Differentiation of Rare Earth Elements in Coal Combustion Products from the Handan Power Plant, Hebei Province, China

Abstract

As a potential source of REY (rare earth elements and yttrium), coal and its products have attracted much attention. In this paper, we aimed to study the enrichment and differentiation of rare earth elements in fly ash with different particle sizes and promote the full recovery and utilization of rare earth elements in fly ash. Our objective was to focus on the REY concentration in feed coal and its combustion products from the Handan Power Plant. We particularly focused on the distribution of REY in relation to different particle sizes, as well as on the state of occurrence (affinity), by applying stepwise chemical extraction and performing examinations using inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results show that the REY content is affected by the mineral composition of coal ash, and REY is more easily enriched in slag and fine ash than coarse ash. In general, the REY content decreased with the decrease in particle size in coarse ash, whereas the REY content increased with the decrease in particle size in fine ash. It was found that the concentration distribution of REY in solid combustion products is as follows: light REY (LREY) > medium REY (MREY) > heavy REY (HREY). This indicates that the enrichment of REY in solid coal products decreases with the increase in atomic number. The results also show that the occurrence state of REY in raw coal and coal ash undergoes no obvious change (residue state > organic/sulfide-bound state > iron–manganese-oxide-bound state > carbonate-bound state > exchangeable state).

1. Introduction

The components of REY (rare earth elements and yttrium) are known as “industrial vitamins” because of their indispensable strategic location in high-tech industries, and widespread use in the petrochemical industry, metallurgy, military, new material synthesis, and many other industrial fields [1,2]. It is challenging to supply rare earth elements because REY resources are limited [1]. However, as a potential source of REY, coal and its products have attracted extensive attention.

Coal is the most commonly used energy resource in China, and it is mainly used to generate thermal power. Due to the unique deposition characteristics of adsorption barriers and reduced barriers in coal seams, REY can be enriched in some coal seams and can even reach the level of mineralization. The REY in coal, coal ash, and coal-related sedimentary rocks show varying degrees of enrichment, and the REY concentrations in a lot of coal or coal ash are equal to or even higher than those in conventional types of REY ores [3,4,5,6,7,8]. After coal combustion, REY are enriched in types of ash such as fly ash, bottom ash, slag, and other coal-burning products. If they can be extracted and utilized, fly ash and other coal-burning products can become potential sources of REY [9]. At present, various studies have described the recovery of REY from fly ash using various physical separation methods or acid–base leaching methods, and, in the best case, it was shown that the recovery rate of REY reached more than 70%, demonstrating that the sustainability of fly ash disposal can be improved [10,11,12,13]. The authors of several articles proposed that compared with various conventional sediments, fly ash usually has a higher proportion of REY [1,2]. It has been estimated that the global average REY content in fly ash from hard coal combustion is 445 µg/g [14]. A great deal of research has been carried out regarding the distribution of REY in coal and the combustion products of various types of coal [15,16]. At the end of the 20th century, Ratafia [17] found that REY are largely retained in solid coal-burning products from coal-fired power plants, which have much higher REY content than the REY content in raw coal. In addition, it was found that the distribution pattern of rare earth elements from low to high atomic number shows a trend of high on the left and low on the right [18,19]. Seredin et al. [20] analyzed cases of extreme REY enrichment in coal and coal ash and discussed the possibility of extracting and recycling non-energy by-products from them. Dai et al. [21] and Hower et al. [22] studied fly ash in Junggar (Inner Mongolia, China) and found that the REY content in fly ash increased with the decrease in the size of fly ash particles. Lanzerstorfer [23] found that the content of rare earth elements is related to the particle size of fly ash to a certain extent, and the finer the particle size, the higher the content level of rare earth elements. Zhang et al. [24] and Pan et al. [25] also observed a similar phenomenon. Previous researchers have conducted extensive studies regarding REY in fly ash, and several achievements have been made in this field. This study is an expansion of previous research. Further study regarding the enrichment and differentiation of rare earth elements in fly ash and an in-depth understanding of the enrichment and differentiation of rare earth elements in fly ash with different particle sizes will aid the design of a more targeted extraction and recovery process. The full recovery and utilization of rare earth elements in fly ash should be promoted. Therefore, in this paper, we selected slag and fly ash collected from the Handan Power Plant as the research object to study the distribution, enrichment and migration rules, and occurrence state of REY in coal combustion products. We evaluated the enrichment degree of REY in specific particle size components, which also has particular significance in guiding the separation and enrichment of rare earth elements and promoting the comprehensive utilization of fly ash.

