FR3080112A1 - PROCESS FOR PACKAGING BORATE WASTE BY CEMENTING - Google Patents

Abstract

The present invention relates to a process for conditioning waste based on borates and alkaline salts by immobilization in a cementing matrix in which said waste is mixed with reagents leading to a cement of zinc phosphate and / or of calcium phosphate, said reagents comprising phosphoric acid H3PO4 in water and ZnO for zinc phosphate cement and CaO for calcium phosphate cement. The present invention also relates to a cementitious matrix comprising waste materials based on borates and alkaline salts obtained by a packaging method according to the invention.

Description

Process for conditioning borate waste by cementing.

The field of the present invention is that of packaging methods for hazardous waste, in particular radioactive waste from the nuclear industry.

More particularly, the present invention relates to cementing which is a technique often used in solid waste conditioning processes or processes for coating waste in solution or in powder form. This cementing process is used to produce heterogeneous (solid waste) or homogeneous (liquid waste) waste packages, well stabilized over time.

These cementing processes to stabilize waste for final storage are relatively simple and easy to implement once the correct formulation of the cement has been found. However, the formulation of cement for incorporating a waste into a cement matrix is not always easy, since many balances between the hydrated phases of the cement and the waste constituents occur which can inhibit the setting times of the cement. cement and / or alter the mechanical strength or impermeability properties of the final cement matrix material.

More particularly still, the present invention relates to the packaging of borate-based waste, in particular in the form of borax and alkaline salts, in particular for the immobilization of borated waste from evaporator concentrates of a nuclear power plant.

Portland cement is the most widely used material for incorporating and stabilizing waste, thereby forming a containment matrix or simply coating the waste particles. Portland cements are hydraulic binders that set and harden under a chemical reaction to water called hydration. When the paste (cement, air and water) is added to the aggregates (sand and gravel, crushed stone or other granular material), it acts as an adhesive and binds the aggregates together to form a mass similar to stone, concrete , the most versatile and widespread artificial material that exists. Portland cement is obtained by reducing to a powder a clinker essentially consisting of hydraulic calcium silicates to which various forms of calcium sulphate (gypsum / plaster), limestone and water are added, as well as various addition products of choice from the manufacturer such as lime, silica, alumina and iron.

The term borate refers to a wide range of molecular compounds made up of boron and oxygen atoms including hydroborates, which also contain the ion or hydroxyl function, as well as various hydrated forms. The most common are the tetraborate ion, B ₄ O ₇ ^2_ and the hydrogenotetraborate ion, HB ₄ O ₇ -. The sodium tetraborate anhydride Na ₂ B ₄ O ₇ is more commonly found in the hydrated state in the form of borax, with the crude formula Na ₂ B ₄ O ₇ "10H ₂ O also called sodium tetraborate decahydrate or hydrated sodium borate

Waste rich in borate ions presents difficulties of immobilization using Portland cement because borate ions inhibit the setting and hardening of Portland cement, leading to unacceptable hardening times, up to several days. While the required setting time of the cement must not exceed 48 hours, preferably between 6 and 24 hours, for an economically acceptable industrial operation.

An object of the present invention is to provide economical and reliable solutions without environmental nuisance for the cementing of borate waste with required setting times of the cement not exceeding 48 hours and properties of mechanical resistance and impermeability suitable for storage at long term.

Two main approaches have been considered to reduce the retarding effect of borate ions, either by chemical pretreatment of waste streams to convert the participating species to other compounds, or by the use of a specific cement.

A chemical pretreatment of known waste consists in adding portlandite Ca (OH) ₂ to Portland cement in order to precipitate borates such as calcium hexahydroborite. This strategy makes it possible to reduce the setting time, but the setting time remains significant and in certain cases up to several weeks / month to harden. In addition, calcium hexahydroborite is rapidly destabilized in the cement matrix, due to a reaction between the tricalcium aluminate, portlandite and borate ions leading to the progressive formation of hydrates of hydrated calcium mono or quadriboroaluminate.

As regards the use of a specific cement to limit the delays in the hydration of borate ions, it has been proposed to use a sulfur-aluminum cement made up of a mixture of Portland cement, aluminous cement and lime, whose objective is to incorporate boron. Studies by Qina Sun et al. (Q. Sun, J. Li, and J. Wang, Solidification of borate radioactive resins using sulfoaluminate cernent blending with zeolite, Nucl. Eng. Des., Vol. 241, no. 12, pp. 5308-5315, 2011), a sulfoaluminous cement-zeolite matrix was used to study the cementation of borate ion exchange resins. Several forms of waste simulated with boron concentrations varied from 9.9 to 44.9 g / L and molar ratios of Na / B from 0.15 to 0.46 were treated by two cement formulations (sulfo-aluminate and Portland) with the addition of Ca ( OH) 2, zeolite and setting accelerator. The experimental results have shown that boron always delays the setting time for the two cement formulas, and the setting end time increases with the decrease in the Na / B ratio and the increase in boron concentration.

According to the present invention, we first researched and studied other alternative cements to Portland cement to finally study more specifically phosphate cements in order to find solutions to this problem of setting time, since certain cements based on phosphates have excellent properties, such as their rapid setting and mechanical properties.

Phosphate cements are synthesized following a rapid reaction of acid-base type, at room temperature, between a liquid or solid acid usually in the form of acid phosphate and a base, usually in the form of a powdered oxide of 'divalent metal oxides such as magnesium, calcium and zinc oxides (CaO, MgO and ZnO) and aluminum oxide (AI2O3) and iron oxide (FeiCh).

The formation of acid-base cements can be summarized according to the following stages:

a) Dissolution of the oxides in an acid solution to form cations,

b) Acid-base reaction between cations and phosphate anions to form a gel,

c) Formation of a connected network which can be crystalline, semi-crystalline or amorphous. This stage depends on several parameters, in particular, the rate of dissolution of the oxides in the acid solution. In the best conditions, the gel crystallizes around the core of each unreacted oxide grain, to form a dense material.

