SCIENTIFIC ANALYSIS OF BROWN COAL FLOTATION USING SURFACTANTS DERIVED FROM INDUSTRIAL WASTE

ABSTRACT

This study addresses the global challenge of improving brown coal beneficiation efficiency while simultaneously utilizing industrial waste as a resource for surfactant production. In terms of methodology, the objectives of the work consist of studying the influence of waste-derived surfactants on the behavior and properties of coals during flotation and their selective separation. Emulsification of the samples was carried out using a high-shear device, followed by detailed analysis using SEM-EDS and FTIR techniques. The main outcomes obtained are the improved hydrophobic-hydrophilic ratio, increased recovery of combustible materials (70–89%), as well as a decrease in mineral contamination. A significant dependence of flotation efficiency on the surface chemistry change was identified. The improved flotation performance is attributed to enhanced adsorption of waste-derived surfactants on coal surfaces, leading to increased hydrophobicity and improved particle–bubble attachment efficiency.

АННОТАЦИЯ

В данной работе исследуется актуальная научная проблема повышения эффективности обогащения бурых углей при одновременной утилизации промышленных отходов. Целью исследования является изучение влияния поверхностно-активных веществ, синтезированных на основе промышленных отходов, на процессы флотации и селективного разделения компонентов угля. Методология включает применение высокосдвиговой эмульсификации, а также комплексный анализ с использованием SEM-EDS и FTIR методов для оценки структурных и поверхностных изменений. Полученные результаты показали, что использование разработанных ПАВ приводит к улучшению гидрофобно-гидрофильного баланса, увеличению извлечения горючей массы до 70–89% и снижению содержания минеральных примесей. Установлена зависимость между эволюцией поверхностной химии и эффективностью флотационного разделения. Сделан вывод о перспективности применения ПАВ на основе промышленных отходов для повышения энергоэффективности и экологичности процессов переработки угля.

Keywords: brown coal, flotation, surfactants, industrial waste, surface chemistry, beneficiation efficiency, selective separation

Ключевые слова: бурый уголь, флотация, поверхностно-активные вещества, промышленные отходы, поверхностная химия, эффективность обогащения, селективное разделение

Introduction

Recently, there is an important task related to the improvement of the effectiveness of using low-grade coals and creating ecological technology for their processing in the energy industry worldwide [1, p. 23975]. In the year 2024, fossil fuels maintained their dominance in electricity production, contributing a combined share of 59.19% [2, p. 14], where coal was the leading fossil fuel energy source, contributing 34.08%, followed by natural gas at 22.30% and oil at 2.81% (Figure 1) [3, p. 68].

Figure 1. Global electricity generation by source in 2024 (share, %)

Thus, the creation of surfactants from waste products and the study of brown coal flotation with the aim of establishing selective separation laws is a great scientific achievement [8, p. 5].

Materials and methods

For the current work, low rank coal samples (2BR-B2 and 2BOMSH-B2) collected from the Angren deposit served as the source materials [9, p. 930]. The physicochemical properties of the coal samples, including ash content, moisture, and elemental composition, were determined prior to flotation experiments to ensure reproducibility and comparability of results.  Industrial wastes produced by the Shurtan Gas Chemical Complex were used for the synthesis of surfactants which were then employed as flotation agents [10, p. 108]. The waste materials mainly consisted of hydrocarbon-rich residues containing surface-active components, which were modified and used as flotation reagents[11, p. 895]. Beneficiation was done through the implementation of gravity-flotation method in a controlled hydrodynamic environment to determine the effectiveness of selective separation. The flotation process was carried out under controlled conditions, including constant air flow rate, pulp density, and agitation speed, to ensure stable hydrodynamic conditions [12, p. 44].

