CLASSIFICATION OF SOLID PARTICLES
Separation depends on the selection of a process in which the behaviour of the material is influenced to a very
marked degree by some physical property.
a sieving method- separated into various size fractions
separation depend on the differences in the behaviour of the particles in a moving fluid-
The size and the density of the particles are the most important factors
Other processes make use of differences in electrical or magnetic properties of the materials or in their surface
properties.
Classifiers –
a. Non Mechanical
b. Mechanical
c. Hydraulic
Gravity settling
Material is introduced in suspension into a tank containing a relatively large volume of water moving at a low
velocity,
The particles soon enter the slowly
moving water and, because the small
particles settle at a lower rate, they are
carried further forward before they reach
the bottom of the tank.
The particles of high terminal falling velocity
collecting near the inlet
The Double cone classifier
A conical vessel, with a second hollow cone of greater angle arranged
apex downwards inside it, so that there is an annular space of
approximately constant cross-section between the two cones.
The liquid level is maintained slightly higher than the overflow
level, so that there is a continuous flow of liquid downwards in
the centre cone.
The elutriator
Material may be separated by means of an elutriator, which
consists of a vertical tube up which fluid is passed at a
controlled velocity.
The particles are introduced, often through a side tube, and
the smaller particles are carried over in the fluid stream
while the large particles settle against the upward current.
Further size fractions may be collected if the overflow
from the first tube is passed vertically upwards through a
second tube of greater cross-section, and any number of
such tubes can be arranged in series.
Rake classifier
Consists of a shallow rectangular tank inclined to
the horizontal, the feed is introduced in the form of
a suspension near the middle of the tank and water
for classifying is added at the upper end, as shown
in Figure.
The heavy material settles to the bottom and is
then dragged upwards by means of a rake
The hydraulic jig
The hydraulic jig operates by allowing material to settle for brief periods so that the particles
do not attain their terminal falling velocities, and is therefore suitable for separating materials
of wide size range into their constituents.
Construction –
Rectangular section tank with a tapered bottom,
divided into two portions by a vertical baffle.
In one section, the plunger operates in a vertical
direction;
The other incorporates the screen over which the
separation is carried out.
In addition, a stream of liquid is fed to the jig during
the upward stroke.
The hydraulic jig
Down Stroke -The particles on the screen are brought into
suspension during the downward stroke of the plunger (Figure 1.32a). As
the water passes upwards the bed opens up, starting at the top, and thus
tends to rise en masse.
Upward stroke- the input of water is
adjusted so that there is virtually no flow
through the bed. During this period differential
settling takes place and the denser material
tends to collect near the screen and the lighter
material above it.
The bottom layer- large particles of the heavy material,
Middle layer - large particles of the lighter material together with small particles
of the heavy material,
Top layer - stratum of small particles of the light material.
MAGNETIC SEPARATION
A Magnetic separation is process in which a magnetic field is employed to remove magnetic materials from feed, or
concentrate ore, or to selectively remove or separate constituent minerals.
By combining the different forces of magnets with gravitational or frictional forces a separation of mineral particle is
possible.
Two or more products are obtained depending on their commercial value, these products are concentrates,
middlings, or tailings.
Materials can be classified into two broad groups,according to whether they are attracted
or repelled by a magnet:
• Diamagnetics (Quartz, Feldspars, Calcite, etc.)
• Paramagnetics (Hematite, Ilmenite, Rutile, Wolframite, Monazite, etc.)
Magnetic head pulley
Magnetic drum separators
Magnetic drum separators are important magnetic separators that removes ferrous metal from
dry bulk products in free-flowing processing systems.
They are self cleaning unit consisting of drums and housing normally of stainless steel.
The magnetic drum separators are available both in single or double drum configuration.
The processing materials enters the top of the magnetic
drum separator and flows across the surface of the
drum. The rotatory drum in the magnetic field captures
the ferrous tramps whereas non ferrous falls free from
the drum into the cleaned material flow. As the drum
rotates, the ferrous metal so captured is carried past
the diverter and released outside of the magnetic field.
Wet Drum Magnetic Separators
A slurry containing the magnetic component is fed
between the rotating magnet drum cover and the casing.
The stationary magnet system has several radial poles
which attract the magnetic material to the drum face,
and the rotating cover carries the magnetic material
from one pole to another, at the same time gyrating the
magnetic particles, allowing the non-magnetics to fall
back into the slurry mainstream. The clean magnetic
product is discharged clear of the slurry tailings.
Operations can be co- or counter-current and the
recovery of magnetic material can be as high as 99.5 per
cent.
