A process for the direct hydrogen reduction of uranyl carbonate to produce uranium oxide

Abstract

The application discloses a method for preparing high-purity uranium dioxide solid by directly hydrogen-reducing leaching solution of carbonated uranyl. The enriched leaching solution is mixed with reaction circulating liquid, and then enters a pre-hydrogenation reaction kettle. Under the action of 1-10.0 MPa hydrogen pressure, a certain amount of hydrogen is dissolved in the reaction liquid. After heat exchange with the precipitated product liquid and steam preheating, the reaction liquid enters a reactor containing catalyst and uranium oxide particles from the bottom through a distribution plate. Under the action of the catalyst or the uranium oxide product particles and hydrogen, the reaction occurs. The precipitated reaction liquid carrying small particle products leaves the reactor from the top end of the reactor, and then is precipitated in a settling tank. The upper clear liquid leaves the reaction system after filtration and raw material liquid heat exchange, and then enters the leaching solution liquid system circulation. The generated uranium dioxide solid leaves the system from the bottom of the reactor, and then is separated by sedimentation. The solid is filtered and dried to serve as the product. The liquid is mixed with the enriched raw material liquid, and then enters the pre-hydrogenation reaction kettle for circulation.

Description

Technical Field

[0001] This invention relates to a process for preparing uranium oxide solid particles by hydrogenation reduction of uranium carbonate, and particularly to a process for pre-hydrogenated liquid-phase catalytic reduction of uranium carbonate. Background Technology

[0002] Nuclear energy is characterized by its cleanliness, low carbon footprint, safety, reliability, and low cost. Therefore, developing a sustainable nuclear energy utilization technology roadmap through innovation in nuclear energy science and technology is of great significance for protecting the environment and addressing climate change.

[0003] The nuclear fuel production process involves leaching uranium from ore, concentrating and precipitating it to produce uranium-containing oxide solids, and then purifying, hydrofluorinating, and fluorinating it to finally obtain nuclear-grade uranium hexafluoride fuel. Currently, my country uses advanced CO2+O2 in-situ leaching technology in uranium mining. The extracted fluid is enriched by separation resin and then leached with a carbonate solution to form a uranyl carbonate solution with a uranyl content of 10-50 g / L. After precipitation with caustic soda, sodium diuranate (commonly known as yellowcake) is produced. The supernatant of the precipitate must be managed as radioactive wastewater. To produce high-purity fuel uranium and sodium diuranate, nitric acid dissolution, extraction and separation of sodium ions, ammonia precipitation and purification to prepare ammonium diuranate solid, followed by roasting to obtain fuel-grade uranium oxide solids, are required. This process has disadvantages such as a long process flow, high emissions of uranium-containing radioactive wastewater (estimated at over 50 tons of wastewater per ton of uranium produced), and high fuel costs. Therefore, in order to solve the problems of high uranium-containing radioactive wastewater discharge, low uranium content in sodium diuranate products, and long and costly production processes in my country's uranium fuel production process, we need to propose a new uranium product production process that can directly obtain high-purity uranium oxide products that can be used for fuel uranium preparation.

[0004] There are two main methods for preparing uranium-containing solids from sodium uranyl carbonate precipitation. One method uses alkaline substances to generate a precipitate containing sodium or ammonium cations, which is then roasted and dried before use. The other method reduces the valence of uranium from hexavalent to tetravalent, causing hydrolysis at a pH greater than 7 and a temperature above 90°C to directly generate uranium oxide solids. This process is characterized by low wastewater production and high product purity. Hydrazine and hydrogen are suitable reducing agents for the liquid-phase reduction of hexavalent uranium under suitable temperature conditions. Hydrazine is highly toxic, and its reduction product is ammonium ions, which are difficult to treat. Hydrogen reduction has the advantages of low cost and water as the product, making it a more ideal choice. However, hydrogen is a flammable and explosive gas, and its application in the radioactive nuclear industry is limited. Safe use of hydrogen is crucial in this process.

