<p>The phase decomposition and microstructure evolutions of vanadium slag during sodium roasting, a critical step in industrial vanadium extraction, have been investigated in detail. The thermodynamic analysis indicates good feasibility for oxidizing the phases in vanadium slag, with liquid phases like sodium metavanadate (NaVO₃) formed. The experimental results show that higher temperatures (800–900&#xa0;°C) facilitate complete decomposition of the original phases in vanadium slag, and promote formation of the liquid phases beneficial for separation of vanadium (V) and iron (Fe). Meanwhile, competition for sodium carbonate (Na₂CO₃) between vanadium spinels and silicates occurs during roasting. Insufficient Na₂CO₃ addition causes generation of water-insoluble calcium and manganese metavanadates. Furthermore, the oxidation of Fe-containing phases (e.g. vanadium spinels and iron olivine) and inner diffusion of sodium ions (Na⁺) in the liquid phases dominate the roasting process. The oxidation of vanadium spinels and iron olivine produces abundant micropores in vanadium slag. Then, Na⁺ diffuses through the micropores and combines with the decomposed intermediates, with NaVO₃, ferric oxide (Fe₂O₃), pseudobrookite (Fe₂TiO₅), acmite (NaFeSi₂O₆), and albite (NaAlSi₃O₈) finally formed at 800&#xa0;°C for 60&#xa0;min (alkali ratio of 1.4). These findings provide valuable insights into both fundamental research and engineering optimizations for vanadium extraction.</p>