Large volumes of waste tailings, combined with slow dewatering rates, create serious tailings management issues in the oilsands. The Alberta oilsand deposits possess an estimated 175 billion barrels of mineable heavy crude oil. Spread over a 140,000-square-kilometre area, these reserves currently supply about 1.5 million barrels of oil per day with plans to double oil production by 2018. Oil extraction involves strip mining of the ore and separating the solid materials from bitumen using hot water. This process utilizes three times more water than the oil produced. The generated waste tailings (a mixture of fine and coarse-grained particles and liquid) are stored in containment facilities with dykes constructed from the coarse fraction of the slurry. The fine portion of the sludge takes decades to dewater after maturing to a solids content of about 30 per cent in the first two years of deposition in the containment facility.
To date, approximately 750 million cubic metres of mature fine tailings (MFT) are stored on-site in the Athabasca region, and this inventory may reach one billion cubic metres by 2014 if the status quo is maintained.
Many treatment technologies, such as physical, natural, and chemical amendments and different types of co-disposal and co-mixing alternatives, have been developed to address these problems. Most of the methods have attained limited success due to one or more of the following reasons: high operational costs; skilled labour shortage; long-time commitment; low energy-efficiency; low bitumen extraction; inconsistency due to tailings variability; difficulty with the removal of supernatant liquid (water sitting above the sediment); detrimental effects on water quality; and low pumping efficiency.
The recent regulatory criteria (which require a 50 per cent MFT reduction by June 30, 2013) have sparked a resurgence in research and development. Dr. Shahid Azam, an associate professor in environmental systems engineering at the University of Regina, is leading a team of researchers to address the MFT issues. The team is conducting research to develop a step-wise dewatering method involving polymer addition, centrifugation, and thin lift drying. Conceptually, the synthetic polymers (commercially available chemical reagents) help bind the fine particles to form large clumps that settle faster under gravity. These particle assemblages are then further dewatered by simulating an increased gravity in the centrifuge. Finally, the centrifuge under-flow product, which still retains a certain amount of moisture, is deposited on the ground in thin layers where it loses additional water due to evaporation.
“Our interrelated approach requires a fundamental understanding of solid-liquid-polymer interactions under high gravity and during drying,” explains Azam.
This step-wise strategy for accelerated tailings dewatering will help reduce the space required to contain the tailings and increase the stability of the containment facilities, as well as increase waste-water recycling and reduce contamination.
Supported by industrial funding and a state-of-the-art test facility, this research has produced very promising results at the laboratory scale and the team is enthusiastic to scale-up their activities in the field. Overall, this research is a significant step toward meeting the regulatory criteria in a sustainable fashion and it provides a cost-effective, environmentally friendly and socially viable solution for the oilsands industry.
Please note, this article was previously published in the 2012 Saskatchewan Oil Report magazine.