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Particle Size and Surface Area Effects on Explosibility Using a 20-L Chamber 

Marcia L. Harris , Michael J. Sapko, Isaac A. Zlochower, Inoka E. Perera and Eric S. Weiss
 

Office of Mine Safety and Health Research,
 

National Institute for Occupational Safety and Health,
 

626 Cochrans Mill Road,
 

Pittsburgh, PA 15236.
 

* Corresponding author. Tel.: +1-412-386-5780, fax: +1-412-386-6595, E-mail: ztv5@cdc.gov 

*

Keywords: Dust Explosion; Mining; Explosion prevention; Particle size; Specific Surface Area 

Abstract 

The Mine Safety and Health Administration (MSHA) specification for rock dust used in 

underground coal mines, as defined by 30 CFR 75.2, requires 70% of the material to pass 

through a 200 mesh sieve (< 75 μm). However, in a collection of rock dusts, 47% were found to

not meet the criteria. Upon further investigation, it was determined that some of the samples did

meet the specification, but were inadequate to render pulverized Pittsburgh coal inert in the 

National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety and 

Health Research (OMSHR) 20-Liter chamber. This paper will examine the particle size 

distributions, specific surface areas (SSA), and the explosion suppression effectiveness of these 

rock dusts. It will also discuss related findings from other studies, including full-scale results 

from work performed at the Lake Lynn Experimental Mine. Further, a minimum SSA for 

effective rock dust will be suggested. 

1.0 Introduction 

Float coal dust, consisting of very fine aerosolized particles, presents a hazard that can contribute 

to a major underground coal mine explosion. In order to mitigate this risk, pulverized rock dust is 

required to be applied to the intake, return, and belt airways (entries). Federal safety regulations 

(30 CFR 75.402 and 30 CFR 75.403) require rock dust to be applied so that the total 

incombustible content of a mine dust sample is not less than 80 percent. 30 CFR 75.2 also 

defines rock dust and requires rock dust to be sized such that 100 percent passes through a 20 

mesh (850μm) screen and 70 percent or more passes through a 200 mesh (75 μm) screen. 

This current particle size specification is so broad that it may not ensure that all rock dust will 

inert at the 80% incombustible level when uniformly mixed with coal dust. Past work (Man and 

Harris 2014) suggests that rock dust particles in excess of 75 μm provide little inerting potential 

and, therefore, do not need to be included in the rock dust supply. A specification of 95% finer 

than 75 μm would ensure that the focus is on particles with the most inerting potential yet within 

grinding mill tolerances for rock dust manufacturers. Furthermore, members of the industrial 

minerals sector have indicated that such a particle size distribution (PSD) is attainable given 

current grinding technology. Given that the PSD of rock dust varies widely, another attribute 

such as specific surface area (SSA) should be considered to ensure that only the most effective 

dust particles are included. 

2.0 Background 

MSHA rock dusting regulations were initially based upon data generated within the Bruceton 

Experimental Mine (BEM) by the U.S. Bureau of Mines (BOM) which suggested that the 
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largest-sized coal dust particle that participated in explosions was 850 μm (Rice et al. 1922). At 

that time, the authors stated that the following circumstances may prevent 20 mesh coal dust 

from propagating: 

1.	 The 20 mesh dust will not mix readily and thoroughly with air due to the weight of the 

coarser particles, 

2.	 The surface area of the coarse particles is less than that of the same weight of fine 

particles, resulting in less surface area for instantaneous oxidation, and 

3.	 The number of the coarse particles is less than that of the same weight of fine particles 

making it probable that the distance between the particles will be greater and thus 

prevent propagation of the flame from particle to particle. 

Since those early BOM tests, other laboratory and experimental mine testing methods were 

developed to d etermine which coal dust particle sizes contribute to explosion propagation and 

which rock dust particle sizes contribute to explosion suppression. Understanding of these 

relationships is critical to properly determining those characteristics of an effective rock dust for 

preventing coal dust explosion propagation. 

