In air swept ball mills, material is swept out of the mill by hot air /gas of significant velocity (5-6 m/s), through separator and coarse fraction is returned to the system for regrinding. In case of bucket elevator mills (gravity discharge mills), ground material is taken by conveyors to a separator where coarse was returned to the mill and ... Gas Velocity Inside Cement Ball Mills. Raw mills usually operate at 7274 critical speed and cement mills at 7476 32 calculation of the critical mill speed g weight of a grinding ball in kg w angular velocity of the mill tube in radialsecond w 2314n60 di inside mill diameter in meter effective mill diameter n revolution per minute in rpm