As an acid source, a solution of an acid phosphate is generally used, in which the bases dissolve moderately. The phosphate acids are numerous and a choice beforehand is necessary for the production of the desired phosphate cement. Ammonium phosphate NH4H2PO4 and diammonium phosphate (NhkhHPO played a major role in the formation of phosphate cement, but they release ammonia during and after cement formation. They are used for exterior applications such as road repair materials but practically no interior application has been found for these products The use of NaH2PO4 leads to the formation of an uncondensed amorphous phase and the final product lacks cohesion and disintegrates in water. Cement tends to develop microcracks over time because the Na + ion is a mobile species which can easily be extracted from the material by aqueous leaching. The compounds AIHsiPO ^ z'HzO and CaCHzPO ^ z'HjO react too quickly and therefore large objects are difficult to manufacture with these precursors.KH2PO4 is the acid phosphate form most used in the production of ets of large size and it allows to produce excellent cements.

Phosphoric acid H ₃ PO ₄ has also been used for the manufacture of phosphate cements. Once dissolved in water, this tri-acid gives up its protons by forming phosphate anions (H ₂ PO ₄ ', HPO ₄ ² and PO4 ³ ').

The basic sources are divalent metal oxides such as magnesium, calcium and zinc oxides (CaO, MgO and ZnO) are the main candidates for the formation of phosphate cement. Aluminum oxide (AI2O3) and iron oxide (FezOs), also have excellent potential for forming these cements.

Magnesium phosphate cements (also called phosphomagnesium cement), that is to say obtained from MgO oxide, have many interesting properties, such as: (very) fast setting, good resistance to compression and tensile, low permeability, good water resistance with a loss of about 10% after immersion in water for 28 days, good heat resistance and good adhesion with Portland cement surfaces.

However, borates have again been used as an effective retarder of the rate of hydration of a phosphomagnesium cement obtained with ammonium or potassium phosphates as an acid source (H. Lahalle et al., Investigation of magnesium phosphate surround hydration in diluted suspension and its retardation by boric acid, Cem. Concr. Res., vol. 87, pp. 77-86, 2016; T. Sugama and LE Kukacka, Characteristics of magnesium polyphosphate cements derived from ammonium polyphosphate solutions, Cem. Concr. Res., Vol. 13, no. 4, pp. 499-506, 1983) in order to properly control the setting time. Besides the retarding effect of borates, several other effects have been observed, in particular borax negatively affects the mechanical behavior of the final material in terms of the compressive strength of magnesium phosphate cements.

The work which has been carried out for the present invention has focused on the incorporation of different waste compositions containing borate ions and alkali salts, in different types of phosphate cements and more particularly magnesium phosphates, calcium phosphates and zinc phosphates. These works were based on preliminary analyzes of mechanical strength of samples, supplemented by analyzes of composition by X-ray diffraction and structural by SEM electron microscopy and soaking tests (parallel with the regulatory leaching test) carried out on some samples.

Thus, according to the present invention, certain cement formulations have been developed for which the structure of the final product obtained is more like a new material due to a chemical reaction between the borate waste and the components of the cement to form the cement matrix. (unlike the known cementitious matrices in which the waste is incorporated by simple mixing without chemical interaction between the waste and the matrix).

The present invention provides formulations of cement matrices of borate and cement waste based on calcium phosphate and zinc phosphate obtained with phosphoric acid as an acid source in the formulation of cement, formulations for immobilizing waste borates from evaporator concentrates and having advantageous properties compared to those of Portiand cement, such as a very fast setting time, good mechanical resistance to compression and traction and also a low permeability to water when the taken place.

More specifically, the present invention provides a process for conditioning wastes based on borates and alkaline salts by immobilization in a cementing matrix in which the reactants leading to a cement are mixed with said wastes, in particular with an aqueous solution of said wastes. of zinc phosphate and / or a calcium phosphate cement, the said reagents comprising phosphoric acid H ₃ PO ₄ in water and ZnO for the zinc phosphate cement and / or respectively CaO for the calcium phosphate cement.

More particularly, said waste is radioactive waste mainly comprising borax at a rate of at least 100 g of borax / kg of waste, in particular from 200 to 750 g / kg, and most often from 300 to 500 g / kg.

More particularly still, the said wastes also contain nitrate, chloride and / or sulfate salts of potassium and / or sodium with each of the molar ratios B / Na, B / K and Na / K of 0.5 to 4, preferably from 1 to 3. We understand that these are the ratios of the moles of atoms of B, Na and K respectively in ions or compounds containing them.

More particularly still, the said wastes also contain sodium nitrate and / or potassium nitrate salts, preferably at an overall concentration of the said sodium nitrate and / or potassium nitrate salts of 50 to 300 g / kg of waste. .

More particularly still, said waste is waste from concentrators of nuclear power plant evaporators containing (a) major components, preferably at a rate of at least 95% by weight comprising borax and potassium and sodium salts chosen from NaNO ₃ and KNO ₃ , and salts KCI and K ₂ SO ₄ , and (b) minority components, preferably in an amount of less than 5% by weight; comprising at least Ca (NO ₃ ) ₂ and Mg (NO ₃ ) ₂ salts.

In some cases the minority components include organic chelating agents such as EDTA and / or traces of metals, at a rate of less than 2% by weight, depending on the type of pretreatment undergone by the waste at the outlet of the evaporators.

Preferably, the rate of incorporation D / C of said waste in said cement is at least 10% by weight, in particular D / C from 10 to 70%, preferably at least 30%, more preferably at least 40 %, with D / C = (mass of waste) x100 / [(mass of oxide) + (volume of phophoric acid x 1.7)].

More preferably, the said phosphate cement comprises a molar ratio M / P of 1 to 1.5 with M = Zn for the zinc phosphate cement and M = Ca for the calcium phosphate cement, from a volume ratio water / phosphoric acid 1.

More particularly still, the said zinc phosphate cement comprises a Zn / P molar ratio less than or equal to 1.5, preferably a Zn / P molar ratio of 1.

More particularly still, the said cement comprises zinc phosphate, the said cement matrix comprising the presence of the Zn ₃ (PO ₄ ) ₂ .4H ₂ O héopite phase. More particularly and most often the héopéite phase is the majority component of said matrix according to DRX analyzes.

More particularly still, the said cement comprises zinc phosphate, the said cement matrix further comprising the presence of boric acid H ₃ BO ₃ .

