Green separation and enrichment of rare and precious metals from spent automotive catalysts via Si collection and electromagnetic directional crystallization

Abstract

Spent automotive catalysts (SACs) are typical hazardous solid waste, and are also important secondary resources for recycling rare and precious metals (RPMs). The current mainstream smelting methods using Cu and Fe as collectors involve carbon emission and a large amount of acid consumption, and especially can only recover platinum group metals (PGMs) from SACs, while the rare earth elements (REEs) and Zr in SACs cannot be recovered and remain in the residual slag. The further recycling of REEs and Zr in the residual slag requires additional methods and involves processes with significant environmental burden. In this study, we proposed a green approach for recycling and concentrating RPMs from SACs: a typical Si waste, silicon ingot slicing sludge waste (SISSW), was used as a new collector to simultaneously recover RPMs (PGMs, REEs and Zr) from SACs to form a Si–RPM alloy, and the largest recovery ratios of Pd, Rh, Zr, Ce, La, and Nd in the SACs were 99.25 ± 0.03%, 99.22 ± 0.34%, 97.92 ± 0.45%, 79.20 ± 0.35%, 68.91 ± 0.12% and 73.11 ± 7.01%, respectively. To achieve the “separation and enrichment” purpose, the Si–RPM alloy was then directly separated into high-purity bulk Si (99.3%), a eutectic Si–Zr alloy (98%) and a “PGMs + REEs” concentrate via the electromagnetic directional crystallization (EDC) method. Finally, the reaction mass efficiency (RME) and environmental factor (EF) of both traditional and new processes were estimated, which showed that the new process can enhance the RME from 7.2% to 34.4% and reduce the EF from 13.6 to 2.3, demonstrating significant environmental benefits.