The Formation and Disintegration of Rafts from Different Aluminas and Fines

Raft formation is a frequently encountered problem during alumina feeding in the Hall-Héroult-process and will delay alumina from being dissolved into the melt. The mechanisms for the formation and disintegration of rafts are however not thoroughly understood yet. The current study investigates the dissolution behavior and raft structure from three different types of secondary alumina in a lab cell, with a particular attention to effect of fines, and involves both sampling of rafts and video recordings of the feeding. The mass loss rate was calculated to vary between −1.57 and −0.42 g min−1 for regular bulk alumina, and −1.15 and −0.06 g min−1 for fines. Rafts created from bulk alumina were flat with a distinct bulge or crater placed in the center of it, while rafts created from fines had a pellet-shaped structure and traces of undissolved alumina in the middle. The observed structure is due to the initial spreading of powder, confirmed by video recordings.