MEND Report PA-2
March 1997

Executive Summary

A new simulation model, MINTOX, has been developed to provide a useful tool for predicting the behaviour of kinetic sulphide mineral oxidation within mine tailings impoundments, and for simulating the subsequent speciation and transport of oxidation products through the tailings and into downstream aquifers. MINTOX includes the major reaction sequences known to control the hydrogeochemistry at many base metal tailings sites. These processes include diffusion of oxygen into the unsaturated zone, diffusion of oxygen into the sulphide mineral grains, sulphide mineral oxidation, acid generation and release of iron, sulphate and heavy metals. Furthermore, the model can simulate the advective-dispersive transport of the mobilized species, accounting for equilibrium speciation and reactive processes including solid mineral dissolution and precipitation.

MINTOX has been tested in both one-dimensional and two-dimensional modes against observed field data from the Nordic Main tailings impoundment near Elliot Lake Ontario (Wunderly et al. 1995, 1996). Simulated depth profiles of selected species, including oxygen and pyrite content, agreed well with observed data, with discrepancies in other phases due primarily to the assumption of local geochemical equilibrium. The two-dimensional simulations of the Elliot Lake site showed reaction sequences and concentration levels consistent with observed or inferred behaviour. MINTOX has also been applied to simulate the geochemical processes occurring at the Nickel Rim tailings impoundment and has provided new insights into processes governing acid generation and neutralization.

Methods to control the rate of sulphide mineral oxidation, and the impact of AMD include reducing the rate of oxygen diffusion into the tailings using a moisture-retaining surface cover, and adding limestone to increase the buffer capacity. MINTOX has simulated the beneficial effects of these types of remediation measures at both the Elliot Lake and Nickel Rim sites. Simulations showed for example, that an increase in moisture content from background levels to saturation effectively restricted the oxidation process. Since most oxidation occurs within 10 years of deposition however, covers appear best suited if emplaced immediately following tailings deposition.

Additional simulations for both the Elliot Lake and Nickel Rim sites were completed to address the effects of adding limestone to the tailings. At Elliot Lake, the results showed significant reductions in heavy metal concentrations as higher pH favours the precipitation of minerals which removes the aqueous species from solution. At Nickel Rim, higher pH and sulphate concentrations were also observed.

A 1D sensitivity analysis based on the Nickel Rim site showed significant variation with diffusion coefficients, fraction of sulphide mineral, initial grain size, and carbonate buffer mineralogy. The simulations suggested a need for determining the influence of spatial variation of physical and chemical properties on AMD evolution, and incorporating uncertainty in the interpretation of results.

Last Modified: 2003-11-26		Important Notices

Français | Contact Us | Help | Search | Canada Site
Home | What's New | CANMET-MMSL | MMS Site | NRCan Site