WO1991005616A1

WO1991005616A1 - Rotary screen for separating fines from low density particulates

Info

Publication number: WO1991005616A1
Application number: PCT/AU1990/000486
Authority: WO
Inventors: Charles Treffner
Original assignee: Charles Treffner
Priority date: 1989-10-13
Filing date: 1990-10-10
Publication date: 1991-05-02

Abstract

A rotary screen for separating fines from expanded polystyrene foam particles comprises a rotary cylindrical screen (15) with its axis substantially horizontal, material to be screened is fed into one end of the screen, a shroud covering an upper part of the screen has its walls sealed to the screen by bristled brush wipers (24), a fan (27) draws air from the shroud, causing air to flow into the screen through its unshrouded lower part, and exhausts same with entrained fines into a filter bag, the residual particles are caused to flow along the screen for discharge at the other end by contact with a stationary Archimedean screw (20) within and co-axial of the screen. The discharged material is received in a vented mixing chamber wherein it is mixed with fresh expandable beads, and fed directly to a foam moulding machine by a second fan drawing air through the mixing chamber.

Description

ROTARY SCREEN FOR SEPARATING FINES FROM LOW DENSITY PARTICULATES

TECHNICAL FIELD

This invention relates to rotary screens or sieves for separating fines, that is to say dust and other small particles, from mixtures of fines and particulate material of a larger particle size. The invention is particularly directed to the separation of fines from low density particulates, such as fragmented expanded polystyrene (EPS) foam.

BACKGROUND ART

It is perceived as being environmentally desirable to recycle EPS foam as much as possible. Thus, it has become customary of late to reduce EPS foam scrap, such as offcuts, damaged product or prior used products, to a particulate condition, and to include a proportion of the particulate material, known as regranulate, in the feed material used for making fresh product.

As is well known, EPS articles are moulded by partially filling a mould cavity with expandable polystyrene beads and heating the beads to cause them to expand to fill the cavity and adhere together to form a rigid foam body. Ideally the regranulate consists of clean, separated, prior expanded beads, but that ideal has not hitherto been realised in practice when utilising known machinery for breaking down the scrap. Normally, regranulate is used straight from the breaking down machinery and comprises a mixture of beads, other more or less bead-sized fragments and fines or dust. DISCLOSURE OF INVENTION

The proportion of regranulate useable in the feed stock is limited. If too much is included, feed guns become choked and moulds tend to gum up, and the present invention arises from the appreciation that these effects are largely due to the fines in the regranulate. Therefore, an object of the present invention is to provide granulate screening apparatus which efficiently removes fines from EPS regranulate stock. It will be appreciated however that the invention is applicable to the separation of fines from any low density particulate material.

There is a highly developed art directed to the separation of fines from particulates utilising rotary screens, where the particulate material is relatively dense, for example cereal grains and the like. Generally, gravity is relied upon to cause the extracted material to fall through the mesh openings of the screen.

Typical prior art patents known to applicant are

French patent 2,41 , 42, and United States patents 2,785,798, 4,043,901, 4,107,034, 4,278,532, and 4,549,660. However, it was found in the development of the present invention that such prior art, gravity dependent, screens were unsatisfactory in operation when applied to very low density granulates, and the object of the invention is to provide a screen type separator adapted for use in relation to such materials.

The invention achieves that object by providing a rotary screen wherein an air stream blows the granulate or the like against the surface of a zone or portion of the screen, against which the particle component is trapped and through which the fines escape with the air stream.

The invention consists in rotary screening apparatus of the kind comprising a rotary, open-ended, cylindrical, perforate screen and means to feed particulate material and fines to be separated . into one end of the screen, for the discharge of fines through the screen perforations and of particulates through the other end of the screen, characterised by sealing means defining two longitudinally extending opposed zones of the curved wall of said screen and fan means causing air to flow into said screen through one said zone and, with entrained fines, from said screen through the other said zone.

For preference said one zone is an under zone and said other zone is an upper zone of the screen surface, so that the air stream lifts the material to be screened against the active or screening zone.

