6 100
wal I,
*% sol. do
Ft Seine |g
§ ob y Jak
y
IA 7
oe 40
a ar a a
DRILL NUMBER
Figure 1. Rotary Drill Dust Summary
os 100
ost Mey 7, 4 30
TZ oust cob Y
~ caf SBimn cn 1 A Jet
op Dee Ao y8
te: y y
. y
Z
y
Vs

DOZER NUMBER
(Note hot bulldorer No. 2 wos not sampled ding
‘dia? control ostesements)

Figure 2. Bulldozer Dust Summary

2 shows the ARD levels above the bulldozer tracks, ARD
levels inside the enclosed cab, and the relative percentage
decrease in ARD levels between the outside and inside of the
cab. Again, multiple-shift dust concentrations for several of
these bulldozers (see Table 2) are represented as time-
weighted averages in the graph. The dust levels generated by
these bulldozers were one order of magnitude lower than the
levels generated by the drills. Although the enctosed cabs on
the bulldozers provided a more inconsistent percentage
reduction in relation to the dust levels generated (between 44
and 100 %), dust levels inside these cabs on average were all
below 0.20 mg/m’. During several of the individual sampling
shifts (see Table 2), the dust levels inside the enclosed cab
were similar or higher than these measured outside the cab,
suggesting that some of the enclosed cab dust may be caused
by re-entrainment from within the cab.

Typically, the silica content of the ARD during these field
surveys was much higher than 5%; therefore, silica dust
exposure is a concern for these highwall drill and bulldozer
operators. Over two-thirds of the ARD measurements met this
study’s silica analysis criteria and are reported in Table 2.
Most of the nonreportable silica content had ARD sample
concentrations less than or equal to 0.1 mg/m’, The reportable
results indicate that nearly three-fourths of the ARD samples
had greater than 20% silica content. A higher percentage of
silica dust was commonly observed near the dust generation
source as compared to the operator’s cab.

Figure 3 shows the average percentage of silica content for
the subset of jointly reported data inside the operator’s cab and
at the generation source for the highwall drili and bulldozer.
As this graph shows the average silica content of the dust
inside the enclosed operator’s cab was less than at the
generation source, especially for the bulldozer. These
differences were significant at the 88% confidence level for

Key
ZZ Outside cab

—

8 8

2
é
T

8.8
T

SILICA CONTENT, pet

6
T

°

ROCK DRILL BULLDOZER

Figure 3. Silica percentages inside and outside the cab

the highwall drill and the 99% confidence level for the
bulldozer, for a one-tail t-test statistic. However, this finding
was unexpected given that prior highwall drill dust studies
have shown that dry or wet control methods reduce the ARD
concentrations, but do not notably change the silica content of
the dust (Organiscak and Page, 1996; Organiscak et al., 1990).
Dust collector reductions achieved at three of the drills during
this study showed that silica percentage in the dust remained
similar for-2 out of the 3 drills studied (drills 1 and 3 in Table
3). Drill 5 had both the dust levels and sitica percentages
reduced, but the suspected reason for the silica content
reduction in the dust is that drilling took place at two very
different areas of the drill bench during these dust
measurements (see Table 3). The notable silica content
differences observed inside and outside the enclosed cabs
during this study are presumed to be due to some of the ARD
present inside these cabs was likely generated by another
lower-silica dust source from within the cab, Dried mud.
and/or dirt buildup was observed on the floor in many of the
cabs and may have been re-entrained by operator movement
and recirculating airflow in the cabs.

These field assessments demonstrate that in order to
achieve low equipment operator exposure to silica dust, all the
engineering controls require quality upkeep with good
operating practices followed. The ARD concentration inside
the operator cabs had a significant positive correlation, at the
99% confidence level, with the ARD concentration at the
generation source (data from Table 2). This positive ARD
correlation explained 54% (correlation squared, r?) of the dust-
level variation between the enclosed operator cab and the
generation source. Since both of these dust levels are directly
related, all the engineering controls need to be operating
effectively to abate operator dust exposure. A particular
example of this dust control interrelationship can be seen by
examining the data from drill 3 at mine C (shown in Table 2).
Dust levels measured next to the shroud and inside the
enclosed cab were similarly reduced by 83% due to the repair
of the collector shroud between working shifts,

Poor operating practices by the equipment operator could
also be detrimental to the cab’s protection from high dust
levels. Several examples of how operator practices can affect
dust levels inside the cab can be seen from MiniRAM data
presented in Figure 4.’ The operator of drill 4 opened the cab
door to collar the next hole, sometimes letting notable
amounts of dust into the cab. Although the operator shut the
cab door during the drilling operation, the air filtration system
consistently took about 7 minutes to adequately remove the
airborne dust that was let into the cab, The other drill
operators (drills 5 and 6)-collared the next hole without
opening the cab door. Better work practices by these
operators helped to maintain lower dust levels in the operator


drill stem and fixed drill table surface. Enclosed cab usage on
the drills maintained dust levels significantly lower than at the
generation source. Dust levels in the operating cabs were
more than 90% lower than dust levels next to the drill table
shroud. The bulldozers generated dust one order of magnitude
lower than the drills, and dust levels inside the enclosed cabs
were consistently maintained under 0.2 mg/m’. Long-term
sampling of several of these highwall drills and bulldozers
showed that the dust levels in drill cabs were frequently higher
than 0.2 mg/m’ and more variable than in the bulldozers. The
bulldozer dust levels infrequently exceeded 0.2 mg/m’. The
higher and more variable dust levels in the drill cabs were
likely caused by drill dust collector performance variations
and/or occasional opening of cab doors during drilling.