The creation of a stable colloidal system is possible. When combining water and hydrocarbon phases, surfactant precursors (0.5-3.0 wt.%) are used; surfactants adsorb at the interface and form a stable emulsion at the critical micelle concentration [13, p. 4,5]. High shear (5,000–20,000 rpm) for 10–15 minutes was selected based on previous studies demonstrating optimal droplet dispersion and enhanced interfacial area, which improves surfactant adsorption efficiency. [14, p. 8]. The phase ratio is kept at 70:30 or 60:40 (water:oil) with pH within 6.5–8.0 at 20–30 °C, corresponding to near-neutral conditions favorable for stable surfactant adsorption and coal surface modification. This reduction in free energy promotes the formation of stable emulsions and enhances interaction between surfactant molecules and coal surfaces. [15, p. 1568]. The emulsions were stabilized for 24-72 hours without phase separation [16, p. 496]. This stability ensures sufficient time for surfactant adsorption onto coal particles prior to flotation [17, p. 1380]. This technology increases surfactant adsorption on the surface of coal particles and ensures a good hydrophobic-hydrophilic balance, increasing the probability of interaction between bubbles and particles during flotation. Therefore, the use of reagents decreases by 15-30 %, and separation efficiency increases (Figure 2).

Figure 2. Schematic illustration of high-shear emulsification process for surfactant-assisted system preparation

The prepared samples were examined using SEM-EDS and FTIR techniques to characterize structural changes, elemental composition, and surface chemistry [18, p. 323]. SEM-EDS analysis was used to evaluate morphological and elemental changes, while FTIR spectroscopy provided information on functional group modifications affecting surface chemistry [19, p. 6]. Furthermore, the flotation behavior was assessed via the examination of combustible matter yield, ash, and sulfur content, which facilitated the identification of surface chemistry-surface chemistry relationships [20, p. 61]. However, most previous studies have focused on conventional surfactants, while the application of waste-derived surfactants remains insufficiently explored. This study aims to fill this gap by investigating the role of industrial waste-based surfactants in modifying coal surface properties and improving flotation selectivity[22, p. 6].

Results and discussion

The received results make it possible to clearly observe the physicochemical processes of brown coal enrichment associated with surfactants. The SEM-EDS study of Angren lignite (2BR-B2) shows considerable differences in the structure and composition depending on the stages of processing. From the images (see Figures 3-5), it can be seen that the raw coal has a heterogenous and porous surface structure with the presence of scattered inclusions. This sample shows a high level of carbon content (65.0 wt.%) along with oxygen (26.4 wt.%) and mineral elements. This morphology indicates a low degree of surface hydrophobicity, which negatively affects flotation efficiency [23, p. 110].

Figure 3. SEM microstructure and corresponding EDS spectrum of raw Angren lignite (2BR-B2)

The mineral-bearing sample (Figure 4) has a more dense and layered structure with an increased proportion of inorganic compounds. The increase in mineral components confirms the presence of hydrophilic phases, reducing selectivity during flotation. It is supported by a low concentration of carbon (42.8 wt.%) and increased oxygen (36.7 wt.%), silicon (10.2 wt.%) and aluminum (8.6 wt.%), which corresponds to the enriched ash-forming minerals such as silica and alumina. The image of flotation sample (Figure 5) demonstrates more porous surface with an increase of carbon content (60.7 wt.%). The increase in carbon content and porosity suggests improved surface hydrophobicity, facilitating better attachment to air bubbles.

Figure 4. SEM microstructure and EDS analysis of mineral-rich fraction after preliminary processing

Moreover, EDS data indicate that sulfur is mainly present in the mineral phase. This trend is reflected in the behavior of flotation performance as well, wherein an increase in water recovery from 5% to 80% improves combustibles recovery from 17% to 94% but increases ash (4 – 59%) and sulfur (5 – 61%) entrapment due to hydrodynamic effects.

Figure 5. SEM microstructure and EDS spectrum of upgraded coal sample after flotation treatment

Selective treatment is seen at intermediate levels of water recovery (40 – 60%), wherein particle-bubble interaction is optimum but excessive entrapment compromises effectiveness. Altogether, both SEM-EDS and flotation results suggest that surfactant-induced operations lead to coal processing improvement through alteration of surface properties.

Figure 6. Influence of water recovery and combustible matter recovery on sulfur and ash-forming mineral recovery to concentrate

The study of flotation effectiveness proves the high level of dependence of the process of hydrodynamics on the selective separation of coal components. It can be noted from Figure 6 that the increase of water content in the froth fraction from about 5% to 80% leads to an essential increase in combustible matter concentration from about 17% to 94%, which shows the increase of the efficiency of particle-bubble interaction. But at the same time, it leads to a proportional increase in sulfur content from 5% to 61% and ash-forming minerals from 4% to 59%, which suggests a considerable part played by mechanical separation. This behavior can be explained by increased entrainment of fine mineral particles at high water recovery levels, leading to reduced selectivity. At moderate water recovery (40–60%), optimal hydrodynamic conditions allow selective attachment of hydrophobic coal particles while minimizing the recovery of ash-forming minerals.

