Calcium carbonate

• Calcium carbonate
• Lime
• Calcium carbonate
• Chalk
• E 170

• A02AC01
• A12AA04

Colorless and odorless solid

Fixed

2.73 g · cm -3

825-899 ° C ( decomposition)

Practically insoluble in water: 14 mg · l-1 (20 ° C)

6450 mg · kg -1 ( LD50, rat, oral)

• -1207.6 KJ / mol ( calcite )
• -1207.8 KJ / mol ( aragonite )

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Calcium carbonate ( technical terminology ), calcium carbonate or in German common name, carbonate of lime, is a chemical compound of the elements calcium, carbon and oxygen with the chemical formula CaCO3. As a calcium salt of carbonic acid, it belongs to the group of carbonates. It is a colorless, crystalline solid, the crystal structure of the ions Ca2 and CO32 - is in the ratio 1:1.

Occurrence

Calcium carbonate is one of the most widely used compounds in the soil, especially in the form of sedimentary rocks. Calcium carbonate occurs in particular in the form of the minerals calcite and aragonite. A further modification of the mineral vaterite CaCO3 which precipitates particularly from supersaturated solutions in the form of microscopic crystals.

Calcium carbonate is the main component of limestone sedimentary formed and the metamorphic marble in living nature it is found in the exoskeleton of crustaceans, corals, clams, snails, and protozoa. The first limestone significant extent created by stromatolites over two billion years ago.

Contrary to popular belief, the bones and teeth of vertebrates contain no calcium carbonate, but also calcium-containing substances hydroxyapatite ( bone ) and additionally fluorapatite in teeth.

Properties

Calcium itself is hardly soluble in pure water. In the presence of dissolved carbon dioxide, however, the solubility increases by more than a hundredfold. In this effect is due to weathering of limestone, with the slightly soluble calcium bicarbonate Ca ( HCO3) 2 forms. Because of its solubility calcium bicarbonate is a part of most natural waters, depending on the rock in different concentrations. The concentration of calcium carbonate in water ( 1 ° dH = 10 mg / l CaO or 17.85 mg / L CaCO3 or 0.18 mmol / l) given in Germany with " degrees German hardness ". In France, the unit " French Degrees " is used, where 1 ° fH = 0.1 mmol / l corresponds to Ca2 or Mg2 ions. In Switzerland, either direct indication of mmol / l or the French hardness will be used. Also by other acidic components of air as well as by nitrification in soil calcium carbonate is dissolved. The reversal of this dissolution process, withdrawal of carbonic acid by increasing the temperature, owe most of their natural occurrence of calcium carbonate formation. The complicated mechanism that is visible on some rivers, such as in the chalk terraces in Pamukkale (Turkey), is defined by the lime- carbonic acid equilibrium.

Modifications in the nature

In nature, calcium carbonate forms various rocks which, although chemically identical, but differ in some respects.

Is a fine, microcrystalline sedimentary rock precipitated by the deposition of carbon dioxide by photosynthetic withdrawal calcite and aragonite shells of fossil microbes, such as coccoliths of coccolithophores and foraminifera shells of, has emerged. Chalk occurs at numerous locations along the European chalk belt to light, from the UK via France to the island of Rügen in northern Germany, and is mined in places. Marl at the bottom of lakes or in silted lake basin consists almost entirely of calcite precipitated. The chalk art is, however, made ​​primarily from gypsum (calcium sulfate).

Is also formed mainly by organisms and is more entrenched than chalk. The accumulation is done either directly or indirectly from the remains of living things, such as deposit, for example, snails, mussels, rock-forming corals and sponges, the calcium carbonate to build external or internal skeletons. Indirectly, it is formed that living things, especially phototrophic, assimilate CO2 and so alkalize the medium, resulting in the precipitation of calcium carbonate. The size of the Carbonatkristalle lies between that of chalk and marble. Large limestone deposits are found for example in the Swabian and Franconian Alb, in the Limestone Alps and the Western Alps, the Himalayas and in many other areas.

