Something about aluminium
June 12 2011: Clay is eroded and weathered rock, and as aluminium is
abundant in most rock bound chemically to oxygen and silicium, it also
constitute a great part of clays.
To purify metallic aluminium from clay is not easy, and only bauxite -
a clay primarily found around the Caribbean - is used.Bauxite contain
around 40% aluminium oxide - Al2O3 or alumina - from where the metal can be extracted through industrial processes.
The fine ground bauxite first will be boiled in water with caustic soda
under pressure in tanks up to 25 meter high where the aluminium oxide
will dissolve as aluminium hydroxide (also called alumina
trihydrate).Insoluble material known as red mud is filtered away, and
the remaining
solution cooled and seeded with fine crystals of aluminium hydroxide
facilitating precipitation of the dissolved aluminium hydroxide.
Remaining caustic soda solution is reused and the obtained aluminium
hydroxide baked in ovens, decomposing to water and now pure Al2O3, after baking a very hard, common salt looking substance.
The refined alumina next is exposed to electrolysis to extract the
aluminium by forcing electrons from a current to the metal ions.Usually
such electrolytic reduction is carried out in water solution
but because of aluminiums high reactivity with water this method is not
possible.Instead alumina is dissolved in a melted mineral, cryolite (Na3AlF6) at a temperature of 10000 C,
in a construction in 1886 named the Hall-Heroult cell after two 23 year
old chemists.Here a 5 V and 100.000 Ampere current is applied over
carbon electrodes, and at the cathode each aluminium ion will accept 3
electrons, liquefy and sink down as molten aluminium (with melting
point 6600 C) while at the anode oxygen ions will react with carbon forming CO and CO2.
Much power is needed in the process, as example will energy consumed
for making a aluminium soda can lit a 60W bulb for 5 hours.Older
aluminium plants are therefore often located near hydroelectric power
plants, in later years also where natural gas is abundant for cheap
power production (The Emirates).The demand for aluminium is
exponential growing with a total production of 10 million tonnes per
year in 1970, 30 in 2005 and 41 million tonnes in 2009.Such a
boom has environmental consequences, and our Labour Organisation (LO)
now support the world leading aluminium company Alcoa if deciding on a
prospective new aluminium plant based on power from natural gas north
in our country, even if this would mean breaking our national goals for
cut in domestic greenhouse gas emissions before 2020.Indeed a complex
world we live in.
Top is image of the beautiful aluminium containing mineral mica.