Навигация
3. Isotopes
Aluminium has nine isotopes, whose mass numbers range from 23 to 30. Only 27Al (stable isotope) and 26Al (radioactive isotope, t1/2 = 7.2 × 105 y) occur naturally; however, 27Al has a natural abundance of 99.9+ %. 26Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of 26Al to 10Be has been used to study the role of transport, deposition, sediment storage, burial times, and erosion on 105 to 106 year time scales.[9] Cosmogenic 26Al was first applied in studies of the Moon and meteorites. Meteoroid fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further 26Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.[10]
4. Natural occurrence
In the Earth's crust, aluminium is the most abundant (8.3% by weight) metallic element and the third most abundant of all elements (after oxygen and silicon).[11] Because of its strong affinity to oxygen, however, it is almost never found in the elemental state; instead it is found in oxides or silicates. Feldspars, the most common group of minerals in the Earth's crust, are aluminosilicates. Native aluminium metal can be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes.[12] It also occurs in the minerals beryl, cryolite, garnet, spinel and turquoise.[11] Impurities in Al2O3, such as chromium or cobalt yield the gemstones ruby and sapphire, respectively. Pure Al2O3, known as Corundum, is one of the hardest materials known.[11]
Although aluminium is an extremely common and widespread element, the common aluminium minerals are not economic sources of the metal. Almost all metallic aluminium is produced from the ore bauxite (AlOx(OH)3-2x). Bauxite occurs as a weathering product of low iron and silica bedrock in tropical climatic conditions.[13] Large deposits of bauxite occur in Australia, Brazil, Guinea and Jamaica but the primary mining areas for the ore are in Ghana, Indonesia, Jamaica, Russia and Surinam.[14] Smelting of the ore mainly occurs in Australia, Brazil, Canada, Norway, Russia and the United States. Because smelting is an energy-intensive process, regions with excess natural gas supplies (such as the United Arab Emirates) are becoming aluminium refiners.
5. Production and refinement
Although aluminium is the most abundant metallic element in the Earth's crust, it is rare in its free form, occurring in oxygen-deficient environments such as volcanic mud, and it was once considered a precious metal more valuable than gold. Napoleon III, emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the other guests had to make do with gold.[15][16] The Washington Monument was completed, with the 100 ounce (2.8 kg) aluminium capstone being put in place on December 6, 1884, in an elaborate dedication ceremony. It was the largest single piece of aluminium cast at the time. At that time, aluminium was as expensive as silver.[17] Aluminium has been produced in commercial quantities for just over 100 years.
Aluminium is a strongly reactive metal that forms a high-energy chemical bond with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the energy required to reduce aluminium oxide (Al2O3). For example, direct reduction with carbon, as is used to produce iron, is not chemically possible, since aluminium is a stronger reducing agent than carbon. However there is an indirect carbothermic reduction possible by using carbon and Al2O3 which forms an intermediate Al4C3 and this can further yield aluminum metal at a temperature of 1900-2000°C. This process is still under development. This process costs less energy and yields less CO2 than the Hall-Héroult process.[18] Aluminium oxide has a melting point of about 2,000 °C. Therefore, it must be extracted by electrolysis. In this process, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. The operational temperature of the reduction cells is around 950 to 980 °C. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a chemical compound of aluminium, sodium, and calcium fluorides: (Na3AlF6). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process of Karl Bayer. (Previously, the Deville process was the predominant refining technology.)
The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the refined alumina is dissolved in the electrolyte, its ions are free to move around. The reaction at the cathode is:
Al3+ + 3 e− → Al
Here the aluminium ion is being reduced. The aluminium metal then sinks to the bottom and is tapped off, usually cast into large blocks called aluminium billets for further processing.
At the anode, oxygen is formed:
Похожие работы
... . The advertising idea is defined. Advertising strategy is however insufficiently clearly stated. We will take advantage of the given reserve for increase of efficiency of an advertising campaign of Open Company "Натали", i. e. we will develop advertising strategy. 3.3 Use of methods of optimization in advertising activity One of optimisation methods in advertising activity is use of ...
... in 1975 together with Paul Alien, his partner in computer language development. While attending Harvard in 1975, Gates together with Alien developed a version of the BASIC computer programming language for the first personal computer. In the early 1980s. Gates led Microsoft's evolution from the developer of computer programming languages to a large computer software company. This transition ...
... , weight, our distinctive marks.That are shown in the blank enclosed. Plz observe all these requirements or we’ll have to refuse the offer with no charges payed. Ex 2: Write a letter for your firm to an English engineering firm, ordering a special machine. Give packing and marking instructions. thx u for offer of april 1.we r very interested in this sort of machine & would like to order 5 ...
... of lithium were produced through the electrolysis of lithium chloride by Robert Bunsen and Augustus Matthiessen.[20] The discovery of this procedure henceforth led to commercial production of lithium metal, beginning in 1923 by the German company Metallgesellschaft AG, which performed an electrolysis of a liquid mixture of lithium chloride and potassium chloride.[20][29] The production and use ...
0 комментариев