WSRC-MS-99-00657

Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161.

Abstract

A batch vitrification process, which utilizes an oxalate precipitate and frit (or cullet), is being developed at the Savannah River Technology Center (SRTC) to immobilize an Am-Cm solution. Prior to being accepted as the baseline flowsheet, numerous laboratory-scale tests were conducted to demonstrate its feasibility and to characterize the general melt behavior of the oxalate/frit system. The effects of frit particle size and oxalate precipitation temperature were the initial focus of these studies. Two technical issues were identified during these initial tests that warranted further study: a volume or bed expansion was observed at approximately 1140°C and "excessive" bubble formation between 1220 - 1250°C. Although high temperature bubble formation does not pose a serious process concern (i.e., longer residence times and/or higher process temperatures minimize bubble retention), the volume expansion is undesirable during processing. The volume expansion may limit the amount of glass that can be produced in a single batch. That is, the batch height may have to be controlled so that the material is contained within the Pt-Rh vessel at all times.

Both the volume expansion and high temperature bubble formation have been linked to the thermal reduction of CeO₂. As part of the oxalate feed, Ce is reduced (3+ state). Upon thermal decomposition of the oxalate under oxidizing conditions, Ce will oxidize (3+ 4+ state) which provides the opportunity for thermal reduction at higher temperatures liberating O₂. Tests using a "Ce-free" oxalate have been performed in which no indication of either the volume expansion or high temperature bubble formation were observed. Complementary studies focused on redox and off-gas related issues provided a fundamental understanding of the melting behavior of the oxalate/frit system and lead to the successful development of the batch vitrification process.

Introduction

Approximately 11,000 L of solution containing isotopes of Am and Cm, currently stored at the Savannah River Site (SRS), will be converted into a high-lanthanide glass for stabilization [1]. Pretreatment operations will be performed to separate actinides and lanthanides from impurities (primarily iron, aluminum, and sodium) and to concentrate the solution to approximately 100 g/L dissolved solids (as equivalent metal oxide) in a nitric acid solution. Approximately 10 wt.% of the total dissolved solids are Am and Cm. The nitric acid-based Am-Cm solution (or in the case of this study the non-radioactive surrogate) is transferred into a reaction vessel. Oxalic acid (8 wt.%) is added to the solution to precipitate the rare-earth elements as oxalates. The precipitate is allowed to settle and the excess liquid is decanted. A dilute oxalic acid wash solution (0.10M) is added to remove excess contaminants (e.g., Fe, Cr) and reduce the acidity of the feed [2]. The precipitate is again allowed to settle and the washwater is decanted. The settled slurry is mixed and then gravity fed to a cylindrical Pt-Rh induction heated vessel. The proper amount of frit is added to result in a final glass composition containing ~35 - 45 wt.% of oxides from the Am-Cm solution. A simplified schematic of the batch process and equipment is shown in Figure 1.

Figure 1.   Schematic diagram of the Am/Cm vitrification process.

The oxalate precipitate slurry and frit are added to the melter vessel and heated to approximately 100°C. This temperature is held until the mixture has dried. The vessel temperature is then raised at a rate between 4 and 8°C/min to a glass temperature between 1350 and 1450°C. During this temperature increase, the remaining nitrate is decomposed and the oxalate precipitate decomposes to the associated rare-earth oxides. The melt is held at temperature for approximately 1 to 4 h to ensure adequate mixing of frit and oxides and is drained from the bottom of the melter by gravity through a drain tube into a stainless steel canister.

The reaction chemistry involved with converting these oxalate feed stocks into glass products determines the potential for volume expansion, bubbling, and other melt and off-gas related phenomena associated with this process. Knowledge of the chemical processes is critical to vitrification plant design, operation, and troubleshooting. The rare earth and actinide oxalates will decompose and release CO and CO₂. Residual nitrates and waters of hydration will also be released during the melting process. Oxidation states will likely change, and ultimately rare earth and actinide elements will be incorporated into the glass. Vienna et al. provide a detailed discussion of the reaction pathways for this system [3].

Prior to accepting this batch process as the baseline flowsheet, numerous laboratory-scale tests were conducted to demonstrate its feasibility and to characterize the general melt behavior of the oxalate/frit system. The effects of frit particle size and oxalate precipitation temperature were the initial focus of these studies. This report discusses the results of these initial studies that were performed to evaluate the feasibility of the oxalate-based batch process. Identification of the mechanism(s) and source(s) of the volume expansion and high temperature bubble formation was a primary objective. Complementary studies focused on redox and off-gas related issues provided a fundamental understanding of the melting behavior of the oxalate/frit system.

