Modes of Occurrence, Migration, and Evolution Pathways of Lithium and Gallium during Combustion of an Al-Rich Coal, Inner Mongolia, China

Abstract

The modes of occurrence, migration, and evolution pathways of lithium (Li) and gallium (Ga) during combustion of an Al-rich coal from Inner Mongolia, China, were investigated using methods of simulated combustion experiments, the sequential chemical extraction procedure (SCEP), and the thermodynamic equilibrium calculation. Mineralogical and chemical compositions of the feed coal and combustion ash were analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy (SEM). The study reveals that Li and Ga are significantly enriched in the ash after combustion, with the contents reaching up to 1086 μg/g and 133 μg/g, respectively. The primary modes of occurrence of Li and Ga in the ash are quartz and aluminosilicates, and sulfides, respectively. Li, in the form of LiAlSi₄O₁₀ (s), primarily occurs in hematite, glass, and quartz below 800 °C. However, it migrates into the glass phase, mullite, and quartz above 1000 °C. On the other hand, Ga exists as Ga₄S₅ (s) and transforms into Ga₂S (g) as the temperature rises from 800 °C to 1000 °C, maintaining this gaseous form until 1200 °C. Ga₄S₅ (s) predominantly occurs in the glass phase at 600 °C, whereas mullite and quartz become its dominant modes of occurrence in industrial combustion ashes and ashes obtained from simulated combustion above 600 °C.

1. Introduction

Lithium (Li) and gallium (Ga), both of which are critical metals, have been applied in technology-driven economies and strategic emerging industries in recent years [1,2,3,4]. The rapid development of aerospace, new energy and electronics industries has led to a substantial increase in the consumption of Li and Ga [5,6,7]. However, their reserves and production capacity are insufficient, resulting in supply–demand imbalance and even trade chain disruption. To satisfy the demands for critical metals, lots of efforts have been devoted to seeking alternative sources and extracting them from seawater [8], coal fly ash [9], and coal gangue [10]. In contrast with other resources, the extraction of critical metals from coal and particularly fly ash has advantages such as high contents of critical metals and sufficient resources due to the enormous fly ash produced after the combustion of coal [11,12,13,14]. In addition, the emission of fly ash has caused increasing concerns in terms of the economic and environmental impacts [15,16,17,18]. Therefore, it is of resource-conscious, economical and environmentally friendly significance to extract critical metals from coal fly ash.

It is essential to determine how these critical metals occur in fly ash and their migration and evolution pathways during coal combustion in order to develop effective, resource-conscious, and economical and environmentally friendly extraction processes. Several works were carried out to reveal the influences of particle size [19,20], coal compositions [21], and co-combustion with ash components [22] on the distribution of Li and Ga through coal combustion. The enrichment and mode of occurrence of critical metals were also explored during coal combustion [23,24,25,26]. It was found that Li in coal ash mostly occurs in the glass phase of aluminosilicates, while Ga is associated with the aluminosilicate phase [24]. However, the migration and evolution pathways of critical metals during combustion at high temperatures has not been fully understood.

In this study, combustion experiments of Al-rich feed coal from Inner Mongolia, China, at various temperatures and sequential chemical extraction were performed to investigate the mode of occurrence and migration of critical metals like Li and Ga in the coal during combustion. Mineralogical and chemical compositions of feed coal and its combustion products were analyzed by a range of conventional techniques. The thermodynamic equilibrium calculation was also used to aid in simulating the evolution of the critical metals.

2. Materials and Methods

2.1. Materials and Chemicals

The feed coal, coal fly ash (FA), and bottom ash (BA) were collected from the Guohua Power Plant in Jungar Banner, Inner Mongolia, China. Hydroxylamine hydrochloride (NH₂OH·HCl, 99%), ammonium acetate (CH₃COONH₄, 99%), and sodium acetate (CH₃COONa, 99.9%) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Magnesium chloride (MgCl₂, 99.9%), hydrogen peroxide (H₂O₂, 99.9%), and hydrogen nitrate (HNO₃, 65%–68%) were supplied by Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Hydrofluoric acid (HF, 40%) was produced by Sinopharm Chemical Reagent Shanghai Co., Ltd. (Shanghai, China). Acetic acid (CH₃COOH, 99.8%) was obtained from Modern Oriental Technology Development Co., Ltd. (Beijing, China). The aforementioned chemicals were guaranteed reagents and were used in the sequential chemical extraction procedure.