More particularly still, the said calcium phosphate cement comprises a Ca / P molar ratio of 1 to 1.5, preferably a Ca / P molar ratio of 1.5.

More particularly still, with the said cement based on calcium phosphate, the said cement matrix comprises the presence of dicalcium phosphate dihydrate or brushite with the chemical formula CaHPO ₄ .2H ₂ O. More particularly, in particular for waste levels of at least minus 40%, most often the brushite phase is the majority component of the said matrix according to analyzes by XRD.

More particularly still, with the said cement based on calcium phosphate, the said cement matrix also comprises the presence of a crystalline phase of KB ₅ O ₈ .4H ₂ O.

Preferably, said cement comprises a zinc phosphate cement, and more preferably a zinc phosphate cement and a calcium phosphate.

The present invention also provides a cement matrix comprising wastes based on borates and alkaline salts obtained by a conditioning process according to the invention.

More particularly still, the said cement matrix is obtained with a said cement comprising zinc phosphate, the said cement matrix comprising the presence of the Zn ₃ (PO ₄ ) ₂ .4H2O hoteite phase and preferably also the presence of boric acid H ₃ BO _3. These components result from the chemical reaction of cement and so-called borate waste.

More particularly still, the said cement matrix is obtained with a said cement comprising calcium phosphate, the said cement matrix comprising the presence of dicalcium phosphate dihydrate of chemical formula CaHPO ₄ .2H ₂ O and preferably also the presence of a KB ₅ O ₈ .4H ₂ O crystalline phase. These components result from the chemical reaction of cement and so-called borate waste.

Other characteristics and advantages of the present invention will emerge more clearly on reading the description which follows, given in an illustrative and nonlimiting manner, with reference to FIGS. 1 to 20 below.

FIG. 1 represents the X-ray diffraction diagram of calcium phosphate cement alone.

FIG. 2 represents the X-ray diffraction diagrams of the materials produced with calcium phosphate cement and the incorporation rates of 20, 50 and 60% of the waste DK3.

FIG. 3 represents the X-ray diffraction diagrams of the materials produced with calcium phosphate cement and the incorporation rates of 20, 40 and 50% of the DKN waste.

Figures 4 to 6 represent SEM images of the material produced with CPCa cement and DK3 waste with incorporation rates of 20% (fig. 4A-4B at two different magnifications), 50% (fig. 5A-5B at two different magnifications) and 60% (fig. 6A-6B at two different magnifications).

Figures 7 to 9 represent SEM images of the material produced with CPCa cement and DKN waste with incorporation rates of 20% (fig.7A-7B at two different magnifications), 40% (fig.8A-8B at two different magnifications) and 50% (fig. 9A-9B at two different magnifications).

FIG. 10 represents the X-ray diffraction diagrams of zinc phosphate cement alone.

FIG. 11 represents the X-ray diffraction diagrams of the materials produced with zinc phosphate cement and the incorporation rates of 30, 50 and 70% of the waste DK3.

FIG. 12 represents the X-ray diffraction diagrams of the materials produced with zinc phosphate cement and the incorporation rates of 30, 50 and 70% of the DKN waste.

FIG. 13 represents the X-ray diffraction diagrams of the materials produced with zinc phosphate cement and the incorporation rates of 30, 50 and 70% of the waste DK4.

Figure 14 shows a SEM image of zinc phosphate cement alone (Z3).

FIGS. 15A-15B represent the SEM images at two different magnifications of the material produced with CPZn cement and 50% of the waste DK3.

Figures 16 to 18 represent SEM images of the material produced with CPZn cement and DKN waste with incorporation rates of 30% (fig.l6A and 16B at two different magnifications), 50% (fig.l7A-17B at two different magnifications) and 70% (fig. 18).

FIGS. 19A and 19B represent the SEM images at two different magnifications of the material produced with CPZn cement and 30% of the waste DK4.

FIGS. 20A and 20B represent the SEM images at two different magnifications of the material produced with CPZn cement and 50% of the waste DK4.

The results of incorporating three types of waste containing borate ions into formulations of cement magnesium phosphate (CPMg), calcium phosphate (CPCa) or zinc phosphate (CPZn) are presented below. The waste has been prepared from borax, sodium nitrate, potassium nitrate and other products so that it is as close as possible to the actual composition of the evaporative concentrate waste and that it best reflects a future industrial process.

The waste is introduced in a mixture with the reagents leading to the formation of the cement. More specifically, phosphoric acid is added to an aqueous solution of the waste and then the metal oxide is added.

1) Protocol for manufacturing the cement matrix:

The waste is mixed with a determined volume of water. Then, 85% phosphoric acid is added to the mixture and the addition is completed by adding the oxide (magnesium, calcium and / or zinc) in several portions with continuous stirring in order to disperse the grains well. oxide in the mixture. Finally, the paste or the solid obtained is left to dry in ambient air. The proportions of the various reagents are explained in the tables (below).

2) The waste incorporation rate is calculated by the following formula:

Incorporation rate = D / C = (mass of waste) x100 / [(mass of powdered oxide) + (volume of phosphoric acid x 1.7)].

The three waste compositions are differentiated by the different proportions of boron, sodium and potassium as shown in Tables IA and IB below in which the following abbreviations are used:

-NB2 = Borax concentration.

-K3 = concentrations in 3 sources of potassium (KNO ₃ , KCI, K ₂ SO ₄ ).

-KN = concentrations of 3 potassium sources (KNO ₃ , KCI, K ₂ SO ₄ ) + concentration of NaNO ₃ .

-K4 = concentrations of 3 potassium sources (KNO3, KCI, K2SO4) + concentration of KNO3. For K4, we replaced NaNO3 by KNO3 with the same molar number (n (Na +) = η (K +)).

Table IA: Nomenclatures and compositions of waste.

