Chemical Engineering August 2014 - 62

Solids Processing
overcome the centrifugal and drag
forces, dropping the coarse particles
out of the airstream. The addition
of air from an external fan assists
in the dispersion and suspension
of particles. The drag force of the
airstream has a greater affect on
fine particles, because of their lower
mass. In this case, the drag force is
greater than the centrifugal force
on the particle. Thus, the fine particles
are swept out of the classifying
zone by the airstream, where they
are held in suspension around the
rotor. The airstream is used to wash
the fine particles out of the material
stream and carry them to the classifier
wheel, where they pass through
the rotor and are discharged as
fines. The air introduced from the
external fan must be balanced with
the solids loading to achieve the optimum
air-to-solids ratio.
These two process variables, air
and solids loading, will be different
for each material, depending on the
material's specific gravity, particle
shape and surface area. Once determined,
control over a product's particle
size is achieved by controlling
the speed of the classifier rotor. The
cut point can be precisely controlled
by increasing or decreasing the rotational
speed of the rotor. Typical
applications for this type of air classifier
include: making size separations
in the range of 5-45 μm (325
mesh); de-dusting of very fine particles;
and generating narrow size
distributions. The air classifier will
classify the particles by surface area
primarily; density classification is
secondary. In general, the classifier
will produce a more efficient separation
when there is an overall broad
distribution of particles in the raw
feed. When the raw feed has a very
narrow particle-size distribution, it
becomes difficult for the classifier
to differentiate between near-size
fines and near-size coarse particles.
Screening equipment
Like air classification, screening or
sieving involves the separation of dry
granular solids according to particle
size. Screening equipment is used in
almost every process that handles
dry particulate matter. Screening
requires relative motion
between the sieve and
the particle mass. In a
few specialized cases,
the sieve is stationary,
but in most commercial
screening applications,
the particle mass flows
over a sieve, wherein
some type of motion is
mechanically applied.
The motion is intended
to enhance both the
flowrate and the passage
of undersize particles
through the sieve. When
vibration is applied to
a screen where there is
a static bed of material
present, a phenomenon
called " trickle stratification "
occurs, causing the
particles to stratify into
layers with finer particles at the bottom
to coarser particles at the top.
The intensity of the vibration affects
the number of times a particle comes
into contact with the screen surface.
The more opportunities a particle
has to come into contact with the
screen opening, the greater the probability
of passage through the screen.
There are different types of motion
that can be applied, depending on
the design of the screening machine,
and each has unique characteristics.
Generally, vibratory screening machines
are typically divided into six
basic categories, as detailed in the
following section.
Gyratory screen. The gyratory
screen is a precision screener that
typically has an operating speed
of around 285 revolutions per minute
(rpm) and a horizontal circular
stroke of 2.5 in. This type of screener
serves a broad range of industries
and is available with multiple screen
decks with a range of surface openings
from 1 in. to 50 mesh.
Straight-line reciprocating screen.
The straight-line reciprocating
screen is a high-capacity precision
screen that normally has operating
parameters of 475 rpm with a
1-in. stroke, zero pitch and a slope
of 6 deg. This type of screener also
serves many industries, as it provides
up to 800 ft2 of deck surface
62 ChemiCal engineering www.Che.Com august 2014
FIGURE 2. Bulk solids are separated and move
through gravity-separation machinery based on terminal
velocity and gravity
with a range of surface openings
from 1 in. to 40 mesh.
High-speed inclined vibrating
screen. The inclined vibrating
screen is a high-speed screen with a
typical operating speed of 1,200 rpm
with ¼-in. vertical circular stroke.
This type of screener is often used
in coal preparation and aggregates,
with deck surfaces ranging in size
from 6 in. to 10 mesh.
High-speed horizontal vibrating
screen. The horizontal vibrating
screen is another type of highspeed
screen that typically has
operating parameters of 850 rpm
with a ½-in. stroke and a 45-deg attack
angle. The horizontal vibrating
screen is used in the same type of
applications as the inclined vibrating
screen. Deck surface openings
range in size from 3 in. to 10 mesh.
High-frequency screen. High
frequency-screens usually employ
vibration that is transmitted to the
screen at an operating speed of 3,000
rpm. Additionally, a burst cycle,
reaching 4,500 rpm, is provided to
control screen blinding. This type of
screen is used for fine-mesh screening,
with deck sizes ranging from
3/16 in. to 325 mesh.
Circular screen. Circular screens,
sometimes referred to as sifters, are
single or multi-deck screeners with
diameters that range from 18-72 in.
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Chemical Engineering August 2014

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