Heavy Metals Volatility during Thermal Plasma Vitrification of Mineral Waste

Abstract

In the developed world, incineration of wastes is widely and increasingly practiced. The incineration produces residues, mainly bottom ash and air pollution control fly ash. Fly ash is especially problematic because of its high heavy-metal content. 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. The major toxic elements in the fly ash are lead, zinc, cadmium, etc., with different volatilities at very high temperature. Consequently, it is necessary to control the volatility of the heavy metals during plasma mineral waste melting and vitrification. 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. A predictive model has been adapted to simulate the noncongruent vaporization of heavy 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.