Thickening

The process of thickening involves the concentration of a slurry, suspension, or sludge, usually by gravity settling. Because concentrated suspensions and/ or fine particle dispersions are often involved, the result is usually not a complete separation of the solids from the liquid but is instead a separation into a more concentrated (underflow) stream and a diluted (overflow) stream. Thickeners and clarifiers are essentially identical. The only difference is that the clarifier is designed to produce a clean liquid overflow with a specified purity, whereas the thickener is designed to produce a concentrated underflow product with a specified concentration (Christian, 1994; Tiller and Tarng, 1995; McCabe et al., 1993).

A schematic of a thickener/clarifier is shown in Fig. 14-5. As indicated in Fig. 14-3, several settling regions or zones can be identified, depending on the solids concentration and interparticle interaction. For simplicity, we consider three primary zones, as indicated in Fig. 14-5 (with the understanding that there are transition zones in between). The top, or clarifying, zone contains relatively clear liquid from which most of the particles have settled. Any particles remaining in this zone will settle by free settling. The middle zone is a region of varying composition through which the particles move by hindered settling. The size of this region and the settling rate depend on the local solids concentration. The bottom zone is a highly concentrated settled

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