Usually a broken iron article is taken to the welding shop for repair. But it is difficult to take broken girders or the craked railway track to the industry for repair. Can the welding equipment be brought to the site for repair ? Name the process, illustrate the principle of its working and name some other practicle applications of this process.

Explanation

The equipment used for on the spot welding is called aluminothermy. In this process, a mixture of aluminium powder and ferric oxide called thermite is ignited in a closed crucible by inserting a burning magnesium ribbon into the ignition mixture consisting of magnesium powder and barium peroxide as shown in the Fig. The ingnition powder burns to produce a large amount of heat needed to initiate the reaction. Since ` Delta _ f G^(@) ` of ` Al_ 2 O _ 3 ` is much more negative than of ` Fe _ 2 O _ 3 , therefore, Al reduces ` Fe_ 2 O _ 3 ` to metallic iron.
` 2 Al ( S) + Fe_ 2 O _ 3 (S) overset (Delta ) to Al _ 2 O _ 3( S) + Fe ( l ) `
This reaction is highly exothermic. As a result, Fe is produced in the molten state and thus can be used to weld heavy machinery of all kinds, such as girders, railway tracks, etc.
Besides ` Fe_2 O_ 3`, Al can also be used to reduce oxides of other metals such as ` Cr_ 2 O _ 3, Mn _ 3 O _ 4 `, etc.
`Cr _ 2 O _ 3 + 2 Al to Al _ 2 O _ 3 + 2 Cr , 3 Mn _3 O _ 4 + 8 Al to 4 Al _ 2 O _ 3 + 9 Mn `