Coal dust particle size survey of US mines 


Michael J. Sapko, Kenneth L.Cashdollar ,Gregory M. Green 


PIII&WQ~Rksameh -toy. NUI~OM/ I~litutefw Ocmpahd Sqfciy and XcaI~h,Pittsburgh, PA, USA

The National Institute for Occupationel Safety and Health (NINSH) and the Mine Safety and Ncalth Administration (MSHA) 

wnductod a joint ~urveyto determine the range of coal particle sizes found in dust samples collected from intake airnap of US coal 

h.The last comprehensivesurvey of this type was perfad in the 1920s.The size of the cod dust is dmnt to tbe amount of rwk 

dust required to inert the coal dust, with more rock dust needed to inert her sks of coal dust. 

Dust sampl#rwere collected by MSHA inspectors from 9cveral mines in each ofMSHA's 10 bituminous Coal Mine Safety and Health 

Districts. Sampla wtre normally wUected in several intaka at each mint. The laboratory analysis procedure3 included acid leaching of 

tk sample to remove the limestom rock dust, sonic sieving todeterminethe dust size, and low-temperaWcarhing of the idfractions 

to mrrect for any remaining incornbustiblc matter. The results indicate that particle sks of mine coal dust in intake airways are finer 

than tholt meaeud in the 19209. T b finer size coal dust in intake airways would require more incombustible matter to bF dFtctivcly 

inertsd than the 65% incombustible spifmi in cumt regulations. 

&yw&: Cool; Mine; pus^ Panick ExploSh 

regulation also requires an additional 1.0% incombustible 

by weight for each 0.1% methane in the ventilating air in 

Despite the worldwide research on coal mine safety. coal intakes and 0.4%additional incombustible for each 0.1% 

mine explosions involving fatalities and injuries still mur. methane in returns. 

Experimental studi~by the Pittsburgh Restarch Eabora- Thc 65% total incombustible content (TIC) required for 

tory1 (PRL) and similar a&encies in other countries have intake airways was based on the measurcd site of coal dust 

shown that mixing n sufficient quantity of inert rock dust found in mines during the 1920s and the amount of rock 

with coal dust will prevent coal dust explosions by acting as dust required to inerl that size of coal in full-r~cale 

a heat sink, The requirements are specified in the Cod experimental mine tats, as summarized by Naw (1W1). 

Mine Hdth and Safety Act of 1969 (USConp#r, 1969) The term "mine sk coal" was adopted in about 1925 and 

and in Title 30, Section 75.403 of the US Codt of FedenI refen to coal dust, all of which passe a US Standard 

Rquhrioar (2006). They mdae that the natioa's 20-mesh sieve (850 pm)and contains 20% minus 200 mesh 

bituminous coal mines maintain an incombustible content (75 p).Thejustification for adopting it is given in Bureau 

of at least 65% in the non-return (intake) airways and at of Mines Technical Paper (TP)464 (Rice & -wold, 

leaart 80% in the return ainvays where the potential for 1929). In the 1920s, representative dust samples were 

accumulation of her float coal dust is greater. The US colIocted from mine passageways that WMF not rock 

dusted, and then they were siad using sieves. TP 464 

states that these ma1 dust samples collected from the mine 

doors had 540% of the matmid less than 200 mesh. TP 

444further indicates that the values were weightad as far as 

possible, and for 80% of the mines, the final values ranged 

from 15% to 25% through 200 mesh. Therefore, coal dust 
requirement for intake roadways (Nagy, 1981; Rice &

Greenwald, 1929). TP 464 states that dust collected from

ribs, roof, and timbers was finer in size, with 40–75% finer

than 200 mesh. TP 464 does not give any additional details

on the total number of mines surveyed or the total number

of samples analyzed for coal particle size. The 80%

incombustible content required for return airways is based

on the finer float coal dust that may be deposited there.

Nagy (1981) defines float coal dust as dust that is finer than

200 mesh.

