Patent Number: 044902875
Section: summary

The present invention relates to the treatment of substances and finds one application in the incorporation of radioactive wastes in a solid material (i.e. solidification and vitrification of radioactive waste) for the purposes of handling and storage. According to one aspect of the present invention there is provided a process for the treatment of a substance contained in a solution or slurry which includes subjecting the solution or slurry to the influence of microwave radiation to produce a fusible dried product, said fusible dried product including a dried form of the substance. According to another aspect of the present invention there is provided a process for the treatment of a substance contained in a solution or a slurry which includes subjecting the solution or slurry to the influence of microwave radiation to produce a fusible dried product, said fusible dried product including a dried form of the substance, and heating the fusible dried product to fuse it. According to a further aspect of the present invention there is provided a process for the treatment of a substance contained in a solution or a slurry which includes treating the solution or slurry to produce a fusible dried product, said fusible dried product including a dried form of the substance, and subjecting the fusible dried product to the influence of microwave radiation to heat and fuse it. The term "solution" as used herein embraces a colloidal solution and a suspension, and the term "slurry" as used herein embraces sludge. Microwave radiation may be used both in the The present invention relates to the treatment of substances and finds one application in the incorporation of radioactive wastes in a solid material (i.e. solidification and vitrification of radioactive waste) for the purposes of handling and storage. According to one aspect of the present invention there is provided a process for the treatment of a substance contained in a solution or slurry which includes subjecting the solution or slurry to the influence of microwave radiation to produce a fusible dried product, said fusible dried product including a dried form of the substance. According to another aspect of the present invention there is provided a process for the treatment of a substance contained in a solution or a slurry which includes subjecting the solution or slurry to the influence of microwave radiation to produce a fusible dried product, said fusible dried product including a dried form of the substance, and heating the fusible dried product to fuse it. According to a further aspect of the present invention there is provided a process for the treatment of a substance contained in a solution or a slurry which includes treating the solution or slurry to produce a fusible dried product, said fusible dried product including a dried form of the substance, and subjecting the fusible dried product to the influence of microwave radiation to heat and fuse it. The term "solution" as used herein embraces a colloidal solution and a suspension, and the term "slurry" as used herein embraces sludge. Microwave radiation may be used both in the production of a fusible dried product and in the fusing thereof. The solution or slurry may be subjected to microwave radiation to an extent such that only the liquid component of the solution or slurry is removed therefrom, or to an extent which in addition to the removal of the liquid component results in the chemical decomposition of at least a portion of the residue obtained when the liquid component has been removed. The dried product may be arranged to contain at least one glass-forming or ceramic-forming component (e.g. silica and borax) capable of forming on its own, or with the dried form of the substance, a glass-like or ceramic material, in which case, after heating to cause fusing, and cooling, the substance will be incorporated in a glass-like or ceramic, solid material. Thus, for example, at least one glass-forming or ceramic-forming component (or a precursor for either of these) may be incorporated in the solution or slurry before production of the fusible dried product (e.g. before the solution or slurry is subjected to microwave radiation where such radiation is used to produce the fusible dried product). Alternatively, at least one glass-forming, or ceramic-forming component (or a precursor for either of these) may be added to the dried product so that after heating to cause fusing and cooling the substance will be incorporated in a glass-like or ceramic solid material. Radioactive wastes for example, uranium, transuranium elements and fission product elements resulting from nuclear fuel reprocessing processes, can be incorporated into ceramic and glass-like solid materials by a process in accordance with the present invention. Such radioactive wastes may comprise solutions containing nitric acid, uranium, transuranium elements and fission products, and corrosion products of reprocessing equipment. It will be appreciated that such wastes may contain, or on drying give rise to, decomposable chemical compounds, for example nitrates. Thus, if the solution or slurry is treated only to the extent such that the liquid component of the solution or slurry is removed therefrom to give a dried product, the compounds will be decomposed during the heating of the dried product to fuse it. Conversely, if the solution or slurry is treated to an extent which also results in the decomposition of the compounds, the heating of the dried product to fuse it will not have also to achieve such decomposition. During the decomposition of the compounds water of crystallisation may be removed. Radioactive wastes containing neutron poisons may also be treated in accordance with the present invention. A further example of a solution or slurry which may be treated in accordance with the present invention is a suspension or slurry of a magnesium compound (e.g. a suspension or slurry derived from fuel element cooling pound waters in certain types of nuclear reactor). Thus, for example, an aqueous suspension of basic magnesium carbonate can be evaporated to dryness by use of microwave radiation to give a fusible dried product and the dried product mixed with glass forming components. Subsequently the mixture of fusible dried product and glass-forming components can be fused by being subjected to the influence of microwave radiation, and cooled to give a glass-like material containing magnesium (e.g. in the form of magnesia). Alternatively