Chemical Engineering November 2016 - 54

Dust Control Technology
tion levels for typical materials.
Many suppliers offer spray bars
and other components aimed at
large-scale saturation. These are
normally simple systems with little
if any control, aimed at flooding the
material in order to keep the dust
down. At times these systems are
updated to provide some customizable
control in an effort to minimize
saturation. Often, these plain water
systems are not sufficient to effectively
control the dust.
FIGURE 11. Atomized mist is one of the few techniques able to control both surface dust and airborne
particulate matter
Otherwise, the material collected by
the central collector requires some
method of disposal, increasing operating
costs.
A better alternative to central collection
for many operations is the
integrated air cleaner, designed to
individually collect dust at the source
by employing a series of smaller,
independently operating units (Figure
6). Unlike central systems, particles
are not stored. The blower
pulls dust-laden air through the filter,
where particles are deposited on the
filter media. Each particle is pulled
into other particles already on the filter,
where they agglomerate into a filter
cake. This cake of larger, heavier
agglomerated particles is periodically
discharged back into the material
stream.
These compact units need only to
contain the blower and the required
filter media, and are designed to be
placed on top of (or close to) the
conveyor system or other pickup
point where dust control is desired.
These integrated systems do not
need to be located outside, as they
do not store dust.
Integrated units pull air from the
settling zone with a blower sized to
remove the excess air from the enclosure.
No ducting means less pressure
drop, reducing overall power
requirements and operating costs.
These integrated air-cleaner designs
feature a pulse-cleaning system,
which uses a short burst of air sent
back through the filter to dislodge
accumulated solid material. Filter
changes are a no-tool procedure
from the clean side of the dust collec54
tor.
These cleaners are easy to install
and maintain, eliminating many of the
issues experienced when using central
collection systems.
Controlling particle size
Air cleaning devices deliver mechanical
agglomeration of dust particles
into heavier particles. Another
method utilized to increase the mass
of dust particles is to add moisture.
Moisture addition increases the
weight and cohesiveness of particles,
thereby creating larger, heavier
particles that are prevented from
ever becoming dust (Figure 7).
Water addition. Water is the most
common and least expensive process
additive to prevent dust. It can
be added in numerous ways, but a
critical question is this: how much
moisture can the material tolerate?
Unfortunately, there's no universal
answer. Every process, every
material, even different loads of the
same material, can exhibit different
characteristics. Adding a known
percentage of moisture may be an
effective solution at a given site,
while a relatively minor fluctuation
could result in significant changes
in physical properties, leading to
thermal penalties, plugged chutes
or quality loss.
The one property solid materials
have in common is a minimum and
maximum moisture content for dust
prevention. Experienced materialhandling-equipment
manufacturers
often have sophisticated testing facilities
to investigate the physical properties
of a material, allowing them to
determine optimum moisture-addiIn
addition, many materials have a
tendency to repel plain water or combine
poorly with it. This often leads to
excess moisture being added to the
process, creating a mess in the plant
and increasing the costs associated
with handling and cleaning up the
material. This excess moisture leads
to additional material carryback,
plugged chutes, belt tracking problems
and housekeeping issues.
Dust-control formulations. In order
to regulate the moisture addition required
to control the dust, chemical
formulations have been engineered
to provide optimum results. Many
of these dust-control products offer
some combination of wetting agents,
binding agents or crusting agents to
improve performance. These additives
improve the ability of the water
droplet to combine with material particles,
thus increasing the mass of
the particles. Dust-control additives
increase particle cohesiveness in
order to get the particles to combine
more readily, improve the longevity of
those bonds, and create a seal that
enhances the long-term effects of
the product.
The individual characteristics of
each application present specific
needs that can be met through a
combination of commercially available
surfactants and tackifiers. The
right match of additives and application
delivers effective control for the
material handling circuit. Many applications
can handle several transfers,
while preventing dust from being created
during stack-out and storage.
Spray techniques. These products
can usually be applied as a spray or
as a foam. Proper application enhances
their effectiveness by ensuring
that they contact as much of the
bulk material as possible. Top-coating
is a common practice and is often
less effective than alternate applicaCHEMICAL
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Chemical Engineering November 2016

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