FR2485510A1 - Selective palladium solvent extn. from nitric acid soln. - esp. from nuclear fuel reprocessing soln., using di:alkyl di:thiophosphoric acid - Google Patents

Abstract

Palladuim is extracted from aq. nitric soln. by solvent extraction using an organophosphorus acid having at least one electron donating sulphur atom. The acid is pref. a dialkyl dithiophosphoric acid, esp. di-2-ethyl-hexyl-dithiophosphoric acid. The palladium can be stripped from the solvent by contact with a reducing agent such as hydrazine to ppte. the palladium. The method is esp. useful for extracting palladium from nuclear fuel reprocessing solns. The solvent is selective for palladium in the presence of ruthenium and gives high palladium recoveries.

Description

The present invention, due to the work of
Messrs Richard FITOUSSI and Claude MUSIKAS
Commission for Atomic Energy and
Miss Sylvie LOURS of the Mechanical Company "PEETERS", has for object a process of extraction of the palladium starting from nitric aqueous solutions, in particular solutions resulting from the reprocessing of the irradiated nuclear fuels.

 In irradiated nuclear fuel reprocessing solutions, palladium is present in a weightable amount because it is a relatively abundant uranium 235 fission product.

Thus, for fuels from light-water reactors, the concentrations of palladium are close to 1300 g per ton of irradiated fuel at 33,000 MW.J 1. Palladium is mainly present in the form of its isotopes 104 and 105 which are not radioactive. However, the separation of palladium is interesting because of the value of this element, and it is also advantageous to separate the palladium from the reprocessing solutions, since this constitutes an annoying element during subsequent purification operations, in particular during the purification of plutonium in the presence of reducing agent.

 Indeed, during the various solvent extraction phases, this element can precipitate in the organic phase in the metallic state, in particular when adding to the solutions a reducing agent during the uranium-plutonium partition.

 Also, it is of great interest to recover palladium quantitatively in the early stages of the reprocessing process of irradiated nuclear fuels.

 However, since the nitric reprocessing solutions contain other elements, in particular ruthenium, it is also desirable to be able to carry out a selective extraction of palladium from nitric solutions simultaneously containing palladium and ruthenium.

 The subject of the present invention is precisely a process for extracting palladium present in aqueous nitric solutions which makes it possible to recover palladium with satisfactory yields, without simultaneously extracting the ruthenium that may be present in the solution.

 The process, according to the invention, for extracting palladium from an aqueous nitric solution, is characterized in that said palladium-containing nitric solution is brought into contact with an organic solvent comprising an organophosphorus acid compound comprising at least one minus one electron donor sulfur atom to extract the palladium in said organic solvent.

 According to the invention, said organophosphorus compound aeide is advantageously a dialkyl dithiophosphoric acid such as di (2-ethylhexyl) -dithiophosphoric acid, dibutyldithiophosphoric acid or bis (2,6-dimethyl-4-heptyl) -dithiophosphoric acid .

 Preferably, di- (2-ethylhexyl) -dithiophosphoric acid is used.

The process as characterized above, has in particular the advantage of leading to high yields of palladium extraction, which was not predictable given the fact that in nitrate solution, palladium is already under the complex form whose ligands can be
NO3, NO2, ND and OH. Also, since this palladium must be extracted into the organic medium also in the form of complex, it could not be expected that by contacting a nitric solution of palladium with an organic solvent comprising an organophosphorus compound atom of electron donor sulfur, the ligands of the aqueous medium are displaced in a wide range of nitric acid concentrations.

 Furthermore, the process of the invention can be implemented to ensure the selective extraction of palladium from a nitric solution simultaneously containing palladium and ruthenium, although this latter element can be extracted using organic solvents. of the same type.

 Indeed, a process for extracting ruthenium from nitric solutions has recently been developed in which a solvent containing an organophosphorus compound with an electron-donating sulfur atom such as di-acid is used as the organic solvent. 2-ethylhexyl) -dithiophosphoric acid. However, according to this process which is the subject of the French patent application No. 7903241 filed on 8/2/79, it is necessary to operate in the presence of compounds such as hydrazine or sulfamic acid to extract the ruthenium.

 Also, the method of the invention has the advantage that it can be used for the treatment of solutions containing ruthenium and palladium simultaneously, since it makes it possible to recover palladium first by leaving the ruthenium in aqueous solution.

 Moreover, the solvent used for the extraction of palladium can be reused further for the extraction of ruthenium after recovering the palladium extracted with this solvent.

This can be achieved by contacting the organic solvent with a reducing agent such as hydrazine to precipitate palladium. For example, the organic solvent containing the palladium is brought into contact with a 0.5 N nitric acid solution containing 0.1 ml of hydrazine; Thus, colloidal particles of Pd are formed which collect at the interphase of the two solutions, and then these particles are separated by filtration or centrifugation.

 For carrying out the process of the invention, the organophosphorus acid compound containing at least one electron-donating sulfur atom is generally diluted in an inert solvent such as dodecane.

 Furthermore, the extraction is advantageously carried out at a temperature of 15 to 500C, preferably at 400C. It is specified that the process of the invention can be implemented in any conventional extraction apparatus such as mixer-settler batteries, pulsed columns, centrifugal extractors, etc.

Other advantages and characteristics of the invention will appear better on reading the following examples given of course by way of illustration and not limitation, with reference to the appended drawing in which:
FIG. 1 is a diagram showing the variations of the partition coefficient D of palladium as a function of time (in minutes) at a temperature of 200 ° C (curve I) and at a temperature of 400 ° C. (curve II),
FIG. 2 is a diagram representing the variations of the partition coefficient D of palladium as a function of the nitric acid content of the aqueous solution, and
FIG. 3 is a diagram showing the variations of the partition coefficient D of palladium as a function of the di- (2-ethylhexyl) -dithiophosphoric acid concentration of the organic solvent.

