Calcium- element of the 4th period and PA-group of the Periodic system, serial number 20. Electronic formula atom [ 18 Ar] 4s 2 , oxidation states +2 and 0. Refers to alkaline earth metals. It has a low electronegativity (1.04), exhibits metallic (basic) properties. Forms (as a cation) numerous salts and binary compounds. Many calcium salts are sparingly soluble in water. In nature - sixth in terms of chemical abundance, the element (the third among metals) is in a bound form. A vital element for all organisms. The lack of calcium in the soil is replenished by the application of lime fertilizers (CaCO 3 , CaO, calcium cyanamide CaCN 2, etc.). Calcium, calcium cation and its compounds color the flame of a gas burner in a dark orange color ( qualitative detection).
Calcium Ca
Silver-white metal, soft, ductile. In humid air, it tarnishes and becomes covered with a film of CaO and Ca(OH) 2. Very reactive; ignites when heated in air, reacts with hydrogen, chlorine, sulfur and graphite:
Reduces other metals from their oxides (an industrially important method is calciumthermy):
Receipt calcium in industry:
Calcium is used to remove non-metal impurities from metal alloys, as a component of light and antifriction alloys, to isolate rare metals from their oxides.
Calcium oxide CaO
basic oxide. The technical name is quicklime. White, highly hygroscopic. Has an ionic structure Ca 2+ O 2- . Refractory, thermally stable, volatile on ignition. Absorbs moisture and carbon dioxide from the air. Reacts vigorously with water (high exo- effect), forms a strongly alkaline solution (hydroxide precipitation is possible), the process is called lime slaking. Reacts with acids, metal and non-metal oxides. It is used for the synthesis of other calcium compounds, in the production of Ca(OH) 2 , CaC 2 and mineral fertilizers, as a flux in metallurgy, a catalyst in organic synthesis, a component of binders in construction.
Equations of the most important reactions:
Receipt CaO in industry– limestone roasting (900-1200 °С):
CaCO3 = CaO + CO2
Calcium hydroxide Ca(OH) 2
basic hydroxide. The technical name is slaked lime. White, hygroscopic. It has an ionic structure Ca 2+ (OH -) 2. Decomposes on moderate heat. Absorbs moisture and carbon dioxide from the air. Slightly soluble in cold water (an alkaline solution is formed), even less so in boiling water. A clear solution (lime water) quickly becomes cloudy due to the precipitation of hydroxide (the suspension is called milk of lime). A qualitative reaction to the Ca 2+ ion is the passage of carbon dioxide through lime water with the appearance of a precipitate of CaCO 3 and its transition into solution. Reacts with acids and acid oxides, enters into ion exchange reactions. It is used in the production of glass, whitewash, lime mineral fertilizers, for soda causticization and softening. fresh water, as well as for the preparation of lime mortars - doughy mixtures (sand + slaked lime + water), serving as a binder for masonry and brickwork, finishing (plastering) walls and other construction purposes. The hardening ("seizure") of such solutions is due to the absorption of carbon dioxide from the air.
Calcium - chemical element Group II with atomic number 20 in the periodic system, denoted by the symbol Ca (lat. Calcium). Calcium is a soft, silvery-gray alkaline earth metal.
20 element of the periodic table The name of the element comes from lat. calx (in genitive case calcis) - "lime", "soft stone". It was proposed by the English chemist Humphry Davy, who isolated metallic calcium in 1808.
Calcium compounds - limestone, marble, gypsum (as well as lime - a product of burning limestone) have been used in construction for several millennia ago.
Calcium is one of the most abundant elements on Earth. Calcium compounds are found in almost all animal and plant tissues. It accounts for 3.38% of the mass of the earth's crust (5th place in abundance after oxygen, silicon, aluminum and iron).
Finding calcium in nature
Due to the high chemical activity of calcium in the free form in nature is not found.
Calcium accounts for 3.38% of the mass of the earth's crust (5th place in abundance after oxygen, silicon, aluminum and iron). Element content in sea water- 400 mg / l.
isotopes
Calcium occurs in nature in the form of a mixture of six isotopes: 40Ca, 42Ca, 43Ca, 44Ca, 46Ca and 48Ca, among which the most common - 40Ca - is 96.97%. Calcium nuclei contain the magic number of protons: Z = 20. Isotopes
40
20
Ca20 and
48
20
Ca28 are two of the five doubly magic number nuclei found in nature.
