In the purification of waste lube oil to give clean lube oil, as shown in figure, the waste lube oil containing $87wt\%$ unwanted solids and $13wt\%$ liquid lube oil is fed to an extractor. Extraction of lube oil from waste lube oil is achieved by contacting the waste oil with hexane. The extractor feed ratio of hexane to waste oil is $3kg$ hexane $\frac{?}{kg}$ waste oil. The unwanted solids are suspended in the liquid and essentially all of the lube oil in the waste lube oil is extracted into the hexane. The extractor effluent passes to a filter. The filter cake contains $75\%$ unwanted solids and the balance lube oil and hexane. The ratio of lube oil to hexane in the filter cake is equal to the ratio of lube oil to hexane in the effluent extractor. The filter cake is discarded and the liquid filtrate is fed to a heated evaporator in which the all of the hexane is vaporized and lube oil remains as a liquid. The hexane vapor is subsequently cooled and condensed, and the liquid hexane condensate is recycled to the extractor.