WO1996023640A1

WO1996023640A1 - Treatment of waste

Info

Publication number: WO1996023640A1
Application number: PCT/GB1996/000224
Authority: WO
Inventors: Anthony Wright
Original assignee: Styromelt Limited
Priority date: 1995-02-03
Filing date: 1996-02-02
Publication date: 1996-08-08
Also published as: AU4546096A; GB9502136D0; GB9602162D0; GB2297553A

Abstract

An apparatus for treating contaminated plastics waste comprises a drum (8) which is mounted for rotation above a heating chamber (10) and is enclosed within an enclosure having lockable front access panels (3). During heating in the heating chamber, the front panels (3) are locked by electronically controlled locks (47) and the enclosure is exhausted by an impeller (7) which draws hot gas from the heating chamber over the drum and thereby sterilises the drum. The plastics waste is held in the heating zone at a temperature and for a period sufficient to disinfect it.

Description

TREATMENT OF WASTE

This invention relates to the treatment of waste material, in particular, waste plastics prior to disposal or recycling.

Plastics in various forms find uses in a wide range of applications, from everyday household items to sophisticated engineering products. Not only have they replaced and improved upon materials formerly used, but they have also made possible industrial and medical applications that would have been impracticable with older technologies.

Examples of plastics materials in common use are polyethylene (LDPE and HDPE) which, in film form, is used in carrier and refuse bags, when injection molded, is used in containers, lids and component parts or, when blow molded, is used to form bottles and various liquid containers.

Polystyrene (PS) and high impact polystyrene (HIPS) are used in a wide range of products, for example, disposable service ware, video cassettes, audio-visual equipment, toys and office machinery and commonly in medical applications, such as syringes, culture dishes, lest tubes and vials. Orientated polystyrene (OPS), characterized by its clarity and strength, is used for blister packs, confectionery wrappings, pastry trays and related food packaging. Expanded polystyrene (EPS) is used extensively for its insulation and shock absorbent properties in the construction industry, custom packaging, fish boxes, agricultural containers for seeds and plants, disposable cups, trays, plates and fast food containers.

Polypropylene (PP) is used, in film form, in general packaging where a clear view of the product is required without discolouration, eg. overwrap for cigarettes, chocolates, cosmetics, clothing and other articles. When injection or blow molded, it finds uses in products such as screw-on caps and closures, including tamper-evident closures for medicines and food etc, parts for small and large appliances, furniture and office equipment, and in medical products such as bottles, syringes, flasks, pipettes and beakers.

There are considerable advantages in the use of plastics in many applications, in that they are relatively inexpensive to manufacture, have advantageous tensile properties and their replacement of natural materials such as wood and paper in for example, furniture and packaging, has undoubtedly contributed toward the conservation of timber resources. However, their disposal poses a major problem due to their inertness especially in relation to their non-biodegradability.

Although some "bio-degradable" plastics have been developed, their use has not become widespread since in many applications this biodegradable property is undesirable as it reduces the lifetime of the product. They are also relatively expensive.

The most common method of disposing of plastics remains landfills which, due to the high volume to low weight ratio of many plastics products, involves high transportation costs and inefficient use of the site capacity. Such sites are often unsuitable for most purposes when the site has been filled because of the risks of collapse and contamination of the site by toxicants.

Moreover, plastics are unsuitable for incineration due to the emission of toxic gases.

Recycling of plastics as of other materials, such as paper and glass, is becoming increasingly common due to increased public environmental awareness. However, such processes involve high costs and very often, the end products are not suitable for use due to contamination of the original plastics by other materials such as metals etc. Decontamination processes are complicated and expensive.

A method and apparatus of treating contaminated plastics waste is known from US-A-5240656. Such a method involves the densification of contaminated waste by causing it to pass through a heating zone to produce molten contaminated plastics and causing the molten waste to flow continuously out of the heating zone under the influence of gravity. However, several problems have been encountered with such a method and apparatus.

The apparatus described in US-A-5240656 is of a complex construction with separate densification and post-densification melt chambers and a downstream cooling and moulding zone. The moulding zone includes a support plate with mould cavities which is guided for reciprocating movement so that when a mould cavity is in alignment with an outlet in the post-densification chamber, molten waste flows into the mould cavity.

The support plate and the individual mould cavities are provided with cooling circuits containing liquid coolants. However, the use of cooling systems in such an apparatus is undesirable as they are expensive, increase maintenance costs considerably and are susceptible to leaks and failure causing the entire system to be shut down.

Contaminants such as pieces of electric wire, metals, glass and in some cases even paper, which are present in the molten waste as it passes out of the melt chamber, can obstruct the reciprocating motion of a valving system at the outlet of the melt chamber, causing it to jam. In order to clear the object which has caused the jam, the plastics waste must remain in a molten condition and hence the heating circuits must be maintained switched on while the offending contaminant is removed. This can be a highly dangerous procedure for the operator as the temperature of the molten plastics can be in the order of 180-200 °C, with the additional risk of toxic fume inhalation if the object has to be removed from inside the melt chamber. This results in the apparatus being inoperative for long periods of time and requiring constant maintenance and expensive replacement of parts which may have been damaged by the obstructing contaminant.

In particular, many problems have been encountered with the treatment of medical and biohazardous waste. These often contain high levels of contaminants such as agar and body fluids which, after the water content of the waste has evaporated, have been found to leave a residue which mixes with the molten plastics. The residue changes the viscosity and forms a congealed substance that does not flow easily through the outlet into the mould cavities beneath. This congealed substance can also jam the valving system. Because of the viscous nature of the waste material it does not flow within the mould to take its shape and, if liquids are still present, these can leak through the mould venting system.

Additionally, operating personnel are placed at risk of contracting contagious and potentially fatal diseases in the treatment of such waste. In the prior art apparatus the molten plastics are irradiated whilst cooling and solidifying in order to achieve disinfection. However, no facility is provided for making the waste safe during the actual treatment process itself and so personnel are still at risk from their working environment, as the loading hopper and melt chambers may be contaminated even after the treatment process has been completed.

Such an apparatus also involves complex procedures to deal with the dangers of fluids leaching from the solidified products, or the possibility of cuts or abrasions from contaminated 'sharps' such as needles and scalpel blades, by gating the molten plastics waste into tough thermoplastic high-temperature outer casings which are then sealed. This process is complex and expensive.

