With the re-emergence of coal workers’ pneumoconiosis as an issue in some Australian mines, recognition and further understanding of the possible contribution of reactive pyrite to lung diseases issues is important to help manage and mitigate risk
The issue of coal workers’ pneumoconiosis (CWP) has re-emerged in some Australian mines (Zosky et al, 2016). Harmful effects from coal dust inhalation have been well documented, but the underlying causes and mechanisms of CWP are still debatable. Several recent epidemiological studies in the United States have shown a general correlation exists between the concentration of pyrite (FeS2) within coal and the prevalence of CWP in miners (Huang et al, 2005; Cohn et al, 2006). The possible role of pyrite as a contributor to the coal dust hazard has not been investigated for Australian coals. It is therefore important that recognition and further understanding of the possible contribution of pyrite to lung disease issues is needed to help manage and mitigate any elevated risk from the hazard of pyritic coal dust.
Correlation between pyrite and prevalence of coal workers’ pneumoconiosis in United States coal miners
Two US studies (Huang et al, 2005; Cohn et al, 2006) have highlighted a possible link between the presence of bioavailable iron from pyrite in a range of US coals to the prevalence of coal workers’ pneumoconiosis. This was based on a correlation between the pyritic sulfur in coal and cases of coal workers’ pneumoconiosis in seven coal mining states in the US (Figure 1).
Both these studies discussed the causal link from a medical perspective, but they did not look at the true physical link in the mine environment. Consequently, the impression is given that all pyrite in coal is capable of being the cause of medical-related dust issues. This is false, as recent studies have shown that for the pyrite in coal to be reactive and oxidise at an appreciable rate it must be present in an appropriate form (size and morphology), otherwise for all intents and purposes the pyrite is unreactive (Beamish and Theiler, 2017). No similar studies have been conducted on Australian coals.
Generating pyritic coal dust
Pyrite in coal can be present in a number of geological circumstances. It may be finely dispersed throughout the coal or present as coatings on cleat planes and in certain circumstances present as layers or lenses parallel to the coal banding (Figure 2a). When the pyrite is in a suitable form that provides both increased surface area and increased porosity for easy air access to reactive sites, it can rapidly oxidise under moist ambient mine conditions. The reaction products vary depending on the amount of moisture present in the environment and the availability of cations from other minerals present in the coal. Generally, hydrated iron sulfate minerals such as roemerite (FeFe2(SO4)4.14H2O), melanterite (FeSO4.7H2O) and rozenite (FeSO4.4H2O) can be formed in substantial quantities. An example of these minerals forming on the surface of reactive pyrite is shown in Figure 2b.
The densities of these minerals, roemerite (2.15), melanterite (1.89) and rozenite (2.195) are significantly less than the original pyrite (5.01) and other minerals normally found associated with coal, such as quartz (2.65). This makes them prone to becoming airborne as part of the coal dust mixture. In fact, the mix of these mineral salts is commonly referred to as ‘fairy dust’ by mine workers due to the way that they can become entrained and sparkle in the mine ventilation air, particularly if they occur in the upper part of a seam that is forming a coal roof.
The fineness of the needle-like crystals (in the order of 1-2 µm as shown in Figures 3a-d) means that they are ideal for lodging in air tracts and piercing lung wall tissue. These minerals are also prone to dissolution chemistry with moisture and can produce significant amounts of sulfuric acid. Contact with the skin can produce a burning sensation.
Findings from United States epidemiological studies
Studies on coal from Pennsylvanian coal regions with a high prevalence of CWP (Zhang et al, 2002; Huang et al, 1998) found that these coals released large quantities of bioavailable iron. This iron availability is capable of causing oxidative stress, which can contribute to cell injury in vitro, and possibly leads to the development of CWP. Cohn et al (2006) studies the implications of bioavailable iron in the form of pyrite as a source of reactive oxygen species (ROS) (ie hydrogen peroxide and hydroxyl radicals), which are capable of degrading nucleic acids. This line of research has been continued by Harrington, Hylton and Schoonen (2012) through experiments using simulated lung fluid to assess the performance of ROS generated from pyrite in contact with the fluid. More recently, Harrington, Tsirka and Schoonen (2013) have examined the role of pyrite in causing inflammatory stress in the A549 lung cell. They concluded that, while inflammatory stress response (ISR) does not increase proportionately with pyrite content in coal, the two coals they studied with the highest pyritic sulfur and bioavailable iron contents generated the greatest ISR. It is most likely that this finding is a result of the lack of recognition given to the form of pyrite present in the coal (reactive or unreactive).
Recognition and quantification of reactive pyrite fairy dust generation for mine planning
Chemical analysis determination of pyritic sulfur content is only a broad-brush indicator, as it does not distinguish between reactive and unreactive pyrite. Therefore it is necessary to supplement this information with visual examination at both the exploration and mining stages of coal mining operations. The general characteristic appearance of ‘furry’ growths resulting from clusters of the needle-like crystals from pyrite oxidation on the coal or immediate roof and floor rocks is a good indicator that reactive pyrite is present. This can be supported with X-ray diffraction analysis of the minerals to confirm their identification. Subsequent disintegration of the coal or immediate roof and floor rocks confirms the oxidation process is ongoing and that the oxidation products are being released into the mine ventilation.
Any future medical studies of the possible role of pyrite as a contributor to the coal dust hazard need to be focused on coals that contain reactive pyrite to maximise the success of identifying the consequences of the hazard and to enable appropriate mitigation strategies to be developed. Further examples and details of the products of reactive pyrite oxidation in the mine environment will be presented by Beamish, Ward and Chalmers (2017) at the upcoming Australian Mine Ventilation Conference 2017, which is being held in Brisbane from 28-30 August.
The author would like to thank Duncan Chalmers for recommending a review of the possible role of reactive pyrite in coal dust hazard issues. Jeff Chen from the University of Queensland helped to obtain the SEM images and Colin Ward from the University of New South Wales provided X-ray diffraction confirmation of the mineral assemblages as part of earlier project work on coal spontaneous combustion.
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