CN113526558B - A kind of method for preparing uranium nitrate by catalytic hydrogenation reduction of uranyl nitrate - Google Patents

Abstract

The invention discloses a method for preparing uranium nitrate (tetravalent uranium) by reducing uranyl nitrate (hexavalent uranium) through catalytic hydrogenation. The gas-liquid mixture is subjected to online dropwise addition of a hydrazine nitrate or hydrazine solution to enable the solution to be converted from an oxidation state to a reduction state and then enter a high-pressure gas-liquid separator for separation, nitrogen oxides in a gas phase are absorbed and separated by alkali liquor, hydrogen enters a high-pressure hydrogen storage tank after being pressurized by a compressor, a liquid-phase product is discharged from the bottom of the separator and enters a normal-pressure liquid-phase product hydrogen replacement tower, dissolved hydrogen in the liquid phase is removed in the tower by using a nitrogen bubbling device, so that the safety of a subsequent process is ensured, and the obtained gas enters a high-pressure tail gas treatment system.

Description

A kind of method for preparing uranium nitrate by catalytic hydrogenation reduction of uranyl nitrate

technical field

The invention relates to a technological process for preparing tetravalent uranium, in particular to a technological process for preparing uranium nitrate (tetravalent uranium) by catalytic hydrogenation reduction of uranyl nitrate (hexavalent uranium).

Background technique

With the large-scale development and application of nuclear power, the clean treatment and efficient recycling of nuclear spent fuel has become the key to the further development of nuclear power. At present, the technology mainly adopted by domestic and foreign reprocessing plants is the Purex process. In this process, the valuable uranium and plutonium elements are first extracted into the organic phase through the extraction unit, so as to be separated from most of the split elements to achieve the purpose of decontamination; then, tetravalent uranium is added as a reducing agent to convert the tetravalent uranium The plutonium is reduced to trivalent and enters the water phase, but the hexavalent uranium remains in the oil phase, thereby realizing the efficient separation and purification of uranium and plutonium. The biggest advantage of this process is that the lipophilic tetravalent plutonium is reduced to hydrophilic trivalent while the newly added tetravalent uranium is oxidized to hexavalent, thus realizing the clean separation of uranium/plutonium without introducing any impurity ions and efficient use. Therefore, in order to realize the above process, in addition to the proper extraction process, it is very important to develop a process for reducing hexavalent uranium to generate tetravalent uranium and related reaction equipment for the development and utilization of the entire process.

In the current technology in my country, the preparation of tetravalent uranium mainly relies on electrolysis, and the content of tetravalent uranium in the prepared feed solution is only 75% (total uranium concentration 200g/l, tetravalent uranium concentration 150g/l). Due to the low concentration of tetravalent uranium, the content of plutonium in the current follow-up process in my country is only 3g/l (France UP3, UP2-800 is 6g/l), which brings great pressure to the subsequent plutonium purification and recycling, increasing system load and processing cost. In addition, the China Institute of Atomic Energy also proposed a process for the preparation of uranium nitrate by catalytic reduction of uranyl nitrate with hydrazine nitrate. In this reaction process, in addition to the reaction with uranyl nitrate as in (1), hydrazine nitrate can also react with uranyl nitrate under the action of a catalyst. The thermal decomposition reaction (2) is carried out. In the relevant reactor, reaction (1) and reaction (2) are a parallel reaction process. If the residence time of the reaction solution is not enough, reaction (1) cannot be carried out completely, and the conversion rate of uranyl nitrate is relatively large, but when the reaction The liquid residence time is too long, and the catalytic decomposition reaction (2) of hydrazine nitrate is carried out transitionally, so that the product liquid changes from a reduced state to an oxidized state, and the tetravalent uranium nitrate generated by the reaction is further oxidized to hexavalent uranyl nitrate, making the hexavalent uranium nitrate. Conversion rates decrease. Therefore, in order to obtain a higher conversion rate of uranyl nitrate in this process, it is necessary to accurately control the residence time of the reaction solution so that the conversion rate of hydrazine nitrate is 30%-40%. This special requirement for the residence time of the reactor, It brings great difficulties to the industrialization of the process.

