A thermite (or aluminothermic) reaction is one in which aluminium metal is oxidized by an oxide of another metal, most commonly iron oxide. (The name thermite is also used to refer to a mixture of two such chemicals.) The products are aluminium oxide, free elemental metal and a great deal of heat. The reactants are commonly powdered and mixed with a binder to keep the material solid and prevent separation.

Although the reactants are stable at room temperature, when they are exposed to sufficient heat to ignite (usually by igniting with a burning magnesium ribbon, but can be done through other methods such as potassium permanganate and glycerine), they burn with an extremely intense exothermic reaction. The products emerge as liquids due to the high temperatures reached (with iron (III) oxide, commonly 3000°C (5432°F) or more). Thermite contains its own supply of oxygen, and does not require any external source (such as air). Consequently, it cannot be smothered and may ignite in any environment (it will burn merrily underwater, for example), given sufficient initial heat. This, combined with the extremely high temperatures generated, makes thermite reactions extremely hazardous unless appropriate precautions are taken.

Thermite reactions have many uses. It was originally used for repair welding in-place such things as locomotive axle-frames where the repair can take place without removing the part from its installed location. Thermite grenades and bombs are used in combat as incendiary devices, able to burn through heavy armor or other fireproof barriers. Thermite can also be used for quickly cutting or welding metal such as rail tracks, without requiring complex or heavy equipment. The mixture has been sold for many years under the trademark name Thermit for use in railroad welding.

This type of reaction when used to purify the ores of some metals is called the Thermite process. The process for obtaining pure uranium was developed as part of the Manhattan Project at Ames Laboratory under the direction of Frank Spedding. It is sometimes called the Ames process.

The thermite reaction can take place by accident in the industrial locations where abrasive grinding and cutting wheels are used with ferrous metals, using aluminium in this situation produces an admixture of oxides which is capable of violent explosive reaction.

When thermite is made using iron (III) oxide, for maximum efficiency it should contain, by mass, 25.3% aluminium and 74.7% iron oxide. (This mixture is sold under the brand name Thermit as a heat source for welding.) The complete formula for the reaction using iron (III) oxide is as follows:

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe;    ΔH = -851.5kJ/mol

Thermite should not be confused with a thermal lance.

History

Thermite was invented in 1893 and patented in 1895 by German chemist Dr. Hans Goldschmidt, in consequence of which the reaction is sometimes called the "Goldschmidt reaction" or "Goldschmidt process". Dr. Goldschmidt was originally interested in producing high purity metals by avoiding the use of carbon in smelting, but soon realised the value in welding. The first commercial application was the welding of tram tracks in Essen, Germany, in 1899. Degussa AG , a corporate descendant of Goldschmidt's firm, is still today one of the world's largest producers of welding thermite.

See also

• Wikibooks: Thermite synthesis

External links

• Thermit Welding LTD
• Video of a thermite reaction
• History of thermite, from Degussa AG, successor of Dr. Goldschmidt's company