Recycling of waste lubricating oil: A review of the recycling technologies with a focus on catalytic cracking, techno-economic and life cycle assessments

Waste lubricating oil (WLO) is a catch-all name for many lubricating oils used in automobiles and machines. WLO has been designated as a hazardous waste in the eight major categories. Toxic substances such as heavy metals, chlorine, and sulphur are produced during the production process of lubricating oils due to the addition of additives or due to high temperatures and oxidation in use, which produce substances such as polycyclic aromatic hydrocarbons and polychlorinated biphenyls that are harmful to humans. The apparent global outlook on the availability of WLO and the issues it poses to environmental and human health if not properly disposed of is a strong motivator for research into the re-use, regeneration, and conversion of WLO into valuable and cleaner goods. This work, for the first time, comprehensively reviews WLO recycling technologies with a focus on catalytic cracking, techno-economics, and life cycle assessments. The commercial status of WLO industries, and the associated challenges and prospects of recycling WLO processing are outlined. The findings in this study suggest that, clay, spent catalysts and magnetite doped with barium and strontium metal ions offer sustainable and low-cost catalysts for WLO cracking, among other catalysts such as zeolites, metals and oxides, ash, carbon materials, and carbonates. There is a need to study WLO catalytic treatment processes' feasibility via techno-economic assessment, and the combination of multi-criteria decision making and life cycle assessment makes a powerful analytical tool to evaluate all the sustainability pillars.