RU2195518C1 - Method of separation of palladium from nitrate solutions (versions) - Google Patents

Abstract

FIELD: separation of palladium on flow- through cathode made from carbon fiber material from aqueous solutions, liquid radioactive waste inclusive. SUBSTANCE: process of separation and cleaning of palladium (Pd) from admixtures, radioactive fission products includes stages of electrochemical deposition of palladium on cathode, washing cathode sediment under voltage and dissolving sediment in strong nitrous acid. Addition of carbamide at concentration of from 0.05 to 0.5 M or nitrate hydroxylamine at concentration of from 0.05 to 0.3 M into starting carbamide solution makes it possible to extract more than 99.99% of palladium (residual content of Pd is lesser than 0.02 mg/l) from solutions at acidity of 0.2 to 4 M of HNO₃, including solutions of complex salt composition, for example, liquid waste of high level of activity after reprocessing of nuclear fuel. EFFECT: increased coefficient of cleaning palladium from fission products and corrosion whose magnitude ranges within 10³-10⁴ for many admixtures contained in liquid waste of high level of activity. 10 cl, 1 dwg, 7 tbl, 24 ex

Description

 The invention relates to the field of liquid radioactive waste management generated during the aqueous regeneration of spent nuclear fuel (SNF). It can also be used in the hydrometallurgy of non-ferrous and noble metals, in the regeneration of palladium from spent catalysts, as well as in preparative chemistry.

 Palladium refers to elements whose content in the earth's crust is very small. At the same time, the need for it is quite large - palladium is widely used in electronics as a catalyst in the chemical industry and in other branches of technology. One of the possible sources of meeting the demand for this metal is liquid waste from spent nuclear fuel regeneration, the Pd content of which is several kg per ton of spent fuel [M. Ozawa et al., Proceed. Intern. Conf. "GLOBAL-97", Yokohama, Japan, 1997, vol. 2, p. 1232-1237]. During water regeneration of spent nuclear fuel, palladium contained in spent fuel along with other fission products (Cs, Sr, etc.) and corrosion (Fe, Ni, Cr, etc.) is concentrated in high-level liquid waste (HLW). The process of separation of palladium from liquid HLW involves not only a more complete extraction of it from such solutions, but also the achievement of a high degree of purification of Pd from corrosion products, and especially radioactive fission products.

 The appropriateness of the separation of Pd from liquid HLW is also due to the need to provide stable conditions for the vitrification of liquid HLW, since in this process palladium (and other platinoids) forms dispersed conductive phases that violate the homogeneity of the glass and interfere with the stable operation of the melter [A.V. Demin, M.I. Fedorova, Yu.I. Matyunin. Atomic Energy, 1996, v. 80, 3, pp. 179-183].

 Various methods are known for the isolation of Pd from nitric acid solutions, of which extraction, sorption and precipitation methods are the most common.

Extraction methods include extracting Pd from an aqueous solution with a solution of an organic solvent (extractant), washing the organic phase with palladium and an aqueous acid solution, and stripping Pd into the aqueous phase with a solution containing compounds that form more stable water-soluble complexes with palladium than the Pd complex with extractant. Tributyl phosphate used as an extractant in the reprocessing of spent nuclear fuel [B.V. Gromov, V.I. Savelyev, V. B. Shevchenko. Chemical technology of irradiated nuclear fuel. - M .: Energoatomizdat, 1983, p.126-133], ineffective for extracting palladium from nitric acid media due to its low distribution coefficient [K. P. Lunichkina, E.V. Renard, V.B. Shevchenko, J. Inorg. Chemistry, 1974, v. 19, 1, p. 205-209], therefore, for the extraction of Pd, organic amines or sulfur-containing organic compounds are usually used, for which the distribution coefficient is substantially greater than 1 [BC Schmidt, N.A. Shorokhov. Atomic Energy. - 1988, t. 64, 2, p. 103-110]. Described by [GH Rizvi et al., Separation Science and Technology, 1996, v. 31, 13, p. 1805-1816] the process of separation of palladium from solutions of 2-6 M HNO ₃ , including extraction of palladium with an organic solution of 0.01 M triisobutylphosphine sulfide in Solvesso-100 diluent, washing the extract with 2 M nitric acid and palladium reextraction with 0.01 M aqueous thiourea.