DK3 DKN DK4 g g g Borax 273.98 Borax 225.98 Borax 225, Nanos 0 NaNOj 160 KNO3 190, KNO3 59.26 KNO3 59.26 KNO3 59.2 KCI 10.6 KCI 10.6 KCI 10.6 K2SO4 12.68 K2SO4 12.68 K2SO4 12.6 C a (N 03) 2 0.18 Ca (NO3) ₂ 0.18 Ca (NO3) 2 0.18 MgCNChh 0.14 Mg (NO ₃ ) ₂ 0.14 Mg (NO ₃ ) 0.14 EDTA 3.1 EDTA 3.1 EDTA 3.1 NB2 / K3 3.19 NB2 / KN 0.92 NB2 / K4 0.82

Table IB: molar ratios of species B, Na and K in the waste compositions

Molar ratio B / Na B / K Na / K B: Na: K DK3 2 3.59 1.79 B> Na> K

DKN 0.77 2.96 3.83 Na> B> K DK4 2 0.88 0.44 K> B> Na

1) TESTS AND RESULTS OBTAINED WITH A PHOSPHOMAGNESIAN CEMENT (“CPMg”)

Preliminary tests were first carried out on the formulation of raw cement (that is to say before incorporation of waste). Tests were then carried out with some constituents of the waste taken separately to assess their individual impact. Following the incorporation of borate, sodium or potassium ions in the phosphate cement, it was found that the Mg / P optimum decreased to 1.5 with an amount of water of I mL per 1 mL of acid phosphoric.

It has been found that the optimal incorporation rate of borax in raw cement is about 25%. The substitution of O.là 0.2 g of borax by NaNO ₃ or KNO ₃ gives the same behavior of the final material. But the total replacement of borax by NaNO ₃ or KNO ₃ weakens the product.

Finally, tests were carried out by integrating increasing quantities of borated waste (DKN, DK3 or DK4) in the formulation.

1.1) Study of the incorporation of DK3 waste

The DK3 waste is incorporated into the magnesium phosphate cement with incorporation rates of approximately 10, 20, 30 and 40% for a Mg / P molar ratio = 1.5. Table 2 below summarizes the tests carried out. For a Mg / P = 2 molar ratio, the materials obtained are all brittle.

The waste incorporation rate is calculated by the following formula:

Incorporation rate = D / C = (mass of waste) xlOO! [(mass of powdered oxide) + (volume of phophoric acid xl.7)].

Table 2: Incorporation of DK3 waste into CPMg cement with Mg / P = 1.5

products H3PO4 Water DK3 waste MgO properties M (g / mol Λ 98,00 18,02 40.30 Flight. ml ml g D / C g Mg / P M20 1 2.00 0.26 10.06 0.90 1.53 Resistant M21 1 2.00 0.52 20.13 0.90 1.53 Less resistant M23 1 2.00 0.52 20.13 0.90 1.53 MgO 1000 “C; Less resistant M22 1 2.00 0.78 30.22 0.90 1.52 Friable M40 1 2.0 1.04 40,20 0.90 1.52 Powder

1.2) Study of the incorporation of DKN waste

Like DK3 waste, DKN waste is incorporated into CPMg cement with D / C incorporation rates of 10, 20, 30 and 40% with a Mg / P molar ratio = 1.5 Table 3 below summarizes the tests performed. For a Mg / P = 2 molar ratio, the materials obtained are also all brittle.

Table 3: Incorporation of DKN waste into CPMg cement with Mg / P = 1.5

products H3PO4 Water DKN waste MgO properties M (g / mol) 98,00 18,02 40.30 testing ml ml g D / C g Mg / P M17 1 2.00 0.26 10 0.90 1.52 Resistant M18 1 2.00 0.53 20 0.90 1.53 less resistant M24 1 2.00 0.52 20 0.90 1.52 Resistant M19 1 2.00 0.78 30 0.90 1.53 Less resistant; Very brittle M47 1 2.0 1.01 39 0.90 1.53 Powder

1.3) Study of the incorporation of DK4 waste

The objective of the study of DK4 waste is to know if we manage to form the K-struvite KMgPO4.6H2O cement phase because the composition of DK4 waste is rich in potassium. In the same way as the two previous wastes, DK4 is incorporated into the CPMg cement with D / C incorporation rates of approximately 10, 20, 30, 40, 50 and 60% with a Mg / P molar ratio = 1.5. Table 4 below summarizes the tests carried out.

Table 4: Incorporation of DK4 waste into CPMg cement

products H3PO4 Water DK4 waste MgO properties M (g / mol) 98,00 18,02 40.30 testing ml ml g D / C g Mg / P M81 1 2.0 0.26 10.5 0.79 1.3 Friable M82 1 2.5 0.26 10.0 0.90 1.5 Less resistant, Friable M83 1 2.5 0.52 20.1 0.90 1.5 Less resistant; Friable M84 1 2.5 0.78 30.1 0.90 1.5 Less resistant; Friable M85 1 2.5 1.04 40.2 0.90 1.5 Very resistant; Friable M86 1 2.5 1.30 50.2 0.90 1.5 Resistant; Friable M112 1 2.0 1.56 60.4 0.90 1.5 Moinsresistant; Friable

The various series of experiments carried out show a significant reduction in the resistance of the materials CPMg + DK3 and CPMg + DKN when the rate of incorporation of waste increases.

However, the materials developed with DK4 give an interesting result after the first weeks of synthesis with an incorporation rate of 40%, but the setting time is too long (about a week).

After a period of approximately three months, it can be seen that the mechanical resistance of all the materials produced with the waste has deteriorated over time, except in the case of pure cement.

Analysis of the fracture facies by SEM (Scanning Electronic Microscope) clearly highlights the difference in the microstructure between the CPMg cement alone and the materials produced with the waste. A continuous cement matrix is observed which plays its role of binder and ensures good cohesion of the material. For the higher waste contents, a decohesion is observed between the grains and the cement matrix. This loss of cohesion is accentuated with the amount of waste.

1.4) Conclusion.

A formulation based on phospho-magnesian cement alone (one of the most studied cements in recent years) has not given encouraging results as to its ability to block borated waste. Its mechanical strength quickly deteriorated as soon as the incorporation rate was increased.

We then looked at other types of phosphate cement, including one based on calcium.

2) TESTS AND RESULTS OBTAINED WITH CALCIUM PHOSPHATE CEMENT (“CPCa”)

2.1) Preparation of calcium phosphate cement alone

For the formulation of calcium phosphate cement alone CPCa, three molar ratios Ca / P 1, 1.5 and 2 were studied by following the same experimental protocol used for the formation of the cement CPMg. The best mechanical behavior is obtained with the two Ca / P ratios of 1 and 1.5. For the following, the Ca / P = 1.5 ratio is kept, for which a part of calcium forms a precursor of the cement in contact with H3PO4 and a second part reacts with the constituents of the waste.

