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Автор: dima
2006-01-25 [#18927]

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Форум: Пиротехника

Page 8 PGI Bulletin No. 46, March, 1985 THE FEW, THE PROUD, THE SULFATES Donald J Haarmann___ Most sulfates are not water soluble, are geologically stable and can be easily and cheaply obtained by mining, rather than having to be produced through complicated and expensive chemical processing. Therefore sulfates pass the first test for possible inclusion in any pyro formula; they are inexpensive. Indeed native sulfates such as barite (BaSO4) and celestite (SrSO4) are the starting materials for other barium and strontium compounds used in fireworks. Sulfates certainly appear attractive because their oxygen content compares favorably with that of metal chlorates, perchlorates and nitrates, as Table 1 illustrates. Also a comparison of the heat evolved from reaction of aluminum and various oxidizing agents again shows that sulfates compare favorably with more familiar pyrotechnic oxidizers. (See Table 2.) Table 1 Percent oxygen contained (percent by weight) for various pyrotechnic oxidizers and sulfates, for the anhydrous compound. However, low cost is not the only criteria for selecting oxidizers for use in fireworks compositions. A quick check of Table 1 reveals several oxidizers with high oxygen content, for instance, calcium, sodium, and ammonium nitrates, sodium chlorate, and magnesium perchlorate. However, of these only sodium nitrate has found use, albeit limited primarily to military pyrotechnics. All of these compounds are hygroscopic and therefore unusable in the real world. In fact, magnesium perchlorate is used as a drying agent under the trade name of "Anhydrone". There can be no doubt that the largest problem concerning the use of sulfates as oxidizing agents is their waters of hydration, for example: Na2SO4-10H2O and CuSO4-5H2O. Although the ten extra oxygen atoms in sodium sulfate raise its total oxygen content from 45% to 70%, this extra oxygen contained in the waters of hydration is not available for productive work. In truth it only gets in the way, since a large amount of heat is required to first remove the water of hydration from a composition's outer surface before the ignition temperature can be reached. Then once the reaction becomes self sustaining, even more heat, produced by a burning star for instance, will be removed from the reaction in the form of vaporized water. (It should be noted that the latent heat of vaporization for water is 540 calories per gram of water at 100° C. This value represents heat that must be supplied by the pyrotechnic reaction to change water at 100° C into steam at 100° C.) There is also the possibility, in magnesium containing compounds, of the water vapor reacting with the magnesium forming hydrogen and magnesium oxide, effectively removing a large amount of fuel, with little gain in heat. In the case of sodium sulfate decahydrate, where 56% of each molecule is water, 31,920 calories of heat would have to be supplied simply to remove all the water of hydration in the form of steam from each 100 grams of sulfate. For example, in a composition using potassium perchlorate as the oxidizer and aluminum as the fuel, 13.3 grams of aluminum and potassium perchlorate would be needed just to remove the water from each 100 grams of sodium sulfate decahydrate, before any useful work (heat and/or light) would be produced! As a further complication, the temperature at which waters of hydration are liberated varies from sulfate to sulfate, e.g., sodium sulfate decahydrate loses all its water at 100° C while manganese sulfate monohydrate does not lose all its water until the temperature reaches 400-450° C! And to really complicate things, manganese(II)sulfate can exist as either mono, tri-, tetra, penta, hexa, or heptahydrate! Although the tetrahydrate is the most common form. However, US Patent 2,885,277 claims to make use of the waters of hydration in magnesium sulfate heptahydrate, MgSO4-7H2O (Epsom salts), to produce hydrogen gas when the sulfate is reacted with magnesium. It is further claimed that this combination will function as either a torch or a salute. It would be well to note that Ellern (1968, p. 272) expresses doubt concerning the safety and utility of such mixtures. The use of sulfates as oxidizers suffers from yet another problem. As Dr. Conkling (in press) has pointed out "In pyrotechnics, the solid liquid transition appears to be of considerable importance in initiating a self propagating reaction. The oxidizing agent is frequently the key component in such mixtures, and a ranking of common oxidizers by increasing melting point bears a striking resemblance to the reactivity sequence for these materials." Unfortunately the melting point of most sulfates is much higher than either chlorates, perchlorates or nitrates. Only four sulfates (manganese, copper, zirconium and iron) have melting points below that of barium nitrate, and these four are well hydrated (tetra or