Occurrence Of Rare Earth Elements Research Articles

Distribution, and mobility of rare earth elements in surface sediment of Gomishan Wetland

Rare earth elements (REEs) are emerging environmental contaminants that pose a threat to ecosystem health. Their accumulation in sediments and potential release into the water column can raise concerns. Understanding the factors influencing REE mobility is crucial. Furthermore, our knowledge of the risk and occurrence of REEs in wetland ecosystems is inadequate. This study investigated REE release from sediments under varying redox conditions (Eh). The different redox potential values were adjusted using a photoanode (Ti/TiO2) and a cathode (graphite). Also, to recognize the relationship between REEs and other parameters, Pearson correlation (PC) was employed. Results showed that increasing Eh initially enhanced REE mobility (until around 300 mV), followed by a decrease at higher Eh values. Among the studied REEs, lanthanum (La), cerium (Ce), and gadolinium (Gd) exhibited the highest release potential into water, with measured concentrations reaching 199.4, 93.1, and 31.2 µg/L, respectively. Moreover, according to several eco-geochemical risk assessment indices, the sediment of the study area was classified as minimal contamination. Statistical analyses revealed that Eh indirectly affects REE mobility by influencing linked changes in pH, salinity, and iron-manganese (Fe-Mn) chemistry, which in turn impacts REE solubility and release from sediments.

Report on Book Release: “Rare Earth Element Occurrences and Deposits of India and Strategies for New Discoveries” by P. Krishnamurthy, Geological Society of India, Bengaluru

Report on Book Release: “Rare Earth Element Occurrences and Deposits of India and Strategies for New Discoveries” by P. Krishnamurthy, Geological Society of India, Bengaluru

A Review of the Occurrence and Recovery of Rare Earth Elements from Electronic Waste.

Electronic waste (e-waste) contains valuable rare earth elements (REEs) essential for various high-tech applications, making their recovery crucial for sustainable resource management. This review provides an overview of the occurrence of REEs in e-waste and discusses both conventional and emerging green technologies for their recovery. Conventional methods include physical separation, hydrometallurgy, and pyrometallurgy, while innovative approaches such as bioleaching, supercritical fluid extraction, ionic liquid extraction, and lanmodulin-derived peptides offer improved environmental sustainability and efficiency. The article presents case studies on the extraction of REEs from waste permanent magnets and fluorescent powders, highlighting the specific processes involved. Future research should focus on developing eco-friendly leaching agents, separation materials, and process optimization to enhance the overall sustainability and efficiency of REE recovery from e-waste, addressing both resource recovery and environmental concerns effectively.

Occurrence of rare earth elements in umbilical cord serum and association with thyroid hormones and birth outcomes in newborns

Rare earth elements (REEs) are emerging contaminants that are increasingly used in high technology products. However, limited information is available regarding exposure to REEs and associated health effects in neonates. This study aimed to investigate the association between REE concentrations and thyroid hormone levels, as well as birth outcomes in 109 newborns in Beijing, China. We measured the concentrations of 16 REEs and thyroid hormones in umbilical cord serum. To assess the impact of exposure to individual REEs and REE mixtures on thyroid hormone levels and birth outcomes, we employed univariate linear regression, least absolute shrinkage and selection operator (LASSO), and weighted quantile sum (WQS) models. We detected 14 REEs at high rates (92.6%–100%), with yttrium exhibiting the highest median (interquartile range) concentration [43.94 (0.33–172.55) ng/mL], followed by scandium [3.64 (0.46–11.15) ng/mL]. Univariate analyses showed that per logarithmic (ln)-unit change of neodymium (Nd) and samarium (Sm) was associated with 0.039 [95% confidence interval (CI): 0.001, 0.007] and 0.031 (95% CI: 0.003, 0.060) increases in free thyroxine (FT4) levels, respectively. Moreover, 14 REEs exhibited significant associations with triiodothyronine (T3) levels, resulting in increases ranging from 0.066 to 0.307. Elevated concentrations of terbium (Tb) [per ln-unit change: −0.021 (95% CI: −0.041, −0.01)] and lutetium (Lu) [-0.023 (95% CI: −0.043, −0.002)] were inversely correlated with birth length in newborns. A further multiple exposure analysis employing the LASSO model identified Sm, Nd, Y, Sc, and Lu as critical factors influencing FT4 and T3 levels. Additionally, WQS analyses showed positive associations between exposure to a mixture of 14 REEs and FT4 (P = 0.046), T3 (P < 0.001), and birth length (P = 0.049). These findings suggest that in utero exposure to REEs might disrupt thyroid hormone homeostasis and impact intrauterine growth. Further studies are warranted to validate these findings and elucidate the underlying mechanisms.

