MOLYBDENUM-RICH SILICATE GLASSES AND METHOD FOR IMPROVING THE SOLUBILITY OF MOLYBDENUM IN A GLASS MELT

Abstract

The present invention relates to a silicate glass comprising from 35 to 45% by mass of silicon dioxide (SiO2), at most 15% by mass of molybdenum trioxide (MoO3), from 4 to 7% by mass of zirconium dioxide (ZrO2), from 1 to 3.5% by mass of phosphorus pentoxide (P2O5), and from 14 to 28% by mass of one or more alkali metal oxides and / or one or more alkaline earth metal oxides, the mass percentages being expressed relative to the total mass of the glass. The present invention also relates to a method for improving the solubility of molybdenum in a glass melt.

Description

Title of the invention: MOLYBDENUM-RICH SILICATE GLASSES AND METHOD FOR IMPROVING THE SOLUBILITY OF MOLYBDENUM IN A GLASS MELT Technical field

[0001] The present invention relates to the general field of vitrification and more particularly to the field of vitrification of nuclear waste.

[0002] Indeed, the present invention provides silicate glasses whose composition, particularly in terms of the quantity of zirconium dioxide (ZrO2) and phosphorus pentoxide (P2O5), makes it possible to obtain a glass cast iron rich in molybdenum without phase separation at the vitrification temperatures used in industrial processes.

[0003] The present invention also relates to a method for improving the solubility of molybdenum in a glass cast iron. STATE OF THE PRIOR ART

[0004] Nuclear waste is classified, according to the applicable regulations, on the one hand, according to the radioactive period of the radionuclides it contains: very short-lived waste (less than 100 days), short-lived waste (or VC i.e. less than 31 years) and long-lived waste (or VL i.e. greater than 31 years) and, on the other hand, according to its initial level of radioactivity (very low-level waste (VLL), low-level waste (LL), intermediate-level waste (IL) and high-level waste (HL)).

[0005] Thus, HA nuclear waste which is always long-lived (HA-VL) contains radionuclides emitting α, [3, and γ radiation and is made up of fission and activation products and minor actinides from the processing of spent fuel or from the spent fuel itself. Intermediate-level waste (ILW), for its part, contains significant quantities of α emitters, generally actinides. This ILW can be short-lived waste (ILW-VC) or long-lived waste (ILW-VL). Among the ILW, only ILW-VL waste is treated by vitrification. The latter comes in particular from rinsing, sanitation and decontamination operations of nuclear installations, nuclear research centres or fuel cycle plants and from sludge from effluent treatment operations.

[0006] Among the nuclear waste management techniques, vitrification is commonly used for HA and MA-VL nuclear waste. This process consists of incorporating, in an amorphous material, i.e. a glass of suitable composition such as sodium alumino-borosilicate, all the elements contained in the nuclear waste. HA and MA-VL as defined above. The radionuclides are incorporated in the form of oxides and are an integral part of the vitreous network.

[0007] In practice, the industrial process of continuous vitrification consists of feeding an induction-heated melting crucible with the calcinate of HA and MA-VL nuclear waste and a vitrification additive (or glass frit). However, there are other vitrification processes such as, for example, processes using direct induction furnaces (cold crucible), indirect induction furnaces (the walls of the furnace heating the glass), electrode furnaces or gas furnaces.

[0008] Molybdenum is an element present in high concentrations in HA waste from the processing of certain old fuels and in MA-VL waste from the cleanup of nuclear installations. To be produced on an industrial scale, the cast iron formed by the mixture comprising the vitrification additives and the MA-VL or HA waste in calcined form must be single-phase. However, molybdenum (Mo) is relatively poorly soluble in borosilicate glasses and tends to cause phase separation of the cast iron beyond 3% mass at 1100°C (which corresponds to 1 to 2% moi in simple borosilicates) in the packaging glasses for HA nuclear waste. Its content in the packaging glasses is therefore limited, which requires the production of a relatively large number of final packages for the immobilization of molybdenum-rich waste.

[0009] Some patents have already addressed the issue of increasing the solubility of molybdenum in nuclear waste conditioning matrices. Thus, patent application CN 110590161 A proposes adding vanadium in the form of vanadium pentoxide (V2O5) to the mixture comprising vitrification additives and nuclear waste [1]. This process cannot take advantage of the vanadium present in nuclear waste since the latter does not contain any, which increases the cost of such a process.

[0010] International application WO 2009 / 039059 A1 [2] proposes, instead of adding the frit containing SiO2, B2O3 and A12O3 to the nuclear waste, to add at least one of the three main constituents of this frit directly as an additive (raw chemical product) in the waste solution, which would limit the formation of secondary phases. For molybdenum in particular, it is indicated that by removing B2O 3 and A12O3 from the frit and introducing them into the waste, the secondary phases of molybdenum redissolve more quickly or form little or not at all. This international application is therefore not aimed at improving the solubility of molybdenum but just at improving the redissolution of the molybdic phases.

