In relation to lowering silicon content of the molten pig iron in a blast furnace, the effect of CaO or MgO addition on the mechanism of the carbothermic reduction of SiO2 has been investigated. Using powdered SiO2-graphite mixtures containing CaO or MgO, the reduction rate has been determined with a thermo-balance in an argon atmosphere at 1873 K. The solid products were SiC, Ca2SiO4 or Mg2SiO4, and the gaseous products were SiO, Ca and Mg. The small additions of CaO and MgO increased the formation rates of SiC and SiO. The formation of SiC and SiO from the mixtures with %CaO/%SiO2 or %MgO/SiO2 above 0.6 was slower than that from 100%SiO2. In the early stage of reaction, the reduction rate of SiO2 was mixed-controlled by the interfacial reactions at both graphite and the silicate particles. As the reduction proceeded, the rate-determining step was shifted to the gaseous diffusion through the Ca2SiO4 or Mg2SiO4 layer left on the surface of the silicate particles.
