in the drawings an elutriator generally designated 1 includes a housing 2 which has an upper part 2a and a lower part 2b . the upper part 2a is connected to the lower part 2b by means of a flexible seal 3 . the provision of the flexible seal 3 permits the upper part 2a to be vibrated relative to the lower part 2b by means of agitators 4 . spring suspension units 5 are provided on mountings 6 which rest on a floor 30 , and the suspension units are attached to the upper part 2a of the housing 2 . the construction of the suspension unit 5 is the same as that described in our earlier united kingdom patent application no . 8313368 . the housing 2 defines a plenum chamber 7 , which plenum chamber underlies distribution means . the distribution means comprises two perforated plates 10 and 11 , and an additional perforated plate 9 . a non - perforated feed plate 8 is provided adjacent the plate 9 . within the plenum chamber 7 a plurality of air diverting plates 12 are arranged so that air can flow between adjacent plates 12 . the plates 12 are bolted to the housing 2 . the orientation of the plates 12 is adjustable . an air supply fan 13 is provided to supply air to the plenum chamber 7 . a hood 14 is arranged above the perforated plates 10 and 11 , and the additional perforated plate 9 , and has an exhaust duct 15 which is connected to a suction fan 31 . a separator 32 which is shown diagrammatically , is provided between the exhaust duct 15 and the suction fan 31 in order to remove the top product from the air flow . the separator 32 may comprise separation apparatus as described in our u . k . pat . no . 1 , 575 , 175 . the top product is collected in a collector 33 . inspection windows 14a are provided in the hood 14 ( not shown in fig1 and 2 ). the fans 13 and 31 may be , for example , high efficiency centrifugal fans , or high pressure axial fans . the plates 10 and 11 are disposed below an elutriation zone 17 , and the additional plate 9 is disposed below a pretreatment zone 16 . an inlet 18 for material 34 to be separated is provided adjacent the feed plate 8 , and an outlet 19 is provided adjacent the elutriation zone 17 . the feed plate 8 is disposed adjacent the pretreatment zone 16 . the way in which the plate 10 is arranged above the plenum chamber 7 is shown in fig4 . it will be appreciated that the plate 11 and the additional plates 8 and 9 are arranged in a similar manner . the plate 10 is supported by support means 20 comprising a support plate 21 which is supported by a support tray 22 . the plate 10 is provided with apertures 24 . the apertures 23 and 24 are of substantially circular cross - section , and the apertures 24 are larger than the apertures 23 . for example , the diameter of the apertures 23 is preferably about 20 thou ( 51 × 10 - 3 cm ). the diameter of apertures 24 is preferably about 3 / 16 inches ( approximately 0 . 47 cm ). the distance between the centres of the apertures 24 may be about 5 / 16 inches ( 0 . 8 cm ). this gives a total aperture area of 33 % of the total area of the plate 21 . the thickness of the support plate 21 may be about 18 swg ( 1 . 2 × 10 - 3 cm ). screws 25 are employed to hold the plate 10 , the support plate 21 , and the support tray 22 together . the screws 25 pass through apertures 25a in the plates 10 and 21 . the apertures 25a in the plate 10 are provided with countersunk portions 25b iso that the heads of the screws 25 lie flush with the plate 10 . screws 26 are provided to secure the support tray 22 to the upper part 2a of the housing 2 , and are arranged to extend through apertures 22 of the support tray . the arrangement of the support plate 21 underneath the distribution plate 10 assists in preventing particles of material to be separated from being caught between the plates 10 and 21 ; this is because the material is less likely to be able to pass through the smaller apertures of the plate 10 . the percentage of the total plate area occupied by the apertures in the additional plate 9 is much less than the percentage of the total plate area occupied by the apertures in the plates 10 and 11 . this causes the air flow velocity through the plates 10 and 11 to be much greater than that through the additional plate 9 . the air flow supplied by the fan 13 is calibrated so that the air velocity through the additional plate 9 is less than the lowest terminal velocity of substantially all of the material to be separated , while the air velocity through the plates 10 and 11 is greater than the terminal velocity of at least part of the material . it is preferred that the cross sectional area of each of the plates 9 , 10 and 11 is substantially the same , and that the number of apertures per square metre in the additional plate 9 is less than in the plates 10 and 11 . for example , the plates 10 and 11 can be provided with 300 apertures per square metre , and the plate 9 can be provided with 81 apertures per square metre . the operation of the elutriator 1 for the application of separating different grades of tobacco material will now be described . air is supplied to the plenum chamber 7 by the air supply fan 13 and flows upwardly therethrough . the air is directed upwardly to the perforated plates 10 and 11 , and to the additional perforated plate 9 , by the air diverting plates 12 . the suction fan 31 draws air from the hood 14 , and the rate of air extracted by the suction fan is adjusted so that it is equal to the rate of air supplied from the air supply fan 13 . in this way air flows through the plates 10 and 11 and the additional plate 9 , through the pretreatment and elutriation zones 16 and 17 , into the hood 14 and up into the exhaust 15 . the provision of an air extraction rate substantially equal to the air supply rate helps to prevent air flowing in or out through the gap between the housing 2 and the hood 14 . the direction of air flow is indicated by arrows a in the drawings . tobacco 34 is fed into the inlet 18 onto the feed plate 8 from where it flows to the additional perforated plate 9 in the pretreatment zone 16 . in the pretreatment zone 16 the tobacco 34 is immediately partially fluidised by the air flowing through the additional plate 9 , and by the vibration supplied by the agitators 4 . in this way a partially fluidised bed is formed in the pretreatment zone 16 in which the tobacco 34 is evenly distributed , and is levelled to a constant depth . it is also stratified because lighter particles migrate towards the top of the bed while heavier particles migrate towards the bottom of the bed . apart from dust particles , substantially none of the tobacco 34 is drawn off into the hood 14 from the pretreatment zone 16 . the tobacco 34 flows from the pretreatment zone 16 to the elutriation zone 17 . in the elutriation zone 17 , the high air velocity causes lighter grades 35 of tobacco 34 to be drawn up through the hood 14 and into the exhaust duct 15 . due to the decrease in cross - sectional area at the exhaust duct 15 the velocity of air increases , thus accelerating the separated tobacco through the exhaust duct . the lighter grades 35 are passed through the separator 32 and are collected in the collector 33 . the heavier grades 36 of tobacco remain adjacent the plates 10 and 11 in a substantially fluidised state until they pass through the outlet 19 , and are collected in collector 37 . it will be appreciated that the operation of the elutriator would be similar for other leaf or fibrous vegetable materials such as tea . the elutriator 1 may be modified , for example , by the provision of two separate hoods each having separate exhaust ducts . one of the hoods can be disposed over the pretreatment zone 16 , and the other can be disposed over the elutriation zone 17 . in this way , the dust particles drawn off in the pretreatment zone 16 are separated from the lighter grades drawn off in the elutriation zone 17 . furthermore , it is not necessary that all the distribution plates in the elutriation zone have the same cross sectional area for air flow . for example it is possible to provide a series of distribution plates with increasing cross sectional areas for air flow so that the velocity of air through successive plates increases . a separate hood can be provided above each plate so that a different grade of material can be drawn off through each hood .
an elutriator for separating different grades of leaf vegetable material comprises an elutriation zone disposed above a flow distributor , and gaseous supply means for supplying a gaseous medium to the elutriation zone through the flow distributor . the flow distributor is adapted to provide a substantially uniform flow of gaseous medium to the elutriation zone , and the velocity of air delivered to the zone is greater than the terminal velocity of at least one grade of the material to be separated .