 Sorption methods for the extraction of palladium are based on passing an aqueous solution of its salt through a layer of a solid (sorbent), which contains active functional groups that form strong compounds with Pd (2+) ions. Described [M.V. Logunov et al., XV Mendeleev Congress on General and Applied Chemistry. - Minsk, May 1993. Abstracts, v. 2, p.240-241] the process of extracting palladium from liquid HLW, including its sorption on oxidized carbon, desorption of palladium with a thiourea solution, reduction of Pd (2+) in desorbate with hydrazine nitrate to palladium mobile, sediment filtration and dissolution of the latter in nitric acid. After 3 cycles of purification of palladium, its yield in the final product was ~ 80%.

The selection of palladium from nitric acid solutions that mimic liquid HLW by precipitating it as a sparingly soluble compound is described in the patent [RF Patent 2147619, cl. With 22 V 11/00, publ. 04/20/2000, bull. eleven]. Palladium was precipitated using a sulfur-containing reagent — rongalite (formaldehyde sulfoxyl acid sodium salt) at [НСО ₃ ] = 0.01-1.4 M and a temperature of 70-80 ^o С. The Pd release completeness in optimal conditions exceeds 90%, and purification from fission and corrosion products on average ~ 10 ³ times.

The main disadvantages of these methods are:
- the formation of secondary, including salt-forming, wastes (spent extractant, aqueous solutions from regeneration of the circulating extractant, filtrates from sorption), increasing the amount of liquid radioactive waste (RAW);
- the use of chemical, usually sulfur-containing, reagents for the deposition and re-extraction (desorption) of palladium, which leads to contamination of the final palladium concentrate with impurities and requires additional cleaning operations;
- insufficiently high capacity of sorbents for palladium (maximum static capacity of the best of the sorbents - aminocarboxylic vinylpyridine cation exchanger brand VPK does not exceed 300 mg Pd per 1 g of sorbent [V.V. Milyutin, S. B. Peskishev, V.M. Gelis, Radiochemistry, 1994, v. 36, 1, pp. 25-28]), as well as their limited radiation and chemical resistance.

The most promising for the extraction of Pd from solutions seems to be the electrochemical method that best meets the current RW management strategy aimed at minimizing the amount of all types of waste in the reprocessing of spent nuclear fuel. This method is implemented in an electrochemical cell (electrolyzer), in which the working (cathode) and auxiliary (anode) electrodes are placed, usually separated by a conductive diaphragm and connected respectively to the negative and positive poles of a direct current source. When passing an electric current at the cathode, palladium ions are reduced to metal:
Pd ²⁺ + 2e -> Pd ⁰ ,
which is deposited on the surface of the cathode. The palladium precipitate is then removed from the cathode either mechanically (manually or by vibration of the cathode), or by electrochemical or chemical dissolution, for example in nitric acid.

 The electrochemical method is widely used in hydrometallurgy to extract Pd and other valuable metals (Cu, Au, Ag, and others) from hydrochloric acid, sulfate, and cyanide solutions. Described [M.S. Igumnov, S.F. Belov, D.V. Drobot. Russian Chemical Journal, 1998, v. 42, 6, p.135-142] methods for the electrochemical separation of Pd from hydrochloric acid solutions formed during hydrometallurgical processing of mineral raw materials (ore wastes). The process is carried out in an electrolyzer with a diaphragm (cation exchange membrane) separating the cathodic and anodic compartments of the electrolyzer. As the cathode, porous carbon-containing, including carbon fiber, materials (UVM) were used, which have a developed surface and a large capacity for recoverable metals, significantly exceeding 1 g of metal per 1 g of cathode mass.

