Experimental Study On Vitrification Of Fly Ashes From Incinerator With A DC Thermal Plasma Torch

In many countries incineration is taken to dispose both municipal solid wastes and medical solid wastes, because it wins an advantage over weight and volume reduction, detoxification and energy recovery. However, a coming second pollution problem attracts more and more attentions, such as pollutant discharge in flue gas and the incinerator residues. Fly ashes as a kind of incinerator residues are high dangerous, for their containing high concentration of heavy metals and the high poisonous PCDD/Fs, and were listed in the hazardous wastes list which was regulated by the State Environment Protecting Bureau.Then a double arcs DC plasma torch was developed to vitrified fly ash and during the vitrification both the decomposing effects of PCDD/Fs and the solidification characteristics of heavy metals were investigated. And some physical and chemistry characteristics of vitrified slags also were studied.An oscillograph, an emission spectrogram and a light intension detector were employed to investigate the behaviors of plasma arc, such as voltage-current characteristics, arc fluctuation, excitated temperature of plasma arc, and the enthalpy and thermal efficiency of plasma jet. And the arc fluctuation and dynamic behavior were experimented under different operating conditions. The results showed that the arc voltage of Argon-Nitrogen plasma was much higher than that of Argon plasma and increased with increase in the proportion of Nitrogen. The fluctuation of plasma jet attributed to the undulation of power supply and the rapid motion of arc root on the anode surface. The temperature of plasma arc exceeded 13500K and the special enthalpy of plasma jet arrived at 5.0 MJ/kg～8.0 MJ/kg.Four kinds of fly ashes, of which two kinds were gotten from MSWI and other two kinds were gotten from medical wastes incinerator, were experimented on their element constitutes and their microstructure by the means of EDS and XRD. TCLP analysis (USEPA Method 1311) was employed on fly ashes in order to analyze the metals leachability and the results showed that the concentration of Zn in the leachate was highest among several kinds of heavy metals and the concentration of Cd in the leachate exceeded State permission level. USEPA Method 1613 was taken to analyze the distribution of PCDD/Fs and the results showed that the concentration of PCDD/Fs in fly ashes from Medical Wastes Incinerator more higher than that in fly ashes from MSWI, and in all the four kinds fly ashes the contribution of 2,3,4,7,8-pentachlorodibenzofuran (2,3,4,7,8-PeCDF) to the I-TEQ value was biggest among the seventeen toxic PCDD/Fs.After fly ashes were vitrified by the plasma arc, the most of PCDD/Fs were decomposed and the destruction rate of the total I-TEQ value was over 98%. A majority of heavy metals was immobilized in the vitrified slag and also the slag showed well retention of heavy metals. The TCLP experiments showed that the concentrations of heavy metals in the leachate were far lower than State permission level.Some additives were used to improve the effect of vitrification of fly ashes. Comparing with CaO, SiO₂ had more advantage of enhancing the destruction rate of PCDD/Fs and solidifying heavy metals in the slag.Then a Scan Electron Microscope, an X-ray diffractometer, Nitrogen adsorption meter and the Archimedes Method were taken to investigate physicochemical properties of the vitrified slag, such as exterior appearance, morphology, porosity and density. The results showed the morphology of vitrified slag was amorphous state which showed the glassy slag of SiO₂, the microstructure of slag was very compact and the special surface of slag minished nearly 95% of fly ashes. The density of slag increased almost three times to five times of fly ashes and the volume reduction of fly ashes was about 60%～80%.