Study of simulated crude oil wastewater for flotation separation of coal gasification slag

Coal gasification slag is a typical coal-based solid waste, and the separation of residual unburned carbon and ash components in coal gasification slag is an important guarantee for its resourceful and high-value utilization. Flotation is the most effective way to separate unburned carbon and tailing from coal gasification slag, but facing the problem of high cost and consumption of collector. Crude oil wastewater contains substances such as long-chain alkanes, cyclic hydrocarbons, aromatic compounds, phenols, ketones, acids, and esters, which are similar to the components of commonly used collectors for coal gasification slag, has the potential to be applied to the flotation separation of coal gasification slag. In this paper, the physical and chemical properties of coal gasification slag and the material composition of crude oil were studied, and simulated crude oil wastewater was employed as a collector to investigate the flotation behavior on coal gasification slag. Results show that, the main substances in coal gasification slag are mullite, fused silica-aluminate, metal oxide and amorphous carbon. The relative content of unburned carbon in coal gasification slag is 25.37%, in addition to methyl and methylene, the surface of coal gasification slag also contains abundant oxygen-containing functional groups such as C—O/C=O/O—C=O, which makes coal gasification slag hydrophilic. Crude oil contains a large amount of structurally complex polycyclic alkanes, long-chain alkanes, and some polar substances. Polycyclic alkanes and aromatic hydrocarbons have a stronger ability to capture unburned carbon than general n-alkanes, and the presence of polar components is also conducive to the flotation separation of coal gasification slag. Therefore, crude oil has a strong collection ability; The flotation results show that, with the stirring rate of 1 800 r/min, slurry concentration of 60 g/L, crude oil wastewater concentration of 40 mg/L, and frother concentration of 30 mg/L, the recovery of unburned carbon reaches 84.62%, and the loss-on ignition of 76.82%; the tailings with loss-on ignition of 4.31%, which achieves the national level of the standard of loss-on ignition ( < 5%); Meanwhile, the oil removal rate of simulated crude oil wastewater is 41.50%, and the chemical oxygen demand is reduced from 219.70 mg/L to 82.77 mg/L, which meets the secondary discharge standard in the national petrochemical industry. The results of FTIR analysis showed that the —CH₃ and —OH stretching vibration peaks indicated by the coal gasification slag were shifted after interacting with the simulated crude oil wastewater, and it could be judged that the crude oil molecules had hydrophobic interactions and hydrogen bonding interactions with the surface of the coal gasification slag, respectively. Zeta potential results shows that the electronegativity of the coal gasification slag surface is enhanced after the action of simulated crude oil wastewater. Combined with the XPS results, it further indicates that hydrogen bonding bridging occurs between the polar oxygenated compounds in the crude oil and the oxygenated functional groups on the coal surface, forming a large three-dimensional spatial structure of the charge body, which makes the surface potential of the coal gasification slag more negative. This paper realizes the recovery of carbon products and the treatment of simulated crude oil wastewater through the co-disposal of coal gasification slag and crude oil wastewater, which has strong industrial application value.