Patent Number: 047724315
Section: description

Examples are now given in order to provide a clearer understanding of the novelty of the process forming the subject of the invention, compared with the state of the art, the first example consisting of an attempt to produce a gelled solution from the teaching of the prior art. EXAMPLE 1 A Conventional Process for the Preparation of Gels, Applied to the Treatment of a Simulated Solution of FP The Solutions On the laboratory scale, a solution of FP was simulated using a typical composition of a real soluticn of FP in the following manner: ______________________________________ Corresponding quantity of oxide Product used Quantity (g) (g) ______________________________________ 1 Al(NO.sub.3).sub.3.9H.sub.2 O 117.6 15.9 Fe(NO.sub.3).sub.3.9H.sub.2 O 146.7 29 Ni(NO.sub.3).sub.2.6H.sub.2 O 19.4 5 Cr(NO.sub.3).sub.2.9H.sub.2 O 26.3 5 Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O 9.4 5.6 NaNO.sub.3 103.6 37.7 2 Sr(NO.sub.3).sub.2 6.7 3.2 CsNO.sub.3 15.2 10.9 Ba(NO.sub.3).sub.2 9.7 5.6 ZrO(NO.sub.3).sub.2.2H.sub.2 O 34.7 15.9 Na.sub.2 MoO.sub.4.2H.sub.2 O 26.4 22.5 Co(NO.sub.3).sub.2.6H.sub.2 O 5.8 1.4 Mn(NO.sub.3).sub.2.4H.sub.2 O 27.7 9.5 Ni(NO.sub.3).sub.2.6H.sub.2 O 18.3 4.6 Y(NO.sub.3).sub.3.4H.sub.2 O 5.5 1.7 La(NO.sub.3).sub.3.6H.sub.2 O 23.7 8.8 Ce(NO.sub.3).sub.3.6H.sub.2 O 24.9 9.3 Pr(NO.sub.3).sub.3.4H.sub.2 O 10.6 4.3 Nd(NO.sub.3).sub.3.6H.sub.2 O 39.6 15.1 ZrO.sub.2 4.6 4.6 Mo 3.5 5.3 U.sub.3 O.sub.8 8.8 8.5 ______________________________________ Group 1 represents the inactive components of the solution of fission products and group 2 represents the FP and the insoluble materials in the same solution. ZrO.sub.2 and Mo remain solid; they simulate the insoluble materials. The total quantity of water added in 2972 g. The simulated solution of FP has a pH of 1.3. The composition of the final glass to be obtained is: ______________________________________ Composition of the glass introduced via ______________________________________ SiO.sub.2 45.5% Ludox B.sub.2 O.sub.3 14% Solution of ATB Al.sub.2 O.sub.3 4.9% Solution of the adjuvant Na.sub.2 O 9.8% and solution of FP ZnO 2.5% " CaO 4.1% " Li.sub.2 O 2% " Active oxides 13.2% Solution of FP Fe.sub.2 O.sub.3 2.9% " NiO 0.4% " Cr.sub.2 O.sub.3 0.5% " P.sub.2 O.sub.5 0.3% " ______________________________________ In the percentage composition shown, it is necessary to allow for the presence of sodium and nickel in the active oxides (originating from group 2 of the solution defined above). Thus, the solutions of the vitrification adjuvant are prepared according to the composition of the glass to be obtained and the composition of the solution of waste to be treated. For this example, the separate solutions of vitrification adjuvant are thus prepared at ambient temperature: ______________________________________ Product used Quantity (g) Quantity taken ______________________________________ Al nitrate solution 60 g of Al(NO.sub.3).sub.3.9H.sub.2 O 41.7 g per 100 cm.sup.3 of water Na nitrate solution 90 g of NaNO.sub.3 per 22.3 g 100 cm.sup.3 of water Zn nitrate solution 180 g of Zn(NO.sub.3).sub.2.6H.sub.2 O 9.1 g per 100 cm.sup.3 of water Ca nitrate solution 265 g of Ca(NO.sub.3).sub.2.4H.sub.2 O 15.2 g per 100 cm.sup.3 of water Li nitrate solution 90 g of LiNO.sub.3 per 12.5 g 100 cm.sup.3 of water ______________________________________ The precursor is Ludox AS40: 40% SiO.sub.2 /60% H.sub.2 O; d.sub.25.degree. C. : 1.30; pH: 9.3; used at ambient temperature. ATB solution: (NH.sub.4).sub.2 0.2B.sub.2 O.sub.3.4H.sub.2 O; 265.2 g dissolved in 663 g of water at 65.degree. C.; pH: 9.2. The Procedure 59 g of ATB solution are placed in a 1 l beaker equipped with a magnetic stirrer (7 cm bar) rotating at 500 rpm, and adjusted to pH 2 by the addition of HNO.sub.3. In another beaker, 56 cm.sup.3 of Ludox are acidified to pH 2 in order to prevent the subsequent precipitation of hydroxides such as Al(OH).sub.3 at pH 5-6 or Zn(OH).sub.2 at pH 4.8. The Ludox solution is introduced into the ammonium tetraborate, with stirring, the reaction taking place at 65.degree. C.