As seen from Figure 6, while the recovery of combustible matter is increasing, the recovery of sulfur and ash is growing proportionally, demonstrating the lack of selectivity at a high level of recovery.

As seen in Figure 7, there is almost a linear relationship between sulfur and ash recovery, which indicates that sulfur is mainly present in the mineral phase. This inference is supported by the data presented in Figure 7, depicting the existence of a linear relationship between total sulfur recovery and pyrite recovery, thus validating the fact that pyritic sulfur is the main sulfur species in the investigated coal sample. The linear correlation between sulfur and ash recovery confirms that sulfur is predominantly associated with mineral phases, particularly pyrite, rather than organic coal matter.

Figure 7. Correlation between sulfur, ash-forming mineral, and pyritic sulfur recovery to concentrate

It should be noted that the best results in terms of separation efficiency can be achieved in the range of moderate water recovery (40–60 %), during which combustible matter recovery ranges from 70 to 89 % without over-flotation of ash and sulfur. Flotation selectivity reduces outside this range due to hydrodynamic effects.

FTIR spectra of the initial Shargun coal and Angren lignite (BOMSHSH-B2) samples, as well as their spectrum after gravity-flotation treatment, exhibit pronounced differences concerning mineralogical and surface features associated with colloid-chemical transformations. The spectrum of initially processed Shargun coal contains a weak line around 2324 cm⁻¹ corresponding to background CO₂. After treatment, the spectrum becomes much more pronounced, with absorption bands at 2324, 2050, and 2002 cm⁻¹ (T ≈ 72-73%), as well as low-frequency bands at 422 and 403 cm⁻¹ (T ≈ 53-54%) (Figure 8).

Figure 8. FTIR spectra (4000–400 cm⁻¹) of Sharg‘un coal  before and after gravity–flotation beneficiation

Analogous trends, albeit somewhat more pronounced, are noted in relation to the Angren lignite. As regards the raw BOMSHSH–B2, the FTIR spectrum features an aliphatic C–H band at 2849.09 cm⁻¹ (transmittance (T) = 82.71%), the presence of a very intense band at 1000.92 cm⁻¹ (T ≈ 68.18%) characteristic of oxygen-containing and mineral-associated structural elements, as well as the existence of the mineral lattice band at 421.88 cm⁻¹ (T ≈ 57.35%) (Figure 9).

before

after

Figure 9. FTIR spectra (4000–400 cm⁻¹) of  Angren lignite (BOMSHSH–B2) before and after gravity–flotation beneficiation

Following the flotation treatment, a shift in the position of the mineral signal to a lower frequency is evident at 420.27 cm⁻¹ with a transmittance of about 72.44%; additionally, the ~1000 cm⁻¹ band experiences a change in its position to 1031.13 cm⁻¹ with T ≈ 72.23%.  The comparative analysis of the physicochemical properties of Angren lignite samples (2BR-B2 and 2BOMSH-B2) before and after beneficiation demonstrates a substantial improvement in fuel quality parameters. As shown in Table, the moisture content decreases from 20–40% to 15–40%, while ash content is significantly reduced from 35–60% to 20–35%, indicating efficient removal of mineral impurities during gravity–flotation treatment. In addition, the transformation of particle size from 1–100 mm to a briquetted form enhances mechanical strength and handling properties. The increase in volatile matter content (V_daf) to 44–55% for 2BR-B2 and 40–55% for 2BOMSH-B2 suggests improved reactivity of the coal matrix. Consequently, the higher heating value (HHV) rises to 28.6 MJ/kg and 29.7 MJ/kg, respectively, while the lower heating value (LHV) reaches 15.9 and 15.7 MJ/kg, confirming a notable enhancement in the energy potential of the material. These changes clearly indicate that beneficiation effectively upgrades low-rank coal into a more efficient fuel suitable for industrial applications.

Table 1.

