WO1999037590A1

WO1999037590A1 - Improvements in and relating to the handling, treatment and storage of waste

Info

Publication number: WO1999037590A1
Application number: PCT/GB1999/000059
Authority: WO
Inventors: John Anthony Schofield
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1998-01-26
Filing date: 1999-01-22
Publication date: 1999-07-29
Anticipated expiration: 2000-07-26
Also published as: AU2172199A

Abstract

The invention provides a method of treatment of materials (2), particularly suited for treating filters, in which the material passes through a feeding stage to a melting stage, the melting stage forming a volume of molten material (58); the feeding stage extending from a location distant from the melt (58) to a location proximal to the melt, and wherein the percentage of solids per unit volume of the feed material is higher at the location proximal to the melt than at the location distal therefrom. It is particularly preferred that mechanical means (102) are used to increase the percentage solids level of the material as it approaches the melt (58). In this way, off-gassing is filtered by the filter material itself and carried back into the melt.

Description

- 1 -

IMPROVEMENTS IN AND RELATING TO THE HANDLING TREATMENT AND STORAGE OF WASTE

This invention concerns improvements in and relating to the handling, treatment and storage of waste, particularly but not exclusively to the handling, treatment and storage of filters, such as HEPA filters.

A wide variety of industrial processes involve and require, at some stage, the filtration of air or other gases to remove suspended solids. Over time the filters inevitably become loaded with the solids and as a consequence require replacement to maintain their efficiency. Removal of suspended solids from gas streams is a requirement of many process stages in the nuclear fuel cycle, for instance. A frequently used filter in such systems is known as the HEPA filter. HEPA filters are generally formed of a mesh of micron diameter fibres, of materials such as borosilicate glass, asbestos or ceramics, mounted in a cardboard, wood or metal support frame. Such filters come in a wide variety of shapes and sizes depending upon their use.

A particular problem in any industry where the materials retained in the filters is toxic or hazardous in any way is that the filters require treatment at the end of their life. At present the expense and difficulty of treating the filters means that they are frequently stored pending a solution. The cost of such storage is significant and this cost is increased still further, in the case of certain radioactive HEPA filters where it is necessary to cement encapsulate the filter as intermediate level waste to provide immobilisation of the waste.

The present invention aims to provide treatment methods, apparatus and techniques which provide an efficient and cost effective method of dealing with waste such as HEPA filters . -2 -

The particular advantages stemming from the present invention are : -

1) a substantial reduction in the volume occupied by the waste following treatment compared with that occupied before treatment;

2) the immobilisation of both non-volatile and volatile contaminants on the filter in the vitrified product of the process;

3) a significant reduction in the level of offgases when compared with prior art treatments;

4) avoidance of secondary waste production as the containment for the waste and the offgas filter form part of the vitrified product;

5) the temperatures involved in the process ensure the destruction of any biological organisms in the feed;

6) the retention and incorporation of particulate material from the feeding process in the product.

According to a first aspect of the invention we provide a method of treating filters, the method including feeding the filters to a melting stage and wherein the filters act as filters for particulate and/or volatile material escaping from the melting stage.

The material may be fed to the melting stage from above .

The filters may provide a filtering action as they pass through a feeding stage.

According to a second aspect of the invention we provide a method of treatment materials comprising the steps of :- feeding the material to be treated through a feeding stage to a melting stage, the melting stage forming a volume of molten material; the feeding stage extending from a location distant from the melt to a location proximal to the melt, the feeding stage containing feed material and wherein the percentage solids per unit volume of the feed material is -3- higher at the location proximal to the melt than at the location distal therefrom.

By providing the feed of the material to the melting stage in these ways, condensation off gasing and particulate material retention is promoted. By reducing the voidage in the feed material as it approaches the melter the flow path of material passing through it generally away from the melter is significantly increased.

Preferably the feed materials comprise glass, ceramics, asbestos or vitreous materials. Borosilicate glass in particular is a preferred component of the feed material. The feed material may include other components such as wood, paper or metal, for instance aluminium. It is, however, preferred that the feed consists completely or substantially completely of non-combustible materials which are easily fragmented, such as glass.

