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Article

The Distribution of Rare Earth Elements in Coal Fly Ash Determined by LA-ICP-MS and Implications for Its Economic Significance

by
Shuliu Wang
1,
Wenhui Huang
2,* and
Weihua Ao
3
1
Department of Finance, Communication University of China, Beijing 100024, China
2
Key Laboratory for Marine Reservoir Evolution and Hydrocarbon Abundance Mechanism, School of Energy Resources, China University of Geosciences Beijing, Beijing 100083, China
3
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, School of Materials Science and Technology, China University of Geosciences Beijing, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(1), 275; https://doi.org/10.3390/su17010275
Submission received: 28 November 2024 / Revised: 27 December 2024 / Accepted: 30 December 2024 / Published: 2 January 2025
(This article belongs to the Special Issue Scientific Disposal and Utilization of Coal-Based Solid Waste)
Browse Figures
Figure 1
<p>The spatial resolution and detection limit of analytical techniques for the elemental composition (modified from [<a href="#B24-sustainability-17-00275" class="html-bibr">24</a>]).</p> "> Figure 2
<p>The mineralogy and LA-ICP-MS analysis spot of different phases in coal fly ash under reflected light. (<b>A</b>) S1, Al-Si-Fe; (<b>B</b>) S2, Fe-oxide; (<b>C</b>) S3, Si-Al; (<b>D</b>) S4, Al-Si-Ca-Ti; (<b>E</b>) S5, Al-Si; (<b>F</b>) S6, Si-Al; (<b>G</b>) S7, Al-Si-Ca; (<b>H</b>) S8, SiO<sub>2</sub>; (<b>I</b>) S9, Fe-oxide; (<b>J</b>) S10, Fe-oxide; (<b>K</b>) S11, SiO<sub>2</sub>; (<b>L</b>) S12, quartz.</p> "> Figure 3
<p>The in situ trace elements of constituents of coal ash.</p> "> Figure 4
<p>The distribution patterns of REE in the aluminosilicates (<b>A</b>), Ca, (Fe)-enriched aluminosilicates (<b>B</b>), Fe-oxides (<b>C</b>), and SiO<sub>2</sub>/quartz (<b>D</b>).</p> "> Figure 5
<p>The distribution of REY of fly ash (<b>A</b>) measured by ICP-MS and that of aluminosilicates, Ca (<b>B</b>), (Fe)-enriched aluminosilicates (<b>C</b>), and Fe-oxides (<b>D</b>) was determined by LA-ICP-MS.</p> "> Figure 6
<p>(<b>A</b>) The contents and proportions of LREY, MERY, and HREY in coal and coal combustion products. (<b>B</b>) The contents of LREY, MERY, and HREY in the fly ash phases. (<b>C</b>) The distribution patterns of REE in feed coal, combustion products, and fly ash phases.</p> "> Figure 7
<p>The distribution of average concentration of REE in China, the U.S., Europe, others, and world coal fly ash.</p> "> Figure 8
<p>The distribution of total coal combustion production (CCP) used by category.</p> ">
Versions Notes

Abstract

:
Coal fly ash represents a potential resource of some critical elements, including rare earth elements (REEs), which are retained and concentrated during coal combustion. Understanding the distribution and modes of occurrence of REEs within fly ash is vital to developing effective recovery methods and enhancing their economic value. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was applied to investigate the in situ elemental constituents of coal fly ash phases, including aluminosilicates, Ca-(Fe)-enriched aluminosilicates, Fe-oxides, and SiO2/Quartz, in order to explore the distribution of REEs in combustion products. LA-ICP-MS results show that V, Cr, and Nb are mainly enriched in Ca-Ti-enriched aluminosilicates with trace element concentrations referenced to the original fly ash composition. Lithium is primarily enriched in SiO2 glassy grains, followed by Ca, (Fe)-enriched aluminosilicates. Co, Ni, and Cu present a concomitant distribution in the Fe-enriched phases, such as Fe-oxides and Fe-enriched aluminosilicates. The chondrite normalized REE distribution patterns show characteristics of LREE enrichment and Eu-negative anomalies in most phases, while the REE patterns of SiO2 glassy grains have a distinct positive anomaly in Sm, Gd, and Dy, coupled with a deficiency in LREEs. Compared to feed coal, elements such as Li, V, Cr, Co, Ni, and Nb and REEs are enriched 2~10 times in various phases of fly ash, with REEs notably concentrated six times higher in aluminosilicates and Ca-Ti-enriched aluminosilicates than the original coal. This study further discusses the feasibility, calibration principles, and advantages of using LA-ICP-MS to determine REE distribution, as well as the economic implications of REE extraction from coal fly ash.

