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Aluminum Foil Manufacturing Process II-Smelting Aluminum

2016-07-12

Smelting

Smelting, which separates the aluminum-oxygen  compound (alumina) produced by the Bayer process, is the next step in extracting pure, metallic aluminum from bauxite. Although the procedure currently used derives from the electrolytic method invented contemporaneously by Charles Hall and Paul-Louis-Toussaint Héroult in the late nineteenth century, it has been modernized. First, the alumina is dissolved in a smelting cell, a deep steel mold lined with carbon and filled with a heated liquid conductor that consists mainly of the aluminum compound cryolite.

Next, an electric current is run through the cryolite, causing a crust to form over the top of the alumina melt. When additional alumina is periodically stirred into the mixture, this crust is broken and stirred in as well. As the alumina dissolves, it electrolytically decomposes to produce a layer of pure, molten aluminum on the bottom of the smelting cell. The oxygen merges with the carbon used to line the cell and escapes in the form of carbon dioxide.

Still in molten form, the purified aluminum is drawn from the smelting cells, transferred into crucibles, and emptied into furnaces. At this stage, other elements can be added to produce aluminum alloys with characteristics appropriate to the end product, though foil is generally made from 99.8 or 99.9 percent pure aluminum. The liquid is then poured into direct chill casting devices, where it cools into large slabs called "ingots" or "reroll stock." After being annealed—heat treated to improve workability—the ingots are suitable for rolling into foil.

An alternative method to melting and casting the aluminum is called "continuous casting." This process involves a production line consisting of a melting furnace, a holding hearth to contain the molten metal, a transfer system, a casting unit, a combination unit consisting of pinch rolls, shear and bridle, and a rewind and coil car. Both methods produce stock of thicknesses ranging from 0.125 to 0.250 inch (0.317 to 0.635 centimeter) and of various widths. The advantage of the continuous casting method is that it does not require an annealing step prior to foil rolling, as does the melting and casting process, because annealing is automatically achieved during the casting process.