A detonator for explosives

Abstract

A detonator for explosives is disclosed comprising a first detonating arrangement that can fire the detonator under direction of a blast controller, and a second detonating arrangement that operates independently of the first detonating arrangement which fires the detonator when sensing a threshold external pressure has been reached. The detonator may be fired by the second detonating arrangement if the threshold pressure is reached, and if the threshold pressure is not reached, the detonator can be fired by the first detonating arrangement. The second detonating arrangement may comprise a high-speed pressure sensor that is a carbon resistor. A method of detonating a bulk explosive is also disclosed.

Description

[0001] A DETONATOR FOR EXPLOSIVES

[0002] FIELD

[0003] This invention relates to a detonator for explosives. The invention also extends to a blasting arrangement for blasting rock and a method of detonating an explosive in a blast hole.

[0004] This invention relates particularly, but not exclusively to, a detonator for detonating explosives located in a host rock in mining operations, and a blasting arrangement and method of detonating bulk explosives in mining operations. It will therefore be convenient to hereinafter describe the invention with reference to this example application. However, it is to be clearly understood that the invention is capable of broader application and may be used in all applications of the detonator for blasting explosives. For example, it may also be used in blasting explosives in for engineering works and in quarries producing rock and aggregate.

[0005] BACKGROUND

[0006] An example prior art detonator for use in mining operations is shown in Figure 1. The detonator comprises a metallic detonator body or detonator housing having a pair of leads or wires entering the detonator body and leading to a trigger in the form of a fuse head. The detonator further comprises an initiator in the form of a primary charge and a secondary charge more remote from the fuse head than the primary charge. In use, a blast signal is transmitted to the fuse head which detonates the primary charge which then, in turn, detonates the secondary charge. The energy generated by detonation of the secondary charge is then transmitted to an associated bulk explosive which supplies the energy required to detonate the bulk explosive optionally with a booster.

[0007] In mining operations, complex patterns of blast holes may be used to break up a host body of rock. A blast pattern may comprise many hundreds of blast holes that are operatively connected to a detonation circuit (either wirelessly or by wires) so that the various blast holes are blasted in a sequence according to a blasting plan. The blasting plan is designed to achieve a most effective oroptimum effective breakage of the host rock and to limit ground vibration during the blasting.

[0008] An example of one type of blasting circuit used to initiate a blast in a mining operation is shown in Figure 2. A blast controller comprises a remote control unit, which is used by operators to effect overall management and control of the blast. In one form, a bus with two main lines extends from the remote control unit to the various blast holes with explosives, one of which is shown in Figure 2. A pair of leg wires branch off from the main bus to each blast hole for transmitting blasting signals to that blast hole. Each pair of leg wires is operatively connected to a local control unit which, in turn, is connected to the detonator which supplies the energy to detonate the bulk explosive.

[0009] Instead, in another type of blasting arrangement that is different to the detonator and blasting circuit which uses wires in Figure 2, wireless detonators are used. With wireless detonators, there is no physical connection from the remote control unit to the local control unit and the detonators. The detonators are self-powered and communicate wirelessly with the control units.

[0010] In both wired and wireless detonators, a blast may be initiated by a remote control unit which sends blast signals to each local control unit which, in turn, initiates detonation of its specific associated blast hole. The blast holes are detonated in a carefully determined blast sequence and each local control unit initiates detonation of its associated detonator at a programmed time. Detonation or firing of the detonator produces shock waves in the bulk explosive that causes a rapid rise in its temperature and triggers its detonation and explosion. When an explosive is detonated in a blast hole, it generates a dynamic pressure wave that moves radially outward through the surrounding ground and rock. The dynamic pressure wave generated by blasting a blast hole can exert an effect on a nearby blast hole that has not yet been detonated.

[0011] By way of background, a bulk explosive requires sensitisation to achieve and maintain detonation. The level of sensitisation required for any given bulk explosive is proportional to the density of the bulk explosive such that a higher density requires a higher level of sensitisation for complete and effective detonation. Dynamic pressure caused by detonating a nearby blast hole acts tocompress voids within a bulk explosive and increase the density of the explosive which, in turn, reduces its sensitivity and increases the energy required to achieve its complete detonation. In some instances, a dynamic pressure wave travelling through the rock from blasting an adjacent blast hole can raise the density of a bulk explosive to a level at which its detonation does not easily occur. When this density is reached, the bulk explosive in the blast hole may not be detonated resulting in a partial or full misfire or a post blast fume.

[0012] In addition to the desensitisation of bulk explosives described above, a high dynamic pressure can cause a detonator to fail by damaging or severing the wires leading to the detonator. It can also physically damage components in the detonator causing it to fail.

[0013] Thus, a high dynamic pressure wave has a possibility of damaging a detonator so that it cannot be fired by a blast controller in sequence according to a blasting plan. This can then result in a live unexploded detonator lying on the ground possibly together with its bulk explosive. This is dangerous because some time later, an excavator or loader clearing a pile of rock may strike the live detonator causing it to detonate. For example, a detonator could be triggered by friction, or impact, with an excavation machine during clearing operations, e.g. by an excavator’s bucket or teeth striking a detonator.

[0014] In yet other cases, a dynamic pressure wave travelling through the rock surrounding a blast hole may be sufficiently large and intense to cause the bulk explosive and / or detonator in the blast hole to fire before it is fired by a blast controller. This is called a sympathetic detonation and is dangerous because it is unpredictable and uncontrolled with a myriad of different outcomes.

[0015] The scenarios discussed above pose significant safety risks and operational challenges as well as additional costs. The mining industry recognises the risks posed by misfires, fuming incidents, and sympathetic detonations, and is constantly looking for ways to mitigate these events. There is an extremely high level of sensitivity in the mining industry to these dangers and accordingly, a high priority to find a solution.Any reference to background art or other prior art in this specification is not an admission that such background art or other prior art is common general knowledge in Australia or elsewhere.

[0016] SUMMARY

[0017] Applicant believes that dynamic pressure in the ground caused by explosive blasts is responsible for sub-optimal outcomes when blasting bulk explosives in a blast hole. These outcomes include misfires, post blast fume, and potentially also sympathetic detonation. Applicant recognises that it would be beneficial if dynamic pressure generated by explosive blasts could be better managed by reducing its influence on unexploded blast holes near to the blast.

[0018] Applicant recognises that it would be beneficial if a detonator could be devised that was able to achieve reliable detonation of an associated bulk explosive in a blast hole even when it was exposed to a high dynamic pressure in the ground due to a nearby explosive blast.

