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Dust Control Handbook for Minerals Processing |
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Chapter 2: Preventing Dust Formation
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Introduction
In any minerals processing facility, dust is generated when ore is
shattered or broken as in dumping, loading, transferring, or
handling. Proper design, selection, and operation of equipment to
minimize ore breakage can therefore reduce dust.
Two primary groups of equipment are used in minerals processing
operations:
- Processing equipment such as crushers, screens, grinding mills, and
dryers
- Bulk material handling equipment such as belt conveyors, screw
conveyors, bucket elevators, feeders, and hoppers
Processing equipment processes the ore into a final product.
Bulk material handling equipment transfers ore between processing equipment and
from the mine to the processing facility.
Processing equipment must be retrofitted with dust control measures;
dust control measures can be implemented when bulk material handling
equipment is designed. Although the choice of equipment for a
specific operation is based on process needs, the use of alternate
equipment, improved equipment design, or sometimes even a change in
process can greatly reduce dust emissions. Simple measures such as
providing shrouds, covers, or enclosures around a dust source can also
help to contain dust emissions or allow the existing dust control system
to operate more efficiently.
The following sections describe the most commonly used processing and
bulk material handling equipment, their major dust emission points, and
measures to prevent or reduce dust generation, emission, and
dispersion. Several common minerals processing operations are also
discussed.
Belt Conveyors
The belt conveyor is one of the major pieces of equipment used to
transfer ore between processing equipment and from one point to another
within a minerals processing facility. It is also one of the most
frequent sources of dust emissions.
Types of Belt Conveyors
There are two primary types of belt conveyors:
- Troughed belt conveyors
- Flat belt conveyors
Troughed belt conveyors are the most common type of conveyor used in
mining and minerals processing operations. Flat belt conveyors are
used mostly in non-mining industries.
Emission Points
Pelt conveyors emit dust from the following four points:
- The tail end, where material is received
- The conveyor skirting
- The return idlers, due to carry-back of fine dust no the return belt
- The head end, where material is discharged
The following measures should be considered in the design or selection of a belt conveyor.
Dust Prevention Measures
Belt Loading
The amount of dust generated at belt conveyor transfer points depends
on the way the material is loaded onto the belt. To reduce dust
generation-
The material should be loaded onto the center of the belt.
The material and the belt should travel in the same direction and at
the same speed, whenever possible.
Impact at Loading Point
A momentary deflection of the belt between two adjacent idlers may
result when ore strikes the belt. As a result, a puff of dust may
leak out under the skirting rubber seal. To prevent dust emissions
at the loading point, adequately spaced impact idlers (1-ft centers)
should be located at transfer points. These will absorb the force of
impact and prevent deflection of the belt between the idlers, thus
preventing dust leakage under the rubber seal.
Conveyor Skirting
Skirtboards are used to keep the material on the belt after it leaves
the loading chute. They are equipped with flat rubber strips that
provide a dust seal between the skirtboards and the moving belt.
The conventional skirtboard design uses vertical rubber strips.
This design is not recommended for the following reasons:
- The vertical rubber seals wear out quickly.
- The rubber must be adjusted constantly to prevent dust leakage, and this is often neglected.
Illustrated is an improved skirting design, which has the following important features:
- The skirtboards are sufficiently high and wide to accommodate both the volume of material and the pressure surges
caused by the inflowing material and induced air.
- Quick-disconnect clamps are used instead of conventional bolts for
fast, easy adjustment of the rubber.
- The flexibility of the inclined skirting rubber allows it to rest on
the moving belt at all times, even when the belt is momentarily
deflected between idlers.
- The inclined skirting rubber is 1/2 in. thick with 60-65 durometer
hardness and provides a greater wear area for increased life.
- The top edges of the skirtboards are covered and sealed with a
self-adhesive neoprene rubber gaskets for a proper dust seal.
A double skirting can be installed in the impact zone if the incoming
material hits the skirting rubber directly, increasing the wear rate.
