The theme of this article is to establish a basis for a physical model of optimization the kinetics of the process caustification (firing).In developing the mathematical model were used available data on the subject. The experimental part was carried out in order to findthe optimal state of firing caustic magnesite at a given temperature and its reactivity to fast speed rotary kiln (FSRK) and the integratedthermal unit (ITA). Both facilities are located in SMZ a.s Jelšava. In this article are only data related to spent magnesite for FSRK.