To comply with regulations, mine personnel periodically

dust the mine intake and return airways with an inert

material, such as pulverized limestone (rock) dust. The

term ‘‘inert’’ in this sense means that the material does not

support combustion. The rock dust is required to be at

least 70% minus 200 mesh. In determining compliance with

the regulation, inspectors from the Mine Safety and Health

Administration (MSHA) periodically collect samples of

deposited dust from various areas in a mine. When samples

are collected in any given mine, they are usually collected at

500 ft intervals along an entry. Generally, a band sample is

collected, which includes dust from the floor, ribs (walls),

and roof at each location in the mine. The inspector then

screens the sample through a 10-mesh sieve while in the

mine, bags the sample, and sends it to MSHA’s laboratory

at Mt. Hope, West Virginia, for determination of the

incombustible content. The fineness of the coal dust

component is not measured and therefore not specifically

considered in assessing the level of dust explosion protection afforded by the 65% inert requirement for intake

airways.

This paper presents the results of a recent coal dust

particle size survey to determine the range of coal particle

sizes found in dust samples collected from intake airways in

50 US coal mines in MSHA’s 10 bituminous Coal Mine

Safety and Health Districts (see Fig. 1). (MSHA District 1

covers anthracite mines in Pennsylvania, which do not

2. Coal particle size effects

Since the amount of rock dust required to inert a mixture

varies with coal particle size (Cashdollar & Hertzberg,

1989; Nagy, 1981; Rice & Greenwald, 1929; Rice, Jones,

Egy, & Greenwald, 1922; Weiss, Greninger, & Sapko,

1989), the measurement of the incombustible concentration

without considering the effect of coal particle size is not, by

itself, sufficient to determine the possible explosion hazard.

The effect of coal particle size on the explosibility is best

illustrated in Fig. 2, which shows the incombustible

required to prevent explosion propagation in large-scale

dust explosion experiments conducted in the NIOSH-PRL

Bruceton Experimental Mine (BEM) and Lake Lynn

Experimental Mine (LLEM). The graph shows the amount

of incombustible required to prevent propagation for

Pittsburgh high volatile bituminous coal dust with

10–80% passing through a 200-mesh sieve (75 mm). Each

of the data points is an individual BEM or LLEM

explosion test. The lower dashed curve shows the amount

to inert based on the older BEM data (Rice & Greenwald,

1929; Rice et al., 1922). The curve is the boundary between

mixtures below that can propagate an explosion and

mixtures above that cannot propagate an explosion. These

are the data used to support the current 65% incombustible

requirement for intake airways. The dotted curve shows the

amount to inert based on the more recent LLEM research

(Sapko, Weiss, Cashdollar, & Zlochower, 2000; Weiss

et al., 1989). The LLEM data show close agreement with

the BEM data for the fine size coal at 80% minus 200 mesh.

For the mine size coal at 20% minus 200 mesh, the data

show that somewhat more rock dust is required to prevent

explosions in the LLEM than in the BEM. The reason may

be that the LLEM is more adiabatic than the BEM because

of the larger cross-sectional area at the LLEM.

Fig. 1. MSHA Coal Mine Safety and Health Districts, identified by

number.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Minus 200 mesh content of coal dust, %

Total Incombustible Content, %

BEM, non-propagation LLEM, non-propagation

BEM, propagation LLEM, propagation

BEM curve LLEM curve

Non-Propagation

Propagation

Fig. 2. Effect of particle size of coal dust on the explosibility.
distribution from various underground coal-mining operations, MSHA coordinated the acquisition of mine dust

samples from 10 Coal Mine Safety and Health Districts.

The dust samples were those collected routinely by mine

inspectors for compliance with 30 CFR 75.403. The

samples were sent to MSHA’s Mt. Hope Laboratory in

WV and analyzed for TIC. The TIC includes the moisture

in the samples, the ash in the coal, and the rock dust. The

incombustible analysis procedure (Montgomery, 2005)

begins with passing the sample through a 20 mesh sieve

(850 mm) and then oven drying the minus 20 mesh material

for 1 h at 105 1C and recording the weight change. The

weight loss constitutes the moisture in the sample. Next,

the dried sample is heated in an oven that is ramped up

over 1.5 h and held at 515 1C for about 2.5 h to burn off the

combustible coal fraction, thereby leaving the incombustible ash. This low-temperature ashing (LTA) burns off the

coal but does not decompose the limestone rock dust. The

amount of the remaining ash plus the as-received moisture

is reported as %TIC. After the MSHA laboratory

completed their incombustible analyses, the remaining

material of the dust samples was sent to NIOSH-PRL for

the coal particle sizing analyses.

