Aluminium

ALUMINIUM, Al (from the Latin word "alumen", which means the name of the alums, which have been used during the ancient times as the mordant during the dyeing and tanning * EN: aluminium; DE: Aluminium; FR: aluminium; ES: aluminio; RU: алюминий), is the chemical element of the Group 3 of the Periodic System by D.I.Mendeleev, its atomic number is 13, its atomic weight is 26.9815. It comprises one stable isotope with the mass number 27. It has been discovered by the Danish scientist H.Oersted in 1825.

Aluminium is the silvery-white light metal. The crystal lattice of aluminium is face-centered cubic with the parameter a = 0.40413 nanometres (4.0413 E). The aluminium of the high purity (99.996%) is characterized by the following physical properties: the density (at 20 degrees Celsius) is 2698.9 kilograms per cubic metre, the temperature of the melting point is 660,24 degrees Celsius, the temperature of the boiling point is 2500 degrees Celsius, the thermal conductivity (at 190 degrees Celsius) is 343 watts per metre-kelvin, the specific heat capacity (at 100 degrees Celsius) if 931.98 joules per kilogram-kelvin, the electrical conductivity with respect to copper (at 20 degrees Celsius) is 65.5%, the coefficient of the thermal expansion (from 20 to 100 degrees Celsius) is 2.39 • 10^-5 per degree Celsius. Aluminium has low strength (tensile strength is 50-60 MPa) and hardness (170 MPa according to Brinell), but high plasticity (50%). Aluminium may be well polished, anodized, and has the high reflectivity (90%). Aluminium is stable under the impact of the various types of the natural waters, nitric acid, and organic acids. Aluminium, which is exposed to the air, coats itself with a thin strong film, which protects the metal from the further oxidation and corrosion.

Under the normal conditions, aluminium has the +3 oxidation state, +1 at the high temperatures, +2 rarely.

Aluminium has strong affinity to oxygen, forming the Al2O3 oxide; aluminium in the powder form burns during the heating within the flow of oxygen, developing the temperature of approximately 3000 degrees Celsius. They use this peculiarity of aluminium during the aluminothermy for the reduction of certain metals from their oxides. At the high temperatures, aluminium combines with nitrogen, carbon, and sulfur, forming respectively the nitride AlN, carbide Al4C3, and sulfide Al2S3. Aluminium does not react with hydrogen; they obtain the hydride (AlH3)x by the indirect pathway. Aluminium easily solves within the alkalis, with separation of hydrogen, and formation of aluminates. Most salts of aluminium are well soluble in water.

Aluminium is one of the most common (after oxygen and silicon) of the elements in the rocks of the Earth crust, namely, 8.8% (by the mass). The maximal content of aluminium has been noted within the sedimentary rocks, namely, 10.45% (by the mass), the content within the intermediate, basic, acidic, and ultrabasic ones is respectively 8.85%, 8.76%, 7.7%, 0.45% (by the mass). There are known hundreds of minerals, within which it is the constituent part as the main or rather common element. Aluminium silicates are the major carriers of aluminium. The minerals with the maximal aluminium content are the corundum, gibbsite, boehmite, diaspore. Bauxites are the main source for the obtainment of aluminium. Besides this, they partially extract aluminium from the high alumina content alkaline rocks (urtites and others) and alunites.

The main peculiarity of the geochemical behaviour of aluminium in the endogenous processes is its fairly uniform distribution within the crystallizing aluminosilicates, namely, the feldspars, micas, amphiboles, and pyroxenes. It is not characteristical for the post-magmatic and hydrothermal formations. Cryolite Na3[AlF6] is the only peculiar, but quite rare mineral of aluminium, which is associated with pegmatites. Aluminium is very weak migrant during the exogenous processes, as the consequence of the high hydrolyzability of its salts, with the precipitation of the poorly soluble hydroxide Al(OH)3, of the poor solubility of its other compounds, of the high crystalline-chemical stability of the aluminium-silicon-oxygen radicals within the aluminosilicates. The main concentrator of aluminium within the exogenous processes is kaolin, which forms itself as the residual product during the process of the weathering of the acidic, intermediate, and basic rocks. Subsequently, during the erosion and redeposition of the kaolinitic crusts of the weathering, aluminium arrives into the sedimentary rocks, mainly clays. Within the particularly contrasting conditions of the weathering (humid tropics, high temperature of the environment), the decomposition within the rocks reaches the stage of the formation of the residual (eluvial) bauxites. Aluminium is scarce within the living organisms and hydrosphere, although there are known the individual organisms, which are the concentrators of aluminium (club mosses, certain species of mollusks). At the same time, within the soils and certain waters, which are rich in organic substance, there is noted the certain migratory mobility of aluminium in the form of the organo-mineral compounds. The peculiar mobility of aluminium is established within the certain volcanogenic-hydrothermal ultra-acidic and acidic solutions. See the "Aluminium ores" and "Bauxites" articles for the major genetic types of the deposits and for the schemes of the benefication.

They obtain the metallic aluminium within the industry by the electrolysis of the solution of the alumina within the molten cryolite or within the melt of the AlCl3; they produce the aluminium of the high purity (99.996%) by the electrolytic refining with the help of the so-called three-layer method. The same technology in principle, but with the usage of the organic electrolytes, permits to achieve the purity of the refined aluminium up to 99.999%.

Thanks to the lightness, sufficient strength, ability to alloy with many other metals, and good electrical conductivity, aluminium finds widespread usage within the electrical technology, and also as the structural material within the mechanical engineering, aviation, construction, and other industries. They use the pure and ultra-pure aluminium within the semiconductor technology, and for the coating of the various types of mirrors. Aluminium has got the usage within the nuclear reactors in association with the relatively low cross section of the absorption of the neutrons. They transport within the tanks and containers, which are made of aluminium, the liquid gases (methane, oxygen, hydrogen), certain acids (nitric, acetic), they store the food, water, oils. They use aluminium as the alloyant within the alloys with Cu, Mg, Ti, Ni, Zn, Fe. In certain cases, aluminium is used for the production of the explosive substances (aluminal, alumotol, and others).