OPP-Wet: Slurry Preparation Plants
Water is first introduced into the Bitumen Production facility within the slurry preparation area of OPP, sometimes referred to as OPP-Wet. Although the feed received from the mine has been "crushed" within the OPP-Dry area, an additional step is required to remove oversized material that may damage downstream equipment. Slurrying occurs through vigorous mixing with hot or warm water. Slurry temperature and density, two critical process variables, are controlled within the slurry preparation area, providing an important first step in the downstream recovery of bitumen.
There are 5 basic types of Slurry Preparation Plants (SPP) in the oil sands:
TYPE 1. Tumblers
TYPE 2. Cyclofeeders
TYPE 3. Mix Boxes
TYPE 4. Rotary Breakers
TYPE 5. Wet-Crushing
Although the equipment used in each Slurry Preparation Plant can be quite different, the basic function of each plant is the same. Regardless of design, the purpose of SPP is as follows:
- vigorously mix the dry oil sands with hot or warm water and well aerate the slurry
- break down the lumps of oil sands, allowing the sand and bitumen to separate from each other and encourage bitumen attachment to air bubbles
- remove any large oversized material or debris that can damage downstream equipment
- create a dense, semi-homogenous slurry that can be pumped to the bitumen extraction plant.
TYPE 1. TUMBLER
The original mine operations at Syncrude and Suncor employed tumblers in their SPP facilities to mix the oil sands and water. Tumblers are basically large rotating drums which can lie flat or be slightly inclined. These drums were on average 5 meters in diameter and up to 30 meters long, providing several minutes of residence time.
Tumblers are basically elongated autogenous mills borrowed from the conventional mining industry. The tumblers rotate slowly allowing the lumps of oil sands to be broken down and mixed with the water, much like a front-loaded washing machine. Dry oil sands, hot water, steam and a small amount of caustic soda (NaOH) are added to the front-end of the drum. Agitation within the tumbler provide aeration, allowing the air bubbles to attach to bitumen particles. This step is critical to bitumen recovery in the downstream extraction step. Caustic soda is a water softening agent that helps liberate fine clays from the bitumen, helping to improve bitumen recovery rates.
Tumblers produce a well-mixed, aerated and pumpable oil sands slurry. The slurry exits the opposite end of the rotating drum and is collected in a pumpbox. Vibrating or stationary trash screens are positioned at the exit of the tumblers, removing oversized material and any undigested lumps of oil sands. These rejects are either hauled away to a waste pile or re-crushed and recycled back into the process. The slurry is then pumped to the bitumen extraction plant.
The original Suncor mine (Lease 86) used horizontal tumblers, each with a capacity of 6,000 tonnes per hour. The tumblers were later decommissioned in 2002. Suncor has since moved to using rotary breakers in all their Slurry Preparation Plants, which remains their SPP technology of choice. Four (4) tumblers were also installed at Syncrude's original Mildred Lake Mine in 1978. Unlike the Suncor tumblers, the Mildred Lake tumblers have a very slight incline.
TYPE 2. CYCLOFEEDER
The next generation of slurry preparation technology at Syncrude's North Mine used two cyclofeeders, which were first developed in the mid-1990s. Cyclofeeders use fast, swirling action to break down the lumps of oil sands. The dry oil sands feed is mixed with hot water to form a pumpable slurry. Unlike the tumblers, cyclofeeders stood upright and rotated at a much faster speed. Dry oil sands, hot water, steam and caustic soda were added to the top of the vessel, allowing the digested slurry to exit from the bottom. Multiple screening decks were installed at the exit of the cyclofeeder, removing any oversized or undigested lumps. The screened slurry was collected in a pumpbox and subsequently pumped to the bitumen extraction plant. The oversized reject material was re-crushed and recycled back into the process.
The faster rotational speed provided better slurry mixing and agitation over the original tumbler design. This allowed for improved breakdown of the oil sands lumps. Better mixing equates to improved liberation of bitumen and higher recovery rates. Cyclofeeders were only used at Syncrude's North Mine, each with a capacity of 6,000 tonnes per hour. The massive conical vessels were almost 5 meters in diameter, lined with a variety of wear-resistant plates and stood about 35 meters tall. Cyclofeeders proved to be high-maintenance and have since been phased out.
