Roasting Extraction of Lithium from Fly Ash: A Study of Influential Parameters and Mechanisms

Abstract

Fly ash consists of significant amounts of lithium, which is an essential resource for developing batteries. This study proposed an efficient method for extracting lithium from fly ash. First, we explored the parameters affecting the activation effect of sodium carbonate roasting and the leaching efficiency of lithium using acid leaching. Additionally, ultrasonic pre-treatment was applied to enhance activation. A further mechanism for the roasting extraction of lithium was symmetrically analyzed. The results showed that ultrasonic treatment at 200 W for 1 h under alkaline leaching conditions (sodium hydroxide solution 4 mol/L, reaction temperature 80 °C, leaching time 2 h, solid–liquid ratio 1 g:30 mL) achieved a lithium leaching rate of 90.74%, surpassing the 84.72% with traditional roasting–alkaline leaching. Under optimal acid leaching conditions (850 °C for reaction of 2.5 h, fly ash-to-sodium carbonate ratio (R_fs) 1:2, sulfuric acid 2 mol/L, reaction temperature 80 °C, solid–liquid ratio 1 g:30 mL, and leaching time 1.5 h), the leaching rate reached 96.62%. With ultrasonic pre-treatment and acid leaching, the highest leaching rate of 98.68% achieved under optimal conditions: reaction temperature 850 °C for 2.5 h, mass R_fs at 1:1.5, sulfuric acid 2 mol/L, reaction temperature 80 °C, solid–liquid ratio 1 g:35 mL, and leaching time 120 min. The study demonstrated that ultrasonic pre-treatment outperforms the traditional method, achieving a higher leaching rate with fewer roasting additives and lower energy consumption.

1. Introduction

Lithium has received wide attention due to its widespread applications in renewable energy and electric vehicles [1]. It is often referred to by various known names, such as the “yeast of industry”, “aerospace alloy”, “white oil”, and “the most promising metal of the 21st century” [2,3,4]. With the global shift in energy structures, it plays an increasingly significant role in human society [5]. Lithium batteries, known for their long life cycle, high energy density, low self-discharge rate, and environmentally friendly characteristics, are widely used in portable electronic devices, electric vehicles, and consumer electronics such as mobile phones. This extensive use has led to substantial consumption of lithium resources [6,7]. As global demand for renewable and clean energy technologies grows, the strategic value of lithium has become more prominent [8]. With increasing awareness of global energy, research into alternative lithium sources has become particularly crucial.

Among various potential lithium resources, the mineral-rich composition of fly ash, especially its lithium content, has attracted growing interest from researchers. Coal fly ash, a by-product of coal combustion in thermal power plants, is one of the most complex and abundant of anthropogenic materials [9]. Studies have shown that a high alumina-coal fly ash sample contains a high lithium concentration, approximately 250–1400 μg/g (Li₂O content: 0.06–0.30 wt%) [10], making its recovery and utilization feasible and valuable. Furthermore, it facilitates the alleviation of the pressure associated with environmental pollution remediation. Today, coal-fired power generation produces over 500 million tons of fly ash annually worldwide, yet only 25% to 30% is repurposed in various industries [11,12].