The feed is preferably of, or comprises, compressible materials .

The feed materials may comprised filters or filter assemblies. The supporting framework and / or spacers for filters are preferably removed prior to feeding. Preferably the feed material comprises HEPA filters, most preferably of any HEPA filters from which the supporting framework and spacers have been removed.

The treatment may comprise a feeding and / or compression and / or melting and / or re-solidification stage for the feed materials. Preferably the method includes a fragmentation stage, and/or a crushing stage, as stages in the feeding stage. The method may include a resolidification stage. The resolidification stage may be provided at the same location as the melting stage. For instance the melting location may subsequently serve as the resolidification stage. A gas tight system is thus provided for the melting and resolidification stages. The change from melting location to resolidification location may occur after a -4- reduction in, or removal of, the heating. One or more of each of such stages may be provided.

The feeding stage may include a fragmentation stage and/or one or more crushing stages. In one embodiment the feeding stage may comprise a crushing stage followed by a fragmentation stage followed by a compression stage.

The crushing stage may comprise two or more opposing rollers, preferably counter rotating, to crush the feed material. Crushing may reduce the volume and/or porosity of the feed material by compaction.

The fragmentation stage may provide a volume and/or porosity reduction in the feed material. The fragmentation stage may consist of or comprise a shredding stage. Two or more opposing shredders may be used to shred the feed materials. The shredders may comprise a series of interlinking blades which are rotated at high speed.

Preferably the material is force fed through one or more of the stages in the feeding stage, for instance the compression stage. Preferably the force feeding compresses the feed material. The compression may arise from deformation of the feed material and/or shredding and/or crushing and/or breakage of the feed material.

The feeding stage, or stages forming the feeding stage, preferably advance feed material therethrough on a substantially linear path. The feed material may be advanced by: the direction of movement of the crushing stage and/or fragmentation stage; and/or by gravity; and/or by rotating feed means; an/or by reciprocating feed means. The feed material may be advanced by rotating feed means, such as a screw feeder, provided in the feeding stage, preferably in the compression stage.

The compression stage may pass feed material from a first location with a first cross sectional area through a second location with a reduced cross sectional area. The transition from first to second cross sectional area may -5- occur in only a portion of the compression stage or occur throughout. The transition may be linear or non-linear.

The feeding stage may provide a body of feed material above the melter stage. The body of feed material may extend upward for at least 50cm, more preferably at least 100cm and ideally more than 150cm. Preferably the body of feed material occupies the path taken by offgases from the melt. The body of feed material may define an average through path which is at least twice the linear extent of the body of feed material. A path length of three times, more preferably five times and ideally 10 times the linear length may be provided. The body of feed material may be provided adjacent and/or contacting the melt stage or spaced therefrom.

The method may include the application of cooling to the feeding stage or one or more of the stages therein.

Preferably gas is drawn from a location in the feeding stage out of the feeding stage and re-cycled to the apparatus in a. preceding part of the feeding stage. Preferably gas is drawn through at least the initial portion of feed material being conveyed towards the melter stage.

Preferably the gas withdrawn from the feed stage is returned to behind the crushing and/or shredding stages relative to the feed stage. In this way a flow of air through the system past the crusher and shredder is provided so giving particulate material suppression.

Preferably the air flow through the one or more stages is between 0.25m³ and 5m³ per minute, and more preferably between 0.5m³ and 2m³ per minute. The air flow may be generated by a centrifugal fan.

Preferably the feeding stage, potentially including its component stages, and the melter stage are provided in a continuous, sealed series of stages. Feed- aterials may be introduced to the stages via a sealable loading area. Preferably a gas lock is provided in the loading area, material being introduced through the outer open seal, the - 6- outer seal being closed before the inner seal is open to allow passage into the apparatus.

The feed material may pass through one or more pre- treatment stages prior to reaching the feeding stage.

The feed material may be pre-treated to remove non- glass, vitreous, ceramic or asbestos components. For filters, support frames and/or spacers may be removed.