[0019] In its broadest form, the invention provides a detonator that detonates when exposed to a dynamic pressure wave travelling through rock so that a bulk explosive in which the detonator is received in a blast hole is detonated.

[0020] An underlying concept is to detonate the detonator and the associated bulk explosive if a threshold external pressure is identified in the surrounding rock independently of a blast controller. The detonator is then fired to achieve complete detonation of the detonator and the bulk explosive. For example, the detonator may be detonated at a time when the dynamic pressure does not affect the detonator or the associated bulk explosive. In one form, the detonator may be detonated rapidly before the dynamic wave can damage the detonator or desensitise the explosive.

[0021] According to one aspect of the invention there is provided a detonator for explosives comprising:

[0022] a first detonating arrangement that can fire the detonator under direction of a blast controller; anda second detonating arrangement that operates independently of the first detonating arrangement which fires the detonator when sensing a threshold external pressure;

[0023] wherein the detonator is fired by the second detonating arrangement if the threshold pressure is reached, and if the threshold pressure is not reached the detonator can be fired by the first detonating arrangement.

[0024] Thus, if the external pressure does not reach the threshold pressure, the detonator will be fired by the first detonating arrangement under direction of the blast controller in a manner like prior art detonators. However, if the external pressure sensed by the second detonating arrangement reaches the threshold pressure before this occurs, the detonator will be pre-emptively fired by the second detonating arrangement independently of the blast controller.

[0025] The detonator may be fired immediately on sensing the threshold pressure. The detonator may be filed with a time delay on sensing the threshold pressure. The detonator may be fired by the second detonating arrangement immediately when sensing the threshold pressure, e.g. where the threshold pressure may damage the detonator.

[0026] For example, if a very high dynamic pressure is sensed, the detonator will be detonated immediately by the second detonating arrangement.

[0027] Instead, when sensing the threshold pressure, the detonator may be fired by the second detonating arrangement after a time delay. For example, the detonator may delay firing the detonator for a defined time interval.

[0028] The time delay may give the dynamic pressure time to pass before the detonator is fired.

[0029] Where an emulsion explosive is being used, the delay in firing the detonator may give the emulsion explosive time to re-sensitise after the dynamic pressure wave has passed.

[0030] The detonator may be programmed to set a time for firing the detonator by the second detonating arrangement. Thus, the detonator may be programmed to firethe detonator immediately after the threshold pressure is reached or after a defined time interval has elapsed.

[0031] The second detonating arrangement may comprise a pressure sensor that is a fast response pressure sensor that responds quickly, or even substantially instantaneously, to a change in sensed pressure. This enables it to rapidly fire the detonator when the threshold pressure is reached.

[0032] The fast response pressure sensor may respond within 0.01 milli-seconds to a change in pressure (called the pressure sensor response time), preferably within 0.05 to 0.09 milli-seconds, more preferably within 0.001 milli-seconds, e.g. within 0.0005 milli-seconds.

[0033] Conveniently, the fast response pressure sensor comprises a resistor, e.g. a carbon or carbon-based resistor. A carbon resistor is inexpensive and can afford to be sacrificed when the detonator is detonated in use.

[0034] The external pressure may comprise the pressure in an external environment outside the detonator. For example, the external pressure may be present in a rock body surrounding the blast hole within which the detonator is received. The external pressure may be a compressive pressure or a tensile pressure. Accordingly, where the term “pressure” is used in the specification, it will be understood to include both compressive pressure and tensile pressure.

[0035] The second detonating arrangement may comprise an electrical circuit, and the pressure sensor may be electrically connected to the electrical circuit. A change in the pressure sensed by the pressure sensor causes a change in the voltage (or potential difference) across the resistor which provides a measure of the external pressure.

[0036] Thus, a change in pressure generates a voltage signal across the electrical circuit that triggers the second detonating arrangement to detonate the detonator when the threshold pressure is reached.

[0037] The threshold pressure may be the pressure at which a bulk explosive with which the detonator is intended to be used or is used, is desensitised and cannot be reliably detonated.An increase in the density of the bulk explosive to 1200 kg / m3may desensitise some explosives such that they cannot be reliably detonated. Some explosives may be desensitised when their density reaches 1300 kg / m3or even 1350 kg / m3. The threshold pressure may be the pressure at which the explosive is at risk of causing physical damage to the detonator or wires extending away from the detonator. The threshold pressure may also be a pressure that risks causing a premature detonation of the detonator.

[0038] The threshold pressure may be 180 MPa. Preferably, the threshold pressure level may be 200 MPa, e.g. 220 MPa.

[0039] Optionally, the threshold pressure may be an external pressure which risks causing a sympathetic detonation of a bulk explosive due to a detonation of an nearby blast hole. This external pressure may be significantly higher than the pressure which causes de-sensitisation of the bulk explosive.

[0040] The detonator may comprise an initiator for energising the bulk explosive when the detonator is fired. The initiator may comprise a primary charge that is detonated first, and a secondary charge that is fired by the energy generated by detonation of the primary charge.

[0041] The detonator may comprise a trigger for firing the initiator to start or trigger the detonation process.

[0042] For an electric detonator, the trigger may comprise a fuse head. Further, for a non-electric detonator, the trigger may comprise a shock tube or safety fuse. Yet further, for an electronic detonator the trigger may comprise a microchip and capacitor. The invention can be practised with all types of detonators.

[0043] The trigger, e.g. fuse head, may be operatively connected to the blast controller by means of wires. Instead, the trigger, e.g. fuse head, may be wirelessly connected to the external blast controller.

[0044] The initiator may be operatively connected directly or indirectly to the first detonating arrangement and the second detonating arrangement.With the first detonating arrangement, the blast controller may be operatively connected, directly or indirectly, to the initiator of the detonator so that it may be triggered by either the first or second detonating arrangement.

[0045] Similarly with the second detonating arrangement, the resistor may be electrically connected, directly or indirectly, to the initiator of the detonator.

[0046] The pressure sensor may further comprise an amplifier for amplifying the voltage measured across the resistor caused for use in processing the pressure information sensed by the sensor.

[0047] The pressure sensor may further comprise a high pass filter for filtering the voltage signal from the resistor.

[0048] The detonator may further comprise a detonator body or housing within which the one or more components of the detonator are received. The detonator body may comprise a strong and robust shell that can withstand contact with other materials.

[0049] The second detonating arrangement may be received in the detonator body. The detonator in this aspect of the invention may include any one or more features, or combination of features, of the detonator defined in any other aspect of the invention.

