Patent Number: 054240425
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to methods and devices for processing waste, especially radioactive, toxic, industrial and household waste generated from factories, nuclear power plants, hospitals, institutions and the like. More particularly, the present invention relates to the treatment of wastes from solid and liquid, including both aqueous and nonaqueous, wastes streams to form a stable, final glass waste product. 2. Discussion of Background Economic and regulatory factors require nuclear power utilities and other generators of radioactive waste to develop and evaluate new technologies and methodologies for improving safety and reducing the costs of operations. Also, reducing the volume of waste generated and improving the stability of the disposed waste is of prime interest within our increasingly environmental-conscious society. Improvements in waste processing operations and procedures can significantly reduce waste management costs, waste volume, and the concern associated with storage and permanent disposal of the final waste. Additionally, existing legislation may require all power plants to store on-site the waste each plant generates if centralized storage or disposal facilities are not available. Thus, improvements in waste processing systems should include not only significant reductions in the volume of the final waste form but also the capability of identifying and separating the waste from each generator so that waste can be returned to the generator for storage. Nuclear power plants, which produce much of the radioactive waste, generate a plurality of waste types typically broken down into three classifications: "dry active waste" (DAW), "wet waste" or ion exchange resins and "liquid waste." Dry active waste includes paper, wood, metal scraps, plastic sheeting, clothing and the like. Currently, dry active waste is processed by low and high density compaction, with compaction ratios of approximately 2:1 to 6:1, depending on its particular composition and the force exerted. Some forms of DAW are currently incinerated with a volume reduction of approximately 50:1. Wet waste includes ion exchange resins, typically in granular or powdered form. Wet waste is currently processed by dewatering (drying) to take out interstitial or free water. Usually, drying does not necessarily include removing water from the resin bead itself. Liquid waste includes organic waste (oils, chemical solutions), which is combustible, and inorganic waste such as aqueous wastes and sludges, inorganic acids, and solutions of boron, NaOH and the like. Organic wastes are currently incinerated or stabilized. Inorganic waste, mostly comprised of aqueous salt solutions, is currently being processed by demineralization, ion exchange, membrane technology and evaporation, all of which are well known in the art of waste processing. Also, aqueous waste is processed by evaporator/dryers to yield concentrated and/or dried, solid waste. Numerous processing methods are known for treating and processing waste generated from power plants and the like, including radioactive wastes generated from nuclear power plants. For example, Bardot et al, in U.S. Pat. No. 4,925,566, describe the use of ultrafiltration, hyperfiltration and demineralization for radioactive liquid elements. Also, the notion of ion exchange and the use of ion exchange resins for radioactive waste processing is well known, as described in U.S. Pat. Nos. 3,520,805, issued to Ryan, and 4,415,457, issued to Shirosaki et al. Ryan describes filtration through ion exchange resin-coated fibers. Shirosaki et al absorb ions in power plant filter backwash onto an ion exchange resin. Several U.S. patents combine additional waste processing methods with the use of ion exchange resins. These patents include U.S. Pat. Nos. 3,773,177, issued to Queiser et al, and 5,158,674, issued to Kikuchi et al. Queiser et al follow the use of ion exchange resins with filtration and drying processes. Similarly, Kikuchi et al treat radioactive liquid wastes using membranes to concentrate the wastes, filtration of oils using active silica, and then incineration of the flammable solids on the active silica. Also, in Macedo et al (U.S. Pat. No. 4,737,316), contaminated liquid is purified by passing it through an ion exchange resin then "sintering" the resin. Another procedure known for use in processing radioactive waste is vitrification, that is, the incorporation of the inorganic portion of the waste into a stable, glass matrix having radioactive elements as part of the glass structure. Vitrification has been studied for decades as a way of stabilizing high level radioactive waste, and a number of patents exist that relate thereto. However, more recently, vitrification has been used with other types of radioactive wastes. For instance, Macedo et al (U.S. Pat. No. 4,737,316) state in their specification that it is well known to form borosilicate glass from the processing of ion exchange resin and glass frit. Incineration is used to reduce the volume of radioactive waste but, because the ash produced from combusting the waste contains radioactive material, further processing of the ash is required to stabilize it. Despite the number of waste processing procedures known for use with hazardous or radioactive wastes from power plants and the like, there exists a need for an effective process system that significantly reduces the volume of all types of waste from nuclear power plants, and produces a stable, final waste product that is