Aluminium dissolution in NaF-AlF₃-Al₂O₃ systems

Abstract

The rate of dissolution of electrolytically deposited aluminium was determined by the method of current reversal chronopotentiometry at a tungsten electrode in NaF−AlF₃−Al₂O₃ melts of varying NaF/AlF₃ molar ratios or cryolite ratios (CR). The temperature was maintained at 1031±3°C and the alumina content at 4 wt%. More accurate data were obtained by introducing delay times of various lengths (at zero current) between the cathodic and anodic current pulses, compared to direct current reversal chronopotentiometry with varying forward (deposition) times. The rate of aluminium dissolution increased with increasing NaF/AlF₃ molar ratio, the curve showing an inflexion in the vicinity of CR=3. This inflexion indicates two dissolution mechanisms, one being predominant depending on the CR. The main reaction in acidic melts (CR<3) may be represented by

$$2Al(l) + AlF_6^{3 - } \rightleftarrows 3Al(I)F_x^{1 - x} + (6 - 3x)F^ - $$

while in basic melts (CR>3)

$$Al(l) + 3Na^ + \rightleftarrows 3Na(soln) + Al(III)$$

is the likely dominant mechanism. For 0.8<CR<5.7 the rate of dissolution is of the order of 10⁻⁷ mol cm⁻²s⁻¹.