Atomic mass of calcium. Calcium as a chemical element, its role

Calcium (Latin Calcium, denoted by the symbol Ca) is an element with atomic number 20 and atomic mass 40.078. It is an element of the main subgroup of the second group, the fourth period of the periodic system of chemical elements of Dmitry Ivanovich Mendeleev. Under normal conditions, a simple substance calcium is a light (1.54 g / cm3) malleable, soft, chemically active alkaline earth metal of a silvery-white color.

In nature, calcium is presented as a mixture of six isotopes: 40Ca (96.97%), 42Ca (0.64%), 43Ca (0.145%), 44Ca (2.06%), 46Ca (0.0033%) and 48Ca ( 0.185%). The main isotope of the twentieth element - the most common - is 40Ca, its isotopic abundance is about 97%. Of the six natural isotopes of calcium, five are stable, the sixth isotope 48Ca, the heaviest of the six and quite rare (its isotopic abundance is only 0.185%), has recently been found to undergo double β-decay with a half-life of 5.3 ∙ 1019 years. The artificially obtained isotopes with mass numbers 39, 41, 45, 47 and 49 are radioactive. Most often they are used as an isotope indicator in the study of the processes of mineral metabolism in a living organism. 45Ca, obtained by irradiating metallic calcium or its compounds with neutrons in a uranium reactor, plays an important role in the study of metabolic processes occurring in soils and in the study of the processes of calcium assimilation by plants. Thanks to the same isotope, it was possible to detect sources of contamination of various grades of steel and ultrapure iron with calcium compounds during the smelting process.

Calcium compounds - marble, gypsum, limestone and lime (a product of limestone firing) have been known since ancient times and were widely used in construction and medicine. The ancient Egyptians used calcium compounds in the construction of their pyramids, and the inhabitants of great Rome invented concrete - using a mixture of crushed stone, lime and sand. Until the very end of the 18th century, chemists were convinced that lime is a simple body. Only in 1789 Lavoisier suggested that lime, alumina and some other compounds are complex substances. In 1808, calcium metal was obtained by H. Davie by electrolysis.

The use of metallic calcium is associated with its high chemical activity. It is used for reduction from compounds of certain metals, for example, thorium, uranium, chromium, zirconium, cesium, rubidium; to remove oxygen and sulfur from steel and some other alloys; for dehydration of organic liquids; for absorption of residual gases in vacuum devices. In addition, metallic calcium serves as an alloying component in some alloys. Calcium compounds are much more widely used - they are used in construction, pyrotechnics, glass production, medicine and many other fields.

Calcium is one of the most important biogenic elements, it is necessary for most living organisms for the normal course of life processes. The body of an adult contains up to one and a half kilograms of calcium. It is present in all tissues and fluids of living organisms. The twentieth element is necessary for the formation of bone tissue, maintaining heart rate, blood clotting, maintaining normal permeability of the outer cell membranes, and the formation of a number of enzymes. The list of functions that calcium performs in organisms of plants and animals is very long. Suffice it to say that only rare organisms are able to develop in an environment devoid of calcium, while other organisms are 38% composed of this element (the human body contains only about 2% calcium).

Biological properties

Calcium is one of the biogenic elements, its compounds are found in almost all living organisms (few organisms are able to develop in an environment devoid of calcium), ensuring the normal course of life processes. The twentieth element is present in all tissues and fluids of animals and plants, most of it (in vertebrates, including humans) is contained in the skeleton and teeth in the form of phosphates (for example, hydroxyapatite Ca5 (PO4) 3OH or 3Ca3 (PO4) 2 Ca (OH) 2). The use of the twentieth element as a building material for bones and teeth is due to the fact that calcium ions are not used in the cell. The concentration of calcium is controlled by special hormones, their combined action preserves and maintains the structure of bones. The skeletons of most invertebrate groups (molluscs, corals, sponges, and others) are built from various forms of calcium carbonate CaCO3 (lime). Many invertebrates store calcium before molting to build a new skeleton or to maintain life in adverse conditions. Animals receive calcium from food and water, and plants - from the soil and in relation to this element are divided into calcephiles and calcephobes.

Ions of this important microelement are involved in blood coagulation processes, as well as in ensuring constant osmotic pressure of the blood. In addition, calcium is necessary for the formation of a number of cellular structures, maintenance of normal permeability of the outer cell membranes, for the fertilization of eggs of fish and other animals, and activation of a number of enzymes (perhaps this circumstance is due to the fact that calcium replaces magnesium ions). Calcium ions transmit excitation to the muscle fiber, causing it to contract, increase the strength of heart contractions, increase the phagocytic function of leukocytes, activate the system of protective blood proteins, regulate exocytosis, including the secretion of hormones and neurotransmitters. Calcium affects the permeability of blood vessels - without this element, fats, lipids and cholesterol would settle on the walls of blood vessels. Calcium promotes the excretion of salts of heavy metals and radionuclides from the body, performs antioxidant functions. Calcium affects the reproductive system, has an anti-stress effect and has an anti-allergic effect.

The calcium content in the body of an adult (weighing 70 kg) is 1.7 kg (mainly in the composition of the intercellular substance of bone tissue). The need for this element depends on age: for adults, the required daily allowance is from 800 to 1,000 milligrams, for children from 600 to 900 milligrams. For children, it is especially important to consume the required dose for intensive growth and development of bones. The main source of calcium intake in the body is milk and dairy products, the rest of the calcium comes from meat, fish, some plant products (especially legumes). The absorption of calcium cations occurs in the large and small intestines, assimilation is facilitated by an acidic environment, vitamins C and D, lactose (lactic acid), as well as unsaturated fatty acids. In turn, aspirin, oxalic acid, estrogen derivatives significantly reduce the absorption of the twentieth element. So, combining with oxalic acid, calcium gives water-insoluble compounds, which are components of kidney stones. The role of magnesium in calcium metabolism is great - with its lack, calcium is "washed out" from the bones and deposited in the kidneys (kidney stones) and muscles. In general, the body has a complex system for storing and releasing the twentieth element, for this reason, the calcium content in the blood is precisely regulated, and with proper nutrition, there is no deficiency or excess. Long-term calcium diet can cause cramps, joint pain, constipation, fatigue, drowsiness, and growth retardation. Prolonged lack of calcium in the diet leads to the development of osteoporosis. Nicotine, caffeine and alcohol are some of the reasons for the lack of calcium in the body, as they contribute to its intensive excretion in the urine. However, an excess of the twentieth element (or vitamin D) leads to negative consequences - hypercalcemia develops, the consequence of which is intense calcification of bones and tissues (mainly affects the urinary system). Prolonged calcium surplus disrupts the functioning of muscle and nerve tissues, increases blood clotting and reduces the absorption of zinc by bone cells. Perhaps the appearance of osteoarthritis, cataracts, problems with blood pressure. From the above, we can conclude that the cells of plant and animal organisms need strictly defined ratios of calcium ions.

In pharmacology and medicine, calcium compounds are used for the manufacture of vitamins, tablets, pills, injections, antibiotics, as well as for the manufacture of ampoules, medical utensils.

It turns out that a fairly common cause of male infertility is a lack of calcium in the body! The fact is that the sperm head has a sagittal formation, which consists entirely of calcium, with a sufficient amount of this element, the sperm is able to overcome the membrane and fertilize the egg, if not enough, infertility occurs.

American scientists have found that a lack of calcium ions in the blood leads to a weakening of memory and a decrease in intelligence. So, for example, from the well-known journal Science News in the United States, it became known about experiments that confirmed that cats develop a conditioned reflex only if their brain cells contain more calcium than blood.

The compound, calcium cyanamide, highly valued in agriculture, is used not only as a nitrogen fertilizer and a source of urea - the most valuable fertilizer and raw material for the production of synthetic resins, but also as a substance with which it was possible to mechanize the harvesting of cotton fields. The fact is that after being treated with this compound, the cotton instantly sheds its foliage, which allows people to leave the cotton picking to machines.

When talking about foods rich in calcium, dairy products are always mentioned, but milk itself contains from 120 mg (cow) to 170 mg (sheep) calcium per 100 g; cottage cheese is even poorer - only 80 mg per 100 grams. Of dairy products, only cheese contains from 730 mg (gouda) to 970 mg (emmental) calcium per 100 g of product. However, poppy is the record holder for the content of the twentieth element - 100 grams of poppy seeds contain almost 1,500 mg of calcium!

Calcium chloride CaCl2, used, for example, in refrigeration plants, is a waste product of many chemical technological processes, in particular, large-scale production of soda. However, despite the widespread use of calcium chloride in various fields, its consumption is significantly inferior to its production. For this reason, for example, near soda plants, whole lakes are formed from calcium chloride brine. Such storage ponds are not uncommon.

In order to understand how much calcium compounds are consumed, it is worth giving just a couple of examples. In the production of steel, lime is used to remove phosphorus, silicon, manganese and sulfur; in the oxygen-converter process, 75 kilograms of lime are consumed per ton of steel! Another example from a completely different area is the food industry. In the production of sugar to precipitate calcium saccharate, the raw sugar syrup is reacted with lime. So, cane sugar usually requires about 3-5 kg ​​of lime per ton, and beet sugar - a hundred times more, that is, about half a ton of lime per ton of sugar!

The "hardness" of water is a series of properties that the calcium and magnesium salts dissolved in it give to water. Stiffness is divided into temporary and permanent. Temporary or carbonate hardness is caused by the presence of soluble bicarbonates Ca (HCO3) 2 and Mg (HCO3) 2 in water. It is very easy to get rid of carbonate hardness - when water is boiled, bicarbonates turn into water-insoluble calcium and magnesium carbonates, precipitating. Permanent hardness is created by sulfates and chlorides of the same metals, but getting rid of it is much more difficult. Hard water is terrible not so much because it prevents the formation of soap suds and therefore washes laundry worse, it is much more terrible that it forms a layer of scale in steam boilers and boiler installations, thereby reducing their efficiency and leading to emergency situations. Interestingly, they knew how to determine the hardness of water even in ancient Rome. Red wine was used as a reagent - its coloring agents form a precipitate with calcium and magnesium ions.

