How does limestone rock dust prevent 


coal dust explosions in coal mines?
 

C.K. MAN AND K.A. TEACOACH 

C.K. Man, and K.A. Teacoach are senior service fellow 

and associate service fellow, respectively, with the Pittsburgh 

Research Laboratory, National Institute for Occupational 

Safety and Health (NIOSH), Pittsburgh, PA. 

Abstract 

Coal dust explosions in underground coal mines are prevented by a generous application of rock dust (usually limestone). If an explosion should occur, the rock dust disperses, 

mixes with the coal dust and prevents fl ame propagation 

by acting as a thermal inhibitor or heat sink. 

To investigate this process in more detail, a number of explosion experiments using various coal dust and limestone 

rock dust mixes have been carried out at the Lake Lynn 

Experimental Mine (LLEM). These were conducted in a 

single entry of about 488 m (1,600 ft) in length and initiated 

with a methane gas explosion. A consistent set of postexplosion floor dust samples were taken along the entry 

after each test. These dust samples have been analyzed in 

the laboratory using thermogravimetric analysis (TGA) 

and solubility to determine how the limestone rock dust 

behaved during the coal dust/rock dust explosions. The 

preliminary results are reported in this paper and indicate 

that the chemistry of limestone plays an important role in 

its capacity to inhibit coal dust explosions.The results also 

show that it may be possible to estimate the intensity of the 

explosion using these conventional methods of analysis. 

Introduction 

Methane gas is a colorless, odorless, flammable gas that is liberated 

naturally from coal seams. Methane 

is particularly dangerous if it reaches 

concentrations between approximately 5% and 15% in air where the 

mixture becomes explosive. Therefore, it is important for mine operators to monitor the level of methane 

in coal mines. Current federal safety 

standards require sufficient ventilation to keep methane 

levels below 1% (Code of Federal Regulations, Title 30, 

Part75-323). 

Explosions in coal mines can be prevented or mitigated by eliminating ignition sources, by minimizing methane 

concentrations through the entries with adequate ventilation and by using barriers to suppress propagating explosions. Methane explosions can also cause subsequent 

large explosions of coal dust, which can have devastating 

consequences. 

To prevent the coal dust from taking part in such an 

explosion, mine operators routinely cover the fl oor, rib 

and roof areas of mine entries with a generous application of inert rock dust. Pulverized limestone rock dust is 

commonly used. The percentage of rock dust is frequently 

checked by mine inspectors. 

Federal law requires that all areas of a coal mine that 

can be safely traveled must be kept adequately rock dusted 

to within 12.2 m (40 ft) of all working faces. In the United 

States, there are regulations for underground coal mines that require 

mine operators to dust mine corridors 

liberally with an inert rock dust, such 

as pulverized limestone, and maintain 

a total incombustible content of at 

least 65% in the non-return (intake) 

and at least 80% in the return areas 

(Code of Federal Regulations, Title 

30, Part 75-403). The regulations also 

require additional rock dust where 

methane is present in any ventilating current and involves 

adding an extra 1% of incombustible material per 0.1% 

methane for air in intakes and 0.4% incombustible per 

0.1% methane in the returns. 

Conventionally, it has been accepted that during an 

explosion, the rock dust disperses, mixes with the coal dust 

and prevents flame propagation by acting as a thermal 

inhibitor or heat sink; i.e., the rock dust reduces the fl ame 

temperature to the point where devolatilization of the 

coal particles can no longer occur; thus, the explosion is 

inhibited. The amount of rock dust required to inert such 

an explosion will depend on the particle size of the inerting 

agent as well as the particle size and composition of the 

coal dust. Pulverized limestone (with a low silica content) 

is commonly used as the rock dust material because it is 

inexpensive and widely available around the world. However, other types of minerals such as dolomite and marble 

dust may also be used. 

This study investigates how limestone rock dust prevents the propagation of coal dust explosions by assessing 

its changes in chemical composition. A number of large 

scale coal dust explosions have been carried out in an experimental mine. A series of post-explosion fl oor samples 

were taken after each experiment and analyzed in the 

laboratory. 

The post-explosion dust sample studies reported here 

were part of a larger 2008 LLEM research study on the 

explosibility of various sizes of coal, which was led by K. 

L. Cashdollar and E. S. Weiss. 

Experimental 

Pittsburgh coal was used in the explosion tests. The 

coal was mined, ground and pulverized at the National 

Institute for Occupational Health (NIOSH) Pittsburgh 

Research Laboratory (PRL), Pittsburgh, PA. 

