Uranyl nitrate purification

Crystallisation was one of the earliest methods used for separation of radioactive microcomponents from a mass of inert material. Uranium X, a thorium isotope, is readily concentrated in good yield in the mother liquors of crystallisation of uranyl nitrate (11), (33), (108). A similar method has been used to separate sulphur-35 [produced by the (n, p) reaction on chlorine-35] from pile irradiated sodium ot potassium chloride (54), (133). Advantage is taken of the low solubility of the target materials in concentrated ice-cold hydrochloric acid, when the sulphur-35 as sulphate remains in the mother-liquors. Subsequent purification of the sulphur-35 from small amounts of phosphorus-32 produced by the (n, a) reaction on the chlorine is, of course, required. Other examples are the precipitation of barium chloride containing barium-1 from concentrated hydrochloric acid solution, leaving the daughter product, carrier-free caesium-131, in solution (21) and a similar separation of calcium-45 from added barium carrier has been used (60). 

Uranium dioxide occurs in mineral uraninite. Purified oxide may be obtained from uraninite after purification. The commercial material, however, also is recovered from other uranium sources. Uranium dioxide is obtained as an intermediate during production of uranium metal (See Uranium). Uranyl nitrate, U02(N03)2, obtained from digesting the mineral uraninite or pitchblende with concentrated nitric acid and separated by solvent extraction, is reduced with hydrogen at high temperatures to yield the dioxide. 

The purification of uranium by precipitation may involve the formation of insoluble oxalate or peroxide complexes.50 In the oxalate method the ore concentrate is dissolved in nitric acid to give a uranyl nitrate solution from which uranyl oxalate is precipitated according to equation (101), leaving the bulk of the impurities in solution. This approach is favoured over dissolution in hydrochloric acid and reduction to UIV prior to U(C204)2 precipitation since it is simpler and... 

The purification of uranium by fractional crystallization involves repeated recrystallizations of U02(N03)2 from water or nitric acid, three usually being sufficient. The composition of the crystalline product may range from U02(N03)v2H20 to U02(N03)2 6H20 depending upon the temperature, acidity and uranyl nitrate concentration of the solution.198... 

In the solvent extraction purification of the uranium ore concentrate, TBP is the extractant of choice although, in the past, Butex, diethyl ether and MIBK have also been used for this purpose. The uranium ore concentrate is first dissolved in nitric acid to produce a solution of uranyl nitrate from which insoluble impurities are separated. The consumption of nitric acid in this step will depend upon the nature of the concentrate, as shown in equations (104)-( 109). Many impurities... 

Teramoto, M., Fu, S.S., Takatani, K., Ohnishi, N., Maki, T., Fukui, T., and Aral, K., Treatment of simulated low level radioactive wastewater by supported Uquid membranes Uphill transport of Ce(III) using CMPO as carrier. Sep. Purif. Tech., 1999, 18 57-69. Gupta, K.K., Manchanda, V.K., Sriram, S., Thomas, G., Kulkami, P.G., and Singh, R.K., Third phase formation in the extraction of uranyl nitrate by iV2V-dialkyl aUphatic amides. Solv. Extr. Ion Exch., 2000, 18 421 39. 

Two processes are employed for the production of uranium(VI) fluoride, namely the wet and dry processes. In both processes uranium(IV) oxide and uranium(lV) fluoride are formed as intermediates. In the wet process the uranium(IV) oxide is produced from the uranium concentrate by way of uranyl nitrate, whereas in the dry process the uranium concentrate is directly reduced to uranium(IV) oxide. The methods of purification used are also different in the wet process the purification proceeds at the uranyl nitrate stage, by solvent extraction, whereas in the dry process the end product uranium hexafluoride is itself distillatively purified. 

