GB2258481A - Mining an underground deposit - Google Patents

Description

A ) n _) ') 1 31 C- - ') L+ 1 A METHOD AND APPAPLATUS E-OR WORITNO AN

UNDERGROUND DEPOSIT BY MEANS OF A TUNNELING MACHINE The invention relates to methods and apparatuses for working an underground deposit under circumstances where the cost of extraction per ton is of secondary importance.

A particularly important application of the invention lies in extracting ores or substances that are too toxic or radioactive for it to be possible to implement conven4,.-.-4 c nal min-ing extraction techniques, particularly when there are 0 geological problems in addition to radioactivity or toxicity.

By way of particularly representative example, mention may be made of working very high grade uranium deposits where the level of radioactivity prohibits conventional methods that require personnel to be present on a permanent basis at the work face, but where a high extraction cost per ton is not prohibitive since it is counterbalanced by yields reaching 100 kg per (metric) ton, or even more.

A problem of the same kind arises with relatively thick deposits having a very high content of a substance of great commercial value., but presenting geological conditions that are particularly difficult, for example when under very high lithostatic or hydrostatic pressure in ground that is not consolidated.

As another application of the invention, mention may also be made of removing dangerous substances for decontamination and/or storage under safe conditions even if said substances are not of commercial value, but where it is essential to eradicate them.

Such circumstances may arise, for example, in the event of underground contamination from a nuclear plant.

Consequently, an object of the invention is to provide a method of working an underground deposit from which extraction is made difficult by the radioactive and/or toxic nature of the deposit and/or by geological conditions, and where the commercial value of the deposit and/or the absolute need to extract it make operating costs per extracted ton of secondary importance.

1 2 To this end, the Present invention provides, in particular, a method comprising the steps of: possibly constituting a watertight co.Efer confining all or part of the deposit by injecting a hydraulic binder and/or by freezing, and possibly draining the deposit; passing throughout the deposit from side to side by directly digging a first tunnel using a turzielina imachinc, ret-urnina the turuneling rn.;,-chin-e after passing through Lhe deposit and U-'-LQa-n a tunnel direct-ly t, 9.1 -L-Q adjacent to the fl-rst, and so on; and if the deposit conta-ins a substance that is radioactive, dangerous, or toxic, reducing the extracted material to a pulp as it is cut, and removing by hydraulic transport.

If the ground is difficult, with a risk of caving in, the gallery may be lined with lining segments immediately behind the tunneling machine. once the tunnel has been completed, it mav be immediatelv backfilled to participate in taking up ground thrust.

Depending on the kinds of difficulties encountered in working, the method will be complex to a greater or lesser extent.

When the difficulties encountered are essentially geological (high hydrostatic and lithostatic pressure), causing severe inQress of water, the problem can be solved by building a barrier in the 'Eorm of lining segments interconnected by seals. with radioactive materials.. the above measures suffice on their own to reduce numerous problems: gamma radiation of external origin is absorbed by the concrete of the lining segments and the skirt of the tunneling machine. Ingress of radon can be reduced by raising the pressure inside the tunnel.

Hydraulic transport as a slurry can be performed in a duct embedded in a shield providing protection against the gamma radiation from the extracted material.

The conditions in which a tunneling machine is used to implement the method of the invention are most unusual and appear on first sight to make economic nonsense. A tunneling machine is designed to dig galleries that are very long and continuous. In contrast, the method of the invention implies 3 long periods of inactivity after each tunnel has been dug. However, this consideration becomes secondary when the commercial value of the extracted material, or when the absolute need to eliminate said material, means that extraction must be performed, since no known solutions, although cheaper, are applicable under the circumstances outlined above.

ir.,,,,c-nti,-.n allso prov i-des apparatus for working the an-a-atus cc,, nriris-ina a tunnelina machine with a point-cutt_er in the forin of ar, excavator on a steerable telescopic arm, the tunneling machine being separable into at least three longitudinal lengths and including a crusher, a pump for pulping the material that has been cut, and for delivering it to a hydraulic transport duct, and a device for installing lining segments for lining the tunnel.

When extracting highly radioactive ore, the tunneling machine includes a skirt of sufficient thickness to reduce the intensity of the radiation. Provision is made to be able to return the tunneling machine in a chamber after passing throughout the deposit, either by rotating it through 1800 about a vertical axis, or else by disassembling it, and reassembling it, in which case the three lengths may themselves be splittable into sectors of about 1200 each.

