GB2454485A - Method of disposing of non-nuclear hazardous waste materials - Google Patents

Abstract

A method of disposing of non-nuclear, inorganic hazardous waste materials, which method comprises compacting a sinterable hazardous waste material into a regular shape to form a hazardous waste core, coating the formed core with a glass composition or a glass precursor composition, and heating the coated core to form a fully dense or relatively low porosity, continuous, inert, water and acid insoluble, non-toxic glassy shell surrounding a fully dense or relatively low porosity core, wherein the glass shell forms a physical barrier to leaching and wherein the glass shell and the core consist of distinct phases.

Description

1 2454485

METHOD OF DISPOSING OF NON-NUCLEAR HAZARDOUS WASTE

MATERIALS

The present invention relates to a method of treating non-nuclear, inorganic, hazardous waste materials to enable such material to be disposed of in landfill as non-hazardous waste. Of particular interest, though not exclusively, are those materials with hazardous properties H41H8 (irritant and corrosive), H5 (harmful), H6 (toxic), Hi 0 (toxic for reproduction), 1-113 and H14 (ecotoxicity) as detailed in the Hazardous Waste Directive, Annex 111 (Directive 91/689/EC). I0

Governments have legislated for what commercial waste products are safe to be disposed of in landfill sites, including the Landfill Directive 1999/31/EC and the European Waste Catalogue (EWC) 2003/33/EC. According to 2003/33/EC, waste is now classified as hazardous, non-hazardous or inert. Some hazardous materials are in the EWC as Absolute Entries whilst other hazardous materials are in the EWC as Mirror Entries. We understand that the classification of "Absolute Entries" cannot be changed from hazardous to non-hazardous for disposal in landfill no matter what processing is undertaken. However, the classification of "Mirror Entries" can be changed from hazardous to non-hazardous, and it is these materials that are the object of the present invention.

Testing of the waste material is important for both classification of material on the EWC (whether a material is hazardous or non-hazardous for paired entries) and for acceptance of material at the landfill site (Waste Acceptance Criteria -WAC analysis). The "leach" test is one of the important tests for hazardous materials; what is "leached" over time from the waste in landfill when it is buried in order to protect ground water (for example BS EN 12457/1, 12457/2, 12457/3 and NEN 7345). Landfill operators will now no longer accept materials at their sites that fail the WAC analysis. The present invention can be used to treat these waste loads so that they pass the WAC analysis, irrespective of whether its classification can be changed from hazardous to non-hazardous.

Any waste that is classified as inert or non-hazardous can be disposed of in landfill relatively easily. For materials classified as hazardous the disposal options are now more difficult and costly and the problem of dealing with hazardous waste disposal affects a wide range of industrial and municipal bodies. Such hazardous materials, particularly inorganic hazardous materials, include lead, cadmium, cobalt and nickel. Undertakings affected include ceramic manufacturers (Pb, Cd, Co, Ni etc.) and those needing to dispose of a wide range of commonly used consumables including fluorescent tubes, batteries (Ni, Cd etc.), electronic materials (Pb solders), computer monitors and television sets. The cost of dealing with such waste disposal can severely impact on industrial profitability and increase the cost of living for the general public.

Various techniques have been proposed for making safe certain commercial waste products including high level waste (HLW) from nuclear energy generating plants, e.g. derived from reprocessing commercial fuel rods. For example, there have been a number of proposals to make radioactive waste more safe by homogeneously dispersing divided particles of the waste in a glassy matrix, such as is disclosed in US patent nos. 4,351,749; 4,759,879; and 5,947,887 and US patent application no. 2004/0267080 and European patent application nos.0139321 and0238744.

More recently, British Nuclear Fuels Limited has developed its Waste Vitrification Process (WVP) and Tetronics has proposed plasma vitrification of incinerator ashes to produce inter alia glass ceramic tile products (see http://www.tetronics.com/Ddf/pIasma vitrification_fact sheet.pdf). However, it is understood that both such techniques produce a homogeneous encapsulated waste product similar to that produced by the prior art suggestions for disposing of radioactive waste.

