Alumina feeding and raft formation: Raft collection and process parameters

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 s during normal operating conditions