Aluminium

Aluminium is one of the most modern metals when compared to the metallurgy that began more than 5000 years ago. It was in the early 19th century that a Danish chemist, Hans Cristian Oersted, isolated the metal for the first time, in 1825, by means of a chemical process that used an amalgam of potassium and aluminium chloride. In 1827, the German chemist Wöhler obtained it in the form of powder by reacting potassium with aluminium chloride and later, in 1845, he identified for the first time the properties of the newly discovered metal, density and lightness, and separated it in the form of small balls.

Aluminium is extremely abundant in the earth's crust (and on the moon), being found in a proportion of approximately 15% and only silica exceeds it.

The most important industrial mineral is "bauxite" with a content of between 55 and 65 alumina (aluminium oxide Al₂O₃), and it is found mainly in the tropics. This mineral was discovered by Pierre Berthier, who named it after the place where he found it, the village of Les Baux de Provence, in Arles, southern France.

Bauxite is found in Spain, but in very small quantities, namely in Teruel, Barcelona, Tarragona and Lleida.

In 1854, Bunsen managed to electrolytically prepare aluminium based on his experiments with the compound sodium aluminium chloride. That same year, Henri Sainte-Claire Deville perfected the procedure and manufactured aluminium for the first time in history by substituting potassium for sodium and he presented it at the Paris exhibition of 1855 in the form of ingots. It can therefore be said that Deville began the industrial production of the metal and this procedure, with slight modifications, was used until 1888, when it was replaced by the electrolytic method.

The founders of the great aluminium industry were the Frenchman Hëroult, the German Kiliani and the American Hall, with the company Aluminium Industrie Aktien Gesellschaft being founded in 1888 by the former. Some forty years after the founding of the aluminium industry, the first notable aluminium production plant, located in Sabiñánigo (Huesca), appeared in Spain in 1929. The current production of primary aluminium in Spain is located in San Ciprian (Lugo), A Coruña and Avilés.

Aluminium is too active a metal to exist on its own and it is found in nature combined with a large amount of minerals, the main ones being bauxite and cryolite. Bauxite is the most important of the aluminium minerals. It is a hydroxide whose composition does not correspond to a certain chemical formula since it is always combined with variable amounts of elements such as iron, silicon and titanium and an undefined quantity of combination water. Its colour varies from dark red to pure white. Cryolite is, alongside bauxite, the most important mineral in the manufacturing of aluminium, its main role being that of alumina flux in electrolytic baths. Cryolite is currently being replaced by cyolite, an artificial aluminium, calcium and calcium fluoride.

Metallurgy | The extraction of aluminium takes place in three stages: mining, refining and reduction.

The bauxite is extracted, washed and dried before being sent to the refinery where the aluminium is separated.

The Austrian chemist Karl Josef Bayer, son of the founder of the Bayer Chemical company, invented the Bayer process for large-scale production of alumina from bauxite. This method is the one most used in the aluminium industry.

Starting with the bauxite, previously subjected to a process of drying and finely grinding the material, it is heated with a concentrated solution of caustic soda (NaOH) to obtain a solution of sodium aluminate (Al0₂Na) and some sodium silicate (Na₂SiO₂).

This solution is filtered and aluminium hydroxide is precipitated (Al(OH)₃), either with carbon dioxide or with a small amount of previously precipitated aluminium hydroxide.

The alumina is reduced to aluminium in electrolytic cells using the Hall-Héroult procedure. In these cells cryolite is used, melted at 980ºC to dissolve the alumina, which, when subjected to electrolysis, separates into aluminium and oxygen. The aluminium goes down to the bottom of the tank where it is periodically extracted and the oxygen is combined with the carbon from the anode to produce CO₂.

By the action of the electric current supplied, the alumina introduced inside the electrolytic cell or furnace is decomposed and according to the laws governing electrolysis, in the negative electrode (cathode) made up of the furnace coating, aluminium is deposited. The metal is extracted from here and pressed in the form of rolled sheets, wads or billets for extrusion or ingots for casting.

According to the same laws, oxygen is produced in the positive electrode (anode), which, due to its high activity, reacts with the carbon of the electrode, forming the gaseous products carbon monoxide and carbon dioxide (CO and CO₂). Due to this reaction, the anode becomes worn, so it has to be replaced periodically. The anode blocks are made of carbon.

To make 1000 Kg. of aluminium, 10,000 Kg of bauxite is required, which produces 500 kg. of alumina, 80 kg. of cryolite, 600 kg. of carbon plus 14,000 kWh of electrical power. Because of its high electricity consumption, aluminium electrolysis plants are set up near places where energy is cheaper, such as hydroelectric or nuclear power plants or in oil-producing countries.

Recycled aluminium | One of the most outstanding properties of aluminium is its recycling capacity. Contrary to what happens with other metals, 100% of the material can be reused. Likewise, this recycling process can be carried out almost indefinitely on the same material, which is why aluminium can be considered to have a practically unlimited useful life.

Another of the most important conditioning factors of this recycling process is that it requires approximately 5% of the energy used to obtain primary aluminium.

On the other hand, the characteristics and properties of the material do not vary with this transformation so the quality of primary and recycled aluminium is the same.

Properties | Aluminium is a very light silver metal. Its atomic mass is 26.9815, it has a melting point of 660°C and a boiling point of 2467°C, and a relative density of 2.7 kg/m³. It is a very electropositive and extremely reactive metal.

Upon contact with air, it is quickly covered with a hard, transparent layer of aluminium oxide that protects it from corrosion.