Rare-earth element (REE) remobilization and fractionation in bauxite zones from sedimentary kaolin deposits, western Georgia (USA), Upper Coastal Plain

Rare-earth elements (REE) are critical minerals that are indispensable for high-tech products and technologies. Significant efforts have been focused on the exploration and identification of new and domestic reserves of REE. This study reports geochemical and mineralogical occurrences of accumulated REE with significant fractions of ion-adsorption clays (IAC) within bauxite-kaolin ore, hosted within sedimentary strata. This deposit type can be found throughout portions of Georgia and eastern Alabama (USA) in the Southeast Upper Coastal Plain. These deposits were developed by high degrees of chemical weathering of older, kaolinite-rich sedimentary strata. Several major changes in whole rock mineralogy and geochemistry occurred, leading to the development of zones of REE depletion, fractionation, and accumulation. Depletions of REE in the bauxite zone were significant, with whole rock contents of ∼50% ΣREE relative to average upper continental crust (UCC). Fractionated REE were observed by low La/YbN values (La/YbN = 0.2) due to depletion in LREE (<40% UCC) with concomitant HREE retention (∼1.1–1.3 times UCC). Strongly positive Ce/Ce* anomalies (Ce/Ce* = 1.3), very low ion-exchangeable Ce contents (Ce/LREE = 0.1), and low solubility of Ce (0.9–1.2%) under reducing conditions compared to all other LREE (4.5–9.9%) were further observed. These characteristics were strong indications of REE fractionation that occurred during formation of the bauxite zone. Underlying the bauxite zone, accumulations of the remobilized REE were substantial. At the base of the lower kaolin zone, elevated ion-exchangeable REE contents were notable (11–33% for total REE), and more specifically 11–12% for LREE, ∼28% for ΣTb-Lu, and ∼ 33% for Y. The elevated ion-exchangeable contents coincided with whole rock contents that were ∼ 250% ΣREE relative to UCC. These REE accumulations occurred within organic-rich strata at the base of the section. This setting represents a significant difference relative to the REE accumulations in profiles for the granite-regolith deposit type. These findings highlighted the movement of dissolved REE, accumulation as sorbed species, as well as the potential role that organic ligands play during REE precipitation and sorption processes. These results permit consideration for further research into REE occurrences in clay deposits of sedimentary origin, which have been underexplored compared to granite-regolith REE deposits.

Organic Ligand-Mediated Dissolution and Fractionation of Rare-Earth Elements (REEs) from Carbonate and Phosphate Minerals.

Global efforts to build a net-zero economy and the irreplaceable roles of rare-earth elements (REEs) in low-carbon technologies urge the understanding of REE occurrence in natural deposits, discovery of alternative REE resources, and development of green extraction technologies. Advancement in these directions requires comprehensive knowledge on geochemical behaviors of REEs in the presence of naturally prevalent organic ligands, yet much remains unknown about organic ligand-mediated REE mobilization/fractionation and related mechanisms. Herein, we investigated REE mobilization from representative host minerals induced by three representative organic ligands: oxalate, citrate, and the siderophore desferrioxamine B (DFOB). Reaction pH conditions were selected to isolate the ligand-complexation effect versus proton dissolution. The presence of these organic ligands displayed varied impacts, with REE dissolution remarkably enhanced by citrate, mildly promoted by DFOB, and showing divergent effects in the presence of oxalate, depending on the mineral type and reaction pH. Thermodynamic modeling indicates the dominant presence of REE-ligand complexes under studied conditions and suggests ligand-promoted REE dissolution to be the dominant mechanism, consistent with experimental data. In addition, REE dissolution mediated by these ligands exhibited a distinct fractionation toward heavy REE (HREE) enrichment in the solution phase, which can be mainly attributed to the formation of thermodynamically predicted more stable HREE-ligand complexes. The combined thermodynamic modeling and experimental approach provides a framework for the systematic investigation of REE mobilization, distribution, and fractionation in the presence of organic ligands in natural systems and for the design of green extraction technologies.

