A method for catalytic reduction of hexavalent uranium to prepare tetravalent uranium solution

Abstract

The present invention provides a method for catalytic reduction of hexavalent uranium to tetravalent uranium, the method comprising using a molecular sieve-supported noble metal catalyst, using hydrazine nitrate or formic acid or formaldehyde as a reducing agent, under the conditions of a reaction temperature of 15-60°C, an acidity of 0.5-1.0 mol / L, a hexavalent uranium concentration of 100-300 g / L, and a reaction pressure of normal pressure, catalytically reducing hexavalent uranium ions to tetravalent uranium ions. In the method provided by the present invention, hexavalent uranium can be effectively reduced to tetravalent uranium under the conditions of a molecular sieve-supported noble metal catalyst, the conversion rate of hexavalent uranium can reach up to 99%, and good stability is shown, with good industrial application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of nuclear fuel post-processing, and in particular to a method for preparing a tetravalent uranium solution by catalytic reduction of hexavalent uranium. Background Art

[0002] Nuclear fuel reprocessing is an important part of the nuclear fuel cycle. It is mainly the process of recovering uranium, plutonium and other valuable elements from used nuclear fuel. At present, nuclear fuel reprocessing plants at home and abroad mainly use the Purex process. The Purex process mainly uses the different extraction capacities of uranium, plutonium and fission products of tributyl phosphate (TBP) to achieve separation. In the uranium-plutonium separation stage, the extraction capacity of trivalent plutonium by TBP is very small, so uranium and plutonium are separated. However, plutonium is tetravalent in the front-end process, so a reducing agent must be added to reduce it to trivalent. When hydrazine is used as a supporting reducing agent, tetravalent uranium (U(IV)) is a better reduction stripping agent for tetravalent plutonium (Pu(IV)). It has the advantages of complete plutonium reduction stripping, no impurities introduced into the system, and fast reaction rate, and is widely used. Therefore, the preparation of tetravalent uranium is of great significance in the purex process.

[0003] At present, the preparation methods of tetravalent uranium mainly include direct electrolysis and catalytic hydrogenation. The electrolysis method is to electrolytically reduce UO2(NO3)2 in nitric acid solution to U(NO3)4 on the electrode. This method has the advantages of simple process and no increase in waste liquid. However, in the actual application process in the reprocessing plant, the conversion rate of hexavalent uranium (U(VI)) is low, generally only 50%-60%, which makes the concentration of tetravalent uranium (U(IV)) low. When it is used as a reducing agent, it will not only lead to the dilution of plutonium products, but also increase the recovery burden of uranium products. The catalytic hydrogenation method is to use high-pressure hydrogen to reduce UO2(NO3)2 solution to U(NO3)4 under the action of a catalyst. The catalytic hydrogenation method has the characteristics of large production capacity and high U(IV) yield, but its equipment is complex, and the high-pressure hydrogen pressure used reaches 10MPa, which is a huge safety hazard for nuclear fuel reprocessing workshops with extremely strong radioactivity. Therefore, it is very important to develop a simple, safe and efficient U(IV) preparation method.

[0004] Using organic reducing agents such as hydrazine and formic acid, U(VI) can be reduced to U(IV) under the action of precious metal catalysts. It has the advantages of mild reaction conditions and simple preparation process, and is a very promising method for preparing U(VI). Li Bin et al. used platinum black as catalyst to study the process conditions of hydrazine reduction of U(VI) to U(IV) in nitric acid system. When the uranium concentration was 0.9 mol / L, the HNO3 concentration was 0.8 mol / L, and the hydrazine concentration was 1 mol / L, the conversion rate of U(VI) could reach more than 90% at 60°C. (Nuclear Chemistry and Radiochemistry, 2013, 35, (1): 24–28). Boltoeva et al. investigated the particle size effect of Pt in the hydrazine reduction of U(VI) to U(IV) in H2SO4, HClO4, and HNO3 systems. It was found that the conversion rate of U(VI) increased with the increase of Pt particle size (Radiochemistry, 2007, 49, 603–606). Anan'ev et al. investigated the reaction performance of hydrazine and formic acid reduction of U(VI) to U(IV) in nitric acid system (Radiochemistry, 2001, 43, 39-43). At present, the research on the hydrazine reduction of U(VI) to U(IV) mainly focuses on the investigation of kinetics and process conditions, while there is little research on the effect of the catalyst itself on the reaction performance.

