Air pollution control
View Patent ↗In a method of controlling air pollution, a combustion gas is passed through a slurry of an alkaline-earth metal sulfide and a redox buffer (e.g., a phosphate), preferably in combination with an alkaline-earth metal carbonate and/or hydroxide. A composition for removing a heavy metal from a combustion gas is also provided, as an aerosolized, aqueous dispersion or slurry of an alkaline-earth metal sulfide and a buffering agent. A solid residue of the air pollution control process is also described.
1. A method for controlling air pollution, comprising: allowing a combustion gas to contact an aqueous, 20-50% w/w solids dispersion of an alkaline-earth metal sulfide and a buffering agent, wherein the buffering agent is selected from the group consisting of phosphoric acid, salts of phosphoric acid, alkaline-earth metal-based pH buffers, and mixtures thereof.
2. The method of claim 1 , wherein the solids have an average particle size of 75 microns or less.
3. The method of claim 1 , wherein the solids have an average particle size of 45 microns or less.
4. The method of claim 1 , wherein the solids have an average particle size ranging from about 45 microns to about 75 microns.
5. The method of claim 1 , wherein the alkaline-earth metal sulfide is selected from the group consisting of calcium sulfide, magnesium sulfide, adducts of calcium sulfide, adducts of magnesium sulfide, mixed calcium-magnesium sulfides, and mixtures thereof.
6. The method of claim 1 ,wherein the buffering agent includes at least one alkaline-earth metal-based pH buffer selected from the group consisting of calcium carbonate, calcium hydroxide, calcium phosphate, magnesium carbonate, magnesium hydroxide, magnesium phosphate, mixed calcium-magnesium carbonates, mixed calcium-magnesium hydroxides, mixed calcium-magnesium phosphates, triple super phosphate, apatite, and mixtures thereof.
7. The method of claim 1 , wherein the buffering agent includes at least one redox buffer capable of stabilizing a mercury sulfide precipitate against water resolubilization.
8. The method of claim 7 , wherein the at least one redox buffer comprises phosphoric acid or a salt of phosphoric acid.
9. The method of claim 1 , wherein the buffering agent comprises triple super phosphate and at least one alkaline-earth metal carbonate or hydroxide.
10. The method of claim 1 , wherein the aqueous mixture includes a hyperdispersant.
11. The method of claim 10 , wherein the hyperdispersant is polymeric.
12. The method of claim 1 ,wherein the aqueous mixture of alkaline-earth metal sulfide and buffering agent comprises a mixture of calcium sulfide, triple super phosphate, and calcium carbonate.
13. The method of claim 12 , wherein the calcium sulfide, triple super phosphate, and calcium carbonate are present in relative amounts of 3:1:2 by weight, respectively.
14. The method of claim 1 , wherein the solids dispersion is prepared by bead milling.
15. The method of claim 1 , wherein the combustion gas is generated in an incinerator or power plant.
16. The method of claim 1 , wherein the combustion gas is generated by a coal-burning furnace.
17. A method for controlling air pollution, comprising
forming an effluent containing at least one heavy metal sulfide by injecting an aqueous, 20-50% w/w solids dispersion into a combustion gas containing at least one heavy metal, the dispersion comprising an alkaline-earth metal sulfide and a buffering agent selected from the group consisting of phosphoric acid, salts of phosphoric acid, alkaline-earth metal-based pH buffers, and mixtures thereof; and
collecting the effluent.
18. The method of claim 17 , further comprising adjusting the pH of the effluent to a pH of about 6 to 8.
19. The method of claim 17 , wherein the alkaline-earth metal sulfide comprises calcium sulfide, the alkaline-earth metal carbonate comprises calcium carbonate, and the phosphate comprises triple super phosphate.
20. The method of claim 17 , wherein the alkaline-earth metal sulfide is selected from the group consisting of calcium sulfide, magnesium sulfide, adducts of calcium sulfide, adducts of magnesium sulfide, mixed calcium-magnesium sulfides, and mixtures thereof.
21. The method of claim 17 , wherein the buffering agent includes at least one alkaline-earth metal-based pH buffer selected from the group consisting of calcium carbonate, calcium hydroxide, calcium phosphate, magnesium carbonate, magnesium hydroxide, magnesium phosphate, mixed calcium-magnesium carbonates, mixed calcium-magnesium hydroxides, mixed calcium-magnesium phosphates, triple super phosphate, apatite, and mixtures thereof.
22. The method of claim 17 , wherein the buffering agent includes at least one redox buffer capable of stabilizing a mercury sulfide precipitate against water resolubilization.
23. The method of claim 22 , wherein the at least one redox buffer capable of stabilizing a mercury sulfide precipitate against water resolubilization comprises phosphoric acid or a salt of phosphoric acid.
24. The method of claim 17 , wherein the mixture of alkaline-earth metal sulfide and buffering agent are injected into the combustion gas through one or more spray nozzles.
25. The method of claim 17 , further comprising dewatering the mixture in the effluent tank to obtain a drained water stream and a concentrated slurry; and filtering the concentrated slurry to obtain a solid residue.
26. The method of claim 25 , wherein the solid residue is an inorganic matrix containing mercury in a substantially nonleachable form.