The Effect Of NaCl On The Volatilization Of Pb And Cd During Waste Incineration

With the advantages of small space occupation,high volume reduction rate and large disposal capacity,waste incineration has gradually become the first choice for municipal solid waste disposal.Municipal solid waste contains semi-volatile heavy metals,which are easy to volatilize and condense in the incineration process to generate sub-micron particles that can penetrate dust removal equipment.NaCl in municipal solid waste can promote the volatilization of semi-volatile heavy metals.In this paper,Pb and Cd were taken as the research objects and the role of NaCl in the volatilization of Pb and Cd in the incineration process was investigated.Firstly,the indirect chlorination effect of NaCl on PbO and CdO was studied.In the nitrogen atmosphere,the temperature above 1000? NaCl reacted with SiO₂ and/or Al₂O₃ and produced chlorine-containing gases.In the oxygen atmosphere,the initial release temperature of chlorine-containing gases was in the temperature range of 700?800? In this temperature range,the release of chlorine-containing gas increased with the increase of proportion of Al₂O₃.In the oxygen with water vapor atmosphere,the initial temperature of chlorine-containing gas was in the temperature range of 700?800?.The accumulative release rate was maximum when the ratio of Si to A1 was 0:1.When the ratio of Si to A1 was 2:1,the accumulative release rate was minimum.The accumulative release rate was in the middle when the ratio of Si to A1 was 1:0.Oxygen with water vapor atmosphere promoted the release of chlorine gas the most,followed by oxygen atmosphere and nitrogen the least.The XRD analysis results of the bottom slag in indirect chlorination test indicated that under nitrogen atmosphere,NaCl did not react with SiO₂ and/or Al₂O₃ when the temperature at 600?,800? or 1000?.In Oxygen atmosphere at 60?,NaCl did not react with SiO₂ and/or Al₂O₃;At 800? and 1000?,NaCl reacted with SiO₂ and/or Al₂O₃.The XRD results of bottom slag in oxygen with water vapor atmosphere were consistent with those under oxygen atmosphere.Then,the direct chlorination effect of NaCl on PbO and CdO was studied.NaCl promoted the direct chlorination of Pb/Cd by the interaction with Si-Al matrix.When the Si/Al ratio was 1:0,NaCl had the most obvious effect on the direct chlorination of Pb/Cd.When Si/Al ratio was 1:0,Pb volatilization initiated in the temperature range of 650?700?.Except the Si/Al ratio of 1:0,under the other Si/Al ratios,Pb volatilization initiated in the temperature range of 800?850?.When Si/Al ratio was 1:0,Cd volatilization initiated in the temperature range of 650?700?.Except the Si/Al ratio of 1:0,under the other Si/Al ratios,Cd volatilization initiated in the temperature range of 750?800?.Under the same reaction conditions,NaCl promoted Pb volatilization greater than Cd volatilization.The reaction of PbO/CdO with Si-Al matrix in direct chlorination process was analyzed.The reaction characteristics of PbO/CdO with Si-Al matrix demonstrated that the direct chlorination of PbO/CdO by NaCl with the participation of Si-Al matrix was completed in two steps.The first step is the reaction of PbO/CdO with Si-Al matrix to generate silicate,aluminate or aluminosilicate.The second step is the chlorination of NaCl with these reaction products to generate PbCl2 and CdCl2.Finally,the indirect and direct chlorination effects of NaCl on PbO and CdO were studied.The chlorination of NaCl was investigated in the atmosphere of oxygen with water vapor.Si/Al ratio of 1:0 was most beneficial to the chlorination of PbO and CdO.When Si/Al ratio was 0:1,NaCl had the worst promoting effect on the chlorination of PbO and CdO.When Si/Al ratio was 2:1,the promoting effect of NaCl on the chlorination of PbO and CdO was between the promoting effects of NaCl on the chlorination of PbO and CdO when the Si/Al ratios were 0:1 and 1:0.The greater the proportion of SiO₂,the greater the promoting effect of NaCl on the chlorination of PbO and CdO.Meanwhile,it was considered that higher temperature was beneficial to the chlorination of PbO/CdO.In the atmosphere of oxygen with water vapor,indirect chlorination and direct chlorination occurred at the same time,and the latter dominated this process.It was considered that the situation in the real atmosphere of the incinerator should be similar with this conclusion.