[0005] At present, some patents have been applied for the preparation of tetravalent uranium solution. The following are several reported patents for detailed description:

[0006] The Chinese patent CN201110097474 is publicly named: A method for preparing a tetravalent uranium solution. The patent reports the use of an organic reducing agent (hydrazine or carboxylic acid and its derivatives) to reduce U(VI) to prepare a U(IV) solution under the action of a Pt catalyst. The catalyst involved in the patent is platinum black, which is very expensive to use directly and has low catalytic efficiency.

[0007] The Chinese patent CN201310743451 is publicly named: A device for preparing tetravalent uranium by electrolytic reduction. The patent reports an improved device for preparing tetravalent uranium by electrolytic reduction, which mainly solves the problem of low U(VI) conversion rate in a diaphragmless electrolysis device. In actual production, the effectiveness of the device needs further verification.

[0008] The Chinese patent CN201910865308 is publicly named: A catalyst, method and application for removing hexavalent uranium by visible light catalytic reduction. The patent reports that the use of ZnFe2O4 catalyst can reduce U(VI) in wastewater to U(IV) under light conditions and generate precipitation. Thereby removing U(VI) ions from the wastewater. The catalyst involved in the patent is ZnFe2O4, which uses a photocatalytic reduction method to remove U(VI) ions.

[0009] At present, there is an urgent need to develop an efficient catalyst to solve the shortcomings of existing problems. Summary of the invention

[0010] One of the technical problems to be solved by the present invention is to solve the problems of harsh reaction conditions and low U(IV) yield in the preparation process of U(IV), and to provide a method for catalytic reduction of U(VI) to prepare U(IV) solution under mild reaction conditions and high U(IV) yield.

[0011] The second technical problem to be solved by the present invention is to solve the problem of catalytic reduction of U(VI) to U(IV) in an acidic system, and to provide a new catalyst having a high U(VI) conversion rate.

[0012] The third technical problem to be solved by the present invention is to adopt the preparation method of the catalyst described in the second technical problem above. The content of each component of the catalyst is controllable, the preparation process is simple and the reliability is good.

[0013] The fourth technical problem to be solved by the present invention is to use the catalyst described in the second technical problem to realize the catalytic reaction process of catalytic reduction of U(VI) to U(IV) in an acidic system.

[0014] In order to solve one of the above technical problems, the present invention adopts the following technical solution: a liquid phase reducing agent is used to reduce hexavalent uranium to prepare a tetravalent uranium solution by using a catalyst. The method is characterized in that: the method uses a noble metal catalyst supported by a molecular sieve; the carrier of the catalyst is a ZSM-5 molecular sieve treated with alkaline solution; the noble metal in the catalyst is any one or two of Pt or Ir, the mass percentage of the noble metal in the catalyst is 0.1%-10% (preferably 0.5-5%), and the rest is the carrier.

[0015] The preparation steps of the catalyst are as follows:

[0016] 1) Carrier pretreatment: prepare an alkaline solution of a certain concentration and heat it to 60-90°C; pour the ZSM-5 molecular sieve powder into the alkaline solution, stir it for 0.5-2h, and then place it in an ice water bath to cool it to 0-40°C; then centrifuge it and wash it with deionized water until it is neutral, and dry it at 80-120°C for 4-24h, and the resulting solid is marked as Z; wherein the Si / Al molarity of the ZSM-5 molecular sieve is 25-50. The alkaline solution is one or more selected from sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution; the concentration of the alkaline solution is 0.1-10 mol / L; the liquid-solid ratio of the alkaline solution to the molecular sieve powder is 3:1-10:1 (ml / g);

[0017] 2) Take any one or both of Pt and Ir soluble salts and dissolve them in deionized water to prepare solution A, where the concentration of noble metal ions is 0.001-0.5 mol / L;

