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Advances in Understanding Metal Electrolysis Processes

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A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Extractive Metallurgy".

Deadline for manuscript submissions: closed (30 June 2022) | Viewed by 32354

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Prof. Dr. Bernd Friedrich

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IME Process Metallurgy and Metal Recycling Department, RWTH Aachen University, 52072 Aachen, Germany
Interests: process technology; metals; recycling; purification; alloying; WEEE; spent batteries; critical materials; circular economy
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Jovan N. Jovićević

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Institute of Chemistry, Technology and Metallurgy, Department of Electrochemistry, University of Belgrade, Njegoševa 12, 110000 Belgrade, Serbia
Interests: electrochemistry; electrochemical deposition of metals; underpotential deposition and alloy formation; metal deposition from nonaqueous electrolytes; metal recycling; nanostructured materials; electrometallurgy; electrochemical engineering; composite materials

Special Issue Information

Dear Colleagues,

The field of metal electrolysis is wide-ranging, from metals production and refinement to galvanic coatings. Even for commodity metals like Cu, Ni, Zn, Pb, Al, Mg many open questions and unresolved issues remain connected to the process, the electrochemical mechanisms and their impact on the efficiency and quality. More and more complex recycling alloys needed to be handled and are subjects of widespread research. Emphases on improvements in energy yields, environmental compatibility and technical feasibility will certainly result in interesting publications.

In this special issue, a further focus is placed on rare metals which as well as their alloys continue to be in high demand due to their ongoing and potential applications in advanced technologies like medical, electronics, and aerospace industries.

The electrodeposition of metals and alloys has, for some time, been a very important approach to the production and recycling of transition metals and rare-earth elements. However, there is a need for technological improvements based on a better understanding of the processes of individual-metal electrodeposition and dissolution. Improving the systematic understanding of electrodeposition processes taking place on the working (cathode) and counter (anode) electrodes during electrolysis is critical for achieving optimizations in these production processes. Since a great majority of the transition metals and rare-earth elements have extremely negative standard potentials in aqueous electrolytes, only nonaqueous ones can be used in their electrodeposition. Finding new and enhancing existing electrolyte options among ionic liquids, molten salts, and deep eutectic solvents (DES) is an additional avenue for improvement. Although not all of their complexities have been completely elucidated, studies of electrochemical metals and alloy deposition and dissolution from these electrolytes have yielded steady progress in understanding and applications. An example that is in line with this progress is a study of underpotential deposition from low-temperature molten salts that elaborates on the methods to synthesize transient metal alloys in thermal conditions several hundred degrees lower than those demanded by their phase diagram. Pushing forward the evolution of these and similar solutions will continue to be of considerable importance.

Prof. Dr. Bernd Friedrich
Prof. Dr. Jovan N. Jovićević
Guest Editors

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Keywords

Electrodeposition
Electrolytes (aqueous electrolytes, ionic liquids, molten salts, deep eutectic solvents (DES))
Coatings
Metals
Rare-earth elements
Alloys
Intermetallics
Metal powders
Composites
Recycling

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Published Papers (11 papers)

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Editorial

Jump to: Research, Review

3 pages, 192 KiB

Open AccessEditorial

Advances in Understanding Metal Electrolysis Process

by Bernd Friedrich, Jovan N. Jovićević, Dominic Feldhaus and Vesna S. Cvetković

Metals 2023, 13(2), 307; https://doi.org/10.3390/met13020307 - 2 Feb 2023

Viewed by 1851

Abstract

Advancements in technologies related to the electrorefining and electrodeposition of metals—as important manufacturing process steps—continue to receive significant attention [...] Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

