Mechanism and kinetics of electric field combined with ultrasound assisted recovering vanadium from unroasted vanadium slag

The roasting-leaching process to recover V from vanadium slag (VS) is limited by energy conservation and toxic gases emission. In this work, electric field combined with ultrasound-assisted leaching (EUL) was employed to directly recover V from VS without roasting. The leaching kinetics and morphology evolution during EUL process were investigated. The obtained results indicated that V in the unroasted VS can be rapidly released under EUL, showing a high V leaching efficiency of 94.64 % after leaching for 120 min. The electric field combined with ultrasound intensification changed the rate determining step of direct sulfuric acid leaching (DSL) from the chemical reaction control to the diffusion and chemical reaction combination control, and significantly decreased the apparent activation energy (Ea) from 52.35 kJ mol-1 to 28.87 kJ mol-1 due to the synergetic effect of ultrasonic cavitation and anodic oxidation. The EUL played the most significant role in reducing slag particle size, enlarging minerals specific surface area and enhancing pore volume than DSL and ultrasound-assisted leaching (UL), which improved V leaching kinetics. The dissolution of original spinel ((Mn, Fe)(V, Cr)2O4) of the VS generated the secondary phases of titanomagnetite (Fe2.5Ti0.5O4), iron oxide (Fe3O4) and chromium spinel (Mn, Fe)Cr2O4, forming an insoluble solid product layer around the original spinel, which inhibited the release of V during the EUL process. The EUL method with no roasting process, no toxic gases emission and high V leaching efficiency is a green and efficient approach for recovering V from the VS.