An expanded model for predicting surface coal mine drill respirable dust

emissions

Steven J. Page*, Randy Reed and Jeffrey M. Listak

*Corresponding author. Email: sep8@cdc.gov

Department of Health and Human Services, Centers for Disease Control and Prevention, National

Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA, USA

Overexposure to airborne respirable crystalline silica dust can cause disabling or fatal

respiratory disease, and mine worker exposure to silica dust continues to be an

ongoing occupational health concern. Exposures of surface coal mine rock drillers to

respirable crystalline silica are of particular concern. On surface coal mine drills,

bailing air flushes the cuttings out of the drill hole. Conveyor belting material is

typically used to fabricate a shroud around the drill deck in an effort to contain the

drill dust so that it can be captured by a collector. Dust leakage from the drill shroud

is usually the worst dust source problem on most drills. The focus of this work is drill

shroud dust leakage and the relationships of various drill parameters on this leakage.

Experimental data were obtained and used in combination with dimensional analysis

to establish these relationships. In general, it is found that airborne respirable dust

(ARD) concentrations vary in a direct relationship with shroud leakage area and in an

inverse relationship with drill deck cross sectional area and shroud height. This work

expands the testing and dimensional analysis previously reported for collector/bailing

air flow ratios ranging from 2:1 to 4:1 to include ratios approaching 1:1. A semi

empirical mathematical model has been developed and expanded to describe ARD

generation on surface coal mine drills. Geometric parameters included are drill deck

height and cross sectional area, shroud leakage associated with the deck shroud, and

the operational parameters of bailing airflow and dust collector airflow. The

relationships can be described by logarithmic functions and yield predictive ARD

values, which fall in the range measured on operating drills for collector/bailing air

flow ratios greater than 2. However, at values of collector/bailing air flow ratios of

approximately 1.1, the amount of ARD shows minimal response, if any, to drill deck

shroud improvements that do not result in near perfect seals. This is a condition that

can occur in actual operation and is a substantially different result than previously

expected and reported. Application of these results should provide mine operators

with sufficient information to determine (1) the relative magnitude of their dust

emissions, (2) where they should focus their efforts to reduce ARD emissions and (3)

the improvement they could reasonably expect to achieve. Given that exposures of

surface coal mine rock drillers to respirable crystalline silica are of particular concern,

substantial reductions of airborne silica dust during drilling may be estimated and

achieved through use of the analysis presented.

Keywords: drill; dust; model; dimensional analysis
1. Introduction

Overexposure to airborne respirable crystalline silica dust can cause serious or fatal

respiratory disease. Exposures of surface coal mine rock drillers to respirable crystalline

silica are of particular concern. In a 1992 Alert on silicosis in rock drillers, the US National

Institute for Occupational Safety and Health (NIOSH) reported on 23 cases of advanced

silicosis (acute, accelerated and chronic) in workers ranging in ages from 25 to 60 with

drilling tenures ranging between 3 and 20 years [1]. Most of the cases involved drill

operators in their 30s and 40s, indicating that high silica exposure levels are associated

with their occupation. A more recent lung X-ray surveillance study of a 664 volunteer

population of surface coal miners showed that the prevalence of silicosis-like

abnormalities was 9% [2]. The two most significant factors associated with these

silicosis-like abnormalities were increasing age and years of drilling experience.

The US Mine Safety and Health Administration (MSHA) permissible dust exposure

for coal mine workers is an 8 h shift average of 2.0 mg m 3 of airborne respirable coal

mine dust, as defined by the Mining Research Establishment (MRE) Criteria [3]. 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 percentage of silica in the dust,

limiting the respirable crystalline silica exposure to a maximum of 100 mg m 3 (MRE

equivalent) for the working shift. Compliance with these respirable dust standards is

expected to significantly reduce a worker’s risk of occupational lung disease throughout an

average life expectancy.

