Investigation of bivariate particle size-density classification and quality differentiation of coal gasification fine slag

Physical separation has inherent limitations in the carbon-ash separation of coal gasification fine slag(CGFS). Strengthening fundamental research on the composition and structural properties of CGFS can enhance separation efficiency. This study investigates the particle size-density classification characteristic of typical GSP entrained-flow coal gasification fine slag. The distribution patterns of residual carbon, macroscopic morphology of ash samples, micro-morphology of particles, and pore structure characteristics are systematically investigated by loss on ignition analysis, XRD mineral analysis, SEM morphology analysis, and N₂ gas adsorption. The results indicate that residual carbon in the CGFS primarily exists as two forms:amorphous carbon and porous carbon. Amorphous carbon accounts for approximately 8-14% of the total fine slag, while porous carbon constitutes about 10%. A clear correlation is observed between the macroscopic morphology of ash samples and their mineral compositions: ash colors ranging from deep red to light red to black correspond to mineral assemblages of hematite/iron-bearing mica, anorthite/cristobalite, and wüstite, respectively. This finding provides a basis for mineral identification in CGFS using visual features. Combining residual carbon content, micro-morphology, and pore structure of classified fractions, we traced the formation pathways of CGFS particles and categorized them into three typical groups: “Amorphous carbon”: Primarily found in fine-sized (0.074-0.25 mm) and low-density (<1.9 g/cm³) fractions, featuring interwoven carbon-ash fusion and a moderate specific surface area. “Porous carbon”: Present in medium-coarse size fractions (0.074-0.25 mm) and medium-high density fractions (1.9-2.3 g/cm³), with well-developed internal pores and a large specific surface area. “Slag particles”: Predominantly in coarse size fractions (>0.25 mm) and high-density fractions (>2.5 g/cm³), possessing the smallest specific surface area. Separation and recovery methods should be tailored to the characteristics of the two residual carbon types.Recovered amorphous carbon can serve as a fuel supplement, porous carbon is suitable for adsorption applications, and slag particles can be purposed in construction materials.