The Micro- and Macrostructure of Alumina Rafts

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 Mosjoen in which raft where collected under different operating conditions. Rafts have been characterized by micro computed X-ray tomography (μCT) to reveal the macroscopic properties i.e. porosity, while energy-dispersive X-ray spectroscopy (EDS) coupled to SEM and XRD has been adopted to identify the chemical composition throughout the raft. Results indicate considerable variations in macrostructure between different samples and also large differences within the same sample, depending upon the vertical position.