Stabilization Of Mercury In Coal-fired Flue Gas Desulfurization Liquid And Gypsum

In this thesis, the competition of several heavy metal ions for precipitating agents (TMT, DTCR and Na2S) against Hg2+ in simulated flue gas desulfurization (FGD) liquid was studied. The results showed that the competitiveness order was Cu2+> Pb2+> Ni2+> Zn2+. Among them, Cu2+ and Pb2+ had the greatest effect on mercury removal. With the increase of their concentrations, the removal rates of Hg2+ by three precipitating agents were all gradually reduced. In the presence of Cu2+ or Pb2+, the increasing pH of solution enhanced mercury removal rate of three precipitating agents. However, when pH>7, the removal rate by DTCR and TMT tended to be stable, while Na2S continued to grow. In addition, the increasing dosing quantity of precipitating agent also significantly improved the removal rate of Hg2+. When the dosing quantity was added from 0.5 to 3.0 times of stoichiometric ratio, the removal rate of Hg2+ increased from 40% to about 85%. Based on the experiment results, DTCR was found to be the most ideal precipitating agent for Hg2+ in the presence of other metals.Stabilization of mercury in FGD-gypsum was done in two aspects. One is to add precipitating agent in the desulfurization slurry, stabilizing the mercury iron in the form of stable precipitate which would attach to FGD-gypsum. The other way is to add precipitating agent (DTCR or TMT) in FGD-gypsum produced by power plant already, inhibiting both re-dissolution and reduction of mercury. The results demonstrated that using different precipitating agents to remove mercury in desulfurization liquid would obviously affect the stability of mercury in FGD-gypsum. When taking NaOH or Na2S as precipitating agent, the mercury usually existed in unstable form in FGD-gypsum and tended to release again in acidic solution. After 24 hours of leaching, the dissolution rate all reached over 1%. While DTCR and TMT could make mercury form stable precipitate, hence led to rather low dissolution rate (only 0.23% and 0.15%); The results also showed that leaching under acidic conditions would increase the re-dissolution and reduction of mercy in FGD-gypsum, while adding precipitating agent could not only greatly reduced the concentration of mercury in leaching solution (nearly declined to 1/10), but also reduce more than 2/3 quantity of mercury reduction. Therefore, adding TMT or DTCR to stabilize the mercury in FGD-gypsum could effectively control secondary pollution of mercury in FGD-gypsum during acid-rain erosion.