Research On The Re-emission Mechanism Of Toxic Trace Elements In FGD Gypsum

The trace element in coal will be released in combustion,captured by Wet Flue Gas Desulphurization?FGD?process,and partly retained in FGD gypsum.The toxic trace elements in FGD gypsum may release to environment again during the utilization/disposal of FGD gypsum and lead to secondary pollution.Seven FGD gypsum samples from typical coal-fired power plants in China were selected in this research and the re-emission characteristics of As,Be,Cd,Co,Cr,,Hg,Mn,Ni,Pb,Sb and Se were studied under natural and industrial conditions of FGD gypsum treatment.The Hg species existing in FGD gypsum indentified by temperature programmed decomposition method were mainly HgS with the average proportion of 87.8%in the seven samples.HgCl₂,Hg₂Cl₂ and Hg₂SO₄ were also identified in FGD gypsum.The results of Bloom sequential chemical extraction indicated that Hg was mainly extracted as strongly complexed phase with the average proportion of 90.79%in the seven samples.The dissolution of HgS in 12 M HNO₃ might lead to the extraction of Hg as strongly complexed phase.The results of BCR sequential chemical extraction shown that Cd,Co and Mn mainly existed as acetic soluble phase,As,Pb,Sb and Se maily existed as reducible phase while Be,Cr and Ni maily existed as residual phase.Detectable amounts of Hg were constantly emitted to atmosphere from FGD gypsum samples in natural condition.The average Hg release rate of the three studied samples in dark at room temperature was 1.15%during the 180-day period test.The Hg release mainly came from the Hg⁰ generated by decomposition and reduction of mercury compounds in the FGD gypsum.Temperature,UV radiation,solid moisture content and air humidity all enhanced the release of Hg from FGD gypsum.However,the enhancement would weaken when too much water was added.The promotion of Hg⁰ generation and diffusion to the atmosphere might be the reason for the enhancement of Hg release by these factors.The Hg release was notable during the calcination of FGD gypsum in industry with the average release rate of 12.10%.The amount of Hg release from FGD gypsum to atmosphere in China at 2013 was about6.56±0.52 t with 0.65±0.05 t from disposal in natural condition and 5.91±0.47 t from industrial condition.The results of leaching experiments according to TCLP and HJ/T 299-2007indicated that the FDG gypsum has no leaching toxicity from the perspective of 11 toxic trace elements.The leaching concentration of Cd and Mn decreased monotonically with the increase of pH value.The leaching concentration of As,Hg and Sb was high under strong acidic condition and alkali condition of pH value of 10.The leaching concentration of Be,Co,Cr,Ni,Pb and Se was high under both acidic and alkaline condition but low near neutral pH value.The leaching rates of these trace elements were high when pH value below 4 or greater than 10.The leaching concentration of Cr,Hg and Pb did not show clear relationship with the increase of liquid solid ratio while the leaching concentration of other elements all decreased monotonically with the increase of liquid solid ratio.The leaching rates of all elements increased with the liquid solid ratio.