Wall effects during settling in cylinders

The phenomena of syneresis and shear densification were analysed through the sedimentation of alumina suspensions. Experiments compared cylinder settling tests with a rod inserted in a cylinder against free settling without a rod for the cases of polymer flocculated and salt coagulated suspensions. Additionally, dye intrusion was used to visualise the final bed structure to identify the influence of cylinder walls and the potential presence of aggregate arching structures. found that, for various cylinder diameters, polymer flocculated alumina suspensions settled more quickly when a rod was placed in the centre of the settling cylinder. This is attributed to shear induced densification of the flocculated aggregates during the settling process, as it was observed that the solids shrank away from the rod, creating a channel which allowed water to escape more easily than when the rod was absent. The reverse result was observed when a rod was placed in an unflocculated/coagulated alumina suspension; the solids stuck to the rod, resembling the syneresis mechanism of isotropic shrinkage towards the rod. Industrially these results are significant since inserting rods into sedimentation devices such as a thickener can result in a flocculated suspension settling further and at a faster rate, such that rods can provide an alternative to mechanical devices such as rakes. trusion tests indicated that there was arching in the network bed, resulting in small pockets of water being trapped in the structure. An equation was derived to predict the maximum diameter arch that could be supported, taking into account the shear yield stress of the suspension. The solution to this equation was supported by observed cavities at the cylinder wall of up to 3 mm. These results suggest that cylinder walls may have a negative effect on the extent to which a suspension dewaters.