Ind. Eng. Chem. Res. 2000, 39, 3157-3168
3157
Advances in Sublimation Separation of Technetium from Low-Specific-Activity Molybdenum-99 Jerry D. Christian,* David A. Petti, Robert J. Kirkham, and Ralph G. Bennett Idaho National Engineering and Environmental Laboratory, Lockheed Martin Idaho Technologies Company, P.O. Box 1625, Idaho Falls, Idaho 83415
Two sublimation techniques have been developed to separate Tc from Mo to support accelerator production of 99Mo/99mTc. After bombardment, the metal target is dissolved in nitric acid, the solution is evaporated, and the precipitated solids are calcined to MoO3. In the first technique, MoO3 is melted to a 0.8 mm layer and oxygen is swept over the 835 °C sample at 30 std cm3/ min. Small amounts of MoO3 vapors transported from the 835 °C sample are deposited above 550 °C onto the walls of a quartz tube in a temperature gradient of 4 °C/cm. Greater than 95% of released technetium oxide is collected in a 5 mL condenser at temperatures between 300 and 25 °C. In the second process, technetium oxide deposited on the surface of fine needle crystals of MoO3 is quantitatively released in an oxygen stream at 650 °C. The crystals are regenerated between milkings by dissolving in NH4OH, evaporating, and drying. In this initial work, greater than 90% product recovery is achieved by both processes during multiple milkings of 14 g Mo samples and a high-purity 99mTc product results at a concentration exceeding 500 mCi/mL in an isotonic saline solution. Introduction and Background Accelerator Production of Molybdenum-99/Technetium-99m. Our laboratory is developing a new technology for producing technetium-99m, a widely used isotope in medical diagnostic imaging.1-3 The approach is to bombard enriched molybdenum-100 with highenergy bremsstrahlung photons produced in an electron accelerator in which the accelerated electrons are impinged on a tungsten foil. 100Mo undergoes a (γ,n) reaction to produce 99Mo at a specific activity of ca. 1.8 Ci 99Mo/g of Mo (3.8 × 10-6 atom fraction) after irradiation. Following irradiation, the metal molybdenum target is converted to the oxide. As 99Mo (t1/2 ) 65.9 h) decays to 99mTc, the technetium product is periodically separated from the molybdenum oxide by one of two sublimation processes termed melt and powder sublimation.1,2,4,5 The production and distribution concept is based on a distribution of electron accelerators, each of which supplies 99mTc to a region with three deliveries from the central facility daily. Details of the concept and of the accelerator production parameters are given by Bennett et al.1 We describe here the scientific basis and experimental results of successful approaches to the sublimation separations and product recovery that address and resolve past difficulties. At the end of approximately 15 milking (separation) cycles over a period of 5 days, the spent molybdenum is reduced in hydrogen and reformed into a metal 100Mo target for irradiation in the accelerator. The overall process concept flowsheet is shown in Figure 12 of ref 1, to which the reader is referred. The basis for, and explanation of, the separations unit operations is given in the current paper. Technetium-99m Separation and Product Requirements. The low specific activity of 99Mo produced * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (208)526-2930.
by (γ,n) reactions on 100Mo requires a compact, highly efficient separation process. The process must achieve high separation factors and quantitative recovery and must yield a high-purity, high-concentration 99mTc product, greater than 500 mCi/mL, in an isotonic saline solution. The standard alumina “cow” used to separate technetium from fission-produced molybdenum (which has a specific activity exceeding 5000 Ci 99Mo/g of Mo) would not be practical for this application. For example, approximately 14 L of eluent would be needed to collect 21 Ci 99mTc, an impractical volume that would also result in a product concentration some 300 times less than is required. Purity requirements for separated and recovered 99mTc are summarized and discussed elsewhere.1 Additional requirements that we consider necessary or desirable for cost-effective production are (a) a separation and 99mTc collection efficiency greater than 80%, (b) a complete elution cycle in 30-60 min, and (c) low losses of 100Mo, e1%, per target cycle. Two radioimpurities that are formed at trace concentrations during accelerator irradiation of 100Mo are isotopes of niobium and zirconium.1 The oxides of Nb and Zr have negligible vapor pressures at the temperatures utilized in the thermochromatographic separation of technetium, so there is very little potential for contamination of the product by radioisotopes of these elements. Historical Sublimation Separation of Technetium and Molybdenum. Sublimation separation relies on the high vapor pressures of technetium oxides (e.g., Tc2O7, TcO3) relative to partial pressures formed as they are released into a flowing gas stream as compared to that of molybdenum trioxide. The differences result in semiquantitative vaporization of technetium at elevated temperatures, while only a very small fraction of molybdenum evaporates. The small amount of molybdenum trioxide evaporated deposits at a higher tem-
10.1021/ie9903792 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/13/2000
3158
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000
Figure 1. Vapor pressures of (MoO3)x and Tc2O7.
