I
GEORGE LONG Aluminum Company of America, Alcoa Research Laboratories, New Kensington, Pa.
Preventing Aluminum Powder Dust Cloud Explosions Flue gas atmospheres, with limited oxygen content, are safe and economical inert media for fluidizing aluminum powders
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PREVENTION of aluminum powder dust cloud explosions by the use of controlled oxygen content of a flue gas environment h a been the subject of a recent study at the Alcoa Research Laboratories. This subject of protective atmospheres has become increasingly important in the aluminum powder industry wheFe processing of these powders has been increased to meet the military and industrial requirements. Enlargement and modernization of production facilities have resulted in a growing interest in fluidized handling of these finely divided combustible materials. To ensure safety of operation, knowledge was required of the limiting oxygen contertt to suppress ignition of these powders, The U. S. Bureau of Mines made extensive studies of the inflammability and explosibility of aluminum powder dust clouds in air 42). Their studies provided information on minimum powder concentration, ignition temperature, ignition energy, rate of pressure rise, and other pertinent data. The Bureau studied the effectiveness of nitrogen and of carbon dioxide separately as inert atmospheres to reduce or eliminate the explosion hazard. They found the maximum allowabfe oxygen in air-carbon dioxide atmospheres to be 7% and 4% for two different ball-milled powders. I n air-nitrogen atmospheres, they found a maximum of 9.5yQ oxygen to be permissible for these aluminum powders (3). The low permissible level of oxygen in carbon dioxide and the relatively high cost of nitrogen justified an investigation of flue gas as an inert atmosphere, Flue gas, having a nominal composition of 11.7% COn and 88.3% NP on a dry basis, is the product of combustion of natural gas. (An actual mass spectrographic analysis of the combustion products from a commercial flue gas generator showed 3% 0 2 , 86% NO, 1% Ar, and 10% (201.) The Bureau of Mines did not study mixtures of carbon dioxide and nitrogen. Extrapolation of their data on nitrogen and carbon dioxide to include mixtures was not recommended because of the
empirical nature of the relationships giving the explosibility of aluminum powders. T o complete the data on protective atmospheres, tests were conducted to evaluate various nitrogen-carbon dioxide mixtures. The limiting oxygen content in these atmospheres, to prevent ignition of three aluminum powders by an electric spark, was determined using equipment and procedures similar to those developed by the Bureau of Mines.
Experimental
Equipment. The apparatus shobvn below, referred to as the Hartmann apparatus, has been fully described in a Bureau of Mines report ( I ) . The explosion chamber consisted of a cylindrical lucite tube, 12 inches long by 23/4 inches inside diameter, with a '/r-inch wall. The bottom of the tube was closed by an aluminum base containing a specially designed cup for holding the powder. An umbrella-shaped head covered the air inlet tube located in the center of
the cup. Air at 10 p.s.i. pressure directed against the umbrella was deflected downward into the cup causing dispersion of the metal powder upward into the chamber. The top of the lucite tube was covered with a blowout diaphragm, usually filter paper. Equipment used by the author differed in three respects from that used by the Bureau. A retractable charging hopper was inserted through the side of the lucite tube to hold the aluminum powder while the tube was being purged with the desired atmosphere. The top of the tube was sealed with a gas-tight blowout diaphragm (Saran Wrap) rather than a porous filter paper to prevent air leakage into the tube. A gas outlet was provided for sampling the atmosphere. A sample of the atmosphere was collected for each experiment. Fifty milliliters of gas were withdrawn and analyzed by an Orsat, using potassium hydroxide and alkaline pyrogallate solutions to determine carbon dioxide and oxygen. Nitrogen was obtained by difference. The precision of an analysis was about +0.4'%.
