Tailings is a mixture of water, sand, clay and silt particles and traces of residual bitumen, which remains after the warm water extraction process separating bitumen from sand.
Some of the fine clay solids called fluid fine tailings (FFT) can take years to settle to a state where reclamation is possible. These particles also trap water, limiting the amount of tailings water available for recycling, and in turn increasing the land needed to store tailings.
A large focus within tailings reduction technology development is management of FFT, which is critical for future reclamation success and reducing the amount of fresh water required to extract bitumen from oil sands.
Getting FFT to a stage where it can be used in reclamation starts with how it is treated prior to being processed through one of the many tailings reduction technologies currently developed.
One common treatment approach is the use of flocculent, a mixing agent that is combined with FFT to solidify the material. Typically, FFT is mixed with flocculent in a static mixer, which does not allow for any modifications to be made to the treatment process to respond to changes in operating conditions.
“A static mixer works well if you know the flow rate and viscosity of the FFT stream,” say Gavin Freeman, Technology Development at Canadian Natural. “But if the quality changes, which it does frequently with FFT, then it impacts the consistency and quality of the treated product. Changes in the quality of treated FFT can affect the pace with which the material can be solidified or dewatered.”
To achieve more consistency in the flocculated material, Albian Sands (formerly Shell and now acquired by Canadian Natural) led a project with Suncor and Teck to explore dynamic flocculation, which allows for more control over the mixing energy and intensity used, based on changes in operational conditions.
FFT material is made up of about 35 per cent solids that retain water, preventing the solids from being able to settle to the bottom of the pond.
“The members of the Tailings EPA have developed a number of technologies that focus on consolidating or dewatering those solids. These include atmospheric fines drying, centrifugation, and thickeners to name a few [link to each case study],” says Gavin. “But in order for any of them to be successful, the FFT must first be treated to promote water and solids separation.”
That’s where flocculation comes in. Flocculants are usually organic compounds, that when added to FFT, cause the solids to clump together, separating them from the water. These larger, heavier particles sink to the bottom, leaving the water at the top. The more effectively treated the FFT is, the better the tailings reduction technologies will work.
In 2014, Shell Albian Sands, Suncor and Teck conducted both laboratory and field tests for flocculation using a dynamic mixer developed by Ledcor Nalco Solutions. At the same time, Albian used static mixers from both its own and Suncor’s operations to establish a baseline to compare against the results of the dynamic mixing tests.
The companies compared the “floc quality” obtained from the two static mixing technologies with that of the dynamic mixer, using yield stress and permeability index targets. “We wanted to make sure the dynamic mixer was tested against both types of mixers currently being used,” says Gavin. “That way we could get an accurate comparison of how this new technology performs against the current approach.
Learn more about dynamic mixers
The current method for adding flocculent to FFT is very simple. A dredge is used to extract FFT from the tailings pond and then it is pumped through a pipe to one of the various tailings reduction technologies that will separate the water from solids. Before it is processed, a flocculent is pumped into the FFT stream through an attachment in the pipe. The turbulence resulting from the FFT being pushed through the pipe helps incorporate the flocculent into the stream.
There is no way to ensure that the flocculent is effectively mixed into the FFT prior to being processed, which can become an issue if the consistency, clay content or volume of material flowing through the pipeline fluctuates.
“In instances where the FFT dredged from the pond contains a large percentage of clay, operators will have to manually adjust the flow rate and the flocculent dosage to ensure the material is effectively treated,” says Kevin Cafferata, Engineering Manager for Ledcor, who worked closely with Albian and the other members on the project.
Ledcor Nalco Solutions provided Albian with a standard dynamic mixer used in a number of other mining industries that allows for more control at the mixing point. It adds flocculent through slits in a rotating paddle to promote better flocculent‐particle interaction while the variable speed drive allows for adjustment of the mixing energy within the pipe. Operators can modify the speed, or rotations per minute (RPM) of the paddles, as well as the flocculent dosage depending on the flow rate and operating conditions.
“While there is an added cost for the application of energy at the mixing point, it’s outweighed by the increase in mixing efficiency, and the resultant consistency of the treated FFT material,” says Kevin.
The results from the laboratory and field test indicate that dynamic mixing creates more consistent flocculation under a range of operational conditions, and therefore a more consistent flocculated material.
“The flocculated FFT had higher yield stress and comparable permeability index when compared to the static mixers,” says Kevin. “The dynamic mixer also produced this higher quality material using a lower dose of flocculent.”
Based on the results of this field pilot, Ledcor Nalco Solutions is currently looking at design improvements to its dynamic mixer to allow it to handle higher FFT and flocculent flow rates. A key component of this work will be through the addition of in-field process instrumentation and control. This will give operators the ability make changes to the process in real time.
“This is an enabling technology,” says Gavin. “Increasing the quality and consistency of treated tailings will increase the performance of our tailings reduction technologies.”
Albian led the dynamic flocculation field pilot at their Muskeg River Mine tailings facilities. Ledcor Nalco Solutions provided and operated the dynamic mixer. Suncor provided an additional static mixer and funding, while Teck contributed both funding and technical expertise.
While there is still development work to be done to achieve full commercialization, COSIA Tailings Director Jonathan Matthews says this is a great example of true collaboration at work. “Each company brings a unique contribution to the project, whether it is in the form of technical expertise, equipment or funding. They each contributed equally to the project’s success.”