Source Of Rare Earth Elements Research Articles

The Origin and Evolution of Rare Earth Element Mineralization in the Muluozhai Deposit (Sichuan, China): Insights from Mineralogical, Trace Element, and Sr-Nd-Pb-C-O-Ca Isotope Data

Abstract Carbonatite complexes are globally significant sources of rare earth elements (REEs); however, mechanisms governing REE deposition in various tectono-lithologic settings, encompassing host rocks, wall rocks, ore-controlling structures, and metasomatism, remain inadequately understood. The Zhengjialiangzi mining camp, situated within the extensive Muluozhai deposit (containing 0.45 million metric tons [Mt] at 4.0 wt % REE2O3) in the northern segment of the Mianning-Dechang belt, Sichuan (southwestern China), is characterized by a complex vein system that evolved within metamorphosed supracrustal rocks of the Yangxin and Mount Emei Formations. The mineralization is coeval with Oligocene intrusions of carbonatite and nordmarkite at ~27 Ma. The major gangue minerals include fluorite, barite (transitional to celestine), and calcite, with bastnäsite serving as the primary host for REEs in all analyzed orebodies. Several other accessory to minor minerals were identified in the ore veins, including some that had not previously been known to occur in the Muluozhai deposits (e.g., thorite and pyrochlore). The stable isotopic (C-O-Ca) and trace element compositions of calcite, along with whole-rock data, suggest that carbonate material was derived from the mantle and subsequently reequilibrated with the Yangxin marbles. The radiogenic isotope (Sr-Nd-Pb) compositions of vein material remained unaffected by wall-rock contamination and suggest a mantle source influenced by crustal recycling, consistent with other REE deposits hosted by carbonatite and nordmarkite in the region. The combined petrographic and geochemical evidence suggests derivation of Muluozhai mineralization from a carbonatitic source and interaction of carbonatite-derived fluids with wall rocks, xenoliths, and early-crystallizing mineral phases, particularly barite.

Microwave-assisted extraction of rare earth elements from phosphogypsum—Effect of hydrogen peroxide addition

Phosphogypsum (PG) is a by-product of phosphate fertilizer manufacturing which is produced in significant amount. It is one of the promising secondary sources of rare earth elements (REEs). The study investigated microwave-assisted extraction (MAE) of yttrium (Y), lanthanum (La), and neodymium (Nd) from the PG, obtained from a fertilizer company in Gresik, Indonesia, using hydrochloric acid (HCl). Extraction efficiency increased as acid concentration increased from 0.25 N to 1.0 N, and increased slightly as reaction temperature increased from 100 °C to 150 °C. When using 1.0 N HCl, solid concentration of 20 g/L, 100 °C, and reaction time of 5 min, complete leaching of REEs was achieved, while 87.0 % of P was leached out. The total energy consumption was 3.74×105 kJ/kg, implying the MAE process was very energy-efficient. The addition of 1 or 2 wt% H2O2, however, decreased leaching efficiency of REEs. It was because the MW/H2O2 system produced hydroxyl radical (•OH), which induced the formation of the passivation layer of zirconium dioxide (ZrO2) on PG surfaces, and hindered REEs leaching. Simulation results revealed that recovery of P via selective precipitation was not feasible due to the interference of calcium (Ca) in the extractant.

RARE-EARTH elements in the topsoils of a Russian industrial city: Sources and human health risk assessment