Research

Jump to: Editorial, Review

12 pages, 1549 KiB

Open AccessFeature PaperArticle

The Behavior of Ruthenium in Copper Electrowinning

by Alexandra Thiere, Hartmut Bombach and Michael Stelter

Metals 2022, 12(8), 1260; https://doi.org/10.3390/met12081260 - 27 Jul 2022

Cited by 2 | Viewed by 2256

Abstract

The recycling of material containing precious metals can lead to the entry of ruthenium into the copper electrowinning process, by so far unknown effects. There, ruthenium is oxidized to highly volatile ruthenium tetroxide. In order to avoid ruthenium losses during electrolysis, the oxidation behavior of ruthenium in copper electrowinning was investigated by testing different oxygen overvoltages using lead alloy and diamond anodes. Furthermore, the temperature and the current density were varied to investigate a possible chemical or electrochemical reaction. The results of the study show that ruthenium is not directly electrochemically oxidized to ruthenium tetroxide at the anode. Especially at anodes with high oxygen overvoltage, the formation of other oxidants occurs parallel to the oxygen evolution in the electrolyte. These oxidants oxidize ruthenium compounds to highly volatile ruthenium tetroxide by chemical reactions. These reactions depend mainly on temperature; the formation of the active oxidants depends on the anodic potential. To avoid ruthenium losses in the copper electrowinning process, anodes with a low anodic potential should be used at low electrolyte temperatures. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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13 pages, 9187 KiB

Open AccessArticle

Influence of Rare Earth Oxide Concentration on Electrochemical Co-Deposition of Nd and Pr from NdF₃-PrF₃-LiF Based Melts

by Vesna S. Cvetković, Dominic Feldhaus, Nataša M. Vukićević, Ksenija Milicevic-Neumann, Tanja S. Barudžija, Bernd Friedrich and Jovan N. Jovićević

Metals 2022, 12(7), 1204; https://doi.org/10.3390/met12071204 - 15 Jul 2022

Cited by 5 | Viewed by 3214

Abstract

The impact of rare earth oxide (REO) concentration on the deposition process and selective recovery of the metal being deposited from a molten fluoride salt system was investigated by applying deposition of Nd and Pr and varying the concentration of REO added to the electrolyte. A ternary phase diagram for the liquidus temperature of the NdF₃-PrF₃-LiF system was constructed to better predict the optimal electrolyte constitution. Cyclic voltammetry was used to record three redox signals, reflecting the processes involving Nd(III)/Nd and Pr(III)/Pr transformations. A two-step red/ox process for Nd(III) ions and a single-step red/ox process for Pr(III) ions were confirmed by square-wave voltammetry. The cyclic voltammetry results indicated the possibility of neodymium and praseodymium co-deposition. In order to sustain higher co-deposition rates on the cathode and to avoid increased production of PFC greenhouse gases on the anode, a low-overpotential deposition technique was used for Nd and Pr electrodeposition from the electrolyte with varying Nd₂O₃ and Pr₆O₁₁ concentrations. Co-deposited neodymium and praseodymium metals were characterized by electron probe microanalysis (EPMA) and X-ray diffraction (XRD) analysis. After electrodeposition, concentration profiles of neodymium and praseodymium were recorded, starting from the cathode surface towards the electrolyte bulk. The working temperature of 1050 °C of the molten fluoride salt basic electrolyte, in line with the constructed phase diagram, was validated by improved co-deposition and led to a more effective deposition process. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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15 pages, 15689 KiB

Open AccessArticle

Model and Mechanism of Anode Effect of an Electrochemical Cell for Nd or (Nd, Pr) Reduction

by Andre Luiz Nunis da Silva, Celia Aparecida Lino dos Santos, Rogério de Melo Riberio de Araújo, Dominic Feldhaus, Bernd Friedrich, Fernando José Gomes Landgraf and Roberto Guardani

Metals 2022, 12(3), 498; https://doi.org/10.3390/met12030498 - 15 Mar 2022

Cited by 3 | Viewed by 2571

Abstract

The anode effect can occur during neodymium and didymium oxide electrowinning, causing a surge in the electrochemical cell voltage, interrupting the process, and increasing the greenhouse gas emissions. In this work, we develop a mathematical model, based on the mass balance of gas bubbles evolving from the anode, to understand the influence of some process parameters on the anode effect. The anode effect occurs due to bubble coverage and limitations on the mass transfer of the oxide species. Variables such as current density, oxide content, viscosity, and electrolyte composition play an important role in the anodic process. Finally, we propose a mechanism for the occurrence of the anode effect during Nd or Di (Nd–Pr) oxide electrolytic reduction based on models used in aluminum electrolysis. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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10 pages, 4097 KiB