One of the well-established American Society for Testing and Materials (ASTM) laboratory 

methods is the use of a 20-Liter (20-L) explosion chamber to test the explosibility of various 

coal dust and rock dust mixtures. Previous data from NIOSH 20-L chamber tests have shown 

that a coal dust (400 g/m3 coal concentration) and rock dust mixture must contain at least 76% 

limestone rock dust to inert the pulverized Pittsburgh coal (PPC) dust which contains 80% 

minus 200 mesh particles (Cashdollar and Hertzberg 1989). This finding was verified at coal 

dust concentrations of 150–700 g/m3

. Dastidar et al. (2001) also tested PPC in a 20-L chamber 

and reported a slightly lower value of 74% rock dust to inert the PPC dust at a dispersed coal 

concentration of 500 g/m3

. In an earlier study, Dastidar et al. (1997) had published an inerting 

value of 77% limestone rock dust associated with a 300 g/m3 PPC concentration. The 

differences were described by the authors as “due to the nature of flame propagation, which is 

probabilistic at limit conditions.” The latter observation reinforces the idea that multiple trials 

are needed to safely conclude that the mixture will remain non-explosive at all coal 

concentrations. 

It is important to note that the 20-L chamber results indicate trends but cannot be directly scaled 

to full-scale results such as those obtained in a nother study performed at the Lake Lynn 

Experimental Mine (LLEM) (Sapko et al. 2000). The differences between the laboratory 

chamber results and the LLEM full-scale results include but are not limited to important 

differences between the dimensions and geometry of the mine and the laboratory chambers, 

differences in the ignition source (pyrotechnic ignitors in the 20-L chamber vs. an initiating 

methane-air explosion in the LLEM), and the manner in which the dust is introduced and 

dispersed. The chamber criterion for explosibility is based on the measured overpressure rise 

whereas the LLEM criterion is based on self-sustained flame propagation beyond the influence 

of the ignition source. Through previous research (Cashdollar 1996, NIOSH 2010), one can 

equate a 75% inerting rock dust concentration given by 20-L tests to an 80% incombustible 

content requirement for mine inerting (at least for Pittsburgh seam coal with a 6% ash content). 

The baselines in both the LLEM and 20-L chamber tests were established using PPC as the coal 
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dust and a reference rock dust (acquired from the same rock dust manufacturer and having 

historically consistent PSDs). 

A recent NIOSH study demonstrated that larger rock dust particles (> 75 μm) are much less 

effective than smaller particles at inerting coal dust as indicated by the large increase in the 

percentage of rock dust required to in ert PPC in both 20-L chamber and 1-m3 chamber tests 

(Man and Harris 2014). Results further indicated that rock dust particles between 250 and 850 

μm (> 60 mesh) did not inert PPC in the 20-L chamber studies. The study also showed that when 

rock dust particles < 38 μm (< 400 mesh) were removed from the particle size distribution, 

inerting was not possible at even a 90% rock dust level. Past research showing the dependence of 

inerting effectiveness on rock dust PSD suggested the need to further quantify this relationship 

using constant volume explosibility studies in the NIOSH 20-L explosion chamber (Man and 

Harris 2014). 

A previous NIOSH investigation of rock dust revealed significant concerns with the material 

used in mines based on the analysis of rock dust samples collected by the Mine Safety and 

Health Administration (MSHA) inspectors from U.S. coal mines in 2010. One concern was the 

frequency of rock dust material in mines not meeting the legal size criterion (70% by weight 

passing through a 200 mesh sieve). In a population of 393 rock dust samples from 278 

underground coal mines, 47% of the rock dust samples failed to m eet the minimum size criterion 

(NIOSH 2011). NIOSH tested these dusts within the 20-L chamber to verify the inadequacy of 

the rock dust that did not meet the definition. Most importantly, some of the rock dusts that did 

meet the current definition did not inert PPC in the 20-L chamber. 

