Basic HTML Version
www.geotechnicalnews.com
Geotechnical News • December 2012
49
WASTE GEOTECHNICS
higher annual rates per area, into deep,
in-pit deposits, relying on self-weight
consolidation to effect further water
release and volume reduction.
A second method employs in-line
flocculation of FFT and discharge
of the flocculated slurry in thin lifts
into cells, where initial dewatering,
effected by flocculation and drain-
age, can increase the solids content to
around 60%. Further water removal
is accomplished via evaporation and
freeze-thaw effects. The volumes
associated with oil sands mining and
the low net evaporation rates in north-
ern Alberta result in large area require-
ments to meet dewatering targets for
reclamation. The dewatered material
can be relocated to overburden cells
after initial dewatering (similar to cen-
trifuge cake), or alternatively, allowed
to dewater further with evaporation
or freeze-thaw to a point where it has
sufficient strength to form an integral
part of a disposal structure.
In-line flocculated FFT can also be
utilized to form deep deposits (e.g., in
a large in-pit cell). Water expressed
from the deposit and precipitation is
decanted from the surface. Surface
dewatering can be assisted by rim-
ditching the perimeter of the deposit
or creating channels on the surface to
direct water to a decant sump. Self-
weight consolidation progressively
increases the solids content of the
deposit, driving water upward through
the deposits (or both upward and
downward if there is bottom drainage).
A fourth dewatering method draws
FFT directly from the extraction pro-
cess (e.g., cyclone overflow), and then
flocculates and thickens the FFT in a
mechanical thickener. Thickening is
generally employed to recover thermal
energy but also has the benefit of
partially dewatering the FFT, produc-
ing thickened tailings (TT). The TT
can be placed in deep deposits, rely-
ing on consolidation for dewatering.
Alternatively, it could be discharged
in thin lifts, in a similar manner to that
described previously.
Blending of sand slurry (typically at
high solids contents) with FFT, using
flocculants or coagulants to attain a
non-segregating mix can also be used
to promote fines capture and dewa-
tering. Once mixed, the material is
then discharged into a deep deposit.
Where the fines are sourced as MFT,
the resulting product is referred to as
composite or consolidating tailings
(CT). Alternatively, where fines are
sourced as TT, the resulting product is
referred to as non-segregating tail-
ings (NST). The key objective of both
methods is to reduce the water content
and produce a sand-dominated mix,
at a moderately high sand to fines
ratio (SFR). This results in a relatively
quick volume reduction and increase
in deposit strength (compared to lower
SFR tailings deposits).
There are essentially four deposit
types that can be created from the fines
management methods under active
development and commercial use.
These deposits include: thin-layered,
fines dominated deposits; deep, fines
dominated deposits; fines-enriched
sand deposits; and water- capped fine
deposits. The full keynote paper (Fair
and Beier, 2012) describes the differ-
ent ways in which these four oil sands
tailings deposit types are produced,
their principal performance factors
and how the deposit performance
can be assessed through the period of
their placement to their readiness for
reclamation.
The Tailings Technical Guide sets out
guidelines for managing FFT through
appropriate treatment and disposal
in a DDA. For each site, operators
must consider land availability and
disturbance, geotechnical conditions,
resource distribution, general site
geology, containment availability and
mine advancement to develop the opti-
mum tailings management strategy.
Conclusions
The OSTC’s Tailings Technical Guide
provides an up-to-date technical
overview of current practice in oil
sand tailings management orientated
towards the different types of deposits
formed and managed using best avail-
able technology. These deposit types
include: thin-layered, fines-dominated
deposits; deep, fines-dominated depos-
its; fines-enriched sand deposits; and
water-capped fine deposits.
The Technical Guide also suggests
updates that would promote better
tailings management given recent
technology developments and changes
in current practice. These changes are
proposed in the context of the original
intent to provide a performance-based
regulation that builds on a foundation
of continuous improvement.
Detailed site-specific mitigation plans
are important to the proposed adap-
tive management approach. In many
cases, the contingencies are still in
a research or developmental stage.
Hence the need for an adaptive man-
agement plan, whereby new insights
are continuously incorporated in future
designs and applications.
Acknowledgements
The authors wishes to acknowledge
the support of the seven member
companies who form the OSTC
(Suncor Energy Inc., Syncrude Canada
Ltd., Shell Canada, Canadian Natural
Resources Ltd, Imperial Oil, Total
E&P Canada Ltd., and Teck Resources
Ltd.) and the expert panel members
for the development of the Techni-
cal Guide (David Carrier III, Richard
Dawson, Gerry DeSorcy, Ross Eccles,
John Errington, Barry Hurndall,
Norbert Morgenstern, Bernie Roth and
John Sobkowicz).
Reference
Fair A. and Beier, N. 2012. Collabo-
ration in Canada’s Oil Sands: Fluid
Fine Tailings Management. 3rd
International Oil Sands Tailings
Conference, December 2 -5, 2012,
Edmonton, Alberta.
Alan E. Fair and Nicholas A. Beier
OSTC/COSIA Tailings EPA,
Edmonton, Alberta, Canada