[0011] Furthermore, there are several scientific publications aimed at increasing the solubility of molybdenum in borosilicate glasses for the conditioning of nuclear waste.

[0012] Prakash et al, 2019 study the homogeneity of simplified borosilicate glasses SiO2-B2 O3-Na2O, with and without phosphorus pentoxide (P2O5) containing increasing amounts of molybdenum trioxide (MoO3), produced at 1000°C for 4h and cooled by slab casting [3]. This homogeneity is studied by X-ray diffractometry (XRD), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) and laser-induced breakdown spectroscopy (LIBS) on post-mortem glasses. A phosphorus-free glass containing 1% of MoO3 leads to a homogeneous glass while at 2% it shows crystallizations. After addition of about 5% of P2O5, no crystallization or phase separation is detected, even with 4% of MoO3.

[0013] Krishnamurthy & Kroeker, 2022, study the structure of simplified SiO2-Na2 O-B2O3-Cs2O-A12O3 glasses containing increasing amounts of P2O5 and MoO3 (by substitution of SiO2), produced at 1100°C for 1 h and cooled at 5°C / min, by nuclear magnetic resonance (NMR) of HB, 23Na, 27Al, 31P and 133Cs [4]. They show that the addition of phosphorus in the glass allows the formation of stable Mo-OP bonds by creating a sub-network that sequesters the alkalis necessary for the charge compensation of the pyro / ortho / metaphosphate units. This sub-network is however at the origin of the phase separation in the melt. The solubility limit of MoO3 in a glass without P2O5 is 1.5% mob. The addition of 2.5% mo of P2O5 then allows the solubility of MoO3 to increase to almost 3% mob, i.e. an increase of almost 50%.At 5% moi of P2O5, although the Na2 xCsxMoO4 compound does not crystallize, there is phase separation between a silicate liquid and a phosphomolybdic liquid which remain amorphous upon cooling. No information on the phase separation temperature of the cast iron is given, knowing that a separated cast iron at the production temperature is unacceptable for industrial vitrification.

[0014] In the two aforementioned articles, the maximum MoO3 contents studied are approximately 4%mob. However, in certain waste conditioning glasses, similar contents are already achieved (the melts must be homogeneous but phase separations on cooling are allowed in the latter) with lower P2O5 contents. In addition, the addition of P2O5 proposed to stabilize the molybdenum is relatively high (5%moi for 4%moi of MoO3 in [3], 2.5%moi for 3%moi of MoO3 in [4]). Since phosphorus is a corrosive element, it is not desirable to increase the P2O5 content in the glasses too much.

[0015] Another study by Schuller et al, 2011, compares, by rheological measurements, the phase separation behavior of four SiO2-Na2O-B2O3-Li2O-CaO-MoO3-P2O5-Al2O3-ZnO-ZrO2-Nd2O3-[Minor oxides] cast irons with increasing MoO3 contents [5]. The glass richest in MoO3 (6.3% moi corresponding to about 12% mass) designated "AMoP" also has an increased ZrO2 content (4.3% moi corresponding to about 7% mass). However, this cast iron is the one with the lowest phase separation temperature (between 1150°C and 1180°C compared to 1240°C for another glass containing 0.1% moi of MoO3 and 2.4% moi of ZrO2 less). Furthermore, phosphorus is at a fixed content in the glasses studied: in the "AMoP" glass, the concentration of P2O5 is 1.9% moi which corresponds to about 4% mass compared to the total mass of this glass.

[0016] The inventors therefore set themselves the goal of proposing a material allowing the confinement of HA and MA-VL nuclear waste rich in molybdenum, said material being easy to implement and not subject to crystallization or phase separation at the temperatures of industrial nuclear waste vitrification processes. Statement of the invention

[0017] The present invention makes it possible to achieve the goal set by the inventors. Indeed, the latter have shown that it is possible to increase the quantity of molybdenum (MoO3) integrated into the glass network at the glass production temperature, by a combined addition of zirconium (in the form of ZrO2) and phosphorus (in the form of P2O5) in the composition of the glass.

[0018] Indeed, during laboratory tests, the inventors have shown that such an addition reduces the phase separation temperature of borosilicate cast irons in a synergistic manner since, with a combined addition of ZrO2 and P2O5, the phase separation temperature drops by more than 60°C, whereas the addition of ZrO2 or P2O5 alone does not modify this temperature or only very little (see experimental section below and in particular Table 2).

