Conversion of Coarse Gibbsite Remaining in Hanford Nuclear Waste

Aug 18, 2014 - Al(OH)3] has been left behind by the current retrieval technology and would ... This study develops a new process to remove this gibbsi...
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Conversion of Coarse Gibbsite Remaining in Hanford Nuclear Waste Tank Heels to Solid Sodium Aluminate [NaAl(OH)4·1.5H2O] Daniel L. Herting, Jacob G. Reynolds,* and W. Blaine Barton Washington River Protection Solutions, LLC, P.O. Box 850, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: The Hanford Site near Richland, WA, has 177 high-level nuclear waste tanks containing 56 million gallons of waste derived from nuclear fuel reprocessing. The site is removing waste from old single-shell tanks, but some gibbsite [γAl(OH)3] has been left behind by the current retrieval technology and would remain in the tanks when grouted on site, if not removed first. This study develops a new process to remove this gibbsite by converting it into water-soluble NaAl(OH)4·1.5H2O in highly concentrated NaOH(aq). Gibbsite was incubated in NaOH(aq) solutions of 8, 11, or 19.4 M concentration at 25 or 50 °C for up to 312 h. Inductively coupled plasma atomic emission spectroscopy and polarized light microscopy were used to monitor the reaction. Gibbsite rapidly converted into NaAl(OH)4·1.5H2O at all NaOH concentrations and temperatures for nonradioactive and real waste gibbsite samples. NaAl(OH)4·1.5H2O initially crystallized with a needle-shaped morphology, but subsequently transformed into a rectangular plate. These results demonstrate that gibbsite can be readily converted into NaAl(OH)4·1.5H2O in concentrated NaOH at ambient temperatures, suggesting that this is a promising technology for removing gibbsite from high-level waste tank heels.

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INTRODUCTION The Hanford Site near Richland, WA, USA, has 56 million gallons of high-level nuclear waste left over from plutonium production.1 This waste is stored in 177 large tanks, although the Site is currently retrieving the waste from much older single-shell tanks into newer double-shell tanks. Aluminumbearing minerals are some of the most prevalent species in the waste.2 Most of the solids in the single-shell tanks are dissolvable saltcake or fine-grained sludge that is easily slurried and pumped to the double-shell tank system.3−5 However, operating experience has found that a small fraction of coarse particles are left behind in the single-shell tanks. The heels of waste tanks retrieved to date have had coarse-grained gibbsite [γ-Al(OH)3] as the primary aluminum-bearing phase.6,7 Example images of coarse gibbsite observed in tank heels at the Hanford Site are shown in the Supporting Information. Cleanup negotiations for the waste storage tanks have resulted in strict volume-based enforceable cleanup targets. To meet these targets, most of the gibbsite heel must be removed from the tanks. Although the gibbsite material is not itself radioactive, it contains within the waste matrix both radioactive and hazardous species that cannot be retrieved without also retrieving the gibbsite. Any waste left in the tanks at the completion of retrieval is expected to be in the closed tanks and becomes part of the source term to the environment. Ongoing work is evaluating the environmental risk of leaving a small waste heel in the tanks after closure.7−11 Nonetheless, as much waste as possible will be removed from the tanks prior to closure. Similar problems with coarse gibbsite have been encountered in retrieved waste tanks at other nuclear waste sites.12 Thus, a method for removing coarse gibbsite particles would have broad applicability. Oxalic acid was used previously to remove small radioactive sludge heels in tanks at the Savannah River Site.13,14 Oxalic acid © 2014 American Chemical Society

