The content of the article

CHROMIUM– (Chromium) Cr, chemical element 6(VIb) of group of the Periodic table. Atomic number 24, atomic mass 51.996. There are 24 known isotopes of chromium from 42 Cr to 66 Cr. The isotopes 52 Cr, 53 Cr, 54 Cr are stable. Isotopic composition of natural chromium: 50 Cr (half-life 1.8 10 17 years) – 4.345%, 52 Cr – 83.489%, 53 Cr – 9.501%, 54 Cr – 2.365%. The main oxidation states are +3 and +6.

In 1761, chemistry professor at St. Petersburg University Johann Gottlob Lehmann, at the eastern foot of the Ural Mountains at the Berezovsky mine, discovered a wonderful red mineral, which, when crushed into powder, gave a bright yellow color. In 1766 Lehman brought samples of the mineral to St. Petersburg. Having treated the crystals with hydrochloric acid, he obtained a white precipitate, in which he discovered lead. Lehmann called the mineral Siberian red lead (plomb rouge de Sibérie); it is now known that it was crocoite (from the Greek “krokos” - saffron) - a natural lead chromate PbCrO 4.

The German traveler and naturalist Peter Simon Pallas (1741–1811) led an expedition of the St. Petersburg Academy of Sciences to the central regions of Russia and in 1770 visited the Southern and Middle Urals, including the Berezovsky mine and, like Lehmann, became interested in crocoite. Pallas wrote: “This amazing red lead mineral is not found in any other deposit. When ground into powder it turns yellow and can be used in artistic miniatures.” Despite the rarity and difficulty of delivering crocoite from the Berezovsky mine to Europe (it took almost two years), the use of the mineral as a coloring agent was appreciated. In London and Paris at the end of the 17th century. all noble persons rode in carriages painted with finely ground crocoite; in addition, the best examples of Siberian red lead replenished the collections of many mineralogical cabinets in Europe.

In 1796, a sample of crocoite came to the professor of chemistry at the Paris Mineralogical School, Nicolas-Louis Vauquelin (1763–1829), who analyzed the mineral, but found nothing in it except oxides of lead, iron and aluminum. Continuing his research on Siberian red lead, Vaukelin boiled the mineral with a solution of potash and, after separating the white precipitate of lead carbonate, obtained a yellow solution of an unknown salt. When treated with lead salt, a yellow precipitate was formed, with mercury salt, a red one, and when tin chloride was added, the solution became green. By decomposing crocoite with mineral acids, he obtained a solution of “red lead acid,” the evaporation of which gave ruby-red crystals (it is now clear that it was chromic anhydride). Having calcined them with coal in a graphite crucible, after the reaction I discovered many fused gray needle-shaped crystals of a metal unknown to that time. Vaukelin noted the high refractoriness of the metal and its resistance to acids.

Vaukelin named the new element chromium (from the Greek crwma - color, color) due to the many multi-colored compounds it forms. Based on his research, Vauquelin was the first to state that the emerald color of some precious stones is explained by the admixture of chromium compounds in them. For example, natural emerald is a deep green colored beryl in which aluminum is partially replaced by chromium.

Most likely, Vauquelin obtained not pure metal, but its carbides, as evidenced by the needle-shaped shape of the resulting crystals, but the Paris Academy of Sciences nevertheless registered the discovery of a new element, and now Vauquelin is rightly considered the discoverer of element No. 24.

Yuri Krutyakov

Chromium is a chemical element with atomic number 24. It is a hard, shiny, steel-gray metal that polishes well and does not tarnish. Used in alloys such as stainless steel and as a coating. The human body requires small amounts of trivalent chromium to metabolize sugar, but Cr(VI) is highly toxic.

Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and used in paints and pigments. The red color of ruby ​​is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (together with sulfuric acid) to clean laboratory glassware. In addition, chromium (VI) oxide is used in the production of magnetic tape.

Discovery and etymology

The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it “Siberian red lead.” Although it was erroneously identified as a compound of lead with selenium and iron, the material was actually lead chromate with the chemical formula PbCrO 4 . Today it is known as the mineral croconte.

In 1770, Peter Simon Pallas visited the site where Lehmann found the red lead mineral, which had very useful properties as a pigment in paints. The use of Siberian red lead as paint developed rapidly. In addition, the bright yellow color of crocont has become fashionable.

In 1797, Nicolas-Louis Vauquelin obtained samples of red. By mixing croconte with hydrochloric acid, he obtained CrO 3 oxide. Chromium was isolated as a chemical element in 1798. Vauquelin obtained it by heating the oxide with charcoal. He was also able to detect traces of chromium in gemstones such as ruby ​​and emerald.

In the 1800s, Cr was primarily used in dyes and tanning salts. Today, 85% of the metal is used in alloys. The remainder is used in the chemical, refractory and foundry industries.

The pronunciation of the chemical element chromium corresponds to the Greek χρῶμα, meaning "color", due to the variety of colored compounds that can be obtained from it.

Extraction and production

The element is produced from chromite (FeCr 2 O 4). About half of the world's ore is mined in South Africa. In addition, Kazakhstan, India and Türkiye are its major producers. There are enough explored deposits of chromite, but geographically they are concentrated in Kazakhstan and southern Africa.

Deposits of native chromium metal are rare, but they do exist. For example, it is mined at the Udachnaya mine in Russia. It is rich in diamonds, and the reducing environment helped produce pure chromium and diamonds.

For industrial metal production, chromite ores are treated with molten alkali (caustic soda, NaOH). In this case, sodium chromate (Na 2 CrO 4) is formed, which is reduced by carbon to the oxide Cr 2 O 3. The metal is produced by heating the oxide in the presence of aluminum or silicon.

In 2000, approximately 15 million tons of chromite ore were mined and processed into 4 million tons of ferrochrome, a 70% chromium-iron alloy, with an approximate market value of US$2.5 billion.

Main characteristics

The characteristics of the chemical element chromium are due to the fact that it is a transition metal of the fourth period of the periodic table and is located between vanadium and manganese. Included in group VI. Melts at a temperature of 1907 °C. In the presence of oxygen, chromium quickly forms a thin layer of oxide, which protects the metal from further interaction with oxygen.