[0019] The originality of the present invention lies in the favorable and unexpected effect of the joint addition of phosphorus and zirconium, which are common elements and sometimes even provided, in part, by the waste to be vitrified. If the favorable effects of zirconium individually [4] and of phosphorus individually [2,3] to lower the phase separation temperature have been seen in certain specific studies on borosilicate molybdenum glasses, the combined effect of phosphorus and zirconium has never been highlighted. In Schuller et al, 2011 [5], the phosphorus is at a fixed content (approximately 4% mass) and its effect coupled with that of zirconium is not shown at all. Furthermore, the skilled person would have been dissuaded from reducing the amount of ZrO2 in the “AMoP” glass described in [5] since, in this publication, glasses containing less ZrO2 exhibit higher phase separation temperatures.

[0020] Phosphorus is, moreover, seen as not very favorable to the reduction of temperature. phase separation in certain articles. This element can be corrosive for the furnace, the increase in the phosphorus pentoxide content is limited, within the framework of the invention, and more particularly between 1 and 3.5% mass, thanks to the increase in that of zirconium dioxide between 4 and 7% mass, unlike the contents implemented in articles [2] and [3].

[0021] Thus, the cast iron produced in the vitrification furnace is homogeneous, even at production temperatures below 1100°C, which facilitates the conduct of the process and reduces corrosion of the furnace. Furthermore, the use of between 1 and 3.5% by mass of P2O5 and between 4 and 7% by mass of ZrO2 makes it possible to limit corrosion and crystallization.

[0022] In other words, the synergy of the combined addition of ZrO2 and P2O5 is favorable to maintaining the homogeneity of molybdenum-rich borosilicate glass cast irons such as those that can be used in the vitrification of certain nuclear waste. By improving the incorporation of molybdenum, the number of packages to be produced is then significantly reduced, which allows significant savings in energy, time, money, warehousing and storage.

[0023] For example, some glasses resulting from the rinsing of fission product tanks contain approximately 8 to 9% by mass of MoO3. An increase of almost 50% by mass in the MoO3 content (from 8 to 12% by mass) in the final glass therefore represents a substantial gain.

[0024] Furthermore, the invention is a solution to the technical problem of the solubility of molybdenum in glass castings that is easy to implement. Indeed, it makes it possible to retain current vitrification processes by only modifying the composition of the glass casting, without modifying the vitrification installations themselves.

[0025] Finally, the present invention relates not only to the glasses used for the treatment of HA and / or MA-VL nuclear waste but applies, more generally, to any glass likely to contain a significant quantity of molybdenum.

[0026] Thus, the present invention relates, first of all, to a silicate glass comprising: - 35 to 45% mass of silicon dioxide (SiO2), - at most 15% by mass of molybdenum trioxide (MoO3), - from 4 to 7% mass of zirconium dioxide (ZrO2), - from 1 to 3.5% mass of phosphorus pentoxide (P2O5), and - from 14 to 28% by mass of one or more alkali metal oxides and / or one or more alkaline earth metal oxides, the mass percentages being expressed relative to the total mass of the glass.

[0027] In the silicate glass according to the invention, the MoO3 is typically present in an amount of between 1 and 15% mass, in particular between 5 and 14% mass, more particularly between 8 and 13% mass.

[0028] In the silicate glass according to the invention, ZrO2 is notably present in a quantity between 4 and 6.5% mass and, in particular, between 4 and 6% mass.

[0029] In the silicate glass according to the invention, the P2O5 is notably present in an amount of between 1 and 3.3% by mass and, in particular, between 1 and 3% by mass.

[0030] By "from 14 to 28% by mass of one or more alkali metal oxides and / or one or more alkaline earth metal oxides" is meant, in the context of the invention, that the quantity of all the alkali metal oxides and / or alkaline earth metal oxides that the silicate glass according to the invention may contain is between 14 and 28% by mass.

[0031] In a first embodiment, the silicate glass according to the invention contains one or more alkali metal oxides chosen from the group consisting of lithium oxide (Li2O), sodium oxide (Na2O), potassium oxide (K2O), rubidium oxide (Rb2O), cesium oxide (Cs2O), and francium oxide (Fr2O). In particular, when the silicate glass according to the invention is a silicate glass for the confinement of HA and / or MA-VL nuclear waste, it may contain one or more alkali metal oxides chosen from the group consisting of lithium oxide (Li2O), sodium oxide (Na2O), potassium oxide (K2O), rubidium oxide (Rb2O), and cesium oxide (Cs2O).

[0032] In a second embodiment, the silicate glass according to the invention contains one or more alkaline earth metal oxides chosen from the group consisting of beryllium oxide (BeO), magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), barium oxide (BaO), and radium oxide (RaO). In particular, when the silicate glass according to the invention is a silicate glass for the confinement of HA and / or MA-VL nuclear waste, it may contain one or more alkaline earth metal oxides chosen from the group consisting of magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO).

