RU2753358C2 - Method for regeneration of silver from actinoid-containing nitric-acid solution - Google Patents

Abstract

FIELD: radiochemistry.

SUBSTANCE: invention relates to the field of radiochemistry and can be used in the production of MOX fuel in the technology of reprocessing spent nuclear fuel (SNF). Regeneration of silver from an actinide-containing nitric acid solution with a nitric acid concentration of 190-380 g / l is carried out by introducing reducing agents, neutralizing the solution, reducing silver to metal, separating the suspension, washing the precipitate and dissolving it. Carbohydrazide and ascorbic acid are used as reducing agents. The molar ratio of carbohydrazide and ascorbic acid to the silver contained in the solution is (2.2÷6.1):1 and (0.9 - 2.0):1. The reduction is carried out in the process of neutralization at a temperature of 15-25°C with 8-15 mol / l with an alkali metal hydroxide solution, which is dosed at a rate of 2-20 vol.% / min to a residual concentration of nitric acid in the solution of 130-224 g / l. After neutralization, the resulting suspension is stirred for 10-30 minutes, and then sedimentation of the metallic silver precipitate is carried out in the bottom of the precipitator for 60-360 minutes. 90-92% of the solution volume is decanted, then the precipitate is washed and dissolved in nitric acid to obtain a nitric acid solution of silver regenerate.

EFFECT: invention makes it possible to extract silver from actinide-containing solutions with an output of more than 84% of silver into the regenerate, when carrying out in one apparatus the operation of precipitation, decantation of mother liquor and washing solutions, dissolving the precipitate to obtain a nitric acid solution of silver regenerate.

8 cl, 32 tbl, 8 ex, 1 dwg

Description

The invention relates to the field of radiochemistry and can be used in the production technology of uranium-plutonium fuel.

From the state of the art, methods for dissolving plutonium dioxide using silver are known, which involve the addition of silver in the nitric acid solution in the form of salts, and do not determine the methods for the subsequent removal of dissolved silver before the subsequent separation of plutonium from the resulting technological products.

There is a known method of dissolving plutonium dioxide [USSR Author's certificate No. 440879 dated 10.11.78], which includes its treatment with hot nitric acid with a concentration of 4 mol / l in the presence of 0.01-0.02 mol / l of catalyst cations with a high redox potential reoxidized with ozone, including silver (II).

There is a known method of dissolving plutonium dioxide, in which the oxidation of the cation-catalyst with ozone is replaced by anodic oxidation [USSR author's certificate No. 224129 of 07/03/1981].

A known method of electrochemical oxidative dissolution of plutonium dioxide, in which the process is carried out in nitric acid with electrolytically generated silver (II) as a catalyst cation. [G.P. Nikitina, Yu.E. Ivanov, Redox dissolution of dioxides of transuranic and rare-earth elements // Proceedings of the Radium Institute named after V.I. V.G. Khlopin, 2014. State Atomic Energy Corporation "Rosatom", JSC "Radium Institute named after V.G. Khlopin "volume XXVI. S. 61-84]. At a silver concentration of 10-100 mmol / l and a nitric acid concentration of 4 mol / l, complete dissolution of plutonium is achieved. Together with plutonium in much lower concentrations, the decay product of its isotopes, americium, passes into the solution. To prevent sedimentation processes at the extraction stage, it is advisable to separate the silver from the obtained actinide-containing products prior to the extraction operation. With a silver content of 4-12 g / l, plutonium, being a macrocomponent, is present in a concentration exceeding the concentration of silver from 2 to 10 times. When processing defective products produced by MOX fuel, the solution can also contain up to 160 g / l of uranium. Thus, the total concentration of actinides in a silver-containing solution can vary over a wide range and amount to 200 g / L or more. At the same time, the content of nitric acid exceeds 190 g / l, and the technological features of the subsequent extraction processing and the possibility of the formation of polymeric hydrolysis forms of plutonium do not allow its reduction to less than 90 g / l. Taking into account the specifics of actinide-containing products for silver regeneration, it is proposed to use nuclear-safe capacitive equipment with minimization of the number of its units and operations. In this regard, it turns out to be advisable to abandon the use of filtering devices, packed columns, as well as centrifuges, and to use only the methods of decantation of the clarified solution at the stage of separating the precipitate of metallic silver. In addition, in addition to the restrictions on reducing the concentration of nitric acid in the silver-containing solution, as well as to the instrumentation of the reduction process, there is a requirement for the grain size of the precipitate, which determines the need to obtain a coarse-crystalline precipitate that also has chemical resistance.

The known method of selective electrolytic reduction and recycling of silver from nitric acid solutions containing silver in the form of Ag (I) [US 6428679 B1 publ. 08/06/2002], including successive stages of electrodeposition of silver in the form of metal Ag (0), and redissolution of precipitated Ag (0), to obtain Ag (I), in which the specified stage of electrodeposition is carried out at a prescribed current, and the specified successive stages of silver electrodeposition and re-dissolution is carried out in one apparatus. The device is an electrolytic unit with one body, without a separate compartment. The initial solution contains Ag (I) at a concentration of 0.01 to 0.1 mol / l, nitric acid from 0.1 to 6 mol / l and an antinitrite agent. Urea (urea) and oxygenated water are used as antinitrite agent. The solution contains one or more elements from the series: uranium, plutonium, americium, neptunium, curium. In this case, the concentration of each of the radioactive elements is less than or equal to 100 mg / l. The dissolution of silver released at the cathode is carried out with a solution of 4 mol / l nitric acid. The advantage of the method is that the process is carried out in one apparatus, which ends with the production of silver regenerate. The disadvantages of this method are: the need to use complex electrolytic equipment for highly active solutions in a remotely serviced area, significant gas separation during the electrolytic process as a result of oxidation of the antinitrite agent, the use of a washing solution in a volume not less than the full volume of the apparatus, low productivity, taking into account the requirements of nuclear safety. In addition, the given concentrations of actinides demonstrate the efficiency of the process only when the silver content in the solution over the sum of actinides is predominant.

Also known is an electrolytic method for the selective extraction and recycling of silver from a nitric acid solution [RU 2171318 C2, publ. 07/27/2001], patented by the same authors in the Russian Federation, including the electrolytic deposition of silver in the form of metallic silver Ag (0) and the subsequent dissolution of the precipitated silver Ag (0) to obtain an Ag (I) solution, characterized in that the electrolytic deposition is carried out at a given current value, while the electrolytic deposition of silver and the dissolution of the deposited silver are carried out in the same device. The method has the same disadvantages and does not demonstrate efficiency in the regeneration of silver from plutonium-concentrated solutions. As in the previous method, the degree of purification of the regenerated silver from the corrosion products contained in the initial solution is not shown.

