Effect of material variability and compacted layers on transfer processes in heterogeneous waste rock piles

Abstract

The heterogeneity of waste rock piles is due to the wide and variable grain size distribution of waste rock and construction methods leading to complex internal structures. The general objective of this work was to better understand the effect of such heterogeneity on the coupled transfer processes acting within waste rock piles producing Acid Mine Drainage (AMD). For this purpose, parametric numerical simulations were conducted with the TOUGH AMD numerical simulator, considering 1) three random spatial distributions of the same material properties to assess the resulting behavior, 2) four ranges of material properties with the same spatial distribution to evaluate the effect of the degree of heterogeneity, and 3) the effect of compacted layers due to circulation of heavy equipment during construction. Results show that fine-grained (denser with lower permeability) material present near the boundary of a pile can limit air entry. Coarse materials promote preferential flow of gas and water vapor. Fine-grained materials beneath the pile surface favor the internal condensation of water vapor and thus minimize water loss. The initiation of secondary gas convection cells requires a minimal degree of heterogeneity, which is closely related to the range of permeability between the coarse and the finer material ratio (k(coarse)/k(fine)). The presence of coarse grained material in the pile does not necessarily lead to more convection and higher AMD production. The magnitude of convection rather depends on the amount of fine-grained material and its distribution in the pile. Results also show that low-permeability compacted layers strongly limit convection. Results thus support waste rock pile construction methods integrating fine-grained materials or compacted layers to minimize AMD production.

Mine Waste Rock: Insights for Sustainable Hydrogeochemical Management

“…However, strategic placement of different materials (e.g., co-location or blending of acid-producing and acid-neutralizing materials) can also be an effective strategy to mitigate acid drainage risks [233]. Geochemical material heterogeneity can cause unpredictable weathering dynamics [115,[234][235][236] and representative sampling is critical to characterize the spatial distribution of mineral reactivity that can significantly affect the overall drainage signature of composite systems. Relevant geological, lithological, and alteration units must be sampled relative to the amounts and particle size of each material [45,132]: inadequate sampling can contribute to substantial variability or incorrect assessment of waste-rock reactivity.…”

“…Surface runoff from waste-rock piles may be deliberately minimized to avoid erosion on the pile batters, in which case it is typically insignificant for the water balance. Yet, certain reclamation strategies rely on maximizing runoff from compacted (subsurface) cover layers to limit percolation into underlying reactive material [236,247]. Material compaction (e.g., in traffic surfaces) can facilitate surface runoff and ponding, especially under flashy precipitation patterns.…”

“…Mass transfer estimates and modeling studies have shown that convective heat and gas transfer can sustain high waste-rock weathering rates [204,274]. The onset and strength of convective transport is determined by the temperature gradient (local atmospheric temperature versus the spatial distribution of reactive waste rock [236,273]) and waste-rock permeability. As a result, parameterization of heat transfer properties is not straightforward [16] and effective heat transfer properties vary widely as a function of porewater saturation and porosity [204,236,266,272].…”

“…The onset and strength of convective transport is determined by the temperature gradient (local atmospheric temperature versus the spatial distribution of reactive waste rock [236,273]) and waste-rock permeability. As a result, parameterization of heat transfer properties is not straightforward [16] and effective heat transfer properties vary widely as a function of porewater saturation and porosity [204,236,266,272]. Further work is required to relate heat transfer mechanisms to waste-rock properties and pile construction methods, and thereby help quantify the contribution of convection to overall weathering rates and ultimately drainage quality.…”

Investigating the Influence of Structure and Heterogeneity in Waste Rock Piles on Mass Loading Rates—A Reactive Transport Modeling Study

“…Materials deposited from these large machines result in random, heterogeneous waste rock placement, though on a larger-scale, materials may be strategically mixed or confined to prescribed zones to meet closure plan strategies (e.g., MEND, 1998;Miller et al, 2006). Construction methods and type of material thus create significant variations in the distribution of grain size and mineralogy (with implications for sulfide content, metal concentrations, and pHbuffering capacity), which affects drainage quality (Smith et al, 1995;Munroe et al, 1999;Fala et al, 2003;Tran et al, 2003;Herasymuik et al, 2006;Stockwell et al, 2006;Lahmira et al, 2017). While the relationships between these characteristics in a pile and the potential to generate acid rock drainage (ARD), as well as related metal release and attenuation have been studied extensively (e.g., Sherlock et al, 1995;Stumm and Morgan, 1996;Nordstrom and Alpers, 1999;Akcil and Koldas, 2006;Amos et al, 2015;Dold, 2017), the influence of heterogeneity and spatial distribution of these characteristics on ARD has received less attention to date.…”