[0033] In a third embodiment, the silicate glass according to the invention contains one or more alkali metal oxides as previously defined and one or more alkaline earth metal oxides as previously defined.

[0034] In particular, the alkali metal oxide(s) and / or the alkaline earth metal oxide(s) that the silicate glass may contain, in particular when the latter is a silicate glass for the confinement of HA and / or MA-VL nuclear waste, are chosen from the group consisting of lithium oxide (Li2O), sodium oxide (Na2O), potassium oxide (K2O), rubidium oxide (Rb2O), cesium oxide (Cs2O), magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO). The quantity of all of these oxides in the silicate glass according to the invention is between 14 and 28% by mass relative to the total mass of the glass.

[0035] The silicate glass according to the invention may further contain at least one element selected from the group consisting of boron trioxide (B2O3), aluminum oxide (or alumina, A12O3), and zinc oxide (ZnO). In one embodiment, the silicate glass according to the invention contains boron trioxide (B2O3) and may be defined as a borosilicate glass. In another embodiment, the silicate glass according to the invention contains boron trioxide (B2O3) and aluminum oxide (A12O3) and may be defined as an alumino-borosilicate glass. In another embodiment, the silicate glass according to the invention contains boron trioxide (B2O3), aluminum oxide (A12O3), and zinc oxide (ZnO).

[0036] When present in the silicate glass according to the invention, the boron trioxide (B2O3) is in an amount of between 12 and 18% by mass relative to the total mass of the glass.

[0037] When present in the silicate glass according to the invention, the aluminum oxide (A12O3) is in an amount of between 2 and 8% by mass relative to the total mass of the glass.

[0038] When present in the silicate glass according to the invention, the zinc oxide (ZnO) is in an amount of between 0.1 and 5% by mass relative to the total mass of the glass.

[0039] Furthermore, the silicate glass according to the invention may further contain at least one element chosen from the group consisting of titanium dioxide (TiO2), vanadium oxide (V2O5), manganese oxide (MnO), manganese dioxide (MnO2), cobalt oxide (CoO), iron oxide (Fe2O3), nickel oxide (NiO), chromium oxide (Cr2O3), copper oxide (CuO), zinc oxide (ZnO), yttrium oxide (Y2O3), niobium oxide (Nb2O5), technetium oxide (TcO2), lanthanum oxide (La2O3), neodymium oxide (Nd2O3), gadolinium oxide (Gd2O3), praseodymium oxide (Pr2O3), (Pr2O3), cerium dioxide (CeO2), selenium dioxide (SeO2), tellurium dioxide (TeO2), uranium dioxide (UO2), thorium dioxide (ThO2), europium oxide (Eu2O3), antimony trioxide (Sb2O3), americium dioxide (AmO2), plutonium dioxide (PuO2), curium dioxide (CmO2), neptunium dioxide (NpO2), ruthenium dioxide (RuO2), rhodium dioxide (RhO2),palladium oxide (PdO), silver oxide (Ag2O), cadmium oxide (CdO), lead oxide (PbO), tin dioxide (SnO2), fluorine in ionic form (F ), chlorine in ionic form (Cl ), sulfur trioxide in ionic form (SO3 ), palladium (Pd), and rhodium (Rh). All or part of the aforementioned elements are present in particular when the silicate glass according to the invention is a silicate glass for the confinement of HA and / or MA-VL nuclear waste. When present, this or these elements are in a total quantity of less than 15% by mass, and in particular less than 10% by mass relative to the total mass of the glass.

[0040] In a first embodiment, the silicate glass according to the invention can easily be prepared by any vitrification technique known to those skilled in the art using a mixture of products, in particular oxides, in the form of powders of which the composition is identical to that of the silicate glass according to the invention.

[0041] In a second embodiment, the silicate glass according to the invention is a silicate glass for the packaging of HA and / or MA-VL nuclear waste.

[0042] In a first method of this second embodiment, the evaporation-calcination step of the HA and / or MA-VL nuclear waste precedes the vitrification step. In this method, a calcination of the HA and / or MA-VL nuclear waste is carried out, to which a calcination aid has optionally been added to obtain a calcinate; in the second step, a vitrification aid is added to this calcinate to melt them in a heated metal pot, for example by induction or by resistors, to obtain a glass melt, then said glass melt is poured and cooled, whereby the silicate glass according to the invention is obtained.