MSHA dust exposure data from 1985 to 1992 (operator and MSHA inspectorcollected samples) showed that the percentage of the designated work position (DWP)

highwall drill dust samples having greater than 5% silica and exceeding the 100 mg m 3

silica limit were 81% and 77%, respectively [4]. A special MSHA inspector sampling

survey of non-designated work positions (NDWP) at surface coal mines showed very

similar silica dust level results for the highwall drill operator as compared to the DWP

sampling data. NDWP work positions are not designated for routine sampling by coal

mine operators and are only sampled by MSHA inspectors. DWPs are work positions that

MSHA has designated for operator sampling. The percentage of NDWP highwall drill

operator samples having greater than 5% silica content and exceeding the 100 mg m 3

silica limit were 81% and 75%, respectively [4]. Although, a recent analysis of the MSHA

data from 2000 to 2006 shows that the percentage of the DWP drill dust samples exceeding

the permissible exposure limit has dropped to 16%, MSHA data still suggests that

overexposure to silica dust is an ongoing surface coal mine dust problem for the highwall

drill operator. The 1992 Alert enabled MSHA to promulgate new dust regulations in 1994

requiring mine operators to provide effective drill dust control, regardless of exposure.

These regulations permitted MSHA to cite a mine operator when a dust control is missing,

not maintained, defective or ineffective, generally based on a visual inspection.

Enforcement of this regulation led to the significant decline in the number of samples

exceeding the standard.

On surface coal mine drills, bailing air flow flushes the cuttings from the hole. The

material is ejected from the hole at ground level with significant velocity. In an attempt to

control/capture the respirable dust emitted from the hole, a deck shroud encloses the area

around the drill deck and an external dust collector is used with its duct inlet typically

located in the upper outboard rear corner of the enclosed deck volume. Frequently, the

deck shroud is made merely by hanging four pieces of rubber belting from the deck. This

obviously leaves gaps at the corner seams for dust to escape. Additionally, the shroud does
not always reach the ground, leaving a gap around the bottom perimeter of the shroud

where dust can escape. Dust leakage from the drill shroud was observed to be one of the

worst dust emission problems on many drills. However, other dust generation sources

were also present on these drills besides the shroud leakage. These 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 out the collector’s exhaust

because of impaired filter capture [5].

The focus of this present work is drill shroud dust leakage and the relationships of

various drilling parameters on this leakage at low dust collector airflows. Experimental

data were obtained on an improved full-scale test facility and used in combination with

dimensional analysis to establish these relationships. This work expands the testing and

dimensional analysis previously reported [6] for collector/bailing air flow ratios ranging

from 2:1 to 4:1 to include ratios approaching 1:1, a condition that can occur in actual

operation. A significantly different result than previously expected and reported was

obtained for airflow ratios below 2:1.

2. Experimental methods

2.1. Test facility

Testing was performed on a full-scale mockup, as previously reported with a detailed

diagram of the test facility [6], of a drill deck and shroud, including drill pipe and drill

hole, enclosed within a large chamber. The following are brief descriptions of each

component and the improvements made since the previous publication.

The drill deck and shroud were constructed of plywood and stud framing with final

dimensions measuring 1.22 m (4 ft) wide by 1.52 m (5 ft) long by 1.22 m (4 ft) high. Each

plywood side panel of the shroud was constructed in hinged segments so that shroud

leakage areas AL corresponding to shroud-to-ground gaps (h) of 51 mm (2 in.), 203 mm (8

in.) and 356 mm (14 in.) could be readily simulated.

The drill pipe and drill hole were simulated by a 152 mm (6 in.) schedule 40 pipe

concentrically mounted inside a 203 mm (8 in.) schedule 80 pipe, respectively. The larger

outside pipe bottom was capped off to simulate the hole bottom and the inside pipe was

suspended in a fixed position to provide adequate clearance between the two pipe ends to

accommodate the bailing air flow rate. The complete pipe assembly extended

approximately 0.61 m (2 ft) above the drill deck and 0.91 m (3 ft) below the floor of the

large test chamber (to be described shortly).

One of the most significant improvements made to the test facility was the

installation of a state-of-the-art air compressor to deliver the bailing air volumetric

flow rate QB. A Kaeser DSD125 compressor capable of delivering 19.5 m3 min 1 @

758 kPa (690 cfm @ 110 psi) was equipped with an oil separator as well as a

refrigeration unit for significant water removal and the maintenance of a low and

constant air temperature. Bailing air was supplied to the top of the inside pipe in two

separate splits. The main split of air was regulated between 13.6 and 14.1 m3 min 1

(482 and 499 ft3 min 1) after bailing air temperature and pressure correction. The

corrections were required to compensate for deviations from the conditions at which

the flow meter was calibrated. The main split temperature and pressure for all the

tests varied from 288 to 297 K (59–758F) and 200 to 214 kPa (29–31 psi), respectively.

