Coal Age

DEC 2012

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r o c k d u st Silica and Respirable Content in Rock Dust Samples JAY F. COLINET AND JEFFREY M. LISTAK Fine coal dust particles, less than 75 microns in diameter, generated by various mining processes can be carried by the ventilating airstream into return entries before depositing onto the surfaces of mine entries [NIOSH 2006]. 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 ยง 75.2) defines rock dust as: Pulverized limestone, dolomite, gypsum, anhydrite, shale, adobe, or other inert material, Table 1: Distribution of Rock Dust Samples by MSHA District * Since the time the samples were collected, District 4 has been separated into Districts 4 and 12. 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 200mesh 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 (SiO 2 ), or, where the Secretary finds that such silica concentrations are not available, which does not contain more Figure 1: Free and combined silica (SiO2) content for 261 rock dust samples. 48 www.coalage.com 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], longestablished 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 Figure 2: Crystalline silica (quartz) content for 261 rock dust samples. December 2012

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