N2H5NO3 _{+ HNO3} + _2UO2 ( _{NO3 )2 =} 2U(OH)( _{NO3 )3 +} 2H2O+ _N2 ( ₁ )

3N ₂ H ₅ NO ₃ +HNO ₃ =4NH ₄ NO ₃ +N ₂ (2)

In the hydrogenation catalytic reduction process disclosed in this patent, hydrogen is used as a reducing agent to react as shown in equation (3) under the action of a catalyst, so that uranyl nitrate is catalytically reduced to uranyl nitrate.

UO ₂ (NO ₃ ) ₂ +HNO ₃ +H ₂ =U(OH)(NO ₃ ) ₃ +H ₂ O (3)

Compared with hydrazine nitrate catalytic reduction, hydrogen catalytic reduction does not have side reactions, and the reducing agent H2 is excessive during the reaction process, which can increase the residence time of the reaction solution in the reactor and ensure that the conversion rate of hexavalent uranium is greater than 98%. , which also reduces the difficulty of process industrialization. In the process of preparing tetravalent uranium by catalytic reduction of hexavalent uranium with hydrazine nitrate, if the thermal decomposition reaction (2) of hydrazine nitrate is included, the conversion of the same molar amount of uranyl nitrate consumes almost the same molar amount of hydrazine and hydrogen, but the cost per mole of hydrazine nitrate It is nearly two orders of magnitude higher than hydrogen, so the process is more economical.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a process for preparing tetravalent uranium.

In order to achieve the above purpose, the present invention proposes a process for preparing uranium nitrate (tetravalent uranium) by catalytic hydrogenation reduction of uranyl nitrate (hexavalent uranium), and the process flow is a mixed solution of uranyl nitrate and nitric acid in a raw material liquid storage tank And the high-pressure hydrogen storage tank is metered by the metering pump and flow meter into the catalytic hydrogenation reactor. Under the action of the catalyst and high-pressure hydrogen, the hexavalent uranium is catalytically reduced to tetravalent uranium, and the gas-liquid mixture of the product liquid and excess hydrogen leaves. After the reactor, after online dropwise addition of hydrazine nitrate or hydrazine, it enters the high-pressure gas-liquid separator 4 for gas-liquid separation, wherein the gas-phase mixture flows out from the top of the separator and enters the tail gas absorption tower, and the NOx produced in the reaction process is passed through the alkali. After the liquid is absorbed and removed, pressurized and circulated into the high-pressure hydrogen gas storage tank for circulation standby, the liquid-phase product flows out from the bottom of the separator 4 and enters the product liquid hydrogen replacement tower, where the liquid-phase dissolved hydrogen is removed by nitrogen replacement. The latter gas phase enters the high-emission waste gas treatment system, and the liquid phase enters the next process as a product.

The main reaction in this process is that uranyl nitrate reacts with hydrogen and nitric acid under the action of a catalyst to generate uranyl nitrate, as shown in formula (3). In addition, in the reaction process, we need to optimize the reaction temperature and reactor pressure to avoid the catalytic reduction of nitric acid to nitrous acid under the action of the catalyst, as shown in the reaction formula (4):

HNO ₃ +H ₂ =HNO ₂ +H ₂ O (4)

Nitrous acid further decomposes:

2HNO ₂ =NO+NO ₂ +H ₂ O (5)

The nitrogen oxides produced in reaction (5) will poison the catalyst during the reaction.

The composition (mass percentage) of the raw material liquid in the storage tank: uranyl nitrate: 5%-40%; nitric acid: 1%-20%; water: balance

The final mass concentration of hydrazine nitrate and/or hydrazine in the gas-liquid mixture after the online dropwise addition is 0%-10% (preferably 1%-5%, more preferably 1%-3%).

Wherein the catalytic hydrogenation reactor can be one or more combinations of a fluidized bed with a heat exchange jacket, a fixed bed or a slurry bed, the operating temperature is -20-100 degrees Celsius, and the operating pressure is 0.1-20Mpa ( Gauge pressure), preferably 2-20MPa

