GB2218701A - Sheathed explosive systems - Google Patents

Abstract

An explosives system comprises an emulsion explosive core surrounded by a viscous sheath material which is preferably a gel, slurry or emulsion and has dispersed therein flame quenching and shock wave attenuating materials. The viscous sheath material and explosive core may be in intimate contact. The sheath material can be inert or comprise explosive materials. The system may be adapted for use in blasting in inflammable atmospheres.

Description

TITLE: SHEATHED EXPLOSIVE SYSTEMS The invention relates to an improved explosive system comprising an emulsion explosive core and an outer sheath material. The outer sheath is, in several forms, a dispersion and may as a preferred embodiment contain explosive materials. More particularly, the explosive system is adapted for use in coal mines.

Around 1930, the concept of enclosing nitroglycerin (NG) explosives in an inert sheath of cooling material was introduced by Lemaire in Belgium. Sodium bicarbonate was almost exclusively used in this respect. A paper sheath covering (shell) was wrapped round and spaced from the explosive core, the powder manually fed into the space and the explosive system vibrated for close packing of the powder. Such vibration and subsequent in transit vibration could produce a density gradient in both the powder sheath and the explosive core (if also a powder). Further improvements led to the sheath being bound as a felt or compressed into hollow cylinders.

Production of these NG permitteds was consequently a very labour intensive process. Furthermore, their use was generally less than fully acceptable due to the vulnerability of the sheath. There was always a danger of the sheath being damaged either in transport or by rough handling, and once the outer sheath covering was punctured, the sodium bicarbonate powder could easily pour out. Since the sheath was responsible for providing the safety of the explosive, a more serious aspect was that the powder sheaths could be deliberately torn and removed entirely from the explosive, resulting in the cartridge detonating with a corresponding reduction in safety. The powder sheathed explosives were eventually withdrawn from the market to be replaced with explosives where the cooling material was incorporated into the explosive itself.

Another form of sheathed explosive is described in Russian patent No. 291086. Here the explosive core is surrounded by a sheath of a 60 to 70% sodium perchlorate solution.

Due to use of a liquid sheath a centring ring is required, presumably to support and retain the explosive core co-axially with the sheath. In this instance, the sheathed explosive can be adapted as a permitted, but presumably only with low velocity of detonation explosives.

Permitted explosives are those explosives which provide a measure of safety when fired in a methan-air environment as found in coal mines. These explosives must pass a series of tests carried out by the relevant Governmental Authority, which in the UK is the Health and Safety Executive at Buxton. A successful testing places the explosive on the permitted list. Further details of the UK tests can be found in Testing Memorandum No. 2, published by the Health and Safety Executive, Harpur Hill, Buxton, Derbyshire.

According to the present invention there is provided an explosive system comprising an emulsion explosive core and a viscous sheath material substantially surrounding the core, and methods of blasting using such systems.

The sheathing material has a viscosity substantially greater than water and it has noticeably different viscous properties. Preferably the viscous sheath material is a dispersion which can take the form of a liquid/liquid dispersion, gas/liquid dispersion, solid/liquid dispersion or solid/solid dispersion.

One suitable type of liquid/liquid dispersion is an aqueous gel. This, like the other dispersions mentioned in the patent, has the advantage of not requiring additional support to retain the sheath in the desirable co-axial position and also minimises loss of the sheath contents if damaged.

Thus, for example, in an aqueous gel, the gelling agent, in this case an isocyanate prepolymer, can be present in about 0.2 to 50% w/w, more preferably 0.5 to 20% w/w (and for at least economic reasons) most preferably about 1-15% w/w.

The desirable limits will, of course, vary depending on the gelling agent/crosslinkers used. Suitable gels can be formed from aqueous gelling agents such as polyacrylamides, guar gums or gelatines.

