EXAMINATION OF DROP IN BATH ACIDITY DUE TO CHANGE-OVER OF ALUMINA QUALITIES IN THE SUNNDAL ALUMINIUM SMELTER, NORWAY

It is well known that alumina qualities with seemingly similar analysis results on the shipment Certificate of Analyses (CoA) may result in different smelter responses. The reasons are often complex and difficult to determine. During a few cases of change-over from alumina quality A to alumina quality B, significant drops in bath acidity have been observed at the Sunndal aluminium smelter. This could not be explained satisfactorily by the CoA or by changes in operating routines.

It was investigated if the reason could be from losses from the bath through additional HF formation and/or reduced HF capture in the dry scrubbers caused by alumina quality B. Primary and secondary alumina samples of both quality A and B were characterised. Potroom operating data and historical data from our alumina shipment and analyses database were also studied.

The results indicate that the drop in bath acidity could be related to certain alumina properties that are not given in the CoA. This includes i) total water content including gibbsite; ii) particle size distribution and the amount of total water in the different particle size fractions; and iii) pore size distribution.

In order to better predict smelter responses, and hence prevent similar operational disturbances in the future, implementation of additional alumina characterisation methods have been initiated.