GB2316402A - Linear polyphenols - their method of preparation and use as sequestering agents in the separation of sodium from caesium in aqueous effluents - Google Patents

Abstract

The invention relates to linear polyphenols, their method of preparation, sequestering agents and a method of separating the sodium from the caesium in aqueous effluents. The aim of the invention is to improve the selectivity between sodium and caesium. This goal is achieved with the help of a process that consists of : ```- adding to the aqueous effluent to be treated, at least one agent for sequestering the caesium with the formula (I) : ```in which n is a whole number from 5 to 10, and then ```- passing said effluent through a charged porous membrane or a nanofiltration membrane by applying a pressure difference between the two opposite faces of said membrane, in such a way that a permeate containing sodium impoverished in caesium and a retained portion enriched in caesium are collected.

Description

LINEAR POLYPHENOLS, THEIR METHOD OF PREPARATION, SEOUESTERING AGENTS AND A METHOD FOR SEPARATING THE SODIUM FROM THE CAESIUM IN AOUEOUS EFFLUENTS DESCRIPTION This invention relates to linear polyphenols, their method of preparation, sequestering agents based on these linear polyphenols and a method for the separation of sodium from caesium in aqueous effluents arising from the reprocessing of spent nuclear fuel elements.

To be more precise, the invention relates to separation techniques using charged porous membranes or nanofiltration membranes in association with selective sequestration 'of the caesium present in the effluents to be treated.

Effluents arising from the reprocessing of spent nuclear fuel elements generally contain large quantities of salts of sodium and traces of radioactive elements such as Sr, Cs, UO2, Ru, etc. Generally these effluents are concentrated through evaporation, then the concentrates are subjected to vitrification in order to store the radioactive elements within a vitreous matrix. However, the presence of sodium, in large quantity, in these elements is harmful for the vitrification. It is therefore necessary to separate the sodium from the radioactive elements before carrying out such a treatment.

For this separation one could envisage using the techniques of precipitation, solid-liquid extraction or liquid-liquid extraction but the employment of such techniques has the disadvantage of leading to the production of other waste products that it will then be necessary to treat.

The use of charged porous membranes or of nanofiltration membranes allows one to selectively concentrate the radioactive elements in an aqueous solution and to separate the sodium without leading to the production of other waste products.

In a general way, the membrane techniques require a semi-permeable membrane which is a barrier situated between two homogeneous media that offers an unequal resistance to the passage of different constituents of a fluid (suspension, solute, solvent). The acting force can be due to a pressure gradient (microfiltration, ultrafiltration, nanofiltration, reverse osmosis), a concentration gradient (dialysis) or a gradient of electrical potential (electrodialysis).

The techniques of nanofiltration use membranes whose cut-off threshold in relation to a neutral solute (defined as the minimum molar mass to have a retention rate equal to 90%) is between 300 to 1000 g. mo .

These membranes have the particular feature of being selective with respect to ions in solution. The membranes used can be organic, mineral or organomineral.

Hence, one knows from document WO-92/06775 of organo-mineral nanofiltration membranes that include an active layer of a polymer such as sulphonated polysulphones, polybenzimidazolones, grafted polyvinylidene fluorides and the perfluorinated ionomers such as NafiontR5. Their cut-off threshold is situated within the range of 300 to 1000 g.mol~l.

Elsewhere, the document FR-A-2 600 264 also describes organo-mineral membranes that include an inorganic porous support and a microporous membrane made of an organic polymer such as the polysulphones, the polyamides, the cellulose esters and the cellulose ethers. These membranes can be used for the demineralisation of water.

Finally, one also knows of a polyaramide membrane marketed under the name FILMTEC NF70 which has a permeability to doubly distilled water of 4.5 l.h1.m 2.bar -.

However, the low selectivity between caesium and sodium obtained with the membranes mentioned above do not allow the efficient separation of these two elements. Whatever the trans-membrane pressure applied or the pH of the solution to be processed, the retention rate of the sodium is always greater or equal to that of the caesium.

Work relating to the selective sequestration of the caesium in relation to an excess of sodium has been carried out on insoluble resins. This work has given prominence to the strong affinity of phenol groups for caesium (H.W. MILLER, G.E. KLINE, JAm. Chem. Soc., (1951), 73, 2741) and notably resins based on resorcinol (S.K. SAMANTHA, M. RAMAWANY, B.M. MISRA, Sep. Sci. Technol. (1992), 27, 255). However, these resins being insoluble in water, one cannot envisage their use as a soluble sequestering agent added to an aqueous solution to be treated.

As a consequence, this invention has the objective of improving the caesium/sodium selectivity of a porous membrane or nanofiltration membrane, through the use of a sequestering agent selective for caesium and soluble in water.