2.2. Methods

2.2.1. Coal Combustion Experiments

The coal samples were treated sequentially by drying, crushing, ball milling, and sieving through a 200-mesh sieve before the experiment. The coal combustion experiments were conducted in an OTF 1200x horizontal tube furnace (Kejing Materials Technology Co., Ltd, Hefei, China) at a range of temperatures from 600 °C to 1200 °C. The detailed experimental process is as follows. Firstly, the alumina boat containing 4 g of feed coal was placed in the constant temperature zone of the furnace. The furnace was then heated at a rate of 10 °C/min to the desired temperature, specifically 600, 800, 1000, and 1200 °C. After that, the feed coal was burned at set temperatures for 2 h in an air stream of 300 mL/min. Finally, the coal ash obtained from combustion at each temperature was collected after the furnace had cooled to room temperature. The combustion experiment was repeated three times at each temperature, and the ash collected from repeated experiments was mixed together for subsequent analysis to ensure precision and accuracy.

2.2.2. Sequential Chemical Extraction

The sequential chemical extraction procedure (SCEP) was employed to analyze the mode of occurrence of Li and Ga in feed coal, FA, BA, and ashes obtained from combustion at different temperatures. Following previous studies [24,26], a five-step procedure (Figure 1) was implemented to determine the relative amounts of the various chemical species of Li and Ga: the ion-exchangeable form, acid soluble form, metal oxides form, sulfide form, and quartz and aluminosilicate form.

Firstly, 1 g of feed coal or coal ash was placed into a conical flask with 15 mL of 1 M MgCl₂. The mixture was then oscillated at 25 °C for 1 h. Afterwards, the mixture was poured into centrifuge tubes. The extract and residual solids were separated by centrifugation at 5000 r/min for 20 min. The obtained supernatant (leaching solution) was used to analyze the ion-exchangeable form of Li and Ga.

In the next two steps, the residual solids obtained in the previous step were added into the 15 mL of 1 M CH₃COONa solution, as well as 15 mL of 0.04 M NH₂OH·HCl in 25% CH₃COOH solution. The mixtures obtained in step 2 and step 3 were oscillated at 25 °C for 5 h and 95 °C for 3 h, respectively. After centrifugation, the supernatants were used for the analysis of acid-soluble and metal oxide forms.

Then, 3 mL of 20% HNO₃ and 8 mL of 30% H₂O₂ were mixed with the residual solids obtained in step 3, and the mixture was oscillated at 85 °C for 5 h. Subsequently, 5 mL of 1 M CH₃COONH₄ in 20% HNO₃ was added to the above mixture. And the mixture was oscillated at 25 °C for 5 h. After centrifugation, the supernatant was employed for the analysis of sulfide form, while the residual solids were used to analyze the quartz and aluminosilicate forms.

Finally, 10 mL of 4% HF was combined with the residual solids obtained in step 4, and the mixture was oscillated at 25 °C for 24 h. Following centrifugation, the supernatant was utilized for the analysis of the glass phase. After the residual solids were dried and digested, they were used for the analysis of mullite and quartz.

