Patent Number: 044902875
Section: description

Referring now to FIG. 1 of the drawings there is shown a process vessel 1, composed of a heat-resistant material (e.g. stoneware clay, zircon or zirconia) surrounded with Vermiculate thermal insulation 2 and silica/alumina thermal insulating bricks 3 and enclosed within a microwave oven 4. The microwave oven 4 is provided with a wave guide 5 for the introduction of microwave radiation and an oven mode stirrer 6. An inlet pipe 7, fabricated from stainless steel and earthed to the oven 4 is provided to connect a feed pump 8 with the process vessel 1. The process vessel 1 is also provided with an outlet pipe 9 for the removal of gas/vapour therefrom, and a product outlet 10 for the passage of fused product to a collector 11 via aperture 12 in the thermal insulation 2 and 3. Microwave chokes 13 are provided at apertures in the microwave oven 4. Additionally there is provided a pressure relief device 14 in the process vessel 1 and a thermocouple temperature indicator 15. In operation solution or slurry to be treated to produce a dried product which is subsequently fused, is drawn from a supply (not shown) by pump 8 and delivered to the process vessel 1 via inlet pipe 7. Microwave radiation from a microwave source (e.g. a Magnetron) is introduced to the oven 4 by means of wave guide 5 and is distributed with the aid of oven mode stirrer 6. Due to coupling of the microwave radiation with solution or slurry in the process vessel 1 heat is generated therein and thermal energy is inhibited from escaping by thermal insulation 2 and 3. Consequently the temperature in the process vessel 1 rises to provide conditions in which the slurry or solution is converted to a dried product and the dried product is fused and runs out of the product outlet 10 to be collected in the collector 11 wherein it may solidify. Vapour and gases (from drying and possibly decomposition of constituents of the solution or slurry) are withdrawn through pipe 9. Due to the fact that inlet pipe 7 is earthed the solution or slurry passing therein is protected from microwave radiation. Thus it is only when solution or slurry leaves the inlet pipe 7 to enter the process vessel 1 that it is subjected to heating. This reduces the risk of blockage due to premature solidification of solution or slurry on its way from the pump 8. It should be noted that during start-up of the apparatus, a solid starting charge of a fusible material capable of coupling with microwave radiation may be placed in the process vessel 1 and subjected to microwave radiation to provide initial heating. The starting charge may be prepared by drying and fusing a sample of solution or slurry to be treated. If desired it can be arranged to permit microwave radiation to couple to the collector 11 to provide heating thereof thereby to promote efficient filling of the collector 11, reduce stresses in the solidifying product by preventing too rapid cooling, and to anneal the product. Referring now to FIG. 2 of the drawings there is shown diagrammatically a fluidised bed vessel 21 having a wave guide 22 for the introduction of microwaves, a solution/slurry inlet 23, a fluidising gas inlet 24, a dried product outlet 25, connected to a melter/receiver 26, and an off-gas outlet 27. An insulating window of microwave transparent material (not shown) may be placed between wave guide 22 and vessel 21. The off-gas outlet 27 is connected to a scrubber bed 28, for containing a particulate solid material, which has means 29 for discharging particulate solid material to the fluidised bed vessel 21. Off-gas outlet 27 and means 29 may be provided by a single piece of apparatus (e.g. a pipe). A particulate solid material inlet 30 is provided for charging the scrubber bed 28 and an off-gas outlet 31 is provided to connect the scrubber bed 28 to a condenser 32. The condenser 32 has cooling-fluid inlet 33 and outlet 34, an outlet 35 for condensate, and a gas outlet 36 which can be connected to a gas clean-up plant (not shown). In operation, particulate solid material is introduced to the fluidised bed vessel 21 via means 29 and is maintained as a fluidised bed (represented as 37) by use of fluidising gas inlet 24. Solution or slurry to be treated is introduced via inlet 23 and microwave radiation (e.g. from a Magnetron source not shown) is directed into the vessel 21 via wave guide 22. Due to the coupling of the microwave radiation with the contents of the fluidised bed 37 the temperature rises thereby to form particles of the solid material coated with a dried product formed from the solution or slurry. The coated solid particles are discharged by means of dried product outlet 25 to melter/receiver 26 wherein they may be fused by heating (e.g. with microwave energy or other means). Off-gases leave vessel 21 via outlet 27 and pass to scrubber bed 28 wherein contaminents in the off-gases are scrubbed out by contact with fresh particulate solid material. Particulate solid material can be passed counter-current to the off-gas and into the vessel 21 via means 29 thereby carrying back contaminants scrubbed from the off-gases. The scrubber-bed 28 can contain a fluidised bed of particulate solid material or a vibrating bed thereof. Fresh particulate solid material is introduced via inlet 30. Off-gases from scrubber bed 28 are passed to condenser 32 (cooled by passing a cooling fluid via 33 and 34) to give a condensate at outlet 35, and gas at outlet 36 for processing in a clean-up plant. In a particular example of the present invention the particulate solid material may comprise spheres (0.01-0.1 mm diameter) of glass formers (e.g. Na, Li, B.sub.2 O.sub.3 and SiO.sub.2) and the solution or slurry may contain radioactive waste, so that in the fluidised bed vessel 21 spheres of glass-formers are produced having a coating of dried product formed from the solution or slurry containing radioactive waste. Thus, after fusing in the melter/receiver (26) a glass-like solid incorporating