Another suitable type of dispersion is a solid/liquid dispersion in the form of a slurry. Preferably the slurry contains a small amount of gelling agent. Preferably it is an aqueous slurry, with the solids loading about at least 10% w/w, more preferably 30 to 70% w/w and most preferably about 45 to 55% solids loading. A preferable amount of gelling agent, if required, is about 5% w/w. The amount of gelling agent required will vary with the type and quantity of solids loading, and with say an 80% w/w solids loading of a flame quenching solid,the gelling agent could be eliminated.

Yet another example of a dispersion of the sheath material would be an emulsion. The emulsion could be of the water-in-oil, melt-in-water or oil-in-water type such as is known in the art but not necessarily exhibit explosive properties i.e. act entirely as an inert barrier.

Preferably the emulsion sheath would comprise explosive ingredients to supplement the explosive force of the core whilst still retaining the safety.

Emulsions with crystallised droplets, so called solid emulsions, would also be useful.

Other types of dispersions, such as gas/liquid dispersions incorporating gas bubbles are described hereinafter.

Preferably the viscous sheath material in whatever dispersive form will comprise one or more of the following: cooling agents such as water or calcium carbonate (which decomposes endothermically) other high heat capacity compounds, quenching or radical scavenging agents such as incombustibles, salts containing water of crystallisation (eg Na2S04), salts volatile at high temperatures (eg NaCl), salts decomposing at high temperatures (eg (NH4)2 S04, NH4C1), sodium bicarbonate. Such solid inert ingredients would be uniformly dispersed through the viscous sheath material. As a further example of this class, a sheath material containing calcium sulphate has given promising results.

Preferably the viscous sheath material will include gas bubbles. For example, a gel such as that made from polyacrylamide, guar gum or gelatine can encapsulate and retain gas bubbles which help to dissipate the shock energy from the core detonation. Such gas microbubbles can be formed in a number of ways.

For example, in situ as a by product of the gelling process such as by isocyanate polymerisation with water; gas bubble generation by NaNO2 and thiourea; or by mechanical entrainment.

Alternatively, microballoons (such as are supplied by 3M), perlite or other low density filler materials could be dispersed throughout the aforementioned dispersions.

The viscous sheath material preferably is also pumpable in the initial stages so that it can be advantageously co-extruded continuously with a pumpable emulsion explosive core. In one form, the viscous sheath material is co-extruded co-axially with the core explosive into a plastic composite skin comprising outer and inner tubes respectively which are then closed to form a cartridge.

The explosive system of the present invention is suitably adapted for use as a permitted explosive. Novel emulsion explosives can be prepared which have all the safety and other advantages of emulsions, and can be adapted to pass the permitteds tests. To date, sheathed emulsions have been made which pass a selected series of the P1, P4 and P5 permitted tests applied in the United Kingdom.

In an especially preferred embodiment of the invention, the inner explosives core covering is eliminated thereby leaving the explosive core and viscous sheath material in intimate contact. The sheath material could therefore be adapted to wholly support the explosive core.

More particularly, an explosive system eliminating the inner core covering is safer in that it is more difficult to remove the sheath material without disturbing the integrity of the explosive core, thus providing a safety explosive system. The sheath material of such an intimate system preferably would be adapted for minimum intermixing and advantageously would be gel-like, but even more so would be an emulsion.

The viscous sheath material has numerous other advantages.

Firstly, viscous sheath materials (especially those incorporating gas bubbles) help to protect the core explosive against rough handling and desensitisation, such as by dampening the shock if the explosive system is dropped. Secondly, the viscous sheath material mitigates fluctuations in temperature and protects the core explosive from oxidation. Thirdly, the viscous sheath material can be adapted to improve the handling characteristics of the whole explosives system. Also in a gelled sheath, a detonator could be inserted without substantial loss of contents.

The viscous sheath material can desirably in certain circumstances comprise explosive materials. Often the required detonation sensitivity of an explosive is acquired at the expense of the explosive bulk density. Thus, glass microbubbles are dispersed in emulsion explosives creating hot spots and increasing the detonation sensitivity, but in the process lowering the density and thereby the bulk strength of the emulsion.

