The results of investigations of beneficiation regularities of Tavsanlt-Omerler lignites in a two-stage consecutive process of heat treatment and magnetic separation are presented. After thermal treatment of lignite samples in a rotary kiln at temperatures ranging from 370 to 650 degreesC, the application of the REMS magnetic separator produces a product higher in calorific value (6727 kcal/kg) and lower in sulphur (1.27 %) and ash (8.5%) content. The mechanisms of the process have been investigated by scanning electron microscopy, chromatography, electron spin resonance, X-ray diffractometry, derivatography and chemical analysis. The efficiency of the beneficiation processes can be explained by the improvement of magnetic properties of pyrite during heat treatment due to oxidation (T < 400C) and decomposition (T > 400 degreesC) of pyrite and compact distribution of part of organic sulphur around pyrite grains. (C) 2001 Published by Elsevier Science Ltd.