an aqueous suspension of a magnesium compound can be coagulated with a silicate-based coagulant (a glass-forming component precursor) and the resulting sludge dried and fused by use of microwave radiation. Glasses themselves may be regarded as supercooled-liquids, but it is intended that the term `solid material` as used herein should include glasses and glass-like materials. With regard to the incorporation of radioactive substances in glass materials reference can be made to our British Pat. Nos. 1,019,373, 1,050,818 and 1,064,583. Microwave heating may be used to produce the fusible dried product and to fuse it, for example, in one continuous operation; alternatively the fusible dried product may be fused by use of other forms of heating, such as resistance heating. A further alternative is for a fusible dried product to be formed by some other drying treatment, such as spray drying, and for microwave radiation to be used in heating the dried product to fuse it. (It will be appreciated that if the other drying treatment used does not result in the decomposition of decomposable chemical compounds in the solution or slurry, microwave radiation may be used to effect decomposition and fusing). Thus, the present invention further provides a process for the fusing of material which comprises subjecting the material to the influence of microwave radiation. Also the present invention further provides a process for the decomposition of a chemical compound which comprises subjecting the chemical compound to the influence of microwave radiation. In a first embodiment of the invention a solution containing a radioactive waste may be contained in vessels of a glass which can combine with oxides present in the dried product, produced from the solution, to form a suitable storage material. In this embodiment a series of such vessels may be passed continuously through a microwave oven, to convert the solution therein to a dried product, and discharged into a melter pot wherein the dried product may be fused with the vessels to form a homogeneous glass. The vessels may be closed by a porous glass filter to contain dust. Glass or quartz are suitable materials for vessels to be used in a microwave field. In a second embodiment of the invention a solution containing radioactive wastes may be applied to a glass fibre tape, or "wick", the tape subsequently passed through a microwave field to produce a dried product on the tape, and the tape and dried product heated to melt them so as to form a homogeneous glass incorporating radioactive waste. The molten glass may be collected directly in a vessel suitable for subsequent storage thereof. In a third embodiment of the invention a stainless steel vessel may be used to contain a radioactive waste solution and constituents of a material for forming a glass-like material, and microwave radiation may be introduced into the vessel by means of a suitable wave guide arrangement for example, through a silica "window", so that the solution can be dried to form a dried product suitable for treatment (either by microwave or other heating) to give a homogeneous glass incorporating radioactive waste. It is believed that a metal vessel itself can constitute a microwave cavity for subjecting a solution or slurry contained therein to microwave radiation. In a fourth embodiment of the invention a solution or slurry to be treated in accordance with the invention (e.g. a solution or slurry of a radioactive waste and glass-forming components to be treated to form a glass-like solid incorporating the radioactive waste), may be fed to a suitable process vessel and the vessel subjected to a microwave field, thereby first to dry the solution to a dried product and then to fuse it. This embodiment offers the possibility for continuous treatment of a solution or slurry and conveniently means are provided for withdrawing fused material from the process vessel at a rate compatible with the rate of introduction of solution or slurry. It is believed that to obtain the temperatures required to effect drying and fusing, it is necessary to provide around the process vessel good thermal insulation which is transparent to microwave radiation (e.g. `Vermiculite`). The process vessel may be made of a ceramic material and may be subjected to microwave radiation by being positioned inside a microwave cavity (oven). Since the heating by microwave radiation tends to be localised in the region of the process vessel (especially where thermal insulation is provided as hereinbefore mentioned) oven walls need not necessarily be made from thick, heat-resisting materials. In a fifth embodiment of the invention a solution or slurry to be treated in accordance with the invention (e.g. a solution or slurry of a radioactive waste and glass forming components to be treated to form a glass-like solid incorporating the radioactive waste) may be subjected to microwave radiation whilst in a fluidised bed. Thus, for example, the production of a dried product can be carried out in a fluidised bed under the influence of microwave radiation and the dried product can be fused in a separate non-fluidised melter/received (which may be heated by microwave radiation or other forms of heating). The microwave radiation may be introduced into the fluidised bed by use of wave guides and by means of a microwave-transparent window (e.g. of quartz). In an example of the fifth embodiment of the invention. applicable to the incorporation of radioactive wastes in glass-like materials, particles (e.g. spheres of 0.01-0.1 mm diameter) of glass formers (e.g. Na, Li, B.sub.2 O.sub.3 and SiO.sub.2) may be fed to a fluidised bed into which a solution or slurry containing radioactive waste is also fed and to which microwave radiation is introduced thereby to produce particles of glass-formers coated with dried product formed from the solution or slurry containing radioactive waste. These coated particles may then be passed to a melter, which may be a final storage vessel, in which they are fused to produce a glass-like material containing the radioactive waste by means of microwave or other heating. Optionally off-gases from the fluidised bed can be scrubbed to remove dust by countercurrent contact with uncoated particles (e.g. spheres) in a second vesssel prior to entering the fluidised bed (e.g. in a second fluidised scrubbing bed or a vibrated scribbing bed). The second vessel used for scrubbing can be conveniently of larger diameter than the fluidised bed to suit gas flow