EP0241365B1 - Process for preparing a borosilicate glass containing nuclear wastes - Google Patents

Abstract

The invention relates to a process for the preparation of a borosilicate glass containing nuclear waste. In this process, an inactive borosilicate matrix is prepared in an aqueous medium by mixing the following: a silica-based gel precursor, a concentrated aqueous solution of a boron compound, and a concentrated aqeuous solution of the vitrification adjuvant, in proportions corresponding to the composition of the final glass minus the waste, with stirring at a high rate of shear, at a temperature of between 20 DEG C. and 80 DEG C., preferably at 65 DEG -70 DEG C., at an acid pH, preferably a pH of between 2.5 and 3.5, so as to form a gelled solution, and the said matrix is heat-treated and the nuclear waste is added at any stage during the said treatment to form, by melting, the final borosilicate glass containing the said waste. The process according to the invention is applied to the treatment of nuclear waste, especially to solutions of fission products.

Description

High-level nuclear waste - such as fission products - or long-lived waste such as actinides is currently immobilized in borosilicate glasses which offer sufficient guarantees of safety for man and the environment.

The French Atomic Energy Commission (CEA) has developed an industrial vitrification process for fission products (PF).

This process (known as AVM) consists in calcining the solution of the fission products, and in sending the calcinate obtained, simultaneously with a glass frit, to a melting furnace.

In a few hours, at a temperature of the order of 1100 _° C., a glass is obtained which is poured into metal containers.

The glass frit is composed mainly of silica and boric anhydride, plus the other oxides (sodium aluminum etc.) necessary for the total formulation calcinate + frit to give a glass which can be produced by known glass techniques and fulfilling the conditions of safety for storage (condition on leaching, mechanical strength, etc.).

In the melting furnace there is digestion of the calcine which is incorporated into the glass structure. The temperature should be chosen high enough to hasten digestion but without having a detrimental effect on the life of the oven.

To limit this drawback, the applicant, instead of preparing the glass from solid constituents in the form of oxides, has developed a process in which the constituents of the glass are mixed in an aqueous medium so as to form a gelled solution.

It is also known that obtaining a glass from a gelled solution (also called "gel route") can be done at temperatures lower than those necessary with the oxides ("oxide route"). The main objective is to manufacture by the gels of glasses having the same formulation as those currently prepared by the oxide route, as the examples will show, but any borosilicate formulation acceptable for packaging waste can be prepared.

• . adiuvant de vitrification : c'est l'ensemble des constituants du verre final autres que les constituants provenant des déchets nucléaires et sauf B et Si. Cet adjuvant ne contient donc pas d'éléments nucléaires actifs. Dans le procédé AVM, il est inclus dans une fritte de verre ; dans le procédé objet de l'invention, c'est une solution aqueuse,
• . verre final : Il s'agit du verre dans lequel les déchets nucléaires sont immobilisés,
• . sol : c'est une solution d'acide orthosilicique ; celui-ci instable évolue en se polymérisant. Les sols commerciaux, tels que le Ludox ^R (du Pont de Nemours), sont des solutions stabilisées dans lesquelles se trouvent des particules de silice en partie hydratées, ces particules colloïdales sont des polymères dont l'évolution a été stoppée mais peut être débloquée par acidification par exemple.
• . solution gélifiée ou gel : c'est une solution homogène, de viscosité variable qui va d'une solution qui s'écoule à une masse figée, selon l'avancement de la polymérisation.

In the following text, the following terms will be used in the sense defined below:

• . vitrification adjuvant: this is all the constituents of the final glass other than the constituents originating from nuclear waste and except B and Si. This adjuvant therefore does not contain active nuclear elements. In the AVM process, it is included in a glass frit; in the process which is the subject of the invention, it is an aqueous solution,
• . final glass: This is the glass in which nuclear waste is immobilized,
• . soil: it is a solution of orthosilicic acid; this unstable evolves by polymerizing. Commercial soils, such as Ludox ^R (from Pont de Nemours), are stabilized solutions in which partially hydrated silica particles are found, these colloidal particles are polymers whose evolution has been stopped but can be released by acidification for example.
• . gelled solution or gel: it is a homogeneous solution, of variable viscosity which goes from a flowing solution to a fixed mass, according to the progress of the polymerization.

A way is known for preparing the gels in an aqueous medium known as the sol-gel method, consisting in using a sol in water and in de-establishing it by modifying the pH, thus causing this solution to gel.

• ZARZYCKI J. - JI of Materials Science 17 (1982) p 3371-3379
• JABRA R. - Revue de Chimie Minérale, t.16, 1979, p 245-266
• PHALIPPOU J. - Verres et Réfractaires, Vol. 35, No. 6, Nov. Déc. 1981.

Relevant publications are:

• ZARZYCKI J. - JI of Materials Science 17 (1982) p 3371-3379
• JABRA R. - Revue de Chimie Minérale, t.16, 1979, p 245-266
• PHALIPPOU J. - Glasses and Refractories, Vol. 35, No. 6, Nov. Dec. nineteen eighty one.

