DE69224230T2 - Explosive containing foamed sensitizer - Google Patents

Description

Field of invention

The present invention relates to explosive compositions and in particular to sensitized explosive compositions.

State of the art

Semi-solid colloidal dispersions of aqueous explosives or blasting agents are well known. These products typically comprise an oxidizing component, usually mainly ammonium nitrate, a propellant component and water. These blasting agents are referred to in the art as explosive slurries (or as water gels) and as emulsion-type explosives.

Explosive slurries typically comprise a discontinuous propellant phase dispersed in a continuous aqueous solution of the oxidizing salt. Thickeners are added to the aqueous phase to increase the viscosity of the explosive or to cause gelation and thus stabilize the structure of the explosive.

Emulsion explosives typically comprise a discontinuous solution of an oxidizing salt dispersed in a continuous fuel phase. Emulsifying agents are generally added to the dispersion to stabilize it.

The addition of additives to both slurries and emulsion explosives to modify the performance of the explosive is also well known. These additives include, for example, the addition of aluminum or ammonium nitrate to the explosive to increase the strength and/or sensitivity of the explosive.

Of particular interest to the invention is the addition of additives to create tiny voids in the explosive, which voids can be used to control the density of the explosive and to increase the sensitivity of the explosive. These sensitized explosives have been described, for example, by Cattermole et al. in U.S. Patent No. 3,674,578; by Bluhm in U.S. Patent No. 3,447,978; by Wade in U.S. Patent No. 4,110,134 and by Clay in U.S. Patent No. 4,181,546.

One method of introducing voids into an explosive is by adding tiny hollow glass balloons to an emulsion explosive. While this method provides a convenient means of creating voids in the explosive, the tiny balloons are relatively expensive and can be difficult to handle due to their low bulk density.

The use of products similar to tiny balloons, which also have particles containing one or a number of gas bubbles, such as tiny inorganic spheres made of glass, sirasu (Japanese volcanic ash), silicic sand or sodium silicate and the like, is also known. These materials suffer from the same disadvantages as tiny glass balloons.

Edamura et al., in US Patent No. 4543137, have disclosed the use of a gas-retaining agent such as that made of expanded polystyrene, expanded polyurethane, and the like. The gas-retaining agents of Edamura et al., similar to the tiny inorganic balloons described above, can have a rigid structure and be brittle and break during handling, or they can be made soft and spongy so that they are more resistant to accidental breakage during handling.

These soft and spongy gas-retaining agents are made by foaming a foaming agent in a thermoplastic resin and curing the thermoplastic resin and thus entrapping gas in the resin structure.

However, this method of introducing gas cavities into the explosive requires the preliminary preparation of a tiny spongy or rigid spherical structure, which is added to the explosive.

The in situ creation of air or gas voids in the blasting agent is an alternative method to the addition of tiny gas-filled balloons and typically involves the addition of a material which reacts in the blasting agent to create a gas bubble. This gas bubble is carried along in the blasting agent due to the viscous nature of the semi-solid blasting agent. The creation of a gas void in the blasting agent by an in situ chemical reaction is referred to in the industry as chemical fumigation.

Chemical fumigation of explosives is well known in the slurry and emulsion explosives industry. In US Patent Nos. 3886010 and 3706607, Thornley and Chrisp respectively describe the use of chemical fumigants such as nitrites, weak acids, hydrazine and Peroxides in explosive slurries and/or emulsion explosives.

While chemical fumigation is practiced in industry, its use is limited due to the difficulty of controlling the reaction rate of the chemical fumigation reaction. The fumigation rate may be insufficient or excessively slow at cold manufacturing temperatures, and excessive at hot manufacturing temperatures, providing uncontrollable wellbore densities.

A third way to introduce gas voids into an explosive is to mechanically stir the explosive composition to entrain a trapped gas void within the explosive. This route has the disadvantage of intensive mechanical stirring of a sensitized explosive and may be subject to poor long-term stability of the explosive as the gas slowly escapes from the explosive.