-70.degree. C. The mixture is stirred (magnetic or mechanical stirrer) for 30 min, the temperature being maintained. To accelerate gelling, a small quantity of dilute aqueous ammonia (0.15 N) is added to bring the pH to 3. Gel formation takes place. Each solution of adjuvant is added separately to the mixture, slowly (dropwise) and with stirring. Stirring is continued for 5 to 10 min. The mixture obtained, which is called the gel, shows no visible precipitation or flocculation. 235 g of the simulated solution of FP are added slowly (dropwise), with stirring. Precipitates are formed. To obtain solidification, the mixture is left to stand at 65.degree. C.-70.degree. C.; at least 20 h are required to give a mass; as soon as this is obtained, it is dried in an oven (90 h at 110.degree. C.) and then melted at between 1000.degree. and 1150.degree. C. Analysis shows that the molybdenum which has deposited has not been included homogeneously in the glass; traces of molybdate are also visible. The glass obtained is not acceptable. With this process, only small quantities of gels (.perspectiveto.100 to 500 ml) could be prepared. Gel could not be obtained with 1 l of solution (precipitation occurs). The solutions of the constituents of the glass have to be introduced separately, or together if they are mutually compatible; precipitation is otherwise observed, making the gel non-homogeneous. The gel and the final glass are not always of good quality. Moreover, with this conventional process, it was never possible to introduce the simulated solution of waste correctly. Precipitation and sedimentation were observed, the consequence being the need for a higher melting temperature and/or a longer digestion time, or the production of an unacceptable final glass. In the tests, a glass of good quality was defined as being a homogeneous glass having no unmelted regions and no bubbles and also showing no traces of molybdate on the surface. The molybdate originating from the solutions of FP actually presents a major problem: part of the active Mo tends to separate out from the solution and deposit, so this phase is not completely dispersed in the mixture and hence is not totally included in the gelled solution. Furthermore, when it diffuses poorly, the molybdenum appears on the surface of the glass in the form of visible yellow traces of molybdate, which are considered to be an indication of inferior quality. EXAMPLE 2 Treatment of a Solution of Fission Products by the Process of the Invention The Solutions These are the same as those of Example 1, except for the solution of vitrification adjuvant. For this example, the solution of vitrification adjuvant is prepared as follows: ______________________________________ Corresponding quantity Product used Quantity (g) of oxide (g) ______________________________________ Al(NO.sub.3).sub.3.9H.sub.2 O 243.6 33.1 NaNO.sub.3 148.4 54.1 Zn(NO.sub.3).sub.2.6H.sub.2 O 91.4 25 Ca(NO.sub.3).sub.2.4H.sub.2 O 170.1 40.4 LiNO.sub.3 91.4 19.8 ______________________________________ Each of the compounds is dissolved in the minimum quantity of water, i.e. a total of 640 g of water at 65.degree. C.; pH: 0.6. The proportions of the elements Al, Na, Zn, Ca and Li are the same as in Example 1. The Device The device used is a conventional turbine having a mixing zone of small volume, in which a propeller with several blades rotates so as to effect mixing at a high rate of shear. It rotates at 2000 rpm in this example. The turbine used for the tests is manufactured by the Company STERMA, the mixing zone has a volume of 1 cm.sup.3 and the thickness of the stirred layer is of the order of mm. For use on the industrial scale in a nuclear environment, some technical improvements will be required, especially as regards the geometry of the blades and the introduction of the solutions; the purpose of these improvements is to facilitate operation in