Tecnologia Química - classificação de partículas

1. Classification
and classifier
April 8
2013
The paper on classification and classifier gives the
basic idea about the particle size separation method
using the principle that different particles have
different settling velocity in a fluid medium (water) by
the process called classification. There are four types
of classifier- 1.Mechanical, 2. Hydraulic, 3. Hydro clone
and 4. Centrifugal air classifiers whose working and
construction along with principle, features, benefits,
etc. are described.
Subodh Rana (367/11)
Md Salim (360/11)
Chandan Kumar (420/11)

2. Paper on Classification and Classifiers
Classification
 Classification is basically a particle size separation method.
 Classification may be defined as the separation of solid particles into two or
more products according to their velocities when falling through a fluid
medium generally water is used as fluid medium.
 Range of particles size: 1µm -800µm.
 Velocity of particles depends on their size, shape and density. Particles
with higher density and larger size will settle down quickly as compared
with particles with lower density and smaller size.
 Limestone, cement, coal industry performs dry classification for separation
of fine particles in which air is used as fluid medium.
Classifier
A classifier is a machine which actually performs process stated above called
classification.
Types of classifier
a) Mechanical Classifiers(Spiral classifier)
b) Hydraulic Classifier
c) Hydro cyclone Classifier
d) Centrifugal air classifier
Principle of classifier
It is based on the different settling velocity of particles in water.

3. MECHANICAL CLASSIFIERS
There are many mechanical classifiers but the two most famous are Rake
classifier and Spiral classifier.
Construction and working
Mechanical classifiers consist of a settling tank with parallel sides and
inclined base, there is a device inside it that facilitates agitation of the pulp
and removal of the settled solid.
The feed pulp is fed into the classifier, solids with high settling velocity
settle at the base of the tank and mechanical rakes and spirals (helical
screws) take away the settled solid from bottom to top and at the same
solids with low settling velocities come out through the water overflow.
Particle Size: - 800-75 microns
Fig. showing spiral classifier at work

4. Fig. showing working sketch of mechanical classifier
Main features
 Adjustable weirs
 Sealed, submerged bearing
 Single spiral range 30cm-220cm
 Overflow collection box
Main benefits
 Energy saving
 Powerful self-contained spiral lifting device
 Continuous spiral raking

5. HYDRAULIC CLASSIFIER
Hydraulic classifiers use the principle of hydraulics and different settling velocity
of solids
Construction and Working
Hydraulic classifiers consist of settling cones which are conical vessels, where pulp
is introduced vertically from the top. Coarse particles settle down and leave the
vessel through the underflow spigot; fine particles leave the vessel with most of
the water overflows.
Hydraulic classifiers consist of a series of column in which water current is
injected from bottom which moves upward and takes away the lighter particles of
low settling velocity. Hydraulic classifiers are widely used in sand and glass
industry.
Particle size: - 2µm-7µm
Fig showing hydraulic classifier

6. Fig. showing working of a hydraulic classifier
Main Features and benefits
 Used for 2-7µm size particles
 Series of columns are used

7. HYDROCYCLONE CLASSIFIER
Hydro cyclone classifier is widely applied in mineral processing for fine
classification (mostly <100µm), often in a closed-circuit with ball mills for flotation
or leaching feed preparation and for special fine final products (kaolin). They are
particularly efficient for fine separation sizes.
Construction and Working
A hydro cyclone is a vessel composed of a cylindrical section with a tangential
feed entrance, joined to a lower conical part. The feed is accelerated and rotates
at high speed within the vessel, transporting the coarse particles by centrifugal
forces to the inner wall, from where it moves down along the conical part and
leaves the vessel through the underflow spigot. The slower settling fine particles
stay in the centre of the fluid, which forms an inner upstream current and leaves
the vessel through the central upper discharge opening. To avoid short-cuts, the
upstream is collected by an adjustable inner piece of pipe, connected to the
overflow outlet.
Particle size: -1-100 µm
Fig showing a hydro cyclone
Main Features and benefits: -
 Mild steel construction
 Maximum operating pressure of 10 kg/cm2
 Efficiently removes fine sand and silt particles of size higher
than 75 microns and specific gravity more than 2.65.

8.  Used in micro irrigation systems to remove sand and silt
particles from irrigation water.
 Dirt can be easily flushed out through dirt collection chamber.
 Easy to clean and maintain.
CENTRIFUGAL AIR CLASSIFIER
These classifiers use centrifugal forces, similar to cyclones, to separate particles at
cut points between 15 and 100 microns.
Construction and working
The feed material is carried into the classifier by the primary inlet flow stream.
Internal baffles apply drag forces to the coarse particles while allowing air to pass
through them for separation of the fines. The heaviest particles drop to the
bottom of the classifier and are discharged through a valve.
Prior to discharge, a secondary air stream enters into the bottom of the classifier
and passes through the material. Particles near the cut point in size are returned
to the classifier chamber. The air stream carrying the fine particles follows a spiral
path to the outlets on each side of the unit.
The two air streams combine and enter a cyclone for final recovery of fine
particles.
A manual adjustment to the secondary air damper will provide a change in the
size of the fines being separated. Because there are no moving parts within the
material flow stream, the need for maintenance is minimized.
Particle size: - 15-100microns

9. Fig. showing mechanism of Centrifugal air classifier
Main features and benefits
 Wide range of cut point adjustability between 15 to 100
micron range
 Increased throughput compared to dynamic rotary classifier,
lower maintenance cost
 High efficiency separation
 Classifier has no moving parts
 Cut point adjustability using interchangeable orifices and
secondary air
 Virtually no internal parts within classifying material flow
stream
 Low cost alternative to dynamic rotary classifier
 Internal Lining – AR Plate or Ceramics available for high
abrasive applications