This project sought to demonstrate that fluidizing magnesium (Mg) powder with liquid spraying could coat Mg powder particles with a binder. Successful operation would greatly diminish or exclude the emission of volatile organic compound (VOCs) into the atmosphere while providing powder that would meet flare requirements when incorporated into a flare composition. The binder would either be dissolved in a VOC with the latter being recycled or would be a melt. Verification of the process would be achieved through the capability to stably fluidize and coat Mg powder, physical examination and analysis of the powder to determine the extent of the coating and handling safety, and radiometric performance of flare pellets. The latter would contain compositions of the coated Mg blended with polytetrafluorethylene (PTFE) powder. The project’s addresses the reduction of environmental and health impacts of energetics manufacturing and specifically to develop solventless processing.
Numerous trials incorporating modifications to the fluidized bed and liquid spraying system were executed to achieve a unit that was capable of producing coated Mg in the quantities necessary for evaluation. The system included a bench top fluidized bed system with 5-liter steel and glass tubs. The former was used initially to enable viewing of the states of fluidization and spraying. A borescope camera system was installed within the metal tub to be able to view inside it. Other key components included hardware for the atomization and spraying of the liquid component, pressurized tank for the storage and flow of the liquid, inlet filter for fluidizing air and outlet filter. The inlet filter was a ‘sandwich’ of two different mesh sizes with a Wurster inlet configuration. Sieving of the Mg powder was found necessary to reduce the particle size range of the powder. The wideness of the range made it essentially impossible to set fluidization conditions that could fluidize all of the loaded powder simultaneously.
Soxhlet extraction was selected for determining the amount of coating on the powder. Morphology on the coated powder to include scanning electron microscope and instrumental particle size had been planned. After mixing the coated powders with PTFE further tasks were to be electrostatic discharge (ESD) sensitivity determination and pressing the composition into pellets. These pellets would be intentionally ignited and burn times and radiometric outputs measured. Composition made through the standard process would also be evaluated to provide a baseline.
Due to the actual effort being greater than originally planned there were insufficient funds to install solvent recycle into the fluidized bed and spray coating system, to spray ethylene-vinyl acetate copolymer (EVA) melt, and to perform morphology and ESD testing.
After optimization, the system was able to produce approximately 413 grams of coated Mg-powder with close to 2% by weight thermoplastic elastomer binder for making flare composition. The material was blended with PTFE and pressed into pellets at three different percentages of Mg content. Pellets were burned in a flare tunnel and burn time and radiometric intensity in the 3– 5 micrometer-region measured. Pellets made from flare mix prepared per the current method (‘uncoated’ Mg) were also tested. The limited data demonstrated the pellets with the coated Mg burned approximately twice as fast as the uncoated samples, and the integrated radiant intensities, a measure of total energy, were approximately the same.
This implied that the fluidized system delivered an equal product and could be a viable, environmentally superior technology. Further work would be needed to optimize the process and equipment. Immediate needs would be the inclusion of solvent recovery, increasing the amount of coating on the Mg powder, a refinement of the Soxhlet extraction technique or replacement for it, fabricating and testing flare pellets closer to the size actually used and conducting morphological analysis and ESD testing. Further, on the spraying of EVA melt and the simultaneous spray coating of Mg and PTF should be attempted.