Page 341 - Chemical Process Equipment - Selection and Design

LID-LIQUID SEPARATION

olid-,Oqud separation is concerned with mechanical
processes for the separation of liquids and finely
divided insoluble solids.

Freely draining solids may be filtered by gravity with horizontal
screens, but often filtration requires a substantial pressure
Much equipmenit for the separation of liquids and finely divided difference across a filtering surface. An indication of the kind of
solids was invented independently in a number of industries and is equipment that may be suitable can be obtained by observations of
of diverse character. These developments have occurred without sedimentation behavior or of rates of filtration in laboratory vacuum
benefit of any but the most general theoretical considerations. Even equipment. Figure 11.1 illustrates typical progress of sedimentation.
at present, the selection of equipment for specific solid-liquid Such tests are particularly used to evaluate possible flocculating
separation appfications is largely a process of scale-up based on processes or agents. Table 11.2 is a classification of equipment
direct experimentation with the process material. based on laboratory tests; test rates of cake formation range from
The nature and sizing of equipment depends on the economic several cm/sec to fractions of a cm/hr.
values and proportions of the phases as well as certain physical Characteristics of the performance of the main types of
properties that influence relative movements of liquids and commercial SLS equipment are summarized in Table 11.3. The
particles. Pressure often is the main operating variable so its effect completeness of the removal of liquid from the solid and of solid
on physical properties should be known Table 11.1 is a broad from the liquid may be important factors. In some kinds of
classification of mechanical processes of solid-liquid separation. equipment residual liquid can be removed by blowing air or other
Clarification is the removal of small contents of worthless solids gas through the cake. When the liquid contains dissolved substances
from a valuable Liquid. Filtration is applied to the recovery of that are undesirable in the filter cake, the slurry may be followed by
valuable solids Prom slurries. Expression is the removal of relatively
small contents of liquids from compressible sludges by mechanical t--.
means.
Whenever feasible, solids are settled out by gravity or with the
aid of centrifugation. In dense media separation, an essentially
homogeneous Equid phase is made by mixing in finely divided solids
(less than 100 mesh) of high density; specific gravity of 2.5 can be
attained with magnetite and 3.3 with ferrosilicon. Valuable ores and
cod are floated away from gangue by such means. In flotation, Critical sedimentaticn
surface active agents induce valuable solids to adhere to gas bubbles
which are skimmed off. Magnetic separation also is practiced when
'd
feasible. Thickeners are vessels that provide sufficient residence
time for settling to take place. Classifiers incorporate a mild raking Time
action to prevent the entrapment of fine particles by the coarser i
ones that are to be settled out. Classification also is accomplished in 1
hydrocyclones with moderate centrifugal action. ... ,.... :.:.: ... . :.:.:.:.=:
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TABLE 11.1. Chief Mechanical Means of F.... . .Ai.. .......... .
Solid-Liquid Separation
1. Settling
a. by gravit
i. in thideners
ii. in classifiers
b. by centrifugal force
c. by air flotation
d. by dense media flotation
e. by magnetic properties
2. Filtration
a. on screens, by gravity
b. on filters
i. byvacuum
ii. by pressure
iii. by centrifugation
3. Expression
a. with batch presses
b. with continuous presses Time
i. screw presses
ii. rolls Figure 11.1. Sedimentation behavior of a slurry, showing loose and
-~ - compacted zones (Osborne, 1981).
discs
iii.