2.2.3. Chemical and Mineralogical Analysis

The proximate analysis of feed coal was conducted on a 801 thermogravimetric analyzer (TGA, LECO Corporation, St. Joseph, MO, USA). The ultimate analysis of feed coal was performed on a FLASH 2000 elemental analyzer (EA, Thermo Fisher Scientific, Bremen, Germany) under CHNS mode and O mode. The contents of major-element oxides in the feed coal, FA, and BA were measured on a Axios Pw4400 X-ray fluorescence (XRF) spectrometer (PANalytical B.V., Almelo, The Netherlands). The mineralogical characteristics of the coal’s low-temperature ash and all the other ash samples were determined by means of a Smartlab SE X-ray powder diffractometer (XRD) (Rigaku Corporation, Akishima, Japan) at a scanning rate of 4°/min in the 2θ range 5~80° using Ni-filtered Cu Kα radiation and operating at 40 kV and 100 mA. The morphology and elemental mappings of the ashes derived from the simulated combustion at various temperatures were characterized by an MIRA LMS scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS, TESCAN Group, Brno, Czech Republic).

The concentrations of Li and Ga in the feed coal, combustion ash, the leaching solutions, and final residues of SCEP were determined using a 7850 inductively coupled plasma mass spectrometry (ICP-MS, Agilent Technologies Inc., Santa Clara, CA, USA). Prior to analyses, a digestion treatment was conducted for each of the samples. The digestion method for solids involved weighing a 50 mg sample into a polytetrafluoroethylene digestion tank, followed by the addition of 1 mL of 40% HF and 0.5 mL of 66% HNO₃. The mixture was then sealed and heated in the oven at a temperature of 185 ± 5 °C for 24 h. After cooling, the digestion tank was removed, and the solution was heated on an electric plate until dry. Subsequently, 0.5 mL of 66% HNO₃ was added, and the solution was heated until dry. Then, 5 mL of 66% HNO₃ and 5 mL of ultrapure water were added, followed by placing the solution back into the oven and heating it at 130 °C for 3 h. After cooling, the solution was transferred to a 50 mL plastic bottle. Lastly, the solution was diluted with ultrapure water to a constant volume of 50 mL, and the contents of Li and Ga in the samples were determined using ICP-MS.

2.2.4. Thermodynamic Calculations

The preferred chemical species of Li and Ga in a closed system under thermodynamic equilibrium in a temperature range of 600 to 2000 °C were calculated using an FactSage software (Version 7.3), jointly developed by Thermfact/CRCT (Montreal, QC, Canada) and GTT-Technologies (Aachen, NRW, Germany). The calculation was based on the occurrence of the four most abundant elements (C, H, N, and S), eight major oxides (Al₂O₃, SiO₂, TiO₂, Fe₂O₃, CaO, K₂O, Na₂O, and MgO) and two studied critical metals (Li and Ga) in the coal sample, as listed in

Table 1. The proximate and ultimate analysis results of the feed coal sample.

Proximate Analysis (wt%)				Ultimate Analysis (wt%)
Mad	Vd	Ad	FCd	C	H	O	N	S
3.03	25.30	37.40	37.30	47.80	3.02	30.6	0.99	0.62

Ad, air dried basis; d, dry basis.

and

Table 2. The contents of major-element oxides (wt%) determined by XRF in the feed coal, FA, and BA samples.

Sample	Al2O3	SiO2	TiO2	Fe2O3	CaO	K2O	Na2O	MgO
Feed coal	36.27	40.04	5.33	4.13	3.85	1.15	0.18	0.21
Fly ash	47.72	41.50	2.87	2.33	3.80	0.50	0.17	0.26
Bottom ash	46.70	42.64	2.26	2.23	4.45	0.49	0.21	0.21

, as well as Figure 2. Additionally, the vapor and condensed phases were assumed to be ideal gas and pure solids, respectively. The air was set to consist of 79% N₂ and 21% O₂, and the pressure was standard atmospheric pressure (1 atm). The excess air ratio (λ) at the combustion chamber inlet was set to 1.2, which is typically selected for coal combustion.