1. Introduction

The average concentration of rare earth elements (REEs) in Chinese coal is approximately twice that of global coal reserves. During coal combustion, REEs can be further enriched by 3~10 times, which provides the possibility of the recovery of REEs. At present, many countries, including the United States (through the NETL and the Department of Energy of U.S.), have carried out investigations into the extraction of REEs from coal fly ash and have made many breakthroughs, making the cost of extracting rare earth elements from coal ash meet industrial standards. According to the Ministry of Natural Resources (MNR) [1], China’s total primary coal production was 4.71 billion tons in 2023, a 3.4% increase from the previous year. Despite the fact that new energy resources are developing rapidly, coal is expected to remain a significant component of China’s energy mix for the foreseeable future. During the coal combustion process, lithophilic trace elements including REEs can be concentrated in coal fly ash [2,3,4,5,6,7,8,9,10,11]. It is estimated that 580 million tons of fly ashes are produced annually in China [9]. The tremendous amount of fly ashes could form a significant source of some metallic and critical elements, such as the reported Al, Ga Li, and REEs [2,12,13,14,15,16,17].
The distribution and modes of occurrence of critical elements in coal fly ash have been widely investigated [18,19]. Methods for determining the modes of occurrence of rare earth elements in coal fly ash can be divided to three parts: bulk elemental analysis, speciation analysis, and direct analysis. The bulk elemental analysis belongs to the category of indirect analysis, including X-ray fluorescence (XRF), Instrumental Neutron Activation Analysis (INAA), inductively coupled plasma-Mass Spectrometry (ICP-MS), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Glow Discharge Mass Spectrometry (GD-MS), and others. Speciation analysis includes float-sink analysis, sequential chemical leaching, and statistical methods and also cannot obtain the modes of occurrence of REE in coal fly ash directly. In recent years, direct in situ elemental characterization methods with various spatial solution have been used to determine the distribution of REY in coal fly ash, including instrumental microanalysis techniques (LA-ICP-MS and SIMS), electron microscopy (SEM-EDS, FIB-SEM, TEM, and EELS), and synchrotron-based techniques (Bulk XANES, UXRF, and uXANES). Concentrations of trace elements of coal fly ash particles are less than 0.1%, so the frequently used elemental analysis methods such as SEM-EDS, WDS, and XRF are not available for these micron-scale particles with level concentrations of n × mg/kg shown in Figure 1. Nowadays, the two in situ trace elemental analysis techniques, Secondary Ion Mass Spectrometry (SIMS) and LA-ICP-MS, are theoretically applicable to the analysis of the in situ elemental composition determination, with their detection limits and scope of application [20] (Figure 1). SIMS analysis had been used to determine the REE contents of coal fly ash phases using a SHRIMP-RG ion microprobe at Stanford University [20]. Kolker suggested that both SIMS and LA-ICP-MS can quantitatively analyze in situ trace elements in coal ash, but SIMS has a lower detection limit than LA-ICP-MS (SIMS > 10 ppb; LA-ICP-MS > 100 ppb) with a smaller applicable radius (SIMS > 1 μm, LA-ICP-MS > 15 μm) [20,21,22,23].
In this study, LA-ICP-MS was selected to determine the content and distribution of trace elements including REEs in different phases of coal fly ash based on the following two premises. (a) The radius of coal ash particles to be tested must be larger than 16 μm to meet the detection limit of LA-ICP-MS. (b) A suitable calibration element must be present in all the particles being analyzed. Compared with SIMS technology, LA-ICP-MS is characterized by a simple sample preparation process, less interference, low blank levels, high sensitivity, and inexpensive cost, if the two premises are satisfied.
This study is aimed to (a) investigate the in situ composition of major and trace elements in coal ash, (b) evaluate the feasibility and advantages of using LA-ICP-MS for trace elements at the single-particle level, (c) determine the distribution of critical elements and REEs in each phase of coal fly ash, and (d) explore the economic significance of extracting REEs from coal combustion by-products.

2. Sample and Methods

The feed coal and coal combustion products including coal fly ash and bottom ash for this study were sampled from the Dalat Banner Power Plant in Baotou City, Inner Mongolia, northern Ordos Basin, the thermal power plant with the largest designed total installed capacity in Asia. The feed coal in the Power Plant was collected from various Jurassic coal mines in the northern margin of the Ordos Basin. The coal, coal fly ash, and bottom ash were first made into a thin section with a thickness of 100 μm. Mineralogy observation and in situ major-oxides and trace elements were determined using optical observation, EPMA, and LA-ICP-MS

2.1. Optical Observation

The internal calibration and external calibration methods were used for the LA-ICP-MS determination. Prior to LA-ICP-MS analysis, the major element compositions of the particles to be detected were analyzed by EPMA. The EPMA thin sections of coal ash were first stuck with resin adhesive and then polished into a thickness greater than 60 μm.

2.2. EPMA

EPMA was performed with an EPMA-1720 electron probe micro-analyzer under operating conditions of accelerating voltage of 15 kV and beam current of 20 nA, which is the most stable experiment condition for EPMA in our ∂. The excitation area of an electron beam is about 1 × 1 μm2 with a detection limit of 0.01%. The detected elements contain Na, Mg, Al, Si, K, Ca, Ti, and Fe (see results in Table 1). Carbon coating treatment was carried out prior to the EPMA in order to increase the conductivity. Additionally, the existence of carbon in the carbon coating may have an influence on the calibration, so the surface carbon coating was wiped off with absolute ethanol before LA-ICP-MS. EPMA was carried out at the Electron Probe Laboratory, Institute of Earth Sciences, China University of Geosciences (Beijing, China).

2.3. LA-ICP-MS

LA-ICP-MS determination was carried with an LSPC-193-SS Type laser ablation system (EAST LASER Corporation) and PQ-MS Elite mass spectrometer (Jena Corporation, Germany). Si and Al are the major constituent elements in coal fly ash. However, in different single fly ash particles, the chemical composition is totally different. Considering the major oxides of coal fly ash, we used external calibration to determine the trace elements compositions during LA-ICP-MS, following the standard of silicate minerals. Internal calibration is not suitable for the process because of the various changes in the chemical composition of coal fly ash. The standard glass NIST610 synthesized by the National Institute of Science and Technology (NIST) of the United States was chosen as the external calibration, and the element Si was used as the internal standard. Almost all particles to be tested (aluminosilicates, Ca, (Fe)-enriched aluminosilicates, SiO2, and Fe-oxides,) contain the element Si [25,26,27], which satisfies the condition with Si as the internal calibration and NIST610 as the external calibration to determine the trace elements. NIST612 and NIST614 were selected as blind samples for monitoring element content results. The recommended values of the NIST series of reference materials were obtained from the GeoReM database (http://georem.mpch-mainz.gwdg.de/). Table 2 shows the specific operating conditions of LA-ICP-MS.
In the online scan mode, LA-ICP-MS began in the state of good La and Th signals with an Th/U signal ratio of about 1 and the oxide yield of ThO+/Th+ less than 0.3%. The isotopes of trace elements were selected as follows: 7Li, 9Be, 11B, 45Sc, 49Ti, 51V, 53Cr, 55Mn, 57Fe, 59Co, 60Ni, 65Cu, 66Zn, 71Ga, 85Rb, 88Sr, 89Y, 90Zr, 93Nb, 95Mo, 115In, 118Sn, 133Cs, 137Ba, 139La, 140Ce, 141Pr, 143Nd, 147Sm, 151Eu, 155Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 173Yb, 175Lu, 179Hf, 181Ta, 182W, 208Pb, 209Bi, 232Th, and 238U. During the test, each spot included 10 s of flushing of the sample tank, 20 s of background acquisition, and 50 s of ablation sampling. Data analysis was calculated by Agilent 7500a off-line workstation in off-line mode. Table 3 and Table 4 show the contents of trace elements and REE from the LA-ICP-MS analysis. LA-ICP-MS was carried out at the Elemental Geochemistry Lab of Institute of Earth Sciences, China University of Geosciences Beijing (CUGB).

