RU2244966C2 - Method and device for cleaning uranyl nitrate from fission products - Google Patents

Abstract

FIELD: radiochemical industry.

SUBSTANCE: proposed method includes crystallization of uranyl hexahydro-nitrate from solution by cooling it down. In the process uranyl nitrate solution is pre-evaporated to uranium concentration of 800-1300 g/l and to nitric acid concentration of 0.5-3.0 mole/l. Crystallization is conducted in concurrent flow of uranyl hexahydro-nitrate crystals formed in the process and mother solution while permanently stirring and cooling solution to 15-30 ^o C. Crystals are separated from mother solution and washed in concurrent flow at 15-30 ^o C in wash solution containing 250-300 g/l of uranium and 3.0-6.0 mole/l of nitric acid. Then crystals are taken out and dried. Device implementing this method has crystallization vat made in the form of externally cooled tube. In addition vertically disposed crystallization vat accommodates blade mixer. Uranyl nitrate solution feed union is disposed in top part of crystallization vat. Bottom part of the latter is placed coaxially with wash-over string and secured in hole of its lid provided with pipe union for joint outlet of mother solution and waste wash solution. Bottom part of wash-over string is joined at certain angle to auger-accommodating inclined tube. Top part of inclined tube is provided with pipe union for taking out dried uranyl hexahydro-nitrate crystals and wash solution feed union. Pipe union for taking out dried crystals is disposed above that wash solution feed union. Pipe union for feeding uranyl nitrate solution is level with wash solution feed union and with water seal.

EFFECT: enhanced cleaning efficiency and economic efficiency of process.

7 cl, 2 ex

Description

The invention relates to the technology of mass transfer countercurrent continuous processes between solid and liquid phases, such as washing, leaching, dissolution, and can be used in radiochemical, chemical, hydrometallurgical and other industries. With the greatest effect, the invention can be used for continuous purification of uranyl nitrate from fission products by crystallization.

A known method of purification of uranyl nitrate [1]. In this method, crystals of uranyl nitrate hexahydrate (GNU) are reduced and recovered from uranium-containing solutions of nitric acid with increasing concentration of nitric acid. The increase in the concentration of nitric acid is carried out while maintaining the temperature of the uranium-containing solution of nitric acid 0-25 ° C and adding to the solution of nitric acid with a concentration of 60-98%, or by evaporation and concentration of the solution, or when preparing nitric acid uranium-containing solutions at temperatures above 25 ° C and cooling the solution to a temperature of 0-25 ° C. The degree of extraction of GNU crystals ranged from 54% to 69%.

The disadvantages of this method include the periodicity of the process, the difficulty of organizing a continuous process for the purification of uranyl nitrate, a large number of auxiliary operations, the issue of cleaning the separated GNU crystals from the mother liquor contaminated with impurities of fission products, the complexity of organizing remote maintenance and control have not been worked out.

A known method of purification of uranyl nitrate [2]. In this method, crystallization of GNU from a solution of 3M HNO _{3 is} carried out by cooling it to a temperature of -20 ° C, while up to 70% of uranium is extracted from the solution after dissolving the fuel.

The disadvantages of this method is the need for cooling of uranium-containing solutions to a temperature below their freezing temperature, which requires the availability of large-scale refrigeration equipment and, as a consequence, a decrease in the efficiency of the process.

A known method of batch cleaning of uranium or plutonium to be returned to the processing of irradiated nuclear fuel and / or fissile materials [3] (prototype). The process of batch cleaning of uranium or plutonium consists in the fact that after the initial purification from impurities, each portion of a solution of uranyl nitrate containing residual amounts of impurities is crystallized and the resulting GNU crystals are separated from the mother liquor, washed and discharged. The initial solution was adjusted to a concentration of 48–480 g / L for uranium, 0.2–2.0 mol / L for nitric acid, and the prepared uranium solution was cooled below 0 ° C for crystallization. The duration from the beginning to the end of crystallization is set between 0.1 and 10 hours. The amount of separated GNU crystals corresponds to less than 80 weight percent of the introduced uranium. The ratio of the volumes of the crystalline phase to the volume of the resulting mother liquor is not more than 0.5. In the mother liquor, the final concentration of nitric acid is set at 6-12 mol / L at room temperature. In this case, co-crystallization of uranyl nitrate with water in the form of ice and / or nitric acid trihydrate is avoided. The resulting mother liquor is separated from the GNU crystals and returned to recycling. The separated crystals are washed for a short time with a solution of nitric acid of the same concentration and at the same temperature as in the separated mother liquor after crystallization is completed. The batch process for the purification of plutonium or plutonium and uranium is carried out in a similar manner.

