Thermit and thermit welding
From LoveToKnow 1911
"AND THERMIT WELDING. THERMIT - Thermit is a mixture of aluminium powder and iron oxide. On ignition the reaction, 8A1+3Fe 3 O 4 =9Fe+4Al 2 O 3, gives a temperature estimated to be between 2,300° and 2,700°C. The reaction, stated in weights, means that 217 parts of aluminium plus 732 parts magnetite (iron oxide) equals 540 parts steel plus 409 parts slag, or approximately 3 parts of aluminium plus 10 parts of magnetite will produce, on combustion, 7 parts of steel. This steel represents about one-half of the original thermit by weight and one-third by volume.
Thermit was discovered by Dr. Hans Goldschmidt of Essen, Germany, in 1895, while trying to reduce chromium and manganese. Dr. Goldschmidt's principal discovery related to a simple and safe method of ignition, as the action of aluminium when mixed with various oxides, sulphides, and chlorides was well known. Fine aluminium will not burn below the temperature of molten cast iron, and previous experimenters had resorted to heating their mixtures in a crucible. This made the initial temperature so high at the moment of ignition that there was an explosion. Dr. Goldschmidt obtained ignition of a cold mixture by means of a barium-peroxide fuse, which was set off by a storm match. Later magnesium powder or ribbon was used, being set off in the same way. A red-hot iron rod may also be used to set off the magnesium, which in turn ignites the thermit. Dr. Goldschmidt's original American patent No. 615,700 was granted March 16 1897, and related principally to the use of aluminium as a reducing agent for the production of carbonfree metals such as cobalt, chromium, magnesium, tungsten, etc., by what is now known as the aluminothermic process.
Thermit is now used considerably in the foundry for purifying iron and steel in the ladle. For this purpose the thermit is placed in a can on the end of a rod and plunged to the bottom of the molten metal. The intense heat generated tends to liberate many impurities which are carried away in the slag. The principal and better-known use, however, is in welding.
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