ISSN 1066-3622, Radiochemistry, 2013, Vol. 55, No. 6, pp. 567–573. © Pleiades Publishing, Inc., 2013. Original Russian Text © Yu.M. Kulyako, S.A. Perevalov, T.I. Trofimov, D.A. Malikov, M.D. Samsonov, S.E. Vinokurov, B.F. Myasoedov, A.Yu. Shadrin, 2013, published in Radiokhimiya, 2013, Vol. 55, No. 6, pp. 481–486.

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Preparation of Uranium Oxides in Nitric Acid Solutions by the Reaction of Uranyl Nitrate with Hydrazine Hydrate

Yu. M. Kulyakoа, S. A. Perevalovа, T. I. Trofimovа, D. A. Malikovа, M. D. Samsonovа, S. E. Vinokurovа, B. F. Myasoedovа, and A. Yu. Shadrinb

а Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia; * e-mail: kulyako@geokhi.ru

b Center for Radioactive Waste and Spent Nuclear Fuel Management and for Decommissioning, Bochvar High-Tech Research Institute of Inorganic Materials, Moscow, Russia

Received February 15, 2013

Abstract—UO2·nH2O formed by thermal denitration of uranyl nitrate in solutions under the action of hydra-zine hydrate can be converted in air to UO3 at 440°С and to U3O8 at 570–800°С, and also to UO2 in an inert or reducing atmosphere at 280–800°С. After the precipitation of hydrated uranium dioxide, evaporation of the mother liquor at 90°С in an air stream allows not only evaporation of water, but also complete breakdown and removal of hydrazine hydrate and NH4NO3. The use of microwave radiation considerably reduces the time re-quired for complete thermal denitration of uranyl nitrate in aqueous solution to uranium dioxide, compared to common convective heating.

Keywords: uranyl nitrate, denitration, hydrazine hydrate, uranium oxides

The end step of modern processes of U recovery from ores and spent nuclear fuel is preparation of uranyl nitrate. From its solutions, uranium is recovered in the form of ammonium diuranate or ammonium uranyl tricarbonate precipitate (ADU or AUC proc-esses), with the subsequent calcination of these com-pounds to UO2 [1, 2]. The drawback of these precipita-tion processes is that mother liquors contain up to tens of mg L–1 U and up to 100 g L–1 nitrate ions. Calcina-tion of these compounds first at ~500°С in air to U3O8 and then at ~800°С in a reducing atmosphere (Ar + Н2) to UO2 [2] is time- (~5 h in each step) and power-consuming. Therefore, for improvement of the process for production of uranium oxide fuel, it is more prom-ising to convert uranyl nitrate in solution to uranium oxide with simultaneous removal or decomposition of nitrate ions. These processes are termed denitration.

Procedures have been described for direct and re-agent (in the presence of reductants) denitration, as a rule, with heating in electric resistance furnaces. Proc-esses are being developed for direct thermal denitra-tion using microwave (MW) radiation [3–7]. It has numerous advantages over convective heating: no con-tact, high heating rate, selectivity, uniform heating

throughout the volume, acceleration of chemical proc-esses, and decreased electric power consumption for performing dehydration, decomposition of hydroxide and salt-forming compounds, synthesis of multicom-ponent substances, and sintering of ceramic [8]. Micro-wave radiation finds increasing use in nuclear chemis-try and technology [9–12].

The following substances are used as reductants in reagent thermal denitration: gaseous hydrogen, formal-dehyde, formic acid, urea, sugar, ethanol, etc. [13–15]. It is also appropriate to combine the process with the subsequent use of microwave radiation for preparing U oxides. In this study, we examined the possibility of preparing U oxides in solution by thermal denitration of uranyl nitrate in the presence of hydrazine hydrate, including the use of MW radiation.

