Aluminium

(Redirected from Aluminum)

HomePage | Recent changes | View source | Discuss this page | Page history | Log in |

Printable version | Disclaimers | Privacy policy

Aluminium (in American English, aluminum; see below) is chemical element number 13 in the periodic table, with symbol Al. It is a soft lightweight metal with a dull silver-gray appearance, caused by a thin layer of oxidation that forms quickly when it is exposed to air and that prevents further corrosion. Although it is an abundant element in the Earth's crust, second only to silicon, it is very rare in its free form, and was once considered a precious metal more valuable than gold. In 1825 the Danish physicist and chemist Hans Christian Orsted produced aluminium for the first time. The invention of the Hall-Héroult process in 1886 made extracting aluminium from minerals inexpensive, and so it is now in common use throughout the world.

Aluminium is one of the few abundant elements that appears to have no function at all in living cells. Though it is not considered as toxic as heavy metals, there is evidence of some toxicity if consumed in excessive amounts. The use of aluminium cookware, popular because of its corrosion resistance and good heat conduction, has not been shown to lead to aluminium toxicity. Excessive consumption of antacids containing aluminium compounds and excessive use of aluminium-containing antiperspirants are more likely causes of toxicity.

The official IUPAC spelling of the element is aluminium; however, Americans and Canadians generally spell and pronounce it aluminum. The element was originally called aluminum, but this was soon after changed to aluminium, to fit better with the other elements (potassium, germanium, etc.) The aluminium spelling then became the most common in both the Britain and the United States; however then the United States changed over time to aluminum for popular purposes. The official name used in the United States for chemistry remained however aluminium until 1926 when the American Chemistry Society changed the spelling to aluminum. In 1990 IUPAC however adopted aluminium as the international standard name for the element. Aluminium is also the name used in French, Dutch, German and Swedish; while Italian uses alluminio, Portugese uses alumínio and Spanish uses aluminio. (The use of these words in these other languages is one of the reasons IUPAC chose aluminium over aluminum.) In 1993, IUPAC recognized aluminum as an acceptable variant, but still prefers the use of aluminium.

The extraction of aluminium

Aluminium is a reactive metal and cannot be extracted from its ore, Bauxite (Al2O3, through reduction with carbon. Instead it is extracted through the process of electrolysis - the metal is oxidised in a solution and then reduced again as a pure metal. The ore needs to be in a liquid state for this to be achieved. However Bauxite has a melting point of 2000'C - which would be a too high temperature to heat it up economically. Instead the Bauxite is dissolved in molten cryolite, which lowers the melting point significantly to about 900'C. This still requires lots of energy and usually aluminium plants have their own power stations nearby.

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

 Al3+ + 3e- -> Al

Here the aluminium is being reduced - the addition of electrons. The aluminium then sinks to the bottom and is tapped off.

The positive cathode oxidises the oxygen from Bauxite, and then it reacts with the carbon electrode to carbon dioxide:

 2O2- -> O2 + 2e-
 O2 + C -> CO2

This cathode needs to be replaced often due to it turning in to carbon dioxide. Despite the cost of electrolysis aluminium is a cheap and widely used metal.


/Talk