Another way of creating gas voids in an explosive is described by Curtin and Yates in British Patent Application No. 2179035, whereby a gas bubble generating agent is added to the explosive before or during subjection of the explosive to a pressure above atmospheric pressure to dissolve at least a portion of the gas present. The explosive is rapidly returned to atmospheric pressure and a fine discontinuous gaseous phase is thereby created in the composition. However, this method of preparation requires the preparation of the sensitized explosive under pressure and thus requires special equipment adapted to the handling of the pressurized explosive.

The concept of incorporating an aqueous suspension based gas-in-liquid foam into an explosive composition was also proposed in GB-A-1270319 and EP-A-0228354.

In light of the problems of the prior art gas void introduction methods, it is an object of the present invention to provide an emulsion disintegrant sensitized by the introduction of gas voids, wherein the gas voids are produced and introduced into the disintegrant in a manner different from those described above.

Summary of the invention

Accordingly, the present invention provides a process for producing an explosive composition from a water-in-oil emulsion which is sensitized by dispersed gas bubbles, wherein the gas bubbles are introduced into the emulsion explosive composition by mixing a gas-in-liquid foam comprising at least 90% by volume of gas into an emulsion explosive composition, wherein the gas in the foam becomes a dispersion of sensitizing gas bubbles in the emulsion explosive composition, wherein the liquid in the gas-in-liquid foam comprises a liquid or liquefiable propellant and a foaming agent.

The term "foam" in this specification and in the claims is used to describe a mass of bubbles dispersed in a liquid. The bubbles are composed of thin flexible films of a liquid with optional viscosity-controlling agents or foaming agent molecules which are used to stabilize the film at the Interface between gas and liquid are absorbed, surrounded and thus separated from each other.

Most of the volume of a foam is in the gas phase, and typically the gas phase comprises at least 90 volume% of the foam.

The foam used in the invention can be produced by introducing or "atomizing" a pressurized gas into a closed vessel containing the pressurized liquid component of the foam, mixing and subsequently removing the pressure from the system to form small gas bubbles in the liquid component. The foam produced can then be added to the basic explosive composition and mixed with it, for example, using a low shear mixer or a static mixer.

The pressurized gas used to form the foam may be any gas compatible with the other explosive components. Preferably the gas is air, carbon dioxide or nitrogen, but any other gas could be used provided that the solubility of the gas in the liquid is adjustable over the entire time and temperature range in which the sensitized explosive is stored prior to use.

Bubbles can also be dispersed in the liquid carrier by mechanical agitation to produce a foam, such as by a high shear mixer such as an Oakes mixer or by mixing the liquid carrier with low shear to entrain gas voids in the liquid carrier. In the process of the invention, mechanical agitation is carried out on a non-explosive liquid carrier and is therefore inherently safer and more effective than a method of directly introducing gas cavities into the explosive composition.

The foams of the invention preferably have a low density so as to effectively reduce the density of the explosive to which the foam is added. Preferably, the foam has a density of less than 0.2 g/ml, more preferably below 0.1 g/ml, and even more preferably below 0.06 g/ml.

Once the foam is created, the liquid phase surrounding each bubble begins to drain, creating a thinner layer at the top of the bubble than at the bottom. Eventually, the thinner layer breaks, causing the bubbles to collapse or gas to escape from the foam, and the carrier liquid to be lost or drained from the foam. The escape of gas is thus related to the drain rate of the foam in that the volume of gas is reduced as liquid drains from the foam. The stability of the foam can therefore be measured by determining the half-life of the foam, where the half-life is the time it takes for half the volume of gas to escape from the foam. The half-life is therefore an indication of the durability of the foam after it has been manufactured.

