Mining Project: Certification Test Protocol Development and Treated Rock Dust Deployment Strategies

Research Summary

30 CFR 75.402 mandates that underground coal mines are to be rock dusted in order to prevent coal dust explosion propagations. Current research by the NIOSH Mining Program has focused on the examination of each element of 30 CFR 75.2 (rock dust definition) and its relevance and applicability in the current mine environment. As a result, desired rock dust attributes such as the appropriate particle size, specific surface area, dispersibility requirements, percent incombustible content, and the maximum allowable silica concentration have been identified. However, until now, there was no standard test protocol by which mine operators can certify the performance of rock dusts.

Current research also identified a means by which rock dust can be treated to prevent caking when wetted and then dried. Caking adversely affects the dispersibility of the rock dust, rendering the rock dust ineffective to inert coal dust. The use of anti-caking additives can prevent rock dust from caking when exposed to water, but further research was needed to assess the deployment of treated rock dust and to evaluate its potential health effects. Since the treated rock dust is readily dispersible, the perception was that it could contribute to the miners’ airborne respirable dust exposure levels. Therefore, research was conducted to assess this potential concern through lab and in-mine studies.

To address this need, this project had eight research aims, as follows:

1. Establish specific surface area relationships between rock dust and coal dust.
2. Examine the application of anti-caking treated rock dust in coal mines and its impact (if any) on the airborne respirable dust concentrations.
3. Evaluate the inerting effectiveness of anti-caking rock by conducting large-scale explosion testing.
4. Reduce the generation of airborne respirable dust by developing foam/slurry rock dust.
5. Quantify the silica content in various size fractions of the rock dust with particular emphasis given to the respirable size fraction.
6. Examine the health effects (if any) of anti-caking treated rock dust through animal testing and toxicological studies.
7. Provide laboratory test protocols for certification of rock dusts.
8. Examine the possibility for eliminating ineffectual particles greater than 75 microns from the rock dust size distribution while maintaining inerting effectiveness. 

Based on large-scale and laboratory-scale explosion test results, NIOSH identified that rock dust should have a minimum surface area of ~2,600 cm²/g in order to inert a coal dust explosion at the 80% incombustible content limit in the absence of methane. Anti-caking rock dust has proven to be as effective or better than regular rock dust in preventing propagating explosions. Homogenous rock dust/coal dust mixtures and layered rock dust/coal dust tests conducted in the Experimental Barbara Mine at the Central Mining Institute in Poland determined that treated rock dust is as effective or better than dry untreated rock dust. When tested in the NIOSH-developed dust dispersion chamber, anti-caking rock dust was found to be readily dispersible even after prolonged exposure to water (up to 6 months).

Field experiments examined the respirable dust contributions due to the deployment of anti-caking treated rock dusts and found that these levels slightly increased when using anti-caking rock dusts. To limit generation of respirable dust particles, foamed rock dusts were examined. When a wet foamed rock dust was applied and allowed to dry, the foamed rock dust generated little or no respirable dust during application and was identified to be better at preventing a propagating explosion than a rock dust slurry (wet rock dust).

NIOSH's Health Effects Laboratory Division conducted a toxicity analysis and concluded that there were no major differences in cellular responses to treated and untreated rock dusts. When classified rock dust size fractions were analyzed for silica, it was concluded that the respirable rock dust size fraction was not significantly enriched with quartz/silica compared to the bulk rock dust.

Finally, this project evaluated test methods by which rock dust can be tested and certified as meeting the desired attributes to inert a propagating coal dust explosion. The NIOSH research team designed and developed a dust dispersion chamber to quantify the relative dispersibility of any rock dust compared to a dry reference rock dust. A known quantity of dust was exposed to a pulse of air having a repeatable pressure and time duration. Using an optical dust probe, the light obscuration provided by the dust cloud suspended in the downwind air stream was measured. This measurement was compared/related to that of the dry reference rock dust (dust historically used in large-scale Lake Lynn Experimental Mine testing).

A common test method used to examine explosibility of a dust is the ASTM E1226 Standard Test Method for Explosibility of Dust Clouds. NIOSH researchers conducted tests using this method to determine the most effective rock dust size fractions that inerts a coal dust explosion. It was found that the size fraction of 20–38 µm rock dust particles inerts coal dust in the 20-L chamber, although rock dust particles <200 mesh (75 µm) are in general most effective in preventing a propagation. To develop a universal 20-L explosion test protocol, NIOSH conducted round-robin testing at three different labs and obtained similar results. The protocol will be published to the mining industry by end of 2020. After conducting cubic meter chamber tests, the NIOSH research team concluded that the 20-L Kuhner chamber is a better apparatus for testing the inerting limits for coal dust.