Of the six naturally occurring calcium isotopes, five are stable. The sixth 48Ca isotope, the heaviest of the six and very rare (its isotopic abundance is only 0.187%), undergoes double beta decay with a half-life of 1.6 1017 years.
In rocks and minerals
Most calcium is contained in the composition of silicates and aluminosilicates of various rocks (granites, gneisses, etc.), especially in feldspar - anorthite Ca.
In the form of sedimentary rocks, calcium compounds are represented by chalk and limestone, consisting mainly of the mineral calcite (CaCO3). The crystalline form of calcite, marble, is much less common in nature.
Calcium minerals such as calcite CaCO3, anhydrite CaSO4, alabaster CaSO4 0.5H2O and gypsum CaSO4 2H2O, fluorite CaF2, apatites Ca5(PO4)3(F,Cl,OH), dolomite MgCO3 CaCO3 are quite widespread. The presence of calcium and magnesium salts in natural water determines its hardness.
Calcium, which migrates vigorously into earth's crust and accumulating in various geochemical systems, forms 385 minerals (fourth place in terms of the number of minerals).
The biological role of calcium
Calcium is a common macronutrient in plants, animals and humans. In humans and other vertebrates, most of it is in the skeleton and teeth. Calcium is found in bones in the form of hydroxyapatite. From various forms calcium carbonate (lime) consists of the "skeletons" of most groups of invertebrates (sponges, coral polyps, mollusks, etc.). Calcium ions are involved in blood coagulation processes, and also serve as one of the universal second messengers inside cells and regulate a variety of intracellular processes - muscle contraction, exocytosis, including the secretion of hormones and neurotransmitters. The concentration of calcium in the cytoplasm of human cells is about 10−4 mmol/l, in intercellular fluids about 2.5 mmol/l.
The need for calcium depends on age. For adults aged 19-50 years and children aged 4-8 inclusive, the daily requirement (RDA) is 1000 mg (contained in approximately 790 ml of milk with a fat content of 1%), and for children aged 9 to 18 years inclusive - 1300 mg per day (contained in approximately 1030 ml of milk with a fat content of 1%). In adolescence, adequate calcium intake is very important due to the intensive growth of the skeleton. However, according to research in the US, only 11% of girls and 31% of boys aged 12-19 achieve their needs. In a balanced diet, most of the calcium (about 80%) enters the child's body with dairy products. The remaining calcium comes from cereals (including whole grain bread and buckwheat), legumes, oranges, greens, nuts. Dairy products based on milk fat (butter, cream, sour cream, cream-based ice cream) contain practically no calcium. The more milk fat in a dairy product, the less calcium it contains. Calcium absorption in the intestine occurs in two ways: transcellular (transcellular) and intercellular (paracellular). The first mechanism is mediated by the action of the active form of vitamin D (calcitriol) and its intestinal receptors. It plays a big role in low to moderate calcium intake. With a higher calcium content in the diet, intercellular absorption begins to play the main role, which is associated with a large calcium concentration gradient. Due to the transcellular mechanism, calcium is absorbed to a greater extent in the duodenum (due to the highest concentration of receptors in calcitriol there). Due to intercellular passive transfer, calcium absorption is most active in all three sections of the small intestine. Calcium absorption is paracellularly promoted by lactose (milk sugar).
Calcium absorption is hindered by some animal fats (including cow's milk fat and beef fat, but not lard) and palm oil. The palmitic and stearic acids contained in such fats fatty acid are cleaved off during digestion in the intestine and in the free form firmly bind calcium, forming calcium palmitate and calcium stearate (insoluble soaps). In the form of this soap with a chair, both calcium and fat are lost. This mechanism is responsible for decreased calcium absorption, reduced bone mineralization, and reduced indirect measures of bone strength in infants with palm oil (palm olein) based infant formula. In these children, the formation of calcium soaps in the intestines is associated with hardening of the stool, a decrease in its frequency, as well as more frequent regurgitation and colic.
