METHOD FOR TREATING ENERGETIC MATERIALS BY HYDROTHERMAL OXIDATION

Description

Field of invention

[0001] The present invention relates to the field of treatment of pyrotechnic materials by hydrothermal oxidation. State of the art

[0002] The management of waste from an energetic materials manufacturing site is a fundamental point of its activities. In the case of the manufacture of pyrotechnic materials, energetic material waste is found in different forms: raw material (liquid, powder, etc.), pasty propellant, reticulated, or waste contaminated by pyrotechnic material. The waste is often treated by open-air burning, a relatively inexpensive treatment method but not very environmentally friendly due to the substances emitted, which are mainly CO, CO 2 , NO, NO 2 , Al 2 O 3 , HCl. It is therefore conventional to treat propellant waste by burning it. Among the propellants, we distinguish propellants that contain a binder, ammonium perchlorate and aluminum; propellants that contain nitramines in addition to these compounds; and propellants with lower levels of ammonium perchlorate and higher levels of nitramine.An alternative to burning propellants containing ammonium perchlorate levels above 50% involves finely grinding the propellants under water, which allows the ammonium perchlorate to be dissolved in water, and the water is then treated biologically. The remaining insoluble part (solid) contains the propellant binder, nitramines and other compounds (plasticizers, stabilizers, additives) which are treated by incineration, either directly or after being mixed with large quantities of inert solids (sand, powdered compound). As you will have understood, this alternative combining biological treatment and then incineration cannot be applied to propellants containing high levels of nitrated compounds that are poorly soluble in water (for example, nitramines of the RDX or HDX type).

[0003] More generally, the incineration of inert energetic materials is carried out by specialized companies, which requires transporting the products to the incineration areas. For this, a classification for the transport of hazardous materials is mandatory but difficult to obtain, because the materials are difficult to classify due to the fact that their nature can vary from one batch to another. One of the possible solutions for incinerating pyrotechnic materials in conventional channels is the dilution of these materials with inert compounds before their transport; however, the mass of the compounds necessary to ensure the inerting of the waste represents between 90 and 99% of the total mass. Consequently, the mass of waste to be treated in fine is multiplied by a factor of between 10 and 100, which has a direct impact on the cost of destruction and on the environment, particularly due to transport.

[0004] Regarding the treatment of pyrotechnic waste, only the General Atomics GATS (General Atomics Total Solution) process appears to be available on an industrial scale (see the document "Analysis of Engineering Design Studies for Demilitarization of Assembled Chemical Weapons at Pueblo Chemical Depot", 2001, ISBN 978-0-309-07607-4 / doi 10.17226 / 10182). This process involves hot hydrolyzing the pyrotechnic waste to be treated, then treating the hydrolyzate by hydrothermal oxidation. This process, however, has the following disadvantages: the hydrolysis stage is relatively long (around 2 hours), for a relatively small quantity of waste to be treated (around 6 kg); the hydrolysis stage requires an air treatment system to treat the gaseous pollutants generated; the requirements for consumables (soda, etc.) and energy are significant; the salts generated by hydrolysis must be extracted before injection into the hydrothermal oxidation system.

[0005] Patent application EP 2 740 529 describes a method for the treatment of toxic waste by hydrothermal oxidation, which method comprises feeding a reactor with a mixture of toxic waste and water, and hydrothermal oxidation of the mixture in the presence of an oxidizing agent. Patent application JP 2002 248455 describes a method for treating toxic substances which implements, in one of its embodiments, a hydrothermal oxidation step. Patent application FR 2 201 922 describes a method for the wet oxidation of liquid or solid combustible materials that are difficult to emulsify or suspend using air. Patent application WO 02 / 30836 describes a method for treating a material by hydrothermal oxidation, in the presence of an oxidizing agent. The article "Waste Management 1998, 18, 539-546" describes the hydrothermal oxidation of hazardous materials produced by / from US Navy ships.

