Mining Contract: Development of a Silica Dust Direct Reading Sampler with Sensitivity for Dust Components and Size

Contract Status & Impact

This contract is ongoing. For more information on this contract, send a request to mining@cdc.gov.

The objective of this contract research is to develop a device and construct a sensor for dust differentiation. A 25-mm personal sampler containing a capacitive sensor, which collects and measures respirable particles, has been designed and has been used to collect dust during drilling activities at the Edgar Experimental Mine. Numerous nano- to micrometer-sized particles were collected. The collected particle agglomerates on the filter primarily measured a few micrometers to submicrometers in size, as analyzed by scanning electron microscopy (SEM). This device will use capacitive sensors to monitor the volume of deposited particles and convert the detection results to a real-time exposure level, and it will use coordinated optical and capacitive sensor responses to determine the total quantity of silica.

The designed sampler will be 37 mm in size. The optical sensor will measure the optical reflectance or transmittance on the deposited particle film, and in conjunction with the capacitive sensor (for mass detection) will provide information on the proportions of silica and carbon (coal) in the dust to aid estimation of their concentrations. When operated during a full work shift, the device will provide real-time mass measurements of respirable dust. When operated for short-term samplings, the collected particles will be analyzed off-line using a microscope to obtain information on particle composition, morphology, size distribution, and number. Under this contract, the contractors will perform the proof-of-concept development.

The proposed exposure-monitoring device provides the following novel features:

1. An optical sensor to differentiate between silica and other types of dust.
2. A capacitive sensor for real-time reading of dust mass and volume.
3. Direct collection of particles in the sampler housing and deposition on a filter without particle loss during passage through the sampling tube or cyclone. The cut-off size (D50, particle size at 50% cumulative probability or overall particle distribution) is determined by the sampler design.
4. Smaller pore-size filters (polycarbonate 0.2 µm) and a lower flowrate (0.3 L/min) than other samplers, to collect more small-sized dust (e.g., measuring a few micrometers or less).

The proposed approach will integrate capacitance- and optical-based methods to quantitate the total particle deposition as well as the relative composition of coal vs. silica dust. Measuring the relative quantity of coal vs. silica dust may be relatively straightforward, given the large differences in reflectance, particularly in the visible region. However, the proposed technique identifies silica and coal dust present as individual particles when the majority of the content is silica or carbon.

Two aspects of accuracy will be addressed in relation to coal or silica dust measurements. The first is the stability (drift) of the sensor due to changes in environmental conditions and the presence of condensation or water droplets. The second is the relative calibration (i.e., determining the relative response to the deposition of various minerals). The relative calibration will be investigated in the present study by quantitating the response to various materials of interest.