The experimental tests and the aspects of the materials are given in table 5.

Table 5: Experimental tests for the synthesis of CPCa cement

products H3PO4 Water CaO Aspects M (g / mol) 98,00 18,02 56.08 testing ml ml g Ca / P

Z4 1 1 0.82 1.0 Very resistant Z5 1 2 1.65 2.0 Limited resistance Z6 1 1.1 1.23 1.5 Very resistant

2.2) Study of the incorporation of two types of waste in calcium phosphate cement

DK3 and DKN wastes are incorporated into calcium phosphate cement 5 with D / C incorporation rates of 10, 20, 30,

40, 50 and 60%, the Ca / P molar ratio of which is 1.5. The same experimental protocol for the tests with CPMg cement is kept. Tables 6 and 7 below summarize the tests carried out.

Table 6 Incorporation of DK3 waste into CPCa cement

elements H3PO4 Water DK3 waste CaO Aspects M (g / mol) 98,00 18,02 56.08 testing ml ml g D / C g Ca / P M6 1 1.50 0.30 10 1.23 1.5 Powder M7 1 1.50 0.59 20 1.23 1.5 Limited resistance M46 1 1.30 0.62 20 1.37 1.67 Resistant M8 1 1.50 0.88 30 1.23 1.5 Resistant M38 1 1.50 1.18 40 1.23 1.5 Resistant - Friable M49 1 1.50 1.18 40 1.23 1.5 Resistant - Friable M39 1 1.50 1.47 50 1.23 1.5 Resistant M50 1 1.50 1.76 60 1.23 1.5 Very resistant

Table 7: Incorporation of DKN waste into CPCa cement

elements H3PO4 Water DKN waste CaO Aspects M (g / mol) 98,00 18,02 56.08 testing ml ml g D / C g Ca / P M14 1 1.20 0.30 10 1.23 1.5 Powder M15 1 1.20 0.59 20 1.23 1.5 Resistant M16 1 1.20 0.88 30 1.23 1.5 Resistant M41 1 1.50 1.18 40 1.24 1.5 Very resistant

M42 1 1.00 1.17 40 1.23 1.5 Very resistant M43 1 0.70 1.47 50 1.23 1.5 Resistant M48 1 1.50 1.47 50 1.23 1.5 Limited resistance M51 1 1.50 1.76 60 1.23 1.5 Friable

For DK3 waste, we observe that the resistance of the materials seems to increase with the proportion of waste incorporated, to reach even a maximum with 60% incorporation rate.

For DKN waste, the resistance of the materials increases with the proportion of waste incorporated up to a maximum of 40%, then it decreases from 50%.

This behavior is unexpected, but it is very interesting because it shows that there is an interaction between calcium and the constituents of waste which is not the case for magnesium phosphate cements.

2.3) Study of the microstructure of materials produced by X-ray diffraction (XRD)

CPCa cement alone and the materials produced with the waste are analyzed by XRD to determine the crystalline phases. The figures below represent the DRX analyzes.

Figure 1 shows the X-ray diffraction pattern of calcium phosphate cement alone.

FIG. 2 shows the X-ray diffraction diagrams of the materials produced with calcium phosphate cement and the incorporation rates of 20, 50 and 60% of the DK3 waste.

FIG. 3 shows the X-ray diffraction diagrams of the materials produced with calcium phosphate cement and the incorporation rates of 20, 40 and 50% of the DKN waste.

The presence of monetite (of formula CaHPO ₄ ) is identified in the CPCa cement alone. But, in the presence of waste, we obtain dicalcium phosphate dihydrate or brushite of chemical formula

CaHPO4.2H2O. Indeed, the neutralization of the second acidity of orthophosphoric acid leads to calcium hydrogen phosphates with an atomic ratio Ca / P = 1, among which is the dicalcium phosphate dihydrate (brushite). The latter is an acidic compound which generally crystallizes in the form of platelets, but which can also take on the appearance of needles.

We note the presence of the brushite phase with the different waste incorporation rates.

We also identify the presence of an unknown crystalline phase KB ₅ O _fi .4H ₂ O in particular from 50% of the waste DK3 in the absence of Na NO3.

2.4) Study of the microstructure of materials produced by scanning electron microscopy (SEM).

The study carried out by MEB of the materials produced with CPCa cement and waste shows that the microscopic structure contains crystals formed on the surfaces of the nodules and strongly entangled between them. This morphology will decrease the porosity, increase the compactness, participate in the cohesion of the material and therefore increase its mechanical resistance (Figures 4 to 9).

With a very high incorporation rate of 60% of waste DK3 (Figure 6) and 50% of DKN (Figure 9), there appear zoning in dark gray, rich in oxygen and potassium (O: K = 72.02: 16.81) , it seems that these zones correspond to the crystalline phase KB ₅ O ₈ .4H ₂ O, which would explain its very high mechanical resistance.

2.5) Conclusion:

Materials with good properties are obtained with DK3 and DKN waste with very high incorporation rates up to 50%, even 60% (limit of tests carried out).

It has been demonstrated that CaO reacts with the waste to form a cement precursor obtained by the addition of H ₃ PO ₄ (namely dicalcium phosphate dihydrate or brushite with the chemical formula CaHPO ₄ .2H ₂ O).

At the end of these results on calcium phosphate cement having very good results, zinc phosphate cement, little or not widespread in the waste treatment environment, was then also studied.

3) TESTS AND RESULTS OBTAINED WITH ZINC PHOSPHATE CEMENT

Zinc phosphate cement (CPZn) is obtained following an acid-base reaction between zinc oxide and phosphoric acid. For this part, we studied the formulation of CPZn cement with variable Zn / P ratios of 0.67, 0.9, 1, 1.5 and 1.76, additions of MgO and / or CaO and water contents varied from 0 to 2.5 depending on the formulation. The experimental protocol is the same as that used for the formation of the CPMg cement (addition of powders (ZnO, MgO and CaO) to the liquid composed of 85% H3PO4 acid and water). The experimental tests and the aspects of the materials are given in table 8.

