A method of separating foreign polymers from a polymer mixture that comprises amorphous PET particles and crystalline PET particles, wherein the softening point of the foreign polymers is lower than that of the crystalline PET particles, and wherein the method includes separating the amorphous PET particles from the polymer mixture, heating the resulting polymer mixture to a temperature between the softening point of the foreign polymers and that of the crystalline PET particles, and separating the foreign polymers from the resulting polymer mixture. Also, a device for separating foreign polymers from a polymer mixture that comprises amorphous PET particles and crystalline PET particles, the device including a first separation unit for separating the amorphous PET particles from the polymer mixture, and a second separation unit for separating the foreign polymers from the resulting polymer mixture.