Use gravity and trenching to accelerate the release of process water from tailings deposit
Cost effectively increase speed of dewatering large amounts of fluid fine tailings with minimal energy input
The tailings ponds required at oil sands mines are large and impact the landscape. Reducing the size and need for tailings ponds, and increasing the speed with which they can be reclaimed, are challenges being addressed by COSIA’s oil sands mining companies. A method called Accelerated Dewatering is being piloted to cost effectively, and with minimal energy input, separate process water from large volumes of fluid fine tailings.
A tailings pond includes the water, sand, clay and residual oil that is left over after oil sands processing. Once in the pond, the sand quickly sinks to the bottom and the water released to the surface is recycled. Below the surface water of the pond, there is a mixture of clay and process water called fine tailings, which takes a long time to settle and solidify naturally. Even after many years, fluid fine tailings (FFT) will still have a semi-solid consistency and it can take decades to separate and dry out which makes the material good for aquatic reclamation. Technologies such as Accelerated Dewatering enable the FFT to be used for a variety of landform designs in reclamation.
The Accelerated Dewatering process involves treating FFT with a flocculent that binds together the small clay particles suspended in water into larger flakes. The clay particles that are bound together into flakes are much larger than the individual clay particles and as a result settle much faster thereby allowing formerly trapped water molecules to flow free. The flocculated FFT is then deposited in a containment area, and as it dries a rim ditch is dug around the deposit. As the sun dries this material, the surface begins to crack. Process water begins to drain out of the cracks, and is captured and drained off via the rim ditch and pumped back into the tailings pond. As the surface dries and becomes denser, it also becomes heavier which further weighs down on the rest of the wet materials inside the containment area accelerating the amount of water squeezing outward through the cracks. The process water is decanted down drainage ditches to a tailings pond, where it becomes available as recycled water for mine operations.
Technology and Innovation
The oil sands industry learned from phosphate mines in Florida, which pioneered this process of using sun and drying to naturally dewater tailings. The first Syncrude field study began in 2009, and it continues to dewater today.
In 2009, Syncrude began its first accelerated dewatering pilot by pumping 80,000 cubic metres of tailings into a test pit. The material settled quickly, and the first 30,000 cubic metres of water was pumped off and recycled. The remaining 50,000 cubic metres of material, which measured as deep as 10 metres at the centre of the pit, would have taken decades to settle out if left alone. A “rim ditch” was excavated in spring and fall around the deposit, which dried and hardened on the surface. Water began running off through cracks in the surface and along the sides exposed to the rim ditch. The more the surface dried, the heavier it became, pushing down on itself. In 2013 the depth of the centre had fallen to 7.3 metres, and the surface has developed adequate strength to permit cracking and drainage. The next step for this dewatered area, if it weren’t a test plot, would be to add further tailings or to go to reclamation.
Learn more about the project
The issue under study now revolves around how the flocculent is best added. It needs to be done in a controlled manner which results in maximum water drainage. A large field trial began August 2012 and was completed in 2013 at Syncrude, focussing on how the flocculent is added to the FFT slurry for optimal dewatering using the Accelerated Dewatering method. Flocculent and FFT mixing is a key enabling technology area that when optimized can result in maximizing the initial water release. The next phase for Accelerated Dewatering at Syncrude would be to repeat the project on a commercial scale, a phase which will have to be undertaken in conjunction with mine planning over time as a mined-out pit would need to be available for a large commercial-scale pilot.
The environmental benefit of Accelerated Dewatering is it provides a practical way of treating large volumes of fluid fine tailings, giving oil sands developers an additional option to speed water release and reclamation. It is an option that is more cost effective than other mechanical dewatering technologies, and allows for low to zero energy use and therefore negligible related GHG emissions. The low cost and low GHG emissions may appeal to companies, allowing those companies to spend research monies on other environmental improvements that may otherwise have gone to tailings dewatering.
This is a Syncrude-led Joint Industry Project, including Canadian Natural, Imperial Oil Canada, Syncrude and Teck Resources.