Experimental

Initial laboratory-scale tests (i.e., Runs A - G) focused on the effects of the particle size (-80,+200 mesh, -14,+30 mesh, and cullet) of the 25SrABS frit (25SrABS is the baseline frit composition established for the Am-Cm vitrification program. This frit contains 25 mass % La₂O₃ but has a high solubility limit of other lanthanide oxides or rare earth oxides. Total lanthanide oxide contents of up to 60% have been demonstrated while still meeting both process and product performance constraints.[4]) and oxalate precipitation temperature (50° and 90°C). Twenty-five (25) grams of oxalate precipitate was added to 23.52 grams of 25SrABS frit (1.0623 gram of oxalate per 1 gram of frit) which targets an approximate 39 wt.% feed loaded glass on an oxide basis. To evaluate the effect of frit particle size and oxalate precipitate temperature various combinations were utilized (see Table I). The various oxalate precipitate and pre-sized frit combinations were subsequently mixed and placed in a 100 ml 95% Pt / 5% Rh crucible. The batches were heated at 4°C/min up to 1350°C. Characteristics of the melting behavior of each batch were recorded during the melt regime. After a 0.5 hour isothermal hold at 1350°C, the crucibles were removed and allowed to cool to room temperature. Visual observations of the resultant glass product were also documented.

Results

Visual observations of Runs A - G (-80,+200 mesh and -14,+30 mesh 25SrABS frit coupled with both the 50 and 90°C oxalate precipitate) are summarized in Table I. Similar melting characteristics were observed regardless of frit particle size or oxalate precipitate temperature utilized. In general, as the temperature increased from room temperature to 500°C, the oxalate/frit batches transitioned from white to a dark green / rust color. The change in batch color being associated with the decomposition of the oxalate which occurs over a temperature span of ~400° - 800°C based on DTA/TGA data. A reduction in batch volume was observed as a result of the oxalate decomposition. Based on visual observations at 1000°C, there was no apparent liquid (or glass) phase present. At 1000°C, Runs A - G were characterized by a "crusty" top surface which visually appeared crystalline.

As temperatures continued to increase, a common denominator for all tests was a volume expansion at ~1140°C. The onset of the volume expansion was characterized by a slight increase in batch height in the center of the crucible which gradually increased (with increased temperature). The volume expansion was estimated to be an approximate 1 - 2x increase in batch height at its maximum. The onset of the volume expansion was consistently observed at ~1140°C for each oxalate/frit system. At ~1200°C, an initial liquid was observed which was followed by a gradual volume reduction. The formation of the initial liquid (or glass) phase may have occurred at a lower temperature.

In concert with the formation of the initial liquid phase, bubbles were observed on the melt surface near the Pt crucible interface between 1200 - 1240°C. As temperatures increased, bubbles accumulated on the melt surface. At 1350°C, the degree of bubble coverage on the melt surface varied from run to run. In some cases, bubbles were observed across the entire melt surface while only a ring of bubbles around the melt/crucible interface was observed in other runs. Although varying degrees of coverage were observed, bubble formation was observed in Runs A - G at temperatures in excess of 1200°C.

After the 0.5 hour isothermal hold at 1350°C, the glasses were homogeneous with respect to crystallization but the melt surface was typically characterized by some degree of very small bubbles on the glass surface. It should be noted that increased residence times and/or increased process temperature these bubbles have been demonstrated to be very effective in eliminating these bubbles (i.e., fining).

Based on these initial tests, no significant differences in the melting behavior of these oxalate / frit mixtures were observed. The volume expansion and high temperature bubble formation was consistently observed in systems containing either size classification (-14,+30 and -80,+200) of the 25SrABS frit and each oxalate precipitation temperature (50° and 90°C).

To address the effect of a larger frit particle size, "cullet" was produced by melting 25SrABS frit at 1450°C and pouring into deionized water. This technique produced cullet which was characterized by a wide range of particle sizes (the majority exceeding 14 mesh) and distribution. Since the results of Runs A - G showed no difference between the 50° and 90°C precipitate, 25SrABS cullet was mixed with 50°C oxalate precipitate and processed under similar conditions (i.e., 4°C/min to 1350°C with a 0.5 isothermal hold). Table II summarizes the results of the 50°C oxalate / cullet tests (Runs H - L). The major difference with the increased particle size (or cullet) is the absence of the volume expansion observed at 1140ºC in the previous runs (see Table I). Again, the volume expansion is undesirable from a processing view point because it may limit the amount of glass that can be produced in a single batch. The minimization (or elimination) of the volume expansion via the use of larger frit particles (i.e., cullet) is a major process advantage. Subsequent testing in the Drain Tube Test Stand (DTTS) and the Cylindrical Induction Melter (CIM) has demonstrated that the use of cullet eliminates (or minimizes) the volume expansion observed with finer particle sizes.[2]. High temperature bubble formation was observed between 1228 and 1300°C during all oxalate / cullet tests.