As a transition element, it reacts with substances in different proportions. Thus, it forms compounds in which it has different oxidation states. Chromium is a chemical element with the basic states +2, +3 and +6, of which +3 is the most stable. In addition, in rare cases conditions +1, +4 and +5 are observed. Chromium compounds in the +6 oxidation state are strong oxidizing agents.

What color is chrome? The chemical element gives the ruby ​​hue. The Cr 2 O 3 used for is also used as a pigment called chrome green. Its salts color glass emerald green. Chromium is the chemical element whose presence makes rubies red. Therefore, it is used in the production of synthetic rubies.

Isotopes

Isotopes of chromium have atomic weights ranging from 43 to 67. Typically, this chemical element consists of three stable forms: 52 Cr, 53 Cr and 54 Cr. Of these, 52 Cr is the most common (83.8% of all natural chromium). In addition, 19 radioisotopes have been described, of which the most stable is 50 Cr with a half-life exceeding 1.8x10 17 years. 51 Cr has a half-life of 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, and for most of them it lasts less than one minute. The element also has two meta states.

Isotopes of chromium in the earth's crust, as a rule, accompany isotopes of manganese, which is used in geology. 53 Cr is formed during the radioactive decay of 53 Mn. The Mn/Cr isotope ratio reinforces other clues about the early history of the Solar System. Changes in the 53 Cr/ 52 Cr and Mn/Cr ratios from different meteorites prove that new atomic nuclei were created just before the formation of the Solar System.

Chemical element chromium: properties, formula of compounds

Chromium(III) oxide Cr 2 O 3, also known as sesquioxide, is one of the four oxides of this chemical element. It is obtained from chromite. The green color compound is commonly called "chrome green" when used as a pigment for enamel and glass painting. The oxide can dissolve in acids, forming salts, and in molten alkali - chromites.

Potassium dichromate

K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as a means for cleaning laboratory glassware from organic matter. For this purpose, its saturated solution is used. Sometimes, however, it is replaced with sodium bichromate, based on the higher solubility of the latter. In addition, it can regulate the oxidation process of organic compounds, converting primary alcohol into aldehyde and then into carbon dioxide.

Potassium dichromate can cause chrome dermatitis. Chromium is likely to cause sensitization leading to the development of dermatitis, especially of the hands and forearms, which is chronic and difficult to cure. Like other Cr(VI) compounds, potassium bichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.

Chromic acid

The compound has the hypothetical structure H 2 CrO 4 . Neither chromic nor dichromic acids occur in nature, but their anions are found in various substances. The “chromic acid” that can be found on sale is actually its acid anhydride - CrO 3 trioxide.

Lead(II) chromate

PbCrO 4 has a bright yellow color and is practically insoluble in water. For this reason, it has found use as a coloring pigment called crown yellow.

Cr and pentavalent bond

Chromium is distinguished by its ability to form pentavalent bonds. The compound is created by Cr(I) and a hydrocarbon radical. A pentavalent bond is formed between two chromium atoms. Its formula can be written as Ar-Cr-Cr-Ar, where Ar represents a specific aromatic group.

Application

Chromium is a chemical element whose properties have given it many different uses, some of which are listed below.

It gives metals corrosion resistance and a glossy surface. Therefore, chromium is included in alloys such as stainless steel, used, for example, in cutlery. It is also used for chrome plating.

Chromium is a catalyst for various reactions. It is used to make molds for firing bricks. Its salts are used to tan leather. Potassium bichromate is used for the oxidation of organic compounds such as alcohols and aldehydes, as well as for cleaning laboratory glassware. It serves as a fixing agent for fabric dyeing and is also used in photography and photo printing.

CrO 3 is used to make magnetic tapes (for example, for audio recording), which have better characteristics than films with iron oxide.

Role in biology

Trivalent chromium is a chemical element necessary for the metabolism of sugar in the human body. In contrast, hexavalent Cr is highly toxic.

Precautionary measures

Chromium metal and Cr(III) compounds are generally not considered a health hazard, but substances containing Cr(VI) can be toxic if ingested or inhaled. Most of these substances are irritating to the eyes, skin and mucous membranes. With chronic exposure, chromium(VI) compounds can cause eye damage if not treated properly. In addition, it is a recognized carcinogen. The lethal dose of this chemical element is about half a teaspoon. According to the recommendations of the World Health Organization, the maximum permissible concentration of Cr (VI) in drinking water is 0.05 mg per liter.

Because chromium compounds are used in dyes and to tan leather, they are often found in soil and groundwater from abandoned industrial sites requiring environmental cleanup and remediation. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.

Element properties

The main physical properties of chromium are as follows:

• Atomic number: 24.
• Atomic weight: 51.996.
• Melting point: 1890 °C.
• Boiling point: 2482 °C.
• Oxidation state: +2, +3, +6.
• Electron configuration: 3d 5 4s 1.

"National Research Tomsk Polytechnic University"

Institute of Natural Resources Geoecology and Geochemistry

Chromium

By discipline:

Chemistry

Completed:

student of group 2G41 Tkacheva Anastasia Vladimirovna 10.29.2014

Checked:

teacher Stas Nikolay Fedorovich

Position in the periodic table

Chromium- element of the side subgroup of the 6th group of the 4th period of the periodic system of chemical elements of D. I. Mendeleev with atomic number 24. Denoted by the symbol Cr(lat. Chromium). Simple substance chromium- hard metal of bluish-white color. Chrome is sometimes classified as a ferrous metal.

Atomic structure

17 Cl)2)8)7 - atomic structure diagram

1s2s2p3s3p - electronic formula

The atom is located in the III period, and has three energy levels

The atom is located in group VII, in the main subgroup - at the outer energy level 7 electrons

Element properties

Physical properties

Chrome is a white shiny metal with a cubic body-centered lattice, a = 0.28845 nm, characterized by hardness and brittleness, with a density of 7.2 g/cm 3, one of the hardest pure metals (second only to beryllium, tungsten and uranium), with a melting point of 1903 degrees. And with a boiling point of about 2570 degrees. C. In air, the surface of chromium is covered with an oxide film, which protects it from further oxidation. Adding carbon to chromium further increases its hardness.

Chemical properties

Chromium is an inert metal under normal conditions, but when heated it becomes quite active.

