AU2017265077A1 - Improved Detonation Pressure Method and Apparatus - Google Patents

Abstract

A melt-cast based explosive booster; the booster yielding an equivalent or improved in-hole initiating performance compared with similar mass commercial boosters; the improved initiating performance dependent on the generation of a planar, or quasi-planar, detonation front at the output face of the booster, wherein the quasi-planar detonation front at the output face of the melt-cast based explosive booster is dependent on selected geometry of the booster, or a selected geometry of a shaped transfer element, or a combination of booster geometry and shaped transfer element geometry, and wherein the selected geometry of the booster includes a separation distance between the explosive end of the detonator of the booster and the output face of the cast volume. {---Top surloace x Transfer Element- - Side surfo.ce t -,o Detonating Cord/ Through Booster aperture 2ottoin £urface FIe Bgstng cap2 aperture Fig. 2

Description

IMPROVED DETONATION PRESSURE AND APPARATUS TECHNICAL FIELD

[0001] The present invention relates to explosive technology and, more particularly although not exclusively, to explosive boosters.

BACKGROUND

[0002] TNT (trinitrotoluene) is used in a vast majority of commercially available boosters, having the unique properties of melting at 80°C, but not becoming thermally unstable until over 200°C. It thus offers a wide margin for error, and hence safety. Owing to TNT’s wax-like melting properties it is used as an energetic binder in compound mixes that add-in higher energy, higher melting point explosives, than TNT, to enhance the performance of the finished article.

[0003] PETN (Pentaerythritol tetranitrate) is one of the more common additives to TNT for booster fill. TNT/PETN blends are commonly referred to as pentolite. Compared to TNT, pentolite has the advantage of improved sensitivity to initiation, velocity of detonation and detonation pressure.

[0004] The disadvantage of pentolite is that its increased sensitivity also makes it much more sensitive to accidental initiation. It also does not have the thermal stability of pure TNT. Essentially, if pentolite is in the molten state then it is decomposing. If held in the molten state for many hours there is a risk of progression to fume off (decomposition releasing oxides of nitrogen) which could then progress to a spontaneous detonation. With increased temperature, which lowers the viscosity of the molten brew and aids processing, the safe time exposure envelope decreases.

[0005] RDX (Cyclotrimethylenetrinitramine / Cyclonite / Hexahydro-l,3,5-trinitro-l,3,5-triazine) offers higher thermal stability than PETN, but the finished RDX/TNT (hexolite) blends are not as sensitive. The lower sensitivity increases process safety, but the finished booster is not innately detonator sensitive. To improve the reliability of detonation transfer from a detonator (i.e. blasting cap) there are various existing mixture and article adaptions that may be introduced (eg. US8127682 incorporated herein by reference, and US4945808). Figure 3 of US8127682 shows a common booster manufacturing method based on a melt-pour casting operation. It is very common that such an inserted transfer element (a bottle containing powdered explosives or a pressing) is used in booster manufacture. To do without such an insert requires a blend of pentolite that uses a high percentage of PETN (typically > 50%). In the interests of the most economical cost of manufacture (which includes process safety) it is typically preferable to use an inserted transfer element over a high PETN percentage.

[0006] A disadvantage in using RDX and PETN is that both are expensive in comparison to TNT.

[0007] A further disadvantage is that for transport in compliance with the UN DG regulations, RDX & PETN have to be wetted (PETN not less than 20% water, RDX not less than 15%.). Transport is prohibited if not desensitised by water. Hence an addition to the booster manufacturing process is either a drying (which then means the handling of a static and impact sensitive material has to be managed) or a wet process has to be developed. A disadvantage with a wet process is that a TNT contaminated water results, which is a toxic waste disposal/treatment issue. A solvable issue, but an additional process and expense.

[0008] It is an object of the present invention to address or at least ameliorate some of the above disadvantages.

