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. 2024 Oct 30;31(54):63210–63224. doi: 10.1007/s11356-024-34943-x

Effects of alkaline extraction on behavior of rare earth elements in coal ashes

Gwang Mok Kim 1,2, Sangwon Park 1, Junhyun Choi 1, Solmoi Park 3, Jeongyun Kim 1,✉
PMCID: PMC11599359  PMID: 39472372

Abstract

The effects of alkaline extraction on the behavior of rare earth elements in coal ashes were investigated in the present study. Independent variables are the concentration of extractant and particle size of coal ashes. Sodium hydroxide was used as an extractant, and the molarity of the solvents varied from 1.0 to 7.0 M. The coal ashes used here were fly and bottom ashes. Fly and bottom ashes were classified into four samples by particle size, with each categorized at both 45 and 300 µm. The particle size distribution and crystalline characteristics of coal ashes, leaching, and the contents of rare earth elements in coal ashes after alkaline extraction were investigated, and the effects of independent variables on the content of rare earth elements in coal ashes were discussed. The test results showed that the dissolution of amorphous phase in coal ashes mainly occurred when the molarity was not more than 3 M, while dissolution and precipitation such as geopolymerization and the formation of zeolite occurred simultaneously when the molarity was more than 5 M. The dissolution and formation of precipitates such as geopolymer in the present study affected the variation of the rare earth element contents in the ashes. Besides, the ashes being smaller in size was more favorable for an increase in the rare earth elements in coal ashes.

Keywords: Rare earth elements, Alkaline extraction, Coal ashes, Enrichment

Introduction

Rare earth elements (REEs), a group of 17 chemically similar elements, have garnered increasing global significance due to their unique properties that make them indispensable in various technological applications (Ramos et al. 2016). From magnets in renewable energy technologies to catalytic converters in automotive systems, REEs play a pivotal role in shaping modern industries (Ferron and Henry 2015). The growing demand for high-tech products underscores the critical importance of ensuring a stable and efficient supply of REEs.

Byproducts resulting from various industrial processes have emerged as valuable reservoirs of REEs, offering a promising avenue for resource recovery and sustainable practices. These byproducts encompass diverse sources, such as coal combustion residues, mining tailings, and industrial waste streams (Amato et al. 2019; Opare et al. 2021). Coal ash generated from circulating fluidized bed combustion processes is one of the representative byproduct sources containing REEs (Mokoena et al. 2022). The recovery of REEs from coal ashes has emerged as a subject of substantial interest due to its potential economic and environmental advantages (Liu et al. 2023). Diverse methodologies, ranging from physical separation to chemical leaching and hydrometallurgical processes, have been explored to extract and concentrate REEs from coal ash (Erust et al. 2023; Pan et al. 2021, 2020).

Alkaline extraction as a method for the recovery of REEs from byproducts has been attempted (King et al. 2018). The alkaline extraction during the recovery process of REEs from coal ashes is typically subjecting the byproducts consisting of the Si-Al system to alkaline solutions, often using sodium hydroxide (NaOH) or ammonium hydroxide (NH₄OH) (Bennett et al. 2019; King et al. 2018). The alkaline environment facilitates the dissolution of amorphous Si-Al structure and REEs present in the byproducts, forming various precipitates (King et al. 2018). This method in previous studies has been employed in combination with an alkali-acidic leaching process (King et al. 2018; Tang et al. 2019). The method is advantageous due to its high selectivity, efficiency, versatility, and environmentally friendly characteristics (Liu and Chen 2021).

The intricate interplay between alkaline solutions and coal ashes can lead to the formation of specific precipitates such as geopolymer and zeolite phases, thereby influencing the distribution and availability of REEs. Geopolymers, formed through the alkaline activation of aluminosilicate materials like fly ash or slag, exhibit a three-dimensional network structure with high surface area and reactivity (Ariffin et al. 2017). The geopolymer in previous studies effectively immobilized various metal ions (Guo et al. 2017). Similarly, zeolites being crystalline aluminosilicate materials with well-defined channels and cages offer an effective means of encapsulating metal ions (Zheng et al. 2019). During zeolite synthesis, metal ions can be incorporated into the zeolitic framework through ion exchange or direct incorporation into the crystal lattice (Farrusseng andTuel 2016). The porous nature of zeolites provides sites for the physical entrapment of metal ions, offering a protective environment (Farrusseng and Tuel 2016). However, few studies on the effects of these precipitates under alkaline extraction conditions on the recovery of REEs have been conducted.

