Combined Gravity-Flotation Approach for Recovery of Unburned Carbon from Thermal Power Plant Slag

Abstract

This study aimed to study unburned carbon recovery from thermal power plant slag using heavy-medium separation and flotation methods. In this context, firstly, the chemical, physical, and mineralogical properties of the slag sample were determined. Based on the data obtained from the chemical, physical, and mineralogical properties of the slag, heavy-medium separation and flotation experiments were carried out. Extensive mineralogical, physical, and microscopic characterization revealed that the slag contains quartz, cristobalite, mullite, hematite, and graphite, and had a porous structure rich in cenospheres and partially unburned coal fragments. The heavy medium separation process showed limited selectivity due to the presence of hollow spherical particles and mineral intergrowth. Thus, the lowest ash content was obtained from the -0.5+0.074 mm fraction with a density of 1.3 g/cm³ in heavy medium separation experiments. Flotation experiments showed that increasing collector dosage improved combustible recovery and reduced ash content, while decreasing pH increased the recovery but also increased the ash levels. The best flotation performance was obtained with 55.0% ash content and 14.4% combustible recovery using 600 g/t of montanol at natural pH (10.1). Overall, the complex mineralogical structure of the slag, particularly the abundance of cenospheres and the intimate association of carbon with inorganic phases, had significantly limited the efficiency of physical separation methods. In conclusion, the combined mineral processing and beneficiation processes, particularly the combination of fine-sized separation methods, are necessary for recovering unburned carbon with higher purity.

Keywords

• Mineral processing
• Thermal power plant slag
• Unburned carbon
• Heavy medium separation
• Flotation

References

1. Behera, S., Mishra, D., Singh, P., Mishra, K., Mandal, S. K., Ghosh, C., Kumar, R., & Mandal, P. K. (2021). Utilization of mill tailings, fly ash and slag as mine paste backfill material: Review and future perspective. Construction and Building Materials, 309, Article 125120. https://doi.org/10.1016/j.conbuildmat.2021.125120
2. Bennehalli, B., Poyil, S. S., Lokesh, B., Nagaraja, S., Basavaraju, S., Rispandi, & Ammarullah, M. I. (2025). A review on the formation, recovery, and properties of coal fly ash (CFA)-derived microspheres for sustainable technologies and biomedical applications. Next Materials, 9, Article 101172. https://doi.org/10.1016/j.nxmate.2025.101172
3. Cangialosi, F., Notarnicola, M., Liberti, L., & Stencel, J. M. (2008). The effects of particle concentration and charge exchange on fly ash beneficiation with pneumatic triboelectrostatic separation. Separation and Purification Technology, 62(1), 240–248. https://doi.org/10.1016/j.seppur.2008.01.031
4. Chen, X., Gu, H. G., Liu, D., & Zhu, R. (2019). The flotation separation of barite-calcite using sodium silicate as depressant in the presence of sodium dodecyl sulfate. Physicochemical Problems of Mineral Processing, 55(2), 346–355. https://doi.org/10.5277/ppmp18136
5. Demir, U., Yamik, A., Kelebek, S., Oteyaka, B., Ucar, A., & Sahbaz, O. (2008). Characterization and column flotation of bottom ashes from Tuncbilek power plant. Fuel, 87(6), 666–672. https://doi.org/10.1016/j.fuel.2007.05.040
6. Deniz, C., Boke, Y., Aydin, O., & Benim, A. (2021). Computational analysis of pulverized coal co-firing with biomass in 150 MWE unit of Tuncbilek thermal power plant. Journal of Thermal Science and Technology, 41(1), 37–50
7. Egemen, E., & Yurteri, C. (1996). Regulatory leaching tests for fly ash: A case study. Waste Management & Research, 14(1), 43–50. https://doi.org/10.1177/0734242x9601400105
8. EPA. (2020). Environmental Protection Agency. Steam electric power generating effluent guidelines. https://www.epa.gov/eg/steam-electric-power-generating-effluent-guidelines

