The metallic element aluminium is the third most plentiful element in the earth’s crust, comprising 8% of the planet’s soil and rocks (oxygen and silicon make up 47% and 28%, respectively).
In nature, aluminium is found only in chemical compounds with other elements such as sulphur, silicon, and oxygen. Pure, metallic aluminium can be economically produced only from aluminium oxide ore.
Metallic aluminium has many properties that make it useful in a wide range of applications. It is lightweight, strong, nonmagnetic, and nontoxic. It conducts heat and electricity and reflects heat and light. It is strong but easily workable, and it retains its strength under extreme cold without becoming brittle. The surface of aluminium quickly oxidises to form an invisible barrier to corrosion. Furthermore, aluminium can easily and economically be recycled into new products.
Background
Aluminium compounds have proven useful for thousands of years. Around 5000 B.C., Persian potters made their strongest vessels from clay that contained aluminium oxide. Ancient Egyptians and Babylonians used aluminium compounds in fabric dyes, cosmetics, and medicines. However, it was not until the early nineteenth century that aluminium was identified as an element and isolated as a pure metal. The difficulty of extracting aluminium from its natural compounds kept the metal rare for many years; half a century after its discovery, it was still as rare and valuable as silver.
In 1886, two 22-year-old scientists independently developed a smelting process that made the economical mass production of aluminium possible. Known as the Hall-Heroult process after its American and French inventors, the process is still the primary method of aluminium production today. The Bayer process for refining aluminium ore, developed in 1888 by an Austrian chemist, also contributed significantly to the economical mass production of aluminium.
In 1884, 125 lb (60 kg) of aluminium was produced in the United States, and it sold for about the same unit price as silver. In 1995, U.S. plants produced 7.8 billion lb (3.6 million metric tonnes) of aluminium, and the price of silver was seventy-five times as much as the price of aluminium.
Raw Materials
Aluminium compounds occur in all types of clay, but the ore that is most useful for producing pure aluminium is bauxite. Bauxite consists of 45-60% aluminium oxide, along with various impurities such as sand, iron, and other metals. Although some bauxite deposits are hard rock, most consist of relatively soft dirt that is easily dug from open-pit mines. Australia produces more than one-third of the world’s supply of bauxite. It takes about 4 lb (2 kg) of bauxite to produce 1 lb (0.5 kg) of aluminium metal.
Caustic soda (sodium hydroxide) is used to dissolve the aluminium compounds found in the bauxite, separating them from the impurities. Depending on the composition of the bauxite ore, relatively small amounts of other chemicals may be used in the extraction
of aluminium. Starch, lime, and sodium sulphide are some examples.
Cryolite, a chemical compound composed of sodium, aluminium, and fluorine, is used as the electrolyte (current-conducting medium) in the smelting operation. Naturally occurring cryolite was once mined in Greenland, but the compound is now produced synthetically for use in the production of aluminium. Aluminium fluoride is added to lower the melting point of the electrolyte solution.
The Manufacturing Process
Aluminium manufacture is accomplished in two phases: the Bayer process of refining the bauxite ore to obtain aluminium oxide, and the Hall-Heroult process of smelting the aluminium oxide to release pure aluminium.
The Bayer process
- 1 First, the bauxite ore is mechanically crushed. Then, the crushed ore is mixed with caustic soda and processed in a grinding mill to produce a slurry (a watery suspension) containing very fine particles of ore.
- 2 The slurry is pumped into a digester, a tank that functions like a pressure cooker. The slurry is heated to 230-520°F (110-270°C) under a pressure of 50 lb/in 2 (340 kPa). These conditions are maintained for a time ranging from half an hour to several hours. Additional caustic soda may be added to ensure that all aluminum-containing compounds are dissolved.
- 3 The hot slurry, which is now a sodium aluminate solution, passes through a series of flash tanks that reduce the pressure and recover heat that can be reused in the refining process.
- 4 The slurry is pumped into a settling tank. As the slurry rests in this tank, impurities that will not dissolve in the caustic soda settle to the bottom of the vessel. One manufacturer compares this process to fine sand settling to the bottom of a glass of sugar water; the sugar does not settle out because it is dissolved in the water, just as the aluminium in the settling tank remains dissolved in the caustic soda. The residue (called “red mud”) that accumulates in the bottom of the tank consists of fine sand, iron oxide, and oxides of trace elements like titanium.
- 5 After the impurities have settled out, the remaining liquid, which looks somewhat like coffee, is pumped through a series of cloth filters. Any fine particles of impurities that remain in the solution are trapped by the filters. This material is washed to recover alumina and caustic soda that can be reused.
- 6 The filtered liquid is pumped through a series of six-story-tall precipitation tanks. Seed crystals of alumina hydrate (alumina bonded to water molecules) are added through the top of each tank. The seed crystals grow as they settle through the liquid and dissolved alumina attaches to them.
- 7 The crystals precipitate (settle to the bottom of the tank) and are removed. After washing, they are transferred to a kiln for calcining (heating to release the water molecules that are chemically bonded to the alumina molecules). A screw conveyor moves a continuous stream of crystals into a rotating, cylindrical kiln that is tilted to allow gravity to move the material through it. A temperature of 2,000° F (1,100° C) drives off the water molecules, leaving anhydrous (waterless) alumina crystals. After leaving the kiln, the crystals pass through a cooler.