Electrode Processes of Iron Species in a Melt MgCl₂-NaCl-KCl-CaCl₂

The equilibrium of reaction FeCl₂+\frac12Cl₂ \mathbin\lower.3ex\hbox\buildrel\textstyle\rightarrow\over \smash\leftarrow\vphantom_\vbox to.5ex\vss FeCl₃ in the melt has been studied. It has been shown that FeCl₃ decomposes almost fully at P_c12→O, and that Fe(Ⅲ)/Fe(Ⅱ)=1.36 at P_C12=latm and t=700℃. The higher the temperature, the smaller the ratio Fe(Ⅲ)/Fe(Ⅱ).
The electrode kinetics of iron species in the melt has been studied by cyclic voltammetry. It has been found that the cathodic reaction Fe (Ⅱ) +2e→Fe(0) is controlled by diffusion of Fe(Ⅱ) in the melt, and that the apparent activity energy of the cathodic reaction is 12.9±4 KJ/mol, and the diffusion coefficient of Fe(Ⅰ)is (4.31 ±0.79) ×10-5cm²/s. Anodic reaction Fe(Ⅱ) -e→Fe(Ⅲ)is also controlled by diffusion and followed by a rapid chemical reaction Fe(Ⅲ) +Cl^-→Fe(Ⅱ) + \frac12l². So, the current efficiency for removing iron by electrolysis should not be effected considerably by the changes of oxidation states of iron species between cathodic and anodic compartment. The above results have been shown to be correct by electrolysis in Lab. scale. Co-deposition of mag-nsium is the main reason of low current efficiency for removing iron by electrolysis. Current efficiency can increase to 70% under following conditions:700℃ cathodic c. d. 0.2A/cm² and P_C12→0 in the cathodic compartment.