BASE CHARGE EXPLOSIVE SYSTEM APPLICATION

BACKGROUND OF THE INVENTION

This invention relates to a detonator.

A molecular explosive is normally used as a base charge in a detonator. The molecular explosive is produced under factory conditions according to an appropriate specification and is then press-loaded into a metallic or non-metallic casing which serves as a housing for a detonator. The explosive charge is usually sensitive to impact, friction, temperature and electrostatic discharge and any of these stimuli can be used to initiate the explosive.

The sensitivity of the explosive charge to the various stimuli dictates a category and class into which the explosive composition is placed for safety compliance purposes. The composition's sensitivity, particularly in respect of impact and friction, constitutes a significant safety aspect which must be addressed. Strict safety rules which apply to the storage, handling, transporting and use of these compositions add significantly to the cost per detonator at the time the detonator is connected in a ready-to-use state in a blasting system.

An object of the present invention is to provide a detonator which substantially increases the degree of safety relating to the storage, handling, transport and use of a detonator, and which can thereby reduce the aforementioned cost.

SUMMARY OF INVENTION

The invention provides a detonator which includes a tubular housing with a bore in which is formed a compartment, a first frangible container inside the compartment, a first quantity of a first material inside the first container, a second frangible container inside the compartment, a second quantity of a second material inside the second container, and an actuating mechanism which is operable to break the first container and the second container thereby to allow the first material to contact the second material and form an explosive composition inside the compartment.

Each container may be in the form of a capsule made from glass, a brittle plastic material or the like. The size of each container determines the quantity of the respective material held in the container.

The actuating mechanism may include a tool which is movable by a user to break the first container and the second container. Preferably the tool is movable to cause mixing of the first material with the second material once the respective containers have been broken.

The detonator may include a safety device to prevent inadvertent breaking of the containers. The safety device may take on any suitable form. For example, at least part of the housing may be encased in a sleeve and only when the sleeve is removed is the actuating mechanism operable. In another form of the invention the safety device comprises a retention member which is engaged with the housing and with the tool thereby to restrict movement of the tool relative to the housing. Once the retention member is disengaged from at least one of the housing and the tool it becomes possible to move the tool relative to the housing.

The tool may include an inner end which is positioned in the compartment, an outer end which is positioned outside of the housing and a link which extends through a wall of the housing and which connects the inner end to the outer end. The outer end may be engageable by a user thereby to cause movement of the inner end to break the first container and the second container. In one form of the invention the tool is movable linearly relative to the housing and the tool and the housing include interengageable formations to cause at least part of the tool to rotate relative to the housing upon said linear movement.

In a different form of the invention at least part of the housing in the compartment is deformable to cause breakage of the containers. The actuating mechanism is then constituted by this deformable part of the housing.

The tubular housing may include a mouth which opens to the bore. The mouth may be connectable to an initiating source of any appropriate kind e.g. a shock tube which forms part of an electrically-based initiating system etc.

A chemical time delay composition may be exposed to the mouth and may be initiated by the initiating source. A primary explosive may be located between the time delay composition and said compartment.

The first material and the second material are selected so that, when mixed, an appropriate base charge explosive composition is formed. Upon mixture a chemical reaction occurs and, preferably, the reaction should be completed within a predefined period which is not more than 10 minutes (this is exemplary and non-limiting).

Conversely, the reaction should be completed in not less than a defined period of say, about 60 seconds. An objective in this respect is to ensure that the chemical reaction is completed within a reasonable period which does not unduly delay placement or application of the resulting composition. The chemical reaction should not take place too rapidly for in this event exothermic heat may be generated which may result in the reaction becoming uncontrollable. There could also be an adverse effect on the safety aspects of the mixing process.

A catalyst may be used to control the reaction rate. The catalyst may be used to initiate the reaction or to accelerate or slow the rate at which the reaction takes place.

In one preferred form of the invention metered quantities of nitric acid and of pentaerythritol (PE) are located in the respective containers. These materials, when mixed, produce PETN (pentaerythritoitetranitrate).

If the reaction is such as to give rise to an unwanted by-product e.g. water, then an appropriate absorbent material may be positioned in the compartment to absorb the by-product.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference to the accompanying drawings in which:

FIG. 1 is a block diagram representation of a prior art technique used for the manufacture of PETN;

FIG. 2 illustrates a detonator, according to the Invention, from one side and in cross-section;

FIG. 3 is a cross-sectional view of the detonator taken on a line 3-3 in FIG. 2;

FIG. 4 is a cross-sectional view of a safety device included in the detonator taken on a line 4-4 in FIG. 2;

FIG. 5 is similar to FIG. 2 but illustrating the detonator while a base charge is being prepared;

FIGS. 6 and 7 are views corresponding to FIGS. 2 and 5 respectively of a different embodiment of the invention; and

FIG. 8 shows another form of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 of the accompanying drawings illustrates a technique used for the manufacture of PETN (pentaerythritoltetranitrate) under factor conditions. PETN is a powerful high explosive which is sensitive to shock and friction, and is widely used as a base charge explosive composition in a detonator.

