Quantification and mitigation of segregation in the handling of alumina in aluminium production

Dyroy, Are (2006) Quantification and mitigation of segregation in the handling of alumina in aluminium production. PhD thesis, University of Greenwich.

Abstract

This thesis addresses the development, evaluation and modelling of an anti-segmentation system (“AS-System”) for use in large silos handling alumina in the aluminium smelting industry. This work is unlike much of what has gone before because it is much more grounded in the technological and economic consequences of segregation for a particular manufacturing process. Segregation of particulates, i.e. separation of components due to differences in properties such as size, density etc., has an extensive literature going back as far as 1915, but this focuses mainly on the segregation process itself and largely ignores the context and the consequences. The consequences of segregation is loss of homogeneity; the impact that has on any given process, is generally not addressed very deeply. Surveying the literature on segregation, and studying the total processes of aluminium production, creates a basis for understanding the importance of segregation for aluminium production, as well as the importance of powder technology in general for this industry.

A method for quantification of segregation in this production process, based on sampling, has been established. This was intended to give the fundamental information necessary for measuring the extent of the problem and the degree of improvement achieved. By placing sampling points along the logistic chain for the alumina, and by sampling these points for a long enough period, information about the influence of the various handling steps on the bulk solids can be identified. Segregation by particle size is the main type of influence, although the work has shown that attrition is another. Standardised statistical expressions have been used for analysing the bottlenecks of the logistic loops, and study of the results has led to a useful way of expressing the level of segregation, the change of segregation level in a handling step (silo filing and discharge), and improvements in this change.

Overall, the AS-System has been shown to be cost-effective in reducing segregation; results measured from the full scale installations show a homogenising factor (reduction in variation of the material being handled) of 1-1-5. As a result of these verifications and the simplicity of the model presented in this thesis, the plant engineer can confidently design a system which will function correctly and make a positive, predictable improvement in the homogeneity of the alumina in his plant.

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