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Efficient Extraction, Separation and Purification of Critical Metal Resources

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A special issue of Separations (ISSN 2297-8739).

Deadline for manuscript submissions: closed (10 March 2026) | Viewed by 8659

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Prof. Dr. Meng Li

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State Key Laboratory of Critical Metals Beneficiation, Metallurgy and Purification, Zhengzhou University, Zhengzhou 450001, China
Interests: comprehensive utilization of critical metallic mineral resources; highly efficient extraction and separation of critical metals and carbon sequestration via mineralization; recycling and resource utilization of solid wastes; design and fabrication of vanadium-based electrode materials
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Dr. Ying Zhang

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Institute of Process Engineering Chinese Academy of Sciences, Beijing, China
Interests: critical metal regeneration from secondary resources; Ti metallurgy
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Dr. Zepeng Lv

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Guest Editor

Zhongyuan Critical Metals Lab, Zhengzhou University, Zhengzhou 450001, China
Interests: short-process clean extraction of critical metals; electrochemical metallurgy and energy conversion technology; efficient separation mechanism for similar critical metals; electrochemical reaction interface and medium design; functional modification of micro/nano materials
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Dr. Fanfan Zhang

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Guest Editor Assistant

Zhongyuan Critical Metals Lab, Zhengzhou University, Zhengzhou 450001, China
Interests: micro- and ultra-fine particles flotation and process intensification; nanobubble and bridging capillary force; critical metallic (minerals) separation and extraction
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Dr. Yun Liu

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Guest Editor Assistant

Zhongyuan Critical Metals Lab, Zhengzhou University, Zhengzhou 450001, China
Interests: critical metal recovery; biosorption; design of metal-binding peptides; biological leaching; biological synthesis of nanoparticles; biological sulfate reduction

Special Issue Information

Dear Colleagues,

Critical metals—including rare earth elements, dispersed (or scattered) metals, and precious rare metals—are indispensable to strategic emerging industries such as new energy and electronics. However, these resources are subject to considerable supply risks and therefore demand heightened attention and protection. Advancing technologies in mineral processing, green extraction, efficient separation, and high-purity preparation of critical metals are vital to improving resource utilization efficiency. These innovations are key to meeting the stringent requirements of high-end applications, including next-generation energy systems and national defense, while ensuring long-term sustainability. In the context of global carbon reduction goals, the development of green, efficient, and environmentally friendly processing technologies has become a research priority in this field.

We cordially invite you to submit your original research articles, communications, or reviews to this Special Issue, which focuses on the efficient extraction, separation, and purification of critical metal resources. Your contributions will play an important role in advancing the science and technology that underpin sustainable and resilient supply chains of critical metals.

Prof. Dr. Meng Li
Dr. Ying Zhang
Dr. Zepeng Lv
Guest Editors

Dr. Fanfan Zhang
Dr. Yun Liu
Guest Editor Assistants

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Separations is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 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

critical metals
extraction
separation
purification
resource recycling
carbon reduction

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Published Papers (8 papers)

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Research

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20 pages, 4403 KB

Open AccessArticle

Effects of Metal Ions on the Flotation of Fluorite and Barite: An Experimental and Mechanistic Investigation

by Ying Wei, Yuqiong Li, Yingchao Liu, Yuxin Guo, Caiyun Li and Wanglin Yang

Separations 2026, 13(3), 85; https://doi.org/10.3390/separations13030085 - 3 Mar 2026

Cited by 1 | Viewed by 427

Abstract

Fluorite (CaF₂) and barite (BaSO₄) commonly occur together in the same deposits. Due to their similar surface chemical properties, their flotation separation is often challenging. In flotation pulps, dissolved metal ions can further interfere with separation and exert a pronounced influence on the flotation behavior of these minerals. This study investigated the effects of metal ions frequently encountered in industrial pulps (Fe³⁺, Al³⁺, Mg²⁺, Ca²⁺, and Zn²⁺) on the floatability of fluorite and barite in a sodium oleate (NaOL) collector system. The aims were to clarify how metal ions affect flotation behavior and to evaluate the feasibility of enhancing fluorite–barite separation via metal-ion regulation. Flotation results showed that, in the NaOL system, the largest floatability difference between fluorite and barite occurred at pH 10. Al³⁺ exhibited the strongest depression on barite while only weakly affecting fluorite flotation. Fe³⁺ and Mg²⁺ caused slight depression of barite, whereas Ca²⁺ and high concentrations of Zn²⁺ (>20 mg/L) promoted barite flotation. Overall, these metal ions had little influence on fluorite flotation. Adsorption measurements indicated that Al³⁺ reduced NaOL adsorption by more than 40% and decreased the contact angle from 35.6° to 23.1°, resulting in a sharp loss of surface hydrophobicity. ICP adsorption tests revealed that Al³⁺ showed the highest uptake on barite surfaces. Density functional theory (DFT) calculations further confirmed that surface SO₄²⁻ groups on barite form strong chemisorption with hydrolyzed Al species (adsorption energy: −436.19 kJ/mol), whereas only weak physisorption occurs on hydroxylated fluorite surfaces (adsorption energy: −43.73 kJ/mol). This study provides insights into the flotation separation of non-metallic minerals dominated by polar ionic bonding and offers practical guidance for efficient fluorite–barite separation under complex ionic environments. Full article

