Pb And Cd Volatilization Characteristic And Mechanism During Waste Incineration

With the increasing of municipal solid waste(MSW) production, many cities have suffered the "junk siege" phenomenon in different degrees. Waste incineration, as a waste treatment technology with the advantages of reduction, harmlessness, recovery, has become the focus in the waste disposal study at present. But waste incineration inevitably brings secondary pollution, especially the heavy metal emission, which limits the application of waste incineration technology. Both experiments and simulated calculations were carried out to analyze the volatilization characteristic of Pb and Cd and the corresponding mechanism in depth.The effects of organic chlorine(PVC) and inorganic chlorine(NaCl) on the dynamic volatilization characteristic of Pb and Cd were investigated in a tube furnace. Through the study of the chlorine release characteristic of NaCl and PVC, the low temperature chlorination analysis of Pb and Cd by PVC and the thermogravimetric and differential scanning calorimetry analysis of the related substances, the mechanism of the effect of organic chlorine and inorganic chlorine on the volatilization characteristic of Pb and Cd were obtained. The experimental results showed that chloride could significantly promote the volatilization of Pb and Cd, and PVC had more significant impact on Pb and Cd volatilization than NaCl. But the mechanisms were different. During the incineration process, PVC released chlorine-containing gas to chlorinate Pb and Cd causing the volatilization promotion of Pb and Cd. NaCl did not release chlorine-containing gas, but directly chlorinated Pb and Cd to form the corresponding chloride causing the volatilization promotion with SiO2 and Al2O3 as medium.The effects of different forms of sulfur(sulfur and sodium sulfate) on the dynamic volatilization characteristic of Pb and Cd were investigated in a tube furnace. Through vulcanization analysis of Pb and Cd and the thermogravimetric and differential scanning calorimetry analysis of the related substances, the mechanism of the effect of sulfur on the volatilization characteristic of Pb and Cd were obtained. The experimental results showed that sulfur reacted with Pb to generate difficult volatile compounds and to inhibit the volatilization of Pb and Cd. Sulfur reacted with Cd weakly. Sulfur combustion in the incineration system enhanced the reducing atmosphere of the system to promote the volatilization of Pb and Cd. Sodium sulfate did not react with Pb and Cd, but absorbed heat and melted to inhibit the volatilization of Pb and Cd weakly.Finally, based on experimental research, HSC chemistry software was used to simulate the effects of atmosphere, temperature, chlorine, sulfur and calcium on the transformation characteristic of Pb and Cd. The simulation results were compared with the experimental results, which could further explain the corresponding mechanisms. Pb and Cd had different forms in the system under reducing atmosphere from those under oxidizing atmosphere. Chlorine could chlorinate Pb and Cd to form the corresponding chloride and to promote the volatilization of Pb and Cd under both oxidizing atmosphere and reducing atmosphere. Sulfur vulcanized Pb and Cd to form the corresponding sulphates under oxidizing atmosphere. However, sulfur vulcanized Pb and Cd to form the corresponding sulfides under reducing atmosphere. Under oxidizing atmosphere, calcium combined with chlorine to prevent the formation of PbCl2 and CdCl2 at low temperature. At high temperature, CaCl2 decomposed, chlorine began to combine with Pb and Cd to form PbCl2 and CdCl2. Calcium combined with sulfur to prevent the formation of PbSO4 and CdSO4 at low temperature. Under reducing atmosphere, calcium always combined with chlorine to prevent the formation of PbCl2 and CdCl2. Calcium did not combine with sulfur and had less effect on the transformation characteristic of Pb and Cd.