perature than do technetium oxides. Figure 1 shows the pertinent vapor pressures6-8 of MoO3 and Tc2O7 that affect the relative vaporization and deposition characteristic temperatures. The key features of this comparison are (1) any MoO3 vapors formed during processing near the melt temperature of ca. 800 °C will be quantitatively deposited by the time the temperature in the carrier gas has decreased to between 600 and 500 °C, before any technetium oxides will saturate and precipitate (for the partial pressures formed during the releases into the flowing gases), and (2) Tc2O7, the most volatile of the technetium oxides, can be condensed out of the gas stream at ambient temperature, again at the approximately 10-6 atm nominal pressure formed in our process described herein. Past approaches to separate Tc from Mo via the sublimation process have met with limited success for MoO3 samples larger than a few hundred milligrams. A summary of prior art and results, with references, is given by Bennett et al.1 Several methods have been investigated or utilized, without a complete understanding of the processes that affected their performances. The methods have included evaporation from bulk quantities of molybdenum trioxide, either as powder or as a melt. Bulk molybdenum trioxide was generally the starting form of molybdenum prior to formation of 99Mo by, for example, neutron activation of 98Mo. This required diffusion of technetium oxide through substantial distances in molybdenum trioxide in order for it to be released. Often the irradiated product was ground to a powder to decrease the diffusion path for technetium oxide, but still the particle size was too large for practical conditions. Also, with sequential milkings, the powder would sinter, resulting in a decreased release. When MoO3 was melted, release of technetium was improved, but the depths of the melt still provided a significant resistance to release. Also, molybdenum trioxide vapors that were condensed using a sharp temperature gradient resulted in small amounts of entrainment of molybdenum oxide transported as a fine powder or aerosol in the technetium product that was recovered near ambient temperature. This sometimes required utilization of a filter, such as a glass wool plug, to capture the molybdenum trioxide; such a physical removal also had limitations in effectiveness.
Other approaches utilized gels that could release technetium at a temperature significantly below the melting point of molybdenum trioxide, by virtue of the fine pore structure of the gels. The gels included binary compounds of molybdenum trioxide with, for example, zirconium oxide or titanium oxide. However, the vapor pressure of MoO3 at the operating temperatures resulted in the eventual loss of molybdenum and breakdown of the favorable structure. Some methods rely on complete vaporization and deposition of MoO3 in a higher temperature zone than the released technetium oxide. This approach has been limited to samples of a few hundred milligrams. New Thermal Separation Techniques. Two current sublimation separation processes developed and demonstrated in our laboratory have addressed the physicochemical aspects that were previously unappreciated or that were not known. These considerations have resulted in overcoming all past difficulties for bulk samples. We term one a melt process and the other a powder process. The specific approaches and features are described in the Experimental Section and Results and Discussion section. Both are the subjects of patents2,5 and are practical approaches for production of a high-purity, high-concentration 99mTc product. They are based on converting the irradiated metal targets to MoO3 by dissolving it in nitric acid, evaporating to dryness, and calcining. A key aspect of both is that they enable utilization of metal molybdenum targets that are converted, after irradiation in the γ beam, to MoO3. This provides a decided advantage for production because Mo metal contains a higher density of “target” 100Mo atoms than does 100MoO3 and results in a higher yield of 99Mo. When the irradiated target is then converted to the oxide, chemical and physical characteristics