[0005] Uranium has a large molecular weight. Even with a uranium content of 50 g / L, its molar concentration is 0.2 mol / L. If the reaction proceeds according to the following formula:

[0006] Na₄[UO₂(CO₃)₃] + H₂ = UO₂ (precipitate) + 2NaHCO₃ + Na₂CO₃

[0007] One mole of uranyl ions has a chemical reaction equivalent to 1 mole of hydrogen gas. Therefore, the amount of hydrogen consumed in 1 L of solution is only 0.2 mol. At a temperature of 90℃ and a pressure of 3.0–4.0 MPa, approximately 0.1 mol of hydrogen gas can dissolve in each L of aqueous solution. If we reduce the uranium concentration to below 25 g / L by circulating the clarified product after precipitation, the dissolved hydrogen concentration will meet the stoichiometric requirements. By increasing the amount of clarified product after precipitation, the excess hydrogen gas can be adjusted to a suitable value, thereby achieving the purpose of uranium reduction.

[0008] Through the above theoretical analysis and extensive experimental research, we have developed a novel process for the pre-hydrogenated catalytic reduction of sodium uranyl carbonate to prepare solid uranium dioxide particles. In this process, the hydrogen required for the reaction is pre-hydrogenated into the reaction liquid and a certain proportion of the circulating liquid. Catalytic hydrogenation reduction occurs in a fixed-bed reactor with a mesh-based or honeycomb ceramic-based catalyst to produce solid uranium oxide. The greatest advantage of this process is that, because the hydrogen is dissolved in the reaction liquid through pre-hydrogenation, the reactor is a single-phase liquid flow reactor. This avoids the dangers of expansion and explosion caused by high-temperature, violent reactions of the gas, even under high pressure. By implementing this process, the effective uranium content per ton of product can be increased from 73% to 88%, and the precipitate can be recycled, reducing wastewater by approximately 50 tons per ton of uranium product compared to traditional sodium hydroxide precipitation. Summary of the Invention

[0009] This invention proposes a process for directly preparing high-purity uranium oxide products by catalytic hydrogenation reduction of uranyl carbonate solution. The main feature of this process is that it utilizes circulating clear liquid and pre-hydrogenation of the reaction liquid to address the safety issues associated with liquid-phase hydrogenation reduction in the field of radiochemical engineering.

[0010] To achieve the above objectives, this invention proposes a process flow for the direct hydrogen reduction of uranyl carbonate to prepare uranium oxide. The process flow is as follows: a reaction liquid with a certain uranium content is metered by metering pump P-1, then mixed with the circulating reaction liquid by pump P-2 and fed into a pre-hydrogenation reactor (flow ratio 1:1 to 1:10). Under high-pressure hydrogen gas (1-10.0 MPa), some hydrogen dissolves in the liquid phase. After metering by pump P-3, the mixture enters heat exchanger H-1 for heat exchange and preheating with the product liquid. Then, it enters heat exchanger H-2 and is preheated to 90-150°C by electricity or steam. After distribution by a distribution plate, the product liquid enters from the bottom of the reactor. In reactor R-1, under the catalytic action of nickel mesh catalyst and the generated uranium oxide solid particles, it reacts with hydrogen to produce solid uranium dioxide particles. The small particles flow upward with the liquid and overflow into the top liquid phase tank V-4 for precipitation. The supernatant is filtered and leaves V-4, then enters the heat exchanger H-1 to preheat the reaction liquid, and then enters the atmospheric pressure hydrogen replacement vessel V-7. The unreacted hydrogen is replaced by nitrogen gas at the bottom and then discharged. The liquid is further separated by sedimentation in the settling tank V-8. The supernatant enters the circulating eluent system for recycling. The solid obtained from precipitation enters the solid-liquid separation process and is dried to form uranium oxide product. Due to gravity, the uranium dioxide solid particles generated and grown in the reaction leave from the bottom of the reactor and enter the liquid-solid separation tank V-5. After mixing with the solid in V-4, they enter the gas replacement vessel V-6. After nitrogen replaces the dissolved hydrogen, the liquid-solid mixture enters the settling separation tank V-3. All the supernatant enters V-2 for recycling. The solid products from V-3 and V-8 enter F-1 for dehydration and filtration, and then enter the drying box D-1 for drying to form uranium oxide solid products.

