Enhanced Dissolution of Platinum Group Metals Using Electroless Iron Deposition Pretreatment
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TRODUCTION
BECAUSE platinum group metals (PGMs) are both corrosion- and heat-resistant with various catalytic properties, they have many applications in the automobile, chemical, electrical, and medical industries.[1] In particular, automobile catalysts, which are installed in the engine exhaust systems of automobiles to detoxify the exhaust gas, account for much of the global demand for Pt, Pd, and Rh. In 2015, automobile catalysts accounted for approximately 40 pct of the worldwide use of Pt, 70 pct of that of Pd, and 80 pct of that of Rh.[1] The increase in automobile production and strictness of environmental regulations is expected to increase the demand for catalytic PGMs. Meanwhile, mineral resources for PGMs are limited to a few locations (e.g., South Africa and Russia), and their production volume is limited.[1–3] Furthermore, the mining and smelting of PGMs are highly energy-consuming and generate large quantities of waste. The total PGM concentration in automobile catalysts is approximately 500 to 5000 mass ppm (0.05 to 0.5 mass pct), which is several orders of magnitude higher than that in natural resources.[2,4,5] Thus, the recovery of PGMs
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: [email protected]. Manuscript submitted May 12, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
from used automobile catalysts is crucial, not only to ensure a steady supply of PGMs, but also to minimize the environmental impact of PGM production. Because PGMs are precious materials, their recovery from spent catalysts is already practiced by many smelting and recycling companies. However, the efficient recovery of PGMs is difficult, because they are present as minor components in geometrically complex structures. Automobile catalysts typically use honeycomb-structured ceramic substrates; fine PGM particles are supported on porous catalyst layers, comprising mainly alumina (Al2O3) and ceria (CeO2) among other oxides, which are coated on the substrate. The total concentration of PGMs to be extracted from catalyst waste is less than 1 mass pct, and the remaining weight is not valuable. Furthermore, the high chemical stability of PGMs impedes their extraction from spent catalysts. The extracted PGMs are purified and separated individually using various hydrometallurgical techniques, such as solvent extraction, precipitation, and ion exchange.[2,6–8] To achieve this, the PGMs in the spent catalysts must be extracted into aqueous solutions (typically of HCl), despite their highly noble characteristics. The PGMs can be dissolved directly from spent catalysts using strong oxidizing acids, such as aqua regia (HCl/HNO3) and hydrochloric acid with chlorine gas (HCl/Cl2).[2,6,7,9,10] However, direct dissolution requires long processing times and highly toxic solutions, and the relatively low recovery rates of PGMs (especially Rh) often limit the applicability of the technique. Further, the method generate
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