2. Materials and Methods

2.1. Materials

We chose to take samples from Handan Power Plant, located in Handan City, Hebei Province, northern China. The plant contains a pulverized coal combustion boiler, and the temperature can reach 1300 °C. We took samples of raw coal and three different coal solid products (Figure 1). The raw coal used in the power plant is taken from the Ordos coalfield and is from Carboniferous and Permian coal measures. The raw coal is mainly mined from coal-bearing strata in the Taiyuan Formation and Shanxi Formation. We prepared the samples by proportionally blending coal and grinding and mixing it sufficiently in the power plant. In order to make full use of the fly ash in the power plant, we screened it and divided it into coarse ash and fine ash. According to the proximate analysis of the coal, the ash yield was 34%, which represents medium to high ash content; the calorific value was 23.43 mJ/kg; the moisture level was 3.35%; the volatile fraction was 13.75% [26]; and the mean total sulfur content was 0.91%. By analyzing the loss on ignition of the coal combustion products, we concluded that the loss on ignition of coarse ash was 9.7%, and that of fine ash was 15.6% [26]. Loss on ignition represents the incomplete combustion of organic matter in coal ash, including the presence of carbon particles. We analyzed the main elements of the prepared samples using an X-ray Fluorescence Spectrometer (XRF) (ARL9800 XRF, Thermo Fisher Scientific, Waltham, MA, USA). Fly ash is a fine coal product captured via electrostatic dust removal, and slag is a hard glass product with large particles, which is collected at the bottom of the boiler. To prevent the cross-contamination of the samples, after collection, we sealed them in plastic bags and stored them in a cool and dry place.

2.2. Analytical Procedures

The collected samples were dried naturally, and the dried samples were divided into two parts. One part was used for sample testing and analysis, and the other part was sealed and stored for further use. By using different sizes of experimental screens, after being dried, the fly ash samples were screened with 50, 60, 80, 100, 150, 200, 250, 300, 350, 400, 500, and 600 mesh, then divided into two categories: coarse ash samples (50 mesh (270 μm) to 250 mesh (58 μm) and fine ash samples (250 mesh (58 μm) to 600 mesh (23 μm). The mineral composition of coal and coal ash was studied by means of X-ray diffraction (XRD) (Rigaku, Japan), scanning electron microscopy (SEM) (HitachiSU8220, Hitachi, Japan), and energy spectrum analysis (EDS). XRD analysis of the coal combustion products and raw coal samples was performed on a SmartLab 9 kW powder diffractometer. The XRD patterns were recorded over a 2θ interval of 3–70°, with a step size of 0.02° and a scanning speed of 2°/min. Then, MDI jade5.0 and Siroquant software were used for the quantitative mineralogical analysis of the mineral compositions of the samples [27]. Additionally, the samples were preprocessed via microwave digestion. The coal (0.25 g) or ash (0.1 g) samples were weighed and placed in dried Teflon beakers. A mixture of 1.5 mL of HNO₃ + 0.5 mL of HF + 0.5 mL of H₂O₂ (3:1:1) was added into the samples, and microwave digestion was carried out at 180 °C for half an hour; then, we added 0.3 mL of HClO₄ to the beakers, which were kept on a hot plate, and the samples evaporated to colloidal shapes similar to the shape of a bean. The pre-treatment was repeated to obtain a sponge-like mass. The mass was dissolved in a 0.25% (w/w) HNO₃ solution and then filtered and eluted to 50 mL with the same solvent; the determination of REY in samples was carried out using inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific, Waltham, MA, USA). In order to study the mode of occurrence of REY in coal and its combustion products, in this paper, we used step-by-step chemical extraction based on Tessier’s five-step continuous extraction method [28].

