The issue of coal workers’ pneumoconiosis (CWP) has recently re-emerged in some Australian mines. 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. The possible role of pyrite as a contributor to the coal dust hazard has not been investigated for Australian coals. 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.
Inferred relationship between pyrite and prevalence of CWP in miners
Two US studies (Huang et al, 2005; Cohn et al, 2006) 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 reasonable 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 to all intents and purposes the pyrite is unreactive (Beamish and Theiler, 2017).
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, 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 Figures 2b-2d.
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 crystals (in the order of 1-2 µm) 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.
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 XRD 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 at the Australian Mine Ventilation Conference in Brisbane in August 2017.
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.
Beamish B B and Theiler J, 2017 (in press). Improved understanding of the role of pyrite in coal spontaneous combustion, in Proceedings 16th North American Mine Ventilation Symposium, (The Society of Mining, Metallurgy and Exploration Inc, Littleton, USA).
Cohn C A, Laffers R, Simon S R, O’Riordan T and Schoonen M A A, 2006. Role of pyrite in formation of hydroxyl radicals in coal: possible implications for human health, Particle and Fibre Toxicology, 3:16.
Huang X, Li W, Attfield M D, Nadas A, Frenkel K and Finkelman R B, 2005. Mapping and prediction of coal workers’ pneumoconiosis with bioavailable iron content in the bituminous coals, Environmental Health Perspectives, 113(8):964-968.