Patent Application
20150053105
The present invention relates to shell for an explosive charge. More specifically, the present invention relates to a shell for a booster. The invention also relates to a booster produced using the shell, to the booster when primed with a detonator and to a method of blasting using the booster.
In commercial mining applications blast holes are drilled, loaded with bulk explosive and the bulk explosive initiated. This is typically done using a so-called booster. This is a separate, relatively small explosive charge that is housed in a shell that is designed to receive a detonator. The detonator typically takes the form of a cylindrical cartridge and includes a base charge at one end. A lead (for signal transmission to fire the detonator) extends from the other end of the detonator. In use, a detonator is inserted into the booster, the booster is positioned in a blast hole and surrounded by bulk explosive. The detonator is then fired thereby triggering detonation of the explosive charge of the booster. In turn, that causes detonation of the bulk explosive.
Manufacture of a booster typically involves casting a molten explosive composition (usually Pentolite) in a suitably designed shell. The explosive composition is typically cast (poured) around metal (e.g. brass) pins suitably positioned within the cavity defined by the booster shell. After the explosive composition has solidified these pins are removed to provide tunnels (passages) that are adapted to receive a detonator. These cast boosters typically have at least two such detonator tunnels extending through the cast composition to allow a detonator to be fed fully down through one tunnel and return up through the other which will have a blind end or stepped end which functions as a stop position for the end of the detonator. The detonator lead (extending out of the top of the booster) is then pulled taut and the booster with detonator (primed booster) is ready to be positioned in a blast hole.
A problem that has been observed with this form of booster design is that as the cast explosive cools and solidifies it shrinks (the shrinkage rate is approximately 7 volume %) and this results in the composition developing shrinkage voids at its upper end, i.e. at the top of the booster. These shrinkage voids can lead to unreliable initiation of the booster because, when loaded in the booster, the detonator is oriented such that the base charge of the detonator is located towards the top of the booster and thus in proximity to any shrinkage voids that will be present. The presence of the voids tend to impair communication of energy from the base charge of the detonator to the cast explosive in the booster, thereby leading to unreliable initiation of the booster.
This problem can be mitigated by minimising the amount of voids present in the cast explosive composition, for example, by casting the explosive composition in stages with at least partial cooling of the composition being allowed between casting stages. In this way voids formed as the composition solidifies can be filled in a subsequent casting stage. However, this multi-stage approach to casting comes at the expense of productivity. The use of metal pins to define the detonator tunnels during casting also adds another step to the manufacturing process.
Against this background it would be desirable to adopt a different approach to the manufacture and use of cast boosters that does not suffer the operational and manufacturing issues noted above.
Accordingly, the present invention provides a booster shell, which comprises:
to The invention also provides a method of making a cast booster by casting a suitable explosive composition in the booster shell of the invention. This is done by delivering molten explosive composition into the chamber of the shell via the inlet at the upper end of the shell. Casting per se is carried out in conventional manner using known compositions and methodology, although it should be emphasised that casting is carried in a single stage. Multi-stage casting is not required.
After the explosive composition has solidified the booster can be primed with a detonator. Conventional cartridge detonators are used. Priming involves insertion of the detonator into the detonator receiving passage from the upper end of the body until the end of the detonator abuts against the stop in the passage. The detonator leads will extend out of the passage and can be accommodated by the detonator lead guide. Depending upon design, it may be necessary to feed the detonator through the detonator lead guide before inserting it into the detonator receiving passage, and this will be discussed in more detail later. The present invention also relates to a primed booster.
Once primed the detonator can be inserted into a blast hole. This is done by “inverting” the booster and feeding it lower end (of the booster body) first into the hole, with the detonator leads extending out of the hole. Bulk explosive can then be delivered into the blast hole and the blast initiated in conventional manner. Consistent with this embodiment the present invention provides a method of blasting which comprises associating a primed booster (in accordance with the invention) with a bulk explosive in a blast hole, and initiating the primed booster by firing of the detonator in the primed booster.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.
