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Based on the phase equilibria of the CaO-SiO 2 -VO x system determined experimentally at 1873 K and the oxygen partial pressure of 6.9910 À11 atm, the isothermal section diagram of the system was constructed. Aside from the simple oxides SiO 2 and V 2 O 3 , three complex crystal phases CaV 2 O 4 , Ca 2 SiO 4 dissolving V 2 O 3 , and CaVO 3 dissolving SiO 2 were found in the present investigated composition range. The solubility of V 2 O 3 in Ca 2 SiO 4 phase can reach 5 mass %, and the CaVO 3 phase dissolves about 5% mass SiO 2 . Furthermore, the thermodynamic activities of VO x and SiO 2 in the determined single liquid region were measured by equilibrating the melts with liquid copper. The activity coefficient of SiO 2 decreases linearly with the increase in the basicity of the melts and is almost irrelevant to the total content of vanadium oxides. The activities of vanadium oxides increase slightly with the increase in the basicity but are mainly determined by the total content of vanadium oxides in the melts. With the increase in the basicity, the activity coefficient of VO 1.5 increases almost linearly, whereas those of VO 2 and VO 2.5 decrease gradually.

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