Thermal engineering studies were carried out on a combined grate-tube furnace at Sokolovsko-Sarbaisky GOK (SSGOKa). The regularities of changes in the basic characteristics of pellets along the length of the rotary kiln are determined. The analysis of the thermal state of individual sections of the furnace (drying, heating, roasting) and the furnace as a whole was carried out. The distribution of temperatures of the gas stream, masonry and pellets along the length of the rotary kiln has been studied. The study of physicochemical processes during roasting of non-fluxed and fluxed pellets is carried out. The temperature boundaries of oxidation of magnetite and the decomposition of carbonates are established. The change in the strength of pellets along the length of the rotary kiln has been studied and the mechanism of their hardening has been considered. It is established that the main indicators obtained on a rotary kiln are higher than on the conveyor-type calciner. As a result of the studies, the basic data for the design of such grate-tube furnace installations with higher productivity and higher metallurgical properties were obtained.