Although the use of cullet does minimize (or eliminate) the volume expansion in this system, the mechanism was still unknown and the reaction pathways were not well understood. Therefore supplemental tests were performed to provide a fundamental understanding of the off-gas and redox related issues in an attempt to isolate the mechanism of the volume expansion and high temperature bubble formation. With the mechanism known, chemical (e.g., frit composition) and/or physical changes (e.g., melter design or heating profiles) could be made, if necessary, to the system and the batch flowsheet could be refined.

Supplemental tests in the development of the batch process included "frit-only" tests, an evaluate of redox as a function of temperature for select multivalent cations associated with the feed, gas analysis of samples from a pilot scale melter, and quartz crucible tests that characterized both off-gas as a function of temperature and confirmed of visual observations of the initial laboratory-scale tests. The results of each of these complementary tests can be used as part of the larger body of data to support the evaluation of the batch process as a viable flowsheet option and also provides critical data to establish a fundamental understanding of the reaction kinetics and phenomena occurring in this system.

Frit-Only Tests

The objective of the "frit-only" tests was to confirm that the source of the volume expansion was associated with the feed material; not the 25SrABS frit composition. These tests utilized both the 25SrABS frit (no surrogate feed materials included) and pre-fabricated glass compositions (containing all or part of the surrogate feed components). Two pre-fabricated glasses fabricated at 1350°C from reagent grade oxides and carbonates were evaluated. The initial pre-fabricated glass targeted a 30 wt.% feed loading (referred to as 50SrABS Hybrid) which contained all the surrogate components associated with the oxalate based tests. The second pre-fabricated glass was Frit 1000. This was a lanthanide-based borosilicate glass containing 6.8 wt% CeO₂ as a surrogate. Pre-sized 25SrABS frit classifications (-14,+30, -80,+200, and cullet) were also evaluated in the absence of oxalate (i.e., no surrogate present).

In these tests, approximately 50 grams of pre-fabricated glass or frit was added to a 100 ml 95% Pt / 5% Rh crucible and heated at 4°C/min to 1350°C or 1450°C (for the 25SrABS frit only). Visual observations were recorded during the melt regime as well as on the resulting glass product. Although no volume expansion was observed in any system (consistent with melting a prefabricated glass or frit), the results were very effective in isolating the source of the high temperature bubble formation. High temperature bubble formation was not observed with the 25SrABS frit up to 1450ºC. However, tests with 50SrABS Hybrid and Frit 1000 resulted in the formation of high temperature bubbles at 1220°C and 1250ºC, respectively. The temperature at which bubbles formed in these tests is very consistent with those observed in Runs A - L (see Tables I and II). The initial hypothesis for the source of bubble formation was thermal reduction of one (or more) of the multivalent oxides associated with the surrogate. A primary candidate was Ce (present in both 50SrABS Hybrid and Frit 1000) which is known to thermally reduce at high temperatures. This hypothesis was confirmed via subsequent testing.

To provide insight into Ce redox (Ce⁴⁺/Ce³⁺) as a function of temperature, samples of the oxalate precipitate were heated at 110°C for 24 hours, 500°C (DTA/TGA data indicates the onset of oxalate decomposition at ~400°C) for 2 hours, and 1300°C for 24 hours under oxidizing conditions. The samples were dissolved in a mixture of hydrofluoric and sulfuric acid at room temperature and an aliquot portion of the stock solution was titrated by (NH₄)₂Fe(SO₄)₂ at potentiometric indication of the equivalence point. A calibration curve of Ce⁴⁺ was established by using a standard ceric sulfate solution. Ce³⁺ was determined by difference using the targeted total Ce concentration.[5] The Ce redox results are summarized in Table III.

