A method for crystallization, separation and purification of uranyl nitrate hexahydrate

Abstract

A method for crystallizing, separating, and purifying uranyl nitrate hexahydrate comprises crystallizing uranyl nitrate hexahydrate crystals from a concentrated uranyl nitrate nitric acid solution, countercurrently washing the crystals, collecting the uranyl nitrate hexahydrate crystals into a collection container while squeezing washing liquid from a container into a washing area, and unloading the washed product. The uranyl nitrate hexahydrate crystals are crystallized from a feed stream under isothermal conditions, washed, and collected. The technical achievement simplifies the process flow, achieving high and independent recovery and product purity indicators. This method can be used in nuclear fuel cycle water metallurgy processes during the refining stage of natural uranium or recycled uranium.

Description

Technical Field

[0001] The invention relates to a heat and mass exchange process in a liquid-solid system and can be used in a nuclear fuel circulation water metallurgy process in the refining stage of natural uranium or regenerated uranium. Background Art

[0002] A method for continuous crystallization of uranyl nitrate hexahydrate and a device for its realization are known (Russian Federation Patent No. 2268510, G21C19 / 46).

[0003] According to this method, the process is carried out in a U-shaped tubular apparatus. The feed stream is introduced on one side of the apparatus to crystallize uranyl nitrate hexahydrate crystals and rinse crystals. The crystals are melted at the bottom of the apparatus. The resulting melt flows under hydrostatic pressure to the other side of the apparatus and exits the production process by gravity. The middle (lower) section of the apparatus is connected to a pulse vibrator to ensure that it is in a pulsed state throughout the process, enhancing the flow in the apparatus.

[0004] Disadvantages of this method: low operation recovery rate and low production efficiency.

[0005] A method for crystallizing, separating, and purifying uranyl nitrate hexahydrate is known (Russian Federation Patent No. 2528399G21C19 / 00), which comprises continuously crystallizing uranyl nitrate hexahydrate crystals from a concentrated uranyl nitrate nitric acid solution, separating the uranyl nitrate hexahydrate crystals from a mother liquor, washing the uranyl nitrate hexahydrate crystals in a countercurrent manner with a washing liquid, discharging the mother liquor and the spent washing liquid, and collecting and unloading the washed uranyl nitrate hexahydrate crystals, wherein the composition of the concentrated uranyl nitrate nitric acid solution and the washing liquid supplied to the process is maintained according to the relationship [UO2(NO3)2]=78.49-k[HNO3], wherein [UO2(NO3)2] and [HNO3] are the concentrations of uranyl nitrate and nitric acid in the liquid phase, respectively, in wt%; 78.49 is the uranyl nitrate concentration in the uranyl nitrate hexahydrate crystals, The unit is wt %; k is a coefficient for determining the position of the operating line of the process, and its value is 1.50-2.24, preferably 1.55-1.80. At the same time, the concentration of uranyl nitrate in the concentrated uranyl nitrate nitric acid solution is not less than 60 wt %, and the concentration in the washing liquid is less than the concentration at the intersection of the operating line and the uranyl nitrate hexahydrate crystallization isotherm. Until there is no uranium, the crystallization process of uranyl nitrate hexahydrate is completed under isothermal conditions and the uranyl nitrate hexahydrate crystals are moved to a washing area. The washed uranyl nitrate hexahydrate crystals are sent to a collection container filled with washing liquid that has been pushed into the washing area by the uranyl nitrate hexahydrate crystals. When the uranyl nitrate hexahydrate crystals fill the collection container, the surface of the uranyl nitrate hexahydrate crystals on the collection container and the inner surface of the valve are melted, and the washed uranyl nitrate hexahydrate crystals are discharged.

[0006] The crystallization and washing of uranyl nitrate hexahydrate crystals are carried out along the same operating line, and the value of the coefficient k is the same for each operation.

[0007] The temperature of the isothermal zone used to complete the crystallization process of uranyl nitrate hexahydrate and move the crystals to the cleaning area was maintained below 10°C.

[0008] The method also provides for the separate discharge of mother liquor and spent wash liquor, as well as the draining of the collection vessel.

[0009] To implement the method, a crystallization separation and purification device for uranyl nitrate hexahydrate includes a vertical crystallizer, a crystal countercurrent washing tower, and is equipped with a paddle mixer, an inlet pipe for feed flow and supplementary washing flow, an outlet pipe for mother liquor and waste washing liquid, and a segmented thermostatic jacket for cooling. The crystallizer and the washing tower are of the same tube type. The bottom of the washing tower is equipped with a multi-position full-bore valve, which connects the pipeline to no less than two collection containers for washed uranyl nitrate hexahydrate crystals in sequence. Each container is equipped with a two-position full-bore valve for unloading the washed uranyl nitrate hexahydrate crystals, a heated discharge joint, and a thermostatic jacket for heating the outer shell, the vertical surfaces of the collection container structural components, and the two-position full-bore valve.