Interaction with non-metals

When heated above 600°C, chromium burns in oxygen:

4Cr + 3O 2 = 2Cr 2 O 3.

Reacts with fluorine at 350°C, with chlorine at 300°C, with bromine at red heat, forming chromium (III) halides:

2Cr + 3Cl2 = 2CrCl3.

Reacts with nitrogen at temperatures above 1000°C to form nitrides:

2Cr + N 2 = 2CrN

or 4Cr + N 2 = 2Cr 2 N.

2Cr + 3S = Cr 2 S 3.

Reacts with boron, carbon and silicon to form borides, carbides and silicides:

Cr + 2B = CrB 2 (possible formation of Cr 2 B, CrB, Cr 3 B 4, CrB 4),

2Cr + 3C = Cr 2 C 3 (possible formation of Cr 23 C 6, Cr 7 B 3),

Cr + 2Si = CrSi 2 (possible formation of Cr 3 Si, Cr 5 Si 3, CrSi).

Does not interact directly with hydrogen.

Interaction with water

When finely ground and hot, chromium reacts with water to form chromium(III) oxide and hydrogen:

2Cr + 3H 2 O = Cr 2 O 3 + 3H 2

Interaction with acids

In the electrochemical voltage series of metals, chromium is located before hydrogen; it displaces hydrogen from solutions of non-oxidizing acids:

Cr + 2HCl = CrCl 2 + H 2;

Cr + H 2 SO 4 = CrSO 4 + H 2.

In the presence of atmospheric oxygen, chromium (III) salts are formed:

4Cr + 12HCl + 3O 2 = 4CrCl 3 + 6H 2 O.

Concentrated nitric and sulfuric acids passivate chromium. Chromium can dissolve in them only with strong heating; chromium (III) salts and acid reduction products are formed:

2Cr + 6H 2 SO 4 = Cr 2 (SO 4) 3 + 3SO 2 + 6H 2 O;

Cr + 6HNO 3 = Cr(NO 3) 3 + 3NO 2 + 3H 2 O.

Interaction with alkaline reagents

Chromium does not dissolve in aqueous solutions of alkalis; it slowly reacts with alkali melts to form chromites and release hydrogen:

2Cr + 6KOH = 2KCrO 2 + 2K 2 O + 3H 2.

Reacts with alkaline melts of oxidizing agents, for example potassium chlorate, and chromium is converted into potassium chromate:

Cr + KClO 3 + 2KOH = K 2 CrO 4 + KCl + H 2 O.

Recovery of metals from oxides and salts

Chromium is an active metal, capable of displacing metals from solutions of their salts: 2Cr + 3CuCl 2 = 2CrCl 3 + 3Cu.

Properties of a simple substance

Stable in air due to passivation. For the same reason, it does not react with sulfuric and nitric acids. At 2000 °C it burns to form green chromium(III) oxide Cr 2 O 3, which has amphoteric properties.

Compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), were synthesized. with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).

Cr(+2) compounds

The oxidation state +2 corresponds to the basic oxide CrO (black). Cr 2+ salts (blue solutions) are obtained by reducing Cr 3+ salts or dichromates with zinc in an acidic medium (“hydrogen at the time of release”):

All these Cr 2+ salts are strong reducing agents, to the point that when standing, they displace hydrogen from water. Oxygen in the air, especially in an acidic environment, oxidizes Cr 2+, as a result of which the blue solution quickly turns green.

Brown or yellow hydroxide Cr(OH) 2 precipitates when alkalis are added to solutions of chromium(II) salts.

Chromium dihalides CrF 2, CrCl 2, CrBr 2 and CrI 2 were synthesized

Cr(+3) compounds

The oxidation state +3 corresponds to the amphoteric oxide Cr 2 O 3 and hydroxide Cr (OH) 3 (both green). This is the most stable oxidation state of chromium. Chromium compounds in this oxidation state range in color from dirty purple (3+ ion) to green (anions are present in the coordination sphere).

Cr 3+ is prone to the formation of double sulfates of the form M I Cr(SO 4) 2 12H 2 O (alum)

Chromium (III) hydroxide is obtained by reacting ammonia with solutions of chromium (III) salts:

Cr+3NH+3H2O→Cr(OH)↓+3NH

You can use alkali solutions, but in their excess a soluble hydroxo complex is formed:

Cr+3OH→Cr(OH)↓

Cr(OH)+3OH→

By fusing Cr 2 O 3 with alkalis, chromites are obtained:

Cr2O3+2NaOH→2NaCrO2+H2O

Uncalcined chromium(III) oxide dissolves in alkaline solutions and acids:

Cr2O3+6HCl→2CrCl3+3H2O

When chromium(III) compounds are oxidized in an alkaline medium, chromium(VI) compounds are formed:

2Na+3HO→2NaCrO+2NaOH+8HO

The same thing happens when chromium (III) oxide is fused with alkali and oxidizing agents, or with alkali in air (the melt acquires a yellow color):

2Cr2O3+8NaOH+3O2→4Na2CrO4+4H2O

Chromium compounds (+4)[

By careful decomposition of chromium(VI) oxide CrO 3 under hydrothermal conditions, chromium(IV) oxide CrO 2 is obtained, which is ferromagnetic and has metallic conductivity.

Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapors.

Chromium compounds (+6)

The oxidation state +6 corresponds to the acidic chromium (VI) oxide CrO 3 and a number of acids, between which there is an equilibrium. The simplest of them are chromium H 2 CrO 4 and dichromium H 2 Cr 2 O 7 . They form two series of salts: yellow chromates and orange dichromates, respectively.

Chromium (VI) oxide CrO 3 is formed by the interaction of concentrated sulfuric acid with solutions of dichromates. A typical acidic oxide, when interacting with water it forms strong unstable chromic acids: chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and other isopoly acids with the general formula H 2 Cr n O 3n+1. An increase in the degree of polymerization occurs with a decrease in pH, that is, an increase in acidity.

The article is devoted to element No. 24 of the periodic table - chromium, the history of its discovery and distribution in nature, the structure of its atom, chemical properties and compounds, how it is obtained and why we need it. The average chromium content in the earth's crust is not high: 0.0083%. This element is probably more characteristic of the Earth's mantle.