The process of preparing calcium for storage is very interesting. Metallic calcium is stored for a long time in the form of lumps weighing from 0.5 to 60 kg. These "pigs" are packed in paper bags, then placed in galvanized iron containers with soldered and painted seams. Tightly closed containers are placed in wooden boxes. Pieces weighing less than half a kilogram cannot be stored for a long time - when oxidized, they quickly turn into oxide, hydroxide and calcium carbonate.

History

Metallic calcium was obtained relatively recently - in 1808, but mankind has been familiar with the compounds of this metal for a very long time. Since ancient times, people have used limestone, chalk, marble, alabaster, gypsum and other calcium-containing compounds in construction and medicine. CaCO3 limestone was most likely the first building material used by humans. It was used in the construction of the Egyptian pyramids and the Great Wall of China. Many temples and churches in Russia, as well as most of the buildings of ancient Moscow, were built using limestone - white stone. Even in ancient times, a person, burning limestone, received quicklime (CaO), as evidenced by the works of Pliny the Elder (1st century AD) and Dioscorides, a doctor in the Roman army, whom he introduced for calcium oxide in his work "On Medicines" the name "quicklime", which has survived to our time. And all this despite the fact that pure calcium oxide was first described by the German chemist I. Then only in 1746, and in 1755, the chemist J. Black, studying the firing process, revealed that the loss of mass of limestone during firing occurs due to the release of carbon dioxide gas:

CaCO3 ↔ CO2 + CaO

Egyptian mortars that were used in the pyramids of Giza were based on partially dehydrated gypsum CaSO4 2H2O or, in other words, alabaster 2CaSO4 ∙ H2O. He is also the basis of all the plaster in the tomb of Tutankhamun. Burnt gypsum (alabaster) was used by the Egyptians as a binder in the construction of irrigation facilities. By burning natural gypsum at high temperatures, Egyptian builders achieved its partial dehydration, and not only water, but also sulfuric anhydride was split off from the molecule. Subsequently, when diluted with water, a very strong mass was obtained, which was not afraid of water and temperature fluctuations.

The Romans can rightfully be called the inventors of concrete, because in their buildings they used one of the varieties of this building material - a mixture of crushed stone, sand and lime. There is a description of Pliny the Elder of the construction of cisterns from such concrete: “For the construction of cisterns, they take five parts of pure gravel sand, two parts of the best slaked lime and fragments of Silex (hard lava) weighing no more than a pound each, after mixing they compact the bottom and side surfaces with blows of an iron rammer ". In Italy's humid climate, concrete was the most resilient material.

It turns out that calcium compounds were known to mankind for a long time, which they widely used. However, until the end of the 18th century, chemists considered lime to be a simple body, only on the eve of the new century began to study the nature of lime and other calcium compounds. So Stahl suggested that lime is a complex body, consisting of earthy and watery principles, and Black established the difference between caustic lime and carbonic lime, which contained "fixed air." Antoine Laurent Lavoisier attributed lime earth (CaO) to the number of elements, that is, to simple substances, although in 1789 he suggested that lime, magnesia, barite, alumina and silica are complex substances, but it will be possible to prove this only by decomposing the "stubborn earth" (calcium oxide). And the first who succeeded was Humphrey Davy. After the successful decomposition of potassium and sodium oxides by electrolysis, the chemist decided to obtain alkaline earth metals in the same way. However, the first attempts were unsuccessful - the Englishman tried to decompose lime by electrolysis in air and under a layer of oil, then calcined lime with metallic potassium in a tube and made many other experiments, but to no avail. Finally, in a device with a mercury cathode, he obtained amalgam by electrolysis of lime, and from it metallic calcium. Pretty soon this method of metal production was improved by I. Berzelius and M. Pontin.

The new element got its name from the Latin word "calx" (in the genitive case calcis) - lime, soft stone. Calx (calx) was called chalk, limestone, in general a bare stone, but most often a mortar based on lime. This concept was also used by ancient authors (Vitruvius, Pliny the Elder, Dioscorides), describing the burning of limestone, lime slaking and the preparation of mortars. Later in the circle of alchemists "calx" meant the product of roasting in general - in particular metals. So, for example, metal oxides were called metallic lime, and the firing process itself was called calcination (calcinatio). In the ancient Russian recipe literature there is the word feces (mud, clay), so in the collection of the Trinity-Sergius Lavra (15th century) it is said: “gather feces, from it they create a crucible for gold”. Only later did the word kal, which is undoubtedly associated with the word "calx", become synonymous with the word manure. In Russian literature at the beginning of the 19th century, calcium was sometimes called the base of the calcareous earth, limestone (Shcheglov, 1830), calcareousness (Job), calcium, calcium (Hess).

Being in nature

Calcium is one of the most common elements on our planet - the fifth in terms of quantitative content in nature (of non-metals, only oxygen is more - 49.5% and silicon - 25.3%) and the third among metals (only aluminum is more common - 7.5% and iron - 5.08%). Clarke (the average content in the earth's crust) of calcium, according to various estimates, ranges from 2.96% by mass to 3.38%, we can definitely say that this figure is about 3%. In the outer shell of the calcium atom, there are two valence electrons, the bond of which with the nucleus is rather fragile. For this reason, calcium is highly reactive and does not occur in free form in nature. However, it actively migrates and accumulates in various geochemical systems, forming about 400 minerals: silicates, aluminosilicates, carbonates, phosphates, sulfates, borosilicates, molybdates, chlorides and others, ranking fourth in this indicator. During the melting of basaltic magmas, calcium accumulates in the melt and is part of the main rock-forming minerals, during the fractionation of which its content decreases during the differentiation of magma from basic to felsic rocks. For the most part, calcium occurs in the lower part of the earth's crust, accumulating in basic rocks (6.72%); there is little calcium in the earth's mantle (0.7%) and, probably, even less in the earth's core (in the iron meteorites of the twentieth element, similar to the core, only 0.02%).

True, the clarke of calcium in stone meteorites is 1.4% (rare calcium sulfide is found), in medium rocks - 4.65%, felsic rocks contain 1.58% calcium by weight. The main part of calcium is contained in silicates and aluminosilicates of various rocks (granites, gneisses, etc.), especially in feldspar - anorthite Ca, as well as diopside CaMg, wollastonite Ca3. In the form of sedimentary rocks, calcium compounds are represented by chalk and limestone, consisting mainly of the mineral calcite (CaCO3).

Calcium carbonate CaCO3 is one of the most widespread compounds on Earth - calcium carbonate-based minerals cover about 40 million square kilometers of the earth's surface. In many parts of the Earth's surface, there are significant sedimentary deposits of calcium carbonate, which were formed from the remains of ancient marine organisms - chalk, marble, limestone, shell rock - all this is CaCO3 with minor impurities, and calcite is pure CaCO3. The most important of these minerals is limestone, more precisely - limestone - because each deposit differs in density, composition and amount of impurities. For example, shell rock is an organic limestone, and calcium carbonate, which has fewer impurities, forms transparent crystals of limestone or Icelandic spar. Chalk is another common type of calcium carbonate, but marble, a crystalline form of calcite, is much less common in nature. It is generally accepted that marble was formed from limestone in ancient geological eras. During the movement of the earth's crust, individual deposits of limestone were buried under layers of other rocks. Under the influence of high pressure and temperature, the process of recrystallization took place, and limestone turned into a denser crystalline rock - marble. Bizarre stalactites and stalagmites are aragonite mineral, which is another type of calcium carbonate. Orthorhombic aragonite is formed in warm seas - the Bahamas, the Florida Keys and the Red Sea basin are formed by huge layers of calcium carbonate in the form of aragonite. Also quite widespread are calcium minerals such as fluorite CaF2, dolomite MgCO3 CaCO3, anhydrite CaSO4, phosphorite Ca5 (PO4) 3 (OH, CO3) (with various impurities) and apatite Ca5 (PO4) 3 (F, Cl, OH) - forms of calcium phosphate, alabaster CaSO4 0.5H2O and gypsum CaSO4 2H2O (forms of calcium sulfate) and others. In calcium-containing minerals, there are impurity elements replacing it isomorphically (for example, sodium, strontium, rare-earth, radioactive and other elements).

A large amount of the twentieth element is found in natural waters due to the existence of a global "carbonate equilibrium" between poorly soluble CaCO3, highly soluble Ca (HCO3) 2, and CO2 in water and air:

CaCO3 + H2O + CO2 = Ca (HCO3) 2 = Ca2 + + 2HCO3-

This reaction is reversible and is the basis for the redistribution of the twentieth element - with a high content of carbon dioxide in the waters, calcium is in solution, and with a low content of CO2, the mineral calcite CaCO3 precipitates, forming powerful deposits of limestone, chalk, marble.

A considerable amount of calcium is part of living organisms, for example, hydroxyapatite Ca5 (PO4) 3OH, or, in another record, 3Ca3 (PO4) 2 Ca (OH) 2 - the basis of the bone tissue of vertebrates, including humans. Calcium carbonate CaCO3 is the main component of the shells and shells of many invertebrates, eggshells, corals and even pearls.

Application

Metallic calcium is rarely used. Basically, this metal (like its hydride) is used in the metallothermal production of hard-to-reduce metals - uranium, titanium, thorium, zirconium, cesium, rubidium and a number of rare-earth metals from their compounds (oxides or halides). Calcium is used as a reducing agent in the production of nickel, copper and stainless steel. Also, the twentieth element is used for the deoxidation of steels, bronzes and other alloys, for removing sulfur from petroleum products, for dehydrating organic solvents, for purifying argon from nitrogen impurities and as a gas absorber in electric vacuum devices. Metallic calcium is used in the production of antifriction alloys of the Pb-Na-Ca system (used in bearings), as well as the Pb-Ca alloy, which is used for the manufacture of electrical cable sheaths. Silicocalcium alloy (Ca-Si-Ca) is used as a deoxidizer and degasser in the production of high-quality steels. Calcium is used both as an alloying element for aluminum alloys and as a modifying additive for magnesium alloys. For example, the addition of calcium increases the strength of aluminum bearings. Pure calcium is also used for alloying lead, which is used for the manufacture of battery plates, maintenance-free starter lead-acid batteries with low self-discharge. Also, metallic calcium is used for the production of high-quality calcium babbitts BKA. With the help of calcium, the carbon content in cast iron is regulated and bismuth is removed from lead, and steel is purified from oxygen, sulfur and phosphorus. Calcium, as well as its alloys with aluminum and magnesium, are used in backup thermal electric batteries as an anode (for example, a calcium chromate element).

