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A Review of Greener Approaches for Rare Earth Elements Recovery from Mineral Wastes
... 4,5 One promising avenue is repurposing previously utilized resources, which, contrary to being permanently lost, can be recovered from waste deposits such as landfills and former mining sites, as well as from discharged gadgets or infrastructure. [6][7][8] sites present a unique opportunity. 9 These sites often have effluents containing residual minerals discarded during past operations. ...
The depletion of high-grade mineral deposits and environmental concerns associated with traditional mining practices necessitate alternative strategies for sourcing critical materials. This study explores the innovative use of reactive laser...
... The importance of rare earth elements (REEs) in modern technology and industry is increasing due to their unique properties, which makes them critical and essential raw materials for the global economy (Rétif et al., 2023). These elements are crucial in high-tech applications, clean energy, medicine, military defence, and agriculture (Tuncay et al., 2024). However, REEs are not yet regulated, are rarely monitored in the environment and their impacts are poorly understood, making them contaminants of emerging concern (Gwenzi et al., 2018). ...
Phosphate fertilizer industries produce a large quantity of phosphogypsum (PG) annually, which permanent storage raises technical and environmental constraints. However, this co-product valorization in civil engineering, particularly in road construction, is a promising solution within a circular economy. To evaluate this option, our experimental study focused on mixtures’ mechanical responses modelling by the Design of Experiment (DOE) method, consisting of PG from the Jorf Lasfar industrial complex treated with a hydraulic binder (cement C/road hydraulic binder HRB) and a granular corrector (sand S/ phosphate tailings T), meeting mechanical criteria of a subgrade/subbase pavement layer material. To do so, the Proctor test was used to find the optimum water content and dry density, and it was optimized using the same DOE method. In addition, environmental assessment, based on leaching tests measured on two mixtures suitable for each use, showed that all measured heavy metals (Cr, Cd, Pb, Cu, As, Zn, Ni) concentrations were far below international standard limits, due to either encapsulation in the crystalline structure of gypsum or thanks to cement treatment, which enabled trace metals to be fixed through the C–S–H formation. In addition, the mechanical strengths of the mixtures were improved, as shown by SEM and XRD investigations. Finally, a life-cycle analysis, based on industrial software, showed that a pavement structure with a sub-base and subgrade course composed of PG mixes reduces energy and water consumption by 50%, and CO2 emissions by 30%.
The importance of rare earth elements as a basis for the development of new technologies or the improvement of existing ones makes their recovery from raw and waste materials necessary. In this recovery, hydrometallurgy and its derivative solvometallurgy play key roles due to their operational characteristics, which are emphasized with the use of ionic liquids. This manuscript reviews the most recent advances (2023 and 2024) in the use of ionic liquids in unit operations (leaching and separation technologies) aimed at the recovery of these valuable and strategic metals. Moreover, a comprehensive review is presented of the use of these chemicals in the development of advanced materials containing some of these rare earth elements.
Previously, proof-of-concept studies have demonstrated that rare-earth elements (REEs) can be preferentially extracted from coal fly ash (CFA) solids using a recyclable ionic liquid (IL), betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]). When the suspension of aqueous solution—IL-CFA—is heated above 65 °C, the majority of REEs will separate from the bulk elements in the solids and partition to the IL phase. Acid stripping of the IL removes REEs and regenerates the IL for reuse in additional extraction cycles. The objective of this study is to showcase the applicability and effectiveness of the optimized method to recover REEs from various CFAs. Six CFA samples with different characteristics (feed coal basins, coal beds, and ash collecting points) and classifications (Class C and Class F) were examined. The process performance was evaluated for a broad range of elements (33 total), including 15 REEs, two actinides, six bulk elements, and 10 trace metals. Results confirmed good recovery of total REEs (ranging from 44% to 66% among the CFA samples) and the recovery process’ high selectivity of REEs over other bulk and trace elements. Sc, Y, Nd, Sm, Gd, Dy, and Yb consistently showed high leaching and partitioning into the IL phase, with an average recovery efficiency ranging from 53.8% to 66.2%, while the other REEs showed greater variability among the different CFA samples. Some amounts of Al and Th were co-extracted into the IL phase, while Fe co-extraction was successfully limited by chloride complexation and ascorbic acid reduction. These results indicated that the IL-based REE-CFA recovery method can maintain a high REE recovery efficiency across various types of CFA, therefore providing a promising sustainable REE recovery strategy for various coal ash wastes.
Bioleaching offers a low-input method of extracting valuable metals from sulfide minerals, which works by exploiting the sulfur and iron metabolisms of microorganisms to break down the ore. Bioleaching microbes generate energy by oxidising iron and/or sulfur, consequently generating oxidants that attack sulfide mineral surfaces, releasing target metals. As sulfuric acid is generated during the process, bioleaching organisms are typically acidophiles, and indeed the technique is based on natural processes that occur at acid mine drainage sites. While the overall concept of bioleaching appears straightforward, a series of enzymes is required to mediate the complex sulfur oxidation process. This review explores the mechanisms underlying bioleaching, summarising current knowledge on the enzymes driving microbial sulfur and iron oxidation in acidophiles. Up-to-date models are provided of the two mineral-defined pathways of sulfide mineral bioleaching: the thiosulfate and the polysulfide pathway.
Bauxite residue (BR) is the main by-product of the alkaline production of alumina from bauxite containing significant amounts of valuable metals such as scandium that belongs to rare-earth elements (REEs), classified by the European Community as critical raw materials (CRMs). BR is considered a hazardous waste due to its huge volume and high alkalinity making its disposal a serious universal environmental problem. The recovery of scandium from Greek BR can be an excellent approach for waste management and resource efficiency of the waste using environmentally friendly biometallurgical methods. In this work, bioleaching of scandium from bauxite residue using the fungus Aspergillus niger was studied. Bioleaching experiments were performed using the Taguchi experimental design, in batch cultures with BR at various pulp densities (1, 5 and 10%, w/v), sucrose concentrations (40, 90 and 140 g/L) and fungus suspension of 2, 4, and 6% v/v under one-step bioleaching condition and subculturing. The highest Sc recovery equal to 46%, was achieved in 20 days at 1% pulp density. Biosorption phenomena were observed during the leaching process. Lactic, acetic, oxalic and citric were the main organic acids identified.
