Spreading rock dust in bituminous coal mines is the primary means of reducing the explosion potential of coal dust that collects during the normal workings of an active coal mine. Accordingly, guidelines have been established by the Mine Safety and Health Administration (MSHA) about the relative proportion of rock dust that needs to be present in both intake and return airways. Specifically, current MSHA regulations require that intake airways contain at least 65% incombustible content and return airways contain at least 80%. The higher limit for return airways was set in large part because fine "float" coal dust (100% < 200 mesh or 75 µm) tends to collect in these airways. These regulations were based on two important findings: a survey of coal dust particle size that was performed in the 1920s and large-scale explosion tests conducted in the U.S. Bureau of Mines' Bruceton Experimental Mine (BEM) using dust particles of that size range to determine the amount of inerting material required to prevent explosion propagation.

Mining technology and practices have changed considerably since the 1920s when the original coal dust particle survey was performed. Also, it has been shown conclusively that as the average size of coal dust particles decreases, the explosion hazard increases. Given these factors, the National Institute for Occupational Safety and Health (NIOSH) and MSHA conducted a joint survey to determine the range of coal particle sizes found in dust samples collected from intake and return airways of U.S. coal mines. Results from this survey show that the coal dust found in mines today is much finer than in mines of the 1920s, presumably due to increased automation and a greater reliance on mining machinery.

Given the results of the recent coal dust particle size survey and large-scale explosion tests, NIOSH recommends a new standard of 80% TIC be required in the intake airways of bituminous coal mines. The survey results indicate that the current requirement of 80% TIC in return airways is still sufficient and appropriate. In addition, NIOSH agrees with and endorses the earlier recommendations that new rock dusting standards should be based on a worst-case scenario (using high volatile coals) with no relaxation for lower volatile coals.

Recent Coal Dust Particle Size Surveys and the Implications for Mine Explosions	RD-inertingOutToExtReview.pdf (801.19kb; 53 pages)
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Public Comment Period

Written comments on the document will be accepted through September 30, 2009 in accordance with the instructions below. All material submitted to NIOSH should reference Docket Number NIOSH-174. All electronic comments should be formatted as Microsoft Word and make reference to docket number NIOSH-174.

Comments will be accepted until 5:00 p.m. ET on September 30, 2009

To submit comments regarding this draft publication, please use one of these options:

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NIOSH Draft Documents for Public Review