It is easy to understand the importance of some of the alumina specifications, such as soda and calcium levels, because of their impact on the materials balance and/or purity of the metal product. However, confusion often arises in discussions on several other properties specified such as the BET surface area, moisture content and particle size distribution. This is because the alumina is used for several purposes within the cell and the different uses place differing emphasis on some of the properties. The uses that must be considered include cover of the anodes to prevent air burn and minimise heat loss, formation of a workable crust on the undersurface of the cover, as an absorbent for removing fluoride emissions and as a volumetrically added feed material that should readily disperse and dissolve in the electrolyte.
Because the emphasis on some of these applications is linked to technology, in this paper we discuss each of the properties in the specification with respect to their relevance for various applications in the cell.