WO1999021793A1

WO1999021793A1 - Method of producing prills of ammonium dinitramide (adn)

Info

Publication number: WO1999021793A1
Application number: PCT/SE1998/001936
Authority: WO
Inventors: Abraham Langlet; Martin Johansson
Original assignee: Försvarets Forskningsanstalt
Priority date: 1997-10-28
Filing date: 1998-10-27
Publication date: 1999-05-06
Also published as: AU9771598A; WO1999021795A1; WO1999021794A1; AU9771798A; SE9703928D0; AU9771698A; SE9703928L; SE512396C2

Abstract

The invention relates to the production of prills of ADN. Homogeneous, pore-free prills are produced by making droplets of ADN solidify at low pressure, preferably under vacuum below 25 mm Hg. The prilling can be carried out in a prill tower or in a liquid nonpolar medium. The nonpolar medium may consist of a plasticiser or a plasticiser with a polymer added for adjustment of the viscosity of the medium. Ultrasonic energy can be supplied to the medium during prilling to obtain an intense dispersion of molten ADN in the medium and, thus, small prills. ADN prills having a particle size below 40 νm can be produced in this way. The invention also concerns a polymodal mixture of ADN prills comprising prills having a particle size below 40 νm produced according to the method.

Description

Method of Producing Prills of Ammonium Dinitramide (ADN)

The invention relates to a method of producing particles, prills, of ADN. More specifically, the invention concerns a method of producing ADN prills having a high homogeneity.

Ammonium dinitramide is an energetic oxidiser which can be used in many applications. One application is that as oxidiser in rocket propellants, which is disclosed in e.g. US 5,498,303. Advantages of using ADN in composite propellants instead of ammonium nitrate are described in, for instance, US 5,498,303 and WO 97/06099.

When producing ADN, the product is obtained in the form of crystal aggregates which are not very suited to be directly mixed with a polymer system for producing a composite propellant. It is therefore desirable to produce prills of ADN. Prills allow a simpler handling system and result in a better rheology in mixing with polymer systems.

A known method of producing prills is in fall towers or prill towers, where molten droplets of the substance are allowed to fall in a tower or vertical duct and, while falling, are cooled so as to solidify into prills. Large amounts of prills of ammonium nitrate for use in ANFO are produced in this fashion and in that case a certain porosity of the produced particles being desirable. When producing ADN prills according to the present invention, homogeneous, pore-free prills are desired instead.

Another previously described method is prilling in liquid phase. The substance is molten and dispersed to droplets in a liquid phase which is inert in relation to the substance. The liquid phase is cooled to solidify the droplets into prills, and these are separated from the liquid phase. Such a method for prilling of ADN is mentioned in Challenges in Propellants and Combustion - 100 Years after Nobel (Conference Proceedings, 1996, pp 627-635); May Lee Chan et al: ADN Propellant Technology.

The prior-art methods for prilling do not result in particles of high homogeneity. For ADN-based propellants, inhomogeneities in the particles result in an increased sensitivity to initiation of detonation. In contrast to, for instance, ammonium per- chlorate, ADN is an oxidiser which without fuel added can be made to detonate. ADN is difficult to initiate by friction but is just as sensitive as hexogen in connection with initiating by impact.

Moreover, with the prior-art methods it is difficult to produce very small particles, i.e. having a diameter of less than 40 μm. To produce suitable polymodal mixtures for compact ADN-based propellants and explosives, it is however desirable to have such small particles. A polymodal mixture consists of a plurality of different particle sizes which are so related to each other as to achieve a high packing density.

A further problem of the method described above for prilling in liquid phase is that the particles must be cleaned with an organic solvent before being used in propellant production.

An overall object of the present invention is to contribute to an improved production of ADN-based propellants and explosives. Thus, a method is provided for producing ADN prills having a high homogeneity. According to an embodiment of the invention, homogeneous ADN prills of a sufficiently small particle size are produced for suitable polymodal mixtures to be prepared. According to a further embodiment of the invention, homogeneous ADN prills are produced in a liquid phase allowing the particles to be directly mixed with a polymer system without any preceding washing.

According to the invention, prills are produced of ADN by preparing droplets of molten ADN, and making the droplets solidify under low pressure, i.e. at a pressure below atmospheric pressure. The pressure should be sufficiently low for dissolved and entrapped gases in the droplets to have time to escape before the droplet solidifies. How low a pressure is required for this varies with the other process conditions, for instance temperature and viscosity of the melt and the time during which the droplet is subjected to the low pressure before solidifying. Normally, the pressure is below 1/5 of the atmospheric pressure, and preferably below 1/10 of the atmospheric pressure.

