Oil sands have become an attractive source of oil recovery to support global demand for oil. Oil sands are large deposits of naturally occurring mixtures of bitumen, water, sand, clays, and other inorganic materials found on the earth's surface. Bitumen is a highly viscous form of crude oil. The largest oil sands deposits are found in Canada and Venezuela. In particular, the Athabasca oil sands deposit is equivalent to 1.6 to 2.7 trillion barrels of oil, and is located in the Canadian provinces of Alberta and Saskatchewan. About 10% to 15% of the economically recoverable Athabasca oil sands deposits can be surface mined. Once the oil sands ore is mined, it is processed to extract the bitumen.
The bitumen must be extracted from the oil sands ore by separating the bitumen from the water, sand and fine clays contained in the oil sands ore. Today, the oil sands ore is prepared and conditioned prior to extracting bitumen. Processes which include open pit mining, crushing the ore, mixing the crushed ore with warm process water and optionally chemicals, and allowing the ore slurry to be conditioned over time with mechanical agitation or through shear experienced in fluid flow. Ore slurry conditioning is accomplished today by pumping the ore slurry through a hydrotransport line to a centralized extraction plant.
The oil sands ore may be mixed with chemicals and water at the front end of the process (preparing and conditioning steps as recited above) and/or added at one or more of the downstream unit operations steps to aid extraction. The oil sands ore is prepared and conditioned to provide the ore in the form of an aqueous slurry. The slurry undergoes extraction in an extraction plant, which comprises several unit operations.
Various tailings streams are produced in a bitumen extraction process. A tailings stream is an aqueous stream (slurry, suspension) containing components requiring further treatment, including for extracting valuable bitumen or for solids removal and/or purification to enable recycle of the water content of the tailings stream. Ultimately, some tailings streams will be deposited in a tailings pond for long periods of time. Coarse solids comprising predominantly sand settle quickly at the bottom of the tailings ponds. The top layer of the pond is clarified with time and gravity to make water that is suitable for re-use in the extraction process. A large middle layer is present that is comprised of water and fine solids which solids settle very slowly. The middle layer ultimately becomes mature fine tailings (MFT).
MFT is a stable composite slurry comprising fine clays and sands, silt, water, and bitumen. MFT has little strength, no vegetative potential and may be toxic to animal life, so it must be confined and prevented from contaminating water supplies. Typically, several years of settling time are required to make MFT.
Thickened tailings, with similar physical properties to MFT, can be produced through chemical thickening. Chemical thickening enables faster recovery of the water and reduces heat energy requirements when this warm water is recycled for use back in the extraction process.
MFT ponds pose an environmental concern. The Energy Resources Conservation Board of Alberta (ERCB) has issued Directive 074, which mandates a reduction of fine tailings ponds and the formation of trafficable deposits for all oil sands operators. MFT then becomes stable with little additional settling or consolidation occurring for decades.
Significant new regulations have been introduced that are forcing operators to develop and change their tailings management processes. Tailings management processes under evaluation and/or development include bioremediation through water capping of tailings ponds (i.e. end pit lakes), composite tailings (MFT plus sand and often a coagulant), non-segregating tailings (thickened tailings plus sand and often a coagulant), dry stackable tailings, paste technology, centrifugation, rim ditching, and several others. Tailings streams that can be generated in oil sands mining operations are known to those skilled in the art. See for example, R. J. Mikula, et al., “Nature and Fate of Oil Sands Fine Tailings,” in Advances in Chemistry, 1996, vol. 251, pp. 677-723; and “Part B Report: A Screening Study of Oil Sands Tailings Technologies and Practices” prepared for Alberta Energy Research Institute by D. W. Devenny, March 2010, available at http://eipa.alberta.ca/media/40994/report %20b%20%20integrated%20oil%20sands%20tailings%20treatment %20technologies%20march%202010.pdf (accessed Dec. 15, 2011). It will be appreciated by those skilled in the art that reference names (terminology) of the tailings streams may vary as oil sands producers vary and processes used to extract oil and address the various tailings streams will also vary.
MFT or fine thickened tailings may be treated with sand and a coagulant such as gypsum and/or carbon dioxide, and optionally lime, to produce consolidated composite tailings (CCT). CCT may be referred to as composite tailings (CT) and non-segregating tailings (NST). Such process is not ideal for several reasons. CCT takes many years to consolidate and strengthen to reach the compressive strength requirements dictated by Directive 074, which sets forth the regulatory requirements. It has been disclosed that CCT may take at least 10 years to consolidate and strengthen to meet the regulatory requirement. CCT dramatically increases the volume of treated fine tailings as compared to the starting MFT or thickened tailings stream. As a result, extra land surface and more containment dykes need to be built to contain the extra treated tailings volume. CCT requires sand to build the dykes and to maintain the correct sand to fines ratio needed in the recipe. Sand shortages and land shortages in mining operations may limit the application of CCT.
