Compression, settled solids

In the top (clarifying) zone the relatively clear liquid moves upward and overflows the top. In the middle zone the solid particles settle as the displaced liquid moves upward, and both the local solids concentration and the settling velocity vary from point to point. In the bottom (compressed) zone, the solids and liquid both move downward at a rate that is determined mainly by the underflow draw-off rate. For a given feed rate and solids... 

The most important design dimensions of a thickener are pool area and depth. The pool area is chosen to be the largest of the three layer requirements. In most cases, only the zone-settling and compression layer requirements need to be considered. However, if the clarity of the overflow is critical, the clarification zone may need the largest area. As to the pool depth, only the compression layer has a depth requirement because the concentration of the solids in the underflow is largely determined by the time detention and sometimes by the static pressure. Thickness of the other two layers is governed only by practical considerations. 

If a gravitationally bound clump forms, the relative velocities of the particles within the clump must be damped before the clump can collapse to form a solid planetesimal. Gas drag and particle-particle collisions will do this, with damping being more effective for small particles. The collapse rate is limited by the pressure of gas within the clump - as particles move inwards, the gas becomes compressed, opposing further collapse (Cuzzi Weidenschilling 2006). The rate of collapse is set by the time required for particles to settle to the center, which may be hundreds of orbital periods for millimeter-sized particles (Cuzzi Weidenschilling 2006). 

Dorr Agitator.—This is an ordinary tank in the center of which is a vertical shaft to the bottom of which are attached two arms with rakes, so that when the shaft is revolved they will rake any solids which settle out towards the center. This shaft in the center is hollow and has openings at the bottom and top and it is made into an air lift by a compressed-air pipe leading in at the bottom. To the top of the shaft are attached two perforated launders which revolve with the shaft and any material coming up through the center of the air lift flows into them and will be distributed over the entire surface of the solution in the tank. 

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