Patent Number: 059616792
Section: description

DETAILED DESCRIPTION OF THE INVENTION The first set of steps, which converts waste feeds into a B.sub.2 O.sub.3 fusion melt inside a glass melter, can be operated as a batch, semibatch, or continuous process. The initial condition for the process is a glass melter filled with a special molten oxidation-dissolution (lead borate) glass, which preferably has a composition of two or more moles of lead oxide (PbO) per mole of B.sub.2 O.sub.3. The B.sub.2 O.sub.3 fusion-melt operations have three steps: (1) feed oxidation, dehalogenation, and oxide dissolution; (2) PbO removal; and (3) lead oxidation. These operations can be carried out sequentially in either a single vessel or in separate process vessels. The process is best described with reference to FIG. 1, which shows a preferred embodiment of the invention. A. Oxidation, Dehalogenation, and Oxide Dissolution of Feed Material Lead oxide and boron oxide are added to the melter to form a dissolution glass. Nuclear waste feeds are added directly to the melter. The ceramic and amorphous components in the feed that are exposed to the molten glass rapidly dissolve into the glass. Molten glasses will generally dissolve most oxides, but the glasses do not dissolve metals or organic material (organics). To dissolve these latter components into the glass, metals and organics must first be oxidized. The PbO in the glass is a strong oxidizing agent and oxidation occurs in situ within the glass melter. If the feed contains organics, the organics are oxidized to CO.sub.2, possibly CO, and steam (H.sub.2 O), and the by-product lead metal sinks to the bottom of the melter. The CO.sub.2, CO and steam exit the melter via the off-gas system. Metals (excluding the noble metals) are oxidized by the PbO in the glass to metal oxides and, subsequently, dissolve into the glass. The lead by-product then sinks to the bottom of the melter. Typical chemical reactions are: 2Al+3PbO.fwdarw.Al.sub.2 O.sub.3 +3Pb.arrow-down dbl. PA1 Pu+2PbO.fwdarw.PuO.sub.2 +2Pb.arrow-down dbl. PA1 C+2PbO.fwdarw.CO.sub.2 .arrow-down dbl.+2Pb.arrow-down dbl. PA1 Zr+2PbO.fwdarw.ZrO.sub.2 +2Pb.arrow-down dbl. PA1 (1) The borate fusion melt is highly soluble in acid. As the B.sub.2 O.sub.3 matrix dissolves, oxides that are soluble in nitric acid dissolve. PA1 (2) The lead-borate oxidation step destroys troubling organics and converts metals to oxides. Thus, the lead-borate processing avoids the need to use some of the nitric acid to oxidize the incoming feed materials to produce oxidized materials that are soluble in the nitric acid. For example, uranium must be fully oxidized to the +6 valence state to be highly soluble in nitric acid. Because oxidation of feeds with nitric acid usually generates large quantities of nitrogen oxides as a by-product, the pretreatment provided by the invention also reduces the size and complexity of the dissolver off-gas system. PA1 (3) The borate fusion melt process further reduces the amount of gas generated by the dissolver off-gas system because volatile materials that would have been released in the acid dissolver are released earlier during the lead-borate dissolution process. When processing spent nuclear fuels (SNF), these volatile materials include tritiated water, xenon, and krypton. PA1 (4) The borate fusion melt dehalogenation step eliminates troublesome halogens. These can interfere with separations and complicate engineering. Halogens mixed with nitric acid are highly corrosive and thus create major problems in terms of equipment corrosion. PA1 (1) The process recycles PbO and excess B.sub.2 O.sub.3 within the process. This feature minimizes final waste volumes and waste quantities. PA1 (2) The process converts some metal components in some feeds into inert, nitric-acid-washed oxides with minimum volumes and mass that are acceptable waste forms. Separation into a clean oxide minimizes the total volume and mass of this waste. The dissolution glass also oxidizes sulfur-containing components to sulfur oxides that exit via the off-gas system. It is to be understood that the dissolution glass of the invention oxidizes everything in the molten mixture except the noble metals. Rapid oxidation and dissolution are the results of the special characteristics of the PbO:B.sub.2 O.sub.3 dissolution glass. At operating temperatures (700-900.degree. C.), the PbO is a powerful oxidizer. However, some metals and other materials form protective oxide coatings. The B.sub.2 O.sub.3 is an effective dissolution agent for oxides. It is used in many welding fluxes and analytical procedures for rapid dissolution of oxides. The combination of the PbO and B.sub.2 O.sub.3 creates the oxidation-dissolution capabilities of this molten glass. The 2PbO:B.sub.2 O.sub.3 glass composition is chosen to maximize chemical reaction rates and maximize solubility of oxides in the melt. B. Separation of Halogens from Feed Materials in the Molten Dissolution Glass The process separates halogens within the feed during feed dissolution. Using as an example a feed containing chlorides, in the dissolution glass, chlorides in the feed will react with the PbO and form lead chlorides (PbCl.sub.2), which are volatile gases at glass melter temperatures and exit to the aqueous sodium hydroxide (NaOH) scrubber. In the scrubber, the PbCl.sub.2 reacts with the NaOH to yield insoluble lead hydroxide Pb(OH).sub.2 ! and soluble NaCl salt. The insoluble Pb(OH).sub.2 is recycled back to the melter, wherein it decomposes to PbO and steam. The aqueous NaCl stream is cleaned and discharged as a chemical waste. Other halogen-containing feeds behave similarly. The PbO should be