GB2231770A - Processing inhomogeneous material such as tobacco - Google Patents

Abstract

A method and apparatus for separating particles having a larger area-to-weight ratio from those having a smaller area-to-weight ratio in a series of steps including moving the mixed particles transversely across a moving gas stream to entrain therein a large proportion of those particles having a larger area-to-weight ratio. An illustrated embodiment is an apparatus comprising a number of modular threshing (4, 8A-C) and classifying (6, 10A-B, 12) units connected by conveyors within housing (20A-G). When used to separated lamina from stem of tobacco leaf, the lamina is entrained in the gas flow and removed on transverse conveyors (16A-D). The stem is removed from an exit (18). The design of each threshing unit is such that all or part of the casing may be slidably removed to reveal the thresher's interior.

Description

APPARATUS AND METHOD FOR PROCSSS INE3CM6NEOUS MXL SUCH AS 3A(XD The invention relates to the processing of inbomogeneous material such as leaf tobacco.

Essentially, preparation of cured tobacco leaves involves stripping the lamina from the stem by a mechanical threshing process followed by classifying and separation of the lamina from the stem parts of the leaf. Both parts are then processed separately before subsequent manufacture of tobacco products. It is essential that the separated products conform to the high standards in respect of extracted lanina particle size and percentage of acceptable stem content within that extracted lamina. Also, from the commercial point of view, maximum yield of both lamina and stem from the whole leaf is required.

According to the invention (which is defined in the claims) there is for example provided a method of progressively threshing the whole tobacco leaf to liberate the lamina from the stem of the leaf. After each threshing stage the free lamina is progressively separated from the threshed product by first separating the particles having a larger surface area-to-weight ratio from those having a smaller surface area-to-weight ratio by their inertia which are pneumatically classified to extract the lamina by being transversely moved into a series of controlled velocity streams of gas such as air. Each gas stream is individally adjusted to control velocity for maxImum entrainment of the stem-free lamina in the area-to-weight ratio group it is classifying.The penultimate gas stream velocity can be adjusted to increase the quantity of lamina extracted with the penalty of increased stem content of that lamina, this product being then reclassified in the ultimate gas stream to reduce the stem content but retain the increased lamina extraction rate and therefore classifier efficiency.

Also, according to the invention there is provided a threshing machine having a roll-away case giving total access to the moving and stationary parts of the machine, thereby facilitating adjustment of clearances between these parts to optimise the threshing performance of the machine.

Furthermore according to the invention there is provided a classifier separating the lamina from stem and stem with lamina attached, comprising a plurality of inertia and pneumatic separating chambers in series, comprising a modular constructed enclosure for each charber, means for providing a classifying gas flow in each chamber, means for projecting the threshed particles transversely to the direction of gas flow therein, means for controlling and adjusting the velocity of the gas flow, means for separating and collecting lamina from the gas flow path via an airlock-controlled outlet, means for collecting and removing the heavy particles (stem and lamina with stem attached) from the chamber and conveying them to the next threshing stage.Preferably, the gas from which the lamina has been separated is returned directly to continue the classifying gas flow.

Conventional processing installations thresh the leaf progressively and between threshing stages use pneumatic classifiers to progressively extract clean lamina from the threshed mixture, i.e. it is always the 'dirty' product which is classified and reclassified. The method of the invention effectively extracts the maximum amount of 'clean' lamina in such pneumatic classifier chamber allowing the remaining 'dirty' product to pass to the next stage of threshing. Given this maximum percentage of clean lamina extracted in chambers other than the last will be contaminated with a unacceptable amount of stem, this contaminated lamina is reclassified to reduce stem content of the lamina collected from the last chamber but retain the high percentage extraction rate.

An embodiment of the invention will now be described by way of example with reference to the drawings, in which: Fig. 1 shows a tobacco threshing and classification line in elevation Fig.2 shows the primary thresher partly in elevation and partly in section Fig.3 show a secondary thresher Fig.4 shows a classifier in elevation Fig.5 shows the preliminary inertial classifying portions of the classifier shown in Fig. 4 on an enlarged scale, and Fig.6 shows the main, pneumatic classifying portions of Fig. 4 on an enlarged scale.

The threshing and classifying line is shown in outline in Fig. 1. An infeed hopper 2 receives the tobacco leaf which is metered to the primary threshing unit 4. The outlet of the thresher 4 is linked, by a conveyor belt within housing 20A, to a first modular classifier assembly 6, from which the 'clean' lamina is removed on a transverse conveyor belt 16A.

