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Dust Control Handbook for Minerals Processing |
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Chapter 1: Dust and Its Control
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What Is Dust?
 Dust consists of tiny solid particles carried by air currents. These articles are formed by a disintegration or fracture process, such as
grinding, crushing, or impact. The Mine Safety and Health
Administration (MSHA) defines dust as finely divided solids that may
become airborne from the original state without any chemical or physical
change other than fracture.
A wide range of particle size is produced during a dust generating
process. Particles that are too large to remain airborne settle
while others remain in the air indefinitely.
Dust is generally measured in micrometers (commonly known as
microns). Some common objects and their size in microns are listed
below.
| |
|
µm |
|
|
|
| |
Red blood corpuscles |
8 |
| |
Human hair |
50-75 |
| |
Cotton fiber |
15-30 |
How Is Dust Generated?
Dust is generated by a wide range of manufacturing, domestic, and
industrial activities. Construction, agriculture, and mining are
among the industries that contribute most to atmospheric dust levels.
In minerals processing operations, dust is emitted-
- When ore is broken by impact, abrasion, crushing, grinding, etc.
- Through release of previously generated dust during operations such as loading, dumping, and transferring
- Through recirculation of previously generated dust by wind or by the movement of workers and machinery
The amount of dust emitted by these activities depends on the physical
characteristics of the material and the way in which the material is
handled.
Types Of Dust
Fibrogenic dust, such as free crystalline silica (FCS) or asbestos, is
biologically toxic and, if retained in the lungs, can form scar tissue and
impair the lungs' ability to function properly.
Nuisance dust, or inert dust, can be defined as dust that contains less
than 1% quartz. Because of its low content of silicates, nuisance
dust has a long history of having little adverse effect on the
lungs. Any reaction that may occur from nuisance dust is potentially
reversible. However, excessive concentrations of nuisance dust in
the workplace may reduce visibility (e.g., iron oxide), may cause
unpleasant deposits in eyes, cars, and nasal passages (e.g., portland
cement dust), and may cause injury to the skin or mucous membranes by
chemical or mechanical action.
From an occupational health point of view, dust is classified by size
into three primary categories:
- Respirable Dust
- Inhalable Dust
- Total Dust
Respirable Dust
Respirable dust refers to those dust particles that are small enough to
penetrate the nose and upper respiratory system and deep into the
lungs. Particles that penetrate deep into the respiratory system are
generally beyond the body's natural clearance mechanisms of cilia and
mucous and are more likely to be retained.
MSHA defines respirable dust as the fraction of airborne dust that
passes a size-selecting device, having the following characteristics:
| Aerodynamic diameter |
Percent |
| µm |
passing |
| (unit density spheres) |
selector |
|
|
| 2.0 |
90 |
| 2.5 |
75 |
| 3.5 |
50 |
| 5.0 |
25 |
| 10.0 |
0 |
Inhalable Dust
The EPA describes inhalable dust as that size fraction of dust which
enters the body, but is trapped in the nose, throat, and upper respiratory
tract. The median aerodynamic diameter of this dust is about 10 µm.
Total Dust
Total dust includes all airborne particles, regardless of their size or
composition.
Why Is Dust Control Necessary?
Although unavoidable in many minerals processing operations, the escape
of dust particles into the workplace atmosphere is undesirable.
Excessive dust emissions can cause both health and industrial problems:
- Health hazards
- Occupational respiratory diseases
- Irritation to eyes, ears, nose and throat
- Irritation to skin
- Risk of dust explosions and fire
- Damage to equipment
- Impaired visibility
- Unpleasant odors
- Problems in community relations
Of greatest concern is the health hazard to workers who are excessively
exposed to harmful dusts. In order to evaluate the severity of
health hazard in a workplace, the American Conference of Governmental
Industrial Hygienists (ACGIH) has adopted a number of standards, commonly
known as threshold limit values (TLVs). These values are used as
guides in the evaluation of health hazards. TLVs are time-weighted
concentrations to which nearly all workers may be exposed 8 hours per day
over extended periods of time without adverse effects. MSHA uses
these TLVs for health hazard evaluation and enforcement.
Health Hazard Factors
Not all dusts product the same degree of health hazard; their
harmfulness depends on the following factors:
- Dust composition
- Chemical
- Mineralogical
- Dust concentration
- On a weight basis: milligrams of dust per cubic meter of air (mg/m3)
- On a quantity basis: million particles per cubic foot of air (mppcf)
- Particle size and shape
- The particulate size distribution within the respirable range
- Fiberous or spherical
- Exposure time
Excessive or long-term exposure to harmful respirable dusts may result
in a respiratory disease called pneumoconiosis. This disease is
caused by the buildup of mineral or metallic dust particles in the lungs
and the tissue reaction to their presence. Pneumoconiosis is a
general name for a number of dust-related lung diseases. Some types
of pneumoconiosis are:
- Silicosis - Silicosis is a form of pneumoconiosis caused by
the dust of quartz and other silicates. The condition of the
lungs is marked by nodular fibrosis (scarring of the lung tissue),
resulting in shortness of breath. Silicosis is an irreversible
disease; advanced stages are progressive even if the individual is
removed from the exposure.
- Black Lung - Black lung is a form of pneumoconiosis in which
respirable coal dust particles accumulate in the lungs and darken the
tissue. This disease is progressive. Although this disease
is commonly known as black lung, its official name is coal worker's
pneumoconiosis (CWP).
- Asbestosis - Asbestosis is a form of pneumoconiosis caused by
asbestos fibers. This disease is also irreversible.
How Is Dust Controlled?
Dust control is the science of reducing harmful dust emissions by
applying sound engineering principles. Properly designed, maintained, and
operated dust control systems can reduce dust emissions and, thus
workers' exposure to harmful dusts. Dust control systems can also
reduce equipment wear, maintenance, and downtime; increase visibility; and
boost employee morale and productivity.
Reducing employee exposure to dust can be accomplished by three major steps:
- Prevention
- Control Systems
- Dilution or Isolation
Prevention
The saying "prevention is better than cure" can certainly be
applied to the control of dust. Although total prevention of dust in
the bulk material handling operation is an impossible task, properly
designed bulk material handling components can play an important role in
reducing dust generation, emission, and dispersion.
Control Systems
After all the necessary preventive measures have been adopted, the dust
still remaining in the workplace can be controlled by one or more of the
following techniques: dust collection systems, wet dust suppression
systems, and airborne dust capture through water sprays.
Dust Collection Systems
Dust collection systems are industrial ventilation principles to
capture airborne dust from the source. The captured dust is then
transported to a dust collector, which cleans the dusty air.
Wet Dust Suppression Systems
Wet dust suppression systems use liquids (usually water) to wet the
material so that it has a lower tendency to generate dust. Keeping
the material damp immobilizes the dust, and very little material becomes
airborne.
Airborne Dust Capture Through Water Sprays
This technique suppresses airborne dust by spraying fine droplets of
water on the dust cloud. The water droplets and dust particles
collide and form agglomerates. Once these agglomerates become too
heavy to remain airborne, they settle from the air stream.
Dilution-Isolation
Dilution Ventilation
This technique reduces the dust concentration in the area by diluting
the contaminated air with uncontaminated fresh air. In general,
dilution ventilation is not as satisfactory for health hazard control or
dust collecting systems; however, it may be applied in circumstances where
the operation or process prohibits other dust control measures.
Isolation
Isolation is another means to protect workers from exposure to harmful
dust. In this technique, the worker is placed in an enclosed cab and
supplied with fresh, clean, filtered air.
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