Oil Sands Tailings: What Needs to be Done
The accumulation of mature fine tailings (MFT) in tailings ponds has been a concern of the oil sands industry for nearly four decades. However, even when a few measures have been implemented, the industry is still using processes that operate at elevated pH, which is the source of tailings with poor settling and consolidation properties.
New extraction processes and thermal efficiencies are needed to produce tailings with acceptable settling and consolidation properties without harming the release water chemistry and fuel quality of bitumen. These methods must be simple, easy to implement, cost effective and have a minimum impact on human health and the environment. But, how can that stage be reached?
A good starting point is to understand the composition of the oil sands deposits. The mineable surface of the McMurray Formation oil sands deposit in Alberta, for instance, consists of a quartzose, uniform, fine to medium grain sized sand. The deposit, however, contains seams and lenses of a weak clayshale varying in thickness from a few millimetres to over a metre. During mining and extraction, the smaller and weaker pieces of clay-shale are broken up and dispersed in the oil sands ore.
Depending on the grade of the ore, the amount of dispersed silt and clay particles varies considerably but averages about 17% by mass of the solids in the tailings stream.
The mineable surface of the McMurray Formation, consists of a quartzose, uniform and fine to medium grain sized sand.
There are four commercial plants producing bitumen from surface minable oil sands, using some version of the Clark Hot Water Extraction (CHWE) process, which was developed in the 1930s by Dr. K.A. Clark and his coworkers at the Alberta Research Council. This process requires seven to nine barrels of water per barrel of bitumen produced. Most of this water, however, is recycled from the tailings pond and only about one third of the required water is fresh water from the Athabasca River. The tailings stream is, therefore, composed of a large amount of water and some bitumen that escapes the extraction process, while the solids are mainly sand. When deposited in a tailings pond, the sand settles out rapidly to form dykes and beaches while, depending on the makeup of the tailings stream and the method of deposition, only one-third to one-half of the fine material is captured or retained in the tailings sand deposits. The majority of the fines separate or segregate during tailings sand deposition and flow into the pond in suspension to form fluid fine tailings. The fine tailings settle in about two years to form mature fine tailings, a fluid tailings about 33% solids content which is almost all fines.
The CHWE process uses caustic NaOH or sodium salts of weak acids as extraction process aids for ore conditioning by maintaining the pH of the ore-water slurry at around 9.5- 10.0. At elevated pH, asphaltic acids that are partly aromatic, containing oxygen functional groups such as phenolic, carboxylic and sulphonic types, become water soluble and act as surfactants reducing the surface and interfacial tensions and promoting the efficiency of bitumen extraction. Chemical additives used in the CHWE process provide optimum conditions to maintain high bitumen extraction efficiency; however, they produce MFT with poor settling and consolidation properties. Further densification of MFT to reach about 5 kPa shear strength acceptable for trafficable land reclamation could take centuries. Also, chemical additives used in the CHWE process cause an increase in the salinity, especially Na+ concentration, of the recycled release water. In fact, release water chemistry plays the key roles in both extraction efficiency and tailings characteristics; therefore, monitoring of the release water chemistry deserves special attention for the overall performance of oil sands plants. The combination of water chemistry, clay mineralogy and organic bitumen result in a fluid fine tailings which have resisted years of reclamation research.
As the capacity of existing plants are increasing and new plants are being constructed, the existing MFT inventory of 800 million m3 will grow at a much faster rate and reach one billion m3 in a few years. MFT is about 85% water by volume and requires large storage ponds for disposal. The existing trend in the accumulation of the MFT inventory is considered a serious environmental liability. The Alberta Energy Resources Conservation Board (ERCB) has issued Directive 074: Tailings Performance Criteria and Requirements for Oil Sands Schemes. This ruling requires operators to: (i) prepare tailings plans and report on tailings ponds annually, (ii) reduce the production of fluid tailings by capturing fines and placing them in a deposit that is trafficable and, (iii) specify dates for construction, use and closure of fluid tailings ponds deposits. The Directive requires that fluid tailings production be reduced to 50% of the existing production rate by 2013. Existing MFT volumes are not specifically addressed in the Directive and even with the compliance of the Directive accumulation of MFT volume will continue to increase. The ERCB is planning to issue further directives to deal with the existing MFT inventory.