was also employed at large scale to remove a sludge heel from Hanford tank 241-C-106, a tank heel with a high concentration of iron rather than aluminum.14 Oxalic acid readily dissolves iron (hydr)oxides,15−17 so oxalic acid was reasonably successful in removing the heel in tank 241-C-106.14 Deutsch et al.,7 however, analyzed samples from tank 241-C-106 after oxalic acid treatment and found that calcium and manganese oxalates precipitated and were left behind in the tank. When the oxalic acid was neutralized with NaOH in the receiving double-shell tank, sodium oxalate precipitated. Complications from Na2C2O4 precipitation have been noted from the oxalate already in the waste,18−21 and these difficulties would be aggravated further by adding oxalic acid for heel removal. Finally, only minimal gibbsite dissolution is obtained at ambient temperatures using oxalic acid,22 and building a system to maintain high temperatures in the old tanks is not practical with the current infrastructure.14 Given these limitations with oxalic acid, another method is required to remove coarse gibbsite from tank heels. Anything added to the tanks to treat the waste will subsequently have to be vitrified as waste, increasing the mass of waste treated and disposed. Consequently, if a chemical must be added to the waste to aid in heel removal, it is desirable for that chemical to be something that would eventually be added to the waste anyway. This would minimize the waste generated. Sodium hydroxide will be added to the Hanford waste to control carbon steel corrosion and to leach out gibbsite from the bulk waste at the Hanford waste treatment plant.18,23 Removing the heel with NaOH would therefore not add to the Received: Revised: Accepted: Published: 13833

April 5, 2014 August 1, 2014 August 16, 2014 August 18, 2014 dx.doi.org/10.1021/ie5014212 | Ind. Eng. Chem. Res. 2014, 53, 13833−13842

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This study used commercially available gibbsite to confirm that gibbsite is transformed into NaAl(OH)4·1.5H2O. The commercial gibbsite was used to obtain a qualitative understanding of the kinetics so that a large range of conditions could be investigated without exposing the laboratory personnel to radiation. A real waste sample containing large-particulate gibbsite was then evaluated for validation. Real waste samples were deemed necessary for validation of eq 1 because minor heel components are reported to interfere with the crystallization of NaAl(OH)4·1.5H2O.34 Additionally, the gibbsite dissolution rate has been reported to be influenced by the history of radiation exposure.35

mass of waste that ultimately must be treated, given that this NaOH would eventually have to be added to the waste for other reasons anyway. Therefore, this study focuses on developing a method to remove gibbsite heels using NaOH. NaOH will eventually be used to dissolve gibbsite and boehmite from the bulk tank waste at elevated temperatures.18,23,24 Gibbsite dissolution, however, is reported to be slow at NaOH concentrations below 5 M at ambient temperatures.25,26 It is hypothesized that reasonable rates can be achieved at ambient temperatures using highly concentrated NaOH(aq), from which sodium aluminate readily precipitates.27−29 It is further hypothesized that gibbsite rapidly dissolves in highly concentrated NaOH(aq) and recrystallizes as sodium aluminate.30−32 If the NaOH concentration were high enough, sodium aluminate might precipitate even as gibbsite was still dissolving through a metathesis reaction. The high water solubility of sodium aluminate suggests that the sodium aluminate solid could be easily redissolved in water after metathesis is complete.27,28 Given the hypothesis above, a process of converting the coarse-grained gibbsite into water-soluble NaAl(OH)4·1.5H2O is being developed. A proposed new reaction scheme (eq 1) is used to convert gibbsite into NaAl(OH)4·1.5H2O in concentrated sodium hydroxide Gibbsite conversion into NaAl(OH)4·1.5 H2O in concentrated NaOH

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MATERIALS AND METHODS Nonradioactive Gibbsite Tests. The nonradioactive gibbsite used in this study was commercially available SH-950 gibbsite from the company Aluminum Pechiney based in Paris, France. Examination of the nonradioactive gibbsite under an optical microscope (Figure 1) gave a mean estimated particle

NaOH(aq) + Al(OH)3 (s) + 1.5H 2O ↔ NaAl(OH)4 ·1.5H 2O(s)

(1)

The NaAl(OH)4·1.5H2O produced is stable in concentrated NaOH(aq) because of the high dissolved sodium concentration. The reason to convert gibbsite into NaAl(OH)4· 1.5H2O is that NaAl(OH)4·1.5H2O is highly water-soluble once the soluble sodium from NaOH is washed out.27,28,33 The dissolution of NaAl(OH)4·1.5H2O in water is expected to rapidly follow the reaction

Figure 1. Polarized light microscope photograph of gibbsite test-bed particles (crossed polars and Red I compensator).