[0050] According to another aspect of the invention there is provided a detonator for explosives comprising:

[0051] a detonating arrangement having a pressure sensor for sensing an external pressure in matter around the detonator,

[0052] wherein the detonator is fired if the external pressure sensed by the sensor reaches a threshold pressure.

[0053] The external pressure may be a pressure outside the detonator that impinges on the detonator. The external pressure may be a transient dynamic pressure wave transmitted through rock surrounding the detonator by an explosion in proximity to the detonator.

[0054] The detonator may be pre-emptively fired in response to the threshold pressure being sensed independently of a blast controller.The detonator may be fired immediately on sensing the threshold pressure or with a time delay on sensing the threshold pressure.

[0055] The pressure sensor may be a fast response pressure sensor that responds within 0.01 milli-seconds to a change in pressure, preferably within 0.05 to 0.09 milli-seconds, more preferably within 0.001 milli-seconds, e.g. within 0.0005 milliseconds.

[0056] The fast response pressure sensor may be a resistor, e.g. a carbon resistor, that indicates a change in voltage in a response to a change in pressure.

[0057] The threshold pressure may be 180 MPa. Preferably, the threshold pressure may be 200 MPa, e.g. 220 MPa.

[0058] The detonator in this aspect of the invention may include any one or more features, or combination of features, of the detonator defined in any other aspect of the invention.

[0059] According to yet another aspect of the invention there is provided a blasting arrangement for blasting rock comprising:

[0060] at least one blast hole formed in a rock;

[0061] a bulk explosive received within the / each blast hole; and

[0062] a detonator as defined in any aspect of the invention received in the / each blast hole for initiating detonation of the bulk explosive when the detonator is fired. Thus, the detonator can be fired in response to sensing a threshold external pressure independently of a blast controller or, where the threshold pressure is not reached, the detonator can be fired under the direction of a blast controller. The detonator may be embedded within the bulk explosive such that the bulk explosive at least partly surrounds the detonator, e.g. fully surrounds the detonator.

[0063] The bulk explosive may be an emulsion explosive, for example, a gas sensitised emulsion explosive comprising gas bubbles that are compressed when the explosive is subjected to dynamic pressure.

[0064] The emulsion explosive may further comprise a physically sensitised emulsion explosive, e.g. comprising plastic or glass micro-bubbles.The dynamic pressure may increase the density of the emulsion explosive due to its gas bubbles being compressed which reduces its sensitivity and thereby increases the energy required to detonate the explosive.

[0065] Once the dynamic pressure wave has passed and the external pressure is reduced, the air bubbles decompress and their density decreases restoring the sensitivity of the bulk explosive. This restores its sensitivity to detonation.

[0066] The bulk explosive may be a packaged explosive. A packaged explosive may comprise an emulsion explosive contained in a package that can be placed in a blast hole. The packaged explosive may contain other components that contribute to the performance of the explosive. For example, the packaged explosive may contain physical sensitisers such as plastic or glass microparticles. It may also contain gas bubbles for sensitising the explosive.

[0067] Instead, the bulk explosive may be a solid explosive comprising solid prills, e.g. solid ammonium nitrate prills.

[0068] Solid prills may be crushed when they are subjected by dynamic pressure in a surrounding rock which increases their density and desensitises them. However, they are generally not able to expand back to their original density when the dynamic pressure wave has passed and the pressure in a surrounding rock has normalised, and consequently the desensitisation of solid prills is not reversible. Thus, if solid prills are desensitised, their sensitivity cannot be restored and they may misfire.

[0069] The blast hole may have an open end and a closed inner end remote from the open end, and the detonator may be positioned closer to the closed inner end than the open end, e.g. the detonator may be positioned adjacent to the closed inner end.

[0070] Applicant has established that the dynamic pressure in a blast hole is greater towards the inner end of the blast hole than the outer end and positioning the detonator towards the closed inner end will cause it to fire when the dynamic pressure at that position reaches the threshold level. This will occur before a part of the bulk explosive that is closer to the open end is desensitised because the dynamic pressure is lower in that position.The blasting arrangement may further comprise a blast controller for controlling the detonation of explosives in each blast hole with the first detonating arrangement.

[0071] The blast controller may comprise a local control unit associated with each detonator, and a remote control unit in communication with each local control unit having overall control of the blasting operation. The remote control unit may have a capability to fire the detonator / s and explode the bulk explosives in the plurality of blast holes.

[0072] The local control unit may communicate with the remote control unit by a bus leading from the remote unit, and the bus may have a pair of leg wires extending from each blast hole to the bus. Instead, in another form, the local control unit may communicate wirelessly with the remote control unit using a wireless protocol.

[0073] The blasting arrangement for blasting rock may comprise a plurality of blast holes formed in the rock, each of which has a bulk explosive received therein, and the blast controller may control the sequence and timing of the detonation of explosives in each of the plurality of blast holes.

[0074] The at least one blast hole formed in a rock may be drilled in the rock, e.g. using rock drills in the manner typically used in mines.

[0075] Optionally, each blast hole may comprise a plurality of detonators and the detonators may be spaced apart from each other in a direction of length of the blast hole.

[0076] The detonator may include any one or more features, or combination of features, of the detonator defined in any other aspect of the invention.

[0077] According to yet another aspect of the invention there is provided a method of detonating an explosive in a blast hole (e.g. a bulk expolsive), the method comprising:

[0078] placing a detonator within the explosive in the blast hole, the detonator comprising:

[0079] a first detonating arrangement that can fire the detonator under direction of a blast controller; anda second detonating arrangement that operates independently of the first detonating arrangement which fires the detonator when sensing a threshold external pressure has been reached; and

[0080] firing the detonator with the first detonating arrangement under the direction of the blast controller unless the second detonating arrangement senses the threshold pressure, in which case the second detonating arrangement independently fires the detonator (i.e. before the first detonating arrangement does so).

[0081] The detonator may be fired immediately on sensing the threshold pressure or with a time delay on sensing the threshold pressure.

[0082] The second detonating arrangement may have a response time of less than 0.01 milli-seconds to a change in pressure, preferably within 0.05 to 0.09 milliseconds, more preferably within 0.001 milli-seconds, e.g. within 0.0005 milliseconds.

[0083] The threshold pressure may be a pressure level that increases the density of the explosive to a point at which the explosive is desensitised to a density at which it cannot be reliably detonated.

[0084] The threshold pressure may be a pressure level that increases density of the bulk explosive to 1300 kg / m3. In some forms, the threshold pressure may be a pressure that increases the density of the bulk explosive to 1400 kg / m3.