Preferably the melting stage converts the feed material into molten material. Energy input to the melting stage may be achieved by one or more of microwave, radio frequency induction or plasma heating. Other forms of energy input may be used.

A preferred form of heating is provided using a microwave generator, linked to the melting stage via a waveguide. Preferably the molten material is retained within a crucible of re-solidified material, most preferably a glass or vitreous material. Preferably the layer of resolidified material is separated from the melter stage perimeter by a layer of unmelted material, for instance glass frit.

The melting stage may be physically distinction from a subsequent stage or stages. For instance the melting stage may be physically distinct from a subsequent resolidification stage. Material may be withdrawn from the melting stage to the subsequent stage or stages.

The melting stage may be continuously or periodically tapped to remove molten material. Tapping may be performed by providing an induction coil and / or plasma heating device in proximity to a lower portion of the molten material. Resolidification of molten material may be permitted in this location to seal the chamber and retain molten material.

An alternative form of heating may be provided by an induction coil, most preferably provided around the periphery of the melter stage. Cooling may be provided for the heating stage, for instance by water cooling. - 7 -

Preferably flow of material through the melting stage is promoted. Flow may be promoted by gravity and/or flow promoting means. The flow promoting means may comprise a helical screw feeder.

The molten material is preferably received in containers and allowed to re-solidify. Steel drums are particularly preferred for this purpose. The drums may be cooled to promote re-solidification.

The melting stage may be physically correspond, at least in part, to a subsequent stage or subsequent stages. For instance the melting stage may physically correspond to a resolidification stage, for the waste material.

The waste material may be melted in the melting stage and allowed to cool and resolidifiy. The material may be withdrawn by detaching the melting location from the process line. Detachment may occur before resolidification, with resolidification occurring at a location removed from the process line. More preferably resolidification, at least in part, occurs before the melting location is detached. In this way off-gasing is minimised.

The melting stage may comprise a container in which the waste is allowed to resolidify, preferably the container intended for long term storage. Steel drums, most preferably stainless steel drums may be employed for the combined melting/resolidification stage. The drums may be actively cooled during the resolidification.

Preferably the heat source in a combined melting stage and resolidification stage location is microwave heating. Preferably the container for the stages is microwave tuned and/or tuneable.

Preferably a vitrified form arises on resolidification.

The waste, from a physically separate melting stage and resolidification stage process and/or from a physically combined melting stage and resolidification stage process, may be received in a lined container. The container may be lined with clean material, for instance at its bottom and side wall(s) . The lining may be of glass, fibreglass or the like .

After the waste has been introduced, the container may be provided with a top portion of clean material. The top portion of material may be provided by introducing a lining, for instance of glass or fibreglass, or by passing a volume of clean material into the melting stage, distinct from the waste material. The clean material may be kept distinct by providing it from a separate batch of molten material or by adding the clean material to the feed to the melting stage after the waste material feed.

A container with the waste physically isolated from its walls by a volume of lining material is a preferred form for the resolidified product.

Preferably the volume occupied by the product of the treatment, compared with the feed to the treatment is reduced by a factor of at least 20, more preferably a factor of at least 50 and ideally a factor of at least 100.

The method may include the introduction of an inert gas to the system to prevent a fire within the system.

Preferably the percentage solids per unit volume of material on entering the feeding stage inlet is between 0.1 and 5%, more preferably between 0.5 and 1.5%.

Preferably the percentage solids per unit volume of material on leaving the compression stage and/or on entering the fragmentation stage is between 0.5 and 15%.

Preferably the percentage solids per unit volume of material on leaving the fragmentation stage and/or on entering the further compression or material advancement stage is between 0.1 and 5%.

Preferably the percentage solids per unit volume of material adjacent the melter stage is between 20% and 50% and - 9- the percentage liquid per unit volume in the melter stage is between 95 and 100%, more preferably between 99 and 100%.

Preferably the percentage solids per unit volume is increased from at most 2% at the feed stage inlet to at least 95% at the melter stage. A increase from at most 1% to at least 98% is preferred.