Comparative analysis of rare earth elements concentrations in domestic dogs and Apennine wolves of Central Italy: Influence of biological, nutritional, and lifestyle factors

Rare Earth Elements (REEs) are strategical elements playing a crucial role in the industry, especially in producing high-tech materials. Therefore, REEs are new contaminants of emerging concerns. However, due to the lack of exposure data on REE occurrence in environmental matrices, especially in European countries, it is still tricky to establish environmental background levels to assess the ecotoxicological risk related to REEs exposure. The present study aimed to evaluate the liver concentrations of REEs in domestic dogs (Canis lupus familiaris) and Apennine wolves (Canis lupus italicus) living in the Abruzzo region, Italy. Moreover, for the scope of the present study, the dog's group was divided according to their sex, age, lifestyle, and diet. Wolves were categorized concerning their sex and genetic characteristics. Liver samples from dogs and wolves were collected during diagnostic necropsies from carcasses, sample mineralization was performed by a microwave digestion system with a single reaction chamber, and simultaneous determination of the presence of REEs was performed by Inductively Coupled Plasma Mass Spectrometer (Q-ICP-MS) using standard mode for all rare earth elements except scandium (Sc) which was acquired in kinetic energy discrimination (KED) mode. Hepatic concentrations of REEs were statistically significantly higher in wolves compared to dogs. Moreover, significant differences in REEs concentrations arose also from the genetic type of wolf, since “pure wolves” had higher liver concentrations of REEs compared to wolf-dog hybrids. Female and adult dogs also showed elevated REEs compared to male and juvenile dogs, while no significant differences were demonstrated for dogs' diet and lifestyle. The results of the present study confirm the exposure of domestic and wild carnivores to REEs, showing also the ability of REEs to accumulate in carnivore livers, suggesting the potential role of this species as an alternative bioindicator.

Distribution and potential contamination assessment of rare earth elements (REE) in Indonesian volcanic soil

Rare earth elements (REE) are essential elements used in various products. Meanwhile, in several countries, REE is feared will pollute the ecosystem. The occurrences of REE in soil depend on the natural influence of the parent material. However, anthropogenic activities could also be responsible for REE enrichment in soil. To understand the effect of volcanic soil parent material (as a natural influence) and anthropogenic activities, we analyzed the enrichment and geoaccumulation factor of REE. The ecological risk index is used to obtain the potential REE contamination into ecosystem. The samples were taken from three Andisols soil profiles (Pangalengan, Lembang, and Wayangwindu) from the same quaternary volcanic rocks in West Java, Indonesia. REE content in Pangalengan soil varied from 0.25mgkg−1 (Tm) to 73.50mgkg−1 (Ce), Lembang soil ranged from 0.57mgkg−1 (Tm) to 77.30mgkg−1 (Ce), and Wayangwindu ranged from 0.17mgkg−1 (Tm) to 77.30mgkg−1 (Ce). The enrichment of REE in all Andisols soils from three different sites (Pangalengan, Lembang, Wayangwindu) was classified as a deficiency to minimal enrichment. The geoaccumulation of REE is classified as moderately contaminated that occurs only in A horizon in Pangalengan soil. The REE enrichment seems closely related to anthropogenic factors, especially in A horizon at Pangalengan. Parent materials weathering is the most dominant factor in REE enrichment in the subsurface horizon for all sites but in the surface horizon in Pangalengan related to agricultural activities. The ecological risk index of REE was smaller than 40 (low risk), except for Lu in Pangalengan soil at moderate risk.