However, compounds of the twentieth element are used much more widely. And first of all, we are talking about natural calcium compounds. One of the most widespread calcium compounds on Earth is CaCO3 carbonate. Pure calcium carbonate is a calcite mineral, and limestone, chalk, marble, shell rock is CaCO3 with minor impurities. Mixed calcium and magnesium carbonate is called dolomite. Limestone and dolomite are mainly used as building materials, pavements, or deacidifying agents. Calcium carbonate CaCO3 is necessary to obtain calcium oxide (quicklime) CaO and calcium hydroxide (slaked lime) Ca (OH) 2. In turn, CaO and Ca (OH) 2 are the main substances in many areas of the chemical, metallurgical and machine-building industries - calcium oxide, both in free form and as part of ceramic mixtures, is used in the production of refractory materials; colossal amounts of calcium hydroxide are needed by the pulp and paper industry. In addition, Ca (OH) 2 is used in the production of bleach (a good bleaching and disinfectant), berthollet salt, soda, and some pesticides to combat plant pests. A huge amount of lime is consumed in the production of steel - to remove sulfur, phosphorus, silicon and manganese. Another role of lime in metallurgy is the production of magnesium. Lime is also used as a lubricant for pulling steel wire and neutralizing waste pickling liquids containing sulfuric acid. In addition, it is lime that is the most common chemical reagent in the treatment of drinking and industrial water (together with alum or iron salts, it coagulates suspensions and removes sediment, and also softens water by removing temporary - bicarbonate - hardness). In everyday life and medicine, precipitated calcium carbonate is used as an acid neutralizing agent, a mild abrasive in toothpastes, a source of additional calcium in diets, a component of chewing gum and a filler in cosmetics. CaCO3 is also used as a filler in rubbers, latexes, paints and enamels, as well as in plastics (about 10% by weight) to improve their heat resistance, hardness, hardness and workability.

Calcium fluoride CaF2 is of particular importance, because in the form of a mineral (fluorite) it is the only industrially important source of fluorine! Calcium fluoride (fluorite) is used in the form of single crystals in optics (astronomical objectives, lenses, prisms) and as a laser material. The fact is that glasses made of calcium fluoride alone are permeable to the entire spectrum. Calcium tungstate (scheelite) in the form of single crystals is used in laser technology and also as a scintillator. No less important is calcium chloride CaCl2 - a component of brines for refrigeration units and for filling tires of tractors and other vehicles. With the help of calcium chloride, roads and sidewalks are cleared of snow and ice, this compound is used to protect coal and ore from freezing during transportation and storage, wood is impregnated with its solution to give it fire resistance. CaCl2 is used in concrete mixtures to accelerate the onset of setting, increase the initial and final strength of concrete.

Artificially obtained calcium carbide CaC2 (when calcined in electric furnaces of calcium oxide with coke) is used for the production of acetylene and for the reduction of metals, as well as for the production of calcium cyanamide, which, in turn, releases ammonia under the action of water vapor. In addition, calcium cyanamide is used for the production of urea - the most valuable fertilizer and raw material for the production of synthetic resins. By heating calcium in a hydrogen atmosphere, CaH2 (calcium hydride) is obtained, which is used in metallurgy (metallothermy) and in the production of hydrogen in the field (more than a cubic meter of hydrogen can be obtained from 1 kilogram of calcium hydride), which is used to fill balloons, for example. In laboratory practice, calcium hydride is used as an energetic reducing agent. Insecticide calcium arsenate, which is obtained by neutralizing arsenic acid with lime, is widely used to combat cotton weevil, codling moth, tobacco worm, and Colorado potato beetle. Important fungicides are lime-sulphate aerosols and Bordeaux mixtures, which are obtained from copper sulphate and calcium hydroxide.

Production

The first to receive metallic calcium was the English chemist Humphrey Davy. In 1808, he electrolyzed a mixture of wet slaked lime Ca (OH) 2 with mercury oxide HgO on a platinum plate that served as an anode (a platinum wire immersed in mercury acted as a cathode), as a result of which Davy obtained a calcium amalgam by removing mercury from it , the chemist obtained a new metal, which he called calcium.

In modern industry, free metallic calcium is obtained by electrolysis of molten calcium chloride CaCl2, the proportion of which is 75-85% and potassium chloride KCl (it is possible to use a mixture of CaCl2 and CaF2) or by alumothermal reduction of calcium oxide CaO at a temperature of 1,170-1,200 ° C. Pure anhydrous calcium chloride required for electrolysis is obtained by chlorination of calcium oxide by heating in the presence of coal or by dehydration of CaCl2 ∙ 6H2O obtained by the action of hydrochloric acid on limestone. The electrolytic process takes place in an electrolysis bath, in which a dry calcium chloride salt and potassium chloride, which is necessary to lower the melting point of the mixture, are placed, free of impurities. Graphite blocks are placed above the bath - an anode, a cast iron or steel bath filled with a copper-calcium alloy acts as a cathode. In the process of electrolysis, calcium transforms into a copper-calcium alloy, significantly enriching it, a part of the enriched alloy is constantly extracted, instead an alloy depleted in calcium (30-35% Ca) is added, at the same time chlorine forms a chlorine-air mixture (anode gases), which subsequently enters the chlorination of lime milk. The enriched copper-calcium alloy can be used directly as an alloy or sent for purification (distillation), where by distillation in vacuum (at a temperature of 1,000-1,080 ° C and a residual pressure of 13-20 kPa), metallic calcium of nuclear purity is obtained from it. To obtain high-purity calcium, it is distilled twice. The electrolysis process is carried out at a temperature of 680-720 ° C. The fact is that this is the most optimal temperature for the electrolytic process - at a lower temperature, the calcium-rich alloy floats to the surface of the electrolyte, and at a higher temperature, calcium dissolves in the electrolyte with the formation of CaCl. In electrolysis with liquid cathodes from calcium and lead alloys or calcium and zinc alloys of calcium with lead (for bearings) and with zinc used in technology are directly obtained (to obtain foam concrete - when the alloy interacts with moisture, hydrogen is released and a porous structure is created). Sometimes the process is carried out with a cooled iron cathode, which only comes into contact with the surface of the molten electrolyte. As calcium is released, the cathode is gradually raised, a rod (50-60 cm) of calcium is pulled out of the melt, protected from atmospheric oxygen by a layer of solidified electrolyte. By the "touch method" calcium is obtained, which is heavily contaminated with calcium chloride, iron, aluminum, sodium, purification is carried out by remelting in an argon atmosphere.

Another method of obtaining calcium - metallothermal - was theoretically substantiated back in 1865 by the famous Russian chemist N.N. Beketov. The alumothermal method is based on the reaction:

6CaO + 2Al → 3CaO Al2O3 + 3Ca

Briquettes are pressed from a mixture of calcium oxide with powdered aluminum, they are placed in a retort made of chromium-nickel steel and the formed calcium is distilled off at 1 170-1 200 ° C and a residual pressure of 0.7-2.6 Pa. Calcium is obtained in the form of steam, which is then condensed on a cold surface. The alumothermal method for producing calcium is used in China, France and a number of other countries. On an industrial scale, the metallothermal method for producing calcium was the first to be used by the United States during the Second World War. In the same way, calcium can be obtained by reducing CaO with ferrosilicon or aluminum silico. Calcium is produced in the form of ingots or sheets with a purity of 98-99%.

There are pros and cons to both methods. The electrolytic method is multi-operational, energy-intensive (for 1 kg of calcium, energy is consumed 40-50 kWh), moreover, it is not environmentally safe, and requires a large amount of reagents and materials. However, the yield of calcium with this method is 70-80%, while with the alumothermal method, the yield is only 50-60%. In addition, with the metallothermal method of obtaining calcium, the minus is that it is necessary to re-distill, and the plus is in the low power consumption, and in the absence of gas and liquid harmful emissions.

Not so long ago, a new method for obtaining metallic calcium was developed - it is based on thermal dissociation of calcium carbide: carbide heated in a vacuum up to 1,750 ° C decomposes with the formation of calcium vapor and solid graphite.

Until the middle of the 20th century, calcium metal was produced in very small quantities, since it was almost never used. For example, in the United States of America during the Second World War, no more than 25 tons of calcium were consumed, and in Germany, only 5-10 tons. Only in the second half of the XX century, when it became clear that calcium is an active reductant of many rare and refractory metals, a rapid growth in consumption (about 100 tons per year) began and, as a result, in the production of this metal. With the development of the nuclear industry, where calcium is used as a component of metallothermal reduction of uranium from uranium tetrafluoride (except for the USA, where magnesium is used instead of calcium), the demand (about 2,000 tons per year) for element number twenty, as well as its production, has increased many times. At the moment, China, Russia, Canada and France can be considered the main producers of calcium metal. From these countries, calcium is sent to the USA, Mexico, Australia, Switzerland, Japan, Germany, Great Britain. Calcium metal prices rose steadily until China began to produce metal in such volumes that an excess of element twentieth appeared on the world market, which led to a sharp decline in prices.

Physical properties

What is metallic calcium? What properties does this element possess, obtained in 1808 by the English chemist Humphrey Davy, a metal whose mass in an adult's body can be up to 2 kilograms?

The simple substance calcium is a silvery white light metal. The density of calcium is only 1.54 g / cm3 (at a temperature of 20 ° C), which is much less than the density of iron (7.87 g / cm3), lead (11.34 g / cm3), gold (19.3 g / cm3) or platinum (21.5 g / cm3). Calcium is even lighter than such "weightless" metals as aluminum (2.70 g / cm3) or magnesium (1.74 g / cm3). Few metals can "boast" with a density lower than that of the twentieth element - sodium (0.97 g / cm3), potassium (0.86 g / cm3), lithium (0.53 g / cm3). In terms of density, calcium is very similar to rubidium (1.53 g / cm3). The melting point of calcium is 851 ° C, the boiling point is 1,480 ° C. Similar melting points (although slightly lower) and boiling points for other alkaline earth metals - strontium (770 ° C and 1,380 ° C) and barium (710 ° C and 1,640 ° C).

