Iron is never found in its elemental metal form in nature. It is nearly always found as a mineral oxide. It is then reduced (oxide removed) through smelting in a furnace. So burning iron is simply reversing the process by oxidizing it again. But this is not necessarily clean. The issue then becomes where did the energy come from to smelt the iron ore? Same issue as with electric cars - where did the energy come from to charge the battery? Coal? natural gas? nuclear?
Why don’t they just burn manure, that way they could be honest and say their green story is bullshit.