FR2973557A1 - Recovering uranium oxide in (uranium, gadolinium)dioxide, comprises adding carbonate solution containing hydrogen peroxide in waste of type (uranium, gadolinium)dioxide and adding acid solution to solution containing uranyl peroxocarbonate - Google Patents

Abstract

Recovering uranium oxide content in (uranium, gadolinium)dioxide, comprises (a) adding a carbonate solution containing hydrogen peroxide in a waste of type (uranium, gadolinium)dioxide contaminated with impurities of metallic oxide type to dissolve uranium in the form of a solution containing a complex of uranyl peroxocarbonate; (b) adding an acid solution to the solution containing the complex; (c) recovering the acid solution and alkaline solution from a supernatant solution; (d) dissolving uranium peroxide in an acid solution; and (e) washing the precipitate of uranium peroxide. Recovering uranium oxide content in (uranium, gadolinium)dioxide by a carbonate solution containing hydrogen peroxide, comprises (a) adding the carbonate solution containing hydrogen peroxide in a waste of type (uranium, gadolinium)dioxide contaminated with impurities of metallic oxide type to selectively dissolve uranium in the form of a solution containing a complex of uranyl peroxocarbonate, where the gadolinium oxide is partially dissolved with the uranium while the metal oxide impurities are insoluble in the carbonate solution; (b) adding an acid solution to the solution containing a complex of uranyl peroxocarbonate comprising ions of gadolinium co-dissolved to precipitate the uranium in the solution in the form of uranium peroxide which is contaminated with ions of gadolinium and absorbing carbon dioxide gas generated during the acidification by an absorber gas with an alkaline solution to recover a solution of carbonate salt; (c) recovering the acid solution and alkaline solution used in step (b) from a supernatant solution after precipitation of uranium peroxide by electrodialysis; (d) dissolving uranium peroxide contaminated by gadolinium prepared in step (b) in an acid solution and adding hydrogen peroxide to the acid solution to precipitate again only uranium peroxide in the solution, while the dissolved ions of gadolinium remain in the solution to purify the contaminated uranium peroxide by gadolinium; and (e) selectively washing the precipitate of uranium peroxide.

Description

1

PROCESS FOR RECOVERING URANIUM OXIDE IN (U, Gd) 02 WASTE USING CARBONATE SOLUTION CONTAINING HYDROGEN PEROXIDE

The present invention relates to a method for recovering uranium oxide in a waste (U, Gd) 02 contaminated with metal oxides by means of a carbonate solution containing hydrogen peroxide. A frit pellet of UO2, wherein U-235 for nuclear fission is concentrated in an amount of 4 wt.% To 5 wt.%, And a frit pellet of (U, Gd) O2i in which Gd203 is used to regulate neutrons in a reactor is included in an amount of 4% by weight to 6% by weight, are used as nuclear fuel for light water reactor. The aforementioned pellets are prepared in the form of a nuclear fuel by mixing various additives in a mixed powder containing UO2 and U3OB and through a series of process steps, that is, ie granulation, addition of a lubricant, compression molding, sintering at a temperature ranging from 1700 ° C. to 1800 ° C. under a hydrogen atmosphere and final grinding process. Uranium-containing wastes, such as sintered pellets based on UO2 or (U, Gd) 02 that can not be used, grinding slurries and powders containing UO2 or (U, Gd) 02 can not be used. to be used, are produced in processes of 2

nuclear fuel preparation previously mentioned. Chromium (Cr), iron (Fe), nickel (Ni), molybdenum (Mo), aluminum (Al) or silicon (Si) oxides are produced in the form of grinding muds from 'an abrasive material and are mixed with waste containing uranium. Gadolinium in (U, Gd) O2 fuel waste is not recycled and is discarded. However, it is increasingly necessary to recover and recycle uranium because of the growing demand for uranium due to increased nuclear power generation. To recover the uranium contained in a type oxide (U, Gd) O2i waste, uranium is conventionally recovered by a process in which only the uranium is separated by extraction with a solvent after dissolving a waste product. uranium oxide in nitric acid at high temperature (about 100 ° C, 10 M HNO3), then the uranium is recovered as a mixed precipitate of ammonia-uranium ((NH4) 2U2O7 or UO42NH4NO3) using ammonia, and U3O8 is finally prepared by thermal decomposition. However, the conventional method described above is very uneconomical and environmentally friendly due to the corrosion of the devices and the production of gaseous NOx during dissolution in nitric acid at high temperature and the production of organic waste. during extraction with a solvent and a large amount of secondary waste, such as ammonia-based nitrogen compounds and environmentally hazardous organic liquid waste. Therefore, the present invention aims to implement a method for recovering the uranium oxide contained in a uranium waste, the process can be carried out at room temperature and to reduce the production of secondary waste. Embodiments of the present invention are intended to provide a method for recovering uranium oxide contained in waste from peroxide-containing metal oxides.

