A coal miner applies rock dust pneumatically. (Photo courtesy of NIOSH)

 

A new type of rock dust offers coal operators a better chance to comply using existing dispersion requirements

By David Berg

Extensive research was conducted to further understand and minimize the risk of coal mine explosions following the Upper Big Branch (UBB) explosion in 2010. As a result of this research, the National Institute for Occupational Safety and Health (NIOSH) found that conventional limestone-based rock dust has a tendency to “cake” when wetted and subsequently dried. That is, the rock dust forms a hardened mass that can’t be dispersed by air. “Caked” rock dust is not effective as an explosion suppressant. Furthermore, rock dust, as defined under 30 CFR § 75.2 of the Federal Mine Safety & Health Act of 1977 (the Mine Act), shall not cake.

Carmeuse, as a supplier of rock dust, became a leading proponent in finding a solution to the rock dust “caking” dilemma. After nearly four years of effort and several collaborative exchanges with the Mine Safety and Health Administration (MSHA) and NIOSH, Carmeuse found a solution: hydrophobic rock dust. Carmeuse will market this product under their MINESAFE brand. The main benefit of hydrophobic rock dust is that it increases safety for coal miners by ensuring air dispersion of rock dust in all mine conditions.

The solution to the caking issue is applying an additive to the rock dust, i.e., surface-treating the rock dust to make it hydrophobic. Carmeuse’s MINESAFE hydrophobic rock dust works similar to conventional rock dust. The big difference is, when exposed to moisture, hydrophobic rock dust does not form a cake, and will readily disperse off the mine surface should an explosion occur. Just like conventional rock dust, hydrophobic rock dust will need to be re-applied on regular intervals to avoid a build-up layer of coal dust on top of the rock dust surface. MINESAFE has successfully passed NIOSH’s 20L explosion chamber testing and it has been applied in NIOSH’s Bruceton experimental mine without caking.

 

Rock Dust Production

Carmeuse is one of the largest limestone-based rock dust producers in North America, with six plants east of the Mississippi River, located in the heart of the Central/Southern Appalachian coal region. Carmeuse is also a rock dust industry stakeholder with MSHA and NIOSH, and since July 2011 has been a leading proponent in the development of the next generation of rock dust technology.

Carmeuse is one of the largest limestone-based rock dust producers in North America, with six plants east of the Mississippi River, located in the heart of the Central/Southern Appalachian coal region. Carmeuse is also a rock dust industry stakeholder with MSHA and NIOSH, and since July 2011 has been a leading proponent in the development of the next generation of rock dust technology.

Rock dust, consisting primarily of pulverized limestone, has been commonly used in the underground bituminous coal mining industry since the turn of the 20th century to prevent the propagation of an explosion. Limestone is an excellent material for rock dusting, as it acts as a heat sink in a fire when scoured from the ribs, roof and floor of an underground coal mine. The limestone becomes entrained in the pressure front of an explosion. This phenomenon is referred to as rock dust “inerting” an explosion. MSHA has recognized the value of rock dust by requiring its use by law.

NIOSH, and specifically, the Office of Mine Safety and Health Research (OMSHR) in Pittsburgh, provides recommendations to MSHA based on OMSHR’s scientific research. Together, MSHA and NIOSH work to keep coal mining as safe as possible.

The UBB explosion in West Virginia killed 29 miners. Upon investigation, the agency determined one of factors in the severity of the explosion was that the mine was not applying a sufficient amount of rock dust. A closer examination of the MSHA final investigation report revealed that the rock dust found in the mine did not meet both the total incombustible content and the sizing requirements of 30 CFR § 75.2. Based on MSHA’s UBB investigation, it was determined that there was almost no rock dust in the tailgate entry of the longwall. Had there been rock dust present in sufficient quantities in this area, MSHA believes that the UBB methane explosion would not have propagated into the resulting coal dust explosion.

In September 2010, MSHA published an emergency temporary standard (ETS) for rock dust, revising the existing standard 30 CFR § 75.403 by requiring mine operators to increase the total incombustible content (TIC) of the combined coal dust, rock dust and other dust from 65% to 80% in all accessible areas of underground bituminous coal mines, and an additional 0.4% for each 0.1% of methane where methane is present in any ventilating current. This ETS became law on June 21, 2011.

 

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Figure 1: Results of MSHA Rock Dust Sample Analysis. Source: IMA-NA Rock Dust Work Group Activities Review WebEx, CaCO3 Section, August 7, 2012.

 

MSHA and NIOSH subsequently embarked on a project to better educate themselves on rock dust, obtaining nearly 400 samples of rock dust across the country to test for compliance to 30 CRF § 75.2. In a population of 393 rock dust samples, which were collected by MSHA personnel from 278 underground coal mines, 47% were found to contain less than the minimum sizing specification of 70% passing through a 200-mesh sieve (see Figure 1). Non-compliant rock dust was found at 51% of the mines sampled. These results indicated to both MSHA and NIOSH that several rock dust suppliers were using rock dust as a possible outlet for undesirable materials.

In October 2011, NIOSH issued Hazard ID 16 (HID 16) on Non-Conforming Rock Dust, stating that rock dust not conforming to the requirements in 30 CFR § 75.2 for particle size and caking properties was being used in U.S. underground coal mines. The announcement identified two concerns with the rock dust being used today; first, rock dust may not meet the sizing requirements, and second, it may form an undesirable “cake” when wetted and dried. NIOSH recommended that mine operators test their supply of rock dust to assure it met 30 CFR § 75.2 and that rock dust suppliers should provide documented assurance to their customers that their rock dust met 30 CFR § 75.2. The agency subsequently issued Program Information Bulletin No. P11-50 on October 27, 2011, essentially repeating the recommendations by NIOSH found in HID 16.

