 5th period. Like the heavy homologue tungsten, it is not attacked by reducing acids (including hydrofluoric acid). That is why molybdenum is used in large quantities for the production of acid-resistant stainless steels and nickel materials. Oxidizing acids such as hot concentrated sulfuric acid, nitric acid or aqua regia lead to high removal rates. Molybdenum is just as unstable in oxidizing alkali melts.

Usage 

In small additions it is used to harden and prevent tempering embrittlement of steel. More than two thirds of the molybdenum produced is used to produce metal alloys such as ferro-molybdenum. The shortage of tungsten in the First World War led to the increased use of molybdenum for the production of high-strength materials. To this day, molybdenum is an alloying element used to increase strength, corrosion and heat resistance. High-performance materials containing molybdenum such as Hastelloy^®, Incoloy^® or Nicrofer^® Many technical processes have made this possible or economically sensible.

Because of its high temperature resistance, molybdenum is used in the manufacture of parts for extreme applications such as B. used in aerospace or metallurgy. In oil processing it is used as a catalyst to remove sulfur.

Due to its layer structure, molybdenum disulfide is an ideal lubricant, even at elevated temperatures. It can be used as a solid, such as graphite, but also suspended in conventional lubricating oils.

Molybdenum can also be found in electronic components. In TFTs (thin-film transistors) it serves as a conductive metal layer and also in thin-film solar cells it is used as a metallic return conductor.

Molybdenum foils serve as a gas-tight current feedthrough in quartz glass, u. a. on halogen lamps and high-pressure gas discharge lamps.

Molybdate are used to impregnate fabrics to make them flame retardant.

Molybdenum is also used in X-ray diagnostics as a target material in the anode. X-ray tubes with molybdenum anode are used because of the lower energy of the characteristic X-rays ( at 17,4 keV and  at 19,6 keV compared to 58 / 59,3 keV or 67,0 / 67,2 / 69,1 keV of tungsten) of molybdenum v. a. used in the examination of the female breast (mammography).

In nuclear medicine, split molybdenum is used in technetium-99m generators. The relatively long-lasting ⁹⁹Mo (HWZ 66 h) breaks down within the RNG into ^99mTc (technetium, half-life 6 h). In this way, this important technetium isotope can be obtained directly on site for research purposes.

physiology 

As a trace element, molybdenum is essential for almost all living organisms, as it is an essential component of the active center of a number of enzymes such as nitrogenase, nitrate reductase and sulfite oxidase. Living beings use enzymes containing molybdenum and the like. a. for purine decomposition and uric acid formation. The bioavailable, i.e. H. the form of molybdenum ingested by organisms is the molybdate ion MoO₄²⁻. This is incorporated into the corresponding enzymes in several steps as molybdenum cofactors. There the Mo atom can switch between the coordination numbers + IV, + V and + VI, and thus catalyze one-electron redox reactions.

Molybdenum is essential for plants. A lack of molybdenum can make a soil sterile, which explains why fertilization with ammonium heptamolybdate increases the yield on such soils. The molybdenum concentration in plants and animals is a few ppm. Molybdenum is a very important trace element, especially for legumes. The bacteria (nodule bacteria) living in symbiosis with legumes are able to bind atmospheric nitrogen with an enzyme containing molybdenum (nitrogenase). You need molybdenum for two processes: fixation of molecular nitrogen and nitrate reduction.

Molybdenum is also essential for human nutrition. The DGE estimate for adolescents and adults assumes 50–100 µg molybdenum as an appropriate daily intake. A molybdenum deficiency does not occur. If high intakes (10–15 mg / day) are achieved - for example through molybdenum-rich soils, gout-like symptoms, joint pain and enlarged liver occur.

However, the molybdenum cofactor deficiency only occurs as a hereditary disease; one of the enzymes that catalyze the biosynthesis of the molybdenum cofactors is mutated.

safety instructions 

Molybdenum dust and compounds such as molybdenum (VI) oxide and water-soluble molybdates exhibit slight toxicity when inhaled or ingested orally.

Tests suggest that molybdenum, in contrast to many other heavy metals, has relatively little toxicity. Acute poisonings are unlikely because of the necessary amounts. In the area of ​​molybdenum mining and production higher molybdenum exposure could occur. So far, however, no cases of illness have become known.

proof 

A qualitative proof of hexavalent molybdenum is possible via the formation of heteropoly acids with phosphate. If phosphoric acid is added to a solution containing molybdate containing sulfuric acid, crystalline molybdenum gel builds up. When the mild reducing agent ascorbic acid is added, it turns a strong blue color (formation of molybdenum blue). At lower concentrations of molybdate there is no precipitation, only a change in color of the solution.

These reactions are also used for the photometric determination of molybdate or phosphate in the trace range. Alternatively, molybdenum can be determined using atomic spectrometry. In polarography, hexavalent molybdenum in sulfuric acid with a concentration of 0,5 mol / l gives two levels at −0,29 and −0,84 V (versus SCE). These are due to the reduction to Mo (V) or Mo (III).

General
Name, symbol, atomic number	Molybdenum, Mo, 42
Series	Transition metals
Group, period, block	6, 5, d
Appearance	gray metallic
CAS number	7439-98-7
Mass fraction of the earth shell	14 ppm
Atomic
atomic mass	95,94 u
Atomic radius (calculated)	145 (190) pm
Covalent radius	154 pm
electron configuration	[Kr] 4d5 5s1
1. ionization	684,3 kJ / mol
2. ionization	1560 kJ / mol
3. ionization	2618 kJ / mol
4. ionization	4480 kJ / mol
Physically
Physical state	fixed
crystal structure	cubic body-centered
density	10,28 g / cm3 (20 ° C)
Mohs hardness	5,5
magnetism	paramagnetic ( = 1,2 10−4)
melting point	2896 K (2623 ° C)
boiling point	4912 K (4639 ° C)
Molar volume	9,38 · 10−6 m3/ mol
Heat of vaporization	600 kJ / mol
heat of fusion	36 kJ / mol
speed of sound	6190 m / s
Electric conductivity	18,2 · 106 A / (V · m)
thermal conductivity	139 W / (m K)
Chemical
oxidation states	2, 3, 4, 5, 6
normal potential	−0,152 V (MoO2 + 4e- + 4 H+ → Mo + 2 H2O)
electronegativity	2,16 (Pauling scale)
isotope
isotope NH t1/2 ZA ZE (MeV) ZP 92Mo 14,84% Sturdy 93Mo {Syn.} 4000 a ε 0,405 93Nb 94Mo 9,25% Sturdy 95Mo 15,92% Sturdy 96Mo 16,68% Sturdy 97Mo 9,55% Sturdy 98Mo 24,13% Sturdy 99Mo {Syn.} 65,94 p.m. β- 1,357 99Tc 100Mo 9,63% 7,3 · 1018 a β-β- 3,034 100Ru
NMR properties
Spin γ in rad * T−1· s−1 Er(1H) fL consider W = 4,7 T in MHz 95Mo 5/2 -1,751 · 107 0,0005 6,52 97Mo 5/2 -1,788 · 107 0,0003 6,65
safety instructions
GHS hazardous substances labeling Danger H and P phrases H: 228 EUH: no EUH rates P: 210 Hazardous Information no danger symbols R and S phrases R: no R phrases S: no phrases

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