Coal Age

DEC 2012

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rock dust continued less than 0.1% for five of the six producers, with a maximum difference of 0.4% for producer D. Size Analysis 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. 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 differ- ence 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 Summary 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. December 2012 51

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