At PRL, the limestone dust was leached from the sample

using hydrochloric acid, and a sieve size analysis was

conducted on the remaining non-leachable residue. In the

laboratory acid leaching method, dilute hydrochloric acid

was added to the dust sample in a beaker and heated on a

hotplate. The acid reacted with the rock dust producing

foam while releasing carbon dioxide (CO2). Additional acid

was added until the foaming stopped. The hotplate kept the

slurry near its boiling point for about 1 h. After the slurry

cooled, the acid-insoluble residue was filtered from the

acid. The solid residue was rinsed with isopropanol and

then transferred to a large evaporating dish. The residue

was dried at 110 1C for 3 h. Agglomerates were broken with

a spatula. The residue consisted of coal plus other insoluble

mineral matter (such as silica from the rock dust and shale

from roof or floor rock in the mine).

The dried residue was classified into the different size

fractions using a sonic siever, which combined two motions

to provide particle separation: a vertical oscillating column

of air and a repetitive mechanical pulse. In addition, the

tops of the sieves were brushed to break up any remaining

agglomerates. The sieves are 8 cm in diameter and include

the following sizes: 20 mesh (850 mm), 30 mesh (600 mm), 40

mesh (425 mm), 50 mesh (300 mm), 70 mesh (212 mm), 100

mesh (150 mm), 140 mesh (106 mm), 200 mesh (75 mm), 270

mesh (53 mm), and 400 mesh (38 mm). After the sieving was

completed, the weight of sample on each sieve was

recorded.

Since the residue from the leaching process contained

other inert mineral matter (such as clay and silica dust) that

did not react with the acid, a correction to the size analysis

PRL to determine the incombustible or non-coal content,

using an LTA method similar to that of the MSHA

laboratory at Mt. Hope. The sieve size analyses were then

corrected for the non-coal content (insoluble mineral

matter) in the three size groupings. The amount of this

insoluble mineral matter in the samples varied greatly, but

was generally in the 20–50% range. For most of the

samples analyzed, the mineral matter was finer in size than

the coal. Therefore, after correction for the mineral matter,

the corrected minus 200 mesh amount would be less than

the original minus 200 mesh amount from the sonic sieving.

There was a wide range of correction values, but a value of

39% minus 200 mesh from the original sieving data might

typically be reduced to 31% minus 200 mesh after

correcting for the mineral matter.

The total size analysis procedure (acid leaching, sieving,

and correction for remaining incombustible matter) was

verified by using prepared mixtures of coal and rock dust.

First, the size of the coal sample was determined by sieving.

Then, samples of coal and rock dust (usually 35% coal and

65% rock dust) were mixed together, and then the rock

dust was leached from the mixture. The residue was then

sieved and corrected via LTA for any remaining incombustible matter in the size fractions. For these prepared

mixtures, the correction was small because amount of

insoluble mineral matter was small. This verification test

was done for samples of Pittsburgh seam high volatile coal,

Blue Creek seam medium volatile coal, and Pocahontas

seam low volatile coal. In general, the size analyses after

leaching were close to the original size analyses, with

agreement to within 1–3% for the amount of minus 200

mesh material.

4. Results

For this study, samples of mine dust from 50 coal mines

in the 10 MSHA bituminous districts were size analyzed.

For each mine, samples were usually collected from two or

more entries. For most analyses, multiple samples from a

mine entry were combined to give an average size

distribution for that entry. Most of the samples were band

samples, but some were floor and rib samples or floor and

roof samples. Table 1 lists the intake coal dust size data by

MSHA Coal Mine Safety and Health District. The table

includes data from a total of 163 combined samples from

50 bituminous coal mines. Columns two and three of the

table list the number of mines and total number of

combined samples per district. The fourth and fifth

columns list the average percent minus 200 mesh (75 mm)

and minus 70 mesh (212 mm) with the associated standard

deviations, respectively. The last column lists the average

and standard deviation for the mass median particle

diameter, which was interpolated from the corrected

sieving data. The averages for all MSHA districts are
Table 2 lists the average sizes for various coal seams or

groups of coal seams. The eastern bituminous coal seams

are those in the Appalachian Mountains from Pennsylvania to Alabama. Only the seams that included samples

from two or more mines are listed. The coal rank is also

listed in the first column, with hvb for high volatile

bituminous, mvb for medium volatile bituminous, and lvb

for low volatile bituminous (ASTM International, 2006).