In light of the above findings and given the need for a more definitive characterization of rock 

dust that is effective for inerting a propagating coal dust explosion, NIOSH researchers 

undertook an investigation of the rock dust particle size effects on explosibility in a 20-L 

chamber. The PSDs of the rock dusts vary greatly with some having multiple peaks in the 

distribution and although sieving can be used to characterize the PSD of rock dusts, the most 

effective particles for inerting lie in the respirable size range and cannot be sieved. To better 

characterize such wide variations, multiple and varying sized sieves would be required and the 

finest size to be assessed would typically be 38 μm or possibly 20 μm (635 mesh sieve not 

widely available commercially). However, the respirable portion of rock dust is the most 

effective and cannot be assessed using sieves. Therefore, in lieu of characterizing rock dust 

solely on the percentage finer than 200 mesh, NIOSH investigated the use of a specific surface 

area (SSA) designation as means to assess inerting effectiveness. The SSA is a calculation of 

outer surface area based upon a spherical approximation given the particle size or width. In this 

paper, the term “explosibility” refers to the ability of an airborne dust cloud and/or gas mixture 

to explode in a confined laboratory chamber or propagate flame within an experimental mine 

after the dust cloud or gas mixture has been initiated by a sufficiently strong ignition source. All 

of the full-scale LLEM explosion tests referenced earlier utilized the same limestone rock dust 

which is referred to herein as the Reference rock dust. Rock dust samples collected by MSHA 

during a survey were tested within the 20-L chamber to demonstrate their inerting abilities. The 

standard PPC dust and Reference rock dust were used for both laboratory and experimental mine 

explosions. 
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3.0 Experimental 

3.1 Particle Size Analyzers 

For a full particle size distribution and SSA, NIOSH used a Beckman Coulter (B-C) LS 13320 

laser diffraction particle size analyzer equipped with a Tornado Dry Powder air dispersion 

system. NIOSH researchers followed the analysis procedure recommended by the manufacturer 

(Beckman Coulter 2011). The laser diffraction data is analyzed by the instrument in terms of 

equivalent spherical scatterers using a Mie scattering algorithm. The volume fraction is 

determined for the various particles sizes, and a specific surface area in terms of area per unit 

volume (cm2/ml) is determined. That area divided by the density of the particles then gives the 

specific surface area (SSA) in units of area per units of mass. The complex refractive index (RI) 

of 1.8 + 0.3i was used for the coal dust analysis and 1.68 + 0.0i was used for the limestone rock 

dusts, where i is the imaginary (absorptive) component. These were average RI values found in 

the B-C manual for carbon and calcium carbonate and were not determined by a separate 

analysis. Control samples of PPC and the Reference rock dust were tested every 30–50 samples 

to confirm proper B-C operation and to detect significant deviations from the typical measured 

average values and uncertainty in their SSAs. The B-C system was the system of choice to use 

for SSA determination. The system requires only a small sample for analysis, is easy to use, 

gives reproducible results, and is not subject to user variability. However, another option is the 

use of an air-jet sieve in conjunction with the Blaine Permeability apparatus (Blaine apparatus). 

The Blaine Apparatus is a simple, low-cost system that can be used as an alternative to obtain 

SSA results. 

A comparison of SSA measurements using the B-C system and the Blaine apparatus for several 

rock dusts are shown in Figure 1. The Blaine air permeability of a packed bed is a standard test 

method based on the Kozeny-Carman equation for permeability of a packed bed of particles to 

determine the fineness of hydraulic cements (ASTM C 204-11; Perry and Green, 1984). 

Following ASTM C 204-11 procedures, the NIOSH manually-operated Blaine apparatus was 

calibrated using the National Institute of Standards and Technology (NIST) Standard Reference 

Material 114q (SRM) (NIST 2001; 2008). The effective SSAs of the samples were compared 

with a standard dust of known SSA (NIST SRM). Despite the small sample size, an R2

-value of 

0.97 between the B-C laser diffraction system (LDS) and the Blaine apparatus (Figure 1) 

suggests the feasibility of using the Blaine apparatus and method as an alternative to an LDS for 

determining minimum SSAs of rock dusts. 

