High-Efficiency Extraction of Lithium and Aluminum from Coal Fly Ash Using Activation-Sintering Technology

Abstract

The objective of this study was to investigate the potential for extracting lithium and aluminum from coal fly ash in depth. The activation-sintering method was used to study how factors like activators, sintering agents, sintering time, leaching concentration, and temperature affect the leaching of lithium and aluminum. A 1:2 Na₂CO₃ activator was proportioned with coal fly ash for primary incineration at a temperature of 1000 °C for 30 min, after which a 3:1 Na₂SO₄ sintering agent was added to be proportioned with coal fly ash for secondary incineration at a temperature of 1000 °C for 30 min. The temperature was then increased to 400 °C for 60 min, after which the lithium and aluminum were leached with a 1% H₂SO₄ solution at 80 °C for 60 min. The leaching process was highly effective, with the lithium and aluminum leached out at rates of approximately 80%.

1. Introduction

Coal fly ash (CFA) is a fine particulate matter formed in the combustion of coal. Its chemical composition is complex, including silicate, alumina, iron oxide, lithium oxide, and numerous other oxides and metal elements. According to the existing research, China’s coal-fired power plants produce approximately 250 to 300 kg of CFA for every 1 t of coal burned [1]. As a significant coal-burning nation, China produces in excess of 600 million tons of CFA annually; yet, its utilization rate is less than 80% [2]. If CFA is not utilized effectively, it will cause ecological damage, and the inhalation of CFA will also pose a potential threat to human health. In this context, the study of valuable elements such as lithium and aluminum in CFA has attracted considerable attention. The in-depth study of its state and extraction process has important theoretical and practical significance.

The complex physical structure of CFA and the chemical structure of crystalline phases such as mullite present significant challenges to the extraction of lithium. The key issue for lithium separation in CFA is the destruction of Si–Al bonds [3]. According to Sun and colleagues, the average value of lithium in Chinese coal is 28.94 μg/g [4], which is significantly higher than the world average of 12 μg/g [5]. The high lithium enrichment was identified in the Carboniferous–Permian coal seams in North China, South China, and Southwest China. The highest mass fraction of lithium in the coal of the Guanbanwusu Mine can reach 710 μg/g, making it a valuable high lithium-associated coal [6]. Currently, numerous techniques exist for extracting lithium from CFA, including acid leaching [7,8], alkali leaching [9], acid–base alternate leaching [10], bioleaching [11], solvent extraction [12], and others. To enhance the leaching rate, some scholars have proposed optimizing the extraction methods, including the addition of chemical reagents [13] and low-temperature activation [14]. These techniques have demonstrated efficacy in improving the extraction rate of lithium. However, the processing method remains complex.

The mass fraction of Al₂O₃ in China’s CFA is generally between 15% and 40%, while the mass fraction of Al₂O₃ in CFA produced from the Pingshuo mine in northern Shanxi and the Jungar mine in west-central Inner Mongolia can reach as high as 50%, which is a high-alumina coal [15]. The key to extracting Al₂O₃ in CFA is to break Si–Al bonds. In order to obtain Al₂O₃ from CFA at a high extraction rate, it is necessary to try to improve the activity of Al₂O₃ and reduce the leaching of other metals. As early as the end of the last century, research began to appear on the leaching of aluminum in CFA. Most of the research employed chemical means, including acid leaching [16,17,18], alkali leaching [19,20], and other methods. Currently, the technical means are more mature. China’s Datang Group employed the pre-silicon removal soda-lime sintering method to establish the inaugural production line with an annual output of 200,000 t. This process not only yielded Al₂O₃ with a purity of up to 94.5% but also eliminated the remaining silica–calcium slag [1]. In light of this research, other scholars have sought to enhance the leaching rate of aluminum by optimizing various techniques, including microwave heating [21], the addition of catalysts [18], high-pressure leaching [17], low-temperature calcination [22], and the integration of multiple methods [23]. All of these methods have the potential to significantly improve the aluminum leaching rate. However, their high cost and harsh calcination conditions limit the process flow.

The extraction technology of lithium and aluminum is still in need of further development to achieve greater efficiency and cleaner production processes. In this study, the activated sintering method was selected for leaching lithium and aluminum based on the aforementioned objective, and the extraction method is described in detail. By conducting a comprehensive analysis of the impact of varying operational conditions on the extraction process, the optimal parameters were identified, resulting in a more effective experimental outcome. We aimed to deeply explore the potential application value of this method in the comprehensive utilization of CFA and provide a reference for the latter.

