Predicting Oil Sands Tailings Behaviour From Clay Content and Water Chemistry

Abstract A variety of tailings handling technologies have been developed over the years to deal with the accumulated mature fine tailings (MFT) and to mitigate storage problems associated with this fluid fine tailings stream. The CT or consolidated tailings process involves chemical amendments to combine the clays and fines in MFT with the coarser sand components to create a nonsegregating mixture that will rapidly consolidate. Variations on this concept involve the use of thickeners to provide an MFT analog at the "end of pipe", without the need for a large tailings/recycle water pond to form and store MFT. Other options include co-deposition of MFT with sand to produce a tailings which retains the strength of a sand deposit, while capturing a significant amount of the fine tailings component. Fundamental limitations to the amount of MFT that can be captured or sequestered are defined by the clay content and water chemistry of the tailings stream. These limitations will be discussed in terms of the basic clay:water interactions and mechanism of strength development in the CT system; along with implications for thickener performance and bulking factors in tailings disposal. Introduction Oil sands tailings can be thought of as being made up of two major solids components: the coarse tailings (sand), and the fluid fine tailings (silts and clays). Historically, the industry has considered these two fractions by arbitrarily dividing the tailings into the sand fraction (+44 micron) and the fines fraction (-44 micron). These definitions were adopted as a matter of convenience since 44 micron is the smallest sieve opening that is used for fractionating samples. Extensive studies on the properties of the fluid fine tailings stream in the last 10 years have clearly shown that the -44 micron size cut off is not useful in characterizing the behaviour of this tailings stream (1–6). The clay content or the -2 micron fraction is a much more useful parameter. The most accurate way to define the fluid fine tailings behaviour is to account for the mineralogy and size distribution of the -2 micron fraction. In general, however, oil sands mineralogy is relatively consistent, and that makes the simple -2 micron, or clay size fraction a useful parameter for characterizing typical tailings streams. Information about this mineral fraction can be obtained using a variety of methods including hydrometer tests and more sophisticated techniques such as the newer generation laser light scattering instruments, quantitative x-ray diffraction, or the monitoring of sedimentation using x-ray adsorption. A very useful short cut technique using the adsorption of methylene blue dye (ASTM C837) has been adapted by Yong and Sethi to correlate with the -2 micron fraction. This calibration developed by Yong and Sethi relates the adsorption of methylene blue to the clay content in oil sands (7,8). All of these methods are useful for describing the behaviour of typical fluid fine tailings streams, as long as they are sensitive to the clay fraction.