Feasibility and mechanism analysis of hydrophobic-hydrophilic separation on the residual carbon from coal gasification fine slag

The coal gasification fine slag is a kind of solid waste that is poorly utilized. The mixing of residual carbon and ash material restricted the utilization of both the residual carbon and the inorganic minerals, or ash material. The separation of carbon and ash is the premise of efficient resource utilization of the gasification slag. Gravity separation shows satisfactory separation effect on the coarse slags of +0.125 mm. In the case of the more abundant fine slags of -0.074 mm, flotation is more frequently used but consumes large quantity of reagents and the ash material are readily entrapped in the froth product of carbon. Moreover, multi-stage is necessary which makes the flowsheet much complicated and the moisture content of the residual carbon product is also high. Hydrophobic-hydrophilic separation (HHS) is a technology integrating separation and dehydration. In this paper, HHS technology was used to separate the residual carbon and the ash material from the coal gasification fine slags produced in Ningxia Coal Industry Company, and the effect of mixing speed, mixing time and amount of hydrophobic liquid on the separation was investigated. The experiment results show that with the increase of mixing speed, the ash content of residual carbon products decreases from 66.35% to 20.80%, the ash content of tailings increases from 79.70% to 98.25%, indicating a significantly increased separation effect. With the increase of mixing time, the ash content of residual carbon products decreased from 33.96% to 28.96%, indicating a slight influence of the mixing time on the separation. With the increase of the amount of hydrophobic liquid, the ash content of residue carbon product decreases from 28.98% to 24.31% and then increases to 26.09%, indicating The hydrophobic liquid in HHS is best at 60% of the sample quality. Compared with froth flotation, HHS can give better separation effect for the carbon and the ash in the gasification slags, with the ash content of the residual carbon products and the ash product below 30% and above 95%, respectively. Employing SEM-EDS, proximate analysis, ultimate analysis, XRF analysis and BET pore structure analysis, the surface morphology changes, the element composition enrichment and the pore structure changes of the raw slags and the HHS concentrate and tailings were analyzed and summarized, which provides a basis for the different utilization of coal gasification fine slag after carbon-ash separation. By measuring the surface properties of both residual carbon and ash material, the hydrophobic-hydrophilic separation mechanism of the carbon and ash in the coal gasification fine slag was analyzed employing Zeta potential analysis, contact angle analysis, XPS analysis and extended DLVO theoretical calculation, which provides a theoretical basis for strengthening the carbon-ash separation in the future. Zeta potential analysis shows that the adhesion between residual carbon and n-heptane oil beads was stronger. After n-heptane treatment, the content of C—C/ C—H functional groups increased by 3.61%, and the contact angle increased from 36.11° to 76.13°, which significantly enhanced the hydrophobicity of carbon surface. By contrast, the contact angle of the ash treated with n-heptane increased from 28.08° to 42.05°, and the surface hydrophobicity was only slightly increased, indicating that n-heptane treatment was helpful to increase the difference of surface hydrophobicity between carbon and ash. The total interaction potential energy between various particles in HHS process was calculated using the extended DLVO theory. It was found that the flocculation occurs earlier between carbon-carbon particles and carbon-big heptane droplets at 60 nm than between ash-ash particles and carbon-ash particles, which resulted in the enrichment of most carbon particles in oil phase and ash particles in water phase, and carbon-ash separation was then realized by phase separation. © 2022 China Coal Society. All rights reserved.