Sedimentation zone settling

Thickener-Basin Depth The pulp depth required in the thickener will be greatly affected by the role that compression plays in determining the rate of sedimentation. If the zone-settling conditions define the area needed, then depth of pulp will be unimportant and can be largely ignored, as the normal depth found in the thickener will be sufficient. On the other hand, with the compression zone controlling, depth of pulp will be significant, and it is essential to measure the sedimentation rate under these conditions. 

In sedimentation zones the flow conditions are laminar. A place is available for the settling of sludges contained in the water to be filtered. 

Figure 2.6 Sedimentation zones during slurry settling (The Jar Test ). A, initial uniform concentration, B, zone of increasing concentration, C, sediment or sludge, D, clear liquor...

FIG. 15-63 Batch settling profile showing four regions a top clarified phase, a sedimentation zone, a dense-packed dispersion zone, and a bottom clarified phase. [Reprinted from Jeelani, Fanoussopoulos, and Hartland, Ind. Eng. Chem. Res., 38(2f pp. 493-501 (1999), with permission. Copyright 1999 American Chemical Society. ] Consult the original article for a detailed description. 

Usnally, there are fonr zones in a thickener the clarification zone, the feed zone, the transition zone, and the compression zone. Not all of these zones will be present for all types of slurries. Liquid flows out of the feed zone, up through the clarification zone, and toward the overflow. Below the feed is the transition zone, where the particles start a downward motion. The concentration just below the supernatant-suspension (or sediment) interface assumes a value dictated by the dominance of either free settling or the presence of a compacting sediment. The region containing the particles offers resistance to liqnid flow and accepts only a portion of the liquid. The remainder reverses direction and exits as overflow. Under steady-state conditions, both solids and liquid fluxes are constant and independent of depth in the snspension and/or sediment zone. No liquid is squeezed out and upward in the sediment unless channels exist. 

Sedimentation characteristics Variation of interface of slurry and clear liquid against time, zone-settling region and compaction region, settling velocity... 

If the feed rate exceeds the maximum of the design, particulate matter are unable to settle out of the normal clarification zone. Hence, there is an increase in the solids concentration, resulting in hindered settling. The result Is a corresponding decrease in the sedimentation rate below that observed for the feed slurry. 

If a clear filtrate is required right from the start it is good practice to form a thin heel that serves as filter medium over the exposed cloth. This is done by either a "cloudy port outlet" that is recirculated or, if solids are settling fast, by allocating the first 20-30 cm to act as a "sedimentation pool" prior to entering the vacuum zone. 

To examine sedimentation in greater detail, let us examine the events occurring in a small-scale experiment conducted batchwise, as illustrated in Figure 9. Particles in a narrow size range will settle with about the same velocity. When this occurs, a demarcation line is observed between the supernatant clear liquid (zone A) and... 

Those particles with sizes d > d" at a given set of conditions (v, p, Pp, and a ) will settle only in the turbulent flow regime. For particles with sizes d < d, d" will settle only when the flow around the object is in the transitional regime. Recall that the transitional zone occurs in the Reynolds number range of 0.2 to 500. The sedimentation numbers corresponding to this zone are 3.6 < S, < 82,500 and 0.0022 < S2 < 1,515. 

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