Preferably the voidage in the melter stage is at most 2%, more preferably 1% and ideally most 0.5% of that in the inlet material to the feed stage.

Preferably the compression stage is between 0.25m and 5m in length, more preferably between 0.35m and 1.5m.

Preferably the compression stage is between 0.05m and 0.75m in width and/or diameter and more preferably between 0.075m and 0.25m.

Preferably the feed material speed through the feed stage is, on average, no more than 5cm per second, more preferably no more than 2cm per second and ideally less than lcm per second.

Preferably the temperature at the melter stage is at least 800°C, more preferably 1000°C and ideally at least 1200°C. Preferably the temperature at the inlet to the feed stage is no more than 200°C, preferably no more than 100°C and ideally no more than 50°C.

According to a third aspect of the invention we provide a melter, provided with an inlet and an outlet, the inlet being connected to a feeding stage, the feeding stage being configured to compact feed material passing therethrough in use, prior to entry to the melting apparatus.

Force feeding means may be provided to encourage flow of the feed material through at least a part of the feeding stage. Reciprocating or rotating force feeding means may be provided. The material may be force fed by a ram, but is more preferably force fed by a screw feeder, for instance a helical screw feeder. A screw feeder of between 5 and 15 turns may be provided. The screw feeder may have a diameter - 10- of between 0.05 and 0.5m, and more preferably 0.075 and 0.2m. The screw feeder may have a length of between 0.25m and 1.5m and more preferably of between 0.4 and 0.75m. Preferably the force feeding means are provided in the feeding stage an elongate passage. Preferably the inlet to the passage is configured as a funnel. In this way collation of the feed material in proximity to the feeding means is assured. Preferably the feeding means extend from the inlet to the elongate portion into proximity with the melter stage.

Preferably at least a portion of the elongate passage is provided with a taper. Preferably the taper reduces the cross sectional area of the elongate passage as it approaches the melter stage. The reduction in cross sectional area may be provided by tapering of one or more walls of the elongate passage. Preferably a funnel type taper is provided for a cylindrical passage. The elongate passage may be provided with a restricted neck portion of lower cross section than a portion of the elongate passage further from the melter stage .

The feeding means may extend into the reduced cross sectional area portion. The feeding means may be tapered or otherwise adjusted in configuration to maintain a spacing between the feeding means and the walls of the elongate passage .

Where the feeding means is a screw feeder a portion of the screw feeder may be provided with a reduced pitch compared with another portion of the screw feeder. Preferably the reduced pitch portion is provided nearer the melter stage than the other pitch portion. The portion with reducing pitch may also be tapered in a radial direction.

The feeding stage may be provided with cooling means, more preferably extending around its periphery. Water cooling means, for instance, water conveying passages, may be provided. - 11 -

The feed stage may be provided with a gas outlet at least part way along the elongate passage towards the melter stage. Preferably gas is drawn through the outlet. Preferably gas is drawn to the outlet through the feed material in the feeding stage in a direction towards the melter stage. Preferably the gas is recycled to the system in a close manner, above the feed material.

Preferably crushing means are provided before the feeding means. Preferably the crushing means comprise opposing rollers.

Preferably a shredding stage is provided before the feeding means, most preferably after a crushing stage. The shredding means may comprise two or more sets of blades. Preferably the blades are interlinking. Preferably the blades are rotated. Preferably the blades are rotated in opposition to one another.

Preferably the gas inlet in the feeding stage is connected to a gas outlet above the shredding and / or crushing means. Preferably the crushing and / or shredding means are provided in a closed circuit vessel.

The melter may be provided with microwave, radio frequency, induction and / or plasma heating means. Other forms of heating means may be provided for the melter.

The melter may be provided with tuning means where microwave heating means are used.

The melter may be provided with induction and / or plasma heating means to tap the melt.

The melter may be an elongate vessel, for instance a cylinder and particularly a right cylinder. One or more induction coils may be provided around the melter.

Preferably the melter is provided with through flow promoting means . The flow promoting means .may be a screw feeder, for instance a helical screw feeder. Preferably the screw feeder extends throughout, at least, a substantial portion of the melter. Preferably the through flow promoting - 12- eans provide churning or other forms of agitation to the molten material in the melter.

