Preparation method of high-purity hydrated uranyl nitrate

Abstract

The invention belongs to the technical field of uranium compounds, and particularly relates to a preparation method of high-purity uranyl nitrate. According to the method, the high-purity uranyl nitrate hexahydrate and uranyl nitrate trihydrate are continuously prepared by taking a ternary system phase diagram as guidance and combining a pH controlled micro-disturbance impurity removal technology and a mother liquor closed-loop circulation technology. In-situ selective removal of amphoteric impurities is achieved through accurate pH oscillation impurity removal, the impurity removal effect is enhanced in cooperation with thermal-state filtration, a target crystal phase is directionally separated out through regulation and control of a phase diagram, mother liquor after centrifugation can be directly reused for a dissolution procedure through split-flow compounding, and therefore full utilization of materials is achieved; and environmental pressure and resource waste caused by discharge of high-acidity waste liquid are avoided. The method provided by the invention ensures high purity of the product and crystal phase stability, realizes flow simplification, high-efficiency utilization of materials and zero wastewater discharge, reduces the comprehensive production cost, has good industrial suitability, and can meet the preparation requirements of high-purity raw materials such as nuclear fuel grade and the like.

Description

Technical Field

[0001] This invention belongs to the field of uranium compound technology, specifically relating to a method for preparing high-purity hydrated uranyl nitrate. Background Technology

[0002] Uranyl nitrate, a key intermediate in the nuclear chemical industry, is widely used in core industrial processes such as nuclear fuel processing, uranium purification and conversion, and spent fuel reprocessing. It is also used as an analytical reagent, oxidant, and film colorant. High-purity hydrated uranyl nitrate can be decomposed at high temperatures to prepare high-purity uranium trioxide, and the purity of uranium trioxide directly depends on the impurity content and crystal phase stability of hydrated uranyl nitrate. Currently, industrial production of hydrated uranyl nitrate to meet high-purity requirements generally relies on complex purification methods such as subsequent extraction and ion exchange. These purification processes have significant technical shortcomings: firstly, the extraction process requires a large amount of organic extractant, which is not only lengthy and cumbersome, increasing production complexity and equipment investment costs, but also easily leaves organic impurities; secondly, multi-step purification operations lead to increased uranium element loss, reduced raw material utilization, and serious environmental treatment pressure and resource waste.

[0003] To simplify the process, some technologies attempt to prepare hydrated uranyl nitrate solely through phase diagram-guided crystallization separation, utilizing the solid-liquid equilibrium characteristics of uranyl nitrate in aqueous solutions with different nitric acid concentrations to directionally precipitate the target crystalline phase. However, existing phase diagram-guided processes still face significant bottlenecks: they cannot effectively remove trace amphoteric metal impurities associated with the raw materials, resulting in insufficient product purity; the mother liquor is often treated by simple dilution followed by reuse or direct discharge, which neither achieves efficient closed-loop material recovery nor prevents batch stability of continuously produced products due to impurity accumulation. Therefore, developing a method based on phase diagram principles that enables in-situ impurity removal, closed-loop mother liquor recycling, and continuous and stable preparation of high-purity hydrated uranyl nitrate has become an urgent technical need in the nuclear chemical industry. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a method for preparing high-purity uranyl nitrate hydrate. Guided by a ternary phase diagram, this method integrates pH-controlled micro-disturbance impurity removal with a closed-loop mother liquor circulation technique to continuously prepare high-purity uranyl nitrate hexahydrate and uranyl nitrate trihydrate. This invention achieves in-situ selective removal of amphoteric impurities through precise pH oscillation, enhanced by hot filtration, and directional precipitation of the target crystalline phase based on phase diagram control. The mother liquor after centrifugation can be directly reused in the dissolution process after fractionation, eliminating the need for additional complex purification treatment. This solves the technical problems of existing uranyl nitrate hydrate preparation processes, which rely on complex extraction and purification, are cumbersome, have low resource utilization, insufficient product purity and batch stability, and cause environmental pollution from high-acid wastewater discharge. It effectively overcomes the bottlenecks of traditional processes, ensuring high product purity and crystalline phase stability while simplifying the process, achieving efficient material utilization and zero wastewater discharge, reducing overall production costs, and possessing good industrial adaptability, meeting the requirements for preparing high-purity raw materials such as nuclear fuel grade.