The disadvantages of this method include the low concentration of the initial solution for uranium and, as a result, a significant amount of processed solutions and the need for multi-stage crystallization, the need for refrigeration equipment, which also reduces the efficiency of the processing process.

For the prototype of the proposed device, the device [4] was selected for the continuous crystallization of nuclear fuel solutions and crystallization purification processes U and Pu with limited criticality, containing an outer pipe mounted horizontally or obliquely, it contains an inner pipe rotated by the drive. Guide vanes are located on the periphery of the inner tube, the outer tube is equipped with nozzles for supplying a solution of uranyl nitrate, acid, refrigerant, and the outlet of the mother liquor and GNU crystals. The solution of uranyl nitrate is cooled in the annulus with the formation of crystals and silt deposits containing crystals. The crystals and the mother liquor are discharged through separate openings, while the mother liquor is captured by crystals.

The disadvantages of this device include low reliability due to the difficulty of protecting the seals located in the crystallization zone, this leads to the need for operations that are dangerous for maintenance personnel, the absence of washing crystals leads to a low degree of purification from fission products, the need for additional refrigeration equipment.

The technical task of the invention is the organization of a continuous process of purification of uranyl nitrate from fission products, increasing the degree of purification of uranium from fission products, a significant reduction in the volume of processed solutions, increasing the efficiency of the crystallization process (the crystallization process is carried out in a positive temperature range), increasing the degree of crystallization of uranyl nitrate, reducing volumes of reagents supplied to the process.

The problem is solved by the presented method and device for its implementation.

The method of purification of uranyl nitrate from fission products involves crystallization of uranyl nitrate hexahydrate from a solution by cooling it, separating the obtained crystals from the mother liquor, washing and unloading, and the uranyl nitrate solution is evaporated to a concentration of 800-1300 g / l with uranium and nitric acid 0 , 5-3.0 mol / l, crystallization is carried out in the direct flow of the formed crystals of uranyl nitrate hexahydrate and the mother liquor with constant stirring and cooling of the solution to a temperature of 15-30 ° C, the crystals formed they are separated from the mother liquor and subjected to countercurrent washing at a temperature of 15-30 ° C with a washing solution coming after washing the unloaded crystals, then the crystals are dried from the washing solution and unloaded, while the washing solution containing 250-300 g / l of uranium and 3.0 -6.0 mol / L nitric acid, is supplied countercurrent to the unloaded crystals, and the spent washing solution together with the mother liquor containing fission products is removed from the process in the contact zone, and the density of the spent washing solution is sub rzhivayut 2-5% greater than the density of the mother liquor.

In a particular embodiment, the purification of uranyl nitrate is carried out in 2-3 steps, using the mother liquor together with the spent wash solution of the next step as a wash solution of the previous step.

A device for the continuous purification of uranyl nitrate from fission products contains a crystallizer made in the form of an external cooling pipe equipped with a nozzle for supplying a uranyl nitrate solution, the mold with a paddle mixer inside, located vertically, while the nozzle for supplying a uranyl nitrate solution is located in it the upper part, the lower part of the mold is mounted coaxially in the washing column and fixed in the hole of its cover, the cover is equipped with a fitting for joint output of the mother liquor of the spent and spent washing solutions connected to a water trap, the lower part of the washing column is connected at an angle to the inclined pipe, inside which the screw is installed, the upper part of the inclined pipe is equipped with a fitting for unloading the dried uranium nitrate hexahydrate crystals and a fitting for supplying the washing solution, and a fitting for unloading the dried crystals placed above the nozzle for supplying a washing solution, and the nozzle for supplying a solution of uranyl nitrate, the nozzle for supplying a washing solution and a water trap are placed on one floor level.

In a particular embodiment, the angle between the wash column and the inclined pipe is 35-45 °.

In a particular embodiment, the nozzle for unloading GNU crystals is connected to two alternately working heated receiving and dispensing tanks.