Coal dust explosion tests 

The full-scale, single-entry explosion experiments were 

carried out at the PRL Lake Lynn Experimental Mine 

(LLEM). Situated about 80 km (50 miles) southeast of the 

PRL, the mine consists of an underground limestone mine 

and surface quarry area.The underground mine section of 

LLEM is a sophisticated underground testing facility for 
conducting large-scale gas and coal dust explosions as well 

as other relevant mine research programs.The entries have 

been designed to physically match those of commercial 

coal mines, making them authentic and full-scale. 

The mine consists of four parallel drifts (A, B, C and 

D), which are approximately 488 m (1,600 ft) long. In addition, C and D drifts are connected by another entry, E drift, 

which is only about 152 m (500 ft) long (Weiss et al. 2006; 

Cashdollar et. al. 2007; Sapko et. al. 2000).There are seven 

perpendicular crosscuts connecting A, B and C drifts. The 

entries are about 6-m (20-ft) wide and about 2-m (6.5-ft) 

high with cross-sectional areas approximately 12 to 13 m2 

(130 to 140 sq ft). 

All of the explosion experiments described in this paper were done as “single-entry” explosions and were carried out in A drift. Prior to these tests, the crosscuts joining 

A and B drifts were sealed (PRL, 2008).A list of the explosion program parameters including details of the coal and 

rock dust mixtures have been summarized in Table 1. 

Table 1 

Summary of large-scale explosion test parameters (a and b indicate two adjacent dusted zones of different 

amounts of rock dust). 

Explosion Ignition Dust zone Coal dust Rock dust Total inert 

test ID zone, ft (m) ft (m) loading, g/m3 (wt.%) (wt.%) 

LLL#511 40 (12) 210a

 (64) 150 a 65, a 67.8, 

 330b

 (100.5) 150 b 80 b 81.5 

LLL#512 40 (12) 300 (91.4) 200 75 77.0 

LLL#513 40 (12) 300 (91.4) 200 80 81.5 

LLL#514 40 (12) 300 (91.4) 200 64 67.0 

LLL#516 40 (12) 300 (91.4) 200 69 71.5 

LLL#517 40 (12) 300 (91.4) 200 71.7 74.0 

LLL#518 40 (12) 300 (91.4) 200 74.4 76.4 

LLL#520 40 (12) 300 (91.4) 200 68.5 71.0 

In test LLL#511, the coal dust loading was 150 g/m3 

(1.2 lbs/linear ft of entry). For all of the other shots, the 

coal dust loading was 200 g/m3 (1.6 lbs/linear ft of entry). 

For each experiment, approximately 50% of the coal and 

rock dust mixture was loaded onto disposable styrofoam 

shelves, suspended on chains hanging about halfway down 

from the roof.The remainder was distributed evenly on the 

mine floor by hand. 

Before each explosion test, the area nearest to the face 

was isolated from the dusted area using a polythene diaphragm.This “ignition zone” was then filled to give a 10% 

methane/air mixture. Two gas samples were routinely taken from a remote location and saved in vacutainers.These 

gas samples were analyzed using gas chromatography after 

the explosion to check the gas content. About 15 m3

 (530 

cu ft) of natural gas is required for an ignition zone of 12 m 

(40 ft). To initiate the explosion, the methane/air mixture 

in the ignition zone was ignited using single point ignition 

(i.e., two electric matches twisted together and located at 

a center point of the face at mid-height). 

It should be noted that the total incombustible or inert 

contents (TIC) of the coal/rock dust mixtures are higher 

than the rock dust content due to the presence of moisture 

and mineral material in the coal. 

After each explosion experiment in the LLEM, dust 

samples were collected from the mine floor at various distances from the face.The samples were sieved through -20 

mesh (850 μm) to remove any debris and contaminants 

before being stored for later analysis. 

FIGURE 1 

TGA heating program. 

Analysis of post-explosion dust samples 

Solubility test. In a coal dust explosion, the limestone 

rock dust may undergo thermal decomposition into calcium oxide and carbon dioxide as shown by Equation 1. 

However, some of the calcium oxide may react with water 

vapor to give calcium hydroxide (Equation 2). Therefore, 

it is important to remove this ambiguity by converting any 

oxide into the hydroxide before doing the tests. 

