The upper section of a Paste Thickener is identical to a High-Rate Thickener and relies on achieving optimum flocculation conditions in the proprietary feed system. The optimum feedwell solids are commonly determined by testwork. Care needs to be taken to ensure exit velocities from the feedwell do not cause excessive shear and fracturing of flocculated particles.
On exiting the feedwell, flocculated solids settle according to their free settling rate to form an unconsolidated bed. The compression effects of the increasing height of the mud bed combined with the action of the rake and pickets to create dewatering channels consolidates the mud bed. In principle, the higher the side wall of the tank, the greater the compression effect and the higher the underflow density.
The high-density mud has a greater resistance to flow, therefore moving the material into the discharge zone requires more force. Thus, the higher the design underflow yield stress, the larger and more powerful both the drive and the rake structure needs to be.
The floor slope on the tank is often increased to provide some assistance to transport the thickened paste, with 30 and 45 degrees being commonly utilized.