Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
3387
Open pit mining for tar sands
This application-is a ~ivision of Application Serial
No. 343,501 filed January 11, 1980, and the invention
relates to improvements in open pit mining for tar sands
and in the layout and method of operation of such a mine.
The world's major oil sands deposits are in Canada,
especially in the Athabasca region of Alberta, and the mining
of such deposits commenced in ernest in 1967 when the Great
Canadian Oil Sands project went on stream. This has now been
followed by the much larger Syncrude project which has
recently begun to produce. A number of other projects are in
the planning stage, as described in "The Oil Sands Open Pit
Mining Review" by V. Srajer published in December 1977 by
the Canada Centre or Mineral and Energy Technology.
Experience to date with the mining of tar sands material
has exposed many unpredicted problems, especially in the
15 ~ handling and transportation of the material and consequently
in the layout of the mine itself. Both capital and running
costs have substantially exceeded estimates and major delays
have~been experienced. Since very large capital expenditures
are involved in any scheme that is to have a reasonable
2~0~ ;prospect of proving economically viable, great care must be
;taken at the planning stage to minimise problems of this
type. Such is the primary objective of the present invention.
One~specific problem related to existing minlng pro-
cedures is the limited reach of the draglines when pit slopes
25~are excavated to a stable incline. One of the primary
:
:: ~: : , :
-- -: . , , ' `:
.
- . .. : . , ~
- ~
338~7
-- 2 --
purposes of dragline mining was based on the belief that it
would permit casting of the overburden and center reject
clear of the mining toe, and thus permit low cost over-
burden removal. This assumption has not proven true, because
the slopes have been unstable at inclines that permitted
back casting of the overburden and center reject. The
stable slope angles also place substantial ore beyond the
reach of the draglines and consequently valuable bitumen is
lost. Mining operations based on draglines have required
major revision by the addition of mobile equipment or bucket
wheel excavators, conveyors, trippers and spreaders to pre-
strip the overburden and transport it long distances around
the mine. Single bench mining then is replaced with double
bench mining in order to reach to the bottom of the ore body
and to clear the center reject from the toe. Double bench
mining requires accelerated initial overburden removal to
make space for the overburden and mid-ore body benches. The
advantages claimed by the dragline scheme, i.e. low cost
overburden removal and minimum initial overburden remova~ are
therefore lost. The operation has therefore lost these
benefits, while incurring the penalty of double handling all
the ore, and high cost long distance transport of overburden
and part of the center reject.
Another specific problem that has plagued existing
schemes has been that of transporting the tar sands material
thereinafter referred to as the "ore") from the mining face
to the extraction plant in which the various bituminous
products are separated from the sand. In the Syncrude layout,
~ for example, as initially planned, the overburden (the surface
- 30 layer of muskeg, rock and oil-poor sand) is first removed to
mine the ore that has been exposed. Bulldozers and front-end
loaders assist draglines in this latter operation. The
mined ore is dumped immediately behind the draglines, re-
claimed by crawler mounted bucket wheel excavators and loaded
onto a conveyor system for delivery to the extraction plant.
Since a conveyor break down would completely halt the
operation and the many adverse operating conditions make such
break downs likely, it is necessary to duplicate the conveyor
,
-
.,
33B'7
3 --
system, even though this may represent a large overcapacity
(even as high as lO0~). Such conveyor systems represent a
significant aspect of the capital costs. They have been
chosen in the past, because they have been thought to be
more economical than other forms of transportation, e.g.
truc]cs, in terms of running expenses, i.e. principally labour
costs. However, apart from the problems of maintenance under
adverse conditions, there is the fact that each conveyor
system continually requires elongation as mining proceeds
and the mining face moves further and further away from
the extraction plant. .
The adverse conditions of which mention has been made
include not only the wide extremes of temperature to which the
equipment is exposed, but also relate to the nature of the ore
itself. Tar sands material tends to adhere to the mining and
transport equipment and to form lumps. The adherence problem
can significantlv decrease the capacity of the equipment.