Notes [0009] The term “comprising” (and grammatical variations thereof) is used in this specification in the inclusive sense of “having” or “including”, and not in the exclusive sense of “consisting only of’.

[00010] The above discussion of the prior art in the Background of the invention, is not an admission that any information discussed therein is citable prior art or part of the common general knowledge of persons skilled in the art in any country.

SUMMARY OF INVENTION

[00011] Accordingly, in one broad form of the invention, there is provided a melt-cast based explosive booster; structured so as to generate, in use, a planar, or quasi-planar, detonation front at the output face of the booster.

[00012] Preferably, the quasi-planar detonation front at the output face of the melt-cast based explosive booster is dependent on selected geometry of the booster.

[00013] Preferably, the quasi-planar detonation front at the output face of the melt-cast based explosive booster is dependent on a selected geometry of a shaped transfer element of the booster.

[00014] Preferably, the quasi-planar detonation front at the output face of the melt-cast based explosive booster is dependent on a combination of booster geometry and shaped transfer element geometry.

[00015] Accordingly, in yet a further broad form of the invention, there is provided a melt-cast based explosive booster; the booster yielding an equivalent or improved in-hole initiating performance compared with similar mass commercial boosters; the improved initiating performance dependent on the generation of a planar, or quasi-planar, detonation front at the output face of the booster.

[00016] Preferably, the quasi-planar detonation front at the output face of the melt-cast based explosive booster is dependent on selected geometry of the booster, or a selected geometry of a shaped transfer element, or a combination of booster geometry and shaped transfer element geometry.

[00017] Preferably, the selected geometry of the booster includes a separation distance between the explosive end of the detonator of the booster and the output face of the cast volume.

[00018] Preferably, the selected geometry of the shaped transfer element includes an explosive lens that partially or completely surmounts the explosive end of the detonator of the booster.

[00019] Preferably, the combination of booster geometry and shaped transfer element includes a minimisation of the separation distance between an explosive end cap of the detonator of the booster and the output face of the cast volume to achieve the quasi-planar detonation front at the output face of the booster.

[00020] Preferably, the selected geometry includes a ratio of the separation distance to an effective diameter of the booster equal to or greater than 3:5.

[00021] Preferably, a bulk of the melt-cast explosive is pure TNT; the booster yielding at least an equivalent in-hole initiating performance to similar mass boosters containing PETN or RDX; the effective diameter based ratio of the separation distance between the end cap of the detonator and the surface of the cast volume being equal to or greater than 3:5.

[00022] Preferably, the transfer element includes an explosive lens that partially or completely surmounts the explosive end of the detonator of the booster.

[00023] Preferably, the selected geometry of the transfer element includes an axially symmetric near-parabolic depression that functions as an explosive lens.

[00024] Preferably, the melt-cast based explosive bulk mixture is more energetic than pure TNT; the booster yielding an improved in-hole initiating performance to similar mass commercial boosters; the booster geometry including an effective diameter based ratio of a separation distance between the explosive end of the detonator and a surface of a cast volume being equal to or greater than 3:5.

[00025] Preferably, the melt-cast based explosive bulk mixture is more energetic than pure TNT; the booster yielding an improved in-hole initiating performance to similar mass commercial boosters; the transfer element including an explosive lens that partially or completely surmounts the explosive end of the detonator of the booster.

[00026] Preferably, the pure TNT booster has a volume defined by an axi-symmetric bottle of length L and equivalent diameter D (twice the square root of the result of the division of the cross-sectional area by Π.) [00027] Preferably, an other than pure TNT booster has a volume defined by an axi-symmetric bottle of length L and equivalent diameter D (twice the square root of the result of the division of the cross-sectional area by Π.) [00028] Preferably, the explosive volume is cast into the axi-symmetric bottle; the casting including an encapsulating cavity for the detonator and a tunnel for detonator wiring.

[00029] Preferably, the casting encapsulates a transfer element located at the blind end of the encapsulating cavity for the detonator.

