Thermodynamics of the CaO–SiO₂–VO_x system at 1873 K under the oxygen partial pressure of 6.9×10⁻¹¹ atm

Abstract

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.

Effects of oxygen partial pressure on the thermodynamics of CaO–SiO2–VOx system at 1873 K

“…To make the deduced formula shown as Equation () usable, the activity coefficients of Ca and V in liquid copper, and , must be known in advance. The activity coefficient of V relative to pure solid vanadium in dilution liquid copper solution at 1873 K has been reported as 16.9, and that of Si relative to pure liquid silicon was reported as 0.016, but no data of Ca are available. Hence, the activity coefficient of Ca in liquid copper was measured firstly through the equilibrium between liquid copper and a CaO (60 mass pct)‐SiO 2 (40 mass pct) melt at 1873 K under the oxygen partial pressure of 6.9 × 10 −11 atm.…”

“…A detailed description of the equipment and the experimental procedure to investigate the phase equilibria of the CaO-SiO 2 -VO x has been presented in a previous publication. 15 For the sake of brevity, only the salient features are given here. The chemical agents CaO, SiO 2 , and V 2 O 3 with purity higher than 99.9 mass% were used as reactants.…”

“…The high-temperature equilibrium was conducted in a vertical resistance furnace, and the schematic diagram of the experimental apparatus can be found in the previous publication. 15 The temperature of the sample was monitored by a W-Re5/W-Re26 thermocouple positioned beneath the bottom the crucible. To control the oxygen partial pressure in the reaction atmosphere, a gas mixture composed of CO, CO 2 , and Ar were adopted.…”

“…Relating to the preparation of some vanadium compounds and the pyrometallurgical extraction process of vanadium, CaO–SiO 2 –VO x is one of the most basic important systems . Hence, the phase equilibria and thermodynamic properties of this system are desirable in controlling the processes …”

“…[11][12][13][14] Hence, the phase equilibria and thermodynamic properties of this system are desirable in controlling the processes. 15,16 Since vanadium is a transition element, vanadium oxide has different oxidation states. Consequently, the oxygen content in the CaO-SiO 2 -VO x system, as well as the valence of vanadium, will not be a constant.…”