The control of the drainage rate and thus the control of the half-life of the foam can be influenced and effectively controlled by adding additives to the foam which stabilize the liquid film around the bubble. If the foam is to be added to the explosive shortly after its manufacture, for example within one to four minutes after manufacture, the foam stability is not as critical as with foams which which are manufactured and added to the explosives only later.

Additives such as high-viscosity polyisobutylene serve to increase the viscosity of the liquid film around the gas bubble. In addition, additional additives such as foaming agents are preferably added to the liquid carrier to support the formation of the foam.

Accordingly, the foams of the invention preferably comprise a gas, a foaming agent, a viscosity controlling agent and a liquid carrier.

The foaming agent provides a film around the gas bubbles to protect them from bursting or collapsing. Typical foaming agents would include materials such as proteins, and more specifically milk proteins, egg proteins, animal proteins, vegetable proteins, fish proteins, and any mixture thereof. The foaming agent may also be a protein derivative or compound product, such as phospholipids, lipoproteins, collagens, hydrolyzed proteins, and globulins. Steroids may also be used as foaming agents.

The foaming agent may also include surfactants such as FC740 (trademark) or FC751 (trademark) which are perfluorinated surfactants, or mixtures of other surfactants. Other foaming agents include lanolin oil, derivatives of succinic anhydride, glycerol monostearate, stearyl octacylene phosphate and long chain alcohols.

Casein is a mixture of proteins extracted from milk solids or soybeans and can be used as such or divided into a water-soluble portion or an oil-soluble portion. fraction, each of which can be used separately as a foaming agent. The casein used may be completely or partially soluble in oil, but is generally dispersible so that it is not detrimental to foam production.

The stability of the foam can also be increased by adding solid particles such as carbon black, talc or other materials known in the art for foam stabilization.

The viscosity in the foam can be controlled by adding viscosity controlling agents such as high viscosity polyisobutylene, butyl rubber, natural rubber, difunctional high molecular weight acids and the like, and mixtures thereof, to the liquid carrier, which results in increasing the viscosity of the liquid component of the foam.

However, an excessively high viscosity of the liquid component of the foam is undesirable because the high viscosity makes the foam difficult to manufacture. Therefore, an optimal level of viscosity control agent is required to ensure that the sag rate of the foam is low, but the foam is also relatively easy to manufacture.

In an oil-based foam, the viscosity of the oil selected also affects the ease with which the foam can be manufactured, as well as the rate of sagging of the foam. The selection of oil therefore depends on, among other things, the foam manufacturing process, mixing conditions, temperature, residence time, pressure, gas type, and the like.

The foam can be added to sensitize any suitable explosive material where gas cavities are advantageous. Explosive materials include in particular emulsion explosives, but also propellants, high heave explosives such as heavy ANFO, modified emulsions, cast explosives, nitroester-based systems and TNT, RDX or NG-based systems.

The liquid carrier used to make the foam is a liquid which is preferably compatible with the continuous phase of the explosive and in which the preferred additives of the foam forming system can be dispersed or dissolved. The liquid carrier may participate in the detonation as a fuel or may comprise an oxidizer or a sensitizer or may be non-reactive.

Preferred liquids for an emulsion explosive are non-aqueous oils and solvents which are miscible with the liquid organic phase. Most preferred, however, are liquids or liquefiable materials which serve as fuels in the explosive reaction. Typically, fuels include, for example, paraffin oil and fuel oil. However, the liquid carrier need not be perfectly compatible with the continuous phase of the explosive, provided that the explosive composition produced remains sufficiently stable to allow for storage stability suitable for the proposed use of the explosive composition.

Emulsion explosives include emulsions with little to essentially no water.

The liquid carrier preferably contributes to the total propellant phase present in the emulsion explosive and can therefore be described as a propellant for the explosive as described above. However, in preparing the foam, it is desirable to minimize the amount of liquid used in preparing the foam in order to maximize the amount of liquid available for the prior formation of the base emulsion explosive.

