RI 9645 REPORT OF INVESTIGATIONS/1998

Investigation of Coal Properties and Airborne

Respirable Dust Generation

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
Report of Investigations 9645

Investigation of Coal Properties and Airborne

Respirable Dust Generation

John A. Organiscak and Steven J. Page

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, PA

October 1998
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ...................................................................................... 1

Introduction ................................................................................... 2

Experimental design ............................................................................. 3

Test facility ................................................................................. 3

Experimental procedure ....................................................................... 3

Sample collection ............................................................................ 4

Experimental results ............................................................................. 6

Data analysis procedures ....................................................................... 6

Product size ................................................................................. 9

ARD generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

ADL and charge effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Dust control and health impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Appendix.—Coal crushing data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

ILLUSTRATIONS

1. Airborne dust size distributions measured at underground continuous mining operations .................. 5

2. ROM coal product size distributions from underground continuous mining operations ................... 6

3. Roll crusher coal product size distributions ..................................................... 8

4. Roll crusher airborne dust size distributions ..................................................... 8

5. MFR relationships with energy and Schuhmann size function parameters .............................. 9

6. MFR relationships with product dust fines, electrostatic field, and specific ARD ........................ 9

7. ADL relationships with specific ARD and electrostatic field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8. Normalized ARD percentage dispersed from product with respect to ADL and electrostatic field . . . . . . . . . . . 10

TABLES

1. Precision of experimental procedures with PRL coal .............................................. 4

2. Underground mine data ..................................................................... 5

3. Constituents of coal seams used in roll crusher experiments ........................................ 7

4. Linear correlations of coal constituents and experimental variables ................................... 7

A-1. Properties of coal samples tested . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

A-2. Experimental data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

A ampere m /s cubic meter per second 3

hr hour mg/kg milligram per kilogram

kg kilogram mg/m milligram per cubic meter 3

kg/hr kilogram per hour rpm revolution per minute

kW kilowatt s second

L/min liter per minute V volt

m meter V/cm volt per centimeter

min minute W#min watt minute

mm millimeter W#min/kg watt minute per kilogram

m/s meter per second )m micrometer

m /min cubic meter per minute % percent 3

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

Health.

To receive additional information about mining issues or other occupational safety and health problems, call 1-800-35-NIOSH

(1-800-356-4674), or visit the NIOSH Home Page on the World Wide Web at http://www.cdc.gov/niosh
INVESTIGATION OF COAL PROPERTIES AND AIRBORNE

RESPIRABLE DUST GENERATION

By John A. Organiscak and Steven J. Page 1 2

ABSTRACT

Laboratory crushing experiments were conducted on a range of low- to high-volatile bituminous coals to

investigate the various factors influencing airborne respirable dust (ARD) generation. This research was

conducted to identify the principles of ARD liberation from the coal product. Five U.S. bituminous coals were

uniformly prepared and processed through a double roll crusher located in a low-velocity wind tunnel.

Experimental factors studied included inherent coal seam constituents, coal grindability, specific energy of

crushing, product size characteristics, dust cloud electrostatic field, and specific ARD generated.

The results of this investigation indicate that a combination of several factors are associated with ARD

generation. One factor is the effect of coal rank, described by the inherent moist fuel ratio, on the product size

characteristics, defined by Schuhmann size function parameters. Another key factor is the effect of air dry loss

(ADL) moisture in the coal seam on the breakage-induced electrostatic field of airborne dust. The effect of

these factors is that different percentages of <10-)m coal particles are dispersed as ARD. A discussion of

electrostatic field principles, coal ADL, and its effect on ARD generation is presented.

Mining engineer. 1

Physicist. 2

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA.
2

INTRODUCTION

Prolonged exposure to airborne respirable coal dust is between these two breakage processes. A negative airborne

responsible for the prevalence of coal workers' pneumoconiosis dust correlation with coal strength (or positive with coal rank)

(CWP) in the United States. Health research studies have iden- was observed for the tumble tests; however, no airborne dust

tified that the severity of CWP is directly related to the amount correlation was observed for the shatter tests. It was concluded

of respirable dust exposure and the coal rank [Attfield and that weaker coals (higher rank coals) had a lower percentage of

Seixas 1995; Attfield and Morring 1992; Hurley and Maclaren dust present in the product fines dispersed during the shatter

1987]. Since enactment of the 2.0 mg/m dust standard in the breakage tests and that airborne dust generated is somewhat re- 3

Federal Coal Mine Health and Safety Act of 1969 (Public Law lated to the violence upon which the particular coal breaks.

91-173), average dust levels for a continuous mining machine Underground and laboratory studies conducted by the

operator were reduced from over 6 mg/m to current levels just former U.S. Bureau of Mines (USBM) in the late 1980s and 3

under the 2.0 mg/m standard [Attfield and Wagner 1992]. The early 1990s showed an opposite correlation to coal rank (low- 3

National Institute for Occupational Safety and Health (NIOSH) to high-volatile bituminous coals studied) and airborne dust

recently determined through its Coal Worker's X-Ray Surveil- generation compared to previously established coal rank and

lance Program that coal miners continue to have an elevated fines production relationships. An underground survey of 20

risk for CWP under the current 2.0 mg/m dust standard and longwalls operating in 16 different bituminous coal seams 3

recommended a 1.0 mg/m dust standard to reduce the preva- throughout the United States indicated that high-volatile, low- 3

lence of CWP [NIOSH 1995]. To achieve a further reduction ash coal seams (lower rank bituminous coals) tended to produce

in the CWP risk, coal mine worker dust exposure needs to be more ARD [Organiscak et al. 1992]. Additional USBM laboranotably reduced. Determination of the key factors involved in tory work on crushing nine bituminous types of 4.8- to 5.7-mmairborne respirable dust (ARD) generation would likely identify sized feed coals through a small roll crusher (38.1-mm-diam

the most influential engineering control strategies needed for rolls spaced 3.2 mm apart) indicated that lower rank coals, as

improving coal mine dust suppression. described by their inherent moist fuel ratio (MFR ˆ (fixed