TYPE 3. MIX BOX
In the late 1990s, Syncrude transitioned from cyclofeeders to mix boxes for its new Aurora Mine. Unlike the swirling action of the North Mine cyclofeeders, the Aurora mix boxes are much simpler in design, basically a steel box equipped with baffle plates. Dry oil sands and warm water are introduced to the top of the mix box, along with a small amount of caustic soda. The oil sands and water fall down the box, hitting each baffle plate on the way down, breaking down the lumps of oil sands. Impact from the abrasive oil sands causes much wear and tear on the steel box and baffle plates. The box and baffles are therefore covered with wear-plates and impact-resistant liners to improve reliability.
The oil sands and water mixture falls into a pumpbox located directly below the mix box. A vibrating screen is positioned below the mix box at the inlet of the pumpbox. These screens remove any oversized material and redirect these "rejects" to a secondary impact crusher. The rejects are then re-crushed and recycled back into the process.
Recycle pumps are used to recirculate the screened slurry back into the pumpbox. This was found to improve oil sands digestion and increase the residence time of the slurry. The oil sands slurry is then pumped to the main extraction plant using large centrifugal slurry pumps.
Although mix boxes are simple in design and cheap to construct, the vibrating screens proved to be horribly unreliable. Impact from the falling chunks of oil sands and the constant shaking of the screen decks puts a lot of wear and tear on the equipment.
The mix boxes also have a very short residence time, in the order of a few seconds, providing insufficient agitation in winter months when the oil sands can be bound in large chunks of ice. The result is a very high reject rate in winter months, up to 5% of the total feed. This represents a large volume of material that needs to be reprocessed and recirculated back into the process.
However, mix boxes are relatively low capital to install and do not require a large footprint. After many years of operation, Syncrude managed to work out many of the kinks and greatly improved reliability of the individual components. Syncrude's mix box technology was later adopted at the CNRL Horizon Mine and the original Kearl Lake SPP facility. Mix boxes are also still in operation at Syncrude's Aurora North mines.
TYPE 4. ROTARY BREAKER
Just as Syncrude moved from cyclofeeders to mix boxes in the late 1990s, Suncor moved from tumblers to rotary breakers at their new Steepbank Mine. Much like tumblers, rotary breakers are inclined rotating drums. The breaker is supported by 4 trunnion rollers, which allow the drum to rotate at a relatively slow speed, typically 10 RPM. The main improvement over tumbler technology are the holes drilled into the breaker shell. The holes are typically 2 to 3" in diameter, allowing the digested oil sands slurry to pass through and be collected in a pumpbox below. Recycle pumps are sometimes used to recirculate the slurry back into the pumpbox providing additional ablation and residence time.
Much like a traditional autogenous mill used in the mining industry, lifters move the feed to the top of the drum, allowing the oil sands lumps to drop to the bottom of the drum. Any large lumps are broken down from the impact of the fall. Rotary breakers are typically 4 to 5 meters in diameter and up to 20 meters long. Warm water sprays are installed inside the length of the rotating drums. The drums are angled, allowing any undigested reject material (anything larger than 2-3" holes) to exit the vessel.
A few years after Suncor's first installation, rotary breakers were also installed at Suncor's Millennium Mine (in 2001) and Shell's Muskeg River Mine (in 2002). Rotary breaker technology was patented jointly by Suncor and Shell and remains the SPP technology of choice for both companies, including the new Fort Hills Mine currently under construction. Although rotary breakers are more capital intensive than the competing mix box technology, the rotating drums have a very predictable maintenance cycle and are not as susceptible to sudden unexpected breakdowns. Since the rotary breakers have built-in holes which screen the slurry, there is no need for external vibrating screens, which have long been the achilles heel of the mix box technology.
TYPE 5. WET CRUSHER
Wet crushing is a Syncrude patented technology which promises to reduce rejects rates and improve bitumen recovery over the traditional mix box design. During wet crushing, water is added while the oil sands is processed through a series of sizers, sequentially breaking-down oversized material and improving digestion of the ore. Wet crushing plants are integrated into the surge bins. The oil sands is fed directly from the apron feeders underneath the bin, eliminating the need for conveyors.
Wet crushing technology is considered a major improvement over the traditional mix box design. The main advantage is that the process does not require any vibrating screens, which have proven to be very high-maintenance and can produce a large volume of rejects in winter (up to 5% of the ore feed). During wet crushing, the gap between each set of sizer rollers are progressively smaller, allowing for better break-down of any chunks of oil sands. The process therefore produces no rejects and does not require a rejects reprocessing plant.
Wet crushing is a very recent innovation in SPP technology found at Syncrude's recently commissioned Mildred Lake Mine Replacement (MLMR) facility and Imperial Oil’s Kearl Lake Expansion plant.