Sometimes the adhering material will break loose and pile up,
preventing further proper operation of the equipment. This
tendency is increased during periods of frost ~hen lumps of
ore can freeze solid. If rubber belts are used, the bitumens
may react chemically with them.
Selection of the best mining techniques should also aim
at carrying out the mining operation as quickly as possible
to minimise the length of time for which the ore is exposed
to frost in winter. In summer, ot1ner problems arise, especially
the stability of the ore and anv e~uipment that is resting on
the ore, if such equipmen~ generates stresses due to vibration
` ~ or other movement. 1~7hen a solid deposit of tar sands material
is disturbed it can change into a sticky viscous mass, the
~ behaviour of which can be further varied by exposure to sun-
; ~ ~ light, rain, snow or simplv to changes in air temperature.
Current mining schemes also require construction of large
dykes within the mine~ out area, which are built at ~reat ex-
pense bv dewatering and compacting tailings sand. The fines
; from the oil sand combine with ~ater to form a sludge requiring
large pond capacity to contain it. Due to the
: `
.. ,... .~ . .
:, . ~ .; '. . :
,
-
- .. - :
. . -
. : . ' ., ., ,:
- :"
~1~3387
-- 4 --
sludge formation, the space required for tailings is 40 %
greater than the original volume of the mined out pit.
The draglines have proven to be poor selective miners.
Consequently large amounts of lean oil sand layers high in
undesirable fines are included in the plantfeed. This
raises ore transport and increases tailings transport
cost, and requires more dike construction to contain the
increased volume of sludge generated.
The mine layout and methods of operation proposed by
the methods and apparatus disclosed herein have been
designed to minimise the adverse impact of many of these
inherently unfavourable factors and thus to provide a
scheme that will be an improvement over previous tar sands
open pit mines, both in respect of the all important issue
of capital costs and in relation to economical and
reliable operation.
The present invention consists of a method of open pit
mining for tar sands ore comprising forming at least two
adjacent expanding mining sectors extending radially away
from a common dump station, including the steps of: (a)
removing overburden from each of said sectors while
leaving a track of unremoved overburden extending between
such sectors, at least some of such overburden being
employed to form a dike extending around the periphery of
at least a first one of said sectors, ~b) mining ore from
said first sector, and (c) subse~uently discharging
tailings from a processing plant into such mined sector
while continuing to mine ore from an adjacent sector.
Other features of the system described herein are
claimed in the parent application and in the divisional
Canadian applications Serial Nos. 391, 396 and 391,397 filed
simultaneously herewith.
An embodiment of the present invention is illustrated
by way of example in the accompanying drawings, in which
.
'
~4338~7
- 4a -
Figure 1 is a schematic plan view of a mine layout;
Figure 2 is a portion of Figure 1 showing initial
operations;
Figures 3 and 4 are views similar to Figure 2 at later
sta~ges in the development of the mine;
Figure 5 is a larger scale plan view of a portion of
the layout seen in Figure 4 showing details of further
development;
Figures 6, 7 and 8 are respectively sections on 6-6;
7-7 and 8-8 in Figure 5;
Figure 9 is a development of Figure 8;
Figure 10 is a section on 10-10 in Figure 5;
Figure 11 is an end perspective view looking down one
of the cuts developed in accordance with the earlier views;
Figure 12 is a view showing use of a mobile conveyor
for reaching beyond the limits of dragline capability; and
Figure 13 is a partial plan view illustrating the
construction of an in-pit dike from center reject.
It should be mentioned that the illustration of open
pit mines on a small scale presents a number of
difficulties that render it impracticable and/or
misleading to adhere
.
. . .