Known design of the apparatus and method for dissolving hazardous waste by electrochemical method in the presence of a catalyst [WO 1989010981 A1, publ. 11/16/98]. The device assumes dissolution of plutonium dioxide by processing it with nitric acid solutions in a catalyzed process of electrochemical dissolution, as well as separating silver from the obtained nitric acid solution of plutonium. The device includes: an anode compartment containing an electrically conductive anode; a cathode compartment containing an electrically conductive cathode; a porous barrier separating the anode compartment from the cathode compartment; and a refrigerant for keeping the catholyte at a temperature lower than that of the anolyte while dissolving the waste in the device. The catalyzed process of electrochemical dissolution of plutonium dioxide is carried out in the anode compartment. Waste containing plutonium dioxide, nitric acid and catalyst is fed into the anode compartment. Nitric acid is fed into the cathode compartment. A potential is applied from the DC source to the anode and cathode. In the process of dissolution, both the anode and the cathode are cooled. Silver is used as a catalyst. The method also includes silver recovery. Upon completion of the process of electrochemical dissolution of plutonium, for the reduction of silver from the obtained nitric acid solution, the polarity of the electrodes is changed. To prevent the formation of NO during the separation of silver at the anode (which becomes the cathode after polarity change), an antinitrite agent is added to the solution. Urea or hydrazine is used as the antinitrite agent. The dissolution of plutonium dioxide and the separation of silver are carried out in one apparatus. The disadvantages of this method are: a decrease in the productivity of the apparatus for dissolved plutonium dioxide, since additional time is required for the regeneration of silver, which is carried out in the same apparatus. In addition, contamination of the silver regenerate with corrosion products that are released on the electrode during the reduction process together with silver is possible, as well as their cyclic accumulation during repeated use of the silver regenerate during the dissolution process.

A known method of processing nitrate actinide-containing solution [RU 2657272 C1, publ. 06/09/2018], including the purification of the initial nitric acid actinide-containing solution from silver by reduction in a solution of silver to metal in the form of a precipitate with carbonic acid dihydrazide, separation of the resulting precipitate from the clarified solution, quantitative decomposition of the reducing agent in the clarified solution and sorption separation of actinides from the clarified solution, which differs in that that the recovery of silver from a solution with carbonic acid dihydrazide is carried out at a nitric acid concentration of 0.4-0.7 mol / l and a temperature of 81-92 ° C, while the clarified solution is corrected with nitric acid to a concentration of 6-8 mol / l and heated to temperature 93-98 ° C. It should be noted that the indisputable advantage of the method is the separation of silver from products concentrated in plutonium. The disadvantages of this method are: a significant increase in the volume of the original solution (with a dilution factor of 12-42), caused by the need to first reduce the nitric acid content to 0.4-07 mol / l (by dilution), and then, after separating silver, bring the acid concentration to 6 -8 mol / l, the need to use filtering devices, the introduction of a significant excess of carbonic acid dihydrazide (carbohydrazide).

A known method for the recovery of silver from the products of electrolytic dissolution of plutonium dioxide CEPOD-process [Final report: the extraction of silver from CEPOD ¹ -anolyte ( ¹ CEPOD - electrochemical dissolution of plutonium oxide in the presence of catalysts) D.L. Blanchard, J.E. Surma et al. Pacific Northwest National Laboratory Richland, pc. Washington 99352 (Final Report: Recovery of Silver from CEPOD Anolyte Solutions) Prepared for the US Departmend of Energy under Contract DE-AC06-76RLO]. An antinitrite agent is introduced into the analyte after the CEPOD process with a nitric acid concentration of 4 mol / L and 0.1 mol / L silver, diluted to a nitric acid concentration of 3 mol / L, and stirred with bubbling air for 10 minutes. Further, from a nitric acid solution diluted to a concentration of 2 mol / l, reduction is carried out with ascorbic acid at its concentration in the solution of 0.05 mol / l. Hydrosylamine or sulfamic acid is used as an antinitrite agent at a concentration of 0.05 mol / l in a solution. Stirring is carried out with bubbling air for 60 minutes. The precipitate of metallic silver is separated by filtration. The plutonium-containing solution with a nitric acid concentration of 2 mol / l is transferred for further processing. The method allows the chemical reduction of silver to metal from moderately acidic media. The disadvantages of this method are: the need for more than two-fold dilution of the initial solution to achieve the acidity of the solution, in which the process of silver recovery is possible, the need to use filtering devices due to the fine precipitate of metallic silver, as well as an increase in the volume of washing solutions required for filter regeneration. In addition, there are no results of silver recovery from solutions with a high (more than 50 g / L) total actinide content. At the same time, the declared amount of antinitrite agent (0.05 mol / l) and ascorbic acid (0.05 mol / l) is insufficient in the presence of variable valence elements in the solution, including plutonium in hexavalent form. The method also does not take into account the effect on the antinitrite agent and ascorbic acid of the residual amounts generated in the electrolytic dissolution of plutonium dioxide of other strong oxidants, such as bivalent silver, as well as pentavalent and hexavalent americium.

Closest to the claimed technical solution is a method for extracting silver from actinide-containing solutions (options) [RU 2618874 C1, publ. 05/11/2017], taken as a prototype, including solution neutralization, silver reduction to metal, separation and dissolution of silver in nitric acid. In the first embodiment, the prototype method is characterized in that the silver-containing solution is neutralized with a non-salt-forming neutralizing agent: hydrazine hydrate or aminoacetic acid to pH 0.8-1.0, aminoacetic acid is added as a complexing agent to ensure the pH of the solution is in the range of 1.0- 2.5, carbohydrazide is introduced as a reducing agent, silver is separated into sedimented solid-phase forms in the volume of solution by increasing the temperature of the reaction medium to 50-80 ° C, followed by dissolving the separated silver in nitric acid. In the second version, the prototype method differs in that the silver-containing solution is neutralized with a non-salt-forming neutralizing agent: hydrazine hydrate or aminoacetic acid to pH 0.8-1.0, aminoacetic acid is added as a complexing agent to ensure the pH of the solution is in the range of 1.0- 2.5, add carbohydrazide as a reducing agent, carry out the isolation of grains of metallic silver on the developed surface of the solid phase of the collector by passing a silver-containing solution with carbohydrazide through a granular packing layer in a column-type apparatus at a temperature of 50-80 ° C, followed by dissolving the separated silver in nitric acid ... The disadvantages of this method are: a significant amount of a non-salt-forming agent required for neutralization, a relatively small grain size of the sediment (8-40 microns), the need for "deep" neutralization of the silver-containing solution, the need to heat the reaction volume and use, in addition to decantation, clarification of the solution on the filtering partition (in option 1 ). In addition, the method does not imply the recovery of silver from plutonium-concentrated nitric acid solutions.

The objective of the invention is the regeneration of silver from actinide-containing solutions with a high content of nitric acid by its reduction to obtain a compact precipitate of metallic silver, which provides separation without the use of special devices (filters, packed columns, centrifuges, etc.), and subsequent dissolution in a minimum volume of nitric acid.

The problem is solved by the fact that in the method of regenerating silver from an actinide-containing nitric acid solution, including the introduction of reducing agents, neutralizing the solution, reducing silver to metal, separating the suspension, washing the precipitate and dissolving it, into an actinide-containing nitric acid solution with a nitric acid concentration of 190-380 g / l reducing agents are introduced - carbohydrazide and ascorbic acid, while the molar ratio of carbohydrazide and ascorbic acid to the silver contained in the solution is (2.2 ÷ 6.1): 1 and (0.9 ÷ 2.0): 1, respectively, the reduction is carried out in the process of neutralization at a temperature of 15-25 ° C 8-15 mol / l with an alkali metal hydroxide solution, which is dosed at a rate of 2-20 vol% / min to a residual concentration of nitric acid in the solution of 130-224 g / l, at the end of the process neutralization, the resulting suspension is stirred for 10-30 minutes, and then in the bottom of the precipitation apparatus sedimentation of the precipitate of metallic silver is carried out into for 60-360 minutes, decant 90-92% of the solution volume, then wash and dissolve the precipitate in nitric acid to obtain a nitric acid solution of silver regenerate.