3. Results and Discussion

3.1. Properties of Feed Coal and Industrial Ash

Table 1 presents the proximate and ultimate analysis results of the feed coal. Table 2 lists the contents of major-element oxides in the feed coal, FA, and BA. As can be seen in Table 2, the primary chemical components of FA and BA in the Guohua Power Plant are SiO₂, Al₂O₃, CaO, TiO₂, and Fe₂O₃, accounting for 98.22% and 98.28%, respectively. These results suggest that the Guohua Power Plant produces Class F FA, characterized by a combined percentage of SiO₂, Al₂O₃, and Fe₂O₃ not less than 70% and a CaO content less than 10% [27]. Class F FA is typically generated through the combustion of high-rank coal [28]; therefore, the anthracite used in the Guohua Power Plant is high-rank coal.

Figure 2 shows the concentrations of Li and Ga in the feed coal, FA, and BA, measured by ICP-MS. As shown in Figure 2, the average concentrations of Li and Ga in the feed coal are 130.18 μg/g and 27.68 μg/g, respectively, which are significantly higher than those in world coal (12 μg/g for Li and 5.8 μg/g for Ga) [29]. During combustion, both Li and Ga undergo significant enrichment in FA and BA. Notably, the Ga content in combustion ash surpasses the cut-off grade of Ga, which is >50 μg/g in coal ash (when the coal thickness exceeds 5 m) [12,29]. Consequently, coal combustion residues represent a potential source for Li and Ga recovery.

3.2. Transformation of Minerals during Coal Combustion

Figure 3 shows the XRD patterns of feed coal (a), FA, BA, and ashes derived from simulated combustion at different temperatures (b).

Table 3. The proportions of various minerals and glass (amorphous) phase in the feed coal and different combustion ashes, determined by XRD and Siroquant analysis (wt%).

Sample	Feed Coal	Fly Ash	Bottom Ash	600 °C Ash	800 °C Ash	1000 °C Ash	1200 °C Ash
Qtz	8.1	2.1	5.2	2.7	3.6	2.7	4.8
Kao	68.3
Bhm	17.6
Py	0.7
Cal	4.4		0.7	1.8
Sd	0.4
Ana	0.5
Rut		0.2	0.2				0.5
Mul		31.1	33.8			20.6	39.3
Crn		4.5	3.4	0.9	0.9	1.9	2.7
Hem				1.3	1.9	1.1
Mag		0.1
Crs			0.2			1	8.2
An					1.5	1.3	0.3
Lm		0.3
Adr				1.8	2.4
Glass		61.6	56.5	91.5	89.6	71.4	44.2

presents the proportions of various minerals and amorphous glass phase in the feed coal and different combustion ashes, determined through XRD and Siroquant analysis (wt%). From Figure 3 and Table 3, the primary minerals in the feed coal are kaolinite, boehmite, quartz, calcite, pyrite, anatase, and siderite. Among these, boehmite and kaolinite are the primary carriers of Ga [30,31,32], as evidenced by the elemental distribution maps in Figure S1, where the elements Si, Al, and O are identified as the dominant constituents within the combustion ashes. A previous study in the Jungar area found that Li in the coal from the Jungar Basin mainly occurs in clay minerals such as kaolinite and chlorite [31,32]. Therefore, Li in the feed coal is expected to occur in clay minerals in this study.

In contrast to the feed coal, its industrial combustion by-products, FA and BA, primarily consist of an amorphous phase and mineralogical phases such as mullite, quartz, corundum, rutile, and magnetite. The transformation of mineralogical phases during coal combustion could be attributed to the thermal decomposition and conversion of kaolinite to mullite (Equation (1)) and boehmite to corundum (Equation (2)) [33,34,35]. For the ashes derived from the simulated combustion at different temperatures, their mineral phases change with increasing temperature. The mineral phases present in the 600 °C ash include quartz, calcite, corundum, hematite, and anhydrite. When the combustion temperature increased to 800 °C, anorthite started to appear in the ash, while calcite was totally broken down and lime formed [36]. The latter reacted with SiO₂ and Al₂O₃ and formed anorthite. Quartz, corundum, and hematite that were formed at 600 °C remained. With a further increase in temperature to above 1000 °C, the minerals in the ash were mostly mullite, quartz, corundum, and rutile, similar to those in the FA and BA produced in the power plant. It is noteworthy that the glass phase content decreased, while the mullite content increased, with rising temperature. Moreover, pyrite disappeared after combustion [37]. This is attributed to the transformation of amorphous SiO₂ into mullite (Equation (3)) and the conversion of pyrite into hematite and magnetite (Equation (4)) [38,39].