The results indicate that Ce, starting almost completely reduced in the oxalate form (Ce⁴⁺/Ce³⁺ = 0.01), oxidized partially (Ce⁴⁺/Ce³⁺ = 0.55) when heat treated to 500°C, and completely reduced (Ce⁴⁺/Ce³⁺ = 0.00) when heat treated to 1300°C. These data are consistent with DTA/TGA data that indicate oxalate decomposition occurring between 400° - 600°C. The oxidation of cerium upon thermal decomposition provides the opportunity for thermal reduction at higher temperatures. The redox analyses indicate full reduction of cerium by 1300°C, which supports the high temperature bubble formation observed in all systems containing the surrogate (i.e., thermal reduction provides a gas source (O₂) for bubble generation).

Two Drain Tube Test Stand (DTTS, a 2.5 " cylindrical pilot scale Pt melter) samples were collected from the melt surface during the vitrification of an oxalate / cullet mixture at approximately 1200°C. The samples were visually characterized as a frothy or foamy glass (i.e., glassy but containing numerous small bubbles). These samples were sent to Corning Engineering Laboratory Services (CELS) for gas analysis by mass spectroscopy via the "crack method". Results of the bubble analyses indicated that the bubbles were "oxygen-rich": 70±5% O₂, 3±1% CO₂, 27±4% N₂ and/or CO (the mass spectrometer used for this analysis could not distinguish between N₂ and CO, which both have a mass of 28 amu). Since at room temperature virtually no CO is in equilibrium with 70% O₂, it is likely that a majority of the mass 28 species is N₂. The present of "oxygen-rich" bubbles in the DTTS samples supports the theory of CeO₂ reduction leading to O₂ generation which results in the high temperature bubble formation.

The reaction chemistry involved with converting these oxalate feed stocks into glass products determines the potential for volume expansion, bubbling, and other melt and off-gas related phenomena associated with this process. Although the source of the high temperature bubble formation has been linked to the thermal reduction of CeO₂, the volume expansion observed in the initial tests at 1140°C (see Table I) have not. The volume expansion could be related to CeO₂ reduction and/or other off-gas related issues. To gain some understanding of the off-gas generated during oxalate decomposition, calcination, and the vitrification processes, the PNNL quartz crucible (QC) furnace was utilized. This furnace allows for visual observations of the batch during the batch-to-glass conversion process as well as "real-time" monitoring off-gas as a function of time (which can then be correlated to temperature) via a GC/MS.[6] Initially, three oxlalate / frit batches were initially tested using the quartz crucible furnace. These initial QC tests utilized a -80,+200 mesh 25SrABS frit with a dried 50°C oxalate precipitate targeting 39 wt% feed loading (on an oxide basis). The batches were heated at approximately 4°C/min to 1350°C. Based on previous tests, the use of the -80,+200 mesh particle size should result in a volume expansion at approximately 1140°C. Table IV shows the major run conditions and observations.

Runs 1017 and 1018 were performed under inert conditions (He / 2% Ar) in an effort of to monitor O₂ liberation (from CeO₂ reduction) as a function of time (or temperature). The intent was to link the on-set of CeO₂ reduction to the volume expansion via the detection of O₂. Although an initial liquid (or glass) phase was observed at 1110° and 1150°C for Runs 1017 and 1018, respectively, there was no evidence of a volume expansion, high temperature bubble formation, or O₂ detection. This was initially surprising since all previous runs with the -80,+200 mesh frit resulted in both phenomena. Coupling the Ce redox data (see Table III) with the results of Runs 1017 and 1018 under inert conditions, the absence of the volume expansion and high temperature bubble formation are easily explained. As part of the oxalate feed, Ce is reduced (3+ state). Upon thermal decomposition of the oxalate (onset at ~400°C) under oxidizing conditions, the Ce will oxidize (3+ 4+ state) and then thermally reduce at high temperatures. Under inert conditions (He / 2% Ar), Ce will remain in the 3+ state and thus there is no thermal reduction at high temperatures and hence no source for the volume expansion or high temperature bubble formation. Runs 1017 and 1018 confirm that the volume expansion and high temperature bubble formation is a result of the thermal reduction of CeO₂. It should be noted the detection of the initial liquid phase in Runs 1017 and 1018 at 1110 and 1150°C, respectively, is consistent with the onset of the volume expansion observed in Runs A - G.

Observations during Run 1018 O₂ (flowing He/21% O₂) indicated a large batch expansion (~ 3× volume) starting at 1075°C and peaking at roughly 1150°C before subsiding. Additionally, bubbles formed in the melt from 1200°C to 1240°C. These quartz crucible furnace observations agree with those made during initial laboratory-scale (see Table I) and pilot plant tests [2], suggesting that the roughly 5 g quartz crucible tests are a reasonable simulation of pilot scale melting. Tests using a "Ce-free" oxalate have been performed in the DTTS and CIM. The results showed no indication of either the volume expansion or high temperature bubble formation that confirm that cerium reduction is the primary source for both phenomena. Vienna et al. provide a detailed discussion of the reaction pathways for this system [3].

