Mechanism of vanadium slag roasting with calcium oxide

• Fayalite is more likely to be oxidized than vanadium spinel thermodynamically.
• Oxidation of Fe2 + contained in the augite to Fe3 + is hindered by diopside.
• Lowering heating rate will improve the vanadium recovery obviously.
• Oxidations of vanadium spinel and augite overlap at heating rate of 3 °C·min− 1.
• Only oxidation of spinel occurs in 657–914 °C at heating rate of 5 °C·min− 1.

Thermodynamic analyses, XRD characterization and non-isothermal oxidation kinetic analyses were conducted to find the mechanism of vanadate formation when vanadium slag was roasted with calcium oxide. The effects of heating rate, added amount of CaO, holding temperature and holding time on oxidation efficiency were investigated. Thermodynamic calculations show that the fayalite is more likely to be oxidized than vanadium spinel, formation of calcium vanadates is easier than that of Mn(VO3)2 and Mg(VO3)2 and the oxidation of Fe2 + in augite is hindered by the presence of diopside. TGA results show that lowering the heating rate can improve the oxidation efficiency of vanadium. The maximum vanadium recovery of 93.3% was achieved when the vanadium slag with a ratio of m(CaO)/m(V2O5) of 0.42 was roasted at 850 °C for 2.5 h. Dynamic heating experiments indicate that oxidations of vanadium spinel and augite overlap within 608–959 °C with a heating rate of 3 °C·min− 1, while only oxidation of spinel occurs within 657–914 °C at 5 °C min− 1. The oxidation was controlled by a 3/2 reaction and a third order reaction, with corresponding overall apparent activation energy values of 140.3 and 247.8 kJ·mol− 1 for the heating rates of 3 °C·min− 1 and 5 °C·min− 1, respectively.