Muckshelves
Muckshelves can be installed in the belt conveyor's material impact zone to-
- Load the material centrally on the belt and keep the belt properly aligned
- Protect the inclined skirting rubber from direct impact with the incoming material
Dust Curtains
Dust curtains are used to contain dust within a conveyor
enclosure. They should be installed at the head, tail, and exit
ends. Dust curtains are made of rubber with 60-65 durometer hardness
and can be hinged at the conveyor's head and exit ends to provide easy
access during maintenance.
Belt Cleaners
Belt Scrapers - A belt scraper should be installed at the head pulley
to dislodge fine dust particles that may adhere to the belt surface and to
reduce carryback of fine materials on the return belt. A scrapings
chute should also be provided to redirect the material removed by the belt
scraper into the process stream or container.
V-Plow - Product spillage or dust leakage may fall on the non carrying
side of the belt and eventually build up on the surfaces of the tail
pulley. This buildup may move the belt laterally and thus make the
skirting rubber seals ineffective. A V-plow installed on the
non-carrying side of the belt will clean the belt and prevent buildup of
material and dust on the tail pulley, thus keeping the belt properly
aligned.
| b=Belt Width (in.) |
A |
B |
C |
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| 24 |
19 |
18 |
6 |
| 30 |
24 |
21 |
8 |
| 36 |
29 |
27 |
9 |
| 42 |
36 |
32 |
11 |
| 48 |
41 |
36 |
12 |
Conveyor Capacity
The belt conveyor should be designed to operate at 7% of its full rated
capacity. This reduces spillage, dust emission, and wear on skirting
rubber seals.
The following measures are suggested to adjust the loading capacity of
existing conveyors:
- Increase the belt speed
- Change the idlers' angle (for example, from 20 to 35 )
- Increase the conveyor width (for example, from 24 to 36 in.)
Transfer Chutes
Transfer chutes transport ore from one piece of equipment to
another. Significant dust generation can result if the transfer
chute is not designed properly.
Dust Prevention Measures
The following points should be considered when designing a transfer
chute:
- The chute should be big enough to avoid jamming of material and
reduce air flow. When fine material and lumps are mixed in
the product stream, the chute depth should be at least three times the
maximum lump size to avoid jamming.
- The chute should be designed so the material falls on the sloping
bottom of the chute and not on the succeeding equipment.
- Wherever possible, the material should fall on a local rockbox or
stonebox rather than on the metal surfaces to
- Reduce dust and noise generation
- Absorb the impact of incoming material
- Reduce wear and abrasion of the chute surfaces
- Reduce the height of material fall
- Reduce dust emission from the backspill rubber seal at the tail end
of the conveyor
Note: When handling fine or abrasive materials, a number of
small steel angles can be welded on the chute bottom to form mini-rockboxes.
The oncoming material slides on the material stored in the
rockboxes, greatly reducing wear and abrasion of the chute bottom.
- Abrupt changes of direction must be avoided to reduce the
possibility of material buildup, material jamming, and dust
generation.
- Curved, perforated, or grizzly chute bottoms should be used when the
product stream consists of fines and lumps. Placing a layer of
fines ahead of the lumps on the belt helps prevent heavy impact of
material on the belt, which reduces belt wear and dust generation.
Other types of chutes are used for controlling dust during bulk material
handling:
- Spiral chutes are used to prevent breakage of fragile or soft material.
- Bin-lowering chutes are used to feed bins and hoppers without
generating large amounts of dust. These chutes consist of a
channel that runs from the discharging equipment down into the
bin. The channel is secured to the sloping side of the
bin. Ore slides down the chute quietly, with minimum
dusting. When the material meets the big side or the surface of
the material in the bin, it leaves the chute from the sides and
spreads out conically.
- Rock ladders are used to prevent breaking and crumbling of
rock. They consist of a steel tower with a series of baffles (or
mini-rockboxes) arranged so the discharged rock never has a free drop
of more than 5 to 6 ft. This lower drop height significantly
reduces dust.
