Rheological Behavior of Oil-Water Emulsions for Steel Rolling Oily Sludge: Effects of Oil Phase Characteristics

Steel rolling oily sludge (SROS) is a highly emulsified, high-viscosity hazardous waste, whose apparent viscosity is governed by its oil–water interfacial structure. To clarify the role of the oil phase, model water-in-oil (W/O) and oil-in-water (O/W) emulsions were prepared using oleic acid and palmitic acid, and their rheological behaviors were systematically investigated via steady-state rheometry, optical and polarized-light microscopy, and quantitative droplet-size analysis. The results revealed that pure oleic acid behaved as a Newtonian fluid, whereas the W/O emulsions exhibited pronounced shear-thinning and yield stress due to the hydrogen-bonding network among water droplets. The O/W emulsions showed nearly Newtonian behavior at low oil content, but their viscosity increased sharply upon phase inversion to W/O. Furthermore, palmitic acid significantly enhanced the viscosity and yield stress of both systems. Droplet-size statistics showed that a moderate amount of palmitic acid minimized droplet size and polydispersity, whereas excess palmitic acid enlarged the droplets. Polarized-light microscopy and macroscopic observations confirmed that this viscosity jump was driven by the precipitation of excess palmitic acid, which formed a rigid crystalline network at room temperature. These findings elucidate the viscosity mechanism of SROS and provide a theoretical basis for improving its flowability and resource utilization.