Currently, coal remains the most crucial energy resource for the foreseeable future in China [13]. Guizhou Province, one of China’s primary coal production bases, hosts numerous thermal power plants [14]. As coal consumption continues to rise, the production of fly ash is also steadily increasing. Efficient extraction of lithium from fly ash could not only mitigate its environmental impact but also provide new momentum for the sustainable development of the local economy. The primary source of lithium products in China is hard rock lithium ores [15]. The common extraction methods for lithium from these ores include the sulfate method, sulfuric acid method, limestone sintering method, chlorination roasting method, soda pressure cooking method, and hydrofluoric acid method [16]. However, 59% of the world’s lithium reserves are stored in salt lake brines [17]. Despite the low extraction cost, the presence of numerous impurities in salt lake brines makes extraction difficult [18]. Consequently, numerous researchers have conducted in-depth and comprehensive analyses of the occurrence and extraction of lithium in fly ash. In the studies on the occurrence of lithium in fly ash, Sun et al. [19] used Time of Flight Secondary Ion Mass Spectrometry to analyze the distribution of lithium, aluminum, and silicon in micro-regions of the coal samples from the Antaibao No. 11 coal seam. They found that lithium-enriched areas coincided with aluminum and silicon enrichment, indicating that aluminosilicate minerals are the primary hosts of lithium. Li et al. [20] discovered that in those fly ash samples that contain a higher alumina content, lithium is predominantly enriched in the amorphous aluminosilicate phase. Hu [21] emphasized the unique distribution of the lithium in these fly ash samples using time-of-flight secondary ion mass spectrometry analysis, revealing that lithium is primarily concentrated in the glass phase of fly ash. These studies have significantly improved the understanding of lithium’s occurrence, laying a foundation for its subsequent extraction. In terms of lithium extraction from fly ash, Yang [22] conducted a detailed comparison between acidic and alkaline roasting. Zang [23] explored the direct acid or alkali leaching of fly ash and found that the chemical properties of raw fly ash are highly stable, achieving a leaching rate of only around 15% at best. However, further investigation into other factors revealed that wet mixing with additives can effectively enhance both the roasting efficiency and leaching efficiency of fly ash under the same additive conditions. Wang [24] investigated the leaching kinetics of fly ash under various conditions by enhancing the roasting and leaching process with microwave assistance. Among these experiments, studies on leaching times of less than 30 min are also one of the directions for further experimental exploration in this work. Bo et al. [25] used NaCl for fly ash roasting and investigated the influencing factors of acid leaching conditions, Xu et al. [26] adopts a novel approach which uses a low-temperature ammonium fluoride activation method to assist leaching, achieving a lithium leaching rate of over 90%. The unique properties of ammonium fluoride have significantly reduced energy consumption during the roasting stage. Fang et al. [27] utilizes a combined room temperature Na₂S₂O₇ mechanochemical activation and Na₂S₂O₈ pressurized leaching technique to recover aluminum and lithium from coal fly ash, obtaining an aluminum leaching rate of 95.58% and a lithium leaching rate of 71.59%. In the stage of extracting lithium from the liquid phase into a final product, Rui [28] examines the effect of the solvent extraction on recovering lithium from hydrochloric acid leachate. Xu et al. [29] synthesizes an adsorption resin to extract lithium from an alkaline solution which has a low lithium concentration, which overcomes adsorbent material loss due to dissolution in an alkaline environment. Zhang et al. [30] develops a manganese dioxide lithium ion-sieve which achieves an exceptional lithium adsorption rate of 100% and a desorption rate of 100%. These studies are crucial for designing efficient extraction methods and optimizing the strategies for utilizing fly ash, which ultimately enhances its economic and environmental value.

This study aims to analyze the method of roasting extraction of lithium, highlighting the influential parameters and the mechanisms for the extraction of lithium from those fly ash samples generated in power plants in Guizhou Province, China. An innovative ultrasonic pre-treatment method was applied to improve the lithium extraction. The effects of the various factors were compared between the new method and the traditional roasting method. The fly ash samples involved in this study were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and X-ray fluorescence (XRF) to analyze the reaction mechanisms and characteristics. The findings tested in this study reveal considerable potential for enhancing our understanding of the process of extracting lithium from fly ash.

2. Experimental

2.1. Materials

The primary reagents used in this study included anhydrous sodium carbonate (Sinopharm Group Co., Ltd., Shanghai, China), sodium hydroxide (Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), and standard sulfuric acid solution (Shenzhen Anzexin Technology Co., Ltd., Shenzhen, China). The fly ash samples were collected from the coal-fired power plants in Guizhou Province, China. After sampling, the fly ash sample was dried in a vacuum drying oven at 105 °C for 2 h for subsequent analysis.