Table 8: Experimental conditions for the formulation tests of CPZn cement alone.

products h ₃ in ₄ Water ZnO MgO CaO Properties (Setting time) M (g / mol) 98,00 18,02 81.41 40.30 56.08 testing ml ml g Zn / P g Mg / P g Ca / P ZI 1 1 1.19 1.00 Very resistant (11 min) Z2 1 0.7 1.19 1.00 Very resistant (8 min) M124 1 0.5 1.20 1.01 Very resistant Z3 1 0 1.19 1.00 Very resistant (4 min) Z8 1 0 1.07 0.90 0.06 0.10 Very resistant Z9 1 1 1.79 1.50 Less resistant Z10 1 1 2.10 1.76 Less resistant Z13 1 2 1.19 1.00 0.59 1.00 Less resistant

Z14 1 2 1.20 1.00 0.83 1.02 Less resistant Z16 1 2 0.80 0.67 0.40 0.67 0.55 0.67 Less resistant Z17 1 2.5 1.79 1.50 0.42 0.51 Less resistant Z21 1 1 1.19 1.00 0.25 0.30 Resistant Z22 1 1 1.19 1.00 0.10 0.12 Very resistant

Analysis of the results shows that the Zn / P molar ratio strongly influences the mechanical appearance of the cement, the best Zn / P ratio is equal to 1. However, the CPZn cement retains its strength with 5 small additions of MgO and CaO (tests Z8 and Z22).

For ratios (Zn + Mg + Ca) / P> 1.1, the materials obtained have poor mechanical behavior as shown in tests Z9 to Z17.

For the ratio Zn / P = 1, an increase in the setting time was noted with the increase in the water content (Zl, Z2 and Z3). Therefore, it can be predicted that the borax structure water can contribute to the delay in setting time.

3.1) Preliminary formatting tests of CPZn cement with borax

A mixture of ZnO powders and borax was added to a liquid consisting of an mL of 85% phosphoric acid (H ₃ PO ₄ ) and a variable volume of water of 0.5 and 1 mL. The results are summarized in Table 9.

Table 9: Preliminary formatting tests of CPZn cement with 20 of borax

products H3PO4 Water Borax ZnO properties M (g / mol) 98,00 18,02 381.4 81.41 testing ml ml g D / C g Zn / P fl 1 1 0.60 20.61 1.19 1.00 Very resistant F3 1 0.5 0.59 20.42 1.19 1.00 Very resistant

F8 1 1 0.60 18.18 1.6 1.34 Very resistant F12 1 1 0.60 19.35 1.4 1.17 Very resistant Z28 1 1 0.74 25,65 1.20 1.00 Very resistant Z35 1 0.5 0.73 25.2 1.20 1.00 Very resistant

This series of borax incorporation experiments gives materials of good resistance with borax contents varying between 18 to 25%.

3.2) Study of the incorporation of the three types of waste in zinc phosphate cement.

Magnesium oxide has been replaced by zinc oxide while keeping the same experimental protocol of the previous tests. One of the three wastes DK3, DKN or DK4 is mixed with a fixed volume of water 10 to 0.5 mL. Then, add an mL of 85% phosphoric acid to the mixture, then add ZnO in several portions with continuous stirring in order to disperse the grains of ZnO well in the mixture. Finally, the paste or the solid obtained is left to dry in ambient air. The incorporation rate of the waste varies from 10 to 70% with a molar ratio of Ζη / P fixed at 1. The three tables 10-12 below summarize the tests carried out.

Table 10: Incorporation of DK3 waste into CPZn cement

products H3PO4 Water DK3 waste ZnO properties M (g / I) 98,00 18,02 81.41 testing ml ml g D / C g Zn / P ml 1 0.50 0.29 10 1.20 1.00 Very resistant M2 1 0.50 0.58 20 1.20 1.00 Very resistant M3 1 0.50 0.87 30 1.20 1.00 Very resistant M79 1 0.50 1.16 40 1.20 1.00 Very resistant M80 1 0.50 1.45 50 1.20 1.00 Very resistant M91 1 0.50 1.74 60 1.20 1.01 Very resistant M117 1 0.50 2.03 70 1.20 1.00 Very resistant

Table 11: Incorporation of DKN waste into CPZn cement

products H3PO4 Water DKN waste ZnO properties M (g / mol) 98,00 18,02 81.41 testing ml ml g D / C g Zn / P M9 1 0.50 0.29 10 1.20 1.01 Very resistant M10 1 0.50 0.58 20 1.20 1.00 Very resistant MH 1 0.50 0.87 30 1.20 1.01 Very resistant M77 1 0.50 1.16 40 1.20 1.01 Very resistant M78 1 0.50 1.45 50 1.20 1.00 Very resistant M90 1 0.50 1.74 60 1.20 1.01 Very resistant M116 1 0.50 2.03 70 1.20 1.00 Very resistant

Table 12: Incorporation of DK4 waste into CPZn cement

products H ₃ PO ₄ Water Waste DK4 ZnO properties M (g / mol) 98,00 18,02 81.41 testing ml ml g D / C g Zn / P MHS 1 0.50 0.29 10 1.20 1.01 Very resistant M120 1 0.50 0.59 20 1.20 1.01 Very resistant M121 1 0.50 0.87 30 1.20 1.01 Very resistant M88 1 0.50 1.16 40 1.20 1.01 Very resistant M89 1 0.50 1.45 50 1.20 1.01 Very resistant M92 1 0.50 1.74 60 1.20 1.01 Very resistant M118 1 0.50 2.03 70 1.20 1.00 Very resistant

Whatever the type of waste, very resistant materials are obtained with very high D / C incorporation rates ranging from 10 to 70%. However, it should be noted that the increase in the rate of incorporation of waste is accompanied by an increase in setting time (limiting factor).

Materials are obtained with good mechanical behavior.

3.3) Study of the microstructure of materials produced by X-ray diffraction (DRX)

The XRD analysis makes it possible to detect the crystal phases present in the samples which already show good mechanical behavior. CPZn cement alone and the materials produced with D / C rates of 30, 50 and 70% of each type of waste are ground by mortar and reduced to powder before being analyzed. The diagrams obtained are processed using two software X'Pert HighScore Plus and Origin. Figures 10 to 13 below illustrate the diffractograms obtained.

