“…V mainly exists in insoluble spinel as V(III) in VS [6] . Several methods have been developed to extract V from VS including sodium salt roasting-water/acidic leaching [7] , [8] , calcification/lime roasting-acidic leaching [9] , [10] , [11] , non-salt roasting-acidic leaching [12] , manganese roasting-acidic leaching [13] , etc. By roasting at high temperature in air atmosphere, the V(III) in spinel can be fully oxidized, converting stable spinel into the leachable forms such as NaVO 3 , Na 4 V 2 O 7 , CaV 2 O 6 , etc.…”