Patent Number: 059603682
Section: description

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS In one embodiment of the present invention, shown in FIG. 1, radioactive, hazardous, or mixed waste feedstocks 1 containing organic carbon compounds are fed to oxidation vessel 2. Nitric acid 3 and phosphoric acid 4 are added to the oxidation vessel 2. Air 5 is not necessary for the operation of the process, but may be optionally pumped in to aid in the oxidation process (in particular, to aid in the recycling of nitric acid) if desired. Heat 6 is added and/or removed as needed to maintain an appropriate oxidation reaction rate. As oxidation of the organic materials occurs in the oxidation vessel, off gases 7 such as carbon monoxide, carbon dioxide, water, HCl, and nitrogen oxides are generated. The nitrogen oxides are optionally converted into nitric acid in nitric acid recovery unit 8. The residual concentrated waste product 9 comprising substantially all of said radioactive or hazardous metal components of the waste feedstock can then be removed from the oxidation vessel and vitrified or ceramified (e.g., by combining with a vitrifying or ceramifying substance or other solidification feed 10) in a melter or other vessel 11 and processed into a final, stable form 12 suitable for disposal in a repository. The present invention is applicable to a wide variety of radioactive, hazardous, and mixed waste starting materials, but is particularly suitable for treatment of low level radioactive and mixed waste containing organic carbon components. Radioactive waste contains at least one radioactive element, such as U, Th, Cs, Sr, Am, Co, Pu, or any other element that is defined in the waste storage or waste disposal art as radioactive. Hazardous waste contains at least one Resource Conservation and Recovery Act (RCRA) listed hazardous material, such as the metals As, Cd, Cr, Hg, Pb, Se, Ag, Zn, and Ni, or a hazardous organic compound. Mixed waste contains both radioactive and hazardous waste components. These radioactive or hazardous materials may contain these elements in the form of metals, ions, oxides, or other compounds, such as organic compounds. Low level waste generally involves a large quantity of waste material and a small amount of radioactive components contaminating the waste material. The non-radioactive, non-hazardous components of the waste are generally organic carbon-containing compounds, and make up the predominant proportion of the waste. The organic carbon components which are oxidized by the process of the present invention are present in the waste as any of a variety of organic compounds. Nonlimiting examples include neoprene, cellulose, EDTA, tributylphosphate, polyethylene, polypropylene, polyvinylchloride, polystyrene, oils, resins, particularly ion exchange resins, and mixtures thereof. The radioactive, hazardous, and mixed waste materials to which the process of the present invention is applied arise from a variety of sources. One source of such waste is job control waste from, e.g., fuel fabrication operations, nuclear power plant maintenance and operations, and hospital, medical, and research operations. This job control waste includes items such as used rubber gloves, paper, rags, glassware, brushes, and various plastics. These items often come into contact with radioactive and/or hazardous material. Although only small quantities of radioactive and/or hazardous material may adhere thereto, large volumes of this material must be disposed of as radioactive or hazardous waste. Another source of radioactive, hazardous, or mixed organic carbon-containing waste is spent organic ion exchange resins used to purify water in fuel fabrication plants, nuclear reactors, and reprocessing plants. These resins are used for the continuous cleaning of water in cooling circuits, as well as the water in nuclear fuel storage basins, where the resins remove ionic corrosion products which have become radioactive when they pass near the reactor core, and fission products of reactor fuel, such as cesium and strontium ions, that have leaked out of the fuel and into the storage basin water. The resins are typically granulated or sulfonated crosslinked divinylbenzenes. Yet another source of radioactive, hazardous, or mixed organic carbon-containing waste suitable for the process of the present invention is the aqueous streams used to clean cooling systems in nuclear power plants. These cleaning streams typically contain EDTA and other organic chelating agents to help remove corrosion from the interior surfaces of piping and other process equipment used to provide reactor cooling water in secondary reactor cooling systems. These cleaning streams typically