This fine coal dust, also known as float dust, can be dispersed back into the airstream by the pressure wave created during an explosion. The suspended coal dust continues to propagate the explosion throughout the mine entries [Harris et al. 2010].
In an effort to protect against coal dust explosions, the Mine Safety and Health Administration (MSHA) requires the application of rock dust throughout mining entries. The rock dust, when dispersed in sufficient quantities into the airstream by the explosion pressure wave, acts as a heat sink and prevents the coal dust flame from propagating down the mine entry [Harris et al. 2009, Cashdollar et al. 2010].
To be effective, the rock dust must meet specific characteristics and must be applied in sufficient quantities, with recent MSHA regulations requiring a minimum of 80% incombustible content [76 Fed. Reg. 35968 (2011)]. Title 30 Code of Federal Regulations Part 75.2 (30 CFR § 75.2) defines rock dust as: Pulverized limestone, dolomite, gypsum, anhydrite, shale, adobe, or other inert material, preferably light colored, 100% of which will pass through a sieve having apertures per linear inch (20 mesh) and 70% or more of which will pass through a 200-mesh sieve having; the particles of which when wetted and dried will not cohere to form a cake which will not be dispersed into separate particles by a light blast of air; and which does not contain more than 5% combustible matter or more than a total of 4% free and combined silica (SiO2), or, where the Secretary finds that such silica concentrations are not available, which does not contain more than 5% of free and combined silica (FCS).
After the 2010 coal mine explosion at the Upper Big Branch mine that claimed the lives of 29 miners [Fiscor 2011], long-established rock dusting practices and the quality of rock dust have undergone increased scrutiny. MSHA personnel collected 444 rock dust samples from coal mines operating in MSHA Districts 2 through 11.1 These samples were grab samples obtained from bags or bulk quantities of rock dust at the mines and were not airborne dust samples. MSHA passed these samples on to researchers at the National Institute for Occupational Safety and Health (NIOSH) for size distribution and silica analyses. A total of 393 samples were analyzed for size distribution with published results showing that 47% of the samples did not contain the minimum specification of 70% passing through a 200-mesh sieve [NIOSH 2011]. After being analyzed for size distribution, the samples were made available for silica analysis.
30 CFR § 75.2 specifies the percentage of FCS that may be contained in compliant rock dust. X-ray fluorescence (XRF) analysis was conducted by an independent laboratory on the grab samples to determine the total FCS content in the rock dust. In addition, NIOSH was interested in determining the crystalline silica content of the rock dust samples to assess the potential for exposure to quartz. X-ray diffraction (XRD) analysis was conducted by the same independent laboratory on the grab samples to quantify the quartz, cristobalite and tridymite found in the rock dust. Funding limitations only allowed for silica analysis of 261 samples. The size distribution data for these rock dust samples was also examined to determine the respirable content (less than 10 microns). Results of these analyses will be summarized and presented in this article.
Rock Dust Samples
Rock dust is mined and processed by many producers around the country that range from large national companies operating mines/quarries in several states to local producers that generate their entire product from one quarry. In addition, mine operators can purchase rock dust that is delivered to the mine in different types of packaging. For example, product can be shipped in bags weighing 40 to 50 lb each, in super-sacks/mega-bags weighing from 1,000 to 2,000 lb each, and/or in bulk tankers. Often times, the same mine will use rock dust delivered in different packaging.
Beginning in 2010 and continuing into 2011, MSHA personnel collected a total of 444 grab samples of rock dust from various mines around the country. Of the 444 rock dust samples, 18 were collected by personnel from the Technical Support group of MSHA, while the remaining 426 were collected by personnel in the MSHA districts. Although these samples were not selected based upon a statistically derived process with regard to MSHA district or producer, the large number of samples can provide a valuable indication of the silica and respirable content found in rock dust being used by coal mines located throughout the various mining regions of the United States.
