About 20 percent of Alberta’s oil sands deposits are close enough to the earth’s surface they can be recovered using open-pit mining techniques. This involves large mechanical shovels that scoop the oil sands ore into trucks for transport to crushers. There, the large clumps of bitumen and sand are broken into smaller pieces. Water is added before this mixture is moved to a plant where the bitumen is separated from the sand so it can be upgraded to create synthetic crude oil. The remaining mixture of sand, silt, clay and water is called tailings. This waste stream is deposited in storage ponds. Once in the pond, the coarse particles of sand quickly sink to the bottom and the water from the top three metres is removed and recycled. At the bottom of the pond is a suspended mixture of fine clay and silt particles and water called fluid fine tailings (FFT). It takes decades for the FFT to settle and solidify.
As a result, FFT is the most difficult tailings management issue facing the oil sands mining industry today because it slows land reclamation and poses an environmental liability.
With funding from COSIA, Natural Resources Canada and Alberta Innovates, researchers at Inline Dewatering Ltd. are meeting this challenge head on through research into cross flow filtration (CFF) technology, which is aimed at preventing the production of FFT.
CFF is designed to remove greater volumes of water from the total tailings stream. This increases the solids content of the total tailings stream to more than 70 per cent, which shows promise for preventing the production of FFT.
The CFF pilot involves feeding the total tailings stream through a simple, closed-circuit pumping loop. The full commercial scale dewatering system is envisioned as an open system, likely discharging to a final disposal area. The current pilot loop is made up of porous or slotted pipe, which filters and dewaters the total tailings stream.
The tailings stream flows along the filter pipe, forming a filter cake on the pipe surface. The pressure within the pipe forces the water droplets through the slots (tiny holes) while the fines remain trapped within the pipe. A half pipe runs parallel underneath the pipe and collects the water dripping out and returns it for recycling.
Stainless Steel Porous Pipe
Stainless Steel Slotted pipe
This thick tailings stream can then be delivered directly to a tailings deposit site rather than deposited in a tailings pond.
Inline Dewatering Ltd.’s laboratories will pilot test a version of CFF and assess how this technology can be scaled-up for use in commercial operations.
Inline Dewatering Ltd.’s work will look at the impact of the tailings properties such as particle size distribution, solids content and bitumen content on filtrate flux and cake porosity. Other filter media including sintered steel and porous plastic pipe will also be evaluated. Properties of the dewatered tailings once they are deposited will be measured to assess segregation behaviour and geotechnical stability.
If successful, CFF would eliminate the creation of FFT thus minimizing the need for tailings ponds at future surface oil sands mining operations. Instead, dewatered fine particles would remain trapped with the sand and then would be directly deposited into stable tailings deposit. CFF could also reduce the volume of FFT stored in existing tailings ponds.
Treating the total tailings stream would also allow operators to capture and reuse the heated water before it is deposited in tailings ponds where it cools down. This would reduce the use of energy and greenhouse gases associated with re-heating the water.
All of COSIA’s Tailings EPA Member companies are involved in this project as well as both provincial and federal governments.