Explosion of Aluminum Powder
Dust Clouds
operated that a given, uniform amount of dust falls continuously. When uniform flow has been attained, t h e cloud in the cylinder is ignited by suitable means. The third general method employs a relatively large closed box, cylinder, or chamber, and attempts to keep t h e added dust in uniform suspension by means of rapidly rotating fans. The container is fitted with an ignition device and a suitable pressure recorder. Various types of oon-
RALPH B. MASON AND CYRIL S. TAYLOR Aluminum Research Laboratories, New Kensington,Pa.
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IKELY divided particles are readily dispersed in air to form a dust cloud which, in many respects, resembles a mixture of gas or vapor with air. Thus, if the dust is flammable and present in high enough concentration, it is possible to obtain explosive mixtures just as is the case with gasoline and other flammable vapors and air (9,11, l a ) . A combustible dust when mixed with air can form an explosive mixture as soon as the concentration exceeds a certain limit. This limiting concentration varies, depending on the chemical composition, fineness, and physico-chemical characteristics of the particular dust under consideration. The results obtained will also vary, depending on the method of ignition and the exact conditions which influence the dust cloud a t the time of ignition. With gases or vapors it is relatively simple to determine the lower limit of flammability, because of the ease of securing uniform mixtures of known composition. Satisfactory data on the lower limits of flammability of many of the commonly known gases and vapors are available in the literature. It is quite different with dusts, for it is difficult to obtain any exact measure of the lower limit of explosibility of a dust in air, and such data as are available are far from concordant. This is especially true in respect to aluminum powder dust clouds. The object of the investigation described in this paper was to determine accurately the lower explosive limit of aluminum powder in air and also to determine the effect of replacing part of the oxygen in the air by carbon dioxide or nitrogen, A study of the various methods which have been used for measuring the lower explosive limit of dust-air mixtures as described in the literature disclosed the experimental difficulties of obtaining, or maintaining, for more than a second or so, a reasonably homogeneous suspension of a definite, known concentration of dust. In general, three different methods of attack have been used in prior laboratory investigations. One method employs a bomb, either the Clement-Frazer apparatus ( 7 ) in its original or modified form, or a bomb of similar design. In such a device a spherical glass or metal bomb (1to 2 liters in capacity) is provided with an ignition device, a pressure indicator or recorder, and a means for introducing or producing a dust cloud. In operation the dust cloud is produced, either ignited after a definite interval or blown directly across a hot, glowing igniter, and the results are measured by the pressure recorder. This general type of apparatus has been used by many investigators, and many ingenious ways have been devised to produce the dust cloud, to ignite it, and to record the results. A number of modifications of this apparatus have been described by Greenwald (7). Another method employs a rotating sieve at the top of a cylinder as a means for producing a uniform concentration of dust throughout the cylinder. The apparatus is SO 626
The lower explosive limit of aluminum powder-dry air mixtures has been found to be approximately 40 mg. of aluminum powder per liter of dry air. The lower limit of oxygen required to make an aluminum dust explosion impossible when the dry air is diluted by carbon dioxide gas is approximately 10 per cent of oxygen by volume. The lower limit of oxygen required 60 permit explosion when nitrogen is the diluent for the dry air is slightly less than when carbon dioxide is used. Two lots of powder with average thicknesses of 0.28 and 0.14 micron, respectively, are employed. The 0.14-micron powder has excellent dispersion and suspension properties, and is believed to be one of the finest, thinnest, and most fluffy powders ever produced. A novel igniter for use in experimental explosion chambers, positive in action and supplying a uniform amount of energy at each operation, was devised in the course of this work.
tainers and fans have been devised and used in the hope khat a really uniform dust concentration would be maintained. However, when dusts are repeatedly driven against a surface, they tend to adhere tenaciously both to the surface and to other particles already attached to them. The inherent weakness of this method is that dust continually separates from the cloud, so that the concentration of the dust-air mixture is always less than the value calculated from the amount of added dust. Therefore, while this third method appears at first to be simple, it is fraught with difficulties and open to much criticism. The apparatus used in the preliminary experiments of the investigation contained in this paper was: patterned somewhat after the one described by Trostel and Frevert (19) in 1924-a modified Clement-Frazer type. -1, spherical aluminum bomb was used, approximately 16 cm. in diame-
JUNE, 19%
INDUST11IAI. AND ENGINEERING CHEMISTRY
ter, having an internal 1-olume of approximately 1900 cc. with tubulures at the bottom and the top. A 2-liter Pyrex flask v-as later substituted for the aluminum bomb, so that the reactions taking place could be observed. It was difficult to obtain a suspension of powder which could be ignited. When the poivtier ITVJS puffed past a red-hot X i c h r o m e
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to hold tile sluminum iiLse wire. Xickel washers are necessary, since the temperatures attained with the igniter were 80 high tliat. the binding posts fused. Some fusion takes place even with the nickel washen. The external cimuit consists of B double-pole switch and B fuse block with renewable fuse links. In operation a Psrnpere fuse link is used in the fuse block, and a fine aluminum fuse wire 0.0042 inrti (0.011 em.) in diameter, which will blow with less current than the rencnal link, is fastened into the two binding posts beiween the nickel washers. Upon closing tlie double-pole switch, the entile- igniter eirouit is shortcircuited acmss a 230-v01t, direct-current, line. The sluminum wire disintemates witii a vivid Bash and a hot arc forms between
