FR2696579A1 - Plutonium-contaminated ion exchange resin treatment - by oxidn. with silver ions in divided electrolysis cell to achieve high plutonium recovery - Google Patents

Abstract

Treatment of Pu-contaminated ion exchange resins involves subjecting the resins, suspended in nitric soln., to oxidn. by Ag2+ ions continuously regenerated by electrolysis, at 40-80 deg.C to destroy the resins and dissolve the Pu, in an electrolysis cell divided by a porous wall into anodic and cathodic compartments. The anodic compartment contains an anode and the resins, suspended in an aq. soln. contg. 3-7 mol/l. nitric acid and 0.05-0.15 mol/l. Ag, and the cathodic compartment contains a cathode and a 7-10 N nitric acid soln. Throughout the electrolysis, 8-12 N nitric acid soln. is introduced into the cathodic compartment and the acidity of the aq. nitric soln. in the anodic compartment is maintained at 3-7 mol/l. by addn. of a weak base. Air is bubbled into the cathodic compartment to convert nitrous acid back into nitric acid. The weak base is CaCO3. Electrolysis is carried out at 10-30 A, while introducing 8-12 N nitric acid into the cathodic compartment at 60-80 ml/hr. During part of the electrolysis time, resin and Ag ions are introduced into the anodic compartment to maintain the resin and Ag ion concns. constant at 100 g/l. and 0.1 mol/l. respectively in the nitric soln. which contains 5 mol/l. HNO3. ADVANTAGE - The process gives Pu recoveries greater than 96% and completely destroys the resins.

Description

Process for treating ion exchange resins contaminated with platonium.
The present invention relates to a method for treating ion exchange resins contaminated with plutonium.

 Contaminated ion exchange resins are organic wastes that can be very rich in plutonium, but in which the plutonium is chemically bound to the resin matrix, which leads to some processing difficulties to recover all of the plutonium.

 Generally, the plutonium is recovered by elution by performing several successive rinses of the resins with appropriate solutions.

However, the overall yield of plutonium extraction is still less than 90X; in addition, this process results in a relatively large volume of plutonium-containing solutions that are not directly purifiable by conventional methods.

 Other methods for the treatment of contaminated organic waste are known, and among these there has recently been developed a process using electrolytically regenerated silver ions as described in EP-A-0 158 555.

 In this document, the plutonium present in waste is oxidized by means of Ag2 + ions in nitric solution and the decontaminated solid waste and a nitric solution containing the plutonium are recovered at the end of the operation.

 The present invention relates to a method for treating contaminated ion exchange resins, also using oxidation by electrolytically regenerated Ag2 + ions, which makes it possible to obtain plutonium recovery efficiencies greater than 96% and to completely destroy the resins.

This process for the treatment of plutonium-contaminated ion exchange resins is characterized by subjecting the ion exchange resins suspended in a nitric solution to oxidation by continuously regenerated Ag2 + ions by electrolysis. a temperature of 400C to 800C, for a time sufficient to destroy the resins and dissolve the plutonium in the solution, operating in an electrolytic cell divided by a porous wall into an anode compartment and a cathode compartment,
The anode compartment containing an anode and
the resins to be treated in suspension in a nitric aqueous solution containing silver having a nitric acid concentration of 3 to 7 mol / l and a silver concentration of 0.05 to 0.15 mol / l, and the cathode compartment containing a cathode and nitric acid 7 to 10N, and in that, throughout the electrolysis, is introduced nitric acid 8. to 12N in the compartment
cathodic and we maintain the acidity of the solution
aqueous nitrate of the anode compartment to a
value of 3 to 7 mol / l by addition of a base
low.

In this process, operating at a
temperature above ambient temperature,
for example at 600C, makes it possible to obtain an oxidation
further ion exchange resins and
to thus achieve the destruction of their skeleton
organic, by recovering more than 96% of the plutonium they contain. This is very advantageous because it limits, on the one hand, the volume of the final waste (resin) to be stored, and on the other hand, a nitric solution containing substantially all of the plutonium that can be treated by conventional processes is obtained. to recover the plutonium.

 In addition, the fact of continuously introducing into the cathode compartment nitric acid at a higher concentration than that of the solution of the anode compartment creates a fLux of material from the cathode compartment to the anode compartment, which makes it possible to avoid that plutonium comes into contact with the cathode and disturbs the reactions.

 However, this addition of nitric acid induces a volume variation in the anode compartment as well as an increase in acidity, but by continuously neutralizing, according to the invention, the excess acidity in the anode compartment by adding With a weak base, optimal operating conditions can be maintained throughout the course of the treatment.