present in at least a stoichiometric amount with respect to the halogens to achieve adequate removal of the halogens. C. Removal of Noble Metals from the Molten Dissolution Glass The noble metals are not oxidized by the PbO. During feed dissolution, the noble metals separate from the glass and dissolve into the lead metal. Noble metals are not soluble in glass but they are highly soluble in lead metal. The noble metals sink to the bottom of the melter in the lead. The noble metals can be separated from the lead by vacuum distillation of the lead or by several other demonstrated processes. Significant quantities of noble metals are found in some lead ores in which the noble metals remain with the lead metal during smelting operations. Consequently, multiple processes for noble metal separation from lead have been developed and deployed. D. Conversion of Molten Dissolution Glass to Borate Fusion Melt Carbon is added to the dissolution glass. This may be done in the same melter, or the xPbO:B.sub.2 O.sub.3 fusion melt (devoid of halogens) may be removed to a separate melter where carbon is then added. Carbon reduces the PbO to lead metal while gaseous CO.sub.2 is produced. All of the PbO is removed from the dissolution glass to produce a B.sub.2 O.sub.3 fusion melt, comprising metal oxides dissolved in B.sub.2 O.sub.3. During this step, it may be necessary to supply additional B.sub.2 O.sub.3, depending upon the feed material, to keep all materials in solution. The solubility limits of certain elements in the B.sub.2 O.sub.3 --PbO dissolution glass may be higher than in just the B.sub.2 O.sub.3 without the PbO. E. Reoxidation of the lead to PbO by Addition of Oxygen Lead is an oxygen carrier in the dissolution process. Oxygen is injected into the molten lead recovered from the lead-borate dissolution step and recovered from the conversion of the dissolution glass to a B.sub.2 O.sub.3 -fusion melt, as can be seen in FIG. 1. Lead is oxidized to PbO. The oxidation reaction is: EQU 2 Pb+O.sub.2 .fwdarw.2PbO The PbO is recycled and used to make the next batch of lead-borate dissolution glass. The option exists to oxidize the lead in the melter by adding O.sub.2 to the melter after removing the B.sub.2 O.sub.3 fusion melt. Because of the corrosive characteristics of the initial dissolution glass during the conversion of feeds to a B.sub.2 O.sub.3 fusion melt, these steps in the process are best carried out in a cold-wall melter in which cooling jackets in the walls produce a "skull" of solidified material that protects the walls from the contents of the melter. The melter(s) can be heated by fossil, induction, plasma arc, or electron-beam systems. Such systems are currently used to melt high-temperature materials (e.g., titanium and superalloys) and produce specialty glasses. F. Removal of B.sub.2 O.sub.3 Fusion Melt The resultant B.sub.2 O.sub.3 fusion melt is poured from the furnace and preferably allowed to solidify before the glassy B.sub.2 O.sub.3 solid is fed to the separations step. Formation of crystalline compounds during solidification is to be avoided because of their slower dissolution rates in nitric acid. The solubility of various oxides in B.sub.2 O.sub.3 -fusion melts is strongly dependent upon the temperature of the melt. With rapid cooling of the melt, higher loadings of oxides can remain dissolved in the B.sub.2 O.sub.3 while forming a solid glassy B.sub.2 O.sub.3 structure. This approach minimizes the B.sub.2 O.sub.3 in the solid and reduces the volume of feed sent to the separations step. With current technology used in research reactor fuel fabrication, the option exists for rapid cooling (up to 10.sup.6 K/sec) and atomization of melts with uniform particles with sizes as small as 50 to 100 microns. G. Recovery of Uranium, Plutonium and Rare Earth Elements Processing of the radioactive waste feed material into a B.sub.2 O.sub.3 fusion melt creates a solid, oxide feed that is optimized for recovering uranium, plutonium, and other elements when using acid-based separation processes such as PUREX and ion exchange. In the process of the invention, the boron oxide fusion melt is solidified, and then dissolved in nitric acid. Prior to the dissolving, boron oxide may be recovered from the fusion melt and recycled back to the glass melter to go into the dissolution glass/waste mixture. After dissolution of the fusion melt in nitric acid, rare earth elements, U, and Pu are recovered from the acid solution by one of several processes such as PUREX or ion exchange. H. Vitrification and Recycle of Boron Oxide The nitric acid-boric acid waste stream resulting after the separation of U, Pu, and rare earths is converted to a waste glass, e.g., borosilicate waste glass, using the traditional vitrification processes. The waste stream is fed to a glass melter simultaneously with glass frit (primarily SiO.sub.2). The nitrates are decomposed to oxides and then converted to glass. This is the standard industrial process for conversion of nitric acid wastes into a high quality waste glass. In some cases, there may be excess B.sub.2 O.sub.3 in the nitric acid-boric acid waste stream. In that event there are three options: 1) Direct Conversion to Glass The waste can be converted to glass using added glass frit to dilute the excess B.sub.2 O.sub.3 in the waste stream. 