, , - ' . - `
:
33~
strictly to the conventions that apply to normal mechanical
engineering drawings. Consequently, in the drawings that
accompany this description some licence has been taken with
the normal rules of drawing, with a view to enhancing clarity
and avoiding showing details that are unimportant to the
inventive concepts.
Figure 1 shows a tar sands ore body 10 that has been
chosen for mining for reasons with which the present invention
is not concerned, e.g. relatively high grade ore, general
accessability of the ore body and other factors. For con-
venience of illustration the ore body 10 has been assumed to
be approximately a semi-circular segment, although obviously
a rectangular body can be chosen and, in practice, natural
conditions will normally dictate a less regular boundary to
the ore body than that shown diagrammatically in Figure 1.
An important consideration in choosing the limits of mining
sectors within an ore body is the placing of limits to
coincide with uneconomical mining zones, so that as much of
the sector perimeters as possible will not require dikes to
protect them from disposed tailings, as is necessary in order
to permit future mining. To provide an indication of the
dimensions typically involved, the radius of the body 10 of
Figure 1 will be of the order of several kilometers in length~
Situated at a location approximately central of the semicircle
there will be set up a processing (extraction) plant 11 which
receives its intake of ore from a common dump station 12.
As a preliminary step in getting the mine into pro-
duction, the overburden and a series of strips of ore will
be removed to form elongated cuts along strips 13 radiating
generally from the common dump station 12. Figure 1 shows
three such strips 13, but this number may vary up or down.
Moreover, in practice, it may be neither feasible nor de-
sirable to form each of the cuts simultaneously. Nevertheless,
one of the important merits of the present method resides in
- 35 the provision of at least two (and preferably more) such
strips that can be in production simultaneously. Hence, for
convenience of description, it will be assumed that the
sectors are formed and subsequently mined approximately
,~
.
: - :
338~
-- 6 --
simultaneously, even though the mining progress in one
sector will normally be somewhat ahead of that in another
and so on. There will normally be at least some measure of
overlapping operation of the sectors.
The angle of divergence between the cuts is shown in
Figure 1 as approximately 60. This angle can be made
smaller or larger, e.g. from 45 to 90, but if it is de-
creased or increased too much, some of the advantages of the
arrangement are lost or become less pronounced.
]0 Each cut is made initially as a slot in the overburdenusing conventional equipment such as large power shovels
and front end loaders loading into a fleet of trucks to
form peripheral spoil dumps 14 which may act as tailings
dikes, as shown in Figure 2, and later described in more
detail. This figure shows the initial formation of a cut
13a, which is approximately half formed by removal in the
radially outward direction of the overburden layer to form
the dump 14. Reference numeral 15 indicates the sloping
edges of the remaining overburden material beside that
which has been removed to reveal the upper surface 16 of
the ore body. In the same figure, the cut 13b is shown in
a later stage of development, all the overburden having
been removed and a first cut having been made into the ore
body. Reference numeral 17 indicates the sloping side
edges of the partially excavated ore body, numeral 17a is
the sloping front edge that is being actively mined to form
the full length of the cut and 17b is the ramp up to the
ground level for removal of the mined ore.
- Figure 3 shows the same cuts at later stages in their
development. Considering first the cut 13a, it will be seen
;that thè first layer ~bench) of ore has been removed for
the full length of the cut and that a second bench beneath
the first is in the process of being removed. The working
face is shown at 18, numeral 19 designating a ramp for
removal of the mined ore up to ground level. The other
portion of Figure 3 serves to illustrate the situation for
the cut 13b when the working face 20 for the second bench
down has reached nearly the end of the cut and work has
~,
.
- ' ': :
.
.
.,
:
.
33~3'7
-- 7 --
begun on widening the cut in the area designated 21. More
specifically this view shows a dragline 22 serving to re-
move the overburden and a shovel 23 removing material from
the ore body. This overburden is dumped at 24 along the
bottom of the mine, while the ore is transported to the dump
12 by trucks 25. The cut 13b also has ramps 26 and 27 for
use by the trucks removing overburden and ore from the
respective sides of the cut.