The effects of alkaline extraction on the behavior of rare earth elements in coal ashes were investigated in the present study. Independent variables are the concentration of extractant and particle size of coal ashes. Sodium hydroxide was used as an extractant, and the molarity of the solvents varied from 1.0 to 7.0 M. The coal ashes used here were fly and bottom ashes. Fly and bottom ashes were classified into four samples by particle size, with each categorized at both 45 and 300 µm. The particle size distribution and crystalline characteristics of coal ashes, leaching, and the contents of rare earth elements in coal ashes after alkaline extraction were investigated, and the effects of independent variables on the content of rare earth elements in coal ashes were discussed.

Research hypothesis and significance

Circulating fluidized bed combustion (CFBC) ash can contain REEs embedded within an aluminosilicate-based frame structure primarily composed of Al and Si components. This framework structure may undergo partial dissolution in an alkaline environment. Such dissolution might lead to a reduction in the quantity of the frame structure, resulting in a relative increase in the concentration of REEs. Additionally, in the conditions, Si and Al components leached from the ashes could form various precipitates, which may facilitate the recovery of some dissolved REEs through encapsulation or electrostatic attraction mechanisms. Consequently, it is hypothesized that alkaline extraction could enhance the concentration of REEs through both the dissolution of the frame structure and the subsequent precipitation process.

The application of alkaline extraction for recovering REEs from CFBC ash is of critical urgency and significance due to its potential to address both environmental and economic challenges. Environmentally, CFBC ashes contain valuable REEs that, if left unrecovered, contribute to hazardous waste and environmental degradation. By utilizing alkaline extraction, we can efficiently extract these REEs, thereby mitigating the environmental impact associated with CFBC ashes disposal and reducing the need for new mining activities. Economically, this method offers a promising solution to enhance resource recovery from existing waste, potentially lowering costs and improving the profitability of REE production. This dual benefit of reducing waste and increasing resource efficiency underscores the pressing need for innovative approaches in REE recovery, making the research both timely and impactful.

Experimental section

Raw materials

The CFBC ashes used in the present study were obtained from the Donghae Bio Power Plant in Korea. Sodium hydroxide (NaOH, Sigma Aldrich, 98%) pellets in an alkali extraction process were used as the extractant in order to dissolve the Si and Al components from the ashes. The ultrapure water (Milli Q, resistivity: 18.2 M Ω·cm) was employed when formulating the alkaline solutions.

Sample preparation and characterization

The CFBC bottom ashes were classified into two samples based on 300 µm in size, while the fly ashes were classified into two samples based on 45 µm in size. For example, the BAA sample represents the bottom ash with a size of more than 300 µm, while the BAB sample represents the ash with a size of lower than 300 µm. Similarly, the FAA sample represents the fly ash with a size of more than 45 µm, while the FAB sample represents the ash with a size of lower than 45 µm. The classification procedure of the CFBC ashes in the present study was as follows: the fly and bottom ashes were crushed in series by a laboratory table-top jaw and stamp mill crushers for required size reduction, and classified ashes were obtained through the standard sieves.

