Ind. Eng. Chem. Res. 2001, 40, 3331-3336
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MATERIALS AND INTERFACES Ultrafine Iron Powder as an Oxygen Adsorbent for Argon Purification. Activation of Iron by Milling P. Pranda,*,† V. Hlavacek,† and M. L. Markowski‡,§ Department of Chemical Engineering, SUNY Buffalo, 218 Furnas Hall, Buffalo, New York 14260, and Praxair Technology Center, Tonawanda, New York 14150
Ultrafine reactive iron powder was prepared by a high-performance milling procedure using commercially available iron powder. The milled iron material was found to have very high affinity to oxygen. The activated powder reacted at very mild conditions even with traces of oxygen in argon. The deactivated iron material can be easily reactivated by reduction in a stream of diluted hydrogen. The activation occurred at mild conditions. The sorption activity of the iron reactive powder does not change substantially over several cycles of adsorption-desorption. Iron powder was characterized by the surface area measured by nitrogen adsorption at -196 °C. Oxygen capacities were determined in an argon gas flow system. The process described here can be scaled up for industrial use. Introduction Ultrafine reactive powders feature high surface area and consequently can adsorb trace gas impurities. These materials represent a very effective class of adsorbents. The laboratory preparation for some of them already was described 70 years ago.1 We learned from our past research that submicron metallic powders exhibit a high affinity toward oxygen. The ability of ultrafine powders to adsorb trace amounts of oxygen provides an interesting low-cost technology to produce ultrapure nitrogen or argon gas at low temperatures ( 300 °C) would be required to reduce all oxygen in the inlet stream to trace levels. Therefore, higher temperatures are necessary to activate more iron surface sites. Runs 9 and 10, as shown in Figure 5, were stopped when full saturation on the outlet side was reached. The corresponding increase of the specific surface area can be explained in two ways. First, when the breakthrough occurs, some residual adsorbed hydrogen could have remained on the iron surface and prevented full nitrogen adsorption at -196 °C. Second, if during the O2 adsorption reaction some surface iron was present as iron oxide, cracks in the oxide layer are formed as a consequence of the different expansion coefficients of the metal and its oxide.15 In both scenarios, the oxygen capacity decreased. Oxygen adsorption efficiency was compared with commercially available BASF material R 3-11.16 The comparison presented in Figure 9 shows that the adsorption capacities of both materials were almost identical. According to our preliminary economical calculation, the price of milled iron powder would be about one-third of the cost of the BASF R 3-11 adsorbent ($13/lb vs $39/lb). We extract from the X-ray analysis that the line broadening occurred for milled iron powder. We did not use this to further characterize the iron powder; however, this type of analysis can be used for evaluating particle size and accumulated strains.17 Decreasing the particle size of milled iron was confirmed by SEM images and by nitrogen adsorption at
-196 °C. The specific surface area after milling for 927 h increased from 0.3 to 26.7 m2/g, and even to 47.1 m2/g after 1400 h of milling. Conclusions High-energy milling is an effective method for making ultrafine iron powder to use in the production of ultrahigh-purity argon. Increasing the surface area of ultrafine iron powder created a high surface area adsorbent that corresponded to a high oxygen loading capacity. With subsequent activation, there was some loss of total oxygen, possibly due to sintering. The iron color remained black during the oxygen loading; this implied that no oxidation to ferric oxide had taken place. We anticipate that the oxygen was predominantly chemisorbed by the active iron surface sites. The sorption capacity was comparable to the BASF-produced material. Our preliminary economic analysis indicates that the material can be manufactured at $13/lb, which makes it very favorable to produce it in larger quantities. The process can be easily scaled up. Gas manufacturers or catalyst material suppliers can use this method to produce ultrahigh-purity gases such as argon. Acknowledgment The authors gratefully acknowledge the funding provided by Praxair, Inc. The UB Foundation through the State University of New York at Buffalo administered the project. Literature Cited (1) Chem. Abstr. 1928, 22, 2299. (2) Gorrie, T. M.; Kopf, P. W.; Toby, S. The kinetics of the reaction of some pyrophoric metals with oxygen. J. Phys. Chem. 1967, 71, 3842. (3) Pranda, P.; Hlavacek, V.; Markowski, M. L. Ultra fine iron powder from organic precursors as oxygen adsorbent in argon purification. Chem. Eng. Sci. 2001, submitted for publication. (4) Moelle, C. H.; Fecht, H. J. Thermal stability of nanocrystalline iron prepared by mechanical attrition. Nanostruct. Mater. 1995, 6, 421. (5) Ding, J.; Miao, W. F.; Pirault, E.; Street, R.; McCormick, P. G. Structural evolution of Fe + Fe2O3 during mechanical milling. J. Magn. Magn. Mater. 1998, 177-181, 933. (6) Nasu, T.; Tokumitsu, K.; Miyazawa, K.; Greer, A. L.; Suzuki, K. Solid-state reduction of iron oxide by ball-milling. Mater. Sci. Forum 1999, 312, 185. (7) Tokumitsu, K. Reduction of metal oxides by mechanical alloying method. Solid State Ionics 1997, 101-103, 25. (8) Munitz, A.; Kimmel, J. C.; Fields, R. J. Balled milling induced bct phase formation in iron and iron alloys. Nanostruct. Mater. 1997, 8, 867.
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(9) Rawers, J.; Cook, D. Influence of attrition milling on nanograin boundaries. Nanostruct. Mater. 1999, 11, 331. (10) Cook, D. C.; Kim, T. H.; Rawers, J. C. Microstructural development of iron powder during attritor ball-milling in nitrogen. Mater. Sci. Forum 1996, 225-227, 533. (11) Shaham, D.; Rawers, J.; Zolotoyabko, E. Structural transformation of iron powders mechanically processed in nitrogen. Mater. Lett. 1996, 27, 41. (12) Dogan, C. P.; Rawers, J. C.; Govier, R. D.; Korth, G. Mechanical processing, compaction, and thermal processing of R-Fe powder. Nanostruct. Mater. 1994, 4, 631. (13) Rawers, J.; Govier, D.; Cook, D. Bct-Fe formation during mechanical processing of bcc-Fe powder. Scr. Metall. Mater. 1995, 32, 1319. (14) Hlavacek, V.; Pranda, P.; Markowski, M. L. Purification
of Argon. U.S. Patent Application filed on Nov 16, 2000. (15) Shevchenko, V. G.; Kononenko, V. I.; Latosh, I. N.; Chulova, I. A.; Lukin, N. V. Effect of the size factor and alloying on oxidation of aluminum powders. Fiz. Goreniya Vzryva 1994, 30, 68. (16) BASF Catalyst R 3-11. Chemical Intermediates. Technical Bulletin of BASF; BASF: Jan 30, 1996. (17) Williamson, G. K.; Hall, W. H. X-ray line broadening from filed aluminum and wolfram. Acta Metall. 1953, 1, 22.
Received for review December 30, 2000 Revised manuscript received May 2, 2001 Accepted May 8, 2001 IE0011324