2. Materials and Methods

2.1. Sources of Test Materials and Analysis

In the experimental work of this thesis, the CFA utilized was sourced from the coal-fired combustion process at the Jungar Power Plant in Inner Mongolia. The fuel employed for this combustion was the 6# coal seam from the Guanbanwusu Mine. To guarantee the representativeness of the samples, they were systematically collected from diverse locations and at various time intervals. After collection, the samples underwent thorough mixing and were then properly stored for subsequent experimental use.

Twenty-five representative samples were collected from the 6# coal seam of the Guanbanwusu Coal Mine in strict accordance with the Sampling Method for Coal Seam Samples (GB/T 482-2008) [24]. The samples were then analyzed for the non-combustible solid substances remaining after the complete combustion of coal, that is, the ash content of the samples. The EDTA complexometric titration method was used to determine the contents of the main chemical components, such as SiO₂, Fe₂O₃, Al₂O₃, CaO, and MgO, in the CFA.

In this paper, the EDTA titration procedures and calculation methods adhered to the standard specified in GB/T 1574-2007 [25], but details are not provided. Astonishingly, the results showed that the mass fraction of Al₂O₃ in the CFA was close to 50%, categorizing it firmly as a typical high-alumina coal fly ash. The detailed data are presented in

Table 1. Contents of the main chemical composition in the samples.

Number	SiO2 (%)	Fe2O3 (%)	Al2O3 (%)	CaO (%)	MgO (%)
1	47.41	5.92	37.93	5.00	3.74
2	52.22	3.39	43.20	0.84	0.35
3	42.74	6.69	41.69	3.53	5.35
4	13.92	4.41	53.77	11.34	16.56
5	57.67	6.66	32.13	0.41	3.13
6	51.31	4.19	42.96	0.79	0.75
7	42.14	4.07	50.40	0.58	2.81
8	20.92	20.24	17.46	11.68	29.70
9	44.13	10.90	40.96	1.70	2.30
10	25.53	24.89	34.56	12.84	2.19
11	32.12	12.86	33.11	7.00	14.91
12	31.85	14.78	36.22	7.85	9.30
13	21.09	5.47	53.22	6.90	13.31
14	48.44	3.42	40.01	0.23	7.90
15	8.86	11.87	23.00	43.01	13.26
16	37.90	2.63	40.78	0.33	18.36
17	22.93	13.34	42.84	9.72	11.17
18	50.31	0.96	39.26	0.01	9.45
19	14.24	4.81	47.87	32.09	0.99
20	35.29	4.27	41.60	0.55	18.29
21	13.54	8.81	56.07	2.05	19.53
22	24.25	3.17	68.40	2.00	2.19
23	44.00	4.88	47.86	0.24	3.02
24	16.40	1.95	66.71	0.51	14.43
25	51.16	2.09	46.33	0.41	0.02

.

To determine the lithium content of the samples, the ash samples were subjected to microwave digestion. Then, they were analyzed by inductively coupled plasma mass spectrometry (TJA Inc., Boston, MA, USA). Samples 7, 16, 18, 23, and 25 were coal seam gangue, while the rest were coal seam samples. This study found that the geometric mean of lithium in the 6# coal of Guanbanwusu Mine reached 279 mg/kg, and in the separated coal gangue, it was 903 mg/kg. The specific measurement results are shown in Figure 1. After removing the separated coal gangue, the ash content of the clean coal was less than 7%. By burning the samples in a muffle furnace (Wuhan Jiecheng Electric Power Technology Co., Ltd., Wuhan, China), it was verified that the loss rate of lithium during the combustion process was within 1%, and almost all of it remained in the CFA and slag. When calculating the lithium content of the coal seam ash and taking the ash content of the clean coal as 7%, the lithium content of the ash could reach 3986 mg/kg. When converted to Li₂O content, it was 0.854%, which reached the industrial grade of pegmatite-independent lithium ore (0.8%, DZ/T 0203-2002) [26].

