Oil sands surface mining in Alberta has generated over a billion cubic metres of waste, known as tailings, consisting of sands, silts, clays, and process-affected water that contains toxic organic compounds and chemical constituents. All of these tailings will eventually be reclaimed and integrated into one of two types of mine closure landforms: end pit lakes (EPLs) or terrestrial landforms with a wetland feature. In EPLs, tailings deposits are capped with several metres of water while in terrestrial landforms, tailings are capped with solid materials, such as sand or overburden. Because tailings landforms are relatively new, past research has heavily focused on the geotechnical and biogeochemical characteristics of tailings in temporary storage ponds, referred to as tailings ponds. As such, the geochemical stability of tailings landforms remains largely unknown. This review discusses five mechanisms of geochemical change expected in tailings landforms: consolidation, chemical mass loading via pore water fluxes, biogeochemical cycling, polymer degradation, and surface water and groundwater interactions. Key considerations and knowledge gaps with regard to the long-term geochemical stability of tailings landforms are identified, including salt fluxes and subsequent water quality, bioremediation and biogenic greenhouse gas emissions, and the biogeochemical implications of various tailings treatment methods meant to improve geotechnical properties of tailings, such as flocculant (polyacrylamide) and coagulant (gypsum) addition.

The novel cationic flocculant, poly(lactic acid) choline iodide ester methacrylate (poly(PLA₄ChMA)), has been shown to provide improved flocculation of 5.0 wt.% mature fine tailings (MFT) diluted in deionized water compared to commercial anionic polymers, with continued dewatering of the sediment occurring as the polymer undergoes partial hydrolytic degradation. However, the elevated dosages (10,000 ppm) required would make the polymer costly to implement on an industrial scale. With this motivation, the impact of MFT loading and the use of process water is explored while comparing the settling performance of poly(PLA₄ChMA) to available commercial alternatives such as anionic FLOPAM A3338. Improved consolidation of 5.0 wt.% MFT diluted with process water could be achieved at reduced dosages (500 ppm) with poly(PLA₄ChMA). However, the final compaction levels after polymer degradation were similar to those achieved with the nondegradable commercial flocculants. Flocculation-filtration experiments with undiluted MFT are also conducted to compare the performance of the polymers. Significantly faster rates of water release were observed with the cationic flocculants compared to FLOPAM A3338, but no improvement in the overall tailings compaction was found either before or after poly(PLA₄ChMA) degradation. Thus, the improved dewatering observed with poly(PLA₄ChMA) in dilute MFT suspensions does not extend to conditions that would be encountered in the field.

Oil sand tailings are a mixture of sand, fines, water, and a residual amount of un-extracted bitumen in varying proportions. Tailings management is highly dependent on the consolidation behavior of the tailings. Although a great deal of work on this sector has been performed to study the consolidation behavior of oil sands tailings, it continues to play a critical role in quantifying the long-term geotechnical stability of tailings storage facilities. A review of large strain consolidation testing that exists, whether in the industry or in academia, has been compiled and presented in this manuscript to illustrate the advantages and drawbacks of measuring consolidation behavior of tailings using these tests. For oil sands mine tailings, it has been concluded that the conventional oedometer consolidation test can result in significant errors in quantifying the consolidation behavior. Conversely, testing procedures such as multi-step loading large strain consolidation tests, large slurry consolidometer tests, centrifuge testing, and seepage induced consolidation tests are widely employed to quantify the consolidation behavior of oil sands tailings.

The dewatering of flocculated high density slurry presents a significant challenge to most mining industries. The new technologies to treat high density slurry require a consistent and robust flocculation method in order to enter the market of tailings management. The flocculation of high density slurry, however, due to its complexity, is always a challenge to be undertaken appropriately and to evaluate the dewatering performance correctly. This paper probes the complexity by using a torque-controlled mixing technique to demonstrate the influence of feed properties, polymer type, polymer dosage, and mixing conditions on dewatering performance. The study shows that flocculant should be dosed at the optimal range to achieve the highest dewatering performance. A full dosage responsive curve including under dosage, optimal dosage, and overdosage is critical to evaluate the dewatering performance of high density slurries and flocculants. The mixing conditions such as mixing speed, mixing time, and geometry of the mixing impeller affect the flocculation efficacy. It was found that the dewatering performance of high density slurry is sensitive to solids content, water chemistry, and clay activity. High sodicity and high clay activity in the high density slurry decreases the dewatering performance. Therefore, it is critical to evaluate flocculants across multiple feeds and dosages with replication in order to select optimal dewatering performance. Using multiple key performance indicators (KPIs) to build technical and economic criteria is also critical for polymer evaluation.

The composition of oil sands tailings is a complex mixture of water, fine clay, sand, silt, and residual bitumen that remains after the extraction of bitumen. Effective tailings disposal management requires an understanding of the mechanisms controlling water movement, surface settlement rates and extents (hydraulic conductivity and compressibility), and strength variation with depth. This investigation examines the self-weight consolidation behavior of oil sands tailings, typically assessed by utilizing large strain consolidation (LSC) methods such as the multi-step large strain consolidation (MLSC) test and seepage-induced consolidation test (SICT). These methods, however, are time consuming and often take weeks or years to complete. As an alternative, centrifuge testing, including both geotechnical beam type and benchtop devices, was utilized to evaluate the consolidation behaviors of three untreated high water content oil sands tailing slurries: two high-plasticity fluid fine tailing (FFT) samples and one low plasticity FFT. The centrifuge-derived compressibility data closely matched the LSC testing compressibility data within the centrifuge stress range. However, the hydraulic conductivity obtained from centrifuge testing was up to an order of magnitude higher than the LSC test results. Comparing centrifuge and large strain modeling results indicates that centrifuge test data demonstrate average void ratios 10–33% lower than those predicted by simulations using LSC parameters, highlighting a notable deviation. To examine the scale effect on result accuracy, validation tests indicated that the benchtop centrifuge (BTC) yielded comparable results to the geotechnical beam centrifuge (GBC) for the same prototype, saving time, resources, and sample volumes in the assessment of tailings consolidation behavior. These tests concluded that the small radius of the benchtop centrifuge had a minimal impact on the results.

Oil sands tailings consist of a combination of sand, fine particles, water, and residual unextracted bitumen in varying ratios. The management of these mine waste tailings is largely influenced by their consolidation behavior. Large strain consolidation testing, such as the multi-step large strain consolidation (MLSC) test, is commonly used to determine consolidation properties but requires considerable time. A benchtop centrifuge (BTC) apparatus was proposed to derive the consolidation parameters of the following three clay-rich oil sands tailings slurries: two samples of high-plasticity fluid fine tailings (FFT) and one of low-plasticity FFT. Comparison with the MLSC tests illustrates that the BTC-derived compressibility data closely matched the MLSC test’s compressibility curve within the BTC stress range. However, the hydraulic conductivity from the BTC test was an order of magnitude higher than that from the MLSC test. The consistency of the BTC method and the validation of scaling laws were confirmed through modeling-of-models tests, showing a consistent average void ratio regardless of the specimen height or gravity scale. The influence of the small radius of the BTC was found to be minimal. The limitations of the BTC in the physical modeling of the consolidation behavior are discussed and their impact on the interpretation of the observed consolidation behavior is addressed. Overall, the BTC test provides a rapid method to gain insight on high-water-content slurries’ large strain consolidation behavior.