CHRONOPOTENTIOMETRIC MEASUREMENTS ON GRAPHITE ANODES IN CRYOLITE-ALUMINA MELTS

At cds above a critical cd, oxygen discharge on carbon in cryolite-alumina melts is limited by diffusion-controlled depletion of dissolved alumina, followed by the anode effect. In melts with <1 wt-% Al2O3, the term iτ 1 2/C is constant for transition times τ from about 8 to 0·1 ms. In melts 2 wt-% Al2O3iτ 1 2/C decreases linearly with increasing current density for transition times from about 8 to 1 ms and attains a constant value at shorter times. This behaviour appears to be caused by a preceding slow dissociation of dissolved alumina. The equilibrium constant for this reaction is found to be 0·60 and the forward and backward rate constants 1400 and 2400 s-1 respectively. The diffusion coefficient of dissolved alumina is 1·5 10-5cm2/s at 1020°C. The theoretical potential/time relationship for irreversible reactions is obeyed at cds below about 15 A/cm2; at higher cds, the potential increases more rapidly than expected. © 1969.