[0006] There is therefore a need for a process for treating energetic materials, and in particular pyrotechnic materials, which is safe, efficient, inexpensive and has a reduced impact on the environment. Summary of the invention

[0007] The invention relates to a method for treating pyrotechnic materials, which comprises the preparation of an aqueous suspension of particles of said materials, said preparation comprising several grindings of said materials including a final grinding in water, then the hydrothermal oxidation of said suspension in a reactor ad hoc. The method according to the invention may provide a step of recycling the water coming from the reactor at the end of the hydrothermal oxidation. Description of the invention

[0008] The present invention relates to a method for treating pyrotechnic materials, and in particular propellants, which comprises: the preparation of an aqueous suspension of particles of energetic materials; the hydrothermal oxidation, in a reactor, of the suspension thus obtained.

[0009] The first step of the process therefore consists of preparing an aqueous suspension of particles of the pyrotechnic materials to be treated. To do this, the said materials must be ground. Several grinding operations are necessary, including one in water: the first grinding operations are typically carried out using knife mills for coarse grinding; the final grinding is then carried out in a tank filled with water and equipped with an impact mill, such as, for example, a mill equipped with a rotor and stator.

[0010] The knife mills are fed in batch mode and can achieve a grinding rate of 30 to 50 kg of energy materials per hour. The final grinding is carried out in a tank filled with water, containing between approximately 10% and approximately 30% by mass of energy materials, to obtain a ground material comprising particles of materials suspended in water. The ground material is then filtered on a filter of suitable technology, for example a vibrating screen, such as those marketed by the company SWECO, in order to (i) eliminate excessively large particles, and (ii) eliminate the grinding water, particularly in the case where the energy waste contains ammonium perchlorate. This phase of extraction of ammonium perchlorate makes it possible to limit the injection of chlorinated elements into the hydrothermal oxidation system and therefore to limit the formation of hydrochloric acid to control corrosion of the system.

[0011] The particle size of the material particles at the end of filtration is advantageously less than approximately 1 mm, very advantageously less than approximately 500 µm.

[0012] In one embodiment of the invention, one or more surfactants are used during the final grinding in order to prevent the agglomeration of particles and the deposition of the generated clumps. Examples of surfactants that may be used in the context of the invention include fire-fighting emulsifiers such as those in the BIO For range marketed by the company BIOex.

[0013] The aqueous suspension thus obtained is then subjected, in a reactor, to a hydrothermal oxidation treatment. The hydrothermal oxidation process consists of subjecting the particles of energetic materials to a treatment with supercritical water, that is to say water at a temperature and pressure above its critical point, i.e. above 221 bars and 374°C. The hydrothermal oxidation is carried out at a pressure between 230 and 280 bars and a temperature between 450° and 600°C. This hydrothermal oxidation reaction is carried out only in the presence of water, under the pressure and temperature conditions mentioned above. In other words, the hydrothermal oxidation reaction is carried out in the absence of any other reagent and in particular in the absence of an oxidizing agent.

[0014] The concentration of pyrotechnic material particles in the injected suspension is from about 1% to about 20% by mass, in particular from about 5% to about 10% by mass. For a reactor with a flow rate of 200 l / h, the mass of pyrotechnic material destroyed is thus between about 10 and about 20 kg / h.

[0015] It is important that the suspension injected into the reactor is homogeneous in order to prevent any blockage and any uncontrolled energy input into the reactor.

[0016] Thus, in one embodiment, the treatment method according to the invention provides for maintaining the particles of pyrotechnic materials in suspension, upstream of the reactor. Those skilled in the art will readily understand that any means making it possible to achieve this aim can be implemented within the framework of the present invention. Advantageously, the suspension of particles of pyrotechnic materials obtained at the end of the filtration is placed in a reactor feed tank, and the particles are maintained in suspension in the tank by the combined action of a sequential or intermittent injection of compressed air and two recirculation loops by pumping the suspension and tangential reinjection at the bottom of the waste storage tank.The compressed air injection is carried out under a plate placed in the center and at the bottom of the tank, which makes it possible to generate a toroidal bubble, which will itself generate an essentially radial movement of the particles. This radial movement acts in a complementary manner to the tangential movement generated by pumping.