Metallic calcium exists in two allotropic modifications: at normal temperatures up to 443 ° C, α-calcium with a cubic face-centered lattice such as copper is stable, with parameters: a = 0.558 nm, z = 4, space group Fm3m, atomic radius 1.97 A, ionic radius Ca2 + 1.04 A; in the temperature range 443-842 ° C β-calcium is stable with a cubic body-centered lattice of the α-iron type, with parameters a = 0.448 nm, z = 2, space group Im3m. The standard enthalpy of transition from the α-modification to the β-modification is 0.93 kJ / mol. The temperature coefficient of linear expansion for calcium in the temperature range 0-300 ° C is 22 10-6. The thermal conductivity of the twentieth element at 20 ° C is 125.6 W / (m K) or 0.3 cal / (cm sec ° C). The specific heat of calcium in the range from 0 to 100 ° C is 623.9 J / (kg K) or 0.149 cal / (g ° C). The specific electrical resistance of calcium at a temperature of 20 ° C is 4.6 10-8 ohm m or 4.6 10-6 ohm cm; temperature coefficient of electrical resistance of element number twenty 4.57 10-3 (at 20 ° C). Calcium elasticity modulus 26 Gn / m2 or 2600 kgf / mm2; tensile strength 60 Mn / m2 (6 kgf / mm2); the elastic limit for calcium is 4 MN / m2 or 0.4 kgf / mm2, the yield point is 38 MN / m2 (3.8 kgf / mm2); relative elongation of the twentieth element 50%; Brinell hardness of calcium 200-300 Mn / m2 or 20-30 kgf / mm2. With a gradual increase in pressure, calcium begins to exhibit the properties of a semiconductor, but does not become it in the full sense of the word (in this case, it is no longer a metal either). With a further increase in pressure, calcium returns to the metallic state and begins to exhibit superconducting properties (the superconductivity temperature is six times higher than that of mercury, and is much higher in conductivity than all other elements). The unique behavior of calcium is similar in many ways to strontium (that is, the parallels in the periodic table are preserved).

The mechanical properties of elemental calcium do not differ from the properties of other members of the family of metals, which are excellent structural materials: metallic calcium of high purity is plastic, well pressed and rolled, drawn into wire, forged and machined - it can be turned on a lathe. However, despite all these excellent qualities of a structural material, calcium is not such - the reason for everything is its high chemical activity. However, one should not forget that calcium is an irreplaceable structural material of bone tissue, and its minerals have been a building material for many millennia.

Chemical properties

The configuration of the outer electron shell of the calcium atom is 4s2, which determines the valency of the twentieth element in the compounds. Two electrons of the outer layer are relatively easily split off from atoms, which are converted in this case into positive doubly charged ions. For this reason, in terms of chemical activity, calcium is only slightly inferior to alkali metals (potassium, sodium, lithium). Like the latter, calcium already at ordinary room temperature easily interacts with oxygen, carbon dioxide and humid air, becoming covered with a dull gray film of a mixture of CaO oxide and Ca (OH) 2 hydroxide. Therefore, calcium is stored in a hermetically sealed vessel under a layer of mineral oil, liquid paraffin or kerosene. When heated in oxygen and air, calcium ignites, burning with a bright red flame, and the basic oxide CaO is formed, which is a white, highly fire-resistant substance with a melting point of about 2,600 ° C. Calcium oxide is also known in the art as quicklime or burnt lime. Also obtained are calcium peroxides - CaO2 and CaO4. Calcium reacts with water with the release of hydrogen (in the series of standard potentials, calcium is located to the left of hydrogen and is able to displace it from water) and the formation of calcium hydroxide Ca (OH) 2, and in cold water the reaction rate gradually decreases (due to the formation of a layer of poorly soluble calcium hydroxide):

Ca + 2H2O → Ca (OH) 2 + H2 + Q

Calcium interacts more vigorously with hot water, violently displacing hydrogen and forming Ca (OH) 2. Calcium hydroxide Ca (OH) 2 is a strong base, slightly soluble in water. A saturated solution of calcium hydroxide is called lime water and is alkaline. In air, lime water quickly becomes cloudy due to the absorption of carbon dioxide and the formation of insoluble calcium carbonate. Despite such violent processes occurring during the interaction of the twentieth element with water, nevertheless, unlike alkali metals, the reaction of interaction of calcium with water proceeds less vigorously - without explosions and ignitions. In general, the reactivity of calcium is lower than that of other alkaline earth metals.

Calcium actively combines with halogens, thus forming compounds of the CaX2 type - it reacts with fluorine in the cold, and with chlorine and bromine at temperatures above 400 ° C, giving CaF2, CaCl2 and CaBr2, respectively. These halides in the molten state form with calcium monohalides of the CaX type - CaF, CaCl, in which calcium is formally monovalent. These compounds are stable only above the melting temperatures of the dihalides (they disproportionate upon cooling with the formation of Ca and CaX2). In addition, calcium actively interacts, especially when heated, with various non-metals: when heated, calcium sulfide CaS is obtained with sulfur, the latter adds sulfur, forming polysulfides (CaS2, CaS4 and others); interacting with dry hydrogen at a temperature of 300-400 ° C, calcium forms a hydride CaH2 - an ionic compound in which hydrogen is an anion. Calcium hydride CaH2 is a white, salt-like substance that reacts violently with water to release hydrogen:

CaH2 + 2H2O → Ca (OH) 2 + 2H2

When heated (about 500 ° C) in a nitrogen atmosphere, calcium ignites and forms nitride Ca3N2, known in two crystalline forms - high-temperature α and low-temperature β. Also, nitride Ca3N4 was obtained by heating calcium amide Ca (NH2) 2 in vacuum. When heated without air access with graphite (carbon), silicon or phosphorus, calcium gives, respectively, calcium carbide CaC2, silicides Ca2Si, Ca3Si4, CaSi, CaSi2 and phosphides Ca3P2, CaP and CaP3. Most of the calcium compounds with non-metals are readily decomposed by water:

CaH2 + 2H2O → Ca (OH) 2 + 2H2

Ca3N2 + 6Н2О → 3Са (ОН) 2 + 2NH3

With boron, calcium forms calcium boride CaB6, with chalcogenes - chalcogenides CaS, CaSe, CaTe. Also known polychalcogenides CaS4, CaS5, Ca2Te3. Calcium forms intermetallic compounds with various metals - aluminum, gold, silver, copper, lead and others. Being an energetic reducing agent, calcium displaces almost all metals from their oxides, sulfides and halides when heated. Calcium dissolves well in liquid ammonia NH3 with the formation of a blue solution, upon evaporation of which ammonia [Ca (NH3) 6] is released - a solid golden compound with metallic conductivity. Calcium salts are usually obtained by the interaction of acid oxides with calcium oxide, the action of acids on Ca (OH) 2 or CaCO3, exchange reactions in aqueous electrolyte solutions. Many calcium salts are readily soluble in water (chloride CaCl2, bromide CaBr2, iodide CaI2 and nitrate Ca (NO3) 2), they almost always form crystalline hydrates. Fluoride CaF2, carbonate CaCO3, sulfate CaSO4, orthophosphate Ca3 (PO4) 2, oxalate CaC2O4 and some others are insoluble in water.

Calcium

CALCIUM-I am; m.[from lat. calx (calcis) - lime] Chemical element (Ca), a silver-white metal that is part of limestone, marble, etc.

◁ Calcium, th, th. K-th salts.

(lat. Calcium), a chemical element of group II of the periodic table, refers to alkaline earth metals. Name from lat. calx, genitive calcis is lime. Silver-white metal, density 1.54 g / cm 3, t pl 842ºC. It easily oxidizes in air at normal temperatures. In terms of prevalence in the earth's crust, it occupies the 5th place (minerals calcite, gypsum, fluorite, etc.). As an active reducing agent, it is used to obtain U, Th, V, Cr, Zn, Be and other metals from their compounds, for the deoxidation of steels, bronzes, etc. It is part of antifriction materials. Calcium compounds are used in construction (lime, cement), calcium preparations are used in medicine.