The secondary split of air was regulated between 0.71 and 0.76 m3 min 1 (25 and

27 ft3 min 1

) after bailing air temperature and pressure correction. The secondary

split temperature and pressure for all the tests varied from 290 to 298 K (63–768F) and
331 to 345 kPa (48–50 psi), respectively. Therefore, the total bailing air flow rate varied

between 14.4 and 14.9 m3 min 1 (508 and 525 ft3 min 1

). This total volumetric airflow

was approximately 4.3 m3 min 1 (150 ft3 min 1

) greater than in tests previously

reported. The secondary air split was used to drive a venturi eductor. This eductor and

secondary air split injected the test dust into the side of the drill pipe near the top to

be mixed with the main split of air.

A dust collector was used to provide exhaust ventilation volumetric flow rate QC

within the deck shroud via ductwork connected to the top of the drill deck in one

corner. This configuration is typical on nearly all rotary drills equipped with a dry dust

collector. Four different collector air flows were evaluated as one of the test parameters

and were obtained by regulation of a bleed air inlet at the collector. A broad range of

parameters was chosen based on parameters typically found on operating rotary drills.

However, the ratio QC/QB 1⁄4 3.0 is usually the maximum design value found on drills

with clean collector filters. The ratio QC/QB 1⁄4 2.0 is typically a much more common

value found in actual operation for dust collectors with loaded filters that should be

replaced. The ratio QC/QB 1⁄4 4.0 was also chosen for testing to ensure that the region

of maximum curvature in the dependent variable was included in the testing protocol,

as well as to evaluate the effect of increasing the dust collector air flow on ARD

concentrations. The ratio QC/QB 1⁄4 1.1 was a new test condition to examine if the

functional relationships for the other airflow ratios are still applicable since this

condition has been observed in actual drill operations. Values of QC/QB � 1, although

possible in practice, would represent a condition with airborne dust escaping the test

chamber.

To provide a suitable dust sampling environment, the drill deck, drill pipe and

shroud were located within a larger chamber measuring 3.05 m (10 ft) wide by 3.66 m

(12 ft) long by 2.44 m (8 ft) high and elevated 0.91 m (3 ft) above ground level. The top

of the chamber was equipped with three adjustable louvers to provide makeup air to

allow for the difference between the bailing airflow into the chamber and the dust

collector airflow out of the chamber. The louvers were adjusted according to the airflow

requirements so that the air velocity in each louver was approximately constant at

1.02 m s 1 (200 ft min 1

) to keep dust from escaping out of the chamber. The chamber

dimensions were chosen based on two requirements: (1) to allow ample room around the

drill deck shroud, and (2) to maintain an average makeup air velocity within the

chamber of no more than 3.24 m min 1 (11 ft min 1

) to minimise the effect on the dust

escaping the shroud.

2.2. Dust generation and test procedures

The commercially available limestone test dust (100% 5 74 mm or 200-mesh) was fed into

the eductor by a Vibra-Screw feeder. Limestone was chosen as a surrogate test dust

because it is much safer to use than silica and has virtually the same density. In reality,

choice of test dust is not critical because dust sampling is performed on an aerodynamic

size classification basis. A change to a smaller auger feed screw in the feeder was the

second improvement in the test facility. This change was made to help improve the

uniformity of the dust feed rate by reducing the void space within the auger as well as

allowing for a faster auger rotation. The auger feed rate was adjusted by a motor

controller to produce an approximately constant feed rate, timed for a 3-min interval. A

tachometer motor voltage output was used to monitor the feed rate during the test. The

feed rate was measured before and after each test, and the two values were averaged to
estimate the feed rate during the test. The feed rate for all tests averaged 4727 (+482) mg

1 min .

