WO2003051793A2

WO2003051793A2 - Method of preparing a sensitised explosive

Info

Publication number: WO2003051793A2
Application number: PCT/ZA2002/000191
Authority: WO
Inventors: Charles Michael Lownds
Original assignee: Sasol Chemical Industries Limited
Priority date: 2001-12-17
Filing date: 2002-11-29
Publication date: 2003-06-26
Also published as: CA2470825A1; AU2002351427A1; EP1458659A2; PE20030685A1; RU2004121964A; BR0215007A; CN1604885A; AR037780A1; WO2003051793A3; MXPA04005834A

Abstract

According to the present invention there is provided a method of preparing a sensitised explosive comprising the steps of providing thermally expandable microspheres (14) which, when expanded, are suitable to reduce the density of an explosive and mixing the microspheres with steam (12) to cause thermal expansion of the microspheres and to provide a resulting stream of wet expanded microspheres. The resulting stream of wet expanded microspheres is then introduced into an explosive (18) to provide a sensitised explosive (23).

Description

METHOD OF PREPARING A SENSITISED EXPLOSIVE

Technical Field

This invention relates to a method of sensitising an explosive by providing expanded microspheres therein.

Background Art

In most fluid explosives or slurry explosives, voids are introduced into the explosives in order to reduce their natural densities and thereby increase their sensitivity to a required level for detonation. This process is known as sensitisation.

Sensitisation is an important part of the art of manufacturing fluid explosives or slurry explosives. Known methods of sensitisation include the use of chemical gassing, whereby gas bubbles are generated at the required rate and to the required extent to effectively sensitise the explosive. Another well-known method is to add so-called mechanical voids,, to an unsensitised explosive. Mechanical voids that are well known include expanded perlite and microspheres. Microspheres may include organic spheres such as polystyrene, or microballoons including glass microballoons or polymeric microballoons. A microballoon comprises a hollow body or shell (usually made of glass or a suitable polymeric material) and a gas contained in the hollow body. In the case of polymeric microballoons the gas is known as a. blowing agent. Explosive manufacturers usually buy pre-expanded microballoons which they then mix with an explosive to produce a sensitised product. Pre-expanded microballoons are prepared by subjecting unexpanded microballoons to heat and other treatments causing them to expand and thus having a reduced density compared to the unexpanded microballoons. Almost all kinds of expansion are too complex for the manufacturer of explosives to carry out with the result that they purchase pre-expanded microballoons. Microspheres such as polystyrene can also be pre-expanded in the same manner to reduce their density.

Several different methods of thermal expansion of polymeric microspheres are known. Known methods of dry expansion involve relatively high capital cost, in particular because of the precautions that must be taken against fire or explosion of the blowing agent (usually a hydrocarbon gas) that is liberated during the expansion. On the other hand steam expansion does not have this hazard because of all the water present in both droplet and vapour form during the entire time when the hydrocarbon gas is liberated. One known method of steam expansion comprises preparing a slurry of microspheres which is fed to a pipe together with steam thereby causing the microspheres to expand. US patent 4,513,106 teaches that a disadvantage of this method is that the expanded particles leaving the pipe have to be cooled directly with water in order not to agglomerate which results in a final product having unsatisfactory high water content. It is stated that if cooling water is not used or is replaced by cooling air or a cooled apparatus mantle, a product of fused particles will be obtained. US 4,513,106 discloses a method of overcoming this problem whereby a slurry of unexpanded microspheres is introduced into a pressure zone to which steam is fed to partly expand the microspheres. The partially expanded microspheres then leave the pressure zone under a pressure drop whereby the spheres are further expanded and accelerated into steam with a velocity of at least 1 meter per second. The accelerated stream is injected into a gas volume whereby the stream is disintegrated and cooled. The expanded non-agglomerated spheres are then separated from the gas. it will be appreciated that this process is a fairly complicated one for avoiding agglomeration of expanded spheres, agglomeration being a common problem where microspheres are thermally expanded.

A further problem with especially pre-expanded microspheres is that they are bulky due to their low density. This results in high transportation costs and high storage costs of the spheres. These spheres, especially glass microballoons, are also difficult to handle. They are prone to fluidise or settle in an unpredictable manner, and are prone to float in the air causing inhalation hazard for the operators handling them.