For each of the rock dust samples collected, MSHA personnel provided information on the mine, rock dust supplier and type of sample (bagged or bulk sample). However, with the large number of individuals involved in the sample collection and inconsistent labeling practices across the rock dust suppliers, the type of recorded information varied somewhat in quality. Consequently, the source of the rock dust was not readily apparent for all samples. NIOSH therefore used the information supplied with each sample and additional information obtained through discussions with rock dust producers to identify the source of each rock dust sample. Twenty-three different rock dust producers were identified. In addition, samples from seven rock dust distributors were identified. However, with the information available, it was not possible to determine the source of the rock dust that was supplied by these distributors. Also, the rock dust producers indicated that distributors have been known to combine rock dust from multiple producers before delivering product to the mines.
Within the samples analyzed for silica, six of the producers had at least 15 samples of their product included in the silica analysis. Examination of these samples can provide some measure of variability in the samples.
Table 1 shows the number of samples that were collected and analyzed for silica content relative to each MSHA district. The samples were selected in an effort to analyze samples from all producers/distibutors and with a mix of bagged and bulk products. As shown, the number of samples collected by MSHA in each district varied substantially. Consequently, the number of samples from each district that was analyzed also varied substantially, but at least 10 samples were analyzed for all but District 11.
XRF analysis was conducted by an independent laboratory with a Bruker AXS S4 Explorer2 X-ray spectrometer (Bruker AXS Inc., Madison, Wis.) to quantify the FCS content found in the rock dust samples. The allowable level for FCS as indicated in the CFR is 4% or less. Figure 1 shows the distribution of samples with respect to FCS content. As shown, only 17 of the samples exceeded 4%, resulting in 93.5% of the samples meeting the allowable level specified in the CFR. Fifteen samples exceeded 4% FCS but were less than 5%, so they could potentially be approved for use by the Secretary of Labor. Two of the samples (0.8%) contained more than 5% FCS and would not meet the requirements in the CFR.
XRD analysis was conducted with a PANalytical X’Pert Pro diffractometer (PANalytical Inc., Westborough, Mass.) to quantify crystalline silica content using modified NIOSH Analytical Method 7500. Figure 2 provides a summary of the XRD analysis for crystalline silica content in the rock dust samples. It should be noted that none of the analyzed samples contained cristobalite or tridymite, with all of the reported crystalline content resulting from quartz.
The sample distribution shows that 69% of the rock dust samples contained 1% or less quartz, while 97.3% contained 2% or less quartz. These levels are well below the 5% quartz threshold3 that is used to determine reduced respirable dust standards in the current coal mine dust regulations [30 CFR § 70.101]*. However, if the rock dust contains significant levels of respirable-sized dust, mine worker exposure to airborne rock dust would increase the potential for exposure to respirable quartz dust.
As previously mentioned, at least 15 samples were analyzed for each of six different producers. However, the number of samples from these producers ranged from 15 to 65 samples. The data from each producer was used to calculate a mean and standard deviation for FCS and quartz. The standard deviation was then used to calculate a 95% confidence interval (CI) around the mean. Figures 3 and 4 illustrate the results for FCS and quartz, respectively.
As expected, variation occurred in the confidence intervals for FCS and quartz from one producer to the next, but this variation was not uniform. It does not appear that the variation is correlated with the levels of FCS or quartz that are present nor with the number of samples. Also, relative size of the confidence interval for FCS appears to be independent of the confidence interval for quartz. For example, the smallest CI for FCS was observed for producer F, but this producer did not have the smallest CI for quartz. Likewise, the largest CI for FCS was found for producer C, but this producer did not have the largest CI for quartz. These results were not surprising, since the rock dust could be produced from multiple quarries that have different rock types.
Silica content was also examined based upon the type of packaging for the rock dust. Rock dust that was delivered in 40- or 50-lb bags was classified as bag dust, while dust delivered in super-sacks/mega bags or bulk mode was classified as bulk. As shown in Figure 5, the difference in FCS was less than 0.1% for three of the producers, with a maximum difference of 0.5% for producer C. Similarly, the difference in quartz was less than 0.1% for five of the six producers, with a maximum difference of 0.4% for producer D.
