Section 5.12 Sedimentation Centrifuges

Centrifuge, differential type, continuous, 316 s/s including flexible connections, explosion-proof motor, variable speed drive, ammeter, tachometer, excluding pumps, starter, flowmeters and control valves. FOB cost 220000 at aqueous feed rated capacity = 2.2 L/s with n = 0.25 for the range 0.03-2.2 and n = 0.38 for the range 2.2-36. L-rM = 1.70. 

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Sedimentation centrifuge Disc type drop diameter > 20 and < 200 pm feed concentration 6 to 60% v/v, suited for low surface tension, density differences > 0.02 Mg/m Solids contamination < 0.1% v/v. Use differential type for drop diameter > 200 pm feed concentration 6 to 60 % v/v, suited for low surface tension, density differences > 0.05 Mg/m. Solids contamination < 0.1 % v/v. Electrodecanter drop diameter 9 to 500 pm feed concentration 0.8 to 8% v/v. Fibrous bed coalescer drop diameter 3 to 75 pm feed concentration < 3 % v/v. see size enlargement Section 9.2. 

To recover liquid in order of preference of settlers, Section 5.8, thickeners. Section 5.10 and sedimentation centrifuges. Section 5.12 clarifier, settler, washing tray thickener, reactortubular bowl centrifuge, batch automatic (horizontal or vertical bowl, disc with intermittent nozzle discharge) continuous disc bowl centrifuge with nozzle discharge. 

Possible option continuous pusher peeler centrifuge vertical basket centrifuge multichamber vertical solid bowl sedimentation centrifuge. Section 5.12 sedimentation centrifuge, Section 5.12... 

For any separator with a size-dependent performance, such as a hydrocyclone, a sedimenting centrifuge or a settling vessel, the grade efficiency varies with particle size, and a graphical representation of this is called the grade efficiency curve (see section 3.2.2). As the value of the grade efficiency has the character of probability, it is sometimes referred to as the partition probability the curve then becomes the partition probability curve or Tromp curve. 

In addition to the now more-or-less standard features and designs of the sedimenting centrifuges described in section 7.3, there have been many refinements and improvements added to the existing technology over the past decade or so. The most important ones are briefly reviewed here, with references given for further details and reading. 

Tubular bowl centrifuges usually incorporate continuous centrate discharge and batch solids recovery, and operate at variable speeds up to 20,000 G. Although primarily intended for liquid-liquid separation, they can also be used for recovery or classification of ultra-fine solids. These sedimenting centrifuges (disc, decanter and tubular bowl) are described further in Section 7. 

A sedimenting centrifuge is formed if the drum-shaped perforated basket centrifugal filter, described in Section 31, is run instead without any perforations in its bowl (basket) walls. Its behaviour mirrors that of the tubular bowl centrifuge, with feed entering below a lip at the top of the bowl, separating liquids forming two layers as they flow down the bowl, to overflow at the bottom from two levels. As with its perforate basket equivalents, the basket can be under driven or over driven, and can have a three-column suspension. 

If a sedimentation experiment is carried out long enough, a state of equilibrium is eventually reached between sedimentation and diffusion. Under these conditions material will pass through a cross section perpendicular to the radius in both directions at equal rates downward owing to the centrifugal field, and upward owing to the concentration gradient. It is easy to write expressions for the two fluxes which describe this situation ... 

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. 

We have already noted that sedimentation and diffusion are opposing processes, the first tending to collect and the second to scatter. Let us now consider the circumstances under which these two tendencies equal each other. Once this condition is reached, of course, there will be no further macroscopic changes the system is at equilibrium. In order to formulate this problem, consider the unit cross section shown in Figure 2.15, in which the x direction is in the direction of either a gravitational or a centrifugal field. Suppose this field tends to pull... 

In the sedimentation equilibrium method, the solution is allowed to come to equilibrium. When this happens, the analysis becomes very similar to that described in the previous section for gravitational fields, with the gravitational work mgdh replaced by mu2rdr, where u is the angular velocity of the centrifuge (in rad sec-1) and r is the distance from the center of the centrifuge. The final result is an equation similar to (12.21)... 

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