The main instruments used in this study included a tube furnace (OTF-1200X-100, Hefei Kejing Materials Technology Co., Ltd., Hefei, China), a solid sample grinder (DM-100, Nanjing Dongmai Scientific Instrument Co., Ltd., Nanjing, China), a circulating water vacuum pump (SHZ-DIII, Yuhua Instrument Manufacturing Co., Ltd., Gongyi, China), a thermostatic water bath magnetic stirrer (DF-101S, Bangxi Instrument Technology Co., Ltd., Shanghai, China), and an ultrasonic cleaner (KQ3200DE digital version, Kunshan Shumei Ultrasonic Instrument Co., Ltd., Kunshan, China).

2.2. Pretreatment Procedures

2.2.1. Ultrasonic Treatment

This procedure serves as a preprocessing step prior to roasting activation. Before the roasting process, 10 mL of ultrapure water was added to the sodium carbonate and fly ash mixture placed in the corundum crucible. The mixture was thoroughly stirred to form a semi-fluid state. The crucible was then placed in an ultrasonic generator and subjected to water bath heating treatment for 60 min until the mixture became a moist solid. Subsequently, the mixture underwent the roasting activation reaction.

2.2.2. Roasting Activation

A basic alkaline roasting method was employed as the foundational activation process in this study. Fly ash was mixed with sodium carbonate. The mixture, prepared at a pre-determined ratio of 1:2, was evenly blended and placed in a corundum crucible. The crucible was then placed in a tube furnace for activation roasting at 850 °C for 150 min. After roasting, a solid product was obtained and pulverized into a uniform powder using a grinder for subsequent leaching experiments.

2.3. Leaching Procedures

A 4 mol/L sodium hydroxide solution and a 2 mol/L sulfuric acid standard solution were used as the leaching agent in this study. One gram of the roasted activated product was weighed and placed into an Erlenmeyer flask. The leaching agent was added at a solid-to-liquid ratio of 1 g:30 mL, and the mixture was stirred in a water bath at 80 °C using a thermostatic magnetic stirrer for 120 min. After reaction, the mixture was filtered using a vacuum filtration device, followed by a drying in a vacuum drying oven. The lithium content in the residual solid was analyzed and compared to the original roasted product to calculate the lithium leaching rate. Due to the minimal residue remaining after acidic leaching, the leachate was retained for analysis. Lithium content was determined using the ICP-OES method.

2.4. Analytical Methods

To elucidate the mechanisms underlying the roasting and leaching processes, the fly ash samples were analyzed in detail. Scanning electron microscopy (SEM, Hitachi (Tokyo, Japan), SU8100), X-ray diffraction (XRD, Bruker (Bavaria, Germany), D8 ADVANCE), and X-ray fluorescence spectroscopy (XRF, PANalytical (Almelo, Herland) AXIOS) were employed to examine the microstructure and mineral composition of the fly ash samples.

3. Results and Discussion

The study of the roasting and leaching techniques for fly ash is critically important to improving lithium recovery rates. These techniques can break the structural bonds that immobilize lithium in fly ash, facilitating its effective release and paving the way for subsequent extraction and purification steps. To identify the optimal roasting and leaching conditions, this study comprehensively investigated multiple key parameters, including leaching time, solid-to-liquid ratio, and ultrasonic treatment.

3.1. Characterization of Original Fly Ash

Before all experiments, a basic characterization analysis was conducted on the raw fly ash samples. From the SEM images of the raw fly ash (Figure 1a,b), it can be observed that the particles are spherical microbeads of varying sizes with smooth surfaces, which are the products of high-temperature combustion. The XRF data (

Table 1. XRF composition of the original fly ash.

