综述：从二次资源和潜在绿色浸出剂角度回收稀土元素的全面评述

《Separation and Purification Technology》：Recovery of rare earth elements: A comprehensive review from the perspective of secondary sources and potentially greener leachants

本综述系统探讨了从采矿尾矿等二次资源中回收稀土元素（REE）的绿色工艺路线，重点评述了预处理（浮选、重力分离、磁选）、浸出（有机酸、低共熔溶剂DES替代无机酸）及后处理（可生物降解沉淀剂）等环节的创新方案，为降低传统湿法冶金环境足迹提供了（深共晶溶剂）等可持续解决方案。

　　

Abstract

Rare earth elements (REEs) are indispensable for advanced technologies, particularly in renewable energy systems and electronics. Conventional extraction from primary sources like bastnaesite, monazite, and xenotime predominantly relies on hydrometallurgical methods, valued for their low energy consumption, operational simplicity, and cost-effectiveness. However, the increasing demand for REEs, coupled with the environmental and supply risks associated with primary mining, has shifted focus towards secondary sources. This review comprehensively examines the recovery of REEs from mining tailings, emphasizing the adoption of greener alternatives to conventional, highly corrosive inorganic acid leachants. The analysis covers effective pretreatment techniques to concentrate REE-bearing minerals, evaluates the performance of environmentally benign leaching agents such as organic acids and deep eutectic solvents (DES), and discusses post-leaching separation methods favoring biodegradable precipitants. The integration of these greener materials and processes aims to enhance the sustainability of REE recovery, minimizing environmental impact while addressing resource scarcity.

Introduction

Rare earth elements (REEs), comprising 17 elements including the 15 lanthanides plus scandium (Sc) and yttrium (Y), are critical components in modern technology. They are categorized into light rare earth elements (LREEs, atomic numbers 57-63) and heavy rare earth elements (HREEs, atomic numbers 64-71), with Sc and Y sometimes considered separately. LREEs are generally more abundant than HREEs. These elements are characterized as soft, silvery metals with high melting points and high reactivity, particularly with oxygen. Cerium (Ce) is the most abundant REE, followed by lanthanum (La) and neodymium (Nd), whereas elements like terbium (Tb) and thulium (Tm) are scarcer, reflecting in their market prices—Ce is relatively inexpensive (~4.7 USD/kg) compared to Sc (~3457 USD/kg). REEs do not occur naturally in pure form but are found in mineral deposits such as bastnaesite (a carbonate-fluoride), monazite, and xenotime (both phosphates), which serve as primary commercial sources.

The demand for REEs has surged over the past two decades due to their essential roles in electronics, alloys, glass manufacturing, catalysts, and crucially, in green technologies like wind turbines, rechargeable batteries, and energy-efficient lighting. The global rare earth market is projected to grow significantly, from USD 5.3 billion in 2021 to USD 9.6 billion by 2026, at a compound annual growth rate of 12.3%. China dominates both reserves (approx. 33%) and production (approx. 60%), prompting other nations to seek supply security through recycling and alternative sources.

Primary mining is unsustainable in the long term, highlighting the importance of a circular economy approach. Secondary sources, such as electronic waste (e-waste) and mining waste (tailings, acid mine drainage, red mud, coal fly ash, phosphogypsum), offer substantial potential for REE recovery. While e-waste often has higher REE concentrations, the vast volumes of mining tailings make them an attractive, independent supply source. Hydrometallurgy is the preferred recovery method due to its lower energy requirements, reduced gas emissions (CO₂, CO), and cost-effectiveness compared to pyrometallurgical or electrochemical processes. However, conventional hydrometallurgy employs corrosive inorganic acids (HNO₃, H₂SO₄, HCl), posing environmental and handling challenges. This review addresses the gap in exploring greener leaching alternatives, specifically deep eutectic solvents (DES), for REE recovery from mining tailings, integrating pretreatment and separation stages from a sustainability perspective.

Rare earth elements

REEs are defined by their position in the periodic table and unique properties. The lanthanide series exhibits a phenomenon known as the lanthanide contraction, where atomic radii decrease from Lanthanum (La) to Lutetium (Lu) due to poor shielding by 4f electrons. This contraction influences their chemical behavior and separation efficiency. REEs predominantly exist as trivalent cations in oxides, silicates, phosphates, and carbonates. Their similar ionic radii and chemical properties make separation challenging, necessitating sophisticated techniques. The strategic importance of REEs spans economic and national security sectors, underpinning their status as critical materials.

Pretreatment methods in the extraction of REEs

Mining tailings are complex mixtures containing valuable REE-bearing minerals like monazite and xenotime alongside gangue minerals such as ilmenite (FeTiO₃), zircon (ZrSiO₄), cassiterite (SnO₂), hematite (Fe₂O₃), quartz (SiO₂), and muscovite. Pretreatment is crucial to concentrate the target minerals and remove impurities, thereby improving the efficiency of subsequent leaching.