Research on rare-earth elements (REEs) in urban soils of Russian industrial cities is extremely limited. This study investigates the potential sources and human health risks of REEs contained in the topsoils of the industrial Russian city of Chelyabinsk. The study also takes into account natural sources of REE as the city is located on the border of granites (Urals) and sedimentary rocks (Western Siberia). We analyzed the concentration and distribution of REEs in the soils of four types of locations: residential courtyards, city parks, roadsides, and industrial locations. The total REE concentrations ranged from 44 to 255 mg/kg, with average concentrations of 140, 124, 113 and 89 mg/kg in the courtyards, roadsides, industrial location and city parks, respectively. The REE content in courtyard soils could be influenced by poor cleaning of fallen leaves. The ratios of light REE (LREE) to heavy REE (HREE) ranged from 9.5 to 10.1, revealing an obvious fractionation between them. The fractionation of LREE and HREE, and the REE/ES (European Shale) pattern showed that REE accumulation in Chelyabinsk soils has been disturbed by human activities. It was shown that the dust from industrial emissions was the main anthropogenic source of REE accumulation in urban soil. The largest amounts of REEs are emitted from an electrometallurgical plant and zinc production plant. Fortunately, the estimated daily intakes of REE from soils for children and adults were well below the safety thresholds. At the same time, in order to prevent social tension and reduce the anthropogenic load on the urban area, it is recommended to use phytoremediation technologies, smart landscaping of industrial and residential areas, more thorough cleaning of fallen leaves and road dust. It is also recommended to move the most dangerous production processes outside the urban area.

Rare earth elements reveal the human health and environmental concerns in the largest tributary of the Mekong river, Northeastern Thailand

The widespread application of rare earth elements (REEs) in contemporary industries and agriculture, has caused emerging contaminant accumulation in aquatic environments. However, there is a limited scope of risk assessments, particularly in relation to human health associated with REEs. This study investigated the provenance, and contamination levels of REEs, further evaluating their environmental and human health risks in river sediments from an agricultural basin. The concentrations of REEs ranged from 30.5 to 347.7 mg/kg, with showing an upward trend from headwater to downstream. The positive matrix factorization (PMF) model identified natural and anthropogenic input, especially from agricultural activities, as the primary source of REEs in Mun River sediments. The contamination assessment by the geoaccumulation index (I-geo) and pollution load index (PLI) confirmed that almost individual REEs in the samples were slightly to moderately polluted. The potential ecological risk index (PERI) showed mild to moderate risks in Mun River sediment. Regular fertilization poses pollution and ecological risks to agricultural areas, manifesting as an enrichment of light REEs in river sediments. Nevertheless, Monte Carlo simulations estimated the average daily doses of total REEs from sediments to be 0.24 μg/kg/day for adults and 0.95 μg/kg/day for children, comfortably below established human health thresholds. However, the risk of REE exposure appears to be higher in children, and sensitivity analyses suggested that REE concentration contributed more to health risks, whether the adults or children. Thus, concerns regarding REE contamination and risks should be raised considering the wide distribution of agricultural regions, and further attention is warranted to assess the health risks associated with other routes of REE exposure.

Application of Plackett-Burman Design in the Screening of Factors Acting on the Sulfuric Leaching Process of Rare Earths from Moroccan Phosphogypsum

As the main waste of the phosphates industry (200–300 million tons per annum produced globally), phosphogypsum (PG) is a major environmental concern (85% of PG is disposed of on land or at sea). The valorization of PG as a secondary source of rare earth elements (REE) is one of the most recent valorization options under investigation. This study investigated the sulfuric acid leaching process of REE from a Moroccan PG. Plackett -Burman design was used to statistically screen significant factors affecting the leaching efficiency of REE named the studied response (Y). Seven factors considered potentially influential were studied as the model variables : Sulfuric acid concentration (X1); Solid to liquid ratio (X2); Drying temperature (X3); Leaching time (X4); Stirring speed (X5); Temperature (X6), Particle size (X7). According to the results, the predicted values of the fitted model were in good agreement with the experimental values by a coefficient of determination (R2) equal to (99 %). drying temperature has a statistically insignificant effect with p=0.336 > 0.05. All the other six variables have significant effects (p < 0.05), but 97,91 % of Y was affected only by the following four factors: X1 (63.71%), X4 (17.94%), X2 (9.98%) , X5 (6.24% ).