The melter may be provided with cooling means, for instance passages carrying a cooling flui.d provided around the periphery of the melter.

Preferably the feeding stage extends away from the melting stage in an at least partially vertical manner. A substantially vertically provided feeding stage is preferred. In this way gravity assistance for the feeding process and compaction can be achieved. Preferably the apparatus is constructed such that material flowing through a pre- treatment stage falls to the material feeding stage. Collection of the material at the material feeding stage may be facilitated by inclined side walls, tapering towards the feeding stage inlet. Most preferably the crushing stage is provided substantially vertically above the shredding stage with the shredding stage being provided substantially vertically above the feeding stage.

The features and options described elsewhere for a physically combined melting stage and resolidification stage apply equally to this aspect.

According to a fourth aspect of the invention we provide a method of treating material in which material is fed from a feeding stage to a melting stage, at least a portion of the feeding stage being provided with a gas flow therethrough, the gas flowing towards the melting stage.

In this way, the drawing of gas down through the feed material promotes retention of particulate matter therein and also provides for cooling of the feed material by the passage of cold gas thereover.

This aspect of the invention may include features, options and possibilities set out elsewhere, in this document.

According to a fifth aspect of the invention we provide apparatus for treating material, the apparatus comprising melting means provided with an inlet and an outlet, the inlet - 13- connecting the melting means to feed means for introducing feed material to the melting means, wherein the feeding means is provided with a further outlet below the level of feed material in the feed means in use, the outlet being connected to gas withdrawal means so as to draw gas through at least a portion of the feed in use.

This aspect of the invention may include features, options and possibilities set out elsewhere in this document.

Various aspects of the invention will now be described, by way of example only, and with reference to the accompanying Figures, in which: -

Figure 1 is a schematic illustration of apparatus according to one embodiment of the present invention;

Figure 2 illustrates an alternative embodiment of the present invention;

Figure 3 schematically illustrates a detail of apparatus from a distinct and separate embodiment of the invention to the first or second embodiments according to the invention;

Figure 4 illustrates a further embodiment of the invention;

Figure 5 illustrates the percentage solids per unit volume of material being fed to the melter, according to the invention with distance; and

Figure 6 illustrates the temperature with distance for the present invention.

The distinct and independent of one another embodiments of the invention used for the purposes of non-limiting illustration only are now described. Whilst the invention is described below in relation to the treatment of HEPA filters the technique is applicable to a wide variety of other highly porous materials, such as filter materials and indeed other - 14- structures, particular where they are formed from glass and other readily meltable materials.

The treatment of glass containing filters is particularly suited to such techniques as the entire material forming the fibre mat can be readily vitrified and indeed itself provides the vitrifying material to a great extent.

EMBODIMENT 1 PRE- REATMENT

The processing method starts by removing the borosilicate filter from its cartridge. The cartridge, which may be wood, metal or cardboard, is discarded and can be cleaned for recycling or burnt or otherwise disposed of.

The removal of the wood, metal or paper skins and / or aluminium spacers which may be present in the filter is advantageously conducted prior to feeding the filters to the melting stage as in this way off gases are reduced significantly. It is these components which contribute principally to off gas generation. In any event these materials can be more readily cleaned or addressed by conventional processing technology than the ultrafine, high porosity filter mats can be. LOADING STAGE

The separated filters (2) are loaded into a shute and introduced through a first door (3) into chamber (4) . Once loaded the door (3) is shut and door (5) below is opened allowing the filter (2) to be pushed down by a rod (6) . CRUSHER STAGE

The mats forming the filters (2) then pass to a slow crushing unit (8) which compresses the very opened structured filters (9B) into a more compact structure (9B) . A pair of opposing rotation rollers are used in this stage. FRAGMENTATION STAGE

From the crushing stage (8) the compressed material passes to a shredding stage (10) provided by a pair of - 15- counter rotating and interlinking shredders (12a & b) . The shredders break the fibre mat up into smaller components which then fall and are collected by inclined sides (14) of the vessel into a feed heap (16) . COMPEESSION STAGE

The feed heap (16) of still relatively open mat structure is drawn downwards by helical screw feeder (18) towards the melting location (20) . The rotary action of the helical feeder and movement downwards cause the feed material to be advanced, gives increasing compaction of the material.

