to mix the pulp in the vessel which pulp in some cases may be even very thick whereby the pulp flows at the pressure of the vessel through the inlet opening 52 of the apparatus to the inlet channel in which the pulp is exposed to the rotational effect of the rotor 10 since the rotor at least partially fluidizes the pulp and is able to accelerate the rotational speed of the pulp close to its own rotational speed the pulp is able to be pressed more freely due to the centrifugal force against the rotor and the wall of the inlet channel to form an annular layer whereby the gas separating from the pulp has ideal conditions to accumulate as bubbles and to drift towards the lower pressure in the center of the rotor at the same time the rotational energy caused by the rotor in the pulp and the centrifugal force generating therefrom enables the increase of the pressure of the pulp at the outlet opening 58 compared with the inlet opening 52 since the pressure is at its lowest close to the flange 20 around the sleeve 16 gas accumulates there and is discharged therefrom through the flow channel 44 to the space 42 behind the flange 20 some pulp is also discharged with the gas to the space 42 whereby the purpose of rear blades 22 in said space 42 is to pump the pulp possibly entrained in the space 42 back to the spiral chamber 56 the gas flows from the space 42 either due to pressure in the space or due to suction connected to the gas separation system through a gap between the annular disc 80 and the second.