CALCIUM (Latin Calcium), Ca (read "calcium"), a chemical element with atomic number 20, is located in the fourth period in group IIA of Mendeleev's periodic system of elements; atomic mass 40.08. Refers to the number of alkaline earth elements (cm. ALKALINE EARTH METALS).
Natural calcium consists of a mixture of nuclides (cm. NUCLID) with mass numbers 40 (in the mixture by weight 96.94%), 44 (2.09%), 42 (0.667%), 48 (0.187%), 43 (0.135%) and 46 (0.003%). Configuration of outer electron layer 4 s 2 ... In almost all compounds, the oxidation state of calcium is +2 (valence II).
The radius of the neutral calcium atom is 0.1974 nm, the radius of the Ca 2+ ion is from 0.114 nm (for coordination number 6) to 0.148 nm (for coordination number 12). The sequential ionization energies of a neutral calcium atom are 6.133, 11.872, 50.91, 67.27 and 84.5 eV, respectively. On the Pauling scale, the electronegativity of calcium is about 1.0. Free calcium is a silvery white metal.
Discovery history
Calcium compounds are found everywhere in nature, so mankind has been familiar with them since ancient times. Lime has long been used in construction (cm. LIME)(quicklime and extinguished), which for a long time was considered a simple substance, "earth". However, in 1808 the English scientist G. Davy (cm. DEVI Humphrey) managed to get new metal out of lime. To do this, Davy electrolyzed a mixture of slightly moistened slaked lime with mercury oxide and isolated a new metal from the amalgam formed at the mercury cathode, which he called calcium (from Latin calx, genus calcis - lime). In Russia for some time this metal was called "lime".
Being in nature
Calcium is one of the most abundant elements on Earth. It accounts for 3.38% of the mass of the earth's crust (5th most common after oxygen, silicon, aluminum and iron). Due to its high chemical activity, free calcium is not found in nature. Most of the calcium is contained in silicates (cm. SILICATES) and aluminosilicates (cm. ALUMOSILICATES) various rocks (granites (cm. GRANITE), gneisses (cm. GNEISS) etc.). In the form of sedimentary rocks, calcium compounds are represented by chalk and limestone, consisting mainly of the mineral calcite (cm. CALCITE)(CaCO 3). The crystalline form of calcite - marble - is much less common in nature.
Calcium minerals such as limestone are quite common. (cm. LIMESTONE) CaCO 3, anhydrite (cm. ANHYDRITE) CaSO 4 and gypsum (cm. GYPSUM) CaSO 4 2H 2 O, fluorite (cm. FLUORITE) CaF 2, apatite (cm. APATITES) Ca 5 (PO 4) 3 (F, Cl, OH), dolomite (cm. DOLOMITE) MgCO 3 · СaCO 3. The presence of calcium and magnesium salts in natural water determines its hardness (cm. HARDNESS OF WATER)... A significant amount of calcium is found in living organisms. So, hydroxylapatite Ca 5 (PO 4) 3 (OH), or, in another notation, 3Ca 3 (PO 4) 2 · Ca (OH) 2 - the basis of the bone tissue of vertebrates, including humans; the shells and shells of many invertebrates, eggshells, etc. are composed of calcium carbonate CaCO 3.
Receiving
Metallic calcium is obtained by electrolysis of a melt consisting of CaCl 2 (75-80%) and KCl or from CaCl 2 and CaF 2, as well as by aluminothermic reduction of CaO at 1170-1200 ° C:
4CaO + 2Al = CaAl 2 O 4 + 3Ca.
Physical and chemical properties
The calcium metal exists in two allotropic modifications (see Allotropy (cm. ALLOTROPY)). Up to 443 ° C, a-Ca with a cubic face-centered lattice is stable (parameter a = 0.558 nm), higher is b-Ca with a cubic body-centered lattice of the a-Fe type (parameter a = 0.448 nm). The melting point of calcium is 839 ° C, the boiling point is 1484 ° C, the density is 1.55 g / cm 3.
The reactivity of calcium is high, but lower than that of all other alkaline earth metals. It easily interacts with oxygen, carbon dioxide and moisture in the air, which is why the surface of metallic calcium is usually dull gray, so in the laboratory, calcium is usually stored, like other alkaline earth metals, in a tightly closed jar under a layer of kerosene.
In the series of standard potentials, calcium is located to the left of hydrogen. The standard electrode potential of the Ca 2+ / Ca 0 pair is –2.84 V, so that calcium actively reacts with water:
Ca + 2H 2 O = Ca (OH) 2 + H 2.
Calcium reacts with active non-metals (oxygen, chlorine, bromine) under normal conditions:
2Ca + O 2 = 2CaO; Ca + Br 2 = CaBr 2.
When heated in air or oxygen, calcium ignites. Calcium interacts with less active non-metals (hydrogen, boron, carbon, silicon, nitrogen, phosphorus and others) when heated, for example:
Ca + H 2 = CaH 2 (calcium hydride),
Ca + 6B = CaB 6 (calcium boride),
3Ca + N 2 = Ca 3 N 2 (calcium nitride)
Ca + 2C = CaC 2 (calcium carbide)
3Ca + 2P = Ca 3 P 2 (calcium phosphide), calcium phosphides of the compositions CaP and CaP 5 are also known;
2Ca + Si = Ca 2 Si (calcium silicide), calcium silicides of the compositions CaSi, Ca 3 Si 4 and CaSi 2 are also known.
The course of the above reactions, as a rule, is accompanied by the release of a large amount of heat (i.e., these reactions are exothermic). In all compounds with non-metals, the oxidation state of calcium is +2. Most of the calcium compounds with non-metals are readily decomposed by water, for example:
CaH 2 + 2H 2 O = Ca (OH) 2 + 2H 2,
Ca 3 N 2 + 3H 2 O = 3Ca (OH) 2 + 2NH 3.
Calcium oxide is typically basic. In the laboratory and technology, it is obtained by thermal decomposition of carbonates:
CaCO 3 = CaO + CO 2.
Technical calcium oxide CaO is called quicklime.
It reacts with water to form Ca (OH) 2 and release a large amount of heat:
CaO + H 2 O = Ca (OH) 2.
The Ca (OH) 2 obtained in this way is usually called slaked lime or milk of lime. (cm. LIME MILK) due to the fact that the solubility of calcium hydroxide in water is low (0.02 mol / l at 20 ° C), and when it is added to water, a white suspension is formed.
When interacting with acidic oxides, CaO forms salts, for example:
CaO + CO 2 = CaCO 3; CaO + SO 3 = CaSO 4.
The Ca 2+ ion is colorless. When calcium salts are added to the flame, the flame turns brick-red.
Calcium salts such as chloride CaCl 2, bromide CaBr 2, iodide CaI 2 and nitrate Ca (NO 3) 2 are readily soluble in water. Fluoride CaF 2, carbonate CaCO 3, sulfate CaSO 4, average orthophosphate Ca 3 (PO 4) 2, oxalate CaC 2 O 4 and some others are insoluble in water.
Of great importance is the fact that, in contrast to the average calcium carbonate CaCO 3, acidic calcium carbonate (bicarbonate) Ca (HCO 3) 2 is soluble in water. In nature, this leads to the following processes. When cold rain or river water saturated with carbon dioxide penetrates underground and falls on limestones, their dissolution is observed:
CaCO 3 + CO 2 + H 2 O = Ca (HCO 3) 2.
In the same places where water saturated with calcium bicarbonate comes out to the surface of the earth and is heated by the sun's rays, the opposite reaction takes place:
Ca (HCO 3) 2 = CaCO 3 + CO 2 + H 2 O.
This is how large masses of substances are transferred in nature. As a result, huge sinkholes can form underground (see Karst (cm. KARST (natural phenomenon))), and beautiful stone "icicles" - stalactites are formed in the caves (cm. STALACTITES (mineral formations)) and stalagmites (cm. STALAGMITS).
The presence of dissolved calcium bicarbonate in water largely determines the temporary hardness of water (cm. HARDNESS OF WATER)... It is called temporary because when boiling water, bicarbonate decomposes, and CaCO 3 precipitates. This phenomenon leads, for example, to the fact that scale builds up in the kettle over time.
The use of calcium and its compounds
Metallic calcium is used for metallothermal production of uranium (cm. Uranium (chemical element)), thorium (cm. THORIUM), titanium (cm. TITANIUM (chemical element)), zirconium (cm. ZIRCONIUM), cesium (cm. CESIUM) and rubidium (cm. RUBIDIUM).
Natural calcium compounds are widely used in the production of binders (cement (cm. CEMENT), plaster (cm. GYPSUM), lime, etc.). The binding effect of slaked lime is based on the fact that over time, calcium hydroxide reacts with carbon dioxide in the air. As a result of the ongoing reaction, acicular crystals of calcite CaCO 3 are formed, which grow into nearby stones, bricks, and other building materials and, as it were, weld them into a single whole. Crystalline calcium carbonate - marble is an excellent finishing material. Chalk is used for whitewashing. Large amounts of limestone are consumed in the production of cast iron, since they allow the refractory impurities of iron ore (for example, quartz SiO 2) to be converted into relatively low-melting slags.
Bleach is very effective as a disinfectant. (cm. BLEACHING POWDER)- "chlorine" Ca (OCl) Cl - mixed chloride and calcium hypochlorite (cm. CALCIUM HYPOCHLORITE) with a high oxidizing capacity.
Calcium sulfate is also widely used, existing both in the form of anhydrous compounds and in the form of crystalline hydrates - the so-called "semi-aqueous" sulfate - alabaster (cm. Aleviz Fryazin (Milanese)) CaSO 4 · 0.5H 2 O and dihydrate sulfate - gypsum CaSO 4 · 2H 2 O. Gypsum is widely used in construction, in sculpture, for the manufacture of stucco and various art products. Plaster is also used in medicine to fix bones in fractures.
Calcium chloride CaCl 2 is used along with table salt for anti-icing road surfaces. Calcium fluoride CaF 2 is an excellent optical material.
Calcium in the body
Calcium is a nutrient (cm. BIOGENIC ELEMENTS), constantly present in the tissues of plants and animals. An important component of the mineral metabolism of animals and humans and the mineral nutrition of plants, calcium performs various functions in the body. As part of apatite (cm. APATITE), as well as calcium sulfate and carbonate, forms a mineral component of bone tissue. The human body weighing 70 kg contains about 1 kg of calcium. Calcium participates in the work of ion channels (cm. ION CHANNELS), carrying out the transport of substances through biological membranes, in the transmission of a nerve impulse (cm. NERVOUS IMPULSE), in the processes of blood coagulation (cm. COLLECTION OF BLOOD) and fertilization. Regulate calcium metabolism in the body calciferols (cm. CALCIFEROLES)(vitamin D). Lack or excess of calcium leads to various diseases - rickets (cm. RICKETS), calcification (cm. CALCINOSIS) and others. Therefore, human food should contain calcium compounds in the required quantities (800-1500 mg of calcium per day). The calcium content is high in dairy products (such as cottage cheese, cheese, milk), some vegetables and other foods. Calcium preparations are widely used in medicine.

encyclopedic Dictionary. 2009 .

Introduction

Properties and uses of calcium

1 Physical properties

2 Chemical properties

3 Application

Getting calcium

1 Electrolytic production of calcium and its alloys

2 Thermal receipt

3 Vacuum-thermal method for obtaining calcium

3.1 Aluminothermic method of calcium recovery

3.2 Silicothermal method of calcium recovery

Practical part

Bibliography

Introduction

Chemical element of group II of Mendeleev's periodic system, atomic number 20, atomic mass 40.08; silver-white light metal. The natural element is a mixture of six stable isotopes: 40Ca, 42Ca, 43Ca, 44Ca, 46Ca and 48Ca, of which the most common 40 Ca (96, 97%).

Ca compounds - limestone, marble, gypsum (as well as lime - a product of limestone firing) were already used in the building industry in ancient times. Until the end of the 18th century, chemists considered lime to be a simple body. In 1789 A. Lavoisier suggested that lime, magnesia, barite, alumina and silica are complex substances. In 1808, G. Davy, subjecting a mixture of wet slaked lime with mercury oxide to electrolysis with a mercury cathode, prepared an amalgam Ca, and after removing mercury from it, he obtained a metal called "Calcium" (from Latin calx, genus calcis - lime) ...

The ability of calcium to bind oxygen and nitrogen made it possible to use it for the purification of inert gases and as a getter (Getter is a substance used to absorb gases and create a deep vacuum in electronic devices.) In vacuum radio equipment.

Calcium is also used in the metallurgy of copper, nickel, special steels and bronzes; they are associated with harmful impurities of sulfur, phosphorus, excess carbon. Alloys of calcium with silicon, lithium, sodium, boron, and aluminum are used for the same purposes.

