In order to extract bitumen from mined oil sands, it is mixed with warm water. Once the bitumen has been removed, a mixture of water, sand, silt, clay and residual bitumen is left over. This mixture is referred to as tailings. The tailings mixture is stored in tailings ponds where the majority of the solids settle to the bottom and the water is recycled back through the bitumen extraction process.
The portion of tailings that does not quickly settle forms a layer in the middle of the pond. This mixture, called fluid fine tailings (FFT) can take decades to form a solid, if left on its own.
The members of COSIA’s Tailings Environmental Priority Area (EPA) have developed a suite of technologies to accelerate the pace at which water is removed from FFT, in order to reduce the foot print of tailings ponds and accelerate the pace of reclamation. Tailings thickeners are being used by a number of operators to reduce FFT and speed reclamation.
This method involves adding a polymer flocculent to the FFT mixture, causing the small particles to aggregate together, which causes them to settle faster. The mixture is then fed into a thickener, which uses gravity to extract the water from the solids so it can be recycled. The thickened tailings product is then transported via pipeline to a dedicated disposal area (DDA) several kilometres away, where it will continue to densify until it is solid enough to be used as reclamation material.
Thickened tailings have a consistency of thick mud. Due to the viscous nature of the material, relatively high velocities are required to achieve turbulent flow. Operating slurry pipelines at high velocities requires significant energy and the pipelines are subject to accelerated wear. Consequently there is significant interest in operating slurry pipelines in laminar flow due to potential energy and wear related cost savings.
Canadian Natural Resources Limited along with Tailings EPA members Imperial, Shell Canada, Suncor Energy, Syncrude Canada and Teck with industry partner Paterson and Cooke has engaged the Saskatchewan Research Council (SRC) to research the behaviour of slurry pipelines operating in laminar flow. The research will be used to improve understanding of laminar flow and work toward the development of a reliable model for laminar operation of slurry pipelines that could be used to design pipelines that can effectively transport thickened tailings. This tool would improve pipeline design and operation, making them more energy efficient, more reliable and reduce costs.
“Currently there is no proven reliable model available to determine the conditions required to successfully operate large diameter slurry pipelines in laminar flow,” says Aref Najafi, Canadian Natural’s Continuous Improvement Engineer. “This could cause sand to settle in the pipeline, leading to blockages.”
In order to develop a more reliable pipeline design method, COSIA members need to have a thorough understanding of the mechanisms which allow for coarse particle transport in laminar flows. The members of the Tailings EPA have commissioned SRC, a COSIA Associate Member, to conduct a study to develop that model.
The researchers at SRC’s Pipe Flow Technology CentreTM (PFTC) are world-leading experts on pipeline slurry behaviour. The PFTC facility is fully equipped to enable full-scale physical modelling of the complex pipe flow behaviour of slurries and crude oil mixtures. The PFTC’s historical work in collaboration with other experts led to the development of the SRC Pipe Flow Model. The model is employed by engineering firms in the design of new pipeline systems in the oil sands and is regarded as an industry standard when it comes to transporting slurries. In fact, Canadian Natural used the model to assist with the design of their Tailings Thickening facilities, which came on line in Q3 of 2015.
"The SRC Pipe Flow Model which was developed over decades of research and recently improved through a multi-client project, is focused on the transport of slurries in the turbulent flow regime," says Lesley McGilp, Manager of SRC's PFTC. "This new study will contribute to the development of a model that oil sands operators can apply to systems involving laminar flows, including the transport of thickened tailings."
“SRC’s PFTC has a 55 year history studying slurry transport in pipes," says Lesley. "The group has been conducting oil sands specific studies for nearly 40 years."
The facility is equipped with a variety of sizes of pipe loops, pumps, tanks, and the associated instrumentation required to test slurry mixtures.
The completion of a wide range of projects for the oil sands and mineral processing industries with a variety of slurries has provided the facility and staff with knowledge and experience in slurry pipeline measurement techniques, data analysis and model development.
Learn more on the Saskatchewan Research Council’s website.
In the upcoming study, an in-depth matrix of pipe loop tests will be conducted with concentrated model slurries over a broad range of conditions comparable to oil sands thickened tailings operations.
“A 100 millimetre diameter pipe loop will be modified to operate in “once-through” mode (as opposed to recirculating mode) to more closely mimic the once through transport of these slurries in actual thickened tailings pipelines,” says Aref “This is important because in order to ensure we have a reliable model, we need to make sure that the behaviour of the slurry is not affected by being recirculated multiple times.”
SRC will also be using an imaging technique called tomography to gain further insight into the fluid mechanics of the flows inside the pipeline. The results of the study will provide a more in-depth understanding of the physical mechanisms governing tailings particle migration in the pipeline and provide the knowledge necessary for an improved predictive model.
“This project is an ambitious undertaking,” says Aref. “Historical work on laminar flow is still limited and the end result may be that further work is required to complete the model development.”
But he thinks it’s worth it. No model of this kind currently exists, but once developed; it will be used alongside the existing SRC Pipe Flow Model, to increase the efficiency of tailings management operations by increasing the energy efficiency of thickened tailings transport. Ultimately, these improvements will contribute to the long-term aim of reducing the size of tailings ponds. The model will also be applicable to other industries transporting thick slurries via pipeline.
“The costs of this project have been divided between seven participants so costs and risks are mitigated while benefits to each participant are realized,” says John Brogly, COSIA Tailings EPA Director. “Sharing project costs and pooling resources and expertise provides a platform for innovative approaches and ideas to grow with minimum cost, maximum speed and efficiency.”
Canadian Natural, along with COSIA Joint Industry Project participants Imperial, Shell, Suncor, Syncrude and Teck are working with the SRC to provide practical insight on operating and transporting thickened tailings slurries. Paterson and Cooke, global specialists on slurry handling and transportation will also be participating in the project as a contributor and an advisor. The research will be conducted by SRC at their PFTC.