Compared with the use in general chemical technology, the electrochemical extraction of Pd from liquid wastes after SNF processing has a number of features. Firstly, the process is carried out in a nitric acid medium, which is reduced at the cathode to nitrous acid HNO ₂ (nitrite ions NO ₂ ^- ). This process competes with the Pd (2+) reduction process, which ultimately leads to a decrease in the efficiency of palladium recovery. Secondly, liquid HLW have an extremely complex salt composition. As noted above, they contain significant amounts of fission and corrosion products (Fe, Mo, Ce, Ru, Zr, etc.), and the ion reduction potentials of some of these elements are in the same region as the Pd ion reduction potentials. For this reason, the presence of fission and corrosion products, especially iron ions, negatively affects the electrochemical extraction of Pd, reducing the completeness of its release from the solution. Thirdly, liquid HLW are characterized by a high level of α-, β- and γ-radiation, the action of which leads to radiation-chemical decomposition of HNO ₃ with the formation of nitrous acid [M.V. Vladimirova, S.V. Kalinina. Radiochemistry, 1989, v.31, 5, p.95-100]). In addition, under the influence of strong radiation fields, destruction of the electrodes, primarily the working electrode (cathode), on which the process of Pd release occurs, can occur.

Known [US Patent 3891741, cl. C 01 G 55/00, 1972] a method for the electrochemical treatment of Pd from HNO ₃ solutions in a cell with a diaphragm with a titanium cathode and platinum titanium anode. According to this method, electrolysis is carried out at a fixed value of the cathode potential φ _к = -0.4B = -0.4 V relative to the standard calomel reference electrode. The completeness of the allocation of palladium from the solution is 99%. Removal of the palladium precipitate from the cathode is carried out by dissolving in 5 M nitric acid.

Described by [Y. Sano et al., Proceed. 5 ^th Intern. Conf. "RECOD-98", Nice, France, 1998, v.3, p. 717-722] a method for the isolation of Pd from nitric acid solutions under galvanostatic conditions (at a constant current strength) on a cathode made of titanium or platinum titanium in an electrolyzer with a cathode and anode chambers separated by a diaphragm (cation exchange membrane). The electrolysis is carried out in the mode of circulation of the palladium solution through the cathode chamber of the electrolyzer (a solution of 0.5-4.5 M nitric acid circulates through the anode chamber) at a temperature of 50 ^o C and a cathode current density of i _k = 5000 A / m ² . When the acidity of the solution was 2.5 M HNO ₃ and the electrolysis duration was 3 hours, the degree of palladium recovery was 99.9%. When using nitric acid solutions simulating liquid HLW in composition and containing, in addition to Pd, also impurities of the main fission and corrosion products, the degree of palladium recovery was 93.5%.

The disadvantages of these electrochemical methods are:
- insufficiently high completeness of palladium extraction from solutions containing fission and corrosion products;
- formation of secondary liquid radioactive waste (spent anolyte);
- the need for heating the working solution, which leads to an increase in energy consumption;
- the use of flat (two-dimensional) cathodes with a small capacity for palladium;
- the use of a high current density, which leads to increased corrosion of the electrodes.

относительно нормального водородного электрода из растворов 1-4 М НNО₃, промывки катодного осадка палладия раствором 1 М НNО₃ под напряжением и растворения осажденного палладия в 6 М азотной кислоте. Максимальная степень извлечения палладия (99,9%) достигается при концентрации HNO₃ в исходном растворе 1 М и уменьшается как при увеличении, так и при уменьшении кислотности. При низкой кислотности (0,2 М HNO₃) выделение палладия из раствора прекращается из-за интенсивного выделения газов на катоде. Коэффициент очистки палладия от примесей при использовании данного метода составляет ~10³ для Fe и редкоземельных элементов, 13 - для Ru и 5 - для Zr.Closest to the proposed technical solution, selected for the prototype, is the method [A. from. USSR 1453954, class C 25 C 1/20, publ. 10/15/94, bull. 19] the electrochemical separation of Pd at the flow-through cathode of porous carbon fiber material of the VINN-250 brand. The process is carried out in an electrolytic cell with a diaphragm and includes the operation of deposition of palladium on the cathode at a fixed value of the cathode potential

relative to a normal hydrogen electrode from solutions of 1-4 M HNO ₃ , washing the cathode deposit of palladium with a solution of 1 M HNO ₃ under voltage, and dissolving the precipitated palladium in 6 M nitric acid. The maximum degree of palladium recovery (99.9%) is achieved with a concentration of HNO ₃ in the initial solution of 1 M and decreases both with increasing and decreasing acidity. At low acidity (0.2 M HNO ₃ ), the release of palladium from the solution ceases due to the intense evolution of gases at the cathode. The coefficient of purification of palladium from impurities using this method is ~ 10 ³ for Fe and rare earth elements, 13 for Ru, and 5 for Zr.