[0085] The threshold pressure may be 180 MPa, e.g. a transient dynamic pressure wave or pulse in the surrounding rock that is 200 MPa or above.

[0086] The method of detonating a bulk explosive in a blast hole may comprise placing the detonator in the bulk explosive closer to a closed inner end of the blast hole than an outer end. Preferably, the detonator is placed adjacent to the inner end. The method may include firing the detonator with a blast controller, e.g. including a remote control unit of the blast controller.

[0087] Firing the detonator may comprise detonating or exploding a bulk explosive within which the detonator is received.The detonator may include any one or more features, or combination of features, of the detonator defined in any other aspect of the invention.

[0088] According to yet another aspect of the invention there is provided a method of resisting a misfire of a detonator and a bulk explosive in a blast hole formed in rock, the method comprising:

[0089] sensing the pressure in the rock at the blast hole, and

[0090] when the sensed pressure reaches a threshold pressure, detonating the detonator independently of detonation by a blast controller.

[0091] The pressure may be in the form of a transient pressure wave caused by a detonation of explosives in another blast hole, e.g. in proximity to the detonator and bulk explosive in said one blast hole.

[0092] The step of detonating the detonator when the sensed pressure reaches a threshold pressure may occur separately and independently of a blasting plan that is implemented by a blast controller.

[0093] In the normal course of events, the blast controller would detonate the detonator and the bulk explosive in accordance with a blasting plan. However, when the threshold pressure is sensed in the surrounding rock, the detonator intervenes and independently detonates the detonator and bulk explosive separate from and outside of the blasting plan.

[0094] The pressure sensor may be a fast response pressure sensor that can fire the detonator before a pressure wave from another explosion can damage the detonator or desensitise the bulk explosive.

[0095] The pressure sensor may be a fast response pressure sensor, e.g. a carbon resistor, that responds within 0.01 milli-seconds to a change in pressure, preferably within 0.05 to 0.09 milli-seconds, more preferably within 0.001 milliseconds, e.g. within 0.0005 milli-seconds.

[0096] In one form of the invention, the threshold pressure may be 200 MPa, in which case, the detonator is fired when the sensed pressure reaches this level.

[0097] The detonator may include any one or more features, or combination of features, of the detonator defined in any other aspect of the invention.According to yet another aspect of the invention there is provided a detonator for resisting a misfire of a bulk explosive in a blast hole formed in a rock, the detonator comprising:

[0098] a pressure sensor for sensing an external pressure in the rock around the blast hole, wherein the pressure sensor causes the detonator to fire when the sensed pressure reaches a threshold pressure, independently of a blast controller.

[0099] The detonator is detonated in this way so that the dynamic pressure in the rock does not adversely affect the detonator and / or bulk explosive and / or interfere with its detonation and explosion.

[0100] The pressure sensor may be a fast response pressure sensor that responds within 0.01 milli-seconds to a change in pressure, preferably within 0.05 to 0.09 milli-seconds, more preferably within 0.001 milli-seconds, e.g. within 0.0005 milliseconds.

[0101] The fast response pressure sensor may be a carbon- or carbon-based resistor. The detonator may include any one or more features, or combination of features, of the detonator defined in any other aspect of the invention.

[0102] BRIEF DESCRIPTION OF THE DRAWINGS

[0103] A detonator and a blasting arrangement for blasting rock in accordance with the invention may be produced in a variety of forms. It will be convenient to hereinafter describe in detail some embodiments of the invention with reference to the accompanying drawings. The purpose of providing this detailed description is to instruct persons having an interest in the subject matter of the invention how to carry the invention into practical effect. However, it is to be clearly understood that the specific nature of this detailed description does not supersede the generality of the preceding summary section. The Detailed Description refers to the accompanying drawings, in which:

[0104] Figure 1 is a schematic drawing of a basic detonator of the type that is known in the prior art;Figure 2 is a schematic drawing of a part of a blasting system for blasting a sequence of blast holes at a mining site;

[0105] Figure 3 is a schematic drawing of a blast hole formed in the ground containing a bulk explosive with a detonator positioned therein;

[0106] Figure 4a is a schematic drawing of an example blast arrangement for blasting rock and / or ore at a mine site and Figure 4b shows a part of the blast arrangement in more detail;

[0107] Figure 5 is a diagrammatic drawing of a first blast hole fitted with two detonators and two pressure sensors alongside a second blast hole that is exploded prior to the first blast hole, and showing the effects of the blast in the second blast hole on the adjacent first blast hole;

[0108] Figure 6 is a schematic diagrammatic drawing of a detonator showing some of the components thereof and how they are operatively connected to each other in accordance with one embodiment of the invention;

[0109] Figure 7 is a basic drawing of a detonator in accordance with one embodiment of the invention for locating in a blast hole with a bulk explosive;

[0110] Figure 8 is a schematic drawing of a blast hole containing a plurality of layers of bulk explosive that are separated by layers of stemming; and

[0111] Figure 9 is a schematic drawing showing how dynamic pressure from an explosion in a lower bench generates dynamic pressure that is sensed in a top bench.

[0112] DETAILED DESCRIPTION

[0113] Figure 1 is a schematic illustration of a detonator 1 of the type known in the prior art. The detonator 1 comprises a detonator body 2 comprising a metallic shell and optionally a reinforcing element radially inward of the metallic shell. The detonator 1 contains a trigger comprising a fuse head 3 that is operatively connected to a local control unit that is in turn operatively connected to two electrical leads 5 leading out of the detonator body 2. The electrical leads 5 are typically then connected to an external remote blast control unit that manages the initiation of the detonator 1 in a manner that is known in the art of explosives.The detonator 1 has an initiator comprising a primary charge 6, e.g. lead azide, in proximity to the fuse head 3 that is detonated by the fuse head 3. The detonator 1 further comprises a secondary charge 7, e.g. PETN, that is detonated by the primary charge 6. When the secondary charge 7 is detonated by the primary charge 6, the energy is transferred to the bulk explosive in the blast hole, optionally through a booster around the detonator, to detonate bulk explosive surrounding the detonator 1 in a blast hole.

[0114] Figure 2 is a diagrammatic representation of a detonator 1 and a blasting circuit for detonating explosives in a blast hole of the type known in the prior art. The detonator 1 is operatively connected to a local control unit 11 that is specific to that specific detonator 1. Two leg wires 5 are operatively connected to the local control unit 11 and lead out of the detonator body 2 to a bus or wiring harness 13. The term “wiring harness” is used more typically in Australia while the term “bus” may be used more commonly in other countries. The bus 13 is operatively connected to a master control unit 15 that is typically remote from the blast holes. The detonator and blasting circuit in Figures 1 and 2 have been described above in the Background section of the specification and do not form part of the invention.

