Southern Coalfield Coal Washery Reject (CWR) Characterisation and Classification, including management strategies for applications in Civil Engineering

With growing emphasis on Environment, Sustainability and Governance (ESG) across the Australian coal mining sector, there is opportunity to reduce the disposal of Coal Washery Rejects (CWR) and increase its beneficial use as an economic resource in both mining and municipal construction and rehabilitation projects, offsetting the need to quarry virgin materials.

Using the Southern Coalfield CWR in NSW as a leading example, this study presents the history of CWR regulatory provisioning; compares CWR to other widely adopted construction and rehabilitation materials; and addresses contemporary data gaps to demonstrate the environmental and geotechnical merits of using CWR in prohibited or ambiguously prescribed engineering applications, including construction of dams, quarry and dredge pond rehabilitation and back-filling quarry voids (including placement in or beneath water, including groundwater). The NSW Environment Protection Authority (EPA) Coal Washery Rejects Order and Exemption 2014 (which regulates the use of CWR in civil engineering applications) is used as a basis for this assessment.

Southern Coalfield CWR has been characterised and compared to other widely adopted construction and rehabilitation materials, using a range of geotechnical, geochemical and environmental laboratory testing methodologies. In general, CWR is held to higher environmental performance thresholds than other routinely used fill materials.

Common constraints to using CWR for engineering purposes relates to its perceived combustion potential and its geochemical behaviour below the water table; however, regulatory controls do not adequately assess or prescribe these risks. The risk management of CWR propensity to combust is elaborated in this study as a complex process involving interrelations between intrinsic geological properties, extrinsic environmental conditions and physical properties (i.e. compaction) when applied to land.

Southern Coalfield CWR is demonstrated as suitable for a range of civil engineering applications with minimal control measures; however, CWR from other regional coalfield may present a hazard if certain risks are not adequately managed. A screening process has been developed that the wider coal industry can adopt to assess the viability of using their CWR in off-site civil engineering applications. However, despite their viability, some CWR materials may not meet the testing thresholds of the NSW EPA Coal Washery Rejects Order and Exemption 2014 (the principal controlling regulation in Australia). As such, this study provides a basis for further assessment including a suite of enhanced characterisation tests recommended to assist demonstrate its suitability.

In addition, where CWR is non-compliant with the NSW EPA Coal Washery Rejects Order and Exemption 2014, it is proposed that operators seek mine specific Order and Exemptions that determine characteristic thresholds specific to adapted site and application risks. Based on regulatory provisions, it is understood that mine specific Order and Exemptions are achievable based on project, volume and end-use specific approvals.