Preventing
AMD by Disposing of Reactive Tailings in Permafrost
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND
Report 6.1
December 1993
Summary
In recent years
the detrimental effect of acid water produced by sulphide oxidization
of tailings and waste rock on the environment was recognized and
is being combatted in southern regions of Canada. There are numerous
abandoned, operating and proposed mines in Canadian permafrost regions.
These northern mines have an additional tool to combat acid generation
by freezing and keeping the tailings and waste rocks in a frozen
state. This tool requires careful evaluation to determine where
and how it may be used economically.
Permafrost
is not homogeneous being greatly dependent on the mean annual air
temperature and physiography that varies across the country. The
permafrost is divided into discontinuous permafrost located near
the 60 degree latitude and continuous permafrost located in the
Arctic region. The permafrost area can also be divided by climatic
regions, namely Boreal, Coridillera and Arctic climatic regions.
The first two represent discontinuous permafrost and the last region,
continuous permafrost.
The mean annual
ground temperature is about 4 degrees Celsius warmer than the mean
annual air temperature. However the ground temperature near the
ground surface fluctuates greatly during the year with the fluctuation
decreasing with depth. Steady temperature is reached some 10 to
15 m below the ground surface. The annual temperature fluctuations
cause a surface layer to thaw annually, called the active zone.
The thickness of the active zone in the continuous permafrost varies
between 0.5 m under thick organics to 10 m under bare rock. In discontinuous
permafrost the depth of this zone may be greater. For a given temperature
regime, the thickness of the active zone is predominantly governed
by the insulation of the organic layer and secondarily by the water
content of the underlying soil/rock stratigraphy. Mineralogy of
the underlying soil/rock has a small influence.
Acid generation
is the result of chemical and biological oxidation of pyrite. Available
data and the RATAP model show that there is a considerable decrease
in the rate of oxidation as the temperature approaches zero degrees
Celsius. The relative rate of oxidation reduces to about 10% of
the relative rate occurring between 25o to 30oC.
However little is lmown about further decrease as the temperature
drops below OoC. Some unfrozen water occurs below OoC.
From geotechnical engineering studies it is known that the unfrozen
water around particles freezes around coarse particles, such as
sand and coarse silt. Around smaller particles a small film of unfrozen
water may still remain to about -5oC or colder depending
on the size and mineralogy of the particles. The volume of unfrozen
water is small and is surrounded by ice. In silt at -31oC
the unfrozen water represents less than 10 percent of the total
weight of water.
Temperature
monitoring at the Lupin mine tailings impoundment provides a good
temperature data base showing the ground temperature changes through
the year in several soil/tailings/bedrock stratigraphies. This data
shows the depth of the active zone to be between 2.5 to 3 m in native
silty sand till and tailings. In natural ground with a 500 mm organic
layer the active zone was measured as 1 rn and in bare fractured
bedrock the active zone was 4 m.
The data from
Lupin located in a cold continuous permafrost environment shows
that about 3 m thick sand/gravel covers are required to bring the
active zone out of the tailings and keep the tailings permanently
frozen. This depth of granular cover is very costly. An alternative
cover could be non-acid generating mine rock that may be more economical
than sand/gravel fills at mines with surplus waste rock. Some savings
would also be achieved by a thinner cover. A more economical solution
may be to design a total surface water containment in the tailings
pond. The total containment would be created by the construction
of frozen core perimeter dykes. In this design the base of the active
zone below the dyke crest would be above the high water levels in
the pond. The raised permanently frozen material could be obtained
by either increasing the height of the dykes or using polystyrene
board insulation within the fill to decrease the fill requirement.
Total surface
water containment may be feasible in most continuous and discontinuous
permafrost regions because of high annual evaporation as compared
to annual precipitation in the north. This design requires to limit
the watershed to the tailings pond and a freeboard to store the
water from extreme snowmelt and precipitation events.
In discontinuous
permafrost regions it may be practically impossible to develop permafrost
in the tailings. In this case total containment with a frozen perimeter
dyke could be developed through artificial means. The most practical
artificial design is the use of thermosyphons and polystyrene board
insulation buried just below the ground surface. In this design
the thermosyphons, non mechanical heat tubes, extract heat from
the ground during the winter and the insulation reduces the heat
re-entry during the summer. Thermosyphons have been used in Alaska
and Canada in road, airfields and building applications.
A marginal
and unproven alternative to develop and maintain permafrost in the
discontinuous permafrost regions may be a convective rock cover.
The principle of the rock cover is that during winter, heat is extracted
from the ground by air convection through the large voids in the
rock and during the summer the air in the voids acts as an insulation
blanket. There is one documented case history from Russia (Robertson
et al. 1982), and one thermal analysis conducted for uranium tailings
in Canada, supporting this principle. Follow up of this design by
further thermal analysis and field installation is recommended because
of its simplicity.
Future work
to obtain further information and develop design principles for
the use of permafrost to prevent tailings acid generation should
be through laboratory and office analyses of sulphide oxidation
in the near below zero temperatures; thermal analyses to improve
the estimated cover thicknesses to keep the tailings frozen and
field installation/monitoring in continuous and discontinuous permafrost
of the recommended designs.
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