2.4. ICP-MS

Trace element data of feed coal, coal fly ash, bottom ash, and magnetic materials collected from fly ash were determined using ICP-MS analysis as the comparative groups for LA-ICP-MS results. The magnetic materials were extracted from coal fly ash with a magnet in absolute ethyl alcohol. A total of 44 trace elements containing REEs in samples were determined with the ThermoFisher ICP-MS (X Series II) produced by Thermo Science, Germany, as Table 3 and Table 4 show. The specific ICP-MS determination procedure followed the elemental analysis methods for coal and ash samples by Dai et al. with ASTM C618-15 [28,29]. ICP-MS analysis was carried out at the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing, China).

3. Results

3.1. Mineralogy in Coal Fly Ash

In this study, the classification of fly ash constituents follows the method outlined with [20], using an Al/Si of 0.5 as the threshold value of Al-enriched aluminosilicates (Al-Si) and Si-enriched aluminosilicates (Si-Al). Based on microscope observations and EPMA, particles in coal fly ash can be divided into four phases: aluminosilicates (Al-Si, with the ratio Al/Si of >0.5; Si-Al, with Al/Si < 0.5), Fe-oxides (Fe/Si > 1), and SiO2/Quartz (Al/Si < 0.1), as shown in Figure 2 with laser ablation pits of LA-ICP-MS. Aluminosilicates, the main constituent in coal ash, consist of Al-Si, Si-Al, Ca-enriched aluminosilicates (Al-Si-Ca), Fe-enriched aluminosilicates, and minor amounts of Ca- and Ti-enriched aluminosilicates (Al-Si-Ca-Ti). Under reflected light, aluminosilicates occurred in semi-transparent glassy particles, of which the surface and inside contains some small grain-sized particles. Fe-oxides and Fe-enriched aluminosilicates are characterized by white spherical particles and regular textures on the surface of the particles, and the values of Fe/Si changes between particles are various. SiO2/Quartz phases are present in two forms: (a) transparent glassy grains; (b) white, non-transparent particles, with characteristics of a certain degree of roundness and sphericity, similar to the quartz in coal. Other fine-grained particles and rarely developed phases need to be determined by EPMA in specific examples.

3.2. Major Element Composition

Aluminosilicates are primarily composed of Al and Si, and a small amount of Ca, Fe, and Mg was determined by EPMA (excluding C and O elements). The weight percentage (wt%) of Al ranges from 10.69% to 34.38%, with an average of 24.92%. The content of Si (wt%) is distributed from 40.66% to 64.37%, with an average value of 49.83%. The Fe-enriched phases (i.e., Fe-enriched aluminosilicates and Fe-oxides) are mainly composed of Al, Si, Fe, and Ca, with a small amount of Ti, but the contents of Al, Si, Ca, and Fe are distributed in variations, especially for Fe (wt%), ranging from 6.37% to 35.79%. The major elements of the SiO2/quartz phase are dominated by Si, followed by Al and a small amount of Ca and Ti.

3.3. Trace Elements Concentration

A total of 34 trace elements, including REEs and Y, were detected in the coal fly ash using LA-ICP-MS as shown in Table 2 and Table 3. Trace elements were distributed in variations among different phases of fly ash. Figure 3 illustrates the in situ distribution of trace elements of aluminosilicates, Fe-oxides, and SiO2 in coal fly ash compared to the ICP-MS test results of bulk coal fly ash. Lithium is the only enriched trace element for the pure aluminosilicates phase, while significant concentrations of Li are also found in the SiO2/quartz phase. Most trace elements represent a similar distribution in the two Fe-enriched phases: Fe-oxides and Fe-enriched aluminosilicates. Cobalt, Ni, and Cu co-exist in the Fe-enriched phases as well as in SiO2/quartz and Ca-enriched aluminosilicates. The majority of the remaining enriched elements (V, Cr, Sr, Nb, Cs, Ba, Th, and U) occur in the Al-Fe-enriched aluminosilicates. Other enriched elements such as Zn, Zr, Hf, Ta, and Pb, are mainly found in the Al-Fe-enriched aluminosilicates, with major phases shown in Figure 3, indicating that they are concentrated in the other un-identified or small-sized particles (such as heavy minerals) in the fly ashes. Among the Fe-oxides and Fe-enriched aluminosilicates, Cr, Zn, and Ba showed distinct concentrations. The concentrations of Co, Ni, and Cu in these two Fe-rich particles are much higher than that of others.