are created that lead to the efficient separation of 99mTc. Finally, MoO3, at the end-of-life depletion of 99Mo, is amenable to recycling into a new metal target, enabling continued utilization of the enriched 100Mo. Experimental Section Dissolution of the Mo Metal Target. An integral part of the accelerator production concept is to convert the approximately 1 cm diameter metal Mo target to MoO3 in order to accomplish the separations. This is accomplished by dissolving it and then evaporating the solvent and calcining the precipitate. The dissolution must be done in a short enough time to be consistent with the overall process cycle requirements. We determined the dissolution kinetics of a Mo rod in two dissolvents that are compatible with the separation and collection of technetium for pharmacy use, 3-13 M HNO3 and 35 wt % (11.5 M) H2O2. The dissolution tests were done by immersing the metal rod into an excess of dissolvent at a measured temperature (28-70 °C) for a period of time such that the dissolvent concentration would not appreciably change during the measurement. The amount of molybdenum dissolved was determined by weight loss, and the penetration rate was calculated from the known dimensions of the cylinder. Melt Milking Tests. Tests with MoO3 and 99Tc were performed. The melt milking experiments were performed in a 2 cm diameter quartz tube placed inside a 90 cm long three-stage clam-shell tube furnace as depicted in Figure 13 of ref 1. We term the ‘milking’
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3159
apparatus a ‘goat’, to distinguish it from the alumina ‘cow’ that is conventionally used for fission-produced 99Mo. MoO (with or without 99Tc) was placed in a flat 3 platinum boat in a widened section of tubing and heated to a melt temperature in the range of 825-850 °C as oxygen was passed over it at a flow rate of 30 std cm3/ min (21 °C, 1 atm basis). After attaining the melt temperature, the O2 flow was adjusted to between 10 and 60 std cm3/min to determine the effect of flow rate on MoO3 and technetium vaporization and transport rates. A constriction was placed at the upstream end of the boat to minimize backdiffusion losses; the design was based on a 30 std cm3/min flow, using the equations of Merten and Bell.9 At the end of a test, the heaters were turned off, the furnace was opened, and the system was allowed to cool. For the tests that used 99Tc, the Tc condenser region was rinsed with a sequence of successively stronger solutions following an initial 0.9 wt % (0.155 M) NaCl to ensure that all of the deposited technetium was captured. The protocol rinse sequence was two saline rinses followed by two rinses with 7 M NH4OH. In some cases, this was followed by a rinse with 3 M HNO3 or 7 M KOH. The molybdenum condenser region, the boat region, and other parts of the quartz goat were rinsed once with 7 M NH4OH. In general, the isotonic saline and ammonia rinses were fully effective at removing the deposited technetium. In all cases, the KOH solution removed any residual technetium. Technetium was determined in the solutions by scintillation counting. A series of experiments were performed in the apparatus to characterize the vaporization, transport, and condensation of MoO3 from the melt down the thermal column with a prescribed temperature gradient. No technetium was added in these experiments. The O2 flow ranged between 10 and 50 std cm3/min, and the total time above 800 °C was 20-60 min. The temperature gradient along the tube was set at 4 °C cm-1 to prevent excessive supersaturation of the molybdenum trioxide vapors as they cooled during transport and to prevent blockage of the tube by a heavy MoO3 deposit. The MoO3 transport mass was determined by weight loss of the boat. In one series of runs, at the end of an experiment, each 5 cm section of the tube was rinsed with 7 M NH4OH to dissolve the deposited MoO3 for analysis by inductively coupled plasma (ICP) spectroscopy. This enabled us to determine the molybdenum deposition profile. In those tests, the analyzed MoO3 recovery was 96.1% of the cumulative mass loss from the boat. The 99Tc-spiked MoO3 samples were prepared as follows. A sample of