Tailings Directive 074 Explained
In 2008, the former ERCB (Alberta Energy Resources Conservation Board, now known as the Alberta Energy Regulator) introduced Directive 074: Tailings Performance Criteria & Requirements for Oil Sands Mining Schemes. Under this new directive, oil sands mining operators are required by law to commit to the reduction of fine fluid tailings and reduce any current inventories. Although the directive itself is quite simple in its objectives, meeting its intent has been challenging, even for the most experienced oil sands mining operator.
Here are some key facts about Directive 074 everyone should know:
WHY WAS DIRECTIVE 074 ENACTED AND HOW IS IT DIFFERENT THAN THE PREVIOUS OIL SANDS TAILINGS REGULATIONS?
Directive 074 was enacted in response to the growing volume of fine tailings and process-affected water accumulated in the oil sands. Historically, oil sands mine operators would store fine fluid tailings (FFT) in the tailings pond and reprocess them after the pond was decommissioned. This led to large volumes of FFT accumulated over time, which would eventually degrade and became more difficult to reprocess. Directive 074 prohibits the long term storage of FFT and requires the operators to reclaim the fines as they progress the mine plan. Oil sands operators are now required by law to capture the FFT within 1 year of deposition and fully reclaim the area in 5 years. Since this is beyond current technology, the mine operators are required to actively undertake research into various different reclamation techniques, share this information with other operators and submit annual progress reports to the Alberta Energy Regulator (AER).
WHY FINE FLUID TAILINGS ARE BAD - REALLY BAD
The oil sands deposit naturally contains a significant amount of fines, ranging from 10-30% depending on the deposit geology. Fines are defined as solid particles with diameters less than 44 microns and are comprised mostly of clay and silt material.
Up to 50% of the fines are captured within the voids of the coarse tailings stream, which is mostly silica sand. This mixture of coarse silica sand and fines settles easily and has good shear strength, making this material ideal for the construction of beaches and dykes.
The remaining 50% of the fines remain suspended in the tailings pond water and have a tendency to form a sludge-like substance termed fine fluid tailings (FFT). If left unprocessed for several years, this layer of fines eventually degrades into mature fine tailings (MFT). Left untreated, it is estimated that MFT can take up to 150 years to fully dewater and settle out. Directive 074 seeks to address the treatment of this fine material and prevent the formation of MFT.
OBJECTIVES OF DIRECTIVE 074
Directive 074 has 3 specific targets:
- At least 50% of the fines in the oil sands deposit are to be captured in dedicated disposal areas (DDAs), which must be formed in a matter that ensures a trafficable deposit.
- Within one year of being deposited, the DDA must have an undrained shear strength of 5 kPa. Any material that does not meet this criteria must be removed and does not count towards the 50% fines capture.
- The DDA must be ready for reclamation within 5 years after active deposition has stopped. The deposit must then meet the strength, stability and structure necessary to establish a trafficable surface, with a minimum shear strength of 10 kPa.
SHEAR STRENGTH EXPLAINED AND WHY IT MATTERS
Shear strength is the amount of force a soil can sustain. Sludge material has a low shear strength and cannot sustain any weight or pressure; this soft material is therefore non-trafficable. A trafficable landscape requires a shear strength high enough so the ground can sustain people and equipment without sinking. The shear strength of fine tailings is mostly a function of water content. A shear strength of 5 kPa equates to approximately 65% solids and 35% water. Removing water from the fines is a pivotal first step to reclaiming the landscape.
DIRECTIVE 074: WHAT IS HOPED TO BE ACHIEVED
- It will minimize and eventually eliminate the long term storage of fine fluid tailings: Under Directive 074, the operator is required to separate the fine tailings stream and dewater immediately, eventually eliminating the need for long-term storage.
- It will create a trafficable landscape and facilitate quicker reclamation of the mine site: Mine operators now have only 5 years to fully reclaim the landscape, measured from the day tailings deposition ends.
- It will reduce the volume of stored process-affected water on site: Since fines tend to trap a large volume of water, this trapped water cannot be recycled back to the process and accumulates in the pond. As the volume of fines grows, the volume of water that needs to be stored also grows. Eliminating the storage of fines therefore also eliminates the need for large volumes of water storage.
