PREPARATION OF HIGH-ENERGY MIXTURES
The most hazardous operations in the high-energy chemistry field involve the mixing of oxidizer and fuel in large quantities, and the subsequent drying of the composition (if water or other liquid is used in the mixing and granulating processes). In these operations,
large quantities of bulk powder are present in one location, and if accidental ignition should occur, there is a good chance that an explosive reaction rate may be reached. For this reason, mixing and drying operations should be iso- lated from all other plant processes, and remote control equip- ment should be used wherever and whenever possible. All high- energy manufacturing facilities should be designed with the idea in mind that an accident will occur at some time during the life of the facility. The plant should be designed to minimize any damage to the facility, to the neighborhood, and most impor- tantly, to the operating personnel.
The manufacturing operation can be divided into several stages:
1. Preparation of the individual components: Materials to be used in the manufacturing process may have to be dried, as well as ground or crushed to achieve the proper particle size, or screened to separate out large particles or foreign objects. Oxidizers should never be processed with the same equipment used for fuels, nor should oxidizers and fuels be stored in the same area prior to use. All materials must be clearly labeled at all times.
2. Preparation o f compositions: This step is the key to proper performance. The more homogeneous a mixture is, the greater its reactivity will be. The high-energy chemist is always walking a narrow line in this area, however. By maximizing reactivity - with small particle sizes and intimate mixing - you are also increasing the chance of accidental ignition during manufacturing and storage. A compromise is usually reached, obtaining a material that performs satisfactorily but is reasonably safe to work with. This compromise is reached by careful specification of particle size, purity of starting materi- als, and safe operating procedures. A variety of methods can be used for mixing. Materials can be blended through wire screens, using brushes. Hand-screen- ing is still used in the fireworks industry, but should never be used with explosive or unstable mixtures. Brushes provide a safer method of screening the oxidizer and fuel together. Materials can also be tumbled together to achieve homogeneity, and this can (and should) be done remotely. Remote mixing is strongly recommended for sensitive explosive compositions such as the "flash and sound" powder used in firecrackers and salutes and the photoflash powders used by the military.
3. Granulation : Following mixing, the powders are often granulated, generally using a small percentage of binder to aid in the process. The composition is treated with water or an organic liquid (such as alcohol), and then worked through a largemesh screen. Grains of well-mixed composition are produced which will retain the homogeneity of the composition better than loose powder. Without the granulation step, light and dense materials might segregate during transportation and storage. The granulated material is dried in a remote, isolated area, and is then ready to be loaded into finished items. Remember: Sizable quantities of bulk powder are present at this stage, and the material must be protected from heat, friction, shock, and static spark.
4. Loading: An operator, working with the minimum quantity of bulk powder, loads the composition into tubes or other containers, or produces pellets for later use in finished items. The making of "stars" - small pieces of color-producing composition used in aerial fireworks - is an example of this pelleting operation.
5. Testing: An important final step in the manufacturing process is the continual testing of each lot of finished items to ensure proper performance. Significant differences in performance can be obtained by slight variation in the particle size or purity of any of the starting materials, and a regular testing program is the only way to be certain that proper performance is being achieved.
A magnesium-containing flare burns with a brilliant white flame in the test tunnel of the Applied Sciences Department, Naval Weapons Support Center, Crane, Indiana. Special instrumentation can measure the intensity of the light output as a function of wavelength. "White light" compositions emit throughout the visible region of the electromagnetic spectrum (380-780 nanometers) , with emission extending into the infrared and ultraviolet regions. Researchers at the Crane facility have performed extensive research on the theory and performance of illuminating flares, especially the sodium nitrate/magnesium system.