Graphical Abstract
Phosphogypsum (PG) waste is a by-product generated from wet-process phosphoric acid (H3PO4) manufacturing during phosphate rock decomposition. Worldwide, the annual production of PG ranges between 100 to 300 million tons, with only a few quantities utilized in several application domains (about 15%), the unused PG is usually discharged into the sea or stocked in large stockpiles with potential serious human and environmental risks. Therefore, in this review article we have studied and discussed the possible alternative ways for PG waste recycling and use. Indeed, this waste material could be considered as a mineral resource of secondary raw materials within the scope of a circular economy. An inclusive bibliographic search, dealing with our review’s objectives, was performed according to the two famous-databases: Web of Sciences and Scopus. After different selecting processes, about 153 articles are found. PG is used in several sectors, including agriculture, as well as in the brick and cement industry, and road construction. Other applications are reported in this study such as PG conversion to valuable products and rare earths elements (REEs) extraction. In the same context and in the sense of reducing greenhouse gasses emissions (GHGs), PG is often used as a calcium source for CO2 mineral sequestration. In addition, different methods of treatment and purification, techno�economic, life cycle and environmental assessment of the PG recycling, and valorization technologies are summarized and reported in this review. Finally, recent technologies used for extracting REEs from PG were investigated. The main results, conclusions, and recommendations reported here could considered as a guide for future studies, and also should be of benefit to scientists, chemists and engineers interested in the utilization/ treatment of PG.
The demand for novel, cost-effective, and environmentally friendly rare earth element and yttrium (REY) sources is essential. The recovery of REY and other valuable components from coal fly ash (CFA) may result in securing alternative resources, decreased disposal costs, and environmental protection, all of which may have positive effects. However, research on the recovery of REY from CFA is underway, and it is still necessary to assess its viability from an economic and environmental standpoint. The authors have reviewed some of the most recent advances in extracting rare earth elements from CFA. However, most techniques reported for the treatment of CFA are still at the laboratory scale. Nevertheless, there are several pathways for industrial-scale applications. Therefore, CFA treatment and the extraction of valuable products from it have considerable potential for reducing both its carbon footprint and environmental burden.
Rare earth elements (REEs) are widely used in electronic devices and renewable energy technology, but their supply is geopolitically-limited and they are extracted by environmentally unsustainable mining practices. Coal fly ash (CFA), which is mostly discarded as waste, has recently gained attention as a potential low-grade REE source, motivating the development of greener and highly specific processes for recovering and enriching REEs. Here we present a proof-of-concept for a novel REE extraction process in which supercritical fluid enhances the ability of tributyl phosphate (TBP) to selectively extract REEs directly from solid CFA matrices. For the first time, we show that supercritical nitrogen and supercritical air can work like supercritical carbon dioxide for selective extraction. Moreover, using a prototype multistage stripping process with an aqueous solution, we collected REEs with concentrations up to 21.4 mg L⁻¹ from the extractant. Our final products contain up to 6.47% REEs, whereas the coal fly ash source initially contained only 0.0234% REEs. Using supercritical fluid, our novel process can recover valuable and critical resources from materials previously considered to be waste.
The release of fine particles during mechanical landfill mining (LFM) operations is a potential environmental pollution and human health risk. Previous studies demonstrate that a significant proportion (40–80% wt) of the content of fine soil-like materials within the size range <10 mm to <4 mm recovered from such operations originate from municipal solid waste (MSW) landfills. This study evaluates the potential health risks caused by emissions from LFM activities. MSW samples recovered from the drilling of four different wells of a closed UK landfill were analysed for physical, chemical, and biological properties to determine the extent of potential contaminant emissions during LFM activities. The results show that fine particles (approximately ≤1.5 mm) accounted for more than 50% of the total mass of excavated waste and contained predominantly soil-like materials. The concentrations of Zn, Cu, Pb, Cd, As, and Cr exceed the permissible limits set by the current UK Soil Guideline Values. The highest geoaccumulation index and contamination factor values for Cu were 2.51 and 12.51, respectively, indicating a moderate to very high degree of contamination. Unsurprisingly, the pollution load index was >1, indicating the extent of pollution within the study area. The hazard quotient values indicated high exposure-related risks for Pb (16.95), Zn (3.56), Cd (1.47), and As (1.46) for allotment land use and As (1.96) for residential land use. The cancer-related risk values were higher than the acceptable range of 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴. The cancer risk factor indicated that Cr and As were the major human health risk hazards.
Acid mine drainage is a legacy environmental issue and one of the largest pollutants in many mining districts throughout the world. In prior work, the authors have developed a process for the recovery of critical materials, including the rare earth elements, from acid mine drainage using a preconcentration step followed by solvent extraction as a concentration and purification technology. As part of the downstream technology development efforts, we have synthesized a suite of ionic liquid extractants that facilitate greater separation factors leading to lower capital costs and reduced environmental impacts. This article provides a comparison of the conventional extractants D2EHPA, EHEHPA and C572 with their respective ionic liquids [c101][D2EHP,c101][EHEHP] and [c101][C572] for the recovery of rare earth elements from acid mine drainage. In the study, laboratory-scale, multi-contact solvent extraction tests were conducted at high and low extractant/dosages. The results show that the ionic liquids varied in performance, with [c101][D2EHP] and [c101][EHEHP] performing poorer than their conventional counterparts and [c101][c572] performing better. Recommendations for further study on [c101][c572] include stripping tests, continuous pilot testing, and techno-economic analysis.
Rare earth elements (REEs) are metals including the 15 lanthanides together with Yttrium and Scandium. China is the leading country in their exploitation and production (∼90%). REEs are necessary for the production of several technological devices. This extended use of REEs has raised concerns about human health safety. In this review, we investigated the hazard of REEs to human health and the main gaps into the knowledge like as the need to develop further focused research activity. We categorized the research papers collected into eight main sections: environmental exposure, association of REEs with health problems, exposure to REEs due to lifestyle, REE exposure through the food chain, Gd contrast agents causing health problems, occupational REE exposure, and cytotoxicity studies of REEs. This review provided information about the exposome of REEs (the exposure of REEs to the human body), the existing research data, and the gaps that require attention and must be further investigated. More than one third of the literature about REE toxicity to human health concerns their cytotoxicity to human cell lines, while hair, blood serum and blood are the most studied matrices. The main results evidenced that REEs can enter human body via several routes, are associated with numerous diseases, can cause ROS production, DNA damage and cell death, and are more toxic to cancer cells than normal cells.
