Abstract
Slime generally should be avoided in the coal flotation process. However, lignite exhibits a tendency for sliming under a flotation liquid-phase disturbance environment. This study innovatively proposes a high-efficiency lignite flotation strategy based on forced shear-induced sliming and preliminary desliming. This approach employs external shear forces to dissolve and disperse gangue minerals in lignite, which are then preremoved prior to flotation. Results demonstrate that the flotation clean coal yield increased from 17.25–36.65% to 39.92%∼55.71%, while the clean coal ash content decreased from 13.8–16.5% to 9.25%∼12.8%, improving lignite upgrading effect. Further characterization revealed that the lignite surface became smoother after sliming and slime removal treatment, which may create favorable flotation interface conditions. The observed larger particle–bubble adhesion angle directly confirms that the sliming and slime removal process promotes particle–bubble mineralization. Therefore, this may be a viable industrial lignite flotation method.
1. Introduction
With the continuous improvement of global environmental awareness, higher technical requirements are required for the efficient and clean utilization of lignite. As the primary fuel in thermal power generation, its inherent ash and sulfur not only significantly reduce thermal energy conversion efficiency but also generate substantial suspended particulate matter and harmful gases during combustion, causing severe environmental hazards. In this context, developing efficient lignite upgrading technologies holds significant strategic importance for achieving low-carbon development. Flotation, which separates particles based on surface property differences, serves as an effective means for ash and sulfur removal. However, the rich oxygen-containing groups on the lignite surface lead to poor floatability.
Current research on lignite flotation primarily focuses on high-efficiency reagents and pretreatment methods. It has been found that emulsification of reagents and the addition of surfactants can improve the flotation effect of oily collectors. , In particular, the combination of multiple reagents can not only save the amount but also improve the flotation recovery. Xia et al. theoretically confirmed the synergistic mechanism between the molecules of compound reagents. Pretreatment techniques, such as grinding, ultrasonic treatment, and thermal processing, can partially remove the oxidized surface layer and reduce porosity, thereby optimizing lignite floatability. Additionally, reverse flotation using depressants to inhibit coal particle flotation has attracted attention.
Slime formation is also a prevalent phenomenon in lignite flotation. The slime generates a large amount of high-ash fine particles, which not only lead to excessive ash content in the clean coal but also negatively impact lignite flotation through slime coating. Yu et al. demonstrates that finer high-ash fine particles are more prone to mechanical entrainment and coating on the coal surface. According to the characteristics of lignite being easy to slime, the strategy of forced shear sliming-slime removal before flotation separation was adopted in this study to reduce the ash content of the flotation feed and reduce the deteriorating influence of fine slime on particle–bubble mineralization during the flotation process.
2. Materials and Experimental Methods
The experimental lignite was sourced from Shengli Coal Mine of China. As shown in Table , the lignite contains 12.15% oxygen, and Fourier-transform infrared spectroscopy (Figure ) revealed that abundant −OH and −C–O exist on the lignite surface, which contributed to its hydrophilic properties. Particle size and density analysis (Tables and ) revealed that the coarse particles exhibit a higher ash content, and the +1.6 g/cm3 density fraction accounts for 14.83% of the total, with an ash content as high as 66.25%, suggesting that the lignite contains a high proportion of gangue minerals with high ash content and demonstrates certain feasibility for sliming.
1. Elemental Analysis of Lignite .
| Cad, % | Nad, % | Oad, % | Had, % | St,ad, % |
|---|---|---|---|---|
| 51.22 | 0.77 | 12.15 | 3.55 | 0.42 |
ad: air-dried basis; t: total
1.
FTIR spectra of lignite.
