Phase Equilibrium of the V₂O₅–Na₂O System

Abstract

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 pO₂ 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 °C to 800 °C. The temperatures and components of some invariant reactions, including Liq → V₂O₅ + Na₂V₁₂O₃₁ (640 °C, 53 mol pct Na₂O), Liq → Na₁₀V₂₄O₆₅ + NaVO₃ (504 °C, 40.2 mol pct Na₂O), Liq → NaVO₃ + Na₄V₂O₇ (540 °C, 59.0 mol pct Na₂O), and a new peritectic reaction at 570 °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.

Graphical Abstract