CaCO3

 + heat → CaO + CO2 (1) 

CaO + H2

O → Ca(OH)2 (2) 

To avoid any effect from the coal, the post-explosion 

dust samples were low temperature ashed (LTA) to remove the combustible material in each of the samples.This 

was carried out by placing about 2 to 3 g (-20 mesh or 850 

μm) of sample into a porcelain crucible and ashing them 

overnight in an oven at 400° C (752° F) for 20 hours. This 

was followed by hydrating the residual inorganic material 

in order to convert any calcium oxide into calcium hydroxide (Equation 2).The hydration process was carried out in 

a modifi ed laboratory desiccator cabinet containing trays 

of water in place of desiccant material, for about 24 hours. 

After the samples were hydrated, they were transferred 

into stoppered glass vials ready for analysis. 

For the solubility experiments, about 0.5 g of a hydrated, ashed sample was mixed with 300 ml of distilled water 

in a 500-ml beaker to allow as much of the soluble component of the sample to dissolve. This was placed in a water 
and ice bath for two hours. Unlike most inorganic salts, the 

solubility of calcium hydroxide increases with a decrease 

in temperature (Handbook of Chemistry & Physics, 1976­

1977). The mixture was then fi ltered using a pre-weighed 

55-mm (2.16-in.) diameter piece of Whatman binder-free 

glass microfi ber filter (Type GF/F) and a Büchner fl ask. 

The sample was left to dry in the laboratory for 48 hours 

then reweighed. The amount of the LTA “ashed” material 

dissolved was calculated by difference (divided by the 

amount tested and multiplied by 100 to give the solubility percentage). A “blank” mixture consisting of 70 wt.% 

analytical grade “pure” calcium carbonate powder with 30 

wt.% PPC was used as a control. 

TGA carbonation test 

The post-explosion samples that were low temperature 

ashed and hydrated were also analyzed in a Perkin Elmer 

TGA7 thermogravimetric analyzer (TGA) with a Perkin 

Elmer TAC/DX thermal analysis controller interface. The 

thermobalance is sensitive to 0.1 μg and was calibrated for 

temperature using the Curie points of nickel and Perkalloy (355.3° C and 596° C or 671.5° F and 1,104° F, respectively). 

The carbonation process involved heating about 30 mg 

of each sample in a platinum crucible in a stream of carbon 

dioxide (Equation 3) and monitoring the mass as a function of temperature. A simple two-step heating program 

was used (Fig. 1). The system was initially purged with 

nitrogen to remove any air in the instrument then heated 

at 20° C/min to 105° C (36° F/min to 221° F) to remove any 

moisture.After holding at temperature for 10 minutes, the 

furnace was ramped at 30° C/min to 900° C (54° F/min to 

1,652° F) and held at temperature for 30 minutes. A constant gas flow rate of 55 ml/min was used for throughout 

each experiment. The gas was switched from nitrogen to 

carbon dioxide only for the second heating sequence. 

Ca(OH)2

 + CO2 → CaCO3 + H2O (3)

Results and discussion 

Results from the solubility tests on the ashed post-explosion floor samples obtained from the experimental mine 

(LLL#511 – LLL#520) are summarized in Table 2. 

Table 2 

Solubility of low temperature ashed post-explosion dust samples.

 Distance, 

Ft (m) #511 #512 #513 #514 #516 #517 #518 #520

 Solubility, wt.% 

108 (32.9) 15.25 7.03 3.34 10.82 5.18 6.93 4.11 5.03 

208 (63.4) 15.63 4.19 1.68 6.02 2.79 4.54 1.86 8.69 

306 (93.3) 5.59 1.28 0.00 0.93 0.63 0.61 0.69 5.77 

404 (123.1) 5.02 0.04 0.00 0.00 0.00 0.64 0.60 7.12 

502 (153.0) 3.08 0.00 0.00 0.00 0.13 0.44 0.90 3.69 

600 (182.9) 0.42 0.02 N/A 0.00 0.00 0.03 0.00 2.34 

699 (213.1) 0.16 0.00 N/A 0.00 N/A N/A N/A 1.47 

807 (246.0) 0.25 N/A N/A 0.00 N/A N/A N/A 1.59 

Since 

the solubility of calcium carbonate is very low, (less than 

1/100th compared to the hydroxide) (Handbook of Chemisty, 1976-1977) the quantity that did dissolve during the 

solubility test can be assumed to be calcium hydroxide. 