Prior research has identified several relationships between carbon ÷ volatile matter) ÷ inherent moisture), also produced

coal rank and dust generation. Laboratory coal comminution more airborne dust [Page et al. 1993]. Although the general

studies have shown a significantly consistent positive correla- airborne dust and coal rank relationships were similar for the

tion between coal rank (described by Hardgrove grindability laboratory and underground studies, differences in the correlaindex (HGI), fuel ratio, vitrinite reflectance, or level of organic tion of particular coal parameters, such as ash, were presumed

metamorphism) and the amount of respirable-sized particles to be an extraneous variable associated with the inherent weakfound in the product [Srikanth et al. 1995; Moore and Bise ness of the coal's cleat (or joint) structure. Others have postu1984; Baafi and Ramani 1979]. These studies show conclusive- lated that coal fragmentation from cutting usually occurs,

ly that either a grinding or crushing process produces more in part, along planes of imperfections (cleats or joints) or weakproduct fines and more respirable sized particles in the product nesses containing mineral matter [Stecklein et al. 1982].

fines for higher rank coals. To identify the underlying factors involved in various relaOther research studies of airborne dust generation and coal tionships observed between various bituminous coals, strength,

rank have shown different relationships compared to the coal product size characteristics, and ARD, the NIOSH Pittsburgh

rank and coal product size conclusion. The National Coal Research Laboratory (PRL) conducted laboratory crushing exBoard's Mining Research Establishment in the United Kingdom periments on larger coal lumps containing inherent planes of

had initially observed discrepancies in airborne dust generation weakness (cleats or joints). This research was conducted to

and the coal product size characteristics with respect to the formulate the principles of ARD liberation from the coal prodbreakage processes of the coal [Knight 1958; Hamilton and uct. Experimental factors studied include inherent coal constitKnight 1957]. Laboratory shatter (drop test) and tumble break- uents, coal HGI, specific energy of crushing, product size

age tests (friability type) were conducted on coals of various characteristics, dust cloud electrostatic field, and specific ARD

ranks mined in the United Kingdom and showed negative cor- generated. This report describes the experimental results of roll

relations between compressive strength and the amount of coal crushing five different U.S. bituminous coals and discusses the

fines produced. Although the higher rank weaker coals (lower air dry loss (ADL) and electrostatic charge effect on ARD

compressive strength coals) consistently produced more product liberation from the coal product.

fines, airborne dust generation differences were observed
3

EXPERIMENTAL DESIGN

The objectives of this research were to (1) develop a re- quantity). Lower velocities permitted dust to escape from the

peatable laboratory testing protocol for generating coal ARD tunnel inlet; therefore, wind tunnel airflow was targeted for

and (2) identify key factors influencing ARD generation, where 0.10 m/s for all of the experiments.

one of the "factors" is different bituminous coals. The intent of

this research was not to conduct experiments with a specialized

coal-cutting or breakage apparatus, but to use commercially

available equipment to accomplish the research objectives. The key elements for reducing crushing experimental error

A double roll crusher was selected to study the primary break- are feed size and feed methods. In development of a reproducage properties of medium-sized coal lumps (approximately ible crushing procedure, both size and feed methods were

50 mm), because it has a small size reduction ratio of 1.5 to 5:1 studied in a series of crushing experiments conducted on 470 kg

(ratio of average feed size to product size) without a significant of Pittsburgh coal obtained from PRL's Safety Research Coal

amount of regrinding [Cummins and Given 1973]. The five Mine. A large batch of PRL coal lumps were jaw-crushed,

different bituminous coals were roll-crushed under uniform screened, and riffled (a sample splitting process) [Taggart 1945]

procedures to investigate the effects of coal MFR, physical coal into 32 representative test samples (14.7 kg by weight) of

strength properties (coal HGI and specific energy of crushing), various feed sizes that would be tested under different feed

coal breakage characteristics (product size distribution), and methods. The sizes to be tested included 50.0 by 25.0 mm, 25.0

dust cloud electrostatic field on specific ARD generation. by 19.0 mm, 19.0 by 12.5 mm, and an equivalent weight threeTEST FACILITY

The experimental test facility was comprised of a roll crusher feeder slowly trickles coal into the crusher, approximately

located in the intake end of a 1.2-m-high by 0.6-m-wide wood- 2 min per sample). The various sized test samples were ranframed, plastic sheath-enclosed wind tunnel 6.1 m long. A dust domly processed through the crusher for the two feed methods,

collector and exhaust fan were located at the discharge end of the yielding four runs for each test condition.

tunnel. The crusher was a 1.1-kW compact double roll crusher The crushing variables studied during these tests included

(79.4-mm-diam rolls) operating at approximately 70 rpm with energy consumption, ARD, dust cloud electrostatic field, and

twenty-four 12.7-mm-high staggered teeth on each roll. An in- Schuhmann size function parameters. Their averages and

ductive current transformer (±0.1 A) was installed to monitor the variations (standard error and coefficient of variation (CV)) are

crusher's current usage. The crusher's operating capacity was shown in table 1. Energy consumption was determined from

227-1,361 kg/hr of up to 101.6 mm feed-size lumps of coal or crusher current, voltage, and time. ARD was determined by

rock material. applying the American Conference of Governmental Industrial

Dust sampling was conducted 3 m downstream of the Hygienists definition of respirable dust to the mass sizes

crusher and approximately 2.4 m upstream of the tunnel transi- collected on the Sierra 298 impactors, sampling rate, and time

tion to the dust collector and exhaust fan. Dust sampling was [Potts et al. 1990]. The electrostatic field was determined by

conducted with two Sierra 298 personal sampling impactors at averaging the field over a time period equal to the crusher

2.0 L/min, each equipped with the standard inlet cowl and operating time plus 10 s. This would allow sufficient time for

positioned at one-half the tunnel height from the floor and one- the generated dust cloud to travel beyond the field meter posithird the tunnel width from a wall. The impactor stages 1 tion. The crusher product was screened for size classification.

through 6 (20-)m through 1.55-)m cut point sizes) were used Schuhmann size function parameters [Schuhmann 1940] dewith the <1.55-)m particle sizes collected on the final filter. An fined in table 1 were determined by nonlinear least squares reMIE RAM-1 sampler continuously monitored the respirable gression of the cumulative size distribution data.

fraction of dust from a 10-mm Dorr-Oliver cyclone located in The experimental results shown in table 1 indicate that the

the middle of the sampling location [Williams and Timko batch-feed method for both the size mix and the 50.0- by

