The recovery of metals from industrial side-streams and process streams is typically performed by solvent extraction, cementation or precipitation, chemical reduction, or electrowinning. However, several industrial process, side- and waste streams do contain also trace amounts of valuable metals such as Ag and Au at lower than ppm scale, making the economics of the recovery challenging.

Recently, we have introduced electrodeposition-redox replacement (EDRR) method as a route to extract Ag from Zn process streams [1], where a successful recovery of Ag was possible even from ppb concentrations. In EDRR method, the base metal (such as Zn) is first deposited during the electrodeposition step, followed by a redox replacement step in the absence of applied potential or current. During this latter step, the deposited less noble metal is spontaneously replaced by a more noble metal (such as Ag) still present in the solution. As a result, Ag is enriched on the electrode.

This paper concentrates on the effect of impurities on the recovery of valuable metals (Ag and/or Au) from solutions typical for hydrometallurgical industry, such as chloride, sulfate and nitrate based solutions. The measurements were performed in simulated process solutions in which metal and impurity concentrations were close to actual solutions of sulfate based Zn process solution [1], Doré slag leaching [2], or cyanide free gold chloride leaching [3]. The morphology and quality of the recovered metal product was determined by SEM-EDS and the EDRR results showed a high selectivity towards Ag and Au in all investigated hydrometallurgical solutions despite of the impurities present in the solution. Moreover, the impurities investigated were shown to speed up EDRR reactions, regardless of the concentration of the impurity - ppb, ppm or even g/L scale.

Acknowledgements

Academy of Finland (NoWASTE - Project No: 297962), EU Framework Program for Research and Innovation Horizon 2020 (MSCA-ETN SOCRATES - Grant Agreement No: 721385) and Finnish Steel and Metal Producers (METSEK-project) are acknowledged for financial support. The research utilized “RawMatTERS Finland Infrastructure” (RAMI, funded by Academy of Finland) based at Aalto University.

References:

[1] P. Halli, H. Elomaa, B. P. Wilson, K. Yliniemi, M. Lundström, ACS Sustainable Chem. Eng. 5 (2017) 10996 - 11004.

[2] P.Halli, S. Jolivet, A. Klöfverskjöld, P. Latostenmaa, B.P. Wilson, M. Lundström(2017) Leaching of Sb from TROF Furnace Doré Slag. In: Zhang L. et al. (eds) Energy Technology 2017. The Minerals, Metals & Materials Series. Springer, Cham.

[3] M. Lampinen, S. Seisko, O. Forsström, A. Laari, J. Aromaa, M. Lundström, T. Koiranen, Hydrometallurgy 169 (2017) 103–111.