[0018] 3) Take the carrier treated with alkali solution, and add or pour the required amount of solution A into Z according to the content of precious metal in the catalyst;

[0019] 4) soaking the mixture in step (3) at room temperature for 0.5 to 48 hours;

[0020] 5) drying the mixture obtained in step (4) at 60-120° C. for 4-24 h;

[0021] 6) calcining the dried mixture at 200-500° C. for 1-8 hours;

[0022] 7) The solid obtained in (6) is reduced and activated by the following process: the reducing gas is H2 (molar purity> 99.9%), and the volume space velocity of the reducing gas is 100-3600h -1 The heating rate from room temperature to the reduction temperature is 1-10°C / min, the reduction temperature is 200-500°C, the pressure is normal pressure, and the reduction time is 1-48h.

[0023] In step 2) of the method, the soluble salt of one or both of Pt and Ir is one or more of the nitrates or chlorides of the corresponding precious metals.

[0024] In order to solve the second of the above technical problems, the present invention adopts the following technical solution:

[0025] The carrier of the catalyst is ZSM-5 molecular sieve soaked in alkali solution; the noble metal in the catalyst is any one or two of Pt and Ir, the mass percentage of the noble metal in the catalyst is 0.1%-10% (preferably 0.5-5%), and the rest is the carrier.

[0026] In order to solve the third of the above technical problems, the present invention adopts the following technical solution:

[0027] The preparation steps of the catalyst are as follows:

[0028] 1) Carrier pretreatment: prepare an alkaline solution of a certain concentration and heat it to 60-90°C; pour the ZSM-5 molecular sieve powder into the alkaline solution, stir it for 0.5-2h, and then place it in an ice water bath to cool it to 0-40°C; then centrifuge it and wash it with deionized water until it is neutral, and dry it at 80-120°C for 4-24h, and the resulting solid is marked as Z; wherein the Si / Al molarity of the ZSM-5 molecular sieve is 25-50. The alkaline solution is one or more selected from sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution; the concentration of the alkaline solution is 0.1-10 mol / L; the liquid-solid ratio of the alkaline solution to the molecular sieve powder is 3:1-10:1 (ml / g);

[0029] 2) Take any one or both of Pt and Ir soluble salts and dissolve them in deionized water to prepare solution A, where the concentration of noble metal ions is 0.001-0.5 mol / L;

[0030] 3) Take the carrier treated with alkali solution, and add or pour the required amount of solution A into Z according to the content of precious metal in the catalyst;

[0031] 4) soaking the mixture in step (3) at room temperature for 0.5 to 48 hours;

[0032] 5) drying the mixture obtained in step (4) at 60-120°C for 4-24h;

[0033] 6) calcining the dried mixture at 200-500° C. for 1-8 hours;

[0034] 7) The solid obtained in (6) is reduced and activated by the following process: the reducing gas is H2 (molar purity> 99.9%), and the volume space velocity of the reducing gas is 100-3600h -1 The heating rate from room temperature to the reduction temperature is 1-10°C / min, the reduction temperature is 200-500°C, the pressure is normal pressure, and the reduction time is 1-48h.

[0035] In step 2) of the method, the soluble salt of one or both of Pt and Ir is one or more of the nitrates or chlorides of the corresponding precious metals.

[0036] In order to solve the fourth technical problem above, the present invention adopts the following technical solution:

[0037] The catalytic process is carried out in a kettle reactor or a fixed bed reactor. The conditions for reducing hexavalent uranium to prepare a tetravalent uranium solution are: an acid concentration (in terms of hydrogen ion concentration) of 0.5-1.0 mol / L, a uranyl ion concentration of 100-300 g / L (in terms of uranium), a reducing agent concentration of 0.5-2.0 mol / L, a reaction temperature of 15-60° C., and a reaction pressure of normal pressure.

[0038] The acidic system is an acidic system formed by one or more of nitric acid, sulfuric acid or perchloric acid; the hexavalent uranium is a uranyl ion of the acid type corresponding to the acidic system, which is one or more of uranyl nitrate, uranyl sulfate and uranyl perchlorate solution; and the tetravalent uranium is a reduction product of the corresponding hexavalent uranium.

