DESENSITIZATION PROCESS FOR ENERGETIC COMPOSITIONS WITH REGENERATION OF REAGENTS

Abstract: Potentially hazardous compositions containing solid combustible fuels, oxidants, nitro-containing, nitramine-containing or nitrato-containing compounds and, in many cases, binders are desensitized by reagents generated from the composition itself by electrolysis at low current and over an extended period of time. According to the disclosed method, the composition is retained in a reaction vessel (8) separate from the electrolysis cell (1), and the aqueous leachant from the composition is circulated in two portions through the two halves (2, 3) of the electrolysis cell (1) to form strong oxidizing and reducing reagents, respectively, which are then circulated in succession through the reaction vessel to decompose the composition. The resulting composition has a lowered sensitivity to initiation by such external influences as inadvertent impact or inadvertent electrostatic discharge.

Description: DESENSITIZATION PROCESS FOR ENERGETIC COMPOSITIONS WITH REGENERATION OF REAGENTS BACKGROUND OF THE INVENTION Compositions containing energetic compounds such as nitratoesters, nitramines and/or other nitro-group-bearing compounds, combustible fuels, oxidants and combinations of these are used for a variety of functions in a wide range of industrial and other types of applications. A problem commonly encountered with the use of such compositions is that they are difficult to dispose of in an ecologically acceptable manner. These compositions have a potential for the accidental or spontaneous initiation of a forceful reaction accompanied by the sudden release of a large amount of energy. Initiation may result from external influences such as an inadvertent impact or an accidental electrostatic discharge, and environmental and safety considerations require such a potential for danger to be reduced or avoided. SUMMARY OF THE INVENTION It has now been discovered that compositions of the type described above can be effectively desensitized, and thus rendered much less susceptible to inadvertent initiation, in a nonhazardous and controlled manner by treatment reagents which are derived from the composition and continuously regenerated, both by electrolysis. In accordance with this invention, an electrolysis cell is used to separately generate strong oxidizing and reducing agents which are successively fed to a reaction vessel containing the energetic composition in slurry form. In the reaction vessel, the oxidizing agents react with the binder material that provides the solid energetic matter with structural integrity and limits access to the other components, such as energetic compounds, oxidizers, fuels, plasticizers, and binding agents. The action of the oxidizing agents exposes these other components for attack by the reducing agents which are then fed to the reaction vessel for reduction of the energetic components to a nonenergetic form. For the most efficient and effective operation, the strong oxidizing and reducing agents are generated in the electrolysis cell by electrolysis of the water-soluble salts which are leached out from the energetic composition itself. Using the composition itself in this manner, no chemicals other than those present in the energetic composition itself are required, except for the optional use of small amounts of additional oxidizing and reducing agents for startup purposes. Regeneration is conveniently achieved by the continuous circulation of the oxidizing and reducing solutions through the electrolysis cell, the cell being divided into half-cells separated by an ion-permeable membrane. This electrolytic regeneration may be continued while the oxidative decomposition is occurring in the slurry, while the reductive decomposition is occurring, or during both the oxidative and reductive stages. Individual retaining tanks for the oxidative and reductive solutions are preferably used, permitting circulation from any one of these tanks to both the appropriate half-cell and the reaction vessel at the same time. The invention is generally applicable to solid energetic compositions. The liquid used to form a slurry of such a composition is one which will promote the transport of ions in response to the electric current, and preferably one which will dissolve one or more of the components of the composition to produce a dissolved electrolyte and facilitate the contact of the composition with the reagents produced by the electrolysis. The efficiency of the process will generally increase as the contact area between the solid and the liquid increases, and thus, higher degrees of maceration, i.e. , smaller solid particles, will generally result in improved efficiencies. Advantages of the invention include the elimination of the need for special solvents otherwise required in the disposal of such materials, the ability of the invention to permit the decomposition of two or more sensitive components simultaneously, the ability to decompose the components with electricity at low current density and voltage, and the ability to conduct the decomposition with simple, readily constructed equipment. Other features, objects and advantages of the invention will become apparent from the description which follows. BRIEF DESCRIPTION OF THE DRAWING The Figure attached hereto is a diagram of an electrolytic cell system in accordance with the invention. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS While the method of the invention is wide and varied in scope, the system shown in the drawing is offered as an illustration, to promote an understanding of the invention. The system shown in the drawing includes an electrolysis cell 1 which is divided into anodic 2 and cathodic 3 half-cells separated by an ion-permeable membrane 4. An anode 5 resides in the anodic half-cell and a cathode 6 resides in the cathodic half-cell, the anode and cathode energized by a conventional power supply 7. The membrane 4 is constructed of any conventional membrane material which permits the passage of ions generated in the electrolysis and yet is capable of withstanding the strong acids and bases produced by the electrolysis reactions and otherwise present in the system. The slurry containing the energetic composition to be desensitized is retained in a separate reaction vessel 8 apart from the electrolysis cell. Other components of the system are an acid storage tank 9 for the acidic oxidizing agent formed in the anodic half-cell; a base storage tank 10 for the basic reducing agent formed in the cathodic half-cell; a circulation pump 11 for circulating the acid solution between the acid storage tank 9 and the anodic half-cell 2; a second circulation pump 12 for circulating the base solution between the base storage tank 10 and the cathodic half-cell 3; third and fourth circulation pumps 13, 14 for circulating the acid and base solutions between their respective storage tanks and the reaction vessel 8; and two three-way valves 15, 16 with shut-off to select which of the two solutions will be circulated through the reaction vessel. The system shown in the drawing may be operated in a variety of ways. The following is a description of a presently preferred method of operation. The reaction vessel 8 is charged with an aqueous slurry of solid propellant material, following maceration of the propellant to a particle size on the order of 0.25 inch (0.64cm) or less. A typical slurry is one having a volume increased to about 1.6 times relative to the dry propellant. The optimal slurry consists of all water-insoluble components of the propellant such as the polymeric binder, plasticizers, nitramines or other energetic components, and aluminum or other fuels, and minimal amounts at most of water-soluble components which have dissolved in the liquid phase. The solids will however contain water-soluble species which are retained in the solids matrix by the binder. The acid and base storage tanks 9, 10 are charged with aqueous solutions of a portion of the water-soluble components of the propellant. The waste water generated by the hydromining and/or maceration procedures is particularly convenient for use in this initial charge of the acid and base storage tanks, since a portion of the water-soluble fraction of the propellant dissolves in the water used in these procedures. This fraction includes, for example, oxidizing agents such as ammonium perchlorate and ammonium nitrate. Once the tanks are loaded, the aqueous solutions in the acid and base storage tanks 9, 10 are simultaneously circulated through anodic and cathodic half-cells 2, 3 of the electrolysis cell, and a current is passed through the cell. During this phase, strong oxidizing agents and acids are generated in the anodic half-cell, and strong reducing agents and alkalies are generated in the cathodic half-cell, and in each case, are circulated through the respective storage tanks. Examples of the types of electrolytic reactions occurring are as follows: In the anodic half-cell: (1) The conversion of ammonium ion to nitrate ion; (2) The conversion of water to hydrogen peroxide; and (3) The generation of hydrogen ions from water and from each of the above two conversions; and In the cathodic half-cell: (1) The conversion of chlorate ion to chloride ion; (2) The conversion of nitrate and nitrite ions to ammonia; and (3) The generation of hydroxyl ions from water and from each of the above two conversions. The electrolysis and circulation of the solutions through the anodic and cathodic half-cells is continued until the pH in the acid storage tank drops to a desired level and the pH in the base storage tank rises to a desired level. In most applications, preferred results are achieved when the desired level in the acid storage tank is about 3.0 or less, preferably about 1.5 or less, and the desired level in the base storage tank is about 8.0 or above, preferably about 9.5 or above. In a presently preferred method, the desired levels are 1.0 or less in the acid storage tank and 10.0 or above in the base storage tank. In applications where waste water does not achieve the desired pH levels on its own, the waste water may be supplemented, or replaced, by materials which will provide stronger acids or bases. Once the desired pH levels are achieved, the two circulation pumps 13, 