White Smoke Production

The processes used to generate white smoke by means of a pyrotechnic reaction include  

Sublimation of sulfur, using potassium nitrate as the oxidizer A fuel-rich ratio of sulfur to KNO3 is used in such mixtures. Caution Some toxic sulfur dioxide gas will be formed. Ignition and use of these mixtures must be done in a well-ventilated area (unless rodent control is a desired goal of the device). 

Volatilization of oil A pyrotechnic reaction produces heat needed to vaporize high molecular weight hydrocarbons. The subsequent condensation of this oil in the air creates a white smoke cloud. With proper selection of a low-toxicity oil, the negative health effects of this smoke are probably the least of all the materials discussed here. 

Formation of zinc chloride (HC smokes) A reaction of the type 

The original HC smoke mixtures (type A) contained zinc metal and hexachloroethane (CjCl ), but this composition is extremely moisture sensitive and can ignite spontaneously if moistened. An alternative approach involves adding a small amount of aluminum metal to the composition, and zinc oxide (ZnO) is used in place of the moisture-sensitive metal. Upon ignition, a sequence of reactions ensues of the following type  

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If this reaction occurs, the chance of spontaneous ignition of the mixture is likely. Therefore, any composition containing both a chlorate salt and an ammonium salt must be considered extremely hazardous. The shipping regulations of the United States Department of Transportation classify any such mixtures as "forbidden explosives" because of their instability [8]. However, compositions consisting of potassium chlorate, ammonium chloride, and organic fuels have been used, reportedly safely, for white smoke production [Ij. 

Boron trifluoride [7637-07-2] (trifluoroborane), BF, was first reported in 1809 by Gay-Lussac and Thenard (1) who prepared it by the reaction of boric acid and fluorspar at duU red heat. It is a colorless gas when dry, but fumes in the presence of moisture yielding a dense white smoke of irritating, pungent odor. It is widely used as an acid catalyst (2) for many types of organic reactions, especially for the production of polymer and petroleum (qv) products. The gas was first produced commercially in 1936 by the Harshaw Chemical Co. (see also Boron COMPOUNDS). 

Smoke pellets are produced in a range of sizes and are commonly used tor the resting of household flues and chimneys. The pellet is ignited and will burn for about 10 seconds producing a dense white smoke. Because this is a combustion process there are obvious restrictions on its use (nonflammable atmo spheres, nonflammable surfaces, etc.). In addition the smoke is buoyant because of the heat generated. The smoke can also be an irritant and/or toxic. The production of smoke cannot be controlled, but pellets are inexpensive, easy to use, and readily available, and the smoke is produced in sufficient quantities to make them useful in the evaluation, for example, of fume cupboards and Ixroths. 

Obscuring Power of White Smokes. The total obscuring power, TOP, of a white smoke agent used for screening purposes, is obtained by multiplying the product of volume, in cubic ft or smoke produced per lb of material, and the reciprocal of the smoke layer, in ft, necessary to obscure the filament of a 40-watt Mazda lamp. The TOP for some white smoke agents, at low altitudes where atm constituents are plentiful, is given in Table 1... 

Titanium (IV) tetrachloride (TiCy produces a dense white smoke-like vapor when exposed to moist air. It is used as smoke screens and for skywriting, as well in theatrical productions where fog or smoke is required for the scene. 

When aluminized AP composite propellant burns, a high mole fraction of aluminum oxide is produced as a combustion product, which generates visible smoke. If smoke has to be avoided, e. g. for miUtary purposes or a fireworks display, aluminum particles cannot be added as a component of an AP composite propellant In addition, a large amount of white smoke is produced even when non-aluminized AP composite propellants bum. This is because the combustion product HCl acts as a nucleus for moisture in the atmosphere and relatively large-sized water drops are formed as a fog or mist This physical process only occurs when the relative humidity in the atmosphere is above about 60%. If, however, the atmospheric temperature is below 260 K, white smoke is again formed because of the condensation of water vapor with HCl produced as combustion products. If the HCl smoke generated by AP combustion cannot be tolerated, the propellant should be replaced with a double-base propellant or the AP particles should be replaced with another... 