[0010] The outlet pipes of the mother liquor and waste washing liquid are tangentially embedded in the pipe of the crystallizer, and the motion vector of the discharge flow is opposite to the motion vector of the mixer blade, and the angle between the axis of each pipe and the axis of the crystallizer pipe does not exceed 45°.

[0011] The technical essence of this method and device is closer to the method and device applied for, so it was selected as the prototype.

[0012] During the testing of the prototype method and the prototype device, some of their shortcomings were discovered.

[0013] According to the prototype method, to prevent contaminated mother liquor from entering the scrubber, crystallization and crystal washing must be performed along the same process line. This requirement requires isothermal conditions during crystallization and crystal delivery to the scrubber to ensure a balanced composition, thereby ensuring a uniform density between the mother liquor and the spent scrubber, thus preventing contamination of the scrubber by the mother liquor. This means that a new scrubber composition must be prepared for any change in the feed stream composition. This requirement is practically impossible to achieve under industrial operating conditions.

[0014] According to the prototype method, uranyl nitrate hexahydrate crystals are cleaned under variable temperature conditions, starting at the crystallization temperature and continuing at elevated temperatures until the cleaning is completed at the ambient medium temperature. This cleaning mode is designed to maintain a high uranium crystallization recovery rate (low-temperature zone) while ensuring the necessary cleaning of the crystals (high-temperature zone). However, under this mode, uranium unexpectedly reverts from the liquid phase to the crystalline phase: as the scrubbing liquid in the scrubbing tower moves upward, the temperature of the scrubbing liquid decreases, reducing the uranium content in the scrubbing liquid. Uranyl nitrate hexahydrate crystallizes on the cleaned crystals, sealing the contaminated surface with a "solidified" layer of crystalline product.

[0015] Therefore, in the prototype process, the conditions for achieving high operating recovery of the product and high purity of the product are interdependent and difficult to achieve together. Summary of the Invention

[0016] Problems to be solved by the invention

[0017] The object of the present invention is to realize a method for separating and purifying uranyl nitrate hexahydrate by crystallization using a scrubbing liquid (solution), wherein the composition of the scrubbing liquid is independent, i.e., not affected by the conditions of a single operating line, and is not related to the composition of the feed stream, and the scrubbing liquid is independent of the operating technology that ensures the integrity of the uranyl nitrate hexahydrate crystallization and purification of the crystals, and the hardware for implementing the method.

[0018] The technical results to be achieved by the method and device of the present application are that the process flow is simplified, and the indicators of operation recovery rate and product purity reach a high level and are independent of each other.

[0019] Means for solving problems

[0020] To achieve the above technical results, the method for crystallization, separation and purification of uranyl nitrate hexahydrate includes crystallizing uranyl nitrate hexahydrate crystals from a concentrated uranyl nitrate nitric acid solution, and the feed stream composition corresponds to the relationship [UN] =78.49-k[НО3], wherein [UN] and [HNO3] are the concentrations of uranyl nitrate and nitric acid, respectively, in wt %; 78.49 is the concentration of uranyl nitrate in the uranyl nitrate hexahydrate crystals, in wt %; and k is a coefficient for determining the position of the process operating line, having a value of 1.50-2.24, preferably 1.57-1.80. The method further comprises separating the uranyl nitrate hexahydrate crystals from the mother liquor, countercurrently washing the crystals, collecting the uranyl nitrate hexahydrate crystals into a collection vessel while squeezing the washing liquid from the vessel into a washing zone, and unloading the washed product, wherein the uranyl nitrate hexahydrate crystals are crystallized from the feed stream under isothermal conditions, washed, and collected, and the original composition of the washing liquid, including the uranium-free component, meets the conditions of the family of operating lines 78.49 = k1[HNO3], where [HNO3] is the nitric acid content of the washing liquid in wt.%, and k1≤k, where k1 is a coefficient for determining the position of the operating line of the washing process, the mother liquor and the spent washing liquid are directed to the supplementary crystallization in the form of a common stream, the supplementary crystallization being carried out at a temperature below the crystallization temperature and in countercurrent motion of the liquid and solid phases, the supplementary crystallized uranyl nitrate hexahydrate crystals being washed together with the crystals prepared from the feed stream, and the mother liquor from the supplementary crystallization being discharged from the process.