The collector airflow velocity pressure for each test was set prior to each test and

recorded for a 5-min interval before and after the test with a pitot tube. The pitot tube was

removed prior to feeding dust into the chamber and during the test. Static pressure in the

collector duct, relative to the inside of the test chamber, was monitored and continuously

recorded for the entire test duration.

Gravimetric samplers using 37 mm filter cassettes and 10 mm Dorr-Oliver cyclones

were used to measure respirable dust levels inside the chamber. The tests were randomised

and followed a two-level factorial design with two centre points using the variables of

collector airflow and leakage area, maintaining constant bailing airflow. Two samplers

were used on each of the four sides of the shroud and located approximately 0.61 m (2 ft)

above the floor level. Previous testing relied on individual air sampling pumps typically

used with the cyclones. This required periodic calibration as well as turning all pumps on/

off at slightly differing times.

The next significant change in the test facility was the method used to regulate sampler

air flow rates. The sampling pumps were replaced with critical orifices and one vacuum

pump to maintain constant flow rate. Consequently, all samplers were turned on/off

simultaneously. The samplers were operated at a flow rate of 2.0 L min 1

. A RAM-1

instantaneous dust monitor was used at one location within the chamber to monitor the

dust concentration during the pre-test dust feed and during the test. The analog output of

the RAM-1 was fed to a strip chart recorder for visual monitoring of the dust

concentration stability. Dust sampling time was fixed at 50 min after verification that the

chamber dust concentration had stabilised. The final chamber dust concentration was

determined from the average of the eight gravimetric samples.

3. Theoretical analysis

The concept of dimensionless parameters, based on variables which are known to be

the most influential, permits a limited amount of experimental data to be applied to

other configurations of geometry, forces, air flows, velocities, etc. However, a detailed

derivation of the dimensional analysis technique known as the Buckingham P theorem

[7] shall not be presented in this work. Also, development of the theoretical analysis

pertaining to this work has been previously published [6] and will not be presented in

this work.

The following variables are known from experience to be the most influential in terms

of drill ARD generation and are defined in this study as:

R 1⁄4 dust feed rate (gm min 1

)

C 1⁄4 ARD concentration emanating from shroud leakage, mg m 3

, the dependent

variable in the analysis

QB 1⁄4 bailing air flow (m3 min 1

)

1⁄4 3 QC collector air flow (m min 1

)

h 1⁄4 open shroud-to-ground gap height (m)

HS 1⁄4 shroud height (m)

HSe 1⁄4 effective shroud height (m), determined by the experimental shroud height HS

minus the leakage height h

AS 1⁄4 cross-sectional area of drill deck shroud (m2

)

AL 1⁄4 leakage area of deck shroud (m2

)
Reiterating, it is assumed that the leakage area AL is restricted to the bottom gap area and

that no leakage occurs at the vertical corners of the deck shroud.

Table 1. Shroud test data.

Test

QC/QB

Target

QC

(m3 min71)

QB (m3

min71)

QC/QB

Actual h (m)

HSe

(m) AL (m2)

R (gm

min71)

ARD

(mg m73)

