Molten Metal, T-Bar, Sow
Almost all aluminum products are produced from molten metal, T-bar,
and sow. T-bar and sow are simply the solid form of molten metal. T-bar
and sow can be remelted to produce extrusions or cast into ingots or
billets, which are then pressed or rolled to form sheet, plate, foil,
wire, rod, and bar. Ingots, billets, and extrusions also can be cast or
drawn directly from molten metal.
Molten metal is produced directly from alumina. Alumina from the refinery is moved to the reduction plant or smelter—long buildings with row after row of reduction cells, or pots, where the pure, white powder is transformed into glistening, molten metal.
Although there are many types of cells, they operate on the same
principle. The pots are actually steel boxes of various sizes, generally
about 20 feet long, 6 feet wide, and about 3 feet deep, and lined with
carbon. The alumina is dissolved in a molten salt called cryolite and
aluminum fluoride in the steel pot, and a carbon electrode (anode) is
lowered into the solution.
Direct electrical current, of between 50,000 and 150,000 amperes,
then flows from the anode through the now molten mixture to the carbon
cathode lining of the pot. Since each pot requires only about 5 volts,
it is customary to connect a hundred or more pots in a series, in rooms
700 feet long or more. Several buildings are often connected in the
electrical current to form a single potline.
The electric current reduces the alumina molecules into aluminum and
oxygen. The oxygen is deposited on the carbon anode, where it combines
with the carbon to form carbon-dioxide. The aluminum, which is heavier
than the cryolite, settles to the bottom of the pot.
The molten aluminum is siphoned into crucibles about once a day, more alumina is added to the pot, and the process continues day and night.
At this point, the molten metal can be cast directly into billets and ingots. (click here for more information on billet and ingot casting) The molten metal can also be cast into T-bar or sow. Once cast, the T-bar and sow can be transported easily from the smelter to the cast house, where it will be remelted and cast into billet or ingot.