By selecting a liquid carrier of appropriate viscosity, such as high viscosity paraffin oil, the use of a viscosity control agent can also be reduced or eliminated.

The foam, once prepared, is added to a basic explosive composition which is an insufficiently or not at all sensitized emulsion explosive. The foam is preferably added to the basic explosive composition shortly after its preparation in order to minimize the need to prepare foams which are stable over longer periods of time. Experience has shown that the use of foams with a half-life of greater than one to four minutes is desirable in order to provide sufficient time for the foam to be mixed into the explosive composition.

The foam is preferably added to the explosive base composition by a low shear mixing technique such as a static mixer or a rubber mixer. During the addition of the foam, the foam is broken up and its gas bubbles are merely dispersed in the explosive base composition. At this stage, there is generally no need for intensive mechanical stirring to introduce additional gas cavities into the explosive composition.

The explosive base composition to which the foam sensitizer is added can be any chemically compatible emulsion explosive whose explosive base compositions are described in the prior art. These explosive base compositions generally consist of a water-in-oil dispersion of an aqueous solution of an oxidizing salt and a propellant.

The oxidizing salt may be any of the oxygenated salts typically used in the industry. These salts include, for example, nitrates, chlorates and perchlorates. Most preferred are salts such as sodium nitrate, calcium nitrate, potassium nitrate and most preferably ammonium nitrate or mixtures thereof. For an emulsion explosive, the oxidizing salt may be melted (e.g. as a eutectic mixture) to provide a liquid which can be dispersed in the fuel as a discontinuous phase or, more preferably, as a concentrated aqueous solution.

The fuel phase may consist of any liquid or liquefiable fuel known in the explosives art and may be identical or different from the fuel used in the manufacture of the foam. Suitable materials include mineral oil, waxes, paraffin oils, benzene, toluene, xylenes and mixtures of petroleum distillates such as gasoline, kerosene and diesel fuel.

An emulsion explosive formed as an embodiment of the invention preferably also includes stabilizing surfactants such as a mixture of sorbitan sesquioleate and a polyisobutylene succinic anhydride (PIBSA) based surfactant. These PIBSA based surfactants are described in Canadian Patent No. 1244463 (Baker). However, any surfactant useful in the emulsion explosives art may be used to prepare the emulsion used in the present invention.

Many suitable conventional emulsifiers have been described in detail in the literature and include, for example, sorbitan esters such as sorbitan sesquioleate, sorbitan monooleate, sorbitan monoalmitate, sorbitan monostearate and sorbitan tristearate, the mono- and diglycerides of fat-forming fatty acids, lecithin from soybeans and derivatives of lanolin such as isopropyl esters of lanolin fatty acid esters, mixtures of higher molecular weight fatty alcohols and wax esters, ethoxylated fatty acid ethers such as polyoxyethylene lauryl ether, polyoxyethylene oleyl ether, polyoxyethylene stearyl ether, polyoxyethylene oleyl laurate and substituted oxazolines such as 2-oleyl-4,4-bis(hydroxymethyl)-2-oxazolines. Suitable mixtures of such conventional emulsifiers may also be selected together with one or more modifiers for use in the compositions of the invention.

The explosive compositions may also comprise additional additives to enhance or modify the properties of the explosive. The use of these additives is well known in the explosives industry and includes the solid additives and sensitizers commonly added to emulsions, for example, aluminum, ferrosilicon, TNT, AN, MAN, PETN and the like. Furthermore, additional sensitizers such as tiny glass balloons may also be used in combination with the foams of the invention.

According to a further embodiment of the invention, a method for producing a sensitized explosive composition according to claim 10 is also provided.

In this embodiment, the foam is prepared by mixing a carrier liquid with a foaming agent to form a foam forming solution and subjecting the solution to mechanical agitation such as in a high shear mixer, low shear mixer, or static mixer or other mixing equipment known to one skilled in the art of foam production, or to pressurized gas atomization or to foaming by a chemical reaction in the foam system.