[00030] In another broad form of the invention, there is provided a method of attaining an equivalent booster yield from a pure TNT explosive booster; the method including the step of meltcasting TNT into an axi-symmetric bottle; the casting including a detonator encapsulating cavity in which a ratio of a separation distance between the explosive end of a detonator fitted to the detonator encapsulating cavity and an upper surface of the booster is equal to or greater than 3:5. [00031 ] Preferably, an explosive transfer element is located within the explosive booster partially or completely surmounting the detonator located in the detonator encapsulating cavity.

BRIEF DESCRIPTION OF DRAWINGS

[00032] Embodiments of the present invention will now be described with reference to the accompanying drawings wherein: [00033] Figure 1 is a cross section of an explosive charge and booster for use in a blasthole according to prior art, [00034] Figure 2 is a cross section of melt-cast explosive material for an explosive booster according to the invention illustrating a preferred geometric relationship, [00035] Figure 3 are diagrammatic representations of hemi-spherically expanding detonating wave fronts at regular progressive time intervals At, as these arrive at an external boundary of a booster assembly for a selection of geometries, [00036] Figure 4 is a diagrammatic representation of an expanding detonating wave front initiated by an explosive lens at regular progressive time intervals At, as these arrive at an external boundary of a booster assembly, [00037] Figure 5 are cross sections of the damage that that may be witnessed by a steel plate placed beneath a functioning booster, [00038] Figures 6 to 8 are graphs showing progress of detonating wave fronts.

DESCRIPTION OF EMBODIMENTS

[00039] In contrast to RDX or PETN, TNT can be safely transported and kept in its raw dry crystal form. It can be fed into a melt process dry and quickly, and safely, melts at low temperature. The present invention provides for a melt-cast TNT based explosive booster yielding an improved in-hole initiating performance when compared with known similar mass commercial boosters.

[00040] As illustrated in Figure 3, the shape of a detonation front 100 as it arrives at the output face or external boundaries 112 of a booster 114 (shown in Figure 2) varies with simple selected geometry. Shown in Figure 3 are cross sections of a simple cylindrical geometry with a common radius. The bulk of the boosters 114, that would encapsulate a detonator are not shown in Figure 3 for clarity. The role of the detonator is geometrically summarised as a central detonation initiation point at a start time of zero.

[00041] Referring now to Figures 1 and 2, it is the upper face 115 of the booster 114 which predominantly interfaces with the bulk of the explosive material 24 which fills a blast hole 26 for example.

[00042] Typically, the detonator encapsulating cavity 116, is created by tunnels made in the booster body 118 during casting of the explosive material 120. Alternately a housing, of plastic or similar, could be manufactured for the puiposes of positioning the detonator and protecting it from adverse impact during a blast-hole loading process.

[00043] While TNT clearly has a lower detonator pressure than the predominant commercially available boosters, it can be shown that this disadvantage can be overcome by adjustments to the booster geometry. By increasing the distance between the top of the detonator (that is, the upper end of the cavity 116) and the upper surface or face 115 of the booster, while neither the velocity of detonation nor the detonation pressure changes, the shape of the detonation front at the output face 115 of the booster does change. The hemi-spherically expanding detonation front progressively approaches a planar detonation front with increasing radius. It is this quasi-planar detonation front that is responsible for the enhanced booster performance. Owing to the insensitivity of TNT, an explosive shaped transfer element 122 is provided, located at the blind end of the encapsulating cavity 116 for the detonator, to ensure reliable progression of the detonation from the output of the detonator to the TNT. Preferably, the transfer element 122 partially or completely surmounts the explosive end of the booster detonator. Preferably, the geometry of the shaped transfer element 122 includes an explosive lens 123 which includes an axially symmetric near-parabolic depression.

