How Aluminum Is Made
Aluminum is produced from bauxite in two major stages:
- alumina (aluminum oxide), a white, granular substance which looks somewhat like sugar, is refined from bauxite in a several-step caustic soda/high temperature process.
- the alumina is electrolytically reduced in molten cryolite to separate it into aluminum and oxygen.
Briefly, to obtain the alumina, the bauxite ore is broken up and screened when necessary to remove stone and similar large impurities. Then it is mixed with a caustic liquor in slurry tanks and pumped to steam-heated digesters, where added caustic dissolves the alumina from the bauxite.
The entire mix is then transferred to setting tanks, where the residue, called red mud collects on the bottom, leaving the alumina in solution on trop. Comparatively clear, but with some very fine particles still suspended as the liquor overflows the setting tank, it now passes through filters to remove the last of the unwanted mud, and is held in large tanks.
When cool, the liquor is seeded with a fine alumina hydrate and air-agitated to precipitate crystals of hydrated aluminum oxide. These are washed, drained, and transferred to high temperature rotary kilns, in which all remaining water is driven from the pure aluminum oxide. When cooled, the alumina is stored for delivery to aluminum reduction plants, or other processors.
The alumina is converted into aluminum at the reduction plant. Typically, this is accomplished in a carbon-lined electrolytic cell in which the electrolyte is molten cryolite (sodium-aluminum fluoride). This is basically the Hall process3. The current flows between a carbon anode and the pot lining (cathode). As alumina is added to the bath, it is separated by electrolysis into oxygen and aluminum. The liberated oxygen combines with the carbon of the anode and is carried away as carbon dioxide gas. The molten aluminum is heavier than the electrolytic solution and settles to the bottom of the reduction cell, from which it is siphoned at appropriate intervals.
The aluminum reduction process operates at low voltage and high amperage. A series of these electrolytic cells or pots connected together by large aluminum electrical bus bars, is called a potline. The molten aluminum is siphoned off into crucibles which are transported by overhead cranes and emptied into furnaces, where other elements are added to produce the desired alloy. The metal is then poured into vertical, direct chill casting devices to produce rolling ingot. Ingot is reduced to re-roll stock gauges on the regular sheet and plate mills, and finally to foil gauges in specialized foil rolling mills.
Re-roll stock also is produced directly from molten metal.
Two pounds of alumina, obtained from four pounds of bauxite, are required to make one pound of aluminum. In addition, 0.6 pound carbon plus small amounts of other materials, are consumed in the process. When commercial production of aluminum began shortly before the turn of the century, 12 kilowatt hours (Kwh) of electrical energy were needed to produce one pound of aluminum.
By the early 1950's, process refinements had reduced the power consumption to about 9. Now, in the late 1970's, the average is less than 8 Kwh/lb, with several newer plants having achieved a 6.5 Kwh/lb energy use. And one pilot plant, now employing a new aluminum smelting process, has consumed less than 5 Kwh/lb average during its first nine months of regular operation. The aluminum industry is continuing to employ energy conservation practices and research at all levels.
This is coiled aluminum of suitable cross section which is used as stock for further rolling into specific foil gauges. It is sometimes called foil stock or sheet stock, but re-roll stock is correct and the standard definition of this material is, A semi-finished rolled product of rectangular cross section in coiled form suitable for further rolling.
Re-roll stock is produced by two processes, as previously mentioned. Still the most widely used is the conventional molten-aluminum-cast ingot/rolled-ingot progression outlined under the Aluminum production section above, and by continuous casting. This produces re-roll directly from molten metal, and will be described briefly in the next section, headed Continuous Casting.
When produced by either of these methods, re-roll stock is usually made in the 0.125 in to 0.250 in thickness range, in various widths. Thickness and length of the re-roll stock determines the length of any given foil gauge that can be rolled from that specific stock. The original width of the re-roll stock, on the other hand, determines the width of the foil rolled from it, regardless of gauge. This phenomenon will be discussed under the Rolling Aluminum Foil.
All re-roll stock is supplied in the O temper, for maximum workability. Most aluminum foil shipped is made from alloy 1145 (99.45% aluminum), and is finished in either the dead soft 1145-O, or the full hard 1145-h19 temper. Depending on the final foil gauge to be produced, the alloy, and the thickness of the re-roll stock initially, the metal must be re-annealed between mill passes, as required, to assure adequate workability for rolling to final gauge.
In a typical continuous casting system for producing re-roll stock, each production line includes substantially the following equipment: melting furnace; holding hearth, to insure a reservoir of molten metal at correct purity and temperature; metal transfer system; a direct-casting unit with suitable stri-feed device; a combination stand with pinch rolls, shear and bridle; and a rewind and coil car unit.
In operation, this system continuously feeds, casts, chills, and coils re-roll stock at production speeds. All of the intermediate steps of conventional foil production-ingot (heating) casting, breakdown soaking (initial heavy rolling), and final mill passes to reduce the metal to re-roll gauges are eliminated. Also, ingot-produced re-roll must be annealed prior to foil rolling, and this is not required of continuous-cast re-roll as it is in an annealed condition as fabricated.