radioactive waste is produced. Referring now to FIG. 3 of the drawings, there is shown a tube 41 a portion of which is located within a microwave oven 42. The tube 41 is provided with an inlet pipe 43 and a gas/vapour outlet 44 and is adapted to contain slugs of glass fibres 45. To permit tube 41 to extend out of the microwave oven 42 apertures 46 and 47 are provided. It will be appreciated that, in accordance with microwave technology, microwave chokes (not shown) may be provided as necessary at apertures 46 and 47 and also where inlet pipe 43 and gas/vapour outlet 44 penetrate the walls of the oven 42. In operation, the slugs of glass fibre 45 are introduced into the tube 41 from the direction 48. Subsequently, solution to be treated is introduced onto a slug 45 via inlet 43, is absorbed therein and subsequently converted to a dried product thereon by application of microwave radiation in the microwave oven 42. (It will be appreciated that microwave radiation is introduced into the microwave oven 42 in a known manner through a wave guide (not shown)). Off-gases produced during the production of the dried product pass through the tube 41 in the direction 49 and therefore pass through, and are filtered by, the "fresh" slugs 45 located in the tube 41 before being discharged therefrom through the gas/vapour outlet 44. Off-gases removed through the outlet 44 can be passed to other treatment apparatus, for example a condensate system, for further treatment. Subsequently a fresh slug 45 is introduced into the tube 41 from the direction 48 with the result that all of the slugs 45 move along the tube in that direction such that "loaded" slugs 45 carrying dried product are thereby moved out of the microwave oven 42 through the aperture 47 and are ultimately discharged from the tube 41. "Loaded" slugs 45 can be discharged from the tube 41 directly to a melting apparatus which may comprise a ceramic melting vessel surrounded by a microwave transparent thermal insulation located in a microwave oven. It will be appreciated that an automatic loading mechanism can be used to introduce fresh glass fibre slugs 45 to the tube 41 in a continuous or semi-continuous manner. It will be appreciated that the present invention is not limited to the treatment of radioactive wastes and that solutions of salts or slurries of non-radioactive substances can be subjected to drying, decomposition and fusion in accordance with the present invention to give a glass-like or ceramic material containing a non-radioactive substance (e.g. in the production of glasses). It will be appreciated that the use of microwave radiation enables the energy applied to be almost wholly absorbed in the matter to be treated thus avoiding the need to pass heat through the walls of containment vessels. The invention will now be further described with reference to the following Examples: EXAMPLE 1 In this example a feed solution simulating a radioactive waste solution was subjected to microwave radiation. The feed solution was a solution/suspension containing nitric acid, 25.7% by weight simulated "waste oxides" (containing some uranium but composed mainly of rare earths, aluminium, iron and magnesium) and the following glass forming components: Na.sub.2 O: 8.3 wt%, Li.sub.2 O: 4.0 wt%, B.sub.2 O.sub.3 : 11.1 wt%, SiO.sub.2 : 50.9 wt%. 126 g of the feed solution were placed in a Pyrex (Reg. Trade mark) beaker and subjected to microwave radiation (from a Magnetron source) in a microwave oven until a dried product was obtained. It was noted that 40 ml of liquid were evaporated in 5 minutes using a power of 750 watts. The beaker and dried product therein were returned to the oven and with the power still set at 750 watts the dried product underwent further decomposition, with the release of nitrous fumes. The temperature rose to bright red heat and the heating was stopped. It was found, after cooling, that the dried product had been converted to a vitreous, glass-like mass. EXAMPLE 2 In this example an apparatus of the type disclosed in FIG. 1 was used to treat a feed solution having the same composition as given in Example 1. For start-up 252 g of preformed fusible dried product (prepared from the solution to be treated) was placed in a vessel, said vessel being surrounded by thermal insulation and situated in a microwave oven (see FIG. 1). Microwave power was applied and increased to a maximum of .about.1.4 KW over a period of 1 hour, and the vessel and fusible product brought up to a temperature of 1020.degree. C. Feed solution as in Example 1 was fed to the vessel initially at 6 ml/min and microwave power maintained at .about.1.4 Kw. Glass flowed from an outlet in the base of the vessel irregularly and was collected in a beaker of water beneath the oven. It is believed that the irregularity of flow was due to the effects of surface tension at the low flow rates used. The main portion of the experiment was conducted with a feed solution flow rate of 7.5 ml/min. It was convenient to end the experiment after .about.9 hours although there was no reason to suppose the process could not have been operated indefinitely. During the experiment the oven was maintained at 1000.degree.-1050.degree. C. and 4.84 liters of feed solution treated to enable 1.344 Kg of glass to be collected (glass production rate 2.14 g/min). EXAMPLE 3 A volume of 400 ml of a suspension of magnesium basic carbonate in water (containing the equivalent of 36 g oxide) was introduced into an alumina tube having a closed bottom end and mounted vertically in thermal insulation. The suspension was subject to microwave radiation (power 1-1.5 Kw) and evaporated to give a fusible dried product. The temperature rose to 970.degree. C. in 80 minutes. At 970.degree. C. glass-forming components were added in the form of a glass frit (200 g) and 20 minutes further application of microwaves took the temperature to 1110.degree. C. at which the contents of the tube was molten. A glass-like solid was obtained on cooling.