Whilst the above techniques and disclosures can be applied to encapsulate non-radioactive hazardous waste so that the encapsulated waste can be disposed of in landfill, only a relatively small volume of waste can be embedded in the glassy matrix per unit volume, so that the capacity of the technique to accommodate waste is relatively low. That is, the glassy matrix dilutes the waste content; the waste is embedded homogeneously within an immobilising medium. This means that the above techniques consume a relatively large amount of raw materials per unit volume to make the glassy matrix. Additionally, as the glass and hazardous species are intimately mixed, the surface of the glass matrix will contain hazardous species.

We have now developed a method of disposing of non-nuclear, hazardous inorganic waste materials that enables such hazardous materials to be disposed of in landfill, i.e. inter a/ia the product meets the relevant European and British "leach" tests. Moreover, the technique can accommodate disposal of a wide range of waste materials including glass containing powders, refractory powders, large bulky items etc. According to one aspect, the invention provides a method of disposing of non-nuclear, inorganic hazardous waste materials, which method comprising compacting a sinterable hazardous waste material into a regular shape to form a hazardous waste core, coating the formed core with a glass composition or a glass precursor composition and heating the coated core to form a fully dense or relatively low porosity, continuous, inert, water and acid insoluble, non-toxic glassy shell surrounding a fully dense or relatively low porosity core, wherein the glass shell forms a physical barrier to leaching and wherein the glass shell and the core consist of distinct phases.

The hazardous waste core may be formed by a variety of methods. These include hot or cold pressing or compaction (uniaxial or isostatic), slip casting, extruding or moulding.

The hazardous waste material can be a fine powder (<100 j.tm), a coarse powder (I OOi.im to 5mm) or granular (5-10mm) or up to 10cm. Where the hazardous waste material comprises solid waste, for example waste cathode ray tubes, desirably such items are first comminuted by crushing in ajaw crusher or roller crusher or hammer mill or similar, followed by ball milling to give a powder/suspension of a desired size distribution in a lubricating liquid. The suspension may be used directly or it may be dried to form a powder before it is treated. Using the method of the present invention, it is possible to mix together different waste streams to optimise cost and performance. One possibility is to mix refractory hazardous waste (for example spent alumina supported catalysts) with a fusible hazardous waste (for example spent cathode ray tubes).

The glass composition or glass precursor composition may be a suspension of coarse or fine ground glass particulates or it may be a sol-gel type material or a silicate-based precursor or a combination of any of these.

The glass coating or the glass precursor coating may be formed on the hazardous waste core by a variety of methods. These include: spraying (wet or dry or electrostatic spraying), painting, dipping, roller coating, curtain coating etc. It is also possible to coat the material by wrapping the hazardous waste core in a tape cast glass precursor.

The resulting glass shell-coated hazardous waste product has good mechanical strength to withstand crush and sheer forces that may be encountered in a landfill. If desired, the mechanical strength of the piece can be improved by employing techniques used in other industries, for example rebars or steel mesh reinforcing in concrete or silicon fibre technology employed in the composite industry. A regular shape can be selected so that it can be manipulated mechanically or robotically or manually handled. The shape can be a cube or rectangular box, to maximise stackability, e.g. to minimise voids between each product.

Alternatively, the regular shape can be a sphere, spheroid, obloid etc. Of course, a sphere shape minimises the surface area the product presents, further limiting opportunities for leaching and reducing the raw material required for the glassy coating. In practice, therefore, the physical shape of the product selected may be a compromise between surface area and processing/handling issues, mechanical strength and landfill packability.

A significant advantage of the present invention is that, unlike the materials prepared by the known vitrification processes mentioned above, no hazardous species or material is present within the glassy surface layer. A second significant advantage of the present invention is that the hazardous material in the core of the body is held within a sintered fully dense or low porosity medium.