[0039] The reducing agent is any one, two or three of hydrazine nitrate, formic acid or formaldehyde.

[0040] The advantages of the present invention are:

[0041] (1) The method for preparing tetravalent uranium by catalytic reduction of hexavalent uranium provided by the present invention has the advantages of mild reaction conditions and high yield of tetravalent uranium.

[0042] (2) The catalyst preparation method provided by the present invention is simple and is conducive to industrial production.

[0043] (3) The catalyst provided by the present invention can reduce U(VI) to U(IV) using hydrazine under acidic conditions.

[0044] In a stirred tank reactor, the conversion of U(VI) can reach 99% within 60 min.

[0045] And it has higher stability. DETAILED DESCRIPTION

[0046] The technical details of the present invention are described in detail by the following examples. It should be noted that the examples are only used to further illustrate the technical features of the present invention, rather than to limit the present invention.

[0047] Catalyst preparation and performance evaluation

[0048] Example 1

[0049] Take 0.4g of sodium hydroxide solid and dissolve it in a beaker with 100ml of deionized water, and place it in a water bath and heat it to 80°C. Take 10g of ZSM-5 molecular sieve with a Si / Al molar ratio of 35, pour it into the above alkaline solution, and continue stirring for 0.5h. After stirring, place the beaker in an ice water bath and cool it to 25°C. After cooling, centrifuge it and wash it with deionized water until it is neutral, and dry it at 80°C for 4h. The resulting sample is marked as Z1.

[0050] Weigh 5.0g of the prepared Z1 carrier, weigh 0.399g of H2PtCl6·6H2O and dissolve it in 5ml of deionized water to prepare a mixed solution, the molar concentration of Pt is 0.154mol / L, add it dropwise to the Z1 carrier and mix it evenly. Soak at room temperature for 12h, dry at 80℃ for 12h, and calcine at 300℃ in air atmosphere for 4h, where the Pt mass content is 3%, recorded as 3Pt / Z1 catalyst.

[0051] Catalyst reduction activation conditions: reducing gas is pure H2 (molar purity> 99.9%), space velocity is 2000h -1 , the heating rate from room temperature to the reduction temperature is 5°C / min, the reduction temperature is 400°C, the pressure is normal pressure, and the reduction time is 4h. The reaction conditions for the catalyst in the stirred tank are: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60°C, stirring rate is 800 rpm, pressure is normal pressure, catalyst dosage is 1.0 g, and reaction liquid is 25 mL. The reaction results are shown in Table 1.

[0052] Example 2

[0053] Take 5.6g of potassium hydroxide solid and dissolve it in a beaker with 100ml of deionized water, and place it in a water bath and heat it to 80°C. Take 10g of ZSM-5 molecular sieve with a Si / Al molar ratio of 25, pour it into the above alkaline solution, and continue stirring for 0.5h. After stirring, place the beaker in an ice water bath and cool it to 25°C. After cooling, centrifuge it and wash it with deionized water until it is neutral, and dry it at 80°C for 6h. The resulting sample is marked as Z2.

[0054] Weigh 5.0g of the prepared Z2 carrier, weigh 0.066g of H2PtCl6·6H2O and dissolve it in 5ml of deionized water to prepare a mixed solution, the molar concentration of Pt is 0.026mol / L, add it dropwise to the Z2 carrier and mix it evenly. Soak at room temperature for 4h, dry at 60℃ for 24h, and calcine at 200℃ in air atmosphere for 1h, where the mass content of Pt is 0.5%, recorded as 0.5Pt / Z2 catalyst.

[0055] Catalyst reduction activation conditions: reducing gas is pure H2 (molar purity> 99.9%), space velocity is 2000h -1, the heating rate from room temperature to the reduction temperature is 5℃℃ / min, the reduction temperature is 300℃, the pressure is normal pressure, and the reduction time is 4h. The catalyst is used in the stirred tank reaction conditions: nitric acid concentration (in terms of hydrogen ion concentration) is 0.5mol / L, uranyl (uranyl nitrate) ion concentration is 100g / L, hydrazine nitrate concentration is 0.5mol / L, temperature is 30℃, stirring rate is 800rpm, pressure is normal pressure, catalyst dosage is 1.0g, and reaction liquid is 25mL. The reaction results are shown in Table 1.

