Chemical Engineering November 2010 - 65

Nuisance dust
collector
(optional)
Feed
System vent
System
fan
Air
supply
Classifier
Rotary
airlock
Coarse
Conveyor
Conveyor
FIGURE 2. Internal-fan air classifiers do not require airlocks,
cyclones or baghouses to collect separated fines
particle size, then air classifiers can be
very effective. However, large particles
with low density can have a similar
mass and weight as some small particles
with high density. This can reduce
the effectiveness of an air classifier's
density separation.
Effects of moisture
Moisture effects are limited by surface
moisture, rather than inherent moisture.
Inherent moisture is naturally
found inside particles of ores, minerals
or stone sand after natural drying
occurs in air. Inherent moisture does
not hinder an air classifier's ability to
remove fine powder or fine dust from
coarse particles. For example, crushed
coal is successfully air classified with
inherent moisture as high as 10%.
Surface moisture, on the other hand,
is found on the surface of ores, minerals
or stone sand and comes from
rainfall or from spraying water in an
aggregate plant or quarry during dust
suppression. Surface moisture is detrimental
to the performance of air classifiers
because the fine particles stick
to the large particles and airflow is not
enough to remove them. When surface
moisture is very high, the water also
centrifuges out and results in equipment
clogging.
The performance of air classifiers in
aggregate plants or quarries is limited
by the surface moisture in the stone
sand. The drier the rock is (1-2%), the
more dust can be removed, often alFIGURE
3. External-fan air classifiers use a variable-speed
rotor with multiple, closely spaced rejector blades for ultrafine
applications
lowing air classifiers to replace water
wash systems altogether. When higher
surface moisture is present in stone
sand (2.5-3.0%), the fines stick to the
rock and larger air classifiers are required
with more airflow than usual
to be effective. When the surface moisture
is very high (3.5-4.0% or more),
the water centrifuges out and results
in equipment clogging.
Pneumatic feeding
Air classifiers can be fed pneumatically
and, in some cases, can be incorporated
into a pneumatic conveying
line. However, in a pneumatic feed
process, particles enter the air classifier
at a much higher velocity than
gravity-fed particles. When these particles
approach the classifier rejector
blades at high velocities, they are
more likely to pass through, which
requires a higher rejecter speed to
stop these oversize particles. Higher
rejector speed can then result in
higher wear and lower efficiency in
fines-removal.
Cyclones or baghouses needed?
There are two categories of mechanical
centrifugal air classifiers: Internal-fan
models and external-fan models.
Internal-fan air classifiers recycle
the air, and therefore, do not require
airlocks, cyclones or baghouses to collect
the separated fines. This design
has a single shaft that controls three
rotating elements - the feed distributing
plate, particle-sizing selector
blades and circulating fan (Figure 2).
The feed distribution plate imposes
centrifugal force on the feed
particles, moving them into the classification
zone. Coarse particles fall
down into the inner cone and exit at
the coarse discharge. The circulating
fan creates an upward draft of air
that carries finer particles away from
the feed and through the selector
blades. Properly sized fine particles
pass through the internal fan still
entrained in air. Fixed vanes recycle
the air back into the classifier, while
the properly sized fine particles drop
out of the airflow and slide down the
fines cone, where they exit.
External-fan air classifiers (Figure
3) require cyclones or baghouses to
collect the separated fines. This design
uses a variable-speed rotor with multiple,
closely spaced rejector blades
for ultra-fine and ultra-efficient applications.
The feed distribution plate
imposes centrifugal force on the feed
particles, moving them into the classification
zone. Coarse particles fall
down into the inner cone and exit at
the coarse discharge point. The external
fan creates a draft of air that
carries finer particles away from the
feed and through the rotor. Properly
sized fine particles, entrained in air,
pass through the rotor and exit the
air classifier. A cyclone or baghouse is
required to recover the classified particles
out of the airflow.
ChemiCal engineering www.Che.Com noVemBer 2010 49
Rotary
airlock
Fines
High-efficiency
cyclone
Rotary
airlock
Nuisance
dust
Exhaust
fan
http://www.Che.Com
Chemical Engineering November 2010

Table of Contents for the Digital Edition of Chemical Engineering November 2010
