Sammendrag
During feeding of alumina into a Hall-Héroult cell, rafts floating on the bath surface may be formed. In this study, rafts were created in a laboratory furnace by adding 4 g secondary alumina in industrial bath. Samples were withdrawn from the bath in a time interval between 30 and 300 s. The experiments show that at 970 ∘ C, rafts will be formed within 30 s, and then slowly dissolve again with a constant rate of 0.8 g/min. Pores were found in the samples, giving extra buoyancy to the raft, thus increasing the floating time. Same experimental setup was used to investigate the effect of preheating of alumina, where it was found that coherent rafts will form up to at least 500 ∘C .