Recycled engine oil has high levels of organic impurities, heavy metals, and carcinogenic compounds, according to work carried out by researchers. The researches have used atomic absorption (AA), inductive couple plasma (ICP) and Fourier transform infrared (FTIR) analyses to spot the differences between virgin and recycled engine oil.

The Royal Scientific Society, Industrial Chemistry Centre, explain that because engine oil undergoes many changes during use it is markedly different, in both chemical and physical properties, from a nice, fresh, virgin engine oil.

The researchers define lubricating engine oils as any fluid used to reduce friction and wear by interposing a film of material between rubbing surfaces. Commonly engine oils consist of two materials – a base oil and chemical additives, such as metallic detergents, ashless dispersants, zinc dithiophosphate, anti-oxidant/anti-wear, friction modifiers, antifoaming agents and pour-point depressants.

In use, engine oil degrades, additives become oxidised, and the fluid accumulates organic and metallic engine residues. As such, engine oils have to be replaced regularly to ensure the clean and efficient operation of the vehicle or machine they are being used to lubricate. It is, of course, possible to recycle used engine oil and refresh it to a near-virgin state.

The recycling processes currently used, however, leave recycled oils with a much higher water content, higher sediment levels, and higher concentrations of organic oxidation products in these oils than virgin oil. Recycled oil is also contaminated with higher levels of iron, cadmium, chromium, lead, and other metals than virgin oil. The presence of halogenated solvents in significant quantities produced by the degradation of chlorine and bromine lead-scavenging additives, is an additional concern in engine oils used with leaded gasoline. Moreover, polynuclear aromatic hydrocarbons (PAHs) are a particular problem given their known carcinogenicity.

Nevertheless, lubricating oils are the most valuable component of crude oil, explain the researchers, so their recycling is important. The researchers hoped to find a straightforward approach to assessing the chemical properties of oils using atomic absorption (AA), inductive couple plasma (ICP) and Fourier transform infrared (FTIR) spectroscopy, and so provide a way to distinguish quickly and easily between recycled and virgin oils.

The first stage of the research involved the use of FTIR spectra of an engine oil sample (mono- and multi-grade) prior to use and in the recycled state. The team observed a broad feature centred at 1716 cm-1 in recycled oil that is not present in virgin oil. This, they explain, is due to the presence of carbonyl-containing degradation products of the oil. They also observed bands at 1732, 1169, 1154 and 1270 cm−1, which they assigned to the polymethacrylate stretching vibrations. This feature of the FTIR spectra allowed them to determine the presence of oil additives, including viscosity modifier and pour-point depressant.

The second stage was to develop the analytical technique, based on AA, for the determination of the levels of wear metals in recycled oil samples. “The determination of metal contents in a lubricating oil seems to be useful in the differentiation of virgin and recycled oil,” the researchers say, “However, very little information concerning this problem is available in the literature.”

The team’s results on real samples show that the recycling processes used for purification of the sample oils are not adequate. This is particularly true in removing, or rather failing to remove, oxidation products and trace elements including lead, iron, and silicon. “The presence of oxidation products and trace metals in engine oils could primarily have a deleterious effect on equipment due to severe erosion,” the team says, “However, the use of more advanced methods to remove such potential pollutants from waste oils involves higher preparation costs, making such oils uncompetitive with virgin oils.”

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