 In these examples, di- (2-ethylhexyl) -dithiophosphoric acid having a degree of purity greater than 95%, diluted in dodecane and as aqueous solution, is used as the organic solvent, a nitric solution containing palladium which is labeled by palladium 103.

 The extraction is carried out in double-walled containers thermostated by circulating water by contacting a volume of aqueous solution with a volume of organic solvent, and stirring the mixture of phases by means of a magnetic bar rotated .

 After extraction, the phases are separated by centrifugation, and each of them is analyzed in order to measure its palladium concentration by counting by means of a "Berthold" proportional counter.

 After this measurement, the partition coefficient D of palladium, which corresponds to the ratio of the concentration of palladium in organic phase to the concentration of palladium in aqueous phase, is determined.

EXAMPLE 1
In this example, the palladium is extracted from an aqueous phase consisting of a 3N nitric acid solution containing 0.005 M palladium (II) using an organic solvent consisting of di (2-ethylhexyl) acid. ) -dithiophosphoric acid (HDEHDTP) diluted with dodecane, the HDEHDTP acid concentration of the organic phase being 0.5 M.

 A volume of aqueous phase is heated with 400C of a volume of solvent and then stirred together to obtain a fine emulsion.

After fifteen minutes, the organic and aqueous phases are separated, 99.5% of the palladium is then in the organic phase.

 The organic phase is then brought into contact with a solution of 0.5 N nitric acid and 0.1 M hydrazine at a rate of one volume per two volumes of solvent. Palladium collects as an insoluble interphase and is recovered by filtration.

 The conditions given in this example are not restrictive and it can be seen in the curves of FIG. 1 to what extent the temperature and stirring time parameters can be varied to obtain a quantitative extraction of palladium.

EXAMPLE 2
In this example, the palladium is extracted from a 1 N aqueous nitric solution containing 0.005 M palladium (II) using an organic phase comprising 0.5 M di- (2-ethylhexyl) -dithiophosphoric acid in dodecane and carrying out the extraction at a temperature of 350C, with a stirring time of 60 min, the ratio of organic phase volumes on aqueous phase being 0.5. After separation of the phases, the extraction is repeated under the same conditions. The two volumes of organic phase are combined and brought into contact with 0.5 volume of 0.5 N nitric acid solution and 0.1 M hydrazine. After filtration the solvent is reused for the recovery of ruthenium.

 The recovery yield of palladium is close to 99%.

 The curve of FIG. 2 shows that these conditions are not restrictive with regard to the acidity of the aqueous phase. In view of this figure, it is found that the palladium extraction mechanism is complex and can not be easily deduced from the shape of the curve obtained.

The shape of this curve is certainly due to kinetic phenomena or competitions between aqueous and organic ligands for the four palladium coordination sites. Indeed, the existence of a single species Pd2 + or
Pd (NO3) 2 in aqueous solution would have resulted in a decrease in the partition coefficient D of the palladium when the acidity increases. Also, it appears that the ligands present in aqueous solution whose affinities for the pd2 + ion increase by
NO3 to NO2 and OH intervene in the extraction mechanism.

 However, it can be seen that palladium can be extracted quantitatively in an organic solvent containing 0.5M di- (2-ethylhexyl) -dithiophosphoric acid, from aqueous solutions having nitric acid concentrations which vary within one hour. wide range.

EXAMPLE 3
In this example, the palladium is extracted from a 3N aqueous nitric acid solution containing 0.005 M palladium (II), at a temperature of 350 ° C., with a stirring time of 60 min, using As the organic phase of 1M di- (2-ethylhexyl) -dithiophosphoric acid diluted in dodecane, the ratio of the organic phase volumes to the aqueous phase is 1. After separation of the phases, the palladium contained in the solvent is recovered. as described in Examples 1 and 2.

 The curve of FIG. 3 shows that these conditions are not limited to the example described.

It shows the variation of the partition coefficient of palladium as a function of the concentration of dialkyl dithiophosphoric acid in the organic phase.

In view of this figure, it can be seen that the slope of the line obtained is substantially equal to 2, which would suggest that the extraction mechanism corresponds to the reaction pattern

<tb> vant <SEP>: <SEP> - <SEP>
<tb> Pd2 + <SEP> + <SEP> 2 (20) 2PS2H <SEP> XPd <SEP><SEP> (RO) 2PS22 <SEP> + <SEP> 2H + <SEP>
<tb> in which the extracted complex satisfies the coordination number 4 which is often observed with the Pu2 + ions.

Claims (7)

 1. A process for extracting palladium from an aqueous nitric solution, characterized in that said palladium-containing nitric solution is brought into contact with an organic solvent comprising an acidic organophosphorus compound comprising at least one donor sulfur atom electrons for extracting palladium in said organic solvent.  2. Method according to claim 1, characterized in that said acid organophosphorus compound is a dialkyl dithiophosphoric acid.  3. Process according to claim 2, characterized in that the dialkyl dithiophosphoric acid is di-2-ethyl-hexyl-dithiophosphoric acid.  4. Method according to any one of claims 1 to 3, characterized in that one operates at a temperature of 15 to 500C.  5. Process according to any one of claims 1 to 4, characterized in that the palladium extracted in said organic solvent is recovered by contacting said organic solvent with a reducing agent to precipitate the palladium.  6. Process according to claim 5, characterized in that the reducing agent is hydrazine.  7. 7. Application of the process according to any one of claims 1 to 6 for the separation of palladium and ruthenium present in a nitric solution.