[0017] The process according to the invention allows for total destruction (≥ 99.8%) of pyrotechnic material particles. This process also has the following advantages: it allows the treatment of a large number of pyrotechnic molecules present in propellants; it is carried out underwater, which minimizes the associated risks; the discharges (gases, liquids, solids) from hydrothermal oxidation have a limited impact on the environment: a) With regard to gases, the main components are carbon dioxide, nitrogen and water, with no impact on local air quality. The residual concentrations and flows of carbon monoxide and nitrogen oxides (traces) are fully compliant with the discharge requirements of French regulations. For comparison, open-air combustion of composite propellant generates a carbon monoxide flow of around 3.10 -3< kg / kg prop . The carbon monoxide discharges from the process of the invention are less than 1.5.10 -5< kg / kg prop , an improvement of more than 99.5%.Similarly, for nitrogen oxides, for burnt quantities greater than several tens of kg, atmospheric emissions from open-air burning are greater than 0.01 kg / kg prop . For the process of the invention, the release is limited to 4.6.10 -5 < kg / kg prop , again an improvement greater than 99.5%. b) As regards liquids, they are essentially made up of water and nitric acid, the latter being compatible with conventional water treatment systems or even nitric acid recovery systems depending on the concentration obtained. c) As regards solids, solid alumina will be generated if the treated waste is an aluminum-based composite propellant. Due to its compactness and mobility, it can be installed as close as possible to the source of waste production, which frees up safety requirements for the transport of waste on public roads and prevents the environmental impact of road transport.

[0018] As mentioned above, hydrothermal oxidation generates water as a liquid waste. This water can be recycled (in a continuous process) either to prepare the pyrotechnic waste suspension (first step of the process) or for hydrothermal oxidation (second step of the process).

[0019] The invention will be better understood with the aid of the following example, given for purely indicative purposes. Example : nitrocellulose waste treatment

[0020] Approximately 20 kg of nitrocellulose were ground by successive grinding processes, first using knife mills and then using a rotor stator type mill in a tank filled with water, which made it possible to obtain, after filtration, a suspension containing nitrocellulose particles with a particle size of less than one mm.

[0021] The nitrocellulose was introduced into the water-filled tank so as to obtain a suspension containing between 10 and 30% solids during grinding.

[0022] The suspension thus obtained was pumped by a diaphragm pump and diluted with demineralized water to obtain a final mixture containing between 3 and 10% nitrocellulose in the form of particles.

[0023] This solid / liquid mixture was set up with suspension systems to ensure a homogeneous mixture (external recirculation and bubbling inside the tank).

[0024] The hydrothermal oxidation treatment of this mixture was carried out with a continuous flow reactor with a feed rate of 200 L / h. The reactor temperature was between 450°C and 550°C, with a feed pressure between 230 and 240 bars.

[0025] No organic matter was detected in the discharge water and the nitrate content in the outlet effluent was between 5 and 10g / l of nitrates.

Claims

1. A process for treating pyrotechnic materials, comprising: - preparing an aqueous suspension of particles of pyrotechnic materials, the preparation comprising crushing the pyrotechnic materials several times, including a final crushing in water; - in a reactor, hydrothermally oxidizing the resulting suspension at a pressure of 230 to 280 bar and at a temperature of 450°C to 600°C, the hydrothermal oxidation reaction being carried out only in the presence of water.

2. The process according to claim 1, wherein one or more surfactant(s) is(are) added during the crushing in water.

3. The process according to claim 1 or 2, wherein the suspension comprises from 1% to 20% by weight of particles of pyrotechnic materials.

4. The process according to claim 1, wherein preparing an aqueous suspension of particles of pyrotechnic materials comprises filtering the crushed pyrotechnic materials.

5. The process according to claim 4, wherein the particle size of the particles of pyrotechnic materials is less than 1 mm, advantageously less than 500 µm.

6. The process according to any one of claims 1 to 5, wherein the particles of pyrotechnic materials are maintained in suspension in a feed tank of the reactor before being subjected to the hydrothermal oxidation reaction.