• - addition d'une solution de Ludox amenée à pH : 2 à une solution aqueuse de tétraborate d'ammonium aqueux amenée aussi à pH : 2
• - mélange sous agitation pendant 1 heure (avec ajout éventuel d'ammonique pour amener le milieu à pH : 3,5 très favorable à la gélification) si la solution obtenue est exempte de précipitation ou de floculation, elle est considéré comme un gel satisfaisant
• - séchage 8 heures à 100_°C puis 15 h à 175_°C sous vide de 0,1 mm Hg
• - pressage à chaud (450 bars-500 à 900_° - 15 min à 5 heures) pour densifier et vitrifier (la fusion est un autre moyen).

The literature describes the preparation of a Si0 ₂ -B ₂ 0 ₃ glass by the sol-gel method:

• - addition of a Ludox solution brought to pH: 2 to an aqueous solution of aqueous ammonium tetraborate also brought to pH: 2
• - mixing with stirring for 1 hour (with possible addition of ammonia to bring the medium to pH: 3.5 very favorable for gelling) if the solution obtained is free of precipitation or flocculation, it is considered to be a satisfactory gel
• - drying 8 hours at 100 _° C then 15 h at 175 _° C under vacuum of 0.1 mm Hg
• - hot pressing (450 bars-500 at 900 _° - 15 min to 5 hours) to densify and vitrify (fusion is another means).

Until now, only binary or ternary glasses have been prepared by this method because the multiplicity of cations present makes it difficult to control gelation and even to obtain it.

• B₂0₃, Si0₂, A1₂0₃, Na₂0, ZnO, CaO, Li₂0, Zr0₂

Thus, to produce a glass of the same composition as the glass frit used in the current vitrification process, it would be necessary:

• B ₂ 0 ₃ , Si0 ₂ , A1 ₂ 0 ₃ , Na ₂ 0, ZnO, CaO, Li ₂ 0, Zr0 ₂

boron makes gelation very difficult (in the HITACHI process described below, boron is added moreover after gel formation), in particular due to the high insolubility of many boron compounds and promotes recrystallization in mixed gels. aluminum promotes precipitation at the expense of gelling, which is opposed to the desired result sodium, calcium and zirconium lead to the formation of crystals which later constitute fragile points which can cause local destruction.

The multiplicity of components therefore leads those skilled in the art to question how to introduce them and their order of introduction.

• - de l'adjuvant de vitrification (AI₂0₃, Na₂0, ZnO, CaO, Li₂C, Zr0₂) plus B₂0₃ et Si0₂
• - de la solution de PF à vitrifier (une vingtaine de cations différents) a conduit les industriels à développer sur la base des gels deux procédés :

The complexity of the components in the vitrification process, with both those

• - vitrification aid (AI ₂ 0 ₃ , Na ₂ 0, ZnO, CaO, Li ₂ C, Zr0 ₂ ) plus B ₂ 0 ₃ and Si0 ₂
• - PF solution to be vitrified (around twenty different cations) has led manufacturers to develop two processes on the basis of gels:

1) Westinghouse and the US Department of Energy have developed a vitrification process for active solutions with preparation of gels but in alcoholic medium (alcogels) - US Patents 4,430,257 and US 4,422,965 -

• - mélange et hydrolyse des constituants inactifs du gel en milieu alcool-eau, les constituants étant introduits sous forme X(OR)_n
  par ex.: Si(OR)₄, B(OR)₃...,R étant un radical organique ou un proton
• - élimination de l'azéotrope eau-alcool et obtention d'un gel sec
• - addition de la solution de déchets nucléaires (le composé final contenant 30-40% au maximum en déchets) ajustée à pH : 4 à 6
• - séchage
• - fusion

Their process can be summarized as follows:

• - mixing and hydrolysis of the inactive constituents of the gel in an alcohol-water medium, the constituents being introduced in X (OR) _{n form}
  e.g. Si (OR) ₄ , B (OR) ₃ ..., R being an organic radical or a proton
• - elimination of the water-alcohol azeotrope and obtaining a dry gel
• - addition of the nuclear waste solution (the final compound containing 30-40% maximum waste) adjusted to pH: 4 to 6
• - drying
• - merger

The gel prepared from compounds X (OR) _n in an alcoholic medium can be obtained more easily because there is no problem of solubility and moreover the peptizing effect of water at high temperature is eliminated by using l 'alcohol.

Major drawback of such a process: the alcoholic environment subject to fire, explosion ... hence the need to eliminate the alcohol before the introduction of nuclear waste, which requires an additional operation that is impractical to implement.

2) The HITACHI process in which the gel is obtained from the solution of PF in a solution of sodium silicate, the boron (in the form B ₂ 0 ₃ ) being added only after gelation, which requires carrying out calcination of the gel at least 600 _° C. for the time necessary (3 h, cited as an example) for the diffusion of boron in the silicate matrix to form the borosilicate structure, the homogeneity of the product remaining a problem.

Publication: UETAKE N. - Nuclear Technology, Vol. 67, Nov. 1984.