Open AccessArticle

Electrodeposition of Indium from an Ionic Liquid Investigated by In Situ Electrochemical XPS

by Zhen Liu, Jun Cheng, Oliver Höfft and Frank Endres

Metals 2022, 12(1), 59; https://doi.org/10.3390/met12010059 - 27 Dec 2021

Cited by 17 | Viewed by 4113

Abstract

The electrochemical behavior and electrodeposition of indium in an electrolyte composed of 0.1 mol/L InCl₃ in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py_1,4]TFSI) on a gold electrode were investigated. The cyclic voltammogram revealed several reduction and oxidation peaks, indicating a complex electrochemical behavior. In the cathodic regime, with the formation of an In-Au alloy, the reduction of In(III) to In(I) and of In(I) to In(0) takes place. In situ electrochemical X-ray photoelectron spectroscopy (XPS) was employed to investigate the reduction process by monitoring the oxidation states of the components during the cathodic polarization of 0.1 mol/L InCl₃/[Py_1,4]TFSI on a gold working electrode under ultra-high vacuum (UHV) conditions. The core electron binding energies of the IL components (C 1s, O 1s, F 1s, N 1s, and S 2p) shift almost linearly to more negative values as a function of the applied cell voltage. At −2.0 V versus Pt-quasi reference, In(I) was identified as the intermediate species during the reduction process. In the anodic regime, a strong increase in the pressure in the XPS chamber was recorded at a cell voltage of more than −0.5 V versus Pt quasi reference, which indicated, in addition to the oxidation reactions of In species, that the oxidation of Cl⁻ occurs. Ex situ XPS and XRD results revealed the formation of metallic In and of an In-Au alloy. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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16 pages, 7239 KiB

Open AccessArticle

Implementation of the Chicot–Lesage Composite Hardness Model in a Determination of Absolute Hardness of Copper Coatings Obtained by the Electrodeposition Processes

by Ivana O. Mladenović, Jelena S. Lamovec, Dana G. Vasiljević-Radović, Rastko Vasilić, Vesna J. Radojević and Nebojša D. Nikolić

Metals 2021, 11(11), 1807; https://doi.org/10.3390/met11111807 - 10 Nov 2021

Cited by 8 | Viewed by 2255

Abstract

The influence of various electrolysis parameters, such as the type of cathode, composition of the electrolyte and electrolysis time, on the morphology, structure and hardness of copper coatings has been investigated. Morphology and structure of the coatings were analyzed by scanning electron microscope (SEM), atomic force microscope (AFM) and X-ray diffraction (XRD), while coating hardness was examined by Vickers microindentation test applying the Chicot–Lesage (C–L) composite hardness model. Depending on the conditions of electrolysis, two types of Cu coatings were obtained: fine-grained mat coatings with a strong (220) preferred orientation from the sulfate electrolyte and smooth mirror bright coatings with a strong (200) preferred orientation from the electrolyte with added leveling/brightening additives. The mat coatings showed larger both measured composite and calculated coating hardness than the mirror bright coatings, that can be explained by the phenomena on boundary among grains. Independent of electrolysis conditions, the critical relative indentation depth (RID) of 0.14 was established for all types of the Cu coatings, separating the zone in which the composite hardness can be equaled with the coating hardness and the zone requiring an application of the C–L model for a determination of the absolute hardness of the Cu coatings. Full article

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17 pages, 4569 KiB

Open AccessArticle

Electrochemical Study of Nd and Pr Co-Deposition onto Mo and W from Molten Oxyfluorides

by Vesna S. Cvetković, Dominic Feldhaus, Nataša M. Vukićević, Tanja S. Barudžija, Bernd Friedrich and Jovan N. Jovićević

Metals 2021, 11(9), 1494; https://doi.org/10.3390/met11091494 - 21 Sep 2021

Cited by 6 | Viewed by 3154

Abstract

Electrodeposition processes of neodymium and praseodymium in molten NdF₃ + PrF₃ + LiF + 1 wt.%Pr₆O₁₁ + 1 wt.%Nd₂O₃ and NdF₃ + PrF₃ + LiF + 2 wt.%Pr₆O₁₁ + 2 [...] Read more.

