Thickeners zones

As particles settle, forming a thickened zone, the void fraction, e, decreases. At the total settling of the slurry, the void fraction is at a minimum, its value depending on the shape of the particles. For example, the minimum void fraction of spherical particles is e, in = 0.2 15 for small crystals, = 0.4. For most systems, the void fraction of a thickened sludge is approximately = 0.6 however, values should be determined experimentally for the specific system. 

This expression is based on our earlier assumption that the time required to thicken the sludge is independent of the interface height of the compression zone. An approximate solution to this expression can be obtained if we assume mj/m to be constant, i.e., an average mass ratio in the thickening zone from top to bottom. Then,... 

Depth of thickening zone. Feed slurry entering the unit at eoneentration F has to reside in the vessel for time /r to reaeh eoneentration S. The sediment therefore has to remain in the thiekening zone for at least a period /r defined by tR = Volume of thiekening zone/Volumetrie flowrate of slurry. 

An approximate value for the depth of the thickening zone is then found by adding the volume of the liquid in the sediment to the corresponding volume of solid, and dividing by the area which has already been calculated, in order to determine the clarifying capacity of the thickener. The required depth of the thickening region is thus ... 

The use of slowly rotating rakes is beneficial in large thickeners to direct the underflow to the central outlet at the bottom of the tank. At the same time the slow motion of the rakes tends to give a gentle agitation to the sediment which facilitates its consolidation and water removal. The height requirement of the rakes must be added to that needed for the thickening zone to achieve the desired underflow concentration. 

Compression zone—In a settling or thickening process, this is the zone B where the thickened sludge from the thickening zone is further compressed, compacted, and consolidated. 

Z] Concentration of solids entering clarifier or thickener [ZJ Solids concentration at thickening zone... 

Section 7.5.3.1B. It is necessary to size a thickening zone so that the upward velocity of the brine does not exceed the settling velocity (relative to brine) at any section in the lower zones. 

A stable boundary between the settling zone and the thickening zone is important for normal continuous operation that is, the rate of introduction of solid to any section in the thickening zone must be equal to the rate of discharge of solid in the underflow (assuming essentially no solids in the overflow). In this region, the velocity of sedimentation depends... 

The Coe/Clevenger method uses experiments run on suspensions of different consistencies ranging from feed concentration to desired underflow concentration. Each solids concentration tested allows calculation of a cross-sectional area by the method described above. This calculated area will exhibit a maximum at some particular concentration, and this is the critical concentration that fixes the cross-sectional area required in the thickening zone. 

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