Abstract
Placement methods and material availability during waste rock pile (WRP) construction may create significant heterogeneities in physical and geochemical parameters (such as grain size, permeability, mineralogy, and reactivity) and influence the internal pile structure. Due to the enormous scale of WRPs, it is difficult to capture the influence of heterogeneities on mine drainage composition and evolution. Although laboratory- or field-scale experimental studies have provided much insight, it is often challenging to translate these results to full scale WRPs. This study uses a numerical modeling approach to investigate the influence of physical and chemical heterogeneities, structure, and scale on the release of acid rock drainage (ARD) through 2D reactive transport simulations. Specifically, the sensitivity of drainage quality to parameters including grain size distribution, sulfide mineral weathering rates, abundance and distribution of primary minerals, and pile structure as a function of construction methods are investigated. The geochemical model includes sulfide oxidation, pH buffering by calcite dissolution, and ferrihydrite and gypsum as secondary phases. Simulation results indicate that the implications of heterogeneity and construction method are scale-dependent; when grain size distribution trends observed in a pile's core are applied to the entirety of a pile, results between push- and end-dumping methods vary substantially—however, predicted drainage for different construction methods become more similar when features such as traffic surfaces, structural variation, and multiple benches are also considered. For all scales and construction methods investigated, simulated results demonstrate that pile heterogeneity and structure decrease peak mass loading rates 2 to 3-fold, but cause prolonged ARD release compared to the homogeneous case. These findings have implications for the economics of planning water treatment facilities for life of mine and closure operations.
Highlights
Numerous industries globally depend on a reliable supply of commodities produced from mining activities
In order to compare results from different simulations, sulfate mass loadings (Table 3 and Figures 6, 7) and pH distributions within the pile (Figure 8) are used to compare the influence of heterogeneities on drainage quality from pyrite oxidation and acid rock drainage (ARD) generation to the homogeneous model
Stylistic models simulated using MIN3PHPC suggest that heterogeneity and pile structure are important to the overall basal drainage evolution observed from a pile
Summary
Numerous industries globally depend on a reliable supply of commodities produced from mining activities. Typical laboratory-based methods (such as humidity-cells) or field-cell experiments do not capture additional structural features such as fining upwards or traffic surfaces (characterized by smaller grain size and lower porosity due to compaction from heavy equipment driving along exposed weathering surfaces during the construction process) To this end, numerical models have proven suitable tools for investigating the influence of physical and chemical heterogeneities (Molson et al, 2005; Fala et al, 2013; Lahmira et al, 2017; Pedretti et al, 2017; Appels et al, 2018; Muniruzzaman and Pedretti, 2020), as well as scale (Wilson et al, 2018; Vriens et al, 2020b) on ARD generation and attenuation processes in WRPs. at the time of writing a sensitivity analysis that examines the roles of physical and chemical parameter distribution, with a particular emphasis on the effect of pile construction methods, structural features, and pile scale, has not been completed
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