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

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.