Silica dust was a concern for the equipment operators at
these surface mines. Over two-thirds of the dust samples had
greater than 20 % silica content. Silica content measured
inside the enclosed cabs was noticeably lower than at the
source of generation outside the cabs for both the highwall
drills and bulldozers. The authors postulate that this
difference in silica content may be due to another source of
dust generation from inside the cab. Mud and dirt were found
on the floor in many of the enclosed operator cabs and were
likely entrained into the recirculating airstream of the cab.
‘The long-term silica content in the dust was quite variable
between operating shifts, with average silica differences
evident between mines, and silica conformity evident between
some of the equipment operated at the same mine. One of the
most significant differences in percent silica content of the
dust was measured between two distinctly different highwall
drills operating at the same mine. This result was surprising
and suggests that some drill design and operating parameters
may affect silica dust generation.

Future research work on silica dust abatement at surface
mining operations should focus on improving some of the
deficiencies present in the drill’s primary dry dust collection
system and developing quality contro! methods to ensure the
integrity of enclosed cab protection for equipment operators.
The major weakness observed with the drill’s primary dry dust
collection system is dust containment and capture above the
collared hole. The collector inlet is commonly located on a
remote side of the shrouded drill table to avoid large drill
cuttings from being drawn into the collector, Any gaps
underneath or in the shrouded drill table permit the fine dust to
escape the remotely located collector inlet, which is multiple-
duct diameters away from most of the shrouded volume.
Development of an improved shroud and/or a drill cuttings
pre-separator collection hood-centrally located over the drill
hole, is needed to improve the containment and inlet capture
over a greater area under the shroud. Enclosed cab air
filtration systems were found to be very effective at
maintaining low dust levels, but quality control methods need
to be developed to quickly identify when these systems are not
being operated under optimal conditions. The essential
operating parameters of these cabs must be identified with the
development of measuring criteria to reliably optimize their
effectiveness.

REFERENCES

Anderson CC (1983). Collaborative tests of two methods for
determining free silica in airborne dust. NIOSH and
USBM Contract Final Report, SRI International, Contract
No.210-79-0059, DHHS (NIOSH) Pub. No. 83-124,157 pp.

Cecala AB, McClelland J and Jankowski RA (1988).
Substantial time savings achieved through computer dust
analysis. American Industrial Hygiene, Vol. 3, No: 7, pp.
203-206.

Maksimovic SD and Page JS (1985). Quartz dust sources
during overburden drilling at surface coal mines. Bureau of
Mines Information Circular, IC 9056, 7 pp.

Mine Safety and Health Administration: MSHA Standard
Method No. P-7, Infrared determination of quartz in
respirable coal mine dust. International Document (1989).

National Institute for Occupational Safety and Health
(NIOSH) (1996). Work-related lung disease surveillance
report 1996. DHHS (NIOSH) Pub. No. 96-134, 447 pp.

National Institute for Occupational Safety and Health
(NIOSH) (1992). Request for assistance in preventing
silicosis and deaths in rock drillers. NIOSH ALERT,
DHHS (NIOSH) Pub. No. 92-107, 15 pp.

Onganiscak, JA. and Page SJ (1996). Assessment of airborne
dust generated from small truck-mounted rock drills.
Bureau of Mines Report of Investigations, RI 9616, 11 pp.

Organiscak JA, Page SJ, and Jankowski RA (1990). Sources
and characteristics of quartz dust in coal mines. Bureau of
Mines Information Circular, IC 9271, 21 pp.

Organiscak JA, Williams KL and Ozanich T (1986).
MINIRAM performance in the coal mining environment,
Published in the Proceedings of the International
Symposium on Respitable Dust in the Mineral Industries,
The Pennsylvania State University, University Park, PA,
October 14-16, 1986, pp. 41-50.

Stauffer JL, Mauger EA, Caulfield JE, Cocalis JC, Conrad
DW, Stricktin KG, Tyson PA, and The Pennsylvania
Partnership for the Elimination of Silicosis (1998).
Screening for silicosis in surface coal miners. Poster
Presentation at a Symposium at the Annual Scientific
Meeting of the American Thoracic Society/American Lung
Association, Chicago, IL, April 23, 1998.

Tomb TF, Gero AJ, and Kogut 3 (1995) Analysis of quartz
exposure data obtained from underground and surface coal
mining operations. Appl. Occup. Environ. Hyg., 10(12),
December, pp. 1019-1026.

U. S. Code of Federal Regulations (1998). Title 30-Mineral
Resources; Chapter I-Mine Safety and Health
Administration, Dep. Labor; Subchapter O-Coal Mine
Safety and Health, Part 70-Mandatory Health Standards-
Underground Coal Mines, Subpart B, Section 70.101; and
Part 71- Mandatory Health Standards-Surface Coal Mines
and Surface Work Areas of Underground Coal Mines,
Subpart B, Section 71.101, U.S. Gov. Printing Office,
Office of Federal Regulations, July 1, 1998.


