Method for the Production of Fine-Grained Explosive Substances

The present invention relates to a method and a device for producing compositions including fine-grained crystalline explosive substances. The overall term explosive substance as used in the present context embraces both low explosives and high explosives. Furthermore, in this context the term low explosive is taken to mean such explosive substances as are normally disintegrated by combustion and which are used for propulsion of rockets of one kind or another or for projectiles in guns, while the term high explosive relates to such explosive substances as are caused in their main use to disintegrate by detonation.

The demand for fine-grained explosives for the production of, for example, nitramine propellant and PBX has increased in recent years. In order to be able to meet this demand, new avenues of approach have been tested for producing fine-grained fractions of old, tried and proven crystalline high explosives such as hexogen or trimethylene trinitramine (RDX), octogen or cyclotetramethylene tetranitramine (HMX) and pentyl or pentaerytrol trinitrate (PETN) and others. In this disclosure, the expression fine-grained crystalline substances is taken to mean such as those having a mean particle diameter (MPD) of less than 20 .mu.m.

The currently applied processes for producing fine-grained high explosives are either doubtful from the standpoint of safety or are impossible from considerations of economy. Numbered among the first group is the currently most widely used method which is based on grinding the high explosive in a mill in the presence of large amounts of liquid, a method which, with the passage of time, has suffered--despite the addition of liquid--from a large number of incidents and consequently can hardly be described as entirely without risk. A further component of this problem structure is that it is difficult to produce sufficient amounts of fine-grained high explosives simply by separating the most finely-grained fractions from each batch of crystalline high explosive. Moreover, the mixing stage itself is an integral part of the production of composite explosives, a stage which, in particular if it must be carried out in the dry state, involves an additional element of risk.

The object of the present invention is to obviate these problems and offer a safe and reliable method of producing desired amounts of fine-grained crystalline high explosives for use directly or together with a binder in PBX, or as a sub-component in a low explosive. Moreover, the present invention allows direct manufacture of a low explosive compound incorporating a fine-grained explosive.

As examples of components included in a low explosive mention may be made of ethylene cellulose (EC), cellulose acetate (CA), cellulose acetate butyrate (CAB), nitrocellulose (NC), nitroglycerine (Ngl), adipates, phthalates, stabilizers and combustion catalysts.

The present invention is based on rapid crystallization and precipitation of relevant crystalline and non-crystalline substances in a steam-driven ejector. Water vapour is suitably used to drive the ejector.

This method has several different advantages besides providing crystals of the desired size, i.e. with a mean particle size of less than 20 .mu.m and a uniform distribution of any other substances as may be included in, for example, a low explosive. The device employed for carrying out the method according to the present invention--this device also being included in the invention as disclosed herein--completely lacks moving parts in the ejector used for precipitation of the substances included and the subsequent cyclone or separator, this providing a simple device in which every risk of overheating of bearings or boiling dry, with all the implicit mechanical risks of ignition, have been entirely eliminated. Moreover, the method and the device according to the present invention are easy to control and may be operated at high capacity. The method and the device according to the invention also enjoy the advantage that they offer direct purification of the solvent in conjunction with the precipitation stage, whereby the solvent will become immediately available for re-use, which obviously is economically advantageous.

According to the invention all relevant components, which may be both pure high explosives and mixtures of other components desired in the end product (which may thus also be a low explosive) and explosive, are dissolved in a suitably vaporizable solvent, such as acetone or methylethyl ketone (MEK), the solution being heated to just below the boiling point of the solvent. The solution is then subjected to positive pressure for two reasons, firstly to prevent it from beginning to boil in the pipes and secondly for reasons of transport engineering. To raise the pressure, use is made of a suitable inert gas such as nitrogen or carbon dioxide. The positive pressure need not be extreme, one or a few atmospheres being sufficient. The hot solution is then fed under positive pressure appropriately via a filter and a flowmeter to the inlet side of a steam-driven ejector. Both the steam flow and the solution flow to the ejector are appropriately regulated by means of controllable valves disposed immediately upstream of the ejector. The diffuser included in the ejector empties in turn into a cyclone.

When the solution is introduced by means of the steam into the diffuser of the ejector, the solvent is vaporized and the components dissolved therein are precipitated very rapidly in the aqueous phase in the form of fine-grained solid particles which are separated from the solvent fumes in the subsequent cyclone. In the cyclone, the precipitated solid components are collected for further processing, while the solvent fumes are conveyed to a condenser for cooling, condensation and collection for recycling and re-use.

