A typical re-refining process is consist of the following steps.
Filtration and de-watering of the waste oil by decantation.
De-watering and light hydrocarbon removal by distillation.
Removal of diesel fraction (C7-C12) by distillation.
Removal of asphaltic fraction by thin film evaporator. High temperature and high vacuum is needed.
After removing water, light hydrocarbons and asphaltic fraction, chemical treatment of the waste oil by solvent extraction or hydrogenation.
Having different cuts of base oil according to their boiling points.