1984]. All of the sampler inlets were faced into the airflow. 25.0-mm feed samples had the lowest amount of measurement

Dust cloud electrostatic field measurements were taken error for all of the crusher variables investigated. ARD had the

immediately downstream of the crusher (within 0.3 m) with a highest variability out of all of the measured variables, but had

Monroe 245 electrostatic field meter and stored on an analog CVs below 20% for the batch-feed method of the size mix and

datalogger. Air velocities were determined from the time it the 50.0- by 25.0-mm samples. Energy consumption measuretook the dust to travel 3 m to the RAM-1 sampling location ments were the least variable for the batch-feed method. The

after crusher startup. Preliminary crushing tests indicated that larger variations in energy consumption measurements for the

the lowest possible wind tunnel air velocity to maximize dust trickle-feed method were likely due to poor resolution of the

concentrations and mass collection was 0.10 m/s (4.5 m /min air very low crushing currents measured. Both the electrostatic 3

EXPERIMENTAL PROCEDURE

size mixture. The two feed methods studied were batch-feed

(crusher self-feeds the batch of coal from its hopper approximately 30 s per sample) and trickle-feed (a separate vibrating
4

Table 1.—Precision of experimental procedures with PRL coal

Feed method

and size Mean Mean Mean Top size s ,

Energy consumption, Electrostatic field, Schuhmann size function

W#min V/cm (all R 's>0.99) ARD, mg/m3

2

( *X/n) ( *X/n) ( *X/n) "a", mm mm

Standard CV Standard CV Standard CV Expoerror (s/ error (s/ error (s/ nent s 

(s/2n) mean) (s/2n) mean) (s/2n) mean) "b"

a

b

Batch:

 Mix . . . . . . . . . . . . . 77.09 1.91 0.05 6.17 0.57 0.18 119.35 8.45 0.07 15.07 0.11 0.88 0.01

 50.0 by 25.0 mm . . 88.00 5.82 0.13 7.38 0.63 0.17 134.55 6.25 0.05 13.78 0.12 0.89 0.01

 25.0 by 19.0 mm . . 94.06 0.80 0.02 5.67 1.23 0.43 112.90 7.87 0.07 14.53 0.15 0.86 0.01

 19.0 by 12.5 mm . . 66.59 4.11 0.12 5.71 1.29 0.45 115.50 16.50 0.14 17.68 0.15 0.89 0.01

Trickle:

 Mix . . . . . . . . . . . . . 35.15 2.38 0.14 4.55 0.79 0.35 118.90 5.69 0.05 15.17 0.17 1.28 0.03

 50.0 by 25.0 mm . . 70.73 7.47 0.21 4.53 0.87 0.38 115.15 10.04 0.09 14.31 0.13 1.12 0.02

 25.0 by 19.0 mm . . 23.90 7.48 0.63 4.65 0.93 0.40 114.00 9.17 0.08 14.75 0.15 1.31 0.03

 19.0 by 12.5 mm . . 22.36 6.12 0.55 3.60 0.77 0.43 77.80 6.38 0.08 16.26 0.18 1.60 0.04

X = variable Schuhmann size function: Y = (X/a) ,b

n = number of measurements where Y = cumulative percent of weight less than X,

s = standard deviation X = size of particles, mm,

CV = coefficient of variation a = Schuhmann top size regression parameter,

b = Schuhmann exponent regression parameter,

s = standard error of regression parameter "a", a

 and s = standard error of regression parameter "b". b

field measurements and Schuhmann size function parameters lumps: >50.0-mm, 50.0 by 25.0 mm, and 25.0 by 19.0 mm.

had low measurement errors for all of the feed method and size Approximately 50 kg of bulk coal was collected each day.

tests. One-half of the test samples were analyzed for coal con- ROM product samples were also collected from shuttle cars

stituents ([ASTM (1996a)], not shown in table 1) and indicated during the cut at the feeder breaker dump point for size analysis.

that CVs for the inherent moisture, ash, volatile, and fixed car- Underground dust sampling was conducted in the intake

bon were all below 10%. These results indicate that representative feed samples were obtained with the feed preparation

procedures.

The batch-feed crushing method of a coal size mix was the

procedure chosen for minimizing the variable measurement error of the remaining coal ARD generation experiments. The

Schuhmann exponent parameters <1 for the batch-feed method

indicate that some regrinding occurs compared with the Schuhmann exponent parameters >1 for the trickle-feed method. This

parallels the <1 Schuhmann exponent parameters observed with

run-of-mine (ROM) coals [Ramani et al. 1987]. The size mix

feed was preferred to the 50.0- by 25.0-mm feed because, as

bulk coal samples are processed (crushed and screened) to make

the feed samples, more feed material can be obtained from a

given amount of bulk coal.

SAMPLE COLLECTION

Three bituminous coals were targeted for sample collection.

These ranged from a low-volatile, high-ash coal (higher rank)

to a high-volatile, low-ash coal (lower rank) with one coal type

in the middle of this range. The bulk coal samples were collected at three continuous miner sections located in the Eagle and

Upper Freeport Seams in West Virginia and the Blind Canyon

Seam in Utah. Three days of coal collection and airborne dust

sampling were conducted at each mining section. On each day,

both coal and dust samples were collected for one mining cut.

The coal was channeled from the entry rib, outby the mined cut.

This coal was hand screened and packaged into three sizes of

and return of the mined cut. Airborne dust was measured with

personal respirable dust samplers, Sierra 298 personal impactors, and a RAM-1 (only at the return location). The operational parameters and dust concentrations measured at the mines

visited are shown in table 2, and the airborne dust size characteristics in the mining return air course are shown in figure 1.