[0043] In a second method of this second embodiment, the evaporation-calcination step of the HA and / or MA-VL nuclear waste also precedes the vitrification step. In this method, calcination of the HA and / or MA-VL nuclear waste is carried out, to which a calcination aid has optionally been added to obtain a calcinate; in the second step, a vitrification aid is added to this calcinate to melt them in a cold crucible where the glass melt is heated by direct induction, then said glass melt is poured and cooled, whereby the silicate glass according to the invention is obtained.

[0044] In a third method of this second embodiment, the evaporation-calcination step of the HA and / or MA-VL nuclear waste and the vitrification step are concomitant. In this one-step method without prior calcination, the liquid supply of the HA and / or MA-VL nuclear waste solution is carried out in a hot pot, a cold crucible or an electrode furnace.

[0045] In the three processes of the second embodiment, it is clear that the composition of the vitrification adjuvant depends on the chemical element contents of the HA and / or MA-VL nuclear waste to be vitrified. Indeed, the composition of the vitrification adjuvant will be chosen so that it makes it possible to obtain a silicate glass for the conditioning of HA and / or MA-VL nuclear waste whose composition is as defined above. This vitrification adjuvant may be a glass frit or a mixture of products, in particular oxides, in the form of powders.

[0046] In the first two methods of the second embodiment, it is possible to add a calcination adjuvant also called a “dilution adjuvant” to the HA and / or MA-VL nuclear waste or during calcination. Examples of such calcination adjuvants include aluminum nitrate and an organic adjuvant such as sugar.

[0047] The HA and / or MA-VL nuclear waste capable of being confined in the silicate glass according to the invention is typically in the form of a nitric aqueous effluent. containing in particular metal or metalloid nitrates, fission products, minor actinides and elements contained in solutions and effluents from nuclear installations in the nuclear fuel cycle or from nuclear research centers.

[0048] In the first two methods of the second embodiment, the first step is a calcination step which can be carried out in a rotating tube heated to a temperature of about 400°C.

[0049] The present invention also relates to the use of zirconium dioxide (ZrO 2) in an amount of 4 to 7% by mass and phosphorus pentoxide (P2O5) in an amount of 1 to 3.5% by mass to improve the solubility of molybdenum in a glass casting comprising - 35 to 45% mass of silicon dioxide (SiO2), - at most 15% by mass of molybdenum trioxide (MoO3), and - from 14 to 28% by mass of one or more alkali metal oxides and / or one or more alkaline earth metal oxides, the mass percentages being expressed in relation to the total mass of the glass cast iron.

[0050] In other words, the present invention relates to a method for improving the solubility of molybdenum in a glass melt comprising - 35 to 45% mass of silicon dioxide (SiO2), - at most 15% by mass of molybdenum trioxide (MoO3), and - from 14 to 28% by mass of one or more alkali metal oxides and / or one or more alkaline earth metal oxides, this method comprising a step of adjusting, in the glass melt, the quantity of zirconium dioxide (ZrO2) to a quantity of 4 to 7% by mass and phosphorus pentoxide (P2O5) to a quantity of 1 to 3.5% by mass, the mass percentages being expressed in relation to the total mass of the glass cast iron.

[0051] Everything that has been previously described for the silicate glass according to the invention also applies to the glass melt from which the latter is obtained. This concerns in particular the quantity of MoO3 and the additional elements that the silicate glass and, therefore, the glass melt from which it is obtained may contain.

[0052] In a first embodiment, this adjustment can be carried out using a mixture of chemical products, in particular oxides, in the form of powders whose composition is identical to that of the silicate glass according to the invention.

[0053] In a second embodiment, the glass cast iron is obtained from a vitrification additive and HA and / or MA-VL nuclear waste such as pre previously defined. In this embodiment, the HA and / or MA-VL nuclear waste may be in the form of a calcine. Since the HA and / or MA-VL nuclear waste or the calcine of an HA and / or MA-VL nuclear waste may contain ZrO2 and / or P2O5, the adjustment consists of choosing a vitrification additive whose composition makes it possible to obtain a glass melt containing from 4 to 7% by mass of ZrO2 and from 1 to 3.5% by mass of P2O5. The vitrification additive is also chosen so that the other elements contained in the glass melt, such as in particular SiO2 and alkali metal oxides and / or alkaline earth metal oxides, make it possible to obtain a silicate glass whose composition is as previously defined. Note that MoO3, for its part, is provided by HA or MA-VL nuclear waste or the calcine of HA or MA-VL nuclear waste.

[0054] Everything that has been previously described for the processes for preparing silicate glass according to the invention also applies to the preparation of glass melting, such as, for example, the nature of the HA and / or MA-VL nuclear waste, the calcines and the vitrification conditions.

