Patent Number: 052884350
Section: summary

FIELD OF THE INVENTION The present invention relates to processes and process systems for incinerating and vitrifying radioactive waste materials. Specifically, the invention relates to the incorporation of radionuclides as ash into a glass matrix and in the case of waste materials containing sulfur compounds, the additional incorporation of the sulfur into a gypsum-based matrix. BACKGROUND OF THE INVENTION The effective disposal of hazardous radioactive waste materials is a continuing problem for industry. These waste materials can take a variety of forms including organic materials, which are combustible, and inorganic materials such as metal oxides, which are not combustible. Typically these wastes include ion exchange resins used in the nuclear industry and various articles of use such as clothing, paper and wood containers, rags, etc. The ion exchange resins are typically porous beads of polystyrene, cross-linked with divinyl benzene. These resins remove such compounds as borate, chloride, carbonate, and cesium ions and other fission and corrosion products from liquid streams. Early systems employed the use of incinerators to burn waste materials. These incinerators had problems associated with ashes, produced by incomplete combustion, entrained in the off-gas. These airborne ashes thus presented an environmental hazard if not properly treated by filtering systems. Also, problems existed with high amounts of noncombustible leachable inorganics left behind in the bottom ash. Various vitrification and incineration processes were employed to resolve the problems of the incineration processes. Such systems are set forth in U.S. Pat. Nos. 5,022,329; 4,666,490; 4,376,070; 4,424,149; 4,297,304; 4,299,611; 4,139,360; 4,020,004; 3,321,409 and in an article titled "Hazardous Waste: Where to Put It? Where Will It Go?", Mechanical Engineering, Sept. 1988, pgs. 70-75. These systems describe the vitrification of the ash product into a glass material to limit the leaching of the radionuclides from the ash using melters and cyclone furnaces. The cyclone furnace design in U.S. Pat. No. 5,022,329 requires the burning of fuels in order to combust the waste material instead of initiating combustion by contacting the waste material with a molten glass pool. The glass pool design is preferred in that the glass is heated by electrodes thereby reducing the dangers associated with the fuel heating of the cyclone furnace. The glass pool designs are deficient in that they do not ensure complete combustion in the plenum area above the glass pool, thus releasing combustible gases in the off-gas stream from the melter. The neutralization and solidification of combustible sulfur compounds which are present in various waste materials is not disclosed in any of these systems. The sulfur compounds that are present in the combustion off-gases pose an environmental problem and should preferably be removed prior to gaseous emission to the atmosphere. The sulfur compounds are a major concern when treating ion exchange waste materials. Other methods of treating ion exchange waste materials is by resin dewatering and resin drying; however, these processes do not attain high levels of volume reduction and the final waste form is not solidified. A need therefore exists to design a safe, efficient system for incinerating and vitrifrying radioactive waste materials. The system should also be designed to provide for proper disposal of sulfur compounds found within the waste material. SUMMARY OF THE INVENTION The present invention relates to processes and process systems for incinerating and vitrifying hazardous radioactive waste material within a melter having an extended heated plenum area designed to ensure combustion residence times of at least 3 seconds at temperatures of at least about 980.degree. C. (1800.degree. F.). The invention is preferably suited for handling radioactive wastes that contain sulfur compounds such as spent ion exchange resins. The process encases radionuclides found within the radioactive waste material into a glass substance and also into a gypsum by-product. Any remaining radionuclides are filtered out of the process off-gases using high efficiency particulate filters to a level acceptable for atmospheric emission. The basic process of the invention is carried out by feeding the waste material along with glass forming material into a melter. It is preferred to feed these materials as an aqueous slurry to decrease the amount of feed material entrained in the gases above the glass pool. The melter has a molten glass pool in its lower portion and an extended plenum area above the glass pool. The waste material enters the melter and contacts the molten glass forming a solid ash and waste gas. The solid ash is retained and incorporated into the glass pool. The waste gas is subjected to substantially complete combustion in the plenum area by contacting the waste gas with oxygen, preferably air, at elevated temperatures of at least about 980.degree. C. (1800.degree. F.). The residence time for the waste gas in the plenum area preferably is at least about 3 seconds, most preferably at least about 5 seconds. The long residence time is advantageous in that the danger of having hazardous uncombusted compounds leaving the process is deceased due to the achievement of a higher level of combustion. When a feed containing sulfur compounds is used, the combustion of the waste gas forms an off-gas that contains sulfur compounds, preferably sulfur dioxide. This off-gas is then contacted with an aqueous solution, preferably in a scrubber, to form sulfuric acid. The sulfuric acid is then contacted with calcium hydroxide to form gypsum and water. The gypsum thus contains a portion of the sulfur compounds originally found in the feed, and a portion of the radionuclides found within a radioactive feed containing sulfur. Various preferred embodiments of the inventive process are provided herein. The process can include the passing of the scrubber off-gas over a surface to allow water particles to collect, preferably in a mist eliminator. This process step helps remove suspended water particles from the scrubber off-gas in preparation for further filtering operations. The aqueous fluid from this step is preferably returned to the process at the scrubber. A further purification of the gaseous stream is accomplished by passing the mist eliminator off-gas through a filter system designed to remove at least about 98% by weight, and preferably at least about 99%, of all particles above about 0.6 microns. The water formed from the gypsum reaction can be advantageously recycled to be added to the feed materials. This embodiment creates a substantially closed loop system for the process water thereby limiting radioactive waste water disposal.