- It will maximize water recycling, reduce fresh water input and improve energy efficiency: The water that remains trapped in the fines must be made up by fresh water from the Athabasca River. By reprocessing the fines immediately (versus storing the fines for many years), the liberated water is now free to be recycled back to the plant and reduces the amount of fresh make-up water required.
- It will reduce the footprint of future and existing tailings ponds and minimize resource sterilization: Mine plans normally include large tailings pond that must be built in order to contain the large volume of fines and trapped water. The oil sands located underneath the pond cannot be processed and is therefore considered "sterilized" or non-recoverable. Reducing the size of the tailings pond in turn reduces the amount of oil sands on the lease that must be sterilized. This increases recovery of the bitumen resource.
MEETING THE TARGETS: COMMON RECLAMATION TECHNIQUES
There are several different methods oil sands mining operators can use to achieve the objectives set out in Directive 074. Some of the more common techniques include:
- Atmospheric Drying: Atmospheric drying first requires the operator to mix the fine tailings stream with a flocculant or polymer. This mixture is then thinly spread over a large surface area within the DDA (dedicated disposal area). Portions of the water trapped within the fines are slowly released and recovered. The remainder of the water evaporates over time as the layer of fines slowly air dries. What remains after the water has evaporated is a dry clay material that can be used as construction material or reclaimed in place. Atmospheric drying can also be used for mature fine tailings or thickened tailings.
- Produce Consolidated Tailings (CT): Tailings from the Extraction plant are pipelined to the CT plant, where cyclones are used to produce a densified coarse tailings stream. This coarse tailings from the cyclone underflow is mixed with a controlled amount of MFT and gypsum creating a non-segregating slurry, known as consolidated tailings (CT). This dense CT stream can be deposited sub-aerially (into the air) or sub-aqueously (below the water surface) without segregating (in other words, the fines remain trapped within the coarse solids and do not separate out). The CT process is one of the oldest methods of fines capture in the oil sands but has a limited success rate. The production of good quality non-segregating tailings requires a very specific sand-to-fines ratio which can be difficult to achieve on a large scale.
- Produce NST (Non-Segregating Tailings): The NST process is identical to the CT process except a thickener is used to produce a fines feedstock. Thickened tailings and coarse sand are mixed to obtain the correct sand-to-fines ratio. MFT can also be added if desired. A coagulant is added to prevent the mixture from separating. The NST stream produced is very dense (i.e., has a low water content) and can be deposited sub-aerially to prevent segregation of the stream. This process requires a well-functioning thickener that produces a high quality, dense underflow with a low water content. As of yet, no operator has successfully produced NST on a large scale. A major part of the stumbling block is getting good quality thickener underflow. Thickeners require very steady operation which can be difficult in a process that have a very variable feed stream.
- Fines Centrifuging: Centrifuging of fines has become increasingly popular among oil sands mining operators as a mechanical method of dewatering the fines. Centrifuges spin at very high speeds which liberate the trapped water from the fines. The centrifuge process produces a paste with a low water content which can be deposited into a DDA. The compacted material can be capped with coke or overburden prior to re-vegetation. Centrifuges are very effective but also very energy and maintenance intensive, making it a very expensive process on a large scale. Syncrude is the first operator to build a large-scale centrifuge plant specifically for tailings reclamation.
- Fines Stacking: Fines stacking involves depositing alternating layering of coarse sand and fine tailings. These fines would have typically been consolidated by a thickener or centrifuge, and therefore already have a relatively low water content (ideally below 50%). The coarse sand acts as a drainage layer, allowing water to be released from the fines layer. Fines stacking has yet to be successfully executed on a commercial scale.
MIXED RESULTS SO FAR
So far, the different techniques employed have been met with limited success, but every operator has been making significant progress. Although the AER recognizes that these targets are very aggressive, the regulator has actively challenged the operators' assumptions on fines content, fines capture and viability of their processes. Since this directive is relatively new (the first full year of required compliance was 2012), there is admittedly still some confusion on the details of how to meet each of the goals. In March of 2015, the Alberta Government abandoned its plans to make Directive 074 legally binding, since it was clear its targets would not be met by any of the operators. However, it remains a valuable framework for tailings management and all operators have continued their efforts to achieve the original objectives spelled out in Directive 074.
