Three scientists were involved in the discovery of molybdenum: first, the Swede Karl Scheele obtained MoO 3 oxide from molybdic acid (1778), then the Frenchman P. Guelm reduced it with coal and obtained a metal with impurities (1782), and after J. Berzelius obtained pure molybdenum as a result of the combination of oxide and hydrogen.
Molybdenum is mined all over the planet, since it is relatively evenly distributed both over the earth's crust and in the waters of the oceans. This element is found both in coal and in oil, but its greatest amount is in feldspars.
Molybdenum: physical properties
Externally, molybdenum is a metal of traditional light gray color. It belongs to the category of refractory, but the purer it becomes softer. The main characteristics of molybdenum:
- density (n.a.) - 10.22 g / cm³
- melting point - 2620°C (2890 K)
- boiling point - 4639°C (4885 K)
- thermal conductivity at 300 K – 138 W/(m K)
Molybdenum: chemical properties
The Mo element is stable up to 400°C, after which it oxidizes. To date, several molybdenum oxides have been obtained, including MoO 3 trioxide, molybdenum (IV) oxide MoO 2, etc. There are also carbides - Mo 2 C and MoC, which are crystalline high-melting substances. 
Molybdenum is present in more than 20 kinds of minerals. The most common can be considered:
- >molybdenite - MoS 2
- molybdite - Fe(MoO 4) 3 nH 2 O
- wulfenite - PbMoO 4
- powellite - SaMoO 4
Molybdenum: where it is applied
The widespread production of molybdenum in the world is primarily due to the needs of the world's metallurgy. This metal acts as an alloying component for most corrosion-resistant and heat-resistant steels. In addition, it is indispensable for giving the metal increased strength characteristics and increasing viscosity. Do not do without molybdenum and manufacturers of light bulbs and high-temperature furnaces. The chemical industry uses Mo and its compounds as catalysts for chemical reactions, pigments for dyes, etc. 
Another area of application of molybdenum is medicine: pure Mo helps doctors diagnose cancer. The same element can be found in the composition of the material for mirrors of high-power gas-dynamic lasers.
Biological role
Molybdenum cannot be called a common element, but it is present in every human body. Moreover, the lack of Mo in the human body can disrupt the most important biological processes, thereby causing serious diseases. It is known that the highest concentration of molybdenum is present in the following products: in milk, liver, cereals, legumes, leafy vegetables.
Molybdenum is important for the processes occurring in the human body: this metal is part of many enzymes, without which normal metabolism is impossible. Let's see what it affects and why it is so important for our health.
Molybdenum is a catalyst for oxidative reactions. To make it more clear what he does, let's give a simple analogy. Imagine that our cell is an internal combustion engine, it receives nutrients and oxygen, which, in general, is similar to gasoline and atmospheric air for an internal combustion engine. But you all probably know that if you just spray gasoline in the air, nothing will happen: you need a spark from the spark plug for the mixture to detonate and give its energy to the engine. The same is true in the cells of our body: oxidative enzymes, such as sulfite oxidase, for example, perform a role similar to ignition in a car engine. They start the process of converting nutrients and oxygen into the energy needed to keep our cells and tissues functioning. As you understand, a car without ignition in the engine will not go anywhere, and a person with inactive oxidative enzymes will not be healthy in any way.
And although the participation of molybdenum in redox reactions is very important for the body, this is not its only role that it performs in the human body. Molybdenum is essential for normal xanthoxidase function.- an enzyme that ensures the processing of nitrogenous compounds in our body. Our body regularly renews its cellular composition, as a result of which a lot of toxins remain, containing excess nitrogen, which is excreted through the kidneys with the help of urea. It is the xanthoxidase enzyme that allows us to turn all this organic debris that accumulates in our body into a form convenient for excretion. To draw an analogy, the work of this enzyme can be compared to garbage collection in the trash bag, which allows you to throw everything at once, rather than taking empty cans and wrappers into the trash bin one by one.
Molybdenum enters the body with food and is absorbed quite easily, depending on the form of intake. from 25 to 80% substance from food. Absorption occurs mainly in the stomach and in the initial sections of the small intestine. The intake of molybdenum from the digestive tract is also strongly affected by the amount of sulfur compounds contained in food, their deficiency greatly complicates the absorption of molybdenum. When it enters the bloodstream, molybdenum is transferred to the liver with the help of special transport proteins, where it is used for the synthesis of enzymes. Molybdenum is excreted mainly by the kidneys, as a result, in the human body, the concentration of molybdenum is highest in the liver, where it is used for the needs of the body, and in the kidneys, through which its excess is excreted. In the blood, molybdenum is evenly distributed between the cellular and liquid parts of the blood. The human body does not accumulate excess molybdenum and removes it through the kidneys and bile.
daily requirement
A person needs per day 75-250 mcg molybdenum, depending on physical activity and body weight.For people over 70 years of age, the need for molybdenum is reduced by about 25% and does not exceed 200 mcg.
According to some experts, the need for molybdenum may be somewhat higher and reach 300-400 mcg.
With a normal diet, our body gets 50 to 100 mcg molybdenum, that is, the usual diet provides the minimum required intake of this trace element.
The largest amount of molybdenum is found in dairy products, dried beans, cabbage, spinach, gooseberries, black currants, liver, kidneys, pastries. Relatively little molybdenum is found in carrots, fruits, sugar, oils, fats, and fish.
Overdose
Molybdenum is relatively non-toxic. For the manifestation of its negative effects, it is necessary to receive a dose equal to 5000 mcg, the lethal dose is 50,000 mcg. It is rather difficult to create an overdose of molybdenum: if pure metal is inhaled or there is a powder, then it will practically not be absorbed. In the case of an overdose of biological compounds containing molybdenum, their absorption also practically stops. The intake of molybdenum is well controlled by our body, and the more it comes in, the less it is absorbed. To gain a toxic dose, you need to eat hundreds of times more than the dangerous dose, the amount of molybdenum. Acute poisoning with molybdenum practically does not occur, chronic overdose of molybdenum in many ways resembles the conditions that occur with copper deficiency. In such a person, growth slows down, anemia develops, nitrogenous slags begin to accumulate in the blood, and gout may develop.deficit
Molybdenum deficiency is a rather rare condition, but it is quite possible in some cases. Typically, these conditions develop in people who receive intravenous nutrition for a long time, for example, in people who are in intensive care or in patients with impaired gastrointestinal tract function.Other causes of molybdenum deficiency can be a rigid vegetarian diet that is not balanced in trace elements, and genetic defects that interfere with normal absorption from the intestine. With a deficiency of molybdenum, the exchange of nitrogenous bases and the adequate binding and excretion of inorganic sulfate compounds suffer.
With chronic molybdenum deficiency, children develop severe congenital pathologies. The normal development of the brain is disturbed, mental retardation develops, vision suffers. It has also been proven that molybdenum deficiency significantly increases the risk of developing esophageal cancer.
Summing up
We can say that molybdenum is a trace element significant for our body and its deficiency leads to serious consequences. However, under normal conditions, it is not worth seriously fearing a deficiency of molybdenum. Our food contains enough of it to meet our daily needs. Problems with its deficiency can occur with relatively exotic diets and in severe conditions in which a person will be forced to switch to intravenous nutrition.Lead and vanadium poisoning can also lead to molybdenum deficiency.
