Sedimenting and Filtering Centrifuges

Theoretical studies (30) comparing the abihty to dewater compressible sohds by sedimenting and filtering centrifuges to pressure filters, have shown that at high G levels, scroU decanters produce drier cakes than pressure filtration. 

Sedimenting and Filtering Centrifuges Under centrifugal force, the sohd phase assumed to be denser than the hquid phase set-... 

Sedimenting and Filtering Centrifuges Under centrifugal force, the solid phase assumed to be denser than the liquid phase settles out to the bowl wall—sedimentation. Concurrently, the lighter, more buoyant liquid phase is displaced toward the smaller diameter—flotation. This is illustrated in Fig. 18-149a. Some centrifuges run with an air core, i.e., with free surface, whereas others run with slurry filled to the center hub or even to the axis in which pressure can be sustained. 

This entry has presented a brief description of sedimenting and filtering centrifuges. The types of separation, separation duties, and modes of operation have been discussed. The basic common and distinct mechanical elements have also been dealt with. References are included for those readers seeking a more in-depth historical, theoretical, as well as practical background on the subject. 

Centrifuges are operated as elassifiers, sedimentors, and filters. Centrifugal classifieation and sedimentation are carried out in solid bowl units (frequently called decanters) as illustrated in Fig. 25. Most decanters are operated on a eontinuous basis. The slurry enters as shown in Fig. 25. The solids settle out and are conveyed toward diseharge by a seroll, whieh is turning at a lower speed than the bowl. The clear liquid moves... 

In a filtering centrifuge, separating sohds from liquid does not require a density difference between the two phases. Should a density difference exist between the two phases, sedimentation is usually at a much more rapid rate compared to filtration. In both cases, the solid and liquid phases move toward the bowl under centrifugal force. The sohds are retained by the filter medium, while the liqmd flows through the cake solids and the filter. This is illustrated in Fig. 18-138/ . 

More detailea descriptions of small-scale sedimentation and filtration tests are presented in other parts of this section. Interpretation of the results and their conversion into preliminary estimates of such quantities as thickener size, centrifuge capacity, filter area, sludge density, cake diyness, and wash requirements also are discussed. Both the tests and the data treatment must be in experienced hands if error is to be avoided. 

The design, construction and application of thickeners, centrifuges and filters is a specialised subject, and firms who have expertise in these fields should be consulted when selecting and specifying equipment for new applications. Several specialist texts on the subject are available Svarovsky (2001), Ward (2000) and Wakeman and Tarleton (1998). The theory of sedimentation processes is covered in Volume 2, Chapter 5 and filtration in Chapter 7. 

Slow filtering 0.1-10 cm/hr pressure filters disc and tubular centrifuges sedimenting centrifuges... 

The filter centrifuge should not be confused with the sedimentation centrifuge widely used in biochemical laboratories. With a sedimentation centrifuge, fine precipitates are not filtered, but are pressed against the walls of the container and thus separated from the liquid. 

Characteristics of the performance of the main types of commercial SLS equipment are summarized in Table 11.3. The completeness of the removal of liquid from the solid and of solid from the liquid may be important factors. In some kinds of equipment residual liquid can be removed by blowing air or other gas through the cake. When the liquid contains dissolved substances that are undesirable in the filter cake, the slurry may be followed by pure water to displace the residual filtrate. Qualitative cost comparisons also are shown in this table. Similar comparisons of filtering and sedimentation types of centrifuges are in Table 11.19. 

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