Recycling of Aluminum Salt Cake: Utilization of Evolved Ammonia
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e salt cake has been considered as a hazardous waste[1] in secondary aluminum production,[2] many processes have been devoted to the recycling of the salt cake in order to mitigate the burden to the environment. However, until now, due to the economic concerns of recovery, the dominant treatment is still landfilling which is affordable under proper control.[3] A typical composition of this waste material is presented in Table I.[4] It is to be noted that the metallic aluminum (about 5 wt.pct) entrapped in the mass is included in the Al2O3 content in Table I. Small amounts of CaF2, AlP, and Al4C3, as well as traces of transition metals or their oxides, like Mn, Cr, etc., are also present in the salt cake. Facing the fact of the increasing cost for landfilling and environmental regulations, it is necessary to look for possible valuable byproducts to increase the recycling value and design new process routes for treating combinations from other waste products from an environmental point of view. The present authors have examined the reaction between the salt cake and water and demonstrated the effect of the chlorides on the hydrolysis of AlN.[5] They had also developed a novel PENG LI, Ph.D Student, State key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China and also with 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, Department of Materials Science and Engineering, Royal Institute of Technology and also with the Institute of Iron and Steel Technology, TU-Bergakademie, Freiberg 09599, Germany. Manuscript submitted September 21, 2012. Article published online December 13, 2012. 16—VOLUME 44B, FEBRUARY 2013
leaching method by which the leaching could be carried out using water, saturated with CO2, which blocks the hydrolysis of AlN present in the salt cake.[4] In this case, the leach residue could be utilized for producing the high value ceramic, SiAlON/MgAlON.[6] The present paper presents an alternative route to the processing of the salt cake by which the ammonia gas evolved during leaching with water could be utilized to form ammonium carbonate solution using CO2-saturated water at a second stage. This solution, which is free from chlorides as described in the present paper, has applications as a fertilizer. Considering the chemical analysis presented in Table I, the ammonium carbonate solution thus produced will be free of other impurities as well. Further, this method also has the advantage of fixing carbon from CO2, which is the subject of many investigations around the world.[7–11] Aluminum nitride would react with water at 373 K (100 °C) according to the following reaction[12]: AlN þ 2H2 O ! AlOOH # þNH
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