The invention will be better understood on reading the following description of a particular embodiment given by way of non-limiting example. The description refers to the accompanying drawings, in which: Figure 1 is a theoretical diagram. in p- erspective showing the operations of preparing a fraction of a deposit, before mining; 30 Figure 2 is a diagrammatic section view showing a first horizontal tunnel being dug through the deposit; Figure 3 is similar to Figure 2 and shows juxtaposed tunnels that have been dug, lined, and backfilled during working; 35 Figure is a diagrammatic vertical section showing the main components of a tunneling machine constituting a particular embodiment of the invention; and 4 Figure 5 is a cross-section through the lined tunnel behind the tunneling machine, during one particular implementation of the method of the invention.

The method described below is usable, in particular, for working an underground deposit of very high grade uranium ore, which exists in the form of an elongate seam in rock that is ricit conso-'iid;- :ted, and that is under very high 1jydrosta-1-1c. and lithostatic pressure.

As ar, example of a deposit having such characteristics, mention may be made of the Cigar Lake deposit in Saskatchewan, Canada, having a mean content of 100 kg per ton, which means that it can be worked at a very low rate, e.g. about 200 tons per day. The surrounding rock is unconsolidated sandy sandstone, with abundant inflow of water. Working must take place at a lithostatic pressure of about 80 bars and a hydrostatic pressure of about 45 bars. The deposit is elongate in shape, being about 100 meters wide.

Before mining the deposit, the first operation to be performed seeks to form a watertight coffer around a longitudinal fraction of the deposit, which coffer can then be drained, the coffer having walls constituted by rock that has been consolidated by freezing and/or by injecting cement grouting, each coffer surrounding the deposit as closely as possible,:Ln pracdc-- at a distance cEE a faq nec---xs.

This is done by initially forming conventional infrastructure giving access to the deposit, which at Cigar Lake is at a depth of 450 meters. Access and ventilation pits are dug and power supply, ventilation, extraction, etc., installations are installed, after which horizontal galleries are dug giving access to the deposit, as are horizontal Jnfrastructure galleries. The infrastructure galleries include at least two galleries 12 extending in the long direction of the deposit, and substantially parallel to the edges thereof. They may be more or less level with the top of the deposit. At least one other gallery 14 is at a depth of a few meters deeper than the deposit.

Two palrallel cross-cuts 1-6 are then dug from the gallery 14 at distances apart that depend on the nature of the ground, but that are generally of the order of 10 meters.

- Vertical holes 18 are then drilled from the various galleries 12 and 14 and the cross-cuts 16 to receive freezing bundles, and possibly also to receive lances for injecting cement- grouting. The vertical holes drilled -f--cflt the crosscut,m 16 and passing through the deposit are always used for freezing.

when these operations and freezing from two cross-cuts have been completed, it can be seen that a coffer has been provided that surrounds a slice of the deposit. This coffer whose outline is referenced C in Figure 2 can then be drained since its central portion has not been frozen. It is from this portion that extraction is performed.

The inside of the coffer is mined by repeating the same sequence of operations. Starting from the galleries 12, two sloping access ways 20 are dug through the frozen rock to gain access to reversing chambers 22. The reversing chambers 22 (Figure 2) are formed at. a suf f icient distance from the ore itself so that the level of radioactivity therein remains acceptable for human activity. A tunneling machine such as described below is assembled in one of the reversing chambers 22 situated immediately above the base of the deposit. The tunneling machine is then put into operation in a direction that is horizontal or oblique relative to the gallery 12 extending towards the other reversing chamber. It passes through a wall of sterile material and then through the deposit until it reaches the opposite reversing chamber 22. In this chamber it can be checked and maintained, by replacing the components that are most liable to wear or to breakdown. It is then moved back up the slope to the gallery 12. The first tunnel is backfilled. A new sloping access way is formed to a reversing chamber situated immediately adjacent to the previous reversing chamber. The sloping access way and the preceding chamber are backfilled, e.g. using the material extracted to form the new sloping access way. The tunneling machine can then be advanced to dig a second tunnel.

6 Once all of the successive tunnels in the length at base level B (Figure 3) have been dug and then backfilled, inclined access ways at a lower slope are then formed to process the higher levels of the deposit in succession.

Because of the prior confinement in a watertight coffer, problems due to ingress of water are avoided. Providing the truinel is lined with lining segments as the tunneling machine a_nd providing seall s are providled between successive rings of segments, and providing the turineling machine itself is watertiaht, the effect of freezing is enhanced and it is ble to work in fragile ground at great depth suffering possL from severe ingress of water under pressure.

while a first coffer is being treated in this way, the following coffer may be prepared, possibly by extending the galleries 12 and 14, by digging cross-cuts 16, and by freezing and/or injection so as to establish a new coffer.

