For FeO-10 wt% SiO//2 slag, the reduction is controlled by chemical reaction in the range where solid SiO//2 does not precipitate and is controlled by FeO transport process after the range where solid SiO//2 precipitates. For FeO-CaO slag, the temperature dependency on reaction rates is negligibly small, and the product is very porous. The reaction takes place in the form of a chain reaction consisting in reduction by CO gas and the Boudouard reaction.