Conveyors within housings 20B-G link alternate threshers 8A-C and classifiers 12A & 12B and final classifier 14. Separated lamina is removed from each classifier on transverse belts 16B, 16C and 16D, and the stem is extracted from the final classifier at exit 18.

The primary threshing unit 4 is shown in Fig.2 and comprises two threshers since it has to haddle the entire throughput of produce. The incoming product to be threshed is divided between the two threshers by means of a flow divider 29a formed by two contra-rotating ribbed drums within the infeed hopper 29. A rotor in each thresher carries fixed blades 30 and is mounted for rotation on a shaft 32 which in turn is mounted in bearings at each end in the base structure of the thresher and carries at one end a pulley by means of which it is driven in rotation by motor 33 through a belt 31.

Adjustment of the tension of each belt 31 is achieved by a screw and nut assembly which enables the motor and its pulley to be moved horizontally.

As the blades 30 of each rotor rotate, they pass between them sets of stationary blades 37 which are pivotally mounted on the support frame of the thresher to enable them to be swung out for cleaning and maintenance.

Beneath each rotor and adjacent to the paths of the rotor blade tips is a fixed basket 34 between the bars of which the threshed product drops to falls onto a conveyor belt 39 which in turn delivers the product onto a conveyor belt 36a of an elevator 36. The specific design of the basket depends on the type of tobacco being processed.

The thresher 4 thus operates in an essentially conventional manner and strips the tobacco leaves into portions which consist partly of pure lamina material, partly pure stem material and the remainder consisting of portions of lamina material still attached to portions of stem material.

The thresher differs from conventional construction in that the enclosure for each rotor, instead of being formed by a fixed enclosure with access doors (which in view of the considerable width of many installations cause difficulties due to their weight and in any case restrict access), as its major portion 38 supported by means of rollers 40 on upper and lower tracks 42a, b on the thresher frame. Thus, whenever it is required to gain access to the interior of the thresher, the complete casing 38 or an upper or lower part 38a or 38b can be rolled aside, thereby providing ready access for the entire interior for example to enable adjustments. As shown in Fig 3, each of the later threshers 8A, 8B and 8C has only a single rotor and one driving motor.The movable part 38 of the casing is shown in full lines in its open position and in dotted lines in its closed working position where it is secured to the fixed part 40a of the casing at the right-hand side of the rotor. Suitable interlocks prevent the thresher driving motors from operating when the casing is open.

Alternate elevating conveyors 20A, 20C, 20E and 20G lift the output material from the respective threshers 4, 8a, 8b and Sc into the inlet of the respective next classifier lOA, lOB, lOC, lOD. Each classifier 10 is assembled from standard sub-units or modules so that the number of classifications to be carried out within each classifier can be chosen as required. Fig. 4, by way of example, shows the classifier lOA. This classifier has two preliminary classification steps 22A and 22B and a full classification step 24A with a lamina reclassification step 24B.

Fig.5 shows the elements of a preliminary classification stage 22. The incoming material to be classified is supplied to the inlet 101 of an impeller 102 in which a rotating paddle wheel 103 projects the material across an enclosure 104. The paddle wheel 103 rotates within the casing 105 of the impeller 102 with a small clearance so as to form with it an airlock in all rotary positions.

A fan assembly 107 mounted at one side of the upper end of the classifier has a fan 108 (the impeller of which is driven through a belt 109 by a motor 110) which draws air into its inlet from the upper end of the enclosure 104 through a duct 112 and a rotating cylindrical screen 113. This air is returned from the outlet of the fan through a duct 115 leading into the lower part 116 of the interior of the classifier.

The fan thus causes a relatively gentle updraught through the enclosure 104 past the outlet of the impeller 102. The air velocity in this region can be adjusted by means of a plate 117 hinged at its upper edge 118 and fixed in position by suitable clamping means (not shown). A window 119 in the casing forming the enclosure 104 enables the operation of this preliminary classifier to be inspected. In operation, the components of the product having a low weight to surface area ratio are entrained in the upward air flow and drawn into the duct 112 to be deposited on the surface of the cylindrical separator 113 and thereafter deposited through an air lock 121 onto the transverse conveyor 16.

The particles having a somewhat greater weight to surface area ratio travel under their momentum across the enclosure to enter the inlet 102 of the next classifying stage which may be however a further preliminary classifier or a main classifier 24 to be described below. The articles of the highest weight-to-surface area ratio, such as stem carrying no lamina, fall through the upward air flow and are deposited onto a conveyor belt 52 on which they are carried to be deposited on the elevating conveyor 36 leading to the next thresher 8.

