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openaire: EC/H2020/721385/EU//SOCRATES The accomplishment of the goals set in the EU Green Deal and the Agenda 2030 for Sustainable Development requires a metallurgical industry that is even more resource-efficient, environmentally friendly and socially responsible than it is today. The European Training Network SOCRATES has taken this up by developing ground-breaking metallurgical processes for the valorisation of industrial intermediate products. This short article summarises the results obtained in the 4-year project and discusses the outcomes in light of the recently adopted EU Green Deal.

Funding Information: This research received funding from the European Union Framework Program for Research and Innovation Horizon 2020 under Grant Agreement No. 721385 (EU MSCA-ETN SOCRATES; project website: https://www.etn-socrates.eu ). The GoldTail (Grant No. 319691, ML) and NoWASTE (Grant No. 297962, KY) projects funded by the Academy of Finland are greatly acknowledged. The authors thank the laboratory personnel at VTT and Metso Outotec for their skillful technical assistance with the experimental work. Publisher Copyright: © 2021, The Author(s). Recently, an emerging electrodeposition-redox replacement (EDRR) method was demonstrated to provide exceptionally efficient gold recovery from cyanide-free hydrometallurgical solutions. However, the effect of electrode material and its corrosion resistance in this process was overlooked, even though the EDRR process is carried out in extremely corrosive, acidic chloride solution that also contains significant amounts of strong oxidants, i.e., cupric ions. In the current study, nickel alloy C-2000, stainless steels 316L and 654SMO, and grade 2 titanium were for the first time critically evaluated as potential cathode materials for EDRR. The particular emphasis was placed on better understanding of the effect of cathode substrate on the overall efficiency of the gold recovery process. The use of a multiple attribute decision-making method of material selection allowed reaching of a well-founded compromise between the corrosion properties of the electrodes and process efficiency of gold extraction. The 654SMO steel demonstrated outstanding performance among the examined materials, as it enabled gold recovery of 28.1 pct after 3000 EDRR cycles, while its corrosion rate (CR) was only 0.02 mm/year.

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.

Despite of being the most common hydrometallurgical process for extraction of gold from ores and concentrates, cyanide leaching is blamed for its hazardous impact on environment and human health. These concerns have given a rise for alternative cyanide-free technologies, such as cupric chloride leaching. However, the state-of-art processes for gold recovery from chloride solutions are facing issues of high reagent consumption and poor selectivity. This article describes an innovative method for recovery of minor concentrations of gold from hydrometallurgical solutions by repetitive Electrochemical Deposition-Redox Replacement (EDRR) cycles. In contrast to conventional carbon-in-leach/resin-in-leach technologies or solvent extraction, the proposed electrochemical method does not require addition of any chemicals in the process and remarkably selective gold recovery can be achieved from concentrated cupric solution by tailoring the process parameters. A number of electrochemical experiments was performed in order to identify the process variables affecting the Au recovery. Results indicate that so called cut-off potential and deposition time were the EDRR parameters having the strongest impact on gold recovery at fixed concentration of Cu and Au ions. The Au content in the metal deposit after 250 EDRR cycles exceeded 75% and the Au:Cu ratio has increased by a factor of 1000, from 1:340 in the solution up to 3.3:1 in the final product. X-ray photoelectron spectroscopy analysis of the cathode surface confirmed full replacement of sacrificial copper with gold. Obtained results prove that the EDRR method can be efficiently used for the recovery of trace amounts of gold from cupric chloride solutions used for cyanide-free gold leaching.

It is becoming ever more clear that the natural resources – especially metals – are gradually being depleted from the Earth’s crust. Therefore, secondary sources such as industrial residues, waste ...