3.2 Dust samples 

3.2.1 Pulverized Pittsburgh Coal 

The pulverized Pittsburgh coal (PPC) dusts used for this study were produced at NIOSH 

OMSHR. The coal was mined on-site from the Safety Research Coal Mine (SRCM), then ground 

and pulverized on-site to produce the pulverized Pittsburgh coal dust. The same SRCM coal 

seam was mined and processed in a similar manner for the various sized Pittsburgh coal dusts 

used during the LLEM explosion tests (NIOSH 2010). The cumulative and differential PSDs of 

the PPC as measured with the B-C are shown in Figure 2. The B-C mass-mean particle size of 

PPC is 61.9 μm with a median particle size of 54.6 μm. PPC has an average calculated SSA of 

240 m2

/kg. Some common size fraction values determined by the B-C instrument and a 

commercial air-jet sieve apparatus are listed in Table 1. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The optical method of particle size determination understates the percentage < 75 μm compared 

to results obtained from sieving methods. The B-C measurement is approximately 10% below 

that of the direct air sieve measurement, or 60% < 200 mesh. This difference is due to 

measurement of oblong particles and the inherent differences within the methods. With an 

oblong particle, the B-C measures the widest dimension of the particle whereas the air-jet sieve 

agitates the particles until the narrowest part of the particle passes through the sieve. The 

percentage difference between the analyzers will be different for each sieve/mesh. Despite such 

differences, the air-jet results are seen to be in line with the B-C analysis. 

3.2.2 Rock Dust Samples 

The samples referred to as MSHA survey samples are rock dust samples collected by MSHA 

from 278 underground coal mines as discussed in the 2011 NIOSH Hazard ID (NIOSH 2011). 

These included a handful of samples that MSHA had collected but which had arrived after the 

Hazard ID was published. All samples were selected from a population of samples collected by 

MSHA inspectors during inspections. These samples were sent to the MSHA National Air and 

Dust Laboratory at Mt. Hope, WV, for cataloging and then sent to NIOSH for analysis. NIOSH 

performed a size analysis on these samples. The samples were gathered from all MSHA 

bituminous coal districts and are believed to be representative of a random cross-sectional 

snapshot of the rock dust available in the operating underground coal mines. The amount of rock 

dust sample collected varied substantially between mines and inspectors which, for some 

samples, limited the number of analyses and 20-L chamber testing that could be conducted. 

3.3 Explosion test chamber 

The NIOSH 20-L explosion chamber was used in this study. This chamber has been extensively 

used as a tool to evaluate the explosibility properties of various dusts prior to and concurrent 

with extensive LLEM full-scale explosion propagation experiments (ASTM E1515-07 2007; 

Cashdollar 1996; 2000; Cashdollar and Hertzberg 1989; Chawla et al. 1996; Dastidar et al. 2001; 

Sapko et al. 2000). Research has shown an ~5% difference in the rock dust content to inert PPC 

in the 20-L chamber compared to that required to prevent flame propagation in the LLEM using 

the same rock dust size distribution (NIOSH 2010)—i.e., ~73% rock dust in the 20-L chamber 

compared to ~78% in the LLEM. 

Detailed descriptions of the 20-L chamber have been previously published (Cashdollar 1996; 

2000; Going et al. 2000). For the 20-L chamber experiments in this paper, 5,000 J electrically 

activated pyrotechnic ignitors were used as the ignition source for testing the explosibilities of 

mixed dusts. A pressure rise ≥ 1 bar (pressure ratio ≥ 2) was used as the criterion for determining 

the occurrence of an explosion during a test. A pressure ratio designation can account for the 

variations in atmospheric pressure. This determination is in accordance with the ASTM test for 

measuring the explosibility of dust clouds (ASTM 2010). A series of three or more tests were 

performed to confirm a non-explosion at each coal dust concentration if sufficient quantities of a 

particular rock dust sample were available. 