Wherein the catalyst in the catalytic hydrogenation reactor can be supported by one or more noble metals in Pt, Ru, Ir or Au on one or more oxides in SiO ₂ , C or TiO ₂ (preferably SiO ₂ or TiO 2 ) One or both of ₂ ) a supported noble metal catalyst formed on a carrier, wherein the noble metal loading is 0.01%-10% (preferably 1%-5%). The uranyl nitrate and nitric acid in the raw material liquid storage tank are mixed with the circulating hydrogen and enter into the high pressure hydrogenation reactor equipped with catalyst through one end of the reactor. The uranyl nitrate is catalytically reduced by hydrogen to uranyl nitrate, excess hydrogen and product nitric acid Uranium flows out of the reactor through the other end of the reactor. The gas-liquid mixture enters the high-pressure gas-liquid separator after dropwise addition of hydrazine nitrate or hydrazine solution online, and the gas phase is decompressed from the top of the separator and then enters the gas-phase tail gas scrubbing tower. Absorption and separation, the hydrogen is pressurized by the compressor and then enters the high-pressure hydrogen storage tank, wherein the liquid-phase product is discharged through the bottom of the separator and enters the normal-pressure liquid-phase product hydrogen replacement tower, in which the nitrogen bubbling device is used in the tower. The dissolved hydrogen in the liquid phase is removed, thereby ensuring the safety of the subsequent process, and the obtained gas enters the treatment system of high-emission tail gas.

Compared with the prior art, the substantial features that the present invention has are:

1. Compared with electrocatalytic reduction of uranyl nitrate to prepare uranium nitrate, the conversion rate of hexavalent uranium can reach more than 98%;

2. Compared with the preparation of tetravalent uranium by catalytic reduction of hydrazine nitrate, it avoids the occurrence of related side reactions and ensures that the conversion rate of hexavalent uranium is greater than 98%;

3. Compared with the existing tetravalent uranium preparation process, the source gas conversion rate is high, the hydrogen cost is low, and it has good economy.

Description of drawings

Fig. 1 is a process flow diagram of the present invention; wherein: 1, raw material liquid storage tank; 2, hydrogen gas storage tank; 3, catalytic hydrogenation reactor; 4, high pressure gas-liquid separation tank; 5, reaction tail gas absorption tower; 6, Product liquid hydrogen replacement tower; 7, lye storage tank.

Detailed ways

The devices used include raw material liquid storage tank, hydrogen gas storage tank, catalytic hydrogenation reactor, high-pressure gas-liquid separation tank, reaction tail gas absorption tower, product liquid hydrogen replacement tower, and lye storage tank for absorption;

The mixed solution of uranyl nitrate and nitric acid in the raw material liquid storage tank is metered by a flow meter, and the hydrogen in the high-pressure hydrogen storage tank is metered by a metering pump and then enters the catalytic hydrogenation reactor. Under the action of uranyl nitrate, the hexavalent uranium is catalytically reduced to tetravalent uranium. After the gas-liquid mixture of the product liquid and excess (relative to uranyl nitrate) hydrogen leaves the catalytic hydrogenation reactor, hydrazine nitrate and/or hydrazine nitrate are added dropwise online. Or hydrazine and then enter into the high-pressure gas-liquid separator (operating pressure is 4Mpa (gauge pressure)) for gas-liquid separation, wherein the gas-phase mixture flows out from the top of the separator and enters the reaction tail gas absorption tower to remove the NOx (X) produced in the reaction process. 1-2) After being absorbed and removed by the lye (the lye is 3M sodium hydroxide), it is pressurized and circulated and returned to the hydrogen gas storage tank for circulation backup, and the liquid-phase product flows out from the bottom of the gas-liquid separator and enters the product In the liquid hydrogen replacement tower, the liquid phase dissolved hydrogen in the tower is removed by nitrogen replacement, and then the gas phase enters the high emission waste gas treatment system and is directly vented, and the liquid phase product is the product.

The top of the reaction tail gas absorption tower is provided with a lye liquid nozzle, which is connected to the lye liquid storage tank through a pump through a pipeline, and the bottom of the reaction tail gas absorption tower is provided with an alkaline liquid return pipeline connected to the lye liquid storage tank.

The top of the liquid hydrogen replacement tower is provided with a tail gas discharge port. A tail gas discharge port with a valve is arranged at the top of the reaction tail gas absorption tower.

Example 1

The processing capacity of the reaction solution is 62.5kg/h, the mass content of UO ₂ (NO ₃ ) ₂ in the raw material liquid storage tank is 0.31 (31%), the content of HNO ₃ is 0.1 (10%) and the content of H ₂ O is 0.59 (59%). %), the concentration of hexavalent uranium is 208g/L; after online dropwise addition of hydrazine nitrate, ensure that the final mass content of hydrazine nitrate in the high-pressure gas-liquid separation tank is ₂ %; the catalyst is a Pt/SiO with an average particle size of 100 microns ( The precious metal loading is 1%). The reactor is a fluidized bed reactor. The flow rate of hydrogen is 180Nm ³ /h, the temperature of the reactor is 30 degrees Celsius, the pressure is 4.0Mpa (gauge pressure), the conversion rate of the outlet uranyl nitrate is 98%, and the content of tetravalent uranium in the product liquid is 204g/L.

