GB1137375A - Method and apparatus for pneumatically grading a powdered product - Google Patents

Abstract

1,137,375. Pneumatic separation. INSTTTUT DE RECHERCHES DE LA SIDERURGIE FRANCAISE. 23 Feb., 1967 [2 March, 1966], No. 8605/67. Heading B2H. Particles of a powdered product to be graded are fed from a hopper 1 via a conduit 4 into the lower portion of a chamber 2 of elongated rectangular form the bottom of which is constituted by a grid 9 through which air is fed by means of a blower 10 via a conduit 12 in such a manner that a fluidised layer 20 of particles is formed from which the finer particles are entrained by the rising air current to be discharged therewith at the top of the chamber 2 while the larger particles remain in suspension in the layer and travel in the direction of the arrow a towards a discharge aperture 22. The length of the chamber 2 is at least 6 times its width. The height of the aperture 22 above the level of the grid 9 is between 5 and 30 cm. so as to maintain the layer 20 at that thickness and the chamber space above the layer 20 is equal to at least 8 times that thickness. The chamber 2 is divided, by means of a partition 7 having an aperture 8 in its lower part, into at least two compartments 5, 6 which at their top are connected to cyclones 30, 34 in which the finer particles are separated from the entraining air and collected in the containers 32, 36. Very fine particles discharging from the cyclones with the air through a common conduit 31 are separated out in a dust-remover, not shown. To facilitate separation in the chamber 2 air is blown through a valved pipe 16 horizontally through the product at its entry into the chamber. Separation is further enhanced by deflectors 18, 19 and obstacles 21 which create turbulence. The air supply conduit 12 is connected to wind-boxes 11a-d which are obliquely set relative to the grid 9 to impart a horizontal component to the air to facilitate movement of the largest particles in the layer 20. This movement may also be facilitated by providing alternatively, or in addition, as shown, a grid comprising a double row of perforated sheets with offset apertures which impart the required horizontal component to the air. Alternatively, an inclined or stepped grid may be used. The air supply conduit 12 is equipped with flow measuring venturis 14 a-d and regulating valves 15 a-d which adjust the air speed so that it increases from the box 11a to the box 11d. Due to this increasing speed, the particles entrained into the cyclone 34 are of a greater size than those entrained into the cyclone 30. The larger particles discharged through the aperture 22 are led on to a conveyor belt 26 via a hopper 23 equipped with level-control indicators 24, 25.