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  Froth Flotation: This technique separates minerals based on differences in surface hydrophobicity. Surfactants are used to make desired minerals hydrophobic, allowing them to attach to air bubbles and be collected as froth. It is effective for fine particles but requires careful control of reagent dosage and pH.
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  Gravity Separation: This method exploits differences in density between REE minerals (e.g., monazite, high density) and lighter gangue minerals (e.g., quartz). Equipment like shaking tables or spirals are used. It is cost-effective and environmentally friendly but may have limited efficiency for very fine particles or minerals with similar densities.
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  Magnetic Separation: This process separates minerals based on their magnetic susceptibility. High-intensity magnetic separators can effectively isolate paramagnetic minerals like monazite from non-magnetic gangue. It is highly selective but is inherently limited to magnetic or paramagnetic components.

The choice of pretreatment depends on the specific mineralogy of the tailings. Often, a combination of these methods is employed to achieve optimal pre-concentration of REEs.

Leaching agents

The leaching stage dissolves REEs from the solid matrix into a solution. The performance of different leaching agents is compared based on recovery efficiency, corrosiveness, environmental impact, and cost.

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  Inorganic Acids: Conventional leachants like sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and nitric acid (HNO₃) are highly effective, achieving high recovery rates under optimized conditions (e.g., temperature, acid concentration, solid-to-liquid ratio, time). However, they are highly corrosive, generate toxic fumes, and produce acidic waste streams that require neutralization, posing significant environmental and safety concerns.
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  Organic Acids: Alternatives such as citric acid, acetic acid, and methanesulfonic acid offer greener profiles. They are less corrosive, biodegradable, and generate fewer toxic by-products. While their leaching efficiency can be slightly lower than inorganic acids, they present a more sustainable option, especially when process parameters are optimized.
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  Deep Eutectic Solvents (DES): DES are emerging as highly promising green leachants. They are typically formed by mixing a hydrogen bond acceptor (HBA), like choline chloride, with a hydrogen bond donor (HBD), such as urea, carboxylic acids, or alcohols. This mixture results in a solvent with a melting point lower than that of its individual components. DES are celebrated for their low volatility, biodegradability, low toxicity, and high selectivity for target metals. Mechanisms for REE extraction in DES can involve complexation or an ion-association mechanism. Their application to REE recovery from mining tailings represents a cutting-edge research area with great potential to reduce the environmental footprint of hydrometallurgical processes.
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  Ionic Liquids: While also considered green solvents, ionic liquids are distinct from DES. They are salts in the liquid state and can be expensive for large-scale applications. Their mechanism often involves complexation with REE ions.

Extracting rare earth elements from leaching solution

After leaching, REEs must be separated and purified from the pregnant leach solution (PLS). Common methods include:

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  Precipitation: This is a straightforward method where a precipitating agent (e.g., oxalic acid) is added to the PLS to form insoluble REE compounds (e.g., oxalates). It is cost-effective, requires simpler equipment, and has a shorter process duration compared to other methods. The use of biodegradable precipitants aligns with green chemistry principles.
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  Solvent Extraction (SX): This technique uses organic solvents to selectively transfer REE ions from the aqueous leach solution to an organic phase, followed by stripping into a new aqueous phase. SX is highly effective for producing high-purity REEs but involves complex multi-stage operations, expensive and often toxic organic solvents, and generates significant waste.
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  Ion Exchange: This method employs solid resin beads that selectively adsorb REE ions from the solution. It is excellent for achieving very high purity but can be slow, expensive, and more suitable for low-concentration solutions.

For greener process integration, precipitation with biodegradable agents is often favored over SX and ion exchange due to its simplicity, lower cost, and reduced environmental impact.

Conclusions

Mining tailings represent a viable and abundant secondary source for REE recovery, aligning with circular economy goals. Successful hydrometallurgical extraction requires effective pretreatment to remove gangue minerals; magnetic separation offers high selectivity for paramagnetic REE minerals, while froth flotation and gravity separation provide complementary benefits. In the leaching stage, inorganic acids, though effective, are being superseded by greener alternatives. Organic acids present a less corrosive option, but deep eutectic solvents (DES) stand out due to their exceptional properties: biodegradability, low toxicity, and high extraction efficiency, positioning them as a key area for future development. For the final separation step, precipitation using biodegradable agents is recommended over solvent extraction and ion exchange, as it offers a more sustainable profile with simpler operation and lower costs. The integration of these greener pretreatment, leaching, and separation techniques paves the way for a more sustainable and environmentally responsible pathway for recovering critical rare earth elements from secondary resources, mitigating the environmental impact of traditional mining and processing.

CRediT authorship contribution statement

Law Yong Ng: Writing – review & editing, Supervision, Funding acquisition, Formal analysis, Conceptualization. Ee Hui Choo: Writing – original draft, Data curation. Amelia Kar Mun Chiang: Writing – review & editing. Ching Yin Ng: Writing – review & editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Law Yong Ng reports financial support was provided by Malaysia Smelting Corporation Berhad. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by Malaysia Smelting Corporation Berhad (MSCB) [UDCC/602-E005663/2024]; Universiti Tunku Abdul Rahman [IPSR/RMC/UTARRF/2024-C1/N01]; and UCSI University [REIG-FETBE-2024/019]. We extend our sincere gratitude to Mr. Wong Kin Nyap, Chief Technical Officer at MSCB, for his invaluable assistance throughout this project.