| Extraction is the process whereby oil or bitumen is removed from the oil sand. The oil sand is processed through apron feeders or feeder bins then into tumblers or conditioning drums. The development of hydrotransport will soon make these tumblers obsolete. In the drums the oil sand is rotated while hot water, about 80 degrees Celsius, and caustic soda are added to form a slurry. This process is referred to as the hot water treatment, a method similar to that developed by Karl Clark in the early part of the 20th century. Heat is used in the hot water treatment to reduce the viscosity or thickness of the bitumen. Caustic soda helps the attachment of bitumen to the air in the froth formation while releasing it from the sand particles. It essentially helps "clean" the bitumen off the sand. The bitumen then forms small globules that are important in the formation of froth. Agitation also aids in the break up of the oil sand. The slurry passes through a series of vibrating screens that separate and reject any rocks or clumps of clay still in the slurry. It is then pumped into separation tanks. |
| The primary separation vessel is a twenty minute process that allows the oil sand slurry to settle out into its various layers. The most important of these layers is the layer of bitumen froth which rises to the top. The sand (or tailings sand) sinks to the bottom. The primary separation vessel is equipped with a rake at the bottom of the vessel. This rake slowly rakes the sand in a downward motion which aids in the separation process. The tailings sand and excess water is pumped into a pipe which carries it to special holding tanks called tailings pond. The tailings sand is used to build dams and dykes around the tailings ponds. The water is treated and is reused and recycled within the plant site. The middle layer (called middlings) consists of bitumen, clay and water. The middlings remain suspended between the sand and the bitumen froth until it is drawn off and put through the secondary separation vessel. The secondary separation vessel extracts the remaining bitumen from the middlings (usually 2% to 4% more bitumen can be extracted). The bitumen froth once again rises to the top of the vessel. |
| The final two steps in the extraction process are the scroll centrifuge and the disc centrifuge. This is an efficient way to extract large volumes of bitumen. By adding a gasoline-like product called naphtha, the bitumen froth is thinned, or made less dense than the water in the froth. This decreases viscosity and aids in the speed of separation. The disc centrifuge rotates much like a cream separator. The bitumen froth remains in the middle, while the clay, water and sand are thrown to the sides of the centrifuge. The water, sand and clay mixture are pumped out as tailings into the tailings pond. Meanwhile, the bitumen is run through a diluent recovery vessel to remove the naphtha and sent on to upgrading. The recovered naphtha is returned to the extraction process. |