According to a preferred embodiment of the invention, the droplets are made to solidify under a vacuum of less than 25 mm Hg, preferably 20-0.5 mm Hg, i.e. a vacuum that can be achieved with a conventional oil vacuum pump.

Prilling can be carried out in a fall tower or prill tower which is designed for the low pressure used in the method. There are several known methods of producing droplets of the molten material. A preferred method is supplying particles of ADN in solid state to the tower and preparing droplets by letting the particles fall through a heated zone in the upper part of the tower. The droplets then fall down through a cooling zone in the tower where they solidify into prills and are discharged from the bottom of the tower via a pressure lock. By this method, both melting and solidifying occur under the low pressure. In the test device that has been used, also the supply system for the solid material, a feed hopper and a feed screw have been included in the low-pressure system.

Another mode of operation according to the invention is to carry out prilling in liquid phase in a reactor of some kind where the pressure is reduced correspondingly. ADN is molten and dispersed into droplets in a nonpolar medium in the reactor. Subsequently the medium is cooled to solidify the droplets into prills which are separated from the medium.

A small amount of surfactant can advantageously be added to the nonpolar medium. The surfactant makes the prills have a smooth and nice surface. The added amount is normally smaller than 5% by weight based on the nonpolar medium. Suitable surfactants are, for instance, laurylium sulphate, stearyl alcohol, sodium stearate, octylamine and dodecylamine.

According to a preferred embodiment, the nonpolar medium consists essentially of a plasticiser, in which case a plasticiser which is normally used in polymer systems in the production of composite propellants and plastic-bonded explosives is selected.

Suitable plasticisers are DOS, DOA, vacuum oil, transformer oil, K-10, BTTN, M/E- NENA, TMETN, Me-NENA, Et-NENA, WM3, FEFO, BDNPF/A, DANPE, DINA, and GAPA. K-10 up to and including GAPA are all energetic plasticisers.

In addition to a plasticiser, the nonpolar medium may contain a polymer for adjusting the viscosity of the medium. The polymer can be, for instance, HTPB or some other polymer that is used in composite propellants or plastic-bonded explosives. A suitable viscosity of the nonpolar medium is 10-250 mPas.

By using a plasticiser as a nonpolar medium, the prills can be directly mixed with a polymer system when producing composite propellants and plastic-bonded explosives without the particles first needing to be washed. The plasticiser and, where appropriate, the viscosity-controlling polymer are selected in consideration of the polymer system with which the prills are to be mixed. The plasticiser also protects the particles from the humidity of the air.

According to another embodiment of the invention, ultrasonic energy is supplied to the nonpolar medium for dispersing the droplets of ADN in the medium. The dispersion can be carried out by means of ultrasonic waves only or ultrasonic waves in combination with mechanical stirring. Alternatively, the mixture of molten ADN in the nonpolar medium can be passed through a flow mixer to which ultrasonic energy is supplied. With the aid of ultrasonic waves, very small prills with a diameter of less than 40 μm can be prepared, which in turn permits the production of suitable polymodal mixtures for compact ADN-based propellants and explosives.

A suitable bimodal mixture of ADN prills may comprise, for instance, a first size fraction having a particle size greater than 80 μm and a second size fraction having a particle size below 40 μm. The greater particle size can be prepared in a fall tower or in liquid phase according to the invention, and the smaller particle size can be prepared in liquid phase by means of ultrasonic waves according to the invention.

The ratio of the particle size in the first size fraction to the particle size in the second size fraction can be e.g. about 5:1. Also the weight ratio of the first to the second size fraction is suitably about 5:1.

Ultrasonic energy can advantageously be used in combination with a nonpolar medium consisting of a plasticiser and, optionally, the addition of a surfactant. The method is excellently suited for continuous production since the resulting emulsion of molten ADN droplets in the nonpolar medium is relatively stable.

The invention will now be exemplified in connection with the accompanying Figures, of which

Fig. 1 is a sectional view of a fall tower for prilling in gas phase according to the invention,

Fig. 2 is a sectional view of a batch reactor for prilling in liquid phase according to the invention, and Fig. 3 is a sectional view of a batch reactor with an ultrasonic transducer for prilling according to the invention.

