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One of mining’s biggest environmental issues is air pollution as a result of suspended dust. Characterizing mine dust is of key importance for monitoring air quality, health of employees and local inhabitants, and environmental effects of mining operations.
Mine activities such as drilling, loading and unloading coal, and coal transport can generate air pollutants. These include total suspended particulate (TSP) matter and smaller particles with diameters of 10 µm or less (PM10). Of these, transport has been recognized as a particularly serious source of TSP and PM10 pollution.
Mine dust can reduce the air quality of an area and effect local populations, for example by silicosis, black lung, and increased mortality rates. The effects are also felt by the local environment, where mine dust can reduce visibility for flora and fauna, affect climate, and affect atmospheric processes. In the interest of protecting the health of employees, addressing environmental concerns, and adhering to government regulations, characterizing mine dust using accessible and accurate methods is of high importance.
What is Electron Microscopy and Why Use It?
Electron microscopy is a microscopy technique to visualize samples at higher resolution than is afforded by light microscopy. Instead of light, it uses a beam of electrons to illuminate the sample and thus allows for characterization of the sample. While commonly applied in biology, it has been a popular method for characterizing mine dust in the past 30 years.
Simple electron microscopy allows images to be acquired at different magnifications, which allow morphological identification of particles and accurate particle counts of both large and small particles, without double counting any particle.
Ongoing developments lead to modifications of methods to continuously increase the quality of electron microscopy. For example, scanning electron microscopy combined with energy-dispersive X-ray (EDX) spectrometry has commonly been used when visualizing mine dust. Here, the electron microscope’s electron beam interacts with the particle surface, emitting X-ray emissions that are detected as a spectrum. Because each of the particle’s elements generate a distinct X-ray type, peaks on the spectrum and their relationship to one another can indicate elemental composition and thus the minerals can be identified.
The electron microscopy method has successfully been implemented to investigate the chemistry and morphology of mine dust particulates. Specifically, electron microscopy coupled with EDX spectrometry gathers data on the composition of particles, dimensions, and shape, from which volume and mass can be calculated. Electron microscopy EDX also allows all these parameters to be measured simultaneously, reducing acquisition time.
Asbestos is another commonly occurring form of mine dust. The term comprises a dangerous group of silicate minerals that can occur as a result of mining such as the famous example of the US town Libby where asbestos and asbestiform spread throughout the town.
Electron microscopy has been one of the main techniques by which airborne asbestos is identified and analyzed. Identification is typically done by morphological analysis: while criteria differ, particles three times longer than their width can be called asbestiforms.
Transmission electron microscopy (TEM) has can be augmented with field emission scanning electron microscopy to improve the resolution and allow more rigorous standards to be imposed on what is and is not considered an asbestiform. This is incredibly valuable when ensuring health of local populations and adherence to regulations.
Applications and Examples
When applied to real world examples, electron microscopy has been successful in characterizing mine dust and monitoring air quality. When applied to northern Colombia, it was able to show that three out of fifteen monitoring stations had higher TSP and PM10 concentrations than the national air quality standards.
Importantly, these three stations were near three population centers, potentially endangering civilian health. However, some particles identified by electron microscopy were highly similar to the area’s soil, making it difficult to specifically assign responsibility to the mines.
The above example highlights the need for continuous development of electron microscopy methods to ensure more precision when characterizing mine dust. Similar situations apply to mine dust in the US, where regulations introduced in 2010 change sampling rules and reduce the allowed dust concentration. With measurements occurring for each shift performed, emphasis is placed on reducing the time it takes to carry out mine dust characterization using electron microscopy and enhance specificity when identifying particulates.
References and Further Reading
- Sellaro R., et al. (2015). A standard characterization methodology for respirable coal mine dust using SEM-EDX. Resources. https://doi.org/10.3390/resources4040939
- Huertas J.I., et al. (2012). Characterization of airborne particles in an open pit mining region. Science of the Total Environment. https://doi.org/10.1016/j.scitotenv.2012.01.065
- Harris K.E., et al. (2007). Discovering the true morphology of amphibole minerals: complementary TEM and FESEM characterization of particles in mixed mineral dust. Modern Research and Educational Topics in Microscopy.