[0011] The reaction equation for this process is as follows:

[0012] [UO2(CO3)3] 4- +H2=UO2(precipitate)+2HCO3 - +CO3 2-

[0013] The reaction process mainly uses a fixed mesh and honeycomb catalyst to catalyze the dissociation of dissolved hydrogen. The dissociated hydrogen ions reduce hexavalent uranyl carbonate ions to generate tetravalent uranium, which is then hydrolyzed in a high-temperature aqueous solution and precipitated. The solid uranium oxide product leaves the reactor after being discharged from the bottom of the reactor. After precipitation, filtration and drying, solid uranium oxide is prepared.

[0014] The feed solution consists of a sodium uranyl carbonate solution with a uranium content of 1-100 g / L and a pH of 7-11. Increasing the uranium content in the feed solution will increase the reaction load, leading to an increase in the uranium content of the finished product at the outlet; decreasing the pH value will decrease the hydrolysis rate.

[0015] The recycling ratio of the reaction liquid to the recycled product liquid is 1:1 to 1:10. Increasing the recycling ratio can improve the solubility of hydrogen and increase the reaction rate, but it also increases the reactor load and preheating energy consumption, thus increasing the production and operation cost per ton of uranium product.

[0016] Reactor R-1 contains one or both of the following: a mesh-based or a honeycomb ceramic-based monolithic catalyst. The mesh-based catalyst can be a nickel mesh or plated with precious metals such as Pt, Ru, Pb, or Ir. The honeycomb ceramic catalyst can be a pre-fabricated honeycomb ceramic impregnation type or a deposition-precipitation-extrusion type catalyst. The operating temperature is 90–150℃ (preferably 100–120℃), and the operating pressure is 1.0–10.0 MPa (gauge pressure) (preferably 2.0–8.0 MPa). Increasing the reaction temperature can increase the hydrogenation and hydrolysis reaction rates, but if the reaction temperature is too high, the generated uranium dioxide solids are prone to form structures on the catalyst surface. Increasing the pressure can improve the reaction rate, but if the pressure is too high, it will increase the cost of the reactor and pump.

[0017] Under pressure, carbon dioxide and high-pressure oxygen convert uranium oxides in the mineral layer into uranyl carbonate solution, which is then carried to the ground by the fluid. After resin adsorption, separation and concentration, a carbonate leaching solution with a uranium content of 1-50 g / L is obtained. In this invention, the enriched eluent is mixed with the circulating reaction liquid and then fed into a pre-hydrogenation reactor. Under a hydrogen pressure of 1–10.0 MPa, a certain amount of hydrogen is dissolved in the reaction liquid. After heat exchange and steam preheating to 90–150°C with the precipitated product liquid, the product liquid enters from the bottom through a distribution plate into a reactor containing a catalyst and uranium oxide particles. The reaction occurs under the action of the catalyst or uranium oxide product particles and hydrogen. The precipitated reaction liquid, carrying small product particles, leaves the reactor at the top and settles in a settling tank. The supernatant is filtered and heat exchanged with the raw material liquid before leaving the reaction system and entering the eluent system for circulation. The uranium dioxide solid generated by the reaction leaves the system at the bottom of the reactor. After sedimentation and separation, the solid is filtered and dried to become the product. The liquid is mixed with the enriched raw material liquid and then circulated in the pre-hydrogenation reactor. The advantages of this process are that it addresses the safety issues of hydrogen-related reactions in radioactive feedstocks by pre-hydrogenating the enriched reaction liquid and precipitate through circulation. Furthermore, except for the pre-hydrogenation reactor, all equipment operates in a single-phase liquid phase, facilitating safe operation at higher pressures and improving reaction efficiency. In the preferred embodiment, the hydrogen pressure is 8.0 MPa, the mass ratio of reaction liquid to circulating liquid is 1:3, the reactor operating temperature is 110°C, and the uranium content of the effluent from the reaction liquid containing 30 g / L uranium is ≤35 mg / L under the action of the nickel mesh catalyst.

