Aluminium

Aluminium is a chemical element with the symbol Al and atomic number 13. It is a light, silvery white metal that is soft, ductile and easy to shape. Aluminium is the most abundant metal in the crust of the Earth, yet it is almost never found as a pure metal in nature, because it bonds strongly with oxygen and other elements. The name is spelled "aluminium" in British English and in the international scientific naming used by IUPAC, while "aluminum" is the standard spelling in American and Canadian English. Both spellings refer to the same element.

Because it is light, strong when alloyed, resistant to corrosion and a good conductor of heat and electricity, aluminium is one of the most widely used metals in the modern world. It appears in aircraft, cars, drink cans, window frames, cooking foil and overhead power lines.

Properties

Aluminium has a density of about 2.7 g/cm3, roughly one third that of steel. This low density is the main reason it is valued in transport and in any product where weight matters. Pure aluminium melts at about 660 degrees C, which is low compared with iron or copper, so it is relatively easy to cast and recycle.

When exposed to air, aluminium quickly forms a very thin layer of aluminium oxide on its surface. This oxide layer is hard and stable, and it protects the metal underneath from further corrosion. The layer can be made thicker on purpose through a process called anodizing, which also allows the surface to be dyed in different colors.

Other notable properties include:

High electrical conductivity. By weight, aluminium conducts electricity better than copper, although copper conducts better for the same cross section.
High thermal conductivity, which makes it useful for heat sinks and cookware.
It is nonmagnetic and does not produce sparks when struck.
It reflects light and heat well, which is why it is used in mirrors, reflectors and insulating foil.
Pure aluminium is fairly soft, so most structural uses rely on alloys.

History

Compounds of aluminium, such as alum, have been used since ancient times for dyeing and medicine, but the metal itself was not isolated until the nineteenth century. The Danish physicist Hans Christian Oersted produced an impure sample in 1825 by heating aluminium chloride with potassium. In 1827 the German chemist Friedrich Wohler improved the method by using sodium and obtained purer aluminium.

In the 1850s the French chemist Henri Sainte-Claire Deville developed a process that allowed small scale industrial production. Even so, the metal remained very expensive. Bars of aluminium were displayed beside the French Crown Jewels at the 1855 Paris Exhibition, and Emperor Napoleon III is said to have reserved aluminium tableware for his most honored guests, while others used gold. This story is widely repeated, but it is best treated as an anecdote.

In 1884 a small pyramid of aluminium was placed as the cap of the Washington Monument in the United States. At the time it was one of the largest single pieces of aluminium ever cast.

The price fell dramatically after 1886, when Charles Martin Hall in the United States and Paul Heroult in France independently discovered an electrolytic method for extracting aluminium. Combined a few years later with the Bayer process for refining ore, the Hall-Heroult process made aluminium cheap enough for everyday use, and it remains the basis of production today.

Production

Uses

Aluminium is used in a very wide range of products:

Transport. Cars, trucks, trains and ships use aluminium to reduce weight and save fuel. In aviation, aluminium alloys have been the main structural material of aircraft for decades.
Packaging. Drink cans, foil, food trays and bottle caps are made from aluminium because it is light, does not rust and protects contents from light and air.
Construction. Window frames, doors, roofing, cladding and curtain walls use aluminium for its durability and low maintenance.
Electrical. Overhead power transmission lines usually use aluminium conductors, often reinforced with steel, because they are lighter and cheaper than copper. See electrical engineering basics for background.
Electronics and household goods. Heat sinks, laptop and phone cases, cookware and appliances often contain aluminium.

Alloys

Pure aluminium is soft, so it is usually mixed with small amounts of other elements such as copper, magnesium, silicon, manganese and zinc. Wrought alloys are grouped into numbered series according to their main alloying element. Two well known examples are:

6061, an alloy of aluminium with magnesium and silicon. It is strong, easy to weld and machine, and is common in frames, bicycle parts and general engineering.
7075, an alloy with zinc as the main addition. It is one of the strongest aluminium alloys and is widely used in aircraft structures and high performance equipment.

Alloys can be further strengthened by heat treatment or by working the metal, which is why the choice of alloy and temper is an important part of materials science and design.

Recycling

Aluminium can be recycled repeatedly without losing its basic properties. Recycling involves melting scrap and casting it again, which is far simpler than extracting new metal from ore. Remelting is commonly reported to use only about 5 percent of the energy needed for primary production. Some material is lost during melting as an oxide residue called dross. According to the Aluminum Association, nearly 75 percent of all aluminium ever produced is still in use today. Drink cans are among the most recycled consumer packaging items in many countries.

Health and environment

Aluminium is widespread in soil, water and food, and people are exposed to small amounts every day. Health agencies generally regard normal dietary exposure as low risk for healthy people. Very high exposure, for example in some industrial settings or in people with serious kidney problems, can cause health concerns, and these situations are managed by medical and occupational guidelines.

The main environmental issues relate to production. Bauxite mining can affect land and ecosystems, the Bayer process produces large amounts of an alkaline waste called red mud, and smelting uses large amounts of electricity and releases greenhouse gases. Recycling and the use of renewable power help to reduce these impacts.

Spelling

The element was named by the English chemist Humphry Davy in the early nineteenth century, who used several forms including "alumium" and "aluminum". The form "aluminium" became standard in Britain and most of the world, matching the "-ium" ending of many other elements. In the United States, "aluminum" became standard during the late nineteenth and early twentieth centuries. IUPAC uses "aluminium" but also recognizes "aluminum" as an acceptable variant.

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