Examples

The invention will now be described with reference to the following examples.

example 1

A fuel (or oil) based foaming solution was prepared with the composition shown in Table 1. All percentages given are by weight unless otherwise stated. Table 1: Oil-based foaming solution

* perfluorinated surfactant and bubble stabilizing agent

A foam was prepared by atomizing the foam-forming solution with a pressurized nitrogen gas stream. A foam was prepared that had a gas volume of more than 890 vol%, a foam density of 0.12 g/mL, and a half-life of more than 45 minutes.

Examples 2 to 5

Emulsion explosive compositions comprising an oil-based foam were prepared in accordance with the invention using the formulations set forth in Table 3. In each example, the foam was prepared according to the procedure and formulation set forth in Example 1.

The oxidizing salt used in these examples was an ammonium nitrate/sodium nitrate mixture.

Paraffin oil was used as the oil phase in each example and a sufficient amount was added to produce a sensitized emulsion explosive with a total oil phase content of 5%. The total oil phase includes surfactants.

In each example, the oxidizing salt solution was added to the oil phase containing a PIBSA-based surfactant and a sorbitan sesquioleate mixture with mixing to prepare an emulsion explosive. Examples 2 and 3 were prepared using low shear mixing and Examples 4 and 5 were prepared by high shear mixing. The foam was dispersed in the emulsion by low shear mixing.

The sensitized emulsion for each example was placed in a 25 mm or 50 mm cartridge to test the blasting ability of the explosive.

The explosive capability of each composition was measured by determining the primer size required to detonate the composition. Therefore, in Table 2, under the blast results for each example, the primer size used and the blast result are given. If measured, the detonation velocity (DG) for a successful blast is given.

Examples 2 and 3 are typical of products that would be sold as "bulk" explosives and therefore would not be expected to have primer sensitivity. Examples 4 and 5 are typical of packaged products and primer sensitivity may be desirable. In all examples, explosives could be made using the foam sensitizer which had acceptable sensitivity for industrial use. Table 2: Blasting results of foamed explosives

¹ An 81/19 mixture (by weight) of ammonium nitrate and water with a fudge point of approximately 60ºC.

² A mixture of paraffin oil (4% of the total explosive weight), a PIBSA-based surfactant (0.66%) and sorbitan sequioleate (0.34%) was added to prepare an emulsion explosive with a total oil phase content of 5%.

³ A 77/11/12 (by weight) mixture of ammonium nitrate, sodium nitrate and water with a fudge point of approximately 75ºC.

&sup4; Blast results are given in grams of PETN in the detonator used for detonation and with the DG for the resulting explosion.

The stability of the manufactured product was measured by storing the explosive composition of Example 5 for 3 months. As shown in Table 2, the composition had an acceptable DG after three months of storage at 22ºC.

Example 6

A "doped" emulsion explosive sensitized by an oil-based foam prepared as in Example 1 was prepared with the following formulation:

i. AN/NN/WATER³ 68%

Oil phase²

ii. Ammonium nitrate prills 29%

iii. Foam 3% Density 1.18 g/ml 100 mm diameter

² A mixture of paraffin oil (4% of the total explosive weight), a PIBSA-based surfactant (0.66) and sorbitan sequioleate (0.34%) was added to prepare an emulsion explosive with a total oil phase content of 5%.

³ A 77/11/12 (weight ratio) mixture of ammonium nitrate, sodium nitrate and water with a fudge point of approximately 75ºC.

The explosive of this example was prepared by first emulsifying the AN/NN/WATER mixture in a mixture of paraffin oil and surfactant using a low shear mixer. The emulsified mixture was subsequently mixed with ammonium nitrate prills to produce a doped emulsion and the oil-based foam was added to sensitize the explosive. The doped emulsion had a DG of 3970 m/s when ignited with a propellant charge of 40 g of pentolite. The sensitivity of the doped emulsion was industrially acceptable for larger diameter applications.

