A salt solution - Molten salts for metallurgy
Dr Carsten Schwandt reviews R&D work in the ﬁeld of molten salts in the Department of Materials Science and Metallurgy at the University of Cambridge, UK.
Processes involving high-temperature molten salts are of huge importance in many large-scale metallurgical industries. The most prominent example is the production of aluminium through the Hall-Héroult process, with a capacity of around 40 million tpa, in which Al2O3 is dissolved in a Na3AlF6 (cryolite) melt and electrolysed with carbon electrodes. Similarly important is the production of magnesium, sodium and lithium through the electrolysis of their molten chlorides. These processes have been developed to a high level of maturity and are unchallenged in industry. Other relevant technologies are hightemperature batteries and the molten carbonate fuel cell.
It may be surprising that the overwhelming presence of the field of molten salts in industry is in stark contrast to its virtual absence in academia. A most notable exception is Professor Derek Fray from the Department of Materials Science and Metallurgy at the University of Cambridge, who for more than four decades has been a world leader in molten salt processes and technologies. Along with his Materials Chemistry Group, he has continually provided key contributions to this area, including the well-known FFC-Cambridge process for the production of metals and a new process for the production of carbon nanomaterial.
Part of the process
The FFC-Cambridge process, named after its inventors, Fray, Farthing and Chen, is a metallurgical method that allows the direct reduction of pure and mixed metal oxides to their corresponding metals and alloys. The process was discovered in an effort to remove oxygen from the metallic α-scale of titanium components, formed during exposure to air at elevated temperatures, by electrochemical polarisation in molten CaCl2. Following the successful deoxidation of the metallic scale, it was found that oxide scales, and indeed entire oxide bodies, could likewise be deoxidised and converted to titanium metal. It was finally realised that the approach is generic and applicable to a vast range of metals and semi-metals, including silicon, zirconium, hafnium, niobium, tantalum, chromium and uranium, as well as many alloys and intermetallics.
The FFC-Cambridge process has key advantages over conventional metallurgical methods. Importantly, the direct reduction of the oxides occurs in the solid state and therefore does not involve any metal deposition steps. This avoids the critical issues of many traditional processes, such as particle nucleation and growth, and dendrite formation. Moreover, the reduction of suitable oxide blends permits the direct preparation of homogeneous alloys, eliminating cumbersome processing steps such as liquid metal mixing or mechanical milling. In this way, alloys can be prepared that are otherwise difficult, or even impossible, to make.
The overall cathode reaction in the FFC-Cambridge process is simply the removal of oxygen from a metal oxide. The precise reaction pathway, however, depends on the particular oxide and may be highly complex. The reduction of TiO2 has been studied in depth and was found to involve the successive formation and decomposition of various binary suboxides and ternary compounds containing calcium. Clearly, this knowledge is crucial to the successful scale-up of the process.
The commercialisation of the FFC-Cambridge process is currently underway at Metalysis Ltd, based in Yorkshire, a spinout company that has raised more than £40 million and employs in excess of 50 staff. After several delays, the production of titanium and tantalum in industrially-relevant quantities is now said to be imminent. The process is also at an advanced stage of development in the nuclear industries, where it is used for the reprocessing of spent nuclear oxide fuels.
One key target of recent research on the FFC-Cambridge process has been the substitution of the consumable carbon anode with a non-consumable one. This simplifies the process, ensures carbon-free products and, with metal and oxygen as the only reaction products, provides green credentials. The search was successful when it was demonstrated that CaTiO3 doped with CaRuO3 is sufficiently conductive and stable to evolve oxygen from the molten salt over extended periods of time. The patent for this material is held by London-based Green Metals Ltd.
The possibility of generating carbon nanomaterial directly from graphite in molten salts has been known since Kroto’s work in the mid-1990s. The method is based on the erosion of graphite by electrochemical polarisation in molten LiCl or NaCl. However, this approach was soon abandoned when it was realised that product quantity was small and product quality poor, owing to the persistent presence of multiple intermixed nanoscopic carbon constituents, such as particles, tubes and fibres, in addition to a fraction of unreacted macroscopic graphite fragments. The molten salt electrolytic method of preparing carbon nanomaterial from graphite was only brought to fruition more than 10 years later in Derek Fray’s Materials Chemistry Group, after fundamental improvements had been introduced with respect to both yield of product and selectivity for carbon nanotubes.
The molten salt electrolytic method of preparing carbon nanomaterial exhibits a number of remarkable features that have the potential to render it a contender for large-scale production of carbon nanotubes or other carbon nanostructures. These include the simplicity and non-toxicity of the process, the inexpensive reactants, and the extraordinary space-time yield exceeding that of gas-based methods by several orders of magnitude. A particularly attractive feature of the process is its ability to generate metal-filled carbon nanomaterial via the addition of suitable metal compounds to the electrolyte.
The molten salt electrolytic process of producing carbon nanomaterial from graphite has now reached scale-up, which is performed in cooperation with Morgan AM&T as the industrial partner and with generous financial support from the Technology Strategy Board. Ongoing activities focus on the construction of a demonstrator cell for the production of plain and metal-filled carbon nanotubes and nanoparticles. Although many uses are envisaged, one intended application is in lithium ion battery electrodes.
The driving force in the case studies presented above is the pressing need to develop viable large-scale production processes that are energy efficient, sustainable and environmentally acceptable. In both cases, impressive progress has been achieved in terms of advancing the science, embarking on scale-up, attracting industry’s attention, and raising funds.
Professor Derek Fray continues to be an active and creative mind in the field of molten salts that he has shaped so uniquely, although, in the present climate, it is difficult to maintain this high level of impact. It is hoped that in the long term the discipline is retained, or adopted, by several UK academic institutions to prevent the decline of this strategically important research, and take advantage of its exceptional role in the development of future materials processes. There are many exciting ideas and concepts warranting exploration, especially in mineral processing, including rare earths, but also in areas such as batteries, fuel cells, inorganic synthesis and carbon capture.
For more information, email Carsten Schwandt, Department of Materials Science and Metallurgy at the University of Cambridge, firstname.lastname@example.org.