The concentration of calcium in the blood due to its importance for a large number vital processes are precisely regulated, and with proper nutrition and sufficient intake of low-fat dairy products and vitamin D, deficiency does not occur. Prolonged deficiency of calcium and/or vitamin D in the diet leads to an increased risk of osteoporosis and causes rickets in infancy.
Excessive doses of calcium and vitamin D can cause hypercalcemia. The maximum safe dose for adults aged 19 to 50 inclusive is 2500 mg per day (about 340 g of Edam cheese).
Compounds of calcium.
CaO- calcium oxide or quicklime, it is obtained by the decomposition of limestone: CaCO 3 \u003d CaO + CO 2 is an oxide alkaline earth metal, so it actively interacts with water: CaO + H 2 O \u003d Ca (OH) 2
Ca(OH) 2 - calcium hydroxide or slaked lime, so the reaction CaO + H 2 O \u003d Ca (OH) 2 is called lime slaking. If the solution is filtered, lime water is obtained - this is an alkali solution, so it changes the color of phenolphthalein to crimson.
Hydrated lime is widely used in construction. Its mixture with sand and water is a good binding material. Under the action of carbon dioxide, the mixture hardens Ca (OH) 2 + CO 2 \u003d CaCO3 + H 2 O.
At the same time, part of the sand and the mixture turns into silicate Ca (OH) 2 + SiO 2 \u003d CaSiO 3 + H 2 O.
The equations Ca (OH) 2 + CO 2 \u003d CaCO 2 + H 2 O and CaCO 3 + H 2 O + CO 2 \u003d Ca (HCO 3) 2 play an important role in nature and in shaping the appearance of our planet. Carbon dioxide in the form of a sculptor and architect creates underground palaces in the strata of carbonate rocks. It is capable of moving hundreds and thousands of tons of limestone underground. Through cracks in rocks, water containing carbon dioxide dissolved in it enters the limestone thickness, forming cavities - castra caves. Calcium bicarbonate exists only in solution. Groundwater moves in the earth's crust, evaporating water under suitable conditions: Ca (HCO3) 2 \u003d CaCO3 + H 2 O + CO 2 , this is how stalactites and stalagmites are formed, the formation scheme of which was proposed by the famous geochemist A.E. Fersman. There are a lot of castra caves in the Crimea. Science studies them speleology.
Used in construction calcium carbonate CaCO3- this is chalk, limestone, marble. All of you have seen our railway station: it is finished with white marble brought from abroad.
experience: blow through a tube into a solution of lime water, it becomes cloudy .
Ca(OH) 2 + CO 2 = CaCO 3 + H 2 ABOUT
Adds acetic acid to the formed precipitate, effervescence is observed. carbon dioxide is released.
CaCO 3 +2CH 3 COOH \u003d Ca (CH 3 SOO) 2 +H 2 O + CO 2
THE TALE ABOUT THE CARBONATE BROTHERS.
Three brothers live on earth
From the Carbonate family.
The older brother is a handsome MARBLE,
Glorious in the name of Karara,
Excellent architect. He
He built Rome and the Parthenon.
Everyone knows LIMESTONE,
That's why it's named like that.
Famous for his work
Building a house behind the house.
Both able and able
The younger soft brother MEL.
How to draw, look
This CaCO 3!
Brothers love to frolic
Burn in a hot oven
CaO and CO 2 are then formed.
It's carbon dioxide
Each of you is familiar with him,
We breathe it out.
Well, this is Sao -
Hot burnt quicklime.
Add water to it
Thoroughly mixing
To avoid trouble
We protect our hands
Cool mixed LIME, but SLAKED!
milk of lime
The walls are whitewashed easily.
The bright house cheered up
Turning lime into chalk.
Hocus pocus for the people:
One has only to blow through the water,
How easy it is
Turned into milk!
Now it's pretty smart.
I get soda
Milk plus vinegar. Ay!
Foam is pouring over the edge!
All in worries, all in work
From dawn to dawn -
These brothers the Carbonates,
These CaCO 3!
Repetition:
CaO– calcium oxide, quicklime;
Ca(OH) 2
- calcium hydroxide (slaked lime, lime water, milk of lime, depending on the concentration of the solution).
