There are two primary branches in SF technology related to ordnance demilitarization: HAZMAT destruction via supercritical oxidation (SCWO), and carbon dioxide-based supercritical fluid extraction (SFE) for purposes of material reutilization (pollution prevention). The use of SCWO has been under study by various researchers, for a variety of applications, over the last decade (see C&E News, Dec. 23, 1991): the DoD has established a pilot plant that uses supercritical water to destroy military toxic wastes. The conventional ammo demil stockpile currently contains 16M pounds of recoverable nitramine-based explosives, with an amount of RDX exceeding 11M pounds (Source: Demil Task Team Study Report for Congress, Aug. 95). This quantity of RDX has a market value of $50M - $100M, and should be considered a potential resource. For the long term, recovery and reutilization of RDX should be the preferred approach.

Solid gun propellants and explosives are currently destroyed by open burning or open detonation (OB/OD). Supercritical fluid recycling would have both economic and environmental advantages over destructive OB/OD processing. Unfortunately, the nitramine ingredients (RDX, HMX) found in many of military explosives and composite LOW Vulnerability Ammunition (LOVA) propellants have insufficient solubility in non-reactive supercritical fluids (e.g., CO₂). The objective of this program is to identify suitable supercritical solvents and processing conditions for the recovery and recycling of the valuable RDX and HMX ingredients used in these munitions.

This research originally involved investigations of the effectiveness of polar "modifiers" in increasing the solubility of RDX in supercritical carbon dioxide. The goal was to identify those compounds which "modifiers" that a) significantly increase the solubility of the RDX in supercritical carbon dioxide, b) do not result in hydrolysis or other chemical degradation of the RDX, and c) can themselves be recycled (supercritical fluid extractors can operate closed-cycle), or are as close as possible to neat carbon dioxide in having negligible environmental impact. In addition to carbon dioxide, several additional polar neat supercritical solvents were investigated for the direct extraction of RDX from LOVA propellant. During FY93 and FY94, polar modifiers were identified which would enhance the solubility of RDX in carbon dioxide by close to two orders of magnitude in the range of 3 mole percent doping.

More recently, emphasis has been placed on the recovery of RDX from explosive munitions such as Comp B (RDX/TNT/Wax). Recent work in our lab has demonstrated that TNT powder can be extracted from RDX powder under relatively mild extraction conditions. Wax should also be much more soluble in supercritical carbon dioxide than is RDX. In this case, it makes more sense to extract these ingredients than the RDX. In this case, we are looking at a process which only involves carbon dioxide as a processing solvent, with no use of organic modifier, although we have not ruled out the use of water as a modifier at a doping (determined by its solubility in carbon dioxide) of around 1 mole percent. By limiting this process to only carbon dioxide and water, this becomes a very environmentally-friendly means for separation of the RDX from the formulated explosive.

The emphasis of our efforts for FY96 is entirely towards explosive formulations, and away from propellants. A lab-scale closed-loop supercritical carbon dioxide extractor has been set up to demonstrate process feasibility on a research series of formulations, including Comp A and Comp B explosives. Since FY96 is the final year for SERDP funding of this project, we will pursue funding through the appropriate government agencies for scale-up and implementation of this work for FY97 and beyond.

Progression is from 6.1 research into solubility relationships and modifiers (through FY94), then 6.2 research into extraction/recycling schemes beginning in FY95. Future plans for technology transfer include a pilot plant demonstration, possibly at the Naval Surface Warfare Center (NSWC), Indian Head, MD, where large-scale supercritical fluid processing facilities are scheduled to be on-line sometime during FY97.

Prevention of pollution associated with the disposal of Army (and Navy) propellants and explosives; associated reduction of life-cycle cost of munitions. Over half of the of the recoverable nitramine-based explosive in the conventional ammo demil stockpile is Comp B, with a RDX market value of $25M - $50M.