On gas bubbles in industrial aluminium cells with prebaked anodes and their influence on the current distribution

The secondary current distribution in industrial aluminium cells with prebaked anodes was calculated, taking into account the gas bubbles. The input data were obtained on the basis of a physical model and data suggested in the literature. The bubbles were modelled in the following manner: (i) as very small bubbles dispersed in a homogeneous layer with higher electrical resistivity than the bulk of the electrolyte, (ii) as large bubbles modelled as discrete slabs with infinite resistivity, and (iii) as a combination of (i) and (ii). The bubble size and the number of bubbles, as well as the resistance of the homogeneous bubble layer, were varied to give an equivalent voltage drop in the range 0.1–0.4 V. Large bubbles (slabs) appeared to have a significant screening effect on the anodic current densities. The anodic current densities between slabs showed local maxima, sometimes reaching twice the value of the working current density (0.75 A cm^–2). The cathodic current densities had local minima underneath the large anodic bubbles, following their position at the anode. Underneath a bubble of 6.1 cm width, the cathodic current density decreased from 0.75 to 0.23 A cm^–2.