General is the same chemical formula Ca(OH) 2 . Difference: lime water is a transparent saturated solution of Ca (OH) 2, and milk of lime is a white suspension of Ca (OH) 2 in water.
CaCl 2
- calcium chloride, calcium chloride;
CaCO 3
- calcium carbonate, chalk, shell rock marble, limestone.
L/R: collections. Next, we demonstrate the collection of minerals available in the school laboratory: limestone, chalk, marble, shell rock.
CaS0 4
∙ 2H 2
0
- hydrated calcium sulfate, gypsum;
CaCO 3
- calcite, calcium carbonate is part of many minerals that cover 30 million km 2 on earth.
The most important of these minerals is limestone. Shell rocks, limestones of organic origin. It goes to the production of cement, calcium carbide, soda, all kinds of lime, in metallurgy. Limestone is the backbone of the construction industry and many building materials are made from it.
Chalk it's not just tooth powder and school chalk. It is also a valuable additive in the production of paper (coated - highest quality) and rubber; in the construction and repair of buildings - as a whitewash.
Marble is a dense crystalline rock. There is color - white, but most often various impurities color it in different colors. Pure white marble is rare and is mainly used by sculptors (statues of Michelangelo, Rodin. In construction, colored marble is used as a facing material (Moscow metro) or even as the main building material of palaces (Taj Mahal).
In the world of interesting "MAUSOLEUM" Taj Mahal ""
Shah Jahan from the Mughal dynasty held in fear and obedience almost all of Asia. In 1629, Mumzat Mahal, Shah Jahan's beloved wife, died at the age of 39 during childbirth on a campaign (it was their 14th child, all of them boys). She was unusually beautiful, bright, intelligent, the emperor obeyed her in everything. Before her death, she asked her husband to build a tomb, take care of the children, and not marry. The saddened king sent his messengers to all big cities, the capitals of neighboring states - to Bukhara, Samarkand, Baghdad, Damascus, in order to find and invite the best craftsmen - in memory of his wife, the king decided to erect the best building in the world. At the same time, messengers sent to Agra (India) plans for all the best buildings in Asia and the best building materials. They even brought malachite from Russia and the Urals. The chief masons came from Delhi and Kandahar; architects - from Istanbul, Samarkand; decorators - from Bukhara; gardeners from Bengal; the artists are from Damascus and Baghdad, and the well-known master Ustad-Isa was in charge.
Together, over 25 years, a melomarble structure was built, surrounded by green gardens, blue fountains and a red sandstone mosque. 20,000 slaves erected this miracle of 75 m (with a 25-storey building). Nearby, he wanted to build a second mausoleum of black marble for himself, but did not have time. He was overthrown from the throne by his own son (2nd, and he also killed all his brothers).
The ruler and ruler of Agra spent the last years of his life looking out of the narrow window of his dungeon. 7 years so the father admired his creation. When his father went blind, his son made him a system of mirrors so that his father could admire the mausoleum. He was buried in the Taj Mahal, next to his Mumtaz.
Those entering the mausoleum see cenotaphs - false tombs. The places of eternal rest of the great khan and his wife are downstairs, in the basement. Everything there is encrusted with precious stones that glow as if alive, and the branches of fabulous trees, intertwined with flowers, adorn the walls of the tomb with intricate patterns. Turquoise-blue lapis lazuli, green-black nephrites and red amethysts processed by the best carvers sing the love of Shah Jahal and Mumzat Mahal.
Every day tourists rush to Agra who want to see the true wonder of the world - the mausoleum of the Taj Mahal, as if hovering above the ground.
CaCO 3 - This construction material the external skeleton of molluscs, corals, shells, etc., egg shells. (illustrations or Animals of the coral biocenosis” and display of a collection of sea corals, sponges, shell rock).
In ancient times, people used calcium compounds for construction. Basically, it was calcium carbonate, which was in the rocks, or the product of its burning - lime. Marble and plaster were also used. Previously, scientists believed that lime, which is calcium oxide, is a simple substance. This misconception existed until the end of the 18th century, until Antoine Lavoisier expressed his assumptions about this substance.
