Magnetic Concentration of Platinum Group Metals from Catalyst Scraps Using Iron Deposition Pretreatment
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TRODUCTION
PLATINUM group metals (PGMs) are widely used in a variety of industrial applications such as catalysts, electrodes, and crucible materials. However, they are expensive and are among the rarest elements in the Earth’s crust. Mineral resources of PGMs are limited in supply and are highly localized in South Africa and Russia.[1–3] Furthermore, the PGM concentration in mineral resources is less than 10 mass ppm (0.001 mass pct), even in the highest grade ores.[3–5] Thus, the mining and smelting of ores generates large quantities of waste and consumes a huge amount of energy. Among the various industrial applications, automobile catalysts account for a large portion of the world’s consumption of Pt, Pd, and Rh.[1–3] The demand for PGMs for automobile catalysts has been steadily increasing over the past 20 years because of increasing automobile production and stricter environmental regulations. Automobile catalysts, which are used in devices for
YU-KI TANINOUCHI, TETSUO WATANABE, 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: [email protected]. Manuscript submitted 19 February 2017. Article published online May 31, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
purifying the exhaust gas from automobiles, usually consist of a honeycomb-structured ceramic substrate and a porous catalyst layer supporting the fine particles of Pt, Pd, and Rh. The total PGM concentration in automobile catalysts is approximately 500 to 5000 mass ppm (0.05 to 0.5 mass pct),[3,6] which is orders of magnitude higher than that in natural mineral deposits. Spent automobile catalysts are therefore the most important secondary resource for PGMs, and their recycling is essential, not only for ensuring a steady supply of PGMs, but also for preserving the limited natural resources. In general, spent automobile catalysts are mechanically pulverized in order to evaluate the PGM content (i.e., evaluate the value of the scrap), and the PGMs are then extracted and recovered by pyrometallurgical or hydrometallurgical recycling processes,[2,3,6–10] as shown in Figure 1. In pyrometallurgical recycling processes, the PGMs in the scrap are extracted and concentrated using a molten metal such as Cu or Fe as the collector, and the ceramic components in the catalyst scrap are removed as slag waste.[11–13] Subsequently, the extracted PGMs are dissolved in an aqueous solution, followed by separation and purification using techniques such as solvent extraction, precipitation, and ion exchange.[3,7] While pyrometallurgical processes usually have high PGM recovery rates and high throughput, they require large-scale equipment and consume a large amount of energy. In hydrometallurgical recycling processes, the PGMs are dissolved directly from the spent catalysts VOLUME 48B, AUGUST 2017—2027
using acids containing strong oxidants (HCl/HNO3, HCl/Cl2, etc.).[7,9] The purification and mutual separation of PGMs are then performed using various hydromet
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