Figure 1 shows the particle size distribution (PSD) curves of the classified ashes. Table 1 shows the chemical composition of classified ashes obtained from XRF. The fly ash mainly consisted of SiO2 and Al2O3. The proportion of the components was approximately 60%. A clear difference between the FAA and FAB samples was not observed. The major components of the bottom ash were also SiO2 and Al2O3, and the chemical composition of the BAA and BAB samples was mostly analogous. The major difference in the chemical composition between the FA and BA samples was the proportion of SiO2. Referring to the ASTM C618, the FAA and FAB samples did not meet the requirement for classification as either C class or F class fly ashes. The SiO2 content in FAA and FAB samples in the present study was less than that in the BAA and BAB samples. The values of oxide contents obtained from XRF are relative. That is, fly ash has a higher CaO content and the value of LOI compared to that of the bottom ash samples. This could affect the lesser content of SiO2 in fly ash samples. In addition, the LOI values of fly ash samples were higher than that of bottom ash samples. In general, the various factors increase the LOI of fly ashes. For example, relatively lower combustion temperature and incomplete combustion could induce the higher LOI of fly ash samples. Table 2 shows the content of rare earth elements in the classified ashes. Yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), and dysprosium (Dy) out of rare earth elements in the present study were selected. The smaller the particle size in the fly ashes was, the higher the content of rare earth elements was. The effects of the particle size on the content of REEs in the BA series were analogous. In previous studies on the characterization of fly ash, fly ashes being smaller particle size were reportedly beneficial to include metal traces, since the smaller particles have a relatively smaller proportion of Si and Al which are the main components of the fly ash (Bogdanović et al. 1995; Campbell et al. 1978). In addition, relatively smaller particles might remain suspended in the flue gas longer, allowing more time (Bogdanović et al. 1995; Campbell et al. 1978). This phenomenon is favorable for the enrichment of metal traces such as REEs in the fly ash. The contents of Ce out of the REEs in the FA and BA samples were the highest, while that of Dy in the samples was the lowest. The total content of REEs was in the order of FAB (352.2 mg/kg), BAB (307.2 mg/kg), FAA (250 mg/kg), and BAA (243.3 mg/kg).

Fig. 1.

Fig. 1

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Particle size distribution curves of classified CFBC ashes

Table 1.

Chemical composition of classified ashes (%)

Samples SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 MnO P2O5 lg. loss
FAA 41.98 20.19 4.00 7.57 1.55 1.74 0.22 1.04 0.05 0.18 19.68
FAB 43.25 20.15 5.02 6.84 1.52 1.62 0.27 1.47 0.06 0.38 17.80
BAA 66.07 20.98 4.30 0.87 1.16 2.44 0.29 0.98 0.05 0.11 2.07
BAB 61.48 23.39 5.07 1.93 1.21 2.09 0.30 1.25 0.06 0.16 2.38
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Table 2.

Content of rare earth elements in pristine CFBC ashes (mg/kg)

Sample Y La Ce Pr Nd Dy Total
FAA 35.8 51.9 104 11.1 41.3 5.9 250
FAB 61.1 71.2 142 15.1 53.9 8.9 352.2
BAA 32.3 50.2 101 11.4 42.8 5.6 243.3
BAB 41.9 63.6 130 13.8 51 6.9 307.2
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Test methods

Alkaline extraction tests were conducted to investigate the leaching behavior of Si and Al components from the classified CFBC ashes. The molarity of the NaOH solution varied from 1.0 to 7.0 M. The reaction temperature and stirring speed for each test were fixed at 80 °C and 300 rpm, respectively. The test procedure was as follows. Each classified ash (selected from FAA, FAB, BAA, or BAB) of 25 g was poured into the NaOH solution of 500 ml. That is, the liquid-to-solid ratio was set at 20:1. Each solution for alkaline extraction tests was stirred for 24 h. After stirring, the solutions to investigate the leached Si and Al concentration were extracted from beakers and were filtered by a syringe filter with a pore size of 0.2 µm to remove the ash particles (Dismic-25cs, Avantec). The leached concentration of Si and Al components from the classified ashes was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 8300, Perkin Elmer). The residual ash samples in beakers were filtered by a glass microfiber filter with a pore size of 1.6 µm (Whatman, GF/A) to conduct laser diffraction particle size analysis, inductively coupled plasma mass spectrometry (ICP-MS) and X-ray powder diffraction (XRD) tests. The filtered samples were immersed in dehydrated ethanol and placed in a low vacuum desiccator to prevent unexpected further reactions. The PSD of the filtered samples under wet conditions was measured via a laser diffraction (LD) particle size analyzer detectable from 10 nm to 5.0 mm. Anhydrous ethanol was used as a solvent in the tests. The content of REEs in filtered samples was measured via ICP-MS (Elan DRC II, Perkin Elmer) after conducting an acid digestion method to completely dissolve the samples. The XRD tests using an Empyrean instrument for the filtrated samples were conducted with CuKα radiation at a tube current, and the samples were scanned with a step size of 0.026°. The Rietveld refinement-based quantitative analysis was conducted to quantify the amorphous content and other phases present in the samples by using TiO2 (99% purity) as an external standard. The scale factor, cell parameters, peak shape functions, and preferred orientation were refined to match the experimentally obtained patterns. The Rwp of the quantitative analysis was in the range of 2.4–4.5 for all the samples.