3. Results and Discussion

3.1. Characteristics of Coal and Its Products

3.1.1. Physical Characteristics of Coal and Ashes

Fly ash can be divided into three forms: spherical bodies, quasi-ellipsoids, and irregular particles with rough surfaces (Figure 2a,b), with spherical particles dominating. Due to the high combustion temperature in power plants, fly ash is generally solidified in a state of suspension in exhaust gas, and the surface tension makes the particles shrink to a minimum and form a sphere. It has been observed that spherical particles have complex crystal structures on their surfaces and inside, while larger particles have fiber structures in their cavities [29]. We found slag to be the most irregular and porous (Figure 2c), which is consistent with the results obtained by Hower [30].

3.1.2. Mineral Composition

The minerals in raw coal are mainly composed of silicates and carbonates, such as quartz (SiO₂), kaolinite (Al₂Si₂O₅(OH)₄), and calcite (CaCO₃), with a lesser amount of pyrite (FeS₂) (Figure 3). Both raw coal and coal combustion products have quartz diffraction peaks, indicating that raw coal and the coal combustion products contain quartz, and the structural crystal of quartz does not change at high temperatures [31]. Fly ash has two phases: crystalline and amorphous, and the main crystalline phases are quartz, mullite (Al_(1+x)Si_(2−x)O_(5.5−0.5x)), and a small amount of magnetite (Fe₃O₄) and pyrite (FeS₂). The amorphous phase is mainly the glass phase. The compositions of coarse ash and fine ash with different particle sizes are the same, and the main phases are mullite and quartz (Figure 4). As kaolinite is converted to mullite at high temperatures, mullite and quartz are the main minerals in coal ash [32]. The average proportion of quartz in coarse ash is 8.53%, that of mullite is 72.07%, and there is a small amount of clay minerals, magnetite, and pyrite. The main crystalline phase in fine ash is mullite; the average proportion of mullite is 72.3%, and the average proportion of quartz is 4.84%. At the same time, fine ash contains a small amount of hematite, pyrite, and other minerals. Except for when the coarse ash size is 200 mesh and the mullite content is 80.5%, the content of mullite in fine ash (69.8−77.7%) is higher than that of coarse ash (66.6−69.1%) overall, which means that fine ash tends to more fully burned in a boiler with a higher combustion temperature, that is, finer particles have a more uniform combustion temperature than coarser particles [33]. There are unburned carbon particles in the coarse ash (Figure 2d), which may derive from fine powders in the coal or fine particles produced via crushing at the initial stage of combustion, which are not burned out in the furnace, or they may be due to the large size of raw coal particles entering the furnace or the occurrence of relatively low combustion efficiency for a short period of time. At the same time, the characteristics of raw coal and the boiler operating load may also affect the content of unburned carbon in coal ash [30,34]. Residual clay minerals and minerals such as pyrite from combustion products may exist in unburned carbon particles. SEM and BSE images of various minerals and unburned carbon in coal combustion products are shown in Figure 5.

3.2. Content Distribution of REY

3.2.1. REY Content in Raw Coal

The content levels of major elements can be used as an auxiliary means to determine the mineral types in coal and coal ash. The major elements of coal combustion products are shown in

Table 1. The chemical compositions of coal combustion products (%).

Major Elements	RC	SA	CA	FA
SiO2	48.43	51.80	43.63	48.03
Al2O3	38.60	32.11	31.56	34.25
Fe2O3	4.29	8.40	13.30	7.45
K2O	1.28	1.22	1.48	1.43
NaO2	0.62	0.30	1.07	0.79
P2O5	0.73	0.25	0.27	0.55
CaO	2.77	3.26	4.09	3.47
MgO	0.87	0.78	0.75	0.94
TiO2	1.19	1.07	2.10	1.74
MnO	0.04	0.10	0.11	0.06
ZrO2	0.08	0.09	0.28	0.17
SrO	0.10	0.09	0.27	0.21
Other elements	1.00	0.53	1.09	0.91

. The oxides in coal combustion products are mainly SiO₂ and Al₂O₃, followed by Fe₂O₃, CaO, TiO₂, MgO, K₂O, Na₂O, and P₂O₅. The sum of Si and Al exceeds 75%, and the ratio of Si and Al is 1.25~1.61. The content of SiO₂ ranges from 43.63% to 51.80%, and the highest proportion in SA (slag) is 51.8%.

