FFC Cambridge process

The FFC Cambridge Process is an electrochemical method in which solid metal compounds, particularly oxides, are cathodically reduced to the respective metals or alloys in molten salts.

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History and invention

The method was invented by three scientists, George Z. Chen, Derek J. Fray and Tom W. Farthing, between 1996 and 1997 in the University of Cambridge, from the names of whom derive the three letters in the name of the process. Chen was the first to discover in late 1996 that oxide scales on titanium foils can be reduced to the metal by molten salt electrochemistry. After seeing the evidence with thick oxide scales, Fray suggested an experiment to reduce small pellets of titanium dioxide powder, which Chen carried out successfully between late 1996 and early 1997. Farthing, who first suggested to electrochemically remove oxygen from titanium metal, later commented on the discovery as "completely out of expectation".

Chemistry of the process

The basic underlying principle of the FFC Cambridge process is that metallic calcium - unlike sodium or magnesium - is quite soluble in its own molten chloride salt: molten calcium chloride dissolves up to a few mole percent calcium metal (3.9 mol% Ca at 900 °C). This molten salt dissolved, molten calcium metal is free to wander about in the melt, including diffusing into and reducing crystalline titanium and other metal oxides. The calcium mobility in the melt is both a blessing and a curse: it's a blessing for providing this calciothermic reduction of titanium dioxide, but a curse when it comes to current efficiency, because the cathode-reduced metallic calcium can also wander back to the anode, and get reoxidised with the evolved gases there. For this reason, the electrolytic production of pure calcium metal from its molten chloride involves an iron rod cathode that must be gradually raised, and a layer of calcium metal deposits as a continuation of the iron rod. Should the calcium rod be left to soak in the molten salt, it would simply wash away as it forms, and recycle to the anode. For this and other economic reasons, the commercial calcium metal production is done via the aluminothermic reduction/calcium vapor vacuum distillation route instead of an electrolytic one, similar to how magnesium is produced via the silicothermic Pidgeon process. Still, considering that the highly uneconomical batch-based Kroll process can take 2-5 days for a single batch to complete, the coulombic current efficiency losses in the FFC Cambridge process — due to ion recycling between cathode and anode, which can be helped by using suitable electrolyte separating diaphragms — might be quite tolerable.

The new method is much simpler in operation and uses less energy than many current industrial technologies, such as the Kroll process, and promises a great potential for cheap production of useful reactive metals such as titanium, zirconium and tantalum. Its other advantage is to produce various metal alloys directly from mixed metal oxide powders, which will offer more saving in energy and operation cost. It is also scientifically interesting because the electrolysis can be carried out on an insulator oxide, such as zirconia and silica. Furthermore, the process can extract pure oxygen gas from oxide based minerals. This is useful, for example, for generating oxygen gas on the Moon from lunar rocks (ilmenite, FeTiO₃) to support space travelling and even living by humans.

References

• G. Z. Chen, D. J. Fray, T. W. Farthing (2000). "Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride". Nature 407: 361-4. doi:10.1038/35030069.
• Ryosuke O. Suzuki (2005). "Calciothermic reduction of TiO₂ and in situ electrolysis of CaO in the molten CaCl₂". Journal of Physics and Chemistry of Solids 66: 461-465.
• Il Park, Takashi Abiko and Toru H. Okabe (2005). "Production of titanium powder directly from TiO₂ in CaCl₂ through an electronically mediated reaction (EMR)". Journal of Physics and Chemistry of Solids 66: 410-413.
• A molten calcium chloride electrolysis reference
• An aluminothermic calcium production reference