The Applicant has developed a method for immobilizing nuclear waste, which does not have the drawbacks of the Westinghouse and Hitachi processes, and in which a borosilicate matrix is prepared in an aqueous medium, the nuclear waste is subsequently added to said matrix at any stage of its treatment, this mixture then being heat treated to obtain a borosilicate glass.

This process therefore has the advantages of working in an aqueous medium and of adding boron at the same time as the formation of the gelled matrix, the boron therefore participating in the structure of the gelled matrix, for this so-called borosilicate.

• - un précurseur de gel à base de silice
• - une solution aqueuse concentrée d'un composé boré
• - une solution concentrée aqueuse de l'adjuvant de vitrification;

The process which is the subject of the invention is characterized in that the borosilicate matrix is prepared by mixing:

• - a silica-based gel precursor
• - a concentrated aqueous solution of a boron compound
• - a concentrated aqueous solution of the vitrification aid;

in the proportions corresponding to the composition of the final glass, minus the waste, with stirring with a high shear rate, at a temperature between 20 and 80 _° C, (65-70 _° C preferably) at an acidic pH (included between 2, 5 and 3, 5 preferably) so as to form a gelled solution, said inactive matrix is heat treated and nuclear waste is added at any stage of said treatment to form by fusion the final borosilicate glass containing said waste.

In the description of the process, a substance containing silica particles, possibly partially hydrolyzed, which is either in the form of a powder which, when dissolved in acid, can produce a sol, or directly in the form of a gel, will be called a gel precursor. 'a floor.

Gel precursors sold commercially and advantageously used in the process can be, for example, a soil such as Ludox ^R (from Pont de Nemours) or Aerosil ^R (Degussa) which is formed by hydrolysis in the gas phase of silicon tetrachloride. In an acidic environment, the Aerosil leads to a soil then to a firm gelled mass.

The Ludox is brought into solution as it is, the Aerosil on the other hand can be used either directly in the form of powder introduced into the mixture (depending on the technology used in particular for stirring) or in solution.

Furthermore, the gel precursor can consist of a mixture of gel precursor, for example in the same operation the silica will be supplied by Ludox and Aerosil.

The gel precursor is placed, in an acidic aqueous medium, according to the process which is the subject of the invention, so that it is transformed into a gelled solution by polymerization from the Si-OH- bonds.

The boron necessary to form the borosilicate structure is supplied by the aqueous solution of a boron compound which is sufficiently soluble. It may for example be ammonium tetraborate (TBA) which has a satisfactory solubility between 50 and 80 ° C (about 300 g / I or 15.1% B ₂ 0 ₃ ). Preferably, the solution is prepared and used at 65-70 _° C. Boric acid may just as well be suitable: solubility of 130 g / I at 65 °, ie 6.5% B203 ..

The solutions used (boron compound and vitrification aid) are concentrated prepared solutions with the aim of rapidly manufacturing a gel and minimizing the amount of water to be evaporated, as will be explained in the description and the examples. It is difficult to give an exact concentration limit for each of the compounds, but the concentration of the solutions can reasonably be situated at least 75% of the saturation concentration.

To prepare the solution of the adjuvant, it is necessary to use compounds containing the desired elements which are soluble in water, at the process temperature, which are compatible with each other, which do not unnecessarily add other ions and whose ions not participating in the structure of the final glass are easily removed by heating. They are, for example, nitrate solutions when nitric FP solutions are treated. The solid compounds are preferably dissolved in the minimum quantity of water so as to minimize the volumes treated and the quantities of water to be evaporated.

The proportions in which these solutions are prepared (with the exception of waste solutions) and mixed depend on the formulation of the final glass desired. It can be considered that the constituent elements of the glass are practically not volatilized and that the composition of the final glass obtained practically corresponds to that of the mixture produced. In the examples, an acceptable glass formulation is indicated. The qualitative and quantitative composition of the vitrification aid is adapted according to the composition of the final glass and that of the waste solution to be treated.

The mixing is carried out between 20 and 80 ° C. The concentrated solution of the boron compound is maintained to avoid precipitation between 50 and 80 ° C. The other solutions are developed at room temperature. It is then possible either to mix the solutions at the temperature at which they are prepared or brought, or to bring all the solutions to a higher temperature.

The latter has the following advantage. After mixing has taken place and the gelled solution has started to form, the polymerization (gelling) takes place during a so-called maturation time. The rise in temperature favors it. It is therefore very advantageous to prepare the mixture between 50 ° C. and 80 _° C. The maturation of the gelled solution takes place, in the process which is the subject of the invention, during drying, at 100-1 050 _° C.

The solutions of the glass constituents have different pH values: the gel precursor in solution is alkaline (Ludox) or acid (Aerosil in nitric solution), the solution of acid vitrification adjuvant, the solution of the acidic boron compound (boric acid) or alkaline (TBA).

In the process described here, the pH of the mixture must be less than 7 and preferably between 2.5 and 3.5. An adjustment of the pH can be undertaken if necessary.

For the solutions used, we have:

In the process which is the subject of the invention, the mixing of the components takes place by simultaneously bringing these components together and agitating them with "a high rate of shear". These components can be brought separately or possibly grouped when they do not react with each other.