Embodiments of the present invention are illustrated with reference to the accompanying non-limiting drawings in which:
In accordance with the present invention the design of the detonator receiving passage of the booster shell means that, on priming, the end of the detonator that includes a base charge will be remote from the upper end of the shell. However, as the explosive composition contained in the booster shell is delivered (cast) into the shell via an inlet at the upper end of the shell, any voids in the explosive composition as a result of shrinkage during solidification will be located at or close to the upper end of the shell. What this means is that there should not be any voids in the cast composition in proximity to the base charge of the detonator. The voids would be present at the upper end of the shell, whereas the base charge of the detonator would be at or close to the lower end of the shell. This avoids the problem highlighted above of unreliable booster initiation. It will be appreciated that the design of the booster shell of the invention enables this desirable outcome.
It is also relevant to note that the detonator receiving passage and detonator lead guide are integrally formed with the body of the booster shell. This enables the casting of explosive composition in the shell to be simplified when compared with the conventional methodology of needing to use removable metal pins to define suitable channels within the cast explosive itself. In the present invention the detonator receiving passage and detonator lead guide are defined by structural features of the shell rather than of the cast explosive composition.
The booster shell of the invention is formed by injection moulding of a plastic material (for example polyethylene or polypropylene) into a suitably configured die/mould. This enables various advantageous design features to be achieved, especially as integrally formed features.
Outer walls of the booster shell should sufficiently thick and robust to withstand intended use. Structures internal to the shell may be formed of thin walls or webs of polymer, although it should be noted that various structures of the shell will come into contact with molten explosive composition during casting of explosive composition into the shell. Materials selection, wall/web thicknesses and design will need to take this into account.
The design of the booster shell should take into account costs and ease of manufacture, as well as ease and practicality of use. To simplify manufacture and assembly it is desirable that the booster shell is made up of the minimum number of component parts. In an embodiment the booster shell is injection moulded as a single piece with the various design features integral to that moulding. In other embodiments the booster shell is made up of a number of simple components that are each injection moulded and that can be assembled with ease to provide a booster shell having the requisite design features. This may offer greater flexibility of design without complicating manufacturing and assembly. The various components may be adapted to be secured together by screwing or by friction fit.
The booster shell of the invention comprises an elongate body portion that defines a chamber. This chamber will house the explosive composition of the booster. The body portion is typically cylindrical (typically the diameter is 30-70 mm). The booster shell is intended to receive and fully enclose a detonator and it is therefore typically 110-140 mm in length. The dimensions of the booster shell may be varied depending upon the energy release, and thus the volume of explosive composition, required. By way of example, the mass of explosive composition contained in the shell may be 50-900 grams.
The booster shell includes at its upper end an inlet which enables explosive composition to is be delivered into the chamber. This will invariably be done by pouring or injecting molten explosive composition (Pentolite for example) through the inlet. The inlet will usually include a cap or bung. This may be secured into the inlet by screw fitting or by friction fit. It is preferred that the entire explosive composition is fully enclosed to reduce exposure to operators and the potential for unintended friction or impact events which could accidentally detonate the explosives.
The booster shell comprises a detonator receiving passage that is adapted to receive a detonator. The passage is intended to fully enclose a detonator along its length and will be sized accordingly. The passage is provided within the chamber defined by the elongate body and extends from the upper end to the lower end of the elongate body. The passage is open at the upper end of the elongate body (booster shell) and includes a detonator stop at or near to the lower end of the passage. This stop may extend fully or partially across the diameter of the passage provided it serves its intended function. The stop may be integral with the passage or it may be a separate component that can be fitted into the end of the passage.
In a preferred embodiment, the end of the detonator receiving passage remote from the detonator stop will include at its upper end a detonator retention means that prevents a detonator inserted into the passage from unintentionally falling out or from being withdrawn, for example when the detonator lead is put in tension as is likely when a primed booster is being loaded in a blast hole. The retention means may comprise a series of (resilient) tabs that extend inwardly across the passage or the inlet to the passage. These tabs are deflected downwardly as the detonator is pushed into the passage and return to their original position after the other end of the detonator has been inserted beyond the tabs.
The booster shell also comprises a detonator lead guide. The function of this is to accommodate the lead of a detonator that is loaded into the booster during priming. The guide may be provided on the outside of the shell, although preferably the guide is provided within the shell as this provides greater protection to the detonator lead. The guide extends from the upper end to the lower of the elongate body, and is usually provided parallel and immediately adjacent to the detonator receiving passage. In an embodiment of the invention priming involves insertion of a detonator into and through the detonator lead guide from below, with the detonator then being inserted and down into the detonator receiving passage. When the guide is intended to allow detonator loading in this way, the diameter of the guide will be sized accordingly. A detonator lead recessed return may be provided between the open ends of the detonator lead guide and the detonator receiving passage. This return may take the form of a “saddle”.