The main object of the present invention is to provide a method and an apparatus for the treatment of plastics waste material, in particular contaminated plastics waste material, prior to disposal or recycling, which overcomes or at least substantially reduces the aforementioned disadvantages.

In summary, the above US patent discloses an apparatus for the treatment of plastics waste, comprising hopper means arranged to receive plastics waste, heating chamber means arranged to heat plastics waste from the hopper means to a flowable condition, said heating heating chamber means being provided with a normally closed outlet for healed flowable plastics, and exhaust means arranged to exhaust fumes or gases from the apparatus.

In one aspect the present invention provides an apparatus as summarised above, characterised in that said exhaust means is arranged to draw hot gas over the surface of said hopper means.

The above US patent also discloses a method of treating plastics waste comprising feeding plastics waste into an entry zone, passing the plastics waste into a heating zone and heating the plastics in said zone to a flowable condition, and exhausting gas from said heating zone.

In another aspect the invention provides a method as summarised above, characterised in that hot gas is exhausted over said entry zone.

Preferred features are defined in the dependent claims.

From another aspect, the invention resides in an apparatus for the treatment of contaminated plastics waste, said apparatus comprising an entry zone for contaminated plastics waste, a healing zone for heating said contaminated plastics waste for a predetermined period of time to a temperature sufficient to cause said contaminated plastics waste to melt and form molten plastics waste having contaminant inclusions, said heating zone being provided with a normally closed outlet in its base; and controllable outlet means in the region of said base of said healing zone, operative to open said outlet in order to allow said molten plastics waste and its contaminant inclusions to pass through said outlet from said heating zone, said controllable outlet means being activated in response lo the expiry of said predetermined period of time.

Expressed in another manner, the invention resides in a method of treating contaminated plastics waste, said method including feeding contaminated plastics waste into an entry zone causing the contaminated plastics waste to pass into a heating zone; heating said contaminated plastics waste in said heating zone to a temperature at which the plastics waste melts and forms molten plastics waste having contaminant inclusions; maintaining said molten plastics waste and its contaminant inclusions within said heating zone at said temperature for a predetermined period of time; and causing said molten plastics waste and its contaminant inclusions to pass out of said heating zone in response to the expiry of said predetermined period of time. Since the particular types of plastics and the levels and forms of contaminants present therein vary from one application to the next, the apparatus and method of this invention ensures that by controlling the period of time for which a particular type of contaminated plastics load is heated and the temperature at which it is heated, a suitable molten product is produced economically for a given contaminated plastics load.

From another aspect, the invention resides in a method of treating contaminated plastics waste, said method including feeding contaminated plastics waste into an entry zone; selecting a particular processing cycle from a plurality of processing cycles in dependence on the nature of said contaminated plastics waste; causing the contaminated plastics waste to pass into a heating zone; heating said contaminated plastics waste in said heating zone to a temperature at which the plastics waste melts and forms molten plastics waste having contaminant inclusions; maintaining said molten plastics waste and its contaminant inclusions within said heating zone at said temperature for a predetermined period of time; and causing said molten plastics waste and its contaminant inclusions lo pass out of said heating zone in response to the expiry of said predetermined period of time.

By provision of a plurality of processing cycles with varying temperature and cycle times, the operator may select the cycle which provides the most efficient and safest manner of treating a particular plastics waste load. Some loads may require specific treatment processes, such as loads with a high liquid content, or loads of potentially hazardous medical waste which require heating at higher temperatures for longer periods of time, than, eg. "dry" or non-hazardous loads, in order to achieve evaporation or sterilization by dry heat. Hence the appropriate processing cycle can be selected in order to achieve these treatment conditions and a more economic and efficient treatment means is provided.

According to a further aspect, the invention provides a method of treating contaminated plastics waste, said method including: feeding contaminated plastics waste into an entry zone; causing the contaminated plastics waste to pass into a heating zone; heating said contaminated plastics waste in said heating zone to a temperature at which the plastics waste melts and forms molten plastics waste having contaminant inclusions; evaporating liquid present in said contaminated plastics waste during said heating so as to achieve a substantially moisture-free environment within said heating zone; maintaining said molten plastics waste and its contaminant inclusions within said heating zone at said temperature for a predetermined period of time; and causing said molten plastics waste and its contaminant inclusions to pass out of said heating zone in response to the expiry of said predetermined period of time.

By ensuring that liquids present in the contaminated plastics waste have evaporated and that the solid contaminants together with the plastics waste have been brought to the required temperature for a predetermined period of time, disinfection by dry heat of both the contaminated plastics waste and the apparatus itself is achieved.

A variety of different figures have been approved by different bodies as acceptable standards for the achievement of sterilization by dry heat:

European Pharmacopoeia (1990)

160 °C 120 minutes

170 °C 60 minutes

180°C 30 minutes

British Pharmacopoeia 160 °C 60 minutes

US Pharmacopoeia

170°C 120 minutes

DHSS (1980) Sterilizers, Health Technical Memorandum No. 10, HMSO, London

160°C 45 minutes 170°C 18 minutes

180°C 7.5 minutes

190°C 1.5 minutes

These values represent the minimum time for which waste must be maintained at a certain temperature in a dry heat environment in order to achieve sterilization. In an apparatus constructed in accordance with, and a method of carrying out the present invention, the plastics waste preferably reaches a minimum temperature of 230° C in the regions near the surfaces of the heating zone and 180 "C in its centre. Thus, since the minimum cycle time is preferably 60 minules, all of these approved standards are exceeded. Hence the solidified plastics waste and the working surfaces and atmosphere of the apparatus are rendered non-hazardous and are thus suitable for disposal in landfills or by recycling processes. This achievement of sterilization during the treatment process also leads to a safer working environment for operating personnel.

In a further aspect, the invention resides in an apparatus for the treatment of contaminated plastics waste, said apparatus comprising: an entry zone for contaminated plastics waste; a heating zone for heating said contaminated plastics waste in order to cause said contaminated plastics waste material to melt and form molten plastics waste having contaminant inclusions, said heating zone having a normally closed outlet in its base; and controllable outlet means in the region of said base of said heating zone, operative to open said outlet allowing said molten plastics waste and its contaminant inclusions lo pass through said outlet means from said heating zone and then to close, said controllable outlet means being provided with a cutting means to slice through contaminant inclusions in the molten plastics waste passing through said outlet as it closes.

