The Stability Of Mercury By Organic Sulfur Chelating Agent Research In Wet Flue Gas Desulfurization Liquors

In this study, considering that Hg2+ in wet flue gas desulfurization (FGD) systems can easily be reduced and then released into atmosphere, causing secondary pollution, macromolecular complex named Sodium Dithiocarbamate (DTCR) was utilized to precipitate Hg2+ in FGD solutions and form stable chelate, inhibiting the reduction of Hg2+ to Hg0.Hg2+-S(IV) system was establihed, the effects of several experimental parameters on the stabilization of Hg2+ were studied, including dose of DTCR-1, initial pH, temperature. Our experimental results indicated that Hg2+ reduction rate decreased sharply while the precipitation efficiency increased rapidly, When DTCR-1 dosage was 1.0 Qth(Theoretical dose),91.1%of Hg2+ was precipitated. When the temperature rose to 50℃, the reduction rate decreased to 6.3% from initial 8.5% while the precipitation efficiency reached 92.7% compared to 86.1% at 30℃, achieving good inhibition performance. Along with the increase of pH value, the Hg2+ reduction efficiency declined from 11.2% to 2.3% whilst the Hg2+precipitation efficiency increased gradually and ultimately maintained at a relatively high level (about 90%). Atan initial pH value of 3, the sulfur atoms in functional groups of DTCR-1 were protonated and those positively charged functional group generated coloumbic repulsion against Hg+, which would reduce their binding capacities with Hg+The effects of several other parameters like concentrations of Cl-,SO42-,NO3-and Ca2+ on the stabilization of Hg2+ were also studied. The results showed that the Hg+ precipitation efficiency gradually declined with an increasing concentration of Cl-, when the concentration of SO42" increased from 0 to 20mM, Hg2+ precipitation gradually increased while the Hg2+ reduction ratio decreased from 14.2% to 2.2%. The efficiency of Hg2+ precipitation remained steady when SO42- concentration further increased. NO3- concentration had an adverse effect on the Hg2+ precipitation while Hg2+ reduction rate saw a slight increase in the circumstance of DTCR-1. When the Ca+ concentration rose from 0 to 10 mM, Hg2+ reduction rate increased from 3.4% to 6.2%, indicating that low concentration of Ca2+ promoted the removal of Hg2+Furthermore, heavy metals, to some extent, inhibited the Hg+ precipitation by DTCR-1. Due to their binding properties with DTCR-1, the inhibition ability of Hg2+ precipitation was as follows:Cu2+>Ni2+>Pb2+>Zn2+。Simulating the conditions of the practical desulfurization solutions, the effects of several experimental conditions on the stabilization of Hg+ were studied, such as dose of DTCR-1, initial pH, temperature. And then concentrations of Cl-,SO42-,NO3-and Ca2+ were also investigated. Our experimental results indicated that Hg2+ reduction rate decreased sharply with increasing dosage of DTCR-1. When DTCR-1 dosage was 1.5Qth, the Hg2+ reduction rate reached a plateau. The Hg2+ precipitation rate improved by adding DTCR-1 with reaction temperature increased and reduction rate was controlled below 5.0%. Along with the increase of initial pH value, the Hg2+ reduction efficiency declined to 1.1% while precipitation efficiency increased gradually between pH 3-5, the Hg2+ removel rate was above 90% when the pH is greater than 5.0. The pH of FGD decreased with adding the concentration of Cl-, stabilizing role of Hg2+ by DTCR-1 was weaken while reduction rate improved. SO42-would react with Hg2+ to generate HgSO3SO42- and therefore damped the formation of HgSO3, and reduction rate of Hg2+ increased. Increasing of NO3- concentration was helpful to the Hg2+ stabilization, the Hg2+ precipitation rate was 89.8% at 50mM NO3-.The increase in Ca2+ concentration led to the gradual increase in the Hg2+ precipitation rate by DTCR-1. With a view to testing the practical application value of Heavy metal chelator DTCR-1, we conducted a pilot study in one Coal-fired power plant in Hangzhou. The effects of precipitating agents (DTCR-1, DTCR-2 and TMT-15) dose, initial pH value on the stabilization of Hg2+ in FGD liquors were studied. Results indicated increment of precipitating agent dosage had a greater impact on removel rate of Hg2+ than pH value of slurry. Meanwhile, stabilization role of Hg2+ by DTCR-1 and DTCR-2 was slightly stronger than the role of TMT-15.Based on the researches above, DTCR-1 was shown to be an effective agent to inhibit Hg2+ reduction and Hg0 re-emission in FGD liquors. More than 80% of Hg2+ was captured by DTCR-1, and the Hg2+ reduction efficiency decreased below 10%, which was provided theoretical basis for industrial application of mercury control in wet FGD systems.