Mining activities are notorious for their environmental impact, with acid mine drainage (AMD) being among the most significant issues. Specifically, AMD has recently been a topical issue of prime concern, primarily due to the magnitude of its environmental, ecotoxicological, and socioeconomic impacts. AMD originates from both active and abandoned mines (primarily gold and coal) and is encountered in Canada, China, Russia, South Africa, USA, and other countries with strong mining industries. Owing to its acidity, AMD contains elevated levels of dissolved (toxic) metals, metalloids, rare-earth elements, radionuclides, and sulfates. Practical and cost-effective solutions to prevent its formation are still pending, while for its treatment, active (driven by frequent input of chemicals and energy) or passive (based on oxidation/reduction), technologies are typically employed with the first being more efficient in contaminants removal, however, at the expense of process complexity, cost, and energy consumption. More recently, and under the circular economy concept, hybrid (combination of active and passive technologies) and particularly integrated (sequential or stepwise treatment) systems have been explored for AMD beneficiation and valorisation. These systems are costly to install and operate but are cleaner production systems since they can effectively prevent pollution and can be used for closed-loop and sustainable AMD management (e.g., zero liquid discharge systems). Herein, an insight into the body of knowledge on AMD treatment, beneficiation (metals/minerals recovery), valorisation (water reclamation), and life cycle assessment (LCA), is comprehensively reviewed and discussed with focus placed on circular economy. Future research directions are provided to introduce reuse, recycle, and resource recovery paradigms and inspire innovation in valorising this toxic and hazardous effluent. Overall, AMD beneficiation and valorisation appears promising since the reclaimed water and the recovered minerals/metals could offset the treatment costs and environmental impacts. However, the main challenges include high-cost, complexity, cross-contamination, and the generation of heterogeneous and highly mineralised sludge.
At present, there are significant knowledge gaps in the research on the resource and environmental effects of rare earth exploitation, especially the carbon emission coefficient. This study applies the life cycle assessment approach to calculate the carbon footprint of producing mixed oxide rare earths using ionic rare earth resources and analyze the sources and influencing factors of the carbon footprint. The results show that the carbon footprint of producing 1 kg of mixed oxide rare earths using ionic rare earths is 17.8~24.3 kg CO2 eq, but its uncertainty is 15.54%; the total carbon footprint from 2012 to 2017 reaches 1.6 × 10⁸~2.19 × 10⁸ kg CO2 eq/year, and after 2018, the carbon footprint decreases to 1.51 × 10⁸~2.07 × 10⁸ kg CO2 eq /year. The total carbon footprint of illegal mining is around 1.50 × 10⁸~1.59 × 10⁸ kg CO2 eq/ year. In principle, the higher the recovery rate, the lower the carbon footprint of 1 kg REO production, but with the increase in the recovery rate, the carbon footprint reduction benefit brought by the increase in the unit recovery rate shows a downward trend. Finally, the new generation of magnesium salt leaching technology, while alleviating ammonia nitrogen pollution in ionic rare earth mines, will increase the carbon footprint of the product.
This work tested an effective and efficient Nd/La extraction and separation via flat-sheet supported liquid membrane. A microporous polypropylene film was used as the solid support for the liquid membrane and P507 as the extractant. The effect of Nd(III) and La(III) concentration in the feed, the concentration of P507, and acid solution concentration in the stripping phase on Nd and La extraction and separation were studied. The results suggest the extraction of La and Nd increases with the initial pH of the feed solution and P507 concentration. Maximum separation factor of 44.25 was obtained at the low P507 concentration of 5% (v/v).
Graphical Abstract
The vulnerability of the rare earth element (REE) supply in a global context of increasing demands entails important economic and political issues, and has encouraged several countries to develop their own REE production projects. This study comparatively evaluated the production of REEs from primary and secondary resources in terms of their sustainability and contribution to the achievement of the Geoethics concept as responsibility towards oneself, colleagues, society, and the Earth system. Twelve categories of potential environmental and social impacts were selected: human health toxicity, global warming or climate change, terrestrial and aquatic eutrophication, acid-ification potential, particulate matter, resource depletion, water consumption, fresh water ecotoxi-city, ionizing radiation, fossil fuel consumption, and ozone depletion. The results showed that the environmental impact of REE production from secondary sources is much lower relative to primary sources. A comparison of conventional and non-conventional REE resources showed that significant impact categories were related to particulate matter formation, abiotic resource depletion, and fossil fuel depletion, which could result from avoiding the tailings disposal before reuse. Based on these findings, governments and stakeholders should be encouraged to increase the recycling of secondary REE sources with Geoethics in mind, in order to balance the high demand of REEs while minimizing the overexploitation of non-renewable resources.
Rare earth elements are a nonrenewable and important strategic resource, and China is rich in these elements. However, the substantial exploitation of these resources has caused the migration, diffusion, transformation and accumulation of pollution sources, which in turn has a profound impact on the ecological environment of mining areas. Accurate evaluations of resource and environmental carrying capacity (RECC) are important for the green development of mining areas. In this paper, the fuzzy comprehensive evaluation method based on the combination of the AHP (Analytic Hierarchy Process) and entropy methods is used to study the RECC of mine areas in terms of both support capacity and pressure. The Bayan Obo mine in Inner Mongolia, the Longnan mine in Jiangxi, the Weishan mine in Shandong, the Mianning mine in Sichuan, the Pingyuan mine in Guangdong, and the Chongzuo mine in Guangxi, which are typical representative mines, were selected for a horizontal comparison. The results show that, with the exception of the Bayan Obo mine, the support index was greater than the pressure index in terms of mining and human activities in all mining areas. The RECC index ranked order for the mining areas was Bayan Obo > Longnan > Mianning > Pingyuan > Weishan > Chongzuo. In addition, an obstacle degree model was used to identify and extract the main factors affecting the ecological quality of the mine sites. The ratio of investment in environmental pollution control to GDP was the most important factor, of all factors, which limited the improvement in the mine support index. Through the above research, we identified the main factors affecting the ecological carrying capacity of each mining area, providing a scientific basis for formulating corresponding environmental regulations and reducing the environmental pollution caused by rare earth mining.
Rare earth elements (REEs) are important raw materials for green technologies. However, REE mining and production uses techniques that are often not environmentally sustainable. Life cycle assessment (LCA) is a well-recognized method for evaluating the environmental impacts of products and technologies. This article provides an overview of the environmental impacts based on published LCA results of primary REE production. Existing major REE deposits (Bayan Obo in China, Mountain Pass in the United States, Mount Weld in Australia, ion-adsorption deposits in several Chinese southern provinces) and currently possible production routes are compared. Alternative minerals, such as eudialyte, are also discussed. The article shows which environmental effects can be minimized by technology optimization and environmental safety strategies. Additionally, some of the environmental impacts discussed, may be difficult to mitigate, as they depend on the mineral type. Activities along the complex process chain of REEs production that have particularly high environmental impacts are identified.
