METHOD FOR CONDITIONING AN ACID WASTE BY CEMENTATION

Abstract

A method for conditioning an acid waste by cementation, wherein the acid waste is chosen among liquids having a pH of no more than 4, semi-liquids having a pH of no more than 4, solids of which the partial or full dissolution in water leads to a solution or suspension having a pH of no more than 4, and mixtures thereof, which method comprises the steps of: a) preparing a cement paste having as components at least: a magnesium phosphate cement and the acid waste, and b) hardening the cement paste thus obtained, and is characterised in that in step a), the cement paste is prepared without subjecting beforehand the acid waste to any treatment consisting in reducing the acidity thereof.

Claims

1. A method for conditioning an acid waste by cementation, the acid waste being a liquid having a pH of no more than 4, a semi-liquid having a pH of no more than 4, a solid of which a partial or full dissolution in water leads to a solution or suspension having a pH of no more than 4, or a mixture thereof, which comprises the steps of: a) preparing a cement paste having as components at least a magnesium phosphate cement and the acid waste, and b) hardening the cement paste thus obtained, and wherein at step a), the cement paste is prepared without subjecting beforehand the acid waste to any treatment consisting in reducing the acidity of the acid waste.

2. The method of claim 1, wherein the magnesium phosphate cement comprises magnesium oxide and a phosphoric acid salt in a Mg/P molar ratio of between 1 and 12.

3. The method of claim 2, wherein the phosphoric acid salt is potassium dihydrogen phosphate.

4. The method of claim 1, wherein the cement paste further comprises at least one superplasticizers or setting retarder.

5. The method of claim 4, wherein the cement paste comprises at least one of hydrofluoric acid, sodium fluoride, boric acid or sodium borate.

6. The method of claim 1, wherein the cement paste further comprises at least one of sand and gravel.

7. The method of claim 1, wherein the cement paste has a water/magnesium phosphate cement mass ratio of 0.10 to 1.

8. The method of claim 1, wherein step a) comprises: i) loading the magnesium phosphate cement and water into a container and mixing the cement and water until a homogeneous mixture is obtained; ii) adding the acid waste in dry, wetted, semi-liquid or liquid form to the container; and simultaneously or successively iii) mixing the mixture obtained at i) with the acid waste until homogenisation, whereby the cement paste is obtained.

9. The method of claim 1, wherein step a) comprises: i) loading the acid waste in dry form into a container and mixing the waste until homogenisation; ii) adding water and the magnesium phosphate cement to the container; and simultaneously or successively iii) mixing the acid waste with the water and the magnesium phosphate cement until homogenisation, whereby the cement paste is obtained.

10. The method of claim 1, wherein step a) comprises: i) loading the acid waste in wetted, semi-liquid or liquid form into a container and mixing the latter until homogenisation; ii) adding the magnesium phosphate cement to the container and mixing the cement with the acid waste until a homogenous mixture is obtained; iii) optionally adding water to the container and, simultaneously or successively, mixing the mixture obtained at ii) with the water until homogenisation, whereby the cement paste is obtained.

11. The method of claim 1, wherein step a) comprises: i) loading the acid waste in wetted, semi-liquid or liquid form into a container and mixing the waste until homogenisation; ii) adding at least one of sand and gravel to the container and mixing the waste with the at least one of sand and gravel until a homogeneous mixture is obtained; iii) adding water and the magnesium phosphate cement to the container; and simultaneously or successively iv) mixing the mixture obtained at ii) with the water and the magnesium phosphate cement until homogenisation, whereby the cement paste is obtained.

12. The method of claim 1, wherein step a) comprises: i) loading the magnesium phosphate cement into a first container and mixing the cement until homogenisation; ii) loading water and the acid waste in dry, wetted, semi-liquid or liquid form into a second container and mixing the water with the waste until a homogenous mixture is obtained; iii) transferring the mixture obtained at ii) from the second container to the first container; and simultaneously or successively iv) mixing the magnesium phosphate cement with the mixture obtained at sub-step ii) until homogenisation, whereby the cement paste is obtained.