Changes in key physicochemical and fuel characteristics of Angren lignite samples (2BR-B2 and 2BOMSH-B2) after beneficiation

This directly explains the improved flotation performance and the observed increase in combustible matter recovery, establishing a clear relationship between surface chemistry evolution and beneficiation efficiency. These data show that the application of surfactants synthesized from industrial waste is quite efficient in improving the quality and effectiveness of flotation. At the same time, it should be noted that this technology requires special attention regarding hydrodynamic processes, which can lead to excessive mineral entrapment at very high levels of coal recovery. Further research must concentrate on improving the surfactant formula, parameters of the flotation process (pH, dispersion, and concentration), and molecular modeling techniques.

Conclusion.

In summary, the presented work illustrates the effectiveness of surfactants obtained from industrial waste in enhancing the flotation performance of brown coal through the modification of its surface properties. By applying gravity-flotation treatment in combination with such surfactants, a noticeable increase in the recovery of combustible matter (from 70% to 89%) and a decrease in mineral impurities were achieved, confirming the efficiency of the proposed approach. Using SEM-EDS and FTIR analyses, the improvement in coal flotation behavior was explained by changes in surface morphology and functional group composition, indicating enhanced hydrophobicity of coal particles. This improvement is attributed to the increased adsorption of surfactant molecules onto the coal surface, which promotes stronger particle–bubble interactions during flotation. In addition, establishing the relationship between surface chemistry evolution and flotation selectivity provides deeper insight into the mechanism of coal beneficiation. It was determined that optimal flotation performance is achieved at moderate water recovery levels (40–60%), where selective separation is maximized and mechanical entrainment of mineral matter is minimized. Overall, the results demonstrate the feasibility of using industrial waste as a cost-effective and environmentally sustainable source for flotation reagents. Compared to conventional surfactants, waste-derived reagents offer additional advantages in terms of resource efficiency and waste valorization, contributing to circular economy principles. Furthermore, the findings highlight the importance of controlling hydrodynamic conditions and interfacial phenomena to achieve high selectivity in coal flotation systems. Future research should focus on optimizing surfactant composition, controlling process parameters under varying hydrodynamic regimes, and applying molecular-level modeling approaches to better understand adsorption mechanisms and interaction energies.

References:

1. Kumar, B., Das, B., Garain, A., Rai, S., Begum, W., Inamuddin, M., ... & Saha, B. (2022). Diverse utilization of surfactants in coal-floatation for the sustainable development of clean coal production and environmental safety: a review. RSC advances, 12(37), 23973-23988.
2. Kucharov, A., Xalilov, S., & To‘Rayeva, X. (2024). Results Of Scientific Analysis Of Coal Processing Products. Journal of Experimental Studies, 2(3), 9-16.
3. Jurayevna, J. D., & Zuvurovich, T. O. (2016). The obtainment of carbon adsorbents and their compositions for cleaning industrial wastewater. Austrian journal of technical and natural sciences, (3-4), 67-70.
4. Muratov, M., Kurniawan, T. A., Eshmetov, R., Salikhanova, D., Eshmetov, I., Adizov, B., ... & Onn, C. W. (2024). Promoting sustainability: Micellization and surface dynamics of recycled monoethanolamine surfactants. Journal of Molecular Liquids, 414, 126010.
5. Kocharov, A. A., Mamanazarov, M. M., Atabekova, D. L., & Toshbobayeva, R. A. (2024). Scientific Analysis of the Ecological Condition of the Soils Around the Angren Coal Mine. In International Congress on Biological, Physical And Chemical Studies (ITALY) (pp. 19-21).
6. Bobojonov, K., Usmanova, K., Smanova, Z., Gafurova, D., & Abdullayeva, M. (2026). Analytical application of sorption–fluorescence methods for the determination of aluminium, beryllium, and lead in environmental samples. International Journal of Environmental Analytical Chemistry, 106(1), 142-162.
7. Azizbek, K., Farkhod, Y., Sanjar, K., & Sukhrob, Y. (2025). Technology for processing coal and converting it into an energy source by extracting gas. Austrian Journal of Technical and Natural Sciences, (3-4), 32-36.
8. Kucharov, A., Xalilov, S., Farxod, Y., Toshboboyeva, R. N., Bekturdiyev, G., Turayeva, K., ... & Golib, T. (2026). A comprehensive technological approach for the selective recovery of aluminum oxide and rare earth elements from coal processing. EUREKA: Physics and Engineering.
9. Buckley, T., Xu, X., Rudolph, V., Firouzi, M., & Shukla, P. (2022). Review of foam fractionation as a water treatment technology. Separation Science and Technology, 57(6), 929-958.
10. Oulkhir, A., Lyamlouli, K., Danouche, M., Ouazzani, J., & Benhida, R. (2023). A critical review on natural surfactants and their potential for sustainable mineral flotation. Reviews in Environmental Science and Bio/Technology, 22(1), 105-131.
11. Lian, F., Li, G., Cao, Y., Zhao, B., Zhu, G., & Fan, K. (2023). Experimental study on the dispersion behavior of a microemulsion collector and its mechanism for enhancing low-rank coal flotation. International Journal of Mining Science and Technology, 33(7), 893-901.
12. Юсупов, Ф. М., Кўчаров, А. А. У., Маманазаров, М. М. У., & Саъдуллаева, Х. Н. К. (2020). Улучшение качества бурых углей марки 2бр-в2 и 2бомсш-б2 с помощью химической обработки. Universum: технические науки, (3-2 (72)), 43-46.
13. Wen, P., Ma, X., Fan, Y., Dong, X., & Chen, R. (2024). Study on the mechanism of anionic composite surfactant pretreatment to promote flotation of clay-rich coal slurry: Interfacial interaction. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 683, 133058.
14. Kucharov, A., Xalilov, S., Alikulova, M., Mamarasulov, T., Turayeva, K., Imanova, G., ... & Farmonov, N. (2025). SCIENTIFIC ANALYSIS OF THE DEVELOPMENT OF NEW TYPES OF FLOTATION REAGENTS USED IN COAL ENRICHMENT. EUREKA: Physics & Engineering, (2).
15. Gao, L. X., Li, X. G., Lyu, X. J., & Zhu, X. N. (2024). Advances and perspectives of green and sustainable flotation of low-rank/oxidized coal: a review. Energy & Fuels, 38(3), 1566-1592.
16. Banat, I. M., Makkar, R. S., & Cameotra, S. S. (2000). Potential commercial applications of microbial surfactants. Applied microbiology and biotechnology, 53(5), 495-508.
17. Zheng, L., Liu, Z., Li, D., Wang, H., & Zhang, Q. (2021). Micromechanism analysis of surfactant wetting of coal based on 13C NMR experiments. Acs Omega, 6(2), 1378-1390.
18. Cheng, G., Zhang, M., Zhang, Y., Lin, B., Zhan, H., & Zhang, H. (2022). A novel renewable collector from waste fried oil and its application in coal combustion residuals decarbonization. Fuel, 323, 124388.
19. Kucharov, A., Xalilov, S., Qurbonov, A., Yaxshiyeva, Y., & Yuldashev, R. (2021). Development of technology for water concentration of brown coal without use and use of red waste in this process as a raw material for colored glass in the glass industry. In E3S Web of Conferences (Vol. 264, p. 01050). EDP Sciences.
20. Yusupov, F. M., Mamanazarov, M. M., Kucharov, A. A., & Saidobbozov, S. S. (2020). Properties of spherical granules based on aluminum oxide. Universum: chemistry and biology, 3, 59-63.
21. Temirov, G. B., Yusupov, F. M., Yusupov, S. K., Baymatova, G. A., Kucharov, A. A., Yodgorov, N., & Nuriddinova, D. Z. (2025). Scientific Dynamics And Global Contributions To Anion Exchange Resin Research (2000–2025): A Comprehensive Bibliometric Study. Rasayan Journal of Chemistry, 18(4).
22. Lian, F., Deng, L., Li, G., Cao, Y., Zhao, B., & Fan, K. (2023). Effective purification of the low-rank coal by the collaboration of the microemulsion collector and the CO2 nanobubbles. Fuel, 339, 127370.
23. Saidmuhamedova, M. Q., Turdiqulov, I. H., Atakhanov, A. A., Ashurov, N. S., Abdurazakov, M., Rashidova, S. S., & Surov, O. V. (2023). Biodegradable Polyethylene-Based composites filled with cellulose Micro-and nanoparticles. Eurasian Journal of Chemistry, 28(2 (110)).