Method for immobilizing a mercury-containing waste

Abstract

A process for immobilizing a mercury-containing waste, which comprises: —stabilizing the mercury of the waste by precipitating the mercury as mercury (II) sulfide; then —encapsulating the waste by cementation, the cementation comprising coating the waste in a cement paste obtained by mixing a composition comprising a powder of at least one binder chosen from hydraulic cements, alkali-activated cements and acid-activated cements, with an aqueous mixing solution, then hardening the cement paste; and which is characterized in that the precipitation of the mercury as mercury (II) sulfide is obtained by reacting the mercury with a thiosulfate in a basic aqueous medium, while stirring and in the presence of a sulfide of an alkali metal, the molar ratio of the thiosulfate to the mercury being at least equal to 1.

Claims

1. A method for immobilizing a waste comprising mercury, which comprises: stabilizing the mercury of the waste by precipitation of the mercury as mercury(II) sulfide; then encapsulating the waste by cementation, the cementation comprising embedding the waste in a cementitious paste obtained by mixing a composition comprising a powder of at least one binder with an aqueous mixing solution, the binder being a hydraulic cement, a base-activated cement or an acid-activated cement, then hardening the cementitious paste; and in which the precipitation of the mercury as mercury(II) sulfide comprises reacting the mercury with a thiosulfate in a basic aqueous medium, under agitation and in the presence of an alkali metal sulfide, with a molar ratio of the thiosulfate to the mercury in the aqueous medium at least equal to 1.

2. The method of claim 1, in which stabilizing the mercury comprises: preparing a suspension by dispersing the waste in an aqueous solution of the thiosulfate under agitation and maintaining the suspension under agitation until the pH of the suspension reaches a value at least equal to 11; then adding the alkali metal sulfide to the suspension under agitation and maintaining the suspension under agitation until all the mercury has precipitated as mercury sulfide.

3. The method of claim 1, in which the molar ratio of the thiosulfate to the mercury is at least equal to 2.

4. The method of claim 1, in which the molar ratio of the alkali metal sulfide to the mercury is at most equal to 1.

5. The method of claim 1, in which the thiosulfate is sodium thiosulfate or potassium thiosulfate.

6. The method of claim 1, in which the alkali metal sulfide is sodium sulfide or potassium sulfide.

7. The method of claim 1, in which stabilizing the mercury comprises: preparing a suspension by dispersing the waste in an aqueous solution of sodium thiosulfate or potassium thiosulfate under agitation, with a molar ratio of thiosulfate to the mercury of 2 to 3, and maintaining the suspension under agitation for a period of 10 hours to 48 hours; adding a first quantity of sodium sulfide or potassium sulfide in solid form to the suspension under agitation, the first quantity being such that the molar ratio of the sulfide to the mercury is from 0.05 to 0.15, and maintaining the suspension under agitation for a period of 10 hours to 48 hours; then adding a second quantity of sodium sulfide or potassium sulfide in solid form to the suspension under agitation, the second quantity being such that the molar ratio of the sulfide to the mercury is from 0.05 to 0.15, and maintaining the suspension under agitation for a period of 48 hours to 96 hours.

8. The method of claim 1, in which the binder is a CEM I, CEM II, CEM III or CEM V cement, a vitrified blast furnace slag, a mixture thereof or a phosphomagnesium cement.

9. The method of claim 8, in which the binder is a CEM I cement or a phosphomagnesium cement.

10. The method of claim 1, in which the composition further comprises a superplasticiser, a setting retarder, or sand.

11. The method of claim 1, in which the composition has a water/binder mass ratio of 0.2 to 1.

12. The method of claim 1, in which stabilizing the mercury and encapsulating the waste are carried out in one container and encapsulating the waste comprises: introducing the binder and the aqueous mixing solution, together or separately, into the container in which the mercury has been stabilized, and mixing the waste with the binder and the aqueous mixing solution until a homogeneous embedding is obtained; and hardening the cementitious paste in the container.

13. The method of claim 1, in which stabilizing the mercury is carried out in a first container and encapsulating the waste is carried out in a second container.

14. The method of claim 13, in which encapsulating the waste comprises: introducing the binder and the aqueous mixing solution into the second container and mixing thereof until a homogeneous cementitious paste is obtained; introducing the waste into the second container and, simultaneously or successively, mixing the cementitious paste and the waste in the second container until a homogeneous embedding is obtained; then hardening the cementitious paste in the second container.

15. The method of claim 13, in which encapsulating the waste comprises: introducing the binder and the waste into the second container and mixing thereof until a homogeneous binder/waste mixture is obtained; introducing the aqueous mixing solution into the second container and mixing the binder/waste mixture with the aqueous mixing solution until a homogeneous embedding is obtained; then hardening the cementitious paste.

16. The method of claim 13, comprising, between stabilizing the mercury and encapsulating the waste, separating the waste from the aqueous medium in which the mercury has been stabilized.