Results and discussion

Particle size distribution of CFBC ashes after alkaline extraction

The PSD curves of the classified CFBC ash samples after alkaline extraction are shown in Fig. 2. The results showed that the PSD curves of all the samples tended to shift toward the left side after alkaline extraction, indicating that the particle size was reduced after the process. This trend was particularly notable in the PSD curve of BAA samples as increasing the molarity of the extractant. The volume density at the peak for the particles ranging from 1.5 to 110 µm increased as the molarity of the extractant increased. The PSD curves of BAB samples showed a similar trend to those of BAA samples. The curves of the BAB samples when the extractant was employed were shifted to the left side compared to that of the pristine BAB sample. The volume density of the particles in the range from 20 to 120 µm was reduced when using an extractant of 1 M and then increased as increasing the molarity. The volume density of the particles in the range from 0.6 to 30 µm in the BAB samples after alkaline extraction was higher compared to the pristine BAB sample.

Fig. 2.

Fig. 2

Fig. 2

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Particle size distribution curves of a FAA, b FAB, c BAA, and d BAB samples after alkaline extraction with concentration of NaOH

The FAA samples exhibited two peaks at 3.0–20 µm and 20–110 µm. It was clearly exhibited that the volume density of the particles ranging from 3.0 to 20 µm in the FAA samples tended to increase when the molarity of the extractant was more than 5 M, while that of the volume density of the particles ranging from 20 to 110 μm was reduced. The FAB samples showed two peaks at 0.2–2 µm and 2–100 µm. The volume density at the peak for the particles ranging from 0.2–2 µm tended to be reduced after alkaline extraction, while the volume density at the peak for the particles ranging from 2 to 100 µm after alkaline extraction tended to increase. The PSD curves of all the samples in Fig. 2 were shifted to the left side after the alkaline extraction process when the molarity of the extractant was not more than 3 M.

It is well known that some components of coal ashes such as Si and Al can be dissolved under alkaline conditions since the bonds in aluminosilicate matrix under a high pH environment could be broken (Wang et al. 2023). The Si and Al components constituting the amorphous phases of coal ashes in previous studies were reportedly dissolved when using a NaOH of more than 2.0 M (Murmu et al. 2023; Nugteren et al. 2001). The dissolved Si and Al under high alkaline conditions can form various precipitates such as geopolymer and zeolites. The geopolymer which are amorphous three-dimensional networks of aluminosilicate structure can be formed owing to the polymerization reaction, and the zeolite being crystalline aluminosilicate phases via the rearrangement of SiO4 and AlO4 tetrahedra into a framework (Król et al. 2017; Längauer et al. 2021; Lee and Lee 2015; Liu et al. 2018; Ma et al. 2023). The aforementioned dissolution and reaction occurring under alkaline conditions could simultaneously affect the PSD of coal ashes, the leached concentration of Si and Al from coal ashes, and the content of the amorphous phase.

Leaching behavior of Si and Al from CFBC ashes induced by alkali activation

The concentrations of Si and Al leached from the classified ashes after alkaline extraction are shown in Fig. 3. For both the FA and BA samples, higher concentrations of Si were leached from the samples with smaller particle sizes (FAB and BAB), especially at 3 M, which can be attributed to a greater reaction surface induced by smaller particle size (Cui et al. 2022). The leached rate of Si from all the samples pronouncedly increased when the molarity of the extractant was between 1 and 3 M. The trend indicated that the dissolution reaction dominantly occurred when the molarity of the extractant was not more than 3 M. The molarity of NaOH promoting the geopolymerization and the formation of zeolites in the previous studies was approximately more than 4 M (Görhan and Kürklü 2014; Hamidi et al. 2016; Rattanasak and Chindaprasirt 2009). Görhan et al. reported that the NaOH of 3 M was not proper to induce the geoplymerization and required a reaction temperature of more than 85 °C (Görhan and Kürklü 2014). For the formation of zeolite, the reaction temperature is generally higher than the temperature used in the present study. The reaction temperature range for the formation of zeolite phases in coal ashes under alkaline conditions was reportedly from 90 to 225 °C (Hui and Chao 2006; Jha and Singh 2014). The BAA samples had the lowest leached concentration of Si among all the samples, while the concentration of Si leached from the BAB samples was comparable to or higher than those from the FA samples. Although the leached Si concentration from the samples generally tended to increase with an increase in the molarity of the extractant, it showed a slight decrease at 7 M of the extractant. The experimental condition used in the present study could induce the occurrence of precipitates such as geopolymer or zeolites when using an extractant of more than 5 M (Görhan and Kürklü 2014, Hamidi et al. 2016). That is, leached Si and Al components from FA samples could be consumed by the formation of the precipitates. In contrast, the leached Si contents from BA samples could not be sufficient for inducing the occurrence of such precipitates.

