Denitration feed, formic acid

As described in Figure 3.7, TRU separation is performed by implementing the DIDPA process on pretreated PUREX raffinates. A front-end denitration step by formic acid is thus required to reduce the nitric acid concentration of the feed down to 0.5 M to allow the TRU elements to be extracted by the cation exchanger di-fvo-dccyl-phosphoric acid (DIDPA). This preliminary step, however, induces the precipitation of Mo and Zr (and thus the potential carrying of Pu), which requires filtration steps. The TRU and Ln(III) elements are coextracted by a solvent composed of the dimerized DIDPA and TBP, dissolved at 0.5 and 0.1 M, respectively, in n-dodecane. The An(III) + Ln(III) fraction is back-extracted into a concentrated 4 M nitric acid solution, whereas Np and Pu are selectively stripped by oxalic acid. 

Americium is separated from iron, chromium, nickel, and other impurities by oxalate precipitation. The Am feed solution for precipitation in the MPPF after formic acid denitration and volume reduction was approximately 2 g Am/L, 14 g Cr/L, 1.2 g Fe/L, and 0.8 g Ni/L in 1M HNO3. Further concentration of the feed solution to >2 g Am/L necessitated evaporation at <85°C because of the potential corrosion of the stainless steel... 

Three simulated tests were run using only nitric and formic acids. In each case, the reaction began promptly and proceeded smoothly. After the second test, the denitrated material was evaporated and additional nitric acid added to simulate the tandem semi-batch operation to be used with actual process solution. At the end of formic acid feed for Test 3, the material was refluxed for 2 hours and evaporated to 2500 L to simulate the final canyon product batch. A final formic acid denitration reduced the acidity of the simulated concentrate to <1M HNO3. 

Figure 2. Formic acid denitration of simulated Am-Cm-NaNOs feed solution...

These tests indicate that to obtain high yields from precipitation, the aluminum concentration of the slurry must be <0.2M (Figure 5). This can best be obtained by denitrating the Am-Cm-Al solution concentrate with formic acid (Figure 3) to about 0.5M HNO3. Then, dilution to an aluminum concentration of <0.5M would yield a feed suitable for oxalate precipitation. 

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