Organic-mineral colloids regulate the migration and fractionation of rare earth elements in groundwater systems impacted by ion-adsorption deposits mining in South China

Ion-adsorption rare earth element (REE) deposits distributed in the subtropics provide a rich global source of REEs, but in situ injection of REEs extractant into the mine can result in leachate being leaked into the surrounding groundwater systems. Due to the lack of understanding of REE speciation distribution, particularly colloidal characteristics in a mining area, the risks of REEs migration caused by in situ leaching of ion-adsorption REE deposits has not been concerned. Here, ultrafiltration and asymmetric flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry (AF4-ICP-MS) were integrated to characterize the size and composition of REEs in leachate and groundwater from mining catchments in South China. Results show that REEs were associated with four fractions: 1) the <1 kDa fraction including dissolved REEs; 2) the 1 – 100 kDa nano-colloidal fraction containing organic compounds; 3) the 100 kDa – 220 nm fine colloids including organic-mineral (Fe, Mn and Al (oxy)hydroxides and clay minerals); 4) the >220 nm coarse colloids and acid soluble particles (ASPs) comprising minerals. Influenced by the ion exchange effect of in situ leaching, REEs in leachate were mostly dissolved (79%). The pH of the groundwater far from the mine site was increased (5.8 – 7.3), the fine organic-mineral colloids (46% – 80%) were the main vectors of transport for REEs. Further analysis by AF4 revealed that the fine colloids can be divided into mineral-rich (F1, 100 kDa – 120nm) and organic matter-rich (F2, 120 – 220nm) populations. The main colloids associated with REEs shifted from F1 (64% ∼ 76%) to F2 (50% ∼ 52%) away from the mining area. For F1 and F2, the metal/C molar ratio decreased away from the mining area and middle to heavy REE enrichment was presented. According to the REE fractionation, organic matter was the predominant component capable of binding REEs in fine colloids. Overall, our results indicate that REEs in the groundwater system shifted from the dissolved to the colloidal phase in a catchment affected by in situ leaching, and organic-mineral colloids play an important role in facilitating the migration of REEs.

Origin of rare earth elements in acid mine drainage traced by strontium and neodymium isotopes

Acid mine drainage (AMD) generated from coal and sulfide mining has attracted much interest due to the high concentrations of rare earth elements (REE) observed. However, the origin of REE in AMD and particularly the mechanism of enrichment in medium REE remain uncertain. The combined study of Sr and Nd isotopes can be used to trace the processes that control the mobility and fractionation of REE in aquatic systems such as AMD. This work reports for the first time worldwide isotopic data of Sr and Nd in AMDs from sulfide mining (Iberian Pyrite Belt, SW Spain) and compare with those in host rocks. Additionally, leaching experiments have been carried out to simulate the water–rock interactions found in these acidic systems. The results obtained strongly suggest that the origin of REE in AMDs from metal mining may be related to the preferential dissolution of shales and felsic rocks. The suitability of Sr and Nd isotopic signatures to trace the REE source in AMD-affected systems has been proven, it provides a new environmental and geochemical prospecting tool which could be useful in different geological water–rock interaction systems.

Study on the material source and enrichment mechanism of REE-rich phosphorite in Zhijin, Guizhou

Rare earth element (REE)-rich phosphorite in the Guizhou region mainly exists in the Doushantuo Formation and Gezhongwu Formation in early Cambrian strata, which are some of the important strata containing phosphorite resources in China. The early Cambrian Zhijin phosphorite in Guizhou Province, China, has high rare earth element and yttrium (REY) contents of up to 2500 ppm, with heavy REY (HREY) contents accounting for ~ 30% of the total REY contents. However, the specific controlling source and environment of phosphorite (especially the REEs in Zhijin phosphorite) are still unsolved. Through field geological investigations; mineralogical, geochemical, Sr–Nd isotope analyses; and tectonic characteristics, the material source, sedimentary environment and seawater dynamics of REEs in phosphorite are studied. It is considered that the REEs enriched in the Zhijin phosphorite are mainly affected by precipitation from hydrothermal fluid. Moreover, from the late Ediacaran to the early Cambrian, the depositional environment from the bottom to the top of the water tended to be hypoxic, and the activity of hot water fluid tended to be strong. The change in redox conditions is closely related to the rise and fall of sea level. Combined with the tectonic background, these results show that the weakly oxidized environment may be an important factor controlling the enrichment of REEs. The enrichment of REEs may be closely related to volcanic hydrothermal activity, later diagenesis and seawater dynamics.