[0005] To address the shortcomings of existing technologies, the present invention adopts the following technical solution: This invention provides a method for preparing high-purity hydrated uranyl nitrate, specifically comprising the following steps: S1, Raw material preparation and dissolution: Prepare an aqueous nitric acid solution and add it to the dissolution reactor. Weigh out uranium oxide that is soluble in nitric acid and add it to the reactor. Dissolve it completely under stirring at 85°C. The tail gas generated during the dissolution process is treated by a condensation reflux device and then sent to an alkali absorption tank for neutralization before being discharged. The condensate is returned to the reactor. After dissolution is completed, the mother liquor is obtained. S2, Evaporation Concentration and pH Shaking for Impurity Removal: The mother liquor is transferred to an evaporation concentration device for constant-temperature evaporation concentration at 85°C. Then, pH shaking is activated to remove impurities, resulting in a high-temperature concentrate. Without cooling, the high-temperature concentrate is directly passed through a sintered metal filter element preheated to 80-90°C to hot-tighten impurities and colloids. The high-purity clear liquid is collected and returned to the crystallization device. The high-purity clear liquid is then subjected to programmed cooling at a rate of 1-1.5°C / min to 25°C, precipitating high-purity uranyl nitrate hexahydrate and obtaining a slurry. S3, centrifugal washing and drying: The slurry is subjected to solid-liquid separation. The slurry is transferred to a centrifuge and solid-liquid separation is performed for 15 min. The solid product and mother liquor are collected. The solid product is washed 3 times with anhydrous ethanol to remove residual mother liquor on the surface. Then it is dried at room temperature of 25℃ for 2 h to obtain high-purity uranyl nitrate hexahydrate. S4, Mother liquor splitting and preparation of uranyl nitrate trihydrate: The mother liquor is split into a first part of mother liquor and a second part of mother liquor. The first part of mother liquor accounts for 60-70% of the total mass of the mother liquor, and the second part of mother liquor accounts for 30-40% of the total mass of the mother liquor, to obtain high-purity uranyl nitrate trihydrate and high-acidity final mother liquor. S5, Closed-loop mixing and recycling of mother liquor: The high-acidity final mother liquor obtained in step S4 is sent to the mother liquor mixing tank and mixed with the second part of the mother liquor to form a mixed liquid. Deionized water is added to adjust the composition of the mixed liquid so that the mass fraction of nitric acid returns to the initial value. This mixed liquid is directly used as the starting nitric acid solution for the next production batch and is put into the dissolution reactor in the first step, thereby realizing a 100% closed-loop circulation of nitric acid medium and unreacted uranium resources, eliminating the generation of uranium-containing nitric acid waste liquid from the source.

[0006] Furthermore, the pH oscillation purification process is as follows: when the mass fraction of nitric acid in the system reaches 60-80% of the target crystallization acidity, the evaporation operation is paused, and then the pH online monitoring is turned on. A 3-8% alkaline regulator is slowly added dropwise through a microburette, controlling the dropping rate, so that the pH value of the system increases controllably by 0.3-1.5 units from the strongly acidic range. This process lasts for 3-10 minutes, and the system is continuously evaporated and concentrated until the mass fraction of nitric acid in the system reaches the target crystallization acidity, forming a high-temperature concentrated solution.

[0007] Furthermore, the alkalinity regulator is a dilute ammonia solution or a urea solution.

[0008] Further, the mother liquor diversion operation is as follows: the first part of the mother liquor is sent to an evaporation and concentration device and evaporated at a constant temperature of 85°C until the mass fraction of nitric acid reaches the target value. Evaporation is paused, and the pH oscillation and impurity removal operation of step S2 is repeated. An alkaline regulator is added to adjust the pH by a slight increase of 0.5 units. After standing for 3 minutes, evaporation is resumed to further remove any possible trace impurities. After impurity removal, evaporation is immediately resumed. By controlling the amount of water evaporated, evaporation continues, and uranyl nitrate trihydrate crystals are precipitated in a directional manner, resulting in a solid-liquid mixture. The solid-liquid mixture is centrifuged for 15 minutes, and the crystals and high-acidity final mother liquor are collected. The crystals are washed three times with anhydrous ethanol and dried for 2 hours to obtain high-purity uranyl nitrate trihydrate.

[0009] Furthermore, the uranium oxide is selected from one of uranium octoxide, uranium trioxide, and uranium dioxide.

[0010] This invention utilizes the principle that uranyl nitrate crystallizes in different crystalline forms in the equilibrium phase regions corresponding to different concentrations of nitric acid aqueous solutions under solid-liquid equilibrium conditions, to achieve the preparation of high-purity uranyl nitrate hexahydrate and high-purity uranyl nitrate trihydrate.

