ALUMINAS AND CELL FEEDING TECHNOLOGIES

Welch, B.J and Grjotheim, K.

In the quarter century since the introduction of automated feeding systems, there has been a considerable range of devices developed, with the present installed smelting capacity utilising the full range. Many of the older devices relied on adding a large mass of alumina at one time through breaking a considerable amount of the cell's crust. In these instances approximately one third of the material entering the electrolyte has been pre-heated on the crust, causing the aluminas to undergo structural changes. This contrasts with modern point feeders where the alumina is usually in the state as delivered from the dry scrubbing systems. Hitherto all feeders have been based on a volumetric delivery system and are reliant on uniform flow and bulk properties. The greatest problem has been sealing the delivery systems.

The basic differences between a range of feeders developed will be discussed with respect to how they relate to important properties for repeatable addition at the required rate and the subsequent alumina dissolution. Their tendency to form sludge and promote alumina structural changes (especially between the dry scrubber and entering the electrolyte) will be discussed. From such a review, it becomes apparent that, while smelters place considerable emphasis on the alumina quality, the feeding systems they have coupled with their operating procedures can play an even more important role, thus masking effects.