Authors: Meiers, G; Pernito, M

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Meiers, G & Pernito, M 2023, 'In mine rock storage facility design, advective oxygen transport is king, but diffusion is queen', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth,

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The concept is that through an established mine rock storage facility (MRSF) design and construction method a reduction in advective oxygen transport is achieved. This limits sulfide oxidation and the generation of stored acidity with a desired advantage of a reduced reliance on the final cover system and / or duration over which water collection and treatment may be required. In general, bottom-up MRSF construction is favoured over top-down construction due to the reduced overall airflow capacity and oxygen supply. Mechanisms of oxygen ingress include diffusion, advection (in air and water), and barometric pumping. It has been generally accepted that advective air transport provides at least an order of magnitude greater oxygen supply than the other noted transport pathways. Advective airflow occurs when the air within the mine rock pores warms up, becoming less dense compared to the surrounding atmosphere. As a result, it starts to rise, drawing in cooler air at the toe of the facility. While the aforementioned is certainly true, this paper provides a more comprehensive assessment of advective and diffusive oxygen transport in a Top-Down and Bottom-Up MRSF. Two-dimensional numerical simulations (oxygen consumption, water, air, and heat transport) were completed using a generalized MRSF and boundary conditions based on an information and literature review and supported by a blind study. In the Top-Down construction advective oxygen transport through the MRSF slope face was approximately five time greater than diffusion. Elevated mine rock pore-oxygen concentrations in the slope face supressed the concentration gradient that drives diffusion. However, across the plateau diffusive oxygen transport exceeded advection. Diffusive oxygen on the plateau accounted for approximately 30% of the total oxygen ingress, driven by a steeper pore-oxygen concentration gradient. Bottom-Up construction reduced the overall oxygen ingress by approximately 40% compared to the Top-Down MRSF. While advective oxygen ingress in the Bottom-Up construction was lower than the Top-Down analysis, this gain was offset by an increase in diffusive oxygen. Results of this analysis highlight that coupled diffusive and advective oxygen transport should be considered in assessment of MRSF construction methods and the ratio of the MRSF slope to the plateau area will have a marked influence on simulated performance. In addition, there is a need to disseminate two-dimensional analysis to a three-dimensional outcome. This is due to bottom-up construction providing an initial overall larger footprint/surface area exposing a greater amount of mine rock over longer time frames to diffusive oxygen, which may moderate the benefit of bottom-up construction.

Keywords: Mine rock storage facility, construction methods, bottom-up and top-down, advection, diffusion, oxygen consumption, sulfide oxidation, mine closure, progressive reclamation

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