Figure 10: X-ray diffraction patterns of zinc phosphate cement alone.

Figure 11: X-ray diffraction diagrams of materials produced with zinc phosphate cement and incorporation rates of 30, 50 and 70% of DK3 waste.

Figure 12 and 13: X-ray diffraction diagrams of the materials produced with zinc phosphate cement and incorporation rates of 30, 50 and 70% of the waste DK4 (Figure 12) and DKN (Figure 13).

DRX analysis shows the majority of two phases in CPZn cement alone: the ZnHPO4 * 3H2O and Zn3 (HPO4) 3.3H2O phases while the hopeite phases: Zn3 (PO4) 2.4H2O and 20 ZnO are present in very small quantities .

When zinc oxide ZnO reacts with phosphoric acid, a zinc hydroxide is formed first, then the acid-base reaction continues:

ZnO - Z (OH) ₂ - Zn (H ₂ PO ₄ ) ₂ '2H ₂ O Zn ₃ (HPO ₄ ) ₃ “3H ₂ O - ZnHPO ₄ -H ₂ O 25 ZnHPO ₄ ' 3H ₂ O -» Zn ₃ ( PO ₄ ) 2'4H ₂ O.

There is a difference between cement formation with CPMg and CPZn. In the case of MgO, the reaction stops when monohydrogen phosphate (MgHPO ₄ '3H ₂ O, newberyite) is formed, while in the case of zinc phosphate cements, the final phosphate product is 30 Zn ₃ ( PO ₄ ) ₂ -4H ₂ O (hopeite). This difference results from the greater solubility of ZnHPO4 compared to that of MgHPO4- (newberyite).

In the presence of waste, the hopeite phase: Zn ₃ (PO ₄ ) ₂ .4H ₂ O is identified in all materials (most often in the majority).

For the three materials produced with 30, 50 and 70% of the DK3 waste, the presence of boric acid is noted. Whereas with DKN and DK4 waste, it is only present from a rate of 70%. Boric acid is considered to be formed by dissolving borax in the phosphoric acid solution according to the following equations:

Na ₂ B ₄ O ₇ .10H2O + 2H ₃ PO ₄ (aq) - 2NaH ₂ PO ₄ (aq) + 4H ₃ BO ₃ (aq) + 5H ₂ O Na ₂ B ₄ O ₇ .10H ₂ O + 2NaH ₂ PO ₄ (aq) - * 4H ₃ BO ₃ (aq) + 2Na ₂ HPO ₄ (aq) + 5H _Z O

The total reaction is as follows:

Na ₂ B ₄ O ₇ .10H ₂ O + H ₃ PO ₄ (aq) - 4H ₃ BO ₃ (aq) + 5H ₂ O + Na ₂ HPO ₄ (aq)

Also, the presence of ZnO is identified from 50% of the DK3 waste. It can be concluded that the presence of the waste inhibits the formation of cement hydrate.

For the three materials produced with 30, 50 and 70% of the DKN waste, the presence of NaNO _{3 is} noted. So, with waste DK4, we logically note the presence of KNO ₃ .

The presence of KNO ₃ , NaNO ₃ and B (OH) ₃ will negatively affect the leaching test.

3.4) Study of the microstructure of the materials produced by scanning electron microscopy (SEM) (Figures 14 to 20).

The SEM study of CPZn cement alone shows the presence of two microscopic structures, one of which is in the form of elongated platelets. In the presence of waste, cracks are observed in the materials. These cracks are not necessarily due to the materials but can appear during the preparation of samples for SEM analysis. We also note the presence of white areas. Elemental analysis of the latter shows that they are rich in Zn and O, therefore it is unreacted ZnO. Elementary analysis highlights the presence of the nitrates KNO ₃ and NaNO ₃ . The spectrum 8 point (Figure 20B) is rich in potassium, oxygen and nitrogen (K: N: O =

27.37: 8.1: 61.77) indicates the presence of KNO ₃ and the spectrum point 36 (Figure 18) is rich in sodium, oxygen and nitrogen (Na: N: O =: 18.22: 54.62) indicates the presence of NaNOs.

Figure 14: SEM images of zinc phosphate cement alone (Z3).

figure 15A-15B: SEM images of material elaborate with of CPZn cement and 50% of waste DK3. figure 16A-16B: SEM images of material elaborate with of CPZn cement and 30% of DKN waste. figure 17A-17B: SEM images of material elaborate with of CPZn cement and 50% of DKN waste.

Figure 18: SEM image of the material produced with CPZn cement and 70% of DKN waste.

Figures 19A-19B: SEM images of the material produced with CPZn cement and 30% of DK4 waste.

Figures 20A-20B: SEM images of the material produced with CPZn cement and 50% of DK4 waste.

3.5) Conclusion:

We obtain very mechanically resistant materials for the 3 types of waste (DK3, DKN or DK4) with very high incorporation rates going here up to 70% and this, with similar setting times whatever the composition of the waste (target <24h).

4) DIP TEST ON CALCIUM PHOSPHATE CEMENT AND ZINC

Simplified leaching tests (soaking) were carried out in order to estimate the water resistance and the release of the elements according to their formulation. A monolith of material is immersed in 30 ml of deionized water, then, the pH is measured with pH paper and the conductivity of the soaking solutions. Table 13 summarizes the measurements obtained.

Table 13: Measurement of pH and conductivity of soaking solutions

Materials pHàlh pH at 24h Conductivity (mS / cm) ZnP alone ZI 3-4 3 2.3 ZnP alone Z3 3-4 3 2.3 ZnP + 30% DK3 M3 4 3 4.1 ZnP + 50% DK3 M80 4-5 4-5 5.1 ZnP + 70% DK3 M117 4-5 5 6.2 ZnP + 30% DKN Mil 4 3 4.3

There is a decrease in the pH of the soaking solutions of the materials produced with CPZn cement. Unlike the case of materials made with CPCa cement where the pH increases.