The thickness of the glassy barrier layer can be varied to optimise performance depending on the nature of the hazardous waste to be treated. Ideally, the total glass additions in the shell-coated product (including any additions to refractory hazardous waste material (see below)) are less than SOwt% of the product, such as less than 25wt% of the product or preferably less than 1 Owt% of the product. For example, a core of 10 cm x 10 cm x 10 cm with a barrier thickness of 5Otm the core will represent 98.9 % of the total weight of the product (assuming that the core and coating are fully dense: core and barrier densities are 3.5 gcm3 and 2.5 gcm3 respectively).

The fired thickness of the barrier layer can be varied to give optimum performance with regards to water and/or acid solubility, mechanical strength and cost.

The barrier glass composition is non-toxic, including lead-free and cadmium-free.

The glass composition as a whole is optimised for a number of properties. These include: Tg (or Tsoft), stiffness, expansion coefficient and water solubility and acid solubility.

The Tg will be optirnised so that the glass sinters to form a fully dense, continuous barrier layer at the lowest possible treatment temperature to minimise processing costs and CO2 emissions. However, it will be appreciated that the shell-coated product may also be formed in a two stage heating process, wherein a first heating stage the sinterable hazardous waste material core is formed and heated to form the fully dense or relatively low porosity sintered core and wherein a second heating stage the sintered core is coated with the glass or glass precursor composition and then heated. The glass should be relatively stiff to maintain the physical shape of the barrier coating so that it does not flow or sag and the integrity of the shell coating is maintained.

The expansion coefficient of the glass can also be tailored to suit the nature of the waste material in the hazardous core, to minimise the likelihood of thermal cracking. If necessary, a "primer layer" may be applied between the core and the coating to minimise any thermal expansion mismatch between the core material and the glass layer. The "primer" may be a preformed glass or a glass precursor or other formulation and may be applied by the same or a different technique as the coating proper.

The glass composition as a whole is optimised to minimise solubility in water and acid as well as to minimise diffusion of any of the hazardous ions/species into the glassy shell layer, e.g. it may be phosphate containing glass or sulphur containing glass. For example, where the hazardous waste material contains lead, a sulphide based glass or a glass composition containing sulphide additions can be used to reduce lead solubility by forming lead sulphide.

Ideally, the hazardous waste material in the core will fuse together to form a fully sintered body without any further additions of fusible material. The temperature and rate of rise at which the coated core is heated is optimised to maintain the physical shape of the product. It should be high enough to allow both the glass particles to fuse together to form a continuous surface coating and for the hazardous core to sinter together to form a fully dense or relatively low porosity body, but low enough that the whole body does not melt and flow together to form a homogeneous glass lens.

Where the hazardous waste material comprises an excess of sinterable material, it is possible that a glass composition with a low Tg is selected for the coating material. Another possibility is to add a refractory filler (for example sand, flint, fireclays, china clay, pyrophyllite or mullite powder etc. in either fine powder form or as a coarse grog) in order to increase the stiffness and maintain the physical shape of the final product after heat treatment.

If the hazardous waste is comprised entirely of refractory material, additions of a glass/frit can be made to the hazardous waste material to allow sintering to occur. The glass fit addition:refractory hazardous waste ratio, the particle size distribution of the hazardous powder and the particle size distribution of the frit may be optimised to maximise the amount of waste to be treated. For hazardous waste comprised entirely of refractory material, or having a very high refractory material content, it may be necessary to use increased processing temperatures to achieve sintering of the hazardous core. Sintering aids may also be used to improve densification and/or lower the processing temperature. Where increased processing temperatures are necessary, a glass composition with a high Tg can be selected so that sintering of the core and coating occur at approximately the same temperature.

Although additions of a glass powder to the refractory hazardous waste will increase the mass of the waste to be disposed of, a benefit of this invention is that the volume of waste to be disposed of by landfill is actually less than that of the powder itself because the material is compacted and the core is fully dense (or has relatively low porosity) after sintering.

A significant advantage of this invention is that there are two mechanisms for reducing the hazardous nature of the waste. The hazardous material is not only encapsulated within a non-hazardous shell, but the hazardous species is also held with a vitrified immobilising medium.

