Usuallya brokenironarticleis takentotheweldingshop for repair. But itis difficultto takebrokengirders or thecrakedrailwaytrackto theindustryfor repair.Cantheweldingequipmentbe broughtto thesite forrepair ? Namethe process,illustratethe principleofitsworkingand name some otherpracticleapplicationsof thisprocess.

Usuallya brokenironarticleis takentotheweldingshop for repair. But iti

1.	Usuallya brokenironarticleis takentotheweldingshop for repair. But itis difficultto takebrokengirders or thecrakedrailwaytrackto theindustryfor repair.Cantheweldingequipmentbe broughtto thesite forrepair ? Namethe process,illustratethe principleofitsworkingand name some otherpracticleapplicationsof thisprocess.
Answer» Solution : Theequipment used for onthe spotwelding is calledaluminothermy.In thisprocess,a mixtureof aluminiumpowderand ferricoxide calledthermiteis IGNITED in aclosedcrucible byinsertinga burningmagnesium ribbonintotheignitionmixtureconsistingof magnesiumpowder and bariumperoxideas shownin the Fig.Theingnition powder burnsto producea largeamountofheatneededto initiatethereaction. Since` Delta_ f G^(@) `of` Al_ 2 O_ 3 `is muchmore negativethan of` Fe _ 2O_ 3,therefore, Al reduces` Fe_ 2O_ 3 `to metalliciron. `2 Al (S)+Fe_ 2O_3(S)overset (DELTA )toAl _ 2O_ 3( S)+Fe ( l ) ` This reactionis highly exothermic. As a result, Fe isproduced in the moltenstateandthuscan beusedto weldheavymachineryof all kinds,suchas girders, railway tracks, etc. Besides` Fe_2O_ 3`, Al can ALSOBE usedto reduceoxidesof othermetalssuch as` Cr_2O_3,MN _3O_4 `,etc. `Cr _2 O_3+2 Al toAl_2O_3 + 2 Cr,3 Mn _3O_ 4 +8Alto4 Al_2O _3+9 Mn `