Furnace and method for operating a furnace

A method for operating a furnace comprising at least two vertical shafts and a transfer channel that connects the at least two vertical shafts, each of the at least two vertical shafts including an inlet for receiving material to be burnt, the inlet being disposed at an upper end of the vertical shaft;

• an outlet for burnt material, the outlet being disposed at a lower end of the vertical shaft, a main burner disposed above the transfer channel;
• a cooling gas inlet disposed below the transfer channel;
• and an additional burner disposed below the transfer channel, the method comprising:
• at least partially calcining the material to be burnt in a fired shaft of the at least two vertical shafts in a main burning zone above the transfer channel at a first temperature and according to a first dwell time;
• and
• thermally aftertreating in an additional burning zone the material that has been at least partially calcined, the additional burning zone disposed between the transfer channel and the additional burner producing a second temperature, the second temperature being greater than the first temperature and according to a second dwell time, the second dwell time being less than the first dwell time,
• wherein the additional burner of one or both of the at least two vertical shafts is/are operated only in the fired shaft or in both of the at least two vertical shafts.