Sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening, i.e.,
Thickeners can be operated in a countercurrent fashion. Applications are aimed at the recovery of soluble material from settleable solids by means of continuous countercurrent décantation (CCD). The basic scheme involves streams of liquid and thickened sludge moving counter-currently through a series of thickeners. The thickened stream of solids is depleted of soluble constituents as the solution becomes enriched. In each successive stage, a concentrated slurry is mixed with a solution containing fewer solubles than the liquor in the slurry and then is fed to the thickener. As the solids settle, they are removed and sent to the next stage. The overflow solution, which is richer in the soluble constituent, is sent to the preceding unit. Solids are charged to the system in the first-stage thickener, from which the final concentrated solution is withdrawn. Wash water or virgin solution is added to the last stage, and washed solids are removed in the underflow of this thickener. The flow scheme for a three-stage CCD system is illustrated in Figure 5. The feed stream, F, is mixed with overflow 02 (from thickener 2) before entering stage 1. The overflow of concentrated solution, O,, is withdrawn from the first stage. The underflow from the first stage, U,, is mixed with third-stage overflow, 03, and fed to the second stage. Similarly, the second-stage underflow, U2, is mixed with wash water and fed to thickener 3.
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