powder containing a 6 × 10-7 Tc/ Mo atom ratio was formed from evaporation and calcination of a solution of 2 M (NH4)2MoO4 in a slight excess of NH4OH in the boat. A series of quench/remelt tests were performed, as explained in the Results and Discussion section, to simulate 99mTc grown internally from decay of 99Mo in the MoO3 sample. First, a solution containing a known amount of 99Tc in 2 M (NH4)2MoO4 (nonradioactive molybdenum) was evaporated to dryness, and the residue was thoroughly dried at 250 °C. It was then rapidly heated above the melting point of MoO3 and immediately air quenched after completely melting the sample. For each test, a control sample was sacrificed after the quick melt and quench for analysis of the
amount of technetium retained. The melt depth of the molten MoO3 in the crucibles (approximately 1 mm) was the same as it would be in the platinum boat of a production milking goat in order to represent the same resistance to transpiration release. After quenching, the integral samples were remelted and held at 835 °C for 30 min. Powder Milking Tests. Initial separation tests were done with NH499TcO4 spiked into a solution of either nitric acid or ammonium hydroxide containing dissolved molybdenum. The solution sample was placed in a platinum crucible that was subsequently placed in a muffle furnace in air and heated to temperatures from 550 to 750 °C for 30 min. The ammonium hydroxide generated powders were prepared by dissolving MoO3 in 5 M NH4OH to a concentration of 2 M molybdenum. The solutions were spiked with NH499TcO4 to a Tc/Mo atom ratio of 4.6 × 10-7, which is typical of the Tc-toMo ratio expected in production. The activity of 99Tc in an aliquot of solution was measured in a liquid scintillation counter. Another aliquot was placed in a glass beaker and evaporated to dryness in air in a hood at room temperature. The residue was redissolved in deionized water and analyzed by liquid scintillation counting. Technetium retained in the residue was 96.4 and 99.6% of the amount in the aliquot in replicate tests. Analysis of the counting error places the results at 100% mass balance at the 95% confidence level, indicating no loss in the evaporation drying process. Nitric acid generated powder samples were prepared by dissolving 0.829 g of Mo metal in approximately 110 mL of 6 M HNO3 at 95 °C. A 99Tc spike of 5.2 × 10-7 g (6.2 × 10-7 Tc/Mo atom ratio) was added prior to the dissolution. Five aliquots were transferred to platinum crucibles for calcination tests, and two aliquots were evaporated at low heat for feed analysis. The feed residue was redissolved in 0.5 M HNO3 and analyzed by liquid scintillation counting. The measured 99Tc concentration was 6.0 × 10-7 times the molybdenum makeup concentration. After heating, the five calcination samples were dissolved in 5 M NH4OH for analysis. This was followed by contacting the platinum crucibles with 6 M HNO3 and counting the solutions. In all cases, the activity in the nitric acid contact was at background, indicating that all of the residual technetium was dissolved in the ammonium hydroxide solution. As discussed in the results on powder separations, ultrafine particles of MoO3 are produced in our precipitation-calcination process. These were characterized for morphology changes during a sequence of simulated powder milking cycles. Nitric acid generated and ammonium hydroxide generated MoO3 powders were prepared as described above. X-ray diffraction determined that the composition of the material following the 250 °C drying step and thereafter at higher temperatures was crystalline MoO3. The powders underwent several series of aging tests with up to 15 simulated milking cycles. Each cycle consisted of heating the test material in a platinum crucible in air up to the 650 °C test temperature for 30 ( 3 min and then slowly cooling for 15-30 min to 350 °C. Additional tests of the same starting materials were taken through three 700 °C aging cycles. Finally, material originating from the Mo metal dissolved in nitric acid was tested at 750 °C. Scanning electron microscopy (SEM) was used to determine if there was any change in the powders after each aging cycle.