The enrichment coefficient (CC: the ratio between the content of REY in coal and the arithmetic mean of REY in the world coal) is used as the judgment standard for the content level of REY in raw coal [35]. Dai et al. [33] proposed the concentration coefficient of trace elements in coal, and its enrichment characteristics can be divided into deficit (CC < 0.5), normal (0.5 < CC < 2), slight enrichment (2 < CC < 5), enrichment (5 < CC < 10), and highly enriched (10 < CC < 100). The REY content in raw coal in power plants is higher than the average value in the world coal (

Table 2. REY contents in raw samples of the studied power plant (μg/g) (RC—raw coal; WC—world coal; CC—raw coal/world coal; Data of world coal from the references [14,22]).

	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y
RC	37.25	62.68	6.68	23.70	4.24	0.79	3.84	0.62	3.22	18.03
WC	11.00	23.00	3.50	12.00	2.00	0.47	2.70	0.32	2.10	8.40
CC	3.39	2.73	1.91	1.97	2.12	1.69	1.42	1.95	1.53	2.15
	Ho	Er	Tm	Yb	Lu	∑LREY	∑MREY	∑HREY	∑REY
RC	0.70	1.89	0.29	1.80	0.27	134.55	26.50	4.95	166.00
WC	0.54	0.93	0.31	1.00	0.20	51.50	13.99	2.98	68.47
CC	1.30	2.03	0.94	1.80	1.37	2.61	1.89	1.66	6.16

). La, Ce, Sm, Y, and Er are slightly enriched in raw coal, while the contents of other elements are normal. The enrichment degree of light REY (LREY) is more obvious than that of medium REY (MREY) and heavy REY (HREY).

3.2.2. Distribution of REY in Coal Combustion Products

Compared with world coal ash, only the levels of La and Ce are higher in coal combustion products than the values in the world coal ash (

Table 3. REY content in raw coal, slag, coarse ash, and fine ash (μg/g) (WCA—world coal ash mean; C/S—coarse ash/slag; F/S—fine ash/slag).

	RC	SA	CA	FA	WCA	C/S	F/S
La	37.25	83.38	73.41	86.66	69	0.88	1.04
Ce	62.68	159.95	139.71	163.30	130	0.87	1.02
Pr	6.68	17.19	14.65	17.06	20	0.85	0.99
Nd	23.70	60.70	51.42	58.91	67	0.85	0.97
Sm	4.24	11.64	9.75	10.75	13	0.84	0.92
Eu	0.79	1.89	1.58	1.70	2.5	0.83	0.90
Gd	3.84	10.16	8.56	9.64	16	0.84	0.95
Tb	0.62	1.74	1.50	1.73	2.1	0.86	0.99
Dy	3.22	8.60	7.64	9.10	14	0.89	1.06
Y	18.03	49.56	40.45	47.87	51	0.82	0.97
Ho	0.70	1.84	1.57	1.90	4	0.85	1.03
Er	1.89	5.04	4.25	5.15	5.5	0.84	1.02
Tm	0.29	0.80	0.66	0.82	2	0.82	1.03
Yb	1.80	4.97	4.07	4.84	6.2	0.82	0.98
Lu	0.27	0.75	0.61	0.75	1.2	0.81	0.99
LREY	134.55	332.86	288.94	336.68		0.87	1.17
MREY	26.5	71.95	59.73	70.04		0.83	1.17
HYEY	4.95	13.4	11.16	13.46		0.83	1.21
REY	166.00	418.21	359.83	420.17		0.86	1.00
LaN/LuN	1.36	1.11	1.21	1.16

). The REY content in the three coal combustion products is more significant than that in raw coal. The ratio of element content in coal ash to slag reflects the distribution and enrichment of REY in coal, slag, coarse ash, and fine ash [36]. If the normal average distribution is 0.85~1.15, most of the coarse ash/slag is at the normal level, and the fine ash/slag is at the normal level. The REY content in fine ash is higher than that in coarse ash. After coal is burned, the REY content in combustion products increases by one to three times, and REY are slightly more enriched in fine ash. It could be that fine ash particles have a finer particle size and a larger specific surface area, and small particles have a higher affinity for REY, which makes some elements condense on the surface of fine ash, resulting in the REY content in fine ash being higher than that in coarse ash [21].