Chromium forms massive and disseminated ores in ultramafic rocks; The formation of the largest chromium deposits is associated with them. In basic rocks, the Chromium content reaches only 2·10-2%, in acidic rocks - 2.5·10-3%, in sedimentary rocks (sandstones) - 3.5·10-3%, in clay shales - 9·10-3 %. Chromium is a relatively weak water migrant: the Chromium content in sea water is 0.00005 mg/l, in surface water -0.0015 mg/l.
In general, chromium is a metal in the deep zones of the Earth.

Today, the total consumption of pure chromium (at least 99% Cr) is about 15 thousand tons, of which about a third is electrolytic chromium. The world leader in the production of high-purity chromium is the English company Bell Metals. The first place in terms of consumption volumes is occupied by the United States (50%), European countries second (25%), and Japan third. The market for chromium metal is quite volatile, and prices for the metal fluctuate widely.

1. CHROME AS A CHEMICAL ELEMENT

Chromium– (Chromium) Cr, chemical element 6(VIb) of group of the Periodic table. Atomic number 24, atomic mass 51.996. There are 24 known isotopes of chromium from 42 Cr to 66 Cr. The isotopes 52 Cr, 53 Cr, 54 Cr are stable. Isotopic composition of natural chromium: 50 Cr (half-life 1.8 10 17 years) – 4.345%, 52 Cr – 83.489%, 53 Cr – 9.501%, 54 Cr – 2.365%. The main oxidation states are +3 and +6.

In 1761, chemistry professor at St. Petersburg University Johann Gottlob Lehmann, at the eastern foot of the Ural Mountains at the Berezovsky mine, discovered a wonderful red mineral, which, when crushed into powder, gave a bright yellow color. In 1766 Lehman brought samples of the mineral to St. Petersburg. Having treated the crystals with hydrochloric acid, he obtained a white precipitate, in which he discovered lead. Lehmann called the mineral Siberian red lead (plomb rouge de Sibérie); it is now known that it was crocoite (from the Greek “krokos” - saffron) - a natural lead chromate PbCrO 4.

The German traveler and naturalist Peter Simon Pallas (1741–1811) led an expedition of the St. Petersburg Academy of Sciences to the central regions of Russia and in 1770 visited the Southern and Middle Urals, including the Berezovsky mine and, like Lehmann, became interested in crocoite. Pallas wrote: “This amazing red lead mineral is not found in any other deposit. When ground into powder it turns yellow and can be used in artistic miniatures.” Despite the rarity and difficulty of delivering crocoite from the Berezovsky mine to Europe (it took almost two years), the use of the mineral as a coloring agent was appreciated. In London and Paris at the end of the 17th century. all noble persons rode in carriages painted with finely ground crocoite; in addition, the best examples of Siberian red lead replenished the collections of many mineralogical cabinets in Europe.

In 1796, a sample of crocoite came to the professor of chemistry at the Paris Mineralogical School, Nicolas-Louis Vauquelin (1763–1829), who analyzed the mineral, but found nothing in it except oxides of lead, iron and aluminum. Continuing his research on Siberian red lead, Vaukelin boiled the mineral with a solution of potash and, after separating the white precipitate of lead carbonate, obtained a yellow solution of an unknown salt. When treated with lead salt, a yellow precipitate was formed, with mercury salt, a red one, and when tin chloride was added, the solution became green. By decomposing crocoite with mineral acids, he obtained a solution of “red lead acid,” the evaporation of which gave ruby-red crystals (it is now clear that it was chromic anhydride). Having calcined them with coal in a graphite crucible, after the reaction I discovered many fused gray needle-shaped crystals of a metal unknown to that time. Vaukelin noted the high refractoriness of the metal and its resistance to acids.

Vaukelin named the new element chromium (from the Greek  - color, color) due to the many multi-colored compounds it forms. Based on his research, Vauquelin was the first to state that the emerald color of some precious stones is explained by the admixture of chromium compounds in them. For example, natural emerald is a deep green colored beryl in which aluminum is partially replaced by chromium.

Most likely, Vauquelin obtained not pure metal, but its carbides, as evidenced by the needle-shaped shape of the resulting crystals, but the Paris Academy of Sciences nevertheless registered the discovery of a new element, and now Vauquelin is rightly considered the discoverer of element No. 24.

In 1798, Lowitz and Klaproth, independently of Vaukelin, discovered chromium in a sample of a heavy black mineral (it was chromite FeCr 2 O 4), found in the Urals, but much north of the Berezovsky deposit. In 1799, F. Tassaert discovered a new element in the same mineral found in southeastern France. It is believed that it was Tassert who first managed to obtain relatively pure metal chromium.

2. CHROME IN NATURE AND ITS INDUSTRIAL EXTRACTION

Chromium is a fairly common element on Earth. Its clarke (average content in the earth’s crust) is 8.3·10–3%. Chromium is never found in a free state. In chromium ores, only chromite FeCr 2 O 4 is of practical importance, which belongs to spinels - isomorphic minerals of the cubic system with the general formula MO·Me 2 O 3, where M is a divalent metal ion, and Me is a trivalent metal ion. Spinels can form solid solutions with each other, therefore, in nature, separately or as impurities to chromite, magnochromite (Mg,Fe)Cr 2 O 4, aluminum chromite Fe(Cr,Al) 2 O 4, chromopicotite (Mg,Fe) are also found. Cr,Al) 2 O 4 - all of them belong to the class of chrome spinels. In addition to spinels, chromium is found in many much less common minerals, for example, melanochroite 3PbO 2Cr 2 O 3, vokelenite 2(Pb,Cu)CrO 4 (Pb,Cu) 3 (PO 4) 2, tarapacaite K 2 CrO 4, ditzeite CaIO 3 ·CaCrO 4 and others.