Is a coarsely crystalline, metamorphic rock that results be recrystallized if chalk, limestone or dolomite under the influence of high temperatures and / or high pressures ( 1000 bar). Large marble reserves are found in North America and Europe, for example in South Tyrol ( Laas ), Austria ( Gummern ), Norway (Molde ) or in Carrara, Italy, home of the pure white statuary from which Michelangelo created his sculptures.

Synthetic calcium carbonate

Synthetic calcium carbonate is called PCC ( Precipitated calcium carbonate English " precipitated calcium carbonate " ) - in contrast to GCC ( ground calcium carbonate engl. " ground calcium carbonate "). It is precipitated by passing carbon dioxide into milk of lime ( calcium hydroxide) and thus obtained as a precipitate:

The precipitation is carried out at a solids content of about 20 %. About the Litigation (temperature, concentration) different crystal modifications can ( crystal morphology ) generated ( " raised " ) are preferred rhombohedral or scalenohedral crystal form. Because you can work with high-purity starting materials, the PCCs are whiter and also have advantages in terms of opacity. Meanwhile generate large paper mills PCC in a " composite " by, recover carbon dioxide that is produced during the combustion in power plants in the form of flue gases back through binding to calcium hydroxide. However, this does not contribute to the reduction of carbon dioxide concentration in the atmosphere ( climate change), because in the previously necessary production of hydrated lime natural limestone must be burned, again using CO2 is released.

Use

Natural calcium carbonate (limestone) is used by in large quantities as raw material for the building materials industry, as an additive in the steel industry, as a mineral fertilizer than feed lime and mineral fillers in various industrial applications such as paper, paints, varnishes, plasters, plastics and backcoats carpets used. A total of over five billion tons of limestone are mined annually. The main use is the production of cement (calcium silicate, calcium aluminate ) and quicklime.

According to the volume sold in the world is the most important calcium carbonate filler. Although more than five percent of the earth's crust is composed of calcium carbonate rocks, a few deposits for the production of fillers are suitable, which should be as white. The largest industrial users of calcium carbonates is white with a lot of over 10 million tonnes ( worldwide) per year, the paper industry, followed by the plastic and building material industry ( plasters and paints ) with a total of another 15 million tons per year. For use in the paper industry, especially as a coating color, especially deposits in France, Italy, Germany, Norway and Austria are mined in Europe, wherein the mineral crushed by wet milling and as a slurry (partly by tanker ) goes on sale.

Calcium carbonate was formerly known as blackboard chalk use, particularly in France called Champagne chalk, consisting of chalk rock, a chemically very pure calcium carbonate. About 55 percent of the chalk sold in Germany today consists of gypsum (calcium sulfate).

Calcium carbonate is approved as a food additive and dye (E 170) and, for example, often used in the baking of bread. For other applications, calcium carbonate is crushed and / or ground and comes in pieces or as a meal in the trade. Natural calcium carbonates are not optimal for some applications, so that synthetic calcium carbonates are used here.

The term hydro - calcite synthetic calcium carbonate is used in water technology for de-acidification of waters with " aggressive carbonic acid ."

Building Materials

By burning of lime produced quicklime. For this by deleting with water hydrated lime (calcium hydroxide Ca ( OH) 2, slaked lime ) is produced. With the carbon dioxide of the air, it reacts to lime and closes the technical Kalkkreislauf. Hydrated lime and lime can be used as plaster or wall coatings such as tadelakt. The first discoverers of this phenomenon were the Romans, who operated on a large scale lime burning.

Lime is sensitive to acids. In the air sulfur oxides contained in an aqueous environment form sulfuric acid ( H2SO4). This turns lime into calcium sulfate ( CaSO4 ) or gypsum ( CaSO4 • 2 H2O). Calcium sulphate containing about 2 g / l, although also slightly water-soluble, but more soluble than calcium carbonate, which leads to a slow leaching of moisture.