3.4. Distribution Mode of Rare Earth Elements and Y

The total contents of REE and Y (∑REY), the ratio of LREE/HREE, and the chondrite normalized REE patterns were used to explore the in situ REE compositions of the four phases in coal fly ash. The ∑REY of aluminosilicates ranges from 60.99 to 1742.81 mg/kg (711.87 mg/kg on average). The average LREE/HREE of aluminosilicates is 91.08, showing strong differentiation. The ∑REY in Ca, (Fe)-enriched aluminosilicates is distributed from 245.79 to 611.59 mg/kg (377.96 mg/kg on average), with an LREE/HREE of 16.22. The ∑REY in Fe-oxide particles occurs in a similar distribution, ranging from 274.47 to 301.93 mg/kg. Some trace elements are enriched in the SiO2 phase, but REEs are rarely to be detected inside SiO2. The contents of ∑REY in both SiO2 glassy grains and quartz particles are low, less than 100 mg/kg, with an average LREE/HREE of 6.32. Figure 4 shows the distribution of REE in the four phases: aluminosilicates, Ca, (Fe)-enriched aluminosilicates, Fe-oxides, and SiO2/quartz. The distribution pattern of most phases are characteristic of LREE-enriched and Eu-negative anomalies, but that of SiO2 features a distinct positive anomaly in Sm, Gd, and Dy. Although the distribution pattern of most phases shows an LREE-type enrichment, the La and Ce of aluminosilicates are apparently higher than those of others.

4. Discussion

4.1. Feasibility

The prerequisite for using LA-ICP-MS to determine the content of trace elements in coal ash is to ascertain that the method is feasible, reliable, and accurate. The determination of trace element compositions of minerals by LA-ICP-MS needs to meet two essential conditions. (a) The sample should contain an internal calibration element with a mass fraction similar to that of the external calibration to correct the sensitivity drift of the instrument, the matrix effect, and the difference in the amount of ablation between the sample and the external calibration [30,31], and (b) the diameter of the LA-ICP-MS laser beam must be smaller than the particle diameter. As we elaborated in Section 2, Si was selected as the internal calibration element for fly ashes. NIST610, a kind of synthetic glass, was selected as the external standard sample due to the lack of standard materials for trace elements in coal ash that can be widely used in LA-ICP-MS analysis. Matrix effect, elemental fractionation (isotope effect), and experimental theoretical error exist, but using Si with the same mass fraction as the internal calibration element can minimize the influence of the matrix effect. The elemental fractionation effect can be minimized by a small wavelength excimer laser (193 nm in this research), a larger laser ablation beam spot, and a lower laser ablation frequency (10 Hz), but cannot be avoided given the limit of detection of a laser ablation beam spot of 32 μm.

4.2. Calibration Method

Determining the mass fraction of Si in minerals can be achieved via two methods. (a) Using the mass fraction of the standard chemical formula of minerals. (b) Prior to LA-ICP-MS, using EPMA to measure the mass fraction of Si, and then calculating with a standard calibration sample [31,32,33]. Method (a) is not available for most fly ash particles due to the complex chemical compositions of each particle. The SiO2 glassy particles in fly ashes are the special case theoretically applicable to Method (a), where we can assume that the SiO2 glassy particles are composed of 100% SiO2 and determine the trace elements in samples with those in Nist610 by the relative calibration value of measured Si (%)/100% Si. However, the determination of trace elements in SiO2 phases with Method (a) is not advisable for the following reasons. (i) The contents or percentages of other major elements in SiO2 phases are usually higher than 5%. The correction value of 3% for all the major elements is reasonable as their contents are higher than 0.1%, but is not available for most trace elements as the concentrations are less than 100 mg/kg. (ii) It is difficult to identify some SiO2 phases with aluminosilicates without energy spectrum results. Therefore, we select internal and external calibration methods (i.e., Method b) to determine the contents of trace elements in fly ash.

4.3. Advantages

With the limit of detection shown in Figure 1, SIMS and LA-ICP-MS are probably the only two viable methods to obtain the in situ concentrations of trace elements including REE. The determination results of LA-IC-MS in this research are compared to the results of SIMS to verify the feasibility and accuracy. The data of SIMS analysis (SHRIMP-RG ion microprobe) are collected from the supplementary file of [21], and the original sample data can be obtained from [8,34,35,36], as shown in Figure 5. Figure 5A shows that the REE of fly ashes determined by ICP-MS analysis are various in their contents but show similar trends in distribution patterns, characteristics of LREE enrichment, and Eu-negative anomalies. The in situ REE distribution of the aluminosilicates, Ca, (Fe)-enriched aluminosilicates, and Fe-oxides determined by LA-IC-MS and SIMS analysis have inherited the features of LREE enrichment and Eu-negative anomalies of the fly ash. The La and Ce of the aluminosilicates determined by LA-ICP-MS are much higher than those analyzed by SIMS experiments, which may be caused by the difference of feed coals. The distinguishing place of LA-IC-MS and SIMS analysis is located on the detection range of Ce, Ho, and Tm. In theory, the detection limit of analysis is much lower than that of the LA-IC-MS analysis. However, the concentrations of the three above REE elements are lower than the detection limit for SIMS analysis but had been determined by LA-ICP-MS analysis with a smooth connection with the adjacent elements. We proposed two explanations for this empty detection. (a) The concentrations of Ce, Ho, and Tm in the detected phases of our samples are higher than the detection limit of LA-ICP-MS analysis and the concentrations of Ce, Ho, and Tm in Figure 5B–D are lower than the detection limit of SIMS analysis. (b) The calibration standard of LA-ICP-MS simulated a value that was originally below the detection limit. Most of the concentrations of REE are larger than 0.1 mg/kg (the lower limit), and a small part of Ho (lower than 0.1 mg/kg) is calculated by the internal standard, so both of the explanations account for the smooth and reasonable distribution patterns of REY.
Compared with the solution-ICP-MS analysis for the average mass fraction of the dissolved samples, LA-ICP-MS can be used to analyze the in situ compositions of trace elements of most phases in fly ashes. Combined with EPMA, the chemical compositions including major elements and trace elements of a single particle are obtained as well as the REE distribution pattern. The LA-ICP-MS in situ analysis is limited by spatial resolution, with a minimum laser beam radius of 16 μm in the LSPC-193-SS at China University of Geosciences Beijing. The radii of coal ash particles vary in this study, ranging from 1 μm to 200 μm based on microscope observation, so those particles with a diameter greater less than 20 μm (4 μm in reserve) are not suitable for LA-ICP-MS analysis. The floor level of 20 μm limits some fine-sized particles like zircon but is permissible for most particles.