an active closed cell. The Procedure The solutions arrive at the turbine separately and simultaneously: ______________________________________ Flow rate Composition of pH T.degree. at T.degree. the solution ______________________________________ Ludox 9.3 20.degree. C. 5.7 kg/h 40% of SiO.sub.2 Ammonium tetra- 9.2 65.degree. C. 4.7 kg/h 21% of anhydrous borate.4H.sub.2 O salt, i.e. 15% of B.sub.2 O.sub.3 Solution of 0.6 65.degree. C. 7 kg/h 40% of anhydrous vitrification salt, i.e. 12% of adjuvant oxides + + Simulated 1.3 18.4 kg/h 14% of anhydrous solution of FP materials, i.e. 6% of oxides ______________________________________ The solutions of vitrification adjuvant and FP are pumped at the indicated flow rate and it is the mixture of these which is sent to the turbine at the overall flow rate of 25.4 kg/h. Thus, there is a flow rate of 36 kg/h of gel. The pH of the gelled solution leaving the turbine is 3. In this test, 12 min sufficed to mix the constituents and produce 7 kg of gelled solution. The following heat treatments were carried out on 3 samples: Test 1 10.5 kg of mixture were concentrated in vacuo in an apparatus manufactured by the Company GUEDU (T: 90.degree. C., P: 630 mm Hg). 6.5 l of water were extracted. The mass recovered (3.5 kg) is calcined for 2 h at 400.degree. C. to give 1.8 kg of product, which is melted at 1050.degree. C. for 5 h. A glass of good quality is obtained. Test 2 5 kg of mixture are dried for 3 days at 105.degree. C. to produce 1.2 kg of dry product; this is then heated for 2 h at 400.degree. C., when it loses 27.5% of its weight. Melting for 5 h at 1025.degree. C. gives 820 g of a glass of good quality which pours well. In this test, it was observed that a gel was obtained en masse in less than 30 min at 105.degree. C. Test 3 3 kg of mixture, spread over a plate with a thickness of 2-3 cm and placed for 8 h in a microwave furnace, gave 550 g of product, which, after 2 h at 400.degree. C. (the temperature being raised uniformly from the drying temperature to 400.degree. C.), reduce to 502 g of calcined product. Melting at 1125.degree. C. takes only 1.5 h (including refining for 1 h) to produce a glass of very good quality which pours well. In conclusion, a gel obtained in this way, treated for 8 h in a microwave furnace, calcined for 2 h at 400.degree. C. and then melted for 1.5 h at 1125.degree. C. (refining for 1 h), leads to a glass of very good quality which is acceptable for the immobilization of nuclear waste and represents a time saving of the order of 3 to 4 h compared with the process currently in use. Replacement of the Ammonium Tetraborate With Boric Acid The ATB solution containing 15% of B.sub.2 O.sub.3 is replaced with an H.sub.3 BO.sub.3 solution containing 6.5% of B.sub.2 O.sub.3, formed by dissolving 130 g of solid boric acid in 1 l of water at 65.degree.-70.degree. C., with stirring (pH=2.7). Consequently the flow rate of boron compound is 10.8 kg/h in H.sub.3 BO.sub.3 solution, the other solutions being conveyed at the same flow rates. This gives about 42 kg/h of mixture, which, when treated in the same way as previously, leads to similar products. EXAMPLE 3 The Treatment of a Soda Effluent Used For Washing At the present time, in the vitrification (AVM) process based on the oxides, it is not possible to treat this soda effluent. In fact, this AVM process uses the vitrification adjuvant in the form of a solid glass frit, a known composition being: ______________________________________ SiO.sub.2 55-60% by weight B.sub.2 O.sub.3 16-18% by weight Al.sub.2 O.sub.3 6-7% by weight Na.sub.2 O 6-7% by weight CaO 4.5-6% by weight ZnO 2.5-3.5% by weight Li.sub.2 O 2-3% by weight ______________________________________ This composition limits the quantity of sodium permissible in the effluent to be vitrified, since the sodium level cannot be increased excessively, thereby lowering the leaching resistance. One might consider reducing the level of sodium in the glass frit, even to zero, so that the final