TECHNICAL PAPER: A shaking idea for oil sands fine tailings management
by Ralph Gunness
This article focuses on oil sands fine tailings management prior to being discharged to a final tailings dam so as to suggest a more sustainable solution.
Miller and others, through a recent series of papers, has ably studied the oil sands fine tailings consolidation process and assessed the efficacy and effectiveness of current oil sand fine tailings management practices. These series of papers set down the logic of why new process management is needed and suggests several practical solutions. This paper works on some of Miller et al’s conclusions and suggests another part solution specifically targeting upstream management of the fine tailings before they enter the tailings dam.
Characterization of oil shale tailings fines
Oil sand fine tailings derived from the Clarke Hot Water Extraction process result in the creation of extremely dispersed, high-void-ratio fine tailings composed primarily of silt, clay, water, and residual bitumen. Depending on the location, the fine tailings are generally composed of clays i.e., 80% kaolinite, 15% illite, 1.5% montmorillonite, 1.5% chlorite, and 2% mixed-layer clays (Fine Tailings Fundamentals Consortium [FTFC], 1995a).
Fine tailings derived from the caustic-based extraction processes typically exhibit used in the oil shale industry exhibit extremely low consolidation rates and shear strengths and require considerable management. Alternative hot water extraction processes (Sury et al. 1993) such as non-caustic bitumen extraction techniques have been developed; an example is the OSLO Hot Water Extraction Process (OHWE). This OHWE extraction processes was developed to improve bitumen (hydrocarbon) recovery and produce tailings with reduced fines.
Presumably, if the processor is not careful the net heat energy required to heat the water in these processes can approach the net energy being extracted or at the very least reduce the net energy being extracted. So implementing a heat recycling step would improve this situation.
These fine tailings are hydraulically deposited into tailing dam confinements and usually have about 30% solids. The fine solids form a lower layer (saturated pore spaces) with a residual water layer lying on top.
In the oil sands refinery process the molecular bonds surrounding the successive clay molecules are stretched and break (by heat, chemical and physical action), releasing the hydrocarbon fractions. The process leaves the fine tailings in a denatured ionic state and containing contaminated water.
Approximately 750 million cubic metres of oil sand fine tailings are currently being stored in tailings ponds (Houlihan & Milan, 2009).
The problem identified by government and others is the fine tailings may exist in this form for some time, even several years. The fines are saturated, and are usually contained behind pond walls of a composite sand nature. This construction in itself raises the question about the phreatic surface within these pond walls and the potential for leakage and failure. However, extensive heat energy losses could occur if the water in the tailings is not recycled; the site is not easily rehabilitated and could be a liability for future generations. Government’s regulatory agencies are requesting proactive management and reclamation practices with respect to these wastes.
The vast quantities and slow consolidation of fine tailings raises environmental and economic questions, particularly in relation to the footprint required to store these vast amounts.
One of the current solutions is to add chemicals to the tailings to form a more solid mass; however, there is considerable chemical energy and cost associated with this methodology and it may not represent a sustainable long term solution. Using this method, if a net energy balance equation over the entire process is carried out, it may prove that the net energy gains are very low.
So researchers have been actively exploring possible improvements in tailings behaviour so as to reduce the current environmental footprint.
My contribution considers that if a significant amount of the net heat energy can be recycled, along with reducing the net amount of water (by recycling the water), and if the fines solids can be made into less of a slurry/gel, then reducing the water in the pore spaces should improve the ability of the fine solids to “stack”. Theoretically, this could also reduce the volume of storage space required, while at the same time providing a more healthy net energy benefit over the entire process and a better more sustainable reclamation process.
How could this be achieved?
In an earthquake, if the pressure is repetitive and the water can’t flow out before the next loading, there is an increase in the hydraulic pressure in the immediate vicinity. The soil matrix loses shear strength along the phreatic boundary and the ability to transfer shear stress dissipates and it flows like a liquid i.e., liquefaction. Examples occur in earthquakes where water and soil is forced to the surface, forming soil/sand boils as it discharges its energy and seeks equilibrium. If this occurs on a slope the whole mass will move to reach equilibrium, usually with disastrous results.
If the saturated mass is shaken similarly to the earthquake scenario, this process is accelerated and the propensity of water presents along the phreatic boundary.
So in theory shear strength is lost and the mass tends to separate, into the liquid and solid phase in order to re-establish equilibrium.
