Cathode reaction mechanism

The electrocalciothermic reduction mechanism may be represented by the following sequence of reactions, where "M" represents a metal to be reduced (typically titanium).

(1) MO
_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

and

(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:

(3) MO
_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⁻