1. Field of the Invention
This invention relates to the recovery of nitramines from aluminized energetic materials. This invention is particularly useful for recovering nitramines from aluminized energetic materials such as solid propellants, explosives, and pyrotechnics.
2. Description of the Related Art
Energetic materials have found widespread use, perhaps no more extensively than in military applications, where energetic materials are used to make composite propellants for ballistic missiles and explosive compositions for munitions and ordnances. An example of a propellant commonly found in rocket motors and missiles is Class 1.1 solid propellants. Like most other energetic materials used in military applications, Class 1.1 solid propellants are formed from a composition comprising a combination of one or more of the following: polymeric binders, plasticizers, ballistic additives, chemical stabilizers, curing agents and catalysts, metal powders, and inorganic and/or organic oxidizers.
Demilitarization in the United States and abroad has created a need for an economical, reliable, non-hazardous, and environmentally friendly method for disposing of the stockpile of surplus tactical missiles and explosives existing worldwide. Additionally, a growing number of larger rocket motors, such as intercontinental ballistic missiles (ICBMs), are being and will have to be demilitarized due to international treaties, such as the START treaties. The disposal of such energetic materials is the subject of various publications and U.S. patents, including U.S. Pat. No. 4,231,822 to Roth and U.S. Pat. No. 4,661,179 to Hunter et al. However, these U.S. patents focus on disposing of explosive materials by xe2x80x9cdesensitizingxe2x80x9d or xe2x80x9cdestroyingxe2x80x9d the materials.
The degradation of energetic materials into an unusable state is not the most economical alternative of disposal, since many energetic materials are both expensive and reusable. For example, one class of organic oxidizer that has found wide acceptance in the rocket propulsion, explosive, and pyrotechnic arts comprises nitramines. Common nitramines include, for example, cyclotetramethylenetetranitramine (also known as HMX and 1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane), cyclotrimethylenetrinitramine (also known as RDX and 1,3,5-trinitro-1,3,5-triaza-cyclohexane), TEX (4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo-[5.5.0.05,903,11]-dodecane), HNIW (also known as CL-20) (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo [5.5.0.05,903,11]-dodecane), and combinations thereof. Nitramines are commonly among the most expensive and highly energetic ingredients of conventional energetic compositions. Further, nitramines are sometimes present in energetic compositions in relatively high concentrations, such as on the order of up to about 50% by weight of solid rocket motor propellants and up to about 98% by weight of explosives. These factors make the successful and efficient recovery of nitramines in high yields for subsequent re-use highly desirable.
A method for the extraction and recovery of nitramine oxidizers from solid propellants is disclosed in U.S. Pat. No. 5,284,995 to Melvin, which discloses the use of a liquid ammonia extraction agent for extracting the nitramines HMX and RDX from rocket motor solid propellants. The use of liquid ammonia in nitramine recovery techniques introduces several complexities and expenses, especially in a closed system, including high capital expenditures required as outlay to obtain equipment capable of operating at the high-pressures (5 to 40 Kpsi) at which liquid ammonia is handled. The presence of liquid ammonia also creates other problems, such as worker safety issues, since contact between the ammonia and human skin can cause severe chemical burns to the handler. Additionally, liquid ammonia is combustible, and presents a severe inhalation hazard if not handled correctly. Another disadvantage of the U.S. Pat. No. 5,284,995 process is that subjecting energetic materials, such as Class 1.1 propellants containing nitramine oxidizers, to the pressurized environments described in the ""995 patent increases the risk of accidental detonation, as well as the accompanying catastrophic consequences that an accidental detonation or explosion often has on human life and property. Yet another disadvantage of the process of U.S. Pat. No. 5,284,995 is that nitramines are dissolved in liquid ammonia, requiring recrystallization of the nitramines. However, the recrystallized nitramines have different particle sizes than the nitramine particles found in the propellant. Also, if recrystallization is not performed under the right conditions, the polymorph of the nitramine changes during recrystallization.
