Tailings are the sand, silt, clay and water found naturally in oil sands that remain following the mining and bitumen extraction process. Tailings are transported by pipeline and deposited in ponds where the majority of the solids – mostly sand – settle to the bottom. The remaining fluid – mostly clay, silt and water – flow to the middle of the pond. This mixture is called Fluid Fine Tailings (FFT). As the solids in the FFT begin to settle it becomes thicker and transforms into a mixture called Mature Fine Tailings (MFT). MFT remain in tailings ponds while the solids continue to very slowly settle and the water separates.
The continuous improvement of tailings management is an integral component of successful oil sands mining operations. Reducing the size and need for tailings ponds, and increasing the speed at which they can be reclaimed are ongoing challenges being addressed by the industry.
To this end, Canadian Natural is adding carbon dioxide (CO2) to tailings lines, before the tailings enter the tailings pond, to increase the rate at which the solids settle to the bottom of the pond. This process is often called dewatering.
In 2015, Canadian Natural began production of non-segregating tailings (NST). NST are tailings that have been significantly treated to form a homogeneous, semi-cohesive mass when placed in the dedicated disposal areas. The treatment process includes the use of cyclones and thickeners.
The coarse sand and thickener underflow are then mixed and combined with CO2 to produce NST. The addition of CO2 to NST increases the yield stress of the slurry mixture and increases fines capture.
Canadian Natural’s tailings technologies work to both reduce the tailings footprint, and energy and greenhouse gas output at its Horizon Oil Sands (Horizon) facility located 70 kilometres north of Fort McMurray.
Canadian Natural developed this technology in partnership with Syncrude and has shared it with COSIA’s member companies.
Since 2009, Canadian Natural has added CO2 into its tailings lines at its Horizon facility to enhance the solids settling rate. CO2 is added into the tailings stream before it enters the pond, where it changes the tailings water pH to the same as river water, allowing the solids – fine clays, silts and sand - to settle more quickly. This enables the increased release of water which can then be recycled and reused in the bitumen extraction process.
Canadian Natural has implemented a NST project which includes the use of CO2.
To create NST, tailings are pumped into an extraction plant containing cyclones that are used to remove the water from the coarse sand, along with thickeners that are used to remove the water from the clay fines. FFT are pumped out of the tailings ponds where they are stored and added to the NST. CO2 is then added to the NST mixture where it acts as a coagulant to assist in the sedimentation of the clay particles. As a direct result of the process, the rock, sand and clay fines bind together and settle to the bottom, allowing water to rise to the top. The solids can then be used in the reclamation process.
NST technology became operational in 2015. Canadian Natural currently purchases waste CO2 to add into their tailings stream, however a new CO2 capture plant is expected to be completed in late 2016 that will be capable of supplying the required CO2 for tailings management. The CO2 capture plant at Horizon will have the capacity to recover 438,000 tonnes of CO2 annually.
The combined environmental benefits of the addition of CO2 to tailings and NST will increase water recycling rates and reduce the volume of fresh water required during bitumen extraction by 30 per cent. Canadian Natural has already seen the surface area of Horizon’s tailings pond reduced to approximately half the size it would have been without these technologies as a result of dewatering the MFT and transitioning it to a settling zone. CO2treated FFT took approximately two years to settle to what conventional oil sands tailings ponds reach in a decade.
With the addition of CO2 to NST, Canadian Natural achieves further environmental benefits. The cyclones and thickeners will decrease the amount of time it takes for water to be released from the tailings mixture, increasing the amount of water available to be recycled. They will also allow warm process water to be recycled before it cools; reducing the amount of energy it takes to reheat the water so that it can be used in the extraction process. This will increase the energy efficiency of the extraction process and reduce greenhouse gas emissions.
The addition of CO2 effectively dewaters NST and will accelerate the reclamation timeframe by providing a trafficable landscape that is solid enough to be used for reclamation.
The use of CO2 in the tailings pond and during the treatment of non-segregated tailings was developed in collaboration with Syncrude Canada Ltd.
Results from the project have been shared with the oil sands industry through COSIA’s Tailings Environmental Priority Area (EPA).