Example 1

Prilling in a fall tower was carried out in a device according to Fig. 1. The actual fall tower consisted of a vertical duct 1 in the form of a vertical stainless steel tube having a length of 4 m and an inner diameter of 6 cm. The upper part of the tube was provided with a heating zone 2, and the lower part with a cooling zone 3. The heating zone was obtained by using an external electric heating jacket 4 on the tube, and the cooling zone by using a cooling jacket 5 for a passing cooling medium. The heating zone occupied about 1/4 of the length of the tube and essentially the entire remaining length constituted the cooling zone. The lower part of the tube was provided with a connection 6 for a vacuum pump. An oil vacuum pump which generated a maximum vacuum of 0.5 mm Hg was used (not shown). The lower end of the tube was provided with a pressure lock 7 for discharging prills from the low-pressure system. The pressure lock consisted of two valves 8, 9 and an intermediate chamber 10 which could be connected to the vacuum pump via a valve 11. The upper end of the tube was provided with a feed system 12 for ADN particles. The feed system consisted of a feed hopper 13 with a feed screw 14 and a connecting part 15 from the discharge end of the screw to the top of the fall tower. The connecting part 15 was equipped with a cooling device 16 to prevent the heat from the heating zone 2 from reaching the feed screw. The feed screw 14 was operated by a motor 17 via a vacuum-tight connection 18 (magnetic power transmission). The feed hopper 13 could be sealed in a gas-tight manner and was, like the screw 14, included in the low-pressure system.

Crystals of ADN of the same size as the desired prills were supplied to the feed hopper 13. The feed hopper was closed and the system was evacuated by the vacuum pump via the connection 6. The ADN crystals were supplied very slowly by means of the screw 14, and the temperature in the heating zone 2 was increased until the crystals melted into droplets while passing the heating zone. The cooling medium passed through the cooling jacket 5 to ensure that the droplets solidified while passing the cooling zone 3. Prills were collected on the bottom of the tube and were discharged via the pressure lock 7. In this way, prills of high homogeneity in sizes from 80 μm and greater were produced. Experiments were made with dif- ferent negative pressures in the fall tower down to 0.5 mm Hg. An improved homogeneity of the prills could be established at a negative pressure corresponding to 1/5 of the atmospheric pressure. A very high homogeneity was achieved at a vacuum of 25 mm Hg and lower.

Example 2

Prilling in liquid phase was carried out in a batch reactor according to Fig. 2 consisting of a cylindrical container 19 with a tight cover 20. The container was provided with a propeller agitator 21 and baffles 22. The container was enclosed by a jacket 23 to be passed by a heating medium and a cooling medium, respectively. The propeller agitator was operated by a motor 24 via a vacuum-tight connection 25 (magnetic power transmission) in the cover 20. A connection for a vacuum pump was arranged through the cover. An oil vacuum pump which generated a maximum vacuum of 0.5 mm Hg was used (not shown).

The reactor was supplied with 5 g of ADN, 700 ml of plasticiser (DOS, dioctyl sebacate) as a nonpolar medium and 200 mg of a surfactant (laurylium sulphate). The reactor was closed and gases were evacuated by means of the vacuum pump. The pressure was kept below 25 mm Hg during the subsequent melting and solidifying process. The mixture was heated to 96°C and kept at this temperature for 5 min during stirring by means of the propeller agitator at a rate of 60 rpm. The mixture was then quickly cooled to 20°C. The resulting prills were filtered off by suction filtration on a paper filter. The prills had a high homogeneity and the particle size was about 200 μm. In repeated experiments, the stirring intensity was increased, whereby prills having a particle size of about 120 μm were produced.

The resulting prills were mixed with a polymer system (HTPB) without any preceding washing and propellant elements were prepared. Propellant elements of high strength were obtained after hardening, without any problems.

Repeated experiments were made with other plasticisers as a nonpolar medium, DOA, vacuum oil, transformer oil, K-10, BTTN, M/E-NENA, TMETN, Me-NENA, Et- NENA, WM3, FEFO, BDNPF/A, DANPE, DINA and GAPA being tested.

Example 3 The reactor according to Fig. 2 was provided with an anchor agitator instead of the propeller agitator 21 and new prillings were carried out in the same way as in Example 2, but with the following changes:

a) As a nonpolar medium use was made of a mixture of DOS and HTPB (Krasol® supplied by Kaucuk A.S.). 24.5 g of HTPB were mixed with DOS to a total volume of 300 ml. 20 g of ADN were added. The pressure was kept at 5-10 mm Hg during the subsequent melting and solidifying process. The stirring rate was 400 rpm.

