Surface studies and dissolution studies of fluorinated alumina

Abstract

A method is described of modified linear sweep voltammetry that gives an accurate and reproducible measure of the dissolution rate of alumina in molten cryolite. The hydrogen fluoride adsorption processes occurring at the surface of alumina were investigated. An Al-F interaction was observed in only one of the hydrogen fluoride adsorbed sample types, that is, the case of the alumina that had dry HF adsorbed after itself being predried. When moisture was already present on the alumina, or water was present in the HF gas stream, no AlF3 formation was observed. Mass spectroscopy of vapour released on heating demonstrated that fluoride was volatilised as Al-F compounds. Stirring speed was found to have a large effect on the dissolution rate of alumina, with slow stirring speeds giving longer dissolution times. For larger superheats (15°C) the rate of dissolution is greater than for smaller (5°C) superheats. Longer dissolution times were observed with higher electrolyte alumina concentration. Dry scrubber alumina gives shorter dissolution times in cryolite than untreated alumina. The dissolution time can be correlated with several properties of the dry scrubber alumina including moisture on ignition, loss on ignition, flow funnel time and percentage fluoride. The dissolution times of dry scrubber alumina were shorter than would be expected when considering the combined effect on the dissolution time of the individual changes which occur with this treatment. When alumina is stored under conditions of high humidity, to increase the adsorbed water content, a significant decrease in the dissolution time occurs. The dissolution time for the laboratory fluorinated aluminas showed only a small decrease from the times for standard alumina. This decrease in the dissolution times can be attributed to the increased moisture content of the alumina which is a consequence of the fluorination procedure. The fluoride itself does not appear to enhance the dissolution rate even though the process of fluorination does. The dissolution times for the alumina which was fluorinated with bath volatiles (NaAlF4) were similar to those obtained for standard alumina. Models for the dissolution of alumina based on heat transfer control and on diffusion control were developed. Both models adequately represented the observed behaviour but it was not possible to state unequivocally which of the models best represents the actual rate controlling mechanism of alumina dissolution.