A high shear rate is defined as agitation delivered by a device rotating at at least 500 rpm and preferably 2000 rpm and for which the thickness of the agitated layer (distance between the agitation blade and the wall of the mixing zone) does not exceed 10% of the diameter of the blade.

This device can be a turbine, for example, for application on an industrial scale. Laboratory tests with a mixer or mechanical stirrer in a narrow beaker have shown sufficient mixing capacity.

In the process which is the subject of the invention, an important advantage not previously obtained by the other gelling techniques is that large amounts of gel can be prepared without difficulty. With a turbine, without being at the limit we reached 40 kg / h of gel very easily.

By mixing, a solution called a gelled solution is obtained, its viscosity and its texture changing over time and going from a fluid solution to a gel.

When mixing with a high shear rate, the thixotropy phenomenon takes place, the viscosity is lowered, a homogeneous dispersion of the particles occurs. Without agitation, this mixture sees its viscosity increase, the ions trapped in the structure can no longer react, the structure is blocked.

The inactive borosilicate matrix thus obtained in the form of a gelled solution is then heat treated, the nuclear waste being added at any stage of said treatment.

Different possibilities for including nuclear waste will now be examined.

The method can be applied to various types of solid and / or liquid nuclear waste. It is particularly suitable for the vitrification of FP solutions alone or with other active effluents, for example the sodium washing solution of tributylphosphate used for the extraction of uranium and plutonium, the sodium washing solution possibly being even be treated alone by this process.

FP solutions are nitric solutions resulting from the reprocessing of fuels, they contain a large number of elements in various chemical forms and a certain amount of insolubles. An example of composition is given below.

The soda effluent is based on sodium carbonate and contains degradation products of tributylphosphate (TBP) caused by washing (example 2). The high sodium content of this effluent must be taken into account when determining the composition of the borosilicate matrix.
Case 1: nuclear waste in solution is added to an inactive borosilicate matrix whose volume has been reduced.

The gelled solution obtained by mixing the constituents under the conditions described is subjected to drying, between 100 and 200 _° C at 100-105 _° C preferably. During this operation, the water evaporates and the volume is reduced. It is possible, for the remainder of the process either to carry out a thorough drying so as to be able to obtain a friable solid product, or to simply be satisfied with a reduction in volume - more rapidly obtained - of 25 to 75% of the initial volume of way to get a paste.

The reduced volume matrix obtained is dispersed and mixed with stirring with the solution of nuclear waste to be treated. It may be advantageous to mix at a temperature between 60 and 100 _° C to reduce the volume of water while mixing.

According to another embodiment, the dried matrix is introduced into the calciner, the waste solution is brought simultaneously to this calciner, the mixing takes place in the calciner which rotates around its longitudinal axis. The product obtained is sent directly to the melting furnace.

Whatever the embodiment, the process has the same characteristics: preparation of the matrix-drying-addition of waste-heat treatment going from a drying temperature to a melting temperature (drying-calcination-melting).

The mixture obtained is dried if necessary (between 100 and 200 _° C at 100-105 _° C preferably) in an oven for example, drying under vacuum is also possible. After drying, a calcination is then carried out between 300 and 500 _° C (350 to 400 _° C preferably) during which the water finishes evaporating and the nitrates decompose in part.

The calcination can be carried out either in a conventional calciner (of the type used in the AVM process) or in a melting furnace of the ceramic melter type for example.

We always finish the decomposition of nitrates during melting, at the entrance of the oven, the product quickly passes from its calcination temperature to its melting temperature, this is the so-called introduction zone - then in the so-called refining where it is at a temperature slightly higher than that of melting and then at the casting temperature. An interesting value is between 1035 _° C and 1100 _° C for which the viscosity of the glass between 200 poises and 80 poises allows the casting of the glass in good conditions.

The melting temperature of the mixture depends on the composition of said mixture. Sodium improves the fusibility of glasses, but it does have the disadvantage of lowering their resistance to leaching.

Also to immobilize nuclear waste, the CEA has developed a formulation of glass that meets the nuclear safety conditions and can be treated by known glass techniques according to the so-called oxide route.

When a mixture having the CEA formulation is prepared in an aqueous medium by the so-called gels route, it is observed that the ripening times are less than those necessary in the so-called oxides route. It is then possible to increase the oven flow rates.

Furthermore, the process which is the subject of the invention makes it possible to vitrify various wastes, in particular wastes rich in sodium, since the composition of the borosilicate matrix is adjusted to the type of wastes treated. Thus for sodium-rich waste a borosilicate matrix low in sodium (or even without sodium) is prepared as will be shown in the examples.

Thus the formulation worked out by the CEA and which gives all satisfactions can be easily obtained with various wastes, other formulations which would be acceptable could just as easily be prepared.

The drying-calcination-melting steps described correspond to heat treatments in certain temperature zones. It is obvious that similar heat treatments in other devices are suitable, as is generally any technique for making glass from the gel. 2nd case: nuclear waste in solution is added to a calcined borosilicate matrix

The borosilicate matrix in the form of a gelled solution is dried (between 100 and 200 _° C, preferably at 100-105 _° C) and then calcined between 300 and 500 _° C, temperature below 400 _° C preferred, in devices similar to those described for the 1st case.

