EFFECT OF ALUMINA PROPERTIES AND OPERATION OF SMELTING CELLS ON THE DISSOLUTION BEHAVIOUR OF ALUMINA
Alumina performs several functions in a modem smelter - it is used to scrub the pot gases to remove fluoride, acts as a thermal insulator on top of the cell and, when added to cryolite-based electrolytes, it is the raw material used to produce aluminium. Alumina is also expected to have good flow and handling properties, and dissolve well in the bath. Unfortunately, it does not always dissolve rapidly and this leads to the formation of "sludge", which creates operating disturbances in the cell; it is difficult to remedy this problem without a basic understanding of the process of alumina dissolution. Consequently, the objective of the work presented in this paper was to develop an apparatus and technique that would allow the dissolution behaviour of powder alumina to be measured as realistically and objectively as possible, and then determine the important factors affecting dissolution.
A series of alumina samples with significantly different alumina properties - such as surface area, phase composition, L.O.I. - were specifically prepared for this work by Alcoa of Australia Ltd. It was found that the properties of alumina had less of an impact than the operating conditions generally. Poor dissolution results more readily from changes in the electrolyte agitation, superheat, dispersion characteristics, and feeder design than from changes in alumina quality. Of other variables studied, the most significant are flowability, bulk density, L.O.I. (300- 1000˚C), and alumina concentration in the bath.
It was found that slow dissolution behaviour resulted primarily from poor feeding and/or dispersion, coupled with poor heat transfer in the first few seconds of the dissolution process.