Example 7

A heavy ANFO explosive sensitized by an oil-based foam prepared as in Example 1 was prepared using the following formulation:

i. AN/NN/WATER³ 50% Oil phase²

ii. Ammonium nitrate prills 47.5%

iii. Foam 2.5%

Density 1.25 g/ml

100mm diameter

² A mixture of paraffin oil (4% of the total explosive weight), a PIBSA-based surfactant (0.66) and sorbitan sequioleate (0.34%) was added to prepare an emulsion explosive with a total oil phase content of 5%.

³ A 77/11/12 (weight ratio) mixture of ammonium nitrate, sodium nitrate and water with a fudge point of approximately 75ºC.

The explosive of this example was prepared by first emulsifying the aninni water mixture in a mixture of paraffin oil and surfactant using a low shear mixer. The emulsified mixture was subsequently mixed with ammonium nitrate prills to produce a heavy ANFO explosive and the oil-based foam was added to sensitize the explosive. The doped emulsion had a DG of 3300 m/s when ignited with a propellant charge of 40 g of pentolite. The sensitivity of the doped emulsion was industrially acceptable for larger diameter applications.

Having described specific embodiments of the invention, it is to be understood that modifications thereto may be suggested by those skilled in the art, and all modifications falling within the scope of the appended claims are intended to be covered.

Claims (11)

1. A process for producing an explosive composition from a water-in-oil emulsion which is sensitized by dispersed gas bubbles, characterized in that the gas bubbles are introduced into the emulsion explosive composition by mixing a gas-in-liquid foam which comprises at least 90% by volume of gas into an emulsion explosive composition, the gas in the foam becoming a dispersion of sensitizing gas bubbles in the emulsion explosive composition, the liquid in the gas-in-liquid foam comprising a liquid or liquefiable propellant and a foaming agent. 2. A process for preparing an explosive composition according to claim 1, wherein the foaming agent is casein or a perfluorinated surfactant or a mixture thereof. 3. A process for preparing an explosive composition according to claim 1, which additionally comprises a viscosity-controlling agent. 4. A process for preparing an explosive composition according to claim 3, wherein the viscosity controlling agent is polyisobutylene or butyl rubber. 5. A process for preparing an explosive composition according to claim 1, wherein the explosive composition comprising an oxidizing salt and a propellant. 6. A process for preparing an explosive composition according to claim 1, wherein the foam comprises a liquid carrier which is miscible with the continuous phase of the emulsion. 7. A process for preparing an explosive composition according to claim 1, wherein the gas-in-liquid foam has a density of less than 0.2 g/ml. 8. A process for preparing an explosive composition according to claim 1, wherein the gas is carbon dioxide or nitrogen. 9. A process for preparing an explosive composition according to claim 1, wherein the gas-in-liquid foam has a half-life of greater than 4 minutes. 10. A process for preparing a sensitized emulsion explosive composition comprising: the preparation of a base emulsion composition by emulsifying an aqueous solution of an oxidizing salt into a liquid or liquefiable fuel; the preparation of a foam consisting essentially of gas-in-liquid, which comprises at least 90% by volume of gas, by mixing a liquid or liquefiable fuel with a foaming agent to form a foaming solution and subjecting the solution to mechanical stirring or to pressurized gas atomization or to foaming by a chemical reaction in the foam system; and mixing the gas-in-liquid foam into the Base emulsion composition for providing an air bubble sensitized explosive. 11. Use of a gas-in-liquid foam containing at least 90% by volume of gas as a sensitizing agent in the preparation of an explosive composition from a water-in-oil emulsion by mixing the gas-in-liquid foam into the emulsion explosive composition, the gas in the foam becoming a dispersion of sensitizing gas bubbles in the emulsion explosive composition, the liquid in the gas-in-liquid foam comprising a liquid or liquefiable propellant and a foaming agent.