Expressed in another manner, the invention resides in a method of treating contaminated plastics waste, said method including: feeding contaminated plastics waste into an entry zone; causing the contaminated plastics waste to pass into a heating zone; heating said contaminated plastics waste in said heating zone to a temperature at which the plastics waste melts and forms molten plastics waste having contaminant inclusions; causing said molten plastics waste and its contaminant inclusions to pass out of said heating zone; and applying a cutting force to the molten plastics waste and its contaminant inclusions as it emerges from said heating zone so as to slice through contaminant inclusions in the molten plastics waste.

Provision of such a cutting means allows contaminants such as wire, glass and metal present in the molten waste passing through the outlet to be sliced through as the controllable outlet means closes the outlet. In this way, jamming of the system by such contaminants is substantially eliminated.

According to yet another aspect, the invention resides in an apparatus for the treatment of contaminated plastics waste, said apparatus comprising: an entry zone for contaminated plastics waste; a heating zone for heating said contaminated plastics waste in order to cause said contaminated plastics waste material to melt to form a molten plastics waste having contaminant inclusions, said heating zone having a normally closed outlet in its base; controllable outlet means in the region of said base of said heating zone, operative to open said outlet allowing said molten plastics waste and its contaminant inclusions to pass through said outlet means from said heating zone and then to close; and obstruction detecting means operable to indicate a condition where said outlet means is obstructed by contaminant inclusions.

Expressed in another manner the invention resides in a method of treating contaminated plastics waste, said method including: feeding contaminated plastics waste into an entry zone; causing the contaminated plastics waste to pass into a heating zone; heating said contaminated plastics waste in said heating zone lo a temperature at which the plastics waste melts and forms molten plastics waste having contaminant inclusions; causing said molten plastics waste and its contaminant inclusions to pass out of heating zone; and detecting the presence of obstructional contaminant inclusions in the molten plastics waste and its contaminant inclusions emerging from said heating zone.

The presence of such an obstruction detecting and indicating means enables the operator to monitor such a condition and ensure that the situation is brought under control as quickly and safely as possible.

According to a preferred embodiment said entry zone comprises a hopper which may be loaded manually or otherwise and is advantageously manufactured from stainless steel to prevent corrosion and to facilitate cleaning. Provision of such a hopper isolates the loading area from the heating zone and so ensures a safer working environment for operating personnel.

In order to further improve safety, the hopper is preferably accessed through a loading panel in the external housing of the apparatus.

The hopper may be provided with a means to detect the level of the load therein. Said detecting means may comprise an ultrasonic sensor. Provision of such a means may be advantageous when the hopper is being fed automatically from a bulk storage hopper or by an air or belt conveyor or when the hopper is being fed manually in batches over a period of time, in order to indicate when maximum load capacity has been reached.

In order to protect operating personnel the hopper is preferably provided with safety means to prevent access to the apparatus during the treatment process. Said safety means advantageously comprises a timer-actuated locking means.

In a first embodiment which is particularly suitable for the treatment of medical and biohazardous waste, said hopper comprises a rotatable drum with an opening through which said hopper is loaded when said opening is in a first position and unloaded when said υpenCing is in a second position. In said first position said opening is preferably in alignment with a loading panel in the housing of the apparatus and in said second position said opening is preferably in alignment with the top of said heating zone. The opening is advantageously provided with a cover means that may be opened and closed manually or otherwise.

A rotary hopper of the abovementioned type, in the form of a self-contained cylindrical drum, facilitates easy loading of the apparatus with minimal risk to the operator, especially where loading does not take place in one step but in small batches which are not treated until a full load is present. Such a design minimizes the surface area of the apparatus in contact with the untreated plastics waste and, in turn, the reduced surface area optimizes Ihe achievement of sterilization of the internal surfaces, by dry heat.

A rectangular hopper is of simpler design and provides greater loading capacity which may be advantageous with loads containing larger objects such as industrial containers etc.

Preferably the temperaure of the hot gas drawn over the hopper is at a temperature of 115 °C lo 175 °C (more preferably 120 °C to 165 °C).

The plastics waste is heated to a sufficient temperature for a sufficient time to permit of sterilization of Ihe contaminants in the plastics waste, whatever those contaminents may be. The whole process takes longer for plastics waste containing liquids than plastics waste containing other contaminents such as metal or glass because evaporation of the liquid must be completed before the temperature can rise to the appropriate sterilisation temperature.

To ensure sterilisation, the plastics waste is preferably heated to a temperature of at least 180 °C for a time of al least 45 minutes.

To facilitate the passage of the load from the hopper into the heating zone, the shape of the opening in the top of the healing zone is preferably adapted to that of the second opening of the hopper. Hence, the load falls easily into the heat chamber without "bridging" i.e blockage of the entrance of the heating zone due to interlocking of partially melted plastic objects to form large partially solid masses, as the rising heat currents from the heating zone cause even large interlocking pieces to deform.

In any of the embodiments hereinbefore described, the hopper may be provided with an additional opening which may be accessed through a panel in the external housing of said apparatus. Said additional panel may preferably be provided with a cover panel which may be opened or closed manually or otherwise. Additional access to the hopper is advantageous should a blockage occur due to the presence of large objects which are unable to pass into or out of the heating zone.

The heating zone of the present invention preferably comprises a melt chamber. The configuration of the melt chamber may be of any suitable form consistent with the achievement of optimum heat transfer conditions. Thus the inner surface of the melting chamber is ideally of circular cross-section.

In order to achieve optimal melting and densification, the melt chamber may be a zonal melt chamber with a plurality of zones at different temperatures which are controlled so that the temperature increases in the direction of movement of said plastics waste from said entry zone to said controlled outlet means. Preferably the melt chamber comprises a plurality of converging zones or sections of decreasing cross-sectional area in the down-stream direction. Said plurality of zones or sections are of generally cylindrical or frusto-conical form or a combination thereof. Such a design facilitates the gradual densification of the plastics waste as it begins to lose its shape in the wider upper zones and gradually become molten as it passes to the narrower lower zones.

Optimum heat transfer is assisted by casting said melt chamber wall of a non-ferrous metal of high conductivity, eg aluminium. All working surfaces within the melt chamber are preferably coated with a high performance heat resistant material which is suitable for continuous temperature ratings in excess of 250 °C and which is chemically resistant to most minerals and organic chemicals. Multiple layer coatings of high performance PTFE are particularly suitable. Insulating the chamber wall is also advantageous as it assists in the maintenance of even heat transfer and prevents undesirable heat loss.

