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16 pages, 9070 KiB

Open AccessFeature PaperArticle

Characterization of Stainless Steel Spent Pickling Sludge and Prospects for Its Valorization

by Fernando Castro, Pedro B. Tavares, Nuno Cristelo, Tiago Teixeira, Joana F. Garcia and Nuno M. G. Parreira

Metals 2022, 12(9), 1539; https://doi.org/10.3390/met12091539 - 17 Sep 2022

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Abstract

Fluorspar is considered a critical raw material for the European Union, due to its industrial uses and lack of sufficient extraction in European countries. It is a source for hydrofluoric acid manufacture, this latter chemical being employed, among other uses, in the pickling [...] Read more.

Fluorspar is considered a critical raw material for the European Union, due to its industrial uses and lack of sufficient extraction in European countries. It is a source for hydrofluoric acid manufacture, this latter chemical being employed, among other uses, in the pickling of stainless steels. From this latter activity, sludge is generated due to the need for used water treatment. In this article, we report a full characterization of this residue, obtained in an industrial plant in Vieira de Leiria, Portugal. Its chemical and mineralogical characteristics were determined, showing that it is mostly a mixture of calcium fluoride and calcium sulfate with some heavy metals content. Thermal behavior allowed us to determine that the material melts at around 950 °C. The influence of calcining operation on the residue was determined, especially concerned with the leachability of some elements. Taking into account the results of the characterization of this residue, some considerations are presented about the potential for the valorization of this industrial residue. Full article

(This article belongs to the Special Issue Recovery of Critical Metals and Materials from Residues)

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Review

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25 pages, 5363 KiB

Open AccessReview

Green and Sustainable Rare Earth Element Recycling and Reuse from End-of-Life Permanent Magnets

by Zara Cherkezova-Zheleva, Marian Burada, Anca Elena Sobetkii (Slobozeanu), Daniela Paneva, Sabina Andreea Fironda and Radu-Robert Piticescu

Metals 2024, 14(6), 658; https://doi.org/10.3390/met14060658 (registering DOI) - 1 Jun 2024

Abstract

Rare earth elements (REEs) are key materials for the development of renewable energy devices such as high-power magnets for wind turbines, electric vehicles, or fuel cells for hydrogen generation, aiming to fulfill the objectives of the European Green Deal for a carbon-neutral economy. [...] Read more.

Rare earth elements (REEs) are key materials for the development of renewable energy devices such as high-power magnets for wind turbines, electric vehicles, or fuel cells for hydrogen generation, aiming to fulfill the objectives of the European Green Deal for a carbon-neutral economy. The increased demand for REEs and their criticality strongly require the improvement of their extraction technologies from primary resources and the enhancement of their circularity reuse rate from secondary resources. The aim of this paper is to focus attention on the possibilities offered by emerging methods such as microwave (MW) treatment and mechanochemistry in waste electric and electronic equipment (WEEE) processing and the reuse of end-of-life (EoL) magnets, directed toward the tailoring of rational REE material flows. The discussed investigation examples explore some key features of conventional and new methods for efficient, environmentally friendly, and scalable REE extraction and reuse, with the final goal of producing recycled NdFeB powders, with potential use in the redesign and fabrication of new REE-based magnets. Full article

(This article belongs to the Special Issue Recovery of Critical Metals and Materials from Residues)

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25 pages, 5093 KiB

Open AccessReview

Efficient Metal Extraction from Dilute Solutions: A Review of Novel Selective Separation Methods and Their Applications

by Soroush Rahmati, Roshanak Adavodi, Mohammad Raouf Hosseini and Francesco Veglio’

Metals 2024, 14(6), 605; https://doi.org/10.3390/met14060605 - 21 May 2024

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Abstract

Notable increases in metal consumption and declining ore grades in recent decades have stressed the significance of dilute solutions as secondary sources of valuable metals. Moreover, environmental considerations and the imperative of sustainable development have further emphasized their treatment. Therefore, finding an efficient [...] Read more.

Notable increases in metal consumption and declining ore grades in recent decades have stressed the significance of dilute solutions as secondary sources of valuable metals. Moreover, environmental considerations and the imperative of sustainable development have further emphasized their treatment. Therefore, finding an efficient solution for separating metals from dilute solutions has attracted the attention of numerous researchers. This paper reviews the purification processes of dilute solutions and highlights key achievements of published research works. Although this study focuses on evaluating the efficiency of recently developed aqueous-phase purification methods, such as immobilized ligands, ionic liquids, and air-assisted solvent extraction, the application of conventional processes to treat these solutions, such as solvent extraction, ion exchange, membranes, chemical precipitation, and adsorption are also briefly outlined. To provide a comprehensive assessment, more than 200 research articles were reviewed, and their key findings are stated in this study. This research contributes to the advancement of knowledge of metal recovery from dilute solutions and sheds light on the dynamic evolution of this field. Full article

(This article belongs to the Special Issue Recovery of Critical Metals and Materials from Residues)

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