Characterisation of Electrochemical Properties for Molten Titanium(IV) Oxide - Sodium Oxide and Expansion into Other Binary Oxide Systems for the Electrolytic Reduction of Valuable Metals

Abstract

Molten oxide electrolysis is a promising route to reduce harmful environmental effects that occur when compared to more common industrial metal extraction techniques. This method can provide a stepping stone to decarbonise metal production and to do this suitable supporting electrolyte ore combinations need to be found. For the application of extracting titanium metal, due to the lack of electrochemical information on complex multi-component titanium metal oxide systems, designing an electrolytic cell and predicting ultra-high temperature (above 1500°C) behaviours becomes difficult. Hence, to gain insight into this electrochemical information a binary system, TiO2 – Na2O, was investigated via experimentation and the use of FactSage 7.2 (a thermodynamic predictive software). This system was chosen as the phase diagram indicated deep eutectics either side of a congruently melting line compound situated at approximately 40 wt% TiO2, hence providing suitable operating temperature and composition ranges. Additionally, FactSage predicted that the relative potentials of the half-cell reactions 2Na2O → 4Na + O2 and TiO2 → Ti + O2, change order when the composition of TiO2 is increased from below the congruently melting line compound, to above, indicating that above approximate 40 wt% TiO2, titanium metal may be favourably reduced first when electrolytically reducing the melt. Lab scale experimental process conditions (temperature and composition) were studied to determine the feasibility of electrolytically reducing titanium metal from the molten oxide mixture. From our research, expansion into other binary oxide systems for different metals of interest, specifically neodymium and tantalum, will be discussed. Specifically, systems that contain congruently melting line compounds with adjacent eutectic reactions were hypothesised to have a similar reordering of the half-cell potentials, favouring the reduction of the desired metals (Nd or Ta). Understanding simplified binary systems will scaffold insight into more complex, industrially-relevant molten oxide mixtures at ultra-high temperatures, potentially contributing to the discovery of new and more sustainable electrochemical pathways for technology-critical metals.