All of the samples gave low solubility values and, as 

expected, the amount of calcium carbonate converted was 

highest near to the face; i.e., by the ignition point of the 

explosion test. 

The solubility of the blank mixture (70 wt% calcium 

carbonate powder plus 30 wt% PPC) was found to be 

about 1.5 wt% or approximately 0.0018 g/100 ml of water. This is close to the published solubility data where 

calcium carbonate has a solubility of 0.00153 g/100 ml of 

water (Handbook of Chemistr y, 1976-1977). The slightly 

higher solubility value observed in the blank sample may 

be attributed to small losses through the fi lter during the 

fi ltration process. 

The post-explosion calcium carbonate rock dust solubility results for explosion test LLL#511 are illustrated in 

Fig. 2.

FIGURE 2 

Solubility of post-explosion dust samples LLL#511. 

The data shows a significant amount of the rock dust 

is soluble in some of the samples.The common trend in all 

the results shows the amount of soluble material is greatest 

near the face and falls rapidly along the entry (Table 2). 

Since the carbonate form is only slightly soluble, the 

majority of the dissolved fraction must be calcium hydroxide, although it should be noted that this may have 

been formed from the oxide during the hydration step 

in the experimental procedure. Nevertheless, this clearly 

indicates that the calcium carbonate does not only act as 

a physical heat sink in the prevention of a coal dust explosion, but also absorbs some of the energy in a calcination 

process to convert it into calcium oxide, which may be 

hydrated by any atmospheric moisture into calcium hydroxide. The degree of solubility indicates the extent of 
this calcination process. It appears this value is quite low 

and only reaches about 15% to 16% (Table 2) for this 

series of explosion tests. It should be noted that out of all 

of the samples tested, the ones that exhibited the largest 

amount of soluble material (more than 10%) were from 

explosion tests containing the least amount of rock dust, 

namely, LLL#511 and LLL#514, which had 65 and 64% 

limestone rock dust, respectively. 

The decomposition of the calcium carbonate rock dust 

into the more water soluble oxide and hydroxide is likely 

to be dependent on the intensity of the explosion test. 

Therefore, the solubility data has been plotted against heat 

flux data obtained during the explosion experiments. Two 

heat flux gauges were placed at 71 m (233 ft) and 108 m 

(355 ft) in the entry and were positioned about 0.05 m (2 

in.) from the rib. The results are given in Fig. 3.

FIGURE 3 

Comparison between heat flux data and rock dust solubility for samples taken at 233 ft (71 m) and 355 ft (108 m). 

 This shows 

a fairly good correlation between the largest amount of 

rock dust dissolved (per explosion) against the peak heat 

flux values measured from the two heat flux gauges. It 

should be noted that there is one distinct outlier in the 

108 m (355 ft) curve, which has been identified as the data 

from LLL#520. 

The TGA apparatus typically produces thermograms. 

The data for LLL#511 are shown in Fig. 4.

FIGURE 4 

TGA thermograms for LLL#511 post-explosion dust 

samples. 

 This shows the 

carbonation reaction to occur between approximately 450 

to 800° C (842° to 1,472° F) or 85 to 97 minutes in the 

diagram. A further increase in temperature results in the 

decomposition of the carbonate material via the calcination process. 

It is slightly more complicated to calculate the amount 

of calcium hydroxide present from this set of data than it 

is to determine those values from simple solubility experiments since molar fractions need to be taken into consideration. If the sample was pure calcium hydroxide and 100% 

of it was converted into calcium carbonate, the resulting 

weight gain would only be about 35.1 wt.%. 

The results from the heterogeneous carbonation experiments in the TGA are summarized in Table 3. Due to 

the fact that the TGA tests require a long time to complete, a few repeat experiments were carried out on the 

first set of samples upon completion of all the tests shown 

in Table 3 to check that the quality of the samples had not 

changed with time. The results of the second experiments 

showed the TGA to give good reproducible data. 

Table 3 also shows the calculated amount of calcium 

hydroxide present in the ashed, hydrated post-explosion 

dust samples. Many of the samples, especially those furthest from the explosion, did not appear to contain any 

calcium hydroxide and this is consistent with the solubility 

data. Conversely, the samples that did react in the TGA 

were all collected near the face. Interestingly, the correlation between the maximum solubility and the TGA is 

not 1:1 (Fig. 5) with the relative solubility values roughly 

double the TGA weight gains.