[0011] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: This invention utilizes the phase diagram characteristics of a ternary system for full-process component and phase region control, enabling precise and directional precipitation of the target hydrate crystal phase, effectively suppressing impurity crystal formation, and ensuring regular crystal form and stable purity of the product. Simultaneously, it achieves continuous stepwise preparation of hexahydrate and trihydrate, improving process integration and production efficiency. This invention introduces pH oscillation for impurity removal during evaporation and concentration. Controllable and subtle acid-base disturbances selectively induce hydrolysis of amphoteric metal impurities in the raw materials, forming colloidal particles without affecting the stable existence of uranyl ions, thus achieving source separation of impurities before crystallization. Through synergy with hot precision filtration, impurity colloids can be effectively retained while the system remains at a high temperature and does not enter the crystallization zone, preventing impurities from redissolving or being encapsulated by crystals during cooling and crystallization. This fundamentally improves the intrinsic purity of hydrated uranyl nitrate products, obtaining high-purity products that meet the requirements of high-end applications. This invention deeply couples impurity removal and purification with mother liquor recycling. The mother liquor after centrifugation can be directly reused in a closed loop after component adjustment, achieving full utilization of materials while avoiding environmental pressure and resource waste caused by the discharge of high-acidity waste liquid. During the recycling process, the mother liquor simultaneously participates in the dynamic balance adjustment of the system's acidity, reducing the excessive addition of external reagents and making the dissolution and reaction system more stable. This invention achieves multiple technical goals: high-purity preparation, simplified process, high resource efficiency, and environmental friendliness. The overall process route is simple, the equipment is highly versatile, and the operation is highly controllable, significantly reducing the complexity of the production process and equipment investment, and possessing stronger industrial adaptability. Attached Figure Description

[0012] Figure 1 The phase diagram of the ternary system for preparing high-purity uranyl nitrate hydrate according to the present invention; Figure 2 This is a flowchart illustrating the preparation process of high-purity hydrated uranyl nitrate according to the present invention. Detailed Implementation

[0013] To enable those skilled in the art to better understand the technical solutions of the present invention and to make the above-mentioned features, objectives, and advantages of the present invention clearer and easier to understand, the present invention will be further described below with reference to embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0014] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to this invention. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this application.

[0015] Unless otherwise specified, all methods described in the following embodiments are conventional. Unless otherwise specified, all materials used in the following embodiments are new materials purchased from the market.

[0016] Figure 1 This is a ternary phase diagram of the ternary system UO2(NO3)2 + HNO3 + H2O at 25°C. The diagram shows the key system points of the preparation method of this invention. Point O, with both x and y axes at 0, represents the pure aqueous phase; point M, with a y-axis of 100.00% and an x-axis of 0, represents the UO2(NO3)2 phase (uranyl nitrate phase); point A represents the composition of the saturated aqueous solution of uranyl nitrate; point E represents the phase boundary between uranyl nitrate hexahydrate and uranyl nitrate trihydrate; and point Q represents the composition of the uranyl nitrate mother liquor. Point P corresponds to the system composition after pH oscillation and impurity removal in step S2, and is the control node before crystallization of uranyl nitrate hexahydrate; point L is the directional crystallization control point of uranyl nitrate trihydrate; point N corresponds to the target control point before crystallization of uranyl nitrate hexahydrate in step S2; M1, M2, and M3 correspond to the target values ​​of nitric acid concentration control at different stages; curve AEF is the solubility boundary line of uranyl nitrate, where segment AE is the solubility boundary of uranyl nitrate hexahydrate and segment EF is the solubility boundary of uranyl nitrate trihydrate.

