FIELD OF INVENTION

This invention relates to a process and apparatus for recycling mixed plastic waste.

BACKGROUND TO INVENTION

Recycling mixed plastics waste is problematic, particularly if the types of plastic vary too widely. Conventionally, mixed plastics are sorted into separate grades or types of plastic and subjected to a recycling process that is suitable for the particular plastic grade and/or type.

Sorting of plastics is labour intensive and can be problematic, particularly when sorting plastic waste mixed in with household and industrial refuse. Applying different recycling processes for recycling different grades and/or types of plastics adds significantly to the cost of recycling.

During conventional recycling of plastics waste, paper labels and non-plastic components attached thereto are removed, and the plastic waste is washed using water to remove excess dirt and other contaminants from the plastic waste. For recycling in conventional thermal extruders, the washed plastic waste must be dried prior to extrusion. The drying of plastic waste after washing and prior to thermal extrusion is both time consuming and adds to the recycling cost.

It is an object of the present invention to ameliorate the above mentioned problems associated with the recycling of mixed plastic waste. SUMMARY OF INVENTION

According to one aspect of the present invention there is provided recycling apparatus for recycling mixed plastic waste, the recycling apparatus comprising a thermal extrusion system including:

a thermal extruder including: a) an elongate barrel having a first end and a second end, the barrel having a feed opening into which a mixture of plastic waste and water is introduced into the barrel, the barrel having a number of heating zones along the length thereof for heating the plastic waste / water mixture, to a temperature at least above the boiling point of water so as to cause the water to change phase to steam, and further above a temperature above the melting point of a predetermined melting point of the plastic in the plastic waste/water mixture; a steam vent opening near the second end of the barrel, through which steam is allowed to vent from the barrel, a water collection reservoir disposed at an operative lower region thereof for collecting excess water which drains from the plastic waste / water mixture;

b) an extrusion head including an extrusion die, near the second end of the barrel;

c) a screw conveyer located within the barrel for conveying the mixture of plastic waste and water along the barrel from the first end thereof to the second end thereof; and

d) a screw drive motor for driving rotation of the screw conveyer.

A reduction gearbox can be provided between the screw drive motor and the screw conveyer. The barrel may include heating bands for heating the plastic waste that is within heating zones of the barrel and in a specification form can include three heating bands spaced apart along the length of the barrel.

The heating bands can comprise a first heating band disposed relatively closer to the first end of the barrel for heating heat the plastic waste to a temperature of about 280°C, a second heating band for heating the plastic waste to a temperature of about 350°C and a third heating band for heating the plastic waste to a temperature of about 250°C, the third heating band being disposed relatively closer to the second end of the barrel, with the second heating band being disposed at a position intermediate the first and second bands.

There can be a filter at the second end of the barrel for filtering contaminants from the molten plastic waste.

According to a further aspect of the present invention there is provided a process for recycling mixed plastic waste, the process including: cutting or grinding mixed plastic waste into pieces; adding water to the mixed plastic waste so as to form a water/plastic waste mixture having a water content of between 10% and 40% by weight; agitating the water/plastic waste mixture so as to blend the water with the plastic waste; heat treating the blended water/plastic waste mixture in a thermal extruder to a temperature above the boiling point of water thereby to cause the water to change phase to steam and further heating the water / plastic waste mixture at least to a temperature above the melting point of a predetermined type of plastic in the plastic waste having the highest melting point; passing steam through the plastic waste; draining excess liquid water from the plastic waste and venting steam from the thermal extruder near a discharge end thereof; and discharging the heat treated plastic waste from the thermal extruder and allowing the plastic waste to cool. In the preferred form the heat treatment of the mixed plastic waste includes the step of heating the mixed plastic waste to a temperature of up to 350 °C. More specifically, the heat treatment may include initially heating the plastic waste to a temperature of about 280°C for a predetermined period of time, thereafter increasing the temperature to a maximum temperature of about 350 °C for a further predetermined period of time and reducing the temperature to 250°C for a predetermined period of time.

The method can include the final step of comminuting the extruded plastic into granules.

DETAILED DESCRIPTION OF THE INVENTION

Further features of the invention are described hereinafter with reference to a non-limiting example of the invention and as illustrated in the single figure of the accompanying drawing which shows a thermal extrusion system in accordance with the invention for recycling mixed plastic waste in accordance with the recycling process in accordance with the invention.