Graphical abstract
Rare earth element (REE) recovery from waste (end-of-line, reusable, recyclable, etc.) should become an essential stream of REE for current demands, making methods to achieve this recovery paramount. This study uses the electrodialytic method (EDR) as an REE extraction method from coal fly ashes. We used different chemicals to assist REE extraction during EDR: distilled water, 0.01M NaNO3, 0.4M Sodium acetate in 1.0M Acetic acid, and 0.5M Citric acid. Citric acid achieved the highest REE extraction/recovery from the four studied solutions: up to 40%. This represents a total recovery of 148 g REE from 1 ton of coal ashes. The citric acid experiment also proved to be energy efficient, using 70Wh per 100g of treated coal ash. The acidic environment provided by the citric acid supplies higher REE migration rates toward the cathode. Once at the cathode compartment, REEs then precipitate at the cathode complexed as Ca- and P-bearing minerals.
Development of the effective technology for recovery of critical rare-earth elements (REEs) from end-of-life permanent magnets is one of the important technological challenges. In this study, chemical and electrochemical leaching of NdCeFeB magnets was investigated in 0.5 mol/L sulfuric acid containing varying concentration of oxalic acid. The influence of H2C2O4 concentration on leaching efficiency and morphological properties of the NdCeFeB surface, as well as on the chemical composition and ζ-potential of the oxalate precipitate particles was discussed. Efficient separation of REEs can be achieved at H2C2O4 concentrations of 0.05–0.20 mol/L and the maximum REE purity was 97.2%. In electrochemical leaching, the leaching rate was substantially higher than in chemical leaching, where blocking of the magnet surface by precipitate layer was observed.
Rare earth elements (REEs) extraction via conventional technologies is exceedingly energy and environmentally intensive. New efficient and sustainable technologies for REE extraction from both primary and secondary resources would be extremely beneficial. This research demonstrated a novel low-temperature electrochemical extraction (LTEE) process for extracting REEs from lignite coal ash (LCA) solutions. The LCA contained 17 different REEs with a concentration ranging from 13 to 1645 ppm. The non-aqueous LCA solution mixed with ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (BMITF) was used for extracting REEs at different electrochemical potentials. The ICP-MS analysis confirmed an overall 26% REEs recovery from the LCA solution with 16 out of the 17 REEs successfully extracted from the LCA solution. This study shows an environmentally benign and energy-efficient REE extraction process that is suitable for coal and coal byproducts.
Rare earth elements (REE) have applications in various modern technologies, e.g., semiconductors, mobile phones, magnets. They are categorized as critical raw materials due to their strategic importance in economies and high risks associated with their supply chain. Therefore, more sustainable practices for efficient extraction and recovery of REE from secondary sources are being developed.
This book, Environmental Technologies to Treat Rare Earth Elements Pollution: Principles and Engineering: presents the fundamentals of the (bio)geochemical cycles of rare earth elements and which imbalances in these cycles result in pollution.overviews physical, chemical and biological technologies for successful treatment of water, air, soils and sediments contaminated with different rare earth elements.explores the recovery of value-added products from waste streams laden with rare earth elements, including nanoparticles and quantum dots.
This book is suited for teaching and research purposes as well as professional reference for those working on rare earth elements. In addition, the information provided in this book is helpful to scientists, researchers and practitioners in related fields, such as those working on metal/metalloid microbe interaction and sustainable green approaches for resource recovery from wastes.
ISBN: 9781789062229 (Paperback)
ISBN: 9781789062236 (eBook)
ISBN: 9781789062243 (ePUB)
Recent years have witnessed an increasing awareness and interest in rare earth elements (REE). These several, usually unfamiliar elements are key components of countless products used in our daily lives. Because of their use in many modern technologies, including those critical for national security, the demand for REEs grows, and so does their production and the need to find new sources and to improve their extraction. This article provides an overview of REEs, their availability, production, and uses, and briefly discusses the future of these valuable and critical metals.
Efficient and sustainable separation of metals is gaining increasing attention, because of the essential roles of many metals in sustainable technologies for a climate-neutral society, such as rare earths in permanent magnets and cobalt, nickel, and manganese in the cathode materials of lithium-ion batteries. The separation and purification of metals by conventional solvent extraction (SX) systems, which consist of an organic phase and an aqueous phase, has limitations. By replacing the aqueous phase with other polar solvents, either polar molecular organic solvents or ionic solvents, nonaqueous solvent extraction (NASX) largely expands the scope of SX, since differences in solvation of metal ions lead to different distribution behaviors. This Review emphasizes enhanced metal extraction and remarkable metal separations observed in NASX systems and discusses the effects of polar solvents on the extraction mechanisms according to the type of polar solvents and the type of extractants. Furthermore, the considerable effects of the addition of water and complexing agents on metal separations in terms of metal ion solvation and speciation are highlighted. Efforts to integrate NASX into metallurgical flowsheets and to develop closed-loop solvometallurgical processes are also discussed. This Review aims to construct a framework of NASX on which many more studies on this topic, both fundamental and applied, can be built.
Mining and mineral exploration has many effects on the surrounding environment. The present study reviews the hydrological and environmental impacts of coal and nonmetal mining operations by mine lifecycle stages and facility patterns. Further, a critical review of regulations and policies in South Korea focusing on the mining-water interaction, conservation, and management was performed to emphasize the current state of legislation in the country. The counties where mining was the primary employer in Gangwon-do province in South Korea were assessed for the mining impact on the community's social life and compared to the non-mining counties in the same province. The results of the comparative study showed the less education, healthcare and employment chances in mining counties than the adjacent counties with no mining activities.
Groundwater is an important source of drinking water in rural areas all over the world. Twenty-one water quality parameters of fifteen drinking wells were studied in a rural area of Ganzhou (South China). The groundwater is dominated by Ca·Mg-HCO3 type, and the Water Quality Index (WQI) ranges from 12.1 to 283.1. The concentrations of groundwater solutes are generally very low, except for Na⁺, Al, Fe, Mn, and arsenic (As) are up to 232.0, 3.22, 1.25, 0.013 mg/L, respectively, which exceeding the drinking standards at several wells. The relatively high As content in groundwater might cause a mean Carcinogenic Risk (CR = 2.08 × 10⁻⁴) to the local people. The mathematical model predicted that the rare earth mining activities would cause a NH4⁺ pollution (concentrations up to 69.9 mg/L) transported at a speed of 200 m per year in the groundwater, resulting in a maximum increase in WQI of 418.9 and 533.2, and in Hazard Quotient (HQ) of 1.04 and 1.32 for the children, at the drinking wells located 1059 m and 6344 m downstream the mining area, respectively. This study is a warning to the local policymakers about the development of green mining technology and effective monitoring and emergency plans for drinking groundwater.