In the upper part (22) of the feeding stage air is drawn down through the feed material (16) by fan (24) connected to an intermediate location (26) . The air is re- circled to the vessel above the crushing stage and passes downwards around the slow crusher (8) and fast shredder (10) and back into the feed heap (16) once more.

Circulation of the air flow in this way suppresses dust formation in the crushing and shredding vessel and so minimises dust scapes when it is necessary to open the loading stage to feed in more filters (2) .

From the mid point of the screw feeder (18) the material continues to pass downward and gets ever closer to the melting stage (20) . Heat rising from the melting stage (20) and off gases generated therein enter the feed stack and elevate the temperature of the material. As a consequence the material softens and gradually melts until on reaching the top of the melt (30) it is practically liquid. The mouth (32) feeding from the helical feed stage (18) into the melt chamber may be tapered (33) to reduce the space occupied by the feed material. MELTING STAGE

The melting stage may take a variety, of forms. In the embodiment of Figure 1 heating is provided by microwave heating. The microwave heater stage (20) consists of a waveguide (40) connecting a microwave source (42) via a - 16-

Quartz window to the vessel (44) . The cavity formed by the vessel (44) is provided with a tuning cavity, not shown, to enable the cavity (44) to be tuned to the microwave source

(42) and so maximise the effectiveness of the heating. Heating provided in this way is centred on the centre of the vessel (44) thereby giving rise to a pool of molten glass

(46) which is retained within a solidified glass layer (48), itself insulated from the vessel (44) by a layer of unmelted glass frit (50) .

The molten glass (46) can periodically or continuously be tapped by using an induction coil (52) to melt the bottom of the glass and so allow drainage to occur through outlet

(54) into collection vessel (56).

The helical feeder stage (18) and / or the heating chamber (44) may be provided with water cooling (60, 62) respectively as required. RESOLIDIFICATION STAGE

Once in the collection vessel (56) the material (58) can be allowed to cool. The treated material can be further processed as desired, for instance by allowing it to cool fully into a vitrified mass which includes any contamination or material present on the filters fed to this process.

EMBODIMENT 2

In the alternative embodiment of Figure 2 equivalent loading, crushing fragmentation and compression stages are provided to convey the material down to the screw feed apparatus (18). Equivalent numbers have been used to designate equivalent parts between the first and second distinct embodiments. COMPRESSION STAGE

In a similar manner to the compression stage of the first embodiment, this part of the feeding stage also compresses the material as well as advancing it. - 17-

The material entering feed material heap (16) is drawn down by rotating helical screw feeder (118). As the screw feeder (118) has a decreasing pitch to its helices as it approaches the melter stage the material held with the feeder (118) is forced to occupy a smaller volume. As a consequence of this the percentage of solids per unit volume increases. MELTER STAGE

In this embodiment, however, a different heating arrangement is used, the source of heat being provided by an induction coil (100) . The coil (100) encloses a vessel including a subsidiary helical mixer screw (102) which promotes feed of material through the heater.

Material exiting the compression stage is drawn down through the induction coil zone by the subsidiary helical mixer screw (102). The induction heating causes the glass to melt and thereby drain from the heating zone into waste vessel (56) through outlet (54) . Once again, the material can be collected, cooled if desired and otherwise treated in accordance with its constituent parts.

The provision of the screw feeder (102) through the induction heating stage is preferred as this allows for the promotion of flow through this stage, the level of heating being less than with microwave heating, for instance. It also facilitates the churning up and mixing of the glass to give even heating and fully incorporate particulate melter present. Another benefit stemming from the screw feeder (108) is that it increases the pressure applied to the melting materials and therefore encourages the retention of volatile material in the glass rather than its off gasing.