[0115] Figure 3 is a schematic drawing of a typical blast hole formed in the ground containing a bulk explosive in which the blast hole is indicated generally by reference numeral 10.

[0116] A detonator 1 is positioned within the bulk explosive 14 towards a blind or inner end of the blast hole 10. A stemming 16, in the form of gel stemming or aggregate stemming, is placed on top of the bulk explosive 14 to confine its blast energy and direct it outward into the surrounding rock 17 to fracture and break up as much of the rock 17 as possible. The detonator 1 detonates the explosives upon receiving a signal from a remote blast control unit (not shown) that is managed externally, e.g. by a blast manager or blast operator. When initiated or triggered, the detonator 1 generates sufficient energy to sensitise the bulk explosive 14 in the blast hole 10 and cause it to detonate. As shown in the drawing, the detonator 1 is positioned deep within the bulk explosive 12.Figure 4a is a schematic drawing of an example blast arrangement for a blasting operation at a mine site. The blast arrangement has a blast pattern indicated generally by reference numeral 38 and comprises many blast holes 10 arranged in an array across a surface of the rock 17 to be blasted. Figure 4a shows more than one hundred blast holes that can be blasted in a single blasting operation. Figure 4b shows a part of blast arrangement in Fig 4A in greater detail. As shown in Figure 4b, the explosives in the various blast holes 10 are operatively connected to a remote blast controller 40 by a bus 42 having leg wires 5 branching off the bus 42 to each of the blast holes 10. The blast controller 40 determines the sequence in which the blast holes 10 of the blast arrangement in Figure 4a will be fired and the time delay between blasting of each of the blast holes 10.

[0117] Figure 5 is a schematic drawing of two blast holes adjacent to each other in a body of rock. In a typical blasting program, different blast holes are timed to detonate at different times and not simultaneously. The reason for detonating the blast holes sequentially is to break up the host rock in the most efficient manner and with the best use of the explosives. Consequently, there is typically a time delay between the detonation of adjacent blast holes.

[0118] In Figure 5, the blast hole on the left is indicated by the reference numeral 10. Another blast hole positioned on the right of the blast hole 10 is indicated by the reference numeral 20. In a blasting plan setting out the sequence in which the various blast holes are blasted, the blast hole 20 may be programmed to blast at a time that is earlier than the blast hole 10. The earlier or prior detonation of the blast hole 20 generates a dynamic pressure that travels outward through the rock in all directions, including in the direction of the unexploded blast hole 10. The pressure wave or pressure pulse which is depicted by arrows 21 in the drawing travels through the host rock to the adjacent blast hole in a short space of time. The pressure wave from the blast hole 20 that reaches the adjacent unexploded blast hole 10 exerts a pulse of high pressure on the detonator 12 and the bulk explosive 14 within the blast hole 10. The dynamic pressure can cause one or more of the following events which potentially have serious consequences:• Misfire of bulk explosive in the unexploded blast hole because it is desensitised by an increase in density;

[0119] • Fuming of the blast hole due to incomplete detonation because the bulk explosive is desensitised by an increase in density;

[0120] • Damage to the detonator such that it no longer works;

[0121] • Severing a leg wire or line from the controller to the detonator so that the detonator cannot be fired by the control unit; and

[0122] • Sympathetic detonation of the bulk explosive in the unexploded blast hole if the dynamic pressure wave is particularly high.

[0123] As discussed in the Background section, the level of sensitisation required for a bulk explosive is proportional to the density of the bulk explosive and the higher the density, the higher is the level of sensitisation that is required. A higher pressure increases the density of the bulk explosive which, requires more energy to detonate the bulk explosive.

[0124] In test work conducted by the Applicant, they identified that one bulk explosive is desensitised when it reaches a density of 1400 kg / m3and cannot be detonated at this level. During blasting, it is therefore important that the density of a bulk explosive in a blast hole does not reach this threshold density.

[0125] Applicant has established that a dynamic pressure caused by blasting an adjacent blast hole moves through the ground as a pressure wave and can raise the density of a bulk explosive above 1400 kg / m3thereby impacting its ability to detonate. Applicant has also identified that a dynamic pressure wave in the surrounding rock greater than 200 MPa can damage a detonator such that it cannot fire the explosive.

[0126] In some test work conducted by the Applicant, a first pressure sensor 30 was placed in a lower region of the bulk explosive and a further pressure sensor 32 was placed at a higher level in the blast hole as shown in Figure 5. The dynamic pressure wave sensed by sensors 30 and 32 is shown in Figure 5. The pressure wave sensed by the lower sensor 30 has a greater amplitude than the pressure wave sensed by the upper sensor 32. The test also showed that the pressure wave has the same duration at its emission in the blast hole which detonated as it does when it arrives at the adjacent blast hole 10.Figure 6 is a diagrammatic representation of some components of a detonator in accordance with one embodiment of the invention showing how they interact with each other.

[0127] The detonator is indicated generally by the reference numeral 50 and comprises a detonator body in the form of a metallic shell housing (not shown) surrounding and enclosing the other components.

[0128] The detonator 50 comprises an initiating arrangement for initiating detonation of the detonator 50 and cause detonation of an associated bulk explosive. A trigger in the form of a fuse head is triggered by an electrical signal. The initiating arrangement further comprises an initiator comprising a primary charge, such as lead azide, and a secondary charge that is PETN that is triggered or initiated when the primary charge is fired. These stages may lead to the generation of sufficient energy and temperature by a detonator 50 to detonate an associated bulk explosive.

[0129] The detonator further comprises a first detonating arrangement or detonating mode that is operatively connected to an external blast controller for detonating the explosive in accordance with a blasting plan. Thus, the first detonating arrangement can fire the detonator under the direction of an external blast controller on receiving a signal from the blast controller. The first detonating arrangement is operatively connected to the initiating arrangement and specifically the fuse head thereof, so that it can fire the detonator under direction of the external blast controller.

[0130] The detonator further comprises a second detonating arrangement or detonating mode that operates independently of the first detonating arrangement. The second detonating arrangement comprises a pressure sensor 54 that is operatively connected to the fuse head of the initiating arrangement. The pressure sensor 54 senses when the external pressure in the surrounding rock reaches a threshold level, and response thereto triggers detonation of the explosive in a blast hole.