The essence of the invention is to provide an activation effect during neutralization of nitric acid on the formation of crystallization centers in the volume of a silver-containing solution, accompanied by homogeneous nucleation of a solid phase with separation of the main amount of metallic silver on the forming centers of crystallization.

The main factor affecting the start of the solid phase formation process is the achievement in the local volume of conditions sufficient to initiate the reduction process with the participation of ascorbic acid and carbohydrazide. The formation of crystallization centers of metallic silver occurs as a result of a short-term decrease in the concentration of nitric acid and the release of thermal energy in the contact zone (on the "mirror" of the silver-containing solution).

The primary introduction of reducing agents into the silver-containing solution, taking into account the thermodynamic features of the process, transfers the system to a metastable state. At the time of the introduction of reducing agents at a nitric acid concentration of 190-380 g / l and a temperature of less than 25 ° C, carbohydrazide and ascorbic acid do not significantly affect the stability of the solution and do not reduce the ionic forms of silver. The nitric acid contained in the silver-containing solution at a concentration of more than 190 g / l reduces the rate of the direct reaction. The intensity of the reduction process is so low that even 3 hours after the introduction of carbohydrazide and ascorbic acid, no significant change in the turbidity of the solution is observed.

Dosing into the reaction volume of an alkali metal hydroxide solution, regardless of the intensity of stirring, does not lead to an instant distribution of the introduced reagent in the volume of the silver-containing solution, but forms a concentration distribution gradient from the reagent injection point with the maximum concentration value at the injection point. Before complete averaging of the neutralizing solution in the reaction volume, a certain period of time is required, the duration of which depends on the geometric parameters of the apparatus and the mixing method. Constant stirring of the reaction volume over time leads to a gradual alignment of the concentration and temperature profiles of the system. At the same time, the risk of the formation of polymeric forms of plutonium (IV) hydroxide as a result of local changes in the content of nitric acid is minimized due to the fact that plutonium passes into the trivalent state during the primary introduction of reducing agents before neutralization.

The interaction of the introduced alkali with nitric acid present in the silver-containing solution begins directly in the contact zone of the reagents. The area of increased concentration of alkali metal hydroxide arising in the local volume in a short time interval leads to deeper, in comparison with the rest of the solution volume, neutralization and insignificant heating from the exothermic effect of the reaction. The effect exerted by the neutralization of nitric acid is quite sufficient to change the thermodynamic state of the system, which leads to the activation of the reduction process. The duration of the time interval during which concentration changes occur in the local volume can be regulated by the rate of supply of the alkali metal hydroxide solution and the intensity of mixing of the medium using rotary (pulsating, bubbling, or other) devices. To initiate the formation of a solid phase, it is sufficient to reduce the concentration of nitric acid by 60 g / L, which reduces the amount of the added neutralizing agent that could be used if neutralization was carried out before the introduction of reducing agents into the silver-containing solution. In this case, neutralization is carried out to a residual content of nitric acid of at least 130 g / l. However, when the content of nitric acid in the original silver-containing solution is more than 320 g / L, neutralization can be carried out up to a value of 224 g / L, as a result of which the concentration of nitric acid decreases by 96-156 g / L.

Due to the fact that the reducing agents are already in the silver-containing solution, diffuse inhibition has practically no significant effect on the course of the reduction process. Silver recovery occurs at a high rate. The resulting decrease in the potential barrier transfers the system to an equilibrium state and is accompanied by an avalanche-like precipitation of a solid phase of metallic silver. After 4 minutes from the moment of complete introduction of the calculated amount of alkali solution, the residual amount of silver in the solution does not exceed 1.06% of the initial amount, after 12 minutes - 0.21%. At the same time, there are no signs of the "silver mirror" effect in the form of deposits on the structural material of the apparatus. The resulting grains of metallic silver are agglomerates with a predominant size of 100-150 microns. When the suspension settles, the separated solid phase within 35-50 minutes is completely sedimented in the bottom of the apparatus in the form of a dense sediment, amounting to 0.6-1.2% of the initial volume of the silver-containing solution.

In a particular case, additional introduction of aminoacetic acid as a modifier into the reduction system, until its molar excess in relation to silver (1.1 ÷ 2.2): 1 is obtained, slightly reduces the rate of sediment maturation. After 4 minutes from the moment of complete introduction of the calculated amount of the alkali solution, the residual amount of silver in the solution does not exceed 4.30% of the initial amount, after 12 minutes - 1.3%, after 30 minutes - 0.8%. The maintenance of aminoacetic acid is aimed at obtaining a coarse crystalline precipitate. Thus, the introduction of aminoacetic acid at a concentration of 2.0-7.5 g / l allows the predominant grain size to be increased to 150-400 microns. In this case, the duration of complete sedimentation during settling of the suspension is reduced to 15-35 minutes.

A system of two different types of reducing agents makes it possible to use their individual properties, which, due to the variation in the molar ratio of ascorbic acid and carbohydrazide in the range (0.2 ÷ 1.2): 1, makes it possible to obtain a coarse sediment with high particle homogeneity. In this case, the molar ratio to the silver contained in the solution of each of them is (0.9 ÷ 2.0): 1 and (2.2 ÷ 6.1): 1, respectively. Due to the kinetic features, the formation of crystallization centers at the initial moment of time is largely due to the action of carbohydrazide, the sensitivity to the concentration of nitric acid and the temperature factor of which is higher, which is observed at the peak moment of a local decrease in the concentration of nitric acid and a short-term increase in temperature. Further precipitation of metallic silver at crystallization centers, accompanied by an increase in their size, is achieved mainly by the action of ascorbic acid, since the reduction of silver with ascorbic acid proceeds by the mechanism of heterogeneous nucleation and tends to precipitate a solid phase on an already existing surface. This stage of the reduction process is less influenced by the content of nitric acid and at the moment of equalization of its concentration with stirring of the reaction volume proceeds quite actively even at high acidity of the solution in the absence of additional heating. At the same time, the reducing ability of carbohydrazide decreases, but its antinitrite effect remains. At the used concentration of carbohydrazide 15.7-33.5 g / l, the reaction of dissolution of the formed precipitate is effectively suppressed. The introduction of aminoacetic acid, which also has antinitrite properties, makes it possible to reduce the concentration of carbohydrazide to 5.9-6.2 g / l and / or ascorbic acid to 5.9-8.0 g / l, with the same result being achieved.

In a particular case, in order to minimize the amount of introduced reducing agents, hydrazine nitrate is used as an antinitrite agent, which is added to an actinide-containing nitric acid solution before adding reducing agents to a concentration in the reaction volume of 2-15 g / l.