US Patent 6,113,715 discloses a method whereby unexpanded microspheres are expanded in situ in an emulsion explosive during the formation of the emulsion explosive i.e. during the emulsification process itself. Although this solves some of the above problems, the process introduces its own problems, amongst others difficulty in controlling the end-point of the expansion, since rapid cooling of the emulsion or emulsion component containing the expanded microspheres to quench the expansion is difficult both because of the large volume of the emulsion or component compared to that of the microspheres and because of the high viscosity of the emulsion or component containing the expanded microspheres. This process is also, potentially hazardous since the high shear emulsification process involves the introduction of significant energy to the emulsion. The emulsion is at least an energetic material and can be explosive. Simultaneous sensitisation of the emulsion while the emulsion is being subjected to high shear can lead to hazardous conditions.

It is accordingly an object of the present invention to provide an alternative method or process of preparing an explosive with expanded microspheres

therein.

Disclosure of the Invention

According to the present invention there is provided a method of preparing a

sensitised explosive comprising - providing thermally expandable microspheres which, when expanded , are suitable to reduce the density of an explosive; mixing the microspheres with steam to cause thermal expansion of the microspheres and to provide a resulting stream of wet expanded microspheres; and - introducing the resulting stream of wet expanded microspheres into an explosive to provide a sensitised explosive.

The term "explosive" as used in this specification includes, in addition to its normal meaning a non-sensitised explosive or explosive pre-cursor which becomes a sensitised explosive when sensitised by the addition of expanded microspheres.

It will be appreciated that the stream of wet expanded microspheres comprises a mixture of expanded microspheres and water in gaseous and/or liquid form. This stream may contain other substances such as air or oil in liquid or vapour form. The volume of air is preferably of the same order as the volume of the expanded microspheres to ensure that pneumatic conveyance is maintained and no blockages occur. The air is also very useful in cleaning unexpanded microsphere slurry out of the system at shutdown, again to avoid blockages due to expansion of the microspheres by residual heat in the metal of the system.

The thermally expandable microspheres may be made of a polymeric material. The polymeric material may comprise a polymeric foam, for example polystyrene. Preferably the microspheres comprise microballoons, preferably microballoons made of a polymeric material. The microballoons may comprise the product sold under the trade name of Expancel.

It will be appreciated that the microspheres are suitable to expand when contacted with steam.

Prior to expansion the microspheres are preferably provided in a suitable carrier fluid, preferably a liquid to provide a slurry, which slurry is then introduced into the steam for expansion. The carrier liquid preferably comprises water. The carrier liquid may also contain oil such as mineral oil or fuel oil. The oil preferably reduces the viscosity of the final product and thereby enables it to remain fluid with volume fractions of microballoons high enough to otherwise impart a paste-like consistency to the sensitised explosive. The carrier liquid may comprise a mixture of oil and water. The carrier liquid may also contain additives which may affect the properties of the explosive; for example, the carrier liquid may contain a cross-linker for a watergel explosive. The volume to volume ratio of unexpanded microspheres to carrier liquid may be any practical ratio, but preferably it is from 1 :1 to 1 :10, preferably it is from 1 :1 to 1 :2. The mass % of dry microspheres in the carrier liquid may be from 10% to 50%, but preferably it is from about 40% to about 45%.

A motive fluid may also be used to propel the microspheres (including a slurry thereof) thereby to introduce the microspheres as a stream into the steam. The motive fluid may comprise a gas, preferably air. In one embodiment, a slurry of microspheres is both atomised and injected into the steam using compressed air.

Preferably the steam is a stream of steam and preferably it is at a temperature of above 105°C, preferably between 110 and 130°C.

Where the microspheres are provided as a slurry the mass ratio of steam to slurry may be any suitable ratio. In one embodiment it may be from 1 :5 to 5:1 , preferably it is about 1 :2.

It will be appreciated that the stream of steam may include one or more other fluids therein, preferably a gas such as air or the like. The one or more other fluids may be introduced into the stream of steam prior to introducing the microspheres into the stream of steam.