Inerting tests conducted with the MSHA survey samples were limited to a PPC concentration of 

400 g/m3 due to limited quantities of the collected rock dust samples. The 400 g/m3 PPC 

concentration was chosen because this is typically the most reactive concentration. The 

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maximum pressure and the rate of rise level off as the oxygen in th e chamber is consumed 

(Cashdollar 1996). For PPC, this leveling or limit occurs at approximately 300 g/m3 coal dust 

concentration with a corresponding maximum pressure of 6.6 bar. This maximum pressure 

remains at approximately 6.6 bar as the coal dust concentration increases from 300 to 8 00 g/m3

. 

Therefore, considering the limited quantities of rock dust samples available, the coal dust 

concentration was held constant at 400 g/m3 to determine if the rock dust was effective in 

inerting the dust mixture with a concentration of 75% rock dust. 

These tests were conducted at 75% rock dust for comparison with the full-scale LLEM explosion 

test results. If the 75% rock dust mixture was explosible, no other inerting tests were conducted. 

If the 75% rock dust mixture was not explosible, additional tests were conducted at the same coal 

concentration until there was insufficient rock dust remaining to continue testing. 

4.0 Results and Discussion 

4.1 Particle Size Analysis 

All experimental laboratory inerting results based on calculated SSAs presented were determined 

using NIOSH’s Beckman Coulter (B-C) model LS 13 320 single wavelength dry powder system. 

The measured average SSA values and standard deviations in the SSA measurements are shown 

in Table 2 for the Reference rock dust and PPC. 

The results of the B-C particle size analyses on the MSHA rock dust survey samples are 

graphically shown in Figure 3 which features a comparison of the B-C-determined SSAs with the 

corresponding percentages of dust finer than 75 μm particle size. It is apparent from the data 

that, although the trending is positive, there is variability in the percentages of dust finer than 75 

μm and their corresponding SSAs (correlation of 0.5, n=401). It should be noted that the SSAs 

determined are the geometric surface areas of the dust treated as equivalent smooth spheres. 

B-C particle size analyses of a random rock dust sample revealed several maxima in the 

differential distribution curve (Figure 4). In addition to a main peak at a greater particle diameter, 

there were one or more peaks at finer particle diameters. It appears as if fine rock dust particles, 

such as those collected by baghouse filters from the pulverizing equipment, had been added back 

into the rock dust supplied to coal mines. While such fine particles would be effective in 

quenching an incipient coal dust explosion, it allows the dust to contain larger (75 to 850 μm), 

likely ineffective, inerting particles while maintaining the legal size requirement for rock dust. 

4.2 Explosibility Tests 

The 20-L explosibility chamber tests were conducted using homogeneous mixtures of 25% 

standard PPC and 75% rock dust (from available MSHA rock dust survey samples1

). 

1 The number of MSHA survey rock dust samples tested was limited to those having sufficient mass 

remaining after quartz analysis, particle size analysis, and wet and dry mechanical sieve analysis. 

The results 

of the 20-L explosibility chamber testing are shown in Figure 5. It appears that the transition 

from explosible to non-explosible occurs when rock dusts have SSA values of approximately 

230 m2

/kg. By comparison, the Reference rock dust used in f ull-scale explosion tests within the 

LLEM had an SSA value exceeding 260 m

2

/kg. This dust consistently inerted explosion tests at 

this facility and in th e 20-L chamber. 
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f(CD) = 1 − f(RD)

For comparison with 30 CFR 75.2, the current requirement of 70% < 200 mesh is added to an 

overlay on the inerting data shown in Figure 5 and displayed in Figure 6. The B-C measurement 

for a 200 mesh fraction is approximately 10% below that of the direct air jet sieve measurement, 

or 60% < 200 mesh as previously mentioned (Table 1). 

Interestingly, there were some samples that met or exceeded the 30 CFR 75.2 specification 

requirement of 70% < 200 mesh particle size but were explosible as noted in Figure 6. These 

samples had SSAs < 230 m2

/kg. On the other hand, some rock dust samples not meeting the 30 

CFR 75.2 specification for particle size were found to b e non-explosible in the 20-L chamber due 

to having SSAs equal to or greater than 230 m

2

/kg. These results suggest the need to i nclude a 

minimum SSA as a key component of effective rock dust. 

