Oxidation-reduction equilibria of vanadium in CaO-SiO2, CaO-Al2O3-SiO2 and CaO-MgO-SiO2 melts

A series of redox studies of vanadium have been carried out in CaO-SiO2, CaO-MgO-SiO2 and CaO-Al2O3-SiO2 melts/slags equilibrated over oxygen partial pressures (pO2) range 10−2–10−9 atm at 1600°C. V2O5 level was varied from 1–5 mol%. Three different melt basicities and alumina contents were investigated. Magnesia content was varied between 3.5 and 4.9 wt%. A newly developed analytical technique based upon electron paramagnetic resonance (EPR) spectroscopy was successfully applied to these melts. Two redox equilibria corresponding to V3+/V4+ and V4+/V5+ pairs followed the O-type redox reaction over the oxygen partial pressure range investigated. Higher oxidation states of vanadium were stabilized with increasing basicity of slags. Two redox pairs coexisted within oxygen pressure region 10−4–10−6 atm. However, redox ratios did not indicate clear trends with increasing V2O5 content in CaO-SiO2-V2O5 system. In CaO-SiO2-Al2O3-V2O5 slags, a slight increase in redox ratios (V3+/V4+) was obvious when alumina quantity was changed from 3.22 to 5.44% at a basicity ratio ∼1.3, indicating an increase in slag acidity. CaO-MgO-SiO2-V2O5 slags showed a sharp decrease in redox ratios (V4+/V5+) between 10−2–10−6 atm with addition of 3.5 wt% MgO, due to increasing free oxygen ions in slags.