Thermit reaction details

Aluminium reacts with iron oxides, particularly ferric oxide, in highly exothermic reactions, reducing the iron oxides to free iron, and forming a slag of aluminium oxide.

3Fe₃O₄+ 8Al = 4Al₂O₃+ 9Fe (3088°C, 719.3kCal↑)

3FeO + 2Al ⇒ Al₂O₃ + 3Fe (2500°C, 187.1kCal↑)

Fe₂O₃ + 2Al ⇒ Al₂O₃ + 2Fe (2960°C, 181.5kCal↑)

The various iron oxides are used in appropriate proportions so as to get the correct resultant quantity and temperature of molten steel. Approximately equal quantities of molten steel and liquid aluminium oxide are separated at about 2400°C, after a few seconds of the exothermic reaction. The iron obtained from such a reaction is soft and unusable as a weld metal for joining rails. To produce an alloy of the correct composition, alloys like ferro-manganese are added to the mixture along with pieces of mild steel, both as small particles, to allow rapid dissolution in the molten iron, to control the temperature and to increase the "metal recovery". Complete slag separation in a short time and better fluidity of the molten metal is achieved by adding compounds like calcium carbonate and fluorspar, etc.

Pre-heating the rail ends (to about 1000°C) is required to help the poured molten metal in washing away the surface oxidation on the rail ends, as otherwise, the molten metal may chill and solidify immediately on coming in contact with cold rail ends, without washing off the surface oxidation.

The procedure of "controlled localized reaction" to keep the thermit mixture ignition under control was invented by Dr Goldschmidt and hence the process is sometimes also known as the Goldschmidt process. Individually patented processes have led to different trade names such as "Thermit", "Boutte", "Argothem", etc.