According to preferred embodiments of the invention transporting means are provided to assist or effect the movement of the particulates from end to end of the screen comprising at least one stationary deflecting element within the screen. That deflecting element is adapted not only to deflect material impinging on the element while moving with the screen in the longitudinal direction of the screen, but also to stir and turn over the material as it flows against the element. For preference the deflector element is an Archimedes screw blade, of somewhat lesser outer diameter than the inner diameter of the screen, extending longitudinally of the screen and being substantially co-axial therewith. That screw blade acts as an auger urging the material along the screen as the screen rotates. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a side elevation of an embodiment of a screening apparatus according to the invention with some side panels removed to show interior components.

Figure 2 is an sectional view taken on line 2-2 of figure 1.

Figure 3 is a plan view of the apparatus of figure 1.

Figure 4 is sectional detail of the mounting arrangements for the screen and deflector element of the apparatus of figure 1 drawn to a larger scale.

BEST MODE OF CARRYING OUT THE INVENTION

The illustrated embodiment of the above-described invention is described in more detail below, with reference to the accompanying drawings.

In this embodiment the screening apparatus is an integral part of mixing and feeding means for the supply of a mixture of winnowed granulate and fresh expandable polystyrene beads directly to the input hopper of a continuous, EPS foam, block moulding machine.

The apparatus as a whole comprises a supporting frame fabricated from steel structural members 10 which, together with sheet metal side and roof panels 11, define three chambers, namely a dust extraction chamber 12, an inlet chamber 13 at one end of the extraction chamber 12 and a mixing chamber 14 at the other end of the extraction chamber 12. A rotary cylindrical screen 15, having mesh walls of sufficient fineness to retain useful regranulate but permit the throughflow of fines, extends effectively from the inlet chamber 13 to the mixing chamber 14« The cylindrical screen 15 is supported at each end by cylindrical, open ended drums 16 integral with three- legged spiders 17.

The spiders 17 and the drums 16 are rotatably mounted upon a fixed shaft 18 co-axial with the screen 15 and extending from a supporting bracket and clamp 19 within the mixing chamber 14 through the extraction chamber 12.

That fixed shaft 18, which may be tubular or, as illustrated, have a tubular central portion, has a deflector element comprising a helical fin 20 wrapped around it which extends as an Archimedean screw, albeit a stationary one, from one end of the screen 15 to the other. The screw has a somewhat lesser diameter than the screen so as not to impede the free rotation of the screen.

The screen spider 17, at the input chamber end, is fixed upon a driven shaft 21 extending through the input chamber 13 from an external belt drive transmission 22 from a motor 31 providing infinitely variable control, within limits, of the speed of rotation of the screen 15. The driven shaft 21, spiders 17 and screen 15 rotate as one, being supported by bearings 32 in a housing 33 on an end wall of the input chamber 13, bearing 34 on one end of the fixed shaft 18 and bearings 35 on the fixed shaft 18 near its other end.

The input chamber 13 is essentially a simple hopper adapted to receive raw regranulate from an elevated storage silo or the like through an inlet chute or duct 23. The driven shaft 21 within it may be provided with radially extending scoops adapted to shovel the raw material between the legs of the associated spider 17 into the drum 16 and thus into the adjacent open end of the cylindrical screen 15 as those parts and the shaft 21 rotate. Felt sealing strips 41 are wrapped upon the drums 16 to prevent leakage where the drums extend through cavity septum walls between the input chamber 13 and the extraction chamber 12 and between the latter and the mixing chamber 14-

The extraction chamber 12 is essentially a shroud covering, say, the upper three quarters of the cylindrical surface of the screen 15. The lower longitudinal edges of the extraction chamber walls are substantially" sealed to the screen surface by sealing means in the form of longitudinal wipers, for example, wipers in the form of bristled brushes 24- Those sealing means remain stationary as the screen rotates and at all times define an under zone 25 of the screen surface and an opposed, somewhat larger upper zone 26 of that surface. Elsewhere the extraction chamber walls are spaced from the cylindrical surface of the screen to provide an air space therebetween.

