ASSESSMENT
OF GAS TRANSFER ANSTO MODEL AT HEATH STEELE MINES
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Project
1.22.1b
July
1997
Executive
Summary
Acid Mine Drainage
(AMD) research under the Mine Environment Neutral Drainage (MEND)
program has been ongoing since 1988 at the Heath Steele Mines site
outside Newcastle, New Brunswick. This research consists of two
distinct projects. The initial study, noted as "Heath Steele
Waste Rock Study" (MEND Project 2.31.1), has been ongoing since
1988 and has as its objective the evaluation of the nature of several
waste rock piles and the examination of the performance of a composite
soil cover on top of pile 7/12 with respect to its effectiveness
in limiting the AMD. In 1990, this study recommended that further
investigative work be carried out at the site to determine the air
permeability of the Heath Steele waste rock piles and to assess
the gas transfer mechanisms present in the piles, this initiated
the "Assessment of Gas TransferANSTO Model at Heath Steele"
project. The objectives of the project were to develop an understanding
of the processes governing the pyrite oxidation rates in acid waste
rock piles at Heath Steele using both the field data and the numerical
model FIDHELM developed by the Australian Nuclear Science &
Technology Organisation (ANSTO), to quantify those processes and
to evaluate the effectiveness of the composite soil cover for acid
waste rock management.
The project
had two major components: field measurements and modeling using
FIDHELM. The field work that included the installation of monitoring
probes permitted in situ measurements of various parameters required
by FIDHELM, namely the oxygen diffusion coefficient, air permeability
and thermal conductivity. The modeling consisted of running FIDHELM
utilizing the Heath Steele data collected in the field program.
The gas transport mechanisms dominating the oxidation rates in the
Heath Steele waste rock piles were identified using observed temperature
and oxygen profiles in conjunction with FIDHELM simulations. A users
manual for FIDHELM was also presented with the report.
The air permeability,
oxygen diffusion coefficient and thermal conductivity were measured
successfully and the values were found to be within the range measured
in four other piles of waste rock at three different mine sites
and in climates ranging from tropical to arctic. FIDHELM was shown
to be a useful tool to assess the various gas transfer mechanisms
in pyritic waste rock piles. Numerical simulations using FIDHELM
with the average measured values of oxygen diffusion coefficient,
thermal conductivity and gas permeability indicate that gas transport
was dominated by diffusion. FIDHELM results also indicate that particular
details in the oxygen profile measured at the Heath Steele waste
rock piles can be explained by the heterogeneity of the pile with
respect to both gas permeability and IOR. The modeling of the pile
with the soil cover (pile 7/12) and the field data indicate that
the composite soil cover is an effective way to reduce the ingress
of oxygen to acid generating waste rock. The data collected to date
from covered pile 7/12 is consistent with the simulations.
While the piles
at Heath Steele are small compared to waste rock piles at many mine
sites around the world, and while the balance of gas transport mechanisms
in these larger piles may be different from those at work in the
piles at Heath Steele, it is also clear that the cover system has
been effective in reducing oxygen ingress and, with this, in reducing
the oxidation rate.
Since it is
possible that advection driven by wind effects is a mechanism in
some of the piles, it is recommended that a program of work be set
up to quantify this effect and to compare measured values with the
results of modeling with a version of FIDHELM modified to accept
space/time varying surface pressure. Continuous monitoring of temperature
and pore gas oxygen concentration profiles throughout the entire
year is also recommended and may provide better understanding of
the chemical processes and gas transfer mechanisms present in waste
rock piles.
This may allow
any temperature dependence of the IOR to be estimated. Monitoring
and simulations using FIDHELM should be used to assess the importance
of pile inhomogeneity on the overall pollutant load from a pile.
Finally, the IOR and the sulphur content of the waste rock from
Heath Steele should be estimated using laboratory testing.
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