In order to provide even heat transfer to said contaminated plastics waste, the melt chamber is preferably provided with at least one electrical heating element which is arranged in a particular configuration and location to provide even heat transfer and avoid any "cold spots". Ideally the or each element is of optimum length to reduce linear loading and give a linear watts density which increases longevity of the heating elements.

In one advantageous configuration, the or each electrical heating element is in the form of a plurality of parallel loops extending around the melt. Alternatively the heating element(s) may be in the form of a helix or a spiral, or a combination of loops and helices or spirals In order to maintain even heat transfer through the chamber wall and into the melt, the heating elements may be arranged to surround the chamber wall and preferably comprise a plurality of loops which may project towards each other like curved fingers in substantially parallel relationship around the wall and stop just short of each other at their ends. Even heat transfer is further enhanced by embedding at least one heating element in the chamber wall at locations which are nearer the inner wall (i.e nearer the melt) than the outer wall.

Advantageously the or each heating element is integrally cast in the metal chamber wall such that there are no inclusions of air around the heating element(s) to result in the formation of local "hot spots" which may cause the element(s) to fail. Cast-in heating elements enable thicker and longer heating elements to be employed and ensure maximum contact with the chamber wall, providing an even heat profile.

Alternatively, or in addition to peripheral heating, heating may be provided in the middle of the melt chamber. The means for introducing heat into the middle region of the melt chamber may comprise a central projection extending upwardly through the centre of said melt chamber and extending across the melt chamber from one wall to the other. To minimize the risk of impeding the passage of the molten plastics waste through the melt chamber, the projection is preferably tapered in the upstream direction.

The projection is advantageously located in the lower region of said melt chamber so as to enhance heat transfer and provide higher temperatures in the lower zones of the melt chamber.

In one advantageous configuration, said projection is located at a position raised above the outlet of the melt chamber so as to minimize the risk of impeding the passage of the molten plastics waste through the outlet, in particular, such waste that contains larger contaminant inclusions such as bottles and cans.

In an alternative configuration which is particularly suitable for loads with a high liquid content, the projection extends upwardly from the base of said melt chamber, dividing the outlet into two separate outlet sections, and facilitating increased heat transfer at the outlet region of the chamber. This arrangement accelerates evaporation of liquids present in the contaminated plastics waste which may have accumulated at the base of the melt chamber.

In its simplest form, the projection preferably comprises at least one web or spider located in the melt chamber, said web dividing the melt chamber into two sections along its length.

In a further preferred form the projection comprises two webs mounted at 90° with respect to each other, said web dividing the melt chamber into four sections along its length. This arrangement increases the surface area of heat transfer into the plastics material.

According to a preferred embodiment of the invention, the controllable outlet means at the base of the melt chamber comprises a gate means which is controlled so as to open at a predetermined time after the treatment process has commenced, allowing the molten plastics waste to pass through an outlet in the base of said melt chamber out of the melt chamber. Said outlet is of such a shape and sufficiently large dimensions that when the outlet is open, said molten plastics with contaminant inclusions can pass through easily. Advantageously, said outlet is of a generally rectangular shape and has dimensions in the order of 240 x 150 mm. In a preferred form, said gate means is controlled by a programmable logic controller.

In a preferred configuration, said gate means is provided with a safety means which causes said gate means to reopen on detection of an obstruction in its travel path. Preferably said safety means comprises a gate travel time detector which causes said gate means to reopen when the time taken for the gate to close (the gate travel time) exceeds a preset value. Safety means preferably also comprises a fault indicating means. To ensure safety of operating personnel, said gate means may be prevented from opening in the absence of a receiving means capable of holding said molten plastics waste after it has emerged through said gate means. Said further safety means may comprise a proximity detector.

According to a preferred embodiment of the invention, the apparatus as hereinbefore described is provided with a receiving means into which the molten plastics waste and contaminant inclusions pass from said heating zone.

In its simplest form, said receiving means may comprise a removable dump tray of any suitable shape and dimensions capable of holding the molten plastics load until it cools and solidifies to form a solidified product or block which supports or encapsulates any contaminants contained therein and preferably has a shape which has been formed during solidification.

To facilitate easy introduction and removal to and from the apparatus, said dump tray is preferably mounted inside a drawer which also serves as an overspill tray. As a safety precaution, the apparatus may be provided with a further receiving means capable of holding the molten plastics waste in the absence of the drawer and dump tray. Said dump tray, drawer and further receiving means are advantageously manufactured from stanless steel when medical and biohazardous waste is being treated or zinc-coated carbon steel when commercial and catering waste is being treated.

In a preferred form, said solidified products are of a generally rectangular shape such as blocks, bricks or slabs, which may be easily stacked. This represents a considerable space saving which is of particular advantage in optimizing the capacity of landfills, storage and transportation facilities.

Since it may be advantageous to produce smaller solidified blocks for easier handling, storage and transportation, said receiving means may be provided with dividers of any suitable shape. For advertising purposes, said receiving means may be provided with a logo or other symbol of identification moulded into its base which is imprinted into the block on solidification.

In another embodiment, said receiving means may be lined with packaging material such as paper or cardboard, so that the solidified blocks can be packaged easily. This may be advantageous in applications where the plastics waste contains 'sharps' such as needles or blades etc which may protrude on solidification and cause injury when handling.

The solidified blocks of densified plastics waste produced by the method and apparatus of the present invention can be recycled and used in several advantageous applications. For example, the solidified blocks have advantageous properties as a fuel element and it has been found that, when added to coal, the calorific value increases favourably. It should be appreciated, however, that only solidified blocks which do not contain contaminants such as, eg. heavy metals, which would be harmful to the environment when used as a fuel, are suitable for recycling to energy. Such solidified blocks may be used for several purposes such as, an aggregate in road building, to produce a "plastic wood" which may be used eg., in the furniture industry, thus helping to conserve natural resources or as a modifier with other plastics lo improve the melt index in the manufacture of injection molded recycable products such as coat hangers.

In order to provide a safer and more pleasant working environment for operating personnel, the entry zone and heating zone may be provided with evacuation means to remove any odours, fumes or gases rising on convection currents and expansion of air. Evacuation is preferably achieved by means of a plenum box with a removable particle filter and exhaust fan connected to a suitable flue.

The apparatus as hereinbefore described may also be provided with an odour controller which preferably operates by injecting a mist of odour neutralizing agents into the entry zone so as to counteract unpleasant odours produced during the treatment process. This may be advantageous in applications where objectionable odours are present, eg. in the treatment of plastic fish containers.