[0021] In individual cases, the crystals prepared from the feed stream and the crystals of the supplementary crystallization are crystallized and washed at a temperature of not less than 20°C.

[0022] In other individual cases, supplementary crystallization was carried out at a temperature not higher than 5°C.

[0023] To achieve this technical achievement, a uranyl nitrate hexahydrate crystallization, separation, and purification device includes a vertical crystallizer and a crystal countercurrent washing tower. The vertical crystallizer and the crystal countercurrent washing tower are in the same pipeline, which is equipped with a paddle mixer, process flow inlet and outlet pipes, a segmented thermostatic jacket, and a multi-position full-bore valve that connects the crystallizer pipeline with no less than two cleaned uranyl nitrate hexahydrate crystal collection containers in sequence, each of which is equipped with a thermostatic device, a discharge device, and a product unloading device. In addition, the device also includes a supplementary crystallizer with a paddle mixer and a thermostatic jacket. The supplementary crystallizer is installed on the mother liquor and washing liquid outlet pipes, and the axis of the supplementary crystallizer is parallel to the axis of the crystallizer.

[0024] Auxiliary studies have shown that the density of the waste washing liquid at any temperature in the "crystallization-cleaning" conjugate process, that is, the uranyl nitrate content equivalent to the uranyl nitrate hexahydrate crystal phase, is higher than the density of the mother liquor when k1 < k, and is lower than the density of the mother liquor when k1 > k. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The figure shows the layout of the crystal crystallization area and the cleaning area according to the proposed method and using the proposed device.

[0026] Figure 2 Cleaning process parameters (prototype method).

[0027] Figure 3 Cleaning process parameters (proposed method). DETAILED DESCRIPTION

[0028] The experimental data as an example are shown in the table below.

[0029] Table. Density of waste washing liquid at 20°C during the crystallization-cleaning process: k = 1.69, p = 1.604 g / cm 3 .

[0030]

[0031] The process of implementing the crystallization separation and purification of uranyl nitrate hexahydrate is as follows.

[0032] The working volume of the device is filled with the washing liquid (50-52 wt% concentration of nitric acid solution, corresponding to the value k1 = 1.57-1.52).

[0033] The original composition of the wash liquid is capable of processing any feed stream with a crystallization operating line coefficient k ≥ 1.57 without mother liquor "falling" into the wash zone.

[0034] The feed stream is then injected into the crystallizer 2 through the pipe 1. In the crystallizer 2, uranyl nitrate hexahydrate crystallization occurs. The crystals enter the cleaning zone 3, and the cleaned crystals enter the alternate working collection container ( Figure 1 not shown in the figure).

[0035] The waste washing liquid and the crystallization mother liquor are passed through the connecting pipe 7 and enter the supplementary crystallizer 6. At a temperature much lower than that of the crystallizer 2, the supplementary crystallizer crystallizes uranyl nitrate hexahydrate from the mother liquor-washing liquid combination.

[0036] The crystals enter the washing zone 3 through the pipe 5 and then enter the uranyl nitrate hexahydrate crystal collection container together with the crystals crystallized from the feed stream.

[0037] From the process description, it can be seen that the shortcomings of the prototype method pointed out in the previous article are overcome in the proposed method.

[0038] Washing liquids with a positive fraction [НNO3] = 50-52 wt% are suitable for processing feed streams with a wide composition range, k = 1.57-1.80 and no mother liquor flowing into the washing zone.

[0039] The operations ensuring high recovery of the crystallization process (recrystallization of uranyl nitrate hexahydrate from the mother liquor-wash solution at low temperature) and product purification (isothermal process of crystallization and washing of crystals prepared from the feed stream and recrystallized crystals at high temperature) are technologically independent and avoid "freezing" of the crystal phase in the washing zone.

[0040] A previously developed mathematical model of the crystallization process of uranyl nitrate hexahydrate (SN Viselov, VI Volk, VA Gaseyev et al., Journal of Engineering Physics, 2017, Vol. 90, 1, pp. 148-158) was used to design a model of the crystallization process in the cleaning zone during production according to the prototype method and the proposed method.

[0041] Given the following conditions: feed flow [UN] = 64 wt%, [HNO3] = 8.57 wt%, (k = 1.69); washing liquid [HNO3] = 50 wt%, (k-1.57); washing zone length 3 m, washing zone pipe inner diameter 3 cm, feed flow consumption 2 kg / h.

