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. 2024 Mar 13;9(12):14336–14342. doi: 10.1021/acsomega.3c10335

Separation and Recovery of Valuable Carbon Components and Li/Ga Metals in Coal Gangue by Using a Flotation Flowsheet

Dan Fang †,‡,§, Yangchao Xia †,‡,*, Yonggai Li †,§,*, Yaowen Xing †,‡, Zhenyong Miao †,§, Xiahui Gui †,‡
PMCID: PMC10976372  PMID: 38559930

Abstract

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Coal gangue (CG), an industrial solid waste with high contents of Li and Ga, has attracted the attention of researchers. However, the utilization of CG remains an economic challenge. Pre-enrichment of Li and Ga by flotation was carried out with a view to improving the comprehensive utilization of CG. Mineral composition, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and elemental composition were used to investigate the embeddedness of each mineral and the mode of elemental occurrence in the CG. The results showed that the main mineral compositions of the CG were kaolinite, quartz, and pyrite. Li and Ga were mainly associated with kaolinite and other clay minerals. Li and Ga had a high correlation with Al2O3 and SiO2, while Li and Ga were highly correlated with SiO2/Al2O3, indicating that Li and Ga may be associated with one or more high-alumina minerals. In addition, flotation tests proved that synergistic sorting of ash impurities and valuable components from the CG was a cost-effective method. The ash content of the final product was increased by 3% under the process of prediscarding concentrate–dissociation–secondary flotation, and the contents of Li and Ga in the final product were also slightly enriched, and the recovery rate of the carrier minerals of Li and Ga can reach 66.1%.

1. Introduction

Coal is one of the main energy sources in many countries, and coal gangue (CG) is a kind of gray-black rock produced in the process of coal processing and washing, with low carbon content and harder than coal, generally accounting for 15%–20% of coal production in China and 25% of industrial waste emissions.1−3 In recent years, with the significant increase in coal mining activities, CG has also become one of the largest industrial wastes.4,5 From 2010 to 2020, China has produced about 5 billion tonnes of CG; thus, CG has become an important environmental issue.6 Traditional treatment methods are based on piling up and landfilling, which leads to a large amount of land occupation, as well as the release of a large amount of minerals in the CG into the soil, thus causing serious environmental pollution.7,8 At present, a carbon-neutral environment is a common goal of mankind, and the spontaneous combustion and conventional utilization processess of CG produce direct or indirect carbon emissions, so there is a need to improve the comprehensive utilization rate of CG in the context of carbon-neutrality and to promote the multielement, multicomponent gradient utilization of CG.9−11

The demand for Li and Ga has been increasing dramatically in recent years with the widespread use of strategic metals, but the current supply of Li and Ga is limited due to the high cost of mining and low abundance, which has led to the need to try to recover Li and Ga from different substances.12−14 It is found that the contents of Li and Ga in CG are abnormally high; therefore, the recovery of Li and Ga from CG provides a new way for the comprehensive utilization of CG.15,16 In order to achieve the maximum utilization of Li and Ga in the CG, the occurrence mode of Li and Ga in coal and its byproducts has been studied.17−19 The study of the coal in the Urinary Basin found that 96.6–99.4% of Li existed in the form of silicates and aluminosilicates, and it was speculated that Li may exist in Illite and kaolinite.20 In the coal of Pingshuo, the contents of Li and Ga in the inorganic components were significantly higher than those in the organic components, and the main contents of Li and Ga were in the silicates of coal samples.21 According to the occurrence mode of Li and Ga in coal and its byproducts, the main methods for extracting Li and Ga are the: roasting method and acid–base leaching method.22−24 However, these methods pose problems, such as high consumption of acid/alkali and low metal recovery.25,26 Therefore, it is necessary to preseparate the noncarrier minerals before roasting and acid leaching to reduce the economic cost of later extraction.

