Abstract: A rotary tubular kiln and a preheater for material supplied to the kiln are connected by a duct leading from one end of the kiln, for conducting waste gases from the kiln to the preheater. A bypass conduit is connected to said duct for diverting part of said waste gases and extends from the duct in such a direction as to divert a stream of waste gases at an angle of at least 90.degree. to the stream of gases flowing through the duct. An upright mixing chamber is connected at its bottom to said bypass conduit to cause said diverted stream of gases to flow upward through said mixing chamber. The mixing chamber has in its lower portion at least one lateral inlet for introducing cooling air into said diverted stream of gases, and the distance between the axis of said duct and a point on the axis of said mixing chamber at the level of said inlet is from 1.5 to 3 times the square root of the quotient of the cross-sectional area of the duct divided by pi.

Abstract: Fine-grained material such as raw ground cement adapted to be fired in a rotary tube furnace is introduced to a vertical preheater and falls through a heating zone located at a level below the level at which the material enters the preheater. A stream of air enters the preheater at a level below the heating zone and passes upwardly at sufficient velocity to entrain the material that has passed through the heating zone, thereby causing such material to pass again through the heating zone. From the preheater the material is delivered to the furnace for final firing.

Abstract: Fine-grained material such as raw ground cement adapted to be fired in a rotary tube furnace is introduced to a vertical perheater and falls through a heating zone located at a level below the level at which the material enters the preheater. A stream of air enters the preheater at a level below the heating zone and passes upwardly at sufficient velocity to entrain the material that has passed through the heating zone, thereby causing such material to pass again through the heating zone. From the preheater the material is delivered to the furnace for final firing.