Oxygen ingress in mine waste: modelling, measurement and design implications for non-acid forming material encapsulation from a tropical environment case

Authors: Fourie, J; du Toit, J; Usher, B Download Paper Open access courtesy of:

DOI https://doi.org/10.36487/ACG_repo/2615_71

Cite As:
Fourie, J, du Toit, J & Usher, B 2026, 'Oxygen ingress in mine waste: modelling, measurement and design implications for non-acid forming material encapsulation from a tropical environment case', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-7, https://doi.org/10.36487/ACG_repo/2615_71



Abstract:
Oxygen availability is a key constraint on sulphide oxidation and oxygen transport is often considered as the rate-limiting step in acid and metalliferous drainage/metal leaching (AMD/ML). Understanding the depth and continuity of the oxic zone is therefore fundamental to inform design decisions that can lower ARD/ML risk especially during the post-closure phase of a mine waste storage facility. Reducing oxygen ingress is one of the few direct levers to mitigate the oxidation of pyrite, which produces ferrous iron, sulphate and acidity. Sulphide mine waste facility designs that achieve lowered oxygen ingress will conceptually have less long‑term contaminant loads and should have more defensible closure performance criteria in closure plans, consistent with leading guidance (e.g. International Network for Acid Prevention 2020). Two complementary approaches for estimating the oxidation zone extent are gas measurements and multiphase reaction and transport modelling, and ideally both should be applied. The objective of a multiphase model is typically to refine the conceptual understanding of oxygen supply mechanisms (diffusion versus advection), and once calibrated, allow screening of mitigation options and landform options before committing to expensive trials. Field measurements typically combine gas ports and oxygen sensors with suction and moisture content probes and lysimeters to capture coupled gas and moisture transport. In a sulphide-rich mine waste facility, mitigation should target the dominant oxygen supply mechanism, which can include promoting water saturation in surficial layers to suppress oxygen diffusion, reducing air permeability to reduce advection, interrupting the development of thermal chimneys and managing heat with interim and final covers and landform designs. The case study presented in this paper assesses an oxygen ingress–informed encapsulation design for a waste rock dump (WRD) at a mine in the tropical Asia-Pacific region.

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