The impact of global climate changes on trace and rare earth elements mobilization in emerging periglacial terrains: Insights from western shore of Admiralty Bay (King George Island, Antarctic)

In the rapidly changing climate, the biogeochemical behaviours of trace elements and Rare Earth Elements (REEs) in emerging periglacial environments assumes profound importance. This study provides pivotal insights into this dynamic by investigating the Antarctic's response to global climate change. The bedrock of King George Island is rich in REEs, with the presence of trace metals (TEs), with the highest concentrations of metals found in ornithogenic soil (∑REE 84.01–85.53 mg∙kg−1 dry weight). REEs in the studied soil, found mainly in igneous rocks, as is indicated by the positive correlation of these elements with sodium and calcium. The TEs released as a result of weathering are leached by water flowing down local watercourses to Admiralty Bay, as indicated by the decreasing results of ∑REE = 11.59 μg∙dm−3 in watercourse water, ∑REE = 1.62 μg∙dm−3 in watercourse pools and ∑REE = 0.66 μg∙dm−3 in the water of Admiralty Bay at the outlet of the watercourse. Water originating from the melting of snow on the glacier also carried REEs (∑REE = 0.14 μg∙dm−3), a fact which suggest the further influx of these elements from atmospheric deposition. The Prasiola crispa turned out to be the most susceptible to the accumulation of REEs (∑ 80.73 ± 5.05 μg g−1) and TEs, with the exception of chromium and zinc, whose concentrations were found to be at their highest in Deschampsia antarctica. In Usnea antarctica, Xanthoria candelaria, and Ceratodon purpureus and Politrichastrum alpinum, a dominant role in the accumulation of REEs was played by HREEs. The determined enrichment factor (EF) indicates that the soil cover is a source of REEs (EFAlgae for ∑REE = 5.07; EFLichen for ∑REE = 6.65; EFBryophyta for ∑REE = 5.04; EFVascular for ∑REE = 4.38), while Ni, As and Pb accumulated in plants may originate from other sources than the soil.

Rare earth element systematics of chimney anhydrite from seafloor hydrothermal vents

Comparative studies of sulphide chemistry have been conducted over the past few decades to characterise seafloor hydrothermal mineralisation, but few studies have been carried out on sulphate minerals. Here we report on a systematic study of the S isotopic composition and rare earth element (REE) chemistry of chimney anhydrite from various types of seafloor hydrothermal vent fields in the Central Indian Ridge, North Fiji Basin, and Tonga intra-oceanic arc. Our results show that sulphate δ34S values of anhydrite cluster around that of seawater sulphate, indicating a significant role for fluid–seawater mixing during anhydrite precipitation. However, some anhydrite has δ34S values lower than that of seawater. This likely reflects sulphate derived from magmatic SO2 disproportionation. Chondrite-normalised REE patterns of the anhydrite are not uniform on the scale of individual grains and/or between each type of hydrothermal vent field. The REE compositions of the anhydrite do not have a clear relationship with the REE compositions of the primary host rocks with which the hydrothermal fluids interacted. This indicates that the REEs are not sensitive indicators of the host rock compositions, even though the host rocks are the dominant source of REEs in the seafloor hydrothermal fluids. In contrast, anhydrite REE concentrations and Eu anomalies are correlated with δ34S values and the redox state of the hydrothermal fluids, respectively. We suggest that relatively REE-enriched anhydrite is formed by efficient leaching of REEs from host rocks by acidic magmatic fluids (i.e., a low pH), whereas a low redox potential of the hydrothermal fluids is the most important factor controlling Eu mobility. The original concentrations of REEs supplied to the hydrothermal fluids can also be modified by fluid boiling and fluid–seawater mixing. The former produces light REE-depleted anhydrite because the availability of chloride ligands decreases in low-density vapor-dominated boiled fluids. In situ geochemical analysis of anhydrite on the intra-grain scale shows that high degrees of fluid–seawater mixing contribute to the relatively low REE concentrations and positive Eu anomalies. As such, our results indicate that the REE systematics of anhydrite are mainly controlled by the combined effects of fluid conditions (i.e., redox state, pH, and Cl concentrations) and sub-seafloor geochemical processes (i.e., magmatic input, fluid boiling, and seawater mixing) rather than the nature of the source materials. In situ analysis of anhydrite grains is a useful tool for constraining variable hydrothermal mineralisation conditions during chimney growth, which cannot be solely inferred from geochemical studies of sulphide minerals, particularly in inactive and relict systems where the hydrothermal fluids cannot be sampled.