Example 2

Different from Example 1 (the rest and process are the same), the mass content of UO ₂ (NO ₃ ) ₂ in the raw material liquid storage tank is 0.20 (20%), the content of HNO ₃ is 0.1 (10%) and the content of H ₂ O is 0.7 (70%), the concentration of hexavalent uranium is 134.2g/L, the conversion rate of export uranyl nitrate is 98%, and the content of tetravalent uranium in the product liquid is 131.5g/L.

Example 3

The difference from Example 2 is that (the rest and process are the same) the mass content of UO ₂ (NO ₃ ) ₂ in the raw material liquid storage tank is that the content of HNO ₃ is 0.2 (20%) and the content of H ₂ O is 0.6 (60%), The concentration of hexavalent uranium is 134.2g/L, the conversion rate of export uranyl nitrate is 98.1%, and the content of tetravalent uranium in the product liquid is 132g/L.

Example 4

The difference from Example 2 is that the final mass content of hydrazine nitrate in the high-pressure gas-liquid separation tank is guaranteed to be 1%; the conversion rate of the outlet uranyl nitrate is 98%, and the content of tetravalent uranium in the product liquid is 131.5 g/L.

Example 5

The difference from Example 2 is that (the rest and process are the same) by online dropwise addition of hydrazine nitrate and hydrazine to ensure that the final mass content of hydrazine nitrate and hydrazine in the high-pressure gas-liquid separation tank is 5%; the transformation rate of outlet uranyl nitrate is 98.1 %, the content of tetravalent uranium in the product liquid is 132g/L.

Example 6

The difference from Example 2 is that the reactor temperature is -20 degrees Celsius and the pressure is 2.0Mpa (gauge pressure), the conversion rate of the outlet uranyl nitrate is 98.1%, and the content of tetravalent uranium in the product liquid is 132g /L.

Example 7

The difference from Example 2 is that the reactor temperature is -20 degrees Celsius and the pressure is 8.0Mpa (gauge pressure), the conversion rate of the outlet uranyl nitrate is 98.1%, and the content of tetravalent uranium in the product liquid is 132g /L.

Example 8

The difference from Example 2 is that the reactor temperature is 40 degrees Celsius, the pressure is 20.0Mpa (gauge pressure), the conversion rate of the outlet uranyl nitrate is 98%, and the content of tetravalent uranium in the product liquid is 131.5%. g/L.

Comparative Example 1

Compared with Example 1, the difference is that (the rest and the process are the same) the reaction temperature is 60 degrees Celsius, the conversion rate of the outlet uranyl nitrate is 90%, and the content of tetravalent uranium in the product liquid is 188g/L, but due to the reaction process. The higher temperature makes the reaction (5) proceed, and the formation of a large amount of nitrogen oxides makes the activity of the catalyst decrease significantly in a short period of time.

Comparative Example 2

Compared with Example 1, its difference is that (the rest and process are the same as it) after the online dropwise addition, it is guaranteed that the final mass content of hydrazine nitrate in the high-pressure gas-liquid separation tank is 0.1%, and the transformation efficiency of the outlet uranyl nitrate is 98%, The content of tetravalent uranium in the product liquid is 120g/L, and the tetravalent uranium generated after leaving the reactor is further oxidized to hexavalent uranium by nitric acid;

Comparative Example 3

Compared with Example 1, the difference lies in that (the rest and process are the same) the mass content of UO ₂ (NO ₃ ) 2 in the raw material liquid storage tank is 0.31 (31%), the content of HNO ₃ is 0.02 (2%) and H The ₂ O content is 0.67 (67%), due to the insufficient concentration of nitric acid in the conversion process, the conversion rate of the export uranyl nitrate is 75%, and the content of tetravalent uranium in the product liquid is 156g/L;

Comparative Example 4

Compared with Example 1, the difference is that (the rest and the process are the same) the catalyst is Pt/Al ₂ O ₃ (the precious metal loading is 1%) with an average particle size of 100 microns, and the conversion rate of the outlet uranyl nitrate is 98 %, the content of tetravalent uranium in the product solution was 204g/L, but the conversion rate of uranyl nitrate decreased significantly with the progress of the reaction due to the dissolution of alumina during the reaction.