Dissolution of NaAl(OH)4·1.5 H2O in water NaAl(OH)4 ·1.5H 2O(s) water

+

−

←→ ⎯ Na (aq) + Al(OH)4 (aq) + 1.5H 2O

size of 56 μm, with a range between 5 and 180 μm. The gibbsite samples were washed with deionized water and then centrifuged. The reason for creating this slurry was to give the gibbsite samples an interstitial liquid that would slightly dilute the concentrated NaOH, just as would happen in the real Hanford tanks. Subsamples of the slurry were prepared for each treatment as described below. Based on aluminum analysis by inductively coupled plasma atomic emission spectroscopy after acid digestion, the subsamples contained an average of 0.242 g of aluminum each. The density of the samples was measured and determined to be 1.8 g/mL. The gibbsite metathesis reactions were performed at two temperatures (25 and 50 °C) and at three different initial NaOH molarities (8, 11, and 19.4 M). At 19.4 M NaOH, three different total NaOH-to-aluminum mole ratios were used. The concentrations of 8 and 19.4 M NaOH were employed because NaOH(aq) is commonly sold in bulk at those concentrations, and 11 M is simply a value between the two. The temperatures were chosen to cover the range of realistic temperatures that could occur in a waste tank heel at the Hanford Site. Table 1 lists the test matrix for the samples. Aliquots (3.6 g or 2.0 mL) of gibbsite slurry were added to each plastic sample bottle used in this study. NaOH was added

(2)

The present study is focused on the reaction in eq 1; eq 2 will be reported in a separate communication. The purpose of the present study is to show that gibbsite can be converted into NaAl(OH)4·1.5H2O at temperatures near ambient in concentrated sodium hydroxide over a time frame reasonable for practical application in the field (i.e., days not months). The second purpose is to show that this is also true of gibbsite from a real nuclear waste heel sample. Most previous works have dissolved gibbsite at much lower NaOH concentrations using temperature to accelerate this admittedly very slow reaction to practical time frames.18,23−26 Given the obstacles in temperature control in >50-year-old buried waste tanks at Hanford, this study examines the possibility of using high NaOH concentration rather than temperature to achieve reasonable rates. Similarly, other researchers have crystallized NaAl(OH)4· 1.5H2O by evaporating sodium aluminate solutions.27,28,30−34 This is the first study that crystallizes NaAl(OH)4·1.5H2O while simultaneously dissolving gibbsite in concentrated NaOH(aq) and demonstrates that this reaction happens over a reasonable period of time. 13834

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Table 1. Test Matrix for 25 and 50 °C Dataa samples A (25 °C) and F (50 °C) B (25 °C) and G (50 °C) C (25 °C) and H (50 °C) D (25 °C) and I (50 °C) E (25 °C) and J (50 °C) a

NaOH molarity

NaOH volume (mL)

NaOH (mmol)