It will be understood that each cut will thus be
1~ widened and deepened on both sides of its centreline by the
taking of successive long~tudinal cuts, the ore being taken
from mining faces that extend approximately transverse to
the longitudinal direction rather than from long faces.
Figure 4 shows later stages in the development of the cuts
13a and 13b, various working faces being shown at 28 to 31
and various ramps at 32 to 38. In some instances, these
ramps may be extensions of shorter ramps formed in the
initial stages, before the mine reached its present depth;
in other cases, new ramps will be formed. The specific
locations of the ramps will, to some extent, be at the dis-
cretion of the engineer controlling the mine development,
but one feature of the ramps is of special importance. It
will be observed from Figure 4 that the ramps all lead on
to one or other of a series of roads or tracks 40 that
2~ radiate from the central area of the ore body adjacent the
dump 12. In Figures 3 and 4 these tracks 40 are shown ex-
tending the full radial distance out from the centre point
to the peripheral spoil dumps 14, but, if preferred, they
can be elongated only as needed. As each cut is widened,
eventually the full length of each track 40 as shown in
Figure ~ will be needed, and the tracks 40 will serve to
define the boundaries between the sectors of the mine.
This use of a common track 40 by the trucks serving
adjacent sides of a pair of adjacent cuts, combined with
3~ the arrangement of the cuts (and hence the tracks) radiating
` from a common dump area provides an important reduction in
the total length of roadways that have to be provided. It
,...
~,
~ ~ '
.
- ~:
:
:,
`` 1~433~
-- 8 --
also shortens the distances that have to be travelled by the
trucks, compared with conventional mine layouts, and hence
represents an important economy. Large capacity trucks, e.g.
350 ton trucks, will be used, in order that the number of
trucks required for the fleet will be kept comparatively low
with consequent limitation to the labour costs involved.
The road 40a seen in Figure 4 is a dragline walkback
road. When the dragline gets to the end of a cut it climbs
up from its chopcut level and walks back to the plant end of
the pit. Such walkback roads will be constructed about every
300 metres and will also serve as haul roads to carry reject
from the upper mining level to the pit end where such reject
could be incorporated into a pit and dike above ground or
dumped into the end of the pit in a dike if neéded to protect
a future extension of mining beyond the present pit perimeter.
Access to the walkback road in the reject haulage case would
be via cross roads from ramps out of the side of the pit.
Figure 5 shows a portion of one of the cuts on a some-
what larger scale, including sections illustrated in Figures
6 to 10 of various operations that take place at various
stages in the development of such a cut or cutso
Figures 5 and 6 combine to show removal by a dragline
41 of overburden 42 and the dumping of such overburden
material at 43.
Figures 5 and 7 combine to show the removal of ore
from an ore face 44 by means of a shovel 45 loading into
trucks 46 that then travel to the dump via a ramp 47 and one
of the tracks 40.
Figures 5 and 8 show similar removàl of ore from an
ore face 48 of a second bench, the trucks using ramp 49 and
a track 40. Any centre reject encountered can be dumped at
50 and moved towards the centre of the cut as shown at 51.
In areas where spoil is cast by dragline onto a ramp left
from mining of the previous cut, the configuration of the
final spoil dump, including centre reject, will vary depend-
ing upon various factors, such as whether the centre reject
is hauled directly to its final position or whether dumped
off the edge of the mining bench, or whether the dragline
~,
~1~33~3~7
g
rehandles centre reject and overburden from a position on
top of the centre reject, or whether from positions on the
lower end of the access ramp.
Figures 5 and 9 show subsequent movement by the dragline
52 of overburden and other waste material from area 53 to
area 54 to provide access to the toe 55 of the ore body,
this being a necessary preliminary to the cutting of a third
bench in the portion of the ore body shown at 56.
