Plutonium peroxide precipitation

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. 

Plutonium Peroxide Precipitation Review and Current Research... 

There are some data that indicate sulfate ion aids in the formation of a more easily filtered plutonium peroxide precipitate. Ganivet (7) found, for instance, that peroxide precipitated from a nitric acid medium containing sulfate had better settling characteristics than precipitates from comparable solutions to which no sulfate was added. However, sulfate in the precipitate is undesirable, because of the corrosive effects it can have on processing equipment. Mainland, et al, (8) showed that by careful control of the precipitation parameters, it was possible to effectively precipitate peroxide in the absence of sulfate. 

A MFi 1 terometer11 was used to measure the fi 1 terabi 1 ity of the plutonium peroxide precipitate. This equipment, designed especially for this work, consisted of a calibrated tube (from a 100 ml buret) attached to a funnel-type support containing a wire screen. A Millipore filter (Solvinert, 1.5ym mean pore size) was supported by this screen. The tip of the funnel extended down into a 500 ml Erlenbeyer filter flask and a constant vacuum of 5.08 cm of Hg was applied through the side arm of the flask. 

The identification of those variables that have a major effect on plutonium peroxide precipitation was done in two ways. The first way used t-test values associated with each variable. The comparative magnitude of these values indicates the relative importance of the variable. The second way involved a subjective evaluation of the relative importance of each of the variables based on a visual comparison of the graphs constructed from the experimental data (Figures 1 through 12 plus a couple of dozen other comparable graphs that could not be included in this paper because of space limitations). The results of the subjective evaluation indicate that only the nitric acid concentration and the rate of hydrogen peroxide addition have a major effect on the relative filtration time. The other four variables influence the... 

Based on results obtained in this investigation, the levels shown in Table IV are recommended for the six major variables in the plutonium peroxide precipitation process. These levels were selected to give the best compromise between the fastest filtration time for the plutonium peroxide precipitate and the lowest concentration of plutonium in the filtrate. 

RECOMMENDED LEVELS FOR MAJOR VARIABLE IN PLUTONIUM PEROXIDE PRECIPITATION... 

The slowest rate of H2O2 addition investigated gives a plutonium peroxide precipitate with the fastest filtration time. The rate of addition has very little effect on the plutonium concentration in the filtrate. 

The recommended levels for each of the six variables are based on experiments in which the plutonium peroxide precipitations were made by a batch process rather than by a continuous process. However, the recommendations would be applicable, basically, to either process. 

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). 

Impure plutonium oxide residues are dissolved in 12M HN03-0.1M HF under refluxing conditions, and then the plutonium is recovered and purified by anion exchange. Plutonium is leached from other residues, such as metal and glass, and is also purified by anion exchange. The purified plutonium eluate from the anion exchange process is precipitated with hydrogen peroxide. The plutonium peroxide is calcined to the oxide, and the plutonium oxide is fluorinated. The plutonium tetrafluoride is finally reduced to the metal with calcium. 

The flowsheet for the FFTF Pu02 production is shown in Figure 2. Briefly, the plutonium metal is converted to an impure oxide by burning the metal in air. This is followed by dissolution of the impure oxide in a 15.6 M HN03 - 0.5 M HF solution. The americium is separated from the plutonium by precipitation of the plutonium as the peroxide. Americium does not form an insoluble peroxide and stays in the filtrate with other cationic impurities. The active peroxide filtrate is slowly dripped into 9 M NaOH. The combination of strong alkali and heat destroys the peroxides and precipitates the americium as the hydroxide. Any residual plutonium in the filtrate, along with other cations, is precipitated also as the hydroxide. The flowsheet for the americium oxide production is shown in Figure 3. 

A prime responsibility of the Rocky Flats Plant since it was built by the Atomic Energy Commission some 25 years ago has been the recovery and purification of plutonium. This recovery and purification has been done using an aqueous process. One of the major steps in that process is the precipitation of plutonium peroxide. This step converts the plutonium from an aqueous to a solid form for further processing and conversion to metal. 

Plutonium peroxide was investigated by Hamaker and Koch, (0 Hopkins, (2) and Koshland, et al. (3) in the 19 0 s and Leary (h) in the early 1950 s. This work showed that the composition of the precipitate varied and often incorporated anions from the solution from which it was precipitated. These investigations, as well as other work (5), also showed that the peroxide precipitate exists in both hexagonal and cubic crystalline forms. Although both forms are compounds of Pu(lV), they have slightly different O /Pu ratios (6). The cubic form can be colloidal and therefore is less suitable for process application than the hexagonal form. The acidity of the solution has an effect on the... 

Based on experience at Rocky Flats and information from the literature, six of the variables deemed most important in the precipitation of plutonium peroxide were selected for investigation. These six, and the ranges over which they were investigated, are identified in Table I. 

VARIABLES AND LEVELS USED IN INVESTIGATING THE PRECIPITATION OF PLUTONIUM PEROXIDE... 

At the end of the digestion time the plutonium peroxide slurry was transferred to the Filterometer. The volume of the slurry was measured to the nearest milliliter and the time required for filtration to the nearest second. The volume of the filtrate was also measured to the nearest milliliter and then diluted with an equal volume of concentrated HNO3. This HNO3 dissolved any plutonium peroxide present and prevented any post precipitation. The filtrate was then analyzed coulometrically for plutonium. From the filtration time, slurry volume, and the plutonium analysis data, the relative filtration time and the... 

The objective of this investigation was to identify those variables that have an effect on the precipitation of plutonium peroxide, rank these variables in order of their effect, and then select levels for each of the variables that will give an optimum precipitation of plutonium peroxide. 

Figure 9.7 Flow sheet for the precipitation of plutonium peroxide. (From Mainland et al. [Ml j and Cleveland [C2], by permission.)...

If the plutonium to be fluorinated is the plutonium peroxide cake, as in one of the processes used at the U.S. Savannah River plant, the air-dried cake is reacted with HF gas at 600°C. The reaction time is quite sensitive to sulfate containment in the oxalate cake, which interferes with fluorination and requires a longer time for reaction of the oxalate with HF. The interfering sulfate is that present due to a sulfuric acid wash of the cation-exchange resin prior to peroxide precipitation. 

The self radiolysis of a solution may change the chemical equilibria of the solution components. For example, the a-decay of plutonium decomposes water in a solution containing 1 mole of Pu, ca. 0.01 mole of H2O2 is produced per day. This hydrogen peroxide can react with the plutonium to form a precipitate of plutonium peroxide. To avoid this precipitation, nitrite ions are added to the solution to react with the hydroxyl radicals formed by the radiolysis and to eliminate the H2O2. 

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