2. Particle Size Composition of Lignite.
| Particle size, mm | Percentage, % | Ash content, % |
|---|---|---|
| 0.50∼0.25 | 18.92 | 23.72 |
| 0.25∼0.125 | 24.68 | 19.10 |
| 0.125∼0.074 | 15.15 | 17.94 |
| 0.074∼0.045 | 11.69 | 17.55 |
| –0.045 | 29.56 | 35.92 |
| Sum | 100.00 | 24.59 |
3. Density Distribution of Lignite.
| Cumulative
floats |
Cumulative
sinks |
|||||
|---|---|---|---|---|---|---|
| Density, g/cm3 | Yield, % | Ash, % | Yield, % | Ash, % | Yield, % | Ash, % |
| –1.3 | 4.13 | 6.11 | 4.13 | 6.11 | 100.00 | 24.08 |
| 1.3–1.4 | 24.57 | 9.44 | 28.70 | 8.96 | 95.87 | 24.85 |
| 1.4–1.5 | 40.07 | 15.93 | 68.76 | 13.02 | 71.30 | 30.16 |
| 1.5–1.6 | 16.41 | 32.31 | 85.17 | 16.74 | 31.24 | 48.42 |
| 1.6–1.8 | 11.12 | 61.29 | 96.29 | 21.88 | 14.83 | 66.25 |
| +1.8 | 3.71 | 81.13 | 100.00 | 24.08 | 3.71 | 81.13 |
| Total | 100.00 | 24.08 | ||||
The sliming device and sliming mechanism are shown in Figure . The experimental conditions were set as follows: fixed sliming time of 10 min, with sludging speeds of 500, 1000, 1500, and 2000 rpm; then a fixed sliming speed of 1500 rpm, with sliming times of 5, 10, and 15 min. The sliming test under the same conditions was conducted twice, and the average values of the two sets of tests were taken as the final results.
2.
Sliming device and sliming mechanism.
Flotation experiments were conducted using a laboratory-scale flotation machine under the following conditions: slurry density of 80 g/L, impeller speed of 1800 rpm, and aeration rate of 0.1 m3/(m2 ·min). Diesel oil served as the collector, while octanol was used as the frother at a fixed dosage of 250 g/t. Two flotation tests were conducted under the same conditions. Surface elemental distribution was analyzed using an X-ray photoelectron spectrometer (XPS), with energy calibration at 284.80 eV. Scanning electron microscopy (SEM) was performed at 2000× magnification to examine surface morphology. During the test, 10 particles were randomly selected for observation, and representative particle morphologies were chosen for comparative analysis.
A self-designed particle–bubble adhesion device was employed to examine the particle–bubble interaction at a single-bubble scale, and the schematic diagram of the device is shown inFigure . In the experiment, 0.5 g of sample was added to a plexiglass trough containing 200 mL of deionized water. A 3.3 ± 0.1 mm diameter bubble was generated at 3 cm below the liquid surface. A CCD camera was utilized to capture the particle–bubble adhesion states at 10, 20, 40, and 80 s. The particle–bubble adhesion angle was measured five times under the same operating conditions, and the photograph closest to the average value was selected as the representative particle–bubble adhesion angle for that condition.
3.
Particle-bubble adhesion test device.
3. Results and Discussion
3.1. Changes in Lignite Particle Size and Surface Properties
As shown in Figure , with increased slime treatment time or stirring speed, the −0.045 mm particles gradually rose, indicating that higher energy inputs intensify the sliming effect. However, excessive extension of treatment time or stirring speed causes fragmentation of the organic matrix of coal. Therefore, an appropriate energy input is critical to balance these effects. The percentage of coarser lignite particles of 0.50∼0.074 mm decreased, while the proportions of 0.074∼0.045 and −0.045 mm particles increased after forced sliming treatment. This indicates that forced shear stirring induced fragmentation and slime formation in coarser lignite particles. Meanwhile, the ash content of 0.50∼0.045 mm particles showed reduction, whereas the ash content of −0.045 mm particles increased, suggesting that gangue minerals in coarser lignite underwent softening and disintegration.
4.