2.2. Extraction Instruments and Main Testing Methods

2.2.1. Test Instruments

An analytical balance, a pulverizer (Zhejiang Huanan Instrument Equipment Co., Ltd., Shaoxing, China), a standard sample sieve (Zhejiang Huanan Instrument Equipment Co., Ltd., Shaoxing, China), a muffle furnace (Wuhan Jiecheng Electric Power Technology Co., Ltd., Wuhan, China), porcelain crucibles (Shanghai Wuxiang Instrument & Meter Co., Ltd., Shanghai, China), nickel crucibles, a dryer, a constant-temperature magnetic stirrer (Shandong Sangze Instrument & Meter Co., Ltd., Jining, China), a water bath (Shanghai Kailang Instrument Equipment Factory, Shanghai, China), a temperature-adjustable electric heating jacket, XRD (Rigaku Corporation, Matsumoto, Japan), ICP-MS (TJA Inc., Boston, MA, USA), AAS (PerkinElmer, Inc., Waltham, MA, USA), a centrifuge, a pre-treatment device, and other instruments were used in this study.

2.2.2. Determination of Alumina Leaching Rate

The determination of alumina was carried out by routine analysis in accordance with the analysis method of coal ash composition in GB/T 1574-2007 [25].

The content of alumina was determined by EDTA complexometric titration. When the pH value is 1.8–2.0, sulfosalicylic acid is used as an indicator, and the solution is titrated with a standard EDTA solution. After that, an excess of EDTA is added to make aluminum and titanium in the solution react with EDTA in a complexation reaction. When the pH is 5.9, xylenol orange is used as an indicator, and an acetic acid zinc solution is used to back-titrate the excess EDTA. After the reaction is complete, a potassium fluoride solution is added to displace the EDTA complexed with aluminum and titanium. Then, it is titrated with a standard acetic acid zinc solution. The initial and final data of the standard acetic acid zinc solution during titration are recorded. The mass fraction of alumina can be obtained according to the following formula:

$$\omega \left(\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\right)={\displaystyle \frac{1.25\times \mathrm{T}\left(\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\right)\times \mathrm{V}}{\mathrm{m}}}-0.638\mathrm{w}\left(\mathrm{T}\mathrm{i}{\mathrm{O}}_2\right)$$

(1)

In the formula:

• ω(Al₂O₃) is the titer of the standard EDTA solution for alumina (mg/mL);
• V is the volume of the standard acetic acid zinc solution consumed by the test solution (mL);
• m is the mass of the ash sample (g); and
• 0.638 is the factor for converting titanium dioxide to alumina.

2.2.3. Determination of Lithium Leaching Rate

In this determination, the standard curve was drawn by flame atomic absorption spectrometry (PerkinElmer, Inc., Waltham, MA, USA) to determine the lithium content of the liquid to be tested so as to determine the final lithium recovery rate.

First, accurately pipette the standard lithium solution (1000 mg/L) to prepare standard lithium solutions at concentrations of 2 μg/mL, 1.5 μg/mL, 1 μg/mL, 0.8 μg/mL, 0.4 μg/mL, 0.2 μg/mL, 0.1 μg/mL, and 0.05 μg/mL. The standard curve is plotted as shown in Figure 2.

Then, on the flame atomic absorption spectrometer, adjust the wavelength to 670.8 nm and the spectral width to 1.4 nm. Ignite the air-acetylene flame. First, adjust the horizontal position and vertical height of the flame head and the burner to ensure that the absorbance of the element to be tested is in the maximum state. When adjusting the position, select the standard solution with the highest concentration in the series for spraying and testing. After the position is adjusted, zero-adjust with distilled water. After zero-adjustment, spray and test the solutions in the order of increasing concentrations in the series. After spraying and testing the standard series concentration solutions, draw a standard working curve according to the absorbance. Next, spray and test the solution to be tested. Before spraying and testing the solution to be tested, zero-adjust with distilled water again. Spray and test each solution to be tested two to three times. In this experiment, three times of spraying and testing times were selected. After the data are stable, take the average value of the three measurement results as the final absorbance of the element to be tested. The content of the element to be tested can be determined according to the drawn standard curve.

2.3. Research on Sintering-Leaching Scheme

CFA contains the major components SiO₂ and Al₂O₃, while lithium is mainly present in the glassy equivalent of CFA and is less abundant in other crystalline phases, such as mullite and quartz [27]. Simultaneous extraction methods of lithium and aluminum can be divided into an acid method, an alkali method, an ammonium sulfate sintering method [28], a sub-molten salt method [29], etc.