[0055] The improvement in the solubility of molybdenum can be appreciated by comparing the phase transition temperature of a glass cast iron comprising i) from 4 to 7% by mass of ZrO 2 but no P2O5, ii) from 1 to 3.5% by mass of P2O5 but no ZrO2 or iii) ZrO2 in a quantity other than 4 to 7% by mass and P2O5 in a quantity other than 1 to 3.5% by mass with the phase transition temperature of a glass cast iron comprising from 4 to 7% by mass of ZrO2 and from 1 to 3.5% by mass of P2O5. This improvement corresponds to a decrease in the phase transition temperature observed for the glass cast iron comprising from 4 to 7% by mass of ZrO2 and from 1 to 3.5% by mass of P2O5.

[0056] Other characteristics and advantages of the present invention will become apparent to those skilled in the art upon reading the examples below given for illustrative and non-limiting purposes, with reference to the appended figures. Brief description of the drawings

[0057] [Fig-1] shows a macroscopic image of the “12Mo-2P” glass.

[0058] [Fig.2] shows the rheograms of the “8.7Mo” glass.

[0059] [Fig.3] shows the rheograms of the “12Mo” glass.

[0060] [Fig.4] presents the rheograms of the “12Mo-7Zr” glass.

[0061] [Fig.5] presents the rheograms of the “12Mo-2P” glass.

[0062] [Fig.6] shows the rheograms of the “12Mo-7Zr-2P” glass according to the invention.

[0063] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS I. Different borosilicate glasses.

[0064] 1.1. Formulations of the different borosilicate glasses.

[0065] A study was carried out on different matrices, based on glasses of condi- tion of simulated and simplified intermediate-level waste, to verify whether the same phenomenon of reduction of the phase separation temperature by addition of ZrO2 was obtained. A first reference glass with a MoO3 content of 8.7% mass (designated "8.7Mo") was first developed. This composition is known and allows molybdenum to be incorporated into a single-phase cast iron at a production temperature between 1050°C and 1200°C.

[0066] Then the MoO3 content was increased to 12% mass (designated “12Mo”) in order to exacerbate the phase separation phenomena and to evaluate the impact of the different oxides on the phase separation temperature of the cast irons produced.

[0067] Thus, at contents of 12% mass of MoO3, a glass similar to the control glass was produced, then a glass with an increased content of ZrO2 only (~7% mass instead of ~3% mass) (designated "12Mo-Zr"), another glass with an increased content of P2O5 only (2% mass instead of ~0.05% mass) (designated "12Mo-P") and finally a coupling between increased content of ZrO2 and P2O5 (respectively ~7% mass and 2% mass) (designated "12Mo-7Zr-2P"). The exact oxide compositions produced are indicated in Table 1 below:

[0068] [Tables 1] Oxides 8.7Mo 12Mo 12Mo-7Zr 12MO-2P 12Mo-7Zr-2P SiO2 46.61 44.59 42.29 43.60 41.37 Na2O 8.38 8.02 7.60 7.84 7.43 B2O3 16.54 15.83 15.01 15.48 14.68 A12O3 3.68 3.53 3.34 3.45 3.27 P2OS 0.05 0.07 0.07 2.00 2.00 Moo3 8.67 12 12 12.00 12.00 ZnO 2.20 2.11 2.00 2.06 1.96 ZrO2 3.16 3.16 7.13 3.09 6.97 CaO 4.95 4.94 4.95 4.83 4.84 Li2O 3.46 3.31 3.14 3.24 3.07 SrO 0.14 0.19 0.19 0.19 0.19 Cs2O 0.32 0.44 0.44 0.43 0.43 BaO 1.83 1.83 1.83 1.79 1.79 Total 100.00 100.00 100.00 100.00 100.00

[0069] 1.2. Preparation processes for different borosilicate glasses.

[0070] For the first three glasses “8.7Mo”, “12Mo” and “12Mo-7Zr”, the process The preparation process was identical. Precursors such as oxide and carbonate powders were weighed, taking into account their respective purities. The powder mixture, with a mass of 600 g, was homogenized using a Turbula® mixer. This mixture was placed in a platinum-rhodium crucible and heated in a tilting furnace to a temperature of 1050°C. The melt was homogenized by mechanical stirring at a speed of 80 rpm during the refining process, which lasted 3 hours. The glass was finally cast onto a plate by tilting the crucible. During casting, a more fluid supernatant phase was distinctly observed for the three glasses. After cooling, the glasses were macroscopically homogeneous and opaque.

