Mirza, Mateen; (2023) Electrochemical Processing in Molten Salts for Refractory Metal Recovery and Battery Materials Recycling. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

With broad consensus on the negative impact of CO₂ emission, there is a global transition to low-carbon energy. Alternative solutions need to be found for the current hydro- and/or pyro-metallurgical recovery of valuable metals. The Fray-Farthing-Chen (FFC) process was the first to demonstrate the potential for the direct recovery of a metal from its metal oxide and has been used as a point of reference in the implementation and development of the ‘fluidised cathode’ molten salt processes for refractory metals recovery applications. Predominance diagrams were initially constructed to predict the electrochemical reduction pathway and to determine the oxide ion concentrations at different electrode potentials. The ‘fluidised cathode’ process was then applied to three metal oxide precursors (TiO₂, Ta₂O₅ and LiCoO₂) to compare cyclic voltammograms and chronoamperograms. This was carried out alongside scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The correlation of these techniques led to the successful recovery of LiTiO₂ from TiO₂ and Co metal from LiCoO₂, an end-of-life battery material, with Faradaic efficiencies greater than 70%. These results demonstrate the advantage of a ‘fluidised cathode’ especially as it incorporates an argon gas agitation mechanism which improves contact between the metal oxide and working electrode. There are, however, several disadvantages in using the mentioned characterisation techniques due to product exposure to air and moisture. To overcome this, an approach was adopted to explore the electrochemical reduction of LiCoO₂ in LiCl-KCl using X-ray computed tomography. For the first time, the electrochemical process was visualised non-destructively in three-dimensions. A key observation of this work was the formation of CO/CO₂ gas which increases the electrolyte resistance of the system and results in negative operational consequences. This pyroprocessing technology has been shown to have much broader applicability than initially thought especially in the processing of battery materials – an area of work that allows for other aspects of operation to be explored.

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