Abstract: The invention intends to separating a methyl derivative of naphthalene (2,6-dimethyl naphthalene or 2-methyl naphthalene) from a mixture material in a high purity more than 98 weight %. The starting material is preliminarily treated at a raised temperature up to 220.degree. c. at the highest and with LHSV of 0.1-6 Hr.sup.-1 in the presence of an acid catalyst usually used for olefin polymerization. The treated material is distillated to remove the polymerized impurities and to raise the naphthalene derivative content at least up to 50 weight %. And then the obtained material is subjected to the crystallization under a pressure of 500-2500 kgf/cm.sup.2 at a temperature of 80.degree.-150.degree. C. (for 2,6-DMN) or 10.degree.-35.degree. C. (for 2-MN).

Abstract: Described herein is a process for separating solid indole in refined form with an extremely high degree of purity from a mixture containing more than 50 wt % of indole, in which a pressure of 700-2500 atms is applied to the mixture in a high pressure vessel, and solid-liquid separation is effected continuedly under pressurized state with an initial temperature in the range of 50.degree.-65.degree. C.