(338c) Mixing and Packing of Fine Particles of Different Sizes

The US Army is interested in uniformly mixing (without segregation) two different sizes (roughly 200-300 µm and 20-30 µm) of fairly dense explosive particles (RDX), which are then packed in weaponry such as burster tubes, grenades, and artillery shells, followed by infusion of a polymerizable liquid which acts as a binder. The particles need to be well-mixed to prevent local hot spots and the packing density needs to be as high as possible so as to pack more explosive particles into the armaments This was accomplished (using aluminum oxide particles as simulants) by adding a small amount of moisture to uniformly mix the particles, and then removing the moisture in a microwave oven to increase the packing density. Adding water and gently mixing in a tumbler greatly improves the homogeneity of the mixture, but actually decreases the packing density. Removing the water, in a second step, keeps the mixture uniform and simultaneously increases the packing density significantly because fine particles coated onto coarse particles due to liquid (water) bridges fall away and fill the interstices or void space between the coarse particles.