Oil-in-water emulsions are used in large amounts in hot-rolling mills of steel industries for cooling and lubrication of contacting metal surfaces. These emulsions are comparatively stable kinetically, and have high specific heats and heat transfer coefficients, and therefore can be used under high temperature and pressure conditions in hot-rolling mills of steel industries. These emulsions used in steel industries are however hazardous to the environment, and cannot be disposed of as such. The Ministry of Environment and Forests has set the permissible limit of 10 mg/l of oil for disposal. These emulsions are notoriously difficult to break. The present work is focused on the treatment of such tough emulsions, by Fe3O4 nanoparticles, in which oil and water are effectively separated. The other conventional processes such as acidification, centrifugation, coagulation, ultrafiltration, UV radiation, ozone treatment, thermal irradiation, and electrolytic method are either energy intensive, time-consuming, costly, or non-eco-friendly. The Fe3O4 nanoparticles were synthesised by a co-precipitation method and characterized by XRD, SEM, and TEM. An experimental setup was built in-house for the treatment of emulsion effluents on a conveyor belt. Experiments were conducted using 1 % (v/v) o/w emulsion and different weight percentages of nanoparticles in an aqueous slurry. The mass balances of oil, water, and nanoparticles were checked and showed good recoveries. This process is economical, eco-friendly, less energy-intensive, and less time-consuming when compared to the other established procedures.