[sycren]

The chemical equation for rust is: 4Fe + 3O2 + 6H2O → 4Fe(OH)3

Where does the hydrogen come from when burning the iron powder since (presumably) water is not part of the burning process?

[yetihehe]

Without water, rust is Fe2O3. It's commonly used as red die for cements and cheapest iron oxide. It's also useful with aluminum to make thermite (which generates VERY high temperatures). Long ago I bought some Fe2O3, it was very fine powder and apparently waste from some chemical process.

Edit - mistook oxides.

[sycren]

Are there any other useful chemical reactions that can utilise this waste product?

[Retric]

Burning Iron is a different process 4Fe + 3 O2 ==> 2 Fe2O3.

Critically, rust forms at much lower temperatures.