An unfired product comprising the hazardous waste core coated with the glass precursor can be referred to as a "green" body or piece, terminology that is standard within the ceramics manufacturing industry. The encapsulated "green", i.e. the coated compacted, hazardous waste material can be formed by a number of methods. It may be a single stage or multiple stage process. Ideally a one step process would be used to minimise cost and energy use.

Plasticisers (for example ball clays, china clays, kaolin, gum arabic or lignosulphonates) and other additives, for example binders (such as clays, silicates, alginates, acrylates, lignins, glucose, starches, cellulose and its derivatives and other thermoplastic resins), rheology controllers (for example DispexTM), mould release agents, can all be used during the formation of both the core and the coat.

It is also possible that the shell-coated waste core product is further treated to enhance stability. The shell-coated piece may be further encapsulated with an organic layer, plastic resin or a different, low temperature glass. The organic layer, plastic resin or low temperature glass can contain additives (e.g. selected from those listed in the previous paragraph) with relatively low thermal stability to further enhance stability.

Apparatus for producing the products of the method of the invention is standard within the advanced ceramics/refractory/whitewares industry. Similarly, apparatus for heating the coated product is also known from these industries. Therefore, it is not essential to design, build and commission unique processing equipment.

The heat treatment step can be carried out in either a gas or electric furnace. The furnace may be directly or indirectly heated. It may be a form of continuous kiln (belt, roller kiln, tunnel kiln) or it may be an intermittent kiln. Ideally, the green piece has sufficient mechanical strength so that it does not need to be placed in a container or crucible but can be fired on a refractory or steel tray. The firing cycle can be optimised to produce minimal thennal cracking and warpage/distortion of the piece whilst still achieving maximum sintering of the core and coat with maximum production output.

For enhanced stability there may be occasions when an extra coating is deposited on top of the fully formed encapsulated piece. These can be applied at room to low temperatures and may be one component or multicomponent systems. For example, for nickel containing hazardous waste, a resin coating system is applied in a continuous manner to the glass coated hazardous core. Contained within the resin system is dimethylglyoxime (DMG) which forms a Ni:DMG complex in a quantitative manner. A resin coating system is used to apply the organic additives such as DMG, as such organic additives would decompose at typical glass processing temperatures. The organic resin system can be used as an extra safeguard to reduce leaching.

Standard analytical techniques (optical microscopy, electron microscopy, analytical electron microscopy etc.) can be used to determine the integrity of the barrier layer and to establish that two distinct phases are formed and maintained at the boundary between the glass barrier layer and the sintered hazardous waste material. In addition, advanced analytical techniques can be used to depth profile the coating; examining the first few atomic layers and then penetrating down through the coating into the bulk. Furthermore, tests to establish whether the product meets the required European and British leach tests for landfill are also readily determined.

In order that the invention may be more fully understood, the following Examples are provided by way of illustration only and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing the process steps of an embodiment according to the present invention; Figure 2 is a photograph of an unfired coated core prepared according to Example I with a section deliberately broken away. A ruler is included for scale; Figure 3 is the product shown in Figure 2 post-firing with the top deliberately broken away; and Figure 4 is an optical micrograph of the cross section seen in Figure 3 clearly showing the two distinct layers.

It is believed that the annotations in the schematic drawings render the drawing self-explanatory.

EXAMPLES

Example 1

In this example, the hazardous waste has already been determined to have good fusibility.

Slip casting can be used as a one stage process, as follows: (i) A mould is selected for the desired shape; (ii) A milled/finely divided aqueous suspension of barrier glass is poured into the mould. The wall of the mould is coated with the barrier glass; (iii) An aqueous suspension of the hazardous waste is poured into the lined mould; (iv) After all the water has been removed from the mould by capillary action, the green body is removed from the mould; (v) If necessary, the piece can be dried further in an oven; and (vi) After drying the piece is heated at the desired temperature and ramp rate to achieve densification of the core and coat.

Plasticisers and other additives can be used during the formation of the core and coat.

Figure 2 shows a coated rectangular box shaped core prepared by slip casting prior to firing according to Example I after step (iv), with the top broken away deliberately to reveal the coating and the hazardous material core (a lead/cadmium enamel).