The inventors have discovered that certain linear polyphenols, soluble in water and of high molecular mass, permit the caesium to be trapped. The latter links itself to a voluminous sequestering agent and does not pass through the nanofiltration membrane.

To this effect the invention relates to linear polyphenols

in which n is equal to 5, 6, 7, 8, 9 or 10. Preferably, n is equal to 5 or 7.

These linear polyphenols can be used as sequestering agents for the caesium present in an aqueous effluent containing numerous radioactive elements.

The invention also relates to the method of preparation of the linear polyphenols mentioned above.

According to the characteristics of the invention, this method includes the steps consisting of a) reacting a dihyroxybenzene with formaldehyde in an acid medium, b) neutralising the precipitate obtained, c) separating the linear polyphenol obtained from the reaction medium.

Finally, these linear polyphenols can be applied to a method that allows one to separate the sodium from the caesium in aqueous effluents arising from the reprocessing of spent nuclear fuel elements According to the characteristics of the invention this method includes the steps consisting of - adding to the aqueous effluent to be treated, at least one caesium sequestering agent, this sequestering agent being a linear polyphenol of formula (I) already mentioned and, - passing. said effluent through a charged porous membrane or a nanofiltration membrane the active layer of which is constituted by a polyaramide, a polysulphone, a sulphonated polysulphone, a polybenzimidazolone, a polyvinylidene fluoride, grafted or not, a polyamide, a cellulose ester, a cellulose ether or a perfluorinated ionomer, by applying a pressure difference between the two opposite faces of said membrane, so as to collect a permeate containing sodium impoverished in caesium and a retained portion enriched in caesium.

The membranes used can be a commercial nanofiltration membrane FILMTEC NF70lR), manufactured by the company Dow Chemicals or one of the membranes described in documents WO-92/06775 or FR-A-2 600 264 incorporated here through a reference. These membranes can be manufactured by the methods described in these documents.

Preferably, in the invention, one of the following membranes is used - 1) a membrane comprising an inorganic porous support made of alumina coated with an intermediate layer of TiO2 and an active layer of perfluorinated ionomer of formula

with m, n and z such that 5 < m < 13.5 n 1000 z = 1, 2, 3.

-2) a polyaramide membrane of the following formula

- 3) a membrane comprising a porous support of alumina coated with a layer of polysulphone or sulphated polysulphone.

In order to implement the method of sodium/caesium separation of the invention, preferably the tangential filtration technique is used which limits the phenomenon of accumulating the retained species on the surface of the membrane, since the circulation of the retained material causes strong turbulence in the neighbourhood of the membrane. Furthermore, this type of filtration permits continuous utilisation.

With this aim, one may use modules in the form of tubes or of parallel plates such as those normally used in this technique. One may also use modules in which flat membranes are wound in a spiral around a perforated hollow tube intended to collect the permeate.

In order to obtain the desired rates of retention, one takes action on the pH of the aqueous effluent to be treated, the pressure difference, the speed of circulation of the retained material and the temperature used.

Generally, the pH of the aqueous effluent is between 7 and 9 since one thereby achieves good rates of retention of caesium with respect to sodium. When the initial effluent to be treated has a pH outside this range, one can adjust it to the desired value by the addition of NaOH or HNO3.

The pressure difference between the two opposite faces of the membrane can vary over a wide range, but good results are obtained with a pressure difference of from 0.2 to 2.5 MPa.

To carry out the method well the flow rate of retained material is also chosen in such a way that the desired separation is achieved and one operates at an ambient temperature or at a lower or a higher temperature between, for example, 5 and 350C.

During the implementation of this method, it is possible to use several modules in series and/or in parallel, by possibly using different membranes in certain modules, certain membranes retaining more specifically the caesium and other membranes allowing other radioactive elements to be trapped so as to obtain, at the end of the operation an aqueous solution of sodium that contains practically no radioactive element and which can thus be released into the environment.

Other characteristics and advantages of this invention will become more apparent on reading the description that follows of an embodiment of the invention given for information purposes which is nonlimitative, this description making reference to the attached drawings in which - Figure 1 is a diagrammatic representation of an installation used for carrying out the method of separation of the invention, - Figures 2, 3 and 4 illustrate respectively the influence of the concentration of sequestering agents (resorcinol, pentaresorcinol and heptaresorcinol) on the rate of retention of a nanofiltration membrane with respect to sodium and to caesium.

- Figure 5 illustrates the influence of the ionic strength and the heptaresorcinol content on the rates of retention of a nanofiltration membrane with respect to sodium and caesium.

The examples that follow illustrate the synthesis of two linear polyphenols according to the invention.