In the production of uranium(lV) oxide in the wet process, the uranium concentrate is first converted into a uranyl nitrate solution with nitric acid. After the purification of the uranyl nitrate by solvent extraction, it can be converted into uranium(IV) oxide by two different routes either by thermal denitration to uranium(VI) oxide which is then reduced to uranium(IV) oxide or by conversion of uranyl nitrate into ammonium diuranate which is reduced to uranium(IV) oxide. Purification proceeds by extraction of the uranyl nitrate hydrate from the acidic solution with tri-n-butylphosphate in kerosene and stripping this organic phase with water, whereupon uranium goes into the aqueous phase. 

Exxon Nuclear Company has conducted various development programs to support the design and licensing of a commercial nuclear fuel reprocessing plant. The uranium conversion portion of the reprocessing plant will use fluidized-bed processes for conversion of uranyl nitrate hexahydrate (UNH) to uranium hexafluoride (UF ). This paper describes the laboratory and pilot plant studies conducted at Oak Ridge National Laboratory (1) for Exxon Nuclear Company on the conversion of UNH to UF, and on the purification of UF. ... 

The next step in purification is separation of uranyl nitrate from the other metallic impurities in the dissolver solution by solvent extraction. Practically aU uranium refineries now use as solvent tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent. The first U.S. refinery used diethyl ether as solvent and later refineries have used methyl isobutyl ketone or organic amines, but practically all have now adopted TBP. It is nonvolatile, chemically stable, selective for uranium, and has a uranium distribution coefficient greater than unity when the aqueous phase contains nitric acid or inorganic nitrates. 

In the Aquafluor process [G4] developed by the General Electric Company, most of the plutonium and fission products in irradiated light-water reactor (LWR) fuel are separated from uranium by aqueous solvent extraction and anion exchange. Final uranium separation and purification is by conversion of impure uranyl nitrate to UFg, followed by removal of small amounts of PuF , NpFg, and other volatile fluorides by adsorption on beds of NaF and Mgp2 and a final fractional distillation. A plant to process 1 MT/day of irradiated low-enriched uranium fuel was built at Morris, Illinois, but was never used for irradiated fuel because of inability to maintain on-stream, continuous operation even in runs on unirradiated fuel. The difficulties at the Morris plant are considered more the fault of design details than inherent in the process. They are attributed to the attempt to carry out aqueous primary decontamination, denitration, fluorination, and distillation of intensely radioactive materials in a close-coupled, continuous process, without adequate surge capacity between the different steps and without sufficient spare, readily maintainable equipment [G5, R8]. 

In the dry uranyl nitrate from the purification process is thermally... 

Purification or refining. The sodium diuranate is then dissolved in hot concentrated 40 wt.% nitric acid, HNO3, to give uranyl nitrate UOjfNOj). The next step is to remove metallic impurities from the uranium by a solvent-extraction process performed on the acidified solution with a mixture of organic solvents such as n-tributyl phosphate (n-C H,)3PO dissolved at... 

Purification of the precipitated concentrates is often carried out by dissolution in nitric acid followed by extraction with tributyl phosphate (TBP) dissolved in kerosene or n-hexane. The extraction is based on the formation of a complex between the uranyl nitrate and TBP according to the equation ... 

For the first purification of plutonium in the processing of irradiated nuclear fuels, an anion-exchange process has been widely used [202]. In this process, complex formation of plutonium(iv) with nitrate is utilized in order to remove the last traces of uranium (present as uranyl(vi)) and fission products (primarily lanthanides). In this system, the maximum sorption of plutonium (iv) occurs at a nitric acid concentration of 7.2 m. The process is run at 60°C. At lower temperatures, the sorption is too slow at higher temperatures, the distribution ratio becomes more unfavorable and the resin is more liable to deteriorate. Under the conditions chosen, neither uranyl(vi) nor lanthanides are sorbed. The elution of plutonium(iv) is readily achieved by dilute (0.7 m) nitric acid. The weak point of the process is the limited resistance of organic ion exchangers to chemical attack and to high doses of radiation, already discussed in Section 21.6.1. These difficulties can be overcome, at least partly, by careful selection of the resin to be used. 

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