Geochemistry of the Leatherwood coal in eastern Kentucky with an emphasis on enrichment and modes of occurrence of rare earth elements

The Middle Pennsylvanian (Bolsovian) Leatherwood coal in the central part of eastern Kentucky is a moderately thick, moderate-S seam that has been one of the major energy resources in the region. Much of the seam profile consists of clarain, dull clarain, and durain with some brighter lithologies at the base of the seam and in the upper portion of the coal at one site. A previous study emphasized petrographic trends. In this investigation, the geochemistry of the coal is examined. Some elements, such as Ge and Ga, are enriched at 2× and 5× the World hard coal averages in the coal lithotypes adjacent to rock (roof, floor, partings). Arsenic and other chalcophile elements (Zn, Co, Ni) tend to be concentrated at the top and bottom of the seam profiles and below partings, generally in the coals with the highest pyritic sulfur content. Manganese enrichments occur in bright clarain lithologies at one site. No coal lithology reaches a 5× the World hard coal average, but samples between the 2× and 5× World hard coal averages tend to be at the top and bottom of the coal seam. Most coal samples tend to have a light rare earth element (LREE) enrichment. The basal lithotype at all sites, though, has a heavy REE enrichment and, via inference from the Zr and TiO2 concentrations, a smaller influx of detrital minerals than the basal lithotypes in many Appalachian coals. While the nature of REE associations is complicated by the high volatile A bituminous coal rank, the relatively high concentrations of the heavy REE suggest an original, albeit possibly not exclusive, organic association for those elements.

Rare earth elements interaction with iron-organic matter colloids as a control of the REE environmental dissemination

Rare earth elements (REE) are highly sought after for advanced technology, in response concerns about their environmental impact have arisen. The mobility and transport of REEs are influenced by their binding to solid surfaces, particularly colloids. With the widespread occurrence of REEs and their potential increase due to climate change, there is growing interest in understanding colloids composed of organic matter (OM) and iron (Fe). The reactivity of these colloids depends on their structural organization and the availability of Fe phase and OM binding sites. The effect of pH on the binding and mobility of REEs in these colloids in response to structural modification of Fe-OM colloids was investigated. REEs are primarily bind to the OM component of Fe-OM colloids, and their mobility is controlled by the response of OM colloids and molecules to pH conditions. At pH 6, the solubilization of small organic colloids (<3 kDa) control the REE pattern and subsequent speciation and mobility. In contrast, at pH 4, Fe-OM colloids bind less amount of REE but aggregate to form a large network. While most REEs remain soluble, those bound to Fe-OM colloids are expected to be immobilized through settlement or trapping in soil and sediment pores.This study supports the idea that colloids control the REE speciation and subsequent dissemination. The findings are particularly relevant for assessing the fate and ecotoxicology of REE in response to changing environmental conditions and increasing REE concentration in natural systems.

Occurrence of rare earth elements in water, sediment, and freshwater fish of diverse trophic levels and feeding ecology: Insights from the Po river (northwest Italy)

To date, the occurrence of rare earth elements (REEs) in freshwater ecosystems has garnered limited attention in the scientific literature. Furthermore, a dearth of data exists regarding their potential bioaccumulation in freshwater fish. To fill this knowledge gap, we studied REEs concentrations in water, sediment, and fish specimens collected along the Po River (northwest Italy) and calculated biota-sediment accumulation (BSAF) and bioconcentration (BCF) factors, while taking into account fish feeding behavior and trophic level effects on the overall content of total REEs (ƩREEs). The fish communities were composed of native and non-native species. Remarkably low concentrations of REEs (<0.0003 mg/L) were detected in the water samples, indicating REEs insolubility. In contrast, sediment samples were found to be a good sink for REEs, with a higher mean ƩREEs recorded for the samples from the Moncalieri station (70.93 mg/kg). Notably, no significant differences in ƩREEs concentration were observed in the muscle tissue of fish samples from the three stations. The highest mean ƩREEs was recorded in the samples from the Murazzi station (0.027 mg/kg). The BSAF was very low, consistently below the unit, indicating an absence of bioaccumulation in fish muscle from sediment. In contrast, the BCF was high for several REEs, mainly for Sc and Y. While feeding ecology did not appear to affect REEs accumulation in muscle, there was a significant negative relationship between the trophic level and ΣREEs, indicating a trophic dilution of REEs from predator (Silurus glanis) to planktivorous (Alburnus arborella) fish. This study provides baseline concentrations, trophic transfers, and patterns of REEs in a river system. Further studies are needed to understand the transfer of REEs to other biotic components of lotic ecosystems.