Example 1 : synthesis of pentaresorcinol In a 250 ml. three-necked flask, 40 g (or 0.36 mol) of resorcinol is dissolved in 40 ml of water acidified with 0.4 g of sulphuric acid (1% by weight with respect to the resorcinol). 9.8 g (0.32 mol) of 37% formaldehyde is added drop by drop to the water (or 0.9 equivalents with respect to the resorcinol) whilst agitating. The temperature of the reaction mixture is kept below 250C by a refrigeration bath. After 12 hours, the precipitate formed is neutralised by about 8 ml of NaOH (0.5 M), then washed three times with water to remove compounds of lower molecular mass.

After drying the residual precipitate in a vacuum oven, 25.5 g of raw product is obtained which is purified on a silica column. The product dissolved in a minimum of acetone is eluted on the column by ethyl acetate. Methanol is used to transport the high molar mass molecules, which are the most retained on the column. 7 g of final product is obtained after drying in the oven under vacuum. This corresponds to a yield of 17.5%. Analysis by gel permeation chromatography (GPC) assessed the molecular mass of the polymer as 550 g/mol (about 5 resorcinol units) corresponding to pentaresorcinol.

Analysis by NMR gave NMR 1H (100 MHz ; acetone) in ppm with respect to the TMS : 3.7 (s,8H) ; 6.3 (b, 5H) ; 7.0 (b, 7H) ; 8.7 (s, 10H) Example 2 : svnthesis of hettaresorcinol The operating protocol is identical to that described in Example 1 and only differs in the following points - the synthesis of heptaresorcinol is carried out starting with 50 g of resorcinol (0.45 mol) dissolved in 50 ml of water acidified by 0.4 g of sulphuric acid.

The quantity of 37% formaldehyde added to the water is 13.6 g (0.45 mol) or one equivalent with respect to the resorcinol.

- the purification of the product is carried out by precipitation in the solvent/non-solvent system acetone/ether. The crude product is dissolved in 100 ml of acetone and is then precipitated in 1 1 of ether.

After five recrystallisations, 44 g of final product is isolated (or a yield of 88% of final product).

GPC analysis assessed the molecular mass of the polymer as 900 g/mol (about 7 resorcinol units(, which corresponds to heptaresorcinol.

The NMR analysis gave NMR 1H (100 MHz ; acetone) in ppm with respect to the TMS . 3.7 (s, 12H) ; 6.5 (b, 7H) ; 7.4 (b, 9H) 9 (s, 14h) - In Figure 1, a tangential filtration module installation has been shown which includes a reservoir 1 containing the effluent 3 to be treated, able to be maintained at an appropriate temperature by a cryostat 5. The effluent to be treated is introduced from the reservoir 1 through a pipe 7, fitted with a pump 9, into the filtration module 11. One extracts from this module 11, on the one hand, the retained material R via the pipe 13 and on the other hand, the permeate P via the pipe 15. The pipes 7 and 13 are fitted respectively with manometers 17 and 19 and the pipe 13 is fitted with a valve 20. Finally, a pipe 21 fitted with a valve 23 links the part of the pipe 7 situated downstream from the pump to the upper part of the reservoir 1.

In the filtration device 11, one can use tubular membranes or spiral modules that comprise two semipermeable membranes wound in a spiral around a hollow perforated support tube, which defines a collection tube for permeate P. These two membranes are held appropriately spaced apart by a spacing grid.

In the method according to the invention, the sequestering agent is added to the effluent 3, inside the reservoir 1, before beginning the method of sequestration/filtration.

Pentaresorcinol and heptaresorcinol are soluble in water at a pH greater than 7. So as to give prominence to the sequestering properties of these two derivatives of resorcinol and their use in the method according to the invention, tests have been carried out using as a membrane, the one sold under the brand name FILMTEC NF70. This one was chosen since it does not separate caesium from sodium at a basic pH and in the absence of a sequestering agent.

The following examples illustrate the results obtained with the separation method of the invention.

Example 3 In this example, an aqueous effluent (pH 9) was treated that included CsNO3 and an excess of sodium (NaNO3 + NaOH), namely 2 g/l Na ions 10 mg/l Cs ions, by using a spiral module fitted with the FILMTEC NF70 membrane. The FILMTEC NF70 membrane has a permeability to double distilled water of 4.5 l.h1.m2 bar1. In addition, a sequestering agent chosen from among resorcinol, pentaresorcinol and heptaresorcinol, was added to the effluent to be treated.

The Na/Cs separation was carried out under the following conditions * AP = 0.5 MPa * T = 250C * flow rate of retained material = 800 l/h The different sequestering agents were added at concentrations of from 0 to 225 equivalents of resorcinol units per atom of Cs. Figures 2, 3 and 4 illustrate the results obtained and show the influence of the concentration and the nature of the sequestering agent on the rates of retention TR of the FILMTEC NF70 membrane.

The rate of retention TR is defined by the following formula TR = [(CO - Cp) / CO ].100 in which CO represents the concentration of the element in the supply and Cp the concentration of the element in the permeate.