In those sites having a significant amount of centre re-
ject, e.g. the proposed Alsands project, the dragline willgo down to the level where the centre reject occurs and, be-
cause it is generally thick and continuous, the dragline
will cast the centre reject directly to the waste rather
than as shown in Figure 8 w~ere it is dumped and then
rehandled as in Figure 9. This is a further important saving,
although there will be times when Figures 8 and 9 will apply.
The actually mining of the Alsands pit will probably be
done in seven lifts to reach the pit bottom, i.e.
Lift No, 1. - dragline casting into pit - thickness
average 7M.
Lift No. 2&3 - 30 yd. hydraulic shovels separating thin
layers of reject and ore. Each lift about
13~1.
Lift No. 4. - 50 yd. electric shovel mining ore averaye
l9M thick.
, Lift No. 5. - dragline mining reject - direct cast into
pit, average 9M thick but up to l9M in some
places.
Lift No. 6. - 20 yd. hydraulic backhoe digging below track
level so as to stay off bottom acquifer sand.
Thickness variable - minimum 3M.
The significance of the 20 cu. yd. hydraulic backhoes
which mine the pit floor is that they permit high production
rate removal of the final pit bottom ore without requiring
the equipment to travel on the basal aquifer sand. They are
travelling on the same rock road laid down for the lowest
50 yd. shovel operation.
Figures 5 and 10 show how ore can be removed from a
3~8~7
-- 10 --
portion of the mine near a track 40, this portion not being
initially mined because of the need for the access ramps,
e.g. the ramp 47, to have a certain length.
Figure 11 provides a more pictorial representation of
some of the typical situations shown in Figures 5 to 10,
and, in particular, shows mining at first and second bench
levels, respectively, i.e. at the ore faces 60 and 61.
Ramps 62 and 63 enable the trucks to reach tracks 40. The
right hand side of the cut shown in Figure 12 illustrates
an area 64 that must be mined by working back towards the
track 40 (Figure 10), an operation that is shown completed
on the left hand side of Figure 11.
The significance of the above-mentioned economy of
roadway length becomes especially clear when it is realised
that the physical properties of the ore often render it
inadequate as a surface that can be relied upon to support
moving and vibrating machinery, such as the trucks and the
crawler-mounted power shovels used in forming the strips.
It is necessary to provide such equipment with a firm
surface on which to work and travel and hence, as the mining
of each strip proceeds, a roadway will be progressively
established in advance of the equipment. Such roadways
are shown in Figures 7, 8 and 9 at 65. Each such roadway
can be constructed in situ of rock from the overburden or
limestone pit floor, or from gravel deposit, or it can take
the form of heavy prefabricated mats that are trucked to
the mine face and laid down ahead of the advancing equip-
ment. Use of such rock roadways is not only desirable in
summer, when the equipment needs firm support, but also in
winter when, even if not needed for support, it provides a
frost protection blanket over the oil sands ore.
The other schemes for mining tar sands ore that have
hitherto been adopted or proposed use draglines for mining
the ore and lifting it to the surface where it is rehandled
by bucket wheel excavators and conveyors. The method pro-
posed herein puts the equipment down into the ore body,
mining in lifts (benches) rather than raising the ore to
the surface. The method uses large power shovels to mine
~ ~3
... :.
.
:i
: . ~ .
3387
-- 11 --
the ore and load it directly into trucks, thus eliminating
a rehandling step. This method also facilitates improved
selection of ore and rejection of waste, because the
operator of the shovel is working right at the mining face
instead of remotely far above it.
Another advantage of using shovels for mining the ore
is that this arrangement provides the freedom to blast
lightly each bench ahead of the mining operation to reduce
excavation costs, tooth wear and other maintenance factors.
This is not possible with the dragline to surface method,
because the high slope mined all at once, on which the
dragline must sit, becomes quite unstable if blasted.
