The effect of alumina properties and smelter operating conditions on the dissolution behaviour of alumina (1990)

Alumina performs several functions in a modern 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 thesis 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. The method developed involved the merging of three different techniques: ● electroanalytical measurement of dissolved oxide concentrations ● recording of the associated thermal phenomena ● visual observation of the interaction of alumina with the bath which were then used to investigate the effect of a range of alumina properties and smelter operating conditions on dissolution behaviour. A series of dissolution parameters was selected to evaluate the relative dissolution behaviour of the different variables. 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. If the flow properties of the alumina were good, alumina flowed easily out of the dropper on to the surface of the bath forming relatively thin but well-distributed rafts. If not, alumina could flow out into a localized area producing very dense rafts which clumped together and eventually sank to the bottom forming sludge. properties such as loss on ignition enhanced the dispersion of the alumina through the release of volatiles, which caused the alumina to "effervesce" on the surface. Conversely, bulk density aggravated the clumping problem as the density of the formed rafts increased with increasing bulk density. Good feeder design can help to counteract deficiencies in the flow properties of an alumina, if the importance of wide coverage and the imparting of a horizontal velocity component to the alumina are kept in mind. It was also found that if the bath agitation was increased, either by increasing the amount of existing stirring or by reducing the bath volume in the feeding area for a given bath velocity, the initial heat transfer to the alumina could be dramatically improved. The presence of bath superheat was important for maintaining optimum heat transfer conditions for fast dissolutions, as alumina was found to dissolve better when a higher proportion of the heat required was supplied from the bulk of the liquid bath as opposed to localized freezing. Increasing the initial alumina concentration in the electrolyte retarded the dissolution process, with the retardation becoming increasingly more significant at higher concentration values. Similarly, reducing the cryolite ratio, which also decreased the alumina solubility, resulted in the dissolution being inhibited in the later stages, as more alumina dissolved. In situations where the mass transfer in the bath was improved, the impact of these concentration effects was minimized. The video recordings were useful for indicating potential operating difficulties with the samples, such as excessive emissions and flow problems. Six different raft formations, characterizing the degree of bath surface coverage and the raft density or cohesion, were identified from the videos and the aluminas were classified accordingly. Preliminary heat balance calculations were performed using the data obtained in this study and were used to estimate the heat of dissolution for α-Al2O3, for a variety of alumina concentrations in cryolite-based electrolytes. The results indicated heat values of 112±15 and 55±5 kJ mol-1 for alumina concentrations in the bath of 0.43 and 2.83 wt% respectively. Further work is necessary to refine these calculations, however. Reacted ore was found to dissolve more slowly than parent virgin alumina but it was difficult to ascertain which property caused the difference - whether bulk density, flowability, influence of HF, or presence of other impurities. As a result, it is recommended that a series of dissolution runs be performed on reacted samples of different compositions to establish which property associated with reacted ore causes the problem.