14 controlling circulation through the reaction vessel 8 are activated, with the three-way valves 15, 16 arranged such that circulation is begun between the acid storage tank 9 and the reaction vessel. The circulation of the oxidizing agents through the electrolytic cell may be suspended while circulation is occurring through the reaction vessel. In the preferred practice of this process, however, the circulation of the oxidizing agents through the reaction vessel is done while the two circulation loops between the acid and base storage tanks and the two halves of the electrolysis cell are still in operation. In the reaction vessel, the strong oxidizing acids from the acid storage tank react with the polymeric binder material and the organic nitro-, nitrato- or nitramine- group-bearing compounds in the propellant to convert these compounds to low molecular weight oxidation products. Oxidation of other components of the propellant such as crosslinkers, plasticizers and stabilizers occurs as well. Gases produced during this procedure are drawn off and scrubbed by conventional means. Simultaneously, the oxidizing agents circulating through the reaction tank are reduced. With circulation of the contents of the acid storage tank 9 through the anodic half cell 2 at the same time, the reduced oxidizing agents are continuously regenerated to maximize their oxidation capabilities in the reaction vessel 8. With the decomposition of the binder material, propellant components initially bound by the binder are liberated and exposed for chemical attack. The three-way valves 13, 14 are then switched to circulate the basic reducing solution from the base storage tank 10 through the reaction vessel 8. The reducing solution reacts with and decomposes any nitrato ester or nitramine not previously oxidized by the acidic oxidizing solution. The products of this decomposition include water-soluble nitrite, nitrate, acetate and formate salts, which are circulated back to the base storage tank 10. As in the oxidation phase, the circulation of basic reducing solution through the cathodic half-cell 3 is preferably continued during the circulation of the same solution through the reaction vessel, thereby providing continuous regeneration of the base. The length of time required for which each of the two phases involving circulation through the reaction vessel is not critical and may vary. Optimal lengths of time will vary with the propellant composition, the particular types of binder material and other components of the composition, the proportions of each and the physical condition of the solid particles in the slurry. In most cases, best results will be achieved by continuing the oxidation phase for from about 2 hours to about 24 hours. The reduction phase may then be performed for a greater or lesser time period. It is presently contemplated that the most typical operation will involve a reduction phase which is from about one-third to about one-fourth the duration of the oxidation phase. The oxidation and reduction cycles may be repeated in alternating manner. In most cases, however, a single cycle of each will be sufficient for desensitization of the propellant. Once the propellant has been desensitized to a condition acceptable for disposal, the pH of the contents of the reactor vessel contents may be adjusted by the addition of supplemental acid or base as needed to achieve a neutral pH. The remaining solids may then be removed from the reaction vessel and incinerated or otherwise disposed of by conventional means. The present invention is applicable to a wide range of compositions of the type described above, including various formulations of propellants and explosives. Examples are single-base propellants, double-base propellants, cast double-base propellants, crosslinked propellants, single-component and multi-component explosives and plastic-bonded explosives. These compositions typically include explosive components, oxidants, fuels, and binders, the latter including both energetic and nonenergetic substances, including fuel-rich and/or oxidizer-rich binders, and other additives such as plasticizers, bonding agents, extenders, catalysts, stabilizers, lubricants and other types of modifiers, fillers and functional substances. Examples of specific energetic components, including oxidizers, are ammonium nitrate (AN), ammonium perchlorate (AP), ammonium picrate, 2,4-diamino-l,3,5-trinitrobenzene (DATB), diazodinitrophenol (DDNP), diethylnitramine dinitrate (DINA), ethylenedinitramine (EDNA), ethylene glycol dinitrate (EGDN), cyclotetramethylene tetranitramine (HMX), lead azide, lead styphnate, mannitol hexanitrate (MN), mercury fulminate, nitrocellulose (NC), nitroglycerin (NG), nitromethane (NM), pentaerythritol tetranitrate (PETN), picric acid (PA), cyclotrimethylene trinitramine (RDX), trinitrophenylmethylnitramine ("Tetryl"), 2,2,2- trinitroethyl 4,4,4-trinitrobutyrate (TNETB), tetrazene, tetranitromethane (TNM), 2,4,6- trinitrotoluene (TNT), and 2-nitrodiphenylamine (2NDPA). Examples of fuels included in these compositions are aluminum and other metals or metal hydrides. Examples of binders and other additives, which