The acid method is used to extract lithium- and aluminum-containing leachates by reacting aluminum with acid to break the Si–Al bond, with the following chemical reaction formula:

Al₂O₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂O

(2)

Na₂SiO₃ + H₂SO₄ → H₂SiO₃ + Na₂SO₄

(3)

2Li⁺ + SO₄²⁻ → Li₂SO₄

(4)

The alkaline method is the same as the acid method, but Na₂CO₃ or NaOH are added, and the chemical reaction formula is as follows:

SiO₂ + CO₃²⁻ → SiO₃²⁻ + CO₂↑

(5)

Al₂O₃ + CO₃²⁻ → 2AlO₂⁻ + CO₂↑

(6)

2Li⁺ + SiO₃²⁻ → Li₂SiO₃

(7)

Li⁺ + AlO₂⁻ → LiAlO₂

(8)

In addition,

SiO₂ + 2OH⁻ → SiO₃²⁻ + H₂O

(9)

Al₂O₃ + 2OH⁻ → 2AlO₂⁻ + H₂O

(10)

2Li⁺ + SiO₃²⁻ → Li₂SiO₃

(11)

Li⁺ + AlO₂⁻ → LiAlO₂

(12)

Based on the research of other studies, the extraction of lithium and aluminum was carried out using methods such as sintering-concentrated acid leaching, sulfate sintering, concentrated alkali leaching, and sodium carbonate sintering.

Parallel experiments were carried out in accordance with the above four methods. The calcination temperature was set at 950 °C, and the calcination time was 1 h. For the detailed steps of the four leaching methods, please refer to the Supplementary Materials. The leaching results are presented in Figure 3.

From an environmental protection perspective, leaching CFA with concentrated acids or alkalis is discouraged. Prior research shows that during alumina extraction from CFA, adding a carbonate activator can boost CFA reactivity, facilitating subsequent reactions. Adding a sulfate sintering agent and sintering can then severely disrupt the Si–Al lattice, greatly increasing alumina dissolution [30].

2.4. Optimized Leaching Scheme

When selecting the extraction scheme, considering the actual production situation, the principles are determined as follows:

(1)

Ensure a high extraction rate. Through in-depth research and analysis of four experimental schemes, it is clear that increasing the leaching rate of metallic lithium is a key point. Since lithium is often trapped within the Si–Al lattice, to improve the leaching rate, it is necessary to release it first. As can be seen from the experimental results, adding a sintering agent can significantly increase the leaching rate of metallic lithium. Therefore, an appropriate amount of sintering agent should be added to the CFA for sintering before leaching.

(2)

Focus on low cost. Cost is an important indicator for evaluating the quality of a process. By studying different schemes, three main approaches to reducing the leaching costs of aluminum and lithium have been identified. First, maximize the recycling of materials to achieve material circular utilization. Second, minimize energy consumption. Third, on the premise of ensuring the extraction rate of the target product, extract other valuable energy sources as much as possible.

(3)

Ensure no pollution. The optimal extraction scheme should not only ensure a high lithium extraction rate and low cost but also guarantee no harm to the environment, achieving zero pollution.

Based on the above-mentioned optimization principles, the extraction scheme is formulated as follows:

(1)

Ignition Decomposition

In a porcelain crucible, weigh a certain quantity of air-dried CFA sample along with a specific mass of activator. Stir the mixture thoroughly to ensure uniform distribution. Subsequently, place the porcelain crucible in a muffle furnace and sinter the blend at a specific temperature for a defined period. Once the sintering process is completed, remove the crucible from the furnace and let it cool down to room temperature.

It has been observed that at a particular roasting temperature, the majority of lithium in CFA is converted into soluble lithium ions. This transformation can be attributed to the dehydration of kaolin and the dehydrogenation expansion of illite and muscovite. Nevertheless, following the initial sintering, chemical forces trap some lithium and aluminum within incompletely broken Si–Al bonds, thereby impeding their full-fledged reaction with the activator.

To facilitate the complete release of lithium and aluminum, it is imperative to thoroughly break the Si–Al bond structure. This necessity renders secondary roasting indispensable. Therefore, a sintering agent must be added to the sample in a pre-determined proportion. Stir the mixture once again to guarantee homogeneity. Place the crucible back into the muffle furnace and perform a second sintering at a certain temperature for a specific duration. After the second sintering process, take out the crucible and allow it to cool down to room temperature. By precisely controlling the parameters of secondary roasting, the complete liberation of lithium and aluminum can be accomplished, which in turn enhances the overall process efficiency and resource utilization.

(2)

Water Leaching and Dissolution

After the sample has cooled, transfer the entire sample into a dry and clean 250 mL wide-mouth bottle. Number the bottles in an orderly sequence. Add distilled water according to a solid-to-liquid ratio of 1:5. Secure the bottle caps tightly. Then, place the well-prepared samples on an electrothermal constant-temperature heating plate. Heat and dissolve the samples at 80 °C for a duration of 1 h.