Effect of forced shear sliming on the particle size distribution: (a) change of −0.045 mm size particles at different times; (b) change of −0.045 mm size particles at different stirring speeds; and (c) overall particle size distribution.
As shown in Figure , the surface of lignite becomes smoother after sliming and slime removal treatment, likely due to the reduction of fine gangue minerals, which is corroborated by XPS analysis. The XPS results indicate that the surface C content increased, while the O, Si, and Al contents decreased for the +0.045 mm particles after the sliming treatment. Conversely, the surface C content decreased, whereas the O, Si, and Al contents increased for the −0.045 mm particles. These findings directly demonstrate that ash-related impurities migrated from coarser to finer particle fractions lead to an increase in the hydrophobicity of the surface of coarser particles.
5.
Surface morphology and element distribution before and after forced shear sliming of +0.045 and −0.045 mm particles.
3.2. Flotation Results after Sliming and Slime Removal Treatment
Forced shear slime treatment leads to a significant enrichment of ash-related impurities in the finer particles. After removing the – 0.045 mm slimes from lignite, flotation tests were conducted and results are shown in Figure . To facilitate comparison, the yield of clean coal after desliming was converted to the yield relative to the whole sample. Compared to untreated lignite, the lignite after sliming and slime removal treatment exhibited markedly enhanced flotation performance, in which the clean coal yield increased dramatically from 17.25–36.65% to 39.92–55.71%. Concurrently, the ash content of the clean coal decreased from 13.8–16.5% to 9.25–12.8%, significantly improving lignite ash removal efficiency. This is of great significance for the industrial upgrading and utilization of lignite. The mechanistic analysis of particle–bubble mineralization during flotation will be discussed in subsequent sections.
6.
Forced shear sliming process and flotation result of lignite before and after sliming and slime removal treatment.
3.3. Effect of Sliming and Slime Removal on Particle–Bubble Interaction
The interactions between particles and bubble surfaces were analyzed via particle–bubble adhesion angle measurements, as illustrated in Figure . For untreated lignite, the adhesion angle increased from 58° to 121°, while for slime-removed lignite, it rose from 80° to 160°, indicating that the sliming and slime removal processes significantly enhanced particle–bubble interactions. Furthermore, collector addition was shown to further promote particle–bubble adhesion. This phenomenon is attributed to the effective removal of high-ash fine mud and improved particle surface hydrophobicity.
7.
(a) Particle–bubble adhesion angle: (a) before sliming; (b) after slime removal; (c) before sliming with collector; and (d) after slime removal with collector.
4. Conclusions
The percentage of coarser lignite particles of 0.50∼0.074 mm decreased, while the proportions of 0.074∼0.045 and −0.045 mm particles increased after forced shear sliming treatment. However, excessive shear can cause fragmentation of organic matter, which may lead to loss of organic matter. The increased fine particles mainly come from gangue minerals containing O, Si, and Al elements. After removing the −0.045 mm slimes from lignite, the flotation clean coal yield increased dramatically from 17.25–36.65% to 39.92–55.71%, and the ash content of the clean coal decreased from 13.8–16.5% to 9.25–12.8%, improving lignite upgrading efficiency, which provides a theoretical reference for the flotation upgrading of lignite. After sliming and slime removal treatment, the particle–bubble adhesion angle increased from 58° to 160°, demonstrating that slime removal significantly enhanced particle–bubble interactions. These findings confirm that forced shear sliming and desliming provide optimal conditions for particle–bubble mineralization.
Acknowledgments
This work was supported by the National Key R&D Program of China (2023YFE0100600), the National Natural Science Foundation of China (52104278), and the China Postdoctoral Science Foundation (2025MD774062).
Y.X.: formal analysis, investigation, writingoriginal draft, and supervision. Y.L.: investigation, validation, and data curation. D.Z.: investigation and data curation. D.G.: methodology
The authors declare no competing financial interest.
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