contain iron, cesium, nickel, chromium, and other stainless steel corrosion and erosion products, some of which have become radioactive due to proximity to the reactor core. Cleaning streams containing EDTA typically exit the cooling system containing iron as the primary metal component. In a nonlimiting example, a suitable waste feedstock material would include solid Pu-contaminated waste of which 60% is combustible, and including, e.g., a mixture of 14% cellulose, 3% rubber, 64% plastic, 9% absorbed oil, 4% resins and sludges, and 6% miscellaneous organics. In one embodiment of the invention, the nitric acid and phosphoric acid are combined together in varying concentrations prior to introduction to the oxidation vessel. In this case, nitric acid, usually added in a concentration of about 0.25 to 1.5 M, is used in a concentrated phosphoric acid media as the main oxidant. In the resulting mixture, nitric acid is generally present in amounts of about 3% to about 7% by weight, phosphoric acid is present in amounts of about 90% by weight, and the balance (typically a few % by weight) is water. Molar quantities of nitric acid may generally be in the range of about 0.03 to about 2.0, and molar quantities of phosphoric acid may generally be in the range of about 12.8 to about 14.77 moles. The large quantity of phosphoric acid retains the nitric acid in the solution well above its boiling point (i.e., the boiling point of concentrated nitric acid), thereby allowing temperatures of up to 200.degree. C. to be used for the oxidation reaction, and is relatively noncorrosive to most types of stainless steel process equipment at room temperature. The temperature of the oxidation reaction may be varied depending on the particular composition of the waste feedstock material. In general, the oxidation reaction is carried out at a temperature of from about 140.degree. C. to about 210.degree. C., more particularly about 160.degree. C. to about 180.degree. C. Most organic compounds can be quantitatively oxidized at temperatures below about 175.degree. C. and pressures below about 5 psig. However, some long chain, saturated hydrocarbyl or halohydrocarbyl compounds like polyethylene, polypropylene, and/or polyvinylchloride, require a contacting temperature in the range of about 185.degree. C. to about 190.degree. C., and a pressure in the range of about 10 to about 15 psig. Organic compounds such as neoprene, cellulose, EDTA, tributylphosphate, and nitromethane have been quantitatively oxidized at temperatures below 180.degree. C. at atmospheric pressure. The concentration of acids and the temperature of oxidation can be varied to obtain reaction rates wherein most organic materials are completely oxidized in under about 1 hour. In general, oxygenated organic materials in the waste feedstock are more easily oxidized than hydrocarbons. While not wishing to be bound be any theory, it is believed that the decomposition of the organic components of the waste material feedstock proceeds by direct oxidation by nitric acid, which is energetically favorable, but very slow due to the difficulties in breaking the carbon-hydrogen bond. It is believed that the oxidation of the organic compounds in the waste feedstock is initiated by dissolved NO.sub.2 and NO radicals in solution. For many types of oxygenated organic compounds, the attack by NO.sub.2 radical can be first order, as shown below. ##STR1## For aliphatic compounds, higher concentrations of NO.sub.2 and NO radicals are needed to obtain comparable oxidation rates. ##STR2## The organic radicals generated are oxidized or nitrated by the various species in solution, according to the following reactions. ##STR3## In some of the reactions, the oxidants and/or catalysts NO.sub.2 .cndot. and NO .cndot. are regenerated. Nitration is a major source of oxidation because radical-radical reactions are relatively fast. In water where strong mineral acids are still abundant, such as 14.8 M (85%) H.sub.3 PO.sub.4, hydrolysis occurs producing an organic carboxylic acid from the nitration products according to the reaction below. EQU RCH.sub.2 NO.sub.2 +H.sub.2 O+H.sub.3 PO.sub.4 .fwdarw.RCO.sub.2 H+H.sub.2 NOH.cndot.H.sub.3 PO.sub.4 .DELTA.H .congruent.