glass (frit+calcinate of soda effluent) has an acceptable sodium level (9 to 11% by weight). However, one is then faced with the difficulty of producing and melting a glass which is poor in sodium (and consequently richer in silica). The present invention makes it possible to produce, with the soda effluent, a borosilicate glass having a composition similar to that which proves totally satisfactory in the AVM process. Moreover, the refining temperature can be considerably lowered or the refining times shortened. For tests, a soda solution was therefore simulated using 120 g of Na.sub.2 CO.sub.3 in 1 l of water (pH=9). The chosen gel precursor is Ludox AS40. The ATB solution contains 312 g/l of ATB.4H.sub.2 O. To obtain a glass having the same composition as that obtained by the AVM process, the following solution of vitrification adjuvant is prepared (amounts are per liter of aqueous solution): ______________________________________ Al(NO.sub.3).sub.3.9H.sub.2 O 209.0 g Ca(NO.sub.3).sub.2.3H.sub.2 O 98.5 g LiNO.sub.3 53.7 g Zn(NO.sub.3).sub.2.6H.sub.2 O 49.7 g Fe(NO.sub.3).sub.3.6H.sub.2 O 73.5 g Mn(NO.sub.3).sub.3.6H.sub.2 O 18.2 g Ba(NO.sub.3).sub.2 5.5 g Co(NO.sub.3).sub.2.6H.sub.2 O 11.3 g Sr(NO.sub.3).sub.2 4.1 g CsNO.sub.3 8.0 g Y(NO.sub.3).sub.3.4H.sub.2 O 71.0 g Na.sub.2 MoO.sub.4.2H.sub.2 O 16.6 g Monoammonium phosphate 2.8 g ______________________________________ The components Fe, Mn . . . phosphate were introduced into this solution so as to give a final glass with a composition similar to that given in Example 2. Each of the solutions is kept in a thermostatically controlled bath (temperature: 65.degree. C.). 4 diaphragm pumps are provided, which have been adjusted beforehand to give the desired flow rates. These solutions are pumped simultaneously into a high-speed mixer (capacity: 1.5 l). The set flow rates are: ______________________________________ ATB solution 0.12 kg/h Adjuvant solution 0.25 kg/h Ludox solution 0.15 kg/h Na.sub.2 CO.sub.3 solution 0.21 kg/h ______________________________________ The process is continued for 1.5 h with vigorous stirring all the time. The contents of the mixer bowl are poured into a beaker and left to stand for 2 h. A virtually solid, homogeneous mass of opalescent color is formed. This mass is spread over a plate to form an approx. 20 to 30 mm thick layer and the plate is placed in an oven heated to 105.degree. C. for 24 h. This gives dry particles of the order of cm.sup.3. These are placed in a calcining furnace and the temperature is raised uniformly to 400.degree. C. over 3 h and maintained for 3 h. The calcinate obtained is crushed into particles of 1 to 3 mm. A Joule-effect electric furnace of sufficient capacity is set to 1150.degree. C. A platinum crucible is filled with a third of the powder prepared and is placed in the furnace. After 30 min, the crucible is filled with another third of the powder; this procedure is repeated with the final third. The crucible is left in the hot furnace for a further 2 h and the contents are then poured onto a plate made of refractory material. The product is annealed at 500.degree. C. for 8 h to give the sample a satisfactory surface, and the temperature is lowered slowly; this produces an intense black plate of glass of perfect visual homogeneity. Chemical analysis gives the following average composition: ______________________________________ SiO.sub.2 45.6% B.sub.2 O.sub.3 14% Al.sub.2 O.sub.3 4.9% Na.sub.2 O 10% CaO 4% Li.sub.2 O 2% Fe.sub.2 O.sub.3 2.9% MnO.sub.2 0.95% BaO 0.55% CoO 0.5% Cs.sub.2 O 1% SrO 0.35% Y.sub.2 O.sub.3 4% MoO.sub.3 2% P.sub.2 O.sub.5 0.3% ______________________________________ This example shows how the composition of the vitrification adjuvant can be adjusted. EXAMPLE 4 Treatment of the Soda Effluent With Aerosil The solutions of vitrification adjuvant, ATB and waste are the same. On the other hand, Aerosil.RTM., marketed by the firm DEGUSSA, will be used instead of Ludox AS40 as the gel precursor. The gel precursor is formed by