3160
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000
The 99Tc goat heating tests were performed with powders containing a 6 × 10-7 Tc/Mo atom ratio that were prepared from evaporation and calcination of a solution that was 2 M Mo and 4.7 M aqueous ammonia, prepared by dissolving MoO3 into aqueous ammonia. For the first two experiments, the powder was evaporated in a platinum boat before placing it into a straight 2.5 cm diameter quartz tube in a tube furnace. In the third experiment, a loading and furnace apparatus that was built for remote operation was used. The solution was pumped into the quartz boat and evaporated to dryness under a reverse flow of oxygen to prevent any aerosol transport to the technetium condenser zone. Following evaporation and calcining to 480 °C, the flow was reestablished in the forward direction and the sample was heated to a releasing temperature. Following a milking, the technetium condenser (380 to 25 °C zone) was washed successively, once with a 0.155 M saline solution, two times with 7 M NH4OH, once with 1 M KOH, and once with water. The remaining MoO3 sample was dissolved in 7 M NH4OH. All of these solutions were analyzed for 99Tc content. For hot milking tests using 99Mo, fission-produced 99Mo, with a specific activity on the order of 20 000 Ci 99Mo/g of Mo, was purchased commercially in the form of a sodium molybdate solution in 0.2 M NaOH. It was shipped in a Nalgene polyethylene bottle. The solution is normally stabilized by the vendor by the addition of NaOCl to prevent precipitate formation that is sometimes observed in polyethylene. However, because of the potential for hypochlorite affecting the technetium volatility in our experiments, we purchased the samples without NaOCl addition. Upon receipt, a half drop of 9.79 M H2O2 (30 wt %) was added to the 0.6 mL sample to redissolve any precipitate that may have formed.10 The samples contained ca. 5.2 Ci Mo-99 when shipped. The as-received Na/Mo atom ratio in the sample was on the order of 45, and the Na/Tc ratio was on the order of 675. Prior to the sublimation experiments, nonradioactive molybdenum was added as a solution of (NH4)2MoO4 to the sample to reduce the specific activity closer to that expected in the accelerator-produced material. This resulted in a Mo-99 specific activity of 0.4 Ci/g equiv at the time of receipt. The addition of the cold molybdenum reduced the Na/Mo ratio by a factor of 37 000 to 0.0012. We were concerned about the possible effect of sodium on the sublimation release of technetium from MoO3. Ro¨sch et al.11 had observed suppressed sublimation release of technetium from cyclotron-irradiated MoO3 mixed with NaOH pellets and, especially, from Na2MoO4, relative to that from MoO3. Therefore, we separated 99% of the sodium from the sample by passing 20 mL of a solution acidified to pH 3 through a 2 g column of acidic alumina that held up the molybdenum, which was subsequently eluted off with 20 mL of 7 M NH4OH. Alternatively, the sample was passed through a Dowex 50W-X8 cation-exchange resin to hold up the sodium. Studies using 99Tc showed that Na/Tc atom ratios up to 300 000 did not affect the quantitative release of technetium at 700 °C from crystals prepared by precipitation from ammonium molybdate solutions with Tc/ Mo atom ratios of 6.25 × 10-6. In those conditions, technetium was likely precipitated as a separate phase on the surface of the MoO3 crystals, and those results do not provide direct information regarding the effect
of sodium on the release of 99mTc grown internally from decaying 99Mo. The hot 99mTc sample milkings were done in the same apparatus as was used for the third 99Tc run. Oxygen was used as the carrier gas. The technetium condensates were collected as described for the 99Tc runs. The first milking for each sample was made from the powder formed from evaporating an ammonium hydroxide solution in the boat. A second set was evaporated from a nitric acid solution to simulate the process used to dissolve metal targets. After the first hot milking, 99mTc was allowed to grow in again to equilibrium, and a sequence of successive dry milkings (without crystal regeneration) was done. Then, the ultrafine powder was regenerated by redissolution in ammonium hydroxide and drying. Once again, this was followed by a sequence of dry milkings