Process
The process typically takes place between 900–1100 °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.
Read more about this topic: FFC Cambridge Process
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