What is the chemical formula for aluminum

Aluminum is an important material due to its chemical resistance, its low density and its mechanical properties. The main share is required for the transport sector, but the construction industry, mechanical engineering and the packaging industry also need the coveted light metal. After oxygen and silicon, aluminum is the third most abundant element in the earth's shell. Friedrich Wöhler was the first to present the pure element in 1827 by reducing aluminum chloride with potassium. At that time, the price of aluminum was higher than that of gold. Base metals such as aluminum or magnesium with a very negative normal potential can be extracted from a molten salt in a highly pure and economically viable manner, but only in an anhydrous environment. The basis for the electrolysis process was laid in 1886 by Charles Martin Hall (1863–1914) in the USA and Paul Héroult (1863–1914) in France, independently of one another. [Literature 1]


The piece of bauxite shown comes from Les Baux / Provence;
today the deposits in Les Baux are no longer worth mining

Bauxite is a common rock that is composed of various aluminum and iron ores. In the past, bauxite was extracted in Les Baux in the south of France, and the ore is named after this place. The most important ore deposits are now in Australia, Guinea, Brazil, Jamaica, India, Guyana and Indonesia. The extraction of aluminum is associated with great effort. During ore processing, aluminum oxide (clay) is enriched from the bauxite. [Literature 3] This is followed by the actual metal extraction with the aid of melt flow electrolysis.
Aluminum oxide has a very high melting point of around 2050 ° C. To reduce this, one gives cryolite Na3AlF6 and other fluorine compounds such as aluminum fluoride AlF3 and calcium fluoride CaF2 in excess. The melting point of the mixture is then around 950 ° C. The alumina-cryolite mixture is located in an iron tub that is lined with carbon or graphite. The anode consists of graphite cylinders, which are immersed in the melt. At a relatively low voltage of 5 to 6 volts, but a very high current flow of 150,000 amperes or more, liquid aluminum settles on the bottom. [Literature 8 and 9] The crust breaker breaks up the crust that has formed and enables replenishment material to be supplied. [Literature 2] The aluminum is sucked off from time to time. The resulting aluminum has a maximum purity of 99.9%. This is brought to a purity of up to 99.999% in further electrochemical refining processes. [References 5 and 7] Oxygen is produced at the graphite anode, which reacts with the carbon of the electrodes to form carbon dioxide and carbon monoxide. Since the electrodes decompose, long, cylindrical rods are used, which are driven further and further into the melt. After consumption, the bars are simply attached so that the process can take place continuously.


The high electrical currents keep the weld pool liquid. However, the overall reaction is endothermic. 1345 kilojoules are required per mole, which means that considerably more energy must be supplied than is released during the reaction. [Literature 5] Half of the energy supplied is required for electrolysis, the other half is used to generate heat so that the molten salt remains hot and liquid. [Literature 8]
Reaction equation 
Reduction at the cathode  4 al3+ + 12 e  4 al
Oxidation on the anode 3 C + 6 O2−  3 CO2 + 12 e  
Overall response 2 al2O3 + 3 C 4 Al + 3 CO2  
To produce one ton of aluminum, four tons of bauxite, up to 0.8 tons of carbon, 4 kilograms of cryolite, up to 20 tons of aluminum fluoride and up to 16,000 kilowatt hours of electricity are used. The small amount of flux is explained by the fact that it can be used again and again in the cycle. [Literature 5] In addition to carbon dioxide, which acts as a greenhouse gas, and poisonous carbon monoxide, the exhaust gases contain very aggressive fluorine compounds. So that these do not get into the environment, the cell is provided with a bell. In a cycle process, the fluorine compounds are adsorbed with the introduced alumina after the cells have been suctioned off, so that they are returned.
A disadvantage of the process is the very high energy consumption. The largest aluminum factory in Europe is located in Iceland, as geothermal energy makes energy available there cheaply. [Literature 6] It is even worthwhile to ship the bauxite from Brazil to northern Iceland and export the aluminum in bars. Australia, China Guinea, Jamaica and India are other countries with large bauxite deposits. The procurement of cryolite, which rarely occurs in nature, is also time-consuming. Since the natural occurrences in Greenland or Canada are relatively modest, the flux is made from hexafluorosilicic acid, which is contained in natural clay deposits. Due to the high overall effort, recycling aluminum is particularly worthwhile.
1 www.aluinfo.de (accessed 1/2012), Gesamtverband der Aluminiumindustrie e.V.
2 Behr / Agar / Jörissen (2010): Introduction to Technical Chemistry, Heidelberg
3 Trueb, Lucien F. (2005): The chemical elements, Stuttgart
4 Roempp (1995): Chemielexikon, Stuttgart
5 Hollemann / Wiberg (2007): Textbook of Inorganic Chemistry, Berlin / New York
6 www.eldey.de (accessed 12/2011): Country portal for Iceland, aluminum industry
7 Hamann / Vielstich (2005): Elektrochemie, Weinheim
8 Glöckner / Jansen et al. (1994): Handbuch der experimental Chemie, Volume 6 Elektrochemie, Cologne
9 Binder (1999): Lexicon of the chemical elements, Stuttgart / Leipzig