Cr2O3 is mixed with requisite amount of Al-powder (this mixture is called thermite mixture) and is placed in a large fire-clay crucible. An intimate mixture of Na2O2 or BaO2 and Mg powder (called ignition mixture or igniter) is placed in a small depression made in the thermite mixture. The crucible is surrounded by sand which prevents the loss of heat by radiation. A piece of Mg ribbon is struck into the ignition mixture and the charge is covered by a layer of fluorspar (CaF2) which acts as a heat insulator. Now Mg-ribbon is ignited so that ignition mixture catches fire and flame is produced, leading to a violent reaction between Mg and BaO2 with the evolution of large amount of heat.
Mg + BaO2 --> BaO + MgO + Heat
Heat produced in the above reaction makes Cr2O3 and AI-powder react together.
Cr2O3 + AI -->2Cr (l) + AI2O3
Molten Cr-metal formed settles down at the bottom of the crucible.
Application:
An application of aluminothermic process has been used for joining the broken pieces of iron (welding). In this process thermite mixture consisting of Fe2O3 and Al-powder in 3 : 1 ratio is placed in a funnel shaped crucible lined internally with magnesite and having a plug hole at its bottom. The thermite mixture is covered with a mixture of BaO2
plus Mg-powder (ignition mixture) in which a piece of Mg ribbon is inserted. The ends of the iron pieces to be welded are thoroughly cleaned and surrounded by a fire-clay mould. When Mg ribbon
is ignited, ignition mixture catches fire and Fe2O3 gets reduced to Fe by Al-powder.
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