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Evolution of microstructure during desiccation of oil sands mature fine tailings
Évolution de la microstructure en séchage des résidus de sables bitumineux
Bajwa T, Simms P.
Department of Civil & Environmental Engineering, University of Carleton, Ottawa, Canada
ABSTRACT: The coupling between desiccation and consolidation is a process with important implications for the management of
soft soils in general and dewatering of fine grained tailings typical of phosphate, bauxite, and oil sands mining in particular. The
management of fine tailings can involve the placement of layers that are allowed to desiccate, and then are subsequently consolidated
by burial under fresh tailings. While desiccation does densify the material, it also changes both the strength and volume change
behaviour of the subsequently consolidated material. This phenomenon is crucial to the oil sands industry, where regulations mandate
that tailings achieve a set undrained strength within 1 year after deposition. To understand the interplay of desiccation and
consolidation, the evolution of microstructure of oil sand fine tailings are tracked through different drying and consolidation paths
using mercury intrusion porosimetry, and non-biased analyses of Environmental Scanning Electron Microscope images. Preliminary
results presented in this paper describe the evolution of microstructure in polymer amended tailings during desiccation. The influence
of flocculant dose on the microstructure appears to lessen as desiccation progresses, but the final microstructure retains a more open
porosity compared to untreated tailings. Résumé
RÉSUMÉ : Le couplage entre la dessiccation et la consolidation est un processus avec des implications importantes pour la gestion
des sols mous en général et le séchage des résidus miniers fins typiques du phosphate, de la bauxite, et des sables bitumineux “ oil
sands” en particulier. La gestion des résidus miniers fins peut comporter le dépôt des couches pour le séchage, et leur consolidation
par enterrement sous les résidus frais. Tandis que la dessiccation densifie le matériau , elle change également la force et le
comportement mécanique du matériau consolidé. Ce phénomène est crucial à l'industrie de sables d'huile, où les règlements exigent
que les résidus atteignent une force non drainée prédéfinie dans un délai de 1 an après dépôt. Pour comprendre l'effet de la
dessiccation et de la consolidation, l'évolution de la microstructure des résidus miniers est étudié pour différents chemins de séchage
et de consolidation utilisant la la porosimétrie au mercure, et des analyses d’images de microscope à balayage électronique. Les
résultats préliminaires présentés dans cet article décrivent l'évolution de la microstructure des résidus modifiés par polymère pendant
la dessiccation. L'influence de la dose de floculant sur la microstructure semble diminuer pendant que la progression de la
dessiccation, mais la microstructure finale maintient une porosité plus ouverte comparée aux résidus non traités
KEYWORDS: Mature fine tailings, polymer, suction, mercury intrusion porosimetry, SEM, desiccation, microstructure
1 INTRODUCTION
The extraction of oil from oil sands deposit result produces
bitumen and tailings. Conventional deposition results in coarse
particles (> 74 microns) settling on the beach, while the fine
fraction of settles and consolidates extremely slowly, retaining a
water content of over 180% after a decade – in this state the
tailings are called mature fine tailings (MFT). Due to the
extraction process, the clays in the fine fraction are highly
dispersed, which results in very low hydraulic conductivity.
Because of the volume of MFT produced, impacts include a
considerable volume of water is lost to the tailings, and a very
large footprint (~200 km^2 for tailings in the FortMcMurray,
Alberta area) and dam constructions costs. In order to
accelerate restoration and water reclamation form these tailings,
the regulator has imposed new rules, that require an increasing
inventory of tailings to achieve specific undrained shear
strengths at scheduled time after deposition, the first target
being 5 kPa after 1 year.
The new regulations have fostered large-scale
experimentation with several techniques to dewater and / or
strengthen MFT. One technique is to mix an anionic polymer
with MFT and re-deposit the amended tailings in relatively thin
lifts (Matthews et al. 2011, Wells et al 2011). The mixing is
done in the pipeline, only a few metres from the deposition
point. This causes aggregation of the clay particles, and results
in decreases in water content by settling down to about 100%
water content (50% solids), or even greater. To reach the
required 5 kPa shear strength, the geotechnical water content
must usually be less than 50% water content To achieve this, the
material may be deposited in thin lifts to facilitate further
dewatering due to evaporation, long-term consolidation, or
drainage.. However, the relative contributions of desiccation
and consolidation to dewatering are not completely understood,
and a better comprehension of the relative effects of each
process on subsequent dewatering behaviour could contribute to
optimizing the overall dewatering process, especially in terms
of required layer thickness, and timing of layer sequencing. This
paper presents some preliminary data on the microstructure of
polymer amended MFT and how it evolves during desiccation.
Data on microstructure is obtained using Mercury Intrusion
Porosimetry (MIP) and Scanning Electron Microscopy (SEM).
1.1 Mercury intrusion porosimetry (MIP):
MIP finds a pore-size distribution for pores ranging from 0.01
up to 100 microns – while this pore-size distribution might not
be the true PSD due to pore accessibility and sample preparation
issues, MIP data is known to exhibit strong correlations to
permeability, consolidation characteristics, and water-retention
behaviour (Simms and Yanful 2005, 2004, Romero and Simms
2008) – it appears to give a good quantitative “fingerprint” of
microstructure. For compacted clays, it is know that volume
change measured by MIP samples is very close to macroscopic
volume change. However, for wetter or slurried clays, it has
been shown that MIP only measures a fraction of the total
porosity, despite use of a rapid freeze drying technique to
dehydrate the samples (Sassinan 2011).
Further details on the methodology of MIP are available in
many other references, such as ASTM 4404-10, Simms and
Yanful (2004) and Romero and Simms (2008), and are not
repeated for reasons of space. All samples were prepared by