17. The method of claim 1, in which the waste comprises earth, rubble, sludge, technological wastes or mixtures thereof.

18. The method of claim 1, in which the waste is a nuclear waste.

19. The method of claim 1, in which the waste comprises mercury in a metal state.

20. The method of claim 1, further comprising, prior to stabilizing the mercury, a treatment for reducing the dimensions of the waste.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 illustrates X-ray diffractogram of the mercury sulfide obtained in an example of implementation of the method of the invention.

(2) FIG. 2 illustrates the evolution of the heat of reaction (or heat of hydration), denoted as Q and expressed in J/g, as a function of the time, denoted as T and expressed in hours, of mortars based on a Portland cement CEM I, with or without the adding of mercury sulfide obtained in an example of implementation of the method of the invention; in this figure, the curves denoted as A and B correspond to two mortars to which respectively 10% and 20% by mass of this mercury sulfide have been added, while the curve denoted as C corresponds to a mortar free of said mercury sulfide.

(3) FIG. 3 illustrates the evolution of the heat of reaction (or heat of hydration), expressed in J/g, as a function of the time, denoted as T and expressed in hours, of mortars based on a phosphomagnesium cement, with or without the adding of mercury sulfide obtained in an example of implementation of the method of the invention; in this figure, the curves denoted as A and B correspond to two mortars to which respectively 10% and 20% by mass of this mercury sulfide have been added, while the curve denoted as C corresponds to a mortar free of the said mercury sulfide.

(4) FIG. 4 illustrates the evolution of the compressive strength, denoted as R and expressed in MPa, of materials resulting from the hardening of mortars, as a function of the mercury sulphide mass content, expressed in %, of these mortars, the mercury sulphide being obtained in an example of embodiment of the method of the invention; in this figure, the symbols .diamond-solid. correspond to the materials resulting from the hardening of mortars based on Portland cement CEM I, while the symbols .square-solid. correspond to the materials resulting from the hardening of mortars based on a phosphomagnesium cement.

(5) FIG. 5 illustrates the evolution of the flexural strength, denoted as R and expressed in MPa, of materials resulting from the hardening of mortars as a function of the mercury sulfide mass content, expressed in %, of these mortars, the mercury sulphide being obtained in an example of embodiment of the method of the invention; in this figure, the symbols .diamond-solid. correspond to the materials resulting from the hardening of mortars based on Portland cement CEM I, while the symbols .square-solid. correspond to the materials resulting from the hardening of mortars based on a phosphomagnesium cement.

DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT

Example 1: Precipitation of the Mercury as Mercury Sulfide in a Basic Aqueous Sodium Thiosulfate/Sodium Sulfide Medium

(6) At ambient temperature (21±2° C.), an aqueous solution of sodium thiosulphate is prepared by dissolution of 6 g of sodium pentahydrate thiosulphate Na.sub.2S.sub.2O.sub.3.5H.sub.2O in 50 ml of deionised water and then adding to this solution 2.04 g of mercury metal Hg(0), under agitation. The mercury is dispersed in the solution in the form of small droplets.

(7) After agitation for a period of 2-3 hours, the solution becomes greyish and its pH, which was 7-8 prior to the addition of mercury, increases until reaching the value of 11-12. These modifications are due to the formation in the reaction medium of mercury thiosulfates of the type Hg(S.sub.2O.sub.3) and/or Hg(S.sub.2O.sub.3).sub.2.sup.2−.

(8) After respectively 24 hours and 48 hours of agitation, 0.20 g of sodium sulfide Na.sub.2S.H.sub.2O is added to the solution, i.e. amounting to a total of 0.40 g.

(9) After 120 hours of agitation, the solution, which is red in colour, is filtered in order to recover all of the solid phase dispersed in this solution.

(10) This solid phase is subjected to X-ray diffraction analysis (XRD). The diffractogram obtained, which is illustrated in FIG. 1, shows that this solid phase is constituted of particles of mercury sulfide crystallised in α or cinnabar form, denoted as α-HgS, which is more stable than the mercury sulfide crystallised in β or metacinnabar form, denoted as β-HgS.

(11) Moreover, the optical microscopic observation of these particles shows that they measure from 5 to 10 μm.

Example 2: Encapsulation of Mercury Sulfide α-HgS in Cementitious Matrices

(12) The mercury sulfide obtained in Example 1 here above is encapsulated in cementitious matrices which are obtained by hardening two types of mortar, respectively M1 and M2, whose composition is presented in Table I here below.