Mode of Occurrence and Distribution of Critical Metal Lithium and Other Trace Elements during Coal Preparation from Jiashun High-Sulfur Coal in Guizhou Province, China

During the Late Permian period, the coal from the Jiashun Mine in Xingren City, Guizhou Province, China, is characterized by a high sulfur (5.84%) and lithium (Li) (94.5 μg/g) content. Lithium is a critical metal in the context of global energy transition. Considering the importance of environmental protection and resource utilization, the mode of occurrence and distribution of trace elements in different coal preparation products were investigated. The obtained results indicated the following: (1) The minerals in Jiashun coal were mainly composed of veined and epigenetic pyrite, quartz, and kaolinite. Most of the minerals could be effectively removed from the cleaned coal through gravity separation. (2) The mode of occurrence of rare earth elements and yttrium (REY) in the coal was relatively complex, and they were mainly found in middlings from gravity separation and in flotation tailings. (3) The mode of occurrence of the trace elements in the coal significantly influenced their removal rate. The removal rate was significantly higher for trace elements removed through gravity separation compared to those removed using flotation. The trace elements mainly found in fine-grained minerals wrapped by organic matter or combined with organic portions had better removal results through flotation. The trace elements that occurred both in the minerals and organic matters had low removal rates (&lt;25%) using two coal preparation methods. (4) Li was more enriched in the middlings from gravity separation (98 μg/g) and in flotation tailings (102 μg/g), reaching the marginal grade (80 μg/g) of Li in coal. (5) Li in Jiashun coal may be derived from intermediate-felsic rocks at the top of the Kangdian Upland and late hydrothermal solutions.

Characterization and pre-concentration of rare earth elements in density fractionated samples from the Waterberg Coalfield, South Africa

The increased global demand for rare earth elements (REEs) from unconventional sources has led to increased interest and research in coal deposits. South Africa has extensive coal resources, but understanding of the concentration, mode of occurrence, and distribution of REEs in the coals and associated sediments is limited. This study aims to assess selected coal samples obtained from an exploration project underway in the Waterberg Coalfield (South Africa) to determine the concentration and mode of occurrence of REEs, and to gain insight into the behaviour of REEs during density fractionation. Density fractionation of coal is considered a pre-concentration method prior to REE extraction. Samples were selected from zones 8, 7, 4, and 3, spanning the Grootegeluk and Vryheid Formations of the Waterberg Coalfield. Density fractionation was conducted at relative densities (RD) of 1.3 to 1.8, providing float (F) and sink (S) products. Five parent and thirty density-fractionated coal samples were characterized using proximate analysis, petrography, X-ray diffraction (XRD), and the concentration of REEs determined using Inductively Coupled Plasma- Mass Spectrometry (ICP-MS) (after microwave digestion).The samples are classified as medium-rank C bituminous coals with variable maceral composition. The dominant minerals are kaolinite and quartz, occurring as both syngenetic and epigenetic phases. The total REY (REEs + yttrium) concentrations range from 45.1 to 389.2 ppm, higher than for world coals (except sample 8H F1.30, REY = 45.1 ppm), and generally higher than the average for Chinese coals. Float products obtained at 1.40 to 1.80 RD show greater variation of light REY (LREY) compared to medium REY (MREY) and heavy REY (HREY), whereas float fractions at 1.30 and sink fractions at 1.80 RD show no distinct enrichment patterns. Although the results suggest a relationship between the concentration of LREY with float fractions obtained between 1.40 and 1.80 RD, pre-concentration using density fractionation generally shows limited benefits for these coals. Ashing of the coals may be more effective in concentrating the REEs and further investigations are therefore required.

A characterization of rare earth elements in coal ash generated during the utilization of Australian coals

ABSTRACT Establishing an alternative resource of rare earth elements (REEs) in coal by-products has been a topic of high interest for the past decade. Despite this, research that characterizes REEs in such materials produced in Australia is lacking. This paper focuses on the concentration and occurrence of REEs in power station coal ashes that represent a suite of major economic coal basins in Australia. The concentration of REEs in coal ash samples was determined by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). Fly ash and bottom ash generated at power stations that utilize coals from the Collie Basin in Western Australia contained elevated REE concentrations. A REE-rich fly ash containing 0.21% total rare earth oxides (REO) was subject to further analysis by Scanning Electron Microscopy with Energy Dispersive X-rays (SEM-EDX) and TESCAN Integrated Mineral Analysis (TIMA). Fine (avg. <5 μm) monazite grains were observed in the fly ash which were typically bound to Al/Si-rich phases. A substantial increase in the aqua regia-extractable fraction of REEs occurred when fly ash samples were milled to an ultrafine particle size. The current investigation has demonstrated that Australian coal ash may represent a promising resource of REEs as the demand for these metals increases.