Table 2 - Dust Control Assessment Data

. Rotary Rock Drill Bulldozer
Mineo: Mine Sampling Date
Drill No. Shroud Dust CabDust | DozerNo. Outside Dust Cab Dust
gfe? (% git? (% mg/m? (% SiO,) mg/m? (% SiO,)
SiO.) Si0,)
1ig97 1 10.65 (48.5) 0.29 (42.9) 1 NS NS
A 1214/97 i 1.43(45.7), 0.84 29.3) 1 0.28 (52.7) 0.11 8.8)
12/10/97 1 1.14 (49.0) —_0.20,29.5) 1 0.07 (NS) 0.00 (NS)
1217/97 2 14.07(NS)___1.2183.1) Il 0.13 68.4) 0.14 24.9)
I 1118/97 3 14,80 (123) 0.57 (13.2) 3 NS NS
B
11/20/97 3 2.52(11.3) 0.09 (NS) 3 NS NS
6/3/98 3 NS NS 3 0.23 (47.6) 0.22 (15.1)
c 6/10/98 8,02 4.56(42.1) (0.53 27.3) 3 0.02 (NS) 0.14 3.8)
7729198 3 482(NS) 0.15 (21.4) 3 0.46 (41.8) 0.14 8.6)
815/98 3 0.92374) 0.07(NS) 3 0.39 (63.2) 0.11 (18.6)
D 714/98 4 1.64(16.7) ___0.10(NS) 4 0,52.(40.0) 0.06 (NS)
nm E 75/98 s $.90(28.9) 0.07 (NS) 5 0.09 (NS) 0.00 (NS)
F 16/98 6 1.04 (27.2)__0.06 (NS) 6 0.45 (24.5) 0.01 (NS)
“Primary dust controls observed to not be operating properly on the dri during a portion oF all of the Smif
*Dust sampling was conducted initially at deill 3 and continued for the same drill operator who move to drill 2 at mine A.
‘NS - No sample was collected or silica percentage was not reported because of low or high sample weights.
Table 3. Drill Dust Collector System Improvements
Drill No. Dust Collector Improvements Pre-Dust Levels Post -Dust Levels % Dust Level
mg/m? (%Si0,) mg/m? (%Si0,) Reduction
‘1 Put drive belt back on dust collector fan 12.03 (49.1) 5.93 (46.6) 51%
4 Replaced torn drill shroud, 14.80 (12.3) 2.52 (11.3) 83%
~ 1/4 of shroud was missing
"5 Reduced shroud-to-ground gap, better vertical 21.43 (42.5) 2.47 (25.9) 88%

positioning

“The collector improvement and the different dust level measurements occurred during the same ship.

“The collector improvement and the different dust level measurements occurred between separate shifts.

‘The collector improvement and the different dust level measurements occurred during the same shift while drilling at notably different locations on the bench.
The pre-dust level sampling was made while the drill was finishing a row of holes in the consolidated overburden at the furthest distance away from the pit
edge. Most of the post-dust level sampling was made while drilting holes in unconsolidated (highly fractured) overburden at the pit edge portion of the bench.

through collector system repairs and corrected on drill 5
through lower vertical drill positioning to close the gap
between the collector shroud and ground. The pre- and post-
dust levels were measured for the collector improvements on
drills 1 and 5 during the same shift, while these measurements
were made during separate shifts for drill no. 3. Table 3
shows the ARD results obtained for these dust controt
improvements (Note: drills 1 and 5 shroud dust levels reported
in Table 2 are represented by time-weighted averages of the
pre- and post-sampling periods measured during the same
shift). Fifty-one to 88% reductions in drill dust generated
were realized because of these changes.

Dust leakage from the drill shroud was observed to be the
worst dust source problem at most of these drills. However,
other dust generation sources were also present on these drills.
They include: dust escaping through the drill stem seal at the
top of the drilling table; dust entrained from the dumping of
collector fines on the mine bench; and dust discharged from
the collector’s exhaust because of impaired filter capture
(Maksimovic and Page, 1985). The dust collectors on drills 1,
2, and 3 had no discharge chutes located near ground level, so
the fines were dumped from approximately 4 feet off the
ground, Drills 4, 5, and 6 had discharge chutes located about

1 foot off the ground. Dust discharged from the collector
exhaust was.a rare problem and was only observed on drill 3
during its first two days of sampling. The amounts of dust
generated from these multiple sources on the drill were likely
included in the dust levels measured next to the drill shroud.
However, the shroud leakage was felt to be the most
significant dust source, because major dust reductions were
realized from improving the dust capture of the shroud.

Although the drills were observed to generate high levels of
respirable dust, the dust levels in their operator cabs were
significantly lower than near their source of generation.
Figure 1 shows the ARD levels next to the collector shroud,
ARD levels inside the enclosed cab, and the relative
percentage decrease in ARD levels between the shroud and
inside the cab for the six drills surveyed. Multiple-shift dust
concentrations for several of these drills (see Table 2) are
represented as time-weighted averages in the graph. These
enclosed operator cabs provided better than a 90 percent
reduction in dust levels generated by the drill. These results
show that enclosed cabs are a key element in controlling the
drill operators’ dust levels,

Bulldozers generated significantly less dust than the drills,

with low dust levels present inside the operators’ cabs, Figure


28} — drill No 5

al H
5, "

08

740 1100 12:40
TIME OF DAY, bes min

Figure 4. Dust levels inside drill cab

cab. At drill 5, the cab dust levels were steadily elevated for
about the first hour of operation (7:52 to 8:49 a.m.), when dust
escaped profusely from the large gap underneath the shroud
(see Table 3). However, the dust levels were notably lowered
next to the shroud (see Table 3) and inside the cab (see Figure
4) after the operator improved the vertical positioning of the
drill to eliminate the gap beneath the shroud (after 8:49 a.m.).
The spikes in the cab dust levels after this period were tikely
caused by the occasional dust leakage observed between the
drill stem seal and the drill table outside the cab. Drill 6 had «
very little dust escaping the dust collection system, and the
operator consistently kept the cab door closed.

