Background of the invention

The invention relates to a method for the production of polystyrene in pellet form on the basis of polystyrene plastic foam items, such as used packaging and the like, so that the polystyre can be re-used either alone or mixed for the production of polystyrene items, said method comprising a heating of the items until they melt in a vessel after which the melted material is passed through a filter to a stringing and pelleting tool, and also a plant for the execution of the method.

The use of polystyrene plastic foam items is strongly in¬ creasing because of the good characteristics of this material.

It is robust and rigid and moreover provides good protection against shock, impact and dropping to the ground by the relatively thick surface layer. Furthermore, it has good heat insulation properties due to the inner cell structure.

Items of this material can be produced in large blocks or sheets, and as moulded items such as those used for packaging.

The items are produced from polystyrene as the only or the main raw material, which is foamed-up in a mould by means of steam and a driving agent.

The large consumption of such plastic foam items gives rise to a considerable waste problem. Consequently, there is a

great need for the re-use of the foam plastic, but the possibilities are limited.

A material for re-use can, however, be produced in a relatively simple manner, namely granulated foam plastic, which is comminuted in a grinder. This granulate can be used as filling for insulation, packaging and similar technical applications. However, the quality and herewith the value of this granulate is not uniform, and the costs in connection with collection, processing and distribution are often greater than the market price for such a granulate.

Therefore, attempts have been made to convert the expanded polystyrene to polystyrene. These attempts include a comminution of the plastic foam, mainly by a cutting pro¬ cess, where simultaneously with the cutting there occurs a heating of the swarf due to the frictional heat developed by the cutting blades.

The cut-up material is hereafter conveyed to an extruder, where it is compressed into melting consistency and led out through an extruding head for the formation of polystyrene strings, which are hereafter pelletted for re-use in a commmonly-known manner.

However, this method has proved to be particularly uneconomic, the reason being that it requires considerable compression work to compress the material for melting consistency. This is because the comminuted material has a low density due to the high cell count. The efficiency of the extruder becomes so low that the re-use price exceeds the market price of the polystyrene raw material.

From German publication no. 2,204,733 there is known a plant for the melting of whole polystyrene items. The plant

comprises a hot oven in which the items are placed.

After a while the items melt, and the melted material leaves the oven and is collected for possible re-use. The purpose of this plant is to reduce the volume of the items in that the melted material takes up only little space.

However, this plant is not suited for effective recovery of the polystyrene, the reason being that the oven is a con- ventional oven with built-in heating elements in the sides of the oven which gives a slow and uneconomic melting. The heat distribution in the items is highly reduced due to the insulating property of the material causing a slow melting- down since melting will take place from the surface.

If, for instance, the material is overheated or the item is not clean, then the melted material will become unsuited for use.

Advantages of the invention

By the method according to the inventionm, where the items are comminuted in a grinder or a similar mechanical device, followed by possible washing and drying before they are conveyed to the melting vessel, there is obtained the advantage that a re-use material immediately suitable as raw material is created in a surprisingly simple manner.

The comminution ensures an effective melting in the vessel, because the heat distribution becomes uniform in that the comminuted material does not create any barriers but allows the heat to pass between the items. This ensures a high capacity at a low energy consumption.

Moreover, at an early point in the process, the material is hereby brought into a form which makes it suitable for

direct stringing by means of simple pump and transport means.

It is hereby possible to produce a re-usable polystyrene in pellet form which can immediately be used in new products or form part of a mixture with new material, and which at its low production cost and high market value can cover the costs of collection of plastic foam items and the process¬ ing of these for re-use, while at the same time the raw material for a new production of polystyrene items is here¬ with made cheaper.

The method according to the invention thus results in a hitherto unattainable step towards a financially profitable re-use of plastic foam items as polystyrene raw material, which solves the problem of waste in a manner beneficial to the environment.

As disclosed in claim 2, by melting-down and maintaining the molten material at temperatures most expedient for the method, the lowest possible cost and therewith production price of the re-usable polystyrene is ensured.

As disclosed in claim 3, the use of a worm conveyor for the last transport towards the tool ensures a suitably uniform delivery of homogenous material to the nozzle head.

