WSRC-TR-2002-00587

D. G. Karraker
Westinghouse Savannah River Company
Aiken, SC 29808

A preliminary study of a process that oxidizes Pu(III) to Pu(IV) in sulfamic-nitric solutions and feeds the resulting solution directly to solvent extraction indicates a probable success for this process change. The gain is the elimination of a cation exchange step in the process in the Modern Pit Processing Facility.

The flowsheet for chemical processing in the projected Modern Pit Processing Facility (MPPF) calls for dissolving plutonium metal in sulfamic acid (H SA), purifying the Pu by solvent extraction with a modified Purex 2nd cycle process and converting the purified Pu to metal by a thermite-type reduction. Dissolving impure Pu metal in H SA has been a routine plant process for many years, but further plant processing requires conversion of the Pu(III) sulfamate solution to a Pu(IV) nitrate solution for solvent extraction. SRP practice has been to dilute the solution 10-fold and oxidize the Pu with NaNO₂ before solvent extraction. However, dilution leads to unacceptably large volumes for the MPPF, which is planned to have geometrically-safe vessels. The alternate plan chosen to reduce storage volumes involves a three-fold dilution, absorption on a cation column, washing the column to remove sulfamate, elution with 5.7M HNO₃-0.3M H SA and oxidation with NaNO₂ to produce a Pu(IV) nitrate solution. Treatment of the cation exchange product with NaNO₂ generates about 0.5 moles (11.2 L) of gas (N₂+ NO₂) per liter of solution.

The aim of this study is to eliminate the cation exchange step. The benefit of eliminating cation exchange is a space saving worth $10-30M and a simplified process. The process requirements are:

1. Oxidize Pu(III) to Pu(IV) without reacting with the H SA
2. Solvent extract from a mixed nitrate-sulfamate feed solution
3. Maintain a sulfate level at 0.2M or below in the solvent extraction feed solution

Reagent chemicals and distilled water were used throughout this study. Plutonium solutions were produced by dissolving a small piece of a 85 wt.% Pu-Al alloy in 1.5M H SA. Tri-n-butyl phosphate(TBP) extractant was mixed from vacuum-distilled Eastman TBP and vacuum-distilled n-dodecane. Pu(VI) tracer was produced by oxidation with excess KMnO_4, destroying the excess with MnSO₄, extraction into 30% TBP and back-extraction into 0.25M HNO₃.

Pu valences were determined from absorption spectra measured with a diode-array spectrophotometer. Pu(VI) distribution coefficients were determined by equilibrating equal volumes of aqueous acid solution with 25% TBP. Samples of the centrifuged phases were analyzed by rad screen alpha counting; aqueous and organic acid concentrations were determined by titration with standard NaOH.

The sulfamate ion is not, per se, a reducing agent. It has specific reaction with nitrite, as:

SO₃NH₂^- + NO₂^- -------® N₂ + SO₄^2- + H₂O

but reacts slowly, if at all, with most oxidizing agents, in particular KMnO₄.

A stoichiometric equivalence of KMnO₄ with 10.5 g/L Pu(III) in 4M HNO₃-0.38M H SA was approached by successive additions of 0.1N KMnO₄ solution until the Pu(III) spectrum had been replaced by the Pu(IV) spectrum. No precipitate formed, and no gas bubbles appeared. The reaction is:

5Pu³⁺ + MnO₄^- + 8H⁺ ------®5Pu⁴⁺ + Mn²⁺ + 4H₂O

The operating problem presented by permanganate oxidation is that an excess of KMnO₄ will further oxidize some of the Pu(IV) to Pu(VI) by

5Pu⁴⁺ + 2MnO₄^- + 2H₂O ---®5PuO₂²⁺ + 2Mn²⁺ + 4H⁺

and a large excess, over 200%, will precipitate MnO₂.

Other possible oxidizing agents were considered, and oxidation by H₂O₂ was explored extensively. There is a rapid initial oxidation by

2Pu³⁺ + H₂O₂ +2 H⁺ ---®2Pu⁴⁺ + 2H₂O

But the oxidation is never complete, and the Pu(IV) is slowly reduced by

2Pu⁴⁺ + H₂O₂ ---®2 Pu³⁺ + 2H⁺ + O₂

The net effect is just the Pu(III)-catalyzed destruction of H₂O₂ .

Since Pu(III) will not extract significantly in 30% TBP, the optimal use of permanganate as an oxidant would be to add a slight excess. A 10% excess would produce a 95% Pu(IV)-5%Pu(VI) feed for solvent extraction. The solvent extraction flowsheet is essentially the Purex 2^nd Pu cycle flowsheet, which has the following streams to the 2A contactor:

The extraction section of the 2A contactor has about 2.7M HNO₃ in the aqueous phase, and the scrub section about 1M HNO_3. The flowsheet safety limit is 20 gPu/L in the organic phase; this concentration of Pu(NO₃)₄·2TBP in the organic phase requires 0.16M TBP of the total 1.09M in 30% TBP. The distribution coefficients for Pu(VI) were determined in 25% TBP(0.93M), the concentration of "free TBP" in the scrub section. Preliminary values are shown in the following table.

Table 1. Pu(VI) Distribution in 25% TBP

These data bracket the conditions of the scrub section, and lead to the prediction that a small amount of Pu(VI) would be recovered by solvent extraction.

Sulfate is produced by the hydrolysis of sulfamate as

SO₃NH₂^- + H₂O ® SO₄^2- + NH₄⁺

Since sulfate complexing of Pu(IV) interferes with Pu(IV) extraction, as a rough rule of thumb, the concentration of sulfate in the 2AF is restricted to less than 0.2M. The hydrolysis of pure sulfamic acid has been previously studied^1,2 and found to be dependent on the temperature and sulfamic acid concentration, however the hydrolysis of H SA in HNO₃ solutions has not been measured. The hydrolysis in H SA solutions can be calculated from the rate law, d[SO₄^2-] /dt = k [HSA], where the brackets indicate molar concentrations, and k=1.45E-03 at 50ºC. The temperature dependence can be expressed by log k = 15.8465 – 5992.2/T, where T is the temperature in degrees K. Representative values are shown in Table 2.

Table 2. Hydrolysis Rates for Sulfamic Acid

The proposed process changes, Pu(III) oxidation by permanganate and feeding a nitric-sulfamate Pu(IV) to solvent extraction, appear practical from this preliminary study. The benefit of reduced process equipment also appears to be a strong incentive to further pursue this approach.

Having said the positives, before implementing the changes to the flowsheet, more complete data for Pu(VI) Do/a, a determination of the hydrolysis rates for H SA in mixed H SA- HNO₃ and a small-scale solvent extraction test are highly desirable to prove the flowsheet.

The treatment of the solvent extraction waste stream (2AW) also needs consideration. This stream will contain ²⁴¹Am, Pu isotopes, nitric and sulfamic acids. In outline, evaporation to recover nitric acid would simultaneously hydrolyze H SA to H₂SO₄, leaving an evaporator bottoms as a H₂SO₄ solution containing Am and Pu. Discharge of this solution to the SRS waste tanks may not be acceptable. Neutralizing this with Ca(OH)₂ and grouting the resulting slurry may be a practical solution.

1. M. E. Cuprey, Sulfamic Acid, a NewIndustrial Chemical, Ind. Eng. Chem., 30, 627 (1938)
2. D. G. Karraker, Hydrolysis of Sulfamic Acid, WSRC-TR-90-78