Abstract: The difference of phase transition during the processes of calcium roasting and acid leaching of vanadium slag has a significant influence on the total vanadium conversion rate.The microstructure and morphology of the vanadium-containing phases,and trend of vanadium during those phases in vanadium slag,clinker and residue were analyzed.The results show that the vanadium spinel with initial smooth and dense polygon structure is gradually oxidized into porous state,and finally forms concavo-convex granular iron oxides and short columnar pseudobrookite.The vanadium element initially enriched in vanadium spinel,gradually transfers into calcium vanadate,iron oxides,pseudobrookite and silicate phases.The most significant change in the acid leaching process is that the concavo-convex vanadium-containing oxides(iron oxides,pseudobrookite and calcium vanadate) in clinker change into hollow pseudobrookite phase step by step,and most of the vanadium has been extracted by sulfuric acid with the residual vanadium mainly remained in iron oxides,pseudobrookite and calcium silicate phases.