Fig.6 shows in elevation two successive main classifying stages. The intermediate product consisting of some stem and some lamina leaf material enters the left-hand stage 24a at 201 where in falls into the paddle roller 203 of an impeller 202 (which may in fact be the impeller 102A of Fig.5). The product is thus ejected into the interior of an enclosure defined by a lower casing 205 and an upper casing 206. Air is drawn upwards within the upper casing 206 by a fan and an air product separator assemble 207 of essentially the same construction as that shown at 107 in Fig.5. However, the returning air from the fan is directed by a conduit 208 into a plenum chanber 209 formed with a series of parallel outlets 210 beneath an inclined conveyor 211 having a slatted conveyor belt 212 through which air can pass from the outlets 210.

This air then passing through the material which falls onto the belt and can carry away light particles which were trapped by heavier particles.

The speed of airflow upwards through the belt 212 and the interior of the casing 206 is adjusted to be sufficient to carry substantially all of the free lamina and those portions of stem which carry appreciable areas of lamina material. The portions of stem which carry no or little lamina, however, remain on the belt 212 and drop off onto the conveyor 52.

To maintain relatively constant air flow speeds across the interior of the casing 206, it is found preferable to include one or possibly two internal baffles 214.

The product which is entrained in the upward air flow within the casing 206 is deposited by the assembly 207 through its airlock-forming outlet paddle roller 221 into the inlet 301 of the next classifying stage 24B. This is of identical construction to the stage 24A being built up from similar modules 205 and 206. The output paddle wheel 321 of the second stage delivers lamina material to the required specification onto the transverse conveyor 16. The conveyor 311 of the second stage delivers product having a high stem content onto the conveyor 52 and thence to the elevating conveyor 36 which delivers it into the next thresher to be re-threshed.

Claims (13)

1. A method of separating particles having a larger area-to-weight ratio from particles having a smaller area-to-weight ratio in a series of steps in which the particles are moved transversely while subjected to a moving stream of gas such as air, wherein the velocity of the stream of gas at the outlet of each stage is sufficiently low for the product entrained therein to contain a sufficiently large proportion of particles having a predetermined large surface area-to-weight ratio, and this product is collected from each step after removing the product from the gas stream.

2. A method according to Claim 1, wherein in each step the gas stream is returned to continue the upwardly moving stream of gas for that step.

3. A method according to Claim 1 or 2, in which in at least the first step the particles are projected across the gas stream from one side thereof and particles which traverse the gas stream are fed directly through an air lock into the next step.

4. A method according to Claim 1 or 2 wherein at least one stage material which falls through the gas stream is deposited onto a moving surface through which upwardly moving gas streams are passed.

5. A method according to any of Claims 1 to 4, wherein, in a penultimate step the gas stream velocity is increased, thereby increasing the amount of particles having a smaller area-to-weight ratio as well as of the particles having a larger area-to-weight ratio, and the collected particles are reclassified in the ultimate step.

6. A method according to any of Claims 1 to 5 wherein the particles which fall through the gas stream fall onto a conveying surface for removal.

7. A method of threshing and classifying tobacco in which tobacco leaves are threshed and lamina portions of the tobacco are separated and collected by a method according to any of Claims 1 to 6 and the remainder is removed by a moving conveyor surface, and alternately rethreshed and classified by methods according to any of Claims 1 to 6.

8. A method of processing tobacco substantially as hereinbefore described with reference to the drawings.

9. A classifier for separating particles having different weight-to-surface area ratios comprising a plurality of pneumatic separator steps in series, comprising an enclosure for each step, means for providing a moving gas flow in each enclosure, means for projecting the particles into the first enclosure transversely to the direction of gas flow therein, means for separating and collecting particles from the gas flow path at an upper outlet of the enclosure, an air-lock controlled outlet for receiving particles which have crossed the enclosure without being entrained and for passing these particles to the next stage, and a conveyor for removing heavy particles which fall towards the lower end of the enclosure.

10. A classifier according to Claim 9, including for each step means for returning the gas to the enclosure.

11. Apparatus for threshing and classifying tobacco comprising a primary thresher, a belt-type conveyor for conveying threshed tobacco to a first multi-step classifier according to Claim 7, and alternate threshers and multistage classifiers for threshing and classifying the material removed by the conveyor of the preceding classifer.

12. Apparatus for thresing the classifying tobacco substantially as hereinbefore described with reference to Fig.l and Figs.3 to 6 of the drawings.

13. A tobacco thresher comprising a housing and at least one bladed rotor mounted for rotation in the housing and cooperating with stationary elements in the housing, wherein an upper part of the housing which part encloses the rotor is supported by rolling means on tracks for movement into an open position allowing free access to the rotor.