The apparatus as hereinbefore described may be conveniently contained within a housing such as a cabinet or casing which is preferably provided with a suitable control panel. Said cabinet may be designed to any form suitable for the application in which it is to be used. Hence, a small compact unit is envisaged for consumer applications such as in fast food outlets whereas a much larger unit may be required for large scale industrial or municipal applications.

The foregoing summary of the invention, as well as the following detailed description of the preferred embodiments, will be better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there are shown in the drawings embodiments which are presently preferred, it being understood, however, that the invention is not limited to the specific arrangements and instrumentalities disclosed.

Figure 1 is a perspective view of one form of housing for an apparatus constructed in accordance with the invention for the treatment of contaminated plastics waste and illustrated in Figures 4 and 5;

Figure 2a is a partial cross-sectional side elevation of the apparatus of Figure 1 to illustrate an embodiment of the invention with a rotary hopper;

Figure 2b is a partial cross-sectional lop plan view of the embodiment of Figure 2a.

Figure 3a is a partial cross-sectional side elevation of another embodiment of the invention with a rotary hopper in the loading position;

Figure 3b is a partial cross-sectional top plan view of the embodiment of Figure 3a.

Figure 4 is a part sectional front view of an apparatus in accordance with the invention, illustrating the main operating components in an assembled condition;

Figure 5 is a part-sectional side elevation of the apparatus of Figure 4;

Figure 6 is an exploded perspective view of some of the operating components of the apparatus of Figures 4 and 5; Figure 7 is a schematic perspective view showing an arrangement of an electrical heating element for a heating zone of the apparatus of Figures 4 and 5;

Figure 8 is a part-sectional side elevation of an electrical resistance heating element for use in the apparatus of Figures 4 and 5;

Figure 9a is a side elevation of the apparatus of Figures 4 and 5, illustrating an embodiment with an alternative hopper;

Figure 9b is a plan view of Figure 9a;

Figure 10a is a side elevation of the apparatus of Figures 4 and 5, illustrating an embodiment with a hopper having additional access facility;

Figure 10b is a plan view on Figure 10a;

Figures 11a and lib are a detailed longitudinal section and a part-transverse section looking from above of the apparatus according lo Figures 4 and 5 illustrating an embodiment with a single central web dividing the lower regions of the heating zone;

Figures 12a and 12b are a detailed longitudinal section and a part-transverse section of the apparatus according to Figures 4 and 5 illustrating another embodiment with two central webs mounted perpendicularly with respect to each other;

Figures 13a and 13b are a detailed longitudinal section and a part-transverse section of the apparatus according lo Figures 4 and 5 illustrating a further embodiment with two central webs mounted perpendicularly with respect to each other and serving to divide the outlet into two separate outlets;

Figures 14 to 17 are perspective views of various types of dump trays which may be used in the apparatus of Figures 4 and 5; Figure 18 is a perspective view of the drawer lined with packaging; and

Figure 19 is a perspective view of a solidified block of plastics waste following removal from the drawer of Figure 18 and having been wrapped in the packing lining that drawer.

Referring to Figure 1 of the drawings, there is shown a housing in the form of a cabinet (1) comprising side panels (2), front panels (3, 4, 5), a top panel (6) and at least one rear panel (not shown). Within the cabinet (1) is a chassis to which the cabinet panels are fixed and which supports the main operating components of the apparatus. The upper front panel (3) covers an opening through which a loading hopper (not shown) within the cabinet may be accessed. Panel (3) may be in the form of two panels hinged to the side panels as is shown in Fig 1, or of any other suitable form, such as single panel hinged to either of the side panels (2) or the top panel (6).

The middle front panel (4) of the cabinet (1) constitutes a control panel which carries various electrical control elements, such as temperature indicators, cycle selectors, switches, indicating lights and circuit breakers, all of which are connected to an electrical control circuit (not shown). The operations effected by the electrical control circuit are controlled by a PLC (Programmable Logic Controller) disposed behind the control panel (4).

The lower front panel (5) which opens outwardly, provides access to a receiving means from which solidified blocks of plastics waste can be removed after the treatment process.

The side panel (2) may also be provided with door panels in its upper region through which a loading hopper and heating zone may be accessed.

In order to remove any odours, fumes or other gases which may be produced from the treatment process, an extraction fan (7) is provided mounted to a plenum box in the rear panel for connection to a suitable flue (not shown). As shown in Figure 3, the plenum box may be provided with a removable particle filter (11).

In Figure 2 the side wall of the chamber is omitted to show the loading hopper, in the form of a stainless steel rotatable cylindrical drum (8) mounted within a stainless steel rectangular chamber having a plenum box (48) at the rear. The rotatable cylindrical drum (8) has an opening (9) in its cylindrical surface through which the contaminated plastic waste is loaded when opening (9) is in alignment with the front upper panel (3) ie when the drum is rotated 240 degrees anticlockwise relative to the orientation shown in Figure 2a. As is best shown in Figure 2b, two front panels (3) are hinged to respective side panels (2) and are kept shut for a predetermined period by timed locks (47) controlled by the above-mentioned PLC. In this manner access to the drum (8) is prevented until the treatment has been completed.

In use, the drum (8) is loaded with bags of biohazardous contaminated plastic waste through the open front panels (3) and is then rotated to the position shown in Figure 2a and the front panels (3) are closed. The contaminated plastics waste is thereby unloaded into a heating zone (10) through the opening (9) when the drum (8) has been turned, either manually or by electrical means, so that the opening (9) is in alignment with the top of the heating zone (10). The plenum box (48) is provided with a removable particle filter (11), electrically heated baffles (49) and air deflectors (50) and connected to an exhaust impeller (7) which exhausts gases from the drum via the particle filter (11). In this manner a negative pressure is provided in the drum and the melt chamber in order lo minimise the risk to the operator work area. During this stage of the treatment all the internal and external surfaces of the drum are maintained at a temperature of self-disinfection by dry heat by the hot air rising from the melt chamber. The small number of microorganisms that may survive during the early stages of the melt process are further treated in the plenum box (48) as the flue gases are drawn through the baffles (49) by the impeller (7). Any fumes or odours are removed and the flue gases are raised to a sterilization temperature. The exhaust outlet of the impeller is connected to a suitable flue (not shown) ending 3 metres above the roof apex and having a vertical discharge cowl to accelerate the exhausting gases to a speed of 15.2 m/s (3,000 ft/min). Figures 3a and 3b show a variant of the embodiment of Figures 2a and 2b wherein an alternative plenum box (48) is shown with a particle filter (11) and an exhaust impeller (7) exhausting into an intermediate stainless steel electrically heated conduit (51). The conduit (51) raises the temperature of the gases lo a sterilization temperature. An insulating layer (not shown) is disposed between the heating element circuit (not shown) and an outer sheath (52) which surrounds the conduit. A four-vane mixing element (53) is located within the bore of the conduit (51) and is arranged to impart a swirling action to the gases in the conduit, which then exhaust into a suitable flue (not shown). The panels (3) are controlled by timed locks (47) in a similar manner to the similar panels shown in Figures 2a and 2b but are hinged to the top panel (6) and front panel (4) respectively in this embodiment.

