Waste solid explosives and gun propellants are destroyed primarily by open pit burning and detonation or incineration. However, extraction and recycling of such explosives and propellants using a non-polluting, inert supercritical fluid (SCF) solvent such as carbon dioxide (CO₂) has economic and environmental advantages. Although the ingredients in composite propellants and explosives demonstrate varying solubility in CO₂, solubility is enhanced when trace amounts of simple polar modifiers are added to the SCF solvent.

The objective of this project is to determine the optimal physical conditions and chemical makeup of an effective SCF CO₂ solvent with added polar modifier using well-established computational chemistry techniques.

The investigation focused on determining modifier properties and physical conditions that enhance the solubility of energetic materials in the SCF solvent using classical molecular simulation. Two complementary theoretical investigations on the properties and effectiveness of polar modified CO₂ SCF solvents were pursued in parallel. The first investigation focused on the actual dynamic event for dissolution of a hexahydro-trinitro-triazine (RDX) crystal in an SCF solvent. Solvation dependence on the physical conditions of the system (far from or close to the critical point of the SCF) were examined. The second investigation focused on determining modifier properties that enhance the solubility of RDX in the SCF solvent using rigorous quantum mechanical methods. A written report detailing predicted optimal conditions for extraction of energetic components from propellants and explosives in SCF CO₂ was developed. In addition to receiving data, the user was supplied with recommendations based on the data analysis from the development of the SCF CO₂ extraction system. Additionally, a program package that allows for prediction of solubility of energetic materials in multi-component SCF CO₂ was developed along with a users manual and a table of recommended parameters for use in applying the model to various types of modifiers and solutes.

The “porting” or “transitioning” of the research-grade software to a user-friendly, black-box suite of computer programs that can be executed on either UNIX or WINDOWS computer platforms was completed. Codes have been benchmarked, and a user's manual has been written. The software allows for the prediction of the solubility of RDX in pure CO₂ or CO₂ modified with one of 37 polar modifier molecules at low concentrations. The software also allows the user to include other polar modifiers that were not incorporated as part of the package submitted.

Results of this effort are being transferred to SERDP's recycling initiative (SERDP project PP-660: “Extraction and Recycling of Low Vulnerability Ammunition Using Supercritical Fuels”) and other DoD demilitarization and recycling initiatives. The program package and manual allow for prediction of solubility of energetic materials in multi-component SCF CO₂.

The solubility of octahedral-tetranitro-tetrazocine (HMX) in SCF CO₂ was investigated and it was observed that, contrary to expectations, HMX solubility was not enhanced by the presence of trinitrotoluene. This apparent discrepancy between theory and experimental results is being studied. This project was completed in FY1998.

The principle benefits resulting from this research include the prevention of pollution associated with disposal of Army and Navy explosives and gun propellants and an associated reduction of life-cycle cost of munitions. Recycling is an alternative to open burning/incineration of gun propellants, which increasingly is restricted, and to incineration, which is not widely available and requires size reduction preprocessing.