$$3{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3·2\mathrm{S}\mathrm{i}{\mathrm{O}}_2·2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{K}\mathrm{a}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{e}\right)\to 3{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3·2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{M}\mathrm{u}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{e}\right)+6{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{g}\right)}+4\mathrm{S}\mathrm{i}{\mathrm{O}}_2$$

(1)

$$2\mathrm{A}\mathrm{l}\mathrm{O}\mathrm{O}\mathrm{H}\left(\mathrm{B}\mathrm{o}\mathrm{h}\mathrm{m}\mathrm{i}\mathrm{t}\mathrm{e}\right)\to {\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3\left(\mathrm{C}\mathrm{o}\mathrm{r}\mathrm{u}\mathrm{d}\mathrm{u}\mathrm{m}\right)+{\mathrm{H}}_2\mathrm{O}$$

(2)

$$2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{G}\mathrm{l}\mathrm{a}\mathrm{s}\mathrm{s}\right)+3{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3\to 3{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3·2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{M}\mathrm{u}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{e}\right)$$

(3)

$$10\mathrm{F}\mathrm{e}{\mathrm{S}}_2\left(\mathrm{P}\mathrm{y}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{e}\right)+21{\mathrm{O}}_2\to 2{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3\left(\mathrm{H}\mathrm{e}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{e}\right)+2{\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4\left(\mathrm{M}\mathrm{a}\mathrm{g}\mathrm{n}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{e}\right)+14\mathrm{S}{\mathrm{O}}_2+3{\mathrm{S}}_2$$

(4)

3.3. Modes of Occurrence of Li and Ga Indicated by SCEP Results

The concentrations of Li and Ga in the combustion ashes, the leaching solutions, and final residues of SCEP, determined by ICP-MS, are listed in Table S1. For FA and BA, the concentrations of Li are the highest during the fifth step of SCEP compared to the results from the other four steps. This suggests that the primary mode of occurrence of Li in industrial combustion products is quartz and aluminosilicates. For the ashes resulting from the simulated combustion at various temperatures, the concentrations of Li in the ashes at 600 °C and 800 °C are relatively higher during the fourth step of SCEP, indicating that Li mainly occurs at these temperatures. However, the concentrations of Li in the ashes at 1000 °C and 1200 °C are the highest during the fifth step of SCEP, showing that Li mainly occurs in quartz and aluminosilicates above 1000 °C, which is consistent with the mode of occurrence of Li in industrial combustion products. Taking into account Figure 3 and Table 3, it can be inferred that Li in the ashes at 600 °C and 800 °C mainly occurs in anhydrite, and Li is primarily present in the glass phase, mullite, quartz, and other aluminosilicates at 1000 °C and 1200 °C.

For FA, BA, and ashes obtained at various temperatures, the concentrations of Ga in the leaching solutions are the highest in the fifth step during SCEP, suggesting that Ga is mostly associated with quartz and aluminosilicate phases. Comparing the result of step 5.1 with that of step 5.2 in Table S1, it can be seen that Ga occurs in mullite and quartz more than in the glass phase for the ashes obtained at temperatures exceeding 600 °C. In addition, the leaching rates (R) of Li and Ga were calculated by dividing the sum of concentrations of Li and Ga obtained from each step of SCEP by its concentration in the corresponding ash, which are 36.3% to 82.8% and 78.0% to 104.7%, respectively. This indicates that the SCEP experiments conducted in this study were reliable.