[0131] This detonation by the second detonating arrangement is separate from, and independently of, detonation initiated by a blast controller (being either a localcontrol unit and / or a remote control unit). In this embodiment, the pressure sensor 54 comprises a carbon resistor or carbon-based resistor that provides a high-speed response time which enables the second detonating arrangement to detonate the detonator before it is affected by the dynamic pressure wave in the rock. The carbon resistor 54 is inexpensive which helps to make the solution viable because the resistor is sacrificed when the detonator is detonated. The second detonating arrangement includes an amplifier 58 for amplifying a voltage signal received from the carbon resistor 54 and a high pass filter 59 for filtering the voltage signal from the resistor 54 to enable the voltage signal to be suitably processed.

[0132] The second detonating arrangement further comprises an electric circuit that electrically connects the components discussed above including the carbon resistor 54 to each other and to the fuse head of the detonator. The detonator 10 is fired by the second detonating arrangement when the voltage signal in the electrical circuit generated by the pressure wave indicates that a threshold external pressure has been reached.

[0133] A carbon resistor can function as a pressure sensor primarily through a piezoresistive effect, where its electrical resistance changes in response to mechanical stress or deformation. Its resistance decreases as external pressure increases which allows it to function as robust, low-cost gauges. When physical pressure is applied to a carbon resistor, the resistor’s physical dimensions change. Compressing the material typically decreases its length and increases its cross-sectional area, which reduces its electrical resistance. In standard carbon composition resistors, conductive carbon particles are embedded in a non-conductive binder (like clay or resin). External pressure forces these particles closer together, creating more conductive pathways and significantly lowering the overall resistance. Further, in advanced carbon materials like carbon nanotubes (CNTs) or graphene, pressure directly alters the material’s atomic structure and electronic bandgap, changing the mobility and density of charge carriers.

[0134] Carbon-based sensors (especially those using carbon fibre or nanotubes) can detect minute pressure changes. This makes them useful as pressure sensorsuseful in high-pressure and high-velocity environments like explosive testing where they can measure shock waves and detonation pressures from explosives. Carbon resistors are robust and can survive and function at extreme pressures ranging from 3 GPa to 8 GPa, where many traditional high-precision sensors would be destroyed. Further, they can be used for in hole pressure measurements in mining because their low cost enables them to be expendable. When a carbon resistor is used in an electric circuit, it may be positioned as one arm of a Wheatstone bridge. This allows for a precise detection of a very small change in resistance by converting it into a measurable output voltage. Typically, the voltage output signal from the Wheatstone bridge is small and may be amplified by passing it through an amplifier and then processed through a data processor to obtain a readable pressure to obtain a pressure value.

[0135] In use, the detonator 50 can be placed in a bulk explosive within a blast hole like that shown in Figure 3. The detonator may be positioned deep within the bulk explosive, preferably towards the closed end of the blast hole, in a position where it can suitably energise and detonate the bulk explosive when it is activated or triggered. The blast hole itself forms part of a blasting arrangement like that shown in Figure 4 where there are literally hundreds of blast holes that are designed to blast in sequence.

[0136] When a blast hole in the blasting arrangement is detonated prior to its adjacent blast holes, it generates a dynamic pressure wave that passes through the rock and bears on the detonator and bulk explosive received in the adjacent blast hole. If this dynamic pressure reaches a threshold pressure level, the carbon resistor 54 will register a voltage signal which causes the second detonating arrangement to fire the detonator. The carbon resistor 54 provides a very fast response time which may be used to fire the detonator 50 before the dynamic pressure has time to adversely impact either the detonator 50 or the bulk explosive. Typically, the detonator 50 may fire and achieve effective detonation of the bulk explosive before it is desensitised. However, if no dynamic pressure at or above the threshold level is sensed by the carbon resistor 54 during the blasting event, then the blast hole will be detonated by the blast controller through the first detonating arrangement in accordance with its blasting plan. This thereby helps to obtainfull detonation of the detonator 50 and bulk explosive in each blast hole and avoid misfires and fuming events.

[0137] Figure 7 is a basic schematic drawing of a cross-section of a detonator in accordance with one example embodiment of the invention.

[0138] The detonator 50 which is contained within a detonator body or housing 2 comprises some of the components of the detonator in Figure 1. The second detonating arrangement of the detonator 50 comprises a carbon resistor 54 forming a pressure sensor that is electrically connected to a fuse head 57. The second detonating arrangement can detonate the detonator 50 independently of a blast controller when the threshold pressure level is reached in the external rock. As shown in the drawing, the pressure sensor 54 is housed in the detonator body or housing 2 and is positioned such that it can sense external pressure in the rock.

[0139] The detonator 50 includes an initiating arrangement comprising the fuse head 57 introduced above, a primary charge, and a secondary charge 58. The detonator further comprises an electronics assembly 55 and a material 56 within the housing 2 surrounding the electronics assembly 55 that are also shown in the drawing.

[0140] Applicant has made some calculations of the speed of travel of a dynamic pressure wave due to an explosive in a blast hole. The pressure wave travels through the rock from the blast hole in which the explosive is detonated at a speed which depends on the VOD (velocity of detonation). Applicant has calculated that a 1.6 MPa pressure wave travels 12.5 m in 12 milli-seconds. Therefore, a fast response detonator with a shorter response time may be used to fire the detonator before the pressure in the rock increases to a level that causes a misfire.

[0141] Figure 8 is a schematic drawing of a blast hole containing a plurality of benched layers of bulk explosive.

[0142] The blast hole 100 comprises three benched layers of bulk explosive 102, 104, and 106 with stemming 108, 110 and packed between the benched layers of bulk explosive 102, 104, 106 and a further stemming 112 on top of the outermostexplosive layer 102 closest to the surface. Each of the benched layers is separated by a layer of stemming 108, 110, or 112 so that the effect of the blast is confined to the bench within which it is received. In practice, the layers of bulk explosive are benched to break up a rock layer while protecting valuable coal seams or mineral layers between the rock layers in a blasting operation.

[0143] As shown in the drawing, each bench or layer of bulk explosive has a detonator 50 contained therein and, in operation, the different benches or layers are detonated at different times according to the blasting plan. This sequential blasting of the different layers or benches results in dynamic pressure caused by detonation of one layer travelling to the other layers. If the dynamic pressure exceeds a ceiling level of 1400 kg / m3, this can cause a desensitisation of an explosive in a blast hole that leads to the problems described above.

[0144] Applicant has found that the use of a detonator as described above with reference to Figures 6 and 7 can avoid desensitisation of a bulk explosive caused by an earlier detonation of an adjacent bench by detonating the detonator with the second detonating arrangement before this occurs. This way the detonator can help to avoid misfires and fuming in benched layouts.