8.0

8.0

64

11.0

8.0

88

19.4

8.0

155

19.4 19.4

4.5 3.3

88 64

morphology of NaAl(OH)4·1.5H2O crystals was identified by the birefringence characteristics.39 With crossed polars and a Red I compensator plate, the birefringence characteristics of the crystals caused them to look blue or yellow/orange, depending on orientation: yellow/orange when parallel to the slow direction of the Red I compensator. Radioactive Samples of Gibbsite. Samples of a small waste heel in tank 241-S-112 were chosen for use in this study because coarse gibbsite was the dominant phase present.7 Tank 241-S-112 was filled with salt cake from the evaporation of Hanford liquid waste in the 1970s.41 Hanford waste contains a large dissolved aluminum load,42 and some of this aluminum was deposited in tank 241-S-112 during salt crystallization. Thermodynamic modeling by Antonyraj et al.43 indicated that gibbsite in the tank would not dissolve when the salt was retrieved by water dissolution. This tank has since been retrieved with water, and as predicted by Antonyraj et al.,43 gibbsite was found in the small heel left in the tank after retrieval.7 Thus, this tank heel sample is ideal for evaluating gibbsite removal using the process described in eqs 1 and 2. The Supporting Information contains a number of SEM images of the gibbsite from this sample, to provide a qualitative feel of the typical particle size. The tank 241-S-112 heel remaining after bulk waste retrieval was sampled using a remote-controlled crawler with a scoop end effector. Six samples were taken from across the tank bottom and then composited in the laboratory. After the small amount of supernatant liquid overlying the composite had been decanted, the approximate water content of the sample was determined by weighing a subsample before and after evaporating to dryness. Mineralogical analysis of the composite heel sample was performed by X-ray diffraction (XRD), PLM, and scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS). XRD was performed on samples that were airdried following vacuum filtration and scanned on the Rigaku Miniflex X-ray diffractometer between 5° and 70° 2θ. JADA version 7.5 software by Material Data Inc. (ICDD PDF-4 + 2008 release) was used for pattern-matching the XRD spectra. The SEM-EDS samples were also vacuum-filtered and then smeared on a polycarbonate filter that was mounted in a SPEX PSEM model scanning electron microscope. For PLM analysis (Nikon eclipse E600), drops of dispersed liquid were placed on glass microscope slides. Detailed descriptions of how these three methods are used on radioactive samples can be found in the works by Reynolds et al.21,44 Subsamples of the composite were centrifuged, and the liquid was decanted for the determination of gibbsite transformation in concentrated sodium hydroxide. To 13.012 g of sample (the largest practical size for handling this radioactive sample) was added 51.727 g of 19.4 M NaOH(aq). The initial sample was about 73% water, so the interstitial liquid diluted the NaOH added by about 15%. The sample was mixed several times, allowed to settle overnight, mixed once more, allowed to settle for 2 h, and then centrifuged (simulating the action of in-tank sluicers on the samples). The centrifuged liquid was decanted, and the total time between caustic addition and decantation was 20 h. This first treatment was designed primarily to remove the interstitial liquid of the samples. All treatments were performed in the hot cell at the Hanford Site, which is kept at 29 °C. After the first treatment, the centrifuged solids were weighed, and a mass of 12.857 g was found. An additional 51.531 g of

Each sample contained 32 mmol of gibbsite.

to each of the bottles according to the matrix shown in Table 1. Five replicate samples were used for each treatment, with one sample for each treatment time. The bottles were mixed until all of the solids appeared suspended and then placed in a shaker bath at the target temperature. Each treatment was examined by polarized light microscopy (PLM) to monitor the rate of metathesis from gibbsite to aluminate (see below). When each bottle reached its allotted treatment time, its reaction was “quenched” by adding 40 mL of 3 M NaOH. The concentration of 3 M NaOH was chosen because this was dilute enough to dissolve sodium aluminate but concentrated enough to keep gibbsite from reprecipitating.28 The bottles were mixed vigorously for 2 min and then filtered. The quench/ mix process dissolves the sodium aluminate but not the gibbsite, based on the low rate of gibbsite dissolution at this temperature and caustic concentration.26 Filtrate samples were analyzed for aluminum by inductively coupled plasma atomic emission spectroscopy. The tests at 50 °C were not incubated as long as the tests at 25 °C because of the higher rate experienced by these samples. Approximately 0.1 mL of sample was taken from each treatment per hour for the first 7 h for optical microscopic analysis. Microscopic analysis was also performed just prior to quenching of the reaction for all quench times longer than 2.5 h. The Olympus BX-51 polarizing light microscope was selected for observing the change in the solid phases during the reaction because samples can be prepared for microscopic analysis in just a few minutes. This meant that there was minimal change in the sample between when the samples were taken and when they were observed.36,37 Based on the birefringence colors of the samples, the morphology, and the temperature of crystallization, NaAl(OH)4·1.5H2O crystals were generated in this study. As described in the Results section, the sodium aluminate crystals formed either needles or rectangular plates. The method of identification by PLM depends on the solid morphology. NaAl(OH)4·1.5H2O is tetragonal and uniaxial.38,39 The optical axis of the plate is normal to the face of the crystal when it is lying flat and is therefore parallel to the light path. This makes the NaAl(OH)4·1.5H2O plates appear isotropic, and thus the same color as the background, when they are lying flat. A Red I compensator plate was used in the microscope between the crossed polars to ensure that the NaAl(OH)4·1.5H2O plates could be seen. At high magnification, a uniaxial interference figure could be observed.40 When tipped slightly on edge, the plates appeared blue/yellow because of the small contribution of the perpendicular refractive index, and when tipped completely on edge, the plates appeared nth-order white because of the full birefringence of the perpendicular refractive index. The anisotropic needle 13835

