(590a) Evaluating the Electrochemical Operating Window for Titanium Metal Reduction Via Molten Oxide Electrolysis

Problem:

Molten oxide electrolysis (MOE) could enable a low-carbon route for sustainable production of metals. All-oxide MOE does not create toxic halide waste, can use electrons generated from renewable energy sources and will not directly produce greenhouse gases if inert anodes are used. MOE has significant technical challenges including electrolyte chemistry, ionic conductivity, and stability; crucible and electrode stability, conductivity, and longevity; and feedstock chemistry.

Titanium was selected as the metal of interest because it has a high strength-to-weight ratio, excellent corrosion resistance, and a high melting temperature. Despite the abundance of Ti-bearing ores, Ti is not widely used due to the expense and complexity of the incumbent production method – the Kroll process. [1] For this reason, we investigated a MOE route to Ti production.

To build a fundamental understanding of the electrolytic operating window a simple binary oxide system was studied. In this work sodia (Na₂O) was experimentally investigated as the supporting electrolyte for titanium oxide reduction to metal. Sodia dissolves oxides of titanium, and the TiO₂-Na₂O pseudo-binary system forms a eutectic, reducing the liquidus temperature over a wide composition range (Figure 1).

Methods:

The FToxid and FactPS databases of FactSage 8.1 [2] were used to make thermodynamic predictions, based on the real solution (i.e. non-ideal) nature of the TiO₂-Na₂O system. The predictions show that a reordering of reduction potentials occurs around the congruently melting line compound (Figure 2). In a reducing (argon + carbon) atmosphere, the predicted complete Ti⁴⁺/Ti and partial Ti⁴⁺/Ti³⁺ reduction potentials were less negative than the Na+/Na reduction potential at 1100°C and . Therefore the TiO₂-Na₂O system is predicted to be suitable for MOE to produce Ti metal.

To confirm the prediction, molten oxide electrolysis was carried out in a vertical tube furnace. Samples were placed in the centre of the hot zone using an internal support structure made of molybdenum (Figure 3) which was contained in an alumina tube inside the vertical tube furnace. Graphite crucibles were used both to contain the sample and to prevent any damage in case of leakage. The alumina tube was purged with argon to prevent oxidation of the internal components. An electrolysis experiment was performed at 1102 ± 5°C on an electrolyte with mol fraction.

After the electrolysis experiment, the furnace was cooled and a cross section of the now solid oxide mixture with the working electrodes still embedded was prepared. The working electrode was analysed via scanning electron microscopy (SEM) and elemental energy-dispersive X-ray spectroscopy (EDS) to determine the extent to which Ti metal had been deposited.

Results:

Ti metal was detected at the platinum working electrode (Figure 4) in a 20 ± 3 μm wide interdiffusion zone (region II) after electrolysis of the melt for 160 minutes at -0.25 V vs Ti reference. Up to 10 ± 1 wt% Ti was identified in the interdiffusion zone, which exceeds the equilibrium solubility of ~2.7 wt% Ti in Pt at 1100°C, [5] hence these are the likely intermetallic phases present in the WE interdiffusion zone (region II).

Ti reduction efficiency was estimated from 10 line-scans around the working electrode cross section. Electrolytic reduction of Ti was confirmed; however, the calculated reduction efficiency for Ti metal was very low (0.06 ± 0.02%). Decreasing the reduction voltage from -0.25 to -0.1 V vs the Ti reference electrode resulted in a reduction efficiency for Ti metal of 0.24 ± 0.08% for . Hence Ti reduction from TiO₂-Na₂O under these conditions is not practically viable.

The low reduction efficiency could be due to co-reduction of sodium. The experiment was performed at mol fraction and the compositional uncertainty was the difference between the mixed precursor (0.54) and XRF analysis of the product material (0.62), which indicated a significant amount of Na vaporisation occurred. Sodium metal was not identified at the electrode because the boiling point is 883°C. Therefore, any sodium metal produced at ~1100°C evaporated and was carried away by the argon.

Implications:

Thermodynamics calculations accounting for the non-ideality of the TiO₂-Na₂O pseudo-binary system correctly predicted that Ti metal would be electrolytically reduced over Na metal under certain conditions. Ti was observed alloying into the Pt working electrode for the experiment conducted at mol fraction. However, despite Ti reduction being confirmed the reduction efficiency was very low (below 0.25%).

While Naâ‚‚O is an unsuitable electrolyte for Ti reduction via MOE, we speculate our work forms the basis for a systematised approach to search for supporting all-oxide electrolytes. The supporting electrolyte must fulfil established criteria [6] such as having: the ability to dissolve the metal of interest; high ionic conductivity; and chemical and material [7] stability. In addition, in our work we looked for a deep eutectic leading to a substantial reduction of the liquidus temperature over a wide range of composition for more readily accessible MOE operating temperatures. Furthermore, we hypothesize that searching for oxide mixtures with a congruently melting line compound which leads to a reordering of reduction potentials can aid the discovery of all oxide mixtures suitable for MOE. However, this approach is limited by a paucity of high temperature data for binary and terniary oxide mixtures and a lack of predictive tools.

This research is funded by New Zealand Ministry of Business, Innovation and Employment (MBIE) contract CONT-76356-ENDSI-UOC.

References:

1. Kroll, W. (1940). The production of ductile titanium. Transactions of the Electrochemical Society, 78(1), 35.

2. Bale, C. W., Bélisle, E., Chartrand, P., Decterov, S. A., Eriksson, G., Gheribi, A. E., ... & Van Ende, M. A. (2016). Reprint of: FactSage thermochemical software and databases, 2010–2016. Calphad, 55, 1-19.

3. Hughes, T. G. (2018). The development of ultra-high temperature experimental capabilities for the electrolytic extraction of titanium from New Zealand steel’s iron slag. Thesis, University of Canterbury.

4. Martin-Treceno, S., Hughes, T., Weaver, N., Marshall, A. T., Watson, M. J., & Bishop, C. M. (2021). Electrochemical study on the reduction of Si and Ti from molten TiO2–SiO2–Al2O3–MgO–CaO slag. Journal of The Electrochemical Society, 168(6), 062502.

5. Biggs, T., Cornish, L. A., Witcomb, M. J., & Cortie, M. B. (2004). Revised phase diagram for the Pt–Ti system from 30 to 60 at.% platinum. Journal of alloys and compounds, 375(1-2), 120-127.

6. Allanore, A. (2014). Features and challenges of molten oxide electrolytes for metal extraction. Journal of The Electrochemical Society, 162(1), E13.

7. Ford, K. T., Marshall, A. T., Watson, M. J., & Bishop, C. M. (2024). Experimental Validation is Always Required for Molten Oxide Electrolysis Laboratory Crucibles. Metallurgical and Materials Transactions B, 1-18.

Figure Captions:

Figure 1: FactSage 8.1 predicted TiO₂-Na₂O phase diagram at unit activity oxygen using the FToxid database. [2]

Figure 2: TiO2-Na2O reduction potentials at 1100°C for mol fraction . Lines are FactSage 8.1 predictions.

Figure 3: Electrochemical cell used in the vertical tube furnace, showing (A) the various individual components of the internal support structure system and (B) the modified area of the molybdenum working electrode in contact with the slag. Adapted from [3,4].

Figure 4: Analysis of the product at the Pt working electrode. a) EDS maps of Na, Ti and Pt, b) secondary electron image indicating line scan in yellow and c) EDS line scan across the interface between the oxide mixture and the Pt working electrode from b).