The Dios process is a bath smelting process, intensively investigated in Japan. Iron oxide is preheated in the first of two fluidized bed reactors in series and pre-reduced to 15-25% in the second reactor using cleaned off-gas from the smelter. In addition, a small amount of coal fines is injected into the smelter off-gas to cool the off-gas and provide additional CO and H2 for pre-reduction. Coal is gravity fed into the smelter. Oxygen is injected into the smelter for combustion of coal and for post combustion.