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Air and COeAir and -K Av. No. of 0 In 0 in AT. Gas Pressure Pressure DetermiGas hlixt. Developed nations Mixture Developed % Atm. 7% Atm. 5.2 0.05 0.05 3 5.2 8.1 0.11 0.11 7 7.6 0.20 9.9 0.16 10 10.4 10.1 0.20 0.19 4 11.5 0.29 3 11.0 0.27 12.5 0.60 0.39 5 12.1 13.5 0.67 2 1.04 13.6 14.7 0.99 16.1 1.28 16.7
7.8 0.07 3 9.8 0.15 11.8 0.13 7 11.8 0.21 13.9 0.28 3 13.7 1.22 15.8 0.34 .. .. 16.4 0.44 Powder B with an average flake thickness of 0.14 micron was w e d in a concentration of 210 mg. of powder per liter of gas. The observed pressures were corrected by deducting a,blank of 0.11 atmosphere for the preasure developed b y the igniter alone in the smaller chamber and a blank of 0.07 atmosphere for the larger chamber, 1 2
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In the gas mixtures containing somewhat less than 15 per cent of oxygen, the firing of the igniter generally produced a burning effect, rather than the usual sharp explosion. A sheet of flame would form around the igniter and progress more or less rapidly t o the ends of the glass tube, the speed depending somewhat upon the percentage of oxygen present in the combustion chamber a t the time. At times the flame progressed rapidly to the ends of the tube; but, as the lower limit of oxygen concentration was approached, the flames traveled rather slowly, a t times taking over 0.25 second to travel 5 inches (12.7 cm.) along a tube P/'S inches (6.7 cm.) in diameter. This burning effect produced an indicator diagram which showed a gradual building up of the pressure throughout the time of burning, instead of the usual sharp rise of pressure characteristic of an explosion. DILUTIOP; WITH KITROGEN.The lower limit of oxygen with nitrogen as the diluent was determined by the same method as that employed with carbon dioxide gas. The dried nitrogen was brought to constant temperature and passed through a flowmeter, redried, and then passed into the tube that had previously been used for the introduction of carbon dioxide gas into the mixing chamber. As before, t h e desired oxygen content was obtained by the proper adjustment of the rates of flow of the air and the nitrogen. I n this case, in making the gas analysis of the mixture, oxygen only was determined. Here again, the oxygen present, as calculated from the flomneter readings, agreed with the volumetric analysis within the limits of analytical error. The data obtained for the lower limit of oxygen required to produce a n explosion, with nitrogen gas used as the diluent, are given in Table I11 and shown graphically in Figure 523.
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Nitrogen is apparently slightly less effective than carbon dioxide for diluting air to prevent an aluminum dust explosion. In making these tests with both carbon dioxide and nitrogen additions, even though no combustion was noticeable, a slight but measurable pressure rise above that obtained by firing the aluminum fuse wire alone was recorded on the indicator paper. This small pressure can readily be accounted for by the fact that in the uniformly dispersed dust cloud within the explosion chamber, a number of fine aluminum particles were always in the immediate vicinity of the aluminum fuse wire igniter. The extremely high temperature produced in this zone by the action of the igniter, when the switch was closed, forced these fine particles to oxidize or burn (local burning), but they did not generate enough energy to ignite the neighboring dust particles. The forced ignition of these relatively few particles generated this slight excess pressure. Only one worker, Gliwitzky ( 6 ) , reported data for the lower limit of oxygen required to produce an explosion with aluminum powder. H e showed that, although the oxygen content had been lowered to 13 per cent, the pressure developed was still high and differed by only a slight amount from that developed in normal air. Just as the publication went to press, a footnote was added, to report that, when the oxygen content had been reduced to 11 per cent by dilution with nitrogen, no explosion could be initiated. That finding is in good agreement with the present results.
Studies Contemplated In the course of this investigation it has been planned to study the effect, on the various limits which were being determined, of the temperature of the air, powder, and apparatus, the effect of humidity, the effect of ultraviolet light, and the effect of static charges. A further study of the effect of powder particle size had also been contemplated. However, in order to obtain the data reported in this paper, almost a thousand determinations have already been conducted. It therefore seems wise to publish the present experimental data without further delay. It is believed that, although some of the items enumerated may have a minor effect on the limits found, none of them will change the final results materially.
Acknowledgment The authors are indebted to H. B. Stere of these laboratories for his assistance in conducting some of the preliminary experiments and also for the photographs.
Literature Cited (1) Bauer, G., Z.ges. Schiess- u. Sprengstofw., 13,272-3 (1918). (2) Coward, H. F., and Hersey, M. D., U. S. Bur. Mines, Rept. Znvestigalions 3274 (1935). (3) Dussen, A. A. van der, Rec. trav. chim., 54, 873-84 (1935). (4) Edwards, J. D . , "Aluminum Paint and Powder," 2nd ed., pp. 20-1, New York, Reinhold Publishing Corp., 1936. Anal. (6) Edwards, J. D., and Mason, R. B., IND. ENQ. CEIEM., Ed., 6, 159-61 (1934). (6) Gliwitzky, W., Z. Ver. deut. Ing., 80, 687-92 (1936). (7) Greenwald, H.P., U. S. Bur. Mines, Bull. 365 (1932). ( 8 ) Griffin, H.K., and Adams, J. R., Mining and Met. Investigations, U. 8. Bur. Mines, Carnegie Inst. Tech., and Mining and Met. Advisory Boards, Codp. Bull. 50,45-70 (1931). (9) Leighton, A.,U.S. Bur. Mines, Tech. Paper 152 (1918). (10) M.atla, W.P. M., Rec. trav. chim., 55, 173-91 (1936). (11) Ritter, F.,Z . Ber. deut. Ing., 74, 145-8 (1930). (12) Trostel, L. J., and Frevert, H. W., Chem. & Met. Eng., 30, 141-0 (1924). RECEIYEDJanuary 9, 1937.
