Antisolvent gibbsite crystallisation from synthetic Bayer liquor

Master Thesis


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University of Cape Town

The current Bayer precipitation of gibbsite is notoriously slow and is considered to be the rate limiting step of the Bayer process. The present industrial reactor configuration involves precipitating gibbsite inside a series of large stainless steel agitated vessels due to the slow precipitation kinetics of gibbsite from pregnant Bayer liquor. The scope of this study is to investigate the effectiveness of using water as an antisolvent to increase the overall gibbsite yield and the associated particle behaviours under isothermal conditions. The experiments were performed in a series of batch experiments which were divided into two subsets. Firstly, the effect of different caustic concentrations present in the anti solvent on gibbsite yield was tested. Secondly, the effect of different seed loading (50 and 100 grams seeds per liter of synthetic liquor) on aggregation and fines formation were investigated. All batch experiments were conducted isothermally at 80°C under a closed system using a baffled stainless steel 316 agitated vessel. The experimental results indicated water to be a feasible medium for the isothermal antisolvent crystallisation of gibbsite from synthetic pregnant Bayer liquor. It was found that the extent of overall gibbsite yield exhibits a power law correlation to the change in the antisolvent caustic concentration with R2 ~1 (R2 = 0.99974). This finding is important as the regressed curve will be used to correlate gibbsite yield as a function of anti solvent purity. From the seed loading experiments, an Increase In seed loading under constant temperature and initial liquor supersaturation resulted in an overall decrease in product particle size due to a rise in inter-particulate attrition. However, crystal growth was not dominant during any period of the experiments with aggregation being more prominent for the system with the lower seed load. Gibbsite morphology determination from SEM revealed hexagonal solids which, in the presence of a higher seed loading indicated signs of fracture due to attrition. Quantitative evidence of this mechanism is shown USIng the discretised population balance method by Bramley et al. (1996) with the aid of the computational technique developed by Hounslow (2005) in Mathematica. Results generated using this method show decreasing aggregation with liquor de supersaturation and increased seed loading. The overall particle behaviours from the current investigation are validated by results obtained from previous gibbsite studies under very similar conditions. Although the gibbsite yield can be increased drastically in the presence of an anti solvent, the average particle size generated from this study is still too small to be commercially feasible due to the increase in initial supersaturation. Thus it is recommended that further particle size optimisation may be conducted inside a fluidised bed reactor where particles generally experience lower shear than inside a conventional agitated vessel.

Includes bibliographical references (leaves 45-49).