[0017] according to Figure 1 As shown, and Figure 2 The preparation process of high-purity hydrated uranyl nitrate of this invention is illustrated in the following examples: Example 1: This example provides a method for preparing high-purity uranyl nitrate hydrate, specifically including the following steps: S1, Raw material preparation and dissolution: Prepare a 15% (w / w) nitric acid aqueous solution (corresponding to...). Figure 1 Uranium trioxide (M1 point) is added to the dissolution reactor. The uranium trioxide is weighed and added to the reactor, and dissolved completely under stirring conditions of 85℃ and 300 r / min. The tail gas generated during dissolution is treated by a condenser and reflux device and then sent to an alkali absorption tank for neutralization before being discharged. The condensate is returned to the reactor. After dissolution is complete, the mother liquor (corresponding to...) is obtained. Figure 1 (Mid-Q point) S2, Evaporation Concentration and pH Shaking for Impurity Removal: The mother liquor is transferred to an evaporation concentration device for isothermal evaporation concentration at 85°C. Then, pH shaking is activated to remove impurities, resulting in a high-temperature concentrated solution. Without cooling, the high-temperature concentrated solution is directly passed through a sintered metal filter element preheated to 80°C to hot-press out impurities and colloids. The high-purity clear liquid is collected and returned to the crystallization device. The high-purity clear liquid is cooled to 25°C at a rate of 1°C / min. The system point is then transferred along the OQ extension line from point M3 to... Figure 1 At point P, uranyl nitrate hexahydrate is precipitated in a directional manner, and a slurry is obtained; S3, Centrifugal Washing and Drying: The slurry is subjected to solid-liquid separation. The slurry is transferred to a centrifuge for solid-liquid separation at 3000 r / min for 15 min. The solid product and mother liquor are collected. Figure 1The solid product was washed three times with anhydrous ethanol (50 mL each time) to remove residual mother liquor on the surface. Then it was dried at room temperature (25 °C) for 2 h to obtain high-purity uranyl nitrate hexahydrate. S4, Mother liquor splitting and preparation of uranyl nitrate trihydrate: The mother liquor is split into a first part of mother liquor and a second part of mother liquor. The first part of mother liquor accounts for 60% of the total mass of the mother liquor, and the second part of mother liquor accounts for 40% of the total mass of the mother liquor, to obtain uranyl nitrate trihydrate and high acidity final mother liquor. S5, Closed-loop compounding and recycling of mother liquor: The high-acidity final mother liquor obtained in step S4 is sent to the mother liquor compounding tank and mixed with the second part of the mother liquor to form a mixed liquid. Material balance is performed through the phase diagram, and deionized water is precisely added to adjust the composition of the mixed liquid so that the mass fraction of nitric acid returns to point M1 on the phase diagram. The mass fraction of nitric acid is 20%, and no additional purification treatment is required. This mixed liquid is directly used as the starting nitric acid solution for the next production batch and is put into the dissolution reactor in the first step, thereby realizing a 100% closed-loop circulation of nitric acid medium and unreacted uranium resources, eliminating the generation of uranium-containing nitric acid waste liquid from the source.

[0018] The pH oscillation purification process is as follows: when the mass fraction of nitric acid in the system reaches the target crystallization acidity ( Figure 1 When the acidity reaches 60% at point M3, evaporation is paused. Then, online pH monitoring is activated, and 3% dilute ammonia solution is slowly added dropwise through a microburette, controlling the dropping rate to allow a controllable micro-increase of 0.3 units in the pH value from the strongly acidic range. This process continues for 3 minutes, triggering the hydrolysis of iron, aluminum, and molybdenum impurities to form hydroxide colloids. Evaporation is immediately resumed after the addition is complete to avoid uranyl ion hydrolysis. Evaporation and concentration continue until the nitric acid mass fraction reaches the target crystallization acidity (M3 point). Figure 1 (M3 point), forming a high-temperature concentrated liquid.

[0019] The mother liquor diversion operation is as follows: the first part of the mother liquor is sent to an evaporation and concentration device and evaporated at a constant temperature of 85°C until the mass fraction of nitric acid reaches point M2 on the phase diagram. Evaporation is paused, and the pH oscillation and impurity removal operation of step S2 is repeated. An alkaline regulator is added dropwise to adjust the pH slightly by 0.5 units. After standing for 3 minutes, evaporation is resumed to further remove trace impurities. The goal of evaporation and concentration is to make the mass fraction of nitric acid in the system reach point M2 on the phase diagram. After impurity removal, evaporation is immediately resumed. By controlling the amount of water evaporated, evaporation continues, causing the system point to shift along the extension line of OE to point N, and then evaporate along the extension line of M2N to point L. Then, it is cooled to 25°C, and uranyl nitrate trihydrate crystals are precipitated in a directional manner, resulting in a solid-liquid mixture. The solid-liquid mixture is centrifuged for 15 minutes at a speed of 3000 r / min. The crystals and the high-acidity final mother liquor are collected. The crystals are washed three times with anhydrous ethanol and dried at room temperature of 25°C for 2 hours to obtain high-purity uranyl nitrate trihydrate.