Another method for recovering ingredients from a pyrotechnic material is disclosed in U.S. Pat. No. 4,098,627 to Tompa et al., in which the pyrotechnic containing a cured polymeric binder is decomposed under mild conditions. The method involves heating the pyrotechnic material to a temperature of from about 50xc2x0 C. to about 160xc2x0 C. in a liquid medium comprising an active hydrogen-containing compound capable of cleaving the chemical bonds contained in the polymer. Representative liquid media include mineral acids such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid and perchloric acid, as well as primary amines, secondary amines, ammonia, and water. The process is expedited by modifying the liquid medium via the addition of an organic solvent. Organic solvents reportedly suitable in the process are toluene, xylene, dioxane, and tetrahydrofuran. The organic solvent functions either to swell the organic polymeric binder present in the pyrotechnic material or to dissolve filler material present in the pyrotechnic material. The decomposition technique is carried out at 80xc2x0 to 120xc2x0 C. In practice, however, these organic solvents raise a host of ecological and safety concerns, including flammability, VOC emissions, environmentally sound and cost-effective waste disposal, and handling expenses. Additionally, U.S. Pat. No. 4,389,265 to Tompa et al. reports that the use of mineral acids and water in the manner prescribed by the ""627 patent produces low yields of about 36%. Indeed, example 5 of the ""627 patent reports that under the basic conditions of its process RDX may be destroyed.
Two additional approaches for reclaiming nitramines from propellants having polymeric binders are disclosed in U.S. Pat. No. 4,389,265 to Tompa et al. The first approach utilizes a solution of 2-aminoethanol in a mixture of an aromatic solvent and an alcohol to dissolve the propellant binder. The 2-aminoethanol breaks down or dissolves the polymeric binder. Examples of aromatic solvents suitable for the first approach include benzene, toluene, xylene, ethylbenzene, and diethylbenzene. Examples of alcohols suitable for the first approach include ethanol, 1-propanol, 2-propanol, and mixtures thereof. The second approach is performed with a solution of a mineral acid other than nitric acid, an organic solvent, and water. For the second approach, examples of suitable mineral acids are hydrochloric, sulfuric and phosphoric acid; examples of organic solvents are acetone, methylethylketone, tetrahydrofuran, and mixtures thereof. The mineral acid and organic solvent combine to break down or dissolve the polymeric binder. After dissolution of the pyrotechnic binder is completed in the 2-aminoethanol or the mineral acid process, the nitramine and metals, if present, are removed by filtration and the nitramine is extracted in acetone. Although relatively high yields are reported in U.S. Pat. No. 4,389,265, the presence of aromatic and organic solvents raises ecological and safety concerns over such issues as flammability, volatile emissions, and waste disposal. Also, the nitramine is recovered with aluminum fuel particulates. Consequently, separation of the aluminum requires dissolving, filtering, and recrystallizing of the nitramine. As mentioned above, recrystallization can cause the polymorph and size of the nitramine particles to change.
U.S. Pat. No. 6,063,960 discloses the recovery of nitroamines and reformulation of by-products. The ""960 patent generally focuses on non-aluminized propellants, with the exception of its mention of VTG-5A and WAY, both of which are aluminized propellants. According to the ""960 patent, the propellants are treated with 60-70% nitric acid in a preferred ratio of nitric acid solution to feed of 1.0:1.0 l/kg. This feed ratio calculates to a weight ratio of less than 1.5:1. Although these conditions are adequate to recover nitramines from non-aluminized propellants containing conventional binders, in the case of an aluminized propellant a substantial proportion of aluminum would not be dissolved under these conditions. Accordingly, in the event that an aluminized propellant were treated by the ""960 process, separation of the aluminum would require additional steps of dissolving, filtering, and recrystallizing of the nitramine.
Thus, it would be a significant improvement in the art to develop a method in which nitramines are recovered from aluminized energetic materials without recrystallizing the nitramines and in which there is no need for the use of either liquid ammonia under increased pressure or hazardous organic solvents that are volatile and/or flammable.
It is, therefore, an object of the present invention to fulfill a long-felt need in the art by providing a nitramine-recovery process capable of achieving the above-discussed improvements in the recovery of nitramines from aluminized energetic materials, especially aluminized propellants, without relying upon either the use of liquid ammonia under increased pressures or the application of hazardous solvents.
