Patent Number: 052778460
Section: summary

This invention relates to processes and furnaces for treating incinerable waste, especially slightly radioactive waste. The particular problem is to treat the industrial waste caused by maintenance and repair work in the active parts of a nuclear installation, such as gloves, overalls, overboots, plastics such as polyethylene flasks containing organic residues, but also operational waste such as resins, organic sludges, oils or emulsions. This waste is made up of organic substances which carry radionuclides. A certain number of methods of incinerating waste of this kind are already known. They all have certain disadvantages, such as the need for preliminary sorting of the waste depending on its lower calorific power, the collection of powdery ash with unreliable packaging, at a later stage, and the production of unburnt matter in the smoke, requiring expensive and not very effective after-burning. The invention relates to a new process which makes it possible to reduce the volume of waste to be packaged and which allows this waste to be packaged, after gasification and/or fusion, without the production of powdery ash and unburnt matter. The operation needs not any regulations of a heating device or the atmosphere in the furnace. The process according to the invention consists in grinding the waste to a particle size of less than 2 mm, carrying it by means of a carrying gas into the lower part of a bath based on molten silica, pouring the bath, which contains the mineral substances, i.e. in particular solid radionuclides in the case of nuclear waste, into a container and leaving the bath to solidify in the container. The ashes and nuclear mineral solids thus remain in the bath, which becomes enriched in solid radionuclides, and which, once it is solidified and stored in the container, takes up much less room than the initial waste. The degradation of the organic chains by pyrolyse results in products with simpler molecules, thus facilitating the total gasification in a preferably oxidising atmosphere above the bath, the combustion gases then being passed downstream where they are purified. The pyrolyse and gasification of the pyrolyse products are independent of the lower calorific powers of the treated waste, which means that there is no need for any preliminary sorting. The volume of the organic carrying waste is converted into a volume of gas which can be released after purification, the residual solids being incorporated in the bath whilst increasing the volume thereof by only a small amount. The reduction in volume is very important and no powdery ash is formed. The total gasification of the pyrolyse products with simpler molecules eliminates the presence of unburnt matter in the smoke. In order to reduce the quantity of gas to be treated downstream of the furnace, it is advantageous that the driving pressure of the carrying gas should be just greater than the pressure corresponding to the height of the column formed by the molten bath. If, as is preferred, only part of the bath is poured into the container, a bath is permanently maintained at the required temperature at the bottom of the crucible to permit continuous treatment without interruption of the energy balance. In order to ensure that the ground waste carried into the bath based on molten silica is thoroughly pyrolysed and that the mineral matter is properly incorporated in the bath by fusion, the height of the bath is 5-40 cm above the intake level of the waste, for a bath temperature of 1000.degree.-1100.degree. C. Similarly, it is preferable that the mass of the bath should represent 0.2 to 6 times the mass flow rate per hour of waste. In order to control the composition of the bath, mineral products may be added to the waste in a quantity and of a kind such that the mineral composition of the waste becomes substantially identical to that of the bath. The latter generally consists of 40-100% by weight of SiO.sub.2 and 0-60% by weight of other metal oxides such as alkali metal oxides and boron oxides serving as fluxes. A flux may also be added to the waste in order to allow the mineral substances contained in the waste to melt at a lower temperature and to ensure that the composition of the mineral substances in the waste is made identical to that of the bath. Preferably, a gas containing oxygen is introduced above the bath in order to bring about the gasification of the pyrolyse products in an oxidising atmosphere above the bath. The simplest carrying gas to be used is air, but the carrying gas for introducing waste into the bath may also be a neutral or a dry gas or a strongly hygroscopic or reducing gas or one which will bring about hypostoichiometric oxidation conditions in the bath. But it is not necessary to change the atmosphere during the process. Advantageously, cryogenic grinding is carried out at a temperature ranging from -120.degree. to -80.degree. C. The invention also relates to a waste treatment furnace comprising a crucible equipped with heating means, a waste intake duct opening into the bottom of the crucible, a duct for extracting a bath opening into the crucible at a higher level than the opening of the waste intake duct, the top of the crucible communicating with a combustion chamber which communicates, by means of a passage which zig-zags towards the top, defined in a vault, with an evacuation chamber, and an intake duct for a gas containing oxygen which opens into the combustion chamber. This furnace can be used according to the invention to treat contaminated waste, the zig-zag passage enabling the pyrolyse gas to be retained in the combustion chamber for a sufficient time to ensure that it is completely burned, whilst preventing these gases from passing directly into the remainder of the installation downstream.