An excess of molybdenum should not be feared: it develops extremely rarely, as a rule, only among workers in metallurgical industries.
By including dairy products, grains, pastries, and bovine liver and kidneys in your diet, you can easily provide yourself with the right level of molybdenum for optimal functioning of your body.
This fairly common trace element does not need special level control, and an additional intake of preparations containing it is not needed for a relatively healthy person who does not have heavy metal poisoning.
DEFINITION
Molybdenum is the forty-second element of the Periodic Table. Designation - Mo from the Latin "molybdaeum". It is located in the fifth period, VIB group. Refers to metals. The core charge is 42.
The main natural compound of molybdenum is molybdenite, or molybdenum luster, MoS 2 - a mineral very similar in appearance to graphite. The total content of molybdenum in the earth's crust is 0.001% (mass.).
In the form of a simple substance, molybdenum is a silvery-white metal (Fig. 1) with a density of 10.2 g / cm 3, melting at 2620 o C. At room temperature, it does not change in air, but when heated, it oxidizes to white MoO 3 trioxide. Hydrochloric and dilute sulfuric acid at room temperature do not act on molybdenum; it dissolves in nitric or hot concentrated sulfuric acid.
Rice. 1. Molybdenum. Appearance.
Atomic and molecular weight of molybdenum
Relative molecular weight of a substance(M r) is a number showing how many times the mass of a given molecule is greater than 1/12 of the mass of a carbon atom, and relative atomic mass of an element(A r) - how many times the average mass of atoms of a chemical element is greater than 1/12 of the mass of a carbon atom.
Since molybdenum exists in the free state in the form of monatomic Mo molecules, the values of its atomic and molecular masses coincide. They are equal to 95.96.
Isotopes of molybdenum
It is known that in nature molybdenum can exist in the form of five six stable isotopes 92 Mo, 94 Mo, 95 Mo, 96 Mo, 97 Mo and 98 Mo. Their mass numbers are 92, 94, 95, 96, 97 and 98, respectively. The nucleus of the atom of the molybdenum isotope 92 Mo contains forty-two protons and fifty neutrons, and the remaining isotopes differ from it only in the number of neutrons.
There are artificial unstable isotopes of molybdenum with mass numbers from 83 to 115, as well as eight isomeric states of nuclei, among which the 93 Mo isotope with a half-life of 4 thousand years is the longest-lived.
molybdenum ions
At the external energy level of the molybdenum atom, there are six electrons that are valence:
1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 4 5s 2 .
As a result of chemical interaction, molybdenum gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:
Mo o -2e → Mo 2+;
Mo o -3e → Mo 3+;
Mo o -4e → Mo 4+;
Mo o -5e → Mo 5+;
Mo o -6e → Mo 6+.
Molecule and atom of molybdenum
In the free state, molybdenum exists in the form of monatomic Mo molecules. Here are some properties that characterize the atom and molecule of molybdenum:
Molybdenum alloys
About 80% of all mined molybdenum is spent on the production of special grades of steel. It is a constituent of many stainless steels; in addition, its introduction contributes to an increase in their heat resistance.
An alloy of molybdenum with tantalum is used to make laboratory glassware used in chemical laboratories instead of platinum.
Examples of problem solving
EXAMPLE 1
| Exercise | Calculate the mass fractions of the elements that make up molybdenum oxide if its molecular formula is Mo 2 O 3 . |
| Solution | The mass fraction of an element in the composition of any molecule is determined by the formula: ω (X) = n × Ar (X) / Mr (HX) × 100%. |
Molybdenum(lat. Molybdaenum), Mo, a chemical element of the VI group of the periodic system of Mendeleev; atomic number 42, atomic mass 95.94; light gray refractory metal. In nature, the element is represented by seven stable isotopes with mass numbers 92, 94-98 and 100, of which 98 Mo (23.75%) is the most common. Until the 18th century, the main mineral Molybdenum molybdenum luster (molybdenite) was not distinguished from graphite and lead luster, since they are very similar in appearance. These minerals were collectively called "molybdenum" (from the Greek molybdos - lead).
The element Molybdenum was discovered in 1778 by the Swedish chemist K. Scheele, who isolated molybdic acid by treating molybdenite with nitric acid. The Swedish chemist P. Gjelm in 1782 was the first to obtain metallic Molybdenum by the reduction of MoO 3 with carbon.
Distribution of molybdenum in nature. Molybdenum is a typical rare element, its content in the earth's crust is 1.1 10 -4% (by mass). The total number of minerals Molybdenum is 15, most of them (various molybdates) are formed in the biosphere. In magmatic processes, Molybdenum is associated mainly with acid magma, with granitoids. There is little Molybdenum in the mantle, and only 2·10 -5% in ultramafic rocks. The accumulation of Molybdenum is associated with deep hot waters, from which it precipitates in the form of molybdenite MoS 2 (the main industrial mineral of Molybdenum), forming hydrothermal deposits. The most important precipitant of Molybdenum from water is H 2 S.
The geochemistry of Molybdenum in the biosphere is closely related to living matter and its decay products; the average content of Molybdenum in organisms is 1·10 -5%. On the earth's surface, especially under alkaline conditions, Mo (IV) is easily oxidized to molybdates, many of which are relatively soluble. In dry climate landscapes, Molybdenum easily migrates, accumulating during evaporation in salt lakes (up to 1-10 -3%) and solonchaks. In humid climates, in acidic soils, Molybdenum is often inactive; fertilizers containing molybdenum are required here (eg for legumes).
There is little Molybdenum in river waters (10 -7 - 10 -8%). Entering the ocean with runoff, Molybdenum partially accumulates in sea water (as a result of its evaporation, Molybdenum is 1 10 -6% here), partially precipitates, concentrating in clayey silts rich in organic matter and H 2 S.
In addition to molybdenum ores, some molybdenum-containing copper and copper-lead-zinc ores also serve as a source of molybdenum.
Physical properties of Molybdenum. Molybdenum crystallizes in a cubic body-centered lattice with a period a = 3.14Å. Atomic radius 1.4A, ionic radii Mo 4+ 0.68A, Mo 6 + 0.62A. Density 10.2 g / cm 3 (20 ° C); t pl 2620 °C; t bales about 4800 °C. The specific heat capacity at 20-100°C is 0.272 kJ/(kg K), i.e. 0.065 cal/(g deg). Thermal conductivity at 20°C 146.65 W/(m K), i.e. 0.35 cal/(cm sec deg). Thermal coefficient of linear expansion (5.8-6.2) 10 -6 at 25-700 °C. Electrical resistivity 5.2·10 -8 ohm·m, that is, 5.2·10 -6 ohm·cm; work function of electrons 4.37 eV. Molybdenum is paramagnetic; atomic magnetic susceptibility -90 10 -6 (20 °C).
The mechanical properties of Molybdenum depend on the purity of the metal and its previous mechanical and thermal processing. So, Brinell hardness is 1500-1600 MN/m 2 , i.e. 150-160 kgf/mm 2 (for sintered bar), 2000-2300 MN/m 2 (for forged rod) and 1400-1850 MN/m 2 (for annealed wire); tensile strength for annealed wire 800-1200 MN/m 2 . Modulus of elasticity Molybdenum 285-300 Gn/m 2 . Mo is more ductile than W. Recrystallizing annealing does not lead to metal brittleness.
