Heavy metals behavior during thermal plasma vitrification of incineration residues

In the developed world, incineration of wastes is widely and increasingly practiced. Worldwide, a total of approximately 100 millions of tons of municipal solid waste (MSW) material is incinerated annually. Incineration of one ton of MSW leads to the formation of 30 to 50 kg of fly ash, depending on the type of incinerator. The waste disposal of these dusts already causes great problems today; they are of low bulk density, they contain high concentrations of hazardous water-soluble heavy metal compounds, organohalogen compounds (dioxines, furanes), sulfur, and chlorinated compounds. Thermal processes, based mainly on electrical arc processes, show great promise: the residues are melted at high temperature and converted in a relatively inert glass. A few tens of plants, essentially in Japan and Taiwan, have been in industrial operation for a few years. To be authorized to be dumped in a common landfill, the glassy product has to satisfy the leaching test procedure to ensure long-term durability. But to satisfy the regulation to be reused, for example as a nonhazardous standard material in road building, the glassy product would probably include contents in some heavy metals lower than critical limits. So today, there are two alternatives: the first one is to improve the heavy toxic metals evaporation to get a 'light' glassy product and to recycle separately the said separated metals; the second is on the contrary to improve the incorporation of a maximum of heavy metals into the vitreous silicate matrix. Whatever, it is highly required to control, in situ and in real time, volatility of these metals during ash melting under electrical arc. The objective of this work was to reach basic data about metals volatility under the plasma column of an electrical arc transferred on the melt: an experiment has been designed to examine the effects of processing conditions, such as melt temperature, melt composition, and furnace atmosphere, upon volatilization and glassy slag properties. A twin-torch plasma system, mounted above a crucible, filled with a known amount of synthetic glass and of toxic elements, has been used as experimental setup to reach basic data about metals volatility under the plasma column of an electrical arc transferred on the melt. Vapors above the melt have been probed by optical emission spectroscopy. Metallic vapors concentrations above the crucible surface and their evolutions with time have been examined; results for lead under various conditions are given; the influence of operating parameters, such as plasma-forming gas composition, has been demonstrated. Off-gases have been analyzed by mass spectrometry. Finally, a predictive model has been adapted to simulate the non-congruent vaporization of metals from the melt. Influences of oxygen partial pressure, of melt surface temperature and of waste chlorine content, on heavy metals depletion rates have been evaluated: results are given for lead and discussed in relation with experimental ones. (author)