4.4. Implications for REY Recovery

REEs are widely used in synthetic materials for aerospace, modern electronic communications, and sophisticated machinery. China is the world’s largest coal producer and the world’s largest producer of REEs, controlling about 89% of the world’s REE production [1]. REE in coal combustion products has been regarded as a potential source [2,11,17,18]. Extracting REEs from fly ash has two obvious advantages compared with traditional REE-enriched ore mining. (a) Fly ash is an easy-to-obtain coal combustion product that can be directly recycled and reused [35,36]. (b) Fly ash is a kind of powder, which makes it an ideal raw material for chemical processing with low processing cost [37]. Therefore, it is necessary to ascertain the distribution and occurrence of REEs in coal fly ashes.
The proportions of LREY, MREY, and HREY of feed coal and coal combustion products such as coal fly ash and coal bottom ash are discussed to explore the distribution and occurrence of REY during the coal combustion process, as Figure 6A shows. The content of REY in fly ash is 383.48 mg/kg, six times higher than that of feed coal (63.20 mg/kg) and slightly lower than that of the world value of 404 mg/kg in coal ash [2]. The content of REY in bottom ash (340.97 mg/kg) is five times more enriched than feed coal, slightly lower than that of fly ash. The distribution of REY in magnetic materials is similar to that of fly ash, with REY content of 386.88 mg/kg. The partition ratio of LREY, MREY, and HREY in feed coal, fly ash, magnetic materials, and bottom ash corresponds to 1:5.8:18.5, 1:4.1:20.7, 1: 5.7:22.9, and 1:5.8:19.0, respectively. The partitions of LREY, MREY, and HREY are approximate between feed coal and bottom ash. The redistribution of REY occurs in the combustion process from coal to fly ash, where the proportion of LREY increases significantly and the proportion of HREY slightly decreases. Among the magnetic materials, MREY and HREY are more concentrated in fly ashes.
From the view of micro-phases in coal fly ashes, the content and proportions of REY in LREY, MREY, and HREY in different phases are different, as Figure 6B shows. With the REY of coal fly ash as a basic reference, REY are concentrated in Si-enriched aluminosilicates and Ca, (Fe)-enriched aluminosilicates. The LREY of fly ash tend to be enriched in aluminosilicates, followed by Ca, (Fe)-enriched aluminosilicates and Fe-oxides. The contents of MREY and HREY are preferentially enriched in Ca-Ti-enriched aluminosilicates, and a certain amount of MREY can be enriched in Fe-enriched phases (i.e., Fe-oxides and Fe-enriched aluminosilicates), even though the content of MREY is much lower than that of LREY. LREY, MREY, and HREY in the SiO2/quartz phase are far lower than the average content of coal ash. The strong LREE enrichment of Si-Al and Si-Al particles are considered to be affected by the feed coal that is the distribution and source of REE in the Jurassic coal in the North Ordos Basin. Dai et al. reported that the Jurassic coal features LREE enrichment as well. La, Ce-rich minerals such as monazite are the main source of the La and Ce in the coals and often occur with clay minerals. During coal combustion, these La, Ce minerals are expected to become a part of Al-Si or Si-Al particles with clay minerals. This is the reason why Al-Si or Si-Al particles in the fly ash featured LREE enrichment.