Chromites are dark or almost black in color, have a metallic luster and usually occur in the form of continuous masses. Chromite deposits are of igneous origin. Its identified resources are estimated in 47 countries and amount to 15 billion tons. The first place in terms of chromite reserves is occupied by South Africa (76% of proven world reserves), where the group of Bushveld deposits is of greatest importance, the content of chrome ore is 1 billion tons. Kazakhstan ranks second in the world in terms of chromite resources (9% of world reserves); chromium ores there are of very high quality. All chromite resources in Kazakhstan are concentrated in the Aktobe region (Kempirsay massif with reserves of 300 million tons); the deposits have been developed since the late 1930s. Zimbabwe ranks third (6% of world reserves). In addition, the USA, India, the Philippines, Turkey, Madagascar, and Brazil have significant chromite resources. In Russia, quite large deposits of chromite are found in the Urals (Saranovskoye, Verblyuzhyegorskoye, Alapaevskoye, Monetnaya Dacha, Khalilovskoye and other deposits).

At the beginning of the 19th century. The main source of chromite was the Ural deposits, but in 1827 the American Isaac Tyson discovered a large deposit of chromium ore on the border of Maryland and Pennsylvania, becoming a monopolist in mining for many years. In 1848, deposits of high quality chromite were found in Turkey, near Bursa. After the depletion of reserves in Maryland, Turkey was the leader in chromite mining, until India and South Africa took over the baton in 1906.

Currently, 11–14 million tons of chromite are mined annually in the world. South Africa occupies the leading place in the production of chrome ore (about 6 million tons annually), followed by Kazakhstan, providing 20% ​​of world needs. Due to the great depth of chrome ore, it is usually mined by open-pit mining (85%), but open pit mining is also sometimes practiced, for example, in Finland and Madagascar. Typically, the mined ores are of fairly high quality and only require mechanical sorting. It is often impractical to enrich chromites, since this can only increase the Cr 2 O 3 content, and the Fe ratio : Cr remains unchanged. The price of chromite on the world market ranges from 40–120 US dollars per ton.

Chrome is a silvery metal with a density of 7200 kg/m3. Determining the melting point of pure chromium is an extremely difficult task, since the slightest impurities of oxygen or nitrogen significantly affect the value of this temperature. According to the results of modern measurements, it is equal to 1907° C. The boiling point of chromium is 2671° C. Absolutely pure (without gas impurities and carbon) chromium is quite viscous, malleable and malleable. At the slightest contamination with carbon, hydrogen, nitrogen, etc. becomes brittle, brittle and hard. At ordinary temperatures it exists in the form of an a-modification and has a body-centered cubic lattice. Chemically, chromium is quite inert due to the formation of a strong thin oxide film on its surface. It does not oxidize in air even in the presence of moisture, and when heated, oxidation occurs only on the surface. Chromium is passivated by dilute and concentrated nitric acid, aqua regia, and even when the metal is boiled with these reagents, it dissolves only slightly. Chromium passivated by nitric acid, unlike metal without a protective layer, does not dissolve in dilute sulfuric and hydrochloric acids, even after prolonged boiling in solutions of these acids; however, at a certain moment, rapid dissolution begins, accompanied by foaming from the liberated hydrogen - from the passive form chromium becomes activated, not protected by an oxide film:

Cr + 2HCl = CrCl 2 + H 2

If nitric acid is added during the dissolution process, the reaction immediately stops - the chromium is again passivated.

When heated, chromium metal combines with halogens, sulfur, silicon, boron, carbon and some other elements:

Cr + 2F 2 = CrF 4 (with an admixture of CrF 5)

2Cr + 3Cl2 = 2CrCl3

2Cr + 3S = Cr 2 S 3

Cr + C = mixture of Cr 23 C 6 + Cr 7 C 3.

When chromium is heated with molten soda in air, nitrates or chlorates of alkali metals, the corresponding chromates (VI) are obtained:

2Cr + 2Na 2 CO 3 + 3O 2 = 2Na 2 CrO 4 + 2CO 2.

Depending on the required degree of metal purity, there are several industrial methods for producing chromium.

Opportunity aluminothermic reduction of chromium(III) oxide was demonstrated by Friedrich Wöhler in 1859; however, this method became available on an industrial scale as soon as it became possible to obtain cheap aluminum. The industrial aluminothermic production of chromium began with the work of Goldschmidt, who was the first to develop a reliable method for regulating the highly exothermic (and therefore explosive) reduction process:

Cr 2 O 3 + 2Al = 2Cr + 2Al 2 O 3.

Previously, the mixture is uniformly heated to 500-600 ° C. Reduction can be initiated either by a mixture of barium peroxide with aluminum powder, or by igniting a small portion of the mixture, followed by adding the rest of the mixture. It is important that the heat released during the reaction is sufficient to melt the resulting chromium and separate it from the slag. Chromium produced by the aluminothermic process usually contains 0.015–0.02% C, 0.02% S and 0.25–0.40% Fe, and the mass fraction of the main substance in it is 99.1–99.4% Cr. It is very fragile and easily ground into powder.

To obtain high-purity chromium, electrolytic methods are used; the possibility of this was demonstrated in 1854 by Bunsen, who subjected an aqueous solution of chromium chloride to electrolysis. Now electrolysis is carried out using a mixture of chromic anhydride or chromoammonium alum with dilute sulfuric acid. The chromium released during electrolysis contains dissolved gases as impurities. Modern technologies make it possible to obtain metal with a purity of 99.90–99.995% on an industrial scale using high-temperature purification in a hydrogen flow and vacuum degassing. Unique methods for refining electrolytic chromium allow you to get rid of oxygen, sulfur, nitrogen and hydrogen contained in the “raw” product.

There are several other less significant ways to obtain chromium metal. Silicothermic reduction is based on the reaction:

2Cr 2 O 3 + 3Si + 3CaO = 4Cr + 3CaSiO 3.

Silicon reduction, although exothermic in nature, requires the process to be carried out in an arc furnace. The addition of quicklime allows you to convert refractory silicon dioxide into low-melting calcium silicate slag.

The reduction of chromium(III) oxide with coal is used to obtain high-carbon chromium intended for the production of special alloys. The process is also carried out in an electric arc furnace.

The Van Arkel-Kuchman-De Boer process uses the decomposition of chromium(III) iodide on a wire heated to 1100° C with the deposition of pure metal on it.

Chromium can also be obtained by the reduction of Cr 2 O 3 with hydrogen at 1500 ° C, the reduction of anhydrous CrCl 3 with hydrogen, alkali or alkaline earth metals, magnesium and zinc.

