Features and Challenges of Molten Oxide Electrolytes for Metal Extraction

Abstract

The electrolytic decomposition of metal oxides to metal and oxygen is an extractive metallurgy principle that, when coupled with carbon-free electricity, drastically mitigates the global warming impact of metal production. The present perspective discusses the electrochemical engineering features of an unconventional electrolyte, molten oxides. A survey of its thermodynamic properties suggests exceptional features, both in terms of applicability to multiple metals and operation at high temperature to produce liquid metal. The review of molten oxides' transport properties indicates that an unprecedented throughput can be envisioned, a promising feature for tonnage production. However, our ability to define the optimal electrolyte composition with regard to energy consumption is rendered limited due to the lack of predictive tools for both of the reviewed properties. A look at the state of the art in electrode materials reveals that quantitative design criteria remain to be developed for both the cathode and the anode. Metals have been essential materials for mankind for more than 2 millennia, and they remain the most important materials in terms of market value. Because of their centrality in the structural framework of the modern world, some metals have developed a commodity status, much like water or food. In the last two decades, an unprecedented increase in the global demand for metals has occurred ( Figure 1a). This trend is expected to continue with the world population predicted to reach 9 billion by 2020.Such growth does not come without challenges, particularly when it comes to the sustainability of primary metal production. Current extraction and processing routes are indeed characterized by large capital and environmental impacts. The former issue leads to difficulty in financing new facilities, particularly in developing countries that are poised to become significant consumers of primary metals. The existing technical paradigm necessitates multi-billion dollar investments that in turn require exceptional profits and short-term metal price stability in order to be supported by financing organizations. The environmental impact, illustrated in Figure 1b via the specific global warming and acidification potentials of key metals, can hinder the implementation of greenfield plants in countries with the most stringent environmental regulations. Additionally these emission potentials constitute a threat to the market in the eventuality of their taxation.The present perspective offers first to reconsider the existing paradigm for the extraction of metals from oxides, which accounts for the majority of metal resources in tonnage and value. In particular, this document focuses on the use of electricity for the direct decomposition of metal oxides. The first section discusses the metallurgical strategies to extract a metal (M) from its oxide (MO) and the characteristics of the direct electrolytic decomposition pathway. In the second section, an argument is made for the use of molten oxides as a possible supporti...

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