CHALLENGES: WHY TAILINGS RECLAMATION ISN'T EASY
- Measurement of total fines volumes: Although it sounds relatively simple, measuring the total amount of fines in the ground isn't easy. The fines content in the mining core samples are reconciled against the amount of fines in each tailings and product stream, and sometimes the numbers don't match. Also, some streams, such as MFT are not 100% below 44 microns and can contain some small amount of sand. This sand fraction needs to be accurately measured and excluded from the 50% fines capture calculation. The AER also requires the operators to report the fines captured in beach sand, trapped in construction cells and any asphaltenes produced from the Froth Treatment plant. Given that some mines process up to 1 million tonnes of material a day and hold millions of tonnes of tailings in storage at any given time, getting an accurate and representative fines count can be a challenging task.
- Measurement techniques used: Particle size distribution can be measured by a variety of methods, such as wet sieve analysis, laser diffraction or the Coulter method. However, different methodologies produce different results. The variances can be especially wide for the ultra fine solids, or material less than 5 microns (mostly clays). Some companies always report more fines in their deposit simply because of the measurement technique used. Although this has never been an issue, it becomes very complicated now that the AER requires a historical profile of the amount of fines processed through the plant. Higher fines in the deposit translates to more fines needing to be captured, which increases the cost of reclamation for the operators. Therefore, an accurate count of fines has become critical and can be a point of disagreement between the operator and the AER.
- Legacy tailings volumes: Since Syncrude and Suncor base mines have been in operation for more than 30 years, these companies have accumulated significant volumes of fluid fines and mature fines. As part of their tailings management plan (prior to Directive 074), each company normally has a number of different ponds where fine tailings are stored, moved around and reprocessed. Since Directive 074 applies to all fine tailings, accurate reporting of all fines contained in all ponds, and agreeing on how fast these fines need to be reclaimed (versus active tailings streams) adds another layer of complexity for the oil sands mine operators.
- Variability of the oil sands feed: Many of the tailings reclamation processes require very specific fractions of sand to fines in order to function properly. The production of consolidated tailings (CT), non-segregating tailings (NST) and proper operation of fines thickeners can be especially finicky. If the fines content is too low, thickener underflow, CT and NST streams will be off-spec and not have the shear stress properties required for eventual reclamation. If there are too many fines, this can lead to CT and NST streams being too soupy and not having enough stability to be successfully deposited. Since oil sands deposits can have a very wide range of particle sizes, this can lead to a large amount of material being off-spec. The AER does not count off-spec material towards the 50% fines capture target and requires this material to be reprocessed until adequate shear stresses are achieved.
- Space constraints for DDAs: Atmospheric drying of fines is one of the more promising technologies for reclamation of fines. In this process, a large volume of fines is thinly spread out over a very large dedicated deposition area (DDA). Thinner layers of fines dry out faster but require more surface area. One problem now commonly being encountered is the space required for these DDAs. Operators now need to find very large clear-cut areas, free of process equipment, close enough to the fine tailings stream but not in an active mining area. Operators must also get approval for each DDA from the AER prior to their use.
- Weather: Atmospheric drying can only happen during the summer months (roughly from April to September). A long hot summer will lead to more evaporation, which is good. A short wet summer will drastically reduce drying rates, which will delay reclamation rates. Weather has now become an important variable in fines dewatering rates.
- Froth Treatment tailings: Up to 10% of the fines in the feed reports to the Froth Treatment tailings stream. Depending on the type of process used, this stream can also have trace amount of solvent or diluent, making the reprocessing of these fines more complicated. The use of thickeners to dewater Froth Treatment tailings has been proposed but has never been done on a commercial scale. The increasing popularity of Paraffinic Froth Treatment also results in a high volume of asphaltenes reporting to the Froth Treatment tailings stream, which can bind with the fines. Consensus on how to capture these fines remains to be resolved within the industry.
THE CONSEQUENCES: WHERE DO WE GO FROM HERE
Operators are required to submit detailed proposals to the AER on how Directive 074 objectives will be met and identify any roadblocks that prevent them from meeting their targets. If targets are not achieved, the operators need to explain exactly why their targets were not met and what will be done in the future to reach the desired goals. Any changes to the originally approved tailings management plan must also be reported and may require an amendment for further approval by the AER.
The AER reserves the right to charge each operator with non-compliance if they deem the operator has not done everything necessary to meet their targets. No operator has yet been charged, keeping in mind Directive 074 is relatively new and requires a reclamation timeframe that has never been achieved before.
All tailings management plans including feedback from the AER is made available to the public on the AER website (www.aer.ca).