Thermal plasma reduction of a SiO2-Al2O3 mixture for metal production was investigated. The understanding of the reaction mechanism would be applied to metal recovery from coal ash. Thermal equilibrium and free energy of formation of the reaction systems were estimated to predict the evaporated species and to evaluate the predominant reactions. Atomic hydrogen plays an important role in the reduction of the SiO2-Al2O3 mixture. Argon and argon-hydrogen plasma jets were used for the treatment of the SiO2-Al2O3 mixture with various compositions. The argon plasma produces gaseous silicon and SiO from the SiO2-Al2O3 mixture, though the recovered amount was very little, while Al2O3 has not been reduced. The argon-hydrogen plasma produces mainly gaseous silicon with some additions of gaseous SiO and aluminum. Both of the argon and the argon-hydrogen plasmas produce the silicon component collected as the fumes having the purity of 98-99% from the SiO2-Al2O3 mixture.