- Telescopic chutes are used to minimize the height of material fall
into stockpiles. The telescopic sections are usually
cable-connected so a winch can lift the sections of the chute.
The lower end of the chute is always kept just clear of the top of the
stockpile to reduce dust.
Enclosures
Enclosures are used to contain dust emissions around a dust
source. They can also lower the exhaust volume requirements of
a dust collection system or help make the existing dust collection system
work more efficiently.
The following guidelines are suggested when designing an enclosure for
a dust source:
- Enclosures should be spacious enough to permit internal circulation of the dust-laden air.
- Enclosures should be arranged in removable sections for easy maintenance.
- A hinged access door should be provided to aid routine inspection and maintenance.
- Dust curtains should be installed at the open ends of the enclosures to contain dust and reduce air flow.
Crushers
Crushers reduce coarse material to a desired size. The crushing
process uses mechanical energy and rubbing to fracture the rock. The
forces applied to rock fragments during crushing processes are-
- Compression force
- Impact Force
Compression is a slow application of force on the rock while impact is
a short, sudden application of force.
All crushers generate dust. Crushers that primarily use impact
forces produce large amounts of fines and dust. Those that primarily
use compression forces produce dust in proportion to the stage of
reduction: dust production increases progressively from first- to
third-stage crushing.
Types of Crushers
There are six main categories of crushers used in minerals processing operations:
- Jaw crushers
- Gyratory crushers
- Cone crushers
- Hammermills
- Impact breakers
- Roll crushers
Jaw, gyratory, and cone crushers primarily use compression forces;
hammermills, impact breakers, and roll crushers use impact forces.
Emission Points
Crushers emit dust primarily from two points:
- Crusher discharge
- Crusher feed
Dust Prevention Measures
Dust control measures are not usually considered in the design of a
crusher. However, the use of shrouds or enclosures for crushers can
contain the dust so that a dust control system can operate more
efficiently. The following measures are recommended:
- A crusher feedbox with a minimum number of openings should be
installed. Rubber curtains should be used to minimize dust
escape and air flow.
- The crusher should be choke fed to reduce air entrainment and dust
emission.
Dust escape at the crusher discharge end can be minimized by properly
designed and installed transfer chutes.
Screens
Screens are used to sort material according to size. The material
fed into a screen is separated into a least two sizes:
- Undersize material, which passes through the screen opening
- Oversize material, which is retained on the screen surface
Although screening can be either wet or dry, dry screening is most
often used in minerals processing operations.
Dust is generated in all dry screening processes. However, the
amount of dust depends on the particle size contained in the ore, the
moisture content, and the type of screening equipment used.
Generally, a screen processing finer material produces more dust.
Also, screens agitated harder and faster produce more dust than those
vibrated more gently and slowly.
Types of Screens
The four most common types of screening equipment are-
- Grizzlies
- Shaking screens
- Vibrating screens
- Revolving screens
Emission Points
In general, screens emit dust from the following points:
The top one-third of the screen surface where incoming material hits
The openings between moving parts (the screen) and stationary parts
(the discharge chutes)
Discharge chutes
Dust Prevention Measures
The rate of dust generated by screens cannot be altered. However,
properly enclosing the screen can reduce dust emissions. A complete
enclosure that can be easily removed for maintenance and inspection should
be used. One commercially available enclosure system consists of a
special rubber cloth, rubber molding, and simple metal hardware. The
rubber cloth can be attached to the screen to provide an almost perfect
dust seal between the screen and the discharge chutes. The top of
the screen can also be enclosed by the rubber cloth to prevent dust
escape. These lightweight rubber covers not only provide a
dust-tight enclosure but also allow easy maintenance, inspection, and
replacement of screening surfaces.
A tight sealing system reduces dust emissions and also minimizes air
flow, which reduces the exhaust volume for the dust collection system
installed downstream.