Electrodeposition processes of neodymium and praseodymium in molten NdF₃ + PrF₃ + LiF + 1 wt.%Pr₆O₁₁ + 1 wt.%Nd₂O₃ and NdF₃ + PrF₃ + LiF + 2 wt.%Pr₆O₁₁ + 2 wt.%Nd₂O₃ electrolytes at 1323 K were investigated. Cyclic voltammetry, square wave voltammetry, and open circuit potentiometry were applied to study the electrochemical reduction of Nd(III) and Pr(III) ions on Mo and W cathodes. It was established that a critical condition for Nd and Pr co-deposition in oxyfluoride electrolytes was a constant praseodymium deposition overpotential of ≈−0.100 V, which was shown to result in co-deposition current densities approaching 6 mAcm⁻². Analysis of the results obtained by applied electrochemical techniques showed that praseodymium deposition proceeds as a one-step process involving exchange of three electrons (Pr(III)→Pr(0)) and that neodymium deposition is a two-step process: the first involves one electron exchange (Nd(III)→Nd(II)), and the second involves an exchange of two electrons (Nd(II)→Nd(0)). X-ray diffraction analyses confirmed the formation of metallic Nd and Pr on the working substrate. Keeping the anodic potential to the glassy carbon working anode low results in very low levels of carbon oxides, fluorine and fluorocarbon gas emissions, which should qualify the studied system as an environmentally friendly option for rare earth metal deposition. The newly reported data for Nd and Pr metals co-deposition provide valuable information for the recycling of neodymium-iron-boron magnets. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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15 pages, 4473 KiB

Open AccessArticle

Towards Understanding the Cathode Process Mechanism and Kinetics in Molten LiF–AlF₃ during the Treatment of Spent Pt/Al₂O₃ Catalysts

by Andrey Yasinskiy, Sai Krishna Padamata, Srecko Stopic, Dominic Feldhaus, Dmitriy Varyukhin, Bernd Friedrich and Peter Polyakov

Metals 2021, 11(9), 1431; https://doi.org/10.3390/met11091431 - 10 Sep 2021

Cited by 3 | Viewed by 2208

Abstract

Electrochemical decomposition of spent catalyst dissolved in molten salts is a promising approach for the extraction of precious metals from them. This article reports the results of the study of aluminum electrowinning from the xLiF–(1-x)AlF₃ melt (x = 0.64; 0.85) containing 0–5 wt.% of spent petroleum Pt/γ-Al₂O₃ catalyst on a tungsten electrode at 740–800 °C through cyclic voltammetry and chronoamperometry. The results evidence that the aluminum reduction in the LiF–AlF₃ melts is a diffusion-controlled two-step process. Both one-electron and two-electron steps occur simultaneously at close (or same) potentials, which affect the cyclic voltammograms. The diffusion coefficients of electroactive species for the one-electron process were (2.20–6.50)∙10⁻⁶ cm²·s^–1, and for the two-electron process, they were (0.15–2.20)⁻⁶ cm²·s⁻¹. The numbers of electrons found from the chronoamperometry data were in the range from 1.06 to 1.90, indicating the variations of the partial current densities of the one- and two-electron processes. The 64LiF–36AlF₃ melt with about 2.5 wt.% of the spent catalysts seems a better electrolyte for the catalyst treatment in terms of cathodic process and alumina solubility, and the range of temperatures from 780 to 800 °C is applicable. The mechanism of aluminum reduction from the studied melts seems complicated and deserves further study to find the optimal process parameters for aluminum reduction during the spent catalyst treatment and the primary metal production as well. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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19 pages, 10434 KiB

Open AccessArticle

Zn-Co-CeO₂ vs. Zn-Co Coatings: Effect of CeO₂ Sol in the Enhancement of the Corrosion Performance of Electrodeposited Composite Coatings

by Marija Riđošić, Nebojša D. Nikolić, Asier Salicio-Paz, Eva García-Lecina, Ljiljana S. Živković and Jelena B. Bajat