[0145] Figure 9 is a schematic drawing showing how dynamic pressure from an explosion in a lower bench of a benched blast hole generates dynamic pressure that is sensed in a top bench of the same blast hole. This can cause problems like those discussed above with reference to Figure 5 where the earlier detonation of one bulk explosive occurred in an adjacent blast hole rather than in the same blast hole.

[0146] By using the detonator described above with reference to Figures 6 and 7, the detonator can be caused to detonate when the external pressure in the rock reaches a threshold or predetermined level before the bulk explosive is desensitised.

[0147] Applicant has conducted some basic test work on benched blast holes by placing a pressure sensor in a top bench of a benched borehole containing at least two separate benches of explosive. The bottom bench was detonated and the sensor in the top bench registered a pressure wave caused by the detonation in the lowerbench. The pressure wave that was sensed in the top bench is graphically illustrated in Figure 9.

[0148] To summarise, in the mining industry, a detonation of a blast hole causes a wave of dynamic pressure which can influence the detonation and blasting of unexploded blast holes that are nearby. More specifically, the dynamic pressure generated by a blast can damage the detonator in a nearby unexploded blast hole so that it does not fire. It can also desensitise a bulk explosive in the nearby blast hole.

[0149] The detonator described in the detailed description with reference to the drawings senses a dynamic pressure wave in the surrounding rock above a threshold pressure level can which inhibit detonation of the bulk explosive or damage the detonator. The detonator can then respond to sensing the threshold pressure and initiates detonation of the detonator which detonates the bulk explosive. This helps to reduce the incidence of misfires and fuming events.

[0150] Applicant is aware of some prior art detonators for detonating bulk explosives. However, they are all fired under the direction of a blast controller in accordance with a blasting plan and do not have any other modes or arrangements for detonation independent of the blast controller. The detonator described in the Detailed Description with reference to the drawings is different because it has a separate detonating arrangement or mode that triggers detonation of the detonator and bulk explosive before a dynamic pressure wave adversely affects the detonator or bulk explosive.

[0151] This detonator has a wide application in quarries, mines and civil engineering works, and Applicant believes it has the potential to be a breakthrough in the detonator arts.

[0152] Another advantage of the Applicant’s detonator is that it can withstand a high dynamic pressure wave without strengthening the detonator body. Modifying a detonator body to withstand high dynamic pressures will inevitably cause additional expense and may result in trade-offs or compromises in the operation of the detonator. It also does not address the issue of desensitisation of the bulk explosive due to the dynamic pressure. The Applicant’s detonator thereforeprovides a more holistic solution which deals with many of the issues caused by dynamic pressure.

[0153] Another advantage is that the Applicant’s solution does not involve changing the sensitivity of the bulk explosives that are used. Such an approach may require some trade-offs and is likely to entail considerable additional cost. Further, it does not deal with some of the problems caused by dynamic pressure discussed above.

[0154] The detonator described above with reference to the drawings provides a solution for emulsion explosives, and for solid prill explosives. Where the product is a gas sensitised emulsion and the pressure rises, the detonator can fire immediately or fire when the emulsion density gets back to normal. In the case where the explosive is a prill / emulsion blend, there will be a different threshold pressure for each product, and the detonator will be programmed to fire immediately. This is only applicable on mines with wet conditions or soft ground.

[0155] Some of the concepts and terms described in this specification are discussed in more detail below.

[0156] A “detonator” is a multi-stage device designed to convert an initial signal into a high-energy shockwave capable of initiating less sensitive secondary explosives. Most detonators, whether electric or non-electric, may be enclosed in a shell or body having a basic cylindrical form.

[0157] Detonators can fail in several ways, ranging from external physical damage to complex internal electronic or chemical malfunctions. These failures typically result in a misfire (failure to detonate) or, more dangerously, an unplanned initiation. For example, excessive pressure from neighbouring explosive charges firing earlier can crush the detonator shell. Further, high impact loads during loading or from adjacent blasts can rupture the internal fuse head, fracture the bridge wire, or cause solder joints to fall off. Yet further, a ground movement from an adjacent charge can physically sever the initiation line (electric wires or shock tube) before the detonator receives its signal from a blast controller.

[0158] A detonator initiates a bulk explosive by providing an intense high-velocity shockwave that forces the stable molecules of the bulk charge to rapidlydecompose. Detonation is achieved by a multi-stage sequence of explosives that amplifies the energy generated with each step. First, an external signal (electrical current, flame, or laser) triggers a small, highly sensitive primary explosive (like lead azide) inside the detonator. The reaction of primary explosive rapidly transitions from burning to a high-velocity detonation, which then sets off a more powerful base or secondary charge (typically PETN or RDX) also within the detonator. This base charge produces a localised, high-pressure shockwave that travels through the detonator’s body or metal shell into the surrounding material. For some bulk explosives (like ANFO), the shockwave from a detonator alone is too weak, in which case the detonator is inserted into a booster which is a larger, more sensitive secondary explosive. The detonator triggers the booster, which then generates a massive shockwave powerful enough to detonate the entire bulk column.

[0159] Most bulk explosives used today are designed to be extremely stable. They are insensitive to common hazards like friction, minor impacts, or even being set on fire, making them safe to transport and handle in large quantities. However, because these bulk materials are so inert, they require a specific high-pressure shockwave, typically around 600 psi or higher to trigger the chemical chain reaction. A detonator is the only practical tool capable of generating this precise level of instantaneous energy.

[0160] Modern blasting, particularly in large scale mining operations, requires exact timing (down to milli-seconds) of each detonation to ensure rock is broken efficiently and vibrations are minimised. The detonators enable this to occur allowing engineers to command the exact moment each bulk charge fires.

[0161] A misfire in the explosives industry refers to the complete or partial failure of a blasting charge to explode or function as intended after initiation. It is considered one of the most hazardous events in mining, construction, and quarrying because it leaves active explosives or detonators in the ground or in blasted rock (the “muckpile”), posing a high risk of unplanned detonation during subsequent work. With a total misfire, the entire charge of explosive fails to ignite or detonate. With a partial misfire, only a portion of the charge detonates, leaving some explosive in the blast hole or scattered in the debris.Fuming in the explosives industry refers to the generation of toxic, non-ideal gaseous byproducts following a detonation. While a “perfect” (oxygen-balanced) detonation produces only harmless gases like nitrogen, carbon dioxide, and water vapor, real-world conditions often cause incomplete chemical reactions that release hazardous “fumes”.