In HID 16, NIOSH highlighted that the requirement for “the particles [of rock dust] 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” was not being met by current commercially available rock dust. That is, the rock dust was “caking” upon exposure to water, then drying, and could not be dispersed by a light blast of air. NIOSH considered this a major problem as the rock dust needs to be readily available during an explosion to extinguish the leading flame. If the rock dust is “caked” on the mine surface, the rock dust would not scour off the surface and participate in inerting the explosion.

In January 2012, MSHA subsequently embarked on an educational tour of numerous rock dust suppliers to determine how rock dust is produced and tested. Carmeuse was one of only four rock dust suppliers who voluntarily made their rock dust production sites available for MSHA inspection.

In September 2012, NIOSH published on their website (www.cdc.gov) that they had 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, OMSHR was investigating 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 should also be able to be distributed using the existing rock dusting equipment. Eight months later, in May 2013, NIOSH reported the following on their website: “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.”

Between May 2012 and April 2014, Carmeuse met with MSHA and NIOSH personnel on 10 separate occasions to review research and discuss rock dust testing results. Carmeuse screened more than 100 prospective “anti-caking agents,” narrowing the list down to two viable candidates. They selected one candidate, MINESAFE, to present to NIOSH, who subsequently evaluated it in the ASTM 20L explosion chamber test and applied it inside their Bruceton experimental mine. MINESAFE passed both tests with flying colors. NIOSH uses 20L explosion chamber data to simulate real-life mine conditions, as they have found excellent correlation between the 20L chamber data and historical data from their former experimental mine at Lake Lynn. Furthermore, MINESAFE has now completed six months of exposure to water and is still readily dispersible in air.

Between May 2012 and April 2014, Carmeuse met with MSHA and NIOSH personnel on 10 separate occasions to review research and discuss rock dust testing results. Carmeuse screened more than 100 prospective “anti-caking agents,” narrowing the list down to two viable candidates. They selected one candidate, MINESAFE, to present to NIOSH, who subsequently evaluated it in the ASTM 20L explosion chamber test and applied it inside their Bruceton experimental mine. MINESAFE passed both tests with flying colors. NIOSH uses 20L explosion chamber data to simulate real-life mine conditions, as they have found excellent correlation between the 20L chamber data and historical data from their former experimental mine at Lake Lynn. Furthermore, MINESAFE has now completed six months of exposure to water and is still readily dispersible in air.

MINESAFE hydrophobic rock dust is manufactured via a one-step, patent-pending process that uses a harmless, non-toxic additive. The amount of additive used is low, usually less than 2% (w/w) rock dust.

Carmeuse presented their MINESAFE test results to MSHA Technical Services and Enforcement personnel at the agency’s headquarters in Arlington, Virginia, during May. The agency was receptive to the new hydrophobic rock dust.

MINESAFE hydrophobic rock dust will cost slightly more than conventional rock dust, as Carmeuse will incur additional raw material and production costs. (Note that those rock dust suppliers who are selling so-called “quarry fines,” limestone dust and other quarry waste materials will likely be unable to meet the anti-caking requirement of 30 CFR § 75.2 without a substantial capital investment.)

 

A Word about Foam-based Rock Dust

Foam-based rock dust is not the answer to the “caking” dilemma. NIOSH has demonstrated that a layer of coal dust the thickness of a piece of paper is enough to sustain an explosion, preferentially scouring and igniting the coal dust on top of the rock dust. If the rock dust has caked, it will not scour off to catch up and extinguish the leading flame of the explosion. The same is true for so-called “foam-based” rock dust. The use of foam-based rock dust has been explored for decades. The foam rock dust has shown to dry on the mine surface to a consistency similar to that of a sponge made of plaster. Once hardened, the foam rock dust will not scour off. Much time, money and effort was invested in foam rock dust immediately after the UBB disaster. Unfortunately, NIOSH has determined that foam will not provide as effective inerting capability in the event of an explosion as dry rock dust.

 

Future Actions

MSHA has modified its rock dust sampling procedures and has recommended continuous rock-dusting in high-dust accumulation areas. While this is a step in the right direction, due to how coal is mined, it is nearly impossible to apply rock dust continuously without causing a highly undesirable disruption in coal production as well as an increased visibility hazard for the miners.

NIOSH will soon publish its recommendations to revise 30 CFR 75.2, for consideration by MSHA.

Carmeuse solved the caking problem with the development of “hydrophobic” rock dust, resulting in MINESAFE rock dust. It meets all the performance criteria as outlined by NIOSH:

  • Prevents caking – dispersible in air after wetting and dried;
  • Does not reduce inerting effectiveness (>80%);
  • Does not add an additional health hazard; and
  • Can be used with existing rock dusting equipment.

Most importantly, MINESAFE will increase safety for coal miners. It meets all of the requirements of 30 CFR § 75.2, negating the need for MSHA to modify the current definition for rock dust.


 

About the Author

David Berg is the marketing manager for Carmeuse North America, a company that manufactures rock dust. He can be reached at: 412-777-0747 (Email: david.berg@carmeusena.com).

 

References

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