An investigation into dust, gases and vapours expulsed during the oxidation of pyritic black shale, and their potential for impacting on employee health

Pyritic black shale (PBS) is occasionally extracted as a waste product of iron ore mining in the Pilbara region of Western Australia. Mineralogical studies such as Brocks, Summons, Buick, and Logan (2003) of the PBS located at Mount Tom Price Operations have established that PBS can contain elevated concentrations of pyrite, carbon, bitumens, polyaromatic hydrocarbons and volatile organic compounds (VOCs). The natural oxidative weathering of PBS can cause exothermic reactions leading to the shale reaching temperatures that cause the expulsion of toxic gases such as hydrogen sulphide, sulphur dioxide, methane and carbon monoxide. Current management of the potential hazards of PBS is based on limiting employees’ working time in PBS areas, using gas detection equipment and wearing respiratory protection for particulates and acid gases. This research investigates the potential risk of adverse health effects for employees working within PBS areas. Three similar exposure groups (SEGs) have work activities located within pits that contain PBS. These groups are classified as: drill and blast; geologists/samplers; and heavy-mobile equipment operators. Historical exposure data for inhalable dust, respirable dust, respirable crystalline silica and toxic gases were obtained for each of the SEGs and analysed using the IHSTAT program. In order to identify what VOCs could potentially be released during the oxidation of PBS, thermal desorption tubes were used to collect positional air samples from burning PBS, and a laboratory trial was conducted where several PBS samples were taken from two waste dumps, crushed, milled and inserted into quartz tubes for thermal desorption. All thermal desorption tubes were heated to 340 °C and 500 °C and gas chromatography mass spectrometry (GCMS) was conducted on the resultant gas samples (WIENV 31). Particulate results show that the only non-conformance with the Safe Work Australia occupational exposure limits (OELs) occurred among heavy mobile equipment operators and their exposure to respirable crystalline silica. The estimated arithmetic mean exposure for the SEG was 0.043 mg/m³ and the estimated 95th percentile was 0.164 mg/m³ compared to the OEL of 0.1 mg/m³. The toxic gas data demonstrate that of the 64 days sampled, hydrogen sulphide exceeded the OEL on 1 day and the short-term exposure limit (STEL) was not exceeded. Sulphur dioxide levels exceeded the OEL on 8 days and on 4 of those days the STEL was also exceeded. All carbon dioxide results were within acceptable ranges. Thermal desorption data showed that benzene, toluene, heptane, cyclohexene, phenol, acetic acid, carbon disulfide and hexane could potentially be produced. The identification of organic vapours indicates the possibility for adverse health impacts not previously considered. Sustained exposure to large quantities of these substances has the potential to produce chronic health effects in the form of cancers and nervous system damage. As a result, recommendations for mining operations in PBS pits are: to provide respiratory protection cartridges that protect against VOCs as well as toxic gases; to carry out sampling for these organic vapours to assess worker exposure; and to maintain existing control measures that limit employees’ exposure to PBS