The main goal of this work is to determine the mineral matter evolution of rubber when old tire is recycled in energy generation. For that, the combustion of a nonspecific mixture of waste rubber tire in an atmospheric fluidized bed reactor is studied in this work. Three different temperatures (750, 800, and 850degreesC) were tested using constant oxygen excess percentage (5%) and gas speed (0.24 m/s). It was observed that combustion efficiency depends on the temperature, but in general, low values are achieved mainly because the oxidation of the carbon black included in rubber tire is a slow gas-solid reaction. Important slag formation was observed after several hours of running the laboratory pilot plant. Big ash particles were generated inside the fluidized bed with particle sizes ranging between 1 and 10 mm. This slag formation caused important operational problems. Transformations of Zn and other inorganic elements are studied by using X-ray diffraction, indicating that the original zincite present in the rubber tire reacts with the silicate phases and is transformed into willemite and hardystonite. However, this reaction is not instantaneous and requires long reaction times.