The airborne dust size distribution data suggest that flooded-bed

scrubber (irrigated-filter dust collector) use on the continuous

mining machine affected the airborne dust measurements. Airborne dust size data obtained in the Eagle Seam while using a

machine-mounted flooded-bed scrubber indicated a noticeably

smaller dust size distribution (mean mass aerodynamic diameter

(MMAD) ˆ 7.5 )m, geometric standard deviation (GSD) ˆ 3.7)

in the return air compared with that at the other two operations

(MMAD ˆ 16.2 and 27.8 )m, GSD ˆ 4.2 and 4.3). The continuous miner operation in the Eagle Seam also had a noticeably

less face air quantity of 2.9 m /s than that at the other opera- 3

tions, suggesting that a notable portion of the face ventilation

and ARD was effectively captured by the flooded-bed scrubber

at this operation. The operation without a flooded-bed scrubber

in the Blind Canyon Seam used the highest ventilation quantity

of 24.0 m /s and had the largest airborne size distribution 3

(MMAD ˆ 27.8 )m, GSD ˆ 4.3). Because flooded-bed scrubbers were used on two of the continuous mining machines

sampled, an ARD generation analysis between mining operations would be unreliable. Therefore, all ARD generation conclusions are based on laboratory crushing experiments using a

comparable operating procedure.
5

Figure 1.—Airborne dust size distributions measured at underground continuous mining operations.

Table 2.—Underground mine data

Underground

parameter

Coal seam

Eagle Upper Freeport Blind Canyon

Seam height, m . . . . . . . . . . 2.4 . . . . . . . . . . . . . . . . . 2.7 . . . . . . . . . . . . . . . . . . . . . . . 2.7.

Cut depth, m . . . . . . . . . . . . . 9.1 . . . . . . . . . . . . . . . . . 7.6 . . . . . . . . . . . . . . . . . . . . . . . 12.2.

Ventilation type .......... Exhaust curtain ....... Exhaust tube ............... Exhaust tube.

Return air quantity, m /s . . . 3 1 2.89 (0.12) ........... 12.55 (0.47) ................ 24.01 (1.02).

Continuous miner dust Cutter boom sprays, Cutter boom sprays, flooded-bed Cutter boom sprays, spray

 controls flooded-bed scrubber, scrubber, radio remote control. fan system, radio remote

radio remote control. control.

Water added to coal, 3.5 (0.4) .............

 percent by weight.2

2.5 (0.5) ................... 4.6 (0.3).

Intake respirable dust 0.56 (0.10) ........... 3

 concentration, mg/m .3

1.59 (0.77) ................. 0.40 (0.22).

Return respirable dust 2.89 (0.12) ........... 3

 concentration, mg/m .3

2.94 (0.37) ................. 4.04 (1.01).

NOTE: All measurements for air quantity and dust concentration are averages, followed by standard error in parentheses.

Measurement taken at the return dust sampling location. 1

Moisture determined from air drying ROM coal samples. 2

Personal dust sampling at 2 L/min. 3

The underground bulk coal samples were processed in the crushing experiments. The ROM samples collected from the

laboratory to obtain two mixed size test samples for each min- three mining operations surveyed were also screened for size

ing cut, for a total of six test samples for each mine. Jaw analysis.

crushing was conducted on the larger lumps to obtain equal The underground ROM product size distributions conportions of feed sizes needed for testing. Riffling was done to firmed that some coal product regrinding occurs during undersplit the coal samples from each cut into equal portions. All of ground coal cutting. The ROM coal product size distributions

the coal test samples were weighed and stored in sealed cans. are shown in figure 2 and are described by Schuhmann exA high-volatile, high-ash bulk coal, collected from the Wadge ponent parameters <1. This is similar to the PRL coal product

Seam in Colorado for another project, was also processed in the size distribution for the batch-feed crushing procedure.

same manner to obtain four additional test samples for the
6

Figure 2.—ROM coal product size distributions from underground continuous mining operations.

EXPERIMENTAL RESULTS

The coal samples were randomly run in the roll crusher test distributions generated for the various coal seams were more

facility using the batch feed of size mix procedure. Experi- consistent than the coal product size distributions.

mental factors studied included inherent coal constituents, coal

HGI, specific energy of crushing, product size attributes, dust

cloud electrostatic field, and specific ARD generated. A small

coal test sample was riffled from the crushed product after

screening to determine the coal constituents by proximate analysis [ASTM 1996a] and to determine the HGI [ASTM 1996b].

The coal constituent and HGI data are included in table A-1 in

the appendix. The specific energy is the energy consumed per

unit weight of the coal sample crushed. The specific ARD is

the total amount of ARD generated in the airstream per unit of

material crushed. The four batch-feed, size-mix tests run on

PRL's Pittsburgh coal were also included in this data analysis.

Thus, 26 tests were completed by the same crushing procedure

on five low- to high-volatile bituminous coals. All of the experimental data are included in table A-2 in the appendix.

Good experimental precision was obtained for each seam

tested. The average coal constituents (with standard errors) for

the five bituminous coal seams tested are shown in table 3.

Low sample variability for each coal seam tested was achieved

with the coal collection and sample preparation procedures.

Figures 3 and 4 show the average coal product and airborne

dust size distributions (with standard error bars) for each of the

coal seams tested. The coal size distributions illustrate that the

various bituminous coals have distinct breakage attributes when

crushed under a uniform process. The airborne dust size

DATA ANALYSIS PROCEDURES

Associations between the experimental variables measured

were primarily analyzed by both parametric correlations and

scatter plot examination. The linear correlations, sample size, and

significance levels between inherent coal constituents, coal HGI,

specific energy of crushing, Schuhmann size function parameters,

dust cloud electrostatic field, and specific ARD generated are

shown in table 4. Multivariate statistical description of ARD with

the variables measured was hindered by the interdependence of

many of the inherent coal constituents and size parameter

variables and by the limited number of data points.

The strongest linear and nonlinear associations of importance

are shown in figures 5 through 8. Specific energy, Schuhmann

size function parameters, amount of dust present in the coal product, dust cloud electrostatic field, and specific ARD are presented

with respect to coal rank, as described by its MFR, in figures 5-6.

Specific ARD and dust cloud electrostatic field are presented with

respect to ADL moisture in the coal in figure 7. Specific ARD

generation is expressed as a normalized variable for the specific

amount of <10-)m dust particles in the coal product (percentage

of respirable dust that becomes airborne) and presented with

respect to ADL and electrostatic field measurements in figure 8.
7

Table 3.—Constituents of coal seams used in roll crusher experiments

Coal seam ADL Ash content Fixed carbon 1 Inherent Volatile

moisture matter 2 2 2 2

Eagle . . . . . . . . . . . 1.0 (0.0) 0.5 (0.0) 7.2 (0.6) 32.2 (0.2) 60.1 (0.5)

Upper Freeport . . . . 1.1 (0.0) 0.3 (0.0) 18.5 (0.8) 16.6 (0.1) 64.7 (0.8)

Blind Canyon . . . . . 3.2 (0.2) 1.4 (0.2) 6.7 (0.8) 43.6 (0.2) 48.2 (0.7)

Wadge . . . . . . . . . . 5.2 (0.2) 2.9 (0.3) 12.9 (0.7) 38.3 (0.4) 45.8 (0.2)

Pittsburgh (PRL) . . . 0.9 (0.1) 0.9 (0.1) 6.0 (0.3) 37.3 (0.2) 55.8 (0.0)

NOTE: All measurements are averages, followed by standard error in parentheses.