3.3. Enrichment of REY

Relative Enrichment Index RE

The relative enrichment index (RE) is introduced to characterize the enrichment of rare earth elements:

$$\mathrm{RE}=\frac{\mathrm{Concentrations} \mathrm{of} \mathrm{elements} \mathrm{in} \mathrm{ash} \times \mathrm{Ash} \mathrm{yield} \mathrm{of} \mathrm{coal}}{\mathrm{Concentrations} \mathrm{of} \mathrm{elements} \mathrm{in} \mathrm{coal} \times 100\%}$$

If the RE is closer to 1, the element does not easily volatilize when coal is burned and is often enriched in solid combustion products. The closer the value of RE is to 0, the more elements are volatilized in the combustion process and the less are enriched in the solid coal products. When RE is 0, all elements evaporate in the combustion process. In order to clearly show the enrichment degree of each element in solid combustion products, the evaluation standard of the enrichment degree is specified (

Table 4. Evaluation criteria of enrichment degree of elements in solid combustion products.

RE ≦ 0.1	0.1 < RE < 0.85	0.85 ≦ RE < 1
Rarely remain in solid combustion products.	Partly remain in solid combustion products.	Mainly remain in solid combustion products.

) [37,38]. The relative enrichment index of REY in three kinds of solid coal products ranges from 0.67 to 0.97 (

Table 5. Relative enrichment index of REY in slag, coarse ash, and fine ash.

	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Yb	Lu
SA	0.76	0.87	0.88	0.87	0.93	0.81	0.90	0.95	0.91	0.93	0.89	0.91	0.94	0.94	0.94
CA	0.67	0.76	0.75	0.74	0.78	0.68	0.76	0.82	0.81	0.76	0.76	0.76	0.77	0.77	0.75
FA	0.79	0.89	0.87	0.85	0.86	0.73	0.85	0.94	0.96	0.90	0.92	0.93	0.97	0.92	0.93

), with low volatility, which is in line with the characteristics of the strong stability of REY. The relative enrichment index of REY in the coarse ash ranges from 0.67 to 0.82, all within the range of 0.1 to 0.85, indicating that some elements are retained in the coarse ash and a small amount of them volatilize with the flue gas. In fine ash, except La and Eu, which are more volatile, the relative enrichment indexes of other REY are in the range of 0.85~0.97, indicating that the other REY are mainly retained in solid coal-burning products (0.85 < RE < 1), and both account for 87% of all REY. By comparing the relative enrichment indexes of coarse ash and fine ash, it is found that the relative enrichment index of fine ash is greater than that of coarse ash, indicating that the enrichment degree of elements increases with the decrease in the particle size of coal ash. In solid combustion products, the enrichment degrees of Tb, Dy, and Tm are generally high, while the enrichment degrees of La and Eu are low. The concentration distribution of REY in combustion products follows the rule of LREY > MREY > HREY (Table 3), indicating that the enrichment ability of REY in fine ash increases with the decrease in atomic number.

3.4. Effect of Fly Ash Particle Size on Migration Rules of REY

3.4.1. Content Distribution of REY

Fly ash is an important component of coal-burning products, and its particle size can affect the content distribution and occurrence state of elements [23]. According to the REY content (

Table 6. REY content in fly ash with different particle sizes (μg/g).

