Mining Topic: Explosion Prevention

What is the health and safety problem?

Explosions in underground mines and surface processing facilities are caused by accumulations of flammable gas and/or combustible dust mixed with air in the presence of an ignition source. While much progress has been made in preventing explosion disasters in coal mines, explosions still occur, sometimes producing multiple fatalities.

What is the extent of the problem?

In the years from passage of the landmark 1969 Federal Coal Mine Health and Safety Act to 2001, the number of fatalities due to underground coal mine explosions had exhibited a general downward trend. From 2001 onward, however, the industry has recorded 59 fatalities and 7 injuries from these occurrences. And, from 2006 to 2011, mine explosions accounted for nearly one-quarter of mining-related deaths. Many of these recent explosions have been due to methane ignitions in abandoned workings that breached the mine seals and extended into the active areas or a deficiency in rock dust related to poor rock dusting practices.

How is the NIOSH Mining program addressing this problem?

The NIOSH Mining Program conducts research to identify and then mitigate the causes of underground coal mine explosions. Research strategies include eliminating ignition sources, minimizing methane concentrations and coal dust accumulations, applying proper dispersible rock dust in sufficient quantity at all locations, using supplemental protection such as passive and active barriers to suppress propagating explosions, understanding the science of explosions, properly designing and evaluating the strength of ventilation structures, and properly managing sealed areas.

Four key components of this explosions research program include:

1. intramural engineering control research;
2. technology transfer and information dissemination;
3. funding of extramural research projects; and improvements to rock dust specifications, rock dusting practices, and
4. sampling and test methods.

The NIOSH Mining Program's ongoing intramural research addresses methane and coal dust explosion problems in the coal mining industries. Laboratory and/or mine-site research is conducted to identify and evaluate improved rock dusting practices and sampling methods and new technologies and structures to mitigate underground coal mine explosions. Control technologies proven to be successful then undergo final evaluation at operating mine sites.

NIOSH Mining is also conducting research to advance the understanding of methane sources, migration and control strategies, interactions of sealed and active gobs with working sections, and improved ventilation methods through a complete understanding of longwall bleeder system effectiveness. A combination of field studies and numerical modeling methods are being used to identify gas migration pathways into working areas, sealed and active gobs, and to mitigate gas emissions using effective engineering controls.

What are the significant findings?

Publications include two documents: “Recommendations for a New Rock Dusting Standard to Prevent Coal Dust Explosions in Intake Airways” and “NIOSH Hazard ID (16): Non-Conforming Rock Dust.” Based on results of an extensive coal dust particle size survey and large-scale explosion testing, NIOSH recommended a new standard of 80% total incombustible content be required in the intake airways of bituminous coal mines in the absence of methane. After NIOSH research revealed that there are finer coal particles in intake airways, the Mine Safety and Health Administration (MSHA) revised the Title 30 Code of Federal Regulations 75.403 for maintenance of incombustible content of rock dust by increasing the incombustible requirement from 65% to 80% in intake airways. Likewise, after NIOSH published its Hazard ID, rock dust manufacturers, suppliers, and mine operators each implemented, as necessary, quality control measures to ensure that the rock dust used in the mines for explosion suppression met the size specifications detailed in the rock dusting standard. Also, after identifying shortcomings in the sample collection procedure, MSHA revised the sampling protocol to focus on the top layer of deposited mine dust.

Current research is focusing on identifying possible strategies to reduce respirable rock dust contributions and to develop test methods by which companies can certify their rock dust. Highlights include:

• An online “Webinar and Workshop on Preventing Coal Dust Explosions in Underground Coal Mines.”
• A new test method to examine dispersibility of a rock dust, "Design and Development of a Dust Dispersion Chamber to Quantify the Dispersibility of Rock Dust."

What are the next steps?

NIOSH Mining funds extramural research in areas that can have a direct impact on the reduction of explosions in mining operations. These extramural research activities include advanced numerical modeling to provide the understanding of the mechanics of coal dust explosions and large-scale testing to determine the ability of newly developed rock dusts to lift and disperse into individual particles.

NIOSH Mining has identified a dispersibility problem involving the effectiveness of rock dust within the mine entries when that rock dust has become wet and then dried; i.e., the rock dust forms a hardened cake that will not disperse. In order to prevent rock dust from caking, NIOSH Mining has investigated the use of additives that would enable the rock dust to maintain its dispersibility. These additives allow the rock dust to maintain dispersibility, but must not contribute to the explosion or introduce a health hazard to the mine personnel. The additive-enhanced rock dust is also able to be distributed using the existing rock dusting equipment.

NIOSH Mining will continue to evaluate new rock dusting techniques as they evolve with the industry. Techniques, such as foamed dusting, will be assessed to ensure that the new methods provide at least the same level or better explosion prevention as the currently accepted practices.

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