Thickener Basin Area

Three key parameters are required for thickener design the thickener basin area (or unit area), the thickener basin depth, and the torque for the rake. These are discussed here. 

Thickener Basin Area (Unit Area). The thickener basin area is determined from the solid flux rates at the critical concentration in the hindered settling regime. By definition, the critical point can be experimentally determined from the solids concentration just prior to the beginning of the compression zone. For a nonflocculated system, the basin area, expressed as the unit area (m /ton/d), Aq, can be calculated using the equation (Osborne, 1990)... 

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. 

The diameter of the circular basin would be 113 ft. As in most thickeners the area is determined by the requirement to thicknn the sludge and not by the requirement to clarify the overflow. Note diet the thickener can be sized from one settling test. In practice, ons should conduct a settling test in at least two different cylinder heights. 

Calculate the minimum area and diameter of a thickener with a circular basin to treat 0.1 m3/s of a slurry of a solids concentration of 150 kg/m3. The results of batch settling tests are ... 

Next, the area required for thickening in die bottom of the basin is dssermined by the method explained by Fitch, where W0 is the initial height of the slurry in the graduated lest cylinder ... 

Slurries exhibiting Type II behavior are sufficiently thick and flocculent that the solids tend to settle as a mass, giving a rather sharp line of demarcation between the clear liquid overflow and the settling solids. For such systems the design is normally controlled by the thickening capability of the basin, although the basin design must be adequate to provide sufficient overflow area to clarify the liquid overflow. 

The size of the settlement basin is sometimes reduced by employing a series of internally mounted plates inclined at about 60° to the horizontal. In a similar manner to the lamella separator (see Section 1.1.2.3), the effective settling area is increased to allow sedimenting solids to slide down the plates and into the collecting well. Other variants include the vertical flow clarifier, the blanket clarifier and the circular clarifier the latter is similar in form to a conventional circular thickener but of a much lighter construction. 

Calculate the minimum area and the diameter of a thickener with a circular basin to treat 0.2 m /s of a slurry which contains 20 pm particles of silica (density 2600 kgW) suspended in water (density 1000kg/m and viscosity 0.001 Ns/m ) at a concentration of 650kg/m. Assume that the slurry cannot be tested and do your calculations on the basis of the Richardson and Zaki equation (equations 18.12 and 18.15 combined), i.e. u u = (notation as in chapter 18). Take the underflow concentration as 1560kg/m and also calculate the underflow volumetric flow rate assuming total separation of all solids. Ans. 1326.5 m, 41.1 m, 0.083 m /s. 

Clarifier design has traditionally been based on the principle of dilute sedimentation. The design calculations of a clarifier include feed well design, sedimentation basin design (providing enough residence time for the separation), and solid flux (G). The other design parameters, such as the torque requirement and clarifier area, are similar to those of the thickeners, which have been discussed in the previous section (3.2.1). 

Calculate the minimum area and diameter of a thickener with a circular basin to treat... 

---