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Ignition was achieved by an electric spark between two tungsten electrodes spaced l/g inch apart. The electrodes were 0.020-inch diameter tungsten wires inserted through the side of the lucite tube, 4 inches from the tube bottom. The spark was generated by discharging electrical condensers of known capacitance. The electrical system was designed so that the energy of the spark could be varied over a wide range by changing the condenser capacitance and voltage. All gases, including compressed air, were obtained from the Matheson Co. except carbon dioxide, which was purchased from the Liquid Carbonic Co. The gases were reported to have minimum purities of 99.97,, and dew points of -75' F. or less. The synthetic flue gas, premixed by the Matheson Go., was analyzed to be 11.201, COS and 88.8y0 Nz, with a dew point of -75" F. Procedure. The Bureau of Mines had established a procedure for testing the explosibility of powders in air using the Hartmann apparatus. This procedure was adopted insofar as powder concentration, spark energy, and cloud dispersion were concerned. In all tests, 1.5 grams of powder were used. This was equivalent to a concentration of 0.13 ounce per cubic foot, compared with the minimum explosive concentration of 0.040 ounce per cubic foot established by the Bureau of Mines in the same apparatus. The spark energy used was 0.20 joule, obtained by discharging five 8-microfarad condensers charged to 100-volt potential. This energy exceeded the minimum value needed to ignite aluminum powders in air. Dispersion of the powder into a dust cloud was achieved by admitting the atmosphere into the explosion chamber at 7 p.s.i. In a typical experiment, the lucite tube was cleaned and firmly attached to the aluminum base. The powder was charged into the hopper which was extended to the center of the explosion chamber. After setting the electrode separation a t '/8 inch, the Saran Wrap diaphragm was secured over top of the chamber, and the gas sampling valve was opened. The gas or gases were metered with flowmeters. These were set a t the predetermined values depending on the composition of the atmosphere desired, and the solenoid valve was opened to permit complete purging of the entire system. The purging time was usually 5 minutes when a change of gas composition was being made. Otherwise, a 2minute purging time was adequate during successive tests with the same atmosphere composition. A 50-ml. gas sample was drawn into the Orsat apparatus while the powder was being dispensed into the cup. After dispensing
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the powder, the charging hopper was retracted to a position flush with the inside of the lucite tube so that it would not impede the dust cloud. O n closing the gas sampling needle valve, the test was ready. During the purging and charging operation, the condensers were being energized. On moving the solenoid switch to the "on" position, the powder was dispersed into a cloud, and 0.2 second later the spark fired. When an explosion occurred, the diaphragm would rupture, accompanied by a flash and a noise. When no explosion occurred, the procedure established by the Bureau of Mines required four successive tests to ascertain that the same conditions would not produce an explosion. Aluminum Powders Tested. The three aluminum powders used in this study were selected from Alcoa's production powders. They were designated as Alcoa hTo. 140, No. 422>and No. ,552 powders. The No. 140 powder was a conventional atomized powder. The particles were tear-drop shaped and varied in particle size as shown above. The mass median particle diameter was 7 microns. The average oxygen content expressed as aluminum oxide was 1% by weight. No. 422 and No. 552 powders were produced by ball-milling which resulted in flaked material as shown above. Since average particle diameter would be a meaningless quantity when applied to thin flakes, the specific surface area was chosen as being indicative of the degree of fineness of these powders. The No. 422 powder contained stearic acid, added as a lubricant during the milling operation. This powder had an average specific surface area of 8.7 square meters per gram. No. 552 powder, milled with oleic acid, had an average specific surface area of 6.0 square meters per gram. Results. More than 140 tests were conducted during the course of this investigation. The results are summarized in the table showing the limiting oxygen content in various atmospheres for each of the three powders. AIR-NITROGEN MIXTURES. The Bureau of Mines determined the explosi-
biliq of aluminum powders in airnitrogen mixtures using the open-spark tube tests. I n those tests: the gas mixture was used as a carrier for the powder over a continuous spark. They stated that 9.5y0 oxygen was safe for all powders they tested in air-nitrogen. Since the Hartmann apparatus differed somewhat from the open-spark cube, it was desirable to compare the results of the two methods. The three aluminum powders were tested in airnitrogen mixtures using the Hartmann apparatus. The limiting oxygen contents were lOy0 for KO. 422 powder. 970for No. 552 powder, and 9% for h-o. 140 powder. These results were in good agreement with the findings of the Bureau of Mines. AIR-CARBON DIOXIDE MIXTURES.Using the open-spark tube test, the Bureau of Mines reported 77, as the limiting
Alcoa No. 140 atomized powder Dark field at 500X
Limiting Oxygen Content of Various Atmospheres t o Prevent Ignition of Aluminum Powder b y Electric Spark Limiting Oxygen C O ~ / N ~ Content, % 0 2 Ratio,Av. 140 422 552
