A Comparative Examination of Fine Particle Solid Suspension in Mechanically Agitated Flat-Bottomed Tanks

Date
2024-12
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Stellenbosch University
Abstract
In this thesis we examine two methodologies for the estimation of full suspension within mechanically agitated tanks and how performance is reduced with decreased power input. Such results are seen to better inform the design choices of leach tanks where a compromise between extractive performance needs to be balanced by cost implications. Leach processes vary widely but the conditions considered in this study include finely ground solids material (P80 ≈ 78 μm), with high solids loading (> 40 % by mass) and impeller placement at and above the settled bed height. Such considerations are made to examine data fits while accounting for practical design considerations made at large scales. Neither methodology has been well established under such conditions. By experimentally determining suspension behaviour both at and below full suspension the relationship between absorbed power and suspension quality are illustrated. As well as considerations to the various system affects which influence the full suspension speed are made. Both Zwietering and Greenwich-Mak-Brown design equations were found to be applicable for the modelling of such systems though the accuracy of the results decreased significantly as process conditions were varied. Both operating point methodologies can be applied to design problems depending on the available system constraints and data. GMB provides estimates without requiring specific geometric properties while Zwietering is more accurate but requires such specifications to be known. Both methodologies will provide improved accuracy if the utilised constants are calculated using similar slurries to the design problems, care must be taken when extrapolation constants. Through the examination of impeller flow patterns in conjunction with settled slurry tests the fillet cross sectional profile was determined. Using the fillet profile a new methodology is proposed for the estimation of settled fillet volumes. By demonstrating the relationship between absorbed power and fillet volume potential reductions in the installed power might be economically justified on a case-by-case basis.
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