control system the second automatic control system which keeps the differential pressure between the gas inlet end pipe 12 of the mill 1 and the gas outlet end pipe 19 of the separator 2 constant comprises the pressure measuring elements 13 and 20 connected to the pipes 12 and 19 a controller 21 which forms the differential pressure and which compares the actual signal with a prescribed value and a variable speed drive arrangement 22 controlled by the controller 21 for influencing the amount of material to be ground which is delivered to the mill 1 the third automatic control system by which the gas temperature at the gas outlet end pipe 19 of the separator 2 is kept constant comprises a temperature measuring element 23 which in the embodiment illustrated acts through a controller on the furnace 7 alternatively the gas temperature may also be kept constant by a control valve in a hot gas supply pipe the function of the fourth automatic control system provided in accordance with the invention is to keep the material level of the storage silo 4 for the ground material constant by varying the hydraulically generated grinding force of the roller mill 1 the signal coming from the pressure gauges 8 is delivered to the controller 9 which in the event of a deviation from the prescribed value varies the hydraulic pressure of the roller mill 1 through the pressure limiting valve 11 shifted by the motor 10 finally in the embodiment illustrated there is a fifth automatic control system.