In accordance with the principles of this invention, the above and other objects are attained by a nitramine-recovery process in which a nitramine-containing aluminized energetic material is treated with aqueous nitric acid having a nitric acid concentration of not more than 55% by weight. This low concentration nitric acid, when used in appropriate ratios relative to the aluminized energetic material to be treated, has the effect of digesting by solvation and/or solvolysis most, if not all, conventional ingredients other than nitramines commonly found in aluminized energetic materials. As referred to herein, the term solvation means the dissolving of a solid into a solvent without chemical reaction. As also referred to herein, the term solvolysis means the dissolving of a solid into a solvent via chemical reaction between the solid and solvent.
At least one mineral acid other than nitric acid is added to the digestion process in order to facilitate digestion of the aluminum. It is also preferred to delay the addition of the mineral acid until sufficient time has passed for the aqueous nitric acid to digest most, if not substantially all, of the binder of the energetic material. The time needed for digesting the binder will depend upon several factors, including the amount of binder in the energetic material, the concentration and amount of aqueous nitric acid, and process conditions, such as temperature. Suitable mineral acids include hydrochloric acid, perchloric acid, sulfuric acid, phosphoric acid, hydrobromic acid, hydroiodic acid. Hydrochloric acid is currently preferred because of its high rate of aluminum digestion. Representative concentrations of mineral acid for use in the aluminum digestion range, for example, from about 1 part by weight to about 5 parts by weight based on 100 parts by weight of the aqueous nitric acid. Such concentrations are usually sufficient to digest at least 99 weight percent of the aluminum. Aqueous nitric acid is preferably not selected as the mineral acid for aluminum digestion. Nitric acid is relatively oxidizing compared to other mineral acids, and will oxide the surface of the aluminum to form aluminum oxide, thereby slowing the rate at which the aluminum is digested.
The nitramines remain substantially insoluble (i.e., are neither solvated nor solvolyzed) in the low concentration nitric acid and mineral acid, and can be separated from the aqueous nitric acid, mineral acid, and digested ingredients in an efficient manner and without incurring great expense.
The recovered nitramines according to the presently preferred embodiment are suitable for recycling into an energetic material, including a solid propellant grain, explosive material, and pyrotechnics (known in the art as PEP materials).
There are several advantages that can be derived from the inventive process. For example, nitramine yields associated with embodiments of the present invention have been found to be much higher than most conventional processes. In particular, nitramine yields are routinely on the order of 90% by weight or greater according to embodiments of the inventive process. Further, the process can be performed, and preferably is performed, free of organic solvents, thus avoiding the ecological and safety concerns and waste disposal and handling expenses of conventional processes. Furthermore, the digested ingredients separated from the nitramine can be recovered and reused as feed stock for commercial blasting agents, thus further reducing the waste disposal concerns and improving the efficiency of the process. The process also does not require volatile digestion agents or high pressurizes that increase the risk of un intentional detonation of the energetic material.
The recovery method of this invention is particularly useful for recovering nitramines from aluminized energetic materials such as solid propellants, explosives, and pyrotechnics, and is especially useful for the recovery of nitramines from aluminized solid rocket motor propellants. While not wishing to be bound by any theory, it is believed that unlike many conventional methods, in which nitramines are typically dissolved in order to separate the nitramines from aluminum and/or other ingredients, in the present invention the aluminum reacts with aqueous nitric acid to form aluminum nitrate Al(NO3)3.9H2O, which is soluble in the aqueous nitric acid and the mineral acid is easily separated from the substantially insoluble nitramine. Additionally, unlike other inorganic acids that react with aluminum to rapidly generate large amounts of hydrogen gas and high temperatures, nitric acid has been found to undergo a much more sedate reaction with aluminum. The reaction of nitric acid with aluminum generates hydrogen gas and heat at manageable rates so as to permit the hydrogen gas and heat to be removed from the digestion vessel. By adding a mineral acid, other than nitric acid, aluminum removal is facilitated and accelerated.