This study entailed the examination of the CG and it was found to be rich in Li and Ga. The occurrence mode of Li and Ga in the CG was investigated. Ultimately, flotation was used to separate the organic carbon and inorganic minerals in the CG to achieve synergistic separation and recovery of ash impurities and valuable components of Li and Ga in the CG. This method reduces the carbon content in the CG, provides better raw materials for the carbon-based materialization of the CG, reduces the cost of Li–Ga extraction and separation, and improves the comprehensive utilization rate of the CG.

2. Materials and Methods

2.1. Experimental Materials

The samples of CG used in this study were collected from the Pingshuo coal mine, Shanxi province, China. The raw lump samples were initially crushed and sieved through a 0.5 mm standard sieve, then, after mixing, shrinking, and drying, put into the sealed bag for storage and reserve. Chemical reagents, such as CH3COONH4, CHCl3, HCl, CHBr3, and others (analytical grade), used in the experiment were supplied by Sinopharm Chemical Reagent Co., Ltd.

2.2. Mineral Composition Analysis

The main mineralogical phases were analyzed by powder X-ray diffraction (XRD, D8 ADVANCE, Bruker), which is a powder diffractometer equipped with a Ni-filtered Cu Kα radiation source and a LynxEye detector. The XRD data were analyzed with Jade 6.5 by Material Data, Inc. (MDI). X-ray fluorescence spectrometry (XRF, Panalytical Axios, Netherlands) was used to determine the major chemical compositions of CG. The polarized light reflection microscope (Zeiss Axio Scope. A1, Germany) was used to analyze the occurrence state and dissociation degree of minerals in the CG sample with reflected light, and the macerals were determined with an optical microscope equipped with an oil-immersion objective.

2.3. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) Measurement

A Bi liquid metal ion source was used to examine the surface composition distribution of coal gangue through a Time-of-Flight Secondary Ion Mass Spectrometer (TOF-SIMS, ION TOF-SIMS 5, USA). A Bi1+ primary ion beam having an energy of 30 keV with an incident angle of 45° was rastered over a 400 μm × 400 μm area on the sample surface for coal gangue analysis. To collect secondary ions, this area was divided into a grid of 256 points × 256 points, the primary ion beam had a spot size of approximately 200 nm, positive secondary ions with mass-to-charge ratios in the range of 0–900 amu were collected, and the bar scale on the distribution map represented the quantity of detected secondary ions.

2.4. Element Content Analysis

The contents of Li and Ga in CG were quantified using pulse counting mode ICP-MS (Agilent 7800, USA). Sample digestion was conducted using a closed high-throughput microwave digestion workstation (JUPITER-B, China). The digestion process involved adding 30 mg of the sample to the digestion vessel, followed by the sequential addition of 6 mL of 65% HNO3, 1 mL of 40% HF, and 1 mL of 70% HClO4. Then, the digestion vessel was placed in an acid trap at 120 °C for 40 min for predigestion to prevent excessive pressure during the microwave digestion process. After predigestion, 2 mL of 40% H2O2 was added, and the mixture was allowed to stand for 10 min. The mixture was introduced into the microwave digestion workstation after air bubbles. The microwave digestion program involved a temperature raised to 150 °C within 10 min in the first stage, then the temperature was further increased to 180 °C within 5 min in the second stage, and the temperature was further raised to 200 °C within 5 min in the third stage, and the fourth stage maintained the temperature at 200 °C for 100 min. After microwave digestion, the digestion solution was placed in an acid trap at 120 °C for 100 min. Finally, the digestion solution was diluted with 5% HNO3.

2.5. Flotation Tests

An XFD-1.0 L single-cell laboratory flotation machine was used for the flotation test. The pulp concentration was 80 g/L, the impeller rotation speed was 1800 rpm, and the aeration quantity was 0.1 m3/h. All tests were conducted at the pH value of the natural pulp. The specific flotation process is shown in Figure 1. Using diesel oil was used as the collector and sec-octanol as the foaming agent. The crushed CG sample was stirred for 2 min, and then the conditioning periods of the collector and foaming agent, respectively, were 2 and 0.5 min. The concentrate was collected in 3 min of flotation time. Later, the concentrate and the tailings were filtered, dried at 80 °C, and weighed. The tailings were ground, using an XMB Φ160 × 200 Intelligent Rod Mill. The grinding concentration was 2%, the grinding time was 10 min, the product after grinding was reflotation, the corresponding concentrate and tailings were collected, dried, and weighed, and the contents of Li and Ga in products were determined.