A measured quantity of nitric acid 10 is added to a measured quantity of pentaerythritol (PE) 12. The solution is mixed in a step 14, in accordance with known parameters. This may lead to the generation of heat. In a step 18 the mixture is allowed to crystallise and thereafter, as may be appropriate, the crystals are re-crystallised (step 20) to allow the resulting crystals to be used effectively in a detonator 22, in a detonating cord (shock tube) 24, or in a booster 26, as the case may be.

PETN is a high explosive and strict regulations apply to the storage, handling, transporting and use of PETN. The detonator of the invention aims to address some of these aspects.

The invention is based, in broad terms, on the incorporation of a two component non-energetic material system into a detonator. The physical construction of the detonator is such that the components can be mixed in situ, when required, to produce an explosive composition inside the detonator.

FIG. 2 illustrates, from one side and in cross-section, a detonator 30 according to one form of the invention. FIG. 3 is a cross-sectional view of the detonator taken on a line 3-3 in FIG. 2, while FIG. 4 is a cross-sectional view taken on a line 4-4 in FIG. 2.

The detonator 30 includes an elongate tubular housing 32 and an actuating mechanism 34.

The housing is made from an appropriate metallic or non-metallic material which, preferably, is rigid. It is possible to make the housing from a flexible or semi-flexible material but without a suitable safeguard this could cause unwanted initiation of the detonator.

The housing 32 defines an inner bore 36 which has a mouth 38 at one end and a wall 40 at an opposing end. A compartment 42 is defined in the housing adjacent the wall 40. Two containers or capsules 44 and 46 respectively are located in the compartment and are fixed in position, as illustrated, using any appropriate technique e.g. a suitable adhesive. The containers may be substantially identical. Each container is frangible and is made from an appropriate material e.g. thin-walled glass, a brittle polymer or the like. The size of each container is determined inter alia by the nature of the material which it houses and the intended application of the detonator. For example, each container has a length 50, in its axial direction, which is of the order of 10 mm. In cross-section, as is shown in FIG. 3, each container may be semi-cylindrical and have a diameter or maximum width 52 of the order of 6.4 mm. These dimensions and this design allow for two of the semi-cylindrical containers to be loaded with ease into the bore 36 which, by way of example, has a diameter 54 of about 7 mm or 8 mm.

The containers 44 and 46 are located on opposing sides of the compartment 42, as shown in FIG. 3, and are axially displaced from each other as is shown in the side view in FIG. 2. Optionally the containers may be additionally physically separated from each other by means of an appropriate spacer 56 which is shown in dotted form in FIG. 2. The spacer, itself, may be made from a frangible material such as glass or a brittle polymer.

The container 44 contains a measured quantity of a first material A while the container 46 contains a measured quantity of a second material B.

A disc-like insert 60 forms one end of the compartment inside the bore. A pointed spigot or spike 62 is rigidly mounted to the insert and extends towards and directly opposes an end of the container 46.

The actuating mechanism 34 is in the form of a tool which has a piston-like function. The actuating mechanism includes a disc 70 which abuts an inner surface of the wall 40. A pointed spigot or spike 72 is fixed to the disc 70 and extends towards and directly opposes an end of the container 44. The wall 40 has a centrally positioned hole 74 and a rod 76 extends through the hole. A greater portion of the length of the rod is outside the detonator housing. An outer end of the rod is connected to a user-engageable handle 78.

A safety device 80 is positioned between the handle 78 and an opposing surface of the wall 40. The safety device includes a small plastic ring 82 with a slot 84 through which the rod 76 can pass to a central location at which the rod engages with a snap fit with the ring. A projection 88 which extends from the ring provides a means whereby a user can grip the safety device.

The safety device is in close fitting engagement with the housing and with the handle and prevents linear movement of the tool relative to the housing.

A charge 90 of a primary explosive abuts the apertured insert 60.

A sleeve 92 abuts the charge 90 and extends towards the mouth 38. The sleeve has an end section 94 with one or more holes 96. A chemical delay composition 98 of a kind known in the art is loaded into the sleeve.

The detonator in the form shown in FIG. 2 is inert and does not fall into any category which is traditionally associated with detonators. As such the detonator 30 is not subjected to the safety regulations which apply to the storage, transport and handling of detonators. The constituents or materials A and B, held inside the detonator, are separated and cannot interact with each other while held in the respective containers.

At a blast site the detonator is connected to a suitable initiating system. As shown in FIG. 2 a shock tube 104, of conventional construction, is engaged with a seal 106, in the form of a plug, and a wall 108 of the housing, at the mouth, is then crimped into position to keep the components connected to each other. This aspect is substantially conventional.