[0020] Example 2: This example provides a method for preparing high-purity uranyl nitrate hydrate, specifically including the following steps: S1, Raw material preparation and dissolution: Prepare a 20% (w / w) nitric acid aqueous solution (corresponding to... Figure 1 Add uranium octoxide (M1 point) to the dissolution reactor. Weigh out uranium octoxide and add it to the reactor. Dissolve it completely at 85℃ and 300r / min with stirring. The tail gas generated during the dissolution process is treated by a condenser and reflux device and then sent to an alkali absorption tank for neutralization before being discharged. The condensate is returned to the reactor. After dissolution is complete, the mother liquor (corresponding to...) is obtained. Figure 1 (Mid-Q point) S2, Evaporation Concentration and pH Shaking for Impurity Removal: The mother liquor is transferred to an evaporation concentration unit for isothermal evaporation concentration at 85°C. Then, pH shaking is activated to remove impurities, yielding a high-temperature concentrated solution. Without cooling, the high-temperature concentrated solution is directly passed through a sintered metal filter element preheated to 85°C to hot-press out impurities and colloids. The high-purity clear liquid is collected and returned to the crystallization unit. The high-purity clear liquid is cooled to 25°C at a rate of 1.5°C / min. The system point is transferred along the OQ extension line from point M3 to... Figure 1 At point P (uranyl nitrate 22.78%, nitric acid 47.36%), uranyl nitrate hexahydrate was precipitated in a directional manner, and a slurry was obtained. S3, Centrifugal Washing and Drying: The slurry is subjected to solid-liquid separation. The slurry is transferred to a centrifuge for solid-liquid separation at 3000 r / min for 15 min. The solid product and mother liquor are collected. Figure 1 At point E (30% uranyl nitrate and 38% nitric acid), the solid product was washed three times with anhydrous ethanol (50 mL each time) to remove residual mother liquor on the surface. Then it was dried at room temperature (25 °C) for 2 h to obtain high-purity uranyl nitrate hexahydrate. S4, Mother liquor splitting and preparation of uranyl nitrate trihydrate: The mother liquor is split into a first part of mother liquor and a second part of mother liquor. The first part of mother liquor accounts for 70% of the total mass of the mother liquor, and the second part of mother liquor accounts for 30% of the total mass of the mother liquor, to obtain uranyl nitrate trihydrate and high acidity final mother liquor. S5, Closed-loop compounding and recycling of mother liquor: The high-acidity final mother liquor obtained in step S4 is sent to the mother liquor compounding tank and mixed with the second part of the mother liquor to form a mixed liquid. Material balance is performed through the phase diagram, and deionized water is precisely added to adjust the composition of the mixed liquid so that the mass fraction of nitric acid returns to point M1 on the phase diagram. The mass fraction of nitric acid is 20%, and no additional purification treatment is required. This mixed liquid is directly used as the starting nitric acid solution for the next production batch and is put into the dissolution reactor in the first step, thereby realizing a 100% closed-loop circulation of nitric acid medium and unreacted uranium resources, eliminating the generation of uranium-containing nitric acid waste liquid from the source.

[0021] The pH oscillation purification process is as follows: when the mass fraction of nitric acid in the system reaches the target crystallization acidity ( Figure 1 When the acidity reaches 70% at point M3, evaporation is paused. Then, online pH monitoring is activated, and 5% dilute ammonia solution is slowly added dropwise through a microburette, controlling the dropping rate to allow a controlled, gradual increase of 1 unit in pH from the strongly acidic range. This process continues for 6 minutes, triggering the hydrolysis of iron, aluminum, and molybdenum impurities to form hydroxide colloids. Evaporation is immediately resumed after the addition is complete to prevent uranyl ion hydrolysis. Evaporation and concentration continue until the nitric acid mass fraction reaches the target crystallization acidity. Figure 1 (M3 point), forming a high-temperature concentrated liquid.

[0022] The mother liquor diversion operation is as follows: the first part of the mother liquor is sent to an evaporation and concentration device and evaporated at a constant temperature of 85°C until the mass fraction of nitric acid reaches point M2 on the phase diagram. Evaporation is paused, and the pH oscillation and impurity removal operation of step S2 is repeated. An alkaline regulator is added dropwise to adjust the pH slightly by 0.5 units. After standing for 3 minutes, evaporation is resumed to further remove any possible trace impurities. The goal of evaporation and concentration is to make the mass fraction of nitric acid in the system reach point M2 on the phase diagram (the mass fraction of nitric acid in the system is 65%). After impurity removal, evaporation is immediately resumed. By controlling the amount of water evaporated, evaporation continues, causing the system point to shift along the extension line of OE to point N, and then along the extension line of M2N to point L. Then, it is cooled to 25°C, and uranyl nitrate trihydrate crystals are precipitated in a directional manner, resulting in a solid-liquid mixture. The solid-liquid mixture is centrifuged for 15 minutes at a speed of 3000 r / min. The crystals and the high-acidity final mother liquor are collected. The crystals are washed three times with anhydrous ethanol and dried at room temperature of 25°C for 2 hours to obtain high-purity uranyl nitrate trihydrate.