Notably the detonator receiving passage and detonator lead guide are each integrally formed with the elongate body of the booster shell. This simplifies manufacture and means that these structures are not formed by moulding of explosive composition around metal pins, as described above.
With respect to the walls defining the detonator receiving passage, if these are too thick this may reduce the ability for a detonator to initiate the booster composition, so it is desirable to have the relevant walls as thin as possible. The walls defining the passage can however be subject to distortion by hot explosive composition during casting. To mitigate this, the detonator receiving passage and detonator lead guide are integral with or attached to a wall of the booster shell. This will provide enhanced structural support to the passage and guide.
It is also preferred that the detonator receiving passage and/or detonator lead guide are integral with the (inner) wall of the booster shell along the entire length of the passage and/or guide. This simplifies mould design and allows walls defining the passage and/or guide to be moulded very thin. This design implies a mould design such that during injection moulding plastic flows along those parts of the mould defining the walls of booster shell while at the same time filling those parts of the mould that define the passage and/or guide. This would not occur if the mould cavities defining the passage and guide were fed from one end only during injection moulding. Preferably, the detonator receiving passage and detonator lead guide are integral with the (inner) wall of the booster shell along the entire length of the passage and guide.
In use hot explosive is cast in the booster shell. After cooling the inlet through which the explosive has been delivered into the shell is closed. Importantly, any voids in the cast composition will be located at the upper end of the cast composition and thus at the upper end of the booster. If the detonator receiving passage does not include an integral detonator stop, a suitable stop is provided in the passage as a separate component as has been described. A detonator can then be inserted into the detonator receiving passage noting here that the base charge at the end of the detonator will be located remote from the end of the booster where any shrinkage voids in the composition will be present. The detonator lead is positioned in the detonator lead guide, the lead extending from the lower end of the booster. On loading into a blast hole, the primed booster is “inverted” and delivered upper end first into a blast hole with the detonator lead extending out of the blast hole. The blast hole can then be charged with bulk explosive. This bulk explosive is initiated using the booster, the booster itself being initiated by the detonator enclosed in it.
In an embodiment of the invention the booster may include a (small) separate sensitiser explosive charge to increase initiation sensitivity. This may be necessary if the (cast) explosive charge contained in the booster is less sensitive to being initiated. A separate sensitiser charge may also be of use depending upon the thickness of plastic wall members (defining the detonator receiving passage, for example) between the base charge of the detonator and the explosive charge contained in the booster. The presence of such wall members can reduce the energy communicated to the explosive charge in the booster when the detonator is fired. In these cases the use of a separate sensitising charge within the booster may be beneficial.
In this embodiment the sensitiser explosive charge may be incorporated into the booster in a sealed and thin-walled container. For example, loose PETN may be contained inside a blow moulded thin-walled plastic bottle which is positioned in the booster shell before casting. The container should be positioned at the lower end of the shell and close to, or in contact with, the wall of detonator receiving passage.
Incorporating a separate sensitising charge in the booster may also render the booster capable of being initiated by use of detonating cord rather than a detonator. In this case low strength detonating cord would typically be used (with a core loading down to about 3.6 g/m). In this embodiment a length of the detonating cord should be provided inside the booster (in the detonator receiving passage and, possible, the detonator lead guide) in close proximity to the separate sensitising charge. How the detonating cord is fed into the booster will depend upon the design of this passage and guide. After priming with detonating cord, the booster is then oriented in a blast hole as described above in relation to a detonator-primed booster.
Embodiments of the invention are discussed below with reference to the accompanying non-limiting drawings.
As shown in
The body portion (2) also includes a groove (10) and the top cap a corresponding projection (11) that enables the top cap (3) and body portion (2) to be fitted together in the correct orientation noting that the inlets (6A,7A) provided by the top cap (3) must align with the detonator receiving passage (6) and detonator lead guide (7) that extend within the body portion (2) of the shell (1) (the passage and guide are not shown in
Embodiments of the present invention include the following advantageous design features:
In terms of manufacturing, a major advantage of the design of the present invention is that all of the above features may be incorporated into a simple design with minimal piece count which allows it to be made at reduced cost to other alternative designs.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012901264 | Mar 2012 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2013/000275 | 3/20/2013 | WO | 00 |