3. APPLICATION OF CHROME IN INDUSTRY

For many decades since the discovery of the metal chromium, only crocoite and some of its other compounds were used as pigments in the manufacture of paints. In 1820, Cochlen proposed the use of potassium dichromate as a mordant for dyeing fabrics. In 1884, the active use of soluble chromium compounds as tannins in the leather industry began. Chromite was first used in France in 1879 as a refractory substance, but its main use began in the 1880s in England and Sweden, when the industrial smelting of ferrochrome began to increase in speed. They were able to obtain ferrochrome in small quantities already at the beginning of the 19th century, so Berthier, back in 1821, proposed reducing a mixture of iron and chromium oxides with charcoal in a crucible. The first patent for the manufacture of chromium steel was issued in 1865. Industrial production of high-carbon ferrochrome began using blast furnaces to reduce chromite with coke. Ferrochrome late 19th century. was of very low quality, as it usually contained 7–8% chromium, and was known as “Tasmanian pig iron” due to the fact that the original iron-chromium ore was imported from Tasmania. The turning point in ferrochrome production came in 1893, when Henri Moissan first smelted high-carbon ferrochrome containing 60% Cr. The main achievement in this industry was the replacement of the blast furnace with an electric arc furnace, created by Moissan, which made it possible to increase the temperature of the process, reduce energy consumption and significantly improve the quality of the smelted ferrochrome, which began to contain 67–71% Cr and 4–6% C. Moissan’s method is still por lies at the basis of modern industrial production of ferrochrome. Chromite reduction is usually carried out in open electric arc furnaces, and the charge is loaded from above. An arc is formed between electrodes immersed in the charge.

Chromium occurs in nature mainly in the form of chromium iron ore Fe(CrO 2) 2 (iron chromite). Ferrochrome is obtained from it by reduction in electric furnaces with coke (carbon):

FeO Cr 2 O 3 + 4C → Fe + 2Cr + 4CO

6) using electrolysis, electrolytic chromium is obtained from a solution of chromic anhydride in water containing the addition of sulfuric acid. In this case, mainly 3 processes take place at the cathodes:

– reduction of hexavalent chromium to trivalent chromium with its transition into solution;

– discharge of hydrogen ions with the release of hydrogen gas;

– discharge of ions containing hexavalent chromium with precipitation of metallic chromium;

Cr 2 O 7 2− + 14Н + + 12е − = 2Сr + 7H 2 O

In its free form, it is a bluish-white metal with a body-centered cubic lattice, a = 0.28845 nm. At a temperature of 39 °C it changes from a paramagnetic state to an antiferromagnetic state (Néel point).

Stable in air. At 300 °C it burns to form green chromium(III) oxide Cr 2 O 3, which has amphoteric properties. By fusing Cr 2 O 3 with alkalis, chromites are obtained

Despite the great importance of high-carbon ferrochrome for the production of many types of stainless steels, it is not suitable for smelting some high-chromium steels, since the presence of carbon (in the form of Cr 23 C 6 carbide, crystallizing along the grain boundaries) makes them brittle and easily susceptible to corrosion. The production of low-carbon ferrochrome began to develop with the beginning of the use of industrial aluminothermic reduction of chromite. Nowadays, the aluminothermic process has been replaced by the silicothermic process (Perrin process) and the simplex process, which consists of mixing high-carbon ferrochrome with partially oxidized ferrochrome powder, subsequent briquetting and heating to 1360 ° C in a vacuum. Ferrochrome prepared by the simplex process typically contains only 0.008% carbon, and briquettes made from it are easily dissolved in molten steel.

The ferrochrome market is cyclical. World production of ferrochrome in 2000 was 4.8 million tons, and in 2001, due to low demand, 3.4 million tons. In 2002, the demand for ferrochrome intensified again. The first place in the world in ferrochrome smelting is occupied by the South African “Big Two” – Xstrata South Africa (Pty) Ltd. (a subsidiary of Xstrata AG) and Samancor Chrome Division (a subsidiary of Samancor Ltd.). They account for up to 40% of the world's ferrochrome smelting. In South Africa and Finland, mainly charge chrome is produced (from the English charge - load coal), containing 52–55% Cr, and in China, Russia, Zimbabwe, Kazakhstan, ferrochrome containing more than 60% Cr. Ferrochrome is used as an alloying additive for low alloy steels. With a chromium content of more than 12%, steel almost does not rust.

The corrosion resistance of iron alloys can be significantly increased by applying a thin layer of chromium to their surface. This procedure is called chrome plating. Chrome-plated layers are well resistant to exposure to humid atmosphere, sea air, tap water, nitric and many organic acids. All chromium plating methods can be divided into two types - diffusion and electrolytic. The Becker-Davies-Steinberg diffusion method involves heating a chrome-plated product to 1050–1100° C in a hydrogen atmosphere, filled with a mixture of ferrochrome and refractory, pre-treated with hydrogen chloride at 1050° C. The CrCl 2 located in the pores of the refractory evaporates and chromes the product. During the electrolytic chromium plating process, metal is deposited on the surface of the workpiece, which acts as a cathode. The electrolyte is often a hexavalent chromium compound (usually CrO 3 ) dissolved in aqueous H 2 SO 4 . Chrome coatings are either protective or decorative. The thickness of protective coatings reaches 0.1 mm; they are applied directly to the product and give it increased wear resistance. Decorative coatings have an aesthetic value and are applied to a sublayer of another metal (nickel or copper), which performs the actual protective function. The thickness of such a coating is only 0.0002–0.0005 mm.

4. BIOLOGICAL ROLE OF CHROME

Chromium is a trace element necessary for the normal development and functioning of the human body. It has been established that only trivalent chromium takes part in biochemical processes. Its most important biological role is the regulation of carbohydrate metabolism and blood glucose levels. Chromium is an integral part of a low-molecular complex - glucose tolerance factor (GTF), which facilitates the interaction of cellular receptors with insulin, thereby reducing the body's need for it. The tolerance factor enhances the action of insulin in all metabolic processes with its participation. In addition, chromium takes part in the regulation of cholesterol metabolism and is an activator of certain enzymes.