Tight control of the sand to fine ratio is required in CCT processes to maintain a non-segregating mixture. If the correct sand to fine ratio is not maintained, the mixture segregates and will never consolidate. Treated material that segregates will need to be re-worked to comply with the regulatory requirement and extra land and time is needed to store off-spec (outside of specifications) CCT.
The NST process using thickened tailings and coarse solids captured directly from the extraction process may produce >40% off-spec NST, that is material that does in fact segregate. Sand and tailings that are needed to make the CCT are supplied from an upstream extraction process. The ratio of sand and fine solids available in real-time from the process will vary depending on the composition of the oil sands ore. In many circumstances, the sand to fine solids ratio needed to make a non-segregating mixture cannot be provided in real-time from the process. Thus, the interaction between extraction and CCT treatment may affect either or both of the upstream extraction process and the downstream tailings treatment process. Finally, gypsum/lime treatment adds calcium ions to the recovered water, which are problematic when this recovered water is re-used in the extraction process.
An alternative process employing centrifuges in conjunction with conventional polymeric flocculants to treat MFT to produce a dewatered MFT stream and recovered water is being evaluated. Centrifuging requires large capital investment in the centrifuge equipment. Also, the centrifuge product cannot be transported through pipeline and must be transported by truck or rail to dedicated disposal areas. R. J. Mikula, et al., “Centrifugation Options for Production of Dry Stackable Tailings in Surface-Mined Oil Sands Tailing Management,” J. Canadian Petroleum Technology, (2009), vol. 48, No. 9, pp. 19-23.
Another challenge for oil sands producers is separation of bitumen from sand and clay fines, as well as dewatering of the sand and clay fines for disposal, for so-called “poor quality ores.” Generally, a poor quality ore, in reference to an oil sands ore, is an ore that contains a lower than average bitumen content, a higher than average fine solids content, or both. Also, ores that contain different types and amounts of natural surfactants than average can be poor quality ores. Ores that have higher ionic strength in their connate water than average can be poor quality ores. Poor quality ores may also be characterized as ores that have undergone a high degree of oxidation. Finally, ores that contain a higher than average percentage of montmorillonite or other fine clays, or a higher proportion of ultra-fine solids less than 2 microns as compared to the total fine solids fraction (<44 microns) can be poor quality ores. The high clay and fine solid content in ores not only hinder the extraction of bitumen, but also the dewatering process of sand and clay fines in downstream tailings operations. Poor quality ores are difficult to extract bitumen from at acceptable yields using conventional methods. In addition, more bitumen is retained in the tailings streams from extraction of poor quality ores, which is sent to the tailings pond as a yield loss. Seem for example, J. Masliyah, et al., “Understanding Water-Based Bitumen Extraction from Athabasca Oil Sands,” Can. J. Chem. Engg. (2004) vol. 82, pp. 628-654.
Poor quality ores can reduce bitumen recovery by 40% or more and are avoided when possible. However, oil sand producers are obligated to process all ores containing 7 wt % bitumen or more with minimum bitumen recovery rates stipulated in the ERCB regulation ID 2001-7. An alternative is to blend poor quality ores with good quality ores so they can be processed more effectively. Another alternative is to apply chemical processing aids, such as sodium hydroxide, with these poor processing ores to improve bitumen recovery. With demand for oil increasing every year, there is a need to mine these poor quality ores and to produce higher yields of bitumen. Tailings streams from these ores should be essentially free of bitumen and separated from water, so the water can be re-used and the solids can be returned to the environment free of bitumen, within environmental limits.
Some producers do not have an economical source of low temperature heat that can be used to raise the temperature of process water. In these instances, there is a strong need to reduce the extraction temperature (for example, less than about 55° C.) to save heat energy and associated greenhouse gas emissions. For example, when an adjacent upgrading facility proximate to the mining operation is not available, there is added cost to supply heat energy for the extraction water. However, higher temperatures increase the efficiency of the extraction process and the recovery yield of the oil. Technology improvement is needed to improve the recovery yield at lower extraction temperatures.
U.S. Patent Publication No. 2010/0101981-A1 discloses a process to extract bitumen from an oil sands ore comprising contacting an aqueous slurry of an oil sands ore with a polysilicate microgel. U.S. Patent Publication No. 2010/0126910-A1 discloses a process to treat a tailings stream comprising contacting a tailings stream with a polysilicate microgel, anionic polyacrylamide, and (i) a multivalent metal compound or (ii) a low molecular weight cationic organic polymer. U.S. Patent Publication No. 2010/0104744-A1 discloses a process to treat a tailings stream comprising contacting a tailings stream with a silicate source, which is a polysilicate microgel or an alkali metal silicate, and an activator, which causes gelation of the silicate source with the tailings stream.
While there have been many advances in the treatment of tailings, there remains a need to improve de-watering (less water in the tailings), consolidation (reduction of volume of the tailings), and strengthening of the tailings and to reduce the need to add fresh water to bitumen recovery processes. There is also a need to reduce or eliminate tailings ponds, including MFT ponds, where the remaining solid can be useful and the water can be recovered and re-used in the process. There is also a need to return the mined area close to its original condition. The present invention meets these needs.
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