-44 In process for producing nitrated organic explosive materials, it is known that water can interfere with nitration of the organic species by nitric acid. In such processes, sulfuric acid is often added to the system to tie up water and keep it from interfering in the nitration reaction. Conversely, in the present process, if the reaction solution is allowed to become sufficiently depleted of water, the phosphoric acid might possibly mimic the activity of sulfuric acid, and prevent the remaining water from denitrating the explosive organic species. If this were to occur, nitrated organic species concentration may build up and possibly cause an explosion hazard. This hazard can be reduced by maintaining sufficient water in the system to denitrate any nitrated organic species. Based upon what is known about sulfuric acid and nitric acid, and based upon past experience with the phosphoric acid and nitric acid system of the present invention, it is believed that any explosion hazard can be minimized by maintaining a maximum temperature of 185 to 190.degree. C. Nitromethane was found to be completely oxidized (101.+-.2%) in a 0.1 M HNO.sub.3 /14.8 M H.sub.3 PO.sub.4 solution, when the water content was maintained during the oxidation. Above 130-150.degree. C., any formed organic hydroperoxides should decompose. In fact, complete oxidation of the organic material usually does not occur until these temperatures are reached possibly due to the formation of the relatively stable organic hydroperoxides. Once carbon chain substitutions begin, hydrogen-carbon bonds on carbon atoms which are also bonded to oxygen are also weakened. As the organic molecules gain more oxygen atoms, they become increasingly soluble in the nitric-phosphoric acid solution. Once in solution, these molecules are quickly oxidized to CO.sub.2, CO, and water. If the original organic compound contains chlorine, hydrochloric acid will also be formed. Relative oxidation rates for various organic compounds in the waste starting material are given below in Table 1. "Fast" oxidation rates denote complete oxidation in less than one hour. "Moderate" oxidation rates denote complete oxidation in 1-3 hours. "Slow" oxidation rates denote complete oxidation in over three hours. TABLE 1 ______________________________________ PRESSURE COMPOUND RELATIVE RATE TEMP. (.degree. C.) (psig) ______________________________________ Neoprene Moderate 165 0 Cellulose Fast 148 0 EDTA Fast 140 0 Tributylphosphate Fast 161 0 Resins Slow 140 0 PE/PP/PVC Slow 161-170 0 PE Moderate 185-190 0 PE Fast 200-205 10-15 PVC Moderate 200-205 10-15 Benzoic Acid Fast 190 0 Nitromethane Fast 155 0 ______________________________________ Typical throughputs for various waste starting materials (at the specified temperature and pressure conditions) are: EDTA (140.degree. C., 0-5 psig) 142 g/L-hr; Cellulose (150.degree. C., 0-5 psig) 90 g/L-hr; Polystyrene resin (175.degree. C., 5-10 psig) 65 g/L-hr; Neoprene (165.degree. C., 0-5 psig) 50 g/L-hr; and Polyethylene (200.degree. C., 10-15 psig) 35 g/L-hr. Since oxidation of plastics is typically slower than the oxidation of other organic materials in a waste feedstock stream, and since plastics often form the predominant component of the waste feedstock stream, plastics oxidation is often the rate limiting step in the processing of waste feedstock streams. In one embodiment of the invention, a catalytically effective amount (e.g., 0.001 M) of Pd(II) or other catalyst is added to the oxidation mixture to reduce the proportion of carbon based off gases that is carbon monoxide. This procedure can result in reduction of CO generation to near 1% of released carbon gases. It is often desirable to recapture nitrogen oxides and convert them back into nitric acid for recycle to the oxidation process, both from a reagent cost standpoint and a pollution reduction standpoint. This can be done using commercially available acid recovery units, and recovery can be improved by introducing air into the oxidation reaction vessel. Air is typically added in amounts that will provide 1-2 moles of O.sub.2 per mole of NO gas produced by the process. Once oxidation is complete and off gases have been removed, the remaining radioactive or hazardous metal components are concentrated in a residual concentrated waste product, which is then removed from the oxidation vessel and placed into a final form where it is immobilized and suitable for long term storage in a suitable repository. Several processes for immobilizing the residual concentrated waste product may be used, including vitrification and ceramification. When vitrification is used, the residual concentrated waste product is introduced into a melter, which may be heated by induction or other methods. The residual concentrated waste may optionally be combined with an additive (such as ferric oxide). The composition of the glass may be varied depending on the composition of the residual concentrated waste product, but typically will involve adding ferric oxide to form an iron phosphate glass. Typically, iron phosphate glasses are processed using ceramic (e.g., silica, alumina, or mullite) or platinum group metal containers. Glasses produced according