Since the action of carbohydrazide primarily causes the reduction of plutonium to the trivalent state, the amount of carbohydrazide introduced into the silver-containing solution is chosen so that the molar excess of carbohydrazide to plutonium is (2.9 ÷ 3.2): 1. At the same time, for the full conversion of plutonium (from various valence forms) to the trivalent state, a 1: 1 ratio is quite sufficient, which is, on average, one third of the total amount of added carbohydrazide. Two thirds of the total amount of added carbohydrazide ensures the reduction of silver to metal and the guaranteed presence of an antinitrite agent at the end of the process. In order to achieve the completeness of the transfer of plutonium to the trivalent state, as well as to provide an excess of the reducing agent necessary for the homogeneous nucleation of metallic silver, in a particular case, preliminary stabilization of plutonium in the tetravalent state is carried out. For this, before adding the reducing agents and hydrazine nitrate, hydrogen peroxide is added to the silver-containing solution to a concentration of 2-11 g / l, the reaction volume is heated to 60 ° C and kept at this temperature for 30 minutes. In addition, the introduction of hydrazine nitrate, hydrogen peroxide, and aminoacetic acid into the silver-containing solution before reducing agents ensures the recovery of the residual amount of divalent silver present in the solution if the product just obtained after electrolytic dissolution is used for silver regeneration.

The combination of the properties of the components of the reduction system provides directional grain formation and allows the deposited metallic silver to accumulate in the bottom of the apparatus in the form of a dense layer. The clarified solution is decanted. The properties of the sediment and the conical shape of the bottom of the apparatus make it possible to remove 90-92% of the clarified solution by decantation. In the process of washing, the sediment does not change the granulometric characteristics, and in the same way is separated from the washing solution. Thus, the combined use of the properties of reagents with reducing and antinitrite properties makes it possible to isolate silver from solutions with moderate acidity (130-224 g / l) without the risk of dissolving the formed precipitate. The resulting precipitate is dissolved in nitric acid 250-441 g / l at a temperature of 50-90 ° C.

The technical result of the invention is the extraction of more than 84% of silver into the regenerate from actinide-containing solutions with a concentration of nitric acid in the initial solution of up to 380 g / l, which makes it possible to carry out precipitation, decantation of the mother liquor and washing solutions, dissolution of the precipitate in one apparatus, to obtain a nitric acid solution of silver regenerate ...

To achieve the technical result in the method of silver regeneration from actinide-containing nitric acid solutions, reducing agents - carbohydrazide and ascorbic acid are introduced into an actinide-containing nitric acid solution with a nitric acid concentration of 190-380 g / l, while the molar ratio of carbohydrazide and ascorbic acid to that contained in 2.2 ÷ 6.1): 1 and (0.9 ÷ 2.0): 1, respectively, the reduction is carried out in the process of neutralization at a temperature of 15-25 ° C 8-15 mol / l with an alkali metal hydroxide solution, which is dosed with at a rate of 2-20 vol.% / min until a residual concentration of nitric acid in a solution of 130-224 g / l, at the end of the neutralization process, the resulting suspension is stirred for 10-30 minutes, and then sediment of metallic silver is carried out in the bottom of the precipitator apparatus within 60-360 minutes, decant 90-92% of the solution volume, then wash and dissolve the precipitate in nitric acid to obtain a nitric acid solution regenerated silver.

The presented range of concentration of nitric acid in the initial actinide-containing nitric acid solution of 190-380 g / l is determined by the conditions of the products formed as a result of electrochemical processes using silver (II). Exceeding the limit of the range turns out to be difficult, since during the electrochemical dissolution of plutonium dioxide, the use of more concentrated nitric acid has a significant effect on the stability of structural materials. The content of nitric acid in actinide-containing solution less than 190 g / l is ineffective for electrochemical dissolution using silver (II).

The presented range of residual concentrations of nitric acid in an actinide-containing nitric acid solution after neutralization of 130-224 g / l is due, on the one hand, to the requirements for the content of nitric acid in the subsequent extraction operation of actinides, on the other hand, by the intensity of the neutralization process with the introduction of an alkali metal hydroxide, sufficient to activate recovery process and achievement of completeness of silver separation. Exceeding the limit of the claimed range is undesirable due to the fact that the claimed completeness of silver separation is not ensured, and the resulting precipitate is small crystals that are prone to agitation. Neutralization of the silver-containing solution to a nitric acid concentration of less than 130 g / l, that is, below the range limit, leads to an unreasonably high consumption of the neutralizing agent and the total volume of the solution after neutralization. In this regard, it is preferable to neutralize to a residual concentration of nitric acid of 150-230 g / l, which is as close as possible to the upper limit of the range.

The presented range of the volume of the solution decanted after silver precipitation is at the level of 90-92% due to the need to quantitatively separate the formed solid phase from the solution. When the range limit is exceeded, the solid phase is captured and carried away with the decanted solution, which, in turn, will require additional clarification of the product before the extraction of actinides. Changing the amount of decanted solution to less than 90% will require more than a twofold increase in the number of washing cycles, which, after combining the main product stream with the washing solution, will lead to a significant increase in the total volume of the solution.

The presented range of the molar ratio of the amount of added carbohydrazide to the silver contained in the product at the level of (2.2 ÷ 6.1): 1 allows initiating the formation of a sufficient number of crystallization centers and obtaining uniform grains. The use of carbohydrazide above the upper limit of the stated range leads to an increase in its complexing properties, a decrease in the proportion of precipitation due to the partial dissolution of the metallic silver precipitate. The use of carbohydrazide below the limit of the stated range leads to the reduction of silver to a greater extent under the influence of ascorbic acid, the appearance of the "silver mirror" effect, expressed in the separation of silver on the walls of the apparatus, and a decrease in the grain size of the precipitate.

The presented range of the molar ratio of the amount of introduced ascorbic acid to the silver contained in the product at the level of (1.1 ÷ 2.0): 1 provides the required completeness of silver separation. The use of ascorbic acid above the limit of the stated range leads to the appearance of the "silver mirror" effect, the deposition of metallic silver on the walls of the apparatus, including in the upper part of the apparatus, where subsequent processing with less than half the volume of nitric acid does not allow its complete dissolution. In this case, there is the formation of backlays of metallic silver in the apparatus and a decrease in its output to the regenerate. The use of ascorbic acid below the limit of the stated range reduces the yield of silver in the regenerate to a level of less than 50% due to the lack of completeness of the reduction process.

The presented range of the molar ratio of the amount of added glycine (aminoacetic acid) to the silver contained in the product at the level of (1.1 ÷ 2.2): 1 provides an increase in the grain size of the precipitate and its uniformity. The use of glycine (aminoacetic acid) above the limit of the stated range can lead to a decrease in the proportion of precipitation due to the enhancement of its complexing properties and partial dissolution of the metallic silver precipitate. The use of glycine below the limit of the stated range does not lead to the desired result from its introduction. Changes in the grain size of the sediment and the rate of its sedimentation does not occur in comparison with if glycine was not introduced into the silver-containing solution. In this case, the introduction of glycine is ineffective.

The presented temperature range at which the actinide-containing nitric acid solution is neutralized, 15-25 ° C, provides the required completeness of silver separation and the achievement of the technical result. When the range limit is exceeded, a partial dissolution of the precipitated silver is observed and a decrease in its yield to the regenerate to a level of less than 80%. When neutralization is carried out at a temperature below 15 ° C, due to the kinetic features of the process, the recovery rate and the completeness of silver release during the neutralization of the silver-containing solution decrease, which leads to a decrease in the yield of silver into the regenerate to a level of less than 75%.

