Thermodynamic properties of MnV2O6 and Mn2V2O7 at high temperatures

MnV ₂O ₆ and Mn ₂V ₂O ₇ are the key compounds during the manganese salt roasting for extracting vanadium from vanadium bearing slag. Thermodynamic properties of MnV ₂O ₆ and Mn ₂V ₂O ₇ are dispensable to get insight into the multi-phase reaction mechanism and clarify the phase evolution. In this study, the MnV ₂O ₆ and Mn ₂V ₂O ₇ compounds were synthesized through solid-state roasting routes under a nitrogen gas atmosphere, and their crystal structure and thermodynamic properties were determined using X-ray powder diffraction, differential scanning calorimetry, and drop calorimetry. The enthalpy increments of MnV ₂O ₆ and Mn ₂V ₂O ₇ were experimentally measured for temperatures ranging from 573 K to 1023 K and 573 K to 1173 K, respectively, using drop calorimetry for the first time. Based on the experimentally measured enthalpy increments, the temperature dependence of the molar heat capacity of MnV ₂O ₆ and Mn ₂V ₂O ₇ was then derived. Furthermore, thermodynamic properties such as enthalpy, entropy, and Gibbs free energy were also calculated. Finally, the Gibbs free energy of the chemical reactions during the roasting of vanadium slag was evaluated, and compared with other roasting techniques.