Chelant screening and refinement tests - Phase I, Task 2. Topical progress report, December 1993--June 1994 Page: 26 of 236
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was achieved. In addition, the total corrosion loss for the eight hour exposure was only 0.46
mils (11.7 um). Therefore, the corrosion acceptance criteria was still met.
In an attempt to improve the uranium dissolution, the follow-on tests were then
performed at a lower pH (2.5-3.2), achieved by adding citric or formic acid to the solvent
formulation (Tests 18, 19, and 20). These organic acids have also been used in the chemical
cleaning industry successfully for removal of deposits from power generating equipment.
HEDTA was used in Test 18 while DTPA was used in Tests 19 and 20. Again testing was
performed at 93*C with 3 mL/L Rodine 31A corrosion inhibitor added. As can be seen from
the data in Appendices B and C, these formulations showed no improvement in uranium
dissolution over the previous tests.
The next series of tests (Tests 21, 22, and 23) concentrated on the three base chelants
at a pH of 5.5. The corrosion coupons were removed from the test set up. Although the ionic
contribution of the metals from corrosion of the coupons had been low, there was a possibility
that they may have interfered with the effectiveness of the solvent system. Therefore Tests 21 -
23 were performed to eliminate this concern. The inhibitor was also removed from the
formulation in the event that it was retarding deposit dissolution. As with the previous tests,
these were performed at 93*C.
As can be seen from the data presented in Appendices B and C, the dissolution of
uranium improved slightly with the removal of the coupons. However, dissolution effectiveness
was still less -than 5% after eight hours exposure. As with the previous tests, most of the
dissolution occurred during the first few hours of exposure. These dissolution results were still
considered to be unacceptable.
One additional test was run using HEDTA at a pH of 2.0; the pH adjustment was made
using nitric acid, since HNO3 is known to be an effective solvent for uranium compounds. As
with the previous three tests, no inhibitor was used. This formulation was evaluated in Test 24.
No corrosion coupons were used in this test. The addition of the HNO3 did not, however, result
in improved dissolution of uranium dioxide.
The effect of increasing chelant concentration to 15% was also evaluated, with no
enhancement of uranium dissolution observed. This testing was performed with DTPA at a pH
of 5.5 in Test 25. No corrosion coupons or corrosion inhibitor were used in Test 25.
A final elevated temperature test was performed (Test 26). This test was a repeat of Test
25 with the addition of ammonium bifluoride (ABF) to the solvent formulation. ABF is a
standard additive to many chemical cleaning formulations and generally improves the dissolution
performance. No corrosion coupons or corrosion inhibitor were present in this test. This was
the first partial success in the screening tests. Uranium dissolution increased significantly (29%
versus 4% comparing Test 26 to Test 25). In addition, uranium appeared to be dissolving
throughout the exposure period. However, total dissolution was still less than 30% after eight
hours exposure, and not considered acceptable. Since the addition of a known activator (ABF)
did not result in acceptable performance, a decision was made to stop the elevated temperature
testing and investigate lower temperature processes. The results of these tests are presented in
the next section.
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Chelant screening and refinement tests - Phase I, Task 2. Topical progress report, December 1993--June 1994, report, July 1, 1995; United States. (https://digital.library.unt.edu/ark:/67531/metadc680875/m1/26/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.