13. The method of claim 1, wherein the cement paste comprises from 5% to 70% by mass of the acid waste.

14. The method claim 1, wherein the acid waste is a waste produced by a nuclear industry.

15. The method of claim 14, wherein the acid waste is: a waste issued from a process of mining extraction of uranium, conversion and enriching of uranium, production of fresh nuclear fuels or treatment of spent nuclear fuels; a waste issued from a decontamination of nuclear cycle equipment and plants, or of nuclear reactors; a waste issued from a remediation-dismantling of nuclear plants; or a mixture thereof.

16. The method of claim 2, wherein the Mg/P molar ratio is between 5 and 10.

17. The method of claim 7, wherein the water/magnesium phosphate cement mass ratio is of from 0.20 to 0.60.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0108] FIG. 1 illustrates the change in setting time, denoted t and expressed in minutes, of mortars based on a magnesium phosphate cement that have been mixed with an aqueous solution either of nitric acid or solely composed of water, as a function of the pH of this aqueous solution; in this Figure, curve A corresponds the onset of mortar setting whilst curve B corresponds to the end of mortar setting.

[0109] FIG. 2 illustrates the change in reaction heat, denoted Q and expressed in J/g, of mortars based on a magnesium phosphate cement that have been mixed with an aqueous solution either of nitric acid or solely composed of water, as a function of time denoted t and expressed in hours; in this Figure, curve A corresponds to a mortar mixed with an aqueous solution comprising 0.1 mol/L of nitric acid (pH 1); curve B corresponds to a mortar mixed with an aqueous solution comprising 3 mol/L of nitric acid (pH≈−0.5), whilst curve C corresponds to a mortar mixed with water.

[0110] FIG. 3 illustrates the change in compression strength, denoted R and expressed in MPa, of mortars based on a magnesium phosphate cement that have been mixed with an aqueous solution either of nitric acid or composed solely of water, as a function of the pH of this aqueous solution.

[0111] FIG. 4 illustrates the curves of differential thermal analysis (or DTA curves) of mortars based on a magnesium phosphate cement that have been mixed with an aqueous solution either of nitric acid or composed solely of water, as a function of the pH of this aqueous solution; in this Figure, the heat flow, denoted Φ and expressed in μV/mg, is given along the Y-axis whilst the temperature denoted Φ and expressed in ° C., is given along the X-axis.

[0112] FIG. 5 gives the X-ray diffraction diagrams of mortars based on a magnesium phosphate cement that have been mixed with an aqueous solution either of nitric acid or composed solely of water; in these diagrams, the letter q indicates the presence of quartz, the letter k indicates the presence of k-struvite, whilst the letter m indicates the presence of magnesium oxide.

[0113] FIG. 6 illustrates the change in compression strength, denoted R and expressed in MPa, of mortars based on a magnesium phosphate cement that have been mixed with an aqueous solution either of sulfuric acid or solely composed of water, as a function of the pH of this aqueous solution.

[0114] FIG. 7 gives the DTA curves of mortars based on a magnesium phosphate cement that have been mixed with an aqueous solution either of sulfuric acid or composed solely of water, as a function of the pH of this aqueous solution; in this Figure the heat flow, denoted Φ and expressed in μV/mg, is given along the Y-axis, whilst the temperature, denoted θ and expressed in ° C., is given along the X-axis.

[0115] FIG. 8 gives the X-ray diffraction diagrams of mortars based on a magnesium phosphate cement that have been mixed with an aqueous solution either of sulfuric acid or composed solely of water; in these diagrams the letter q indicates the presence of quartz, the letter k indicates the presence of k-struvite, whilst the letter m indicates the presence of magnesium oxide.

[0116] FIG. 9 illustrates the change in compression strength, denoted R and expressed in MPa, as a function of time denoted t and expressed in days, of a first mortar based on a magnesium phosphate cement comprising an acid sludge, and for comparison a second mortar only differing from the first in that it is devoid of acid sludge; in this Figure, the curves A and B correspond to two different samples of the first mortar whilst curve C corresponds to a sample of the second mortar.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

Example 1: Cementation of Nitric Acid

[0117] A first series of mortars was prepared having the composition and characteristics given in Table 1 below.

TABLE-US-00001 TABLE I Components Mg/P water/cement sand/cement (mass %) (mol/mol) (m/m) (m/m) MgO (DBM 90) 26 5 0.30 1 KH.sub.2PO.sub.4 17 Borax  1 Sand CV32 (Sibelco) 43 Water 13

[0118] For doing that, the solid constituents of these mortars (i.e. MgO, KH.sub.2PO.sub.4, borax and sand) were first mixed together in a mixer for 2 minutes to obtain a homogenous mixture, and the mixture thus obtained was mixed with an aqueous mixing solution for 30 seconds at slow speed, then 30 seconds at rapid speed and finally for 1 minute at slow speed.