With, for example, a gelled sheath, variable amounts of explosive material can be dispersed to augment, on detonation of the explosive core, the explosive power of the system. The reactivity of the sheath may be adjusted so that it is essentially unreactive when unconfined but reacts to augment the power of the system when confined.

Materials of this type are particularly useful as P4, PS and P4/5 permitteds.

A desirable viscous sheath material with explosive properties would be an emulsion. An emulsion core/emulsion sheath would typically have a more sensitive, less dense and thus less powerful core, and a less sensitive, more dense and more powerful sheath. Such an emulsion/emulsion twin explosive system desirably would be also adapted as a permitted explosive.

Like the inert sheath, the explosive emulsion sheath would also protect the more sensitive core emulsion from rough handling and other external factors described previously such as temperature fluctuations.

In another example of the emulsion core/sheath principle the sensitivity of the core and the sheath material could be reversed. Therefore, in an emulsion/emulsion twin explosive system, the sheath would incorporate more glass microbubbles or other sensitising voids, and so would be more sensitive and less dense than the core. Furthermore, such an explosive system could be initiated by a standard detonating cord attached to its outer surface. With, for example, existing single emulsion explosive systems, the emulsion must sacrifice some bulk strength to be fuse or cord sensitive. However, a more dense emulsion core, on detonation of the more sensitive sheath would augment the explosive power of the system.

The reactivity of the core and sheath emulsions can be controlled in a number of ways: Firstly, the density of the sheath can be altered by changing the amount of aeration/microballoons in the sheath. Fewer hot spots would result in decreased reactivity.

Secondly, the type of microballon could be changed. A different void material could be used between core and sheath. Small efficient voids to ensure detonation in the core whilst the sheath contains inefficient/large voids to reduce velocity of detonation (VOD) and reactivity.

Thirdly, the chemical reactivity of the emulsion could be reduced by increasing the water content or ratio of metal nitrate salts to ammonium nitrate in the aqueous solution.

The viscous sheath preferably is also pumpable at process temperatures of around 9O0C and extruded continuously. In this instance, thixotropic liquids would be especially useful in that they would easily flow under the shear of the extruder, but thicken up once within the sheath covering.

The invention will now be illustrated by way of the following examples. Examples 2, 4, 5, 7, 9, 10, 11, 12 and 15 (1, 3, 6 and 8 are comparative examples) illustrate how explosive systems of the invention can be adapted to pass one or more of the permitted tests, while examples 13, 14 and 15 illustrate how to increase the bulk strength of an emulsion explosive while still retaining a desirable detonation sensitivity.

The emulsion explosives were prepared by standard processes well known in the art. The core explosive was packed into a standard plastic tubing such as is know in the art, which was located co-axially within a similar outer sheath skin and into which was extruded the sheathing material. Both skins were then sealed.

EXAMPLE 1 (Comparative example) 260g of core explosive of formulation A, diameter 25mm, was fired unsheathed in the P5 series (i)* configuration. Five ignitions were obtained from five shots.

* HSE Buxton, Derbyshire, UK-defined test.

Formulation A Ammonium Nitrate 66.15 Sodium Nitrate 13.23 Water 11.24 Surfactant 1.35 Oil 0.91 Wax 3.83 GMB 3.29 EXAMPLE 2 570g of a sheathed combination comprising 25mm explosive core (formulation A), and a 6mm gelled water sheath was submitted for P5 series (i) tests. The gelled water sheath was prepared by dissolving 15 parts water gellant 35-0019 into 85 parts cold water. Water gellant 35-0019 is available from Freeman Chemicals Limited, Ellesmere Port, South Wirral, L65 OHB, England, and is one of a range of diisocyanate prepolymers which can be used to form stable gels with water. No ignitions were obtained from a total of 25 shots fired.