9. Groppo, J., Robl, T., & Hower, J. C. (2004). The beneficiation of coal combustion ash. Geological Society, London, Special Publications, 236(1), 247–262. https://doi.org/10.1144/gsl.sp.2004.236.01.15
10. Guttikunda, S. K., & Jawahar, P. (2014). Atmospheric emissions and pollution from the coal-fired thermal power plants in India. Atmospheric Environment, 92, 449–460. https://doi.org/10.1016/j.atmosenv.2014.04.057
11. Huang, Y., Takaoka, M., & Takeda, N. (2003). Removal of unburned carbon from municipal solid waste fly ash by column flotation. Waste Management, 23(4), 307–313. https://doi.org/10.1016/s0956-053x(02)00069-7
12. Hwang, J. Y., Sun, X., & Li, Z. (2002). Unburned carbon from fly ash for mercury adsorption: I. Separation and characteristics on unburned carbon. Journal of Minerals & Materials Characterization & Engineering, 1, 39–60
13. Izquierdo, M., & Querol, X. (2011). Leaching behaviour of elements from coal combustion fly ash: An overview. International Journal of Coal Geology, 94, 54–66. https://doi.org/10.1016/j.coal.2011.10.006
14. Kosivtsov, Y. Y., Chalov, K., Sulman, M. G., Lugovoy, Y., Novichenkova, T., Petropavlovskaya, V., Gadzhiev, S., & Popel, O. (2021). Use of ash and slag waste from thermal power plants as an active component of building materials. Chemical Engineering Transactions, 88, 331–336. https://doi.org/10.3303/cet2188056
15. Kravchenko, J., & Lyerly, H. K. (2018). The impact of coal-powered electrical plants and coal ash impoundments on the health of residential communities. North Carolina Medical Journal, 79(5), 289–300. https://doi.org/10.18043/ncm.79.5.289
16. Külaots, I., Hurt, R. H., & Suuberg, E. M. (2004). Size distribution of unburned carbon in coal fly ash and its implications. Fuel, 83(2), 223–230. https://doi.org/10.1016/s0016-2361(03)00255-2
17. Kumar, A., Paul, B., & Singh, S. (2010). A study on backfill properties and use of fly ash for highway embankments. International Journal of Engineering, Science and Technology, 2(7).
18. Kurama, H., & Kaya, M. (2008). Usage of coal combustion bottom ash in concrete mixture. Construction and Building Materials, 22(9), 1922–1928. https://doi.org/10.1016/j.conbuildmat.2007.07.008
19. Li, L., Liu, J., Li, X., Peng, Z., Han, C., Lian, W., Xue, B., Gao, C., Zhang, Q., & Huang, W. (2024). Physicochemical characteristics of residual carbon and inorganic minerals in coal gasification fine slag. Molecules, 29(16), Article 3956. https://doi.org/10.3390/molecules29163956
20. Liu, K., Jiang, J., Takasu, K., Zhang, Y., Zu, K., Gao, W., Yu, R., & Yao, W. (2024). Carbon removal flotation performance and economic analysis of different coal fly ash using waste fried oil as a collector. Separation and Purification Technology, 354, 129131. https://doi.org/10.1016/j.seppur.2024.129131
21. Ma, G., Huang, H., Liu, J., Dong, L., Bilal, M., Shao, H., & Tao, D. (2023). Effects of carrier particles on flotation removal of unburned carbon particles from fly ash. Powder Technology, 434, Article 119247. https://doi.org/10.1016/j.powtec.2023.119247
22. Maroto-Valer, M. M., Taulbee, D. N., & Hower, J. C. (1999). Novel separation of the differing forms of unburned carbon present in fly ash using density gradient centrifugation. Energy & Fuels, 13(4), 947–953. https://doi.org/10.1021/ef990029s
23. Mohsen, M. O., Aburumman, M. O., Diseet, M. M. A. A., Taha, R., Abdel-Jaber, M., Senouci, A., & Taqa, A. A. (2023). Fly ash and natural pozzolana impacts on sustainable concrete permeability and mechanical properties. Buildings, 13(8), Article 1927. https://doi.org/10.3390/buildings13081927
24. Nanda, B., & Rout, S. (2021). Properties of concrete containing fly ash and bottom ash mixture as fine aggregate. International Journal of Sustainable Engineering, 14(4), 809–819. https://doi.org/10.1080/19397038.2021.1920641
25. Oktay, Z. (2009). Investigation of coal-fired power plants in Turkey and a case study: Can plant. Applied Thermal Engineering, 29(2–3), 550–557. https://doi.org/10.1016/j.applthermaleng.2008.03.025
26. Özdemir, O., & Çelik, M. S. (2002). Characterization and recovery of lignitic fly ash byproducts from the Tuncbilek power station. Canadian Metallurgical Quarterly, 41(2), 143–150. https://doi.org/10.1179/cmq.2002.41.2.143
