9
4
Chemistry of Pyrotechnics
2.
The composition can easily and safely be manufactured,
handled, transported, stored, and used, assuming nor-
mal treatment and the expected variations in temperature.
3.
Storage lifetime is acceptable, even in humid conditions,
and there is reasonably low toxicity associated with both
the starting materials and reaction products.
These requirements seem rather simple, but they do restrict
or eliminate a number of potential starting materials. These com-
pounds must either be deleted from our "acceptable" list or spe-
cial precautions must be taken in order to use them. Examples
include
Potassium dichromate (K
2
Cr
2
O
7
):
This is a strong oxidizer,
but it only contains 16% oxygen by weight. It has a cor-
rosive effect on the mucous membranes, and its toxicity
and suspected carcinogenicity suggest the use of alternate
oxidizers.
Ammonium perchlorate (NH,,ClO,,):
This is a good oxidizer, and
can be used to make excellent propellants and
colored
flames.
However, it is a
self-contained oxidizer-fuel sys-
tem (much like ammonium nitrate). The mixing of NH
4
+
(fuel) and C1O
a
-
(oxidizer) occurs at the ionic level.
The
potential for an explosion cannot be ignored. Conclusion:
if this material is used, it must be treated with respect
and minimum quantities of bulk powder should be pre-
pared.
Magnesium metal
(
Mg) : This is an excellent fuel and produces
brilliant illuminating mixtures.
The metal is water-reactive
however, suggesting short shelf-life and possible sponta-
neous ignition if magnesium-containing mixtures become
wet.
Conclusion: replace magnesium with the more stable
aluminum (or possibly titanium) metals. If magnesium gives
the best effect, coat the metal with an organic, water-re-
pelling material.
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,
Pyrotechnic Principles
95
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. Ox-
idizers 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 al-
ways walking a narrow line in this area, however. By maxi-
mizing reactivity - with small particle sizes and intimate mix-
ing - 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. Ma-
terials 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