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.