In accordance with the present invention, there is provided a process for recovering uranium oxide contained in (U, Gd) O2 by means of a carbonate solution containing hydrogen peroxide, the process comprising: ) the addition of a carbonate solution containing hydrogen peroxide to a waste (U, Gd) O2 contaminated with metal oxide impurities to selectively dissolve the uranium (U) in the form of a uranyl peroxocarbonate complex containing solution, wherein gadolinium oxide (Gd) is partially dissolved with uranium while metal oxide impurities are insoluble in the carbonate solution; B) adding an acidic solution to the solution containing a typical contaminated uranyl peroxocarbonate (U, Gd) O2 complex of a solution of hydrogen. According to a 4 comprising co-dissolved Gd ions to precipitate the uranium in the solution in the form of uranium peroxide UO4 which is contaminated with Gd ions and the absorption of gaseous carbon dioxide generated during the acidification by a gas absorber with an alkaline solution to recover a carbonate salt solution; c) recovering the acid solution and the alkaline solution used in step b) from a supernatant solution after precipitation of uranium peroxide by electrodialysis; d) dissolving the Gd-contaminated uranium peroxide prepared in step b) in an acid solution and adding hydrogen peroxide to the acid solution to again precipitate only the uranium peroxide in the solution , while the dissolved Gd ions remain in the solution, to purify the Gd-contaminated uranium peroxide; e) and washing the precipitated uranium peroxide selectively. According to a particular characteristic of the process of the invention, the carbonate solution containing hydrogen peroxide of step a) has a pH ranging from about 10 to about 13 and contains hydrogen peroxide. at a concentration of from about 0.1 M to about 3.0 M. According to another characteristic, an acid is added in step b) to adjust the pH in a range between about 0 and about 4. The acid may be any acid selected from the group consisting of nitric acid, hydrochloric acid and sulfuric acid. According to yet another characteristic of the invention, the reaction between the carbon dioxide and the alkaline solution of step b) is carried out by means of a gas absorber in which the alkaline solution circulates. In addition, the acid solution used in step d) contains nitric acid, its temperature is in the range of about 70 ° C to about 90 ° C and its concentration is in the range from about 1 M to 2 M. According to other particular characteristics of the process of the invention: the carbonate recovered in step b) is used in the form of a carbonate solution in step a); step c) is carried out by an electrodialysis method by means of a cation exchange membrane and an anion exchange membrane; the acid solution and the alkaline solution recovered in stage c) are used respectively as acid solution and alkaline solution in stage b); steps d) and e) are repeated twice or more than twice.

The aforementioned objects and other objects, features and other advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Figure 1 schematically illustrates the entire a method of recovering uranium oxide according to the present invention; Figure 2 schematically illustrates a method of step b) according to the present invention; Figure 3 schematically illustrates a method of step c) according to the present invention; Figure 4 is a graph showing changes in uranium and hydrogen peroxide concentrations in a solution over time; Fig. 5 is a graph showing the recovery yield of carbonate in a gas absorber over time; and Figure 6 is a graph showing the concentrations of HNO3 and NaOH recovered from a NaNO3 solution over time in an electrodialysis apparatus. The present invention provides a process for recovering pure uranium oxide from (U, Gd) O2 using a carbonate solution containing hydrogen peroxide, the process comprising: a) adding a carbonate solution containing hydrogen peroxide to a waste (U, Gd) O2 contaminated with metal oxide impurities to selectively dissolve uranium (U) in the form of a solution containing a peroxocarbonate complex uranyl, wherein gadolinium oxide (Gd) is partially dissolved with uranium while metal oxide impurities are insoluble in the carbonate solution; B) adding an acid solution to the solution containing a uranyl peroxocarbonate complex comprising co-dissolved Gd ions to precipitate the uranium in the solution in the form of uranium peroxide UO4 which is contaminated by Gd ions and the absorption of gaseous carbon dioxide generated during acidification by a gas absorber with an alkaline solution to recover a carbonate salt solution; c) recovering the acid solution and the alkaline solution used in step b) from a supernatant solution after precipitation of uranium peroxide by electrodialysis; d) dissolving the Gd-contaminated uranium peroxide prepared in step b) in an acid solution and adding hydrogen peroxide to the acid solution to again precipitate only the uranium peroxide in the solution , while the dissolved Gd ions remain in the solution, to purify the Gd-contaminated uranium peroxide; e) and washing the precipitated uranium peroxide selectively. Hereinafter, each step of the present invention is described in detail.

