Hazardous or toxic waste destruction or containment – Destruction or containment of radioactive waste – Chemical conversion to a stable solid for disposal
Patent
1996-09-13
1998-06-23
Mai, Ngoclan
Hazardous or toxic waste destruction or containment
Destruction or containment of radioactive waste
Chemical conversion to a stable solid for disposal
110345, 422120, 422159, G21F 900
Patent
active
057714738
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns the cleaning of gases produced by the incineration of lightly radioactive waste, especially although not exclusively the treatment of gases from an incinerator for melting and vitrifying waste, such as waste generated by the nuclear industry, hospitals and universities.
The incineration of lightly radioactive waste produces gases containing water vapor, acid pollutants such as hydrogen halogenides, solid particles, some of which are soluble, and radioactive heavy metals, all of which must be extracted from the gases before they are returned to the atmosphere.
One treatment of these gases known in itself entails cooling them by means of a heat recovery device to a temperature compatible with passing them through a filter adapted to retain the solid particles and then treating the gases from which the dust has been removed in a gas scrubbing plant to remove the acid pollutants and some of the gaseous heavy metals, before the gases are returned to the atmosphere. The gases from incinerators for melting and vitrifying waste are at a high temperature, up to 1 250.degree. C., and the heat recovery device must be especially designed and constructed from heat and corrosion resistant materials which are costly. To overcome this drawback it has been proposed to cool the gases by diluting them with air but this solution has the disadvantage of increasing the quantity of gases to be treated. Moreover, cooling the gases before they are sent to the filters causes adsorption of the radioactive heavy metals on the solid particles calling for special precautions in packaging and handling them when they have been extracted from the gases. The prior art plants thus produce a large volume of radioactive solid residues that are costly to handle and store. Moreover, the solid particles that have adsorbed the radioactive heavy metal particles contaminate all of the plant on the outlet side of the filter, and the gaseous radioactive heavy metals contaminate the gas scrubbing equipment, and when these plant items are replaced they are radioactive, calling for particular precautions in dismantling, transportation and disposal.
Publication FR-A-2 408 196 describes a waste treatment process that includes, among other steps, a step of cooling and condensing particles contained in these gases, the wet particles being separated by passing through a screen after which the gases are heated and filtered.
The present invention is intended to reduce the cost of treating the gases and to improve the performance of the gas treatment by reducing the residual levels of pollutants in the cleaned gases, and achieves this aim through a plant characterized in that it includes:
a cooler-condenser adapted to cool the gases before filtration or collection of the solid particles to a temperature below their dew point temperature to capture the radioactive heavy metals in the condensates at the same time as the acid pollutants and soluble solid particles contained in the gases,
a unit for treating the condensates to precipitate the radioactive heavy metals to recover a radioactive precipitate and an aqueous solution,
a salt crystallization unit for crystallizing salts contained in said aqueous solution and to recover water to be recycled in the plant,
a heater for increasing the temperature of the gases on leaving the cooler-condenser, and
an absolute filter for recovering the solid particles on the outlet side of the heater before the cleaned gases are returned to the atmosphere.
The solid particles conveyed by the gases are recovered by the filter after the radioactive heavy metals have been removed from the gases and are therefore not radioactive or only slightly radioactive, requiring no special precautions for handling them, and can be passed to the incinerator to be melted and vitrified. Crystallization of the treated neutralized condensate salts eliminates liquid waste and produces salts that can be used industrially and water that can be recycled into the gas circuit. The final waste resulting form the treatment of
REFERENCES:
patent: 4482479 (1984-11-01), Kamiya et al.
patent: 4666490 (1987-05-01), Drake
Patent Abstracts of Japan, vol. 13, No. 269, Jun. 21, 1989.
Touchais Dominique
Yu Dehui
Mai Ngoclan
Service Protection Environnement Ingenierie et Construction "SPE
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