Chemical properties of Molybdenum. Molybdenum is stable in air at ordinary temperatures. The onset of oxidation (tint color) is observed at 400 °C. Starting from 600 °C, the metal rapidly oxidizes with the formation of MoO 3 . Water vapor at temperatures above 700 °C intensively oxidize Molybdenum to MoO 2 . Molybdenum does not chemically react with hydrogen until it melts. Fluorine acts on Molybdenum at normal temperature, chlorine at 250 °C, forming MoF 6 and MoCl 6 . Under the action of sulfur and hydrogen sulfide vapors above 440 and 800 °C, respectively, disulfide MoS 2 is formed. With nitrogen Molybdenum above 1500 °C forms a nitride (probably Mo 2 N). Solid carbon and hydrocarbons, as well as carbon monoxide (II) at 1100-1200 ° C interact with the metal to form Mo 2 C carbide (melts with decomposition at 2400 ° C). Above 1200 °C, Molybdenum reacts with silicon, forming MoSi 2 silicide, which is highly stable in air up to 1500–1600 °C (its microhardness is 14 100 MN/m 2).
Molybdenum is somewhat soluble in hydrochloric and sulfuric acids only at 80–100°C. Nitric acid, aqua regia and hydrogen peroxide slowly dissolve the metal in the cold, quickly - when heated. A good solvent for molybdenum is a mixture of nitric and sulfuric acids. Tungsten does not dissolve in a mixture of these acids. Molybdenum is stable in cold alkali solutions, but corrodes somewhat when heated. The configuration of the outer electrons of the atom Mo 4d 5 5s 1, the most characteristic valence is 6. Compounds of 5-, 4-, 3- and 2-valent Molybdenum are also known.
Molybdenum forms two stable oxides - MoO 3 (white crystals with a greenish tint, mp 795 ° C, bp 1155 ° C) and MoO 2 (dark brown). In addition, intermediate oxides are known, corresponding in composition to the homologous series Mo n O 3n-1 (Mo 9 O 26, Mo 8 O 23, Mo 4 O 11); all of them are thermally unstable and above 700 °C decompose with the formation of MoO 3 and MoO 2 . MoO 3 oxide forms simple (or normal) Molybdenum acids - H 2 MoO 4 monohydrate, H 2 MoO 4 H 2 O dihydrate and isopoly acids - H 6 Mo 7 O 24, HMo 6 O 24, H 4 Mo 8 O 26 and others . Salts of normal acids are called normal molybdates, and polyacids are called polymolybdates. In addition to those mentioned above, several molybdenum peracids are known - H 2 MoO X (x - from 5 to 8) and complex heteropoly compounds with phosphoric, arsenic and boric acids. One of the common salts of heteropoly acids is ammonium phosphoromolybdate (NH 4) 3 [P (Mo 3 O 10) 4] 6H 2 O. Of the halides and oxyhalides of Molybdenum, MoF 6 fluoride (t pl 17.5 ° C, t bp 35 °С) and chloride MoCl 5 (t pl 194 °С, bp t 268 °С). They can be easily purified by distillation and are used to produce high purity molybdenum.
The existence of three Molybdenum sulfides, MoS 3 , MoS 2 and Mo 2 S 3 , has been reliably established. The first two are of practical importance. The disulfide MoS 2 occurs naturally as the mineral molybdenite; can be obtained by the action of sulfur on Molybdenum or by fusing MoO 3 with soda and sulfur. The disulfide is practically insoluble in water, HCl, diluted with H 2 SO 4 . Decomposes above 1200 °С with the formation of Mo 2 S 3 .
When hydrogen sulfide is passed into heated acidified solutions of molybdates, MoS 3 precipitates.
Getting Molybdenum. The main raw materials for the production of molybdenum, its alloys and compounds are standard molybdenite concentrates containing 47-50% Mo, 28-32% S, 1-9% SiO 2 and impurities of other elements. The concentrate is subjected to oxidative roasting at 570-600 °C in multi-hearth furnaces or fluidized bed furnaces. The roasting product - cinder contains MoO 3 contaminated with impurities. Pure MoO 3 necessary for the production of metallic Molybdenum is obtained from cinder in two ways: 1) by sublimation at np and 950-1100 °C; 2) by a chemical method, which consists in the following: the cinder is leached with ammonia water, transferring Molybdenum into solution; Ammonium polymolybdates (mainly paramolybdate 3(NH 4) 2 O 7MoO 3 nH 2 O) are isolated from the ammonium molybdate solution (after cleaning it from Cu, Fe impurities) by neutralization or evaporation followed by crystallization; by calcining paramolybdate at 450-500 ° C, pure MoO 3 is obtained, containing no more than 0.05% impurities.
Molybdenum metal is obtained (first in the form of a powder) by the reduction of MoO 3 in a stream of dry hydrogen. The process is carried out in tube furnaces in two stages: the first - at 550-700 °C, the second - at 900-1000 °C. Molybdenum powder is converted into a compact metal by powder metallurgy or smelting. In the first case, relatively small blanks are obtained (section 2-9 cm 2 with a length of 450-600 mm). Molybdenum powder is pressed in steel molds under pressure of 200-300 MN/m 2 (2000-3000 kgf/cm 2). After preliminary sintering (at 1000–1200°C) in a hydrogen atmosphere, the blanks (rods) are subjected to high-temperature sintering at 2200–2400°C. The sintered rod is processed by pressure (forging, broaching, rolling). Larger sintered blanks (100-200 kg) are obtained by hydrostatic pressing in elastic shells. Billets of 500-2000 kg are produced by arc melting in furnaces with a cooled copper crucible and a consumable electrode, which is a package of sintered rods. In addition, electron beam melting of Molybdenum is used. For the production of ferromolybdenum (alloy; 55-70% Mo, the rest Fe), which serves to introduce Molybdenum additives into steel, the reduction of calcined molybdenite concentrate (calcine) with ferrosilicon in the presence of iron ore and steel chips is used.
The use of molybdenum 70-80% of the extracted Molybdenum goes to the production of alloy steels. The rest is used in the form of pure metal and alloys based on it, alloys with non-ferrous and rare metals, as well as in the form of chemical compounds. Metal Molybdenum is the most important structural material in the production of electric lighting lamps and vacuum devices (radio tubes, generator lamps, X-ray tubes, and others); Molybdenum is used to make anodes, grids, cathodes, filament holders in electric lamps. Molybdenum wire and strip are widely used as heaters for high temperature furnaces.
After mastering the production of large billets, molybdenum began to be used (in its pure form or with alloying additions of other metals) in cases where it is necessary to maintain strength at high temperatures, for example, for the manufacture of parts for rockets and other aircraft. To protect Molybdenum from oxidation at high temperatures, parts are coated with Molybdenum silicide, heat-resistant enamels, and other methods of protection. Molybdenum is used as a structural material in nuclear power reactors, since it has a relatively small thermal neutron capture cross section (2.6 barn). Molybdenum plays an important role in the composition of heat-resistant and acid-resistant alloys, where it is combined mainly with Ni, Co, and Cr.
