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Exposure to Silica Dust on Continuous Mining Operations Using Flooded-Bed Scrubbers Description of Hazard

HID 1
Exposure to Silica Dust on Continuous Mining Operations Using
Flooded-Bed Scrubbers
Description of Hazard
Silicosis, a debilitating and potentially deadly lung disease, develops through inhalation of excessive
concentrations of respirable crystalline silica dust. In underground coal mines, continuous miner operators have
a high risk of overexposure to silica, with approximately 25% of compliance dust samples exceeding the
permissible exposure limit. Continuous miners often extract high-silica-content rock from the coal seam or the
surrounding strata. Large quantities of silica dust can be generated during cutting and can become entrained in
the ventilating air, which can carry the dust to the breathing zones of mine workers. Over the last 15 years, the
application of flooded-bed scrubbers (fan-powered dust collectors) on continuous miners has increased as mine
operators expand efforts to reduce airborne dust concentrations. During the last several years, the density of the
filter media in scrubber filter panels has been reduced to improve scrubber airflow. However, the reduction in
filter density suggests that dust collection efficiencies are lower and thus expose mine workers to higher
concentrations of respirable silica dust.
Flooded-bed scrubbers capture dust-laden air from the cutting
face, carry this air through ductwork on the miner, and pass the
air through a filter panel that is wetted with water sprays (see
Figure 1). As dust particles impact and travel through the filter
panel, they mix with water droplets and are removed from the
airstream by a mist eliminator. The cleaned air is discharged
from the scrubber back into the mine environment. The density
and type of media used in a filter panel influence the dust
collection efficiency and air-moving capacity of a scrubber.
Optimum flooded-bed scrubber performance is achieved when
all of the dust-laden air at the cutting face is drawn into the
scrubber and a high percentage (+90%) of the respirable dust is
removed from this air.
Filter media in the original flooded-bed scrubber design
consisted of 40 layers of stainless steel wire mesh secured in a flat filter panel. Panels with pleated filter areas
and reduced filter densities were developed to increase the airflow of fixed-capacity scrubbers. Filters
containing 30, 20, 15 or 10 layers of stainless steel wire mesh were fabricated. The 20-layer panel is the most
common filter now being used. In addition, panels containing filter media other than stainless steel have been
developed and are also being used by the coal mining industry.
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Recommendations For Prevention
A flooded-bed scrubber test facility was
constructed to measure the respirable silica
collection efficiency of 10-, 15-, 20- and
30-layer wire mesh filter panels. Two filter
panels containing alternative filter media
(nonwoven synthetic fiber and an array of
nylon brushes) were also evaluated. Operating
conditions for both low- and high-capacity
scrubbers were evaluated by supplying air
velocities of 2,200 feet per minute (fpm) or
3,500 fpm at the scrubber filter. As shown in
Figure 2, collection efficiencies of 69% to
84% were obtained with the 20-layer filter.
The 30-layer and the synthetic filters had the
densest filter media, which resulted in the best
silica collection. Efficiencies of 91% to 95%
were measured for these panels. The lowest
collection efficiencies, 58% to 76%, were
found with the 10-layer mesh filter. All filter
panels exhibited improved silica collection at
the high scrubber air velocity.
Figure 2.
Respirable silica collection for scrubber filter panels.
Overall performance of a flooded-bed
scrubber depends on the collection efficiency
of the filter panel and the amount of
ventilating air drawn into the unit. Increasing
filter density improved silica collection but
also reduced the quantity of air that is drawn
through the scrubber. The silica collection
efficiency and air-moving capacity of each
filter panel were used to determine the relative
overall performance of the scrubber. Figure 3
shows the respirable silica concentrations
found in the scrubber discharge air with each
filter panel. These results indicate that the
30-layer wire mesh panel and the synthetic
filter were the most effective in reducing silica
dust. Use of the other filter panels resulted in
higher concentrations of dust in the discharge
air, particularly when simulating low-capacity
scrubbers.
Figure 3.
Relative silica concentration in scrubber discharge.
2 On the basis of these findings, NIOSH recommends that the 30-layer stainless steel wire mesh or the synthetic
filter panels be used in flooded-bed scrubbers to improve silica collection. Collection efficiency also improved
when higher air velocities were present in the scrubber, regardless of the filter panel being tested. The 10-layer
wire mesh filter exhibited the poorest performance, and its use should be avoided.
For More Information
Acknowledgments
For additional information about flooded-bed
scrubbers or for information about other
mining-related dust control technology, call the
National Institute for Occupational Safety and
Health (NIOSH), Pittsburgh Research
Laboratory at (412) 892-4213.
August 1997
The principal contributors to this Hazard Identification are
Jay F. Colinet, Mine Safety and Health Division, and
Jerome P. Flesch, Education and Information Division,
NIOSH.
DHHS (NIOSH) Publication No. 97-147
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