Aluminium

Aluminium or aluminum (Symbol Al) (see the spelling section below) is a silvery and ductile member of the poor metal group of chemical elements. Its atomic number is 13. Aluminium is found primarily as the ore bauxite and is remarkable for its resistance to oxidation (due to the phenomenon of passivation), its strength, and its light weight. Aluminium is used in many industries to make millions of different products and is very important to the world economy. Structural components made from aluminium are vital to the aerospace industry and very important in other areas of transportation and building in which light weight, durability, and strength are needed.

Natural occurrence

Although Al is an abundant element in Earth's crust (7.5% -> 8.1%, it has not been confirmed), it is very rare in its free form and was once considered a precious metal more valuable than gold (It is said that Napoleon III of France had a set of aluminium plates reserved for his finest guests. Others had to make do with gold ones.) It is therefore comparatively new as an industrial metal and has been produced in commercial quantities for just over 100 years.

Related Topics:
Precious metal - Gold - Napoleon III of France - Industrial metal

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Aluminium was, when it was first discovered, extremely difficult to separate from the rocks it was part of. Since the whole of Earth's aluminium was bound up in the form of compounds, it was the most difficult metal on earth to get, despite the fact that it is one of the planet's most common. The reason is that aluminium is oxidized very rapidly and that its oxide is an extremely stable compound that, unlike rust on steel, does not flake off. The very reason for which aluminium is used in many applications is why it is so hard to produce.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Recovery of this metal from scrap (via recycling) has become an important component of the aluminium industry. Recycling involves simply melting the metal, which is far less expensive than creating it from ore. Refining aluminium requires enormous amounts of electricity; recycling it requires only 5% of the energy to produce it. A common practice since the early 1900s, aluminium recycling is not new. It was, however, a low-profile activity until the late 1960s when the exploding popularity of aluminium beverage cans finally placed recycling into the public consciousness. Sources for recycled aluminium include automobiles, windows and doors, appliances, containers and other products.

Related Topics:
Recycling - Electricity - 1900s - Beverage can

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Aluminium is a reactive metal and it is hard to extract it from its ore, aluminium oxide (Al2O3). Direct reduction, with e.g. carbon, is not economically viable since aluminium oxide has a melting point of about 2000 °C. Therefore, it is extracted by electrolysis — the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. By this process, the actual operational temperature of the reduction cells is around 950 to 980 °C. Cryolite was originally found as a mineral on Greenland, but by has been replaced by a synthetic cryolite. Cryolite is a mixture of aluminium, sodium, and calcium fluorides: (Na3AlF6). The aluminium oxide (a white powder) is obtained by refining bauxite, which is red since it contains 30 to 40% iron oxide. This is done using the so-called Bayer process. Previous to this, the process used was the Deville process.

Related Topics:
Aluminium oxide - O - Carbon - Electrolysis - Cryolite - Sodium - Calcium - Fluoride - Bauxite - Bayer process - Deville process

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The electolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium.

Related Topics:
Wöhler process - Aluminium chloride - Potassium

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The electrodes used in the electrolysis of aluminium oxide are both carbon. Once the ore is in the molten state, its ions are free to move around. The reaction at the negative cathode is

Related Topics:
Electrode - Carbon - Cathode

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

: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 gas is formed:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:2O2- → O2 + 4e-

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

This carbon anode is then oxidized by the oxygen. The anodes in a reduction must therefore be replaced regularly, since they are consumed in the process:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:O2 + C → CO2

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Contrary to the anodes, the cathodes are not consumed during the operation, since there is no oxygen present at the cathode. The carbon cathode is protected by the liquid aluminium inside the cells. Cathodes do erode, mainly due to electrochemical processes. After 5 to 10 years, depending on the current used in the electrolysis, a cell has to be reconstructed completely, because the cathodes are completely worn.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

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 (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.

Related Topics:
Electrolysis - Hall-Héroult - Kilowatt-hour - MJ

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Electric power represents about 20 to 40% of the cost of producing aluminium, depending on the location of the aluminium smelter. Smelters tend to be located where electric power is plentiful and inexpensive, such as South Africa, the South Island of New Zealand, Australia, China, Middle-East, Russia, Iceland and Quebec in Canada.

Related Topics:
South Africa - South Island - New Zealand - Australia - China - Middle-East - Russia - Iceland - Quebec - Canada

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

China is currently (2004) the top world producer of aluminium. The Economy of Suriname might be the economy most dependent on its aluminium exports.

Related Topics:
China - 2004 - Economy of Suriname

~ ~ ~ ~ ~ ~ ~ ~ ~ ~