The process for recycling mixed plastic waste is particularly adapted for recycling mixed plastic waste consisting of two or more of the following plastics:

Polyethylene (LDPE, MDPE and HDPE), Polyvinyl chloride (PVC), Polypropylene (PP), Polystyrenes (ASA, ABS, EPS, PS), thermosets and composites, Polyamide and industrial thermoplastics.

The recycling process broadly includes the steps of:

cutting or grinding mixed plastic waste into pieces having a size of between 4mm and 20mm; adding water to the mixed plastic waste so as to form a water/plastic waste mixture having a water content of between 10% and 40% by weight;

agitating the water/plastic waste mixture so as to blend the water with the plastic waste;

heat treating the blended water/plastic waste mixture in a thermal extruder to a temperature above the boiling point of water thereby to cause the water to change phase to steam and further heating the water / plastic waste mixture at least to a temperature above the melting point of a predetermined type of plastic in the plastic waste having the highest melting point;

allowing steam to pass through the plastic waste;

allowing excess liquid water to drain from the plastic waste and steam to be vented from the thermal extruder near a discharge in thereof; and

discharging the heat treated plastic waste from the thermal extruder and allowing the plastic waste to cool.

In a typical plastic recycling process in accordance with the invention, the heat treatment includes initially heating the plastic waste to a temperature of about 280°C for a predetermined period of time and thereafter increasing the temperature to a maximum temperature of about 350 °C for a further predetermined period of time and reducing the temperature to 250°C for a predetermined period of time.

The process includes grinding the extruded plastic into granules. The use of steam in the recycling process prevents plastics having relatively lower melting point from being burned. Furthermore, the use of steam in the recycling process forms pockets of water within the molten plastic, which subsequently evaporates leaving behind internal spaces within the resulting extruded plastic mass. The spaces provide the resultant plastic granules with a porous structure which has a number of advantages including that the spaces make the plastic granules lighter and render it suitable for use in a variety of applications, including as a replacement for drainage stones for planters, and surface cover for soil or other growing media, as a growth medium in hydroponics or aquaponics, as a construction material for concrete mixes and the like, wherein the porous plastic granules lighten concrete structures and can also enhance thermal and sound insulation properties of structures of concrete or other materials into which the plastic granules are incorporated.

With reference to the drawing, a thermal extrusion system 10 in accordance with the invention is shown which is for recycling mixed plastic waste in accordance with the invention. The system 10 comprises, broadly, a thermal extruder 12 and a screw drive motor 14.

The thermal extruder includes an elongate barrel 18 and a screw conveyor 20 which is located within the barrel for conveying the plastic waste/water mixture along the barrel. The motor 14 is coupled to the screw conveyor via reduction gearbox 16.

The system includes a hopper 22 containing the mixed plastic waste to be recycled. The hopper is located at a first end of the barrel for feeding the plastic waste/water mixture into the thermal extruder via a feed opening in the barrel. The barrel includes a water collection reservoir disposed along the length of the barrel at an operative lower region thereof for collecting excess water which drains from the plastic waste / water mixture. A water drainage outlet 24 is disposed near the first end of the barrel for draining the excess water from the barrel. The applicant envisages that the barrel will be inclined slightly to allow excess water which collects in the water collection reservoir to flow towards the outlet 24.

Before being fed to the hopper the waste plastics is cut into pieces having dimensions of 4 to 20 mm.

The barrel includes a steam vent opening 26 near a second end of the barrel, through which steam is allowed to vent from the barrel. The barrel further includes three heating band spaced along the length of the barrel. More specifically, the heating bands comprise a first heating band 28.1 disposed relatively closer to the first end of the barrel for heating heat the plastic waste to a temperature of about 280°C, a second heating band 28.2 for heating the plastic waste to a temperature of about 350°C and a third heating band 28.3 for heating the plastic waste to a temperature of about 250°C, the third heating band being disposed relatively closer to the second end of the barrel, with the second heating band being disposed at a position intermediate the first and second bands.

The thermal extrusion includes a filter screen 40 at the second end of the barrel for filtering contaminants from the molten plastic waste. The filter screen is supported within a filter plate 36 which is mounted between a pair of mounting plates 34 and 38. The mounting plate 34 is screwed onto a threaded formation 32 at an end region 30 of the barrel at the second end thereof. The mounting plate 38 is provided with a discharge pipe end 44 through which a tube of the extruded plastic waste is discharged from the thermal extruder. In the drawing, detail A shows the mounting plates and the filter plate in an exploded view.

After discharge from the thermal extruder the tube of plastic waste is comminute by grinding it into granules as described above.