(This article belongs to the Special Issue Efficient Extraction, Separation and Purification of Critical Metal Resources)

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16 pages, 2652 KB

Open AccessArticle

Separation and Recovery of Fe and REEs from a Hydrochloric Acid Leachate of NdFeB Waste Using Aliquat 336-Based Solvent Extraction

by Lushuai Yao, Zishuai Liu, Zhihui Zhao, Qianwen Li, Enhao Li and Huiyang Lin

Separations 2026, 13(2), 70; https://doi.org/10.3390/separations13020070 - 16 Feb 2026

Viewed by 608

Abstract

Neodymium–iron–boron (NdFeB) waste represents a valuable secondary source of rare earth elements (REEs). However, existing recovery technologies face several challenges, such as the difficulty of selectively recovering REEs, the generation of large volumes of secondary iron-rich slag, and an overall low level of comprehensive resource utilization. In this study, Aliquat 336 was applied for the selective extraction and separation of REEs and iron (Fe) from hydrochloric acid leachate derived from NdFeB waste. Experimental results showed that under optimized conditions—specifically, a 15% Aliquat 336 concentration, an organic-to-aqueous phase ratio of 1:2, and a 2 min extraction time—Fe extraction efficiency reached 99.93% after three-stage countercurrent extraction, while REEs were predominantly retained in the aqueous phase. Subsequent oxalic acid precipitation of the raffinate yielded RE₂(C₂O₄)₃·10H₂O with a purity of 99.60%. Moreover, under stripping conditions of 2 mol/L NaOH, a phase ratio of 2:1 (aqueous to organic), and a 2 min contact time, over 99.21% of Fe was stripped after three-stage countercurrent stripping, resulting in Fe(OH)₃ with a purity of 99.26%. The extraction mechanism followed an anion-exchange process: under high chloride ion concentrations, Fe³⁺ formed anionic FeCl₄⁻ complexes, which were exchanged with Cl⁻ ions in Aliquat 336 and transferred into the organic phase, whereas RE³⁺ cations remained in the aqueous phase, enabling efficient separation of Fe and REEs. These findings provide important insights for improving the comprehensive utilization of NdFeB waste and promoting the green and sustainable development of secondary rare earth resource recycling. Full article

(This article belongs to the Special Issue Efficient Extraction, Separation and Purification of Critical Metal Resources)

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15 pages, 4247 KB

Open AccessArticle

Mechanism of Selective Extraction and Separation of Vanadium and Aluminum from Oxalic Acid Leachate of Shale: Experimental Investigation and DFT Calculations

by Zhihui Zhao, Zishuai Liu, Hui He, Qianwen Li, Heng Luo, Wenbin Liu and Yancheng Lv

Separations 2026, 13(2), 45; https://doi.org/10.3390/separations13020045 - 26 Jan 2026

Cited by 1 | Viewed by 556

Abstract

Oxalic acid serves as an environmentally benign leaching agent, exhibiting strong reducing and complexing capabilities. In the oxalic acid leachate derived from vanadium-bearing shale, aluminum ions are present as major impurities. Achieving efficient and deep separation of vanadium from aluminum remains a key technical challenge. This study investigates the selective separation of vanadium and aluminum from oxalic acid leaching solutions using solvent extraction with Aliquat 336, supported by density functional theory (DFT) calculations. Experimental results demonstrate that, under optimized conditions, Aliquat 336 enables effective separation of vanadium from aluminum. DFT analysis elucidates the molecular-level interaction mechanism, revealing that the binding affinity of Aliquat 336 for [VO(C₂O₄)₂]²⁻ (ΔG = −287.96 kJ/mol) is significantly stronger than for [Al(C₂O₄)₂]⁻ (ΔG = −186.68 kJ/mol). These results provide a solid thermodynamic basis for the observed selectivity and establish a robust theoretical framework for developing high-efficiency separation processes. This work thus clarifies, for the first time, the mechanistic foundation of vanadium–aluminum separation in oxalic acid systems. Full article

