Oxidation Studies of SiAlON/MgAlON Ceramics with Fe 2 O 3 and CaO Impurities, Part II: Phase Evolution
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INTRODUCTION
DURING secondary aluminum smelting, the flux usually containing equal moles of sodium chloride and potassium chloride and minor amount of fluoride is added to protect the aluminum melt. After skimming, salt cake is generated with a low amount of metallic phase, which normally consists of 22 wt pct NaCl, 11 wt pct KCl, 44 wt pct Al2O3, 7 wt pct AlN, 7 wt pct SiO2, 4 wt pct MgO, and other impurities like CaO and Fe2O3. Recycling of the salt cake is difficult as AlN in the material gets hydrolyzed leading to ammonia evolution, which is highly undesirable from an environmental viewpoint. The current authors have developed[1] a successful leaching process by which the salts can be leached out using CO2-saturated water, keeping AlN in the leach residue unhydrolyzed. After leaching, the salt
PENG LI, Ph.D. Student, is with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China, and with the Department of Materials Science and Engineering, Royal Institute of Technology, 10044 Stockholm, Sweden. MEI ZHANG, Professor, is with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing. Contact e-mail: [email protected] LIDONG TENG, Doctor, is with the Department of Materials Science and Engineering, Royal Institute of Technology. SESHADRI SEETHARAMAN, Professor, is with the Department of Materials Science and Engineering, Royal Institute of Technology, and with the Institute of Iron and Steel Technology, TU-Bergakademie, 09599 Freiberg, Germany. Manuscript submitted May 20, 2012. Article published online January 4, 2013. 220—VOLUME 44B, FEBRUARY 2013
cake in CO2-saturated water solution, a chemical analysis of the leach residue is presented in Table I. The residue after leaching is a potential precursor for synthesizing SiAlON/MgAlON ceramic material, which has a number of high-temperature applications. Thus, the value addition to the residue, which otherwise is an environmental hazard, is very significant. The authors have already successfully synthesized SiAlON/MgAlON ceramic material from the salt cake leach residue. To test the suitability of the synthesized ceramic material for high-temperature applications, it is necessary to investigate the oxidation resistance of the synthesized products, with a view to examine the effect of impurities on the oxidation resistance. The main impurities in the salt cake leach residue are Fe2O3 and CaO, the presence of which may have serious impact on the performance of SiAlON/MgAlON, especially oxidation resistance. Fe2O3 addition has been reported to weaken the oxidation resistance.[24] The addition of CaO caused the shrinkage during the synthesis and liquid formation during the oxidation above 1673 K (1400 °C). In the current work, the oxidation resistance of the SiAlON/MgAlON ceramic material, produced from the salt cake leach residue, was investigated to arrive at the oxidation mechanism and the effect of the impurities on the oxidation resistance. The oxidation behavior
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