Metal oxides having very high melting points, e.g. oxides of $Cr , Mn , Ti , Mo, Fe$ etc. are difficult to reduce by carbon reduction. These can be reduced by thermite reduction using $Al$ powder as reducing agent.

Alumino-thermite process: Certain Oxides of $Mn$, Cr do not reduce by carbon due to very high melting point. They are reduced by $Al$.

$Cr _2 O _3+2 Al \rightarrow 2 Cr + Al _2 O _3$

$3 Mn _3 O _4+8 Al \rightarrow 9 Mn +4 Al _2 O _3$

So $Al$ is used as reducing agent.