In order to illustrate the distribution of various modes of occurrence of Li and Ga, the relative percentage of each mode of occurrence was calculated through dividing the concentration determined from each step of the SCEP by the total concentration across five steps, as shown in Figure 4. In Figure 4a, for the FA and BA samples, the primary modes of occurrence of Li are strongly associated with quartz and aluminosilicates, accounting for up to 61.75% and 66.28% of the total Li content during SCEP, respectively. For ashes derived from the simulated combustion at different temperatures, the modes of occurrence of Li at 600 °C are associated with sulfides (70.90%), metal oxides (12.97%), and quartz and aluminosilicates (9.24%). As the combustion temperature increased, the modes of occurrence of Li changed. At 800 °C, Li is associated with sulfides (51.15%), quartz and aluminosilicates (29.99%), and metal oxides (13.05%). At 1000 °C, Li mainly occurs in quartz and aluminosilicates, accounting for 85.29%, with a smaller occurrence in sulfides (7.74%). At 1200 °C, Li occurs primarily in quartz and aluminosilicates, accounting for up to 89.92%. Overall, the proportions of sulfides and metal oxides decrease as the combustion temperature rises, while those of quartz and aluminosilicates increase. Below 800 °C, sulfides are the main modes of occurrence of Li, while quartz and aluminosilicates emerge as the dominant modes of occurrence of Li for the ashes above 1000 °C and industrial combustion products.

As depicted in Figure 4b, quartz and aluminosilicates are the predominant mode of occurrence of Ga for all the ashes. For the FA sample, the quartz and aluminosilicate mode of Ga accounted for 56.55%, while the sulfide mode accounted for 34.13%. For the BA sample, these percentages were 78.19% and 15.66%, respectively. For the ashes at different temperatures, the percentages of quartz and aluminosilicate mode and sulfide mode were 47.00% and 49.46% at 600 °C, and 65.82% and 35.06% at 800 °C. As the temperatures increased to 1000 °C and 1200 °C, the quartz and aluminosilicate modes accounted for up to 95.37% and 96.28%, respectively.

A comparative analysis between the glass phase, and mullite and quartz was conducted in order to further elaborate on the quartz and aluminosilicate modes of Li and Ga. These were obtained from step 5.1 and step 5.2 of the SCEP, respectively, as depicted in Figure 4c,d. Figure 4c reveals that, excluding the ash at 1000 °C, the glass phase modes of Li are predominantly higher in other ashes, particularly reaching 91% and 81% in ashes at 600 °C and 800 °C, respectively. Conversely, Figure 4d illustrates a gradual increase in the proportions of mullite and quartz modes of Ga as the combustion temperature rose.

Combining the information from Figure 3 and Figure 4, it can be inferred that during the combustion of the feed coal, Li migrates into anhydrite, hematite, glass phase, and quartz below 800 °C, while it transfers into glass phase, mullite, and quartz above 1000 °C. Ga in the 600 °C ash primarily occurs in the glass phase, while mullite and quartz are the dominant occurrence modes of Ga in the industrial combustion ashes and ashes obtained from the simulated combustion above 600 °C. The occurrence modes and migration pathway of Li and Ga during coal combustion are schematically represented in Figure 5.

3.4. Evolution of Li and Ga Based on Thermodynamic Calculation

Figure 6 illustrates the equilibrium distribution of Li and Ga during coal combustion based on thermodynamic calculations. In Figure 6a, Li primarily occurred in the form of aluminosilicate (LiAlSi₄O₁₀, s) at combustion temperatures ranging from 600 °C to 1500 °C. As the temperature increased to 1900 °C, LiAlSi₄O₁₀ (s) gradually transformed into LiOH (g) through a two-step reaction, as shown in Equations (5) and (6). Additionally, a portion of the LiOH (g) reacts with hydrogen gas from the pyrolysis of organic matter in coal, leading to the production of Li (g) at temperatures ranging from 1700 °C to 2000 °C, as shown in Equation (7). The calculations at the temperatures between 600 °C and 1500 °C also indicated the presence of other chemical species of Li, including LiH (g), LiN (g), LiO (g), Li₂O (g), Li₂O₂ (g), LiOH (g), (LiOH)₂ (g), LiON (g), LiONa (g), and Li₂SO₄ (g), the total molar percentage of which was less than 0.001%. These gaseous Li compounds are not included in Figure 6a due to their negligible amounts.

