What is the health and safety problem?

Accumulations of combustible dust in coal mines create the risk of large-scale explosions that can result in multiple deaths and traumatic injuries. The explosion hazard can be effectively controlled through the application and mixing of quality rock dust, such as limestone dust, to render inert the combustible coal dust generated during the mining and transport of coal. Recent studies have uncovered a number of concerns related to rock dusting. (1) How much rock dust is needed to effectively inert the coal dust? (2) How can researchers more efficiently and more effectively determine the potential explosibility of coal and rock dust mixtures? (3) Is the rock dust meeting the quality standards necessary to provide proper inerting? (4) How do mine operators keep the rock dust from caking and reducing its effectiveness?

What is the extent of the problem?

Rock dusting problems are universal to the coal industry and applicable to all operating underground bituminous coal mines. An explosion can propagate with as little as 0.01 inches of float coal dust on top of pure rock dust, suggesting the need to simultaneously apply rock dust as the mining face advances.

How is OMSHR addressing this problem?

The Office of Mine Safety and Health Research (OMSHR) has a comprehensive long-range research program to investigate problems related to the detection, prevention, and suppression of coal dust explosions. OMSHR researchers have conducted large-scale and laboratory-scale experiments to examine the influences of coal dust and rock dust particle size and concentrations needed for the prevention of coal dust explosions. Researchers have conducted extensive in-mine studies to determine the average size of coal dust particles found in intake airways that result from modern mining methods. These particles found in intake airways have been compared to the average coal particle size upon which the original 65% rock dust inerting requirement was based.

OMSHR researchers are investigating the inadequacies of current dust sampling and analysis procedures for detecting and mitigating potentially explosible accumulations of coal dust. Current dust sampling and collection procedures are being evaluated at several underground mines and the results examined to determine if there are shortcomings in the current assessment procedures.

OMSHR is also investigating the variability that occurs between different rock dusts to determine if this hinders the ability of the rock dust to inert explosions. Mine Safety and Health Administration (MSHA) regulations currently require 70% of the rock dust to pass through a 200 mesh sieve. OMSHR is conducting research to determine if the current rock dust supplies are meeting this specification as well as the federally mandated caking and silica content requirements.

What are the significant findings?

Through large-scale testing, OMSHR researchers determined that 80% total incombustible content when mixed with coal was needed to prevent dust explosions in intake airways. The Coal Dust Explosibility Meter (CDEM) was developed and commercialized for use by mine operators and mine inspectors to assess the potential explosibility of a coal dust-rock dust mixture. The CDEM can be used by mine operators to manage their day-to-day rock dusting practices.

OMSHR also issued a Hazard ID on non-conforming rock dust to alert the industry that some rock dusts were not meeting the particle size and caking specifications set forth in MSHA regulations. OMSHR determined that rock dusts not conforming to these requirements were being used in U.S. underground coal mines. The use of non-conforming rock dust reduces the protection from potential dust explosions.

What are the next steps?

If rock dust absorbs moisture and forms a cake, it is unlikely to be scoured and entrained in the pressure front of an explosion, thus reducing the ability of the rock dust to inert the coal dust and prevent the explosion from propagating. Studies are underway to identify the key factors that contribute to the caking properties of rock dust. Preliminary results indicate that the presence of ultra-fine particles less than 10 microns may be one factor causing caking. OMSHR is investigating the best method to determine the caking tendency of rock dust and methods to prevent caking, such as the addition of anti-caking agents.

Federal law now requires that an incombustible content of 80% be maintained within 40 feet of the face, making wet rock dusting amenable to some operations to avoid respirable dust exposures. Wet rock dusting is performed by mixing rock dust with water and spraying it on the roof and ribs. However, once the wet mixture dries, the rock dust may cake, preventing it from being adequately dispersed to prevent a propagating explosion. New technologies have emerged which employ a similar wet dusting technique where small amounts of additives are mixed into the rock dust and water slurry before spraying onto the exposed mine surfaces. The additives ensure that the resulting dry layer is able to be scoured by a pressure front. However, no required testing of these additives currently exists to determine if they function properly and if they are equivalent in behavior to a dry rock dust application.

Layering of coal dust on top of rock dust can defeat all rock dusting efforts. OMSHR is working to determine what steps can be taken to avoid this layering issue and to avoid undusted or poorly dusted areas close to the working areas. As a part of these rock dusting best practices, dispersion of coal dust/rock dust relative to ventilation practices also needs to be further investigated.

The work in this topic area is supported by the NIOSH Mining Fire Fighting and Prevention program. See the NIOSH Mining Products page for software, guides, training materials or other items related to this topic.