Experimental and Numerical Analysis of a Sedimentation Forming Compressible Compacts

AbstractBatch sedimentations of the mineral talc suspended in water at various initial concentrations resulted in compacts that displayed compression, and compression with channel formation. During the experiments the local concentration was deduced by means of local electrical resistance measurement. The technique provided concentrations that integrated throughout the vessel to give masses that matched the known initial mass employed to within ±5%. Two types of channel zones were observed: soft and hard, the former appeared to be due to the liquid inertia of water discharging from the latter. The region within and above the soft channel zone diluted from the initial concentration, and this caused the visible interface between the suspension and the supernatant to accelerate. The top of the hard channel zone followed the line of constant solids concentration representing the first significant increase in concentration over the initial suspension. A finite difference numerical model of sedimentation matched the experimental data, including the data determined below the visible interface, with very high precision for the talc suspensions exhibiting compression with insignificant channeling. The implicit model was implemented on a conventional computer spreadsheet package and rapidly converged. The model did not employ a function for hydraulic permeability, instead a linear function between the so-called Kozeny “constant” (or coefficient) and concentration was used. In order to provide an accurate numerical model for compressible sedimentation with significant channel formation, the hydraulic permeability needs to be augmented, or the Kozeny coefficient reduced, and the dilution above the channel zone must be predicted. These should be achieved in a way that is general to all sedimentations of a given type of material, and not specific to only one starting concentration. Experimental and numerical results also indicate that the buoyancy force experienced by the solids is adequately described by the density difference between the solids and the suspending liquid, and not the density difference between the solids and the suspension.