DOI: 10.1134/S1066362213060015

EXPERIMENTAL

We used uranyl nitrate hexahydrate (UNH) UO2(NO3)2·6H2O and hydrazine hydrate (HH) N2H5OH of chemically pure grade. After adding HH to a uranyl nitrate solution in polypropylene test tubes with hermetic stoppers and formation of a yellow

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RESULTS AND DISCUSSION amorphous suspension, the test tubes with the suspen-sion were placed in a thermostat at ~90°С. To deter-mine the U oxidation state in the course of heating, the test tubes with the suspension were taken out from the thermostat at regular intervals, and samples of the agi-tated suspension were taken. The precipitates separated from the aqueous phase by centrifugation were dis-solved in 4 M HCl and in a mixture of 6 M HNO3 with 0.1 M HF, and the solutions were analyzed by spectro-photometry and radiometry. The suspension was kept in a thermostat until the amorphous precipitate was converted to a rapidly settling finely crystalline black precipitate. This precipitate was separated from the mother liquor, washed with water, dried in air at 60°С, and subjected to X-ray phase and thermal gravimetric analyses. To determine the oxidation state of U, the precipitate was dissolved in the above acids, and the solutions were analyzed by spectrophotometry and radiometry.

In denitration of U solutions by MW heating, the suspension was placed in a special glass reaction cell of a Discover SP-D MW installation (CEM Corp., USA). The samples were permanently in the homoge-neous high-density radiation field owing to the self-tuning MW cavity ensuring the maximal efficiency of MW treatment of the substrates. The ActiVent sensor for automatic monitoring and control of pressure in the reaction vessel and the IR temperature sensor had a feedback with a magnetron, which allowed control of the fed power to maintain and reproduce the preset process parameters. The reaction mixture was stirred with a built-in magnetic stirrer. The system was con-trolled with a computer with SynergyD software. A PC1010 compressor (Senco, Taiwan) was used for cooling the system after MW heating and for the proc-ess completion.

The U concentration in solutions was determined radiometrically with an Alpha Analyst α-ray spec-trometer (Canberra, USA). X-ray phase analysis was performed with an automated diffractometric complex. Mathematical processing and graphical presentation of the experimental results and substance identification were performed with TRFA software including JCPDS database. Thermal gravimetric and differential thermal analyses were performed with an Exstar TG/DTA7200 device. Spectrophotometric measurements were made with a Unicam UV-340 spectrophotometer. A com-bined glass electrode (Hanna Instrument HI 931 400) calibrated against buffer pH standards (pH 1–13, Merck) was used for pH monitoring.

Reaction of Uranyl Nitrate with Hydrazine Hydrate in Solutions

On adding HH, taken in an excess relative to U, to a uranyl nitrate solution ([U] ~50–200 g L–1), an amor-phous suspension is formed. Keeping the suspension at 90°С results in a change in the color and state of the suspension. In 15 min after adding HH to the initial uranyl nitrate solution, the suspension is yellow-gray. After keeping for 1 and 15 h, the suspension becomes black but does not noticeably settle (at the total solu-tion volume of 10 mL, the suspension volume is ap-proximately 8 mL). After 2 days, the amorphous sus-pension transforms into a rapidly settling precipitate consisting of fine black particles and occupying the volume of ~0.8 mL.

Experiments have shown that the process can be performed not only in aqueous but also in alcoholic solution. In this case, the time in which the amorphous suspension occupying much volume transforms into a compact settling precipitate decreases to approxi-mately 1 day. At room temperature, the uranyl nitrate denitration also occurs but takes longer time: approxi-mately 5 days in alcoholic solution and 10 days in aqueous solution.

The change in the color and volume of the suspen-sion is due to the reduction of U(VI) to U(IV) in an amorphous compound, uranyl hydrazinate, with hydra-zine incorporated in this compound, with transforma-tion of uranyl hydrazinate into hydrated U oxide or into a mixture of U oxides. Similar transformation of yellow uranyl hydroxylaminate into black U oxide due to reduction of U(VI) to U(IV) at ~300°С was de-scribed previously [16, 17]. Under the conditions of this study, the U reduction leading to its denitration occurs in solution at considerably lower temperature. This fact opens prospects for direct preparation of a powder of hydrated U oxide after removal of the mother liquor by evaporation, with simultaneous de-composition of nitrate ions present in the solution.

