4 Aluminium Fundamentals

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ALUMINIUM
FUNDAMENTALS

Aluminium – number 13 in the periodic table is the most abundant metal in the earth's crust.

75% of the aluminium produced since 1888 is still in use

95+% of all Aluminium in vehicles is already recycled

Aluminium is an excellent conductor for electricity and heat. Hence, transmission cables are generally made from aluminium.

Aluminium forms an ultra-thin, but stable oxide layer on the surface, which prevents further oxidation the material (other than steel)

The weight of Aluminium is only 1/3 of the weight of Steel

Aluminium is a rather young metal compared to the other important metallic elements. Nevertheless, with a share of 8%, it is one of the most common chemical elements in the earth's crust and therefore only behind oxygen and silicon. The widespread occurrence in nature suggests an earlier exploration of this metal than only in the past two centuries.

In 1807, the British Scientist Sir Humphrey Davy named the metal aluminium without producing any aluminium. It was Hans Christian Oersted, a Danish chemist, who produced the first aluminium in 1825. He heated potassium amalgam with aluminium chloride and distilled the mercury from the resultant aluminium amalgam, thus achieving a small lump of aluminium.

Frederick Woehler from Germany reduced aluminium chloride with potassium and received small aluminium particles.

Frenchman Henri Sainte-Claire Deville improved the process by using sodium chloride - aluminium chloride fluxes to fuse the aluminium particles produced in Woehler’s process.

With the help of Napoleon, Deville opened the first aluminium factory near Paris in 1859. The price of the metal was still very high at that time and therefore attracted much scientific interest until two researchers independently filed their patents for the electrolytic production of aluminium in molten salt in 1886. The principle of this so called Hall-Héroult process is based on the fact that alumina is soluble in a molten cryolite salt, like sugar in coffee. The electrolysis of this salt leads to a separation of aluminium and oxygen ions, whereby liquid aluminium is generated at the cathode. There are two major industrial applications for this Hall-Heroult process today, the Soederberg and the pre-baked process.

The Soederberg process uses the heat from the electrolysis process to make a self-baked monolithic anode from carbon paste and pitch, while the pre-baked process requires petroleum coke-based anodes that were previously baked at more than 1,100 °C. The latter is superior today and thus the basis of modern, newly developed cell technology. The industrial production and use of aluminium is increasing exponentially due to its enormous energy saving potential in industrial applications. In 2001 the production of primary aluminum reached 25 million tons with a consumption of 25 million tons. Demand has more than doubled in the past 20 years and will reach 65 million tons in 2021. In the past 120 years, the process has improved dramatically, reduced energy consumption and increased current efficiency, while the basic process has remained basically unchanged. Charles Hall recorded an energy consumption of 55 MWh/t aluminium in 1886. This figure was reduced to 25 MWh/t by the middle of the 20th century, while today we achieve energy consumption of less than 13 MWh/t in the best smelting plants. The world average is still over 15 MWh/t. These numbers indicate the DC energy consumption during the electrolytic melting process.

Aluminium Production in Eastern Germany around 1960:

Anode Change (left) and Metal Tapping (right)

Some scholars have suggested limited production of aluminium metal may have occurred as long as 2000 years ago! In his famous encyclopedia "Historia Naturalis" Pliny the Elder mentions in a story a familiar sounding silvery metal:

"One day a goldsmith in Rome was allowed to show the Emperor Tiberius a dinner plate of a new metal. The plate was very light, and almost as bright as silver. The goldsmith told the Emperor that he had made the metal from plain clay. He also assured the Emperor that only he, himself, and the Gods knew how to produce this metal from clay. The Emperor became very interested, and as a financial expert he was also a little concerned. The Emperor felt immediately, however, that all his treasures of gold and silver would decline in value if people started to produce this bright metal of clay. Therefore, instead of giving the goldsmith the regard expected, he ordered him to be beheaded."

While there is obviously no way of testing the truth behind this story (Pliny's Historia Naturalis is not known for its scientific accuracy!) the similarities are interesting. Indeed, almost 2000 years later another Emperor, Napoleon III, used aluminium plates and cutlery to serve the King of Siam at a state banquet. Aluminium was then a rare and precious metal and less important guests had to eat from plates of pure gold.