Some screen manufacturers provide sheet-metal covers to enclose the top
of the screen. These covers are effective when properly
maintained. However, they do not provide a dust seal between the
moving screen surfaces and the stationary chutes.
Storage Bins and Hoppers
Bins and hoppers are used to store ore temporarily. They act as
buffers to absorb the surge between unloading and consumption.
The ore is fed into the bin by various equipment, such as conveyors,
elevators, and screens. The material is normally discharged from
bins and hoppers through gravity or vibrating feeders.
Emission Points
Bins and hoppers primarily emit dust from-
- Feed openings
- Discharge feeder
- Inspection doors
Dust Prevention Measures
Dust emissions during feeding operations can be minimized by-
- Installing a bin-lowering chute.
- Completely enclosing the bin or hopper. When bins and hoppers
are enclosed completely, an inspection door or a bin-level detector
should be installed so the material level can be monitored.
Dust emissions during material discharge can be minimized by-
- Installing a telescopic chute.
- Installing a loading spout.
Such spouts are sophisticated versions of the telescopic chute and are used to load and stack ore
into bags, trucks, and railroad cars. The spouts apply three basic principles of dust control:
1. Containment
2. Dust capture close to the source
3. Preventing air flow caused by the falling material
The falling material is enclosed by a flexible duct, acting as a
chute, which retracts as the height of the material pile
increases. The duct also prevents air flow during free fall of material between
the chute and stockpile. The generated dust
is captured by the same flexible duct and is conveyed, countercurrent
tot he material flow, to a dust collector.
Another method commonly used to reduce dust emission is to transfer
dusty air through a vent into an adjoining bin or silo. Of course,
this assumes that multiple bins are used, that the bins are totally
enclosed, and that the adjoining bin has room for the displaced air to
expand.
Bucket Elevators
A typical bucket elevator consists of a series of buckets mounted on a
chain or belt that operates over head and foot wheels. A steel
casing usually encloses the entire assembly. The buckets are loaded
by scooping up material from the boot (bottom) or by feeding material into
the buckets. Material is discharged as the bucket passes over the
head wheel.
Types of Bucket Elevators
- Centrifugal discharge
- Positive discharge
- Continuous discharge
Emission Points
Pucket elevators emit dust from two points:
- The boot, where material is fed
- The head wheel, where material is discharged
Dust Prevention Measures
The steel casing that encloses the buckets and chain assembly contains
dust effectively unless there are holes or openings in the casing.
Emissions at the boot of the bucket elevator can be reduced by proper
design of a transfer chute between the feeding equipment and the
elevator. Dust production can be reduced significantly by keeping
the height of material fall to a minimum and by gently loading material
into the boot of the elevator.
Proper venting to a dust collector, as well as proper enclosures and
chutes between the elevator discharge and the receiving equipment, will
control dust emission at the discharge end of the bucket elevator.
Feeders
Feeders are relatively short conveyors used to deliver a controlled
rate of ore to the processing equipment.
Although dust is emitted from all types of feeders, the amount of dust depends on-
- The kind of material being handled
- The size of material
- The degree of agitation of the material
Types of Feeders
- Apron
- Belt
- Reciprocating
- Vibrating
- Disc
Emission Points
Dust emission points from a feeder are-
- At the tail end, where material is received
- At the head end, where material is discharged
Dust Prevention Measures
Reduce dust production during feeding operations by-
- Enclosing feeders as much as possible
- Selecting an oversize feeder or a feeder that produces less agitation of the ore
Screw Conveyors
Screw conveyors are one of the oldest and simplest types of equipment
used to move ore. They consist of a conveyor screw rotating in a
stationary trough. Material placed in the trough is moved along its
length by rotation of the screw.
Emission Points
Screw conveyors emit dust primarily from-
- The inlet, where material is received
- Leaks in the trough cover
- Worn-out troughs
Dust Prevention Measures
Normally, screw conveyors are totally enclosed except at the ends,
where emissions can be controlled by proper transfer chute design.