4.5. Implications for Economic Significance

In order to reduce the pollution of soil, water sources, and air caused by the random stacking of fly ash, fly ash has been widely used in various fields, such as building materials, soil improvement, mine backfilling, and water or waste gas purification. Numerous studies have suggested using fly ash as a raw material to produce value-added products (i.e., high-value utilization), such as molecular sieves and the recovery of critical elements (Al, Ga, REEs, and Li) [37,38,39]. In China, the utilization rate of fly ash is about 70%, but the high-value utilization rate of fly ash is only about 5%. Besides, the accumulation of unused fly ash continues to increase, with up to 3 billion tons of fly ash in China remaining unused, resulting in environmental pollution and resource waste. From the perspective of utilizing fly ash in China, extracting REEs from fly ash has three points of economic significance [40].
Firstly, it will greatly improve the high-value utilization of fly ash and make up for the situation of insufficient supply of traditional strategic resources. Figure 7 shows the distribution and average concentration of REEs in coal fly ash in China, the U.S., Europe, others, and worldwide. The concentration of LERY and total REY in coal fly ash in China (371.5 ppm) and the U.S. (340.8 ppm) are very close to that in worldwide coal (331.3 ppm), while the MREY concentration in China (86.5 ppm) and the U.S. (100.5 ppm) are much higher than that in worldwide coal (29.2 ppm).
Secondly, by increasing the utilization rate of fly ash, the economic losses caused by environmental pollution during fly ash stacking can be reduced, including the pollution control fees for air and soil caused by strong winds and the pollution control fees for soil and groundwater caused by heavy metals (Hg, As, Se, Pb, Cr, Cd, etc.) due to infiltration [39]. Based on the data of the U.S. Energy Information Administration, Figure 8 shows the coal combustion product production in the U.S. by category. Among them, 3/4 of the CCP were used in the concrete related products and blended cements [41].
Lastly, it would provide an opportunity for reducing land occupation and resource waste caused by stacking, while also reducing the cost of landfilling and storing fly ash. Storing one ton of fly ash requires a processing fee of approximately USD 2–3. According to China’s current reserves of about 3 billion tons of fly ash, the total processing cost is approximately USD 7~9 billion. The usage percentage of coal in electric power could be reduced, but coal consumption will not decrease in coming years, particularly in China. The tremendous coal combustion production and the proper extraction method of REEs from coal fly ash address the issue of insufficient supply of traditional strategic resources.
However, there exists some technological and economic barriers in increasing the recovery of REEs from fly ash as well as balancing economic costs and environmental benefits to achieve industrial extraction [42,43]. Even for HREY-rich coal, although coal combustion can further enrich REY and other elements in the ash, the total amount of various key metals including REY in the coal ash is low. REY in coal ash are mostly present in a glassy state (Al-Si and Si-Al), and it is necessary to strengthen the research on the reaction and reconstruction mechanism between REY in coal and other mineral phases during the coal combustion process and realize the efficient extraction of REY from the level of coal combustion processes to the subsequent extraction method. Besides, the current extraction and utilization efficiency is low, and the leaching and separation of REY in coal ash still faces problems such as low efficiency and high reagent consumption [38] while realizing the recovery and extraction of REEs and other key metals. As for China and the U.S., due to the high economic value and application prospect of MREY, more attention should be paid to the evaluation and screening of MREY content in coal fly ash during the process of rare earth recovery. For coal fly ash with high MREY concentration, more efficient techniques need to be developed. Given their similar situations, China and the U.S. could strengthen international cooperation and exchanges during this process, jointly promote the development and application of the recovery of REE, and realize the sharing and mutual benefit of REY resources.
The primary reason for the low high-value utilization rate is that complex process technology limits the extraction of rare earth elements from coal. Not all combustion products of raw coal can be used as raw materials for the extraction of key metals. The cost of the extraction process is also high, including the treatment of harmful elements [44]. There are also transportation and storage costs incurred throughout the entire process. Considering that coal in China is mainly used for thermal power generation, three management strategies were proposed to a better usage of the high-value utilization rate in this study based on the content of rare earth elements or other key metal elements in the raw coal. (1) Feed coal with REY in fly ash >550 ppm that can be industrially utilized as a combustion product is mainly concentrated in Inner Mongolia, Shanxi Province, and Guizhou Province in China. Power plants can be established around coal, and the generated electricity can be directly connected to the national power grid to reduce coal transportation costs, in addition to establishing a chemical plant for extracting REY near power plants with good transportation and power infrastructure to form an industrial cluster effect and increase the added value of coal resources. (2) For coal rich in REY but in which the content of rare earth elements in the combustion products cannot meet current industrial extraction standards, a raw material storage base will be established on a provincial basis for future utilization of this part of coal. This approach not only reduces logistical costs but also provides strong support for the subsequent integration of upstream and downstream resource industry chains. (3) For coal poor in REY, other forms of utilization can be found such as concrete materials like, of which blended cement is the primary choice. In addition, this is only an example of the utilization of REY in coal. Elements such as Li, Ga, Nb, and U are often associated with REY to form coal-hosted polymetallic deposits. The utilization of fly ash formed by these coals requires a more comprehensive development and utilization plan, including investment of funds, the site selection of power plants, the setting of extraction processes and steps, and the selection of waste treatment technologies. In summary, extracting REY from coal fly ash still faces many problems, but from the perspective of technology and energy reserves, this research path is sustainable.

5. Conclusions

The trace elements are distributed in coal fly ash phases with strong differentiation. Compared with the content of trace elements in coal ash, most of the Co, Cu, and Ni are co-enriched in the Fe-enriched phases (i.e., Fe-oxides and Fe-enriched aluminosilicates), followed by the SiO2/quartz phase. Li, V, and Cr are widely distributed in the SiO2 phase as well, but REY are rarely detected inside it. V, Sr, Nb, Ba, Th, and U are concentrated in the Ca-Ti-enriched aluminosilicates. Zn, Zr, Hf, Ta, and Pb are deficient in the detected phases but still are distributed in the fly ashes, indicating that these elements are concentrated in the other un-identified or small-sized particles (such as heavy minerals). The above-mentioned trace elements are difficult be detect in the aluminosilicates, but LREY, especially La and Ce, and MREY are mainly concentrated in aluminosilicates. A small amount of LREY can be enriched in Fe-enriched phases. From feed coal to coal combustion, REE and Y can be enriched in both bottom ash and fly ashes with enrichment coefficients of five and six times, respectively. During the combustion process, LREY tends to be concentrated in fly ashes, whereas MREY and HREY are more enriched in the bottom ashes. The magnetic materials selected from fly ash show a similar LREY/MREY/HREY as bottom ash, but the REY contents are still consistent with the fly ash. LA-ICP-MS has its own advantages for determining the REE distribution in coal fly ash, with in situ quantitative characterization and high spatial resolution. But the work necessary to determine the types of particles and their major element contents is difficult and time-intensive. Fly ash particles with a grain size larger than 50 μm are convenient for characterization using LA-ICP-MS. REY in coal ash are mostly present in a glassy state (Al-Si and Si-Al), so separating the glassy state particles from the coal fly ash may greatly reduce the costs. It is also necessary to strengthen the research on the reaction and reconstruction mechanisms between REY in coal and other mineral phases during the coal combustion process and realize the efficient extraction of REY from the level of the coal combustion process to the subsequent extraction method.

Author Contributions

Conceptualization, S.W. and W.H.; methodology, S.W.; formal analysis, S.W. and W.A.; investigation, S.W.; resources, W.H.; data curation, S.W.; writing—original draft preparation, S.W.; writing—review and editing, W.H. and W.A.; funding acquisition, W.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (No. 42272201).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

We thank Jianxun Wu for his help with sampling. We especially thank Li Su of the Elemental Geochemistry Lab at CUGB for her technological guidance and operation of LA-ICP-MS.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviation

Phases in coal fly ash. Aluminosilicates (Al-Si phase, with the ratio Al/Si of >0.5; Si-Al phase, with Al/Si < 0.5), Fe-oxides phase (Fe/Si > 1), SiO2/Quartz phase (Al/Si < 0.1). Ca-enriched aluminosilicates phase (Al-Si-Ca; with Ca content>%), Fe-enriched aluminosilicates phase (with Fe content > 5%), and minor amounts of Ca- and Ti-enriched aluminosilicates (Al-Si-Ca-Ti with Ca content > 5% and Ti content > 2%). Characterization methods. X-ray fluorescence (XRF); Instrumental Neutron Activation Analysis (INAA); Inductively coupled plasma-Mass Spectrometry (ICP-MS); Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Glow Discharge Mass Spectrometry (GD-MS); Fourier transform infrared reflection (FTIR); Laser Induced Breakdown Spectroscopy (LIBS); Scanning electron microscopy-energy dispersive x-ray analysis (SEM-EDS); Secondary Ion Mass Spectrometry (SIMS). Laser Ablation-ICP-MS (LA-ICP-MS).