[00044] Experiments have shown that with a booster geometry in which the distance from detonator tip at the upper end of the cavity 116 to booster face 115 (or surface of the explosive volume), to the diameter of the booster is in the ratio of equal to or greater than 1:2 provides optimal booster performance in absence of the shaped transfer element including an explosive lens.

[00045] Referring again to Figure 2, for a booster 50mm in diameter, there should be a minimum of 25mm of explosive material above the tip of the detonator. Experiments to date suggest that 3:5 is a “sweet spot” with consideration of the additional amount of explosive required to achieve a significantly improved booster output (compared to shallower aspect ratios like 1:5). That is, the selected geometry includes a ratio of the separation distance to an effective diameter of the booster equal to or greater than 3:5. Increasing the ratio further will see additional output improvement but with diminishing returns. Increasing the ratio beyond 1:1 will see negligible improvement over the 4:5 performance. Owing to the vagaries of geology dominated in-ground bulk explosives behaviour the experimental validation has been biased towards the reproducible damage that the booster face 115 does to a steel plate 117 of known thickness and specification. Figure 5 illustrates the improved booster output that results fiom the selected geometry against a steel plate.

[00046] The generation of the quasi-planar detonation front at the output face of the booster may be enhanced by the selected geometry of the shaped transfer element. An axially symmetric nearparabolic depression acts as an explosive lens if the detonation velocity of the transfer element exceeds the detonation velocity of the melt cast fill of the booster. The precise geometry of the nearparabolic depression is defined by the relative detonation velocities of the two explosive components. Figure 4 illustrates the effect of the addition of an explosive lens 123 to the development of the detonation front. As per Figure 3 the role of the detonator is geometrically simplified as a central detonation initiation point at a start time of zero.

[00047] The combination of booster geometry and shaped transfer element geometry acting as an explosive lens enables a minimisation of the separation distance between an explosive end cap of the detonator of the booster and the output face of the cast volume to achieve the quasi-planar detonation front at the output face of the booster.

[00048] The axially symmetric near-parabolic depression acts as a shock wave focussing lens 123 if the depression is preserved as an air space rather than being filled with the melt cast material. This arrangement promotes the reliable initiation of the melt cast fill when the transfer element chemistry is of marginal detonation velocity and pressure. However, the enhancement of the generation of quasi-planar detonation front is lost if the air-space shock wave focussing lens shaped transfer element geometry is employed. That is, the combination of booster geometry and shaped transfer element geometry acting as a shock wave lens does not enable a minimisation of the separation distance between an explosive end cap of the detonator of the booster and the output face of the cast volume to achieve the quasi-planar detonation front at the output face of the booster. The development of the quasi-planar detonation front at the output face of the booster becomes totally reliant on the booster’s external geometry. This approach still has merit for booster manufacture where there are performance limiting options for the construction and ingredients of the transfer element.

[00049] By this means a low cost, high process safety, explosive booster fill (such as TNT) may be made into a booster that generates sufficient output to provide reliable detonation transfer to the bulk explosives used to fill the blast hole, typically ANFO (Ammonium Nitrate - Fuel Oil), ammonium nitrate based emulsions & watergels, or ANFO/emulsion blends.

[00050] As of yet unconfirmed, but a performance improvement of explosive in the blast-hole should be achieved with a geometry optimised booster. This translates into more efficient and effective rock breakage, particularly at the booster placement end of the hole (commonly referred to as the “toe”) where rock confinement is greatest.

[00051] The graph of Figure 6 shows expected the characteristic of in-hole detonation behaviour with a typical pentolite/hexolite booster using an aspect ratio of 1:5 to 2:5.

[00052] Figure 7 shows the expected characteristic performance for a TNT booster using the same range of aspect ratios, that is 1:5 to 2:5.

[00053] Figure 8 shows the expected characteristic performance for a TNT/pentolite/hexolite booster using aspect ratios between 3:5 and 1:1.

[00054] The booster of the invention may be a pure TNT booster or an other than pure TNT booster. In either case the volume of the booster is defined by an axi-symmetric bottle of length L and equivalent diameter D (twice the square root of the division of the cross-sectional area by π).