2) Mixing with Other Wastes The waste steam can be fed to a glass melter along with other waste streams. The nitric acid stream from the acid-borate dissolution step provides the necessary B.sub.2 O.sub.3 to make borosilicate glass for both waste streams. In contrast, the traditional nitric acid separation processes creates waste streams with no B.sub.2 O.sub.3 ; hence, B.sub.2 O.sub.3 must be added to these waste streams when they are being converted to borosilicate glass. In the United States, there are large facilities to convert nitrate wastes in storage (primarily high-level radioactive wastes) to glass for disposal. These facilities are likely sites for deployment of this invention to process miscellaneous wastes. At such sites, the quantities of nitric acid wastes from processing miscellaneous wastes would be small compared to nitrate wastes that are currently being converted to glass. The B.sub.2 O.sub.3 -rich nitric acid wastes could be simultaneously converted to glass along with existing wastes, and the B.sub.2 O.sub.3 -rich acid waste could provide some of the needed B.sub.2 O.sub.3 for the glass-conversion step. 3) Separation of B.sub.2 O.sub.3 From Waste Stream The B.sub.2 O.sub.3 can be separated from the acid waste stream, after removal of the U, Pu and/or rare earths, and recycled back to the front of the process. There are several options for separation of B.sub.2 O.sub.3 depending upon the purity desired for the B.sub.2 O.sub.3. The commercial borate industry has various separation techniques. In addition, borates are used in pressurized water reactors as a soluble neutron absorber. Multiple technologies have been developed to recover borates from the reactor aqueous coolant. System Configuration and Equipment The B.sub.2 O.sub.3 fusion-melt process steps can be configured as batch, semibatch, or continuous operation. The preferred option will depend upon the scale of operation and other factors. In a batch operation all of the major steps (except off-gas processing) are performed in a single vessel in a sequence of four steps over a period of time. At the start of the process, B.sub.2 O.sub.3 and PbO are added to the melter to form a dissolution glass. As waste feed is added to the melter, feed oxidation, dehalogenation, and oxide dissolution simultaneously occur in the molten mixture with buildup of lead metal at the bottom of the melter. After feed dissolution, carbon is added for conversion of the dissolution glass to a B.sub.2 O.sub.3 fusion melt. The B.sub.2 O.sub.3 fusion melt is poured from the melter and the molten lead metal is left in the bottom of the melter. The solidified B.sub.2 O.sub.3 fusion melt is sent to the separations process. A new batch of dissolution glass is made in the melter by oxidizing the lead metal with O.sub.2 and adding B.sub.2 O.sub.3 to the melter. The cycle is then repeated. There is off-line recovery of any noble metals that build up in the lead over time. In a semibatch or continuous operation, the lead metal is drained from the melter as it is produced and it is reoxidized off-line. FIG. 2 shows a schematic drawing of a vessel used for a continuous process. There are also continuous process options for large-scale operations. The separations and vitrification steps use existing equipment designs. The B.sub.2 O.sub.3 fusion melt step is preferably carried out in a cold-wall melter because of the corrosive characteristics of the initial dissolution glass. The dissolution glass will dissolve all materials except noble metals and the molten lead will dissolve noble metals. Cold-wall melters have cooling jackets in the wall to produce a "skull" of solidified material that protects the wall from the melter contents. Cold-wall melters are used industrially to melt high-temperature materials (e.g., titanium and superalloys) and to produce ultrapure materials (e.g., glass for fiber optics). Russia, France, and the United States are modifying such equipment for processing various radioactive wastes. Batch size may be as large as hundreds of kilograms for miscellaneous fissile materials (MFMs) with low fissile material concentrations. In Europe, cold-wall melters are currently being developed for throughputs of up to 800 kg/h. There are multiple heating methods available, known to those of skill in the art. The process of the invention produces a boron oxide fusion melt which provides a superior feed material to be used in an aqueous separations process, particularly to recover U, Pu, and rare earths from radioactive waste, industrial or other wastes. Some of the advantages of the process are: Further advantages include the minimization of waste generation. Some of the features which accomplish this are: Additionally, the process of the invention has the capability to recover key elements from the waste or convert the waste directly into borosilicate glass. The initial process steps produce a lead-borate dissolution glass. From this dissolution glass, a boron oxide fusion melt is produced that, in turn, allows recovery of valuable elements. Alternatively, the lead-borate dissolution glass can be turned into a borosilicate waste glass for direct disposal of the material as shown in FIG. 1 (the alternative end point). For some wastes, it will not be clear whether recovery of selected elements is required for waste management and/or is economically viable. Some feeds are complex, heterogeneous mixtures that are difficult and expensive to analyze. After such feeds are converted to a homogeneous lead-borate dissolution glass, simple analytical tests can determine the concentration of valuable elements in the glass. At such time, a decision can be made as to whether recovery of valuable elements is economically worthwhile. While preferred embodiments of the present invention have been illustrated and described, it will be understood that changes and modifications can be made therein without departing from the invention in broader aspects. Various features of the invention are defined in the following claims.