The disadvantages of the prototype are:
- not a wide range of nitric acid concentration, at which a high degree of palladium recovery is achieved;
- insufficiently high degree of purification from impurities - fission products;
- the practical impracticability of the method on an industrial scale due to problems with maintaining a constant value of the potential on the entire surface of the porous electrode (cathode).

The technical task of the invention is to find a chemical compound, the introduction of which in a nitric acid solution of palladium eliminates the negative effect of nitrithione (nitrous acid) and iron ions in the process of electrochemical deposition of palladium. This compound should be sufficiently resistant to decomposition in the HNO ₃ solution and should not restore Pd (2+) in the solution to a metallic state (in this case, the precipitate is “palladium black”, which will complicate the process and lead to the formation of secondary radioactive waste due to the necessity of carrying out the operations of filtering the solution, washing the precipitate and converting palladium to the metallic state).

 The technical result when using the inventions is to increase the completeness of the extraction of palladium from nitric acid solutions, including liquid high-level waste, increase the degree of purification of palladium from impurities, including fission products contained in liquid HLW, to extend the working range of the concentration of nitric acid during electrodeposition of palladium , lack of solid and liquid, including saline, secondary radioactive waste.

 The technical result according to the first embodiment of the invention is solved by conducting electrochemical deposition of palladium from nitric acid solutions, including from liquid radioactive waste after regeneration of spent nuclear fuel on a volume-porous carbon fiber cathode in an electrolyzer without separation of the cathode and anode spaces from a solution with a concentration of nitric acid from 0, 2 to 4 M, preferably from 0.5 to 3.5 M, to which carbamide is added in a concentration of from 0.05 to 0.5 M, preferably from 0.05 to 0.3 M, washing under the voltage of the cathode o charge with a solution of 1-3 M nitric acid containing from 0.05 to 0.5 M urea, preferably from 0.1 to 0.3 M, and dissolving the cathode deposit in nitric acid with a concentration of from 4 to 6 M.

 The technical result according to the second embodiment of the invention is realized by conducting electrochemical deposition of palladium from nitric acid solutions, including from liquid radioactive waste after regeneration of spent nuclear fuel on a volume-porous carbon-fiber cathode in an electrolyzer without separation of the cathode and anode spaces from a solution with a concentration of nitric acid from 0.2 up to 3.5 M, preferably from 0.5 to 3 M, to which hydroxylamine is added in a concentration of from 0.05 to 0.3 M, washing under voltage of the cathode deposit with a solution of 1-3 M nitric acid Ota, containing from 0.05 to 0.3 M hydroxylamine and cathode dissolution precipitate in nitric acid with a concentration from 4 to 6 M.

 The drawing shows a General diagram of the installation for electrochemical metal separation for both variants of the proposed method. The installation consists of an electrolyzer with a flowing cathode made of VINN-250 UVM, a direct current source, a container for the initial solution, and also a pump for supplying the solution to the electrolyzer.

где [Pd] _иcx и [Рd]_кон - концентрация Pd в исходном (до электролиза) и конечном (после электролиза) растворах соответственно.The degree (completeness) of the extraction of palladium from the solution, α, is calculated by the formula:

where [Pd] and _cx and [Pd] _con are the concentration of Pd in the initial (before electrolysis) and final (after electrolysis) solutions, respectively.

The following are examples of carrying out the invention in both cases. All experiments were carried out in the galvanostatic mode of electrolysis at room temperature (20 ± 2 ^o C). The content of Pd and other metals in the solutions was analyzed by the plasma spectral-emission method.

 Example 1. Electrochemical precipitation of palladium from nitric acid solutions according to the first embodiment (in the presence of urea).

