Filter Thickeners

Separation Efficiency. Similarly to other unit operations in chemical engineering, filtration is never complete. Some soflds may leave in the hquid stream, and some Hquid will be entrained with the separated soHds. As emphasis on the separation efficiency of soHds or Hquid varies with application, the two are usually measured separately. Separation of solids is measured by total or fractional recovery, ie, how much of the incoming solids is coUected by the filter. Separation of Hquid usually is measured in how much of it has been left in the filtration cake for a surface filter, ie, moisture content, or in the concentrated slurry for a filter-thickener, ie, solids concentration. 

Washing by successive dilution is used when the soHds are separated into a slurry, such as in filter thickeners. The soHds, thickened into a small amount of mother Hquor, are diluted into a wash Hquid and then separated again, diluted, separated, etc until clean of mother Hquor. The consumption of the wash Hquid can be reduced in countercurrent washing systems, sometimes referred to as countercurrent decantation. Cocurrent dilution washing, however, can be built into some dynamic filter-thickeners such as the Escher-Wyss filter. 

Total submergence is used in the vacuum disk filter thickener (Eig. 13) in which the cake discharge, by backwashing with filtrate, occurs as each sector passes through the lowest point of the slurry tank. 

Thickening Pressure Filters. The most important disadvantage of conventional cake filtration is the declining rate due to the increased pressure drop caused by the growth of the cake on the filter medium. A high flow rate of Hquid through the medium can be maintained if Httle or no cake is allowed to form on the medium. This leads to thickening of the slurry on the upstream part of the medium filters based on this principle are sometimes called filter thickeners. 

Figure 27. Components of a staged, thin-film filter/thickener.

In the second stage, the suspension is concentrated in a thickener. Although gravity thickeners dominate the field, cross-flow filter thickeners, hydrocyclones, and electrophoretic devices can be also used. A large fraction of the liquid can be removed economically in thickeners, thereby leading to smaller units in the third stage. 

FIGURE 22.48 Shriver continuous cross-flow filter thickener. 

Figure 19, Principal components of a staged, thin-cake filter thickener.

Many filter-thickeners are simple settling tanks or decanters. Thickeners of this type are generally large and bulky and have relatively slow rates. Centrifuges have a greater driving force but, in general, are expensive to operate and can deliver cloudy overflow if fine particles are present. 

Washing water conditioning upstream and/or downstream from the washed filter. Thickening of washing effluents usually by flotation, or by sedimentation if the oil/SS ratio in the raw water is too low-... 

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