Spreading of Alumina and Raft Formation on the Surface of Cryolitic Bath

It is a well-known phenomenon in aluminum industry that alumina powder, fed into the electrolyte, tends to stay afloat on the surface of the bath. This hinders dispersion and direct contact of most of the powder with the electrolyte, therefore delays dissolution. In addition, large clusters of alumina particles sintered together during raft formation might lead to the agglomerate piercing through the bath-aluminum interface, which should be avoided. Since the interference between raft formation and alumina dissolution is significant, it deserves more attention. Several experiments were conducted in a small carbon crucible in which cryolitic bath was melted and alumina of different properties was fed. The injections were recorded by a video camera above the bath. The spreading of the powder on the surface, the infiltration of powder by the bath as well as the disintegration or the sinking of the raft was observed and the results analyzed.