[0023] Example 3: This example provides a method for preparing high-purity uranyl nitrate hydrate, specifically including the following steps: S1, Raw material preparation and dissolution: Prepare a 35% (w / w) nitric acid aqueous solution and add it to the dissolution reactor. Weigh out uranium dioxide and add it to the reactor. Dissolve it completely under stirring conditions at 85℃ and 300 r / min. The tail gas generated during the dissolution process is treated by a condensation reflux device and then sent to an alkali absorption tank for neutralization before being discharged. The condensate is returned to the reactor. After dissolution is completed, the mother liquor is obtained (corresponding to...). Figure 1 (Mid-Q point) S2, Evaporation Concentration and pH Shaking for Impurity Removal: The mother liquor is transferred to an evaporation concentration unit for isothermal evaporation concentration at 85°C. Then, pH shaking is activated to remove impurities, yielding a high-temperature concentrated solution. Without cooling, the high-temperature concentrated solution is directly passed through a sintered metal filter element preheated to 90°C to hot-press out impurities and colloids. The high-purity clear liquid is collected and returned to the crystallization unit. The high-purity clear liquid is cooled to 25°C at a rate of 1.5°C / min. The system point is transferred along the OQ extension line from point M3 to... Figure 1 At point P, uranyl nitrate hexahydrate crystals were precipitated in a directional manner, and a slurry was obtained; S3, Centrifugal Washing and Drying: The slurry is subjected to solid-liquid separation. The slurry is transferred to a centrifuge for solid-liquid separation at 3000 r / min for 15 min. The solid product and mother liquor are collected. Figure 1 The solid product was washed three times with anhydrous ethanol (point E) to remove residual mother liquor on the surface. Then it was dried at room temperature (25°C) for 2 hours to obtain high-purity uranyl nitrate hexahydrate. S4, Mother liquor splitting and preparation of uranyl nitrate trihydrate: The mother liquor is split into a first part of mother liquor and a second part of mother liquor. The first part of mother liquor accounts for 60% of the total mass of the mother liquor, and the second part of mother liquor accounts for 40% of the total mass of the mother liquor, to obtain uranyl nitrate trihydrate and high acidity final mother liquor. S5, Closed-loop compounding and recycling of mother liquor: The high-acidity final mother liquor obtained in step S4 is sent to the mother liquor compounding tank and mixed with the second part of the mother liquor to form a mixed liquid. Material balance is performed through the phase diagram, and deionized water is precisely added to adjust the composition of the mixed liquid so that the mass fraction of nitric acid returns to point M1 on the phase diagram. The mass fraction of nitric acid is 20%, and no additional purification treatment is required. This mixed liquid is directly used as the starting nitric acid solution for the next production batch and is put into the dissolution reactor in the first step, thereby realizing a 100% closed-loop circulation of nitric acid medium and unreacted uranium resources, eliminating the generation of uranium-containing nitric acid waste liquid from the source.

[0024] The pH oscillation purification process is as follows: when the mass fraction of nitric acid in the system reaches the target crystallization acidity ( Figure 1 When the acidity reaches 80% at point M3, evaporation is paused. Then, online pH monitoring is activated, and an 8% urea solution is slowly added dropwise through a microburette, controlling the dropping rate to allow a controllable micro-increase of 1.5 units in the pH value from the strongly acidic range. This process continues for 10 minutes, triggering the hydrolysis of iron, aluminum, and molybdenum impurities to form hydroxide colloids. Evaporation is immediately resumed after the addition is complete to avoid uranyl ion hydrolysis. Evaporation and concentration continue until the nitric acid mass fraction reaches the target crystallization acidity. Figure 1 (M3 point), forming a high-temperature concentrated liquid.

