Effect of Unburned Carbon on Chloride-induced Volatilization of Heavy Metals from Molten Fly Ash

Abstract

The effect of unburned carbon on the chloride-induced volatilization of heavy metals was investigated using model and molten fly ashes, in the temperature range of 873-1173 K under N₂ gas flowing condition.
As a result, it was found that the volatilization ratio of lead and zinc from the molten fly ash increased with an increase in the temperature. Almost 100% of the lead and zinc were volatilized from the molten fly ash at 1173K. However, the volatilization ratio of lead and zinc from the model ash remained 85% and 25% respectively. It was found that metal oxides in the molten fly ash were converted to metal chlorides of PbCl₂, Pb₂OCl₂, ZnCl₂ and reduced metals of Pb, Zn, Cu by parallel reaction of chlorination with CaCl₂ and reduction with unburned carbon.
Reduction of metal oxides in the molten fly ashes was envisaged in terms of carbon equivalent ratio R_c. Almost 100% of the lead was volatilized at R_c=1.25-13.63 at 1173K. The volatilization ratio of 100% for zinc was also achieved at R_c=13.63. By contrast, the volatilization ratio of copper decreased with an increase in the R_c, resulting in less than 5% of copper volatilization at the R_c greater than 4.0.