Geochemistry of Palaeoproterozoic Kajrahat Limestone, Vindhyan Supergroup, central India: insights into depositional conditions and sources of rare earth elements

Geochemistry of Palaeoproterozoic Kajrahat Limestone, Vindhyan Supergroup, central India: insights into depositional conditions and sources of rare earth elements

Rare Earth Elements in Sediments from the Laptev Sea Shelf: Insight into Sources and Distribution Factors

The study of rare earth elements (REEs) in marine sediments is a powerful geochemical tool for determining depositional processes and sediment provenance, as well as for understanding paleoenvironmental changes. In this context, we present REE, some major and trace elements, grain size, and mineralogy data on surface and core sediments, which were collected in different areas of the eastern Laptev Sea Shelf (LSS; Arctic Ocean). The primary objective of this paper was to assess the principal controlling factors influencing REE concentration and their vertical to lateral distribution. The total REE content (ΣREE) ranged from 139 ppm to 239 ppm within the studied sediment samples, predominantly consisting of silt. The normalized REE distribution patterns, based on North American Shale Composite (NASC) standard, exhibited an enrichment in light REE (LREE) when compared to heavy REE (HREE), which is similar to that in Lena River suspended particulate matter. The primary sources of REEs in the eastern LSS were both the suspended particulate matter from the Lena River and sediments originating from the coastal ice complex. The spatial distribution of REEs was primarily contingent upon the distance from sediment sources and prevailing hydrological conditions and was generally characterized by a decrease in REE concentration seaward. There was a moderate positive correlation between ΣREE and mean grain size in the studied surface sediment. However, this relationship was specific only for surface samples and was not found in the sediment cores, indicating that sediment grain size does not play a significant role in the REE vertical distribution. The strong positive correlation between ΣREE and Al, K, Ti, V, Cr, Zr, Hf, and Th suggests that REEs are hosted by not only heavy but also clay minerals. The vertical fluctuations of LREE/HREE, Eu/Eu*, (La/Lu)N can point at variable dominance of one or another REE source for during sediment accumulation.

Geochemical characteristics and environmental implication of rare earth elements in sediments from the Three Gorges Reservoir, China

The construction and operation of the Three Gorges Dam occluded sediment transportation in the Yangtze River. However, the sources, transport processes, and environmental impacts of these sediments on the Three Gorges Reservoir (TGR) remain unclear. Here, we used rare earth elements (REEs) to trace the transport pathways of sediments in the TGR, China. Geochemical characteristics including the chemical composition and fractionation, temporal and spatial distribution, and potential sources of REEs were also evaluated in this study. The individual REEs concentration in the TGR sediments followed the Oddo-Harkins rule, with the mean REEs value of 207.33 μg/g. REEs concentrations in the midstream were higher than those in the upstream and downstream of the TGR. Statistical analysis showed that water impoundment phase had no significant influence on the distribution of REEs. TGR sediments are mainly derived from terrigenous detrital particulates, characterized by a distinctive enrichment in light REEs, with its percentage higher than 90 % of the total REEs. The significant positive correlation among the REEs confirmed that they are co-existed and shared the similar sources. Multiple provenance analysis approaches using discriminant function analyses, provenance indices, and La/Yb-La/Sm-Gd/Yb ternary diagrams further indicated that the REEs in sediments originated from the weathering of mudstone in the basin of TGR. After periodic water level fluctuation for more than six years, the chemical compositions of REEs in TGR sediments slightly differed from those of the Yangtze River sediments before TGR construction, but were similar to those of the downstream of the Yangtze River. Therefore, this study indicated that the construction and operation of the TGR changed the chemical compositions and the origin of the sediments in the Yangtze River, which can provide useful insights into the transport pathways of TGR sediments and their impacts on the fluvial environment.

