MOSET

HID1

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

Description of Haz.

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-aden air at the cutting face is
drawn into the scrubber and a high percentage
(+90%) of the respirable dust is removed from
this air.

Figure 1. Continuous miner with flooded-bed scrubber.

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.

% U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
f Public Health Service
z Centers for Disease Control and Prevention

National institute for Occupational Safety and Health Conran £9 ete, CONTROL


Recommendations for Preventi

A flooded-bed scrubber test facility was constructed
to measure the respirable silica collection efficiency
of 10-, 15-, and 30-layer wire mesh filter panels.
Two filter panels containing altemative 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.

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, particulary when simulating
low-capacity scrubbers.

Collection Efficiency, %

bo
1O-ayer {Slayer 20-layer Brush Synthetic 30-la
Filter Panel

Figure 2. Respirable silica collection for scrubber filter panels.

‘Scrubber Airflow
High

HB Low

08

06

04

02

Relative silica concentration

10-layer 15layer 20-layer Brush Synthetic 30-layer

Figure 3. Relative silica concentration in scrubber discharge.

On the basis of these findings, NIOSH recommends that the 30-layer stainless stee! 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

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.

Acknowledgements

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.