OPTICAL EMISSION SPECTROSCOPY AS A CONTROL TOOL TO IMPROVE FLY-ASH PLASMA VITRIFICATION

The development of industrial processes for vitrification of fly-ash from gas cleaning facilities is strongly imposed by regulations. For authorization of glassy products to be dumped in a common landfill or to be reused as a non hasardous standard material in road building, they will have not only to satisfy the leaching test procedure ensuring their long term durability, but also to contain certain heavy metals in quantities lower than critical limits. Consequently, it is highly required to control, in situ and in real time, volatility of these heavy metals during fly-ash melting and vitrification by plasma.
A twin torch plasma system, mounted above a cold crucible, filled with a known synthetic glass, has been used. A new optical emission spectroscopy method has been tested to examine metallic vapors above the melt. A numerical model has been developed : it is based on calculations of plasma volume radiation for spectral lines of the plasma forming gas and of metallic vapors using plasma temperature profiles along the observation direction. These temperature profiles are evaluated using measured intensities of spectral lines of the plasma forming gas. Metallic vapor concentration in the plasma above the melt is deduced from the ratio of the metal/gas lines intensities.
Distribution of lead vapor concentration along the crucible surface and concentration evolution with time have been examined. Also, the influence of operating parameters, such as plasma forming gas composition, has been demonstrated. The elemental analysis of the glass has been measured by Scanning Electronic Microscopy (SEM) with Energy Dispersive Spectrometry (EDS). The contribution of some elements diffusion within the melt to the entire volatility processus is taken into consideration. According to the data obtained, steep variations of the volatility of the elements depend strongly on redox potential of the gas phase near the melted surface.
A predictive model has been adapted to simulate the non-congruent vaporization of heavy metals, especially of lead, from the melt. The actions of the melt surface temperature, of the oxygen partial pressure, and of the melt composition have been evaluated.