Lime miningIN early XIX century, the English scientist Humphrey Davy discovered calcium in its pure form using electrolysis. Moreover, he received calcium amalgam from slaked lime and mercury oxide. Then, after distilling the mercury, he obtained metallic calcium.
The reaction of calcium with water is violent, but is not accompanied by ignition. Due to the abundant release of hydrogen, the plate with calcium will move through the water. A substance is also formed - calcium hydroxide. If phenolphthalein is added to the liquid, it will turn bright crimson - therefore, Ca(OH)₂ is a base.
Ca + 2H₂O → Ca(OH)₂↓ + H₂
The reaction of calcium with oxygen
The reaction of Ca and O₂ is very interesting, but the experiment cannot be performed at home, as it is very dangerous.
Consider the reaction of calcium with oxygen, namely, the combustion of this substance in air.
Attention! Do not try to repeat this experience yourself! you will find safe chemistry experiments you can do at home.
Let us take potassium nitrate KNO₃ as a source of oxygen. If calcium was stored in a kerosene liquid, then before the experiment it must be cleaned with a burner, holding it over a flame. Next, calcium is dipped into KNO₃ powder. Then calcium with potassium nitrate must be placed in the flame of the burner. Potassium nitrate decomposes into potassium nitrite and oxygen. The released oxygen ignites the calcium, and the flame turns red.
KNO₃ → KNO₂ + O₂
2Ca + O₂ → 2CaO
It is worth noting that calcium reacts with some elements only when heated, these include: sulfur, boron, nitrogen and others.
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 preparation
3 Vacuum-thermal method for obtaining calcium
3.1 Aluminothermic method of calcium reduction
3.2 Silicothermic method of calcium reduction
Practical part
Bibliography
Introduction
Group II chemical element periodic system Mendeleev, atomic number 20, atomic mass 40.08; silver-white light metal. A natural element is a mixture of six stable isotopes: 40Ca, 42Ca, 43Ca, 44Ca, 46Ca and 48Ca, of which 40 is the most common Ca (96.97%).
Ca compounds - limestone, marble, gypsum (as well as lime - a product of burning limestone) have been used in construction since ancient times. Until the end of the 18th century, chemists considered lime to be a simple substance. 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 of Ca, and after driving mercury out of it, he obtained a metal called "Calcium" (from Latin calx, genus case calcis - lime) .
The ability of calcium to bind oxygen and nitrogen made it possible to use it for cleaning inert gases and as a getter (A getter is a substance that serves 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. For the same purposes, calcium alloys with silicon, lithium, sodium, boron, and aluminum are used.
In industry, calcium is obtained in two ways:
) By heating a briquetted mixture of CaO and Al powder at 1200 ° C in a vacuum of 0.01 - 0.02 mm. rt. Art.; released by the reaction:
CaO + 2Al = 3CaO Al2O3 + 3Ca
Calcium vapor condenses on a cold surface.
) By electrolysis of a melt of CaCl2 and KCl with a liquid copper-calcium cathode, an alloy of Cu - Ca (65% Ca) is prepared, 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 has also been developed for obtaining calcium by thermal dissociation of calcium carbide CaC2.
Calcium is very common in nature in the form of various compounds. In the earth's crust, it occupies the fifth place, accounting for 3.25%, and is most often found in the form of limestone CaCO 3, dolomite CaCO 3MgCO 3, gypsum CaSO 42H 2O, Phosphorite Ca 3(PO 4)2 and fluorspar CaF 2, not counting a significant proportion of calcium in the composition of silicate rocks. Sea water contains an average of 0.04% (wt.) calcium.
In this term paper the properties and application of calcium are studied, as well as the theory and technology of vacuum-thermal methods for its production are also 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 an oxide on its surface. It is a ductile metal harder than lead. Crystal cell ?-form Ca (stable at ordinary 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 stable hexagonal ?-form. mp 851 °C, tbp 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 s °C); specific heat capacity (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). Modulus of elasticity 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 plastic, well pressed, rolled and can be machined.
