Circular basin thickeners

Typical uses Larger scale thickening and deliquoring of solids from relatively dilute suspension. 

Typical particle size and feed concentration range 0.1-500 pm and 20% w/w. 

Tanks with a diameter smaller than 25 m are usually formed from steel and have flat bottoms with rake arms at an angle less than 10°. Larger tanks between 25 and 200 m diameter are normally made from a combination of concrete and steel and employ rakes designed to match the angle of the conical bottom. Circular thickeners are frequently constructed to large scales and can be used to raise suspension concentration prior to another solid/liq-uid separation process. 

Particularly large volumes of slowly settling slurries (which are also of low value) may be thickened in lagoons if land is not at a premium. Lagoons usually need to be lined to prevent seepage and are rarely an environmentally 

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Circular basin thickener Circuiar high capacity thickener... 

Figure 1.2 Circular basin thickener showing rakes, drive head and walkway.

The circular class of high capacity thickener is similar in general form to a conventional circular basin thickener, but cylinder diameters are limited to between 4 and 18 m (see Figure 1.4). Units are constructed from steel and include a cylindrical top portion, an inverted cone bottom section and an angled rake system mounted on a central drive head. Suspension throughputs are limited to about 4000 m h"k... 

The conventional, circular basin thickeners are constructed of steel up to about 25 m in diameter or concrete up to 200 m in diameter. The floor is... 

The most common thickener is the circular basin type shown in Figure 7. After treatment with flocculant, the feed stream enters the central feed well which dissipates the stream s kinetic energy and disperses it gendy into the thickener. The feed finds its height in the basin where its density matches the density of the inside suspension and spreads out at that level. SoHds concentration increases downward in an operating thickener giving stabiHty to the process. 

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 ... 

Circular basin clarifiers are most commonly fed through a centrally located feed well. The overflow is led into a trough around the periphery of the basin. The bottom gendy slopes to the center and the setded solids are pushed down the slope by a number of motor-driven scraper blades that revolve slowly around a vertical center shaft. This design closely resembles a conventional thickener. Like thickeners, circular clarifiers can be stacked in multitray arrangements to save space. Some juice clarifiers are also arranged in this way. 

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. 

Alternative designs for gravity thickeners (a) circular-basin continuous thickener, (b) deep-cone thickener, and (c) lamella thickener. 

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. 

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

As in most other iron chelate-based redox processes, settling is the preferred first-stage sulfur separation method used in the LO-CAT process (Quinlan, 1991). In the LO-CAT process, the usual approach is to combine the settler and the oxidizer in one single dual purpose vessel. The settler section is designed as a circular-basin type thickener. It has three operating zones clarification, zone settling, and compression. The settler diameter is a function of the solution circulation rate, which is determined by the sulfur production rate. 

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. 

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