requirements. In another arrangement, given by way of example, off-gases from a vessel in which a solution or slurry is treated in accordance with the present invention (e.g. in accordance with the fourth embodiment hereinbefore disclosed) may be treated to remove dust by means of a chamber containing baffles. Conveniently the chamber may be subjected to microwave radiation (e.g. by being enclosed in the same microwave cavity as the vessel in which the solution or slurry is treated or in an adjacent microwave cavity). Thus, off-gases may be passed through the chamber so that dust in the off-gases collects on the baffles and is subsequently melted off due to microwave heating. The melted dust may be united with fused product from the vessel in which the solution or slurry is treated. In the chamber microwave heating may be achieved by the coupling of the dust with microwave radiation. It is believed that microwave heating may be achieved by arranging that the baffles also couple with microwave radiation. Gases leaving the chamber may be passed to other treatment apparatus (e.g. a condenser). In a sixth embodiment of the invention, a solution or slurry to be treated in accordance with the invention (e.g. a solution or slurry of a radioactive waste to be treated to form a glass-like solid incorporating the radioactive waste) is fed to a slug of glass fibre and absorbed therein, and a microwave field applied to produce a dried product on the slug; subsequently the slug loaded with dried product can be passed to a melting apparatus and fused to produce a molten glass-like material incorporating the radioactive waste. In one example of the immediately foregoing embodiment a train of slugs of glass fibre are positioned in a tube which can be subjected to the influence of microwave radiation. Solution or slurry to be treated is fed to a slug, absorbed therein and converted to a dried product by means of microwave radiation. Thus the slug becomes "loaded" with dried product. A fresh slug of glass fibre can be inserted into the end of the tube remote from that containing the loaded slug such that the train of slugs is pushed along the tube and the loaded slug is ejected from the other end of the tube. The loading and ejection of further slugs then proceeds in the same manner such that slugs are treated sequentially. By arranging for off-gases produced during the production of the dried product to be removed from a point on the tube which is disposed towards the end of the tube remote from that containing the loaded slugs, the off-gases can be made to pass through, and thereby be filtered by, successive "clean" slugs of glass fibre. It will be appreciated that since the train of slugs is pushed along the tube, the filtering slugs are continually being renewed. A melting apparatus suitable for use in connection with the immediately foregoing embodiment of the invention may comprise a ceramic melting vessel surrounded by microwave transparent thermal insulation located in a microwave cavity. The "loaded" slugs carrying dried product can be ejected directly to the melting apparatus if desired. It will be appreciated that in the immediately foregoing aspect of the invention the use of glass fibre can obviate the need to supply glass-forming components in the solution or slurry and hence reduce the total amount of solution or slurry to be processed. At least partial decomposition of chemical compounds in the solution or slurry (e.g. partial denitration in the case of a radioactive waste containing nitric acid) can take place during production of dried product on the slug. According to a further aspect the present invention provides apparatus for carrying out a process in accordance with the invention. Thus, according to a further aspect the present invention provides apparatus for the treatment of a substance contained in a solution or slurry comprising means for subjecting the solution or slurry to the influence of microwave radiation to produce a fusible dried product, said fusible dried product including a dried form of the substance. In a further aspect the invention also provides apparatus for the treatment of a substance contained in a solution or a slurry comprising means for treating the solution or slurry to produce a fusible dried product, said fusible dried product including a dried form of the substance, and means for heating the fusible dried product to fuse it wherein: (i) the means for treating the solution or slurry comprises means for subjecting the solution or slurry to the influence of microwave radiation; and/or PA1 (ii) the means for heating the fusible dried product to fuse it comprises means for subjecting the fusible dried product to the influence of microwave radiation. In one embodiment apparatus comprises a vessel, thermal insulation around the vessel, means for introducing solution or slurry to be treated to the vessel, means for subjecting the vessel to microwave radiation to produce a fusible dried product from the solution or slurry and to fuse the fusible dried product. Preferably means are provided for withdrawing fused dried product from the vessel. In another embodiment apparatus comprising a fluidised bed vessel, means for feeding solution or slurry to be treated to the fluidised bed vessel, means for feeding fluidising gas to the fluidised bed vessel, means for feeding particles to the fluidised bed vessel, means for introducing microwave radiation to the fluidised bed vessel and means for withdrawing fusible dried product from the fluidised bed vessel, the arrangement being such that the fusible dried product can be formed on the particles and removed therewith from the fluidised bed vessel. The apparatus of the immediately foregoing embodiment may have a second vessel containing particles wherein dust can be removed from off-gases from the fluidised bed vessel by countercurrent contact with the particles. In a further embodiment apparatus comprises means for feeding a solution or slurry to be treated to a slug of glass fibre to be absorbed therein and means for applying a microwave field to the slug to produce dried product on the slug. The apparatus of the immediately foregoing embodiment may have a melting vessel for fusing the slug and dried product to produce a molten glas-like material, and means for feeding the slug and dried product to the melting vessel. The invention also provides a fusible dried product or a fused fusible dried product whenever prepared by a process in accordance with the present invention.