Dissolution Behavior of Rhodium in the Na 2 O-SiO 2 and CaO-SiO 2 Slags
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DUCTION
RHODIUM, one of the major platinum group metals (PGMs), is extensively used in specialized applications, e.g., as a catalyst for toxic gas conversion in automobiles, as a material for molten glass containers, and as a component in electronic devices. Because of the worldwide scarcity of rhodium mineral resources, in recent years, rhodium has been increasingly recycled from scrap.[1] Since rhodium is chemically inert, the separation of rhodium from scrap using a liquid base metal by the pyrometallurgical process is considered as one of the major recovery methods.[2–4] In general, the process is carried out at temperatures ranging from 1373 K to 1973 K (from 1100 °C to 1700 °C) in either a reducing or an oxidizing atmosphere. Theoretically, rhodium has the higher metal stability at high temperatures than most base metals, and hence, it is possible to collect rhodium in the liquid metal in the reducing atmosphere. Nevertheless, to maximize the rhodium recovery in the oxidizing atmosphere, the loss of rhodium by the dissolution into slags must be minimized. Thus, the conditions of the separation process must be effectively controlled. Accordingly, the knowledge of the dissolution behavior of rhodium in molten slags is essential. CHOMPUNOOT WIRASERANEE, Graduate Student, is with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1138656, Japan. Contact e-mail: [email protected] TORU H. OKABE and KAZUKI MORITA, Professors, are with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Manuscript submitted February 27, 2012. Article published online February 26, 2013. 584—VOLUME 44B, JUNE 2013
The dissolution behaviors of some precious metals such as platinum, gold, and ruthenium in molten slags in the oxidizing atmosphere have been reported earlier. Nakamura et al.[5,6] measured the solubility of platinum in various oxide melts such as the Na2O-SiO2, the CaOSiO2 and the CaO-Al2O3 systems. The solubility of platinum in the melts increased with increasing oxygen partial pressure and temperature. It was suggested that platinum exists as a platinum cation (Pt2+) in the acidic melts and as a platinate ion (PtO2 2 ) in the basic melts. Swinbourne et al.[7] measured the solubility of gold in various metallurgical slag systems including the PbOSiO2 binary slags. They found that the solubility of gold increased with increasing oxygen partial pressure and with the content of PbO in the slags. It was proposed that gold dissolves into the basic slags as an AuO4 2.5 ion. Recently, Shuto et al.[8] examined the dissolution behavior of ruthenium in molten slags at temperatures ranging from 1373 K to 1873 K (from 1100 °C to 1600 °C) using the Na2O-SiO2, the CaO-SiO2, and the Na2O-SiO2Al2O3 slag systems. It was suggested that ruthenium dissolves into the slags as an acidic oxide (RuO 2 ) by an exothermic reaction. In the case of rhodium, however, the actual dissolution behavior has not yet bee
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