The invention relates to a process for operating a mixing kneader, comprising one or more shafts on whose surfaces are disposed kneading bars and which are surrounded by a casing, comprising the following steps: (a) supplying reactants at an addition site in the mixing kneader, (b) converting the reactants in an exothermic reaction, the reaction at first forming a coherent kneadable intermediate, (c) tearing and dividing the coherent kneadable intermediate to form a product, the exothermic reaction optionally continuing during the tearing and division, (d) withdrawing the product at a withdrawal site in the mixing kneader, wherein at least one of the shafts and/or the casing is heated to a temperature above 20° C. during the operation of the mixing kneader.