Thermodynamic properties of aqueous solutions of vanadate salts

Vanadium is an important metal in modern society, predominantly used as an alloying element in steels to enhance mechanical strength. Other important applications can be found in various sectors, including batteries, catalysts, ceramics and even medicine. It is increasingly produced from secondary raw materials, such as slags from the metal industry, through processes combining pyro- and hydrometallurgy.

Understanding the aqueous chemistry of vanadium is important for developing greener production routes, improving existing processes, and evaluating the environmental impacts of vanadium pollution. The aim of this thesis was to develop a basis for modelling the thermodynamic properties of aqueous solutions of vanadate salts, focusing on the key binary systems NaVO3 – H2O and NH4VO3 – H2O.

Solubility and freezing point depression data for the above systems were experimentally determined and used with other literature data to develop and validate a model for the computation of various thermodynamic properties, including osmotic and activity coefficients, enthalpy changes, solubility and freezing point depression. The model was based on spectroscopic observations reported in the literature, and the non-ideality of the solution phase was modelled with the Pitzer equation. The extension of the model to ternary systems was investigated by modelling vanadate salt solubility in two such systems. Furthermore, the model was applied to estimate the level of saturation of ammonium vanadate in laboratory-scale vanadium recovery experiments performed with authentic materials.

The results obtained in this thesis extend the database of physical properties of aqueous vanadate salt solutions and enable the thermodynamically consistent calculation of various properties of these systems. This thesis also shows that spectroscopically determined equilibrium constants offer a useful starting point for the thermodynamic modelling of aqueous vanadate salt solutions.