Occurrence of rare earth elements (REEs) and trace elements (TEs) in feathers of adult and young Gentoo penguins from King George Island, Antarctica

Penguins are sentinel species for marine pollution, but their role as potential biovectors of REEs or TEs to ecosystems has been poorly studied. The present study analyzed (ICP-MS) feathers of young and adult Gentoo penguins from Fildes Bay, for 63 elements (including 15 REEs). Most of the REEs were present at very low levels, ranging from 0.002 (Lu) to 0.452 (Sm) μg g−1 d.w., several orders of magnitude lower than TEs. The content of TEs varied widely, with Al, Fe, Zn, Sr, Ba, Ti and Mn as the seven having the highest concentrations in the feathers of both age groups. The results show that P. papua deposits REEs and TEs through the feathers on the penguin rockery, whose potential actual impacts and long-term fate in remote regions need deeper research. This work presents essential baseline data that will be useful for further studies on Antarctic penguins.

Determination of Scandium (Sc), Yttrium (Y), and Rare-Earth Elements (REEs) in Mafic and Ultramafic Rock Powder by a Modified and Validated Digestion Protocol and Inductively Coupled Plasma – Mass Spectrometry (ICP-MS)

The global demand for rare earth elements (REEs) has been associated with the increased use of renewable energy technologies. However, due to the low occurrence of REEs in economic sources, such as geologic samples, it is essential to employ suitable sample preparation methods and measurement techniques that allow for reliable quantification. While acid digestion procedures have been conventionally used to free metals from silicate rocks, few studies have evaluated these methods for quantitative measurement of REEs, including Sc and Y in mafic-ultramafic rocks. This study involves validating a modified digestion procedure to facilitate the determination of Sc, Y, and REEs in mafic-ultramafic matrices. Three reference materials (RMs) (OKUM, BHVO-1, and JB-1a) were used to assess the performance of this modified method. The calculated percent error (< 10%) for most elements was in good agreement with the reference values, except La in OKUM, Er in BHVO-I, and Eu, Gd, Tb, Ho, and Er in JB-1a. The precision of the modified method was satisfactory, with values generally below 5%. The smooth chondrite normalized REE patterns indicate good quality data. The t-values of the studied elements were lower than the t-critical value (± 12.71), suggesting no significant differences between the analytical and Intertek values. Thus, the modified method could be used as a viable alternative in determining REEs in Ni laterite samples by ICP-MS.

Characterization of industrially pre-treated waste printed circuit boards for the potential recovery of rare earth elements

Rare earth elements (REE) are classified as critical raw materials and the environmental impact of mining them is of growing concern. The recovery of REE from electronic waste (e-waste) could offer a more sustainable practice. Waste printed circuit boards (WPCBs) are an important resource in the e-waste stream due to their content of valuable materials. However, data regarding the concentration and distribution of REE in WPCBs is very limited. The aims of this research were: (a) to analyse the chemical composition of comminuted WPCBs prior to processing (industrially pre-treated) with emphasis on REE, and (b) to determine the distribution of REE and other metals in different size fractions of the pre-treated WPCBs. The samples were supplied by commercial e-waste recycling companies, which makes them representative of the e-waste processing industry in the UK. Correlation between elemental concentrations and particle size was analysed using Spearman’s rank correlation. Most REE concentrations were inversely correlated to the particle size. Concentrations of Y, La and Gd were found up to a thousand times higher in the smaller particle size compared with coarser particles. However, most of base metals including Cu, Sn, Pb and Zn did not show this trend. The present study highlights the occurrence of REE in comminuted WPCBs, and fine fractions as potential sources of these critical elements, currently not recovered during recycling process. A cost-effective sieving step is proposed to enrich the REE content for further recovery, prevent the possible loss of REE and maximize the total material recovered from WPCBs. • REE concentrations are inversely correlated to the particle size in comminuted WPCBs. • Concentrations of REE are up to a thousand times higher in finest fractions (<0.25 mm). • Fine fractions can contain over 60% of the total mass of some REE. • Simple and cost-effective sieving step can enhance yield of REE for recovery purposes. • Comminuted WPCBs are an untapped resource for REE recycling.