Particle Size/Mesh	Element Content
	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Yb	Lu	REY	LaN/LuN
CA 50	86.0	162	17.4	61.7	11.7	1.88	10.1	1.67	8.48	47.3	1.79	4.89	0.78	4.65	0.71	421	1.22
CA 60	83.3	156	16.9	59.9	11.3	1.82	9.96	1.74	8.85	46.9	1.83	4.89	0.75	4.62	0.72	409	1.16
CA 80	82.8	158	17.1	60.1	12.0	2.03	11.3	1.96	9.81	50.3	1.98	5.15	0.81	4.80	0.73	419	1.13
CA 100	82.5	164	17.1	60.9	11.3	1.86	9.97	1.70	8.60	45.3	1.78	4.83	0.78	4.58	0.72	416	1.15
CA 150	68.5	136	14.4	51.2	9.39	1.47	8.13	1.40	6.74	38.0	1.49	4.02	0.63	3.76	0.58	345	1.17
CA 200	67.2	128	13.4	46.8	8.48	1.38	7.40	1.30	6.54	34.6	1.39	3.77	0.58	3.61	0.56	325	1.21
CA 250	65.7	132	14.1	50.3	10.1	1.64	9.61	1.68	8.32	43.0	1.67	4.31	0.67	3.90	0.60	348	1.09
CA > 250	83.6	161	16.9	60.0	11.1	1.76	9.85	1.71	8.59	47.1	1.77	4.81	0.77	4.53	0.70	414	1.19
FA 250	54.2	103	10.6	36.1	6.42	1.04	5.49	0.97	5.09	27.7	1.10	3.05	0.47	2.97	0.43	259	1.25
FA 300	92.2	168	17.9	61.7	11.5	1.80	9.97	1.75	9.19	49.5	1.94	5.31	0.85	5.15	0.76	438	1.22
FA 350	65.9	124	13.0	44.3	8.04	1.28	6.85	1.20	6.31	34.3	1.35	3.71	0.58	3.57	0.55	315	1.20
FA 400	81.7	157	16.6	57.3	10.6	1.72	9.42	1.64	8.45	47.6	1.79	4.80	0.80	4.75	0.71	405	1.15
FA 500	101	187	19.8	68.2	12.6	2.03	11.0	1.96	10.2	56.4	2.20	5.96	0.96	5.79	0.88	486	1.15
FA 600	104	190	20.0	68.9	12.9	2.06	11.1	1.98	10.3	56.1	2.20	6.06	0.97	5.76	0.87	493	1.19
FA > 600	105	197	20.6	71.4	13.0	2.08	11.6	2.04	10.6	57.4	2.28	6.21	1.00	5.93	0.92	507	1.15

), rare earth elements are divided into three groups, and it can be seen that the variation trend of the REY content in each group is similar (Figure 6), revealing that the elements in each group show greater similarity in terms of their properties [39,40]. The REY content in coarse ash decreases with the decrease in particle size when the particle size is larger than 200 mesh, and an inflection appears when the particle size is 200 mesh. The change trend changes and the REY content increases with the decrease in particle size. In general, the REY content decreases with the decrease in particle size in coarse ash. The REY content in fine ash increases with the decrease in particle size overall, the particle size appears to exhibit an abnormal peak at 300 mesh, and in the ash with the smallest particle size (>600 mesh), the REY content is the highest. However, there is an inflection point in the trend at 200 mesh and 300 mesh for coarse ash and fine ash, respectively. According to Table 3, the average content of rare earth elements in fine ash is 420.17 μg/g, which is higher than the average content of rare earth elements in coarse ash (359.83 μg/g). This is because the specific surface area, surface activity, and adsorption capacity of fine particle ash are stronger than those of coarse particle ash, which can affect the distribution and migration of elements in the combustion process [41], indicating that the REY content is affected by the particle size of combustion products.

3.4.2. Distribution Patterns of REE in Fly Ash with Particle Sizes

The geochemical parameters of REE reflect the relative differentiation degree of REE. We used the standardization of chondrite meteorite to study the distribution patterns of REE (Figure 7 and Figure 8). The REE distribution patterns of coarse ash, fine ash, and slag with different particle sizes are consistent with the distribution patterns of raw coal (Figure 7), that is, the REE distribution characteristics of raw coal are preserved after coal combustion, and all of them are wide and slow “V-shaped” curves with a high left and low right and an obvious Eu negative anomaly. The variation range of δEu in coal ash with different particle sizes varies from 0.51 to 0.54, with an average of 0.53, indicating an obvious negative anomaly of Eu. The negative anomaly of Eu is inherited from the source rock [42,43]. (La/Yb)_N varies from 11.12 to 12.41 in different particle sizes, indicating that LREE is enriched and HREE is relatively deficient.

