Suspensions zone settling

In flocculated or concentrated suspensions, zone settling occurs (Fig. 7.28). In the region... 

Equations 20 and 21 apply only to free, particulate separation not affected by flocculation or aggregation and for all particles having the same density. Under gravity field, few shear forces exist and have little effect on floes. Flocculation may be strong to change behavior of settling suspension to that of zone settling (i.e., all particles in slurry settle en masse at the same velocity irrespective of their size). 

In gravitational sedimentation the panicle concentration in the measurement zone remains equal to the initial concentration until the largest particle present in the suspension has settled through the zone. The concentration of particles in the measurement zone at time t represents panicles whose size is less than or equal to the Stokes diameter d< u>kes- rhe Stokes diameter is inversely proportional to the square root of time as shown by Equation 34-13. Therefore, a plot of concentration versus f/5j,okes represents the undersize distribution. 

The design and scale-up of conventional gravity thickeners is usually based upon batch settling tests on suspensions in the zone settling regime as... 

Figure 3.4 T)rpe 1 batch settling. Zones A, B and S are zones of constant concentration. Zone A is a clear liquid zone B is a suspension of concentration equal to the initial suspension concentration zone S is a suspension of settled bed or sediment concentration...

A variation to the top-feed dmm filter is the dual dmm filter which uses two dmms of the same size in contact with each other and rotating in opposite directions. The feed enters into the V-shaped space formed on top of the two dmms and the cake that starts forming initially contains coarser particles due to the settling which takes place in the feed zone. This is beneficial to the clarity of the filtrate because the coarser particles act as a precoat. Erom the point of view, however, of the final moisture content of the cake the stratification of the soHds in the cake may lead to somewhat wetter cakes. Utilization of the area of the dmms is poor since there are dead spaces under the two dmms. The primary appHcation of the dual dmm filters is in dewatering coarse mineral or coal suspensions at feed concentrations greater than 200 kg/m. ... 

In the Jar Test , an initially uniform, homogeneous suspension of partieles slurries settles out under the foree of gravity into distinet zones, as illustrated in Figure 2.6. 

The DTB, crystallizer has a relatively slow-speed propeller agitator located within a draft-tube which draws a fine-crystal suspension up to a boiling zone of wide cross-sectional area, as shown in Figure 3.3(i). The fine-crystal magma then passes through an annular zone in which an additional baffle is located. Liquor flow continues upwards at low velocity while crystals settle out and fall to the base of the vessel. Liquor from the external pumped loop provides an up-... 

Dunes and sand plains form where strong winds carry sand grains in saltation over short distances. Particles finer than sand are transported in suspension and over greater distances until they settle as loess , predominantly in the steppe regions adjacent to the desert zone. 

The top surface of the suspension should always be sufficiently far below the level of the overflow weir to provide a clear zone deep enough to allow any entrained particles to settle out. As they will be present only in very low concentrations, they will settle approximately at their terminal falling velocities. Provided that this requirement is met, the depth of the thickener does not have any appreciable effect on its clarifying capacity. 

Several workers(68,69,70) have studied the sedimentation of suspensions formed of particles of two different sizes, but of the same densities. The large particles have higher settling velocities than the small ones and therefore four zones form, in order, from the top downwards ... 

Equation 5.113 gives the rate of fall of the interface between zones (b) and (c) that is the apparent rate of settling of the zone of mixed particles. The velocity of fall // of the interface between zones (a) and (b) is the sedimentation rate of the suspension composed only of fine particles, and will therefore depend on the free-falling velocity u<)S and the concentration Cf of this zone. ... 

On the above basis, the differences between the velocities of the two types of particles should increase with concentration and, consequently, the concentration of the two separated zones should become increasingly different. This is found to be so at volumetric concentrations up to 20 per cent. At higher concentrations, the effect is not observed, probably because the normal corrections for settling velocities cannot be applied when particles are moving in opposite directions in suspensions of high concentration. 

---