Plutonium oxalate precipitation

Oxalates. Stable oxalates of Pu(III), Pu(IV), and Pu(VI) are known. However, only the Pu(III) and Pu(IV) oxalates are technologically important (30,147). Brilliant green plutonium(III) oxalate [56609-10-0] precipitates from nitric acid solutions containing Pu(III) ions upon addition of oxaUc acid or sodium oxalate. The composition of the precipitate isPu2(C20 2 10H2O. A homogeneous oxalate precipitation by hydrolysis of diethyl oxalate at... 

The complete chemistry of plutonium 1 iquid-to-solid conversion processes, especially peroxide and oxalate precipitation, should be further studied. Research and development of direct thermal denitration methods should also be pursued. 

Precipitation Processes. Plutonium peroxide precipitation is used at Rocky Flats to convert the purified plutonium nitrate solution to a solid (14) the plutonium peroxide is then calcined to Pu02 and sent to the reduction step. The chemistry of the plutonium peroxide precipitation process is being studied, as well as alternative precipitation processes such as oxalate, carbonate, fluoride, and thermal denitration. The latter method shows the most promise for cost and waste reduction. 

The process sequence currently used for waste salts (except those containing aluminum for which no process currently exists) is shown in Figure 1. The process includes (1) dilute hydrochloric acid dissolution of residues (2) cation exchange to convert from the chloride to the nitrate system and to remove gross amounts of monovalent impurities (3) anion exchange separation of plutonium (4) oxalate precipitation of americium and (5) calcination of the oxalate at 600°C to yield americium oxide. 

A continuing problem with the cation exchange process as used in production operations is that it has not been sufficiently selective and therefore allows considerable carryover of the MSE salt constituents and impurities with the plutonium and americium. This isn t serious with plutonium since plutonium can be subsequently purified by anion exchange. For americium, however, the subsequent recovery process is oxalate precipitation which is less selective and carries some of the impurities into the final product. 

Plutonium Finishing. The separated plutonium was processed to Pu02 by conventional cation resin exchange, oxalate precipitation, and calcination methods. 

The plutonium metal feed stock contained about 5 ppm natural lead which was not removed by the process. Recovery of americium in the finishing process (oxalate precipitation and calcination) averaged 98.5%. Most of the residual chromium contaminant was removed from the oxalate in decanted supernate and washes. The finished oxide product purity exceeded specifications i.e., >95% Am02 By selective blending, impurities in the shipped product, predominantly lead and nickel, were kept below 2%. 

Plutonium was stripped from the solvent with hydroxylamine, concentrated further by cation exchange, precipitated as plutonium oxalate, and calcined to the oxide. 

During plutonium reconversion in a reprocessing plant, oxalate supernatant is normally generated during plutonium precipitation by oxalic acid as plutonium oxalate. This is further converted into plutonium oxide. This waste solution will have the following composition uranium (U) 5 gm dm , plutonium (Pu) 25 mg dm, mthenium (Ru ° ) 0.0032 mCi dm, cesium (Cs ) 0.003 mCi dm , nitric acid 3 M, and H2C2O4 0.1 M. 

In their manufacture uranium(IV) oxide is mixed with the appropriate quantity of plutonium(IV) oxide, the mixture pressed into pellets and then sintered (termed coprocessing in the USA). Uranium(IV) oxide is produced by one of the above-described processes and plutonium(IV) oxide from the aqueous nitrate solution produced during reprocessing by precipitating it as plutonium oxalate and calcining the oxalate. 

The PuFs process does not attain the degree of decontamination from cationic impurities that can be achieved in peroxide or oxalate precipitation, but it is acceptable vriien the plutonium nitrate feed contains no more than a few hundred parts of uranium and aluminum per million of plutonium. This process has been in routine use at the U3. plant at Savaimah River [B6,03]. 

Plutonium dioxide, prepared by direct calcination of Uie nitrate or calcination of the peroxide or oxalate precipitates, can be chlorinated to PuQ by HQ-Hj, gaseous CCI4, or phosgene (COCI2), the latter resulting in the most rapid reaction. 

Precipitation of plutonium oxalate by adding oxalic acid ... 

Oxalates. Precipitation of plutonium oxalate from dilute acidic solution... 

The contractual end product for uranium is the nitrate solution. If a utility wants to re-use it, it has to be converted to the hexavalent form or to the oxide form, which can be used in MOX fuels, but, in general, it is more attractive to use natural or depleted manium. The plutonium cannot be economically stored in the nitrate form because critical masses or concentrations in aqueous solutions can become very small. For this reason, the plutonium solution is converted to oxide powder by treating it with a hot oxalic acid, causing the plutonium to precipitate as Pu(lV) oxalate. When dried, it is calcined while modestly annealed, the product being Pu(lV) oxide (PUO2). This is weighed (with a typical uncertainty of 0.1%) and put in storage containers. 

Plutonium. The plutonium nitrate product must be converted to MO fuel if it is to be recycled to lightwater reactors. Whether from a plutonium nitrate solution or a mixed U/Pu nitrate solution, the plutonium is typically precipitated as the oxalate and subsequendy calcined to the oxide for return to the fuel cycle (33). 

The plutonium extracted by the Purex process usually has been in the form of a concentrated nitrate solution or symp, which must be converted to anhydrous PuF [13842-83-6] or PuF, which are charge materials for metal production. The nitrate solution is sufficientiy pure for the processing to be conducted in gloveboxes without P- or y-shielding (130). The Pu is first precipitated as plutonium(IV) peroxide [12412-68-9], plutonium(Ill) oxalate [56609-10-0], plutonium(IV) oxalate [13278-81-4], or plutonium(Ill) fluoride. These precipitates are converted to anhydrous PuF or PuF. The precipitation process used depends on numerous factors, eg, derived purity of product, safety considerations, ease of recovering wastes, and required process equipment. The peroxide precipitation yields the purest product and generally is the preferred route (131). The peroxide precipitate is converted to PuF by HF—O2 gas or to PuF by HF—H2 gas (31,132). 

C minimises plutonium loss ia the filtrate and decreases filtering time (166). Heating the compound to ca 270°C ia air or to 460°C ia absence of air results ia the formation of PUO2. Yellow-green Pu(IV) oxalate [26588-74-9] Pu(C20 2 6H20, precipitates from acidic solutions of Pu(IV) upon addition... 

The eflBciency of these methods of co-precipitation was studied for plutonium, only, by comparing the chemical recovery of the original yield monitor with that of a second (and different) plutonium isotope yield monitor added to the acid solution of the co-precipitated hydroxide or oxalate (6). This second monitor shows all losses following the coprecipitation step. The eflBciency of the hydroxide precipitation for plutonium extraction was in the range 70-80%. That for the oxalate coprecipitation was typically 75-85%. 

---