[0119] Six different aqueous mixing solutions were used, namely: [0120] five solutions comprising nitric acid in respective proportions of 0.003 mol/L (pH≈2.5), 0.01 mol/L (pH 2), 0.1 mol/L (pH 1), 1 mol/L (pH 0) and 3 mol/L (pH≈−0.5); and [0121] a solution solely composed of water (pH 7) to provide a reference mortar.

[0122] The mortars were subjected to: [0123] setting time measurements, performed with a Vicat instrument in accordance with standard NF EN 196-3+A1 (Methods of testing cement. Part 3: Determination of the setting time and soundness); and [0124] measurements of reaction heat (or heat of hydration) over a period of 150 hours, performed using a Langavant calorimeter in accordance with standard NF EN 196-9 (Methods of testing cements. Part 9: Heat of hydration, semi-adiabatic method).

[0125] After hardening, they were also subjected to: [0126] measurements of compression strength, performed using a mortar press on prismatic test specimens of 4 cm×4 cm×16 cm, in accordance with standard NF EN 196-1 (Methods of testing cements. Part 1: Determination of mechanical strengths); and [0127] differential thermal analyses (DTA).

[0128] The results of these measurements and DTA are shown in FIGS. 1 to 4.

[0129] FIGS. 1 to 3 show that the presence of nitric acid in the aqueous mixing solutions:

[0130] 1° has no notable negative impact on the setting time of mortars for solutions having a pH equal to or higher than 2 (i.e. a concentration of nitric acid equal to or lower than 0.01 mol/L); on the other hand, an increase in setting time is observed for solutions having a pH equal to lower than 1 (cf. FIG. 1);

[0131] 2° leads to a reduction in the heat of hydration of mortars when the acid concentration of the aqueous solution is increased (cf. FIG. 2); and

[0132] 3° induces a reduction in the compressive strength of mortars but that, irrespective of the nitric acid concentration, the compressive strength obtained is greater than 8 MPa which represents the desired minimum value of compressive strength (cf. FIG. 3).

[0133] FIG. 4 shows that a first endothermal peak, positioned between 120° C. and 135° C. and corresponding to dehydration of k-struvite (representing the binder phase of magnesium phosphate cements derived from the reaction between MgO and KH.sub.2PO.sub.4), is common to all the mortars even if it is ascertained that the mass loss associated with this peak becomes more and more smaller as the nitric acid concentration of the aqueous mixing solution increases.

[0134] After hardening, the mortars were also characterized by X-ray diffraction (XRD).

[0135] As shown in FIG. 5, the reference mortar is composed of the following crystalline phases: [0136] a phase corresponding to quartz (subscript q), which represents the main phase of the mortar and is derived from the sand, [0137] a phase corresponding to k-struvite (subscript k) previously mentioned, and [0138] the remaining MgO (subscript m) since only about 15 mass % of the MgO added to the mortar are consumed at the time of k-struvite formation.

[0139] FIG. 5 also shows that on and after a nitric acid concentration of 0.1 mol/L (pH 1), characteristic peaks of potassium nitrate (KNO.sub.3) occur at the following 2θ angle values: 27.2°; 27.6°; and 34.1°.

[0140] No trace of KH.sub.2PO.sub.4 is observed in the XRD diagram of the reference mortar, suggesting that this compound is fully consumed at the time of k-struvite formation.

[0141] Therefore, the addition of nitric acid to a mortar at the time of preparation thereof induces the formation of potassium nitrate.

[0142] The present example shows that the cementation of highly to very highly acidic waste produced by industrial processes using nitric acid, such as aqueous effluents derived from the refining of natural uranium concentrates or from the treatment of spent nuclear fuels, can be carried out directly, i.e. without any prior treatment of this waste intended to reduce the acidity thereof, and without shortening the setting time and without increasing the reaction heat.

[0143] A light reduction in mechanical properties is observed with an increase in nitric acid concentration. This is due to the fact that, since potassium dihydrogen phosphate reacts with nitric acid, it is partially consumed by this reaction and is hence less available to react with the magnesium oxide and to form k-struvite with the latter. An increase in the amount of phosphoric acid salt, in this case KH.sub.2PO.sub.4, incorporated in the cement paste, mortar or concrete should be sufficient to overcome this phenomenon.