The method concerning the actual getting of the ore is described in greater detail further on.

The tunneling machine used may have the general structure shown in Figure 4, comprising three longitudinal lengths 28, 30, and 32 suitable for being assembled together and for being disassembled. Each of these lengths may itself be designed to be dismountable into a plurality of angular sectors, e.g. three sectors, so as to reduce the size and the weight of components that cannot be taken apart during transport. For the same reason, a compact tunneling machine will generally be used, having a skirt of small diameter, e.g. about 4 meters (m). Each of its three lengths may also be relatively short, e.g. having a length of about 1.50 m.

The front length includes a skirt and constitutes an assembly that can be directed by actuators or jacks 34, which assembly is closed at its rear end by a watertight partition provided with observation ports (the partition could alternatively belong to the middle length 30). The length 28 carries a point-action cutter apparatus constituted in the example shown by a telescopic arm excavator 36 terminated by a cutting disk 38 actuatable to project from the length 28. The 7 excavator is associated with a block-breaking apparatus 39 serving to reduce blocks to a size of a few centimeters, should that be required, and with a device for injecting water under pressure. The ore that has been cut in this way is thus conveyed in the form of fragments that are small enough in size to be entrained by a conveyor, as described below.

The front length 28 is advantageously designed to recei4vc a removable diaphragm 41 suitable for isolating the cutting device from the work face after the telescopic arm 316 ',,as been retracted, thereby making mechanical intervention possible. The diaphragm may be connected to a safety cap 40 provided at the top of this length.

The middle length 30 constitutes the pusher. Consequently it comprises, inside the skirt, a ring of hydraulic actuators 42 (e.g. twenty-four actuators) each having a plunger 44 designed to bear against lining segments that are already in place. This length also carries a conveyor for transporting the ore and constituted in the example shown by a sloping extractor screw 46. The screw raises the fragments into a crusher 48 which reduces them to a grain size that is small enough to enable them to be transported in the form of a pulp. A circulation-feed mixer-pump unit 50 takes the crushed ore and forces it through a non- return valve 52 to the inlet of a tube 54 connected to a transport duct. The central length 30 may also include a hydraulic unit 56 providing the hydraulic fluid under pressure required for driving the actuators, the pump excavator, and possibly also the transport screw, the crusher, and the pump.

The rear length 32 also includes a sealing skirt 58 slidable within the most recently installed lining segment and for providing sealing. It may also contain an erector for installing lining segments, which is not described since it may be of conventional structure.

In the embodiment shown, the erector is designed to install successive lining segments 62 after the plungers 44 have been retracted, which segments are conveyed by tackle 64 carried by a beam fixed to the rings of lining segments as the 8 tunneling machine advances. The segments may be arranged in the manner shown in Figure 5, i. e. there nW be- five tnn segments, numbered 1 to 5, a key 68, and two counter keys 70.

The tackle may be designed also to install lengths 72 of flooring, each length of flooring extending axially over the same length as a lining segment, and serving to provide a flat floor andR also to define passages. one of the passages --n.ay receive a duct 74 for conveying the ore and connectable to a telescopic tube 76 that connects with the tube 54. Other LU passages 78 may be used for various utilities, e.g. for receivinq cables.

Depending on the nature of the ore extracted, additional precautions of varying severity need to be taken. In particular, water seals may be interposed between the lining segments in the same ring and between successive rings of lining segments and between the last ring and the skirt 58, thereby avoiding inflows, and in a uranium mine avoiding the lined tunnel being invaded by radon dissolved in water. A second seal, e. g. a sheet of lead may be provided to absorb the gamma radiation from the surrounding rock, with which there is no direct communication in any event. Injection holes 8 may be provided through the installed lining segments.

An airlock is advantageously provided at the entrance to the gallery in sterile rock. This makes it possible to raise the pressure in the gallery, thereby further countering infiltration of radon from the ore.

Control of the advancing tunneling machine does not require personnel to be present inside the tunnel, unless an accident occurs. A video camera may be provided in the middle length to film the working face. Control personnel can thus remain in the reversing chamber. The presence of personnel is required only for installing the lining segments after each advance through a length equal to the length of the segments. This stage takes up less than one hour on station. It takes place while the personnel is protected by the rings of lining segments that have already been installed and by the steel of the skirt of the tunneling machine. Lining segments having a 9 thickness of about 30 cm provide satisfactory protection. The steel duct contained in the flooring 72 provides protection against radiation from the pulped ore conveyed by the water along the tubes 42 and 44 and along the duct 74.

