The dual role of MgO in phase transformation and dissolution within the FeV2O4-CaO-MgO system: Toward efficient vanadium extraction

The calcium‑magnesium composite roasting – acid leaching process represents a sustainable alternative for vanadium extraction from vanadium slag, yet its underlying mechanism remains insufficiently understood. This study systematically investigates the phase transformation and dissolution behavior within the FeV 2 O 4 –CaO–MgO system to clarify the role of MgO. Results reveal its critical dual role: it promotes the formation of acid-soluble calcium‑magnesium vanadate (Ca 5 Mg 4 V 6 O 24 ), yet concurrently facilitates the formation of insoluble magnesium‑iron vanadates (e.g., Mg 2 FeV 3 O 11 ), thereby severely compromising leaching efficiency. The formation of the low-melting-point compound CaMgV 2 O 7 can also inhibit extraction via phase encapsulation. As a result, vanadium recovery demonstrates a pronounced non-monotonic dependence on MgO content, with a well-defined efficiency maximum under optimal conditions. These findings elucidate the phase-transformation mechanism governing leaching performance and establish a fundamental basis for the rational design of efficient vanadium recovery processes.