Mining Contract: Continuous Float Dust Monitor

Contract Status & Impact

This contract is complete. To receive a copy of the final report, send a request to mining@cdc.gov.

The airborne presence and surface accumulation of float coal dust in underground coal mines poses a serious safety hazard to miners. Unless inerted properly, float coal dust can be re-entrained and aerosolized into the airstream by the pressure wave of a methane explosion. To control float coal dust, mines apply inerting rock dust to achieve mixtures of at least 80% inerting material and 20% coal dust. Compliance with 30 CFR 75.403 requires that samples be taken from the mine floor, roof, ribs, and suspended items and sent off for analysis, which results in the percentage of incombustible material. After receiving these results, a mine will then know whether more or less rock dust is required.

A necessary input to an automated, programmable rock duster capable of distributing rock dust concentrations “on demand” is the concentration of float coal dust in real time; however, no sensor technology for this purpose currently exists. While instruments are available to measure coal dust in real time, these units are not intended to be used under the same conditions as a float dust monitor. Specifically, a float dust monitor must be able to measure both very low and very high (0–1,000 milligrams per cubic meter) concentrations of float dust. Because float coal dust has a larger size range than respirable mine dust (sub-75 microns compared to sub-10 microns), a sensor must be able to avoid dust losses within a unit’s internal components. It must also be robust against excessive dust loading associated with float coal and rock dust that will inevitably occur in a permanent sensor installation.

Under this contract, Akita Innovations designed, fabricated, and tested a continuous float dust monitor in the OMSHR longwall gallery at the Pittsburgh Research Laboratory in Pittsburgh, PA, with the goal of optimizing the design to provide the necessary sensitivity, dynamic range, battery lifetime, and real-time responsiveness. After investigating more than five different measurement techniques, Akita chose to pursue a light transmission/extinction method using a multi-pass optical absorbance cell. A benchtop working prototype was built and tested. It exhibited excellent sensitivity and linearity when exposed to glass slides with various representative concentrations of float coal dust.

A second prototype was constructed and tested at OMSHR’s longwall gallery using various dust types. Results of this testing demonstrated that the second prototype did not have the ability to distinguish between coal and rock dust. However, it did differentiate between dust particles and water spray droplets. By far, the most significant issue with the current CFDM is that dust rapidly accumulates on the mirrors, reducing their reflectivity to the point where the instrument no longer generates valid data. This must be resolved prior to any further advancement of this research. Additionally, before the instrument can be used as an input to an automated, programmable rock duster, an algorithm which relates the CFDM output to an actual dust concentration must be developed. Finally, although designed to consider intrinsically safe requirements, the current prototype was not constructed to be intrinsically safe. An intrinsically safe prototype will need to be constructed before testing in an underground coal mine is possible.