Method for immobilizing a mercury-containing waste

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.