The trough cover is usually fastened by nuts and bolts. However,
to maintain a proper dust seal, a self-adhesive neoprene rubber gasket
should be installed. Many manufacturers provide two-bar flanges and
formed-channel cross members that make a continuous pocket around the
trough. The flange-cover sections are set in this channel.
Once the channel section is filled with dust, an effective dust seal is
created.
Abrasive materials can wear out screw conveyor troughs quickly unless a
special coating or abrasion-resistant material is used for the trough.
Pneumatic Conveyors
Pneumatic conveyors are tubes or ducts through which material is moved
by pressure or vacuum (suction) systems.
Pressure systems can be either high or low pressure. Low-pressure
systems operate at pressures obtainable from a fan; high-pressure systems
use a compressed-air source. When material is fed into a pressure
system, the airstream immediately suspends is and conveys it to a cyclone-
or filter-type collector. The conveying air then escapes through the
cyclone vent or a filter.
Vacuum systems offer clean, efficient pickup of material from rail
cars, trucks, or bins and hoppers for unloading into other types of
equipment. Cyclone receivers or filters are used at the end of such
systems to separate the material.
 
Emission Points
Since pneumatic systems are totally enclosed, dust emissions do not
usually occur unless the system has work-out areas.
Dust Prevention Measures
Because maximum wear in the conveying ductwork occurs at elbows, long
radius elbows made of heavy gauge material should be used. The
elbows can also be lined with refractory or ceramic material to further
reduce the war and abrasion.
In low-pressure pneumatic systems, dust may leak through joints.
Self-adhesive neoprene gaskets should be used at all joints to provide a
dust-tight seal.
Grinding Mills
Grinding and pulverizing reduce ore to a desired fineness for further
treatment.
In its basic form, a grinding mill consists of a horizontal,
slow-speed, rotating cylindrical drum. Rod, pebble, and ball mills
are the most common types of grinding mills used in minerals processing
operations. Steel rods, balls, or pebbles roll freely inside the
drum during rotation to provide the grinding action.
Emission Points
Dust is emitted from a grinding mill-
- At the inlet, where material is fed
- At the outlet, where material is discharged
Dust Prevention Measures
Most grinding mills are fed by a belt conveyor or a feeder. The
ground ore is discharged to a screen, conveyor, or elevator.
Properly designed enclosures and chutes, as well as rubber dust seals
between moving and stationary components, should be installed at the feed
and discharge ends to minimize dust emissions.
Dryers
Dryers remove water or other volatile material from solid substances
primarily by introducing hot gases into a drying chamber. The hot
air readily absorbs moisture from the material.
Rotary dryers, flash dryers, spray dryers, and tray and compartment
dryers are a few of the many types of dryers. However, in minerals
processing operations, rotary dryers are the most commonly used.
Rotary dryers consist of a horizontally inclined rotating
cylinder. Material is fed at one end and discharged at the other
end. In direct-type rotary dryers, hot gases move through the
cylinder in direct contact with the material, either with or against the
direction of its flow. The cylinder is equipped with flights, which
lift the material and shower it down through the hot gas stream. This type of dryer has a potential for high levels of dust emission. In an indirect-type rotary dryer, heat is applied by combustion gases on
the outside of the cylinder or through steam tubes inside the
cylinder. This type of dryer has much less tendency to emit
dust. It is usually used when continuous drying of powdery or fine
material is needed.
Emission Points
Dust emission can be a problem in any dryer in which material is
agitate or stirred. Dust is emitted only from the discharge end.
A study on rotary dryers by the Barber Greene Company concluded that
the dust carryout increased proportionately to the square of the exhaust
gas volume.
Dust Prevention Measures
The hot, dust-laden gases from the dryer are carried to a dust
collector, and dust normally does not escape unless the ventilation system
is work out or improperly maintained.