Claims (17)

1. A melt-cast based explosive booster; the booster yielding an equivalent or improved in-hole initiating performance compared with similar mass commercial boosters; the improved initiating performance dependent on the generation of a planar, or quasi-planar, detonation front at the output face of the booster.

2. The explosive booster of claim 1 wherein the quasi-planar detonation front at the output face of the melt-cast based explosive booster is dependent on selected geometry of the booster, or a selected geometry of a shaped transfer element, or a combination of booster geometry and shaped transfer element geometry.

3. The explosive booster of claim 2 wherein the selected geometry of the booster includes a separation distance between the explosive end of the detonator of the booster and the output face of the cast volume.

4. The explosive booster of claim 2 or 3 wherein the selected geometry of the shaped transfer includes an explosive lens that partially or completely surmounts the explosive end of the detonator of the booster.

5. The explosive booster of any one of claims 2 to 4 wherein the combination of booster geometry and shaped transfer element includes a minimisation of the separation distance between an explosive end cap of the detonator of the booster and the output face of the cast volume to achieve the quasi-planar detonation front at the output face of the booster.

6. The explosive booster of any one of claims 2 to 5 wherein the selected geometry includes a ratio of the separation distance to an effective diameter of the booster equal to or greater than 3:5.

7. The explosive booster of any one of claims 2 to 6 wherein the selected geometry includes an axially symmetric near-parabolic depression that functions as an explosive lens.

8. The explosive booster of any one of claims 1 to 8 wherein a bulk of the melt-cast explosive is pure TNT; the booster yielding at least an equivalent in-hole initiating performance to similar mass boosters containing PETN or RDX; the effective diameter based ratio of the separation distance between the end cap of the detonator and the surface of the cast volume being equal to or greater than 3:5.

9. The explosive booster of claim 8 wherein the transfer element of includes an explosive lens that partially or completely surmounts the explosive end of the detonator of the booster.

10. The explosive booster of any one of claims 1 to 9 wherein a melt-cast based explosive bulk mixture is more energetic than pure TNT; the booster yielding an improved in-hole initiating performance to similar mass commercial boosters; the booster geometry including an effective diameter based ratio of a separation distance between the explosive end of the detonator and a surface of a cast volume being equal to or greater than 3:5.

11. The explosive booster of claim 3 to 10 wherein the melt-cast based explosive bulk mixture is more energetic than pure TNT; the booster yielding an improved in-hole initiating performance to similar mass commercial boosters; the transfer element including an explosive lens that partially or completely surmounts the explosive end of the detonator of the booster.

12. The explosive booster of claim 11 wherein the pure TNT booster has a volume defined by an axi-symmetric bottle of length L and equivalent diameter D (twice the square root of the result of the division of the cross-sectional area by Π.)

13. The explosive booster of claim 11 wherein an other than pure TNT booster has a volume defined by an axi-symmetric bottle of length L and equivalent diameter D (twice the square root of the result of the division of the cross-sectional area by Π.)

14. The explosive booster of claim 12 or 13 wherein the explosive volume is cast into the axi-symmetric bottle; the casting including an encapsulating cavity for the detonator and a tunnel for detonator wiring.

15. The explosive booster of claim 14 wherein the casting encapsulates a transfer element located at the blind end of the encapsulating cavity for the detonator.

16. A method of attaining an equivalent booster yield from a pure TNT explosive booster; the method including the step of melt-casting TNT into an axi-symmetric bottle; the casting including a detonator encapsulating cavity in which a ratio of a separation distance between the explosive end of a detonator fitted to the detonator encapsulating cavity and an upper surface of the booster is equal to or greater than 3:5.

17. The method of claim 16 wherein an explosive transfer element is located within the explosive booster partially or completely surmounting the detonator located in the detonator encapsulating cavity.