Patent Number: 046541730
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A number of large cations, including Tl.sup.+, AG.sup.+, Ca.sup.+, nitron [4,5-dihydro-2,4-diphenyl-5-(phenylimino)-1,2,4-triazole] and (C.sub.6 H.sub.5).sub.4 As.sup.+, form precipitates with pertechnetate anion that are insoluble, or only slightly soluble, in aqueous solution. But solubility correlations, based on a common anion, between compounds comprising different cations are notoriously erratic. Accordingly it cannot be predicted a priori whether a given cation complex with the pertechnetate anion will be sufficiently insoluble to be useful in a process for removing technetium as described above, particularly when technetium is present in concentrations typical of the Savannah River Plant salt solution, i.e., in the range of about 2.5.times.10.sup.-5 to 1.times.10.sup.-4 M (average of about 6.times.10.sup.-5 M) or approximately 35-200 mCi per liter of waste solution. It was surprising, therefore, to discover that tetraphenylphosphonium ion (TPP.sup.+) not only forms a pertechnetate salt which is essentially insoluble in aqueous solution, but also that precipitation of the pertechnetate-phosphonium complex could be effected even under conditions of high pH and/or low Tc-99 content that are characteristic of a waste salt solution like that produced by the Savannah River Plant. More specifically, in simulated waste solutions containing 3.times.10.sup.-5 M pertechnetate (see Table 1), the addition of 4.2.times.10.sup.-4 M tetraphenylphosphonium chloride (TPPCl) resulted in the precipitation of approximately 96% of the pertechnetate, providing a decontamination factor (DP) of 30, where DF is defined as the ratio of technetium activity measured before precipitation to that measured after precipitation. TABLE 1 ______________________________________ COMPOSITION OF SIMULATED SALT SOLUTION Concentration Component (molar) ______________________________________ Na.sup.+ 5.6 K.sup.+ .015 NO.sub.3.sup.- 2.3 NO.sub.2.sup.- .70 OH.sup.- (free) 1.3 CO.sub.3.sup.2- .20 AlO.sub.2.sup.- .38 SO.sub.4.sup.2- .17 F.sup.- .017 Cl.sup.- .025 SiO.sub.3.sup.2- .0045 CrO.sub.4.sup.2- .0039 MoO.sub.4.sup.2- .00051 C.sub.2 O.sub.4.sup.2- (oxalate) .029 PO.sub.4.sup.3- .012 TPB.sup.- (tetraphenylborate) .001 TcO.sub.4.sup.- 3.0 .times. 10.sup.-5 ______________________________________ It was found that the DF for the simulated waste solutions was directly dependent on the amount of TPPCl added. A DF of 10, for example, required 0.053 grams of TPPCl per liter of waste solution. As described below, the DF values obtained with actual waste solutions (see Table 2) were somewhat lower than those achived with simulated solutions. Thus, to obtain a 90% removal of technetium required the addition of 0.29 grams of TPPCl per liter of actual waste solution. TABLE 2 ______________________________________ AVERAGE COMPOSITION OF DECONTAMINATED SALT SOLUTION Major Non-Radioactive Major Components Radioactive Components Concentration Concentration Component (molar) Radionuclide (mCi/l) ______________________________________ Na.sup.+ 5.0 Tc-99 50 NO.sub.3.sup.- 2.0 Ru-106 50 OH.sup.- 1.2 Cs-137 25 NO.sub.2.sup.- .62 Sr-90 .9 AlO.sub.2.sup.- .34 I-129 .25 CO.sub.3.sup.2- .15 SO.sub.4.sup.2- .023 F.sup.- .015 PO.sub.4.sup.3- .011 TPB.sup.- .002 (tetraphenyl- borate) ______________________________________ Preferably, technetium precipitation by addition of TPP.sup.+ in accordance with the present invention is accomplished in a batch process. An exemplary arrangement for batch processing of a waste stream to remove Tc-99 using the present invention is shown schematically in FIG. 1. Typically, waste solution from which cesium-137 and strontium-90 has been removed via the process disclosed by Lee et al is pumped from a storage tank 1, first to one of two or more precipitation tanks 2 and then, alternatively, to the other tank(s). For a system comprising two precipitation tanks, as shown in FIG. 1, the cycle of alternatively filling the tanks could extend, for example, over about eight hours. After the first tank has been filled, and while filling of the second tank is in progress, a batch precipitation of Tc-99 is carried out by addition of a water soluble tetraphenylphosphonium salt, such as a soluble TPP halide salt (e.g., TPP chloride, TPP bromide and TPP fluoride) or TPP hydroxide, to the solution in the first tank. The resulting slurry of pertechnetate precipitate is then separated for further processing, as elaborated below, and the procedure repeated in the other tank(s) in succession. A precipitation process within the present invention is now described in greater detail, with reference to a batch processing setup as shown in FIG. 1; (1) Actual waste solution, preferably partially decontaminated by the removal of cesium and strontium, is fed into a precipitation tank (2) which could contain, for example, some 7,200 gallons of solution based on an instantaneous processing rate of 15 gallons per minute. (The "instantaneous processing rate" corresponds to the maximum rate achieveable at any given time; the average processing rate, which includes downtime, might be the range of about 10 gallons per minute over an entire year.) (2) A water soluble potassium salt is then optionally added to precipitate tetraphenylborate (TPB) ion present by virtue of a prior addition of sodium tetraphenylborate in accordance with Lee et al. A 45% KOH solution (11 molar) is suitable for this purpose, with about two gallons of the 11M solution required per 720 gallons of the waste solution shown in Table 2. When the KOH and waste solutions are thoroughly mixed over a period of about one-half hour, precipitation of potassium tetraphenylborate (KTPB) is rapid and no additional time for ripening of the crystals comprising the precipitate is required to obtain crystals of a size amenable to easy filtering. (3) TPP.sup.