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.
