Alumina, alongside with electricity and carbon, is the raw material used for production of aluminium in the Hall-Héroult process. An efficient dissolution process is important to acquire stable conditions for the cell, resulting in lower energy consumption. Under certain conditions, the alumina will not dissolve upon addition but remains afloat on the bath surface as a so called raft. The conditions under which the rafts form are still not fully understood, although it is likely that their behaviour is influenced by operational conditions which in turn depend upon bath and alumina properties. In order to obtain more knowledge on the conditions for raft formation, an industrial measurement campaign was performed at Alcoa Mosjøen in which raft behaviour was recorded alongside collection of bath and alumina samples as well as the rafts themselves. The current paper describes the procedure utilized for data collection together with an analysis of bath and alumina properties, aiming to correlate these with raft flotation times. Raft floating times were found to vary between 5 and 140 seconds during normal operating conditions.

As alumina particles are fed to aluminium reduction cells, a frozen layer of bath is typically formed on the particle surface, due to the relatively low bath superheat. For particles in close proximity, platelets with frozen bath can be formed, resulting in agglomerates (rafts) containing solidified bath and alumina. The formation, flotation and break up of these agglomerates is determined by macroscopic properties (i.e. size, density etc.) which in turn is related to microscopic properties, i.e. how grains are interconnected. The formation of rafts delays the dissolution of alumina and thus adversely influences the conditions in the pot. In order to obtain more knowledge on the conditions for raft formation, an industrial measurement campaign was performed at Alcoa Mosjøen in which raft where collected under different operating conditions. Rafts have been characterized by micro computed