1.2 Chemical properties
Calcium is an active metal. So at 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)
With hydrogen, nitrogen, sulfur, phosphorus, carbon and other non-metals, calcium reacts 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 \u003d Ca 3R 2(calcium phosphide) (6)
Ca + 2 C \u003d CaC 2 (calcium carbide) (7)
Calcium interacts slowly with cold water, and very vigorously with hot water, giving a strong base Ca (OH) 2 :
Ca + 2 H 2O \u003d Ca (OH) 2 + H 2 (8)
Being an energetic reducing agent, calcium can take away oxygen or halogens from oxides and halides of less active metals, i.e. it has restorative properties:
Ca + Nb 2O5 = CaO + 2 Nb; (9)
Ca + 2 NbCl 5= 5 CaCl2 + 2 Nb (10)
Calcium reacts vigorously with acids with the release of hydrogen, reacts with halogens, with dry hydrogen to form CaH hydride 2. When calcium is heated with graphite, CaC carbide is formed 2. Calcium is obtained by electrolysis of molten CaCl 2or aluminothermic reduction in vacuum:
6СаО + 2Al = 3Ca + 3CaO Al2 ABOUT 3 (11)
Pure metal is used to reduce Cs, Rb, Cr, V, Zr, Th, U compounds to metals, for steel deoxidation.
1.3 Application
Calcium finds an ever-increasing use in various industries production. Recently he has acquired great importance as a reducing agent in the production of a number of metals.
Pure metal. Uranium is obtained by reducing uranium fluoride with calcium metal. Titanium oxides, as well as oxides of zirconium, thorium, tantalum, niobium, and other rare metals can be reduced with calcium or its hydrides.
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, so it is used to purify lead from bismuth.
Calcium is added to various light alloys. It contributes to the improvement of the surface of the ingots, fineness and reduction of oxidizability.
Bearing alloys containing calcium are widely used. Lead alloys (0.04% Ca) can be used to make cable sheaths.
Antifriction alloys of Calcium with lead are used in engineering. Calcium minerals are widely used. So, limestone is used in the production of lime, cement, silicate brick and directly as a building material, in metallurgy (flux), in the chemical industry for the production of calcium carbide, soda, caustic soda, bleach, fertilizers, in the production of sugar, glass.
Chalk, marble, Icelandic spar, gypsum, fluorite, etc. are of practical importance. Due to the ability to bind oxygen and nitrogen, calcium or calcium alloys with sodium and other metals are used to purify 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 drop of metal that wets the cathode is formed, which, when the cathode is slowly and evenly raised, is removed from the melt with it and solidifies. In this case, the solidifying drop is covered with a solid film of electrolyte, 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, obtaining a double alloy of lead - sodium, obtaining a ternary alloy of lead - sodium - calcium, and diluting the ternary alloy with lead after removing salts. The reaction with calcium chloride proceeds according to the equation
CaCl 2 + Na 2Pb 5=2NaCl + PbCa + 2Pb (12)
· Carbide process: the basis for obtaining a lead-calcium alloy is 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 material for vacuum thermal process
The raw material for the thermal reduction of calcium oxide is lime obtained by roasting limestone. The main requirements for raw materials are as follows: lime must be as pure as possible and contain a minimum of impurities capable of being reduced and converted into metal along with calcium, especially alkali metals and magnesium. Calcination of limestone should be carried out until the carbonate is completely decomposed, but not before it is sintered, since the reducibility of the sintered material is lower. The fired product must be protected from absorption of moisture and carbon dioxide, the release of which during recovery reduces the performance of the process. The technology of burning limestone and processing the burnt product is similar to the processing of dolomite for the silicothermic method of obtaining magnesium.
.3.1 Aluminothermic method of calcium reduction
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. The 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).
Practical use So far, he has found only an aluminothermic method for obtaining calcium, since it is much easier to reduce 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. Voynitsky, the formation of pentacicium trialuminate is predominant in the reaction:
CaO + 6Al = 5CaO 3Al 2O3 + 9Ca. (17)
Latest research, A. Yu. Taits and AI Voynitsky found 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 according to the following scheme:
CaO + 6Al = 2 (3CaO Al 2O 3)+ 2CaO + 2Al + 6Ca
(3CaO Al 2O 3) + 2CaO + 2Al = 5CaO 3Al 2O 3+ 3Са
CaO + 6A1 \u003d 5CaO 3Al 2O 3+ 9Ca
Since oxide reduction occurs with the release of vaporous calcium, and the remaining reaction products are in a condensed state, it is possible to easily 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. The relationship between temperature and the equilibrium vapor pressure of calcium is given below. The free energy of reaction (17), calculated for temperatures 1124-1728°K, is expressed as
F T \u003d 184820 + 6.95T-12.1 T lg T.