The use of supported liquid membrane extraction for recovery and separation of rare-earth elements (REEs) has been investigated. Experiments have been carried out using the different configurations: (1) standard hollow fiber supported liquid membrane operation (HFSLM), (2) renewal liquid membrane operation (HFRLM), and (3) emulsion pertraction technology (EPT). The experiments were performed in pilot scale using a hollow fiber module with a mass transfer surface area of 8 m2. Synthetic feed solution was used with compositions based on a process for recovery of REE from an apatite concentrate. The total concentration of REE in the feed was varied from 1 to 22 mM REE and the pH was varied in the range 1.5–3.2. Di(2-ethylhexyl) phosphoric acid (D2HEPA) diluted in kerosene, 10% (v/v), was used as the organic membrane solution, and 3 M HCl was used as stripping solution. In supported liquid membrane extraction, the extraction performance is governed by both the kinetics of REE transport through the membrane and by thermodynamics. The effect of feed composition on the selectivity and transport of REE through the liquid membrane have been investigated. The results show that the liquid membrane is more selective toward the heavy REE at lower pH values and higher REE concentration. HFRLM shows a higher transport rate than HFSLM, while the HFSLM configuration gives a higher selectivity toward individual REE. The membrane performance in HFSLM configuration rapidly decays with time, while in the HFRLM and EPT configurations, the performance is much more stable. Possible mechanisms for decaying membrane performance are discussed, and gel formation is identified as being of significant importance. Gel formation is observed at an organic loading above ∼46% for Nd, 38% for Y, 46% for Dy, and 65% for Er. The work performed in this study serves as an initial step to demonstrate that HFRLM and EPT can provide stable operation and be feasible options for processing of REE liquors. A process flow diagram for the recovery of the REE, present in the apatite concentrate, in three fractions is proposed based on the results from this study.
There is a growing demand for advancing products and renewable technologies worldwide that rely on rare earth elements (REEs), including those directly necessary for a low-carbon energy transition, national security applications, and consumer electronics. This study focuses on current nature-based biological methods (i.e., bioleaching and biosorption) for REEs extraction from electronic wastes (e-wastes) and ore deposits. Comprehensive narrative and systematic reviews of bioleaching and biosorption extraction methods are performed to identify their sustainability challenges and benefits, and highlight the potential pathways that would address the existing gaps. From the narrative review, it is evident that biological methods for REEs extraction are more environmentally friendly than conventional methods currently used in the REE mining industry (e.g., acid leaching and solvent extraction). From the systematic review, it is clear that bioleaching and biosorption research has been a rapidly growing field of interest over the last 10 years, particularly for precious metals extraction (e.g., copper and gold). From both reviews, it is apparent that REEs extraction from domestic ore deposits alone is inadequate, and sustainable REEs recovery from e-wastes is also necessary to meet the growing REEs demand. It is concluded that targeted mixed REEs extraction for specific products can be a potential pathway for sustainable REEs extraction from both ore and e-wastes that would reduce separation costs and emissions from the associated use of harsh chemicals. It is further concluded that nature-based biological REE extraction solutions offer an opportunity to generate significant socio-economic and environmental benefits.
Rare earth elements (REEs) have been increasingly exploited for crucial new technologies, and their massive use in the past decades has significantly increased their environmental concentrations. In this article, we have tried to answer the question as to whether or not the wide use of REEs, including nanoparticles, in agriculture and medicine may pose a health risk to the population through diet. For this reason, information on their biological role and potential toxicity to living organisms has been summarised. The fate of REEs in the aquatic and terrestrial environment, such as surface water, soil, soil-plant systems and animals, was described. Particular emphasis was placed on their uptake by plants and animals, translocation between species and thus their entrance into the human food chain. For a better understanding of REEs bioavailability and toxicity, their physicochemical properties, such as e.g. solubility, oxidation state, chemical form, and coordination with ligands are discussed. Data on the estimated daily intake and the presence of REEs in the human body were also compiled. In our concluding remarks we identified gaps in knowledge about the impact of REEs on the population through diet and predicted future research needs in this area.
Acid mine drainage (AMD), formed by the instability of sulfides, typically generates acidity and releases potentially toxic elements and sulfate to the environment, among other pollutants. An example is the group of rare earth elements (REE) that may have high toxic behavior. This toxicity leads to degradation of soils, water reservoirs and rivers, promoting serious risks for the ecosystems. So, the main goal of the present work is to study the hydrochemical properties of a system with mine-influenced waters during the rainy season, focusing on the origin, evolution/behavior, and concentration of REE. The study area is the São Domingos mining complex, located in one of the largest metallogenetic provinces in the world (Iberian Pyrite Belt), known by the evidences of AMD contamination. The obtained results reveal extraordinarily low pH (0.4), high electrical conductivity, reaching 26,200 μS/cm, and high values of sulfate and acidity. Regarding the REE, the determined concentration exceeded that observed in normal pH of neutral freshwaters by 2–3 times the order of magnitude. The results revealed that Y and Ce are distinguished in practically all sampled sites, due to its higher concentrations, with maximum values of 221.8 and 166.9 μg/L. In general, the concentrations increase as the water pH decreases. The statistical analysis indicates that REE elements may have a common origin, mutual dependence, and similar behavior during transport with typical AMD elements and composition of host rocks. Most samples show enrichment in middle REE (MREE) (Gdn/Lun), like the classic signature of AMD. In turn, colloids and AMD-precipitates may be participating in the incorporation of these elements. Therefore, due to potential risk of impacts on ecosystems, REE are a topic of relevant interest for future studies in order to assist monitoring processes and help government decisions related to water quality management.
This paper presents results of the study on the applicability of deep eutectic solvents (DES) for the recovery of rare earth elements (REE) from coal flyash (CFA) assaying REE 0.22%. The combination of DES used was choline chloride (ChCl) with lactic acid (LA) and ChCl with para toluene sulphonic acid monohydrate (pTSA). Besides the synthesis and characterisation of the DES systems, detailed study on their flow behaviour as a function of temperature and viscosity was also made. Screening studies indicated better leaching performance when the molar ratio of ChCl:pTSA and ChCl:LA were 1:1 and 1:2 respectively. Influence of various process parameters like temperature, solid to liquid ratio, reaction time and dilution ratio of DES with water on the leaching efficiency of REE and other impurity ions was investigated using the best molar ratio combination identified for the two systems. Performance of the DES systems were compared with that of individual hydrogen bond donors (HBD) namely, LA and pTSA, hydrogen bond acceptor (HBA) viz. ChCl as well as with H2SO4. The dissolved REE from the DES medium were selectively and quantitatively recovered by chemical precipitation. The DES systems gave REE leachability of about 85–95%. The mixture of ChCl:pTSA(1:1) showed higher leach recovery and faster kinetics compared to ChCl:LA(1:2). Both DES systems gave a decisive 5–8% higher leachability than the individual HBDs (LA and pTSA) and HBA (ChCl) which constitute the respective eutectic. Comparison of leaching performance of REE using DES with that of H2SO4 indicated significant enhancement, about 35%, with the former over the inorganic acid. A probable leaching mechanism of REE with the DES used is also proposed. Precipitation of REE from the DES medium was near complete with oxalic acid and NaF reagents.
