Mechanism and kinetics of the anodic reaction in cryolite melts. I. The influence of CaF2 (5 wt%) at different Al2O3 content

Calcium fluoride is one of the two main components of the cryolite-based el ectrolyte in aluminium electrolysis. By Electrochemical Impedance Spectrosc opy (EIS) the influence of CaF2 upon the kinetics and mechanism of the anod ic reaction in 4 cryolite-alumina melts containing 2, 4, 6 and 8 wt% of alu mina and 5 wt% of CaF2 was determined at 1000 degrees C. The addition of 5 wt% of CaF2 to a cryolite melt with a certain alumina content increases the overpotential of the anodic reaction as compared to the cryolite-alumina s aturated (CRAS) melt. The Tafel slope increases from around 0.1 V/dec., to 0.17 V/dec indicating that at low alumina content the second charge transfe r step is the rds, whereas at higher alumina content this control is shifte d towards the first step. The effective rate constants of both charge trans fer steps are increasing with the alumina content and this dependence resem bles the concentration dependence of Al2OF62- and Al2O2F42- with alumina co ntent. It can be concluded that the CaF2 acts rather as a complexing agent. The diffusion coefficient of the Al2OF62- anionic species decreases as a f unction of the alumina content, due to the increase of the melts viscosity.