Reported as percent-weight on an as-received basis. 1

Reported as percent-weight on an as-determined basis (weight percentages determined without 2

 the ADL).

Table 4.—Linear correlations of coal constituents and experimental variables

Parameter energy of top exponent Coal Electrostatic Specific

HGI field ARD 

Specific Schumann Schuhmann

roll crusher size "a" "b" 1 1 

ADL:2

 Correlation coefficient . . .

 Sample size . . . . . . . . . .

 Significance level . . . . . .

 ̃0.650  ̃0.102 0.739 0.759  ̃0.843 0.155

24 26 26 26 26 26

0.001 0.620 0.000 0.000 0.000 0.451

Inherent moisture:3

 Correlation coefficient . . .

 Sample size . . . . . . . . . .

 Significance level . . . . . .

 ̃0.685 0.027 0.737 0.869  ̃0.801  ̃0.040

24 26 26 26 26 26

0.000 0.896 0.000 0.000 0.000 0.847

Ash content:3

 Correlation coefficient . . .

 Sample size . . . . . . . . . .

 Significance level . . . . . .

0.726  ̃0.770  ̃0.549  ̃0.392 0.214  ̃0.182

24 26 26 26 26 26

0.000 0.000 0.004 0.047 0.293 0.372

Volatile matter:3

 Correlation coefficient . . .

 Sample size . . . . . . . . . .

 Significance level . . . . . .

 ̃0.956 0.666 0.907 0.750  ̃0.611 0.250

24 26 26 26 26 26

0.000 0.000 0.000 0.000 0.001 0.219

Fixed carbon:3

 Correlation coefficient . . .

 Sample size . . . . . . . . . .

 Significance level . . . . . .

0.871  ̃0.353  ̃0.932  ̃0.847 0.778  ̃0.206

24 26 26 26 26 26

0.000 0.077 0.000 0.000 0.000 0.312

Fuel ratio:4

 Correlation coefficient . . .

 Sample size . . . . . . . . . .

 Significance level . . . . . .

0.988  ̃0.679  ̃0.902  ̃0.820 0.672  ̃0.175

24 26 26 26 26 26

0.000 0.000 0.000 0.000 0.000 0.392

Inherent moist fuel

 ratio (MFR):5

 Correlation coefficient . . .

 Sample size . . . . . . . . . .

 Significance level . . . . . .

0.986  ̃0.681  ̃0.881  ̃0.834 0.674  ̃0.134

24 26 26 26 26 26

0.000 0.000 0.000 0.000 0.000 0.514

Specific energy of roll

 crusher:

 Correlation coefficient . . .

 Sample size . . . . . . . . . .

 Significance level . . . . . .

 ̃0.647 1.000 0.423 0.360  ̃0.188  ̃0.069

24 26 26 26 26 26

0.001 0.000 0.032 0.071 0.358 0.738

Schuhmann top size "a":1

 Correlation coefficient  ̃0.935 0.423 1.000 0.803  ̃0.752 0.370

Sample size . . . . . . . . . . . .

 Significance level 0.000 0.031 0.000 0.000 0.000 0.063

24 26 26 26 26 26

Schuhmann exponent "b": 1

 Correlation coefficient  ̃0.882 0.360 0.803 1.000  ̃0.866  ̃0.122

Sample size . . . . . . . . . . . .

 Significance level . . . . . .

24 26 26 26 26 26

0.000 0.071 0.000 0.000 0.000 0.554

Schuhmann size function Y ˆ (X/a)b. 1

Reported as percent-weight on an as-received basis. 2

Reported as percent-weight on an as-determined basis (weight percentages determined without the ADL). 3

Fuel ratio is defined as fixed carbon ÷ volatile matter. 4

Inherent moist fuel ratio is defined as fuel ratio ÷ inherent moisture. 5
8

Figure 3.—Roll crusher coal product size distributions.

Figure 4.—Roll crusher airborne dust size distributions.
9

Figure 5.—MFR relationships with energy and Schuhmann size

function parameters.

Figure 6.—MFR relationships with product dust fines,

electrostatic field, and specific ARD.

PRODUCT SIZE

Results of the roll crushing tests show that the higher MFR specific energy, and Schuhmann size function parameters indicate

coals are significantly correlated with HGI and a smaller product that nonlinear associations exist between some of these variables.

sized distribution. Table 4 shows the linear correlations of in- One notable nonlinear association is between the coal's MFR and

herent coal seam parameters and experimental variables meas- crusher specific energy (see figure 5A). Specific energy seems to

ured. All of the inherent coal constituents are significantly have a direct relationship for MFRs below 1 (two western U.S.

correlated to HGI and Schuhmann size function parameters coals tested) and an indirect relationship for MFRs above 1 (three

(significance levels < 0.05). The MFR had one of the highest eastern U.S. coals tested). An apparent physical difference that

linear correlations to HGI and Schuhmann size function parame- may explain the inconsistent energy relationship between these

ters. Noticeably lower correlations were observed between roll two coal groups (eastern and western) is that the eastern coals

crusher specific energy and many of the coal constituents, MFR,

and Schuhmann size function parameters, which suggests that

some unquantified factor(s) in this experiment influenced the coal

crushing specific energy.

Scatter plot examination of coal constituents, MFR, crusher
10

Figure 7.—ADL relationships with specific ARD and electrostatic field.

