In the extraction of bitumen from oil sand using the Clark Hot Water Process, large volumes of a poorly consolidating fine tails are produced. This material will remain in suspension indefinitely and poses a problem for ultimate reclamation. Syncrude is examining various options for the disposal of this material. As part of an integrated approach, one method under evaluation is the storage of the fine tails sludge in the mined out pits and capping it with a layer of clean water. The capping layer will effectively isolate the fine tails from mixing and will sustain a viable aquatic ecosystem. Laboratory and field experimental results are presented to show the feasibility of this wei landscape option as an environmentally acceptable reclamation method for fine tails. Chemical and biological developments of the capping water are summarized and projections for the evolution of the resulting water body are given.
At the present operating oil sands plants in northeastern Alberta, bitumen is extracted from the oil sand using the hot water flotation method. In this process large quantities of water are required. As a result, large volumes of fluid waste are produced and large tailings ponds are required for their containment (Figure 1). Most of the water in the tailings slurries transported to this pond are released to a surface zone of clarified water which is recycled for plant usage. But, about 15% of the tailings water remains tied up with the fine tails fraction of the extraction tailings to form a relatively stable suspension of solids (clays, silts and fine sands) with water and unrecovered hydrocarbons.
At Syncrude, about 0.15 - 0.20 m3 of this fine tails sludge per tonne of oil sand processed are produced (Figure 2). The coarser solids (>22 µm) settle relatively quickly to form the dykes and beaches that contain the tailings pond (MacKinnon, 1989). About 40–50% of the fine tails fraction (<22 µm) of the oil sand solids enter the tailings pond as a thin slurry, where it undergoes settling and consolidation. A clarified released or "free" water surface zone develops over the slowly densifying fine tails zone. Consolidation of the fine tails is very slow(Kessick, 1979, Yong et al, 1963, Scott et al, 1985, MacKinnon, 1989). In the environment of the tailings pond, some of this material may be essentially non settling and as a consequence it poses problems with regard to ultimate reclamation.
Two general approaches for the reclamation of the fine tails are being examined, one aimed at producing a dry landscape, the other a wet landscape. Dry landscape options are based on accelerating the dewatering of the fines suspension (flocculation, freeze-thaw, evaporation, evapotranspiration) or mixing the fines with clay overburden or with sand to produce a deposit on which vegetation can be established (Scott et al, 1984, Dusseault et al, 1989, Johnson et al, 1989, Lord and Isaac, 1989, Mimura and Lord, 1991). The wet landscape option is based on the disposal of the fine tails in the mined-out pit as a fluid which will be covered with a layer of water.