(This article belongs to the Special Issue Efficient Extraction, Separation and Purification of Critical Metal Resources)

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20 pages, 4803 KB

Open AccessArticle

Process Intensification and Multivariable Interaction Analysis for Platinum Extraction from Spent Catalysts Using Ultrasound-Assisted Leaching

by Xiaoping Zhu, Xiaolong Sai, Chuan Liu, Yige Yang, Bei Ren, Ke Shi, Shaobo Wen and Hao Pan

Separations 2026, 13(1), 22; https://doi.org/10.3390/separations13010022 - 6 Jan 2026

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Abstract

The efficient recovery of platinum (Pt) from spent catalysts is of critical strategic importance to alleviate resource scarcity and supply chain dependencies. This study developed an ultrasonic-assisted leaching process using a H₂SO₄-NaCl system for Pt extraction from spent petroleum catalysts. Single-factor experiments were first conducted to identify the preliminary effects of key parameters. Subsequently, Response Surface Methodology (RSM) based on a Box–Behnken design was employed to model and optimize the interactive effects of ultrasonic power, sulfuric acid concentration, leaching time and NaCl concentration. The results demonstrated that ultrasonic power had the most significant influence on Pt leaching efficiency. The optimized conditions were determined as ultrasonic power of 300 W, H₂SO₄ concentration of 60%, leaching time of 100 min, and NaCl concentration of 0.10 mol/L. Under these optimal parameters, the Pt leaching rate reached approximately 99.8%, validating the model’s high accuracy and reliability. This work provides an efficient and stable technical pathway for the sustainable recycling of platinum from secondary resources. Full article

(This article belongs to the Special Issue Efficient Extraction, Separation and Purification of Critical Metal Resources)

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15 pages, 1927 KB

Open AccessArticle

Enhanced Elution of Residual Ammonium from Weathered Crust Elution-Deposited Rare Earth Ore Tailings by Ferric Chloride

by Xiaoyan Wu, Jian Feng, Xianping Luo, Fang Zhou and Ruan Chi

Separations 2025, 12(11), 301; https://doi.org/10.3390/separations12110301 - 1 Nov 2025

Viewed by 514

Abstract

The existence of residual ammonium in weathered crust elution-deposited rare earth ore (WREO) tailings will cause serious environmental pollution, and it is necessary to remove it from the ore body. In this work, ferric chloride was applied as the eluent, and the effects of the ferric salt concentration, liquid/solid ratio, and the eluting temperature on the ammonium removal process were investigated. The results indicated that ferric chloride demonstrated a significant capability to eliminate residual ammonium (RA) from rare earth (RE) tailings. The optimal conditions identified for this process included a ferric salt concentration of 0.06 mol/L, a liquid/solid ratio of 2:1, and a temperature of 25 °C. Under optimal conditions, the removal efficiency of RA by ferric chloride was measured at 97.47%. The NH₄⁺ concentration in the final stage leachate was determined to be 1.85 mg/L, which satisfies the environmental standards. Kinetic analysis revealed an internal diffusion-controlled elution mechanism for RA in the RE ore tailings, with a reaction order of 0.28 and an activation energy of 13.36 kJ/mol. FT-IR characterization results showed that most of the RA salts were effectively removed. This study establishes a feasible approach to remove RA from RE ore tailings, thereby laying a theoretical foundation for this process. Full article

(This article belongs to the Special Issue Efficient Extraction, Separation and Purification of Critical Metal Resources)

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13 pages, 3106 KB

Open AccessArticle

Recovery of Rare Earth Elements from Waste Phosphate-Based Phosphors Containing Glass by Pre-Enrichment—Alkali Roasting Method

by Yufang Qin, Shaochun Hou, Chenghong Liu, Burenbayaer Borjigin, Xuejie Zhang, Chunlei Guo and Bo Zhang

Separations 2025, 12(11), 293; https://doi.org/10.3390/separations12110293 - 26 Oct 2025

Cited by 1 | Viewed by 1529

Abstract

Based on the physicochemical properties of waste phosphate-based rare earth phosphors containing glass, this paper proposes a novel recovery method for rare earth elements (REEs) that integrates pre-enrichment, alkali roasting, and enhanced leaching. Initially, preliminary enrichment of REEs was achieved through sieving to remove silicon (from glass components) and pickling to reduce calcium content (originating from calcium phosphate compounds). The enriched material was then subjected to alkaline roasting, followed by washing for impurity removal, hydrochloric acid leaching, and finally oxalic acid precipitation to extract the rare earth elements. Experimental results demonstrate that the overall recovery rate of rare earth oxides (REO) reached 96.6%, indicating highly efficient extraction and separation of REEs from the waste phosphors. Furthermore, the mechanism of the alkali roasting process was investigated via differential thermal analysis (TG-DSC). Microstructural and phase changes in the waste phosphors before and after roasting were systematically characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that green phosphor (REPO₄) was converted into rare earth oxides and water-soluble sodium phosphate under alkaline roasting conditions. The Na₃PO₄ could be effectively removed through washing, while the rare earth elements were retained in the form of oxides within the washed residue. This study provides an important theoretical foundation and technical approach for the efficient recovery of rare earth resources from waste phosphate-based phosphors. Full article

