Suspensions settling of particles

There are a large number of processes in the chemical industries that handle a variety of suspensions of solid particles in liquids. The application of filtration techniques for the separation of these heterogeneous systems is sometimes very costly. If, however, the discrete phase of the suspension largely contains settleable particles, the separation can be effected by the operation of sedimentation. The process of sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening,... 

The flow problems considered in previous chapters are concerned with homogeneous fluids, either single phases or suspensions of fine particles whose settling velocities are sufficiently low for the solids to be completely suspended in the fluid. Consideration is now given to the far more complex problem of the flow of multiphase systems in which the composition of the mixture may vary over the cross-section of the pipe or channel furthermore, the components may be moving at different velocities to give rise to the phenomenon of slip between the phases. 

Fine suspensions are reasonably homogeneous and segregation of solid and liquid phases does not occur to any significant extent during flow. The settling velocities of the particles are low in comparison with the liquid velocity and the turbulent eddies within the fluid are responsible for the suspension of the particles. In practice, turbulent flow will always be used, except when the liquid has a very high viscosity or exhibits non-Newtonian characteristics. The particles may be individually dispersed in the liquid or they may be present as floes. 

Khan, A. R. and Richardson, J. F. Chem. Eng. Comm. 78 (1989) 111. Fluid-particle interactions and flow characteristics of fluidized beds and settling suspensions of spherical particles. 

Although some colloids settle out into two separate phases if left standing long enough, others persist indefinitely a suspension of gold particles prepared by... 

Figure 1 shows the volume contraction observed in the settling of a 35% suspension of quartz particles. Also shown is the sediment height of a column of such a suspension of quartz particles. As the sediment volume approaches a constant value (the final compaction volume), the volume contraction ceases as expected. Similar results are obtained with uniformly sized polystyrene spheres. The volume contraction effect is reversible when the polystyrene spheres were resuspended, the total volume increased to the original value as the vicinal hydra-... 

Suspension characterization begins with characterization of the constituent particles and ions in solution. Bulk suspension behavior, settling, particle aggregation, and rheological properties are all dependent on the chemical properties of the particles and the nature of the suspension fluid. The wide array of analytical methods for materials char-... 

If there is a narrow distribution of particle sizes then sedimentation is experimentally very simple. A dilute suspension of the particles is shaken in a tall graduated cylinder. After a few seconds the suspension becomes stagnant and the particles start to settle at a constant (terminal) velocity. A clear layer of liquid forms at the top of the cylinder and grows as the particles continue to settle. The velocity of the downward movement of the interface between the clear liquid and suspension is v, which can readily be obtained using a stopwatch and the cylinder graduations. 

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