Stockpiles
Large volumes of processed material are stored in open or enclosed
stockpiles. Open stockpiles are normally used when the material size
is large. Enclosed stockpiles are used when material is either very
fine or must be stored dry. Stockpiles are considered active when
material is continuously removed or added. They are considered
inactive when material is not added or withdrawn for long periods. All types of stockpiles can be significant dust source.
Emission Points
Generation of dust emissions from stockpiles is due to-
- The formation of new stockpiles
- Wind erosion of previously formed piles
During formation of stockpiles by conveyors, dust is generated by wind
blowing across the stream of falling material and separating fine from
coarse particles. Additional dust is generated when the material
hits the stockpile.
Dust Prevention Measures
Dust from stockpiles can be reduced through the following measures:
- Minimizing height of free fall of material and providing wind protection using-
- Stone ladders, which consist of a section of vertical pipe into
which stone is discharged from the conveyor. At different levels,
the pipe has square or rectangular openings through which the material
flows to form the stockpile. In addition to reducing the height of
free fall of material, stone ladders also provide protection against
wind.
- Telescopic chutes, in which the material is discharged to a
retractable chute. As the height of the stockpile increases or
decreases, the chute is raised or lowered accordingly. Although
some free fall of material from the end of the chute to the top of the
stockpile occurs, proper design of the chute can keep the drop to a
minimum.
- Stacker conveyors, which operate on the same principle as
telescopic chutes. The conveyor has an adjustable hinged boom that
raises or lowers it according to the height of the stockpile.
- Minimizing wind erosion of the stockpile by-
- Locating stockpiles behind natural or manufactured windbreaks
- Locating the working area on the leeward side of the active piles
- Covering inactive piles with tarps or other inexpensive materials
- Minimizing vehicle traffic on or around the stockpile
- Using specialized equipment such as a reclaimer to minimize the
disturbance of the stockpile or providing a tunnel underneath to
reclaim the material.
Haul Roads
Haul roads are used in minerals processing operations to transport ore
from the quarry to the processing plant, as well as within the
facility. Large amounts of dust can be generated during this
process.
Emission Points
Dust emissions from hauling operations vary, depending on-
- The condition of the road surface
- The volume and speed of vehicle traffic
Dust Prevention Measures
Dust emissions from haul roads can be minimized by-
- Spraying the soil frequently with water, chemicals, oil, or other stabilizing agents
- Paving the haul roads (Note: Paved roads should be cleaned and
vacuumed periodically to remove accumulated soil and dust.)
- Reducing traffic volume by replacing small haul vehicles with larger ones.
- Reducing and strictly enforcing traffic speed
Truck and Railroad Car Dumping
Ore is dumped or unloaded from trucks or railroad cars in most minerals
processing facilities.
Emission Points
Dust generation and emission during the dumping of ore are caused by-
- Dumping large volumes of material in a relatively short time (3 to
10 seconds), which displaces an equal volume of air carrying fine dust
particles
- Wind spreading the dust
Dust Prevention Measures
Enclosures should be used to contain dust during dumping. A
sufficiently large enclosure will contain most of the dust, as well as aid
internal recirculation of the dust-laden air.
Enclosures for railroad- car dumping operations must have openings at
each end to allow cars to enter and exit. However, these openings
can crate a wind-tunnel effect. To minimize this effect, the size of
these openings should be kept as small as possible with shrouds, rubber
curtains, etc. Shrouds or rubber curtains should also be used for
other types of enclosures to reduce the area through which air can escape
or enter.
In addition to the above design guidelines, the following operational
measures are suggested:
- Increase the amount of open area through which the material
flows. This measure will reduce the escape of dust through
unused areas.
- Decrease the amount of open area through which the material
flows. This measure will reduce the escape of dust through
unused areas.
Power Handling and Packing
Pulverized material, such as sand, silica flour, hydrated lime, or
other powdery material, is normally shipped either in bulk quantities
through truck or railroad cars or in small volumes using paper bags,
drums, or barrels. Although the process of filling a bag, drum, or
barrel is simple, packing fine material can be an extremely dusty
operation.