In a particular embodiment, the lower part of the mold is installed in the wash column to a depth equal to 2-3 times the diameter of the mold.

In a particular embodiment, the nozzle for unloading the dried crystals is placed above the nozzle for supplying the washing solution at a distance equal to 3-4 diameters of the inclined pipe.

The invention is illustrated in the drawing, which shows a schematic vertical section of a device.

The device comprises a crystallizer 1 located vertically, with external cooling 2, made in the form of a pipe 3 with a nozzle for supplying a solution of uranyl nitrate 4, located in the upper part. A paddle mixer 5 is installed inside the pipe. The lower part of the mold pipe is placed in the washing column 6 to a depth equal to 2-3 times the diameter of the mold pipe and is fixed in the hole of the cover of the washing column 7 coaxially with the latter. The lid is equipped with a nozzle 8 for discharging the spent washing and mother liquor connected to the hydraulic lock 9. The washing column in the lower part is connected at an angle to the inclined pipe 10, inside which the screw 11 is installed. The upper part of the inclined pipe is equipped with a fitting 12 for unloading the separated GNU crystals and a fitting 13 for supplying a washing solution. The nozzle for unloading crystals separated from the wash solution is placed above the nozzle for supplying the wash solution at a distance equal to 3-4 diameters of the inclined pipe. The nozzle for supplying a washing solution, the nozzle for supplying a solution of uranyl nitrate and a water seal are located at the same level. The fitting for unloading GNU crystals is connected to two alternately working heated receiving and dispensing tanks 14.

The device operates as follows.

After evaporation to a concentration of 800–1300 g / L and nitric acid 0.5–3 mol / L through uranium, the solution continuously enters through the supply nozzle 4 into the pipe 3 of the mold 1 with external cooling 2, in which, with constant stirring, the paddle stirrer 5 is cooled to a temperature of 15-30 ° C.

The resulting GNU crystals along the way with the mother liquor move down and at the exit point from the crystallizer they separate from the mother liquor. The crystals, under the action of gravitational forces, continue to move downward into the coaxial crystallizer flushing column 6 through the hole in its cover 7, passing through which, they are collected in its lower part, from which they are continuously discharged by the screw 11 through the unloading nipple 12 into the heated receiving and dispensing tank 14. Collected crystals in this vessel, after they are heated to a temperature above 60 ° C, they are fed to the next crystallization stage. The washing solution is continuously fed through the upper fitting 13 of the inclined pipe 10 and it moves to the lower part of the washing column in a countercurrent to the crystals rising by the screw. Upon reaching the wash zone, the promo solution rises countercurrently to the falling crystals, then it meets the mother liquor at the outlet of the wash zone, carries the latter along with it (since the mother liquor is less in specific gravity than the wash) and exits through the nozzle 8 of the cover of the wash column and water seal 9 of the process.

The number of nascent GNU crystals in the mold, as well as its grain size, depends on the temperature gradient in the mold and on the residence time of the crystals in the mold. The degree of washing of the GNU crystals depends on their size and their residence time in the wash zone.

Execution examples

Example 1

For studies on the crystallization of uranyl nitrate, a mother liquor of 320 g / l uranium and 3.75 mol / l nitric acid was prepared, which corresponds to a saturated solution at room temperature. GNU crystals were introduced into a part of the mother liquor to a ratio of solid and liquid phases equal to 1.5: 1, and a solution of zirconium nitrate (as an imitator of fission products). After heating the system to 65 ° C, the aqueous solution had the composition: 800 g / l of uranium, 8 g / l of zirconium and 1.7 mol / l of nitric acid. The solution was slowly cooled and the crystalline phase was allowed to stand for 2 hours, after which the mother liquor and crystals were separated by filtration. Wet crystals were washed with stirring with the initial engineered solution with a ratio of T: L = 1.5: 1, the phases after settling were separated by filtration. The crystalline phase contained 1300 g / l of uranium and 0.8 g / l of zirconium. Thus, after a single crystallization and washing, the relative zirconium content decreased by 15-17 times. After the second crystallization-washing cycle, the crystalline phase contained 1300 g / l of uranium and 0.012 g / l of zirconium. The relative content of zirconium decreased by 65 times. After the third crystallization-washing cycle, zirconium is not detected in the uranium product.

