6. CONTROL MEANS IN COMBINATION WITH A SHAFT FURNACE COMPRISING MEANS CONSTITUTING A VERTICALLY EXTENDING HEAT TREATING CHAMBER HAVING A LOWER COMPRESSIBLE FLUID INLET END AND AN UPPER COMPRESSIBLE FLUID DISCHARGE END, AND THROUGH WHICH PELLET-LIKE MATERIAL FLOWS DOWNWARDLY, SAID CHAMBER HAVING AN UPPER CONTINUOUS PORTION OF SUBSTANTIALLY CONSTANT CROSS-SECTIONAL AREA FROM ITS FLUID DISCHARGE END AND EXTENDING THROUGHOUT A SUBSTANTIAL ADJACENT PORTION OF ITS VERTICAL EXTENT, HAVING AN INTERMEDIATE CONTINUOUS PORTION IMMEDIATELY ADJACENT SAID UPPER PORTION AND HAVING AT LEAST SUBSTANTIALLY AS GREAT A CROSS-SECTIONAL AREA AS SAID UPPER CONTINUOUS CHAMBER PORTION, AND HAVING A LOWER PORTION EXTENDING TO ITS FLUID INLET END, AND MEANS EFFECTIVE TO CAUSE A COMPRESSIBLE FLUID TO FLOW UPWARDLY THROUGH THE LOWER PORTION OF THE CHAMBER COUNTERCURRENT TO, AND IN HEAT EXCHANGE RELATIONSHIP RELATIVE TO THE PELLET-LIKE MATERIAL, SAID CONTROL MEANS COMPRISING, IN COMBINATION, MEANS FOR WITHDRAWING A STREAM OF THE COMPRESSIBLE FLUID FROM THE LOWER END OF SAID INTERMEDIATE PORTION OF SAID CHAMBER, MEANS FOR RETURNING A SECOND STREAM OF A COMPRESSIBLE FLUID TO THE LOWER END OF SAID UPPER PORTION OF SAID CHAMBER, AND MEANS EFFECTIVE TO CONTROL THE WEIGHT PER UNIT OF TIME FLOWING IN EACH OF THE FIRST AND SECOND STREAMS AT LEAST SUBSTANTIALLY AS HIGH AS THE WEIGHT PER UNIT OF TIME OF COMPRESSIBLE FLUID FLOWING UPWARDLY THROUGH THE CHAMBER BELOW THE REGION OF WITHDRAWAL OF THE FIRST STREAM.