Atmospheric Controls on Gas Flow Directions in a Waste Rock Dump

Lahmira, Belkacem; Lefebvre, René; Hockley, Daryl; Phillip, M.

doi:10.2136/vzj2014.03.0032

Cited by 12 publications

(11 citation statements)

References 37 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…S4). The observed significance of O 2 advection supports previous work showing that even small pressure gradients (<1 Pa m −1 ) can cause significant viscous fluxes in porous media (Elberling et al, 1998) and that advection can be a dominant O 2 supply mechanism in coarse waste rock (Amos et al, 2009; Pantelis et al, 2002; Lahmira et al, 2014; Pantelis and Ritchie, 1991). At the same time, the advective O 2 transport appears to more strongly depend on waste‐rock porosity and pore‐water saturation than diffusion (e.g., diffusion and advection vary 5 and 10 orders of magnitude, respectively, as a function of saturation), which is a reflection of the fact that the effective air permeability, like D eff , varies greatly with particle size and pore‐water saturation (Supplemental Table S2).…”

Section: Resultssupporting

confidence: 87%

Localized Sulfide Oxidation Limited by Oxygen Supply in a Full‐Scale Waste‐Rock Pile

Vriens

Arnault

Laurenzi

et al. 2018

Vadose Zone Journal

View full text Add to dashboard Cite

Open-pit mines produce large amounts of waste rock that is placed on-site, often in tall, mixed-composition piles. These waste-rock piles pose environmental risks because their weathering (i.e., sulfide oxidation) may generate acidic and metalladen drainage. However, the reaction and transport limitations controlling sulfide oxidation remain poorly described at field scales. Here, we present comprehensive multiyear data from two instrumented boreholes in an operational waste-rock pile at the Antamina mine in Peru. Localized but significant (?20 m) sections of reactive waste rock with up to 20% (w/w) sulfide existed at depths of up to 100 m in the pile, and their oxidation with rates of up to 1 × 10 −7 kg S m −3 s −1 completely consumed pore-gas O 2 and generated temperatures >40°C. Using mass-transport calculations, we show that advective rather than diffusive O 2 ingress controlled the sulfide oxidation rates in reactive regions of the waste-rock pile. Sulfide weathering rates in the less reactive zones of the pile were not limited by O 2 ingress, independent of the gas transport mechanism, with the exception of fine-grained waste rock at or near complete water saturation. Our results demonstrate the pronounced effects of physical and mineralogical heterogeneity on O 2 supply to waste-rock piles and controls of gas transport on waste-rock weathering rates.Abbreviations: AP, acid production potential; bgs, below ground surface; NP, neutralization potential.Waste rock is typically the largest waste fraction produced by open-pit mines, with millions of tonnes produced around the world annually (Lottermoser, 2010). Waste rock is often stored on-site in mixed-composition, heterogeneous piles or "dumps" that can be hundreds of meters high. The weathering of these exposed waste-rock piles bears significant environmental risks because the oxidation of reactive sulfide minerals, if present, can produce acidic drainage with high metal concentrations (Amos et al., 2015;Dold, 2017) The rate of waste-rock weathering, and thus ultimately the drainage quality from waste-rock piles, is determined by a number of intricately coupled processes, including chemical-reaction kinetics (Singer and Stumm, 1970;Strömberg and Banwart, 1994), biological catalysis (Johnson and Hallberg, 2003;Blackmore et al., 2018), gas transport (Ritchie, 2003), and hydrological flow (Amos et al., 2015;Nordstrom et al., 2015).During waste-rock weathering, the oxidation of sulfidic gangue minerals consumes O 2 and H 2 O and releases acidity and heat, whereas the acid-buffering dissolution of carbonate minerals (when present) releases CO 2 . It has been previously observed that sulfide oxidation in waste-rock piles can lead to locally elevated temperatures >60°C (Lefebvre et al., 2001b), virtually complete O 2 depletion (Dold, 2017;Parbhakar-Fox and Lottermoser, 2015), and, in carbonate-rich waste-rock piles, to CO 2 levels >2% (v/v) (Lorca et al., 2016). Fast reaction kinetics and relatively slow mass-and heat-transport mechanisms in unsaturated waste-rock ...

show abstract

Section: Resultssupporting

confidence: 87%

Localized Sulfide Oxidation Limited by Oxygen Supply in a Full‐Scale Waste‐Rock Pile

Vriens

Arnault

Laurenzi

et al. 2018

Vadose Zone Journal

View full text Add to dashboard Cite

show abstract

“…These studies have demonstrated that the processes are coupled and their analysis cannot be done separately. Subsequently, research in this area has focused on developing environmental protection methods (Wilson et al, 2000b;Fala et al, 2005Fala et al, , 2006Aubertin et al, 2005;Aubertin, 2013;Martin et al, 2005;Molson et al, 2005Molson et al, , 2008Dawson et al, 2009;Raykaart and Hockley, 2009;Lefebvre et al 2011;Lahmira et al 2014aLahmira et al , 2014bBroda et al, 2015). For new sites, the choice of a waste rock pile construction method is related to practical considerations and economic constraints, in addition to environmental issues (Wels et al, 2003).…”

