M. A. Baich
Westinghouse Savannah River Company
Aiken, SC 29808

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The antifoam agent IIT B52 was found to have no detectable effect upon the rate of phenylboric acid (PBA) reaction during acid hydrolysis of a tetraphenyl borate (TPB) simulant slurry containing 2000 ppm of the antifoam agent. Analysis of reactor contents at the completion of feeding revealed no detectable quantity of the antifoam agent. Analysis of all process streams at the completion of the hydrolysis cycle also revealed no detectable quantity of the antifoam agent. The antifoam agent, IIT B52 appears to rapidly decomposes in the feed slurry and/or during acid hydrolysis of the TPB feed.

Should the Small Tank TPB process is selected as the Salt Processing process, then future work is recommended to identify, quantify, and evaluate the impact of the decomposition products of IIT B52 antifoam agent. Future studies should incorporate all components and factors that are known to affect the properties of the precipitate slurry and the process chemistry: mercury, sludge solids, and radiation dose during slurry storage time.

Keywords: Antifoam, KTPB, STTP

One of the alternatives to processing the highly radioactive salt solutions in the SRS Waste Tanks is to precipitate the radioactive cesium with sodium tetraphenylborate and then concentrate and wash the precipitate slurry for subsequent processing in the Defense Waste Processing Facility (DWPF). This alternative salt disposition process is called the Small Tank Tetraphenylborate Precipitation process (STTP). In the STTP precipitation process, soluble ions of cesium, potassium and ammonium are precipitated as insoluble TPB (tetraphenylborate) salts. Strontium, uranium, and plutonium are sorbed on solid monosodium titanate (MST). The resulting slurry, which now contains most of the radionuclides as insoluble solids, is filtered to concentrate the solids. After washing the solids to reduce the concentration of soluble sodium salts in the slurry, the precipitate is stored until it can be further processed and incorporated into glass in the DWPF. The decontaminated salt solution or filtrate is transferred to Z Area for processing and disposal as Saltstone.¹

In recent tests of the precipitation process using actual radioactive waste material excessive foaming was observed.² Foaming was also observed in testing at ORNL using slurry spiked with radioactive cesium.³ Foaming during the precipitation, concentration and washing steps using simulants was also observed at SRTC.⁴ As a result of these experiences with foam generation during proposed STTP process steps, an investigation into finding suitable antifoam/defoam agents that can eliminate or mitigate the consequences of foam generation during normal operations of the proposed STTP process was undertaken.

Antifoam experts at Illinois Institute of Technology (Drs. D. T. Wasan and A. D. Nikolov) were contracted to develop and recommend three potential antifoam/defoam agents to be tested in laboratory scale demonstrations of the precipitation, concentration and washing steps using simulate waste. Antifoam agent IIT B52 has been adopted for use in the STTP process, but the full impact of its use has not been evaluated.

Key in this evaluation is how does the use of IIT B52 affect the hydrolysis kinetics of phenylboric acid (PBA). Complete hydrolysis of PBA is required to meet several process requirements. First, Precipitate Hydrolysis Aqueous must be below 52 ppm in PBA to insure that a flammable mixture can not be formed in the Chemical Process Cell (CPC). Second, the highest possible aromatic carbon removal from the Precipitate Hydrolysis Aqueous is required to minimize the buildup of organic materials in the Chemical Process Cell. Third, PBA has been shown to slow the hydrolysis of diphenylmercury leading to higher than allowed mercury content in the organic product from hydrolysis.⁵

Precipitate slurry used for batches (1) "No Antifoam-475", and (2) "IIT B52 antifoam-475" of this study was previously prepared for use in foam column testing, but was never used.

Note: Batch name expresses the use of antifoam and the target copper concentration.

This slurry contained monosodium titanate (MST) adsorbent, but no sludge solids or diphenylmercury. No irradiation of the slurry had been performed to simulate storage under radioactive conditions. Table A-1 presents the analysis and batch calculations for preparation of the pre-reaction heel for this slurry.