4 OPERATIONAL DUST VARIANCE

Long-term dust sampling was continued for eight months at
three drills and three bulldozers operated by Mining Company
lin West Virginia. Table 4 shows the daily equipment
operator cab dust data during this period and the data
summary for each piece of equipment. Figures 5 and 6 show
the Box-and-Whisker Plots for the equipment ARD
concentrations and silica dust contents, respectively. These
Box-and-Whisker Plots display the data frequency
distributions in 4 equal areas (25 percent for each box and
whisker) around a median, with outliers shown as individual
points. Two drills (1 & 2) were alternately used at mine A.
Bulldozer no. 2 at mine A was sampled for one day when
bulldozer no. 1 was not operated. Drill 3 and bulldozer no. 3
were moved to mine C after mine B ran out of coal reserves.

The long-term data show that enclosed operator cab dust
levels tended to be higher and more variable in the drill than in

+ the bulldozer. The data summary in Table 4 shows that two of
the highwall drills (drills 1 & 2) had average dust levels more
than double the third drill (drill 3) and all the bulldozers
(dozers 1, 2, and 3). All the drills also had higher relative dust
level variations than the bulldozers. The drills had
coefficients of variation (CV = Std. Dev./Average) above 50%
while the dozers had CV’s below 50%.

Figure 5 shows the median and 25 percentile (boxes and
whiskers) ARD data ranges for the three highwall drills and
bulldozers. This figure illustrates that drills | and 2 had the
highest and the widest range of dust levels. A suggested factor
for these wide ranges in ARD levels is that the cab doors were
observed to be left open occasionally during drilling on some
of the sampling shifts. On drill 1, the door was sometimes left
open because the cab had no air conditioning filtration system.
Drill 2 also had a non-continuous shroud design with a
hydraulically activated door on the front of the shroud and this
door did not reliably seal the collared hole. Drill 3 had a
noticeable improvement in its cab dust levels when the
damaged shroud (outlier point in Figure 5) was replaced with a

18
1.6b 4
Nab 4

2 ab 4

> ob 4

g o8t 4

Rost + 4
oat 4

“Lo Te

DRILL ORL DRILL DOZER DOZER DOZER
NOT NO.Z M3 NOT NOZ NOS

Figure 5. Multiple box-and-whisker plot of drill and dozer
dust levels

continuous wrap-around enclosure (see Table 3, discussed
earlier). The range of dust levels for drill 3 remained below
that of the other two drills with the new shroud. The normal
climatic seasons (autumn—dry, winter and spring--wet, and
summer--dry) of West Virginia were not observed to have any
significant effect on the dust levels inside the enclosed cabs
during this study. Dust leveis appear to be arbitrary through
the clirnatic seasons of dust sampling in Table 4,

The silica dust sample analyses show that noticeable silica
content differences exist between the individual highwall drills
and the individual bulldozers sampled, with some conformity
evident between the equipment operated at the same mine.
More than three-fourths of the ARD samples had reportable
silica percentages. Better than two-thirds of these reportable
samples contained more than 20% silica. The silica data and
the average silica content summaries for the equipment are
shown in Table 4.

Figure 6 shows the range of percent silica content in the
dust for the equipment sampled. Highwail drills 1 and 2 and
bulldozers 1 and 2 were operated at mine A, with the widest
range of silica content measurements in the dust. Highwall
dritf 3 and bulldozer 3 were operated at mines B and C, with
the lowest and least variable silica content measurements as
compared to the equipment at mine A. Mines B and C were
geographically close as compared to their location with mine
A

One unexpected and significant difference in the silica
content of the dust was observed between drills 1 and 2 at
mine A. The silica content in the dust generated by drill | was
significantly higher than for drill 2 at the 99% confidence level
(one-tailed t-test). This was an unexpected outcome, because
the drills were alternately used in close proximity at the mine

60 T T T T t T

f (Bl 3

DRILL ORILL DRILL OOZER DOZER DOZER
WO1 N02 NOS NOt NOZ NOS

Figure 6. Multiple box-and-whisker plot of drill and dozer
silica content


7 f f t
Field assessment of control techniques and long-term dust variability
for surface coal mine rock drills and bulldozers

J.A.Organiscak & S.J.Page
National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA, USA

ABSTRACT: Airborne respirable dust surveys were conducted at six surface coal mines to investigate the effectiveness of dust
control methods used on rotary rock drilis and bulldozers. Dust controls commonly used on drills include a dry dust collection
system, exhausting from a shrouded area around the collared hole, and an enclosed drill operator cab, filtering airborne dust from the
cab. Bulldozers are also typically equipped with an enclosed operator cab as a dust control measure. Airbome respirable dust
sampling was conducted near each equipment’s source of generation and inside its enclosed cab. Silica analysis was performed on
the dust samples to determine the percent silica content in the dust generated at the six mining operations. An additional eight-
month follow-up of dust sampling was also conducted in the enclosed operator cabs of several rock drills and bulldozers. Data were
analyzed for long-term variability of accepted control methods to abate dust and silica levels.