spaced approximately 1 day apart. The reconstitution of the powder was done when it was shown that the milking efficiency decreased after the first separation or two (see the Results and Discussion section). It was not possible to perform a mass balance of 99Tc by scintillation counting because 103Ru (and smaller contributions from 106Ru and 137Cs) in the 99Mo sample overwhelmingly interfered. Instead, the technetium content was measured by ICP mass spectroscopy (ICPMS) to provide approximate mass balance information. Results and Discussion Technetium Oxide Vaporization and Deposition Chemistry. A brief discussion of the literature and thermodynamics of technetium oxide vaporization is pertinent to our observations regarding the vapor deposition behavior described in the following sections. There is substantial disagreement in the literature regarding the oxidation state of vapor species of technetium that may exist, especially of that released from heated MoO3. A volatile oxide described by Fried et al.12 and thought by them to be TcO3 is believed by Rard13 to more likely be Tc2O7. There is considerable additional evidence, however, that lower oxide vapor species of technetium, TcO2(g), Tc2O5(g), and TcO3(g), can be formed under certain conditions. When present, they condense onto quartz surfaces at much higher temperatures than a corresponding partial pressure of Tc2O7 would suggest. TcO2 has been prepared and characterized in the solid state, and evidence exists for its vapor species. Efforts to prepare, identify, and characterize the solid TcO3 have been unsuccessful, although thermodynamic properties have been estimated for it.14-19 Migge20 has derived PO2-temperature regions of stability for the condensed phases of TcO2 and TcO3 from a combination of the estimated thermodynamic properties. They indicate a limited range of stability for TcO3. Above 500 °C, TcO3 disproportionates to TcO2 and Tc2O7. At a lower temperature of 85 °C, for example, stability is predicted in the range of 3 × 10-14-4 × 10-10 Torr of O2. Outside this range, either TcO2 or Tc2O7 is stable. Thus, one understands the difficulty experienced in attempts to prepare the condensed phase of TcO3. Tachimori et al.21 determined deposition temperatures onto a quartz tube of the oxide vapor species of 99mTc released from irradiated MoO between 600 and 3 800 °C in flowing streams of He, air, or O2. In He, deposition occurred in the range 400-320 °C. In air, deposition occurred from 320 °C to ambient temperature, and in O2, it occurred near room temperature. The
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000 3161
original deposits in air, with additional time, revolatilized and transported to the ambient temperature zone where they redeposited. This did not happen when He was used as the transporting gas. Their observations led to the hypothesis that the species released is TcO2(g). It gradually oxidizes up to Tc2O7(g), with the oxidation occurring more rapidly in O2 than in air. As discussed below, it is unlikely that the vapor pressure of TcO2 was sufficient to release and transport technetium from MoO3. The species that was released and deposited in the elevated temperature zones was more likely TcO3. Sekine et al.22 separated 95mTc from niobium targets irradiated with R particles by heating the target at 1100 °C in an oxygen flow. The slowly released technetium oxide was condensed on the walls of the quartz tube outside the furnace region, indicating that the 2 h release time resulted in oxidation to Tc2O7. The deposited technetium was collected as a pertechnetate solution by washing the wall with water. Eichler and Domanov23 released technetium from UO2 at 1100 °C and studied the deposition temperature profile onto quartz. They attribute the depositing vapor species to TcO3. The deposition occurred over the temperature range of approximately 600 to 100 °C, with the peak at about 400 °C. Ro¨sch et al.11 found that technetium released from irradiated MoO3 or Nb foil in air deposited (adsorbed) onto a quartz tube at 400 ( 40 °C. They attributed the vapor species to TcO3(g). Steffen and Ba¨chmann24 studied the adsorption, onto quartz granules, of TcOx vapor species formed during the release of technetium in O2 at 1500 °C from irradiated uranium or gold foils. They attributed observed deposits of