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A luminum is electrolytically extracted
from alumina. It is made by igneous electrolysis of aluminum oxide which is found in
larger concentrations within bauxite ore. Bauxite is a mixture of the hydroxides of
aluminum, together with other impurities such as oxides of iron, titanium, and silicon.
B auxite is produced by the weathering and
change of aluminum silicate rocks usually found in tropical and semitropical regions where
climate has produced an accelerated weathering process. Bauxite is not a rare ore and is
widely available in the US, the Caribbean, Europe and more specially Australia, Guinea,
Venezuela. Approximately 4 pounds of ore are required to produce 1 pound of aluminum
metal.
T he process used almost universally to
purify bauxite is the Bayer process, which separates
aluminum hydrate from the bauxite and then uses a calcination process to convert it to
oxide of aluminum, which has 2 aluminum and 3 oxygen atoms.
T he aluminum oxide is dissolved in
electric furnaces, (resembling melting pots) in a molten bath of sodium-aluminum fluoride
at 940 to 980 degrees centigrade (1725 to 1800 degrees fahrenheit). Using a method
developed independantly by P. HEROULT (in France) and by C. M. HALL (in the U.S) in 1886,
the furnace pots are made of carbon lined steel and used as a cathode. Other carbon
electrode is used as cosummable anodes, and current introduced through these pairs of
carbon electrodes electrolytically separates the aluminum and also provides the heat
necessary to keep the bath molten. With electricity applied, the oxygen in the ore
combines with the carbon in the anode, leaving at the bottom of the pot or vessel 99.9%
pure molten aluminum.
T he molten aluminum is removed
periodically by vacuum suction from the bottom of the furnace or "cell" as it
collects. It takes approximately 6 to 7 kilowatt hours of electricity to make each pound
of aluminum, and for that reason aluminum production is concentrated in areas of the world
where electricity is relatively cheap.
T he molten aluminum, once siphoned off,
is poured into molds to form what is known as a primary ingots. If alloying with other
metals is desired, the molten aluminum is transferred to a furnace where pure alloying
elements or master alloys (concentrated alloys within an aluminum base) are added to
produce the desired aluminum alloy. The alloyed aluminum is then poured into molds to make
primary aluminum ingots. |