The highwall rock drill was the major and most variable dust source as compared to the bulldozer, generating dust fevels, on
average, one order of magnitude higher than dust levels of the bulldozer. Four of the six drills surveyed had dust containment and
capture problems at the shrouded drill table above the hole. Repairs or modifications to three of the drill dust collection systems
were shown to reduce dust levels by more that 50% next to the shrouded drill table. The enclosed operator cabs provided more than
90% and more than 40% lower dust levels than at the dust source for the drill and bulldozer, respectively. Long-term sampling of
several of these highwall drills and bulldozers showed that the dust levels in the drill cabs were frequently higher than 0.2 mg/m? and
more variable than in the bulldozers. The bulldozer dust levels were frequently below 0.2 mg/m’, Future research should focus on
improving some of the deficiencies present in the drill’s primary dust collection system, and developing quality control methods to
ensure the integrity of enclosed cab protection for equipment operators.

1 INTRODUCTION

Overexposure to airborne respirable crystalline silica dust
can cause serious or fatal respiratory disease. Although
mortality rates attributed to silicosis have dramatically
dropped in the general U.S. population (for ages 15 and over)
from 7.9 deaths per million people in 1968 to 1.3 deaths per
million people in 1992, eight states still have notably higher
silicosis-related mortality rates as compared to the general
U. S. population (NIOSH, 1996). The eight states with
silicosis-related mortality rates above 3 deaths per million.
people for 1991-1992 are Vermont, Pennsylvania, Ohio,
West Virginia, Wisconsin, Utah, Colorado, and New Mexico
(NIOSH, 1996). These states are associated with a long
history of mining and mineral processing activities, The
most frequent occupation (16.0%) recorded on silicosis-
related death certificates between 1991-1992 was mining
machine operators, with coal mining being the first (9.5%)
and metal mining being the third (8.6%) most frequently
associated industries (NIOSH, 1996). Therefore, mine
worker exposure to silica dust continues to be an ongoing
occupational health concem.

Exposure of surface coal mine rock drillers to respirable
crystalline silica is of particular concen. In a 1992 NIOSH
Alert on silicosis in rock drillers, the National Institute for
Occupational Safety and Health (NIOSH) reported 23
individual cases of advanced silicosis (acute, accelerated, and
chronic) ranging in workers from 25 to 60 years of age, with
drilling tenures ranging between 3 and 20 years (NIOSH,
1992). Most of the cases involved drill operators in their
30's and 40's, clearly indicating that high silica-exposure
levels are associated with their occupation (NIOSH, 1992).
A more recent lung x-ray surveillance study of a 664-
volunteer population of surface coal miners revealed that the
prevalence of silicosis-like abnormalities was 9% (Stauffer et

al., 1998). The two most significant factors associated with
these silicosis-like abnormalities were increasing age and
years of drilling experience.

The Mine Safety and Health Administration (MSHA)
permissible dust exposure for coal mine workers is a shift
average of 2.0 mg of airborne respirable coal mine dust per
cubic meter of air (2.0 mg/m? as defined by the Mining
Research Establishment (MRE) Criteria (U. S. Code of
Federal Regulations, 1998). If the airborne respirable dust
(ARD) sample contains more than 5% crystalline silica, the
dust standard is reduced to the quotient of 10 divided by the

crystalline silica exposure to a maximum of 100 g/m?
(MRE equivalent) for the workirig shift. Compliance with
this respirable dust standard is expected to significantly
reduce a worker's risk of occupational lung disease

“throughout an average life expectancy.

MSHA'’s dust enforcement program includes both
inspector and coal mine operator dust sampling. MSHA’s
surface coal mine dust program focuses its sampling efforts
at designated work positions (DWP’s). These are particular
areas or occupations that have been historically shown to
either exceed | mg/m’ of respirable dust or have high silica
exposure. The local MSHA official has the authority to
classify DWP’s based on an operation’s dust sampling
history or to classify non-designated work positions
(NDWP’s) based on a history of competent dust abatement.
The most frequently sampled and classified DWP’s at
surface coal mines are highwall drill and bulldozer operators.
MSHA dust exposure data from 1985 to 1992 showed that
the percentage of the highwall drill dust samples (taken from
DWP’s) having greater than $% silica and exceeding the 100
pig/m? silica limit were 81% and 77%, respectively (Tomb et
al., 1995), Bulldozer operator dust samples (taken from.
DWP’s) having greater than 5% silica and exceeding the 100


