Sodium Diuranate and Sodium Aluminosilicate Continuous Precipitation Testing Results
R. A. Pierce and R. A. Peterson
Westinghouse Savannah River Company
Aiken, SC 29808
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This study investigated the simultaneous continuous crystallization of sodium diuranate and sodium aluminosilicate. The primary conclusions from these tests follow.
· Continuous evaporation of aluminum and silica bearing sodium hydroxide solutions produced sodium aluminosilicate solids of identical crystal patterns to those found in the 2H evaporator system.
· Use of longer residence times facilitated the crystallizations of sodium diurante.
· During batch tests, crystallization of aluminosilicate produces additional crystallization of sodium diurante.
In the spring of 2000, SRTC performed testing to determine the solids formed from the evaporation of silica/alumina/uranium bearing hydroxide salt solutions. These tests indicated that under batch evaporation conditions, that silica deficient sodium aluminosilicate solids are formed. In addition, these solutions supersaturated with respect to uranium. Additional testing was recommended to determine if continuous operation produced significantly different solids. In particular, these tests were to investigate the required conditions to produce solids similar to those found in 1997 in the gravity drain line from the 242-16H (2H) evaporator system. Analysis of the material found in the drain line identified both sodium aluminosilicate and sodium diuranate. A later sample of solids from the 2H evaporator pot again indicated the presence of sodium aluminosilicate and sodium diuranate. Concentration Storage and Transfer Engineering (CSTE) requested SRTC to investigate the nature of the co-precipitation and deposition of these species under continuous evaporation conditions.
Feed solutions were prepared containing 1 M NaOH and 1 M NaNO3. In addition, these solutions contained between 1163 and 6000 mg/L of aluminum (added as Al(NO3)3-9H2O) and between 50 and 920 mg/L of silica (added as Na2SiO3-9H2O). These feed solutions also typically contained between 1.3 and 8.0 mg/L of uranium (added as uranyl nitrate) and between 0.01 and 0.79 mg/L plutonium.
Prior to the start of each continuous evaporation test, the evaporator pot was filled with a concentrated starting solution. These starting solutions were 3 M NaOH and 3 M NaNO3. In addition, these solutions contained between 14000 and 17000 mg/L of aluminum and 150 to 800 mg/L of silica. These starting solutions also typically contained between 0.1 and 10.0 mg/L of uranium and between 0.04 and 0.4 mg/L of plutonium.
The NaOH was weighed out and gradually dissolved into water. Next, while the solution of water and NaOH was still warm from the heat of dissolution, the sodium metasilicate was weighed, added, and dissolved into solution. Similarly, while the solution was still warm, the appropriate amount of aluminum nitrate was weighed and gradually dissolved into solution. Last, sodium nitrate was weighed and added. Some of the solutions prepared above were supersaturated in Al and Si.
The solutions were stored overnight. The next day, the solutions were transferred to a radioactive hood, and the appropriate amounts of U and Pu solutions were added. Sufficient Pu and U were added to achieve saturation, resulting in a precipitate. The solution was mixed for 4-6 hours to establish equilibrium. The solution was then decanted (leaving the Pu and U precipitate behind).
The starting pot solution had the Pu and U added directly to the bottle. The bottle was capped, shaken and stored. The starting pot solution was decanted directly into the evaporator pot; the residual Pu and U solids remained in the bottom of the bottle.
For batch evaporation tests, researchers prepared a 2 M sodium (i.e., 1 M NaOH and 1 M NaNO3) solution containing approximately 2000 mg/L of soluble aluminum. Personnel then saturated the initial solution with uranium and added ~ 300 mg/L soluble silica. Note that 300 mg/L was used because this was the maximum amount that could be readily dissolved in the simulant matrix.
Masterflex peristaltic pumps were used to pump feed solution into the evaporator pot and bottoms solution from the pot. The pumps were calibrated for the specified flowrate prior to the experiment. Due to some slight variance from the target value, the pumps were occasionally adjusted to maintain the proper flow rates. The feed solution was metered from a graduated cylinder to ensure that the pump rates met the specified test rates. Similarly, the bottoms were collected in a graduated cylinder.
The evaporator pot was heated from the bottom using a hotplate, and on the sides using heating tape. The heat settings were determined prior to the experiments. These were also adjusted slightly to maintain the proper evaporation rate. An overheads condenser was cooled with process cooling water to condense the vapors coming off the evaporator. The overheads were collected in a graduated cylinder as means of measuring the evaporation rate.
Experimental Protocol – Continuous Evaporator Tests
At the start of the experiment, the starting pot solution (400 mL) was transferred to the evaporator pot. The feed pump line was then filled with feed solution. Next, the hot plate and heating tape were turned on to heat the system; the cooling water was also turned on. The system reached boiling in approximately 10 minutes. When condensate began to drip into the condensate collection graduated cylinder (about 15-18 minutes), the feed pump and bottoms pump were turned on. The feed, evaporation, and bottoms removal rates were then controlled throughout the experiment to maintain the evaporator pot volume within 10% of its initial volume. Samples were periodically withdrawn from the bottoms removal stream for analyses. The test duration was typically set to be 5-8 residence times of the evaporator system. The residence times studied were between 1-8 hours.