Hence the logarithmic dependence of the equilibrium elasticity of calcium vapor (mm Hg)
Lg p \u003d 3.59 - 4430 \ T.
L. Pidgeon and I. Atkinson determined experimentally the equilibrium vapor pressure of calcium. A detailed thermodynamic analysis of the reduction reaction 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:
lgp Ca(1) \u003d 8.64 - 12930\T mm Hg
lgp Ca(2) \u003d 8.62 - 11780\T mm Hg
lgp Ca(3 )\u003d 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 elasticity of calcium vapor in systems (1), (2), (3), (3), mm Hg.
Temperature °С Experimental 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 under the most favorable conditions. This is consistent with observations, since pentascalcium trialuminate and tricalcium aluminate predominate in the residues of the charge after the reduction of calcium oxide with aluminum.
Equilibrium elasticity data 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 Rost system must be below the equilibrium P equals , i.e., the inequality Р equals >P ost , and the process must 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) is achieved.
According to the above interpretation of the chemistry of the process, the optimal composition is the mixture designed for the formation of 5CaO 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 much (10-20%), otherwise this will adversely affect other process indicators. With an increase in the degree of aluminum grinding from particles of 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 affected by the mode of charge briquetting. A mixture of lime and aluminum powder should be briquetted without binders (to avoid outgassing in a vacuum) at a pressure of 150 kg/cm 2. At lower pressures, the use of aluminum decreases due to segregation of molten aluminum in overly porous briquettes, and at higher pressures, due to poor gas permeability. The completeness and speed of recovery also depend on the packing density of the briquettes in the retort. When laying them without gaps, when the gas permeability of the entire charge is low, the use of aluminum is significantly reduced.
Figure 2 - Scheme for obtaining calcium by vacuum-thermal method.
Technology of alumino-thermal way
The technological scheme for the production of calcium by the aluminothermic 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., capable of evaporating and turning into condensate. This must be taken into account when choosing grades of recycled aluminum.
According to the description of S. Loomis and P. Staub, in the USA, at the New England Lime Co. plant in Canaan (Connecticut), calcium is obtained by the aluminothermic method. Lime of the following typical composition is used, %: 97.5 CaO, 0.65 MgO, 0.7 SiO 2, 0.6 Fe 2Oz + AlOz, 0.09 Na 2O+K 2Oh, 0.5 the rest. The calcined product is ground in a Raymond mill with a centrifugal separator, the fineness of grinding is (60%) minus 200 mesh. As a reducing agent, aluminum dust is used, which is a waste in the production of aluminum powder. Burnt lime from closed hoppers and aluminum from drums are fed to the dosing scales and then to the mixer. After mixing, the mixture is briquetted in a dry way. At the mentioned plant, calcium is reduced in retort furnaces, which were previously used to obtain magnesium by the silicothermic method (Fig. 3). Furnaces are heated with generator gas. Each furnace has 20 horizontal retorts made of refractory steel containing 28% Cr and 15% Ni.
Figure 3 - Retort furnace for calcium production
Retort length 3 m, diameter 254 mm, wall thickness 28 mm. Reduction occurs 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 pumped out 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 up to 1200°. After 12 hours. after loading, the retorts are opened and unloaded. The resulting calcium has the form 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 sleeve. The average 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 - are found in the range of 0.005-0.04%. A. Yu. Taits and A. I. Voinitsky used a semi-factory electric vacuum furnace with coal heaters to obtain calcium by the aluminothermic method and achieved a degree of aluminum utilization of 60%, a specific aluminum consumption of 0.78 kg, a specific charge consumption of 4.35 kg, respectively, and a specific electricity consumption 14 kWh per 1 kg of metal.