The STP volume ratio of air to dry steam may be form 0 to 5:1 , preferably about

1 :1.

The microspheres and stream of steam may be mixed in any suitable manner but preferably they are mixed by introducing both the microspheres and the steam into a tubular member. An eductor may be used to introduce the microspheres and steam into the tubular member or other mixing device. Alternatively, the microspheres (preferably in the form of a slurry and mixed with a motive fluid) may be sprayed into the stream of steam and preferably the resulting mixture is introduced into a tubular member where further mixing of the microspheres and steam take place.

The stream of expanding microspheres and water is preferably conveyed at very high velocity down the tubular member. The high velocity usually ensures high turbulence in the tubular member, thus ensuring good heat transfer from the steam to the microspheres, which may result in almost instantaneous expansion of the microspheres.

The rate of steam supply and the physical arrangement of the tubular member into which the microspheres and the steam and optionally the air are injected determines the pressure in the resulting stream of wet expanded microspheres. This mixture preferably contains saturated steam and its temperature is therefore determined by the pressure. The temperature profile to which the expanding microspheres are subjected can be controlled by controlling the

pressure profile.

The residence time of the microspheres in the steam and the maximum temperature of the stream of wet expanded microspheres is in use controlled to achieve the desired extent of expansion. The explosive may comprise an explosive which normally includes water therein.

In one embodiment of the invention the explosive may comprise a fluid explosive (preferably a liquid based explosive) such as a gel explosive or an emulsion explosive. The explosive may comprise a watergel or a water-in-oil emulsion.

Alternatively the explosive may comprise a slurry explosive, preferably a pumpable composition. The slurry explosive may comprise a mixture of a fluid explosive and a granular material. The granular material may comprise an ammonium nitrate product, preferably ammonium nitrate fuel oil (ANFO) but it may also comprise other products such as granular nitrate, a granular propellant or even a granular or particulate explosive such as TNT.

The explosive is preferably provided at a temperature which quenches the

expansion of the expanded microspheres when received therein.

Accordingly the explosive is preferably provided at a temperature below the

temperature at which expansion of the microspheres will take place (the

expansion temperature). Preferably the explosive is at a temperature below

80°C. In the case of a watergel the temperature of the explosive may be at ambient temperature. In the case of an emulsion the temperature of the

explosive may be from ambient temperature to about 60 to 75°C. Last

mentioned temperature range is the normal manufacturing temperature of an

emulsion. It will be appreciated that the explosive may preferably also serve

to prevent or reduce agglomeration of the expanded microspheres.

The mass % of expanded microspheres in the sensitised explosive may be

from 0.03 to 0.4%, but is preferably from 0.06 to 0.1 %. The volume of the

stream of wet expanded microspheres and optionally air may comprise

about 5% to about 100% of the volume of the explosive, but will preferably

be about 10 to about 30% by volume of the explosive.

The stream of wet expanded microspheres is preferably introduced directly into the explosive without intermediate processing, especially intermediate processing such as drying or filtering to separate fluid, especially water, from the expanded microspheres.

The explosive and expanded microspheres are preferably thoroughly mixed and any mixing means suitable for safe use with explosives may be considered. Known mixing means include single-screw augers, twin-screw mixers, ribbon blenders, bowl mixers and the like. The inventor has found that convenient and effective mixing between the explosive and stream of expanded microspheres can be achieved by introducing the stream of expanded microspheres and a stream of the explosive into a tubular member. Barriers may be provided in the tubular member to improve the mixing process. Preferably however, the tubular member is non-linear to cause a change of direction of the streams as they flow through the tubular member. The tubular member may comprise a curved member, such a hose or pipe. Preferably the tubular member comprises a straight pipe with one 90 degree bend therein, preferably a short radius elbow. Preferably the bend is located towards the exit end of the tubular member.

Preferably both the stream of wet expanded microspheres and the explosive are introduced in substantially the same direction into the tubular member. Preferably the microspheres and the stream of steam are introduced co-Iinearly or co-axially into the tubular member. In one embodiment of the invention thus is achieved by means of a co-axial eductor.

The process preferably comprises a continuous process.