The apparatus of Figure 3 is used in a similar manner to the apparatus of Figure 2 and in particular the front panels (3) are shut before discharging the contents of the drum into the melt chamber by rotating the drum and suction is applied to the drum by the impeller in order to ensure a negative pressure in the apparatus and to ssfeguard the operator work area. During the melting of the plastic waste the internal and external surfaces of the drum as well as the internal surface of the panels (3) are maintained at a temperature sufficient lo ensure disinfection by dry heat.

In the embodiment of Figures 4 to 6, the heating zone comprises a melt chamber (10) in the form of a single open volume with a series of sub-zones (12, 13, 14, 15) of decreasing cross-section in the down-stream direction. The upper sub-zone (12), of largest cross-seclion, is a frusto-conical portion. This merges into the next sub-zone (13) which is a cylindrical portion of uniform cross-section. The next sub-zone (14) is also a frusto-conical portion which merges into the sub-zone (15) of smallest cross-section which again is cylindrical and of uniform cross-section.

An integral wall-like projection (16) extends upwardly through the hollow interior of sub-zones (14, 15) of the melt chamber (10). The projection (16) constitutes a central web which extends from a position in the lowest sub-zone (15) to split the sub-zones (14, 15) into two D-shaped sections along its length, extending upwardly through the centre of the melt chamber (10) and across its diameter from one wall to the other.

The melt chamber (10) is manufactured from heavy duty cast aluminium to optimize heat transfer by conductivity and radiation and is provided with integrally cast-in electrical resistance heating elements (17, 18). Moreover, all working surfaces of the melt chamber (10) and the web (16) are provided with high-temperature polymeric coatings of PTFE having thicknesses in the range of 50 - 75 microns.

The electrical heating elements (17, 18) comprises spirally or helically wound turns which are also relatively closely spaced together and whose ends are connected to the electrical control circuit. The heating elements (17) are located closer to the inner surfaces of the cast wall portions of the melt chamber (10) than to the outer surfaces of those wall portions, in order lo maximize heat transfer into the interior of the melt chamber (10).

As can be seen from Figure 7, the heating elements (17) in the walls of the melt chamber (10) comprise a plurality of loops (17a), each having electrical connections (17b) at their open ends for connection to an electrical power source. The loops (17a) extend like fingers around the chamber (10) to terminate with the closed ends (17c) of the alternate loops adjacent each olher to maximize heat transfer further. The loops (17a) extend circumferentially around the melt chamber (10) in parallel with one another.

The web (16) is also provided with an integrally cast-in electrical resistance tubular heating element (18). This is arranged in the form of two loops (18a) extending in the plane of the web, which loops change direction (18c) and continue around the diameter of the lower sub-zones of the melt chamber (10). The ends (18b) of the looped heating element are also connected to the electrical control circuit. By means of the heated web, heat is brought lo the middle or central region of the lower sub-zones (14,15) of the melt chamber (10).

Referring to Figure 8, each electrical resistance heating element (17, 18) is a tubular sheathed element and comprises a heat conductive metal outer tube (19) and an inner axially extending spiral resistance wire (20), eg. of nichrome, with an insulating packing material (21) such as magnesium oxide powder between the wire and the tube. The insulating powder is compressed by reducing the diameter of the tube after assembly

In order to sense the temperature of the melted plastics within the melt chamber (10), the chamber walls are provided with thermocouple-type melt probe access points (not shown) which are spaced from each other, through which temperature sensing probes (not shown) can project into the chamber (10). Preferably such probes would be located in each sub-zone of the melt chamber (10) and connected to the temperature indicators of the electrical control circuit; because of their spacing the probes can attain useful temperature differentials to facilitate efficient operation.

An insulation jacket (22) surrounds most of the melt chamber as is shown by the chain lines in Figures 4 and 5.

At the base of the melt chamber a dump gate (23) is provided to facilitate exit of the molten plastics waste from the melt chamber (10). The dump gate (23) is driven by a simple linear actuator mechanism (24), and is controlled by the PLC so that it opens automatically after a preset time which is dependent on the cycle selected by the operator. The dump gale (23) is also controlled by the PLC so that the time taken for it lo travel from an open position to a closed position, or vice versa, is measured, and if this time exceeds a preset value the dump gate (23) reopens and a fault indicating light is activated. The dump gate (23) itself is manufactured from tool steel of about 10mm thickness and is machined to have a sharp blade-like edge capable of slicing through most contaminant inclusions in the molten plastics waste.

As can be seen from Figures 4 to 6, the dump gate (23) is guided by eccentric cam followers (25) on a slide rail (26). Spacers (27) are provided between the melt chamber (10) and the slide rail (26) and insulating blocks (28) are provided between the slide rail (28) and the cabinet housing (1). Spacers (27), slide rails (26), and insulating blocks (28) are all provided with holes for bolts and are all fixed together to the base of the melt chamber by bolts (not shown). Melt chamber (10) is provided with jacking screws (not shown) to set the correct clearance for the dump gate (23) to slide between the chamber (10) and the slide rails (26).

A drawer (29) containing a dump tray (30) is provided beneath the dump gate (23) into which the mollen plastics waste and any contaminant inclusions fall when the dump gate (23) opens. The drawer (29) is accessed by pulling down the front lower panel (5) and acts as an overspill tray should the capacity of the dump tray (30) be insufficient for the molten plastics load or should no dump tray (30) be present. Figure 14 is a perspective view of the dump tray (30) containing molten plastics waste after dumping from the melt chamber (10).

