Sedimentation, or thickening, involves increasing the solids content of a slurry or suspension by gravity settling in order to effect separation (or partial separation) of the solids and the fluid. It differs from the gravity settling process that was previously considered in that the solids fraction is relatively high in these systems, so particle settling rates are strongly influenced by the presence of the surrounding particles. This is referred to as hindered settling. Fine particles (10 mm or less) tend to behave differently than larger or coarse particles (100 mm or more), because fine particles may exhibit a high degree of flocculation due to the importance of surface forces and high surface area. Figure 12-1 shows a rough illustration of the effect of solids concentration and particle/fluid density ratio on the free and hindered settling regimes.

A. Hindered Settling

A mixture of particles of different sizes can settle in different ways, according to Coulson et al. (1991), as illustrated in Fig. 14-3. Case (a) corresponds to a suspension with a range of particle sizes less than about 6:1. In this case, all the particles settle at about the same velocity in the "constant composition zone'' (B), leaving a layer of clear liquid above. As the sediment (D) builds up, however, the liquid that is "squeezed out'' of this layer serves to further retard the particles just above it, resulting in a zone of variable composition (C). Case (b) in Fig. 14-3 is less common and corresponds to a broad particle size range, in which the larger particles settle at a rate significantly greater than that of the smaller ones, and consequently there is no constant composition zone.

The settling characteristics of hindered settling systems differ significantly from those of freely settling particles in several ways:


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