Two notable efforts have been made to reduce MFT production and accumulation. The first one was the production of composite tailings (CT). To produce CT, coarse tailings are pipelined from the extraction plant to the CT plant, where they are cycloned to produce an underflow densified coarse tailings stream. The densified coarse tailings stream is combined with MFT and gypsum (CaSO4) to form CT. The gypsum or another coagulant is required to prevent the fines from segregating from the sand when the CT is deposited. The CT product is then transported hydraulically to a designated tailings disposal facility. Once deposited in the pond, the mixture dewaters relatively rapidly, leaving a soft deposit that is capped using coarse tailings sand to create a trafficable surface for reclamation. Although CT production technology can accelerate tailings densification and reclamation, the continuous accumulation of Ca2+ and SO4 2- ions in the recycle water detrimentally affects bitumen extraction efficiency. CT production has been commercially used for about two decades by Suncor Energy Inc. and Syncrude Canada Ltd. The overall tailings balance, however, shows that CT production does not entirely reduce the MFT inventory, since additional MFT is produced from the cyclone overflow effluent. Also, the CT process does not improve the thermal energy efficiency of the oil sands plants because of the discharge of warm cyclone overflow water into the tailings pond. Furthermore, CT production may cause H2S emissions from the tailings ponds by anaerobic reduction of SO42- by the residual bitumen in the tailings.
The second significant effort towards reducing MFT production was the development of a non-caustic, low temperature, or low energy, extraction (LTE) process to replace the CHWE process. Since no additive is used in the LTE process, dispersion of clay size particles in the ore-water slurry is reduced. As a result, settling and consolidation properties of the tailings are improved. The non-caustic LTE process can incorporate thickeners to thicken cyclone overflow (about 10% solids, >50% fines) to thickened tailings (>40% solids, >50% fines) and the cyclone overflow water is recycled back to the extraction process as warm as possible to save thermal energy. This non-additive LTE process improves the thermal efficiency of the plant and, as well, reduces the environmental impact of oil sands plants by producing tailings with improved settling characteristics. A non-additive LTE process was implemented at two commercial plants, but it attained lower than expected bitumen extraction efficiency. To boost the extraction efficiency the non-additive LTE processes have been modified to some version of the CHWE process by increasing both temperature and pH.
However, the oil sands industry is still using the CHWE process, which, as stated above, operates at elevated pH, causes an increase in the salinity of the recycled release water and produces tailings with poor settling and consolidation properties. Novel bitumen extraction and tailings disposal processes are needed to attain high bitumen extraction efficiency, especially for the processing of low grade ores with low bitumen and high fines contents. They must also improve the thermal efficiency and produce tailings with acceptable settling and consolidation properties without harming the release water chemistry and fuel quality of bitumen.
Oil sands operators have submitted their plans to comply with Directive 074, but each oil sands operator has his or her own plan. Production of CT with CaSO4 or CO2 additives to prevent segregation remains one of the major technologies to reduce MFT accumulation.
Shell Energy Canada is planning the implementation of nonsegregating tailings (NST) on a commercial scale, the timing of which recognizes the need for testing, compilation, data analysis and detailed design for a commercial facility.
Syncrude Canada Ltd. is proposing the implementation of MFT centrifuging technology to complement CT as a fines management technology to reduce the MFT inventory. Centrifuging MFT accelerates the release of water from the MFT and produces a soft, clay-rich soil termed “centrifuge cake.” Syncrude Canada proposes to sand cap the resulting centrifuge cake deposit for reclamation as a dry landscape feature. The implementation of MFT centrifuging technology is proposed to be executed in three stages, commencing in 2012 and in increased capacity in 2018.
Canadian Natural Resources Ltd. is in the start-up phase of operations and has proposed to develop and implement fines capture technology with a target of 2015. Two technology options are being investigated; the use of thickeners to make NST and dewatering MFT by treating it with an organic polymer and CO2.
Suncor Energy Inc. is proposing Tailings Reduction Operations (TRO) to reduce MFT accumulation as an alternative to CT production. The TRO process is based on the addition of a polymer flocculent to MFT and the spreading of it in a thin layer to dry. The product of the Tailings Reduction Operation is a clay material that may be reclaimed in-place or re-handled to mine dumps. The technologies proposed by the oil sands players have appeared as novel and ambitious processes for the management of their MFT inventory. The cost effectiveness and practicality of these proposed technologies appear to be their foremost constraints.
All companies envision the use of end pit lakes at mine closure. Any fluid tailings remaining at this time will be returned to the mined pits and capped with water. It is planned that these lakes will, with time, become viable ecosystems that will sustain plant and aquatic life. Environmental concerns exist with reference to this concept and end pit lakes have not yet been approved by the ERCB. Syncrude Canada Ltd. has been conducting long-term field tests to develop and prove out this technology, but the future remains a mystery.
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