Aluminium既提煉方法....

Aluminium is a reactive metal that is difficult to extract from ore, aluminium oxide (Al2O3). Direct reduction — with carbon, for example — is not economically viable since aluminium oxide has a melting point of about 2,000 °C. Therefore, it is extracted by electrolysis; that is, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. By this process, 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the reduction cells is around 950 to 980 °C. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance.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 mixture of aluminium, sodium, and calcium fluorides: (Na3AlF6). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process. (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 ore is in the molten state, its ions are free to move around. The reaction at the cathode — the negative terminal — is

Al3+ + 3 e- → Al
Here the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off.

At the positive electrode (anode), oxygen is formed:

2 O2- → O2 + 4 e-
This carbon anode is then oxidised by the oxygen, releasing carbon dioxide. The anodes in a reduction must therefore be replaced regularly, since they are consumed in the process:

O2 + C → CO2
Unlike the anodes, the cathodes are not oxidised because there is no oxygen present at the cathode. The carbon cathode is protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.

Aluminium 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 world-wide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced from alumina. (52 to 56 MJ/kg). The most modern smelters reach approximately 12.8 kW·h/kg (46.1 MJ/kg). Reduction line current for older technologies are typically 100 to 200 kA. State-of-the-art smelters operate with 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, 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. (Nearly all the power for aluminium smelting in Iceland comes from the heat vents upon which the island sits.

Aluminum is mainly extracted from an ore called bauxite which is rich in Al2O3. We can melt the aluminum into sodium hydroxide to form the Na[Al(OH)4] in a bit below 200C. The impurities cannot melt and they can be filtered out. Afterwards, we can do a electrolysis to deposit the pure aluminum metal with this liquid at 950C.

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