Aluminum goes
way beyond the can

The ancient Greeks and Romans knew the aluminum compound alum (one of several hydrated aluminum sulfates of a univalent cation such as potassium, sodium or ammonium) and used it both as an astringent and as a mordant in dyeing.

The astringent properties of aluminum compounds are used today in antiperspirants and styptic applications.

Although pure aluminum has metallic properties, its place at No. 13 in the periodic table puts it just on the metallic side of the border between metal and nonmetal.

Aluminum is a highly reactive chemical element, so it never occurs uncombined in nature. Its primary occurrence is as major constituent of the mineral feldspar, the most commonly occurring mineral on Earth, in which it is chemically bonded tightly to oxygen and silicon atoms.

Yet feldspar is unstable and breaks down chemically in the presence of slightly acidified water from carbon dioxide that dissolves in rain from the atmosphere.

The weathering process is slow in the human time but rapid on a geological time frame.

When feldspar weathers, the end product is clay, which under the extremes of warm and moist tropical climates further weathers to form bauxite, a general term for hydrated aluminum oxides (alumina), iron oxides, silica and other impurities.

In 2003 most of the world's production of bauxite, just over 160 million tons, came from the Australian outback, Guinea and Jamaica. Brazil and Venezuela also contribute significant amounts. Of this, U.S processing plants smelt up to 6.5 million tons yearly.

Aluminum's metallic properties make it suitable for many applications.

It is lightweight, nonmagnetic and nonsparking. It is second among metals in malleability and sixth in ductility. It has a high strength-to-weight ratio and resists corrosion.

In order to isolate pure aluminum, the impurities must be removed from the bauxite.

The first commercial extraction of alumina from bauxite is attributed to Henri Sainte-Claire Deville in 1854.

Charles Hall perfected the first commercial production of metallic aluminum in 1888 by processing of cryolite, an aluminum-bearing mineral of very limited distribution found mostly in Greenland.

The Hall process uses electrolysis (passing an electric current through molten cryolite) to separate the aluminum metal from the sodium and fluorine that also comprise the cryolite.

In the same year that Hall developed the electrolytic process, Karl Joseph Bayer described what is now known as the Bayer process to convert bauxite to alumina. As a result of the two processes, aluminum is an everyday commodity rather than a precious metal.

The Bayer process mixes ground bauxite into a liquid solution of sodium hydroxide, where steam under pressure slowly dissolves the bauxite.

Alumina in the bauxite reacts with the sodium hydroxide to form sodium aluminate.

A sequence of processes reduces the pressure and temperature and precipitates alumina, leaving behind the iron oxides, silica and other impurities. After the precipitate is removed from the tank, it is washed, then heated in a kiln to dry. The final product is a commercially pure alumina.

The alumina has many uses, most of which is smelted into aluminum metal.

The tendency for aluminum to form strong chemical bonds requires large amounts energy to extract it from ore, and mining it has heavy environmental costs.

Bauxite normally lies within 100 feet of the surface, so mining typically consists of scraping away the top layers, leaving scars on the landscape.

Smelting still uses a variation of Hall's electrolytic process using cryolite as a flux to melt alumina as 150,000 amperes at 3 volts direct current (450,000 watts) is sent through carbon electrodes immersed in the mixture.

Compare this with the typical 200 ampere, 220 volts (44,000 watts) available in totality in a modern home.

To make a single soda can requires the equivalent of a quart of gasoline.

The amount of alumina produced is about half the weight of the original bauxite, and ultimately an aluminum can costs more than the soda that will fill it.

The process also produces carbon dioxide and perfluorocarbons (PFCs) as byproducts. Both are greenhouse gases.

PFCs are one of the most powerful greenhouse gases with a heat-trapping potential 6,500 to 9,200 times greater than carbon dioxide.

Recycled aluminum is much less energy-intensive. Three cans can be made from recycled aluminum with the same amount of energy needed to produce just one can from ore.

Recycling accounts for an estimated 75 percent of all aluminum in use, and 51.2 percent of aluminum cans were recycled in 2004.

According to Anheuser-Busch, in 2003 the amount of energy saved from recycling aluminum cans was equivalent to 15 million barrels of oil.

There is an increasing variety of uses for aluminum, the two most obvious being soda cans and aluminum foil.

More and more aluminum is replacing iron as the material of choice for framing material in the construction industry and as automotive body and engine parts.

Although aluminum is even more reactive with oxygen than iron, unlike rust (iron oxide), aluminum oxide is a hard, durable material that forms a protective coating on exposed aluminum.

Thermite, a powerful explosive, is a mixture of aluminum and iron oxide.

When aluminum atoms "steal" oxygen atoms from iron oxide, a large amount of heat is released as the iron oxide is reduced to metallic iron.

Aluminum is an important component in refrigeration equipment, electrical transmission lines and airframe construction. Many consumer goods are at least partially made of aluminum: pots and pans, boats, edging for gardens, fasteners and even horseshoes, just to name a few.

Alumina is itself a useful product. Its hardness is second only to diamond and is useful as a grit for sanding and grinding.

It occurs naturally in the crystalline form known as corundum, which if colored and transparent is known as ruby or sapphire, depending on the color.

There is a certain amount of irony that the most abundant metal in Earth's crust is the most difficult to extract, yet, in its most abundant form as alumina, occurs as precious gemstones when nature processes it in just the right way.

It illustrates the beauty of nature and its laws, that elements such as aluminum and oxygen exist at all, but even more so that their properties allow for such a variety of uses, both practical and esthetic.