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19.4 M NaOH was added to the centrifuge cone. The sample was mixed regularly over 8 days before centrifugation and decantation of the supernatant. The resulting solids were then analyzed by PLM, SEM-EDS, and XRD.

percent complete =

where CAl is the aluminum concentration in the slurry, Vs is the volume of the slurry, and mAl is the mass of aluminum in the slurry. These figures demonstrate that the rate of gibbsite conversion increases with initial NaOH molarity at constant solid-to-liquid ratio, as indicated by the increase in percent complete with NaOH molarity on the plot. Tables 2 and 3 show that the reactions proceed faster at higher liquid-to-solid ratios at a constant 19.4 M NaOH initial concentration, as indicated by the higher aluminate molarities in the quench solutions in the samples as the liquid-to-solid ratio increases. Thus, these results indicate that the strongest NaOH concentration (19.4 M) possible should be used in the real tanks to optimize the rate. Comparison of the 25 and 50 °C results in Figures 2 and 3 shows that the percent complete is higher at the higher temperature, indicating that the conversion occurs considerably faster as the temperature is raised. The rate intriguingly slows after about 100 h. The consumption of hydroxide (through eq 1) is undoubtedly a contributing factor to this slowdown, given that the rate is clearly dependent on the hydroxide concentration. Although the rate was much higher at 50 °C than at 25 °C, NaAl(OH)4·1.5H2O needles formed within 1 h even at 25 °C. The PLM images in Figures 4−8 show small (2−10 μm in length) NaAl(OH)4·1.5H2O needles that formed in this hour. All of these samples show some small NaAl(OH)4·1.5H2O needles, although the needles are only about 2 μm in length in samples D and E. This shows that the crystallization of NaAl(OH)4·1.5H2O starts rapidly in these samples (less than 1 h) even at 25 °C. Figure 9 shows the progress of the metathesis reaction in sample C, which was the sample with the highest quantity of NaOH per mole of aluminum at 25 °C (nearly 5:1 mole ratio). In Figure 9a, at 1 h of incubation, the large out-of-focus gibbsite particles are still mostly intact, and the tiny blue/orange sodium aluminate crystals in their needle habit are in focus. Approximately 10% of the gibbsite had metathesized into NaAl(OH)4·1.5H2O at this point. Figure 9b shows treatment C at 48 h of incubation, where the conversion had progressed to approximately 82%. The remaining gibbsite particles look

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RESULTS Nonradioactive Gibbsite. Table 2 reports the aluminate molarity in each of the 25 °C test samples, and Table 3 reports Table 2. Aluminate Molarity (Solid plus Liquid), 25 °C Data sample time (h)

A

B

C

D

E

2.5 7.5 48 120 312

0.02 0.04 0.15 0.28 0.51

0.04 0.09 0.43 0.78 1.37

0.25 0.71 2.59 2.59 3.15

0.30 0.81 2.73 3.82 4.03

0.33 0.79 2.58 3.74 4.21

Table 3. Aluminate Molarity (Solid plus Liquid), 50 °C Data sample time (h)