+ is then added in the form of an aqueous concentrate of a water soluble tetraphenylphosphonium compound. TPPCl is preferred in this regard, but other soluble TPP compounds, such as tetraphenylphosphonium hydroxide, can be used, for example, if elimination of chloride is necessary to ameliorate corrosion. After addition of TPP.sup.+ to a concentration preferably in the range of about 7.times.10.sup.-4 M to 2.times.10.sup.-3 M, the batch solution is agitated for about one-half hour or more in the tank to assure complete mixing of the solution and precipitation of the phosphonium complex. The batch can be sampled at this stage and analyzed for technetium content. If the DF value thus determined is too low, more TPP.sup.+ -contributing precipitating agent can be added. For the above-mentioned preferred range of TPP.sup.+ concentration, the corresponding range for DF is between about 10 and 130. If preliminary decontamination in accordance with Lee et al is not carried out, and precipitation (2) of TPB ion therefore not effected, somewhat smaller phosphonium crystals may be obtained. If TPB ion is present, filtration of the resulting KTPB precipitate formed in step (2) is not required, as the presence of KTPB actually enhances technetium recovery in the present invention, presumably by aiding in the filtration of tetraphenylphosphonium pertechnetate (TPPTcO.sub.4). For reasons not fully understood, the efficiency of technetium removal is also improved if the concentration of the initial salt solution, as reflected by sodium ion content, is adjusted (e.g., by allowing water to evaporate during storage) to a level higher than 5.6M sodium ion, preferably up to about 7M sodium. In addition, the volume of the TPPTcO.sub.4 precipitate obtained is reduced by up to 40% or more by carrying out step (3) at the higher salt concentration. The pertechnetate precipitate, which forms as a slurry at the bottom of the precipitation tank, can be removed and concentrated for easier storage. Preferably, concentration of the precipitate is accomplished by cross-flow filtration, as disclosed by Martin et al, "In-tank Precipitation Process for Decontamination of Water Soluble Radioactive Waste" in 1 Waste Management '84 291 (Univ. Arizona 1984), the contents of which are incorporated herein by reference. In the setup shown in FIG. 1, cross-flow filtration is carried out by pumping the pertechnetate slurry through a sintered metal pipe (3), such that filtered waste solution "weeps" through the sintered metal and leaves the slurry behind. The dewatered slurry is returned to the tank and recycled back through the pipe until the slurry is concentrated to about 10-15% solids (approximately 35 to 50 gallons for a 7200-gallon batch). From the original waste solution to a concentrated slurry of about 10% solids, the overall concentration factor for technetium, using cross-filtration, is about 155:1. Higher concentrations are possible with other methods, such as bed filtration or centrifugation, that allow for greater removal of water. But cross-flow filtration requires no moving parts aside from the pump components, and hence offers the advantages of operational simplicity and reliability. After the pertechnetate slurry has been sufficiently concentrated, it can be retained in a slurry storage tank 4, from which the slurry is transferred periodically to another facility for incorporation into glass or for further recovery processing. The filtered waste solution is collected from the sintered metal pipe 3 and retained in a holding tank 5; it can be transferred from there to a saltstone disposal facility. As noted above, both simulated and actual waste solutions were processed, in accordance with the present invention, to remove Tc-99 by the addition of TPP.sup.+. More specifically, 10 ml aliquots of waste solution, simulated or actual, were each placed in 25 ml polyethylene containers. To each container, a sufficient amount (approximately 0.1 ml) of 0.32M KOH solution was added to precipitate TPB ion after thorough agitation over several seconds. About 0.05 ml of TPPCl solution (0.088M) was then added to each container; the containers were capped and shaken for one hour. The resulting slurry in each container was filtered through a cellulose filter (0.2 micron nominal pore size), and the filtrate was analyzed for technetium by a standard scintillation counting method. The observed DF values for Tc-99 at differing TPP.sup.+ concentrations are shown, for both simulated and actual waste solutions, in FIG. 2. The present invention permits the high efficiency precipitation, from relatively Tc-poor, caustic waste streams, of technetium in a form that is non-toxic and easily stored.