The resulting metal, with the exception of magnesium impurity, was distinguished by a 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 Silicothermic reduction method calcium
The silicothermic method is very tempting; the reducing agent is ferrosilicon, the reagent is much cheaper than aluminum. However, the silicothermic process is more difficult to implement than the aluminothermic one. The reduction of calcium oxide by silicon proceeds according to the equation
CaO + Si = 2CaO SiO2 + 2Ca. (18)
The equilibrium elasticity of calcium vapor, calculated from the values of free energy, is:
°С1300140015001600Р, mm Hg st0.080.150.752.05
Therefore, in a vacuum of the order of 0.01 mm Hg. Art. 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 silicoaluminum proceeds somewhat easier, in which both aluminum and silicon of the alloy serve as reducing agents. It has been established experimentally that reduction with aluminum predominates at the beginning; moreover, 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 form, the reduction reaction of calcium oxide with silicoaluminum is expressed by the following equation:
mSi + n Al + (4m +2 ?) CaO \u003d m (2CaO SiO 2) + ?n(5CaO Al 2O3 ) + (2m +1, 5n) Ca.
Research by A. Yu. Taits and A. I. Voinitsky found 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.; silicoaluminum containing 60-30% Si and 32-58% Al (the rest is iron, titanium, etc.) reduces calcium oxide with a metal yield of approximately 70% at temperatures of 1350-1400 ° in a vacuum of 0.01-0.05 mm Hg . Art. Experiments on a semi-factory scale proved the fundamental possibility of obtaining calcium on lime with ferrosilicon and silicoaluminum. The main hardware difficulty is the selection of a lining that is resistant to this process.
When solving this problem, the method can be implemented in industry. Decomposition of calcium carbide Production of metallic calcium by decomposition of calcium carbide
CaC2 = Ca + 2C
should be regarded as promising. In this case, graphite is obtained as the second product. W. Mauderly, E. Moser, and W. 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 \u003d 1.35 - 4505 \ T (1124 - 1712 ° K),
lgp ca \u003d 6.62 - 13523 \ T (1712-2000 ° K).
Apparently, commercial calcium carbide decomposes at much higher temperatures than follows from these expressions. The same authors report thermal decomposition of calcium carbide in compact pieces at 1600-1800°C in a vacuum of 1 mm Hg. Art. The yield of graphite was 94%, calcium was obtained in the form of a dense coating 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, the latter was heated in a vacuum of 0.3-1 mm Hg. Art. at a temperature of 1630-1750°. The resulting graphite differs from Acheson's in larger grains, higher electrical conductivity, and lower bulk density.
3. Practical part
The daily outflow of magnesium from the electrolyzer for a current of 100 kA was 960 kg when the bath was fed with magnesium chloride. The voltage on the cell jester 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 kode;
)Daily filling MgCl 2into the electrolyzer, provided that the loss of MgCl 2 occur mainly with sludge and sublimation. Amount of sludge 0.1 per 1 ton of Mg containing MgCl 2 in sublimation 50%. The amount of sublimation is 0.05 t per 1 t of Mg. The composition of the poured magnesium chloride, %: 92 MgCl2 and 8 NaCl.
.Determine the current output at the cathode:
m etc =I ?k mg · ?
?=m etc \I ?k mg \u003d 960000\100000 0.454 24 \u003d 0.881 or 88.1%
.Determine the amount of Cl received per day:
x \u003d 960000g \ 24 g \ mol \u003d 40000 mol
Converting to volume:
х=126785.7 m3
3.a) We find pure MgCl 2, for the production of 960 kg Mg.
x \u003d 95 960 \ 24.3 \u003d 3753 kg \u003d 37.53 tons.
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 \u003d 960 kg - mass of sludge per day.
Per day 96 kg of sludge: 96 0.35 (MgCl2 with sludge).
c) losses with sublimates:
kg - 1000 kg
kg sublimates: 48 0.5 = 24 kg MgCl 2 with sublimates.
All you need to fill in Mg:
33.6+24=3810.6 kg MgCl2 per day
Bibliography
Fundamentals of Metallurgy III
<#"justify">metallurgy of Al and Mg. Vetyukov M.M., Tsyplokov A.M.
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