With a calcination temperature less than or equal to 400 _° C. a friable gel is obtained, which facilitates its dispersion in the waste solution, moreover this gel has in this zone a maximum specific surface, indeed from 400 _° C. sintering begins and closes the porosity.

The calcined matrix obtained is dispersed and mixed with the solution of the waste to be treated.

As before, it is advantageous to operate at more than 60 _° C, preferably 100-105 _° C, so as to dry during mixing.

This operation of mixing the calcined matrix with the waste solution can be carried out in a reactor or else in the calciner itself. In the latter case, the calciner is supplied with the solution of the FPs and the calcined matrix separately brought into the desired proportions. From then on, the operation takes place at around 200 _° C at the entrance to the calciner to progress to around 400 _° C.

In the reactor, a stirring device makes it possible to mix the substances, in the calciner it is its own rotation around its longitudinal axis which ensures the mixing.

The mixture obtained (calcined matrix + waste) is subjected to a heat treatment (drying, calcination, fusion) under the conditions already described to form a glass. 3rd case: the waste is in solid form

The case where nuclear waste solution was added to the calcined borosilicate matrix. It is equally conceivable to bring the waste in solid form, calcinate for example.

This process has the advantage of being able to be implemented immediately in current production chains by allowing the adaptation of the vitrification adjuvant to the treated waste (as will be shown in Example 3).

It is also possible to add the waste in solid form, calcinate for example, to the dried matrix.

The following examples will illustrate the invention.

Example 1: 1st case: . The solutions

On a laboratory scale, a FP solution was simulated from a typical composition of a real FP solution as follows:

Group 1 represents the inactive elements of the FP solution and group 2 simulates the active elements of this same solution and the insolubles.

Zr0 ₂ and Mo remain solid, they simulate the insoluble materials in suspension contained in the solution. The total amount of water added is 2.972 g. The simulated FP solution has a pH: 1.3.

The final glass composition to be obtained is:

In the percentage composition presented, account must be taken of the presence of Na and Ni in the active oxides (group 2 of the FP solution defined above).

Thus the solution of the vitrification aid is prepared according to the composition of the glass to be obtained and that of the waste solution to be treated.

For this example, the vitrification aid solution is prepared as follows:

Each of the compounds is dissolved in the minimum amount of water, ie a total of 640 g of water at 65 ° C; pH: 0.6.

• 0 des particules : 21 nm
• d25°C : 1,30 -
• pH 9,3 utilisé à température ambiante

The precursor agent is Ludox AS40: 40% Si0 ₂ - 60% H ₂ 0

• 0 of particles: 21 nm
• d25 ° C: 1.30 -
• pH 9.3 used at room temperature

The TBA solution: (NH ₄ ) ₂ 0.2B ₂ 0 ₃ , 4H ₂ 0, 265.2 g dissolved in 663 g of water at 65 ° C - pH: 9.2.

. The device

A conventional turbine is used comprising a mixing zone of small volume in which a propeller with several blades rotates so that a mixture with a high rate of shear is produced. In this example, it rotates at 2000 rpm.

The turbine used for the tests is manufactured by the company STERMA, the mixing zone has a volume of 1 cm ³ and the thickness of the agitated layer is of the order of mm.

Procedure

The solutions arrive separately and simultaneously at the turbine:

Thus 36.5 kg / h of borosilicate matrix are prepared. 1.7 kg are spread on a plate with an average thickness of 2 cm and then placed in an oven for 48 hours at 100-105 _° C. 0.6 kg of dry matrix is obtained.

1.6 1 of simulated FP solution are placed in a 3 l container fitted with a rotating mechanical agitator, the dried matrix is poured regularly while stirring.

The mixture obtained is stirred for approximately 30 min then dried at 100-105 _° C in an oven, on a plate, calcined for 2 hours at 400 _° C and finally melted for 5 hours at 1050 _° C. The glass obtained (0.5 kg) obeys acceptability criteria.

In the tests, a glass of good quality was defined as being a homogeneous glass, not presenting unfonders and bubbles and moreover not showing on the surface traces of molybdate.

Indeed, the molybdate coming from FP solutions poses a major problem: part of the active Mo tends to separate from the solution and sediment so that this phase is not completely dispersed in the mixture therefore it is not fully included in the gelled solution. In addition, molybdenum when it diffuses badly appears on the surface of the glass in the form of traces of yellow and visible molybdate which are considered to be an index of lower quality of the glass.

The chemical analysis of the glass obtained also shows that the components have practically not volatilized, so that one can consider that the composition of the mixtures (borosilicate matrix then matrix + waste) practically corresponds to that of the final glass.

Example 2: 2nd case Trial 1

3.7 kg of the borosilicate matrix from the turbine (preparation according to example 1) are dried for 20 hours at 100-105 _° C on plates in an oven. They are then placed in an oven in which the temperature is gradually raised over 2 hours to 350 ° C and calcined for 2 hours at 350 _° C. The product obtained is brittle and is in the form of fragments a few mm in diameter (2-3 mm on average).