easily manageable for storage, transporting, disposal and the like. SUMMARY OF THE INVENTION According to its major aspects and broadly stated, the present invention is a device and method for processing all kinds of waste including toxic, industrial, household and the like, but especially the three main radioactive waste streams generated by power plants, hospitals and the like. In particular, it is a system applicable for processing all kinds of waste, especially radioactive waste, both solid and liquid forms, including vitrification to immobilize radioisotopes in a stable, final waste product. This system has several subsystems, including a feed conditioning subsystem for conditioning each type of waste, a feed preparation subsystem for blending all of the waste types, a feed melter chamber with an upper thermal zone and a lower melting zone, a glass handling subsystem for packaging and storing the final product, and an off-gas cleaning and control subsystem. The conditioning subsystem conditions the waste feed by shredding tile dry active waste, drying tile bead and powdered ion exchange resins, and concentrating the aqueous waste. The conditioned waste can then be blended in tile feed preparation subsystem to produce a waste feed having a consistent BTU value per unit mass when combusted and incorporated with glass formers into the waste if necessary. In the feed melter chamber, combustible, organic waste is oxidized and noncombustible waste and decomposition products are incorporated into the melted glass formers. The molten glass is cooled and put into a suitable container. The particulate carried in the off-gas resulting from destruction of the waste is captured in the off-gas cleaning and control subsystem and can be returned to the feed melter chamber to be incorporated into the melt or can be solidified with a blender-dryer in the feed conditioning subsystem. The first component of the feed conditioning subsystem is a feed inventory and handling conveyor and shredder for shredding the dry active wastes. Shredding the dry active wastes promotes better blending of dry active wastes with the other waste types, reduces the size to enable faster and more efficient burning and minimizes potential damage to the melter from large, heavy objects. The second component of the feed conditioning subsystem involves means for drying the resin wastes by removing the interstitial or free water from among the granular and powdered ion exchange resins. Such resin drying can be effected by a number of known methods, including the method disclosed in U.S. Pat. No. 4,952,339, which is commonly assigned. The third component of the feed conditioning subsystem involves means for concentrating aqueous wastes by separating some of the liquid from the dissolved salts and particulate suspended in it. Methods for reducing the liquid volume of aqueous wastes include sophisticated filtering systems based on membrane technology such as microfiltration and hyperfiltration (also known as reverse osmosis). Also, drying systems using a rotary blender-dryer to evaporate water, can be used for liquid volume reduction of aqueous wastes. The feed preparation subsystem mixes conditioned DAW, dried resins and concentrated aqueous waste with glass forming materials prior to feeding them into the reciter chamber. Wastes can be processed separately or as a blended feed depending upon radiation dose levels, BTU value or other criteria. Resin drying and concentrating of the aqueous wastes takes place typically at the generator's facility, but the shredding of DAW generally takes place away from the generator's facility. Accordingly, shredders can be used to assist in mixing the waste types and serve the two functions of blending and shredding. Also, depending on the particular constituency of the waste types, some of the waste can be fed directly into the melter chamber along with glass formers without blending. It is desirable to deliver a waste feed to the melter chamber that has a consistent BTU and radioactivity content when combusted to improve the operational efficiency of the melter chamber operation and ultimately produce a highly stable and more uniform final glass product. Shredders can include a low speed shear type, a high speed impact type, as well as rotary conveyor screws. The melter chamber, which has an upper thermal zone and a lower melting zone, receives the waste feed from the feed preparation subsystem. The thermal zone oxidizes the organic constituents within the waste feed in converting the organic and inorganic constituents into ash and off-gas. The melting zone uses a heated vessel to controllably melt the glass formers and combine them with the ash and the noncombustible material. The melting zone of the melter chamber is connected to the glass handling subsystem. In the preferred embodiment, the glass handling subsystem cools the molten glass received from the melter and packages it accordingly. Alternatively, the molten glass may be cooled as it moves along a conveyor that causes the molten glass to form globules (marbles) as it cools for ease in recycling glass in the future or by a fluid cooled bath prior to being packaged in the appropriate containers. The thermal zone of the melter chamber is connected to the off-gas cleaning and control subsystem. The off-gas cleaning and control subsystem captures a portion of the off-gas for recycling to the melter chamber and scrubs the remainder of the off-gas for stack emitting. An important feature of the present invention is the combination of aqueous