As disclosed in claim 4, the mounting of electrical heating elements in and around the individual machine parts ensures the best possible temperature at the lowest possible operational costs.

As disclosed in claim 5, the building-in of non-return valves in the process line safeguards the flow of material against reflux, hereby avoiding a non-homogeneous flow of the molten material.

As disclosed in claim 6, by using a stringing tool with a number of adjacent nozzle openings which are oriented in a uniform manner in relation to the delivery flow of molten material, a configuration with a high capacity is obtained.

Finally it is expedient, as disclosed in claim 7, to insert a store vessel, in which the material can be stored in case that the molten material piles up.

The drawing

An example of an embodiment according to the invention will now be described in closer detail with reference to the drawing, where

fig. 1 shows a plant in schematic form,

fig. 2 shows a part of the plant comprising melting vessel, heating vessel and stringing tool,

fig. 3 shows a section through the coarse filter and the non-return valve seen in the direction III-III in fig. 2,

fig. 4 shows the non-return valve seen in the direction

IV-IV in fig. 3,

fig. 5 shows a section through the fine filter seen in the direction V-V in fig. 2,

fig. 6 shows a section through the stringing tool seen in the direction VI-VI in fig. 2,

fig. 7 shows an enlarged cross-section through a nozzle opening seen in the direction VII-VII in fig. 6,

fig. 8 shows the melting vessel seen from above,

fig. 9 shows a part-section through the melting vessel,

fig. 10 shows the melting vessel seen from below, and

fig. 11 shows the positioning of the heating elements on the filters, the heating vessel, the worm convey¬ or and the stringing tool.

Description of the example embodiment

An example of the execution of the method is shown in fig. 1.

The plant comprises a receiving station for the polystyrene plastic foam items 1 which, as indicated by the arrow, are fed into a grinder 2 where they are comminuted into small pieces to give them a form in which they can be transported through the plant, and which does not fill more space than necessary.

After this comminution, the material can be sorted for impurities, either in a commonly-known separator and/or by washing and subsequent drying. Such cleaning can be necessary when the items contain elements which may be in¬ jurious for the plant or the re-use, or which reduce the effectivity of the subsequent comminution and stringing out in the plant.

The comminuted material is conveyed via a channel 3 to a silo 4 which serves as an intermediate store.

The silo is provided with a ventilation channel 5 and, at the bottom, with a commonly-known control arrangement, for example a blower or other conveyor, for the further

transport of the comminuted material via a channel 6 to a separator, preferably in the form of a cyclone 7.

The gases mixed with and/or suspended in the air are led via an exhaust channel 8 through a carbon filter 9 to an outlet channel 10 and out to the surroundings.

The solid constituents of the comminuted material from the cyclone 7 are then led via a channel 18 down into a melting vessel 11, which will be described in more detail with reference to figs. 2, 8, 9 and 10.

In fig. 2 the melting vessel 2 is seen from the outside, and it will be seen how the cyclone 7 is mounted directly on the closed upper cover of the vessel. Through this cover there extends a stub for the exhaust channel 8 which, as indicated in fig. 1, leads to the outlet channel 10 via a dust filter 48.

In fig. 2 is also seen a hatch 12 for inspection and clean¬ ing of the inside of the vessel.

A pair of level sensors 13 extend through the upper cover for the regulation of the supply to the vessel.

The funnel-shaped bottom 20 of the vessel 11 is connected lowermost to a stub 22 with a flange to which a filter 14 is mounted.

The vessel is provided with an insulated jacket as shown in figs. 8 and 9, under which jacket electrical heating ele¬ ments 16 extend around the side. Through the upper cover there are inserted a number of electrical heating rods 17 which, as indicated in fig. 8, are evenly distributed in the interior of the vessel. These heating rods 17 have connection pipes 19 which extend up through the cover.

In the bottom 20 of the vessel 11 there is also provided a number of heating elements 21 which, as indicated in fig. 10, extend from the outside and in towards the outlet channel 22 for the vessel 11.

In this channel 22 there can be mounted a shut-off valve in the form of a slide-valve 23 or the like.