A proximity device (not shown) which is mounted between the dump gate (23) and the drawer (29) communicates with the PLC which controls the dump gate (23) so that the dump gate (23) will not operate and so remain closed unless the drawer (29) is in position and the dump tray (30) is empty.

A catchment tray (31) is provided as an additional safety precaution in case of leakage of the molten plastics or other liquids when the drawer (29) has been removed.

The drawer (29) and both the dump and catchment trays (30, 31) are manufactured from stainless steel or from zinc-coated carbon steel when the apparatus is used in commercial or catering applications.

The apparatus is provided with an odour controller (32) comprising an electric motor driven compressor which siphons liquid odour-neutralising agents from a reservoir and pipes it to a syphon air atomising nozzle strategically placed in the hopper to inject a mist of the odour-neutralising agent into the hopper so as to mask objectionable smells.

Figures 9 and 10 show further possible constructions of the loading hopper (8).

In the embodiment of Figure 9, the hopper (8) comprises a rectangular chamber with a generally square-shaped opening (54) located directly above a circular-shaped opening (33) in the heating zone (10). The contaminated plastics waste is loaded into the hopper (8) through the upper front panel (3).

Figure 10 represents a variant of the embodiment of Figure 9, where additional access lo the hopper (8) and heating zone (10) is provided through an opening (34) in a side wall of the loading hopper (8). This opening (13) is provided with door panels (35, 36) which can be accessed through corresponding door panels in the side panel of the cabinet housing. The hopper (8) is provided with an ultrasonic sensor (55) to detect when maximum load capacity of the apparatus has been reached.

Like the embodiment of Figure 9, this embodiment is provided with a plenum box (48) which communicales with the hopper (8) via a removable particle filter (11) and an impeller (7) which exhausts the hopper when the front panel (3) is shut. The hot air rising from the melt zone during this process is used to heat the interior of the hopper and the panel (3) lo a sterilization temperature whilst a negative pressure is maintained by the impeller (7) lo prevent escape of microorganisms or harmful gases past the panel (3) into the operator work area.

This hopper is particularly suitable for automatic loading through loading port 56, eg from a bulk storage silo, or an air or belt conveyor.

Figures 11 lo 13 show further possible configurations of the web (16) (of the embodiment of Figures 4 to 6) within the melt chamber.

With reference to Figures 11a and lib, a single web (37) is located in the lower two zones (14, 15) of the melt chamber (10) in a position raised above the outlet (38) of the melt chamber.

The configuration of Figures 12a and 12b differs from that of Figures 11a and lib in that, two webs (39, 40) are mounted perpendicularly with respect to each other in a position raised above the outlet (38) of the melt chamber (10). This arrangement improves heat transfer into the centre of the melt. In Figures 13a and 13b, two webs (41, 42) are mounted at perpendicularly with respect to each other with one below the other so that the lower web (41) is located directly on Ihe outlet (38) and divides the outlet (38) into two sections. This arrangement improves the rate of evaporation of any liquids that have accumulated in this region.

Figures 15 to 17 show alternative embodiments of the dump tray (30).

In Figure 15, the base of the dump tray (30) is provided with a relief pattern (43), such as a company logo or other identification symbol, which is imprinted into the plastics block on solidification.

In Figures 16 and 17, the dump tray (30) is provided with dividers (44) to provide smaller individual blocks on solidification.

In Figure 18, the drawer (29) is lined with preformed cardboard packing (45) and is useful for plastics waste loads which may contain 'sharps' allowing the solidified block to be packaged on cooling.

Figure 19 shows a packaged solidified block (46) after removal from the drawer (29) of Figure 18. The sides of the cardboard packing (45) have been folded down to comple le the protection against sharps.

The operation of an apparatus constructed in accordance with the present invention will now be described with reference to appropriate ones of the aforementioned drawings.

The apparatus is switched on and the melt chamber (10) is preheated for approximately 45 minutes to a minimum temperaiure of 180 °C, a load indicating lamp in the control panel shows that the apparatus is ready for loading. On reaching this minimum temperature setting, the extraction fan (7) is automatically activated. The contaminated plastics waste is loaded into the hopper (8), either manually, as loose items, or pre-contained within a plastics refuse bag or a plastics container. A particular processing cycle is selected depending on the nature of the load to be treated and a "START CYCLE" button is activated. The load indicating lamp now switches off and the loading panel (3) is locked automatically.

The contaminated plastics waste passes from the hopper (8) into the upper sub-zones of the melt chamber (10) where the heat causes the plastic bag or container to melt and its load to fall out. The plastic items begin to lose their shape and gradually become molten, passing through the successively converging zones of the melt chamber (10). Any liquids present in the plastics waste evaporate off or, in the case of loads with a high liquid content, most of the liquid will initially fall to the base of the melt chamber (10) where, as a result of the higher temperature in this region due to the zonal heating system and its smaller cross-sectional area, evaporation occurs rapidly. As a result, the atmosphere within the melt chamber (10) becomes moisture-free and the conditions required for disinfection by dry heat of both the plastics waste and the interior of the apparatus are achieved.

The plastics waste with its contaminant inclusions remain inside the melt chamber (10) and continues to melt and densify until the cycle time of the processing cycle selected by the operator has elapsed. The dump gate (23) then opens for a fixed time period. The molten plastics and its contaminant inclusions pass through the outlet in the base of the melt chamber (10) into the dump tray (30) beneath. After the fixed time period has elapsed, the dump gate (23) begins to close and, by means of its blade-like edge, exerts a cutting force on the molten plastics waste and contaminant inclusions therein, so as to slice through contaminants in its path.

In the case of the dump gale (23) becoming blocked due lo the presence of contaminant obstructions in its travel path and the inability of the blade lo cut through certain contaminants such as hard metals etc, the dump gate (23) reopens and a fault indicator is activated. In most cases, the obstructional contaminant will fall through the outlet into the dump tray (30) on reopening of the dump gate (23) but, should this not occur, the operator can take the necessary steps to remove the obstruction.

When the dump gate (23) is fully closed, the load indicating lamp will switch on again to indicate that the apparatus is ready to accept another load. A further indicating lamp on the control panel is also activated to show that the dump tray (30) requires emptying. If the dump tray (30) is not emptied before completion of the cycle time of the next load, the dump gate (23) will fail lo open and the lamp on the control panel indicating that the tray requires emptying flashes so that the operator is aware of the situation and removes the loaded dump tray.