technetium to TcO2 deposited in the range 800-500 °C and TcO3 deposited from 280 to 400 °C. The authors state that TcO3 decomposes to TcO2 as it deposits. The adsorptive deposition occurred from a partial pressure calculated from their reported operating parameters to be on the order of 4 × 10-12 Torr. This is convincing evidence of strong adsorptive bonds for the condensing species formed on the quartz surface. Cobble has prepared and measured the vapor pressure of Tc2O7.6,7 Direct observation of TcO3 vapor has been made,25 although its thermodynamic properties are not known. No information exists on its vapor pressure. Gibson25 also observed the vapor species Tc2O5 and believes that it must be considered in modeling and predicting the high-temperature vapor transport of technetium. Although solid TcO2 has been prepared (e.g., Muller et al.26), only qualitative information is available on its vapor pressure at elevated temperatures. Experimental observations support contentions that TcO2 vaporizes incongruently by disproportionation to Tc and TcO3(g) or Tc2O7(g) and, also, that it vaporizes and condenses congruently. The former argument is supported by the fact that Gibson, in a mass spectrometric study of vapors above technetium oxide, showed the presence of all highly oxidized vapors but never TcO2(g), whether the sample was exposed to O2 or vacuum. This suggests vaporization of TcO2 by disproportionation to a nonvolatile reduced form and Tc2O7.25 It has been observed that, in vacuo, sublimation of TcO2 begins at 900 °C to the extent that black deposits can be seen outside the heated zone; at 1100 °C, approximately 50% of a 20 mg sample sublimes from a Pt boat in 1 h.27 X-ray diffraction patterns of the residual sample, of the deposited
Table 1. Dissolution Rate of Mo Metal Rods in Excess HNO3 or H2O2 dissolvent
temp (°C)
penetration rate (cm/h)
3 M HNO3 3 M HNO3 6 M HNO3 6 M HNO3 13 M HNO3 13 M HNO3 11.5 M H2O2 11.5 M H2O2 11.5 M H2O2
28 70 29 70 28 70 29 54 70
1.17 × 10-3 2.38 × 10-2 2.14 × 10-2 9.10 × 10-2 2.38 × 10-4 9.48 × 10-4 2.64 × 10-4 6.52 × 10-3 1.70 × 10-3
material in the cooler zone, and of TcO2 prepared by thermal decomposition of NH4TcO4 showed identical patterns, suggesting that TcO2 sublimes in vacuo without decomposition or disproportionation. If the higher oxide Tc2O7 had volatilized from a disproportionation process (which should have left Tc metal in the residue), it likely could not have decomposed to TcO2 upon deposition in the cooler region except under very high vacuum conditions and would have required the presence of Tc (or some other reducing agent). Less information is available to assess whether TcO3(g) would dissociate in small residual oxygen pressures, and this vaporizing species cannot be ruled out, except by the concern that it should have left elemental Tc in the residue. While this latter question may be avoided by considering that even a trace of oxygen would have oxidized the metal to TcO2, one might, in such a case, suspect even further oxidation to Tc2O7(g) to be possible but perhaps not completely. Muller et al.26 observed considerable volatility of TcO2 at 950 °C and above. A mixture of Tc and TcO2 was held in a sealed tube at 1250 °C and then quenched. The volatilized portion of the sample that had deposited in the cooler region was shown by X-ray analysis to consist of Tc, TcO2, and an intermediate phase, while the nonvolatilized portion was primarily Tc. The presence of Tc in the vapor deposit can be explained by the deposition reaction 7TcO2(g) ) 3Tc + 2Tc2O7(g) or, alternatively, by 7TcO3(g) ) Tc + 3Tc2O7(g). From these observations, one may estimate a measurable vapor pressure (perhaps on the order of 10-3 Torr) in the vicinity of 1000 °C. At temperatures in the vicinity of 800 °C, the vapor pressure would begin to become very small and would become vanishingly small at 600 °C. Migge20 used the standard values and free-energy function of TcO2 reported by Brewer and Rosenblatt28 to derive a maximum vapor pressure of TcO2 at 897 °C [TcO2(c) ) TcO2(g)] of 8 × 10-14 Torr. He reported this to be consistent with the observed volatility. While the situation regarding TcO2 vaporization is uncertain, evidence as a whole argues against it as a vaporizing species in our experiments. First, if it vaporized congruently, the vapor pressure in the vicinity of 650 °C would likely be much too small to account for our observations. Second, if it vaporized incongruently, only a fraction of technetium in MoO3 would have vaporized, with the balance being Tc. We observed virtually quantitative vaporization. Molybdenum Metal Dissolution. The results of the rate studies, expressed as a penetration rate of Mo metal, are summarized in Table 1. Excess 11.5 M H2O2 is significantly less reactive than 6 M HNO3 in dissolving molybdenum metal. Therefore, our dissolution tests focused on nitric acid to achieve an acceptable dissolu-
