The Ilmenox process is the most recent approach that NASA considered worthwhile testing for liberating oxygen from lunar materials. The Ilmenox process is derived from the FFC-Cambridge process (Chen et al. 2000), which was developed at the University of Cambridge for the electro-deoxidation of metals and metal oxides. The FFC process was invented in a search for ways of using molten salt electrochemistry to remove the oxide scale from titanium artefacts that forms when titanium is heated in air. It warrants a brief excursion through the metallurgy of titanium making to fully appreciate the relevance and scope of the FFC process and hence realise the potential of the Ilmenox process.
The existing process for titanium extraction was invented and developed by Kroll in the 1930s to 1940s and later named after him. The process comprises two steps, the carbo-chlorination of titanium dioxide to titanium tetrachloride, and the magnesium reduction of the titanium tetrachloride to titanium metal (e. g., Habashi 1997). The process is laborious and time-consuming and, by modern standards, grossly environmentally unfriendly. Kroll himself was reported to have stated that within fifteen years an electrolytic route would replace his process.
Conventional thinking was that in order to win titanium via an electrolytic process, it would be necessary to dissolve a titanium salt in a molten salt, such as sodium chloride, and electrolyse it. At the cathode, titanium ions are reduced to titanium, which deposits; and at the anode, chloride ions are oxidised to chlorine, which exits as a gas. However, it has proven to be exceptionally intricate to win solid titanium from a molten salt, because the metal product always occurs in the
form of a very fine powder which is both difficult to harvest and prone to oxidation. Moreover, the efficiency of this type of electrolytic process is poor, owing to titanium existing in several oxidation states in the molten salt and being able to set up parasitic shuttle reactions. Many tens of millions of dollars have been spent in vain on attempts to develop an electrolytic process for titanium winning.
The concept of the FFC process is favourable in that the starting material is not a titanium salt but titanium dioxide that is abundantly and cheaply available. The FFC process uses an electrolytic cell at around 900°C, in which a porous sintered titanium dioxide body is made the cathode in an electrolyte of molten calcium chloride. A potential is applied, which is sufficiently high to decompose the cathode such that oxide ions are expelled into the electrolyte, but also sufficiently low not to decompose the electrolyte. This means the reduction of the oxide in the cathode proceeds without the deposition of metal from the electrolyte onto the cathode (Fray et al. 1999; Chen et al. 2000). The cathodic reaction in the FFC process may hence be construed as the direct ionisation of oxygen, and the cathodic product is the de-oxidised metal that is left behind:
TiO2 + 4 e -= Ti + 2 O2- (7.9)
The anodic reaction in the FFC process is the discharge of oxide ions from the electrolyte. When a carbon material is used as the anode, the anodic reaction product is carbon oxides that are released from the cell as a gas:
C + O2- = CO + 2 e – and/or C + 2 O2 -= CO2 + 4 e – (7.10a, b)
The key features of the FFC process are that the titanium is always in the solid state and forms within the cathode, while the molten salt electrolyte is a transport medium for oxide ions and is not consumed. A schematic of an FFC cell is shown in Fig. 7.4. Usefully, as the titanium product is not deposited from a salt, it is relatively compact and inert and can readily be retrieved from the cell. Overall, the FFC process is not an incremental improvement of pre-existing technologies but a novel approach in the field of electrowinning. Furthermore, it is a generic method with manifold applications for the winning of other metals and the synthesis of alloys and intermetallics from suitable oxide blends (Fray 2001, 2002).
In order to scale-up the FFC process toward the production of industrial quantities of titanium, it was necessary to identify and optimise all the relevant process parameters. To that end, a series of fundamental studies was conducted at the University of Cambridge that extended over a period of several years. This rigorous research program led to a full understanding of the individual reaction steps in the cathode, the transference properties of the electrolyte, and the reactions at the anode. These studies are now published (Schwandt and Fray 2005; Fray et al. 2006; Alexander et al. 2006, 2011; Schwandt et al. 2009). The scientific results, along with the practical experience, gained in these studies have made possible the successful production of titanium in pilot plants (Schwandt et al. 2010).