Additional experiments were conducted to f urther quantify the effect of rock dust SSA on 

explosibility within the 20-L chamber using various controlled size classifications of a local 

limestone rock dust supply (Reference rock dust) previously used in full-scale LLEM explosion 

inerting studies (NIOSH 2010). The classified size fractions had SSAs ranging from 49 to 446 

m

2

/kg. While the results are not conclusive due to the single PPC concentration used, they do 

indicate the sensitivity of inerting efficiency to the rock dust SSA. 

An inerting index or limit, Z, is defined as the mass ratio of rock dust to coal dust. Figure 7 

shows the relationship between Z and measured rock dust SSA. These data exhibited a good fit 

to the following exponential expression (R2 

= 0.98): 

z = 385.55 ∗ ssA-0.638

The inerting limit Z increases as rock dust SSA decreases. This indicates that greater quantities 

of rock dust are needed to inert as the average rock dust particle size increases. Rock dust with 

an average SSA of 446 m

2

/kg (Z = 1.9) required about 65% rock dust to inert the PPC, while 

rock dust with an average SSA of ~49 m

2

/kg required about 90% (Z = 9.0) to inert the PPC. As 

expected, finer rock dust particles are seen to b e more effective in inerting as compared to l arger 

particles. 

Previous large-scale research results in the BEM, LLEM, and in la boratory studies using the 20­

L chamber have determined an experimental uncertainty of approximately ± 3% inert content 

(Sapko et al. 2000; Cashdollar 1996; Richmond et al. 1975). The simplest way to illustrate this 

3% uncertainty when viewing the 20-L data in Figure 7 is to assume a worst-case scenario where 

a nominal 75% rock dust is actually 72%. A 72% rock dust mixture (28% coal dust) corresponds 

to a Z-value of 2.57, yielding an SSA value of approximately 260 m

2

/kg. 

Another way of viewing this uncertainty and its effect on a conservative value for the minimum 

rock dust SSA specification is to consider the variation in th e Z value arising from the fraction of 

rock dust in the mixture. Assuming variables f(CD) and f(RD) represent the fractions of coal dust 

and rock dust, respectively, the following expressions hold: 

and 
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z

f(RD) = 1 + z

[(1 + z)dz − zdz] df(RD) = (1 + z)2

dz = (1 + z)2df

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Approximating the uncertainty in f(RD) as a differential, df, then: 

and 

With 75% rock dust and 25% coal dust, Z=3. Given an experimental uncertainty of 3%, df = 0.03 

and then dZ = 0.48. Hence, Z−dZ = 3.0 − 0.48 = 2.52. 

Using the expression in Figure 7 yields an approximate SSA value of 260 m

2

/kg, similar to that 

obtained graphically in the previous discussion. 

5.0 Conclusions 

In this study, NIOSH adopted a specific surface area (SSA) designation (surface area per unit 

mass) as a means to improve uniformity of rock dust particle size distributions, in lieu of relying 

solely on the percentage finer than 200 mesh (75 μm). The overall data showed a good 

correlation between the SSA measurements and the effectiveness of the rock dusts in suppressing 

a coal dust explosion. The study also showed that it is critical to specify a minimum SSA to 

ensure an effective rock dust, since some rock dusts that met the current particle size 

specifications of the 30 CFR 75.2 failed to inert the coal dust in the 20-L chamber. 

Combining findings from this study with those from recent NIOSH publications (Man and Harris 

2014; NIOSH 2010; NIOSH 2011), the following conclusions can be drawn: 

•	 Dust particle size has the greatest influence on the propagation (coal dust) and inhibition 

(rock dust) of dust explosions. 

•	 Samples collected from the MSHA rock dust survey (as discussed in the 2011 NIOSH 

Hazard ID), were multi-modal, and several samples appeared to have wide variations in 

the amount of effective finer particles. 

•	 Rock dust particles from 200 mesh to 60 mesh are largely ineffective in inerting coal dust 

explosions. 