[0025] The mother liquor diversion operation is as follows: the first part of the mother liquor is sent to an evaporation and concentration device and evaporated at a constant temperature of 85°C until the mass fraction of nitric acid reaches point M2 on the phase diagram. Evaporation is paused, and the pH oscillation and impurity removal operation of step S2 is repeated. An alkaline regulator is added dropwise to adjust the pH slightly by 0.5 units. After standing for 3 minutes, evaporation is resumed to further remove any possible trace impurities. The goal of evaporation and concentration is to make the mass fraction of nitric acid in the system reach point M2 on the phase diagram (the mass fraction of nitric acid in the system is 65%). After impurity removal, evaporation is immediately resumed. By controlling the amount of water evaporated, evaporation continues, causing the system point to shift along the extension line of OE to point N, and then along the extension line of M2N to point L. Then, it is cooled to 25°C, and uranyl nitrate trihydrate crystals are precipitated in a directional manner, resulting in a solid-liquid mixture. The solid-liquid mixture is centrifuged for 15 minutes at a speed of 3000 r / min. The crystals and the high-acidity final mother liquor are collected. The crystals are washed three times with anhydrous ethanol and dried at room temperature of 25°C for 2 hours to obtain high-purity uranyl nitrate trihydrate.

[0026] The difference between Comparative Example 1 and Example 2 is that the pH shaking impurity removal step is omitted, while the rest is exactly the same as Example 2.

[0027] The difference between Comparative Example 2 and Example 2 is that the mother liquor diversion step is omitted, while the rest is exactly the same as Example 2.

[0028] The difference between Comparative Example 3 and Example 2 is that the latter uses traditional extraction purification + crystallization, while the rest is exactly the same as Example 2.

[0029] Experimental example: 1. Chemical Purity: High-purity uranyl nitrate hydrate prepared in Examples 1-3 and Comparative Examples 1-3 of this invention was used as the sample. The mass was accurately weighed, dissolved in an appropriate amount of deionized water by ultrasonication, cooled to room temperature, and quantitatively transferred to a volumetric flask. The solution was then diluted to the mark, shaken well, and allowed to stand until clear to obtain the sample solution. Simultaneously, a blank control solution was prepared to eliminate reagent and environmental interference. A certain volume of the sample solution was transferred to an Erlenmeyer flask, and a buffer solution was added to adjust the acidity to the applicable titration range. The potassium dichromate titration method was used for testing, and the consumption of the standard titration solution was recorded. The test was performed in triplicate, and the average value was taken. Based on the stoichiometric ratio of the titration reaction, the concentration and consumption of the standard titration solution, and the sample mass, the main content of uranyl nitrate was calculated. Combined with the detection results of trace impurities (iron, aluminum, molybdenum, etc.), the final purity of the product was verified, and the results are recorded in Table 1.

[0030] 2. Yield: Using the preparation steps of Examples 1-3 and Comparative Examples 1-3 of this invention, the raw material uranium oxide octoxide was accurately weighed, and the mass fraction of uranium in the raw material was determined by chemical analysis. The total mass of uranium in the raw material was calculated. The dried uranyl nitrate hexahydrate and uranyl nitrate trihydrate products were collected, and their total mass was weighed. The mass fraction of uranium in each product was determined, and the total mass of uranium in the two products was calculated. Based on the total mass of uranium in the raw material and the total mass of uranium in the two products, and after deducting the loss of measurable materials (tail gas entrainment, equipment residue), the total uranium yield was calculated; the results are recorded in Table 1.

[0031] 3. Recycling Rate: High-purity hydrated uranium nitrate was prepared according to the preparation methods of Examples 1-3 and Comparative Examples 1-3 of the present invention. Based on the dissolution reaction equation and the amount of feed, the theoretical number of moles of nitric acid consumed was calculated. After the closed-loop circulation was stably operated, the mass and concentration of fresh concentrated nitric acid required to maintain the circulation for each batch were recorded, and the number of moles was calculated. The nitric acid recycling rate was then recorded in Table 1.

[0032] Table 1: Performance test table of high-purity uranyl nitrate hydrate prepared in this invention

[0033] Table 1 shows that the purity of the high-purity uranyl nitrate hydrate prepared according to the preparation methods of Examples 1-3 of this invention is significantly higher than that of Comparative Examples 1-3. This indicates that the coupled step of pH shaking + hot filtration brings about a deep purification effect and solves the purity bottleneck of simple physical crystallization. Comparative Example 1 does not use shaking for impurity removal and cannot effectively remove impurities. The traditional extraction method in Comparative Example 3 easily leaves trace amounts of organic reagents. The yields of Examples 1-3 are comparable to those of Comparative Example 1, indicating that the impurity removal step of this invention achieves deep purification without causing significant loss of the main metal uranium. In Comparative Example 2, the mother liquor is directly discharged, and uranium resources are lost with the waste liquid, causing the yield to plummet to 89.1%, demonstrating the key role of closed-loop recycling in improving the yield. The multi-step extraction operation in Comparative Example 3 results in uranium entrainment loss, which is much lower than that of Examples 1-3. The recycling rate of Examples 1-3 compared to Comparative Examples 1-3 directly demonstrates the resource-efficient utilization and green environmental protection characteristics of this invention.

