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. It is thought that this process will eventually be capable of producing metals or alloys more efficiently than by current conventional processes, such as titanium by the Kroll process. Producing tantalum can also make use of this process.
The FFC Cambridge process was developed by George Chen, Derek Fray and Tom Farthing between 1996 and 1997 at the University of Cambridge. (The name FFC derives from the first letters of their last names.) They reduced oxide scales on titanium foils, as well as small pellets of titanium dioxide powder, to the metal by molten salt electrochemistry. The technology has since been commercialised in the form of an intellectual property company known as Metalysis, which is based in Sheffield, UK. A similar process was patented in 1904 as German patent 150557.
The process typically takes place between 900 and 1100 °C, with an anode (typically carbon) and a cathode (oxide being reduced) in a bath of molten CaCl2. Depending on the nature of the oxide it will exist at a particular potential relative to the anode, which is dependent on the quantity of CaO present in CaCl2. The cathode is then polarised to a more negative voltages versus the anode. This is simply achieved by imposing a voltage between the anode and cathode. When polarised to more negative voltages the oxide releases oxygen ions into the CaCl2 salt, which exists as CaO. To maintain charge neutrality, as oxygen ions are released from the cathode into the salt, so oxygen ions must be released from the salt to the anode. This is observed as CO or CO2 being evolved at the carbon anode. In theory an inert anode could be used to produce oxygen.
When negative voltages are reached, it is possible that the cathode would begin to produce Ca (which is soluble in CaCl2). Ca is highly reductive and would further strip oxygen from the cathode, resulting in calciothermic reduction. However, Ca dissolved into CaCl2 results in a more conductive salt leading to reduced current efficiencies.
Cathode reaction mechanism
The electro-calciothermic reduction mechanism may be represented by the following sequence of reactions.
x+ x Ca → M + x CaO
When this reaction takes place on its own, it is referred to as the "calciothermic reduction" (or, more generally, an example of metallothermic reduction). For example, if the cathode was primarily made from TiO then calciothermic reduction would appear as:
TiO + Ca → Ti + CaO
Whilst the cathode reaction can be written as above it is in fact a gradual removal of oxygen from the oxide. For example, it has been shown that TiO2 does not simply reduce to Ti. It, in fact, reduces through the lower oxides (Ti3O5, Ti2O3, TiO etc.) to Ti.
The calcium oxide produced is then electrolyzed:
(2a) x CaO → x Ca2+ + x O2−
(2b) x Ca2+ + 2x e− → x Ca
(2c) x O2− → x/2 O2 + 2x e−
Reaction (2b) describes the production of Ca metal from Ca2+ ions within the salt, at the cathode. The Ca would then proceed to reduce the cathode.
The net result of reactions (1) and (2) is simply the reduction of the oxide into metal plus oxygen:
x→ M + x/2 O2
Anode reaction mechanism
The use of molten CaCl2 is important because this molten salt can dissolve and transport the O2− ions to the anode to be discharged. The anode reaction depends on the material of the anode. Depending on the system it is possible to produce either CO or CO2 or a mixture at the carbon anode:
- C + 2O2− → CO2 +4 e−
- C + O2− → CO + 2 e−
However, if an inert anode is used, such as that of high density SnO2, the discharge of the O2− ions leads to the evolution of oxygen gas. However the use of an inert anode has disadvantages. Firstly, when the concentration of CaO is low, Cl2 evolution at the anode becomes more favourable. In addition, when compared to a carbon anode, more energy is required to achieve the same reduced phase at the cathode. Inert anodes suffer from stability issues.
- 2O2− → O2 + 4 e−
- "Titanium". The Essential Chemical Industry - online. the University of York, York, UK.: CIEC Promoting Science. 15 January 2015. Retrieved 30 July 2015.
- Fray, D. J.; Chen, G. Z.; Farthing, T. W. (2000). "Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride". Nature 407 (6802): 361–4. doi:10.1038/35030069. PMID 11014188.
- Metalysis Official Website
- DRP 150557 "Verfahren der Gewinnung von Titan aus seinen Sauerstoffverbindungen auf elektrolytischem Wege". dpma.de
- Rideal, Eric Keightley (1919) Industrial Electrometallurgy, Including Electrolytic and Electrothermal Processes. D. Van Nostrand co. p. 137
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- Molten calcium chloride electrolysis reference – 1911 Encyclopædia Britannica – Calcium
- X. Ge, X. Wang, S. Seetharaman (2009). "Copper extraction from copper ore by electro-reduction in molten CaCl2–NaCl". Electrochimica Acta 54 (18): 4397–4402. doi:10.1016/j.electacta.2009.03.015.