to the present invention should contain no less than about 20% Fe.sub.2 O.sub.3 by weight. Fabrication is difficult if the iron content exceeds 45% (by weight as Fe.sub.2 O.sub.3). Approximately 4-8% by weight of alkali oxide and about 2-4% by weight of alkaline earth metal oxide is desirably used to help ensure waste solubility. The balance of the system is phosphorus pentoxide P.sub.2 O.sub.5), and the total P.sub.2 O.sub.5 content should not be less than about 50% by weight. All percentages are based upon the final glass composition. The phosphate glasses are typically melted at temperatures between about 1050.degree. C. and about 1300.degree. C., more particularly between about 1080.degree. C. and 1200.degree. C. If the melt is stirred, a typical residence time of less than about 1 hour is used. A static melt typically remains in the melter for a residence time of between about 1 and 4 hours. For example, spent cationic and anionic exchange resins (e.g., sulfonated divinylbenzene polymer, quaternary amine divinylbenzene polymer, or resorcinol resins) suitable for use in purifying water in nuclear facilities can be oxidized according to the present invention by dissolving the resin in the mixed acid oxidizing solution, and the resulting reduced volume product immobilized as a homogeneous glass by adding glass forming additives including 25% by weight of Fe.sub.2 O.sub.3, 15% by weight Na.sub.2 HPO.sub.4 .cndot.7H.sub.2 O, and 3% by weight of BaCl.sub.2 .cndot.2H.sub.2 O at a melt temperature of 1150.degree. C., to yield a glass which provides a two fold volume reduction. The residual concentrated waste product may also be immobilized in the form of a ceramic, such as magnesium phosphate or ferric phosphate ceramic. These ceramics are formed by acid-base reactions between inorganic oxides and the phosphoric acid solution exiting the oxidation vessel. Phosphate ceramics have low temperature setting characteristics, good strength, and low porosity, and can be produced from readily available starting materials. For instance, a magnesium phosphate ceramic can be made by combining calcined MgO with the phosphoric acid residual waste solution from the oxidation vessel with thorough mixing. The reaction between the acid mixture and the MgO is slightly exothermic, but cooling of the reaction vessel is generally not required. The resulting slurry is poured into a mold and allowed to set. Magnesium phosphate ceramics allow for a relatively high waste loading and a chemically stable, high strength final form. As a nonlimiting example, a magnesium phosphate ceramic may be formed from a mixture of about 33.5 wt % H.sub.3 PO.sub.4, about 16.5 wt % H.sub.2 O, about 42.5 wt % MgO, and about 7.5 wt % H.sub.3 BO.sub.3, where the percentages are based upon the final magnesium phosphate ceramic composition. Since the residual waste solution typically may contain 50-70 wt % H.sub.3 PO.sub.4 (based upon the residual waste solution), the amounts of water, magnesium oxide, and boric acid may be suitably adjusted to approximate the above composition. It should be understood that the particular composition of the magnesium phosphate ceramic is not critical to the invention, and variations from the above composition are within the scope of the invention. EXAMPLES The following Examples 1 through 7 were conducted using the following procedures: A glass reaction vessel was charged with a mixture of nitric acid and phosphoric acid. Palladium catalyst was also added to help convert CO to CO.sub.2. TEFLON fittings and VITON o-rings were used to help create gas seals. The system temperature and pressure were measured using standard methods. Example 1 2.15 grams of disodium EDTA was added to 34 mL of mixed nitric and phosphoric acid containing 1.5 molar HNO.sub.3 and 13.3 molar H.sub.3 PO.sub.4 at 140.degree. C. and atmospheric pressure. Completion of oxidation was measured by converting any carbon monoxide produced to carbon dioxide and monitoring the total amount of carbon dioxide produced and comparing this amount to the theoretical yield of carbon dioxide based upon the amount of EDTA added to the reaction mixture. Complete oxidation of the organic materials occurred in less than one hour. Example 2 A solution of 480 mL of disodium EDTA (16.6% by weight) and Fe.sub.2 O.sub.3 (4.1% by weight) in water (79.3% by weight) was gradually added to 100 mL of mixed nitric and phosphoric acid, the nitric acid concentration of which varied between 0.25 molar and 1 molar, and phosphoric acid concentration of which varied between 14.55 molar and 13.8 molar, at 165.degree. C. and atmospheric pressure. After 8 hours, the resulting residual concentrated waste solution was heated at 200.degree. C. to