HF Formation Upon Addition of Different Industrial Aluminas to Cryolitic Baths

The aluminium smelter at Hydro Sunndal has experienced problems with an unexpected increased loss of AlF3 from the electrolyte when using a certain quality of alumina. One hypothesis considered was that the fluoride loss was a direct result of unusually high HF formation when the quality was added to the cryolitic melt. This work has set out to study the HF formation potential of three different industrial alumina qualities. Alumina A was an alumina quality that worked well in the smelter. Alumina B was the alumina quality where the aforementioned drop in AlF3 was observed and alumina C, made by the same producer, controversially did not to exhibit the same loss at smelter. The present work has been divided into two parts: a characterisation section, where the aluminas have been characterised by techniques such as LOI, TGA, DSC and XRD, and an experimental section, where the industrial aluminas were added to a cryolitic melt. The HF formation and the corresponding H2O concentration were measured in-situ using a tunable diode laser during the alumina additions to the cryolitic melt. Through this work it has been found that both primary and secondary alumina A form less HF than alumina B and C. The LOI, TGA and XRD characterisation has shown that alumina B and C contain a substantial amount of gibbsite, whilst the gibbsite level in alumina A is negligible. A clear correlation between the quantity of moisture found through LOI characterisation and HF formation has been found. It has also been shown that all types of moisture found through LOI testing contribute to HF formation; both physisorbed and chemisorbed types, as well as structural hydroxyl. From the present work it cannot be explained why alumina B caused a drop in AlF3 in the electrolyte, whilst alumina C did not. One explanation postulated is that alumina C has better scrubber efficiency than alumina B. Examining this hypothesis has been outside the scope of this master work. If variations in scrubber efficiency for the two aluminas fail to explain the drop in AlF3, the solution may be found in parameters outwit the alumina quality e.g. weather conditions at the time of production or storage conditions at the calciner