 According to a preferred embodiment of the process of the invention, more air is bubbled into the cathode compartment to recombine the nitrous acid, produced by electrolysis in this compartment, nitric acid.

In fact, in the cathode compartment, the nitric acid is converted by electrolysis into nitrous acid HNO 2, but this can be recombined with the oxygen of the air to regenerate nitric acid according to the following reaction scheme:

 Thus, the supply of nitric acid 8 to 12N in the cathode compartment can be reduced.

In addition, the gaseous saturation of the solution of the cathode compartment by the oxygen and the nitrogen of the air decreases the solubility coefficient of the nitrous acid. This makes it possible in particular to avoid the parasitic reaction described below between nitrous acid and the Ag2 + ions.

 According to the invention, the weak base added in the anode compartment to neutralize the excess acidity is preferably constituted by a solid product soluble in the solution and easily removable so as not to generate additional dilution.

 By way of example of such a base, mention may be made of calcium carbonate, a part of which is eliminated in the form of CO 2.

 To implement the process of the invention, the ion exchange resins to be treated are preferably first subjected to grinding in order to increase the exchange surfaces between the resin and the nitric solution of dissolution.

 The treatment parameters such as the current intensity and the rate of introduction of nitric acid 8 to 12N into the cathode compartment are chosen according to the cell used and the other treatment parameters.

 Generally, good results are obtained with current intensities of 10 to 30 A, and rates of introduction of HNO 3 from 60 to 80 ml / h.

Other characteristics and advantages of the invention will appear better on reading the description which follows, given of course by way of illustration and not limitation with reference to the appended drawing in which
FIG. 1 schematically represents an installation for implementing the method of the invention, and
- Figure 2 schematically shows another installation for implementing the method of the invention.

 In Figure 1, there is shown in vertical section an installation for implementing the method of the invention discontinuously.

 This installation consists of an electrolysis cell (1) divided by a porous wall (3) into an anode compartment (5) and a cathode compartment (7).

The anode compartment (5) contains
The anode (9) constituted for example by a Pt grid and an agitator (11), while the cathode compartment comprises the cathode (13) and air supply means (15). In the anode compartment, the resin suspension (16) can be introduced into a nitric solution containing silver ions as well as the weak base, for example calcium carbonate, through the introduction orifice (17).

 The cathode compartment (7) can be fed continuously with nitric acid via the pipe (19).

 The cell (1) is also provided with heating means (not shown in the drawing) to bring the temperature to the desired value.

Similarly, the cathode (13) and the anode (9) are connected to an electric power generator which is not shown in the drawing either.

 The operation of this installation is described below.

 The cell (1) is first filled with the starting nitric solution containing in suspension the resin to be treated and silver nitrate at the desired concentration and the cathode compartment (7) is filled with nitric acid, for example The current is passed through the cell and the cathode compartment (17) is continuously supplied with 8N nitric acid. Similarly, to maintain the processing parameters such as the resin concentration, the silver concentration and the nitric acidity at optimum values, the anode compartment is still fed with resin and nitrate of silver and the weak base, for example CaCO3, to neutralize excess acidity.

 Under these conditions, the volume of the nitric solution and the Pu concentration in the anode compartment increase steadily.

 When the desired Pu concentration is reached, the supply of resin and silver nitrate is stopped, but electrolysis is continued to completely destroy the resin. When the solution is dyed dark brown, that is to say there is more reduction of Ag2 +, the process is stopped. The electrolysis cell (1) is then emptied, the collected solution is filtered, and the weight of the residue and the plutonium content of the solution are determined.

 The following examples illustrate the results obtained with this installation.

 Examples 1 to 4

In these examples, plutonium-contaminated ion exchange resins are treated under the following conditions: - nitric solution of
departure in the comparison
anodic strength 5mol / l HN03 - Temperature 600C - Concentration in
resin in the com
anodic content 100g / l - AgN03 concentration 0.1mol / l - CaCO3 flow 0.1 to 0.3moL / h - Current intensity 20A
The results obtained, as well as the treatment times, the feeds in HN03 of the cathode compartment, the plutonium contents of the starting resins and the quantities treated are given in the attached table.

 In Examples 2 to 4, as the starting nitric solution in the anode compartment, the filtered solution obtained in the previous example was used after having adjusted its HNO 3 content to 5 mol / l by addition of CaCO 3.

 The results in the table show that a 98-99% resin kill efficiency and plutonium recovery efficiencies of greater than 99% can be achieved by operating for extended periods of time.