F

G

H

I

J

1.5 4.8 30 102 170

0.31 0.66 1.42 1.53 1.66

0.55 1.26 2.54 2.70 2.76

2.57 2.87 2.81 3.07 3.17

3.42 4.11 4.56 4.37 4.90

2.74 4.94 5.76 5.32 5.70

CAlVs × 100 mAl

the same data for the 50 °C tests. The 3 M NaOH used to quench the reaction and dissolve the sodium aluminate does not allow for a distinction to be made between aluminate that was initially dissolved and aluminate that was precipitated as NaAl(OH)4·1.5H2O. Therefore, the aluminate molarity values in Tables 2 and 3 represent the sum of the aluminate produced (solid + liquid) per liter of the original prequench slurry. These values, along with the slurry volume and original gibbsite concentration, were used to calculate the percentages complete for the reaction converting gibbsite to sodium aluminate for samples A−C (Figure 2) and F−H (Figure 3)

Figure 2. Percent complete as a function of initial NaOH molarity at 25 °C and 8 mL of liquid per 32 mmol of gibbsite. 13836

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Figure 3. Percent complete as a function of initial NaOH molarity at 50 °C and 8 mL of liquid per 32 mmol of gibbsite.

Figure 4. Optical microscope image of sample A (8 M NaOH, 64 mmol total NaOH) after 1 h.

Figure 6. Optical microscope image of sample C (19.4 M NaOH, 155 mmol total NaOH) after 1 h.

Figure 5. Optical microscope image of sample B (11 M NaOH, 88 mmol total NaOH) after 1 h.

Figure 7. Optical microscope image of sample D (19.4 M NaOH, 88 mmol total NaOH) after 1 h. The light spots that can just barely be seen are sodium aluminate.

13837

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of the gibbsite from this sample, to provide a qualitative feel of the typical particle size. The gibbsite particles in the tank 241-S112 heel samples generally had diameters between 30 and 500 μm. The radioactive waste sample was treated with 19.4 M NaOH for 8 days because this seemed to be a reasonable time period based on the qualitative metathesis kinetics shown in Figure 2. Figure 12 shows the XRD pattern of the sample after this 8-day treatment with 19.4 M NaOH and indicates that NaAl(OH)4·1.5H2O was the predominant solid present, with no gibbsite identified. These results clearly indicate that 19.4 M NaOH can completely turn gibbsite from real nuclear waste tank heels into NaAl(OH)4·1.5H2O in a reasonable period of time (8 days). PLM was used to confirm the NaAl(OH)4·1.5H2O identified by XRD. Figure 13 shows a PLM image of the sodium aluminate crystals. Comparison of the plates in Figure 13 to the NaAl(OH)4·1.5H2O plates observed in Figure 9 demonstrates that these crystals are NaAl(OH)4·1.5H2O. The fact that NaAl(OH)4 ·1.5H 2O was found by PLM supports the conclusion that the NaAl(OH)4·1.5H2O observed by XRD is not just an artifact of XRD sample preparation. Of note in Figure 13 is that there are small holes in the centers of the square NaAl(OH)4·1.5H2O crystals. These holes are caused by a small drop of water that was deliberately added to the microscope slide. This is a diagnostic test for NaAl(OH)4·1.5H2O.39 NaAl(OH)4·1.5H2O plates exhibit the curious behavior of dissolving at the center first, followed by the edges.39 This is an indication of how water-soluble NaAl(OH)4· 1.5H2O is. Sodium aluminate is not very soluble in 19.4 M NaOH because of the high sodium concentration, but sodium aluminate easily dissolves in water.28 The next step in the treatment process is to dissolve the NaAl(OH)4·1.5H2O in water for removal from the tank. This second step, foreshadowed by Figure 13, will be the focus of future work by the authors.