waste streams and vitrification. Although vitrification of radioactive waste is known, the feeding of partially concentrated aqueous wastes, wastes that are less than 20% concentrated, is believed to be new. Another important feature of the present invention is the waste feed melter chamber. The melter chamber is dual purpose. Some conventional melters allow for some destruction of organics but are not designed or intended for significant destruction. The present melter is so designed in order to accommodate the large fraction of DAW and organic liquid wastes entering the melter. Furthermore, the heated vessel portion of the melter chamber is designed to be easily replaceable so that processing is not interrupted for long periods of time while the entire melter chamber is replaced. The present invention features a disconnect mechanism that detaches the heating vessel from the melting zone. The heating vessel can be quickly and easily interchanged to optimize waste processing or refractory replacement. All melter types can be adapted for use with the disconnect system, including: refractory lined, Joule heated electrode melters; induction-heated cold wall crucible melters; induction-heated warm wall crucible melters; in-can melters having induction or resistance heating; and slagging cold wall melters. Finally, unlike conventional melters, the heated vessel in the preferred embodiment of the present invention is heated by induction rather than by Joule heater electrodes. An inductively heated melter is simpler in design and easier to maintain and can heat to higher temperatures than one heated with electrodes. In addition, most melters operate in a reducing mode that can generate liquid metals that are not readily incorporated into a glass matrix. Also, a hotter oxidizing melter is better suited for waste having a high metal content because it further reduces the amount liquid metal that forms in the melt. Induction heating allows more efficient, uniform and controlled heating and melting rates than conventional electrode melter chambers. Also, direct heating is possible since the induction field directly heats the melter liner, metal and glass matrix. As a result, any undissolved metal remaining in the melt chamber can be melted or consumed by a short heatup to a higher temperature, thereby minimizing refractory temperature during normal operations and eliminating electrode loss common in Joule heated melters. Also, induction melting has inherent stirring characteristics that provide better glass homogeneity. The depth of penetration of the induction field can be increased by selection of the induction coil power supply frequency. Cold or warm wall induction heated crucibles are preferred, as glass can significantly corrode (dissolve) typical suscepting crucible materials such as silicon carbide, whereas a warm or cold wall crucible uses a solidified layer of glass against the inner wall for optimum corrosion protection. Still another feature of the present invention is the system for feeding of enriched oxygen into the melter chamber. Most melters operate in reducing mode, which can generate liquid materials that are not readily incorporated into the glass matrix. The present invention feeds an air/oxygen mixture into the melter chamber to aid in thermal oxidation and control. Also, the use of an enriched air/oxygen improves combustion kinetics, allows higher temperature operation and greatly reduces off-gas volumes. The improved oxygen atmosphere in the melter of the present invention enhances the production of glass soluble metal oxides instead of liquid metals that can cause catastrophic failure of most melters. Since the air/oxygen mixture has enriched oxygen, approximately half of this feed can be returned to the melter chamber from the off-gas if desired. The recycled gas preheats the fresh oxygen/air mixture but does not dilute the amount of oxygen available for oxidation below appropriate levels. Carbon dioxide and nitrogen in the recycle gas can be removed using conventional cyrogenic or adsorption technology to provide oxygen-enriched recycle gas. The major purpose of recycle gas is to control or moderate melter temperatures and to allow reuse of excess oxygen in the off-gas. Yet another feature is the use of off-gas cleaning and pollution control components within the waste processing system. The off-gas cleaning and pollution control components collectively remove radioactive or undesirable particulate and acid gases from off-gas resulting from combustion during waste processing. The advantage of this feature is that harmful pollutants are removed from the off-gas, thus producing cleaner stack emissions. Another feature is the use of a liquid-cooled conveyor in an alternative embodiment of the present invention. The liquid-cooled metal belt conveyor rapidly transports and cools the glass moving from the melter chamber to the storage containers. The conveyor's movement and rapid cooling cause the molten glass to form globules (marbles) that are eventually placed in the storage containers. Another feature of the present invention is the shredding device for shredding dry active waste and, when desired, mixing DAW with ion exchange resins and concentrated liquid waste. The shredder makes the DAW easier to mix and increases it combustion efficiency. Other features and advantages of the present invention will be apparent to those skilled in the art from a careful reading of the Detailed Description of a Preferred Embodiment presented below and accompanied by the drawings.