Some compounds of Molybdenum are used in the technique. So, MoS 2 is a lubricant for rubbing parts of mechanisms; Molybdenum disilicide is used in the manufacture of heaters for high-temperature furnaces; Na 2 MoO 4 - in the production of paints and varnishes; Molybdenum oxides are catalysts in the chemical and petroleum industries.
Molybdenum in the body of plants, animals and humans is constantly present as a microelement involved mainly in nitrogen metabolism. Molybdenum is necessary for the activity of a number of redox enzymes (flavoproteins) that catalyze the reduction of nitrates and nitrogen fixation in plants (there is a lot of Molybdenum in legume nodules), as well as the reaction of purine metabolism in animals. In plants, Molybdenum stimulates the biosynthesis of nucleic acids and proteins, increases the content of chlorophyll and vitamins. With a lack of Molybdenum, legumes, oats, tomatoes, lettuce and other plants fall ill with a special type of spotting, do not bear fruit and die. Therefore, soluble molybdates in small doses are introduced into microfertilizers. Animals usually do not lack Molybdenum. An excess of Molybdenum in the feed of ruminants (biogeochemical provinces with a high content of Molybdenum are known in the Kulunda steppe, Altai, the Caucasus) leads to chronic molybdenum toxicoses, accompanied by diarrhea, exhaustion, impaired copper and phosphorus metabolism. The toxic effect of Molybdenum is removed by the introduction of copper compounds. Excess Molybdenum in the human body can cause metabolic disorders, bone growth retardation, gout, etc.
Molybdenum (Latin Molybdenum, denoted by the symbol Mo) is an element with atomic number 42 and atomic weight 95.94. It is an element of a secondary subgroup of the sixth group, the fifth period of the periodic table of chemical elements of Dmitry Ivanovich Mendeleev. Together with chromium and tungsten, molybdenum forms a subgroup of chromium. The elements of this subgroup differ in that their outer electron layer of atoms contains one or two electrons, this determines the metallic nature of these elements and their difference from the elements of the main subgroup. Molybdenum under normal conditions is a transitional refractory (melting point 2620 ° C) light gray metal with a density of 10.2 g / cm3. In many ways, the mechanical properties of molybdenum depend on the purity of the metal and its previous mechanical and thermal processing.
There are 31 known isotopes of molybdenum from 83Mo to 113Mo. Stable are: 92Mo, 94Mo - 98Mo. In nature, the forty-second element is represented by seven isotopes: 92Mo (15.86%), 94Mo (9.12%), 95Mo (15.70%), 96Mo (16.50%), 97Mo (9.45%), 98Mo (23.75%) and 100Mo (9.62%) with half-life = 1.00 1019 years. The most unstable isotopes of element #42 have half-lives less than 150 ns. Radioactive isotopes 93Mo (half-life 6.95 h) and 99Mo (half-life 66 h) are isotope tracers.
Molybdenum in the form of the mineral molybdenite (molybdenum disulfide - MoS2) was known to the ancient Greeks and Romans for a very long time. For many centuries, molybdenite, or as it is also called, molybdenum luster, was not distinguished from galena (PbS lead luster) and graphite. The fact is that all these minerals are very similar in appearance, in addition, they are all capable of leaving a mark on paper. Therefore, until the 18th century, these minerals were called the same: "Molybdaena", which in Greek means "lead".
The first to suggest that all these three minerals are independent substances was the Swedish chemist F. Kronstedt. After 20 years, another Swedish chemist K. Scheele took up the study of molybdenite. After dissolving the mineral in concentrated nitric acid, he obtained a white precipitate, which he called molybdic acid. Assuming that the metal can be obtained by calcining this white precipitate with pure coal, but not having the necessary equipment (furnace), Scheele suggested that another chemist, Gjelma, who had such a furnace, conduct an experiment. The result of the experiment was to obtain molybdenum carbide, which both scientists took for a metal, which they called molybdenum. J. Ya. Berzelius was destined to obtain a relatively pure metal, establish its atomic weight and describe some of the properties in 1817.
Most of the mined molybdenum (80-85%) is consumed as an alloying element in the production of special grades of steel. Molybdenum is a constituent of many stainless steels, in addition, the addition of this element helps to increase the heat resistance of these steels. Alloys alloyed with the forty-second element are used in aviation, rocket and nuclear technology, and chemical engineering. In its pure form, the metal is used in the manufacture of parts for electronic lamps and incandescent lamps (anodes, grids, cathodes, filament holders, etc.), molybdenum wire and tape are used as heaters for high-temperature furnaces. Some compounds of the forty-second element have also found wide application. So molybdenum anhydride is widely used as a positive electrode in lithium current sources, MoS2 is a lubricant for rubbing parts of mechanisms, some molybdenum oxides are catalysts in the chemical and oil industries.
Scientists have found that molybdenum is constantly present in the body of plants, animals and humans as a microelement involved mainly in nitrogen metabolism. The forty-second element is necessary for the activity of a number of redox enzymes necessary for metabolic processes in plants and animals.
Biological properties
The forty-second element is one of the most important trace elements in the nutrition of humans, animals and plants, it is necessary for the normal development and growth of organisms, and affects the reproduction of plants. The content of molybdenum in the green mass of plants is about 1 mg per kilogram of dry matter. This element is necessary for the activity of a number of redox enzymes (flavoproteins) that catalyze the reduction of nitrates and nitrogen fixation in plants (there is a lot of molybdenum in the nodules of legumes, common in bacteria and archaea). In addition, in plants, the forty-second element stimulates the biosynthesis of nucleic acids and proteins, increases the content of chlorophyll and vitamins.
With a lack of molybdenum, tomatoes, legumes, oats, lettuce and other plants fall ill with a special type of spotting, do not bear fruit and die. For this reason, it is necessary to introduce soluble molybdates into microfertilizers in small amounts. So, in one of the experimental farms in New Zealand, it was found that when small doses of molybdenum salts are introduced into the soil, it increases the yield of clover and alfalfa by about a third. Further agricultural studies have shown that micro-amounts of molybdenum increase the activity of nodule bacteria, as a result of which plants absorb nitrogen better. It was also found that molybdenum is best absorbed on acidic soils, and on red soils and burozems rich in iron, the effectiveness of molybdenum is minimal.
The physiological effect of molybdenum on animal and human organisms was first established in 1953, with the discovery of the effect of this element on the activity of the xanthine oxidase enzyme. Molybdenum makes the work of antioxidants, including vitamin C, more efficient, and it is also an important component of the tissue respiration system, enhances the synthesis of amino acids, and improves the accumulation of nitrogen. The forty-second element is an integral part of a number of enzymes (xanthine oxidase, aldehyde oxidase, sulfite oxidase, etc.) that perform important physiological functions, in particular, the regulation of uric acid metabolism. The lack of molybdenum in the body is accompanied by a decrease in the content of xanthine oxidase in the tissues, from which anabolic processes “suffer”, a weakening of the immune system is observed.