In industry, calcium is obtained in two ways:

) Heating a briquetted mixture of CaO and Al powder at 1200 ° C in a vacuum of 0.01 - 0.02 mm. rt. Art .; emitted by reaction:

CaO + 2Al = 3CaO Al2O3 + 3Ca

Calcium vapor condenses on a cold surface.

) The Cu - Ca alloy (65% Ca) is prepared by electrolysis of the CaCl2 and KCl melt with a liquid copper-calcium cathode, from which calcium is distilled off at a temperature of 950 - 1000 ° C in a vacuum of 0.1 - 0.001 mm Hg.

) A method for producing calcium by thermal dissociation of calcium carbide CaC2 has also been developed.

Calcium is abundant in nature in the form of various compounds. In the earth's crust, it ranks fifth, accounting for 3.25%, and is most often found in the form of CaCO limestone 3, dolomite CaCO 3Mg CO 3, gypsum CaSO 42H 2O, phosphorite Ca 3(PO 4)2 and fluorspar CaF 2, not counting the significant proportion of calcium in the composition of silicate rocks. Sea water contains on average 0.04% (wt.) Calcium.

In this course work, the properties and use of calcium are studied, as well as the theory and technology of vacuum-thermal methods for its production are considered in detail.

. Properties and uses of calcium

.1 Physical properties

Calcium is a silvery-white metal, but tarnishes in air due to the formation of oxide on its surface. It is a ductile metal that is harder than lead. Crystal cell ?-form Ca (stable at ambient temperature) face-centered cubic, a = 5.56 Å ... Atomic radius 1.97 Å , ionic radius Ca 2+, 1,04Å ... Density 1.54 g / cm 3(20 ° C). Above 464 ° C, the hexagonal ?-the form. tp 851 ° C, bp 1482 ° C; temperature coefficient of linear expansion 22 10 -6 (0-300 ° C); thermal conductivity at 20 ° C 125.6 W / (m · K) or 0.3 cal / (cm · sec · ° C); specific heat (0-100 ° C) 623.9 J / (kg K) or 0.149 cal / (g ° C); electrical resistivity at 20 ° C 4.6 10 -8ohm m or 4.6 10 -6 ohm cm; temperature coefficient of electrical resistance 4.57 · 10-3 (20 ° C). Elastic modulus 26 Gn / m 2(2600 kgf / mm 2); tensile strength 60 MN / m 2(6 kgf / mm 2); elastic limit 4 MN / m 2(0.4 kgf / mm 2), yield strength 38 MN / m 2(3.8 kgf / mm 2); elongation 50%; Brinell hardness 200-300 Mn / m 2(20-30 kgf / mm 2). Calcium of sufficiently high purity is ductile, well pressed, rolled and amenable to cutting.

1.2 Chemical properties

Calcium is an active metal. So, under normal conditions, it easily interacts with atmospheric oxygen and halogens:

Ca + O 2= 2 CaO (calcium oxide) (1)

Ca + Br 2= CaBr 2(calcium bromide). (2)

Calcium reacts with hydrogen, nitrogen, sulfur, phosphorus, carbon and other non-metals when heated:

Ca + H 2= CaH 2(calcium hydride) (3)

Ca + N 2= Ca 3N 2(calcium nitride) (4)

Ca + S = CaS (calcium sulfide) (5)

Ca + 2 P = Ca 3R 2(calcium phosphide) (6)

Ca + 2 C = CaC 2 (calcium carbide) (7)

Calcium reacts slowly with cold water, and very vigorously with hot water, giving a strong base Ca (OH) 2 :

Ca + 2 H 2O = Ca (OH) 2 + H 2 (8)

Being an energetic reducing agent, calcium can remove oxygen or halogens from oxides and halides of less active metals, i.e., it has reducing properties:

Ca + Nb 2O5 = CaO + 2 Nb; (nine)

Ca + 2 NbCl 5= 5 CaCl2 + 2 Nb (10)

Calcium reacts vigorously with acids with the evolution of hydrogen, reacts with halogens, with dry hydrogen to form CaH hydride 2... When heated, calcium with graphite forms CaC carbide 2... Calcium is produced by electrolysis of molten CaCl 2or aluminothermic reduction in vacuum:

6СаО + 2Al = 3Ca + 3CaO Al2 O 3 (11)

Pure metal is used for the reduction of Cs, Rb, Cr, V, Zr, Th, U compounds to metals, for the deoxidation of steels.

1.3 Application

Calcium finds increasing application in various industries. Recently, it has acquired great importance as a reducing agent in the preparation of a number of metals.

Pure metallic. uranium is obtained by reduction of uranium fluoride with metallic calcium. Calcium or its hydrides can be used to reduce titanium oxides, as well as oxides of zirconium, thorium, tantalum, niobium, and other rare metals.

Calcium is a good deoxidizer and degasser in the production of copper, nickel, chromium-nickel alloys, special steels, nickel and tin bronzes; it removes sulfur, phosphorus, carbon from metals and alloys.

Calcium forms refractory compounds with bismuth, therefore it is used to purify lead from bismuth.

Calcium is added to various light alloys. It helps to improve the surface of the ingots, fineness and reduce oxidation.

Bearing alloys containing calcium are widespread. Lead alloys (0.04% Ca) can be used for the manufacture of cable sheaths.

Antifrictional alloys of Calcium with lead are used in technology. Calcium minerals are widely used. So, limestone is used in the production of lime, cement, silicate bricks and directly as a building material, in metallurgy (flux), in the chemical industry for the production of calcium carbide, soda, sodium hydroxide, bleach, fertilizers, in the production of sugar, glass.

Of practical importance are chalk, marble, Icelandic spar, gypsum, fluorite, etc. Due to the ability to bind oxygen and nitrogen, calcium or calcium alloys with sodium and other metals are used for the purification of noble gases and as a getter in vacuum radio equipment. Calcium is also used to produce hydride, which is a source of hydrogen in the field.

2. Getting calcium

There are several ways to obtain calcium, these are electrolytic, thermal, vacuum-thermal.

.1 Electrolytic production of calcium and its alloys

The essence of the method lies in the fact that the cathode initially touches the molten electrolyte. At the point of contact, a liquid metal droplet that wets the cathode well is formed, which, when the cathode is slowly and uniformly raised, is removed from the melt together with it and solidifies. In this case, the solidifying drop is covered with a solid electrolyte film, which protects the metal from oxidation and nitriding. By continuously and carefully lifting the cathode, the calcium is drawn into the rods.

2.2 Thermal preparation

calcium chemical electrolytic thermal

· Chloride process: the technology consists of melting and dehydrating calcium chloride, melting lead, producing a lead-sodium double alloy, obtaining a lead-sodium-calcium ternary alloy and diluting the ternary alloy with lead after removing the salts. The reaction with calcium chloride proceeds according to the equation

CaCl 2 + Na 2Pb 5 = 2NaCl + PbCa + 2Pb (12)

· Carbide Process: Lead-calcium alloy production is based on the reaction between calcium carbide and molten lead according to the equation

CaC 2+ 3Pb = Pb3 Ca + 2C. (13)

2.3 Vacuum-thermal method for obtaining calcium

Raw materials for vacuum thermal method

The raw material for thermal reduction of calcium oxide is lime obtained by calcining limestone. The basic requirements for raw materials are as follows: lime should be as pure as possible and contain a minimum of impurities that can be reduced and converted into metal along with calcium, especially alkali metals and magnesium. Calcination of limestone should be carried out before the complete decomposition of the carbonate, but not before its sintering, since the reducibility of the sintered material is lower. The fired product must be protected from the absorption of moisture and carbon dioxide, the release of which during recovery reduces the process performance. The technology for calcining limestone and processing the calcined product is similar to the processing of dolomite for the silicothermal method for producing magnesium.

.3.1 Aluminothermic method of calcium recovery

The diagram of the temperature dependence of the change in the free energy of oxidation of a number of metals (Fig. 1) shows that calcium oxide is one of the most durable and difficult to reduce oxides. It cannot be reduced by other metals in the usual way - at a relatively low temperature and atmospheric pressure. On the contrary, calcium itself is an excellent reducing agent for other difficult-to-reduce compounds and a deoxidizing agent for many metals and alloys. Reduction of calcium oxide with carbon is generally impossible due to the formation of calcium carbides. However, due to the fact that calcium has a relatively high vapor pressure, its oxide can be reduced in vacuum with aluminum, silicon or their alloys according to the reaction

CaO + Me? Ca + MeO (14).

So far, only the aluminothermic method of obtaining calcium has found practical application, since it is much easier to restore CaO with aluminum than with silicon. There are different views on the chemistry of the reduction of calcium oxide with aluminum. L. Pidgeon and I. Atkinson believe that the reaction proceeds with the formation of calcium monoaluminate:

CaO + 2Al = CaO Al 2O3 + 3Ca. (15)

V. A. Pazukhin and A. Ya. Fisher indicate that the process proceeds with the formation of tricalcium aluminate:

CaO + 2Al = 3CaO Al 2O 3+ 3Ca. (16)

According to A.I.

CaO + 6Al = 5CaO 3Al 2O3 + 9Са. (17)

The latest research by A. Yu. Taits and A. I. Voinitskiy has established that the aluminothermic reduction of calcium proceeds stepwise. Initially, the release of calcium is accompanied by the formation of 3CaO AI 2O 3, which then reacts with calcium oxide and aluminum to form 3CaO 3AI 2O 3... The reaction proceeds as follows:

CaO + 6Al = 2 (3CaO Al 2O 3) + 2СаО + 2Аl + 6Са

(3СаО Al 2O 3) + 2СаО + 2Аl = 5СаО 3Al 2O 3+ 3Ca

CaO + 6A1 = 5CaO 3Al 2O 3+ 9Са

Since the reduction of oxide occurs with the release of vaporous calcium, and the rest of the reaction products are in a condensed state, it is easy to separate and condense it in the cooled sections of the furnace. The main conditions necessary for the vacuum-thermal reduction of calcium oxide are high temperature and low residual pressure in the system. Below is the relationship between temperature and the equilibrium vapor pressure of calcium. The free energy of reaction (17) calculated for temperatures of 1124-1728 ° K is expressed

F T = 184820 + 6.95T-12.1 T log T.