The process may be used in a fixed explosives manufacturing plant to sensitise an explosive in the manufacturing process. Such explosive may be one suitable for use in bulk or packaged form. The process may also be used at a site remote from manufacture of the explosive, for example at a storage site, to sensitise an as yet non-sensitised explosive. For example a non-sensitised explosive may be sensitised as it is pumped from a delivery vehicle into a storage tank. Alternatively, the process may be used on an explosives blending and delivery vehicle in order to sensitise the explosive before it is pumped to a delivery point, e.g. a borehole. This method may be used in addition to but preferably in place of the more common current method of chemical gassing for sensitisation.

The invention also relates to a sensitised explosive prepared according to the method substantially as set out above.

The invention will now be further described by means of the following non- limiting examples and drawings wherein:

Figure 1 is a diagrammatic drawing of an apparatus suitable for carrying out a method of sensitising an explosive according to the present invention;

Figure 2 is a diagrammatic drawing of another apparatus for carrying out a method of sensitising an explosive according to the present invention; and

Figure 3 is a diagrammatic drawing of yet another apparatus for carrying out a method of sensitising an explosive according to the present

invention. Referring now to Figure 1 , an apparatus 10 suitable for preparing a sensitised explosive according to the invention comprises an eductor 11 defining an inlet 12 an outlet 13 and a suction port 14. A tubular member in the form of an expansion pipe 15 is connected to the outlet 13 of the eductor 11 and to a suction port 16 of a second eductor 17. The pipe 15 has a length of approximately 1 meter and a diameter of 75mm. The eductor 17 also defines an inlet 18 and an outlet 19. A hose 20 is connected to the outlet 19. The hose 20 is curved to have a general S-shape, and the inlet 21 of the hose 20 is not in line with the outlet 23 of the hose 20. The hose is 1.6m long with a 100mm diameter.

Referring now to Figure 2, an alternative apparatus100 suitable for carrying out a method of sensitising an explosive according to the invention comprises a container 101 for an aqueous slurry of unexpanded microballoons and a pump

102 for pumping the slurry of unexpanded microballoons into an orifice sprayer

103 through an inlet 104. Air is introduced into the orifice sprayer 103 through the inlet 105 and this air serves as a motive fluid to propel the microspheres into a mixing tee 106. Air and a stream of steam are introduced into the mixing tee through the inlet 107. The mixing tee has a diameter of 25mm. The resulting mixture of steam, air and microballoons then flow from_. the mixing tee 106 into a tubular member in the form of an expansion pipe 108 with a length of 1 ,5m and a diameter as shown in Table 2. The microballoons expand in the expansion pipe 108 in the presence of the steam to form a stream of wet expanded microballoons. The expansion pipe 108 also serves as a suction port of an inline and co-axial Penberthy eductor 109. In use an explosive is introduced into the eductor 109 through the inlet 110 which explosive serves as the motive fluid for the eductor 109. The eductor 109 is coupled to a tubular member 111 in the form of a pipe and the stream of expanded wet microballoons from the expansion pipe 108 and explosive introduced through the inlet 110 are introduced co-axially to flow in the same direction into the tubular member 111 wherein they are mixed thoroughly. The tubular member 111 comprises a pipe (length of 1 ,5m and diameter of 76mm with a 90°C short radius elbow towards the exit end 113 of the member 111. A hose 114 is coupled to the tubular member 111 for delivering the mixture of sensitised explosive to a suitable point e.g. a truck, storage tank or borehole etc.

Referring now to Figure 3, an alternative apparatus 200 suitable for carrying out a method of sensitising an explosive according to the invention is shown. The apparatus 200 is very similar to the apparatus 100 of Figure 2 and the same reference numbers are used to denote corresponding parts. However, in this case the orifice sprayer 104 (of Figure 2) is replaced with an eductor 201. The eductor 201 defines an inlet 202 through which air (as the motive fluid is .introduced). The slurry of unexpanded microballoons is sucked into the eductor 201 through the suction port 203. The eductor 201 is coupled to the mixing tee 106. In this case only steam (not steam and air) enters the mixing tee 106 through the inlet 107.