The presented concentration range of the alkali metal hydroxide solution used for neutralization of 8-15 mol / l (in the particular case of a sodium hydroxide solution of 320-600 g / l) makes it possible to minimize the amount of neutralizing agent introduced into the reaction volume. When the range limit is exceeded, excessive neutralization of nitric acid occurs in the contact zone with an actinide-containing nitric acid solution, as a result of which the risk of initiating the irreversible formation of polymer forms of plutonium present in the solution and its coprecipitation with a precipitate of metallic silver increases. In this case, it is possible that some of the plutonium may enter the recycled silver, deplete the target component of the product entering the extraction processing, as well as increase the risk of plutonium accumulation in the form of sediment in the cone of the vessel, which makes the requirements for ensuring nuclear safety more stringent. With a decrease in the concentration of alkali metal hydroxide less than 8 mol / l, a negative factor is an increase in the volume of the solution after neutralization and, as a consequence, an increase in the load on the subsequent extraction processing.

The presented range of dosing rate of alkali metal hydroxide solution 2-20 vol.% / Min provides concentration changes in the local volume in the contact zone (on the “mirror” of the silver-containing solution) without the risk of actinides precipitating into the solid phase. An increase in the rate of entry of the neutralizing flow into the reaction volume by more than 20 vol% / min, similar to an increase in the concentration of alkali metal hydroxide, leads to the initiation of the formation of polymeric forms of plutonium hydroxide present in the actinide-containing nitric acid solution and their coprecipitation with a precipitate of metallic silver. A decrease in the rate of entry of the neutralizing flow into the reaction volume of less than 2 vol.% / Min does not allow to precipitate more than 82% of the silver contained in the solution due to insufficient neutralization intensity in the contact zone (on the “mirror” of the silver-containing solution).

In the selected range of the duration of the precipitate formation within 10-30 minutes, the required completeness of silver separation and the formation of grains with a high degree of homogeneity are ensured. Reducing the holding time to less than 10 minutes increases the amount of fines and adversely affects the sedimentation rate. An increase in the holding time for more than 30 minutes does not affect the structure of the sediment and is impractical due to a decrease in the productivity of the technological scheme. The duration of the deposition of metallic silver in the presented range of 60-360 minutes ensures complete clarification of the solution, and the formation of a dense layer of metallic silver sediment in the bottom of the precipitator apparatus. If the settling time is reduced to less than 60 minutes, there is a risk of incomplete sedimentation of the particles and entrapment of the sediment with the solution separated during decantation. An increase in the settling time of more than 360 minutes is impractical due to a decrease in the productivity of the technological scheme.

In the selected concentration range of hydrazine nitrate 2-15 g / l, introduced into the reaction volume as an antinitrite agent, the exclusion of the action of nitrous acid is achieved. At a concentration of less than 2 g / L, the possibility of spontaneous dissolution of the metallic silver precipitate is comparable to that if hydrazine nitrate was not introduced. Exceeding the range limit leads to an increased consumption of the reagent with the already achieved effect from the introduction of a smaller amount.

In the selected range of hydrogen peroxide concentration of 2-11 g / L, which is added before the introduction of reducing agents, complete stabilization of plutonium in the tetravalent state is ensured at its concentration up to 60 g / L. Residual hydrogen peroxide is removed by heating the reaction volume to 60 ° C for 30 minutes. With a decrease in the concentration of hydrogen peroxide less than 2 g / l, the process does not proceed completely. An increase in the concentration of hydrogen peroxide more than 11 g / L increases the risk of the formation of peroxide precipitates and the introduction of sparingly soluble forms of plutonium into the precipitate of metallic silver. In addition, the introduction of an excess amount of hydrogen peroxide requires an increase in the duration of the decomposition of its excess, which will be accompanied by intense gas separation.

In the selected concentration range of nitric acid used to dissolve the separated and washed precipitate of metallic silver, 250-441 g / l, the process is carried out at a temperature of 50-95 ° C. The volume of nitric acid is 20-50% of the volume of the actinide-containing nitric acid solution (at the time of the end of neutralization). This technique makes it possible to obtain a solution of silver in the form of a nitrate salt in an equal or less volume of solution than the volume of the initial product taken into the apparatus. Silver regenerate is suitable for reuse in electrolytic dissolution processes without additional adjustment of the solution, or with some dilution. When the upper limit of the range is exceeded, the content of nitric acid in the regenerate turns out to be excessive relative to the proportion to silver, which causes difficulties in preparing a working solution for electrolytic dissolution of plutonium dioxide. A decrease in the concentration of nitric acid to less than 250 g / L upon dissolving a precipitate of metallic silver leads to a solution that requires further strengthening with nitric acid in order to obtain a product suitable for reuse for the electrolytic dissolution of plutonium dioxide (defective products of MOX fuel production).

The proposed method for the regeneration of silver from an actinide-containing nitric acid solution is implemented in accordance with the block diagram shown in the figure, in the following sequence. In the original actinide-containing nitric acid solution with a silver concentration of 4.1-12.5 g / l and 190-380 g / l of nitric acid with controlled mixing intensity, add carbohydrazide, and then ascorbic acid until their concentration in the solution is 1.1-33, 5 g / l and 4.1-19.4 g / l, respectively.

In a particular case, before the introduction of carbohydrazide and ascorbic acid, a solution of aminoacetic acid is added until its concentration in the reaction volume is 2-9 g / L

In a particular case, before the introduction of carbohydrazide and ascorbic acid, a solution of hydrazine nitrate is added until its concentration in the reaction volume is 2-15 g / l.

In a particular case, before the introduction of carbohydrazide and ascorbic acid, as well as hydrazine nitrate, a solution of hydrogen peroxide is added to the silver-containing solution until its concentration in the reaction volume is 2-11 g / l, the reaction volume is heated to 60 ° C and kept at this temperature for 30 minutes ... Next, an 8-15 mol / l alkali metal hydroxide solution is dosed into the reaction volume in a continuous flow at a rate of 2-20 vol.% / Min into the solution, into which reducing and antinitrite agents are added, in the interval of 5-60 minutes.

In a particular case, a sodium hydroxide solution with a concentration of 320-600 g / l is used. The neutralization process is carried out at a temperature of 15-25 ° C, using, in a particular case, forced cooling through an external jacket.

Upon completion of neutralization, the suspension is stirred for 10-30 minutes, and then, for 60-360 minutes, a precipitate of metallic silver is sedimented in the bottom of the precipitator, the clarified solution is decanted. The formed precipitate of metallic silver is washed once with a portion of water in the amount of 15-20% of the volume of the actinide-containing nitric acid solution (at the time of the end of neutralization). The suspension is defended in the same way for 60-360 minutes and the same volume of the clarified washing solution is decanted.

A solution of 250-441 g / l of nitric acid is added to the separated and washed precipitate of metallic silver in an amount of 20-50% of the volume of the actinide-containing nitric acid solution (at the time of the end of neutralization). Dissolution is carried out at a temperature of 50-95 ° C and constant stirring. The resulting solution (silver regenerate) with a concentration of silver nitrate 6.9-9.3 g / l is returned to the technological scheme for dissolving plutonium dioxide. The decantate is combined with the wash solution. The resulting combined solution, after preliminary stabilization of the valence form of plutonium, is sent to the extraction processing.