Solvent extraction is one of the most useful methods for the separation and concentration of metal ions after leaching and solid–liquid separation. The overall process involves the selective extraction (or loading) of a metal ion from an aqueous phase into an organic phase containing an organic extractant and subsequent recovery, or stripping of the extracted metal back into an aqueous phase. The chemistry of the various classes of reagents that can be used to extract metal ions from the aqueous phase into a suitable organic phase and subsequent recovery from the organic phase has been described. The equilibria that govern the extent of extraction and stripping are described quantitatively in terms of distribution isotherms and coefficients.
The various methods of contacting the phases in multistage units are introduced and the mathematics quantitatively developed to illustrate the advantages of each method. Graphical methods can be used in common cases in which the distribution coefficient is not a constant with the extent of loading or stripping.
The equipment used to accomplish the separations is described with particular emphasis on the mixer-settler as the most common choice. The quantitative description of both mixer and settler characteristics has been outlined and important operating parameters emphasized. Column contactors are also described. A number of industrial operating solvent extraction circuits for a number of metals have been introduced with the emphasis on the configuration of the units and justification of operating procedures to deal with various impurity metal ions.
Increasing consumption of toxic and non-renewable materials forces researchers to replace them with less dangerous, bio-based, and renewable materials. Green chemistry principles are one of the worldwide strategies for reducing the hazardous materials consumption in chemical applications. Although membrane technology has been considered a convenient method for environmental issues such as wastewater treatment, pharmacy, medicine, food and etc., it was addressed as an important technology that should be revised in its fabrication and applications. Polymeric membrane fabrication is the considered section that should satisfy the principles of green chemistry since a wide range of toxic solvents has been generally used for polymer dissolution. Many investigations have been implemented to replace them with less toxic or bio-based solvents mentioned as green solvents. This review is prepared to cover the recent works implemented for membrane fabrication using green solvents as a major solvent for polymer dissolution. The most eminent classification of green solvents can be assigned as water, bio-sourced solvents, ionic liquids (ILs), deep eutectic solvents (DESs), green synthetic organic solvents, and supercritical fluids (SCF). In addition, in this review, the Hansen solubility parameters (HSP) of some DESs are calculated and their possibility as green solvents for polymeric membrane fabrication is indicated.
The traditional mining processes of rare earth elements (REEs) are accompanied by the production of a large number of acid mine drainage rich in REEs. A wide-adaptive, low-cost and environmentally friendly biosorbent is an attractive technology to enrich and recycle REEs from the liquid wastes. To construct a broad-spectrum and efficient biosorbent, a novel REEs-binding protein Lanmodulin (LanM) is successfully displayed on the cell surface of a fungus, Yarrowia lipolytica, for the first time, and the adsorption capacities for various REEs are studied. The LanM-displayed Y. lipolytica shows significantly enhanced adsorption capacities for multiple REEs, achieving the highest reported values of 49.83±2.87 mg Yb /g DCW, 50.38±1.46 mg Tm /g DCW, 49.94±3.61 mg Er /g DCW and 48.72±3.09 mg Tb/g DCW, respectively. Moreover, the LanM-displayed Y. lipolytica possesses a high selectivity for REEs over other common metal cations and excellent suitability under acidic conditions. The kinetics and equilibrium analysis of biosorption processes agree well with the pseudo-first kinetic and Langmuir isotherm model. Based on the FTIR and SEM-EDS analysis, the chelation with phosphate/carboxylate groups dominates the Yb binding in LanM-displayed cells, and LanM enhances the adsorption performances by introducing more binding sites with high selectivity towards a wide range of REEs. Thus, the LanM-displayed Y. lipolytica investigated in this study exhibits prosperous potential for the enriching/removal of REEs from acid mine drainage.
Industrial production of sulfuric acid requires a high amount of energy and in remote regions such as mineral extraction pits, in situ production becomes unfeasible. This work proposes an alternative for the production of H2SO4 that uses sulfur-oxidizing microorganisms. The production capacity of this acid in a mesophilic (30 °C) and a thermophilic (65 °C) condition was studied by three collected consortia from acid mine drainage and compared with Acidithiobacillus thiooxidans. It was found that At. thiooxidans presented higher sulfuric acid production than the consortia collected and, therefore, it was selected for the bioleaching of rare earths elements (REEs) from phosphogypsum (PG). Due to the acid consumption of phosphogypsum, the two-step bioleaching condition resulted in higher REEs extraction (98% Nd, 60% Ce, 58% La, and 62% Y) when compared to one-step bioleaching (28% Nd, 17% Ce, 18% La, and 30% Y). The process was performed on a reactor scale and it was possible to extract 55.0% of the REEs contained in 300 g of waste and concentrate them into 0.922 g of rare earth oxalates, with a final yield of 52.5%, showing that the proposed bioprocess has potential application even in remote areas due to its low energy consumption when compared to traditional processes.
The excessive production of bauxite residue (red mud) in the Bayer process is one of the major challenges amongst alumina producers. The Pedersen process is known as a combination of smelting reduction of bauxite and leaching treatment of the produced slag for alumina production, and the process also produces an inert bauxite residue (grey mud), which is more suitable for further processing compared to the red mud. This paper describes the life cycle assessment for two alumina production processes, the current commercialized Bayer process and the alternative Pedersen process, based on the complete mass and energy balance simulation. Process simulation and environmental software are applied to quantify the metallurgical processes and their related environmental effects. By assuming the same treatments for the bauxite residues, it is found that, with the production of pig iron, the Pedersen process shows the best performance in mineral resource scarcity but requires a higher energy demand and generates twice the amount of bauxite residue compared to the Bayer process. The final, relative performance in climate change impact depends on the carbon intensity of energy and the bauxite ore composition.