Figure 8. Optical microscope image of sample E (19.4 M NaOH, 64 mmol total NaOH) after 1 h. The light spots that can just barely be seen are sodium aluminate.

broken up or fragmented. In Figure 9c (312 h of incubation), the needles have been largely replaced by the rectangular plate habit of NaAl(OH)4·1.5H2O. The cause of the transition from needles to plates is not completely known but might be because the plates were considerably larger than the needles and so had less surface energy. The metathesis reaction was much faster at 50 °C. As shown in Figure 3, the reaction was >80% complete in just 5 h in the samples with 19 M NaOH added. The effect of NaOH concentration was evident at 50 °C as it was at 25 °C. Sample H, the sample with the highest NaOH per aluminum ratio, was the only one that formed nearly 100% rectangular plate crystals, all other samples being dominated by the needle habit. The other samples simply might not have been incubated long enough to form the plates. Radioactive Sample. XRD revealed the predominance of gibbsite with small amounts of thermonatrite (Na2CO3.H2O) in the tank 241-S-112 heel sample prior to NaOH treatment (Figure 10). Deutsch et al.7 also found predominantly gibbsite in subsamples from this same sampling event. Apparently, the subsample they analyzed did not have sufficient thermonatrite to be observed by their XRD analysis, but they did see a “Na− Al−O” phase by SEM-EDS on a carbon-coated sample. A Na− C−O-containing species was also observed in the present SEMEDS analysis (Figure 11), with Al showing up in the spectrum from the neighboring gibbsite. This SEM-EDS-observable Na− C−O phase is presumably the thermonatrite observed by XRD. The Supporting Information contains a number of SEM images

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DISCUSSION The results in this study show that coarse gibbsite heels can be converted into water-soluble NaAl(OH)4·1.5H2O in just 8 days at ambient temperatures (25 °C) at the highest NaOH concentration. This was demonstrated using commercially procured gibbsite and validated using gibbsite from a real tank waste heel sample. Eight days is a reasonable time frame for treating a tank heel at the Hanford Site, given the benefits of heel removal. The rate was even higher at 50 °C. In the rare case where 8 days would be too long for operations staff, preheated caustic could be added to the tank. The metathesis

Figure 9. Progression of the reaction in sample C (25 °C, 19.4 M initial NaOH concentration). 13838

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Figure 10. X-ray diffraction pattern of tank 241-S-112 heel sample.

Figure 11. SEM image and associated EDS spectrum of cemented gibbsite particles from tank 241-S-112 heel.

occurred in just 5 h at 50 °C, so using preheated caustic would likely increase the rate substantially even if the temperature could not be maintained for more than a few hours. In-tank heating might be available at other nuclear waste sites.12 Of note is that the rate of conversion increases substantially as the NaOH concentration increases (Figures 2 and 3). For instance, increasing the starting NaOH concentration from 8 to 11 M increased the fraction metathesized only from 10% to 20% at 125 h, but the fraction metathesized was 80% at 19.4 M NaOH. The reason for this increase cannot be described

mechanistically at present. The structure and speciation of NaOH−NaAl(OH)4−H2O solutions is much different at very high NaOH concentrations than at 3−5 M NaOH concentrations.45−48 The coordination numbers of both sodium and hydroxide decrease at high NaOH concentrations because of the deficit of water available for ion hydration.45,48 Additionally, a much larger fraction of dissolved aluminum is in the form of the aluminate dimer [Al2O(OH)6] rather than the monomer at high NaOH concentrations.46,47 Consequently, it is not surprising that this dramatic change in solution structure 13839

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Figure 12. X-ray diffraction pattern of tank 241-S-112 heel sample after treatment with 19.4 M NaOH for 8 days.