Example 1

During the experiment, we used a nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel, containing silver - 8.5 g / l, plutonium - 37.1 g / l, americium - 10 mg / l, uranium - 158 g / l, nitrogen acid 364 g / l.

Silver regeneration was carried out using a precipitating reactor made of quartz glass in the form of a one-piece structure. The precipitating reactor was a vertical cylindrical apparatus with a conical bottom having an apex angle of 60 °. The ratio of diameter to height is 1: 2 (at a height of 100 mm). The working volume was 111 ml. The introduction of reagents was carried out with a peristaltic pump at a rate of 0.5 ml / min, and the decantation of products was carried out at a rate of 2.5 ml / min. The inner diameter of the communications was 1.1 mm. The apparatus is equipped with an overhead rotary stirrer through a sealed coupling. The stirrer rotation speed was varied in the range of 250-400 rpm. The volume of the thickened suspension (sediment) after removal of the clarified solution by decantation was in the range of 8-12 ml. Complete emptying of the apparatus was carried out from the bottom level, directly adjacent to the top of the cone of the bottom part of the apparatus. The unreported volume was 0.2-0.4 ml. Gaseous products were removed through a blow off pipe in the upper part of the apparatus. The processes of sediment formation, sedimentation, decantation, and washing were observed through the transparent wall of the apparatus.

In the reactor precipitator received 70 ml of nitric acid solution from electrolytic dissolution of defective products produced by MOX fuel (obtained 120 hours after dissolution). 28 ml of a solution of carbohydrazide with a concentration of 108 g / l (at a concentration of nitric acid of 31.5 g / l) and 8.7 ml of a 200 g / l solution of ascorbic acid were introduced into the apparatus. With constant stirring, the exposure was carried out for 10 minutes and then 3.4 ml of 325 g / L sodium hydroxide solution was introduced into the mirror at a rate of 0.3 ml / min to a residual concentration of nitric acid of 223 g / L. Neutralization was carried out at a temperature of 15 ° C and constant thermostating of the reaction volume. The total volume of the solution after neutralization was 110 ml. The concentrations of the components of the solution after neutralization, and the molar ratio between the reagents are shown in tables 1, 2. The suspension was stirred for 20 minutes and then settled for 120 minutes. The clarified solution (decantate) in the amount of 99 ml was decanted. Within 20 minutes, the precipitate of metallic silver was washed with 20 ml of water while stirring with a rotary stirrer at a speed of 250 rpm, the clarified wash solution was defended and decanted similarly to the silver reduction process. The washing solution in the amount of 20 ml was connected to the decanter and passed to the subsequent extraction processing. The apparatus received 50 ml of a 265 g / l solution of nitric acid, and at a temperature of 75 ° C, the formed precipitate of metallic silver was dissolved. The resulting solution with a silver concentration of 9.2 g / L was returned to the laboratory electrochemical cell for use in the next cycle of dissolution of defective products from the production of MOX fuel. To determine the completeness of the transfer of silver with the regenerate and the absence of silver plugs in the precipitator reactor, a control washing of the apparatus was carried out at a temperature of 95 ° C with a nitric acid solution of 440 g / L, taken in a volume of 100% of the volume of the silver-containing solution (at the time of the end of neutralization). The distribution of silver by technological products is shown in Table 3.

94.8% of silver was transferred to the regenerate. As a result of processing from 70 ml of actinide-containing solution, 61 ml of regenerated silver was obtained. The multiplicity of the increase in the volume of the solution according to the results of processing was 2.6. The distribution of components in processed products is shown in Table 4.

Example 2

When carrying out the experiment, 64 ml of a nitric acid solution from electrolytic dissolution of defective products produced by MOX ^- fuel was used in the same way as in Example 1. The experimental conditions were the same as in Example 1, with the exception of the feed rate and the amount of sodium hydroxide solution introduced. 8.5 ml of a 325 g / l sodium hydroxide solution was injected at a rate of 1.7 ml / min until a residual nitric acid concentration of 187 g / l was obtained. Neutralization was carried out at a temperature of 25 ° C and constant thermostating of the reaction volume. The description of the conditions and the results of the experiment are shown in tables 5, 6, 7, 8.

95.4% of silver was transferred to the regenerate. As a result of processing from 64 ml of actinide-containing solution, 56 ml of regenerated silver was obtained. The multiplicity of the increase in the volume of the solution according to the results of processing was 2.7. In example 2, in comparison with example 1, the efficiency of silver regeneration is shown, regardless of the rate of entry into the reaction volume of the neutralizing agent and the "depth" of neutralization (within the stated range).

Example 3

During the experiment, 63 ml of a nitric acid solution was used from electrolytic dissolution of defective products produced by MOX fuel in the same way as in example 1. The experimental conditions were the same as in example 1, with the exception of the feed rate and the amount of sodium hydroxide solution introduced. 12.8 ml of 325 g / L sodium hydroxide solution was introduced at a rate of 0.26 ml / min until a residual concentration of nitric acid of 158 g / L was obtained. The description of the conditions and the results of the experiment are shown in tables 9, 10, 11, 12.

95.1% of silver was transferred to the regenerate. As a result of processing from 63 ml of actinide-containing solution, 63 ml of silver regenerate was obtained. The multiplicity of the increase in the volume of the solution according to the results of processing was 2.8. The output of silver in the regenerate of more than 94.0% is achieved both by neutralization to a residual concentration of 2.5 mol / l (example 3) and to a residual concentration of 3.5 mol / l (example 1).

Example 4

During the experiment, 65.5 ml of a nitric acid solution was used from the electrolytic dissolution of defective products produced by MOX fuel in the same way as in Example 1. The experimental conditions were the same as in Example 1, with the exception of the additional introduction of an aminoacetic acid solution before neutralization, a decrease in the amount of ascorbic acid, the duration of stirring and settling of the resulting suspension. ... 5.4 ml of 150 g / L aminoacetic acid solution, 24.4 ml of carbohydrazide solution with a concentration of 108 g / L (at a nitric acid concentration of 31.5 g / L) and 4.3 ml of 200 g / l solution of ascorbic acid. With constant stirring, the exposure was carried out for 5 minutes and then 8.4 ml of 325 g / l sodium hydroxide solution was introduced into the mirror at a rate of 0.5 ml / min to a residual concentration of nitric acid of 187.9 g / l. The total volume of the solution after neutralization was 108 ml. The suspension was stirred for 10 minutes and then allowed to stand for 60 minutes. The conditions and results of the experiment are shown in tables 13, 14, 15, 16.

94.1% of silver was transferred to the regenerate. As a result of processing from 65.5 ml of actinide-containing solution, 65 ml of regenerated silver was obtained. The multiplicity of the increase in the volume of the solution according to the results of processing was 2.8.

Example 5

During the experiment, a nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel was used, containing silver - 4.0 g / l, plutonium - 6.2 g / l, americium - 2.3 mg / l, nitric acid 190 g / l ... The apparatus was used analogously to example 1.

