BACKGROUND ART In the granulation of plastics waste in a granulator mill, a main fraction of plastics particles is produced where the individual particles are of relatively large size, often of the order of magnitude of one or a few mm. In addition, smaller particles or pure dust are also produced where the individual particle may be of extremely small size. The dust particles or the smaller particles often adhere to the larger particles as a result of purely mechanical forces, but also as a result of electrostatic bondings.

Conventionally, the granulate is conveyed with dust admixed therein from the granulator mill via a conduit to a cyclone where the granulate is separated from the air current carrying the granulate. Because of the slight mass of the small particles, their separation from the air current will be defective in the cyclone, for which reason a certain fraction of the particles departs with the exhaust air flow from the cyclone.

Those dust particles and otherwise smaller particles which adhere to the granulate particles accompany them in the separation and, as a result, wind up among the granulate. If this latter is handled without specific safety measures, the dust particles will be separated from the granulate and spread into the ambient surroundings, which may give considerable pollution problems. A fan is often placed between the granulator mill and the cyclone for positively advancing the air current which passes the cyclone and which entrains the granulate and dust particles with it. This implies that the cyclone is under excess pressure in relation to the ambient surroundings and also to components placed downstream of the cyclone. The

smallest leakage in the system entails that dust constantly flows out into the ambient surroundings which, in particular in clean environments, is totally unacceptable.

PROBLEM STRUCTURE The present invention has for its object to realise an environment in connection with the granulation of plastics waste where the pollution problems by the spread of dust are in principle solved. In particular, the present invention has for its object to realise a cyclone possessing superior separation capability and in which small and light particles are reliably led out together with added air without being mixed into the coarser fractions. Finally, the present invention relates to a solution which is simple and economical in manufacture and operation.

SOLUTION The object forming the basis of the present invention will be attained in respect of the method if this is characterised in that a partial vacuum in relation to the ambient surroundings is maintained in the air current carrying the dust.

The object forming the basis of the present invention will be attained in respect of the apparatus if this is characterised in that the fan is disposed to create, in relation to the ambient surroundings, a partial vacuum in a flow region which includes at least a part of the interior of the granulator mill, the cyclone, the filter and conduits interconnecting these components.

Finally, the object forming the basis of the present invention will be attained in respect of the dust separator if this is characterised in that a separator chamber of larger cross sectional area than the outlet of the cyclone for larger particles is disposed therebeneath, that a deflector device is disposed in the separator chamber to be impinged upon by falling, larger particles and dust carried thereby, and that an inlet for air is disposed beneath the deflector device whereby there is created an air current through the flow of falling, larger particles and dust.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The present invention will now be described in greater detail hereinbelow, with reference to the accompanying Drawings. In the accompanying Drawings: Fig. 1 schematically illustrates a plant for separating granulate and dust from an airborne flow from a granulator mill; and Fig. 2 shows, on a larger scale, those components which are employed for separating the granulate and the dust.

DESCRIPTION OF PREFERRED EMBODIMENT In Fig. 1, reference numeral 1 relates to an arrow which represents a conduit from the outlet side of a granulator mill (not shown) to the inlet 2 to a cyclone 3. In its upper regions, the cyclone is somewhat unconventional, as will be apparent below, but has a centrally located outlet 4 on the upper end of the cyclone. To the outlet 4, there is connected a conduit 5 which runs to the inlet side of a filter 6 whose downstream side is connected via a conduit 9 to a fan 7 with an outlet 8.

It will be apparent from the foregoing that the fan is placed at the terminal end of the series of components employed for separating, on the one hand, granulate and, on the other hand, for separating dust which is produced in the granulator mill not shown on the Drawings. It should also be observed that the air current which the fan 7 generates not only flows through the cyclone 3 but also through the granulator mill so that at least certain parts of its interior are under a partial vacuum in relation to the ambient surroundings. The same conditions naturally apply also to the cyclone 3, the conduits 5 and 9, as well as the filter 6 and the intake side of the fan 7. In that, in this manner, the system is kept under partial vacuum, dust can hardly- regardless of particle size-escape into the ambient surroundings since, in the event of leakages, air would flow into the system and thereby entrain with it any possible dust.