Geochemical and Isotopic (Nd, Sr) Tracing of the Origin of REE Enrichment in the Cambrian Georgina Basin Phosphorites

AbstractPhosphorites of the Georgina Basin (northern Australia) are an established economic source of phosphate and have recently been recognized to be a potential source of rare earth elements (REE). Previous bulk‐rock geochemistry work focused on the eastern margin of the basin revealed that phosphorites from the southern region have significantly higher (up to 0.5%) REE contents than equivalent prospects further to the north. In this study, we examine the origin of REE enrichment in the Georgina Basin phosphorites using an integrated geochemical, petrographic, mineralogical, and isotopic (Sm‐Nd and Sr) approach. The trace element geochemistry of primary phosphate minerals is consistent with a seawater origin for the REEs in phosphorites, with minimal input from the underlying basement rocks. Variations in total REE concentrations are controlled largely by the seawater composition, depositional environment, and the texture of the phosphorite (grainstone vs. mudstone phosphorite). Grainstone phosphorite yields on average higher concentrations of REEs compared to the mudstone phosphorite, likely due to the higher surface‐to‐mass ratio of the grainstone phosphorite resulting in the uptake of higher concentrations of REEs during coastal reworking. The formation of the low‐REE northern and central deposits took place during early transgression in oxidized, shallow supratidal to subtidal platformal environments. During the subsequent main transgressive event, fault‐controlled topography in the southern domain allowed phosphogenesis along basement highs to be facilitated along a redox boundary, where more saline and REE‐rich deep anoxic bottom waters, along with Fe‐cycling and bacterial mediation played a major role in the enrichment of REEs in the southern phosphorites.

Influence of Clay Particle Interactions on Leaching Percolation in Ion-adsorption-type Rare Earth Ores

Ion-adsorption-type rare earth ores (IRE-ores) are a vital source of rare earth elements globally. The percolation properties of the ore body significantly determine the ease of rare earth extraction using in situ leaching processes. This paper investigates the interactions among clay particles in ion-adsorption-type rare earth ores within aqueous solutions, utilizing methods such as mineral particle aggregation and settling and acid–base adsorption on mineral surfaces. Based on these analyses, this paper elucidates the influence of solution properties on the percolation process of ion-type rare earth ores during leaching. The results indicate that the electrostatic attraction between minerals in aqueous solutions is pronounced, enhancing the interaction and resulting in a notable instability of the aggregates. The aggregation of minerals impacts the viscous effect of water bound to the mineral surface on the solution, thereby affecting the percolation rate. Extremely low concentrations of SO42− and CH3COO− can act as surface modifiers, reducing the electrostatic attraction between particles and consequently increasing the leaching percolation rate.

Spatial Variability of Rare Earth Elements in Groundwater in the Vicinity of a Coal-Fired Power Plant and Associated Health Risk.