3.4.3. Influence of Occurrence State on Element Differentiation

In this study, we used sequential chemical extraction experiments to study the occurrence of REY in coal and its combustion products (Figure 9). The method was divided into a five-step extraction procedure and was applied to classify chemicals into the following five types [28]: ion-exchange form, acid-soluble form, metal oxide form, organic or sulfide form, and silicate and aluminosilicate form. The leaching solution of every procedure was analyzed similar to the digestion solution with ICP-MS explained previously. The parameter, leaching ratio (R_L) was introduced to characterize the occurrence of REY [44,45].

$${\mathrm{R}}_{\mathrm{L}}=\frac{{\mathrm{C}}_{\mathrm{n}}{\mathrm{V}}_{\mathrm{n}}}{{\mathrm{C}}_0\mathrm{M}} \times 100\%$$

where C₀ is the REY content of the original sample, C_n (n = 1, 2, 3, 4, 5) is the REY content of each step sample, M is the mass (1 g) of the coal or its combustion products, and V_n is the constant volume (50 mL).

REY, LREY, MREY, and HREY have almost the same distribution of occurrence states, the contents of exchangeable-form carbonate and bounded-form carbonate are very small and almost zero, and the proportion of the residue state is dominant. Eskenazy [46] suggested that the stability of organic compounds formed by REY and organic matter increases with the increase in atomic number. Hence, the organic compounds formed by HREY are also more stable. There is no significant difference in the proportion of REY in raw coal and ash (residue state > organic/sulfide-bound state > iron–manganese-oxide-bound state > carbonate-bound state > exchangeable state), while more than 95% of REY in slag are in the residue state. The occurrence characteristics of all REY are the same as those of total REY [47]. REY mainly occur in the residual state in raw coal, coarse ash, and fine ash, followed by the organic/sulfide-bound state, iron and manganese oxide binding state, and the exchangeable state and carbonate binding state, which account for a very low proportion. This is because the minerals in the raw coal of the power plant are mainly aluminosilicate minerals, and in the process of combustion, aluminosilicates will become molten when heated and will cover other occurrences of rare earth, resulting in the silicate and aluminosilicate forming as the main occurrence state of rare earth elements in an absolutely dominant position [48]. The residual state is mainly silicate and aluminosilicate form; therefore, it is inferred that there is a strong correlation between Si, Al, and REY in fly ash, and REY mainly occur in the Si and Al matrix. According to the XRD data, the carrier minerals of Si and Al in coal products are mainly mullite and quartz. Mullite is a secondary mineral produced by burning the clay minerals of aluminosilicate in coal [49], meaning the content of rare earth elements may be closely related to the mullite–quartz phase. The proportion of REY in slag is dominant in the residual state, and the proportion of the residual state in slag is higher than that in raw coal, both of which are above 93%. The proportion of the organic matter/sulfide-bounded form in raw coal, coarse ash, and fine ash is about 10%, which may be related to the residual unburned carbon and sulfate minerals after coal combustion [1,50]. The low bound state of Fe-Mn oxides in combustion products indicates that Fe-Mn oxides have a small adsorption effect on REY during combustion. The REY content in raw coal and all coal products is very small in exchangeable and carbonate-bound states, with a proportion below 0.05%, and the high REY content in the residue state indicates that REY do not easily migrate to the environment [18,51].

4. Conclusions

(1)

The REY content in raw coal fed to the Handan Power Plant is higher than the average content in the world coal, and the concentration distribution of REY in raw coal and solid combustion products follows the order of LREY > MREY > HREY. Compared with coarse ash, REY are more easily enriched in slag and fine ash. There is no difference in the distribution patterns of REE in all the samples with different particle sizes, showing “V-shaped” curves with negative Eu.

(2)

After coal combustion, the levels of REY content are obviously different in coal ash with different particle sizes. In general, the REY content decreases with the decrease in particle size in coarse ash, whereas the REY content increases with the decrease in particle size in fine ash. However, there is an inflection point in the trend at 200 mesh and 300 mesh for coarse ash and fine ash, respectively.

(3)

The occurrence state of REY in raw coal and coal ash does not change to an obvious degree (residue state > organic/sulfide-bound state > iron–manganese-oxide-bound state > carbonate-bound state > exchangeable state). The proportion of REY in the residue state is above 93% in the combustion product slag. The residual state is mainly the aluminosilicate combined state. It is inferred that there is a strong correlation between Si, Al, and REY in fly ash, and REY may occur in mullite, which is the main carrier mineral of Si and Al in coal-burning products.