Claims (8)

1. A method for preparing uranium nitrate by reducing uranyl nitrate through catalytic hydrogenation,

the adopted device comprises a raw material liquid storage tank, a hydrogen gas storage tank, a catalytic hydrogenation reactor, a high-pressure gas-liquid separation tank, a reaction tail gas absorption tower, a product liquid hydrogen displacement tower and an absorption alkali liquor storage tank;

metering a mixed solution of uranyl nitrate and nitric acid in a raw material liquid storage tank (1) by a flowmeter, metering hydrogen in a high-pressure hydrogen storage tank (2) by a metering pump, then feeding the hydrogen into a catalytic hydrogenation reactor (3), catalytically reducing hexavalent uranium in uranyl nitrate into tetravalent uranium under the action of a supported noble metal catalyst and hydrogen in the catalytic hydrogenation reactor (3), after a product liquid and a gas-liquid mixture of hydrogen which is excessive relative to uranyl nitrate leave the catalytic hydrogenation reactor, dropwise adding hydrazine nitrate and/or hydrazine on line, feeding the mixture into a high-pressure gas-liquid separator (4) for gas-liquid separation, wherein the gas-phase mixture flows out from the top end of the separator, enters a reaction tail gas absorption tower, NOx generated in the reaction process is absorbed and removed by alkali liquor, is subjected to pressure circulation, and then returns to enter a hydrogen gas storage tank for circulation and standby, wherein X in the NOx is 1-2, and a liquid-phase product flows out from the bottom of the gas-liquid separator (4) and enters a product liquid-hydrogen gas displacement tower (6), after liquid-phase dissolved hydrogen in the tower is removed by nitrogen displacement, the gas phase enters a high-level discharge waste gas treatment system and is directly discharged, and a liquid-phase product is a product;

wherein the raw material liquid in the raw material liquid storage tank (1) comprises the following components in percentage by mass: uranyl nitrate: 5% -40%; nitric acid: 5% -20%; the balance of water;

and (2) dropwise adding hydrazine nitrate and/or hydrazine on line, wherein the final mass concentration of the hydrazine nitrate and/or hydrazine in the dropwise added mixture is 1-10%.

2. The process according to claim 1, wherein the catalytic hydrogenation reactor (3) is one or more of a fluidized bed with a heat exchange jacket, a fixed bed or a slurry bed, and the operating temperature is-20 to 50 ℃ and the operating pressure is 0.1 to 20 Mpa;

the treatment capacity of the reaction solution is 62.5kg/h, and the flow rate of the hydrogen is 160-240Nm ³ /h。

3. The process of claim 1 wherein the catalyst in the catalytic hydrogenation reactor is supported on SiO by one or more noble metals of Pt, Ru, Ir, or Au ₂ C or TiO ₂ Wherein the noble metal loading is 0.01% to 10%.

4. The method of claim 1, wherein the alkaline solution is 1-5M sodium hydroxide.

5. The process according to claim 1, wherein the high pressure gas-liquid separator (4) is operated at a pressure of 0.1 to 20Mpa gauge.

6. The method of claim 1, wherein an alkali liquor spray head is arranged at the top in the reaction tail gas absorption tower, the alkali liquor spray head is connected with an alkali liquor storage tank (7) through a pipeline via a pump, and an alkali liquor return pipeline connected with the alkali liquor storage tank (7) is arranged at the bottom of the reaction tail gas absorption tower.

7. The method of claim 1, wherein the top of the liquid hydrogen displacement column (6) is provided with a tail gas discharge port.

8. The method according to claim 1, characterized in that the feed liquid in the feed liquid storage tank (1) comprises the following components in percentage by mass: uranyl nitrate: 20% -35%; nitric acid: 8% -20%; the balance of water; dropping hydrazine nitrate and/or hydrazine on line, wherein the final mass concentration of the hydrazine nitrate and/or hydrazine in the mixture after dropping is 1-5%;

the loading amount of the noble metal in the catalyst is 1-5%;

the operation temperature of the catalytic hydrogenation reactor (3) is 20-40 ℃, the operation pressure is 2-8Mpa, and the flow rate of hydrogen is 180-200Nm ³ /h；

The operating pressure of the high-pressure gas-liquid separator (4) is 2-8 Mpa.

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