Recycling and characterization of used engine lubricating oil

Used engine lubricating oil contains degradation products and wear-derived metals that require controlled treatment before any recovery option can be considered. This study evaluated five used-oil samples (three gasoline-engine oils and two diesel-engine oils) and examined a staged process comprising settling, magnetic separation, filtration, dewatering, acetic-acid treatment, solvent flocculation, and treatment with two synthesized non-ionic surfactants. Physicochemical properties were measured using standardized methods for density (15 °C), kinematic viscosity (100 °C), flash point, pour point, carbon residue, and ash content, while elemental concentrations were determined by ICP-MS. For surfactant-1-treated samples, gasoline-engine oils showed densities of 0.8714-0.8904 g cm−3, kinematic viscosities of 9.51-13.59 mm2 s−1, and ash contents of 0.010-0.014 wt%, whereas diesel-engine oils showed densities of 0.8934-0.8954 g cm−3, viscosities of 13.16-20.26 mm2 s−1, and ash contents of 0.011-0.012 wt%. Treatment reduced Fe from 40.93 to 28.56 ppm, Zn from 47.26 to 38.83 ppm, and Al from 10.50 to 8.15 ppm; however, sulfur remained high at 407.39 ppm, and no engine, bench, anti-wear, moisture-balance, or long-term stability tests were performed. The results therefore support physicochemical improvement and partial contaminant removal, but they do not establish that the regenerated oil is suitable for direct reuse in engines; operational validation and additive rebalancing are required.