Precipitate feed slurry used for batches (3) "No Antifoam-950", and (4) "IIT B52 antifoam-950" of this study was previously prepared for use in foam column testing. This slurry contained monosodium titanate (MST) adsorbent, but no sludge solids or diphenylmercury. No irradiation of the slurry had been performed to simulate storage under radioactive conditions. Table A-2 presents the analysis and batch calculations for preparation of the pre-reaction heel for this slurry.

Two identical experimental rigs were set up in a hood in lab 109 in 772-T. The equipment used is summarized in Figure 1. A feed to heel ratio of 2.25 was used similar to that used in the Precipitate Hydrolysis Experimental Facility (PHEF); [320 gallon heel with 720 gallon feed batch.] The mass of feed was set at 1000 grams.

The precipitate slurry was mixed continuously on a magnetic stirrer plate and pumped into the reaction vessels using calibrated Masterflex Model 7550-90 pump controllers with Model 7518-10 pump heads and a #16 silicone tubing. This tubing material was chosen because it is chemically resistant to caustic solutions.

Two identical rigs used a Servodyne Model 50003-30 agitator with a Model 50000-00 controller. The nitrogen purge is designed to exclude oxygen from the vapor space, preventing a flammable mixture from accumulating in the vessels or offgas system.

All experimental runs were performed with the following common experimental conditions:

1. Target feed mass of 1000 gm, (No water flush performed)
2. Target feed time of 100 minutes, i. e. 10 ml/min feed rate
3. Target final acidity of 0.25 molar,
4. Feed temperature of 90 deg C,
5. Five hour hold period at 90 deg C,
6. Condenser water temperature of 10 deg C,
7. Nitrogen purge of 3.36 sccm, (40 sccm during heat-up to purge vessel)
8. Agitation rate of 170 rpm,
9. Aqueous boiling time of 3 hours.

The target copper concentration for batches 1-2 was 475 ppm while the target copper for batches 3-4 was 950 ppm. Table 1 presents the Pre-reaction heel make-up.

If required, the IIT B52 antifoam was added to the precipitate just prior to the start of heat-up at the 2000 ppm level. Antifoam agent was 75% in ethanol dated 9/14/2000 was used in all experimental runs. This level of antifoam represents that amount calculated to be in the feed if it remains stable and does not wash out of the slurry.⁶

Two sets of experiments were completed for a total of 4 hydrolysis experiments.

Since the data from the first two runs indicated incomplete conversion of PBA during the 5 hour hold period, the target copper concentration was increased to 950 ppm for the second series of runs.

NOTE: A target copper concentration of 950 ppm represents the current design basis for the proposed STTP process.

The final acidity of this run was 0.321 molar and the benzene generation was visibly less than that expected based upon slurry analysis, indicating poor mixing of the carboy prior to transfer of the batch of precipitate slurry into the feed vessel. The feeding of low solids slurry would consume less acid and lead to a higher final acidity compared to the target of 0.25 molar. The higher acid content would tend to increase the PBA reaction rate during the 90 deg C five hour hold period. A linear plot of the natural log of the PBA concentration versus time produced a first order reaction rate constant of 0.41/hrs for this run.

The final acidity for the antifoam run was 0.186 molar. The benzene generation for this run was visibly greater than that expected based upon slurry analysis. Poor mixing of the carboy prior to transfer of the slurry for the first batch would lead to higher solids content during the second transfer into the feed vessel and consume more acid during feeding. A linear plot of the natural log of the PBA concentration versus time produced a first order reaction rate constant of 0.27/hrs for this run.

Note: Instrumentation failure prevented analysis of process streams for IIT B52 antifoam agent.

The final acidity for the no antifoam run was 0.247 molar. The benzene generation for this run was about that expected based upon slurry analysis. A linear plot of the natural log of the PBA concentration versus time produced a first order reaction rate constant of 1.12/hrs for this run. The PBA concentration at the end of the 5-hour hold was 29 ppm. This is approximately the detection limit of the instrument used for the analysis. Organic product collected was about 64 grams.

