Large out-at-pit ponds are required tor the disposal of tailings from the oil sands open pit mines. Present methods of tailings disposal have largely evolved to provide safe engineering designs. Increasing numbers of environmental concerns must, however, be addressed in the d:!sign of future tailings disposal systems. Also, inevitable building of some tailings ponds over possible mineable ore reserves has been the subject of recent criticism. Any future tailings disposal systems must, therefore, be designed also with this concern in mind.
This paper describes the current practice for tailings disposal in the open pit mining of oil sands and investigates possible alternate concepts that could be employed to mitigate some of the above concerns.
It is currently estimated that 25 billion barrels (1) of recoverable oil exists under less than 500 feet of overburden in the Athabasca oil sands region located in Northern Alberta, Canada (Figure 1). Because or the shallow cover, this oil can be recovered using open pit mining technology. There are presently two commercial scale operations in existence; one more is proposed and several others are being considered. It is envisaged that some day as many as 20 oil sands plants may be operating in the Athabasca region.
Typically, tailings are produced from the oils sands plants at the daily rates of 250,000 to 300,000 tons (dry weight) in hydraulic streams. By nature of the oil sands deposit and inherent in the hot water process used for extraction of bitumen is the fact that bulking factors of the order 40 to 50 percent must be accommodated resulting in some of the world's largest tailings disposal schemes. This paper presents a state-of-the-art description of the procedures currently used which have evolved in this industry for the design and operation of these tailings disposal schemes. Some of the innovative techniques presently being considered for further design optimizations in terms of cost effectiveness, environmental concerns and resource conservation aspects are also discussed.
The oil bearing zone in the Athabasca oil sands region is known geologically as the McMurray Formation. It is a cretaceous sandstone and the oil migrated into the formation late in its geological history. The formation is quite stratified and lenticular in nature resulting in a rather irregular distribution of bitumen content.
Of particular relevance to tailings disposal is the fact that sand is the predominant solids component in the Athabasca oil sands deposit. It constitutes on the average some 85 to 90 percent of the solids fraction. The sand generally includes a small silt-clay (fines) fraction but is interspersed with beds and lenses of silt and clay varying from less than one inch to several feet in thickness.
In the Hot Water Extraction Process, no grinding of the oil sand feed is required. The feed is simply mixed with hot water and steam in rotating tumblers to yield a thick aerated slurry.