It is not completely established, but it is assumed that molybdenum plays an important role in the process of incorporation of fluoride into tooth enamel, as well as in the stimulation of hematopoiesis. With a lack of molybdenum in the body of animals, the ability to oxidize xanthine to uric acid is impaired, the excretion of uric acid and inorganic sulfates decreases, and the growth rate decreases. Animals develop xanthine kidney stones. Deficiency of molybdenum can lead to a decrease in the breakdown of cellulose and excessive accumulation of copper in the body, up to copper intoxication. All these phenomena can be eliminated by adding molybdenum to the diet. In humans, molybdenum deficiency manifests itself in the form of hypouricemia, hypermethioninemia, hyperoxypurinemia, hypouricosuria and hyposulfaturia, progressive mental disorders (up to coma).
It has been established that the compounds of the forty-second element enter the body with food. During the day, 75-250 mcg of molybdenum enters the body of an adult with food, which is the necessary daily intake of this microelement. Molybdenum supplied with food in the form of soluble complexes is easily absorbed - 25-80% of the element supplied with food is absorbed in the human gastrointestinal tract. Further, approximately 80% of the molybdenum that enters the bloodstream binds to proteins (primarily to albumins) and is transported throughout the body. The concentrators of the forty-second element are the liver and kidneys. Molybdenum is excreted mainly with urine and bile. The accumulation of molybdenum in the body of mammals does not occur. The main suppliers of molybdenum to the body are dried beans, milk and dairy products, organ meats, cruciferous, gooseberries, black currants, cereals and pastries. Despite the fact that molybdenum is a rare element, cases of its deficiency in the human body are rare.
An excess of molybdenum in the body leads to metabolic disorders, bone growth retardation. Xanthine oxidase accelerates nitrogen metabolism in the body, in particular purine metabolism. As a result of the breakdown of purines, uric acid is formed. If there is too much of this acid, then the kidneys do not have time to remove it from the body, salts dissolved in this acid accumulate in the joints and muscle tendons. The joints begin to hurt, gout develops. An excess of molybdenum in the feed of ruminants leads to chronic molybdenum toxicosis, accompanied by diarrhea, exhaustion, impaired copper and phosphorus metabolism. To reduce the toxic effect of molybdenum on the body, it is necessary to reduce the intake of molybdenum-rich products, carry out symptomatic treatment, use those drugs and dietary supplements for food that contain copper and sulfur (methionine, unithiol, sodium thiosulfate, etc.).
It turns out that molybdenum is able to influence the body not only directly - as an important trace element, but also indirectly - as a component of the soil. So, in the north of China there is a place Lin Xian (Lin Xian), it is located in the province of Honan (Honan). This place is known as the area with the highest percentage of esophageal cancer among the local population. What is the reason for such an anomaly? The answer came from a thorough study of the soil. It turned out that the lands of Ling Xian are poor in the forty-second element, the presence of which is necessary for the normal functioning of nitrogen-fixing bacteria. The fact is that the restoration of nitrates introduced into the soil is carried out by them with the help of the molybdenum-dependent enzyme nitrate reductase. The lack of molybdenum reduces the activity of the enzyme, which is only enough to reduce nitrate not to ammonia, but to nitrosamines, which are known to have high carcinogenic activity. The introduction of molybdenum fertilizers into the soil significantly reduced the percentage of morbidity in the population. Similar endemic diseases have also been reported in South Africa.
It is interesting that the molybdenum mine, which was developed in the 30s of the XX century and located on one of the spurs of the Takhtarvumchorr ridge (Kola Peninsula), is now a frequently visited tourist route. There is only one horizon in the mine, which has three entrances at an altitude of 600 meters above sea level. A little below the entrance to the adit there is a steam engine, which once supplied steam through pipes to the miners' jackhammers. By the way, both the steam engine and the supply pipes - everything has been preserved. The route is small, about three kilometers of drifts, and part of the mine is flooded.
Mysterious spirals of tungsten, molybdenum and copper are a controversial and not completely explained by modern science phenomenon in the form of small (from 3 microns to 3 mm) objects found in the Subpolar Urals. For the first time such finds appeared in 1991, during the exploration, which was carried out in the area of the Naroda River in sand samples examined for the presence of gold. Later, similar finds were repeatedly found in the Subpolar Urals in the area of the Naroda, Kozhim and Balbanyu rivers, as well as in Tajikistan and Chukotka. The uniqueness of the finds is their age. Dating objects is very difficult due to the fact that most of them were found in alluvial deposits.
The exception is two finds made in 1995 in the wall of a quarry in the area of the lower reaches of the Balbanyu River. Examination of the rocks in which molybdenum springs were found gave a vague result - from 20,000 to 318,000 years! Many hypotheses have been put forward about these findings: the spirals are of alien origin and may be the product of extraterrestrial nanotechnology brought to Earth several thousand years ago; mysterious springs - artificial objects, but not ancient, but modern, caught in the rocks from the surface of the earth. The generally accepted theory is the opinion of Nikolai Rumyantsev, Doctor of Geological and Mineralogical Sciences, Honored Geologist of Russia, about the natural origin of "springs" - a form of native tungsten.
Molybdenum is not a coin metal, however, the similarity of "coins" (they have no denomination) or medallions are sold by Metallium, there are other medal-tokens sold by manufacturing companies (they also mine the metal) of molybdenum.
Another fantastic hypothesis indirectly related to molybdenum is the version of the extraterrestrial origin of life on Earth. One of the arguments of this theory: "the presence of extremely rare elements in terrestrial organisms means that they are of extraterrestrial origin." Molybdenum is contained in the earth's crust in an insignificant amount, and its role in the metabolism (metabolism) of terrestrial organisms is significant. At the same time, it is noted that the so-called "molybdenum stars" with a high content of molybdenum are known, which are the original "plantations" of microorganisms brought to Earth!
However, this phenomenon is explained from the standpoint of evolutionary biochemistry, for example, the earth's crust contains very little phosphorus, and phosphorus is an essential component of nucleic acids, which, along with proteins, are essential for life; in addition, higher nervous activity is also very closely related to phosphorus. In addition, the Japanese scientist Egani determined that the total content of molybdenum on Earth is really low, but its percentage in sea water is twice as high as that of chromium. On this occasion, Egani writes: "The relative abundance of this element in sea water confirms the widely accepted view that life on Earth originated in the primordial ocean."
Story
Even the ancient Greeks noticed that some minerals are able to leave a gray mark on paper. Based on this fact, they combined a number of substances that were completely dissimilar in properties under one name - "Molybdaena", which in Greek means lead, which itself is quite capable of writing on paper. Blame for this confusion and the similarity of lead-gray molybdenite with galena and graphite. The softness of these minerals allowed them to be used as pencil leads, although if you look closely, the molybdenum luster leaves a greenish-gray color on paper, in contrast to the gray color of graphite or lead luster. These factors, plus the similarity of the Greek names for lead "ó" and galena "o", caused the misconception about the similarity of the three minerals (PbS - galena, MoS2 - molybdenite and graphite) from ancient times smoothly migrated to the Middle Ages. This situation continued until the 18th century.
The first who wanted to break the "vicious circle" was the famous Swedish chemist and mineralogist Axel Fredrik Cronstedt (1722-1765). In 1758, he suggested that in fact graphite, molybdenite (MoS2 - molybdenum luster) and galena (PbS - lead luster) are three completely independent substances. However, on this assumption, the progress towards the truth was completed.