Step a) according to the present invention consists in adding a carbonate solution containing hydrogen peroxide to a waste (U, Gd) 02 contaminated with metal oxide impurities, which is the raw material for the present invention. invention for selectively dissolving uranium. 8

In order to separate the uranium from the oxide (U, Gd) O2 mixed with impurities, such as grinding sludges, in step a) of the present invention, the metal oxide impurities and most of the gadolinium oxide are not dissolved, whereas uranium is selectively dissolved by means of an alkaline solution containing carbonate and hydrogen peroxide, although gadolinium oxide is partially dissolved with uranium . At this time, a pH value of the carbonate solution may be in the range of 10 to 13. In the carbonate solution having a pH of 10 to 13, the metal oxide impurities (Fe, Ni, Mo, Al, Si, etc.), such as grinding sludge, and gadolinium oxide are practically not dissolved, and even if the aforementioned oxides are dissolved in the alkaline solution, the metal impurities precipitate in the form of metal hydroxides by hydrolysis because of their very low solubility in the solution. As a result, most of the metal oxide impurities remain in an insoluble form in the carbonate solution. On the other hand, the oxide UO2 is oxidized by hydrogen peroxide and combines with carbonate ions in the carbonate solution to form complex ions of uranyl peroxo. The generally known UO 2 dissolution process follows a route identical to that represented by the following reaction formula 1.

Reaction Formula 1 UO2 - UO2 + x UO2, 33 -> UO3 -> (UO22 +) surface (002 (CO3) 34-) mass 9 (where x is a prime number satisfying the following relationship: 2 <2 + x <2, 33). When the uranium dissolution process is described in more detail, a non-stoichiometric oxide remaining at the grain boundary, which is generated during sintering during UO2 fuel fabrication, is first oxidized then the crystalline UO2 of the mass is oxidized to H02.33 (H2O7).

UO2,33 is a mixed state consisting of U6 + and U4 + or U5 + and U4 +, and the decrease of U4 + in UO2 occurs by the formation of UO2,33. When the pentavalent oxide in UO2 is finally oxidized to UO3i UO3 is converted to uranyl ion UO22 + if a solution in contact with a UO2 surface is acidic and changes to UO2 (OH) W3 ion if the solution is alkaline. When the solution is a carbonate solution, the previously mentioned ions combine with CO32 + to form uranyl tricarbonate ions UO2 (CO3) 3 - 4 and dissolve in the solution. In addition, when the carbonate solution contains hydrogen peroxide (H2O2), the uranium is dissolved in the form of a complex of uranyl peroxocarbonate ([UO2 (02) Y (CO3) X] 2-2x -2Y, where x / y = 1/2, 2/1 and 3/0 when y = 0, 1 and 2), as shown by the following reaction formula 2, and the uranyl peroxocarbonate complex is very soluble by UO2 ratio (0O3) 3_4 which is a uranium carbonate complex forming when hydrogen peroxide is not present. Therefore, the carbonate solution in step a) of the present invention may contain hydrogen peroxide, and in this situation the hydrogen peroxide may be included at a concentration ranging from 0.1 M to 3, 0 M. When hydrogen peroxide is included at a concentration below 0.1 M, a quantity

Sufficient uranyl peroxocarbonate complex is not formed because the hydrogen peroxide in the solution disappears rapidly. When hydrogen peroxide is included at a concentration greater than 3 M, the total volume of the solution

increases because of the addition of a large volume of hydrogen peroxide solution and, consequently, the concentration of the uranium finally dissolved in the solution decreases without modifying the dissolution rate and the solubility of 002.