Hence, the logarithmic dependence of the equilibrium vapor pressure of calcium (mm Hg)

Lg p = 3.59 - 4430 \ T.

L. Pidgeon and I. Atkinson determined experimentally the equilibrium vapor pressure of calcium. A detailed thermodynamic analysis of the reduction of calcium oxide with aluminum was carried out by I.I.Matveenko, who gave the following temperature dependences of the equilibrium pressure of calcium vapor:

Lg p Ca (1) = 8.64 - 12930 \ T mm Hg

Lg p Ca (2) = 8.62 - 11780 \ T mm Hg

Lg p Ca (3 )= 8.75 - 12500 \ T mm Hg

The calculated and experimental data are compared in table. 1.

Table 1- The effect of temperature on the change in the equilibrium pressure of calcium vapor in systems (1), (2), (3), (3), mm Hg

Temperature ° C Test data Calculated in systems (1) (2) (3) (3 )1401 1451 1500 1600 17000,791 1016 - - -0,37 0,55 1,2 3,9 11,01,7 3,2 5,6 18,2 492,7 3,5 4,4 6,6 9,50,66 1,4 2,5 8,5 25,7

It can be seen from the data presented that interactions in systems (2) and (3) or (3 ") are in the most favorable conditions. This is consistent with observations, since pentacalcium tri-aluminate and tricalcium aluminate predominate in the charge residues after reduction of calcium oxide with aluminum.

The data on the equilibrium elasticity show that the reduction of calcium oxide with aluminum is possible at a temperature of 1100-1150 ° C. To achieve a practically acceptable reaction rate, the residual pressure in the Growth system must be below the equilibrium pressure P equal , i.e., the inequality P equal > P ost , and the process should be carried out at temperatures of the order of 1200 °. Studies have established that at a temperature of 1200-1250 °, high utilization (up to 70-75%) and low specific consumption of aluminum (about 0.6-0.65 kg per kg of calcium) are achieved.

According to the above interpretation of the chemistry of the process, the optimal composition is the charge calculated for the formation of 5СаО 3Al in the residue 2O 3... To increase the degree of use of aluminum, it is useful to give some excess of calcium oxide, but not too large (10-20%), otherwise it will negatively affect other indicators of the process. With an increase in the degree of aluminum grinding from particles 0.8-0.2 mm to minus 0.07 mm (according to V. A. Pazukhin and A. Ya. Fisher), the use of aluminum in the reaction increases from 63.7 to 78%.

The use of aluminum is also influenced by the charge briquetting mode. A mixture of lime and powdered aluminum should be briquetted without binders (to avoid gas evolution in vacuum) at a pressure of 150 kg / cm 2... At lower pressures, the use of aluminum decreases due to segregation of molten aluminum in excessively porous briquettes, and at higher pressures, due to poor gas permeability. The completeness and speed of recovery also depend on the density of the briquettes in the retort. When laying them without gaps, when the gas permeability of the entire cage is low, the use of aluminum is significantly reduced.

Figure 2 - Scheme of obtaining calcium by vacuum-thermal method.

Aluminum thermal process technology

The technological scheme for the production of calcium by the aluminothermal method is shown in Fig. 2. Limestone is used as a raw material, and aluminum powder prepared from primary (better) or secondary aluminum is used as a reducing agent. Aluminum used as a reducing agent, as well as raw materials, should not contain impurities of easily volatile metals: magnesium, zinc, alkalis, etc., which can evaporate and turn into condensate. This must be taken into account when choosing grades of secondary aluminum.

According to the description of S. Loomis and P. Staub, in the USA at the plant of New England Lime Co. in Canaan (Connecticut), calcium is obtained by the aluminothermic method. Lime of the following typical composition is used,%: 97.5 CaO, 0.65 MgO, 0.7SiO 2, 0.6 Fe 2Oz + AlO3, 0.09 Na 2O + K 2Oh, 0.5 is the rest. The fired product is ground in a Raymond mill with a centrifugal separator, the fineness of grinding is (60%) minus 200 mesh. Aluminum dust is used as a reducing agent, which is a waste in the production of aluminum powder. Burnt lime from closed hoppers and aluminum from drums are fed to a batching scale and then to a mixer. After mixing, the charge is dry briquetted. At the mentioned plant, calcium is reduced in retort furnaces, which were previously used to obtain magnesium by the silicothermal method (Fig. 3). The furnaces are heated with generator gas. Each furnace has 20 horizontal retorts made of refractory steel containing 28% Cr and 15% Ni.

Picture 3- Retort oven for calcium production

Retort length 3 m, diameter 254 mm, wall thickness 28 mm. Recovery takes place in the heated part of the retort and condensation occurs in the cooled end protruding from the speech. The briquettes are introduced into the retort in paper bags, then the condensers are inserted and the retort is closed. Air is evacuated by mechanical vacuum pumps at the beginning of the cycle. Then the diffusion pumps are connected and the residual pressure is reduced to 20 microns.

The retorts are heated to 1200 °. After 12 hours. after loading, the retorts are opened and unloaded. The resulting calcium has the shape of a hollow cylinder of a dense mass of large crystals deposited on the surface of a steel sleeve. The main impurity in calcium is magnesium, which is reduced in the first place and is mainly concentrated in the layer adjacent to the liner. On average, the content of impurities is; 0.5-1% Mg, about 0.2% Al, 0.005-0.02% Mn, up to 0.02% N, other impurities - Cu, Pb, Zn, Ni, Si, Fe - occur in the range of 0.005-0.04%. A. Yu. Taits and A. I. Voinitskiy used a semi-factory electric vacuum furnace with coal heaters to obtain calcium by the aluminothermal method and achieved a degree of aluminum utilization of 60%, specific consumption of aluminum 0.78 kg, specific consumption of charge, respectively, 4.35 kg and specific electricity consumption 14 kW / h per 1 kg of metal.

The resulting metal, with the exception of magnesium impurities, was of relatively high purity. On average, the content of impurities in it was: 0.003-0.004% Fe, 0.005-0.008% Si, 0.04-0.15% Mn, 0.0025-0.004% Cu, 0.006-0.009% N, 0.25% Al.

2.3.2 Silicothermal recovery method calcium

The silicothermal method is very tempting; the reducing agent is ferrosilicon, the reagent is much cheaper than aluminum. However, the silicothermal process is more difficult to carry out than the aluminothermal one. The reduction of calcium oxide by silicon proceeds according to the equation

CaO + Si = 2CaO SiO2 + 2Ca. (eighteen)

The equilibrium vapor pressure of calcium, calculated from the values ​​of free energy, is:

° С 1300140015001600Р, mm Hg. st0,080,150,752,05

Therefore, in a vacuum of the order of 0.01 mm Hg. Art. the reduction of calcium oxide is thermodynamically possible at a temperature of 1300 °. In practice, to ensure an acceptable speed, the process should be carried out at a temperature of 1400-1500 °.

The reduction reaction of calcium oxide with silicon-aluminum proceeds somewhat easier, in which both aluminum and silicon of the alloy serve as reducing agents. It has been established by experiments that reduction with aluminum predominates in the beginning; and the reaction proceeds with the final formation of bCaO 3Al 2Oz according to the scheme outlined above (Fig. 1). Silicon reduction becomes significant at higher temperatures when most of the aluminum has reacted; the reaction proceeds with the formation of 2CaO SiO 2. In summary, the reduction reaction of calcium oxide with aluminum silico is expressed by the following equation:

mSi + n Al + (4m +2 ?) CaO = m (2CaO SiO 2) + ?n (5СаО Al 2O3 ) + (2m +1, 5n) Ca.

Studies by A. Yu. Taits and A. I. Voinitskiy have established that calcium oxide is reduced by 75% ferrosilicon with a metal yield of 50-75% at a temperature of 1400-1450 ° in a vacuum of 0.01-0.03 mm Hg. Art .; silicon-aluminum containing 60-30% Si and 32-58% Al (the rest is iron, titanium, etc.), reduces calcium oxide with a metal yield of about 70% at temperatures of 1350-1400 ° in a vacuum of 0.01-0.05 mm Hg ... Art. Experiments on a semi-factory scale have proven the fundamental possibility of obtaining calcium from lime with ferrosilicon and aluminum silico. The main instrumental difficulty is the selection of a rack under the conditions of this lining process.

When solving this problem, the method can be implemented in industry. Decomposition of calcium carbide Obtaining metallic calcium by decomposition of calcium carbide

CaC2 = Ca + 2C

should be attributed to the promising methods. In this case, graphite is obtained as a second product. V. Mauderly, E. Moser, and V. Treadwell, having calculated the free energy of formation of calcium carbide from thermochemical data, obtained the following expression for the vapor pressure of calcium over pure calcium carbide:

ca = 1.35 - 4505 \ T (1124 - 1712 ° K),

lgp ca = 6.62 - 13523 \ T (1712-2000 ° K).

Apparently, commercial calcium carbide decomposes at significantly higher temperatures than it follows from these expressions. The same authors report the thermal decomposition of calcium carbide in compact lumps at 1600-1800 ° in a vacuum of 1 mm Hg. Art. The graphite yield was 94%, calcium was obtained in the form of a dense deposit on the refrigerator. A. S. Mikulinsky, F. S. Morii, R. Sh. Shklyar to determine the properties of graphite obtained by the decomposition of calcium carbide, heated the latter in a vacuum of 0.3-1 mm Hg. Art. at a temperature of 1630-1750 °. The resulting graphite differs from the Acheson graphite in larger grains, higher electrical conductivity and lower bulk density.

3. Practical part

The daily pouring of magnesium from the electrolyzer with a current strength of 100 kA was 960 kg when the bath was fed with magnesium chloride. The voltage at the joke of the electrolyzer is 0.6 V. Determine:

)Current output at the cathode;

)The amount of chlorine obtained per day, provided that the current output at the anode is equal to the current output at the ktode;

)Daily filling of MgCl 2into the electrolyzer, provided that the loss of MgCl 2 occur mainly with sludge and fumes. The amount of sludge 0.1 per 1 ton of Mg containing MgCl 2 in the sublimate 50%. The amount of sublimate is 0.05 tons per 1 ton of Mg. The composition of the poured magnesium chloride,%: 92 MgCl2 and 8 NaCl.

.Determine the current output at the cathode:

m NS = I ? K Mg · ?