Figure 8.—Normalized ARD percentage dispersed from product

with respect to ADL and electrostatic field.

tested tended to have a more distinguishable cleat system than the

western coals (eastern coals had more planes of weakness than the

western coals). The amount of specific ARD generated from roll crushing

MFR also has strong relationships between the Schuhmann was not conclusively correlated with any one of the coal consize function parameters, exhibiting a direct linear relationship to stituents, MFR, amount of fines produced, or specific energy, but

smaller dust particles in the coal product. These breakage involved the amount of ADL moisture in the coal and the dust

attributes can be observed in plots of the coal's MFR and the cloud electrostatic field. Table 4 shows dust cloud electrostatic

Schuhmann size function parameters (see figures 5B and 5C). As field and specific ARD correlations with coal seam constituents,

the MFR increases, both Schuhmann size function parameters MFR, crusher specific energy, and Schuhmann size function padecrease, resulting in a finer coal product size distribution with rameters. The electrostatic field had significant correlations to

more fine particles in the coal product. This direct relationship most of the coal constituents, MFR, and the Schuhmann size

between MFR and respirable-sized dust particles in the coal function parameters of the crushed coal; the strongest correlations

product (defined as <10 )m) is shown in figure 6A. The specific were to the ADL and inherent moisture constituents. Insignificant

amount of <10-)m dust particles in the product is determined linear ARD correlations were observed for all of the coal confrom the Schuhmann size function parameters measured for each stituents, MFR, Schuhmann size function parameters, and crusher

test. This analysis assumed that the weight of <10-)m coal par- specific energy consumption. Figure 6 shows that, although the

ticles can be reasonably projected by the Schuhmann size function MFR is directly related to the amount of <10-)m fines in the

parameters. All of the Schuhmann size function parameters are product (see figure 6A) and the dust cloud electrostatic field (see

efficient (R 's > 0.99) for size data between 250 )m and 12.5 mm. figure 6B), it is not associated with specific ARD generated (see 2

Others have shown this same direct effect between coal rank and figure 6C). Others have observed similar coal rank discrepancies

product fines [Srikanth et al. 1995; Moore and Bise 1984; Baafi between the amount of fine coal particles produced and the

and Ramani 1979].

ARD GENERATION
ˆ  ̄

11

amount of airborne dust generated [Knight 1958; Hamilton and dust as opposed to producing fresh airborne dust from comKnight 1957]. minution. The electrostatic fields measured in this study and

Scatter plot analysis of specific ARD data with all of the coal shown in figure 7B suggest that, although some coal types may

constituents and crushing variables indicates that the best syste- exhibit close to net neutral fields, the particle charging asmatic association observed with specific ARD was the amount of sociated with some eastern coal types has a net positive polarity

ADL moisture in the coals tested (see figure 7A). ADL also had due to primary breakage.

the strongest correlation with electrostatic field measurements The above results indicate that no single factor is decisively

(see figure 7B). ADL moisture is the free water present in the associated with ARD generation; rather, ARD generation is

coal's internal fracture structure, with a normal vapor pressure of likely the result of several interrelated factors. These factors inwater [Leonard 1979]. Inherent moisture is bound water in the in- volve coal fines production, ADL moisture, and/or dust cloud

ternal pore structure of the coal, with a vapor pressure lower than electrostatic field, all of which are associated with coal MFR.

normal [Leonard 1979]. Inherent moisture in the coals tested Due to the limited number of tests and the interdependence of

showed a less systematic association with specific ARD, but also these measured variables, statistical model building was prohad a strong linear correlation to electrostatic field. hibitive. To provide some insight into ARD produced, the speThe graph of the ADL moisture and specific ARD (fig- cific ARD generated was normalized for the specific amount of

ure 7A) shows that the correlation is separated into two distinct <10-)m dust particles in the product.

groups of data: a group below (eastern coals) and above Figure 8 shows the percentage of respirable dust that be-

(western coals) 2% ADL moisture. The eastern coals below 2% comes airborne from the <10-)m product fines compared with

ADL had considerable variability in specific ARD generated for ADL moisture and dust cloud electrostatic field, respectively.

a very small change in ADL moisture. Western coals had a less These graphs indicate that the amount of product fines that besensitive and negative association between specific ARD and come airborne as respirable dust is positively correlated (0.839)

ADL moisture. with ADL moisture (figure 8A) and negatively correlated

ADL moisture and dust cloud electrostatic field also have ( ̃0.802) with electrostatic field (figure 8B). This analysis insimilar data groupings (see figure 7B). The electrostatic field dicates that although higher MFR coals produce more fines,

measurements for eastern coals are much more variable for a they seem to have a smaller percentage of respirable particles

small range in ADL moisture compared with those for the that become airborne because of some effect associated with

western coals tested. Coal types with <2% ADL moisture coal ADL moisture and/or dust cloud electrostatic field. We

(eastern coals) also tended to generate higher electrostatic fields postulate that the ADL moisture would likely influence the rein their airborne dust clouds compared with coals having >2% sultant electrostatic charge of the dust fines immediately after

ADL moisture (western coals). Other researchers, e.g., Polat coal breakage by providing leakage paths for charge dissipation.

et al. [1993], have shown that there are almost equal numbers The electrostatic field could reflect the strength of dust fines atof positively and negatively charged particles for coals, tachment to larger particles before airborne entrainment and/or

resulting in a net neutral dust cloud. However, these past reflect airborne agglomeration of respirable size particles to

research results may be indicative of the process of redispersing larger dust particles.

DISCUSSION

ADL AND CHARGE EFFECT

The data represented in table 4 and in figures 5-7 show

complex interactions between the numerous interrelated

factors measured. Although distinct trends are observed, l ˆ 0,1,2,3

there are not sufficient data to formulate a predictive model.