A solution containing 2.0 M HNO ₃ , 50 mg / L Pd (2+), 1 g / L Fe (3+) and 0.2 M urea is placed in a receiving container. The same solution is poured into the electrolyzer, voltage is supplied to the electrodes from a direct current source, and at the same time, a solution from a receiving tank with a linear velocity of 2.34 cm / min begins to be dosed into the electrolyzer by a pump, and voltage from a direct current source is applied to the electrodes. During the electrolysis, a constant current density value of 100 A per 1 m ^{2 of the} overall surface of the carbon fiber cathode is maintained. The solution leaving the electrolyzer enters the receiving tank and from there goes back to the electrolysis, i.e. Pd deposition is carried out under conditions of continuous electrolyte circulation between the receiving tank and the electrolyzer. After electrolysis for 20 min, the Pd content in the receiving tank was ≤0.02 mg / L, and the completeness of palladium extraction was α≥99.96%. In a similar experiment, but in the absence of urea, palladium does not deposit on the cathode for 60 minutes of electrolysis.

Examples of electrochemical deposition of Pd from nitric acid solutions according to the first embodiment at other concentrations of HNO ₃ are given in Table 1.

From the data in table. Figure 1 shows that the introduction of urea into the initial solution makes it possible to extract more than 99.9% of palladium (its residual concentration in the solution is less than 0.02 mg / l) from solutions containing iron ions with acidity up to 4 M HNO ₃ . The urea concentration required for the complete extraction of palladium is from 0.05 to 0.5 M CO (NH ₂ ) ₂ and increases with increasing concentration of HNO ₃ . The lower limit of urea concentration (0.05 M) is the minimum necessary value to neutralize nitrous acid, which is formed from nitric acid either during electrolysis or under the influence of radioactive radiation. The upper limit of the concentration of urea is limited by its solubility, which is about 0.5, 0.4, and 0.3 M CO (NH ₂ ) ₂ with [НNО ₃ ] = 3.0, 3.5, and 4.0 M, respectively ( at 20 ± 2 ^o C).

The lower limit of the working concentration of HNO ₃ (0.2 M) is limited by the hydrolysis of iron ions due to alkalization of the solution in the cathode layer, as well as a significant increase in terminal voltage at this acidity. At the highest concentration of HNO ₃ (4 M), intense gas evolution is observed, which negatively affects the hydrodynamics of the electrolyzer. Given these circumstances, the preferred range for the electrochemical deposition of palladium from nitric acid solutions is the acidity range of 0.5-3.5 M HNO ₃ .

 Example 13. Electrochemical precipitation of palladium from nitric acid solutions according to the second embodiment (in the presence of hydroxylamine).

A solution containing 3.5 M HNO ₃ , 50 mg / L Pd (2+), 1 g / L Fe (3+) and 0.2 M hydroxylamine nitrate is placed in a receiving container. In the electrolyzer, the pump begins to dose the solution from the receiving tank with a linear velocity of 2.34 cm / min, and the voltage from the DC source is applied to the electrodes. During the electrolysis, a constant current density value of 135 A is maintained per 1 m ^{2 of the} overall surface of the carbon fiber cathode. The solution leaving the electrolyzer enters the receiving tank and from there goes back to the electrolysis, i.e. Pd deposition is carried out under conditions of continuous electrolyte circulation between the receiving tank and the electrolyzer. After electrolysis for 30 min, the Pd content in the receiving tank was ≤0.02 mg / L, and the completeness of palladium extraction was α≥99.96%. In a similar experiment, but in the absence of hydroxylamine, palladium does not precipitate at the cathode for 60 minutes of electrolysis.

In the table. Figure 2 shows examples of the electrochemical deposition of Pd from nitric acid solutions according to the second embodiment at other concentrations of HNO ₃ . From the results in table 2 it is seen that the introduction of hydroxylamine into the solution before electrolysis allows you to remove more than 99.9% of palladium in the range of concentrations of HNO ₃ from 0.2 to 3.5 M from solutions containing iron ions.