•	 Rock dust particles < 38 μm are more effective in inerting coal dust. 

•	 The inerting effectiveness of rock dust is correlated to the SSA of the rock dust. Results 

from this study suggest the need to include a minimum SSA as a critical specification for 

effective rock dust. 

These findings show that rock dust is most effective for inerting propagating coal mine dust 

explosions if the particle size is at least 95 percent finer than 200 mesh or 75 μm, and more 

importantly has a minimum surface area of 260 m2

/kg. 

6.0 Acknowledgements 

The authors acknowledge NIOSH OMSHR Physical Science Technicians, James Addis, Linda 

Chasko, and Jarod Myers, whose contributions made the particles size analyses and the 20-L 

chamber tests possible. 
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7.0 Disclaimer 

Mention of any company or product does not constitute endorsement by the National Institute for 

Occupational Safety and Health. The findings and conclusions in this paper are those of the 

authors and do not necessarily represent the views of NIOSH. 

8.0 References 

ASTM C 204-11 (2011). Standard test method for Fineness of Hydraulic Cement by AirPermeability Apparatus. In: Annual Book of ASTM Standards, vol. 04.01. West Conshohocken, 

PA: ASTM International. 

ASTM E1226-10 (2010). Standard test method for explosibility of dust clouds. In: Annual Book 

of ASTM Standards, vol. 04.01. West Conshohocken, PA: ASTM International. 

ASTM E1515-07 (2007). Standard Test Method for Minimum Explosible Concentration of 

Combustible Dusts. In: Annual Book of ASTM Standards, vol. 14.02. West Conshohocken, PA: 

ASTM International. 

Beckman Coulter (2011). Instruction for use, LS 13 320 laser diffraction particle size analyzer. 

Beckman Coulter, PN B05577AB, October 2011, 

https://www.beckmancoulter.com/wsrportal/page/techdocSearch?sortColumn=documentName& 

sortOrder=asc&startIndex=1&searchByURL=no&searchType=searchByProduct&marketSegme 

ntId=1000355&productLineId=1000227&productSeriesId=1002187&productId=1002190&docu 

mentCategoryId=6&languageCode=en&pageSize=10&downloadTechDocFileFormat=exe&acti 

veCategoriesFlag=false accessed 03/27/2014. 

Cashdollar K (1996). Coal dust explosibility. Journal of Loss Prevention in t he Process 

Industries, 9:65–76. 

Cashdollar KL (2000). Overview of dust explosibility characteristics. Journal of Loss Prevention 

in the Process Industries. 13:183–199. 

Cashdollar KL, Hertzberg M (1989). Laboratory study of rock dust inerting requirements: effects 

of coal volatility, particle size, and methane addition. In: Proceedings of the 23rd International 

Conference of Safety in Mines Research Institutes. U.S. Department of the Interior, Bureau of 

Mines, Washington, DC, September 11–15, 1989, 965–977. 

CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, Office 

of the Federal Register. 

Chawla N, Amyotte PR, Pegg MJ (1996). A comparison of experimental methods to determine 

the minimum explosible concentration of dusts. Fuel 75:654–658. 

Dastidar AG, Amyotte PR, and Pegg MJ (1997). Factors influencing the suppression of coal dust 

explosions. Fuel Vol. 76, no. 7, pp. 663–670. 
10
 

Dastidar A, Amyotte P, Going J, Chatrathi K (2001). Inerting of coal dust explosions in 

laboratory- and intermediate-scale chambers. Fuel 80:1593–1602. 

Going JE, Chatrathi K, and Cashdollar KL (2000). Flammability limit measurements for dusts in 

20-L and 1-m3 vessels. Journal of Loss Prevention in th e Process Industries 13 (2000) pp. 209– 

219. 

Man CK and Harris ML (2014). Participation of large particles in coal dust explosions, Journal 

of Loss Prevention in t he Process Industries 27 (2014), pp. 49–54. 