EXAMPLE 3 (Comparative example) 465g of core explosive of formulation A, diameter 25mm, was fired unsheathed in the P5 series (ii) configuration. Five ignitions were obtained from five shots.

EXAMPLE 4 1020g of the sheathed combination comprising the core from example 3 surrounded by 6mm sheath of gelled water was submitted for P5 series (ii) testing. No ignitions were obtained from a total of eight shots.

In 25/37mm diameter combinations, although the package was shown to achieve the required levels of safety to pass the P5 tests (see examples 2 and 4) the gap propagation results could be further improved by increasing the core diameter. An increase to 28/37mm diameter combination maintains the levels of incendivity below that required to pass the P5 tests (see examples 7 and 9) whilst improving gap sensitivity. An example of this improvement in gap sensitivity is shown below in example 5.

EXAMPLE 5 A lm column of sheathed explosive as described in example 7 made up of three cartridges was fired in accordance with the requirements for continuity of detonation (COD). The explosive detonated the full length of the column and therefore passed the COD test.

EXAMPLE 6 (Comparative example) 350g of core explosive of formulation A, diameter 28mm, was fired unsheathed in the P5 series (i) configuration. Four ignitions were obtained from four shots.

EXAMPLE 7 570g of a sheathed explosive comprising 350g of core explosive (formulation A), diameter 28mm, surrounded by a gelled water sheath (as prepared in example 2) approximately 4.5mm thick (total cartridge diameter 37mm) was prepared for P5 gallery testing. The cartridges were submitted for testing in the series (i) condition. No ignitions were obtained from 20 shots in series (i) test.

Density of the explosive WAS l.l0gcm3. Sheath density was 0.89gem'3.

EXAMPLE 8 (Comparative example) 630g of core explosive, formulation A, diameter 28mm, was tested in the P5 series (ii) configuration. Ignitions were obtained from all five shots.

EXAMPLE 9 1020g of a 28/37mm combination of explosive core formulation A, surrounded by gelled water sheath was submitted for PS series (ii) tests. No ignitions were obtained in a total of eight shots.

EXAMPLE 10 A sheathed composition comprising 25mm explosive core (formulation A) surrounded by 6mm sheath of calcium sulphate plaster slurry was tested in the P5 series (i) and (ii) tests. The explosive passed the tests by giving no ignitions in either test.

EXAMPLE 11 A sheathed composition comprising 28mm explosive core (formulation A) surrounded by a 4.5mm sheath of composition B gave no ignitions when tested in the P5 series (i) and (ii) tests.

Composition B Calcium Carbonate 47.5 Water 47.5 Gelatine 5.0 Density 1.45gcm-3 EXAMPLE 12 This example illustrates the ability to meet the P4* level of safety with a sheathed explosive. The example comprises a 25mm core of explosive of composition C surrounded by a 6mm sheath covering of composition B. This sheathed combination passes the test criterion for P4 Break Test II* giving no ignitions at 227g.

Composition C Ammonium Nitrate 59.82 Sodium Nitrate 11.96 Water 10.16 Surfactant 1.22 Fuel 4.29 Sodium Chloride 8.05 GMB 4.50 Density 1.OOgcm'3 EXAMPLE 13 The example consists of a 50mm inner core of explosive of formulation D surrounded by 17.5mm of explosive more sensitive than D (formulation E). The total cartridge diameter is 85mm. This cartridge has a sensitive sheath which can be initiated by standard detonating cord running along its surface. The core material is designed to have a higher energy density than the outer sheath, therefore the combination the advantage of greater bulk strength relative to an explosive totally comprised of composition E coupled with cord sensitivity.

