How does waste mineral oil become base oil again?

If an engine oil is black after 10,000 kilometres, that does not mean the oil is “finished”. The carrier part — the base oil — largely stays the same; what is used up are the additives: detergents, antioxidants, anti-wear agents. Refining is the work of removing those additive residues and the contamination, leaving the base oil behind.

What is in used oil

A used mineral oil typically contains: base oil (most of the weight), spent additive residue, combustion soot, metal particles from wear, water, light hydrocarbons such as fuel dilution, and dust picked up from outside.

Each is separated by a different method — and the order matters.

Line stages

Settling. Incoming oil is left to stand. Heavy particles and free water drop to the bottom. This is a cheap, energy-free step; skipping it makes everything after it harder.

Dewatering. Controlled heating removes water and light hydrocarbons. Water must go first because it flashes off in the distillation column and causes foaming and carry-over.

Vacuum distillation. Lowering the pressure lowers the boiling point, so the oil separates into fractions without degrading at high temperature. Fractions of different viscosity are collected separately.

Treatment. Remaining polar compounds and colour bodies are captured on clay or a similar adsorbent. Some plants use hydrogen treatment instead.

Analysis and blending. Viscosity, flash point, water and acid number of the output are measured. The result determines what the product is fit for.

Why mixing matters so much

Refining is built on separating the non-oil from the oil. When antifreeze, brake fluid, solvent or an oil-water emulsion enters the system, that separation gets harder; in some cases the whole batch becomes unrefinable and has to go to energy recovery instead.

In practice: one person tipping antifreeze into an oil drum can zero out the value of the 200 litres in it. Drums should therefore be labelled, and it should be clear who pours what.

In numbers

Roughly, a tonne of waste oil in a suitable grade yields around 600–700 kg of base oil; the rest is light fractions, water and sediment. Burn the same oil and nothing remains but the energy.