Only twenty years later - in 1778 - the chemical composition of molybdenite was again interested. And again it was a Swedish chemist - Carl Wilhelm Scheele. The first thing Scheele did was boiling molybdenum luster in concentrated nitric acid, as a result of which the chemist obtained a white precipitate of "special white earth" (Wasserbleyerde). He called this earth molybdic acid (Acidum Molybdaenae). In the time of Karl Wilhelm, "earths" were called anhydrides, that is, the combination of an element with oxygen, in other words, "acid minus water." The absence of this knowledge did not prevent the scientist from suggesting that metal from the "earth" could be obtained by calcining the latter with pure coal. However, without the necessary equipment (Scheele did not have a suitable furnace), the scientist could not conduct the experiment on his own.
Devoted only to science, Scheele, without any feeling of envy, sent a sample of molybdic acid to another Swedish chemist, Peter Jacob Hjelm, in 1782. In turn, he finally manages to restore it with coal and get a metal bead (smelted metal obtained by fusing a mineral or ore with soda or other fluxes). However, it was only heavily contaminated molybdenum carbide. The fact is that when molybdenum trioxide MoO3 is calcined with coal, it is impossible to obtain pure molybdenum, because it reacts with coal, forming carbide. Nevertheless, scientists rejoiced. Scheele congratulated his colleague: "I am glad that we now have the metal - molybdenum." Thus, in 1790, the new metal received a foreign name, because the Latin molibdaena comes from the ancient Greek name for lead - μολνβδος. This is a well-known paradox - it is difficult to find metals more dissimilar than molybdenum and lead.
Relatively pure metal was obtained only in 1817 - after the death of both discoverers. The honor of such a discovery belongs to another famous Swedish chemist, Jens Jakob Berzelius. He reduced molybdenum anhydride not with carbon, but with hydrogen and obtained really pure molybdenum, established its atomic weight and studied its properties in detail.
Molybdenum of industrial purity was obtained only at the beginning of the 20th century.
Being in nature
According to various sources, the content of molybdenum in the earth's crust ranges from 1.1∙10-4% to 3∙10-4% by weight. Molybdenum does not occur in free form; in general, the forty-second element is poorly distributed in nature. According to the classification of the Soviet geochemist V.V. Shcherbina, elements that are less than 0.001% in the earth's crust are considered rare, therefore, molybdenum is a typical rare element. However, the forty-second element is distributed relatively evenly. About twenty minerals of molybdenum are known in nature, most of them (various molybdates) are formed in the biosphere. Ultrabasic and carbonate rocks contain the least amount of molybdenum (0.4 - 0.5 g/t).
It is noted that the concentration of molybdenum in rocks increases with increasing SiO2, because in magmatic processes, molybdenum is associated mainly with acid magma and granitoids. The accumulation of molybdenum is associated with deep hot waters, from which it precipitates in the form of molybdenite MoS2, forming hydrothermal deposits. The most important precipitant of the forty-second element from waters is H2S. Molybdenum is found in sea and river water, plant ash, coal and oil. Moreover, the content of the forty-second element in sea water ranges from 8.9 to 12.2 μg / l - depending on the ocean and water area. A general phenomenon can only be considered that the waters near the coast and the surface layers are much poorer in molybdenum than the deep layers of the ocean. The waters of the oceans and seas contain the forty-second element more than river waters. The fact is that, acting with river runoff, molybdenum partially accumulates in sea water, and partially precipitates, concentrating in clay silts.
The most important minerals of molybdenum are molybdenite (MoS2), povelite (CaMoO4), molybdo-scheelite (Ca(Mo,W)O4), molybdite (xFe2O3 yMoO3 zH2O) and wulfenite (PbMoO4). Molybdenite or molybdenum luster is a mineral from the sulfide class (MoS2), it contains 60% molybdenum and 40% sulfur. A small amount of rhenium is also found - up to 0.33%. Most often, this mineral is found in greisen, less often pegmatite deposits, in which it is associated with wolframite, cassiterite, topaz, fluorite, pyrite, chalcopyrite, and other minerals. The most important accumulations of molybdenite are associated with hydrothermal formations, and are especially widespread in quartz veins and silicified rocks.
The average content of molybdenum in ores of large deposits is 0.06-0.3%, small - 0.5-1%. As an associated component, the forty-second element is extracted from other ores with a molybdenum content of 0.005% or more. In addition, molybdenum ores are distinguished by the mineral composition and shape of the ore bodies. According to the latter criterion, they are subdivided into skarn (molybdenum, tungsten-molybdenum and copper-molybdenum), vein (quartz, quartz-sericite and quartz-molybdenite-wolframite) and vein-disseminated (copper-molybdenum, quartz-molybdenite-sericite, copper porphyritic with molybdenum). Previously, quartz vein deposits were of paramount industrial importance, but in modern times they have almost all been worked out. Therefore, vein-disseminated and skarn deposits acquired paramount importance.
More recently, the United States of America was rightfully considered the world leader in reserves and production of molybdenum ores, where molybdenum-containing ores are mined in Colorado, New Mexico, Idaho and a number of other states. However, recent discoveries of new rich deposits have brought China to the forefront, where seven large provinces are engaged in mining. While the US is still the leader in molybdenum production, China's booming economy could soon lead that country to the top spot in the production of the forty-second element. Other countries with large reserves of molybdenum ores include: Chile, Canada (the territory of British Columbia), Russia (seven developed deposits), Mexico (La Caridad deposit), Peru (Tokepala mine), many CIS countries, etc.
Application
The main consumer of molybdenum (up to 85%) is metallurgy, where the lion's share of the mined forty-second element is spent on obtaining special structural steels. Molybdenum significantly improves the properties of alloyed metals. The additive of this element (0.15-0.8%) significantly increases the hardenability, improves the strength, toughness and corrosion resistance of structural steels, which are used in the manufacture of the most critical parts and products.
Molybdenum and its alloys are refractory materials, and this quality is simply necessary in the manufacture of shells for the head parts of rockets and aircraft. Moreover, the use of such alloys is possible both as an auxiliary material - thermal screens, separated from the main material by thermal insulation, and as the main structural material. Although molybdenum is inferior to tungsten and its alloys in terms of strength characteristics, however, in terms of specific strength at temperatures below 1,350-1,450 ° C, molybdenum and its alloys take first place, and titanium-molybdenum alloys have a temperature limit of operation of 1500 ° C!
It is because of this that molybdenum and niobium, as well as their alloys, which have a higher specific strength up to 1,370 °C compared to tantalum, tungsten and alloys based on them, are most widely used in the manufacture of skin and frame elements of rockets and supersonic aircraft. From heat-resistant steels alloyed with the forty-second element, shells of rockets and capsules returning to earth, honeycomb panels of spacecraft, heat shields, heat exchangers, wing edge skins and stabilizers in supersonic aircraft are made. In addition, molybdenum is used in steels intended for some parts of ramjet and turbojet engines (injector flaps, turbine blades, tail skirts, rocket engine nozzles, control surfaces in rockets with solid fuel). Materials operating under such conditions require not only high resistance to oxidation and gas erosion, but also high long-term strength and impact resistance. All these indicators at temperatures below 1370 °C are met by molybdenum and its alloys.