The calcined (1 kg) and ground matrix (= 300 - 400 µ) is dispersed in the FP solution (3 kg) with simple stirring (magnetic stirrer 30-45 min). The mixture is calcined for 4 hours at 400 _° C. after baking for 34 hours at 120 _° C., then melted at 1125 _° C.

An introduction of 40 min and a refining over 1 hour lead to a good quality glass.

Trial 2

This test relates to the treatment of subsequently acidified washing soda effluent.

Currently, in the vitrification process (AVM) from oxides, it is not easy to treat this effluent alone.

• Si0₂ 55-60 % en poids
• B₂O₃ 16-18 % en poids
• Al₂O₃ 6-7 % en poids
• Na₂0 6-7 % en poids
• CaO 4,5-6 % en poids
• ZnO 2,5-3,5 % en poids
• Li₂0 2-3

Indeed, this AVM process uses the vitrification adjuvant in a form of solid glass frit, a known composition is:

• Si0 ₂ 55-60% by weight
• B ₂ O ₃ 16-18% by weight
• Al ₂ O ₃ 6-7% by weight
• Na ₂ 0 6-7% by weight
• CaO 4.5-6% by weight
• ZnO 2.5-3.5% by weight
• Li ₂ 0 2-3

If we used this composition to vitrify the soda effluent, we would be in front of a glass very rich in sodium.

One could think of reducing the sodium content of the glass frit and even bringing it to zero so that the final glass (frit + calcine of soda effluent) has an acceptable sodium content (9 to 11% by weight). But then, we are faced with the difficulty posed by the development and fusion of a glass low in sodium (and consequently richer in silica).

The present invention makes it possible to manufacture a borosilicate glass with the soda effluent having a composition close to that giving any satisfaction in the AVM process. In addition, the ripening temperature can be significantly lowered or the ripening times shortened.

For the tests, a sodium solution was therefore simulated with 100 g Na ₂ CO ₃ in one liter of water. The TBA solution: 312 g / I TBA, 4H ₂ 0. The boric acid solution: 130 g / I (6.5% B ₂ 0 ₃ ) - pH = 2.7.

• Al(NO₃)₃,9H₂O 209,0 g
• Ca(NO₃)₂,3H₂O 98,5 g
• LiNOs 53,7 g
• Zn(NO₃)₂,6H₂O 49,7 g
• Fe(NO₃)₃,6H₂O 73,5 g
• Mn(NO₃)₃,6H₂O 18,2 g
• Ba(NO₃)₂ 5,5 g
• Co(NO₃)₂,6H₂O 11,3 g
• Sr(N03)2 4,1 g
• CsN0₃ 8,0 g
• Y(NC₃)₃,4H₂O 71,0 g
• Na₂MoO_4,2H₂O 16,6 g
• Phosphate monoammonique 2,8 g

To obtain a glass having a composition close to that obtained by the AVM process, the following vitrification aid solution is prepared for one liter of aqueous solution:

• Al (NO ₃ ) ₃ , 9H ₂ O 209.0 g
• Ca (NO ₃ ) ₂ , 3H ₂ O 98.5 g
• LiNOs 53.7 g
• Zn (NO ₃ ) ₂ , 6H ₂ O 49.7 g
• Fe (NO ₃ ) ₃ , 6H ₂ O 73.5 g
• Mn (NO ₃ ) ₃ , 6H ₂ O 18.2 g
• Ba (NO ₃ ) ₂ 5.5 g
• Co (NO ₃ ) ₂ , 6H ₂ O 11.3 g
• Sr (N03) 2 4.1 g
• CsN0 ₃ 8.0 g
• Y (NC ₃ ) ₃ , 4H ₂ O 71.0 g
• Na ₂ MoO _4, 2H ₂ O 16.6 g
• Monoammonium phosphate 2.8 g

In this solution, the components Fe, Mn ... phosphate were introduced so as to obtain a final glass with a composition close to that given in the previous examples.

On the other hand, instead of using Ludox AS40 as a gel precursor, we will take the Aerosil R marketed by the company DEGUSSA. The gel precursor is formed by gradually pouring, with stirring, the Aerosil into water acidified with 3N HNO ₃ (pH: 2.5) so as to obtain a solution of 150 g of silica per liter.

There are 3 diaphragm pumps that have been adjusted beforehand to obtain the desired flow rates.

• Les débits réglés sont :
  • . Solution TBA.......0,57 l/h 65_°C ou bien solution H₃BO₃ 1,25 l/h 65_°C
  • . Solution adjuvant ..1,15 l/h 65°C
  • . Solution Aérosil...2 I/h 20_°C

The following solutions are sent simultaneously by the pumps to a high speed mixer (capacity 1.5 liters) at the flow rates and temperatures indicated:

• The debits set are:
  • . TBA solution ....... 0.57 l / h 65 _° C or H ₃ BO _{3 solution} 1.25 l / h 65 _° C
  • . Adjuvant solution. 1.15 l / h 65 ° C
  • . Aerosil solution ... 2 I / h 20 _° C

The borosilicate matrix obtained in the form of a gelled solution is dried for 24 hours at 105 _° C. and then calcined for 3 hours at 350 _° C. Solid particles having a large specific surface are removed from the oven, varying from one test to another but always close to 50m ² / g. After cooling, these particles are poured into the effluent to be treated and stirred for 2 h. A gelatinous mass is formed which is dried at 105 _° , calcined at 400 ° C and finally melts at 1150 _° C.