are also part of the fuel, are polysulfides, polyurethanes, polybutadienes, triacetin, resorcinol, and graphite. These lists are not exhaustive, but merely illustrative of the types of materials included in compositions which can be treated in accordance with this invention. Solid compositions are first formed into a slurry, preferably an aqueous slurry, prior to placement in the reaction vessel and treatment according to the present invention. Propellant grains are typically removed from rocket motors by hydromining, i.e. , the loosening and breaking up of the grain by jets of high-pressure water. To prepare the grain for processing in accordance with the present invention, the broken grain pieces are then recovered and macerated by conventional techniques, and combined with water to form the slurry. The macerated particles are preferably less than about 1.0 inch (2.54cm) in diameter, and most preferably about 0.25 inch (0.635cm) in diameter or less. The wastewater from the hydromining may be used to form the slurry, or may be used as the initial charge for the retaining tanks in those embodiments where retaining tanks are used, or both. While aqueous slurries are the most convenient, the liquid used to form the slurry can be any liquid capable of conducting an electric current ionically. Polar liquids capable of dissolving salts, acids or bases to form an ionically conducting electrolyte are preferred. It is also preferred that the liquid be one which will partially dissolve one or more of the active components of the composition, i.e. , those which are the source of the detonation risk. This will help leach out some of the active component and enhance its decomposition. Examples of polar liquids other than water and aqueous media in general are low molecular weight alcohols such as methanol, ethanol, propanol, isopropanol, butanol and isobutanol, and ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone. Other possibilities and examples will readily occur to those skilled in the art. Water is preferred for purposes of low cost, safety and ease of use. The amount of liquid used to form the slurry is also not critical, and will be selected primarily on the basis of practical considerations of equipment scale, and ease of handling, processing and transferring. In most cases, the proportion of liquid actually used will range from about 30% to about 90% by volume of the slurry, with amounts between about 50% and about 75% preferred. In the presently preferred practice of the invention, sufficient liquid is added to produce a slurry with a volume of 1.6 times the volume of the dry energetic composition. While the method of performing the electrolysis included in the process of the present invention is not critical, and may be varied widely while still obtaining acceptable and effective results, it is preferred that a low current density be used over an extended period of time. The term "current density" is used herein to denote the amount of current per unit area of electrode surface. In processing cells where the two electrodes differ significantly in surface area, the surface area used in determining the current density is that of the electrode which offers the highest resistance to current flow. The process is to be conducted under such conditions of time, temperature and current density that the reactions which take place occur in a non-self-propagating manner, i.e. , are not subject to spontaneous acceleration but are driven essentially entirely by the electric current. The optimum or preferred current for any particular application of this invention will depend on the scale of the process, including the amount of material to be treated, the size of the equipment, and the time period available for the treatment. In most cases, however, effective results are obtained with a current density not exceeding about 0.30 amps/cm 2 , preferably not exceeding about 0.20 amps/cm 2 . Currents as low as 0.01 amps/cm 2 will be useful and practical in certain small scale systems. The preferred range for most systems is therefore about 0.01 amps/cm 2 to about 0.20 amps/cm 2 , with about 0.01 amps/cm 2 to about 0.03 amps/cm 2 particularly preferred. In many cases, it will be advantageous to gradually decrease the applied electrode potential (and hence the current) as desensitization proceeds. As the concentration of an energetic compound decreases, the applied potential may be lowered in the direction of the minimum activation potential, since progressively less reducing agent is required. Best results will be achieved by adjusting the potential at intervals to the lowest potential that will maintain the maximum negative slope for the depletion curve. The temperature is not critical, the only consideration being that the temperature itself not create a hazardous situation or cause any substantial amount of vaporization. While the rate of desensitization increases with increasing temperature, the invention is readily and adequately conducted at ambient or room temperature, i.e. , 20 to 25 °C. Cooling of the system during the process is generally not required, and the temperature will frequently rise due