COMPARISON OF BUBBLE NOISE OF SODERBERG POTS AND PREBAKE POTS

In previous work [1] we analyzed the relationship between bubble noise, bath height, and anode quality with data mostly from Soderberg pots. We have now acquired additional data for both Soderberg pots and for prebake pots. From an accidental current increase from 127 to 145 kA over 2 days at a Soderberg plant, we can see both the immediate increase of bubble noise proportional to the square of the current, and the gradual decline of bubble noise due to increasing crack area in the anode. For prebakes, however, the response is different. In the short run the bubble noise is increasing when bath height goes up, because of increased adhesion when pressure goes up, but after 1-2 hours the bubble noise has fallen to a value lower than before the bath height went up. This happens because less cur-rent flows in the central regions when there is more gas there, and more current flows further away from the central region, giving a steeper slope of the anode closer to the boundary. In fact, the Iona term decline of bubble noise for prebake pots is very similar to the decline found for Soderberg pots when bath height goes up. Because of the opposite short term and long term response, the correlation between bath height and bubble noise of prebake pots can be positive for rapid height changes and negative for height changes with a bigger time spacing.