The upper limit of the working concentration of HNO ₃ (3.5 M at ~ 20 ^o C) is determined by the stability of hydroxylamine in a solution of nitric acid. At higher acidity (4 M HNO ₃ ), hydroxylamine decomposes when it is added to the initial nitric acid solution to form water and gaseous nitrous oxide. The lower limit of the working concentration of HNO ₃ (0.2 M) is limited by the hydrolysis of iron ions due to alkalization of the solution in the cathode layer and an increase in terminal voltage. With this in mind, the preferred range for the allocation of palladium in the presence of iron ions is the acidity range of 0.5-3.5 M HNO ₃ .

 Below are examples of the electrochemical separation of palladium from nitric acid solutions of complex salt composition, simulating liquid HLW in composition, in an electrolyzer with a flow cathode made of VINN-250 carbon fiber material.

 Example 21. The electrochemical separation of Pd in the second embodiment (using hydroxylamine) from nitric acid solutions simulating highly active liquid wastes.

A solution containing 3.0 M HNO ₃ , palladium (2+), fission products and corrosion, and 0.2 M hydroxylamine nitrate (the composition of the solution is given in Table 3) is placed in a receiving container. The same solution is poured into the electrolyzer, voltage is supplied from the direct current source to the electrodes, and at the same time, the solution begins to be dosed into the electrolyzer by the pump from the receiving tank with a linear velocity of 2.34 cm / min. In the process of electrolysis, which is carried out under conditions of continuous circulation of the electrolyte between the receiving capacity and the electrolyzer, a constant current density of 100 A per 1 m ^{2 of the} overall surface of the carbon fiber cathode is maintained. At the end of electrolysis, the duration of which is 2 hours, the solution is removed from the electrolyzer and the content of components is determined in it (Table 3). According to the analysis, the completeness of Pd extraction at the electrodeposition stage exceeds 99.99%.

 The cathode with the precipitate located on it is washed with water in a circulation mode for 2 hours at a constant voltage at the electrodes of 4V. The washing solution is removed from the electrolyzer and the cathode precipitate is dissolved by circulating a solution of 6 M nitric acid through the electrolyzer for 2 hours. After dissolution, the content of Pd and other components is determined in the solution and the coefficients of Pd purification from fission products and corrosion are calculated, given in Table 3 .

 Example 22. The electrochemical separation of Pd in the second embodiment (using hydroxylamine) from nitric acid solutions simulating highly active liquid wastes.

A solution containing 3.0 M HNO ₃ , palladium (2+), fission products and corrosion, and 0.2 M hydroxylamine nitrate (the composition of the solution is given in Table 4) is subjected to sequential electrochemical extraction of palladium, washing the cathode deposit and dissolving the cathode deposit as described in example 3. All process parameters are kept the same as in example 3, except that a solution of 3 M HNO ₃ containing 0.2 M hydroxylamine nitrate is used to rinse the cathode deposit after the precipitation of Pd, and the washing itself is carried out while maintaining constant th current density of 100 A / m ^2.

 The results obtained (Table 4) show that the completeness of Pd extraction during its electrodeposition exceeds 99.99%, and the purification coefficients of Pd from most impurities are higher than when using water as a washing solution (Table 3).

 Example 23. The electrochemical separation of Pd in the first embodiment (using urea) from nitric acid solutions simulating highly active liquid wastes.

A solution containing 3.0 M HNO ₃ , palladium (2+), fission and corrosion products and 0.2 M urea (the composition of the solution is given in Table 5) is subjected to sequential electrochemical extraction of palladium, washing the cathode deposit and dissolving the cathode deposit, as described in example 4. All process parameters are kept the same as in example 4, except that the electrodeposition of palladium is carried out at a current density of 166 A / m ² , and a solution of 3 M HNO _{3 is} used to rinse the cathode deposit after Pd deposition, containing 0.2 M urea.

 The results obtained (Table 5) show that the completeness of Pd extraction during its electrodeposition exceeds 99.99%, and the purification coefficients of Pd from most impurities are higher than when using water as a washing solution (Table 3).

 Example 24. The electrochemical separation of Pd in the first embodiment (using urea) from nitric acid solutions simulating highly active liquid wastes.