However, a descriptive mathematical representation based on m ˆ 2,3,4

established physical laws is suggested. This mathematical

representation involves the association observed between n ˆ 1,2,3

specific ARD and ADL. From Figure 7A, it is observed that

the data, despite the absence of a peak at an ADL of 2%, a,b ˆ constants

could follow a set of serpentinelike curves having the functional form
ˆ %Ò  ̄

(

%Ò  ̄

12

The special case l ˆ 1, m ˆ 2, n ˆ 2 is the serpentine function. where F(a) ˆ F(a{MFR}) ˆ some function that represents the

Although it was studied and named by Sir Isaac Newton in dipole charge separations to be de1701, the serpentine had been studied earlier by de L'Hôpital pendent on the airborne dust cloud

and Huygens in 1692. These serpentinelike functions have concentration (higher concentrasignificant physical importance in that they recur frequently in tions imply smaller mean particle

solutions that describe gravitational and electrostatic fields. separation), presumed to be afBecause of the same inverse square law (differing only by fected by the coal MFR;

constants) of both gravitational and electrostatic fields, these

functional forms can be obtained for certain volumetric mass and G(q) ˆ G(q{ADL}) ˆ some function that represents the

and charge distributions. As an example, the electrostatic field dipole charge magnitude, preof an electric dipole has the electric field E at points along the sumed to be affected by the coal

perpendicular bisector of the dipole axis (x) given by ADL.

l ˆ 0 ARD generated and the charge-associated variable ADL, as

m ˆ 2 dynamic charge distribution and lack of data about the ADL ˆ

n ˆ 3 other parameters that may be significant are required.

where Ò ˆ permittivity of free space, 0

2a ˆ charge separation, Given that the coal product size characteristics and the perand q ˆ charge magnitude. affect the amount of ARD generated, aspects of these coal propFor distances where x»a, the essential properties of this charge tory research has consistently shown that reducing the amount

distribution, defined as 2aq ˆ p, the electric dipole moment, of energy expended on comminution and/or increasing bit penenter only as a product. This means that if E is measured at etration of cutting a given coal will increase the coal product

various distances from the dipole, q and 2a can never be size distribution and reduce the amount of ARD generated

deduced separately, but only as the product 2aq. If q were [Pomeroy 1963; Kurth et al. 1975; Strebig and Zeller 1975;

doubled and a simultaneously halved, E at large distances from Roepke et al. 1976]. Several field studies substantiated that

the dipole would not change. If the dust clouds generated by changing coal-cutting systems to increase bit penetration rethe crusher in these tests are considered to consist of particles of duced ARD [Brooker 1979; Ludlow and Jankowski 1984].

positive and negative charge (not necessarily equal in mag- Thus, a key element to dust control is improving the energy efnitude), then E may be considered to be a vector sum of many ficiency of cutting systems to increase product size and reduce

electric dipoles having distributions of a and q. The resultant ARD generation.

field could have a serpentinelike functional form. Another element of dust control involves the suppression

In the presently described tests, E was measured at a fixed and/or capture of ARD. Research has shown that coal wetting

point with the generated three-dimensional dust cloud passing reduces ARD generated from breakage; however, the wetting

over the fixed measuring point. This is the equivalent of meas- effectiveness varies for coal rank, degrees of mixing, and time

uring E at various points r along the same direction of motion after application [Knight 1958]. Past research on water adfor a fixed dust cloud charge distribution in each test. Assum- ditives (surfactants) to improve coal wettability and ARD suping varying degrees of charge state for the different coal pression has resulted in mixed and inconclusive results [Kost

samples crushed, the variations in E measured might be ex- et al. 1981]. A more recent laboratory surfactant study on coal

pected to have a form similar to wettability and airborne dust capture from water sprays has

In a fashion similar to the previously discussed characteristic of p, the same field can be obtained at a different point

for variations in q (determined in large part by the ADL) merely

by adjusting r. Therefore, a serpentinelike functional relationship can be reasonably expected to exist between the specific

shown in figure 7A. However, due to the complexity of the

2% value, further investigations of additional coal seams and

DUST CONTROL AND HEALTH IMPACTS

centage of <10-)m dust particles entrained into the air both

erties should be exploitable for ARD control. Previous laborashown that a coal powder sink test may be a good initial screening tool for surfactant selection, but does not necessarily predict

its airborne dust capture effectiveness [Kim and Tien 1993].

Additional laboratory research on water spray droplet charging
13

with respect to concentration of cationic surfactants showed a health is lung dust deposition. Previous research has shown that

strong correlation between water droplet charge and airborne lung deposition of aerosol particles increases directly with the

coal dust capture [Polat et al. 1993]. These research findings aerosol's charge properties [Melandri et al. 1983]. Prior coal

suggest that coal wettability is not the only key element in- mine worker health studies have shown an increased prevalence

volved in spray dust suppression effectiveness; electrostatic of CWP for higher rank coals [Attfield and Seixas 1995; Attcharge properties of the coal dust cloud and spray droplets may field and Morring 1992; Hurley and Maclaren 1987]. This coal

be another influential factor involved in a surfactant's ability to rank and CWP relationship may be in part related to the insuppress or capture ARD. crease in dust cloud charging properties of higher rank biOne final aspect that the electrostatic charge properties of tuminous coals observed in this research study and the increase

coal dust observed in this study may have on coal mine worker in lung deposition observed by Melandri et al. [1983].

CONCLUSIONS

A reproducible laboratory coal-crushing test procedure for noticeably influenced by the coal cleat structure. No sigARD generation was developed, and ARD generation was re- nificant correlations were observed between specific energy

lated to multiple factors associated with coal rank. A coal feed and either dust cloud electrostatic field or specific ARD. The

mix of equal portions of 50.0- by 25.0-mm, 25.0- by 19.0-mm, amount of ADL moisture in the coal had the best association

and 19.0- by 12.5-mm coal sizes processed through a double with both the dust cloud electrostatic field and the specific

roll crusher in a wind tunnel provided the least amount of error ARD generated. Examination of the specific ARD normalized

in the laboratory measurements. Experimental factors studied for the specific amount of <10-)m product fines indicates that

were inherent coal constituents, coal HGI, specific energy of the net effect of coal MFR, fines production, ADL moisture,

crushing, product size attributes, dust cloud electrostatic field, and/or electrostatic field properties result in different perand specific ARD generation. Five low- to high-volatile U.S. centages of product fines becoming ARD. Results from the

bituminous coals were studied using the coal-crushing pro- five coals tested show that the higher MFR bituminous coals

cedure developed. had lower percentages of ARD generation per amount of dust

Results of these crushing experiments indicate that a com- fines produced with higher dust cloud electrostatic field

bination of several factors are associated with specific ARD values. Future research should focus on determining if coal

generation. MFR was directly correlated with more coal dust charging properties can be exploited by surfactant

product fines and dust cloud electrostatic field, but did not have application to improve water spray dust suppression and if

the same conclusive correlation with specific ARD. Specific worker CWP is in part influenced by higher dust-charging

energy had a nonlinear association with MFR and seemed to be properties of higher MFR coals.

