Hydrolysis of late-washed, irradiated tetraphenylborate slurry simulants I: Phenylboric acid hydrolysis kinetics Page: 10 of 54
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Westinghouse Savannah River Company WSRC-TR-95-0099
Savannah River Technology Center Revision 0
Page 2 of 46
concentration in the Precipitate Hydrolysis Aqueous (PHA) product was less than 1 mg/L in
eighteen of the nineteen experiments, including all the experiments that processed feeds .
containing mercury. The phenylboric acid concentration in the PHA product was 64 mg/L in run
92-18, which exceeds the TSR limit for DWPF. This run processed a feed containing no
mercury and only 14 mg/ nitrite and used only 800 ppm copper catalyst. An experiment
performed using the identical feed but with 950 ppm copper catalyst produced PHA containing
less than one mg/L, successfully meeting the TSR limit for PHA in the DWPF. These data are
valuable because there is a lack of any comparable data from operations in large scale
experimental facilities, such as the Precipitate Hydrolysis Experimental Facility (PHEF) at TNX,
for determining the kinetics of phenylboric acid hydrolysis during processing of late-washed,
irradiated tetraphenylborate slurries containing mercury.
Hydrolysis of the tetraphenylborate anion to boric acid and organic products (primarily benzene)
proceeds through a sequential series reaction network that ultimately must convert phenylboric
acid to boric acid, benzene, and other organic byproducts to achieve complete hydrolysis.
Hydrolysis of phenylboric acid requires removal of the last phenyl group from the boron atom
that was in tetrahedral coordination to four phenyl groups in the tetraphenylborate molecule.
This is generally acknowledged to be the slowest step in the reaction network representing the
complete hydrolysis of tetraphenylborate, requiring temperatures of about 150 *C in the absence
of any catalyst'. Although many metals exhibit some catalytic effects on the rate of phenylboric
acid hydrolysis, copper is by far the most active identified catalyst2 and is used to achieve
acceptable reaction rates and conversions of phenylboric acid at ambient pressures and
temperatures of about 90 - 101*C3.
Phenylboric acid hydrolysis kinetic data during acid hydrolysis of potassium tetraphenylborate
precipitates were reported earlier by Bannochie et al4, 5. These studies were performed primarily
with unirradiated tetraphenylborate precipitate feed slurry simulants characteristic of the feeds
used in DWPF Cold Chemical runs and in the Precipitate Hydrolysis Experimental Facility
(PHEF) at TNX. The low-nitrite feed simulants contained the expected levels of nitrite from the
late washing operation, but these precipitate slurries were not actually washed and hence
contained excess alkali. The feed simulants also omitted certain radiolysis products in addition
to mercury, all of which are anticipated in the precipitate feed. A single data set was reported for
hydrolysis of a tetraphenylborate precipitate feed simulant that contained mercury and was
irradiated to 1.9 E+08 rads before late washing to 0.01 M nitrite4' 5. This work concluded that
when diphenylmercury was present the hydrolysis of phenylboric acid was no longer first order
in the phenylboric acid concentration and that only after hydrolysis of phenylboric acid was
completed was the disappearance of diphenylmercury possible. Inspection of the original data,
contained in Figure 2 of this report, indicates that even with diphenylmercury present the
reaction was first order for about 1 - 1.5 hours, during which the majority (about 90%) of the
phenylboric acid was hydrolyzed. Hence, the data were consistent with a reaction mechanism
that starts out first order in phenylboric acid and changes order once a second reaction
mechanism is initiated, perhaps involving diphenylmercury cleavage to generate phenylmercuric
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Marek, J.C. Hydrolysis of late-washed, irradiated tetraphenylborate slurry simulants I: Phenylboric acid hydrolysis kinetics, report, February 10, 2000; South Carolina. (https://digital.library.unt.edu/ark:/67531/metadc705729/m1/10/: accessed May 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.