Modeling Pyrite Oxidation in Arid Environments

Abstract

Understanding future pit lake water quality has become an important element in permitting open pit mines, due to the potential long-term water quality impacts on ecological receptors. Quantifying pyrite oxidation kinetics is an integral part of this analysis. To date, constraints resulting from low moisture content in the arid regions of the southwestern U.S. that host many large, disseminated deposits have not been considered. In this study, laboratory humidity cells routinely used to simulate pyrite reactivity were found to overestimate pyrite oxidation rates in arid environments by a factor of >2 compared to the same material emplaced in the field. Increasing particle size also resulted in decreasing reactivity, with 16-64 mm diameter grains <50% as reactive as 4-16 mm diameter grains. A pyrite oxidation model that incorporates both fracture and porous media pyrite oxidation using site-specific data, e.g., wall rock geometry, moisture content, etc., was developed to simulate wall rock reactivity in arid environments. The results demonstrated a 3-fold reduction in oxidized wall rock thickness in arid environments compared to assumptions of constant 100% moisture, with a concomitant reduction in solute loading to the incipient pit lake.

Mine Wastes

“…A vuggy texture and intense fracturing of the wall rocks will greatly aid sulfide oxidation. This in turn reduces the solute loads entering the incipient acid pit lake (Fennemore et al 1998). Concentrations of dissolved metals may reach saturation levels.…”

Geochemistry of unsaturated soil systems: Aqueous speciation and solubility of minerals and gases in capillary solutions

“…Indeed, Hecht and Kölling (2002) performed the complete leaching of the pyrite contained in an unsaturated column of soil consisting of silty sand for a period of around 200 days. Furthermore, Fennemore et al (1998) and Hollings et al (2001) have demonstrated experimentally that the size of the grains, and therefore the specific surface of pyrite, is also a parameter that affects the reactivity of the mineral to oxidation. Fig.…”

Physical, Hydrologic, and Aqueous Chemical Characterization of the Ore Knob Tailings Pile (Ashe County, North Carolina, USA)

“…Water serves as both the reaction medium and as an important reactant (Evangelou and Zhang 1995;Rose and Cravotta 1998). In arid environments, oxidation rates may be 2-3 times lower than rates in humidity cells due to reduced moisture content and larger particle size (Fennemore et al 1998). Sulfur-and iron-oxidizing bacteria can catalyze the kinetic reaction of pyrite oxidation and increase the oxidation rate 30-300 fold (Nordstrom and Alpers 1999).…”