Molybdenum and its alloys are used in parts that operate in vacuum for a long time, as a structural material in nuclear power reactors, for the manufacture of equipment operating in aggressive environments (sulfuric, hydrochloric and phosphoric acids). To increase the hardness, molybdenum is introduced into cobalt and chromium alloys (stellites), which are used for surfacing the edges of parts made of ordinary steel that are subject to wear (abrasion). Since molybdenum and its alloys are stable in molten glass, it is widely used in the glass industry, for example, for the manufacture of glass melting electrodes. At present, molybdenum alloys are used to make molds and cores for injection molding machines for aluminum, zinc and copper alloys. A molybdenum-tungsten alloy paired with pure tungsten is used to measure temperatures up to 2,900 °C in a reducing atmosphere.
In its pure form, molybdenum is used in the form of a tape or wire, as heating elements in high-temperature (up to 2,200 °C) induction furnaces. Molybdenum sheet and wire are widely used in the radio-electronic industry (as a material for the anodes of radio lamps) and X-ray engineering for the manufacture of various parts of electronic lamps, X-ray tubes and other vacuum devices.
Numerous compounds of the forty-second element have also found wide application. MoS2 disulfide and MoSе2 diselenide of molybdenum are used as lubricants for rubbing parts operating at temperatures from -45 to +400 °C. In addition, molybdenum disulfide is added to motor oil, where it forms friction-reducing layers on metal surfaces. Molybdenum hexafluoride is used in the deposition of metallic molybdenum on various materials. MoSi2 molybdenum disilicide is used in the manufacture of heaters for high-temperature furnaces, Na2MoO4 is used in the production of paints and varnishes. Molybdenum telluride is a very good thermoelectric material for the production of thermoelectric generators. Many compounds of the forty-second element (sulfides, oxides, molybdates) are good catalysts for chemical reactions, and are also part of pigment dyes and glazes.
Production
Initially, molybdenum ores are enriched, for which the flotation method is used, based on the different surface wettability of minerals with water. The finely divided ore is treated with water with the addition of a small amount of a flotation agent, which enhances the difference in wettability of the ore mineral particles and gangue. Air is intensively blown through the resulting mixture; at the same time, its bubbles stick to the grains of those minerals that are worse wetted. These minerals are carried along with air bubbles to the surface and thus separated from the waste rock.
The molybdenum concentrate enriched in this way contains 47-50% of molybdenum itself, 28-32% of sulfur, 1-9% of SiO2, in addition, there are impurities of other elements: iron, copper, calcium and others. The concentrate is subjected to oxidative roasting at a temperature of 560-600 °C in multi-hearth furnaces or fluidized bed furnaces. In the presence of rhenium in the concentrate during firing, volatile oxide Re2O7 is formed, which is removed together with furnace gases. The roasting product is the so-called "calcine" - contaminated with MoO3 impurities.
Pure MoO3, necessary for the production of metallic molybdenum, is obtained from the cinder in two ways. The first is sublimation at a temperature of about 1000 ° C, the second is a chemical method, in which the cinder is leached with ammonia water. In this case, molybdenum goes into solution (ammonium molybdate). The solution is purified from impurities of copper, iron and other elements, then by neutralization or evaporation and subsequent crystallization, ammonium polymolybdates are isolated - mainly paramolybdate (NH4)6Mo7O24 4H2O. After that, by calcining ammonium paramolybdate at 450-500 ° C, pure MoO3 is obtained containing no more than 0.05% impurities.
It happens that instead of roasting, the molybdenum concentrate is decomposed with nitric acid, while the resulting molybdic acid MoO3 ∙ H2O is precipitated, which is dissolved in ammonia water and ammonium paramolybdate is obtained. A certain proportion of the forty-second element remains in the primary solution, from which molybdenum is extracted by ion exchange or extraction. During the processing of low-grade concentrates, which contain 10-20% molybdenum, cinders leach Na2CO3, CaMoO4 used in ferrous metallurgy is precipitated from the resulting Na2MoO4 solutions. By another method, using ion exchange or liquid extraction, the Na2MoO4 solution is transferred to the (NH4)2MoO4 solution, from which ammonium paramolybdate is then isolated.
By reducing pure MoO3 in a stream of dry hydrogen, metallic molybdenum is obtained (in powder form). The process is carried out in tube furnaces in two stages: the first at a temperature of 550-700 °C, the second at 900-1,000 °C.
Compact molybdenum is produced mainly by powder metallurgy or smelting. The powder metallurgy method consists in pressing the powder into a workpiece and sintering the workpiece. Molybdenum powder is pressed in steel molds under a pressure of 0.2-0.3 MPa (2000-3000 kgf / cm2), then sintered first at 1,000-1,200 ° C in a hydrogen atmosphere - preliminary sintering, the purpose of which is to increase strength and electrical conductivity of the rods, and then at 2200-2400 °C - high-temperature sintering. As a result, relatively small workpieces are obtained (with a cross section of 2–9 cm2 and a length of 450–600 mm). The resulting blanks (sintered rods) are pressure treated (forging, broaching, rolling). To obtain larger blanks, arc melting is used, which makes it possible to obtain ingots weighing up to two tons. Melting in arc furnaces is carried out in a vacuum. An arc is ignited between the cathode (package of sintered molybdenum rods) and the anode (cooled copper crucible). The cathode metal is melted and collected in the crucible. Due to the high thermal conductivity of copper and the rapid removal of heat, molybdenum hardens.
To obtain highly pure molybdenum, melting in an electron beam (electron beam melting) is used. The heating of a metal by an electron beam is based on the conversion of the greater part of the kinetic energy of electrons into heat when they collide with the surface of the metal. Melting is carried out in a high vacuum, which ensures the removal of impurities that evaporate at the melting temperature (O, N, P, As, Fe, Cu, Ni, and others). After such melting, the purity of the metal exceeds 99.9%.
A promising method for the production of molybdenum by the aluminothermic reduction of MoO3, the ingots obtained by this method are refined by vacuum melting in arc furnaces. In addition, molybdenum is obtained by reduction of MoF6 or MoCl5 with hydrogen, as well as electrolytically in salt melts. For the production of ferromolybdenum (an alloy of 55-70% Mo, the rest Fe), which serves to introduce additives of the forty-second element into steel, the reduction of calcined molybdenite concentrate (calcine) with ferrosilicon in the presence of iron ore and steel chips is used.
Physical properties
Molybdenum is a light gray metal. However, its appearance largely depends on the method of obtaining. Compacted (sintered) molybdenum without processing (in the form of a rod and blanks for molybdenum rolling) is a rather dark metal, traces of oxidation are allowed. Compact rolled (in the form of ingots, wire or sheets) metal after processing is of various colors: from dark, almost black, to silver-faded (mirror). The color depends on the processing method: turning, grinding, chemical cleaning (etching) and electropolishing. Molybdenum, obtained in the form of a mirror (decomposition) - shiny, but gray. Powdered forty-second element has a dark gray color.