Section: Introductionmentioning

confidence: 99%

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

Lahmira

Lefebvre

Aubertin

et al. 2017

Journal of Contaminant Hydrology

Self Cite

View full text Add to dashboard Cite

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.

show abstract

“…Lefebvre et al (2001b) demonstrated the occurrence of convection for low‐permeability material with limited temperature increases, even for a limited pile height. In addition, Lahmira et al (2014) showed that convection can occur even in cold and covered piles. However, the fine‐grained nature and relatively high moisture content of the uppermost waste‐rock layer, which includes the traffic surface, inhibits the development of convection cells and therefore limits the contribution of convective gas transport (Lefebvre et al, 2001b).…”

Section: Discussionmentioning

confidence: 99%

“…Pore‐gas composition in full‐scale waste‐rock dumps has been monitored in the past (Harries and Ritchie, 1985; Lundgren, 2001; Lefebvre et al, 2001a; Lahmira et al, 2014). Multilevel monitoring wells installed in operational or reclaimed dumps have provided depth‐discrete information on pore‐gas composition and related parameters (Linklater et al, 2005; Sracek et al, 2006).…”

mentioning

confidence: 99%

Spatial and Temporal Fluctuations of Pore‐Gas Composition in Sulfidic Mine Waste Rock

Lorca

Mayer

Pedretti

et al. 2016

Vadose Zone Journal

View full text Add to dashboard Cite

The oxidation of sulfide minerals and pH-neutralization reactions in unsaturated mine waste rock produce changes in the pore-gas composition, leading to substantial deviations from atmospheric conditions. We studied the temporal and spatial changes in pore-gas composition, temperature, and volumetric water content (VWC) in an experimental pile during a 2-yr period. The pile was constructed by end dumping and is exposed to a marked two-season (wet-dry) climate. The grain-size segregation of waste rock occurring during end-dump construction significantly affected the spatial distribution of pore-gas composition within the pile. Results from continuous monitoring also demonstrated the role of VWC fluctuations on pore-gas composition at the study site. Oxygen decreased during the wet season, while CO 2 concentrations showed the opposite behavior. Gaseous diffusion was inferred as an important O 2 supply mechanism for this experimental waste-rock pile. In addition, wind-induced gas advection and convection probably contributed to O 2 ingress into the basal regions of the pile. Intrinsic oxidation rates were estimated based on one-dimensional reaction-diffusion modeling, showing pronounced variations between the top and bottom of the pile. These differences can be attributed to low reactivity in the basal region of the pile related to the coarse-grained nature of the material and the omission of O 2 -supply mechanisms other than diffusion. The results demonstrate the complex interactions between physical and chemical heterogeneities in mine waste rock and contribute to an improved understanding of oxidation reactions and metal release that occur in sulfidic waste rock.Abbreviations: TP, tipping phase; VWC, volumetric water content.Open-pit mining produces large amounts of waste rock containing variable amounts of sulfide minerals. This waste rock is deposited in storage facilities, under predominantly unsaturated conditions, where it can interact with water and O 2 derived from the atmosphere. Weathering of the sulfide minerals can lead to acid generation and metal release. The quality of the drainage depends strongly on the rate at which the sulfides are oxidized with time and the amount of pH-buffering minerals present in the system (Amos et al., 2015). Understanding the processes controlling metal release at the scale of operational facilities is required to make informed decisions for mine closure plans.Oxidation of sulfide minerals in mine waste generates acidity, liberating metals and releasing sulfate, which negatively affects the quality of the drainage water. The oxidation reactions consume O 2 derived from the atmosphere, and the O 2 availability within the pile provides a major control on the progress and extent of sulfide mineral weathering. For example, the oxidation of pyrite can be described as 22 2 22 4 FeS 3.5O H O 2H Fe 2SO ++ -Core Ideas • Grain-size segregation during end dumping affects the pore-gas distribution. • Seasonal variations in precipitation affect O 2 ingress and CO 2 egress. • Hot spots...

show abstract

Atmospheric Controls on Gas Flow Directions in a Waste Rock Dump

Cited by 12 publications

References 37 publications

Localized Sulfide Oxidation Limited by Oxygen Supply in a Full‐Scale Waste‐Rock Pile

Localized Sulfide Oxidation Limited by Oxygen Supply in a Full‐Scale Waste‐Rock Pile

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

Spatial and Temporal Fluctuations of Pore‐Gas Composition in Sulfidic Mine Waste Rock

Contact Info

Product

Resources

About