FIGURE 5 

Comparison between post-explosion rock dust solubility and TGA carbonation (only the maximum values of 

each explosion test is shown). 

 The low TGA values may 

be due to the competing decomposition reaction, which 

becomes increasingly significant at high temperatures (Lee 

et. al. 1993). It has been reported that the heterogeneous 

carbonation reaction is relatively slow (Sun et. al. 2008; 

Lee 2004; Murty et. al. 1994; Wang et. al. 2008) but can 

be accelerated with water vapor which acts as a catalyst 

(Wang et. al. 2008).These factors may also account for the 

low conversion values observed. The data given in Fig. 5 

suggest that only about 50 percent of the calcium hydroxide has reacted in the TGA. Nevertheless, there appears to 

be a good correlation to show that the soluble component 

of the post-explosion limestone rock dust is either calcium 

oxide or calcium hydroxide and, critically, not the original 

limestone. 

Conclusions 

This research has shown that limestone rock dust acts 
more than a simple heat sink when stopping a coal dust 

explosion. A small fraction of it decomposes during an 

explosion. From the limited number of tests carried out 

so far, there appears to be a good correlation between the 

amount of calcium carbonate, which decomposes, and the 

magnitude of the explosion. 

TGA has been shown to be a useful instrument. The 

TGA carbonation data supports the solubility data well 

and gives a similar trend. However, it is a time-consuming 

test and requires more skill in the interpretation of the 

data. 

Table 3 

Summary of TGA carbonation data on the post-explosion dust samples collected at various distances. 

Explosion Distance, Mass Calculated mass Explosion Distestance Mass Calculated mass 

test ID ft (m) gain (%) of Ca(OH) (%) 2 test ID ft (m) gain % of Ca(OH) (%) 2

LLL#511 	 108 (32.9) 1.5 4.3 
 LLL#516 	 108 (32.9) 0.6 1.7 


208 (63.4) 2.4 6.8 
 208 (63.4) 0 0
 

306 (93.3) 0.4 1.1 306 (93.3) 0 0 

404 (123.1) 0.5 1.4 404 (123.1) 0 0 

502 (153.0) 0.2 0.6 502 (153.0) 0 0 

600 (182.9) 0 0 600 (182.9) 0 0 

699 (213.1) 0 0 LLL#517 	 108 (32.9) 1.1 3.1
 

807 (246.0) 0 0 208 (63.4) 0.5 1.4


LLL#512 	 108 (32.9) 1.1 3.1 
 306 (93.3) 0 0
 

208 (63.4) 0.1 0.3 
 404 (123.1) 0 0
 

306 (93.3) 0 0 
 502 (153.0) 0 0
 

404 (123.1) 0 0 
 600 (182.9) 0 0
 

502 (153.0) 0 0 
 LLL#518 	 108 (32.9) 0.6 1.7
 

600 (182.9) 0 0 
 208 (63.4) 0.1 0.3
 

699 (213.1) 0 0 
 306 (93.3) 0 0


LLL#513 	 108 (32.9) 0.1 0.3 
 404 (123.1) 0 0
 

208 (63.4) 0.1 0.3 
 502 (153.0) 0 0
 

306 (93.3) 0 0 
 600 (182.9) 0 0
 

404 (123.1) 0 0 
 LLL#520 	 108 (32.9) 0.7 2.0
 

502 (153.0) 0 0 
 208 (63.4) 1.1 3.1


LLL#514 	 108 (32.9) 0.9 2.6 
 306 (93.3) 0.7 2.0
 

208 (63.4) 0 0 
 404 (123.1) 0.8 2.3
 

306 (93.3) 0 0 
 502 (153.0) 0 0
 

404 (123.1) 0 0 
 600 (182.9) 0 0
 

502 (153.0) 0 0 
 699 (213.1) 0 0
 

600 (182.9) 0 0 
 807 (246.0) 0 0


 699 (213.1) 0 0 
 

 807 (246.0) 0 0 


Acknowledgments 

The authors would like to thank K. Cashdollar and E. 

Weiss for planning and conducting the Lake Lynn explosion experiments, and to D. Sellers and F. Karnack for collecting the post-explosion mine dust samples. 

Disclaimer 

The findings and conclusions in this report have not 

been formally disseminated by the National Institute for 

Occupational Safety and Health and should not be construed to represent any agency determination or policy. 

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