$$2\mathrm{L}\mathrm{i}\mathrm{A}\mathrm{l}{\mathrm{S}\mathrm{i}}_4{\mathrm{O}}_{10}\left(\mathrm{s}\right)+9\mathrm{C}\mathrm{a}\mathrm{O}\left(\mathrm{s}\right)\stackrel{1500~1900 °\mathrm{C}}{\to }{\mathrm{L}\mathrm{i}}_2\mathrm{O}\left(\mathrm{s}\right)+\mathrm{C}\mathrm{a}\mathrm{O}·{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+8\mathrm{C}\mathrm{a}\mathrm{O}·\mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{s}\right)$$

(5)

$${\mathrm{L}\mathrm{i}}_2\mathrm{O}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\stackrel{1500~1900 °\mathrm{C}}{\to }2\mathrm{L}\mathrm{i}\mathrm{O}\mathrm{H}(\mathrm{g})$$

(6)

$$2\mathrm{L}\mathrm{i}\mathrm{O}\mathrm{H}\left(\mathrm{g}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)\stackrel{1700~2000 °\mathrm{C}}{\to }2\mathrm{L}\mathrm{i}\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}(\mathrm{g})$$

(7)

From Figure 6b, Ga existed as sulfide (Ga₄S₅, s) at combustion temperatures between 600 °C and 800 °C. Ga₄S₅ (s) transformed into Ga₂S (g) with increasing temperature from 800 °C to 1000 °C, as seen in Equation (8). Subsequently, Ga remained in the form of Ga₂S (g) at 1000~1300 °C. As the temperature further increased from 1300 to 1900 °C, Ga₂S (g) was gradually reduced to Ga (g), as shown in Equation (9). No quartz and aluminosilicates were obtained in the thermodynamic equilibrium calculation of Ga. When only the elements of Si, Al, O, and Ga were used for the calculation, the result showed separate aluminosilicates and Ga-related oxides. The disagreement with the experimental result might be due to the complexity of the isomorphism reaction between Ga and Al during coal combustion, as well as the sulfur affinity and inactivity of Ga compared to Li.

$${\mathrm{G}\mathrm{a}}_4{\mathrm{S}}_5\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{800~1000 °\mathrm{C}}{\to }2{\mathrm{G}\mathrm{a}}_2\mathrm{S}\left(\mathrm{g}\right)+3\mathrm{S}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(8)

$${\mathrm{G}\mathrm{a}}_2\mathrm{S}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{1300~1900 °\mathrm{C}}{\to }2\mathrm{G}\mathrm{a}\left(\mathrm{g}\right)+\mathrm{S}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

(9)

4. Conclusions

In summary, kaolinite and boehmite in the feed coal are the primary carriers of Li and Ga. Li and Ga become significantly enriched in the ashes after combustion, reaching the highest content of 1085.54 μg/g and 132.67 μg/g, respectively. The primary modes of occurrence of Li and Ga in the combustion ashes are quartz and aluminosilicates, and sulfides.

Lithium and Ga exist in the form of LiAlSi₄O₁₀ (s), Ga₄S₅ (s), and Ga₂S (g) and exhibit a dynamic migration pattern during combustion. Li migrates from kaolinite into anhydrite, hematite, glass, and quartz below 800 °C, whereas it occurs as the glass phase, mullite, and quartz as the temperature rises above 1000 °C. Gallium transfers from kaolinite and boehmite into the glass phase at 600 °C and migrates into mullite and quartz above 600 °C. At temperatures ranging from 800 °C to 1000 °C, Ga₄S₅ (s) transforms into Ga₂S (g), maintaining its gaseous form until 1200 °C. This comprehensive understanding of the modes of occurrence, migration, and evolution pathways of Li and Ga during combustion provides valuable insights for their potential recovery from coal combustion residues.