Phase Equilibrium of the V2O5-Na2O System

Knowledge of the equilibrium phase relationship in the V2O5-Na2O system is critical and essential for the sodium salts roasting process of slag-bearing vanadium, and those fundamental studies can be used to accelerate industrial optimization and update the process. Therefore, phase equilibrium and quenching and thermal analysis experiments on the V2O5-Na2O binary system were carried out in the present work under pO(2) of 10(-3.5) atm. The V2O5-Na2O system has 7 primary phase fields including two simple oxides (V2O5 and Na2O), and 5 binary compounds (Na2V12O31, Na10V24O65, NaVO3, Na4V2O7, and Na3VO4). The existence of Na2V12O31 and Na10V24O65 was experimentally confirmed and then characterized by EPMA, no extensive compounds or solid solutions were detected in the present study. The liquidus lines were experimentally determined in most of the primary phase fields in the temperature range from 500 degrees C to 800 degrees C. The temperatures and components of some invariant reactions, including Liq -> V2O5+ Na2V12O31 (640 degrees C, 53 mol pct Na2O), Liq -> Na10V24O65 + NaVO3 (504 degrees C, 40.2 mol pct Na2O), Liq -> NaVO3 + Na4V2O7 (540 degrees C, 59.0 mol pct Na2O), and a new peritectic reaction at 570 degrees C with 32 mol pct Na2O were rechecked, and Liq -> Na2V12O31 -> Na10V24O65 was also reconstructed. The large amount of new phase equilibrium data proposed in this work will serve as reliable input for future thermodynamic reassessments of the V2O5-Na2O system within the CALPHAD framework to extensively improve the current V2O5-based thermodynamic databases. [GRAPHICS]