morphology for NaAl(OH)4·1.5H2O was observed previously by Sprauer and Pearce28 as well as Fricke and Jucaitis33 at 25 °C. Cao et al.31 reported a plate morphology, whereas You et al.32 showed micrographs of NaAl(OH)4·1.5H2O shaped like a set of plates that were so highly twinned they looked like a ball. In their studies, NaAl(OH)4·1.5H2O was crystallized by evaporating aqueous sodium aluminate solutions. To the authors’ knowledge, the present work is the first to report the observance of the needle morphology transforming into the plate morphology over time (in this case, within 312 h). Both the Bayer process (used in the alumina refining industry) and the bulk nuclear waste aluminum leaching process dissolve gibbsite in much lower NaOH concentrations than 19.4 M.18,23,24,49 In the Bayer process, a high NaOH concentration is not used because it could cause problems with excessive silica solubility.50 Most nuclear waste does not contain much of silica (noting the lack of any silicates observed here). High NaOH concentrations are not used in the bulk removal of gibbsite from Hanford waste because the high viscosity of highly concentrated solutions would hinder the subsequent filtration process.51 The facts that silica dissolution is not a concern for waste heels and the heel removal solutions are not filtered enables the use of high sodium hydroxide concentrations here. The high NaOH concentration, in turn, allows for gibbsite metathesis even at ambient temperatures (Figure 2).

Figure 13. PLM image of solid sodium aluminate after digestion of tank 241-S-112 heel sample in 19.4 M NaOH. All of the crystals in this photograph are sodium aluminate; several prominent crystals are labeled.

would have an effect of some sort on the rate of gibbsite metathesis. Here, it is demonstrating that the rate is enhanced. Figure 9 illustrates a change in morphology of NaAl(OH)4· 1.5H2O over time. Previous observations of the needle morphology for NaAl(OH)4·1.5H2O were documented by Herting39 as well as Fricke and Jucaitis.33 The rectangular plate 13840

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In addition to increasing the conversion rate, high NaOH concentrations are also desirable because they minimize the amount of water in the Hanford double-shell tank system, which, in turn, minimizes the amount of space taken up by the waste. The present results show that gibbsite metathesis to NaAl(OH)4·1.5H2O can be complete within 8 days at ambient temperatures using elevated NaOH concentrations instead of temperature. An 8-day leach step would be too slow for a continuous process run on bulk waste sludge, but it is a reasonable length of time for removing tank heels. Removal of tank heels is only required prior to tank closure. Figure 9 indicates that coarse gibbsite is broken up into smaller shards after just a few days. Thus, an alternative to this process could be simply to use concentrated NaOH to break up the gibbsite into smaller shards that can be more easily pumped from the tank using standard slurry-handling technologies. The process developed here is an alternative to oxalic acid cleaning. Oxalic acid has limited success with Hanford gibbsitebased heels unless impractical elevated temperatures are employed.22 Oxalic acid treatment resulted in the precipitation of calcium and manganese oxalate from solution, which were left behind in the tank when the Hanford tank farm contractor retrieved tank 241-C-106.7 In contrast, no solids that are difficult to dissolve were created by treating these samples with concentrated NaOH(aq). The sodium oxalate created by oxalic acid cleaning also creates problems for downstream treatment processing18,20 and adds to the mass of waste that must be vitrified. In contrast, the present process adds only NaOH, a chemical that would eventually have to be added to the waste for corrosion control and bulk waste leaching anyway.

CONCLUSIONS This study has determined that gibbsite readily converts into NaAl(OH)4·1.5H2O within a few days in concentrated NaOH(aq) solution, even at 25 °C. NaAl(OH)4·1.5H2O is highly water-soluble.27,28 A wide range of metathesis conditions were investigated using commercially procured gibbsite. Those results allowed for the selection of a time (8 days) and NaOH concentration (19.4 M before dilution by interstitial liquid) to test a real waste sample. The real waste was shown to convert into NaAl(OH)4·1.5H2O in this 8-day period, the product being confirmed by PLM and XRD. This study has successfully demonstrated that coarse gibbsite from a real nuclear waste tank heel can be converted into solid NaAl(OH)4·1.5H2O over a reasonable time period in concentrated NaOH. These results thus verify the first part of a new process to remove coarse gibbsite particles from nuclear waste tank heels. ASSOCIATED CONTENT

S Supporting Information *

SEM images of Gibbsite from Hanford Tank 241-S-112 Heel. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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*E-mail: [email protected]. Tel.: 509-373-5999. Fax: 509-373-3883. Notes

The authors declare no competing financial interest. 13841

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