Because a great deal of oil sand deposits lie deeper than 75 meters, a specific method is required to recover the resource. The most common way to extract bitumen from oil sands is through a process known as steam assisted gravity drainage (SAGD). The technique involves two horizontal wells that are drilled into the deposit. These wells can extend for miles in all directions. Through one well, steam is continuously injected to heat the bitumen. This reduces the viscosity enough so the oil product can be pumped to the surface through the other well in much the same way as crude oils. SAGD is used to extract approximately 90 percent of the oil sands deposits in the major regions of Canada.
The bitumen produced via SAGD contains a significant amount of water from the steam condensate. Since the quality of the water required for discharge is high and the availability of raw water is limited, the choice is most often made to recycle this water.
Since high amounts of silica are common in water at SAGD operations, there is a desire to preserve as much heat as possible. This conserved heat is used to reduce the silica. A major benefit of the heat recovery is the saved energy in cold climates.
Water treatment begins with the separation of free oil. Several WesTech products are used in this process. The first part is done either in a conventional rectangular oil/water separator or in a circular oil/water separator. Once the free oil has been removed, the effluent flows to a dissolved gas flotation (DGF) unit where the dispersed oil is floated to the top by means of dissolved gas bubbles. Both these units are gas tight to prevent the release of volatile organic compounds (VOCs) and to preserve as much heat value as possible.
Warm Lime Softener
The effluent of the DGF unit is pumped to a warm lime softener. As the name implies, this unit employs traditional lime softening run at an elevated temperature of 140 degrees Fahrenheit. At this elevated temperature the removal of silica is greatly enhanced. This removal rate can be as high as 80-90 percent.
In addition, the lime softening reduces any hardness and acts as a final oil removal step to ensure there is no oil contamination of downstream processing. From the warm lime softening, the water is pumped through dual media filters to remove any suspended solids in the lime softening effluent. This water then goes through weak acid cation units for further polishing prior to the steam generators.
The other common way to recover bitumen from oil sands is through open pit mining of deposits near the surface. This process is similar to strip mining for coal or other minerals.
Mining shovels remove the oil sand and load it into large trucks. The trucks carry the oil sands to mobile crushers. The crushed material is then stockpiled for the next step.
The oil sands broken up in these crushers are then fed to rotary breakers with the addition of hot water to remove rocks and other debris. The resulting slurry is pumped through a pipeline and chemicals are added as required. The slurry is sent to a primary separator where it is classified into three distinct cuts —the overflow, the middle means, and the underflow.
The middle means are sent to flotation units where the floating material is recovered and returned to the head of the primary separator. The underflow from the flotation units is combined with the primary separator underflow and sent to a trash screen. The oversized material from the screen is washed and is returned to the mine via pipeline to fill in mined-out areas. The undersized material is sent to a further bank of flotation units. Floated material off the secondary flotation units is also recovered to the head of the primary separator while the underflow is sent to the tailings thickener.
The overflow from the primary separator is sent for processing via steam heating of the bitumen. Bitumen is deficient in hydrogen and must be upgraded to synthetic crude oil specification in order to be acceptable feedstock for refineries. This is done by the addition of hydrogen or the rejection of carbon, or both. Upgrading uses natural gas as a source of heat and steam for processing and also as a source of hydrogen. Other hydrocarbons, such as naphtha, may also be used for upgrading.
In the tailings thickeners, the suspended solids are settled to a sludge that is sent to a horizontal vacuum belt filter for dewatering. The filtrate from the horizontal belt filter is returned to the head thickener for reprocessing. The dried cake from the horizontal belt filter is sent to tailings piles or landfills for disposal.
The overflow of the tailings thickener is water that is recovered for recycling back into the circuit. This is not solely due to restrictions on water usage. It is therefore critical that treatment processes involving water recovery in reuse are employed in this application. The combination of tailings thickener(s) and vacuum dewatering equipment results in maximum water recovery.