A process and apparatus for condensing solid materials such as phosphates which tend to sinter and adhere when heated in a reactor wherein the reactants are introduced into a trough-like reactor provided with at least one set of rotatable agitating paddles. Heat is supplied to the reactor by passing a heating medium, such as a molten salt or a molten metal, through one or more heating jackets on the exterior of the reactor housing. The agitating paddles press the solid material in a thin layer against the wall of the reactor housing, and the material in the layer absorbs heat from the heating medium and sinters to a hard layer. One paddle of each set of paddles is provided with a stripping knife which moves along a path closely adjacent to the exterior wall of the reaction chamber to strip sintered material from the wall of the reactor housing. Heat is also transferred to the interior of the reactor housing by circulating the peeled material through the interior region of the reactor housing.