How Aluminium Is Made

In many ways, aluminium is the perfect metal. It is strong, light, resistant to heat and corrosion and a good conductor of electricity. On top of that it is plentiful and inexpensive.

Aluminium is also the most abundant metal in the Earth’s crust, and the third most abundant element after oxygen and silicon. However, it was not until 1809 that English chemist Sir Humphry Davy formally identified and named it.

Today, aluminium is the most commonly used metal in the world after iron and steel. Aluminium is a vital component of almost every part of our lives, from the vehicles we drive to the packaging of our food.

Aluminium is at its most versatile when it’s combined with other metals to form aluminium alloys. The alloying process gives aluminium improved properties to suit a range of applications.

How Aluminium Is Made

The first stage in making aluminium is locating the aluminium ore.

Finding the aluminium ore

Aluminium tends to combine with other elements and rarely exists in nature in its pure metallic form. Aluminium compounds are found in most common rock types including clay, slate, shale, granite and anorthosite.

The most important aluminium ore is bauxite, a rock containing about 52% aluminium oxide with impurities of iron oxide, silica, and titania. Bauxite is commonly found in deposits on or close to the Earth’s surface throughout many parts of the world, including Europe, Asia, Australia and South America.

Mining aluminium

Geologists locate bauxite deposits by taking samples and conducting investigatory drilling. When deposits are found, they are mined in open pits. The earth is blasted loose and the bauxite is extracted using power shovels or draglines.

90% of all mined bauxite is made into alumina to be smelted into aluminium. The remaining 10% is used for other purposes including the manufacture of abrasives, furnace linings, and proppants for the oil industry. It takes 4 tons of high-quality bauxite to produce 2 tons of alumina, from which 1 ton of aluminium can be made.

Refining the bauxite

The bauxite is refined using the Bayer process which was first developed by Karl Joseph Bayer in 1888. The Bayer process has four steps: digestion, clarification, precipitation and calcination.

Digestion

The bauxite is ground, mixed with caustic soda and pumped into pressure tanks where steam heat and pressure is applied. This causes the caustic soda to react with the aluminium compounds in the bauxite to form a solution of sodium aluminate. The unwanted impurities are left behind in what is known as red mud.

Clarification

Next, the sodium aluminate solution is passed through blow-off tanks where the pressure is reduced to atmospheric pressure. The red mud is removed with the use of clarifying agents and cloth filters. The clarified solution is then cooled in heat exchangers and pumped into tall silos.

Precipitation

Aluminium hydroxide seed crystals are added to the sodium aluminate solution to cause precipitation. During this process, the aluminium becomes solid. This results in large aluminium crystals which are filtered and washed to remove water and other impurities.

Calcination

Now the aluminium hydroxide crystals are subjected to calcination, a thermal treatment process where the supply of air is controlled. Rotary kilns are used to heat the crystals to temperatures over 960° C which removes any remaining impurities, leaving a fine white powder known as alumina, or aluminium oxide.

Aluminium Smelting

Smelting is the process during which aluminium is extracted from the alumina. This is carried out by the Hall-Héroult process, which was invented in 1886 by Charles Martin Hall and Paul Héroult.

The smelting takes place in steel reduction pots filled with molten electrolyte, where carbon anodes are used to pass an electric current through the electrolyte. Alumina is then added to the molten surface. The electric current deposits molten aluminium which can be collected and siphoned off.

The molten aluminium is then poured into molds to form foundry ingot. At this stage it’s 99.8% pure. Now it can be further refined to produce superpure aluminium or used for alloying with other metals.

Superpure aluminium

Superpure aluminium of high purity (99.99%) is soft and lacks tensile strength. However, it is corrosion-resistant and an excellent conductor of electricity. Superpure aluminium is used in chemical equipment, electronic components, and to make gasoline.

Aluminium alloys

Most aluminium is alloyed with other elements. By alloying the aluminium, its hardness and strength can be significantly improved. Common aluminium alloys are aluminium-manganese (used in beverage containers), aluminium-magnesium (used in appliances and utensils), aluminium-magnesium-silicon (used in buildings and vehicles), and aluminium-copper (used in aircraft).

Recycling aluminium

Aluminium can be endlessly recycled without losing its quality. This makes it one of the most environmentally friendly metals on the planet. Incredibly, most of the aluminium ever produced is still being used today.