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Geotechnical News • March 2012
WASTE GEOTECHNICS
(in-pit lake) or dewatered separately
creating a cohesive, silty clay deposit.
The industry is trending toward
managing the fine stream with various
chemical, physical and environmental
dewatering techniques. In short, the
objective is to dewater the fines stream
sufficiently to meet the Directive 074
performance criteria.
In addition to Directive 074 require-
ments, the chosen tailings manage-
ment process should satisfy both
operational and reclamation/closure
goals (Hyndman and Sobkowicz,
2010). As the mining operations
proceed, tailings should be reclaimed
progressively, thereby limiting the
accumulation of fluid fine tailings that
would require containment, leaving
remediation to the end of mine life.
Fluid containment structures should be
limited to a minimum (i.e., only what
is required for effective tailings man-
agement). Meeting these operational
goals would allow the operator to pro-
ceed with reclamation and return the
mine site back to the public, thereby
achieving the reclamation goals.
Essentially, operators would be able
to avoid tailings ponds/dams in the
closure landscape that would require
ongoing maintenance (in the order
of decades); transform the tailings
deposits into geotechnically stable
landforms that are resistant to natural
processes and are self-sustaining both
physically and environmentally; and
ensure these landforms and features
are integrated into the future natural
ecosystem successfully.
Physical/mechanical dewatering
methods include centrifugation and
high rate thickeners (thickened tailings
[TT]) or paste thickeners (paste thick-
ener [PT]). In centrifugation opera-
tions, MFT is dredged from a tailings
pond, diluted and then mixed with a
polyacrylamide flocculant. The floccu-
lated fines stream is then processed in
the centrifuge and dewatered to nearly
55 wt% (water content [WC] of 80%)
prior to deposition. The fines stream
for thickeners would come directly
from the extraction process rather than
from the MFT pond. Sand-depleted
tailings streams or cyclone overflow
(COF) would be flocculated and
dewatered in the thickeners prior to
deposition at solids contents reaching
60 wt% (WC of 67%). Alternatively,
the fines may be dewatered through
a combination of chemical addition
and strategic deposition. Polymer
solutions are injected directly into the
transfer pipeline containing dredged
MFT or COF (dilution is optional).
This process is termed “in-line floc-
culation” or in-line thickened tailings
(ILTT). Two depositional techniques
are available for the flocculated fine
material. The mixture (well above the
liquid limit) can be discharged onto
a gently sloped beach in thin layers
where initial dewatering occurs due
to shear, settlement and drainage from
the flocs, followed by environmental
dewatering (desiccation, freeze/thaw).
The flocculated tailings may also be
discharged into large depositional cells
(>10 m deep) to promote self-weight
consolidation and environmental
dewatering via the surface. As water
is released to the surface, active water
management is required via decant
structures and mechanical channel-
ing (perimeter ditching) to promote
further dewatering and development of
strength. This later depositional tech-
nique is referred to as rim-ditching or
accelerated dewatering. Each of these
fines management techniques involves
some form of polymer or chemical
addition to promote dewatering and
strength gain. However, it is important
to note that the dewatered fines are
being placed at water contents above
their natural liquid limit (LL).
Implications of fines
management techniques
The performance measure for fine-
grained deposits in Directive 074
requires an undrained shear strength
(Su) of 5 kPa one year after deposi-
tion. For typical MFT, this would
require dewatering from water
contents of 233% to below their LL
(a Liquidity Index [LI] of 0.6). Figure
2 depicts this relationship between
LI and remolded Su for typical MFT
from the Albian Sands and Syncrude
mining operations. Additionally,
Figure 2 contains data for natural
clay deposits as reported in Locat and
Demers (1988) and Mitchell and Soga
(2005). As can be seen, the relation-
ship (Equation 1) proposed by Locat
and Demers (1988) provides a good
overall fit for typical MFT.
Equation 1.
Su
remolded
= (19.8/LI)
2.44
Figure 4. Sensitivity of oil sands fine tailings.