Molybdenum crystallizes in a cubic body-centered lattice with a period a = 0.314 nm, z = 2. Atomic radius 1.4 A, ionic radii Mo4+ 0.68 A, Mo6+ 0.62 A. The forty-second element belongs to refractory metals with a melting point 2620° C and boiling point - 4639°C. Only tungsten (about 3400°C), rhenium (about 3190°C) and tantalum (3000°C) have higher melting points. The density of molybdenum is 10.2 g/cm3, which is comparable to the density of silver (10.5 g/cm3), the Mohs scale defines its hardness as 5.5 points. The specific heat capacity of molybdenum at 20-100 °C is 0.272 KJ/(kg K), i.e. 0.065 cal/(g deg). The thermal conductivity at 20 ° C for the forty-second element is 146.65 W / (m K), that is, 0.35 cal / (cm sec deg). Thermal coefficient of linear expansion (5.8-6.2) 10-6 at 25-700 °C. After examining the physical properties of the forty-second element, scientists found that the metal has a negligible coefficient of thermal expansion (about 30% of the expansion coefficient of copper). When heated from 25 to 500 ° C, the dimensions of the molybdenum part will increase by only 0.0000055 of the original value. Even when heated above 1,200 °C, molybdenum barely expands. This property has played an important role in electrovacuum technology.
Molybdenum is paramagnetic, its atomic magnetic susceptibility is approximately equal to 90 10-6 (at 20 °C). Electrical resistivity 5.2 10-8 ohm m, i.e. 5.2 10-6 ohm cm; work function of electrons 4.37 eV. The transition temperature to the superconducting state is 0.916 K. Molybdenum is a good conductor of electricity, in this parameter it is only three times inferior to silver. However, its electrical conductivity is higher than that of iron, nickel, platinum and many other metals.
Molybdenum is a malleable and ductile metal and is a transition element. As with a number of other metals, the mechanical properties are determined by the purity of the metal and the previous mechanical and heat treatment (the purer the metal, the softer it is). The presence of impurities increases the hardness and brittleness of the metal. So, when contaminated with nitrogen, carbon or sulfur, molybdenum, like chromium, becomes brittle, hard, brittle, which makes it difficult to process. In a completely pure state, compact molybdenum is ductile, malleable and malleable, quite easily subjected to stamping and rolling. The strength characteristics of molybdenum at high temperatures (but not in an oxidizing environment) exceed the strength of most other metals. For the sintered molybdenum rod, the Brinell hardness is 1500-1600 MN/m2, i.e. 150-160 kgf/mm2. For forged bar - 2000-2300 Mn/m2; for annealed wire - 1400-1850 Mn/m2. In terms of strength, molybdenum is somewhat inferior to tungsten, but it is more ductile, more easily amenable to both mechanical processing and pressure processing. In addition, recrystallizing annealing does not lead to metal brittleness. The metal, like its alloys, is characterized by a high modulus of elasticity (285–300 GPa), a low thermal neutron capture cross section (which makes it possible to use it as a structural material in nuclear reactors), good thermal stability, and a low thermal expansion coefficient.
Despite the many advantages of the forty-second element associated with its physical and mechanical properties, it also has a number of disadvantages. These include a small amount of molybdenum scale; high fragility of its compounds; low plasticity at low temperatures. In addition, the presence of impurities of carbon, nitrogen or sulfur makes the metal hard, brittle and brittle, which greatly complicates its processing.
Chemical properties
In air at room temperature, molybdenum is resistant to oxidation. A sluggish reaction with oxygen begins at 400°C (the so-called tint colors appear), at 600°C the metal begins to actively oxidize with the formation of MoO3 trioxide (white crystals with a greenish tint, tmelt 795°C, tbp 1 155°C), which is possible also obtained by oxidation of molybdenum disulfide MoS2 and thermolysis of ammonium paramolybdate (NH4)6Mo7O24 4H2O.
At temperatures above 700 ° C, the forty-second element interacts intensively with water vapor, forming MoO2 dioxide (dark brown). In addition to the above two oxides, molybdenum also forms a number of oxides intermediate between MoO3 and MoO2, which correspond in composition to the homologous series MonO3n-1 (Mo9O26, Mo8O23, Mo4O11), however, all of them are thermally unstable and above 700 ° C decompose with the formation of MoO3 and MoO2. MoO3 oxide forms simple (normal) molybdenum acids - H2MoO4 H2O dihydrate, H2MoO4 monohydrate and isopoly acids - H6Mo7O24, HMo6O24, H4Mo8O26 and others.
Molybdenum does not chemically interact with hydrogen until melting. However, when the metal is heated in hydrogen, some gas absorption occurs (at 1000 ° C, 0.5 cm3 of hydrogen is absorbed in one hundred grams of molybdenum) with the formation of a solid solution. With nitrogen, molybdenum above 1500 °C forms a nitride, the probable composition of which is Mo2N. Solid carbon and hydrocarbons, as well as carbon monoxide CO (II) at 1 100-1 200 ° C interact with the metal to form Mo2C carbide, which melts with decomposition at 2 400 ° C approximately.
Molybdenum reacts with silicon to form disilicide MoSi2 (dark gray crystals do not dissolve in water, hydrochloric acid, H2SO4, decompose in a mixture of HNO3 with hydrofluoric acid), which is highly stable in air up to 1500-1600 ° C (its microhardness is 14 100 Mn /m2). When the forty-second element interacts with selenium or H2Se vapor, molybdenum diselenide of composition MoSe2 (dark gray substance with a layered structure) is obtained, decomposes in vacuum at a temperature of 900 ° C, does not dissolve in water, HNO3 is oxidized. Under the action of sulfur and hydrogen sulfide vapors above 440 and 800 °C, respectively, graphite-like disulfide MoS2 is formed (practically insoluble in water, hydrochloric acid, diluted H2SO4). MoS2 decomposes above 1200 °C to form Mo2S3.
In addition to it, molybdenum forms three more compounds with sulfur, obtained only artificially: MoS3, Mo2S5 and Mo2S3. Mo2S3 sesquisulfide (gray needle-shaped crystals) is formed by rapidly heating the disulfide to 1700 ... 1800 ° C. Molybdenum penta- (Mo2S5) and trisulfide (MoS3) are dark brown amorphous substances. In addition to MoS2, only MoS3 is practically used. With halogens, the forty-second element forms a number of compounds in different oxidation states. Fluorine acts on molybdenum at ordinary temperature, chlorine at 250°C, forming MoF6 and MoCl6, respectively. With iodine, only molybdenum diiodide MoI2 is known. Molybdenum forms oxyhalides: MoOF4, MoOCl4, MoO2F2, MoO2Cl2, MoO2Br2, MoOBr3 and others.
In sulfuric and hydrochloric acids, molybdenum is slightly soluble only at 80-100 ° C. Nitric acid, aqua regia and hydrogen peroxide slowly dissolve the metal in the cold, quickly - when heated. Well dissolves molybdenum a mixture of nitric and sulfuric acids. The metal dissolves in hydrogen peroxide to form peroxo acids H2MoO6 and H2MoO11. Molybdenum is stable in hydrofluoric acid, but quickly dissolves in its mixture with nitric acid. In cold solutions of alkalis, molybdenum is stable, but is somewhat corroded by hot solutions. The metal is intensively oxidized by molten alkalis, especially in the presence of oxidizing agents, forming salts of molybdic acid.