The mid-eastern seams are those in Illinois, Indiana, and

western Kentucky. These seams are known by different

names in different states, as listed in the table. The western

coal seams include various high volatile C bituminous

(hvCb) coals in Colorado or Utah. The coal samples from

the Hazard #4 seam in Kentucky and the Blue Creek seam

in Alabama are the finest in size, with 40% less than 200

mesh. These variations with coal seam may be related to

the friability of the coal.

of the coal particles in a coal and rock dust mixture.

Underground coal mining technology has changed significantly since the 1920s when data were collected on the

particle size of coal dust distributed in mine passageways.

Coal mining has become highly mechanized, and this has

resulted in increased coal production rates. Particle size can

vary with coal seam type, as shown in Table 2. In addition

to TIC and methane concentration, the coal dust particle

size should be considered as an essential part of the

explosibility assessment strategy in underground coal

mines. The present coal size study indicates that the coal

dust in intake airways of US mines is finer than that

measured in the 1920s (Rice & Greenwald, 1929). Based on

the inerting data from the Bruceton and Lake Lynn

Experimental Mines, the present size of coal in intake

airways would require more incombustible to be effectively

inerted than the 65% incombustible specified in current

regulations.

A complementary study is being undertaken to collect

and analyze dust samples in a selection of mines to assess

the feasibility of a hand-held instrument to evaluate the

explosibility hazard. NIOSH in collaboration with MSHA

(Sapko & Verakis, 2006) has devised a prototype hand-held

instrument, based on optical reflectivity, which can provide

a direct assessment of the potential explosibility of a

coal and rock dust mixture, which is independent of

coal dust particle size. The field studies will be used to

evaluate the practicality for in situ assessment of dust

mixture explosibility with the newly developed hand-held

instrument.

Acknowledgments

The authors acknowledge the contributions of H.

Verakis of MSHA (Triadelphia, WV) for his valuable

Table 1

Average coal sizes from mines in 10 MSHA Safety and Health Districts

District No. of

mines

No. of

samples

200

mesh

(%)

70

mesh

(%)

Dmed

(mm)

2 5 13 2974 6075 161722

3 7 19 33710 6379 147745

4 4 15 3177 62710 156740

5 4 10 33710 68713 141740

6 4 21 3277 6077 155735

7 5 16 35710 6378 135748

8 4 10 2775 5877 172739

9 6 12 2774 5975 168726

10 4 26 2974 6175 151725

11 7 21 3879 74715 122742

Table 2

Average coal particle size from various coal seams

Coal seams No. of mines No. of samples 200 mesh (%) 70 mesh (%) Dmed (mm)

Eastern bituminous coal seams

Pittsburgh, hvb 9 26 3378 6377 143732

Upper or lower Kittanning, hvb 2 5 2575 5376 197737

Alma, upper Elkhorn #1 or #3, hvb 3 18 3377 6176 149734

Hazard #4, hvb 2 8 40713 6977 104745

Pocahontas #3, lvb 2 8 3576 6477 138733

Pratt coal seam, hvb 2 6 3176 63711 155734

Blue Creek coal seam, mvb 5 15 41714 79724 109747

Mid-eastern bituminous coal seams

Springfield, Illinois #5, or W. Kentucky #9 3 16 2975 6277 150731

Herrin, Illinois #6, or W. Kentucky #11 3 12 2673 5974 164724

Western bituminous coal seams

Various hvCb seams in Colorado 3 7 2773 5775 174727

Various hvCb seams in Utah 3 5 2675 6177 164729
samples in coal mines throughout the USA. The authors

also acknowledge the laboratory assistance of M. Conroy,

K. Helfrich, L. Leslie, and K. Vaughan of NIOSH, PRL in

the acid-leaching and size analysis of the dust samples.

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 or the Mine Safety and Health Administration.

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