Fig. 3.

Fig. 3

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Leached concentrations of a Si and b Al components from classified CFBC ashes after alkaline extraction with concentration of NaOH

The leached concentration of Al from the samples was also mainly affected by the particle size. The FAB sample showed the highest leached concentration of Al, while the BAA sample showed the lowest leached concentration of Al. Similar to the Si leaching behavior, an increase in the molarity of the extractant of lower than 5 M in all the samples also tended to increase the leached concentration of Al, while the leached concentration of Al when the molarity of the solution was 7 M tended to be slightly reduced.

Behaviors of rare earth elements in CFBC ashes induced by alkali activation treatments

The REE contents in the classified ashes after alkaline treatment are shown in Fig. 4. For the samples with a bigger particle size (FAA and BAA), an increase in the molarity of the extractant led to a reduction or a lesser increase in the content of REEs in comparison with the FAB and BAB samples. The content of REEs in the FAA sample when treated with the extractant of more than 3 M was clearly reduced. For example, Dy content in the FAA sample was reduced up to approximately 20% compared to that in the pristine FAA sample. The content of REEs in the FAA sample when using the extractant of 3 M was reduced compared to that in the pristine FAA sample, indicating that the dissolution reaction leached the REEs as well as aluminosilicate glassy phases. Liu et al. reported that the REEs in coal fly ashes were present as a form of REE oxides, REE phosphates, apatite, zircon, and REE-bearing glassy phase (Liu et al. 2019). That is, partial REEs were probably distributed in the outer amorphous areas of the FAA sample and the dissolution of the areas reduced the REE contents in the sample when using an extractant of 3 M (Jun et al. 2023; Li et al. 2022). However, further study is needed to investigate the distribution and leaching characteristics of REEs in the sample. The content of REEs in the BAA sample tended to slightly increase due to the alkaline extraction.

Fig. 4.

Fig. 4

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Normalized contents of rare earth elements in a FAA, b FAB, c BAA, and d BAB samples after alkaline extraction with concentration of NaOH

The content of REEs in the FAB and BAB samples clearly increased after alkaline extraction. The content of REEs in the samples tended to increase as the molarity of the extractant increased. Furthermore, the enrichment of the REE content in the BAB sample when the molarity of the extractant was not more than 3 M was higher than that in the BAA samples. This is attributable that leaching of Si and Al occurred well due to the relatively smaller particle size compared to that of the BAA sample, and the amorphous content of the BAB samples was higher than that of the BAA sample. That is, the mineralogical and physical properties of the BAB sample were favorable for inducing dissolution.

The increase in the REEs in the BAB samples was particularly notable. The content of REEs such as Y and Dy increased up to approximately 40% compared to that in the pristine BAB sample.

Crystalline characteristics of CFBC ashes induced by alkali activation treatments

The XRD patterns of FA and BA samples after alkaline extraction are shown in Fig. 5. The pristine FAA sample exhibited peaks corresponding to halloysite (Al2Si2O5(OH)4), laumontite (Ca(AlSi₂O₆)₂·4H₂O), syngenite (K₂Ca(SO₄)₂·H₂O), thernardite (Na₂SO₄), vaterite (CaCO3), calcite (CaCO3), quartz(SiO₂), mullite (3Al₂O₃2SiO₂), maghemite (Fe2O3), hematite (Fe2O3), anhydrite (CaSO₄), and brucite (Mg(OH)₂). The FAA sample after alkaline extraction with 5 M and 7 M solution showed new peaks corresponding to nahcolite (NaHCO3), gaylussite (Na₂Ca(CO₃)₂·5H₂O), and hemicarbonate (Ca4Al2(OH)13(CO3)0.5·5.5H2O), which are attributed to the intercalation of calcium carbonates phase into the gaylussite and hemicarbonate. The formation of hemicarbonate signifies the dissolution of the amorphous phase of the pristine ash resulting in the formation of a new product. The pristine FAB sample showed a mineral composition similar to that of FAA, gaylussite was only present in the sample with 5 M and hemicarbonate was found in the sample with 7 M only. In both FAA and FAB samples, the intensity of syngenite became weaker with an increase in the molarity of the solution, suggesting a higher extent of dissolution of the ash.

