Chemical Engineering August 2014 - 61

Solids Processing
Dry Separation Methods
Separating bulk solids via air classification,
Matt Mayo, Chris Meadows
and Celie Reid
Triple/S Dynamics, Inc.
T
he chemical process industries
(CPI) have many applications
that require classifying
or separating solid
materials. Examples of separation
or classification processes include
those operations that attempt to isolate
specific material fractions according
to particle size, scalp off the
coarse fraction of a material stream,
de-dust the fine fraction of a material
stream or remove the contamination
from a material stream. For
instance, dry separation is required
in recycling processes for heavy metals,
depicted in Figure 1. Dry separation
methods can basically be broken
down into three main types of
mechanical separation procedures:
air classification, screening and
specific-gravity separation.
Process engineers are routinely
challenged with the ubiquitous
problem of how to most effectively
and efficiently perform particle sizing,
along with addressing concerns
about the removal of unwanted contaminants
from a material stream.
This article provides background on
the different techniques and equipment
used for dry separation, and
also guides engineers in addressing
some specific dry-separation issues.
Air classification
The development of air-classification
equipment, like most processing
equipment, is an ongoing
process. As such, newly introduced
equipment has been designed to
offer better classification efficiency
with the ability to produce increasingly
fine products. Rising demand
for finer products and more closely
controlled particle-size distributions
are the driving forces behind
the development of high-efficiency,
centrifugal-type air classifiers. In
recent history, almost all design
improvements have been the result
of an increased understanding of
screening or gravity separation is ubiquitous in many
industries - an understanding of these processes
is crucial to solids-handling engineers
FIGURE 1. A common dry separation process is the recycling of heavy metals,
such as copper
aerodynamic principles, as well as
particles' behavior in an airstream
when subjected to various forces.
Other areas of concern for classifier
improvements involve achieving
greater classification efficiency
while delivering higher capacities
and finer products. These improvements
result in greater control over
the classification process, because
improved designs tend to be more
sensitive to changes in process parameters,
such as rotor speed, air
flowrate and particularly feedrate.
Despite the complexity of airclassification
equipment, all air
classifiers operate under the same
basic laws of physics. There is an
established balance of forces -
gravitational, centrifugal and drag
forces are integral to air-classification
processes. It is difficult to precisely
evaluate the interaction of all
the forces within the classifier and
their effect on the particles. However,
centrifugal force (Fc) and drag
force (Fw) are the two main opposing
forces under which particles
are subjected. Expressions for centrifugal
and drag forces are given in
Equations (1) and (2), respectively.
Fc dP Rn π 5 400/,
=× ×× ×32 3
pp
Fw dg Vrad
=× ××3πη
p
where:
dp
= particle diameter
Pp = particle density
ηg
= viscosity of the gas (air)
Vrad = radial speed of the gas (air)
R = radius of classifier wheel
n = speed of classifier wheel (rpm)
In general, the centrifugal forces
within the air classifier are typically
imparted by a high-speed rejecter
rotor, also known as a classifier
wheel. The particles that are
introduced into the classifier are
accelerated by the classifier's mechanically
driven rotor. The rotor
allows for very high air and particle
acceleration, resulting in high
centrifugal forces. The ability to
disperse the particles through centrifugal
spin is of the utmost importance
in achieving efficient air
classification. The coarse particles
are affected by centrifugal forces
and move in the outward direction.
As they move toward the outer edge
of the vortex, their peripheral velocity
will decrease and gravity will
ChemiCal engineering www.Che.Com august 2014 61
(1)
(2)
http://www.Che.Com
Chemical Engineering August 2014

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