Coal Age

DEC 2012

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rock dust continued Figure 3: Mean and 95% confidence interval for free and combined silica (FCS). 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 ยง 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 December 2012 Figure 4: Mean and 95% confidence interval for quartz content. 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/megabags 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 infor- mation 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. Silica Analysis 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 49

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