Elements			Compounds
Element	Conc	Unit	Element	Conc	Unit
Si	38.758	wt%	SiO2	45.239	wt%
Al	25.744	wt%	Al2O3	30.777	wt%
Fe	18.364	wt%	Fe2O3	11.23	wt%
Ca	5.204	wt%	CaO	3.56	wt%
K	4.58	wt%	TiO2	3.524	wt%
S	3.802	wt%	K2O	2.253	wt%

) and XRD pattern (Figure 1c) indicate that the main components of the fly ash are mullite-quartz phases, SiO₂ glass phases, and iron microbead phases. Additionally, the EDX mapping images of the raw fly ash (Figure 2a–g) reveal the distribution patterns of the main elements in the fly ash. It is evident that the primary composition is a mixture of SiO₂ and Al₂O₃, with other elements present in smaller amounts. The results also show that the mullite phase and the glass phase are intermixed.

3.2. Roasting Effects

3.2.1. Sodium Carbonate Addition Ratio

This study examined the leaching performance of the roasted products by mass R_fs during the roasting process. The results are shown in Figure 3. The roasting duration was set at 2.5 h, with a peak temperature of 850 °C. The leaching was conducted with a NaOH concentration of 4 mol/L, a solid-to-liquid ratio of 1 g:30 mL, a leaching temperature of 80 °C, and a leaching duration of 120 min. The R_fs was increased from 1:1 to 1:4. The leaching rate increased with the sodium carbonate addition ratio. The steepest increase occurred between 1:1 and 1:2, while the efficiency plateaued as the R_fs exceeded 1:2. At a R_fs of 1:4, the highest leaching rate of 89.32% was achieved, which was only 4.6% higher than that at a R_fs of 1:2 (84.72%). The main reason for the increase may be attributed to a complete reaction between the fly ash and sodium carbonate as the addition ratio increased. However, when the addition ratio exceeds 1:3, the contact between sodium carbonate and fly ash had been sufficiently uniform, and further addition of sodium carbonate no longer significantly enhanced the reaction. Considering the cost of sodium carbonate, the optimal R_fs of 1:2 was selected.

3.2.2. Roasting Time

This study investigated the roasting effect on the leaching rate of lithium. The experimental conditions were fixed at a mass ratio of fly ash to sodium carbonate of 1:2, a roasting temperature of 850 °C, an NaOH concentration of 4 mol/L, a solid–liquid ratio of 1 g:30 mL, an extraction temperature of 80 °C, an extraction time of 120 min, and a roasting time in the range of 0.5 h to 3 h. The results are shown in Figure 4.

As shown in the Figure 4, the leaching rate is proportional to the roasting time, with a rapid increase between 0.5 h and 2 h, followed by a slow rise from 2 h to 3 h. The leaching rate reached its peak at 87.44% after 3 h. After a reaction time of 3 h, a further increase of the roasting time would not improve the roasting effect or the leaching rate significantly.

3.2.3. Roasting Analysis

The SEM results of the fly ash samples after roasting are shown in Figure 5a–c. Figure 5a,b shows the roasted fly ash, with indistinct particle edges and particles bonded together by sodium carbonate. The sodium carbonate disrupts the vitreous structure within the fly ash, compromising its particle integrity and enhancing the reactivity for the subsequent leaching process.

Additionally, the XRD analysis in Figure 1c reveals that the primary phases of the fly ash before roasting are mullite, quartz, and a small quantity of iron-rich microspheres. After roasting with sodium carbonate, the XRD analysis in Figure 5c shows that nepheline and magnetite phases were formed. Furthermore, with an R_fs of 1:2 in conventional roasting, residual sodium carbonate was observed. Therefore, the possible pathway for the reaction in the sodium carbonate roasting process is as follows:

Na₂CO₃ → Na₂O + CO₂↑

(1)

xAl₂O₃·ySiO₂ + nNa₂O → nNaAlSiO₄

(2)

SiO₂ + Na₂O → Na₂SiO₃

(3)

3.3. Ultrasonic Pretreatment Effects

3.3.1. Comparison Analysis

This study examined the leaching effect of lithium from the roasting products at different R_fs and roasting durations. A comparison was conducted to assess the impact of ultrasonic treatment, both with and without its applications, prior to the roasting process. The results are shown in Figure 6a.

