WO2006003409A1

WO2006003409A1 - Artificial aggregate comprising plastic material

Info

Publication number: WO2006003409A1
Application number: PCT/GB2005/002586
Authority: WO
Inventors: Howard Jeffery-Wright; Howard Lloyd Robinson
Original assignee: Tarmac Limited
Priority date: 2004-07-02
Filing date: 2005-07-01
Publication date: 2006-01-12

Abstract

A method of manufacturing a building material comprises providing a plastics material, shredding the plastics material, providing mineral fines, feeding the shredded plastics material and the mineral fines to a mixer and mixing the plastics materials and the mineral fines in the mixer at an elevated temperature, wherein at least 80% by mass of the shredded plastics material fed to the mixer as a particle size of less than 30mm.

Description

ARTIFICIAL AGGREGATE COMPRISING PLASTIC MATERIAL

This invention relates to a building material incorporating a plastics material and to a method of manufacturing such a material.

There have been various proposals for making building materials, for example, an aggregate material, including plastics material obtained from recycled waste material.

GB 2 291 419A, US 5,702,199 and US 6,000,877 all provide examples of such proposals.

It is an object of the present invention to provide building materials and methods of manufacturing building materials that provide improvements over existing materials and methods. Such improvements may be represented, for example, by a better or more reliable quality of the final product or by enabling a product of the same quality to be obtained more economically.

According to a first aspect of the invention there is provided a method of manufacturing a building material comprising the following steps: providing plastics material, shredding the plastics material, providing mineral fines, feeding the shredded plastics material and the mineral fines to a mixer and mixing the plastics material and the mineral fines in the mixer at an elevated temperature, wherein at least 80 per cent by mass of the shredded plastics material fed to the mixer has a particle size of less than 30mm. Where reference is made herein to particle size, it should be understood that the dimension given represents the longest dimension of the particle. The particles of shredded plasties material may be in the form of elongate flakes and in this case a flake has a particle size of less than 30mm if the longest dimension of the flake is less than 30mm.

We have found that by shredding the plasties material to the small size defined above, the quality of the final product can be improved and its appearance enhanced. In a typical process some but not all the plastics material melts in the mixer. Shredding the material to a small size promotes a uniform melting in the mixer of plastics material with a sufficiently low melting point and reduces the size of any plastics material that does not melt, improving the physical properties and appearance of the product.

Where reference is made herein to "shredding" it should be understood that the term is to be construed broadly as covering any physical process that provides smaller particles of plastics material from larger ones.

Preferably the mineral fines have a moisture content of less than 10 per cent by mass.

We have also found that the method is much improved by keeping the moisture content of the mineral fines (defined as the mass of water as a percentage of the total mass of the moist fines) below 10 per cent. If the moisture content of the fines exceeds 10 per cent the heating of the materials in the mixer can be unduly affected. The mineral fines may, if necessary, be dried to reduce their moisture content before they are fed into the mixer.

Preferably the shredded plastics material fed to the mixer is of a size such that at least 80 per cent by mass of the material has a particle size of less than 20mm. The particle size of the mineral fines can be chosen to suit a particular application. Preferably at least 80 per cent by mass of the mineral fines fed to the mixer have a particle size of less than 4mm. Mineral fines are commonly divided in the industry into two groups: a first group referred to as "dust" and comprising principally particles in a size range of 4mm down to about 63 microns, and a second group referred to as "filler" and comprising principally particles of less than 63 microns in particle size. In the present invention the mineral fines may be dust and/or filler. Thus at least 80 per cent by mass of the mineral fines fed to the mixer may have a particle size greater than 63 microns; in this case the mineral fines would be principally dust rather than filler. At least 30 per cent by mass of the mineral fines fed into the mixer may have a particle size greater than 63 microns; in this case the mineral fines may be principally dust or a mixture of dust and filler. A further possibility within the scope of the invention is that at least 80 per cent by mass of the mineral fines fed to the mixer have a particle size less than 63 microns; in this case the mineral fines will be principally filler.

After the mixing step one or more further processing steps are preferably carried out to provide a product of the method.

In one group of methods, the mixed plastics material and the mineral fines are compressed into agglomerates, for example, ovoids. The ovoids may have a particle size of 20 to 50mm. The mixed plastics material and the mineral fines may be compressed, for example at a pressure of more than 10 tonnes/in². Thereafter the ovoids may be crushed, for example, using a hammer mill, to produce graded aggregates. The hammer mill may be operated at a low speed to reduce the amount of fines generated during the milling; alternatively a different kind of crushing process may be employed. It is preferred that the proportion of fines in the product immediately after crushing is less than 30 per cent by mass and more preferably less than 20 per cent by mass. If the crushing step results in an aggregate having more fines than desired, some or substantially all of those fines may be removed by sieving. If desired, some or all of the fines that are removed may be returned to the mixer.

