Mechanism and kinetics of the anodic reaction in cryolite melts. II. The influence of AlF3 (11 wt%) at different Al2O3 content

Aluminium trifluoride is the main additive to the cryolite-based electrolyt e in aluminium electrolysis. By Electrochemical Impedance Spectroscopy (EIS ) the influence of AlF3 upon the kinetics and mechanism of the anodic react ion in 4 cryolite-alumina melts containing 2, 4, 6 and 8 wt% of alumina and 11 wt% of AlF3 was determined at 1000 degrees C. The addition of 11 wt% of AlF3 to a cryolite melt with a certain alumina content increases the overp otential of the anodic reaction as compared to cryolite-alumina saturated ( CRAS) melt. The Tafel slope increases from around 0.12 V/dec. to 0.24 V/dec ., which indicates that at low alumina content the second charge transfer s tep is the rds, whereas at higher alumina content this control is shifted t owards the first step. The effective rate constants of both charge transfer steps are increasing with the alumina content. It can be concluded that th e influence of the added AlF3 is directly related to the participation of t his reagent in the first charge transfer step, in which it is a product. Th e diffusion coefficient of the Al2OF62- anionic species decreases as a func tion of the alumina content, due to the increase of the viscosity of the me lts studied.