NIOSH (2010). Report of Investigations 9679: Recommendations for a new rock dusting 

standard to prevent coal dust explosions in intake airways. By Cashdollar KL, Sapko MJ, Weiss 

ES, Harris ML, Man CK, Harteis SP, Green GM: U.S. Department of Health and Human 

Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety 

and Health, DHHS (NIOSH) Publication No. 2010-151, p. 48. 

NIOSH (2011). Non-conforming rock dust. U.S. Department of Health and Human Services, 

Centers for Disease Control and Prevention, National Institute for Occupational Safety and 

Health, DHHS (NIOSH) Publication No. HID 16, p. 4. 

NIST (2001). Particle size characterization. By Jillavenkatesa A, Dapkunas SJ, Lum SH: U.S. 

Department of Commerce, National Institute of Standards and Technology, NIST Recommended 

Practice Guide, Special Publication 960-1. 

NIST (2008). Certificate of Analysis, Standard Reference Material 114q, Portland Cement 

Fineness Standard. Gaithersburg, MD 20899, Certificate Issue Date: 15 September 2008. 

Perry PH and Green DW (1984). Perry’s Chemical Engineers’ Handbook, Sixth Edition, 

McGraw-Hill Book Company, pp 20–98. 

Rice GS, Jones LM, Egy WL, and Greenwald HP (1922). Coal-dust explosion tests in the 

experimental mine 1913 to 1918, inclusive. Pittsburgh, PA: U.S. Department of the Interior, 

Bureau of Mines, Bulletin 167. 

Richmond JK, Liebman I, Miller LF (1975). Effect of rock dust on explosibility of coal dust. 

Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 8077. 

Sapko MJ, Weiss ES, Cashdollar KL, Zlochower IA (2000). Experimental mine and laboratory 

dust explosion research at NIOSH. Journal of Loss Prevention in th e Process Industries 13:229– 

242. 
 

Table 1 Common size fraction designations of PPC 

Dry Air-jet Sieve B-C 

Mesh Size μm % < % < 

635 20 23.7 22.4 

400 38 42.8 38.4 

200 75 81.6 69.8 

60 250 100.0 100.0 

20 850 100.0 100.0 

Table 2 SSA and particle density of PPC and Reference rock dust using the B-C system 

Sample Average SSA, m 2

/kg Std. Dev., m 2/kg Particle Density, g/cc 

PPC (n = 14) 239.4 ± 15.7 1.3 

Reference rock dust (n = 

37) 

265.1 ± 11.9 2.7 
Figures: 

Figure 1 – C omparison of SSA results from the B-C system and the Blaine apparatus 

Figure 2 – A representative PSD of PPC by B-C LDS and air-jet sieving used in 2 0-L chamber 

experiments. Data used is listed in Table 1 

Figure 3 – C omparison of the B-C laser diffraction system (LDS) measured SSAs with the 

percentage < 75 μm from MSHA rock dust survey samples 

Figure 4 – A n examples PSD of a rock dust having more than 1 m axima in the differential 

distribution curve 

Figure 5 – E xplosibility results from 20-L chamber tests using selected MSHA rock dust survey 

samples 

Figure 6 – C omparison of current minimum percentage < 200 mesh particle size specification of 

30 CFR 75.2 with 20-L explosibility chamber inerting results 

Figure 7 – R esults using classified rock dusts of 20-L chamber inerting limits, Z, and minimum 

rock dust SSA to inert PPC. The PPC has an SSA of 244 m2

/kg 
Figure 1 

Figure 2
 
 

 

Figure 3 

Figure 4
 
 

 

Figure 5 

Figure 6
 
Figure 7
 
Highlights: 

•	 The rock dust particle size distribution widely varies as well as the associated specific surface 

area. 

•	 Some rock dusts meeting the U.S. particle size requirements do not inert PPC in the 20-L 

chamber. 

•	 20-L chamber tests were conducted with several rock dusts. 

•	 The inerting effects of specific surface area are examined. 

•	 A specific surface area of 260 m2

/kg is a more effective inerting specification than the 

percentage of material passing through 200 mesh.