IS CRYSTALLINE SILICA BIOAVAILABILITY IN COAL DUST EXPLAINED BY COAL MINERALOGY?

Quartz is present in coal dust either as a component of the coal itself, or as a contaminant, mobilized when crystalline seams adjacent to coal seams are mined. Silicosis is observed where > ~10 % of the coal dust is freshly-fractured contaminant quartz. If the quartz is derived from the coal itself, the relationship between silica and disease becomes unclear and the risk varies among mines. Several studies have suggested that much coal-derived silica has its surface occluded by aluminosilicates (clay) and toxicologists have shown that clay inhibits quartz toxicity in vitro. These studies imply that the level of occlusion may depend on coal rank (low rank implies the presence of clays). In this pilot study, we build on Wallace et al’s (1994) work by testing whether the level of occlusion of the quartz surface in coal dust is related to coal mineralogy.

Field emission scanning electron microscopy with energy dispersive spectroscopy (FE-SEM-EDS) was used to study sections through coal dust particles sourced from China (Xuan Wei County) and the USA (Penn. State Coal Bank), to identify the presence of coatings on the silica particles. The Xuan Wei sample derives from an area of high lung cancer mortality and this Permian coal has particularly high quartz (13.5 wt.%). The 5 USA samples were picked from the PSCB database by SiO₂ (51-80 wt. %) and Al₂O₃ (10-34 wt. %) contents of dry high temp. ash, with high SiO₂ likely indicative of high free quartz and Al₂O₃ being a proxy for clay content. In each sample, inhalable quartz grains were identified by EDS (Si, O peaks) and, where the grains were free (i.e. not part of an aggregate with coal/clay), high-resolution EDS transects were taken from the resin towards the particle core (over ~5 µm).

The coal samples with high Al₂O₃ contained more clay than the low Al₂O₃ samples and free quartz was hard to identify, usually being bound in aggregates; where identified, the quartz invariably contained peaks of Al at the surface or obvious clay rims could be observed. The low Al₂O₃ samples contained easily identifiable free quartz where surfaces were mostly free of occlusion by aluminosilicates.

These results imply that the potential quartz-related pathogenicity of coal dust could be rapidly assessed by clay quantification, with further work allowing a threshold for occlusion-free quartz in coal dust to be developed.