Electrostatic Separator
Electrostatic precipitator generally used to
separate particulate matter that is easily
ionized from a gas stream by using electrostatic
charges. The principal actions are the charging
of dust particles and forcing them to the
collector plates. According to e.m.f. gradient
the charge particle migrates and is attracted to
collecting electrodes. Negatively charged
particle are attracted towards the positive
electrode and positively charged particles to
the negative electrodes.
CYCOLNE SEPARATOR
Cyclone separators provide a method of removing particulate matter from air or other gas streams at low cost and low
maintenance.
Cyclones are basically centrifugal separators, consists of an
upper cylindrical part referred to as the barrel and a lower
conical part referred to as cone.
They simply transform the inertia force of gas particle flows to
a centrifugal force by means of a vortex generated in the
cyclone body.
The particle laden air stream enters tangentially at the top of
the barrel and travels downward into the cone forming an
outer vortex. The increasing air velocity in the outer vortex
results in a centrifugal force on the particles separating them
from the air stream.
When the air reaches the bottom of the cone, it begins to flow
radially inwards and out the top as clean air/gas while the
particulates fall into the dust collection chamber attached to
the bottom of the cyclone.
Efficiency of cyclone separator –
The efficiency of a cyclone collector is related to the pressure drop across the collector. This is
an indirect measure of the energy required to move the gas through the system. The pressure
drop is a function of the inlet velocity and cyclone diameter.
small cyclones are more efficient than large cyclones. Small cyclones, however, have a higher
pressure drop and are limited with respect to volumetric flow rates.
Arrange smaller cyclones in series and/or in parallel to substantially increase efficiency at lower
pressure drops
The cut diameter-
The cut diameter of the cyclone is defined as the size of the particles collected with 50%
collection efficiency. It is an indicator of the size range of particles that can be collected. It is a
convenient way of defining as it provides information on the effectiveness for a particle size
range. A frequently used expression for cut off diameter is
SEDIMENTATION
This is the separation of a liquid from particles suspended in the liquid
The deposition by settling of a suspended material
1) Produce a clarified (free of suspended solids) effluent.- Clarification e.g. water purification
2) Produce a highly concentrated solid sludge stream – Thickening
Batch Sedimentation Test
The particles zone B settle at a uniform rate at the start, and a
clear liquid zone A appears. The height of z drops at a constant
rate.
Zone D also begins to appear, which contains the settled
particles at the bottom and zone C is a transition layer whose
solids content varies from that in zone B to that in zone D.
After further settling, zone B and C disappear. Then
compression first appears; this moment is called the critical
point
During compression, liquid is expelled upward from zone D and
the thickness of zone D decreases.
Factors Affecting the Sedimentation Rate
Height of Suspension –Not affects
Diameter of vessel – If diameter of vessel > by 100 times diameter of
particle
Concentration of suspension – Higher is the concentration slower is the
sedimentation
Application of batch settling test to Thickeners
Thickener Area Calculation
Q0 Y U C s
A
uT f
• where A = area (m2)
Q0 = feed rate of suspension (m3/s)
Y = mass ratio liquid to solid in feed
U = mass ratio liquid to solid in underflow
C = particle volume fraction (1-ε)
ρs = density of solid (kg/m3)
uT = terminal velocity at conc. C (m/s)
ρf = density of liquid (kg/m3)
Thickeners
The basic components are the
same:
• Tanks
• Drive Support Structure
• Drive Assembly
• Rake mechanism
• Feed Well
• Overflow Arrangement
• Underflow Arrangement
Tanks Tanks or Basins are constructed of such material
as steel, concrete, wood, compacted earth.
Drive Support Structure Bridge supported thickeners are
common in diameters of 30m, the maximum being 45m.
Drive Assembly
The drive assembly provides: ● The force to move the rakes through the thickened pulp and move settled solids to the point of
discharge. ● The support mechanism which permits it to rotate. ● Adequate reserve capacity to withstand upsets and
temporary overloads. ● A reliable control which protects the mechanism from the damage when a major overload
The drive assembly provides:
● The force to move the rakes through the thickened pulp and move settled solids to the point of discharge.
● The support mechanism which permits it to rotate.
● Adequate reserve capacity to withstand upsets and temporary overloads.
● A reliable control which protects the mechanism from the damage when a major overload
Rake Mechanism
Aids in thickening the pulp by disrupting bridged
floccules, permitting trapped fluid to escape and allowing
the floccules to become more consolidated. Rake
mechanisms are designed for specific application, usually
having two long rakes with an option for two short rake
arms. Assists in moving the settled solids to the point of
discharge.
They offer the following advantages
over other thickeners:
• Ability to transfer loads to the tank
periphery
• Ability to give a denser and more consistent
underflow concentration with a single draws
off point
• A less complicated lifting device;
• Fewer structure members subject to mud
accumulation;
• Access to the drive from both ends of the
bridge; and
• Lower cost for units smaller than 30m in
diameters
Center Column Thickener
