Basic HTML Version
34
Geotechnical News • September 2013
www.geotechnicalnews.com
WASTE GEOTECHNICS
Pham’s doctoral research at the
University of Alberta investigated the
impact of heat transfer within the sul-
phide waste rock piles on ARD. “The
chemical reactions of the sulphuric
minerals in the [Diavik] waste rock
depend on temperature due to bacterial
activity,” says Pham. “The lower the
temperature, the fewer the chemical
reactions.” The research team found
that chemical reactions in the waste
rock piles also release heat into the
environment and thaw the permafrost
beneath the piles, which have implica-
tions for ARD. “For example,” Pham
says, “if a waste rock pile is placed in
a discontinuous permafrost area, the
increase in ground temperatures due to
warming can trigger the oxidation of
sulphide minerals causing ARD.”
Most solutions to mitigate ARD are
designed for mines in temperate
regions. However, permafrost regions
experience phenomena such as freeze-
thaw cycles, frost heave, thaw consoli-
dation and the presence of ground ice
(e.g. in the form of ice wedges) which
have a significant impact on how
northern mining projects must address
ARD. Air convection cover (ACC) is
one method to produce rapid cool-
ing in waste rock piles and to reduce
ARD, which was investigated during
the research project.
In an ACC system, a waste rock pile
is covered with a low permeability
soil layer with high moisture content
(e.g. fine-grained soils) and over-
laid with a high permeability coarse
non-acid generating rock layer that
is relatively dry. The moist soil layer
constrains the heat from the active
waste rock within the pile, while the
coarse rock layer allows cold air to
penetrate and cool the pile via natural
air convection during the cold winter.
During the summer, the coarse rock
layer and the frozen fine-grained layer
act as insulators, keeping the pile
cool, relative to outside temperatures.
Pham conducted numerical simula-
tions to understand the ability of ACC
to keep waste rock piles cool in the
order of several decades. Based on his
simulations for an 80 m high waste
rock pile, ACC can maintain frozen
conditions on a waste rock pile for
100 years, considering climate warm-
ing, which can aid in reducing the
risks of ARD. “We’re using natural
processes to mitigate the environmen-
tal impact of mining,” says Pham.
Other research studies in the Diavik
Waste Rock Project focused on the
geochemistry of the waste rock piles,
water flow through the waste rock into
the surrounding terrain and wind-
induced gas transport. Many findings
from this extensive research program
hold important lessons and consid-
erations for planning future mining
projects in northern Canada.
Acknowledgements
The Diavik Waste Rock Project was
a joint research program between the
University of Waterloo, University of
British Columbia and the University
of Alberta supported by the Natural
Sciences and Engineering Research
Council of Canada, the International
Network for acid Prevention, the
Mine Environment Neutral Drainage
Program, the Canadian Foundation
for Innovation and Rio Tinto (Diavik
Diamond Mines Inc.).
References
Bailey, B.L., Smith, L.J.D., Blowes,
D.W., Ptacek, C.J., Smith, L. &
Sego, D.C. (in press) The Diavik
Waste Rock Project: Persistence
of contaminants from blasting
agents in waste rock effluent.
Applied Geochemistry. doi: http://
dx.doi.org/10.1016/j.apgeo-
chem.2012.04.008
Chi, X., Amos, R.T., Stastna, M.,
Blowes, D.W., Sego, D.C. &
Smith, L. (in press) The Diavik
Waste Rock Project: Implications
of wind-induced gas transport.
Applied Geochemistry. doi: http://
Figure 2. Permafrost distribution (modified by Rekacewicz 2005).
Figure 3. Location of Diavik
Diamond Mine (Pham 2013).