This study investigated the occurrence and distribution of rare earth elements (REEs), including 14 lanthanoids, scandium (Sc), and yttrium (Y), in groundwater around a large coal-fired thermal power plant (TPP). The ICP-MS technique was used to analyze 16 REEs in groundwater samples collected from monitoring wells. REE concentrations ranged from 59.9 to 758 ng/L, with an average of 290 ng/L. The most abundant was Sc, followed by La, accounting for 54.2% and 21.4% of the total REE concentration, respectively. Geospatial analysis revealed the REE enrichment at several hotspots near the TPP. The highest REE concentrations were observed near the TPP and ash landfill, decreasing with the distance from the plant and the landfill. REE fractionation ratios and anomalies suggested the Light REE dominance, comprising over 78% of the total REEs. Correlation and principal component analyses indicated similar behavior and sources for most REEs. Health risk assessment found hazard indices (HI) of 1.36 × 10-3 and 1.98 × 10-3 for adults and children, respectively, which are far below the permissible limit (HI = 1). Likewise, incremental lifetime cancer risks (ILCR) were all below 1 × 10-6. Nevertheless, ongoing ash disposal and potential accumulation in the environment could elevate the REE exposure over time.

Using machine learning to identify indicators of rare earth element enrichment in sedimentary strata with applications for metal prospectivity

Rare earth elements (REE), classified as critical minerals which are crucial for clean energy technologies, face soaring demand. While economic deposits are found in limited geologic environments including carbonatites and ion-adsorption clays, unconventional, secondary sources such as those from sedimentary basins could hold potential to meet this increased demand. Coal and its associated combustion by-products, phosphorites, oil sands tailings, and formation waters have all garnered interest for REE recovery, yet they remain significantly underexplored. Accordingly, new tools for data analysis and optimization such as machine learning can assist in mineral prospectivity, with these tools being subject to rapid proliferation in the Earth sciences.This work leverages compositional data analysis principles and machine learning to probe geochemical relationships and predict REE abundances in sedimentary lithologies using unsupervised (correlation, principal component, and cluster analysis) and supervised (regression, support vector machine, random forest, and boosting) machine learning models. These three unsupervised models display similar results, with REE typically being associated with incompatible elements (e.g., Th, Nb, and Hf). Gradient boosting, Adaboost, and Random Forest had the highest performance for predicting REE concentrations, with Th and P commonly being the most important predictor variables. Identifying geochemical indicators of REE enrichment that may be used to assist in discovering potentially exploitable REE resources based on existing data, as well as increasing the understanding of metal behaviour in sedimentary systems, is a step forward in understanding novel secondary and unconventional REE sources. Although REE concentrations from these sources are generally lower than primary ore deposits, the amount of available feedstock, potentially simpler, cheaper, and less environmentally taxing extraction processes, and the added benefit of remediating waste streams and contributing to the circular economy make these sources alluring.

Rare Earth Element Characteristics of Shales from Wufeng-Longmaxi Formations in Deep-Buried Areas of the Northern Sichuan Basin, Southern China: Implications for Provenance, Depositional Conditions, and Paleoclimate.

To explore the sedimentary environment and the background of the source area of organic-rich shales in the Wufeng-Longmaxi Formations in the northern Sichuan Basin, samples from Well XX1 in the area were subjected to geochemical testing and analysis of organic carbon content, trace elements, and rare earth elements (REEs). The results show that the total content of REE (ΣREE) of the shale in the Wufeng-Longmaxi Formations varied from 183.08 to 234.66 μg/g with an average of 212.59 μg/g, which is significantly higher than the content of the North American shale composite. The fluctuations in the total amount of REEs in the shale of the Wufeng-Longmaxi Formations reflect certain differences in the geochemical conditions of the Upper Ordovician-Lower Silurian shale. The ratios of LREE/HREE, LaN/YbN, LaN/SmN, and GdN/YbN and the distribution of normalized REE patterns indicate that the source supply or sedimentary structural background may have changed during the shale deposition period of the Wufeng Formation, while the shale deposition period of the Longmaxi Formation may be in a relatively stable source supply and sedimentary structural background. There is no significant correlation between δCe and ΣREE, and the obviously negative Eu abnormity and the weak Ce abnormity indicated that the diagenesis had a limited impact on REEs. Geochemical parameters such as values of ∑REE, δEu, δCe, Ceanom, and LaN/YbN indicate that the climate during the Wufeng-Longmaxi Formation shale deposition period was warm and humid, and the shale was deposited mainly in the suboxic-anoxic water environment. The deposition rate was stable and slow, providing good conditions for the production and preservation of organic matter. At the same time, this shows that the water environment of Wufeng Formation is more anoxic and reductive than that of Longmaxi Formation, which is more conducive to the preservation of organic matter. The correlation between ΣREE and the content of Sc, Ti, Cr, Co, Zr, Nb, Th, Hf, Ta, and other elements indicates that the sources of REEs in the shale of Wufeng and Longmaxi Formations in the study area are similar, mainly terrestrial clasts, and some may come from the sea. The REE distribution pattern shows that the shale provenance of the Wufeng-Longmaxi Formations mainly comes from the upper crust. The La/Yb-∑REE diagram shows that the sediment-parent rocks are mainly early sedimentary rocks and these sediment-parent rocks have granite provenance characteristics. Compared to La/Yb, LREE/HREE, LaN/YbN, and other REE characteristic parameters, it is inferred that the tectonic background of the study area is dominated by passive continental margin.