The most manifest advantages inherent in the method and device according to the present invention may be summarized as follows:

1. No handling of dry high explosive. (This may be batched moistened with water or alcohol.)

2. No grinding.

3. Small crystals of the desired size.

4. The crystal size and distribution may be controlled.

5. Possibilities of maintaining small tolerances in the composition of such products as contain both low explosive compound mass and crystalline high explosive.

6. The product can be obtained in water.

7. The product can be obtained phlegmatized.

8. Possibilities for only one flow to subsequent processing in a continuous process.

This should be compared with a conventional continuous process in which all raw material flows must be controlled most carefully and in which both grinding and different mixing stages must be included, which creates problems in terms of safety, composition and process control.

The present invention, as defined in the appended claims, will now be described in greater detail with particular reference to the accompanying schematic drawings and the subsequent examples.

In the accompanying drawings:

FIG. 1 schematically illustrates a device for carrying out the method according to the present invention; and

FIG. 1a is a schematic view of a detail design of the steam ejector.

The device shown schematically in the drawing for producing fine-grained explosive substances containing one or more different substances consists of three different dissolving vessels 1, 2 and 3, each fitted with an agitator 4, 5 and 6. In addition, each dissolving vessel is provided with a first adjustable inlet 7, 8 and 9 for the solvent and a second adjustable inlet 10, 11 and 12 for the solid component which is to be included in the final product and which thus may consist of one or more explosive substances and possibly also other substances. Finally, each dissolving vessel is fitted with a third inlet 13, 14 and 15 for an inert gas such as nitrogen, enabling the closed vessels provided with heating devices 34 may be placed under a certain positive pressure. The dissolving vessels 1, 2 and 3 communicate by means of bottom valves 16, 17 and 18, with a main supply pipe 19 which, in turn, includes a filter 20 and an adjustable flow valve 21. The latter runs out into an ejector 22 in the inlet 23 thereof, which is disposed at right angles to the feed direction of the ejector (see detail FIG. 1a ), for material treated in the ejector. The ejector is further provided with an inlet 25 fitted with a control valve 24 for water vapour. Since the water vapour acts as a driving medium in the ejector, the inlet 25 is disposed in the feed direction of the ejector.

The diffuser included in the ejector is designated 26. This flows out into a cyclone 27 where the fine-grained product is separated from the solvent fumes which leave the cyclone via an upper outlet aperture 28 for the solvent fumes while the solid components leave the cyclone via a lower outlet aperture 29.

In the Figure, further processing of the thus obtained product is indicated by the vessel 30, while the solvent, which thus leaves the cyclone via the outlet aperture 28 then follows a pipe 31 to the cooler 32 where it is condensed and is then conveyed in condensed form to the collection vessel 33 whence it may be recycled as required via the pipe 35 to the dissolving vessel 1, 2 and 3, respectively.

The example described above includes three dissolving vessels, but there may be either only one or several vessels, depending on how the necessary output stock is best prepared from them.

In the examples referred to below, the device sketched in the drawing is employed, the requisite number of dissolving vessels being employed in each particular case.

Method: 6 kg of hexogen were added to 60 l of methylethyl ketone and 20 l of water under agitation. The mixture was heated under agitation to 60.degree.-70.degree. C. until complete dissolution of the high explosive had been obtained.

The solution was pressurized with nitrogen gas (1 atm positive pressure) and was subsequently fed to the ejector at an adjusted flow rate of 4 l/min. At the same time, water vapour at a vapour pressure of 3 kg/cm.sup.2 was supplied to the ejector.

Result: After crystallization in the ejector, separation from the solvent in the cyclone and dewatering, 5.9 kg of hexogen with a mean particle size of 8 .mu.m was obtained. 90% of the solvent could be recovered in the condenser.

Method: 7.06 kg of moist hexogen (15% moisture), 0.95 kg of cellulose acetate butyrate (CAB), 0.600 kg of tributyl citrate (TBC), 0.315 kg of nitrocellulose (NC) and 0.032 kg of centralite were added to 60 kg of watersaturated (approx. 12%) methylethyl ketone. The mixture was heated to approx. 70.degree. C. under agitation and the dissolving vessel was pressurized with nitrogen gas (1 atm), the mixture being fed to the ejector at a flow rate of 3 kg/min and at a vapour pressure of approx. 3 kg/cm.sup.2. The precipitated product was washed with water and dried.

Result: After drying, 7.6 kg of low explosive compound were obtained. Approx. 80% of the solvent could be recovered.