Trash to Treasure: Recovery of Trace Platinum from Industrial Process Solutions

Abstract

Concerns about metal scarcity are becoming more prevalent due to the depletion of the world’s natural resources and increased demand of metal productions. Therefore, more efficient and innovative methods for metal recovery are required. Among the traditional hydrometallurgical methods [1], the electrochemical recovery of metals - like electrowinning - provide a state-of-the-art basis for sustainable metals recovery through the use of renewable electricity. In electrowinning, the metals are recovered by electrodeposition from concentrated and purified leaching solutions. Nevertheless, the process solutions in the final stages of hydrometallurgical or pyro-hydrometallurgical operations still contain precious metals Ag, Au, and platinum group metals at very low concentrations (< mg L-1 or < μg L-1). For example, platinum - a critical element in photocatalysts for the hydrogen economy, electronics and medical applications – can also be recovered from such trace concentrations to prevent both material loss and to provide additional value. Recently, electrodeposition and redox replacement (EDRR) - method has been applied for selective precious metal recovery from lower-grade raw materials-hydrometallurgical process solutions [2]. During EDRR, a sacrificial metal with less nobility is first deposited through electrodeposition, then the precious metal is recovered onto the electrode surface through RR reaction, driven by the difference in reduction potentials between the two metals. EDRR can be tailored through operational parameter selection, thereby allowing an optimized purity of recovered metals and even controllable preparation of metal coatings, nanoparticles, high-value-added functional surfaces (photocatalyst, corrosion resistant) directly from industrial process solutions [2].In the course of previous studies, it has been observed that metal reduction may also occur via aqueous reduction close to the electrode surface [3]. If actual electrodeposition of sacrificial species into metallic form is not indispensable before the metal recovery in the RR step, process with lower energy consumption and higher purity end-product may be realized via aqueous reduction (EAR). In the current study, multivalent iron was selected as reductive species during EAR-process to recover dissolved platinum. Specifically, dissolved Fe(III) in chloride-based media is first electrochemically reduced to a lower oxidation state Fe(II), then in the RR step, the produced aqueous Fe(II) species serve as reductant to trigger the consequent deposition of Pt on the electrode. As a result, the EAR method succeeded in recovering platinum on the electrode. Chloride concentration was shown to have a substantial impact on the platinum recovery, in terms of both recovery efficiency and structure of deposits, which is suggested to be derived from the Fe and Pt complexation in chloride-based media. The recovery of Pt based on EAR also varies with EAR process parameters, e.g., the applied potential and time to reduce Fe(III), or RR time. The influence of other species (concentrations) and the pH in solution were systematically studied as well, further paving the way to apply EAR method in other metal recovery system. Acknowledgements This work was supported by the Academy of Finland project EARMetal (LC, KY, ML: 339979 and LC, KY, JV: 342080). The RawMatTERS Finland Infrastructure (RAMI) funded by the Academy of Finland and based at Aalto University is also acknowledged. Reference [1] M.E. Wadsworth, J.D. Miller, Hydrometallurgical Processes. In: H.Y. Sohn, M.E. Wadsworth (Eds), Rate Processes of Extractive Metallurgy, Springer, Boston, 1979. https://doi.org/10.1007/978-1-4684-9117-3_4[2] L. Cui, K. Yliniemi, J. Vapaavuori, M. Lundström,Recent developments of electrodeposition-redox replacement in metal recovery and functional materials: A review, Chem Eng J. (2023). https://doi.org/10.1016/j.cej.2023.142737 [3] I. Korolev, S. Spathariotis, K. Yliniemi, B.P. Wilson, A.P. Abbott, M. Lundström, Mechanism of selective gold extraction from multi-metal chloride solutions by electrodeposition-redox replacement, Green Chem. 22 (2020) 3615–3625. https://doi.org/10.1039/D0GC00985G