Recovery of Platinum Group Metals from Spent Catalysts Using Iron Chloride Vapor Treatment
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UCTION
PLATINUM group metals (PGMs) are expensive and relatively scarce elements in the Earth’s crust. The mineral resources of PGMs are predominately localized in South Africa and Russia.[1–4] Furthermore, their extraction and subsequent treatment generate large amounts of waste and consume a lot of energy. PGMs are widely used in various industrial applications because of their unique catalytic properties and excellent heat and corrosion resistances. In particular, automobile catalysts account for a large fraction of the global demand for Pt, Pd, and Rh.[1] Therefore, the recovery of PGMs from spent automobile catalysts is important not only in terms of ensuring their steady supply, but also minimizing the environmental burden. The recycling of PGMs from spent catalysts has been actively practiced by many smelting and recycling companies. However, the efficient recovery of these metals is difficult because of their relatively low concentrations in the scrap and high chemical stability.
YU-KI TANINOUCHI and TORU H. OKABE are with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Contact e-mail: taninou@ iis.u-tokyo.ac.jp Manuscript submitted December 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Automobile catalysts mostly consist of honeycomb-structured ceramic substrates with porous catalyst layer. The ceramic substrates are composed of cordierite (Mg2Al4Si5O18) or similar chemically stable oxides. The catalyst layers that support fine PGM particles consist of alumina (Al2O3) and other oxides such as ceria (CeO2) and zirconia (ZrO2). The total concentration of PGMs in the catalysts is approximately 0.05–0.5 mass pct (500–5000 ppm).[5] Usually, the catalysts recovered from used automobiles are first mechanically pulverized to evaluate their PGM contents and then subjected to pyrometallurgical and/or hydrometallurgical processes for PGM extraction (see Figure 1).[2–5] In commercial applications, pulverized catalysts are often smelted with a collector metal such as liquid Fe or Cu; hereby, PGMs are concentrated in the metallic phase, while the ceramic components of the spent catalysts are removed as slag waste.[6,7] Subsequently, the PGMs extracted in the collector metal are dissolved in an aqueous solution followed by their separation and purification using various techniques such as solvent extraction, precipitation, and ion exchange.[8,9] The benefits of the pyrometallurgical processing method include large PGM recovery rates and high throughput. However, its practical implementation requires the use of largescale equipment and high amount of energy. It should also be noted that PGMs could be dissolved directly from spent catalysts in strong oxidizing acids, such as
aqua regia and hydrochloric acid with chlorine gas, which is typically performed using a small-scale plant with relatively low energy consumption. However, the applicability of this method is often limited by the low PGM recovery rates, longer processing times, and generation of large vol
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