Rare earth metals (REM) have applications in multitudinous fields such as the medicinal industry, nuclear technology, and agricultural implementations. Recently, this has caused considerable interest within the community due to its growing demand which leads scientists to explore effective, economical, and safer ways of extraction from primary and secondary sources. This review is of pivotal importance as it summarizes the requirements of REM worldwide, leaching of REM from ores, recycling of REM from scrap materials, end-life-products, and industrial wastes. This represents an important topic to explore as it demonstrates existing REM production and effective recovery pathways, and highlights the critical challenges such as the harsh harmful leaching technologies used to recover REM from primary and secondary sources. A comprehensive classification of REM, hydrometallurgical extraction, bioleaching, separation techniques, and sustainable approaches to supporting a circular economy has been extensively discussed. This factual and comprehensive overview of past, present and future aspects of REM recovery technologies will be helpful to researchers in developing efficient leaching pathways for REM recovery.
In recent years, the global demand for scandium has increased due to its essential role in the high-technology industry. Red mud leachates constitute a significant source of scandium and an efficient and environmentally friendly approach that attracted considerable interest. In this study, H3PO4 activated biochar (P40s) is manufactured from pitaya peel (PP) as a novel material to be used for scandium adsorption and recovery from red mud leachates. The material was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption isotherms, X-ray photoelectron spectroscopy (XRF), zeta potential, and element analysis (such as C, H, O, N and P) to characterize adsorbents. The results indicated that the phosphorus-containing functional groups were successfully introduced to improve the physicochemical properties of biochar. The pseudo-second-order kinetic model and Freundlich isotherm fit experimental data, verifying that the scandium adsorption by activated biochar is dominated by chemisorption and belongs to multilayer sorption. The maximal adsorption capacity of 750°C calcinated H3PO4 activated biochar (P40-750) was 20.77 mg/g. Notably, the adsorbent was better for selective recovery of Sc³⁺ from Al³⁺, Fe³⁺, Dy³⁺, Y³⁺ coexistence ions. The separation coefficients (SF) of scandium for aluminum, iron, dysprosium, and yttrium are 137.03, 27.04, 77.74, and 208.25 times respectively, at pH3. Importantly, scandium is efficiently extracted from the leaching solution of red mud with an 83% recovery rate. Meanwhile, P40-750 exhibited a stable scandium adsorption capacity after 5 adsorption/desorption cycles. Overall, P40-750 could be considered a low-cost and environment-friendly biosorbent for recovering scandium from red mud leaching solution.
Electro-sorption has been used for water purification and desalination, but little research on the recovery of rare earth element (REE) in aqueous solution. In this work, sodium diphenylamine sulfonate (SDAS) modified the activated carbon (AC) on carbon felt (CF) is used to recovery the La(III), Nd(III) and Ce(III) ions by electro-sorption. Moreover, this electrode can be refreshed under the reversal voltage without any other chemical reagent. After 500 min of reaction, the adsorption capacity of La(III) ions reaches nearly 600 mg g⁻¹, which remains stable in three cycles. Due to the synergistic effect of chemical and physical absorption, the electro-sorption of La(III) ions on SDAS/AC-CF electrode follows Langmuir isotherm, the pseudo-first-order and pseudo-second-order models. Moreover, the adsorption capacities of Nd(III) and Ce(III) ions after nearly 180 min of reaction are 275 mg g⁻¹ and higher than 366 mg g⁻¹, respectively. Herein, white flocculent La(OH)3, purple flake-type purple Nd(OH)3 and brownish yellow spherical CeO2 are obtained after electro-sorption. This method is simple without the pollution of other chemical reagents. Therefore, it has revealed potential application value in the recovery of REE from wastewater.
Coal combustion generates almost 40% of world's electricity. However, it also produces 1.1 billion tons of coal combustion residues (CCR) annually, half of which end up in landfills. Although current regulations require proper lining and monitoring programs, the ubiquitous old, abandoned landfills are often not lined nor included in these programs. In addition, the total number of coal ash disposal sites and their status in the world is unknown. Therefore, this article reviews the environmental damage caused by CCR and three commonly used risk assessment methodologies: leaching assessment, groundwater assessment, and toxicity testing.
Leaching methods are usually the first step in coal ash risk assessment, however, a large number of methods with different parameters make a comparison of data difficult. Groundwater pollution is commonly detected near coal ash disposal sites, but other anthropogenic activities may also exist nearby. Therefore, multivariate statistical methods and isotope traces should be used to differentiate between different sources of pollution. So far, both stable (δ¹⁸O, δD, δ¹¹B, δ³⁴S, δ⁷Li) and radiogenic (⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁷Pb) isotopes have been successfully used as coal ash pollution tracers. Coal ash also negatively affects biota, reduces the diversity of organisms, affects children's health, and increases the risk for developing various diseases. Toxicity studies are great for early screening of coal ash safety; however, they provide no insights into mechanisms causing the adverse effects.
Future directions are also proposed, such as the development of new ‘low-level’ detection methods for coal ash pollution and sustainable and selective method for recovery of critical elements.
Pre-concentration and separation processes of Rare Earth Elements (REEs) were investigated in terms of several factors. Nitric acid leachate of e-waste was first pre-treated by increasing pH and filtering through microfiltration. For pre-concentration, pre-treated leachate was concentrated by nanofiltration. While a 70% permeate recovery ratio was kept constant, the rare earth elements concentrations were triplicated under optimum conditions. A flat sheet supported liquid membrane process with a polyvinylidene fluoride (PVDF) support membrane was successfully used to extract REEs from the pre-treated leachate. Of the two extractants evaluated, bis-2-ethylhexyl phosphoric acid (D2EHPA) displayed a higher REE separation efficiency than did di-2,4,4,-trimethylpentyl phosphinic acid (Cyanex 272). However, Cyanex 272 separated Sc more selectively. For direct membrane solvent extraction (MSX) and MSX with pre-concentration, pre-treatment pH and D2EHPA concentrations were optimized at values of 1.5 and 15%, respectively. When comparing the results of MSX for direct and pre-concentrated configurations, it was seen that REEs and HREEs recoveries were increased 10% and 30% in MSX with pre-concentration at the end of single-stage MSX. Pre-concentration not only increased the MSX process efficiency but also enabled acid recovery from nanofiltration permeate. A more environmentally friendly and economical process scheme was proposed, including acid recovery from both NF filtrate and post-MSX leaching residue by two different membrane distillation configurations.