The selectivities Cs/Na are calculated for the different derivatives of resorcinol and for the different concentrations of sequestering agents. The selectivity S Cs/Na is defined by the following formula : S Cs/Na = (100 - TRNa) / (100 - TRCs) with TRCs and TRNa representing respectively the rates of retention of caesium and of sodium and TRCs > TRNa . These results are given below in Table 1 TABLE 1

Number of Resorcinol Pentaresorcin Heptaresorcin equivalents S Cs/Na ol ol of resorcinol S Cs/Na S Cs/Na units / Cs 0 1.05 1.06 1.08 25 1.05 1.24 1.19 50 1.05 1.34 1.34 75 1.05 1.54 1.51 One observes that whatever the concentration of non-polymerised resorcinol, the Cs/Na selectivity remains unchanged (see Figure 2).

The calculation of the retention rates TR of resorcinol (UV dose at X = 270 nm) shows that the passage through the FILMTEC NF70 membrane of this type of compound increases with the concentration of the latter.

The polymerisation of the resorcinol produces molecules of greater molar masses which are better retained by the membrane while remaining soluble in water. One obtains a good Cs/Na selectivity.

Furthermore, the flow of the neighbouring membrane of 9.3 l.h~l.m~2 is not modified by the nature and the content of sequestering agents based on resorcinol.

Example 4 In this example, the influence of the ionic strength of the solution on the Cs/Na selectivity was studied using the FILMTEC NF70(R) membrane.

An aqueous effluent (pH 9) including 2 g/l of Na+ ions (0.09 M) or 10 g/l of Na+ ions(0.43 M) and 10 mg/l of Cs+ ions was treated. The adjustment to pH 9 was carried out with sodium hydroxide. The operating conditions were identical to those in example 3. The number of equivalents of resorcinol units per atom of Cs varied from 0 to 550.

The results obtained are illustrated in Figure 5.

The Cs/Na selectivities for different ionic strengths and different concentrations of heptaresorcinol are collected together in Table 2.

TABLE 2

S Cs/Na Number of [NaNO3] = 0.09 M [NaNO3] = 0.43 M equivalents of ([Na ] = 2 g/l ([Na ] = 10 g/l resorcinol units / Cs 300 2.27 1.5 500 1.74 Increasing the ionic strength causes a decrease in the Cs/Na selectivities and the rates of retention.

However the selectivity with the 0.43 M NaNO3 is still 1.74 for 500 equivalents of resorcinol units in relation to the Cs.

Claims (11)

1. Linear polyphenols of formula (I)

in which n is a whole number from 5 to 10.

2. A linear polyphenol according to Claim 1, characterised in that n is equal to 5.

3. A linear polyphenol according to Claim 1, characterised in that n is equal to 7.

4. A method of preparation of a linear polyphenol of the following formula (I)

in which n is a whole number from 5 to 10 inclusive, characterised in that it includes steps consisting of a) reacting a dihyroxy benzene with formaldehyde in an acid medium, b) neutralising the precipitate obtained, c) separating the linear polyphenol obtained from the reaction medium.

5. A caesium sequestering agent characterised in that it contains at least one linear polyphenol according to Claim 1.

6. A method of separating the sodium from the caesium in aqueous effluents arising from the reprocessing of spent nuclear fuel elements, characterised in that it includes the steps consisting of - adding to the aqueous effluent to be treated, at least one caesium sequestering agent, this sequestering agent being a linear polyphenol of formula (I) given in Claim 1 and then, - passing said effluent through a charged porous membrane or a nanofiltration membrane the active layer of which is constituted by a polyaramide, a polysulphone, a sulphonated polysulphone, a polybenzimidazolone, a polyvinylidene fluoride, grafted or not, a polyamide, a cellulose ester, a cellulose ether or a perfluorinated ionomer, by applying a pressure difference between the two opposite faces of said membrane, so as to collect a permeate containing sodium impoverished in caesium and a retained portion enriched in caesium.

7. A method according to Claim 6, characterised in that the charged porous membrane or the nanofiltration membrane comprises an inorganic porous support of alumina covered with an intermediate layer of TiO2 and an active layer of perfluorinated ionomer of formula

with m, n and z such that 5 < m < 13.5 n 1000 z = 1, 2, 3.

8. A method according to Claim 6, characterised in that the nanofiltration membrane is a polyaramide membrane of the following formula

9. A method according to Claim 6, characterised in that the charged porous membrane includes a porous support of alumina coated with a layer of polysulphone or sulphated polysulphone.

10. A method according to Claim 6, characterised in that the aqueous effluent has a pH of between 7 and 9.

11. A method according to Claim 6, characterised in that the pressure difference between the two opposite faces of the membrane is from Oi2 to 2.5 MPa.