[0042] Temperature conditions

[0043] Prototype method: crystallization temperature 0°C, cleaning temperature 25°C (cleaning start) - 0°C (cleaning end).

[0044] The proposed method: crystallization temperature is 25°C, the temperature for washing crystals prepared from the feed stream and for supplementary crystallization is 25°C, and the supplementary crystallization temperature is 0°C.

[0045] Comparable parameters for the cleaning process are given in Figure 2 (prototype method) and Figure 3 (method provided).

[0046] Length of the cleaning zone pipe – from the start to the end of the cleaning process.

[0047] Compare the temperature, the uranyl nitrate content in the washing solution in wt%, the effective diameter of the crystalline particles, and the volume fraction of the solid phase.

[0048] Figure 2 and Figure 3 The simulation results shown in Figure 2 demonstrate that the temperature-dependent cleaning process (prototype method) is expected to be accompanied by a return of uranyl nitrate from the liquid phase to the crystalline phase, with a 7.7% increase in the diameter of the crystal particles and a 24.8% increase in the volume of the particles. The listed indicators are the result of the solid phase "solidifying" on the surface of the crystal wash, which negatively affects the cleaning quality and reduces the purity grade of the product.

[0049] Industrial applicability

[0050] When the process is carried out according to the provided method, the crystal phase recovery rate is higher than that of the prototype method (12.3% solid phase volume ratio, while the solid phase volume ratio in the prototype method is 9.1%) due to process-independent supplementary crystallization.

[0051] The cleaning parameters according to the provided method (temperature, uranyl nitrate content in the washing solution, size of the crystal particles) are stable, which proves that the product has not reversed and has not blocked the cleaned surface.

[0052] Therefore, all the comparable parameters mentioned above demonstrate the advantages of the proposed method.

Claims

1. A method for crystallizing, separating, and purifying uranyl nitrate hexahydrate, comprising crystallizing uranyl nitrate hexahydrate crystals from a concentrated uranyl nitrate nitric acid solution, wherein the feed stream components correspond to the relationship [UN] = 78.49 - k [HNO3], wherein [UN] and [HNO3] are the concentrations of uranyl nitrate and nitric acid, respectively, expressed in wt%; 78.49 is the uranyl nitrate concentration in the uranyl nitrate hexahydrate crystals, expressed in wt%; and k is a coefficient for determining the position of a process operating line, having a value of 1.50-2.

24. The method further comprises separating the uranyl nitrate hexahydrate crystals from a mother liquor, washing the crystals in a countercurrent manner, collecting the uranyl nitrate hexahydrate crystals into a collection container while squeezing a washing liquid from a container into a washing area, and unloading the washed product, wherein: Uranyl nitrate hexahydrate crystals are crystallized from a feed stream under isothermal conditions, washed, and collected, wherein the original composition of the wash liquor, including the uranium-free fraction, meets the condition of the family of operating lines: 78.49 = k1 [HNO3], where [HNO3] is the nitric acid content of the wash liquor expressed in wt.%, and k1 ≤ k, where k1 is a coefficient for determining the position of the operating line for the wash process, wherein the mother liquor and the spent wash liquor are directed as a single stream to supplemental crystallization, wherein the supplemental crystallization is conducted at a temperature below the crystallization temperature and in countercurrent motion of the liquid and solid phases, wherein the supplemental crystallized uranyl nitrate hexahydrate crystals are washed together with the crystals prepared from the feed stream, and the mother liquor from the supplemental crystallization is discharged from the process.

2. The method according to claim 1, characterized in that The crystals prepared from the feed stream and the crystals of the supplemental crystallization are crystallized and washed at a temperature of not less than 20°C.

3. The method according to claim 1, characterized in that The supplementary crystallization is carried out at a temperature not higher than 5°C.

4. A uranyl nitrate hexahydrate crystallization, separation, and purification apparatus comprising a vertical crystallizer and a crystal countercurrent washing tower, wherein the vertical crystallizer and the crystal countercurrent washing tower are in the same pipeline, the pipeline being equipped with a paddle mixer, process inlet and outlet pipes, a segmented thermostatic jacket, and a multi-position full-bore valve connecting the crystallizer pipeline to at least two collection containers for cleaned uranyl nitrate hexahydrate crystals, each container being equipped with a thermostat, a discharge device, and a product unloading device, characterized in that: The device comprises a supplementary crystallizer with a paddle mixer and a constant temperature jacket. The supplementary crystallizer is installed on a mother liquid and washing liquid outlet pipe, and the axis of the supplementary crystallizer is parallel to the axis of the crystallizer.

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