Reaction Formula 2 - UO2 + xCO32 + yH2O2 + 2yOH -> [UO2 (O2) y (CO3) x] 2 2x2Y + 2yH2O + 2e- (where x / y = 1/2, 2/1 and 3 / 0 when y = 0, 1 and 2).

Step b) according to the present invention consists in adding an acidic solution to the uranium solution containing the uranyl peroxocarbonate complex prepared in step a), in which

gadolinium coexist, to precipitate uranium in the form of UO4 in the solution of uranyl peroxocarbonate complex in which the Gd ions are entrained in the precipitate of 004, and to absorb the gaseous carbon dioxide generated during

acidification with an alkaline solution for 11

recover a solution of carbonate salt in a gas absorber. The carbonate-containing species are interchangeable depending on the pH value of the solution, as shown by the following reaction formula 3. Therefore, when the pH value of the solution containing the uranyl peroxocarbonate complex is adjusted by adding the acid solution to the uranium solution, a uranyl peroxocarbonate complex ion in the solution is decomposes and precipitates as uranyl peroxide (UO4), as shown by the following reaction formula 4, and at the same time, the carbonate ion separates from the uranyl peroxocarbonate complex and a free carbonate ion in the solution becomes converts to carbon dioxide and leaves the solution in the gaseous state. At this time, the pH value of the solution containing the uranyl peroxocarbonate complex can be adjusted in a range from 0 to 4. When the pH value is greater than 4, uranyl peroxide U04 does not precipitate. enough. The acid added to adjust the pH value may be nitric acid, hydrochloric acid or sulfuric acid, but the acid is not limited thereto.

Reaction Formula 3 H 2 CO 3 (production of gaseous CO 2) HCO 3 -H CO 32 -Reaction Formula 4 UO 2 (O 2) x (CO 3) y 2 -2x-2y + mH + + 2H 2 O -3 UO 2 (O 2) 4H 2 O + y H 2 CO 3 (where x / y is 1/2, 2/1 or 3/0 when y is 0, 1 or 2, and in this case, m is 4, 6 or 8) The gaseous carbon dioxide produced at this time is absorbed in an alkaline solution for it is recovered in the form of a carbonate salt solution. The reaction between the carbon dioxide and the alkaline solution can be carried out by means of a gas absorber, in which an alkaline solution, such as sodium hydroxide solution (NaOH), flows down to from the upper part of the gas absorber, a process for recovering the carbonate salt solution accompanying the UO4 precipitation being shown in FIG.

When step b) of the present invention is described on the basis of Figure 2 and a solution containing a uranyl peroxocarbonate complex is acidified with an acid solution in a uranium decarbonization bath, the species containing carbonate of the solution containing a uranyl peroxocarbonate complex is converted to carbon dioxide and removed from the solution, and the uranium and peroxo ion-containing species that remain precipitate in a very poorly soluble UO4 form. The carbon dioxide gas evacuated from the solution is introduced through the lower part of the gas absorber filled with filler balls having a diameter of 1 mm to allow a gas-liquid contact and is in contact with an alkaline solution. flowing down from the top of the gas absorber to be recovered under the 13

form of a solution of carbonate salt, which is removed from the gas absorber by its lower part. The recovered carbonate salt solution exiting the gas absorber can be reused by being recycled in step a) and the production of secondary waste can be significantly reduced by the previously described method. Step c) according to the present invention consists in eliminating the gadolinium ions contained in a supernatant after the precipitation of U04 in step b) and in recovering the acid solution and the alkaline solution used during step b). Step c) can be carried out by means of an electrodialysis method using a cation exchange membrane and an anion exchange membrane, an example of this method being shown in FIG. c) of the present invention is described on the basis of Figure 3, a trace amount of residual Gd3 + ions, NO3- anions generated from an acid and Na + cations from a carbonate salt solution is included in the supernatant solution discharged in step b). When the solution is introduced between a cation exchange membrane and an anion exchange membrane of an electrodialysis reactor, the Na + cations pass through the cation exchange membrane and join to the OH produced by the electrolysis of the water in a cathode chamber to form NaOH, and the NO3 - anions pass through the anion exchange membrane and join the H + produced by the electrolysis of water in an anode chamber to give back HNO3. 14