In order to ready the detonator for use the safety device 80 is released. A user grips the portion 88 and tugs on it to move the rod 76 through the gap 84. The rod 76 is then exposed. The user can then grip the handle 78 and push the rod 76 into the compartment 42. When this happens the spike 72 is driven into the container 44, and the container 46 is driven onto the spike 62. The two containers are fractured in the process and the materials A and B are released and come into contact with each other. It is then possible for a user to rotate the handle 78 relative to the housing. The spikes 62 and 72 agitate the compositions A and B inside the compartment and the components are therefore suitably mixed—see FIG. 5.

A chemical reaction occurs. The nature of the reaction is dependent on the nature of the materials A and B. Use can be made of a catalyst of any appropriate kind which is held in the compartment and which is released, when necessary, to influence the reaction of the materials. The catalyst can be loaded into a capsule or container which is similar to the capsules 44 and 46. One of these containers may be compartmentalised to house the material A or B, as the case may be, and the catalyst. The invention is not limited in this respect.

To make the detonator usable in a practical and safe manner the reaction which takes place between the materials A and B should be completed within acceptable limits of exothermic heat generation and within a reasonable time. Preferably the reaction is completed within a period of, say, not more than 10 minutes. The reaction should not be completed too rapidly, say, in not less than 60 seconds. The objective in this respect is to ensure that the reaction is completed within an acceptable time so that the resulting explosive composition can be used without undue delay. The reaction should not be completed too rapidly for in this event excess heat may be generated which may lead to the reaction becoming uncontrolled or the safety of the mixing step could be compromised.

A correct selection of the materials A and B ensures that the resulting explosive composition (the outcome of the chemical reaction) is sensitive to heat and friction to such an extent that the composition can readily be initiated by either of these stimuli.

During the chemical reaction large volumes of gas are not produced. By-products which may be generated by the chemical system do not affect the performance of the explosive composition.

In one implementation the material A is a measured quantity of graded pentaerythritol (PE). Care is taken under laboratory conditions to ensure that the characteristics of the pentaerythritol are precisely and accurately determined. Similar care is taken with the material B which, in order to react with the pentaerythritol, is a measured quantity of nitric acid complying with tightly controlled specifications.

When the capsules which hold the PE and nitric acid are broken the constituents are released and, much in the manner shown in FIG. 1, PETN is produced. The characteristics of the PE and nitric acid are accurately specified and controlled and the PETN which is formed, by mixing of the ingredients, has a crystal structure which is well suited for use in a detonator.

The detonator 30 is used in a conventional manner in that when the shock tube 104 is initiated a shock wave front is propagated into the bore 36. The delay composition 98 is ignited via the effects of the shock wave passing through the holes 96. At the end of a time delay period determined by the burning of the composition 98 the primary explosive 90 is ignited and this causes initiation of the PETN as a secondary explosive. This leads to initiation of an explosive mixture located in a borehole in which the detonator is placed.

The invention can be implemented in different ways. As indicated in broad terms, the invention is based on the incorporation of a two component non-energetic material system into a detonator housing.

FIG. 5 shows the safety device 80 detached from the housing 32. FIG. 5 also illustrates a different form of a safety device designated 80A and shown in dotted outline. The safety device 80A comprises a sheath or sleeve which fits closely around one end of the housing and which is bonded to the handle 78. The actuating mechanism 34 is only operable if a portion of the sleeve marked X, which surrounds the housing, is peeled free from the housing. When this occurs the handle 78 can be moved to drive the rod 76 into the compartment. The handle can also be rotated to cause further mixing of the ingredients A and B.

FIGS. 6 and 7 show another version of the actuating mechanism designated 34A. Like components to those shown in FIGS. 2 and 5 bear like reference numerals and only differences between the two forms of construction are described. The rod 76, in the modified actuating mechanism 34 has a rough external thread 110 over a portion of its length extending from the handle 78. A corresponding rough thread 112 is formed in the wall 40.

When the safety device 80 is released the rod 76 can be moved linearly into the compartment to cause the spikes 62 and 72 to break the respective containers 44 and 46. At this point the handle 78 is rotated to cause the threads 110 to engage with the threads 112. The spikes are thereby rotated and mixing of the constituents A and B is ensured.

As shown in FIG. 8 it is possible to make the housing 32 or at least a portion 32A thereof which surrounds the compartment 42 pliable so that with the application of sufficient force to the housing is deformed and forces are transmitted to the containers 44 and 46 to break the containers. The actuating mechanism is thus formed by the deformable housing, or part thereof. Each container then releases its material content into the compartment and intimate mixing of the materials can take place. Care should be taken however that forces which are applied to the housing during handling and transport do not cause inadvertent breaking of the containers. A suitable safeguard is to locate a rigid sleeve 100 over the flexible portion 32A of the housing which forms the compartment 42. It is then only possible to depress the compartment and break the containers once the sleeve is removed. This arrangement dispenses with the spikes 62, 72.

A primary benefit of the invention lies in its capability to reduce, materially, the application of rigid safety regulations which relate, generally, to the storage, handling, transport and use of conventional detonators.