Size distribution data was obtained by analyzing the grab samples with a Beckman Coulter Counter LS 13 320 analyzer (Beckman Coulter Inc., Brea, Calif.), equipped with a Tornado Dry Powder System. This instrument used laser diffraction technology to quantify the size distribution of the rock dust samples by volume. To determine the respirable fraction, the volume of dust less than 9.8 µm (the size closest to 10 µm as measured by the instrument) was summed for each rock dust sample. Data from the rock dust size analysis for 260 samples (data for one sample not available) is plotted in Figure 6. This figure shows that 87.7 % of the samples contained more than 20% respirable-sized particles, with nearly 30% of the samples containing greater than 30% respirable dust. Past research has shown that only a small portion of the respirable coal dust produced during mining becomes airborne [Ramani et al. 1987]. Similarly, not all of the respirable dust within these rock dust samples will be present as airborne respirable dust when the rock dust is applied in the mining entries. However, with samples containing this large proportion of respirable-sized particles and identified quartz in the samples, the potential for airborne rock dust to add to the respirable quartz exposure of mine workers does exist.
The Code of Federal Regulations defines specific size and silica limitations for rock dust that is used in the underground coal mining industry. MSHA collected 444 grab samples of rock dust from mines located in Districts 2 through 11 and made these samples available to NIOSH for analysis. XRF and XRD analyses were completed on 261 rock dust samples, representing 23 producers and seven distributors.
Results from the XRF analysis show that 93.5% of the analyzed samples meet the CFR requirement of containing 4% or less of FCS. Another 5.7% of the samples contained between 4.1 and 5% FCS, which can be approved for use by the Secretary of Labor. Consequently, only 0.8% of the samples analyzed contained FCS above the maximum allowable limit.
Fifteen or more samples of rock dust were analyzed for six different producers. These samples were delivered to the mines in 40- or 50-lb bags or in bulk quantities. Comparison of the percentage of FCS and quartz content for the bagged and bulk products showed that these percentages were within 0.1% of each other for eight of 12 comparisons, with a maximum difference of 0.5% for FCS and 0.4% for quartz in the four remaining samples.
Although not specified in the CFR, NIOSH was interested in quantifying the quartz content in the rock dust samples and had XRD analysis completed on these grab samples. Even though this analysis was not specifically conducted on the respirable fraction of the grab samples, the presence of quartz and a high respirable content would provide the potential for quartz inhalation by mine workers when exposed to airborne rock dust. A summary of these results showed that 97.3% of the samples contained less than 2% quartz. This quartz content is relatively low when compared to the 5% quartz level that must be exceeded in airborne respirable coal mine dust samples to result in a reduced respirable dust standard. However, size analysis of the rock dust samples showed that 87.7% of the samples contained over 20% respirable-sized dust, while 29.6% of the samples contained over 30% respirable dust. Not all of the respirable-sized dust in these samples will be present as airborne respirable dust when the rock dust is applied in mine entries. Nonetheless, mine operators should be aware of the potential exposure to respirable rock dust containing quartz and take steps to minimize or eliminate the time that mine workers are exposed to airborne rock dust. For workers required to apply the rock dust, it may also be appropriate for them to wear respiratory protection while completing this task.
Appreciation is expressed to Dr. Chi Man, senior service fellow, and Gregory Green, physical science technician, of the Fires and Explosions Branch of NIOSH for their efforts in completing the size analysis of the rock dust samples.
The findings and conclusions in this report are those of the authors and do not necessarily represent the views of NIOSH.
76 Fed. Reg. 35968 . Maintenance of incombustible content of rock dust in underground coal mines; final rule.
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1 Samples were collected before MSHA divided District 4, creating a new District 12.
2 Mention of any company or product does not constitute endorsement by the National Institute for Occupational Safety and Health (NIOSH).
3 When a respirable dust sample contains greater than 5% quartz, the 2 mg/m3 respirable dust standard is reduced by dividing the % quartz into 10 (e.g., 10÷10% quartz = 1 mg/m3 standard).
* Code of Federal Regulations. See CFR in references.
Colinet is a senior scientist and Listak is a mining engineer with the NIOSH Office of Mine Safety and Health Research in Pittsburgh. Colinet can be reached at: 412-386-6825 (Email: firstname.lastname@example.org).