to the electric current itself. In most cases, the rise will not be sufficient to require temperature control. In the preferred practice of the invention, the temperature is maintained at a level below about 140°F (60°C). The process can in fact be operated at room temperature. The electrodes may be constructed of any of the materials which are known for use as electrodes. The actual material to be used may be varied widely. Selection of the material for any particular application, however, will be influenced by a number of factors. For example, preferred materials will generally be those which are the least susceptible to degradation from the passage of electric current. In certain systems, furthermore, the preferred materials will be those which are inert to the electrochemical reactions which will occur during the process. In certain other systems, it will be preferable to use electrodes which themselves become reduced or oxidized during the process. In still other systems, it will be preferable to use electrodes which absorb reactants or products of the electrochemical reactions occurring in the process. With these considerations in mind, examples of types of materials from which the electrodes can be formed are metals, graphite, metal oxides and conducting polymers. Examples of specific metals are copper, silver, aluminum, platinum, titanium and zinc. Examples of metal oxides are PbO 2 (lead dioxide), MnO 2 (manganese dioxide) and NiFe 2 O 4 (nickelous ferric oxide). Examples of conducting polymers are polyaniline, polyacetylene and polypyrrole. Each type of electrode will offer advantages for particular types of compositions being treated. For example, in systems where electrolysis results in hydrogen evolution, metals with high hydrogen overpotentials (also referred to as "high hydrogen overvoltages") may be used to reduce or eliminate the release of gaseous hydrogen. For systems where oxidation of the electrode may occur at the anode, metal oxides or noble metals are preferred in order to preserve the anode. Other reasons and motivations and the appropriate selections in each case will be apparent to those skilled in the art. The configuration and spacing of the electrodes and the design and construction of the electrolysis cell are not critical, and may be varied according to the particular needs of the system. The spacing between the ion-permeable membrane and either of the two electrodes will preferably be within the range of about 0.03 inch to about 0.3 inch (0.076cm to 0.76cm), and most preferably about 0.1 inch (0.25cm). The cell itself may be constructed of any inert material capable of withstanding the operating conditions and pH of the materials treated and used in the process. Nonconductive materials of construction such as plastic will generally be the most preferred, although a wide range of other materials may be used as well. Alternatively, the cell may be constructed of a conducting material with the cell walls serving as one of the electrodes. Solid deposits forming on the electrodes may be periodically or intermittently removed to maximize the electrode surface area to optimize the efficiency of the current flow. Further optional features include temperature detectors and voltage detectors, which may be placed on or near the electrodes or at any location in the cell, as well as pH probes. Electrolysis may be conducted using any of a variety of electric current profiles. The actual type of current may be varied, although certain types may be preferable for treating certain compositions. In general, alternating current, direct current or pulsed current may be used. For alternating current, the frequency may vary and is not critical. For pulsed currents, each pulse will be direct current. The pulse duration however may vary. A computer is particularly useful for control of pulse switching and duration. The degree to which the composition is decomposed in the practice of the invention is also noncritical and may vary. In cases where the composition is being treated for purposes of disposal and must meet specific requirements or conform to regulations before being disposed of, it is only necessary that the composition be decomposed to a sufficient degree that such requirements or regulations be met. It will generally not be necessary to proceed to substantially full decomposition of those components which present a hazard, i.e. , to a degree where at most only trace amounts are present. In most cases, for example, it will be acceptable and sufficient to convert at least 30%, preferably at least about 70%, and preferably at least about 90%, of the nitro groups of the composition to amino groups. Likewise, it will in most cases be acceptable and sufficient to reduce at least about 30% , and preferably at least about 70% , of the oxidant contained in such compositions. The apparatus and methods depicted and described herein are merely illustrative. Modifications, variations, and alternative arrangements and designs which, although differing from that described above, still embody the basic concepts and spirit of the invention will be readily apparent to those skilled in the art.