Example 1

Utilising the apparatus of Figure 1

A slurry of unexpanded, thermally expandable polymeric microballoons was prepared by mixing (on a mass basis) 10 parts unexpanded microballoons of the type sold under the trade name of Expancel 551 WU with 5.7 parts of a carrier liquid in the form of water. This mixture comprised 45% dry weight Expancel 551 WU and 55% by weight water. The volume ratio of Expancel 551 WU: water was 1 :1.33. The Expancel 551 WU was in the form of a wet cake_. containing about 29% water.

The slurry of microballoons was pumped at a rate of 3.2kg/minute into the suction port 14 of the eductor 11 of the apparatus 10 of Figure 1. The motive fluid of the eductor 11 was a stream of steam at a temperature of about 115°C and a nominal pressure of O.δbar gauge pressure flowing at 1.0kg/minute through the inlet 12 of the eductor 11. The steam was nominally dry steam with little if any liquid water. An unknown amount of air, estimated to be approximately 0.45kg/minute, was sucked into the eductor 11 through the

suction port 14. The slurry of unexpanded microballoons was thus introduced into a stream of steam causing thermal expansion of the microballoons thereby providing a resulting stream of water wet expanded microspheres flowing through the ^' expansion pipe 15. The stream of water wet expanded microspheres comprised expanded microspheres, steam, liquid water in droplet form and air.

The stream of wet expanded microspheres was sucked into the eductor 17 through the suction port 16. A non-sensitised emulsion explosive in the form of a water-in-oil emulsion explosive entering the eductor 17 through the inlet 18 at a rate of 10OOkg/minute provided the motive fluid for the eductor 17. The resulting sensitisation by microspheres turned the non-sensitised emulsion explosive into a sensitised emulsion explosive.

The non-sensitised emulsion explosive comprised 8.5% (mass by mass) continuous fuel phase of a suitable mixture of mineral oil (obtained under the name of MODEF from Continental Nitrogen & Resources) and an emulsifier of the PlBSAtype (obtained underthe name NBX2000Afrom Nelson Bros.) The ratio of these in the fuel phase was 7:1 mass oil: mass PIBSA-type emulsifier. The discontinuous aqueous phase comprised an 82% (mass by mass) ammonium nitrate solution. The emulsion explosive had a viscosity of about 20 000 cp at 10rpm at a temperature of 75°C. The temperature of the emulsion explosive was about 75°C when introduced into the eductor 17. In this arrangement where the explosive is used as the motive fluid in the eductor 17, the pressure at the exit of the expansion pipe (15) is less than atmospheric pressure. Because the stream issuing from the pipe (15) contains a mixture of steam and water in equilibrium, the drop in pressure to below atmospheric pressure is believed to cause evaporative cooling of the mixture to below the boiling point of water at atmospheric pressure i.e. to below 100°C. This provides an important safety feature for the process, although it is not essential. The relatively huge mass of explosive compared to the mass of the hot stream of steam and microballoons will cause substantially immediate cooling. It can easily be shown, as one skilled in the art will realise, that the temperature rise of the final product of this process can easily be limited to a few degrees. It is thus relatively easy to control temperatures at all points in the explosive to safe limits.

in this manner the stream of wet expanded microspheres was introduced into the emulsion explosive and they mixed thoroughly in the hose 20. The resultant mixture flowing from the outlet 23 may be received in any suitable vessel or may be introduced directly into a borehole.

The density of the emulsion explosive prior to introduction of the expanded microspheres was 1 ,3 g/cc and after the introduction of the expanded microspheres the emulsion explosive had a density of 1 ,24 g/cc. Examples 2 to 10 Utilising the apparatus of Figures 2 and 3

The apparatus of Figures 2 and 3 respectively were used to conduct further experiments.

The slurry of unexpanded thermally expandable polymeric microballoons contained 40% (by mass) microballoons and 60% (by mass) water with a volume ratio of about 1 :1.5. Microballoons of the type sold under the trade name Expancel 007WU was used. [This wet unexpanded material is a kind of filter cake. It contained about 30% (by weight) water, varying from batch to batch and printed on the container; that figure is used to calculate what wet weight ratios to use to get 40% dry weight].

The emulsion explosive was the same as used in example 1.

