Aluminium dissolves in sodium hydroxide to yield hydrogen gas and aluminates of the form [Al(OH)4]–.
+ 2NaOH (aq)
+ 3H2 (g)
Occurrence and Production of Aluminium
Metal production and refinement
Although aluminium is the most abundant metallic element in Earth’s crust (believed to be 7.5% to 8.1%), it is very 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 ones!
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 2,000oC. It is extracted by electrolysis; 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 980oC. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance.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.
The electrolytic process replaced the Wohler 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+ + 3e– –> 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:
2O2- –> O2 + 4e–
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-Heroult 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 kilowatt-hours per kilogram of aluminium produced from alumina. (52 to 56 MJ/kg). The most modern smelters reach approximately 12.8 kWh/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.
Recovery of the metal via recycling has become an important facet of the aluminium industry. Recycling involves melting the scrap, a process that uses only five percent of the energy needed to produce aluminium from ore. Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public consciousness.
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, China, the Middle East, Russia, Quebec and British Columbia in Canada, and Iceland.
In 2004, the People’s Republic of China was the top world producer of aluminium. Over the last 50 years, Australia has become a major producer of bauxite ore and a major producer and exporter of alumina. Australia produced 62 million tonnes of bauxite in 2005. The Australian deposits have some refining problems, being high in silica but have the advantage of being shallow and relatively easy to mine.
Isotopes of Aluminium
26Al [13 neutrons]
Half life: 7.17 x 105 years [ beta+ ]
Decay Energy: 1.17 MeV
Decays to 26Mg.
Half life: 7.17 x 105 years [ Electron Capture ]
Decay Energy: ? MeV
Decays to 26Mg.
Half life: 7.17 x 105 years [ Gamma Radiation ]
Decay Energy: 1.8086 MeV
Decays to ?.
Meteorite research has also shown that 26Al was relatively abundant at the time of formation of our planetary system. Most meteoriticists 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.
27Al [14 neutrons]
Stable with 14 neutrons