A Comprehensive Review of Global Waste Lubricating Oil Re-Refining Technologies: Mechanisms, Performance, and Environmental Footprint

The global demand for lubricants, driven by industrial expansion and a growing vehicle fleet, has led to a significant increase in the generation of waste lubricating oil (WLO). This waste presents a formidable environmental challenge, particularly in developing economies where indiscriminate disposal leads to severe contamination of water, soil, and air with toxic and carcinogenic compounds. In response to these pressures, the re-refining industry has emerged as a crucial component of a global circular economy, transforming this hazardous waste into a valuable resource. This article presents an in-depth comparative analysis of the diverse technologies employed globally for the re-refining of WLO. It systematically examines processes such as acid clay, hydrogenation, solvent extraction, and thermal methods, evaluating their respective mechanisms, technical performance, and environmental footprints. The analysis highlights a clear technological evolution from environmentally problematic methods, such as the acid-clay process, toward more sophisticated and sustainable approaches like hydrogenation and advanced distillation. It systematically describes their mechanisms, evaluates their performance in terms of product quality and yield, and assesses their environmental footprints. The findings and analysis demonstrate that this combined process effectively removes contaminants, producing lubricating base oil and diesel of comparable quality to commercial-grade products. This study not only reaffirms the technical viability of WLO re-refining but also underscores its vital role in achieving resource conservation, economic resilience, and environmental preservation on a global scale.