Example 2: Cementation of Sulfuric Acid

[0144] A second series of mortars was prepared having the composition and characteristics given in Table 1 above, following the same operating protocol as indicated in Example 1 but using as mixing solution: [0145] three aqueous solutions comprising sulfuric acid in respective proportions of 0.1 mol/L (pH 1), 1 mol/L (pH 0) and 3 mol/L (pH≈−0.5); and [0146] a solution composed solely of water (pH 7), also to provide a reference mortar.

[0147] The mortars were subjected to measurements of setting time performed in the same manner as in Example 1 and, after hardening, to compressive strength measurements and DTA also performed in the same manner as in Example 1.

[0148] The results of the setting time measurements are given in Table II below, whilst the results of the compressive strength measurements and DTA are given in FIGS. 6 and 7.

TABLE-US-00002 TABLE II Setting time (min) pH Onset End 7 27.4 42.4 1 17 30 0 42 72 ≈−0.5 27 47

[0149] This Table and FIGS. 6 and 7 show that the presence of sulfuric acid in the mortars does not have any notable impact on the setting time of the mortars or on their compressive strength or on the amount of k-struvite formed during hardening of the mortars.

[0150] After hardening, the mortars were also characterized by XRD.

[0151] As shown in FIG. 8, the presence of sulfuric acid in a mortar does not appear to have any visible effect on the crystalline phases of the mortar, even with a sulfuric acid concentration of 3 mol/L. No phase onset is ascertained.

Example 3: Cementation of a Sludge Containing Hydrofluoric Acid

[0152] A sludge containing hydrofluoric acid was cemented proceeding as follows.

[0153] First, corrosion products in the form powdery flakes and comprising on average: 17.6 mass % of fluorine, 4.4 mass % of nickel, 9.8 mass % of iron, 15.6 mass % of uranyl fluoride (UO.sub.2F.sub.2) and 33.3 mass % of uranium tetrafluoride (UF.sub.4), were mixed with water in a mass ratio of 1 to prevent any dispersion of the flakes into the surrounding atmosphere.

[0154] An acid sludge was obtained of pH 2 since the UF.sub.4 contained in the flakes reacts with water to release hydrofluoric acid following the equation:

UF.sub.4+2H.sub.2O.fwdarw.UO.sub.2+4HF.

[0155] A first mortar was then prepared having the composition and characteristics given in Table III below.

TABLE-US-00003 TABLE III Components Mg/P water/cement sand/cement (mass %) (mol/mol) (m/m) (m/m) Sludge (flakes + water) 35 5.1 0.52 0.5 MgO (DBM 90) 24 KH.sub.2PO.sub.4 16 Borax 1 Sand CV32 (Sibelco) 20 Additional water 4

[0156] For doing that, the solid constituents of the mortar (i.e. MgO, KH.sub.2PO.sub.4, borax and sand) and the additional water were first mixed together in a mixer until homogenisation, after which the acid sludge was added to the mixer and the whole was mixed until homogenisation.

[0157] As reference, a second mortar was prepared of same composition and characteristics as the first mortar with the exception that it was free of corrosion products, i.e. flakes.

[0158] No notable difference was observed in terms of setting time and reaction heat between the first and second mortars.

[0159] After hardening of the mortars, the latter were cut into samples of cubic shape with sides of 4 cm and these samples were subjected to compressive strength tests using a manual press.

[0160] The results of these tests are given in FIG. 9 where the curves A and B correspond to two different samples of the first mortar, whilst curve C corresponds to a sample of the second mortar.

[0161] The water/cement mass ratio of the first and second mortars was high since it is 0.52, the effect of which is to reduce the compressive strength, an effect which is notoriously known for all cement materials and, in particular, for materials based on magnesium phosphate cements.

[0162] On the other hand, FIG. 9 shows that the presence of a highly acidic sludge in the first mortar has no notable impact on the values of compressive strength obtained for this mortar.

CITED REFERENCES

[0163] [1] C. Utton and I. H. Godfrey, Report by the National Nuclear Laboratory NNL (09) 10212, 29 Jan. 2010 [0164] [2] International application PCT WO 2004/075207