[0071] The last two glasses "12Mo-2P" and "12Mo-7Zr-2P" were also produced by mixing oxide and carbonate powders. Taking into account the purities of each oxide, each powder was weighed to obtain a final glass mass of 200 g. These powders were mixed and then introduced into a platinum-gold crucible. The mixture was heated to 1050°C at 300°C / h in a muffle furnace. The glasses were refined for 3 hours at 1050°C and then cast onto a plate. The recovered glasses were then crushed and then returned to the muffle furnace at approximately 700°C and heated again to 1050°C for 3 hours. The melts are then cast onto a plate and the final glasses are obtained. This double melting aims to homogenize the melt and eliminate any unmelted matter that may remain during a first melting, without stirring. During the casting of the "12Mo-2P" glass, a supernatant phase, appearing more fluid, was observed.For the "12Mo-7Zr-2P" glass, during the first casting, no supernatant phase was observed. During the second casting, a small supernatant phase was observed this time. These two cast and cooled glasses are, like the first glasses, opaque and milky white ([Fig. 1]).

[0072] II. Phase separation temperature for different borosilicate glasses.

[0073] IL 1. Calculation methods.

[0074] The phase separation temperature of these cast irons was determined by rheological measurements. This temperature is the temperature below which the "molten glass" liquid is no longer stable but separates into two more stable liquids: a silicate liquid and a molybdate liquid. Above this temperature, the cast iron is homogeneous.

[0075] Rheological measurements make it possible to determine the viscosity of the glass as a function of the shear rate at different temperatures (>800°C), with an uncertainty of 10%. They are carried out on a setup combining a rheometer with a high-temperature furnace. The rheometer controls the rotation speed of the rotor, which then imposes a shear rate (linked by a calibration constant to the rotation speed of the rotor) on the melt. The torque exerted by the molten glass, linked to the stress by another calibration constant, is then measured and the viscosity is given by the ratio stress / shear rate. Analyzing the viscosity of the glass as a function of the shear rate, at a given temperature, makes it possible to define its rheological behavior. The latter is representative of its homogeneity: - if the viscosity does not depend on the shear rate, the glass is homogeneous, its behavior is said to be “Newtonian”, or - if the viscosity depends on the shear rate, the glass is heterogeneous, its behavior is said to be “non-Newtonian”.

[0076] It is then possible to highlight the presence of several liquid phases in the glass. Measurements carried out at several temperatures, from the highest temperatures to the lowest temperatures, then make it possible to determine the temperature at which the glass becomes heterogeneous, i.e. the phase separation temperature.

[0077] Furthermore, the viscosity of homogeneous cast irons follows a VFT law (for “Vogel-Fulcher-Tamann”) of the type as a function of temperature:

[0078] [Math.l]

[0079] where q is the viscosity of the melt at temperature T. A, B and T0 are adjustment constants.

[0080] The temperature at which a deviation from the VFT law occurs is also linked to the phase separation temperature. The methodology adopted was to retain, as the phase separation start temperature, the temperature at which at least one of the two criteria is met.

[0081] II.2. Results.

[0082] Approximately 55 g of glass were weighed and then remelted at around 1250°C in the furnace associated with the rheometer. Thus, the rheological measurements start on a homogeneous melt whose behavior is Newtonian. The imposed shear rate increases from 0 to 100 s 1 then decreases from 100 to 0 s1. The temperature is lowered in steps of 50°C, 20°C or 10°C to determine the phase separation temperature as precisely as possible. The rheograms of each glass are presented in Figures 2 to 6.

[0083] The phase separation temperatures measured by this method are shown in Table 2 below:

[0084] [Tables2] Glass 8.7Mo 12Mo 12Mo-7Zr 12MO-2P 12Mo-7Zr-2 P Temperature (phase separation) 1040 + 10°C 1070 + 10°C 1070 + 10°C 1060 + 15°C 1000 + 15°C

[0085] The results obtained with the “12Mo” glass clearly show that molybdenum is the cause of the increase in the phase separation temperature. The more MoO3 a glass is loaded with, that is to say, the higher the waste incorporation rate, the more difficult it is to obtain a homogeneous melt.

[0086] An addition of ZrO2 alone (“12Mo-7Zr” glass) or of P2O5 alone (“12Mo-2P” glass) does not change or changes very little the phase separation temperature of the melt.

[0087] However, a combined addition of ZrO2 and P2O5 (glass "12Mo-7Zr-2P") lowers the phase separation temperature by more than 60°C. This reduction then makes it possible to prepare glasses richer in MoO3 at a constant production temperature. References

[0088] [1] Demande de brevet CN 110590161 A-publiée le 20 décembre 2019.

[0089] [2] Demande internationale WO 2009 / 039059 Al publiée le 26 mars 2009.

[0090] [3] Prakash et al, 2019, “Studies on modified borosilicate glass for enhancement of solubility of molybdenum”, Journal of Non-Crystalline Solids, vol. 510, pages 172-178.

