. "2. Hydrolysis Tests of Energetic Materials." Analysis of Engineering Design Studies for Demilitarization of Assembled Chemical Weapons at Pueblo Chemical Depot. Washington, DC: The National Academies Press, 2001.
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Analysis of Engineering Design Studies for Demilitarization of Assembled Chemical Weapons at Pueblo Chemical Depot
reclaim potentially valuable energetic components such as TNT, RDX, and HMX for reuse (Garrison, 1994). However, many of the R3 processes are still being evaluated for economic feasibility in a number of DOD demonstration programs (Newman et al., 1997; Marinkas et al., 1998; Goldstein, 1999). According to Mitchell (1998), “In 1998, approximately 60% of the 100,000 tons of demilitarization surplus ordnance were disposed in a way which enabled at least some of the material to be recovered and recycled.”
Some energetic materials do not lend themselves to recovery and recycling, either because the economics of the process are unfavorable or because the material properties are unfavorable. Nitrocellulose-based propellants and materials containing nitrate ester plasticizers are not suitable feedstocks for an R3 program because of their long-term instability. Compositions containing these ingredients always include a stabilizer to prevent catastrophic self-heating as the materials age. However, the degradation of the propellants and the presence of impurities in aging energetics of this type make them poor candidates for the economical recovery of energetic components. These materials are still destroyed by OB/OD. Research has been done to evaluate the potential of demilitarized gun propellants for a variety of uses, such as sensitizers for commercial slurry explosives and boiler fuels (Machacek, 2000).
The demilitarization of small items, such as igniters and fuzes, is routinely accomplished in an APE-1236 furnace, a rotary kiln in which the devices are heated until the energetic material decomposes thermally. The amount of material that could be recovered from these items is small, and the energetic materials themselves, especially detonators, are often quite sensitive. Because of their sensitivity, attempting to disassemble the items would be more hazardous than disassembling main-charge explosives. Therefore, these items are either intentionally “functioned” (i.e., actuated) or thermally decomposed.
Alternative technologies to OB/OD for items that contain energetic materials not worth recovering are being explored but are not widely used. Confined burning, a process in which the gaseous and condensed products of combustion can be captured and treated before release, is being used at some sites around the country. Hydrolysis of energetics as a means of disposal is being used at the Hawthorne Army Depot.
Several other technologies (e.g., molten-salt destruction) are being used at research and development sites (e.g., Eglin Air Force Base and Strauss Avenue Thermal Treatment Plant) to destroy energetic materials, but these technologies are not an integral part of DOD’s plan for the demilitarization of obsolete munitions.
CAUSTIC HYDROLYSIS OF ENERGETIC MATERIALS
Caustic hydrolysis of energetic materials has been investigated as an alternative technology to the OB/OD method. Newman (1999)1 published a review of the known chemistry of caustic hydrolysis of energetic materials used in assembled chemical weapons, and recent work on the destruction of aromatic nitro compounds (TNT and tetryl) by alkaline hydrolysis has been reported (Bishop et al., 2000).
The chemistry of caustic hydrolysis takes advantage of the susceptibility of the functional groups commonly found in energetic materials to attack by hydroxide ion, which yields products that are essentially nonenergetic. Caustic hydrolysis decomposes energetic materials to organic and inorganic salts, soluble organic compounds, and various gaseous effluents. Partial hydrolysis of some energetic materials, particularly materials with aromatic ring systems, may lead to ill-defined oligomeric materials with low solubility in either aqueous or organic solvents.
The rate of reaction depends on, among other things, the concentration of the energetic compound in solution or, for heterogeneous reactions, on the surface area of the solids being hydrolyzed. An important factor in determining the rate of destruction is the phase of the compound in the hydrolysis reactor. The compounds of interest may be divided into three classes:
compounds that are liquids at normal reactor temperatures (e.g., 2,4,6-TNT and nitroglycerin)
compounds that are solids at normal reactor temperatures (e.g., RDX and tetryl)
polymeric materials (usually nitrocellulose)
TNT has low solubility in aqueous solutions and forms an emulsion with hot caustic solution. Thus, because the TNT is molten, the size of the droplets in the emulsion is determined not by the size of the granules in the original feedstock but by the degree of agitation in the hydrolysis reactor, as well as the presence of any surfactant. For Composition B, the size of the RDX particles in the reactor will reflect the size that was used in manufacturing the Composition B.2 During manufacturing, a small fraction of the RDX dissolves in molten TNT, but the remainder is suspended in the TNT matrix. Therefore, when the TNT is remelted, the original RDX particles can be recovered. Thus, the RDX particle size does not depend on the size of the Composition B pieces fed into the reactor but on the size of the original RDX particles mixed into the TNT, typically between 10 µm and 1 mm. The particle distribution may be skewed toward the larger particles because the smaller particles dissolve more rapidly in the TNT.
This information can also be found in condensed form in Appendix E of the initial ACW I Committee report (NRC, 1999).
Composition B contains 1 percent wax. Depending on the nature of the wax, some long-chain fatty acids may be present, which act as surfactants. Hydrolysis of the plasticizers in M28 propellant may also release phthalate salts, which can aid in the emulsification of TNT when M28 and Composition B are processed together.