[0162] Some causes of misfires and fuming include damaged shock tubes, and damaged detonators; high-pressure shockwaves from a nearby earlier blast can crush the “hot spots” (sensitisers) in an adjacent charge, making it inert (dead-pressed); and ground movement from a neighbouring charge can physically sever the initiation line (wires or tubes) before the signal can reach the detonator. Visual indications of a misfire or fume include exposed detonator wires, leg-wires, bulk explosive material in the face or muck pile, an inadequate heave or a duller “thud” when the explosive is detonated, or a noxious yellow or orange smoke (nitrogen dioxide).

[0163] Sympathetic detonation (SD or SYDET), also known as “flash over”, is an initiation of an explosive charge (the acceptor) caused by the nearby explosion of another charge (the donor). This occurs without a direct priming mechanism, such as a detonator, between the two charges. The initiation of the acceptor charge may be triggered by the following stimuli: A high-pressure shock wave may propagate through a medium (air, water, or ground) and impact the acceptor charge causing it to initiate. Shock waves travel faster and more effectively through rock than through air, increasing the likelihood of SD over greater distances. Further, highly sensitive explosives (e.g. nitroglycerine-based) are more prone to sympathetic detonation than insensitive explosives. Further, larger donor charges produce stronger shock waves.

[0164] In this specification, the term “carbon resistors” shall be understood to include carbon nanotubes and carbon fibres, and these materials may offer higher sensitivity than traditional carbon composition resistors.

[0165] In this specification, the term “comprising” is intended to denote the inclusion of a stated integer or integers, but not necessarily the exclusion of any other integer, depending on the context in which that term is used. This applies also to variants of that term such as “comprise” or “comprises”.It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. All such modifications and variations thereto, as would be apparent to persons skilled in the art, are deemed to fall within the broad scope and ambit of the invention as is set forth herein. Changes in detail or structure may be made without departing from the basic elements of the invention as defined in the following claims.

Claims

CLAIMS:

1. A detonator for explosives comprising:a first detonating arrangement that can fire the detonator under direction of a blast controller; anda second detonating arrangement that operates independently of the first detonating arrangement which fires the detonator when sensing a threshold external pressure;wherein the detonator is fired by the second detonating arrangement if the threshold pressure is reached, and if the threshold pressure is not reached the detonator can be fired by the first detonating arrangement.

2. A detonator for explosives according to claim 1, wherein the second detonating arrangement comprises a pressure sensor that is a fast response pressure sensor.

3. A detonator for explosives according to claim 2, wherein the fast response pressure sensor responds within 0.001 milli-seconds, preferably within 0.0005 milli-seconds to a change in pressure.

4. A detonator for explosives according to claim 2 or claim 3, wherein the second detonating arrangement comprises an electrical circuit, and the pressure sensor comprises a carbon resistor electrically connected to the electrical circuit.

5. A detonator for explosives according to any one of claims 1 to 4, wherein the threshold pressure is the pressure at which a bulk explosive with which the detonator is intended to be used or is used, is desensitised and cannot be reliably detonated.

6. A detonator for explosives according to any one of claims 1 to 4, wherein the threshold pressure is the pressure which risks causing physical damage to the detonator.7 A detonator for explosives according to any one of claims 1 to 6, wherein the threshold pressure is 200 MPa.

8. A detonator for explosives according to any one of claims 1 to 7, comprising an initiator for energising the bulk explosive when the detonator is fired, and the initiator comprises a primary charge that is triggered first, and a secondary charge that is fired by detonation of the primary charge.

9. A detonator for explosives according to any one of claims 1 to 8, wherein the detonator is fired immediately on sensing the threshold pressure.

10. A detonator for explosives comprising:a detonating arrangement having a pressure sensor for sensing an external pressure in matter around the detonator,wherein the detonator is pre-emptively fired when the sensed external pressure reaches a threshold pressure.

11. A detonator for explosives according to claim 10, wherein the pressure sensor is a fast response pressure sensor that responds within 0.001 milliseconds, preferably within 0.0005 milli-seconds to a change in pressure.

12. A detonator for explosives according to claim 10 or claim 11 , wherein the threshold pressure is 180 MPa, preferably 200 MPa.

13. A blasting arrangement for blasting rock comprising:at least one blast hole formed in a rock;a bulk explosive received within the / each blast hole; anda detonator as defined in any one of claims 1 to 13, received in the / each blast hole for initiating detonation of the bulk explosive when the detonator is fired.

14. A blasting arrangement for blasting rock according to claim 13, wherein the bulk explosive is an emulsion explosive.

15. A blasting arrangement for blasting rock according to claim 13, wherein the bulk explosive is a packaged explosive16. A blasting arrangement for blasting rock according to claim 13, wherein the bulk explosive is a solid explosive in the form solid ammonium nitrate prills.

17. A blasting arrangement for blasting rock according to any one of claims 13 to 16, wherein the blast hole has an open end and a closed inner end remote from the open end, and the detonator is positioned closer to the closed inner end than the open end.

18. A blasting arrangement for blasting rock according to any one of claims 13 to 17, comprising a plurality of blast holes formed in the rock each of which has a bulk explosive received therein, and a blast controller for controlling the sequence and / or timing of detonation of the explosives in each of the plurality of blast holes.

19. A method of detonating an explosive in a blast hole, the method comprising:placing a detonator within the explosive in the blast hole, the detonator comprising:a first detonating arrangement that can fire the detonator under direction of a blast controller; anda second detonating arrangement that operates independently of the first detonating arrangement which fires the detonator when sensing a threshold external pressure has been reached; andfiring the detonator with the first detonating arrangement under the direction of the blast controller unless the second detonating arrangement senses the threshold pressure, in which case the second detonating arrangement independently fires the detonator.

20. A method of detonating a bulk explosive in a blast hole according to claim 19, wherein the second detonating arrangement has a response time of less than 0.001 milli-seconds, preferably within 0.0005 milli-seconds to a change in pressure.

21. A method of detonating a bulk explosive in a blast hole according to claim 20, wherein the threshold pressure is a pressure level that increases the density of the explosive to a point at which the explosive is desensitised to a density at which it cannot be reliably detonated.

22. A method of detonating a bulk explosive in a blast hole according to claim 21, wherein the threshold pressure is a pressure that increases density of the bulk explosive to 1300 kg / m3.

23. A method of detonating a bulk explosive in a blast hole according to any one of claims 19 to 22, wherein the threshold pressure is 200 MPa.

24. A method of detonating a bulk explosive in a blast hole according to any one of claims 19 to 23, comprising placing the detonator in the bulk explosive towards a closed inner end of the blast hole.

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