[0091] [4] Krishnamurthy & Kroeker, 2022, “Improving Molybdenum and Sulfur Vitri fication in Borosilicate Nuclear Waste Glasses Using Phosphorus: Structural Insights from NMR”, Inorg Chem., vol. 61, pages 73-85.

[0092] [5] Schuller et al, 2011. “Liquid-Liquid Phase Séparation Process in Borosilicate Liquids Enriched in Molybdenum and Phosphorus Oxides”, J. Am. Ceram. Soc., vol. 92, pages 447-454.

Claims

Claims

1. Silicate glass comprising: - from 35 to 45% by mass of silicon dioxide (SiO2), - at most 15% by mass of molybdenum trioxide (MoO3), - from 4 to 7% by mass of zirconium dioxide (ZrO2), - from 1 to 3.5% by mass of phosphorus pentoxide (P2O5), and - from 14 to 28% by mass of one or more alkali metal oxides and / or one or more alkaline earth metal oxides, the mass percentages being expressed relative to the total mass of the glass.

2. Silicate glass according to claim 1, characterized in that the MoO3 is present in an amount of between 1 and 15% by mass, in particular between 5 and 14% by mass, more particularly still, between 8 and 13% by mass.

3. Silicate glass according to claim 1 or 2, characterized in that said alkali metal oxide(s) and / or said alkaline earth metal oxide(s) are selected from the group consisting of lithium oxide (Li2O), sodium oxide (Na2O), potassium oxide (K2O), rubidium oxide (Rb2O), cesium oxide (Cs2 0), magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO).

4. Silicate glass according to any one of claims 1 to 3, characterized in that said silicate glass further contains at least one element chosen from the group consisting of boron trioxide (B2O3), aluminum oxide (or alumina, A12O3), and zinc oxide (ZnO).

5. A silicate glass according to any one of claims 1 to 4, characterized in that said silicate glass further contains at least one element selected from the group consisting of titanium dioxide (TiO2), vanadium oxide (V2O5), manganese oxide (MnO), manganese dioxide (MnO2), cobalt oxide (CoO), iron oxide (Fe2O3), nickel oxide (NiO), chromium oxide (Cr2O3), copper oxide (CuO), zinc oxide (ZnO), yttrium oxide (Y2O3), niobium oxide (Nb2O5), technetium oxide (TcO2), lanthanum oxide (La2O3), neodymium oxide (Nd2O3), gadolinium oxide (Gd2O3), praseodymium (Pr2O3), cerium dioxide (CeO2), selenium dioxide (SeO2), tellurium dioxide (TeO2), uranium dioxide (U02), thorium dioxide (ThO2), europium oxide (Eu2O3), antimony trioxide (Sb2O3), americium dioxide (Am02), plutonium dioxide (PuO2), curium dioxide (CmO2), neptunium dioxide (NpO2), ruthenium dioxide (RuO2), rhodium dioxide (RhO2), palladium oxide (PdO), silver oxide (Ag2O), cadmium oxide (CdO), lead oxide (PbO), tin dioxide (SnO2), ionic fluorine (F), ionic chlorine (Cl), ionic sulfur trioxide (SO3), palladium (Pd), and rhodium (Rh).

6. Use of zirconium dioxide (ZrO2) in an amount of 4 to 7% by mass and phosphorus pentoxide (P2O5) in an amount of 1 to 3.5% by mass to improve the solubility of molybdenum in a glass cast iron comprising - from 35 to 45% by mass of silicon dioxide (SiO2), - at most 15% by mass of molybdenum trioxide (MoO3), and - from 14 to 28% by mass of one or more alkali metal oxides and / or one or more alkaline earth metal oxides, the mass percentages being expressed relative to the total mass of the glass cast iron.

7. A method for improving the solubility of molybdenum in a glass cast iron comprising - from 35 to 45% by mass of silicon dioxide (SiO2), - at most 15% by mass of molybdenum trioxide (MoO3), and - from 14 to 28% by mass of one or more alkali metal oxides and / or one or more alkaline earth metal oxides, said method comprising a step of adjusting, in the glass cast iron, the amount of zirconium dioxide (ZrO2) to an amount of 4 to 7% by mass and phosphorus pentoxide (P2O5) to an amount of 1 to 3.5% by mass> the mass percentages being expressed relative to the total mass of the glass cast iron.

8. Method according to claim 7, characterized in that said glass melt is obtained from a vitrification additive and high-level (HL) and / or long-lived intermediate-level (LL-IL) nuclear waste.

9. Method according to claim 7 or 8, characterized in that said HA and / or MA-VL nuclear waste is in the form of an aqueous nitric effluent containing in particular metal or metalloid nitrates, fission products, minor actinides and elements contained in the solutions and effluents from nuclear installations of the nuclear fuel cycle or from nuclear research centers.

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