Figure 1.

Figure 1

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Flowchart of flotation tests.

3. Results and Discussion

3.1. Mineral Composition Analysis

The results of the mineral composition analysis of CG are shown in Figure 2. The mineral phases of CG contained kaolinite, boehmite, pyrite, and quartz. Because the diffraction characteristic peaks of kaolinite were much stronger than those of others, kaolinite was the main mineral phase component of CG. The main chemical components of CG were Al2O3 and SiO2, with contents of 44.855% and 42.719%, respectively. The ratio of Al2O3/SiO2 in the CG was about 1.05, which belonged to the high alumina–silicon ratio CG specified in the National Standard Classification of Gangue in China (GB/T 29 162–2012). In addition, Fe2O3 accounted for 4.824%, and other components were SO3, TiO2, K2O, etc.

Figure 2.

Figure 2

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Results of mineral composition analysis. (a) XRD patterns and (b) XRF results.

The mineral distribution characteristics of CG samples under Polarized light microscopy are shown in Figure 3. The surface of the CG sample was relatively rough and had abundant pores and cracks. The occurrence characteristics of gangue minerals in the CG sample were observed by polarized light microscopy, and the results showed that the gangue mineral occurrence forms were complex. Therefore, further dissociation of CG was required in subsequent flotation tests.

Figure 3.

Figure 3

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Mineral distribution characteristics of the coal gangue samples. (a) Coal–kaolinite intergrowth, (b) coal–pyrite intergrowth, (c) coal–kaolinite–pyrite intergrowth, and (d) quartz monomer (magnification: 500 ×).

3.2. Statistical Correlation Analysis of the Occurrence Mode of Li and Ga

The contents of Li and Ga in CG are listed in Table 1. It was found that there were high concentrations of Li and Ga in the samples, which were 344.47 μg/g and 30.47 μg/g, respectively, significantly exceeding the typical levels of Ga (6.55 μg/g) and Li (31.8 μg/g) detected in Chinese coal; therefore, these CG samples can be used as a substitute to extract Li and Ga.27

Table 1. Contents of Li and Ga in the Coal Gangue.

element Li Ga
content (μg/g) 344.47 30.47
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The results of statistical correlation analysis between Li–Ga and ash content, Al2O3, SiO2, and SiO2/Al2O3 in CG are shown in Figure 6. When R2 > 0.9 is a significant correlation, when 0.8 < R2 < 0.9 is a high correlation, and when 0.6 < R2 < 0.8 was a moderate correlation.20 Therefore, in Figure 4a, the contents of Li and Ga in CG were highly correlated with the ash content, indicating that Li and Ga had strong inorganic affinity. The contents of Li and Ga were moderately correlated with the Al2O3 content (Figure 4b) and SiO2 content (Figure 4c), indicating that Li and Ga were present in aluminosilicate minerals. When the ratio of SiO2/Al2O3 shows a relatively strong negative correlation, the elements may be associated with one or more Al-containing minerals, such as kaolinite and chlorite group minerals. In the CG samples, the contents of Li and Ga were highly correlated with SiO2/Al2O3 (Figure 4d), suggesting that Li and Ga may be associated with certain high-Al aluminosilicate phases in addition to Si-containing phases.28−30

Figure 6.

Figure 6

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Yield, ash content of each product in flotation of a coal gangue.

Figure 4.

Figure 4

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Results of statistical correlation analysis between Li–Ga and (a) ash content, (b) Al2O3, (c) SiO2, and (d) SiO2/Al2O3.