[0056] Example 3

[0057] Take 32g of sodium hydroxide solid and dissolve it in a beaker with 100ml of deionized water, and place it in a water bath and heat it to 80°C. Take 10g of ZSM-5 molecular sieve with a Si / Al molar ratio of 50, pour it into the above alkaline solution, and continue stirring for 2h. After stirring, place the beaker in an ice water bath and cool it to 25°C. After cooling, centrifuge it and wash it with deionized water until it is neutral, and dry it at 120°C for 6h. The resulting sample is marked as Z3.

[0058] Weigh 5.0g of the prepared Z3 carrier, weigh 0.665g of H2PtCl6·6H2O and dissolve it in 5ml of deionized water to prepare a mixed solution, the molar concentration of Pt is 0.257mol / L, add it dropwise to the Z3 carrier and mix it evenly. Soak at room temperature for 48h, dry at 120℃ for 12h, and calcine at 500℃ in air atmosphere for 2h, where the Pt mass content is 5%, recorded as 5Pt / Z3 catalyst.

[0059] Catalyst reduction activation conditions: reducing gas is pure H2 (molar purity> 99.9%), space velocity is 2000h -1 , the heating rate from room temperature to the reduction temperature is 5℃ / min, the reduction temperature is 450℃, the pressure is normal pressure, and the reduction time is 4h. The catalyst is used in the stirred tank reaction conditions: nitric acid concentration (in terms of hydrogen ion concentration) is 1.0mol / L, uranyl (uranyl nitrate) ion concentration is 300g / L, hydrazine nitrate concentration is 1.5mol / L, temperature is 50℃, stirring rate is 800rpm, pressure is normal pressure, catalyst dosage is 1.0g, and reaction liquid is 25mL. The reaction results are shown in Table 1.

[0060] Example 4

[0061] Take 1.2g of lithium hydroxide solid and dissolve it in a beaker with 100ml of deionized water, and place it in a water bath and heat it to 80°C. Take 10g of ZSM-5 molecular sieve with a Si / Al molar ratio of 35, pour it into the above alkaline solution, and continue stirring for 1.0h. After stirring, place the beaker in an ice water bath and cool it to 25°C. After cooling, centrifuge it and wash it with deionized water until it is neutral, and dry it at 100°C for 24h. The resulting sample is marked as Z4.

[0062] Weigh 5.0g of the prepared Z4 carrier, weigh 0.402g of H2IrCl6·6H2O and dissolve it in 10ml of deionized water to prepare a mixed solution, the molar concentration of Ir is 0.156mol / L, add it dropwise to the Z4 carrier and mix it evenly. Soak at room temperature for 48h, dry at 120℃ for 12h, and calcine at 300℃ in air atmosphere for 2h, wherein the mass content of Ir is 3%, recorded as 3Ir / Z4 catalyst.

[0063] Catalyst reduction activation conditions: reducing gas is pure H2 (molar purity> 99.9%), space velocity is 2000h -1 , the heating rate from room temperature to the reduction temperature is 5°C / min, the reduction temperature is 400°C, the pressure is normal pressure, and the reduction time is 4h. The reaction conditions for the catalyst in the stirred tank are: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60°C, stirring rate is 800 rpm, pressure is normal pressure, catalyst dosage is 1.0 g, and reaction liquid is 25 mL. The reaction results are shown in Table 1.

[0064] Example 5

[0065] Take 2.8g of potassium hydroxide solid and dissolve it in a beaker with 30ml of deionized water, and place it in a water bath and heat it to 80°C. Take 10g of ZSM-5 molecular sieve with a Si / Al molar ratio of 25, pour it into the above alkaline solution, and continue stirring for 2h. After stirring, place the beaker in an ice water bath and cool it to 25°C. After cooling, centrifuge it and wash it with deionized water until it is neutral, and dry it at 120°C for 4h. The resulting sample is marked as Z5.