The formation of uranyl hydrazinate is confirmed by the results of titration of a uranyl nitrate solution with an HH solution (Table 1, Fig. 1a).

As can be seen, on adding HH to a uranyl nitrate solution, the U(VI) optical density at 420 nm increases (pH ~3). Similar change in the U(VI) optical density was observed previously in the reaction of uranyl ni-trate with hydroxylamine [17].

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On adding the first portions of HH to a uranyl ni-trate solution, the pH does not change noticeably (Table 1, solutions 1 and 2). As the HH concentration is increased further to the value close to the uranyl ni-trate concentration (Table 1, solution 3), the solution becomes turbid owing to the formation of a compound of uranyl ion with hydrazine. In the process, pH does not change noticeably. On adding the next portion of HH, pH of the solution increases to ~5, and U quantita-tively precipitates from the solution in the form of an amorphous yellow substance darkening with time. Ra-diometric analysis shows that the degree of U precipi-tation exceeds 99.98%. It was shown previously [18–20] that a series of heavy metals react with excess HH in solutions to give complex compounds in the form of amorphous precipitates. Table 1 (solution 4) shows that, when the hydrazine concentration in solution ex-ceeds the uranyl ion concentration by a factor of 2, U fully precipitates from the solution according to the spectrophotometric data (Fig. 1a, spectrum 4). In the process, free HH absorbing at 1060 nm appears in the solution (Fig. 1b).

Apparently, the precipitate formed by uranyl ion with hydrazine has the composition [UO2(N2H4)2(NO3)2]· nН2О, similar to the compositions of the known zinc and copper complexes: [Cu(N2H4)2(NO3)2]·2Н2О and [Zn(N2H4)2(NO3)2]·3Н2О [18, 19].

As already noted, on keeping of an amorphous sus-pension of uranyl hydrazinate at ~90°С, it transforms into a rapidly settling precipitate consisting of fine par-ticles. This is due to the reduction of U(VI) to U(IV) in the uranyl hydrazinate phase with the formation of hy-drated U oxide and then of UxOy in the solution.

Table 1. Variation of the hydrazine concentration and of pH in 0.077 M uranyl nitrate solution on adding HH

Solution no. Hydrazine concentration,a M рН Note 0 0 2.1 Initial uranyl nitrate solution 1 0.034 2.9

All U is in solution 2 0.067 2.9 3 0.079 3.1 U partially precipitates 4 0.168 ~5 All U is in precipitate

а The concentration of hydrazine contained in the added HH solution is given.

Identification of the U Oxidation State in UxOy Formed by Denitration Heating of an Amorphous Suspension

of Uranyl Hydrazinate

It was interesting to determine the U oxidation state in the product formed in the reaction of uranyl nitrate with HH and in the subsequent thermolysis. It is

known [21] that UO2 dissolves neither in dilute nor in concentrated HCl, and U3O8 dissolves in these acid solutions insignificantly. Both oxides are soluble in a mixture of 6 M HNO3 with 0.1 M HF, and UO3 readily dissolves both in HCl and in HNO3.

Figure 2 shows the spectra of the solutions obtained after the contact of the UxOy precipitate separated from the mother liquor by centrifugation with 4 M HCl or with a mixture of 6 M HNO3 and 0.1 M HF. The fol-lowing specific features of the dissolution behavior of this compound were revealed.

In contrast to the UO2 sample prepared by the stan-dard procedure, which does not dissolve in 4 M HCl, the UxOy sample did dissolve, though slowly. This is due to the fact that standard UO2 is prepared at 800°С, and it has stable crystalline structure. The compound

Fig. 1. (a) Spectra of solutions in spectrophotometric titra-tion of a 0.077 M uranyl nitrate solution with hydrazine hydrate (spectrum no. corresponds to solution no. in Table 1) and (b) spectra of centrifuged mother liquor nos. 3 and 4.

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Fig. 2. Spectra of U solutions obtained by dissolving the UxOy precipitates isolated from the suspension (1) in 4 M HCl and (2) in a mixture of 6 M HNO3 with 0.1 M HF.

UxOy isolated from the solution at ~90°С, apparently, does not have such a structure and therefore slowly dissolves in HCl.