Samples of the starting solutions were analyzed using ICP-ES and ICP-MS. Precipitation was occasionally observed in these sample vials. Liquid samples from the bottoms stream were removed as a function of time throughout the experiments. Some samples contained solids. The liquid portions were analyzed using ICP-ES and ICP-MS. Solids from the bottoms were also sampled periodically and analyzed using both XRD and SEM.
Experimental Protocol – Batch Evaporator Tests
During batch evaporator tests, researchers evaporated the samples to achieve a two-fold concentration (i.e., reduce volume by 50% through evaporation). During the evaporation, personnel obtained samples periodically At this point, researchers added a concentrated silica bearing stream to return the soluble silica concentration to roughly 300 mg/L. Evaporation then continued until the volume had again been reduced by half. Sample analyses included aluminum and silicon determination by Inductively Coupled Plasma Emission Spectroscopy (ICP-ES) and uranium measurement by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). The test was repeated under similar conditions with the exception that frit was added to the solution after 50% volume reduction.
Figure 1 is a plot of the typical solution phase aluminum and silica measurements. Notice that the soluble aluminum starts out higher than the target at the end of the 3 fold evaporation and decreases to a steady state value. Also note that as indicated above, the aluminum was typically fed in 10 fold excess relative to the silica (assuming 1-1 molar ratio of aluminum to silica in the sodium aluminosilicate). Thus, one expects the aluminum concentration to be relatively unaffected by the formation of the aluminosilicate.
Note, however, that the silica is significantly below the anticipated target (the target is based on the assumption of a simple 3 fold concentration of the feed solution in the absence of crystallization). Thus, inspection of figure 1 indicates that aproximately 75% of the silica is precipitated to form sodium aluminosilicate. Note, however, that a significant amount of silica (~ 200 mg/L) remains in solution. Thus, the crystallization of aluminosilicate occurs in the presence of fairly high silica concentrations. Clearly, if the silica concentration in the feed solution were significantly lower, the formation of aluminosilicate could be forestalled.
Figure 1. Typical Aluminum and Silica Concentration in Continuous Evaporator Effluent
Figure 2 is a XRD pattern for solids typical of those formed in this test. Inspection of this figure indicates that the primary solids are sodium aluminum nitrate silicate hydrate. These are the same solids that were observed in the 2H gravity drain line and pot samples. Note that due to the low concentration of uranium in these solids, no X-ray pattern appears for the uranium phase.
Figure 2. XRD Pattern for Typical Solids from Continuous Evaporator Test
Figure 3. Sem Image for Typical Solids from Continuous Evaporator Test.
These solid samples were also submitted for analysis by SEM. Figure 3 contains an image of typical solids obtained from these tests. Inspection of this image indicates the presence of three distinct materials. Surface analysis of the bright spots labeled A and C indicate that these solids contain primarily uranium (see Figure 4). Some plutonium may also be present in these spots. Analysis of the darker areas indicates that these portions of the surface are primarily sodium aluminosilicate (with equal parts aluminum and silica). These results indicate that these tests have successfully replicated the solids formed in the 2H evaporator system. Note, however, that in all cases, no significant deposit on the evaporator was observed. All solids formed during these tests remained primarily suspended and were easily pumped from the evaporator pot. This result is significantly different that the observations for the 2H evaporator system. However, these results may simply reflect the differing geometries of the two systems.
Figures 5 and 6 provide typical soluble uranium and plutonium profiles for these tests. The soluble fractions of uranium in Figure 5 changes with time as it reaches a steady state. Inspection of these graphs indicates that (as indicated by the SEM images) uranium did precipitate but that little or no plutonium is precipitated during these tests.
Figure 4. Surface Analysis from Spot C in Figure 3.
Figure 5. Uranium Concentration Profile during a Typical Continuous Evaporation Test.
Figure 6. Plutonium Concentration Profile during a Typical Continuous Evaporation Test.
Figure 7. Impact of Silica Addition during Testing on Uranium Crystallization
Figure 7 contains a plot of the quantity of silica and uranium removed due to crystallization during a batch evaporator test. Inspection of this figure indicates that the second addition of silica was coincidental with an additional crystallization of uranium. (Note that a small additional amount of uranium appeared to crystallize prior to the addition.) These results suggest that the crystallization of the aluminosilicate coincides with the crystallization of the diuranate. Figure 7 also contains the data from the addition of frit during evaporation. Inspection of this data indicates that the addition of frit did not produce any significant uranium crystallization. (Note that while some crystallization did occur, the quantity crystallized was within the analytical uncertainty of the measurements.) This test indicates that the presence of frit does not significantly increase the crystallization of diuranate, and thus no further testing with frit was warranted.
Continuous evaporator tests were performed and produced crystallized solids of essentially the same form and composition as those observed in the 2H evaporator system. The resultant solids consisted of primarily sodium aluminosilcate solids with a small amount of uranium and plutonium solids on the surface of these solids. The actinide solids formed a distinct, separate crystalline phase (as was observed in the 2H evaporator system). Batch evaporator tests indicated that the crystallization of sodium aluminosilicate can facilitate the crystallization of sodium diuranate.