Beneath the cyclone 3, there is provided a conventional discharge sluice 10 whose purpose is to discharge such granulate as was separated in the cyclone without allowing any air flow either into or out from the cyclone 3. This discharged granulate is then transferred to a

receptacle container located beneath the discharge sluice 10 or is conveyed further for re-use with the aid of an air current flowing in a conduit system.

Since the granulate which departs from the discharge sluice 10 can be handled under atmospheric pressure and, for example, be poured or blown via a conduit to the infeed of an injection moulding machine, it will readily be perceived that even minor quantities of dust in the granulate would cause major pollution problems in the ambient surroundings. For this reason, it is vitally important that the granulate which departs from the discharge sluice 10 is dust-free as far as this is humanly possible. It will be apparent from Fig. 1 that there is disposed, beneath the outlet 12 of the cyclone 3 for larger particles or granulate, a dust separator 11 which in turn is placed over an inlet conduit 13 to the discharge sluice 10.

As was mentioned above, in the cyclone larger particles and granulate are separated from the air current which enters in via the conduit 1 in the upper region of the cyclone 3. In the lower region of the cyclone at the outlet 12, the granulate or large particles are no longer airborne, but these fall down under the action of gravity in order, in due course, to arrive in the discharge sluice 10. The dust separator 11 is disposed to separate, from the flow of downwardly falling large particles or granulate, dust particles which may possibly float freely in the air, but also separate such dust particles as adhere to the downwardly falling granulate particles. These released small particles and dust particles are conveyed upwards through the central region of the cyclone and depart via the outlet 4 of the cyclone, the conduit 5 and to the filter 6 where the dust and small particles are separated and accumulated. A relatively dust-free current of air then passes from the downstream side of the filter, through the conduit 9, through the fan 7 and out via the outlet 8.

In order to realise an air current through the lower region of the cyclone 3 and upwards through the central region of the cyclone and out to the outlet 4, there is provided, beneath the dust separator 11, an inlet 14 via which air from the ambient surroundings can be sucked in under the action of the fan 7. The air volume sucked in via the inlet 14 carries the dust and small particles released in the dust separator 11 upwards through the cyclone and to its outlet 4.

As was intimated above and as is apparent from Fig. 2, the cyclone is, at least as regards its upper regions, somewhat unconventional in design. In a conventional manner however, it has a tangentially directed inlet 2 which discharges in an annular flow space between a cylindrical

outer wall 15 and a cylindrical inner wall 16. The cylindrical inner wall 16 has a downwardly directed opening which discharges in the conical section 17 of the cyclone 3. Upwardly, the space inside the inner wall 16 discharges in the outlet 4 of the cyclone.

That which distinguishes the present cyclone from conventional cyclones is the fact that the cylindrical inner wall 16 extends considerably further down in the cyclone than was previously the case. Thus, the inner wall extends down into the conical region 17 of the cyclone so far that the radial distance between the inner wall 16 and the conical section 17 is considerably smaller than the radial distance between the inner wall 16 and the cylindrical outer wall 15, or, if this were to be absent, the corresponding radial distance at the height level of the inlet 2. Preferably, this radial distance at the lower end of the inner wall 16 is of the order of magnitude of 0.25 to 0.5 of the radial distance at the upper region of the inner wall 16.

Further, there is disposed, inside the inner wall 16, one or more plates 23 whose purpose is to prevent or reduce rotation of the flow which takes place with the major direction upwards inside the inner wall. In one embodiment, use is made of one such plate 23 which is placed in an axial diametric plane to the inner wall 16. The plate 23 has a lower edge which, in the vertical direction, is located at the lower end of the inner wall 16 and an upper edge which is located a distance up which approximately corresponds to the diameter of the inner wall 16, which is approximately twice as large as the diameter of the lower outlet 12 of the cyclone.

As will be apparent from Fig. 2, the dust separator 11 has a separator chamber 18 which is approximately in the form of two frustoconical shells which are turned to face with their large ends towards one another. Possibly, as is the case in Fig. 2, a small cylindrical band may be placed between both of the large ends. In accordance with that disclosed above, the separator chamber is rotation-symmetrical and is coaxial with the cyclone 3 and its outlet 12.