penta). Melting points are summarized in Table 3. It is evident that getting compositions based on sulfates as oxidizers to ignite while not impossible ... is not going to be easy. There can be no doubt that it is going to take an extremely hot ignition source! Copper sulfate with its low melting point looks like a prime candidate but again, the water of hydration is a problem. Exposed to moist air, CuSO4 becomes CuSO4-H2O, and when wetted, CuSO4-5H2O. Also, because copper sulfate is water soluble, it is seldom found in native form (chalcanthite). Therefore it is manufactured from copper metal and sulfuric acid, and as a result fails the first test, it is not cheap. It is also not safe with chlorates. Although certainly attracting because of their low cost oxygen content, sulfates have for the most part, not been employed as oxidizing agents. However, them have found their niche in strobe formulas. Vander Horck (1974) reported on several formulas using calcium and copper sulfates demonstrated to him by Bob Winokur who later (Winokur, 1974) made additional comments about them. Further Dr. Shimizu (1981) presents several strobe ("twinkler") formulas using sulfates, i.e., strontium, barium, sodium and calcium. Advantage is taken of the great difficulty of igniting and then sustaining ignition in sulfate based compositions. Therefore flashes of light are produced each time the sulfate reaches its melting point or decomposition temperature, burning commences and shortly thereafter extinguishes only to repeat, producing the strobe light effect. Sulfates have long been used in color flame compositions more for their metal than oxygen content. However, for the most part, the color produced by sulfate based compositions not containing metal fuels such as aluminum or magnesium, will be found to be less than satisfactory, since only metal fuels are capable of producing the high temperatures necessary to melt or decompose most sulfates. The use of various sulfates is detailed below: Copper sulfate: In older literature, e.g. Kentish (1878) compositions for blue flames can be found using copper sulfate and potassium chlorate, where the copper ion is used to produce the blue color. THIS COMBINATION IS DANGEROUS. Safer and more effective blue formulations are available. Barium sulfate: Troy Fish (1981) recommends the use of barium sulfate in parlon bound green stars. He notes that as a result of barium sulfate's extreme insolubility (0.000413 grams per 100 ml of boiling water!), it is one of the few nontoxic barium compounds. I have been able to locate only seven formulas using barium sulfate, and all seven use either magnesium, aluminum or magnalium. Calcium sulfate: Despite the many obstacles noted above, calcium sulfate hemihydrate (plaster of Paris) [CaSO4- 1/2H2O] has been used as an oxidizer in fireworks and pyrotechnics: In combination with sodium and barium nitrate in white light compositions (Ellern, 1968, formulas 36, 37 and 38), as an incendiary when combined with aluminum (US Patent 2,424,937, Vol. 3 of the "Black Book", 1982), or aluminum and magnesium sulfate (US Patent 4,381,207), and when compounded with aluminum, Teflon, and sulfur (US Patent 4,349,396) as a metal cutting torch. Calcium sulfate combined with either aluminum or magnesium has been suggested as a "flash report" mixture! (Sanford, 1974) This sulfate is found in pink tableau fire or star compositions using potassium perchlorate as the oxidizing agent. Weingart (1947) has the only modern for mula I have been able to locate that uses calcium sulfate without either aluminum, magnesium or magnalium. Potassium sulfate: The Technico Chemical Receipt Book 1896 long ago recommended the use of potassium sulfate in blue compositions. There is only one modern formula using potassium sulfate, Dr. Shimizu's white "twinkler" using magnalium as the metal fuel. Strontium sulfate: This sulfate had long ago been used in the production of red or purple flames. However, there are no formulas using strontium sulfate in Lancaster, Ellern or Weingart. There are however, three "twinkler" formulas in Shimizu using strontium sulfate. All three contain magnalium. Sodium sulfate: I have been able to locate only four formulas using sodium sulfate, all by Dr. Shimizu, who uses sodium sulfate in combination with magnalium for yellow strobe stars. Manganese sulfate: Perhaps the most interesting use of sulfate is the addition of manganese sulfate (MnSO4 H2O) to aluminum sodium nitrate flare compositions. Farnell et al.(1972) discovered that this compound alters "the decomposition of sodium nitrate to form oxides of nitrogen rather than its normal decomposition products of nitrogen and oxygen." This change results in a 55% decrease in burning rate, a 155% increase in luminous output, and a 466% increase in luminous efficiency! Although not a mainstays of the fireworks trade, sulfates have found employment along with the proverbial kitchen sink, used frying pans, oil of spike and philosopher's wool!!!

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