Eco-friendly utilization of waste oil for preparing coal water slurry: Additive suitability and dispersion mechanism

•Industrial waste oil was recycled for eco-friendly utilization.•The optimal additive and composition of waste-oil-coal-water slurry were determined.•Viscosity reduction effect of Tween 80 in the waste oil-coal-water slurry was best.•The interface properties of coal particles, oil, and water were studied.•Dispersion and stabilization mechanism for waste oil-coal-water slurry was proposed.