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Ludlow J, Jankowski RA [1984]. Use lower shearer drum speeds to Ramani RV, Mutmansky JM, Bhaskar R, Qin J [1987]. Fundamental

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440-447.
15

APPENDIX.—COAL CRUSHING DATA

Table A-1.—Properties of coal samples tested

Coal seam ADL HGI Sample Inherent Ash Volatile Fixed

No. moisture content matter carbon

1

222 2

Eagle ........... 1 1.89 0.57 5.60 32.45 61.38 62

Eagle ........... 2 0.93 0.64 5.41 32.35 61.60 60

Eagle ........... 3 0.97 0.55 8.24 32.39 58.82 55

Eagle ........... 4 0.94 0.58 9.24 31.37 58.81 55

Eagle ........... 5 1.11 0.47 7.25 32.00 60.28 59

Eagle ........... 6 1.03 0.42 7.54 32.47 59.57 58

Upper Freeport .... 1 1.20 0.29 17.64 16.38 65.69 91

Upper Freeport .... 2 1.11 0.29 17.32 16.17 66.22 91

Upper Freeport .... 3 1.10 0.34 16.33 16.94 66.39 89

Upper Freeport .... 4 1.05 0.37 17.85 16.54 65.24 87

Upper Freeport .... 5 1.16 0.32 21.26 16.34 62.08 89

Upper Freeport .... 6 1.08 0.27 20.40 16.96 62.37 89

Blind Canyon ..... 1 3.82 1.42 5.49 43.47 49.62 49

Blind Canyon ..... 2 3.40 1.83 5.31 43.63 49.23 49

Blind Canyon ..... 3 3.10 1.21 10.32 42.54 45.54 48

Blind Canyon ..... 4 2.79 0.83 8.09 44.32 46.76 46

Blind Canyon ..... 5 3.35 1.34 6.02 43.72 48.92 48

Blind Canyon ..... 6 2.87 1.84 5.09 44.03 49.04 47

Wadge .......... 1 4.57 3.75 11.49 38.41 46.35 42

Wadge .......... 2 5.55 2.43 12.22 39.46 45.46 42

Wadge .......... 3 4.96 3.06 13.45 37.54 45.95 43

Wadge .......... 4 5.55 2.50 14.49 37.71 45.30 43

Pittsburgh (PRL) . . 1 0.77 0.98 5.40 37.89 55.73 NA

Pittsburgh (PRL) . . 2 0.74 1.00 5.87 37.48 55.65 NA

Pittsburgh (PRL) . . 3 1.13 0.73 6.66 36.80 55.81 51

Pittsburgh (PRL) . . 4 1.12 0.84 6.15 37.17 55.84 51

NA Data not available.

Reported as percent-weight on an as-received basis. 1

Reported as percent-weight on an as-determined basis (weight percentages determined without the ADL). 2
16

Table A-2.—Experimental data

Coal seam weight, time, quantity, top size "a", level, Sample Crusher Schuhmann static

No. current, A exponent "b" field,

Feed Crush Air Schuhmann ARD

kg min m /min mm mg/m

1

3

ElectroV/cm

2

3

Eagle ........... 1 12.652 0.47 0.18 49.8 15.119 0.786 112.8 11.07

Eagle ........... 2 12.624 0.47 0.24 49.8 14.954 0.782 112.4 13.36

Eagle ........... 3 13.894 0.50 0.27 45.3 15.178 0.808 72.0 12.90

Eagle ........... 4 13.811 0.52 0.24 47.5 15.266 0.826 116.0 11.33

Eagle ........... 5 13.608 0.53 0.16 47.5 15.278 0.811 76.4 12.29

Eagle ........... 6 13.608 0.50 0.25 47.5 15.268 0.804 77.6 11.90

Upper Freeport .... 1 11.725 0.37 0.13 47.5 13.863 0.627 146.0 10.86

Upper Freeport .... 2 11.762 0.35 0.07 43.0 13.892 0.618 180.0 9.52

Upper Freeport .... 3 11.618 0.37 0.16 49.8 14.318 0.636 131.2 8.08

Upper Freeport .... 4 11.644 0.33 0.13 49.8 14.272 0.630 158.8 8.08

Upper Freeport .... 5 11.766 0.37 0.19 58.9 14.351 0.646 128.8 8.73

Upper Freeport .... 6 11.737 0.37 0.13 47.5 14.235 0.627 162.0 6.27

Blind Canyon ..... 1 12.176 0.53 0.18 47.5 15.855 0.828 55.2 12.12

Blind Canyon ..... 2 12.179 0.48 0.21 45.3 15.909 0.842 64.0 11.40

Blind Canyon ..... 3 12.576 0.53 0.16 47.5 16.211 0.854 108.8 18.81

Blind Canyon ..... 4 12.573 0.55 0.25 49.8 16.357 0.857 53.6 13.14

Blind Canyon ..... 5 11.239 0.52 0.17 47.5 16.243 0.861 100.4 14.70

Blind Canyon ..... 6 11.236 0.52 0.21 45.3 16.187 0.851 90.8 13.03

Wadge .......... 1 12.651 0.53 0.12 49.8 16.510 1.108 24.0 10.12

Wadge .......... 2 12.665 0.52 0.14 56.6 15.973 1.100  ̃28.8 7.37

Wadge .......... 3 12.031 0.52 0.17 43.0 15.975 1.083  ̃25.2 7.26

Wadge .......... 4 12.012 0.47 0.18 43.0 15.975 1.083  ̃9.2 8.28

Pittsburgh (PRL) . . 1 14.634 0.57 0.27 52.1 14.893 0.883 114.2 6.67

Pittsburgh (PRL) . . 2 14.674 0.57 0.27 47.5 15.031 0.878 139.2 4.67

Pittsburgh (PRL) . . 3 14.658 0.55 0.30 52.1 15.281 0.872 99.2 7.33

Pittsburgh (PRL) . . 4 14.675 0.53 0.32 56.6 15.094 0.871 124.8 6.00

Crusher current with coal material minus the baseline crusher current without coal material (480 V). 1

ARD level measured over a 3-min sampling period at 2 L/min. 2
DHHS (NIOSH) Publication No. 98-160

October 1998

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