3.3. In-Situ Analysis of Li and Ga Occurrence

The chemical maps obtained by TOF-SIMS analysis can provide information about the distribution of elements in the CG samples, and it is an effective technique for detecting light elements and heavy metals.31 As shown in Figure 5, a 400 μm × 400 μm region of the sample was analyzed, and Li and Ga were found to be closely related to Al and Si, suggesting that Li and Ga predominantly occurred in aluminosilicate minerals. This may be because Li and Ga are lithophilic elements with a high affinity for oxygen and are easily soluble in silica–aluminum salt melts.32 This conclusion was consistent with the results of previous analyses.

Figure 5.

Figure 5

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TOF-SIMS chemical maps of Al, Si, C, Li, Ga, and Fe.

3.4. Li and Ga Flotation Pre-Enrichment Tests

Through the previous analysis results, it was known that most of Li and Ga were combined with inorganic minerals, and the minerals in the CG were embedded seriously, so it was proposed to use flotation to synergistically separate and recover the ash impurities and valuable components of Li and Ga in the CG. The yield and ash content of each product of flotation are shown in Figure 6, which shows that compared with the original product, the ash content in tailings 2 had been improved to 3%, indicating that the process can effectively separate the organic carbon and ash impurities in the CG.

The Li–Ga flotation pre-enrichment results are shown in Figure 7. Most of Li and Ga were associated with kaolinite, which was enriched in tailings 2 by flotation, and a small amount of Li and Ga was thrown off together with organic carbon (this part was negligible). As can be seen from the results, compared to the contents of Li and Ga in the original product, the content of Li in tailings 2 was enriched by a factor of 1.02, the content of Ga was enriched by a factor of 1.21, and the recovery rate of the carrier minerals of Li and Ga can reach 66.1%. Therefore, this process can achieve the synergistic flotation separation and recovery of ash impurities and valuable components Li and Ga in the CG, and this is expected to achieve the utilization of all coal gangue components.

Figure 7.

Figure 7

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Results of Li–Ga flotation pre-enrichment. (a) Li and (b) Ga.

4. Conclusion

A comprehensive study of the occurrence mode and sorting method of Li and Ga pre-enrichment in the CG. The results showed that the main minerals in the CG were kaolinite, quartz, pyrite, and boehmite, while Li and Ga were mainly associated with aluminum-containing minerals, minerals, such as kaolinite, and noncarrier minerals were quartz and pyrite. The results of the statistical correlation analysis showed that Li and Ga had a high correlation with Al2O3 and SiO2, while Li and Ga were highly correlated with SiO2/Al2O3, indicating that Li and Ga may be associated with one or more high-alumina minerals. The TOF-SIMS results showed that the distributions of Li and Ga were highly correlated with the distributions of Al and Si, suggesting that Li and Ga were related to the aluminosilicate minerals (kaolinite). The results showed that the ash content of the final product was increased by 3% under the process of prediscarding concentrate–dissociation–secondary flotation, and the contents of Li and Ga in the final product were also slightly enriched, in which the enrichment ratio of Li was 1.02, that of Ga was 1.21, and the recovery rate of the carrier minerals of Li and Ga can reach 66.1%. Compared with the process of direct dissociation and flotation, the process added a prediscarding concentrate process. The remaining tailings were dissociated and refloated, which can reduce the grinding time, and thus, saved the costs. The results of this paper showed that the pre-enrichment of Li and Ga in CG is an effective method, and it can also provide a new way for coal-based materialization of CG.

Acknowledgments

This work was funded by the National Key Research and Development Program of China (No. 2021YFC2902602), Jiangsu Natural Science Foundation-Youth Foundation (BK20210505), National Natural Science Foundation of China (Grant no. 52104278), and China Postdoctoral Science Foundation (2023M733769).

Author Contributions

∥ D.F. and Y.X. contributed equally to this work. D.F., Y.X., Y.L., Y.X., Z.M., and X.G. conceived and designed the experiments. D.F. performed the experiments, analyzed the data, and wrote the paper.

The authors declare no competing financial interest.

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