The effect of calcination on the occurrence and leaching of rare earth elements in coal refuse

Coal refuse, as one of the solid wastes with huge yield, is a promising source of rare earth elements (REE) given the demand-supply gap, zero-cost construction and policy-related subsides. In this paper, the mechanism and effects of calcination on the occurrence and leaching of REE in coal refuse were examined by sequential chemical extraction procedure, characteristic analysis, and acid leaching test. The results showed that the calcination promotes the transformation of rare earth from silicate & aluminosilicate form to metal oxide form, and then, turns back with the continuous increase of temperature. The characteristic analysis indicated that the sample calcined at 400 ℃ began to undergo a dehydration reaction, and kaolinite began to transform into metakaolin. The transformation reaction may be completed at 600 ℃ when REE was released from the mineral lattice. Metakaolin was transformed to the amorphous phase as the temperature continued to rise to 900 ℃. The transition of kaolinite is one of the most important reasons for the variation of REE occurrence, resulting in the change of leaching rate. Leaching of REE in coal refuse took advantage of the proper calcination and reached up to 98 % at the optimal temperature ranging from 600 ℃ to 900 ℃. This theoretical work gives an implication for low-temperature combustion assist to REE recovery from coal refuse of which the calorific value meets the requirements of the fluidized bed.

Highly-efficient and sequential recovery of rare earth elements, alumina and silica from coal fly ash via a novel recyclable ZnO sinter method

A novel sinter method using ZnO as the activator instead of the conventional Na2CO3/CaCO3, (NH4)2SO4, and K2S2O7 was developed to achieve efficient sequential extraction of rare earth elements (REEs), alumina (Al), and silica (Si) from coal fly ash (CFA). Up to 93.3% Si, 87.1% REEs (70.7% Ce, 82.5% La, 83.2% Gd, 87.1% Nd, 62.3% Dy, and 81.7% Y), and 92.9% Al were extracted from CFA, respectively. Moreover, 93.1% of the ZnO activator was efficiently recycled, and the yield of red mud was only 14.9%. X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) results showed that the speciation transformation of Al/Si during CFA/ZnO roasting was as follows: mullite, quartz, amorphous Al2O3, and SiO2 → Zn0.75Al1.5Si1.5O6, kyanite and willemite → gahnite and quartz/cristobalite solid solutions. The change in the REEs occurrence mode hinted at the migration of most REEs in aluminosilicates forms with Si during roasting, and disassociation with Si into the acid-soluble form after alkali leaching. These results indicate that the coupling of Al-Si-REE in CF was broken by this ZnO sinter method, promoting the sequential and efficient extraction of REEs, Al, and Si from CFA. This study provides a green and efficient strategy for element recovery from CFA, substantially reducing residues and favoring REEs concentration.

Characterization and Leaching Kinetics of Rare Earth Elements from Phosphogypsum in Hydrochloric Acid

The characterization and leaching mechanism of REEs from phosphogypsum (PG) in HCl was studied in-depth. REEs contained in the PG were 208 ppm, of which Y, La, Ce, and Nd were the four most abundant elements. The modes of occurrence of rare earth elements (REEs) in the PG were quantified using the sequential chemical extraction (SCE) method. Among the five REE occurrence species, the metal oxide form accounted for the largest proportion, followed by the residual, organic matter, and ion-exchangeable fractions, and REEs bound to carbonates were the least. From the comparison of the distributions of REEs and calcium in different occurrence states, it can be determined that REEs contained in the PG were mainly present in the residue state (existed in the gypsum lattice) and the metal oxide state (easily leached). The leaching results show that the suitable leaching conditions were acid concentration of 1.65 mol/L, S/L ratio of 1/10, and reaction temperature of 60 °C. At the condition, the maximum leaching efficiency for ∑REE was 65.6%, of which the yttrium leaching rate was the highest and reached 73.8%. Importantly, A new kinetic equation based on the cylindrical shrinking core model (SCM) was deduced and could well describe REE leaching process from PG. The apparent activation energy for ∑REE leaching was determined to be 20.65 kJ·mol−1.