(This article belongs to the Special Issue Efficient Extraction, Separation and Purification of Critical Metal Resources)

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15 pages, 2024 KB

Open AccessArticle

Ionic Speciation and Coordination Mechanisms of Vanadium, Iron, and Aluminum in the Oxalic Acid Leachate of Shale

by Qing Xiong, Zishuai Liu, Qianwen Li, Huiyang Lin, Xuekun Tang and Xianping Luo

Separations 2025, 12(9), 235; https://doi.org/10.3390/separations12090235 - 1 Sep 2025

Cited by 4 | Viewed by 1704

Abstract

The oxalic acid leachate of vanadium-bearing shale (OALS) is a complex system in which the ion states and coordination mechanisms of the primary metallic elements—vanadium, iron, and aluminum—are not fully understood. This study investigated the ionic speciation and coordination mechanisms of vanadium, iron, and aluminum in OALS. The results indicate that vanadium predominantly existed as VO(C₂O₄)₂²⁻ anions, iron as Fe(C₂O₄)₂⁻ and Fe(C₂O₄)₃³⁻ anions, and aluminum as Al(C₂O₄)₂⁻ and Al(C₂O₄)₃³⁻ anions. The coordination reaction processes and equations of various oxalate complexes were examined. Regardless of whether the molar ratio was 1:1 or 1:2, the iron–oxalate complex exhibited the lowest reaction Gibbs free energy (ΔG), with values of −5343.69 and −1470.72 kJ/mol, respectively. The aluminum–oxalate complex followed, with ΔG values of −5169.23 and −1318.87 kJ/mol, respectively. The vanadium–oxalate complex displayed the highest reaction ΔG, at −2760.65 and −714.12 kJ/mol, respectively. Therefore, the coordination mechanism of vanadium, iron, and aluminum with oxalate ions in OALS is such that iron coordinated with oxalate first, followed by aluminum, and finally vanadium. The research results have important guiding significance for the purification, enrichment, and coordination mechanisms of complex solutions. Full article

(This article belongs to the Special Issue Efficient Extraction, Separation and Purification of Critical Metal Resources)

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Review

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16 pages, 2173 KB

Open AccessReview

A Review of the Application of Oxalic Acid in Hydrometallurgical Processes

by Muling Sheng, Zishuai Liu, Zhihui Zhao, Qianwen Li, Wenbin Liu, Heng Luo and Yancheng Lv

Separations 2026, 13(2), 66; https://doi.org/10.3390/separations13020066 - 12 Feb 2026

Cited by 1 | Viewed by 1629

Abstract

Conventional hydrometallurgical processes typically employ inorganic acids as leaching agents; however, these processes are frequently associated with significant environmental pollution and suffer from poor metal selectivity. Oxalic acid, as a green alternative leaching agent, demonstrates considerable application potential owing to its mild acidity, strong reducing capability, and superior complexing properties. This paper presents a systematic review of recent advances in the application of oxalic acid in hydrometallurgy, encompassing the coordination chemistry between oxalic acid and metal ions, its role as a selective leaching agent, and strategies for handling multicomponent oxalate-rich solutions. Furthermore, the industrial prospects of oxalic acid-based leaching technologies are discussed. Research indicates that oxalic acid exhibits high selectivity and efficient leaching performance for critical metals—including vanadium, lithium, cobalt, nickel, and gallium—from both primary ores and solid secondary resources. The underlying leaching mechanism primarily involves the formation of stable chelation complexes between oxalate anions and high charge-density metal ions, or valence state modulation via reduction, enabling selective dissolution and separation of target metals. In multicomponent oxalate systems, where metals predominantly exist as anionic complexes, established enrichment and purification approaches include anion exchange extraction, as well as precipitation techniques based on valence adjustment and double salt crystallization. To advance the industrial implementation of oxalic acid leaching technologies, further in-depth investigation is required into the recycling mechanisms of oxalic acid and the fundamental reaction pathways governing leaching and metal recovery processes. Full article

(This article belongs to the Special Issue Efficient Extraction, Separation and Purification of Critical Metal Resources)

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