Claims: WHAT IS CLAIMED IS: 1. A process for treating a composition containing one or more energetic compounds selected from the group consisting of nitro-group-bearing, nitramine- group-bearing and nitrato-group-bearing compounds, bound together by a polymeric binder, to substantially reduce the sensitivity of said composition to inadvertent initiation, said process comprising: (a) contacting said composition with an aqueous liquid to extract water-soluble components therefrom, thereby forming an aqueous solution of water-soluble components of said composition, and dividing said aqueous solution into first and second portions; (b) combining the remainder of said composition with an aqueous liquid to form a slurry; (c) circulating said first portion of said aqueous solution through an anodic half-cell, and said second portion through a cathodic half-cell separated from said anodic half-cell by an ion-permeable barrier, while passing an electric current across said anodic and cathodic half-cells through said ion-permeable barrier, to generate an acidic solution of oxidizing agents in said anodic half-cell and a basic solution of reducing agents in said cathodic half-cell; (d) contacting said slurry with said acidic solution in a reactor vessel to cause oxidative decomposition of solid binder materials in said slurry, thereby exposing energetic compounds retained by said binder for chemical attack; and (e) once said energetic compounds are exposed, contacting said slurry with said basic solution to cause reductive decomposition of said energetic components thus exposed. 2. A process in accordance with claim 1 in which step (c) comprises circulating said first portion of said aqueous solution between said anodic half-cell and a first retaining vessel, and circulating said second portion between said anodic half-cell and a second retaining vessel. 3. A process in accordance with claim 2 in which step (d) comprises circulating said acidic solution between said first retaining vessel and said reactor vessel, and step (e) comprises circulating said basic solution between said second retaining vessel and said reactor vessel. 4. A process in accordance with claim 3 in which step (b) is continued during the performance of step (c). 5. A process in accordance with claim 3 in which step (b) is continued during the performance of steps (c) and (d). 6. A process in accordance with claim 2 in which step (d) is begun only when the pH in said first retaining vessel drops to about 3.0 or below. 7. A process in accordance with claim 2 in which step (d) is begun only when the pH in said first retaining vessel drops to about 1.5 or below. 8. A process in accordance with claim 2 in which step (e) is begun only when the pH in said second retaining vessel rises to about 8.0 or above. 9. A process in accordance with claim 2 in which step (e) is begun only when the pH in said second retaining vessel rises to about 9.5 or above. 10. A process in accordance with claim 1 in which said electric current of step (c) has a current density of from about 0.01 amps/cm 2 to about 0.20 amps/cm 2 .

Inventors: CHIN GEORGE, SMITH REX M, WAGNER PATRICK J, WONG MICHAEL K, CHIN, GEORGE, SMITH, REX, M., WAGNER, PATRICK, J., WONG, MICHAEL, K.
Applicants: AEROJET GENERAL CO, AEROJET-GENERAL CORPORATION
Application year: 1992
Main classification: C02F1
IPC codes: A62D3, A62D101, C02F1