Abstract: A process for purifying waste oil comprising the steps of admixing with the waste oil an aqueous solution of an alkali metal silicate, and maintaining the waste-oil-silicate solution mixture at a temperature of from 140 to about 200 degrees Fahrenheit, wherein from about 0.001 to about 0.8 pounds of a silicate selected from the group consisting ofM.sub.4 SiO.sub.4, M.sub.2n (SiO).sub.n, and M.sub.2 SiO.sub.5,wherein n is an integer of from about 1 to about 2, and M is an alkali metal, are mixed with each gallon of said waste oil, wherein said aqueous solution of the alkali metal silicate contains from about 1.0 to about 10.0 percent (by weight of said solution) of said silicate, and wherein from about 1.0 to about 10.0 percent (by volume of said waste oil) of said aqueous solution of said silicate is added to said waste oil.