The proposed mechanism for the volume expansion has been linked to the overlap of two key processes: the softening of the 25SrABS frit and the release of O₂ via the thermal reduction of multivalent oxides in the feed. As temperatures increase in the oxalate / frit system, thermal decomposition occurs at approximately 500ºC. Under oxidizing atmospheres, the multivalent oxides (reduced in the oxalate) oxidize creating the potential for thermal reduction at higher temperatures. At approximately 1140ºC, the 25SrABS frit beings to soften while thermal reduction of the oxidized multivalent oxides is occurring. With the softening of the 25SrABS frit, a highly viscous liquid is formed which traps the O₂ being liberated from thermal reduction. The result is an increase in batch volume until sufficient liquid phase is formed with a viscosity that allows for the trapped bubbles to escape. Once a liquid phase, bubble formation can still be observed until the thermal reduction of the multivalent oxides is complete (approximately 1300ºC in this system). Again, increased residence times and/or processing temperatures have shown to be effective in eliminating bubble from the final product.

Summary

A batch vitrification process, which utilizes an oxalate precipitate and frit (or cullet) is being developed at the Savannah River Technology Center (SRTC) to immobilize an Am-Cm solution. Prior to being accepted as the baseline flowsheet, numerous laboratory-scale tests were conducted to demonstrate its feasibility and to characterize the general melt behavior of the frit/oxalate system. The effects of frit particle size and oxalate precipitation temperature were the initial focus of these studies. The results of these initial tests indicated no significant difference in the melting behavior of batches using various combinations of -80,+200 or -14,+30 mesh 25SrABS frit coupled with 50° or 90°C oxalate precipitate. Two technical issues were observed during these initial tests that warranted further study. A volume or bed expansion was observed at approximately 1140°C and "excessive" bubble formation between 1220 - 1250°C.

The Ce redox studies indicated that Ce, starting almost completely reduced in the oxalate form (Ce⁴⁺/Ce³⁺ = 0.01), oxidized partially (Ce⁴⁺/Ce³⁺ = 0.55) when heat treated to 500°C, and completely reduced (Ce⁴⁺/Ce³⁺ = 0.00) when heat treated to 1300°C. These data are consistent with DTA/TGA data that indicate oxalate decomposition occurring between 400° - 600°C. The oxidation of cerium upon thermal decomposition provides the opportunity for thermal reduction at higher temperatures (i.e., thermal reduction provides a gas source (O₂) for bubble generation).

No volume expansion or high temperature bubble formation was observed in the oxalate / frit quartz crucible tests performed under inert conditions (2% He/ Ar). Under inert conditions (He / 2% Ar), Ce will remain in the 3+ state throughout the decomposition, calcination, and vitrification processes and thus there is no thermal reduction at high temperatures. Therefore, the gas source to cause the volume expansion or high temperature bubble formation is not present.

Observations during Run 1018 O₂ (flowing He/21% O₂) indicated a large batch expansion (~ 3× volume) starting at 1075°C and peaking at roughly 1150°C before subsiding. Additionally, bubbles formed in the melt from 1200°C to 1240°C. These quartz crucible furnace observations agree with those made during initial laboratory-scale and pilot plant tests [2], suggesting that the roughly 5 g quartz crucible tests are a reasonable simulation of pilot scale melting. Tests using a "Ce-free" oxalate have been performed in the DTTS and CIM. The results showed no indication of either the volume expansion or high temperature bubble formation that confirm that cerium reduction is the primary source for both phenomena
Laboratory, pilot, and full-scale testing has demonstrated that the volume expansion can be minimized (or eliminated) through the use of large particle sized frit (i.e., 25SrABS cullet). Tests using a "Ce-free" oxalate have been performed in the DTTS and CIM. The results showed no indication of either the volume expansion or high temperature bubble formation that confirm that cerium reduction is the primary source for both phenomena.

References

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5. H. Li, J.D. Vienna, P. Hrma, M.J. Schwieger, D.E. Smith, and M. Gong, "Borosilicate Based Glasses for Immobilization of Plutonium-Bearing Materials, Ceramic Transactions, Volume 72, pp. 399 - 408 (1996).
   
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