Hindered settling rate, effect

This diameter should have no effect on slurry hindered-settling rate certainly up to 400 g Th/kg H2O concentration and possibly even higher. 

Fig. 4-12. Effect of thorium sulfate on hindered settling rates of oxide slurries.

Fig. 4 20. Effect of angle of container inclination on TI1O2 slurry hindered settling rate. Slurry concentration 300 g Th/kg H2O, container diameter 1.63 in.

As the solids concentration in the suspension increases, interparticle distances become smaller and the particles start to interfere with each other. If the particles are not uniformly distributed, the effect may be a net increase in settling velocity, because the return flow due to volume displaced will predominate in particle-sparse regions (i.e., cluster formation). The cluster formation effect is significant only in suspensions that are nearly monodispersed. In practice, most suspensions are poly-dispersed and the clusters in such suspensions are short-lived. As a result, the settling rate steadily deteriorates with the increase in solids concentration because of the return flow being more uniform this is known as hindered settling. 

The relation between concentration of the discharged centrifuge solids and feed flow rate, via residence time within the machine, can be illustrated by considering the effect of hindered settling on the suspension in a bowl-type centrifuge. 

Rietema s theory does not take into account the radial fluid flow, it neglects any effects of inertia, it takes no account of hindered settling at higher concentrations and it assumes any influence of turbulence to be negligible. A more recent version of the residence-time theory, the so-called bulk model due to HoUand-Batt °, does take into account the radial fluid flow. He simply used the hold-up time of the liquid in the cyclone (flow rate per cyclone volume) as the residence time, average radial fluid velocity (flow rate per wall area of the cyclone) and a general continuity equation for two-dimensional flow to derive an expression for the cut size. 

Design variables are not the only ones to affect the cyclone number. While the effects of pressure, flow rate, viscosity and the densities of the fluid and particles are included, the strong influence of the feed concentration and of underflow orifice control are not. Most of the theories only apply to low solids concentrations, some (Holland-Batt, Schubert and Neesse) offer a correction for hindered settling and others consider to some extent the effect of the underflow-to-throughput ratio (Bradley) or the limited capacity of the underflow orifice (Schubert and Neesse). Once again, the effect of solids concentration for the purpose of scale-up is best described by dimensionless correlations derived from pilot tests and this is shown in some detail later. 

---