[0066] Weigh 5.0g of the prepared Z5 carrier, weigh 0.067g of H2IrCl6·6H2O and dissolve it in 5ml of deionized water to prepare a mixed solution, the molar concentration of Ir is 0.026mol / L, add it dropwise to the Z5 carrier and mix it evenly. Soak at room temperature for 24h, dry at 80℃ for 24h, and calcine at 200℃ in air atmosphere for 1h, where the mass content of Ir is 0.5%, recorded as 0.5Ir / Z5 catalyst.

[0067] Catalyst reduction activation conditions: reducing gas is pure H2 (molar purity> 99.9%), space velocity is 2000h -1 , the heating rate from room temperature to the reduction temperature is 5℃ / min, the reduction temperature is 300℃, the pressure is normal pressure, and the reduction time is 3h. The catalyst is used in the stirred tank reaction conditions: nitric acid concentration (in terms of hydrogen ion concentration) is 0.7mol / L, uranyl (uranyl nitrate) ion concentration is 150g / L, hydrazine nitrate concentration is 0.7mol / L, temperature is 50℃, stirring rate is 800rpm, pressure is normal pressure, catalyst dosage is 1.0g, and reaction liquid is 25mL. The reaction results are shown in Table 1.

[0068] Example 6

[0069] Take 20g of sodium hydroxide solid and dissolve it in a beaker with 50ml of deionized water, and place it in a water bath and heat it to 80°C. Take 10g of ZSM-5 molecular sieve with a Si / Al molar ratio of 50, pour it into the above alkaline solution, and continue stirring for 2h. After stirring, place the beaker in an ice water bath and cool it to 25°C. After cooling, centrifuge it and wash it with deionized water until it is neutral, and dry it at 80°C for 6h. The resulting sample is marked as Z6.

[0070] Weigh 5.0g of the prepared Z3 carrier, weigh 0.670g of H2IrCl6·6H2O and dissolve it in 5ml of deionized water to prepare a mixed solution, the molar concentration of Ir is 0.260mol / L, add it dropwise to the Z6 carrier and mix it evenly. Soak at room temperature for 24h, dry at 120℃ for 12h, and calcine at 500℃ in air atmosphere for 2h, wherein the mass content of Ir is 5%, recorded as 5Ir / Z6 catalyst.

[0071] Catalyst reduction activation conditions: reducing gas is pure H2 (molar purity> 99.9%), space velocity is 2000h -1 , the heating rate from room temperature to the reduction temperature is 5℃ / min, the reduction temperature is 400℃, the pressure is normal pressure, and the reduction time is 4h. The catalyst is used in the stirred tank reaction conditions: nitric acid concentration (in terms of hydrogen ion concentration) is 1.0mol / L, uranyl (uranyl nitrate) ion concentration is 300g / L, hydrazine nitrate concentration is 1.5mol / L, temperature is 60℃, stirring rate is 800rpm, pressure is normal pressure, catalyst dosage is 1.0g, and reaction liquid is 25mL. The reaction results are shown in Table 1.

[0072] Example 7

[0073] Take the 3Pt / Z1 catalyst activated by reduction in Example 1. The reaction conditions in the stirred tank are as follows: sulfuric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl sulfate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60°C, stirring rate is 800 rpm, pressure is normal pressure, catalyst dosage is 1.0 g, and reaction liquid is 25 mL. The reaction results are shown in Table 1.

[0074] Example 8

[0075] Take the 3Pt / Z1 catalyst activated by reduction in Example 1. The reaction conditions in the stirred tank are: perchloric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl perchlorate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60°C, stirring rate is 800 rpm, pressure is normal pressure, catalyst dosage is 1.0 g, and reaction liquid is 25 mL. The reaction results are shown in Table 1.

[0076] Example 9

[0077] Take the 3Pt / Z1 catalyst activated by reduction in Example 1. The reaction conditions in the stirred tank are: nitric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, formic acid concentration is 1.0 mol / L, temperature is 60°C, stirring rate is 800 rpm, pressure is normal pressure, catalyst dosage is 1.0 g, and reaction liquid is 25 mL. The reaction results are shown in Table 1.

