Enhanced Dissolution of Rh from RhZn 3 Formed Through Zn Vapor Pretreatment
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RHODIUM (Rh) is one of the platinum group metals (PGMs) with useful properties such as high chemical stability, heat resistance and catalytic activity. One of the most important uses of Rh is as an automotive catalyst, which accounts for more than 80 pct of the total demand of the metal.[1] Rh production, however, is low even at the global level, and its price is even higher than platinum (Pt). Therefore, the recovery of Rh from a wasted catalyst is an important issue. In automotive catalysts, Rh is supported on ceramics with high surface areas, e.g., cordierite with honeycomb structure and alumina, together with other PGMs.[2] PGMs used in this way are recovered by a pyrometallurgical process,[3] in which the used catalytic converters are melted at a temperature above 1600 K (1327 °C). PGMs separate from oxide, are concentrated into a metallic phase through this process, and then are refined from the metallic phase by dissolution into an aqueous solution and various subsequent separation processes. The recovery process including the high-temperature treatment requires a great deal of energy and generates slag originating from the catalyst support. To reduce the involved environmental load and cost, the development of sounder processes is required. Alternatively, PGMs may be separated from ceramics by a hydrometallurgical process because metals can be dissolved selectively in aqueous solutions. However, strong oxidizing agents are required to dissolve PGMs, so that the environmental load involved in the leaching is formidable. The dissolution of PGMs can be enhanced when they are alloyed with other metals,[4–7] and thus, slightly HIDEAKI SASAKI, Project Research Associate, and MASAFUMI MAEDA, Professor, are with the International Research Center for Sustainable Materials, Institute of Industrial Science, University of Tokyo, Meguro, Tokyo 153-8505, Japan. Contact e-mail: hideakis@iis. u-tokyo.ac.jp Manuscript submitted November 18, 2010. Article published online February 22, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
soluble PGMs such as Rh have often been alloyed prior to leaching. Suitable alloying elements are copper, lead, nickel, zinc (Zn), and so on.[4] Taking advantage of this finding, a recovery process of PGMs using metal vapor was proposed. In the process, used products containing PGMs, e.g., automotive catalysts, are exposed to metal vapor as a pretreatment of leaching. If the PGMs form an alloy or compound, then the environmental load involved in the subsequent leaching can be reduced. Among metals, Zn is considered most suitable as the metal vapor. One reason is that Zn has high vapor pressure and its gas can be supplied easily. In addition, unlike some active metals such as magnesium and calcium, Zn does not react with the ceramics used as catalyst supports. It, therefore, reacts selectively with PGMs without generating unnecessary reactions. Reactions between Rh and Zn, however, have not been reported, so that it is unclear what kind of Rh-Zn compound would be formed during the Zn vapor tre
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