Rare earth elements (REEs) are vital for the technology, military, and defense industries. They have been recognized as critical due to potential scarcity, supply constraints, and lack of minable concentrations. Therefore, alternative sources are needed to meet the demand and continue manufacturing rare earth-dependent products. However, the environmental prospect of rare earth mining was not investigated enough, and comprehensive studies are lacking. It demands serious consideration as toxic radionuclides are seen in the same mineralization as rare earths regardless of their primary or secondary sources. The concentration of these hazardous trace elements may be elevated as a result of extraction and beneficiation processes. Unless proper separation and disposal are performed, these radionuclides accumulate on the surface of the soil or integrate with aquatic systems, which consequently raise environmental and health concerns. This review manuscript compiled the environmental impact and aspect of rare earth extraction processes while addressing separation techniques for these radioactive materials from rare earths, emphasizing selective precipitation, solvent extraction, and solid-phase separation.
Supercritical carbon dioxide (sc-CO2) is an emerging green solvent for recovery of metals from secondary resources. The sc-CO2 is a non-polar solvent; thus, a suitable chelating agent is required to accommodate the polarity difference between the solute and solvent during the extraction of metal ions with organometallic compounds. This study investigates four different organophosphorus ligands including triethyl phosphate (TEP), tri-n-butyl phosphate (TBP), tributyl phosphine oxide (TBPO), and trioctyl phosphine oxide (TOPO), and their effect on the extraction of neodymiumNeodymiumfrom NdFeB magnetNdFeB magnet in sc-CO2. A COSMO-vac model is developed to estimate solubility of the four agents in sc-CO2. The results show that the order of solubility is TEP > TBPO ~ TBP > TOPO. The coordination environment of neodymium was determined by UV–Vis spectroscopy, resulting a TEP:Nd = 1, TBP:Nd = 3, TBPO:Nd = 3, and TOPO:Nd = 4 coordination chemistry. The highest extraction for neodymium is achieved with TEP, because of low coordination number which results in less hydrophobic interactions between aliphatic functionalities. Also, the smaller micellar assemblies have a higher solubility in sc-CO2; thus, a higher extraction efficiency is achieved.
Recycling of wasteSupercritical fluid extraction electrical and electronic equipment (WEEE) has been receiving significant attention around the world. Here, we develop an environmentally sustainable process that uses supercritical carbon dioxide as the solvent along with a small volume of tributyl-phosphate (TBP) nitric acid adduct as the chelating agent to recover rare earth elementsRare earth elements (REEs) from fluorescent lamp waste. We show that mechanical activation using oscillation millingOscillation milling increases extraction efficiency. We elucidate the process mechanism by characterizing the solids before and after the process using transmission electron microscopyTransmission Electron Microscopy (TEM) and X-ray photoelectron spectroscopyX-ray Photoelectron Spectroscopy (XPS). We show that Al3+ and Ca2+ cations from the Al2O3 and Ca5(PO4)3OH (hydroxyapatite) present in the fluorescent lamp waste result in competing reactions with REEs with TBP-HNO3 adduct; thus, REERare earth elements extractions from real fluorescent lamp waste are less than what has been reported from synthetic feeds. Management of fluorescent lamp waste leads to sustainability of biosphere and circular economy.
Domestic Rare Earth Element sources and production are limited in the United States and currently rely on final processing overseas. Increasing demand and resource security domestically has led to significant investigation into rare earth element domestic resources. Much of this work focuses on unconventional potential ore stocks, including coal and coal byproducts. This investigation focuses on coal byproducts generated as ash from coal burning power stations. Wyoming's Powder River Basin hosts the largest U.S. coal stocks for energy production, providing approximately 40% of all thermal coal mined in the U.S. In this effort, in section I, we have studied coal byproducts for rare earth element concentrations and compare these data to current alternative resource knowledge. We find that coal byproducts in this investigation are consistently high enough in rare earth element concentration (above the current Department of Energy 300 ppm cutoff grade) to warrant consideration as a promising potential resource. Rare earth element behavior within the host coal seams is also considered in an effort to better understand resource prospecting and ore body description. In Section II, we evaluate the economic feasibility of rare earth extraction from Powder River Basin coal byproducts using net present value analysis and the rare earth concentrations data from Section I. We calculate the break-even ash-to-oxide output and input unit costs for four coal stations in the Powder River Basin. All four stations have break-even unit costs that are higher than the mine-to-oxide operating cost reported for a traditional rare earth element mine. This is a promising result, especially given that it is more costly to refine rare earths from mined material than from ash. The results are highly sensitive to rare earth prices: given low long-term prices, none of the four stations can feasibly break even. Section III summarizes federal policy considerations in rare earth element resource development. The history of policy development, most recent focus on rare earth element specific funding legislation, paired with section I and II herein, suggest a robust opportunity for development of Wyoming based coal byproducts as a partial solution to current domestic rare earth element short falls and strategic needs.
Limited natural resources and a continuous increase in the demand for modern technological products, is creating a demand and supply gap for rare earth elements (REEs) and Sc. There is therefore a need to adopt the sustainable approach of the circular economy system (CE). In this review, we defined six steps required to close the loop and recover REEs, using a holistic approach. Recent statistics on REEs and Sc demand and the number of waste generations are reported and studies on more environmentally friendly, economic, and/or efficient recovery processes are summarized. Pilot-scale recovery facilities are described for several types of secondary sources. Finally, we identify obstacles to closing the REE loop in a circular economy and the reasons why secondary sources are not preferred over primary sources. Briefly, recovery from secondary sources should be environmentally and economically friendly and of an acceptable standard concerning final product quality. However, current technologies for recovery from for secondary sources are limiting and technology needs will vary depending on the source type. The quality/purity of the recovered metals should be proven so that they do not result in any adverse effects on the product quality, when they are being used as secondary raw material. In addition, for industrial-scale facilities, process improvements are required that consider environmental conditions.
The study of the kinetics of REEs from secondary resources and the understanding of the mechanisms governing the leaching reactions are fundamental aspects for the design and the optimization of industrial processes. Despite their interest, the kinetic aspect of these valuable elements from secondary resources has not been reviewed yet. Therefore, a deep understanding of the major phenomena involved in the elementary stages of REEs leaching is extremely crucial and would allow setting up a comprehensive methodology for kinetics investigation.
This review provides a state of the art on various kinetics approaches describing the leaching kinetics and mechanisms of REEs from secondary resources notably low-grade minerals, industrial residues as well as end-of-life products. Different existing kinetics models applied for the leaching were reviewed and deeply discussed. Meanwhile, comparative study of different leaching mechanisms was also realized for a better understanding of the REEs leaching reactions. More importantly, the effect of different leaching parameters on the kinetics was discussed for further optimization and development of REEs extraction processes from secondary resources. In addition, the comparison of different REEs secondary resources was provided in terms of content, availability and economic opportunities to target the most potentially and economically viable resources.