In all cases, the solid residue (very fine powder) is very weak. A qualitative analysis of this residue shows that the majority elements are Si, Ag, Fe and the minority elements are
B, Mg, Al, Ca, Na, Sn. In the solutions the majority elements are Ag and Na; the minority elements are Ca, Fe, Si, Pb, Mg and Al.

 The results of these examples also confirm the fact that the solution obtained in one example can be used to treat a new batch of ion exchange resins and thus increase the plutonium concentration of the solutions obtained at the end of treatment.

 Thus, with this method, it is possible to reduce 1 kg of contaminated resin containing 2 to 4% of Pu to 5 g of solid residue.

 Furthermore, the solutions obtained can be treated by conventional methods for recovering the plutonium they contain, for example by extraction of plutonium in an organic solvent consisting of tributyl phosphate diluted in dodecane.

 In Examples 1 to 4, samples were taken in the anode compartment (5) and in the cathode compartment (7) every 3 hours, and the acidity of the aqueous solution was determined in the anode compartment and its concentration of plutonium, and the possible presence of plutonium in the solution of the cathode compartment (7).

 In all the examples, no presence of Pu in the cathode compartment was observed, which proves that the flow of acid between the two compartments is sufficiently intense to prevent the passage of the chemical entities from the anode compartment to the cathode compartment.

 On the other hand, the concentration of plutonium regularly increases in the anode compartment and then decreases at the end of the operation as a result of dilution with HN03.

Indeed, in the process of the invention, the oxidizing agent, that is to say the Ag2 + ions participate in two competing reactions which are
The attack of the resin and the dissolution of plutonium. These two reactions do not have the same kinetics, the plutonium is put in solution more quickly than the resin is destroyed.

 Also, at the end of the operation, the reaction only concerns the destruction of the resin without appreciable additional solubilization of plutonium.

 Regarding the nitric acidity in the anode compartment, it remains in the vicinity of 5mol / l of HN03. Neutralization with CaCO3 is therefore effective.

 Moreover, by repeating Example 1, without addition of CaCO 3, it was found that the evolution of the amount of acid in the anode compartment was linear, which proves that the acid losses, by evaporation and reaction. at the cathode, are carried out regularly. Also, according to the process of the invention, it is possible to neutralize the excess acidity of the anode compartment by addition of a weak base such as calcium carbonate, at a constant rate.

 Comparative exercise 1.

 In this example, 200 ml (1209) of an ion exchange resin which has been dried beforehand and ground in two macerations for 2 hours each in 200 ml of nitric acid at 0.5 mol / l each followed by a rinse per 100 ml of water are subjected to 200 ml (1209). water for 2h.

 After these operations, the plutonium contents of the solutions and the resin are determined.

The results obtained are as follows:
Pu Yield
(in mg) (in%)
First elution (solutions) 63,92 59
Second elution (solutions) 25.0 23
Resin 19.6
The total plutonium extraction efficiency is only 82% while it exceeds 95% with the process of the invention.

 Moreover, at the end of the operation, about 120 g of solid resin to be stored are recovered.

 In Figure 2, there is shown another installation for implementing the method of the invention, adapted to a semi-continuous treatment of the resins.

 In this installation, two electrolysis cells (1A) and (1B) identical to that of FIG. 1 are used. Also, the same reference numbers have been used to designate the constituents of the cells (1A) and (1B) already. referenced in Figure 1.

 A liquid circuit (31, 35, 37) provided with a settling tank 33 equipped with an agitator 34 makes it possible to feed the cell (1B) with the solution coming from the cell (1A).

 In order to make this system work, the resin in suspension in a nitric solution, for example at a concentration of 100 g / l, with 5 mol / l of HNO 3 and 0, is placed in the anode compartment of the first electrolysis cell (1A). 1 mol / l of Ag, then a current intensity of 20 A is passed to 600 ° C., by continuously feeding the anode compartment (5) with resin, silver nitrate and calcium carbonate, and feeding it also continuously cathode compartment with HN03 12N at a flow m.

 When the volume V in the cell (1A) is reached, some of the anode compartment resin and resin mixture (5) is withdrawn through line (31) at a flow rate (m + q), and then liquid of this mixture in the settling tank (33), for recycling a flow rate q of solution containing the resin particles thus separated in the first electrolysis cell (1A) through the pipe (35), and introducing the separated solution, at a flow rate m, through the pipe (37) in the second electrolysis cell (1B).