In another group of methods, the mixed plasties material and the mineral fines are extruded. Further material, which may or may not be plastics material, may be added to the mixture of the plastics material and the mineral fines prior to extrusion. The further material may be selected from the group comprising tyre waste, fibres and mineral fines, but other materials can also be added. The mixing of the further materials prior to extrusion may be carried out in a Z-Blade mixer.

The temperature of the material in the mixer is preferably controlled. Preferably the temperature of the material remains below 200⁰C since at higher temperatures harmful gases are liable to be released from the plastics material. The temperature of the walls of the mixer may be higher than the temperature of the material in the mixer and may even be higher than 200⁰C. In that case the residence time of the materials in the mixer must of course be restricted. Preferably the temperature of the mixer is monitored and preferably it is also controlled. In addition or instead, the temperature of the product leaving the mixer may be monitored and the process may be adjusted (for example, by altering the temperature of the mixer or by altering the residence time in the mixer) to control the temperature of the product leaving the mixer. The temperature of the product leaving the mixer is preferably not less than 16O⁰C. For most applications the temperature of the product leaving the mixer is preferably in the range of 170⁰C to 195°C and more preferably in the range of 175⁰C to 195°C. This good control of the temperature of the product leaving the mixer allows a product of consistent quality to be produced.

The plastics material may be obtained from any suitable source but in an especially preferred embodiment of the invention the plastics material is plastics waste material. It is also possible, if desired, to incorporate hazardous waste material either in the plastics waste material provided at the beginning of the method, or by adding such material during the extrusion step. Examples of such hazardous waste materials that may be incorporated are incinerator flue ash and sewage sludge. The waste material may become encapsulated within the mixture and in that way rendered harmless.

The method of the invention may be carried out with a wide variety of plastics materials. It is preferable that a component of the plastics material melts at a temperature below 200⁰C. Typically this component is represented principally by polyolefin plastics materials. The plastics material may include other plastics materials which do not melt in the mixer. Even a relatively small proportion of the component of low melting point may be sufficient: for example, in some applications a proportion by mass of at least 5 per cent in the final product may be adequate while in other applications a proportion by mass of at least 10 per cent is preferred.

The proportion of plastics waste material to fines can also be varied over a relatively wide range. For example, we have made useful products from a mixture by mass of 80 per cent fines to 20 per cent plastics waste material and also of 40 per cent fines to 60 per cent plastics waste material. In the case of a material having only 20 per cent by mass of plastics waste material, it is preferred that a relatively large proportion (at least 25 per cent by mass) melts in the mixer, whereas in the case of a material having 60 per cent by mass of plastics waste material, a much lower proportion of the plastics waste material is required to melt.

We have found that it is especially advantageous if the plastics waste material has a relatively high proportion of plastics material. Preferably the proportion of plastics material in the waste material that is fed to the mixer is at least 80 per cent by mass and more preferably more than 90 per cent by mass. We have found that whilst a wide variation in the composition of the plastics materials can be accommodated in many applications, it is disadvantageous for there to be a large proportion of non-plastics material. If necessary the plastics waste material may be pre-treated to increase the proportion of plastics material to above 80 per cent and preferably to above 90 per cent.

The moisture content of the mineral fines fed to the mixer is preferably less than 5 per cent, by mass.

Products of the method of the invention may be used in a wide range of applications including: asphalt, concrete blocks, aggregates or pourable concrete. The products may be shaped e.g. extruded, to any desired shape to form articles such as paviours or kerbs as well as articles for use in other applications outside the building industry, for example, any article usually made from a pure plastics material.

There may also be advantage in including other components in the final products of the method. In particular, for some applications of the product it is especially advantageous to include a fire retardant. That fire retardant may be added at an appropriate stage of the method; for example, it may be added to the plastics material or to the fines before they are fed into the mixer, it may be fed separately to the mixer or it may be mixed with the mixture taken from the mixer. Examples of fire retardants that may be employed are aluminum trihydroxide and antimony trioxide.

The use of plastics waste material having a high proportion of plastics material is an especially significant feature of the present invention and one which may be employed even with larger particles of shredded plastics material than those required according to the first aspect of the invention. Thus, according to a second aspect of the invention, the present invention provides a method of manufacturing a building material comprising the following steps: providing plastics waste material, shredding the plastics waste material, providing mineral fines, feeding the shredded plastics waste material and the fines to a mixer and mixing the plastics waste material and the fines in a mixer at an elevated temperature, wherein the proportion of plastics material in the plastics waste material that is fed to the mixer is at least 80 per cent by mass, based on the mass of the plastics waste material.

The method according to the second aspect of the invention may also incorporate any of the optional and preferred features described above with reference to the first aspect of the invention.

