Environmentally Conscious Production of Composite Propellants and Explosives

Resodyn has joined with the Naval Air Warfare Center – Weapons Division, in an effort to develop technologies that will assist DoD in limiting waste streams associated with manufacture of energetic materials under a SERDP-funded effort.

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Specifically, the effort will develop a manufacturing process using  the new Continuous Acoustic Mixer Clean in Place (CAM-CIP) in production of various explosives, rocket propellants, and pyrotechnic formulations for military use. The proposed technology will reduce overall waste streams and improve worker safety associated with energetic supply-chains and manufacture of millions of pounds (annually) of energetic material.

Ultimately, this effort will quantify the extent to which the implementation will improve safety and occupational health, and reduce environmental impacts. It is anticipated that use of the technology will significantly reduce downstream waste on a production scale commensurate with current production operations.

The continuous resonant acoustic mixing (RAM) process has been successful thus far in mixing surrogate polymer-bonded explosive (PBX) material. As part of the development effort, a CAM-CIP has been designed, manufactured, and qualified at Resodyn. The CAM-CIP was designed to mix energetic material continuously using a RAM 5 platform, but now with an ability to control temperature of the mixed material.  This capability improves on the overall design and allows for more highly complex chemistries to be controlled during manufacture. The CAM-CIP module is part of a continuous mixing process, as shown in Figure 1, that has since completed 50+ continuous mixing runs.

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Figure 1. Continuous Acoustic Mixing Process Setup.

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Figure 2. Images of CAM-CIP Plates Before (Top) and After (Bottom) the CIP Process.

To measure the consistency of the material produced by the CAM-CIP process, samples were collected from the exit of the CAM-CIP every 2 minutes, then the solids loading was measured using Thermal Gravimetric Analysis (TGA). The Relative Standard Deviation (RSD) of the solids loading over time is typically between 0.5% and 1.5% depending on the material rheology and material addition process indicating a process that is in control. Furthermore, a process with an RSD of 0.5% can also be described as being significantly better than a 3-sigma process when compared to the PBX specification.

The CAM-CIP was also designed as the central component of a CIP process. The CIP process has been optimized to clean surrogate material from the CAM-CIP with 100% cleaning efficiency using an aqueous detergent solution. Before and after CIP images of a CAM-CIP plate are shown in Figure 2. The CIP capability of the CAM-CIP will allow energetics manufacturers to reduce their environmental footprint by eliminating the use of organic solvents from their legacy cleaning processes.

The process reduces the amount of waste produced during energetic manufacturing. The absolute amount of wasted energetic material is small (nominally half the volume of the CAM-CIP or 5 kg of energetic material) providing an economic as well as an environmental benefit. Continuous processing also provides manufacturers an ability to limit production only to the amount of material required, preventing wasted material from batch overruns.

The continuous RAM mixing process also allows the amount of material wasted during cleaning to be decoupled from the amount of material produced, providing additional economic benefit during larger production runs. The amount of detergent-solution used in the scrubbing cycle is limited to only 3.6 Liters (L), and the amount of rinse water used is 4.7 L. As more material is mixed, the amount of CIP waste created remains constant; therefore, the ratio of CIP waste to mixed material produced decreases at a rate proportional to the inverse of the production rate. Figure 3 compares the amount of PBX and cleaning waste generated during legacy mixing processes versus the CAM-CIP process.

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Figure 3. Legacy Mixing and Cleaning Waste Compared to CAM‑CIP Waste as a Function of PBX Produced.

The CIP process also protects workers from exposure to organic solvents and associated vapors that are generally toxic as well as hazardous. The improvement in personnel safety is a large incentive to developing CIP processes, but the reduced amount of personal protective equipment (PPE) and cleaning articles (wash clothes etc.) that require disposal provides environmental and cost benefits as well.

The program is now entering Phase III, which involves transferring the CAM-CIP to NAWCWD China Lake to demonstrate the continuous mixing process and CIP using energetic material on the RAM 5.

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