Abstract
Water availability is beginning to impact oil sands development and, as a
result, several technologies to increase the percentage of recycled water are
being evaluated. One such option being re-evaluated is the use of centrifuges
to produce dry tailings that can accommodate overburden and soil replacement.
Previous evaluations of centrifuge performance to capture water from the clay
and silt tailings (mature fine tailings) components demonstrated some success
but, at the time, at unacceptable costs. A better appreciation of the long-term
costs of mature fine tailings storage has prompted a re-evaluation of
centrifuge technology. The use of additives to improve centrifuge performance
has significantly improved the results that can be achieved. Aside from the
obvious positive environmental benefit of reclaiming the fluid fine (mature
fine) tailings, the increase in the amount of water recycled will reduce the
demand for fresh water from the Athabasca River. This paper discusses a
laboratory-scale study of the water chemistry and clay/silt feed properties
affecting centrifuge performance, as well as the results of a 20 tonne per hour
pilot.
Introduction
The term dry stackable tailings is commonly used in oil sands tailings
management to mean a mineral stream left over from the bitumen extraction
process which can be stored without the need for dikes or other fluid
containment structures. The use of water in the bitumen extraction process
results in mineral tailings streams in the form of slurries or suspensions that
require containment. In general, surface mined oil sands tailings fall into
three categories: sand tailings, fine tailings and froth treatment tailings.
Although the froth treatment tailings have important environmental implications
due to their contamination with solvents or diluents from the froth treatment
process, they are generally combined with the other tailings prior to discharge
into the recycle water area (tailings settling pond). It is typically a
straightforward process to create dry stackable tailings from the sand
tailings, and they are often used to build the containment for the remaining
fluid fine tailings.
Another common practice in the surface mined oil sands industry is to define
sand as mineral particles larger than 44 micrometres ( µm). Fines are therefore
smaller than 44 µm, and this definition can be useful when discussing the
long-term tailings properties during mine planning. It has been demonstrated
very clearly, however, that it is the clay content that determines all of the
important properties of the fine tailings or fluid fine tailings (or mature
fine tailings when the solids content exceeds about 25 wt%)(1-3).
Over the entire mine, of course, the average clay-to-fines ratio is fixed and
the fines content can be a useful approximation tool for large-scale planning
purposes and the prediction of sand volumes and the fine tailings volume that
may require containment.
The clay content, often expressed as a clay-to-fines ratio, can vary across an
oil sands lease by as much as a factor of four. It is therefore essential that
the clay content, clay-to-water ratio or clay-to-fines ratio be understood in
order to predict the properties of the fluid fine tailings on a daily or
monthly basis.
© 2009. Her Majesty the Queen in Right of Canada, as represented by the
Minister of Natural Resources
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History
- Original manuscript received:
3 April 2008
- Meeting paper published:
17 June 2008
- Revised manuscript received:
27 July 2009
- Manuscript approved:
4 August 2009
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