Fig. 5.

Fig. 5

Fig. 5

Fig. 5

Fig. 5

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XRD patterns of a FAA, b FAB, c BAA, and d BAB series after alkaline extraction (Q quartz, C calcite, V vaterite, T thernardite, L laumontite, S syngenite, E ettringite, N nahcolite, G gaylusite, B brucite, A anhydrite, Ba bassanite, Ha halloysite, Mu mullite, Mg maghemite, Hc emicarbonate, He hematite)

The XRD patterns of BAA and BAB samples before alkaline extraction revealed that they have a mineral composition similar to that of FAA and FAB, consisting of halloysite, laumontite, syngenite, thernardite, vaterite, calcite, quartz, mullite, and hematite. While the mineralogical change upon alkaline extraction was insignificant in the BAA samples, the BAB samples with 5 M solution showed new peaks due to zeolite such as chabazite Na2Al2Si4O12·6H2O) and natrolite (Na2Al2Si3O10·2H2O).

The amorphous contents of the samples were obtained by quantitative XRD analysis and summarized in Fig. 6. The amorphous contents of the pristine FAA and FAB samples were similar at 63.1 and 60.9%, respectively, and experienced a decrease to 56% when the molarity of the solution was 3 M. Both samples exhibited an increase in the amorphous contents when using an extractant of more than 5 M. In addition, the trend of the results on the PSD, leached concentration of Si and Al, and the content of amorphous phases in the ashes was quite different when using the extractant of more than 5 M. The PSD curves of all the samples when using an extractant of 7 M were shifted to the right side compared to that of those samples when using an extractant of 5 M, indicating that the particle size relatively increased. The leached concentration of Si and Al when using an extractant of 7 M was mostly reduced compared to that when using an extractant of 5 M. The amorphous contents when using an extractant of more than 5 M mostly increased, except for the BAB sample. It can be inferred from the results that the formation of precipitates occurred when using an extractant of more than 5 M. As aforementioned, the geopolymerization in coal ashes has been reportedly promoted when the molarity of the NaOH was more than 4 M (Görhan and Kürklü 2014; Hamidi et al. 2016). The geopolymer is an amorphous phase being aluminosilicate gel charge-balanced by alkali ions such as Na+ (Costa et al. 2021; Provis 2014). That is, the leached Si and Al from coal ashes when using an extractant of more than 5 M was possibly consumed by the geopolymerization, and thereby the contents of amorphous phases increased (Zheng et al. 2019).

Fig. 6.

Fig. 6

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Amorphous phase of a FA and b BA series obtained by quantitative XRD analysis after alkaline extraction

Meanwhile, the amorphous content in the FAA sample was higher compared to that in the FAB sample, although the FAB sample has a relatively higher reactivity due to the higher specific surface area. The formation of crystalline such as zeolite might be reportedly favorable when Si/Al ratio in a solution was relatively higher under the identical temperature and molarity of NaOH conditions (Chen et al. 2019; Arioz et al. 2020). The formation of zeolite generally consumes more Si components compared to the formation of geopolymer (Ma et al. 2021). The Al concentration leached from the FAB sample was clearly higher than those of other samples. That is, the formation of the crystalline phases in the solution with the FAB sample could be promoted since the reactivity of the FAB sample was higher than that of the FAA sample. Consequently, the higher content of amorphous phase in the FAA sample compared to that in the FAB sample was attributable to the formation characteristics of zeolite and geopolymer. The amorphous contents of BAA and BAB samples tended to be reduced when using an extractant of not more than 3 M, while the contents of BA samples tended to increase when using of more than 5 M.