(13) TABLE-US-00001 TABLE I Sand/Binder Mortar Cement Composition (m/m) W/B M1 Portland CEM I 52.5N CP2 3 0.50 (CEM I) (HOLCIM) + sand CV32 (SIBELCO) + water M2 Phospho- MgO - DBM 90 (RICHARD 1 0.30* magnesian BAKER HARRISON) + (MKP) KH.sub.2PO.sub.4 + borax + sand CV32 (SIBELCO) + water Mass ratio MgO/KH.sub.2PO.sub.4 = 1.47 *W/B = mass ratio water/(MgO + KH.sub.2PO.sub.4 + borax)

(14) In order to do this, the mercury sulfide is added to the mixture of the solid constituents of the mortars, at a level of 10% or 20% by mass relative to the total mass of the mortars, and then, after homogenisation, the mixing water is added. The mixing of the mortars is carried out according to the rules defined in the standards in force for the preparation of typical standard mortars for the measurements of mechanical resistance.

(15) The setting time, as determined by means of a Vicat setting time tester according to the standard EN 196-3+A1 (Methods of testing cement. Part 3: Determination of setting times and soundness), as well as the maximum temperature reached during hydration, as determined under Langavant semi-adiabatic conditions according to the standard EN 196-9 (Methods of testing cement. Part 9: Heat of hydration—semi-adiabatic method), of the mortars thus added of mercury sulfide are shown in Table 2 here below.

(16) By way of comparison, also indicated in this table are the Vicat setting time and the maximum hydration temperature obtained for mortars M1 and M2 free of mercury sulfide α-HgS.

(17) TABLE-US-00002 TABLE 2 Setting time Maximum hydration Mortar Start (min) End (min) temperature (° C.) M1 180 223 46.7 M1 + 10% of α-HgS 131 244 47.4 M1 + 20% of α-HgS 167 227 48.8 M2 19 26 76.2 M2 + 10% of α-HgS 21 32 68.4 M2 + 20% of α-HgS 17 24 65.3

(18) Moreover, FIGS. 2 and 3 illustrate the evolution of the heat of reaction (or heat of hydration), denoted as Q and expressed in J/g, as a function of the time, denoted as t and expressed in hours, of the various mortars, FIG. 2 corresponding to the mortars M1 (curve C), M1+10% of α-HgS (curve A) and M1+20% of α-HgS (curve B) and FIG. 3 corresponding to the mortars M2 (curve C), M2+10% of α-HgS (curve A) and M2+20% of α-HgS (curve B).

(19) Table 2 and FIGS. 2 and 3 show that, for a given type of mortar (M1 or M2), the adding of mercury sulfide α-HgS in the mortar does not substantially modify either the setting time of this mortar or the heating up that it undergoes over the course of hydration.

(20) The materials resulting from the hardening of the mortars M1, M1+10% of α-HgS, M1+20% of α-HgS, M2, M2+10% of α-HgS, and M2+20% of α-HgS are subjected to compressive and flexural strength tests according to the standard NF EN 196-1 (Methods of testing cement. Part 1: Determination of mechanical strength).

(21) The results of the compressive strength tests are illustrated in FIG. 4, while the results of the flexural strength tests are shown in FIG. 5. In these figures, which show the strength obtained, denoted as Q and expressed in MPa, as a function of the mercury sulfide α-HgS mass content of the mortars of, expressed in %, the symbols .diamond-solid. correspond to the materials resulting from the hardening of the mortars M1, M1+10% of α-HgS, and M1+20% of α-HgS, while the symbols .square-solid. correspond to the materials resulting from the hardening of the mortars M2, M2+10% of α-HgS, and M2+20% of α-HgS.

(22) These figures show that, for a given type of mortar (M1 or M2), the adding of mercury sulfide α-HgS in the mortar does not substantially modify the mechanical properties of the material resulting from the hardening of this mortar.

(23) The materials resulting from the hardening of the mortars M1, M1+10% of α-HgS, M1+20% of α-HgS, M2, M2+10% of α-HgS, and M2+20% of α-HgS are also subjected to leaching tests according to the standards XP CEN/TS 15862 (Leaching on monoliths) and NF EN 12457-2 (Leaching on fragments).

(24) The main operating conditions for these tests are presented in Table 3 here below.

(25) TABLE-US-00003 TABLE 3 Leaching on monoliths Leaching on fragments (XP CEN/TS 15862) (NF EN 12457-2) Leachate Ultrapure water Ultrapure water Sample sizes ≥40 mm in all directions granularity <4 mm Volume of 12 cm.sup.3/cm.sup.2 — leachate/Surface area of a sample Volume of — 10 L/kg leachate/Mass of a sample Time of contact of 24 hours 24 hours samples/leachate

(26) At the end of the 24 hours of leaching, the leachates are filtered on a 0.45 μm membrane filter using a vacuum filtration device and then the eluates are analysed by plasma torch atomic emission spectrometry (ICP-AES).

(27) These analyses show that all the eluates have a mercury concentration of less than 0.01 part per million (ppm), which corresponds to maximum leaching values of 0.005 mg/kg for monolithic tests and 0.1 mg/kg for fragment tests, that is to say leaching values well below the regulatory thresholds as set by ANDRA.

REFERENCES CITED

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