The conductivity of the soaking solutions of the materials produced with CPZn + DK3 is lower than those obtained with DKN and 10 DK4 for the three incorporation rates, which would tend to prove better resistance of the formulations with DK3 (especially at 50%).

The materials produced with CPCa cement retain the same mechanical behavior after a 24-hour immersion in water. Additional tests of longer duration have confirmed this apparent good resistance to leaching.

5) GENERAL CONCLUSION

The cement formulations for blocking borated waste (3 types of formulation) presented above have highlighted the following aspects.

A formulation based solely on a phosphomagnesium cement (phosphate cement most studied in waste treatment) does not make it possible to obtain significant incorporation rates, the mechanical strength observed degrading relatively quickly with the increase in the rate of waste.

A formulation based solely on a calcium phosphate cement has satisfactory results in terms of mechanical strength for high incorporation rates (40 to 60% depending on the formulation of the waste). The tests have highlighted what is akin to the formation of a new material and not to a simple incorporation of the waste material into the cement material.

A formulation based solely on zinc phosphate cement gives very good mechanical strength results even for very high waste incorporation rates (70%). Leaching tests have shown better resistance of the formulation rich in borates (DK3). However, these tests also demonstrate an innovative character since this cement had not been tested until then for the treatment of borated waste.

In view of the results obtained, it appears interesting to use formulations of calcium phosphate and zinc cements, in particular for waste evaporator concentrates from nuclear reactors of the CNPEs type.

To date, these borated evaporator concentrates are processed in 2 ways at EDF:

-Either by cementing directly with Portland cement;

-Either by sending for incineration before cementing the ashes (preferred method to date),

The production of borated waste represents at EDF alone around 200 tonnes per year, expressed in equivalent 50 OOOppm of boron.

The advantage of the formulations according to the invention is that they have:

- an interesting potential for reducing the total volume of waste (by increasing the incorporation rate, we would improve the LIF (volume reduction factor) which is close to 0.1 for currently cementing (i.e. cementing is done by adding reagents and therefore the final volume of the treated waste is 10 times greater than the volume of the initial waste), and

- an economically attractive alternative to incineration which is itself costly (transport cost + incineration cost + final cementing cost of the ashes (LIF of 10)).

Claims (17)

1. Process for conditioning wastes based on borates and alkaline salts by immobilization in a cementing matrix in which is mixed with said wastes, reagents leading to a zinc phosphate cement and / or a calcium phosphate cement, the said reagents comprising phosphoric acid H ₃ PO ₄ in water and ZnO for the zinc phosphate cement and / or respectively CaO for the calcium phosphate cement. 2. Method according to claim 1, characterized in that said waste is radioactive waste mainly comprising borax in an amount of at least 100g of borax / kg of waste, preferably from 200 to 750g / kg. 3. Method according to claim 2, characterized in that said waste also contains nitrate, chloride and / or sulfate salts of potassium and / or sodium with each of the molar ratios B / Na, B / K and Na / K from 0.5 to 4, preferably from 1 to 3. 4. Method according to one of claims 1 to 3, characterized in that said waste also contains salts of sodium nitrate and / or potassium nitrate, preferably at an overall concentration of said salts of sodium nitrate and / or potassium nitrate from 50 to 300g / kg of waste. 5. Method according to one of claims 1 to 4, characterized in that said waste is waste concentrates from evaporators of nuclear power plants containing (a) major components, preferably at a rate of at least 95% in weight, comprising borax and potassium and sodium salts chosen from NaNO ₃ and KNO ₃ , and salts KCI, and K ₂ SO ₄ , and (b) minority components, preferably in an amount of less than 5% by weight; comprising at least Ca (NO ₃ ) ₂ and Mg (NO ₃ ) ₂ salts. 6. Method according to one of claims 1 to 5, characterized in that the rate of incorporation D / C of said waste in said cement is at least 10% by weight, more preferably at least 30% with D / C = (mass of waste) x100 / [(oxide mass) + (volume of phophoric acid x 1.7)]. 7. Method according to one of claims 1 to 6, characterized in that said phosphate cement comprises a M / P molar ratio of 1 to 1.5 with M = Zn for zinc phosphate cement and M = Ca for cement calcium phosphate, from a water / phosphoric acid volume ratio of 1. 8. Method according to one of claims 1 to 6, characterized in that said zinc phosphate cement comprises a Zn / P molar ratio less than or equal to 1.5, preferably a Zn / P molar ratio of 1. 9. Method according to one of claims 1 to 8, characterized in that the said cement comprises zinc phosphate, the said cement matrix comprising the presence of the Zn ₃ (PO ₄ ) ₂ .4H ₂ O hoteite phase . 10. The method of claim 9, characterized in that said cement comprises zinc phosphate, said cement matrix further comprising the presence of boric acid H ₃ BO ₃ . 11. Method according to one of claims 1 to 10, characterized in that the said calcium phosphate cement comprises a Ca / P molar ratio of 1 to 1.5 preferably a Ca / P molar ratio of 1.5. 12. Method according to one of claims 1 to 11, characterized in that the said cement comprises calcium phosphate, the said cement matrix comprising the presence of dicalcium phosphate dihydrate of chemical formula CaHPO ₄ .2H ₂ O: 13. The method of claim 12, characterized in that said cement comprises calcium phosphate, said cement matrix further comprising the presence of a crystalline phase of KB ₅ O ₈ .4H ₂ O. 14. Method according to one of claims 1 to 13, characterized in that said cement comprises a zinc phosphate cement, and more preferably a zinc phosphate cement and a calcium phosphate. 15. Cement matrix comprising waste based on borates and alkaline salts obtained by a conditioning process 5 according to one of claims 1 to 14. 16. Cement matrix according to claim 15, characterized in that said cement comprises zinc phosphate, said cement matrix comprising the presence of hopnite phase of Zn ₃ (PO ₄ ) ₂ .4H ₂ O and preferably also the presence of boric acid H ₃ BO ₃ . 10 17. Cement matrix according to claim 15 or 16, characterized in that the said cement comprises calcium phosphate, the said cement matrix comprising the presence of dicalclic phosphate dihydrate of chemical formula CaHPO ₄ .2H ₂ O and preferably also the presence of a crystalline phase of KB ₅ O ₈ .4H ₂ O.