[0018] The essential features of this invention compared to existing technologies are as follows:

[0019] 1. Compared with my country's current sodium diuranate (yellowcake) process, its uranium content can reach 88%, and wastewater discharge can be reduced by more than 50 tons per ton of uranium product;

[0020] 2. Compared with the ammonia precipitation method for preparing U3O8 abroad, it does not require a calcination process, and the process flow is shorter and the cost is lower;

[0021] 3. Compared with the preparation of U3O8 by oxidation and precipitation with H2O2, the cost of hydrogen gas used is much lower than that of H2O2, and less chemical reagents are consumed;

[0022] 4. Compared with existing hydrogenation multiphase catalytic reduction processes, this method avoids the use of gaseous hydrogen in the reactor, thus solving the safety issues of hydrogen use in the radioactive chemical industry. Attached Figure Description

[0023] Figure 1 This is a process flow diagram of the present invention; wherein: P-1 is a reaction liquid metering pump; P-2 is a circulating liquid metering pump;

[0024] P-3 Feed liquid metering pump; V-1 Pre-hydrogenation reactor; V-2 Circulating liquid buffer tank; V-3 Reactor bottom settling tank; V-4 Reactor top outlet tank; V-5 Reactor bottom outlet tank; V-6 Gas replacement reactor; V-7 Reactor top hydrogen replacement reactor; V-8 Reactor top outlet settling tank; Reactor (R-1); F-1 Filter; D-1 Dryer; First heat exchanger (H-1); Second heat exchanger (H-2). Detailed Implementation

[0025] The equipment used includes a pre-hydrogenation reactor, a catalytic hydrogenation reactor, top and bottom outlet tanks, top and bottom atmospheric pressure liquid dissolution hydrogen replacement tanks, top and bottom product liquid sedimentation tanks, circulating liquid buffer tanks, solid phase product filters, and drying ovens, etc.

[0026] A reaction liquid with a certain uranium content is metered by reaction liquid metering pump P-1 and then mixed with circulating liquid metered by circulating liquid metering pump P-2 before entering the pre-hydrogenation reactor V-1 (the flow-to-volume ratio of reaction liquid to circulating liquid is 1:1 to 1:10). Hydrogen gas from a hydrogen source is introduced into the pre-hydrogenation reactor. Under the action of high-pressure hydrogen gas of 1 to 10.0 MPa, some hydrogen dissolves in the liquid phase material in the pre-hydrogenation reactor. The liquid phase material in the pre-hydrogenation reactor is metered by raw material metering pump P-3 and then enters the first heat exchanger H-1 for heat exchange and preheating with the product liquid. After that, it enters the second heat exchanger H-2 and is preheated to 90 to 150°C by electric heating or steam heating. The gas then enters reactor R-1 from the bottom of the reactor. After being distributed by the distribution plate, it reacts with hydrogen under the combined catalytic action of the nickel mesh catalyst and the generated solid uranium oxide particles to form solid uranium dioxide particles. The small particles flow upward with the liquid and overflow from the top of reactor (R-1) into the top outlet tank V-4 for precipitation. The clear liquid in the top outlet tank V-4 is filtered and then leaves the top outlet tank V-4 to enter the first heat exchanger H-1 to preheat the reaction liquid. After that, it enters the hydrogen replacement vessel V-7 at the top of the reaction liquid. The unreacted residual hydrogen is replaced by nitrogen gas introduced into the lower part or bottom of the hydrogen replacement vessel (V-7) before being discharged.

[0027] The liquid in the hydrogen replacement vessel (V-7) at the top of the reaction liquid is further separated by sedimentation in the discharge settling tank V-8 at the top of the reactor. Part of the clear liquid enters the circulating liquid buffer tank (V-2) for recycling, and part of it leaves the device or system of this invention and returns to the uranium leaching and concentration system for recycling. The solid obtained by precipitation enters the solid-liquid separation in filter F-1 and is then dried by dryer D-1 to form uranium oxide product. Due to gravity, the uranium dioxide solid particles generated and grown by the reaction leave from the bottom of reactor R-1 and enter the bottom discharge tank V-5 of the reaction liquid. They mix with the solid particles that leave from the bottom of the top discharge tank V-4 of the reactor and enter the bottom discharge tank V-5 of the reaction liquid. Then, they leave from the bottom of the bottom discharge tank V-5 of the reaction liquid and enter the gas replacement vessel V-6. After the dissolved hydrogen is replaced by nitrogen gas introduced into the lower part or bottom of the gas replacement vessel V-6, the liquid-solid mixture in the gas replacement vessel V-6 enters the bottom settling separation tank V-3 of the reactor. All the clear liquid in the bottom settling separation tank V-3 of the reactor enters the circulating liquid buffer tank V-2 and is used as circulating liquid. The solid obtained by precipitation enters the filter F-1 for dehydration and filtration, and then is dried by the dryer D-1 to form uranium oxide solid product.