The separator chamber has a largest diameter in the central region in the vertical direction and this diameter is greater than the diameter of the outlet 12 of the cyclone 3, but is also greater than the diameter of the inlet conduit 13 to the discharge sluice 10.

Interiorly in the separator chamber 18, there is disposed a deflector device 19 which may also be considered as being composed of two conical shells which are turned to face with their

large ends towards one another. The diameter of the deflector device 19 is equal to or greater than the inner diameter in the outlet 12 of the cyclone 3. The deflector device 19 is placed concentrically in the separator chamber 18 and thereby also concentric in relation to the outlet 12 of the cyclone and the inlet 13 of the discharge sluice.

The inlet conduit 14 to the inlet 13 of the discharge sluice has its centre line placed in a diametric plane to the inlet 13 and has a downwardly angled portion 20 located most proximal the inlet and discharging in the inlet 13. For regulating the air current which passes in through the inlet 14, there is provided a regulator valve 21 which controls the volume flow of air sucked in through the inlet 14.

The dust separator functions in the following manner. Via the inlet 2 a mixture of air, granulate and small particles and dust flows into the cyclone 3. The air and these particles begin to rotate with increasing speed of rotation the further down in the conical region of the cyclone the come. Gradually as the larger particles impinge on the wall of the conical region, the rotation speed is retarded and the particles chute or slide along the inner surfaces of the cyclone down to the outlet 12 whence they fall under the action of gravity down into the separator chamber 18 where they strike the deflector device 19. At least the greater proportion of the falling granulate particles chute along the upper side of the deflector device 19 and depart therefrom at the peripheral edge 22 of the deflector device in order subsequently (once again under the action of gravity) to fall down and possibly strike the lower wall of the separator chamber 18 under the deflector device 19. As a result, there will be formed a curtain-like flow of granulate and larger particles from the peripheral edge and possibly the region diametrically outside this edge down to the lower wall of the separator chamber 18 and the inlet 13 to the discharge sluice 10. The air which is sucked in via the inlet 14 passes in a direction upwards and is forced to pass straight through the falling flow of granulate and large particles. Initially, the flow takes place at least partly radially outwards under the deflector device 19, past its periphery 22 and then at least partly radially inwards over the deflector device. This implies that the air current passes twice through the curtain of falling granulate, once under the deflector device 19 and once over it. As a result, the air volume sucked in via the inlet 14 will entrain with it both loose dust particles in the flow of granulate particles, but will also tear loose small particles ands dust particles adhering to the granulate particles, these small particles and dust particles being borne upwards by the air current centrally through the

cyclone 3 and out through the outlet 4 of the cyclone. The granulate which in due course arrives in the discharge sluice 10 is as good as completely free of dust and small particles.

As a result of the downward drawing of the inner wall 16 far down into the conical section 17 of the cyclone, the distance in the vertical direction which the dust-carrying air current must pass in an upward direction will be considerably shorter than it would have been in a conventional cyclone. Further, the plate 23 may be expected to reduce the turbulence or rotation in the upwardly flowing air current in the central regions of the cyclone.

According to the present invention, it is possible further to amplify the dust separating effect by employing two separator chambers 18 with deflector devices 19 placed therein above one another. Between the separator chambers, there is a short conduit section dimensioned analogous with the outlet 12 and the inlet conduit 13.

The above-described dust separating process can, to some degree, be controlled by a regulation of the air flow entering via the valve 21.

In the foregoing, the separator chamber 18 has been described as composed of two frustoconical shells. However, it also falls within the scope of the present invention that the separator chamber 18 is approximately spherical or approximately discus-shaped.

Further regulation possibilities reside in an adaptation of the peripheral diameter (at 22) of the deflector device 19 in relation to the inner diameter in the separator chamber 18 at its central seen in the vertical direction.

Alternative embodiments of the deflector device 19 are also conceivable and, thus, this may conceivably be composed of approximately spherical hemispheres or be generally approximately discus-shaped.