(3)

Determination of Aluminum and Lithium

Filter the samples contained in the wide-mouth bottle. Quantify the filtrate obtained. Take 10 mL of the filtrate and utilize flame atomic absorption spectrometry to determine the lithium content. For the remaining liquid, employ the EDTA complexometric titration method to determine the aluminum content.

2.5. Univariate Tests

To increase the extraction rates of lithium and aluminum from CFA and optimize the utilization of CFA resources, the impacts on the leaching rates of lithium and aluminum were studied by selecting the ratios of the activator to the sintering agent during the sintering process, along with the concentration, temperature, and time of the leaching agent during the leaching process. This selection was based on previous research and comparisons. Throughout the entire experiment, it is essential to ensure that other variables remain constant.

2.5.1. Activator and Sintering Agent

The function of the activator is to break the Si–Al bond in the CFA and release the lithium and aluminum inside. The leaching of lithium and aluminum is greatly affected by the ratio of the activator. The comparative results of the 5:1, 2:1, and 1:1 ratios are shown in Figure 4.

The function of the sintering agent is to facilitate the liberation of lithium and aluminum. The adjustment of different ratios of the sintering agent can enhance the liberation of lithium and aluminum. The results of the comparison are presented in Figure 5.

The trend of the leaching rate of lithium and aluminum can be clearly discerned in the figure. After comprehensive consideration of the leaching rate of lithium and process cost, the mass ratio of CFA to the Na₂CO₃ activator was determined to be 2:1, and the mass ratio of CFA to the Na₂SO₄ sintering agent was 1:3.

2.5.2. Determination of Leaching Agent Concentration

In this test, acid leaching was employed to extract lithium and aluminum. The following figure illustrates the impact of varying leaching agent concentrations on the leaching efficacy of lithium and aluminum.

As illustrated in Figure 6, the leaching agent exerts a more pronounced influence on the leaching rate of aluminum. From the standpoint of cost, the 1% H₂SO₄ solution is deemed to be the optimal selection.

2.5.3. Determination of Leaching Temperature

It can be observed that different leaching temperatures may lead to different leaching rates. To illustrate this, a sintered specimen was added to a 1% H₂SO₄ solution, which was heated at different temperatures. The resulting leaching results are shown in Figure 7.

As illustrated in Figure 7, the integrated leaching rate is highest when the leaching temperature is increased to 80 °C, and the value will decrease if the temperature continues to increase or decrease. Therefore, the optimal leaching temperature is 80 °C.

2.5.4. Determination of Leaching Time

An experiment was conducted to determine the effect of varying leaching times on the leaching rate at a specific leaching temperature. The results are presented in Figure 8.

As illustrated in the line graph, the leaching rate initially increased with time, reaching a peak after 60 min. However, after this point, the rate declined. This suggests that the optimal leaching time is 60 min.

2.6. Orthogonal Tests

The optimal conditions for leaching aluminum were determined by a one-way test, as follows: a 1:2 ratio of Na₂CO₃ activator to CFA; a 3:1 ratio of Na₂SO₄ sinter to CFA; a 1% H₂SO₄ solution for leaching; and a temperature of 80 °C for leaching for 60 min.

The results of the one-way test demonstrated that while aluminum leaching exhibited relatively stable performance under the optimal conditions, lithium leaching exhibited a pronounced fluctuating trend when the one-way conditions were varied. This is attributed to the fact that the lithium present in the CFA is not solely concentrated in the amorphous substance but is also distributed or attached to various minerals in different forms [31]. Therefore, in order to determine superior lithium leaching conditions, orthogonal tests were chosen for systematic design and analysis.

In this experiment, the primary incineration temperature and incineration time and the secondary incineration temperature and incineration time were selected as the four factors to be studied. These factors were subjected to a three-level four-factor test to determine their effect on the leaching rate of lithium.

The remaining conditions were maintained at the optimal leaching conditions derived from the one-way test. Orthogonal tests were conducted using the L₉(3⁴) orthogonal table, resulting in a total of nine tests. The final results are presented in

Table 2. Result of orthogonal experiment.