pouring the Aerosil gradually, with stirring, into water acidified with 3N HNO.sub.3 (pH: 2.5), so as to give a solution containing 150 g of silica per liter. The flow rates are adjusted to the values indicated: ATB: 0.37 kg/h PA0 Adjuvant: 0.75 kg/l PA0 Aerosil: 1.3 kg/h PA0 Na.sub.2 CO.sub.3 solution: 0.63 kg/h The procedure is identical to Example 1 above in all respects except the drying step, which is accomplished in a vacuum oven; this makes it possible to reduce the time to 4 h. The result is the same. The two glasses cannot be distinguished. In particular, the same chemical analysis is found (within the limits of experimental error). The two Examples 3 and 4 illustrate the invention as applied to the treatment of the soda effluent with different gel precursors, but they do not imply a limitation. In particular, they can be combined to vary the procedure, without going outside the framework of the invention, for example by introducing the silica in the form of both Ludox and Aerosil simultaneously. In Examples 3 and 4, the neutralized soda effluent was treated on its own. It is obviously advantageous to treat the unneutralized soda effluent (i.e. in the form in which it leaves the extraction units) at the same time as the solutions of FP, which contain nitric acid, so as not to consume excessive amounts of nitric acid and increase the volumes of waste. To do this, the water used to scrub the nitrous vapors, which contains nitric acid, is added to the soda effluent to neutralize it, the resulting liquid being mixed with the solution of FP in fixed proportions. The solution of vitrification adjuvant will then be adapted to this treatment. All the solutions were prepared in the minimum quantity of water--they are close to saturation point--so as not to increase the drying times, the volumes of liquid to be handled or the active gaseous discharges, since the water to be evaporated off is contaminated by radioisotopes and the operator is obliged to treat the said discharges. For reasons of pumping or flows, it may be necessary to dilute these solutions more, but this has no adverse effect on the process. Furthermore, in the examples, the boron compound used is ammonium tetraborate tetrahydrate, thereby affording easier comparison with the prior art. However, in the existing vitrification plants, the use of ATB presents problems as regards the treatment of the gaseous effluents rich in ammonia and nitrous vapors, which are liable to recombine to produce ammonium nitrate, this being dangerous under certain conditions. For these reasons, boric acid is preferred under the conditions of the process forming the subject of the invention. Thus, the description clearly shows that the process developed by the Applicant Company differs from the HITACHI process described in the prior art in that all the components of the final glass are introduced simultaneously to form a gelled solution. In contrast to the HITACHI process, the boron is introduced before rather than after gel formation. It therefore forms part of the structure right from the start, whereas, in the HITACHI process, it is dispersed in the previously produced silicate structure. The Applicant Company is of the opinion that the gelled solution produced by the process according to the invention forms more rapidly than the compounds can react with one another to give a precipitate. The gelled solution obtained has the structure of the desired final glass and the ions can no longer migrate in this solution. It is in fact considered that, during mixing under the conditions indicated, the phenomenon of thixotropy occurs so that a homogeneous dispersion of the ions is produced. After this mixing stage, the viscosity of the solution increases, trapping the ions in the medium. They are no longer able to react (precipitation, sedimentation etc.) and the medium is "frozen". According to the Applicant Company, this effect is due to the choice of solutions used and the method of stirring employed to mix them.