 In the cell (lu), the remaining organic particles which are troublesome for the subsequent reextraction of the plutonium, are totally degraded by electrolysis. In this second cell (1B), 12N HN03 is introduced into the cathode compartment at a very low flow rate n and CaCO 3 into the anode compartment to neutralize excess acidity, as well as silver.

 When the volume V 'is reached, the solution of the anode compartment is withdrawn at a flow rate (mon) through the pipe 39 and is stored in the tank 41 for subsequent plutonium recovery treatments that can be carried out directly on this solution .

 The resin supply of the plant is then stopped when the quantity of insoluble residues in the cell (1A) is such that the electrodissolution efficiency drops significantly.

After this stop, the electrolysis is continued in the cell (1A) until the solution becomes dark brown. The cell is then emptied, the solution is filtered, the insolubles are recovered and the filtered solution is poured into
The storage tank 41.

 This installation is advantageous because it can operate continuously.

BOARD

Resin <SEP> Pu <SEP> in <SEP> Food <SEP> Time <SEP> Residue <SEP> Pu <SEP> in <SEP> Rt <SEP> Rt
<tb> Ex. <SEP> (g) <SEP> resin <SEP> HNO3 <SEP> (h) <SEP> (g) <SEP> residue * <SEP> destruction <SEP> recovered (g) <SEP><SEP> 7) <SEP> (mg) <SEP> resins <SEP> (%) <SEP> tion <SEP> Pu (%)
<tb> 1 <SEP> 109 <SEP> 1.6 <SEP> 8N <SEP> 19h15min <SEP> 8.9 <SEP> 25.8 <SEP> 91.9 <SEP> 98.4
<tb> 120ml / h <SEP> (0.29%)
<tb> 2 <SEP> 147 <SEP> 3.7 <SEP> 12N <SEP> 43h30min <SEP> 0.7 <SEP> 1.1 <SEP> 99.52 <SEP> 99.96
<tb> 80ml / h <SEP> (0.16%)
<tb> 3 <SEP> 150 <SEP> 3.8 <SEP> 12N <SEP> 28h40min <SEP> 1.8 <SEP> 3.7 <SEP> 98.86 <SEP> 99.90
<tb> 68ml / h <SEP> (0.19%)
<tb> 4 <SEP> 163 <SEP> 3.5 <SEP> 12N <SEP> 46h50min <SEP> 1.3 <SEP> 2,3 <SEP> 99.2 <SEP> 99.93
<tb> 68ml / h <SEP> (0.2%)
<tb> mass determined after 4 washes with 25ml of 0.5N HNO3

Claims (9)

 A process for the treatment of plutonium-contaminated ion exchange resins, characterized in that the ion exchange resins suspended in a nitric solution are subjected to oxidation by continuously regenerated Ag2 + ions by electrolysis. at a temperature of 40 to 800C, for a time sufficient to destroy the resins and dissolve the plutonium in the solution, operating in an electrolytic cell divided by a porous wall into an anode compartment and a cathode compartment, the anode compartment containing an anode and the resins to be treated suspended in a nitric aqueous solution containing silver, having a nitric acid concentration of 3 to 7 mol / l and a silver concentration of 0.05 to 0.1 molol / l and the cathode compartment containing a cathode and nitric acid 7 to 10 N, and in that, throughout the duration of the electrolysis, nitric acid 8 to 12N is introduced into The cathode compartment and the acidity of the aqueous nitric solution of the anode compartment is maintained at a value of 3 to 7 mol / l by addition of a weak base.  2. Method according to claim 1, characterized in that air is bubbled into the cathode compartment to recombine the nitrous acid, produced by electrolysis in this cathode compartment, nitric acid.  3. Method according to any one of claims 1 and 2, characterized in that the weak base is CaCO 3 calcium carbonate.  4. Method according to any one of claims 1 to 3, characterized in that one carries out the electrolysis under a current intensity of 10 to 30 A.  5. Method according to any one of claims 1 to 4, characterized in that the rate of introduction of nitric acid 8 to 12 N in the cathode compartment is 60 to 80 ml / h.  6. Method according to any one of claims 1 to 5, characterized in that during a portion of the duration of the electrolysis is introduced resin and silver ions in the anode compartment to maintain substantially constant resin concentrations and silver nitrate solution of the anode compartment.  7. Method according to any one of claims 1 to 6, characterized in that the resin concentration is about 100g / l.  8. Method according to any one of claims 1 to 7, characterized in that the silver concentration is about 0.1mol / l.  9. Method according to any one of claims 1 to 8, characterized in that the concentration of HNO3 of the aqueous solution of the anode compartment is about 5mol / l.