Level	Factors				Lithium Leaching Rate/%
	A	B	C	D
1	600	30	400	30	74.53
2	600	60	500	60	75.99
3	600	90	600	90	57.89
4	800	30	500	90	70.41
5	800	60	600	30	63.69
6	800	90	400	60	72.59
7	1000	30	600	60	90.16
8	1000	60	400	90	76.99
9	1000	90	500	30	69.14
K1	208.41	235.1	224.11	207.36	T = 651.39
K2	206.69	216.67	215.54	238.74
K3	236.29	199.62	211.74	205.29
K1/3	69.47	78.37	74.7	69.12	Y = 72.38
K2/3	68.9	72.22	71.85	79.58
K3/3	78.76	66.54	70.58	68.43
R	29.6	35.48	12.37	33.45
Factors	B > D > A > C
Optimal Solution	1000 °C	30 min	400 °C	60 min

.

A review of the data in the above table reveals that the most influential factors are primary incineration time and secondary incineration time. This may be attributed to the incineration time being excessively long, which resulted in a reduction in lithium release. The primary incineration temperature and secondary incineration temperature also appear to be significant. From the results of the lithium leaching rate, which reached a maximum of 90.16%, it can be concluded that the optimal conditions are those with the highest indicators. Accordingly, the optimal program is defined as a primary incineration temperature of 1000 °C, with a duration of 30 min, and a secondary incineration temperature of 400 °C, with a duration of 60 min.

The results were verified by integrating the single-factor experiment and the orthogonal experiment. Three parallel experiments were conducted under optimal experimental conditions, and the experimental results are presented in Figure 9.

It was discovered, through three repeated verification experiments under these experimental conditions, that this process exhibits good repeatability. Specifically, the average leaching rate of lithium reached 90.19%, and the average leaching rate of aluminum reached 91.38%. These results indicate that the process parameters are reliable.

3. Results and Discussion

3.1. Optimal Leaching Scheme

Based on the results of the above experimental data, the specific scheme is as follows:

(1)

Pulverization and Sintering of CFA

A certain mass of CFA samples was pulverized and sieved to 200 mesh in a grinder, and the sample was prepared for sintering.

For the primary sintering process, a mixture was prepared by placing CFA and the Na₂CO₃ activator in a nickel crucible at a mass ratio of 2:1. This nickel crucible was transferred into a muffle furnace and the primary sintering operation was performed at 1000 °C for a duration of 30 min. Once completed, the crucible was removed from the furnace and allowed to cool down to room temperature. Subsequently, the cooled sintered material was pulverized.

For the secondary sintering, the Na₂SO₄ sintering agent was added to the CFA that had undergone primary sintering, maintaining a mass ratio of 1:3. The two components were thoroughly blended. Then, this well-mixed mixture was placed into a porcelain crucible and the secondary sintering was conducted within the muffle furnace. The sintering temperature was set to 400 °C and the process was maintained for 60 min. After the secondary sintering was over, the porcelain crucible was taken out and allowed to cool to room temperature; finally, the resulting substance was pulverized. This pulverized product served as the sample prepared for the subsequent leaching process.

(2)

Leaching of Samples

Upon cooling, all the samples from this experiment were transferred into a dry and clean 250 mL wide-mouth bottle. Distilled water was then added to the bottle at a solid-to-liquid ratio of 1:5. After capping the bottle, the sample–water mixture was placed on an electrothermal constant-temperature heating plate. The mixture was heated at 80 °C for 1 h to achieve dissolution.

(3)

Precipitation of Lithium and Aluminum

A variety of techniques can be used to extract alumina from CFA. The underlying principle is to destroy the Si–O–Al bond and enhance Al₂O₃ activity. The activation-sintering method adopted in this experiment is similar to the ammonium sulfate sintering method, but the destination of lithium in the subsequent alumina precipitation process is not clear; so, it is necessary to find out the problem of the destination of lithium in the process of alumina precipitation.

The experiment was conducted in five distinct groups. The first four groups were subjected to direct leaching with weak alkali reagents, resulting in pH values of 4, 7, 9, and 11, respectively. The fifth group was treated with an excess of strong alkali, resulting in a pH of 13. The comparative test results are presented in Figure 10.

The first group of filtrates was then treated with a weak alkali reagent in order to adjust the pH value to 11. This resulted in the generation of a white precipitate, which was identified as the Al(OH)₃. The proportion of lithium in the precipitate was then determined, and it was found to be 64.95%, in accordance with the original filtrate.

The chemical composition of the precipitate was determined by adjusting the pH of the leaching solution and observing the color of the precipitate. The first set of tests yielded a yellow-brown precipitate, predominantly composed of Fe(OH)₃. The second, third, and fourth groups of tests produced yellowish white precipitates (Figure 11), primarily a mixture of Fe(OH)₃ and Al(OH)₃. However, the fifth set of tests exhibited a notable change in color, initially transitioning from yellowish brown to yellowish white and subsequently returning to yellowish brown.

