Aluminium — страница 4

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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the reduction cells is around 950 to 980 °C. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance.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: 2 O2− → O2 + 4 e− This carbon anode is then oxidized by the oxygen, releasing carbon dioxide: O2 + C → CO2 The anodes in a reduction cell must therefore be replaced regularly, since they are consumed in the process. Unlike the anodes, the cathodes are not

oxidized because there is no oxygen present, as the carbon cathodes are protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes and metal movement. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear. World production trend of aluminiumAluminium electrolysis with the Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The worldwide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced (52 to 56 MJ/kg). The most modern smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg). (Compare this to

the heat of reaction, 31 MJ/kg, and the Gibbs free energy of reaction, 29 MJ/kg.) Reduction line currents for older technologies are typically 100 to 200 kA; state-of-the-art smelters[19] operate at about 350 kA. Trials have been reported with 500 kA cells. Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Smelters tend to be situated where electric power is both plentiful and inexpensive, such as South Africa, Ghana, the South Island of New Zealand, Australia, the People's Republic of China, the Middle East, Russia, Quebec and British Columbia in Canada, and Iceland.[20] Aluminium output in 2005In 2005, the People's Republic of China was the top producer of aluminium with almost a one-fifth world share,

followed by Russia, Canada, and the USA, reports the British Geological Survey. Over the last 50 years, Australia has become a major producer of bauxite ore and a major producer and exporter of alumina.[21] Australia produced 62 million tonnes of bauxite in 2005. The Australian deposits have some refining problems, some being high in silica but have the advantage of being shallow and relatively easy to mine.[22] 6. Recycling Aluminium is 100% recyclable without any loss of its natural qualities. Recovery of the metal via recycling has become an important facet of the aluminium industry. Recycling involves melting the scrap, a process that requires only five percent of the energy used to produce aluminium from ore. However, a significant part (up to 15% of the input material) is

lost as dross (ash-like oxide).[23] Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public awareness. In Europe aluminium experiences high rates of recycling, ranging from 42% of beverage cans, 85% of construction materials and 95% of transport vehicles.[24] Recycled aluminium is known as secondary aluminium, but maintains the same physical properties as primary aluminium. Secondary aluminium is produced in a wide range of formats and is employed in 80% of the alloy injections. Another important use is for extrusion. White dross from primary aluminium production and from secondary recycling operations still contains useful quantities of aluminium which can be extracted industrially.[25] The process