[0078] Example 10

[0079] Take the 3Pt / Z1 catalyst activated by reduction in Example 1. The reaction conditions in the stirred tank are: nitric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, formaldehyde concentration is 1.0 mol / L, temperature is 60°C, stirring rate is 800 rpm, pressure is normal pressure, catalyst dosage is 1.0 g, and reaction liquid is 25 mL. The reaction results are shown in Table 1.

[0080] Embodiment 11

[0081] Take the 3Pt / Z1 catalyst activated by reduction in Example 1. Reaction conditions in the fixed bed: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 15°C, liquid space velocity is 0.3 h -1 , the pressure was normal pressure, the amount of catalyst was 20 g, and the flow rate of the reaction liquid was 0.2 ml / min. The reaction results are shown in Table 1.

[0082] Comparative Example 12

[0083] Take 5.0g ZSM-5 molecular sieve with Si / Al molar ratio of 200, weigh 0.399g H2PtCl6·6H2O and dissolve it in 5ml deionized water to prepare a mixed solution, the molar concentration of Pt is 0.154mol / L, add it dropwise to the above carrier and mix it evenly. Soak at room temperature for 12h, dry at 80℃ for 12h, and calcine at 300℃ in air atmosphere for 4h, where the mass content of Pt is 3%, recorded as 3Pt / Z0 catalyst.

[0084] Catalyst reduction activation conditions: reducing gas is pure H2 (molar purity> 99.9%), space velocity is 2000h -1 , the heating rate from room temperature to the reduction temperature is 5℃ / min, the reduction temperature is 400℃, the pressure is normal pressure, and the reduction time is 4h. The reaction conditions for the catalyst in the stirred tank are: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8mol / L, uranyl (uranyl nitrate) ion concentration is 210g / L, hydrazine nitrate concentration (in terms of hydrogen ion concentration) is 1.0mol / L, temperature is 60℃, stirring rate is 800rpm, pressure is normal pressure, catalyst dosage is 1.0g, and reaction liquid is 25mL. The reaction results are shown in Table 1.

[0085] Comparative Example 13

[0086] Take 5.0g SiO2 carrier, weigh 0.399g H2PtCl6·6H2O and dissolve it in 5ml deionized water to make a mixed solution, the molar concentration of Pt is 0.154mol / L, add it dropwise to the Z1 carrier and mix it evenly. Soak at room temperature for 12h, dry at 80℃ for 12h, and calcine at 300℃ in air atmosphere for 4h, where the mass content of Pt is 3%, recorded as 3Pt / Si catalyst.

[0087] Catalyst reduction activation conditions: reducing gas is pure H2 (molar purity> 99.9%), space velocity is 2000h -1 , the heating rate from room temperature to the reduction temperature is 5°C / min, the reduction temperature is 400°C, the pressure is normal pressure, and the reduction time is 4h. The reaction conditions for the catalyst in the stirred tank are: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60°C, stirring rate is 800 rpm, pressure is normal pressure, catalyst dosage is 1.0 g, and reaction liquid is 25 mL. The reaction results are shown in Table 1.

[0088] Example result analysis:

[0089] From the data analysis in Table 1, it can be seen that when using the molecular sieve carrier after alkali treatment to prepare Pt-based or Ir-based catalysts, when using hydrazine nitrate, formic acid, formaldehyde, etc. as reducing agents under acidic conditions, the U(VI) conversion rate in the U(VI) to U(IV) reaction is higher than 90%. In the 3Pt / Z1 catalyst, when hydrazine nitrate is used as a reducing agent, the U(VI) conversion rate can reach 99%. Moreover, in a fixed bed reactor, the 3Pt / Z1 catalyst also shows good reaction performance in the reduction of U(VI) to U(IV) by hydrazine nitrate, and the U(VI) conversion rate can reach 99%. However, the 3Pt / Z0 catalyst prepared by the molecular sieve with a Si / Al ratio of 200 has a U(VI) conversion rate of only 63% within 180 minutes, which is equivalent to the 3Pt / Si catalyst prepared by SiO2. The results show that the catalyst prepared by using ZSM-5 molecular sieve with suitable Si / Al ratio and loading Pt and Ir has good performance in the reaction of U(VI) to U(IV) under acidic conditions when hydrazine nitrate, formic acid, formaldehyde, etc. are used as reducing agents.

