Underground » Health and Safety
Timely monitoring and reporting of personal exposure levels of respirable coal dust (RCD) and respirable crystalline silica (RCS) is crucial to dust exposure management for protecting coal mine workers from mine dust lung disease. The current gravimetric method needs 1-2 weeks to get exposure results due to off-site analysis. The latest personal dust monitor (PDM3700) is a real-time mass-based respirable dust monitor in the coal mines. Its introduction significantly improves monitoring of RCD exposure and eliminates the delays in the current gravimetric method.
For timely RCS measurement, the National Institute for Occupational Safety & Health (NOISH) has developed a field-based silica analysis method using a compact Fourier transform infrared (FTIR) spectrometer to directly analyse RCS of loaded dust over a traditional gravimetric PVC filter without ashing and redeposition, which is called direct-on-filter (DoF) method. As the PDM uses a glass fibre filter for dust sampling, the FTIR analytic method cannot be directly applied to silica analysis due to interference. Moreover, the current PDM filter assembly cannot accommodate the DoF silica analysis due to the blockage of the IR beam by the filter holder.
The project aimed to develop a new type of PDM filter with a non-silica filter material and a novel filter assembly structure to enable the DoF silica analysis of the PDM-collected dust sample with a field-based FTIR method. The project work was commenced with investigation of structure of commercial glass fibre PDM filter from scanning electron microscopy (SEM) observations of disassembled filter, followed by design of preliminary new filter assembly for non-silica filter membranes screening. Ten types of non-silica membranes with various pore sizes and woven structures were chosen as the candidate filter membranes for fabrication of new PDM filter assemblies, labelled as A1-A10. The prepared new filter assemblies were then installed in a PDM3700 unit for preliminary screening through conducting instrument diagnostics. Diagnostic failures were observed over new filter assemblies made from five of candidate filter membranes, due to high differential pressure and/or low frequency.
Parallel dust sampling experiments with a laboratory dust testing chamber were undertaken using the existing commercial PDM glass fibre filter and those new non-silica filter assemblies that passed the PDM instrument diagnostic test. The sampling performance, in terms of the total dust mass and real-time dust concentrations (15 and 30-min rolling time), of the commercial and new filter assemblies were compared based on the results of laboratory parallel sampling. In result, the membrane of filter assembly A9 was chosen for further studies of performance improvement and development of the non-silica filter prototype. This is because that the A9 filter possesses the greatest dust loading capacity and exhibits the dust sampling performance most comparable to that of commercial PDM filter. The dust sampling performance of filter assembly A9 was further improved by optimisation of exposed surface area of dust collecting layer.
Tremendous efforts were then devoted to new filter prototype development for fabricating the optimum new non-silica filter employing the determined filter membrane, to enable readily filter assembly/disassembly and practical DoF silica analysis in a compact filed-based FTIR spectrometer.
Parallel dust sampling experiments using commercial PDM filters and the new non-silica filters were conducted with the laboratory dust test chamber to obtain dust samples with various total dust mass (0.34-7.29 mg) and silica concentrations (1.23-3.49 wt%). The dust sampling results demonstrated that the new non-silica filter achieves both real-time dust concentrations and total dust mass very close to those of the commercial PDM filters tested in a same run of parallel dust sampling experiment, with a major difference less than 5%.
The dust samples collected by commercial PDM filters were redeposited onto a PVC membrane for silica analysis using the FTIR method, while all the new filter collected dust samples were examined with the DoF silica analysis method using a compact field-based FTIR spectrometer. After DoF silica measurement some of sampled new non-silica filters were also redeposited onto a PVC membrane for FTIR silica analysis. A clear linear relationship between silica contents measured by the DoF and redeposition methods was established for correction of DoF silica analysis results. The corrected DoF silica analysis results are comparable to the measured (with a redeposition method) silica content of dust sample collected by the commercial PDM filter, with a majority variation below 15%.
The developed novel non-silica filter prototype allows easy assemble and disassemble of filter components, and the dust-collecting membrane layer with loaded dust can be intactly removed after dust sampling for the subsequent rapid DoF silica analysis using a compact field-based spectrometer. Therefore, the PDM3700 equipped with the newly developed non-silica filter is able to achieve both real-time respirable dust monitoring throughout the sampling shift and the DoF RCS analysis silica at the end of the sampling shift. Patent protection for this novel non-silica filter prototype is being pursued and patent filing is currently underway. Once the patent is filed, the detailed information on the filter structure and dust collecting membrane of this filter prototype will be updated.
The project has successfully developed a novel non-silica filter prototype for PDM3700 to enable DoF measurement of the RCS content of PDM-sampled dust.