The content of REEs when using an extractant of 7 M mostly increased compared to that when using an extractant of 5 M. It has been reported in previous studies that geopolymer exhibited immobilization of heavy metals due to encapsulation, chemisorption, and adsorption, substitution for Al3+ sites. Among them, the physical encapsulation mainly contributed to the immobilization of heavy metal ions (Ji and Pei 2019; Phair andVan Deventer 2001). Ji and Pei reported that polycondensation reaction consuming reactive Si and Al during geoplymerization process led to the encapsulation of heavy metals via the coating effect of oligomeric gel (Ji and Pei 2019). Zhang et al. reported that geopolymer can immobilize heavy metals via the formation of chemical bonds or the electrostatic attraction on the [AlO4]- sites (Zhang et al. 2022). That is, the geopolymerization could contribute to the increase in the REE contents in coal ashes when using an extractant of 7 M. It should be noted that the amorphous contents in the BAB sample tended to slightly increase when using the extractant of not more than 5 M, while that in the BAB sample with the extractant of 7 M was exceptionally reduced. The reduction in the amorphous content could be induced by the formation of crystallines such as chabazite, halloysite, and kaolinite which was observed in the XRD patterns of the BAB sample with the extractant of 7 M. The formation of chabazite, halloysite, and kaolinite could also affect an increase in the REE content of the BAB sample with 7 M.

Overall, dissolution of the amorphous phase was the dominant reaction when using an extractant of not more than 3 M. The dissolution reaction of the larger samples (FAA and BAA) negatively affected the content of REEs in the samples, indicating that a part of REEs could distributed in the outer amorphous areas of the larger samples. That is, a relatively smaller particle size was more favorable for the enrichment of REEs in coal ashes, while the formation of precipitates was dominant when using an extractant of more than 5 M. The precipitates such as geopolymer possibly contributed to the enrichment of REEs in coal ashes.

Concluding remarks

The effects of alkaline extraction on the enrichment of REEs in the classified CFBC ashes were investigated in the present study. The fly and bottom ashes were classified based on the particle size of 45 µm and 300 µm, respectively. The molarity of the extractant varied from 1.0 to 7.0 M. The particle size distribution, leaching of reactive Si and Al components, and crystalline characteristics of the ashes due to the alkaline extraction were systematically investigated along with the enrichment characteristics of REEs in the ashes after alkaline extraction. The main findings drawn in the present study were as follows.

  1. The PSD curves of the ashes when using an extractant of not more than 3 M were shifted to the left side compared to that of the corresponding pristine curves, indicating that the size of the ash samples was reduced.

  2. The leached concentration of the Si and Al components from the ashes increased when using an extractant of not more than 5 M, while that from the ashes was reduced when using an extractant of 7 M. The amorphous contents of the ashes decreased when using an extractant of not more than 3 M, while that of the ashes mostly increased when using an extractant of more than 5 M.

  3. The content of REEs in the ashes in the FAB and BAB samples tended to increase as the molarity of the extractant increased, while that in the FAA sample was reduced as the molarity of an extractant increased.

  4. The dissolution and formation of precipitates such as geopolymer in the present study simultaneously affected the variation of the REE contents in the ashes. The effect of the dissolution and the formation of precipitates on the REE contents could vary depending on the particle size and the molarity of the extractant.

The results obtained in the present study can contribute to broadening fundamental knowledge of the enrichment of REEs on CFBC ashes via the alkaline extraction process. In terms of industrial applicability, the alkaline extraction used in the present study could be helpful to enrich the REEs in the CFBC ashes. Furthermore, the alkaline extraction reduced the particle size of CFBC ashes, indicating that the specific surface area increased. The increase in the surface area in practice could promote the leaching of REEs from the treated ashes under acidic conditions.

Acknowledgements

This work was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science and ICT of Korea (GP2022-002).

Author contribution

Gwang Mok Kim: conceptualization, data collection, interpretation, manuscript writing. Sang Won Park and Junhyun Choi: analysis, and literature review. Solmoi Park and Jeongyun Kim: supervising, verification, manuscript writing.

Funding

This work was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science and ICT of Korea (GP2022-002).

Data availability

Data are available from the corresponding author upon reasonable request.

Declarations

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

All authors approved of the final version prior to submission.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s Note

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