27. Pan, J. R., Huang, C., Kuo, J., & Lin, S. (2008). Recycling MSWI bottom and fly ash as raw materials for Portland cement. Waste Management, 28(7), 1113–1118. https://doi.org/10.1016/j.wasman.2007.04.009
28. Ryabov, Y. V., Delitsyn, L. M., Ezhova, N. N., & Sudareva, S. V. (2019). Methods for beneficiation of ash and slag waste from coal-fired thermal power plants and ways for their commercial use (a review). Thermal Engineering, 66(3), 149–168. https://doi.org/10.1134/s0040601519030054
29. Şahbaz, O., Öteyaka, B., Kelebek, Ş., Uçar, A., & Demir, U. (2008). Separation of unburned carbonaceous matter in bottom ash using Jameson cell. Separation and Purification Technology, 62(1), 103–109. https://doi.org/10.1016/j.seppur.2008.01.005
30. Şahbaz, O., Uçar, A., Emer, Ç., & Karagüzel, C. (2024). Advanced techniques on fine and coarse particle flotation. In Advances in material research and technology (pp. 55–76). Springer. https://doi.org/10.1007/978-3-031-49175-7_3
31. Sidek, N., Ma, H., Rahman, A. S. A., & Ruzilan, P. E. (2021). Physical properties and shear strength of boiler slag. Acta Mechanica Malaysia, 4(1), 01–04. https://doi.org/10.26480/amm.01.2021.01.04
32. Sobhy, A., & Tao, D. (2013). High-efficiency nanobubble coal flotation. International Journal of Coal Preparation and Utilization, 33(5), 242–256. https://doi.org/10.1080/19392699.2013.810623
33. IEA (2025). International Energy Agency. New IEA market report sees increasing competition from other power sources, though developments in China’s electricity sector will remain key for coal’s prospects. Global coal demand has reached a plateau and may well decline slightly by 2030. https://www.iea.org/news/global-coal-demand-has-reached-a-plateau-and-may-well-decline-slightly-by-2030
34. Uçurum, M., Toraman, Ö. Y., Depci, T., & Yoğurtçuoğlu, E. (2011). A study on characterization and use of flotation to separate unburned carbon in bottom ash from Çayirhan power plant. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 33(6), 562–574. https://doi.org/10.1080/15567030903117638
35. Unver, I. K., & Terzi, M. (2018). Distribution of trace elements in coal and coal fly ash and their recovery with mineral processing practices: A review. Journal of Mining and Environment, 9(3), 641–655
36. Vassilev, S. V., Menendez, R., Alvarez, D., Diaz-Somoano, M., & Martinez-Tarazona, M. (2003). Phase-mineral and chemical composition of coal fly ashes as a basis for their multicomponent utilization: 1. Characterization of feed coals and fly ashes. Fuel, 82(14), 1793–1811. https://doi.org/10.1016/s0016-2361(03)00123-6
37. Wang, G. C. (2016). Nonmetallurgical slags. In The utilization of slag in civil infrastructure construction (pp. 63–85). Woodhead Publishing
38. Wang, L., Tao, Y., Su, B., Wang, L., & Liu, P. (2022). Environmental and health risks posed by heavy metal contamination of groundwater in the Sunan coal mine, China. Toxics, 10(7), Article 390. https://doi.org/10.3390/toxics10070390
39. Williams, O., Newbolt, G., Eastwick, C., Kingman, S., Giddings, D., Lormor, S., & Lester, E. (2016). Influence of mill type on densified biomass comminution. Applied Energy, 182, 219–231. https://doi.org/10.1016/j.apenergy.2016.08.111
40. Xing, Y., Guo, F., Xu, M., Gui, X., Li, H., Li, G., Xia, Y., & Han, H. (2019). Separation of unburned carbon from coal fly ash: A review. Powder Technology, 353, 372–384. https://doi.org/10.1016/j.powtec.2019.05.037
41. Yamık, A., & Dogruoz, A. (2008). Recovery of unburned carbon by conventional flotation of bottom ashes from Tuncbilek thermal power plant. Journal of the Southern African Institute of Mining and Metallurgy, 108(3), 171–177
42. Yang, L., Li, D., Zhang, L., Yan, X., Ran, J., Wang, Y., & Zhang, H. (2020). On the utilization of waste fried oil as flotation collector to remove carbon from coal fly ash. Waste Management, 113, 62–69. https://doi.org/10.1016/j.wasman.2020.05.045
43. Yatsenko, E. A., Goltsman, B. M., Izvarin, A. I., Kurdashov, V. M., Smoliy, V. A., Ryabova, A. V., & Klimova, L. V. (2023). Recycling ash and slag waste from thermal power plants to produce foamed geopolymers. Energies, 16(22), Article 7535. https://doi.org/10.3390/en16227535
44. Yoon, R. H., & Luttrell, G. H. (1989). The effect of bubble size on fine particle flotation. Mineral Processing and Extractive Metallurgy Review, 5(1-4), 101–122. https://doi.org/10.1080/08827508908952646