Abstract
The paper addresses the basic mechanisms leading to the low-voltage anode effect (LVAE) as well as the high-voltage anode effect (HVAE) in aluminium electrolysis cells. The root cause of any anode effect is too low alumina concentration at the anode. By statistical treatment based on a Gaussian distribution of the alumina concentration in the bath, it was found that the onset of a HVAE takes place at higher average alumina concentration if there is a large standard deviation. By calculating the anode potential using a pragmatic numerical model, it was found that a LVAE can take place at a part of the anode as small as 0.001 m2, while the rest of the anode is at the normal anode reaction, which makes it extremely difficult to discover. It was suggested that the on-and-off current pattern observed during LVAE can be explained by alternation between blocking of the surface (AE) and normal electrolysis, induced by non-stationary mass transfer of alumina.