Combustion with complete gasification occurs when an AP pyrolant is composed of l2ip(0.86) and bi3I((0.14). The mass fraction of hydrogen chloride (HCl) among the combustion products is about 0.3. It is well known that HCl molecules combine with water vapor in the atmosphere to generate a white smoke. It is for this reason that AP pyrolants act as white smoke generators in a humid atmosphere. 

HCl molecules form visible white fog when water vapor is present in the atmosphere. An HCl molecule acts as a nucleus, becoming surrounded by HjO molecules, which forms a fog droplet large enough to be visible. When the combustion products of an AP composite propellant are expelled from a rocket nozzle into the atmosphere, a white smoke trail is seen as a rocket projectile trajectory whenever the relative humidity of the air is above about 40%. Furthermore, if the temperature of the atmosphere is below 0 °C (below 273 K), the HjO molecules generated among the combustion products form a white fog with the HCl molecules even if the relative humidity is less than 40 %. Thus, the amount of white fog generated by the combustion of an AP composite propellant is dependent not only on the humidity but also the temperature and pressure of the atmosphere. 

Thus, AP is a valuable oxidizer for formulating smokeless propellants or smokeless gas generators. However, since the combustion products of AP composite propellants contain a relatively high concentration of hydrogen chloride (HCI), white smoke is generated when they are expelled from an exhaust nozzle into a humid atmosphere. When the HCI molecules diffuse into the air and collide with H2O molecules therein, an acid mist is formed which gives rise to visible white smoke. Typical examples are AP composite propellants used in rocket motors. Based on experimental observations, white smoke is formed when the relative humidity exceeds about 40 %. Thus, AP composite propellants without any metal particles are termed reduced-smoke propellants. On the other hand, a white smoke trail is always seen from the exhaust of a rocket projectile assisted by an aluminized AP composite propellant under any atmospheric conditions. Thus, aluminized AP composite propellants are termed smoke propellants. 

A good white smoke can be obtained by the formation of zinc chloride, ZnCl 2, from a reaction between zinc metal and a chlorinated organic compound (the chlorine-containing species serves as the oxidizer). Reaction products that strongly attract moisture (such as ZnCl 2) will have an enhanced smoke effect in humid atmospheres. The burning of elemental phosphoms, producing... 

Phosphorus is available in two forms, white (or yellow) and red. White phosphorus appears to be molecular, with a formula of P,. It is a waxy solid with a melting point of 44°C, and ignites spontaneously on exposure to air. It must be kept cool and is usually stored under water. It is highly toxic in both the solid and vapor form and causes burns on contact with the skin. Its use in pyrotechnics is limited to incendiary and white smoke compositions. The white smoke consists of the combustion product, primarily phosphoric acid (H 3PO,). 

In either event, the products are ZnCl 2, CO, and A12O 3 The zinc oxide cools and whitens the smoke by consuming atomic carbon in an endothermic reaction that occurs spontaneously above 1000°C (equation 8.6). The reaction with aluminum (equation 8.4 or 8.8) is quite exothermic, and this heat evolution controls the burning rate of the smoke mixture. A minimum amount of aluminum metal will yield the best white smoke. Several "HC" smoke compositions are listed in Table 8.4. 

FIGURE 3 When magnesium burns in air, it gives off a lot of heat and light. The white powdery product contributes to the smoke. 

Economic Aspects—The economic aspects, from the standpoint of efficient use of fuel, have been touched upon but briefly. It is clear that black smoke is the result of ineffective coal combustion. As a matter of fact, effective combustion of fuel may actually represent a saving and has been the most pertinent argument for the elimination of black smoke. However, the use of powdered coal with complete combustion and the production of a white smoke or fly-ash is equally detrimental. When we consider that approximately 15 to 20 percent of coal is ash, and if this is poured out continuously from an industrial chimney day after... 

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