The regenerated NaOH and HNO3 are recycled respectively to the gas absorber of step b) and the uranium precipitation-decarbonization bath, and are reused respectively in the form of an alkaline solution and an acidic solution. . As a result, the production of secondary waste can be significantly reduced. Step d) of the present invention consists in dissolving the Gd-contaminated uranium peroxide which precipitated in step b) in an acid solution to separate the uranium ions and the gadolinium ions and to add hydrogen peroxide to selectively precipitate uranium peroxide, while the dissolved Gd ions remain in the solution so that the Gd-contaminated U04 can be purified. Since gadolinium is partially dissolved when the (U, Gd) 02 type waste is dissolved in the carbonate solution containing hydrogen peroxide in step a), gadolinium ions coexist as impurities in a solution. containing a uranium peroxocarbonate complex. Since the solution of gadolinium ions is trapped in the pores of the U04 precipitate, which form on the surfaces and within the U04 precipitates particles when the uranium of the solution containing a uranyl peroxocarbonate complex precipitates under While the carbon dioxide gas is evacuated by adding an acid to the uranium solution, pure UO4 can not be recovered. Step d) is intended to solve this problem.

The gadolinium contaminated 004 is purified using the following characteristics; gadolinium is very soluble in acids, UO4 dissolves easily as 0022+ in a heated acidic solution and 0022+ ions precipitate as 004 when hydrogen peroxide exists in excess in an acid solution containing 0022+ Therefore, when gadolinium-contaminated 004 is dissolved in a heated acidic solution, uranium and gadolinium dissolve respectively as UO22 + and Gd3 + in the acidic solution. Then, when hydrogen peroxide is added to the solution, the 0022+ ions precipitate again as UO4, while the Gd3 + ions remain in the acidic solution because of their high solubility in the acid. Therefore, the gadolinium-contaminated 004 can be purified by the above method. The acid used in step d) of the present invention may be nitric acid at a concentration of 1 M to 2 M, and its temperature may be 70 ° C to 90 ° C. When the temperature is below 70 ° C, the redissolution of 004 is not facilitated in the previously mentioned nitric acid concentration range, and when the temperature is above 90 ° C, it may be difficult to control the dissolution of 004 because of the boiling of the solution. On the other hand, the precipitation mechanism of 0022+ dissolved ions in the acid solution to obtain 004 is in accordance with the following reaction formula. 16

Reaction Formula 5 UO22 + + 2H2O2 + 4H2O - UO2 (02) 4H20 + 4H +

Step e) according to the present invention consists in selectively washing the precipitate, and the washing can be carried out by any method since the washing does not modify the characteristics of the purified 004. Furthermore, steps d) and e) can be repeated twice or more than twice in order to sufficiently remove contaminating gadolinium 004. A nuclear fuel material, U3O8, can be obtained by the heat treatment of pure 004 recovered by means of to the previously described process.

Hereinafter, the present invention is described with reference to experimental examples. However, the following experimental examples are only presented to give examples of the effects of each step in the present invention, and the scope of the present invention is not limited to these examples.

Experimental Example 1 Measurement of the Degree of Dissolution of 002 in a Carbonate Solution Containing Hydrogen Peroxide The following experiments were conducted to confirm the direct dissolution of a (U, Gd) O 2 type oxide powder in a carbonate solution containing hydrogen peroxide. 17

1.67 g of powder (U, Gd) O 2 (average particle size: 10 μm) was added to 100 ml of a solution of 0.5 M Na 2 CO 3 and H 2 O 2 at 1.0 M at room temperature. , and changes in the concentration of dissolved uranium and dissolved hydrogen peroxide were measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES, Jobin Yvon JY 38 Plus) and a reflectoquant (Merck RQflex plus 10). The oxide UO2 and Gd203 (U, Gd) O2 were respectively dissolved in the form of a red uranyl peroxocarbonate complex and in the form of Gd3 *. The results are shown in FIG. 4. As shown in FIG. 4, with respect to the dissolution of UO 2 in the carbonate solution containing hydrogen peroxide, it can be understood that the dissolution rate and the final concentration of the Dissolved uranium is greatly affected by the concentration of hydrogen peroxide in the solution. That is, it can be understood that the UO2 dissolves rapidly and completely in the carbonate solution as long as hydrogen peroxide is present and the dissolution stops when the hydrogen peroxide is present. in the carbonate solution is exhausted. Therefore, it can be understood that when a type (U, Gd) O 2 waste is added to a carbonate solution containing hydrogen peroxide in step a) of the present invention, the uranium dissolves in the solution. .