In its broader aspects, the invention may comprise any of a wide variety of features. Accordingly, in a third aspect of the invention there is provided a method of manufacturing a building material comprising the following steps: providing plastics material, shredding the plastics material, providing mineral fines, feeding the shredded plastics material and the mineral fines to a mixer, and mixing the plastics material and the mineral fines in the mixer at an elevated temperature, the method being further characterized by one or more of the following features: i) the plastics material is plastics waste material and the proportion of plastics material in the plastics waste material that is fed to the mixer is at least 80 per cent by mass, based on the mass of the plastics waste material; ii) the mineral fines fed to the mixer consist substantially of dust; iii) at least 30 per cent by mass of the mineral fines fed to the mixer have a particle size greater than 63 microns; iv) at least 80 percent by mass of the mineral fines fed to the mixer have a particle size of less than 4mm; v) at least 80 per cent by mass of the shredded plastics material fed to the mixer have a particle size less than 30mm; vi) the materials are heated to a temperature in the range of 170⁰C to 195°C during the mixing; and/or vii) a fire retardant is incorporated in the materials in the mixer.

By way of example, various methods embodying the invention will now be described.

The method of a first embodiment of the invention may conveniently be regarded as comprising the following steps:

1. Providing a plastics waste material.

2. Shredding the plastics waste material.

3. Providing mineral fines. 4. Feeding the shredded plastics waste material and the mineral fines into a mixer and mixing the plastics waste material and the mineral fines in the mixer at an elevated temperature.

5. Compressing the heated mixture into ovoids.

6. Crushing the ovoids to form an aggregate. Each of the steps above will be described in more detail below.

The plastics waste material is delivered as bulk mixed plastics material and comes from sources such as agricultural waste, general consumer waste, vehicle manufacturing or dismantling waste or other sources. The plastics waste material comprises some plastics materials, typically polyolefins, which melt at a temperature below 195°C and other plastics materials with higher melting points (above 200⁰C). About 95 per cent by mass of the waste material is represented by plastics materials, the remaining small fraction being foreign material such as wood, ferrous and non-ferrous metals.

The plastics waste material is passed through a shredder which may be of a design known per se and is then fed through a hammer mill with a 20mm output mesh. The resulting material is in the form of flakes of an elongate shape having a particle size of the order of 20mm or less.

The mineral fines may be obtained from various sources including surplus quarry fines, canal dredgings, gravel pit settling lagoons and china clay waste. The fines have a particle size of less than 4mm and a moisture content of less than 10 per cent. A small amount of foreign material in the mineral fines can be tolerated.

The shredded plastics waste material and the mineral fines are fed to a mixer which may be of a kind known per se. In the described embodiment the mixer is a twelve-paddle horizontal shaft type mixer operating at approximately 60rpm with an oil-heated jacket. The oil temperature in the jacket is maintained at about 260⁰C. In the described embodiment a batch mixing process is adopted with each batch comprising about 200kg of material. The fines are fed into the mixer first via a side access hatch, the mixer is then started and a conveyor feeds in the shredded plastics waste material through a top-loading door. As mixing takes place the materials are heated and any moisture is driven off. After a mixing time of typically 6 to 8 minutes, the materials have reached a temperature of between 175°C and 195°C, preferably between 185⁰C and 195°C, with some of the plastics materials, in particular polyolefins such as polyethylene, having melted, thereby binding the mixture into a loose blend which is then removed from the mixer. If the dwell time in the mixer is too short the mixture will not bind together or will be very coarse and "granular". If the dwell time in the mixer is too long the mixture becomes more cohesive and there is a risk of its temperature becoming so high that some polymers break down and release toxic fumes.

The mixture taken from the mixer is discharged to a conveyor system that feeds a press such as a briquette press. The briquette press is of a type known per se and provides moulds in which 38mm ovoids can be formed by applying an appropriate pressure. The ovoids are then allowed to cool.

The cooled ovoids are then fed to a crushing machine, again of a type known per se, where they are crushed to provide a graded aggregate. The crushing machine is designed to produce a relatively low proportion of fines. If desired, the aggregate material can be screened to remove some or all of the fines.

In a particular example of the invention, a blend of 60 parts by mass of limestone fines and 40 parts by mass of General Plastics Waste was mixed for 7 minutes. The mixture reached a temperature of 170⁰C and after pressing, crushing and screening was used to form concrete blocks.

In other particular examples of the invention, a blend of 60 parts by mass of gritstone fines and 40 parts by mass of automotive waste plastic was mixed for 6 to 7 minutes and reached temperatures in the range of 164-188°C. After pressing, crushing and screening an aggregate with particle sizes in the range of 4mm to 10mm was obtained and subsequently used as 10 per cent by volume replacement of the aggregate in an asphalt mix.

In an alternative embodiment of the invention, the mixture removed from the mixer is fed to a Z-BIade mixer, additional material such as tyre waste or fibres is added and the product is then extruded via a ram system and fed to a mould. The extruded product may be moulded to any desired shape.

