Study On The Transformation And Stabilization Of Mercury In Wet Desulfurization Slurry

Using wet flue gas desulfurization(WFGD)system has been recognized as one of potential technologies for mercury pollutants removal.WFGD system can control SO2 in the flue gas efficiently,at the same time it also can remove Hg2+ in the flue gas.However,the trapped Hg2+ by the slurry might be reduced into Hg0 because of the reducibility of slurry.Hg0 will release into gas phase and cause the secondary pollution.Furthermore,the mercury in the slurry might undergo the complex migration and re-distribution among gas,liquid and solid phases.The mercury in the solid fraction(gypsum)will release again during the utilization of WFDG gypsum.In view of above issuses,the investigations on the transformation and stabilization of mercury speciatio-ns in WFGD slurry and gypsum were carried out in this paper.The main works is as followed.Firstly,the reduction of Hg2+ into Hg0 was simulated on the laboratory condition.The effect of various factors on the mercury re-emission in the slurry and the distribut-ion of mercury in the gas,liquid and solid fraction was studied.Increasing in the temperature resulted in the higher Hg2+,reduction and promoted the mercury retained in the gypsum.Increased pH value also could control the mercury re-emission.As the main factor causing the Hg2+ reduction,S(IV)had a strong effect on the mercury re-emission in the slurry,and with the increasing of its concentration,the rate of Hg2+reduction was decreased.But the compound from the reaction between the S(IV)and Hg2+ attached on the surface of gypsum,thus the mercury content in the solid phase increased.Similarly,Cl' in the slurry could reacted with mercury into stable and insoluble HgClX2-x,ClHgSO3-and C12HgSO2-3.At the same time,with the increasing of Cl-concentration,the Hg2+ reduction reduced but the distribution in the gypsum increased.Secondly,this paper conducted the study on the effect of different additives on the mercury re-emission in the slurry,especially Na2S,2,4,6-trimarcaptotiazine,trisodium salt nonahydrate(TMT-18),sodium dithiocarbamate(DTCR-2)and Fenton reagent.Experimental results indicated that these four additives could effectively inhibited Hg2+reduction then the rate of Hg0 in the gas phase decreased.Na2S,TMT-18 and DTCR-2 can capture the mercury in the slurry by forming stable chelates with Hg2+,but the chelates will attach on the gypsum,so the mercury concentration in the gypsum increased.At the same time,the distribution of Hg2+ in the liquid fraction decreased after the reaction.The optimal amounts of Na2S,TMT-18 and DTCR-2 were 1 time,3 times and 1 time stoichiometric ratios in this study.And the pH value of slurry affected the rate of Hg0 reduction with TMT-18 and DTCR-2.High pH value favored the control of mercury reduction.Fenton reagents also can inhibit the re-emission of mercury in the slurry with free radicals ·OH and HO2·.These free radicals have a strong oxidization,so they can oxidize Hg0 into Hg2+.At the same time,the production Fe(OH)3 promoted the deposition of mercury in the slurry.At last,the release of Hg from two WFGD gypsum samples under heat treatment and acidic conditions during the utilization was simulated.During the heat treatment process,when the temperature was lower than 150?,the releasing rate of mercury from two samples of gypsums was all about 15%.The mercury released from gypsums increased sharply with the increasing of temperature.In addition,the increasing of heating rate promoted the Hg release from gypsums slightly.In the leaching experiment which simulated the gypsums eroded by acid rain,the results showed that when the Liquid/Solid(L/S)ratio was low,the concentration of mercury in the leaching solution was very high for two samples.The concentrations were 0.63?g/L and 0.14?g/L for two samples when L/S was 10/1.While with the L/S ratio increased,the leaching system became unsaturated,but the total amount of mercury extracted from gypsum only had a little change.It means that the increasing of extraction dose decreased the concentrati-on of mercury in leaching solution.When the pH value of the leaching solution increas-ed from 3 to 6,the concentration of extracted mercury decreased.The type of acid rain was simulated by different ratio of SO2-4:NO3-.It was found that the acid rain type had negligible effect on Hg leaching.