As can be seen from the figure, without ultrasonic pre-treatment for the roasting process the leaching rate reached its maximum at an R_fs of 1:2, but in the presence of ultrasonic treatment, the maximum as value almost reached at an R_fs of 1:1, and the leaching rate was also slightly higher than the former, in the range of 87.39% to 90.74%. The ultrasonic treatment resulted in more uniform mixing of the fly ash and sodium carbonate. As a result, the leaching rate increased, which required less additive. The leaching rate at different roasting times was analyzed. A possible impact of ultrasonic pre-treatment in reduction of energy utilization of the roasting process was observed. The results are shown in Figure 6b. It shows that while at a roasting time 1.5 h, the leaching rate was higher than that with the ultrasonic pre-treatment, ultrasonic pre-treatment resulted in a higher leaching rate in the roasting process for a reaction of 1.5 h and the maximum rate was achieved at 89.04% for roasting reaction of 2 h. At a roasting time over 2.5 h, the leaching rates for the both methods were similar, which suggests that ultrasonic treatment has potential in saving energy. Roasting time for 2 h was recommended for a better leaching in the presence of ultrasonic pre-treatment.

3.3.2. Ultrasonic Treatment Analysis

As shown in Figure 7a–d, a comparison of the fly ash samples was made between a sole sodium carbonate roasting product and an ultrasonic-assisted roasting one. In Figure 7b, the sole sodium carbonate roasting product with R_fs at 1:2 reveals the presence of numerous unreacted sodium carbonate crystals as characterized by densely packed elongated particles that were formed by melting and subsequent crystallization cooling. In contrast, the ultrasonic-assisted roasting product at the same 1:2 ratio (Figure 7c) exhibits a similar structure, which was comparable to that of a sole sodium carbonate roasting product at an R_fs 1:1.5 (Figure 7a). Additionally, at the 1:1 ratio with ultrasonic pre-treatment (Figure 7d), there was almost no residual sodium carbonate observed. Furthermore, comparing Figure 5c and Figure 7e, the characteristic peak of sodium carbonate disappeared in the ultrasonic-assisted roasting product at the ratio of 1:1.5. It confirms that ultrasonic pre-treatment significantly improved roasting activation efficiency and contributed to reducing the amount of sodium carbonate additive.

3.4. Acid Leaching Effects

3.4.1. Effect of Leaching Temperature

This study investigated the variation of the leaching rate of fly ash at different leaching temperatures in the range of 40 °C to 90 °C. The experiment was conducted at a mass R_fs of 1:2, a roasting temperature of 850 °C, aroasting duration of 2.5 h, a sulfuric acid concentration of 2 mol/L, a solid–liquid ratio of 1 g:30 mL, and a leaching time of 120 min. The results are shown in Figure 8. As shown in Figure 8, the highest leaching rate of 91.52% reached 40 °C. The fluctuation in the leaching rate was relatively small throughout the whole temperature, suggesting that temperature has a minor impact on the acidic leaching rate.

3.4.2. Effect of Solid–Liquid Ratio

With an R_fs of 1:2, a maximum roasting temperature of 850 °C, a roasting duration of 2.5 h, a sulfuric acid concentration of 2 mol/L, a leaching temperature of 80 °C, and a leaching duration of 120 min, this study examined the leaching rate of lithium of fly ash at various solid–liquid ratios in the range of 1:10–1:45. The results are shown in Figure 9. As shown in Figure 9, when the solid–liquid ratio was adjusted from 1 g:10 mL to 1 g:45 mL in increments of 5 mL, the leaching rate was found to be proportional to the solid–liquid ratio up to a point of 1 g:35 mL. Specifically, the leaching rate remained relatively stable between 1 g:15 mL and 1 g:30 mL. The highest leaching rate of 93.08% was achieved at a solid–liquid ratio of 1 g:35 mL. However, beyond this ratio, the leaching rate began to decrease. This decrease was attributed to the product of silicates. At excessively high solid–liquid ratios, the reaction generated a significant amount of silicate, which entrapped the fly ash residue and caused it to adhere to the walls of the reaction vessel. Consequently, some fly ash was unable to react with the acid solution, resulting in a diminished leaching effect.