Two types of mini-bulk-packing processes commonly used in minerals processing are-
- Bagging
- Barrel or drum filling
Bagging
This process uses bagging machines commonly known as packers. Several different types of packers, such as fluidized auger or screw type,
belt or sling type, drop type, and impeller type, are available. However, the two most commonly used packers in the minerals processing
industry are-
- Open-mouth (drop-type) packers using sewn or adhesive-sealed bags
- Spout fluidizing packers used with seal-valve bags
The open-mouth type uses gravity to fill sewn or adhesive-sealed bags.
The material is fed into a weigh hopper, and when the correct
weight is reached, the feed gate closes and the contents drop into the
open-mouth bag.
The spout fluidizing packer uses compressed air to force the material
through a nozzle into a paper bag equipped with a seal valve. When
the desired weight is reached, the compressed air supply is cut off and
the feed stopped. The internal pressure of the contents of the bag
then forces the valve to close.
Barrel or Drum Filling
Sometimes the final product is shipped in barrels, drums, or
containers. The material is fed into the receptacle by gravity.
Emission Points
Considerable amounts of dust may be emitted during bagging
operations. The following are common occurrences:
- Dust emitted while filling the bag. Material and dust
spillage may occur because of the compressed air used to pressurize
the bag.
- Bag surface dust. The dust emitted during bagging
extremely fine products may stick to the outer surfaces of the
bag. This dust may become airborne during later handling,
conveying, loading, or stacking of bags. Unfortunately, attempts
to clean the bags by air brushing have been unsuccessful.
- Poor bag quality. Poor bag quality or improper storage
may result in broken bags, leaky valves, or weak bag seams.
Leaks from the valve and poorly glued seams may be continuous source
of dust as the bag is moved from the bagging machine to the loading
point.
- Spillage from nozzles. Spillage from the nozzle of
spout fluidizing packers is another major dust source. The bag
is usually discharged from the machine immediately after the flow of
product into the bag is stopped. However, fluidized material
remaining in the nozzle area continues to discharge, which results in
spillage. This spillage may also prevent the bag valve from
sealing properly, which may cause another dust source as the bag is
handled.
Dust generation and emission during barrel- or drum- filling operations
result primarily from-
- Displaced air carrying fine dust
- Spillage of material during the filling process and subsequent
handling of drums and barrels
Dust Prevention Measures
Dust emissions caused by bagging machines can be minimized in the
following ways:
- The bag should be properly attached to the spout to reduce dust
leakage or material spillage.
- Where possible, the bag's outer surfaces should be wetted to prevent
the surface dust from becoming airborne.
Dust emissions due to poor bag quality can be minimized by
- Following proper bag specifications
- Eliminating sharp obstructions during bag-handling processes
- Using proper compressed-air pressures to fill the bag, which can
significantly reduce the number of broken bags
- Storing bags at recommended temperatures and humidity levels to
prevent drying and cracking
In the case of spout fluidizing packers, the following additional
measures are suggested:
- The fluidized air in the bag should be vented to control dust
emissions during filling. This can be done by using bags with
perforations staggered through the layers of bag paper. This
eliminates a direct path for the product and reduces dust spillage,
while maintaining adequate air relief through the bag. The
porosity of the paper further aids air relief and has a filtering
effect.
- The nozzle should be cleaned before the bag is released from the
machine to prevent spillage of material from the nozzle.
Cleaning is accomplished by injecting a short, high-velocity,
low-volume blast of compressed air at the rear of the nozzle to
fluidize the remaining material in the nozzle and force it into the
bag. (This procedure is useful only for fine granular material,
such as whole grain sand. For finely ground material such as
ground silica, a specialized nozzle developed by the Bureau of Mines
may be used.)
- Dust generation and emission during drum or barrel-filling operations
can be minimized by-
- Providing a cover with a flexible chute attached to the storage bin
- Enclosing the operation as much as possible to contain the dust
- Reducing the rate of discharge of the material
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