Figure 3 shows the coated core of Figure 2 after firing (product of step vi) which has been broken open to reveal the coating and the hazardous core (the crack visible was caused by the force of breaking the piece open). It can be seen that the material has sintered and is fully dense. An optical micrograph of the cross section seen in Figure 4 clearly shows the two distinct layers and that good densification has been achieved.

The results of a rudimentary leach test performed on the product produced by step (vi) are provided below. For the test, the piece was immersed in 4% acetic acid solution for 24 hours. This test is based in a standard leach test developed for determining metal leaching from surface decoration, e.g. decals, on tableware by simulating contact with acidic foodstuffs such as vinegar or lemon juice. The piece was then washed with water (the washings were collected in the acid solution) and the acetic acid made up to volume. Analysis was carried out on the acid solution using Inductively Coupled Plasma spectroscopy. For comparison, results are given in Table I for: 1. The untreated hazardous waste powder 2. The hazardous waste powder after vitrification without encapsulation 3. The encapsulated hazardous powder.

The results are quoted in mg leached per gram of hazardous material: Table 1 -Results of 4% Acetic Acid Leach Test Test Piece Leach Results -Leach Results -

LEAD CADMIUM

Microgram/gram Microgram/gram 1. Untreated hazardous waste powder 25,000 3775 2. Vitrified hazardous waste -no 25 5 encapsulation 3. Encapsulated hazardous waste <0.5 0.05 It can be seen that the encapsulated hazardous waste has reduced leaching over the starting hazardous waste powder by approximately 50,000 for lead and 75,000 for cadmium.

Encapsulation has reduced leaching over the vitrified waste by 50 times for lead and 100 times for cadmium. It should be noted that the encapsulated test piece had a surface area 15 times greater than that of the vitrified waste piece.

Example 2

In this example, the hazardous waste has already been determined to have good fusibility.

Uniaxial pressing is used as a two stage process, as follows: $ge1 (a) Load a die with waste powder; (b) Press; and (c) Eject pressed piece from die.

Stage 2 (d) Set up spray line with barrier glass composition (a suspension of preformed glass particles milled to the correct particle size distribution); (e) Spray coat the pressed hazardous waste with barrier glass precursor; (f) Dry green body; and (g) After drying the piece is heated at the desired temperature and ramp rate to achieve densification of the core and coat Plasticisers and other additives can be used during the formation of the core and coat.

In both Example I and Example 2, an increase in packing efficiency is obtained over the starting powder. In general, densification increases of two to five times can be achieved by slip casting or pressing.

Example 3

In this example, the hazardous waste has already been determined to have good fusibility.

Uniaxial pressing can be used as a two stage process, as follows: Stage I The hazardous body is formed by uniaxial pressing as in Example 2.

Stage 2 (a) Set up spray line with barrier glass precursor (the precursor is a so! gel which will form the desired glass composition on a suspension on heating); (b) Spray coat the pressed hazardous waste obtained from Step I with barrier glass precursor; (c) Dry green body; and (d) After drying the piece is heated at the desired temperature and ramp rate to achieve densification of the core and coat.

Example 4

In this example, the hazardous waste has already been determined to have good fusibility.

a. A core of hazardous material is made by slip casting (with or without plasticiser and binders); b. A glass barrier layer (made of preformed glass particles) is applied by spray coating; and c. After drying the piece is heated at the desired temperature and ramp rate to achieve densification of the core and coat.

Exampte S In this example, the hazardous waste has already been determined to have good fusibility.

a. A core of hazardous material is made by slip casting (with or without plasticiser and binders); b. A barrier glass precursor (the precursor is a sol gel which will form the desired glass composition on heating) is applied by spray coating; and c. After drying the piece is heated at the desired temperature and ramp rate to achieve densificat ion of the core and coat.

Example 6

In this example, the hazardous waste has already been determined to have good fusibility.

Uniaxial pressing can be used as a two stage process, as follows: Stage I (a) The hazardous body is formed by uniaxial pressing as in Example 2.