Experimental Example 2 Measurement of the Degree of Dissolution of Gadolinium in a Carbonate Solution Containing Hydrogen Peroxide The following experiments were performed to measure the degree of dissolution of gadolinium in a carbonate solution containing hydrogen peroxide. 1.67 g of oxide Gd2O3-UO2 to 4 by weight of a waste (U, Gd) 02, which contained oxides of Fe, Ni, Cr and Al produced in a grinding process during of the manufacture of a nuclear fuel, was dissolved in 100 ml of a solution of 0.5 M Na2CO3 and 1.0 M H2O2 at room temperature for 1 hour, then the solubility of each element was measured. using ICP-AES. The results are shown in the following Table 1.

Table 1 Element U Gd Fe, Ni, Cr, Al Concentration at 14,127 87.7 Below the dissolved limit of detection (ppm) by ICP-AES The UO2 component of (U, Gd) O2 was completely dissolved in the solution. However, as can be seen in Table 1, the impurities, i.e. the oxides of Fe, Ni, Cr and Al, mixed in the waste powder were not dissolved in the carbonate solution and, therefore, were not detected in the solution. Even if the concentration of gadolinium in the solution should have been around 19

At 588 ppm, if Gadolinium from Gd2O3-UO2 oxide at 4 by weight of the waste had completely dissolved, the measured concentration was 87.7 ppm, as can be seen in Table 1. This means that approximately 15% of the Gd2O3 of the 4% by weight Gd2O3-UO2 oxide had dissolved. Since a portion of the gadolinium oxide has dissolved in the carbonate solution containing hydrogen peroxide and some of the dissolved gadolinium is entrained in the precipitate of 004 when the solution containing the peroxocarbonate complex of uranyl and gadolinium ions are acidified to precipitate uranium in the form of 004 in step b), it can be understood that a step of purifying the recovered uranium is necessary, such as step d) of the present invention.

Experimental example 3

Measurements of 004 precipitation yield and carbonate recovery yield in the gas absorber The following experiments were performed to confirm the yield of 004 precipitation and the carbonate recovery yield in step b ) Of the present invention. A solution containing a uranium peroxocarbonate complex having a 0.5M carbonate concentration Na 2 CO 3 was introduced at a flow rate of 1.0 ml / min in a uranium precipitation-decarbonization bath designed as shown in FIG. 2, and a 1.0 M nitric acid solution

was introduced by a pH regulator so that the pH value of the decarbonization precipitation bath was 2.5 ± 0.2. 1.0 M NaOH was flowed at a flow rate of 1.0 ml / minute into a gas absorber having an internal diameter of 2.8 cm and a length of 42 cm, and filled with glass beads having a diameter of 1 mm. Carbonate ions existed as carbonic acid (H 2 O 3) when the pH value was less than or equal to 4, but were unstable in the solution and removed from it as gaseous carbon dioxide. Therefore, the carbonate ions of the uranyl peroxocarbonate complex were converted to gaseous carbon dioxide in the uranium precipitation-decarbonization bath having a pH value of 2.5 and totally removed from the solution, and at least 99% of the uranium in the solution containing the uranium peroxocarbonate complex precipitated as UO4r as shown in Reaction Formula 4. The U04 precipitate contained about 35 ppm gadolinium because the coexisting gadolinium ions in the solution containing the uranyl peroxocarbonate complex were entrained in the 004 during precipitation. The Gd content in the once purified UO4 precipitate was reduced to less than 0.35 ppm by dissolving the Gd-contaminated UO4 in acid and reprecipitating the 004 by adding hydrogen peroxide to the solution, as well as step d) of the present invention. 21

The liberated carbon dioxide gas was completely absorbed as a solution of Na2CO3 in the gas absorber, and the previous result can be confirmed by FIG. 5. Therefore, it can be understood that UO4 precipitates sufficiently to step b) of the present invention and that the carbon dioxide generated can be completely recovered again in the form of a carbonate salt solution.

Experimental example 4

Electrodialysis Experiments The following experiments were performed to determine the effectiveness of a method of removing gadolinium ions remaining after precipitation of U04 in step b) of the present invention from a solution and recovering an acid and an alkaline.

