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IMPACT OF CALCINER TECHNOLOGIES ON SMELTER GRADE ALUMINA MICROSTRUCTURE AND PROPERTIES

Perander, L, Klett, C., Wijayaratne, H., Hyland, M., Stroeder, M. and Metson, J.

To examine Smelter Grade Alumina (SGA) microstructure and better understand its impact in smelter operations, it is essential to evaluate structural ordering on several different scales. This requires advanced and often multiple analytical techniques. By analysing cross sectioned alumina particles using both Environmental Scanning Electron Microscopy and Transmission Electron Microscopy, features relating to the calcination history and the transformation reactions can be observed. Microstructure is significantly influenced by calciner technology and operations, and has a range of impacts on the performance of the material in the aluminium smelter.

Laboratory and pilot-plant experiments suggest that modern calcination operations, with high heating rates and short residence times, lead to large structural variations not only between particles, but also within them. The high heating rates can cause a boehmite-like shell to form which results in a slight variation in calcination pathway for the core and the shell of the particle. The shell formation also results in high internal vapour pressures which can lead to an explosive shattering of the particles, thus contributing to the generation of fines. Alpha alumina formation is not surprisingly closely tied to growth morphology, and seems to follow the same pattern as the growth rings revealed in gibbsite cross sections. This suggests that local structure is important in the nucleation and transformation reactions and ultimately the formation and location of alpha alumina within particles.