The final acidity for the antifoam run was 0.247 molar. The benzene generation for this run was about that expected based upon slurry analysis. A linear plot of the natural log of the PBA concentration versus time produced a first order reaction rate constant of 1.27/hrs for this run. The PBA concentration at the end of the 5-hour hold was below the detection limit of about 29 ppm. Organic product collected for this batch was also about 64 grams, indicating a comparable mass of TPB was fed in both batches.

The antifoam agent IIT B52 was added to the precipitate slurry at a concentration 2000 ppm approximately 20 minutes prior to the start of feeding. A sample of the reactor taken at the completion of feeding showed no detectable B52. (Detection limit 15 ppm.) Samples of the decanter at the completion of the process cycle also showed no detectable quantity of B52 antifoam agent. The IIT B52 antifoam agent appears to rapidly decompose in the precipitate hydrolysis process.

NOTE: Samples of each of the process streams were spiked with a known amount of IIT B52 antifoam and analyzed to insure that IIT B52 could be detected in each of the process streams. Analysis of these blind standards demonstrated that IIT B52 could be identified, but less than 100% recovery was achieved. This indicates that the IIT B52 is decomposing rapidly (5-10 minutes) even at room temperature.

The Phenyl boric acid reaction rates observed at a target final copper concentration of 475 ppm did not lead to complete reaction of the PBA during the 5 hour hold period at 90°C. The experimental run containing 2000 ppm of IIT B52 antifoam in the feed had a lower rate constant than the run performed without the antifoam. However, this difference can be attributed to the difference in final acid concentration between the two runs.

The Phenyl boric acid reaction rates observed at a target final copper concentration of 950 ppm demonstrated complete reaction of the PBA during the 5 hour hold period at 90°C. The experimental run, with the higher target copper concentration containing 2000 ppm of IIT B52 antifoam in the feed had a slightly higher rate constant compared to the run performed without the antifoam.

While direct comparison of the reaction rate constants determined in this study with previous studies is not possible, the numerical values are similar and the reaction rates observed at 950 ppm copper would allow all process requirements to be achieved.

The addition of 2000 ppm of IIT B52 antifoam to the precipitate slurry has no detrimental effects upon the PBA hydrolysis kinetics. Decomposition of IIT B52 in the precipitate hydrolysis process is rapid, with no detectable quantity of the antifoam agent in any of the product streams. Operation of the Precipitate Hydrolysis process with IIT B52 antifoam and a target copper concentration of 950 ppm will allow product and cycle time requirements to be met.

This study has demonstrated the complete decomposition of IIT B52 antifoam agent in the precipitate hydrolysis process, but the impact of these decomposition products upon down stream processes has not been determined. Therefore, future work to identify and track these decomposition products through the entire melter feed preparation process is recommended. Future testing should incorporate mercury compounds and sludge solids in the test simulant. Irradiation of the test simulant is also recommended.

1. R. A. Dimenna, et. al., Bases, "Assumptions, and Results of the Flowsheet Calculations for the Decision Phase Salt Disposition Alternatives", WSRC-RP-99-00006 Revision 0 1999.
2. R. A. Peterson and J. O. Burgess, "The Demonstration of Continuous Stirred Tank Reactor Operations with High Level waste", WSRC-TR-99-00345, Revision 1, February 15, 2000.
3. D. D. Lee and J. L. Collins, "Continuous-Flow Stirred-Tank Reactor 20-L Demonstration Test: Final Report, ORNL/TM-1999/234.
4. Personal communication with Mike Poirier about producing KTPB slurry.
5. C. J. Bannochie, "Technical Bases for Precipitate Hydrolysis Process (U)" WSRC-TR-92-458, Rev 1.
6. M. A. Baich, Task Technical Plan "Phenylboric Acid Hydrolysis Kinetics with IIT B52 Antifoam" WSRC-RP-2000-00988, Rev. 0 , December 8, 2000.