Example 2

Tests of the device on a model product — an initial solution of 1000 g / l of uranium, 8.2 g / l of zirconium, 3.35 mol / l of nitric acid, were fed into the cooling tube of the mold with a flow rate of 150-170 ml / h As a washing solution, a solution of 360 g / l of uranium and 4.55 mol / l of nitric acid was used, which was fed into the screw part of the mold with a flow rate of 60-65 ml / h. The unit worked for 9 hours; 1,500 ml of the solution was processed. Coming out washed crystals of GNU contained 1360 g / l of uranium and 0.01 g / l of zirconium. The relative zirconium content decreased by 1100 times, which corresponded to two crystallization-washing cycles in the static mode.

Thus, tests of the model installation on the model product showed that the uranyl nitrate purification process was continuous, the yield of GNU crystals was more than 80% of the uranium content in the initial solution, the uranium reflux was 3-5%, after the third crystallization at the initial weight ratio Zr / U = 10 ^-2 zirconium is not detected in the uranium product; the Zr / U ratio in the mother liquor is less than 10 ^-5 .

Sources of information

1. Japan Patent No. 10167730, 1998-06-23, G 21 C 19/46, C 01 G 43/00.

2. Approach to the extreme safety in a nuclear fuel reprocessing system in mild aqueous solutions. Asanuma N., Asono Y., Tomiyasu H., Mizumachi K., Ikeda Y., Asou M., Hanzawa M. JAERJ-Conf. 1999, No. 99-004 (Pt 1). p.192-201. English

3. US patent No. 4759878, 1988, G 21 C 19/46 (prototype).

4. Japan patent No. 7140294, 1995-06-02, G 21 C 19/46, B 01 D 9/02, C 01 G 43/00 (prototype).

Claims (7)

1. The method of purification of uranyl nitrate from fission products, including crystallization of uranyl nitrate hexahydrate from a solution by cooling it, separating the obtained crystals from the mother liquor, washing and unloading, characterized in that the uranyl nitrate solution is pre-evaporated to a concentration of 800-1300 g on uranium / l and nitric acid 0.5-3.0 mol / l, crystallization is carried out in the direct flow of the formed crystals of uranyl nitrate hexahydrate and the mother liquor with constant stirring and cooling the solution to a temperature of 1 5-30 ° C, the crystals formed are separated from the mother liquor and subjected to countercurrent washing at a temperature of 15-30 ° C with a washing solution containing 250-300 g / l of uranium and 3.0-6.0 mol / l of nitric acid, then the crystals unload and drain from the wash solution, and the spent wash solution together with the mother liquor containing fission products is removed in the contact zone, and the density of the spent wash solution is maintained 2-5% higher than the density of the mother liquor. 2. The method according to claim 1, characterized in that the purification of uranyl nitrate from fission products is carried out in 2-3 stages. 3. A device for cleaning uranium nitrate from fission products, containing a crystallizer made in the form of an external cooling pipe equipped with a uranyl nitrate solution inlet chuck, characterized in that the crystallizer inside which a paddle mixer is installed is located vertically, while the nozzle for supplying the solution uranyl nitrate is located in its upper part, the lower part of the crystallizer is mounted coaxially in the washing column and fixed in the hole of its cover, the cover is equipped with a fitting for joint output uterine and spent washing solutions connected to a water trap, the lower part of the washing column is connected at an angle to the inclined pipe, inside which a screw is installed, the upper part of the inclined pipe is equipped with a nozzle for unloading the dried crystals of uranium nitrate hexahydrate and a nozzle for supplying the washing solution, and a nozzle for unloading the drained crystals placed above the nozzle for supplying a washing solution, and the nozzle for supplying a solution of uranyl nitrate, the nozzle for supplying a washing solution and a water seal at the same level. 4. The device according to claim 3, characterized in that the angle between the wash column and the inclined pipe is 35-45 °. 5. The device according to claim 3, characterized in that the fitting for unloading crystals is connected to two alternately working heated receiving and dispensing containers. 6. The device according to claim 3, characterized in that the lower part of the mold is installed in the wash column to a depth equal to 2-3 diameters of the mold. 7. The device according to claim 3, characterized in that the nozzle for unloading the dried crystals is placed above the nozzle for supplying the washing solution at a distance equal to 3-4 diameters of the inclined pipe.