Control of Hg0 re-emission from gypsum slurries by means of additives in typical wet scrubber conditions

The effect of different additives on the re-emission of Hg0 in wet limestone-based flue gas desulfurization (WFGD) conditions is assessed. Because re-emission of elemental mercury from limestone slurries is dependent on the pH of the gypsum slurry, re-emission of elemental mercury was evaluated for different additives using gypsum slurry with a constant pH value of 7. The additives investigated included oxidizing and reducing agents (Fenton reagents and sodium thiosulfate) and also chelating agents (2,4,6 trimercaptotriazine trisodium and sodium hydrosulfide). Fenton reagents were added to prevent the reduction of oxidized mercury species as H2O2 was the precursor used to convert sulfite ions into sulfate or to oxidize Hg0. Thiosulfate were added so that mercury could be retained in the gypsum at low concentrations as insoluble HgS, while mercury is being stabilized in the liquid fraction at higher additive doses. The addition of 2,4,6 trimercaptotriazine trisodium (TMT) and NaHS in low doses prevented the reduction of oxidized mercury species as insoluble HgS and Hg3 (TMT) precipitate onto the gypsum slurry. TMT was found to be most effective additive for Hg2+ capture.