The common test parameters used in examples 2 to 10 are provided in table 1.

Table 1 : Common Tests Parameters

When the apparatus of Figure 2 was utilised the rate of air supply through

the orifice sprayer 103 was estimated to be 0.7kg/min and the rate of air

supply to the inlet 107 of the mixing tee 106 was about the same. When

the apparatus of Figure 3 was used the rate of air supply to the inlet port

202 of the educator 201 was estimated to be about 0.7kg/min.

The results obtained are provided in Table 2 Table 2

The quality of dispersion of the microballoons in the sensitised emulsion explosive of examples 2 to 10 was extremely uniform in all cases. No broken balloons were visible under the microscope, nor were any agglomerates visible; the microballoons were spherical and separate.

It will be appreciated that by introducing the expanded microspheres directly into the explosive the explosive quenches the expansion process and also prevents agglomeration of the expanded microspheres. It is accordingly not required to use expensive cooling equipment. It is also not necessary to introduce cooling liquids which would dilute the expanded microspheres. The introduction of such cooling liquids could reduce the dry content of the expanded microspheres below levels acceptable for use in explosives. Drying equipment would then be required to reduce the cooling liquid to an acceptable . level.

According to the present invention the wet or aqueous expanded microspheres can be introduced directly into the explosive without the necessity of intermediate processing such as filtering or drying, intermediate storage, metering of expanded microspheres, etc.

It will also be appreciated that the water level in the sensitised explosive can be controlled by controlling the amount of water in the expanded microspheres and the amount of water provided in the emulsion explosive prior to introduction of the expanded microspheres. Since the expanded microspheres are introduced directly into the explosive, less water is required to prevent excessive agglomeration of the expanded microspheres and thus rendering it easier to control and limit the amount of water introduced into the explosive.

The process also has the advantage that low density expanded microspheres do not have to be transported, stored or handled. Accordingly the difficulties associated therewith are avoided.

It will be appreciated that many variations in detail are possible without thereby departing from the scope and spirit of the invention.

Claims

1. A method of preparing a sensitised explosive comprising - providing thermally expandable microspheres which, when expanded, are suitable to reduce the density of an explosive; mixing the microspheres with steam to cause thermal expansion of the microspheres and to provide a resulting stream of wet expanded microspheres; and introducing the resulting stream of wet expanded microspheres into an explosive to provide a sensitised explosive.

2. The method of claim 1 wherein the thermally expandable microspheres comprise microballoons made of a polymeric material.

3. The method of either one of claims 1 or 2 wherein, prior to expansion , the microspheres are provided in a suitable carrier liquid to provide a slurry, which slurry is then introduced into the steam for expansion.

4. The method of claim 3 wherein a motive fluid in the form of a gas is used to propel the slurry of microspheres.

5. The method of any one of the preceding claims wherein the steam is a stream of steam.

6. The method of any one of the preceding claims wherein the steam is at a temperature of above 105°C.

7. The method of any one of the preceding claims wherein the microspheres and steam are mixed by introducing both the microspheres and the steam into a tubular member.

8. The method of any one of the preceding claims wherein the explosive comprises a fluid explosive selected from the group consisting of a gel explosive and an emulsion explosive.

9. The method of claim 8 wherein the explosive comprises a water-in-oil emulsion.

10. The method of any one of claims 1 to 7 wherein the explosive comprises a slurry explosive.

11. The method of any one of the preceding claims wherein the explosive is provided at a temperature below the temperature at which expansion of the microspheres take place thereby quenching the expansion of the expanded microspheres when the expanded microspheres are received in the explosive.

12. The method of claim 11 wherein the explosive is provided at a temperature below 80°C.

13. The method of any one of the preceding claims wherein the explosive and expanded microspheres are mixed by introducing them as streams into a non-linear tubular member causing them to flow through the non-linear tubular member.

14. The method of claim 13 wherein the non-linear tubular member comprises a curved pipe.

15. The method of either one of claims 13 or 14 wherein both the stream. of wet expanded microspheres and the explosive are introduced co-linearly into the tubular member.

16. A sensitised explosive prepared according to the method of any one of the preceding claims.