?= m NS \ I ?K Mg = 960,000 \ 100,000 0.454 24 = 0.881 or 88.1%

.Determine the amount of Cl received per day:

x = 960000g \ 24 g \ mol = 40,000 mol

We translate into volume:

x = 126,785.7 m3

3.a) Find pure MgCl 2, for the production of 960 kg Mg.

x = 95 960 \ 24.3 = 3753 kg = 37.53 t.

b) losses with sludge. From the composition of magnesium electrolyzers,%: 20-35 MgO, 2-5 Mg, 2-6 Fe, 2-4 SiO 2, 0.8-2 TiO 2, 0.4-1.0 C, 35 MgCl2 .

kg - 1000 kg

m shl = 960 kg - mass of sludge per day.

96 kg of sludge per day: 96 0.35 (MgCl2 with sludge).

c) losses with sublimates:

kg - 1000 kg

kg sublimates: 48 0.5 = 24 kg MgCl 2 with sublimates.

In total, you need to fill in Mg:

33.6 + 24 = 3810.6 kg MgCl2 per day

Bibliography

Fundamentals of Metallurgy III

<#"justify">metallurgy Al and Mg. Vetyukov M.M., Tsyplokov A.M.

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Natural compounds of calcium (chalk, marble, limestone, gypsum) and products of their simplest processing (lime) have been known to people since ancient times. In 1808, the English chemist Humphrey Davy subjected to electrolysis wet slaked lime (calcium hydroxide) with a mercury cathode and obtained calcium amalgam (an alloy of calcium with mercury). From this alloy, having distilled mercury, Davy obtained pure calcium.
He also suggested the name of the new chemical element, from the Latin "calx" meaning the name of limestone, chalk and other soft stones.

Being in nature and receiving:

Calcium is the fifth most abundant element in the earth's crust (more than 3%), forms many rocks, many of which are based on calcium carbonate. Some of these rocks are of organic origin (shell rock), showing the important role of calcium in wildlife. Natural calcium is a mixture of 6 isotopes with mass numbers from 40 to 48, with 40 Ca accounting for 97% of the total. Other calcium isotopes have also been obtained by nuclear reactions, for example, radioactive 45 Ca.
To obtain a simple calcium substance, electrolysis of melts of its salts or alumothermy is used:
4CaO + 2Al = Ca (AlO 2) 2 + 3Ca

Physical properties:

A silvery-gray metal with a cubic face-centered lattice, significantly harder than alkali metals. Melting point 842 ° C, boiling point 1484 ° C, density 1.55 g / cm 3. At high pressures and temperatures, about 20K goes into the superconducting state.

Chemical properties:

Calcium is not as active as alkali metals, however it must be stored under a layer of mineral oil or in tightly sealed metal drums. Already at ordinary temperatures, it reacts with oxygen and nitrogen in the air, as well as with water vapor. When heated, it burns in air with a red-orange flame, forming an oxide with an admixture of nitrides. Like magnesium, calcium continues to burn in an atmosphere of carbon dioxide. When heated, it reacts with other non-metals, forming compounds that are not always obvious in composition, for example:
Ca + 6B = CaB 6 or Ca + P => Ca 3 P 2 (and also CaP or CaP 5)
In all its compounds, calcium has an oxidation state of +2.

The most important connections:

Calcium oxide CaO- ("quicklime") a white substance, an alkaline oxide, vigorously reacts with water ("quenched"), turning into a hydroxide. Received by thermal decomposition of calcium carbonate.

Calcium hydroxide Ca (OH) 2- ("slaked lime") white powder, slightly soluble in water (0.16 g / 100 g), strong alkali. A solution ("lime water") is used to detect carbon dioxide.

Calcium carbonate CaCO 3- the basis of most natural minerals of calcium (chalk, marble, limestone, shell rock, calcite, Icelandic spar). Pure white or colorless substance. crystals, When heated (900-1000 C) decomposes, forming calcium oxide. Not p-rim, reacts with acids, is able to dissolve in water saturated with carbon dioxide, turning into bicarbonate: CaCO 3 + CO 2 + H 2 O = Ca (HCO 3) 2. The reverse process leads to the appearance of deposits of calcium carbonate, in particular such formations as stalactites and stalagmites
It also occurs naturally in dolomite CaCO 3 * MgCO 3

Calcium sulfate CaSO 4- a white substance, in nature CaSO 4 * 2H 2 O ("gypsum", "selenite"). The latter, upon careful heating (180 C), transforms into CaSO 4 * 0.5H 2 O ("burnt gypsum", "alabaster") - a white powder, when mixed with water again forms CaSO 4 * 2H 2 O in the form of a solid, sufficiently strong material. Slightly soluble in water, in excess of sulfuric acid it is able to dissolve, forming hydrosulfate.

Calcium phosphate Ca 3 (PO 4) 2- ("phosphorite"), insoluble, under the action of strong acids passes into more soluble calcium hydro- and dihydrogen phosphates. Feedstock for the production of phosphorus, phosphoric acid, phosphoric fertilizers. Calcium phosphates are also included in apatites, natural compounds with the approximate formula Ca 5 3 Y, where Y = F, Cl, or OH, respectively fluoro-, chloro-, or hydroxyapatite. Along with phosphorite, apatite is part of the skeleton of many living organisms, incl. and a person.

Calcium fluoride CaF 2 - (natural .:"fluorite", "fluorspar"), an insoluble substance of white color. Natural minerals have a variety of colors due to impurities. Glows in the dark when heated and under UV irradiation. Increases the fluidity ("fusibility") of slags when receiving metals, which is due to its use as a flux.

Calcium chloride CaCl 2- colorless. krist. in-in well-r-rim in water. Forms crystalline hydrate CaCl 2 * 6H 2 O. Anhydrous ("fused") calcium chloride is a good desiccant.

Calcium nitrate Ca (NO 3) 2- ("calcium nitrate") colorless. krist. in-in well-r-rim in water. An integral part of pyrotechnic compositions, which gives the flame a red-orange color.

Calcium carbide CaС 2- reacts with water, to-tami forming acetylene, for example: CaC 2 + H 2 O = C 2 H 2 + Ca (OH) 2

Application:

Metallic calcium is used as a strong reductant in the production of certain hard-to-reduce metals ("calciothermy"): chromium, REE, thorium, uranium, etc. In the metallurgy of copper, nickel, special steels and bronzes, calcium and its alloys are used to remove harmful impurities of sulfur, phosphorus, excess carbon.
Calcium is also used to bind small amounts of oxygen and nitrogen in high vacuum and inert gas purification.
Neutron-excess 48 Ca ions are used for the synthesis of new chemical elements, for example, element No. 114,. Another calcium isotope, 45 Ca, is used as a radioactive label in studies of the biological role of calcium and its migration in the environment.

The main area of ​​application of numerous calcium compounds is the production of building materials (cement, building mixtures, drywall, etc.).

Calcium is one of the macronutrients in the composition of living organisms, forming the compounds necessary for building both the internal skeleton of vertebrates, and the external many invertebrates, the shell of eggs. Calcium ions are also involved in the regulation of intracellular processes, causing blood clotting. Lack of calcium in childhood leads to rickets, in the elderly - to osteoporosis. The source of calcium is dairy products, buckwheat, nuts, and vitamin D contributes to its absorption. In case of a lack of calcium, various drugs are used: calcex, calcium chloride solution, calcium gluconate, etc.
The mass fraction of calcium in the human body is 1.4-1.7%, the daily requirement is 1-1.3 g (depending on age). Excessive intake of calcium can lead to hypercalcemia - the deposition of its compounds in the internal organs, the formation of blood clots in the blood vessels. Sources:
Calcium (element) // Wikipedia. URL: http://ru.wikipedia.org/wiki/Calcium (date of access: 03/01/2014).
Popular library of chemical elements: Calcium. // URL: http://n-t.ru/ri/ps/pb020.htm (3.01.2014).

DEFINITION

Calcium- the twentieth element of the Periodic Table. Designation - Ca from the Latin "calcium". Located in the fourth period, IIA group. Refers to metals. The core has a charge of 20.

Calcium is one of the most abundant elements in nature. It contains approximately 3% (mass) in the earth's crust. It occurs in the form of numerous deposits of limestone and chalk, as well as marble, which are natural varieties of calcium carbonate CaCO 3. There are also large quantities of gypsum CaSO 4 × 2H 2 O, phosphorite Ca 3 (PO 4) 2 and, finally, various calcium-containing silicates.

In the form of a simple substance, calcium is a malleable, rather hard white metal (Fig. 1). In air, it quickly becomes covered with a layer of oxide, and when heated, it burns out with a bright reddish flame. Calcium reacts with cold water relatively slowly, but it quickly displaces hydrogen from hot water, forming hydroxide.

Rice. 1. Calcium. Appearance.

Atomic and molecular weight of calcium

The relative molecular weight of a substance (M r) is a number that shows how many times the mass of a given molecule is more than 1/12 of the mass of a carbon atom, and the relative atomic mass of an element (Ar) is how many times the average mass of atoms of a chemical element is more than 1/12 the mass of a carbon atom.

Since in the free state calcium exists in the form of monatomic Ca molecules, the values ​​of its atomic and molecular weights coincide. They are equal to 40.078.

Calcium isotopes

It is known that in nature calcium can be found in the form of four stable isotopes 40 Ca, 42 Ca, 43 Ca, 44 Ca, 46 Ca and 48 Ca, with a clear predominance of the isotope 40 Ca (99.97%). Their mass numbers are 40, 42, 43, 44, 46 and 48, respectively. The nucleus of the 40 Ca isotope isotope contains twenty protons and twenty neutrons, and the rest of the isotopes differ from it only in the number of neutrons.

There are artificial isotopes of calcium with mass numbers from 34 to 57, among which the most stable is 41 Ca with a half-life of 102 thousand years.

Calcium ions

At the outer energy level of the calcium atom, there are two electrons that are valence:

1s 2 2s 2 2p 6 3s 2 3p 6 4s 2.

As a result of chemical interaction, calcium gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:

Ca 0 -2e → Ca 2+.

Molecule and atom of calcium

In a free state, calcium exists in the form of monatomic Ca molecules. Here are some properties that characterize the calcium atom and molecule:

Calcium alloys

Calcium serves as an alloying component in some lead alloys.

Examples of problem solving

EXAMPLE 1