[0028] The raw material solution consists of uranyl carbonate solution with a uranium content of 1-100 g / L and a pH value of 7-11.

[0029] The reactor (R-1) is a fixed bed with a single-phase liquid flow and an internally fixed metal mesh or honeycomb ceramic catalyst. The operating temperature is 90–150℃; the reaction liquid flow rate is 50 kg / h; the circulating liquid flow rate is 50–500 kg / h (optimal flow rate is 150–300 kg / h); and the hydrogen pressure is 1–10.0 MPa (optimal is 2.0–8.0 MPa).

[0030] The filter (F-1) can be one or more of the following: plate and frame filter, rotary filter, or scraped disc filter.

[0031] The filter inside the top outlet tank (V-4) of the reactor is a sintered stainless steel metal filter capable of filtering particles with a diameter ≤5 micrometers.

[0032] Gas replacement vessel V-6 and hydrogen replacement vessel (V-7) at the top of the reaction liquid are equipped with venting gas outlets with valves. A drain port with a valve is located at the bottom of circulating liquid buffer tank V-2.

[0033] Example 1

[0034] The reaction liquid processing capacity is 62.5 kg / h, the feed liquid is a sodium uranyl carbonate solution with a uranium content of 30 g / L and a pH value of 10.0, and the circulating liquid flow rate is 125 kg / h; the reactor uses a nickel mesh with a wire diameter of 0.5 mm and a mesh size of 10 mesh as the catalyst, and the operating temperature is 90℃; the hydrogen pressure in the pre-hydrogenation reactor is 3.0 MPa; the uranium content of the outlet reaction liquid is 50 mg / L, and the average particle size is 25 micrometers.

[0035] Example 2

[0036] Same as Example 1, except that (the rest of the process is the same) the concentration of sodium uranyl carbonate feed solution is 100 g / L, the uranium content of the outlet reaction liquid is 102 mg / L, and the average particle size is 42.5 micrometers.

[0037] Example 3

[0038] Same as Example 1, except that (the rest of the process is the same) the hydrogen pressure is 8.0 MPa, the uranium content of the outlet reaction liquid is 10 mg / L, and the average particle size is 45 micrometers.

[0039] Example 4

[0040] Same as Example 1, except that (the rest of the process is the same) the circulating liquid flow rate is 312.5 kg / h; the uranium content of the outlet reaction liquid is 5 mg / L and the average particle size is 15 micrometers.

[0041] Example 5

[0042] Same as Example 1, except that (the rest of the process is the same) a nickel mesh plated with platinum catalyst is used; the uranium content of the outlet reaction liquid is 15 mg / L and the average particle size is 30 micrometers.

[0043] Example 6

[0044] The difference from Example 1 is that (the rest of the process is the same) the reactor temperature is 120°C, the uranium content of the outlet reaction liquid is 30 mg / L, and the average particle size is 105 micrometers.

Claims

1. A method for preparing uranium oxide by direct hydrogen reduction of uranyl carbonate, A reaction liquid with a certain uranium content is metered by a reaction liquid metering pump (P-1) and then mixed with circulating liquid after being metered by a circulating liquid metering pump (P-2) before entering the pre-hydrogenation reactor (V-1). The flow-to-volume ratio of the reaction liquid to the circulating liquid is 1:1 to 1:

10. Hydrogen from a hydrogen source is introduced into the pre-hydrogenation reactor. Under the action of high-pressure hydrogen (1-10.0 MPa), some hydrogen dissolves in the liquid phase material in the pre-hydrogenation reactor. The liquid phase material in the pre-hydrogenation reactor is metered by a raw material metering pump (P-3) and then enters the first heat exchanger (H-1) for heat exchange and preheating with the product liquid. After entering the second heat exchanger (H-2), it is preheated to 90-150°C by electric heating or steam heating and then enters the reactor (R-1) from the bottom of the reactor. After being distributed by a distribution plate, it reacts with hydrogen under the co-catalytic action of a nickel mesh catalyst and the generated uranium oxide solid particles to produce solid carbon dioxide. Uranium particles, small particles, flow upward with the liquid and overflow from the top of the reactor (R-1) into the top outlet tank (V-4) to settle. The upper clear liquid in the top outlet tank (V-4) is filtered and then leaves the top outlet tank (V-4) and enters the first heat exchanger (H-1). After the heat exchanger preheats the reaction liquid, it enters the hydrogen replacement vessel (V-7) at the top of the reaction liquid. Nitrogen gas is introduced into the lower part or bottom of the hydrogen replacement vessel (V-7) to replace the unreacted residual hydrogen gas and then it is vented. The liquid in the hydrogen replacement vessel (V-7) at the top of the reaction liquid is further settled and separated in the top discharge settling tank (V-8) of the reactor. Part of the upper clear liquid enters the circulating liquid buffer tank (V-2) for recycling and part leaves the device or system. The solid obtained from the sedimentation enters the filter (F-1) for solid-liquid separation and is then dried in the dryer (D-1) to form uranium oxide products. Due to gravity, the uranium dioxide solid particles generated and grown by the reaction leave from the bottom of the reactor (R-1) and enter the bottom discharge tank (V-5) of the reaction liquid. They mix with the solid particles that leave from the bottom of the top discharge tank (V-4) of the reactor and enter the bottom discharge tank (V-5) of the reaction liquid. Then, they leave from the bottom of the bottom discharge tank (V-5) of the reaction liquid and enter the gas replacement vessel (V-6). After the dissolved hydrogen is replaced by nitrogen gas introduced into the lower part or bottom of the gas replacement vessel (V-6), the liquid-solid mixture in the gas replacement vessel (V-6) enters the bottom settling separation tank (V-3) of the reactor. All the clear liquid in the bottom settling separation tank (V-3) of the reactor enters the circulating liquid buffer tank (V-2) and is used as circulating liquid. The solid obtained by precipitation enters the filter (F-1) for dehydration and filtration, and then is dried by the dryer (D-1) to form uranium oxide product. The reaction solution is composed of uranyl carbonate solution with a uranium content of 1~100g / L and a pH value of 7~11.

2. The method according to claim 1, wherein the reactor (R-1) is a fixed bed with liquid single-phase flow containing an internally fixed metal mesh-based or honeycomb ceramic-based catalyst, and the operating temperature is 90~150℃; the reaction liquid flow rate is 50-100kg / h, the circulating liquid flow rate is 50~500kg / h, and the hydrogen pressure is 1~10.0MPa.

3. The method according to claim 2, wherein the circulating liquid flow rate is 150-300 kg / h and the hydrogen pressure is 2.0-8.0 MPa.

4. The method according to claim 1 or 2, wherein the catalyst is one or more of the following: a nickel mesh, an iron mesh, or a metal mesh or an integral honeycomb ceramic catalyst with a surface plated with one or more noble metals selected from Pt, Pb or Ru, which has hydrogenation activity.

5. The method according to claim 1, wherein the filter (F-1) is one or more of a plate and frame filter, a rotary filter, or a scraped disc filter.

6. According to the method of claim 1, the filter in the liquid outlet tank (V-4) at the top of the reactor is a sintered stainless steel metal filter with a filtration diameter ≤ 5 micrometers.

7. The method according to claim 1, wherein a venting gas outlet with a valve is provided at the top of the gas replacement vessel (V-6) and the hydrogen replacement vessel at the top of the reaction liquid (V-7).

8. The method according to claim 1, The upper clear liquid in the top discharge settling tank (V-8) of the reactor leaves the device or system and is returned to the uranium leaching and concentration system for recycling.

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