Abstract: A calcining apparatus comprising a kettle, a heating chamber for heating the outside of the kettle, a stack for exhausting gas from the heating chamber, a duct connected at one end to the stack and having the other end extending for a limited distance into the contents of the reaction chamber of the kettle for introducing a portion of the stack gas directly into the calcining reaction mixture, and means for propelling the stack gas through the duct and into the reaction chamber of the kettle, thereby directly adding heat to the reaction mixture and accomplishing fuel conservation by recapturing and utilizing a portion of the heat from the stack gas.