Its part of sedimentation which is covered under the Mechanical operations. It contains the gravity sedimentation, clarifier and classifiers, sink and float method, gravity and sorting classifiers, differential settling methods etc....
Its just gives basic concept of sedimentation.
2. Gravity Settling Processes
Two types of sedimentation
1. Gravity settling and 2. Centrifugal settling
Particles heavier than the suspending fluid may be removed from a gas or
liquid in a large settling box or settling tank. ( fluid velocity is low and the
particles have ample time to settle out.)
Industrial separators nearly all provide for the continuous removal of settled
solids.
The separation may be partial or very nearly complete.
A settler that removes virtually all the particles from a liquid is known as a
clarifier.
whereas a device that separates the solids into two fractions is called a
classifier.
3. GRAVITY CLASSIFIERS
Separate particles on the basis of size and the density of the fine particles
(the same as that of the larger ones.)
Working- By adjusting the upward velocity of the liquid so that it is smaller
than the terminal settling velocity of acceptably large crystals, this device
carries unwanted fine crystals back to the crystallizing zone for further
growth.
Mechanical classifiers are used in closed-circuit grinding, especially in
metallurgical operations.
Here the relatively coarse particles are called sands and the slurry of fine
particles is called slimes.
Sufficient time is provided to allow the sands to settle to the bottom of the
device; the slimes leave in the effluent liquid.
4. SORTING CLASSIFIERS
Devices that separate particles of differing densities are known as sorting
classifiers.
They use two principal separation methods—sink-and-float and differential
settling.
5. Sink-and-float methods
A sink-and-float method uses a liquid sorting medium, the density of
which is intermediate between that of the light material and that of the
heavy.
Then the heavy particles settle through the medium, and the lighter ones
float, and a separation is thus obtained.
This method is also called heavy fluid separation.
Separation - independent of the particle size.
Used to treat relatively coarse particles, greater than 10-mesh.
Medium - Halogenated hydrocarbons and CaC12 solutions and pseudo
liquid consisting of a suspension in water of fine particles of a heavy
mineral
7. Differential settling methods
Differential settling methods utilize the difference terminal velocities that
can exist between substances of different density.
disadvantage the method is that since the mixture of materials to be
separated covers a rani of particle sizes, the larger, light particles settle at
the same rate as the small, heavy ones and a mixed fraction is obtained.
But this problem can be solved.
Let of two materials A and B settling through a medium density p. Let
material A be the heavier; e.g., component A might be galena (specific
gravity 7.5) and component B quartz (specific gravity 2.65).
The terminal velocity of a particle of size Dp and of density pp settling under
gravity through a medium of density p is given .
8. Continue…
the equal- settling diameter ratio is given by,
By using this plotting of terminal velocity vs avg. particle size
Its clear that from above eq., the
sharpness of separation is
improved if the density of the medium is
increased.
It is also clear from Fig. that the mixed
fraction can be reduced or eliminated by
closer sizing of the feed.
For example, if the size range of the feed
is from Do to Dp4 in Fig.,
complete separation is possible.
9. CLARIFIERS AND THICKENERS
Gravity separation under hindered settling conditions is often used to
convert a dilute slurry of fine particles into a clarified liquid and a
concentrated suspension.
This process is carried tanks called thickeners or clarifiers.
The concentrated suspension, or sludge, may have to be filtered to produce
a drier product, but the cost of the filtration step is much lower than if the
original slurry had been filtered directly.
The clarified liquid is free or nearly free of suspended particles, and it may be
reused as process water or discharged as waste.
