Rotary mixing apparatus particularly suitable for treatment of foundry sand having a drum mounted for rotation about an axis which is horizontal, or at least approximately horizontal, operates to treat bulk material such as foundry sand which is placed within the rotating drum. Stationary deflection apparatus is arranged within the drum in the upper range thereof and a rotating impact rotor is mounted within the drum so that as the drum rotates and the material contained therein is propelled into the upper region of the drum it is deflected by the deflection apparatus and formed into a downwardly falling jet which is directed at the rotating impact rotor.