3.4.3. Effect of Leaching Time

This study investigated the effect of the leaching time (0.5 h to 3 h) on the leaching rate of lithium. The experiment was conducted at an R_fs of 1:2, a temperature of 850 °C, a roasting duration of 2.5 h, a sulfuric acid concentration of 2 mol/L, a solid–liquid ratio of 1 g:30 mL, and a leaching temperature of 80 °C. The results are shown in Figure 10. As shown in Figure 10, when the leaching time was less than 1.5 h, the leaching rate remained relatively stable. At 1.5 h, the leaching rate achieved its highest value of 96.62%. Beyond this point, the leaching rate fluctuated, initially decreasing and then slightly increasing, but these changes were not significant. The optimal leaching time is recommended to be 1.5 h for costs.

3.4.4. Influence of Leaching Agent Concentration

This study investigated the effect of the sulfuric acid concentrations (1 mol/L to 6 mol/L) on the leaching rate of lithium at an R_fs of 1:2, a roasting temperature of 850 °C, a roasting duration of 2.5 h, a leaching temperature of 80 °C, a solid-to-liquid ratio of 1 g:30 mL, and a leaching duration of 120 min. The results are shown in Figure 11. As shown in Figure 11, the leaching rate initially increased and then decreased, where the maximum of 92.00% was reached at a sulfuric acid concentration of 3 mol/L. This phenomenon was related to the silicate product. When the sulfuric acid concentration is too high, the generated silicate precipitates adsorb onto the surface of the fly ash and attach to the reaction vessel walls, leading to a poor leaching. In addition, a sulfuric acid concentration beyond 4 mol/L yields a large amount of translucent gel-like precipitate inside the reaction vessel. This precipitate was identified as silicate. During reaction, the mixed precipitate of silica and silicates would hinder some roasted product reactions, thus resulting in a decrease in leaching rate when the acid concentration is too high.

3.4.5. Acid Leaching Process Characterization Analysis

The SEM results of the residues after acidic leaching are shown in Figure 12a,b. The residues exhibited more extensive particle destruction, and almost no spherical residues were found within the large particles. Additionally, the background of the residues appeared to be fine and smooth, and little large size residue existed. Herein, the residues implied the finely textured material enveloping disrupted roasted fly ash particles.

The XRD pattern of the residues (see Figure 12c) shows a broad amorphous diffraction peak. XRF analysis (see

Table 2. XRF composition of the residues after acid leaching.

Elements			Compounds
Element	Conc	Unit	Element	Conc	Unit
Si	70.429	wt%	SiO2	74.604	wt%
S	25.513	wt%	SO32−	23.353	wt%
Cl	0.808	wt%	Al2O3	0.506	wt%
Ca	0.77	wt%	TiO2	0.373	wt%
Ti	0.749	wt%	CaO	0.357	wt%
Fe	0.611	wt%	Fe2O3	0.256	wt%
Al	0.381	wt%	Cl−	0.256	wt%

) further confirms that the finely textured residues consisted of a mixture of silica and sulfites. This suggests that the reaction produced a significant amount of silicic acid, showing the effectiveness of acid leaching experiment in our study. Based on the lithium extraction results and the characterization analysis, the following reaction equations were derived:

2NaAlSiO₄ + 4H₂SO₄ = Na₂SO₄ + Al₂(SO₄)₃ + 2H₂SiO₃ + 2H₂O

(4)

H₂SiO₃ = H₂O + SiO₂

(5)

3.5. Orthogonal Experiment Analysis

Based on the results of the single-factor experiments, another experiment with four-factors at three-levels was conducted to optimize the leaching effects. These factors included R_fs (denoted as A), acid leaching time (denoted as B), sulfuric acid concentration (denoted as C), and solid-to-liquid ratio (denoted as D), and the design results of factors and levels are shown in

Table 3. Factor and level design results based on an orthogonal experiment.