form 60 mL of iron phosphate ceramic. Example 3 1.01 grams of cellulose was added to 32 mL of mixed nitric and phosphoric acid having a concentration of 1.5 molar HNO.sub.3 and 13.3 molar H.sub.3 PO.sub.4 at 155.degree. C. and 0-2 psig. Complete oxidation of the organic components occurred in less than one hour. Example 4 0.12 grams of polyethylene was added to 25 mL of mixed nitric and phosphoric acid having a concentration of 0.25 molar HNO.sub.3 and 14.55 molar H.sub.3 PO.sub.4 at 200.degree. C. and 10-15 psig. Complete oxidation of the organic components occurred in less than two hours. Example 5 0.15 grams of polyvinylchloride was added to 25 mL of mixed nitric and phosphoric acid having a concentration of 0.25 molar HNO.sub.3 and 14.55 H.sub.3 PO.sub.4 at 190.degree. C. and 10-15 psig. Complete oxidation of the organic components occurred in approximately two hours. Example 6 4.01 grams of divinylbenzene ion exchange resin was added to 200 mL of mixed nitric and phosphoric acid having a concentration of 1 molar HNO.sub.3 and 13.8 molar H.sub.3 PO.sub.4 at 175.degree. C. and 5-10 psig. Complete oxidation of the organic components occurred in less than two hours. Example 7 360 mL of radioactively contaminated ion exchange resin was gradually added to 100 mL of mixed nitric and phosphoric acid whose concentration varied between 0.25 molar and 1.0 molar HNO.sub.3 and 14.55 molar and 13.8 molar H.sub.3 PO.sub.4. The resulting residual concentrated waste solution was then combined with ferric oxide (30% by weight), NaCO.sub.3 (5% by weight), Na.sub.2 O (5% by weight), BaO (2% by weight) and P.sub.2 O.sub.5 (balance) and heated to 1150.degree. C. for about 1.5 hours to form 60 mL of iron phosphate glass. Example 8 Approximately 120 mL of spent resin used in the cleaning basin water from the reactor facilities at Savannah River Site were dissolved in 100 mL of the mixed acid solution of Example 7. Analyses of the resin solution indicated that it contained the species shown below in Table 2 TABLE 2 ______________________________________ SPECIES CONTENT ______________________________________ Al 130 ppm B 11.1 ppm Ca 451 ppm Cd 2.7 ppm Cr 9.3 ppm Cu 6.7 ppm Fe 191 ppm Mg 31 ppm Na 6582 ppm Ni 22.4 ppm P 174,260 ppm Si &lt;2.7 ppm Zn 16.5 ppm Cl.sup.- 1776 ppm F.sup.- 274 ppm NO.sub.3.sup.- 27,236 ppm PO.sub.4.sup.3- &lt;1000 ppm SO.sub.4.sup.2- 15,865 ppm alpha 9.4 * 10.sup.4 dpm/mL Beta/Tritium 3.1 * 10.sup.5 dpm/mL Cs-137 6.29 * 10.sup.-2 .mu.Ci/mL Tritium 2.31 * 10.sup.-2 .mu.Ci/mL ______________________________________ The resulting oxidation solution was mixed with glass forming additives BaCl.sub.2 .cndot.2H.sub.2 O, Fe.sub.2 O.sub.3, and Na.sub.2 BPO.sub.4 .cndot.H.sub.2 O and heated to 1150.degree. C. at a rate of approximately 5.degree. C./minute, and melted at 1150.degree. C. for 4 hours to form a homogeneous black glass having the composition set forth below in Table 3. TABLE 3 ______________________________________ OXIDE AMOUNT (WT %) ______________________________________ Al.sub.2 O.sub.3 2.649 B.sub.2 O.sub.3 0.013 BaO 2.796 CaO 0.262 Cr.sub.2 O.sub.3 0.162 Fe.sub.2 O.sub.3 34.007 La.sub.2 O.sub.3 0.023 Na.sub.2 O 0.233 Nd.sub.2 O.sub.3 0.142 NiO 0.066 P.sub.2 O.sub.5 58.383 PbO 0.173 SiO.sub.2 0.199 SrO 0.007 Total 99.116 ______________________________________ A gamma PHA of this glass indicated a Cs-137 content of 4.22*10.sup.-2 .mu.Ci/g, or a total of 1.181 .mu.Ci. Based on the analyses of the spent resin, indicating that 6.29*10.sup.-2 .mu.Ci/mL or a total of 1.037 .mu.tCi of Cs-137 were present in the solution stabilized in the glass, Cs-137 was retained in the glass. Standard PCT leaching tests were performed on the glass, resulting in an average measured release of 0.031 g/L P, 0.002 g/L Ba, 3.104 g/L Na, and 0.000 g/L Fe, at a measured leachate pH of 6.00. These values are much lower than the EA accepted value for HLW borosilicate glass. A TCLP extraction using a modified EPA protocol was performed to determine the amount of RCRA metal leaching. The modification consisted of using ground glass, approximately 150 .mu.m, instead of the specified &lt;1 cm glass specimen size, and was made due to the small amount of glass produced and the conservative results that would be obtained by using a large leaching surface area. Results indicated that Ba was the only metal to leach in an amount (1.049 ppm) above the analytical detection limits. This amount is much lower than any of the EPA allowable limits. It will be apparent to those skilled in the art that many changes and substitutions can be made to the specific embodiments disclosed herein without departing from the spirit and scope of the present invention as set forth in the claims.