The present scheme also has the advantage that it is not
seriously affected by a deep ore body. The draglines can
go do~m to lower levels when required and thus can clear
overburden and centre reject from the ore toe in any amount
and condition normally encountered.
The present method also provides a much improved likeli-
hood of a continuous supply of ore to the dump and hence the
extraction plant, without the need for the duplication of
equipment that is necessary when using interdependent equip-
ment. A few extra trucks will be provided as replacements
when others are temporarily out of service, but there is no
need to duplicate the fleet in the manner that is necessary
when conveyors are used.
The present method proposes to use large hydraulic shovels
in the upper portion of the oil sand where there are many
thin layers of ore and reject interbedded. The hydraulic
shovels, due to the characteristics of their digging
operation, are able to selectively mine layers of one metre
or less, thus sending only high quality feed with low fines
content to the extraction plant.
An important element in the present method is the use of
a firm surface at the working level of each bench so that
the shovels and trucks have adequate support. While the
literature comments on the problem presented by the physical
properties of tar sands material, the practical approach to
the problem in the past has been to use low bearing pressure
~ ~N
' ., ~ , . .
'
~:
1~3387
- 12 -
equipment such as draglines and bucket wheels.
It has been found in practice that the draglines fre-
quently cannot reach to the bottom of tihe ore body, and in
fact often fall far short of it. This is because the
limited horizontal reach o' the draglines is inadequate to
reach the toe of tle excavation slope when a stable angle
of repose is excavated. Also the digging angle for the
bucket becomes unfavorable beyond certain depths, and, at
still greater depths, the limits of cable lengths are ex-
ceeded. T.~hen the excavation slope angle is increased tofacilitate dragline reach, the slopes often fail, thus
causing mined ore and the underlying bank to collapse back
into the pit. This results in production losses, and
permanent ore losses as they flow beyond the limit of reach
for recovery. A two bench dragline operation may alleviate
certain problems, but then all overburden must be excavated
and transported around the pit using bucket wheels, conveyors
trippers and spreadexs or mobile equipment, at much greater
cost than direct disposal of the waste materials from the
face to the pit. Adoption of the present method of nutting
the equipment down directly on the ore body surface, while
rendering such surface firm by one OL the means discussed
above, allows direct single handling of ore, instead of
100~ rehandling, and direct handling of overburden and
centre reject to the mined out pit, or at worst with some
conditions at the greatest depth, partial low cost dragline
casting of rehandled material.
~ In the present method the rock used in constructing
; ~ the roads on each bench level is lowered from bench to
bench as the face is advanced by 22 cu. yd~ draglines.
Alternativel~, the material can be lowered by a hydraulic
shovel lifting the rock off the top of its advancing face
(which was the floor of the previous excavation level) or
by thé hydraulic backhoes sitting above the advancing face.
Another method which can be used is to employ the large
dragline excavating the thick centre reject zone. It will
lift the rock off the top level of its excavation and place
it on the bottom of its excavation zone.
............ . .
,- . : ,
. . . .
~' . ~ - -~' .. ' .
,
- ' ~ .
331~
- 13 -
A very valuable aspect of the presently proposed method
resides in the economy arising out of the radial layout
feeding common roads or tracks. Initially, mining takes
place in the vicinity of the dump, but, as mining and
widening progresses, the locations where the cuts open into
each other will recede from the dump. As this happens, there
is formed and progressively extended between each pair of
cuts a common track (or road or ramp) that can be used by
the trucks serving both cuts. When it is borne in mind
that, generally speaking, all tracks used as roadways will
require to be provided with a firm surface to avoid rutting
of the surfaces and delays arising from trucks sinking into
the ore body, it will be apparent that shortening of the
haulage routes will represent a large reduction in con-
struction and operating costs. The radial system herein
disclosed represents a significant shortening of such routes,
when compared with those necessary to haul ore out of con-
ventionally laid out open pit mines employing independent
parallel cuts (see, for example, Canadian patent No. 982,162
issued January 20, 1976 to Robert D. Hendry).