NB 1 1.1 16.4 14.6 1.13 0.051 1.168 0.24 4.7 13.01

NB 15 1.1 16.6 14.7 1.13 0.051 1.168 0.24 5.2 10.39

NB 18 1.1 16.5 14.8 1.11 0.051 1.168 0.24 4.1 6.99

NB 7 1.1 16.5 14.7 1.12 0.203 1.016 0.96 4.0 12.22

NB 11 1.1 17.8 14.7 1.22 0.203 1.016 0.96 4.8 13.19

NB 22 1.1 17.6 14.7 1.19 0.203 1.016 0.96 5.2 10.16

NB 19 1.1 17.5 14.7 1.19 0.356 0.864 1.68 4.3 9.69

NB 27 1.1 17.2 14.6 1.17 0.356 0.864 1.68 4.3 8.61

NB 36 1.1 15.5 14.7 1.05 0.356 0.864 1.68 5.2 10.71

NB 2 2.0 29.0 14.5 2.02 0.051 1.168 0.24 5.1 3.30

NB 5 2.0 29.1 14.7 1.95 0.051 1.168 0.24 5.2 2.21

NB 25 2.0 29.1 14.7 1.98 0.051 1.168 0.24 4.5 1.59

NB 20 2.0 29.7 14.9 2.00 0.203 1.016 0.96 4.6 3.96

NB 29 2.0 28.9 14.6 1.99 0.203 1.016 0.96 4.6 3.53

NB 31 2.0 27.5 14.6 1.87 0.203 1.016 0.96 4.1 4.15

NB 4 2.0 28.7 14.5 2.00 0.356 0.864 1.68 4.8 7.71

NB 14 2.0 27.9 14.7 1.88 0.356 0.864 1.68 4.9 5.62

NB 21 2.0 22.7 14.4 1.58 0.356 0.864 1.68 6.2 9.40

NB 6 3.0 43.0 14.4 3.01 0.051 1.168 0.24 4.8 0.29

NB 9 3.0 42.8 14.5 2.98 0.051 1.168 0.24 5.6 0.40

NB 12 3.0 42.2 14.6 2.91 0.051 1.168 0.24 4.8 0.88

NB 28 3.0 43.4 14.5 3.00 0.203 1.016 0.96 4.3 1.49

NB 32 3.0 42.6 14.4 2.99 0.203 1.016 0.96 4.3 1.90

NB 33 3.0 42.8 14.5 2.97 0.203 1.016 0.96 4.6 2.41

NB 16 3.0 41.8 14.6 2.87 0.356 0.864 1.68 4.0 2.49

NB 26 3.0 41.9 14.6 2.89 0.356 0.864 1.68 4.8 2.19

NB 35 3.0 43.6 14.6 3.00 0.356 0.864 1.68 4.8 2.62

NB 3 4.0 56.0 14.5 3.88 0.051 1.168 0.24 4.7 0.10

NB 13 4.0 55.8 14.6 3.81 0.051 1.168 0.24 4.7 0.11

NB 23 4.0 54.2 14.6 3.75 0.051 1.168 0.24 5.1 0.09

NB 8 4.0 57.4 14.5 4.02 0.203 1.016 0.96 4.8 1.05

NB 10 4.0 57.0 14.4 4.01 0.203 1.016 0.96 4.4 1.09

NB 30 4.0 56.3 14.6 3.90 0.203 1.016 0.96 4.2 1.45

NB 17 4.0 55.1 14.7 3.76 0.356 0.864 1.68 5.2 1.84

NB 24 4.0 53.6 14.6 3.70 0.356 0.864 1.68 4.9 1.81

NB 34 4.0 56.2 14.8 3.83 0.356 0.864 1.68 4.6 1.74

 Published reports

demonstrate that this condition can be achieved in practice [8,9]. The convention used

for the analysis and presentation of data will assume that HS is the respective effective

height HSe. From dimensional analysis [6], the P parameters are determined to be

P1 1⁄4 QCC=R; P2 1⁄4 QB=QC; P3 1⁄4 HS=AL

0:5

; and P4 1⁄4 AS=AL ð1Þ 

The grouped data in Table 1 can be presented in the form [6]

P2 � P1 1⁄4 a lnðP3Þ þ b ð2Þ 
or

P2 � P1 1⁄4 c lnðP4Þ þ d; ð3Þ 

as shown in Figures 1 and 2.

Determination of parameters a, b, c, d in Equations (2) and (3) by regression of the

data then allows these equations to be solved for the ARD concentration C in units of mg

m 3 for any arbitrary dust generation rate R. In this manner, the effect of each variable

QB, QC, HS, AS, AL on the ARD, taken independently of the other variables, can be

determined.

Although the ‘‘true’’ functional relationship between the Q parameters is not known,

the logarithmic functional form of Equations (2) and (3) was chosen based on several

considerations. First, P2 * P1 can reasonably be deduced to be a strictly decreasing

monotonic function of P3 and P4. Second, since AS 4 0, HS 4 0, and AL has an upper

limit defined by the condition of the absence of a shroud, both P3 and P4 must possess

lower limits P3LL 4 0 and P4LL 4 0. Third, the logarithmic form can provide a more

uniform rate of change of the dependent variable than a power function, and its use is not

without precedence in aerosol science [10].

Figure 1. ARD concentration P parameter as a function of the deck height to shrould leakage area

ratio with 95% confidence bands.