Levels	Factors
	A/g:g	B/min	C/mol·L−1	D/g:mL
1	1:1	60	2	1:30
2	1:1.5	90	3	1:35
3	1:2	120	4	1:40

. The experiment was examined by roasting and ultrasonic treatment. The orthogonal experiment test runs were shown in

Table 4. Test runs of the orthogonal experiment.

	Levels	Factors				Lithium Leaching Rate/%
Numbers		A	B	C	D
1		1	1	1	1	98.67
2		1	2	3	2	88.36
3		1	3	2	3	93.78
4		2	1	3	3	88.24
5		2	2	2	1	96.12
6		2	3	1	2	98.68
7		3	1	2	2	91.14
8		3	2	1	3	91.11
9		3	3	3	1	98.04
K1		280.81	278.05	288.47	292.83	T = 844.15
K2		283.04	275.59	281.04	278.19
K3		280.30	290.51	274.64	273.13
k1		93.60	92.68	96.16	97.61	Y = 93.79
k2		94.35	91.86	93.68	92.73
k3		93.43	96.84	91.55	91.04
R		0.91	4.97	4.61	6.57
Impact Order		D > B > C > A
Optimal Level		D1B3C1A2
Optimum Condition		1:1.5; 120 min; 2 mol/L; 1 g:30 mL

Note: k_n = K_n/3. T = Sum of lithium leaching rate. Y = mean value of k_n.

. Other conditions were fixed at a roasting temperature of 850 °C, a roasting duration of 150 min, and a leaching temperature of 80 °C.

Experiments were arranged using the L₉(3⁴) orthogonal table, as shown below:

Based on the orthogonal experiment results, the solid–liquid ratio holds the utmost significance. Specifically, a decrease in the solid–liquid ratio corresponds directly to an increase in the lithium leaching rate. Following this, leaching time emerges as the second most influential factor, closely trailed by sulfuric acid concentration. Conversely, the roasting R_fs exhibits the least impact. The findings from orthogonal experiments further reinforce the conclusions derived from single-factor studies. Notably, excessively high acid concentrations and volumes result in the formation of silica precipitates, which subsequently impede the leaching process of lithium from fly ash. Similarly, under ultrasonic treatment conditions, the leaching rate attains the maximum value at a R_fs of 1:1.5, with minimal differences observed in leaching rates between ratios of 1:1, 1:2, and the optimal ratio.

In investigating the factors influencing the roasting pretreatment process, this study initially adopted a conventional alkali leaching scheme. However, subsequent study revealed that the leaching rate achieved through the alkali leaching method was inferior to that of acid leaching. Consequently, further exploration of the single-factor effects of alkali leaching was deemed unnecessary in the later stages of this study.

4. Conclusions

This study offers the valuable insights into the lithium leaching from the fly ash samples of those power plants in Guizhou Province, China. The findings revealed that the extraction of the lithium from the fly ash samples was efficient with acid leaching in conjunction with ultrasonic treatment and sodium carbonate roasting. It reduced roasting additives and shorter roasting times, and promoted a circular economy. In detail, ultrasonic pretreatment is able to make the reaction more sufficient between sodium carbonate and fly ash and residual sodium carbonate existed. It could address the weakness of alkali leaching in lithium extraction. At the R_fs 1:1.5, it was able to achieve a leaching rate at 90.74%, which was significantly higher than the 78.60% using conventional method without ultrasonic assistance. With acid leaching and ultrasonic pre-treatment, the leaching rate at 98.68% was able to achieve, which was also feasible for cost. Overall, this study provides a new mechanism for extracting of lithium from fly ash.