The chromium content in the human body is 6–12 mg. There is no exact information about a person’s physiological need for this element; in addition, it strongly depends on the nature of the diet (for example, it increases greatly with an excess of sugar in the diet). According to various estimates, the daily intake of chromium in the body is 20–300 mcg. An indicator of the body's supply of chromium is its content in the hair (the norm is 0.15–0.5 mcg/g). Unlike many microelements, the chromium content in body tissues (with the exception of the lungs) decreases as a person ages.

The concentration of the element in plant foods is an order of magnitude lower than its concentration in mammalian tissues. The chromium content in brewer's yeast is especially high; in addition, it is found in noticeable quantities in meat, liver, legumes, and whole grains. Chromium deficiency in the body can cause a diabetes-like condition, contribute to the development of atherosclerosis and disruption of higher nervous activity.

Already in relatively small concentrations (fractions of a milligram per m 3 for the atmosphere), all chromium compounds have a toxic effect on the body. Particularly dangerous in this regard are soluble compounds of hexavalent chromium, which have allergic, mutagenic and carcinogenic effects.

Poisoning with chromium and its compounds occurs during their production; in mechanical engineering (galvanic coatings); metallurgy (alloying additives, alloys, refractories); in the manufacture of leather, paints, etc. The toxicity of chromium compounds depends on their chemical structure: dichromates are more toxic than chromates, Cr (VI) compounds are more toxic than Cr (II), Cr (III) compounds. The initial forms of the disease are manifested by a feeling of dryness and pain in the nose, sore throat, difficulty breathing, cough, etc.; they can go away when contact with Chromium is stopped. With prolonged contact with chromium compounds, signs of chronic poisoning develop: headache, weakness, dyspepsia, weight loss and others. The functions of the stomach, liver and pancreas are impaired. Possible bronchitis, bronchial asthma, diffuse pneumosclerosis. When exposed to Chromium on the skin, dermatitis and eczema can develop. According to some data, chromium compounds, mainly Cr(III), have a carcinogenic effect.
chrome plating A decrease in chromium content in food and blood leads to a decrease in growth rate, an increase

Ripan R., Ceteanu I. Inorganic chemistry, vol. 2. – M.: Mir, 1972.

Chromium(lat. Cromium), Cr, chemical element of group VI of the periodic system of Mendeleev, atomic number 24, atomic mass 51.996; bluish-steel colored metal.

Natural stable isotopes: 50 Cr (4.31%), 52 Cr (87.76%), 53 Cr (9.55%) and 54 Cr (2.38%). Of the artificial radioactive isotopes, the most important is 51 Cr (half-life T ½ = 27.8 days), which is used as an isotope indicator.

Historical reference. Chromium was discovered in 1797 by L. N. Vauquelin in the mineral crocoite - natural lead chromate PbCrO 4 . Chrome got its name from the Greek word chroma - color, paint (due to the variety of colors of its compounds). Independently of Vauquelin, Chromium was discovered in crocoite in 1798 by the German scientist M. G. Klaproth.

Distribution of Chromium in nature. The average content of Chromium in the earth's crust (clarke) is 8.3·10 -3%. This element is probably more characteristic of the Earth's mantle, since ultramafic rocks, which are believed to be closest in composition to the Earth's mantle, are enriched in Chromium (2·10 -4%). Chromium forms massive and disseminated ores in ultramafic rocks; The formation of the largest chromium deposits is associated with them. In basic rocks, the Chromium content reaches only 2·10 -2%, in acidic rocks - 2.5·10 -3%, in sedimentary rocks (sandstones) - 3.5·10 -3%, in clay shales - 9·10 -3 %. Chromium is a relatively weak aquatic migrant; Chromium content in sea water is 0.00005 mg/l.

In general, Chromium is a metal in the deep zones of the Earth; stony meteorites (analogues of the mantle) are also enriched in Chromium (2.7·10 -1%). Over 20 chromium minerals are known. Only chrome spinels (up to 54% Cr) are of industrial importance; in addition, Chromium is contained in a number of other minerals, which often accompany chromium ores, but are not of practical value themselves (uvarovite, volkonskoite, kemerite, fuchsite).

Physical properties of Chromium. Chrome is a hard, heavy, refractory metal. Pure Chrome is ductile. Crystallizes in a body-centered lattice, a = 2.885Å (20 °C); at 1830 °C it is possible to transform into a modification with a face-centered lattice, a = 3.69 Å.

Atomic radius 1.27 Å; ionic radii of Cr 2+ 0.83 Å, Cr 3+ 0.64 Å, Cr 6+ 0.52 Å. Density 7.19 g/cm3; t pl 1890 °C; boiling point 2480 °C. Specific heat capacity 0.461 kJ/(kg K) (25°C); thermal coefficient of linear expansion 8.24·10 -6 (at 20 °C); thermal conductivity coefficient 67 W/(m K) (20 °C); electrical resistivity 0.414 μΩ m (20 °C); the thermal coefficient of electrical resistance in the range of 20-600 °C is 3.01·10 -3. Chromium is antiferromagnetic, specific magnetic susceptibility 3.6·10 -6. The Brinell hardness of high-purity Chromium is 7-9 Mn/m2 (70-90 kgf/cm2).

Chemical properties of Chromium. The external electronic configuration of the Chromium atom is 3d 5 4s 1. In compounds it usually exhibits oxidation states +2, +3, +6, among them Cr 3+ is the most stable; Individual compounds are known in which Chromium has oxidation states +1, +4, +5. Chromium is chemically inactive. Under normal conditions, it is resistant to oxygen and moisture, but combines with fluorine to form CrF 3 . Above 600 °C it interacts with water vapor, giving Cr 2 O 3; nitrogen - Cr 2 N, CrN; carbon - Cr 23 C 6, Cr 7 C 3, Cr 3 C 2; sulfur - Cr 2 S 3. When fused with boron, it forms boride CrB, and with silicon it forms silicides Cr 3 Si, Cr 2 Si 3, CrSi 2. Chromium forms alloys with many metals. The interaction with oxygen is quite active at first, then slows down sharply due to the formation of an oxide film on the metal surface. At 1200 °C the film is destroyed and oxidation proceeds quickly again. Chromium ignites in oxygen at 2000 °C to form the dark green oxide of Chromium (III) Cr 2 O 3. In addition to oxide (III), other compounds with oxygen are known, for example CrO, CrO 3, obtained indirectly. Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chromium chloride and sulfate and release hydrogen; Regia vodka and nitric acid passivate chromium.