The radial roads laid out remain in the same location
for many years and consequently are amenable to setting up
trolley lines, so that, as the haul distances increase on
the main road, electric drive trucks may switch to electricity
from the power grid, rather than using their own diesel
electric power.
It is known to rotate the working face of an open pit
mine about a central collection point at which a crusher is
located. See, for example, United States patent No. 4,103,972
issued August 1, 1978 to Boris J. Kochanowsky. The problems
;; ~ solved by this prior arrangement (so-called "screw-mining
method") are essentially different from those faced in the
handIing of tar sands ore in the present method. The main
transportation system used by Kochanowsky consists of a
series of conveyors operating in underground tunnels and
these would be subject to the difficulties discussed above,
if they were used for the transportation of tar sands ore.
While the screw-mining method uses trucks to transport the
':
.. i .
...... . . . .
: ' '
:, ' ~
.. . . - : :.
,. ..
~ :
.
191 3;~37
- la _
ore the short distances between -the mining face and the
turning point area, the major transportation distances are
taken care of by the conveyors, and thus the important
feature of the present method, namely shortening of truck
haulaye routes, was not of concern to Kochanowsky. In
addition, it will be noted that in the screw-mining method
the face being mined is a long radial face. sy contrast, in
the present method, in which two or more separate and
diverging cuts are formed substantially simultaneously and
are subsequently caused to open to a common track, the
mining faces each extend basically transversely to the
longitudinal direction of the respective cut, an arrange-
ment that is much better suited to a tar sands mine in view
of the very extensive area involved and the much larger
volumes of ore (as well as overburden) that must be handled.
Another advantage of the presently disclosed system
lies in the mobility of the trucks at the dump station
where the ore is reclaimed for the extraction plant. In
existing tar sands mining systems, the plant feed line
storage is limited by the inflexibility of the stacker
system. On the other hand, trucks can dump anywhere so
that a long dumping area becomes possible, including dumping
from a high protective embankment with heated sides and,
if necessary, overhead radiant heaters in winter. A large
number of feed points using reclaim tunnels easily allows
any desired quantity of line storage without limiting the
feed quantity and introducing the related problems that occur
ir. the present systems.
Another advantageous feature of the new method is its
mobility. It can be moved relatively cheaply from area to
area to take advantage of smaller bodies of high grade ore.
Another feature of the present system is the manner in
which it facilitates an improved procedure for the handling
of tailings. The processing plant 11 produces large
quantities of tailings (a sand and water slurry), indeed
the volume of these tailings exceeds the volume of the
original ore by 40~ due to the compact nature of the latter
r` J
.
.
. ' .
'
. .
3387
-- 15 -
and the production of cla~ and water sludge. Disposal of
the tailings is a major problem in tar sands mines and hence
the economies that the present method affords in this
resard are important. To aid the explanation of the present
method Figures 3 and 4 show schemes in which the two
widened cuts or "sectors" having diverging walls are mined
approximately simultaneously. In practice, it may be ad-
vantageous to develop one sector predominantly in advance
of another, while maintaining at least some overlap of
operations to optimise utilisation of the equipment. In
any event, it is possible and indeed likely, that a first
sector will be completely mined out while the other sectors
are still being developed. This situation enables tailings
from the processing plant to be discharged directly into
the adjacent end (i.e. radially inner end) of the mined-out
sector without the need to construct expensive standard
dikes built from tailings. This is because the selective
mining of thin layers of lean tar sand from the upper beds
by hydraulic type shovels provides dike material hauled by
truck and placed within the perimeters of the sectors as
mined, thus providing containment dykes for the tailings
and protection of the roadway area between the cectors when
they are ultimately mined.
The ability to separate the high fines content, lean
tar sand at the mine results in a high bitumen and lo~?er
fines content feed to the extraction plant, with a consequent
substantial reduction in annual tailings volume with
considerably less production of high volume sludge.