Since each pass of the tunneling machine takes place over a short period of time, maintenance performed each time the bui.L,.ie--,-'Ln- machine occupies, t reversin th j- k chaniber makes e. rsk or17 breakdown very low. Nev,:rtheless. in the event of necessity, the diaphragm in the front compartment car, be installed quickly and mechanically and monitored by a workman who remains in the middle length. The diaphragm then isolates the compartment contained in the front length from the working face. After the equipment has been decontaminated and the radon contained in the compartment has been pumped out, the operator can enter said compartment through a sealed door provided in the partition and can perform the action required.

On leaving the ore, the tunneling machine passes through a sterile portion of rock before reaching the reversing chamber. Cutting through sterile rocK cleans the front structures and the skirts of the tunneling machine. Such cleaning may be associated with decontamination using water.

Once the tunneling machine has reached the reversing chamber, it is turned through 1800 to drill a new tunnel in the opposite direction. It can be turned about simply by providing a turntable in the reversing chamber. Reversing may also be performed by disassembling 1Ehe lengths of the tunneling machine and then reassembling them- in the reverse condition. For example, a tunneling machine that is 4 meters in diameter having a point-acting cutter using a diamond carrying ring can 30 easily cut 10 tons per hour with a speed of advance of 0.20 m/hour in some kinds of groiind. The lengths of flooring may themselves be 21. meters wide and they may receive a transport duct having a diameter of 150 mm. Stacking of circular tunnels would leave about 9% of the deposit in place. However in practice the cutting apparatus sweeps through a zone of larger section than the tunneling machine and such excess cutting suffices to work the deposit almost completely.

1

Claims (1)

1/ A method of working an underground deposit by means of a tunneling machine, characterized by the steps of: traversing through the deposit from side to side by directly digging a first tunnel using a tunneling machine; reversing the tunneling machine after passing through the deposit and digging a tunnel d-Lrectlv ad'acent to th and so on; an,', if the de-oosit j L, lle - -L - - contains a substance that is radioactive., danaerous,....r toxic, reducing the extracted material to a pulp as it is cut and removing it by hydraulic transport.

2/ A method according to claim 1, further lining the tunnel with rings of lining segments behind the tunneling machine.

0 3/ A method according to claim 2, further comprising connecting the lining segments together and to the skirt of the tunneling machine by waterproof seals.

4/ A method according to any preceding claim, characterized in that the tunnel is isolated by an airlock and the pressure in the tunnel is raised.

S/ A method according to any one of claims 2 to 4, further comprising removing the extracted material by hydraulic transport in a duct having a retractable tube (76) provided in the tunneling machine and lengths of duct (74) contained in flooring (72) installed on the bottom lining segments.

6/ A method according to any preceding claim, characterized in that after passing through the deposit, the tunneling machine is reversed in a reversing chamber (22) on a turntable or by being disassembled and then reassembled.

7/ Apparatus for working underground deposits, the apparatus comprising a tunneling machine with a pointcutter in the form of an excavator on a steerable telescopic arm, the tunneling machine being separable into at least three longitudinal 11 lengths (28, 30, 32) and including a crusher (48), a pump for pulping the material that has been cut, and for delivering it to a hydraulic transport duct (42, 44, 74), and a device for installing lining segments (62) for lining the tunnel.

8/ Apparatus according to claim 7, characterized in that the front lerig"-1b- (28) con.tai ns the excavator and is separate-d by a sea-led partition from the rrtidd"Le lengthl (30,1 which _includes a lifting conveyor, the crusher (48) and the pump, and in 1Chat- the rear length (32) contains an erector for installing prefabricated lining segments.

9/ Apparatus according to claim 7 or 8, characterized in that the middle length contains a ring of hydraulic advance actuators (42); a conveyor (46) for taking the ore to the crusher (48), said pump, and a telescopic tube (54) that connects with said duct.

10/ A device according to claim 7, 8, or 9, characterized in that the front length (28) is designed to receive a diaphragm and is separated from the middle length by a sealed partition.

11/ A method of working an underground deposit by means of a tunneling machine substantially as hereinbefore described with reference to the accompanying drawings.

12/ Apparatus for working underground deposits substantially as hereinbefore described with reference to the accompanying drawings.