As the degree of oxidation increases, the acidic and oxidizing properties of Chromium increase. Derivatives of Cr 2+ are very strong reducing agents. The Cr 2+ ion is formed at the first stage of the dissolution of Chromium in acids or during the reduction of Cr 3+ in an acidic solution with zinc. Oxide hydrate Cr(OH) 2 upon dehydration turns into Cr 2 O 3. Cr 3+ compounds are stable in air. They can be both reducing and oxidizing agents. Cr 3+ can be reduced in an acidic solution with zinc to Cr 2+ or oxidized in an alkaline solution to CrO 4 2- with bromine and other oxidizing agents. Hydroxide Cr(OH) 3 (or rather Cr 2 O 3 nH 2 O) is an amphoteric compound that forms salts with the Cr 3+ cation or salts of chromous acid HC-O 2 - chromites (for example, KS-O 2, NaCrO 2). Compounds Cr 6+: chromic anhydride CrO 3, chromic acids and their salts, among which the most important are chromates and dichromates - strong oxidizing agents. Chromium forms a large number of salts with oxygen-containing acids. Chromium complex compounds are known; Cr 3+ complex compounds, in which Chromium has a coordination number of 6, are especially numerous. There is a significant number of Chromium peroxide compounds

Getting Chrome. Depending on the purpose of use, Chromium of varying degrees of purity is obtained. The raw material is usually chrome spinels, which are enriched and then fused with potash (or soda) in the presence of atmospheric oxygen. In relation to the main component of ores containing Cr 3 +, the reaction is as follows:

2FeCr 2 O 4 + 4K 2 CO 3 + 3.5 O 2 = 4K 2 CrO 4 + Fe 2 O 3 + 4CO 2.

The resulting potassium chromate K 2 CrO 4 is leached with hot water and the action of H 2 SO 4 turns it into dichromate K 2 Cr 2 O 7 . Next, by the action of a concentrated solution of H 2 SO 4 on K 2 Cr 2 O 7, chromic anhydride C 2 O 3 is obtained or by heating K 2 Cr 2 O 7 with sulfur - Chromium (III) oxide C 2 O 3.

The purest Chromium in industrial conditions is obtained either by the electrolysis of concentrated aqueous solutions of CrO 3 or Cr 2 O 3 containing H 2 SO 4, or by the electrolysis of Chromium sulfate Cr 2 (SO 4) 3. In this case, Chromium is released on a cathode made of aluminum or stainless steel. Complete purification from impurities is achieved by treating Chromium with especially pure hydrogen at high temperatures (1500-1700 °C).

It is also possible to obtain pure Chromium by electrolysis of CrF 3 or CrCl 3 melts in a mixture with sodium, potassium, calcium fluorides at a temperature of about 900 ° C in an argon atmosphere.

Chromium is obtained in small quantities by reducing Cr 2 O 3 with aluminum or silicon. In the aluminothermic method, a preheated mixture of Cr 2 O 3 and Al powder or shavings with oxidizing agent additives is loaded into a crucible, where the reaction is excited by igniting the mixture of Na 2 O 2 and Al until the crucible is filled with Chromium and slag. Silicothermic chromium is smelted in arc furnaces. The purity of the resulting Chromium is determined by the content of impurities in Cr 2 O 3 and in Al or Si used for reduction.

Chromium alloys - ferrochrome and silicon chromium - are produced on a large scale in industry.

Application of Chromium. The use of Chrome is based on its heat resistance, hardness and corrosion resistance. Most of all, Chromium is used for smelting chromium steels. Aluminum- and silicothermic chromium is used for smelting nichrome, nimonic, other nickel alloys and stellite.

A significant amount of Chromium is used for decorative corrosion-resistant coatings. Powdered Chromium is widely used in the production of metal-ceramic products and materials for welding electrodes. Chromium in the form of Cr 3+ ion is an impurity in ruby, which is used as a gemstone and laser material. Chromium compounds are used to etch fabrics during dyeing. Some Chromium salts are used as a component of tanning solutions in the leather industry; PbCrO 4 , ZnCrO 4 , SrCrO 4 - like art paints. Chromium-magnesite refractory products are made from a mixture of chromite and magnesite.

Chromium compounds (especially Cr 6+ derivatives) are toxic.

Chromium in the body. Chromium is one of the biogenic elements and is constantly included in the tissues of plants and animals. The average content of Chromium in plants is 0.0005% (92-95% of Chromium accumulates in the roots), in animals - from ten thousandths to ten millionths of a percent. In planktonic organisms, the accumulation coefficient of Chromium is enormous - 10,000-26,000. Higher plants do not tolerate Chromium concentrations higher than 3-10 -4 mol/l. In leaves it is present in the form of a low-molecular complex not associated with subcellular structures. In animals, Chromium is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), and carbohydrates (a structural component of the glucose-resistant factor). The main source of Chromium in animals and humans is food. A decrease in chromium content in food and blood leads to a decrease in growth rate, an increase in blood cholesterol and a decrease in the sensitivity of peripheral tissues to insulin.

Poisoning with Chromium and its compounds occurs during their production; in mechanical engineering (galvanic coatings); metallurgy (alloying additives, alloys, refractories); in the manufacture of leather, paints, etc. The toxicity of chromium compounds depends on their chemical structure: dichromates are more toxic than chromates, Cr (VI) compounds are more toxic than Cr (II), Cr (III) compounds. The initial forms of the disease are manifested by a feeling of dryness and pain in the nose, sore throat, difficulty breathing, cough, etc.; they can go away when contact with Chromium is stopped. With prolonged contact with chromium compounds, signs of chronic poisoning develop: headache, weakness, dyspepsia, weight loss and others. The functions of the stomach, liver and pancreas are impaired. Possible bronchitis, bronchial asthma, diffuse pneumosclerosis. When exposed to Chromium on the skin, dermatitis and eczema can develop. According to some data, chromium compounds, mainly Cr(III), have a carcinogenic effect.