Table 4 - Eight-Month Dust Level History For One Mining Company

Rotary Rock Drill Bulldozer
Sampling Date
Mine,Drilt No, Cab Dust Levels Mine, Dozer No. Cab Dust Levels
mg/m’ (SiO,) mg/m? (Si0,)
11/18/97 B,3 0.57 (13.2) B,NS NS
1119/97 AA 0.29 (42.9) ; A,NS NS
1120197 B3 0.09 (NS) B,NS NS
12/4/97 Al 0.54 (29,3) Al 0.11 G88)
12/1097 Al 0.20 29,5) A,NS NS
1217197 A2 1.21 33.1) Ad 0.14 (24.9)
123097 A2 0.65 (7.4) ALNS NS
1/7/98 A2 0.78 21.1) Al 0.12 (12.6)
114/98 A.2 0.49 (NS) AL 0.14 (NS)
1/2198 Ay2 1.18 (24.7) AL 0.03 (NS)
1/28/98 At 0.45 (44.0) AL 0.07 (NS)
214/98 A2 0.28 (20.6) AL 0.03 (NS)
2/11/98 A2 1.36 (40.1) Al 0.13 (11,2)
2/18/98 A2 0.70 (23.2) 42 0.21 20.6)
225/98 A,2 1.34 (38.2) At 0.14 38.6)
314/98 A,2 0.48 (24,1) Al 0.14 (41.4)
3/11/98 A2 0.48 (19,7) Al 0.11 (NS)
3/18/98 A,NS NS Al 0.13 2.7)
3/26/98 Al 1.51 (48.9) Al 0.17 (28.0)
4/1198 Al 0.51 (47.1) Ay 0.04 (NS)
418/98 AT 0.27 (28,3) Al 0.20 32.0)
4/15/98 AL 0.94 (28.0) Al 0.09 (NS)
422198 A2 0979.5) Al 0.08 (NS)
4729198 A2 1.62.37.) AL 0.06 (NS)
516/98 A2 039 21.6) Al 0.15 G73)
5/13/98 A,NS NS Al 0.14 (26.6)
5/21/98 C3 0.20 (24.1). C.NS NS
5127198 C,NS NS C3 0.19(123)
6/3198 CNS NS C3 0.22 (15.1)
6/1098 C3&A,2 0.53 27.3) C3 0.14.8)
6/18/98 A? 0.55 (22.0) Al 0.20 (44.6)
6/24/98 C3 0198.7) Al 0.16 (46.1)
718198 A2 0.37 (6.0) Add 0.07 (NS)
75198 A2 0.35 (8.4) C3 0.09 (NS)
722198 A2 0.31 9.4) Al 0.08 (NS)
7129198 C3 0.15 (21.4) ©,3 0.14 (8.6)
8/5/98 C3 0.07 (NS) C3 0.11 (18.6)
AL Average = 0.59 (36.9) Al Average = 0.11 G19)
7 Std. Dev. = 0.05 (11.1)
DATA AQ ‘Average = 0.78 (23.3) A2 Avernge = 0.21 (20.6)
SUMMARY 7 Std, Dev. = 0.41 (11,3) 7 Std, Dev. =NA (NA)
Average = 0.21 (16.9) Average = 0.15 (11.7)
BaG3 Std. Dev. = 0.18 (7.1) o3 Std. Dev, = 0.05 (5.7)

NS- No sample was collected or silica percentage was not reported because oftow or high sample welghis.

during the eight-month sampling period. One likely 5 CONCLUSIONS
explanation for this silica dust difference is the actual design
dissimilarities between these drills. Drill 1 had no air Results of the surface mine field assessments show that the
conditioning system for its enclosed cab, less bailing air accepted dust control technologies have the ability to achieve
capacity, less collector capacity, and less-pull down thrust than lower silica dust level requirements for highwall drill and
drill 2. Although the individual effects of these dissimilarities bulldozer operators. To achieve these low dust levels, all the
could not be quantified, the overall observation was that driif 1 engineering controls require quality upkeep with good
had more silica dust content in its operator cab than drill 2. operating practices followed. The highwall drill was the
‘These long-term sampling data indicate that ARD major and most variable dust source as compared to the
concentrations and silica dust content can be highly variable at bulldozer. The drill’s primary dust collector system was
surface coal mining operations. The drills had higher dust observed to have several inherent weaknesses, with dust
levels with more variations (0.62 mg/m’ average with a 0.07 to generation problems evident at four of the drills studied. The
1.51 mg/m? range) than the bulldozers (0.12 mg/m? average most common problem was drill dust containment and capture
with a 0.03 to 0.22 mg/m’ range). Also, the silica content from under the shrouded drill table. Drill dust commonly
varied notably at each mine for all the equipment sampled. escaped through gaps in or around the shroud and through the
Thus, accepted dust control methods and good equipment drill stem seal at the top of the drill table. When containment
operating procedures must be diligently practiced daily to problems with the shrouded drill table were rectified,
consistently achieve low ARD and silica dust concentrations. significant dust reductions were realized. These containment

problems were not always easily corrected, because of uneven
drilling surfaces or hard- to-seal areas between the rotating


g/m? silica limit were 68% and 60%, respectively (Tomb et
al., 1995).

A special MSHA inspector sampling survey of non-
designated work positions (NDWP’s) at surface coal mines
showed very similar silica dust level results for the highwall

drill operator and somewhat lower silica dust level resutts for -

the bulldozer operator as compared to the DWP sampling
data. The percentage of NDWP highwall drill operator
samples having greater than 5% silica content and exceeding
the 100 jag/m? silica limit were 81% and 75%, respectively
(Tomb et al., 1995). The NDWP bulldozer operator samples
having greater than 5% silica content and exceeding the 100
pag/m’ silica limit were 56% and 47%, respectively (Tomb et
al., 1995). Therefore, MSHA data suggest that
overexposure to silica dust is an ongoing surface coal mine
dust problem for highwall drill and bulldozer operators.