3162
Ind. Eng. Chem. Res., Vol. 39, No. 9, 2000
tion rate. The best rate obtained in these tests was a 0.091 cm/h penetration rate, in 6 M HNO3 at 70 °C. The Arrhenius extrapolation to 90 and 95 °C respectively predicts 0.164 and 0.188 cm/h at these temperatures. The optimization of the HNO3 concentration was not completed. At very high concentrations, the rate decreases dramatically, probably as a result of oxidation of the molybdenum surface by the highly oxidizing potential. Parenthetically, we note that a similar effect might be occurring at the 11.5 M concentration of H2O2. The rates in 13 M HNO3 and in 11.5 M H2O2 at the same temperatures are comparable. Future testing at lower H2O2 concentrations might reveal increased rates for that solvent, but until the tests are performed, any such effects can only be speculated. A subsequent experiment using 7 M HNO3 at 97100 °C (slightly below boiling) verified the Arrhenius extrapolation and the expectation that a concentration slightly greater than 6 M may result in an increased rate. In this test, Mo was dissolved to a concentration of 0.135 M, resulting in a stable solution of ca. 6 M HNO3 at the finish. The overall penetration rate was 0.21 cm/h (36 mg/cm2‚min). We use 0.2 cm/h as the basis for flowsheet evaluations. The minimum required time for dissolution of a cylinder will be the time to penetrate half of the shorter of the two dimensions of diameter and length. Thus, for a 1 cm diameter by 1.8 cm long baseline pellet, the diameter will determine the dissolution time (2.5 h). One 99Tc distribution concept is to segment a 1.2 cm diameter target into five wafers of 0.5 cm length; these would be dissolved in 1.25 h. These are practical times. Also, a powder-pressed accelerator target will be porous and expected to dissolve faster than the arc-melted rod used in the tests. Melt Process. Two major improvements, and several minor process changes, resulted in overcoming past difficulties of poor recovery and product contamination. The most important operational modification was to melt MoO3 to a very thin layer, less than 1 mm deep. Practical equipment dimensions resulted for a 21 g sample with a 0.8 mm thickness. This resulted in ready diffusion and release of the technetium. The temperature was maintained near the melting point to limit the vapor pressure of MoO3 and minimize vapor transport of molybdenum. Nevertheless, there is sufficient vapor pressure to result in some mass transport. An optimum oxygen carrier gas flow rate was maintained (at approximately 30 std cm3/min) that was a balance between keeping the time required to transport technetium to the collection condenser to less than 10 min and minimizing the transport of saturated MoO3 vapors. The low flow rate also enabled collection of technetium in a small-diameter condenser of volume substantially less than 5 mL, the maximum volume allowed to obtain a product concentration of 500 mCi/mL. The second major improvement was to gradually decrease the temperature from the sample boat as the vapors were transported down the tube toward the product condenser. The temperature gradient of 4 °C/ cm over a 70 cm range allowed the MoO3 vapors to become saturated during transport to the walls and condense into long, adherent crystals on the walls ahead of the technetium condenser. Others29 have utilized a rather steep temperature gradient that caused the formation of fine MoO3 powder in the carrier gas that resulted in some entrainment of MoO3 in the technetium
Table 2. Corrosion of Materials by Molten MoO3 at 850 °Ca material
composition (wt %)
platinum-rhodium platinum iridium gold Inconel 72 Hastelloy G-30
90 Pt, 10 Rh 100 Pt 100 Ir 100 Au 55.9 Ni, 43.3 Cr 28.0-31.0 Cr, 13-17 Fe, 4-6 Mo, 1.0-2.4 Cu,
