Dodecylamine enhanced coal gasification fine slag flotation and its molecular dynamics simulation

Coal gasification fine slag (CGFS) is produced by the raw gas flow through the top of the gasifier, and is formed after preliminary washing and purification, precipitation, and dehydration. The CGFS cannot be directly used as building materials or in other industries because of its higher ignition loss than the standard of building mixed raw materials. Neither can it be used for direct combustion because of low carbon content, high moisture content and low calorific value. Currently, its treatment is mainly landfill-based disposal which causes environmental pollution and carbon resources waste. The utilization of CGFS is becoming increasingly urgent and its premise is the separation of ash material and residual carbon. Froth flotation is a common method used for separating fine materials. In this study, kerosene (D1) and a dodecylamine-kerosene mixture (D2) were used as collectors and the flotation performance of CGFS sampled from Ningxia Coal Industry Company was investigated. The results indicate that D2 is better than D1 in that it gives higher yield of concentrate, ash of tailings, and combustible recovery. Pore structure analysis (BET) and scanning electron microscopy energy dispersive spectroscopy (SEMEDS) were used to characterize the flotation products, which provided a basis for the separate utilization of carbon ash from CGFS. In this paper, a certain percentage of functional groups were grafted onto graphene based on the results of XPS analysis to replace the residual carbon model, and the D2 collector is proved to be more efficient than D1 by two models. Molecular dynamics (MD) simulations was used to simulate the adsorption morphology of D1 and D2 on the surface of residual carbon and the adsorption morphology of D2 on the surface of residual carbon with four functional groups. The results show that D2 has stronger adsorption energy with residual carbon than D1, and the electrostatic forces play a certain role. The adsorption energy of D2 with four functional groups decreases from carboxyl, carbonyl, hydroxyl, to ether bonds. Some dodecylamine molecules help kerosene to form smaller oil droplets that act on the carbon residue surface through van der Waals (vdW) force and electrostatic force, while other dodecylamine molecules form hydrogen bonds with oxygen-containing functional groups on the carbon surface to cover some hydrophilic points. This significantly increases the hydrophobicity of the residual carbon and enhances its flotation effect.