Environmental Behavior Of Heavy Metals During The Thermal Treatment Of Residues From Imported Weee Dismantling

The fast and prosperous development of Waste Electrical and Electronic Equipment (WEEE) dismantlingindustry has brought considerable economic benefits for the developing countries, but it also has caused environmental pollution to a certain extent.Among all the pollution, the improper disposal of the residues was a bottleneck of WEEE dismantling industry itself. To counter this phenomenon, this paper will focus on the dismantling residues,analyse the background pollution, and employ an environmentally friendly process of preliminaryand themial treatment in both plot and pilot scale to study the environmental behavior of the heavy medals during the dismantling. The results indicated that the heavy metals were distributed in the tiny components, and the concentration of Cu、Zn、Pb in the dismantling residues was7180mg/kg,2783mg/kg,1954mg/kg respectively. Recycling can be an effective way to decrease the releasing risk of heavy metals. The optimal parameters of "sieving&washing" were obtained and applied to the pretreatment of heavy metals reduction. After been sieved and washed, the remained concentration of Cu、Zn、Pb、Ni、Cd in the dismantling residues was reduced to733mg/kg,1391mg/kg,1080mg/kg,71.5mg/kg and18.1mg/kg respectively, and the undersize components and the sediments can be recycled directly. The results of the further thermal treatment (plot scale) indicated that the operation condition of "7min of pyrolysis under650℃,5min of gasification under850℃, and10min of vitrification under1400℃" can minimize the emission of the heavy metals. In addition, the chemical speciation of heavy metals in the thermal treatment mostly was in the form of residuals and it was a stable environmental behavior. The results of the pilot scale test proved that after the treatment of pyrolysis, gasification, vitrification, the concentration of Cu, Zn, Pb in the bottom ash was1.5,2.4,2.5times of their input concentration, and the Zn, Pb, Cd in the fly ash was3.3,2.2,5.4oftheir input concentration. The migration behavior of heavy metals in the thermal treatment system was determined byevaporation, melting point and entrainment of the airflow, etc.. Cd showed the most obvious tendency of migration in this polit scale system, and Pb showed the highest amount of emission. After1t of dismantling residues was processed in this system,0.7g of Cu,4.67g of Zn,5.24gof Pb,0.034g of Ni, and0.011g of Cd will be discharged in the environment as gaseous state. This paper defines the emission and migration of heavy metals in the process ofpyrolysis, gasification and vitrification, and provides atheoretical foundation of the follow-up engineering treatment of the dismantling residues.