• SiO₂ 45,6 %
• B₂O₃ 14 %
• Na₂0 10 %
• Al₂O₃ 4,9 %
• Ca04%
• Li₂0 2%
• Fe₂0₃ 2,9 %
• MnO₂ 0,95 %
• BaO 0,55%
• CaO 0,5 %
• CS₂0 1 %
• SrO 0,35 %
• Y₂O₃ 4 %
• MoO₃ 2 %
• P₂0₅ 0,3%

A chemical analysis gives the following average composition:

• SiO ₂ 45.6%
• B ₂ O ₃ 14%
• Na ₂ 0 10%
• Al ₂ O ₃ 4.9%
• Ca04%
• Li ₂ 0 2%
• Fe ₂ 0 ₃ 2.9%
• MnO ₂ 0.95%
• BaO 0.55%
• CaO 0.5%
• CS ₂ 0 1%
• SrO 0.35%
• Y ₂ O ₃ 4%
• MoO ₃ 2%
• P ₂ 0 ₅ 0.3%

Example 3: 3rd case Trial 1

• . solution TBA à 15 % B₂O₃ 0,75 l/h ou bien solution H₃BO₃ à 6,5 % B₂O₃ 1,7 l/h
• . solution Aérosil à 150 g Si0₂/< 1,3 I/h
• . solution Adjuvant à 12% oxydes 0,75 l/h.

In a 2 1 mixer, we simultaneously bring in 1/2 hour:

• . 15% TBA solution B ₂ O ₃ 0.75 l / h or H ₃ BO _{3 solution} at 6.5% B ₂ O ₃ 1.7 l / h
• . Aerosil solution at 150 g Si0 ₂ / <1.3 I / h
• . Adjuvant solution at 12% oxides 0.75 l / h.

1.4 kg of mixture are obtained, dried at 100-105 _° C in an oven on a plate, then calcined for 3 hours at 350 ° and finally melted.

320 g of this inactive calcined matrix are added to 135 g of FP calcinate and roughly mixed. It takes 2 h of melting at 1100 _° C. to obtain 300 g of glass of the desired composition (that of Examples 1 and 2).

This example shows that one can prepare a calcined gel having the same composition as the glass frit used in the AVM process.

Trial 2

We want to vitrify a mixture of PF + soda effluent solution.

To do this, a calcined matrix is prepared having a composition similar to the glass frit of the AVM process except for sodium: the sodium oxide content is reduced from 7% to 2.6%.

The vitrification aid solution will have the following composition:

• - comme source de silice : le Ludox AS 40
• - comme source de bore : une solution d'acide borique contenant 130,5 g pour 1000 g d'eau, maintenue à 60°C.

We will use to complete the matrix

• - as a source of silica: Ludox AS 40
• - as a source of boron: a boric acid solution containing 130.5 g per 1000 g of water, maintained at 60 ° C.

• solution d'adjuvant de vitrification : 5 kg/h
• solution de Ludox : 9,5 kg/h
• solution d'acide borique : 5,8 kg/h

The following flows are sent simultaneously to the turbine with three pumps

• vitrification aid solution: 5 kg / h
• Ludox solution: 9.5 kg / h
• boric acid solution: 5.8 kg / h

Almost 20 kg of a gel are recovered in one hour, which is dried on a plate in an oven at 100-105 _° C and then calcined at 400 _° C (with gradual rise in temperature and leveling out at 200 _° C ).

We obtain a solid mass composed of irregular pieces of a few cm ³ . It is ground to regularize, and sieved to 2.5 mm.

• Si0₂ 61,6 (% en poids)
• B₂O₃ 19 (% en poids)
• Na₂0 2,7 (% en poids)
• A1₂0₃ 4,5 (% en poids)
• ZnO 3,4 (% en poids)
• CaO 5,5 (% en poids)
• Li₂0 0,75 (% en poids)

An analysis of this calcined product gives

• Si0 ₂ 61.6 (% by weight)
• B ₂ O ₃ 19 (% by weight)
• Na ₂ 0 2.7 (% by weight)
• A1 ₂ 0 ₃ 4.5 (% by weight)
• ZnO 3.4 (% by weight)
• CaO 5.5 (% by weight)
• Li ₂ 0 0.75 (% by weight)

It can be seen that this analysis is very close to the formulation of the standard frit used in the AVM process for all the constituents, except sodium.

The silica to boric anhydride ratio is equal to 3.244 in the theoretical formula and to 3.242 in the calcined gel.

The silica to alumina ratio is 13.75 in the theoretical formulation and 13.69 in the calcined gel.

On the other hand, the silica / sodium ratio is equal to 8.407 in the theoretical formulation and to 22.82 in the calcined gel.

