Re-release And Speciation Analysis Of Mercury In Byproducts Of Coal-fired Power Plants

Coal-fired power plants are the main cause of mercury emissions from human emissions.Trace elements of coal are burned with coal at high temperature through the furnace,and then pass through the flue gas purification device along with the flue gas.Some of the heavy metals are enriched in the coal-fired by-products,and some of them are emitted into the environment with the flue gas.With the introduction of the flue gas cleaning device,the increased output of desulphurized gypsum and fly ash has greater resource utilization value.Desulfurized gypsum particles are finer and have a higher taste,and can replace natural gypsum to produce wallboards.Fly ash can be used for the production of cement and concrete.During the process of resource utilization and storage of coal-fired by-products,there may be processes such as washing with acid rain and heating,and some of the mercury will be released.Therefore,the study of the form of mercury in heavy metals from coal-fired by-products provides theoretical guidance for the secondary resource utilization and storage.In this regard,many scholars at home and abroad have proposed various methods to study the form of mercury.Among them,the BCR continuous extraction method is widely used.Many scholars at home and abroad used this method to study the mercury form of FGD gypsum,but the recovery was relatively low.This article thinks that the low recovery ratio may be due to:(1)Hg0 release occurs when aqueous solutions are added to the solution;(2)Hg0 release occurs during heating in a water bath(90℃).The BCR method measures the mercury content in the leach solution.Because Hg0 is insoluble in water,this part of Hgo cannot be measured.The first design experiment verified the presence of Hgn release after addition of the aqueous solution,followed by the proposed improved BCR method.By selecting five kinds of desulfurized gypsum and three types of fly ash samples,BCR experiments and improved BCR experiments were conducted to obtain a more accurate form of mercury in coal by-products.Comparing the results of the two extraction methods,it was found that there may be significant mercury reduction during the heating process.Design experiments verified the presence of Hg0 release during the water bath heating(90℃)step.This study shows that aqueous leaching can promote the release of Hg0.In the process of adding aqueous solution,about 30%and 20%Hg0 of desulfurized gypsum and fly ash respectively escaped.Analysis of the speciation of mercury in desulfurized gypsum and fly ash shows that desulfurized gypsum has a strong ability to migrate mercury.More than 90%of the available mercury in desulfurized gypsum is mercury that may migrate out of the environment.The F5 residue is the most stable form and is not easily escaped from desulfurized gypsum or fly ash.Compared with desulfurized gypsum,fly ash has a higher proportion of F5(residual mercury),compared to fly ash has a higher environmental stability.Water bath heating can promote Hg0 release.In this experiment,approximately 25%and 20%Hg0 of the desulfurized gypsum and fly ash respectively escaped during the water bath heating.The amount of Hg0 released may be related to the content of Hg2Cl2 and HgCl2 in the sample.Comparing the experimental results of the two extraction methods,it can be found that the "improved BCR method" for desulfurized gypsum can increase the recovery ratio from about 50%to about 100%.The use of "improved BCR method"for fly ash can increase the recovery ratio from approximately 70%to approximately 100%.The "improved BCR method" can effectively improve the recovery ratio,can more accurately understand the form of mercury,and help to assess the mercury migration ability of the desulfurized gypsum and fly ash during subsequent resource utilization or during storage.