Special Issue "Electrorefining in Sustainable Metals Production"

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

Deadline for manuscript submissions: closed (30 September 2021) | Viewed by 8511

Special Issue Editor

Aalto University, School of Chemical Engineering, Department of Chemical and Metallurgical Engineering, PO Box 16100, FI-00076 Aalto, Finland
Interests: hydrometallurgy; electrolysis processes; secondary and complex raw materials; circular economy; corrosion engineering

Special Issue Information

Dear Colleagues,

The steadily growing demand of raw materials is strengthened by urbanization, and the transition towards renewable energy systems and electrical vehicles requires increasing amounts of non-ferrous and specialty metals. Increasing demand puts pressure on metals production processes. To make metals production more sustainable, it is necessary to use resources efficiently and at the same time develop processes that can treat both complex, low-grade primary materials as well as secondary materials. These aims shall be realized without excessive environmental load.

Hydrometallurgy is often used for the production of non-ferrous, noble, and specialty metals, and in hydrometallurgical processes electrorefining is often the final step in the production. Electrorefining is efficient in the production of pure metals because only very small amounts of metallic impurities end up in the cathodes. However, electrorefining is often a large and slow operation requiring physical space, water, and electrical energy. Good maintenance of the process is essential to secure high product quality. This includes, for example, additive control, electrolyte purification, and cathode starting sheet maintenance.

The general focus of this Special Issue of Metals is on papers related to the improvement of production rate, improvement of energy usage, and methods to ascertain product quality. Papers that address the challenges caused by the increasing use of secondary raw materials are encouraged. In this Special Issue, the concept of electrorefining is not limited to traditional aqueous systems, and papers on non-aqueous systems such as ionic liquids and molten salt electrolysis are very welcome.

Dr. Jari Aromaa
Guest Editor

Manuscript Submission Information

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Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • hydrometallurgy
  • electrorefining
  • process development
  • non-ferrous metals
  • noble metals
  • metallurgical engineering
  • sustainability
  • circular economy

Published Papers (6 papers)

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Editorial

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Editorial
Electrorefining in Sustainable Metals Production
Metals 2022, 12(3), 372; https://doi.org/10.3390/met12030372 - 22 Feb 2022
Viewed by 677
Abstract
Electrorefining of metals was developed in the second half of the 19th century [...] Full article
(This article belongs to the Special Issue Electrorefining in Sustainable Metals Production)