According to the research, both iron and aluminum play an indispensable role in the unidirectional enrichment of lithium. The addition of alkaline reagents to the leaching solution allows for the adjustment of the pH, which in turn facilitates the precipitation of Fe(OH)₃ and Al(OH)₃. This precipitation is more conducive to the unidirectional enrichment of lithium. Concurrently, to preclude the introduction of residual impurities into the precipitation, the pH was set to 7–8. This ensured the complete precipitation of aluminum while minimizing the precipitation of other metal ions under these environmental conditions. Given the acidic nature of the reaction environment and the concomitant requirement for a substantial alkaline reagent input, it was determined that the pH should be adjusted to approximately 6 by adding NaOH initially, followed by the addition of ammonia hydroxide to achieve a pH between 7 and 8. The precipitate transformed from yellow-brown to yellow-white. Subsequently, it was allowed to stand undisturbed for a certain period. Afterward, the mixture was filtered, and then, the obtained precipitate was washed thoroughly with water.

(4)

Aluminum–Lithium Separation

After the aluminum precipitation process, a mixture containing aluminum, lithium, and iron ions was obtained through filtration. The key is to separate lithium from aluminum. When Fe(OH)₃ is heated below 500 °C, it completely dehydrates into Fe₂O₃. The specific chemical formulas are as follows. It is prone to decompose during drying. However, when the temperature is not high enough, the decomposition is incomplete, meaning there is a gradual loss of water. Al(OH)₃ releases crystal water at 200 °C and loses all crystal water at 300 °C.

$$2{\mathrm{Fe}\left(\mathrm{OH}\right)}_3\stackrel{∆}{\to }{\mathrm{Fe}}_2{\mathrm{O}}_3 + 3{\mathrm{H}}_2\mathrm{O}$$

(13)

$$2{\mathrm{Al}\left(\mathrm{OH}\right)}_3\stackrel{∆}{\to }{\mathrm{Al}}_2{\mathrm{O}}_3 + 3{\mathrm{H}}_2\mathrm{O}$$

(14)

Based on the known information, three groups of samples were selected. The test procedures described above were carried out in accordance with the pre-determined conditions, and the data were meticulously recorded. The precipitated mixtures of aluminum, lithium, and iron ions from different groups were taken and incinerated at 500 °C, 600 °C, and 700 °C, respectively, for 30 min. After that, they were taken out, crushed, and ground. Then, distilled water was added, and the mixtures were shaken, washed, and filtered to obtain aluminum oxide and iron oxide precipitates, as well as the filtrate. The results of the assay are shown in Figure 12.

The test results show that when the incineration temperature starts from 500 °C, with the increase in temperature, the lithium washing rate is not affected. From the results of the above analysis, lithium and aluminum separation can be used under the condition of 500 °C incineration for 30 min, washing, and filtration. This process can effectively improve the lithium washing yield.

(5)

Lithium Extraction

The Li₂SO₄ concentration in the solution was relatively low, which resulted in a low lithium precipitation rate. In order to enhance the concentration of Li₂SO₄, a preliminary lithium-containing solution treatment may be conducted through the evaporation and concentration of Li⁺ enrichment. Once the Li₂SO₄ concentration in the solution attains a specific value, the evaporation process should be terminated, resulting in the formation of a lithium-containing concentrate, which is then filtered for subsequent lithium precipitation.

To determine the precise temperature for the evaporation experiment, the entire concentrate was evaporated to obtain the solid product. At this point, the solid was primarily composed of crystalline sodium sulfate salt. An XRD diffraction test analysis was conducted on the solid component (Figure 13). By studying the solid and its solubility differences, the temperature required for evaporation could be determined.

In the XRD analysis results, it can be seen that a large number of diffraction peaks appear between 20 and 40°, which are the evaporated Na₂SO₄ crystals. The objective of the evaporation test was to remove a substantial quantity of washing water from the solution, thereby ensuring that the precipitation of Na₂SO₄ and Li₂SO₄ crystals would not be affected. The solubility of Na₂SO₄ and Li₂SO₄ in water at varying temperatures was examined (Figure 14).