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Figure 1. The spatial resolution and detection limit of analytical techniques for the elemental composition (modified from [24]).
Figure 1. The spatial resolution and detection limit of analytical techniques for the elemental composition (modified from [24]).
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Figure 2. The mineralogy and LA-ICP-MS analysis spot of different phases in coal fly ash under reflected light. (A) S1, Al-Si-Fe; (B) S2, Fe-oxide; (C) S3, Si-Al; (D) S4, Al-Si-Ca-Ti; (E) S5, Al-Si; (F) S6, Si-Al; (G) S7, Al-Si-Ca; (H) S8, SiO2; (I) S9, Fe-oxide; (J) S10, Fe-oxide; (K) S11, SiO2; (L) S12, quartz.
Figure 2. The mineralogy and LA-ICP-MS analysis spot of different phases in coal fly ash under reflected light. (A) S1, Al-Si-Fe; (B) S2, Fe-oxide; (C) S3, Si-Al; (D) S4, Al-Si-Ca-Ti; (E) S5, Al-Si; (F) S6, Si-Al; (G) S7, Al-Si-Ca; (H) S8, SiO2; (I) S9, Fe-oxide; (J) S10, Fe-oxide; (K) S11, SiO2; (L) S12, quartz.
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Figure 3. The in situ trace elements of constituents of coal ash.
Figure 3. The in situ trace elements of constituents of coal ash.
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Figure 4. The distribution patterns of REE in the aluminosilicates (A), Ca, (Fe)-enriched aluminosilicates (B), Fe-oxides (C), and SiO2/quartz (D).
Figure 4. The distribution patterns of REE in the aluminosilicates (A), Ca, (Fe)-enriched aluminosilicates (B), Fe-oxides (C), and SiO2/quartz (D).
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Figure 5. The distribution of REY of fly ash (A) measured by ICP-MS and that of aluminosilicates, Ca (B), (Fe)-enriched aluminosilicates (C), and Fe-oxides (D) was determined by LA-ICP-MS.
Figure 5. The distribution of REY of fly ash (A) measured by ICP-MS and that of aluminosilicates, Ca (B), (Fe)-enriched aluminosilicates (C), and Fe-oxides (D) was determined by LA-ICP-MS.
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Figure 6. (A) The contents and proportions of LREY, MERY, and HREY in coal and coal combustion products. (B) The contents of LREY, MERY, and HREY in the fly ash phases. (C) The distribution patterns of REE in feed coal, combustion products, and fly ash phases.
Figure 6. (A) The contents and proportions of LREY, MERY, and HREY in coal and coal combustion products. (B) The contents of LREY, MERY, and HREY in the fly ash phases. (C) The distribution patterns of REE in feed coal, combustion products, and fly ash phases.
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Figure 7. The distribution of average concentration of REE in China, the U.S., Europe, others, and world coal fly ash.
Figure 7. The distribution of average concentration of REE in China, the U.S., Europe, others, and world coal fly ash.
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Figure 8. The distribution of total coal combustion production (CCP) used by category.
Figure 8. The distribution of total coal combustion production (CCP) used by category.
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Table 1. Contents of major elements of coal fly ash phases determined by EPMA analysis.
Table 1. Contents of major elements of coal fly ash phases determined by EPMA analysis.
NumberPhaseONaMgAlSiCaTiFeAl/SiFe/Si
1Fe-enriched Al-Si51.85 1.786.2812.7112.430.496.370.490.50
2Fe-oxides40.60 0.703.4010.332.31 35.790.333.46
3Si-Al60.960.41 1.0827.520.53 0.240.040.01
4Ca-Ti-enriched Al-Si58.76 17.0918.188.432.010.170.940.01
5Al-Si51.41 4.624.535.780.19 4.680.780.81
6Si-Al62.47 0.715.3227.042.420.360.440.200.02
7Ca-enriched Al-Si63.34 0.150.4327.7410.67 0.160.020.01
8SiO239.06 0.3653.300.43 0.670.010.01
9Fe-oxides51.74 0.614.4311.322.560.6823.080.392.04
10Fe-oxides40.57 1.763.694.19 50.000.4813.55
11SiO260.960.41 1.0821.530.18 0.240.040.01
12Quartz58.21 1.050.6931.210.531.643.410.030.16
Table 2. Operation conditions of LA-ICP-MS for determining trace elements of coal fly ashes.
Table 2. Operation conditions of LA-ICP-MS for determining trace elements of coal fly ashes.
ICP-MSLaser Ablation System
ICP-MS TypePQ-MS EliteLaser typeLSPC 193 SS
RF Power1500 WWavelength193 nm
Scan modeE-scanEnergy70 mJ
Cool gas flow (Ar)16.05 L/minFrequency10 Hz
Auxiliary gas flow (Ar)0.95 L/minGas flow (He) 0.8 L/min(He)