[0034] In summary, the method for preparing high-purity uranyl nitrate hydrate described in this invention is guided by a ternary phase diagram and organically integrates pH-controlled micro-disturbance impurity removal technology with a closed-loop mother liquor recycling process. This constructs a comprehensive, synergistic preparation system encompassing raw material dissolution, directional impurity removal, phase region control, and resource recovery. This method achieves in-situ selective removal of amphoteric impurities through precise pH oscillation without affecting the stability of uranyl ions. Combined with hot filtration to enhance impurity removal, and relying on phase diagram characteristic points for directional crystallization and continuous preparation of uranyl nitrate hexahydrate and uranyl nitrate trihydrate, the mother liquor after centrifugation is recombined and adjusted, and can be directly recycled back to the raw material dissolution process in a closed loop, eliminating the need for additional complex purification treatment and forming a green production model with complete material recycling. Through a combination of simple physical operation and precise control, this method achieves multiple objectives—simplified process, efficient resource utilization, and environmental friendliness—while ensuring high product purity and crystal phase stability.

[0035] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing high-purity uranyl nitrate hydrate, characterized in that, The high-purity uranyl nitrate hydrate includes high-purity uranyl nitrate hexahydrate and high-purity uranyl nitrate trihydrate, and the preparation method specifically includes the following steps: S1, Raw material preparation and dissolution: Prepare a nitric acid aqueous solution, weigh out uranium oxide and add it to the nitric acid aqueous solution, heat and stir to dissolve, and obtain uranyl nitrate mother liquor; S2, Evaporation Concentration and pH Shaking for Impurity Removal: The uranyl nitrate mother liquor is evaporated and concentrated, and then pH shaking is turned on to remove impurities, resulting in a high-temperature concentrated liquid. The liquid is then hot-filtered, high-purity clear liquid is collected, and the mixture is cooled in a programmed manner to obtain a mixed slurry. S3, centrifugal washing and drying: solid-liquid separation of the mixed slurry to precipitate high-purity uranyl nitrate hexahydrate and mother liquor; S4, Mother liquor splitting and preparation of high-purity uranyl nitrate trihydrate: The mother liquor is split into a first part of mother liquor and a second part of mother liquor to obtain high-purity uranyl nitrate trihydrate and high-acidity final mother liquor; S5, Closed-loop compounding and recycling of mother liquor: The high-acidity final mother liquor obtained in step S4 is mixed with the second part of the mother liquor to form a mixed solution. Deionized water is added to adjust the mass fraction of nitric acid to the initial value to obtain a regenerated nitric acid solution, which is then reused in step S1.

2. The method for preparing high-purity uranyl nitrate hydrate according to claim 1, characterized in that, In step S2, the pH oscillation impurity removal process is as follows: when the mass fraction of nitric acid in the system reaches 60-80% of the target acidity value, evaporation is paused, pH online monitoring is turned on, and an alkaline regulator is slowly added dropwise to make the pH value of the system increase by a controllable micro-increase of 0.3-1.5 units. Then, evaporation continues until the mass fraction of nitric acid in the system reaches the target crystallization acidity.

3. The method for preparing high-purity uranyl nitrate hydrate according to claim 1, characterized in that, The hot filtration process involves passing the high-temperature concentrate through a filter preheated to 80-90°C. The cooling process involves cooling the high-purity liquid to 25°C at a rate of 1-1.5°C / min.

4. The method for preparing high-purity uranyl nitrate hydrate according to claim 1, characterized in that, In step S4, the first portion of mother liquor accounts for 60-70% of the total mass of the mother liquor, and the second portion of mother liquor accounts for 30-40% of the total mass of the mother liquor; The mother liquor diversion operation is as follows: the first part of the mother liquor is evaporated and concentrated until the nitric acid concentration reaches the target value. Then, the pH shaking and impurity removal operation is repeated, and the evaporation continues until the predetermined crystallization point. After hot filtration, the liquid is cooled and crystallized, and the solid and liquid are separated.

5. The method for preparing high-purity uranyl nitrate hydrate according to claim 1, characterized in that, The uranium oxide is selected from one of uranium octoxide, uranium trioxide, and uranium dioxide.

6. The method for preparing high-purity uranyl nitrate hydrate according to claim 2, characterized in that, The alkalinity regulator is a dilute ammonia solution or a urea solution.

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