Dust controls commonly used on highwall drills include a
dry dust collection system, exhausting from a shrouded area
around the collared hole, and an enclosed drill operator cab,
filtering airborne dust from the cab air. Bulldozers also use
an enclosed operator cab as a dust control measure. Field
assessment of these dust control practices was conducted to
examine their dust abatement strengths and weaknesses. Six
highwall drilling machines and five bulldozers were
evaluated for their dust control effectiveness at six surface
coal mines, Three of the drills and two of the bulldozers
were sampled weekly over an eight-month period to
determine the mitigated long-term dust level variability
inside the operator cab. Silica analysis was also performed
on the dust samples to determine the amount of silica dust
generated from these mining operations. This paper
describes these field studies, analyzes their results, and
identifies any deficiencies found in the dust control practices
used,

2 FIELD STUDY SAMPLING PROCEDURES

Field assessments of highwall drills and bulldozers included
airborne respirable dust (ARD) sampling with documentation
of dust control practices utilized. Area ARD sampling was
conducted near each equipment’s source of dust generation
and inside its enclosed cab to examine the effectiveness of
the dust control practices used. Four personal gravimetric
dust samplers were located inside the dust plume escaping
the drill shroud and inside the enclosed cab. Four personal
gravimetric dust samplers were located above the bulldozer
tracks on both sides of the cab and inside the enclosed cab of
the bulldozer. Each personal sampler utilized a 10-mm Dorr
Oliver nylon cyclone classifier', operating at 2.0 liters/tnin.
The respirable dust was deposited on a MSA 37-mm coal
dust filter cassette. All the dust concentrations measured by
these personal samplers adhere to the Atomic Energy
Commission (AEC) criteria for respirable dust and are not
reported as MRE equivalent concentrations (U. S. Code of
Federal Regulations, 1998).

Dust sampling was conducted during most of the working
shift, but sampler times were less for some locations having
high dust concentrations. The dust samplers were run
continuously for about 7 hours inside the operator cabs and
outside the bulldozer cabs where dust concentrations were
expected to be less than 1 mg/m’. Since dust concentrations
in the dust plume near the drill shroud were notably higher
than at the other sampling locations, sampling times were

‘Mention of any company name or product does not
constitute endorsement by the National Institute for
Occupational Safety and Health.

commonly between 3 and 6 hours for this location.
Sampling times were varied in an effort to keep the amount
of the respirable dust mass collected within the
recommended range for silica analysis, as discussed below.

The dust mass from the cassette filters was analyzed for
crystalline silica by MSHA’s P-7 infrared spectrophotometer
method (MSHA, 1989). The P-7 procedure was found to be
a NIOSH-classified B method of less than 25% error within
the true value in a single laboratory (Anderson, 1983).
Preparation of dust samples for testing involves low-
temperature ashing of the filter cassettes and re-depositing
the residue mass (ash) on another filter for infrared analysis.
The tested and recommended net dust mass collected on a
coal mine filter cassette for the P-7 method is between 0.5
and 2 mg with a minimum silica mass of 25 1g (Anderson,
1983). Because the P-7 method testing had at least twice as
much or more combustible coal dust mass than
noncombustible mineral matter deposited on the cassette
filters (Anderson, 1983), this method is expected to provide
similar precision for a dust mass as low as 0,25 mg of
essentially noncombustible mineral matter with 25 yg of
silica. Surface coal mine dust generated from highwall drills
and bulldozers during overburden removal is expected to
have little or no combustible coal dust, so the silica content
(percent) in the ARD measurements is reported for at least
0.25 mg of dust mass and 25 j2g of silica, Many of the
cassette filters did not have 0.25 mg of dust mass, so
multiple filters from the same sampling location were
composited for silica analysis. Some dust levels were so low
that even the composite filter mass did not meet the authors’
minimum reportable range, Also, silica analysis for some
filter cassettes with high weight gains was not conducted
because numerous splits of the sample would be required to
obtain a dust mass in the recommended mass range for this
analytical technique.

Supplemental airborne dust sampling was also conducted
with instantaneous dust monitors. A MiniRAM
instantaneous dust instrument connected to a Metrosonics
331 data logger was operated in the operator's cab to
examine real-time dust level variation during the shift. The
MiniRAM measures respirable dust by light-scattering
techniques and was operated in the passive sampling mode
(Organiscak et al., 1986). The Metrosonics 331 data logger
records analog output from instruments and is downloaded to
a persona! computer for data analysis (Cecala et al, 1988).
These instantaneous dust data are expressed in relative
MiniRAM units to identify corresponding dust-level changes
to particular operational events during the shift.