Research

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Article
Effective Deoxidation Process of Titanium Scrap Using MgCl2 Molten Salt Electrolytic
Metals 2021, 11(12), 1981; https://doi.org/10.3390/met11121981 - 08 Dec 2021
Cited by 1 | Viewed by 1688
Abstract
To overcome the scarcity and resource limitations of Ti metal, deoxidation of Ti scrap was conducted through electrolytic refining and chemical reaction with MgCl2 molten salt electrolysis. The oxygen concentration in Ti scraps was decreased by the electrochemical and chemical reactions generated [...] Read more.
To overcome the scarcity and resource limitations of Ti metal, deoxidation of Ti scrap was conducted through electrolytic refining and chemical reaction with MgCl2 molten salt electrolysis. The oxygen concentration in Ti scraps was decreased by the electrochemical and chemical reactions generated by the applied voltages. The optimized conditions for the process were derived by controlling the conditions and parameters by decreasing the thermodynamic activity of the reactants. The correlation between the deoxidation efficiency and the behavior of the voltage and current was confirmed by setting the conditions of the electrolysis process in various voltage ranges. In addition, the correlation between the presence of impurities and the measured oxygen concentration was evaluated. The surface element analysis result indicated that the salt that was not removed contained a certain amount of oxygen. Thus, the removal efficiencies of impurities and particles by deriving various post-treatment process conditions were analyzed. The results confirmed that the most stable and efficient current was formed at a specific higher voltage. Moreover, the best deoxidation result was 2425 ppm, which was 50% lower than that of the initial Ti scrap. Full article
(This article belongs to the Special Issue Electrorefining in Sustainable Metals Production)
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Article
Modelling the Effect of Solution Composition and Temperature on the Conductivity of Zinc Electrowinning Electrolytes
Metals 2021, 11(11), 1824; https://doi.org/10.3390/met11111824 - 13 Nov 2021
Cited by 3 | Viewed by 1364
Abstract
Zinc electrowinning is an energy-intensive step of hydrometallurgical zinc production in which ohmic drop contributes the second highest overpotential in the process. As the ohmic drop is a result of electrolyte conductivity, three conductivity models (Aalto-I, Aalto-II and Aalto-III) were [...] Read more.
Zinc electrowinning is an energy-intensive step of hydrometallurgical zinc production in which ohmic drop contributes the second highest overpotential in the process. As the ohmic drop is a result of electrolyte conductivity, three conductivity models (Aalto-I, Aalto-II and Aalto-III) were formulated in this study based on the synthetic industrial electrolyte conditions of Zn (50–70 g/dm3), H2SO4 (150–200 g/dm3), Mn (0–8 g/dm3), Mg (0–4 g/dm3), and temperature, T (30–40 °C). These studies indicate that electrolyte conductivity increases with temperature and H2SO4 concentration, whereas metal ions have negative effects on conductivity. In addition, the interaction effects of temperature and the concentrations of metal ions on solution conductivity were tested by comparing the performance of the linear model (Aalto-I) and interrelated models (Aalto-II and Aalto-III) to determine their significance in the electrowinning process. Statistical analysis shows that Aalto-I has the highest accuracy of all the models developed and investigated in this study. From the industrial validation, Aalto-I also demonstrates a high level of correlation in comparison to the other models presented in this study. Further comparison of model Aalto-I with the existing published models from previous studies shows that model Aalto-I substantially improves the accuracy of the zinc conductivity empirical model. Full article
(This article belongs to the Special Issue Electrorefining in Sustainable Metals Production)
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Article
Copper Cathode Contamination by Nickel in Copper Electrorefining
Metals 2021, 11(11), 1758; https://doi.org/10.3390/met11111758 - 02 Nov 2021
Cited by 2 | Viewed by 1704
Abstract
Nickel behavior has a significant role in the electrorefining of copper, and although it has been extensively studied from the anode and electrolyte point of view over the past decades, studies on nickel contamination at the cathode are limited. In the current paper, [...] Read more.
Nickel behavior has a significant role in the electrorefining of copper, and although it has been extensively studied from the anode and electrolyte point of view over the past decades, studies on nickel contamination at the cathode are limited. In the current paper, three possible contamination mechanisms—particle entrapment, electrolyte inclusions and co-electrodeposition—were investigated. Copper electrorefining (Cu-ER) was conducted at the laboratory scale, and the cathodes were analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and flame atomic absorption spectroscopy (AAS). Particle entrapment was studied by adding NiO and Fe2O3 to the system to simulate nickel anode slime, and the experiments were replicated with industrial anode slime material. The possibility of electrolyte entrapment due to nodulation was explored through the addition of graphite to produce nodules on the cathode. Co-electrodeposition was analyzed by experiments that utilized a Hull cell. The results indicate that particle entrapment can occur at the cathode and is a major source of the nickel contamination in Cu-ER, whereas nickel compounds were not shown to promote nodulation. Inclusions of bulk electrolytes within the surface matrix were observed, proving that electrolyte entrapment is possible. As co-electrodeposition of Ni in Cu-ER is thermodynamically unlikely, these experimental results also verify that it does not occur to any significant extent. Full article
(This article belongs to the Special Issue Electrorefining in Sustainable Metals Production)
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Article
Production of Fe–Ti Alloys from Mixed Slag Containing Titanium and Fe2O3 via Direct Electrochemical Reduction in Molten Calcium Chloride
Metals 2020, 10(12), 1611; https://doi.org/10.3390/met10121611 - 30 Nov 2020
Cited by 3 | Viewed by 1300
Abstract
High-purity intermetallic β-Ti (FeTi4) and FeTi alloys were prepared via molten salt electrolysis from a titanium-containing waste slag and Fe2O3 mixture using molten CaCl2 salt as the electrolyte. The mixed slag powders were pressed into a pellet [...] Read more.
High-purity intermetallic β-Ti (FeTi4) and FeTi alloys were prepared via molten salt electrolysis from a titanium-containing waste slag and Fe2O3 mixture using molten CaCl2 salt as the electrolyte. The mixed slag powders were pressed into a pellet that served as a cathode, while a graphite rod served as an anode. The electrochemical process was conducted at 900 °C with a cell voltage of 3.1 V under an inert atmosphere. The formation process of the alloys and the influence of the Ti:Fe atomic ratio on the product were investigated. With an increased proportion of Ti, the phase of the product changed from FeTi/Fe2Ti to FeTi/FeTi4, and different structures were observed. At a Ti:Fe ratio of 1.2:1 in the raw slag, an alloy with a sponge-like morphology and a small amount of FeTi4 were obtained. During the initial stages of electrolysis, a large amount of intermediate product (CaTiO3) was formed, accompanied by an abrupt decrease in current and increase in particle size. The current then increased and Fe2Ti alloy was gradually formed. Finally, as the reaction process extended inside the pellet, the current remained stable and the product mainly contained FeTi and FeTi4 phases. The observed stages, i.e., CaTiO3(TiO2) → Fe2Ti(Ti) → FeTi(FeTi4), were consistent with the thermodynamic analysis. Full article
(This article belongs to the Special Issue Electrorefining in Sustainable Metals Production)
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Article
The Optimum Electrolyte Parameters in the Application of High Current Density Silver Electrorefining
Metals 2020, 10(12), 1596; https://doi.org/10.3390/met10121596 - 28 Nov 2020
Cited by 3 | Viewed by 1075
Abstract
Increasing silver production rate has been a challenge for the existing refining facilities. The application of high current density (HCD) as one of the possible solutions to increase the process throughput is also expected to reduce both energy consumption and process inventory. From [...] Read more.
Increasing silver production rate has been a challenge for the existing refining facilities. The application of high current density (HCD) as one of the possible solutions to increase the process throughput is also expected to reduce both energy consumption and process inventory. From the recently-developed models of silver electrorefining, this study simulated the optimum electrolyte parameters to optimize the specific energy consumption (SEC) and the silver inventory in the electrolyte for an HCD application. It was found that by using [Cu2+] in electrolyte, both objectives can be achieved. The suggested optimum composition range from this study was [Ag+] 100–150 g/dm3, [HNO3] 5 g/dm3, and [Cu2+] 50–75 g/dm3. HCD application (1000 A/m2) in these electrolyte conditions result in cell voltage of 2.7–3.2 V and SEC of 0.60–1.01 kWh/kg, with silver inventory in electrolyte of 26–39 kg silver for 100 kg per day basis. The corresponding figures for the conventional process were 1.5–2.8 V, 0.44–0.76 kWh/kg, and 15.54–194.25 kg, in respective order. These results show that, while HCD increases SEC by app. 30%, the improvement provides a significant smaller footprint as a result of a more compact of process. Thus, applying HCD in silver electrorefining offers the best solution in increasing production capacity and process efficiency. Full article
(This article belongs to the Special Issue Electrorefining in Sustainable Metals Production)
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