90 ml of a nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel (obtained 96 hours after dissolution) was taken into the precipitator reactor and was taken into the precipitator reactor. The apparatus was filled with 0.5 ml of 455 g / L hydrazine nitrate solution and 1.8 ml of 150 g / L aminoacetic acid solution, 6 ml of carbohydrazide solution with a concentration of 108 g / L (at a nitric acid concentration of 31.5 g / L) and 3.1 ml of 200 g / l ascorbic acid solution. With constant stirring, the exposure was carried out for 30 minutes and then 3.6 ml of 600 g / L sodium hydroxide solution was introduced into the mirror at a rate of 0.3 ml / min to a residual concentration of nitric acid of 131 g / L. Neutralization was carried out at a temperature of 20 ° C and constant thermostating of the reaction volume. The total volume of the solution after neutralization was 105 ml. The concentrations of the components of the solution after neutralization and the molar ratio between the reagents are shown in Tables 17, 18. The suspension was stirred for 10 minutes and then settled for 60 minutes. The clarified solution (decantate) in an amount of 94 ml was decanted. For 20 minutes, the precipitate of metallic silver was washed with 18 ml of water while stirring with a rotary stirrer at a speed of 400 rpm, the clarified wash solution was defended and decanted similarly to the silver reduction process. The washing solution in the amount of 18 ml was connected to the decanter and transferred to the subsequent extraction processing. The apparatus received 40 ml of a 265 g / l solution of nitric acid, and at a temperature of 95 ° C, the formed precipitate of metallic silver was dissolved. The resulting solution with a silver concentration of 6.9 g / L was returned to the laboratory electrochemical cell for use in the next cycle of dissolution of rejected products from the production of MOX fuel. The conditions and results of the experiment are shown in tables 17, 18, 19, 20.

97.6% of silver was transferred to the regenerate. As a result of processing, 51 ml of regenerated silver was obtained from 90 ml of an actinide-containing solution. The multiplicity of the increase in the volume of the solution according to the results of processing was 1.8.

Example 5 shows the possibility of reducing the amount of reducing agents (carbohydrazide and ascorbic acid) introduced in the case of preliminary introduction of hydrazine nitrate in combination with aminoacetic acid and subsequent neutralization with a more concentrated sodium hydroxide solution (600 g / l). The use of a more concentrated alkali solution makes it possible to reduce the total volume of products obtained as a result of silver regeneration.

Example 6

When carrying out the experiment similarly to example 1, a nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel was used, containing silver - 8.5 g / l, plutonium - 37.1 g / l, americium - 10 mg / l, uranium - 158 g / l, nitric acid 364 g / l. Silver was regenerated using a precipitator reactor made of steel 112Kh18N10T (stainless). The precipitator reactor was a vertical cylindrical apparatus with a conical bottom, having an apex angle of 90 °, and also equipped with a split cover and an outer jacket. The ratio of diameter to height is 1: 2 (at a height of 100 mm). Working volume - 115 ml. The introduction of reagents was carried out with a peristaltic pump at a rate of 0.5 ml / min, and the decantation of products was carried out at a rate of 2.5 ml / min. The inner diameter of the communications was 0.89 mm. The device is equipped with a bubble line, which is a steel tube 10 × 2 mm with a perforated polymer tip. The zone of dispersion of the gas flow is located at a distance of 15 mm from the top of the cone of the bottom part of the apparatus. The flow rate of bubbling air was varied in the range of 40-60 ml / min. The products were dispensed by decanting the solution through a branch pipe located at a distance of 20 mm from the top of the cone of the bottom part of the apparatus. For this reason, the volume of the thickened suspension (sediment) after removal of the clarified solution by decantation was in the range of 8-12 ml. Complete emptying of the apparatus was carried out from the bottom level, directly adjacent to the top of the cone of the bottom part of the apparatus. The non-dispensed volume was 0.7-0.9 ml. Gaseous products were removed through a blow off branch pipe in the lid of the apparatus. The introduction of the reagents was carried out through the same communication as the dispensing of the products. The conjugation of the fluoroplastic cover and along the groove in the wall of the body of the apparatus ensured the tightness of the apparatus up to an overpressure of 0.7 kg / cm ² . The temperature regime in the reaction volume was maintained at a level of 20 ± 0.5 ° C by circulating the heat carrier from the thermostat through the outer jacket.

In the reactor precipitator received 70 ml of nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel (obtained 20 hours after dissolution). The apparatus was sequentially introduced into 26.9 ml of a carbohydrazide solution with a concentration of 108 g / l (at a nitric acid concentration of 31.5 g / l), 9 ml of a 200 g / l solution of ascorbic acid. With constant stirring, the exposure was carried out for 10 minutes and then 9 ml of 325 g / L sodium hydroxide solution was injected "onto the mirror" at a rate of 0.5 ml / min to a residual concentration of nitric acid of 189 g / L. Neutralization was carried out at a temperature of 18 ° C and constant thermostating of the reaction volume. The total volume of the solution after neutralization was 115 ml. The concentrations of the components of the solution after neutralization and the molar ratio between the reagents are shown in tables 21, 22. The suspension was stirred for 10 minutes and then settled for 60 minutes. The clarified solution in the amount of 103 ml was decanted. Within 10 minutes, the precipitate of metallic silver was washed with 23 ml of water while stirring with bubbling air, the clarified washing solution was defended and decanted similarly to the silver reduction process. The wash solution in the amount of 23 ml was connected to the decanter and transferred to the subsequent static extraction of the target components. The apparatus received 57 ml of a solution of 265 g / l of nitric acid, and at a temperature of 75 ° C, the formed precipitate of metallic silver was dissolved. The resulting solution with a silver concentration of 7.3 g / L was returned to the laboratory electrochemical cell for use in the next cycle of dissolution of defective products from the production of MOX fuel. The distribution of silver by technological products is shown in Table 23.

84% of silver was transferred to the regenerate. As a result of processing from 70 ml of actinide-containing solution, an equivalent volume of 69 ml of silver regenerate was obtained. The content of silver in the regenerate up to 3.8% of actinides (of their initial content) does not limit the reuse of the solution containing the regenerated silver. The multiplicity of the increase in the volume of the solution according to the results of processing was 2.5. The distribution of components in processed products is shown in Table 24.

Example 7

During the experiments, a nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel was used, containing silver 7.7 g / l, plutonium - 22.0 g / l, americium - 5.9 mg / l, uranium - 65 g / l, nitric acid 323 g / l. Silver regeneration was carried out using a precipitator reactor similar to example 6.