Pipelines for discharging the tailings can be laid
along the inside of one of the common tracks. When mining
of an adjacent sector (or sectors) reaches the common track,
it too is mined.
To explain the foregoin~ more specifically in relation
to the particular layout chosen for illustration, assume
that 3 sectors are mined consecutively from right to left.
The overburden from sector 1 ~right) initial cut goes to a
dike 14 at the remote perimeter of the proposed pit. Over-
burden from the second sector later goes similarly to its
. ~ .
... . .
.
~ . . . .
. ,: , .
338t7
- 16 -
remote perimeter. When the third sector is mined, its
overburden from the centre (initial) cut goes adjacent to
the lower right hand road to form a dike. Sec~or 2 is
mined while sector l is backfilled with tailings. Sector 3
centre cut is placed adjacent to the lower right hand road
to form a dike prior to completion of mining of sector l,
provided that the area adjacent is not mineable.
During the mining of sector l lean tar sand from the
upper lean beds of the deposit are placed progressively
into the mined out pit following behind the mining advance
with a dyke wherever a face or roadway must be protected
for future mining. Upon completion of mining of sector l
the roadway between sectors l and 2 is also mined. Back-
filling by tailings proceeds in sector l by discharging of
tailings close to the plant, with pipelines fanning out and
being extended as required to fill sector l.
Dike building with reject, where necessary, proceeds
behind mining through sectors 2 and 3, with tailings being
placed in each sector after it is mined out and enclosed
by a dike.
The mining area is always protected from dike failure
by an intervening roadway of tar sand, which is not true
of tar sand mining methods employed to date. Of course
dike failure is not expected, but the safety feature is
there.
It is apparent that very short pumping distances and
low cost tailings disposal are possible by these methods,
as compared to building rectangular dyke ponds out of
densified tailings around mined out areas following
conventional tar sand mining operations.
Figure 12 shows the manner in which a mobile conveyor
70 can be used for reaching beyond the limit of reach of a
dragline, a need that may arise as the pit becomes widened.
The upper 30 metres of some portions of the oil sands
deposit contains numerous thin beds of centre reject. At
times, ideally up to 5090 of this zone should be rejected.
This volume of reject, along with surface overburden and
lower centre reject, is too great to be cleared beyond the
: _i
33~37
- 17 -
toe by draglines. In condition~ where this up~er reject
cannot be advantageousl~I used in dike construction for
future tailings containment, it will be desirable to use
the conve~or 70 which is a type of stacking or spreading
conveyor mounted on a walking dragline base, or crawlers,
and fed by a beltwagon, which in turn is loaded through
an attached hopper by a hydraulic shovel which is selectively
mining the reject and ore layers, the ore layers being
loaded into trucks for delivery to the extraction plant.
Under certain conditions such a unit may be desirable for
placing surface overburden on the pit bottom in a similar
manner. Such a method has not previously been used in oil-
sand mining and represents a method that can overcome the
reach limitations of the dragline.
Figure 13 illustrates portion of a widened sector or
pit and shows the building of a dike 71 from upper centre
reject which would be excavated by a hydraulic shovel and
hauled in by truck, either from the end of the dike which
would connect to the sector roads, or from any desired
point along the length of the dike, whenever the placing
and later removal of this connecting causeway was cheaper
than the extra cost of hauling to the end and back to the
advancing face of the dike. A hydraulic shovel has the
capability to selectively mine thin interbedded layers of
oil sand and reject, but has not to date been used for
this purpose. The pit floor level is shown at 72.
It is estimated that the capital cost of a large
scale installation such as proposed herein would be some
hundreds of millons of dollars cheaper than an installation
of the same capacity constructed in the conventional
manner. Correspondin~ savings can also be expected in
maintenance and operating costs.
.,
'
'
-' , . ` ~
' ': ':
: ~
