is made initially to fig1 which shows the device of the invention fitted to a fluidization enclosure 1 which is itself known and in particular which is known for possessing a fluidization grid 2 see fig3 through which a gas is blown via openings 3 to form a fluidized bed 4 of particulate material the enclosure 1 includes an opening 5 preferably situated close to the fluidization grid 2 and extended to the outside by a tube 6 the tube is preferably horizontal and a duct 7 is connected thereto so that their geometrical axes intersect the duct 6 is vertical or steeply sloping close to the vertical and opens out into the tube 6 via an opening 8 situated in the bottom of the tube 6 the diameter of the duct 7 may be equal to or less than the diameter of the tube 6 the tube 6 constitutes a guide jacket for a moving closure device for the opening 5 this device comprises a piston 9 whose diameter is slightly less than the inside diameter of the tube 6 assuming their sections are circular in order to enable it to slide properly therein while providing adequate sealing the piston 9 is fixed to a rod 10 which constitutes the rod of an actuator 11 carried at the far end of the tube 6 serving to control operation of the piston 9 along the tube 6 the figure shows an electrically controlled valve 12 which symbolizes the control member for the actuator 11 said electrically controlled valve being the actuator of a.