4. Results

Table 1 summarises the test parameters and results, categorised by QC/QB and AL, for

thirty-six tests. Figures 1(a) to (d) and 2(a) to (d) summarise the functional relationships
0:5 between the dependent parameter P2 * P1 and the independent parameters P3 1⁄4 HS=AL

and P4 1⁄4 AS/AL, respectively, and for the four ratios of QC/QB. The non-linear

regressions were generally efficient with R2 values ranging from 0.80 to 0.95 and the

dashed lines are the regression 95% confidence bands. It is observed that for any values of

parameters and an arbitrary material feed or generation rate R, it is possible to predict the

ARD concentration. Table 2 summarises the solution for the ARD concentrations C1, C2,

C3 and C4 for the parameter values tested. The values of QC 1⁄4 15, 30, 40 and 60 m3

min 1 approximate the values of QC experimentally tested in Table 1. In general, the

function inversions for P3 and P4 at QC/QB 1⁄4 2 are shown in Figures 3 and 4 for the four

values of QC. The y-axes of Figures 3 and 4 intersect the x-axes at the value at which the

independent variable is a minimum. This corresponds to the maximum value of AL, which

represents no deck shroud.

Figure 2. ARD concentration P parameter as a function of the deck area to shrould leakage area

(ratio) with 95% confidence bands.

5. Discussion

5.1. Change of model for QC/QB 1⁄4 1.1

It has been previously reported [6] that, given the operational parameters of this work,

extrapolation of the curves in Figures 3 and 4 to the minimum values of the P

parameters would result in ARD values between 25 and 30 mg m 3

. Moreover,

extrapolating the previously reported data for values of QC/QB 5 2 would indicate

similar decreases in ARD for increases in the P parameters. However, it is seen from

Figures 1 and 2 that this is not necessarily the case. Figures 1(a) and 2(a) clearly
.

1 7 min mg 6365 1⁄4 R analysis, dimensional by predicted concentrations dust Respirable 2. Table

 

AS/AL AS/AL

QC/QB Regression Regression QC1 QC2 QC3 QC4 C1 C2 C3 C4

73 73 73

 

73 Target AS/AL coefficient constant (m3 min71) (m3 min71) (m3 min71) (m3 min71) (mg m ) (mg m ) (mg m ) (mg m )

1.1 1.11 70.68 34.29 15 30 40 60 15.97 7.99 5.99 3.991.1 1.94 70.68 34.29 15 30 40 60 15.79 7.90 5.92 3.95

1.1 7.74 70.68 34.29 15 30 40 60 15.35 7.68 5.76 3.84

 2.0 1.11 76.54 19.64 15 30 40 60 16.11 8.05 6.04 4.03 2.0 1.94 76.54 19.64 15 30 40 60 13.00 6.50 4.88 3.25 2.0 7.74 76.54 19.64 15 30 40 60 5.31 2.66 1.99 1.33 3.0 1.11 73.29 8.28 15 30 40 60 10.12 5.06 3.80 2.53 3.0 1.94 73.29 8.28 15 30 40 60 7.78 3.89 2.92 1.94

 3.0 7.74 73.29 8.28 15 30 40 60 1.97 0.99 0.74 0.49 4.0 1.11 72.61 5.64 15 30 40 60 9.12 4.56 3.42 2.28 4.0 1.94 72.61 5.64 15 30 40 60 6.65 3.32 2.49 1.66 4.0 7.74 72.61 5.64 15 30 40 60 0.51 0.25 0.19 0.13

0 5 0 5 HS=AL HS=AL

 

Regression Regression 0 5

 

 

 


 

 

 

 

 

 

 

 

 

 

HS=AL coefficient constant

1.1 0.67 70.67 33.76 15 30 40 60 15.88 7.94 5.96 3.97

1.1 1.04 70.67 33.76 15 30 40 60 15.75 7.87 5.90 3.94

1.1 2.39 70.67 33.76 15 30 40 60 15.49 7.74 5.81 3.87

 2.0 0.67 710.3 15.24 15 30 40 60 16.48 8.24 6.18 4.12 2.0 1.04 710.3 15.24 15 30 40 60 12.62 6.31 4.73 3.15 2.0 2.39 710.3 15.24 15 30 40 60 5.34 2.67 2.00 1.33 3.0 0.67 75.07 6.05 15 30 40 60 10.32 5.16 3.87 2.58 3.0 1.04 75.07 6.05 15 30 40 60 7.47 3.73 2.80 1.87