Metals 2021, 11(5), 704; https://doi.org/10.3390/met11050704 - 25 Apr 2021

Cited by 8 | Viewed by 2273

Abstract

Electrodeposition and characterization of novel ceria-doped Zn-Co composite coatings was the main goal of this research. Electrodeposited composite coatings were compared to pure Zn-Co coatings obtained under the same conditions. The effect of two ceria sources, powder and home-made sol, on the morphology and corrosion resistance of the composite coatings was determined. During the electrodeposition process the plating solution was successfully agitated in an ultrasound bath. The source of the particles was found to influence the stability and dispersity of plating solutions. The application of ceria sol resulted in an increase of the ceria content in the resulting coating and favored the refinement from cauliflower-like morphology (Zn-Co) to uniform and compact coral-like structure (Zn-Co-CeO₂ sol). The corrosion resistance of the composite coatings was enhanced compared to bare Zn-Co as evidenced by electrochemical impedance spectroscopy and scanning Kelvin probe results. Zn-Co doped with ceria particles originating from ceria sol exhibited superior corrosion resistance compared to Zn-Co-CeO₂ (powder) coatings. The self-healing rate of artificial defect was calculated based on measured Volta potential difference for which Zn-Co-CeO₂ (sol) coatings exhibited a self-healing rate of 73.28% in a chloride-rich environment. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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14 pages, 4035 KiB

Open AccessFeature PaperArticle

Electrodeposition of Aluminium-Vanadium Alloys from Chloroaluminate Based Molten Salt Containing Vanadium Ions

by Vesna S. Cvetković, Nataša M. Vukićević, Dominic Feldhaus, Ksenija Milicevic-Neumann, Tanja S. Barudžija, Bernd Friedrich and Jovan N. Jovićević

Metals 2021, 11(1), 123; https://doi.org/10.3390/met11010123 - 10 Jan 2021

Cited by 2 | Viewed by 2531

Abstract

The Al-V alloys were synthetized by potentiostatic electrodeposition onto a glassy carbon electrode from equimolar AlCl₃ + NaCl bath containing vanadium ions at 200 °C. The alloy deposits were characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The deposits were identified as Al₃V and AlV₃ alloys. It was found that intermetallic alloys were synthetized during aluminium underpotential deposition onto vanadium metal that was previously deposited on the glassy carbon electrode by diffusion-controlled overpotential deposition. Alloys were the result of solid-state interdiffusion between the initially deposited vanadium and the subsequently deposited aluminium. As a source to secure a constant concentration of vanadium in the electrolyte during deposition, vanadium anodic dissolution, and VCl₃ melt addition were studied. The effect of vanadium ion concentration in the electrolyte on the composition and the surface morphology of the obtained deposits was investigated. The results indicate that controlled vanadium and aluminium codeposition could be a further step to the successful development of an advanced technology for Al₃V and AlV₃ alloy synthesis. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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Review

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22 pages, 16756 KiB

Open AccessReview

Correlation of Morphology and Crystal Structure of Metal Powders Produced by Electrolysis Processes

by Nebojša D. Nikolić, Vesna M. Maksimović and Ljiljana Avramović

Metals 2021, 11(6), 859; https://doi.org/10.3390/met11060859 - 24 May 2021

Cited by 19 | Viewed by 3209

Abstract

In this review paper, morphologies of metal powders produced by the constant (potentiostatic and galvanostatic) regimes of electrolysis from aqueous electrolytes are correlated with their crystal structure at the semiquantitative level. The main parameters affecting the shape of powder particles are the exchange current density (rate of electrochemical process) and overpotential for hydrogen evolution reaction. Depending on them, various shapes of dendrites (the needles, the two-dimensional (2D) fern-like, and the three-dimensional (3D) pine-like dendrites), and the particles formed under vigorous hydrogen evolution (cauliflower-like and spongy-like particles) are produced by these regimes of electrolysis. By decreasing the exchange current density value, the crystal structure of the powder particles is changed from the strong (111) preferred orientation obtained for the needle-like (silver) and the 2D (lead) dendrites to the randomly orientated crystallites in particles with the spherical morphology (the 3D dendrites and the cauliflower-like and the spongy-like particles). The formation of metal powders by molten salt electrolysis and by electrolysis in deep eutectic solvents (DESs) and the crystallographic aspects of dendritic growth are also mentioned in this review. Full article

(This article belongs to the Special Issue Advances in Understanding Metal Electrolysis Processes)

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