EMBODIMENT 3

In the embodiment of the invention now described, and illustrated in Figure 3. The loading stage, crushing stage and fragmentation stage may be provided as before, but the compression stage is altered to give further compaction. - 1 8 -

By providing a reduced neck portion between the end of the feeding stage and the melting stage, further reduction in the voidage in the feed material is achieved.

This reduced neck location is illustrated more clearly in Figure 3. In this schematic cross section, the heating stage (100) comprises a vessel wall (102) which contains a volume of molten material (104) . The top (106) of the molten material is in contact with greatly softened feed material. The progress of the feed material towards the melt (104) is promoted due to gravity and due to the helical feed system (108) which is rotated within the feed chamber (110).

The walls of the feed chamber (110) are provided with cooling passages (112) for the passage of water therethrough.

EMBODIMENT 4

In the embodiment illustrated in Figure 4, the initial stages (loading, crushing, fragmentation, compression) are provided as before in the Figure 1 embodiment. The significant variation in this form of the invention is the combination of the melting and resolidification stages.

The melting stage 200 consists of a stainless steel drum 202 with an inlet 204 connected to the compression stage so as to receive feed material 206. The drum 202 is provided adjacent a microwave source 208 provided with a waveguide 210 for channelling microwaves into the drum 202. The drum 202 is rendered tunable through the use of adjacent tuning stub 212. This ensures that as the impedance changes the microwaves remain tuned and give optimised heating of the central portion 214 of the drum 202.

Before introduction to the process from preparation stage 216, empty drums 218 are provided with a bottom 220 and side 222 lining of fibreglass, to give prepared drums 224.

A prepared drum 224 is attached to the compression stage and material 206 is introduced as heating is applied. The preferential nature of the heating gives melting of the - 19- central portion 214 which contains the feed material 206, but does not cause melting of the lining 220, 222. The cooler exterior also ensures that this part is more viscous, both factors giving a lower vapour pressure at the outside of the central portion and so promoting retention of off gases during the melting stage.

When sufficient feed material 206 has been introduced to the compression stage, a volume of clean glass material is introduced. This is feed to the drum 202 at the last moment, when the molten material has reached its desired level, and forms a cool clean top lining for the drum contents. This stage generally coincides with the turning off of the heat source and the beginning of cooling for the waste.

Over time the drum 202 contents cool. The cooler outside and the hotter inside result in the formation of a solid shell to the molten contents first. Again this shell reduces the amount of off gases which can escape.

After sufficient cooling has occurred the finished drum 202 is detached and joins other finished drums 230 destined for storage. A new prepared drum 224 is connected to the compression stage and the process repeated.

Operation of Invention

During heating in these embodiments and in other variations according to the invention, it is desirable to suppress as far as possible the flow of off gases from the melt stage up through the apparatus. In this way as much as possible of the off gas material is retained in the produce and escapes from the apparatus are minimised. Any hazard which may arise from the conversion of hazardous material into gaseous form can therefore be avoided.

To achieve this aim, the present invention employs the material heap (16) and subsequent stack of material in the helical feeder stage (18) as a condensing location for off gases. The temperature within this zone is kept as low as -20- possible to facilitate condensation of off gases. The feeding means is used to promote their consequential recycling into the melt.

It is also an important aspect of the compression process in the feed stage that this promotes the retention of any particulate matter which might be given off from the feed materials, for instance suspended in the up flow of off gasing and gives a long tortuous gas flow path. Again these effects are assisted by the filter material present as a pile in the path taken by off gases from the melter stage.

Feed material in the upper portion of the feed material cavity is relatively open in its structure.

As the material is compressed an increased percentage solids per unit volume arises.

As a consequence of this compression a relatively closed mat of feed material is generated and presents a tortuous through path to material encountering it.

A still further increase in the percentage of solids per unit volume occurs as the material advances further from as compaction is assisted by the softening of the material due to the increased temperatures.

By the time the material is just above the melt surface the level of solids has increased substantially due to melting and compression of the feed material due to its softening.