 3.0 2.39 75.07 6.05 15 30 40 60 2.09 1.05 0.78 0.52 4.0 0.67 74.03 3.87 15 30 40 60 9.35 4.67 3.50 2.34 4.0 1.04 74.03 3.87 15 30 40 60 6.32 3.16 2.37 1.58 4.0 2.39 74.03 3.87 15 30 40 60 0.62 0.31 0.23 0.16

 


 

 

 

 
indicate that QC/QB 1⁄4 1.1 would require reducing shroud leakage by amounts greater

than achieved in this work. Doing so would require a near-perfect shroud seal or,

alternatively, near zero leakage.

Figure 3. Predicted ARD concentration as a function of the deck height to shrould leakage area

ratio for QC/QB 1⁄4 2 and four values of QC (m3 min71

).

Figure 4. Predicted ARD concentration as a function of the deck area to shrould leakage area ratio

for QC/QB 1⁄4 2 and four values of QC (m3 min71

).

5.2. Practical usage of the model

The practical usefulness of any model representation is only achieved if it can be readily

applied by the mine operator or engineer. Although this topic has been previously

presented [6], it is presented again here because of its importance in enabling the mine

operator or engineer to protect the health of surface miners by reducing dust emissions.

Figures 3 and 4 present such a representation. By measuring a few basic parameters and

using the equations presented in the figures, there are two methods of estimating the
relative severity of a drill’s dust emissions as well as how much of a relative reduction can

be obtained by changing any given parameter. Using Figure 4 for example, the parameters

that must be measured or estimated are (1) the drill deck shroud area AS, (2) an

approximate amount of shroud leakage area AL or a range for the leakage area, and (3)

the bailing air flow rate, QB. QB is usually known from the drill manufacturer. It should be

noted that QC is perhaps the more difficult parameter to measure and that dust collector

specifications should not be used since collector air flow specifications are made under

ideal conditions with unloaded filters. However, it will be observed that all four curves for

differing QC follow the same trend and, as a result, it is not mandatory to know this value.

The calculated value of ARD is not important because this is a relative value and the

important consideration is where on the curve the drill operates. Therefore, as a first

approximation, all that must be known is the ratio AS/AL. If a more precise estimate is

needed, then it is necessary to measure QC. Making this measurement will allow applying

the correct equation in Figure 4 as well as determining QC/QB. Knowing this ratio will

allow the user to estimate on a more absolute basis the magnitude of the drill dust

emissions using Figures 1 or 2.

These determinations will indicate the long-term average improvement that can be

expected from either increasing the collector air flow (installing a larger collector, for

example) or reducing the amount of shroud leakage.

6. Conclusions

A semi-empirical mathematical model has been developed and expanded to describe ARD

generation on surface coal mine drills. Geometric parameters included are drill deck height

and cross-sectional area, shroud leakage associated with the deck shroud, and the

operational parameters of bailing airflow and dust collector airflow. The relationships

appear to be described by logarithmic functions and yield predictive ARD values which

fall in the range measured on operating drills for collector/bailing air flow ratios greater

than 2. However, at values of collector/bailing air flow ratios of approximately 1.1 the

amount of ARD shows minimal response, if any, to drill deck shroud improvements that

do not result in near-perfect seals. Application of these results should provide mine

operators with sufficient information to determine (1) the relative magnitude of their dust

emissions, (2) where they should focus their efforts to reduce ARD emissions, and (3) the

improvement they could reasonably expect to achieve. Given that exposures of surface

coal mine rock drillers to respirable crystalline silica are of particular concern, significant

reductions of airborne silica dust during drilling may be estimated and achieved through

use of the analysis presented.

7. Disclaimer

The findings and conclusions in this report are those of the authors and do not necessarily

represent the views of the National Institute for Occupational Safety and Health

(NIOSH). Mention of any company or product does not constitute endorsement by

NIOSH.

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