Stage 2 (b) A film is made of preformed glass particles within a plasticised resin film by tape casting; (c) A piece of film is cut to the correct dimensions and used to wrap the hazardous core; and (d) The coated core is heat treated at the desired temperature and ramp rate to achieve densification of the core and coat.

Example 7

In this example, the hazardous waste has already been determined to have good fusibility (a) A thin walled tube (made of lead free glass) is capped at one end using a plug made of preformed glass powder; (b) The tube is then filled with hazardous powder waste to approximately 95 % full (vibration can be used to aid compaction); (c) A second cap is then applied to the tube using a plug made of preformed glass powder; and (d) The glass tube is then heated in a furnace to a temperature at which the glass tube softens and the hazardous waste sinters. An electric tube furnace can be used to carry out the heat treatment. If desired, an induction furnace, similar to that used in zone refining can also be used to carry out the heat treatment.

Example 8

In this example, the hazardous waste has been determined to have poor intrinsic fusibility.

(i) A suspension is created by ball milling together 60 wt% hazardous waste, 37 wt% preformed glass and 3 wt% bentonite clay. A milling ratio of 7 parts solids: 3 parts medium is used to achieve an intimate dispersion.

(ii) A mould is selected for the desired shape; (iii) A milled/finely divided aqueous suspension of barrier glass is poured into the mould.

The wall of the mould is coated with the barrier glass; (iv) An aqueous suspension of the hazardous waste (see i) is poured into the lined mould; (v) After all the water has been removed from the mould by capillary action the green body is removed from the mould; (vi) If necessary, the piece can be dried further in an oven; and (vii) After drying the piece is heated at the desired temperature and ramp rate to achieve densification of the core and coat.

Plasticisers and other additives can be used during the formation of the core and coat.

Example 9

In this example, the hazardous waste has already been determined to have good fusibility.

Uniaxial pressing is used to form the hazardous core and part if not all of the coating.

(a) The die cavity of the press is filled with a predetermined amount of glass barrier powder (circa 10 % volume fill); (b) Hazardous waste material is then added to the die cavity (circa 90 % volume fill); (c) A second quantity of glass barrier powder is then used to fill the die cavity; (d) The piece is then compacted (the hazardous powder is now sandwiched between two layers of barrier powder; (e) The piece is removed from the die; (1) The remaining 4 sides of the hazardous waste core are coated by spraying; and (g) The piece is heated at the desired temperature and ramp rate to achieve densification of the core and coat.

Example 10

In a variation on the method according to Example 9.

(a) After step (e) in Example 9 the piece is turned to expose two fresh faces to the die; (b) The die cavity is filled with the predetermined amount of glass barrier powder; (c) The compressed piece is reinserted into the cavity; (d) A second portion of glass barrier powder is added to the cavity; (e) A second compaction step is then carried out; (f) The hazardous core is removed from the die (now has 4 sides coated with barrier glass); (g) The piece is rotated again so that the two exposed faces are in the correct alignment; (h) Steps (b) to (f) are repeated; (i) The piece is heated at the desired temperature and ramp rate to achieve densification of the core and coat.

Example 11

A boron nitride bat is placed on top of a stainless steel metal tray (typically 316 or 314 grade stainless). The "green body", for example produced by step (iv) of Example I or step (f) of Example 2 is placed on top of the boron nitride bat. The tray is then treated in an electric belt furnace.

On exiting the furnace, the sintered block is easily removed from the tray. The sintered block has reduced in size (due to sintering) but maintains its shape. Cross sectioning shows that the piece consists of two discrete zones as desired.

Example 12 -Glass Coatin2 Composition A typical lead free glass composition used for encapsulating the waste is given below.

These are the final oxide levels in the formed glass: Alumina 8.2 % Bismuth oxide 1.3 % Calcium oxide 7.2 % Iron oxide 0.07 % Potasssium oxide 1.4 % Magnesium oxide 0.03 % Sodium oxide 3.7% Silica 64.3 % Titania 0.04 % Zinc oxide 6.7 % Zirconia 6.9 %.

The glass is formed by fitting at 1500°C for 2 hours and quenching into running water.