While applying a bath voltage of 15 V while 100 ml of 0.5 M NaNO3 solution containing 100 ppm Gd3 * were circulated between the cation and anion exchange membranes of a reactor. electrodialysis and that initial solutions of 0.1 M NaOH and 0.1 M HNO 3 were respectively circulated in a cathode chamber and an anode chamber, the concentrations of HNO 3 i in NaOH and NaNO 3 in the anode chamber, the cathode chamber and a feed solution were measured by means of an automatic acid-alkali titration device. The results are 22

6. As shown in FIG. 6, it can be understood that the Na + and NO 3 ions are substantially regenerated in the form of 0.5 M HNO 3 and 0.5 M NaOH solutions by means of a reaction. electrolysis of water, while the Na + and NO 3 ions of the feed solution between the two ion exchange membranes move respectively to the cathode chamber and the anode chamber during the previous experiment. In this situation trivalent gadolinium anions do not pass through the cation exchange membrane and are discharged out of the reactor. Therefore, it can be understood that gadolinium can be removed from the solution from step b) through step c) of the present invention, and that an acid and an alkali can be recovered and used at step b). According to the present invention, a process is carried out in an alkaline condition with a carbonate and at room temperature, therefore the problems related to the safety of the steps and to the corrosion of the devices are minimized and the environment is better respected thanks to the minimization of the production of secondary waste through the reuse of carbonate, acid and alkali that are recovered.

While the preferred embodiments of the present invention have been described for purposes of illustration, those skilled in the art will understand that various modifications, additions and substitutions are possible, without departing from the scope and scope of the invention. spirit of the invention as described in the appended claims.

Claims (10)

REVENDICATIONS1. Process for recovering the uranium oxide contained in (U, Gd) O2 by means of a carbonate solution containing hydrogen peroxide, characterized in that it comprises the following steps: a) the addition of a carbonate solution containing hydrogen peroxide to a waste (U, Gd) 02 contaminated with metal oxide impurities to selectively dissolve the uranium (U) in the form of a solution containing a complex uranyl peroxocarbonate, wherein gadolinium oxide (Gd) is partially dissolved with uranium while the metal oxide impurities are insoluble in the carbonate solution; b) adding an acidic solution to the solution containing a uranyl peroxocarbonate complex comprising co-dissolved Gd ions to precipitate the uranium in the solution in the form of uranium peroxide U04 which is contaminated with Gd ions and the absorption of gaseous carbon dioxide generated during acidification by a gas absorber with an alkaline solution to recover a carbonate salt solution; c) recovering the acid solution and the alkaline solution used in step b) from a supernatant solution after precipitation of uranium peroxide by electrodialysis; d) dissolving the Gd-contaminated uranium peroxide prepared in step b) in an acid solution 24 and adding hydrogen peroxide to the acid solution to again precipitate only the uranium peroxide in the solution, while the dissolved Gd ions remain in the solution, to purify the Gd-contaminated uranium peroxide; and e) washing the precipitated uranium peroxide selectively. The process according to claim 1, wherein the carbonate solution containing hydrogen peroxide of step a) has a pH in a range of from about 10 to about 13 and contains hydrogen peroxide at a concentration ranging from about 0.1 M to about 3.0 M. The process according to claim 1, wherein an acid is added in step b) to adjust the pH in a range from about 0 to about 4. The process according to claim 3, wherein the acid is any acid selected from the group consisting of nitric acid, hydrochloric acid and sulfuric acid. The process according to claim 1, wherein the reaction between the carbon dioxide and the alkaline solution in step b) is carried out by means of a gas absorber in which the alkaline solution circulates. The process according to claim 1, wherein the acidic solution used in step d) contains nitric acid, its temperature is in the range of about 70 ° C to about 90 ° C and its concentration is in the range of about 1 M to 2 M. The process according to claim 1, wherein the carbonate recovered in step b) is used in the form of a carbonate solution in step a). The method of claim 1, wherein step c) is performed by an electrodialysis method using a cation exchange membrane and an anion exchange membrane. 9. The method of claim 1, wherein the acid solution and the alkaline solution recovered in step c) are used respectively as acid solution and alkaline solution in step b). The method of claim 1, wherein steps d) and e) are repeated twice or more than twice.