The spectrum of the solution of UxOy in 4 M HCl

Fig. 3. (a) X-ray diffraction pattern of the UxOy sample obtained under the conditions of thermal heating, (b) its TG and DTA curves, and (c) X-ray diffraction pattern of the same UxOy sample after its heating to 800°С in air. (b) Sample weight 389.166 mg, final weight 379.880 mg, Δm = –9.286 mg ≈ 2.39%.

(Fig. 2, spectrum 1) contains only the absorption bands characteristic of U(IV). This fact indicates that the compound formed by thermolysis of the amorphous suspension of uranyl hydrazinate is UO2.

Standard UO2 dissolves in a mixture of 6 M HNO3 with 0.1 M HF, with complete oxidation of U(IV) to U(VI). The compound obtained in our experiments dissolves in this acid mixture at similar rate. However, in dissolution of UxOy, in contrast to UO2, uranium remains in the tetravalent state in 6 M HNO3, despite oxidizing properties of HNO3. We intend to study the causes of this phenomenon in the future.

X-ray Phase and Thermal Gravimetric Analyses of UxOy Obtained by Denitration Thermolysis

of an Amorphous Uranyl Hydrazinate Suspension

The UxOy samples were additionally studied by X-ray diffraction and differential thermal analysis (TG and DTA curves). The results are shown in Fig. 3. The reflections in the X-ray diffraction pattern of the initial sample (Fig. 3a) coincide with the published data for UO2 [22]. Hence, the compound UxOy formed by ther-molysis of uranyl hydrazinate is UO2. The differential thermal analysis of this compound shows that, starting from ~60°С, the black UxOy sample loses weight on heating owing to the removal of water of hydration that remained in the sample after drying at 60°С. The weight loss is complete at 280°С (Fig. 3b), with the sample becoming dark gray.

Further heating to 440°С is accompanied by the weight gain (Fig. 3b). The sample becomes light brown, which is associated with the transformation of UO2 into UO3 in air. Further heating of this sample from 440 to 570°С leads to gradual weight loss and to a change in the color to dark gray, and on heating to 800°С the sample becomes black. The final weight loss is due to elimination of a part of bound oxygen from UO3 and its transformation into black U3O8 (Fig. 3c).

The above-described structural transformations and the corresponding changes in the color of the UxOy sample are fully consistent with the results of X-ray phase analysis (Fig. 4).

Heating of the initial UO2·nH2O from 25 to 280°С does not change the positions of the reflections in the X-ray diffraction pattern typical of UO2 (Fig. 3a). The X-ray diffraction pattern of the substance cardinally changes on heating to 440°С. In this case, the UO2 re-

End

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flections disappear, and only an X-ray amorphous phase obtained by oxidation of weakly crystallized UO2 to X-ray amorphous UO3 in air is detected. Heat-ing of the powder to 570°С leads to the formation of the U3O8 crystalline phase, as does the heating to 800°С. Heating of UxOy in an inert or, the more so, in a reducing atmosphere (Ar + 10% H2) surely leads to the formation of UO2 whose crystallinity will increase with an increase in the heating temperature. The latter factor is very important for preparing a UO2 powder with ceramic properties, which is necessary in the pro-duction of oxide nuclear fuel for NPP.

Thus, it can be assumed that the thermal denitra-tion, or transformation of uranyl nitrate into UO2 in solution under the action of HH, follows the equation

Fig. 4. X-ray diffraction patterns of samples of U oxides after heating in air at (a) 280, (b) 440, (c) 570, and (d) 800°С.

where х is a small excess of the reagent relative to the reaction stoichiometry.

UO2(NO3)2·6H2O + (2 + х)N2H4·H2O → UO2·2H2O + N2

+ 2NH4NO3 + 6H2O + хN2H4·H2O, (1)

Separation of UO2·nH2O from the Mother Liquor by Evaporation in a Heated Air Stream

After the separation of the UO2 precipitate, a small excess of HH and the NH4NO3 formed in the reaction remain in the mother liquor with pH ~11. Changes of some characteristics of the mother liquor in the course of its evaporation are presented in Table 2 and Fig. 5.