Regardless of the technology used, slurry preparation plants normally target a specific operating density, carefully measuring the amount of water added to the oil sands. A higher density is more desirable since it helps reduce water consumption and improves break down of the oil sands. However, if the density is too high, the centrifugal pumps may not be able to handle the slurry. Failure of the pumping system is considered a very serious event since it can lead to a prolonged plant outage.
SPPs normally target an operating density of 1500 to 1580 kg/m³. The mass of oil sands sent to each SPP plant is normally measured by weightometers installed on the conveyor belts. Water addition is measured by flowmeters installed on the water supply lines. A density of 1600 kg/m³ is the most that can be handled by a centrifugal pump. Slurry density is measured by densitometers (or density meters) installed on the slurry pipelines.
Operating temperature is also very important operating variable. Process water used to slurry the oil sands usually varies from 70 to 90°C. In general, hotter is better for the process, particularly in winter months where the bitumen can be bound with fines and clays within the chunks of ice.
REJECTS HANDLING CIRCUITS
Rejects are defined as any oversized material which does not pass through the screens or rotary breaker openings. Removal of this oversized material is important since these large, hard objects can damage downstream equipment. During summer months, rejects are comprised mostly of waste rock, petrified wood and other debris from the mine site. However, during winter months, chunks of ice bound with oil sands can make up a majority of the rejects stream. Discarding the rejects in this case can lead to significant bitumen losses, up to 5% of the total feed. This represents a serious waste of resources.
There are two options for ovesized reject material:
- Re-crush the rejects and recycle them back into the process, or
- Discard the rejects completely and haul them away to a waste pile.
Rejects reprocessing plants normally consist of impact crushers, such as hammer mills, which crush the oversized material into much smaller pieces. The crushed solids are then mixed with water and pumped back into the process. If all the rejects are reprocessed, then there are no bitumen losses in the SPP.
The biggest disadvantage of rejects reprocessing is the re-introduction of fines or clays, which are often bound with chunks of oil sands. Clay lenses can be harder than the rest of the deposit, often ending up in the rejects pile. Since clays are very harmful to bitumen recovery, some operators prefer not to reprocess the rejects and instead just haul the material to a waste pile.
It is important to note that mix boxes produce a lot more rejects than rotary breakers. Rotary breakers have a higher residence time, allowing the chunks of ice and oil sands to break down and pass through the openings. In contrast, mix boxes provide much less mixing and have very short residence times. Lumps of ice and oil sands do not break down easily and get rejected by the vibrating screens. As a result, all SPPs using mix box technology reprocess their rejects, otherwise bitumen losses in winter would be too high. However, if installing rotary breakers, the reprocessing of rejects is optional, as losses in winter are considerably lower.
Wet-Crushing plants are the only SPP technology which do not produce a separate rejects stream.
CAUSTIC SODA: A KEY PROCESS AID
Almost all Slurry Preparation Plants use caustic soda or sodium hydroxide (NaOH) as a water softening agent.
An important step in bitumen extraction is the separation of bitumen from the fine clays. An oil sands deposit with a high fines content is highly undesirable. Since fines tend to stick to the bitumen, they contaminate the bitumen, lowering product quality. Fines also prevent the bitumen from attaching to air bubbles and floating to the top of the gravity separation vessel. This reduces overall bitumen recovery.
One way to prevent fines from sticking to the bitumen is to raise the pH of the slurry (or lower the slurry acidity). At higher pH values, the fines become negatively charged and tend to repel each others, avoiding agglomeration and improving the efficiency of the separation process. A deposit with more fines will benefit from operating at a higher pH. A deposit with very little fines may require no pH modifications at all.
The general rule is to add enough caustic soda to raise the slurry pH to about 9. Note that too much caustic soda is harmful for tailings ponds since it slows down the settling of fines. Since the normal pH of the slurry is about 8, a very small amount of caustic soda is required to reach optimal slurry pH. Overdosing offers no process benefit and should be avoided if possible.
THE NEXT STEP: HYDROTRANSPORT
Once an oil sands slurry is produced, it is pumped to the downstream bitumen extraction plant, where aerated bitumen floats to the top of a gravity separation vessel and is ultimately recovered. The connecting pipeline between the SPP and Extraction plant can be several kilometres long, allowing for further digestion and aeration of the oil sands, much more than can be provided by the SPP which has a relatively short residence time. These long pipelines, known as Hydrotransport Lines, have been shown to improve bitumen liberation and improve recovery rates. Hydrotransport is a technology developed and patented by Syncrude.