CA1143387A - Open pit mining for tar sands - Google Patents

Abstract

Abstract:
The layout of an open pit mine for tar sands ore facilitates the handling of the ore by employing two or more cuts that diverge from each other while extending away from a common dump adjacent a processing (extraction) plant.
Successive parallel and adjacent strips of overburden and ore are removed, whereby each cut is widened and deepened.
As a result, each adjacent pair of cuts open to a common track. Initially, this takes place in the vicinity of the dump, but, as mining proceeds a track (or ramp) is formed that is common to the two cuts while leading from each cut to the dump. This provision of a common track shortens the haulage routes involved. The ore is mined with large power shovels and loaded into large trucks, this equipment working down on the ore body. For stability, a firm surface (roadway) is formed on the ore to support the shovels and trucks. A
major advantage of the system is flexibility of movement between pits and lack of restriction in mining depth and much improved ability to separate ore and reject. The handling of tailings is also substantially facilitated, and the over-all capital cost of a large scale installation, as well as the operating and maintenance costs, can be expected to be hundreds of millions of dollars cheaper than an installation of the same capacity constructed in the conventional manner.

Description

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 :

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338~7 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 : `
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~1~3387 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 .
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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 ,~

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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 ~, .
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33~3'7 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 ,...
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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 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 ... :.

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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

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1~3387 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.

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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 ':
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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

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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 . ~ .

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338t7 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 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.

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Claims (3)

Claims:

1. A method of open pit mining for tar sands ore comprising forming at least two adjacent expanding mining sectors ex-tending 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) subsequently discharging tailings from a processing plant into such mined sector while continuing to mine ore from an adjacent sector.

2. A method according to claim 1, wherein three such sectors are cleared of overburden and mined substantially consecutively, the second sector being mined while the first sector is filled with tailings and the third sector being mined while the first and second sectors are filled with tailings.

3. A method according to claim 1 or 2, wherein the removal of overburden and the mining of ore from each sector is carried out by initially removing longitudinal strips extending radially along such sector from the dump station and subsequently widening the sector by removing overburden and ore from further strips adjacent and parallel to the initial strips, whereby adjacent sectors open to a track of unremoved overburden common to such sectors, said track providing a route for hauling ore from both sectors to the dump station.