A solution containing 3.0 M HNO ₃ , palladium (2+), fission and corrosion products and 0.2 M urea (the composition of the solution is given in Table 6) is subjected to sequential electrochemical extraction of palladium, washing the cathode deposit and dissolving the cathode deposit, as described in example 5. All process parameters are kept the same as in example 5, except that a solution of 1 M HNO ₃ containing 0.2 M urea is used to rinse the cathode deposit after Pd deposition.

 The results obtained (Table 6) show that the completeness of Pd extraction during its electrodeposition exceeds 99.99%, and the purification coefficients of Pd from most impurities are higher than when using water as a washing solution (Table 3).

The results in examples 21-24 show that the application of the proposed method for both options — using both urea and hydroxylamine — allows almost completely (more than 99.99%, residual Pd content less than 0.02 mg / l) to isolate palladium from solutions that simulate liquid HLW from spent nuclear fuel reprocessing and obtain a high degree of Pd purification (10 ³ -10 ⁴ times) from most fission and corrosion products.

The use of a solution of HNO ₃ containing carbamide or hydroxylamine for washing the cathode deposit allows more than 10-fold increase in the purification of palladium from iron, rhodium and rare-earth elements and 2-5 times - from Mo, Zr, Cs and Ru compared with washing with water a solution not containing urea or hydroxylamine.

 Table 7 gives a comparison of the main indicators of the electrochemical separation of palladium from nitric acid solutions according to the prototype and the proposed method.

The main technical results when using the invention:
1. The increase in the completeness of the extraction of palladium from nitric acid solutions, including from liquid high-level waste.

 2. An increase in the degree of purification of palladium from impurities, including from fission products contained in liquid HLW.

 3. Extension of the working range of nitric acid concentration during electrodeposition of palladium.

 4. Lack of solid and liquid, including saline, secondary radioactive waste.

Claims (10)

 1. The method of separation of palladium from nitric acid solutions, including from liquid radioactive waste after regeneration of spent nuclear fuel, including electrochemical deposition of palladium from nitric acid solutions at the cathode, washing under voltage of the cathode deposit with nitric acid solution and dissolving the cathode deposit in nitric acid, characterized in that from 0.05 to 0.5 M urea, preferably from 0.05 to 0.3 M, the concentration of nitric acid in the starting material is introduced into the nitric acid solution for electrochemical precipitation of palladium the solution is maintained in the range from 0.2 to 4 M, preferably from 0.5 to 3.5 M, and from 0.05 to 0.5 M urea, preferably from 0.1 to 0, is introduced into the solution for washing the cathode deposit 3M.  2. The method according to p. 1, characterized in that the concentration of nitric acid in the solution for washing the cathode deposit is set in the range from 1 to 3 M.  3. The method according to p. 1, characterized in that the concentration of nitric acid in the solution for dissolving the cathode deposit is set in the range from 4 to 6 M.  4. The method according to p. 1, characterized in that as the cathode use volume-porous carbon fiber materials.  5. The method according to p. 1, characterized in that the process is carried out in an electrolyzer without separation of the cathode and anode spaces.  6. The method of separation of palladium from nitric acid solutions, including from liquid radioactive waste after regeneration of spent nuclear fuel, including electrochemical deposition of palladium from nitric acid solutions at the cathode, washing under voltage of the cathode deposit with nitric acid solution and dissolving the cathode deposit in nitric acid, characterized in that from 0.05 to 0.3 M hydroxylamine is introduced into the nitric acid solution for electrochemical precipitation of palladium, the concentration of nitric acid in the initial solution is maintained in the range from 0.2 to 3.5 M, preferably from 0.5 to 3.0 M, and from 0.05 to 0.3 M hydroxylamine are introduced into the solution for washing the cathode deposit.  7. The method according to p. 6, characterized in that the concentration of nitric acid in the solution for washing the cathode deposit is set in the range from 1 to 3 M.  8. The method according to p. 6, characterized in that the concentration of nitric acid in the solution for dissolving the cathode deposit is set in the range from 4 to 6 M.  9. The method according to p. 6, characterized in that as the cathode use volume-porous carbon fiber materials.  10. The method according to p. 6, characterized in that the process is carried out in an electrolyzer without separation of the cathode and anode spaces.

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