| As a type of silicon-alumina solid waste,fly ash can be used to prepare ceramsite fracturing proppant by partially substituting bauxite due to the similar chemical composition,which contributes to the comprehensive utilization of fly ash resources.However,heavy metal elements in fly ash will be released during high-temperature calcination,causing harm to a certain extent.In this paper,the optimal preparation process of ceramsite proppant were determined by means of X-ray diffraction（XRD）,scanning electron microscope（SEM）and universal testing machine and other micro and macro analysis and testing methods.Then,by adding single heavy metal oxides,the mechanism of migration,transformation,solidification and volatilization of heavy metals in proppant at elevated temperature were studied,providing scientific guidance for reducing environmental pollution caused by the migration of harmful elements during high-temperature disposal of solid waste.The main contents include:（1）As the fly ash content decreases,the calcination temperature and milling time increase,which reduces the crushing rate of ceramsite proppant and increases the bulk density.In addition,the increase of calcination temperature and the extension of milling time significantly improve the sphericity of the proppant.The low-density ceramsite proppant with a bulk density of 1.22 g/cm3 and a crushing grade of 3K was prepared at 20%fly ash content,1400℃calcination temperature and 8h milling time.（2）The migration and solidification mechanisms of heavy metals mixed with different mass fractions（0.5%,1.0%,1.5%,2.0%）in the solid phase of ceramsite proppant as well as the influence on the proppant properties were studied.The experimental results show that copper irons promote the formation of mullite phase and partially reacts with S in the raw material to form Cu2S.Zinc irons react with other irons to form gahnite Zn Al2O4,which reduces the proppant strength.The chromium irons partially solidify into the mullite phase,resulting an increase of proppant strength.Lead irons has no obvious effect on the phase composition of ceramsite proppant,and the lead irons remaining in the solid phase is mainly distributed in the mullite phase.The cadmium irons wil be reacted and form Cd Al2Si2O8,which consumes quartz and increases the proppant strength.The order of heavy metal solidification rate is Cd>Zn>Pb>Cu>Cr.The heavy metal leaching concentration of ceramsite proppant under 2.0%heavy metal content all met the standard requirements.（3）The enrichment and volatilization rate of heavy metals were studied by using high temperature tube furnace and isokinetic sampling device.Pb,Cd and Zn are mainly volatilized and enriched at the front end of the collection device,accounting for 100%,89.1%and 94.3%of the total volatilization respectively;Sulfate element is mainly absorbed by the absorption liquid at the back end of the flue gas collection system,and its volatilization rate increases with the rise of content.The order of heavy metal volatilization rate is Pb>Cd>Zn.A mathematical model of heavy metal volatilization was established based on Fick’s second law,and the uncertainty of the model was less than 15%.The volatilization rate increased rapidly at the initial stage of calcination,and the growth rate of volatilization rate gradually slowed down with the prolongation of calcination time. |
