preheater 4 39 composed of four cyclone stages 1 39 2 39 3 39 and 43 as well as with a separate calcinator combustion chamber 44 a tertiary air line 45 bridges across the rotary kiln 8 and entirely independent of the latter on the gas side is supplied with hot combustion air recovered through the cooler 42 the calcinator 44 is equipped with a fuel supply device 46 and in accordance with the present invention has an independent start up burner 47 at the entry end of the tertiary air line 45 into the calcinator 44 the tertiary air line 45 itself is likewise equipped with a start up burner 48 which in the start up state can be driven at full capacity or at partial capacity in the operating state at the discharge end 49 of the tube cooler 42 there is provided a separate and independent heater in the form of a burner 50 the rotary kiln 8 as in the previously described forms is provided with a primary burner 18 in the case of the system illustrated in fig3 upon starting up from the cold state the burners 18 50 48 and 47 are ignited at substantially the same time and slowly brought up to temperature the system in accordance with conventional practice is equipped at several locations with temperature monitors and when the various stages have.