In the embodiments of the hoppers shown in Figures 2, 3 and 9 which are normally loaded manually, any trapped fumes present within the hopper (8) that may be harmful to the operator are evacuated on completion of a processing cycle, before the hopper can be accessed through the loading panel (3). The loading panel (3) is then unlocked and the apparatus is ready lo accept another load.

If Ihe molten plastics material is of a sufficiently flowable consistency it will spread, taking the shape of the dump tray (30) or, if a divider (44) is used, the shape of the divider sections. The molten plastics and contaminant inclusions are allowed to cool in Ihe dump tray (30) until Ihey solidify. This may take place inside the apparatus, or the dump tray (30) containing the molten material may be removed from the apparatus and allowed to cool elsewhere. The plastics waste will shrink slightly on cooling, facilitating easier removal from the dump tray (30), and the resultant solidified blocks or briquettes of densified plastics waste can be packaged or simply removed in their solidified form. If desired, the solidifying process can be accelerated by removing the dump tray and its molten load from the apparatus and placing in a tank of cold water.

If the molten plastics material is of a more viscous nature such as in the case of medical waste containing agar, it may not take the shape of the dump tray (30) but will cool and solidify in an amorphous mass within the dump tray (30).

The apparatus is preferably provided with a shut-down cycle which when actuated, allows the apparatus to shut-down automatically either at the start of or during a cycle. This facility allows the apparatus to continue its cycle while unattended eg. at the end of a working shift. The apparatus continues normal operation until completion of the cycle when it automatically switches off the power to the heating elements (17, 18). The extraction fan (7) will also switch off automatically when the temperature inside the apparatus has reached its minimum value.

While particular embodiments of ihe present invention have been described with reference to the accompanying drawings, it should be appreciated that various modifications may be made without departing from the scope of the invention. For example, although the invention is described primarily in relation to the treatment of contaminated plastics waste, it should be understood that the invention is equally applicable to the treatment of uncontaminated plastics waste.

The invention also extends to any novel combination or sub-combination disclosed herein.

Claims

1. An apparatus for the treatment of plastics waste, said apparatus comprising:

a) hopper means (8) arranged to receive plastics waste;

b) heating chamber means (10) arranged to heat plastics waste from the hopper means to a flowable condition, said heating heating chamber means being provided with a normally closed outlet for heated flowable plastics, and

c) exhaust means (7,48,51) arranged to exhaust fumes or gases from the apparatus,

characterised in that said exhaust means (7,48,51) is arranged to draw hot gas over the surface of said hopper means (8).

2. An apparatus according lo claim 1 wherein said exhaust means (7,48,51) communicates with the interior of said hopper means and is arranged to draw hot gas from said heating chamber means over the interior surface of said hopper means (8) and to depressurise the interior of the hopper means.

3. An apparatus according to claim 1 or claim 2 wherein said hopper means (8) is enclosed within an enclosure having an openable cover means (3, 56) for providing access to the hopper means.

4. An apparatus according to claim 3 wherein said cover means (3, 56) is controlled by electrically controlled locking means (47) which are arranged to prevent opening of the cover means during a heating cycle.

5. An apparatus according to any preceding claim wherein said hopper means is movable between a first position in which it can receive plastics waste and a second position in which it discharges plastics waste to said heating chamber means (10).

6. An apparatus according to any preceding claim wherein said exhaust means (7) is provided with heating means (49,50; 51,53)) downstream of the hopper means (8) for destroying microorganisms exhausted from the apparatus.

7. An apparatus according to claim 6 wherein said heating means comprises a heated labrynth (49,50).

8. An apparatus according to claim 6 wherein said heating means (51,53) is arranged to apply a swirling motion to the exhaust gases.

9. An apparatus according to any preceding claim wherein the melt chamber means (10) is a zonal melt chamber with a plurality of zones at different temperatures which are controlled so that the temperature increases in the direction of movement of said plastics waste to said outlet.

10. An apparatus according to any preceding claim wherein said outlet is provided with controlled gating means (23).

11. An apparatus according to any preceding claim wherein said heating chamber means (10) is provided with controllable outlet means which is arranged to release flowable plastics material therefrom after a predetermined heating cycle.

12. An apparatus according lo any preceding claim wherein said heating chamber means (10) is provided with web means (16) in its interior which divides the flow of heated plastics material.

13. An apparatus according to any preceding claim wherein a cutting means (23) is arranged to cut contaminated inclusions in the heated flowable plastics waste.

14. An aparatus according to claim 13 wherein said cutting means (23) is located adjacent said outlet.

15. An apparatus according to any preceding claim wherein in use, said hot gas is at a temperature of 115 °C to 175 °C.

16. An apparatus according to any preceding claim wherein in use, said plastics waste plastics waste is heated to a temperature of at least 180 °C for a time of at least 45 minutes.

17. A method of treating plastics waste comprising:

a) feeding plastics waste into an entry zone (8),

b) passing the plastics waste into a heating zone (10) and heating the plastics in said zone to a flowable condition, and

c) exhausting gas from said heating zone

characterised in that hot gas is exhausted over said entry zone (8).

18. A method according to claim 17 wherein said plastics waste is contaminated with biohazardous contaminants and said hot gas is at a temperature sufficient to sterilise the entry zone (8).

19. A method according to claim 18 wherein said hot gas heats said entry zone to a temperature of 115 ° C to 175 ° .

20. A method according to claim 18 wherein said hot gas heats said entry zone to a temperature of 120 °C lo 165 °C.

21. A method according to any of claims 17 to 20 further comprising the step of detecting obstructional contaminant inclusions in the heated flowable plastics waste.

22. A method according to any of claims 17 to 21 further comprising the step of cutting through solid inclusions in said plastics waste.

23 A method according to any of claims 17 to 22 comprising the step of blocking access to said entry zone while the plastics waste is being heated to a flowable condition.

24. A method according to any of claims 17 to 23 wherein said plastics waste is maintained in said healing zone for a period and at a temperature sufficient to disinfect the plastics waste by dry heat.

25. A method according to claim 24 wherein a heating cycle for the plastics waste in said heating zone is selected in dependence upon the nature of the plasics waste.

26. An apparatus according to any preceding claim wherein in use, said plastics waste plastics waste is heated to a temperature of at least 180 °C for a time of at least 45 minutes.

27. A method according to any of claims 17 to 26 when performed using apparatus as claimed in any of claims 1 to 16.