The sodium content is 7% in the theoretical formula and 2.7% in the calcined gel.

This makes it possible to treat a vitrification mixture of PF solution + soda effluent while maintaining a normal sodium content for the final glass as shown in the following example.

To 10 liters of the solution simulating the FPs (as described in Example 1), 2500 g of a sodium nitrate solution at 100 g / kg are added simulating the sodium effluent. (Sodium nitrate is used because the solution simulating FP does not, unlike reality, contain free nitric acid).

It is dried at 105 _° C on a dish in an oven and then calcined in a small oven at 400 _° C, obtaining a grain powder of a few millimeters which represent the calcinate of (PF + soda effluent) and which we call calcinate.

375 g of said calcinate are thoroughly mixed in the dry with 1000 g of the calcined gel.

It is loaded several times into a crucible placed in an oven regulated at 1100 ° C. A complete fusion of 5 hours is followed by a pouring. Very slight mottling is observed on the surface, no doubt corresponding to traces of molybdate, but quite acceptable.

An analysis shows that the glass contains 10.2% Na ₂ 0 for 46% of silica, ie a silica to sodium ratio of 4.5 whereas this ratio is equal to 4.56 in the standard formulation of the final glass.

This example shows the possibility of producing at will a calcined gel having a composition which is difficult to obtain in the form of a glass frit, and in particular the possibility of manufacturing a calcined gel low in sodium making it possible to vitrify at the same time the solution of the FPs and the soda effluent.

In the tests presented, the concentrated solutions have been prepared, some are even close to saturation, so as not to increase the drying times and the volumes of liquid to be handled. It may be necessary, without damage to the process, to further dilute these solutions, in particular for pumping and flow issues.

The process developed by the applicant is therefore different from the processes previously described, in particular the Westinghouse process.

The Applicant believes that it has succeeded in preparing in an aqueous medium a borosilicate matrix ready to be used for the treatment of nuclear waste by the solutions used and the method of agitation used.

Agitation with a high shear rate makes it possible to obtain a thixotropic mixture and homogeneity. As soon as the agitation ceases, the viscosity increases and the polymerization develops rapidly, thus blocking the ions before they react (precipitation, sedimentation for example).

The process which is the subject of the invention has an important advantage during its industrial exploitation in a nuclear environment: the matrix is prepared in an inactive environment, so that this whole part of the process is outside the rigid constraints which are essential to observe in an active environment, conventional technologies in the chemical industry can be used as they are.

In addition, the second part of the process (heat treatment with introduction of waste) can use virtually as is the current production lines already installed and working with oxides.

Claims (12)

1. Process for the preparation of a borosilicate glass containing nuclear waste, characterized in that

a) an inactive borosilicate matrix is prepared in an aqeuous medium by mixing the following:

- a silica-based gel precursor,

- a concentrated aqueous solution of a boron compound, and

- a concentrated aqueous solution of the vitrification adjuvant, in proportions corresponding to the composition of the final glass minus the waste, with stirring at a high rate of shear, at a temperature of between 20_°C and 80_°C, preferably between 65_°C and 70_°C, and at an acid pH, preferably a pH of between 2.5 and 3.5, so as to form a gelled solution, and

b) the said matrix is heat-treated and the nuclear waste is added at any stage during the said treatment in order to form, by melting, the final borosilicate glass containing the said waste.

2. Process according to claim 1, characterized in that the mixture for preparing the inactive matrix is effected with a stirrer which rotates at more than 500 rpm, preferably 2000 rpm, and in which the thickness of the stirred layer does not exceed 10% of the diameter of the stirrer.

3. Process according to claim 2, characterized in that the mixture for preparing the inactive matrix is effected in an apparatus selected from the group comprising a turbine and a mixer.

4. Process according to claim 1, characterized in that the gel precursor is a sol.

5. Process according to claim 1, characterized in that the gel precursor is a Ludox^R.

6. Process according to claim 1, characterized in that the gel precursor is Aerosil^R.

7. Process according to claim 1, characterized in that the boron compound is ammonium tetraborate.

8. Process according to claim 1, characterized in that the boron compound is boric acid.

9. Process according to claim 1, characterized in that the inactive matrix is dried at between 100 and 200_°C, preferably at 100-105_°C, and then calcined at between 300 and 450_°C, preferably at a temperature below 400_°C, characterized in that said calcinate is dispersed in the aqueous solution of nuclear waste and mixed by stirring, and in that said mixture is dried, calcined and then melted to form the final glass.

10. Process according to claim 1, characterized in that the inactive matrix is dried at between 100 and 200_°C, preferably at 100-105_°C, in that said dry gel is brought into contact with the aqueous solution of waste, with stirring, and in that said mixture is dried, calcined and then melted to form the final glass.

11. Process according to claim 9 or 10, characterized in that the dried or calcined matrix and the solution of waste are introduced separately into the calciner, and in that the mixing, drying and calcination are effected in said calciner.

12. Process according to claim 1, characterized in that the solution of waste is calcined and in that the inactive matrix, selected from the group comprising dried or calcined matrices, and the calcinate of the waste are introduced separately into a melting furnace to form the final glass.