Carrier gas flow (Ar)0.93 L/minSpot size32 μm
Table 3. Concentrations of trace elements of coal fly ash phases determined by LA-ICP-MS analysis.
Table 3. Concentrations of trace elements of coal fly ash phases determined by LA-ICP-MS analysis.
NumPhaseLiVCrCoNiCuZnRbSrZrNbCsBaHfTaPbThU
1Fe-enriched Al-Si55.77 53.70 44.24 487.49 433.83 108.03 35.70 4.86 1696.89 84.62 17.25 0.96 301.88 2.10 1.15 8.41 15.40 9.14
2Si-Al47.68 60.33 46.82 8.59 23.26 21.81 9.11 7.82 2015.32 890.82 31.75 1.04 942.07 21.51 2.03 12.44 23.43 6.67
3Ca-Ti-enriched Al-Si35.07 946.10 631.09 86.94 144.71 101.81 108.81 46.16 6162.41 174.29 65.83 5.77 817.24 5.57 2.38 60.22 86.58 46.32
4Al-Si297.20 15.16 14.51 7.55 15.36 28.34 11.92 19.55 306.77 10.34 1.50 1.63 122.15 0.34 0.12 14.79 2.33 0.93
5Si-Al22.12 2.82 6.32 1.61 18.52 8.51 5.90 29.31 349.55 0.60 0.16 0.17 109.88 0.03 0.03 2.46 0.15 0.05
6Ca-enriched Al-Si16.01 68.09 53.49 28.38 36.05 17.85 5.66 3.43 1363.97 61.54 10.16 0.17 301.90 1.66 0.66 1.02 11.74 3.71
7SiO28.34 3.88 8.82 2.18 25.70 7.50 5.56 0.67 0.23 0.61 0.03 0.21 0.92 0.09 0.01 0.08 0.02 0.02
8Fe-oxides39.56 43.87 70.17 339.64 1157.04 48.56 121.78 9.30 1605.27 54.35 7.87 0.79 384.96 1.65 0.61 15.84 22.27 6.29
9SiO2792.42 903.83 2777.77 215.42 2495.56 713.57 458.53 60.19 37.10 61.06 8.82 19.03 76.28 4.69 1.73 31.10 3.83 3.31
10Quartz14.81 5.31 12.26 3.26 36.46 10.47 6.30 1.98 10.66 5.28 6.11 0.31 13.47 0.51 0.33 0.36 0.20 0.41
11Feed coal5.28 20.06 22.16 9.53 10.80 8.98 10.57 13.19 408.37 25.30 2.60 1.27 116.26 0.72 0.33 5.28 3.07 1.15
12Fly ash42.84 133.10 79.35 40.94 48.93 62.94 183.78 31.87 1383.86 469.23 27.01 3.10 634.33 11.41 2.21 83.59 14.12 15.69
13Magnetic materials 35.47 118.52 119.13 59.81 79.86 75.45 217.14 59.57 1232.80 318.40 19.98 4.00 805.15 7.61 1.75 53.72 22.91 12.87
14Bottom cinder39.13 87.85 69.30 32.79 38.36 32.87 28.24 46.45 1629.98 199.33 17.67 3.19 681.49 5.96 2.07 18.49 18.99 6.05
Table 4. Concentrations of REY of coal fly ash phases determined by LA-ICP-MS analysis.
Table 4. Concentrations of REY of coal fly ash phases determined by LA-ICP-MS analysis.
NumPhaseLaCePrNdSmEuGdTbDyHoErTmYbLuY
1Fe-enriched Al-Si51.30106.0010.2839.257.271.445.961.026.331.393.450.483.230.4238.66
2Si-Al637.531038.615.5319.733.580.663.250.513.380.752.270.363.020.5023.13
3Ca-Ti-enriched Al-Si86.63162.8018.8384.3618.964.5421.563.5522.614.9113.581.9213.471.84152.03
4Al-Si93.52187.781.425.511.250.312.130.513.900.962.770.422.680.3628.29
5Si-Al18.4041.210.090.450.110.050.110.020.100.020.050.010.050.010.33
6Ca-enriched Al-Si59.2097.3610.1037.327.271.554.860.643.450.651.690.221.640.2419.60
7SiO20.160.210.040.130.120.040.180.020.170.020.050.010.090.020.09
8Fe-oxides47.6896.2610.5741.869.212.147.981.418.752.054.710.685.060.6462.94
9SiO27.1310.311.3913.3413.223.5021.490.795.641.093.080.554.381.048.28
10Quartz5.3111.910.772.620.770.190.380.040.220.030.100.020.180.031.11
11Feed coal10.4720.112.349.521.780.431.890.251.550.310.950.130.900.139.92
12Fly ash59.02159.2815.2961.8211.992.4812.191.7110.202.005.910.805.380.7634.65
13Magnetic materials 55.38121.1213.2050.4710.082.2710.621.538.721.795.210.734.760.7054.40
14Bottom ash66.62144.7615.3560.8311.182.3311.691.569.001.755.150.714.750.6950.52
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Wang, S.; Huang, W.; Ao, W. The Distribution of Rare Earth Elements in Coal Fly Ash Determined by LA-ICP-MS and Implications for Its Economic Significance. Sustainability 2025, 17, 275. https://doi.org/10.3390/su17010275

AMA Style

Wang S, Huang W, Ao W. The Distribution of Rare Earth Elements in Coal Fly Ash Determined by LA-ICP-MS and Implications for Its Economic Significance. Sustainability. 2025; 17(1):275. https://doi.org/10.3390/su17010275

Chicago/Turabian Style

Wang, Shuliu, Wenhui Huang, and Weihua Ao. 2025. "The Distribution of Rare Earth Elements in Coal Fly Ash Determined by LA-ICP-MS and Implications for Its Economic Significance" Sustainability 17, no. 1: 275. https://doi.org/10.3390/su17010275

APA Style

Wang, S., Huang, W., & Ao, W. (2025). The Distribution of Rare Earth Elements in Coal Fly Ash Determined by LA-ICP-MS and Implications for Its Economic Significance. Sustainability, 17(1), 275. https://doi.org/10.3390/su17010275

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