This variation in solids level is significant in minimising off gas escape and maximising its condensation, as well as the retention of particulate matter. As the level of solids increase the spacing between the fibres of the filter material, for instance, reduces and the ease with which particulate material can pass through is reduced as a result. In effect an increasingly fine filter is provided as the compaction progresses. These benefits result in the vast majority of particulate material which tries to escape from the melter stage being retained relatively low down. -21 -

Just as the passage of solids is restricted then so is the passage of gases up and away from the melt. This effect, caused by the resistance to flow due to the tortuous path the gas has to take results in low gas flow rates away from the melt. This in turn means that heat is only conveyed into the lower part of the feed material. As a consequence the temperature gradient up the feed material is relatively steep. Rapid cooling with distance results in volatile materials within the gas stream condensing quickly and in the lower part of the feed material, so facilitating their return to the melt.

It is significant in this regard that the present invention provides a gradual feed of the material from the feed heap where it is still of a high porosity through to the melt itself. By providing this gradual transition, the compaction and condensation effects occur. Rapid pushing of the filters down into a melt, for instance, does not achieve this effect as the material undergoes a rapid transition from very porous structure to molten without achieving any condensation or collection effect.

The variation in percentage solids of the feed as it progresses down the screw feeder, or equivalent feed stage, is illustrated schematically in Figure 4. The porosity curve (200) is illustrated from above the level of the screw feeder stage, equivalent to feed heap (16), zone K into zone L where the screw thread feeder advances the feed and compresses if slightly due to physical deformation and minor levels of breakage in the feed material. This stage slightly thereby increasing the level of solids to a limited degree.

Upon further progress and compression, stage M, the percent of solids begins to increase quite markedly. Greater levels of physical deformation and breakage occur here.

The increase in percentage of solids becomes even more marked in zone N at which stage the feed is in close -22- proximity to the melt and indeed is itself largely softened and partially melted by the elevated temperatures.

On reaching the X axis, equivalent to the melt itself, the porosity decreases to practically zero.

The decrease in up gas flow also leads in turn to a decreased rate of heat transfer up the column and so to the type of temperature profile illustrated in Figure 5.

In Figure 5 the temperature plot profile (300) is at the temperature of the melt itself at the X axis. Within the zone N the temperature decreases very rapidly with distance due to the tortuous path the gases have to flow along and the amount of material taking up heat in this zone.

The effect continues up the apparatus with ever decreasing temperatures, but with the majority of the decrease in temperature occurring in the lower part, such that a relatively long cold column of feed material exists.

Vitrification of the waste according to the invention, particularly for HEPA filters, allows the volume of the waste to be reduced by a factor of up to 100, whilst fully and permanently immobilising organic and particulate materials trapped on the filter elements during their useful life.

The HEPA filters fed to this process may for instance be HEPA filters which have been used in the decontamination of buildings, apparatus and the like. Laser ablation of such surfaces is a preferred technique for removing any contamination present on their surfaces, however, it does generate a significant level of airborne particulate waste as a result. HEPA filters offer an excellent way of retaining such particulate matter due to their micron sized fibre structure.

Claims

-23 - CLAIMS :

1. A method of treatment materials comprising the steps of :- feeding the material to be treated through a feeding stage to a melting stage, the melting stage forming a volume of molten material; the feeding stage extending from a location distal from the melt to a location proximal to the melt, the feeding stage containing feed material and wherein the percentage solids per unit volume of the feed material is higher at the location proximal to the melt than at the location distal therefrom.

2. A method according to claim 1 in which the percentage solids per unit volume of material at the proximal location is between 0.1 and 5%.

3. A method according to claim 1 or claim 2 in which the percentage solids per unit volume of material proximal at the melter stage is between 20 and 50%.

4. A method according to any of claims 1 to 3 in which the feeding stage includes a compression stage, the material being advanced therethrough by rotating feed means .

5. A method according to any preceding claim in which the feeding stage includes a fragmentation stage and crushing stage before the distal location.

6. A method according to any preceding claim in which gas is drawn from a location in the feeding stage out of the feeding stage and recycled to the process in a part thereof preceding the feeding stage. -24-

7. A method according to any preceding claim in which the feed material comprises or consists of HEPA filters.