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

Abstract

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 ...

Mobilization of Metal(oid) Oxyanions through Circumneutral Mine Waste-Rock Drainage

“…In a minority of field barrels bearing intrusive PAG waste rock with NPR < 1 (3 out of 49 barrels), drainage pH values reached pH < 3 after 5–10 years (Figure 2). The observed release of elevated sulfate and metals such as Fe, Cu, or Zn (concentrations >100 mg/L; Figure S2) from PAG compared to NAG waste rock (concentrations <1 mg/L) aligns with previous research at Antamina 2,52,54,55 and the general notion that weathering of predominantly high-sulfide, low-carbonate waste rock can generate acidic drainage conditions that mobilizes sulfate and metals at high levels. 1,5…”

Mine Waste Rock: Insights for Sustainable Hydrogeochemical Management

“…As a result, sulfide oxidation and bulk weathering rates reported from laboratory and field experiments vary by orders-of-magnitude across sites (Table 3). Because it is virtually impossible to resolve all molecular-scale mineralogical heterogeneity, waste-rock reactivity or weathering rates are typically presented through representative bulk parameters such as drainage loads (i.e., net sulfate leaching) or oxygen consumption or heat production rates [192][193][194]. Similarly, the estimation of the acid-producing versus acid-neutralizing nature of waste-rock material is often based on bulk laboratory tests (static testing or acid-base accounting (ABA)) rather than (or complementary to) microscale mineralogical analyses.…”

“…In addition to diffusion, pressure gradients can result in advective gas transport in porous media [263]. Estimates of advective gas transport using e.g., pneumatic heads and Darcy's law for gas, have shown that barometric fluctuations can be relevant for gas transport particularly through more permeable coarse rubble zones at the bases of waste-rock piles [194,243,264,265].…”

“…Both diffusive and advective gas transfer are strongly controlled by the properties of the waste rock, including its permeability, degree of porewater saturation, and spatial heterogeneity therein [194,266]. Various empirical and semi-empirical formulae to estimate diffusivity and permeability from particle size distributions or porosity ranges exist [267][268][269] but these often invoke tortuosity or constrictivity parameters that are poorly defined [266].…”

“…Oxygen consumption from sulfide oxidation and CO 2 production from carbonate dissolution may induce poregas pressures and compositions that differ from atmospheric conditions, particularly in large waste-rock piles with reactive material [194,198,199,260]. In smaller-scale (laboratory) experiments with less-reactive waste rock, poregas variations are typically neglected yet rarely experimentally determined.…”