Example 13 -Sol-Gel Glass Precursor Coating Composition Tetraethoxysilane Si(OC2H5)4) was hydrolysed in the presence of water and hydrochloric acid (Si(OC2H5)4:H20:HCI) with molar ratios 1:16:0.002 at 50°C. Fumed silica (Aerosil� 0X50), ultrasonically dispersed in methanol, was then added followed by Al(NO3)39H2O and finally gelation was achieved by addition of hydrofluoride acid (HF) catalyst at 50°C.

Claims (23)

1. CLAIMS: 1. A method of disposing of non-nuclear, inorganic hazardous waste materials, which method comprising compacting a sinterable hazardous waste material into a regular shape to form a hazardous waste core, coating the formed core with a glass composition or a glass precursor composition and heating the coated core to form a fully dense or relatively low porosity, continuous, inert, water and acid insoluble, non-toxic glassy shell surrounding a fully dense or relatively low porosity core, wherein the glassy shell forms a physical barrier to leaching and wherein the glass shell and the core consist of distinct phases.
2. 2. A method according to claim 1, wherein the compacting and coating steps are performed simultaneously.
3. 3. A method according to claim I or 2, wherein the compacting step comprises slip casting a suspension of hazardous waste material.
4. 4. A method according to claim 1 or 2, wherein the compacting step comprises hot or cold pressing or uniaxial or isostatic compaction of hazardous waste material.
5. 5. A method according to claim I or 2, wherein the compacting step comprises extruding or moulding of hazardous waste material.
6. 6. A method according to any preceding claim, wherein the coating step comprises spraying, painting, dipping, roller coating, curtain coating or coating using a tape cast film.
7. 7. A method according to any preceding claim, comprising the step of adding a glass and/or frit to a refractory hazardous waste material to form the sinterable hazardous waste material.
8. 8. A method according to any of claims Ito 6, comprising the step of adding a refractory filler to the hazardous waste material prior to the compacting step.
9. 9. A method according to claim 8, wherein the refractory filler is a china clay, a sand, flint, fireclay, pyrophyllite or mullite powder.
10. 10. A method according to any preceding claim, comprising applying a second layer of glass composition or glass precursor composition or a resin system to a hazardous waste core coated with a first glass shell and heating the resulting coated product in a second heating step to produce a glass shell or a resinous shell comprising a second glass layer or a resinous shell fused to a first glass layer.
11. 11. A method according to claim 10, wherein the glass in the second layer has a Tg that is significantly lower than the first glass layer.
12. 12. A method according to any preceding claim, wherein a preformed glass or glass precursor is applied to the formed core as a primer layer and the glassy shell layer proper is applied thereover, wherein an expansion coefficient of the primer layer glass is intermediate an expansion coefficient of the waste material and the glassy shell layer.
13. 13. A method according to any preceding claim, wherein the hazardous waste material comprises a fine powder of Dmax <l00xm.
14. 14. A method according to any of claims ito 12, wherein the hazardous waste material comprises particles of 100l.tm to 10cm.
15. 15. A method according to any preceding claim, wherein the or each barrier layer comprises an additive that combines with a hazardous inorganic metal in the waste material to form a precipitate to prevent leaching of the metal.
16. 16. A method according to any preceding claim, wherein the glassy layer comprises alumino silicate-containing glass, bismuth-containing glass, phosphate-containing glass, sulphur-containing glass or sulphide-containing glass.
17. 17. A method according to claim 14, wherein the hazardous waste material comprises lead and the glassy layer comprises a suiphide-based glass or a glass composition containing sulphide additions.
18. 18. A method according to claim 14 when appendant to claim 10, wherein the hazardous waste material comprises nickel and the resin outer layer comprises dimethylglyoxime.
19. 19. A product obtainable by the method according to any preceding claim, wherein the total glass additions are less than SOwt% of the product.
20. 20. A product according to claim 19, wherein the glass additions forms less than 25wt% of the product.
21. 21. A product according to claim 19, wherein the glass additions forms less than lOwt% of the product.
22. 22. A method substantially as described herein with reference to the accompanying drawings.
23. 23. A product substantially as described herein.