The channel 22 opens out into a filter 14 which is shown in section in fig. 3. The filter element 24 is inclined so that it slopes in the direction of flow. The element can be cleaned as required through an openable hatch or cover 49 in the valve housing.

The material melts when it is heated in the vessel 11. This requires a temperature of around 200-230°C, which is achieved by means of the electrical heating elements 16, 17 and 21 in and around the vessel 11, and heating elements in the form of a jacket 25, see fig. 9, which surrounds the filter 14.

The heating elements ensure a rapid melting-down of the material, which they maintain at a viscosity suitable for the further flow of material through the plant.

For this purpose there is used a pump 26, see figs. 2 and 11, which can be a piston pump or a rotary pump, in that said pump 26 presses the melted material through the filter element 24 and a subsequent non-return valve 27 which, as shown in fig. 3, can be a spring-loaded ball valve. This non-return valve safeguards against reflux and gives rise to an effective suction from the vessel and pumping through the filter 14.

Through a channel 28, the melted and filtered material is pumped through a further filter 29 which, as shown in fig.

5, can be a commonly-known automatic filter. Since this is a finer filter, it requires more frequent cleaning and re¬ placement, which can be effected via an actuator 31 which changes the filter elements 30 as required. A non-return valve 27 can also be mounted in the direction of flow after the fine filter 29.

Hereafter, the channel 28 leads to a heating vessel 32.

As shown in fig. 11, heating jackets 35 are provided around the channel 28, the fine filter 29 and the non-return valve 27.

As shown in fig. 2, the heating vessel 32 is provided with a level control 33 in the form of a float 34 with a pulse generator which can activate a pair of contacts for the upper and the lower levels of melted material in the vessel.

The object of the vessel 32 is to ensure a suitable store of melted material for the subsequent stringing process.

If such a store is not required, that is when the volume of melted material corresponds to the strung out volume, the melting vessel 32 can be omitted, either by providing the plant with a not shown bypass from the filters to the conveyor for the pelleting device, or by not providing a melting vessel 32 at all.

The vessel is connected to an exhaust channel 8. Finally, electrical heating bands 36 are mounted around the sides and the bottom of the vessel 32.

From the heating vessel 32, the melted material is conveyed through an outlet 37 to a conveyor arrangement which, as seen in figs. 2 and 11, is preferably a worm conveyor 38

which is driven by a motor 39. This conveyor arrangement feeds the melted material forward to a stringing tool, as shown in figs. 6 and 7.

This tool comprises a nozzle head 40 with a number of nozzle openings 41 which are shown on an enlarged scale in fig. 7.

All of the nozzle openings 41 extend at right angles to the nozzle head, which results in the best flow of melted material out through the openings when the material is pressed through them, in that the material will then be strung out.

To ensure the best condition of the melted material, an additional heating element in the form of a jacket 42 is mounted around the stringing tool.

The temperature of the melted material through the channel 28, the heating vessel 32 and the conveyor 38 is lower or around 200°C. The heat supply can be slightly higher around the stringing tool in order to ensure an advantageous stringing-out, the temperature of which can be a little higher.

Under the stringing tool 40 there is placed a belt conveyor 43 on which the strung-out polystyrene strings, as indi¬ cated by the arrow 45, can be conveyed further.

As indicated in fig. 1, exhaust hoods 44 can be suspended above the conveyor belt 43, said hoods being connected to the exhaust channel 8. Moreover, for the quick cooling of the polystyrene strings 45, jets (not shown) can be employed for the spray-cooling of the strings on the conveyor belt. The cooling can also be effected by allowing the strings to pass through a cooling bath.

The conveyor belt 43 carries the styrene strings 45 to a commonly-known pelleting tool 46, from which the pelleted polystyrene can be sifted and possibly vacuum-cleaned for the removal of dust before the regenerated polystyrene 47 can leave the plant for re-use in a new production.

The efficiency of the plant is high because of the effective melting-down and short production time through the plant from when the polystyrene plastic foam is intro- duced until the pelleting of the polystyrene granulate is completed.

Furthermore, the consumption of energy for the transport and pumping through the plant is remarkably low, partly because of the expedient viscosity brought about by the melting, and partly because of the suitability of the melted material for pressing out through the nozzle head.