[0090] Table 1 Reaction performance of hydrazine reduction of hexavalent uranium to tetravalent uranium on different catalysts under acidic conditions

[0091]

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Claims

1. A method for preparing tetravalent uranium by catalytic reduction of hexavalent uranium, characterized in that: The method uses a molecular sieve-supported noble metal catalyst; the catalyst carrier is a ZSM-5 molecular sieve that has been soaked in an alkaline solution; the noble metal in the catalyst is any one or two of Pt or Ir, the mass percentage of the noble metal in the catalyst is 0.1%-10%, and the rest is the carrier; The steps for preparing the catalyst are as follows: 1) Carrier pretreatment: prepare an alkaline solution of a certain concentration and heat it to 60-90°C; pour ZSM-5 molecular sieve powder into the alkaline solution, stir it for 0.5-2 h, and then place it in an ice water bath to cool it to 0-40°C; then centrifuge it and wash it with deionized water until it is neutral, and dry it at 80-120°C for 4-24 h. The obtained solid is marked as Z; wherein the Si / Al molar ratio of the ZSM-5 molecular sieve is 25-50; the alkaline solution is one or more selected from sodium hydroxide solution, potassium hydroxide solution, and lithium hydroxide solution; the concentration of the alkaline solution is 0.1-10 mol / L; the liquid-solid ratio of the alkaline solution to the molecular sieve powder is 3:1-10:1 (ml / g); 2) Take any one or both of Pt and Ir soluble salts and dissolve them in deionized water to prepare solution A, where the concentration of noble metal ions is 0.001~0.5mol / L; 3) Take the carrier treated with the alkaline solution, and add or pour the required amount of solution A into Z according to the content of the precious metal in the catalyst; 4) Soaking the mixture in step 3) at room temperature for 0.5 to 48 h; 5) drying the mixture obtained in step 4) at 60-120° C. for 4-24 h; 6) calcining the dried mixture at 200-500°C for 1-8 h; 7) The solid obtained in 6) is reduced and activated by the following process: the reducing gas is H2 with a molar purity of >99.9%, and the volumetric space velocity of the reducing gas is 100-3600 h -1 The heating rate from room temperature to the reduction temperature is 1-10 ℃ / min, the reduction temperature is 200-500 ℃, the pressure is normal pressure, and the reduction time is 1-48 h.

2. The method according to claim 1, characterized in that: The mass percentage of precious metals in the catalyst is 0.5~5%.

3. The method according to claim 2, characterized in that: In step 2), the soluble salt of one or both of Pt and Ir is one or more of the nitrates or chlorides of the corresponding noble metals.

4. The method according to claim 1, characterized in that: The catalyst is used for catalytic reduction of hexavalent uranium to prepare tetravalent uranium in an acidic system.

5. The method according to claim 4, characterized in that: The reducing agent is any one, two or three of hydrazine nitrate, formic acid or formaldehyde.

6. The method according to claim 4, characterized in that: The acidic system is an acidic system formed by one or more of nitric acid, sulfuric acid or perchloric acid; the hexavalent uranium is a uranyl ion of the acid type corresponding to the acidic system, which is one or more of uranyl nitrate, uranyl sulfate and uranyl perchlorate solution; and the tetravalent uranium is a reduction product of the corresponding hexavalent uranium.

7. The method according to any one of claims 4 to 6, characterized in that: The catalytic process is carried out in a kettle reactor or a fixed bed reactor. The conditions for reducing hexavalent uranium to prepare tetravalent uranium solution are: the acid concentration is 0.5-1.0 mol / L based on hydrogen ion concentration, the uranyl ion concentration is 100-300 g / L based on uranium, the concentration of the reducing agent is 0.5-2.0 mol / L, the reaction temperature is 15-60°C, and the reaction pressure is normal pressure.

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