During these field surveys, the make and model of
equipment and dust control methods being used were
documented. This documentation included identification of
equipment sampled, classification of control methods used,
and inspection of the controls’ operating status. Because
actual dust control operating parameters were difficult to
measure in the field, their specified capacities are reported.
Dust collector and cab airflow parameters were especially
difficult to measure because of numerous elbows and area
changes in the duct work. The only reliable measurements
made during the surveys were bailing airflow on some of the
drills. Bailing airflow is compressed airflow directed at the
bit in order to remove cuttings from the hole. This airflow
was measured with a calibrated limited orifice flow device
placed on the drill steel with the bit removed. Because the
drill had to be taken out of production and the bit removed to
install the instrument, opportunities to acquire this
measurement on all drills were limited. However,
measurements were made on one of each drill make, and


Table 1 - Surface Mine Equipment Studied

Mine Equipment “Make/Model General Description Dust Controls
Drill No. 1 Drill Tech/ « Truck-mounted drill «= 11,200 ofin dry collector
D40K + 6.75" diameter hole + Enclosed weather cab, no air cleaning
+ = "400 fin bailing airflow system
Drill No. Ingersol-Rand/ + Track-mounted drit! + = 3,000 cfim dry collector
2&3 DMSOE + 6.75" diameter hole + Enclosed cab with air cleaning system
+ = 1500 fim bailing airflow
Drill No. ReedDrill/ + Track-mounted drill + = 3,000 cfim dry collector
4&5 SKA4Si + 7.875" diameter hole + Enclosed cab with air cleaning system
+ = "750 cfm bailing airflow
Drill No. 6 ReedDrill/ + Track-mounted drill + = 3,000 cfm dry collector
SK50i + 7.875" diameter hole + Enclosed cab with air cleaning system
+ = 750 fin bailing airflow .
Dozer No. Caterpitiar/ + Elevated track-drive sprocket + Enclosed cab with air cleaning system
1,4,5,&6 DION + U-shaped dozer blade
Dozer No.2 Caterpitlar/ + Usshaped dozer blade + Enclosed cab with air cleaning system
DoH
Dozer No. 3 Caterpillar! + Elevated track-drive sprocket + Enclosed cab with air cleaning system
DIIN + U-shaped dozer blade

*Mention of any company name or product does not constitute endorsement by the National Institute for Occupational Safety and Health.

‘Measurement made on this equipment model.
‘Measurement made on one of these same models.

these measurements were assumed to be similar for alike
drills.

Frequent visual inspections of the dust control practices
‘were made during the shift and any operational problems
with these practices were documented. Several critical
problems were observed with the primary dry dust collection
systems used on some of the highwall drills. Efforts were
made to rectify these operational problems during the shift or
between shifts, and additional gravimetric dust samplings
were conducted to measure the net effect of the control
improvement near the source of generation. The
instantaneous dust levels measured inside the cabs were
useful for identifying the significance of equipment operator
practices that allowed dust into the cabs.

Additional intermittent long-term dust sampling was
conducted in the operator cabs of three drills and two
bulldozers to examine the variability of accepted control
methods designed to abate dust levels. At several mine sites,
airborne respirable dust sampling was conducted one day each
week over an 8-month period in the enclosed operator cab of a
highwall drill and bulldozer. Four personal dust samplers
(described earlier) were placed in each cab and were operated
for nearly 7 hours or more during the sampling shift. Dust
samples were analyzed for silica by the P7 method as
described earlier. Because this study was conducted with a
small coal mine operator, only select pieces of equipment
were being used during each week at the operator’s different
mine sites. Usually coal loading occurred at one mine site
while overburden removal occurred at the other mine site.
Only the highwall drill and bulldozer involved with
overburden removal at either mine sight could be sampled.
Therefore, variability data were collected for several pieces of
equipment, sampled at various intermittent time intervals.
Visual inspection of the dust control practices and recording of
climatic conditions were also documented for each sampling
shift.

3 DUST CONTROL ASSESSMENT RESULTS

Dust control field assessments were completed on six highwall
drills and five bulldozers. Two coal mining companies were

cooperative in providing access to their equipment at six
different surface coal mining sites. These six surface mines
were located in the Appalachian mountains of West Virginia,
Kentucky, and Tennessee, All the mines used the truck
haulback strip mining method, where front end loaders load
the blasted overburden into trucks to be hauled and dumped
onto the other side of the existing pit. Bulldozers are used to
push the overburden into the previously mined-out pit.

Table 1 shows the equipment surveyed and the control
practices used for each piece of equipment. The highwall
drills and bulldozers sampled usually operated remotely from
each other and were not considered to be contributing dust
sources to one other. The highwall drills forced compressed
air through the drill stem and bit (bailing air) to flush the
cuttings and dust out of the collared hole. Dry dust collection
systems were used on the drills to capture the dust from.
undemeath the shrouded drill table. These drills also used
enclosed cabs to provide the operator additional protection
from the drill dust. All the dritls except drill no. | had
positive air filtration systems conditioning the enclosed cab.
All the bulldozers also had enclosed cabs with air filtration
systems. These cab filtration systems re-circulated and
filtered most of the inside cab air with some additional make-
up air drawn in through an inlet filter.

Table 2 shows the ARD survey data for the two coal,
companies, six surface mines, and the various pieces of
equipment studied. As these data show, the highwall drills
generated significantly more ARD than the bulldozers.
During four of the sampling days the primary dust collection
system on drills 1, 2,3, and 5 were observed to be
malfunctioning, generating some of the highest respirable dust
concentrations during these surveys. The collector fan on
drill 1 was found not to be operating because the drive belts
were either broken or slipped off the drive pulleys. Drill 3 had
a badly damaged collector shroud with about one-fourth of the
shroud material missing. Drills 2 and 5 had dust escaping
underneath the shroud due to sloped and uneven ground
conditions. Drill 2 was operated at the edge of the drill bench
during a good portion of the shift, making it difficult to seal
the area under the shroud.

The dust control problems were rectified on drills 1 and 3