A centrifugal fan 27 is mounted on or adjacent the extraction chamber 12 with its inlet port 3 communicating with that air space and its outlet 37 ducted to a dust filter bag (not shown). Thus, when the fan 27 is operating, air is drawn through the exposed under zone 25 of the screen wall, through the regranulate within the screen, through the upper zone 26 of the screen wall and the said air space into the fan 27. It is then expelled through the filter bag. As the air travels through the regranulate it picks up the dust and fines which are entrained with the air and eventually held within the filter bag. As the screen 15 rotates, which may be at a speed of from say 60 rpm to 150 rpm, depending on the required throughput, the regranulate is sucked against the upper zone 26 and taken along by its rotation relative to the screw. At the same time it is turned over and tumbled to be thoroughly exposed to the air stream for the efficient entrainment of the dust and fines. Some particles held clear of the screw blade 20 against an upper part of the screen by the air flow get blown off as the part of the screen in question moves past a brush 24 clear of the extraction chamber and is exposed to inflowing air. This also contributes to the general churning and mixing of the material within the screen.

The cleaned or treated regranulate discharges from the screen 15 into the mixing chamber 14« Fresh beads from a bead silo or the like are also discharged into the mixing chamber 14 at a predetermined rate by a screw conveyor 28 or other conventional means. The regranulate and the beads are mixed together by swirling air currents in the mixing chamber 14« To that end a second centrifugal fan 29 may be provided with its inlet port 39 opening into the interior of the mixing chamber and its outlet 38 ducted to the input hopper (not shown) of a block moulding machine.

An adjustable vent opening 30 is provided in the roof or a wall of the mixing chamber 14- That opening 30 admits an air stream which both mixes the chamber contents and provides an entraining stream for the discharge of the mixed contents through the fan 29.

The size of the vent opening is adjustable by means of a sliding damper 40. The size of the vent opening 30 is selected to ensure that the air pressure differential between the pressure in the mixing chamber 14 and in the outlet end of the screen 15 is suited to the prevailing conditions. The differential has to be such that, on one hand, the mixing chamber fan 29 does not suck material from the screen before it has been adequately cleaned and, on the other hand, such that the screen is not clogged by the extraction chamber fan 27 sucking material back from the mixing chamber 14 against the flow of material in the screen. In practice it has been found that there is little difficulty in establishing the correct setting by observation of the apparatus in operation and that once the vent setting has been established to suit the back pressures of the silos and filter bag little or no further adjustment is required unless and until there should be major changes in those components of the system.

Claims

1. A rotary screening apparatus of the kind comprising a rotary, open-ended, cylindrical, perforate screen (15) and means (23,13,17) to feed particulate material and fines to be separated into one end of the screen, for the discharge of fines through the screen perforations and of particulates through the other end of the screen, characterised by sealing means (24) defining two longitudinally extending opposed zones (25,26) of the curved wall of said screen and fan means (27) causing air to flow into said screen through one said zone (25) and, with entrained fines, from said screen through the other said zone (26).

2. A screening apparatus according to claim 1 wherein said one zone (25) is an under zone and said other zone (26) is an upper zone.

3. A screening apparatus according to claim 1 further characterised by a stationary deflector element (20) extending longitudinally within said screen, whereby material is directed towards the output end of the screen.

4. A screening apparatus according to claim 3 wherein said deflector element is an Archimedean screw.

5. A screening apparatus according to claim 1 further characterised in that the particulates are discharged directly into a mixing chamber, in that fresh make up beads are also fed into said mixing chamber and in that a second fan means (29) draws air through said mixing chamber for the entrainment and discharge of mixed material therewith.

6. A screening apparatus according to claim 5 wherein said mixing chamber incorporates an adjustable vent for the admission of air thereto.

7. A screening apparatus according to any one of claims 1 to 6 wherein said sealing means comprise wipers in the form of bristled brushes.