In the precipitator reactor was taken 70 ml of nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel (obtained 24 hours after dissolution) was taken into the precipitator reactor. The apparatus received successively 3.5 ml of 455 g / l of hydrazine nitrate solution until a concentration in the reaction volume of 14.3 g / l, 16.2 ml of a carbohydrazide solution with a concentration of 108 g / l (at a nitric acid concentration of 31.5 g / l) was obtained. l), 9 ml of 200 g / l ascorbic acid solution. With constant stirring, the exposure was carried out for 3 minutes and then 11.4 ml of 325 g / l sodium hydroxide solution was introduced into the mirror at a rate of 1.0 ml / min to a residual concentration of nitric acid of 157 g / l. Neutralization was carried out at a temperature of 25 ° C and constant thermostating of the reaction volume. The concentration of the components of the solution after neutralization and the molar ratio between the reagents are shown in tables 25, 26. The total volume of the solution was 110 ml. The suspension was stirred for 30 minutes and then allowed to stand for 80 minutes. The clarified solution in the amount of 99 ml was decanted. Within 15 minutes, the precipitate of metallic silver was washed with 20 ml of water while stirring with bubbling air, the clarified wash solution was defended and decanted as in the process of silver reduction. A wash solution in an amount of 20 ml was connected to the decanate and, after the plutonium was stabilized in the tetravalent state, was transferred to the subsequent extraction operation of the target components. The apparatus received 50 ml of a solution of 441 g / l of nitric acid, and at a temperature of 50 ° C, the formed precipitate of metallic silver was dissolved. The resulting solution with a silver concentration of 7.6 g / L was returned to the laboratory electrochemical cell for use in the next cycle of dissolution of rejected products from the production of MOX fuel. The distribution of silver by technological products is shown in Table 27.

85.2% of silver was transferred to the regenerate. As a result of processing from 70 ml of an actinide-containing solution, a regenerated silver in a volume of 61 ml was obtained. The multiplicity of the increase in the volume of the solution according to the results of processing was 2.6. The distribution of components in processed products is shown in Table 28.

Example 8

During the experiments, we used a nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel, containing silver - 7.5 g / l, plutonium - 30.0 g / l, americium - 8.1 mg / l, uranium - 72 g / l , nitric acid 290 g / l. Regeneration of silver was carried out using a precipitator reactor similar to example 6, with the exception of the stirring method. The solution in the reaction volume was stirred throughout the entire process using a rotary stirrer at a speed of 350 rpm.

56 ml of nitric acid solution from the electrolytic dissolution of defective products produced by MOX fuel (obtained 5 hours after dissolution) was taken into the precipitator reactor and was taken into the precipitator reactor. The apparatus received 1.5 ml of a 30% hydrogen peroxide solution, the solution was kept for 20 minutes, then heated to 60 ° C and kept at this temperature for 30 minutes. The solution was cooled to 30 ° C at a rate of 0.5 ° C / min, and 2 ml of a 455 g / L solution of hydrazine nitrate, 17.5 ml of a carbohydrazide solution with a concentration of 108 g / L (at a nitric acid concentration of 31.5 g / l). The contents of the apparatus were cooled to 18 ° C at a rate of 0.2 ° C / min and 7 ml of a 200 g / L ascorbic acid solution were introduced. With constant stirring, the exposure was carried out for 20 minutes and then 5.8 ml of 325 g / l sodium hydroxide solution was introduced into the mirror at a rate of 0.3 ml / min to a nitric acid concentration of 154 g / l. Neutralization was carried out at a temperature of 18 ° C and constant thermostating of the reaction volume. The concentrations of the components of the solution after neutralization and the molar ratio between the reagents are shown in Tables 29, 30. The total volume of the solution was 90 ml.

The suspension was stirred for 30 minutes and then allowed to stand for 360 minutes. The clarified solution in the amount of 81 ml was decanted. Within 30 minutes, the precipitate of metallic silver was washed with 15 ml of water while stirring with bubbling air, the clarified wash solution was defended and decanted similarly to the silver reduction process. The washing solution in the amount of 15 ml was connected to the decanate and, after the plutonium was stabilized in the tetravalent state, was transferred to the subsequent static extraction of the target components. The apparatus received 40 ml of a solution of 441 g / l of nitric acid, and at a temperature of 50 ° C, the formed precipitate of metallic silver was dissolved. The resulting solution with a silver concentration of 8.0 g / L was returned to the laboratory electrochemical cell for use in the next cycle of dissolution of defective products from the production of MOX fuel. The distribution of silver by technological products is shown in Table 31.

93.2% of silver was transferred to the regenerate. As a result of processing from 56 ml of actinide-containing solution, a regenerated silver in a volume of 49 ml was obtained. The multiplicity of the increase in the volume of the solution according to the results of processing was 2.6. The distribution of components in processed products is shown in Table 32.

Example 8 demonstrates the possibility of silver recovery from plutonium dioxide freshly obtained after electrochemical dissolution (rejected product from the production of MOX fuel). The silver backfill obtained in the apparatus can be reduced to a level of 1.2-1.4% by increasing the temperature from 50 ° C to 95 ° C when it is dissolved with nitric acid and increasing the duration of the process to 60 minutes. The minimum amount of silver in the apparatus was estimated as 0.4-1.0%, which is largely determined by the presence of stagnant zones in the apparatus, and not by deposits on the walls of the apparatus.

Claims (8)

1. A method for the regeneration of silver from an actinide-containing nitric acid solution, including the introduction of reducing agents, neutralizing the solution, reducing silver to metal, separating the suspension, washing the precipitate and dissolving it, characterized in that the actinide-containing nitric acid solution with a nitric acid concentration of 190-380 g / l reducing agents are introduced - carbohydrazide and ascorbic acid, while the molar ratio of carbohydrazide and ascorbic acid to the silver contained in the solution is (2.2 ÷ 6.1): 1 and (0.9 ÷ 2.0): 1, respectively, the reduction is carried out in the process of neutralization at a temperature of 15-25 ° C 8-15 mol / l with an alkali metal hydroxide solution, which is dosed at a rate of 2-20 vol% / min to a residual concentration of nitric acid in the solution of 130-224 g / l, at the end of the process neutralization, the resulting suspension is stirred for 10-30 minutes, and then in the bottom of the precipitation apparatus sedimentation of the precipitate of metallic silver is carried out in flow 60-360 minutes, decant 90-92% of the solution volume, then wash and dissolve the precipitate in nitric acid to obtain a nitric acid solution of silver regenerate. 2. A method according to claim 1, characterized in that the actinide-containing nitric acid solution contains plutonium with a concentration of 6.2-63 g / l. 3. A method according to claim 1 or 2, characterized in that the molar ratio of carbohydrazide to plutonium contained in the solution is (2.9 ÷ 3.1): 1. 4. The method according to claim 1, characterized in that, prior to introducing the reducing agents, glycine is added to the actinide-containing nitric acid solution to obtain a molar excess with respect to silver (1.1 ÷ 2.2): 1. 5. The method according to claim 1, characterized in that before introducing the reducing agents into the actinide-containing nitric acid solution, hydrazine nitrate is added at a concentration of 2-15 g / l. 6. The method according to claim 1 or 6, characterized in that before adding the reducing agents and hydrazine nitrate, hydrogen peroxide is added to the silver-containing solution at a concentration of 2-11 g / l, the reaction volume is heated to 60 ° C and maintained at this temperature for 30 minutes. 7. The method according to claim 1, characterized in that the metallic silver precipitate is washed once with a portion of water in the amount of 15-20% of the volume of the silver-containing solution at the end of neutralization, followed by settling the suspension and decanting the same volume of the clarified washing solution. 8. The method according to claim 1, characterized in that the precipitate of metallic silver is treated for 30-60 minutes with a portion of 250-441 g / l of nitric acid solution taken in an amount of 20-50% of the volume of the silver-containing solution at the time of the end of neutralization, when temperature 50-95 ° C.

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