High Temperature Flash Calcining Of Kaolin And Its Adsorption Characteristics Of Heavy Metal Pb

Kaolin can adsorb heavy metals at high temperatures.After being added to the furnace,Kaolin promotes the migration of heavy metal vapor to large particles that are easily trapped through chemical reaction and chemisorption,it can reduce heavy metal emissions from combustion,gasification and other processes of coal,biomass and garbage combined with dust removal equipment.Although,the researchers have worked on this technology for a long time,the adsorption characteristics and mechanisms are not interpreted well until now,especially in the aspect of the participation of water vapour.The structure distortion caused by dehydroxylation of kaolin surface should be closely related to the adsorption characteristics of heavy metals.In this paper,the characteristics of kaolin flash calcination in the furnace were studied firstly by downflow furnace with the temperature range of 800-1300? and residence time range of 0.5-3s.The hydroxyl groups of kaolin were divided into three types:E type(easy to remove,about 50%),D type(difficult to remove,about 40%)and U type(unable to remove,about 10%).The activation of hydroxyl groups was considered to be the first step in the removal of E-type and D-type hydroxyl groups.The kinetics model of dehydroxylation at 900-1200? was established based on the Arrhenius equation,with the apparent activation energy of 140kJ/mol and the pre-exponential factor of 1.32×106s-1.During the flash calcination at 800?,part of ?-coordinated Al in kaolin converted to V-coordinated Al for the removal of E-type hydroxyl groups and the formation of meta-kaolinite.At 1000?,more V-coordinated Al was produced with the removal of D-type hydroxyl groups,and some converted to ?-coordinated Al.When the temperature rose to 1200?,mullite was produced and part of V-coordinated Al converted to IV-coordinated Al and ?-coordinated Al.Based on this,this paper analyzed the flash calcination characteristics and possible effects of kaolin in waste incinerator and pulverized coal furnace.Then,the effect of water vapour on dehydroxylation and structural distortion characteristics of kaolin.It was found that water vapour converted to surface hydroxyl groups,resulting in the inhibition of dehydroxylation.This inhibitory effect increased with the increase of water vapour concentration.In addition,water vapour inhibited the formation of mullite,enhance the expansion of kaolin particles,and increased the specific surface area.It was speculated that the high concentration of water vapour in the waste incinerator enhanced the adsorption of Pb vapour(in the form of PbCl2 mainly)significantly,while the low concentration of water vapour in the pulverized coal furnace inhibited the adsorption of Pb vapour(in the form of PbO mainly).Finally,the adsorption characteristics of PbCl2 vapour and PbO vapour was investigated with the molar ratio of one Pb two 7 kaolin.The mechanism was analyzed based on the high temperature flash calcination characteristics of kaolin.The experiment confirmed the role of water vapour.The experiment results demonstrated the effect of water vapour in theory,which was that,water vapour can enhance the adsorption of PbCl2 vapour,but can inhibit the adsorption of PbO vapour.The increase of temperature enhanced the adsorption of PbCl2 vapour and PbO vapour.Water vapour and temperature have no significant effect on the adsorption products,which composed of Pb silicates and aluminosilicates.The morphology of products depended on the amount of Pb adsorption and temperature.The particles of PbCl2 adsorption product were formed by the bonding of thin and small sheets,while that of PbO adsorption products were multi-particle combination structure formed by the bonding of thick and large sheets.The reason is that the adsorption amount of PbO was greater than that of PbCl2,and the sheet melting was more significant.Combining the adsorption characteristics,product characteristics and high-temperature structural distortion characteristics of PbCl2 vapour and PbO vapour,the mechanisms of adsorption of Pb vapour by kaolin in the waste incinerator/pulverized coal furnace were comprehensively described,including the steps of participation/inhibition of water vapor at the microscopic angle and the collapse of the pore structure at the macroscopic angle.At 1000? and the water vapour concentration of 20%,the PbCl2 adsorption efficiency was 34%;at 1300? and the water concentration of 10%,the PbO adsorption efficiency was 78%.Considering that alkali metals in the furnace would also react with kaolin,it was speculated that the capture efficiency of Pb was less than 34%and 78%,resepectively in waste incinerator and in pulverized coal furnace.