Applying a cyclic induced wave loading (vibration) to the tailings slurry, in theory, will reduce the pore space volume, increasing the hydraulic pressure, decreasing the shear pressure reducing the effective shear stress along a new phreatic surface, leading to the explusion of the water. As the water exits, the shear strength of the remaining mass increases, resulting in a beaching effect. This action increases the propensity not to slump and theoretically increases the “beachability” of the remaining mass.
How is this theoretical construct best applied in the field?
The tailings exiting from the oil sand extraction process (usually as a slurry/gel in a pipe) could be directed to a vibrating live bottom floor table to induce the liquid/solid phase separation. The solids clumping and moving along the table as the liquid fraction passes through the table are collected separately. A conveyor belt could be used to transfer the solid mass to a purpose-built tailings dam or for further beneficial use such as a base material for the manufacture of concrete or bricks, using the waste heat from the oil sand extraction process.
With the liquid fraction being directed to separate treatment to recapture the heat energy, techniques such as centrifugal force (centrifuge or hydrocyclones) or filters can be applied to separate the liquid from the super fines prior to conventional waste water treatment and polishing.
* Ralph Gunness has degrees in Business Management, Environmental Studies, Training, & Agricultural Science. A Fellow of the Australasian Institute of Mining and Metallurgy, Gunness has completed and given many OHS & other training courses associated with the mining, petroleum and gas industries. He is an accredited auditor. Gunness recently launched an online waste water management course on EduMine called Sustainable Waste Water Management for the Energy and Mining Industries. The course has been designed to deliver basic water management fundamentals for the resources extraction industry.
Cymerman, G.J. and Kwong, T. 1995. Improvements in the oil recovery flotation process at Syncrude Canada Ltd. In Processing of Hydrophobic Minerals and Fine Coals: Proceedings of the 1st UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing, Vancouver, B.C., 20–24 August 1995. Edited by J.S. Laskowski and G.W. Poling. Canadian Institute of Mining, Metallurgy and Petroleum, Montre´al, Que. pp. 319–328.
Kasperski, K.L. 2001. Review of research on aqueous extraction of bitumen from mined oil sands. CANMET Western Research Centre, Natural Resources Canada, Devon, Alta. Division report CWRC 01-17 (CF).
Miller, W. G. (2010). Comparison of Geoenvironmental Properties of Caustic and Noncaustic Oil Sand Fine Tailings (2010). Unpublished doctoral thesis, University of Alberta, Edmonton, Alberta.
Miller, W. G., Scott, J. D., & Sego, D. C. (2009). Flume deposition modelling of caustic and noncaustic oil sand tailings. Canadian Geotechnical Journal, 46, 679-693.
Miller, W. G., Scott, J. D., & Sego, D. C. (2010a). Influence of the extraction process on the characteristics of oil sands fine tailings. CIM Journal, 1(2), 93-112.
Miller, W. G., Scott, J. D., & Sego, D. C. (2010b). Effect of extraction water chemistry on the self-weight consolidation of oil sands fine tailings. Manuscript accepted by CIM Journal for publication.
Miller, W. G., Scott, J. D., & Sego, D. C. (2010c). Effect of extraction water chemistry on the consolidation of oil sands fine tailings. CIM Journal, 1(2), 113-129.
Miller, W. G., Scott, J. D., & Sego, D. C, Influence of extraction process and coagulant addition on thixotropic strength of oil sands fine tailings, CIM Journal | Vol. 1, no. 3
Houlihan, R., & Milan, H. (2009) Influence of extraction process and coagulant addition on thixotropic strength of oil sands fine tailings ERCB tailings Directive 074. Paper presented at the Canadian Prairie Air and Waste Management and northern Section 2009 Annual Conference, Edmonton, AB. Shaw, R.C., Czarnecki, J., Schramm, L.L., and Axelson, D. 1994.
Shaw, R.C., Schramm, L.L., and Czarnecki, J. 1996. Suspensions in the hot water flotation process for Canadian oil sands. In Suspensions: fundamentals and applications in the petroleum industry. Edited by L.L. Schramm. American Chemical Society
Books, Washington, D.C. pp. 639–675. Sury, K.N., Paul, R. Dereniwski, T.M,. and Schulz, D.G, 1993.
Sury, K.n., Paul, R., Dereniwski, T. M., & Schulz, D. G. (1993, April). Next generation oilsands technology: The new OSLO processes (pp. 1- 36). Proceedings of the Fine Tails Symposium presented at the Oil Sands: Our Petroleum Future Conference, Edmonton, AB.