Evaporation of the alkaline mother liquor contain-

ing HH at 90°С in an air stream leads to the break-down and removal of HH (Fig. 5, spectrum 4) owing to its oxidation with atmospheric oxygen to N2, NH3, and H2O. This leads to the formation of a neutral precipi-tate of NH4NO3 (Table 2, solution 4), which decom-poses at 250–270°С in accordance with the reaction

(2) NH4NO3 = N2O + 2H2O.

The decomposition of NH4NO3 can also be per-formed in the presence of formaldehyde (Н2СO) with the formation of environmentally acceptable off-gases: N2 and СО2.

Preparation of U Oxides from Uranyl Nitrate in Aqueous Solution in the Presence of Hydrazine

Hydrate Using Microwave Radiation

Table 2. Variation of characteristics of the mother liquor in the course of evaporation

Solution no.

Evaporation time, min

Weight of solution or its residue, g

Volume, mL

рН

1 0 11.1729 11 ~11 2 45 7.0385 ~6 – 3 90 1.9691 ~1 – 4 200 0.0608а 11 7.6b

а Dry residue. b After dissolving the dry residue in 11 mL of water.

As already noted, MW radiation as a heating tool has certain advantages over common convective heat transfer [8]. Therefore, we examined the possibility of using MW radiation for denitration thermolysis of a

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uranyl nitrate solution in the presence of HH. To this end, a test tube with a suspension of uranyl hydrazinate was placed into a Discover MW installation described in detail in the Experimental. Heating was performed in the autoclave mode at the magnetron power of 200 W, temperature of 150°С, and pressure of 4 atm. The process was complete in 30 min and yielded a well-settling fine black powder-like suspension of UxOy. The isolated product was analyzed by X-ray dif-fraction and thermal gravimetric analyses. The results are given in Fig. 6.

As can be seen, the X-ray diffraction patterns and thermograms (Fig. 6) of the compound prepared in the solution under the action of MW radiation fully coin-cide with the results of analysis of the substance ob-tained by denitration thermolysis of the suspension at 90°С in an oven (Fig. 3). Hence, in both cases we ob-tained UO2. However, with MW radiation, the time required for the UO2 formation decreased from 2 days to 30 min.

Thus, U oxides can be prepared directly in solutions by obtaining hydrated UO2 in the course of thermal denitration of uranyl nitrate in solution under the ac-tion of HH, including the process performed in an MW radiation field, in accordance with the reaction

Fig. 5. Spectra of the mother liquor in the course of its evaporation (spectrum no. corresponds to solution no. in Table 2).

Fig. 6. (a) X-ray diffraction pattern of the UxOy sample obtained under the conditions of MW heating, (b) its TG and DTA curves, and (c) X-ray diffraction pattern of the same UxOy sample after its heating to 800°С in air. (b) Sample weight 182.341 mg, final weight 176.233 mg, Δm = –6.188 mg ≈ 3.35%.

(3) [UO2(N2H4)2(NO3)2]·nН2О → UO2·nН2О + N2 + 2NH4NO3.

The structure of the dioxide obtained corresponds to the structure of UO2 forming under standard condi-tions by prolonged calcination of U salts at 500– 800°С. This fact allows the method that we developed to be recommended for commercial production of ura-nium oxide fuel for NPP nuclear reactors.

ACKNOWLEDGMENTS

The authors are grateful to Dr. Sci. (Chem.) A.M. Fedoseev and Dr. Sci. (Chem.) A.A. Bessonov (Frumkin Institute of Physical Chemistry and Electro-chemistry, Russian Academy of Sciences) for the as-sistance in performing X-ray phase and thermal gra-vimetric analyses and to Dr. Sci. (Chem.) I.V. Kub-rakova and Researcher E.M. Toropchenova (Vernad-sky Institute of Geochemistry and Analytical Chemis-try, Russian Academy of Sciences) for the opportunity to perform experiments on a Discover SP-D MW in-stallation.

The study was supported by the Ministry of Educa-tion and Science of the Russian Federation (agreement

TG

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