Note: Descriptions are shown in the official language in which they were submitted.
CA 02397574 2002-08-12
METHOD OF BARGE MINING TAR SANDS
Field of the Invention
This invention relates to the mining of tar sands for the purpose of
extracting oil therefrom.
Background of the Invention
The Athabasca tar sands consist of vast areas of sand infused with bitumen.
The sand is
often overlain by various types of overburden such as muskeg, clay, sand,
gravel, fill and
lean tar sand. Limestone bedrock typically lies underneath the tar-sand, but
sometimes
coal is the underlying material.
Where possible, the tar sand deposits are harvested by open-pit mining. The
overburden
is stripped off and used for various purposes. Muskeg is often stockpiled
separately for
future land reclamation, some materials are used to construct haul roads or
dykes for tailing
ponds, and the remaining materials are placed in waste dumps.
Following the stripping, the tar sands are excavated and delivered to sizers
that break down
any large lumps. The ore is then conveyed to a coarse ore stockpile, or surge.
It is fed
from the surge via a reclaim conveyer to the preparation plant, where it
blended with hot
water and agitated to form a slurry. The slurry is pumped to an extraction
plant. In older
operations, the surge may be eliminated; the sized material is fed directly to
the preparation
plant via a conveyer.
At the extraction plant, bitumen is separated from the sand. The bitumen is
then pumped
to an upgrader for processing, while the sand and water are pumped back to
tailing ponds,
usually mined-out pits.
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The mining of tar sands and extraction of bitumen is a very costly process.
Four reasons
for the high costs are the use of trucks, the use of ground-engaging tools,
the need for
ripping and blasting during the winter, and the blocking of the conveyances by
frozen lumps
of sand.
Tar sands are difficult to drive on once disturbed, since it has little shear
strength and tends
to become badly rutted. Trucks have trouble getting through it even in good
weather, and
in wet weather they often become stuck. Sand and limestone are often used to
strengthen
frequently-travelled areas; this is not only costly, but has only limited
success.
Tar sands are very abrasive, and tightly packed. Digging is therefore very
costly, as
ground-engaging tools must be frequently replaced.
During the winter, most of the tar sands are frozen, necessitating expensive
ripping, drilling
and/or blasting. In addition, the coarse ore piles downstream of the sizers
frequently
become plugged with frost lumps. The sizer must then be stopped, and the surge
piles
excavated. Older feed systems without surges have continual spillage of rocks
and frost
lumps, causing costly clean-up year-round, and especially during the winter.
A less costly method of mining tar sands is required.
Summary of the Invention
An object of the present invention is to provide a reduced-cost method for
mining tar sands.
The tar sand pits are flooded with water and mined in a series of cells with
dredging barges
rather than trucks. The overburden may be stripped prior to the flooding by
traditional
methods, or after flooding, by dredging. The low permeability of tar sand
minimizes
leakage and reduces the likelihood of environmental damage. If necessary,
permeable
areas can be sealed with membranes, grout or even bitumen.
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The barges are fitted with conventional dredging equipment, such as a cutter-
head system
for loosening tightly packed material. The excavation proceeds in horizontal
layers at the
bottom of the pond, the water level being regulated by pumping water back and
forth
between the mining pond and tailing ponds.
With the use of barges, the need for trucks to drive on unstable tar sands for
excavation
purposes is eliminated or reduced. This translates into savings in cost.
Once each cell has been mined down to the bedrock level, it is reflooded to
allow the barge
to navigate into the next cell and begin excavation. A number of cells may
operate
concurrently at various stages of completion, with one or more dredges
operating in each.
In this way, ore grade may be controlled by adjusting and blending flows from
different
depths, and throughput can be maintained by having spare capacity to handle
moves,
breakdowns and refits.
Berms left between cells may be mined later by truck and shovel or by dredge.
Completed
cells are used for tailings and waste disposal. Discharge pipes from the
dredges are
floated in the ponds during the summer months, and carried by the ice during
the winter.
Once extracted, the ore is pumped to the mixing plant for further processing,
or directly to
the extraction plant via a pipeline downstream of the mixing plant.
A distinct cost advantage of this process is the delivery of warm material
during the winter.
The ore being mined will be kept flooded with water, so that its temperature
will never be
lower than 0°C. This may make a difference of up to 30 or 40°C
compared to current
practices.
The recent practice of pumping the tar sand in warm slurry to the extraction
plant has the
advantage of causing a partial separation of sand and oil particles in the
pipeline en route
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CA 02397574 2002-08-12
to the extraction plant. The dredge mining method enhances this effect by
increasing the
distance of slurry travel from the pit itself all the way to the extraction
plant.
In order to increase slurry temperature, the mined slurry may have to be
partially dewatered
at the preparation plant, with surplus water being returned to the pit and
additional hot
water being added to the sand for the journey to the extractor. Alternatively,
warm tailings
water may be piped directly to the cutter head, or a reduced-scale heating and
mixing
system may be installed on the barge. In any case, there is a reduction in
energy costs
due to the higher average ore temperature.
Further savings are realized through a lowering of ore cut-off grade. Since
the cost of
mining is significantly reduced with this method, it becomes cost-effective to
mine a lower
grade of ore. Accordingly, more of the lean tar sand may be classified as ore,
thus
enlarging the reserve and lowering the amount of stripping required.
Overall, significant cost reductions in the order of $2.00 per barrel are
anticipated.
Brief Description of the Figures
Figure 1 is a flow chart illustrating the traditional open-pit mining method
(prior art).
Figure 2 is a flow chart illustrating the flooded-pit dredging mining method.
Detailed Description of the Figures
In Figure 1, the pit (1 ) is excavated with heavy-duty ground-engaging
equipment (2). Ore
is transported by trucks (3) to a sizer (4) for breakage of large ore lumps.
The ore is
conveyed to a surge (5), and then is conveyed to a preparation plant (6) where
it is mixed
with water to form a slurry. The slurry is pumped to an extraction plant (7),
where sand and
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bitumen are separated. Bitumen is pumped to an upgrader for processing, while
sand and
water are pumped to tailing ponds.
In Figure 2, the pit (1 ) is filled with water (8) and excavated with a dredge
(9). The ore,
along with some water, is conveyed directly to a preparation plant (6), where
the cooler
water may be replaced with hot water. The slurry is pumped to an extraction
plant (7),
where sand and bitumen are separated. Bitumen is pumped to an upgraderfor
processing,
while sand and water are pumped to tailing ponds.
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