The solubility of Na₂SO₄ showed an increase with rising temperature. Due to the low solubility of sodium sulfate at low temperatures, freezing and cooling caused sodium sulfate to precipitate and increased the concentration of lithium in the solution (Figure 15); the precipitated sodium sulfate crystalline hydrate was recycled after treatment to reduce the production cost. The mother liquor containing lithium was evaporated and concentrated at 50 and 100 °C until saturation with crystals was reached, at which point stirring and cooling to room temperature was initiated. The precipitated trace crystals were then filtered off, and the resulting filtrate was microfrozen at −5 °C. A substantial quantity of salt was subsequently analyzed, and the precipitated crystals were filtered off once more. The evaporation–concentration–frozen sodium precipitation process was repeated cyclically until the concentration of lithium oxide in the solution reached 20 to 45 g per liter. At this point, CO₃²⁻ was introduced into the lithium concentrate to precipitate lithium, and the resulting solid product was obtained. The flowchart design is presented in Figure 16.

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Aluminum Extraction

Following the aforementioned steps, a mixture of Fe₂O₃ and Al₂O₃ was produced. The NaOH solution was then added to the mixture, producing a NaAlO₂ solution. The Fe₂O₃ did not participate in the reaction and could be filtered out. The resulting NaAlO₂ solution was then carbonized by the addition of CO₂, which resulted in the precipitation of Al(OH)₃, leaving behind a Na₂CO₃ solution. The latter solution could be recycled after concentration. The specific operation flow is shown in Figure 17.

3.2. Validation of Test Results

The preceding analysis indicates that the optimal conditions are as follows: The optimal conditions were found to be a 1:2 ratio of Na₂CO₃ activator to CFA; a 3:1 ratio of Na₂SO₄ sintering agent to CFA; a 1% H₂SO₄ solution for leaching; a temperature of 80 °C leaching for 60 min; and primary incineration. The optimal program calls for a temperature of 1000 °C and a time of 30 min for the first incineration, followed by a temperature of 400 °C and a time of 60 min for the second incineration. To ensure the reliability of the results, three samples were subjected to parallel testing. The final results are presented in Figure 18.

The results of the repeated tests, as shown in Figure 18, indicate that the process is effective and that the average leaching rate of lithium is 80.7% and the average leaching rate of aluminum is 85.4%. Therefore, the scheme is reasonable.

3.3. Discussion

This study aimed to comprehensively extract lithium and aluminum from the CFA of Guanbanwusu. The experiment was divided into two major parts (sintering-leaching and the extraction of lithium and aluminum) and followed the optimization principles of high extraction rate, low cost, and no pollution.

The sintering-leaching part has the advantages of simultaneously leaching aluminum and lithium, low equipment requirements, and easy process control. However, it has the problem of a large amount of sintering agent usage. The extraction part of lithium and aluminum successfully achieved the comprehensive extraction of lithium and aluminum from CFA. Nevertheless, the parameters of the extraction process still need to be optimized to further improve the extraction rate.

Using power plant CFA as the raw material, through orthogonal and single-factor experiments, and based on the research of acid- and alkali-based sintering processes, an optimal extraction scheme of acid–base sintering-acid leaching method is proposed. Under the optimal experimental conditions, the average extraction rate of lithium reached 80.7%, and the average extraction rate of aluminum was 85.4%.

This paper focuses on experimental research on the extraction of lithium and aluminum from CFA, achieving the resource utilization of CFA, strengthening the degree of comprehensive utilization, opening up new source routes for lithium and aluminum, and providing theoretical support for the comprehensive exploration, development, and utilization of coal. Currently, the experiment is in the laboratory stage with a relatively small sampling amount. In the follow-up, the experimental scale should be expanded to conduct large-batch extraction tests, aiming to obtain pure lithium carbonate products and promote the transformation of this technology from the laboratory to practical production applications.

4. Conclusions

The optimal conditions were as follows: a 1:2 ratio of the Na₂CO₃ activator to CFA, a primary combustion temperature of 1000 °C for 30 min, a 3:1 ratio of the Na₂SO₄ sintering agent to CFA, a secondary incineration temperature of 400 °C for 60 min; a 1% H₂SO₄ solution for leaching, a temperature of 80 °C, and leaching for 60 min. The results show that under the above conditions, the leaching of lithium and aluminum is remarkable, and both can achieve a leaching rate of approximately 80%.

In conclusion, the efficient extraction of lithium and aluminum from CFA can be achieved by optimizing the process parameters and rationally designing various conditions. This provides a new avenue for the sustainable utilization of waste resources, which is of significance for further research into the utilization of resources and the production of clean energy.