Claims

1. A method of manufacturing a building material comprising the following steps: providing plastics material, shredding the plastics material, providing mineral fines, feeding the shredded plastics material and the mineral fines to a mixer and mixing the plastics material and the mineral fines in the mixer at an elevated temperature, wherein at least 80 per cent by mass of the shredded plastics material fed to the mixer has a particle size of less than 30mm.

2. A method according to claim 1, in which the mineral fines have a moisture content of less than 10 per cent by mass.

3. A method according to claim 1 or 2, in which at least 80 per cent by mass of the shredded plastics material fed to the mixer has a particle size of less than 20mm.

4. A method according to claim 1 , 2 or 3, in which the mineral fines consist substantially of dust.

5. A method according to any preceding claim, in which at least 80 per cent by mass of the mineral fines fed to the mixer have a particle size of less than 4mm.

6. A method according to any preceding claim, in which at least 80 per cent by mass of the mineral fines fed to the mixer have a particle size greater than 63 microns.

7. A method according to any preceding claim, in which at least 30 per cent by mass of the mineral fines fed to the mixer have a particle size greater than 63 microns.

8. A method according to claim 1 , 2 or 3, in which the mineral fines consist substantially of filler.

9. A method according to claim 8, in which at least 80 per cent by mass of the mineral fines fed to the mixer have a particle size of less than 63 microns.

10. A method according to any preceding claim, further including the step of compressing the mixed plastics material and the mineral fines into agglomerates.

11. A method according to claim 10, in which the agglomerates are ovoids.

12. A method according to claim 11, in which the ovoids have a particle size in the range of 20 to 50mm.

13. A method according to any of claims 10 to 12, further including the step of crushing the agglomerates to produce graded aggregates.

14. A method according to claim 13, in which less than 30 per cent by mass of the particles of the aggregates have a size of less than 4mm.

15. A method according to any of claims 1 to 9, in which the mixed plastics material and the mineral fines are extruded.

16. A method according to any preceding claim , in which the temperature of the materials remain below 200⁰C during the mixing.

17. A method according to claim 16, in which the materials are heated to a temperature in the range of 170⁰C to 195⁰C during the mixing.

18. A method according to any preceding claim, in which the plastics material comprises a plurality of different plastics materials.

19. A method according to any preceding claim, in which the plastics material is a plastics waste material.

20. A method according to claim 19, in which the proportion of plastics material in the plastics waste material that is fed to the mixer is at least 80 per cent by mass, based on the mass of the plastics waste material.

21. A method according to claim 19, in which the proportion of plastics material in the plastics waste material that is fed to the mixer is at least 90 per cent by mass, based on the mass of the plastics waste material.

22. A method according to any preceding claim, in which a fire retardant is incorporated in the materials in the mixer.

23. A method of manufacturing a building material comprising the following steps: providing plastics waste material, shredding the plastics waste material, providing mineral fines, feeding the shredded plastics waste material and the fines to a mixer and mixing the plastics waste material and the fines in a mixer at an elevated temperature, wherein the proportion of plastics material in the plastics waste material that is fed to the mixer is at least 80 per cent by mass, based on the mass of the plastics waste material.

24. A method according to claim 23, in which the mineral fines have a moisture content of less than 10 per cent by mass.

25. A method according to claim 23 or 24, in which the proportion of plastics material in the plastics waste material that is fed to the mixer is at least 90 per cent by mass, based on the mass of the plastics waste material.

26. A method according to claim 23, 24 or 25, including any of the further features specified in any of claims 2 to 22.

27. A method of manufacturing a building material comprising the following steps: providing plastics material, shredding the plastics material, providing mineral fines, feeding the shredded plastics material and the mineral fines to a mixer, and mixing the plastics material and the mineral fines in the mixer at an elevated temperature, the method being further characterized by one or more of the following features: i) the plastics material is plastics waste material and the proportion of plastics material in the plastics waste material that is fed to the mixer is at least 80 per cent by mass, based on the mass of the plastics waste material; ii) the mineral fines fed to the mixer consist substantially of dust; iii) at least 30 per cent by mass of the mineral fines fed to the mixer have a particle size greater than 63 microns; iv) at least 80 percent by mass of the mineral fines fed to the mixer have a particle size of less than 4mm; v) at least 80 per cent by mass of the shredded plastics material fed to the mixer have a particle size less than 30mm; vi) the materials are heated to a temperature in the range of 170⁰C to 195°C during the mixing; and/or vii) a fire retardant is incorporated in the materials in the mixer.

28. A method according to claim 27, in which the mineral fines that are fed to the mixer have a moisture content of less than 10 per cent by mass.

29. A method of manufacturing a building material, the method being substantially as herein described.

30. A product obtained from a method according to any preceding claim.