Formulation D Formulation E Ammonium Nitrate 64.5 Ammonium Nitrate 58.1 Sodium Nitrate 13.65 Sodium Nitrate 16.0 Water 12.0 Water 7.8 Oil 0.6 Methylamine Nitrate 9.7 Surfactant 1.5 Oil 0.87 Wax 3.0 Surfactant 1.55 Aluminium 4.00 Wax 3.10 GMB 1.00 GMB 2.90 Density 1.25 Density 1.00 gem 3 9cam'3 EXAMPLE 14 This is essentially a reversal of example 13 and similarly gives a useful increase in bulk density over an 85mm cartridge of explosive E. The example consists of a 50mm explosive core of composition E surrounded by a 17.5mm explosive sheath of composition D. The total cartridge diameter is 85mm. This cartridge has a sensitive core capable of being initiated by a conventional detonator/primer arrangement.

EXAMPLE 15 This is an example of achieving permitted safety in gas using the explosive-explosive system. The cartridge consists of 25mm inner core of explosive of formulation F surrounded by a 6mm sheath of explosive of composition G.

Total cartridge diameter is 37mm. The explosive meets the Pl* criterion of safety in methane-air.

Formulation F Formulation G Ammonium Nitrate 64.34 Ammonium Nitrate 49.65 Sodium Nitrate 13.27 Calcium Nitrate 10.54 Water 13.80 Water 12.55 Oil 0.90 Oil 0.52 Surfactant 1.75 Surfactant 1.75 Wax 3.50 Wax 2.50 GMB 2.44 Sodium Chloride 19.87 (solid) Density 1.12gem'3 GMB 2.50 Density 1.17gcm-3

Claims (7)

1. CLAIMS 1. An explosive system comprising an emulsion explosive core and a viscous sheath material surrounding said core.
2. 2. An explosive system as defined in claim 1 wherein the viscous sheath material is a dispersion.
3. 3. An explosive system as defined in claim 2 wherein the viscous sheath dispersion is an aqueous gel or emulsion containing a substantial solids loading, for example, around 50% by weight.
4. 4. An explosives system as defined in claim 1 wherein the explosive core and viscous sheathed material are in intimate contact.
5. 5. An explosives system as defined in any one of the preceding claims wherein the viscous sheath material has dispersed therein flame quenching, shock wave absorbing or radical scavenging materials and/or microbubbles.
6. 6. An explosives system as defined in any one of the preceding claims wherein the viscous sheathed material comprises explosive materials capable of augmenting the explosive force of the system, for example, wherein the viscous sheathed material and core are different oil-in-water or water-in-oil explosive emulsions the explosive emulsion sheath being more dense and less sensitive to detonation than the explosive emulsion core, or visa versa.
7. 7. An explosives system substantially as described herein and as illustrated by the examples.

   7. An explosives system substantially as described herein and as illustrated by the examples.

   8. A method of blasting in an inflammable gas/air atmosphere wherein an explosive system as claimed in any of claims 1 to 7 above is used so composed and structured that there is attentuation by the viscous sheath material of gas ignition tendency upon core explosion.

   Amendments to the claims have been filed as follows 1. An explosive system comprising an emulsion explosive core and an agueous viscous sheath material surrounding said core that has microbubbles incorporated therein.

   2. An explosive system as defined in claim 1 wherein the viscous sheath material is a dispersion.

   3. An explosive system as defined in claim 2 wherein the viscous sheath dispersion is an aqueous gel or emulsion containing a substantial solids loading, for example, around 50% by weight.

   4. An explosives system as defined in claim 1 wherein the explosive core and viscous sheathed material are in intimate contact.

   5. An explosives system as defined in any one of the preceding claims wherein the viscous sheath material has dispersed therein flame quenching, shock wave absorbing or radical scavenging materials and/or microbubbles.

   6. An explosives system as defined in any one of the preceding claims wherein the viscous sheathed material comprises explosive materials capable of augmenting the explosive force of the system, for example, wherein the viscous sheathed material and core are different oil-in-water,or water-in-oil explosive emulsions the explosive emulsion sheath being more dense and less sensitive to detonation than the explosive emulsion core, or visa versa.