Atmosphere Type 02-N2
Air-Flue Gas Oz-N*-COa Air-COn 02-co2
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Alcoa No. 4 2 2 powder Flakes lying flat (above). Cross-section flakes (below). Dark field at 500 X
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ALUMINUM POWDER oxygen content for No. 422 powder and 49ib for No. 552 powder in airCOa atmospheres. Using the Hartmann apparatus, the author’s results showed the limiting oxygen contents as 7% for No. 422 powder, 870 for No. 552 powder, and 7% for No. 140 powder. There is no obvious explanation for the two-fold difference between the Bureau’s value and our value for No. 5 5 2 powder. T h e results for No. 422 are in excellent agreement. The Bureau did not test No. 140 powder. AIR-FLUE GAS MIXTURES.Having established the agreement between the present results and those of the Bureau of Mines with air-carbon dioxide and air-nitrogen mixtures, the three powders were tested in air-flue gas mixtures to determine the limiting oxygen content where no explosions occurred. For No. 422 powder, there were no explosions at the atmosphere composition of 10% 0 2 , 5.8% COZ, and 84.2% Nz. Explosions occurred when the oxygen was increased to 1 1 . 2 y ~ . The value of 10% was chosen as the limiting oxygen allowable in air-flue gas mixtures for No. 422 powder. The +0,4% precision of the Orsat analysis did not permit establishing narrower limits. The limiting oxygen content was 12% for No. 552 powder and 9% for No. 140 powder. The atmosphere compositions were 12% 0 2 , 4.8T0 COZ, 83.2% N2 and 9.2% 0 2 , 84.4% Nz, 6.401, COz. OTHER GAS MIXTURES.The three aluminum powders were tested in oxygen-carbon dioxide and in air-50% C 0 2 and 50% N r m i x t u r e s for the
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Limiting oxygen content for aluminum powder combustion in 02-N2-COe mixtures
purpose of establishing a ternary plot of the limiting oxygen content in any combination of oxygen, carbon dioxide, and nitrogen. The data are plotted in triangular coordinates as shown below. Values of oxygen above the line drawn through the points are in tbe explosion region while those below the line are in the no-explosion region.
Discussion The effectiveness of nitrogen, carbon dioxide, and mixtures of the two as protective atmospheres for safe handling of aluminum powders has been evaluated, Nitrogen was more effective than carbon dioxide, inasmuch as the limiting oxygen concentration was higher for nitrogen than carbon dioxide. A flue gas atmosphere, as produced from combustion of natural gas, was for all practical purposes as effective as nitrogen. The behavior of No. 552 powder was somewhat anomalous in showing a higher limiting oxygen value in flue gas than in nitrogen. There are a number of factors which could have contributed to this inconsistency. For example, the dispersion of this particular powder could have been different from the others. Also, No. 552 is probably less uniform than the other powders tested. Because of these factors, this slight inconsistency was not considered significant enough to warrant further investigation. Although in commercial practice flue gas would be used as a purging medium for replacement of air, the limiting values were expressed in terms of the oxygen concentration because oxygen can be easily monitored. I n terms of air concentration, the limiting values would be 47.8y0 air in flue gas for No. 422 powder, 57.5% for No. 552 powder, and 43y0 for No. 140 powder. This means that the air content of processing equipment such as ball mills or blenders need be reduced by about 60y0to achieve a safe working atmosphere. A further
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interpretation could be added-ie., that the flue gas generator could be set to operate with excess air to ensure complete combustion of the fuel, thus eliminating the possibility of having unburnt combustible gases in the protective atmosphere. There are a number of parameters that affect the inflammability and explosibility of powder dust clouds. All of these have by no means been interrelated or for that matter recognized. Therefore, in the strictest sense, the results of the present investigation should relate only to the powders tested. However, the degree of fineness, particularly in the subsieve range, has a Iarge effect on inflammability. Application of these results to other aluminum powders having similar particle size distribution could be done with confidence. Certainly, if the “safe” atmosphere were limited to perhaps half of the observed limiting oxygen value, no safety hazard would be incurred on applying these results to similarly sized aluminum powders.
Acknowledgmenl The author is pleased to acknowledge the kind cooperation of John Nagy, Murray Jacobson, and Henry Dorsett, Jr., of the Explosives and Physical Sciences Division of the Bureau of Mines a t Pittsburgh, Pennsylvania, for their helpful discussions on powder explosions.
literature Cited (1) Dorsett, H.,Jr., Jacobson, M., Nagy, J., Williams, R., “Laboratory Equipment and Test Procedures for Evaluatinc Explosibility of Dusts,” U. S. Bureau df Mines Report R. I. 5624,1960. (2) Hartmann, I., Nagy, J., Brown, H. R., “‘Inflammability and Explosibility of Metal Powders,” U. S . Bureau of Mines Report R. I. 3722, 1943. (3) Nagy, J., Jacobson, M., Dorsett, H., Jr., U. S. Bureau of Mines, Pittsburgh, private communication, April 28, 1960. RECEIVED for review February 14, 1961 ACCEPTEDMarch 31, 1961 VOL. 53, NO. 10
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OCTOBER 1961
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