The produced prills became relatively large.

Example 3a was repeated with a nonpolar medium consisting of a mixture of 31.5 g of HTPB and DOS to a total volume of 300 ml. The stirring rate was 600 rpm. The product was filtered off, washed with methylene chloride and screened. The total amount of prills was 15.3 g and had the following size distribution:

X>350 μm 2.1 g

250> X>180 μm 3.9 g

180> X 0.2 g

c) Example 3 b was repeated with a stirring rate of 800 rpm. Prills of good quality with a size distributed round 60 μm were produced.

d) Example 3c was repeated with 30 g of ADN in the nonpolar medium. In this case, the prills became smaller than in Example 3c and were of good quality.

Example 4

Prilling in liquid phase by supplying ultrasonic energy was carried out in a batch reactor according to Fig. 3. The reactor was the same as in Example 2, except that it was provided with a magnetic agitator 27 and an ultrasonic transducer 28 which was arranged through the cover 20 and which was immersed in the medium in the reactor. Equivalent components in Figs 2 and 3 have been given the same reference numerals. In the same way as in Example 2, 5 g of ADN, 700 ml of plasticiser (DOS) and about 200 mg of a surfactant (laurylium sulphate) were supplied to the reactor which was then closed and gases were evacuated by means of the vacuum pump. The pressure was kept below 25 mm Hg during the subsequent melting and solidi- fying process. The mixture was heated to 96°C and kept at this temperature for 5 min during stirring by means of the magnetic agitator. The ultrasonic transducer was operated at its maximum for 3 min and was then turned off, and the mixture was quickly cooled to 20°C. The resulting prills were filtered off from the plasticiser. Prills of high homogeneity with a particle size below 40 μm were produced. Also prills with a greater particle size could be produced by lowering the intensity of the ultrasonic treatment.

Claims

Claims:

1. A method of producing prills of ADN, characterised by the steps of preparing droplets of molten ADN, and making the droplets solidify under low pressure.

2. A method as claimed in claim 1 , characterised in that the pressure is below 1/5 of the atmospheric pressure.

3. A method as claimed in claim 1 , characterised in that the pressure corresponds to a vacuum of below 25 mm Hg, preferably 20-0.5 mm Hg.

4. A method as claimed in claim 1 , characterised in that the prilling is earned out in a fall tower.

5. A method as claimed in claim 4, characterised by producing the droplets by making particles of ADN fall through a heating zone in the upper part of the tower.

6. A method as claimed in claim 4 or 5, characterised by making the droplets solidify by passing a cooling zone in the tower.

7. A method as claimed in claim 1 , characterised in that the prilling is carried out in liquid phase.

8. A method as claimed in claim 7, characterised by melting ADN and dispersing it to droplets in a nonpolar medium; cooling the medium for solidifying the droplets into prills, and separating said prills from the medium.

9. A method as claimed in claim 8, characterised by adding a surfactant to the nonpolar medium.

10. A method as claimed in claim 8, characterised in that the nonpolar medium has a viscosity of 10-250 mPas.

11. A method as claimed in claim 8, characterised in that the nonpolar medium essentially consists of a plasticiser.

12. A method as claimed in claim 11 , characterised in that the plasticiser is selected from the group consisting of DOS, DOA, vacuum oil, transformer oil, K-10, BTTN, M/E-NENA, TMETN, Me-NENA, Et-NENA, WM3, FEFO, BDNPF/A, DANPE, DINA and GAPA.

13. A method as claimed in claim 11, characterised in that the nonpolar medium contains a polymer for adjusting the viscosity of the medium.

14. A method as claimed in claim 13, characterised in that the polymer is HTPB.

15. A method as claimed in claim 8, characterised in that ultrasonic energy is supplied to the medium for dispersing the droplets of ADN in the medium.

16. A method as claimed in claim 15, characterised in that prills having a particle size of less than 40 ╬╝m are produced.

17. A method as claimed in claim 15, characterised in that said ultrasonic energy is supplied to the medium by means of an ultrasonic transducer which is immersed in the medium.

18. A polymodal mixture of ADN prills comprising prills having a particle size of less than 40 ╬╝m produced according to claim 15.