Mechanical Activation as a Stage of Coal Sample Preparation in the Analysis of Rare Earth Elements Content by Inductively Coupled Plasma Mass Spectrometry

Background: Brown coals are recognized as promising sources of rare earth elements (REEs). Rare earths are present in both the mineral and organic parts of brown coal. Objectives: This study was conducted to investigate the influence of preliminary mechanical activation in the process of sample preparation prior to analyzing the concentrations of rare earth elements in brown coal samples of various origins and compositions Methods: Four coal samples from different deposits in Russia were selected for the study. Samples were treated with mechanical activation, without reagents, or mechanochemical activation, with humic acids added externally as reagents. X-ray phase analysis was carried out with the selected samples. The quantities of rare-earth elements present in the samples were studied by the method of high-sensitivity inductively coupled plasma mass spectrometry (ICP-MS). Results: It was found that the mechanical activation of coal before dissolution in a mixture of nitric and hydrofluoric acids leads to an increase in the determined concentration of rare earth elements. For this study, the expediency of using only nitric acid as an optimal solvent for the elemental analysis of coal samples was shown. The total concentration of all REE after dissolution of nitric acid and mechanochemical activation with humic acid reached 2456 g/t in Vanchin coal, 968 g/t in Azeysky coal, and 24 g/t and 150 g/t in Itatsky and Spetsugli coals, respectively. Conclusion: Mechanical activation and mechanochemical treatment can greatly help to facilitate sample preparation of natural objects, such as coals for elemental analysis, but in some cases, only a change of solvent is sufficient. When developing technology for concentrating rare earth elements from coal involving grinding, it is necessary to take into account the fact that mechanical activation of coal changes its tendency to dissolve, which may affect the results of the analysis and should be taken into account during experiments.

The distribution and enrichment characteristics of rare earth elements between coals and coal ashes from four coal-fired power plants.

Coal and coal ash (slag and fly ash) have emerged as important potential sources for rare earth elements (REY). The distribution and enrichment of REY during coal combustion from coal-fired power plants are of great significance for the recovery of REY. In this study, the concentration of REY in coal and coal ash from four coal-fired power plants in China was determined using microwave digestion-inductively coupled plasma mass spectrometry. The distribution and enrichment characteristics of REY in coal and coal ash were studied. The coal ash contains 310-683 μg g-1 of REY, and the proportion of critical REY was greater than 30%, which is higher than the extraction concentration (300 μg g-1) recommended by the United States Department of Energy, indicating significant REY recovery potential. The concentration of REY in coal was positively correlated with ash yield, which could be explained by the fact that REY are mainly distributed in minerals. Coal and coal ash show similar REY distribution patterns, indicating that REY were not fractionated during the coal combustion. In addition, the enrichment coefficient of REY in fly ash was greater than that in slag, which revealed that REY were mainly enriched in fly ash. Therefore, the REY in fly ash has more recycling potential.