Research Into Environmental Compatibility Of Desulfurization Gypsum In Mode Ecosystem Method

Flue gas desulfurization gypsum（FGD gypsum）is the largest industrial byproduct of FGD process.The utilization of FGD is one of the key links to optimize FGD process.However,the environmental compatibility in the utilization of FGD gypsum should be focused on.This paper investigated distribution and migration of impurity components and heavy metal components in FGD gypsum by the ecological model method.Further,the environmental compatibility of FGD gypsum was assessed.It also provided basic data for the safe practical utilization of FGD gypsum.There were two typical samples used in the research,which were from Wuhan region and Zhejiang region respectively with identifications of FA1 and FA2 each.Leaching experiment was carried out to study the migration rules of chromium,arsenic,nickel,and lead by column leaching mode and batch leaching mode.These two modes represented two natural environments of rainfall and surface water.The standard method of Tessier was used to analysis the occurrence forms of heavy metals components in FGD gypsum.The analytical results showed that the main parts of heavy metals components were transferable,while the stable form was the lowest proportion in FGD gypsum.Moreover,there were different proportions of different heavy components occurrence in the researched samples FA1 and FA2.So different heavy metal components presented different transferable characteristics.The samples of FA1 and FA2 were used.And the two main factors,leaching time and leachate pH value,were considered.Experimental results indicated that the transferable characteristics of heavy metals components were the combined action of occurrence forms,leaching time and leachate pH value.Long leaching time and low pH value promoted the migration in chromium,arsenic,nickel,and lead components.Leaching experiments of sample FA1 showed that the leaching rate of different heavy metal components in descending sequence was lead,chromium,nickel,and arsenic in batch leaching mode,and lead,arsenic,chromium and nickel in column leaching mode.Leaching experiments of sample FA2 showed that the leaching rate of different heavy metal components in descending sequence was chromium,nickel,arsenic,and lead in batch leaching mode,and arsenic,lead,nickel and chromium in column leaching mode.Heavy metal components in FGD gypsum could migrate in leaching experiments.The content of heavy metal in leachate almost exceeded the national standard.Therefore,FGD gypsum could bring risk to water environment.Thermal induced desorption method was used to study the emission rule of mercury components in nitrogen gas.Thermal desorbed mercury components at different temperature ranges were absorbed by Ontario’s water method and further quantitative analyzed by atomic fluorescence spectrometry.Experimental results showed that the main emission temperature ranges were from 250 ℃ to 450 ℃ and from 650 ℃ to 850 ℃ with the similar emission rules in samples FA1 and FA2.The Hg²⁺ components emission increased with temperature increasing,and then decreased,and further increased.Hg0 components reached to the peak in temperature range from 250 ℃ to 450 ℃.So there was a risk of re-emission especially in calcinations process of FGD gypsum at high temperature.Efflorescence of FGD gypsum was investigated under different curing conditions to explore the infiltration mode of salt in gypsum block.The distributions of sodium and magnesium components in gypsum block were determined.The phenomenon of efflorescence was the main drawback caused by sodium and magnesium components migrating from inner to surface in FGD gypsum blocks.Three main factors of curing conditions,soluble salt content and water-gypsum ratio were considered in experiments.Experimental results showed that the higher soluble salt content and the lower water-gypsum ratio,the more severe efflorescence.The soluble salt crystallized first at edge of the block.The reason was that soluble salt would migrate to the surface of gypsum blocks with the evaporation of moisture,and finally precipitate.This research showed that there existed exchange,migration and emission between impurity components and heavy metal components and environment.All these components from FGD gypsum to environments in the process of practical utilization and stock in environment of FGD gypsum would lead to micro-local pollution.However,the long term cumulative effect by the micro-local pollution could not be neglected.